Over the centuries, humanity has improved the design of heating furnaces, which were originally intended to burn solid fuels that were available everywhere. In this regard, little has changed, and today in the 21st century, with the availability of gas and liquid fuel, we often turn to traditional heating technologies. Somehow it becomes easier on the heart if in a modern house, in addition to central heating, there is also a good stove in reserve. Well, traditional baths cannot do without the heat of a wood-burning stove.
To operate a wood-burning stove efficiently and safely, the stoker needs to know the intricacies of burning solid fuels. Many people today no longer remember how to properly light a stove, but experiments in this matter are extremely undesirable. IN this material We will try to cover the topic of solid fuel combustion as much as possible.
Solid fuel means firewood, coal, anthracite, coke, peat, etc. In traditional furnaces, all this is burned in a layered manner, with or without grates. Fuel is periodically loaded into the furnace, and the resulting slag is removed. The layer combustion method is cyclic in nature. The closed cycle has several stages:
Each of these stages has its own thermal regime, while the indicators during fuel combustion are constantly changing. To ensure optimal thermal conditions of the furnace, it is necessary to periodically add a new portion of fuel (layer). The moment of loading a new layer is determined individually and depends on many factors. Let us consider the stages of layer-by-layer combustion of solid fuel in more detail.
Warming up and drying layer is accompanied by heat absorption, i.e. is endothermic in nature. The heat supplier is the starting flame from thin dry wood or already ignited fuel, as well as the hot walls of the firebox.
Ignition and smoldering stage occurs with increasing heat release. Excessive air intake into the firebox during this period is undesirable, since it will cool the flue gases, and, therefore, the chimney will heat up longer. The air dampers at the stage of ignition and smoldering should only be slightly open, and it is desirable that cold air be supplied only to the ignition zone.
Combustion stage needs large volumes of air oxygen, because this process is nothing more than the oxidation of hydrocarbons. Flame heating is increasing, and, in fact, is limited only by the amount of incoming oxygen. If the chimney cross-section is insufficient, the flame may escape from the air supply openings. In such a situation, there is only one way out - immediately open the chimney valve completely and close the air supply. When the air supply decreases, the flames become longer and can even penetrate the chimney, which will be a sign of underburning. It is obvious that the supplied air in the flame combustion mode must be divided into two controlled flows. The primary stream will be fed directly into the firewood, depending on the volume, increasing or decreasing the rate of release of volatile substances; and the secondary one - to the flame torch, to regulate the completeness of combustion of volatile substances, i.e. the length of the flames. An increase in the intensity of the secondary flow leads to a reduction in the length of the latter until it disappears, but the burning rate of firewood does not slow down. However, the firepower of a wood fire is actually not as great as it seems. It is capable of heating the walls of the firebox of a metal furnace no higher than 300-400°C.
Burning coals ensures that the metal firebox is heated red-hot - this is the most exothermic stage. The heat release effect increases with increasing supply of primary air (passing through the layer). Secondary air on at this stage not needed. Coals will burn out faster if you feed raw lumps of wood into the firebox: a gasification reaction of the coal with water vapor will occur. If the firewood is damp, then the combustion and smoldering stages occur almost simultaneously.
Types of fuel chambers and the process of burning wood
In the simplest fireplace-type stove with a solid hearth, the combustion process takes place with excess air, since the area of the open portal is usually 8-15 times larger than the cross-sectional area of the chimney. Due to the fact that large volumes of sucked air do not allow the fireplace pipe to heat above 60-80°C, the draft in them is significantly less than in stoves with a door (250-400°C).
If the fireplace insert is equipped with a door and a vent with a damper, then its efficiency will significantly increase. However, this design has a serious drawback - excessive smoke in the chamber, when opened, smoke rushes out. You can reduce smoke by moving the pipe as far forward as possible, but then it will block the top of the stove, which is used to heat water or stones. A compromise solution in this case could be an inclined shelf with the pipe located at the rear. The shelf will create maximum draft right next to the door, when opened, the upward flow will suck in the smoke, preventing it from escaping. This design is good for long-term burning, because... the air flows along the floor, falling under the firewood, and in the smoke circulation area it mixes well with volatile substances, ensuring the completeness of their combustion. 
