8.2.1 Forest Industries

Wood is the first biomass resource which has been used by human beings as fuel for cooking for the last few centuries. Wood is one of the oldest energy sources and has been in constant use throughout the modern era, despite the widespread adoption of other types of fuels. Even now, more than 2 billion people depend on wood energy for cooking and/or heating, in households in developing countries. It represents one-third of the global renewable energy consumption, making wood the most decentralized energy in the world. Forests are a major source of woody biomass. However, woody materials can also be sourced from agriculture. For example, fast-growing tree species such as hybrid willow (Salix) and poplar have been developed for the production in agricultural settings (i.e. grown as row crops on farms).

Today, wood energy has become an important source of green energy because of climate change and energy security concerns. Wood energy is considered as a climate neutral and socially viable source of renewable energy, provided the trees harvested as biomass are re-planted as fast as the wood is burnt. In such condition, the new trees absorb the carbon dioxide produced by the combustion of old trees, and the carbon cycle theoretically remains in balance. Thus, no extra carbon is added to the atmospheric balance sheet. Biomass can only maintain carbon balance if fast-growing crops are grown on otherwise unproductive land; in this case, the re-growth of the plants offsets the carbon produced by the combustion of the crops. But cutting or clearing forests for energy, either to burn trees or to plant energy crops, releases carbon into the atmosphere that would have been remained stored if the trees had remained untouched.

Biomass for fuel can be gathered as waste or grown in fields. The most common biomass resources are therefore from agricultural and forestry. Forest residues and wood wastes represent a large potential resource for energy production and include forest residues, forest thinnings, and primary mill residues [2]. But the energy potential of waste biomass is relatively small compared to the virgin biomass as an energy resource. The key to the large-scale production of energy, fuels, from biomass is to grow suitable virgin biomass species in an integrated biomass-production conversion system (IBPCS) at costs that enable the overall system to be profitable.

Even though the costs for these fuels are usually greater than that of coal, they reduce the fuel price risk by diversifying the fuel supply. These also result in significantly lower sulfur dioxide and nitrogen oxide emissions compared to coal and can easily be cofired. [3]

8.2.2 Forest Residues

Forest residues are the biomass material remaining in forests from harvest operations that are left in the forest after stem wood removal, such as branches, foliage and roots. Harvesting may occur as thinning in young stands or cutting in older stands for timber or pulp that also yields tops and branches usable for biomass energy production. Harvesting operations usually remove only 25–50% of the volume, leaving the residues available as biomass for energy. Stands damaged by insects, disease or fire are additional sources of biomass. Forest residues normally have low density and fuel values that keep transport costs high, and so it is economical to reduce the biomass density in the forest itself.

Since lumber mills and other processing facilities use timber of certain quality, forest residues are left in the forest. These residues could be collected after a timber harvest and used for energy purposes. Typically, forest residues are either left in the forest or disposed of via open burning through forest management programs. The primary advantage of using forest residues for power generation is that an existing collection infrastructure is already available to harvest wood in many areas.

8.2.3 Agriculture Residues

Agriculture residues are traditionally considered as “trash”, or agricultural waste is increasingly being viewed as a valuable resource. Agricultural residues such as straw, rice husk, coconut shell, groundnut shell and sugar cane bagasse are an excellent alternative to using virgin wood fibre as they have many advantages. These are available in abundance and are renewable. Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields while processing produces fibrous bagasse, both of which are good sources of energy.

Rice, wheat, sugar cane, maize (corn), soybeans and groundnuts are just a few examples of crops that generate considerable amounts of residues. Following the harvest of these agricultural crops, residues such as crop stalks, leaves and cobs are left in the field. A segment of these residues could potentially be collected and combusted to produce energy. These residues constitute a major part of the total annual production of biomass residues and are an important source of energy for both domestic and industrial purposes. However, transportation of residues can be expensive and not highly energy efficient.

