List of Abbreviations and Nomenclature

3.1 Introduction

Hydrocarbon gases in a reservoir are called a natural gas or simply a gas. Natural gas is found in several different types of rocks including sandstone, coal seams, and shales. Thus, the generic term natural gas applies to gases commonly associated with petroliferous (petroleum-producing, petroleum-containing) geologic formations. Natural gas occurs under pressure in underground cavities. It can be found in petroleum reservoirs or in other reservoirs as the sole occupant. The term natural gas refers to hydrocarbon-rich gas; it is a gaseous fuel that is found in oil fields, gas fields, and coal beds (Speight, 2007; Carroll, 2010).

Natural gas generally contains high proportions of methane, is the lightest hydrocarbon (a single-carbon hydrocarbon compound), and is from the group of paraffins (CH₄); its content may range from 50 to 90%. Some of the higher molecular weight paraffins (C_nH_2n+2) generally contain up to six carbon atoms, such as ethane, propane, butane, pentane, and, all the isomers of butane and pentane may also be present in small quantities (Table 3.1). The hydrocarbon constituents of natural gas are combustible, but non-flammable, non-hydrocarbon components can be also found in small amounts and are regarded as contaminants. There is no single composition of components which might be termed typical natural gas. Before its refining, carbon dioxide (CO₂), hydrogen sulfide (H₂S), and mercaptans (thiols; R–SH), as well as trace amounts of other constituents may be present. Sulfur content is among the most important characteristics of natural gas, as well as of crude oil. Sulfur in natural gas is usually found in the form of hydrogen sulfide (H₂S), which can reach up to 30% by volume. However, it should be noted that corrosive hydrogen sulfide can make natural gas extremely toxic (Chapter 1) and must be fastidiously removed throughout natural gas treatment. Moreover, H₂S causes an irritating, rotten egg smell in concentrations above 1 ppm and at concentrations above 10 ppm the toxicological exposure limits are exceeded (Chapter 1).

Methane and ethane constitute the bulk of the combustible components, while carbon dioxide (CO₂) and nitrogen (N₂) are the major noncombustible (inert) components. However, in its purest form, such as the natural gas that is delivered to the consumer, it is almost pure methane. Trace amounts of rare gases, such as helium, may also occur and certain natural gas reservoirs are a source of these rare gases. Just as petroleum can vary in composition, so can natural gas. Differences in natural gas composition occur between different reservoirs and two wells in the same field may yield gaseous products that are different in composition. Natural gas can be classified according to its constituents (Table 3.2) (Mokhatab et al., 2006; Speight, 2007, 2014; El-Gendy and Speight, 2016).

There are several general definitions that have been applied to natural gas. Thus, lean gas is gas in which methane is the major constituent. Dry natural gas contains <0.1 gallon (1 gallon, US, = 264.2 m³) of gasoline vapor (higher molecular weight paraffins) per 1000 ft³ (1 ft³ = 0.028 m³). Wet gas contains considerable amounts of the higher molecular weight hydrocarbons (paraffins), in fact, more than 0.1 gal/1000 ft³. Sour gas contains hydrogen sulfide, whereas sweet gas contains very little, if any, hydrogen sulfide. Residue gas is natural gas from which the higher molecular weight hydrocarbons have been extracted and casing head gas is derived from petroleum, but is separated at the separation facility at the wellhead (El-Gendy and Speight, 2016).

Associated or dissolved natural gas occurs either as free gas or as gas in the solution in the petroleum. Gas that occurs as a solution in the petroleum is dissolved gas, whereas the gas that exists in contact with the petroleum (gas cap) is associated petroleum gas (APG), while the non-associated gas is never linked to another product (El-Gendy and Speight, 2016).

In addition, natural gas condensate is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields (Mokhatab et al., 2006; Speight, 2014). As the name implies, the higher molecular weight constituents (typically hydrocarbon such as pentane, hexane, heptane, octane, and even nonane, and decane) condense out of the raw gas if the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas. The natural gas condensate is also referred to as simply condensate, or gas condensate, or sometimes natural gasoline because the hydrocarbon constituents fall within the gasoline boiling range. Additionally, condensate may contain additional impurities such as hydrogen sulfide (H₂S), thiols (RSH, also called mercaptans), and carbon dioxide (CO₂) (El-Gendy and Speight, 2016).

There are three sources for raw natural gas: crude oil, gas, and condensation wells. Natural gas that comes with crude oil is generally referred to as associated gas or wet natural gas. Natural gas created from gas and condensation wells, during which there is very little or no oils termed, is non-associated or free gas. Gas wells differ from condensation wells, since the former type produces raw natural gas only, while the latter produces natural gas along with very light liquid hydrocarbon, known as natural gasoline as a results of its high octane number.

The first drilling of a gas well was done by William Hart, the “Father of Natural Gas”, in 1821 in Fredonia, United States. Natural gas was discovered as a result of prospecting for crude oil drilling. Throughout the 19^th century, natural gas was used locally as a source of light due to the lack of a safe structure for long-distance gas transport. After World War II, natural gas was extensively used because of the advances in engineering that allowed the construction of safe, reliable, long-distance pipelines for gas transportation (Speight, 1993, 2007; Kidnay et al., 2011).

In its pure state, natural gas is colorless, shapeless, and odorless. It is a combustible gas and it gives off a significant amount of energy when burned (Speight, 2015). It is considered to be an environmentally friendly, clean fuel when compared with other fossil fuels such as coal and crude oil. The combustion of fossil fuels, other than natural gas, leads to the emission of enormous amounts of compounds and particulates that have negative impacts on human health (EIA, 1999).

Since natural gas is a reliable, cleaner-burning fuel, flexible, and plentiful, it underpins growing domestic and export production sectors. Moreover, natural gas is a very safe source of energy once transported, stored, and used. Consequently, now days it is used to generate electricity and to power appliances such as heaters and stoves. Not only for residential, commercial, and industrial heating, but also in many industrial processes including making fertilizers, glass, steel, plastics, paint, fabrics, ammonia production, and many other products. In the petrochemical industry, it is used as a feedstock or raw material, e.g. in the production of ethylene. Hydrogen, sulfur, and carbon black can be also produced using natural gas (Schoell, 1983; Mokhatab et al., 2006; Tabak, 2009; Faramawy et al., 2016).

Globally, natural gas accounts for 23.7% of primary energy consumption (BP Statistical Review of World Energy, 2015). The expected growth of the worldwide natural gas demands is 1.9% each year over the BP Energy Outlook (2015). Demand for natural gas will rise nearly 50% to 190 tcf in 2035, compared to 130 tcf for now. Gas demand in the forecasting period will be mainly driven by non-OECD countries, with growth averaging 3% p.a. to 2030, with the greatest demand coming from Asia (4.6% p.a.) and the Middle East (3.9% p.a.). Of the major sectors globally, growth is fastest in power (2.6% p.a.) and industry (2% p.a.) which matches with historic patterns. The usage by the power and industrial sectors accounts for over 80% of the expansion within the world demand for natural gas. Furthermore, compressed natural gas used in transport is confined to 2% of global transport fuel demand in 2030, with a three-fold increase from today’s level (BP, 2016).

Natural gas proved reserves in the Middle East and the Europe & Eurasian regions account for 75% of whole world’s reserves (Figure 3.1). However, 40% of the world’s natural or associated gas reserves, currently identified as remaining to be produced, representing over approximately 2600 trillion cubic feet (tcf), are sour with both H₂S and CO₂ present most of the time. Among these sour reserves, more than 350 tcf contain H₂S in excess of 10% and almost 700 tcf contain over 10% CO₂ (Lallemand et al., 2012; International Energy Agency, IEA, 2013; BP, 2016).

It is worthy to know that natural gas can also be formed through the degradation of organic matters by microorganisms, such as methanogens, that biologically break down organic matters to produce methane (biogas). They are found in areas near the surface of the earth that are devoid of oxygen. The produced methane, therefore, may sometimes escape into the atmosphere. However, sometimes, it may be trapped and probably recovered. Notably, methanogens also live in the intestines of most animals, as well as humans. In this regard, the production of methane (biogas) by livestock is a relevant contributor to the release of greenhouse gases into the atmosphere (Heinz, 2008). Thus, biogas is considered a renewable energy that can be used directly as a fuel (its heating value is between 15 and 30 MJ/Nm³) or as a raw material for the production of synthesis gas and/or hydrogen. Its main constituents are methane (CH₄) and carbon dioxide (CO₂), but biogas, like natural gas, also contains significant quantities of undesirable compounds (i.e. contaminants), such as hydrogen sulfide (H₂S), ammonia (NH₃), and siloxanes (Table 3.3). The percentage of those contaminants depends on the biogas source (Kwaśny et al., 2015; Barbusiński and Kalemba, 2016). Table 3.4 summarizes the sulfur (S) content of some typical biogas substrates in percent of fresh matter (Naegele et al., 2013).

Typically, methane content in biogas was reported to range between 35 and 75% vol. (Kwaśny et al., 2015). However, it is also reported that the biogas mainly comprises of methane (60–70 %) and carbon dioxide (30–40 %) (Wu et al., 2016). The H₂S content can vary between 100 to about 10,000 ppmv (0.0001–1 %vol) (Abatzoglou and Boivin, 2009). In other studies it is reported to range between 1–12 g/m³ (Wu et al., 2016) and 0.1–3 % (Li et al., 2016). Naegele et al. (2013) reported that critical H₂S can be formed in biogas under anaerobic conditions by sulfur and sulfate reducing bacteria from animal manure and renewable energy crops, while Yang et al. (2015) attributed the formation of H₂S during the anaerobic digestion process by specific microorganisms, such as sulfate-reducing bacteria (SRB), due to the existence of sulfur-containing compounds in substrates. Sometimes, mercaptans (such as CH₃SH) are detected in biogas, as they result from the anaerobic fermentation of S-bearing organic molecules (i.e., proteins). Thus, one of the main barriers that slow down the application and commercialization of biogas as an energy source is the existence of H₂S gas during its production process. Same as natural gas, these contaminates should be removed, as they are toxic, corrosive, have bad smell, and generate harmful environmental emissions, for example SO₂ and H₂SO₄. In a hydrocarbon-based fuel cell system, as one of the most potential and suitable energy conversion devices for generating electricity for both mobile vehicles and stationary power plants including residential applications, the catalysts used, such as reforming catalysts and water–gas–shift catalysts are poisoned by the produced H₂S (Song, 2002). Biogas can be used for power generation, as fuel for vehicles, or for incorporation into the natural gas network, but H₂S concentration should be no more than 300, 15, and 20 mg/m³, respectively (Wu et al., 2016).

Biogas, natural gas, synthesis gas, and Claus process tail gas may contain H₂S. For large-scale (>15 tons S/ day) gas treatment, amine absorbers and Claus plants can be used to remove H₂S. For smaller quantities, liquid redox systems, based on reaction with iron chelates, are used. In recent years, biological techniques have been applied more frequently in waste gas treatment systems because they eliminate many of the drawbacks of the classical physico-chemical techniques (Madox, 1985). Some of the disadvantages of these classical methods for gas treatment are that they require relatively large investments and operational costs (e.g. special chemicals, equipment corrosion, high pressures, and temperatures) and they require special operational safety and health procedures (Gadre, 1989). In addition, they may produce waste products (e.g. spent chemical solutions or spent activated carbon). On the other hand, biodesulfurization (BDS) processes can proceed at around ambient temperatures and atmospheric pressure, thus eliminating the need for heat and pressure, reducing energy costs to a minimum. Moreover, different kinds of bacteria proved high efficiency in different bioreactor setups (Sublette and Sylvester, 1987a,b; Cline et al, 2003; Amirfakhri et al, 2006; Ma, et al, 2006; Syed et al, 2006; Aroca, et al, 2007). H₂S can be converted into elemental sulfur (S⁰) via microbial processes. Various groups of organisms can oxidize sulfur compounds under aerobic or anaerobic conditions and reduce them to S⁰.

Energy has become a crucial factor for humanity to continue economic growth and maintain a high standard of living, especially after the inauguration of the 55 industrial revolutions in the late 18^th and 19^th centuries (International Energy Agency EIA, 2011). According to the IEA report, the world will need 50% more energy in 2030 than today of which 45% will be accounted by China and 57% by India (Shahid and Jama, 2011; Atabani et al., 2011). Consequently, there is a worldwide increase and demand for renewable energy sources. That comes with the increase in awareness about environmental protection. As a complementary or alternative for natural gas, biogas has attracted considerable attention within the scientific community (Mao et al., 2015; Horváth et al., 2016). For that reason, this chapter will cover, briefly, the research efforts on biodesulfurization of both natural gas and biogas.

3.2 Natural Gas Processing

The term gas processing is generally used to cover CO₂ removal, H₂S removal, water removal, hydrocarbon dew pointing, and gas sweetening. Gas sweetening may be a generic term for sulfur removal. Sometimes the term ‘gas conditioning’ is employed rather than gas processing. The actual practice of natural gas processing can be quite complex, but usually involves four main processes to remove the associated impurities oil and condensate removal, water removal, separation of natural gas liquids, and sulfur and carbon dioxide removal, as shown in Figure 3.2.

Carbon dioxide, hydrogen sulfide, and other contaminants are usually found in natural gas streams. CO₂, when combined with water, creates carbonic acid which is corrosive. CO₂ additionally reduces the British thermal unit (BTU) value of gas and, in concentrations of more than 2% or 3%, the gas is unmarketable. H₂S is an extremely toxic gas, causing a lot of environmental and health problems and it is also hugely corrosive to equipment (Chapter 1). The recovered hydrogen sulfide gas stream may be vented to atmosphere, flared in waste gas flares or modern smokeless flares, incinerated for sulfur removal and utilized for the production of elemental sulfur or sulfuric acid. If the recovered H₂S gas stream is not to be utilized as a feedstock for commercial applications, the gas is sometimes passed to a tail gas furnace during which the H₂S is oxidized to SO₂ and is then passed to the atmosphere out a stack.

Currently, the primary method of SO₂ disposal from flue gas is a scrubbing process with calcium-based sorbents (Bo et al., 2007), which results in huge amounts of waste. Currently, other adsorbents are used, for example CuO supported on alumina, titania, and zirconia showed a direct relationship between the adsorption capacity of the sorbents and the surface area of the support. Also, the capacity and efficiency of the materials were directly related to the CuO content. Finally, increasing the temperature enhanced the adsorption capacity and the efficiency of the sorbents (Flores et al., 2008). The Cu supported on alumina (CuO/γ–Al₂O₃) is reported to express higher ADS of simulated flue gas with high amounts of SO₂, relative to individual CuO and γ–Al₂O₃ (Qing-chun et al., 2015).

Moreover, the Ni impregnation on activated carbon and acid treated activated carbons was reported to enhance the ADS of SO₂ because nickel oxides can easily change their valence state, promote the redox cycle, and have also good oxidation performance because of their variable d electronic structure. Apart from the supported phase, carriers strongly affect the surface morphology and electronic structure of the metal particle, which can directly affect the stability and catalytic activity of catalysts (Efremenko and Sheintuch, 2003). Upon the ADS of SO₂ from a simulated gaseous mixture in a fixed bed flow microreactor, Ni- catalysts supported on untreated and acid treated coal-activated carbons were prepared by excessive impregnation method. The ADS activity of the prepared catalysts decreased in the following order: Ni/AC–HNO₃ > Ni/AC–H₂SO₄ > Ni/AC > AC (Guo et al., 2012). This was related to the specific surface area of those catalysts, where the specific surface area of the original activated carbons was 723 m²/g, which was less than that of the activated carbons treated with nitric (831 m²/g) and sulfuric acid (803 m²/g). The increment of specific surface area with acid treatment is due to the elution of some impurities on the AC by acid, especially with HNO₃ because of its high oxidative ability. Although, the specific surface area of Ni/AC was lower than that of AC, recording 697 m²/g, due to the presence of Ni, the ADS increased. The decrease in specific surface area is due to the presence of Ni species on activated carbons, which blocks some of the microporous surfaces. Moreover, acid treatment is known to affect the surface of functional groups of AC. Guo et al. (2012) noticed that the AC treated with nitric acid and sulfuric acid had significant increments in the less acidic C=O and C-O functional groups, which contributed in the enhancement of the ADS performance of the catalysts. Ni and NiO species coexist as microcrystals on the activated carbons before desulfurization, but after desulfurization Ni and NiO species disappear and Ni₂O₃ is observed, indicating that Ni species could be involved in the desulfurization reaction.

H₂SO₄ can be washed with excess condensed H₂O to recover the active sites and Ni₂O₃ can be retransformed into Ni and NiO and release O₂ in the H₂SO₄ medium. Thus, a new cycle of adsorption and oxidation of SO₂ and production and elution of H₂SO₄ begins. This continuous operation makes the removal of SO₂ possible.

The simultaneous desulfurization and denitrification are significant trends in the field of flue gas purification to reduce the cost of flue gas purification. Yi et al. (2007) reported the preparation of highly active absorbent fly ash, lime, and a few oxidizing NaClO₂ additives in which 93.7% and 65.5% removal of SO₂ and NO were achieved, respectively, at the optimal reaction temperature, additive quantity, humidity, and Ca/(S + N) molar ratio with this process shown to be approximately 60, 1.6%, 4.46%, and 1.2, respectively. The orthogonal experiments indicate that Ca/(S + N) is the main factor that influences the effectiveness of desulfurization and denitrification, which is followed by temperature and content of the absorbents.

Davini (2001) reported the good adsorption capacity of vanadium-impregnated palm shell activated carbon for SO₂ relative to Ni-impregnated and Fe-impregnated palm shell AC. Sumathi et al. (2010) also reported the preparation of metal modified palm shell activated carbon by impregnation with vanadium and cerium metal oxides (V₂O₅ and CeO₂), which expressed good removal of SO₂ and NO in gas streams. This was mainly attributed to the powerful oxidizing activity of cerium and vanadium (that is, the redox conversion between Ce³⁺ and Ce⁴⁺ and V⁵⁺ and V³⁺) and their oxygen storage properties which could bring new oxygen surface groups, such as C–O, C=O, and COOH on the surface of the palm shell AC (Kaspar et al., 1999; Tian et al., 2009) where the maximum recoded adsorption capacity of 121.7 mg SO₂/g cerium modified AC and 3.5 mg NO/g cerium modified AC was at optimum conditions of 10% loading of cerium and at 150 °C. It was also observed that the cerium modified AC expressed higher adsorption capcity towards SO₂ and NO at temperatures ranging 100–300 °C than the vanadium modifed AC. However, it was also noticed that lower temperatures are more favorable for the removal of SO₂, while higher temperatures are more favorable for NO removal. The good removal of NO and SO₂ has been described by adsorption and reduction of cerium and vanadium metal in presence of oxygen, throughout the chemisorption of SO₂ and NO on the catalyst surface, then the transfer of oxygen from the catalytic sites to the carbon active sites and, finally, the desorption of oxygen from the carbon surface (Illan-Gomez et al., 1999; Zhu et al., 2000). Moreover, the capability of cerium and vanadium to develop the reduction of NO to N₂ would have indirectly contributed to the highest breakthrough time of NO in this system.

At relatively low temperatures (150–250 °C), metal doped carbons express high catalytic activity due to the disassociation of NO chemisorption, which is accompanied with N₂O and N₂ evolution and oxygen accumulation on the catalyst surface (Mehandjiev et al., 1997). However, CeO₂ capability for the storage and release of O₂ through the redox shift between Ce⁴⁺ and Ce³⁺ under oxidation and reduction conditions, is low at this relatively low temperature range (Qi and Yang, 2003). This would explain the moderate removal of NO from simulated flue gas at relatively low temperatures (<150 °C). However, at higher temperatures the Ce-impregnated palm shell AC will be in a more active state to oxidize and reduce NO. Not only this, but it is also known that at high temperatures, AC itself could decompose NO and reduce it to N₂ (Mehandjiev et al., 1996). Further, higher temperature (>150 °C) means less water accumulation on the sorbent surface, allowing the metals to be more active. But, at higher temperatures (>250 °C) SO₂ molecules lose their kinetic energies, making the adsorption an exothermic process, which would indirectly lessen the adsorption of SO₂ onto the pores of metal impregnated AC (Guo and Lua, 2002).

Pyrolusite is an ordinary and economical mineral resource, with main metal oxide components of MnO₂, Fe₂O₃ and a few other transition metals. It was used to modify walnut shell-derived column activated carbon by blending method. Then, the activity of the prepared adsorbents was compared with that of MnO₂- and F₂O₃- modified activated carbons to study and investigate the advantages of pyrolusite addition to AC in the development of proper physiochemical properties and desulfurization activity compared to single metal oxide addition (Fan et al., 2013). The desulfurization experiments showed that pyrolusite loaded carbons performed the best toward the removal of SO₂. Upon the optimal dosage of additives, the maximum sulfur capacity of activated carbon loaded by pyrolusite, MnO_2, and Fe₂O₃ were 227.8, 157.8, and 140.6 mg/g, which were 84.0, 27.5, and 13.6% higher than that of blank activated carbon, respectively. Thus, the desulfurization activity of prepared samples with the optimal additive dosage was in the order: pyrolusite-AC5 > MnO₂–AC10 > Fe₂O₃–AC2 > AC (the number after AC, represents the dosage of the loaded metal oxide). The decrease of sulfur capacity with the increase of the additive loading ratio might be related to the pore blockage caused by excess additive loading amounts. However, the remarkable performance of pyrolusite loaded activated carbon is related not only with its better development of texture property, higher surface area, larger pore volume, and relatively larger content of surface functional groups, but also the synergistic catalytic oxidation of SO₂ by manganese, iron, and other transition metals in pyrolusite. It was concluded from that study that the modification of AC from agricultural wastes by pyrolusite using a simple blending method is a very promising technique for improving the sulfur capacity of AC. This is due to its remarkable desulfurization activity, low cost, and relatively easy preparation process.

The proper ratio for metal loading on AC promotes the formation of specific porosity due to the reaction between the added metal oxides and the carbon substrate during carbon gasification process. However, excess addition of metal particles may promote aggregation and the formation of large crystals during preparation which would result in pore blockage and surface area decrease (Lee et al. 2002; Nguyen-Thanh and Bandosz 2003). Fan et al. (2014) reported the preparation of metal oxide modified activated carbon from walnut shell. The transition metal oxides powders (Co₂O₃, Ni₂O₃, CuO and V₂O₅) were blended with walnut shell carbon chars in presence of hot coal tar, which served as the major binder, and carboxymethyl cellulose (CMC) and/or polyvinyl butyral resin (PVB), which served as accessorial binders, then were activated at 900°C in the presence of CO₂ for 2 h. The prepared column activated carbons were named AC, AC-Co, AC-Ni, AC-Cu, and AC-V based on the metal oxides that were used for modification. A weight percentage attached refers to the metal oxide dosage. The pure AC prepared from walnut shells proved a good ADS capacity of 123.8 mg SO₂/g. The best metal oxide loading percentage was 2% and recorded an ADS capacity of 133.3, 147.5, 172.4, and 181.2 mg SO₂/g of AC-Co2, AC-Ni2, AC-Cu2, and AC-V2, respectively (where 2 refers to the percentage of metal oxide loading, i.e. 2%), which were 7.7, 19.1, 39.3, and 46.4 % higher than the AC ADS sulfur capacity, respectively. It is worth noting that for the 2 wt.% metal loading, the change of surface area and pore volume of AC was not evident. Thus, the development of texture properties was not the main reason for their better ADS. Thus, the increase in ADS capacity was related to the improved generation of oxygenate functional groups on the carbon surface with metal oxide additions. Also, the increment of basic functional groups is beneficial for the SO₂ adsorption process on AC due to the binding affinity. Thus, since AC-Cu2 possessed the highest basic functional group content, it exhibited good desulfurization activity. Moreover, the catalytic cycle depends on the change between chemical valences of the metal species. The complex metal species component in AC-V2 could partially explain its higher sulfur capacity compared to the other metal modified activated carbons. Thus, the improvement of sulfur capacity mainly resulted from the additional basic functional groups that were generated after modification and the high catalytic activities of the active metal oxides that were formed in the presence of O₂ during the desulfurization process. According to Doornkamp and Ponec (2000), the oxidation state of the added metal oxides is partly reduced during carbon activation. In the oxidative catalytic reaction, the gaseous oxygen is first absorbed onto these reduced and intermediate state metal species and converted into lattice oxygen. Active metal oxides with much higher catalytic activity are then formed by the reaction between the reduced and intermediate state metal species and the lattice oxygen atoms. For example, for the V₂O₅ modified AC V³⁺, V^2+, and V⁺ species coexist on the carbon matrix after activation (Li et al., 2009). Next, these active metal oxides reacted with SO₂ in the desulfurization system to form SO₃. This process was followed by H₂SO₄ generation with H₂O. However, the resulting metal species in their low oxidation state would be oxidized by O₂ in the desulfurization system to form active metal oxides again. Briefly, this catalytic cycle depends on the change between chemical valences of the metal species.

Song et al. (2014) examined the feasibility of the application of activated carbon from waste tea in desulfurization and denitrification of gas stream. The results showed that less graphitization, lower microspore size, and a more nitrogenous basic group of the adsorbent enhanced its desulfurization ability. When well-developed mesopores were present in the adsorbent, the NO removal efficiency was decreased while more nitrogenous basic groups promoted the removal rate of NO. However, upon the simultaneous removal of SO₂ and NO, competing adsorption occurred. The removal efficiencies of SO₂ and NO increased in the presence of oxygen and steam in the flue gas. The adsorption of SO₂ and NO onto waste tea activated carbon, in absence of oxygen and steam, was physosoprtion, while the steam and oxygen promoted the chemical adsorption of SO₂.