To emphasize flaming combustion, secondary air is introduced into the flow of volatile substances. The implementation of this firewood burning mode is also helped by designs with a grate. They are good, first of all, because they provide oxygen supply to any area of the layer. However, a large amount of incoming air reduces the temperature of the walls of the smoke channel, and, consequently, draft and convective heat transfer. This phenomenon can be minimized by covering the periphery of the grate with a bottom, leaving the purge area only in the center.
Any grate is suitable for burning wood. If necessary, you can make them yourself from reinforcement or rod. But for burning coal you will need cast iron grates, the cross-sectional shape of which is close to triangular. This shape prevents slag from clogging the cracks between the grates. The grate bars should be placed along the firebox so that you can poke the coal with a poker. Cast iron grates are available for both coal and firewood. The latter have thinner grates and narrower gaps between them.
Grate furnaces are capable of developing greater power, but keep them from acceleration not easy. When the air supply coefficient is equal to one, the walls of the furnace are heated to red-hot temperatures, and the firewood begins to become increasingly gasified. The flame becomes so large that it enters the pipe and in this case it is necessary to increase the air supply, which in turn causes even greater gasification and heating. The stove will calm down on its own only after volatile substances have left the wood stack. The combustion of coals can then be easily adjusted. 
It is important to understand that the main reason furnace acceleration acceleration is achieved by metal walls heated to a high temperature, which no longer take away the heat from the firewood, while the latter begin to heat themselves. You can prevent the furnace from accelerating if you keep the pipe damper only half open when firing, and when characteristic gas pops begin to be heard from the firebox, open the firebox door slightly and at the same time open the pipe completely. Due to the sudden appearance of excess air, the walls of the stove will begin to cool, and when they stop glowing, it will be possible to close the firebox door and the air intake. The chimney is closed halfway again. This will cause the oven to smoothly switch to smoldering mode.
An important point that influences the acceleration of the stove is the portion of the installed firewood. To reduce the likelihood of runaway conditions, firewood should be added in small portions of 1 to 3 kg at a time. Moreover, the larger the diameter of the log, the greater the mass of the bookmark can be. By adjusting the air supply, you should try to prevent the walls from overheating. Overclocking the furnace is dangerous, first of all, because it can lead to warping or burning of the metal parts of the furnace.
First of all, the lower part of the walls of the firebox suffers from acceleration. If a metal stove accelerates over and over again, then the walls can be protected from the inside with refractory bricks to a height of 20-30 cm. It would be a mistake to line the walls from the outside, because... this will lead to even stronger heating of the metal. The problem of acceleration is completely eliminated by the water jacket - the boiler. However, if we talk about sauna stoves, then this solution is suitable not for saunas, but for hammam.
Burnouts through the firebox or hidden cracks are really dangerous during spontaneous acceleration of a metal stove. If during normal combustion mode they work as air intake holes, then in acceleration mode they will become “nozzles” through which burning volatile substances will burst out.
Features of combustion of solid fuels
Combustible gases and tar vapors (the so-called volatiles), released during the thermal decomposition of natural solid fuel during its heating, mixing with the oxidizer (air), at high temperatures burn quite intensively, like ordinary gaseous fuel. For this reason, burning fuels with a high yield of volatiles (firewood, peat, shale) does not cause difficulties, unless, of course, the ballast content in them (humidity plus ash content) is not so high as to become an obstacle to obtaining the temperature required for combustion.
Combustion time of fuels with medium (brown and coals) and small (lean coals and anthracites) yield of volatiles is practically determined by the reaction rate on the surface of the coke residue formed after the release of volatiles. The combustion of this residue also ensures the release of the main amount of heat.
