The owners of the patent RU 2553748:
The invention relates to thermal power engineering and can be used in furnaces and heat generators of various types that use organic fuel for combustion.
There is a known method for efficient fuel combustion by separating gas (combustion reaction products), for example, a method for separating gases using membranes with permeate purge to remove CO 2 from combustion products according to patent 2489197 (RU) BAKER Richard (US), WIGMANS Johannes Gee (US) and others .
The implementation of this combustion method is carried out in several stages: a carbon dioxide capture stage, a membrane gas separation stage, working in combination with compression and condensation to obtain a product from carbon dioxide in the form of a liquid, and a purge-based stage in which incoming air or oxygen is used for combustion as purge gas. The disadvantage of this method is its complexity in implementation, since it includes many additional steps of the standard type, such as heating, cooling, compression, condensation, pumping, various types of separation and / or fractionation, as well as monitoring pressures, temperatures, flows, etc. etc., with this method, the capture of carbon dioxide occurs from the exhaust stream formed by the combustion of fuel diluted with ballast gases, which therefore has a lower temperature.
The closest technical solution (prototype) is the method of burning solid fuel in household heating stoves according to patent 2239750 (RU), authors Ten V.I. (RU) and Ten G.Ch. (RU), Patent owner Ten Valeriy Ivanovich (RU) .
This method includes loading fuel onto the grate of the furnace, creating draft in its working space, ignition and combustion of fuel with the removal of combustion products into the atmosphere, regulation of draft and the amount of combustion products removed from the furnace by slightly opening the blower and chimney dampers.
The disadvantage of this method of burning solid fuel is its complexity in implementation, due to the breakdown of the process into a number of separate periods, in each of which the fuel is re-ignited, brought to the intensive combustion mode, and after reaching the set temperature of the furnace, the combustion process is switched to the attenuation mode, then ignition is performed again using sophisticated automation and using already liquid or gaseous fuels. The disadvantage of these and other similar methods of fuel combustion is the mixing of combustion products, heat sources (CO 2 and H 2 O), in the reaction zone, into a single stream with ballast gases (nitrogen, excess air, etc.), which worsen the conditions for fuel combustion and use of the released heat (useful heat is taken away and carried out into the atmosphere).
The present invention aims to improve the conditions for fuel combustion and increase the amount of thermal energy released by the fuel.
The technical result of the proposed method is to increase the efficiency of furnaces and heat generators by burning combustible gases in the middle zone of the furnace bell and removing ballast gases from the combustion zone, as well as by exposing hot carbon to superheated steam.
The proposed method of fuel combustion is illustrated by graphic material, where the following designations are accepted: 1 - combustion reaction zone; 2 - blower (ash pan); 3 - supply of primary air for ignition, maintenance of combustion and gasification of fuel (volatile combustible gases); 4 - combustion chamber with fuel; 5 - hydrocarbon (volatile gases); 6 - supply of secondary air to the combustion zone for burning volatile combustible gases; 7 - harmful non-combustible ballast gases not involved in combustion; 8 - superheated steam supply; 9 - useful hot products - heat carriers, carbon dioxide and water vapor; 10 - heat exchange zone; 11 - grate; 12 - gas outlet from the furnace hood.
The proposed method is carried out as follows. Solid fuel is loaded onto the grate 11, it is ignited, while primary air enters through the blower 2 and the grate 11. Then, after ignition, secondary air 6 enters the bell directly into the combustion zone for burning volatile combustible gases. As a result of the combustion reaction, a mixture of unrelated gases arises: incandescent carbon dioxide and water vapor and conditionally cold ballast gases - excess air and released nitrogen in its composition (excess air with a high nitrogen content). The peculiarity of the bell design is that during the combustion reaction, the resulting gases are separated in it. Hot gases rise up, giving off thermal energy to the bell, and cold particles of ballast gases fall down through the lower temperature zones of the bell. Fuel combustion reactions are expressed by known combustion equations. The ratios of the substances entering into the reaction are maintained, as well as their composition. That is, carbon C, hydrogen H 2 with oxygen O 2 enter into the reaction in an amount determined by chemical equations:
other substances cannot react. The combustion reaction occurs in the combustion zone between hydrocarbon and oxygen without the participation of ballast gases, while the nitrogen released from the air in the composition of excess air, as less heated, is pushed out through the lower part of the cap (the outlet pipe is not shown in the diagram). After heating the combustion chamber and the presence of hot carbon in it, superheated water vapor 8 is fed into the hood below the secondary air supply zone. As a result of the interaction of carbon with water vapor at high temperatures, combustible gases arise in accordance with the known chemical equations
at low temperature with a total positive thermal effect, which enhance the process of fuel combustion and increase heat transfer from it. The implementation of the proposed method of fuel combustion will increase the efficiency of furnaces and heat generators. The proposed method is quite simple to implement, does not require sophisticated equipment and can be widely used in industry and at home.
