Assignment ……………………………………………………………………… ..3
Introduction ………………………………………………………………… ... 4
Theoretical part
1. Features of solid fuel combustion ……………………… ..... 6
2. Combustion of fuel in chamber furnaces…. ………………………… .9
3. The place and role of solid fuel in the energy sector of Russia …………… ..12
4. Reduction of ash particles emissions from boiler furnaces by constructive and technological methods …………………… 14
5. Ash collection and types of ash collectors ……………………. …… .15
6. Cyclonic (inertial) ash collectors… .. …………………… ..16
Calculated part
1. Initial data ………………………………………………… .18
2. Calculation of the elementary composition of the working fuel ………………… ..19
3. Calculation of masses and volumes of fuel combustion products during combustion in boiler rooms ……………………………… ... ………………………… ..19
4. Determination of the height of the pipe H ……………………………. ………… 20
5. Calculation of dispersion and standards of maximum permissible emissions of harmful substances into the atmosphere ……………………………………….… 20
6. Determination of the required degree of purification ……………………….… 21
Justification for the choice of a cyclone ………………………………………… ..22
Applied devices ……………………………………………. …… 23
Conclusion ……………………………………………………………… .24
List of used literature …………………………………… ... 26
Exercise
1. Determine the elementary composition of the working fuel based on the specified design characteristics of solid fuels.
2. Using the results of clause 1 and the initial data, calculate the emissions and volumes of combustion products of solid particles A, sulfur oxides SO x, carbon monoxide CO, nitrogen oxides NO x, the consumption of gases entering the chimney under operating conditions of the boiler plant.
3. Based on the results of clause 2 and the initial data, determine the diameter of the chimney mouth. 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 atmospheric air taking into account the background concentration (C m + C f) with sanitary and hygienic standards (MPC), if MPC CO = 5 mg / m 3, MPC NO 2 = 0.085, MPC SO 2 = 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 M emission of any substance exceeds the design standard (MPE).
8. To develop and justify the methods and devices used for the treatment of waste hazardous substances.
Theoretical part
Introduction
Industrial production and other types economic activity humans are accompanied by the release of pollutants into the environment.
Boiler installations that use the combustion of solid, liquid and gaseous fuels to heat water for heating systems cause significant damage to the environment.
The main source negative impact energetics are the products formed during the combustion of fossil fuels.
The working mass of fossil fuel consists of carbon, hydrogen, oxygen, nitrogen, sulfur, moisture and ash. As a result of complete combustion of fuels, carbon dioxide, water vapor, sulfur oxides (sulfur dioxide, sulfuric anhydride and ash). Sulfur oxides and ash are among the toxic ones. In the core of the torch of high-power combustion chamber boilers, partial oxidation of nitrogen in the fuel air occurs with the formation of nitrogen oxides (nitrogen oxide and dioxide).
Incomplete combustion of fuel in furnaces can also form carbon monoxide CO 2, hydrocarbons CH 4, C 2 H 6, as well as carcinogenic substances. The products of incomplete combustion are very harmful, however, when modern technology incineration, their formation can be eliminated or minimized.
The highest ash content is found in oil shale and brown coal, as well as some types of coal. Liquid fuels have low ash content; natural gas is an ashless fuel.
Toxic substances emitted into the atmosphere from chimneys of power plants have a harmful effect for the whole complex of wildlife and the biosphere.
A comprehensive solution to the problem of protecting the environment from the effects of harmful emissions during the combustion of fuels 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 effective methods and methods for cleaning waste gases.
The most effective solution environmental issues at the present stage - the creation of technologies close to waste-free. At the same time, the problem is solved at the same time rational use natural resources, both material and energy.
Features of solid fuel combustion
Combustion of solid fuel includes two periods: heat preparation and actual combustion. In the process of thermal preparation, the fuel is warmed up, dried, and at a temperature of about 110, the pyrogenetic decomposition of its constituent 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 environment and fuel particles. The course of processes during the period of thermal preparation is associated with the absorption of heat, mainly for heating, drying of fuel and thermal decomposition of complex molecular compounds.
The combustion itself begins with the ignition of volatile substances at a temperature of 400-600, and the heat released during the combustion process provides accelerated heating and ignition of the coke residue.
Combustion of coke begins at a temperature of about 1000 and is the most time-consuming process.
