Big encyclopedia of oil and gas. Combustion of liquid and solid fuels
Hello! Depending on the conditions of the combustion process, a larger or smaller proportion of the starting substances may enter into the reaction. To fully utilize the chemical energy of the fuel, it is necessary to bring the combustion reaction of the fuel almost to completion. Under conditions of industrial combustion of fuel, equilibrium of combustion reactions is rarely achieved due to the short amount of time it takes for combustion reactions to occur.
The process of combustion of liquid and solid fuels in combustion theory is called heterogeneous combustion, since it occurs in an inhomogeneous (heterogeneous) system. If a mixture of gases burns, then the combustion is called homogeneous.
When liquid fuel burns in the combustion chamber, fuel evaporates from the surface of the droplets. The resulting fuel vapors, due to the high temperature in the furnace, undergo thermal decomposition and quickly burn at the surface of the particles. Under these conditions, the rate of the combustion process is determined by the intensity of fuel evaporation. In order to increase the total surface area of the droplets, liquid fuel, when supplied to the combustion chamber, is subjected to fine atomization using nozzles (the surface area increases several thousand times). The heavy fractions that have not evaporated from the droplet undergo thermal decomposition (cracking), resulting in the formation of dispersed carbon, which imparts a glow to the flame.
The combustion process of solid fuel can be divided into two stages. After evaporation of moisture from the fuel, combustion of volatile substances occurs, which are released as a result of thermal decomposition of the fuel. Then the combustion of the solid residue (coke) begins. When the fuel is heated very quickly, both stages overlap each other, since some of the volatile substances burn together with the carbon of the coke.
The coke is partially gasified, and the resulting gaseous products, consisting mainly of carbon monoxide CO, are burned in the combustion chamber. Combustion of a solid fuel particle occurs not only from its surface, but also in the volume due to the penetration of oxygen into the pores. In this case, a boundary (laminar) layer of gas is formed on the surface of the particle, in which the oxygen content decreases and the content of gasification and combustion products (CO and CO2) increases. This boundary layer of gas prevents the supply of oxygen, and the rate of the combustion reaction will depend on the rate of diffusion of the oxidizer through the boundary layer. To increase the combustion intensity, the speed of the oxidizer (air) relative to the surface of the fuel particles is increased, which reduces the thickness of the boundary layer.
The fuel combustion process is also significantly affected by mineral impurities (ash content). As carbon burns out, a layer of ash forms on the surface of the fuel particles. At a low ash softening temperature and a high ash content, this layer envelops (slags) the fuel particles and worsens the combustion process. To remove ash build-up during layer combustion of fuel, scooping is performed, that is, loosening the fuel layer.
In powerful modern boilers, solid fuel is burned in suspension. Pieces of fuel are pre-ground in special mills, which increases their specific surface area several hundred times. A mixture of fuel dust and air is fed into the combustion chamber, where the fuel ignites and burns in a gas-air flow. Fuel combustion also occurs in two stages, but the combustion time of a fuel particle is significantly reduced. This combustion method makes it possible to intensify the combustion process, as well as completely mechanize all production operations. Spanish literature: 1) Khzmalyan D.M., Kagan Ya.A. Combustion theory and combustion devices, Moscow, “Energy”, 1976; 2) Thermal engineering, Bondarev V.A., Protsky A.E., Grinkevich R.N. Minsk, ed. 2nd, "Higher School", 1976.
Topic 15. SOLID AND LIQUID FUELS AND THEIR COMBUSTION
15.1. Calculation of combustion of solid and liquid fuels
To calculate the combustion processes of solid and liquid fuels, a material balance of the combustion process is compiled.
The material balance of the combustion process expresses the quantitative relationships between the initial substances (fuel, air) and the final products (flue gases, ash, slag), and the heat balance is the equality between the incoming and outgoing heat. For solid and liquid fuels, the material and heat balances are per 1 kg of fuel, for the gaseous phase - per 1 m 3 of dry gas at normal conditions(0.1013 MPa, O °C). Volumes of air and gaseous products are also expressed in cubic meters, normalized to normal conditions.
When burning solid and liquid fuels, combustible substances can oxidize to form oxides of varying degrees of oxidation. The stoichiometric equations for the combustion reactions of carbon, hydrogen and sulfur can be written as follows:
When calculating the volumes of air and combustion products, it is conventionally assumed that all combustible substances are completely oxidized with the formation of only oxides with highest degree oxidation (reactions a, c, d).
From equation (a) it follows that for the complete oxidation of 1 kmol of carbon (12 kg), 1 kmol, i.e. 22.4 m 3 , of oxygen is consumed and 1 kmol (22.4 m 3) of carbon monoxide is formed. Accordingly, for 1 kg of carbon, 22.4/12 = 1.866 m 3 of oxygen will be required and 1.866 m 3 CO 2 will be formed. 1 kg of fuel contains C p /100 kg of carbon. For its combustion, 1.866 C p /100 m 3 of oxygen is required and during combustion 1.866 C p /100 m 3 CO 2 is formed.
Similarly, from equations (c) and (d), the oxidation of combustible sulfur (μ s = 32) contained in 1 kg of fuel will require (22.4/32) S p l /100 m 3 of oxygen and the same volume of SO 2 will be formed . And the oxidation of hydrogen () contained in 1 kg of fuel will require 0.5·(22.4/2.02) N p /100 m 3 of oxygen and (22.4/2.02) N p /100 m will be formed 3 water vapor.
Summing up the obtained expressions and taking into account the oxygen in the fuel ( 
), after simple transformations we obtain a formula for determining the amount of oxygen theoretically required for the complete combustion of 1 kg of solid or liquid fuel, m 3 /kg:
In the process of complete combustion with the theoretically required amount of air, gaseous products are formed, which consist of CO 2, SO 2, N 2 and H 2 O - oxides of carbon and sulfur are dry triatomic gases. They are usually combined and denoted by RO 2 = CO 2 + SO 2.
