Biogas

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Pipes carrying biogas (foreground), natural gas and condensate

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, green waste and energy crops^[1]. This type of biogas comprises primarily methane and carbon dioxide. The other principal 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 used 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 can be compressed, much like natural gas, and used to power motor vehicles and in the UK for example is estimated to have the potential to replace around 17% of vehicle fuel ^[2]. Biogas is a renewable fuel, so it qualifies for renewable energy subsidies in some parts of the world.

[edit] Production

(LFG)^[3] 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 wet organic waste decomposing under anaerobic conditions in a landfill.^[4] 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. The lower explosive limit is 5% methane and the upper explosive limit is 15% methane.^[5] 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 may significantly contribute 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.

Sweden produces biogas from confiscated alcoholic beverages.^[6]

[edit] 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 ^[8].

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 (Si_xO_y) 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.

[edit] Applications

Biogas can be utilized for electricity production on sewage works ^[9], in a CHP gas engine, where the waste heat from the engine is conveniently used to heat the digestoer; cooking, 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 and is a much more effective displacer of carbon dioxide than the normal use in on site CHP plants^[10].

Methane within biogas can be concentrated via a biogas upgrader to the same standards as fossil 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^[11][12].

Bates, an inventor, lived in Devon, UK, modified his car to run on biogas. A short documentary film called 'Sweet as a Nut' in 1974, talks through the simple process and benefits of running a car on biogas, at which point he had run his car for 17 years on gas he had produced by processing pig manure. The conversion was simply made with an adapter attached to the combustion engine.

[edit] Scope and potential quantities

In the UK, sewage gas electricity production is tiny compared to overall power consumption - a mere 80 MW of generation, compared to 70,000 MW on the grid. Estimates vary but could be a considerable fraction from digestion of ^[13].

[edit] In developing nations

In India biogas produced from the anaerobic digestion of manure in small-scale digestion facilities is called Gober gas; it is estimated that such facilities exist in over 2 million households. 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. Some designs use vermiculture to further enhance the slurry produced by the biogas plant for use as compost ^[14]. Biogas is used extensively throughout rural China and where wastewater treatment and industry coincide. ^[15] (See Bioenergy in China)

In most Indian villages, manure is in regular supply

The Biogas Support Program in Nepal has installed over 150,000 biogas plants in rural areas, ^[16] and in 2005 won an Ashden Award for their work ^[17].

Vietnam’s Biogas Programme for Animal Husbandry Sector has led to the installation of over 20,000 plants throughout that country. ^[18]

Biogas is also in use in rural Costa Rica.^[19]

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. ^[20]

In Rwanda, the Kigali Institute of Science and Technology has developed and installed large-scale biogas plants at prisons to treat sewage and provide gas for cooking ^[21].

[edit] Deenabandhu Model (India)

This is a new model of Biogas unit popular in India. The word means "helpful for the Poor".^{[citation needed]} The unit usually has a capacity of 2 to 3 cubic metres. It is constructed using bricks or by a ferrocement mixture. The unit is subsidised by the Ministry of Non Conventional Energy Sources of the Government of India. A subsidy of Rupees 3500 per plant is provided to any body who constructs such a unit. A turn key agent fee of Rs 700 is provided to the approved mason for maintenance of the unit for three years. The total cost of construction of a 2 cubic meter unit comes to Rs 18000 for the brick model and Rs 14000 for the Ferrocement model.

[edit] Legislation

The European Union presently has some of the strictest legislation regarding waste management and landfill sites called the Landfill Directive.^{[citation needed]} 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.

[edit] Gober gas

The airtight circular cylinder of a gober gas plant, which moves up and down depending upon the collection of gas

Gober gas (also spelt as "Gobar gas", from the Hindi word "gober" for cow dung) is biogas generated out of cow dung. In India, gober gas is generated using countless household micro plants (an estimated more than 2 million). In Pakistan the concept is also quickly growing. The Government of Pakistan provides 50% funds for the construction of moveable gas chamber biogas plants^{[citation needed]}. The gober gas plant 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 water or wastewater. The gas pipe is connected to the kitchen fire place through control valves. The flammable methane gas generated out of this is largely odourless and smokeless. The residue left after the extraction of the gas is used as fertiliser. Owing to its simplicity in implementation and use of cheap raw materials in the villages, it is often quoted as one of the most environmentally sound energy source for the rural needs.^{[22][23] [24]}

[edit] See also

Sustainable development portal

Energy portal

• Anaerobic digestion
• Biodegradability
• Bioenergy
• Biofuel
• Biohydrogen
• Landfill gas monitoring
• MSW/LFG (municipal solid waste and landfill gas)
• Natural gas vehicle
• Renewable energy
• Renewable natural gas
• Tables of European biogas utilisation
• Waste management
• Food waste in the United Kingdom

[edit] References

[edit] External links

• An Introduction to Biogas, University of Adelaide
• Biogas purification
• Biogas in rural Costa Rica
• Selected chapters from The Complete Biogas Handbook
• Biogas from manure and waste products - Swedish case studies
• Case studies on biogas plants in developing countries
• Organic farming and Biogas - presentation by the Danish Agricultural Advisory Service
• Biogas introduction by the Agency for Renewable Resources

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