8.2.4 Energy Crops

Dedicated energy crops are another source of woody biomass for energy. These crops are fast-growing plants, trees or other herbaceous biomass which are harvested specifically for energy production. Crops such as switchgrass, hybrid poplars (cottonwoods), hybrid willows and sugar cane are being studied for their ability to serve as energy crops for fuel. These crops have the greatest potential for dedicated energy use over a wide geographic range. One of their great advantages is that they are short rotation crops; they re-grow after each harvest, allowing multiple harvests without having to re-plant. Corn and sorghum serve a dual purpose as they can be grown for fuel, with the leftover by-products being used for other purposes, including food. Similarly, sugar cane crops are used for food (sugar) and energy in the form of ethanol. Oil-bearing trees such as soya bean, corn, sunflower, soybean, castor and wild plants such as jatropha and karanj are used to produce biodiesel. Sugar cane is a major source for bioethanol. Jatropha is a shrub that grows up to 5 m in height. It can be planted even in desert and can be grown in places where no crop is grown normally. These can be used for energy production after 2–3 years.

Karanj is another wild tree which is generally used to stop soil corrosion along highways and canals. The natural growth of karanj is along coasts and river banks in India and Myanmar. It is now also grown in the Philippines, Malaysia, Australia and Seychelles. Miscanthus (commonly known as Elephant Grass) is a high-yielding energy crop that grows over 3 m tall, resembles bamboo and produces a crop every year without the need for re-planting. The rapid growth, low mineral content and high biomass yield of Miscanthus increasingly make it a favourite choice as a biofuel, outperforming maize (corn) and other alternatives. Biodiesel can be produced from non-food feedstock. This means that the feedstock does not serve to diminish food supply. It also means that feedstock used for biodiesel can be shielded from global commodity prices.

Thus, a significant and increasing fraction of agricultural land worldwide is likely to be dedicated to the production of energy crops in the near future. This is a matter of concern that cultivation of energy crops might reduce land availability for food production.

8.2.5 Food and Industrial Wastes

While there is an obvious need to minimize the generation of wastes in cities and to reuse and recycle them, the technologies for recovery of energy from wastes can play a vital role in mitigating the problems. Besides recovery of substantial energy, these technologies can lead to a substantial reduction in the overall waste quantities requiring final disposal, which can be better managed for safe disposal. Food processing wastes are being used throughout the world as biomass feedstocks for energy generation. These wastes include house garbage, also called municipal solid waste (MSW), trash that comes from plant or animal products, food waste from restaurants. Many food materials are processed at some stage to remove components that are inedible or not required such as peel/skin, shells, husks, cores, pips/stones, fish heads, pulp from juice and oil extraction. Through a process called co-digestion, many treatment plants are enhancing AD with organic waste. Adding fat, oil and grease (FOG), as well as food waste, to AD accelerates this process, producing more methane gas for beneficial use and reducing the amount of solid waste conveyed to landfills.

Lawn clippings and leaves are all examples of biomass trash. Animal farms are also producing wastes that are not converted into fuels at present. These wastes are contributing to environmental degradation because many municipalities have not yet used them in energy conversion. Because of the necessity to generate energy locally, biomass resources such as MSW and sewage will become important in the future. MSW can be used to produce energy by either burning MSW in waste-to-energy plants or capturing biogas.

At an MSW combustion facility, MSW is first sorted, and items that can be recycled are removed from the waste. The material that is left is sent into a combustion chamber to be burnt. The heat released from burning is used to convert water to steam. The steam is then sent to a turbine generator to produce electricity.

8.4.1 Combustion

Biomass conversion into heat energy is still the most popular and perhaps the most efficient process using combustion. The most common application of biomass energy in developing countries is its use as a source of heat for cooking, sometimes called traditional biomass use. The burning of biomass in air, that is, combustion, is used over a wide range of outputs to convert the chemical energy stored in biomass into heat, mechanical power or electricity. Combustion of biomass produces hot gases at temperatures around 800–1000°C. It is possible to burn any type of biomass, but in practice, combustion is feasible only for biomass with a moisture content <50%, unless the biomass is pre-dried. High-moisture-content biomass is better suited to biological conversion processes. The heat is then used in a manufacturing process or to raise steam in a boiler which can drive a steam turbine to generate electricity. Net bio-energy conversion efficiencies for biomass combustion power plants range from 20% to 40%. The higher efficiencies are obtained with systems over 100 MW or when the biomass is co-combusted in coal-fired power plants.

