A boiler is a mechanical device that uses water or other energy to heat water into hot water or steam.
The boiler includes two parts, the boiler and the boiler. The original meaning of the boiler refers to the water container heated on the fire, and the furnace refers to the place where the fuel is burned. The hot water or steam generated in the boiler can directly provide the required heat energy for production and living, can also be converted into mechanical energy through a steam power plant, or can be converted to electrical energy through a generator.
The boilers that provide hot water are called hot water boilers, and they are mainly used in daily life. There are also a few applications in industrial production. Steam generating boilers are called steam boilers, also called steam generators, often referred to simply as boilers, and are an important part of steam power plants. They are mostly used in thermal power plants, ships, locomotives and industrial and mining enterprises.
Boilers are subjected to high temperature and high pressure, and safety issues are very important. Even small boilers have serious consequences in the event of an explosion. Therefore, there are strict laws and regulations for the selection of boiler materials, design calculations, manufacturing and inspections. Boiler Development Boiler development is divided into pots and furnaces.
In the first half of the 18th century, the steam pressure used by British coal mines, including Watts' initial steam engine, was equal to atmospheric pressure. In the second half of the 18th century, steam was used that was higher than atmospheric pressure.
In the 19th century, the commonly used steam pressure increased to about 0.8 MPa. In line with this, the earliest steam boiler was a large-diameter cylindrical vertical shell containing water, which was later changed to a horizontal shell and fired in a brick furnace below the shell. As the boiler becomes bigger and bigger, in order to increase the heated area, a fire tube is installed in the pot shell and the fire is fired at the front end of the fire tube. The smoke comes out from behind the fire tube and is discharged through the brick flue to the chimney and heats the outside of the pot shell. Fire tube boiler. At the beginning, only one fire pit, called a single fire pit boiler or a Connex boiler, was installed. It was later added to two fire pits, called double fire pit boilers or Lancashire boilers. Around 1830, fire tube boilers emerged after mastering the production of high-quality steel tubes and expanding tube technology. Some fire tubes are installed in the shell and constitute the main heating surface of the boiler. Fire (flue gas) flows through the tubes. Install as many fire tubes as possible below the water line in the shell, which is called the horizontal flame-retardant tube boiler. Its metal consumption is low, but it requires a lot of masonry.
In the middle of the 19th century, water pipe boilers appeared. The heating surface of the boiler is the water pipe outside the pot shell instead of the pot shell and the fire tube inside the pot shell. The increase in the heated area and steam pressure of the boiler is no longer limited by the diameter of the shell, which helps to increase the boiler evaporation and steam pressure. The cylindrical shell in this type of boiler was renamed the drum, or steam drum. In the initial stage, only water pipes were used for the water pipe boilers, and the pressure and capacity of the direct water pipe boilers were limited. At the beginning of the 20th century, steam turbines began to develop. They required boilers with higher capacity and steam parameters. Straight-pipe boilers can no longer meet the requirements. With the development of manufacturing processes and water treatment technologies, bent pipe boilers have emerged. It started with a multi-drum type. With the application of water-cooled walls, superheaters and economizers, as well as the improvement of steam and water separation elements inside the drum, the number of drums is gradually reduced, saving both metal and boiler pressure, temperature, capacity and efficiency. . The former fire-tube boilers, fire-tube boilers, and water-tube boilers belonged to the natural-circulation boilers. Vapors in the ascending and descending pipes were affected by different heating conditions, resulting in poor density and natural flow.
In the development of natural circulation boilers, the application of once-through boilers began in the 1930s and the use of auxiliary circulating boilers began in the 40s. The auxiliary circulating boiler, also called the forced circulation boiler, is developed on the basis of natural circulation boilers. A circulation pump is installed in the descending pipe system to strengthen the water circulation on the evaporating heating surface. There is no drum in the direct current boiler. The feed water is sent from the feed pump to the economizer, and the superheated steam is sent to the steam turbine via the water cooling wall and the superheater. The flow resistance of each part is overcome by the feed pump.
After the Second World War, these two types of boilers developed faster because the generator set required high temperature, high pressure and large capacity at that time. The purpose of the development of these two types of boilers is to reduce or eliminate the use of drums. Small diameter pipes can be used as heating surfaces and heating surfaces can be arranged more freely. With the advancement of automatic control and water treatment technologies, they have gradually matured.