When burning solids and liquid fuels theoretical volumes of combustion products, m 3 /kg, are calculated using equations (15.1) taking into account the content of the corresponding components in the fuel and air.
The volume of triatomic gases in accordance with equations (15.1, a and b)
Theoretical volume of water vapor 
, m 3 /kg, consists of the volume obtained from the combustion of hydrogen, equal to (22.4/2.02)·(H p /100), the volume obtained from the evaporation of fuel moisture, equal to 
, and the volume introduced with air: 
,
- specific volume of water vapor, m 3 /kg; ρ in = 1.293 kg/m 3 - air density, d in = 0.01 - moisture content in the air kg/kg. After transformations we get:
The actual volume of air V may be greater or less than the theoretically required volume, calculated from the combustion equations. The ratio of the actual volume of air V to the theoretically required V 0 is called the air flow coefficient α = V/V 0 . For α > 1, the air flow coefficient is usually called excess air ratio.
For each type of fuel, the optimal value of the excess air coefficient in the firebox depends on its technical characteristics, combustion method, firebox design, method of formation of the combustible mixture, etc.
The actual volume of combustion products will be greater than the theoretical one due to nitrogen, oxygen and water vapor contained in the excess air. Since air does not contain triatomic gases, their volume does not depend on the excess air coefficient and remains constant, equal to the theoretical one, i.e. 
.
The volume of diatomic gases and water vapor (m 3 / kg or m 3 / m 3) is determined by the formulas:
When burning solid fuels, the concentration of ash in flue gases (g/m3) is determined by the formula
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Where 
- the proportion of fuel ash carried away by gases (its value depends on the type of solid fuel and the method of its combustion and is taken from the technical characteristics of the furnaces).
The volume fractions of dry triatomic gases and water vapor, equal to their partial pressures at a total pressure of 0.1 MPa, are calculated using the formulas
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All formulas for calculating volumes are applicable when complete combustion of fuel occurs. The same formulas are also applicable for incomplete combustion of fuel with sufficient accuracy for calculation, if they are not exceeded standard values given in technical specifications firebox
15.2. Three stages of solid fuel combustion
The combustion of solid fuel has a number of stages: heating, drying of the fuel, sublimation of volatiles and formation of coke, combustion of volatiles and coke. Of all these stages, the decisive one is the stage of combustion of coke residue, i.e. the stage of carbon combustion, the intensity of which determines the intensity of fuel combustion and gasification as a whole. The decisive role of carbon combustion is explained as follows.
First, the solid carbon contained in the fuel is the main combustible component of almost all natural solid fuels. For example, the heat of combustion of anthracite coke residue is 95% of the heat of combustion of the combustible mass. With an increase in the yield of volatiles, the share of the heat of combustion of the coke residue decreases and in the case of peat amounts to 40.5% of the heat of combustion of the combustible mass.
Secondly, the stage of combustion of coke residue turns out to be the longest of all stages and can take up to 90% of the total time required for combustion.
And thirdly, the coke combustion process is crucial in creating thermal conditions for the occurrence of other stages. Hence, basis The correct construction of a technological method for burning solid fuels is the creation of optimal conditions for the carbon combustion process.
In some cases, secondary preparatory stages may determine the combustion process. For example, when burning highly moist fuel, the drying stage may be decisive. In this case, it is rational to strengthen the preliminary preparation of fuel for combustion, for example, by using a technological combustion method with drying the fuel with gases taken from the furnace.
Powerful steam generators consume large quantities fuel and air. For example, for a 300 MW steam generator, the fuel consumption - anthracite pellet - is 32 kg/s, and the air consumption is 246 m 3 /s, and in the steam generator of an 800 MW unit, 128 kg of Berezovsky coal and 555 m 3 of air are consumed every second. In some cases, pulverized coal steam generators use liquid or gas fuel as a backup.
The process of combustion of pulverized fuels takes place in the volume of the combustion chamber in flows of large masses of fuel and air, to which combustion products are mixed.
The basis for combustion of pulverized fuels is the chemical reaction of the combustible components of the fuel with oxygen in the air. However, chemical combustion reactions in the combustion chamber occur in powerful dust-gas-air flows over extremely a short time(1-2 s) residence of fuel and oxidizer in the combustion chamber. These reactions occur under conditions of strong mutual influence with simultaneously occurring physical processes. Such processes are:
The process of movement of the components of the combustible mixture of gas and solid dispersed substances supplied to the combustion chamber in a system of jets, turning into a flow and spreading in the limited space of the combustion chamber with the development of vortex flows, which together make up the complex structure of the aerodynamics of the furnace;
Turbulent and molecular diffusion and convective transfer of starting substances and reaction products in a gas flow, as well as the transfer of gas reagents to dispersed particles;
Heat exchange in gas streams of combustion products and the initial mixture and between gas streams and the fuel particles contained in them, as well as the transfer of heat released during chemical transformation in the reacting medium;
Radiation heat exchange of particles with a gaseous medium and dust-gas-air mixture with screen surfaces in the combustion chamber;
Heating of particles, sublimation of volatiles, their transfer and combustion in a gas volume, etc.
Thus, the combustion of coal dust is a complex physical and chemical process, consisting of chemical reactions and physical processes occurring under conditions of mutual connection and mutual influence.
15.3. Layer, flare and cyclone methods of burning solid fuels
Combustion devices of boilers can be layered - for burning lump fuel and chamber - for burning gaseous, liquid and solid pulverized fuel.
Some of the options for organizing combustion processes are presented in Fig. 15.1.
Layer furnaces come with a dense and fluidized bed, while chamber furnaces are divided into flare and cyclone.
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Rice. 15.1. Schemes for organizing combustion processes |
When burning in a dense layer, the combustion air passes through the layer without disturbing its stability, i.e. The gravity force of fuel particles is greater than the dynamic pressure of air.
When burning in a fluidized bed, due to the increased air speed, the stability of the particles in the layer is disrupted, they go into a “boiling” state, i.e. become suspended. In this case, intensive mixing of fuel and oxidizer occurs, which contributes to the intensification of the combustion process.
