Question

Explain thermal power plant with diagram.

Answer 1

Thermal power plant 

Thermal power plants (referred to as thermal power plants ) can generate large amounts of electricity continuously. In many countries, most of the electricity is provided by thermal power plants.

Thermal power plants (except magneto-fluid generators ) convert the thermal energy generated by combustion into mechanical energy through various rotating machines and then drive the generator . The prime mover is usually a steam engine or a gas turbine . In some smaller power plants, it is also possible to use an internal combustion engine . They all generate electricity by using high pressure, high pressure steam, or gas pressure drops through the turbine to low pressure air or condensate.

Thermal power plants generate many other byproducts and generate many environmental impacts. According to the principle of the Carnot cycle, there is always a part of the waste heat that is discharged to the atmosphere through the cooling tower or cooled by natural rivers and other bodies of water. After the combustion of fossil fuels, the flue gas will be emitted to the atmosphere. The main components of the flue gas are carbon dioxide , water vapor, and other components such as nitrogen, nitrogen oxides, and sulfur oxides. If it is a coal power plant, it will also have pulverized coal. Ash , mercury and so on. Residues from coal combustion must also be removed from the boiler, and some can be recycled to make building materials.

Thermal power plants are the main source of carbon dioxide emissions. This large amount of greenhouse gas emissions in the last 100 years has led to global warming . Every time it is generated, brown coal emits about three times as much carbon dioxide as natural gas, and black coal is twice as much as natural gas. It is not realistic to try to store the carbon in emissions until 2025. Although global warming has entered a “suspend period” in the past decade, recent research shows that this may be due to the large amount of sulfur emissions from China's thermal power plants (mainly coal-based fuel), among other research. Mentioned in recent years that the deep oceans have absorbed the temperature of shallow seawater, so the temperature rise seems to be slowing down, but there is no guarantee how long this will last [2] .

Name discrimination 

A thermal power plant literally refers to a power plant that has a combustion process involved. However, there is no corresponding vocabulary in English. Fossil -fuel power stations are commonly used to describe fossil-fuel -fired power plants ( fossil fuel power plants ), and subdivisions can use names such as coal-fired. In theory, a biomass -fired power plant can also be called a thermal power plant in Chinese, but in practice the fossil fuel power plant is generally referred to as a thermal power plant.

Thermal power plants generally refer to power plants that use steam to generate electricity, such as most fossil-fired power plants (however, gas turbines do not generate electricity from steam), nuclear power plants, and geothermal power stations. Sometimes it also refers to all power plants that use thermal engines to generate electricity. Power plants that use gas turbines or internal combustion engines can also be counted. However, in actual use, thermal power plants are often referred to as thermal power plants. In another case, the use of thermal power plants and power plants refer to cogeneration power plants and power generation plants, respectively.

Basic concepts 

In thermal power plants, the chemical energy stored in various fossil fuels such as coal , fuel oil , natural gas, or oil shale is gradually converted to heat energy , mechanical energy , and finally to electric energy for supply to the four parties. A thermal power plant is a highly complex system with a construction cost of up to 1,300 U.S. dollars per kilowatt , or about 500 million U.S. dollars for a 500MW power plant. A power plant often has multiple power generation units to save manpower and material resources. Most of the world's thermal power plants use fossil fuels that exceed nuclear , geothermal , biomass, or solar power stations.

Thermal energy to mechanical energy 

The second law of thermodynamics shows that any closed thermodynamic cycle can only partially convert the heat energy generated by combustion into work . The remaining heat, called waste heat , must be vented to a cooler environment to return to the beginning of the cycle. The proportion of heat discharged to the cold source must be equal to or greater than the ratio of the absolute temperature of the cold source and the absolute temperature of the heat source (burner). Increasing the boiler temperature can increase efficiency, but it also adds to the complexity of the design—mainly the need to use higher levels of alloys, which increases the cost of the furnace. Waste heat cannot be converted to mechanical energy unless there is a cooler cooling system. However, waste heat can be exported in cogeneration power plants to heat buildings, produce hot water or be used to heat materials in industry, for example in certain refineries or chemical synthesis plants.