In the case of supercritical pressure, the once-through boiler is the only boiler that can be used. In the 1970s, the largest single-unit capacity was a 27 MPa pressured 1300 MW generator set. Later, a compound circulating boiler composed of an auxiliary circulating boiler and a direct current boiler was developed. In the development of the boiler, the type of fuel has a great influence on the hearth and combustion equipment. Therefore, it is not only required to develop various furnace types to adapt to the combustion characteristics of different fuels, but also to increase the combustion efficiency to save energy.
In addition, the technical improvement of furnaces and combustion equipment also requires the minimization of pollutants (sulfur oxides and nitrogen oxides) in the boiler exhaust fumes. Early boiler shell boilers use fixed grate, high quality coal and firewood, and coal and Slag removal is done manually. After the appearance of the straight-tube boiler, the mechanized grate began to be used, and the chain grate was widely used. The air supply under the grate will not be segmented.
In the early days, the furnace was low and its combustion efficiency was low. Later, people recognized the role of the volume and structure of the furnace in the combustion, built up the furnace, and used the furnace arch and secondary air, thereby improving the combustion efficiency. When the power of the generator set exceeds 6 megawatts, the size of the grate above these layers is too large, the structure is complicated, and it is difficult to arrange. Therefore, in the 20th century, the use of a room-combustion furnace and combustion of pulverized coal and oil in the chamber combustion furnace was started. The coal is pulverized into coal dust by a coal mill and injected into the furnace with a burner. The capacity of the generator set is no longer limited by the combustion equipment. Since the beginning of the Second World War, utility boilers have used almost all room furnaces.
The pulverized coal furnace manufactured in the early years used a U-shaped flame. The flow of pulverized coal from the burner drops first in the furnace and then turns to rise. Later, a swirling burner was placed on the front wall. The flame formed an L-shaped torch in the hearth.
As the capacity of the boiler increases, the number of swirl burners also begins to increase, and can be arranged on both sides of the wall, as well as on the front and rear walls. In about 1930, DC burners were installed in the four corners of the furnace and were mostly tangentially burned. After the Second World War, oil was cheap and many countries began to widely use oil-fired boilers. The degree of automation of oil-fired boilers is easy to increase.
After the oil price increase in the 1970s, many countries turned to use coal resources again. At this time, the capacity of the power station boiler is also getting larger and larger. It is required that the combustion equipment can not only complete combustion, be stable in ignition, be reliable in operation, and be of low load performance, but must also reduce pollutants in the exhaust fumes. Combustion of coal (especially lignite) power station boilers using staged combustion or low temperature combustion techniques, that is, delay the mixing of pulverized coal and air or smoke in the air to slow down the combustion, or spread the burner to suppress the furnace Temperature, not only can inhibit the formation of nitrogen oxides, but also reduce slagging. The boiling combustion method is a kind of low-temperature combustion. In addition to the solid fuel with high ash content that can be burned, limestone can also be incorporated into the fluidized bed for desulfurization.
Boiler operation Boiler parameters are the main indicators of boiler performance, including boiler capacity, steam pressure, steam temperature, feedwater temperature, and so on. Boiler capacity can be expressed in terms of rated evaporation or maximum continuous evaporation. The rated evaporation is the amount of steam continuously produced per unit of time at the specified outlet pressure, temperature and efficiency. The maximum continuous evaporation is the amount of steam that can be continuously produced in a unit of time at the specified outlet pressure and temperature. The steam parameters include the steam pressure and temperature of the boiler, usually referred to as the superheater, the superheated steam pressure at the outlet of the reheater, and the temperature without the superheater and reheater, ie the saturated steam pressure and temperature at the outlet of the boiler. Feedwater temperature refers to the inlet water temperature of the economizer. When there is no economizer, it refers to the inlet temperature of the drum. Boilers can be classified according to different methods.
