8.1 Introduction

Coupling sustainable biomass with emerging clean energy technologies can provide transformational opportunities for low-carbon energy and bio-based products manufacturing in developing low-carbon economy. Changing supply and demand dynamics in the energy sector and especially recent developments of increased supply of transition fuels, demand for energy resources worldwide, the potential negative impacts of climate change, and the need to develop a sustainable and resilient national low-carbon energy supply will require alternatives to existing fossil fuel resources. Recent horizontal drilling achievements in national gas supplies when compared with coal and oil may seem to reduce the CO₂ emissions; however, the assessments show that natural gas domination would still heat up the planet [1]. To accelerate switch from fossil transition fuels to low-carbon energy sources, solutions are critically needed to contribute to sustainability goals and provide for today's energy demands while conserving limited resources for future generations. The development and deployment of new and innovative clean energy systems may offer solutions that are environmentally sustainable, economically viable, and socially equitable. To achieve this result, it is critical to understand, test, scale up, and prove existing and emerging clean energy technologies and verify “sustainable clean energy pathways.”

8.2 Sustainable Biomass for Sustainable Development

The term sustainable development is an important concept, and its definition was first introduced in the Brundtland Report in 1987 [2]. In 2015, United Nations released 17 sustainable development goals as part of the 2030 Agenda forSustainable Development [3]. These 17 goals include various target areas spanning from ending poverty and achieving nutritional food supply for all by sustainable agriculture to ensuring reliable and affordable access to clean water and energy. The agenda is a plan of action for people, planet, and prosperity. These 17 goals were set to help to understand the interdependent nature of these pillars, act on effectively, and help guide decision-making, innovation, and deployment of technologies that bring us closer to sustainably developed clean energy systems.

There is a critical need to find alternatives to meet the increasing demand for energy, to accelerate economic growth by market transformation, and to mitigate climate change. Aggressive energy planning at the international, national, state, and local government levels is needed that are based on reasonably achievable targets [4]. Solutions will require complex strategies for both energy demand and supply that simultaneously and rigorously reduce emissions and eliminate other environmental concerns such as ecodiversity and healthy habitats while promoting a low-carbon economy. Demand strategies include aggressive programs at all levels for energy conservation and efficiency from home heating and lighting to manufacturing and transportation. On the supply side, increases in the use of renewable power and fuel will be critical and will include sustainable biomass (e.g., purpose-grown wood and other energy crops, agricultural waste, and biomass waste) options, among other renewables. Promoting sustainable clean energy pathways can provide possible solutions to the growing energy problem and achieve overall sustainable development goals.

8.3 Biorefineries and Bioenergy Conversion Pathways

8.3.2 Biomass-to-Bioenergy and Bio-products Conversion Pathways

Biomass conversion into power, heat, fuels, and bio-products varies based on the specific feedstock and is generally categorized into two major conversion pathways – biochemical and thermochemical (Figure 8.1).

8.3.2.1 Biochemical Conversion Processes

Biochemical processes include anaerobic digestion (AD) of organic waste (food waste, agricultural waste, some high moisture animal manure, yard clippings, and sludge) to generate biogas and by-products that can be utilized for soil remediation and production of liquid fuels from lignocellulosic feedstocks through acid and/or enzymatic hydrolysis. Also, production of first-generation biofuels through fermentation can be classified as a biochemical conversion technology. In this chapter, the transesterification process is included in the chemical/biochemical conversion category, though some researchers prefer to categorize it as a physiochemical conversion technology. Additionally, this chapter is intended to provide information on non-food biomass and organic waste-to-bioenergy and bio-products pathways such as direct fermentation of cornstarch and other starches that can be utilized as food sources and transesterification of soy bean oil and other straight vegetable oils that can serve as food sources, including palm oil into biodiesel production, are not discussed. However, waste starch to alcohols and waste oils to biodiesel pathways are included.

8.3.2.1.1 Anaerobic Digestion Pathway

AD occurs in an oxygen-depleted environment through a series of chemical reactions by specific species of microorganisms. The microorganisms digest the organic feedstocks and break them down in stages and produce biogas [17]. Suitable feedstocks for AD include organic waste, namely, food waste from institutional, commercial, industrial, and residential sources wastewater/sewage sludge and farm animal waste, particularly dairy farm waste. Controlled AD requires an airtight vessel that can serve as a digester. An anaerobic digester has to maintain a minimum temperature of 20 °C to promote bacterial activity. Higher temperatures of up to 60 °C are more beneficial as it shortens processing time, thus allowing the digester to handle a larger volume of organic feedstocks [17]. Based on the type of bacteria and its optimal digestion temperature, the ADs can be classified as thermophilic and mesophilic. A mesophilic bacterium is a microorganism that performs best between 35 and 45 °C and “thermophilic” bacterium is a microorganism that performs best between 50°C and 60 °C [17, 18]. The biogas (also known as digester gas) is actually a mixture of gases, with methane and carbon dioxide making up more than 90% of the total volume. In addition, biogas typically contains small amounts of hydrogen sulfide (H₂S) (see Figure 8.2).

The first option is to site the facility near a transfer station that accepts unsorted waste. Following mechanical separation of metals, plastics, and glass, the remaining organic fraction of the waste can be pulped and fed directly into the AD system for biogas generation. Second, ADs can be colocated within existing landfills to utilize the waste-management infrastructure already in place. This approach can help existing landfills increase gas generation and collection efficiencies and also serves as an important component of an efficient waste management operation. Third, if colocation with landfills and transfer stations is not feasible, AD applications can be developed as stand-alone facilities. Finally, ADs colocated with wastewater treatment facilities can utilize source separated organic waste generated by the facility, since the AD technology is already in use.

It is important to recognize that every link of the biogas economic value chain needs to be profitable and environmentally sustainable, as well as socially acceptable. For example, to prevent unpleasant odors from affecting surrounding areas, AD systems may be required to implement certain safeguards such as installation of bio-filters and granular activated carbon (GAC) filters and/or maintain negative pressure in the AD chambers during operation. Similarly, to reduce noise and associated emissions pollution due to trucking organic waste to the AD system, the facility should be constructed in close proximity to the sources of the organic waste streams.

8.3.2.1.1.1 Biochemical Reactions in Anaerobic Digestion

The conversion of organic waste to biogas is accomplished through four stages of biochemical reactions, which are known as hydrolysis, acidogenesis, acetogenesis, and methanogenesis [19, 20].

1. 1.

   Hydrolysis: The first stage of AD breaks down the organic waste feedstock into its components. In this stage the following conversion reactions occur:

   • Lipids → fatty acids
   • Polysaccharides → monosaccharides
   • Protein → amino acids

   These reactions are catalyzed by enzymes excreted by bacteria such as cellulase, protease, and lipase [21]. If the biomass polymer is complex, the hydrolysis phase occurs slowly. Therefore, high lignin-containing biomass, such as woody waste, is not a feasible feedstock for AD conversion. Carbohydrates, however, can be converted easily to simple sugars [19]. The chemical formula of an organic waste mixture can be presented as C₆H₁₀O₄, and the hydrolysis stage can convert the mixed organic waste into simple sugars [21, 22]:

   
2. 2. Acidogenesis: During this stage, acid-forming reactions occur, and products of hydrolysis are converted into primarily short chain acids such as lactic, formic, propionic, butyric, or succinic acids. In addition, ketones and alcohols are formed.
3. 3. Acetogenesis: This stage converts long fatty acids formed from the hydrolysis of lipids to acetate through carbohydrate fermentation. CO₂ and H₂ are also produced at this stage. The role of hydrogen as an intermediary is of critical importance to AD reactions in this stage.
4. 4. Methanogenesis: During this stage, methane is produced. This reaction is very sensitive to pH changes. Optimum pH levels are neutral to slightly alkaline, and if the pH level becomes too acidic, essential microorganisms cannot survive and carry out the desired reactions [21, 22].