Xiao-Li et al. (2014) reported the preparation of Fe catalysts supported on activated carbon, untreated and treated by HNO_3, using an ultrasound-assisted incipient wetness method. The desulfurization activity of the prepared catalysts was evaluated at a fixed bed reactor under a simulated flue gas. The results revealed that the adsorption capacity of the AC to SO₂ was 74 mg/g, which increased upon acid treatment to reach 84 mg/g. That was attributed to the increase of surface oxygen functional groups on activated carbon after HNO₃ treatment. The Fe-loading further increased the activity of the AC to reach 244 mg SO₂/g. This was related to iron species (active site) which promote the transformation of the adsorbed SO₂ into other stable forms. The ADS capacity was further increased for Fe-loaded-acid treated AC (Fe/AC-HNO₃) reaching 322 mg/g, owing to the different functional groups. The increase of mesopores volume is in the order AC < AC-HNO₃ < Fe/AC < Fe/AC-HNO₃. The increase of the mesopores may enhance adsorption abilities of samples, especially for large molecules of adsorbents, as SO₂ molecules. Furthermore, the average pore sizes of Fe/AC (or Fe/AC-HNO₃) slightly increased compared with that of AC (or AC-HNO₃), which was attributed to the aggregation of Fe species. The larger pore size provides a fast transporting channel for gas molecules (Li et al., 2013) which is consequently conducive to the SO₂ adsorption. The Fe₃O₄ species were found to be the main active phases in the prepared samples and expressed excellent desulfurization activity at 353 K, but Fe₂(SO₄)₃ was detected after desulfurization (Xiao-Li et al., 2014).

The adsorption of SO₂ on the surface of AC, in the presence of O₂ and H₂O, occurs as follows (Bagreev et al., 2002):

While on the basis of free-radical mechanism (Fu et al., 2007), the adsorption of SO₂ on the surface of Fe₃O₄ can occur as follows:

As detected from the above equations, upon the establishment of the Fe²⁺ and Fe³⁺ redox cycle, Fe₃O₄ reacts with generated H₂SO₄ on the carbon producing Fe₂(SO₄)₃. The generated Fe₂(SO₄)₃ gathered around the micropore mouth and surface and partially or totally blocked the micropores, leading to the increase in mass transfer resistance, which was the main reason for the deactivation of iron supported on activated carbon. This was proven by the SEM-analysis. The Fe₃O₄ on the carbon surface was considered the active center for SO₂ oxidation, whereas micropores are related with the diffusion of reactive molecules. When the pores are blocked by the generated Fe₂(SO₄)₃, which have larger molecular volume than Fe₃O₄, active sites are covered and isolated from SO₂ and O₂. Therefore, the blockage of Fe₂(SO₄)₃ may be the main reason for the deactivation of iron supported on activated carbon (Xiao-Li et al., 2014).

Dube et al. (2015) investigated the use of bagasse ash as a siliceous material and ammonium acetate as a hydrating agent to enhance the removal of sulfur dioxide (SO₂) in a low temperature dry flue gas desulfurization (FGD) process using magnesium-based sorbent, where response surface methodology (RSM) was used to determine the effects of hydration temperature and time, amount of ammonium acetate, and bagasse ash on the surface area of the sorbent. A polynomial model was developed to relate the preparation variables to the sorbent surface area. The Brunauer-Emmet-Teller (BET) surface area results showed that the sur face area increased from 56 m²/g to 219 m²/g when the bagasse ash and ammonium acetate were used. This enhanced the ADS of SO₂ in the dry FGD process. The desulfurization experiments performed using a thermogravimetric analyzer (TGA) and (SO₂) removal efficiency reached approximately 99.9% within 2 h. The desulfurization reaction kinetics between SO₂ and the magnesium hydrated sorbent was described by an unreacted shrinking core model (USCM). The model showed the formation of a nonporous shielding layer that stopped further occurrence of gas (SO₂)/solid (magnesium sorbent) reaction and that product layer was determined to be the rate limit. Moreover, the scanning electron microscope (SEM) results illustrated the plugging of pores with the reaction products (for example magnesium hydrated sulfate silicate) during the sulfation process that led to a non-porous surface. The amount of bagasse ash was found to display the greatest influence on the surface area, followed by the hydration temperature and hydration period, respectively, while the amount of ammonium acetate showed a slight influence. The surface area increased as the amount of bagasse ash, hydration temperature, and hydration time increased, but a minimal surface increment occurred when more ammonium acetate was used. The USCM model elucidated that the reaction would have taken place throughout three sequential steps: (1) diffusion of SO₂ onto the sorbent surface, (2) reaction SO₂ and the sorbent on the sorbent surface, and (3) diffusion of SO₂ through the product (ash) layer towards the surface of the unreacted core.

The following sequential mechanism could explain why the hydrating agents enhance the sorbent surface area (Fillipou et al., 1999; Birchalm et al., 2001; Rocha et al., 2004):

Finally, the dissociation of magnesium complexes occurs and the magnesium hydroxide precipitation in the bulk of the solution formed due to the super-saturation. This is the main reason why hydrating agents improve the surface area of sorbents to be used in FGD.

While the following sequential reactions could explain why the siliceous additive, that is the bagasse ash, enhances the sorbent surface area (Dube et al., 2015):

Coconut-shell-based activated carbon was reported to exhibit good desulfurization activity for the simultaneous removal of H₂S and SO₂ from simulated Claus tail gas performance under a feed gas of H₂S (20,000 ppmv), SO₂ (10,000 ppmv), and N₂ (balance). The concentrations of H₂S and SO₂ in the simulated Claus tail gas reduced to less than 10 mg/m³. The breakthrough sulfur capacity of coconut-shell-AC was 64.27 mg of S/g of sorbent at an adsorption temperature of 30 °C and a gas hourly space velocity of 237.7 h^–1. Micropores with sizes of around 0.5 nm in the AC were found to be the main active centers for adsorption of H₂S and SO₂, whereas mesopores showed little desulfurization activity for deep removal of H₂S and SO₂. Both physical and chemical adsorption coexisted in the process of desulfurization. The majority of sulfides were removed by physical adsorption and 11% of the sulfur compounds existing in the form of elemental sulfur (ca. 20 atom %) and sulfate (ca. 80 atom %) were derived from chemical adsorption. The mechanism of H₂S and SO₂ adsorption on the AC can be briefly summarized as follows: H₂S and SO₂ are first adsorbed on AC by physical adsorption and then partially oxidized to elemental sulfur and sulfate, respectively, by the oxygen adsorbed on AC. At the same time, the Claus reaction between H₂S and SO₂ occurs. The prepared AC was able to completely regenerate using water vapor at 450 °C with a stable breakthrough sulfur capacity during five adsorption–regeneration cycles (Shi et al., 2015).

Isik-Gulsac (2016) studied the effects of relative humidity, carbon dioxide (CO₂), methane (CH₄), oxygen (O₂) presence, and gas hourly space velocity (GHSV) on H₂S adsorption dynamics of KOH/CaO impregnated activated carbon. The presence of water, O₂, and lower GHSV has beneficial effects on the activated carbon performance. However, CO₂ decreases the adsorption capacity due to its acidic characteristics and competition in adsorption between CO₂ and H₂S on the basic sites of activated carbon. The best adsorption capacity is obtained as 13 wt % in KOH/CaO impregnated activated carbon, in a CH₄ (60%)/CO₂ (38%)/O₂ (2%) gas atmosphere at ambient temperature, a relative humidity of 90, and 5000 h^–1 GHSV. The specific surface area, micropore area, average pore diameter, total pore volume, and external surface area were found to be significantly decreased after the ADS. That was attributed to the deposition of the sulfur or sulfurous products of catalytic oxidation in the pores, which consequently decreases the surface area.

If H₂S and CO₂ are present in wet conditions, the following occurs:

As observed in the reactions, CO₂ transport is facilitated by bicarbonate ions and H₂S is similarly transported in the form of bisulfide ions. In sufficiently thin films, diffusion times are so short that negligible amounts of CO₂ are converted via the slow first two reactions that are responsible for the production of bicarbonate. However, steady-state carbonate, bicarbonate, and hydrogen ion concentrations are affected by the presence of carbon dioxide and the extent of reaction responsible for dissociation of H₂S is a function of pH. Thus, although the first two reactions have negligible effects upon CO₂ transport, their occurrence affects the permeation of H₂S because of their influence on the pH (Meldon and Dutta, 1994). Additionally, Bandosz (2002) and Bouzaza et al. (2004) stated that high-valent sulfur compound formation is promoted and H₂S dissociation is limited in an acidic environment, thus the removal of sulfur decreases.

Isik-Gulsac (2016) illustrated the mechanism for ADS of H₂S on KOH/CaO impregnated activated carbons as follows:

Physical or ionic adsorption of H₂S on a porous carbon surface in dry or wet conditions

should be taken into consideration and the basic environment favors the formation of S^2–, which may promote the oxidation process and formation of elemental sulfur and partial conversion into sulfuric acid (Adib et al., 1999).

Chemical adsorption of H₂S on metal oxides with sulfur formation:

In the presence of CO₂, reactions between KOH and CO₂ also occur which weaken the catalytic activity of the impregnated activated carbon for ADS.

The presence of oxygen enhancing the rate of ADS should also be taken into consideration because oxygen and H₂S adsorb on the free active sites of the carbon surface (C_f) and then react catalytically with the formation of elemental sulfur, sulfur dioxide, or sulfuric acid (Puri 1970; Choi et al. 2008):

Moreover, in the presence of oxygen and humidity, CaSO₄ can be formed on the surface of activated carbon (Seredych et al., 2008):

Villegas (2016) reported the application of activated carbon prepared from different agricultural wastes including tropical bamboo, central American mahogany seed shells, Honduran mahogany seed shells, mamey zapote seeds, and corncobs in cleaning the combustion gases of diesel engines with CO, SO₂, NO_2, and H₂S, throughout a physical adsorption process.

Liu et al. (2017) reported the preparation of nitrogen-doped coconut shell activated carbon catalysts were prepared by urea or melamine impregnation followed by heat treatment and used for the removal of methyl mercaptan (CH₃SH) in a fixed bed quartz reactor. The AC sample has a specific surface area of 1508 m²/g, a micropore volume of 0.571 cm³/g, and a total pore volume of 0.807 cm³/g. Nitric acid oxidation increased the total pore volume and decreased the micropore volume and specific surface area. This was attributed to the erosion and pore-widening effects of nitric acid treatment. Also, there was a small recorded decrease in the micropore volume with a small recorded increase in the total pore volume in the AC modified with urea, while the melamine-modified AC recorded a decrease in the micropore and total pore volumes. This was attributed to the introduced nitrogen-containing species, which create steric hindrances and partially prevent the access of nitrogen molecules into the micropores. The SEM analysis proved the porous structure of the prepared catalysts. Moreover, it is worth noting that the acid treatment with HNO₃ increased the acidic groups on the surface of AC, while the nitrogen doping increased the number of basic groups. The nitrogen in acidified AC by HNO₃ (ACO) is mainly in the form of nitrate and nitric oxides, which were introduced by the nitric acid treatment. The melamine-modified activated carbons (ACM or ACOM) samples have more nitrogen than those modified with urea (ACU or ACOU). This was attributed to the high content of nitrogen in the melamine molecule and its conversion to melamine resins at high temperatures. The dominant nitrogen species on the surface of nitrogen-doped activated carbons (ACU or ACM) are pyridinic-nitrogens. However, nitric acid oxidation leads to higher relative contents of pyridine-N-oxide and less pyrrolic-nitrogen. This was attributed to the nitric acid oxidation which increased the acidic groups and created more active sites on the surface of activated carbon, which is beneficial for nitrogen incorporation into the carbon matrix in a pyridinic-like nitrogen configuration. In addition, a small amount of pyridinic-nitrogen can be converted into pyridine-N-oxide. The ACM or ACOM contained lower relative amounts of pyridinic-nitrogen and quaternary-nitrogen than the corresponding ACU or ACOU. However, the melamine modification provided more nitrogen. Thus, the overall contents of pyridinic-nitrogen and quaternary-nitrogen in the activated carbons increased when melamine was used as a precursor. The CH₃SH capacity of the original activated carbon (AC) was only 161.8 mg/g. Nitrogen-doping increased the ADS capacity and the highest breakthrough CH₃SH capacity of approximately 602.1 mg/g was achieved by the ACOM sample with a surface nitrogen content of 4.41%. This was attributed to the pyridinic-nitrogen and quaternary-nitrogen which are known to be beneficial to the oxidation of S- and N- containing compounds because of their strong electron transfer abilities (Bashkova et al., 2003; Bashkova and Bandosz, 2009; Sun et al., 2013). Pyridinic- nitrogen can act as a Lewis basic site with a lone electron pair, promoting the dissociation of CH₃SH to thiolate ions. The breakthrough CH₃SH capacity of AC in the absence of moisture is at least three times smaller than that in the presence of moisture, while the breakthrough CH₃SH capacity of ACOM, under dry air conditions, is almost twice that measured without oxygen. Thus, water and oxygen are beneficial to remove CH₃SH. A thin water film can be formed on the carbon surface in the presence of water (Sun et al., 2013) where CH₃SH dissolves into the water film and dissociates to the thiolate ion, which can be further oxidized to CH₃SSCH₃. In the absence of moisture, CH₃SH is first adsorbed onto the carbon surface and then the adsorbed CH₃SH can be oxidized to CH₃SSCH₃, which can be easily removed from the carbon surface. Thus, the regeneration of the adsorbent was easy and can be reused for four successive cycles retaining a CH₃SH capacity of 88.33%. Pyridinic-nitrogen and quaternary-nitrogen occupy the high-energy states (Bashkova et al., 2005). The electrons could be transferred from the thiolate ion to the adsorbed oxygen, forming thiolate radicals and superoxide ions because the two types of nitrogen groups enhance the ion-exchange properties of the carbons. These superoxide ions can react with water forming the active hydroxyl radicals. All of these species could also facilitate oxidation. Finally, water and CH₃SSCH₃ are formed and stored in the pores. Considering the presence of water and active sites, such as hydroxyl radicals and oxygen radicals, CH₃SSCH₃ can be further oxidized into methyl methane thiosulfonate and methanesulfonic acid.

The aforementioned examples briefly promote the efficiency of using agricultural wastes as feasible alternatives for granular activated carbons preparation for the bioadsorptive desulfurization (BADS) of gas.

3.3 Desulfurization Processes

As already explained, there are several reasons why sulfur components need to be removed from natural gas before the gas may be ‘shipped’. Even when further processing is to be done at site, sometimes sulfur removal up front will be required. This is very much the case in downstream plants where catalysts are used for processing. How sulfur components are removed will be subjected to local conditions and requirements. Engineers should always attempt to make the processing as cost efficient as possible under the constraints imposed.

Natural gas can be classified according to the amount of sulfur content (generally H₂S) within the produced gas (Rojey et al., 1997; Carroll, 2010; Kidnay et al., 2011, Speight, 2015 and Faramawy et al., 2016). In this classification, the natural gas could be sweet or sour. Sweet gas contains no or a negligible quantity of H₂S, whereas sour gas contains unacceptable quantities of H₂S (more than 5 mg/Nm³) (Maddox, 1974). There are several technologies available for removing H₂S from natural gas. They may be classified in six main classes, as illustrated in Figure 3.3.

3.3.1 Scavengers

There is no accurate definition of scavengers, however, the term is employed to explain chemicals injected into the gas stream to react with the H₂S, mainly without side reactions. The removal or scavenge of acid gas impurities, like CO₂ and H₂S, from industrial gas streams is a significant operation in natural gas processing, hydrogen purifying, refinery off-gas treating, and synthesis gas for ammonia and methanol manufacturing. The industrial gas streams containing acid gas impurities must be purified in order to meet the requirements of the gas mixtures sequential processing (e.g., avoiding catalyst damage) or environmental regulation (exhaust of the gas mixtures as off-gas) (Lu et al., 2006). The available methods to scavenge the gases can be largely divided into physical, chemical, and biochemical (Satoh et al., 1988). These chemicals may be injected into pipelines or applied in special scavenging towers. They are generally used once the H₂S level is below 200 ppm (Dalrymple et al., 1994). According to Dalrymple et al. (1994), H₂S scavengers can be categorized into three categories (Figure 3.4).

A scavenger ought to meet a number of criteria to be effective, such as the product should be able to efficiently remove all sulfide species under field condition, these being pH, pressure, and temperature and the reaction should be reliable, speedy, and irreversible. Moreover, the reaction products should be simple to dispose to the environment, the chemical should not exhibit incompatibilities with different components of the fluids, the chemical overdosing should not give rise to system difficulties, the chemical should not be corrosive, the chemical should be safe to handle and nonpolluting to the environment, and the chemical should be readily available and cost-efficient (Nassar et al., 2016).

Different scavengers are employed where the water-soluble scavengers are among the foremost common scavengers and are often the product of choice for applications at temperatures below 200 °F (Nagi, 1997). Oil-soluble scavengers are employed in high-temperature applications or when the water tolerance of the hydrocarbon is a problem (Garrett et al., 1988). Metal-based scavengers answer the particular wants of very high temperature and high-H₂S concentration applications. These additives are used at temperatures above 177 °C to create thermally stable products and are able to achieve H₂S reduction levels other than H₂S scavengers (Speight, 1993).

The H₂S scavenge from a gas stream can be carried out by adsorption onto a solid surface, catalytic oxidation, and absorption by a liquid solution (amine/alkaloamine) (Polychronopoulou et al., 2005). However, many problems exist with the catalysts employed in these processes including the cost of renewing the inactivated catalysts, as well as the generation of secondary substances causing pollution and high energy requirement (Wubs et al., 1991).

McKinsey (2003) published results on cow-manure compost as H₂S removal media. The data was rather inconclusive. The removal efficiency reported was around 80% for a gaseous stream containing 1500 ppmv H₂S. The removal rate was estimated to be 16–118 g H₂S/m³ solids/h for residence times (empty bed) of 100 s. He did not have sufficient data to distinguish between the physical, chemical, and biological mechanisms of H₂S retention. The main utility of this work is the fairly comprehensive presentation of available technologies, including those essentially employed in scavenging H₂S in the oil industry on a large scale (solvent-based absorption and solid oxide scavengers). In addition, this review fairly well covers the technical and market survey in the field.

Kandile et al. (2014) prepared three H₂S scavengers by reacting monoethanolamine with formaldehyde in different ratios (1:1, 2:1 and 2:3) to give MF1, MF2, and MF3, respectively. The efficiency of scavengers increased with an increasing reaction time up to 50 min. Also, as concentration of scavengers and temperature increased, the removal efficiency of the scavengers increased. By comparing the efficiency of the prepared products with the commercial products EPRI-710 and EPRI 730, it was found that MF3 exhibited a similar efficiency comparing with the commercial scavenger EPRI 730 (currently used in the field) at different concentrations and temperatures.

One method that is usually used to overcome the issues related to the chemical treatment of H₂S, is the oxidation into elemental sulfur employing a metal chelating agent in the form of a liquid catalyst; this methodology uses metal ions, like Fe²⁺, and different chelating agents, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), that are non-toxic, and thus, have no environmental pollution during the removal of H₂S (Husein et al., 2010). Another methodology is sorbent injection into the gasifier. In situ desulfurization can be accomplished through the employment of (regenerable) transition metal oxides (Yumura and Furimsky, 1985). However, many of such processes are of limited efficiency and have high-energy costs. The employment of enormous amounts of solvents and catalysts in traditional removal processes increase the cost of treatment (Fuda et al., 1991). The standard methods for H₂S removal include amine aqueous solution absorption-Claus process (Stirling, 2000). The Claus process has been most typically used to remove H₂S from natural gas or refinery plants. Claus plants typically convert 94–98% of sulfur compounds within the feed gas into elemental sulfur. Also, one of the more frequently used biological treatment methods of removing H₂S involves a combination of biological and chemical process, using the bacteria that oxidize iron from Fe (II) to Fe (III), Thiobacillus ferrooxidans. When this bacterium produces oxidized iron, this oxidized iron can oxidize H₂S to elemental sulfur and then T. ferrooxidans turns the Fe (II), again, into Fe (III) (Sasaoka et al., 1992).

In another process, Bio-SR process bacteria can treat sour gas within chemical plants and refiners (Satoh et al., 1988), the basis of the process being explained in Figure 3.5. A solution of ferric sulfate contacts sour gas in an absorber. The solution absorbs hydrogen sulfide and oxidizes it to elemental sulfur. At the same time, the ferric sulfate is reduced to ferrous sulfate.

Elemental sulfur is removed from the solution by a separator. Then, the solution goes to a bio-reactor. This is where the bacteria do their work. Upon contact with air, the bacteria oxidize the ferrous sulfate back to ferric sulfate (regeneration step).

The oxidized solution is then recycled to the absorber to repeat the cycle. In this closed cycle, there is no waste and no special chemical.

Based on a similar principle, using chelated ferric ion for oxidation of H₂S to elemental sulfur, followed by a microbial process of regeneration of ferric ions using a mixed culture of iron oxidizing bacteria has been reported (Rai and Taylor, 1996). T. ferroxidans, for example, can be used to desulfurize petroleum and natural gas with the reaction being carried out in a closed vessel containing substrate mixed with a bacterial suspension (Das et al., 1993).

To decide which method to use, many factors should be considered, including the required extent of H₂S removal, the gas composition, temperature, volume, and pressure, and, further, the impact of sulfur recovery on the process economics and/or the environment (Wang, 2008).

3.3.2 Adsorption

It is doable to get rid of H₂S from natural gas by adsorption on a variety of materials, for instance zeolite molecular sieves. These are generally used to remove the last traces of water, CO_2, and H₂S upstream of liquefied natural gas (LNG) condensation trains. A general method conception is shown in Figure 3.6.

Adsorption/oxidation by carbonaceous surfaces was widely studied as a technique to manage H₂S pollution. Boki and Tanada (1980) studied the adsorption of H₂S onto activated carbon surfaces at three different temperatures using a vacuum system. The authors discovered that a little quantity of H₂S that was chemisorbed attenuated with the decrease in temperature. Aranovich and Donohue (1995) and Bagreev et al. (2000) compared experimental adsorption isotherms for various adsorbates onto microporous adsorbents with those predicted using the Dubinin-Radushkevich (DR) equation. They found that the adsorption of H₂S follows the pore filling mechanism. These results indicate that the adsorption of H₂S is mainly physical when processed under dry and anaerobic conditions, where surface functionalities exhibit no impact on the adsorption process. Similar studies, with fifteen different activated carbons, showed that lower average pore size leads to stronger interaction between H₂S and, also, the carbon surface, which is due to the increase in adsorption potential (Bagreev et al., 2000). All of those studies agree that physical adsorption of H₂S dominates under ideal conditions (i.e. vacuum, dry, anaerobic, and low temperatures).

Upon the application of activated carbon (AC) for the removal of H₂S, it is usually dosed with KI or sulfuric acid (H₂SO₄) to increase the reaction rate. Before entering the carbon bed, 4–6 % air is added to the biogas and H₂S is catalytically converted to elemental sulfur and water in biological filters as follows:

The elementary sulfur is then adsorbed by the AC. Usually, the best efficiency is obtained at pressures of 700–800 kPa and temperatures of 50–70 °C that can be achieved through heat generation during compression. In a continuous process, the system consists of two vessels (Hagen et al., 2001; Wellinger and Lindberg, 2005; Ryckebosch et al., 2011): one vessel for adsorption and the other for regeneration. Regeneration can be performed with hot nitrogen (inert gas) or steam. The sulfur is vaporized and, after cooling, liquefied at approximately 130 °C. However, typically, the AC is replaced rather than regenerated (Hagen et al., 2001; Wellinger and Lindberg, 2005; Ryckebosch et al., 2011).

Bandosz (2002) summarized that activated carbons as hydrogen sulfide adsorbents under field conditions needed the correct combination of surface chemistry and carbon porosity; a more acidic environment encourages the formation of high-valent sulfur compounds and reduces the H₂S removal capacities. A basic environment favors the formation of elemental sulfur and increases the capability of sulfur removal. Also, pre-adsorbed water can enhance H₂S dissociation and removal from the gas stream.

Cal et al. (2000) carried out systematic experimental studies with unmodified and modified activated carbons at 550 °C and showed that both HNO₃ oxidation and Zn impregnation improved H₂S adsorption capability.

Since modification of surface chemistry is probably going to be effective for improving H₂S removal capability by activated carbons, different impregnates were studied. Bagreev and Bandosz (2002) evaluated the impact of NaOH on adsorption of H₂S and mentioned that the H₂S removal capacities were dominated by the presence of NaOH and were not sensitive to surface area and pore structure. Other reports on NaOH (Chiang et al., 2000; Tsai et al., 2001) or K₂CO₃ (Przepiorski et al., 1998, 1999) impregnation also showed vital improvement within the H₂S removal capability. It is believed that the presence of alkaline chemicals facilitates the dissociation of H₂S on carbon surfaces.

The ADS-activity of CuO-AgO sorbents calcined at 700°C has been studied where CuO was used as a main active material, AgO was used as an additive material, and 25 wt% SiO₂ was used as a support material. The ADS activity and the activation energy of sorbent were found to be decreased as the content of AgO increased. The maximum ADS of a simulated gas recorded 14.95 g sulfur/100 g sorbent at 550°C using sorbent containing 1 wt% AgO (Lee et al., 2005).

Yazdanbakhsh et al. (2014) reported the H₂S breakthrough capacity of copper exchanged Engelhard Titanosilicate-2 (ETS-2). The adsorbent efficiency remained unchanged up to 950 °C, but at a lower temperature, <750 °C, the adsorption capacity at breakthrough was 0.7 mol H₂S/mol copper, while at >750 °C the capacity of the adsorbent is halved. The change in H₂S capacity was attributed to the reduction of Cu²⁺ by the produced H₂ from the thermal dissociation of H₂S.

Liu et al. (2015a) reported an efficient hybrid adsorbent/photocatalytic composite (TiO₂/zeolite) for the H₂S removal and SO₂ captured by coating TiO₂ on the surface of cheap natural zeolite with an ultrasonic-calcination way. The TiO₂/zeolite expressed the highest H₂S removal capacity and lowest SO₂ emission, compared with the single zeolite adsorption and TiO₂ photo-catalysis. Moreover, the H₂S removal capacity and SO₂ capture capacity of TiO₂/zeolite were found to be enhanced in the presence of moisture in the biogas.

