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name might suggest, air is mixed with the gas prior to reaching the burner in a pre- aerated flare, whereas air is mixed at the burner in a diffusion flare (Wheeler, 2000). Since on-farm agricultural anaerobic digesters are in their infancy in Ontario, there are currently no regulations requiring an emergency flare. The methane released is considered to be analogous to methane emissions from a typical manure storage. The matter, however, is currently under review (Hilborn, 2005). Internal Combustion Engines The use of internal combustion (IC) engines with biogas is long established and reliable (Wellinger and Lindberg, 2001). IC engines are sub-divided into two categories: compression engines, and spark ignition engines. Both types of engine may be converted to run on the biogas produced by anaerobic digesters. Biogas operation of compression engines is known as “dual fuel” operation because a small amount of diesel fuel is combined with the gas for ignition purposes. Spark ignition engines are operated on a mixture of biogas and air, as ignition is caused by a spark p lug. Dual Fuel Engines – Simply stated, in a dual fuel engine, a mixture of biogas and air (mixed in an external device) is sucked into the engine chamber and ignited with a small amount of diesel fuel. The amount of this pilot fuel needed for ignition is approximately 10 to 20% of the amount needed for operation on diesel fuel alone (von Mitzlaff, 1988). This extra fuel represents an extra cost in running the engine. However, the dual fuel engine can operate on anywhere from zero to 85% biogas, which can be advantageous during digester start-up as well as during periods of low biogas production. A limitation to the dual fuel engine is that the fuel injectors can overheat when the flow of diesel fuel is reduced to 10 to 15% of normal (von Mitzlaff, 1988). Furthermore, Bari (1996) found that dual fuel engines “ran harshly” with biogas containing more than 40% CO2. Spark Ignition Engines - The carburetor of a typical spark ignition engine must be replaced with a biogas-air mixer. Converted SI engines show a 15 to 20% decrease in power due to the decrease in volumetric efficiency and the flame speed of gas-air compared to gasoline-air mixtures (Henham and Makkar, 1998). An increase of compression ratio and an advancement in spark timing can decrease this power loss a certain amount. However, the loss can never be fully regained and the changes to compression ratio are permanent. Therefore the engine cannot be run on gasoline in periods of little or no biogas production. Biogas consisting of 60% methane is considered a medium weak gas and causes engine power reductions of approximately 20%, compared to 10% for natural gas and 5% for LPG (von Mitzlaff, 1988). IC engines are at risk of corrosion from the presence of H2S (similar to the heat boiler). It is for this reason that levels of H2S less than 1000 ppm are recommended (Wellinger and Lindberg, 2001). In surveying digester experience in the United States prior to 1998, Lusk (1998) found that H2S corrosion of engines was quite common in cases of no pre-treatment. Nevertheless, the United States Environmental Protection Agency states that gas treatment is not necessary given a regular maintenance schedule (AgSTAR, 2004). Indeed, on one Colorado hog farm, gas is used directly out 12PDF Image | Electricity and Heat Production Using Biogas from the Anaerobic Digestion of Livestock Manure - Literature Review
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