This is determined by the fact that part of the oxygen in the zone near the surface of the particle is consumed for the combustion of flammable volatiles and its remaining concentration has decreased, in addition, heterogeneous reactions are always inferior in speed to homogeneous ones for substances homogeneous in chemical activity.
Eventually total duration combustion of a solid particle is mainly determined by combustion of the coke residue (about 2/3 of the total burning time). In young fuels with a high yield of volatile substances, the coke residue is less than half of the initial particle mass, therefore, their combustion (with equal initial sizes) occurs rather quickly and the possibility of underburning decreases. Old types of solid fuels have a large coke residue close to the initial particle size, the combustion of which takes up the entire residence time of the particle 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 is mainly, and for a number of solid fuels, almost entirely consists of carbon (from 60 to 97% of the organic mass of the fuel). Taking into account 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 sufficiently high intensity, but a thin gas layer (boundary layer) remains directly at the particle surface, 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. It burns out combustible gas components released from the carbon surface during chemical reaction.
Diffusion, kinetic and intermediate combustion regions are distinguished. In the intermediate and especially in the diffusion region, intensification of combustion is possible by increasing the supply of oxygen, by activating the flow of an oxidizer to blow the burning fuel particles. At high flow rates, the thickness and resistance of the laminar layer at the surface decrease and the oxygen supply increases. The higher this speed, the more intensive 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 combustion zone.
A similar effect in terms of intensifying combustion is achieved by reducing the particle size of pulverized fuel. Small particles have a more developed heat and mass transfer with environment... Thus, as the particle size of the pulverized fuel decreases, the kinetic combustion region expands. An increase in temperature leads to a shift towards the region of diffusion combustion.
The area of purely diffusive combustion of pulverized fuel is limited mainly by the torch core, which differs most high temperature combustion, and the afterburning zone, where the concentrations of the reactants are already small and their interaction is determined by the laws of diffusion. Ignition of any fuel starts at relatively low temperatures, in conditions of sufficient oxygen, i.e. in the kinetic region.
In the kinetic region of combustion, the decisive role is played by the rate of the 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 area is negligible.
Over the centuries, mankind has improved the design of heating stoves, in which it was originally intended to burn the universally available solid fuel. In this regard, little has changed, and today, in the 21st century, in the presence of gas and liquid fuels, we often turn to traditional heating technologies. It somehow becomes easy on the heart if in a modern house, in addition to central heating, there is also a good stove in reserve. Well, traditional baths cannot do without the heat of a wood-burning stove.
To operate a wood-burning stove efficiently and safely, a stoker needs to know about the intricacies of burning solid fuels. Many today no longer remember how to properly heat the stove, however, experiments in this matter are extremely undesirable. V this material we will try to cover the topic of solid fuel combustion as much as possible.
Solid fuel means firewood, coal, anthracite, coke, peat, etc. In traditional stoves, all of this is burned in a layer method on grates or without them. Fuel is periodically loaded into the furnace, and the resulting slag is removed. Layer combustion is cyclical. The closed loop has several stages:
Each of these stages has its own thermal regime, while the indicators are constantly changing during fuel combustion. To ensure the optimal thermal regime of the oven, it is necessary to periodically put new portion fuel (layer). The moment of loading a new layer is determined on an individual basis and depends on many factors. Let us consider the stages of layer-by-layer combustion of solid fuel in more detail.
Warming up and drying the layer is accompanied by heat absorption, i.e. is endothermic. The heat supplier is the flame of the starting bookmark made of thin dry wood or already ignited fuel, as well as the hot walls of the firebox.
Stage of ignition and smoldering occurs with increasing heat release. Excessive intake of air into the furnace during this period is undesirable, since it will cool flue gases, and, therefore, the chimney will heat up longer. Air dampers at the stage of ignition and smoldering should only be slightly open, while it is desirable that cold air was supplied only to the ignition zone.
Combustion stage needs to large volumes oxygen in the air, because this process is nothing more than the oxidation of hydrocarbons. Flame heating is increasing, and, in fact, is limited only by the amount of oxygen supplied. If the chimney cross-section is insufficient, then the flame can be knocked out of the air supply holes. In such a situation, there is only one way out - immediately open the chimney damper completely and shut off the air supply. When the air supply is reduced, the flames become longer and may even penetrate the chimney, which is a sign of underburning. Obviously, the supplied air in the flame combustion mode must be divided into two controlled streams. The primary stream will be fed directly into the wood, depending on the volume, increasing or decreasing the rate of release of volatiles; and the secondary - to the flame, to adjust the completeness of combustion of volatile substances, i.e. the length of the flames. An increase in the intensity of the secondary flow leads to a reduction in the length of the latter until it disappears, but the rate of firewood burning does not slow down. However, the firepower of a wood fire is actually not as great as it seems. It is capable of heating the walls of the firebox of a metal stove no higher than 300-400 ° С.
