Big encyclopedia of oil and gas. Combustion of solid fuel

In heating boilers, solid fuel is burned in a layer mainly on manual grates with manual operation. AT recent times they begin to introduce mechanical fireboxes of the "shoveling bar" type. The main elements of the furnace for burning solid fuel in heating boilers with manual operation are a grate that supports a layer of lumpy fuel through which the air necessary for combustion passes, and a furnace space in which combustible volatile substances burn. With a constant draft, the amount of air passing into the furnace of the fuel layer in the period between loads increases constantly as a result of burning the layers and reducing its resistance. Of the air that enters the furnace, part is used for burning solid fuel in the layer, and part is used for the combustion of volatile substances in the furnace space, and some air remains unused.

The fuel loaded on the burning bed first dries up, then the combustion process begins, during this period, due to a lack of air, incomplete combustion of the fuel may occur, which disappears as the process of coking a portion of the fuel goes out. By the end of the period between fuel loads in a thin layer burns mainly. Typically, this burning period is characterized by complete combustion of the fuel with a large excess of air.

Thus, in the first moments after the fuel is loaded onto the grate, its combustion occurs with chemical incompleteness, and at the end of the combustion process, with increased excess air and, consequently, with increased heat loss with the exhaust gases. Therefore, the correctly chosen thickness of the fuel layer ensures the minimum amount of heat loss from the chemical completeness of combustion and from the outgoing gases with a minimum excess of air. These conditions can best be created with more frequent loading of fuel in small portions.

The last circumstance should be emphasized, since the machinists often ignore it, and as a result, the atmosphere is polluted with carbon monoxide. The periods between the loading of fuel, for example anthracite, should be 10-15 minutes, for the rest even less. Control over the correctness of the chosen thickness of the fuel layer is carried out either with the help of gas analyzers, according to the readings of which the completeness of combustion and excess air are estimated, or visually by the color of the flame in the absence of devices. Visually, the chemical incompleteness of combustion is determined by the degree of transparency of the smoke, and excess air is determined by the shape and color of the torch. Complete combustion of solid fuel with a small excess of air gives a transparent flame, straw-yellow in color. With a large excess of air, the flame, without changing its transparency, becomes short. With incomplete combustion, the flame, remaining long, turns red and dark layers appear on it. Incomplete combustion of solid fuels, which have a low yield of volatile substances, is manifested in the blue tongues of burning carbon monoxide that appear above the fuel layer.

Solid fuel combustion may involve the use of low quality coals. But this leads to a sharp decrease in the efficiency of boilers and, as a result, to excessive fuel consumption and the construction of additional boiler houses, or, with a rationed supply of fuel, to a shortage of heat to consumers and, in addition, to air pollution not only by products of incomplete combustion, but also by well-aimed particles of unburned fuel (entrainment ).

High humidity (over 30%) with high ash content (over 35%) worsens the combustion process and reduces the efficiency of boilers. The Academy of Public Utilities named after K. D. Pamfilov, on the basis of dialysis experimental data, found that for efficient combustion hard coal and anthracites in cast-iron and steel boilers limiting ash content. Boiler operation experience shows that the maximum size of coal lumps should not exceed 50 mm.

Brown coal can be used in cast-iron boilers in the form of briquettes, so it is necessary to speed up the solution of the complex technical problem of upgrading them at the mining site to obtain semi-coke, tar, gas, briquetting semi-coke with the addition of resin as a binder. Thus, for cast-iron boilers it is necessary: ​​to use coal according to the size of pieces of two classes: 13-25 and 25-50 mm: replace brown coal with coal and anthracite; when solving the problem of industrial upgrading at the mining site, use coal in the form of briquettes; use coals with a moisture content of not more than 8% and a fines content of not more than 20%.

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. Thus, 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 the 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 a large number 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 the combustion of 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 to fire stoves and fireplaces, which in its own way 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 fall asleep big chunks coal.

After the coal ignites, 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 (warming up, evaporation of moisture, 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 air is additionally 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 reducing 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.

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The combustion process of solid fuel also consists of a number of successive stages. First of all, mixture formation and thermal preparation of the fuel, including drying and volatile release, take place. The resulting combustible gases and coke residue, in the presence of an oxidizing agent, are further burned to form flue gases and a solid non-combustible residue - ash. The longest stage is the combustion of coke - carbon, which is the main combustible component of any solid fuel. Therefore, the combustion mechanism of solid fuel is largely determined by the combustion of carbon.

