Solid fuel combustion methods. Flare combustion. Fuel combustion methods. Types of combustion devices, their characteristics Methods of gas combustion
Holders of the patent RU 2553748:
The invention relates to heat power engineering and can be used in furnaces and heat generators of various types that use fossil fuel for combustion.
There is a known method of efficient combustion of fuel by separating gas (combustion reaction products), for example, Method of gas separation using membranes with purging of permeate to remove CO 2 from combustion products according to patent 2489197 (RU) BAKER Richard (US), VIDZHMANS Johannes Gee (US) et al.
The implementation of this combustion method is carried out in several stages: the stage of capturing carbon dioxide, the stage of membrane separation of gases, working in combination with compression and condensation to obtain a product from carbon dioxide in the form of a liquid, and a stage based on blowing, in which incoming air or oxygen is used for the furnace. as a purge gas. The disadvantage of this method is its complexity in implementation, since it includes many additional stages of a standard type, such as heating, cooling, compression, condensation, pumping, various types of separation and / or fractionation, as well as monitoring pressures, temperatures, flows, etc. with this method, the capture of carbon dioxide occurs from the waste stream formed by the combustion of fuel diluted with ballast gases, which therefore has a lower temperature.
The closest technical solution (prototype) is a Method for burning solid fuel in household heating furnaces according to patent 2239750 (RU), the authors of Ten V.I. (RU) and Ten G.Ch. (RU), Patent holder Ten Valery Ivanovich (RU).
This method includes loading fuel onto the grate of the furnace, creating traction in its working space, igniting and burning fuel with the removal of combustion products into the atmosphere, regulating the thrust and the amount of combustion products removed from the furnace by slightly opening the blower and chimney flaps.
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 an intensive combustion mode, and after reaching a predetermined furnace temperature, the combustion process is transferred to a damping mode, then ignition is performed again with the help of sophisticated automation and using already liquid or gaseous fuel. 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 flow with ballast gases (nitrogen, excess air, etc.), which worsen the conditions for fuel combustion and use of the released heat (useful heat is taken and carried out into the atmosphere).
The proposed 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 adopted: 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 that do not participate in combustion; 8 - supply of superheated steam; 9 - useful hot products - heat carriers, carbon dioxide and water vapor; 10 - heat exchange zone; 11 - grate; 12 - outlet of gases from the furnace bell.
The proposed method is carried out as follows. Solid fuel is loaded onto the grate 11, it is ignited, while the 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 combustion of 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 an increased nitrogen content). The peculiarity of the bell structure is that during the combustion reaction, the resulting gases are separated. Hot gases rise upward, giving off thermal energy to the bell, while cold particles of ballast gases go down through the bell zones with a low temperature. Fuel combustion reactions are expressed by well-known combustion equations. The ratios of the reacting substances are maintained, as is their composition. That is, carbon C, hydrogen H 2 with oxygen O 2 enter into the reaction in the amount determined by the chemical equations:
other substances cannot react. The combustion reaction takes place in the combustion zone between hydrocarbon and oxygen without the participation of ballast gases, while nitrogen released from the air in the composition of excess air, as less heated, is pushed out through the lower part of the bell (the outlet pipe is not shown in the diagram). After warming up the combustion chamber and the presence of incandescent carbon in it, superheated water vapor 8 is supplied to the bell below the secondary air supply zone. As a result of the interaction of carbon with water vapor at high temperatures, flammable gases arise in accordance with the well-known chemical equations
at low temperatures with a total positive thermal effect, which enhance the process of fuel combustion and increase heat transfer from it. 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 complex equipment and can be widely used in industry and in everyday life.
SOURCES OF INFORMATION
1. Patent of the Russian Federation No. 2489197, IPC B01D 53/22 (2006.01). Gas separation method using membranes with permeate purging to remove carbon dioxide from combustion products. Patentee, MEMBRANE TECHNOLOGY AND RESERCH, INC. (US).
2. Patent of the Russian Federation No. 2239750, IPC F24C 1/08, F24B 1/185. A method of burning fuel in household heating stoves. The patentee is Ten Valery Ivanovich.
