Beatrice Lang
Coal is a fossil fuel that has been used to generate electricity since the industrial revolution. The process of turning coal into electricity involves several stages, starting with mining the coal from underground, then crushing it into a fine powder to facilitate combustion. The coal particles are then converted into gas through heating in a gasifier, and the gas is cleaned and cooled before being directed to a gas power plant for electricity generation. The electricity produced is then distributed to households and industries, while the waste heat is discharged. This complex process is illustrated in diagrams that show the various stages and facilities involved in coal-fired power generation.
| Characteristics | Values |
|---|---|
| Coal source | Underground mines |
| Coal transportation | Trucks |
| Coal preparation | Crushed into a fine powder to increase surface area and facilitate combustion |
| Conversion to gas | Gasifiers/gasification |
| Gas cleaning | Removal of impurities |
| Electricity generation | Gas power plants, combustion chambers of boilers, steam power cycles |
| Electricity transmission | Power line grids |
| Electricity consumption | Households, industries |
| By-products | Heat, solid waste |
| Environmental impact | CO2 emissions, NOx emissions |
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What You'll Learn
Coal extraction and crushing
The process of turning coal into electricity starts with coal mining. Coal is a fossil fuel that has been used to generate electricity since the industrial revolution. It is extracted from the ground or from mines, using a variety of different methods. The two most common methods are underground mining and surface mining.
Underground mining is used to reach coal seams that are buried deep underground. This method involves using room-and-pillar mining or longwall mining techniques. In room-and-pillar mining, a set of entries are driven into a block of coal and connected by cross-cuts, forming pillars that may be extracted or left in place depending on mining conditions. Longwall mining is an advanced technique that is used to mine a long wall of coal that is a few kilometres long and several hundred meters wide. Continuous mining machines, auxiliary fans, and roof bolters are used in this process, which is one of the noisiest and requires administrative and engineering controls to reduce noise exposure.
Surface mining, on the other hand, involves removing the ground covering the coal seam (the overburden) to expose the coal for extraction. This can be done through contour strip mining, area strip mining, or mountaintop removal techniques. In contour strip mining, bulldozers or power shovels are used to remove large amounts of ground material in terraced strips to extract coal from a seam on a hill. Area strip mining is similar but is used on flat terrain to extract deposits spread over a large area. Mountaintop removal, as the name suggests, involves removing the mountaintop to expose the coal seam and then disposing of the mining overburden in adjacent "valley fills".
Once the coal has been extracted, it is transported to a coal preparation plant for crushing and breaking. This process reduces the coal to an acceptable size for treatment in the plant. Various devices can be used for this purpose, such as feeder breakers, rotary breakers, hammer mills, and roll crushers. The crushed coal is then screened into different sizes and stored in silos or stockpiled before and after cleaning.
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Coal conversion to gas
The gasifier is a crucial component of the coal-to-gas conversion process. It converts the crushed coal particles into a gas mixture through heating. This gas mixture typically comprises carbon monoxide and hydrogen and trace amounts of other gases like carbon dioxide and hydrogen sulfide. The resulting gas is known as raw syngas.
The raw syngas undergoes further processing to remove the trace gases, producing clean syngas. This clean syngas is now suitable for conversion to liquids. One method is the Fischer-Tropsch process, where syngas is transformed into liquid hydrocarbons. Alternatively, the syngas can be converted into methanol, which serves as a basis for gasoline and plays a role in removing carbon dioxide from the raw syngas.
The gasification process can also be modified, such as in the COGAS process, which includes the gasification of char. The TOSCOAL process, similar to the TOSCO II oil shale retorting process, employs hot recycled solids for heat transfer. However, the resulting coal tars and oils often require further treatment before they can be used as motor fuels.
After gasification and any subsequent processing, the gas is directed to a gas power plant for electricity generation. This process produces excess heat and solid waste as byproducts. The electricity generated is then supplied to households and industries for various applications.
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Gas cooling
The gas cooling stage is an important part of the coal-to-electricity process. After the coal is crushed and transported to the gasifier, it is converted into a gas through heating. This gas then needs to be cooled before it can be transported to the gas power plant for electricity generation. This cooling process is done in a separate cooler, and it produces waste heat and solid waste as byproducts. The gas cooling process is essential to prepare the gas for the next stage of the coal-to-electricity process, ensuring that it is at the right temperature and pressure for safe and efficient transportation and use.
Cooling systems are a critical component of power plants, as they remove the large amount of heat generated during the coal-to-electricity process. This heat removal is necessary to condense the steam used to drive the turbine generators. Traditionally, power plants relied on water sources such as rivers and lakes for cooling, but in recent years, there has been a shift towards dry cooling systems that use little to no water. This transition is driven by the need to reduce water consumption and the increasing pressure on water resources.
Dry cooling systems have higher capital costs and require more energy to operate, resulting in lower overall power plant efficiency. However, they offer significant water savings, with some systems using up to 95% less water than traditional wet cooling systems. The type of cooling system used can vary depending on the power plant's fuel source, with natural gas combined-cycle plants, for example, tending to favour dry cooling due to their lower cooling requirements.
