A fossil-fuel power plant is a power plant that burns fossil fuels such as coal, natural gas orpetroleum (oil) to produce electricity.
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 plants (except for MHD generators) have some kind of rotating machinery to convert the heat energy ofcombustion into mechanical energy, which then operate an electrical generator. The prime mover may be a steam turbine, a gas turbine or, in small isolated plants, a reciprocating internal combustion engine. Some thermal plants have the intermediate step of using the heat from combustion to produce steam, reducing overall efficiency of electricity production. All plants use the drop between the high pressure and temperature of the steam or combusting fuel and the lower pressure of the atmosphere or condensing vapor in the steam turbine.
Byproducts of power thermal plant operation need to be considered in both the design and operation. Waste heat due to the finite efficiency of the power cycle, when not recovered and sold as steam or hot water, must be released to the atmosphere, often using a cooling tower, or river or lake water as a cooling medium, especially for condensing steam. The flue gas from combustion of the fossil fuels is discharged to the air; this contains carbon dioxide and water vapor, as well as other substances such asnitrogen, nitrogen oxides, sulfur oxides, and (in the case of coal-fired plants) fly ash and mercury. Solid waste ash from coal-fired boilers must also be removed, although some coal ash can be recycled for building materials.
Fossil fueled power stations are major emitters of greenhouse gases (GHG) which according to theconsensus of scientific organisations are a major contributor to the global warming observed over the last 100 years. Brown coal emits 3 times as much GHG as natural gas, black coal emits twice as much. Efforts exist to use carbon capture and storage of emissions but these are not expected to be available on a commercial scale and economically viable basis by 2025.
[edit]Basic concepts
In a fossil fuel power plant the chemical energy stored in fossil fuels (such as coal, fuel oil, natural gas oroil shale) and oxygen of the air is converted successively into thermal energy, mechanical energy and, finally, electrical energy for continuous use and distribution across a wide geographic area. Each fossil fuel power plant is a highly complex, custom-designed system. Construction costs, as of 2004, run toUS$1,300 per kilowatt, or $650 million for a 500 MWe unit[citation needed]. Multiple generating units may be built at a single site for more efficient use of land, natural resources and labor. Most thermal power stations in the world use fossil fuel, outnumbering nuclear, geothermal, biomass, or solar thermal plants.
[edit]Conversion of chemical energy to heat
The complete combustion of fossil fuel using air as the oxygen source is summarized in the following chemical reaction, assuming the nitrogen remains inert:
where stoichiometric coefficients x and y depend on the fuel type. A simple word equation for this chemical reaction is:
Depending on temperature and flame parameters during combustion, however, some of the nitrogen can be oxidized, producing various nitrogen oxides. Other, unintended, products of combustion are sulfur dioxide coming from sulfur impurities (predominantly in coal).
[edit]Conversion of heat into mechanical energy
The second law of thermodynamics states that any closed-loop cycle can only convert a fraction of the heat produced during combustion into mechanical work. The rest of the heat, called waste heat, must be released into a cooler environment during the return portion of the cycle. The fraction of heat released into a cooler medium must be equal or larger than the ratio of absolute temperatures of the cooling system (environment) and the heat source (combustion furnace). Raising the furnace temperature improves the efficiency but also increases the steam pressure, complicates the design and makes the furnace more expensive. The waste heat cannot be converted into mechanical energy without an even cooler cooling system. However, it may be used in cogeneration plants to heat buildings, produce hot water, or to heat materials on an industrial scale, such as in some oil refineries, cement plants, andchemical synthesis plants.
[edit]Fuel transport and delivery
Coal is delivered by highway truck, rail, barge or colliership. Some plants are even built near coal mines and coal is delivered by conveyors. A large coal train called a "unit train" may be two kilometers (over a mile) long, containing 100 cars with 100 tons of coal in each one, for a total load of 10,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the winter months when power consumption is high. A large thermal power plant such as the one in Nanticoke, Ontario stores several million tons of coal for winter use when the lakes are frozen.
Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a unit train takes about three hours.
Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel-electric-drive trucks.
A collier (cargo ship carrying coal) may hold 40,000 tons of coal and takes several days to unload. Some colliers carry their own conveying equipment to unload their own bunkers; others depend on equipment at the plant. Colliers are large, seaworthy, self-powered ships. For transporting coal in calmer waters, such as rivers and lakes, flat-bottomed vessels called barges are often used. Barges are usually unpowered and must be moved by tugboats or towboats.