Burning coals provides heating of the metal firebox red-hot - this is the most exothermic stage. The effect of heat release increases with increasing primary air supply (passage through the bed). Secondary air is not needed at this stage. Coals will burn out faster if raw chocks are fed into the furnace: a reaction of coal gasification with water vapor will occur. If the wood is damp, then the stage of burning and smoldering occurs almost simultaneously.
Types of fuel chambers and the process of burning wood
In the simplest fireplace insert with a deaf hearth, the combustion process takes place with excess air, since the area open portal usually 8-15 times the chimney cross-sectional area. Due to the fact that large volumes of sucked air do not allow the chimney to heat up above 60-80 ° C, the draft in them is much less than in ovens with a door (250-400 ° C).
If the fireplace insert is equipped with a door and a blower with a damper, then its efficiency will significantly change upward. However, this design has a serious drawback - excessive smoke from the chamber, when opened, the smoke escapes. Smoke can be reduced by moving the pipe as far forward as possible, but then it will block the top of the stove used to heat water or stones. A compromise solution in in this case there may be a sloping shelf when the pipe is rearward. The shelf will create maximum traction at the door itself, when opened, the upward flow will suck in the smoke, preventing it from escaping. This design is good for long burning since the air goes along the hearth, getting under the firewood, and in the area of the smoke circulation it mixes well with volatile substances, ensuring the completeness of their combustion. 
For emphasis on fiery combustion, secondary air inlets into the flow of volatiles are used. Implementation this regime firewood combustion is also helped by grate structures. They are good, first of all, in that they provide oxygen supply to any area of the layer. However, a large amount of incoming air reduces the temperature of the walls of the flue duct, and, consequently, the draft and convective heat transfer. This phenomenon can be minimized by covering the periphery of the grate with a hearth, leaving the purge area only in the center.
Any grates are suitable for burning firewood. If necessary, you can make them yourself from fittings or a rod. But for burning coal, you will need cast iron grates, the sectional shape of which is close to triangular. This shape does not allow the slag to clog the gaps between the grates. The grates should be placed along the firebox so that you can shove the coal with a poker. Cast iron grates are available for both coal and firewood. In the latter, the grates are thinner, and the gaps between them are narrower.
Grizzly furnaces are capable of developing high power, however, keep them from overclocking not easy. When the air supply coefficient is equal to unity, the walls of the furnace are heated to red, and the firewood begins to gasify in an increasing manner. The flame becomes so much that it enters the pipe and in this case it is required to increase the air supply, which in turn causes even more gasification and heating. The stove will calm down on its own only after the volatiles have escaped from the woodwork. Combustion of coals after that already lends itself well to regulation. 
It is important to understand that the main reason oven acceleration overclocking are metal walls heated to a high temperature, which no longer take away the heat of the firewood, while the latter begin to heat themselves. It is possible to prevent the stove from accelerating if, when firing, keep the chimney damper open only halfway, and when characteristic gas pops are heard from the firebox, open the firebox door slightly and at the same time fully open the chimney. From the sudden appearance of excess air, the walls of the stove will begin to cool down, and when they stop glowing, it will be possible to close the firebox door and the air intake. The chimney is again half-covered. From this, the furnace will smoothly go into the smoldering mode.
An important point affecting the acceleration of the stove is a portion of the laid firewood. To reduce the likelihood of overclocking conditions, firewood should be loaded in small portions from 1 to 3 kg at a time. Moreover, the larger the diameter of the log, the greater the weight of the bookmark can be. By adjusting the air supply, you must try to prevent overheating of the walls. Furnace acceleration is dangerous, first of all, because it can lead to warping or burnout of the metal parts of the furnace.
First of all, it suffers from overclocking Bottom part firebox walls. If a metal furnace accelerates from time to time, then the walls can be protected from the inside refractory bricks to a height of 20-30 cm. It will be a mistake to cover the walls from the outside. this will lead to an even stronger heating of the metal. The overclocking problem is completely eliminated by the water jacket - the boiler. However, if we talk about sauna stoves, then this solution is not suitable for saunas, but for a hammam.
