Features of combustion of solid fuel. fuel burning

The combustion process of solid fuel can be represented as a series of successive stages. First, the fuel warms up and moisture evaporates. Then, at a temperature above 100 °C, the pyrogenic decomposition of complex macromolecular organic compounds and the release of volatile substances begin, while the temperature at which the volatiles begin to emerge depends on the type of fuel and the degree of its carbonification (chemical age). If the ambient temperature exceeds the ignition temperature of volatile substances, they ignite, thereby providing additional heating of the coke particle before it ignites. The higher the yield of volatiles, the lower their ignition temperature, while the heat release increases.

The coke particle is heated by the heat of the surrounding flue gases and heat release as a result of the combustion of volatiles and ignites at a temperature of 800÷1000 °C. When solid fuels are burned in a pulverized state, both stages (combustion of volatiles and coke) can overlap each other, since the heating of the smallest coal particle occurs very quickly. In real conditions, we are dealing with a polydisperse composition of coal dust, therefore, at each moment of time, some particles only begin to warm up, others are at the stage of volatile release, and still others are at the stage of combustion of the coke residue.

The process of combustion of a coke particle plays a decisive role in assessing both the total burning time of the fuel and the total heat release. Even for fuel with a high volatile yield (for example, brown coal near Moscow), the coke residue is 55% by weight, and its heat release is 66% of the total. And for a fuel with a very low volatile yield (for example, DS), the coke residue can be more than 96% of the weight of the dry initial particle, and the heat release during its combustion, respectively, is about 95% of the total.

Studies of the combustion of coke residue have revealed the complexity of this process.

When carbon is burned, there are two possible primary direct heterogeneous oxidation reactions:

C + O 2 \u003d CO 2 + 34 MJ / kg; (fourteen)

2C + O 2 \u003d 2CO + 10.2 MJ / kg. (15)

As a result of the formation of CO 2 and CO, two secondary reactions:

oxidation of carbon monoxide 2CO + O 2 \u003d 2CO 2 + 12.7 MJ / kg; (16)

reduction of carbon dioxide CO 2 + C \u003d 2CO - 7.25 MJ / kg. (17)

In addition, in the presence of water vapor on the heated surface of the particle, i.e. in the high-temperature region, gasification occurs with the release of hydrogen:

C + H 2 O \u003d CO + H 2. (eighteen)

Heterogeneous reactions (14, 15, 17, and 18) testify to the direct combustion of carbon, accompanied by a decrease in the weight of the carbon particle. Homogeneous reaction (16) proceeds near the surface of the particle due to oxygen diffusing from the surrounding volume and compensates for the decrease in the temperature level of the process that occurs as a result of endothermic reaction (17).

The ratio between CO and CO 2 at the surface of a particle depends on the temperature of the gases in this region. So, for example, according to experimental studies, at a temperature of 1200 ° C, the reaction proceeds

4C + 3O 2 \u003d 2CO + 2CO 2 (E \u003d 84 ÷ 125 kJ / g-mol),

and at temperatures above 1500 ° C

3C + 2O 2 \u003d 2CO + CO 2 (E \u003d 290 ÷ 375 kJ / g-mol).

Obviously, in the first case, CO and CO 2 are released in approximately equal amounts, while with an increase in temperature, the volume of released CO is 2 times higher than CO 2 .

As already noted, the burning rate mainly depends on two factors:

1) speed chemical reaction , which is determined by the Arrhenius law and grows rapidly with increasing temperature;

2) oxidant supply rate(oxygen) to the combustion zone due to diffusion (molecular or turbulent).

In the initial period of the combustion process, when the temperature is still not high enough, the rate of the chemical reaction is also low, and there is more than enough oxidizer in the volume surrounding the fuel particle and at its surface, i.e. there is a local excess of air. No improvement in the aerodynamics of the furnace or burner, which leads to an intensification of the supply of oxygen to the burning particle, will affect the combustion process, which is retarded only by the low rate of the chemical reaction, i.e. kinetics. This - kinetic combustion region.

