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Combustion process 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 solid non-flammable 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. Fireboxes operating at high blood 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 a fuel, the more 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 in volatile substances in a number of processes and technological schemes it is necessary to know the combustion characteristics of hydrocarbon gases. The mechanism and kinetics of homogeneous combustion reactions are discussed in Chap. In addition to the secondary reactions mentioned above, the list 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.  

The combustion process of 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 volatile release, and others are at the stage of combustion of 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 the 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 furnace), 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 n And T G– respectively, the temperature of the particle surface and the surrounding flue gases.

The combustion of solid fuel lying motionless on the grate with top loading of fuel is shown in Fig. 6.2.

At the top of the layer after loading there is fresh fuel. Below it is burning coke, and directly above the grate is slag. These layer zones partially overlap each other. As the fuel burns out, it gradually passes through all zones. In the first period after fresh fuel enters the burning coke, its thermal preparation occurs (warming up, evaporation of moisture, release of volatiles), which consumes part of the heat released in the bed. In Fig. Figure 6.2 shows the approximate combustion of solid fuel and the temperature distribution along the height of the fuel layer. The region of the highest temperature is located in the coke combustion zone, where the main amount of heat is released.

The slag formed during fuel combustion flows droplets from the hot pieces of coke towards the air. Gradually the slag cools and, already in a solid state, reaches the grate, from where it is removed. The slag lying on the grate protects it from overheating, heats it up and evenly distributes the air over the layer. The air passing through the grate and entering the fuel layer is called primary. If there is not enough primary air for complete combustion of the fuel and there are products of incomplete combustion above the layer, then additional air is supplied to the space above the layer. This air is called secondary.

With the top supply of fuel to the grate, bottom ignition of the fuel and counter movement of gas-air and fuel flows are carried out. This ensures efficient ignition of the fuel and favorable hydrodynamic conditions for its combustion. Primary chemical reactions between fuel and oxidizer occur in the hot coke zone. The nature of gas formation in the burning fuel layer is shown in Fig. 6.3.

At the beginning of the layer, in the oxygen zone (K), in which intensive oxygen consumption occurs, carbon oxide and carbon dioxide CO 2 and CO are simultaneously formed. Towards the end of the oxygen zone, the O 2 concentration decreases to 1-2%, and the CO 2 concentration reaches its maximum. The temperature of the layer in the oxygen zone increases sharply, having a maximum where the highest concentration of CO 2 is established.

In the reduction zone (B) there is practically no oxygen. Carbon dioxide reacts with hot carbon to form carbon monoxide:

Along the height of the reduction zone, the CO 2 content in the gas decreases, and CO increases accordingly. The reaction between carbon dioxide and carbon is endothermic, so the temperature drops along the height of the reduction zone. If there is water vapor in the gases in the reduction zone, an endothermic decomposition reaction of H2O is also possible.

The ratio of the amounts of CO and CO 2 obtained in the initial section of the oxygen zone depends on the temperature and changes according to the expression

where E co and E CO2 are the activation energies of formation of CO and CO 2, respectively; A - numerical coefficient; R - universal gas constant; T - absolute temperature.
The temperature of the layer, in turn, depends on the concentration of the oxidizer, as well as on the degree of air heating. In the reduction zone, the combustion of solid fuel and the temperature factor also have a decisive influence on the ratio between CO and CO 2. With increasing temperature of the reaction CO 2 +C=P 2 CO shifts to the right and the content of carbon monoxide in the gases increases.
The thicknesses of the oxygen and reduction zones depend mainly on the type and size of pieces of burning fuel and temperature conditions. As the fuel size increases, the thickness of the zones increases. It has been established that the thickness of the oxygen zone is approximately three to four times the diameter of the burning particles. The reduction zone is 4-6 times thicker than the oxygen zone.

