Energy from Biomass PDF

# Chapter 78 Energy from Biomass Biomass is organic matter produced by plants, both terrestrial and aquatic, and their derivatives. It includes forest crops and residues, crops grown specifically for their energy content on "energy farms", and animal manure. Biomass can be considered a renewable e...

# Chapter 78 Energy from Biomass Biomass is organic matter produced by plants, both terrestrial and aquatic, and their derivatives. It includes forest crops and residues, crops grown specifically for their energy content on "energy farms", and animal manure. Biomass can be considered a renewable energy source because plant life renews and adds to itself every year. It can also be considered a form of solar energy as the latter is used indirectly to grow these plants by photosynthesis. There are a variety of ways of obtaining energy from biomass. These may be broadly classified as direct methods and indirect methods. ## Direct Methods Raw materials that can be used to produce biomass energy are available throughout the world in various forms: * Forest wood and wastes * Agricultural crops and residues * Residential food wastes * Industrial wastes * Human and animal wastes * Energy crops Raw biomass has a low energy density based on their physical forms and moisture contents and their direct use are burning them to produce heat for cooking. The twin problems of traditional biomass use for cooking and heating are the energy inefficiency and excessive pollution. ## Indirect Methods Biomass can also be used indirectly by converting it either into electricity and heat or into a convenient usable fuel in solid, liquid, or gaseous form. The efficient conversion processes are as follows: 1. **Thermochemical conversion** takes two forms: gasification and liquefaction. Gasification takes place by heating the biomass with limited oxygen to produce low heating value gas or by reacting it with steam and oxygen at high pressure and temperature to produce medium heating value gas. The latter may be used as fuel directly or used in liquefaction by converting it to methanol (methyl alcohol CH3OH), or ethanol (ethyl alcohol CH3CH2OH) or it may be converted to high heating value gas. 2. **Biochemical conversion** takes two forms. Anaerobic digestion and fermentation. Anaerobic digestion involves the microbial digestion of biomass. An anaerobe is a micro-organism that can live and grow without air or oxygen, it gets its oxygen by the decomposition of matter containing it. It has already been used on animal manure but it also possible with other biomass. The process takes place at low temperature up to 65°C, and requires a moisture content of at least 80 per cent. It generates a gas consisting mostly of CO2 and methane (CH4) with minimum impurities such as hydrogen sulfide. The gas can be burned directly or upgraded to synthetic natural gas by removing the CO2 and the impurities. The residue may consist of protein rich sludge that can be used as animal feed and liquid effluents that are biologically treated by standard techniques and returned to the soil. Fermentation is the breakdown of complex molecules in organic compound under the influence of a ferment such as yeast, bacteria, enzymes, etc. Fermentation is a well-established and widely used technology for the conversion of grains and sugar crops into ethanol. ## Biomass Conversion Technologies We can divide Biomass conversion technologies into Wet & Dry processes ### Wet Processes (Thermal Process) 1. **Anaerobic digestion**: Biogas is produced by the bacterial decomposition of wet sewage sludge, animal dung or green plants in the absence of oxygen. The natural decay process anaerobic decomposition can we speeded up by using a thermally insulated, air tight tank with a stirrer unit and heating system. The gas collects in the digester tank above the slurry and can be piped off continuously. Each kilogram of organic material (dry weight) can be expected to yield 450-500 litres of biogas (9-12 MJ) at atmospheric pressure in a modem batch or continuous feed unit-one and a half to two digester volumes of gas per day. The residue left after digestion is valuable fertilizer. It is also rich in protein and could be dried and used as animal feed supplement. 