8.3.2.1.1.2 Anaerobic Digestion Reactors

AD reactors can be classified by the total solids (TS) content of the slurry in the digester. Low solids (LS) reactors contain less than 10% TS, medium solids (MS) reactors contain about 15–20% TS, and high solids (HS) processes range from 22% to 40% TS. The disadvantage of LS reactors is the large quantity of water in the slurry that requires large reactor volumes and expensive posttreatment dewatering technology at the end of the process. HS reactors are more robust and have high organic loading capacity. In addition to being categorized by the solids content, AD rectors can be further classified as

1. 1. Single stage: Single-stage low solids (SSLS) or single-stage high solids (SSHS)
2. 2. Multistage: Multistage low solids (MSLS) or multistage high solids (MSHS)
3. 3. Batch: Single-stage batch, sequential batch, and hybrid batch

Single-stage AD processes utilize a single reactor for both acidogenic and methanogenic stages. Depending on the solids content, they can either be SSLS or SSHS. HS applications in single-stage reactors are more efficient but more expensive due to the need for conveyor belts and powerful pumps. However, the pretreatment is less problematic than LS applications. Single-stage reactors both in LS and HS applications do not require mechanical devices for stirring and mixing.

Multistage AD processes occur in two stages with hydrolysis, acidogenesis, and acetogenesis occurring in the first stage and methanogenesis occurring in the second stage. The two stage process results in an increased rate of hydrolysis in the first stage and longer retention time in the second stage, which allows for optimum growth rate of microorganisms and maximized methanogenesis. These reactors can also be classified as MSLS or MSHS. The LS and HS application differences are similar to the single-stage applications.

Batch type reactors are operated by loading the reactor vessel with the materials in a slurry form to be decomposed, allowing the AD reactions take place and emptying the remaining biomass fully or partly after the completion of the reactions. The remaining biomass in the reactor following the AD reactions can be used to activate the subsequent batch operation. Batch reactors can be single-stage batch, sequential batch, or hybrid batch. Batch process offers the advantages of being technically simple, inexpensive, and robust [20–24].

Landfill Gas and Digester Gas as Renewable Natural Gas (RNG)

AD of organic waste can occur in digesters and landfills to produce anaerobic digester gas (biogas) and landfill gas, respectively. Both gases are a mixture of methane (CH₄), carbon dioxide (CO₂), and other impurities based on the feedstock. These gases are also called renewable natural gas (RNG) [25] if they contain 50–65% methane. The organic waste AD to biogas generation concept can be applied to clean power generation, or it can be utilized as CNG in transportation. Additionally, the solid and liquid residue following AD can be utilized as compost. Utilization of landfill gas for power and heat generation can help displace fossil fuels and reduce the adverse effects of fossil fuel utilization, such as air emissions that include criteria pollutants and GHG emissions. However, widespread utilization of landfill gas and digester gas from sewage sludge/wastewater treatment to power generation pathways is being challenged by the presence of volatile methyl siloxanes (siloxanes), which damage power generation equipment. Siloxanes are found in wastewater and the solid wastes deposited in landfills. They are formed as a result of the hydrolysis of polydimethylsiloxanes (PDMS). PDMS are a class of organosilicon compounds used in the manufacturing of industrial products and household products, such as personal hygiene and healthcare products [26]. When landfill gas or other forms of RNG are combusted in gas turbines, boilers, microturbines, or internal combustion engines to generate power, siloxanes convert into silicon dioxide (SiO₂) and deposit in the various parts of the power generation equipment. This results in high maintenance costs, prolonged downtime and even serious equipment failure. Deposition on turbine blades, heat exchangers, and emission control equipment can result in reduced or total loss of heat transfer efficiency, equipment failure, and possible poisoning in catalytic converters, requiring expensive repairs and service interruptions [27]. The level of siloxanes varies depending on the feedstock source. Concentrations can be as high as 140 mg/m³, which is significantly above the 15 mg/m³ limit recommended by most equipment manufacturers [28]. Current siloxane removal technologies can be categorized as wet or dry removal technologies. These technologies include adsorption onto activated carbon or silica gel, chilling, refrigeration, and condensation or liquid scrubbing by absorption into solvents [29]. These technologies have been reported to be moderately effective [29]. Biological removal of siloxanes not only is reported to be possible but also poses significant challenges [30]. In addition to these technical difficulties, other important factors in siloxane removal are environmental impacts and cost of the technology.

8.3.2.1.2 Transesterification Pathway

The transesterification reaction converts oils and fats (triglycerides) into biodiesel and glyceride by-products. A triglyceride, by definition, contains a glycerine molecule with three long-chain fatty acids attached (Figure 8.3). When converting triglycerides into esters, alcohol reacts with the fatty acids to form mono-alkyl esters, namely, biodiesel and glycerol. Methanol and ethanol are widely used alcohols in this process. The resulting biodiesel products can be defined as fatty acid methyl ester (FAME) and fatty acid ethyl ester (FAEE), respectively [31–34]. The conversion reaction takes place utilizing catalysts such as sodium hydroxide or potassium hydroxide.

8.3.2.1.2.1 Transesterification of Waste Oils and Lipids from Non-food Sources into Biodiesel

Biodiesel is a synthetic diesel-like fuel produced from vegetable oils by the transesterification process described above and is an alternative to petroleum–diesel. In addition to plant oils (soybean oil, canola oil, cottonseed oil, palm oil), biodiesel can be produced from recycled cooking greases or oils (e.g., yellow grease, brown grease), animal fats (beef tallow, pork lard), or lipids extracted from high-lipid-forming aquatic species (i.e., algae and duckweed species) [31, 34–37].

Biodiesel blends easily and completely with fossil diesel fuel at any concentration. Currently, most diesel engines are able to run-on blends up to B20 (20% biodiesel and 80% fossil diesel fuel by volume) with few or no engine modifications [32, 33, 36, 37].

8.3.2.1.2.2 Waste–Oils (Yellow Grease and Brown Grease) to Biodiesel

Yellow grease is defined as waste vegetable oil or animal fats collected after cooking. Yellow grease cannot be used directly as a replacement for fossil diesel fuel unless the engine is specifically designed to combust it [35, 36]. Converting yellow grease into biodiesel results in better engine performance. Yellow grease may contain several types of fats, including animal fats, and contains high concentrations of free fatty acids (FFA) (up to 10%) [24] that pose significant processing problems in alkali-catalyzed processes. Prior to the transesterification process, yellow grease and animal fats need to be treated by an acid-esterification step to remove the FFA. Removal is accomplished by the addition of extra alkaline catalysts or homogeneous acid catalysts, causing esterification of the FFA. Another treatment approach is converting FFA in yellow grease into monoglycerides and then proceeding with transesterification.