During flare combustion, fuel burns in the volume of the combustion chamber, for which solid fuel particles must have a size of up to 100 microns.
During cyclonic combustion, fuel particles under the influence of centrifugal forces are thrown onto the walls of the combustion chamber and, being in a swirling flow in a high-temperature zone, burn out completely. A particle size larger than during flaring is allowed. The mineral component of the fuel in the form of liquid slag is continuously removed from the cyclone furnace.
15.4.Features of combustion of liquid fuel
Each liquid fuel, just like any liquid substance, at a given temperature has a certain vapor pressure above its surface, which increases with increasing temperature.
When a liquid fuel with a free surface is ignited, its vapor contained in the space above the surface ignites, forming a burning torch. Due to the heat emitted by the torch, evaporation increases sharply. In a steady state of heat exchange between the torch and the liquid mirror, the amount of evaporating, and therefore burning, fuel reaches its maximum value and then remains constant over time.
Experiments show that when burning liquid fuels with a free surface, combustion occurs in the vapor phase; the torch is installed at some distance from the surface of the liquid and a dark strip is clearly visible, separating the torch from the edge of the crucible with liquid fuel. The intensity of radiation from the combustion zone onto the evaporation mirror does not depend on its shape and size, but depends only on the physicochemical properties of the fuel and is a characteristic constant for each liquid fuel.
The temperature of a liquid fuel at which vapors above its surface form a mixture with air that can ignite when an ignition source is applied is called the flash point.
Since liquid fuels burn in the vapor phase, in a steady state the combustion rate is determined by the rate of evaporation of the liquid from its mirror.
The combustion process of liquid fuels with a free surface occurs as follows. In a steady-state combustion mode, due to the heat emitted by the torch, the liquid fuel evaporates. Air from the surrounding space penetrates into the upward flow of fuel, which is in the vapor phase, through diffusion. The mixture obtained in this way forms a burning torch in the form of a cone, spaced 0.5-1 mm from the evaporation mirror. Steady combustion occurs on the surface where the mixture reaches a proportion corresponding to the stoichiometric ratio of fuel and air. This assumption follows from the same considerations as in the case of diffusion gas combustion. The chemical reaction occurs in a very thin layer of the flame front, the thickness of which does not exceed a few fractions of a millimeter. The volume occupied by the torch and combustion zone is divided into two parts: inside the torch there are vapors of flammable liquid and combustion products, and outside the combustion zone there is a mixture of combustion products with air.
The combustion of liquid fuel vapors rising inside the torch can be represented as consisting of two stages: the diffusion supply of oxygen to the combustion zone and the chemical reaction itself occurring in the flame front. The speeds of these two stages are not the same; the chemical reaction at the high temperatures occurring occurs very quickly, while the diffusion supply of oxygen is a slow process that limits the overall combustion rate. Therefore, in in this case combustion occurs in the diffusion region, and the combustion rate is determined by the rate of oxygen diffusion into the combustion zone.
Since the conditions for the supply of oxygen to the combustion zone during the combustion of various liquid fuels from the free surface are approximately the same, it should be expected that the rate of their combustion relative to the flame front, i.e., to the side surface of the torch, should also be the same. The greater the evaporation rate, the greater the length of the torch.
A specific feature of the combustion of liquid fuels from a free surface is a large chemical underburning. Each fuel, which is a carbon compound when burned from a free surface, has a chemical underburning value characteristic of it, which is, %:
for alcohol......... 5.3
for kerosene........ 17.7
for gasoline........ 12.7
for benzene......... 18.5.
The picture of the occurrence of chemical underburning can be presented as follows.
Vaporous hydrocarbons, when moving inside a cone-shaped torch up to the flame front while in the region of high temperatures in the absence of oxygen, undergo thermal decomposition until the formation of free carbon and hydrogen.
The glow of the flame is caused by the presence of free carbon particles in it. The latter, having become heated due to the heat generated during combustion, emit more or less bright light.
Part of the free carbon does not have time to burn and is carried away in the form of soot by combustion products, forming a smoky torch.
In addition, the presence of carbon causes the formation of CO.
High temperature and low partial pressure of CO and CO 2 in combustion products favor the formation of CO.
The amounts of carbon and CO present in combustion products determine the amount of chemical underburning. The higher the carbon content in liquid fuel and the less it is saturated with hydrogen, the greater the formation of pure carbon, the brighter the torch, the greater the chemical underburning.
Thus, studies of the combustion of liquid fuels from a free surface have shown that:
1) combustion of liquid fuels occurs after their evaporation in the vapor phase. The burning rate of liquid fuels from the free surface is determined by the rate of their evaporation due to the heat emitted by the combustion zone, under a steady state of heat exchange between the torch and the evaporation mirror;
2) the rate of combustion of liquid fuels from the free surface increases with increasing temperature of their heating, with the transition to fuels with a higher radiation intensity of the combustion zone, lower heat of vaporization and heat capacity and does not depend on the size and shape of the evaporation mirror;
3) the intensity of radiation from the combustion zone onto the evaporation mirror burning from the free surface of the liquid fuel depends only on its physicochemical properties and is a characteristic constant for each liquid fuel;
4) the thermal stress of the front of the diffusion plume above the surface of evaporation of liquid fuel practically does not depend on the diameter of the crucible and the type of fuel;
5) combustion of liquid fuels from a free surface is characterized by increased chemical underburning, the magnitude of which is characteristic of each fuel.
Keeping in mind that the combustion of liquid fuels occurs in the vapor phase, the combustion process of a drop of liquid fuel can be represented as follows.
A drop of liquid fuel is surrounded by an atmosphere saturated with vapors of this fuel. A combustion zone is established near the drop along the spherical surface. The chemical reaction of the mixture of liquid fuel vapors with the oxidizer occurs very quickly, so the combustion zone is very thin. The burning rate is determined by the slowest stage - the rate of evaporation of the fuel.
In the space between the drop and the combustion zone there are vapors of liquid fuel and combustion products. In the space outside the combustion zone there is air and combustion products.
