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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Thermal power plant. History of invention and production
Directory / The history of technology, technology, objects around us Thermal power plant (thermal power plant) - a power plant that generates electrical energy by converting the chemical energy of the fuel into mechanical energy of rotation of the shaft of the electric generator.
At thermal power plants, the thermal energy released during the combustion of fossil fuels (coal, peat, shale, oil, gases) is converted into mechanical and then into electrical energy. Here, the chemical energy contained in the fuel goes through a complex path of transformations from one form to another to obtain electrical energy. The conversion of energy contained in the fuel at a thermal power plant can be divided into the following main stages: the conversion of chemical energy into thermal energy, thermal energy into mechanical energy, and mechanical energy into electrical energy. The first thermal power plants (TPPs) appeared at the end of the 1882th century. In 1883, the TPP was built in New York, in 1884 - in St. Petersburg, in XNUMX - in Berlin. Most of the TPPs are thermal steam turbine power plants. On them, thermal energy is used in a boiler unit (steam generator).
One of the most important elements of the boiler unit is the furnace. In it, the chemical energy of the fuel is converted into thermal energy during the chemical reaction of the combustible elements of the fuel with atmospheric oxygen. In this case, gaseous combustion products are formed, which perceive most of the heat released during the combustion of the fuel. In the process of heating the fuel in the furnace, coke and gaseous, volatile substances are formed. At a temperature of 600-750 °C, volatile substances ignite and begin to burn, which leads to an increase in the temperature in the furnace. At the same time, the combustion of coke begins. As a result, flue gases are formed that exit the furnace at a temperature of 1000-1200 °C. These gases are used to heat water and produce steam. At the beginning of the XIX century. to obtain steam, simple units were used, in which heating and evaporation of water were not distinguished. A typical representative of the simplest type of steam boilers was a cylindrical boiler. For the developing electric power industry, boilers were required that produce high-temperature and high-pressure steam, since it is in this state that it gives the greatest amount of energy. Such boilers were created and they were called water tube boilers. In water tube boilers, flue gases flow around pipes through which water circulates, heat from flue gases is transferred through the walls of the pipes to water, which turns into steam.
Modern steam boiler works as follows. The fuel burns in a furnace with vertical pipes near the walls. Under the influence of the heat released during the combustion of fuel, the water in these pipes boils. The resulting steam rises into the boiler drum. The boiler is a thick-walled horizontal steel cylinder filled with water up to half. Steam is collected in the upper part of the drum and exits it into a group of coils - a superheater. In the superheater, the steam is additionally heated by flue gases leaving the furnace. It has a temperature higher than that at which water boils at a given pressure. Such steam is called superheated. After leaving the superheater, the steam goes to the consumer. In the boiler ducts located after the superheater, flue gases pass through another group of coils - a water economizer. In it, water before entering the boiler drum is heated by the heat of flue gases. Downstream of the economizer, along the flue gas path, air heater pipes are usually placed. In it, the air is heated before being fed into the furnace. After the air heater, flue gases at a temperature of 120-160 °C exit into the chimney. All working processes of the boiler unit are fully mechanized and automated. It is served by numerous auxiliary mechanisms driven by electric motors, the power of which can reach several thousand kilowatts. Boiler units of powerful power plants produce steam of high pressure - 140-250 atmospheres and high temperature - 550-580 °C. The furnaces of these boilers mainly burn solid fuel, crushed to a pulverized state, fuel oil or natural gas. The transformation of coal into a pulverized state is carried out in pulverized plants. The principle of operation of such an installation with a ball drum mill is as follows. The fuel enters the boiler room via belt conveyors and is discharged into the bunker, from which, after automatic scales, it is fed by a feeder to the coal mill. The grinding of fuel takes place inside a horizontal drum rotating at a speed of about 20 rpm. It contains steel balls. Hot air heated to a temperature of 300-400 °C is supplied to the mill through a pipeline. Giving part of its heat to fuel drying, the air is cooled to a temperature of about 130 ° C and, leaving the drum, carries the coal dust formed in the mill into the dust separator (separator). The dust-air mixture freed from large particles leaves the separator from above and goes to the dust separator (cyclone). In the cyclone, coal dust is separated from the air, and through the valve enters the coal dust bunker. In the separator, large dust particles fall out and return to the mill for further grinding. A mixture of coal dust and air is fed into the boiler burners. Pulverized coal burners are devices for supplying pulverized fuel and the air necessary for its combustion into the combustion chamber. They must ensure complete combustion of the fuel by creating a homogeneous mixture of air and fuel. The furnace of modern pulverized coal boilers is a high chamber, the walls of which are covered with pipes, the so-called steam-water screens. They protect the walls of the combustion chamber from sticking to them from slag formed during fuel combustion, and also protect the lining from rapid wear due to the chemical action of slag and the high temperature that develops when fuel is burned in the furnace. The screens perceive 10 times more heat per square meter of surface than the other tubular heating surfaces of the boiler, which perceive the heat of flue gases mainly due to direct contact with them. In the combustion chamber, coal dust ignites and