3. Mäkelä K. Stoves and fireplaces. Reference manual. Translated from Finnish. Moscow: Stroyizdat, 1987.
4. Ginzburg D.B. Solid fuel gasification. State publishing house of literature on construction, architecture and building materials. M., 1958.
A method of fuel combustion in furnaces having a bell with a fuel combustion chamber and a grate, including loading fuel, igniting and burning fuel due to the primary air entering through the blower, characterized in that the movement of gases in the bell is carried out without using the draft of the pipe, with the possibility of accumulating of hot gases in the upper part of the bell, while secondary air is supplied to the bell, directly to the combustion zone, while hot gases rise upward, giving thermal energy to the bell, and cold particles of ballast gases go down through the bell zones with a low temperature, after the chamber is heated combustion into it, below the secondary air supply, superheated water vapor is supplied to the incandescent carbon and combustible gases are obtained.
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The invention relates to heat power engineering and can be used in furnaces and heat generators of various types that use fossil fuel for combustion. The technical result is an increase in the efficiency of furnaces and heat generators. The method of fuel combustion in furnaces having a bell with a fuel combustion chamber and a grate, includes loading fuel, igniting and burning the fuel due to the primary air entering through the blower. The movement of gases in the bell is carried out without using the draft of the pipe, with the possibility of accumulating hot gases in the upper part of the bell. In this case, secondary air is fed into the bell, directly into the combustion zone. Hot gases rise upward, giving off thermal energy to the bell, while cold particles of ballast gases go down through the bell zones with a low temperature. After warming up the combustion chamber, superheated water vapor is supplied to the incandescent carbon into it, below the secondary air supply, and combustible gases are obtained. 1 ill.
5.1. Solid fuel combustion methods
5.2. Combustion of liquid fuels
5.2.1. Fuel oil quality.
5.2.2. Problems of preparation of fuel oil for combustion
5.2.3. Problems with the use of fuel oil in boiler houses and thermal power plants
5.3. Combustion of gaseous fuels
5.3.1. Gas preparation
5.3.2. Features of the natural gas combustion process
5.3.3. Combustion of gaseous fuels
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
Incineration methods. The combustion device, or firebox, is the main element of a boiler unit or a fired industrial furnace and serves to burn fuel in the most economical way and convert its chemical energy into heat. Fuel combustion occurs in the furnace, part of the heat of combustion products is transferred to heating surfaces located in the combustion zone, as well as a certain amount of focal residues (ash, slag) is captured. 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 solid fuel combustion are usually used: layer, flare (chamber), vortex and fluidized bed combustion (Fig.5.5). Each of these methods has its own characteristics concerning the basic principles of the organization of aerodynamic processes occurring in the combustion chamber. For combustion of liquid and gaseous fuels, only the flare (chamber) combustion method is used.
Layer method. The combustion process by this method 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 meets a stationary or slowly moving layer of fuel during its movement and, interacting with it, turns into a flow of flue gases.
An important feature of layered furnaces is the presence of a fuel reserve on the grate, tied to its hourly consumption, which allows the primary regulation of the furnace power only by changing the amount of supplied air. The fuel supply on the grate also ensures 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 solid fuel combustion will be used for a long time in boiler houses of small and medium power generation.
In fig. 5.6 6 shows the schematic diagrams of layered furnaces. In layered combustion, the air required 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 gaseous products of thermal decomposition of the fuel and fine fuel particles removed from the layer burn above the layer. Combustion products together with excess air from the furnace enter the boiler gas ducts.
Layer furnaces are widely used in boilers of small and medium power. They are divided according to several classification criteria. Depending on the method of maintenance, there are manual fireboxes (see Fig.5.6, a), non-mechanized, semi-mechanized (see Fig.5.6, b, c) and mechanized (see fig. 5.6, d, e). Shown in Fig. 5.6 layer furnaces 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; d - in a fluidized bed.
Furnaces with a fixed grate and movinga layer of fuel flowing along it(see fig. 5.6, b, d).
Furnaces with a bed moving together with the grateI eat fuel(see fig. 5.6, e).