The typical thermal efficiency of an industrial generator is approximately 33% for coal or fuel, and up to 50% for a gas-fired combined cycle power plant. Peak power plants are generally less efficient because they cannot always be operated in optimal design conditions (eg, too low a temperature) [3] .

The theoretical maximum efficiency of converting thermal energy into useful work is the Carnot cycle , so all practical thermal power plants cannot exceed this limit. Fuel cells are not so limited because they do not use this principle to generate electricity.

Coal 

Coal is the most important raw material for thermal power generation, while China , India , the United States and other politically stable countries have many coal mines, unlike oil and natural gas, which rely heavily on the volatile Persian Gulf imports, so their supply is relatively stable. However, if you want to replace or even replace oil or natural gas, coal must be converted, especially considering that the car or heater must also be modified to adapt to the new fuel. In addition, if the exhaust gas is not recovered, coal will produce more pollution than oil or natural gas , especially if it counts greenhouse gases such as carbon dioxide .

At the same time, the mining of solid coal mines is more dangerous and highly polluting. It is necessary to send excavation pits to send people to open mines, or to excavate large areas with explosives on certain terrains. China's Shanxi Province is the largest concentration of coal in the world and the province produces coal. With more than 12 billion tons, the mined area formed by coal mining has reached 20,000 square kilometers, equivalent to an area of 1/8 of that in Shanxi Province. The Chinese government has established a comprehensive reform pilot area for national resource-based economy transformation and development to start rectifying the coal industry. Improve mining safety and equipment efficiency and reduce pollution.

The world's largest thermal power plant is the Inner Mongolia Tuoketuo Power Generation Company ; the world’s most energy-efficient generating unit is the two 1 million-kw China-made ultra-supercritical coal-fired generating units of the Shanghai Waigaoqiao Third Power Generation Company. It is also the first power plant in the world to reduce the actual coal consumption to 280 g/kWh. [4]

Fuel delivery 

Coal is transported via highway trucks , railways , ships or coal slurry pipelines. Some power plants are even built directly on the edge of coal mines and send coal by conveyor belts. The large coal train trains can be up to two kilometers in length and contain 100 carriages, each carrying more than 100 tons of coal, for a total of 10,000 tons. Large power plants need at least one such train every day at full load. In peak periods of electricity use, such as the hottest summer and the coldest winter (depending on the local climate), power plants even need three to five such trains a day. For example, the , the largest coal-fired power plant in North America, in Ontario , requires the storage of millions of metric tons of coal for each winter's icing period.

Power stations sometimes use fuel oil. Fuel oil can be transported by pipelines, oil tankers, etc. The oil is first stored in a cylindrical drum in the power plant. Higher viscosity oils may need to be heated before combustion.

Natural gas-fired power stations are often built next to natural gas pipelines, or they are transported on dedicated pipelines. Where no natural gas is produced, the natural gas needs to be liquefied from the production site, transported to a natural gas receiving station by a LNG ship, and then transported by pipeline to the power plant.

Fuel treatment 

The coal is first crushed to pieces no larger than 2 inches (5 cm) in length and then transported to the power plant's stockyard for continuous input through conveyor belts . It can be used up to 3600 tons per hour.

For coal-fired power plants, coal blocks are pulverized in a coal mill before entering the boiler and then blown into the furnace with preheated air. A 500 MW power plant may have six such mills (one for standby), which can provide more than 200 tons per hour into the boiler at full load. In an unburnt coal-fired power plant, 2-inch (51-mm) coal is sent directly to the active grate or cyclone (a burner that burns large chunks of fuel efficiently) through silos.

As for fuel oil and natural gas power stations, fuels do not require special treatment. Liquid natural gas and refined petroleum products can be sent directly to the power station to join the unit's fuel tank.

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