The size of combustion system has a very wide range from a few kilowatts of thermal input such as a single gas ring for cooking to huge coal-fired combustion boilers with inputs of 3–5 GW in a single unit.

8.4.2 Gasification

Gasification is a thermochemical process where solid biomass is converted into gaseous fuel without leaving any solid residue. It is a well-established technology and was extensively used in World War II to power vehicles for transport. Gasification is the conversion of biomass into a combustible gas mixture by the partial oxidation of biomass at high temperatures, typically in the range of 800–900°C. The main components of this gas are CO, H₂, CO₂, CH₄, H₂O and N₂. However, a variety of tars are also produced during the gasification reaction. The partial oxidation can be carried out using air, oxygen, steam or a mixture of all of these. Air gasification produces poor quality of gas in terms of heating value. Oxygen gasification provides better quality gas in terms of energy contents. The production of syngas from biomass allows the production of methanol and hydrogen, each of which may have a future a fuel for transportation. Figure 8.2 shows a schematic diagram that illustrates the biomass gasification technology.

The conversion of biomass by gasification into a fuel is most suitable for use in an IC engine.

Basic chemical reactions in the gasification process for producing syngas are

8.2

8.3

8.4

8.5

8.6

Since there is an interaction of air or oxygen and biomass in the gasifier; these are classified according to the way air or oxygen is introduced in it as downdraft, updraft, cross-draft and fluidized bed. In an updraft gasifier, air or oxygen passes through the biomass from the bottom and the combustible gases come out from the top.

The updraft gasifier is the simplest type of reactor for the gasification of biomass that is easy to operate and has high conversion efficiency, although it produces high levels of tar. Because it has high tar content, these are not suitable for internal combustion engine. Updraft gasifiers shown in Fig. 8.3 achieve the highest efficiency as the hot gas passes through fuel bed and leaves the gasifier at a low temperature. The sensible heat given by gas is used to pre-heat and dry fuel. The gas produced has practically no ash content.

In downdraft gasifiers shown in Fig. 8.4, gas is drawn from the bottom of the reactor while the hottest reaction zone is in the middle. It is suitable for a variety of biomass materials. The biomass fuel enters through the hopper and flows down, gets dried and pyrolyzed before being partially combusted by the gasifying media entering at the nozzles. In the downdraft gasifier, air contacts the pyrolyzing biomass before it contacts the char and supports a flame. The heat from the burning volatiles maintains the pyrolysis. When this phenomenon occurs within a gasifier, the limited air supply in the gasifier is rapidly consumed, so that the flame gets richer as pyrolysis proceeds. At the end of the pyrolysis zone, the gases consist mostly of about equal parts of CO₂, H₂O, CO and H₂. It is possible to distinguish four separate zones in this gasifier:

• Drying zone
• Pyrolysis zone
• Oxidation zone
• Reduction zone

Solid fuel is introduced at the top. In the drying zone, the drying of the fuel takes place. As the fuel moves down, it enters the pyrolysis zone. Here large molecules (e.g. cellulose, hemicellulose and lignin) break down into medium-size molecules and carbon (char) during the fuel heating. Then the products of the pyrolysis process flow downwards into the hotter zones of the gasifier. Some of these are burnt in the combustion zone, and the rest (depending on the residence time in the hot gasifier zone) will break down to even smaller molecules of hydrogen, methane, carbon monoxide, ethane, ethylene and so on.

