1. What is the role of the boiler?
Answer: The heat released by the combustion of the fuel in the furnace is used to heat the feed water to produce steam that meets the specified parameters (temperature, pressure) and quality, and send it to the steam turbine for work.
2. What is 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 represented by Pamb. When the pressure is higher than the atmospheric pressure, the value indicated by the pressure gauge is called the gauge pressure, expressed in Pe, that is, P=Pe+Pamb.
When the absolute pressure of the working fluid is lower than the atmospheric pressure, the value indicated by the pressure gauge, called vacuum, is represented by Pv, that is, Pv=Pamb-P.
3. What are the five states when water vapor is formed at a constant pressure?
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℃? why?
Answer: Yes, because the saturation temperature increases with the saturation pressure.
5. What are the three basic ways of heat transfer?
Answer: Heat conduction, convection, heat radiation.
6. Why are thermal insulation materials porous?
Answer: Because there is air in the hole, the thermal conductivity of the air is very small and inactive.
7. Why can't the lubricating oil temperature be too high?
Answer: Because the oil temperature is too high, the viscosity is reduced, the oil film becomes thin, or the thickness is uneven, and a continuous and uniform oil film cannot be formed, which will increase the vibration of the unit and endanger the safety of the unit. In order to maintain proper liquid viscosity, the outlet temperature of general lubricating oil is controlled at about 45°C.
8. What is water hammer? How to prevent and eliminate it?
Answer: When the liquid moves in the pressure pipeline, due to some external disturbance, the speed of the liquid movement suddenly changes, causing repeated and rapid changes in the pressure in the pipeline. This phenomenon is called water hammer.
9. What are the types of vane pumps and fans? What is its working principle and scope of use?
Answer: Vane pumps and fans are divided into centrifugal, axial and mixed flow. The working principles are: centrifugal force; lift; part is centrifugal force and part is lift. The scope of application is: small flow, high head; large flow, low head; large flow, low head.
10. What are the losses in centrifugal pumps and fans?
Answer: mechanical loss, volume loss, activity loss.
11. Briefly describe the production process of thermal power plants.
Answer: The coal is sent to the boiler for combustion, and the heat released will heat the feed water and evaporate into saturated steam. After being heated by the heater, it becomes superheated steam with a certain temperature and pressure, which is sent to the steam turbine to perform work. The high-speed steam flow drives the steam turbine rotor and drives power generation. The rotors of the machine rotate together to generate electricity.
The steam that has completed the work in the steam turbine is discharged into the condenser and condensed into water. After being boosted by the condensing water pump, it flows through the low-pressure heater and is heated by the extraction steam of the steam turbine and then sent to the deaerator for heating and deoxygenation. After the feed water pump is boosted, it is further heated by the high-pressure heater and then sent back to the boiler. The production process of a thermal power plant is a process of continuously repeating the above-mentioned cycle.
12. What three types of energy conversion exist in the production process of thermal power plants?
Answer: In the boiler, the chemical energy of the fuel is converted into heat energy; in the steam turbine, the heat energy is converted into mechanical energy; in the generator, the mechanical energy is converted into electrical energy. Boilers, steam turbines and generators are called the three main engines of thermal power plants.
13. How dangerous is the temperature difference between the upper and lower walls of the steam drum that exceeds the limit?
Answer: During the startup and shutdown of the boiler, the temperature of the upper wall of the steam drum is generally higher than that of the lower wall, so that a temperature difference is formed between the upper and lower walls of the steam drum.
The upper wall has a high temperature and there is a tendency to expand, but due to the low temperature of the lower wall, the expansion of the upper wall is prevented. The upper wall of the steam drum bears compressive stress, while the lower wall bears tensile stress, so that the steam drum will be deformed upward. Form a greater thermal stress.
When the temperature difference between the upper and lower walls of the steam drum exceeds the limit, it will cause micro cracks in the weld on the steam drum.
14. What equipment does the boiler consist of?
Answer: The power plant boiler is composed of the boiler body equipment and auxiliary equipment.
The boiler body equipment includes the steam and water system and the combustion system, as well as the furnace wall and the boiler frame.