2. **Fermentation**: Ethanol (ethyl alcohol) is produced by the fermentation of sugar solution by natural yeasts. After about 30 hours of fermentation the brew (or 'beer') contains 6-10% alcohol and this can readily be removed by distillation. Traditionally, the fibrous residues from plant crops like sugar cane bagasse have been burnt to provide the heat. Suitable feed stocks include crushed sugar cane and beet, fruit etc. Sugar cane also be manufactured from vegetable starches and cellulose, maize, wheat grain, or potatoes, for example, must be ground or pulped and then cooked with enzymes to release the starch and convert it to fermentable sugars. Cellulose materials like wood, paper waste or straw, require harsher pretreatment typically milling and hydrolysis with hot acid. One tonne of sugar will produce upto 520 litres of alcohol; a tonne of grain, 350 litres and a tonne of wood, an estimated 260 to 540 litres. After fermentation, the residue from grains and other feed stuffs contains high protein content and is a useful cattle feed supplement. The hydrolysis and distillation steps require a high energy input; for woody feedstocks direct combustion or pyrolysis is probably more productive at present, although steam treatment and new low energy enzymatic hydrolysis techniques are under development. The energy requirement for distillation is also likely to be cut dramatically. Alcohol can be separated from the beer by many methods which are now under intensive development. These include solvent extraction, reverse osmosis, molecular sieves and use of new desiccants for alcohol drying. 3. **Chemical reduction**: Chemical reduction is the least developed of the wet biomass conversion processes. It involves pressure cooking animal wastes or plant cellulosic slurry with an alkaline catalyst in the presence of carbon monoxide at temperatures between 250°C and 400°C. Under these conditions the organic material is converted into a mixture of oils with a yield approaching 50%. If the pressure is reduced and the temperature increased, the product is a high calorific value gas. | Conversion Process | Solids | Principles | Products Liquid | Gases | Further Treatment | Conversion Process | |---|---|---|---|---|---|---| | Wet Anaerobic Digestion | | | | Methane and carbon dioxide | Carbon dioxide removal | Methane | | Fermentation | | | Ethanol | Ethanol | Distillation | Ethanol | | Chemical Reduction | Char | | Mixture of oils | | Fractional distillation | Hydrocarbon liquids | | Thermal Liquefaction Process | Char | | Pyroligneous acids oils and tars | Fuel gas | Fractional distillation | | | Gasification | Char | | | Fuel gas | Steam reforming and/or shift reaction | Methane or Methanol | | Steam-gasification | Char | | | Methane | | Higher Alcohols | | Hydrogenation | | | Mixture of oils | Methane | Fractional distillation | Hydrocarbon Liquids | | Oil Extraction | | | Vegetable oil | | Esterification | Diesel Substitute | ### Dry Processes 1. **Pyrolysis**: A wide range of energy rich fuels can be produced by roasting dry woody matter like straw and wood chips. The material is fed into a reactor vessel or retort in a pulverised or shredded form and heated in the absence of air. Air would cause the products of pyrolysis to ignite. As the temperature rises the cellulose and lignin break down to simpler substances which are driven off leaving a char residue behind. This process has been used for centuries to produce charcoal. The end products of the reaction depends critically on the conditions employed; at lower temperatures - around 90'C organic liquid predominates, whilst at temperatures nearer 1000°C a combustible mixture of gases results. The vapours are condensed from the gas stream and these separate into a two phase- the aqueous phase (pyroligneous acid) contains a soup of water-soluble organic materials like acetic acid, acetone and methanol (wood alcohol'); the non-aqueous phase consists of oils and tars. These crude products can be burnt (with some difficulty), but it is usually more profitable to up-grade them to premium fuels by conventional refining techniques. Other pyrolysis products include fuel gas-essentially carbon-monoxide and hydrogen and carbon char. The gas is generally burnt to maintain the temperature of the reactor; the char can be manufactured into briquetts for use as solid fuel. Pyrolysis can also be carried out in the presence of small quantities of oxygen ('gasification'), water ('steam gasification') or hydrogen ('hydrogenation'). a. **Gasification**: Pyrolysis of wet biomass produces fuel gas and very little liquid. An alternative technique for maximising gas yields is to blow small quantities of air or oxygen into the reactor vessel and to increase the temperature to over 