What Is a Thermal Power Station?

Thermal power (thermoelectric power generation) is a power generation method that uses the thermal energy generated by combustibles during combustion and converts it into electrical energy through a power generation device. China has abundant coal resources. In 1990, it produced 1.09 billion tons of coal, of which only 12% was used for power generation. Thermal power still has great potential.

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Thermal power is the main power generation method in China.

There are three types of energy conversion processes in thermal power: fuel

The method of generating electricity by using energy contained in combustibles is collectively referred to as thermal power generation. According to the power generation method, thermal power generation is divided into coal-fired turbine power generation, oil-fired turbine power generation,

According to their role, there are those that supply electricity alone and both generate electricity and heat (

Thermal power generation principle

Thermal power generation generally refers to the general term for the method of heating water by using the thermal energy generated by the combustion of combustible materials to make water into high temperature and high pressure water vapor, and then driving the generator with water vapor. Power plants that use combustibles as fuel are collectively referred to as thermal power plants ^[2] .

The main equipment systems of thermal power plants include: fuel supply systems, water supply systems, steam systems, cooling systems, electrical systems and other auxiliary processing equipment.

Most coal-fired power plants use coal as a primary energy source and use belt transfer technology to deliver treated pulverized coal to the boiler. The pulverized coal fires and heats the boiler to turn the water in the boiler into water vapor. After one heating, the water vapor enters the high-pressure cylinder. In order to improve the thermal efficiency, the water vapor should be reheated, and the water vapor enters the medium pressure cylinder. The steam from the medium-pressure cylinder is used to propel the turbo-generator to generate electricity. Lead from the medium pressure cylinder into the symmetrical low pressure cylinder. Part of the steam that has been used for work is extracted from the middle section and supplied to brother companies such as oil refining and fertilizers, and the rest flows through the condenser water-cooled to become saturated water at about 40 degrees as reuse water. Saturated water at about 40 degrees passes through the condensate pump and passes through the low-pressure heater to the deaerator. At this time, saturated water at about 160 degrees is deoxidized through the deaerator and fed into the high-pressure heater by the feedwater pump. The device uses reheated steam as heating fuel, and finally flows into the boiler for reuse. The above is a production process.

Thermal power generation process

The thermal power generation process varies depending on the prime mover used. In the steam turbine power generation method, the basic process is to first send fuel to the boiler and air at the same time. The boiler injects chemically treated feedwater. The heat energy released by the combustion of the fuel is used to turn the water into high-temperature and high-pressure steam. Drive generators to generate electricity. The combined heat and power method is to use the exhaust gas (or special extraction steam) of the prime mover to supply heat to industrial production or residential life. In the gas turbine power generation method, the basic process is to use a compressor to press the compressed air into the combustion chamber, mix it with the injected fuel, and atomize it to combust, forming high-temperature gas into the gas turbine to expand the work, pushing the turbine blades to rotate and driving power generation. Machine power generation. In diesel engine power generation, the basic process is to use a fuel injection pump and injector to inject fuel into the cylinder at high pressure to form a mist, which is mixed with air and combusts to drive the diesel engine to rotate and drive the generator to generate electricity.

Thermal power efficiency

In terms of thermal power, gas and steam turbine power plants have achieved the highest energy efficiency to date-more than 60%. Due to the very short start-up time, this type of power plant is best suited to supplement the natural power fluctuations caused by wind power. Higher energy efficiency can be achieved with combined heat and power plants-over 90%.

Thermal power

According to the production process of thermal power generation, its basic components include a combustion system, a soda water system (there is no such system for gas turbine power generation and diesel power generation, but these two have a small proportion in thermal power generation), electrical systems, and control systems.

Thermal power combustion system

It is mainly composed of the combustion chamber (ie, furnace) of the boiler, air supply device, coal (or oil, natural gas) device, and ash and slag discharge device. The main function is to complete the combustion process of the fuel and release the energy contained in the fuel in the form of thermal energy, which is used to heat the water in the boiler. The main processes are flue gas process, ventilation process, ash discharge and slag discharge process. The basic requirements for the combustion system are: complete combustion as much as possible, so that the boiler efficiency is 90%; ash discharge meets the standards.

Thermal power soda water system

It is mainly composed of feed water pump, circulation pump, feed water heater, condenser, deaerator, water wall and pipeline system. Its function is to use the combustion of fuel to turn water into high-temperature and high-pressure steam and circulate water. The main processes are soda water process, make-up water process, cooling water process, etc. The basic requirement for the soda water system is to minimize the soda water loss; use extraction steam to heat the condensate as much as possible to increase the water temperature.

Thermal Power Generation Electrical System

Mainly by the main wiring of the power plant, turbo-generators, main transformers, power distribution equipment, switchgear, generator leads, factory junctions, factory transformers and reactors, factory motors, security power supplies, battery DC systems and communications Equipment, lighting equipment, etc. The basic function is to ensure that power is supplied to the load or power system according to the power quality requirements. The main processes include power supply processes and plant power processes. The basic requirements for the electrical system are safe and reliable power supply; flexible scheduling; good adjustment and operation functions to ensure the quality of power supply; the ability to quickly remove faults and avoid accidents from expanding.