As the combustion process proceeds, heat is released, the temperature increases, and, consequently, the rate of the chemical reaction, which leads to a rapid increase in oxygen consumption. Its concentration at the surface of the particle steadily decreases, and in the future the combustion rate will be determined only by the diffusion rate of oxygen into the combustion zone, which is almost independent of temperature. This - diffusion combustion area.

IN transitional combustion region the rates of a chemical reaction and diffusion are quantities of the same order.

According to the law of molecular diffusion (Fick's law), the rate of diffusion transfer of oxygen from the volume to the particle surface

where is the coefficient of diffusion mass transfer;

And are, respectively, the partial pressures of oxygen in the volume and at the surface.

The oxygen consumption at the particle surface is determined by the rate of the chemical reaction:

, (20)

where k is the reaction rate constant.

In the transition zone in the steady state

,

where
(21)

Substituting (21) into (20), we obtain an expression for the combustion rate in the transition region in terms of the consumption of the oxidizer (oxygen):

(22)

where
is the effective rate constant of the combustion reaction.

In the zone of relatively low temperatures (kinetic region)
, Consequently, k ef = k, and expression (22) takes the form:

,

those. oxygen concentrations (partial pressures) in the volume and at the surface of the particle differ little from each other, and the burning rate is almost completely determined by the chemical reaction.

With an increase in temperature, the rate constant of a chemical reaction grows according to the exponential Arrhenius law (see Fig. 22), while molecular (diffusion) mass transfer weakly depends on temperature, namely

.

At a certain temperature value T*, the rate of oxygen consumption begins to exceed the intensity of its supply from the surrounding volume, the coefficients α D And k become comparable values ​​of the same order, the oxygen concentration at the surface begins to noticeably decrease, and the burning rate curve deviates from the theoretical kinetic combustion curve (Arrhenius law), but still noticeably increases. An inflection appears on the curve - the process passes into an intermediate (transitional) combustion region. The relatively intensive supply of the oxidant is explained by the fact that, due to a decrease in the oxygen concentration near the surface of the particle, the difference in the partial pressures of oxygen in the volume and near the surface increases.

In the process of combustion intensification, the oxygen concentration near the surface practically becomes equal to zero, the supply of oxygen to the surface weakly depends on temperature and becomes almost constant, i. α D << k, and, accordingly, the process passes into the diffusion region

.

In the diffusion region, an increase in the burning rate is achieved by intensifying the process of mixing the fuel with air (improvement of burner devices) or by increasing the speed of blowing the particle with an air stream (improving the aerodynamics of the furnace), as a result of which the thickness of the boundary layer near the surface decreases and oxygen supply to the particle is intensified.

As already noted, solid fuel is burned either in the form of large (without special preparation) pieces (stratified combustion), or in the form of crushed (fluidized bed and low-temperature vortex), or in the form of the smallest dust (flare method).

Obviously, the largest relative speed blowing of fuel particles will be in stratified combustion. With vortex and flare combustion, fuel particles are in the flue gas flow, and the relative velocity of their blowing is much lower than in a stationary layer. Proceeding from this, it would seem that the transition from the kinetic to the diffusion region should occur first of all for small particles, i.e. for dust. In addition, a number of studies have shown that a coal dust particle suspended in a gas-air mixture flow is blown so weakly that the released combustion products form a cloud around it, which greatly inhibits the supply of oxygen to it. And the intensification of heterogeneous combustion of dust in the flare method was presumably due to an exceptionally significant increase in the total reacting surface. However, the obvious is not always true. .

The supply of oxygen to the surface is determined by the laws of diffusion. Studies on the heat transfer of a small spherical particle in a laminar flow revealed a generalized criterion dependence:

Nu = 2 + 0.33Re 0.5.

For small coke particles (at Re< 1, что соответствует скорости витания мелких частиц), Nu → 2, т.е.

.

There is an analogy between the processes of heat and mass transfer, since both are determined by the movement of molecules. Therefore, the laws of heat transfer (Fourier and Newton-Richmann laws) and mass transfer (Fick's law) have a similar mathematical expression. The formal analogy of these laws allows us to write in relation to diffusion processes:

,

where
, (23)

where D is the molecular diffusion coefficient (similar to the thermal conductivity coefficient λ in thermal processes).