burns in the gas stream carrying it. Boiler furnaces that burn gaseous or liquid fuels are also chambers covered with screens. A mixture of fuel and air is supplied to them through gas burners or oil burners. The device of a modern high-capacity drum boiler unit operating on coal dust is as follows. Fuel in the form of dust is blown into the furnace through the burners, together with part of the air necessary for combustion. The rest of the air is supplied to the furnace preheated to a temperature of 300-400 °C. In the furnace, coal particles burn on the fly, forming a torch, with a temperature of 1500-1600 °C. Non-combustible impurities of coal turn into ash, most of which (80-90%) is removed from the furnace by flue gases formed as a result of fuel combustion. The rest of the ash, consisting of stuck together particles of slag, accumulated on the pipes of the furnace screens and then detached from them, falls to the bottom of the furnace. After that, it is collected in a special shaft located under the firebox. The slag is cooled in it with a jet of cold water, and then it is carried out by water outside the boiler unit by special devices of the hydraulic ash removal system. The walls of the furnace are covered with a screen - pipes in which water circulates. Under the influence of heat radiated by a burning torch, it partially turns into steam. These pipes are connected to the boiler drum, which is also supplied with water heated in the economizer. As the flue gases move, part of their heat is radiated to the screen tubes and the temperature of the gases gradually decreases. At the exit from the furnace, it is 1000-1200 °C. With further movement, the flue gases at the outlet of the furnace come into contact with the tubes of the screens, cooling down to a temperature of 900-950 °C. In the gas duct of the boiler, tubes of coils are placed, through which steam passes, formed in the screen pipes and separated from the water in the boiler drum. In coils, the steam receives additional heat from the flue gases and superheats, i.e. its temperature becomes higher than the temperature of water boiling at the same pressure. This part of the boiler is called the superheater. After passing between the pipes of the superheater, flue gases with a temperature of 500-600 ° C enter the part of the boiler in which the pipes of the water heater or water economizer are located. Feed water with a temperature of 210-240 °C is supplied to it by a pump. Such a high water temperature is achieved in special heaters that are part of the turbine plant. In the water economizer, water is heated to the boiling point and enters the boiler drum. The flue gases passing between the pipes of the water economizer continue to cool and then pass inside the pipes of the air heater, in which the air is heated due to the heat given off by the gases, the temperature of which is then reduced to 120-160 °C. The air required for fuel combustion is supplied to the air heater by a blower fan and is heated there to 300-400 °C, after which it enters the furnace for fuel combustion. The flue, or outgoing, gases leaving the air heater pass through a special device - an ash catcher - for ash removal. Purified exhaust gases are emitted into the atmosphere through a chimney up to 200 m high by a smoke exhauster. The drum is essential in boilers of this type. Through numerous pipes, a steam-water mixture from the furnace screens enters it. In the drum, steam is separated from this mixture, and the remaining water is mixed with feed water entering this drum from the economizer. From the drum, water passes through pipes located outside the furnace into prefabricated collectors, and from them into screen pipes located in the furnace. In this way, the circular path (circulation) of water in drum boilers is closed. The movement of water and steam-water mixture according to the scheme drum - outer pipes - screen pipes - drum occurs due to the fact that the total weight of the steam-water mixture column filling the screen pipes is less than the weight of the water column in the outer pipes. This creates a pressure of natural circulation, providing a circular movement of water. Steam boilers are automatically controlled by numerous regulators, which are supervised by the operator. The devices regulate the supply of fuel, water and air to the boiler, maintain a constant water level in the boiler drum, the temperature of superheated steam, etc. The devices that control the operation of the boiler unit and all its auxiliary mechanisms are concentrated on a special control panel. It also contains devices that allow remotely performing automated operations from this shield: opening and closing all shut-off devices on pipelines, starting and stopping individual auxiliary mechanisms, as well as starting and stopping the entire boiler unit as a whole. Water-tube boilers of the described type have a very significant drawback: the presence of a bulky, heavy and expensive drum. To get rid of it, steam boilers without drums were created. They consist of a system of curved tubes, at one end of which feed water is supplied, and superheated steam of the required pressure and temperature exits from the other, i.e., water passes through all heating surfaces once without circulation before it turns into steam. Such steam boilers are called once-through. The scheme of operation of such a boiler is as follows. Feed water passes through the economizer, then enters the lower part of the coils, located helically on the walls of the furnace. The steam-water mixture formed in these coils enters the coil located in the boiler flue, where the conversion of water into steam ends. This part of the once-through boiler is called the transition zone. The steam then enters the superheater. After exiting the superheater, the steam is directed to the consumer. The air required for combustion is heated in the air heater. Once-through boilers allow you to get steam with a pressure of more than 200 atmospheres, which is impossible in drum boilers. The resulting superheated steam, which has a high pressure (100-140 atmospheres) and a high temperature (500-580 ° C), is able to expand and do work. This steam is transferred via main steam pipelines to the machine room, where steam turbines are installed. In steam turbines, the potential energy of steam is converted into mechanical energy of rotation of