Features of combustion of solid fuel. Combustion of liquid and solid fuels
Solid fuels include wood, peat and coal. The combustion process of all types of solid fuels has similar features.
Fuel must be placed on the grate of the furnace in layers, observing the combustion cycles - such as loading, drying, heating the layer, burning with the release of volatile substances, burning out residues and removing slags.
Each stage of fuel combustion is characterized by certain indicators that affect the thermal regime of the furnace.
At the very beginning of drying and heating of the layer, heat is not released, but, on the contrary, is absorbed from the heated walls of the firebox and unburned residues. As the fuel heats up, gaseous combustible components begin to be released, burning in the gas volume of the furnace. Gradually, more and more heat is released, and this process reaches its maximum during the combustion of the coke base of the fuel.
The combustion process of fuel is determined by its qualities: ash content, humidity, as well as the content of carbon and volatile combustible substances. In addition, the correct choice of the furnace design and fuel combustion modes is important. So, when burning wet fuel, a significant amount of heat is expended on its evaporation, due to which the combustion process is delayed, the temperature in the firebox rises very slowly or even decreases (at the beginning of combustion). Increased ash content also slows down the combustion process. Due to the fact that the ash mass envelops combustible components, it limits the access of oxygen to the combustion zone and, as a result, the fuel may not burn completely, so that the formation of mechanical underburning increases.
The intensive combustion cycle of a fuel depends on its chemical composition, that is, the ratio between volatile gaseous components and solid carbon. First, volatile components begin to burn, the release and ignition of which occurs at relatively low temperatures (150-200 ° C). This process can continue for quite a long time, because there are a lot of volatile substances that differ in their chemical composition and ignition temperature. All of them burn in the above-layer gas volume of the firebox.
The solid components of the fuel remaining after the release of volatile substances have the highest combustion temperature. As a rule, they are based on carbon. Their combustion temperature is 650-700 ° C. Solid components burn in a thin layer located above the grate. This process is accompanied by the release of a large amount of heat.
Of all types of solid fuels, firewood is the most popular. They contain a large amount of volatile substances. From the point of view of heat transfer, birch and larch wood is considered the best. After burning birch firewood, a lot of heat is released and a minimal amount of carbon monoxide is formed. Larch firewood also gives off a lot of heat; when they burn, the furnace array heats up very quickly, which means that they are consumed more economically than birch ones. But at the same time, after burning firewood from larch, a large amount of carbon monoxide is released, so you need to be careful about manipulating the air damper. A lot of heat is also emitted by oak and beech firewood. In general, the use of certain firewood depends on the presence of a nearby forest area. The main thing is that the firewood is dry, and the chocks are of the same size.
What are the features of burning wood? At the beginning of the process, the temperature in the firebox and gas ducts rises rapidly. Its maximum value is reached in the stage of intense combustion. During combustion, a sharp decrease in temperature occurs. To maintain the combustion process, constant access to the furnace of a certain amount of air is necessary. The design of household stoves does not provide for the presence of special equipment that regulates the flow of air into the combustion zone. For this purpose, a blower door is used. If it is open, a constant amount of air enters the furnace.
In batch furnaces, the air requirement varies depending on the stage of combustion. When there is an intensive release of volatile substances, there is usually not enough oxygen, so the so-called chemical underburning of the fuel and the combustible gases emitted by it is possible. This phenomenon is accompanied by heat losses, which can reach 3-5%.
At the stage of afterburning of residues, the opposite picture is observed. Due to an excess of air in the furnace, gas exchange increases, which leads to a significant increase in heat loss. According to studies, up to 25-30% of heat is lost together with the exhaust gases during the afterburning period. In addition, due to chemical underburning, volatile substances settle on the inner walls of the firebox and gas ducts. They have low thermal conductivity, so the useful heat transfer of the furnace is reduced. A large amount of sooty substances leads to a narrowing of the chimney and a deterioration in draft. Excessive buildup of soot can also cause a fire.
Peat, which is the remains of decayed plant matter, has a chemical composition similar to firewood. Depending on the method of extraction, peat can be carved, lumpy, pressed (in briquettes) and milled (peat chips). The moisture content of this type of solid fuel is 25-40%.
Along with firewood and peat, coal is often used for heating stoves and fireplaces, which in its chemical composition is a combination of carbon and hydrogen and has a high calorific value. However, it is not always possible to purchase really high-quality coal. In most cases, the quality of this type of fuel leaves much to be desired. An increased content of fine fractions in coal leads to compaction of the fuel layer, as a result of which the so-called crater combustion begins, which is uneven in nature. When burning large pieces of coal, it also burns unevenly, and with excessive moisture in the fuel, the specific heat of combustion is significantly reduced. In addition, such coal is difficult to store in winter, because coal freezes under the influence of sub-zero temperatures. To avoid such and other troubles, the optimal moisture content of coal should be no more than 8%.