As follows from formula (23), the diffusion mass transfer coefficient α D is inversely proportional to the radius of the particle. Consequently, with a decrease in the size of the fuel particles, the process of oxygen diffusion to the surface of the particle is intensified. Thus, during the combustion of coal dust, the transition to diffusion combustion is shifted towards higher temperatures (despite the previously noted decrease in the particle blowing velocity).

According to numerous experimental studies conducted by Soviet scientists in the middle of the twentieth century. (G.F.Knorre, L.N. Khitrin, A.S.Predvoditelev, V.V.Pomerantsev and others), in the zone of normal furnace temperatures (about 1500 ÷ 1600 ° C), the combustion of the coke particle is shifted from the intermediate zone to diffusion, where the intensification of oxygen supply is of great importance. In this case, with an increase in the diffusion of oxygen to the surface, the deceleration of the burning rate will begin at a higher temperature.

The combustion time of a spherical carbon particle in the diffusion region has a quadratic dependence on the initial particle size:

,

where r o is the initial particle size; ρ h is the density of the carbon particle; D o , P o , T o are, respectively, the initial values ​​of the diffusion coefficient, pressure, and temperature;
is the initial oxygen concentration in the furnace volume at a considerable distance from the particle; β - stoichiometric coefficient, which establishes the correspondence of the weight consumption of oxygen per unit weight of burned carbon at stoichiometric ratios; T m– logarithmic temperature:

where T P And T G are, respectively, the temperatures of the particle surface and the surrounding flue gases.

Due to the growing popularity of solid fuel boilers, a huge number of potential buyers of this equipment are interested in the question of what type solid fuel give preference as the main one, and, depending on the decision made, order one or another type of heating equipment.

The main indicator of any fuel, not only solid fuel, is its heat transfer, which is ensured by the combustion of solid fuel. In this case, the heat transfer of solid fuel is directly related to its type, properties and composition.

some chemistry

The composition of solid fuels includes the following substances: carbon, hydrogen, oxygen and mineral compounds. When it burns fuel, carbon and hydrogen combine with atmospheric oxygen (the strongest natural oxidizing agent) - a combustion reaction occurs with the release of a large amount of thermal energy. Further, gaseous combustion products are removed through the smoke exhaust system, and solid combustion products (ash and slag) fall out as waste through the grate.

Accordingly, the main task facing the designer of solid fuel heating equipment is to ensure the longest possible burning of a solid fuel stove or solid fuel boiler. At this point in time, some progress has been made in this area - long-burning solid fuel boilers operating on the principle of upper combustion and the pyrolysis process have appeared on sale.

Calorific value of the main types of solid fuel

• Firewood. On average (depending on the type of wood) and humidity from 2800 to 3300 kcal / kg.
• Peat - depending on humidity from 3000 to 4000 kcal / kg.
• Coal - depending on the type (anthracite, brown or fiery) from 4700 to 7200 kcal / kg.
• Pressed briquettes and pellets - 4500 kcal / kg.

In other words, the process of burning solid fuels of various types is accompanied by a different amount of released thermal energy, therefore, the choice of the main type of fuel should be taken very responsibly - be guided in this matter by the information specified in the operational documentation (passport or Operating Instructions) for one or another solid fuel equipment.

Brief description of the main types of solid fuels

Firewood

The most affordable, therefore, the most common type of fuel in Russia. As already mentioned, the amount of heat generated during combustion depends on the type of wood and its moisture content. It should be noted that when using firewood as fuel for a pyrolysis boiler, there is a humidity limit, which in this case should not exceed 15-20%.

Peat

Peat is the compressed remains of rotted plants that lie in the soil for a long time. According to the method of extraction, high and low peat are distinguished. And according to the state of aggregation, peat can be: carved, lumpy and pressed in the form of briquettes. In terms of the amount of thermal energy released, peat is similar to firewood.

Coal

Coal is the most "high-calorie" type of solid fuel, which requires a special ignition technology. In the general case, in order to kindle a stove or boiler on coal, you first need to kindle the firebox with firewood and only then, load coal (brown, fiery or anthracite) on well-burned firewood.