8.3.2.1.2.3 Brown Grease

Brown grease can be defined as grease from restaurant grease traps and sewage grease and can serve as a viable feedstock to produce biodiesel [33]. It generally has higher FFA than yellow grease (∼ >20%) and can also be used for animal feed. Brown grease is gelatinous at ambient temperatures and has high odor levels. Pumped trap grease is typically disposed of at wastewater treatment plants or landfills. Trap grease can contain contaminants such as pesticides, fungicides, and herbicides, polycyclic hydrocarbons, detergents cleaners, trace metals, aflatoxins (from molds), and/or perchloroethylene from dry cleaning operations [32, 36]. Biodiesel production from brown grease involves more steps than yellow grease and, prior to transesterification, requires physical refining as follows [36]:

1. 1. Degumming: Brown grease consists of a high amount of phosphatides that cause gumming. Degumming treatment involves hydration with water or orthophosphoric acid.
2. 2. Bleaching: This step removes residues of phosphoric acid generated by the degumming process.
3. 3. Caustic refining, steam stripping, solvent extraction, and adsorption can be applied as needed to improve the physical properties of brown grease to make it more easily utilized in biodiesel production [36–38].

8.3.2.1.2.4 Non-food Lipids to Biodiesel

Lipids from aquatic species including algal species are receiving considerable interest as a feedstock for biodiesel production. Conversion of aquatic species into biofuels will be discussed in Section 8.3.2.1.4.

8.3.2.1.3 Hydrolysis of Biomass and Organic Wastes into Cellulosic Ethanol

Due to controversial concerns created by ethanol production from corn feedstocks, such as food-to-fuel pathways and iLUC emissions, there has been great interest in finding new conversion pathways for non-food biomass feedstocks. Therefore, biomass-to-sugars pathways are emerging to utilize cellulose and hemicellulose contents of biomass as possible fuel production options. Wood waste, agricultural waste, forestry residue, non-recycled organic waste, and purpose-grown energy crops such as switch grass and poplar are viable feedstocks for this pathway (Table 8.1) [39].

Lignocellulosic material	Cellulose (%)	Hemicellulose (%)	Lignin (%)
Hardwood stems	40–55	24–40	18–25
Softwood stems	45–50	25–35	25–35
Nut shells	25–30	25–30	30–40
Corn cobs	45	35	15
Grasses	25–40	35–50	10–30
Paper	85–99	0	0–15
Wheat straw	30	50	15
Sorted refuse	60	20	20
Leaves	15–20	80–85	0
Cotton seed	80–95	5–20	0
Newspaper	40–55	25–40	18–30
Waste papers	60–70	10–20	5–10
Primary wastewater solids	8–15	—	—
Solid cattle manure	1.6–4.7	1.4–3.3	2.7–5.7
Coastal bermuda grass	25	35.7	6.4
Switch grass	45	31.4	12
Swine waste	6.0	28	0

Source: Prasad et al. 2007 [39]. Reproduced with permission of Elsevier.

8.3.2.1.3.1 Acid Hydrolysis

Acid hydrolysis is usually performed by adding mineral acids such as sulfuric acid (H₂SO₄) or hydrochloric acid (HCl). The concentrated acid ruptures the cellulose chain by breaking the hydrogen bonding. Once the cellulose is de-crystallized, it forms a homogenous gelatine and becomes extremely susceptible to hydrolysis [40–42]. Water is used to dilute this mixture at modest temperatures (40–60 °C) to extract xylose (C5 sugars) and glucose (C6 sugars). The sugars then undergo a standard fermentation process to produce ethanol. In addition to ethanol, acid hydrolysis processes can be designed to produce butanol and levulinic acid for other fuel and chemical needs. Acid hydrolysis applications require reactors that are resistant to corrosion, since the concentrated acid is highly corrosive [40]. Acid hydrolysis is also used in a dilute form to pretreat biomass to prepare for enzymatic hydrolysis.

8.3.2.1.3.2 Enzymatic Hydrolysis

Enzymatic hydrolysis is performed by using cellulase enzymes to convert cellulosic feedstocks (especially those high in lignin) into monomeric sugars that are subsequently fermented into ethanol using the following processes:

1. 1. Pretreatment: The first step in enzymatic hydrolysis is pretreatment of biomass to free the cellulose fraction by disrupting the lignin. The goal of the pretreatment process is to break down the lignin structure and disrupt the crystalline structure of cellulose so that the acids or enzymes can easily access and hydrolyze the cellulose [43, 44]. Pretreatment methods can be categorized as physical (milling and grinding), physicochemical (steam pretreatment/autohydrolysis, hydrothermolysis, and wet oxidation), chemical (alkali, dilute acid, oxidizing agents, and organic solvents), biological, electrical, or a combination of these methods [44].
2. 2. Enzyme application step: The most common enzyme types are cellulase enzymes which are used to hydrolyze cellulose into glucose. Also, hemicellulases and other enzymes can be used to release sugars from different parts of biomass as follows:
   • Cellulases are used to hydrolyze cellulose to glucose.
   • Xylanases are used to hydrolyze xylan into xylose. Removal of xylan from lignocelluloses increases the accessibility of cellulose for enzymatic hydrolysis.
   • Peroxidases and laccases are two effective groups of enzymes that are used for degradation of the lignin component of biomass, which increases accessibility to cellulose enzymes. These agents serve as efficient lignocellulose-degrading enzymes, which are necessary in order to reduce the cost of enzymes required in the biomass-to-ethanol process [43–47].

8.3.2.1.4 Advanced Biofuels Production from Aquatic Biomass

In addition to development of advanced fuels from cellulosic feedstocks and the organic fraction of municipal solid waste, a number of other viable feedstocks are emerging for biofuels production. Aquatic species-to-biofuels can be categorized as third-generation advanced biofuels. Currently, aquatic species-to-biofuel pathways are not commercialized, and they are in their infancy in terms of research, development, and deployment. However, early research shows that these feedstocks hold potential for advanced biofuels production. There are many benefits (described below) associated with algae and other aquatic plants, such as duckweed species, that would make “aquatic plants to biofuels” a viable pathway for manufacturing transportation fuels and bio-based products. In addition, these plants are suitable for CO₂ fixation and wastewater treatment [48].

8.3.2.1.4.1 Algae to Biofuels

The term algae can refer to microalgae, cyanobacteria (blue-green algae), and macroalgae (seaweed) [49]. Algae are single-cell aquatic organisms that photosynthesize [50–53]. Algal production can offer high biomass yields per acre, and it has been demonstrated that algae cultivation can minimize or avoid competition with arable land and nutrients used for conventional agriculture practices [49]. Some algal strains are capable of doubling their mass several times per day. In some cases, more than half of that mass consists of lipids or triacylglycerides [54]. Algae cultivation does not always require freshwater and can utilize wastewater or saline water. In addition, algae can recycle carbon from CO₂-rich emissions from stationary sources including power plants and other industrial emitters [49, 54, 55]. Depending on the type, algae can be utilized for both ethanol and biodiesel production.