The throat allows maximum mixing of gases in a high-temperature region, which aids tar cracking. Since the volatile matter in the fuel gets cracked within the reactor, the output gas is almost tar-free. However, the gas, as it comes out of the reactor, contains small amounts of ash and soot. They can produce as much as 20% char, but in most cases, the char content is 2–10%. While production of char reduces the quantity of energy contained in the syngas, it can be used as a fuel (charcoal) and again burnt in the gasifier. Char can also be used for soil amendment. Because char often has a high value, gasifiers are sometimes operated to produce high quantity of char with reduction in gas production. The gas from the downdraft gasifiers can be cleaned to very high purity such that it can be used in IC engines or for direct heating applications where the purity of gas is most important.

Downdraft gasifiers are of two types, single throat and double throat. Single-throat gasifiers are used for stationary applications, whereas double-throat ones are used in automotive.

The cross-draft-type gasification is one of the simplest types of gasification. These were adapted for the use of charcoal as a fuel. They have certain advantages over the updraft and downdraft gasifiers, and unlike these types, the ash bin, fire and reduction zone in cross-draft gasifiers are separated. This reactor operates on a small scale and virtually produces no tar. The reactor for this gasification is much as the updraft gasifier in that the fuel enters from the top and the thermochemical reaction occurs progressively as this fuel descends into the reactor. As shown in Fig. 8.5, the main difference is that the air enters the gasifier from the side of the reactor, instead of from the top or the bottom. Normally, an inlet nozzle is used to bring the air to the centre of the combustion zone as shown in the figure. The velocity of the air as it enters the combustion zone is considerably higher in this design, which creates a hot combustion zone. The start-up time for this reactor is relatively short, and high temperatures can be attained using this type of gasification.

Cross-draft gasifiers respond rapidly to load changes (it takes less time to start the gasifier). They are normally simpler to construct and more suitable for running engines than the other types of fixed-bed gasifiers. However, they are sensitive to changes in biomass composition and moisture content.

Fluidized-bed gasifier is unique among the biomass gasifiers in one important capability: biomass fuel in any particle size range, any moisture content and any ash or grit content can be gasified. The fluidizing process is the forced flow of gaseous constituents through a stacked height of solid particles. At high-enough gas velocities, the gas/solid mass exhibits fluid-like properties, thus the term fluidized-bed.

A fluidized-bed gasifier is shown in Fig. 8.6. Air, steam and mixture of oxygen and steam are commonly used fluidization media. The bed is usually composed of sand, limestone, dolomite or alumina. There are two types of fluidized-bed gasifiers: bubbling and circulating. Bubbling-bed reactors operate at relatively lower gas velocities (less than 1 m/s) compared to circulating-bed reactors that operate at gas velocities of 3–10 m/s. Bubbling fluidized-bed gasifier consists of a horizontal air distributor with an array of bubble caps. This provides the fluidizing air to the lower furnace bed material. The bubble caps are closely spaced so that airflow is distributed uniformly over the furnace plan area. The lower furnace is filled with sand or other non-combustible material such as crushed limestone or bed material from prior operation. Airflow is forced upwards through the material, and the bed expands. The airflow through the bed is very uniform due to a high number of bubble caps.

The circulating fluidized-bed furnace has a flat-floor horizontal air distributor with bubble caps. It provides fluidizing air to the lower furnace bed material. The bubble caps are closely spaced for uniform air distribution over the furnace plan area. 50–70% of the total combustion air enters the furnace through the bubble caps with the balance injected through over-fire air (OFA) ports. Air is blown through a bed of solid particles at a sufficient velocity which drags them upwards with the gas flow and keeps these in a state of suspension. The flow of gas into the reactor must be sufficient to float the coal particles within the bed but not so high as to entrained them out of the bed. However, as the particles are gasified, they will become smaller and lighter and separated in a cyclone and then recycled to the bottom of the fluidized bed. The maximum operating point of the reactor is limited by the melting point of the bed material and is usually between 800 and 900°C. Such systems are less sensitive to fuel variations but produce larger amounts of tar and dust. They are more compact but also more complex and usually used at larger scales.