1000°C. This causes part of the feed to burn. Fuel gas from air-blown gasifiers has a low calorific value (around 5 MJ/m3) and may contain up to 40% inert nitrogen gas overall yields of 80-85% can be expected. Fuel gas from oxygen-fed systems has a medium calorific value (10-20 MJ/m³). This gas can either be burnt or converted into substitute natural gas (methane) or methanol by standard catalytic processes. Methanol yields of around 50% can be achieved from biomass. b. **Steam-gasification**: Methane is produced directly from woody matter by treatment at high temperatures and pressures with hydrogen gas. The hydrogen can be added or, more commonly, generated in the reactor vessel from carbon monoxide and steam. Recent analyses suggest that steam gasification is the most efficient route to methanol. Net energy yields of 55% can be achieved although higher yields are likely in the future as the technology is developed. c. **Hydrogenation**: Under less severe conditions of temperature and pressure (300-400°C and 100 atmospheres), carbon monoxide and steam react with cellulose to produce heavy oils which can be separated and refined to premium fuels. ## Biomass Gasification Biomass gasification is a process of partial combustion in which solid biomass usually in the form of pieces of wood or agricultural residue is processed into a combustible gas mixture. Gasification, which is incomplete combustion of carbonaceous fuels, can be represented with the following sub-stoichiometric equation. Biomass + air → carbon monoxide (CO) + carbon dioxide (CO2) + methane (CH4) + hydrogen (H₂) + nitrogen (N2) + water vapour. Gasification produces a synthesis gas with usable energy content is produced by gasification in which biomass is heated with less oxygen than that needed for complete combustion. As a result, a gaseous mixture of carbon monoxide (CO), carbon dioxide (CO₂), methane (CH4), hydrogen (H₂), and nitrogen (N₂) called producer gas is obtained. Producer gas can be used: 1. In internal combustion engines (both compression and spark ignition) 2. As substitute for furnace oil in direct heat applications and 3. To produce, infeconomically viable way, methanol # Methanol Methanol is an extremely attractive chemical that is useful both as fuel for heat engines as well as chemical feedstock for industries. Since any biomass material can undergo gasification, this process is much more attractive than ethanol production or biogas where only selected biomass materials can produce the fuel. Gasification processes involved with biomass are as follows: 1. **Drying of fuels**: It is the process of drying biomass before it is fed into gasifier. 2. **Pyrolysis**: It is a process of breaking down biomass into charcoal by applying heat to bio- mass in the absence of oxygen. 3. **Combustion**: All the heat required for different processes of gasification are made available from combustions. 4. **Cracking**: In this process, breaking down of large complex molecules (such as tar) takes place when heated into lighter gases. 5. **Reduction**: Oxygen atoms are removed in this process from the combustion products (hydro-carbon) molecules and returning them to combustible form again. ## Low Temperature Gasification When gasification of biomass is carried out at 750°C to 1,100°C, it is referred to as low temperature gasification. The gas produced has relatively high level of hydrocarbons. It is used directly to either burn for steam production and generation of electricity or cleaned and used in internal combustion engine or combined heat power (CHP). The producer gas is a mixture of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), methane (CH4), and nitrogen from air. The gas mixture composition depends on gasifiers. ## High Temperature Gasification It is carried out in temperature range of 1,200°C–1,600°C and gas product is referred to as syn- thesis gas (Syngas). It contains high proportion of CO and H2 and is convertible to high quality synthetic diesel biofuel compatible for use in diesel engines. # Composition and Properties of Producer Gas The producer gas is affected by various processes as above mentioned, and hence one can expect variations in the gas produced from various biomass sources. The composition of producer gas is highly dependent upon the inputs to the gasifier and gasifier design. Table below lists the composition of gas produced from various sources. Nitrogen affects the maximum dilution of gas and almost 50%-60% of gas is composed of noncombustible