Fuel vapor diffuses into the combustion zone from the inside, and oxygen diffuses from the outside. Here these components of the mixture enter into a chemical reaction, which is accompanied by the release of heat. From the combustion zone, heat is transferred outward and to the drop, and combustion products diffuse into the surrounding space and into the space between the combustion zone and the drop. However, the mechanism of heat transfer does not yet seem clear.
A number of researchers believe that the evaporation of a burning drop occurs due to molecular heat transfer through a stagnant boundary film at the surface of the drop.
As the droplet burns out due to a decrease in the surface, the total evaporation decreases, the combustion zone narrows and disappears when the droplet is completely burned out.
This is how the combustion process proceeds of a drop of completely evaporating liquid fuels, which is at rest in environment or moving with it at the same speed.
The amount of oxygen diffusing to the spherical surface, other things being equal, is proportional to the square of its diameter, therefore, establishing a combustion zone at some distance from the drop causes a higher rate of its combustion compared to the same particle of solid fuel, during the combustion of which the chemical reaction practically takes place on the surface itself .
Since the burning rate of a drop of liquid fuel is determined by the evaporation rate, its burnout time can be calculated based on the equation heat balance its evaporation due to the heat received from the combustion zone.
Since the combustion of liquid fuels occurs after their evaporation in the vapor phase, its intensification is associated with the intensification of evaporation and mixture formation. This is achieved by increasing the evaporation surface by spraying liquid fuel into tiny droplets and mixing the resulting vapor well with air while evenly distributing fine fuel in it. These two tasks are performed using burners with nozzles that spray liquid fuel into air streams supplied to the chamber firebox through the burner air guides.
The air required for combustion is supplied to the mouth of the nozzle, captures the finely atomized liquid fuel and forms a non-isothermal flooded jet in the combustion chamber. The jet, spreading, heats up due to the entrainment of high temperature combustion products. The smallest droplets of liquid fuel, heating up due to convective heat exchange in the jet, evaporate. Heating of atomized fuel also occurs due to their absorption of heat emitted by flue gases and hot lining.
In the initial section and especially in the boundary layer of the jet, the intense heating of the torch causes rapid evaporation of droplets. Fuel vapors, mixing with air, create a gas-air combustible mixture, which, when ignited, forms a torch.
Thus, the combustion process of liquid fuel can be divided into the following phases: atomization of liquid fuel, evaporation and formation of a gas-air mixture, ignition of the combustible mixture and combustion of the latter.
The temperature and concentration of the gas-air mixture vary across the cross section of the jet. As you approach the outer boundary of the jet, the temperature rises and the concentration of the components of the combustible mixture decreases. The speed of flame propagation in a steam-air mixture depends on the composition, concentration and temperature and reaches its maximum value in the outer layers of the jet, where the temperature is close to the temperature of the surrounding flue gases, despite the fact that here the combustible mixture is highly diluted with combustion products. Therefore, ignition in an oil flame begins at the root from the periphery and then spreads deep into the jet over the entire cross-section, reaching its axis at a significant distance from the nozzle, equal to the movement of the central jets during the time of flame propagation from the periphery to the axis. The ignition zone takes the form of an elongated cone, the base of which is located at a small distance from the outlet section of the burner embrasure.
The position of the ignition zone depends on the speed of the mixture; the zone occupies a position in which at all its points an equilibrium is established between the speed of flame propagation and the speed of movement. The central jets, which have the highest speed, attenuate as they move through the combustion space, determining the length of the ignition zone by the place where the speed drops to the absolute value of the flame propagation speed.
The combustion of the main part of vaporous hydrocarbons occurs in the ignition zone, which occupies the outer layer of the torch of small thickness. The combustion of high molecular weight hydrocarbons, soot, free carbon and unevaporated liquid fuel droplets continues beyond the ignition zone and requires a certain space, determining the total length of the torch.
The ignition zone divides the space occupied by the torch into two areas: internal and external. In the internal region, the process of evaporation and formation of a flammable mixture occurs.
In the internal region, vaporous hydrocarbons are subjected to heating, which is accompanied by oxidation and splitting. The oxidation process begins at relatively low temperatures - about 200-300°C. At temperatures of 350-400°C and above, the process of thermal splitting occurs.
The process of oxidation of hydrocarbons favors the subsequent combustion process, since this releases a certain amount of heat and increases the temperature, and the presence of oxygen in the composition of hydrocarbons promotes their further oxidation. On the contrary, the process of thermal decomposition is undesirable, since the high molecular weight hydrocarbons formed in this process are difficult to burn.
Of the petroleum fuels, only fuel oil is used in the energy sector. Fuel oil is a residue from the distillation of oil at a temperature of about 300°C, but due to the fact that the distillation process does not occur completely, fuel oil at temperatures below 300°C still releases a certain amount of lighter vapors. Therefore, when a sprayed jet of fuel oil enters the furnace and is gradually heated, some of it turns into vapor, and some can still be in a liquid state even at a temperature of about 400°C.
Therefore, when burning fuel oil, it is necessary to promote the flow oxidative reactions and prevent thermal decomposition at high temperatures in every possible way. To do this, all the air necessary for combustion should be supplied to the root of the torch. In this case, the presence large quantity oxygen in the internal region will, on the one hand, favor oxidative processes, and on the other, lower the temperature, which will cause the splitting of hydrocarbon molecules more symmetrically without the formation of a significant amount of difficult-to-burn high molecular weight hydrocarbons.
The mixture resulting from the combustion of fuel oil contains steam and gaseous hydrocarbons, liquid heavier products, as well as solid compounds formed as a result of the splitting of hydrocarbons (i.e., all three phases - gaseous, liquid and solid). Vapor and gaseous hydrocarbons, mixing with air, form a flammable mixture, the combustion of which can occur in all possible ways combustion of gases. The CO formed during the combustion of liquid droplets and coke burns similarly.