Starch-Forming Algae for Ethanol Production

Cyanobacteria and macroalgae primarily produce carbohydrates, which can be a viable source for advanced ethanol production. Some types of cyanobacteria grow very quickly, and the quantity of biomass can double in less than 10 h. Despite recent advancements, a comprehensive understanding of the carbon metabolism of cyanobacteria is still not sufficient to guide researchers in the selection of suitable species for fuel production. Macroalgae and seaweed are also high in carbohydrates, which can be converted into various fuel types including ethanol [49]. A major obstacle of bioethanol production from microalgae is degrading the hard cell walls that surround the starch particles. Degrading and removing the cell wall improves access and enzyme conversion [54].

Lipid-Forming Algae for Biodiesel Production

Many algae are rich in lipids, which can serve as an excellent feedstock for biodiesel production. Strains have been identified that produce greater than 80% of their dry weight as lipids [55, 56]. Most algae species are capable of converting CO₂ and water to O₂ and biomass through photosynthesis. This type of cultivation is usually called “photoautotrophic cultivation” and can be achieved in open ponds, in raceways, in closed photobioreactors, or on special growth membranes. Photobioreactors use much less water than open ponds but have strict temperature requirements, which can result in high cooling costs.

Biodiesel from algae is receiving renewed attention because it does not follow a food-to-fuel pathway as first-generation feedstocks, such as soybean oil or rapeseed oil. Microalgae typically will produce more oil, consume less space, and can be grown on land unsuitable for traditional agriculture. However, environmental implications such as excessive water usage and fertilizer inputs still need to be optimized. Microalgae to fuels pathways need to also include the reutilization of the algal waste in environmentally sustainable and economically feasible ways [48, 49, 56–58].

8.3.2.1.4.2 Duckweed to Biofuels

The Lemnaceae (duckweed) is a small floating aquatic plant that usually blooms through vegetative budding [59]. Duckweed is a monocotyledon with 37 species distributed among five genera (Landoltia, Lemna, Spirodela, Wolffia, and Wolfiella). Duckweed can grow in wastewater with high nutrient concentrations in an environmentally friendly manner. Because of its tolerance of high nutrient levels and excellent nutrient uptake, it has been studied as a tertiary treatment of municipal and industrial wastewater, as well as for nutrient recovery from swine wastewater [60]. Based on the species and growing conditions, duckweed starch content may reach up to 75% (dry weight basis) [59, 61]. The high starch content of duckweed makes the plant a potential feedstock for bioethanol production [60, 61]. A compositional analysis indicates that duckweed cell wall material is rich in cellulose and pectin and contains little lignin. This also makes duckweed enzymatic conversion attractive because it may need significantly reduced pretreatment and enzyme applications [61–63].

8.3.2.2 Thermochemical Conversion Processes of Biomass

Thermochemical technologies that can be used to convert sustainable biomass (purpose-grown energy crops, wood chips, algae, duckweed) and organic waste (agricultural waste, forestry waste, farm waste) into energy forms (heat, power, and fuels) can be categorized as combustion, gasification, and pyrolysis. Thermochemical conversion has several advantages, including the possibility of converting all the major components of biomass, namely, cellulose, hemicellulose, and lignin. Thermochemical conversion is typically utilized with solid, low moisture biomass. The diverse nature of biomass feedstocks, agricultural wastes, dedicated energy crops, forestry wastes, and agro-industrial wastes can reduce efficiency if the most suitable conversion technology is not selected.

8.3.2.2.1 Direct Combustion

Direct combustion is the most common methodology to convert solid biomass feedstocks into heat and power or combined heat and power (CHP) [64]. Direct combustion is a chemical reaction between the biomass fuel and pure oxygen or air that produces carbon dioxide and water along with heat release. Direct combustion systems oxidize biomass to generate hot flue gas that can be used for heat or fed into a boiler to generate steam. The steam can then be used in a steam turbine to generate electricity [65, 66]. Various forms of biomass fuels such as wood logs, chips, pellets, saw dust, forest residues, and straw are commonly used in small combustion appliances such as wood stoves, pellet burners, pellet boilers, wood log boilers, and wood chip boilers [64]. Industrial use of biomass for energy is primarily solid biomass especially industrial, forestry, and agricultural residues in the form of pellets. Traditionally, pellet mills are typically located near sawmills and utilize the saw dust waste to form pellets. However, large pellet mills also use wood residues and in some cases wood logs as raw material, but the feedstock is mainly local [64, 67]. Biomass fuel properties are the defining parameters for the technology to combust them. The most important fuel properties for combustion are the proximate analysis and heating value of biomass. Proximate analysis provides fixed carbon (char), volatile, moisture, and ash content assessment of biomass, while ultimate analysis examines elemental percentages of C, H, O, N, S, and Cl in a given biomass sample (Table 8.2).

Biomass type	Ultimate analysis (db, % w/w)					Proximate analysis (% w/w)				LHV (MJ/kg)
	C	H	O	N	S	Ash	VM	FC	M
Cedar wood	51.10	5.90	42.50	0.12	0.02	0.3	80–82	18–20	—a	19.26
Wood sawdust	46.2	5.1	35.4	1.5	0.06	1.3	70.4	17.9	10.4	18.81
Olive oil residue	50.7	5.89	36.97	1.36	0.3	4.6	76	19.4	9.5	21.2
Rice husk	45.8	6.0	47.9	0.3	—	0.8	73.8	13.1	12.3	13.36
Rice straw	38.61	4.28	37.16	1.08	0.65	12.64	65.23	16.55	5.58	14.40
Pine sawdust	50.54	7.08	41.11	0.15	0.57	0.55	82.29	17.16	—a	20.54
Spruce wood pellet	49.3	5.9	44.4	0.1	—	0.3	74.2	17.1	8.4	18.5
Coffee husk	46.8	4.9	47.1	0.6	0.6	1.0	74.3	14.3	10.4	16.54
Coffee ground	52.97	6.51	36.62	2.8	0.05	1.0	71.8	16.7	10.5	22
Larch wood	44.18	6.38	49.32	0.12	—	0.12	76.86	14.86	8.16	19.45
Grapevine pruning waste	46.97	5.8	44.49	0.67	0.01	2.06	78.16	19.78	—a	17.91
Jute stick	49.79	6.02	41.37	0.19	0.05	0.62	76–78	21.4–23.4	—a	19.66
Sugar-cane bagasse	48.58	5.97	38.94	0.2	0.05	1.26	67–70	28.74–30.74	—a	19.05
Com cob	40.22	4.11	42.56	0.39	0.04	2.97	71.21	16.11	9.71	16.65
Peach stone	51.95	5.76	40.7	0.79	0.01	0.65	81.3	18.1	8.53	21.6
Wheat straw	46.1	5.6	41.7	0.5	0.08	6.1	75.8	18.1	—a	17.2
Cotton stem	42.8	5.3	38.5	1.0	0.2	4.3	72.3	15.5	7.9	15.2
Straw	36.57	4.91	40.70	0.57	0.14	8.61	64.98	17.91	8.5	14.6
Camphor wood	43.43	4.84	38.53	0.32	0.1	0.49	72.47	14.75	12.29	17.48
Beech wood	48.27	6.36	45.2	0.14	—	0.8	81	18	—a	19.2
Switch grass	47	5.3	41.4	0.5	0.1	4.6	58.4	17.1	20	18.7

Source: Adapted Alauddin et al., 2010 [75].