Firebox burnouts or hidden cracks are really dangerous during spontaneous acceleration of a metal stove. If during normal combustion they will work as air intake holes, then in the acceleration mode they will become "nozzles" through which the burning volatiles will start to escape.
To category: Ovens
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 the furnace.
The designs of heating furnaces have been created for a long time and were intended to burn solid fuel in them. Only in more late period began to create structures designed for the use of liquid and gaseous fuel... In order to use these valuable species most effectively in existing furnaces, it is necessary to know how the combustion of these fuels differs from the combustion of solid fuels.
In all stoves, solid fuel (firewood, different kinds coal, anthracite, coke, etc.) is burned on the grates in a layered method, with periodic loading of fuel and cleaning the grates from slag. The layered combustion process has a clear cyclical nature. Each cycle includes the following stages: fuel loading, drying and heating of the bed, release of volatiles and their combustion, combustion of fuel in the bed, afterburning 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, that is, 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 heats up, the release of gaseous combustible components begins and their burnout in the gas volume. At this stage, heat generation in the furnace begins, which gradually increases. Under the influence of heating, combustion 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, a low-intensity afterburning of combustible substances takes place. It is known that the role and influence of individual stages of the layered combustion cycle depends on the following indicators of the quality of solid fuel: humidity, ash content, the content of volatile combustible substances and carbon in the combustible.
mass.
Let us consider how these components affect the nature of the combustion process in the layer.
Fuel humidification has a negative effect on combustion, since a part of the specific heat of combustion of the fuel must be spent on evaporation of moisture. As a result, the temperatures in the firebox decrease, the combustion conditions deteriorate, 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 the fuel does not burn out, a so-called mechanical underburning is formed.
Researches of scientists have established that the ratio of the content of volatile gaseous substances and solid carbon in solid fuel has a great influence on the nature of the development of combustion processes. Volatile combustible substances begin to evolve from solid fuels at relatively low temperatures, starting from 150-200 ° C and above. Volatiles are varied in composition and differ in different outlet temperatures; therefore, 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 layer.
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 starts last. It flows directly in a thin layer of the grate, and due to the intense heat release, high temperatures develop in it.
A typical picture of the temperature change 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 firebox there is a rapid increase in temperatures in the firebox and chimneys. a sharp decline temperatures inside the stove, 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 ratio is at = 1.5-2, and at the afterburning stage, the duration of which reaches 25-30% of the furnace time, the excess air ratio reaches at = 8-10. In fig. 2 shows how the excess air ratio changes during one combustion cycle on the grate of three types of solid fuels: wood, peat and coal in a typical batch heating furnace.
Rice. 1. Change in the temperature of flue gases in different sections of the heating stove with solid fuel firing 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 (in front of the stove damper)
From fig. 2 shows that the excess air ratio 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 insufficient for their complete combustion, and at the stages of preliminary heating and afterburning of combustible substances, the amount of air is several times higher than the theoretically required one.
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, there is an increased loss of heat with exhaust gases 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. Operating experience a large number heating stoves shows; that as a result of chemical underburning of intensely emitted volatile substances, amorphous carbon is deposited in the form of soot on the inner walls of the furnace and chimneys.
Rice. 2. Change in the 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 transfer from 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 flammable.
It is clear from what has been said that the unsatisfactory performance of the layer process is largely due to the uneven evolution of volatile substances over time.
In layered combustion of high-carbon fuels, the combustion process is concentrated within a rather thin fuel layer, in which high temperatures develop. The combustion of pure carbon in the bed is self-regulating. This means that the amount of reacted (burned) carbon will correspond to the amount of oxidizer (air) supplied. Therefore, at constant flow air will be constant and the amount of fuel burned. The change in the heat load should be made by regulating the air supply VB. For example, with an increase in VB, the amount of fuel burned increases, and a decrease in UB will cause a decrease in the thermal performance of the layer, and the value of the excess air ratio will remain stable.
However, the combustion of anthracite and coke is associated with the following difficulties. For the possibility of creating high temperatures, the thickness of the layer 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., the actual combustion takes place. The entire overlying layer serves as a thermal insulator of the burning part of the layer, which protects the combustion zone from cooling due to the radiation of heat onto the walls of the firebox.
