30-01-2013, 11:12 AM
Geothermal electricity
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INTRODUCTION
Geothermal electricity is electricity generated from geothermal energy. Technologies in use include dry steam power plants, flash steam power plants and binary cycle power plants. Geothermal electricity generation is currently used in 24 countries[1], whilegeothermal heating is in use in 70 countries.[2]
Estimates of the electricity generating potential of geothermal energy vary from 35 to 2,000 GW.[2] Current worldwide installed capacity is 10,715 megawatts (MW), with the largest capacity in the United States (3,086 MW),[3] Philippines, and Indonesia.
Geothermal power is considered to be sustainable because the heat extraction is small compared with the Earth's heat content.[4] The emission intensity of existing geothermal electric plants is on average 122 kg of CO2 per megawatt-hour (MW•h) of electricity, about one-eighth of a conventional coal-fired plant.[5]
Resources
Enhanced geothermal system1:Reservoir 2ump house 3:Heat exchanger 4:Turbine hall 5roduction well 6:Injection well 7:Hot water to district heating 8orous sediments 9:Observation well 10:Crystalline bedrock
The earth’s heat content is 1031 joules.[2] This heat naturally flows to the surface by conduction at a rate of 44.2 terawatts, (TW,)[16] and is replenished by radioactive decay at a rate of 30 TW.[4] These power rates are more than double humanity’s current energy consumption from primary sources, but most of this power is too diffuse (approximately 0.1 W/m2 on average) to be recoverable. The Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) to release the heat underneath.
Electricity generation requires high temperature resources that can only come from deep underground. The heat must be carried to the surface by fluid circulation, either throughmagma conduits, hot springs, hydrothermal circulation, oil wells, drilled water wells, or a combination of these. This circulation sometimes exists naturally where the crust is thin: magma conduits bring heat close to the surface, and hot springs bring the heat to the surface. If no hot spring is available, a well must be drilled into a hot aquifer. Away from tectonic plate boundaries the geothermal gradient is 25-30°C per kilometre (km) of depth in most of the world, and wells would have to be several kilometres deep to permit electricity generation.[2] The quantity and quality of recoverable resources improves with drilling depth and proximity to tectonic plate boundaries.
Biogas
Biogas typically refers to a gas produced by breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal faeces, and kitchen waste can be converted into a gaseous fuel called biogas. Biogas originates from biogenic material and is a type of bio fuel.
Biogas is produced by the anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and crops.[1] Biogas comprises primarily methane (CH4) and carbon dioxide(CO2) and may have small amounts of hydrogen sulphide (H2S), moisture andsiloxanes.
The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen. This energy release allows biogas to be used as a fuel. Biogas can be used as a fuel in any country for any heating purpose, such as cooking. It can also be used in anaerobic digesters where it is typically used in a gas engine to convert the energy in the gas into electricity and heat.[2] Biogas can be compressed, much likenatural gas, and used to power motor vehicles. In the UK, for example, biogas is estimated to have the potential to replace around 17% of vehicle fuel.[3] Biogas is arenewable fuel, so it qualifies for renewable energy subsidies in some parts of the world. Biogas can also be cleaned and upgraded to natural gas standards when it becomes bio methane.
In the Indian subcontinent
In India, Nepal, Pakistan and Bangladesh 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 thousands in Pakistan, particularly North Punjab, due to the thriving population of livestock. It has become a popular source of fuel in many parts of Nepal. 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.[43] In order to create awareness and associate the people interested in biogas, an association "Indian Biogas Association" (www.biogas-India.com)[44]is formed. The “Indian Biogas Association” aspires to be a unique blend of; nationwide operators, manufacturers and planners of biogas plants, and representatives from science and research. The association was founded in 2010 and is now ready to start mushrooming. The sole motto of the association is “propagating Biogas in a sustainable way”.
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% methane,[11] which for reactors with free liquids can be increased to 80-90% methane using in-situ gas purification techniques[12] As-produced, biogas also contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for both water vapor content and thermal expansion is easily done via a simple mathematic algorithm[13] which yields the standardized volume of dry biogas.
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) orsilicates (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.