^a – Dry Basis

VM, volatile matter; FC, fixed carbon; M, moisture.

8.3.2.2.1.1 Biomass Combustion Basics

Although combustion of biomass is considered an oxidation process, it consists of a complex series of thermal decomposition reactions including drying, devolatilization, gasification, char formation, combustion, and gas phase reactions. When biomass is heated its components start to hydrolyze, oxidize, dehydrate, and pyrolyze with increasing temperatures, forming combustible volatiles, tarry substances, and char. The decomposition products of cellulose, hemicellulose, and lignin first undergo partial and complete oxidation reactions. Composition and physiochemical properties of biomass are the determining parameters for duration and rate of the steps of the oxidation process [67, 68].

Particle formation and uncontrolled emissions from biomass combustion are raising growing health concerns about using direct combustion as a technology option. Research shows that combustion of biomass is a common source of particulate matter (PM) and gaseous emissions such as polycyclic aromatic hydrocarbons (PAH), volatile organic compounds (VOC), carbon monoxide, nitrogen oxide, and sulfur oxide [66, 70, 71]. Based on the biomass fuel type and combustion conditions, particle formation can be categorized as soot particles, organic particles, and ash particles. In any size application, emissions should be controlled for pollutants and meet air quality regulations. Particle control technologies include

• Cyclones
• Multicyclones
• Core separators
• Cartridge filters
• Electrostatic precipitator (ESP)
• Fabric filters (baghouses)
• Rotating particle separators

8.3.2.2.1.2 Types of Direct Combustion Boilers

Direct combustion to electricity applications can utilize two types of systems, namely, fixed-bed and fluidized-bed combustion.

• Fixed-bed combustion systems burn biomass on a grate or underfeed stokers and release the hot flue gases into a heat exchanger system to generate steam. When primary air passes through the fixed bed, biomass drying, partial gasification, and initial charcoal combustion take place. The combustible gases then are burned in a separate zone. Underfeed stokers feed biomass into the combustion chamber by screw conveyors from below and transfer biomass upward on a grate.
• Fluidized-bed combustion utilizes an incombustible suspended bed (usually sand) where the biomass is burned to release flue gas. Fluidized-bed boilers can be categorized based on the fluidization type, namely, bubbling or circulating fluidized beds. In bubbling fluidized-bed applications, the bed particles are suspended by the primary air at comparatively lower fluidization velocities (∼1–3 m/s), whereas in circulating bubbling fluidized-bed designs, higher gas velocities (∼3–6 m/s) are applied and solids leave the combustion zone. Additional bed materials are reintroduced by circulating the material again [67, 68]. The fluidized-bed application increases heat transfer to achieve improved efficiency and close to complete oxidation reactions with reduced emissions. However, this method has greater standby power needs than fixed-bed boilers [64].

8.3.2.2.1.3 Co-firing Biomass

Co-firing biomass with fossil fuels is receiving increased attention as a way to reduce the adverse effects of coal burning for energy, including GHG mitigation. Research reviews report over 100 successful co-firing demonstrations in 16 countries that use biomass species including herbaceous woody and waste biomass in various types boilers (tangential, wall, and cyclone fired) [68, 72, 73]. Co-firing biomass with coal can generate a substantial reduction in NO_x, SO_x, and GHG emissions from direct coal combustion. Generally, up to 15% biomass can efficiently be co-fired with coal without drastic reduction in efficiency or retrofitting [74].

8.3.2.2.2 Gasification (Syngas Platform)

Gasification is a thermochemical conversion process in which carbonaceous substances such as biomass are partially oxidized and converted into gaseous products [69, 75, 76]. The product gas, commonly referred to as producer gas or syngas, consists mainly of hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), water (H₂O), and small amounts of hydrocarbons such as methane (CH₄) [72, 75, 76]. Depending on the feedstocks used, the syngas can also contain alkali, metal compounds, sulfur, nitrogen, carbonyl sulfide, ammonia, and cyanide (HCN). Following gasification, syngas needs to be cleaned prior to further utilization. Syngas is best used as a fuel for stationary power and heat generation or to manufacture a range of fuels, chemical intermediates, and end products [69].

Gasification is one of the promising technologies that are being widely researched to convert sustainable biomass and waste biomass into energy forms, as it adds value to low or negative-value feedstocks into marketable fuels and products [69].

8.3.2.2.2.1 Biomass Gasification Basics

The chemistry of biomass gasification consists of four consecutive reactions that take place in a partial oxidation process. These reactions usually overlap without any definitive limits (Figure 8.4) [69, 75–77]. Providing a valid theory to describe the entire biomass gasification process is very complex due to the variety of biomass feedstocks that can be used and their specific characteristics, reaction thermodynamics, pressure, and temperature conditions [78]. In addition to air, pure oxygen, and steam can be used as a gasifying agent. It has been shown that gasification with air results in a gaseous product with low to medium heating value (4–7 MJ/Nm), whereas gasification with oxygen or steam results in a medium heating value (10–14 MJ/Nm) [75]. Although steam gasification leads to higher amounts of H₂ yields, it requires higher operating temperatures to vaporize water and is proven to be an expensive approach. Research indicates that a mixture of air/steam at variable ratios would contribute to more efficient syngas production [79, 80]. In summary, the following operating parameters define efficient gasification of biomass: the type of gasifying agent, gasifying agent/biomass ratio, biomass moisture content, residence time, reaction temperature, pressure, air–fuel ratio, and catalyst additives.

Biomass gasification stages: As previously mentioned, the gasification process for biomass takes place in four stages: drying, pyrolysis, oxidation, and reduction (see Figure 8.4). A description of each stage is as follows:

• Drying: The moisture content of biomass is highly variable and can range from 30% to 60%. During the drying stage, the moisture level of the biomass feedstock should be reduced to 10–15%.
• Pyrolysis: In this stage, the breakdown of biopolymers occurs at 150–400 °C along with the formation of char and condensable and noncondensable gases. This stage is influenced by type and particle size of biomass, heating rate and maximum temperature, residence time, and pressure.
• Oxidation: During oxidation, the char, formed in the pyrolysis stage, is oxidized to CO₂ and water. This stage releases a large amount of heat as a result of the oxidation of carbon and hydrogen.
• Reduction: The sub-stoichiometric amount of oxygen will initiate partial oxidation of carbon releasing carbon monoxide. In addition, several reduction reactions occur as follows:
  • C + H₂O → CO + H₂
  • C + CO₂ → 2CO
  • CO₂ + H₂ → CO + H₂O
  • C + 2H₂ → CH.

8.3.2.2.2.2 Gasifier Types

A gasifier is a chamber in which the biomass and gasifying agent are mixed at certain ratios and at certain temperatures and in some applications with inert materials such as catalysts. Design and operation of gasifiers require an understanding of the effects of various biomass characteristics and operating conditions on the performance of the system. Gasifier designs have been researched for more than a century and are classified by [77, 81]

• Gasifying agent: Air-blown, steam, and oxygen
• Heating type: Direct or auto-thermal, indirect
• Reactor pressure: Atmospheric or pressurized
• Reactor design: Fixed bed, fluidized bed, and entrained flow

The most common classification of gasifiers is based on the design of the reactors [69, 75–78].

