Biogas

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Biogas, also called digester gas, typically refers to methane produced by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste, or any other biodegradable feedstock, under anaerobic conditions. Biogas is also called swamp gas and marsh gas, depending on where it is produced. The process is popular for treating many types of organic waste because it provides a convenient way of turning waste into electricity, decreasing the amount of waste to be disposed of, and of destroying disease causing pathogens which can exist in the waste stream. The use of biogas is encouraged in waste management because it does not increase the amount of carbon dioxide in the atmosphere, which is responsible for much of the greenhouse effect, if the biomass it is fueled on is regrown. Also, methane burns relatively cleanly compared to coal.

Processing of the biodegradable feedstock occurs in a anaerobic digester, which must be strong enough to withstand the buildup of pressure and must provide anaerobic conditions for the bacteria inside. Digesters are usually built near the source of the feedstock, and several are often used together to provide a continuous gas supply. Products put into the digester are composed mainly of carbohydrates with some lipids and proteins. Bananas produces a large amount of biogas in a short amount of time.

More recently, developed countries have been making increasing use of gas generated from both wastewater and landfill sites. Landfill gas production is incidental and usually nothing is done to increase gas production or quality. There are indications that slightly wetting the waste with water when it is deposited may increase production, but there is a concern that gas production would be large at first and then drop sharply.

Even if not used to generate heat or electricity, landfill gas must be disposed of or cleaned because it contains trace volatile organic compounds (VOCs), many of which are known to be precursors to photochemical smog. Because landfill gas contains these trace compounds, the United States Clean Air Act, and Part 40 of the Federal Code of Regulations, requires landfill owners to estimate the quantity of VOCs emitted. If the estimated VOC emissions exceeds 50 metric tons, then the landfill owner is required to collect the landfill gas, and treat it to remove the entrained VOCs. Usually, treatment is by combustion of the landfill gas. Because of the remoteness of landfill sites, it is sometimes not economically feasible to produce electricity from the gas.

Biogas digesters take the biodegradable feedstock, and convert it into two useful products: gas and digestate. The biogas can vary in composition typically from 50-80% methane, with the majority of the balance being made up of carbon dioxide.

The digestate comprises of lignin and cellulose fibres, along with the remnants of the anaerobic microorganisms. This digestate can be used on land as a soil amendment, to increase moisture retention in soil and improve fertility.

If biogas is cleaned up sufficiently, biogas has the same characteristics as natural gas. More frequently, it is burned with less extensive treatment on site or nearby. If it is burned nearby, a new pipeline can be built to carry the gas there. If it is to be transported long distances, laying a pipeline is probably not economical. It can be carried on a pipeline that also carries natural gas, but it must be very clean to reach pipeline quality. Water (H_{2_{O), hydrogen sulfide (H_{2_{S) and particulates are removed if present at high levels or if the gas is to be completely cleaned. Carbon dioxide is less frequently removed, but it must also be separated to achieve pipeline quality gas. If the gas is to be used without extensively cleaning, it is sometimes cofired with natural gas to improve combustion. Feeding biogas into the natural gas network is particularly interesting for climates where the waste heat of a biogas powered power plant cannot be used during the summer.}}}}

In some cases, landfill gas contains siloxanes. Burning converts them to silica particles, which tend to clog conventional combustion engines. Stirling engines are more resistant against them.

Biogas cleaned up to pipeline quality is called renewable natural gas, and in this form can be used in any application that natural gas is used for. Such applications include distribution via the natural gas grid, electricity production, space heating, water heating and process heating. If compressed, it can replace compressed natural gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells.

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