Reaction occurring at the interface between two phases(in this case on the surface of a coke piece) called heterogeneous. It consists of at least two sequential processes: the diffusion of oxygen to the surface and its chemical reaction with the fuel (almost pure carbon remaining after the release of volatiles) at the surface. Increasing according to Arrhenius' law, the rate of a chemical reaction at high temperature becomes so great that all the oxygen supplied to the surface immediately reacts. As a result, the burning rate turns out to depend only on the intensity of oxygen delivery to the surface of the burning particle through mass transfer and diffusion. It is practically no longer affected by both the process temperature and the reaction properties of the coke residue. This mode of heterogeneous reaction is usually called diffusion. Combustion can be intensified in this mode only by intensifying the supply of the reagent to the surface of the fuel particle. This is achieved using different methods in different fireboxes.
Layer fireboxes. Solid fuel, loaded in a layer of a certain thickness onto a distribution grid, is ignited and blown (most often from bottom to top) with air (Fig. 28, a). Filtering between pieces of fuel, it loses oxygen and is enriched with oxides (CO 2, CO) of carbon due to the combustion of coal, the reduction of water vapor and carbon dioxide by coal.
Rice. 28. Schemes for organizing combustion processes:
A- in a dense layer; b - in a dusty state; _V - in a cyclone furnace;
G - in a fluidized bed; IN- air; T, V - fuel, air; ZhSh - liquid slag
The zone within which oxygen almost completely disappears is called oxygen zone; its height is two to three times the diameter of the fuel pieces. The gases leaving it contain not only CO 2, H 2 O and N 2, but also flammable gases CO and H 2, formed both due to the reduction of CO 2 and H 2 O by coal, and from volatiles released from coal. If the height of the layer is greater than the oxygen zone, then the oxygen zone is followed by a reduction zone, in which only the reactions CO 2 + C = 2CO and H 2 O + C = CO + H 2 occur. As a result, the concentration of flammable gases escaping from the layer increases as its height increases.
In layered fireboxes, they try to keep the layer height equal to or greater than the height of the oxygen zone. To afterburn the products of incomplete combustion (H 2 , CO) leaving the layer, as well as to afterburn the dust carried out from it, additional air is supplied to the combustion volume above the layer.
The amount of burned fuel is proportional to the amount of supplied air, however, an increase in air speed above a certain limit violates the stability of the dense layer, since air breaking through the layer in certain places forms craters. Since polydisperse fuel is always loaded into the layer, the removal of fines increases. The larger the particles, the faster air can be blown through the layer without compromising its stability. If we take for rough estimates the heat of “combustion” of 1 m 3 of air under normal conditions at α in = 1 equal to 3.8 MJ and understand it as w n air flow per unit grate area (m/s) reduced to normal conditions, then the thermal voltage of the combustion mirror (MW/m 2) will be
q R = 3.8W n / α in(105)
Combustion devices for layer combustion are classified based on the method of supplying, moving and screwing the fuel layer on the grate. In non-mechanized furnaces, in which all three operations are carried out manually, no more than 300 - 400 kg/h of coal can be burned. The most widely used in industry are fully mechanized layer fireboxes with pneumomechanical throwers and a chain return grid (Fig. 29). Their feature is fuel combustion on a grate continuously moving at a speed of 1-15 m/h, designed in the form of a conveyor belt web driven by an electric motor. The grate web consists of individual grate elements mounted on endless hinge chains driven by “stars”. The air required for combustion is supplied under the grate through the gaps between the grate elements.