As a result of oxidative reactions in the combustion zone, useful heat is released according to the reaction
c + o2-> co.
However, at high temperatures of the layer in its upper zone, reverse reductive endothermic reactions occur, proceeding with the absorption of heat, according to the equation
C02 + 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; therefore, 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 above, it is clear that in any batch furnace operating on solid fuel, a non-stationary combustion process takes place, which inevitably reduces the efficiency of the furnaces in operation.
Great importance for economical operation, the furnace is of solid fuel quality.
According to the standards for household needs, mainly bituminous coals (grades D, G, Zh, K, T, etc.), as well as brown coals and anthracites are isolated. 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 coals and up to 20% for anthracite, moisture content is 6-15% for bituminous coals and 20-45% for brown coals.
Furnace devices of household stoves do not have means of mechanizing the combustion process (regulating the supply of blast air, shuraing the layer, etc.), therefore, for efficient combustion in furnaces, sufficiently high requirements must be imposed on the quality of coal. A significant part of the coal is supplied, however, unsorted, raw, with quality characteristics (moisture, 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. KD Pamfilov determined the annual material damage caused by the supply of low quality coals. Calculations have shown that material damage due to incomplete use of fuel is approximately 60% of the cost of coal mining. It is economically and technically feasible to enrich the fuel in the places of its production to a conditioned state, since additional expenses for enrichment will amount to approximately half of the specified amount of material damage.
An important quality characteristic of coal that affects the efficiency of its combustion is its fractional composition.
At increased content in fines fuel, it becomes denser and closes the gaps in the burning fuel layer, which leads to crater combustion, which is uneven 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 significant amount little things.
On the other hand, overuse large pieces coal (more than 100 mm) also leads to crater combustion.
The moisture content of the 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 subzero temperatures it freezes. To prevent freezing, the moisture content of coal should not exceed 8%.
Harmful component in solid fuel sulfur is, since the products of its combustion are sulfur dioxide SO2 and sulfur dioxide S03, which have strong corrosive properties, and are also very toxic.
It should be noted that in batch kilns, although less efficient, raw coals can still be burned satisfactorily; for long-burning furnaces, these requirements must be strictly met in full.
In ovens continuous action, in which liquid or gaseous fuel is burned, the combustion process is not cyclical, but continuous. The flow of fuel into the furnace occurs evenly, due to which a stationary combustion mode is observed. If, when burning solid fuel, the temperature in the firebox of the stove fluctuates within wide limits, which adversely affects the combustion process, then when burning natural gas shortly after turning on the burner, the temperature in the combustion chamber reaches 650-700 ° C. Further, it constantly increases over time and reaches 850-1100 ° C at the end of the furnace. The rate of temperature rise in this case is determined by the thermal stress of the furnace space and the time of the furnace firing (Fig. 25). The combustion of gas is relatively easy to maintain at a constant excess air ratio, which is accomplished by means of 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 loss with exhaust gases and to achieve the operation of the furnace with a high efficiency, reaching 80-90%. The efficiency of a gas furnace is stable over time and is significantly higher than that of a solid fuel furnace.
Influence of the fuel combustion mode and the size of the area of the heat-perceiving surface of the smoke circulation on the efficiency of the furnace. Theoretical calculations show that the thermal efficiency of the heating furnace, i.e. the value thermal efficiency, depends on the so-called external and internal factors. External factors include the value of the area of the heat-transferring outer surface S of the stove in the zone of the firebox and flue gas circulations, wall thickness 6, thermal conductivity coefficient K of the material of the walls of the stove and heat capacity C. The larger the value. S, X and less than 6, the better the heat transfer from the walls of the furnace to the ambient air, the more completely the gases are cooled and the higher the efficiency of the furnace.
Rice. 3. Change in the temperature of combustion products in the firebox of a gas heating stove, depending on the intensity of the combustion space and the time of the fire
The internal factors include, first of all, 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 incompleteness of combustion and mechanical incompleteness of combustion. 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 QIV value for a firebox of a given design depends on the fuel consumption.
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 conditions burning. 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 entrained in the gas ducts, the combustion process stops and chemical underburning of the fuel appears. In the same way, with excessive fuel consumption, some of it does not have time to burn and remains on the grate, which leads to mechanical underburning. Thus, in order for the heating stove to have the maximum efficiency, it is necessary that its firebox operate with the optimal thermal voltage.