It should be borne in mind that the use of solid fuel for heating household stoves is quite troublesome, especially if the house is large and heated by several stoves. In addition to the fact that a lot of effort and material resources are spent on harvesting and a lot of time is spent on bringing firewood and coal to the stoves, about 2 kg of coal, for example, is poured into the blower, from which it is removed and thrown away along with the ash accumulating there.
In order for the process of burning solid fuels in domestic stoves to be as efficient as possible, it is recommended to proceed as follows. Having loaded firewood into the firebox, you need to let it flare up, and then fill it with large pieces of coal.
After the coal is ignited, it should be covered with a finer fraction with moistened slag, and after a while, a moistened mixture of ash and fine coal, which has fallen through the grate into the blower, is placed on top. In this case, the fire should not be visible. A stove flooded in this way is capable of giving off heat to the room for a whole day, so that the owners can safely go about their business without worrying about constantly maintaining the fire. The side walls of the furnace will be hot due to the gradual combustion of coal, evenly giving off its thermal energy. The top layer, consisting of fine coal, will burn out completely. Inflamed coal can also be sprinkled on top with a layer of pre-moistened waste coal briquettes.
After firing the furnace, you need to take a bucket with a lid, it is better if it is rectangular in shape (it is more convenient to choose coal from it with a scoop). First you need to remove a layer of slag from the firebox and throw it away, then pour a mixture of fine coal with ash into a bucket, as well as burn and ash, and moisten all this without stirring. Place about 1.5 kg of fine coal on top of the resulting mixture, and 3-5 kg of larger coal on top of it. Thus, the simultaneous preparation of the furnace and fuel for the next kindling is carried out. The described procedure must be repeated constantly. Using this method of burning the furnace, you do not have to go out into the yard every time to sift the ashes and burn.
Combustion of solid fuel, motionless lying on the grate, with the top fuel loading is shown in fig. 6.2.
At the top of the layer after loading is fresh fuel. Under it is burning coke, and directly above the grate - slag. These zones of the layer partially overlap each other. As the fuel burns out, it gradually passes through all zones. In the first period after the supply of fresh fuel to the burning coke, its thermal preparation takes place (heating, moisture evaporation, release of volatiles), which consumes part of the heat released in the layer. On fig. 6.2 shows the approximate combustion of solid fuel and the temperature distribution along the height of the fuel layer. The area of the highest temperature is located in the coke combustion zone, where the main amount of heat is released.
The slag formed during the combustion of fuel flows in droplets from the red-hot pieces of coke towards the air. Gradually, the slag cools and, already in a solid state, reaches the grate, from where it is removed. The slag lying on the grate protects it from overheating, heats it up and evenly distributes air over the layer. The air passing through the grate and entering the fuel layer is called primary. If there is not enough primary air for complete combustion of the fuel and there are products of incomplete combustion above the layer, then additional air is supplied to the above-layer space. Such air is called secondary.
When the fuel is fed from the top to the grate, the bottom ignition of the fuel and the oncoming movement of the gas-air and fuel flows are carried out. This ensures efficient ignition of the fuel and favorable hydrodynamic conditions for its combustion. The primary chemical reactions between the fuel and the oxidizer take place in the hot coke zone. The nature of gas formation in the layer of burning fuel is shown in Fig. 6.3.
At the beginning of the layer, in the oxygen zone (K), in which oxygen is intensively consumed, carbon oxide and carbon dioxide CO 2 and CO are simultaneously formed. By the end of the oxygen zone, the concentration of O 2 decreases to 1-2%, and the concentration of CO 2 reaches its maximum. The temperature of the layer in the oxygen zone rises sharply, having a maximum where the highest CO 2 concentration is established.
In the reduction zone (B), oxygen is practically absent. Carbon dioxide reacts with hot carbon to form carbon monoxide:
Along the height of the reduction zone, the content of CO 2 in the gas decreases, and CO increases accordingly. The reaction of interaction of carbon dioxide with carbon is endothermic, so the temperature decreases along the height of the reduction zone. In the presence of water vapor in gases in the reduction zone, an endothermic decomposition reaction of H 2 O is also possible.
The ratio of the amounts of CO and CO 2 obtained in the initial section of the oxygen zone depends on temperature and varies according to the expression
where E co and E CO2 are the activation energies of formation, respectively, of CO and CO 2; A - numerical coefficient; R is the universal gas constant; T is the absolute temperature.
