Several ways of thermal power generation

The electric power industry is an important basic industry for the development of the national economy and society. Electricity is not only a means of production that promotes the development of the national economy, but also an indispensable means of living in people’s lives. The main power generation methods in my country are thermal power and hydropower, which account for more than 90% of the total installed capacity. Thermal power generation has the longest history, and the electricity it provides accounts for more than 70% of all power generation methods in my country. Although in recent years, with the development of new energy, its proportion has declined, but for a long period of time in the future, thermal power will still be the main power generation method in my country.

The production process of thermal power generation can be briefly summarized as: the chemical energy of fuel (such as coal, oil, natural gas, etc.) is converted into heat energy of water vapor through combustion in a boiler, and then the heat energy of water vapor is converted into heat energy in a steam turbine. The mechanical energy drives the generator, which is finally converted into electrical energy through the action of the generator and output outwards. It can be seen that the thermal power generation system is composed of boilers, steam turbines, generators and other main equipment and other auxiliary equipment.

There are various categories of thermal power generation. According to the power generation method, it can be divided into gas turbine power generation, gas-steam combined cycle power generation, internal combustion engine power generation and steam turbine power generation (usually what we call thermal power generation, mainly refers to steam turbine power generation); according to the different fuels, it can be divided into combustion Coal power generation, oil-fired power generation, gas power generation, waste heat power generation, etc.; according to the difference of steam pressure and temperature, it can be divided into medium and low pressure power generation, high pressure power generation, ultra-high pressure power generation, subcritical pressure power generation, supercritical pressure power generation, and ultra-supercritical pressure power generation Power generation, etc.; depending on the output energy, it can be divided into condensing power generation and thermal power.

my country has abundant coal reserves and is the main fuel used in thermal power plants. At present, domestic thermal power mainly uses steam turbines to generate electricity. The following takes a coal-fired condensing steam power plant as an example (as shown in Figure 1) to introduce the process and equipment of the thermal power generation system.

Several ways of thermal power generation
Figure 1 Schematic diagram of the production process of a coal-fired condensing power plant

Thermal power generation systems mainly include combustion systems, soda systems, electrical systems, and control systems.

The combustion system consists of a fuel system, an air supply system, etc., including coal conveying, coal grinding, combustion, air smoke, ash and slag. First, the pretreated coal is sent to the boiler raw coal bunker through the coal conveying device, and then sent to the coal grinder through the coal feeder to be ground into pulverized coal, and then transported by air through the burner and sent to the combustion chamber. At the same time, the cold air is sent to the air preheater by the fan and heated into hot air, part of which is used as a desiccant in the pulverized coal preparation system and transported pulverized coal into the furnace (primary air), and the other part directly enters the burner ( Secondary air). When pulverized coal and air are fully mixed and burned in the furnace, a large amount of high-temperature flue gas is released. Then, the high-temperature marriage gas conducts intense heat exchange with the water-cooled walls arranged on the four walls of the furnace and the superheater above the furnace. The large particles of ash and slag formed after the combustion of pulverized coal are discharged from the bottom of the furnace, and the small particles of fly ash flow up into the flue with the flow of marriage gas. When the flue gas flows through the convection superheater, reheater, economizer and air preheater, convective heat exchange is performed, and the heat of the flue gas is transferred to the steam, water and air flowing in the tube. After the heat exchange, the flue gas passes through the dust collector to remove most of the fly ash, and after passing through the flue gas purification device (desulfurization, denitration), it is sent to the chimney through the induced air and discharged to the atmosphere.

The steam and water system consists of boilers, steam turbines, deaerators, condensers, high and low pressure heaters, condensate pumps and feed water pumps, including make-up water system, water supply system and circulating water system. After the raw water is strictly desalinated by the chemical water treatment system, it enters the desalinated water tank (make-up water), and enters the water supply system after deoxidizing by the deaerator. The boiler feed water is pressurized by the feed water pump and sent to the economizer in the tail flue. After the feed water is heated in the economizer to close to saturation, it is guided through a duct to the steam drum arranged on the top of the furnace. The water in the steam drum descends along the downpipe outside the furnace wall to the lower header of the water wall, and is distributed to the parallel water wall pipes through the lower header. Saturated water in the water wall accepts the radiation heat outside the tube to become a steam-water mixture flowing upwards, and is introduced into the steam drum through the duct. The steam-water mixture entering the steam drum is separated by the steam-water separator, and the separated water is again with the economizer incoming water. It enters the water wall pipe through the downcomer and the lower header to be heated to complete the next cycle. The saturated steam leaving the outside leads to the superheating system. After being superheated to the specified temperature, it is sent to the high-pressure cylinder of the steam turbine through the main steam pipe to expand and perform work. After the temperature returns to the middle and low pressure cylinders of the steam turbine, it continues to expand and do work.

The electrical system consists of generators, excitation devices, auxiliary power systems and booster substations. The terminal voltage and current of the generator vary with the capacity, the rated voltage is generally between 10-24kV, and the rated current can reach more than 20kA. A small part of the electrical energy generated by the generator is stepped down by the factory transformer, and then supplied to the blower, coal mill, water supply pump and other factory-use electrical equipment through the factory-use power distribution device. After the main transformer is boosted, it is sent to the power grid via high-voltage power distribution devices and transmission lines.

The control system is composed of various control devices to ensure the safe, reasonable and economical operation of each system.