The simplest layered furnace with a fixed grate and manual operation (see Fig.5.6, a) It is used for burning all types of solid fuels. Boilers of only very low steam capacity - 0.275 ... 0.55 kg / s (1 ... 2 t / h) are equipped with such furnaces.
In a firebox with a fixed inclined grate (see Fig.5.6, b) fuel, as it burns, moves along the grate under the action of gravity. These furnaces are used for burning wet fuels (wood waste, sod peat) under boilers with a steam capacity of 0.7 ... 1.8 kg / s (2.5 ... 6.5 t / h).
In a semi-mechanized firebox (see fig. 5.6, v), The fuel is supplied to the stationary grate by means of a spreader 5. In these furnaces, coal and brown coals, sorted anthracite are burned under boilers with a steam capacity of 0.55 ... 2.8 kg / s (2 ... 10 t / h).
The simplest mechanized firebox is a firebox with a rustling bar (see Fig.5.6, G). It consists of a fixed lattice lattice, along the entire width of which the bar 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 layered firing is a mechanical chain grate (see fig. 5.6, e). The mechanical chain grate is made in the form of an endless grate, moving together with a layer of burning fuel lying on it. Each new portion of fuel entering the grate follows the layer of fuel. The grate speed can be changed depending on the fuel consumption (boiler operating mode) from 2 to 16 m / h. These furnaces are used to burn sorted anthracite and non-caking coals with moderate moisture content and ash content and the release of volatile substances. Have T = 10 ... 25%. Existing modifications of furnaces with chain grates make it possible to use them for combustion of other fuels as well. Furnaces with chain grates are installed under boilers with a steam capacity of 3 ... 10 kg / s (10.5 ... 35 t / h) and above.
Flare method. In contrast to the layered process, this process (See Fig.5.5, b) 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 homogeneity of the burning torch, and, consequently, the gas-air flow with the fuel suspended in it, the solid fuel particles are ground to a pulverized state, to sizes measured in microns (from 60 to 90% of all particles have a size less than 90 microns). Liquid fuel is pre-sprayed in nozzles into very small droplets so that the droplets do not fall out of the stream and have time to completely burn out in a short time in the furnace. Gaseous fuel is fed into the furnace through the 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 changes in the mode. The power of the furnace can be adjusted 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 pulverization system and in the form of dust is blown into the furnace, where it burns in a suspended state. Grinding of the fuel 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 layer furnaces, which significantly increases the volume of furnace spaces.
Dust preparation from lump 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 working 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 grains 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 is left on a sieve with a mesh size of 90 microns, and all the rest of the dust has passed through the sieve.
The optimum fineness of grinding (fineness) is determined by the total factor: the minimum power consumption for grinding fuel and losses from mechanical underburning. The fineness of grinding depends on the reactivity of the fuel, characterized mainly by the yield of volatile substances. The higher the volatile content of the fuel, the coarser the grind.
The grinding properties of the fuel are characterized by the grindability coefficient, (for anthracite Klo = 1; for lean coal TO lo = 1.6; For brown coal near Moscow, Cl 0 = 1.75).
The main disadvantages of this scheme are the lack of a dust supply, which reduces the reliability of the boiler, and the strong wear of the mill fan, through which all the coal dust is passed.
In fig. 5.9 is a diagram of dust preparation with an intermediate hopper. Its difference is 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 settles, and then sent to the intermediate hopper 9. Dust from the cyclone into the hopper descends through the valves (flashers) 8, which open when a certain portion of dust is pressed on them. Air with residual fine dust is sucked out of the cyclone by a mill fan 12 and is pumped into the primary air pipeline, where, in turn, dust from the intermediate hopper enters with the help of auger or paddle dust feeders 10. The dust preparation scheme with an intermediate hopper, as the most flexible and reliable, has become the most widespread.
Various types of mills are used to grind fuel. The choice of the type of mill depends on the grinding characteristics of the fuel, the yield of volatiles and the moisture content of the fuel. Distinguish between low-speed and high-speed mills.