In a torch, droplets are ignited due to convective heating; A combustion zone is established around each drop. The burning of a drop is accompanied by chemical underburning in the form of soot and CO. Drops of high-molecular hydrocarbons, when burned, produce a solid residue - coke.
The solid compounds formed in the torch - soot and coke - burn in the same way as heterogeneous combustion of solid fuel particles occurs. The presence of heated soot particles causes the torch to glow.
Free hydrocarbon and soot in a high temperature environment can burn if there is enough air. In the case of a local lack of air or an insufficiently high temperature, they do not burn completely with a certain chemical incompleteness of combustion, turning the combustion products black - a smoky torch.
The afterburning zone of gaseous products of incomplete combustion and solid particles, following the combustion zone, increases the total length of the torch.
Chemical underburning, characteristic of the combustion of liquid fuels from a free surface when burning them in a torch, can and should be reduced to almost zero by appropriate regime measures.
Thus, to intensify the combustion of fuel oil, good atomization is necessary. Preheating the air and fuel oil promotes gasification of the fuel oil, therefore it will favor ignition and combustion. All air required for combustion should be supplied to the root of the torch. In this case, the rational design of the burner air guide device, the correct installation of the nozzle and the appropriate configuration of the burner embrasure must ensure good mixing of the atomized fuel with air, as well as mixing in the burning torch and especially in its final part. The temperature in the torch must be maintained at a sufficiently high level and to ensure intensive completion of the combustion process at the end of the torch it must not be lower than 1000-1050°C.
The torch must be provided with sufficient space for the development of the combustion process, since in the event of contact of combustion products (before completion of the combustion process) with the cold heating surfaces of the steam generator, the temperature can drop so much that unburnt particles of soot and free carbon contained in the gases, as well as high-molecular hydrocarbons will not be able to burn.
The process of burning an oil torch in a swirling jet proceeds similarly to the case considered with a direct-flow jet. With swirling motion, a rarefaction zone is created on the jet axis, causing an influx of hot combustion products to the root of the torch. This ensures stable ignition.
The use of the centrifugal effect in mechanical and rotating nozzles leads to a break in the continuous flow. The liquid inside the nozzle outlet takes the form of a hollow cylinder filled with vapors and gases. The emulsion flows out of the nozzle, forming a liquid film in the form of an opening hyperboloid. In the direction of motion, the cross-section of the hyperboloid increases, and the liquid film thins, begins to pulsate and, finally, breaks up into fast-moving droplets, which undergo further crushing in the flow.
In steam nozzles, primary crushing is carried out due to the kinetic energy of steam flowing from the nozzle nozzle. The droplets of primary crushing acquire the speed of the steam jet, usually corresponding to the critical speed.
15.5.Fuel combustion and environmental protection
15.5.1. Ferrous metallurgy as a source of environmental pollution
A metallurgical plant producing 1 million tons of steel per year emits 350 tons of dust, 400 tons of carbon monoxide and 200 tons of sulfur dioxide into the atmosphere per day. Of the total emissions, metallurgical plants account for 20% of dust emissions, 43% of carbon monoxide, 16% of sulfur dioxide and 23% of nitrogen oxides. The sinter plant and thermal power plant have the most emissions. Of the total emissions of a metallurgical plant, the sinter plant produces 34% of dust, 82% of sulfur dioxide, 23% of nitrogen oxides. The thermal power plant emits 36% of dust. Thus, the sinter plant and the thermal power plant together emit about 70% of the plant’s total dust emissions into the atmosphere.
A distinction is made between the purification of gases from suspended solid particles (dust) and the capture of harmful gaseous substances using chemical gas purification methods. Currently, the purification of gases emitted into the atmosphere from harmful gaseous substances is almost never used (and not only in our country), with the exception of coke production, where such purification is widespread due to the need to capture a number of valuable substances.
In factories ferrous metallurgy, mainly carry out mechanical purification of gases from dust. Based on the operating principle, the cleaning methods used are divided into dry and wet. Wet dust collectors allow, at the same time as collecting dust, to partially purify gases from sulfur dioxide (SO 3). However, these dust collectors increase water consumption and require the use of devices for its purification.
15.5.2.Apparatuses for dry mechanical gas purification
They are divided into dust collectors and filters. In turn, dust collectors are divided into gravitational and inertial. Gravity dust collectors have dust chambers various designs. In these dust collectors, dust sedimentation occurs mainly under the influence of gravity. Inertial forces here have little influence on the process of extracting dust from the gas flow.
Figure 15.2 shows a diagram of a radial dust collector. Dusty gas enters it through the central flue, which in the bunker reduces its speed and changes the direction of movement by 180 0. The dust contained in the gas, under the influence of gravity and inertia, settles in the bunker, and the gas is removed in a purified form.
Gravity dust collectors are effective in removing dust particles larger than 100 microns, i.e. fairly large particles.
In inertial (centrifugal) dust collectors (Fig. 15.3), dust particles are acted upon by an inertial force that occurs when the gas flow turns or rotates. Since this force significantly exceeds the gravitational force, smaller particles are removed from the gas flow than during gravitational cleaning.
An example of such a dust collector is a cyclone, which removes dust particles larger than 20 microns from a gas flow. The dusty gas flow is introduced into the upper part of the cyclone body through a pipe located tangentially relative to the body. The flow is acquiring rotational movement, heavy dust particles are thrown by inertial forces towards the walls of the cyclone and, under the influence of gravity, fall into the bunker, and the purified gas is removed from the cyclone.
Filters (Fig. 15.4) are devices that provide fine gas purification. According to the type of filter element, they are divided into filters with a fibrous filter element, with a fabric, granular, metal-ceramic, and ceramic filter element. A typical example is filters with a woven filter element: made of natural and synthetic fabrics or metal woven, withstanding temperatures up to 600 0 C.
Regeneration of the fabric filter is carried out by back blowing with compressed air.
The dusty gas passes through the hose fabric, leaving dust particles on it, and is removed from the filter purified. Dust settles into the hopper as it accumulates on the fabric. When the resistance of the fabric increases significantly, the fabric sleeve is cleaned of dust by back blowing air.
