K category: Furnaces
Main features of fuel combustion processes
Heating stoves can use solid, liquid and gaseous fuels. Each of these fuels has its own characteristics that affect the efficiency of stove use.
The designs of heating furnaces were created over a long period of time and were intended to burn solid fuels. Only in a later period did designs begin to be created that were designed to use liquid and gaseous fuels. In order to most effectively use these valuable types in existing furnaces, it is necessary to know how the combustion processes of these fuels differ from the combustion of solid fuels.
All stoves contain solid fuel (wood, different kinds coal, anthracite, coke, etc.) is burned on grates in a layered manner, with periodic loading of fuel and cleaning of the grate from slag. The layer 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, post-burning of residues and, finally, removal of slag.
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 a 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. Then, as the layer heats up, the release of gaseous combustible components begins and their combustion in the gas volume begins. At this stage, heat release in the firebox 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, the heat release gradually decreases, and in the final stage 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, content of volatile combustible substances and carbon in the fuel
mass.
Let us consider how these components influence the nature of the combustion process in the layer.
Humidifying the fuel has a negative effect on combustion since part of the specific heat of combustion of the fuel must be spent on the evaporation of moisture. As a result, temperatures in the firebox decrease, combustion conditions worsen, and the combustion cycle itself is prolonged.
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 air oxygen from accessing them. As a result, the combustible mass of fuel does not burn out, so-called mechanical underburning is formed.
Research by scientists has 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 fuel at relatively low temperatures, starting from 150-200 ° C and above. Volatile substances are varied in composition and differ in different release temperatures, so the process of their release is extended over 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 occurs directly in a thin layer of the grate, and due to intense heat generation, high temperatures develop in it.
A typical picture of temperature changes in the furnace and flues during the combustion cycle of solid fuel is shown in Fig. 1. As you can see, at the beginning of the firebox there is a rapid increase in temperature in the firebox and chimneys. At the post-burning stage, there is a sharp decrease in temperature inside the furnace, especially in the firebox. Each stage requires a certain amount of combustion air to be supplied to the firebox. However, due to the fact that what enters the furnace constant quantity air, at the stage of intense combustion the excess air coefficient is at=1.5-2, and at the post-burning stage, the duration of which reaches 25-30% of the combustion time, the excess air coefficient reaches at=8-10. In Fig. Figure 2 shows how the excess air coefficient changes during one combustion cycle on a grate of three types of solid fuel: firewood, peat and coal in a typical batch heating furnace.
Rice. 1. Change in the temperature of flue gases in various sections of a heating furnace when firing 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 (in front of the stove damper)
From Fig. 2 shows that the excess air coefficient in furnaces operating with periodic loading of solid fuel continuously changes.
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 preheating 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 intense release of volatile substances, chemical underburning of the released combustible gases occurs, and when the residues are burned, there are increased heat losses with the exhaust gases due to an increase in the volume of combustion products. Heat losses with chemical underburning are 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 stoves shows; that as a result of chemical underburning of intensely released volatile substances, amorphous carbon in the form of soot is deposited on the internal walls of the firebox and chimneys.
Rice. 2. Change in excess air coefficient during the solid fuel combustion cycle
Since soot has low thermal conductivity, its deposits increase the thermal resistance of the furnace walls and thereby reduce the useful heat transfer 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 flammable.
From the above it is clear that the unsatisfactory performance of the layer process is largely explained by the uneven release of volatile substances over time.
During layer combustion of high-carbon fuels, the combustion process is concentrated within a fairly 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 supplied oxidizer (air). Therefore, with a constant air flow rate, the amount of fuel burned will also be constant. Changing the heat load should be done 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 thermal productivity of the layer, and 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 when burning 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 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 reduction endothermic reactions occur, occurring with heat absorption, according to the equation
С02+С2СО.
As a result of these reactions, carbon monoxide CO is formed, which is a flammable gas with a fairly 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 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 operated furnaces.
Great importance for economical operation, the stove has the quality of solid fuel.
According to the standards, mainly hard coals (grades D, G, Zh, K, T, etc.), as well as brown coals and anthracites are distinguished for domestic needs. According to 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 hard coals and up to 20% for anthracite, humidity - 6-15% for hard coal and 20-45% for brown coal.
Combustion devices of household furnaces do not have means of mechanizing the combustion process (regulating the supply of blown air, scissoring the layer, etc.), therefore, for efficient combustion in furnaces, fairly high requirements must be placed on the quality of coal. A significant part of the coal, however, is supplied unsorted, ordinary, with quality characteristics (moisture, ash content, fines content) significantly lower than those provided for by the standards.
The combustion of substandard fuel occurs imperfectly, with increased losses from chemical and mechanical underburning. Academy of Public Utilities named after. K. D. Pamfilova determined the annual material damage caused as a result of the supply of low-quality coal. Calculations have shown that material damage caused by incomplete use of fuel amounts to approximately 60% of the cost of coal production. It is economically and technically feasible to enrich fuel at the sites of its production to a conditioned state, since the additional costs of enrichment will amount to approximately half of the specified amount of material damage.
An important qualitative characteristic of coal that 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 is uneven over the area of the layer. For the same reason, brown coals, which tend to crack when heated and produce a significant amount of fines, are burned worse than other types of fuel.
On the other hand, the use of excessively large pieces of coal (more than 100 mm) also leads to crater combustion.
The moisture content of coal, generally speaking, does not impair the combustion process; however it reduces specific heat combustion, combustion temperature, and also complicates the storage of coal, since at sub-zero temperatures it freezes. To prevent freezing, the moisture content of coals should not exceed 8%.
The harmful component in solid fuel is sulfur, since its combustion products 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, raw coals, although less efficient, can still be burned satisfactorily; For long-burning furnaces, these requirements must be strictly met in full.