Increasing the blast intensity has virtually no effect on the thickness of the zones. This is explained by the fact that the rate of the chemical reaction in the layer is much higher than the rate of mixture formation and all incoming oxygen instantly reacts with the very first rows of hot fuel particles. The presence of oxygen and reduction zones in the layer is characteristic of the combustion of both carbon and natural fuels (Fig. 6.3). With an increase in the reactivity of the fuel, as well as a decrease in its ash content, the thickness of the zones decreases.

The nature of gas formation in the fuel layer shows that, depending on the organization of combustion, either practically inert or combustible and inert gases can be obtained at the exit from the layer. If the goal is to maximize the conversion of fuel heat into physical heat of gases, then the process should be carried out in a thin layer of fuel with an excess of oxidizer. If the task is to obtain flammable gases (gasification), then the process is carried out with a layer developed in height and with a lack of oxidizer.

Combustion of fuel in the boiler furnace corresponds to the first case. And the combustion of solid fuel is organized in a thin layer, ensuring maximum oxidation reactions. Since the thickness of the oxygen zone depends on the size of the fuel, the larger the size of the pieces, the thicker the layer should be. Thus, when burning fines of brown and hard coals(particle size up to 20 mm) the layer thickness is maintained at about 50 mm. With the same coals, but in pieces larger than 30 mm, the layer thickness is increased to 200 mm. The required thickness of the fuel layer also depends on its humidity. The higher the moisture content of the fuel, the greater the reserve of burning mass in the layer must be in order to ensure stable ignition and combustion of a fresh portion of fuel.

Combustion of solid fuel takes place in two stages: thermal preparation; combustion itself.

At the first stage, the fuel is heated and dried. At 100 C, pyrogenetic decomposition of fuel components begins with the release of gaseous volatile substances. (Zone I). The duration of this process depends on the fuel moisture content, particle size, and heat exchange conditions between fuel particles and the combustion environment.

Fuel combustion begins with the ignition of volatiles (zone II). t in this zone is 400-600 C. During combustion, heat is released, which ensures accelerated heating and ignition of the coke residue. (Two necessary conditions for fuel to burn: temperature and a sufficient amount of oxidizer. In any firebox there are 2 inputs: one for fuel, and the other for oxidizer)

This process occurs in tenths of seconds. Volatiles burn from 0.2 to 0.5 seconds. Q is released when t 800-1000 - zone III begins. Coke combustion begins at a temperature of 1000 C and occurs in region III. This process is long. 1 – Tgas environment around the particle. 2 –Tthe particle itself . I– thermal preparation zone,II– combustion zone of volatile substances,III– combustion of coke particles.

III – heterogeneous process. The speed depends on the speed of the oxygen supply. The burning time of a coke particle is from ½ to 2/3 of the total burning time (from 1 to 2.5 s) - depends on the type and size of the fuel. In young fuels, the carbonization process has not been completed, with a large yield of volatiles. Coke residue< ½ начальной массы частицы. Горение идет быстро, возможность недожога низкая. У стар. топ. большой коксовый остаток, ближе к начальн размерам частиц. Время горения 1 мм ~ 1-2,5 с. Кокс остаток С = 60-97% массы топлива органического. 1 – surface of coke particle, 2 – narrow laminar layer with thickness δ, 3 – turbulent flow zone.

Oxygen is supplied from the environment to the carbon particle due to turbulent diffusion, which has high intensity, but near the surface of the particle there is a thin gas layer (2), where the supply of the oxidizer is subject to the laws of molecular diffusion (lam sl) - it inhibits the supply of oxygen to the surface of the particle. In this layer, the combustion of flammable gas components released from the carbon surface during chemical reactions occurs.

The amount of oxygen supplied per unit time to a unit surface of a particle through turbulent diffusion is determined by:

GOK = A(SPOT - SSL) (1) , A – set of turbulent mass transfer. The same amount of oxygen diffuses through the submerged layer due to molecular diffusion:

GOK = Dδ (SSL – SPOV) (2) D – set of dif-and h/w ​​submerged layer δ. SSL = GOK* δ D+ SPOV, GOK = A(SPOT – GOK* δ D– SPOV) , ​​GOK = A*( S POT – SPOV ) 1+ AδD = ( S POT – SPOV ) 1 A + δ D = αD*(SPOT – SPOV), 1 A + δ D= αD – generalized diffusion rate constant.

