Biogas

Biogas can be used after appropriate gas cleanup as a fuel for engines, gas turbines, fuel cells, boilers, industrial heaters, other processes, or for the manufacturing of chemicals. Before landfilling, treatment or stabihzation of biodegradable materials can be accomplished by a combination of anaerobic digestion followed by aerobic composting. 

The same types of anaerobic bacteria that produced natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large qnantities of oxygen into the atmosphere. Anaerobic bacteria break down or digest organic material in the absence of oxygen and produce biogas as a waste product. 

Respiration refers to those biochemical processes in which organisms oxidize organic matter and extract the stored chemical energy needed for growth and repro- 

Under aerobic conditions, the pyravate is oxidized to CO and H O via the tricarboxylic acid or Krebs cycle and the electron transport system. The net yield for glycolysis followed by complete oxidation is 38 moles ATP per mole glucose, although there is evidence that the yield for bacteria is 16 moles ATP per mole glucose (Aiba et al., 1973). Thus, 673 kcal are liberated per mole glucose, much of which is stored as ATP. 

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 40-60% (by volume). Fats and oils can yield biogas with 70% methane content. 

There are several potential industrial applications for the CMS membranes some which are close to market, others which may be more future applications. This may be a function of both the volume of the gas streams, and/or challenging process conditions. Closest to market is the upgrading of biogas to vehicle fuel and separation of air by the use of carbon membranes. 

The purification of natural gas by removal of CO2 (natural gas sweetening) is, in principle, the same separation process as for upgrading of biogas, although it 

10 Biorefineries and Systems of Industrial Ecxtlogy for Utilizing Biomass 

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Sweetening. Another significant purification appHcation area for adsorption is sweetening. Hydrogen sulfide, mercaptans, organic sulfides and disulfides, and COS need to be removed to prevent corrosion and catalyst poisoning. They ate to be found in H2, natural gas, deethanizer overhead, and biogas. Often adsorption is attractive because it dries the stream as it sweetens. 

The initial biogas recovered is an MHV gas and is often upgraded to high heat value (HHV) gas when used for blending with natural gas suppHes. The aimual production of HHV gas ia 1987, produced by 11 HHV gasification facihties, was 116 x 10 m of pipehne-quaUty gas, ie, 0.004 EJ (121). This is an iacrease from the 1980 production of 11.3 X 10 m . Another 38 landfill gas recovery plants produced an estimated 218 x 10 m of MHV gas, ie, 0.005 EJ. Additions to production can be expected because of landfill recovery sites that have been identified as suitable for methane recovery. In 1988, there were 51 sites ia preliminary evaluation and 42 sites were proposed as potential sites (121). 

Biogas is produced by the bacteria in the form of small bubbles these float upward through the sludge bed/blanket, providing a good mixing action. When the biogas reaches the top of the reactor, it is collected and used as a fuel. 

Biogas is composed primarily of methane (GH ) and carbon dioxide. Biogas is a by-product from anaerobic bacteria breaking down organic material. Large amounts of biogas can be released from areas such as... 

Economics for generating electricity from biogas can be favorable. Landfill gas from municipal solid waste can supply about 4 percent of the energy consumed in the United States. In 1997, a total of 90 trillion Btus were generated by landfill gas, about 3 percent of total biomass energy consumption. 

Pohland, F. G. Harper, S.R. InAnaerobkDigestkn 1985 Quangzhou China, China State Biogas Association, 1985 pp 41-82. 

Legrand, R. Warren, C. S. "Biogas generation from commimity-derived wastes and biomass in the U.S. " Paper presented at the Tenth Annual Energy-Sources Technology Conf. and Exhib. ASME Dallas, TX, 1987. 

This process is an extension of the anaerobic treatment of waste diseussed in Chapter 2, and is also similar to the natural process operating in landfill sites, which evolves methane. By treatment of biomass with bacteria in the absence of air a gas rich in methane can be produced a typical digester may produce over 300 m of gas containing over 50% methane per tonne of dry biomass. The economics of biogas generation for use as a fuel are currently unfavourable. The plants that do exist have been built because of the need to treat waste such as sewage sludge. 

Cuellar and Webber (2008) estimated that manure from 95,000,000 animal units (1000 pounds of animal) in the United States could produce approximately 1% of total U.S. energy consumption. Conversion of the biogas into electricity could produce 2.4 0.6% of annual electricity consumption with reduction of 3.9 2.3% of annual GHG emissions from electricity generation in the United States. 

Singh, S. P. and Prerna, P. (2009). Review of recent advances in anaerobic packed bed biogas reactors. Renew. Sustain. Rev. 13,1569-1575. 

In developing countries, additional advantages of using CWs can be obtained. They may provide economic benefits and could encourage small communities to maintain natural wastewater treatment systems. The production of plant biomass can provide economic returns to communities through production of biogas, animal feed, compost, and fiber for paper according to the type of pollutant.46... 

The decomposition of solid urban waste in landfills is essentially a result of microbiological processes and, therefore, the production of biogas and leachate are both directly related to the activity of microorganisms. It has been demonstrated that large variations in leachate quality exist for different landfills, but also at different locations within the same landfill.6... 

Savings in energy costs biogas production makes positive energy balance... 

Deublein D, Steinhauser A. Biogas from Waste and Renewable Resources. Weinheim Wiley-VCH 2008. 

Office of the Leading Group for the Popularisation of Biogas (OLGPB) in Sichuan Province, Peoples Republic of China. A Chinese Biogas Manual. 1978. 

Juarez-Rodriguez et al. applied the sludge derived from anaerobically digested cow manure in the production of biogas (methane-air), to maize (Zea mays L.) cultivated in a nutrient-low, alkaline-saline soil with EC 9.4 dS m 1 and pH of 9.3. The results showed that the C02... 

Schniirer A, Jarvis A. Microbial Handbook for Biogas Plants, Swedish Waste Management U2009 2010 03. 

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