Biogas can be obtained from several sources. There are a number of processes for converting biomass into gaseous fuels such as methane or hydrogen. One uses plant and animal wastes in a fermentation process leading to biogas from which the de­sired fuels can be isolated. This technology is established and in widespread use for waste treatment. Anaerobic digestion of biowastes occurs in the absence of air, the resulting gas, called biogas, is a mixture consisting mainly of methane and carbon dioxide. Biogas is a valuable fuel that is produced in digesters filled with feedstock like dung or sewage. The digestion is allowed to continue for a period of 10 d to a few weeks. A second process uses algae and bacteria that have been genetically modified to produce hydrogen directly instead of the conventional biological energy carriers. Finally, high-temperature gasification supplies a crude gas, which may be transformed into hydrogen by a second reaction step. This process may offer the highest overall efficiency.

Anaerobic digestion (AD) is a bacterial fermentation process that is sometimes employed in wastewater treatment for sludge degradation and stabilization. This is also the principal process occurring in the decomposition of food wastes and other biomass in landfills. The AD operates without free oxygen and results in biogas containing mostly CH4 and CO2 but frequently carrying other substances such as moisture, hydrogen sulfide (H2S), and particulate matter that are generally removed prior to use of the biogas. The AD is a biochemical process for converting biogenic solid waste into a stable, humuslike product. Aerobic conversion uses air or oxygen to support the metabolism of the aerobic microorganisms degrading the substrate. Aerobic conversion includes composting and activated sludge wastewater treatment processes. Composting produces useful materials, such as mulch, soil additives and amendments, and fertilizers.

AD is known to occur over a wide temperature range from 283 to 344 K. It re­quires attention to the nutritional needs of the facultative and methanogenic bacteria degrading the waste substrates as well as maintenance of reasonable temperatures for those bacteria. The carbon/nitrogen (C/N) ratio of the feedstock is especially important. Biogas can be used after appropriate gas cleanup as a fuel for engines, gas turbines, fuel cells, boilers, industrial heaters, other processes, and the manu­facture of chemicals. AD is also being explored as a route for direct conversion into hydrogen.

Cellulose and hemicelluloses can be hydrolyzed into simple sugars and amino acids that are consumed and transformed by fermentive bacteria. Lignin is refrac­tory to hydrolysis and generally exits the process undigested. In fact, lignin may be the most recalcitrant naturally produced organic chemical. Lignin polymers are cross-linked carbohydrate structures with molecular weights on the order of 10,000 atomic mass units. As such, lignin can bind with or encapsulate cellulose, mak­ing that cellulose unavailable to hydrolysis and digestion. Lignin degradation (or deligniflcation of lignocellulosics) in nature is due principally to aerobic fllamen — tous fungi that decompose the lignin in order to gain access to the cellulose and hemicelluloses.

For anaerobic systems, methane gas is an important product. Depending on the type and nature of the biological components, different yields can be obtained for different biodegradable wastes. For pure cellulose, for example, the biogas product is 50% methane and 50% carbon dioxide. Mixed waste feedstocks yield biogas with methane concentrations of 40 to 60% (by volume). Fats and oils can yield biogas with 70% methane content.

Anaerobic digestion functions over a wide temperature range from the so-called psychrophilic temperature near 283 K to extreme thermophilic temperatures above 344 K. The temperature of the reaction has a very strong influence on the anaero­bic activity, but there are two optimal temperature ranges in which microbial ac­tivity and biogas production rate are highest, the so-called mesophilic and ther­mophilic ranges. The mesophilic regime is associated with temperatures of about 308 K, a thermophilic regime of about 328 K. Operation at thermophilic tempera­ture allows for shorter retention time and a higher biogas production rate; however, maintaining the high temperature generally requires an outside heat source because anaerobic bacteria do not generate sufficient heat. Aerobic composting can achieve relatively high temperatures (up to 344 K) without heat addition because reaction rates for aerobic systems are much higher than those for anaerobic systems. If heat is not conducted away from the hot center of a compost pile, then this could cause thermochemical reactions that might lead to spontaneous combustion if sufficient oxygen reaches the hot areas. Managed compost operations use aeration to pro­vide oxygen to the bacteria but also to transport heat out of the pile. The molec­ular structure of the biodegradable portion of the waste that contains proteins and carbohydrates is first broken down through hydrolysis. The lipids are converted to volatile fatty acids and amino acids. Carbohydrates and proteins are hydrolyzed to sugars and amino acids. In acetogenesis, acid forming bacteria use these byproducts to generate intermediary products such as propionate and butyrate. Further micro­

bial action results in the degradation of these intermediary products into hydrogen and acetate. Methanogenic bacteria consume the hydrogen and acetate to produce methane and carbon dioxide.

After the first 6 d of digestion, methane production from manure increases expo­nentially, after 16 d it reaches a plateau value, and at the end of day 20, the digestion reaches the stationary phase. For wheat straw and mixtures of manure and straw the rates of digestion are lower than that of manure.

The maximum daily biogas productions are between 4 and 6 d. During a 30-d digestion period, approx. 80 to 85% of the biogas is produced in the first 15 to 18 d. This implies that the digester retention time can be designed to 15 to 18 d instead of 30 d. For the first 3 d, methane yield is almost 0% and carbon dioxide generation is almost 100%. In this period, digestion occurs as fermentation to carbon dioxide. The yields of methane and carbon dioxide gases are 50/50 at day 11. At the end of day 20, digestion reaches the stationary phase. The methane content of the biogas is in the range of 73 to 79% for the runs, the remainder being principally carbon dioxide. During digestion, the volatile fatty acid concentration is lower and the pH higher. The pH of the slurry with manure increases from 6.4 initially to 6.9 to 7.0 at the maximum methane production rate. The pH of the slurry with wheat straw is around 7.0 to 7.1 at the maximum methane production rate.

The first methane digester plant was built at a leper colony in Bombay, India, in 1859 (Meynell 1976). Most of the biogas plants utilize animal dung or sewage. A schematic of biogas plant utilizing cow dung is illustrated in Figure 3.5 (Balat 2008). AD is a commercially proven technology and is widely used for treating high-moisture-content organic wastes including +80 to 90% moisture. Biogas can be used directly in spark-ignition gas engines (SIGEs) and gas turbines. Used as a fuel in SIGEs to produce electricity only, the overall conversion efficiency from biomass to electricity is about 10 to 16% (Demirbas 2006).