SOURCES OF INFORMATION
1. Patent of the Russian Federation No. 2489197, IPC B01D 53/22 (2006.01). Gas separation method using membranes with permeate purge to remove carbon dioxide from combustion products. Assignee, MEMBRANE TECHNOLOGY AND RESEARCH, INC. (US).
2. Patent of the Russian Federation No. 2239750, IPC F24C 1/08, F24B 1/185. The method of burning fuel in household heating stoves. Patentee Ten Valery Ivanovich.
3. Myakelya K. Stoves and fireplaces. Reference manual. Translation from Finnish. Moscow: Stroyizdat, 1987.
4. Ginzburg D.B. Gasification of solid fuel. State publishing house of literature on construction, architecture and building materials. M., 1958.
A method for burning fuel in furnaces having a hood with a fuel combustion chamber and a grate, including fuel loading, ignition and combustion of fuel due to primary air entering through the blower, characterized in that the movement of gases in the hood is carried out without using pipe draft, with the possibility of accumulation hot gases in the upper part of the bell, while secondary air is supplied to the bell, directly into the combustion zone, while hot gases rise upwards, giving off thermal energy to the bell, and cold particles of ballast gases fall down through the lower temperature zones of the bell, after heating the chamber combustion into it, below the supply of secondary air, superheated water vapor is fed to hot carbon and combustible gases are obtained.
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The invention relates to thermal power engineering and can be used in furnaces and heat generators of various types that use organic fuel for combustion. The technical result is an increase in the efficiency of furnaces and heat generators. The method for burning fuel in furnaces having a hood with a fuel combustion chamber and a grate includes loading fuel, ignition and combustion of fuel due to primary air entering through a blower. The movement of gases in the bell is carried out without the use of pipe draft, with the possibility of accumulating hot gases in the upper part of the bell. At the same time, secondary air is supplied to the bell directly into the combustion zone. Hot gases rise up, giving off thermal energy to the bell, and cold particles of ballast gases fall down through the lower temperature zones of the bell. After heating the combustion chamber, below the secondary air supply, superheated water vapor is supplied to the hot carbon and combustible gases are obtained. 1 ill.
FUEL COMBUSTION METHODS.TYPES OF FURNACE DEVICES.
The combustion device, or furnace, being the main element of the boiler unit, is designed to burn fuel in order to release the heat contained in it and obtain combustion products with the highest possible temperature. At the same time, the furnace serves as a heat exchange device in which heat is transferred by radiation from the combustion zone to the colder surrounding heating surfaces of the boiler, as well as a device for trapping and removing some of the focal residues during the combustion of solid fuel.
According to the method of fuel combustion, furnace devices are divided into layered and chamber. In layer furnaces, solid lump fuel is burned in a layer, in chamber furnaces - gaseous, liquid and pulverized fuel in a suspended state.
In modern boiler plants, three main methods of burning solid fuels are usually used (Fig. 14): layered, torch, vortex.
Layer fireboxes. Furnaces in which stratified combustion of lumpy solid fuel is carried out are called stratified. This furnace consists of a grate supporting a layer of lumpy fuel and a furnace space in which combustible volatile substances are burned. Each furnace is designed to burn a specific type of fuel. The designs of furnaces are diverse, and each of them corresponds to a certain method of combustion. The performance and efficiency of the boiler plant depend on the size and design of the furnace.
Rice. 14. Schemes of fuel combustion processes: a - layered, 6 - torch, c - vortex
Layered furnaces for burning various types of solid fuels are divided into internal and external, with horizontal and inclined grates.
Thins located inside the brickwork of the boiler are called internal, and those located outside the brickwork and additionally attached to the boiler are called remote.
Depending on the method of fuel supply and the organization of maintenance, layer thinners are divided into manual, semi-mechanical and mechanized.
Manual furnaces are those in which all three operations - fuel supply to the furnace, its skimming and removal of slag (focal residues) from the furnace - are performed by the driver manually. These fireboxes have a horizontal grate.
Semi-mechanical fireboxes called those in which one or two operations are mechanized. These include mine
inclined grates, in which the fuel loaded into the furnace manually, as the lower layers burn out, moves along the inclined grates under the action of its own weight.
Mechanized furnaces they are called those in which the supply of fuel to the furnace, its skimming and removal of focal residues from the furnace.
Ryas 15 Schemes of furnaces for burning solid fuel in a layer.
a - with a manual horizontal grate, b - with a caster on a fixed layer, c - with a screwing bar, d - with an inclined grate, e - vertical, f - with a forward running chain grate, g - with a reverse running grate with a caster mechanical drive without manual intervention of the driver.
Fuel enters the furnace in a continuous stream.