Rice. 29. Scheme of a firebox with a pneumomechanical thrower and a chain return grid:
1 - grate cloth; 2 - drive sprockets; 3 - layer of fuel and slag; 4 – 5 - caster rotor; 6 - belt feeder; 7 - fuel bunker; 8 - combustion volume; 9 - screen pipes; 10 - 11 - furnace lining; 12 - back seal; 13 - windows for air supply under the layer
Flare furnaces. In the last century, only coal that did not contain fines (usually a fraction of 6 - 25 mm) was used for combustion in layered furnaces (and there were no others then). The fraction smaller than 6 mm - staub (from the German staub - dust) was waste. At the beginning of this century, a pulverized method was developed for its combustion, in which coals were crushed to 0.1 mm, and difficult-to-burn anthracites were crushed even finer. Such dust particles are carried away by the gas flow, the relative speed between them is very small. But their combustion time is extremely short - seconds and fractions of seconds. For this reason, with a vertical gas velocity of less than 10 m/s and a sufficient height of the furnace (tens of meters in modern boilers), the dust has time to completely burn on the fly as it moves along with the gas from the burner to the outlet of the furnace.
This principle forms the basis of torch (chamber) fireboxes, into which finely ground combustible dust is blown through burners along with the air necessary for combustion (see Fig. 28, b ) similar to how gaseous or liquid fuels are burned. Moreover, chamber fireboxes are suitable for burning any fuel, which is their big advantage over layer fireboxes. The second advantage is the ability to create a firebox for almost any arbitrary power. For this reason, chamber furnaces now occupy a dominant position in the energy sector. At the same time, dust cannot be burned stably in small furnaces, especially under variable operating conditions; therefore, pulverized coal furnaces with a thermal power of less than 20 MW are not made.
The fuel is crushed in milling devices and blown into the combustion chamber through pulverized coal burners. The transporting air blown in along with the dust is usually called primary air.
During chamber combustion of solid fuels in the form of dust, volatile substances, released during the heating process, burn in a torch as gaseous fuel, which helps to heat the solid particles to the ignition temperature and facilitates stabilization of the torch. The amount of primary air must be sufficient to burn volatiles. It ranges from 15 - 25% of the total amount of air for coals with a low yield of volatiles (for example, anthracite) to 20 - 55% for fuels with a high yield (brown coals). The rest of the air necessary for combustion (it is called secondary) is supplied to the firebox separately and mixed with dust during the combustion process.
In order for dust to ignite, it must first be heated to a high enough temperature. Along with it, naturally, it is necessary to heat the air transporting it (i.e., the primary) air. This can be done only by mixing hot combustion products into the flow of dust suspension.
Good organization of combustion of solid fuels (especially difficult-to-burn, with low volatile yield) is ensured by the use of so-called snail burners (Fig. 30).
Rice. 30. Direct-flow volute burner for solid pulverized fuel: IN- air; T, V - fuel, air
Coal dust with primary air is fed into them through a central pipe and, thanks to the presence of a divider, exits into the furnace in the form of a thin annular jet. Secondary air is supplied through the “snail”, strongly swirls in it and, exiting into the firebox, creates a powerful turbulent swirling torch, which ensures the suction of large quantities of hot gases from the core of the torch to the mouth of the burner. This accelerates the heating of the fuel mixture with primary air and its ignition, i.e., it creates good flame stabilization. The secondary air mixes well with the already ignited dust due to its strong turbulization. The largest dust particles burn out during their flight in the gas flow within the combustion volume.
When flaring coal dust, at any given time there is an insignificant supply of fuel in the furnace - no more than a few tens of kilograms. This makes the flare process very sensitive to changes in fuel and air consumption and, if extremely important, allows almost instantaneous changes in the furnace productivity, as when burning fuel oil or gas. At the same time, this increases the requirements for the reliability of supplying the furnace with dust, since the slightest (a few seconds!) interruption will lead to the extinguishing of the torch, which is associated with the danger of an explosion when the supply of dust is resumed. For this reason, pulverized coal furnaces usually have several burners installed.