8.4.3 Pyrolysis

Pyrolysis is a thermochemical decomposition process in which the organic material is converted into a carbon-rich solid and volatile matter by heating the biomass in the absence of air to around 500°C (Fig. 8.7). Pyrolysis is a high-temperature process in which biomass is rapidly heated in the absence of oxygen. As a result, it decomposes to generate mostly vapours and aerosols and some charcoal. The products that are formed in this process are water, charcoal, methane, hydrogen, carbon dioxide and carbon monoxide. Figure 8.7(b) shows the production of charcoal using pyrolysis. The charcoal or coke is generally of high carbon content and may contain around half the total carbon of the original organic matter.

Pyrolysis is the simplest and certainly the oldest method of processing biomass to produce charcoal, a better combusting fuel. This requires relatively slow reaction at very low temperatures to maximize the solid yield. More recently, studies on the mechanisms of pyrolysis have suggested ways of substantially changing the proportions of the gas, liquid and solid products by changing the rate of heating, temperature and residence time.

Pyrolysis can be performed at relatively small scale and at remote locations which enhance the energy density of the biomass resource and reduce transport and handling costs. Temperature is the most important factor for the product distribution of pyrolysis; the best temperature for the production of the pyrolysis products is between 625 and 775 K. At lower process temperature, the product is mainly charcoal. At high temperature, the biomass will mainly produce gases, and moderate temperature is optimum for producing liquids. For highly cellulosic biomass feedstocks, the liquid fraction usually contains acids, alcohols, aldehydes, ketones, esters, heterocyclic derivatives and phenolic compounds.

Pyrolysis processes can be categorized as slow pyrolysis or fast pyrolysis. Fast pyrolysis is currently the most widely used pyrolysis system. Fast pyrolysis process is shown in Fig. 8.8 and can be described as: a process in which organic materials are rapidly heated to 450–600°C in the absence of air. Under these conditions, organic vapours, pyrolysis gases and charcoal are produced.

Fast pyrolysis is characterized by high heating rates and short vapour residence times. This generally requires a feedstock prepared as small particle sizes and a design that removes the vapours quickly from the hot solids. There are a number of different reactor configurations that can achieve this, including ablative systems, fluidized beds, stirred or moving beds and vacuum pyrolysis systems. A moderate (in pyrolysis terms) temperature around 500°C is usually used. The main product of fast pyrolysis is 60–80% bio-oil and takes only seconds for completion. In addition, it gives 20% char and 20% gas. The essential features of a fast pyrolysis are as follows:

• Dry feedstock: (less than 12% moisture)
• Small biomass particles: <3 mm
• Biomass heating time of 1–2 s
• Moderate temperatures (400–500°C)
• Vapour residence time of 1 s.

The slow pyrolysis has been known for a long time and is characterized by longer residence time of 30 min to days and lower temperature range. Its main product is charcoal, and the equipment are available for small- as well as large-scale production.

The pyrolysis process is an endothermic reaction and consumes energy. In terms of energy demand in pyrolysis, the main factor is the water content of the starting biomass. The heat of vaporization of pure water is 2.26 kJ/g at 100°C, while the chemical energy content of wood is only about 18.6 kJ/g. If the wood has high moisture content, then the net energy yield of the pyrolysis process will be very low because the energy necessary for the pyrolysis and gasification processes comes mainly from combustion of one or more of the products of pyrolysis.

8.4.4 Liquefaction

Liquefaction is a thermochemical conversion process of organic material into liquid bio-crude and co-products. Depending on the process, it is usually conducted at moderate temperatures (300–400°C) and pressures (10–20 MPa) with added hydrogen or CO as a reducing agent. Unlike coal, the biomass is “wet”, or at least wetter than coal, and can be processed as an aqueous slurry. Liquefaction of biomass can be hydrothermal or gaseous. In hydrothermal liquefaction, water simultaneously acts as reactant and catalyst, and this makes the process significantly different from pyrolysis. Hydrothermal liquefaction is pyrolysis in hot-compressed water of around 300°C and 10 MPa. Biomass is converted into gas, liquid and solid, similarly to common pyrolysis in gas phase.