Saimura et al. (2017) reported the preparation of dual-layered paper-structured catalysts separately containing manganese oxides (MnO_x) and nickel/magnesium oxides (Ni/MgO) for sequential desulfurization and methane-steam reforming, respectively. The porous paper-structured MnOx adsorbents could remove H₂S down to 7 ppm or lower at 400 °C for 81 min from simulated gas, resulting in higher H₂S adsorption capacity, as compared to commercial powdered manganese-based adsorbents. That system was designed by stacking MnO_x papers upstream and Ni/MgO papers downstream in one reactor as a dual-layered form that enabled the continuous direct hydrogen production of 2174 ppm H₂S-containing simulated biogas at 400 °C, whereas single-layered Ni/MgO papers immediately lost their catalytic activity due to the H₂S poisoning. The Mn-based adsorbents are promising for capturing H₂S impurities in crude biogas, since manganese oxides (MnO_x) can react with H₂S and are stable even at high temperatures (400–1000 °C) (Westmoreland et al., 1977; Bakker et al., 2003).

Several studies on zeolites (Hernández et al., 2011; Kwaśny et al., 2015; Kwaśny and Balcerzak, 2016) show that zeolites have low efficacy of adsorption of H₂S as compared to activated carbons.

Li et al. (2012) reported the preparation of nano-ZnO by a homogeneous precipitation method and activated carbon from coconut shell, which was pretreated by two different acids: HNO_s and HCl. Then, AC-loading nano-ZnO was prepared by mechanical hybrid method. Then, the ADS capacity of the prepared AC-loading nano-ZnO was tested against H₂S gas. The load of activated carbon should be proper and the activated carbon loaded nano-ZnO reported to achieve the most effective desulfurization activity when the mass ratio of ZnO and AC is 0.3 (Li et al., 2012) since excessive nano-ZnO powders could accumulate in the material surface and block the pores of the AC so that specific surface area and pore volume of material are reduced, hence the desulfurization properties. The desulfurization property of AC loading nano-ZnO desulfurizer modified by hydrochloric acid is better than that modified by nitric acid and the breakthrough time is up to 110 min. This is due to the strong oxydic ability of HNO₃, which reduces the specific surface area of the AC and, thus, its ADS activity. The most important thing is that the modification of nano-ZnO, with AC, did not alter its structure.

Phooratsamee et al. (2014) reported the preparation of AC from palm oil shells by carbonization of raw material in an inert atmosphere at 600 °C, followed by activation of char product by ZnCl₂, then, finally, alkali impregnation of activated carbon by NaOH, KI, and K₂CO₃ solutions. Then, the activity of the prepared AC was tested for H₂S absorption from biogas. The K₂CO₃-AC showed the highest surface area and the pore volume of 741.71 m²/g and 0.4210 cm³/g, respectively. Thus, it showed the best performance for biosorption of H₂S. The results of this research showed that H₂S adsorption on alkaline impregnated activated carbon (K₂CO₃-AC and KI-AC) was higher than commercial activated carbon. This positive effect is mainly due to the chemical reaction in which H₂S reacts with alkali at carbon surface to produce alkali hydrogen sulfilde (AHS) and alkali sulfide. However, NaOH-AC showed lower adsorption activity than that of commercial AC, due to the less residual water vapor content which blocks the carbon surface.

In another study, activated carbon treated with Na₂CO₃ was reported to be the most effective sorbent, showing a breakthrough time of 1222 min at 0.5 ppm that is twice the time of the untreated-AC (Micoli et al., 2014). Furthermore, Monteleone et al. (2011) reported that activated carbons impregnated with metal salts express high adsorption capacity due to the combination of micro-porosity and oxidative properties. Osorio and Torres (2009) reported a study dealing with biogas purification coming from the anaerobic digestion of sludges in a wastewater treatment plant. The purification apparatus contains scrubbing towers and filters of activated carbon at the end of the line. The H₂S in flow concentrations were quite high (~ 2000 ppm), while the effluent biogas from the scrubbing towers presented a H₂S concentration <1 ppm, zero, or undetectable values after adsorption of active carbons filters.

Aslam et al. (2015) reported the preparation of AC from waste oil fly ash (OFA) which is produced in large quantities from power generation plants through combustion of heavy fuel oil, applying physicochemical treatments using a mixture of HNO₃, H₂SO₄, and H₃PO₄ acids to remove non-carbonaceous impurities. The acid treated OFA was then activated by CO₂ at 990 °C. The physicochemical treatments of OFA increased the surface area from 4 to 375 m²/g. The AC is further treated with HNO₃ and NH₄OH solutions in order to attach the carboxylic and amine groups on the surface, respectively. Then, its activity was tested for the removal of H₂S from a synthetic natural gas by carrying out breakthrough curves. The NH₄OH-treated AC was found to be more effective for H₂S removal than acid-treated AC, recording a maximum adsorption capacity of 0.3001 mg/g for NH₄OH functionalized AC with 86.43% regeneration efficiency. The obtained results indicated that the presence of more acidic functionalities on the surface reduces the H₂S adsorption efficiency from the gas mixture.

As listed before, biogas desulfurization through the adsorption process is an attractive method due to its simple process and low starting cost. Currently available adsorbents, such as activated carbon, metal/metal oxide, and zeolite, are the most applied adsorbents for effective biogas desulfurization. Kaolinite is a naturally occurring material that has been used as an adsorbent material for several organic compounds. However, the presence of metal oxides in kaolin, such as aluminium (Al₂O₃), silica (SiO₂), iron (Fe₂O₃), potassium (K₂O), magnesium (MgO), and TiO, makes it possible to be used as H₂S adsorbent. Abdullah et al. (2017) studied the activity of a calcined kaolinite in ADS of H₂S using a fixed bed reactor in comparison with commercial activated carbon, commercial zeolite A, and pure zinc oxide. That recorded 0.087, 51.68, 2.1, and 58.30 mg H₂S/g sorbent, respectively. Moreover, as many other adsorbents, it was observed that the contact time between the gas and adsorbent is the key factor in determining the adsorption capacity. Dhage et al. (2008) reported that the increase in the gas flow rate reduces the contact time and, consequently, reduces the adsorption capacity. Furthermore, the increase in temperature increased the ADS capacity. That recorded 26 mg H₂S/100 g of adsorbent at 80 °C. It was also noted that at 30°C, the physical adsorption was dominant, meanwhile at a higher temperature, the mechanism of H₂S removal was governed by chemical adsorption. Thus, the presence of metal oxides on the kaolinite surface is suggested to play an important role in the ADS of H₂S.

The modification of a copper based metal-organic framework, (MOF-199) by incorporating activated carbon (AC) during its synthesis under hydrothermal conditions, has been reported (Shi et al., 2017) where the XRD, SEM, and nitrogen adsorption results showed that the synthesized composites, with an amount of AC of less than 2% (MAC-2), had a more ordered crystallinity structure, as well as higher surface area, than the parent MOF-199. MAC-2 exhibited a maximum uptake of 8.46 and 8.53% for H₂S and CH₃SCH₃, respectively, which increased by 51 and 41% compared to that of MOF-199. This was attributed to the increased micropores and the number of copper metal sites resulted from AC incorporation. The pyridine adsorption infrared spectroscopy (Py-IR) confirmed the chemisorption process of H₂S, which led to the formation of CuS as well as the destruction of the MOF structure, whereas reversible chemisorption was found for CH₃SCH₃ adsorption, as testified by thermogravimetric- mass spectroscopy (TG-MS) and fixed-bed regeneration. However, there was a recorded obviously different increasing slope of the CH₃SCH₃ concentration curve, along with the corresponding color change during temperature programmed desorption which indicated that both physisorption and weak chemisorption were involved. Moreover, the used MAC-2 was almost totally regenerated by nitrogen purge at a temperature of 180 °C, with 94% of breakthrough capacity recovered after 3 cycles. This good regeneration behavior promoted MAC as a promising sorbent for CH₃SCH₃ removal.

3.3.3 Liquid Redox Processes

Liquid redox processes employ aqueous-based solutions containing metal ions, usually iron, which are capable of transferring electrons in reduction-oxidation (redox) reactions. The best familiar of these processes is the Stretford process that uses vanadium, valence state five, as an oxidant (Andrews, 1989).

The use of vanadium possesses different environmental problems; in addition, this process needs a large absorption tower and a large oxidation tank to regenerate the solution and the precipitated sulfur which is produced from H₂S. Thus, it is not an immediate choice for platform use. To overcome this problem using the redox principle many researchers make use of iron as an oxidant. The most publicized is the LO-CAT (presently owned by Merichem company), which is licensed by Gas Technology Products LLC. In this process, a non-toxic, chelated iron catalyst is employed to accelerate the reaction between H₂S and oxygen to form elemental sulfur (Hardison, 1991).

(1)

As implied by its generic name, liquid redox, all of the reactions in the LO-CAT process occurs in the liquid phase in spite of the fact that equation (1) is a vapor phase reaction. In the process, the sour gas is contacted in an absorber with the aqueous, chelated iron solution where the H₂S is absorbed and ionized into sulfide and hydrogen ions (Figure 3.7)

(2)

Spent absorbent is pumped to the oxidizer for regeneration where oxygen in the air is employed as an oxidizing agent. There is a more recent version where the absorbent is enclosed within the oxidant unit. Additionally, a more recent conception is the Japanese Process Bio-SR that uses an unchelated form of iron which is regenerated microbially and it will be mentioned later. Shell also offers a microbiological process known as Paques (Benschop et al., 2002; O’Brien et al., 2007).

Successful operation of the liquid redox processes depends on a proper understanding of the fundamentals. The present development efforts may lead to improved processes in the future. Once their increased competitiveness opens a larger market, more development money is likely to be spent giving, hopefully, even better processes. The application range for these redox processes is generally 1–20 tons of sulfur per day and they are applicable to a vast range of gases.

In dry oxidation processes, the sulfur is removed from hydrogen sulfide with the employment of iron oxide (eq. 3). Iron oxide is used in the form of iron fillings, iron pellets, iron sponge, or steel wool (Shannon, 2000; Hansen, 2006).

(3)

The sulfur removal efficiency by iron oxide ranges from 0.20–0.716 kg of H₂S for every 1 kg of iron oxide (Wellinger and Linberg, 2000; Magomnang and Villanueva, 2014).

As shown in Figure 3.8, the sour gas from the gas holder/tank passes through the inlet section on which mass flow, temperature, humidity, and initial concentration of the H₂S from the sour gas is being measured. It passes through the first stage purifying chamber and is passed through another until it reaches the last chamber. Upon exiting the respective chambers, the concentration of the H₂S is measured until it reaches the third stage chamber.

LO-CAT^® (US Filter/Merichem) and SulFerox^® (Shell/Dow) processes are currently the available chelated-Fe H₂S removal technologies. LO-CAT^® can effectively treat any stream containing sulfur and, consequently, any biogas (Rouleau and Watson, 2014). Its typical economic niche is the removal needs of more than 200 kg of S/day. As an example for the real field application of Lo-CAT^®, US Filter/Merichem company installed a LO-CAT^®II H₂S oxidation system in Broward County, Floridato to treat up to 5 Nm³/s of landfill gas containing up to 5000 ppmv H₂S. That led to capturing about 300 kg S/day (Abatzoglou and Boivin, 2009). While the SulFerox^® license is jointly handled by Dow and Shell, Dow licenses the technology externally and Shell markets the process among its own company divisions. Institut Français du Petrole (IFP) has one such license and Gaz Integral Enterprise of France, a company like IFP, markets the SulFerox^® process (Le Gaz Intégral, 2000) for S removal rates between 100 and 20,000 kg/day with high CO₂/H₂S ratios. Although CO₂ is not significantly removed, approximately 50–90% of mercaptans can be removed in either low- or high- pressure applications. The S-removal with SulFerox^® costs around $0.24–$0.3/kg.

3.3.4 Claus Plants

The sulfur species in natural gas after its removal is usually in the form of H₂S. The most common means of recovering the sulfur contained in hydrogen sulfide is the Claus process. This process can recover 93–99% of the sulfur contained in its feed. Recovery depends upon feed composition, age of catalyst, and number of reactor stages.

The gas leaving the Claus plant is referred to as tail gas and is burnt to convert the remaining H₂S, that is fatal at low levels, to sulfur dioxide which has an abundant toxic limit. The off-gas stream is ventilated to the atmosphere or sent to a tail gas recovery plant.

3.3.4.1 Classic Claus Plant

The Claus process (Figure 3.9) is the most important gas desulfurizing process, recovering elemental sulfur from gaseous H₂S. It was developed by Carl Friedrich Claus in 1883. The H₂S containing feed gas is fed to the Claus furnace along with air. In this furnace, about one-third of the H₂S is converted to SO₂. The Claus technology can be divided into two process steps.

Thermal step: Hydrogen sulfide-laden gas reacts in a sub-stoichiometric combustion at temperatures above 850 °C, such that elemental sulfur precipitates within the downstream process gas cooler. The H₂S content and the concentration of other combustible components (hydrocarbons or ammonia) determine the location where the feed gas is burned. Claus gases (acid gas) with no further combustible contents, apart from H₂S, are burned in lances surrounding a central muffle by the following chemical reaction:

Gases containing ammonia, like the gas from the refinery’s sour water stripper (SWS), or hydrocarbons are converted in the burner muffle. Enough air is injected into the muffle for the complete combustion of all hydrocarbons and ammonia. The air to acid gas ratio is controlled so that, in total, 1/3 of all H₂S is transformed to SO₂. This ensures a stoichiometric reaction for the Claus reaction. The separation of the combustion processes ensures a correct dosage of the desired air volume required as a function of the feed gas composition. To minimize the process gas volume or get higher combustion temperatures, the air demand may also be covered by injecting pure oxygen. Many technologies utilizing high-level and low-level oxygen enrichment are available in the industry, which needs the employment of a special burner within the reaction furnace for this process option. Usually, 60 to 70% of the whole quantity of elemental sulfur produced in the process is obtained within the thermal process step.

The main portion of the hot gas from the combustion chamber flows through the tube of the process gas cooler and is cooled down so that the sulfur formed in the reaction step condenses. The heat given off by the process gas and the condensation heat evolved are utilized to produce medium- or low-pressure steam. The condensed sulfur is removed at the gas outlet section of the process gas cooler. A little portion of the process gas may be routed through a bypass inside of the process gas cooler. This hot bypass stream is added to the cold process gas through a three-way valve to regulate the inlet temperature needed for the first reactor.

Catalytic step: The Claus reaction continues in the catalytic step with activated aluminum (III) or titanium (IV) oxide and serves to boost the sulfur yield. H₂S reacts with the SO₂ formed during combustion in the reaction furnace and results in gaseous, elemental sulfur. This is often the known as a Claus reaction:

The catalytic recovery of sulfur consists of three sub-steps: heating, catalytic reaction, and cooling plus condensation. These three steps are normally repeated a maximum of three times where an incineration or tail-gas treatment unit (TGTU) is added downstream of the Claus plant and only two catalytic stages are usually installed.

The first process step in the catalytic stage is that the gas heating process. It is necessary to prevent sulfur condensation in the catalyst bed, which might result in catalyst fouling. The desired bed operating temperature within the individual catalytic stages is achieved by heating the process gas in a re-heater until the desired operating bed temperature is reached. Many strategies of reheating are used in industry:

• Hot-gas bypass involves mixing the two process gas streams from the process gas cooler (cold gas) and the bypass (hot gas) from the first pass of the waste-heat boiler.
• Indirect steam reheaters, where gas also can be heated with high-pressure steam in a heat exchanger.
• Gas/gas exchangers, where the cooled gas from the process gas cooler is indirectly heated from the hot gas coming out of an upstream catalytic reactor in a gas-to-gas exchanger.
• Direct-fired heaters wre fired reheaters utilizing acid gas or fuel gas, which is burned sub-stoichiometrically to avoid oxygen breakthrough which can damage Claus catalysts.

The recommended operating temperature of the first catalyst stage is typically 315 °C to 330 °C (bottom bed temperature). The high temperature in the first stage also helps to hydrolyze carbonyl sulfide (COS) and carbon disulfide (CS)₂, which is formed in the furnace and would not otherwise be converted in the modified Claus process.

The catalytic conversion is maximized at lower temperatures, but care must be taken to make sure that each bed is operated above the dew point of sulfur. The operating temperatures of the subsequent catalytic stages are usually 240 °C for the second stage and 200 °C for the third stage (bottom bed temperatures).

In the sulfur condenser, the process gas coming from the catalytic reactor is cooled to between 150 and 130 °C. The condensation heat is used to generate steam at the shell side of the condenser. Before storage, liquid sulfur streams from the process gas cooler, the sulfur condensers, and from the final sulfur separator are routed to the degassing unit where the gases (primarily H₂S) dissolved in the sulfur are removed.

The tail gas from the Claus process, still containing combustible components and sulfur compounds (H₂S, H₂ and CO), is either burned in an incineration unit or further desulfurized in a downstream tail gas treatment unit. The ‘Tail Gas’ is a key point here. There is unconverted H₂S in this tail gas and that must be taken care of. The tail gas treatment is a process field of its own and is discussed further next.

3.3.4.2 Split-Flow Claus Plant

The classic Claus process must have a really high concentration of H₂S to make the plant work. For H₂S concentrations in the range of 25 to 40%, the split flow configuration can be utilized. During this process, the feed is split and one third or more of the feed goes to the furnace and the remainder joins the furnace exit gas before entering the first catalytic converter. Once two thirds of the feed are bypassed, the combustion air is adjusted to oxidize all the H₂S to SO₂ and, consequently, the necessary flame temperature can be maintained. The split-flow process has two constraints:

• Sufficient gas should be bypassed so the flame temperature is more than approximately 927 °C.
• The maximum bypass is only two thirds, as a result of one third of the H₂S must be reacted to form SO₂.

Thus, if air preheating is employed with the split-flow configuration, gases with as little as 7% H₂S can be processed.

3.3.4.3 Oxygen Enrichment Claus Plant

In this system, oxygen is provided to the Claus plant through a sparger into the air line to the reaction furnace. The oxygen can also be injected directly into the furnace through a lance or burner or feed air can be fully replaced by oxygen through a burner in the furnace. In some cases, complete feed air replacement can double Claus plant capacity. The resulting higher oxygen concentration increases the operating temperature of the furnace, which leads to steadier operation and greater ammonia destruction. As a result, the Claus reaction furnace maximizes its ability to combust the oxygen and air mixture and the H₂S produced throughout processing works with system converters and condensers to recover the sulfur. Increased oxygen also means lower nitrogen volumes which reduces gas volume in tail gas treatment.

3.3.4.4 Claus Plant Tail Gas

The tail gas treating unit converts the tiny quantity of sulfur compounds (<5%) that were not converted in the sulfur recovery unit (SRU) into H₂S and recycles them back to the SRU for additional processing. The foremost well-known tail-gas treatment technology is at the SCOT (Shell Claus Off-gas Treatment) plant (Kohl and Nielsen, 1997). The SRU tail gas is heated and sent to the catalytic reactor where, essentially, all of the sulfur compounds are converted into H₂S. The gas from the catalytic reactor is cooled in the waste heat exchanger and the quench tower. Excess water is removed in the cooling process and is sent to the sour water stripper. The cooled gas is then sent to the absorber column, where amine removes the H₂S and some of the CO₂ in the gas stream. The remaining gas (vent gas) is sent to the thermal oxidizing unit. The rich amine from the absorber is heated in the lean/rich exchanger and fed to the regenerator column. Steam, generated in the re-boiler, heats the amine and removes the H₂S and CO₂ from the amine. The lean amine from the stripper is cooled in the lean/rich exchanger and the lean solvent cooler and returned to the absorber. Amine has a natural affinity for both CO₂ and H₂S allowing this to be a very efficient and effective removal process. There are many various amines employed in gas treating:

• Monoethanolamine (MEA): Used in low pressure natural gas treatment applications requiring stringent outlet gas specifications
• Diethanolamine (DEA): Used in medium to high pressure treating and does not require reclaiming as MEA and DGA systems do
• Methyldiethanolamine (MDEA): Has a higher affinity for H₂S than CO₂, which allows some CO₂ “slip” while retaining H₂S removal capabilities
• Diisopropylamine (DIPA)
• Aminoethoxyethanol /diglycolamine (DGA)
• Formulated special solvents

MEA is the strongest base of the group and, as such, is the most reactive with acid gases. Many chemical reactions are involved and the reversibility of these reactions is the basis for the cyclic MEA scrubbing process. The SCOT absorption process is designed to absorb as little CO₂ as possible. The H₂S and CO₂ removed from the amine is cooled (and water removed) within the overhead condenser and recycled to the sulfur recovery unit for additional processing into sulfur.

The Shell Claus Off-Gas Treating (SCOT) process, which was announced to the industry in late 1972, was developed by Royal Dutch Shell laboratories in the Netherlands and is licensed in the U.S. by Shell Development and can be viewed as the industry standard classical process for small scale gas treatment. The SCOT process takes place in essentially three stages: heating and reducing all S-compounds to H₂S, cooling and quenching, and H₂S absorption, stripping, and recycling (Figure 3.10).

The reactions take place at 165–175 °C and the hydrogenated tail gas is cooled in the waste heat boiler, generating medium pressure steam, and then further cooled in a quench tower. The cool gas enters the absorber where it is contacted with a 25–50 wt.% methyldiethanolamine (MDEA) aqueous solution to absorb H₂S selectively. The rich MDEA solution flows down the stripper where the absorbed H₂S and CO₂ are stripped by upward rising steam. The regenerated solvent from the bottom of the stripper is recycled back to the absorber after cooling to 30–48 °C. H₂S and CO₂ leave the stripper at the top and recycle to the front-end Claus plant.

The SCOT reactor contains a bed of cobalt-nickel or cobalt-molybdenum catalyst that allows the reducing atmosphere to hydrogenate or hydrolyze most of the S-compounds contained in the heated tail gas feed to H₂S according to the following reactions:

3.3.5 Absorption/Desorption Process

Chemical absorption is mainly based on the chemical affinity of H₂S for metallic cations. This process can be categorized to those involving oxidation of S^2– to S⁰ and those based on the capture of S^2– through precipitation of its metallic salts, owing to their very low K_sp (water solubility product). Another option, which belongs to the second category, is the capture by aqueous alkaline solutions which rapidly react with diffused H₂S, but it is of low interest in industrial application due to the high reactivity of CO₂ with alkaline solutions and less selectivity towards H₂S. Not only this, but its main drawback comes from the high consumption of relatively expensive alkaline reactants (i.e. NaOH or CaO) throughout the capturing of CO₂.

The absorption/desorption processes supported with amines are those most widely used even for treatment of relatively low H₂S levels. There are many available processes and they may be divided into three categories, according to the absorbent: (1) physical absorption processes, (2) absorption combined with reversible reaction by carbonate solutions, and (3) amine solutions. A typical process based on an amine solution is shown in Figure 3.11. In this process, the acidic components react with an alkanolamine absorption liquid via an exothermic, reversible reaction in a gas/liquid contactor. These processes have the ability to remove a large quantity of H₂S economically and CO₂ may also be controlled if necessary. When CO₂ and H₂S are present, the following chemical reactions occur in an aqueous MDEA solution (Zare and Mirzaei, 2009):

The absorber operates from ambient pressure up to 70 bar and from 25 to 70 °C. The energy consuming desorption of the acid gases is carried out at around 130 °C and at pressures from ambient up to 3 bar. Desorption pressure may not necessarily be lower than the absorption pressure (e.g. tail gas treatment); this depends on the requirements of the connected Clausplant. MDEA-plants process up to 40,0000 Nm³/h feed gas in a single train (Bolhàr-Nordenkampf et al., 2004).

For accurate plant design, it is of great importance to be able to predict the mass transfer behavior in the absorption and desorption column. Reactions which take place in the liquid phase can be divided, in principle, into two groups: reactions where equilibrium is controlled and reactions that are kinetically determined. The chemical reactions determine the composition of the different ion species in the liquid phase and, therefore, the enhancement of mass transfer. Equilibrium reactions are fast enough to assume chemical equilibrium throughout the entire liquid phase (Markus et al., 2004). An equilibrium approach for the absorption is not suitable, if predictive capabilities of the model are necessary, as it is the case for selective H₂S and/or CO₂ removal in alkanolamine-systems. This can only be achieved using a rate-based non-equilibrium model. It is based on the mass and heat transfer between the liquid and vapor phase occurring on a height-increment of the structured and random packing, respectively. Mass and energy balances are connected by rate-equations across the interface using the two-film theory to calculate the transfer rates (Bolhàr-Nordenkampf et al., 2004).

3.3.6 Biodesulfurization

Major problems in oil and gas operations result from the biogenic formation of H₂S in the reservoir. The presence of H₂S results in increased corrosion, iron sulfide formation, higher operating costs, reduced revenue, and constitutes a serious environmental and health hazard. Linkous (1993) reported a technology that removes and prevents the formation of biogenic H₂S which is based on the principle of ‘Biocompetitive Exclusion’ where the addition of low concentrations of a water soluble nutrient solution selectively stimulates the growth of an indigenous microbial population, thereby inhibiting the detrimental Sulfate Reducing Bacteria (SRB) population, which causes the generation of the H₂S. The versatility and low cost of this novel technology offers the petroleum industry a practical and cost effective methodology for the control of H₂S in oil and gas wells.

As mentioned before, H₂S contamination may be treated by biochemical, chemical, and physical methods (Burgess, 2001). Many physicochemical processes, like the dry gas reduction-oxidation (redox) process, liquid redox processes, and the liquid adsorption process, are usually used for desulfurization of gases containing H₂S. However, they have high capital costs, demand large energy inputs, and lead to the generation of secondary hazardous wastes (Pandey and Malhotra, 1999). Several physical means of controlling the formation of acid-mine drainage have been improved, but they have not been very successful. Therefore, efforts have been directed towards biological processes for the removal of H₂S that are characterized by small capital costs and low energy requirements.