During pulverized combustion of fuels, high temperatures (up to 1400-1500 °C) develop in the torch core, located near the burner mouth, at which the ash becomes liquid or doughy. The adhesion of this ash to the walls of the furnace can lead to their overgrowing with slag. For this reason, combustion of pulverized fuel is most often used in boilers where the furnace walls are closed with water-cooled pipes (screens), near which the gas is cooled and the ash particles suspended in it have time to harden before contacting the wall. Powdered combustion can also be used in furnaces with liquid slag removal, in which the walls are covered with a thin film of liquid slag and molten ash particles flow down in this film.
The volumetric heat stress in pulverized coal furnaces is usually 150-175 kW/m 3 , increasing in small furnaces to 250 kW/m 3 . With good mixing of air and fuel, it is accepted α in=1.2÷1.25; q fur= 0.5÷6% (large numbers - when burning anthracite in small fireboxes); q chemical= 0 ÷1%.
In chamber furnaces, after additional grinding, it is possible to burn coal waste generated during their enrichment at coke plants (industrial product), coke screenings and even finer coke sludge.
Cyclonic furnaces. A specific combustion method is carried out in cyclone furnaces. They use fairly small particles of coal (usually finer than 5 mm), and the air required for combustion is supplied at enormous speeds (up to 100 m/s) tangentially to the cyclone generatrix. A powerful vortex is created in the furnace, drawing particles into a circulation movement in which they are intensively blown by a flow. As a result of intense combustion in the furnace, temperatures close to adiabatic (up to 2000 °C) develop. Coal ash melts, liquid slag flows down the walls. For a number of reasons, the use of such furnaces in the energy sector was abandoned, and now they are used as technological ones - for burning sulfur to produce SO 2 in the production of H 2 SO 4, roasting ores, etc. Sometimes fire neutralization is carried out in cyclone furnaces Wastewater, i.e., burning out the harmful substances 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, air nitrogen begins to noticeably oxidize according to the reaction N 2 + O 2 = 2NO. A certain amount of NO is also formed from nitrogen contained in the fuel. Nitrogen oxide released along with flue gases into the atmosphere is further oxidized to highly toxic dioxide NO 2. In the USSR, the maximum permissible concentration of NO 2 (MPC), safe for human health, in the air of populated areas is 0.085 mg/m 3 . To ensure this, large thermal power plants have to build tall chimneys that disperse flue gases over the largest possible area. At the same time, 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 quantity harmful emissions per unit of heat released during fuel combustion. For example, in the USA, large enterprises are allowed to emit 28 mg of nitrogen oxides per 1 MJ of combustion heat. In the USSR, emission standards for different fuels range from 125 to 480 mg/m3.
When burning fuels containing sulfur, toxic SO 2 is formed, the effect of which on humans is also cumulative with the effect of NO 2.
These emissions cause the formation of photochemical smog and acid rain, which have a harmful effect not only on people and animals, but also on vegetation. In Western Europe, for example, such rains kill a significant part of coniferous forests.
If there is not enough calcium and magnesium oxide in the fuel ash to bind all SO 2 (usually a two- or three-fold excess is needed compared to the stoichiometry of the reaction), limestone CaCO 3 is added to the fuel. Limestone at temperatures of 850-950 °C intensively decomposes into CaO and CO 2, but gypsum CaSO 4 does not decompose, i.e. the reaction does not occur from right to left. However, toxic SO 2 is bound to harmless, practically 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: coal preparation tailings, coal mining dumps, numerous wastes from the pulp and paper industry and other sectors of the national economy. It is paradoxical, for example, that the “rock”, which is piled up in huge waste heaps near coal mines, often spontaneously ignites and pollutes the surrounding space with smoke and dust for a long time, but it cannot be burned either in layer or chamber furnaces due to the high ash content. In layered fireboxes, ash, sintered during combustion, prevents the penetration of oxygen to the fuel particles; in chamber fireboxes, it is not possible to obtain the high temperature required for stable combustion.
The urgent and extreme importance of developing waste-free technologies that has arisen for humanity has raised the question of creating combustion devices for burning such materials. They became fireboxes with a fluidized bed.