In continuous furnaces, in which liquid or gaseous fuel is burned, the combustion process is not cyclical, but continuous. Fuel enters the furnace evenly, ensuring a stationary combustion mode. If, when burning solid fuel, the temperature in the furnace firebox fluctuates widely, which adversely affects the combustion process, then when burning natural gas, soon after turning on the burner, the temperature in the combustion chamber reaches 650-700 ° C. Then it constantly increases over time and reaches 850-1100 °C at the end of the firebox. The rate of temperature increase in this case is determined by the thermal stress of the combustion space and the furnace firing time (Fig. 25). Gas combustion is relatively easy to maintain at a constant excess air ratio, which is achieved using an air damper. Thanks to this, when burning gas in a furnace, a stationary combustion mode is created, which allows minimizing heat loss with exhaust gases and achieving operation of the furnace with high efficiency, reaching 80-90%. The efficiency of a gas furnace is stable over time and is significantly higher than that of solid fuel furnaces.
The influence of the fuel combustion mode and the size of the heat-receiving surface of the smoke circulation on the efficiency of the furnace. Theoretical calculations show that the thermal efficiency of a heating furnace, i.e. the value of thermal efficiency, depends on the so-called external and internal factors. TO external factors include the area of the heat-releasing outer surface S of the furnace in the area of the firebox and smoke circulation, wall thickness 6, thermal conductivity coefficient K of the furnace wall material and heat capacity C. The larger the values. S, X and less than 6, the better the heat transfer from the furnace walls to the surrounding air, the gases are more completely 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 furnace depending on the tension of the combustion space and the combustion time
Internal factors include, first of all, the efficiency of the firebox, which depends mainly on the completeness of fuel combustion. In periodic heating furnaces there are almost always heat losses due to chemical incomplete combustion and mechanical underburning. These losses depend on the perfection of the organization of the combustion process, determined by the specific thermal voltage of the combustion volume Q/V. The QIV value for a firebox of a given design depends on the consumption of burned fuel.
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 internal walls of the firebox heat up weakly, and the temperatures in the combustion zone are insufficient for efficient combustion of fuel. As Q/V increases, the temperatures in the combustion volume increase, and when a certain Q/V value 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 flues, their combustion process stops and chemical underburning of the fuel appears. In the same way, if fuel consumption is excessive, part of it does not have time to burn and remains on the grate, which leads to mechanical underburning. Thus, in order for a heating stove to have maximum efficiency, it is necessary that its firebox operates with optimal thermal voltage.
Heat loss in 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 thermal voltage of the firebox, the volume of gases passing through the chimneys can change significantly due to changes in the excess air coefficient at, which is the ratio of the actual air flow entering the firebox to the theoretically required amount. For a given value of QIV, the value of am can vary within very wide limits. In conventional periodic heating furnaces, 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 period of fuel preparation and at the stage of post-burning of residues, the am value 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 they remain in the smoke circulation system decreases and, as a result, heat losses with exhaust gases increase.
In Fig. Figure 4 shows graphs of the efficiency of a heating furnace depending on various parameters. In Fig. Figure 4a shows the efficiency values 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. Figure 4, b shows the dependence of the efficiency of a heating furnace on the area of the internal surface of the smoke circulation S, from which it can be seen that as S increases from 1 to 4 m2, the efficiency increases from 65 to 90%.
In addition to the listed factors, the efficiency value depends on the furnace firing time t (Fig. 4, c). As x increases, the inner walls of the furnace are heated to a higher temperature and the gases are correspondingly cooled less. Therefore, as the duration of the fire increases, the efficiency of any heating stove decreases, approaching a certain minimum value characteristic of a stove of a given design.
Rice. 4. Dependence of the efficiency of a gas heating furnace on various parameters a - on the excess air coefficient for the area of the internal surface of the smoke circulation, m2; b - on the area of the internal surface of the smoke circulation at various excess air ratios; c - on the duration of the fire for different areas of the internal surface of the smoke circulation, m2
Heat transfer of heating stoves 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 massiveness of the masonry (its storage capacity) change accordingly. For example, when the thickness of the walls decreases, their thermal resistance decreases, the heat flow increases, and at the same time the dimensions of the furnace decrease. However, reducing the thickness of the walls of periodic furnaces operating on solid fuel is unacceptable for the following reasons: with periodic short-term combustion, the internal 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 above permissible limits; after combustion stops, due to intense heat transfer from the outer walls to the environment, the furnace will quickly cool.
At large values of M, the room temperature will vary over a wide range over time and be outside the permissible limits. On the other hand, if the stove is laid out with too thick walls, then in a short period of combustion 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 internal surface of the chimneys, which receives heat from the gases, and the area of the outer surface of the stove, which transfers heat, increases to the surrounding air, as a result of which the temperature of the outer surface of the stove will be too low for effective heating of the room. Therefore, there is an optimal wall thickness (1/2-1 brick) at which the mass of a periodic furnace accumulates a sufficient amount of heat during combustion 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 fuel in heating stoves, a continuous combustion mode is quite achievable, so with continuous combustion 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 ensuring a certain storage value, but on the basis of considerations of the strength of the masonry and ensuring proper durability.
The effect of converting the furnace from batch to continuous firing is clearly visible from Fig. 5, which shows the change in temperature of the inner surface of the firebox wall in the case of periodic and continuous firing. With periodic firing, after 0.5-1 hour, the inner surface of the firebox wall heats up to 800-900 °C.
Such sudden heating after 1-2 years of operation of the furnace often causes cracking of bricks and their destruction. This mode, however, is forced, since a decrease in the heat load leads to an excessive increase in the duration of the firebox.
With continuous combustion, fuel consumption is sharply reduced and the heating temperature of the firebox walls is reduced. 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 kilns are usually lined with refractory bricks, while the fireboxes of continuous kilns do not need lining, since the temperature on their surface does not reach the fire resistance limit of ordinary red brick (700-750 °C).
Consequently, with continuous combustion, brickwork is used more efficiently, 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.
Draft in furnaces. In order to force flue gases to pass from the firebox through the smoke circulation of the stove to the chimney, overcoming all the local resistances encountered along the way, it is necessary to expend a certain force, which must exceed these resistances, otherwise the stove will smoke. This force is usually called the traction force of the furnace.