BDS, via the injection of ambient air into the gas headspace in the digester, is the most widely used process for internal desulfurization applied in 90% of all biogas plants and followed by the addition of iron salts to the fermenting substrate. Depending on the

proportion of oxygen injected into the gas phase, H₂S is oxidized into elementary sulfur, sulfate-S, or sulfite-S. A reformation of S into H₂S by an unintended return of accumulated degradation products from the colonization surfaces into the fermentation substrate is one of the crucial drawbacks of this method. Recently, external-BDS methods have become much more popular and find their way into the market as it solves a lot of the drawbacks of the internal-BDS. Several methods for external-BDS have been reported, such as two-stage bio-scrubbers and one stage-bio-trickling filters.

In two-stage bio-scrubbers, the H₂S in the raw gas is absorbed, first, by contacting with water in a scrubber and, afterwards, biologically degraded in a bioreactor (van Groenestijn and Hesselink, 1993). A further option is a one-stage bio-trickling filter where the raw biogas passes through a fixed bed reactor filled with plastic packing materials. Washing water is trickled periodically over the fixed bed to promote a biofilm generation on the packing material enabling the immobilization of microorganisms. Dissolved H₂S is oxidized by certain microbial groups to elementary sulfur and sulfate (Naegele et al., 2013; Barbusiński and Kalemba, 2016). Moreover, treatment of gaseous emissions contaminated with H₂S by biotrickling filters inoculated with single cultures of sulfur oxidizer bacteria exhibit several advantages over physicochemical methods, such as shorter adaptation times, meaning a shortening and even absence of the bacterial lag phase and a higher BDS efficiency (Syed et al., 2006; Aroca et al., 2007).

One of the most common technologies for the biological treatment of sour gas is the THIOPAQ™ process (Figure 3.12) (Cline et al, 2003). It removes H₂S from gaseous streams by absorption into a mild alkaline solution, under a pressure of up to 75 barg, in a trayed or packed column (which is preferred) gas/liquid contactor, which is followed by the oxidation of the absorbed sulfide to elemental sulfur by a consortium of naturally occurring, colorless bacteria, Thiobacilli, in the THIOPAQ™ reactor. The volume of the bioreactor usually ranges from 5–2500 m³ and operates at atmospheric pressure at a temperature of around 30 °C. Beasley and Abry (2003) reported that virtually complete conversion of H₂S to elemental sulfur (>99.999%) can be achieved.

The following reactions may occur in the absorber at feed gas pressure using caustic soda as a solvent:

The sulfide formed in the absorber is removed in the bioreactor, where the following reactions may occur at atmospheric pressure and under aerobic conditions:

Thiobacillus bacteria in the bioreactor can be considered equivalent to small catalyst particles that gain energy required for their growth from the conversion of the sulfide to elemental sulfur. Moreover, the caustic used to absorb H₂S gas is regenerated in the bioreactor and recycled back to the absorber and the produced elemental sulfur is gravimetrically separated in a decanter. In order to avoid accumulation of sulfate ions, a continuous bleed stream from the bioreactor is required.

The biodesulfurization rate of H₂S depends on several parameters which could be better controlled with a recirculated water phase (pH, temperature, addition of nutrients, removal of accumulated salts, etc.) (Smet et al., 1998). A wide range of pH was reported especially for H₂S removal in bio-filters. Gabriel and Deshuesses (2003) reported that the decrease in pH to pH2 resulted in an increased hydrogen sulfide degradation rate up to 99%. Jin et al. (2005) recorded 95% BDS at pH2. However, an optimum pH of 6 was reported for autotrophic bacteria. Smet et al. (1998) reported that within the genus of the sulfur compound bacteria Thiobacillus some species are found to favor pH-neutral environments and others favor low pH values. An insufficient availability of oxygen in the bioreactor would lead to an incomplete removal of H₂S (Gadre, 1989). An optimal stoichiometric demand of 4–6% (v/v) air in biogas is required for complete oxidation. However, in another study, the optimal desulfurization performance was obtained at 8–12% air fed into the biogas flow, but higher air application rates resulted in lower CH₄ content in the biogas and a lower calorific value (Naegele et al., 2013). Furthermore, low and higher temperature are not desirable and approximately 30–40 °C is the optimum. The desirable bacteria to be used in a bioprocess to convert H₂S to S⁰ should possess the subsequent basic features: reliable capability of converting H₂S to S⁰, minimum nutrient inputs, and simple separation of S⁰ from the biomass (Rattanapan and Ounsaneha, 2011). Moreover, a variety of bacteria are capable of H₂S oxidation and, hence, serve as potential candidates for gas desulfurization technology (Gadre, 1989).

3.3.6.1 Photoautotrophic Bacteria

The photosynthetic van Niel reaction is a well-studied anaerobic, inorganic acid gas bioconversion (van Niel, 1931):

Genera of the family Chlorobiaciae and Chromatiaciae catalyze this reaction and the reaction has been optimized for desulfurization of acid gas effluent containing H₂S, H₂, and CO₂. Fed-batch and chemostat cultures have shown that simultaneous control of molar flow rates of incoming gases and bioreactor photon flux is important to the optimization policy of the van Niel reaction (Maka and Cork, 1990). However, the requirement for light as an energy source in this process is a severe economic disadvantage.

Studies on microbial ecology related to phototrophic bacteria have shown that a species of green sulfur bacteria (GSB), Cholorobium limicola, (Larsen, 1952) is the best suited for sulfide removal and satisfies the criteria for a desirable bacterium (Syed and Henshaw 2003). GSB are non-motile and deposit elemental sulfur extracellularly. This feature makes GSB suitable where the recovery of elemental sulfur from sulfide-containing wastewater is desired (Syed et al., 2006). A schematic diagram of the anaerobic acid gas bioconversion process is shown in Figure 3.13. In contrast of the Claus process, this system does not need oxygen because sulfide is anaerobically oxidized to elemental sulfur in the presence of CO₂ according to the van Niel reaction. The accumulation of sulfur compounds in the system reduced the total conversion of H₂S to elemental sulfur. However, the oxidation of H₂S to sulfate was suppressed when the reactor was operated with high H₂S concentrations and high space velocity (defined as the number of reactor volumes of feed gas that can be treated per hour).

A gas-fed batch reactor is a stirred tank type reactor (Figure 3.14) continuously or intermittently operated for the gas phase and cyclically operated for the liquid phase (nutritive solution). The microorganisms can be suspended in the solution or immobilized on different media (i.e. strontium alginate beads (Kim and Chang 1991)).

Kim and Chang (1991) immobilized cells of Chlorobium limicola in a strontium-alginate matrix for transforming H₂S into elemental sulfur. They also addressed the problems of sulfate accumulation due to the undesirable side reaction:

The accumulation of sulfur and sulfate was found to be dependent on the light energy and feed rate of H₂S. The intensity of the light source was varied, keeping the exposed area of the reactor constant at loading rates of 32 or 64 mg S^2–/h/ L. A high irradiance resulted in sulfide removal, over half of which was converted to sulfate. At low irradiance, sulfide was not fully oxidized and accumulated in the reactor. Between these limits, a state was found in which neither sulfide nor sulfate was found in the reactor, while elemental sulfur and thiosulfate (S₂O₃^2–) were the only products. For white light (380 to 900 nm), this optimum state resulted in 60% of the influent H₂S becoming S⁰ and 40% becoming S₂O₃^2–. For infrared light (700 to 900 nm), the optimum favored sulfur production was 97:3 (S⁰:S₂O₃^2–). The optimum for infrared light also occurred at a lower irradiance level (219 W/m²) than white light (406 W/m²). These findings demonstrated the superiority of infrared light as a light source to produce elemental sulfur using GSB. It was suggested that light uptake by the transparent strontium alginate was easier because of light scattering and easy light permeation into the interior space of the beads. Thus, the maximum oxidation rate of 3.8 mmol (H₂S)/L h, with negligible SO₄^2– accumulation, was achieved with immobilized cells. This experiment also tried to optimize light energy input by using light emitting diodes (LEDs) with a peak wavelength at 710 nm in a batch-fed stirred tank reactor. Experiments were performed separately using an incandescent bulb, LED710, and a combination of LED710 and a fluorescent bulb as the light sources and their individual performances during H₂S removal per unit luminous flux, (mmol/min) per g protein/L, and per W/m² were evaluated. They found that the maximum performances using LED710 and LED710 with a fluorescent bulb were 18.7 and 14.1 times the performance of a reactor with an incandescent bulb, respectively. Kim et al. (1996) investigated the performance of LEDs in a plate type photo-bioreactor. They observed that the maximum performance per unit luminous flux while using LEDs was 31 times that of an incandescent bulb. This efficiency was achieved by only supplying light within the wavelength range where absorption by bacteria was at a maximum.

In addition to the above-described system, a two-step bioprocess for the removal of sulfate from waste streams has been proposed (Jensen and Webb, 1995) (Figure 3.15). The first step of the process is bio-catalyzed by a strictly anaerobic acetate-degrading and sulfate reducing microorganism, for example Desulfobacter postgateii, while the second step is catalyzed by the strictly anaerobic photoautotroph Chlorobium limicola.

Theoretically, only 25% of the total acetate required in the first step can be produced by Chlorobium. The Chlorobium reactor does not require steam sterilization, as constant flushing with toxic concentrations of H₂S inhibits the growth of potential contaminating microorganisms. Studies on the optimization of Desulfobacter for sulfate reduction, utilizing a whole-cell immobilized Desulfobacter chemostat, have been reported by Grimm et al. (1984). The major disadvantages in the use of photosynthetic bacteria on a large scale are mainly attributed to their anaerobic nature and their requirement for radiant energy and, hence, extremely large transparent surface area.

Basu et al. (1996) used a continuous stirred tank reactor equipped with a sulfur separator to remove hydrogen sulfide from a gas stream containing 2.5% H₂S at 1 atmospheric pressure. When a sulfide loading rate was 94.4 mg/h/L, H₂S conversion by Chlorobium thiosulfatophilum ranged from 53% to 100% at a gas retention time of 12.2 and 23.7 min, respectively. The sulfur recovered from the process by gravity separation was 99.2% of the theoretical yield. The separation of elemental sulfur from the bioreactor contents is essential to attain its value as a chemical industry feedstock.

Henshaw et al. (1997) tested the separation of the sulfur by settling, filtration, and centrifugation. Centrifugation achieved the best separation results with 90% of the elemental sulfur and 29% of the bacteria were removed from the suspension. They noted that a continuous-flow suspended growth bioreactor system for sulfide removal/sulfur recovery requires two separation stages: one to separate S⁰ from the bioreactor effluent and one to separate biomass from the liquid product of the primary separator.

A fixed-film reactor can reduce the requirement for two separators since the biomass remains in the reactor. Two types of phototube reactors are shown in Figures 3.16a and b. These are tubular type reactors that are operated continuously. The reactor can be horizontally oriented (Figure 3.16a), having several passes or spirals to improve the residence time in the reactor (Kobayashi et al., 1983) or can be vertically oriented, as presented in Figure 3.16b (Henshaw et al., 1999 and Syed and Henshaw, 2003). The material of the tube is transparent to light and impermeable to oxygen (Syed and Henshaw, 2003) and the bacteria develops on the inner wall of the tube reactor (fixed-film reactor).

Kobayashi et al. (1983) used a phototube reactor in which a sulfide containing reactor was passed through a 12.8 m long, 3.2 mm ID Tygon tube which was immersed in an illuminated water bath. The tube was able to achieve 95% sulfide removal in about 24.6 min while operated at a sulfide loading rate of 67 mg/h/L, postulating that a vertical attached growth configuration could eliminate or significantly reduce the problem of sulfur accumulation encountered by Kim and Chang (1991).

3.3.6.3 Chemotrophic Bacteria

Natural gas and biogas can be biologically desulfurized in additional units represented mainly by bio-filters, bio-trickling filters, and bio-scrubbers or directly into the anaerobic reactor by applying micro-aerobic conditions. All these processes are based on the S-cycle, and more specifically, in H₂S oxidation. In the aforementioned extra units H₂S is solubilized in a humid packed bed where aerobic species of sulfide-oxidizing bacteria (SOB) are immobilized and grown as a biofilm in the presence of O₂ (Noyola et al., 2006). Elemental sulfur (S⁰) and SO₄^2– are the thermodynamically stable by-products of biological H₂S oxidation, which have been proposed to proceed through several intermediates. Duan et al. (2005) suggested the following pathway of chemoautotrophs:

Tang et al. (2009) reported the main reactions that can be carried out by SOB:

According to the above equations, Gibbs free energy (∆G) is negative which means the reactions can happen spontaneously. At this point, it should also be noted that H₂S oxidation in biological systems occurs concurrently with chemical reactions (van der Zee et al., 2007), where S₂O₃^2– is the main by-product (Janssen et al., 1995).

3.3.6.3.1 Thiobacilli

A number of chemotrophs are suitable for the biodesulfurization (BDS) of gases containing H₂S. These bacteria grow by using inorganic carbon (CO₂) as a carbon source and chemical energy from the oxidation of reduced inorganic compounds such as H₂S. In the presence of reduced organic carbon sources (glucose, amino acids, etc.), some of these bacteria (so-called mixotrophic microorganisms) can grow heterotrophically using the organic carbon as a carbon source and an inorganic compound as an energy source (Prescott et al., 2003).

BDS of gases containing H₂S by chemotrophs occurs under aerobic conditions with O₂ as an electron acceptor or in anaerobic conditions with alternative electron acceptors (e.g. nitrate), depending on the type of bacteria (Prescott et al., 2003; Syed et al., 2006).

Thiobacillus sp. is widely used in studies of the degradation of H₂S and other sulfur compounds by biological processes (Sublette and Sylvester 1987a,b; Chung et al. 1996; Cha et al. 1999 and Oyarzún et al. 2003). These bacteria have the ability to grow under various environmental stress conditions, such as oxygen deficiency, acid conditions, etc. Many Thiobacillus sp. have acidophilic characteristics and are able to develop in conditions of low pH (1–6).

Thiobacillus ferroxidans is reported to oxidize ferrous ions at 30 °C and pH 2.2 at a rate 500,000 times as fast as the rate at which oxidation would occur in the absence of the bacteria (i.e in chemical oxidation) (Satoh et al., 1988). In another microbial desulfurization process developed up to laboratory scale, where the direct conversion of H₂S to water soluble sulfate ions has been reported, the culture used was facultative anaerobic and autotrophic bacteria, Thiobacillus denitrificans (Sublette and Sylvester, 1987a,b).

Gadre (1989) reported a fixed-film bioreactor for H₂S removal from biogas (2% H₂S) using chemoautotrophic bacteria likely to be Thiobacilli. At a maximum volumetric removal rate of 3.2 mmol (H₂S)/L/h, the removal efficiency of 69.5% was not satisfactory. Insufficient oxygen availability in the bioreactor was thought to be the reason. The employing of fixed-film bioreactors, according to Gadre (1989), would be simple and effective since they have many advantages over other advanced bioreactors, such as rapid initial start-up, ability to withstand shock loadings, rapid restart after long shutdowns, elimination of mechanical mixing and biomass recycling, and better stability and efficiency. Removal of carbon disulfide and hydrogen sulfide from the exhaust gases of a cellulose filament manufacturing plant has been carried out on a pilot scale. Large volumes of exhaust gas, with a comparatively low concentration of H₂S and CS, were treated in a fixed bed bioreactor equipped with polypropylene or polyethylene packing material carrying immobilized microorganisms identified mainly as Thiobacillus. Figure 3.17 shows a schematic diagram of the process. The crude gas flows counter currently to a water-activated sludge suspension through the reactor. The oxidation products (SO₄^2–, H⁺) are removed continuously from the reactor and neutralized by the addition of NaOH. The sulfate formed is removed from the settler by continuous addition of water and the surplus of sludge is also continuously harvested from the settler.

Cadenhead and Sublette (1990) described a study of H₂S oxidation by Thiobacillus thioparus, Thiobacillus versutus, Thiobacillus neopolitanus, and Thiobacillus thiooxidans the purpose of which was to determine whether any of these Thiobacilli offered clear advantages over T. denitrificans in the aerobic oxidation of H₂S. None of the organisms utilized were found to offer a clear advantage over T. denitrificans; all Thiobacilli showed lower biomass yields and lower NH₄⁺ utilization than those reported by Sublette (1987) for aerobic oxidation of H₂S by T. denitrificans in similar batch conditions. No comparisons were made on the basis of oxidation rates.

Nishimura and Yoda (1997) used a multiple bubble-tray airtight contact tower (bio-scrubber) to scrub H₂S from the biogas produced by an anaerobic wastewater treatment process, as shown in Figure 3.18a. A two-reactor system (a gas-liquid contact tower and an aeration tank) was employed to separate the oxidation process from the absorption process to prevent air from mixing with the biogas. In the contact tower, H₂S from the biogas was absorbed into the mixed liquor and subsequently oxidized to sulfate by sulfur oxidizing bacteria after returning to the aeration tank. When treating 2000 ppm of H₂S in 40 m³/h of biogas, more than 99% removal efficiency was achieved.

Kraakman et al. (2011) divided the bioreactors for gas treatment into two groups: turbulent and laminar contactors, The power consumption in laminar contactors systems (BTF) can be 1 or 2 orders of magnitude lower than in turbulent contactors. For the BTF, the nozzle at the top can disperse water evenly and form a thin liquid film on the surface of the carriers, which is good for H₂S transferring from gas phase to liquid phase. For the BBC however, the biogas enters from the bottom of the reactor to form bubbles and the diameter of the bubbles is about 1 mm between microbubbles and small bubbles. Smaller bubbles are good for mass transfer because they increase the gas-liquid contact area, while the turbulence of gas-liquid is weakened when the bubble is smaller. Thus, it is necessary for the BBC with smaller bubbles to adopt extra measures to improve the mixing of gaseous pollutants and carriers with biofilm.

Soreanu et al. (2008a; 2009) reported a maximum EC of 10.6–14.5 g H₂S/m³/h with 74–100 % of removal efficiency in an anoxic BTF.

Shell International Oil Products developed a process that uses naturally occurring Thiobacillus bacteria to remove hydrogen sulfide from natural gas by converting it to sulfur (Van Grinsven, 1999). Thiobacillus thiooxidans has a great tolerance for acidic conditions and can grow at a pH < 1 (Takano et al., 1997; Devinny et al., 1999).

A full scale plant located northeast of Brooks, Alberta, Canada uses the Shell-Paques process for natural gas desulfurization (Benschop et al., 2002). H₂S is removed from a gaseous stream by absorption into a sodium carbonate/bicarbonate solution. The sulfide containing scrubbing liquid is treated in the bioreactor where it is mostly converted biologically to elemental sulfur. The bioreactor is supplied with a nutrient stream, air, makeup water, and sodium hydroxide. It is reported that normally less than 3.5% of the sulfide is converted to sulfate and a continuous bleed stream is required to avoid accumulation of sulfate. A compost filter is used to treat the trace H₂S present in the spent air from the bioreactor. Less than 4 ppmv effluent H₂S concentration is achieved when treating natural gas containing 2000 ppmv H₂S.

A bio-filter is a three phase bioreactor (gas, liquid, solid) made with a filter bed that has a high porosity, high buffer capacity, high nutrient availability, and high moisture retention capacity to ensure that the target microorganisms can grow on it, as shown in Figure 3.18b (Jorio and Heitz, 1999; Elias et al., 2002; Daustos et al., 2005). The contaminated gas is continuously fed in the bio-filter, while a nutrient solution is discontinuously added. Various types of bio-filter media have been used by researchers. Representative cases are discussed below.

Chung et al. (1997) used Thiobacillus novellus in a bio-filter for H₂S oxidation under mixotrophic conditions. A removal efficiency of 99.6% was achieved and the products were sulfate (83.6%) and sulfite (12.6%). Little conversion of sulfide to elemental sulfur was achieved.

Shareefdeen et al. (2002) reported the operation of a commercial bio-filter for the treatment of an air stream containing hydrogen sulfide, ammonia, dimethyl sulfide, methanethiol, and ethylamine. This proprietary wood-based (BIOMIX™) bio-filter achieved 96.6% removal of H₂S at an inlet concentration of 1.07 mg/m³.

Elias et al. (2002) used packing material made up of pig manure and sawdust for bio-filtration purposes. More than 90% H₂S removal efficiency was attained at a loading rate of 45 g/m³/h. No nutrient was added to the system and the porosity of the packing material decreased from 23.1 to 12.9%. However, this change in porosity did not affect the removal efficiency significantly and it was claimed that the bio-filter could be easily cleaned by flushing water through the inlet. The main by-product of the biodegradation process was sulfur (82% of total sulfur accumulation) accompanied by sulfates and thiosulfates (<18%).

Schieder et al. (2003) described the “BIO-Sulfex” bio-filter to remove H₂S from biogas, which uses thiobacteria attached on fixed bed material. The biomass was aerated and the filter was flushed with nutrient containing liquid to remove sulfur from the system. Six BIO-Sulfex modules to treat biogas containing up to 5000 ppm H₂S were operated at flowrates of 10 to 350 m³/h with 90% or more H₂S removal achieved.

Clark et al. (2004) operated a pilot-scale agricultural bio-filter to reduce odors from a swine manure treatment plant’s exhaust air. Bio-filters were packed with polystyrene particles and peat moss (3:1 ratio by volume). The packing volume was 1.89 m³. The gas flowrate was 100 L/s. The inlet load contained 2–60 ppm hydrogen sulfide and 2–30 ppm ammonia. The addition of nutrients did not play an important role in the overall system performance. Average odor reduction was approximately 38% without addition of nutrients and 45% when nutrients were added. Increasing the temperature had a favorable effect during the acclimatization phase only.

The working principle of a bio-trickling filter is the same as for a biofilter except that the packed bed is continuously trickled over by an aqueous phase nutritive solution as shown in Figure 3.18c (Cox and Deshusses, 2001).

Cox and Deshusses (2001) used two laboratory scale bio-trickling filters (BTF) made of polypropylene inoculated with biomass from a toluene biodegrading filter operating at pHs of 4.5 and 7.0 to treat H₂S and toluene in a gas stream. There was no significant difference between the performances of the two reactors in terms of H₂S removal. At an inlet concentration of approximately 50 ppmv, complete consumption of H₂S was observed. However, the removal efficiency decreased to 70–80% when the inlet concentrations were raised to 170 ppmv. High removal efficiency for H₂S, in comparison to other reduced sulfur compounds, was obtained by Gabriel and Deshusses (2003) using Thiobacillus sp. in a bio-trickling filter (BTF). For inlet H₂S concentrations as high as 30 ppmv, the typical removal efficiency was 98%. Methyl mercaptan, carbonyl sulfide, and carbon disulfide removal efficiencies were 67, 44, and 35% at inlet concentrations of 67, 193, and 70 ppbv, respectively.

Soreanu et al. (2005) developed a laboratory-scale bio-trickling system, as shown in Figure 3.18c, in order to remove H₂S from digester biogas under anaerobic conditions. In these experiments, polypropylene balls inoculated with anaerobically digested sludge were used as packing material in the bioreactor (packing volume of 0.0062 m³, 90% volume free). Sodium sulfite was added in the nutritive solution as an oxygen scavenging agent. Nitrate was used as an electron acceptor in the absence of oxygen. Removal efficiency greater than 85 % was achieved for an H₂S inlet concentration of 500 ppm and a gas flowrate of 0.05 m³/h. Of particular interest, inhibition of the biological process by trace amounts of O₂ was noticed when a nitrate solution was used as the sole nitrogen/nutrient source.

Li et al. (2016) investigated the influence of the molar ratio of sulfide/nitrate (S/N) on biogas desulfurization performance in a bio-trickling filter (BTF) and a bio-bubble column (BBC). The results showed that with the decrease of the S/N ratios from 3.6 to 0.7, the removal efficiencies of H₂S increased from about 66 to 100%, while the removal of nitrate decreased from 100 to 70% in the two bioreactors. Thus, the BTF has a better and more stable desulfurization performance than the BBC does which could be attributed to their different gas-liquid contacting modes. After the startup period, the removal efficiencies of H₂S were above about 95% in the BTF at different S/N ratios. However, in the BBC, the removal efficiencies of H₂S were about 78, 85, and 100% when the S/N ratios were about 2.4, 1.2, and 0.7, respectively. With the increase of the S/N ratios from 1.0 to 2.5 in the BTFs, the removal of H₂S in biogas was affected slightly, while the percentages of the produced sulfate decreased evidently. In addition, different supplying methods of nitrate wastewater, i.e. intermittent and continuous, did not affect the removal of H₂S significantly, while the intermittent addition of nitrate wastewater increased the percentages of sulfate and denitrification performance. The maximum elimination capacity (EC) of H₂S recorded 54.5 g H₂S/m³/h in the BTF.

3.3.6.3.1.a. Thiobacillus Denitrificans

Sublette and Sylvester (1987a) employed the anaerobic growth of T. denitrificans on H₂S in a continuous stirred-tank reactor. Although the removal of H₂S recorded no more than 3%, a maximum volumetric productivity of 2.3 mmol (H₂S)/L/h was recorded. They concluded that the efficiency of the process was low and the reactor volumes required for field applications would be impractical. Biomass concentration and the quality of the environment of the cells were identified as the two most important variables in maximizing volumetric productivity while maintaining reactor stability.

Sublette (1990) reported the application of the above process on a pilot scale for the treatment of a biogas from an anaerobic digester. The bioreactor consisted of a bubble column that received a gas feed of biogas plus compressed air. The off gas from the bubble column contained >9% oxygen. The contamination of biogas or methane by O₂ can be hazardous due to the explosive nature of such mixtures. The volumetric productivity was just 1.1 mmol (H-S oxidized)/L/h. However, a concentration of only 0.15% H₂S in the biogas made the T. denitrifcans reactor volumetrically feasible.