For startup or auxiliary purposes, the plant may use fuel oil as well. Fuel oil can be delivered to plants bypipeline, tanker, tank car or truck. Oil is stored in vertical cylindrical steel tanks with capacities as high as 90,000 barrels' worth (14,000 m³, or about 5 million US gallons). The heavier no. 5 "bunker" and no. 6 fuels are typically steam-heated before pumping in cold climates.
Plants fuelled by natural gas are usually built adjacent to gas transport pipelines or have dedicated gas pipelines extended to them.
[edit]Fuel processing
Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (5 cm) in size. The coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts at rates up to 4,000 tons/hour.
In plants that burn pulverized coal, silos feed coal pulverizers (coal mills) that take the larger 2-inch pieces, grind them to the consistency of face powder, sort them, and mix them with primary combustion air which transports the coal to the furnace and preheats the coal to drive off excess moisture content. A 500 MWe plant will have six such pulverizers, five of which can supply coal to the furnace at 250 tons per hour under full load.
In plants that do not burn pulverized coal, the larger 2-inch pieces may be directly fed into the silos which then feed the cyclone burners, a specific kind of combustor that can efficiently burn larger pieces of fuel.
[edit]Feedwater heating and deaeration
The feedwater used in the steam boiler is a means of transferring heat energy from the burning fuel to the mechanical energy of the spinning steam turbine. The total feedwater consists of recirculatedcondensate water and purified makeup water. Because the metallic materials it contacts are subject tocorrosion at high temperatures and pressures, the makeup water is highly purified before use. A system of water softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter. The makeup water in a 500 MWe plant amounts to perhaps 20 US gallons per minute (1.25 L/s) to offset the small losses from steam leaks in the system.
The feedwater cycle begins with condensate water being pumped out of the condenser after traveling through the steam turbines. The condensate flow rate at full load in a 500 MWe plant is about 6,000 US gallons per minute (0.38 m³/s).
The water flows through a series of six or seven intermediate feedwater heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and gaining temperature at each stage. Typically, the condensate plus the makeup water then flows through a deaerator[2][3] that removes dissolved air from the water, further purifying and reducing its corrosivity. The water may be dosed following this point with hydrazine, a chemical that removes the remaining oxygen in the water to below 5 parts per billion (ppb). It is also dosed with pH control agents such as ammonia ormorpholine to keep the residual acidity low and thus non-corrosive.
[edit]Boiler operation
The boiler is a rectangular furnace about 50 feet (15 m) on a side and 130 feet (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.
Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22 MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine.
Plants designed for lignite (brown coal) are increasingly used in locations as varied as Germany,Victoria, and North Dakota. Lignite is a much younger form of coal than black coal. It has a lower energy density than black coal and requires a much larger furnace for equivalent heat output. Such coals may contain up to 70% water and ash, yielding lower furnace temperatures and requiring larger induced-draft fans. The firing systems also differ from black coal and typically draw hot gas from the furnace-exit level and mix it with the incoming coal in fan-type mills that inject the pulverized coal and hot gas mixture into the boiler.
Plants that use gas turbines to heat the water for conversion into steam use boilers known as HRSGs,Heat Recovery Steam Generators. The exhaust heat from the gas turbines is used to make superheated steam that is then used in a conventional water-steam generation cycle, as described in Gas turbine combined-cycle plants section below.
[edit]Steam turbine generator
The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. As steam moves through the system and loses pressure and thermal energy it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. The entire rotating mass may be over 200 tons and 100 feet (30 m) long. It is so heavy that it must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. This is so important that it is one of only five functions of blackout emergency power batteries on site. Other functions are emergency lighting, communication, station alarms and turbogenerator lube oil.
Superheated steam from the boiler is delivered through 14–16-inch (360–410 mm) diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4 MPa) and to 600 °F (320 °C) in temperature through the stage. It exits via 24–26-inch (610–660 mm) diameter cold reheat lines and passes back into the boiler where the steam is reheated in special reheat pendant tubes back to 1,000 °F (540 °C). The hot reheat steam is conducted to the intermediate pressure turbine where it falls in both temperature andpressure and exits directly to the long-bladed low pressure turbines and finally exits to the condenser.