The bed temperature, in turn, depends on the concentration of the oxidizing agent, as well as on the degree of air heating. In the reduction zone, the combustion of solid fuel and the temperature factor also have a decisive influence on the ratio between CO and CO 2 . With an increase in the reaction temperature of CO 2 + C \u003d P 2, CO shifts to the right and the content of carbon monoxide in gases increases.
The thicknesses of the oxygen and reduction zones depend mainly on the type and size of pieces of burning fuel and the temperature regime. As the fuel size increases, the thickness of the zones increases. It has been established that the thickness of the oxygen zone is approximately three to four diameters of the burning particles. The recovery zone is 4-6 times thicker than the oxygen zone.
An increase in the blast intensity has practically no effect on the thickness of the zones. This is explained by the fact that the rate of the chemical reaction in the layer is much higher than the rate of mixture formation, and all incoming oxygen instantly reacts with the very first rows of hot fuel particles. The presence of oxygen and reduction zones in the layer is typical for the combustion of both carbon and natural fuels (Fig. 6.3). With an increase in the reactivity of the fuel, as well as with a decrease in its ash content, the thickness of the zones decreases.
The nature of gas formation in the fuel layer shows that, depending on the organization of combustion, either practically inert or combustible and inert gases can be obtained at the exit from the layer. If the goal is to maximize the conversion of fuel heat into physical heat of gases, then the process should be carried out in a thin layer of fuel with an excess of oxidizer. If the task is to obtain combustible gases (gasification), then the process is carried out with a layer developed along the height with a lack of an oxidizing agent.
The combustion of fuel in the boiler furnace corresponds to the first case. And the combustion of solid fuel is organized in a thin layer, which ensures the maximum course of oxidative reactions. Since the thickness of the oxygen zone depends on the size of the fuel, the larger the size of the pieces, the thicker the layer should be. So, when burning brown and black coals (up to 20 mm in size) in a layer of fines, the layer thickness is maintained at about 50 mm. With the same coals, but in pieces larger than 30 mm, the layer thickness is increased to 200 mm. The required thickness of the fuel layer also depends on its moisture content. The greater the moisture content of the fuel, the greater the amount of burning mass in the layer should be in order to ensure stable ignition and combustion of a fresh portion of the fuel.
Hello! Depending on the conditions of the combustion process, a larger or smaller proportion of the starting materials may enter into the reaction. For the full use of the chemical energy of the fuel, it is necessary to bring the combustion reaction of the fuel almost to the end. Under conditions of industrial combustion of fuel, the equilibrium of combustion reactions is rarely achieved due to the small amount of time for the combustion reactions to occur.
The process of burning liquid and solid fuels in the theory of combustion is called heterogeneous combustion, since it proceeds in an inhomogeneous (heterogeneous) system. If a mixture of gases burns, then combustion is called homogeneous.
When burning liquid fuel in the combustion chamber, the fuel evaporates from the surface of the droplets. Due to the high temperature in the furnace, the resulting fuel vapors undergo thermal decomposition and quickly burn out at the surface of the particles. Under these conditions, the rate of the combustion process is determined by the intensity of fuel evaporation. In order to increase the total surface of droplets, liquid fuel, when fed into the combustion chamber, is subjected to fine atomization using nozzles (the surface increases several thousand times in this case). The heavy fractions that have not evaporated from the droplet are subjected to thermal decomposition (cracking), as a result of which dispersed carbon is formed, which gives the glow to the flame.
The combustion process of solid fuel can be divided into two stages. After evaporation of moisture from the fuel, the combustion of volatile substances occurs, which are released as a result of thermal decomposition of the fuel. Then the burning of the solid residue (coke) begins. When the fuel is heated very quickly, both stages are superimposed on each other, since part of the volatile substances burns together with the carbon of the coke.
The coke is partially subjected to gasification, and the resulting gaseous products, consisting mainly of carbon monoxide CO, are burned in the furnace space. Combustion of a solid particle of fuel occurs not only from its surface, but also in volume due to the penetration of oxygen into the pores. In this case, a boundary (laminar) layer of gas is formed on the surface of the particle, in which the oxygen content decreases and the content of gasification and combustion products (CO and CO2) increases. This boundary layer of gas prevents the supply of oxygen, and the rate of the combustion reaction will depend on the rate of diffusion of the oxidant through the boundary layer. To increase the intensity of combustion, the speed of the oxidizer (air) relative to the surface of the fuel particles is increased, which reduces the thickness of the boundary layer.