The number of inputs depends on αD and the difference between the concentrations of the flow and surface. The supply of oxygen to the reacting fuel surface is determined by the diffusion rate and oxygen concentration in the flow and on the reacting surface.

In a steady-state combustion mode, the amount of oxygen supplied to the reaction surface by diffusion is equal to the amount of oxygen that reacted with this surface.

ωР = αД(SPOT – SPOV) . At the same time, the burning rate: ωГ = k*SPOT, if they are equal, then it can determine: ωГ = 1 1 K + 1 α D* WITHSWEAT= kG*SPOT. KG = 1 1 K + 1 α D = K * α D α D + K (*) – reduced combustion constant. 1 k G = 1 K + 1 α D– generalized resistance to the combustion process. 1/k – kinetic resistance, determined by the intensity of chemical combustion; 1/αD – physical (diffusion) resistance – depends on the intensity of the oxidizing agent supply.

Depending on the resistance, the kinetic and diffusion regions of heterogeneous combustion are distinguished.

I – kinetic region (ωG = k*SPOT), II – intermediate region, III – diffusion region (ωG = αD*SPOT)

According to Arrhenius' law, the rate of a chemical reaction depends on temperature. αD (constant differential) responds poorly to temperature. At temperatures less than 800-1000 C, the chemical reaction proceeds slowly, despite the excess O2 near the solid surface. In this case, 1/k is a large value - combustion is inhibited by the kinetics of p-i (t is small) and the region is called Kinetic combustion region. (1/k >> 1/αD) . k<<αД, kГ ~k (*) – Because the flow is sluggish, the oxygen supplied by diffusion is not consumed and its concentration at the reaction surface is approximately equal to the concentration in the flow ωГ = k*SPOT - this is the combustion rate in the kinetic region.

The combustion rate in the kinetic region will not change with an increased supply of oxygen, by improving aerodynamic processes (regionI), but depends on the kinetic factor, namely temperature. Supply of ok-la >> consumption - the concentration remains almost unchanged. As t increases, the reaction rate increases, and the concentration of O2 and C decreases. Further t leads to an increase in the combustion rate and its value is limited by the lack of O2 supply to the surface and insufficient diffusion. Oxygen concentration at the surface →0.

The combustion region in which the rate of the process depends on diffusion factors is called Diffusion areaIII. Here k>>αД ( From * ): kG~αD. The rate of combustion diffusion is limited by the delivery of O2 to the surface and its concentration in the flow.

The diffusion and kinetic regions are separated by intermediate zone II, where the rate of oxygen supply and the rate of chemical reaction are approximately equal to each other. How smaller sizes solid fuel, the greater the heat and mass transfer area.

In areas II and III, combustion can be enhanced by supplying oxygen. At high speeds, the resistance and thickness of the laminar layer increases and the supply of oxygen increases. The higher the speed, the more intensely the fuel mixes with O2 and the higher t the transition from kinetic to industrial occurs, then to the differential region. As the particle size decreases, the area of ​​kinetic combustion increases, since small-sized particles have more developed heat and mass transfer with the environment.

D1>d2>d3 , v1>v2>v3

D – particle size of pulverized fuel, v – speed of mixing fuel with air – speed of fuel supply

Ignition of any fuel begins at relatively low t when the amount of fuel is available (I). Pure differential combustion III is limited by the flame core. An increase in temperature leads to a shift to the region of diffusion combustion. The diffusion combustion zone is located from the core of the torch to the afterburning zone, where the concentration of reactants is low and their interaction is determined by the laws of diffusion.

Thus, if combustion occurs in the diffusion or intermediate region, then with a decrease in the size of particles of pulverized fuel, the process shifts towards kinetic combustion. The region of purely diffusion combustion is limited. This is observed in the core of the flame with the maximum combustion temperature. Outside the core, combustion occurs in the kinetic or intermediate region, which is characterized by a strong dependence of the combustion rate on temperature.

