Combustion of solid fuel: features and characteristics of the main types. Big encyclopedia of oil and gas

The combustion process of solid fuel can be represented as a series of successive stages. First, the fuel heats up and moisture evaporates. Then, at temperatures above 100 ° C, pyrogenic decomposition of complex high-molecular organic compounds and the release of volatiles begin, while the temperature of the beginning of the release of volatiles depends on the type of fuel and the degree of its coalification (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 the temperature of their ignition, while the heat release increases.

The coke particle heats 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 the heating of the smallest coal particle occurs very quickly. In real conditions, we are dealing with a polydisperse composition of coal dust, therefore, at each moment of time, some particles only begin to warm up, others are at the stage of release of volatiles, and others are at the stage of combustion of the coke residue.

The combustion process of a coke oven particle plays a decisive role in assessing both the total combustion time of the fuel and the total heat release. Even for fuels 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, DS), the coke residue can be more than 96% of the weight of the dry initial particle, and the heat release during its combustion, respectively, is about 95% of the total.

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

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

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

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

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

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

reduction of carbon dioxide CO 2 + C = 2CO - 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. (eighteen)

Heterogeneous reactions (14, 15, 17, and 18) indicate direct combustion of carbon, accompanied by a loss in weight of the carbon particle. A homogeneous reaction (16) proceeds near the surface of a particle due to oxygen diffusing from the surrounding volume and compensates for the decrease in the temperature level of the process arising as a result 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. So, for example, according to experimental studies, at a temperature of 1200 ° C, the reaction proceeds

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).

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

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

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

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

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

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

V 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 the particle

where - coefficient of diffusion mass transfer;

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

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

, (20)

where k Is the reaction rate constant.

In the transition zone in a steady state

,

where
(21)

Substituting (21) into (20), we obtain an expression for the combustion rate in the transition region in terms of the 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 eff = k, and expression (22) takes the form:

,

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

.

At a certain value of the temperature T *, the rate of oxygen consumption begins to exceed the intensity of its supply from the surrounding volume, the coefficients α D and k become comparable quantities of the same order, the oxygen concentration at the surface begins to decrease noticeably, and the burning 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 goes into an intermediate (transitional) combustion region. The relatively intensive supply of the oxidizing agent is explained by the fact that due to a decrease in the oxygen concentration at the surface of the particle, the difference in the partial pressures of oxygen in the bulk and at the surface increases.

In the process of intensification of combustion, the oxygen concentration at the surface practically becomes zero, the oxygen supply to the surface weakly depends on the temperature and becomes practically constant, i.e. α D << k, and, accordingly, the process goes over to the diffusion region

.

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

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

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

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

Nu = 2 + 0.33Re 0.5.

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

.

There is an analogy between the processes of heat and mass transfer, since both are determined by the movement of molecules. Therefore, the laws of heat transfer (Fourier and Newton-Richman 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 coefficient of molecular diffusion (similar to the coefficient of thermal conductivity 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 the fuel particles, the process of oxygen diffusion to the particle surface is intensified. Thus, during the combustion of coal dust, the transition to diffusion combustion shifts towards higher temperatures (despite the previously noted decrease in the blowing speed of particles).

According to numerous experimental studies carried out by Soviet scientists in the middle of the twentieth century. (G.F. Knorre, L.N. Khitrin, A.S. Predvoditelev, V.V. Pomerantsev and others), in the zone of ordinary furnace temperatures (about 1500 ÷ 1600 ° C), diffusion, where intensification of oxygen supply is of great importance. In this case, with an increase in oxygen diffusion to the surface, the deceleration of the combustion rate begins 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- the density of the carbon particle; D o , P o , T o- respectively, the initial values ​​of the diffusion coefficient, pressure and temperature;
- the initial concentration of oxygen in the furnace volume at a considerable distance from the particle; β - stoichiometric coefficient, which establishes the correspondence of the weight oxygen consumption per unit weight of the burned carbon at stoichiometric ratios; T m- logarithmic temperature:

where T NS and T G- respectively, the temperature of the particle surface and the surrounding flue gases.

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

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

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

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

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

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

The combustion process of solid fuel can be conditionally divided into three stages, which are successively superimposed on each other.

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

Intensification of the solid fuel combustion process and a significant increase in the degree of ash collection are achieved in cyclone furnaces. C, in which the ash melts and the liquid slag is removed through the tap holes in the lower part of the combustion device.

