Biogas
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. Biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and energy crops.[1] This type of biogas comprises primarily methane and carbon dioxide. Other types of gas generated by use of biomass is wood gas, which is created by gasification of wood or other biomass. This type of gas consist 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.
History
Ancient Persians observed that rotting vegetables produce flammable gas. In the 13th century, the traveller Marco Polo noted the Chinese used covered sewage tanks to generate power, while biogas technologies were also referred to by 17th century author Daniel Defoe.[3]
In 1859, an anaerobic digestion plant was built to process sewage at a Bombay leper colony. Biogas has been used in the UK since 1895, when gas from sewage was used in street lamps across the city of Exeter.[4]
[edit]Production
Biogas is practically produced as landfill gas (LFG) 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. During the process, an air-tight tank transforms biomass waste into methane producing renewable energy that can be used for heating, electricity, and many other operations that use any variation of an internal combustion engine, such as GE Jenbacher gas engines.[5] There are two key processes: Mesophilic and Thermophilic digestion.[6]
Landfill gas is produced by wet organic waste decomposing under anaerobic conditions in a landfill.[7][8] The waste is covered and mechanically compressed by the weight of the material that is deposited from above. This material prevents oxygen exposure thus allowing anaerobic microbes to 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.[9] 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 landfill gas' impact in global warming, volatile organic compounds (VOCs) contained within landfill gas contribute to the formation of photochemical smog.
[edit]Composition
| Typical composition of biogas[10] | ||
| Compound | Chem | % |
|---|---|---|
| Methane | CH4 | 50–75 |
| Carbon dioxide | CO2 | 25–50 |
| Nitrogen | N2 | 0–10 |
| Hydrogen | H2 | 0–1 |
| Hydrogen sulfide | H2S | 0–3 |
| Oxygen | O2 | 0–0 |
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 [11] or higher using in situ purification techniques[12] As-produced, biogas also contains water vapor, with the fractional water vapor volume a function of biogas temperature; correction of measured volume for water vapor content and thermal expansion is easily done via algorithm.[13]
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. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means.
Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are currently available.[14]
[edit]Applications
Biogas can be utilized for electricity production on sewage works,[15] in a CHP gas engine, where the waste heat from the engine is conveniently used for heating the digester; cooking; space heating; water heating; and process heating. If compressed, it can replacecompressed natural gas for use in vehicles, where it can fuel an internal combustion engineor fuel cells and is a much more effective displacer of carbon dioxide than the normal use in on-site CHP plants.[2]
Methane within biogas can be concentrated via a biogas upgrader to the same standards as fossil natural gas(which itself has had to go through a cleaning process), and becomesbiomethane. If the local gas network allows for this, the producer of the biogas may 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.[16][17]
Biogas has also powered automobiles. In 1974, a British documentary film entitled Sweet as a Nut detailed the biogas production process from pig manure, and how the biogas fueled a custom-adapted combustion engine.[18][19]
[edit]Benefits
By using biogas, many advantages arise. In North America, utilization of biogas would generate enough electricity to meet up to three percent of the continent's electricity expenditure. In addition, biogas could potentially help reduce global climate change. Normally, manure that is left to decompose releases two main gases that cause global climate change: nitrous dioxide and methane. Nitrous dioxide warms the atmosphere 310 times more than carbon dioxide and methane 21 times more than carbon dioxide. By converting cow manure into methane biogas via [anaerobic digestion]], the millions of cows in the United States would be able to produce one hundred billion kilowatt hours of electricity, enough to power millions of homes across the United States. In fact, one cow can produce enough manure in one day to generate three kilowatt hours of electricity; only 2.4 kilowatt hours of electricity are needed to power a single one hundred watt light bulb for one day.State Energy Conservation Office (Texas). "Biomass Energy: Manure for Fuel." State Energy Conservation Office (Texas). State of Texas, 23 April 2009. Web. 3 October 2009. Furthermore, by converting cow manure into methane biogas instead of letting it decompose, we would be able to reduce global warming gases by ninety-nine million metric tons or four percent.Webber, Michael E and Amanda D Cuellar. "Cow Power. In the News: Short News Items of Interest to the Scientific Community." Science and Children os 46.1 (2008): 13. Gale. Web. 1 October 2009.
The 30 million rural households in China that have biogas digesters enjoy 12 benefits: saving fossil fuels, saving time collecting firewood, protecting forests, using crop residues for animal fodder instead of fuel, saving money, saving cooking time, improving hygienic conditions, producing high-quality fertilizer, enabling local mechanization and electricity production, improving the rural standard of living, and reducing air and water pollution.China Biogas" "China Biogas.
[edit]Biogas upgrading
Raw biogas produced from digestion is roughly 60% methane and 29% CO2 with trace elements of H2S, and is not high quality enough if the owner was planning on selling this gas or using it as fuel gas for machinery. The corrosive nature of H2S alone is enough to destroy the internals of an expensive plant. The solution is the use of a biogas upgrading or purification process whereby contaminants in the raw biogas stream are absorbed or scrubbed, leaving 98% methane per unit volume of gas. There are four main methods of biogas upgrading, these include water washing, pressure swing absorption, selexol absorption and chemical treatment. The most prevalent method is water washing where high pressure gas flows into a column where the carbon dioxide and other trace elements are scrubbed by cascading water running counter-flow to the gas. This arrangement can deliver 98% methane with manufacturers guaranteeing maximum 2% methane loss in the system. It takes roughly between 3-6% of the total energy output in gas to run a biogas upgrading system.