Layered furnaces for burning solid fuels (Fig. 15) are divided into three classes:
fireboxes with a fixed grate and a layer of fuel that lies motionless on it, which include a firebox with a manual horizontal grate (Fig. 15, a and b). All types of solid fuels can be burned on this grate, but due to manual maintenance, it is used under boilers with a steam capacity of up to 1-2 t / h. Furnaces with casters, into which fresh fuel is continuously mechanically loaded and scattered over the surface of the grate, are installed under boilers with a steam capacity of up to 6.5-10 t / h, furnaces with a fixed grate and a layer of fuel moving along it (Fig. 15, c, guide), which include fireboxes with a screwing bar and fireboxes with an inclined grate. In furnaces with a screwing bar, the fuel moves along a fixed horizontal grate with a special bar of a special shape, which reciprocates along the grate.
They are used for burning brown coal under boilers with a steam capacity of up to 6.5 t / h
in furnaces with an inclined grate, fresh fuel loaded into the furnace from above, but as it burns under the action of gravity, slides into the lower part of the furnace.
Such furnaces are used for burning peat wood waste under boilers with a steam output of up to 2.5 t / h. High-speed mine furnaces of the V.V. t / h furnaces with moving mechanical grates (Fig. 15, f and g) of two types: forward and reverse.
The forward running chain grate moves from the front wall towards the rear wall of the furnace. Fuel flows to the grate by gravity. The reverse chain grate moves from the rear to the front wall of the firebox. Fuel is supplied to the grate by a thrower. Furnaces with chain grates are used for burning hard, brown coal and anthracite under boilers with a steam output of 10 to 35 t / h.
Chamber (torch) furnaces. Chamber furnaces (Fig. 16) are used for burning solid, liquid and gaseous fuels. In this case, solid fuel must be pre-ground into a fine powder in special pulverizing installations - coal-pulverizing mills, and liquid fuel must be sprayed into very small drops in oil nozzles. Gaseous fuel does not require pretreatment.
The flare method makes it possible to burn a wide variety of low-grade fuels with high reliability and efficiency. Solid fuels in a pulverized state are burned under boilers with a steam capacity of 35 t/h and above, and liquid and gaseous fuels are burned under boilers of any steam capacity.
Chamber (torch) furnaces are rectangular prismatic chambers made of refractory bricks or refractory concrete. The walls of the combustion chamber are covered from the inside with a system of boiler pipes - furnace water screens. They represent an effective heating surface of the boiler, which perceives a large amount of heat emitted by the torch, at the same time they protect the lining of the combustion chamber from wear and destruction under the influence of the high temperature of the torch and molten slag.
According to the method of slag removal, flare furnaces for pulverized fuel are divided into two classes: with solid and liquid ash removal.
The furnace chamber with solid slag removal (Fig. 16, a) has a funnel-shaped shape from below, called cold funnel 1. Drops of slag falling from the torch fall into this funnel, solidify due to the lower temperature in the funnel, and granulate. into individual grains and through the neck 3 enter the slag receiving device 2. The furnace chamber b with liquid slag removal (Fig. 16, b) is performed with a horizontal or slightly inclined hearth 7, which has thermal insulation in the lower part of the furnace screens to maintain a temperature exceeding the temperature melting ash. The molten_ slag that has fallen from the torch to the hearth remains in a molten state and flows out of the furnace through tap-hole 9 into the slag-receiving bath 8 filled with water, hardens and cracks into small particles.
Furnaces with liquid slag removal are divided into single-chamber and two-chamber.
In a two-chamber furnace, it is divided into a fuel combustion chamber and a combustion products cooling chamber. The combustion chamber is reliably covered with thermal insulation to create a maximum temperature in order to reliably obtain liquid slag.
Flare furnaces for liquid and gaseous fuels are sometimes made with a horizontal or slightly inclined hearth, which is sometimes not shielded. The location of the burners in the combustion chamber is done on the front and side walls, as well as in its corners. Burners are direct-flow and swirling.
The method of fuel combustion is selected depending on the type and type of fuel, as well as the steam output of the boiler unit.
The combustion device or furnace is the main element of the boiler unit or fire furnace and serves to burn fuel in the most economical way and convert it in the most economical way and convert its chemical energy into heat. There are the following main methods of solid fuel combustion: 1) stratified; 2) flare (chamber); 3) vortex; 4) combustion in a fluidized bed. For burning liquid and gaseous fuels, only the flare method is used. 1. Layered method - the combustion process is carried out in layered furnaces. Layer furnaces can be divided into 3 groups: 1) furnaces with a fixed grate and a dense layer of fuel that is still lying on it. As the speed of the fuel passing through the layer of fuel increases. The latter may become boiling. Such a layer of fuel burns more intensively due to an increase in the contact surface with air. 2. Furnaces with a fixed grate and layers of fuel moving along it. 3. Furnaces with a layer of fuel moving along with the grate.
1 - ash pan; 2 - grate; 3 – fuel layer; 4 - combustion chamber; 5 - lance for air supply; 6 - window for fuel supply.
The fire chamber is intended for combustion of all types of fuel.