The generator, 30 feet (9 m) long and 12 feet (3.7 m) in diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor— no permanent magnets here. In operation it generates up to 21,000 amps at 24,000 volts AC (504 MWe) as it spins at either 3,000 or 3,600 rpm, synchronized to the power grid. The rotor spins in a sealed chamber cooled with hydrogengas, selected because it has the highest known heat transfer coefficient of any gas and for its lowviscosity which reduces windage losses. This system requires special handling during startup, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that the highlyexplosive hydrogen-oxygen environment is not created.
The power grid frequency is 60 Hz across North America and 50 Hz in Europe, Oceania, Asia (Koreaand parts of Japan are notable exceptions) and parts of Africa.
The electricity flows to a distribution yard where transformers step the voltage up to 115, 230, 500 or 765 kV AC as needed for transmission to its destination.
[edit]Steam condensing
The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycleincreases. The condenser is usually ashell and tube heat exchangercommonly referred to as a surface condenser. Cooling water circulates through the tubes in the condenser's shell and the low pressure exhaust steam is condensed by flowing over the tubes as shown in the adjacent diagram. The tubing is designed to reduce the exhaust pressure, avoid subcooling the condensate and provide adequate air extraction. Typically the cooling water causes the steam to condense at a temperature of about 35 °C (95 °F) and that creates an absolute pressure in the condenser of about 5–7 kPa (1.5–2.1 inHg), i.e. a vacuum of about 95 kPa (28 inHg) relative to atmospheric pressure. The large decrease in volume that occurs when water vapor condenses to liquid creates the low vacuum that helps pull steam through and increase the efficiency of the turbines. The limiting factor is the temperature of the cooling water and that, in turn, is limited by the prevailing average climatic conditions at the power plant's location (it may be possible to lower the temperature beyond the turbine limits during winter, causing excessive condensation in the turbine).
From the bottom of the condenser, powerful condensate pumps recycle the condensed steam (water) back to the water/steam cycle.
The heat absorbed by the circulating cooling water in the condenser tubes must also be removed to maintain the ability of the water to cool as it circulates. This is done by pumping the warm water from the condenser through either natural draft, forced draft or induced draft cooling towers (as seen in the image to the right) that reduce the temperature of the water by evaporation, by about 11–17 °C (20–30 °F) - expelling waste heat to the atmosphere. The circulation flow rate of the cooling water in a 500 MWeunit is about 14.2 m³/s (225,000 US gal/minute) at full load.[4]
The condenser tubes are made of brass or stainless steel to resist corrosion from either side. Nevertheless they may become internally fouled during operation by bacteria or algae in the cooling water or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many plants include an automatic cleaning system that circulates sponge rubber balls through the tubes to scrub them clean without the need to take the system off-line.
The cooling water used to condense the steam in the condenser returns to its source without having been changed other than having been warmed. If the water returns to a local water body (rather than a circulating cooling tower), it is tempered with cool 'raw' water to prevent thermal shock when discharged into that body of water.
Another form of condensing system is the air-cooled condenser. The process is similar to that of aradiator and fan. Exhaust heat from the low pressure section of a steam turbine runs through the condensing tubes, the tubes are usually finned and ambient air is pushed through the fins with the help of a large fan. The steam condenses to water to be reused in the water-steam cycle. Air-cooled condensers typically operate at a higher temperature than water cooled versions. While saving water, the efficiency of the cycle is reduced (resulting in more carbon dioxide per MW of electricity).
[edit]Diagram
[edit]Stack gas path and cleanup
- see Flue gas emissions from fossil fuel combustion and Flue gas desulfurization for more details
As the combustion flue gas exits the boiler it is routed through a rotating flat basket of metal mesh which picks up heat and returns it to incoming fresh air as the basket rotates, This is called the air preheater. The gas exiting the boiler is laden with fly ash, which are tiny spherical ash particles. The flue gas contains nitrogen along with combustion products carbon dioxide, sulfur dioxide, and nitrogen oxides. The fly ash is removed by fabric bag filters or electrostatic precipitators. Once removed, the fly ash byproduct can sometimes be used in the manufacturing of concrete. This cleaning up of flue gases, however, only occurs in plants that are fitted with the appropriate technology. Still, the majority of coal fired power plants in the world do not have these facilities.[citation needed] Legislation in Europe has been efficient to reduce flue gas pollution. Japan has been using flue gas cleaning technology for over 30 years and the US has been doing the same for over 25 years. China is now beginning to grapple with the pollution caused by coal fired power plants.