Thermal power generation control system

It is mainly composed of boilers and auxiliary systems, steam turbines and auxiliary systems, generators and electrical equipment, and auxiliary systems. The basic function is to implement automatic adjustment and control of each production link of the thermal power plant in order to coordinate the working conditions of each part, so that the entire thermal power plant operates safely, rationally and economically, reduces labor intensity, improves productivity, and can quickly and correctly perform faults. Handle to avoid causing accidents. The main work flow includes the start-stop of the steam turbine, the automatic speed-up control process, the boiler's combustion control process, the fire protection system control process, the thermal measurement and control process, the automatic removal of electrical faults, and the ash and slag removal automation process.

Thermal power

Electric power is an important energy source for the development of the national economy, and thermal power generation is the main method for producing electrical energy in China and many countries in the world. Then the steam enters the turbine along the pipeline to expand and do work, which drives the generator to rotate at high speed, thereby generating electricity. Finally, it was pumped back to the boiler and repeatedly participated in the above cycle. Obviously, there are three types of energy conversion processes in this thermal power plant:

Thermal power station boiler

Power generation boilers are called power station boilers. Compared with industrial boilers used in other factories, power station boilers have the following obvious characteristics: large capacity of power station boilers; high steam parameters of power station boilers; high degree of automation of power station boilers, and its operations have basically achieved mechanization and automation to adapt to the load The ability to change is very strong, up to more than 90, and the thermal efficiency of industrial boilers is mostly between 60 and 80.

Coal for thermal power stations

The coal used in thermal power plants is usually called thermal coal. The classification method is mainly based on the dry ash-free volatile matter of coal.

Preparation of pulverized coal for thermal power generation

The pulverized coal used for pulverized coal furnace combustion is irregular fine coal particles ground by a coal mill. The average particle size is 0.05 to 0.01 mm, and the majority of particles are less than 20 to 50 m (microns). Because the pulverized coal particles are small and the surface is large, they can absorb a large amount of air, and have properties that are not found in ordinary solids-fluidity. The smaller the particle size of the pulverized coal, the smaller the moisture content, and the better its fluidity. However, if the pulverized coal particles are too small or too dry, the phenomenon of pulverized coal self-flow will occur, making the working characteristics of the coal feeder unstable, and the boiler The adjustment operation in operation caused difficulties. In addition, pulverized coal is oxidized by contact with oxygen, and pulverized coal may ignite under certain conditions. In the pulverizing system, pulverized coal is transported by gas. As soon as a mixture of gas and pulverized coal meets a spark, the ignition source will expand and generate a large pressure, which will cause the pulverized coal to explode.

The fineness of the pulverized coal used in the boiler should be determined by the following conditions: It is desirable that the pulverized coal be ground finer in terms of combustion, so that the amount of air supply can be appropriately reduced, so that

The loss is reduced; from the aspect of the pulverizing system, it is desired that the pulverized coal be ground coarsely, thereby reducing the power consumption and metal consumption of the pulverized coal. Therefore, when selecting the fineness of pulverized coal, the sum of the above losses should be minimized. The fineness of pulverized coal with a small total loss is called "economic fineness". It can be seen that the fineness of pulverized coal can be appropriately larger for coal types with higher volatile and flammable coal, or for pulverizing equipment with more uniform grinding of pulverized coal particles, and some intensified combustion boilers to save grinding. Coal energy consumption. Due to the different degrees of softness and hardness of various coals, their wear resistance is also different, so the economic fineness of each coal is also different.

Pulverized coal combustion

The pulverized coal made by the pulverized coal preparation system enters the furnace through a pulverized coal burner. The burner is the main combustion equipment of the pulverized coal furnace. The burner has three functions: one is to ensure that the pulverized coal gas flows into the furnace to ignite quickly after being injected; the other is to enable the primary and secondary air to be strongly mixed to ensure that the pulverized coal is fully burned; After the pulverized coal gas flows into the furnace through the burner, the combustion process of coal begins. The three phases of the combustion process are generally the same as those of other furnace types. The difference is that the pre-combustion stage and the combustion stage of this type of furnace are short in time, while the burn-out stage is relatively long.