[edit]Biogas gas-grid injection
Gas-grid injection is the injection of biogas into the methane grid (natural gas grid). Injections includes biogas:[20] until the breakthrough ofmicro combined heat and power two-thirds of all the energy produced by biogas power plants was lost (the heat), using the grid to transport the gas to customers, the electricity and the heat can be used for on-site generation [21] resulting in a reduction of losses in the transportation of energy. Typical energy losses in natural gas transmission systems range from 1–2%. The current energy losses on a large electrical system range from 5–8%.[22]
[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 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. However, countries such as the United Kingdom and Germany now has legislation in force that provide farmers with long term revenue and energy security.[23][24]
[edit]Development around the world
In 2007 an estimated 12,000 vehicles were being fueled with upgraded biogas worldwide, mostly in Europe.[25]
[edit]In the United States
With the many benefits of biogas, it is starting to become a popular source of energy and is starting to be utilized in the United States more. In 2003 the United States consumed 147 trillion BTU of energy from "landfill gas", about 0.6% of the total U.S. natural gas consumption.[25] Methane biogas derived from cow manure is also being tested in the U.S. According to a 2008 study, collected by theScience and Children magazine, methane biogas from cow manure would be sufficient to produce 100 billion kilowatt hours enough to power millions of homes across America. Furthermore, methane biogas has been tested to prove that it can reduce 99 million metric tons of greenhouse gas emissions or about 4% of the greenhouse gases produced by the United States.[26]
In Vermont, for example, biogas generated on dairy farms around the state is included in the CVPS Cow Power program. The Cow Power program is offered by Central Vermont Public Service Corporation as a voluntary tariff. Customers can elect to pay a premium on their electric bill, and that premium is passed directly to the farms in the program. In Sheldon, Vermont Green Mountain Dairy has provides renewable energy as part of the Cow Power program. It all started when the brothers who own the farm, Bill and Brian Rowell, wanted to address some of the manure management challenges faced by dairy farms, including manure odor, and nutrient availability for the crops they need to grow to feed the animals. They installed an anaerobic digester to process the cow and milking center waste from their nine hundred and fifty cows to produce renewable energy, a bedding to replace sawdust, and a plant friendly fertilizer. The energy and environmental attributes are sold. On average the system run by the Rowell brothers produces enough electricity to power three hundred to three hundred fifty other homes. The generator capacity is about three hundred kiloWatts.[27]
In Hereford, Texas cow manure is being used to power an ethanol power plant. By switching to methane bio-gas, the ethanol power plant has saved one thousand barrels of oil a day. Overall, the power plant has reduced transportation costs and will be opening many more jobs for future power plants that will be relying on biogas.[28]
[edit]In the United Kingdom
In the UK, sewage gas electricity production is tiny compared to overall power consumption - a mere 80 MW of generation, compared to 70 GW on the grid.[29][clarification needed]. There are currently less than 50 non-sewage landfill plants in the UK.[30]
[edit]In the Indian subcontinent
In Pakistan and India biogas produced from the anaerobic digestion of manure in small-scale digestion facilities is called gobar gas; it is estimated that such facilities exist in over two million households in India and in hundreds of thousands in Pakistan, particularly North Punjab, due to the thriving population of lifestock . 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 fireplace 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. One type of these system is the Sintex Digester. Some designs use vermiculture to further enhance the slurry produced by the biogas plant for use as compost.[31]
The Deenabandhu Model is a new biogas-production model popular in India. (Deenabandhu means "friend of the helpless.") The unit usually has a capacity of 2 to 3 cubic metres. It is constructed using bricks or by a ferrocement mixture. The brick model costs approximately 18,000 rupees and the ferrocment model 14,000 rupees, however India's Ministry of Non-conventional Energy Sources offers a subsidy of up to 3,500 rupees per model constructed.[citation needed]
[edit]In developing nations
Domestic biogas plants convert livestock manure and night soil into biogas and slurry, the fermented manure. This technology is feasible for small holders with livestock producing 50 kg manure per day, an equivalent of about 6 pigs or 3 cows. This manure has to be collectable to mix it with water and feed it into the plant. Toilets can be connected. Another precondition is the temperature that affects the fermentation process. With an optimum at 36 C° the technology especially applies for those living in a (sub) tropical climate. This makes the technology for small holders in developing countries often suitable.
Depending on size and location, a typical brick made fixed dome biogas plant can be installed at the yard of a rural household with the investment between 300 to 500 US $ in Asian countries and up to 1400 US $ in the African context. A high quality biogas plant needs minimum maintenance costs and can produce gas for at least 15–20 years without major problems and re-investments. For the user, biogas provides clean cooking energy, reduces indoor air pollution and reduces the time needed for traditional biomass collection, especially for women and children. The slurry is a clean organic fertilizer that potentially increases agricultural productivity.
Domestic biogas technology is a proven and established technology in many parts of the world, especially Asia.[32] Several countries in this region have embarked on large-scale programmes on domestic biogas, such as China[33][34] and India. The Netherlands Development Organisation, SNV,[35] supports national programmes on domestic biogas that aim to establish commercial-viable domestic biogas sectors in which local companies market, install and service biogas plants for households. In Asia, SNV is working in Nepal,[36] Vietnam,[37] Bangladesh,[38] Cambodia,[38]Lao PDR,[39] Pakistan[40] and Indonesia,[41] and in Africa in Rwanda,[42] Senegal, Burkina Faso, Ethiopia,[43] Tanzania,[44] Uganda and Kenya.
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