Coal-fired power station
A coal-fired power station or coal power plant is a thermal power station which burns coal to generate electricity. Worldwide there are about 2,500 coal-fired power stations, on average capable of generating a gigawatt each. They generate about a third of the world's electricity, but cause many illnesses and the most early deaths per unit of energy produced, mainly from air pollution. World installed capacity doubled from 2000 to 2023 and increased 2% in 2023.
A coal-fired power station is a type of fossil fuel power station. The coal is usually pulverized and then burned in a pulverized coal-fired boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines that turn generators. Thus chemical energy stored in coal is converted successively into thermal energy, mechanical energy and, finally, electrical energy.
Coal-fired power stations are the largest single contributor to climate change, releasing approximately 12 billion tonnes of carbon dioxide annually, about one-fifth of global greenhouse gas emissions. China accounts for over half of global coal-fired electricity generation. While the total number of operational coal plants began declining in 2020, due to retirements in Europe and the Americas, construction continues in Asia, primarily in China and Southeast Asia. The profitability of some plants is maintained by externalities, as the health and environmental costs of coal production and use are not fully reflected in electricity prices. However, newer plants face the risk of becoming stranded assets. This is a particular risk in Southeast Asia, one of the world regions with the highest growth in coal power plants.
The UN Secretary General has called for OECD nations to phase out coal-fired generation by 2030, and the rest of the world by 2040.
History
The first coal-fired power stations were built in the late 19th century and used reciprocating engines to generate direct current. Steam turbines allowed much larger plants to be built in the early 20th century and alternating current was used to serve wider areas.Transport and delivery of coal
Coal is delivered by highway truck, rail, barge, collier ship or coal slurry pipeline. Generating stations are sometimes built next to a mine; especially one mining coal, such as lignite, which is not valuable enough to transport long-distance; so may receive coal by conveyor belt or massive diesel-electric-drive trucks. A large coal train called a "unit train" may be 2 km long, containing 130–140 cars with around 100 tonnes of coal in each one, for a total load of over 10,000 tonnes. 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 hottest summer or coldest winter months when power consumption is high.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.
A collier may hold 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. For transporting coal in calmer waters, such as rivers and lakes, flat-bottomed barges are often used. Barges are usually unpowered and must be moved by tugboats or towboats.
For start up or auxiliary purposes, the plant may use fuel oil as well. Fuel oil can be delivered to plants by pipeline, tanker, tank car or truck. Oil is stored in vertical cylindrical steel tanks with capacities as high as. The heavier no. 5 "bunker" and no. 6 fuels are typically steam-heated before pumping in cold climates.
Operation
As a type of thermal power station, a coal-fired power station converts chemical energy stored in coal successively into thermal energy, mechanical energy and, finally, electrical energy. The coal is usually pulverized and then burned in a pulverized coal-fired boiler. The heat from the burning pulverized coal converts boiler water to steam, which is then used to spin turbines that turn generators. Compared to a thermal power station burning other fuel types, coal specific fuel processing and ash disposal is required.For units over about 200 MW capacity, redundancy of key components is provided by installing duplicates of the forced and induced draft fans, air preheaters, and fly ash collectors. On some units of about 60 MW, two boilers per unit may instead be provided. The hundred largest coal power stations range in size from 3,000 MW to 6,700 MW.
Coal processing
Coal is prepared for use by crushing the rough coal to pieces less than in size. The coal is then transported from the storage yard to in-plant storage silos by conveyor belts at rates up to 4,000 tonnes per hour.In plants that burn pulverized coal, silos feed coal to pulverizers that take the larger 5 cm pieces, grind them to the consistency of talcum powder, sort them, and mix them with primary combustion air, which transports the coal to the boiler furnace and preheats the coal in order to drive off excess moisture content. A 500 MWe plant may have six such pulverizers, five of which can supply coal to the furnace at 250 tonnes per hour under full load.
In plants that do not burn pulverized coal, the larger 5 cm pieces may be directly fed into the silos which then feed either mechanical distributors that drop the coal on a traveling grate or the cyclone burners, a specific kind of combustor that can efficiently burn larger pieces of fuel.
Boiler operation
Plants designed for lignite are used in locations as varied as Germany, Victoria, Australia, 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.Ash disposal
The ash is often stored in ash ponds. Although the use of ash ponds in combination with air pollution controls decreases the amount of airborne pollutants, the structures pose serious health risks for the surrounding environment. Power utility companies have often built the ponds without liners, especially in the United States, and therefore chemicals in the ash can leach into groundwater and surface waters.Since the 1990s, power utilities in the U.S. have designed many of their new plants with dry ash handling systems. The dry ash is disposed in landfills, which typically include liners and groundwater monitoring systems. Dry ash may also be recycled into products such as concrete, structural fills for road construction and grout.
Fly ash collection
is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters located at the outlet of the furnace and before the induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos and stored on site in ash ponds, or transported by trucks or railroad cars to landfills.Bottom ash collection and disposal
At the bottom of the furnace, there is a hopper for collection of bottom ash. This hopper is kept filled with water to quench the ash and clinkers falling down from the furnace. Arrangements are included to crush the clinkers and convey the crushed clinkers and bottom ash to on-site ash ponds, or off-site to landfills. Ash extractors are used to discharge ash from municipal solid waste–fired boilers.Flexibility
Effective energy policy, law and electricity markets are essential for grid flexibility. While the flexibility of some coal-fired power stations can be enhanced, they generally offer less dispatchable generation than most gas-fired power plants. A key aspect of flexibility is low minimum load; however, certain flexibility upgrades for coal plants may be more costly than deploying renewable energy sources with battery storage.Coal power generation
two-thirds of coal burned is to generate electricity. In 2020 coal was the largest source of electricity at 34%. Over half of global coal-fired generation in 2020 occurred in China, and coal provided approximately 60% of electricity in China, India and Indonesia.Globally in 2020, 2,059 GW of coal-fired capacity was operational, with 50 GW newly commissioned and 25 GW under construction, while 38 GW was retired.
By 2023, global coal power capacity had increased to 2,130 GW, largely due to 47.4 GW of additions in China.
While some nations pledged to transition away from coal power at the 2021 United Nations Climate Change Conference through the Global Coal to Clean Power Transition Statement, significant challenges persist, especially in developing countries such as Indonesia and Vietnam.