1. 1. Fixed-bed gasifiers are also known as moving bed reactors. Depending on the air flow in the chamber, they can be further grouped as updraft fixed-bed, downdraft fixed-bed, cross-draft fixed-bed, and open-core fixed-bed gasifiers. These work by piling the biomass on top of a grate inside the gasification chamber, and the fuel bed moves slowly down the reactor as the gasification reactions proceed. These types of gasifiers are simple, inexpensive, and proven to be easy to operate. However, they produce gas with low energy content.
2. 2. Fluidized-bed gasifiers utilize a hot, suspended inert bed where biomass is fed and gasified. Fluidized-bed technology offers better performance and yield since it provides better heat and mass transfer for efficient conversion of various types of biomass and wastes with different compositions and heating values (Table 8.2) [75, 82]. Research has shown that fluidized-bed reactors are promising for lignocellulosic biomass due to their lower gasification temperature requirements [75]. Fluidized-bed reactors generally operate at 800–1000 °C to prevent ash from becoming too sticky and depositing on the internal parts of gasifiers. Depending on the operational mode of the heat transfer medium in the gasification chamber, fluidized-bed gasifiers can be further grouped as circulating fluidized-bed, bubbling fluidized-bed, and twin-bed fluidized-bed gasifiers.
3. Bubbling fluidized-bed gasifiers are the simplest and most cost-effective type of continuous gasification [75]. They provide high rates of heat transfer between the bed material and the biomass to generate uniform syngas with a low amount of tar. However, it has been reported that because of back mixing of solids, high conversion is not attainable and improved mixing is required. In some cases, slow oxygen diffusion rates may occur, which can hinder gasification efficiency [75, 76].
4. Circulating fluidized-bed gasifiers operate in a way such that the solids (biomass and bed material) move in a circulating manner to avoid bubble formation, which prevents the gasifying agent from bypassing the bed and solids. High thermal inertia and vigorous mixing enables gasification of various types of biomass; it is therefore one of the recommended technologies for large-scale biomass gasification [69, 76, 77].
5. Twin-bed fluidized-bed gasifiers use two separate fluidized-bed reactors. The biomass is gasified with steam in the first reactor and remaining char is transported into the second reactor and combusted fully with air to supply heat for the first reactor [69, 75, 76, 78].
6. 3. Entrained-flow gasifiers were originally designed for coal gasification for the integrated gasification combined cycle (IGCC). They operate in high temperatures around 1400 °C and with a pressure of 20–80 bar. The fuel feed for the gasifier is a slurried powder that facilitates inexpensive feeding of solid fuel. These reactors consume 20% more oxygen than dry feeding due to their high draft demand [69, 79–81]. Oxygen rapidly reacts with volatile materials and the char and produces exothermic reactions that raise the temperature above the melting point of the ash and completely destroys the tars [79, 80]. The high temperatures also result in a higher level of conversion of carbon [83].

8.3.2.2.2.3 Syngas Cleaning

In order to reduce the tar formation in syngas, the operating parameters should be optimized. Biomass tar is a light hydrocarbon and phenolic mixture, and it can be converted into gaseous products through steam reforming and various types of catalysts that can enhance the conversion. Catalysts are categorized into three groups: (i) naturally occurring catalysts (i.e., dolomite, olivine); (ii) metal catalysts (i.e., nickel and alkali metals); and (iii) alkalis (i.e., potassium, potassium hydroxide (KOH), potassium bicarbonate (KHCO3)). In some cases the char also is used in reforming the syngas [84–86]:

8.1

8.2

The main gas cleaning systems for the removal of dust, particles, and tars from syngas are as follows [72, 82]:

• Cyclones
• Ceramic, textile bag filters
• Rotating particle separators
• ESP
• Scrubbers

8.3.2.2.2.4 Syngas for Power Generation and Liquid Fuels Manufacturing

The syngas generated by biomass gasification can be utilized for both power and heat generation and also converted into second-generation biofuels and chemicals.

Biomass Gasification for Power and Heat Generation

Although, the gasification of biomass is proven to be a fuel for small-scale power plants and heat generation, gasification is yet to be optimized to be technically and economically viable on a larger scale. Since biomass gasification takes place at lower temperatures than biomass combustion, it reduces potential ash-related problems. However, gasification in power generation plant operations are more complex than combustion, and the processes are more sensitive to operating parameters. Most biomass gasification applications demonstrated to date for electric power have power output less than 10 MW [87]. Biomass is also being investigated for co-gasification with coal to reduce fossil-based CO₂ emissions. Analysis of IGCC oxy-co-gasification of straw and coal have shown that up to 10% biomass co-gasification in an oxygen-blown entrained flow gasifier was technically feasible [88, 89]. Low heating value clean syngas is suitable for use in internal combustion engines, gas turbines, or other applications requiring a high quality gas. The clean gas mixture can also be utilized in microturbine systems and fuel cells. Fuel cells can utilize the hydrogen fraction of syngas with oxygen to generate electricity and heat in the presence of an electrically conductive electrolyte medium. In addition, the carbon monoxide and methane fractions of the syngas can be converted into hydrogen via dry reforming and water gas shift reactions [84, 90–93]:

Fischer–Tropsch (FT): Biomass Gasification for Advanced Liquid Fuels Manufacturing

Biomass gasification end products of syngas can be converted into various long-chain hydrocarbons to displace fossil-based counterparts. The conversion reaction is widely known as the Fischer–Tropsch (FT) reaction, which was developed originally for coal gasification into liquid fuels [94, 95]:

Liquid hydrocarbons such as biomass-based diesel, kerosene, and gasoline production can be optimized by temperature, pressure, and ratio of H₂ to CO. The desired ratio should be close to 2 : 1, and sometimes water gas-shift reaction can be performed to achieve the desired H₂ to CO ratios. Cobalt is the widely used catalyst for FT conversions at pressures of 20–40 bar and temperatures between 180 and 250 °C [84, 96].

Syngas from biomass gasification can be converted into ethanol, methanol, and dimethyl ether (DME) as alternative advanced fuels. In addition, syngas can also be chemically and biologically converted into bio-based products including organic acids, alcohols, and polyesters [84].

8.3.2.2.3 Pyrolysis

The term pyrolysis refers to heating of materials in the absence of an oxidizing agent, causing thermal decomposition of polymer structures into smaller molecules. Pyrolysis reactions occur at lower temperatures of 400–600 °C compared with 800–1200 °C in gasification reactions. Pyrolysis conversion yields condensable vapors that subsequently produce liquid products, noncondensable gases (H₂, CO, CO₂, CH₄, and light hydrocarbons), and solid char [97–99]. The reactor types and operating conditions impact the yields of the various products. Depending on the heating rate, pyrolysis reactions can be classified as slow or fast/flash pyrolysis with different yields. Slow heating rates provide higher char yields because slow heating and longer residence time in a reactor results in slow decomposition. In fast pyrolysis, the rapid heat transfer and short residence times produce higher percentages of liquid products. Both types of reaction conditions also generate noncondensable gaseous products such as C1–C4 hydrocarbons. Biomass pyrolysis is emerging as a thermochemical technology that has the potential to serve as a viable conversion pathway of sustainable biomass into sustainable fuels and chemicals to displace petroleum-based counterparts, bio-char to sequester carbon, and sustainable management for biomass waste. Viable feedstocks for biomass pyrolysis include wood residues (mill residues, sawdust, urban wood, and forest thinning), agricultural residues (i.e., corn stover, wheat straw, rice hulls, sugarcane and sorghum bagasse, and low moisture animal waste), and energy crops (i.e., switch grass, hybrid poplar, and hybrid willow) [100–102].