The combustion process of solid fuels can be conditionally divided into the following stages: heating and evaporation of moisture, sublimation of volatiles and the formation of coke, combustion of volatiles and coke, and slag formation. When burning liquid fuel, coke and slag are not formed, when burning gaseous fuel There are only two stages - heating and combustion.

The combustion process of solid fuel can be divided into two periods: the period of fuel preparation for combustion and the combustion period.

The combustion process of solid fuel can be divided into several stages: heating and evaporation of moisture, sublimation of volatiles and the formation of coke, combustion of volatiles, and combustion of coke.

The process of combustion of solid fuel in a stream at elevated pressures leads to a decrease in the dimensions of the combustion chambers and to a significant increase in heat stresses. High-pressure furnaces are not widely used.

The combustion process of solid fuel has not been theoretically studied enough. The first stage of the combustion process, leading to the formation of an intermediate compound, is determined by the course of the process of dissociation of the oxidizer in the adsorbed state. Next comes the formation of a carbon-oxygen complex and the dissociation of molecular oxygen to an atomic state. The mechanisms of heterogeneous catalysis as applied to the oxidation reactions of carbon-containing substances are also based on the dissociation of the oxidant.

The combustion process of solid fuel can be conditionally divided into three stages, successively overlapping each other.

The combustion process of solid fuel can be considered as a two-stage process with blurred boundaries between two stages: primary incomplete gasification in a heterogeneous process, the rate of which depends mainly on the speed and conditions of air supply, and secondary - combustion of the evolved gas in a homogeneous process, the rate of which depends mainly on from kinetics chemical reactions. The more volatiles in the fuel, the more more its rate of combustion depends on the rate of ongoing chemical reactions.

The intensification of the combustion process of solid fuel and a significant increase in the degree of ash capture are achieved in cyclone furnaces. C, at which the ash melts and the liquid slag is removed through tapholes in the lower part of the combustion device.

The basis of the combustion process of solid fuel is the oxidation of carbon, which is the main component of its combustible mass.

For the process of combustion of solid fuels, the combustion reactions of carbon monoxide and hydrogen are of undoubted interest. For solid fuels rich in volatile substances, in a number of processes and technological schemes it is necessary to know the combustion characteristics of hydrocarbon gases. The mechanism and kinetics of homogeneous combustion reactions are discussed in Chap. In addition to the secondary reactions mentioned above, the list of them should be continued with heterogeneous reactions of decomposition of carbon dioxide and water vapor, the reaction of carbon monoxide conversion with water vapor, and a family of methane formation reactions that proceed at noticeable rates during gasification under high pressure.

K category: Furnaces

The main features of fuel combustion processes

Heating furnaces can use solid, liquid and gaseous fuels. Each of these fuels has its own characteristics that affect the efficiency of using furnaces.

The designs of heating furnaces were created for a long time and were intended for burning solid fuels in them. Only in more late period constructions designed for the use of liquid and gaseous fuels began to be created. To make the most of these valuable species in existing kilns, it is necessary to know how the combustion processes of these fuels differ from the combustion of solid fuels.

All stoves use solid fuel (wood, different kinds coal, anthracite, coke, etc.) is burned on the grate in a layered manner, with periodic loading of fuel and cleaning of the grate from slag. The layered combustion process has a clear cyclic character. Each cycle includes the following stages: loading of fuel, drying and heating of the layer, release of volatile substances and their combustion, combustion of fuel in the layer, burnout of residues and, finally, removal of slags.

At each of these stages, a certain thermal regime is created and the combustion process in the furnace occurs with continuously changing indicators.
The primary stage of drying and heating the layer is of the so-called endothermic nature, i.e., it is accompanied not by the release, but by the absorption of heat received from the hot walls of the firebox and from unburned residues. Further, as the layer is heated, the release of gaseous combustible components and their burning out in the gas volume begins. At this stage, heat release in the furnace begins, which gradually increases. Under the influence of heating, the burning of the solid coke base of the layer begins, which usually gives the greatest thermal effect. As the layer burns out, the heat release gradually decreases, and in the final stage there is a low-intensity afterburning of combustible substances. It is known that the role and influence of individual stages of the stratified combustion cycle depends on the following solid fuel quality indicators: moisture content, ash content, content of volatile combustible substances and carbon in the fuel.
mass.

Let us consider how these components affect the nature of the combustion process in the layer.