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

For the combustion of solid fuel, the reactions of combustion of carbon monoxide and hydrogen are of absolute interest. For solid fuels rich in volatile substances, 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 considered in Ch. In addition to the above secondary reactions, the list of them should be continued with heterogeneous reactions of decomposition of carbon dioxide and water vapor, the reaction of conversion of carbon monoxide with water vapor, and a family of methane formation reactions, which proceed at noticeable rates during gasification under high pressure.

In connection with the increasing popularity of solid fuel boilers, a huge number of potential buyers of this equipment are interested in the question of which type of solid fuel to give preference as the main one, and depending on the decision made to order this or that type of heating equipment.

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

A little chemistry

Solid fuel contains the following substances: carbon, hydrogen, oxygen and mineral compounds. When fuel is burned, carbon and hydrogen combine with atmospheric oxygen (the strongest natural oxidizer) - a combustion reaction takes place with the release of a large amount of thermal energy. Further, gaseous combustion products are removed through the flue system, and solid combustion products (ash and slag) fall out as waste through the grate.

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

Calorific value of the main types of solid fuels

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

In other words, the combustion process of solid fuels of various types is accompanied by different amounts of heat energy released, therefore, the choice of the main type of fuel should be very responsible - be guided in this matter by the information specified in the operational documentation (passport or Operating Instructions) for this or that solid fuel equipment.

Brief description of the main types of solid fuels

Firewood

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

Peat

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

Coal

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

Briquettes and pellets

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

Benefits of briquettes and pellets

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

Combustible gases and vapors of tar (the so-called volatile), emitted during the thermal decomposition of natural solid fuel in the process of heating, mixing with an oxidizer (air), at high temperatures burn quite intensively, like ordinary gaseous fuel. Therefore, the combustion of fuels with a high yield of volatiles (firewood, peat, oil shale) does not cause difficulties, unless, of course, the ballast content in them (moisture 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 bituminous coals) and low (lean coals and anthracite) volatiles is practically determined by the reaction rate on the surface of the coke residue formed after the volatiles are released. The combustion of this residue also provides the release of the main amount of heat.

Reaction at the interface of 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 fuel (almost pure carbon remaining after the release of volatiles) on the surface. Increasing according to Arrhenius's law, the rate of a chemical reaction at high temperatures 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 by means of mass transfer and diffusion. It practically ceases to be influenced by both the process temperature and the reaction properties of the coke residue. This mode of a 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 in different furnaces by different methods.

Layer furnaces. Solid fuel, loaded with a layer of a certain thickness on the distribution grid, is ignited and blown through (most often from the bottom up) with air (Fig. 28, a). Filtered between pieces of fuel, it loses oxygen and is enriched with carbon oxides (CO 2, CO) due to coal combustion, reduction of water vapor and carbon dioxide by coal.

Rice. 28. Diagrams of the organization of combustion processes:

a- in a dense layer; b - in a dusty state; _v - in a cyclone furnace;

G - in a fluidized bed; V- air; T, B - fuel, air; ZhSh - liquid slag

The zone within which oxygen almost completely disappears is called oxygen; its height is two to three diameters of pieces of fuel. The gases leaving it contain not only CO 2, H 2 O and N 2, but also combustible gases CO and H 2, formed both due to the reduction of CO 2 and H 2 O by coal, and from the volatiles emitted from the coal. If the height of the layer is greater than that of 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 take place. As a result, the concentration of combustible gases leaving the layer increases as its height increases.

In layered furnaces, the height of the layer is tried to be kept equal to or greater than the height of the oxygen zone. For afterburning the products of incomplete combustion (H 2, CO) coming out of the layer, as well as for afterburning the dust removed from it, additional air is supplied to the furnace volume above the layer.