The occurrence of traction force is illustrated in the diagram (Fig. 6). Flue gases formed in the firebox, being lighter compared to 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 significantly heavier than the column of gases. If we draw a conventional vertical plane through the combustion door, then right side it will be acted upon (pressed) by a column of hot gases with a height from the middle of the fire 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 the right one, since the density of cold air is greater than hot air, so the left column will displace the flue gases filling the chimney, and gases will move in the system in the direction from higher pressure to lower pressure, i.e. side of the chimney.
Rice. 5. Change in temperature 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 operation of a chimney 1-burner door; 2- firebox; 3 - column of outside air; 4 - chimney
The effect of the draft force, therefore, is that, on the one hand, it forces hot gases to rise upward, and on the other hand, it forces outside air to pass into the firebox for combustion.
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 the fuel occur, including drying and release of volatiles. The resulting flammable gases and coke residue, in the presence of an oxidizer, then burn to form flue gases and a solid non-combustible residue - ash. The longest stage is the combustion of coke - carbon, which is the main combustible component of any solid fuel. Therefore, the combustion mechanism of solid fuel is largely determined by the combustion of carbon.
The combustion process of solid fuel can be divided into the following stages: heating and evaporation of moisture, sublimation of volatiles and formation of coke, combustion of volatiles and coke, 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 preparing the fuel for combustion and the combustion period.
The combustion process of solid fuel can be divided into several stages: heating and evaporation of moisture, sublimation of volatiles and formation of coke, combustion of volatiles, combustion of coke.
The process of burning solid fuel in a flow at elevated pressures leads to a reduction in the dimensions of the combustion chambers and to a significant increase in thermal stress. Furnaces operating at high pressure are not widely used.
The combustion process of solid fuel has not been theoretically studied enough. The first stage of the combustion process, leading to the formation of an intermediate compound, is determined by the dissociation of the oxidizing agent in the adsorbed state. Next comes the formation of a carbon-oxygen complex and the dissociation of molecular oxygen to the 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 oxidizing agent.
The combustion process of solid fuel can be divided into three stages, sequentially superimposed on each other.
The combustion process of solid fuel can be considered as a two-stage process with vaguely defined boundaries between two stages: primary incomplete gasification in a heterogeneous process, the rate of which depends mainly on the speed and conditions of the air supply, and secondary - combustion of the released gas in a homogeneous process, the rate of which depends mainly on the kinetics of chemical reactions. The more volatiles there are in the fuel, the more to a greater extent its combustion rate depends on the rate of chemical reactions occurring.
Intensification of the combustion process of solid fuel and a significant increase in the degree of ash collection are achieved in cyclone furnaces. C, at which the ash melts and liquid slag is removed through tapholes in the lower part of the combustion device.
The basis of the combustion process of solid fuel is the oxidation of carbon, which is the main component of its combustible mass.
For the combustion process of solid fuels, the combustion reactions of carbon monoxide and hydrogen are of obvious interest. For solid fuels rich volatile substances, in a number of processes and technological schemes it is necessary to know the combustion characteristics of hydrocarbon gases. The mechanism and kinetics of homogeneous combustion reactions are discussed in Chap. In addition to the secondary reactions mentioned above, the list 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 methane formation reactions that occur at noticeable rates during gasification under high pressure.
Combustion process solid fuel can be represented as a series of sequential stages. First, the fuel warms up and moisture evaporates. Then, at temperatures above 100 °C, pyrogenic decomposition of complex high-molecular organic compounds and the release of volatile substances begin, while the temperature at which volatiles begin to release depends on the type of fuel and the degree of its carbonization (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 warms up due to 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 burning solid fuel in a pulverized state, both stages (combustion of volatiles and coke) can overlap each other, since heating of the smallest coal particle occurs very quickly. In real conditions, we are dealing with a polydisperse composition of coal dust, so at each moment of time some particles are just beginning to warm up, others are at the stage of becoming volatile, and others are at the stage of burning coke residue.
The combustion process of a coke particle plays a decisive role in assessing both the total fuel combustion time and the total heat release. Even for fuel with a high yield of volatiles (for example, brown coal near Moscow), the coke residue is 55% by weight, and its heat release is 66% of the total. And for fuel with a very low volatile yield (for example, AS), the coke residue can account for more than 96% of the weight of the dry initial particle, and the heat release during its combustion, accordingly, is about 95% of the total.
Studies of combustion of coke residue have revealed the complexity of this process.
When burning carbon, there are two possible primary direct heterogeneous oxidation reactions:
C + O 2 = CO 2 + 34 MJ/kg; (14)
2C + O 2 = 2CO + 10.2 MJ/kg. (15)
As a result of the formation of CO 2 and CO, two processes can occur secondary reactions:
oxidation of carbon monoxide 2CO + O 2 = 2CO 2 + 12.7 MJ/kg; (16)
reduction of carbon dioxide CO 2 + C = 2СО – 7.25 MJ/kg. (17)
In addition, in the presence of water vapor on the hot surface of the particle, i.e. in the high-temperature region, gasification occurs with the release of hydrogen:
C + H 2 O = CO + H 2. (18)
Heterogeneous reactions (14, 15, 17 and 18) indicate direct combustion of carbon, accompanied by a decrease in weight of the carbon particle. The homogeneous reaction (16) occurs 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 consequence of the endothermic reaction (17).
The ratio between CO and CO 2 at the particle surface depends on the temperature of the gases in this area. For example, according to experimental studies, the reaction occurs at a temperature of 1200 °C
4C + 3O 2 = 2CO + 2CO 2 (E = 84 ÷ 125 kJ/g-mol),
and at temperatures above 1500 °C
3C + 2O 2 = 2CO + CO 2 (E = 290 ÷ 375 kJ/g-mol).
It is obvious that in the first case, CO and CO 2 are released in approximately equal quantities, whereas with increasing temperature, the volume of CO released is 2 times greater 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 increases rapidly with increasing temperature;
2) oxidizer supply speed(oxygen) to the combustion zone due to diffusion (molecular or turbulent).