T. denitrificans have also been used for the oxidation of sulfide (H₂S, HS^-, S^2-) in sour gas to sulfate. A sulfide-tolerant strain of T. denitrifcans was co-immobilized with CaCO₃ in calcium alginate beads and contacted with sour gas under anaerobic conditions in a packed-bed column. The co-immobilized CaCO₃ has three functions; CaCO₃ acts as a buffer neutralizing the acid by-product of sulfide oxidation. Acting as a buffer, CaCO₃ converts to HCO₃^– and CO₂, which serve as carbon sources to support the growth of T. denitrijcans. Ca²⁺-generated internally into the beads maintains the mechanical stability of the beads.

Buisman et al. (1990) tested three different continuous-flow reactor configurations: fixed-film CSTR (stirred-tank reactor), bio-rotor (a rotating cage containing reticulated polyurethane biomass support particles partly immersed in the reactor liquid), and a fixed-film up-flow reactor. For the up-flow and bio-rotor reactors, 95 to 100% sulfide removal efficiencies were achieved for loading rates up to 500 mg H₂S/h/L. The removal efficiency decreased rapidly above this loading rate. At 938 mg/h/L (biorotor) and 1040 mg/h/L (up-flow) loadings, sulfide removal efficiencies were 69 and 73%, respectively. At a 500 mg/h/L sulfide loading rate, the stirred-tank reactor’s removal efficiency was approximately 62%.

Thiobacillus denitrificans is able to grow facultatively on reduced sulfur compounds by reducing nitrate (NO₃^-) to nitrogen gas (N₂) (Lampe and Zhang 1996; Kleerebezem and Mendez 2002). The oxidation of H₂S by T. denitrificans has also been applied in a two-stage microbial process for the removal of sulfur dioxide from a gas with net oxidation to sulfate. In reactors in a series, SO₂ was reduced to H₂S in the first stage by Desulfovibrio desulfuricans. The H₂S was then stripped with nitrogen and sent to a second stage where it was oxidized to SO₄^–2 by T. denitrifcans. A sulfur balance demonstrated complete reduction to H₂S in the first stage and complete oxidation of H₂S to SO₄^–2 in the second stage.

Malhautier et al. (2003) used two laboratory scale bio-filters packed with granulated digested sludge. One unit was fed mainly with H₂S and the other unit with NH₃. Complete H₂S removal (100%) and 80% NH₃ removal efficiency occurred. However, the authors concluded that the oxidation of high levels of H₂S might have a negative effect on the growth and activity of nitrifying bacteria.

Ma et al. (2006) described a biological removal of high concentrations of H₂S using the immobilized Thiobacillus denitrificans with peat moss, wood chip, ceramic, and granular activated carbon (GAC), separately, as shown in Figure 3.19. A GAC bioreactor had significant potential to treat H₂S odor gas as GAC provides a more uniform surface area and good resistance to crushing, allowing better operational control in areas such as gas adsorption capacity, gas distribution, and pressure drop. In addition, GAC provided higher bacterial adsorption capacity than other inert carriers and could be regenerated. The GAC bioreactor achieved an average 96.8% removal efficiency of H₂S at the inlet concentrations of 110–120 mg/L of H₂S during a 160-day operating period. No significant acidification phenomenon occurred in this system during H₂S treatment because its main product was determined to be elemental sulfur.

3.3.6.3.1.b. Thiobacillus Thioparus

Tanji et al. (1989) described a system for the simultaneous removal of low concentrations of DMS, methyl mercaptan (MM), and H₂S from malodorous gases. Thiobacillus thioparus TK-m cells were immobilized on cylindrical porous polypropylene pellets and contacted with a sulfur-containing gas in a packed tower. Up to 95% removal of H₂S was achieved at a rate of 0.73 mmol/L/h. DMS was relatively less decomposable both by immobilized and freely dispersed T. thioparus TK-m. The removal rate of MM was intermediate. It was stressed that when treating large quantities of gas at low concentrations, a reduction in pressure drop was important to economize the operating costs.

Cho et al. (1992b) presented the first report of the application of an isolated microorganism to a practical deodorizing system. The capacity of Thiobacillus thioparus DW44 to remove hydrogen sulfide, MT, DMS, and DMDS from exhaust gas was displayed in a pilot-scale peat bio-filter. A schematic diagram of the peat bio-filter is shown in Figure 3.20. The moisture content of the peat was controlled at 6–70% by spraying with effluent water from a wastewater treatment plant. The temperature inside the bio-filter was kept above 8 °C at all times by preheating the inlet gas. No acclimation period was needed to reach such a high efficiency in the removal of the gases. The presence of heterotrophic bacteria and fungi utilizing organic substances extracted from the peat by the supplied wastewater did not seem to adversely affect the activity of T. thiopurus DW44.

Chung et al. (1996) immobilized Thiobacillus thioparus CH11 with Ca-alginate producing pellet packing material for the bio-filter. At a 28 s optimal retention time, the H₂S removal efficiency was more than 98%. Elemental sulfur or sulfate was produced depending on the inlet H₂S concentration.

Kim et al. (2002) investigated the simultaneous removal of H₂S and NH₃ using two bio-filters, one packed with wood chips and the other with granular activated carbon (GAC). A mixture of activated sludge (as a source of nitrifying bacteria) and Thiobacillus thioparus (for sulfur oxidation) was sprayed on the packing materials and the drain solution of the bio-filter was recirculated to increase the inoculation of microorganisms. Initially, both of the filters showed high (99.9%) removal efficiency. However, due to the accumulation of elemental sulfur and ammonium sulfate on the packing materials, removal efficiency decreased over time to 75 and 30% for H₂S and NH₃, respectively.

Oyarzún et al. (2003) used peat for the filter bed of a bio-filtration system inoculated with Thiobacillus thioparus. Supplemental nutrients were added and the initial moisture content was adjusted to 92%. The pH was also adjusted to 6.0. Full removal was achieved when fed with 355 ppm H₂S at 0.03 m³/h. The removal efficiency decreased with increasing inlet H₂S concentrations and a maximum removal capacity of 55 g/m³/h was obtained.

Aroca et al. (2007) performed a comparative study on the removal of hydrogen sulfide in bio-trickling filters inoculated with Thiobacillus thioparus (ATCC23645) and Acidithiobacillus thiooxidans (ATCC19377). That proved the acid bio-trickling filter inoculated with A. thiooxidans perfoms better H₂S removal with the advantage being that the system does not require an exhaustive pH control of the liquid media, recording 370 g S/m³/h at 45 sec of residence time and 405 g S/m³/h of inlet load (91% of efficiency), while a maximum recorded removal capacity of the biotrickling filter inoculated with T. thioparus was 14 g S/m³/h at 30 g S/m³/h of inlet load and 47% removal efficiency at a residence time of 26 sec within pH range of 5.5–7.0.

Tóth et al. (2015) employed a sulfur-oxidizing bacteria, Thiobacillus thioparus (immobilized on Mavicell B support), to develop a micro-aerobic bio-trickling filter reactor for the efficient elimination of H₂S from synthetic biogas as shown in Figure 3.21. To test the capability of this particular strain in an oxygen-limited atmosphere, a fixed bed reactor was operated under 0.25–5.0 vol.% O₂ concentrations and its H₂S decomposing ability was statistically evaluated. It was found that the system achieved 100% H₂S elimination efficiency when at least 2.5 vol.% oxygen was provided.

The biological oxidation process of the absorbed sulfides to elemental sulfur by Thiobacillus thioparus TYY-1 was studied in an airlift-loop reactor (effective volume of 20 L) (Liu et al., 2015b). The air aeration quantity was found to be the key influence factor of the desulfurization rate and elemental sulfur production rate. The best treatment effect of aeration was obtained at 120–160 L/h. The hydraulic remain time was found to be also important and the optimum hydraulic remain time was found to be 4–6 h under the influent concentration of S^2– for 200 mg/L. With these conditions after 20 days of operation, the results showed superior performance of the bioreactor for the desulfurization rate and elemental sulfur production rate, where the conversion products were mainly sulfur and the production rate of SO₄^2– was low. The removal efficiency of sulfide was more than 99.5% while the maximum yield of sulfur was 88% approximately.

In the raw natural gas or biogas, there are also some volatile organic sulfur compounds (VOSCs) such as methanethiol, dimethyl disulfide, dimethyl sulfide, carbon disulfide, and carbonyl sulfide (Mata-Alvarez and Llabrés, 2000; Böresson, 2001; Sheng et al., 2008). These VOSCs could also be absorbed by the alkaline adsorbents and make a significant effect on the activity of sulfide-oxidizing bacteria (SOB) (Lobo et al., 1999). Carbon disulfide was a common ingredient in sour natural gas, biogas, and some tail gases of plants. Currently, carbon disulfide is found to have a negative effect on desulfurization, but little research was done to investigate the inhibitory effect of carbon disulfide on the BDS process. As an example, Kim et al. (2005) reported the concentrations of carbon disulfide in landfill gas from four landfill sites to be in the range from 25 to 5352 ppb, where the highest ratio of H₂S to CS₂ was about 6:1, which was highly toxic to the strain Thiobacillus thioparus. Ziyu et al. (2014) have investigated the effect of different carbon disulfide concentrations (0.01, 0.05, 0.1, 0.15, and 0.2%) on the growing and resting cells of Thiobacillus thioparus CGMCC 4826, which was isolated from the effluent of sulfate reducing the bioreactor, and found that this strain could oxidize thiosulfate to elemental sulfur and sulfate.

Although the carbon disulfide slightly dissolved in the water, little carbon disulfide could obviously inhibit the growth of cells. Carbon disulfide at a concentration of 0.01% has significantly inhibited the growth of cells, but has hardly affected the BDS efficiency of resting cells. Although carbon disulfide at concentration of 0.05% had a negative effect on the BDS efficiency of resting cells, the effect of inhibition could be relieved by the increased density of resting cells. Therefore, 0.05% was chosen to be the critical value of carbon disulfide for BDS. For the resting cells, the parameters of the Michaelis-Menten equation were calculated by the method of Lineweaver-Burk. The V_max of BDS was decreased from 27.93 to 14.0 S₂O₃^2– mg/L/h and the K_m was increased from 0.264 to 0.884 mM with the concentration of carbon disulfide rising up from 0.0 to 0.1%, so, under the optimized BDS-process, it was necessary to adjust the concentration of carbon disulfide in the absorbent below 0.05% by renewing the absorbent. Finally, these results showed that the growth of cells was sensitive to carbon disulfide and the resting cells had a resistance to the low level of carbon disulfide (0.05%).

3.3.6.3.1.c. Thiobacillus Thiooxidans

Berzaczy et al. (1990) patented a microbiological conversion process for the removal of sulfur-containing pollutants such as H₂S, CS₂, COS, thioalcohols, thioethers, and thiophenes in a waste gas, especially from cellulose fiber manufacture. In this process, Thiobacillus thiooxidans cells immobilized on commercially available packing material are contacted with the gas in a packed-bed reactor. The cells are kept moist at all times by spraying with a nutrient solution. The metabolic products (mainly H₂SO₄) draining from the reactor are neutralized by the addition of lye and lime water in two stages. CaSO₄ precipitates and is removed as a waste product. The supernatant is returned for internal circulation to minimize loss of chemicals. To prevent salt concentration in the cycle liquid, a corresponding quantity of salt solution is drawn from the neutralization vessel and replaced with fresh water.

Duan et al. (2006) used biological activated carbon as a novel packing material to achieve a performance enhanced bio-filtration process in treating H₂S through an optimum balance and combination of adsorption capacity with the biodegradation of H₂S by Thiobacillus thiooxidans, immobilized on the material, as shown in Figure 3.22. The biofilm was mostly developed through culturing the bacteria in the presence of carbon pellets in mineral media. In two identical laboratory scale bio-filters, one was operated with biological activated carbon (BAC) and another with virgin carbon without bacteria immobilization. Various concentrations of H₂S (up to 125 ppmv) were used to determine the optimum column performance. A rapid startup (a few days) was observed for H₂S removal in the bio-filter. At a volumetric loading of 1600 m³/h (at 87 ppmv H₂S inlet concentration), the elimination capacity of the BAC (181 g H₂S m³/h) at removal efficiency of 94% was achieved. If the inlet concentration was kept at below 30 ppmv, high H₂S removal (over 99%) was achieved at a gas retention time as low as 2 s, a value which is shorter than most previously reported for bio-filter operations.

3.3.6.3.1.d. Thiobacillus Ferrooxidans

Neumann et al. (1990) described a method for the removal of H₂S from biogas utilizing Thiobacillus ferrooxiduns cells in a packed bed. The microorganisms are immobilized on peat, brushwood or pruning, refuse compost, or rubber. Air is fed continuously with gas to the reactor for oxidation of H₂S to S or SO₄^2–. Part of the clean gas/air mixture is recycled to the reactor inlet. To maintain a residual O₂ concentration in the purified gas below 3.0%, the feeding of air is adjusted by a controller. The controller receives a continuous signal from an oxygen probe on the outlet gas line, calculates a response compensating for the dynamics of the bio-filter system, and regulates the position of a control valve on the air line. The bed is kept moist at all times by a recirculating nutrient solution which also removes S/SO₄^2– and keeps the pH in its optimal range of 1.7–3.2. There is, however, no mention of flow rates or oxidation rates. This process applies to a situation, often encountered in practice, where the amount and composition of biogas are not constant. Only by adjusting the air flowrate to the amount and composition of biogas can the generation of an explosive mixture be avoided.

A promising alternative for the microbial treatment of H₂S containing gases is a Japanese method utilizing T. ferrooxidans as a means of reducing the costs of H₂S removal (Magota et al., 1988). A layout of the BIO-SR process scheme is shown in Figure 3.5.

The process is basically dependent on the reaction of H₂S gas injected into a ferric sulfate solution in an absorber, producing a precipitate of elemental sulfur:

Depending on the gas flowrate and efficiency required, several types of absorbers are suitable, such as jet scrubbers, bubble-cap towers, or packed towers. Elemental sulfur is separated and recovered from the reduced solution of ferrous sulfate in a separator.

The sulfur separators can include settlers, filter presses, and sulfur melters, depending on the quality of elemental sulfur required.

The reactant, ferric sulfate, is regenerated from the ferrous sulfate solution by biological oxidation in an aerated bioreactor using T. ferrooxidans cells:

The oxidized solution is then recycled to the absorber to repeat the cycle. A distinct advantage of the process is that the reaction of H₂S with ferric sulfate is so rapid and complete that there remains no danger of discharging toxic waste gas. An H₂S removal efficiency of more than 99.99% has been attained in an existing commercial plant.

In general, the operating cost of the BIO-SR process is one-third that of conventional processes though the capital cost is only slightly lower. This gives an overall cost for the BIO-SR process, which is about 50% of the cost of conventional processes. Serious disadvantages of other microbial processes for H₂S removal are avoided in the BIO-SR process. H₂S does not inhibit the bacteria and SO₄^2– does not accumulate in the system. Also, contamination of the purified gas with O₂ is prevented.

Son and Lee (2005) improved a hybrid reactor by combining a chemical reduction reactor and a biological oxidation reactor to remove the toxic effect of H₂S on the cells and to enhance the H₂S removal rate. The microbial cells were immobilized on the surface of curdlan particles in order to enhance the Fe(II) oxidation rate through repeated fed-batch operation. As a result, the iron oxidation rate was four times faster than that obtained with the free cells. Iron solution, oxidized in an oxidation reactor by Thiobacillus ferrooxidans, was fed into the iron reduction reactor and the reduced iron solution was recycled into the iron oxidation reactor. The X-ray diffractometer (XRD) data indicated that iron was precipitated along with elemental sulfur at the high concentration of H₂S, resulting in the iron oxidation rate being decreased with increasing reaction time.

Lin et al. (2013) used a pilot-scale chemical–biological process to remove H₂S from biogas, as shown in Figure 3.23. The inlet H₂S concentration was 3542 ppm and a removal efficiency of 95% was achieved with a gas retention time of 288 s. Purified biogas with an average of 59% CH₄ was collected for power production.

3.3.6.3.2 Other Examples of Chemotrophic Bacteria

Mesa et al. (2002) described a bio-scrubber system which can be integrated into a system to remove H₂S from biogas by a combination of chemical and biological processes. H₂S removal can be achieved by absorption in a ferric sulfate solution producing ferrous sulfate and elemental sulfur. Ferric sulfate can be regenerated by biological oxidation using Acidithiobacillus ferrooxidans. This study investigated the oxidation of ferrous iron by ferrooxidans which was immobilized on a polyurethane foam support where the support particles placed in an aerated column. Low cost polyurethane was chosen for being macroporous, having a large surface for microbial growth, and offering lower diffusion resistance to substrate transfer. However, ferric precipitates were accumulated on the support and on the air diffusers, which necessitated periodic interruptions of the process for cleaning. Precipitation, air supply, and chemical cost are the potential constraints for this process.

Beggiatoa sp. and Thiothrix sp., both microaerophilic σ-proteobacteria, have mixotrophic nutritional functions because they are able to degrade H₂S using inorganic and organic (e.g. acetate) energy sources (Howarth et al., 1999; Prescott et al., 2003). Pseudomonas acidovorans and Pseudomonas putida are other mixotrophs which degrade both H₂S and organosulfur compounds (Chung et al., 2001; Oyarzún et al., 2003).

Chung et al. (2001) used bio-filters packed with co-immobilized cells Pseudomonas putida CH11 and Arthobacter oxydans CH8 for the removal of H₂S and NH₃, respectively, which are often present in off-gases of a livestock farm. In the 5–65 ppm range, H₂S and NH₃ removal efficiencies were greater than 96%. However, at higher concentrations, H₂S and NH₃ showed inhibitory effects on H₂S removal. They also assessed the environmental risk associated with the release of bacteria when treating large volumes of waste gases; the exhaust gas contained small amounts of bacteria (<19 CFU/m³ in all cases) and was considered safe.

Sercu et al. (2005) studied the aerobic removal of hydrogen sulfide using a bio-trickling filter (BTF) packed with 1 L-polyethylene rings inoculated with Acidithiobacillus thiooxidans ATCC-19377. The inlet H₂S concentration was varied between 400 and 2000 ppm and the airflow rate was varied between 0.03 and 0.12 m³/h. However, the system performance was not affected by changing the operational conditions and a maximal removal efficiency of 100% was obtained. During the experiment, the pH of the nutritive solution decreased to 2–3, but this did not affect the process performance.

Other studies that combined chemical and biological processes for both H₂S elimination and ferric iron regeneration by Acidithiobacillus ferrooxidans have been reported (Giro et al., 2006; Alemzadeh et al., 2009; Ho et al., 2013). These processes are based on two reactions, as follows: the inlet H₂S is first oxidized with a ferric iron solution and yields elemental sulfur and the reduced ferrous iron is then re-oxidized by A. ferrooxidans in the biological process.

Cheng et al. (2013) investigated the integration of chemical and biochemical processes for desulfurization of simulated natural gas containing hydrogen sulfide (H₂S) using polyurethane foam as a support for the immobilization of Acidithiobacillus ferrooxidans. A good biological oxidation performance with a maximum oxidation rate of ferrous iron of 4.12 kg/m³/h was recorded. That proved the effectiveness of the immobilizing matrix. The chemo-biochemical process is illustrated in Figure 3.24. A bubble column absorber was used as a chemical oxidation unit. Inlet and outlet were provided at the bottom and the top for untreated and treated simulated natural gas, respectively. The immobilized cell bioreactor was a glass column with a working volume of 0.56 L, where fresh modified 9K medium (Fe²⁺ = 8.5 kg/m³) or reduced iron solution from the absorber was influx at the bottom of the bioreactor. Air was supplied with an air compressor through a filter and fed in at the bottom of the bioreactor. H₂S was produced by an equimolar reaction of Na₂S and H₂SO₄. After being introduced by passing nitrogen (99.9% v/v) over an H₂SO₄ solution where a solution of Na₂S was dripped, the simulated natural gas was introduced into the absorber and the gas flow rate was adjusted by a gas flowmeter. Elemental sulfur was retained through a sedimentation basin. Such chemo-biochemical processes (Figure 3.24) achieved removal efficiencies of about 80% when treating high concentrations of H₂S (15,000 ± 100 ppmv). This capacity was higher than those reported in literature, compared to 77 g H₂S/m³/h (Pagella and De Faveri,2000), 250 g H₂S/m³/h (Pandey et al., 2003), 22 g H₂S/m³/h (Son and Lee, 2005), and 100 g H₂S/m³/h (Li et al., 2008).

Ho et al. (2013) used the chemical–biological process, as shown in Figure 3.25, using iron-tolerant A. ferrooxidans CP9 that maintained the balance of the Fe²⁺/Fe³⁺ ratio and reached an H₂S removal efficiency of 98%. In the pilot-scale operations, the addition of glucose improved the biogas purification efficiency by increasing the cell density and ferrous oxidation efficiency. The H₂S loading reached 65.1 g S/m³/h (3.3-fold higher than the laboratory-scale condition) with a removal efficiency of 96%. In addition, the factors of high tolerance for iron ions at 20 g/L and the rapid ferrous iron oxidation ability of A. ferrooxidans CP9 were important in maintaining the balance of Fe²⁺/Fe³⁺ concentrations. Although the exotic microbes appeared during the 311 d operation, the A. ferrooxidans CP9 cell density remained more than 10⁸ CFU/g. These results clearly demonstrate that the chemical–biological process is a feasible method for removing a high H₂S concentration from biogas.

Vikromvarasiri et al. (2017) evaluated the efficiency of a bio-trickling filter inoculated with Halothiobacillus neapolitanus NTV01 (HTN) on H₂S removal from synthetic biogas. HTN is an obligatory chemolithoautotrophic bacteria able to tolerate and metabolize high sulfide concentrations. The HTN was isolated and purified from activated sludge and is a sulfur oxidizing bacteria able to degrade H₂S and thiosulfate to elemental sulfur and sulfate, respectively. Operational parameters in a short term operation were varied as follows: gas flow rate (0.5–0.75 LPM), inlet H₂S concentrations (0–1500 ppmv), and liquid recirculation rate (3.6–4.8 L/h). The maximum elimination capacity was found as 78.57 g H₂S/m³ h, which had a greater performance than the previous studies.

Nazari et al. (2017) reported the isolation of Gram negative, motile, aerobic, and obligately chemolithoautotrophic and non-spore forming Halothiobacillus sp. ISOB 14 from contaminated soil with sulfur compounds for its ability to oxidize thiosulfate and used it as an electron donor. It showed a unique ability and high potential for usage in the removal of hydrogen sulfide. This bacterium also grows faster in a Postgate medium with 0.7% thiosulfate than in sulfur and sodium sulfide. Under optimum conditions, ISOB 14 showed the capability to remove thiosulfate at 100% after 18–24 h and produce sulfate up to 89.14% and 93.14% after 24 h and 72 h, respectively.

3.3.7 Other Approaches Concerning the Biodesulfurization of Natural Gas

Considerable efforts are still required concerning the packed media based biotechnologies; though effective, they have strict requirements in terms of both monitoring and maintenance due to the bacteria’s high sensitivity to fluctuations in operational conditions, which translates into costs.

Recently, biogas biodesulfurization (BDS) with high concentrations of H₂S (>1000 ppmv) is mainly done by bio-filtration systems (Fortuny et al., 2008; Montebello et al., 2012). However, since biogas is initially oxygen-free, this means that the oxidants need to be supplied into a biogas BDS system for the oxidization of H₂S. Based on different electron acceptors, i.e. O₂ or NO₃^–/NO₂^–, BDS can be classified into two categories: aerobic and anaerobic processes (Li et al., 2016). Currently, biogas aerobic desulfurization has mainly been applied, which requires a stoichiometric oxygen level depending on the inlet concentrations of H₂S in biogas (Tang et al., 2004; Vanderzee et al., 2007; Fdz-Polanco et al., 2009; Fortuny et al., 2011), but it is important to control the oxygen dosage in order to avoid reaching high concentrations of oxygen in biogas for safety reasons and because the residual oxygen or air in biogas after desulfurization can lead to a dilution of methane concentration, which will affect the further use of biogas (Fortuny et al., 2008; Soreanu et al., 2009). This can be solved upon the usage of NO₃^–/NO₂^– as electron acceptors during the BDS process. Moreover, biogas desulfurization could be coupled with wastewater denitrification when wastewater containing NO₃^-/NO₂^- is supplied (Soreanu et al., 2008a; Deng et al., 2009; Jing et al., 2009; Turker et al., 2011).

During anaerobic processes, the S/N molar ratio is the key parameter to control the level of H₂S oxidation, i.e. the end desulfurization products. At low concentrations of NO₃^–, H₂S is oxidized into S⁰, while in the presence of enough concentrations of NO₃^–, H₂S is completely transformed to SO₄^2–. The anaerobic biological utilization of H₂S as an energy source for lithoautotrophic organisms can be described with the following reactions (Soreanu et al., 2008b):

Zhou et al. (2015) also found that the S⁰ is the dominant desulfurization product at high inlet loading rates of H₂S, while the sulfate is dominant at low loading rates in a micro-aerobic BTF.

Soft sulfur bacteria are typical anaerobic autotrophic microorganisms which can use CO₂ to produce microbial biomass and transfer H₂S to elemental sulfur or sulfate in the presence of inorganic nutrients and illumination (Wu et al., 2016).

As previously mentioned, the evolved H₂S during the biogas fermentation process is removed after the anaerobic digestion process through different techniques, for example installing a set of additional desulfurization systems, such as iron oxide adsorption/oxidation, activated carbon adsorption, bio-filtration, etc. Although these methods can effectively remove H₂S from biogas, they have lots of disadvantages, such as large area required, high operation costs, and complex technological processes. Not only this, but the production of H₂S during the biogas fermentation process is reported to inhibit the anaerobic digestion process, thus reducing biogas production and leadig to poor biogas quality (Lar and Li, 2009); some novel desulfurization methods need to be developed to overcome these problems. One of these methods is in-situ H₂S removal during the biogas fermentation, which has some advantages, such as not requiring a set of additional desulfurization systems, ease of operation, simple process, and high efficiency (Jiang et al., 2014).

