Biogas Energy
In recent years, the importance of biogas energy has risen manifold and has become universal. This is due to the realization that biogas capture and utilization has great potential in controlling global warming. By capturing biogas wherever it is formed,
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Biogas and Biogas Energy: An Introduction
Abstract “Biogas” is the name popularly used to denote the flammable mixture of gases that are generated when organic material undergoes anaerobic decomposition. The mixture contains 40–70% (usually 55–65%) methane, carbon dioxide, and traces of other gases. “Biogas” has good calorific value and can be directly used as fuel or indirectly used to generate electricity. In this chapter a general introduction to “biogas” is provided, and steps involved in its formation are described. The factors which influence the sustainability and efficiency of anaerobic digestion – hence biogas production – are also briefly discussed.
1.1 What is Biogas? When organic matter – such as food, plant debris, animal manure, sewage sludge, biodegradable portions of municipal solid waste, etc. – undergoes decomposition in the absence of free oxygen, it normally generates a gas which consists of 40–70% methane, the rest being mostly carbon dioxide with traces of other gases. If ignited, this gas burns cleanly (i.e., gives off no soot or foul smell) similar to liquefied petroleum gas (LPG) or compressed natural gas (CNG). This gas is commonly called “biogas” which is an inexact and imprecise term because the gas which is produced by aerobic decomposition (carbon dioxide) is also “biogas” in the sense that it is also a result of biodegradation just as the other biogas is. But the word “biogas” has come to be used exclusively to denote the combustible CH4–CO2 mixture (besides traces of other gases) that is generated by the anaerobic decomposition of organic matter. Biogas has good calorific value, though lesser than LPG and CNG (Table 1.1). It must be mentioned that a mixture of CH4 and CO2 is not the only gas possible by anaerobic degradation of organic matter. Of the two, methane is produced only if methanogenic bacteria are involved in the anaerobic decomposition. Under different conditions, and with other species of anaerobic micro-organisms, gases such as hydrogen
T. Abbasi et al., Biogas Energy, SpringerBriefs in Environmental Science 2, DOI 10.1007/978-1-4614-1040-9_1, © Tasneem Abbasi 2012
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1 Biogas and Biogas Energy: An Introduction
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Table 1.1 Comparison of the calorific values of various fuels (MNRE 2011) Fuel Calorific value (approximate) Natural gas 8,600 kcal m−3 Liquefied petroleum gas 10,800 kcal kg−1 Kerosene 10,300 kcal kg−1 Diesel 10,700 kcal kg−1 Biogas 5,000 kcal m−3
and hydrogen sulphide may be generated instead of methane. But methanogenic bacteria occur very commonly in nature and in most instances anaerobic digestion does result in the generation of the predominantly CH4–CO2 mixture which is widely referred as “biogas.” Since the early years of the twentieth century, developing countries, notably China and India, had recognized the value of obtaining biogas from animal dung as a source of energy for the rural poor. From 1950s onwards these countries have made particularly strong efforts to popularize the use of “biogas plants.” But till the start of the 1970s,
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