Fluidized (or boiling) is usually called a layer of fine-grained material blown from bottom to top with gas at a speed exceeding the stability limit of the dense layer, but not sufficient to remove particles from the layer. The intense circulation of particles in the limited volume of the chamber creates the impression of a rapidly boiling liquid, which explains the origin of the name.
A dense layer of particles physically blown from below loses stability because the resistance to the gas filtering through it becomes equal to the weight of the column of material per unit area of the supporting grid. Since aerodynamic drag is the force with which gas acts on particles (and, accordingly, according to Newton’s third law, particles act on gas), then if the resistance and weight of the layer are equal, the particles (if we consider the ideal case) rest not on the lattice, but on the gas.
The average particle size in fluidized bed furnaces is usually 2-3 mm. They correspond to the operating speed of fluidization (it is taken 2-3 times higher than w to) 1.5 ÷ 4 m/s. This determines according to the area of the gas distribution grid for a given thermal power of the firebox. Volume thermal stress q v taken approximately the same as for layer fireboxes.
The simplest firebox with a fluidized bed (Fig. 31) is in many ways reminiscent of a layer firebox and has many common structural elements with it. The fundamental difference between them is that intensive mixing of particles ensures constant temperature throughout the entire volume of the fluidized bed.
Rice. 31. Diagram of a fluidized bed furnace: 1 - unloading of ash; 2 - air supply under the layer; 3 - fluidized bed of ash and fuel; 4 - air supply to the caster; 5 - caster rotor; 6 - belt feeder; 7 - fuel bunker; 8 - combustion volume; 9 - screen pipes; 10 - sharp blast and entrainment return; 11- furnace lining; 12 - heat-receiving tubes in a fluidized bed; IN - water; P- steam.
Maintaining the temperature of the fluidized bed within the required limits (850 - 950 °C) is ensured in two different ways. In small industrial furnaces burning waste or cheap fuel, significantly more air is supplied to the bed than is essential for complete combustion, setting α in ≥ 2.
With the same amount of heat released, the temperature of the gases decreases as the α in, for the same heat is spent on heating a large number of gases.
In large energy units, this method of reducing the combustion temperature is uneconomical, because the “excess” air leaving the unit also carries away the heat spent on heating it (losses with exhaust gases increase - see below). For this reason, pipes are placed in fluidized bed furnaces of large boiler units. 9 and 12 s the working fluid (water or steam) circulating in them, which receives an extremely important 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 compared to a traditional one. Fuel burns stably when its content in the fluidized bed is 1% or less; the remaining 99% With unnecessary - ash. Even under such unfavorable conditions, intensive mixing does not allow ash particles to block combustibles from oxygen access (unlike a dense layer). In this case, the concentration of combustibles turns out to be the same throughout the entire volume of the fluidized bed. To remove ash introduced with fuel, part of the bed material is continuously removed from it in the form of fine-grained slag - most often it is simply “drained” through holes in the hearth, since the fluidized bed is capable of flowing as a liquid.
Furnaces with circulating fluidized bed. Recently, second-generation furnaces with the so-called circulating fluidized bed have appeared. A cyclone is installed behind these fireboxes, in which all unburnt particles are captured and returned back to the firebox. However, the particles become “locked” in the furnace-cyclone-furnace system until they burn completely. These fireboxes are highly economical, not inferior to the chamber combustion method, while maintaining all the environmental benefits.
Fluidized bed furnaces are widely used not only in the energy sector, but also in other industries, for example, for burning pyrites to produce SO 2, roasting of various ores and their concentrates (zinc, copper, nickel, gold-containing), etc. (From the point of view of combustion theory, roasting, for example, zinc ore according to the reaction 2ZnS + 3O 2 = 2ZnO + 2SO 2 is the combustion of this specific ʼʼfuelʼʼ, which proceeds, like all combustion reactions, with the release of large amounts of heat.) Fluidized bed furnaces are widely used, especially abroad, for fire neutralization (i.e. combustion) of various hazardous industrial wastes (solid, liquid and gaseous) - sludge from wastewater clarification, garbage, etc.