Where required by law, the sulfur and nitrogen oxide pollutants are removed by stack gas scrubbers which use a pulverized limestone or other alkaline wet slurry to remove those pollutants from the exit stack gas. Other devices use catalysts to remove Nitrous Oxide compounds from the flue gas stream. The gas travelling up the flue gas stack may by this time have dropped to about 50 °C (120 °F). A typical flue gas stack may be 150–180 metres (500–600 ft) tall to disperse the remaining flue gas components in the atmosphere. The tallest flue gas stack in the world is 419.7 metres (1,377 ft) tall at the GRES-2 power plant in Ekibastusz, Kazakhstan.
In the United States and a number of other countries, atmospheric dispersion modeling[5] studies are required to determine the flue gas stack height needed to comply with the local air pollution regulations. The United States also requires the height of a flue gas stack to comply with what is known as the "Good Engineering Practice (GEP)" stack height.[6][7] In the case of existing flue gas stacks that exceed the GEP stack height, any air pollution dispersion modeling studies for such stacks must use the GEP stack height rather than the actual stack height.
Block quote
[edit]Supercritical steam plants
Above the critical point for water of 705 °F (374 °C) and 3,212 psi (22.15 MPa), there is no phase transition from water to steam, but only a gradual decrease in density. Boiling does not occur and it is not possible to remove impurities via steam separation. In this case a supercritical steam plant is required to utilise the increased thermodynamic efficiency by operating at higher temperatures. These plants, also called once-through plants because boiler water does not circulate multiple times, require additional water purification steps to ensure that any impurities picked up during the cycle will be removed. This purification takes the form of high pressure ion exchange units called condensate polishers between the steam condenser and the feedwater heaters. Subcritical fossil fuel power plants can achieve 36–40% efficiency. Supercritical designs have efficiencies in the low to mid 40% range, with new "ultra critical" designs using pressures of 4,400 psi (30 MPa) and dual stage reheat reaching about 48% efficiency.
Older nuclear power plants must operate below the temperatures and pressures that coal fired plants do. This limits their thermodynamic efficiency to the order of 30–32%. Advanced designs, such as theAdvanced gas-cooled reactor and the Supercritical water reactor, operate at temperatures and pressures similar to current coal plants, producing comparable efficiency.
[edit]Gas turbine combined-cycle plants
One type of fossil fuel power plant uses a gas turbine in conjunction with a heat recovery steam generator (HRSG). It is referred to as acombined cycle power plant because it combines the Brayton cycle of the gas turbine with the Rankine cycle of the HRSG. The thermal efficiency of these plants has reached a record heat rate of 5690 Btu/kWh, or just under 60%, at a facility in Baglan Bay, Wales. [8]
The turbines are fueled either with natural gas orfuel oil. While more efficient and faster to construct (a 1,000 MW plant may be completed in as little as 18 months from start of construction), the economics of such plants is heavily influenced by the volatile cost of natural gas. The combined cycle plants are designed in a variety of configurations composed of the number of gas turbines followed by the steam turbine. For example, a 3-1 combined cycle facility has three gas turbines tied to one steam turbine. The configurations range from (1-1), (2-1), (3-1), (4-1), (5-1), to (6-1)
Simple-cycle gas turbine plants, without a steam cycle, are sometimes installed as emergency or peaking capacity; their thermal efficiency is much lower. The high running cost per hour is offset by the low capital cost and the intention to run such units only a few hundred hours per year.