15.5.3.Electric precipitators
Electric precipitators (Fig. 15.5) are devices for fine gas purification. The principle of operation of these filters is based on the force interaction of charged particles with each other and with metal electrodes. You know that like-charged particles repel, and unlike-charged particles attract. In an electric precipitator, dust particles entering an electric field are charged and then, under the influence of interaction forces with the precipitation electrodes, they are attracted to them, deposited on them and lose their charge. As an example, consider the operation of a tubular electrostatic precipitator. The filter consists of a housing and a central electrode, the design of which is not disclosed in the diagram. The filter housing is grounded. The central electrode consists of plates, some of which are connected to the housing, and the other part is insulated from it.
Electrodes insulated and connected to the body alternate. A potential difference of about 25-100 kV is created between them. The magnitude of the potential difference is determined by the geometry of the electrodes and the greater the distance between them. This is due to the fact that the electrostatic precipitator operates if there is a corona discharge between the electrodes.
Gas passing between the electrodes is ionized. Dust particles interact with ions, acquire a negative charge and are attracted to the collecting electrodes. When dust particles settle on the electrodes, they lose their charge and partially fall into the hopper.
The filter is periodically cleaned by shaking or washing. The filter is switched off during cleaning.
When working with blast furnace gas, the filter is washed every 8 hours for 15 minutes. The maximum temperature of the purified gas should not exceed 300 0 C. Working temperature purified gas 250 0 C. Electrode height up to 12 m.
An electric precipitator cleans the gas from dust particles with sizes smaller than 1 micron.
15.5.4. Wet gas cleaning
In wet cleaning devices, dusty gas is washed with water, which makes it possible to separate a significant part of the dust.
Scrubbers of various designs and turbulent gas scrubbers are most widely used in ferrous metallurgy.
Scrubbers (Fig. 15.6) are units in which dusty gas rises towards the irrigation water. For corrosion protection internal surfaces scrubbers are lined with ceramic tiles. The maximum gas temperature in the scrubber is 300 0 C. Scrubber dimensions: diameter - 6-8 m, height - 20-30 m. Water consumption - 1.5-2 kg/m 3 of gas. Scrubbers perform semi-fine dust removal.
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Rice. 15.6. Scrubber circuit |
A high-speed gas scrubber (Fig. 15.7) is an effective fine cleaning device, used both independently and for preparing gas before an electric precipitator. Consists of a spray pipe and a droplet eliminator cyclone. Captures dust particles up to 0.1 microns in size. Gas capacity 40,000 m 3 /h or more. The specific consumption of irrigation water is 0.15-0.5 kg/m 3 . The gas speed in the neck of the spray pipe is 40-150 m/s.
The principle of operation of a high-speed gas scrubber is based on the capture in a cyclone of small dust particles weighed down by water wetting them. Wetting of dust particles is carried out in a spray pipe.
In conclusion, it should be noted that dust with particles larger than 10-20 microns is well captured in most gas cleaning devices. To remove dust with particles smaller than 1 micron, only fine cleaning devices are suitable: porous filters, electric precipitators, high-speed gas scrubbers.
Combustion process solid fuel can be represented as a series of sequential stages. First, the fuel warms up and moisture evaporates. Then, at temperatures above 100 °C, pyrogenic decomposition of complex high-molecular organic compounds and the release of volatile substances begin, while the temperature at which volatiles begin to release depends on the type of fuel and the degree of its carbonization (chemical age). If the ambient temperature exceeds the ignition temperature of volatile substances, they ignite, thereby providing additional heating of the coke particle before it ignites. The higher the yield of volatiles, the lower their ignition temperature, while the heat release increases.
The coke particle warms up due to the heat of the surrounding flue gases and heat release as a result of the combustion of volatiles and ignites at a temperature of 800÷1000 °C. When burning solid fuel in a pulverized state, both stages (combustion of volatiles and coke) can overlap each other, since heating of the smallest coal particle occurs very quickly. In real conditions, we are dealing with a polydisperse composition of coal dust, so at each moment of time some particles are just beginning to warm up, others are at the stage of becoming volatile, and others are at the stage of burning coke residue.
The combustion process of a coke particle plays a decisive role in assessing both the total fuel combustion time and the total heat release. Even for fuel with a high yield of volatiles (for example, brown coal near Moscow), the coke residue is 55% by weight, and its heat release is 66% of the total. And for fuel with a very low volatile yield (for example, AS), the coke residue can account for more than 96% of the weight of the dry initial particle, and the heat release during its combustion, accordingly, is about 95% of the total.
Studies of combustion of coke residue have revealed the complexity of this process.
When burning carbon, there are two possible primary direct heterogeneous oxidation reactions:
C + O 2 = CO 2 + 34 MJ/kg; (14)
2C + O 2 = 2CO + 10.2 MJ/kg. (15)
As a result of the formation of CO 2 and CO, two processes can occur secondary reactions:
oxidation of carbon monoxide 2CO + O 2 = 2CO 2 + 12.7 MJ/kg; (16)
reduction of carbon dioxide CO 2 + C = 2СО – 7.25 MJ/kg. (17)
In addition, in the presence of water vapor on the hot surface of the particle, i.e. in the high-temperature region, gasification occurs with the release of hydrogen:
C + H 2 O = CO + H 2. (18)
Heterogeneous reactions (14, 15, 17 and 18) indicate direct combustion of carbon, accompanied by a decrease in weight of the carbon particle. The homogeneous reaction (16) occurs near the surface of the particle due to oxygen diffusing from the surrounding volume and compensates for the decrease in the temperature level of the process that occurs as a consequence of the endothermic reaction (17).
The ratio between CO and CO 2 at the particle surface depends on the temperature of the gases in this area. For example, according to experimental studies, the reaction occurs at a temperature of 1200 °C
4C + 3O 2 = 2CO + 2CO 2 (E = 84 ÷ 125 kJ/g-mol),
and at temperatures above 1500 °C
3C + 2O 2 = 2CO + CO 2 (E = 290 ÷ 375 kJ/g-mol).