Losses of heat to the environment from the walls of the firebox do not reduce the efficiency of the stove, since the heat is spent on useful heating of the room.
The second important internal factor is the flue gas flow rate Vr. Even if the stove operates at the optimal value of the heat voltage of the firebox, the volume of gases passing through the chimneys can change significantly due to a change in the excess air coefficient at, which is the ratio of the actual flow rate of air entering the firebox to its theoretically required amount. At a given value of QIV, the value of am can vary over a very wide range. In conventional heating furnaces of periodic action, the value of am during the period of maximum combustion can be close to 1, i.e., correspond to the minimum possible theoretical limit. However, during the preparation of the fuel and at the stage of afterburning of the residues, the value of am in batch furnaces usually increases sharply, often reaching extremely high values - of the order of 8-10. With an increase in am, the volume of gases increases, their residence time in the smoke circulation system decreases and, as a consequence, heat losses with exhaust gases increase.
In fig. 4 shows the graphs of the dependence of the efficiency of the heating furnace on various parameters. In fig. 4, a shows the values of the efficiency of the heating furnace depending on the values of at> from which it can be seen that with an increase in at from 1.5 to 4.5, the efficiency decreases from 80 to 48%. In fig. 4, b shows the dependence of the efficiency of the heating furnace on the size of the area of the inner surface of the smoke circulation 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 these factors, the value of the efficiency depends on the duration of the furnace firing t (Fig. 4, c). As x increases, the inner walls of the furnace are heated to a higher temperature and the gases are accordingly cooled less. Therefore, with an increase in the duration of the firebox, the efficiency of any heating furnace decreases, approaching a certain minimum value characteristic of a furnace of this design.
Rice. 4. Dependence of the efficiency of a gas heating furnace on various parameters a - on the excess air ratio with the area of the inner surface of the smoke circulation, m2; b - from the area of the inner surface of the smoke turnover at different air excess ratios; c - on the duration of the furnace for different areas of the inner surface of the smoke flow, m2
Heat transfer of heating furnaces and their storage capacity. In heating furnaces, the heat that must be transferred by the 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 accumulating capacity) change accordingly. For example, as the thickness of the walls decreases, 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 intermittent furnaces operating on solid fuel is unacceptable for the following reasons: with intermittent short-term heating, the inner surfaces of the firebox and chimneys are heated to high temperatures and the temperature of the outer surface of the furnace during periods of maximum combustion will be higher than the permissible limits; after the cessation of combustion due to the intense heat transfer of the outer walls to the environment, the furnace will quickly cool down.
At large values of M, the room temperature will vary over a wide range over time and go beyond the permissible standards. On the other hand, if you lay out the stove too thick-walled, then in a short period of heating its large mass will not have time to warm up and, in addition, with the thickening of the walls, the difference between the area of the inner surface of the chimneys, which receives heat from gases, and the area of the outer surface of the stove, which transfers heat, increases. ambient air, as a result of which the temperature of the outer surface of the oven will 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 outer surfaces of the furnace is achieved for normal heating of the room.
When using liquid or gaseous fuels in heating furnaces, it is quite achievable continuous mode combustion, therefore, with a continuous fire, there is no need for heat accumulation due to an increase in the masonry mass. The process of heat transfer from gases to the heated room is stationary in time. Under these conditions, the wall thickness and massiveness of the furnace can be selected not on the basis of providing a certain accumulating value, but from considerations of the strength of the masonry and ensuring proper durability.
The effect of switching the furnace from a batch to a continuous furnace is clearly seen from Fig. 5, which shows the change in the temperature of the inner surface of the firebox wall in the case of a periodic and continuous furnace. With a periodic firebox, after 0.5-1 hours, the inner surface of the firebox wall heats up to 800-900 ° C.
Such a sharp heating already after 1-2 years of operation of the furnace often causes cracking of bricks and their destruction. Such a mode, 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 seen from Fig. 27, with continuous firing, for most brands of coal, the wall temperature rises from 200 only to 450-500 ° C, while with periodic firing it is much higher - 800-900 ° C. Therefore, fireboxes of intermittent furnaces are usually lined with refractory bricks, while fireboxes of continuous furnaces do not need lining, since the temperature on their surface does not reach the refractoriness limit of ordinary red bricks (700-750 ° C).