[edit]Environmental impacts
The world's power demands are expected to rise 60% by 2030.[9] With the worldwide total of active coal plants over 50,000 and rising,[10] the International Energy Agency (IEA) estimates that fossil fuels will account for 85% of the energy market by 2030.[9]
World organizations, and international agencies like the IEA are concerned about the environmental impact of burning fossil fuels, and coal in particular. The combustion of coal contributes the most to acid rain and air pollution, and has been connected with global warming, due to the chemical composition of coal and the difficulties of removing the impurities from this solid fuel prior to its combustion. Acid rain is caused by the emission of nitrogen oxides and sulfur dioxide into the air. These themselves may be only mildly acidic, yet when they react with the atmosphere, they create acidic compounds (such assulfurous acid, nitric acid, and sulfuric acid) that fall as rain, hence the term acid rain. In Europe and the U.S.A., stricter emission laws and decline in heavy industries have reduced the environmental hazards associated with this problem, leading to lower emissions after their peak in 1960s.
[edit]Carbon dioxide
Electricity generation using carbon based fuels is responsible for a large fraction of carbon dioxide (CO2) emissions worldwide; and for 41% of U.S. man-made carbon dioxide emissions.[11]. Coal combustion in thermal power stations result in greater amounts of carbon dioxide emissions per unit of electricity generated while natural gas produces the least for fossil fuels. Carbon dioxide is produced and released to the atmospheres from natural sources such as volcanoes, biological breakdown and respiration by living things. Carbon dioxide is absorbed and converted during photosynthesis by plants and dissolved into water, such as the oceans. Increasing concentrations of CO2 assist plant growth and are radiative forcing.
Increased concentration of carbon dioxide in the atmosphere assist climate change including global warming; concern over the rate of climate change has led to targets to stabilise or reduce carbon dioxide and other greenhouse gas (GHG) emissions by between 25 and 40% by 2020.[12]Fossil fueled, especially coal-fired, plants make reductions difficult. Emissions may be reduced through more efficient and higher combustion temperature and through more efficient production of electricity within the cycle.Carbon capture and storage (CCS) of emissions from coal fired power stations is another alternative but the technology is still being developed and will increase the cost of fossil fuel based production of electricity. CCS may not be economically viable, unless the price of emitting CO2 to the atmosphere rises.
[edit]Particulate matter
Another problem related to coal combustion is the emission of particulates that have a serious impact on public health. Power plants remove particulate from the flue gas with the use of a bag house orelectrostatic precipitator. Several newer plants that burn coal use a different process, Integrated Gasification Combined Cycle in which synthesis gas is made out of a reaction between coal and water. The synthesis gas is processed to remove most pollutants and then used initially to power gas turbines. Then the hot exhaust gases from the gas turbines are used to generate steam to power a steam turbine. The pollution levels of such plants are drastically lower than those of "classical" coal power plants.[13]
Particulate matter from coal-fired plants can be harmful and have negative health impacts. Studies have shown that exposure to particulate matter is related to an increase of respiratory and cardiac mortality.[14] Particulate matter can irritate small airways in the lungs, which can lead to increased problems with asthma, chronic bronchitis, airway obstruction, and gas exchange.[14]
There are different types of particulate matter, depending on the chemical composition and size. The dominant form of particulate matter from coal-fired plants is coal fly ash, but secondary sulfate and nitrate also comprise a major portion of the particulate matter from coal-fired plants.[15] Coal fly ash is what remains after the coal has been combusted, so it consists of the incombustible materials that are found in the coal.[16]
The size and chemical composition of these particles affects the impacts on human health.[14][15]Currently coarse (diameter greater than 2.5 μm) and fine (diameter between 0.1 μm and 2.5 μm) particles are regulated, but ultrafine particles (diameter less than 0.1 μm) are currently unregulated, yet they pose many dangers.[14] Unfortunately much is still unknown as to which kinds of particulate matter pose the most harm, which makes it difficult to come up with adequate legislation for regulating particulate matter.[15]
There are several methods of helping to reduce the particulate matter emissions from coal-fired plants. Roughly 80% of the ash falls into an ash hopper, but the rest of the ash then gets carried into the atmosphere to become coal-fly ash.[16] Methods of reducing these emissions of particulate matter include:
The baghouse has a fine filter that collects the ash particles, electrostatic precipitators use an electric field to trap ash particles on high-voltage plates, and cyclone collectors use centrifugal force to trap particles to the walls.[16]
[edit]Radioactive trace elements
As most ores in the Earth's crust, coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment leads to radioactive contamination. While these substances are present as very small trace impurities, enough coal is burned that significant amounts of these substances are released. A 1,000 MW coal-burning power plant could release as much as 5.2 tons/year of uranium (containing 74 pounds (34 kg) of uranium-235) and 12.8 tons/year of thorium. The radioactive emission from this coal power plant is 100 times greater than a comparable nuclear power plant with the same electrical output; including processing output, the coal power plant's radiation output is over 3 times greater.[17]
[edit]Greening of fossil fuel power plants
At present, several methods exist to improve the efficiency of fossil fuel power plants. A frequently used and cost-efficient is by converting them to run on a different fuel. This includes conversions as biomass and waste.[18][19][20] Conversions to waste-fired power plants have the benefit that they can be used to eliminate existing landfills. In addition, waste fired power plants can be equipped with material recovery, allowing again additional environmental gain.
