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Biogas can loosely refer to a variety of Landfill Gas (dirty gas) or Digester gas (sour gas) produced from the breakdown of organic matter. This could be from any number of biodegradable materials such as sewage or municipal waste, animal manure, plant waste, or landfill decomposition. These types of biogas are principally comprised of methane (50 to 70%) and carbon dioxide (35 to 45%), with small amounts of oxygen and nitrogen. The energy content of the biogas mixture is proportionate to the amount of methane contained therein. Natural gas (which is mostly methane and may contain a smidgen of ethane, propane, butane, etc.) has energy value of about 900 to 1000 Btu per cubic foot. Biogas energy value is dependent on the methane levels with the corresponding energy values as mentioned above (so about 500 to 700 Btu per cubic foot). For internal combustion engines, different biogases offer their own challenges as to selecting the proper spark plug. Each Biogas has its own contaminants such as siloxanes, solids, sulfurs, halides, and other corrosive acids. Typically only Iridium-Alloy, Platinum-Alloy or Platinum-Iridium blends on the center and ground electrodes are suggested for use. Secondary heat sources burning biogas such as boilers or kilns are not as dependent on precious metals on their igniter plugs.
Wood can also be gasified and referred to as biogas, but through a significantly different process.
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