In the initial period of the combustion process, when the temperature is not yet 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 firebox or burner, leading to an intensification of the supply of oxygen to the burning particle, will affect the combustion process, which is inhibited only by the low rate of the chemical reaction, i.e. kinetics. This - kinetic combustion region.
As the combustion process progresses, heat is released, the temperature increases, and, consequently, the rate of the chemical reaction increases, which leads to a rapid increase in oxygen consumption. Its concentration at the surface of the particle is steadily decreasing, and in the future the burning rate will be determined only by the rate of oxygen diffusion into the combustion zone, which is almost independent of temperature. This - diffusion combustion area.
IN transition region of combustion the rates of chemical reaction and diffusion are of the same order of magnitude.
According to the law of molecular diffusion (Fick's law), the rate of diffusion transfer of oxygen from the volume to the surface of a particle
Where 
– coefficient of diffusion mass transfer;
And 
– respectively, partial pressures of oxygen in the volume and at the surface.
Oxygen consumption at the particle surface is determined by the rate of the chemical reaction:
,
(20)
Where k– reaction rate constant.
In the transition zone in a steady state
,
where 
(21)
Substituting (21) into (20), we obtain an expression for the combustion rate in the transition region in terms of oxidizer (oxygen) consumption:
(22)
Where 
is the effective rate constant of the combustion reaction.
In a zone of relatively low temperatures (kinetic region) 
, hence,
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 combustion rate is almost completely determined by the chemical reaction.
With increasing temperature, the rate constant of a chemical reaction increases according to the exponential Arrhenius law (see Fig. 22), while molecular (diffusion) mass transfer weakly depends on temperature, namely
.
At a certain temperature T*, the rate of oxygen consumption begins to exceed the intensity of its supply from the surrounding volume, coefficients α D And k become commensurate values of the same order, the oxygen concentration at the surface begins to noticeably decrease, and the combustion rate curve deviates from the theoretical curve of kinetic combustion (Arrhenius’s law), but still increases noticeably. An inflection appears on the curve - the process moves into the intermediate (transition) combustion region. The relatively intensive supply of oxidizer is explained by the fact that due to a decrease in the oxygen concentration at the surface of the particle, the difference between the partial pressures of oxygen in the volume and at the surface increases.
In the process of combustion intensification, the oxygen concentration at the surface becomes practically equal to zero, the supply of oxygen to the surface weakly depends on temperature and becomes almost constant, i.e. α D << k, and, accordingly, the process goes into the diffusion region
.
In the diffusion region, an increase in the combustion rate is achieved by intensifying the process of mixing fuel with air (improving burner devices) or increasing the speed of blowing the particle with an air flow (improving the aerodynamics of the firebox), as a result of which the thickness of the boundary layer at the surface decreases and the supply of oxygen to the particle is intensified.
As already noted, solid fuel is burned either in the form of large (without special preparation) pieces (layer combustion), or in the form of crushed particles (fluidized bed and low-temperature vortex), or in the form of fine dust (flare method).
Obviously the greatest relative speed blowing of fuel particles will occur during layer combustion. With vortex and flare combustion methods, fuel particles are in the flue gas flow, and the relative speed of their blowing is significantly lower than under stationary bed conditions. Based on this, it would seem that the transition from the kinetic region to the diffusion region should occur first for small particles, i.e. for dust. In addition, a number of studies have shown that a coal dust particle suspended in a flow of gas-air mixture 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 during the torch method was presumably explained solely by a 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 flowing around a laminar flow have 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 coefficient of diffusion mass transfer α D is inversely proportional to the radius of the particle. Consequently, with a decrease in the size of fuel particles, the process of oxygen diffusion to the particle surface intensifies. Thus, during the combustion of coal dust, the transition to diffusion combustion shifts towards higher temperatures (despite the previously noted decrease in the rate of particle blowing).
According to numerous experimental studies conducted by Soviet scientists in the mid-twentieth century. (G.F. Knorre, L.N. Khitrin, A.S. Predvoditelev, V.V. Pomerantsev, etc.), in the zone of normal combustion temperatures (about 1500÷1600 °C) the combustion of a coke particle shifts from the intermediate zone to diffusion, where intensification of the oxygen supply is of great importance. In this case, with an increase in the diffusion of oxygen to the surface, the inhibition of the combustion 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– initial particle size; ρ
h– density of the carbon particle; D o , P o , T o– respectively, the initial values of the diffusion coefficient, pressure and temperature; 
– initial oxygen concentration in the combustion 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– respectively, the temperature of the particle surface and the surrounding flue gases.
Combustible gases and tar vapors (the so-called volatiles), released during the thermal decomposition of natural solid fuel during its heating, mix with the oxidizer (air), and at high temperatures burn quite intensively, like ordinary gaseous fuel. Therefore, burning fuels with a high yield of volatiles (firewood, peat, shale) does not cause difficulties, unless, of course, the ballast content in them (humidity plus ash content) is not so high as to become an obstacle to obtaining the temperature required for combustion.
The combustion time of fuels with medium (brown and hard coals) and low (lean coals and anthracites) volatile yield is practically determined by the reaction rate on the surface of the coke residue formed after the release of volatiles. The combustion of this residue also releases the main amount of heat.
Reaction occurring at the interface between two phases(in this case on the surface of a coke piece) called heterogeneous. It consists of at least two sequential processes: the diffusion of oxygen to the surface and its chemical reaction with the fuel (almost pure carbon remaining after the release of volatiles) at the surface. Increasing according to Arrhenius's law, the rate of a chemical reaction at high temperature becomes so great that all the oxygen supplied to the surface immediately reacts. As a result, the burning rate turns out to depend only on the intensity of oxygen delivery to the surface of the burning particle through mass transfer and diffusion. It is practically no longer affected by both the process temperature and the reaction properties of the coke residue. This mode of heterogeneous reaction is called diffusion. Combustion in this mode can be intensified only by intensifying the supply of the reagent to the surface of the fuel particle. This is achieved using different methods in different fireboxes.