nitrogen. The use of oxygen instead of air will be beneficial for gasification with due regards to the costs of oxygen. Nevertheless, production of a high energy quality methanol may justify the cost of oxygen.. On an average, 1 kg of biomass produces about 2.5 m³ of producer gas at S.T.P and consumes about 1.5 m³ of air for combustion. For complete combustion of wood, about 4.5 m³ of air is required. The average energy conversion efficiency of gasifiers is defined as: _ης= Calorific value of gas / Average calorific value of 1 kg of fuel_ The average gas temperature produced by gasifier is about 300°C-400°C and it may even attain a higher temperature of approximately 500°C, if partial combustion of gas is taking place. Partial combustion of biomass can be eliminated by increasing air flow rate higher than the design value. | Bio Mass Feed | Gasifier | % Volume | Calorific value MJ/m³ | |---|---|---|---| | Charcoal | Downdraft | CO: 28-31, H2: 5-10, CH4: 1-2, CO2: 1-2, N2: 55-60 | 4.60-5.65 | | Wood with 12-20% | Downdraft | CO: 17-22, H2: 16-20, CH4: 2-3, CO2: 10-15, N2: 55-50 | 5.00-5.86 | | Wheat straw pellets | Downdraft | CO: 14-17, H2: 17-19, CH4: 11-14 | 4.50 | | Coconut husks | Downdraft | CO: 16-20, H2: 17-19.5, CH4: 10-15 | 5.80 | | Coconut shells | Downdraft | CO: 19-24, H2: 10-15, CH4: 11-15 | 7.20 | | Pressed Sugarcane | Downdraft | CO: 15-18, H2: 15-18, CH4: 12-14 | 5.30 | | Charcoal | Updraft | CO: 30, H2: 19.7, CH4: 3.6, N2: 46 | 5.98 | | Corn cobs | Downdraft | CO: 18.6, H2: 16.5, CH4: 6.4 | 6.29 | | Rice hulls pelleted | Downdraft | CO: 16.1, H2: 9.6, CH4: 0.95 | 3.25 | | Cotton stalks cubed | Downdraft | CO: 15.7, H2: 11.7, CH4: 3.4 | 4.32 | # Theory of Gasification Gasification may be considered as a special case of pyrolysis where destructive decomposition of biomass (wood wastes) by heat is converted into charcoal, oils, tars, and combustible gas. It is referred to as the partial combustion of solid fuel (biomass) and takes place at temperatures of about 1,000°C. The reactor used for gasification is called a gasifier. The complete combustion of biomass produces biomass gasses that generally contain nitrogen, water vapour, carbon dioxide, and surplus of oxygen. However, in gasification (with incomplete combustion), as shown in Figure, product gas contains gases such as carbon mono oxide (CO), hydrogen (H₂), and traces of methane and non-useful products such as tar and dust. The production of these gases is obtained by the reaction of water vapour and carbon dioxide through a glowing layer of charcoal. Thus, the key to gasifier design is to create conditions such that: 1. Biomass is reduced to charcoal 2. Charcoal is converted at suitable temperature to produce CO and H2 Typically, the volumetric composition of biomass-based producer gas is as follows: * CO: 20%-22%, * H2: 15%-18%, * CH4: 2%-4%, * CO2: 9%-11% and * N2: 50%-54%. # Gasifier Biomass gasifier may be considered as a chemical reactor in which biomass goes through several complex physical and chemical processes and producer or syngas is produced and recovered. There are two distinct types of gasifier: 1. **Fixed bed gasifier**: In this gasifier, biomass fuels move either countercurrent or concurrent to the flow of gasification medium (steam, air, or oxygen) as the fuel is converted to fuel gas. They are relatively simple to operate and have reduced erosion. Since there is an interaction of air or oxygen and biomass in the gasifier, they are classified according to the way air or oxygen is introduced in it. There are three types of gasifier as shown in Figure. [Image Descrition: A image of different types of gasifier.] a. **Downdraft gasifiers**: In the downdraft gasifier, the air is passed from the layers in the downdraft direction. Single throat gasifiers are mainly used for stationary applications, whereas double throat gasifier is used for varying loads as well as automotive purposes. b. **Updraft gasifiers**: Updraft gasifier has air passing through the biomass from bottom and the combustible gases come out from the top of the gasifier. c. **Cross draft gasifiers**: It is a very simple gasifier and is highly suitable for small outputs. With slight variation, almost all the gasifiers fall in the abovementioned categories. The choice of one type of gasifier over other is dictated by the fuel, its final available form, its size, moisture content, and ash content. Table 9.2 lists the comparative features of various types of fixed bed gasifiers. 