Coal for thermal power generation

The pulverized coal furnace of the power plant has a wide application range for coal types. It can be designed to burn lignite with high volatility or anthracite with low volatility. However, for an installed boiler, it is impossible to burn various volatile coals, because it is limited by the type of burner and the structure of the furnace. The quality indicators of coal for power generation are:

Volatile matter. It is the primary indicator to determine the characteristics of coal ignition. The higher the volatile content, the easier it is to catch fire. According to the design requirements of the boiler, the value of coal volatile content should not change too much, otherwise it will affect the normal operation of the boiler. For example, after the original design burns low-volatile coal and burns high-volatile coal, the flame center approaches the burner outlet, and the furnace may be shut down due to burnout of the burner; However, burning low-volatile coal will cause incomplete combustion due to fire too late, and even cause a flameout accident. Therefore, the coal supply should be based on the original design of volatile coal or similar coal. Ash content. The ash content will reduce the flame propagation speed, delay the ignition time, unstable combustion, and reduce the furnace temperature. moisture. Moisture is one of the harmful substances in the combustion process. It absorbs a large amount of heat during the combustion process and has a much greater impact on combustion than ash. Calories. For the heat generation is an important basis for boiler design. Because the pulverized coal of the power plant is highly adaptable to coal types, as long as the calorific value of the coal is generally consistent with the design requirements of the boiler. Ash melting point. Because the temperature of the flame center of the pulverized coal furnace is more than 1500 ° C, at this high temperature, the coal ash is mostly softened or fluid. Sulfur content of coal. Sulfur is a harmful impurity in coal. Although it has no effect on the combustion itself, its content is too high, which causes serious corrosion to equipment and environmental pollution. Therefore, the sulfur content of coal used in power plants cannot be too high, and the maximum requirement is generally not to exceed 2.5. ^[1]

Thermal power production process

The main production systems of thermal power plants include soda water systems, combustion systems and electrical systems, which are now described as follows:

Thermal power soda water system

The steam-water system of a thermal power plant is composed of boilers, turbines, condensers, high and low pressure heaters, condensate pumps and feedwater pumps. It includes steam-water circulation, chemical water treatment and cooling systems.

Water is heated into steam in the boiler, and is heated by the heater to become superheated steam, and then enters the turbine through the main steam pipe. As the steam continues to expand, the high-speed flowing steam pushes the blades of the turbine to rotate the generator.

In order to further improve its thermal efficiency, part of the steam that has been used for work is extracted from some intermediate stages of the steam turbine to heat the feed water. This feed water recuperation cycle is used in modern large steam turbine units. In addition, a reheating cycle is also used in the ultra-high pressure unit, that is, the steam that has performed a certain amount of work is extracted from the exit of the high-pressure cylinder of the steam turbine, and the steam that has been used for the work is completely extracted, sent to the boiler reheater, and then the gas is introduced. The medium-pressure cylinder of the turbine continues to expand for work, and the steam sent from the medium-pressure cylinder is sent to the low-pressure cylinder to continue the work. During the continuous work of steam, the pressure and temperature of the steam are continuously reduced, and finally discharged into the condenser and cooled by cooling water to condense into water. The condensed water is concentrated in the lower part of the condenser, heated by the condensate pump to low pressure, and then deaerated by the deaerator. The water pump sends the pre-heated deaerated water to the high-pressure heater, and the heated hot water enters the boiler, and then In the superheater, the water that has been heated to superheated steam is sent to the steam turbine to perform work, so that the work is repeated continuously.

The steam and condensed water in the soda water system, because there are a lot of pipes to be cleared and also need to pass through a lot of valve equipment, this will inevitably produce running, falling, dripping, leaking and other phenomena, which will cause more or less water loss, Therefore, we must continuously add chemically treated demineralized water to the system. These make-up water is generally added to the deaerator.

Thermal power combustion system

The combustion system is composed of coal transportation, coal grinding, coarse and fine separation, powder discharge, powder feeding, boiler, dust removal, desulfurization and so on. It is conveyed from the coal yard by the belt conveyor, through the electromagnet, the coal crusher, and then to the coal hopper in the coal warehouse, and then passes through the coal feeder into the coal mill for grinding. The ground coal powder passes through the air preheater. The hot air coming from the coal blows the pulverized coal to a coarse and fine separator. The coarse and fine separator sends qualified pulverized coal (the unqualified pulverized coal is sent back to the coal mill), and then sends it to the powder silo through the powder discharge machine. The burner is sent to the boiler for combustion. The flue gas is discharged through the electric dedusting to remove dust, and then the flue gas is sent to the desulfurization device. The gas discharged from the slurry spray is sent to the flue through the suction fan and discharged into the sky. ^[2]

Thermal Power Generation System

The power generation system is composed of a secondary exciter, a magnetic disk, a main exciter (spare exciter), a generator, a transformer, a high-voltage circuit breaker, a booster station, and a power distribution device. The power is generated by the secondary exciter (permanent magnet). The current from the secondary exciter is rectified by the excitation disk and then sent to the main exciter. After the main exciter sends electricity, it passes through the voltage regulator and the deactivation switch through the carbon brush. It is sent to the generator rotor. When the generator rotor rotates its stator coil, a current is induced. The strong current is divided into two lines through the generator outlet. One is sent to the factory electrical transformer, and the other is sent to the SF6 high-voltage circuit breaker. The SF6 high-voltage circuit breaker is sent to the grid. ^[6]