Layer fireboxes. Solid fuel, loaded with a layer of a certain thickness onto a distribution grid, is ignited and blown (most often from bottom to top) with air (Fig. 28, a). Filtering between pieces of fuel, it loses oxygen and is enriched with oxides (CO 2, CO) of carbon due to the combustion of coal, the reduction of water vapor and carbon dioxide by coal.
Rice. 28. Schemes for organizing combustion processes:
A- in a dense layer; b - in a dusty state; _V - in a cyclone furnace;
G - in a fluidized bed; IN- air; T, V - fuel, air; ZhSh - liquid slag
The zone within which oxygen almost completely disappears is called oxygen zone; its height is two to three times the diameter of the fuel pieces. The gases leaving it contain not only CO 2, H 2 O and N 2, but also flammable gases CO and H 2, formed both due to the reduction of CO 2 and H 2 O by coal, and from volatiles released from coal. If the height of the layer is greater than the oxygen zone, then the oxygen zone is followed by a reduction zone, in which only the reactions CO 2 + C = 2CO and H 2 O + C = CO + H 2 occur. As a result, the concentration of flammable gases escaping from the layer increases as its height increases.
In layered fireboxes, they try to keep the layer height equal to or greater than the height of the oxygen zone. To burn the products of incomplete combustion (H 2 , CO) leaving the layer, as well as to burn the dust carried out from it, additional air is supplied to the combustion volume above the layer.
The amount of burned fuel is proportional to the amount of air supplied, however, an increase in air speed above a certain limit violates the stability of the dense layer, since air breaking through the layer in certain places forms craters. Since polydisperse fuel is always loaded into the layer, the removal of fines increases. The larger the particles, the faster air can be blown through the layer without compromising its stability. If we take for rough estimates the heat of “combustion” of 1 m 3 of air under normal conditions at α in = 1 equal to 3.8 MJ and understand it as w n air flow per unit grate area (m/s) reduced to normal conditions, then the thermal voltage of the combustion mirror (MW/m 2) will be
q R = 3.8W n / α in(105)
Combustion devices for layer combustion are classified depending on the method of supplying, moving and screwing the fuel layer on the grate. In non-mechanized furnaces, in which all three operations are carried out manually, no more than 300 - 400 kg/h of coal can be burned. The most widely used in industry are fully mechanized layer furnaces with pneumomechanical throwers and a chain return grid (Fig. 29). Their feature is fuel combustion on a grate continuously moving at a speed of 1-15 m/h, designed in the form of a conveyor belt web driven by an electric motor. The grate canvas consists of individual grate elements mounted on endless hinge chains driven by “stars”. The air required for combustion is supplied under the grate through the gaps between the grate elements.
Rice. 29. Scheme of a firebox with a pneumomechanical thrower and a chain return grid:
1 - grate cloth; 2 - drive sprockets; 3 - layer of fuel and slag; 4 – 5 - caster rotor; 6 - belt feeder; 7 - fuel bunker; 8 - combustion volume; 9 - screen pipes; 10 - 11 - furnace lining; 12 - back seal; 13 - windows for air supply under the layer
Flare furnaces. In the last century, only coal that did not contain fines (usually a fraction of 6 - 25 mm) was used for combustion in layered furnaces (and there were no others then). The fraction smaller than 6 mm - staub (from the German staub - dust) was waste. At the beginning of this century, a pulverized method was developed for its combustion, in which coals were crushed to 0.1 mm, and difficult-to-burn anthracites were crushed even finer. Such dust particles are carried away by the gas flow, the relative speed between them is very small. But their combustion time is extremely short - seconds and fractions of seconds. Therefore, with a vertical gas velocity of less than 10 m/s and a sufficient height of the furnace (tens of meters in modern boilers), the dust has time to completely burn on the fly as it moves along with the gas from the burner to the outlet of the furnace.
This principle forms the basis of torch (chamber) fireboxes, into which finely ground combustible dust is blown through burners along with the air necessary for combustion (see Fig. 28, b ) similar to how gaseous or liquid fuels are burned. Thus, chamber fireboxes are suitable for burning any fuel, which is their big advantage over layer fireboxes. The second advantage is the ability to create a firebox for almost any arbitrary power. Therefore, chamber furnaces now occupy a dominant position in the energy sector. At the same time, dust cannot be burned stably in small furnaces, especially under variable operating conditions, so pulverized coal furnaces with a thermal power of less than 20 MW are not made.
The fuel is crushed in milling devices and blown into the combustion chamber through pulverized coal burners. The transport air blown in along with the dust is called primary air.
During chamber combustion of solid fuels in the form of dust, volatile substances, released during the heating process, burn in a torch as gaseous fuel, which helps to heat the solid particles to the ignition temperature and facilitates stabilization of the torch. The amount of primary air must be sufficient to burn volatiles. It ranges from 15 - 25% of the total amount of air for coals with a low yield of volatiles (for example, anthracite) to 20 - 55% for fuels with a high yield (brown coals). The rest of the air necessary for combustion (it is called secondary) is supplied to the firebox separately and mixed with dust during the combustion process.
In order for dust to ignite, it must first be heated to a high enough temperature. Along with it, naturally, it is necessary to heat the air transporting it (i.e., the primary) air. This can be done only by mixing hot combustion products into the flow of dust suspension.
Good organization of combustion of solid fuels (especially difficult-to-burn, with low volatile yield) is ensured by the use of so-called snail burners (Fig. 30).
Rice. 30. Direct-flow volute burner for solid pulverized fuel: IN- air; T, V - fuel, air
Coal dust with primary air is fed into them through a central pipe and, thanks to the presence of a divider, exits into the furnace in the form of a thin annular jet. Secondary air is supplied through the “snail”, strongly swirls in it and, exiting into the firebox, creates a powerful turbulent swirling torch, which ensures the suction of large quantities of hot gases from the core of the torch to the mouth of the burner. This accelerates the heating of the fuel mixture with primary air and its ignition, i.e., it creates good flame stabilization. The secondary air mixes well with the already ignited dust due to its strong turbulization. The largest dust particles burn out during their flight in the gas flow within the combustion volume.