2. **Fluidized bed gasifier**: In fluidized bed gasifier, an inert material (such as sand, ash, or char) is utilized to make bed and that acts as a heat transfer medium # Use of Biomass Gasifier The output of a biomass gasifier can be used for a variety of direct thermal applications such as cooking, drying, heating water, and generating steam. It can also be used as a fuel for internal combustion engines to obtain mechanical shaft power or electrical power. If used as a fuel for internal combustion engines, it has to be cleaned first for complete removal of particulate material and tar. A cleaning system consisting of cyclone, a scrubber, and a filter is used for the purpose. If the engine is a spark-ignition engine, it can operate with producer gas alone. The gas is sucked from the gasifier and cleaner unit by the engine suction along with a proportionate amount of air. It is then compression-ignition engine, as it operates in the 'dual-fuel' mode. Here, the engine sucks in a mixture that is compressed, and a small amount of diesel sprayed in. Combustion initiates with the diesel droplets and then spreads to the mixture of the gaseous fuel and air. The phrase 'dual-fuel' implies that both diesel and producer gas are simultaneously used. ## Advantages and disadvantages of Fixed Bed Gasifiers | Comparative features | Updraft Gasifier | Downdraft Gasifier | Cross-draft Gasifier | |---|---|---|---| | | It works on coal, briquettes, and other fuels (fuel flexibility). Comparatively low-quality gas having tar and particulate matter. | It works on woody biomass and charcoal (fuel specific). High quality gas. | Type of fuel usage restricted to only low ash fuels such as wood, charcoal, and coke. Good quality gas. | | | Suitable for thermal applications. Gas is drawn out of the gasifier from the top of the fuel bed, while the gasification reactions take place near the bottom. The air comes in at the bottom and produced syngas leaves from the top of the gasifier. It tolerates higher ash content, higher moisture content, and greater size variation in fuel. | Suitable for power (IC engines) and thermal applications. Air is introduced into downward flowing packed bed or solid fuels and gas is drawn off at the bottom. Hence, fuel and gas move in the same direction. It is sensitive to ash content, moisture content, and size variation in fuel. | Suitable for heat and power applications. Air enters from one side of the gasifier, and fuel is released from the opposite side. Flexible gas production.| India is one of the leading countries in the world in the field of biomass gasification. Biomass gasifier systems are available in a wide range of capacities and standard facilities for testing and evaluating gasifiers have been set up. For thermal applications, systems with outputs ranging from 60,000 to 5 × 106 kJ/h are available, while for electrical power generation, systems with outputs ranging from 3 to 500 kW are also available. The largest biomass gasification system produces 5 × 106 kJ/h (1,450 kW) output in the thermal mode and 500 kW in the electric power generation mode. It uses biomass in the form of wood blocks (25 to 100 mm long and up to 70 mm in diameter) at the rate of 500 kg/h and produces 1,250 m³/h of gas. The internal combustion engine is a compression ignition engine operating in the dual-fuel mode. It uses only 25% of the diesel normally required by the engine if operating with diesel alone. # Liquid Fuels When compared to gaseous fuels such as producer gas or biogas, liquid fuels are somewhat harder to obtain from biomass sources. One of the methods is the production of methanol from wood or straw. The process involves the gasification of plant matter followed by chemical synthesis. Another method is the conversion of certain food grains and crops such as sugarcane, maize, cassava and tapioca by fermentation into ethanol. When blended with petrol, ethanol is good alternate fuel for automotive engines. This fact has received considerable attention as a means of overcoming the oil crisis. However, if one examines the requirement of land for growing the agricultural products concerned, it is obvious that the method can be of substantial benefit only to a country having a large surplus of land. For this reason, Brazil has adopted this method on a large scale and produces significant amounts of ethanol for use as an alternate fuel. However, the position in India is quite different since the availability of land is limited. In plants, algae and certain types of bacteria, the photosynthetic