Briquettes and pellets

This is a new type of solid fuel, which differs in the size of individual elements. Briquettes are larger and pellets are smaller. The starting material for the manufacture of briquettes and pellets can be any "combustible" substance: wood chips, wood dust, straw, nut husks, peat, sunflower husks, bark, cardboard and other "mass" combustible substances that are freely available.

Advantages of briquettes and pellets

• Environmentally friendly renewable fuel with high calorific value.
• Long burning due to the high density of the material.
• Convenient and compact storage.
• The minimum amount of ash after combustion is from 1 to 3% of the volume.
• Low relative cost.
• The possibility of automating the process of the boiler.
• Suitable for all types of solid fuel boilers and domestic heating stoves.

Solid fuels include wood, peat and coal. The combustion process of all types of solid fuels has similar features.

Fuel must be placed on the grate of the furnace in layers, observing the combustion cycles - such as loading, drying, heating the layer, burning with the release of volatile substances, burning out residues and removing slags.

Each stage of fuel combustion is characterized by certain indicators that affect the thermal regime of the furnace.

At the very beginning of drying and heating of the layer, heat is not released, but, on the contrary, is absorbed from the heated walls of the firebox and unburned residues. As the fuel heats up, gaseous combustible components begin to be released, burning in the gas volume of the furnace. Gradually, more and more heat is released, and this process reaches its maximum during the combustion of the coke base of the fuel.

The combustion process of fuel is determined by its qualities: ash content, humidity, as well as the content of carbon and volatile combustible substances. In addition, the correct choice of the furnace design and fuel combustion modes is important. So, when burning wet fuel, a significant amount of heat is expended on its evaporation, due to which the combustion process is delayed, the temperature in the firebox rises very slowly or even decreases (at the beginning of combustion). Increased ash content also slows down the combustion process. Due to the fact that the ash mass envelops combustible components, it limits the access of oxygen to the combustion zone and, as a result, the fuel may not burn completely, so that the formation of mechanical underburning increases.

The intensive combustion cycle of a fuel depends on its chemical composition, that is, the ratio between volatile gaseous components and solid carbon. First, volatile components begin to burn, the release and ignition of which occurs at relatively low temperatures (150-200 ° C). This process can continue for quite a long time, because there are a lot of volatile substances that differ in their chemical composition and ignition temperature. All of them burn in the above-layer gas volume of the firebox.

The solid components of the fuel remaining after the release of volatile substances have the highest combustion temperature. As a rule, they are based on carbon. Their combustion temperature is 650-700 ° C. Solid components burn in a thin layer located above the grate. This process is accompanied by the release of a large amount of heat.

Of all types of solid fuels, firewood is the most popular. They contain a large amount of volatile substances. From the point of view of heat transfer, birch and larch wood is considered the best. After burning birch firewood, a lot of heat is released and a minimal amount of carbon monoxide is formed. Larch firewood also gives off a lot of heat; when they burn, the furnace array heats up very quickly, which means that they are consumed more economically than birch ones. But at the same time, after burning firewood from larch, a large amount of carbon monoxide is released, so you need to be careful about manipulating the air damper. A lot of heat is also emitted by oak and beech firewood. In general, the use of certain firewood depends on the presence of a nearby forest area. The main thing is that the firewood is dry, and the chocks are of the same size.

What are the features of burning wood? At the beginning of the process, the temperature in the firebox and gas ducts rises rapidly. Its maximum value is reached in the stage of intense combustion. During combustion, a sharp decrease in temperature occurs. To maintain the combustion process, constant access to the furnace of a certain amount of air is necessary. The design of household stoves does not provide for the presence of special equipment that regulates the flow of air into the combustion zone. For this purpose, a blower door is used. If it is open, a constant amount of air enters the furnace.

In batch furnaces, the air requirement varies depending on the stage of combustion. When there is an intensive release of volatile substances, there is usually not enough oxygen, so the so-called chemical underburning of the fuel and the combustible gases emitted by it is possible. This phenomenon is accompanied by heat losses, which can reach 3-5%.