The amount of fuel burned is proportional to the amount of air supplied, however, an increase in air velocity beyond a certain limit violates the stability of the dense layer, since air, breaking through the layer in some places, forms craters. Since polydisperse fuel is always loaded into the bed, fines carryover increases. The larger the particles, the faster air can be blown through the layer without disturbing 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 mean w n reduced to normal conditions air consumption per unit area of ​​the grate (m / s), then the heat stress of the combustion mirror (MW / m 2) will be

q R = 3.8W n / α in(105)

Furnace devices for layered combustion are classified depending on the method of supply, movement and skimming of 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 widespread in the industry are fully mechanized layered furnaces with pneumatic-mechanical throwers and a reverse chain grate (Fig. 29). Their feature is the combustion of fuel 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 fabric consists of individual grate elements fixed 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. Diagram of a firebox with a pneumatic-mechanical thrower and a return chain grate:

1 - grate cloth; 2 - drive "stars"; 3 - fuel and slag layer; 4 – 5 - spreader rotor; 6 - belt feeder; 7 - fuel bunker; 8 - furnace volume; 9 - screen pipes; 10 - 11 - furnace lining; 12 - rear 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 at that time). Fraction finer than 6 mm - spear (from the German staub - dust) was a waste. At the beginning of this century, a pulverized method was developed for its combustion, in which the coals were crushed to 0.1 mm, and the hard-to-burn anthracites were even smaller. Such grains of dust are carried away by the gas flow, the relative velocity between them is very low. But the time of their combustion is extremely short - seconds and fractions of seconds. Therefore, with a vertical gas velocity of less than 10 m / s and a sufficient furnace height (tens of meters in modern boilers), the dust has time to completely burn out on the fly while moving along with the gas from the burner to the exit from the furnace.

This principle is the basis of flare (chamber) furnaces, into which finely ground combustible dust is blown through the burners together with the air necessary for combustion (see Fig. 28, b ) similar to how gaseous or liquid fuels are burned. Thus, chamber furnaces are suitable for burning any fuels, which is their great advantage over layered ones. The second advantage is the ability to create a furnace for any practically arbitrarily high power. Therefore, chamber furnaces now occupy a dominant position in the energy sector. At the same time, dust cannot be stably burned in small furnaces, especially with variable operating modes, therefore, pulverized coal furnaces with a thermal power of less than 20 MW are not made.

The fuel is crushed in mill devices and blown into the combustion chamber through pulverized coal burners. The conveying air blown in together with the dust is called the primary air.

In the chamber combustion of solid fuels in the form of dust, volatile substances, released during its heating, are burned in the torch as a gaseous fuel, which contributes to the heating of solid particles to the ignition temperature and facilitates the stabilization of the torch. The amount of primary air must be sufficient to burn volatiles. It constitutes from 15 - 25% of the total amount of air for coals with a low yield of volatiles (for example, anthracites) to 20 - 55% for fuels with a large yield (brown coals). The rest of the air necessary for combustion (it is called secondary) is fed into the furnace separately and mixed with dust during the combustion process.

In order for the dust to ignite, it must first be heated to a sufficiently high temperature. Together with it, naturally, it is necessary to heat and transport it (i.e., primary) air. This manages to be done only by admixing incandescent combustion products to the dust suspension stream.

A good organization of the combustion of solid fuels (especially hard-to-burn, with a low release of volatiles) is ensured by the use of so-called snail burners (Fig. 30).

Rice. 30. Coiled-coil burner for solid pulverized fuel: V- air; T, B - fuel, air

Coal dust with primary air is fed into them through a central pipe and, due to the presence of a divider, enters the furnace in the form of a thin annular jet. Secondary air is supplied through the "snail", swirls strongly in it and, entering the firebox, creates a powerful turbulent swirling torch, which ensures the suction of large amounts of incandescent gases from the torch core to the burner mouth. This accelerates the heating of the mixture of fuel with primary air and its ignition, i.e., it creates a good stabilization of the flame. Secondary air mixes well with already ignited dust due to its strong turbulence. The largest dust grains burn out during their flight in the gas flow within the furnace volume.

When coal dust is flared, at each moment of time, there is an insignificant supply of fuel in the furnace - no more than several tens of kilograms. This makes the flare process very sensitive to changes in fuel and air consumption and allows, if necessary, to almost instantly change the furnace performance, as when burning fuel oil or gas. At the same time, this increases the requirements for the reliability of the supply of dust to the furnace, because the slightest (in a few seconds!) Interruption will lead to the extinguishing of the torch, which is associated with the danger of explosion when dust is supplied again. Therefore, in pulverized coal furnaces, as a rule, several burners are installed.

