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Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Biogas originates from biogenic material and is a type of biofuel. One type of biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure or sewage, municipal waste, and energy crops. This type of biogas is comprised primarily of methane and carbon dioxide. The other principle type of biogas is wood gas which is created by gasification of wood or other biomass. This type of biogas is comprised primarily of nitrogen, hydrogen, and carbon monoxide, with trace amounts of methane.
The gases methane, hydrogen and carbon monoxide can be combusted or oxidized with oxygen. Air contains 21% oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a low-cost fuel in any country for any heating purpose, such as cooking. It can also be utilized in modern waste management facilities where it can be used to run any type of heat engine, to generate either mechanical or electrical power. Biogas is a renewable fuel and electricity produced from it can be used to attract renewable energy subsidies in some parts of the world.
Production
Depending on where it is produced, biogas can also be called swamp, marsh, landfill or digester gas. A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops.
Biogas can be produced utilizing anaerobic digesters. These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste.
Landfill gas is produced by organic waste decomposing under anaerobic conditions in a landfill. The waste is covered and compressed mechanically and by the weight of the material that is deposited from above. This material prevents oxygen from accessing the waste and anaerobic microbes thrive. This gas builds up and is slowly released into the atmosphere if the landfill site has not been engineered to capture the gas. Landfill gas is hazardous for three key reasons. Landfill gas becomes explosive when it escapes from the landfill and mixes with oxygen within lower and higher explosive limits. The methane in biogas forms explosive mixtures in air. The lower explosive limit is 5% methane and the upper explosive limit is 15% methane.[1] The methane contained within biogas is 20 times more potent as a greenhouse gas than carbon dioxide. Therefore uncontained landfill gas which escapes into the atmosphere significantly contributes to the effects of global warming. In addition to this volatile organic compounds (VOCs) contained within landfill gas contribute to the formation of photochemical smog.
Composition
The composition of biogas varies depending upon the origin of the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55-75%CH4 [3].
In some cases biogas contains siloxanes. These siloxanes are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica (SiO2) or silicates (SixOy) and can also contain calcium, sulfur, zinc, phosphorus. These white mineral deposits build to a surface thickness of several millimetres and must be removed by chemical or mechanical means.
| Matter | % |
|---|---|
| Methane, CH4 | 50-75 |
| Carbon dioxide, CO2 | 25-50 |
| Nitrogen, N2 | 0-10 |
| Hydrogen, H2 | 0-1 |
| Hydrogen sulfide, H2S | 0-3 |
| Oxygen, O2 | 0-2 |
Applications
Biogas can be utilized for 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.
Methane within biogas can be concentrated to the same standards as natural gas, when it is, it is called biomethane. If the local gas network permits it the producer of the biogas may be able to utilize the local gas distribution networks. Gas must be very clean to reach pipeline quality, and must be of the correct composition for the local distribution network to accept. Carbon dioxide, Water, hydrogen sulfide and particulates must be removed if present. If concentrated and compressed it can also be used in vehicle transportation. Compressed biogas is becoming widely used in Sweden, Switzerland and Germany. A biogas-powered train has been in service in Sweden since 2005.[4][5]
Bates' and his biogas car were the subject of a short documentary film called 'Sweet as a Nut' in 1974, at which point he had run his car for 17 years on gas he had produced by processing pig manure. Bates, an inventor lived in Devon, UK and in the film talks through the simple process and benefits of running a car on biogas. The conversion was simply made with an adapter attached to any combustion engine.
Biogas in Developing Nations
In India biogas produced from the anaerobic digestion of manure in small-scale digestion facilities is called Gober gas. In India biogas is generated at an estimated 2 million+ household facilities. The digester is an airtight circular pit made of concrete with a pipe connection. The manure is directed to the pit, usually directly from the cattle shed. The pit is then filled with a required quantity of wastewater. The gas pipe is connected to the kitchen fire place through control valves. The combustion of this biogas has very little odour or smoke. Owing to simplicity in implementation and use of cheap raw materials in villages, it is one of the most environmentally sound energy sources for rural needs.
Biogas is used extensively throughout rural China and where wastewater treatment and industry coincide. [6]
The Biogas Support Program in Nepal has installed over 100,000 biogas plants in rural areas. [7]
Vietnam’s Biogas Programme for Animal Husbandry Sector has led to the installation of over 20,000 plants throughout that country. [8]
Biogas is also in use in rural Costa Rica.[9]
In Colombia experiments with diesel engines-generator sets partially fuelled by biogas demonstrated that biogas could be used for power generation, reducing elecricity costs by 40% compared with purchase from the regional utility. [10]
Legislation
The European Union presently has some of the strictest legislation regarding waste management and landfill sites called the Landfill Directive. The United States legislates against landfill gas as it contains these VOCs. The United States Clean Air Act and Title 40 of the Code of Federal Regulations (CFR) requires landfill owners to estimate the quantity of non-methane organic compounds (NMOCs) emitted. If the estimated NMOC emissions exceeds 50 tonnes per year the landfill owner is required to collect the landfill gas and treat it to remove the entrained NMOCs. Treatment of the landfill gas is usually by combustion. Because of the remoteness of landfill sites it is sometimes not economically feasible to produce electricity from the gas.
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