This process can be performed through two techniques. The first is a micro-aerobic desulfurization process, in which a small amount of oxygen/air is supplied into anaerobic digesters so that the growth of SRB can be inhibited seriously and, at the same time, the introduced oxygen could react with the generated H₂S producing elemental sulfur. The other method is in-situ desulfurization throughout the addition of desulfurizers (such as iron-based oxidants) into the anaerobic digesters (Ripl and Fechter, 1991; Zhong et al., 2004; Su et al., 2012).

Diaz et al. (2011) reported reduction in evolved H₂S when the oxygen or air was supplied into the bioreactor. Kobayashi and Li (2011) also developed a self-agitated anaerobic reactor with the addition of air, in which about 99 % of H₂S in biogas could be removed.

As mentioned before, H₂S present in biogas can be oxidized to S⁰ or SO₄^2– using nitrate and nitrite. Both nitrate and nitrite are normally available in most wastewater treatment plants and could be used to oxidize H₂S depending on the molar loading ratio of wastewater and biogas. However, a control approach is required in order to minimize the fluctuations in inlet and outlet H₂S concentrations in biogas and the oxidation potential of the wastewater used.

Turker et al. (2011) proved that combining sulfide removal with nitrate or nitrite removal not only allows the control of H₂S, but also improves nitrogen removal via autotrophic denitrification without using a carbon source. Biogas desulfurization was integrated with nitrogen removal in an industrial wastewater treatment plant (Turker et al., 2011) and a control scheme (Figure 3.26) to monitor and control the concentration of hydrogen sulfide, combined with autotrophic denitrification process was developed. The mixture of nitrate and nitrite from a nitrification plant was used as a source of electron acceptors to oxidize sulfide in biogas. The performance of the process was monitored by an oxidation–reduction potential (ORP) sensor and the control scheme was developed to improve fluctuations in sulfide load to the bio-scrubber for stable operation, where the control scheme has been developed for biogas desulfurization using ORP under industrial conditions. The redox potential was maintained at about +50 to +100 mV in the activated sludge plant to monitor the performance of the nitrification process. The redox potential in the bio-scrubber was related to sulfide removal from biogas. More than 90% of H₂S was removed from the biogas with simultaneous nitrogen removal at wastewater/biogas ratios between 2 and 3. The process control algorithm was developed based on the measurement of redox potential in the effluent of the bio-scrubber. The effluent redox potential was related to sulfide removal and greater than 90% sulfide removals were correlated with approximately –100 mV effluent redox potentials. This value was used as a controlled variable and an influent wastewater flowrate containing a mixture of nitrate and nitrite was the manipulated variable. Therefore, the combination of anaerobic treatment, biogas production, and biogas cleaning with aerobic treatment could improve biogas desulfurization. This allows the integration of sulfur and nitrogen cycles to alleviate sulfur emissions. Thus, the advantage of the method developed by Turker et al. (2011) is that it combines sulfide removal with nitrogen removal which is normally required in most industrial wastewater treatment systems.

In another study concerning the simultaneous removal of sulfide and nitrite, Doğan et al. (2012) declared that bio-oxidation of sulfide under denitrifying conditions is a key process in the treatment of gas and liquids that are contaminated with sulfide and nitrite. A lab-scale continuous flow stirred tank reactor (CFSTR) was operated with nitrite as the electron acceptor for the evaluation of the effects of loading rates, hydraulic retention time (HRT), and substrate concentrations on the performance of the autotrophic denitrification process. The influent sulfide concentration was maintained at 0.16 kg/m³, the HRT was decreased from 8.4 to 2 h and, for the entire study period, the sulfide removal efficiency was above 80% for the loading rates that ranged from 0.47 to 2.16 kg S^2–/m³day. However, lower influent loading of NO₂^–-N that corresponded to the stoichiometric ratios was used and the nitrite removal efficiency was close to 100%.

Klok et al. (2012) reported the application of haloalkaliphilic sulfide-oxidizing bacteria in gas lift bioreactors inoculated at oxygen-limiting levels, that is below an O₂/H₂S mole ratio of 1, where sulfide was oxidized to elemental sulfur and sulfate. This suggested that the bacteria reduced NAD⁺ without the direct transfer of electrons to oxygen and that this is most likely the main route for oxidizing sulfide to elemental sulfur, which is subsequently oxidized to sulfate in oxygen-limited bioreactors. This pathway is called the limited oxygen route (LOR). Biomass growth under these conditions is significantly less abundant at higher oxygen levels.

Ramos et al. (2012) developed a new biotechnological process for the removal of H₂S from biogas. The desulfurization conditions present in micro-aerobic digesters were reproduced inside an external chamber called a micro-aerobic desulfurization unit (MDU) as shown in Figure 3.27. A 10 L-unit was inoculated with 1 L of digested sludge in order to treat the biogas produced in a pilot digester. After 128 d of incubation under optimum conditions, the average removal efficiency was 94%. Microbiological analysis confirmed the presence of at least three genera of sulfide-oxidizing bacteria. Approximately 60% of all the H₂S oxidized was recovered from the bottom of the system in the form of large solid S⁰ sheets with 98% w/w of purity. Therefore, this system could become a cost-effective alternative to the conventional bio-techniques for biogas desulfurization.

Wu et al. (2016) reported the in-situ removal of H₂S by the microsupply of oxygen during anaerobic batch fermentation with rice straw as a raw material under mesophilic and thermophilic conditions at 35 and 55 °C, respectively. There were no obvious changes in biogas production and methane concentration with the addition of limited oxygen in both mesophilic and thermophilic fermentation. However, the suitable oxygen supply was found to be 2.0 ~ 4.0 times of theoretical demand under which the average desulfurization efficiency could be over 92% and the oxygen residues content complied with the requirements of biogas used as car fuel or incorporated into the gas network (no more than 0.5%). Moreover, the total biogas production increased slightly compared with the control experiment without oxygen. This was explained as follows: the hydrogen sulfide concentration is greatly reduced if more oxygen is being supplied, which reduces toxicity and increases methane production. It may also be attributed to the limited oxygen supply, causing facultative anaerobic bacteria in the fermentation system to use oxygen for hydrolysis which produces carbon dioxide or other gases. Upon the usage of air instead of oxygen, there was no obvious difference in biogas production, but the nitrogen content was higher compared to the case of pure oxygen. The desulfurization efficiency reached 93.9% and the nitrogen content could be controlled at about 1.69% when the air supply was 2.0 times that of theoretical air demand. Thus, air can be used as an oxygen source in the desulfurization of biogas for its utilization (such as combined heat and power (CHP)).

The practical suitability of the fixed-bed trickling bioreactor (FBTB) system could be proven while avoiding the disadvantages of internal biological desulfurization methods. Naegele et al. (2013) investigated the removal of hydrogen sulfide from biogas by external BDS in a full scale fixed-bed trickling bioreactor (FBTB) at a research biogas plant with a given output of 96 m³ biogas/h and an H₂S concentration ranging between 500 ppm and 600 ppm (1 ppm = 1 cm³/m³) on average. The FBTB column has been designed to hold a packing volume of 2.21 m³ at a gas retention time of 84 seconds being loaded at an average of 32.88 g H₂S/m³/h. It was found that the temperature has a detectable effect, while the change in pH and air ratios have a little effect on BDS efficiency. The highest H₂S removal efficiency of 98% was recorded at a temperature range of 30–40 °C with a great decline to 21–45% at low temperatures ranging from 5–25 °C. Although different oxygen contents did not have a significant effect on desulfurization efficiency, 0.5% oxygen is recommended for general plant safety. It should be noted that, operation at pH7 caused a high consumption of operation resources, whereas pH2 imposed particularly high requirements for anti-corrosion.

In an attempt to enhance the biogas-BDS efficiency in bio-trickling filters, Chaiprapat et al. (2015) investigated the effects of triple stage and single stage bio-trickling filters with oxygenated liquid recirculation under the acidic condition at a pH down to 0.5. The packed bed of triple stage bio-trickling filters was divided into 3 stages (lower, middle and upper); each stage was 10 cm high with an attempt to improve the distribution of the dissolved O₂ within the bed. Single stage bio-trickling filters (S-BTF) with an equal size of total packed bed volume was operated for comparison with deigned triple stage bio-trickling filters (T-BTF) (Figure 3.28).

The coconut husk, which has been used as bed media, was inoculated with native wastewater microorganisms in order to establish an initial active biofilm. Coconut husk was used due to its rough surface, moisture storage capacity, inexpensiveness, and abundant availability. Triple stage bio-trickling filters (T-BTF) achieved a higher H₂S elimination capacity and removal efficiency of 175.6 g H₂S/m³/h and 89.0 %, than that in the case of applying the single stage bio-trickling filters which recorded 159.9 g H₂S/m³/h and 80.1%, respectively. This study clearly concluded that the O₂/H₂S ratio has a great impact on performance and can be manipulated by the distribution of the recirculating liquid flow. The oxidation of H₂S to elemental sulfur (S⁰) requires only one-fourth of the O₂ needed for the conversion to sulfate. Thus, the production of sulfuric acid is enhanced with the increment of oxygen availability.

The T-BTF has superiority in H₂S removal and sulfuric acid recovery relevant to the S-BTF because of the achieved more uniform distribution of dissolved oxygen in a step feed mode that could improve the performance of T-BTF over S-BTF. Moreover, the T-BTF has a potential for effective biogas clean up under highly acidic conditions (Chaiprapat et al., 2015).

Pirolli et al. (2016) designed a simple and low maintenance bio-trickling filter (BTF) for desulfurization of the swine wastewater-derived biogas stream (Figure 2.29). The newly designed bio-trickling filter was filled with polypropylene bio-balls as supporting material for the microorganism’s biofilm fixation. BTF was continuously fed with wastewater effluent from an air sparged nitrification-denitrification bioreactor installed down-gradient from an up-flow anaerobic sludge bed reactor UASB-type digester. Thus, the swine wastewater was used as a source of electron acceptors, nutrients, and H₂S oxidizing bacteria inoculum to overcome the system’s complexity and maintenance. Based on the measured consumption of NO₂^–-N and NO₃^–-N and the accumulation of SO₄^2– and S⁰ over time, the biological H₂S removal in the BTF system was assumed to be carried out by chemolitotrophic denitrification.

The predominant microbial population in the biofilm was found to be the hydrogenotrophic methanogens Methanosarcinales, Methanobacteriales, and Methanomicrobiales, and the latter represented the highest concentration. This was corroborated with the observed strong correlation between CO₂ removal and high CH₄ production as a source of renewable energy.

The use of wastewater effluent from denitrification bioreactors as feeding solution for BTF, which contains sufficiently high concentrations of NO₂^– and NO₃^–, are known to exert toxic or inhibitory effects on methanogens. However, in that study, the presence of NOx at concentrations of 783 mg/L did not affect the methanogenic activity. Methanogens are also known to be highly sensitive and inhibited by low concentrations of oxygen. However, the presence of dissolved oxygen, as high as 2.2 mg/L, also did not affect the methanogenesis process. All of these added to the advantages of using this new design. Moreover, the desulfurization efficiency obtained with the new designed bio-trickling filter was 99.8% with a maximum elimination capacity of 1,509 g/m³/h. The SO₄^2– and S⁰ were the major metabolites produced from biological conversion of H₂S, with an average increase in methane production of ≈3.8 ± 1.68 g/m³ in the filtered gas stream throughout 200 days of BTF operation.

Biodesulfurization of biogas has been largely studied under aerobic conditions (Fortuny et al., 2008, 2011; Ramírez-Sáenz et al., 2009; Chaiprapat et al., 2011) with very few studies carried out under anoxic conditions (Soreanu et al., 2009; Baspinar et al., 2011). Although, the advantage of using anoxic BTFs over aerobic BTFs is that the biogas is not diluted with air and, therefore, the methane (CH₄) concentration is not reduced (Chaiprapat et al., 2011; Montebello et al., 2012). Furthermore, the electron acceptor mass transfer limitation is negligible because the nitrate solubility is very high [91.2 g/100 g at 25 °C] (Haynes, 2012). Consequently, for the production of pipeline grade methane, anoxic biofiltration is a more feasible technology as a pretreatment for H₂S removal than the more commonly used aerobic BTFs (Fernández et al., 2013).

Fernández et al. (2014) studied the anoxic biofiltration process for H₂S removal from biogas using a BTF packed with open-pore polyurethane foam (OPUF) (Figure 3.30) to increase the elimination capacity (EC) and obtain a deeper understanding of the influence of the operating variables.

The BDS process was carried out throughout 620 days with a continuous supply of biogas. A BTF with a volume of 2.4 L (working volume bed) was packed with OPUF cubes. The biogas was produced by two up-flow anaerobic sludge bed reactors (UASB) of 200 L. In order to increase and set different H₂S concentrations, the biogas was passed through an H₂S generating column. Three nitrate mineral media were used to test three types of nitrate sources: Ca(NO₃)₂·4H₂O, NaNO₃, and KNO₃. Biofilm formation was performed in-situ in the BTF (day 1–35). The results revealed that OPUF has great properties as a carrier for anoxic biofiltration and it can reach a critical EC of 130 g S/m³/h (%BD ≈ 99 %) under the following conditions: inlet loads below 130 g S^2-/m³/h, temperature of 30 °C, sulfate concentration below 33 g/L, a pH between 7.3 and 7.5, and a trickling liquid velocity higher than 4.6 m/h. In regard to nitrite concentration, high concentrations were reached without showing an inhibitory effect on the process, so nitrite could be used instead of nitrate as an electron acceptor source.

Ripl and Fechter (1991) employed iron oxides to control H₂S in anaerobic sludge through the conversion of H₂S into insoluble FeS without the inhibition of the anaerobic activity. Charles et al. (2006) reported the addition of an FeCl₃ solution (Fe³⁺/S = 5:1) into a digester as an effective controller for H₂S emission throughout the precipitation of FeS. Su et al. (2012) reported the good efficiency of Fe(OH)₃ as a deslfurizer in an anaerobic digester. The supply of O₂ or air will need an external power and the amount of O₂ or air must be strictly controlled in order to avoid its inhibition on methanogens, decrease in methane content in biogas, and even the safety problem (Khanal et al., 2003; Ramos and Fdz-Polanco, 2014). Moreover, the ferric agents (i.e. the iron-based oxidants) are relatively expensive when they are applied in large amounts in the practical in-situ desulfurization process. Therefore, it is very necessary to explore a novel in situ desulfurizer for biogas desulfurization with low cost and high efficiency.

Natural iron ores, such as hematite and limonite, contain a high quantity of iron oxides and are widely available with low cost. Some natural iron ores have been adopted as the adsorbents in the H₂S abatement process and good desulfurization results were obtained (Sasaoka et al., 1993). Five kinds of iron ores, limonite, hematite, manganese ore, magnetite, and lava rock were investigated as being suitable for in-situ desulfurization during anaerobic digestion in bioreactors for biogas production (Zhou et al., 2016). These five ores showed good desulfurization for the evolved H₂S in biogas and ranked in the following increasing order: magnetite < lava rock < manganese ore < hematite < limonite. Thus, limonite expressed the best performance for H₂S removal. As limonite dosages increased (10–60 g/L), the contents of H₂S in biogas evidently decreased in the digesters with different initial sulfate concentrations (0–1000 mg/L). Not only this, but the amount of biogas from the digesters with limonite and hematite was slightly higher than that from the digesters without iron ore, which proved the promotion of biogas production due to hydrogen sulfide removal which leads, consequently, to the decrease of the H₂S inhibition on methanogenesis. Thus, limonite in the digesters expressed three functions: adsorption, oxidation, and precipitation, resulting in multiple control on H₂S in biogas (Figure 3.31). However, the manganese ore had a negative effect on biogas yield, although it effectively controlled the H₂S content in biogas. This was attributed to the production of MnCO₃. As manganese captures the produced CO_2, it produced the insoluble MnCO₃ precipitates which would lead to a flocculation with the enzyme and, thus inhibit microbial activity (Zhong et al., 2004).

References

Abatzoglou, N., Boivin, S. (2009) A review of biogas purification processes. Biofuels, Bioproducts and Biorefining. 3: 42–71.

Abdullaha, A.H., Mat, R., Abd Aziz, A.S., Roslanc, F. (2017) Use of kaolin as adsorbent for removal of hydrogen sulphide from biogas. Chemical Engineering Transactions. 56: 763–768.

Adib, F., Bagreev, A., Bandosz, T.J. (1999) Effect of pH and surface chemistry on the mechanism of H2S removal by activated carbons. Journal of Colloid and Interface Science. 216(2): 360–369.

Alemzadeh, I., Kahrizi, E., Vossoughi, M. (2009) Bio-oxidation of ferrous ions by Acidithioobacillus ferrooxidans in a monolithic bioreactor. Journal of Chemical Technology and Biotechnology. 84: 504–510.

Amirfakhri, J., Vossoughi, M., Soltanieh, M. (2006) Assessment of desulfurization of natural gas by chemoautotrophic bacteria in an anaerobic baffled reactor (ABR). Chemical Engineering and Processing. 45: 232–237.

Andrews E.M. (1989) Stretford Chemistry and Analytical Methods Handbook. GRI Report. Project 5086–253–1275. PB89–173595. GRI/GTI (Gas Technology Institute). Des Plaines. IL.

Aranovich G.L., Donohue, M.D. (1995) Adsorption-isotherms for microporous adsorbents. Carbon. 33:1369–1375.

Aroca, G., Urrutia H., Núñez, D., Oyarzún, P., Arancibia, A., Guerrero, K. (2007) Comparison of the removal of hydrogen sulfide in biotrickling filters inoculated with Thiobacillus thiorapus and Acidithiobacillus thiooxidans. Electronic Journal of Biotechnology. 10(4): 514–520.

Asai S., Konishi Y., Yabu T. (1990) Kinetics of absorption of hydrogen sulfide into aqueous ferric sulfate solutions. American Institute of Chemical Engineers Journal. 36: 1331–1338

Aslam, Z., Shawabkeh, R.A., Hussein, I.A., Al-Baghli, N., Eic, M. (2015) Synthesis of activated carbon from oil fly ash for removal of H₂S from gas stream. Applied Surface Science. 327: 107–115.

Atabani, A.E., Silitonga, A.S., Irfan, A.B., Mahlia, T.M.I., Masjuki, H.H., Mekhilef, S. (2012) A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renewable and Sustainable Energy Reviews. 16(4): 2070–2093.

Atabani, A.E., Badruddin, I.A., Mekhilef, S., Silitonga, A.S. (2011) A review on global fuel economy standards, labels and technologies in the transportation sector. Renewable and Sustainable Energy Reviews. 15: 4586–4610.

Bagreev A, Bashkova S, Bandosz TJ. (2002) Adsorption of SO₂ on activated carbons: the effect of nitrogen functionality and pore seizes. Langmuir.18: 1257–1264.

Bagreev A., Bandosz T.J. (2000) Study of hydrogen sulfide adsorption on activated carbons using inverse gas chromatography at infinite dilution. Journal of Physical Chemistry B. 104: 8841–8847.

Bagreev, A., Bandosz, T.J. (2002) A role of sodium hydroxide in the process of hydrogen sulfide adsorption/oxidation on caustic impregnated activated carbons. Industrial and Engineering Chemistry Research. 41: 672–679.

Bakker, W.J.W., Kapteijn, F., Moulijn, J.A. (2003) A high capacity manganese-based sorbent for regenerative high temperature desulfurization with direct sulfur production: conceptual process application to coal gas cleaning. Chemical Engineering Journal. 96: 223–235.

Bandosz, T.J. (2002) On the adsorption/oxidation of hydrogen sulfide on activated carbons at ambient temperatures. Journal of Colloid and Interface Science. 246(1): 1–20.

Barbusiński, K., Kalemba, K. (2016) Use of biological methods for removal of H₂S from biogas in wastewater treatment plants – A review. Architecture Civil Engineering Environment. 1: 103–111.

Bashkova, S., Bagreev, A., Bandosz, T.J. (2003) Adsorption/Oxidation of CH₃SH on Activated Carbons Containing Nitrogen Langmuir. 19(15): 6115–6121.

Bashkova, S., Bagreev, A., Bandosz, T.J. (2005) Catalytic properties of activated carbon surface in the process of adsorption/oxidation of methyl mercaptan. Catalysis Today. 99(3/4): 323–328.

Baspinar, A.B., Turker, M., Hocalar, A., Ozturk, I. (2011) Biogas desulphurization at technical scale by lithotrophic denitrification: Integration of sulphide and nitrogen removal. Process Biochemistry 46(4): 916–922.

Basu R., Clausen E.C., Gaddy J.L. (1996) Biological conversion of hydrogen sulfide into elemental sulfur. Environmental Progress & Sustainable Energy. 15: 234–238.

Beasley, T., Abry, R.G.F. (2003) Managing hydrogen sulfide the natural way. In: Papers of a Canadian Institute conference on the latest techniques for managing sour gas operations, Calgary, AB (Canada), 26–27 May 2003, Canadian Institute Conferences, [400] pages.

Bedell S.A., Kirby L.H., Buenger C.W. McGaugh M.C. (1988) Chelates role in gas treating. Hydrocarbon Process. 2: 63–66.

Benschop A., Janssen A., Hoksberg A., Seriwala M., Abry R., Ngai C. (2002) The shell-Paques/THIOPAQ gas desulphurization process: Successful startup first commercial unit. http://www.paques.nl.

Berzaczy L., Niedermayer E., Kloimstein, L., Windsperger. A. (1990) Biological exhaust gas purification in the rayon fiber manufacture (The Waagner-Biro/Glanzstoff Austria Process). Chemical and Biochemical Engineering Quarterly. 2: 201–203

Biogas composition, Biogas renewable energy – Information website on biogas, online access: http://www.biogas-renewable-energy.info/biogas_composition.html.

Birchalm, V.S.S., Rocha, S.D.F., Ciminelli, V.S.T. (2001) The effect of magnesite calcination conditions on magnesia hydration. Minerals Engineering. 13: 1629–1633.

Bo, Z., Yan, L., Yu-Qun, Z., Hui-Ling, T., Xiao-Wen, Z., Chang-He, C. (2007) Mass transfer and kinetics study on the sulfite forced oxidation with manganese ion catalyst. Korean Journal of Chemical Engineering. 24(3): 471−476.

Boki K., Tanada S. (1980) Adsorption of hydrogen sulfide on activated carbon. Chemical & Pharmaceutical Bulletin. 28: 1270–1275.

Bolhàr-Nordenkampf M., Friedl A., Koss U., Tork T. (2004) Modelling selective H₂S absorption and desorptionin an aqueous MDEA-solution using a rate-based non-equilibrium approach. Chemical Engineering and Processing. 43: 701–715.

Böresson, G. (2001). Inhibition of methane oxidation by volatile sulfur compounds (CH3SH and CS2) in landfill cover soils. Waste Management & Research. 19:314–319.

Bouzaza, A., Laplanche, A., Marsteau, S. (2004) Adsorption–oxidation of hydrogen sulfide of activated carbon fibers: Effect of the composition and the relative humidity of the gas phase. Chemosphere. 54(4): 481–488.

BP (2016) statistical review of world energy. 65^th edition. https://www.bp.com/content/dam/bp/pdf/energy-economics/statistical-review-2016/bp-statistical-review-of-world-energy-2016-full-report.pdf.

BP Energy Outlook 2035. (2015) BP p.l.C. London, United Kingdom, 64^th edition (February). https://www.bp.com/content/dam/bp/pdf/energy-economics/energy-outlook-2015/bp-energy-outlook-2035-booklet.pdf.

BP Statistical Review of World Energy. (2015) BP p.l.C. London, United Kingdom, 64^th edition (June). https://www.bp.com/content/dam/bp/pdf/energy-economics/statistical-review-2015/bp-statistical-review-of-world-energy-2015-full-report.pdf.

Buenger C.W., Kushner D.S. (1988) The SulFerox process – Plant design considerations. Proceeding of the 67^th Annual Convention GPA. Dallas. TX. March 14–16. p. 56–62.

Buisman C.J., Wit B., Lettinga G. (1990) Biotechnological sulphide removal in three polyurethane carrier reactors: stirred reactor, biorotor reactor and upflow reactor. Water Research. 24: 245–251.

Burgess J.E., Parsons S.A., Stuetz M. (2001) Developments in odor control and waste gas treatment biotechnology: a review. Biotechnology Advances. 19: 35–63.

Cadenhead P., Sublette K.L. (1990) Oxidation of hydrogen sulfide by Thiobacilli. Biotechnology and Bioengineering. 35: 1150–1154.

Cal M.P., Strickler B.W., Lizzio A.A. (2000) High-temperature hydrogen sulfide adsorption on activated carbon I. Effects of gas composition and metal addition. Carbon. 38: 1757–1765.

Carroll J.J. (2010) Acid gas injection and carbon dioxide sequestration. John Wiley & Sons Inc., Scrivener publishing LLC. Massachusetts. USA.

Cha J.M., Cha W.S., Lee J.H. (1999) Removal of organosulphur odour compounds by Thiobacillus novellas SRM, sulphur-oxidizing microorganisms. Process Biochemistry. 34: 659–665.

Chaiprapat S., Charnnok B., Kantachote D., Sung S. (2015) Bio-desulfurization of biogas using acidic biotrickling filter with dissolved oxygen in step feed recirculation. Bioresource Technology. 179: 429–435.

Chaiprapat, S., Mardthing, R., Kantachote, D., Karnchanawong, S. (2011) Removal of hydrogen sulfide by complete aerobic oxidation in acidic biofiltration. Process Biochemistry. 46(1): 344–352.

Charles, W., Cord-Ruwisch, R., Ho, G., Costa, M., Spencer, P. (2006) Solutions to a combined problem of excessive hydrogen sulfide in biogas and struvite scaling. Water Science and Technology. 53(6): 203–210.

Cheng, R., Zhou, W., Lin, L., Xie, T., Zhang, Y. (2013) Removal of high concentrations of H₂S from simulated natural gas by Acidithiobacillus ferrooxidans immobilized on polyurethane foam. Journal of Chemical Technology and Biotechnology. 88(5): 975–978.