During the pulverized combustion of fuels in the torch core located near the burner mouth, high temperatures develop (up to 1400-1500 ° C), at which the ash becomes liquid or pasty. The adhesion of this ash to the walls of the furnace can lead to their overgrowth with slag. Therefore, the combustion of pulverized fuel is most often used in boilers where the walls of the furnace are closed with water-cooled pipes (screens), around which the gas is cooled and the ash particles suspended in it have time to solidify before contacting the wall. Pulverized combustion can also be used in liquid ash removal furnaces, in which the walls are covered with a thin film of liquid slag and molten ash particles flow down in this film.

The heat voltage of the volume in pulverized coal furnaces is usually 150-175 kW / m 3, increasing in small furnaces up to 250 kW / m 3. With good mixing of air with fuel, α in= 1.2 ÷ 1.25; q fur= 0.5 ÷ 6% (large numbers - when burning anthracite in small furnaces); q chem= 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 furnaces. A specific combustion method is carried out in cyclone furnaces. They use rather small particles of coal (usually finer than 5 mm), and the air necessary for combustion is supplied at tremendous speeds (up to 100 m / s) tangentially to the cyclone generatrix. A powerful vortex is created in the firebox, which entrains the particles in a circulation movement, in which they are intensively blown by the 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 power industry was abandoned, and now they are used as technological ones - for burning sulfur in order to obtain SO 2 in the production of H 2 SO 4, roasting ores, etc. Sometimes in cyclone furnaces, fire neutralization of wastewater is carried out , i.e., burning out the harmfulness contained in them due to the supply of 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, nitrogen in the air 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. Nitric oxide, emitted together with flue gases into the atmosphere, is further oxidized in it to the highly toxic dioxide NO 2. In the USSR, the maximum permissible concentration of NO 2 (MPC), safe for human health, in the air of settlements is 0.085 mg / m 3. To ensure it, at large thermal power plants, it is necessary to build tall chimneys, scattering 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, for large enterprises, emissions of 28 mg of nitrogen oxides per 1 MJ of calorific value are allowed. In the USSR, emission standards for different fuels are from 125 to 480 mg / m 3.

When fuels containing sulfur are burned, toxic SO 2 is formed, the effect of which on humans is also added to the effect of NO 2.

These emissions are responsible for the formation of photochemical smog and acid rain, which adversely affect not only people and animals, but also vegetation. In Western Europe, for example, a significant part of coniferous forests perishes from such rains.

If in the ash of the fuel calcium and magnesium oxides are not enough to bind all SO 2 (usually a two- or three-fold excess of it is needed in comparison with the stoichiometry of the reaction), CaCO 3 limestone is added to the fuel. Limestone at temperatures of 850-950 ° C intensively decomposes into CaO and CO 2, and gypsum CaSO 4 does not decompose, that is, the reaction does not go from right to left. Thus, toxic SO 2 is bound to harmless, practically water-insoluble gypsum, 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 preparation, dumps during 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 into huge waste heaps near coal mines often ignites spontaneously 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 furnaces, ash, sintering during combustion, prevents oxygen from penetrating to the fuel particles; in chamber furnaces, it is not possible to obtain the high temperature required for stable combustion in them.

The urgent need for the development of waste-free technologies that has arisen before mankind has raised the question of creating furnace devices for burning such materials. They are fluidized bed furnaces.

Fluidized (or boiling) is called a layer of fine-grained material blown from the bottom up with gas at a rate exceeding the stability limit of a dense layer, but insufficient to carry particles out of the layer. The intense circulation of particles in a 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 its stability because the resistance to the gas filtering through it becomes equal to the weight of a column of material per unit area of ​​the supporting grid. Since the aerodynamic drag is the force with which the gas acts on the particles (and, accordingly, according to Newton's third law, the particles on the gas), when the resistance and weight of the layer are equal, the particles (if we consider the ideal case) do not rely on the lattice, but on the gas.

The average particle size in fluidized bed furnaces is usually 2-3 mm. They correspond to the working speed of fluidization (it is taken 2-3 times more than w to) 1.5 ÷ 4 m / s. This determines in accordance with the area of ​​the gas distribution grate at a given heat output of the furnace. Volume heat stress q v take about the same as for layered furnaces.

The simplest fluidized bed furnace (Fig. 31) resembles a layered one in many ways and has many structural elements in common with it. The fundamental difference between them lies in the fact that intensive mixing of the particles ensures a constant temperature throughout the volume of the fluidized bed.