Humidification of the fuel has a negative effect on combustion, since a part of the specific heat of combustion of the fuel must be spent on the evaporation of moisture. As a result, the temperatures in the firebox decrease, combustion conditions worsen, and the combustion cycle itself is delayed.

The negative role of the ash content of the fuel is manifested in the fact that the ash mass envelops the combustible components of the fuel and prevents the access of air oxygen to them. As a result, the combustible mass of fuel does not burn out, the so-called mechanical underburning is formed.

Research by scientists has found that big influence the nature of the development of combustion processes is affected by the ratio of the content of volatile gaseous substances and solid carbon in the solid fuel. Volatile combustible substances begin to be released from solid fuels at relatively low temperatures, starting from 150-200 ° C and above. Volatile substances are diverse in composition and differ different temperatures exit, so the process of their release is extended in time and its final stage is usually combined with the combustion of the solid fuel part of the bed.

Volatile substances have a relatively low ignition temperature, since they contain many hydrogen-containing components, their combustion occurs in the above-layer gas volume of the firebox. The solid part of the fuel remaining after the release of volatile substances consists mainly of carbon, which has the highest ignition temperature (650-700°C). The combustion of the carbon residue begins last. It flows directly in the thin layer of the grate, and due to intense heat release, high temperatures develop in it.

A typical pattern of temperature changes in the furnace and gas ducts during the solid fuel combustion cycle is shown in fig. 1. As you can see, at the beginning of the furnace, there is a rapid increase in temperatures in the firebox and chimneys. At the stage of afterburning, there is a sharp decrease in temperature inside the furnace, especially in the firebox. Each of the stages requires the supply of a certain amount of combustion air to the furnace. However, due to the fact that a constant amount of air enters the furnace, at the stage of intensive combustion, the excess air coefficient is at = 1.5-2, and at the afterburning stage, the duration of which reaches 25-30% of the furnace time, the excess air coefficient reaches at = 8-10. On fig. 2 shows how the excess air coefficient changes during one combustion cycle on a grate of three types of solid fuels: firewood, peat and coal in a typical batch heating furnace.

Rice. 1. Flue gas temperature change in various sections of the heating furnace when burning with solid fuel 1 - temperature in the firebox (at a distance of 0.23 m from the grate); 1 - temperature in the first horizontal chimney; '3 - temperature in the third horizontal chimney; 4 - temperature in the sixth horizontal chimney (before the furnace damper)

From fig. 2 it can be seen that the coefficient of excess air in furnaces operating with periodic loading of solid fuel is continuously changing.

At the same time, at the stage of intensive release of volatile substances, the amount of air entering the furnace is usually not enough for their complete combustion, and at the stages of preheating and afterburning of combustible substances, the amount of air is several times higher than theoretically required.

As a result, at the stage of intensive release of volatile substances, chemical underburning of the released combustible gases occurs, and during the afterburning of residues, increased heat losses with the exhaust gases occur due to an increase in the volume of combustion products. Heat loss with chemical underburning is 3-5%, and with exhaust gases - 20-35%. However, the negative effect of chemical underburning is manifested not only in additional heat losses and a decrease in efficiency. Experience in operating a large number of heating furnaces shows; that as a result of chemical underburning of intensely released volatile substances, amorphous carbon in the form of soot is deposited on the inner walls of the furnace and chimneys.

Rice. 2. Change in excess air ratio during the combustion cycle of solid fuel

Since soot has a low thermal conductivity, its deposits increase the thermal resistance of the furnace walls and thereby reduce the useful heat output of the furnaces. Soot deposits in chimneys narrow the cross section for the passage of gases, impair draft and, finally, create an increased fire hazard, since soot is combustible.

From what has been said, it is clear that the unsatisfactory indicators of the layered process are largely due to the uneven release of volatile substances over time.

In the layered combustion of high-carbon fuels, the combustion process is concentrated within a rather thin fuel layer, in which high temperatures develop. The combustion process of pure carbon in the layer has the property of self-regulation. This means that the amount of reacted (burnt) carbon will correspond to the amount of oxidant (air) supplied. Therefore, when constant expense air, the amount of fuel burned will also be constant. The change in the heat load must be made by regulating the air supply VB. For example, with an increase in VB, the amount of burned fuel increases, and a decrease in HC will cause a decrease in the heat output of the layer, while the value of the excess air coefficient will remain stable.

