Trystan Casey
Coal is converted into electricity through a multi-step process. Firstly, a machine called a pulverizer grinds the coal into fragments and powder. The ground coal is then burned in a furnace, producing heat energy. This heat energy transforms water into steam, and the pressure from the steam turns the blades of a turbine. The spinning turbine then transforms the heat energy into mechanical energy, which is used to power a generator. Within the generator, a copper coil converts the mechanical energy into electrical energy. The amount of coal used to generate electricity depends on the efficiency of the generator or power plant and the heat content of the coal.
| Characteristics | Values |
|---|---|
| Process | Coal is ground into fragments and powder, then burned with hot air to produce heat energy, which is converted into mechanical energy by spinning turbines. |
| Use | Coal is used as a fuel for combustion to generate electricity, with coal accounting for about 19.5% of U.S. electricity generation in 2022. |
| Efficiency | The efficiency of coal-to-electricity conversion depends on the heat rate of the generator or power plant and the heat content of the coal. |
| Syngas | Coal can be converted into synthetic natural gas (syngas), which can be used to produce electricity and hydrogen. |
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What You'll Learn
Coal combustion
The first step in coal combustion is establishing the overall stoichiometry of the reaction, which can be challenging due to coal's molecular complexity. In direct combustion, coal undergoes a self-supporting exothermic reaction, where the interaction of chemical and physical processes results in the production of thermal energy. The physical processes involved include the transport of matter and energy, such as the conduction of heat, the diffusion of chemical species, and the bulk flow of gases.
The combustion of coal has been used for electricity generation for a long time. The coal is burned to release energy in the form of heat, which is then used to produce steam. This high-pressure steam is directed into a turbine containing propeller-like blades, causing the turbine shaft to rotate at high speed. A generator mounted on one end of the turbine shaft consists of carefully wound wire coils, which are essential for electricity generation.
The efficiency of coal combustion processes is crucial, with newer power plants utilising improved combustion technologies, such as pulverised coal (PC) combustion, to increase efficiency. Additionally, there is a growing trend towards more efficient and environmentally friendly methods of coal combustion. For instance, the use of low-sulfur coal or post-combustion cleanup of off-gases can help reduce emissions and minimise environmental impact.
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Heat energy conversion
The steam enters the turbine at high pressure, pushing against thousands of propeller-like blades. This force causes the turbine shaft to rotate at high speed. To convert this mechanical energy into electrical energy, a generator is mounted at one end of the turbine shaft. This generator consists of carefully wound wire coils.
The amount of coal required to generate a kilowatthour (kWh) of electricity depends on the efficiency or heat rate of the generator or power plant. This heat rate is measured in British thermal units (Btu) per kWh. By dividing the heat rate by the fuel heat content (in Btu per physical unit), we can calculate the amount of fuel required to generate one kWh of electricity.
Additionally, coal can be converted into synthetic fuels, or synfuels, by heating it in large vessels. An example of this process is observed at the Great Plains Synfuels Plant in North Dakota, where coal is converted into synthetic natural gas, or syngas. Syngas, derived from coal, can be used for electricity production.
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Kinetic energy from steam
The conversion of coal into electricity involves multiple processes, one of which involves utilising the kinetic energy from steam. Firstly, coal is burned within a boiler to create high-pressure steam. This steam is then passed into a turbine containing thousands of propeller-like blades, causing the turbine shaft to rotate at high speed. This phenomenon is known as the kinetic energy of steam.
The turbine shaft, to which the blades are attached, begins to rotate at high speed due to the kinetic energy imparted by the steam. This mechanical energy is then utilised to drive a generator, which is typically mounted at one end of the turbine shaft. The generator consists of carefully wound wire coils that play a vital role in the next step of electricity generation.
The kinetic energy from the steam, transferred to the turbine, ultimately results in the rotation of the generator's components. This mechanical energy is then converted into electrical energy through the principles of electromagnetic induction. The specific process and calculations involved in this conversion are based on established electrical engineering principles and equations.
While the explanation provided focuses on the role of kinetic energy from steam, it is important to recognise that the overall process of converting coal to electricity involves various other steps and considerations. These include the specific characteristics of the coal, the design of the boiler, turbine, and generator, as well as the overall efficiency of the power plant.
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Turbine rotation
The conversion of coal to electricity involves several processes, with turbine rotation being a critical one. This rotation is achieved through the use of steam, which is generated by burning coal. The steam is directed through a turbine, causing it to rotate at high speed. This mechanical energy is then converted into electrical energy through the use of a generator.
The turbine itself is a complex piece of machinery, consisting of thousands of propeller-like blades arranged in a specific pattern. These blades are mounted on a rotor shaft, which serves as the axis of rotation. When the high-pressure steam is introduced, it pushes against these blades, exerting a force that results in the rotation of the entire shaft.
The rotor shaft is connected to a generator, which is typically mounted at one end. This generator houses carefully wound wire coils that play a crucial role in the conversion process. As the shaft rotates, it spins the generator, and this mechanical energy is transformed into electrical energy through the interaction of the wire coils with the magnetic field created by the rotating shaft.
The efficiency of this energy conversion process depends on various factors, including the design of the turbine, the pressure and temperature of the steam, and the speed of rotation. Optimizing these parameters can improve the overall efficiency of the power plant.
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Electricity generation
The process of converting coal to electricity involves several steps, and the principle has remained largely unchanged since Thomas Edison's coal-fired power plant began operating in 1882. Coal is a fossil fuel that has been used to generate electricity since the Industrial Revolution.
The first step in the process is the combustion of coal. The coal is burned to heat water, creating steam. This step is common to other thermal power stations that use fuel sources such as gas. The efficiency of this step can be measured by the heat rate, or the amount of energy used to generate one kilowatt-hour (kWh) of electricity. A lower heat rate indicates higher efficiency. For example, a heat rate of 7,500 British thermal units (Btu) would correspond to an efficiency of 45%.
The steam generated in the first step is then used to spin a turbine. The heavy spinning turbines in coal-fired power plants have rotating speeds that sync with the grid's frequency, providing important stabilising properties to the electricity system.
Finally, the spinning turbine produces electricity. This process has been the primary method of electricity generation in Britain, peaking in the early 1990s, when coal accounted for over 60% of the energy mix. However, due to the decarbonisation of the electricity system, coal's role has been declining, with a target to phase out coal usage by the end of 2024.
To reduce carbon emissions, the implementation of carbon capture and storage (CCS) technologies, combined with high-efficiency, low-emissions (HELE) technologies, can significantly reduce CO2 emissions from coal-fired power plants. HELE technologies, when combined with CCS, can decrease CO2 emissions by up to 90%, making them an essential factor in the transition to a low-carbon future.
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Frequently asked questions
The energy content of coal is given in terms of KiloJoules (kJ) per kilogram (kg) of coal as the Gross calorific value (GCV) or the Higher Heating value (HHV) of coal. This value can vary from 10,500 kJ/kg to 25,000 kJ/kg depending on the quality and type of coal.
A power plant converts coal to electricity in two stages. First, high-pressure steam is passed into a turbine containing thousands of propeller-like blades. The steam pushes these blades, causing the turbine shaft to rotate at high speed. Second, a generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils.
Since coal has a heat value of 20,000 kJ/kg, to produce one kW.hr we require (10765 / 20000) 0.538 kg of coal.