Standard grate type RPK- Consists of grate, typed in several rows and planted shafts of rectangular section. When the shafts are rotated through a rotation angle of 30 0, the rows of grates are tilted at the same angle, and through the gaps formed, the slag from the grate spills into the ash pan. Lattices have dimensions in width from 900 to 3600 mm and in length from 915 to 3660 mm. The most common type of layer furnace is a mechanized layer furnace with a chain mechanical transmission. The mechanical grate is made in the form of an endless grate moving the depth of the furnace together with the layer of burning fuel lying on it. The fuel passes through all stages of combustion and is poured into the slag bunker in the form of dust. The grating speed can be changed depending on the fuel consumption from 2 to 16 m/h. These furnaces are used for the combustion of sorted anthracite with a particle size of up to 40 mm. A feature of layered furnaces is the presence of a supply of fuel on the grate, which allows you to adjust the power of the furnace by changing the amount of air supplied and ensures the stability of the combustion process. The layered method is not suitable for large power plants, and in small and medium power plants this method is widely used. 2. Torch method. In contrast to the layered one, it is characterized by the continuity of movement in the furnace space of fuel particles along with the flow of air and combustion products, in which they are in suspension. The figure shows a chamber furnace with flaring fuel combustion. It consists of a burner 1. combustion chamber 2, boiler pipes 3, rear screen pipes 4, slurry funnel 5. Pre-crushed fuel in the form of coal dust and a gas mixture are fed into the burner 1, secondary air is blown in through a series of holes. The gas-air flow with suspended particles of solid fuel is ignited at the outlet of the burner into the furnace 2. In the combustion chamber, the fuel burns out with the formation of a burning torch. The heat released during the combustion of fuel in the form of radiation and convection is transferred to the water circulating in the boiler pipes and pipes of the rear screen. The remainder of the burnt fuel enters the slag funnel, and then is discharged. The main advantage of this combustion method is the possibility of creating powerful furnaces with a steam capacity of up to 2000 t/h and the possibility of economical and reliable combustion of ash, wet and waste fuels under boilers of various capacities. The disadvantages of this method include: 1) The high cost of the pulverization system; 2) High consumption of electrical energy for grinding; 3) Somewhat lower thermal loads of the combustion chamber than in layered furnaces, which contributes to the condition of volumes of furnace spaces. Dust preparation from lumpy fuel consists of the following operations: 1. Removal of metal objects from the fuel with the help of magnetic separators. 2. Crushing of large pieces of fuel in crushers up to a size of 15-25 mm. 3. Drying and grinding of fuel in special mills and classification of fuels. 4. Classification. For crushing large pieces, you can use ball, roller, cone crushers. As grinding equipment in the pulverization system, low-speed ball drum mills, high-speed hammer mills with axial and plate supply of the drying agent are used. Round and slot burners are used to burn pulverized fuel. They are placed in front of the front wall of the furnace, opposite on the side walls, as well as at the corners of the furnace. For frontal and counter spraying, round turbulent burners are used, creating a short torch.
5.1. Solid fuel combustion methods
5.2. Combustion of liquid fuels
5.2.1. Oil quality.
5.2.2. Problems of fuel oil preparation for combustion
5.2.3. Problems when using fuel oil in boiler houses and CHP
5.3. Combustion of gaseous fuels
5.3.1. Gas treatment
5.3.2. Features of the natural gas combustion process
5.3.3. Combustion of gaseous fuel
5.3.4. Gas-burners
5.4. Combined burners
5.5. Flame control devices
5.6. Gas analyzers
5.7. Examples of gas burners
5.7.1. BK-2595PS
5.7.3.BIG-2-14
5.8. Removal of combustion products.
5.1. Solid fuel combustion methods
Burning methods. The combustion device, or furnace, is the main element of a boiler unit or an industrial fire furnace and serves to burn fuel in the most economical way and convert its chemical energy into heat. The combustion of fuel takes place in the furnace, the transfer of part of the heat of the combustion products to the heating surfaces located in the combustion zone, as well as the capture of a certain amount of focal residues (ash, slag). In modern boiler units and furnaces, up to 50% of the heat released in the furnace is transferred to the heating surfaces by radiation. In furnace technology, the following main methods of burning solid fuels are usually used: stratified, flare (chamber), vortex and fluidized bed combustion (Fig. 5.5). Each of these methods has its own characteristics regarding the basic principles of the organization of aerodynamic processes occurring in the combustion chamber. For burning liquid and gaseous fuels, only the flare (chamber) method of combustion is used.
layered method. The combustion process in this way is carried out in layered furnaces.
(see fig. 5.5a ), having a variety of designs. The layered combustion process is characterized by the fact that in it the air flow encounters a stationary or slowly moving layer of fuel during its movement and, interacting with it, turns into a flue gas flow.
An important feature of layered furnaces is the presence of a fuel supply on the grate, linked to its hourly consumption, which allows for primary regulation of the furnace power only by changing the amount of air supplied. The stock of fuel on the grate also provides a certain stability of the combustion process.