It is obvious that in the first case, CO and CO 2 are released in approximately equal quantities, whereas with increasing temperature, the volume of CO released is 2 times greater than CO 2.
As already noted, the burning rate mainly depends on two factors:
1) chemical reaction rate, which is determined by the Arrhenius law and increases rapidly with increasing temperature;
2) oxidizer supply speed(oxygen) to the combustion zone due to diffusion (molecular or turbulent).
In the initial period of the combustion process, when the temperature is not yet high enough, the rate of the chemical reaction is also low, and there is more than enough oxidizer in the volume surrounding the fuel particle and at its surface, i.e. there is a local excess of air. No improvement in the aerodynamics of the firebox or burner, leading to an intensification of the supply of oxygen to the burning particle, will affect the combustion process, which is inhibited only by the low rate of the chemical reaction, i.e. kinetics. This - kinetic combustion region.
As the combustion process progresses, heat is released, the temperature increases, and, consequently, the rate of the chemical reaction increases, which leads to a rapid increase in oxygen consumption. Its concentration at the surface of the particle is steadily decreasing, and in the future the burning rate will be determined only by the rate of oxygen diffusion into the combustion zone, which is almost independent of temperature. This - diffusion combustion area.
IN transition region of combustion the rates of chemical reaction and diffusion are of the same order of magnitude.
According to the law of molecular diffusion (Fick's law), the rate of diffusion transfer of oxygen from the volume to the surface of a particle
Where 
– coefficient of diffusion mass transfer;
And 
– respectively, partial pressures of oxygen in the volume and at the surface.
Oxygen consumption at the particle surface is determined by the rate of the chemical reaction:
,
(20)
Where k– reaction rate constant.
In the transition zone in a steady state
,
where 
(21)
Substituting (21) into (20), we obtain an expression for the combustion rate in the transition region in terms of oxidizer (oxygen) consumption:
(22)
Where 
is the effective rate constant of the combustion reaction.
In a zone of relatively low temperatures (kinetic region) 
, hence,
k ef = k, and expression (22) takes the form:
,
those. oxygen concentrations (partial pressures) in the volume and at the surface of the particle differ little from each other, and the burning rate is almost completely determined by the chemical reaction.
With increasing temperature, the rate constant of a chemical reaction increases according to the exponential Arrhenius law (see Fig. 22), while molecular (diffusion) mass transfer weakly depends on temperature, namely
.
At a certain temperature T*, the rate of oxygen consumption begins to exceed the intensity of its supply from the surrounding volume, coefficients α D And k become commensurate values of the same order, the oxygen concentration at the surface begins to noticeably decrease, and the combustion rate curve deviates from the theoretical curve of kinetic combustion (Arrhenius’s law), but still increases noticeably. An inflection appears on the curve - the process moves into the intermediate (transition) combustion region. The relatively intensive supply of oxidizer is explained by the fact that due to a decrease in the oxygen concentration at the surface of the particle, the difference between the partial pressures of oxygen in the volume and at the surface increases.
In the process of combustion intensification, the oxygen concentration at the surface becomes practically equal to zero, the supply of oxygen to the surface weakly depends on temperature and becomes almost constant, i.e. α D << k, and, accordingly, the process goes into the diffusion region
.
In the diffusion region, an increase in the combustion rate is achieved by intensifying the process of mixing fuel with air (improving burner devices) or increasing the speed of blowing the particle with an air flow (improving the aerodynamics of the firebox), as a result of which the thickness of the boundary layer at the surface decreases and the supply of oxygen to the particle is intensified.
As already noted, solid fuel is burned either in the form of large (without special preparation) pieces (layer combustion), or in the form of crushed particles (fluidized bed and low-temperature vortex), or in the form of fine dust (flare method).
Obviously the greatest relative speed blowing of fuel particles will occur during layer combustion. With vortex and flare combustion methods, fuel particles are in the flue gas flow, and the relative speed of their blowing is significantly lower than under stationary bed conditions. Based on this, it would seem that the transition from the kinetic region to the diffusion region should occur first for small particles, i.e. for dust. In addition, a number of studies have shown that a coal dust particle suspended in a flow of gas-air mixture is blown so weakly that the released combustion products form a cloud around it, which greatly inhibits the supply of oxygen to it. And the intensification of heterogeneous combustion of dust during the torch method was presumably explained solely by a significant increase in the total reacting surface. However, the obvious is not always true .
The supply of oxygen to the surface is determined by the laws of diffusion. Studies on the heat transfer of a small spherical particle flowing around a laminar flow have revealed a generalized criterion dependence:
Nu = 2 + 0.33Re 0.5.
For small coke particles (at Re< 1, что соответствует скорости витания мелких частиц), Nu → 2, т.е.
.
There is an analogy between the processes of heat and mass transfer, since both are determined by the movement of molecules. Therefore, the laws of heat transfer (Fourier and Newton-Richmann laws) and mass transfer (Fick's law) have a similar mathematical expression. The formal analogy of these laws allows us to write in relation to diffusion processes:
,
where 
, (23)
where D is the molecular diffusion coefficient (similar to the thermal conductivity coefficient λ in thermal processes).
As follows from formula (23), the coefficient of diffusion mass transfer α D is inversely proportional to the radius of the particle. Consequently, with a decrease in the size of fuel particles, the process of oxygen diffusion to the particle surface intensifies. Thus, during the combustion of coal dust, the transition to diffusion combustion shifts towards higher temperatures (despite the previously noted decrease in the rate of particle blowing).
According to numerous experimental studies conducted by Soviet scientists in the mid-twentieth century. (G.F. Knorre, L.N. Khitrin, A.S. Predvoditelev, V.V. Pomerantsev, etc.), in the zone of normal combustion temperatures (about 1500÷1600 °C) the combustion of a coke particle shifts from the intermediate zone to diffusion, where intensification of the oxygen supply is of great importance. In this case, with an increase in the diffusion of oxygen to the surface, the inhibition of the combustion rate will begin at a higher temperature.