Mineral impurities (ash content) also significantly affect the process of fuel combustion. As the carbon burns out, a layer of ash forms on the surface of the fuel particles. At a low ash softening temperature and a high ash content, this layer envelops (slags) the fuel particles and worsens the combustion process. To remove ash build-up during layered combustion of fuel, skinning is performed, that is, loosening of the fuel layer.
In powerful modern boilers, solid fuel is burned in suspension. Pieces of fuel are pre-ground in special mills, which increases their specific surface by several hundred times. A mixture of fuel dust and air is fed into the combustion chamber, where the fuel ignites and burns in the gas-air stream. Fuel combustion also proceeds in two stages, however, the combustion time of a fuel particle is significantly reduced in this case. This method of combustion allows intensifying the combustion process, as well as fully mechanizing all production operations. Use Literature: 1) Khzmalyan D.M., Kagan Ya.A. Theory of combustion and furnace devices, Moscow, Energia, 1976; 2) Heat engineering, Bondarev V.A., Protsky A.E., Grinkevich R.N. Minsk, ed. 2nd, "Higher School", 1976.
Combustion of fuel is a process of oxidation of combustible components that occurs at high temperatures and is accompanied by the release of heat. The nature of combustion is determined by many factors, including the method of combustion, furnace design, oxygen concentration, etc. But the conditions for the flow, duration and final results of combustion processes largely depend on the composition, physical and chemical characteristics of the fuel.
Fuel composition
Solid fuels include hard and brown coal, peat, oil shale, wood. These types of fuels are complex organic compounds formed mainly by five elements - carbon C, hydrogen H, oxygen O, sulfur S and nitrogen N. The composition of the fuel also includes moisture and non-combustible minerals, which form ash after combustion. Moisture and ash are the external ballast of the fuel, while oxygen and nitrogen are the internal ones.
The main element of the combustible part is carbon, it causes the release of the greatest amount of heat. However, the greater the proportion of carbon in the composition of solid fuel, the more difficult it is to ignite. During combustion, hydrogen releases 4.4 times more heat than carbon, but its share in the composition of solid fuels is small. Oxygen, not being a heat-generating element and binding hydrogen and carbon, reduces the heat of combustion, and therefore is an undesirable element. Its content is especially high in peat and wood. The amount of nitrogen in solid fuel is small, but it is capable of forming oxides harmful to the environment and humans. Sulfur is also a harmful impurity, it emits little heat, but the resulting oxides lead to corrosion of the boiler metal and air pollution.
Technical characteristics of the fuel and their effect on the combustion process
The most important technical characteristics of the fuel are: calorific value, volatile matter yield, properties of non-volatile residue (coke), ash content and moisture content.
Fuel combustion heat
The heat of combustion is the amount of heat released during the complete combustion of a unit mass (kJ / kg) or volume of fuel (kJ / m3). Distinguish between higher and lower calorific value. The higher one includes the heat released during the condensation of vapors contained in the combustion products. When fuel is burned in boiler furnaces, the exhaust gases have a temperature at which moisture is in a vapor state. Therefore, in this case, the lowest calorific value is used, which does not take into account the heat of condensation of water vapor.
The composition and net calorific value of all known coal deposits are determined and are given in the calculated characteristics.
Yield of volatile substances
When a solid fuel is heated without air access, under the influence of high temperature, water vapor is first released, and then the thermal decomposition of molecules occurs with the release of gaseous substances, called volatile substances.
The release of volatile substances can occur in the temperature range from 160 to 1100 °C, but on average - in the temperature range of 400-800 °C. The temperature at which volatiles begin to emerge and the amount and composition of gaseous products depend on the chemical composition of the fuel. The chemically older the fuel, the lower the yield of volatiles and the higher the temperature at which they begin to evolve.
Volatile substances provide earlier ignition of the solid particle and have a significant effect on the combustion of the fuel. Fuels young in age - peat, brown coal - easily catch fire, burn out quickly and almost completely. Conversely, low volatile fuels such as anthracite are more difficult to ignite, burn much more slowly, and burn incompletely (with increased heat loss).
Properties of non-volatile residue (coke)
The solid part of the fuel that remains after the release of volatiles, consisting mainly of carbon and a mineral part, is called coke. The coke residue can be, depending on the properties of the organic compounds included in the combustible mass: sintered, weakly sintered (destroyed upon impact), powdery. Anthracite, peat, brown coal give a powdery non-volatile residue. Most bituminous coals sinter, but not always strongly. A sticky or powdery non-volatile residue is produced by coals with a very high yield of volatiles (42-45%) and with a very low yield (less than 17%).
The structure of the coke residue is important when burning coal in grate furnaces. When flaring in power boilers, the characteristics of the coke are of little importance.