At the stage of afterburning of residues, the opposite picture is observed. Due to an excess of air in the furnace, gas exchange increases, which leads to a significant increase in heat loss. According to studies, up to 25-30% of heat is lost together with the exhaust gases during the afterburning period. In addition, due to chemical underburning, volatile substances settle on the inner walls of the firebox and gas ducts. They have low thermal conductivity, so the useful heat transfer of the furnace is reduced. A large amount of sooty substances leads to a narrowing of the chimney and a deterioration in draft. Excessive buildup of soot can also cause a fire.

Peat, which is the remains of decayed plant matter, has a chemical composition similar to firewood. Depending on the method of extraction, peat can be carved, lumpy, pressed (in briquettes) and milled (peat chips). The moisture content of this type of solid fuel is 25-40%.

Along with firewood and peat, coal is often used for heating stoves and fireplaces, which in its chemical composition is a combination of carbon and hydrogen and has a high calorific value. However, it is not always possible to purchase really high-quality coal. In most cases, the quality of this type of fuel leaves much to be desired. An increased content of fine fractions in coal leads to compaction of the fuel layer, as a result of which the so-called crater combustion begins, which is uneven in nature. When burning large pieces of coal, it also burns unevenly, and with excessive moisture in the fuel, the specific heat of combustion is significantly reduced. In addition, such coal is difficult to store in winter, because coal freezes under the influence of sub-zero temperatures. To avoid such and other troubles, the optimal moisture content of coal should be no more than 8%.

It should be borne in mind that the use of solid fuel for heating household stoves is quite troublesome, especially if the house is large and heated by several stoves. In addition to the fact that a lot of effort and material resources are spent on harvesting and a lot of time is spent on bringing firewood and coal to the stoves, about 2 kg of coal, for example, is poured into the blower, from which it is removed and thrown away along with the ash accumulating there.

In order for the process of burning solid fuels in domestic stoves to be as efficient as possible, it is recommended to proceed as follows. Having loaded firewood into the firebox, you need to let it flare up, and then fill it with large pieces of coal.

After the coal is ignited, it should be covered with a finer fraction with moistened slag, and after a while, a moistened mixture of ash and fine coal, which has fallen through the grate into the blower, is placed on top. In this case, the fire should not be visible. A stove flooded in this way is capable of giving off heat to the room for a whole day, so that the owners can safely go about their business without worrying about constantly maintaining the fire. The side walls of the furnace will be hot due to the gradual combustion of coal, evenly giving off its thermal energy. The top layer, consisting of fine coal, will burn out completely. Inflamed coal can also be sprinkled on top with a layer of pre-moistened waste coal briquettes.

After firing the stove, 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.

Task………………………………………………………………………..3

Introduction……………………………………………………………………...4

Theoretical part

1. Features of solid fuel combustion ………………………..... 6

2. Combustion of fuel in chamber furnaces ….………………………….9

3. Place and role of solid fuel in the energy sector of Russia ……………..12

4. Reducing emissions of ash particles from boiler furnaces by constructive and technological methods……………………………………………………14

5. Ash collection and types of ash collectors…………………….…….15

6. Cyclone (inertial) ash collectors…..……………………..16

Settlement part

1. Initial data…………………………………………………….18

2. Calculation of the elemental composition of the working fuel…………………..19
3. Calculation of the masses and volumes of fuel combustion products during combustion in boiler houses ……………………………………………………………………..19

4. Determining the height of the pipe H…………………………….…………20

5. Calculation of dispersion and standards for maximum permissible emissions of harmful substances into the atmosphere……………………………………….…20

6. Determination of the required degree of purification……………………….… 21

Rationale for choosing a cyclone………………………………………………..22

Applied devices……………………………………………. ……23

Conclusion………………………………………………………………….24

List of used literature……………………………………...26

The task

1. According to the given design characteristics of solid fuels, determine the elemental composition of the working fuel.

2. Using the results of paragraph 1 and the initial data, calculate the emissions and volumes of combustion products of particulate matter A, sulfur oxides SO x , carbon monoxide CO, nitrogen oxides NO x , the flow rate of gases entering the chimney under operating conditions of the boiler plant.

3. Based on the results of paragraph 2 and the initial data, determine the diameter of the mouth of the chimney. Determine the height of the pipe H.