In the context of coal-to-electricity processes, gas cooling plays a vital role in optimizing the efficiency of the power plant. By effectively removing waste heat, the gas cooling stage ensures that the gas is ready for the next steps in electricity generation. This stage also highlights the environmental considerations of the coal-to-electricity process, as water consumption and waste management are crucial factors in the overall impact of the power plant.
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Electricity generation
The process of generating electricity from coal starts with mining the coal from underground. The coal is then crushed into a fine powder to increase its surface area and facilitate combustion. This process is known as pulverised coal combustion (PCC). The powdered coal is then transported to a gasifier, where it is converted into gas through heating. This gas is then cooled in a separate cooler, generating solid waste as a byproduct. The cleaned gas is directed to a gas power plant, where it is used to generate electricity. The electricity is then distributed to households and industries, and the excess heat produced is used for various applications.
During the combustion process, the powdered coal is blown into the combustion chamber of a boiler, where it is burned at high temperatures. The hot gases and heat energy produced convert water in tubes lining the boiler into steam. This high-pressure steam is passed into a turbine containing thousands of propeller-like blades, causing the turbine shaft to rotate at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. When these coils are rapidly rotated in a strong magnetic field, electricity is generated.
The electricity generated in this process is transformed into higher voltages of up to 400,000 volts for efficient transmission via power line grids. When the electricity nears the point of consumption, such as homes, it is transformed into safer voltages of 100-250 volts used in the domestic market.
To address the environmental impact of coal-fired power plants, there is a focus on minimising fuel consumption and reducing emissions. Technology-specific factors, such as the efficiency of power plants and the presence of pollution control systems, play a significant role in emission levels. Additionally, Integrated Gasification Combined-Cycle (IGCC) technology is used to produce electricity and reduce emissions. In IGCC systems, coal is reacted with oxygen and steam to produce a syngas composed mainly of hydrogen and carbon monoxide. The carbon is captured and separated from the hydrogen gas before combustion, allowing for the removal of up to 90% of CO2 from the power plant.
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Electricity transmission
The process of turning coal into electricity involves several stages, beginning with coal mining and culminating in power delivery to households and industries. After the coal is mined and crushed into fine pieces, it is transported to a gasifier, where it is burned at high temperatures to turn it into gas. This gas is then cooled and purified before being utilised to produce electricity in a power plant. Finally, the electricity is transmitted to homes and industries.
The electricity produced in the power plant is transported from the generator by connector cables to a switchyard. At the switchyard, the voltage is increased to a higher level (up to 400,000 volts) to enable efficient transmission over long distances through power line grids. This high voltage ensures that electricity can be delivered economically and efficiently to areas of consumption.
As the electricity travels through the transmission lines, it eventually reaches the point of consumption, such as residential areas. Before it can be safely utilised in homes, the voltage needs to be reduced to the standard domestic voltage. This is typically done by transforming the electricity to safer voltages of around 100-250 volts, suitable for household appliances and electrical systems.
The transmission of electricity is a critical aspect of the power grid infrastructure. It allows for the distribution of electricity from centralised power plants to distant locations, ensuring that even remote areas can have access to electricity. The efficient transmission of electricity ensures a stable and reliable supply of power to industries, businesses, and households, contributing to the overall functioning of modern society.
To ensure a stable and reliable electricity supply, it is essential to maintain and optimise the transmission infrastructure. This includes regularly inspecting and maintaining power lines, transformers, and other electrical equipment. By investing in advanced technologies and smart grid solutions, the transmission process can be further improved, enhancing the overall efficiency and reliability of the electricity supply chain.
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Frequently asked questions
Coal is a fossil fuel that has been used to generate electricity since the Industrial Revolution. The process of turning coal into electricity starts with mining coal from the ground and transporting it to a power plant. The coal is then finely ground into a powder, which is burned in a furnace. The heat from the furnace generates steam that turns turbines, which then generate electricity.
The Powder River Basin in Wyoming and Montana is the biggest coal deposit by volume, with 1.07 trillion short tons of in-place coal resources, 162 billion short tons of recoverable coal resources, and 25 billion short tons of economic coal resources as of 2013.
Coal-fired power plants produce high levels of greenhouse gas emissions, specifically carbon dioxide (CO2). Technologies such as carbon capture and storage (CCS) aim to reduce these emissions by capturing CO2 before it is released into the atmosphere and either using it in other industries or storing it underground.
Coal has played a significant role in electricity generation in Britain, but the country is now phasing out coal and gas in favour of more sustainable alternatives. Britain is introducing new innovations and technologies to manage the stabilising properties that coal and gas provide to the electricity system.
After electricity is generated in a power plant, it is transformed into higher voltages (up to 400,000 volts) for efficient transmission via power line grids. When it nears the point of consumption, such as homes or industries, the electricity is transformed into safer voltages (100-250 volts) for domestic use.