Hydrothermal processing is divided into three separate processes, depending on the temperature of the operating conditions. At temperatures below 520 K, it is known as hydrothermal carbonization. The main product is a hydrochar which has a similar property to that of a low-rank coal. At intermediate temperature ranges between 520 and 647 K, the process is defined as hydrothermal liquefaction resulting in the production of a liquid fuel known as biocrude. Biocrude is similar to petroleum crude and can be upgraded to the whole distillate range of petroleum-derived fuel products. At higher temperatures above 647 K, gasification reactions start to dominate, and the process is defined as hydrothermal gasification, resulting in the production of a synthetic fuel gas.

8.5.1 Fermentation

Fermentation is a chemical process where glucose breaks down to form ethanol and CO₂. Ethanol production from sugar is the most simple and common process and makes use of yeast for conversion into ethanol which is a one-step fermentation process. Ethanol was used as fuel in combustion engines as early as the 18th century. But at present, it is used mainly as an additive to gasoline to give clean burning and good octane properties However, substantial gains made in fermentation technologies now make the production of ethanol for use as a petroleum substitute and fuel enhancer both economically competitive and environmentally beneficial.

One of the most promising fermentation technologies used recently is the “Biostil” process which uses centrifugal yeast reclamation and continuous evaporative removal of the ethanol. It is a biological conversion process through the action of certain specific yeasts or enzymes produced by microbes that act on the sugar, starch or cellulosic components and convert these into ethanol and carbon dioxide. Ethanol is separated from other components using distillation process. The mixture is heated so that the ethanol boils off and can be condensed back to liquid by cooling. Ethanol from sugar or starchy crops is termed the “first-generation” ethanol as opposed to “second-generation” ethanol that comes from cellulosic biomass. A wide variety of carbohydrates containing raw materials have been used for the production of ethanol by fermentation process. These raw materials are classified under three major categories: (i) sugar-containing crops – sugar cane, wheat, beet root, fruits, palm juice and so on; (ii) starch-containing crops – grain such as wheat, barely, rice, sweet sorghum, corn and root plants such as potato, cassava; (iii) cellulosic biomass – wood and wood waste, cedar, pine, wood, agriculture residues and so on. Recent advances in the use of cellulosic feedstock may allow the competitive production of alcohol from woody agricultural residues and trees to become economically competitive in the medium term.

Cellulosic ethanol is chemically identical to first-generation ethanol. However, it is produced from different raw materials via a more complex process (cellulose hydrolysis). In contrast to first-generation bioethanol, which is derived from sugar or starch produced by food crops (e.g. wheat, corn, sugar beet, sugar cane), cellulosic ethanol may be produced from agricultural residues (e.g. straw, corn stover), other lignocellulosic raw materials (e.g. wood chips) or energy crops (Miscanthus, switchgrass, etc.). Conversion of starch and cellulosic biomass into ethanol requires an additional step. First, sets of microbes are required to break down the starch components into sugars. In the next step, sugars are converted into ethanol via fermentation by yeast. The most common microbes for starch conversion into sugars are certain microbes that produce enzymes to break down starch into sugars. The production of second-generation biofuels is non-commercial at this time, although pilot and demonstration facilities are being developed. With the commercialization of these second-generation biofuels, there will be significant reduction in CO₂ production. They also do not compete with food crops, and some types can offer better engine performance.

The following chart shows the various processes to produce ethanol from sugary, starchy and lignocellulosic biomass. The ethanol concentration during fermentation is around 10–18% by volume and will have to be distilled to generate high-purity ethanol and dehydrated to obtain 100% pure ethanol.

The theoretical ethanol production from pure sugar (e.g. pure glucose) is illustrated in Eq. (8.7).