Boilers can be divided into industrial boilers, power station boilers, marine boilers, and locomotive boilers according to their purposes. According to the boiler outlet pressure, boilers can be divided into low pressure, medium pressure, high pressure, ultra high pressure, subcritical pressure, and supercritical pressure boilers; The flow path of flue gas can be divided into fire tube boiler, fire tube boiler and water tube boiler. Fire tube boiler and fire tube boiler are also collectively referred to as boiler shell boiler. According to the circulation method, they can be divided into natural circulation boiler and auxiliary circulation boiler (ie, forced circulation Boiler), once-through boiler and compound circulating boiler; According to the combustion method, the boiler is divided into chamber furnace, layer furnace and boiling furnace.
In the water vapor system, the feed water is heated in the heater to a certain temperature, and then enters the economizer through the feed water pipe. After further heating, it is sent to the drum, mixed with the pan water, and then descends along the descending pipe to the water wall inlet header. The water absorbs the radiant heat of the furnace in the water wall tube to form a soda mixture and reach the drum through a rising pipe. The water and steam are separated by a steam and water separation device. The separated saturated steam flows from the upper part of the drum to the superheater, continues to absorb heat and becomes superheated steam at 450°C, and is sent to the steam turbine.
In combustion and smoke systems, the blower sends air to the air preheater to a certain temperature. The pulverized coal finely pulverized in the pulverizer is carried by a part of the hot air from the air preheater and injected into the furnace through the burner. The mixture of pulverized coal and air emitted by the burner is mixed with the rest of the hot air in the furnace to generate a large amount of heat. After the combustion of hot flue gas flow through the furnace, slag tube bundle, superheater, economizer and air preheater, and then through the dust removal device to remove the fly ash, and finally sent to the atmosphere by the induced draft fan to the chimney.
The structure of the boiler The overall structure of the boiler consists of two parts, the boiler body and auxiliary equipment. The main parts of the boiler, furnace, drum, burner, waterwall superheater, economizer, air preheater, frame and furnace wall, constitute the core part of the steam production, which is called the boiler body. The two main components in the boiler body are the furnace and the drum.
Furnace, also known as the combustion chamber, is the space where fuel is burned. The solid fuel is placed on the grate, and the fire pit burning furnace is called layer burning furnace, also known as the fire bed furnace; the liquid, gas or powdered solid fuel is injected into the combustion chamber and the combustion chamber is called the chamber. Furnace, also known as the fire room furnace; the furnace that the air holds the coal particles to make it boil, and is suitable for burning inferior fuels, is called the boiling furnace, also called the fluidized bed furnace; using the air flow to make the coal particles rotate at high speed, And the intensely fired cylindrical furnace is called a cyclone furnace.
The cross-section of the furnace is generally square or rectangular. The fuel is burned in the furnace to form flames and high-temperature flue gas, so the furnace walls around the furnace consist of high-temperature resistant materials and heat-insulating materials. Water wall tubes are often laid on the inner surface of the furnace wall, which not only protects the furnace wall from burning, but also absorbs a lot of radiant heat from the flame and high-temperature fumes.
Furnace design requires full consideration of the characteristics of the fuel used. Each boiler should use the originally designed fuel as much as possible. When fuels with different characteristics are used, the economic efficiency and reliability of boiler operation may be reduced.
The drum is a cylindrical vessel that receives the water from the economizer, connects the circulation loop, and delivers saturated steam to the superheater in natural circulation and multiple forced circulation boilers. The drum is made of high-quality thick steel plate and is one of the heaviest components in the boiler. The main function of the drum is storage of water, separation of steam and water, exclusion of salt water and mud in the pan during operation, and avoiding boiler water containing high concentrations of salt and impurities from entering the superheater and steam turbine with steam. The internal devices of the drum include steam separation and steam cleaning devices, water distribution pipes, sewage discharge and dosing equipment. The function of the steam separation device is to separate the saturated steam and moisture from the water wall and minimize the small water droplets carried in the steam. Common baffles and gap baffles are used as coarse separation elements in medium and low pressure boilers; in addition to widely used in various types of cyclone separators, medium pressure boilers are used for coarse separation, but also use louver windows, steel wire mesh, or even steam, etc. Further separation. The drum is also equipped with monitoring and protection facilities such as water level gauges and safety valves.