However, the combustion of anthracite and coke is associated with the following difficulties. To be able to create high temperatures, the layer thickness during the combustion of anthracite and coke is maintained sufficiently large. In this case, the working zone of the layer is its relatively thin lower part, in which exothermic reactions of carbon oxidation with atmospheric oxygen take place, i.e., combustion itself occurs. The entire overlying layer serves as a thermal insulator for the burning part of the layer, protecting the combustion zone from cooling due to heat radiation to the walls of the firebox.

As a result of oxidative reactions, useful heat is released in the combustion zone according to the reaction
c+o2->co.

However, at high temperatures of the layer in its upper zone, reverse restorative endothermic reactions proceed with the absorption of heat, according to the equation
CO2+C2CO.

As a result of these reactions, carbon monoxide CO is formed, which is a combustible gas with a rather high specific heat of combustion, so its presence in the flue gases indicates incomplete combustion of the fuel and a decrease in the efficiency of the furnace. Thus, to ensure high temperatures in the combustion zone, the fuel layer must have a sufficient thickness, but this leads to harmful reduction reactions in the upper part of the layer, leading to chemical underburning of the solid fuel.

From the foregoing, it is clear that in any batch furnace operating on solid fuel, an unsteady combustion process takes place, which inevitably reduces the efficiency of the furnaces in operation.

Great importance for economical, operation of the furnace has the quality of solid fuel.

According to the standards for domestic needs, mainly black coals (grades D, G, Zh, K, T, etc.), as well as brown coal and anthracites, are distinguished. By the size of the pieces, coals should be supplied in the following classes: 6-13, 13-25, 25-50 and 50-100 mm. The ash content of coal on a dry basis ranges from 14-35% for bituminous coal and up to 20% for anthracite, the moisture content is 6-15% for bituminous coal and 20-45% for brown coal.

Furnaces of household stoves do not have the means of mechanizing the combustion process (controlling the supply of blast air, layer skimming, etc.), therefore, for efficient combustion in furnaces, rather high requirements must be imposed on the quality of coal. A significant part of the coal is supplied, however, unsorted, ordinary, with quality characteristics (in terms of moisture content, ash content, fines content) significantly lower than those stipulated by the standards.

The combustion of substandard fuel is imperfect, with increased losses from chemical and mechanical underburning. Academy of Public Utilities. K. D. Pamfilov was determined annual material damage caused by the supply of low quality coal. Calculations showed that the material damage caused by incomplete use of fuel is approximately 60% of the cost of coal mining. It is economically and technically expedient to enrich the fuel in the places of its production to a standard state, since the additional costs for enrichment will amount to approximately half of the indicated amount of material damage.

An important qualitative characteristic of coal, which affects the efficiency of its combustion, is its fractional composition.

With an increased content of fines in the fuel, it becomes denser and closes the gaps in the burning fuel layer, which leads to crater combustion, which has an uneven character over the area of ​​the layer. For the same reason, brown coals are burned worse than other types of fuel, which tend to crack when heated to form a significant amount of fines.

On the other hand, the use of excessively large pieces of coal (more than 100 mm) also leads to crater combustion.

Humidity of coal, generally speaking, does not impair the combustion process; however, it reduces specific heat combustion, combustion temperature, and also complicates the storage of coal, since at sub-zero temperatures it freezes. To prevent freezing, the moisture content of coal should not exceed 8%.

Sulfur is a harmful component in solid fuels, since the products of its combustion are sulfur dioxide S02 and sulfur dioxide S03, which have strong corrosive properties, and are also very toxic.

It should be noted that in batch furnaces ordinary coals, although less efficiently, can still be burned satisfactorily; for long-burning furnaces, these requirements must be categorically met in full.

In continuous furnaces, in which liquid or gaseous fuels are burned, the combustion process is not cyclic, but continuous. The flow of fuel into the furnace occurs evenly, due to which a stationary combustion mode is observed. If during the combustion of solid fuel the temperature in the firebox of the furnace fluctuates over a wide range, which adversely affects the combustion process, then during combustion natural gas shortly after turning on the burner, the temperature in the furnace space reaches 650-700 °C. Further, it constantly increases with time and reaches 850-1100 °C at the end of the furnace. The rate of temperature increase in this case is determined by the thermal stress of the furnace space and the time of furnace burning (Fig. 25). Gas combustion is relatively easy to maintain at constant factor excess air, which is carried out using an air damper. Due to this, when gas is burned in the furnace, a stationary combustion mode is created, which makes it possible to minimize heat losses with exhaust gases and achieve high efficiency furnace operation, reaching 80-90%. The efficiency of a gas stove is stable over time and is significantly higher than that of solid fuel stoves.