To grind anthracite and bituminous coals with a small release of volatile substances, burned by boilers of medium and large steam capacity, low-speed ball drum mills are used (Figure 5.10). The main advantages of a drum mill are good controllability of grinding fineness and grinding reliability. The disadvantages of these mills include: loudness, high cost, increased specific power consumption, significant noise accompanying the operation of the mill.
There are two types of high-speed mills: hammer mills and fan mills.
The fan mill (MB) is designed for grinding mainly high-moisture brown coals and milled peat. Fireboxes with MV are used in boilers of average productivity. The grinding body MV is a massive impeller 1 (Fig. 5.12) with a rotational speed of 380 ... 1470 rpm, located in an armored case 6.
Vtheir 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 of "residence 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 much is necessary for their complete burnout, regardless of the duration of the stay of the combustion products in the furnace.
They burn rather small particles of coal (usually finer than 5 mm), and the air necessary for combustion is supplied with huge (up to 100 m / s) speeds tangentially to the cyclone generatrix - A powerful vortex is created in the furnace, involving the particles in the circulation movement, in which they are intensively blown by the flow (see Fig.5.5, v).
A significant specific surface area of small particles, large values of the mass transfer coefficients between the flow and the particles, high concentrations of fuel in the chamber ensure 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 ° С) develop in the furnace. Coal ash melts, liquid slag, flowing down the walls, slows down the movement of particles adhering to its surface, which further increases the speed of their washing with a stream, and hence the coefficient of mass transfer.
Since the centrifugal effect decreases with an increase in the 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 cyclonic combustion chambers are mainly used, for example, for the combustion of sulfur (in order to obtain SO 2 - raw material for the production of H 2 SO 4; in this case, the heat of combustion is also used), for the melting and roasting of ores and non-metallic materials (for example, phosphorites) etc. Recently, in cyclone furnaces, fire neutralization of wastewater is carried out, i.e., the combustion of harmful impurities contained in them is carried out due to the supply of additional (usually gaseous or liquid) fuel.
In combustion chambers, in which fuel is burned at high temperatures, a large amount of extremely toxic nitrogen oxides are formed. The maximum permissible concentration (MPC) N0, 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) combustion in a fluidized bed, when stable and complete combustion of bituminous and brown coals it is possible to provide at 750 ... 950 "C.
Fluidized bed incineration. A layer of fine-grained material, blown from the bottom up by air at a speed exceeding the stability limit of a dense layer, but not sufficient to carry particles out of the layer, creates circulation. Intense circulation of particles in a limited volume of the chamber creates 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 combustion method in a fluidized (fluidized) bed (see Fig. 5.5, d) is, in a sense, intermediate between a layer and a 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.
Combustion of fuel is a chemical process of combining its combustible elements with atmospheric oxygen, which occurs at high temperatures and is accompanied by the release of a significant amount of heat. Depending on the type of fuel distinguish homogeneous, heterogeneous combustion and pulsating (PULSAR). Homogeneous combustion occurs in volume (in mass), while the fuel and oxidizer are in the same state of aggregation (for example, gaseous fuel and air). Heterogeneous combustion occurs at the interface between two phases, that is, during the combustion of solid and liquid fuels. There are two types of combustion: in a lump fuel layer and in a pulverized fuel flare (layered and flared combustion methods). Gaseous and liquid fuels are burned only in a flare. The method of supplying air to the fuel is essential when burning it in a flare. The total combustion time t is determined by the time of mixture formation td and the time of the chemical combustion reactions tc. Since it is possible to overlap these stages of the processes, the total combustion time is t = td + tc.
