2015年9月18日星期五
2015年9月17日星期四
Fossil-fuel power station
A fossil-fuel power station is a power station which burns fossil fuel such as coal, natural gas or petroleum to produce electricity. Central station fossil-fuel power plants are designed on a large scale for continuous operation. In many countries, such plants provide most of the electrical energy used. Fossil-fuel power stations have machinery to convert the heat energy of combustion into mechanical energy, which then operates an electrical generator. The prime mover may be a steam turbine, a gas turbine or, in small plants, a reciprocating internal combustion engine. All plants use the energy extracted from expanding gas, either steam or combustion gases.
By products of thermal power plant operation must be considered in their design and operation. Waste heat energy, which remains due to the finite efficiency of the Carnot, Rankine, or Diesel power cycle, is released directly to the atmosphere or river/lake water, or indirectly to the atmosphere using a cooling tower with river or lake water used as a cooling medium. The flue gas from combustion of the fossil fuels is discharged to the air. This gas contains carbon dioxide and water vapor, as well as other substances such as nitrogen oxides (NOx), sulfur oxides (SOx),mercury, traces of other metals, and, for coal-fired plants, fly ash. Solid waste ash from coal-fired boilers must also be removed. Some coal ash can be recycled for building materials.
Fossil fueled power stations are major emitters of CO2, a greenhouse gas which according to a consensus opinion of scientific organizations is a contributor to global warming. Per unit of electric energy, brown coal emits nearly two times as much CO2 as natural gas, and black coal emits somewhat less than brown. Carbon capture and storage of emissions is not currently available.
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Classes of Biomass fired boiler for power system
Biomass fired boiler for power systems divided into co-fired, gasification, and modular systems.
Co-firing involves substituting biomass for a portion of coal in an existing power plant furnace. It is the most economic near-term option for introducing new biomass power generation. Because much of the existing power plant equipment can be used without major modifications, cofiring is far less expensive than building a new biopower plant. Compared to the coal it replaces, biomass reduces sulphur dioxide (SO2), nitrogen oxides (NOx), and other air emissions. After "tuning" the boiler for peak performance, there is little or no loss in efficiency from adding biomass. This allows the energy in biomass to be converted to electricity with the high efficiency (in the 33-37% range) of a modern coal-fired power plant.
Biomass gasifiers operate by heating biomass in an oxygen-limited environment where the solid biomass breaks down to form a flammable gas. The producer gas can be cleaned and filtered to remove problem chemical compounds. The producer gas can be used in more efficient power generation systems called combined-cycles, which combine gas turbines and steam turbines to produce electricity. The efficiency of these systems can reach 60 percent. Additionally, gasifiers are sometimes located next to existing coal or natural gas boilers and used to fire or supplement the fuels to these boilers.
Modular systems employ some of the same technologies mentioned above, but on a smaller scale that is more applicable to villages, farms, and small industry. These systems are now under development and could be most useful in remote areas where biomass is abundant and electricity is scarce. There are many opportunities for these systems in developing countries.
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