Influence of the fuel combustion mode and the size of the area of ​​the heat-receiving surface of the smoke circuits on the efficiency of the furnace. Theoretical calculations show that the thermal efficiency of a heating furnace, i.e., the value of thermal efficiency, depends on the so-called external and internal factors. To external factors include the area of ​​​​the heat-releasing outer surface S of the furnace in the area of ​​\u200b\u200bthe firebox and smoke circulation, wall thickness 6, thermal conductivity coefficient K of the material of the furnace walls and heat capacity C. The larger the values. S, X and less than 6, the better the heat transfer from the walls of the furnace to the surrounding air, the gases are more fully cooled and the higher the efficiency of the furnace.

Rice. Fig. 3. Change in the temperature of combustion products in the firebox of a gas heating furnace, depending on the intensity of the furnace space and the time of combustion

The internal factors primarily include the value of the efficiency of the firebox, which depends mainly on the completeness of fuel combustion. In heating furnaces of periodic action, there are almost always heat losses from chemical incomplete combustion and mechanical underburning. These losses depend on the perfection of the organization of the combustion process, determined by the specific thermal stress of the furnace volume Q/V. The value of QIV for a firebox of a given design depends on the consumption of the fuel being burned.

Research and operating experience have established that for each type of fuel and firebox design there is an optimal Q / V value. At low Q/V, the inner walls of the firebox warm up weakly, the temperatures in the combustion zone are insufficient for efficient fuel combustion. With an increase in Q/V, the temperatures in the furnace volume increase, and when a certain value of Q/V is reached, optimal combustion conditions are achieved. With a further increase in fuel consumption, the temperature level continues to rise, but the combustion process does not have time to complete within the firebox. Gaseous combustible components are carried away into the gas ducts, the process of their combustion stops and chemical underburning of the fuel appears. In the same way, with excessive fuel consumption, part of it does not have time to burn out and remains on the grate, which leads to mechanical underburning. Thus, in order for the heating furnace to have maximum efficiency, it is necessary that its firebox operate with optimal thermal stress.

Heat loss in environment from the walls of the firebox do not reduce the efficiency of the furnace, since the heat is spent on useful heating of the room.

The second important internal factor is the flue gas flow Vr. Even if the furnace operates at the optimal value of the thermal stress of the firebox, the volume of gases passing through the chimneys can vary significantly due to a change in the excess air coefficient am, which is the ratio of the actual air flow entering the furnace to the theoretically required amount. For a given value of QIV, the value of am can vary over a very wide range. In conventional batch heating furnaces, the value of a in the period of maximum combustion can be close to 1, i.e., correspond to the minimum possible theoretical limit. However, during the period of fuel preparation and at the stage of afterburning of residues, the value of am in batch furnaces usually increases sharply, often reaching extremely high values ​​- about 8-10. With an increase in at, the volume of gases increases, the time of their stay in the smoke circulation system is reduced and, as a result, heat losses with the exhaust gases increase.

On fig. 4 shows graphs of the dependence of the efficiency of the heating furnace on various parameters. On fig. 4, a shows the values ​​of the efficiency of the heating furnace depending on the values ​​of am, from which it is clear that with an increase in am from 1.5 to 4.5, the efficiency decreases from 80 to 48%. On fig. 4b shows the dependence of the efficiency of the heating furnace on the area of ​​the inner surface of the smoke circuits S, from which it can be seen that with an increase in S from 1 to 4 m2, the efficiency increases from 65 to 90%.

In addition to the above factors, the efficiency depends on the duration of the furnace furnace t (Fig. 4, c). As x increases, the inner walls of the furnace are heated to a higher temperature and the gases, respectively, are cooled less. Therefore, with an increase in the duration of the furnace, the efficiency of any heating furnace decreases, approaching a certain minimum value characteristic of a furnace of this design.

Rice. Fig. 4. Dependence of the efficiency of a gas heating furnace on various parameters a - on the coefficient of excess air at the area of ​​​​the inner surface of the smoke circuits, m2; b - from the area of ​​​​the inner surface of the smoke circuits at various coefficients of excess air; c - from the duration of the furnace at various areas of the inner surface of the smoke circuits, m2

Heat transfer of heating furnaces and their storage capacity. In heating furnaces, the heat that must be transferred by flue gases to the heated room must pass through the thickness of the furnace walls. With a change in the thickness of the walls of the firebox and chimneys, the thermal resistance and the massiveness of the masonry (its storage capacity) change accordingly. For example, with a decrease in the thickness of the walls, their thermal resistance decreases, the heat flux increases and, at the same time, the dimensions of the furnace decrease. However, a decrease in the thickness of the walls of batch furnaces operating on solid fuel is unacceptable for the following reasons: during periodic short-term combustion, the internal surfaces of the firebox and chimneys heat up to high temperatures and the temperature of the furnace outer surface during periods of maximum combustion will be above the permissible limits; after the combustion ceases due to the intense heat transfer of the outer walls to the environment, the furnace will cool rapidly.