A device designed to burn fuel is called firebox... Classification: by the way of fuel combustion- layered, chamber (flare) and cyclonic; only solid fuel is burned in the layer, and in other cases - solid, liquid and gaseous; by fuel supply mode- with periodic and continuous supply; in relation to the boiler- internal, i.e. located inside the boiler, remote, arranged outside the heated surface of the boiler; by the way of fuel supply and organization of service- manual, semi-mechanical and mechanical. Burners for layered fuel combustion can be of the following types: a) furnaces with a fixed grate and a layer of fuel lying motionless on it; b) furnaces with a fixed grate and a layer of fuel moving on it; c) furnaces with a moving grate, moving the layer of fuel lying on it . Manual firebox with a horizontal stationary grate allows burning all types of solid fuel with manual maintenance of loading, slagging and slag removal operations; it is used in boilers with a steam capacity of 1-2 t / h. Fire chambers with rustling bar: when moving forward, it moves fuel from the loading hopper to the depth of the furnace and dumps slag from the grate, and during the return stroke it agitates the fuel layer. a- manual with a horizontal grate; b - firebox with a thrower on a fixed bed; v- firebox with rustling bar; G- a firebox with an inclined grate; d- firebox of the Pomerantsev system; e - a firebox with a chain mechanical grate; f- the same reverse and thrower; s- chamber furnace for pulverized fuel; To- furnace for burning liquid and gaseous fuels Inclined grate furnaces... In them, the fuel is loaded into the furnace from above, as it burns under the influence of gravity, it slides into the lower part of the furnace, creating an opportunity for new portions of fuel (2.5-20 t / h) to enter the furnace. High-speed shaft furnaces of V. V. Pomerantsev's system They are used for burning sod peat under boilers with a steam capacity of up to 6.5 t / h. Moving grate furnaces... These include furnaces with a forward and reverse mechanical chain grate. The forward-running chain grate moves from the front wall of the furnace to the rear one, while the fuel flows by gravity onto the grate. (10-150 t / h). In chamber furnaces the fuel is burned in the form of coal dust. It is fed mixed with air into the furnace, where it burns in suspension. Chamber furnaces for liquid and gaseous fuel. Direct-flow and vortex burners are used. The operation of the furnaces is characterized by the following indicators: thermal power, heat loads of the grate and furnace volume, efficiency.
1 TYPES OF FUEL
Solid fuel - flammable substances, the main component of which is carbon. Solid fuels include coal and brown coal, oil shale, peat and wood. Fuel properties are largely determined by its chemical composition - the content of carbon, hydrogen, oxygen, nitrogen and sulfur. The same amounts of fuel give different amounts of heat during combustion. Therefore, to assess the quality of the fuel, its calorific value is determined, that is, the largest amount of heat released during the complete combustion of 1 kg of fuel (the highest calorific value of coal). Basically, solid fuels are used to obtain heat and other types of energy, which are spent on obtaining mechanical work. In addition, more than 300 different chemical compounds can be obtained from solid fuel with appropriate processing (distillation); processing of brown coal into valuable types of liquid fuel - gasoline and kerosene is of great importance.
Briquettes
Briquettes are solid fuel formed in the process of compressing waste from the woodworking process (shavings, chips, wood dust) as well as household waste (straw, husks), peat.
Fuel briquettes are convenient for storage, no harmful binders are used in the manufacture, therefore this type of fuel is environmentally friendly. During combustion, they do not spark, do not emit foul gas, they burn evenly and smoothly, which ensures a sufficiently long combustion process in the boiler chamber. In addition to solid fuel boilers, they are used in home fireplaces and for cooking (for example, on the grill).
There are 3 main types of briquettes:
1. RUF-briquettes. Formed rectangular bricks.
2. NESTRO-briquettes. Cylindrical, can also be with holes inside (rings).
3. Rini& Kau - briquettes. Faceted briquettes (4,6,8 sides).
Advantages of fuel briquettes:
Environmentally friendly.
Long and convenient storage. Thanks to heat treatment, they are not susceptible to fungal attack. And thanks to the formation, they are conveniently used.
Long and even burning is due to the high density of the briquettes.
High calorific value. Almost twice as high as that of ordinary firewood.
Constant combustion temperature. Due to the uniform density.
Cost effective.
Minimum ash content after burning: 1-3%
Pellets or fuel pellets.
Essentially the same production principle as for briquettes. Lignin (plant polymer) is used as a binder.
The materials are the same as for briquettes: bark, shavings, straw, cardboard. First, the raw material is crushed to the state of pollen, then, after drying, a special granulator forms granules of a special shape from the mass. Used in pellet heating boilers. Prices for this type of solid fuel are the highest - this is due to the complexity of production and popularity with buyers.