The soda water system is mainly composed of economizer, steam drum, downcomer, water wall, superheater, reheater, header, etc.
The combustion system is mainly composed of furnace, flue, burner, air preheater, etc.
Boiler auxiliary equipment mainly includes ventilation equipment, coal conveying equipment, pulverizing equipment, water supply equipment, dust removal equipment, ash removal equipment, automatic control equipment, water treatment equipment and boiler accessories (safety door, water level gauge, soot blower, thermal instrument) Wait.
15. Briefly describe the working process of the boiler.
Answer: The working process of the boiler includes the working process (process) of the combustion system and the working process (process) of the steam-water system.
The working process of the combustion system is as follows: the coal is sent from the coal conveyor to the raw coal hopper, and then sent to the coal mill through the coal feeder to be ground into pulverized coal; at the same time, a stream of hot air from the air preheater is also sent to the coal mill. The pulverized coal is dried and transported out, into the coarse powder separator.
In the coarse powder separator, the coarse powder in the different grids is separated and sent back to the coal mill for grinding. Qualified pulverized coal air flow is used as primary air and sent into the furnace for combustion through the primary air nozzle of the burner;
In order to ensure the complete combustion of the fuel, another hot air from the air preheater is used as the secondary air and sent into the furnace from the secondary air nozzle of the burner to support combustion and disturbance.
The fuel is burned in the furnace, and the flue gas produced by the combustion flows through the water wall, superheater, reheater, economizer and air preheater in turn. The heat of the flame and flue gas heats the working fluid or air in the heating surface, and the flue gas After leaving the boiler to be dusted by the dust collector, the flue gas is sent to the chimney by the induced draft fan and discharged to the atmosphere.
The slag is crushed and sent to the ash plant together with the ash removed by the dust collector.
The working process of the steam water system is (take the natural water circulation boiler as an example): the feed water first enters the economizer, absorbs the heat of the flue gas at the boiler tail, and the feed water is heated and sent to the steam drum; the water in the steam drum is sent to the water cooling through the downcomer Header under the wall, supplying the water wall;
The water absorbs the radiant heat of the flame and flue gas in the water wall, and evaporates part of the water to form a steam-water mixture, and then returns to the steam drum for steam-water separation, and the separated water enters the downcomer for water circulation;
The separated saturated steam is led from the top of the steam drum, heated by a heater to superheated steam with a certain temperature, and then sent to the steam turbine to perform work.
16. What are boiler capacity and steam parameters?
Answer: The boiler capacity is the boiler evaporation capacity, which is the basic characteristic data reflecting the amount of energy produced by the boiler. Large boilers are divided into rated evaporation capacity and maximum continuous evaporation capacity.
Boiler rated evaporation capacity (BECR) refers to the amount of steam specified when rated steam parameters, rated feed water temperature, using design fuels and ensuring thermal efficiency.
The maximum continuous evaporation capacity (BMCR) refers to the maximum evaporation capacity that can be reached per hour when the rated steam parameters, the rated feed water temperature, and the designed fuel are used for long-term continuous operation, in t/h.
The steam parameters are the steam pressure (MPa) and temperature (°C) at the outlet of the superheater.
17. According to the activity mode of the working fluid in the evaporating heating surface, what are the 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 circulation boilers.
18. What are the elemental analysis components of coal?
Answer: The elemental analysis components of coal include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), moisture (M) and ash (A). Among them, carbon, hydrogen, and sulfur are combustible components; sulfur, moisture, and ash are the main harmful components.
19. What are the industrial analysis components of coal?
Answer: The industrial analysis components of coal include: moisture, volatile matter, fixed carbon and ash. Among them, volatile matter and fixed carbon are combustible components. The amount of volatile matter has a significant impact on the occurrence and development of the coal combustion process. The more volatile matter content, the easier it is to fire and the more complete the combustion.