At large values ​​of M, the room temperature will change over time over a wide range and go out of allowable norms. On the other hand, if you lay out the stove too thick-walled, then in a short period of burning, its large array will not have time to warm up and, in addition, with the thickening of the walls, the difference between the area of ​​\u200b\u200bthe inner surface of the chimneys, which receives heat from gases, and the area of ​​\u200b\u200bthe outer surface of the furnace, which transfers heat, increases ambient air, causing the outside temperature of the oven to be too low to effectively heat the room. Therefore, there is such an optimal wall thickness (1/2-1 brick), at which the mass of the batch furnace accumulates a sufficient amount of heat during the furnace and at the same time a sufficiently high temperature of the external surfaces of the furnace is reached for normal heating of the room.

When using liquid or gaseous fuels in heating furnaces, a continuous combustion mode is quite achievable, therefore, with continuous combustion, there is no need for heat accumulation due to an increase in the masonry array. The process of heat transfer from gases to a heated room is stationary in time. Under these conditions, the thickness of the walls and the massiveness of the furnace can be chosen not on the basis of providing a certain storage value, but on the basis of the strength of the masonry and ensuring proper durability.

The effect of switching the furnace from batch to continuous is clearly seen in Fig. 5, which shows the change in the temperature of the inner surface of the wall of the firebox in the case of periodic and continuous combustion. With periodic fire, after 0.5-1 hour, the inner surface of the wall of the firebox heats up to 800-900 °C.

Such a sharp heating after 1-2 years of operation of the furnace often causes cracking of bricks and their destruction. Such a regime, however, is forced, since a decrease in the heat load leads to an excessive increase in the duration of the furnace.

With continuous combustion, the fuel consumption is sharply reduced and the heating temperature of the walls of the firebox decreases. As can be seen from fig. 27, with continuous combustion for most grades of coal, the wall temperature rises from 200 to only 450-500 ° C, while with periodic combustion it is much higher - 800-900 ° C. Therefore, the fireboxes of batch furnaces are usually lined with refractory bricks, while the fireboxes of continuous furnaces do not need lining, since the temperature on their surface does not reach the refractory limit of ordinary red brick (700-750 ° C).

Consequently, with continuous firing, brickwork is used more efficiently, the service life of furnaces is greatly increased, and for most grades of coal (excluding anthracites and lean coals), it is possible to lay out all parts of the furnace from red brick.

Thrust in ovens. In order to force the flue gases to pass from the firebox through the furnace chimneys to the chimney, overcoming all the local resistances encountered in their path, it is necessary to expend a certain effort, which must exceed these resistances, otherwise the furnace will smoke. This effort is called the thrust force of the furnace.

The emergence of traction force is illustrated in the diagram (Fig. 6). The flue gases generated in the firebox, being lighter than the surrounding air, rise up and fill the chimney. The column of outside air opposes the column of gases in the chimney, but, being cold, it is much heavier than the column of gases. If we draw a conditional vertical plane through the furnace door, then with right side it will be acted upon (pressed) by a column of hot gases with a height from the middle of the furnace door to the top of the chimney, and from the left - by a column of outside cold air of the same height. The mass of the left column is greater than the right one, since the density of cold air is greater than hot air, so the left column will displace the flue gases filling the chimney, and gases will move in the system in the direction from higher pressure to lower, i.e. in side of the chimney.

Rice. 5. Temperature change on the inner surface of the wall of the firebox a - the thermostat is set to the lower limit; b - the thermostat is set to the upper limit

Rice. 6. Scheme of operation of the chimney 1-furnace door; 2- firebox; 3 - outdoor air column; 4 - chimney

The action of the draft force thus consists in that, on the one hand, it causes the hot gases to rise upwards, and, on the other hand, it forces the outside air to pass into the firebox for combustion.

average temperature gases in the chimney can be taken equal to the arithmetic mean between the temperature of the gases at the inlet and outlet of the chimney.