There are the following types of this solid fuel:
Processing of round timber of hard and soft tree species into pellets.
Peat pellets
Pellets obtained from the processing of sunflower husks.
Straw pellets
The advantages of pellets:
Environmentally friendly.
Storage. Due to special production technologies, pellets can be stored directly in the open air. They do not swell, do not become covered with fungus.
Long and even burning.
Low cost.
Due to their small shape, the pellets are suitable for boilers with automatic loading.
Wide range of applications (boilers, stoves, fireplaces)
Firewood
Pieces of wood intended for obtaining heat by burning in boilers for heating with solid fuels, fireboxes intended for firewood. For convenience, the length of the logs is most often 25-30 cm. For the most effective use, "the lowest possible moisture level is required. For heating, it is necessary to burn as slow as possible. Also, in addition to heating, firewood can be used, for example, in boilers for solid fuel. Best for these parameters. deciduous species are suitable: oak, ash, hazel, hawthorn, birch .. Worse - coniferous firewood, as they contribute to the deposition of resin and have a low calorific value, while quickly burn out.
Firewood is presented in two types:
Sawed.
Chipped.
2 FUEL COMPOSITION
For the formation of coal, an abundant accumulation of plant matter is necessary. In ancient peat bogs, starting from the Devonian period, organic matter accumulated, from which fossil coals were formed without oxygen. Most of the commercial deposits of fossil coal date from this period, although there are also younger deposits. The oldest coals are estimated to be about 350 million years old. Coal is formed when rotting plant material accumulates faster than bacterial decomposition occurs. An ideal environment for this is created in swamps, where stagnant water, depleted in oxygen, interferes with the vital activity of bacteria and thereby protects the plant mass from complete destruction? At a certain stage of the process, the acids released during the process prevent further bacterial activity. This is how peat is formed - the initial product for the formation of coal. If then it is buried under other sediments, then the peat is compressed and, losing water and gases, is converted into coal. Under the pressure of 1 kilometer thick layers of sediment, a layer of brown coal 4 meters thick is obtained from a 20-meter layer of peat. If the depth of burial of plant material reaches 3 kilometers, then the same layer of peat will turn into a layer of coal 2 meters thick. At a greater depth, about 6 kilometers, and at a higher temperature, a 20-meter layer of peat becomes an anthracite layer 1.5 meters thick. As a result of the movement of the earth's crust, the coal seams experienced uplift and folding. Over time, the raised parts were destroyed due to erosion or spontaneous combustion, and the lowered ones remained in wide shallow basins, where coal is at least 900 meters from the earth's surface.
Brown coals. They contain a lot of water (43%) and therefore have a low calorific value. In addition, they contain a large amount of volatile substances (up to 50%). Formed from dead organic residues under load pressure and under the influence of elevated temperatures at depths of about 1 kilometer.
Coals. They contain up to 12% moisture (3-4% internal moisture), therefore they have a higher calorific value. They contain up to 32% volatile substances, due to which they are quite flammable. Formed from brown coal at depths of about 3 kilometers.
Anthracites. Almost entirely (96%) are carbon. They have the highest calorific value, but are poorly flammable. Formed from coal and in the form of oxidesBUT NS. They refer to the harmful components of combustion products, the amount of which should be limited.
Sulfur - found in solid fuels as organic compoundsSOand pyriteS xthey are combined into volatile sulfurS l... Sulfur is also included in the fuel in the form of sulfurous salts - sulphates - which are incapable of burning. Sulfate sulfur is usually referred to as fuel ash. The presence of sulfur significantly reduces the quality of solid fuels, since sulfurous gasesSO 2 andSO 3 combining with water, they form sulfuric acid - which in turn destroys the metal of the boiler, and getting into the atmosphere harms the environment. It is for this reason that the sulfur content in fuels - not only in solid ones - is highly undesirable.
Ash - fuel is a ballast mixture of various minerals remaining after the complete combustion of the entire combustible part of the city. Ash directly affects the quality of fuel combustion - it reduces combustion efficiency.