In order to assess performance and improve design, boilers often undergo heat balance tests. The method of calculating the boiler thermal efficiency directly from the effective use of energy is called positive balance. The method of back-calculating efficiency from various heat losses is called counterbalance. When considering the actual benefits of the boiler room, not only the thermal efficiency of the boiler but also the energy consumed by the auxiliary boiler is taken into consideration. When the unit mass or unit volume of fuel is completely burned, the air demand calculated from the chemical reaction is called the theoretical air amount. In order to make the fuel have more chance to burn in contact with oxygen in the furnace, the actual amount of air sent into the furnace is always greater than the theoretical air volume. Although more air can be added to reduce the heat loss from incomplete combustion, the heat loss from the flue gas will increase, and the sulfur oxide corrosion and nitrogen oxide formation will also be exacerbated. Therefore, efforts should be made to improve the combustion technology so as to make the combustion in the furnace complete with a minimum excess air ratio.
The dust (including fly ash and carbon black), sulfur and nitrogen oxides contained in the boiler flue gas are substances that pollute the atmosphere. When not purified, the emission index can reach several to several dozen times of the environmental protection regulations. Measures to control the discharge of these substances include pre-combustion treatment, improved combustion technology, dust removal, desulfurization, and denitrification. With high chimneys, the concentration of pollutants in the atmosphere near the chimney can only be reduced. The forces used in the dust removal of flue gas include gravity, centrifugal force, inertial force adhesion, and sonic waves and static electricity. The separation of gravity sedimentation and inertial force is generally adopted for coarse particles. Centrifugal force is often used to separate the electrostatic precipitator and bag filter with high dust removal efficiency. The water droplets in the wet and Venturi-water film precipitators can adhere to fly ash, and the dust removal efficiency can also absorb gaseous pollutants.
Since the 1950s, people have been working hard to develop comprehensive utilization of slag, which is beneficial to people. Such as the use of ash to produce cement, brick and concrete aggregates and other building materials. In the 1970s, hollow microspheres were also extracted from fly ash as fire-resistant insulation materials. The future development of boilers will further increase the thermal efficiency of boilers and power stations; reduce equipment costs for unit power of boilers and power stations; increase the operational flexibility and automation level of boiler units; develop more boiler types to suit different fuels; Auxiliary equipment operating reliability; reduce pollution to the environment.
The system issued the instruction by the inverter to automatically start the first pump operation, the system detects the water pressure of the water supply pipe. When the frequency of the inverter rises to the power frequency, if the water pressure does not reach the set pressure value, the system automatically sets the first motor. Switch to the power frequency direct power supply, and drive the second pump to run by the inverter. If the inverter runs to the power frequency state, the pressure of the water supply pipe still does not reach the set pressure value. The system automatically switches the second pump to the power frequency. Direct power supply, and then the third drive run by the inverter, and so on, until the pressure reaches the set value. If the amount of water required by the boiler is reduced, the frequency conversion control system can automatically reduce the frequency converter's operating frequency. If the frequency of the frequency converter fails to meet the requirements, the frequency converter will automatically switch to the previous pump for variable frequency operation, and so on. The essence of the constant pressure water supply control system is: always use a frequency converter to automatically adjust the pump speed, switching time is determined by the difference between the actual pressure of the pipe network and the set pressure, while ensuring the pressure of the pipe network is dynamically constant. It is worth noting that in order to prevent the inverter from stopping the alarm or other faults causing the pump not to turn will cause the boiler to be deficient in water, a feedback device should be added to ensure the normal operation of the inverter.
In addition, the boiler's water supply system also includes deaerator pressure control and deaerator water level control. The deaerator pressure control is mainly to ensure that the deaerator port has enough steam pressure to deoxidize the demineralized water. It is a single closed-loop control loop. The input parameter is the deaerator pressure output parameter to control the deaerator inlet valve. The deaerator water level control is mainly to ensure that there is enough water in the deaerator to supply the boiler. This is a single closed loop control loop input parameter, and the deaerator water level output parameter controls the deaerator inlet valve.
Boiler Combustion Control System Although the choice of the combustion process automatic adjustment system is related to the type of combustion and the supply system, the combustion method, and the connection method of the boiler and the load, the tasks of the automatic adjustment of the combustion process are the same.