process results in the release of molecular oxygen and the removal of carbon dioxide from the atmosphere that is used to synthesize carbohydrates (oxygenic photosynthesis). Other types of bacteria use light energy to create organic compounds but do not produce oxygen (anoxygenic photosynthesis). Photosynthesis provides the energy and reduced carbon required for the survival of virtually all life on our planet, as well as the molecular oxygen necessary for the survival of oxygen consuming organ-ism. In addition, the fossil fuels, currently being burned to provide energy for human activity, were produced by ancient photosynthetic organisms. # Biogas Biogas is a clean, non-polluting, and low-cost fuel. It contains about 50%-70% methane, which is inflammable. A methane gas molecule has one atom of carbon and four atoms of hydrogen (CH4) and is the main constituent of popularly known biogas. A colourless, odourless, inflammable gas also been referred to as sewerage gas, clear gas, marsh gas, refuse-derived fuel (RDF), sludge gas, gobar gas (cow dung gas), and bio energy. It produces about 9,000 kcal of heat energy per cubic metres of gas burnt and specifically used for cooking, heating, and lighting. The composition of biogas is shown in Table, which mainly composed of 50% to 70% methane (CH4), 30% to 40% carbon dioxide (CO2), and traces of other gases. Biogas is lighter than air by about 20% and has an ignition temperature in the range of 650°C to 750°C burns with clear blue flame similar to that of liquefied petroleum gas (LPG) and burns with 60% efficiency in a conventional biogas stove. Its calorific value is 20 MJ/m³. Its equivalence with other energy and fuels are as follows: A 1,000 cubic feet of processed biogas is equivalent to about: 1. 600 cubic feet of natural gas 2. 6 gallons of diesel oil 3. 5.2 gallons of gasoline 4. 6.4 gallons of butane | Substances | Symbol | % | |---|---|---| | Methane | CH4 | 50-70 | | Carbon dioxide | CO2 | 30-40 | | Hydrogen | H2 | 5-10 | | Nitrogen | N2 | 1-2 | | Water vapour | H2O | 0.2-0.3 | | Hydrogen sulphide | H2S | Minute traces | Biogas can be produced by either anaerobic digestion with anaerobic bacteria, which digest material inside a closed system, or by fermentation of biodegradable materials. # Materials Used for Bio-gas Generation Feed stock materials. The following organic matter rich feed stocks are found feasible for their use as input materials for biogas production: * **Animal wastes**: * Cattle dung, urine, goat and poultry droppings, slaughter house wastes, fish wastes, foetus wastes, leather and wood wastes, sericulture wastes elephant dung, piggery wastes etc. * **Human wastes**: * Faeces, urine and other wastes emanating from human occupations. * **Agricultural wastes**: * Aquatic and terrestrial weeds crop residue, stubbles of crops, sugarcane trash, spoiled fodder, bagasse, tobacco wastes oilcakes fruit and vegetable processing wastes, press mud, cotton and textile wastes, spent coffee and tea wastes. * **Waste of aquatic origin**: * Marine plants, twigs, algae, water hyacinth and water weeds. * **Industrial wastes**: * Sugar factory, tannery, paper etc. The following three marine plants (waste of aquatic origin) are considered promising for biomass production. * (i) Water hyacinth (ii) Algae and (iii) Ocean kelp. # Anaerobic Digestion Anaerobic digestion consists broadly of three phases: 1. **Insoluble biodegradable materials**, e.g., cellulose, polysaccharides and fats, are broken down to soluble carbohydrates and fatty acids. This occurs in about a day at 25°C in an active digester. 2. **Acid forming bacteria** produce mainly acetic and propionic acid. This stage likewise takes about one day at 25°C. 3. **Methane forming bacteria** slowly, in about 14 days at 25°C, complete the digestion to-70% CH4,- 30% CO2 with trace amounts of H2 and perhaps H2S. H2 may play an essential role, and indeed some bacteria (e.g., Clostridium) are distinctive in producing H2 as the final product. The methane forming bacteria are sensitive to pH, and conditions should be mildly acidic (pH 6.6 to 7.0) and certainly not below pH 6.2. Nitrogen should be present at 10% by mass of dry input, and phosphorus at 2%. A golden rule for successful digester operation is to maintain constant conditions of temperature and suitable input material. As a result a suitable population of bacteria is able to become established to suit these conditions. When comparison of methane percentage from different organic matter was done for example cow dung. Poultry dropping and dairy waste scum, then best result was observed in dairy waste. 