Questions:
1. What are the main types of solid fuels?
2. What is ash?
3 FUEL APPLICATION
The use of coal is diverse. It is used as a household, energy fuel, raw material for the metallurgical and chemical industries, as well as for the extraction of rare and trace elements from it. Liquefaction (hydrogenation) of coal with the formation of liquid fuel is very promising. For the production of 1 ton of oil, 2-3 tons of coal are consumed, some countries almost completely provided themselves with fuel due to this technology. Artificial graphite is obtained from coal.
Brown coal outwardly differs from coal by the color of a line on porcelain plastic - it is always brown. The most important difference from bituminous coal is its lower carbon content and significantly higher VOC and water content. This explains why brown coal burns more easily, gives more smoke, smell, as well as the aforementioned reaction with caustic potassium and produces little heat. Due to its high water content for combustion, it is used in powder, into which it inevitably turns during drying. The nitrogen content is significantly inferior to coal, but the sulfur content is increased.
The use of brown coal - as a fuel, brown coal is used in many countries much less than coal, however, due to its low cost in small and private boiler houses, it is more popular and sometimes takes up to 80%. It is used for pulverized combustion (during storage, brown coal dries up and crumbles), and sometimes the whole. In small provincial CHP plants, it is also often burned for heat. However, in Greece and especially in Germany, lignite is used in steam power plants, generating up to 50% of electricity in Greece and 24.6% in Germany. The production of liquid hydrocarbon fuels from brown coal by distillation is spreading at a high speed. After distillation, the residue is suitable for the production of soot. Combustible gas is extracted from it, and carbon-alkali reagents and methane-wax (mountain wax) are obtained. In scanty quantities, it is also used for crafts.
Peat is a combustible mineral formed in the process of natural withering away and incomplete decay of marsh plants in conditions of excessive moisture and difficult air access. Peat is a product of the first stage of the coal educational process. The first information about peat as a "combustible soil" used for cooking dates back to the 26th century AD.
Sedimentary rock of plant origin, composed of carbon and other chemical elements. The composition of coal depends on age: anthracite is the oldest, coal is younger, and the youngest brown. It has different moisture content depending on aging; the younger, the more moisture. Coal in the process of burning pollutes the environment, plus it is sintered into slag and deposited on the grates in the boiler. This prevents normal combustion.
Questions:
Fuel application?
Is fuel combustion harmful to the environment, and which type is the most ?
4 WAYS OF FUEL BURNING
There are three ways of fuel combustion: layer, flare or chamber and vortex.
1 - grate; 2 - igniter door; 3 - loading door; 4 - heating surfaces; 5 - combustion chamber.
Figure 4.1 - Layered furnace scheme
This drawing shows a layered method of fuel combustion, where a layer of lumpy fuel lies motionless on the grate and is blown with air.
The layered method is used to burn solid fuels.
And here is shown a flare and vortex method of fuel combustion.
1 - burner; 2 combustion chamber; 3 - lining; 4 - furnace screen; 5 - ceiling-mounted radiant steam superheater; 6 - scallop.
Figure 4.2 - Chamber furnace
Figure 4.3 - Vortex fuel combustion
With the flare and vortex method, all types of fuel can be burned, only solid fuel is preliminarily subjected to breaking, turning it into dust. When fuel is burned, all heat is transferred to the combustion products. This temperature is called the theoretical combustion temperature of the fuel.
In industry, continuous boilers are used to burn solid fuels. The principle of continuity is supported by a grate, to which solid fuel is constantly supplied.
For a more rational combustion of fuel, boilers are being built that are capable of burning it in a dusty state. Liquid fuels are burned in the same way.
Questions:
What is the most rational combustion method?
Explain the advantages of the chamber combustion method.
5 OPERATING PROCESSES IN BOILERS
Work processes in boilers:
Steam formation
In boiler plants, processes such as the formation of steam occur:
The conditions under which steam is formed in boilers are constant pressure and continuous heat supply.
Steps in the vaporization process: water heating to saturation temperature, vaporization and steam heating to a predetermined temperature.
Even in boilers, one can observe the corrosion of heating surfaces:
The destruction of metal under the influence of the environment is called corrosion.
