boiler

Overview

Boilers are mechanical devices that use heat from fuel or other energy sources to heat water into hot water or steam. The original meaning of the pot refers to the water container heated on the fire. The furnace refers to the place where the fuel is burned. The boiler includes two parts, the pot and the furnace. The hot water or steam generated in the boiler can directly provide the required heat energy for industrial production and people's life. It can also be converted into mechanical energy through steam power devices, 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, often referred to simply as boilers, and are used in thermal power plants, ships, locomotives and industrial and mining enterprises.

Boiler development

The development of boilers 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 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, technical improvements in furnaces and combustion equipment also require the minimization of pollutants (sulfur oxides and nitrogen oxides) in boiler exhaust fumes.

In the early years, boiler shell boilers used fixed grate, high quality coal and firewood were used, and both coal addition and slag removal were manually operated. 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.

Boiler work

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, and continues to absorb heat to become a certain temperature of superheated steam (at present, the main steam temperature of most 300MW and 600MW units is about 540°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.

Boiler structure

The overall structure of the boiler includes two major parts of 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.

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.