4. Determine the most expected concentration C m (mg / m 3) of harmful substances: carbon monoxide CO, sulfur dioxide SO 2, nitrogen oxides NO x, dust, (ash) in the surface layer of the atmosphere under unfavorable dispersion conditions.

5. Compare the actual content of harmful substances in the atmospheric air, taking into account the background concentration (C m + C f) with sanitary and hygienic standards (MPC), if MPC CO \u003d 5 mg / m 3, MPC NO 2 \u003d 0.085, MPC SO 2 \u003d 0, 5 mg/m 3 , MPC dust = 0.5 mg/m 3 .

7. Determine the required degree of purification and give recommendations for reducing emissions if the actual emission M of any substance exceeds the calculated standard (MAL).

8. Develop and justify the methods and devices used for the treatment of waste hazardous substances.

Theoretical part

Introduction

Industrial production and other types of human economic activity are accompanied by the release of pollutants into the environment.

Significant damage to the environment is caused by boiler plants that use the combustion of solid, liquid and gaseous fuels when heating water for heating systems.

The main source of the negative impact of the energy sector is the products formed during the combustion of fossil fuels.

The working mass of organic fuel consists of carbon, hydrogen, oxygen, nitrogen, sulfur, moisture and ash. As a result of the complete combustion of fuels, carbon dioxide, water vapor, sulfur oxides (sulfur dioxide, sulfuric anhydride and ash) are formed. Sulfur oxides and ash are among the toxic ones. In the core of the torch of high-power furnace boilers, partial oxidation of nitrogen in the fuel air occurs with the formation of nitrogen oxides (nitrogen oxide and nitrogen dioxide).

With incomplete combustion of fuel in furnaces, carbon monoxide CO 2, hydrocarbons CH 4, C 2 H 6, as well as carcinogens can also be formed. The products of incomplete combustion are very harmful, but with modern combustion technology, their formation can be eliminated or minimized.

Oil shale and oil shale have the highest ash content. brown coals, as well as some varieties hard coal. Liquid fuel has a low ash content; natural gas is an ashless fuel.

Toxic substances emitted into the atmosphere from the chimneys of power plants have a harmful effect on the entire complex of wildlife and the biosphere.

A comprehensive solution to the problem of protecting the environment from the effects of harmful emissions from fuel combustion in boiler units includes:

· Development and implementation of technological processes that reduce emissions of harmful substances due to the completeness of combustion of fuels, etc.;

· Implementation of effective methods and ways of purification of waste gases.

The most effective way to solve environmental problems at the present stage is the creation of technologies that are close to waste-free. At the same time, the problem of rational use of natural resources, both material and energy, is being solved.

Features of solid fuel combustion

The combustion of solid fuel includes two periods: thermal preparation and actual combustion. In the process of thermal preparation, the fuel is heated, dried, and at a temperature of about 110, pyrogenetic decomposition of its components begins with the release of gaseous volatile substances. The duration of this period depends mainly on the moisture content of the fuel, the size of its particles and the conditions of heat exchange between the surrounding combustion medium and fuel particles. The course of processes during the period of thermal preparation is associated with the absorption of heat mainly for heating, drying the fuel and thermal decomposition of complex molecular compounds.

Combustion itself begins with the ignition of volatile substances at a temperature of 400-600, and the heat released during combustion provides accelerated heating and ignition of the coke residue.

The combustion of coke begins at a temperature of about 1000 and is the longest process.

This is determined by the fact that part of the oxygen in the zone near the surface of the particle is used up for the combustion of combustible volatile substances and its remaining concentration has decreased, in addition, heterogeneous reactions are always inferior in speed to homogeneous reactions for substances that are homogeneous in chemical activity.

As a result, the total burning time of a solid particle is mainly determined by the burning of the coke residue (about 2/3 of the total burning time). For young fuels with a high yield of volatile substances, the coke residue is less than half of the initial mass of the particle; therefore, their combustion (with equal initial sizes) occurs quite quickly and the possibility of underburning is reduced. Old types of solid fuels have a large coke residue close to the initial particle size, the combustion of which takes the entire time the particle stays in the combustion chamber. The combustion time of a particle with an initial size of 1 mm is from 1 to 2.5 s, depending on the type of initial fuel.