Ash content
Solid fuel contains the largest amount of non-combustible mineral impurities. This is primarily clay, silicates, iron pyrites, but also ferrous oxide, sulfates, carbonates and iron silicates, oxides of various metals, chlorides, alkalis, etc. can also be included. Most of them fall during mining in the form of rocks between which coal seams lie, but there are also mineral substances that have passed into fuel from coal formers or in the process of converting its initial mass.
When fuel is burned, mineral impurities undergo a series of reactions, resulting in the formation of a solid non-combustible residue called ash. The weight and composition of the ash is not identical to the weight and composition of the mineral impurities in the fuel.
The properties of the ash play an important role in the organization of the operation of the boiler and furnace. Its particles, carried away by the products of combustion, at high speeds abrade the heating surfaces, and at low speeds are deposited on them, which leads to a deterioration in heat transfer. Ash carried into the chimney can harm the environment, in order to avoid this, the installation of ash collectors is required.
An important property of ash is its fusibility; there are refractory (above 1425 ° C), medium-melting (1200-1425 ° C) and low-melting (less than 1200 ° C) ash. Ash that has passed the melting stage and turned into a sintered or fused mass is called slag. The temperature characteristic of ash fusibility is of great importance for ensuring reliable operation of the furnace and boiler surfaces, the correct choice of gas temperature near these surfaces will prevent slagging.
Moisture is an undesirable component of the fuel; along with mineral impurities, it is ballast and reduces the content of the combustible part. In addition, it reduces the thermal value, since additional energy is required for its evaporation.
Moisture in fuel can be internal or external. External moisture is contained in capillaries or held on the surface. With chemical age, the amount of capillary moisture decreases. Surface moisture is the greater, the smaller the pieces of fuel. Internal moisture enters organic matter.
Fuel combustion methods depending on the type of furnace
The main types of furnace devices:
- ply,
- chamber.
Layer fireboxes designed for burning large-sized solid fuels. They can be with a dense and fluidized bed. When burning in a dense layer, the combustion air passes through the layer without affecting its stability, that is, the gravity of the burning particles exceeds the dynamic head of the air. During combustion in a fluidized bed, due to the increased air velocity, the particles go into a "boiling" state. In this case, an active mixing of the oxidizer and fuel occurs, due to which the combustion of the fuel is intensified.
AT chamber furnaces burn solid pulverized fuel, as well as liquid and gaseous. Chamber furnaces are divided into cyclone and flare. During flaring, coal particles should be no more than 100 microns, they burn out in the volume of the combustion chamber. Cyclonic combustion allows larger particles, under the influence of centrifugal forces they are thrown onto the walls of the furnace and completely burn out in a swirling flow in the high temperature zone.
Fuel combustion. Main stages of the process
In the process of solid fuel combustion, certain stages can be distinguished: heating and evaporation of moisture, sublimation of volatiles and the formation of coke residue, combustion of volatiles and coke, and slag formation. Such a division of the combustion process is relatively arbitrary, since although these stages proceed sequentially, they partially overlap each other. Thus, the sublimation of volatiles begins before the final evaporation of all moisture, the formation of volatiles occurs simultaneously with the process of their combustion, just as the beginning of the oxidation of the coke residue precedes the end of the combustion of volatiles, and the afterburning of coke can go on after the formation of slag.
The flow time of each stage of the combustion process is largely determined by the properties of the fuel. The coke combustion stage lasts the longest, even for fuels with a high volatile yield. A variety of regime factors and design features of the furnace have a significant impact on the duration of the stages of the combustion process.
1. Preparation of fuel before ignition
The fuel entering the furnace is heated, as a result of which, in the presence of moisture, it evaporates and the fuel dries. The time required for heating and drying depends on the amount of moisture and the temperature at which the fuel is supplied to the combustion device. For fuels with a high moisture content (peat, wet brown coals), the stage of heating and drying is relatively long.
Fuel is fed into layered furnaces at a temperature close to the environment. Only in winter, if coal freezes, its temperature is lower than in the boiler room. For combustion in flare and swirl furnaces, the fuel is subjected to crushing and grinding, followed by drying with hot air or flue gases. The higher the temperature of the incoming fuel, the less time and heat is needed to heat it up to the ignition temperature.
Drying of the fuel in the furnace occurs due to two heat sources: the convective heat of the combustion products and the radiant heat of the torch, lining, slag.
In chamber furnaces, heating is carried out mainly due to the first source, that is, mixing combustion products with the fuel at the place of its input. Therefore, one of the important requirements for the design of devices for introducing fuel into the furnace is to ensure intensive suction of combustion products. A higher temperature in the furnace also contributes to a decrease in heating and drying time. For this purpose, when fuels are burned with the onset of volatile emissions at high temperatures (more than 400 ° C), incendiary belts are made in chamber furnaces, that is, screen pipes are covered with refractory heat-insulating material in order to reduce their heat absorption.