The combustion time of a spherical carbon particle in the diffusion region has a quadratic dependence on the initial particle size:
,
Where r o– initial particle size; ρ
h– density of the carbon particle; D o , P o , T o– respectively, the initial values of the diffusion coefficient, pressure and temperature; 
– initial oxygen concentration in the combustion volume at a considerable distance from the particle; β
– stoichiometric coefficient, which establishes the correspondence of the weight consumption of oxygen per unit weight of burned carbon at stoichiometric ratios; T m– logarithmic temperature:
Where T P And T G– respectively, the temperature of the particle surface and the surrounding flue gases.
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 goal 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 fine brown and hard coals (up to 20 mm in size) in a layer, 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.
During the intake process, a fresh charge of the fuel mixture enters the combustion chamber, and it begins to mix with the residual gases located there. The mixing process continues during the compression stroke, when after the appearance of a spark at the electrodes of the spark plug, the combustion process begins. As a result of the appearance of a spark, a certain volume of plasma is formed and a flame core is formed, which can spread in the unburned charge of the fuel mixture. The ignition process and the initial stage of combustion, at which the flame core is formed, are determined mainly by chemical reactions and the properties of the fuel mixture. Moreover, the initial stage of combustion is more sensitive to the characteristics of the flows of burning gases in and around the combustion zone. When the flame core becomes large enough, it gradually develops into a developed, spreading flame. The process of flame propagation is usually determined by the laws of fluid and gas mechanics; Depending on the characteristics of the gas flow and the composition of the charge of the fuel mixture, chemical phenomena may also be of significant importance at this stage. In the end, the flame covers almost the entire mixture, and at the final stage of the combustion process near the walls it slowly fades and is extinguished as a result of heat removal into the walls. The process of post-burning of unburned gases after extinguishing the flame is a diffusion process.
The entire combustion process is a transient process, but based on the above brief description, it can be divided into the following stages according to the development of the combustion zone:
1. ignition;
2. flame formation;
3. flame spread;
4. extinguishing the flame.
This division is suitable for normally occurring combustion processes in the absence of such phenomena as misfire, incomplete combustion or detonation. These phenomena disrupt the normal combustion process, and the possibility of their occurrence characterizes the maximum operating conditions of the engine under given conditions. Since different processes play a decisive role in each of the four stages of combustion, these stages will be discussed separately in the following sections.
In heating boilers, solid fuel is burned in a bed mainly on manual grates with manual operation. Recently, mechanical fireboxes of the “swirling strip” type have begun to be introduced. The main elements of the furnace for burning solid fuel in heating boilers with manual operation are the grate, which supports a layer of lump fuel, through which the air necessary for combustion passes, and the combustion space, in which flammable volatile substances are burned. With constant draft, the amount of air passing into the furnace layer of fuel in the period between loads constantly increases as a result of burning out the layers and reducing its resistance. Of the air entering the furnace, part is used for the combustion of solid fuel in the bed, part is used for the combustion of volatile substances in the combustion space, and a certain amount of air remains unused.
The fuel loaded onto the burning layer first dries out, then the combustion process begins; during this period, due to a lack of air, incomplete combustion of the fuel may occur, which disappears as the coking process of the fuel portion dies out. By the end of the period between loadings, the fuel in the thin layer burns mostly. Typically this combustion period is characterized by complete combustion of the fuel with a large excess of air.
Thus, in the first moments after loading fuel onto the grate, its combustion occurs with chemical incompleteness, and at the end of the combustion process - with increased excess air and, consequently, with increased heat loss with the exhaust gases. Therefore, the correctly selected thickness of the fuel layer ensures a minimum amount of heat loss from the chemical completeness of combustion and from the exhaust gases with a minimum excess of air. These conditions can best be created by loading fuel in small increments more frequently.
The last point should be emphasized, since drivers often ignore it, and as a result, atmospheric pollution with carbon monoxide occurs. The periods between loading fuel, for example anthracite, should be 10-15 minutes, for others even less. Monitoring the correctness of the chosen thickness of the fuel layer is carried out either using gas analyzers, according to the readings of which the completeness of combustion and excess air are assessed, or visually by the color of the flame in the absence of instruments. Visually, the chemical incompleteness of combustion is determined by the degree of transparency of the smoke, and excess air is determined by the shape and color of the torch. Complete combustion of solid fuel with a small excess of air produces a transparent flame of straw-yellow color. With a large excess of air, the flame, without changing its transparency, becomes short. With incomplete combustion, the flame, remaining long, turns red and dark layers appear on it. Incomplete combustion of solid fuel with a low yield of volatile substances is manifested in blue tongues of burning carbon monoxide appearing above the fuel layer.
Combustion of solid fuels may involve the use of low quality coals. But this leads to a sharp decrease in the efficiency of boilers and, as a consequence, to excessive consumption of fuel and the construction of additional boiler houses, or, with a rationed supply of fuel, to a lack of heat delivery to consumers and, in addition, to atmospheric pollution not only with products of incomplete combustion, but also with fine particles of unburned fuel (entrainment ).
High humidity (over 30%) with high ash content (over 35%) worsens the combustion process and reduces the efficiency of boiler operation. The Academy of Public Utilities named after K. D. Pamfilov, based on Dialysis of experimental data, established that for the effective combustion of coal and anthracite in cast iron and steel boilers, the maximum value of ash content is. Boiler operating experience shows that the maximum size of coal pieces should not exceed 50 mm.
Brown coals can be used in cast iron boilers in the form of briquettes, so it is necessary to speed up the solution to the complex technical problem of upgrading them at the mining site to obtain semi-coke, tar, gas, and briquet the semi-coke with the addition of resin as a binder. Thus, for cast iron boilers it is necessary to: use coal according to the size of pieces of two classes: 13-25 and 25 - 50 mm: replace brown coal with hard coal and anthracite; when solving the problem of industrial upgrading at the mining site, use coal in the form of briquettes; use coals with a moisture content of no higher than 8% and a fines content of no more than 20%.