In the conditions of modern furnace technology, the layered method of fuel combustion is outdated, since its various schemes and options are unsuitable or difficult to adapt to large power plants. However, layered methods of burning solid fuels will be used for a long time in boiler houses of small and medium-sized power generation.
On fig. 5.6 6 schematic diagrams of layered furnaces are shown. With the layered combustion method, the air necessary for combustion is supplied from the ash pan. 1 to the fuel layer 3 through the free section of the grate 2. In the combustion chamber 4 above the layer, gaseous products of thermal decomposition of the fuel and small particles of fuel removed from the layer burn. Combustion products, together with excess air from the furnace, enter the boiler flues.
Layer furnaces are widely used in boilers of small and medium power. They are divided according to several classification criteria. Depending on the maintenance method, there are fireboxes with manual operation (see Fig. 5.6, a), non-mechanized, semi-mechanized (see Fig. 5.6, b, c) and mechanized (see Fig. 5.6, d, e). Presented in fig. 5.6 layer fireboxes can be divided into three groups
Rice. 5.5. Solid fuel combustion methods
a - in a dense layer; b - in a dusty state; c - in a cyclone furnace; g - in a fluidized bed.
Fireboxes with a fixed grate and movinglayer of fuel flowing over it(see fig. 5.6, b, d).
Fireboxes with a layer moving together with the grateI eat fuel(see fig. 5.6, e).
The simplest layered furnace with a fixed grate and manual maintenance (see Fig. 5.6, a) it is applied to combustion of all types of solid fuel. Such furnaces are equipped with boilers of only very low steam output - 0.275 ... 0.55 kg / s (1 ... 2 t / h).
In a furnace with a fixed inclined grate (see Fig. 5.6, b) As the fuel burns, it moves along the grate under the action of gravity. These furnaces are used for burning wet fuels (wood waste, lumpy peat) under boilers with a steam output of 0.7 ... 1.8 kg / s (2.5 ... 6.5 t / h).
In a semi-mechanized furnace (see Fig. 5.6, v), fuel is supplied to the fixed grate by means of a caster 5. In these furnaces, hard and brown coals, sorted anthracite are burned under boilers with a steam output of 0.55 ... 2.8 kg / s (2 ... 10 t / h).
The simplest mechanized firebox is a firebox with a screw bar (see Fig. 5.6, G). It consists of a fixed elk grating, along the entire width of which a plank slides. b wedge-shaped section. The bar makes reciprocating movements using a special device. These furnaces are used for burning brown coal under boilers with a steam capacity of up to 2.8 kg / s (10 t / h).
The most common type of mechanized layer furnace is the furnace with a mechanical chain grate (see Fig. 5.6, e). The chain mechanical grate is made in the form of an endless grate moving along with a layer of burning fuel lying on it. Each new portion of fuel entering the grate moves after the fuel layer. The grate movement speed can be changed depending on the fuel consumption (boiler operation mode) from 2 to 16 m/h. At T = 10...25%. Existing modifications of furnaces with chain grates allow them to be used for combustion of other fuels as well. Fireboxes with chain grates are installed under boilers with a steam output of 3...10 kg/s (10.5...35 t/h) and above.
flare method. In contrast to the layered process (See Fig. 5.5, b) is characterized by the continuity of movement in the combustion space of fuel particles together with the flow of air and combustion products, in which they are in suspension.
To ensure the stability and uniformity of the burning torch, and, consequently, the gas-air flow with the fuel suspended in it, solid fuel particles are ground to a dusty state, to sizes measured in microns (from 60 to 90% of all particles have a size of less than 90 microns). Liquid fuel is pre-sprayed in the nozzles into very fine droplets so that the droplets do not fall out of the flow and have time to burn out completely in a short time in the furnace. Gaseous fuel is supplied to the furnace through burners and does not require any special preliminary preparation.
A feature of flare furnaces is an insignificant supply of fuel in the combustion chamber, which is why the combustion process is unstable and very sensitive to mode changes. It is possible to regulate the power of the furnace only by simultaneously changing the supply of fuel and air to the combustion chamber. During flare combustion (Fig. 5.7, solid fuel is preliminarily crushed in the pulverizing system and blown into the furnace in the form of dust, where it burns in suspension. Fuel grinding sharply increases its reaction surface, which contributes to better combustion.
The disadvantages of this method include the high cost of equipment for the dust preparation system, power consumption for grinding, lower specific heat loads of the combustion chamber (approximately twice) than with layered furnaces, which significantly increases the volume of furnace spaces.
Dust preparation from lumpy fuel consists of the following operations:
removal of metal objects from the fuel using magnetic separators;
crushing large pieces of fuel in crushers;
drying and grinding of fuel in special mills.