When flaring coal dust, at any given time there is an insignificant supply of fuel in the furnace - no more than a few tens of kilograms. This makes the flare process very sensitive to changes in fuel and air consumption and allows, if necessary, to change the furnace productivity almost instantly, as when burning fuel oil or gas. At the same time, this increases the requirements for the reliability of supplying the furnace with dust, since the slightest (a few seconds!) interruption will lead to the extinguishing of the torch, which is associated with the danger of an explosion when the supply of dust is resumed. Therefore, as a rule, several burners are installed in pulverized coal furnaces.
During pulverized combustion of fuels, high temperatures (up to 1400-1500 °C) develop in the torch core, located near the burner mouth, at which the ash becomes liquid or doughy. The adhesion of this ash to the walls of the furnace can lead to their overgrowing with slag. Therefore, the combustion of pulverized fuel is most often used in boilers where the furnace walls are closed with water-cooled pipes (screens), near which the gas is cooled and the ash particles suspended in it have time to harden before contacting the wall. Powdered combustion can also be used in furnaces with liquid slag removal, in which the walls are covered with a thin film of liquid slag and molten ash particles flow down in this film.
The thermal volumetric voltage in pulverized coal furnaces is usually 150-175 kW/m 3 , increasing in small furnaces to 250 kW/m 3 . With good mixing of air and fuel, it is accepted α in=1.2÷1.25; q fur= 0.5÷6% (large numbers - when burning anthracite in small fireboxes); q chemical= 0 ÷1%.
In chamber furnaces, after additional grinding, it is possible to burn coal waste generated during their enrichment at coke plants (industrial product), coke screenings and even finer coke sludge.
Cyclonic fireboxes. A specific combustion method is carried out in cyclone furnaces. They use fairly small particles of coal (usually finer than 5 mm), and the air necessary for combustion is supplied at enormous speeds (up to 100 m/s) tangentially to the cyclone generatrix. A powerful vortex is created in the furnace, drawing particles into a circulation movement in which they are intensively blown by a flow. As a result of intense combustion in the furnace, temperatures close to adiabatic (up to 2000 °C) develop. Coal ash melts, liquid slag flows down the walls. For a number of reasons, the use of such furnaces in the energy sector was abandoned, and now they are used as technological ones - for burning sulfur to produce SO 2 in the production of H 2 SO 4, roasting ores, etc. Sometimes fire neutralization of wastewater is carried out in cyclone furnaces , i.e., burning out the harmful substances contained in them by supplying additional (usually gaseous or liquid) fuel.
Fluidized bed furnaces. Stable combustion of a pulverized coal torch is possible only at a high temperature in its core - not lower than 1300-1500 °C. At these temperatures, air nitrogen begins to noticeably oxidize according to the reaction N 2 + O 2 = 2NO. A certain amount of NO is also formed from the nitrogen contained in the fuel. Nitrogen oxide, released along with flue gases into the atmosphere, is further oxidized in it to highly toxic dioxide NO 2. In the USSR, the maximum permissible concentration of NO 2 (MPC), safe for human health, in the air of populated areas is 0.085 mg/m 3 . To ensure this, large thermal power plants have to build tall chimneys that disperse flue gases over the largest possible area. However, when a large number of stations are concentrated close to each other, this does not help.
In a number of countries, it is not the MPC that is regulated, but the amount of harmful emissions per unit of heat released during fuel combustion. For example, in the USA, large enterprises are allowed to emit 28 mg of nitrogen oxides per 1 MJ of combustion heat. In the USSR, emission standards for different fuels range from 125 to 480 mg/m3.
When burning fuels containing sulfur, toxic SO 2 is formed, the effect of which on humans is also cumulative with the effect of NO 2.
These emissions cause the formation of photochemical smog and acid rain, which have a harmful effect not only on people and animals, but also on vegetation. In Western Europe, for example, such rains kill a significant part of coniferous forests.
If there is not enough calcium and magnesium oxide in the fuel ash to bind all SO 2 (usually a two- or three-fold excess is needed compared to the stoichiometry of the reaction), limestone CaCO 3 is mixed into the fuel. Limestone at temperatures of 850-950 °C intensively decomposes into CaO and CO 2, but gypsum CaSO 4 does not decompose, i.e. the reaction does not occur from right to left. Thus, toxic SO 2 is bound to harmless, practically insoluble gypsum in water, which is removed along with the ash.
On the other hand, in the process of human activity, a large amount of combustible waste is generated, which is not considered fuel in the generally accepted sense: “tailings” of coal processing, dumps from coal mining, numerous wastes from the pulp and paper industry and other sectors of the national economy. It is paradoxical, for example, that the “rock” that is piled up in huge waste heaps near coal mines often spontaneously ignites and pollutes the surrounding space with smoke and dust for a long time, but it cannot be burned either in layer or chamber furnaces due to the high ash content. In layered fireboxes, ash, sintered during combustion, prevents the penetration of oxygen to the fuel particles; in chamber fireboxes, it is not possible to obtain the high temperature required for stable combustion.
The urgent need for humanity to develop waste-free technologies has raised the question of creating combustion devices for burning such materials. They became fireboxes with a fluidized bed.
Fluidized (or boiling) is called a layer of fine-grained material blown from bottom to top with gas at a speed exceeding the stability limit of the dense layer, but not sufficient to remove particles from the layer. The intense circulation of particles in the limited volume of the chamber creates the impression of a rapidly boiling liquid, which explains the origin of the name.
A dense layer of particles physically blown from below loses stability because the resistance to the gas filtering through it becomes equal to the weight of the column of material per unit area of the supporting grid. Since aerodynamic drag is the force with which gas acts on particles (and, accordingly, according to Newton’s third law, particles act on gas), then if the resistance and weight of the layer are equal, the particles (if we consider the ideal case) rest not on the lattice, but on the gas.
The average particle size in fluidized bed furnaces is usually 2-3 mm. They correspond to the operating speed of fluidization (it is taken 2-3 times higher than w to) 1.5 ÷ 4 m/s. This determines according to the area of the gas distribution grid for a given thermal power of the firebox. Volume thermal stress qv taken approximately the same as for layer fireboxes.