When burning fuel in a layer, the role of each type of heat source is determined by the design of the furnace. In furnaces with chain grates, heating and drying are carried out mainly by the radiant heat of the torch. In furnaces with a fixed grate and fuel supply from above, heating and drying occur due to combustion products moving through the layer from bottom to top.
In the process of heating at a temperature above 110 °C, the thermal decomposition of organic substances that make up the fuels begins. The least durable are those compounds that contain a significant amount of oxygen. These compounds decompose at relatively low temperatures with the formation of volatile substances and a solid residue, consisting mainly of carbon.
Fuels young in chemical composition, containing a lot of oxygen, have a low temperature of the beginning of the release of gaseous substances and give a greater percentage of them. Fuels with a low content of oxygen compounds have a low yield of volatiles and a higher ignition temperature.
The content of molecules in solid fuels, which are easily decomposed when heated, also affects the reactivity of the non-volatile residue. First, the decomposition of the combustible mass occurs mainly on the outer surface of the fuel. With further heating, pyrogenetic reactions begin to occur inside the fuel particles, the pressure increases in them and the outer shell breaks. When burning fuels with a high volatile yield, the coke residue becomes porous and has a large surface area compared to a dense solid residue.
2. The combustion process of gaseous compounds and coke
Actually, the combustion of fuel begins with the ignition of volatile substances. During the preparation of fuel, branched chain reactions of oxidation of gaseous substances occur; at first, these reactions proceed at low rates. The released heat is perceived by the furnace surfaces and partially accumulates in the form of energy of moving molecules. The latter leads to an increase in the rate of chain reactions. At a certain temperature, oxidation reactions proceed at such a rate that the heat released completely covers the heat absorption. This temperature is the ignition temperature.
The ignition temperature is not a constant, it depends both on the properties of the fuel and on the conditions in the ignition zone, on average it is 400-600 °C. After ignition of the gaseous mixture, further self-acceleration of the oxidation reactions causes an increase in temperature. To maintain combustion, a continuous supply of oxidizing agent and combustible substances is necessary.
The ignition of gaseous substances leads to the enveloping of the coke particle with a fire shell. The combustion of coke begins when the combustion of volatiles comes to an end. The solid particle is heated to a high temperature, and as the amount of volatile substances decreases, the thickness of the boundary burning layer decreases, oxygen reaches the hot carbon surface.
The combustion of coke begins at a temperature of 1000 °C and is the longest process. The reason is that, firstly, the oxygen concentration decreases, and secondly, heterogeneous reactions proceed more slowly than homogeneous ones. As a result, the burning time of a solid fuel particle is determined mainly by the burning time of the coke residue (about 2/3 of the total time). For fuels with a high volatile yield, the solid residue is less than ½ of the initial mass of the particle, so they burn quickly and the possibility of underburning is low. Chemically old fuels have a dense particle, the combustion of which takes almost the entire time in the furnace.
The coke residue of most solid fuels mainly, and for some types, entirely consists of carbon. The combustion of solid carbon occurs with the formation of carbon monoxide and carbon dioxide.
Optimal conditions for heat release
The creation of optimal conditions for the combustion of carbon is the basis for the correct construction of a technological method for burning solid fuels in boiler units. The following factors can influence the achievement of the highest heat release in the furnace: temperature, excess air, primary and secondary mixture formation.
Temperature. Heat release during fuel combustion significantly depends on the temperature regime of the furnace. At relatively low temperatures, incomplete combustion of combustible substances occurs in the core of the torch, carbon monoxide, hydrogen, and hydrocarbons remain in the combustion products. At temperatures from 1000 to 1800-2000 °C, complete combustion of the fuel is achievable.
excess air. The specific heat release reaches its maximum value at complete combustion and an excess air coefficient equal to one. With a decrease in the excess air ratio, the heat release decreases, since the lack of oxygen leads to the oxidation of a smaller amount of fuel. The temperature level decreases, the reaction rates decrease, which leads to a sharp decrease in heat release.
Increasing the excess air coefficient more than one reduces heat release even more than the lack of air. Under real conditions of fuel combustion in boiler furnaces, the limit values of heat release are not reached, since there is incomplete combustion. It largely depends on how the mixing processes are organized.
Mixing processes. In chamber furnaces, primary mixture formation is achieved by drying and mixing the fuel with air, supplying part of the air (primary) to the preparation zone, creating a wide-open flame with a wide surface and high turbulence, and using heated air.