Topic 12. Furnaces of the chemical industry. Schematic diagram of a fuel furnace. Classification of furnaces in the chemical industry. Main types of furnaces, features of their design. Heat balance of furnaces
Chemical industry furnaces. Schematic diagram of a fuel furnace
An industrial furnace is an energy-technological unit designed for heat treatment of materials in order to give them the necessary properties. The source of heat in fuel (flame) furnaces are various types of carbon fuel (gas, fuel oil, etc.). Modern furnace installations are often large mechanized and automated units with high productivity.
The greatest importance for choosing the technological mode of the process is optimal temperature technological process, which is determined by thermodynamic and kinetic calculations of processes. Optimal temperature conditions process are the temperature conditions under which maximum productivity for the target product in a given furnace is ensured.
Typically, the operating temperature in the furnace is slightly lower than optimal; it depends on the fuel combustion conditions, heat exchange conditions, insulating properties and durability of the furnace lining, thermophysical properties of the processed material, etc.
Posted on ref.rf
factors. For example, for firing kilns the operating temperature is in the range between the temperature of active oxidation processes and the sintering temperature of the firing products. The thermal regime of a furnace is understood as a set of processes of heat inertia, heat of mass transfer and mechanics of media that ensure heat distribution in the technological process zone. The thermal regime of the technological process zone determines the thermal regime of the entire furnace.
The operating mode of furnaces is greatly influenced by the composition of the gas atmosphere in the furnace, which is necessary for the correct flow of the technological process. For oxidative processes, the gas environment in the furnace must contain oxygen, the amount of which ranges from 3 to 15% or more. It is typical for a restorative environment low content oxygen (up to 1-2%) and the presence of reducing gases (CO, H 2, etc.) 10-20% or more. The composition of the gas phase determines the conditions for fuel combustion in the furnace and depends on the amount of air supplied to combustion.
The movement of gases in a furnace has a significant impact on the technological process, combustion and heat transfer, and in furnaces, fluidized bed or vortex furnaces, the movement of gases is the main factor in stable operation. The forced movement of gases is carried out by smoke exhausters and fans.
The speed of the technological process is affected by the movement of the material undergoing heat treatment.
The furnace installation diagram includes the following elements: a combustion device for burning fuel and organizing heat exchange; furnace working space to perform the target technological mode; heat exchange devices for regenerating heat from flue gases (heating gas, air); recovery plants (recovery waste boilers) for using the heat of flue gases; traction and blowing devices (smoke exhausters, fans) to remove combustion of fuel and gaseous products of heat treatment of materials and supply air to burners, nozzles under the grate; cleaning devices (filters, etc.).
Features of combustion of solid fuels - concept and types. Classification and features of the category "Features of combustion of solid fuels" 2017, 2018.
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 under 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 solid and liquid fuels, the 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
 |
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 correct construction The technological method of burning solid fuels is to create 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 of fuel and air. For example, for a 300 MW steam generator, the fuel consumption - anthracite pellet - is 32 kg / s, and the air 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 physical and chemical properties 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, limiting the overall combustion rate. Consequently, 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) the 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 of a drop of completely evaporating liquid fuel occurs, being at rest in the 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 heat balance equation for 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 considerable 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 short 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 absolute value 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, causing total length 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 occurrence of oxidative reactions and in every possible way prevent thermal decomposition at high temperatures. To do this, all the air necessary for combustion should be supplied to the root of the torch. In this case, the presence of a large amount of 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. 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, coloring 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.
Ferrous metallurgy plants 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 of 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. Based on the type of filter element, they are divided into filters with a fibrous filter element, with a fabric filter element, granular filter element, metal-ceramic filter element, 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. The operating temperature of the purified gas is 250 0 C. The height of the electrodes is 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. To protect against corrosion, the internal surfaces of the scrubber are lined 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.