20. What are the types of coal used in power plants? What are the characteristics of combustion?
Answer: The coal used in power plants is divided into anthracite, lean coal, bituminous coal and lignite according to the volatile content of the dry ash-free base. Among them, anthracite (Vdaf≤10%) has a high calorific value, but it is not easy to catch fire and burns slowly;
Lean coal (10%<vdaf<20%); bituminous coal (vdaf=20%～40%), which easily catches fire and burns, and has a higher calorific value; lignite (vda>40%) catches fire and burns easily, but has a higher calorific value Low. In addition, there are washed coal, peat, oil shale and coal gangue.
21. What are the three stages of the pulverized coal combustion process?
Answer: Preparatory stage, burning stage, burnout stage before fire.
22. What are the conditions for rapid and complete combustion of pulverized coal?
Answer: Quite high furnace temperature, suitable air volume, good mixing of fuel and air, and sufficient burning time.
23. What are the functions of the primary, secondary, and tertiary winds?
Answer: The function of the primary air is to carry the pulverized coal into the furnace and meet the needs of the volatile matter to catch fire.
The role of the secondary air is to support combustion and to disturb and enhance combustion.
The function of the tertiary air is to send the exhaust gas separated by the fine powder separator into the furnace to reduce pollution and improve the thermal efficiency of the boiler.
24. What is the function of the coal mill? What are the types? What are the commonly used coal mills?
Answer: The function of the coal pulverizer is to crush and grind a certain size of coal into pulverized coal.
Coal pulverizers are divided into low-speed coal mills (14-25r/min), medium-speed coal mills (50-300r/min), and high-speed coal mills (500-1500r/min) according to their rotational speed.
Commonly used low-speed coal mills are ball mills; medium-speed coal mills are bowl-type coal mills, MPS coal mills, etc.; high-speed coal mills are fan mills.
25. What is the task of the milling system? What are the forms?
Answer: The task of the pulverizing system is to grind the raw coal into pulverized coal, dry it to a certain degree, and then send it into the furnace to burn through the burner. Commonly used pulverizing systems are divided into two categories: direct blowing type and intermediate storage type.
26. What are the main equipment of the pulverizing system?
Answer: Raw coal bunker, coal feeder, coal mill, coarse powder separator, powder exhauster, air locker, intermediate storage silo system also has fine powder separator, coal powder silo, powder feeder, etc.
27. What is the function of the burner? What are the main types?
Answer: The function of the burner is to send fuel and air required for combustion into the furnace and form a certain air flow structure, so that the fuel can catch fire quickly and stably; supply air in time, so that the fuel and air are fully mixed in the furnace to achieve complete combustion.
According to the characteristics of the outlet airflow, the burners are divided into two categories: direct current burners and swirl burners. Direct current burners generally adopt four-corner layout and tangential combustion; swirl combustion generally adopts one-wall layout or two-wall layout.
28. Briefly describe the function and ignition method of the ignition device.
Answer: The function of the ignition device is to ignite the pulverized coal when starting; it burns stably under low load.
Ignition mostly adopts the hierarchical ignition method, that is, the high-energy igniter ignites the fuel first, and then the oil ignites the pulverized coal flow.
29. How is the natural water cycle formed?
Answer: In the natural water circulation loop, the water wall absorbs the radiant heat of the furnace flame and flue gas, so that part of the water evaporates to form a steam-water mixture; while the downcomer is not heated, and the inside of the pipe is water.
Therefore, the density of the water in the downcomer is greater than the density of the steam-water mixture in the water wall, and a pressure difference (the weight difference between the liquid columns on both sides) is generated on both sides of the lower header. This pressure difference will push the working fluid to move upward in the water wall. It moves downward in the downcomer to form a natural water circulation.
30. What are the main methods of steam purification?
Separation of steam and water to reduce the amount of water carried by steam. Commonly used primary separation elements include cyclone separators, turbine separators, etc.; secondary separation elements commonly used corrugated plate separators.
Steam cleaning to reduce the salt dissolved by the steam and the salt in the water. Commonly used flat-hole plate cleaning device. Note: Steam cleaning is generally only used on high-pressure and ultra-high-pressure boilers.
Boiler blowdown to control the salt content of the boiler water. Boiler blowdown is divided into continuous blowdown and regular blowdown. The purpose of continuous sewage discharge is to continuously discharge part of the pot water, suspended matter and grease during operation to maintain a certain quality of the pot water; the location is around the evaporation surface.