8.7

The density of pure ethanol is about 0.789 g/cm³. Thus, 180 g of pure glucose is needed to produce 92 g of pure ethanol, while producing 88 g of carbon dioxide or 1.54 kg sugar is required to produce 1 l of ethanol. The various processes used in fermentation are shown in Fig. 8.9.

Potential ethanol yields of some sugar and starchy crops in l/kg are shown in Table 8.1.

Distillation: To increase the concentration of ethanol, distillation is required which is heat intensive usually supplied by crop residues. Bioethanol can be used to produce ethanol gas, a clean burning fuel that consists of bioethanol bound in a hydrated cellulose-thickening agent.

To produce ethanol from cellulosic biomass, it is hydrolyzed by enzymes into glucose sugar or by chemical method using sulfuric acid which is then fermented to form ethanol.

8.5.2 Anaerobic Digestion

AD is a process that uses bacteria to convert animal slurries, silage, food processing and other organic wastes to methane-rich biogas. An aerobic digester is a large sealed vessel from which air is excluded. Anaerobic bacteria in the absence of oxygen are used to break down the organic matter of biomass, and during the conversion, a mixture of methane and carbon dioxide gases is produced. When functioning well, the bacteria convert about 90% of the feedstock energy content into biogas (containing about 60% methane), which is a readily useable energy source for cooking and lighting. In this process, organic compounds are solubilized and hydrolyzed by the microbial action to smaller compounds and volatile fatty acids (VFAs), which are then degraded into methane and carbon dioxide. The sludge produced after the manure has passed through the digester is non-toxic and odourless. In addition, it has lost relatively little of its nitrogen or other nutrients during the digestion process, thus making a good fertilizer.

AD can occur at two main temperature ranges: mesophilic conditions, between 20 and 45°C, usually 35°C and thermophilic conditions, between 50 and 65°C, usually 55°C.

AD is a commercially proven technology and is widely used for treating high-moisture-content organic wastes, that is, +80% to 90% moisture. Biogas can be used directly in spark ignition gas engines and gas turbines and can be upgraded to higher quality, that is, natural gas quality, by the removal of CO₂. Used as a fuel in spark ignition gas engines (s.i.g.e.) to produce electricity only, the overall conversion efficiency from biomass to electricity is about 10–16%. As with any power generation system using an internal combustion engine as the prime mover, waste heat from the engine oil and water-cooling systems and the exhaust could be recovered using a combined heat and power system. Figure 8.10 shows an AD system also often referred to as “biogas systems”. Depending on the system design, biogas can be combusted to run a generator producing electricity and heat (called a co-generation system), burned as a fuel in a boiler or furnace or cleaned and used as a natural gas.

There are three stages of AD that include stage I, hydrolysis and acidogenesis; stage II, acetogenesis; and stage III, methanogenesis. In AD, hydrolysis and acidogenesis are the essential first step where polymeric materials such as carbohydrates, proteins and lipids are hydrolyzed into smaller, water-soluble compounds such as sugars, amino acids and long-chain fatty acids (LCFA) by enzymes produced by the microorganisms. In general, hydrolysis is a chemical reaction in which the breakdown of water occurs to form H+ cations and OH− anions. The hydrolytic activity is of significant importance in wastes with high organic content and may become rate limiting. Chemicals can be added during this step in order to decrease the digestion time and provide a higher methane yield. Thus, in hydrolysis and acidogenesis, sugars, amino acids and fatty acids produced by microbial degradation of biopolymers are successively metabolized by fermentation end products such as lactate, propionate, acetate and ethanol by other enzymatic activities which vary tremendously with microbial species.

In the second stage, acetogenic bacteria, also known as acid formers, produce an acidic environment in the digestive tank to convert the products from the first stage into simple organic acids, carbon dioxide and hydrogen. The principal acids produced are acetic acid, butyric acid, propionic acid and ethanol. While acidogenic bacteria further break down the organic matter, it is still too large and unusable for the ultimate goal of methane production, so the biomass must next undergo the process of acetogenesis.