the steam turbine rotor. In turn, the rotor is connected to the rotor of the electric generator. The principle of operation and the device of a steam turbine are discussed in the article "Electric Turbine", so we will not dwell on them in detail. The steam turbine will be the more economical, i.e., the less heat it will consume for each kilowatt-hour generated by it, the lower the pressure of the steam leaving the turbine. To this end, the steam leaving the turbine is not directed into the atmosphere, but into a special device called a condenser, in which a very low pressure is maintained, only 0,03-0,04 atmospheres. This is achieved by lowering the temperature of the steam by cooling it with water. The steam temperature at this pressure is 24-29 °C. In the condenser, the steam gives up its heat to the cooling water and, at the same time, it condenses, i.e., it turns into water - condensate. The temperature of the steam in the condenser depends on the temperature of the cooling water and the amount of this water consumed for each kilogram of condensed steam. The water used to condense the steam enters the condenser at a temperature of 10-15 °C, and leaves it at a temperature of about 20-25 °C. Cooling water consumption reaches 50-100 kg per 1 kg of steam. The condenser is a cylindrical drum with two end caps. Metal boards are installed at both ends of the drum, in which a large number of brass tubes are fixed. Cooling water passes through these pipes. Between the tubes, flowing around them from top to bottom, steam from the turbine passes. The condensate formed during the condensation of steam is removed from below. During the condensation of steam, the transfer of heat from the steam to the wall of the tubes through which the cooling water passes is of great importance. If there is even a small amount of air in the steam, then the heat transfer from the steam to the tube wall deteriorates sharply; the amount of pressure that will need to be maintained in the condenser will also depend on this. Air that inevitably enters the condenser with steam and through leaks must be continuously removed. This is carried out by a special apparatus - a steam jet ejector. For cooling in the condenser of the steam that has worked out in the turbine, water from a river, lake, pond or sea is used. The consumption of cooling water at powerful power plants is very high and, for example, for a power plant with a capacity of 1 million kW, is about 40 m3 / s. If water is taken from the river to cool the steam in the condensers, and then, heated in the condenser, is returned to the river, then such a water supply system is called once-through. If there is not enough water in the river, then a dam is built and a pond is formed, from one end of which water is taken to cool the condenser, and heated water is discharged to the other end. Sometimes, to cool the water heated in the condenser, artificial coolers are used - cooling towers, which are towers about 50 m high. The water heated in the turbine condensers is supplied to trays located in this tower at a height of 6-9 m. Flowing out in jets through the holes of the trays and splashing in the form of drops or a thin film, the water flows down, while partially evaporating and cooling. The cooled water is collected in a pool, from where it is pumped to the condensers. Such a water supply system is called closed. We examined the main devices used to convert the chemical energy of fuel into electrical energy in a steam turbine thermal power plant. The operation of a coal-burning power plant is as follows. Coal is fed by broad gauge railway trains to the unloading device, where, with the help of special unloading mechanisms - car dumpers - it is unloaded from the cars onto belt conveyors. The stock of fuel in the boiler room is created in special storage tanks - bunkers. From the bunkers, the coal enters the mill, where it is dried and ground to a pulverized state. A mixture of coal dust and air is fed into the boiler furnace. When coal dust is burned, flue gases are produced. After cooling, the gases pass through the ash catcher and, having been cleaned of fly ash in it, are thrown into the chimney. Slags and fly ash from the ash collectors that have fallen out of the combustion chamber are transported by water through channels and then pumped to the ash dump. Combustion air is supplied by a fan to the boiler air heater. Superheated steam of high pressure and high temperature obtained in the boiler is fed through steam pipelines to the steam turbine, where it expands to a very low pressure and goes to the condenser. The condensate formed in the condenser is taken by the condensate pump and fed through the heater to the deaerator. The deaerator removes air and gases from the condensate. Raw water that has passed through the water treatment device also enters the deaerator to make up for the loss of steam and condensate. From the deaerator feed tank, feed water is pumped to the water economizer of the steam boiler. Water for cooling the exhaust steam is taken from the river and sent to the turbine condenser by a circulation pump. The electrical energy generated by the generator connected to the turbine is discharged through step-up electrical transformers through high voltage power lines to the consumer. The power of modern thermal power plants can reach 6000 megawatts or more with an efficiency of up to 40%. Thermal power plants can also use natural gas or liquid fuel gas turbines. Gas turbine power plants (GTPPs) are used to cover electrical load peaks. There are also combined-cycle power plants in which the power plant consists of steam turbine and gas turbine units. Their efficiency reaches 43%. The advantage of thermal power plants in comparison with hydroelectric power plants is that they can be built anywhere, bringing them closer to the consumer. They run on almost all types of fossil fuels, so they can be adapted to the type that is available in the area. In the mid 70s of the XX century. the share of electricity generated at thermal power plants was approximately 75% of the total generation. In the USSR and the USA it was even higher - 80%. The main disadvantage of thermal power plants is a high degree of environmental pollution with carbon dioxide, as well as a large area occupied by ash dumps. Author: Pristinsky V.L.
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