The purpose of regular sewage discharge is to discharge the slag and other sediments in the boiler water regularly during operation; the location is at the bottom of the header under the water wall.
31. How many stages are there in the steam-water separation process in the steam drum?
Answer: The steam-water separation process in the steam drum is generally divided into two stages: the first stage is the coarse separation stage (also known as one-time separation). Its task is to eliminate the kinetic energy of the steam-water mixture and initially separate steam and water;
The second stage is the fine separation stage (also known as the secondary separation). Its task is to separate the small water droplets carried in the steam and to draw the steam uniformly from the upper part of the steam drum.
32. What are the functions of the steam drum?
Answer:
①The steam drum is the key to the connection and the rough dividing point of the three processes of heating, evaporation and overheating;
②The steam drum has a certain heat storage capacity, which can function as a heat accumulator and a water accumulator when the load changes, which can alleviate the speed of steam pressure changes, which is beneficial to the regulation of boiler operation;
③The steam drum is equipped with a steam-water separation device, a steam cleaning device, a sewage device and a chemical dosing device, etc., which can improve the steam quality;
④The steam drum is equipped with accessories such as pressure gauges, water level gauges and safety valves. The steam drum is equipped with accidental water discharge devices to ensure the safe operation of the boiler.
33. What is the function of the water wall? What are the types and where are they used?
Answer: The water wall is the only heating surface in the evaporation system. It is generally arranged around the inner wall of the furnace. Its main functions are:
① Absorb the radiant heat of the furnace, so that the water partly evaporates into saturated steam;
②Protect the furnace wall and simplify the structure of the furnace wall;
③Save metal and reduce boiler cost.
The main types of water wall are smooth tube type (mainly used for small and medium-sized boilers), membrane type (mainly used for modern large boilers), pin type (mainly used for fixing the gas belt), internal threaded tubular type (mainly used for Asian boilers) High heat load area and water wall outlet area of critical and above pressure boilers).
34. What is the function of the superheater and what types are there?
Answer: The function of the superheater is to heat the saturated steam produced by the boiler into superheated steam with a certain temperature, and then send it to the steam turbine for work.
According to different heat transfer methods, superheaters are divided into: convection superheaters, radiant superheaters and semi-radiative superheaters.
The convection superheater is arranged in the convection flue. Generally, there are four types of cocurrent, countercurrent, mixed flow, and double countercurrent; the radiant superheater is arranged above the furnace or on the furnace wall, generally there are ceiling superheaters, screen superheaters, and wall superheaters. The semi-radiative superheater is arranged at the outlet of the furnace, and generally a screen superheater is used.
Generally, medium and low pressure boilers have only convection superheaters; high pressure and above boilers use "radiant-semi-radiative-convection" combined superheaters.
35. What is the role of the reheater? What are the types?
Answer: The function of the reheater is to return the steam discharged from the high-pressure cylinder of the steam turbine to the boiler, reheat it into reheated steam with a certain temperature, and send it to the low-pressure cylinder of the steam turbine for work.
The main types of reheaters are: convection, radiant and semi-radiative.
The convection type is arranged in the convection flue; the radial type is arranged on the furnace wall above the furnace; the semi-radial type is arranged at the furnace outlet.
Generally, ultra-high pressure boilers only use convection type; modern subcritical pressure and
The reheater of the upper boiler mostly adopts "radiant-semi-radiative-convection" series combined reheater.
36. What is the function of the economizer? What are the types?
Answer: The economizer uses the heat of the flue gas from the tail of the boiler to heat the boiler feed water. Its functions are:
①Save fuel; ②Reduce boiler cost; ③Improve the working conditions of the steam drum.
Economizers are divided into boiling type and non-boiling type according to the state of export working fluid. Medium and low pressure boilers mostly use boiling type; ultra-high pressure and above pressure boilers use non-boiling type.