Chiang H.L., Tsai J.H., Tsai C.L. Hsu Y.C. (2000) Adsorption characteristics of alkaline activated carbon exemplified by water vapor, H2S, and CH3SH gas. Separation Science and Technology. 35: 903- 918.

Cho K.S., Hirai M., Shoda M. (1992a) Degradation of hydrogen sulfide by Xanthomonas sp. strain DY44 isolated from peat. Applied and Environmental Microbiology. 158: 1183–1189.

Cho K.S., Hirai M., Shoda. M. (1992b) Enhanced removal efficiency of malodorous gases in a pilot-scale peat biofilter inoculated with Thiobacillus thioparus DW44. Journal of Fermentation and Bioengineering. 73: 46–50.

Choi, Y., Lee, J.W., Jang, S.C., Ahn, B.S., Choi, D.K. (2008) Adsorption dynamics of hydrogen sulfide in impregnated activated carbon bed. Adsorption. 1494/5): 533–538.

Chung Y.C., Huang C., Li C.F. (1997) Removal characteristics of H₂S by Thiobacillus novellous CH₃ biofilter in autotrophic and mixotrophic environments. Journal of environmental science and health. 5: 1435–1450.

Chung Y.C., Huang C., Tseng C.P. (1996) Operation optimization of Thiobacillus thioparus CH11 biofilter for hydrogen sulfide removal. Journal of Biotechnology. 52: 31–38.

Chung Y.C., Huang C., Tseng C.P. (2001) Biological elimination of H₂S and NH₃ from waste gases by biofilter packed with immobilized heterotrophic bacteria. Chemosphere 43: 1043–1050.

Clark O.G., Edeogu I., Feddes J., Coleman R.N., Abolghasemi A. (2004) Effects of operating temperature and supplemental nutrients in a pilot-scale agricultural biofilter. Canadian Biosystems Engineering. 46: 6.7–6.16.

Claus C.F. (1883) Invention concerning the production of sulfur resp. sulfurous acid from hydrogen sulfide. German Patent 28758.

Cline, C., Hoksberg, A., Abry, R., Janssen, A. (2003) Biological process for H₂S removal from gas streams: The Shell-Paques/THIOPAQTM gas desulfurization process, LRGCC 2003 Conference Proceedings, 1–17.

Cox H.H., Deshusses A.M. (2001) Co-treatment of H₂S and toluene in a biotrickling filter. Chemical Engineering Journal. 3901: 1–10.

Dalrymple D.A., Trofe T.W., Leppin D. (1994) Gas industry assesses new ways to remove small amounts of “H₂S” and “H₂S” scavenger selection index updated. Oil & Gas Journal. 92: 54–60.

Das, A., Roy, P., Mishra, A.K., (1993) Desulphuring of natural gas and petroleum oil by autotrophic Thiobacillus ferrooxidans. Letters in Applied Microbiology. 16(3): 164–166.

Dastous P.A., Soreanu G., Nikiema J., Heitz M. (2005) Biofiltration of three alcohols on a mature bed compost. In 2005 A&WMA Annual Conference Proceedings CD-ROM, Paper #1038. Pittsburgh, PA: Air & Water Management Association. Pittsburgh.

Davini, P. (2001) SO₂ and NO_x adsorption properties of activated carbons obtained from a pitch containing iron derivatives. Carbon. 39: 2173–2179.

Deng, L., Chen, H., Chen, Z., Liu, Y., Pu, X., Song, L. (2009) Process of simultaneous hydrogen sulfide removal from biogas and nitrogen removal from swine wastewater. Bioresource Technology. 100: 5600–5608.

Devinny S.J., Deshusses A.M., Webster S.T. (1999) Biofiltration for Air Pollution Control. Boca Raton, FL: CRC/Lewis Publishers.

Dhage P., Gaur V., Tatarchuk B.J. (2008) Promoted ZnO sorbents for low temperature H₂S removal: Influence of temperature, support surface area, pore size and pore. volume, 2008 AIChE Spring National Meeting, 6–10 April 2008, New Orleans, LA, United States, paper 231c.

Díaz I., Fdz-Polanco M. (2011) Robustness of the microaerobic removal of H2S from biogas. Water Science & Technology. 65: 1368–1374.

Díaz I., Pérez S.I., Ferrero E.M. Fdz-Polanco M. (2010) Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters. Bioresource Technology. 102: 3768–3775.

Diaz, I., Perez, S.I., Ferrero, E.M., Fdz-Polanco, M. (2011) Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters. Bioresource Technology. 102(4): 3768–3775.

Doğan, E.C., Türker, M., Dağaşan, Arslan, A. (2012) Simultaneous sulfide and nitrite removal from industrial wastewaters under denitrifying conditions. Biotechnology and Bioprocess Engineering. 17(3): 661–668

Doornkamp, C., Ponec, V. (2000) The universal character of the Mars and Van Krevelen mechanism. Journal of Molecular Catalysis. A: Chemical. 162: 19–32.

Duan H., Yan, R., Koe, L.C. (2005) Investigation on the mechanism of H₂S removal by biological activated carbon in a horizontal biotrickling filter. Applied Microbiology and Biotechnology. 69: 350–357.

Duan H., Koea L.C., Yan R., Chen, X. (2006) Biological treatment of H₂S using pellet activated carbon as a carrier of microorganisms in a biofilter. Water Research. 40: 2629–2636.

Dube, G., Osifo, P., Rutto, H. (2015) Preparation of bagasse ash/MgO/ammonium acetate sorbent properties and modeling their desulphurization reaction. Environmental Progress and Sustainable Energy. 34(1): 23–31.

Duissenov, D. (2013). Production and processing of sour crude and natural gas – challenges due to increasing stringent regulations. Master Thesis. Department of Petroleum Engineering and Applied Geophysics. Norwegian University of Science and Technology.

Efremenko, I. Sheintuch, M. (2003) Carbon-supported palladium catalysts. Molecular orbital study. Journal of Catalysis. 214: 53–67.

EIA. Energy Information Administration (1999) Natural Gas 1998: Issues and Trends. Office of Oil and Gas. U.S. Department of Energy. Washington. p. 49–71.

El-Gendy N.Sh., Speight J.G. (2016) Handbook of Refinery Desulfurization. CRC Press, Taylor and Francis Group, LLC, 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487–2742, USA.

Elias A., Barona A., Arreguy A., Rios J., Aranguiz I., Peñas J. (2002) Evaluation of a packing material for the biodegradation of H₂S and product analysis. Process Biochemistry. 37: 813–820.

Fan, L., Chen, J., Guo, J., Jiang, X., Jiang, W. (2013) Influence of manganese, iron and pyrolusite blending on the physiochemical properties and desulfurization activities of activated carbons from walnut shell. Journal of Analytical and Applied Pyrolysis. 104: 353–360.

Fan, L., Jiang, X., Jiang, W., Guo, J., Chen, J. (2014) Physicochemical properties and desulfurization activities of metal oxide/biomass-based activated carbons prepared by blending method. Adsorption. 20: 747–756.

Faramawy S., Zaki T., Sakr A. (2016) Natural gas origin, composition, and processing: A review. Journal of Natural Gas Science and Engineering. 34: 34–54

Fdz-Polanco, M., Diaz, I., Perez, S.I., Lopes, A.C., Fdz-Polanco, F. (2009) Hydrogen sulphide removal in the anaerobic digestion of sludge by micro-aerobic processes: pilot plant experience. Water Science and Technology. 60: 3045–3050.

Fernández, M., Ramírez, M., Gómez, J. M., and Cantero, D. 2014. Biogas biodesulfurization in an anoxic biotrickling filter packed with open-pore polyurethane foam. Journal of Hazardous Materials. 264: 529–535.

Fernández, M., Ramírez, M., Pérez, R. M., Gómez, J. M., Cantero, D. (2013) Hydrogen sulphide removal from biogas by an anoxic biotrickling filter packed with Pall rings. Chemical Engineering Journal. 225: 456–463.

Fillipou, D., Katiforis, N., Papassiopi, N., Adams, K. (1999). On the kinetics of magnesia hydration in magnesium acetate solutions. Journal of Chemical Technology and Biotechnology. 74: 322–328.

Flores, R., Rodas, A., Palma, C., Melendez, Y.I., Melendez, X.S., Gonzalez, L.L. (2008) Thermogravimetric studies on the effect of support in sorbent-materials applied to flue gas desulfurization. Fuel. 87(7): 1115−1126.

Fortuny, M., Baeza, J. A., Gamisans, X., Casas, C., Lafuente, J., Deshusses, M. A., Gabriel, D. (2008) Biological sweetening of energy gases mimics in biotrickling filters. Chemosphere. 71(1): 10–17.

Fortuny, M., Gamisans, X., Deshusses, M. A., Lafuente, J., Casas, C., Gabriel, D. (2011) Operational aspects of the desulfurization process of energy gases mimics in biotrickling filters. Water Research. 45(17): 5665–5674.

Fu, H.B., Wang, X., Wu, H.B., Yin, Y., Chen, J.M. (2007) Heterogeneous uptake and oxidation of SO₂ on iron oxides. The Journal of Physical Chemistry C. 111: 6077–6085.

Fuda K., Palmer A.D., Sears P., Blais D., Furimsky E. (1991) Chemical changes occurring during sulphidation and regeneration of iron-containing sorbents. Fuel. 70:100–106.

Gabriel, D., Deshusses, M.A. (2003) Retrofitting existing chemical scrubbers to biotrickling filters for H₂S emission control. Proceedings of the National Academy of Sciences of the United States of America. 100(11): 6308–6312.

Gadre, R.V. (1989) Removal of hydrogen sulfide from biogas by chemoautotrophic fixed-film bioreactor. Biotechnology and Bioengineering. 34(3): 410–414.

Garrett R.L., Carlton L.A., Denekas M.O. (1988) Methods for field monitoring of oil-based drilling fluids for hydrogen sulphide and water intrusions. SPE Drilling Engineering. 3: 296–302.

Giro, M.E.A., Garcia, O. Jr., Zaiat, M. (2006) Immobilized cells of Acidithiobacillus ferrooxidans in PVC strands and sulfide removal in a pilot-scale bioreactor. Chemical Engineering Journal. 28: 201–207.

Grimm D.T., Cork D.J., Uphaus R.A. (1984) Optimization of Desulfobacter for sulfate reduction. Development in Industrial Microbiology. 24: 709–716.

Guo, J.; Lua, A.C. (2002) Microporous activated carbons prepared rom palm shell by thermal activation and their application to Sulphur dioxide adsorption. Journal of Colloid Interface Science. 251: 242–247.

Guo, J.-X., Liang, J., Chu, Y.-H., Sun, M.-C., Yin, H.-Q., Li, J.-J. (2012) Desulfurization activity of nickel supported on acid-treated activated carbons. Applied Catalysis A: General (421/422): 142–147.

Hagen, M., Polman, E., Jensen, J., Myken, A., Jonsson, O., Dahl, A. (2001) Adding gas from biomass to the gas grid. Swedish Gas Center, Malmo Sweden, July 2001.Pp. 144. Report SCG118. http://www.dgc.eu/sites/default/files/filarkiv/documents/R0105_biomass_gas_grid.pdf.

Hansen C. (2006) Biogas Scrubbing System (Eliminating Corrosive Hydrogen Sulfide Gas). The Local Energy Farms Network – Reliable Renewable Energy for a Post Carbon World. Post Carbon Institute. http://www.energyfarms.net/node/1034.

Hardison L.C. (1991) Recent developments in acid gas treatment using the auto circulation LO-CAT H2S approach. AIChE Spring National Meeting, Houston, TX, April 7–11, Session 16c.

Haynes, W.M. (2012). Physcial constants of inorganic compounds, in: W.M. Haynes (Ed.), CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL.

Heinz-Wolfgang H. (2008) Industrial Gases Processing. WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim. ISBN: 978-3-527-31685-4.

Henshaw P.F., Bewtra J.K., Biswas N. (1997) Biological conversion of sulfide to elemental sulfur. Indian Journal of Engineering & Material Sciences. 5: 202–210.

Henshaw P.F., Medlar D., McEwen J. (1999) Selection of a support medium for a fixed-film green sulfur bacteria reactor. Water Research. 33: 3107–3110.

Henshaw P.F., Zhu W. (2001) Biological conversion of hydrogen sulfide to elemental sulfur in a fixed-film continuous flow photo-reactor. Water Research. 35: 3605–3610.

Hernández S.P., Scarpa, F., Fino, D., Conti, R. (2011) Biogas purification for MCFC application. International Journal of Hydrogen Energy. 36(13): 8112–8118.

Ho, K.L., Lin, W.C., Chung, Y.C., Chen, Y.P., Tseng, C.P. (2013) Elimination of high concentration hydrogen sulfide and biogas purification by chemical-biological process. Chemosphere. 92: 1396–1401.

Horváth, I.S., Tabatabaei, M., Karimi, K., Kumar, R. (2016) Recent updates on biogas production – a review. Biofuel Research Journal 10: 394–402.

Howarth R., Unz F.R., Seviour M.E., Seviour J.R., Blackall L., Pickup, W.R., Jones, G.J., Yaguchi J., Head I. (1999) Phylogenetic relationships of filamentous sulfur bacteria (Thiothrix spp. and Eikelboom type 021N bacteria) isolated from wastewater treatment plants and description of Thiothrix eikelboomii sp. nov., Thiothrix unzii sp. nov., Thiothrix fructosivorans sp. nov. and Thiothrix defluvii sp. nov. International Journal of Systematic Bacteriology. 49: 1817–1827.

Husein M.M., Patruyo L., Pereira-Almao P., Nassar N.N. (2010) Scavenging H₂S (g) from oil phases by means of ultra-dispersed sorbents. Journal of Colloid and Interface Science. 342:253–260.

Illan-Gomez, M.J., Raymund-Pinero, E., Garcia-Garcia, A., Linares-Solano, A., de Salinaz-Martinez-Salinaz, L. (1999) Catalytic NO_x reduction by carbon supporting metals. Applied Catalysis B: Environmental. 20: 267–275.

International Energy Agency, IEA (2011) Oil marketer reports, Total OECD12 month moving average demand Vs 160 year. 1year growth. Pans, France: Organization for economic Co-operation and development. https://www1.oecd.org/environment/cc/48073739.pdf.

International Energy Agency, IEA (2013) Resources to reserves. Oil, gas and coal technologies for the energy markets of the future. https://www.iea.org/publications/freepublications/publication/Resources2013.pdf

Isik-Gulsac, I. (2016) Investigation of impregnated activated carbon properties used in hydrogen sulfide fine removal. Brazilian Journal of Chemical Engineering. 33(4): 1021–1030.

Janssen A.J., Sleyster R., van der Kaa C., Jochemsen A., Bontsema J., Lettinga G. (1995) Biological sulphide oxidation in a fed-batch reactor. Biotechnology and Bioengineering. 47: 327–333.

Jensen A.B., Webb C. (1995) Treatment of H₂S-containing gases: A review of microbiological alternatives. Enzyme and Microbial Technology. 17:2–10.

Jiang, H., Li, T., Zhou, H. (2014) Method for determining desulfurizer addition in in-situ desulfurization process of methane. CN Patent CN104195183-A.

Jin, Y., Veiga, M.C., Kennes, C. (2005) Effects of pH, CO₂, and flow pattern on the autotrophic degradation of hydrogen sulfide in a biotrickling filter. Biotechnology and Bioengineering. 92(4): 462–471.

Jing, C., Ping, Z., Mahmood, Q. (2009) Simultaneous sulfide and nitrate removal in anaerobic reactor under shock loading. Bioresource Technology. 100: 3010–3014.

Johansen S.S., Licht D., Arvin E., Mosbaek H., Hansen A.B. (1997) Metabolic pathways of quinoline, indole and their methylated analogs by Desulfobacterium indolicum (DSM 383). Applied and Environmental Microbiology. 47: 292–300.

Jorio H., Heitz M. (1999) Treatment de l’air par biofiltration. Canadian Journal of Civil Engineering. 26: 402–424.

Kandile N., Razek T., Al-Sabagh A., Khattab M. (2014): Synthesis and evaluation of some amine compounds having surface active properties as H2S scavenger; Egyptian Journal of Petroleum; 23: 323–329.

Kaspar, J., Fornasiero, P., Granziani, M. (1999) Use of CeO₂-based oxides in the three-way catalysis. Catalysis Today. 50: 285–298.

Khanal, S., Shang, C., Huang, J. (2003) Use of ORP (oxidation-reduction potential) to control oxygen dosing for online sulfide oxidation in anaerobic treatment of high sulfate wastewater. Water Science and Technology. 47(12): 183–189.

Kidnay A.J., Parrish W.R., McCartney D.G. (2011) Fundamentals of Natural Gas Processing, second ed. CRC Press. Taylor and Francis Group. LLC.

Kim B.W., Chang H.N. (1991) Removal of hydrogen sulfide by Chlorobium thiosulfatophilum in immobilized cell and sulfur settling free-cell recycle reactors. Biotechnology Progress. 7:495–500.

Kim H., Kim J.Y., Chung S.J., Xie. Q. (2002) Long-term operation of a biofilter for simultaneous removal of H₂S and NH₃. Air & Waste Management Association. 52: 1389–1398.

Kim Y.J., Kim B.W. Chang H.N. (1996) Desulfurization in a plate-type gas-lift photobioreactor using light emitting diodes. Korean Journal of Chemical Engineering. 13: 606–611.

Kim, K.H., Choi, Y.J., Jeon, E.C., Sunwoo, Y. (2005). Characterization of malodorous sulfur compounds in landfill gas. Atmospheric Environment. 39: 1103–1112.

Kleerebezem R., Mendez R. (2002) Autotrophic denitrification for combined hydrogen sulfide removal from biogas and post-denitrification. Water Science and Technology.45: 349–356.

Kleinjan W. (2005) Biologically Produced Sulfur Particles and Polysulfide Ions: Effects on a Biotechnological Process for the Removal of Hydrogen Sulfide from Gas Streams. PhD Thesis. Wageningen University. Wageningen. The Netherlands.

Klok, J.B.M., Van Den Bosch, P.L.F., Buisman, C.J.N., Stams, A.J.M., Keesman, K.J., Janssen, A.J.H. (2012) Pathways of sulfide oxidation by haloalkaliphilic bacteria in limited-oxygen gas lift bioreactors. Environmental Science and Technology. 46(14): 7581–7586.

Kobayashi H.A., Stenstrom M., Mah R.A. (1983) Use of photosynthetic bacteria for hydrogen sulfide removal from anaerobic waste treatment effluent. Water Research. 17: 579–587.

Kobayashi T., Li Y.Y., Kubot A.K., Harada H., Maeda T., Yu H.Q. (2012) Characterization of sulfide-oxidizing microbial mats developed inside a fullscale anaerobic digester employing biological desulfurization. Applied Microbiology and Biotechnology. 93: 847–857.

Kobayashi, T., Li, Y.Y. (2011) Performance and characterization of a newly developed self-agitated anaerobic reactor with biological desulfurization. Bioresource Technology. 102(10): 5580–5588.

Kohl A., Nielsen R. (1997) Gas Purification. 5^th Edition. Gulf Publishing Company. Houston. USA.

Kraakman, N.J.R., Rocha-Rios, J., van Loosdrecht, M.C.M. (2011) Review of mass transfer aspects for biological gas treatment. Applied Microbiology and Biotechnology. 91: 873–886.

Kwaśny, J., Balcerzak, W. (2016) Sorbents used for biogas desulfurization in the adsorption process. Polish Journal of Environmental Studies. 25(1): 37–43.

Kwaśny, J., Balcerzak, W., Rezka, P. (2015) Application of zeolites for the adsorptive biogas desulfurization. Technical Transactions Environmental Engineering. 3-Ś: 40–45.

Lallemand, F., Perdu, G., Maretto, C., Weiss, C., Magne-Drisch, J., Lucquin, A.-C. (2012) Solutions for the treatment of highly sour gases. In Gas Treatment/Processing, Digital Refining, Gas 2012, April 2012. http://www.digitalrefining.com/article/1000356.

Lampe D.G., Zhang T.C. (1996) Evaluation of sulfur-based autotrophic denitrification. In Proceedings of the HSRC/ WERC Joint Conference on the Environment. Great Plains/ Rocky Mountain Hazardous Substance Research Center. http://www.engg.ksu.edu/HSRC/96Proceed/lampe.pdf.

Lar, J.S., Li, X. (2009) Removal of H₂S during anaerobic bioconversion of dairy manure. Chinese Journal of Chemical Engineering. 17(2):273–277.

Larsen H. (1952) On the culture and general physiology of the green sulfur bacteria. Journal of Bacteriology. 64: 187–196.

Le Gaz Intégral (2000) The SulFerox process. http://www.gazintegral.com/sulferox/sulferox.htm.

Lee, H.-S., Kim, J.-Y., Yu, J.-K., Kil, I.-S., Kim, D.-H., Lee T.-J., Rhee, Y.-W. (2005) A Study of desulfurization ability and activation energy for CuO-AgO sorbents. Korean Journal of Chemical Engineering. 22(6): 889–893.

Lee, Y.W., Park, J.W., Choung, J.H., Choi, D.K. (2002) Adsorption characteristics of SO₂ on activated carbon prepared from coconut shell with potassium hydroxide activation. Environmental Science and Technology. 36: 1086–1092.

Li Y, Zhu CL, Lu T, Guo ZP, Zhang D, Ma J, Zhu, S. (2013) Simple fabrication of a Fe₂O₃/carbon composite for use in a high-performance lithium ion battery. Carbon. 52: 565–573.

Li, F., Yan, B., Jiang, W., Gong, Z. (2012) Preparation of activated carbon loading nano-ZnO and desulfurization properties under room temperature. In: Preparation of activated carbon loading nano-ZnO and desulfurization properties under room temperature. Zhang, W., ed., AISC 162, Springer-Verlag, Berlin, Heidelberg. Pp. 177–183.

Li, J.J., Kobayashi, N., Hu, Y.Q. (2009) Performance of V₂O₅/AC activatedwith the flue gas on SO₂ removal. Journal of Environmental Engineering. 4: 176–187.

Li, X., Jiang, X., Zhou, Q., Jiang, W. (2016) Effect of S/N ratio on the removal of hydrogen sulfide from biogas in anoxic bioreactors. Applied Biochemistry and Biotechnology. 180: 930–944.

Li, Z., Sun, T., Zhu, N., Cao, X., Jia, J. (2008) Comparative study of using different materials as bacterial carriers to treat hydrogen sulfide. Applied Microbiology and Biotechnology. 81: 579–588.

Lin W.C., Chen Y.P., Tseng C.P. (2013) Pilot-scale chemical–biological system for efficient H₂S removal from biogas. Bioresource Technology. 135: 283–291.

Linkous, C.A. (1993) Sulfur recovery from oil and gas refinery waste streams using semiconductor particulates. Waste Management. 13(5/7): 527–528

Liu, C., Zhang, R., Wei, S, Wang, J., Liu, Y., Li, M., Liu, R. (2015a) Selective removal of H₂S from biogas using a regenerable hybrid TiO₂/zeolite composite. Fuel. 157: 183–190.

Liu, Q., Ke, M., Liu, F., Yu, P., Hu, H., Li, C. (2017) High-performance removal of methyl mercaptans by nitrogen-rich coconut shell activated carbon. RSC Advances. 7: 22892- 22899.

Liu, Q.S., Xu, B., He, J.L., Sun, R., Chang, H.G. (2015b) Optimal conditions for bio-oxidation of biodesulfurization in natural gas sweetening process using Thiobacillus. Advanced Materials Research. 1104: 57–60

Lobo, R., Revah, S., Viveros-Garcia, T. (1999) An analysis of a trickle-bed bioreactor: Carbon disulfide removal. Biotechnology and Bioengineering. 63: 98–109.

Lu J., Zheng Y., He D. (2006) Selective absorption of H₂S from gas mixtures into aqueous solutions of blended amines of methyldiethanolamine and 2-tertiarybutylamino-2-ethoxyethanol in a packed column. Separation and Purification Technology. 52:209–217.

Ma, Y.-L., Yang, B.-L., Zhao, J.-L. (2006) Removal of H₂S by Thiobacillus denitrificans immobilized on different matrices. Bioresource Technology. 97(16): 2041–2046.

Maddox R.N. (1974) Gas and Liquid Sweetening. Second ed. John M. Campbell Ltd. Petroleum Series. Norman. OK.

Magomnang M., Villanueva P. (2014) Removal of Hydrogen Sulfide from Biogas using Dry Desulfurization Systems; International Conference on Agricultural, Environmental and Biological Sciences. Phuket (Thailand). http://dx.doi.org/10.15242/IICBE.C414016 65.

Magota H., Shiratori Y. (1988) Treatment of sour natural gas containing hydrogen sulfide. Japanese Patent 63,205,124.

Maka A., Cork. D. (1990) Quantum efficiency requirements for an anaerobic photobioreator. Journal of Industrial Microbiology and Biotechnology. 5: 337–354.

Malhautier L., Gracian C., Roux C.J., Fanlo L.J., Le Cloirec. P. (2003) Biological treatment process of air loaded with an ammonia and hydrogen sulfide mixture. Chemosphere. 50: 145–153.

Mao, C., Feng, Y., Wang, X., Ren, G. (2015) Review on research achievements of biogas from anaerobic digestion. Renewable and Sustainable Energy Reviews. 45: 540–555.

Markus B.N., Anton F., Ulrich K., Thomas T. (2004) Modelling selective H₂S absorption and desorption in an aqueous MDEA-solution using a rate-based non-equilibrium approach. Chemical Engineering and processing. 43: 701–715.

Mata-Alvarez, J., Llabrés, S.M. (2000). Anaerobic digestion of organic solid wastes: An overview of research achievements and perspectives. Bioresource Technology. 74: 3–16.

McKinsey Z.S. (2003): Removal of hydrogen sulfide from biogas using cowmanure compost, Thesis presented to the Faculty of the Graduate School of Cornell University; 1–104.