In general, acetogenesis is the creation of acetate, a derivative of acetic acid, from carbon and energy sources by acetogens. These microorganisms catabolize many of the products created in acidogenesis into acetic acid, CO₂ and H₂. Acetogens break down the biomass to a point at which methanogens can utilize much of the remaining material to create methane as a biofuel.

The final step in the AD process is the conversion of methane from the final products of acetogenesis as well as from some of the intermediate products from hydrolysis and acidogenesis by the action of methane-producing microbes (methanogenesis).

There are two general pathways involving the use of (i) acetic acid and (ii) carbon dioxide, to produce methane in methanogenesis: the main mechanism to create methane in methanogenesis is the path involving acetic acid. The conversion of hydrogen into methane utilizing the carbon dioxide produced during the process is called hydrogenotrophic (Eq. (8.9)). These reactions are shown in Eqs (8.8) and (8.9), respectively, including the estimated amounts of products formed by volume (mole basis) and by weight (kg).

8.8

8.9

Throughout this entire process, large organic polymers that make up the biomass are broken down into smaller molecules by chemicals and microorganisms. Upon completion of the AD process, the biomass is converted into biogas, namely carbon dioxide and methane, as well as digestate and wastewater [9]. The composition of solid residue remaining after digestion depends on the system and original feedstock, but it contains much of nitrogen and other plant nutrients from the original feedstock and can be used as a fertilizer and soil conditioner.

8.5.3 Anaerobic Digestion Technologies Suitable for Dairy Manure

The increase in production and concentration of intensive livestock operations along with increased urbanization of rural regions has resulted in greater awareness and concern for the proper storage, treatment and utilization of livestock manure. In particular, the management, treatment and disposal of liquid and solid manure at dairy operations are receiving increased attention. The optimal manure management system should provide a sustainable approach designed to minimize environmental impacts and maximize resource recovery. AD under controlled conditions offers a manure treatment [10, 11] solution that not only stabilizes the wastewater but also produces a significant amount of energy in the form of biogas, controls odours, reduces pathogens, minimizes environmental impact from waste emissions and maximizes fertilizer nutrient and water recovery for reuse. To increase the methane production, manure from dairy cows can be co-digested with additional substrates such as agricultural residues and food-processing waste as shown in Figure 8.11.

The main equipment for dairy manure digestion system are digester and manure separator. Many of the problems associated with the handling and the storage of animal slurry containing a high proportion of the solid material can be overcome by mechanical separation. The products of separation are an easily handled solid and readily pumped liquid. The output of liquid and fibre, together with the dry matter content of both fractions, depends on the dry matter content of the raw slurry delivered to the separator; however, it has been found that separation provides about 25% solids and 75% liquid from raw dairy manure. Separation can be used as a first stage in a biological treatment system.

A digester is a device for optimizing the AD of biomass and/or animal manure, mainly used to recover biogas for energy production. It is a containment vessel designed to exclude air and promote the growth of methane bacteria. Commercial digester types include complete mix, continuous flow (horizontal or vertical plug flow, multiple tank and single tank) and covered lagoon. It may be designed to heat or mix the organic material. Complete-mix digesters are the most flexible of all digesters as far as the variety of wastes that can be accommodated. Wastes with 2–10% solids are pumped into the digester, and the digester contents are continuously or intermittently mixed to prevent separation. Complete-mix digesters are usually aboveground, heated, insulated round tanks. Mixing can be accomplished by gas recirculation, mechanical propellers or circulation of liquid. Advantages of anaerobic digestion processes are as follows:

1. 1. Renewable combustible gas is produced.
2. 2. The digestion of solids removes objectionable odours after the conversion process.
3. 3. A highly valued sludge is co-produced, which is considered an excellent source of nutrients and has fertilizer value.
4. 4. There is reduction in the amount of wastes to handle at the end of the conversion process.
5. 5. The waste resources are contained in an enclosed vessel, preventing spoilage and generation of odour.