Consequently, with continuous firing, brickwork is more efficiently used, the service life of the furnaces is greatly increased and for most brands of coal (excluding anthracite and lean coals) it is possible to lay out all parts of the furnace from red brick.
Furnace draft. In order to force the flue gases to pass from the firebox through the smoke flow of the stove to the chimney, overcoming all local resistances encountered on their way, it is necessary to expend a certain effort, which must exceed these resistances, otherwise the stove will smoke. This effort is commonly referred to as the thrust force of the furnace.
The appearance of the traction force is illustrated in the diagram (Fig. 6). The flue gases formed in the firebox, being lighter in comparison with the surrounding air, rise upward 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 combustion door, then on the right side it will be acted upon (pressed) by a column of hot gases from the middle of the furnace door to the top of the chimney, and on the left - a column of external cold air of the same height. The mass of the left column is greater than that of the right one, since the density of cold air is greater than that of hot air, therefore the left column will displace flue gases filling the chimney, and gases will move in the system in the direction from more pressure to the smaller one, i.e. towards the chimney.
Rice. 5. Temperature change on the inner surface of the firebox wall a - the thermostat is set to the lower limit; b - the thermostat is set to the upper limit
Rice. 6. Scheme of chimney operation 1-furnace door; 2- firebox; 3 - a column of outside air; 4 - chimney
Thus, the action of the traction force consists in the fact that, on the one hand, it makes hot gases rise upward, and on the other hand, it forces outside air go into the firebox for burning.
The average temperature of 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.
- Main features of fuel combustion processes
Page 1
The combustion process of solid fuel also consists of a number of successive stages. First of all, mixture formation and thermal preparation of fuel take place, including drying and release of volatiles. The resulting combustible gases and coke residue in the presence of an oxidizing agent are further burned with the formation of flue gases and a solid non-combustible residue - ash. The longest is the stage of combustion of coke - carbon, which is the main combustible component of any solid fuel. Therefore, the combustion mechanism of solid fuels is largely determined by the combustion of carbon.
The combustion process of solid fuel 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 the formation of slag. 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 preparation of the fuel for combustion and the period of combustion.
The combustion process of solid fuel can be conditionally divided into several stages: heating and evaporation of moisture, sublimation of volatiles and the formation of coke, combustion of volatiles, combustion of coke.
The combustion of solid fuel in a stream at elevated pressures leads to a decrease in the dimensions of combustion chambers and to a significant increase in heat stresses. Furnaces working with high blood pressure, did not receive wide distribution.
The combustion process of solid fuel has not been studied theoretically enough. The first stage of the combustion process, leading to the formation of an intermediate compound, is determined by the course of the dissociation process of the oxidant 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, which are successively superimposed on each other.
The combustion process of solid fuel can be considered as a two-stage process with loosely defined 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 from the kinetics of chemical reactions. The more volatiles in the fuel, the more its combustion rate depends on the rate of the ongoing chemical reactions.
An intensification of the solid fuel combustion process and a significant increase in the degree of ash collection are achieved in cyclone furnaces. C, in which the ash melts and the liquid slag is removed through the tap holes 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 combustion of solid fuel, the reactions of combustion of carbon monoxide and hydrogen are of absolute interest. For solid fuels rich in volatile substances, it is necessary to know the combustion characteristics of hydrocarbon gases in a number of processes and technological schemes. The mechanism and kinetics of homogeneous combustion reactions are considered in Ch. In addition to the above secondary reactions, the list of them should be continued with heterogeneous reactions of decomposition of carbon dioxide and water vapor, the reaction of conversion of carbon monoxide with water vapor and a family of reactions of methane formation, which proceed at noticeable rates during gasification under high pressure.
Combustion of solid fuel (coal dust) includes two periods: thermal preparation and combustion itself (Fig. 4.5).
In the process of thermal treatment (Fig. 4.5, zone I), the particle heats up, dries up, and at temperatures above 110 ° C, thermal decomposition of the initial fuel substance begins with the release of gaseous volatiles. The duration of this period depends mainly on the moisture content of the fuel, its particle size, heat transfer conditions and is usually tenths of a second. 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, therefore, heating the particle during this time is running slow.
Combustion itself begins with the ignition of volatile substances (Fig. 4.5, zone II) at a temperature of 400 ... 600 ° C, and the heat released during their combustion provides accelerated heating and ignition of solid coke residue. Combustion of volatile substances takes 0.2 ... 0.5 s. With a large yield of volatiles (brown and young coal, shale, peat), the released heat of their combustion is sufficient to ignite the coke particle, and with a low yield of volatiles, it becomes necessary to additionally warm the coke particle from the surrounding incandescent gases (zone III).
