Questions and answers on power plant boilers

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1. What is the function of a boiler? Answer: The heat released by the combustion of fuel in the furnace is used to heat the feed water, generating steam that meets specified parameters (temperature, pressure) and quality standards, which is then sent to the turbine to produce power. 2. What are absolute pressure, gauge pressure, and vacuum? Answer: The actual pressure of the working fluid itself is called absolute pressure, denoted by P. The local atmospheric pressure is denoted as Pamb. When the absolute pressure is higher than atmospheric pressure, the value indicated by the pressure gauge is called gauge pressure, denoted as Pe; that is, P = Pe + Pamb. When the absolute pressure of the working fluid is lower than atmospheric pressure, the value indicated by the pressure gauge is called vacuum, denoted as Pv, that is, Pv = Pamb – P. 　　 3. What are the five states that occur during the constant-pressure formation of water vapor? Answer: unsaturated water state, saturated water state, wet saturated steam state, dry saturated steam state, superheated steam state. 4. Is there unsaturated water at 300°C? Why? Answer: Yes, because the saturation temperature increases as the saturation pressure increases. 5. What are the three basic modes of heat transfer? Answer: Heat conduction, convection, and thermal radiation. 6. Why are insulation materials all porous? Answer: Because there is air inside the hole; air has a very low thermal conductivity and remains stationary. 7. Why can’t the lubricating oil temperature be too high? Answer: Due to excessively high oil temperature, the viscosity decreases, causing the oil film to become thinner or uneven in thickness. This prevents the formation of a continuous and uniform oil film, which can increase the vibration of the unit and pose a threat to its safety. To maintain an appropriate liquid viscosity, the outlet oil temperature of general lubricants is typically controlled at around 45°C. 8. What is water hammer? How to prevent and eliminate it? Answer: When a liquid flows through a pressurized pipe, due to some external disturbance, its flow velocity suddenly changes, causing repeated and abrupt changes in pressure within the pipe. This phenomenon is known as water hammer. 9. What are the different categories of vane pumps and fans? What is its working principle and scope of use? Answer: Vane pumps and fans are classified into centrifugal, axial flow, and mixed-flow types. The working principles are as follows: centrifugal force ； lift ； Part of it is centrifugal force, and part is lift. Their application ranges are respectively: low flow rate, high head ； High flow rate, low head pressure ； High flow rate, low head. 10. What are the internal losses in centrifugal pumps and fans? Answer: Mechanical losses, volumetric losses, and kinetic losses. 11. Briefly describe the production process in a thermal power plant. Answer: Coal is fed into the boiler to burn; the heat released heats the water, causing it to evaporate into saturated steam. After being heated in a superheater, this steam becomes superheated steam at a certain temperature and pressure. It is then sent to the turbine to generate power, with the high-speed steam flow driving the turbine rotor and, in turn, the generator rotor, thereby producing electricity. The steam that has done work in the turbine is discharged into the condenser, where it condenses into water. After being pressurized by the condensate pump, this water flows through the low-pressure heater; it is then heated using steam extracted from the turbine before being sent to the deaerator for further heating and deoxygenation. Finally, after being pressurized by the feed pump, it passes through the high-pressure heater before being returned to the boiler. The production process in a thermal power plant is one that continuously repeats the aforementioned cycle. 12. What are the three types of energy conversion that occur during the production process in thermal power plants? Answer: In a boiler, the chemical energy of the fuel is converted into thermal energy ； In a steam turbine, thermal energy is converted into mechanical energy ； In a generator, mechanical energy is converted into electrical energy. Boilers, steam turbines, and generators are known as the three main components of a thermal power plant. 13. What are the hazards of an excessive temperature difference between the upper and lower walls of the steam drum? Answer: During the startup and shutdown of a boiler, the temperature of the upper wall of the drum is generally higher than that of the lower wall, resulting in a temperature difference between the upper and lower walls of the drum. The upper wall has a high temperature and tends to expand, but the low temperature of the lower wall prevents this expansion. As a result, the upper wall of the steam drum is subjected to compressive stress while the lower wall is under tensile stress; this causes the steam drum to bend upward, generating significant thermal stress. When the temperature difference between the upper and lower walls of the steam drum exceeds the allowable limit, microcracks may appear in the welds on the steam drum. Over time, this can lead to leaks in the steam drum, causing damage to equipment and posing risks to human safety. 