8.3.2.2.3.1 Biomass Pyrolysis Basics

Pyrolysis of biomass is a complex process and is influenced by many different reaction parameters. Due to the complex nature of biomass, the fundamentals of pyrolytic decomposition reactions of biomass are still not completely understood. Consequently, a number of pyrolytic studies of biopolymers (lignin, cellulose, and hemicellulose) are being conducted to further understand the mechanisms of biomass pyrolysis [102, 103]. Biomass pyrolysis products are a complex combination of products as a result of the individual pyrolysis processes for cellulose, hemicellulose, and lignin, each of which has its own kinetic characteristics [103]. However, it is not clear that studying the thermal decomposition of a small number of model compounds such as cellulose, hemicellulose, and lignin is sufficient to model the wide range of biomass species existing in nature and in waste biomass. In addition, the complex interactions between different organic and inorganic biomass components have to be considered to understand the detailed decomposition pathways of biomass and wastes [103–105].

Plasma Gasification

Plasma gasification is an emerging technology that can be used to convert waste material and biomass materials into an energy source. Acceptable feedstocks include organic and plastics components of municipal solid waste, textiles, wood, and rubber. In addition, plasma gasification can convert hazardous waste, hospital solid waste and contaminated materials, sewage sludge, and low level radioactive waste. This technology basically is an incomplete oxidation of hydrocarbon components of waste and generation of a combustible such as mixture of carbon monoxide and hydrogen (syngas) and other minor gases [106]. Syngas can be used for either direct power generation or generation of renewable hydrogen or other fuels where applicable. Plasma gasification also results in compaction and vitrification of solid wastes after gasification. This achieved by the melting of inorganic material by high plasma temperatures eliminating voids and binding hazardous metals in a ceramic matrix (e.g., a silicate) with low leach rates.

Plasma occurs in any gas in which at least part of the atoms or molecules are partly or fully ionized [106]. Thermal plasmas are generated either by an electric arc or by a radiofrequency induction discharge. In waste treatment, arc plasmas dominate because they are relatively insensitive to changes in process conditions. High temperature treatment of waste material provides the following:

• Rapid heating and reaction start-up
• High heat and reactant transfer rates
• Smaller installation size for given waste throughput
• Melting of high temperature materials

A typical plasma system for treatment of solid wastes consists of a plasma reactor with collection of the metal and slag at the bottom and a power supply. In order to eliminate fugitive emissions, plasma gasification reactors should be operated under negative pressure. Plasma gasification temperatures may reach to 2000–30 000 °C [106–108].

A number of biomass pyrolysis studies have shown that the pyrolysis reaction may occur as follows [103]:

Experimental and modeling research confirms that increased heating rates reduce the char yield during biomass conversion. The distribution of products between bio-oil, char, and gas on a mass basis for slow heating rates is approximately 30%, 35%, and 35%, respectively, whereas under fast/flash heating conditions.,the product distribution can be altered dramatically. Under the fast/flash pyrolysis conditions, bio-oil, char, and gas yields can reach up to 75%, 12%, and 13%, respectively. Pyrolysis oils produced during the decomposition of biomass can decompose further through secondary reactions. In order to maximize the oil yield and reduce the char formation, the reactor design and optimization efforts should minimize the secondary reactions [105, 109–114].

8.3.2.2.3.2 Pyrolysis Reactor Types

Pyrolysis reactions are based on the rate of the pyrolytic conversion, and reactor design can be classified as follows [102, 103, 115, 116]:

1. 1. Slow pyrolysis in fixed-bed pyrolysis reactors
2. 2. Fast pyrolysis in:
   • Fluidized beds, both bubbling and circulating
   • Ablative (biomass particles move across hot surface), vacuum, and transported beds without a carrier gas.

Slow pyrolysis in fixed-bed reactors: Slow pyrolysis reactions are generally performed in a fixed-bed reactor where the biomass or waste is placed on a fixed-support bed in the reactor as a batch operation. The reactor is usually heated slowly to final temperatures below 500 °C. Slow heating rates coupled with long solid and gas residence times maximize the yield of char via secondary coking and re-polymerization reactions [98, 115, 116]. Slow pyrolysis is receiving renewed attention due to carbon capture and storage benefits to mitigate climate change. If a feedstock for slow pyrolysis is clean, untreated biomass, the char may also have soil remediation benefits.

Fast/flash pyrolysis: In order to maximize bio-oil yields, efficient heat and mass transfer is necessary during the pyrolysis of materials. Fast pyrolysis occurs during rapid heating (1000–10 000 °C/s) and at high temperatures (∼650 °C) under inert conditions. Increased pyrolysis heating rates and increased surface area result in better heat transfer and increased mass transfer rates. Biomass rapidly depolymerizes to form vapors (a mixture of light hydrocarbons and oxygenates), which ultimately condense to form bio-oil [100, 116]. Also, faster gas feeding rates will reduce vapor residence times in the hot pyrolysis zone, which will minimize secondary cracking and re-polymerization reactions and consequently increase the bio-oil yield. In order to achieve efficient conversion of biomass to liquid products, various types of fast/flash pyrolysis reactors have been developed and tested. In addition to pyrolysis reactor designs, biomass pretreatment for moisture removal, size reduction, and efficient continuous feeding into the pyrolysis zone are important parameters that can be optimized to achieve economically and technically feasible pyrolysis operations.

Fluidized-bed reactors: Biomass conversion into high oil yields requires high heating rates, short vapor residence times, and rapid cooling of the vapors. These characteristics can be achieved in fluidized-bed reactors [117]. Fluidized-bed reactors were developed for the petroleum and chemical process industries and have been in use for more than 50 years [118]. Fluidization technology is a process in which solids are made to behave like a fluid by feeding gas or liquid upward through the solid-filled reactor [119]. Fluidized-bed reactors are used for fast/flash pyrolysis to obtain efficient heat transfer rates from the heated bed material to biomass particles in order to achieve high heat transfer and efficient conversion before biomass particles become charred. The most commonly tested fluidized-bed reactors for biomass pyrolysis are bubbling and circulating-bed reactors. Silica sand is widely used as a fluidized-bed material [120]. Literature has also shown in situ fractional fast pyrolysis with fluidized catalytic beds [121–123].

