Power station boiler utility boiler
Boiler in thermal power plants that provides steam to steam turbine generator sets. Including boiler body and supporting auxiliary machines. After the fuel is burned in the furnace of the boiler, heat transfer is carried out through the metal wall to convert the water in the boiler into steam with specified parameters (temperature and pressure) and quality. Then the steam is sent to the steam turbine , and the generator is driven by the turbine to generate electricity.
Power plant boiler is one of the three major equipments of thermal power plants. (2006) The world's thermal power generation accounts for about 65% of the total power generation, and about 75% in China. Therefore, power plant boilers have an important impact on the development of the national economy, both in terms of their role and energy consumption.
1. Composition and principle
mainly consists of two parts: soda system and combustion system.
1.1. Soda and water system
feed water absorbs heat emitted from fuel combustion in the boiler. After three stages of preheating, vaporizing and overheating, it becomes superheated steam with specified parameters and quality. In order to improve the efficiency of steam power cycle, it is necessary to go through the fourth stage to overheat again. The superheated steam is sent to the high-pressure cylinder of the turbine, and after expansion, it cools down and returns to the boiler reheater, and then heats to a certain temperature, and then is sent to the medium-pressure cylinder of the turbine and the low-pressure cylinder of the to to continue expanding and performing work.
Preheating, vaporizing, overheating and re-superheating are carried out in economizer , evaporation heating surface (water-cooled wall, slag condensing tube , convection tube bundle), superheater and reheater , respectively.
1.2. Combustion system
Power station boiler fuel can be solid material, liquid fuel and gas fuel. The fuel characteristics determine the different preparation, supply and combustion methods of fuel.
Coal is the main fuel of thermal power plants, and coal-fired boilers in power stations are mainly suspended and fluidized bed combustion methods.
① Combustion system of pulverized coal boiler. The raw coal is heated and dried into coal powder by coal mill . There are two ways to send coal powder into the furnace and mix and burn with hot air from air preheater . One is a direct blowing type, that is, the coal powder grounded by the coal mill is sent directly into the furnace to burn; the other is an intermediate storage type, that is, the coal powder grounded by the coal mill is first sent to the coal powder bin for storage, and then the coal powder is sent into the furnace to burn with a powder feeder. The high-temperature flue gas generated by combustion flows through the furnace and the heating surfaces in the convection flue and transfers heat to water and air. When the flue gas leaves the boiler, the temperature is lower, and then it is discharged into the atmosphere through the dust collector , desulfurization and denitrification device, induced fan and chimney.
②Fluidized bed combustion system. The raw coal is broken into coal particles of a certain particle size, and enters the furnace through the feeder and is strongly mixed and calcified with the gas flowing in the furnace at high speed, a solid two-phase fluid. The main difference with the pulverized coal boiler is that the particle concentration in the fluidized bed combustion chamber is much greater than that in the pulverized coal boiler.
2. Structural type of boiler body
The structural type of the body mainly depends on factors such as fuel characteristics, boiler capacity and steam parameter . Common ones include Π type, tower type and box type (Figure 1).
2.1.Πtype
is inverted U-shaped, suitable for boilers of various capacity and various types of fuels. It is the most widely used form in power plant boilers. The boiler has a low height, is easy to install and lift, the heating surface is easy to arrange, and the heating surface at the tail is more convenient to repair. Fans, desulfurization and denitrification devices, dust collectors, etc. can all be placed on the ground but cover a large area.
2.2. Tower type
is suitable for burning multi-ash inferior bituminous coal and lignite . No turning flue can reduce the local wear of fly ash on the heating surface. The coal dust pipeline and burner are conveniently arranged, and the floor area is small. However, the furnace body is high, and when air preheater, dust collector, induced air fan and other equipment are placed on the top of the furnace, it is difficult to support the boiler frame and the installation and maintenance are more complicated.
2.3. Box type
is suitable for medium and large-capacity fuel and gas boilers . Except for the air preheater, each heating surface is arranged in a box furnace body. It has a small external dimension, simple structure, small footprint, good sealing performance, and is more convenient to connect the boiler and the steam turbine.The disadvantage is that the manufacturing process requirements are high and the installation and maintenance are difficult.
3. Burner and its layout
Burner is the main combustion equipment of power station boilers. Its function is to ensure that the fuel and combustion air are fully mixed, and timely ignite and stable combustion. According to its outlet air flow retention, it can be divided into DC burners and cyclone burners. The layout of
burner mainly adopts three methods: front wall layout, front and rear wall hedging layout and four corner layout. When arranged in the first two ways, a cyclone burner is generally used. The advantage is that the pulverized coal pipeline is relatively simple, but it is not suitable for fuels with low volatile and high ash content.
Four corner arrangement means that the DC burner is arranged at the four corners of the furnace. The geometric axis of the burner outlet air flow is tangent to one or several imaginary circles in the center of the furnace. The advantage is that the combustion is relatively stable and is suitable for coal (including bituminous coal, lignite and lean coal), oil, gas and other fuel boilers. The disadvantage is that the air duct layout is more complicated.