In layered furnaces, the task of primary mixture formation is to supply the required amount of air to different combustion zones on the grate.
In order to ensure the afterburning of gaseous products of incomplete combustion and coke, processes of secondary mixture formation are organized. These processes are facilitated by: supply of secondary air at a high speed, creation of such aerodynamics, in which uniform filling of the entire furnace with a torch is achieved and, consequently, the residence time of gases and coke particles in the furnace increases.
3. Slag formation
In the process of oxidation of the combustible mass of solid fuel, significant changes also occur in mineral impurities. Low-melting substances and alloys with a low melting point dissolve refractory compounds.
A prerequisite for the normal operation of boiler units is the uninterrupted removal of combustion products and the resulting slag.
During layered combustion, slag formation can lead to mechanical underburning - mineral impurities envelop unburned coke particles or viscous slag can block air passages, blocking the access of oxygen to the burning coke. To reduce underburning, various measures are used - in furnaces with chain grates, the time spent by slag on the grate is increased, and frequent skimming is carried out.
In layered furnaces, the slag is removed in a dry form. In chamber furnaces, slag removal can be dry and liquid.
Thus, fuel combustion is a complex physical and chemical process, which is influenced by a large number of different factors, but all of them must be taken into account when designing boilers and combustion devices.
Task………………………………………………………………………..3
Introduction……………………………………………………………………...4
Theoretical part
1. Features of solid fuel combustion ………………………..... 6
2. Combustion of fuel in chamber furnaces ….………………………….9
3. Place and role of solid fuel in the energy sector of Russia ……………..12
4. Reducing emissions of ash particles from boiler furnaces by constructive and technological methods……………………………………………………14
5. Ash collection and types of ash collectors…………………….…….15
6. Cyclone (inertial) ash collectors…..……………………..16
Settlement part
1. Initial data…………………………………………………….18
2. Calculation of the elemental composition of the working fuel…………………..19
3. Calculation of the masses and volumes of fuel combustion products during combustion in boiler houses ……………………………………………………………………..19
4. Determining the height of the pipe H…………………………….…………20
5. Calculation of dispersion and standards for maximum permissible emissions of harmful substances into the atmosphere……………………………………….…20
6. Determination of the required degree of purification……………………….… 21
Rationale for choosing a cyclone………………………………………………..22
Applied devices……………………………………………. ……23
Conclusion………………………………………………………………….24
List of used literature……………………………………...26
Exercise
1. According to the given design characteristics of solid fuels, determine the elemental composition of the working fuel.
2. Using the results of paragraph 1 and the initial data, calculate the emissions and volumes of combustion products of particulate matter A, sulfur oxides SO x , carbon monoxide CO, nitrogen oxides NO x , the flow rate of gases entering the chimney under operating conditions of the boiler plant.
3. Based on the results of paragraph 2 and the initial data, determine the diameter of the mouth of the chimney. Determine the height of the pipe H.
4. Determine the most expected concentration C m (mg / m 3) of harmful substances: carbon monoxide CO, sulfur dioxide SO 2, nitrogen oxides NO x, dust, (ash) in the surface layer of the atmosphere under unfavorable dispersion conditions.
5. Compare the actual content of harmful substances in the atmospheric air, taking into account the background concentration (C m + C f) with sanitary and hygienic standards (MPC), if MPC CO \u003d 5 mg / m 3, MPC NO 2 \u003d 0.085, MPC SO 2 \u003d 0, 5 mg/m 3 , MPC dust = 0.5 mg/m 3 .
7. Determine the required degree of purification and give recommendations for reducing emissions if the actual emission M of any substance exceeds the calculated standard (MAL).
8. Develop and justify the methods and devices used for the treatment of waste hazardous substances.
Theoretical part
Introduction
Industrial production and other types of human economic activity are accompanied by the release of pollutants into the environment.
Significant damage to the environment is caused by boiler plants that use the combustion of solid, liquid and gaseous fuels when heating water for heating systems.
The main source of the negative impact of the energy sector is the products formed during the combustion of fossil fuels.
The working mass of organic fuel consists of carbon, hydrogen, oxygen, nitrogen, sulfur, moisture and ash. As a result of the complete combustion of fuels, carbon dioxide, water vapor, sulfur oxides (sulfur dioxide, sulfuric anhydride and ash) are formed. Sulfur oxides and ash are among the toxic ones. In the core of the torch of high-power furnace boilers, partial oxidation of nitrogen in the fuel air occurs with the formation of nitrogen oxides (nitrogen oxide and nitrogen dioxide).