75 to 79 methane percentage found in dairy waste biogas while in cow dung, biogas was only 65%. # Process Stages of Anaerobic Digestion The biological and chemical stages of anaerobic digestion are shown in Figure. These are divided into the following four main stages. The four main stages are explained as follows. **Hydrolysis** The process of breaking large biomass organic chains into their smaller constituent parts such as sugar, fatty acids, and amino acids and dissolving the smaller molecules into solution is called hydrolysis. This process assists bacteria in anaerobic digesters to access the energy potential of the material. Hydrolysis of these high-molecular-weight polymeric components of biomass completes the first step in anaerobic digestion. Hydrogen and acetate products of first stage are directly used by methanogens. Other molecules with a chain length larger than that of acetate (e.g. volatile fatty acids) must first be catabolized into compounds and then used by methanogens. **Acidogenesis** Acidogenesis is the biological process in which the remaining components are broken down by acidogenetic (fermentative) bacteria. It creates voltaic fatty acids together with ammonia, carbon dioxide, and hydrogen sulphide, and other by-products. **Acetogenesis** In this stage of anaerobic digestion, simple molecules created through the acidogenesis phase are further digested to produce more acetic acid, carbon dioxide, and hydrogen. **Methanogenesis** Finally, the process of biogas production is completed by methanogenesis. In this stage of anaerobic digestion, the methanogens use intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water which makes the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pH values. A simplified generic chemical equation for the overall processes outlined earlier is as follows: C6H12O6-3CO2 + 3CH4 The remaining indigestible material cannot be used by microbes and any dead bacterial remains constitute the digestate. # Factors affecting Biodigestion 1. **pH or Hydrogen ion concentration**. pH of the slurry changes at various stages of the digestion. In the initial acid formation stage in the fermentation process, the pH is around 6 or less and much of CO2 is given off. In the latter 2-3 weeks time, the pH increases as the volatile acid and N2 compounds are digested and CH4 is produced. To maintain a constant supply of gas, it is necessary to maintain a suitable pH range in the digester. The digester is usually buffered if the pH is maintained between 6.5 to 7.5. In this pH range, the micro-organisms will be very active and biodigestion will be very efficient. If the pH range is between 4 and 6 it is called acidic. If it is between 9 and 10 it is called alkaline. Both these are deterimental to the methanogenic (Methane production) organisms. It should always be remembered that there should not be any sudden upset in the pH by the addition of any material which is likely to cause an imbalance in the bacterial population. The ideal pH valves for digestion of sewage solids are reported to be in the range 7 to 7.5. But a slightly higher value of 8.2 has been reported to be optimum for digestion of raw animal or plant wastes. 2. **Temperature**. Methanogenic bacteria work best at a temperature of between 35° to 38°C. The fall in gas production starts at 20°C and stops at a temperature of 10°C. At one experiment 2.25 cu m of gas was produced from 4.25 m³, of cattle dung everyday when the digester temperature was 25°C. When the temperature was raised to 28.3°C, the gas production increased by 50% to 3.75 cu ml day. There are two significant temperature zones in anaerobic digestion. These have been studied in some detail for digestion of sewage sludges for 90% digestion. Figure shows the time required for 90% digestion at various temperatures, and the two temperature zones. It has been established that two types of micro organisms, mesophilic and thermophilic are responsible for digestion at the two temperature range. The optimum mesophilic temperature lies at about 35°C while the optimum thermophilic temperature is around 55°C. The gas production starts falling very steeply when the temperature goes below 20°C and almost stops at 10°C. Generally it is easier to maintain the temperature of the digester at the mesophilic range rather than at the thermophilic range. In addition to ambient temperature, other weather conditions also influence the gas generation viz: * (a) Wind velocity (chill factor) * (b) Sun shine directly available to keep the dome at the optimum temperature. * (c) Type of food given to cattle (in case of Gobar gas generation). 