Mehandjiev, D., Bekyarova, E. Khristova, M. (1997) Study of Ni-impregnated active carbon II. Catalytic behavior in NO conversion. Journal of Colloid Interface Science. 192: 440–446.

Mehandjiev, D., Khristova, M., Bekyarova, E. (1996) Conversion of NO on Co-impregnated active carbon catalysts. Carbon. 34: 757–762.

Meldon, J.H., Dutta, A. (1994) Analysis of ultimate permselectivity for H₂S over CO₂ in alkaline solutions. Chemical Engineering Science. 49(5): 689–697.

Mesa M.M., Macías M., Cantero D. (2002) Biological iron oxidation by Acidithiobacillus ferrooxidans. Chemical and Biochemical Engineering Quarterly. 16: 69–73.

Micoli, L., Bagnasco, G., Turco, M. (2014) H₂S removal from biogas for fueling MCFCs: new adsorbing materials. International Journal of Hydrogen Energy. 39:1783–1787.

Mokhatab S., Poe W.A., Speight J.G. (2006) Handbook of Natural Gas Transmission and Processing. Elsevier Inc. USA.

Montebello, A.M., Fernández, M., Almenglo, F., Ramírez, M., Cantero, D., Baeza, M., Gabriel, D. (2012). Simultaneous methylmercaptan and hydrogen sulfide removal in the desulfurization of biogas in aerobic and anoxic biotrickling filters. Chemical Engineering Journal. 200: 237–246.

Montebello, A.M., Fernández, M., Almenglo, F., Ramírez, M., Cantero, D., Baeza, M., Gabriel, D. (2012) Simultaneous methylmercaptan and hydrogen sulfide removal in the desulfurization of biogas in aerobic and anoxic biotrickling filters. Chemical Engineering Journal. 200: 237–246.

Monteleone, G., De Francesco, M., Galli, S., Marchetti, M., Naticchioni, V. (2011) Deep H₂S removal from biogas for molten carbonate fuel cell (MCFC) systems. Chemical Engineering Journal. 173: 407–414.

Naegele, H.-J., Lindner, J., Merkle, W., Lemmer, A., Jungbluth, T., Bogenrieder, C. (2013) Effects of temperature, pH and O2 on the removal of hydrogen sulfide from biogas by external biological desulfurization in a full scale fixed-bed trickling bioreactor (FBTB). International Journal of Agricultural and Biological Engineering. 6(1): 69–81.

Nagl G. (1997) Controlling H₂S emission. Chemical Engineering. 104: 125–128.

Nassar I.M., Noor M.R., Morsi R.E., Abd El-Azeim A., Hashem A.I. (2016) Renewable and Sustainable Energy Reviews. 65: 101–112.

Nazari J. (2017) Isolation, identification, and characterization of a novel chemolithoautotrophic bacterium with high potential in biodesulfurization of natural or industrial gasses and biogas. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects. 39(10): 971–977.

Neumann W., Riickauf H., Volk N., Michael A., Forkmann, R. (1990) Holzemann Metallverarbeitung GmbH; Technische Hochschule Merseburg “Carl Schlorlemmer”. (Ger) Method for removal of hydrogen sulphide from a combustible waste gas. European Patent EP 402,704.

Nguyen-Thanh, D., Bandosz, T.J. (2003) Effect of transition-metal cations on the adsorption of H₂S in modified pillared clays. Journal of Physical Chemistry. B. 107: 5812–5817.

Nishimura S., Yoda M. (1997) Removal of hydrogen sulfide from an anaerobic biogas using a bio-scrubber. Water Science & Technology. 36: 349–356.

Noyola A., Morgan-Sagastume J.M., López-Hernández J.E. (2006) Treatment of biogas produced in anaerobic reactors for domestic wastewater: odor control and energy/resource recovery. Reviews in Environmental Science and Bio/Technology. 5: 93– 114

O’Brien M., Wentworth C., Lanning, A., Engert T. (2007) Shell-Paques biodesulfurization process directly and selectively removes H2S from high pressure natural gas – start-up report. Proceeding of the Laurance Reid Gas Conditioning Conference. Norman. OK.

Osorio, F., Torres, J.C. (2009) Biogas purifiation from anaerobic digestion in a wastewater treatment plant for biofuel production. Renewable Energy. 34: 2164–2171.

Oyarzún P., Arancibia F., Canales C., Aroca E.G. (2003) Biofiltration of high concentration of hydrogen sulphide using Thiobacillus thioparus. Process Biochemistry. 39: 165- 170.

Pagella, C., De Faveri, D.M. (2000) H₂S gas treatment by iron bioprocess. Chemical Engineering Science. 55: 2185–2194.

Pandey R.A., Malhotra S. (1999) Desulfurization of gaseous fuels with recovery of elemental sulfur: an overview. Critical reviews in environmental science and technology. 29: 229–268.

Pandey, R.A., Malhotra, S., Rajvaidya, A.S., Sharma, S., Peshwe, S. (2003) Bench scale evaluation of a chemo-biochemical process for desulphurization of gaseous streams containing hydrogen sulphide. Water Air Soil Pollution. 146: 243–260.

Phooratsamee, W., Hussaro, K., Teekasap, S., Hirunlabh, J. (2014) Increasing adsorption of activated carbon from palm oil shell for adsorb H₂S from biogas production by impregnation. American Journal of Environmental Sciences. 10(5): 431–445.

Pirolli, M., Busi da Silva, M.L., Mezzari, M.P., Michelon, W., Prandini, J.M., Soares, H.M. (2016) Methane production from a field-scale biofilter designed for desulfurization of biogas stream. Journal of Environmental Management. 177: 161–168.

Polychronopoulou K., Cabello G.F., López G.M., Fierro L.G., Bakas T. Efstathiou A.M. (2005) Novel Fe–Mn–Zn–Ti–O mixed-metal oxides for the low-temperature removal of H₂S from gas streams in the presence of H₂, CO₂, and H₂O. Journal of Catalysis. 236:205–220.

Postgate J.R. (1968) The sulphur cycle. In Inorganic Sulphur Chemistry. ed. G. Nickless. Amsterdam. Netherlands: Elsevier Publishing Company. P. 259–279.

Prescott M.L., Harley P.J., Klein A.D. (2003) Microbiology, 5^th edition. NY: McGraw-Hill Companies. New York. USA.

Przepiorski J., Oya A. (1998) K₂CO₃-loaded deodorizing activated carbon fiber against H₂S gas: Factors influencing the deodorizing efficiency and the regeneration method. Journal of materials science letters. 17: 679–682.

Przepiorski J., Yoshida S., Oya, A. (1999) Structure of K₂CO₃-loaded activated carbon fiber and its deodorization ability against H₂S gas. Carbon. 37: 1881–1890.

Puri, B.R. (1970) In Chemistry and Physics of Carbon. Vol. 6, (Ed., P.A. Thrower), Marcel Dekker, New York. Pp. 191–282.

Qi., G., Yang, R.T. (2003) Performance and kinetics study for low-temperature SCR of NO with NH₃ over MNO_x-CeO₂ catalysts. Journal of Catalysis. 217: 434–441.

Qing-chun, Y., Yong, D., Fei, W., Yue-bin, F., Bin, Y., Bao-qiang, X., Da-chun, L. (2015) Comparison of desulfurization kinetics of copper oxide sorbent. Journal of Central South University. 22: 2902−2908.

Rai, C., Taylor, M. (1996) Microbial sweetening of sour natural gas using mixed cultures. Environmental Progress. 15(1): 6–11.

Ramírez-Sáenz, D., Zarate-Segura, P.B., Guerrero-Barajas, C., García-Peña, E.I. (2009) H2S and volatile fatty acids elimination by biofiltration: Clean-up process for biogas potential use. Journal of Hazardous Materials. 163(2/3): 1272–1281.

Ramos I., Díaz I., Fdz-Polanco M. (2012) The role of the headspace in hydrogen sulfide removal during microaerobic digestion of sludge. Water Science & Technology. 66: 2258–2264.

Ramos, I., Fdz-Polanco, M. (2014) Microaerobic control of biogas sulphide content during sewage sludge digestion by using biogas production and hydrogen sulphide concentration. Chemical Engineering Journal. 250: 303–311.

Rattanapan C., Ounsaneha W. (2012) Removal of Hydrogen Sulfide Gas using Biofiltration – a Review. Walailak Journal of Science and Technology. 9: 9–18.

Ripl, W., Fechter, L. (1991) Treating settled anaerobic sludge at base of water bodies–with additives which eliminate or convert toxic pollutants, esp. iron oxide to convert hydrogen sulphide. DE Patent DE4005064 A1.

Rocha, S.D.F., Mansur, M.B., Ciminelli, V.S.T. (2004) Kinetics and mechanistic analysis of caustic magnesia hydration. Journal of Chemical Technology and Biotechnology. 79: 816–821.

Rojey A., Jaffret C., Cornot-Gandolphe S., Durand B., Julian S., Valais M. (1997) Natural Gas: Production, Processing. Transport. Editions Technip. Paris.

Rojey, A. (1997) Natural gas: production, processing, transport. Institut Franpcais Du Pbetrole Publications.

Rouleau, W., Watson, J. (2014) LO-CAT^®: A flexible hydrogen sulfide removal process. http://www.merichem.com/LO-CAT-Flexible-H2S-Removal-Process

Ryckebosch, E., Drouillon, M., Vervaeren, H. (2011) Techniques for transformation of biogas to biomethane. Biomass and Bioenergy. 35: 1633–1645.

Saimura, A., Shiratori, Y., Kitaoka, T. (2017) Dual-layered paper-structured catalysts for sequential desulfurization and methane-steam reforming of simulated biogas containing hydrogen sulfide. Journal of Materials Science. 52: 314–325.

Sasaoka E., Ichio T, Kasaoka S. (1992) High-temperature H₂S removal from coal-derived gas by iron ore. Energy and Fuels. 6:603–608.

Sasaoka, E., Sakamoto, M., Ichio, T., Kasaoka, S., Sakata, Y. (1993) Reactivity and durability of iron oxide high temperature desulfurization sorbents. Energy and Fuels. 7(5): 632–638.

Satoh, H. Yoshizawa, J., Kametanl, S. (1988) Bacteria help desulfurize gas. Hydrocarbon Processing. 76-D-76-F.

Schaack J.P., Chan F. (1989) H₂S scavenging: Formaldehyde-methanol, metallic oxide agents head scavengers list processes. Oil and Gas Journal. 87, 51–55; 81–82; 45–48; 90–91. 4 parts.

Schieder D., Quicker P., Schneider R., Winter H., Prechtl S., Faulstich M. (2003). Microbiological removal of hydrogen sulfide from biogas by means of a separate biofilter system: Experience with technical operation. Water Science & Technology. 48: 209–212.

Schoell M. (1983) Isotope techniques for tracing migration of gases in sedimentary basins. Journal of the Geological Society. 140: 415–422.

Sercu B., Núñez D., Langenhove V.H., Aroca G., Verstraete W. (2005) Operational and microbiological aspects of a bioaugmented two-stage biotrickling filter removing hydrogen sulfide and dimethyl sulfide. Biotechnology and Bioengineering. 90: 259–269.

Seredych, M., Strydom, C., Bandosz, T.J. (2008) Effect of fly ash addition on the removal of hydrogen sulfide from biogas and air on sewage sludge based composite adsorbents. Waste Management. 28(10): 1983–1992.

Shahid, E.M., Jama, J. (2011) Production of biodiesel: a technical review. Renewable and Sustainable Energy Reviews. 25(9): 4732–4745.

Shannon R. (2000) Biogas. conference proceedings http://www.rosneath.com.all/ipc6/ch08/shannon2/.

Shareefdeen Z., Herner B., Wilson S. (2002) Biofiltration of nuisance sulfur gaseous odors from a meat rendering plant. Journal of Chemical Technology & Biotechnology. 77: 1296–1299.

Sheng, Y.Q., Chen, F.Z., Wang, X.M., Sheng, G.Y., Fu, J.M. (2008) Odorous volatile organic sulfide in wastewater treatment plants in Guangzhou, China. Water Environment Research. 80: 324–330.

Shi, L., Yang, K., Zhao, Q., Wang, H., Cui, Q. (2015) Characterization and mechanisms of H₂S and SO₂ adsorption by activated carbon. Energy and Fuels. 29(10): 6678–6685.

Shi, R.-H., Zhang, Z.-R., Fan, H.L., Zhen, T., Shangguan, J., Mi, J. (2017) Cu-based metal-organic framework/activated carbon composites for sulfur compounds removal. Applied Surface Science. 394: 394–402.

Sisani, E., Cinti, G., Discepoli, G., Penchini, D., Desideri, U., Marmottini, F. (2014) Adsorptive removal of H₂S in biogas conditions for high temperature fuel cell systems. International Journal of Hydrogen Energy. 39(36): 21753–21766.

Smet, E., Lens, P., Van Langenhove, H. (1998) Treatment of waste gases contaminated with odorous sulfur compounds. Critical Reviews in Environmental Science and Technology. 28(1): 89–117.

Son H.J., Lee J.H. (2005) H₂S removal with an immobilized cell hybrid reactor. Process Biochemistry. 40: 2197–2203.

Song, C. (2002) Fuel processing for low-temperature and high temperature fuel cells: challenges, and opportunities for sustainable development in the 21st century. Catalysis Today. 77: 17–49.

Song, L., Zhang, B., Deng, W. (2014) Application of activated carbon from waste tea in desulfurization and denitrification. Huan Jing Ke Xue. 35(10): 3674–82 (Article in Chinese).

Sonta H., Shiratori T. (1990) Method of treating H₂S containing gases. Dova Mining Co. Ltd. US Patent 4,931,262, 1990.

Soreanu G, Al-Jamal M, Béland M. (2005) Biogas treatment using an anaerobic biosystem. In Proceedings of the 3^rd Canadian Organic Residuals and Biosolids Management Conference, June 1–4, 2005. Calgary, AB. Pp. 502–513.

Soreanu, G., Beland, M., Falletta, P., Edmonson, K., Seto, P. (2008a) Laboratory pilot scale study for H₂S removal from biogas in an anoxic biotrickling filter. Water Science and Technology. 57: 201–207.

Soreanu, G., Béland, M., Falletta, P., Ventresca, B., Seto, P. (2009) Evaluation of different packing media for anoxic H₂S control in biogas. Environmental Technology. 30(12): 1249–1259.

Soreanu, G., Seto, P., Béland, M., Edmonson, K., Falletta, P. (2008b). Investigation on the use of nitrified wastewater for the steady-state operation of a biotrickling filter for the removal of hydrogen sulphide in biogas. Journal of Environmental Engineering and Science. 7: 543–552.

Speight J.G. (1993) Gas Processing: Environmental Aspects and Methods. Butterworth-Heinemann Ltd., Oxford, England.

Speight J.G. (2015) Liquid fuels from natural gas. In: Lee, S., Speight, J.G., Loyalka, S.K. (Eds.), Handbook of Alternative Fuel Technologies. second ed. Taylor and Francis Group. LLC. CRC Press. p. 157–178.

Speight, J.G. (2007) Natural Gas: A Basic Handbook. GPC Books, Gulf Publishing Company, Houston, TX.

Speight, J.G. (2014). The Chemistry and Technology of Petroleum, 5^th Edition. CRC Press, Taylor & Francis Group, Boca Raton, FL.

Stanier R.Y., Ingraham J.L., Wheelis M.L., Painter P.R. (1986) The Microbial World, 5^th edition. Englewood Cliffs. NJ. Prentice-Hall. Inc.

Stirling D. (2000) The sulfur problem cleaning up industrial feedstocks. First ed. The Royal Society of Chemistry. Cambridge. UK.

Su, L., Zhang, M., Shi, X. (2012) In situ odor control of sewage of sludge by addition of ferric hydroxide. Environmental Pollution Control. 34(12): 1–2.

Sublette K.L. (1988) Production of microbial biomass protein from autotrophic fermentation of hydrogen sulfide. Biotechnology and Bioengineering. 32: 408–409.

Sublette, K.L., Sylvester, N.D. (1987a) Oxidation of hydrogen sulfide by Thiobacillus denitrificans: desulfurization of natural gas. Biotechnology and Bioengineering. 29(2): 249–257.

Sublette. K.L. (1990) Microbial treatment of sour gases for the removal and oxidation of hydrogen sulphide. Gas Separation & Purification. 4: 91- 96.

Subletteb, K.L., Sylvester, N.D. (1987b) Oxidation of hydrogen sulfide by mixed cultures of Thiobacillus denitrificans and heterotrophs. Biotechnology and Bioengineering. 29(6): 759–761.

Sumathi, S., Bhatia, S., Lee, K.T., Mohamed, A.R. (2010) Performance of palm shell activated carbon impregnated with CeO₂ and V₂O₅ catalyst in simultaneous removal of SO₂ and NO₂. Journal of Applied Sciences. 10(2): 1052–1059.

Sun, F., Liu, J., Chen, H., Zhang, Z., Qiao, W., Long, D., Bashkova, S., Bandosz, T.J. (2009) The effects of urea modification and heat treatment on the process of NO₂ removal by wood-based activated carbon. Journal of Colloid and Interface Science. 333(1): 97–103.

Sun, W., Xia, C., Xu, M., Guo, J., Wang, A., Sun, G. (2013) Distribution and abundance of archaeal and bacterial ammonia oxidizers in the sediments of the Dongjiang River, a drinking water supply for Hong Kong. Microbes and Environments. 28(4): 457–465.

Syed M.A., Henshaw P.F. (2003) Effect of tube size on performance of a fixed-film tubular bioreactor for conversion of hydrogen sulfide to elemental sulfur. Water Research. 37: 1932–38.

Syed M.A., Henshaw P.F. (2005) Light emitting diodes and an infrared bulb as light sources of a fixed-film tubular photobioreactor for conversion of hydrogen sulfide to elemental sulfur. Journal of Chemical Technology & Biotechnology. 80:119–123.

Syed, M., Soreanu, G., Falleta, P., Beland, M. (2006) removal of hydrogen sulfide from gas streams using biological processes – A review. Canadian Biosystems Engineering. 48: 2.2–2.14.

Tabak, J. (2009). Natural Gas and Hydrogen. Facts on File. New York. USA.

Takano B., Koshida M., Fujiwara Y., Sugimori K., Takayangi S. (1997) Influence of sulfur-oxidizing bacteria on the budget of sulphate in Yugama crater lake. KusatsuShirane volcano. Japan. Biogeochemistry. 38: 227–253.

Tang K., Baskaran V., Nemati M. (2009) Bacteria of the sulphur cycle: an overview of microbiology, biokinetics and their role in petroleum and mining industries. Biochemical Engineering Journal. J. 44: 73–94.

Tang, Y.Q., Shigematsu, T., Ikba, Morimura, S., Kida, K. (2004). The effects of micro-aeration on the phylogenetic diversity of microorganisms in a thermophilic anaerobic municipal solid-waste digester. Water Research. 38: 2537–2550.

Tanji Y., Kanagawa T., Mikami, E. (1989) Removal of dimethyl sulfide, methyl mercaptan. and hydrogen sulfide by immobilized Thiobacillus thioparus TK-m. Journal of Fermentation and Bioengineering. 67: 280–285.

Tian, L., Li, C., Li, Q., Zeng, G., Gao, Z., Li, S., Fan, X. (2009) Removal of elemental mercury by activated carbon impregnated with CeO₂. Fuel. 88: 1687–1691.

Tóth G., Nemestóthy N., Bélafi-Bakó K., Vozik D., Bakonyi, P. (2015) Degradation of hydrogen sulfide by immobilized Thiobacillus thioparus in continuous biotrickling reactor fed with synthetic gas mixture. International Biodeterioration and Biodegradation, 105: 185–191.

Tsai J.H., Jeng F.T., Chiang H.L. (2001) Removal of H₂S from exhaust gas by use of alkaline activated carbon. Adsorption. 7: 357–366.

Tseng, H.H., Wey, M.Y., Fu, C.H. (2003) Carbon materials as catalyst supports for SO₂ oxidation: catalytic activity of CuO–AC. Carbon. 41(1): 139–149

Turker M., Baspinarb A., Hocalara Akif (2011): Monitoring and control of biogas desulphurization using oxidation reduction potential under denitrifiying conditions. Journal of Chemical Technology and Biotechnology. 87: 682–688.

U.S. Department of Energy (1988) Handbook on Biogas Utilization, Georgia Institute of Technology. Atlanta. Georgia. https://rirdc.infoservices.com.au/downloads/12–056

van der Zee F.P., Villaverde S., García-Encina P.A., Fdz-Polanco F. (2007) Sulphide removal by moderate oxygenation of anaerobic sludge environments. Bioresource Technology. 98: 518–524.

Van Grinsven, P. (1999) Shell unveils gas biodesulfurization process. Oil and Gas Journal. Pp. 32.

van Groenestijn, J.W., Hesselink, P.G.M. (1993) Biotechniques for air pollution control. Biodegradation. 4(4): 283–301.

van Groenestijn, J.W., Kraaman, N.J.R. (2005) Recent developments in biological waste gas purification in Europe, Chemical Engineering Journal. 113: 85–91.

Van Niel C.B. (1931) On the morphology and physiology of the purple and green sulphur bacteria. Archives of Microbiology. 3: 1–112.

Vanderzee, F.P., Villaverde, S., Garcia, P.A., Fdz-Polanco, F. (2007) Sulfide removal by moderate oxygenation of anaerobic sludge environments. Bioresource Technology. 98: 518–524.

Vikromvarasiri N., Champreda V., Boonyawanich S., Pisutpaisal N. (2017) Hydrogen sulfide removal from biogas by biotrickling filter inoculated with Halothiobacillus neapolitanus. International Journal of Hydrogen Energy.1–9.

Villegas, P.G. (2016) Preliminary study of activated carbon filters for pollutants removal in diesel engines. International Journal of Sustainable Energy and Environmental Research. 5(2): 31–45.

Wang D. (2008) Breakthrough behavior of H₂S removal with an iron oxide based CG-4 adsorbent in a fixed-bed reactor. MSc. Saskatoon. SK: Department of Chemical Engineering, University of Saskatchewan.

Weiland P. (2010) Biogas production: current state and perspectives. Applied Microbiology and Biotechnology. 85: 849–860.

Wellinger A., Linberg A. (2000) Biogas Upgrading and Utilization-IEA Bioenergy Task 24. International Energy Association. Paris. France. p. 2000

Wellinger, A., Lindberg, A. (2005) Biogas upgrading and utilisation. IEA Bioenergy Task 24: Energy from Biological Conversion of Organic Waste (2005). http://www.iea-biogas.net/files/daten-redaktion/download/publi-task37/Biogas%20upgrading.pdf.

Westmoreland, P.R., Gibson, J.B., Harrison, D.P. (1977) Comparative kinetics of high-temperature reaction between hydrogen sulfide and selected metal oxides. Environmental Science and Technology. 11: 488–491.

Wu, M., Zhang, Y., Ye, Y., Lin, C. (2016) In situ removal of hydrogen sulfide during biogas fermentation at microaerobic condition. Applied Biochemistry and Biotechnology. 180: 817–825.

Wubs H.J., Beenackers C.M., Krish R. (1991) Absorption of hydrogen sulfide in aqueous solutions of iodine – a critical review. Chemical Engineering Science. 46:703–706.

Xiao-Li, L., Jia-Xiu, G., Ying-Hao, C., De-Ming, L., Hua-Qiang, Y., Ming-Chao, S., Yavuz, R. (2014) Desulfurization performance of iron supported on activated carbon. Fuel. 123: 93–100.

Yang, S., Tang, Y., Gou, M., Jiang, X. (2015) Effect of sulfate addition on methane production and sulfate reduction in a mesophilic acetate fed anaerobic reactor. Applied Microbiology and Biotechnology. 99(7): 3269–3277.

Yazdanbakhsh, F., Blasing, M., Sawada, J.A., Rezaei, S., Muller, M., Baumann, S., Kuznicki, S.M. (2014) Copper exchanged nanotitanate for high temperature H₂S adsorption. Industrial and Engineering Chemistry Research. 53: 11734–11739.

Yi, Z., Xiaoying, M., Zhouyan, C., Shuang-chen, M., Lidong, W. (2007) “Experimental study on simultaneous desulfurization and denitrification by highly active absorbent” Proceedings of International Conference on Power Engineering-2007, October 23–27, 2007, Hangzhou, China. Pp. 771–721.

Yumura M., Furimsky E. (1985) Comparison of calcium oxide, zinc oxide, and iron (III) oxide hydrogen sulfide adsorbents at high temperatures. Industrial & Engineering Chemistry Process Design & Development. 24:1165–1168.

Zare A.H., Mirzaei, S. (2009) Removal of CO₂ and H₂S using Aqueous Alkanolamine Solusions; International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering. 3: 50–59.

Zhong, J., Xue, Y., Chen, J. (2004) Effect of cobalt and manganese on activity of anaerobic sludge in treatment of waste water from production of purified terephthalic acid. Industrial Water and Wastewater. 35(3): 2004–2006.

Zhou, Q., Jiang, X., Li, X., Jiang, W. (2016) The control of H₂S in biogas using iron ores as in situ desulfurizers during anaerobic digestion process. Applied Microbiology and Biotechnology. 100: 8179–8189.

Zhou, Q., Liang, H., Yang, S., Jiang, X. (2015) The removal of hydrogen sulfide from biogas in a microaerobic biotrickling filter using polypropylene carrier as packing material. Applied Biochemistry and Biotechnology. 175: 3763–3777.

Zhu, Z., Liu, Z., Liu, S., Niu, H., Hu, T., Liu, T., Xie, Y. (2000) NO reduction with NH₃ over an activated carbon-supported copper oxide catalysts at low temperatures. Applied Catalysis B: Environmental. 26: 25–35.

Ziyu, S., Qiang, L., Dan, W., Hang, T., Maohua, Y., Jianmin, X. (2014) Inhibition of carbon disulfide on bio-desulfurization in the process of gases purification. African Journal of Biotechnology. 13(16): 1739–1744.