Combustion of coke (Fig. 4.5, zone IV) begins at a temperature of about 1000 ° C and is the most prolonged process. This is determined by the fact that part of the oxygen in the zone near the surface of the particle is consumed for the combustion of flammable volatiles and its remaining concentration has decreased, in addition, heterogeneous reactions are always inferior in speed to homogeneous ones for substances homogeneous in chemical activity.
As a result, the total burning time of a solid particle (1.0 ... 2.5 s) is mainly determined by the combustion of the coke residue (about 2/3 of the total burning time). For fuels with a high yield of volatile substances, the coke residue is less than half of the initial mass of a particle, therefore, their combustion at different initial sizes occurs rather quickly and the possibility of underburning decreases. Fuels older in age have a dense coke particle, the combustion of which takes almost the entire time spent in the combustion chamber.
The coke residue of most solid fuels mainly, and for a number of solid fuels entirely, consists of carbon (from 60 to 97% of the mass of a particle). Taking into account 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 sufficiently high intensity, however, a thin gas layer (boundary layer) remains directly at the particle surface, the transfer of the oxidant through which is carried out according to the laws of molecular diffusion (Fig. 4.6). This layer significantly inhibits the supply of oxygen to the surface. It burns out combustible gas components released from the particle during thermal decomposition. The amount of oxygen supplied per unit time to a unit surface area of a particle by means of turbulent diffusion is determined by the formula
In (4.16) and (4.17) C POT is the oxygen concentration in the flow surrounding the particle; C SL - the same on the outer boundary of the boundary layer; With POV - the same on the fuel surface; δ is the thickness of the boundary layer; D is the coefficient of molecular diffusion through the boundary layer; A is the coefficient of turbulent mass transfer.
The joint solution of equations (4.16) and (4.17) leads to the expression
 | 4.18a |
| 4.18b |
in which
 | 4.19 |
Generalized diffusion rate constant.
From formula (4.18) it follows that the supply of oxygen to the reacting surface of the solid fuel is determined by the constant of the diffusion rate and the difference in oxygen concentrations in the flow and on the reacting surface.
In a steady combustion process, the amount of oxygen supplied by diffusion to the reaction surface is equal to the amount reacted on the surface as a result of a chemical reaction. Hence, the reaction rate of combustion of carbon from the surface K s is found from the equality of the mass rates of two processes - diffusion supply and consumption of oxygen on the surface as a result of the chemical reaction
In accordance with the Arrhenius law, the process temperature is the determining parameter of the rate of a chemical reaction. The diffusion rate constant k D changes slightly with increasing temperature (see Fig. 4.1, a), while the reaction rate constant k p has an exponential dependence on temperature.
At a relatively low temperature (800 ... 1000 ° C), the chemical reaction proceeds slowly, despite the excess of oxygen near the solid surface, since k D >> k P. In this case, combustion is inhibited by the kinetics of the chemical reaction, therefore this temperature zone is called the kinetic combustion region ...
On the contrary, at high combustion temperatures (above 1500 ° C) and combustion of coal dust, the value of k P >> k D and the combustion process is inhibited by the conditions of oxygen supply (diffusion) to the particle surface. The region of diffusion combustion corresponds to these conditions. The creation in this zone of the torch temperatures of additional conditions for mixing the burning mixture (increasing the value of k D) promotes the acceleration and deepening of fuel burnout.
A similar effect in terms of intensifying combustion is achieved by reducing the particle size of pulverized fuel. Particles of small size have a more developed heat exchange with the environment and, thus, a higher value of k D. An increase in temperature leads to a shift in the oxidation process to the region of diffusion combustion.
The area of purely diffusive combustion of pulverized fuel is characteristic of the torch core, which is characterized by the highest combustion temperature, and the afterburning zone, where the concentrations of reactants are already small and their interaction is determined by the laws of diffusion. Ignition of any fuel begins at relatively low temperatures, in conditions of sufficient oxygen, i.e. in the kinetic region. In this area of combustion, the decisive role is played by the rate of the chemical reaction, which depends on factors such as the reactivity of the fuel and the level of temperature. The influence of aerodynamic factors in this combustion area is negligible.