With operating moisture W R < 20 % сушка топлива производится в мельнице одновременно с процессом размола, для чего в мельницу подается горячий воздух из воздухоподогревателя котла. Температура воздуха доходит до 400 °С, и он одновременно служит для выноса пыли из мельницы.
When grinding the fuel, dust particles with a size of 0 ... 500 microns are formed. The main characteristic of dust is the fineness of its grinding, which, according to GOST 3584-53, is characterized by a residue on sieves with cells of 90 and 200 microns, designated R 90 and R 2 oo. So, R 90 = 10% means that 10% of the dust remains on the sieve with a mesh size of 90 microns, and all the rest of the dust has passed through the sieve.
The optimal fineness of grinding (fineness) is determined by the total factor: the minimum power consumption for fuel grinding and losses from mechanical underburning. The fineness of grinding depends on the reactivity of the fuel, characterized mainly by the release of volatile substances. The higher the content of volatile substances in the fuel, the coarser the grind.
The grinding properties of the fuel are characterized by the grinding capacity coefficient, (for anthracite Klo = 1; for lean coal TO lo = 1.6; For brown coal near Moscow Kl 0 = 1.75).
The main disadvantages of this scheme are the lack of dust supply, which reduces the reliability of the boiler, and the heavy wear of the mill fan, through which all coal dust is passed.
On fig. 5.9 shows the scheme of pulverization with an intermediate hopper. Its difference lies in the fact that a cyclone is placed behind the separator 6, into which the finished dust is sent. In the cyclone, 90 ... 95% of the dust is separated from the air and settled, and then sent to the intermediate hopper 9. Dust from the cyclone descends into the bunker through valves (flashing lights) 8, which open when a certain portion of dust is pressed on them. Air with fine dust residue is sucked out of the cyclone by a mill fan 12 and is injected into the primary air pipeline, which, in turn, receives dust from the intermediate hopper using screw or blade dust feeders 10. The pulverization scheme with an intermediate hopper, as the most flexible and reliable, has become the most widely used.
Mills of various types are used for grinding fuel. The choice of mill type depends on the grinding characteristics of the fuel, the yield of volatile substances and the moisture content of the fuel. Distinguish between low-speed and high-speed mills.
For grinding anthracite and bituminous coals with a low yield of volatile substances, burned by boilers of medium and high steam output, low-speed ball drum mills (SPM) are used (Fig. 5.10). The main advantages of the drum mill are the good adjustability of the fineness of grinding and the reliability of grinding. The disadvantages of these mills include: bulkiness, high cost, increased specific power consumption, significant noise that accompanies the operation of the mill.
There are two types of high-speed mills: hammer mills and fan mills.
The fan mill (MB) is intended for grinding mainly high-moisture brown coals and milled peat. Fire chambers with MW are used in medium-capacity boilers. The grinding body of the MW is a massive impeller 1 (Fig. 5.12) with a rotation speed of 380 ... 1470 rpm, located in an armored case 6.
Vwhimsical way. V In the considered flare furnaces, fuel particles burn in the volume of the furnace on the fly. The duration of their stay in the furnace space does not exceed the time "stay of the combustion products in the furnace and is 1.5 ... 3 s. In cyclone furnaces, which are designed to burn finely crushed fuel and coarse dust, large particles of coal are in suspension for so long how long it is necessary for their complete burnout, regardless of the length of stay of the combustion products in the furnace.
Rather small particles of coal (usually smaller than 5 mm) are burned in them, and the air necessary for combustion is supplied at huge (up to 100 m / s) speeds tangentially to the generatrix of the cyclone. they are intensively blown by the flow (see Fig. 5.5, v).
Significant specific surface area of small particles, large values of mass transfer coefficients between the flow and particles, high concentrations of fuel in the chamber provide high heat stresses in the furnace volume (q= 0.65... 1.3 MW/m 3 at a= 1.05... 1.1), as a result of which temperatures close to adiabatic (up to 2000 °C) develop in the furnace. Coal ash melts, liquid slag, flowing down the walls, inhibits the movement of particles adhering to its surface, which further increases the speed of their washing by the flow, and hence the mass transfer coefficient.
Since the centrifugal effect decreases with increasing radius of the cyclone, the diameter of the latter usually does not exceed 2 m, which makes it possible to obtain a thermal power of 40...60 MW.
In our country, technological cyclone combustion chambers are mainly used, for example, for burning sulfur (in order to obtain SO 2 - raw materials for the production of H 2 SO 4; the heat of combustion is also used), for melting and roasting ores and non-metallic materials (for example, phosphorites) etc. Recently, in cyclone furnaces, fire disposal of wastewater has been carried out, that is, the burning of harmful impurities contained in them due to the supply of additional (usually gaseous or liquid) fuel.
In combustion chambers in which fuel burns at high temperatures, a large amount of extremely toxic nitrogen oxides is formed. The maximum allowable concentration (MAC) of NO, safe for human health, in the air of settlements is 0.08 mg/m 3 .