3. **Total Solid Content**. The cow dung is mixed usually in the proportion of 1: 1 (by weight) in order to bring the total solid content to 8-10%. The raw cow dung contains 80-82% of moisture. The balance 18-20% is termed as total solids. The adjustment of total solid content helps in digesting the material at the faster rate, and also in deciding the mixing of the various crop residues weeds plants etc. as feed stocks in biogas digester. 4. **Loading Rate**. Loading rate is defined as the amount of raw material (usually kg of volatile solids) fed to the digester per day per unit volume. Most municipal sewage treatment plants operate at a loading rate of 0.5 to 1.6 kg of volatile soilds per m³ per day. If a digester is loaded with too much raw material at a time, acids will accumulate and fermentation will stop. The main advantage of higher loading rate is that by stuffing a lot into a little space, the size and therefore the cost of the digester can be reduced. 5. **Seeding**. Although the bacteria required for acid fermentation and methane fermentation are present in the cow dung, their numbers are not large. While the acid formers proliferate fast and increase in numbers, the methane formers reproduce and multiply slowly. It would be advantageous to increase the number of methane formers by artificial seeding with a digested sludge that is rich in methane formers. But beyond a certain seed concentration, the gas production will decrease, due to reduction of raw cow dung solids fed to the digester. 6. **Carbon Nitrogen ratio of the input material**. Besides carbon the quantity of N2 present in the wastes is a crucial factor in the production of biogas. All living organisms require nitrogen to form their cell proteins from a biological view point, a digester is a culture of bacteria feeding upon and converting organic wastes. The elements of carbon (in the form of carbon hydrates) and nitrogen (as protein, ammonia nitrates, etc.) are the main food of anaerobic bacteria. Carbon is used for energy and nitrogen for building the cell structure. The bacteria use up carbon about 30 times faster than they use up nitrogen. Carbon and nitrogen should be present in the proper proportion. Other conditions (temperature, pH etc.) being favourable, a carbon, nitrogen (C/N) ratio of 30 (i.e., 30 times more carbon than nitrogen) will permit digestion to proceed at an optimum rate. When there is too much carbon in the raw wastes, nitrogen will be used up first and carbon left over. This will make the digester slow down and come to a stop. In this case the bacteria will not be able to use all the carbon present and the breaking down of the organic matter will be inefficient. On the other hand if there is too much nitrogen, the carbon soon becomes exhausted and fermentation stops. The nitrogen left over will combine with hydrogen to form ammonia. This can kill or inhibit the growth of bacteria specially the methane producers. The optimum Carbon-Nitrogen ratio that best suits for maximum microbiological activity is 30: 1. 7. **Nutrients.** The major nutrients required by the bacteria in the digester are, C, H2, O2, N2, P and S, of these nutrients N2 and P are always in short supply and therefore to maintain proper balance of nutrients an extra raw material rich in phosphorus (night soil) and N2 (chopped leguminous plants) should be added along with the cow dung to obtain maximum production of gas. # Construction Parts of Biogas Plants Figure below shows various parts of typical biogas plant. It is a brick and cement structure having the following five sections: 1. Mixing tank 2. Digester tank 3. Dome or gas holder 4. Inlet chamber 5. Outlet chamber [Image Description: A image of a diagram of a biogas plant.] ## Mixing Tank It is the first part of biogas plants located above the ground level in which the water and cow dung are mixed together in equal proportions (the ratio of 1:1) to form the slurry that is fed into the inlet chamber. ## Digester Tank It is a deep underground well-like structure and is divided into two chambers by a partition wall in between. It is the most important part of the cow dung biogas plants where all the important chemical processes or fermentation of cow dung and production of biogas takes place. The digester