4. Water circulation method
The circulation of working fluid in the evaporation system of the power station boiler includes four methods: natural cycle, forced cycle, DC and composite cycle.
4.1. Natural cycle
Relying on the pressure difference generated by the density difference between the working fluid in the descending tube and the rising tube of the evaporation system. It is most commonly used for boilers from low pressure to high pressure to subcritical pressure, but the pressure in boiler drum is generally limited to less than 20 megapas.
4.2. Forced circulation
In addition to relying on the density difference between the water and soda mixture, the working fluid mainly relies on the pressure head of the circulation pump installed between the downward pipe and the rising pipe. Therefore, the arrangement of the evaporation heating surface is relatively free, and the diameter of the pot drum may be smaller. Mainly used in boilers with subcritical pressure.
4.3. The DC boiler
water feed depends on the pressure head of feed pump , and the heating surfaces at all levels are finally all superheated steam of the required pressure and temperature through heating, evaporation and superheating. The DC boiler does not have a boiler drum. The boiler has a small water capacity and corresponding heat storage capacity, and the salt brought into the boiler cannot be removed by continuous sewage discharge, so the requirements for automatic control and water treatment are very high. Widely used in units above high pressure, it can be applied to supercritical pressure and above parameters.
4.4.Composite cycle
combines the DC boiler and the forced circulation boiler, and adds a circulation pump in the DC boiler steam system. Suitable for subcritical and supercritical pressure boilers. The soda system of composite cycle boiler has many arrangements.
At high load, the circulating pump is used as boost pump , and the system operates in the DC boiler mode. When recirculation is put into recirculation below a certain load, the flow rate through the water-cooled wall is the sum of the feed water flow rate and the recirculation flow rate. The characteristic of this system is that it reduces the difference in flow velocity in the water-cooled wall under high and low loads, which is conducive to low load operation, and the flow resistance at high loads is not too large.
A composite circulation system with subcritical pressure, also known as low cycle rate boiler . In this system, the evaporation heating surface outlet is equipped with an steam and water separator . The cyclic ratio K≤2 at the rated load is generally between 1.3 and 1.8. Compared with pure DC boilers, the evaporation system of the low-cyclic ratio boiler has a smaller resistance and is more suitable for transformer operation. Moreover, the diameter of the separator used is much smaller than that of a general boiler.
5. Development trend
Power station boilers are: improve steam parameters, increase stand-alone capacity, environmental protection and clean coal combustion, garbage boilers, etc.
5.1. Improve steam parameters
Increase steam pressure and superheated steam temperature, which can improve the efficiency of the power station steam turbine generator set. Taking the overheating and reheating steam temperature of the subcritical unit both at 538℃ as an example, when the steam temperature is kept unchanged and the outlet pressure is increased from 16.5 MPa to supercritical 24.2 MPa, the power plant efficiency can be improved by 1.8%. If the temperature remains unchanged, the pressure will be further increased, and the efficiency of the power station will not be much improved.
If the outlet steam pressure remains unchanged by 24.2 megapas and the steam temperature is increased from 538°C to 621°C, the power plant efficiency can be improved by 3.7%. When the steam pressure is 30-31 megapas, the steam temperature is 580-600℃, the unit efficiency can reach 45%-49%.
With the successful development of three types of high-strength heat-resistant steel with good process performance, high-parameter power plant boilers have been widely used.
5.2. Increasing the stand-alone capacity
Increasing the stand-alone capacity can reduce the equipment cost of the unit power (kilowatts) of the power station, but the equipment cost does not decrease proportionally with the increase of the stand-alone capacity. The development direction of power plant boilers is generally to increase the capacity of a single machine, but when the capacity exceeds a certain value, the economy will be reduced. It is still under discussion to what extent the maximum capacity of a single machine should be increased.
5.3. Environmental protection and clean coal combustion
The flue gas emitted from the power station boiler contains toxic and harmful substances such as sulfur oxide , nitrogen oxides and carbon dioxide , and the emissions account for about one-quarter of the total amount of various pollutants.
Clean coal combustion technology can control the formation of sulfur oxides and nitrogen oxides during the burning process, or remove sulfur oxides and nitrogen oxides from flue gas to ensure that pollutants emissions meet environmental protection requirements.
Since the 1990s, circulating fluidized bed boiler (CFB), coal combined cycle units using advanced combustion technology [Pressure fluidized bed coal-fired combined cycle (PFBC-CFB) and integrated coal gasification combined cycle (IGCC)] have been widely used and are developing towards large-scale development.
Whether from the perspective of improving power station efficiency or from the control of pollutant emissions, clean coal burning technology will be a development direction for power station boilers.
5.4. Garbage Boiler
Boiler fueled by urban domestic waste or solid waste. It aims to reduce, harmless and resource utilization of urban garbage. The capacity parameters and efficiency of the boiler are not high, but the social benefits are obvious.
The garbage resource rate in developed countries has reached 50%, and China is still in its infancy. Due to the strict emission control of waste power plants and the high price of flue gas treatment equipment, the garbage power plants have affected their development to a certain extent.
excerpted from: "China Encyclopedia (2nd Edition)" Volume 5, China Encyclopedia Press , 2009