14. What equipment does a boiler consist of? Answer: A power plant boiler consists of the boiler main equipment and auxiliary equipment. The main boiler equipment includes the steam-water system and combustion system, as well as the furnace wall and boiler frame. The steam system mainly consists of a economizer, a steam drum, downcomers, water wall, superheater, reheater, header, etc. The combustion system mainly consists of a furnace, flue ducts, burners, air preheaters, etc. The auxiliary equipment for boilers mainly includes ventilation equipment, coal conveying equipment, powder preparation equipment, water supply equipment, dust removal equipment, ash removal equipment, automatic control equipment, water treatment equipment, as well as boiler accessories (safety valves, level gauges, soot blowers, thermal instruments), etc. 15. Briefly describe the working process of a boiler. Answer: The operating process of a boiler includes the operation process (flow) of the combustion system and the operation process (flow) of the steam and water system. The operation process of the combustion system is as follows: Coal is delivered to the raw coal hopper by a coal conveyor belt, and then fed into the coal grinder through a coal feeder to be ground into coal powder ； At the same time, a stream of hot air from the air preheater is also sent into the coal grinder to dry the coal powder and carry it out to the coarse powder separator. In the coarse powder separator, the coarse powder separated by the partition is taken out and sent back to the coal grinder for further grinding. The qualified coal powder stream, acting as primary air, is fed into the furnace for combustion through the primary air nozzles of the burner ； To ensure complete combustion of the fuel, another stream of hot air from the air preheater is used as secondary air, which is fed into the furnace through the secondary air nozzles of the burner to assist combustion and promote mixing. Fuel burns in the furnace, and the flue gases generated by this combustion flow sequentially through the water wall, superheater, reheater, economizer, and air preheater. The heat from the flames and flue gases heats the fluid or air present in the heating surfaces. After passing through the dust collector to have the dust removed, the flue gases are sent to the chimney by an exhaust fan and released into the atmosphere. After being crushed, the slag, together with the ash removed by the dust collector, is pumped to the ash plant using an ash pump. The working process of the steam system is as follows (taking a natural circulation boiler as an example): The feed water first enters the economizer, where it absorbs heat from the flue gases at the rear of the boiler; after being heated, the feed water is sent to the steam drum ； The water in the drum is sent via downcomers to the lower header of the water wall, where it supplies the water wall ； Water in the water wall absorbs the radiant heat from the flames and flue gases in the furnace, causing some of the water to evaporate and form a vapor-water mixture. This mixture then returns to the steam drum where vapor and water are separated; the separated water proceeds to the downcomer for further water circulation ； The separated saturated steam is drawn out from the top of the drum; after being heated in a heater to become superheated steam at a certain temperature, it is sent to the turbine to generate power. 16. What are boiler capacity and steam parameters? Answer: The boiler capacity refers to the boiler’s evaporation rate, and it is the basic characteristic data that indicates the amount of energy the boiler can produce. Large boilers are further divided into rated evaporation capacity and maximum continuous evaporation capacity. The boiler’s rated evaporation capacity (BECR) refers to the amount of steam specified under rated steam parameters, rated feedwater temperature, use of the designed fuel, and with guaranteed thermal efficiency. The maximum continuous evaporation rate (BMCR) refers to the maximum evaporation rate that can be achieved per hour under rated steam parameters, rated feedwater temperature, and the use of the designed fuel during long-term continuous operation, expressed in t/h. The steam parameters are the steam pressure (MPa) and temperature (°C) at the superheater outlet. 17. Based on the way the working fluid moves within the evaporation heating surface, what are the different types of power plant boilers? Answer: Natural circulation boilers, controlled circulation boilers (also known as forced circulation boilers or auxiliary circulation boilers), once-through boilers, and combined cycle boilers. 18. What are the elemental components of coal? Answer: The elemental components of coal are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), moisture (M), and ash (A). Among them, carbon, hydrogen, and sulfur are the combustible components ； Sulfur, moisture, and ash are the main harmful components. 19. What are the components of an industrial analysis of coal? Answer: The components of industrial analysis for coal include moisture, volatile matter, fixed carbon, and ash. Among them, volatile matter and fixed carbon are the combustible components. The amount of volatiles has a significant impact on the occurrence and progression of coal combustion; the higher the volatile content, the easier it is for ignition to occur and the more complete the combustion. 20. What are the categories of coal used in power plants? What are the characteristics of combustion? Answer: Coal used in power plants is mainly classified into anthracite, lean coal, bituminous coal, lignite, etc., based on its volatile matter content on a dry, ash-free basis. Among them, anthracite (Vdaf≤10%) has a high calorific value, but is difficult to ignite and burns slowly ； Low-grade coal (10%)