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of the digester and lubrication oil is changed as often as every ten days (Anon. 2004c). Converted diesel engines are thought to be more capable of handling H2S levels than spark ignition engines (Wellinger and Lindberg, 2001). Pressurization - Biogas does not require pressurization befo re use in an IC engine. Digester-produced pressures are usually adequate (Anon, 2004a). Efficiency - Electrical efficiencies of IC engines (coupled with generators) vary. Some studies report efficiencies as high as 29% for spark ignition engines and 31% for dual fuel engines (Wellinger and Lindberg, 2001). The US EPA assumes a range of 18 to 25%, depending on engine design and load factor (AgSTAR, 2004). In independent testing of one IC engine/generator pair, the Greenhouse Gas Technology Center found an efficiency of only 19.7% (AgSTAR, 2004). With so many available engines and generators, it is not surprising to find a wide range of electrical efficiencies. Thermal energy may be recovered by means of a heat recovery system, thereby increasing the efficiency of the system. For example, by passing exhaust gas through heat exchanger units, low pressure steam can be produced. A thermal jacket surrounding the engine can collect thermal energy in the form of hot water. This thermal jacket water can reach temperatures of 125°C (Willingham and Pipattanasomporn, 2003). Total thermal efficiency can reach 65%. Recall the example of 100 m3 of biogas with a theoretical heat value of 650 kWh/ m3. Consider an engine/generator set that is 20% efficient at producing electricity and 40% thermally efficient. In this case, the total system efficiency is 60% - approximately 130 kWh of electricity and 260 kWh of heat are produced. IC Engine Emissions - IC engine emissions from biogas are similar to those of natural gas. However, the higher level of CO2 in biogas decreases cylinder temperatures. This helps to decrease NOx emissions (Huang and Crookes, 1998). Typical values, compared to other electricity generators, can be found in the Emissions section of this report. Gas Turbines Gas turbines are essentially internal combustion engines that operate with rotary instead of reciprocating action. There are three main components to a gas turbine: the compressor, the combustor, and the power turbine. Ambient air is compressed and added to the fuel in the combustor. The mixture is ignited and the resulting hot gases are directed towards the power turbine. The power turbine section allows the gases to expand over the turbine fins, thereby rotating the shaft. A portion of the power created by the shaft is commonly used to drive the gas turbine’s internal compressor. The balance is used to drive an external load (e.g. a generator). Unfortunately, gas turbines are only competitive at creating electricity at scales greater than 800 kW (Wellinger and Lindberg, 2001). Most farm-based anaerobic digesters would not produce enough biogas to power a generator of this magnitude, making turbines impractical for the majority of farms. 13PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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