Discussion on economic operation technology of boiler
The operation of boiler sets largely determines the economy of the whole power plant. The main index to measure the economy of coal-fired power plants is the coal consumption of power supply. The size of power supply coal consumption depends on power generation coal consumption and auxiliary power consumption rate, and the main factor affecting power generation coal consumption is boiler efficiency. Therefore, it is of great significance to study the economic operation mode of power plant boilers to improve the economy of power plants.
Keywords boiler, economy, coal burning
1, overview. Boiler is an important heat energy supply equipment in national economy. Electric power, textile, paper making, food, machinery, metallurgy, chemical industry and industrial and civil heating all need boilers to provide a lot of heat energy. Boiler is a combustion equipment that converts the chemical energy of fuel into heat energy. It provides good combustion conditions as much as possible, releases the chemical energy of fuel to the maximum extent and converts it into heat energy, which is used to heat the water in the pot.
2. Classification of boilers. Boilers are divided into the following categories in different ways: household boilers, industrial boilers, power station boilers and hot water boilers according to their uses. According to the classification of fuel used in boilers, it can be divided into coal-fired stoves, oil-fired stoves and gas-fired stoves. According to the classification of combustion methods, it can be divided into: layer combustion furnace, chamber combustion furnace and boiling (fluidized bed) furnace between them. According to the existence of steam drum, it can be divided into drum boiler and once-through boiler. According to the classification of steam pressure, it can be divided into low-pressure boilers, medium-pressure boilers, secondary high-pressure boilers, high-pressure boilers, ultra-high-pressure boilers, subcritical pressure boilers and supercritical pressure boilers. According to the classification of boiler water circulation mode, it can be divided into natural circulation boiler, forced circulation boiler and compound circulation boiler.
3. Application of boiler. The hot water or steam generated by the boiler can directly provide the heat energy needed for production and life, and can also be converted into mechanical energy by steam power plant or into electrical energy by generator. Boilers that provide hot water are called hot water boilers, which are mainly used in life and have a small number of applications in industrial production. The boiler that produces steam is called steam boiler, which is an important part of steam power plant and is mostly used in thermal power plants, ships, locomotives and industrial and mining enterprises.
4, the structure of the boiler. Boiler is the main component of thermal power generation equipment. The main components in the boiler, such as furnace, drum, burner, water wall superheater, economizer, air preheater, frame and furnace wall, constitute the core part of steam production, which is called boiler body. Is it by? Marijuana? And then what? Furnace? It consists of two parts. ? Marijuana? It is a steam-water system, and its main task is to attract the heat released by fuel, so that water can be heated and evaporated, and finally become hot water or superheated steam with certain heat energy. It consists of economizer, drum, downcomer, header, water wall, superheater and reheater, and their connecting pipes and valves. Furnace, also known as combustion chamber, is the space where fuel burns. The main function of steam drum is to store water, separate steam from water, and remove brine and sludge from boiler water during operation to prevent boiler water with high salt and impurity concentration from entering superheater with steam.
5. The working principle of the boiler. Boiler mainly has the following systems to complete the chemical energy of fuel and make the steam have enough kinetic energy (taking pulverized coal boiler as an example): steam-water system, flue gas system, fuel (pulverized coal and auxiliary fuel) system, pulverizing system, ash removal system, etc. The pulverizing system is used to grind qualified pulverized coal and store it in the powder bin. Through the powder feeder, it is sent into the furnace by primary air for combustion. Coal powder and high temperature flue gas are fully mixed and burned in the furnace, which heats the water wall and produces a large amount of high temperature flue gas at the same time. The flue gas cooled by radiation, semi-radiation and semi-convection of low-temperature and high-temperature superheaters at all levels is discharged into the atmosphere from the chimney after the dust in the flue gas system reaches the standard by electrostatic precipitator and bag dust collector, and the feed water in the furnace is absorbed by all levels to form high-temperature and high-pressure steam. The slag produced by pulverized coal combustion is transported out through the ash removal system.
6. Maintenance of the boiler. In the daily operation of the boiler, the auxiliary machines of each system operate normally. Attention should be paid to maintaining all parameters within the allowable range, strictly controlling the pressure and temperature exceeding the standard, and regularly discharging pollutants to maintain the qualified soda quality and prolong the service life of the equipment. After the boiler is shut down, it still needs maintenance. The method of boiler maintenance is to reduce boiler corrosion by minimizing dissolved oxygen in boiler water and leakage of external air. The most common maintenance methods are generally wet maintenance, nitrogen replacement, drying and anti-corrosion maintenance.
7. Economic operation of boiler. The operation of boiler sets largely determines the economy of the whole power plant. The main index to measure the economy of coal-fired power plants is the coal consumption of power supply. The size of power supply coal consumption depends on power generation coal consumption and auxiliary power consumption rate, and the main factor affecting power generation coal consumption is boiler efficiency. Therefore, it is of great significance to study the economic operation mode of power plant boilers to improve the economy of power plants.