The simplest firebox with a fluidized bed (Fig. 31) is in many ways reminiscent of a layer firebox and has many common structural elements with it. The fundamental difference between them is that intensive mixing of particles ensures constant temperature throughout the entire volume of the fluidized bed.
Rice. 31. Diagram of a fluidized bed furnace: 1 - unloading of ash; 2 - air supply under the layer; 3 - fluidized bed of ash and fuel; 4 - air supply to the caster; 5 - caster rotor; 6 - belt feeder; 7 - fuel bunker; 8 - combustion volume; 9 - screen pipes; 10 - sharp blast and entrainment return; 11- furnace lining; 12 - heat-receiving tubes in a fluidized bed; IN - water; P- steam.
Maintaining the temperature of the fluidized bed within the required limits (850 - 950 °C) is ensured in two different ways. In small industrial furnaces that burn waste or cheap fuel, significantly more air is supplied to the bed than is necessary for complete combustion, setting α in ≥ 2.
With the same amount of heat released, the temperature of the gases decreases as the α in, for the same heat is spent on heating a large number of gases.
In large power generating units, this method of reducing the combustion temperature is uneconomical, because the “excess” air leaving the unit also carries away the heat spent on heating it (losses with exhaust gases increase - see below). Therefore, in the fluidized bed furnaces of large boiler units, pipes are placed 9 and 12 s a working fluid (water or steam) circulating in them, which receives the required amount of heat. Intensive “washing” of these pipes with particles ensures a high heat transfer coefficient from the layer to the pipes, which in some cases makes it possible to reduce the metal consumption of the boiler compared to a traditional one. Fuel burns stably when its content in the fluidized bed is 1% or less; the remaining 99% With unnecessary - ash. Even under such unfavorable conditions, intensive mixing does not allow ash particles to block combustibles from oxygen access (unlike a dense layer). In this case, the concentration of combustibles turns out to be the same throughout the entire volume of the fluidized bed. To remove ash introduced with fuel, part of the bed material is continuously removed from it in the form of fine-grained slag - most often it is simply “drained” through holes in the hearth, since the fluidized bed is capable of flowing as a liquid.
Fireboxes with circulating fluidized bed. Recently, second-generation furnaces with the so-called circulating fluidized bed have appeared. A cyclone is installed behind these fireboxes, in which all unburnt particles are captured and returned back to the firebox. Thus, the particles are “locked” in the furnace-cyclone-furnace system until they burn completely. These fireboxes are highly economical, not inferior to the chamber combustion method, while maintaining all the environmental benefits.
Fluidized bed furnaces are widely used not only in the energy sector, but also in other industries, for example, for burning pyrites to produce SO 2, roasting of various ores and their concentrates (zinc, copper, nickel, gold), etc. (From the point of view of combustion theory, roasting, for example, zinc ore according to the reaction 2ZnS + 3O 2 = 2ZnO + 2SO 2 is the combustion of this specific “fuel” , which proceeds, like all combustion reactions, with the release of large amounts of heat.) Fluidized bed furnaces are widely used, especially abroad, for the fire neutralization (i.e., combustion) of various hazardous industrial wastes (solid, liquid and gaseous) - wastewater clarification sludge, garbage, etc.
Topic 12. Furnaces of the chemical industry. Schematic diagram of a fuel furnace. Classification of furnaces in the chemical industry. Main types of furnaces, features of their design. Heat balance of furnaces
Chemical industry furnaces. Schematic diagram of a fuel furnace
An industrial furnace is an energy-technological unit designed for heat treatment of materials in order to give them the necessary properties. The source of heat in fuel (flame) furnaces are various types of carbon fuel (gas, fuel oil, etc.). Modern furnace installations are often large mechanized and automated units with high productivity.
The optimal temperature of the process, which is determined by thermodynamic and kinetic calculations of the processes, is of greatest importance for choosing the technological mode of the process. The optimal temperature regime of the process is the temperature conditions under which maximum productivity for the target product in a given furnace is ensured.
Typically, the operating temperature in the furnace is slightly lower than optimal; it depends on the fuel combustion conditions, heat exchange conditions, insulating properties and durability of the furnace lining, thermophysical properties of the processed material, and other factors. For example, for firing kilns the operating temperature is in the range between the temperature of active oxidation processes and the sintering temperature of the firing products. The thermal regime of a furnace is understood as a set of processes of heat inertia, heat of mass transfer and mechanics of media that ensure heat distribution in the technological process zone. The thermal regime of the technological process zone determines the thermal regime of the entire furnace.
The operating mode of furnaces is greatly influenced by the composition of the gas atmosphere in the furnace, which is necessary for the correct flow of the technological process. For oxidative processes, the gas environment in the furnace must contain oxygen, the amount of which ranges from 3 to 15% or more. A reducing environment is characterized by a low oxygen content (up to 1-2%) and the presence of reducing gases (CO, H 2, etc.) 10-20% or more. The composition of the gas phase determines the conditions for fuel combustion in the furnace and depends on the amount of air supplied to combustion.
The movement of gases in a furnace has a significant impact on the technological process, combustion and heat transfer, and in furnaces, “fluidized bed” or vortex furnaces, the movement of gases is the main factor in stable operation. The forced movement of gases is carried out by smoke exhausters and fans.
The speed of the technological process is affected by the movement of the material undergoing heat treatment.
The furnace installation diagram includes the following elements: a combustion device for burning fuel and organizing heat exchange; furnace working space to perform the target technological regime; heat exchange devices for regenerating heat from flue gases (heating gas, air); recovery plants (recovery waste boilers) for using the heat of flue gases; traction and blowing devices (smoke exhausters, fans) to remove combustion of fuel and gaseous products of heat treatment of materials and supply air to burners, nozzles under the grate; cleaning devices (filters, etc.).