Rice. 31. Scheme of a fluidized bed furnace: 1 - ash discharge; 2 - air supply under the layer; 3 - fluidized bed of ash and fuel; 4 - air supply to the spreader; 5 - spreader rotor; 6 - belt feeder; 7 - fuel bunker; 8 - furnace volume; 9 - screen pipes; 10 - sharp blowing and return of entrainment; 11- furnace lining; 12 - heat-absorbing pipes in a fluidized bed; V - water; NS- steam.

Maintaining the temperature of the fluidized bed within the required range (850 - 950 ° C) is provided in two different ways. In small industrial furnaces that burn waste or cheap fuel, significantly more air is fed into the bed than is necessary for complete combustion, setting α at ≥ 2.

With the same amount of heat released, the gas temperature decreases as α in, for the same heat is spent on heating a large amount of gases.

In large power generating units, this method of reducing the combustion temperature is uneconomical, because the "extra" air, leaving the unit, also carries away the heat spent on heating it (losses with exhaust gases increase - see below). Therefore, pipes are placed in furnaces with a fluidized bed of large boilers 9 and 12 s the working fluid circulating in them (water or steam), which perceives the required amount of heat. Intensive "washing" of these pipes with particles provides a high coefficient of heat transfer from the layer to the pipes, which in some cases makes it possible to reduce the metal consumption of the boiler in comparison with the traditional one. Fuel burns steadily when its content in the fluidized bed is 1% or less; the rest 99% with superfluous - ash. Even under such unfavorable conditions, intensive mixing does not allow ash particles to block combustible from access to oxygen (in contrast to a dense layer). In this case, the concentration of fuels is the same throughout the volume of the fluidized bed. To remove the ash introduced with the fuel, part of the bed material is continuously removed from it in the form of fine-grained slag - most often it simply "drains" through the holes in the hearth, since the fluidized bed is capable of flowing like a liquid.

Circulating fluidized bed furnaces. Recently, second generation furnaces with the so-called circulating fluidized bed have appeared. A cyclone is installed behind these furnaces, in which all unburned particles are captured and returned back to the furnace. Thus, the particles are “trapped” in the firebox-cyclone-furnace system until they are completely burnt out. These furnaces have high efficiency, which is 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 pyrite in order to obtain SO 2, roasting of various ores and their concentrates (zinc, copper, nickel, gold-bearing), etc. (From the point of view of the theory of combustion, roasting, for example, zinc ore according to the reaction 2ZnS + 3O 2 = 2ZnO + 2SO 2 is the combustion of this specific "fuel" , proceeding, like all combustion reactions, with the release of large amounts of heat.) Widespread, especially abroad, fluidized bed furnaces have found for fire neutralization (i.e., incineration) of various hazardous production wastes (solid, liquid and gaseous) - sewage clarification sludge, garbage, etc.

Topic 12. Ovens of the chemical industry. Schematic diagram of a fuel furnace. Classification of furnaces in the chemical industry. The main types of furnaces, features of their design. Heat balance of ovens

Chemical industry furnaces. Schematic diagram of a fuel furnace

An industrial furnace is an energy technology unit designed for heat treatment of materials in order to give them the necessary properties. Various types of carbon fuels (gas, fuel oil, etc.) serve as a source of heat in fuel (flame) furnaces. Modern kiln installations are often large mechanized and automated units with high productivity.

The most important for the selection of the technological mode of the process is the optimum temperature of the technological process, which is determined by thermodynamic and kinetic calculations of the processes. The optimum temperature regime of the process is the temperature conditions under which the maximum productivity of the target product in a given furnace is ensured.

Usually, the operating temperature in the furnace is somewhat lower than the optimal one; it depends on the conditions of fuel combustion, heat exchange conditions, insulation properties and durability of the furnace lining, thermophysical properties of the material being processed, and other factors. For example, for kilns, the operating temperature is in the range between the temperature of active oxidation processes and the sintering temperature of the fired products. The thermal regime of the furnace is understood as a set of processes of inertia of heat, heat of mass transfer and mechanics of media that provide heat distribution in the zone of the technological process. The thermal regime of the process zone determines the thermal regime of the entire furnace.