Kinetic and intermediate combustion regions also occur in the ignition zone of the dust-air flow, and the combustion of fuels of all types with preliminary mixture formation occurs in the diffusion or intermediate region.

K category: Furnaces

Main features of fuel combustion processes

IN heating stoves solid, liquid and gaseous fuels can be used. Each of these fuels has its own characteristics, which 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. To make the most effective use of these valuable species in existing furnaces, you need to know how the combustion processes of these fuels differ from the combustion of solid fuels.

All stoves contain solid fuel (wood, various types 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 relatively low temperature ignition, 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 feeding into the furnace a certain amount air for combustion. However, due to the fact that a constant amount of air enters the furnace, 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 furnace 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 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 (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 negative action chemical underburning manifests itself not only in additional heat losses and a decrease in efficiency. Operating experience large quantity 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, when constant flow air, 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 thickness of the layer when burning anthracite and coke is maintained sufficiently large. At the same time work area 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 recovery reactions in the upper part of the layer, leading to chemical underburning of 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 value 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, moisture content is 6-15% for hard coals and 20-45% for brown coals.

Combustion devices household stoves they do not have means of mechanizing the combustion process (regulating the supply of blown air, layer scuffing, 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 utilities them. K. D. Pamfilova determined the annual material damage caused as a result of the supply of coal low quality. 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 advisable to enrich fuel at the places of its production to a conditionable state, since the additional costs of enrichment will amount to approximately half of the specified amount of material damage.

Important qualitative characteristics coal, affecting 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 when 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 furnaces long burning These requirements must be strictly met in full.

In the ovens continuous action, in which liquid or gaseous fuel is burned, the combustion process is not cyclical, but continuous. Fuel flows into the furnace evenly, which ensures a stationary combustion mode. If, when burning solid fuel, the temperature in the firebox of the furnace 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 makes it possible to minimize heat loss with exhaust gases and achieve operation of the furnace with high efficiency, reaching 80-90%. Efficiency gas oven stable over time and significantly higher than solid fuel stoves.

The influence of the fuel combustion mode and the size of the area 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 thermal efficiency, depends on the so-called external and internal factors. External factors include the size 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 greater the value. 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. At further increase fuel consumption, the temperature level continues to rise, but the combustion process does not have time to complete within the firebox. Gaseous combustible components are entrained 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 spend in the smoke circulation system decreases and, as a result, heat losses with flue 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 a heating furnace depending on the values ​​of at, from which it can be seen that with an increase in at from 1.5 to 4.5, the efficiency decreases from 80 to 48%. In Fig. 4, b shows the dependence of the efficiency of the heating furnace on the area inner surface 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, with increasing combustion duration, the efficiency of any heating furnace decreases, approaching a certain minimum value characteristic of a furnace 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 - from the area of ​​the internal surface of the smoke circulation at various coefficients excess air; 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 will leave acceptable standards. 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, as the walls thicken, 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 ambient 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 such optimal thickness walls (1/2 - 1 brick), in which the mass of the intermittent furnace accumulates a sufficient amount of heat during combustion and at the same time sufficient high temperature external surfaces of the stove 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 a heated room is stationary in time. Under these conditions, the wall thickness and massiveness of the furnace can be selected based not on ensuring a certain storage value, but on considerations of the strength of the masonry and ensuring proper durability.

The effect of converting the furnace from batch firing 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 combustion, 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 furnaces are usually lined fire brick, while the fireboxes of continuous furnaces do not need lining, since the temperature on their surface does not reach the fire resistance limit of ordinary red brick (700-750 ° C).

Therefore, with continuous combustion, it is more efficiently used brickwork, 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 furnace to the chimney, overcoming all those encountered on their way local resistance, 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 traction force is, therefore, that, on the one hand, it forces hot gases to rise upward, and on the other hand, it forces outside air 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