Bubbling fluidized-bed pyrolysis occurs when the bed material is fluidized by a carrier inert gas fed from the bottom of the reactor to make the bed act as a fluid. The important parameters are appropriate particle size for biomass and the bed fluidizing media. The carrier gas flow rate is important to ensure that the bed material gets fluidized properly but does not get carried away above the free board section of the reactor. Vapor residence times are controlled by the carrier gas flow rate. In order to avoid having the char catalyze vapor cracking reactions, the residence time should be optimized and char should be removed from the bed section as quick as possible [100]. Bubbling fluidized-bed reactors have been reported to be easy to operate and are reliable.

Circulating fluidized-bed pyrolysis reactors provide more efficient heat transfer and shorter residence times. These types of reactors are different from bubbling fluidized beds since there is a secondary reactor following the pyrolysis chamber to combust char and return the hot sands back into the first pyrolysis chamber. These reactors require fine biomass particles since larger biomass particles may get combusted in the secondary reactor, potentially reducing bio-oil yields [100, 115, 118].

Ablative, vacuum, and transported bed pyrolysis reactors: Ablative pyrolysis reactors are designed to convert larger particle size (i.e., 2 cm) biomass into bio-oils. Vacuum pyrolysis reactors were developed to achieve a slow pyrolysis process with higher liquid yields by avoiding secondary cracking reactions. Transported bed reactors do not require a carrier gas since biomass and sand are fed from the bottom of a cone reactor and spin upward to the top of the reactor. Literature reports scalability issues and mechanical problems with this type of pyrolysis reactors. These reactors have been tested at smaller sizes and proved to be unscalable [115, 118, 124].

Fractional catalytic pyrolysis: Biomass feedstocks are composed of structural (cellulose, hemicellulose, and lignin) and nonstructural (extractive) components that have distinct chemical properties [119]. The literature suggests that selectively converting biomass components to end products is possible through fractional catalytic pyrolysis [97, 118, 119]. Fractional pyrolysis is defined as a selective in situ conversion of biopolymers into desired fuels and chemicals without going through secondary separation and upgrading processes. Zeolite catalysts are widely tested catalysts for fractional pyrolysis to generate stable pyrolysis oils with minimal re-polymerization reactions and can potentially be processed in a conventional petroleum refinery [97, 118, 119].

Bio-oil from biomass pyrolysis: Bio-oils via pyrolysis can be produced from a variety of biomass feedstocks including forest and agricultural residues (i.e., bagasse, rice straw, and hulls) and dedicated energy crops (i.e., switch grass, aspen, poplar). The bio-oil yield, based on the feedstock and conversion technology, is in the range of 70–90% by weight. Feedstock with high lignin content, such as bark, may yield lower amounts of bio-oil in the range of 60–65%. Bio-oils have long been considered an alternative to petroleum-based fuels. However, since pyrolysis causes bond cleavage and produces small fragments of biomass polymers, most of the original oxygen in the biomass feedstock is retained as oxygenated compounds in the bio-oil [116, 121, 124, 125]. Bio-oil usually contains 45–50% weight oxygen, depending on the moisture level of the feedstock. Also, most bio-oil analyses show that bio-oils appear to be acidic with a pH of 2.0–3.0 [116, 126–128]. The chemistry of bio-oils can be tailored by changing thermal conditions and using catalytic treatments that influence characteristics of end products. High temperatures can increase the severity of cracking, which may result in lower molecular weight liquids and increased gas products. Higher temperatures also cause dehydrogenation/aromatization reactions to take place, resulting in higher levels of PAH. Both the reaction conditions and catalyst properties are critical in maximizing the desired product selectivity. Research suggests that high heating rates and high catalyst to feed ratio favor aromatic production over coke formation [129]. Fast pyrolysis appears to be a viable technology for producing olefins and aromatic monomers from carbohydrates if the reaction conditions are optimized [100, 115, 128]. Recent research also suggests that converting biomass and its components to BTX and other petroleum-based platform chemicals such as ethylene and propylene is possible by catalytic pyrolysis conversion [130–133]. The advantages include low production costs, high thermal efficiency, and potential CO₂ reductions. Various researches on producing pyrolysis oils from agricultural wastes and residues and municipal solid wastes have shown that pyrolysis appears to be a promising technology but requires further research and demonstration [134–138].

8.4 Conclusions

Sustainable biomass-to-bioenergy pathways, including bio-power, bio-heat, advanced biofuels, and bio-based products are emerging as viable alternatives to displace petroleum-based fuels and products. While promoting biomass as a feedstock, it is important to ensure that it does not follow a food-to-fuel pathway that can further stress food resources and negatively impact biodiversity. In addition, new fuels and products should have smaller carbon footprints then the petroleum-based counterparts that they intend to displace. Existing and emerging biomass and waste conversion technologies for power, fuels, and bio-products should be tested, verified, and scaled up from bench to pilot and demonstration scales before full commercial trials are implemented in order to ensure that the technology achieves its energy goals and is economically and environmentally sustainable.

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Further Reading/Resources

1. Black & Veatch (2008) Advanced Biopower Technology Assessment prepared for Massachusetts Division of Energy Resources.
2. Davis, R., Tao, L., Tan, E.C.D., Biddy, M.J., Beckham, G.T., Scarlata, C., Jacobson, J., Cafferty, K., Ross, J., Lukas, J., Knorr, D., and Schoen, P. (2013) Process Design and Economics for the Conversion of Lignocellulosic Biomass to Hydrocarbons: Dilute-Acid and Enzymatic Deconstruction of Biomass to Sugars and Biological Conversion of Sugars to HydrocarbonsPDF. NREL report no. NREL-TP-5100-60223, 147 pp.
3. Dutta, A., Dowe, N., Ibsen, K.N., Schell, D.J., and Aden, A. (2010) Economic comparison of different fermentation configurations to convert corn stover to ethanol using Z. mobilis and Saccharomyces. Biotechnol. Prog., 26 (1), 64–72. NREL report no. JA-510-45764 (January 2010).
4. Dutta, A. and Phillips, S.D. (2009) Thermochemical Ethanol via Direct Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass PDF. NREL report no. TP-510-45913,144 pp.
5. Dutta, A., Talmadge, M., Hensley, J., Worley, M., Dudgeon, D., Barton, D., Groendijk, P.. Ferrari, D., Stears, B., Searcy, E.M., Wright, C.T., Hess, J.R. (2011) Process Design and Economics for Conversion of Lignocellulosic Biomass to Ethanol: Thermochemical Pathway by Indirect Gasification and Mixed Alcohol Synthesis PDF. NREL report no. TP-5100-51400, 187 pp.
6. French, R. and Czernik, S. (2010) Catalytic pyrolysis of biomass for biofuels production. Fuel Process. Technol. 91(1), pp. 25–32.
7. Granatstein, D., et al. (2009) Use of Biochar from the Pyrolysis of Waste Organic Material as a Soil Amendment. Washington State University, Sustaining Agriculture and Natural Resources, C0800248.
8. Humbird, D., Davis, R., Tao, L., Kinchin, C., Hsu, D., Aden, A., Schoen, P., Lukas, J., Olthof, B., Worley, M., Sexton, D., and Dudgeon, D. (2011) Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn StoverPDF. NREL report no. TP-5100-47764, 147 pp.
9. Wright, M., et al. (2010) Techno-Economic Analysis of Biomass Fast Pyrolysis to Transportation Fuels. Technical report. NREL/TP-6A20-46586.