To sum up, the automatic adjustment system for the combustion process has three major tasks:
1 Maintain a constant steam pressure. The change in the steam pressure indicates that the steam volume of the boiler does not correspond to the steam consumption of the load, and the amount of fuel must be changed accordingly in order to change the steam volume of the boiler.
2 Ensure the economy of the combustion process. When the amount of fuel changes, the amount of air to be supplied must be adjusted accordingly so that it can be matched with the amount of fuel to ensure a high degree of economical combustion process.
3 Adjust the amount of induced draft and the amount of air supply to ensure that the furnace pressure is constant.
The combustion regulation system generally has three adjusted parameters, steam pressure p, flue gas oxygen content a, and furnace negative pressure pt. There are generally 3 adjustments. They are the fuel quantity M, the air supply quantity F and the draft quantity Y. The adjustment target of the combustion regulation system may be a grate motor or a fuel valve depending on the type of fuel. For the air supply volume and the induced air volume, it is generally a baffle actuator or frequency converter. The combustion regulation system is a multi-parameter variable regulation system. This type of regulation system usually simplifies it into three mutually associative, closely coordinated but independent 3 single-variable systems. For ease of analysis, we analyze the following three systems. The three systems are a steam pressure regulation system that maintains a constant boiler fuel pressure, a supply air regulation system that maintains the boiler's economical combustion rate, and a negative pressure regulation system that maintains the negative pressure of the furnace and the stability of the furnace negative pressure.
Characteristics of Vapor Pressure Regulatory Objects The main causes of vapor pressure changes are changes in the amount of fuel and steam load. Its dynamic characteristics are as follows.
1 The characteristics of steam pressure variation under disturbance of fuel quantity When the steam load is constant, such as boiler fuel quantity (B), there is a step disturbance of ΔB. At this time, the soaring curve of steam pressure is shown in Figure 4(a). . At this point, the object does not have self-balancing capability, and has a large hysteresis and inertia. However, if the opening of the steam valve at the outlet of the boiler does not change, the steam flow will also change when the steam pressure changes due to the fuel volume disturbance. As the vapor pressure changes, the increase in steam flow voluntarily limits the change in vapor pressure, so the subject has the ability to balance. The soaring steam pressure curve at this time is shown in Figure 4(b).
2 The characteristics of steam pressure variation under steam load disturbance The dynamic characteristics of steam pressure change under the disturbance of load step also have the following two conditions: When using steam valve step disturbance, the object shows self-balancing capacity without delay, but There is a large inertia, and there is an initial leap proportional to the change in the valve. The soaring curve is shown in Figure 4(c). When a gas step is used to perturb, the soaring curve is shown in Figure 4(d). At this point, the object has no self-balancing ability. If the amount of fuel entering the boiler is not increased in time, the steam pressure will continue to decrease.
The characteristics of the air supply automatic adjustment object The work of the air supply regulation system is good or bad. It directly affects the change of the air excess coefficient of the hearth, that is, the oxygen content of the discharged flue gas. The main disturbance that causes changes in the air excess coefficient is the ratio of the fuel volume to the supply air volume. The dynamic characteristics of the object under the disturbance of air volume have a large self-balancing capacity, and there is almost no delay and inertia, which is approximately a proportional link. When the amount of fuel is disturbed, there is a slight delay after the transportation and combustion process. Due to the air supply system, there is almost no delay and inertia. Therefore, in the case of adequate fuel, the amount of air supply will be more directly reflected in the steam pressure of the boiler. So how can we ensure the right combination of air flow and fuel volume, here we introduce the concept of wind and coal ratio. Wind-to-coal ratio is the maximum value of coal that can be burned at the current air flow rate. The wind-to-coal ratio in the control is mainly based on the current air volume to limit the speed of the grate, to prevent the coal from being unable to fully burn due to insufficient air volume. This parameter has great significance for saving coal and environmental protection. Because if it can not be fully burned, it will lead to an increase in the carbon content of the coal slag, which is a waste of coal. At the same time, it will also cause an increase in the carbon content of the flue gas to affect the discharge.