Corrosion from the side of combustion products is called external, and from the side of the heated medium - internal.
There is low temperature and high temperature corrosion.
To reduce the destructive force of corrosion, it is necessary to monitor the water regime of the boiler. Therefore, raw water before use forboiler feed is pretreated in order to improve its quality.
Boiler water quality is characterized by dry residue, total salt content, hardness, alkalinity and content of corrosive gases
Sodium cation filter - where the water is purified
Deaerator - corrosive agents, air oxygen and carbon dioxide are removed.
Samples of pipes that have corroded outside and inside.
Corrosion of heating surfaces
Internal corrosion of steam and hot water boilers is mainly of the following types: oxygen, steam-water, alkaline and sub-sludge.
The main appearance of oxygen corrosion is ulcers, usually with iron oxides.
Steam-water corrosion is observed during the operation of boilers with increased thermal loads. As a result of this corrosion, on the inner surfaces of the wall tubes and brittle damage in the places where the boiler water is evaporated.
Pits are formed as a result of undersludge corrosion.
External corrosion can be low temperature and high temperature.
Low temperature corrosion can occur when any fuel is burned. High-temperature corrosion can occur when burning fuel oil.
Combustion devices or furnace is the main element of a boiler unit or firing furnace and serves for burning fuel in the most economical way and converting it in the most economical way and converting its chemical energy into heat. There are the following main methods of solid fuel combustion: 1) layered; 2) flare (chamber); 3) vortex; 4) combustion in a fluidized bed. For combustion of 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 lying motionless on it. With an increase in the speed of the fuel passing through the fuel layer. The latter can 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 fuel layers moving along it. 3. Furnaces with a layer of fuel moving together 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 furnace is designed to burn all types of fuel.
Standard grate type RPK- Consists of grate bars, recruited in several rows and mounted on shafts of rectangular cross-section. When the shafts are turned by a turning angle of 30 0, the rows of grates are inclined at the same angle, and through the formed gaps, slag from the grate spills into the ash pan. The gratings are 900 to 3600 mm wide and 915 to 3660 mm long. The most common type of stacker is a mechanized stacker with a mechanical chain drive. The mechanical grate is made in the form of an endless grate cloth moving the depth of the furnace together with a 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 speed of the grating can be changed depending on the fuel consumption from 2 to 16 m / h. These furnaces are used to burn sorted anthracite with a lump size of up to 40 mm. A feature of layered furnaces is the presence of a fuel supply on the grate, which makes it possible to regulate the power of the furnace by changing the amount of supplied air and ensures the stability of the combustion process. The layered method is not suitable for large power plants, and in low and medium power plants, this method is widely used. 2. Flare method. In contrast to the layered one, it is characterized by the continuous movement of fuel particles in the combustion space together with the flow of air and combustion products, in which they are in suspension. The figure shows a chamber combustion chamber with fuel flaring. It consists of burner 1. combustion chamber 2, boiling pipes 3, pipes of the rear screen 4, slurry funnel 5. Pre-crushed fuel in the form of coal dust and the gas mixture are fed into burner 1, secondary air is blown into the same place through a series of holes. A gas-air flow with suspended particles of solid fuel is ignited at the exit from the burner to the furnace 2. In the combustion chamber, the fuel burns to form a burning torch. The heat released during fuel combustion in the form of radiation and convectively 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 dust preparation system; 2) High consumption of electrical energy for grinding; 3) Slightly lower thermal loads of the combustion chamber than in layered furnaces, which contributes to the condition of the volume of combustion spaces. Dust preparation from lump fuel consists of the following operations: 1. Removal of metal objects from fuel using magnetic separators. 2. Crushing large pieces of fuel in crushers 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. The grinding equipment in the pulverization system is low-speed ball drum mills, high-speed hammer mills with axial and disc drying agent supply. For the combustion of pulverized fuel, round and slot burners are used. They are placed in front of the front wall of the firebox, oppositely on the side walls, as well as in the corners of the firebox. For frontal and backward spraying, circular turbulent burners are used that create a short torch.
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