Regardless of the conversion, in order to become a truly green fossil fuel power plant, Carbon Capture and Storage may be implemented. It infers that the CO2 is captured for CCS. This method allows any fossil fuel power plants to be converted to a emissionless power plant. Examples of a CCS fossil fuel power plant includes the Elsam power station near Esbjerg, Denmarkt. [21]
[edit]Clean coal
"Clean coal" is the name attributed to a process whereby coal is chemically washed of minerals and impurities, sometimes gasified, burned and the resulting flue gases treated with steam, with the purpose of removing sulfur dioxide, and reburned so as to make the carbon dioxide in the flue gas economically recoverable. The coal industry uses the term "clean coal" to describe technologies designed to enhance both the efficiency and the environmental acceptability of coal extraction, preparation and use[22], but has provided no specific quantitative limits on any emissions, particularly carbon dioxide. Whereas contaminates like sulfur or mercury can be removed from coal, carbon cannot be effectively removed while still leaving a usable fuel, and "clean" coal plants without carbon sequestration and storage do not significantly reduce carbon dioxide emissions.
[edit]Combined Heat and Power - CHP
CHP is the practice of using power station reject heat, nearly always fossil fueled, to heat processes or large areas of housing in which latter case it is referred to as CHPDH or CHP District Heating, using buried insulated pipes. While rejecting heat at a higher than normal temperature to enable building heating lowers overall plant efficiency, the extra fuel burn is more than offset by the reduction in fossil fuel that would otherwise be used for heating buildings. This technology is widely practiced in for example Denmark, other Scandinavian countries and parts of Germany. Calcualtions show that CHPDH is the cheapest method of carbon emissions reductions.[23]
[edit]Alternatives to fossil fuels
Alternatives to fossil fuel power plants include nuclear power or solar power and other renewable energies(see non-carbon economy). Natural gas is a cleaner fossil fuel than coal and petroleum since its combustion emits very little sulfur dioxide and does not emit particulate matter or radioactive materials.
However, coal is abundant in the world and generally cheaper for power generation. Most of thedeveloping world uses coal for its power production, including countries such as China and India.
[edit]Relative cost by generation source
When comparing power plant costs, it is customary to start by calculating the cost of power at the generator terminals by considering several main factors. External costs such as connections costs, the effect of each plant on the distribution grid are considered separately as an additional cost to the calculated power cost at the terminals.
Initial factors considered are:
- Capital costs (including waste disposal and decommissioning costs for nuclear energy)
- Operating and maintenance costs
- Fuel costs (for fossil fuel and biomass sources, and which may be negative for wastes)
- Likely annual hours per year run or load factor( maybe 30% for wind energy, but 90% for nuclear energy)
- Offset sales of heat for example in Combined Heat and Power District Heating / CHP/DH.
These costs are brought together over the life of the plant - which could be 100 years for hydro, and 20 years for wind,and 30 – 50 years for fossil plant, using discounted cash flow here.[24] and here [25]
In general large fossil plants are attractive due to their low initial capital costs - typically around £750 - £1000 per kWe compared to perhaps £1500 per kw for onshore wind.
Inherently renewables are on a decreasing cost curve, while non-renewables are on an increasing cost curve.[26] In 2009, costs are comparable between wind, nuclear, coal, and natural gas, but CSP - concentrating solar power, and PV - photvoltaics are somewhat higher.
There are often additional costs particularly for renewables in terms of increased grid interconnection to allow for diversity of weather and load,back up and load management costs, but these have been shown in the pan - European case to be quite low, showing that overall for example wind energy costs about the same as present day power.[27]
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