Since the formation of nitrogen oxides significantly decreases with decreasing temperature, in recent years, power engineers have shown increasing interest in the so-called low-temperature (as opposed to high-temperature - with a temperature of 1100 ° C and above) fluidized bed combustion, when stable and complete combustion of hard and brown coals it is possible to provide at 750 ... 950 "C.
Burning in a fluidized bed. A layer of fine-grained material, blown from bottom to top with air at a speed exceeding the stability limit of a dense layer, but insufficient to carry particles out of the layer, creates circulation. The intense circulation of particles in the limited volume of the chamber gives the impression of a rapidly boiling liquid. A significant part of the air passes through such a layer in the form of bubbles, strongly mixing the fine-grained material, which further enhances the resemblance to a boiling liquid and explains the origin of the name.
The method of combustion in a fluidized (fluidized) bed (see Fig. 5.5, d) is, in a certain sense, intermediate between layer and chamber. Its advantage is the ability to burn relatively small pieces of fuel (usually smaller than 5 ... 10 mm) at an air speed of 0.1 ... 0.5 m / s.
Fluidized bed furnaces are widely used in industry for burning pyrites in order to obtain SO 2, roasting various ores and their concentrates (zinc, copper, nickel, gold), etc.
There are three methods of fuel combustion: layered, in which the fuel in the layer is blown with air and burned; flare, when the fuel-air mixture burns in a torch in a suspended state when moving through the combustion chamber, and vortex (cyclone), in which the fuel-air mixture circulates along a streamlined contour due to centrifugal forces. Torch and vortex methods can be combined into a chamber method.
Process stratified combustion of solid fuels occurs in a fixed or fluidized bed (fluidized). In a fixed layer (Fig. 2.6, a) pieces of fuel do not move relative to the grate, under which the air necessary for combustion is supplied. In a fluidized bed (Fig. 2.6, b) particles of solid fuel under the action of the velocity pressure of air intensively move one relative to the other. The flow velocity at which the stability of the layer is violated and the reciprocating motion of particles over the lattice begins is called critical. The fluidized bed exists within the velocity range from the beginning of fluidization to the mode of pneumatic transport.
Rice. 2.6. Fuel combustion schemes: a– in a fixed layer; b– in a fluidized bed; v– flare once-through process; G– vortex process; d– the structure of the fixed layer during fuel combustion and the change a, O 2 , SO, SO 2 and t according to the layer thickness: 1 - lattice; 2 - slag; 3 – burning coke;
4 - fuel; 5 - superficial flame
On fig. 2.6, d the structure of the fixed layer is shown. Fuel 4 poured onto the burning coke is heated. The released volatiles burn out, forming a superficial flame 5. The maximum temperature (1300 - 1500 °C) is observed in the area of combustion of coke particles 3. Two zones can be distinguished in the layer: oxidative, a > 1; restorative, a< 1.
In the oxidizing zone, the reaction products of fuel and oxidizer are both SO 2 and SO. As air is used, the formation rate SO 2 slows down, its maximum value is reached with an excess of air a = 1. In the reduction zone, due to an insufficient amount of oxygen (a< 1) начинается реакция между SO 2 and burning coke (carbon) to form SO. Concentration SO in combustion products increases, and SO 2 decreases. The length of the zones depending on the average size d to fuel particles is as follows: L 1 = (2 – 4) d to; L 2 = (4 – 6) d to. For zone lengths L 1 and L 2 (in the direction of their decrease) are affected by an increase in the content of volatile combustibles, a decrease in ash content A r, rising air temperature.
Since in zone 2, except for SO contained H 2 and CH 4 , the appearance of which is associated with the release of volatiles, then for their afterburning, part of the air is supplied through blow nozzles located above the layer.
In a fluidized bed, large fuel fractions are in suspension. The fluidized bed can be high temperature and low temperature. Low-temperature (800 - 900 °C) fuel combustion is achieved by placing the heating surface of the boiler in a fluidized bed. In contrast to the fixed bed, where the particle size of the fuel reaches 100 mm, the fluidized bed burns crushed coal with d to£25mm.
The layer contains 5 - 7% of the fuel (by volume). The heat transfer coefficient to the surfaces located in the layer is quite high and reaches 850 kJ/(m2×h×K). When burning low-ash fuels, to increase heat transfer, fillers are introduced into the layer in the form of inert granular materials: slag, sand, dolomite. Dolomite binds sulfur oxides
(up to 90%), which reduces the likelihood of low-temperature corrosion. The lower temperature level of the gases in the fluidized bed helps to reduce the formation of nitrogen oxides during combustion, the release of which into the atmosphere pollutes the environment. In addition, slagging of the screens, i.e., sticking of the mineral part of the fuel on them, is excluded.
A characteristic feature of the circulating fluidized bed is the approach to the operation of the bed in the mode of pneumatic transport.