37. What is the function of the air preheater? What are the types?
Answer: The air preheater uses the heat of the flue gas from the tail of the boiler to heat the air. Its main functions are:
①Reduce the exhaust gas temperature of the boiler and improve the efficiency of the boiler;
②Increase the furnace temperature, improve the ignition and combustion conditions of the fuel, and reduce the loss of incomplete combustion; ③Save the metal and reduce the cost of the boiler;
④It is used for drying coal powder, which is beneficial to the work of the pulverizing system;
⑤The exhaust gas temperature is reduced, and the working conditions of the induced draft fan are improved.
There are two main types of air preheaters: tube type and rotary type. Tube type is generally used for small and medium boilers; large boilers mainly use heating surface rotary air preheaters.
38. What is the role of the blower?
Answer: The function of the blower ensures the supply of air required for the combustion of the boiler fuel.
39. What is the role of the induced draft fan?
Answer: The function of the induced draft fan is to suck out the flue gas produced by the combustion of the fuel in the furnace, and send it to the chimney after dust removal and desulfurization, and then exhaust it to the atmosphere.
40. What are the main items of steam quality monitoring?
Answer: Sodium, silicon dioxide, copper and iron.
41. What are the boiler feed water indicators?
Answer: Hardness, silica, dissolved oxygen, iron content, copper content, sodium content, PH value, electrical conductivity, hydrazine, oil content, total carbonic acid salt content, etc.
42. What is the standard coal consumption rate for power generation?
Answer: The amount of standard coal consumed by the steam turbine generator unit for every 1kw·h of electricity produced.
43. What losses exist in the production process of thermal power plants? Which is the biggest loss?
Answer: There are boiler losses, pipeline losses, steam turbine losses, and cold source losses during the production process of thermal power plants. Among them, the loss of the cold source is the largest.
44. What are the common boiler protection projects?
Answer: Drum water level high and low protection (on-line boiler water cut protection), main steam pressure high protection, furnace fire extinguishing protection, reheater wall temperature high protection, reheated steam temperature high protection, emergency shutdown protection, etc.
45. What are the commonly used temperature measuring components?
Answer: Thermocouple (platinum rhodium-platinum, nickel-chromium-nickel silicon, nickel-chromium-copper-nickel, copper-constantan) (principle: a closed loop composed of two conductors of different materials, when the two contact points have different temperatures Thermoelectric potential), thermal resistance (platinum resistance, copper resistance, which converts temperature changes into resistance changes).
46. What are the commonly used load measuring components?
Answer: The liquid column pressure gauge (U-shaped tube, single tube) converts the pressure signal into a change in the height of the liquid column, and the elastic pressure gauge (diaphragm, diaphragm, spring tube, bellows) converts the pressure signal into an elastic element The displacement.
47. What are the commonly used flow measurement instruments?
Answer: Throttle pressure-reducing flowmeter (differential pressure flowmeter converts the flow signal into the pressure difference before and after the throttling element. The throttling element is an orifice plate and a nozzle. The pressure method is angle joint pressure and flange pressure. ), oval gear flow meters, impeller water meters, scraper flow meters, etc.
48. What are the commonly used water level measuring instruments?
Answer: Electric contact type water level gauge, in-situ water level gauge (mica water level gauge, two-color water level gauge), differential pressure water level gauge (converts the change of water level into a balanced container to output a differential pressure signal of positive and negative pressure head).
49. What kind of measurement is used to measure the oxygen content of the flue gas? What is the purpose?
Answer: With zirconia oxygen meter, zirconia can convert the change of oxygen content into oxygen concentration difference potential.
In order to ensure that the pulverized coal is in a good combustion state, a good ratio of pulverized coal and air is required, and the excess air coefficient is used to evaluate the excess air coefficient. The size of the excess air coefficient is related to the oxygen content of the flue gas.
In the air supply adjustment system, the oxygen content is a given value by adjusting the amount of air supply to ensure economical combustion.
50. What is the adjusted amount and the adjusted amount of common control systems?
Boiler drum feed water control system: drum water level, feed water flow
Superheater outlet steam temperature control system: superheater outlet steam temperature, desuperheating water flow
Combustion control system: boiler load or main steam pressure, coal feed rate
Air supply control system: flue gas oxygen content, air supply
Induced air control system: furnace negative pressure, induced air volume