With incomplete combustion of fuel in furnaces, carbon monoxide CO 2, hydrocarbons CH 4, C 2 H 6, as well as carcinogens can also be formed. The products of incomplete combustion are very harmful, but with modern combustion technology, their formation can be eliminated or minimized.
Combustible shale and brown coal, as well as some grades of hard coal, have the highest ash content. Liquid fuel has a low ash content; natural gas is an ashless fuel.
Toxic substances emitted into the atmosphere from the chimneys of power plants have a harmful effect on the entire complex of wildlife and the biosphere.
A comprehensive solution to the problem of protecting the environment from the effects of harmful emissions from fuel combustion in boiler units includes:
· Development and implementation of technological processes that reduce emissions of harmful substances due to the completeness of combustion of fuels, etc.;
· Implementation of effective methods and ways of purification of waste gases.
The most effective way to solve environmental problems at the present stage is the creation of technologies that are close to waste-free. At the same time, the problem of rational use of natural resources, both material and energy, is being solved.
Features of solid fuel combustion
The combustion of solid fuel includes two periods: thermal preparation and actual combustion. In the process of thermal preparation, the fuel is heated, dried, and at a temperature of about 110, pyrogenetic decomposition of its components begins with the release of gaseous volatile substances. The duration of this period depends mainly on the moisture content of the fuel, the size of its particles and the conditions of heat exchange between the surrounding combustion medium and fuel particles. The course of processes during the period of thermal preparation is associated with the absorption of heat mainly for heating, drying the fuel and thermal decomposition of complex molecular compounds.
Combustion itself begins with the ignition of volatile substances at a temperature of 400-600, and the heat released during combustion provides accelerated heating and ignition of the coke residue.
The combustion of coke begins at a temperature of about 1000 and is the longest process.
This is determined by the fact that part of the oxygen in the zone near the surface of the particle is used up for the combustion of combustible volatile substances and its remaining concentration has decreased, in addition, heterogeneous reactions are always inferior in speed to homogeneous reactions for substances that are homogeneous in chemical activity.
As a result, the total burning time of a solid particle is mainly determined by the burning of the coke residue (about 2/3 of the total burning time). For young fuels with a high yield of volatile substances, the coke residue is less than half of the initial mass of the particle; therefore, their combustion (with equal initial sizes) occurs quite quickly and the possibility of underburning is reduced. Old types of solid fuels have a large coke residue close to the initial particle size, the combustion of which occupies the entire time the particle stays in the combustion chamber. The combustion time of a particle with an initial size of 1 mm is from 1 to 2.5 s, depending on the type of initial fuel.
The coke residue of most solid fuels mainly, and for a number of solid fuels, almost entirely consists of carbon (from 60 to 97% of the organic mass of the fuel). Considering that carbon provides the main heat release during fuel combustion, let us consider the dynamics of combustion of a carbon particle from the surface. Oxygen is supplied from the environment to the carbon particle due to turbulent diffusion (turbulent mass transfer), which has a fairly high intensity, however, a thin gas layer (boundary layer) remains directly at the surface of the particle, the transfer of the oxidizer through which is carried out according to the laws of molecular diffusion.
This layer significantly inhibits the supply of oxygen to the surface. In it, the combustion of combustible gas components released from the carbon surface during a chemical reaction takes place.
Allocate diffusion, kinetic and intermediate region of combustion. In the intermediate and especially in the diffusion region, intensification of combustion is possible by increasing the supply of oxygen, by activating the blowing of the burning fuel particles with an oxidizer flow. At high flow rates, the thickness and resistance of the laminar layer near the surface decrease and the supply of oxygen increases. The higher this speed, the more intense the mixing of fuel with oxygen, and the higher the temperature, the transition from the kinetic to the intermediate zone, and from the intermediate to the diffusion zone of combustion occurs.
A similar effect in terms of combustion intensification is achieved by reducing the particle size of pulverized fuel. Particles of small size have a more developed heat and mass exchange with the environment. Thus, with a decrease in the particle size of pulverized fuel, the region of kinetic combustion expands. An increase in temperature leads to a shift to the region of diffusion combustion.
The region of pure diffusion combustion of pulverized fuel is limited mainly by the flame core, which has the highest combustion temperature, and the afterburning zone, where the concentrations of reactants are already low and their interaction is determined by the laws of diffusion. The ignition of any fuel begins at relatively low temperatures, in conditions of a sufficient amount of oxygen, i.e. in the kinetic area.
In the kinetic region of combustion, the decisive role is played by the rate of a chemical reaction, which depends on such factors as the reactivity of the fuel and the temperature level. The influence of aerodynamic factors in this combustion region is insignificant.