Because of the different combustion conditions, temperature levels, pollution discharge and heat exchange States and auxiliary power consumption of heating surfaces at all levels, the operating economy of fuel in the furnace is also different. Careful combustion adjustment tests must be carried out to obtain the best operating conditions under various loads as the basis for daily operation adjustment, so as to ensure the good economic operation of boiler units. In operation, we should master the characteristics of burner, the ratio of air volume, the adjustment of primary air pulverized coal concentration and air volume according to the change of coal type, and emphasize the scientific adjustment of combustion conditions to make the combustion in the furnace in the best state. In order to make the fuel fully mixed with oxygen in the furnace, the actual amount of air sent into the furnace is always greater than the theoretical amount of air. Although more air can reduce the heat loss of incomplete combustion, the heat loss of exhaust gas will increase, which will also aggravate the corrosion of sulfur oxides and the generation of nitrogen oxides. Therefore, in addition to reasonable air-powder ratio, adjustment of flame plumpness and proper flame burning center, we should also improve the combustion technology according to the performance test of the boiler, and strive to make the combustion in the furnace complete with the minimum excess air coefficient.
Pulverized coal boilers usually take the following measures to improve the economic performance of boilers:
7. 1 Reasonable coal blending to ensure coal quality. Carefully mix all kinds of coal, reduce the sharp change of coal burning, and maintain the basic stability of operating parameters.
7.2 Adjust the fineness of pulverized coal reasonably. The fineness of pulverized coal is the main factor affecting the combustible content of fly ash. The fineness of economic pulverized coal should be selected according to thermal test.
7.3 Control the appropriate excess air coefficient. Pulverized coal combustion needs enough oxygen, but too much cold air will reduce the temperature level in the furnace and increase the exhaust air. Reasonable excess air coefficient should be determined according to combustion adjustment test and coal type.
7.4 Pay attention to combustion adjustment. Combustion condition, temperature level and the difficulty of pulverized coal ignition in the furnace directly affect the combustible content of ash.
In order to check the performance and improve the design, the boiler often needs to carry out heat balance test. The method to calculate the thermal efficiency of boiler directly from the effective utilization of energy is called positive balance, and the method to calculate the efficiency from various heat losses is called anti-balance. When considering the actual benefit of the boiler, we should not only look at the thermal efficiency of the boiler, but also consider the energy consumed by the boiler auxiliary equipment. When the fuel per unit mass or volume is completely burned, the air demand calculated according to the chemical reaction is called theoretical air quantity. In order to make the fuel have more opportunities to contact with oxygen and burn in the furnace, the actual amount of air sent into the furnace is always greater than the theoretical amount of air. Although more air can reduce the heat loss of incomplete combustion, the heat loss of exhaust gas will increase, which will also aggravate the corrosion of sulfur oxides and the generation of nitrogen oxides. Therefore, efforts should be made to improve the combustion technology and strive to make the combustion in the furnace complete with the minimum excess air coefficient.
8. Dust (including fly ash and unburned coal powder), sulfur and nitrogen oxides contained in discharged boiler flue gas are all substances that pollute the atmosphere, and their emission indexes can reach several times to dozens of times the limit indexes of environmental protection laws and regulations without purification. Measures to control the emission of these substances include pretreatment before combustion, improvement of combustion technology, dust removal, desulfurization and denitrification. With the help of chimneys, the concentration of pollutants in the air near chimneys can only be reduced, and pollutants cannot be completely eradicated. The forces used in flue gas dust removal include gravity, centrifugal force, inertia force, adhesion, sound wave and static electricity. Generally, coarse particles are separated by gravity settlement and inertia force, while electrostatic precipitator and bag precipitator are often separated by centrifugal force under large capacity, which has high dust removal efficiency. Wet and warm? The drip film in water film dust collector can adhere to fly ash, which has high dust removal efficiency and can adsorb gaseous pollutants. In order to achieve high dust removal efficiency, coal-fired units usually adopt multi-stage dust removal, electric dust removal, bag dust removal and so on. Through desulfurization and out-of-stock, the indexes of flue gas meet the requirements of national standards.
9. Development of boilers. Boilers will develop in the direction of further improving the thermal efficiency of boilers and power stations in the future; The unit power equipment cost of boilers and power stations will be further reduced; It will greatly improve the operation flexibility and automation level of boiler units; More boiler varieties will be developed to adapt to different fuels; Will continue to improve the operation reliability of boiler units and their auxiliary equipment; We will do our best to take measures to reduce environmental pollution.
References:
[1] Zhang Aicun. Operation adjustment and economic analysis of coal-fired boilers in power plants [D]. Graduation thesis of North China Electric Power University, 2003.