Chamber method of solid fuel combustion carried out mainly in powerful boilers. In chamber combustion, ground to a pulverized state and pre-dried solid fuel is fed with part of the air (primary) through the burners into the furnace. The rest of the air (secondary) is introduced into the combustion zone most often through the same burners or through special nozzles to ensure complete combustion of the fuel. In the furnace, pulverized fuel burns in suspension in a system of interacting gas-air flows moving in its volume. With greater grinding of the fuel, the area of the reacting surface increases significantly, and, consequently, the chemical reactions of combustion.
A characteristic of the grinding of solid fuel is the specific area F pl dust surface or the total surface area of dust particles weighing 1 kg (m 2 /kg). For spherical particles of the same (monodispersed) size, the value F pl is inversely proportional to the particle diameter.
In fact, the dust obtained during grinding has a polydisperse composition and a complex shape. To characterize the quality of grinding polydisperse dust, along with the specific surface area of the dust, the results of its sifting on sieves of various sizes are used. According to the sieving data, a grain (or grinding) characteristic of dust is built in the form of a dependence of residues on a sieve on the size of sieve cells. The most commonly used indicators of residues on sieves of 90 microns and 200 microns R 90 and R 200 . Preliminary preparation of fuel and heating of air provide burnout of solid fuel in the furnace for a relatively short period of time (several seconds) of dusty air flows (torches) in its volume.
Technological methods of organizing combustion are characterized by a certain input of fuel and air into the furnace. In most pulverizing systems, the transport of fuel to the furnace is carried out by primary air, which is only a part of the total amount of air required for the combustion process. The supply of secondary air to the furnace and the organization of its interaction with the primary are carried out in the burner.
The chamber method, unlike the layer method, is also used for burning gaseous and liquid fuels. Gaseous fuel enters the combustion chamber through the burner, and liquid fuel enters the combustion chamber in pulverized form through nozzles.
Layer fireboxes
The fixed bed firebox can be manual, semi-mechanical or mechanical with a chain grate. Mechanical firebox called a layered furnace device in which all operations (fuel supply, slag removal) are performed by mechanisms. When servicing semi-mechanical furnaces, manual labor is used along with mechanisms. There are furnaces with a straight line (Fig. 2.7, a) and reverse (Fig. 2.7, b) by the movement of gratings 1, driven by sprockets 2. The fuel consumption supplied from the hopper 3 is regulated by the installation height of the gate 4 (see Fig. 2.7, a) or the speed of movement of dispensers 7 (Fig. 2.7, b). In gratings with a reverse stroke, fuel is supplied to the web by mechanical casters 8 (Fig. 2.7, b, c) or pneumatic (Fig. 2.7, G) type. Small fractions of fuel burn in suspension, and large fractions - in a layer on the grate, under which air is supplied 9. Heating, ignition and combustion of the fuel occur due to the heat transferred by radiation from the combustion products. Slag 6 with the help of a slag remover 5 (Fig. 2.7, a) or under the action of its own weight (Fig. 2.7, b) enters the slag bunker.
The structure of the burning layer is shown in fig. 2.7, a. Region III burning coke after zone II heating of incoming fuel (zone I) is located in the central part of the lattice. There is also a recovery area. IV. The uneven degree of fuel combustion along the length of the grate leads to the need for a sectional air supply. Most of the oxidant must be fed into the zone III, a smaller amount - to the end of the coke reaction zone and a very small amount - to the zone II preparation of fuel for combustion and zone V slag burning. This condition is met by a stepped distribution of excess air a 1 along the length of the grate. The supply of the same amount of air to all sections could lead to increased excess air at the end of the grate web, as a result of which it would not be enough to burn coke (curve a 1) in the zone III.
The main disadvantage of furnaces with chain grates is the increased heat loss from incomplete combustion of the fuel. The scope of such gratings is limited to boilers with steam output D= 10 kg/s and fuels with volatile \u003d 20% and reduced humidity.
Fluidized bed furnaces are characterized by reduced emissions of such harmful compounds as NO x, SO 2, a low probability of screen slagging, the possibility (due to the low temperature of the gases) of saturation of the furnace volume with heating surfaces. Their disadvantages are increased incompleteness of fuel combustion, high aerodynamic resistance of the grate and layer, and a narrow range of regulation of the steam output of the boiler.
Rice. 2.7. Schemes of operation of chain grates and types of fuel dispensers: a, b- furnaces with forward and reverse gratings, respectively; v, G– mechanical and pneumatic casters;
1 - lattice; 2 - stars; 3 - bunker; 4 - gate; 5 - slag remover; 6 - slag; 7 - fuel dispenser; 8 - caster; 9 - air supply; I – zone of fresh fuel; II – fuel heating zone;
III - area of combustion (oxidation) of coke; IV - recovery zone; V - fuel burning zone
The layered method of fuel combustion is characterized by relatively low rates of the combustion process, its reduced efficiency and reliability. Therefore, he did not find application in boilers of high productivity.