Over the centuries, humanity has improved the design of heating furnaces, which were originally intended to burn solid fuels that were available everywhere. In this regard, little has changed, and today in the 21st century, with the availability of gas and liquid fuel, we often turn to traditional heating technologies. Somehow it becomes easier 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, the stoker needs to know the intricacies of burning solid fuels. Many people today no longer remember how to properly light a stove, but experiments in this matter are extremely undesirable. In 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 furnaces, all this is burned in a layered manner, with or without grates. Fuel is periodically loaded into the furnace, and the resulting slag is removed. The layer combustion method is cyclic in nature. The closed cycle has several stages:
Each of these stages has its own thermal regime, while the indicators during fuel combustion are constantly changing. To ensure optimal thermal conditions of the furnace, it is necessary to periodically add a new portion of fuel (layer). The moment of loading a new layer is determined individually 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 layer is accompanied by heat absorption, i.e. is endothermic in nature. The heat supplier is the starting flame from thin dry wood or already ignited fuel, as well as the hot walls of the firebox.
Ignition and smoldering stage occurs with increasing heat release. Excessive air intake into the firebox during this period is undesirable, since it will cool the flue gases, and, therefore, the chimney will heat up longer. The air dampers at the stage of ignition and smoldering should only be slightly open, and it is desirable that cold air be supplied only to the ignition zone.
Combustion stage needs large volumes of air oxygen, 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 incoming oxygen. If the chimney cross-section is insufficient, the flame may escape from the air supply openings. In such a situation, there is only one way out - immediately open the chimney valve completely and close the air supply. When the air supply decreases, the flames become longer and can even penetrate the chimney, which will be a sign of underburning. It is obvious that the supplied air in the flame combustion mode must be divided into two controlled flows. The primary stream will be fed directly into the firewood, depending on the volume, increasing or decreasing the rate of release of volatile substances; and the secondary one - to the flame torch, to regulate 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 burning rate of firewood 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 furnace no higher than 300-400°C.
Burning coals ensures that the metal firebox is heated red-hot - this is the most exothermic stage. The heat release effect increases with increasing supply of primary air (passing through the layer). Secondary air is not needed at this stage. Coals will burn out faster if you feed raw lumps of wood into the firebox: a gasification reaction of the coal with water vapor will occur. If the firewood is damp, then the combustion and smoldering stages occur almost simultaneously.
Types of fuel chambers and the process of burning wood
In the simplest fireplace-type stove with a solid hearth, the combustion process takes place with excess air, since the area of the open portal is usually 8-15 times larger than the cross-sectional area of the chimney. Due to the fact that large volumes of sucked air do not allow the fireplace pipe to heat above 60-80°C, the draft in them is significantly less than in stoves with a door (250-400°C).
If the fireplace insert is equipped with a door and a vent with a damper, then its efficiency will significantly increase. However, this design has a serious drawback - excessive smoke in the chamber, when opened, smoke rushes out. You can reduce smoke by moving the pipe as far forward as possible, but then it will block the top of the stove, which is used to heat water or stones. A compromise solution in this case could be an inclined shelf with the pipe located at the rear. The shelf will create maximum draft right next to the door, when opened, the upward flow will suck in the smoke, preventing it from escaping. This design is good for long-term burning, because... the air flows along the floor, falling under the firewood, and in the smoke circulation area it mixes well with volatile substances, ensuring the completeness of their combustion. 
To emphasize flaming combustion, secondary air is introduced into the flow of volatile substances. The implementation of this firewood burning mode is also helped by designs with a grate. They are good, first of all, because 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 smoke channel, and, consequently, draft and convective heat transfer. This phenomenon can be minimized by covering the periphery of the grate with a bottom, leaving the purge area only in the center.
Any grate is suitable for burning wood. If necessary, you can make them yourself from reinforcement or rod. But for burning coal you will need cast iron grates, the cross-sectional shape of which is close to triangular. This shape prevents slag from clogging the cracks between the grates. The grate bars should be placed along the firebox so that you can poke the coal with a poker. Cast iron grates are available for both coal and firewood. The latter have thinner grates and narrower gaps between them.
Grate furnaces are capable of developing greater power, but keep them from acceleration not easy. When the air supply coefficient is equal to one, the walls of the furnace are heated to red-hot temperatures, and the firewood begins to become increasingly gasified. The flame becomes so large that it enters the pipe and in this case it is necessary to increase the air supply, which in turn causes even greater gasification and heating. The stove will calm down on its own only after volatile substances have left the wood stack. The combustion of coals can then be easily adjusted. 
It is important to understand that the main reason furnace acceleration acceleration is achieved by metal walls heated to a high temperature, which no longer take away the heat from the firewood, while the latter begin to heat themselves. You can prevent the furnace from accelerating if you keep the pipe damper only half open when firing, and when characteristic gas pops begin to be heard from the firebox, open the firebox door slightly and at the same time open the pipe completely. Due to the sudden appearance of excess air, the walls of the stove will begin to cool, and when they stop glowing, it will be possible to close the firebox door and the air intake. The chimney is closed halfway again. This will cause the oven to smoothly switch to smoldering mode.
An important point that influences the acceleration of the stove is the portion of the installed firewood. To reduce the likelihood of runaway conditions, firewood should be added in small portions of 1 to 3 kg at a time. Moreover, the larger the diameter of the log, the greater the mass of the bookmark can be. By adjusting the air supply, you should try to prevent the walls from overheating. Overclocking the furnace is dangerous, first of all, because it can lead to warping or burning of the metal parts of the furnace.
First of all, the lower part of the walls of the firebox suffers from acceleration. If a metal stove accelerates over and over again, then the walls can be protected from the inside with refractory bricks to a height of 20-30 cm. It would be a mistake to line the walls from the outside, because... this will lead to even stronger heating of the metal. The problem of acceleration is completely eliminated by the water jacket - the boiler. However, if we talk about sauna stoves, then this solution is suitable not for saunas, but for hammam.
Burnouts through the firebox or hidden cracks are really dangerous during spontaneous acceleration of a metal stove. If during normal combustion mode they work as air intake holes, then in acceleration mode they will become “nozzles” through which burning volatile substances will burst out.