The operating mode of the furnaces is greatly influenced by the composition of the gas atmosphere in the furnace, which is necessary for the correct course of the technological process. For oxidation processes, the gas environment in the furnace must contain oxygen, the amount of which ranges from 3 to 15% or more. The reducing environment is characterized by a low oxygen content (up to 1-2%) and the presence of reducing gases (CO, H2, 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 for combustion.

The movement of gases in a furnace has a significant effect 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 sustainable operation. Forced movement of gases is carried out by smoke exhausters and fans.

The speed of the technological process is influenced by the movement of the material undergoing heat treatment.

The scheme of the furnace plant includes the following elements: a combustion device for burning fuel and organizing heat exchange; working space of the furnace to fulfill the target technological regime; heat exchangers for heat recovery of flue gases (heating of gas, air); utilization plants (baked waste heat boilers) for utilizing the heat of flue gases; traction and blowing device (smoke exhausters, fans) for removing combustion of fuel and gaseous products of heat treatment of materials and supplying air to burners, nozzles under the grate; cleaning devices (filters, etc.).

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

Fresh fuel is in the upper part of the layer after loading. Burning coke is located under it, and slag is located directly above the grate. These zones of the layer partially overlap each other. As the fuel burns out, it gradually passes through all zones. In the first period after the fresh fuel arrives at the burning coke, its thermal preparation takes place (warming up, moisture evaporation, volatiles release), for which part of the heat released in the layer is spent. In fig. 6.2 shows the approximate combustion of solid fuel and the temperature distribution over the height of the fuel layer. The area of ​​the highest temperature is located in the coke combustion zone, where the main amount of heat is released.

The slag formed during the combustion of the fuel flows in droplets from the hot pieces of coke towards the air. Gradually the slag cools down 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 up and evenly distributes the air over the layer. The air passing through the grate and entering the fuel layer is called primary air. If there is not enough primary air for complete combustion of the fuel and there are incomplete combustion products above the layer, then air is additionally supplied to the above-layer space. This air is called secondary air.

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

At the beginning of the layer, in the oxygen zone (K), in which there is an intensive consumption of oxygen, carbon monoxide and carbon dioxide CO 2 and CO are simultaneously formed. Towards the end of the oxygen zone, the concentration of О 2 decreases to 1–2%, and the concentration of СО 2 reaches its maximum. The temperature of the layer in the oxygen zone rises sharply, having a maximum where the highest concentration of CO 2 is established.

In the reduction zone (B), oxygen is practically absent. Carbon dioxide reacts with hot carbon to form carbon monoxide:

Along the height of the reduction zone, the content of CO 2 in the gas decreases, and the CO content increases accordingly. The reaction of interaction of carbon dioxide with carbon is endothermic, so the temperature drops along the height of the reduction zone. In the presence of water vapor in the gases in the reduction zone, an endothermic decomposition reaction of H 2 O is also possible.

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

where Е с and Е СO2 - activation energies of formation of СО and СО 2, respectively; A is a numerical coefficient; R is the universal gas constant; T is the absolute temperature.
The temperature of the layer, in turn, depends on the concentration of the oxidizing agent, as well as on the degree of heating of the air. 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 an increase in the reaction temperature, 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 the pieces of burning fuel and the temperature regime. With an increase in the size of the fuel, the thickness of the zones increases. It was found that the thickness of the oxygen zone is approximately three to four diameters of the burning particles. The recovery zone is 4-6 times thicker than the oxygen zone.

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

The nature of gas formation in the fuel layer shows that, depending on the organization of combustion, either practically inert or combustible and inert gases can be obtained at the outlet of the layer. If the goal is to maximize the conversion of fuel heat into physical heat of gases, then the process should be carried out in a thin layer of fuel with an excess of oxidizer. If the task is to obtain combustible gases (gasification), then the process is carried out with a layer developed in height with a lack of an 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, which ensures the maximum course of oxidative reactions. Since the thickness of the oxygen zone depends on the size of the fuel, the larger the size of the pieces, the thicker the layer should be. So, when burning brown and bituminous coals (up to 20 mm in size) in a layer of fines, the layer thickness is maintained at about 50 mm. With the same coals, but in pieces more than 30 mm in size, the layer thickness is increased to 200 mm. The required thickness of the fuel layer also depends on its moisture content. The higher the moisture content of the fuel, the greater the reserve of the burning mass in the layer must be in order to ensure stable ignition and combustion of a fresh portion of the fuel.