The coke residue of most solid fuels mainly, and for a number of solid fuels, almost entirely consists of carbon (from 60 to 97% of the organic mass of the fuel). Considering that carbon provides the main heat release during fuel combustion, let us consider the dynamics of combustion of a carbon particle from the surface. Oxygen is supplied from the environment to the carbon particle due to turbulent diffusion (turbulent mass transfer), which has a fairly high intensity, but a thin gas layer (boundary layer) remains directly at the surface of the particle, the transfer of the oxidizer through which is carried out according to the laws of molecular diffusion.

This layer significantly inhibits the supply of oxygen to the surface. In it, the combustion of combustible gas components released from the carbon surface during a chemical reaction takes place.

Allocate diffusion, kinetic and intermediate region of combustion. In the intermediate and especially in the diffusion region, intensification of combustion is possible by increasing the supply of oxygen, by activating the blowing of burning fuel particles with an oxidizer flow. At high flow rates, the thickness and resistance of the laminar layer near the surface decrease and the supply of oxygen increases. The higher this speed, the more intense the mixing of fuel with oxygen, and the higher the temperature, the transition from the kinetic to the intermediate zone, and from the intermediate to the diffusion zone of combustion occurs.

A similar effect in terms of combustion intensification is achieved by reducing the particle size of pulverized fuel. Particles of small size have a more developed heat and mass exchange with the environment. Thus, with a decrease in the particle size of pulverized fuel, the region of kinetic combustion expands. An increase in temperature leads to a shift to the region of diffusion combustion.

The region of pure diffusion combustion of pulverized fuel is limited mainly by the flame core, which has the highest combustion temperature, and the afterburning zone, where the concentrations of reactants are already low and their interaction is determined by the laws of diffusion. The ignition of any fuel begins at relatively low temperatures, in conditions of a sufficient amount of oxygen, i.e. in the kinetic area.

In the kinetic region of combustion, the decisive role is played by the rate of a chemical reaction, which depends on such factors as the reactivity of the fuel and the temperature level. The influence of aerodynamic factors in this combustion region is insignificant.

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. It was only in a later period that structures designed for the use of liquid and gaseous fuels began to be created. In order to make the most efficient use of these valuable species in existing furnaces, it is necessary to know how the combustion processes of these fuels differ from the combustion of solid fuels.

In all furnaces, solid fuel (wood, various types of 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 layered combustion cycle depends on the following quality indicators of solid fuel: 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.

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 be released from solid fuels at relatively low temperatures, starting from 150-200 ° C and above. Volatile substances are diverse in composition and differ in different outlet temperatures, 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 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 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. Figure 2 shows how the excess air coefficient changes during one combustion cycle on a grate for three types of solid fuels: wood, 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, at a constant air flow, 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 above, 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.

Of great importance for the economical operation of the furnace is 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 means of mechanizing the combustion process (regulation of 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 public utilities them. K. D. Pamfilov, the annual material damage caused as a result of the supply of low-quality coal was determined. Calculations have shown that 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 enrichment costs 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 coal is burned worse than other types of fuel, which tends 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 the specific heat of combustion, the combustion temperature, and also complicates the storage of coal, since it freezes at sub-zero temperatures. 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 furnace firebox fluctuates over a wide range, which adversely affects the combustion process, then when natural gas is burned, soon after the burner is turned on, 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 a constant excess air ratio, which is carried out with the help 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 losses with exhaust gases and achieve furnace operation with high efficiency, 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. 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 value. 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 oven is running at optimal value 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 its 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 may 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%.

except listed factors the value of 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 vary over time over a wide range and go out of acceptable norms. On the other hand, if the stove is laid out 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 ​​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, 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 array of the batch furnace accumulates a sufficient amount of heat during the furnace and at the same time sufficient heat external surfaces of the furnace 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 stoves is greatly increased, and for most grades of coal (excluding anthracites and lean coals), it is possible to lay out all parts of the stove 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 a conventional vertical plane is drawn through the furnace door, then on the 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 on the left - 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 that of 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.

The average temperature of the 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