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considerations associated with the manure handling system. In general, it is agreed that methane emissions from a lagoon increase with corresponding increases in temperature (Mangino et al, 2002; Moller et al, 2004b). However, Moller et al. (2004b) found the actual potential methane emission reduction to be greater than the MCF predicted. This discrepancy is explained by aerobic processes near the surface of the lagoon occurring simultaneously with anaerobic processes below. The MCF only accounts for emissions produced by anaerobic processes. The report claimed a loss in methane production potential of 4.3 to 6.6 % after 15 days and 7.7 to 11.9% after 30 days storage (Moller et al, 2004b). Composition of Biogas Raw biogas can contain 55 to 70% methane , though 60 to 65% is typical. It contains 30 to 45% carbon dioxide, and trace amounts of H2S, NH3, and H2. The composition of the gas depends largely on the composition of the substrate. The biogas can be used directly or it may be purified before use. Due to the hot, humid environment inside the digester, the resulting biogas is saturated with water vapour. At a temperature of 35°C and relative humidity of 100%, a cubic metre of gas will contain 40 g of water. This vapour can interfere with pressure reducers, boiler orifices, and other devices as well as reduce the energy value of the biogas (Persson et al, 1979). Condensate traps are an effective way of removing the moisture. Hydrogen sulfide (H2S) can also cause problems when using biogas, as it is corrosive even in small concentrations. To avoid corrosion, H2S levels should not exceed 500 to 700 ppm for use in conventional internal combustion engines. Unfortunately, H2S concentrations can reach as high as 6800 ppm. As a result, removal techniques may be needed. Biological removal is accomplished by adding a small amount of air to the gas (2 to 6% by volume). When this is done, micro-organisms convert the hydrogen sulfide to sulfur and sulfate. The sulfate forms sulfuric acid in solution which can be very corrosive. This process is often completed in a separate holding tank where the acid can be neutralized readily. This can be dangerous, as biogas is explosive when air is added in the range of 6 to 12 % by volume. Hydrogen sulfide can be removed chemically using either iron chloride or iron oxide. Dissolved iron chloride is added to the digestate, thereby reacting with the hydrogen sulfide to form iron sulfide. This process is very effective at reducing high concentrations of H2S but less capable of keeping levels low (Wellinger and Lindberg, 2001). This is a relatively expensive solution (Wellinger and Lindberg, 2001; Poulsen, 2003). Iron oxide (rust) also reacts with H2S to produce iron sulfide. Biogas is often passed through chambers of iron oxide pellets to achieve a reaction. The pellets can be regenerated by e xposure to air and re-used. Water scrubbing may be used to remove both hydrogen sulfide and carbon dioxide. Since these two gases are more soluble in water than methane, they dissolve. This leaves a high concentration of gaseous methane. However, the spent water is corrosive and either must be regenerated or replaced. 10PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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