Furnace negative pressure automatically adjusts the characteristics of the object The furnace vacuum negative pressure automatically adjusts the dynamic characteristics of the object is better, but the perturbation channel has a very short flying time, and the flying speed is very fast. Based on the above analysis of the adjustment objects of the combustion system, the following analysis of the three control tasks of the combustion automatic control system is carried out. The general control flow chart of the combustion process control system is shown in Fig. 5(a). The steam pressure transmitter first obtains the signal after being filtered and compares it with the set steam pressure to determine the adjustment direction of the blower PI regulator. And size, calculate the output size of the blower inverter through the blower PI adjustment unit. At the same time, the signal is output to the wind-to-coal ratio calculation unit, and the maximum output value of the grate under the current air volume is calculated accordingly. Then the difference signal of steam pressure is sent to the PI regulator of the grate, and the output of the grate converter is calculated through the PI adjusting unit of the grate. After the wind-coal ratio limit, the output to the grate converter.
In the actual debugging process, we often set the proportional coefficient in the blower PI regulation to be larger than the PI unit of the grate, which can ensure that the blast system is more sensitive to the steam pressure than the grate. It has been proved that the steam pressure of the boiler under control by this method is stable and has a correspondingly high degree of variation in the steam load. Ash contains low carbon content. The size of the negative pressure in the furnace has a great influence on energy saving. The negative pressure is large, the heat taken away by the flue gas is large, the heat loss increases, the coal consumption increases, and the ideal operating state should be in a state of slight negative pressure. It can significantly increase the residence time of suspended coal particles in the hearth, increase the settlement, reduce the fly ash, and make the coal fully burn to improve the thermal efficiency. However, due to load changes, it is necessary to change the amount of coal to be fed and the amount of air to be supplied, and the amount of induced draft should also be changed accordingly to ensure the stability of the furnace negative pressure. However, because the system has a certain lag time, the negative pressure of the furnace is caused to avoid the change of the blast. In the fluctuations, the system introduces the blast signal as a feedforward signal to advance the induced draft fan. The general control flow chart of the furnace negative pressure control system is shown in Fig. 5(b). The adjustment principle is relatively simple and it is a single closed-loop regulation system. Its input volume is the negative pressure output of the furnace, which is the inducer, and the amount of blast air is introduced at the same time. As a feed forward signal. In addition, each system loop has two manual operation modes. In order to realize bumpless switching, the system introduces the feedback value of each control object. In manual operation, the PLC output will automatically track the feedback of the control object when switching to the automatic status. It can perform bumpless switching and make the system transition to the automatic state smoothly. Fourth, the boiler control system composition structure Above we have a brief analysis of the control system of the boiler control system, based on the above analysis, we know that the construction of a reliable, intelligent follow-up intelligent control system is to ensure the safety of boiler production. The boiler control system is a typical multivariable, purely delayed, and strongly coupled control system. If the control strategy and software implementation can not solve the multivariate solution relationship and the lag response problem well, then the implementation of the intelligent boiler control system is transformed. It will also fail to achieve the desired goal. In the design of the control system, we adopt the distributed control concept of centralized control decentralized driving (P-T scheme) and divide the control system into three layers: a) Information management layer: Complete the key technical data of the system, set the real-time data and operating status The monitoring and control, viewing of historical data, recording and printing of data reports, alarm and fault prompt processing, etc.; mainly composed of IPC, configuration development software, application programs, communication modules, etc.; b) Control layer: mainly completes various control action commands, sampling and processing of real-time data, linkage expression of chained actions, implementation of control algorithms, automatic processing of abnormal phenomena, etc.; mainly by the switch of programmable logic controller (PLC). Modules, analog modules, intelligent PID regulators, frequency converters, PLC applications, etc. c) Equipment layer: It mainly receives control commands from the PLC, performs corresponding actions or provides corresponding detection data. Mainly by the circuit breaker, AC contactor, pressure transmitter, temperature transmitter, flow transmitter, electric switch valve, analog signal isolation distributor and other components. V. Concluding remarks In summary, the transformation of the boiler control system has a good market development space and investment income prospects, and it is worthy of widespread promotion. It can not only achieve the purpose of safe production through automation control technology, but also can save electricity and save energy and make the emissions more environmentally friendly. In short, the computer automation control of boilers is the trend of the development of the boiler industry, and it is also a development direction for the country and the people.