3.2 Combustion gaseous fuel
Minimum temperature at which the mixture ignites is called the ignition temperature. The value of this temperature for various gases is not the same and depends on the thermophysical properties of combustible gases, the fuel content in the mixture, ignition conditions, heat removal conditions in each specific device, etc.
Combustible gas mixed with an oxidizing agent burns in a torch. There are two methods of gas combustion in a flare - kinetic and diffusion. In kinetic combustion, the gas is premixed with an oxidizer before combustion begins. The gas and oxidant are first fed into the mixing device of the burner. Combustion of the mixture is carried out outside the mixer. In this case, the combustion rate should not exceed the rate of combustion chemical reactions tburn = tchem.
Diffusion combustion occurs in the process of mixing combustible gas with air. The gas enters the working volume separately from the air. The speed of the process will be limited by the rate of mixing of gas with air thot = tphys.
Strong point diffusion method of combustion can be called its following properties:
High stability of the flame when changing thermal loads;
No flashover of flame;
Temperature uniformity along the length of the flame.
The disadvantages of this diffusion method of combustion include:
Probability of thermal decomposition of hydrocarbons;
The need for large furnace volumes;
Low intensity of combustion, the probability of incomplete combustion of gas.
The kinetic method of combustion is characterized by the fact that a gas-air mixture completely prepared inside the burner is supplied to the place of combustion, burning in a short torch with a blue transparent cone. Thus, the combustion of fuel is carried out on the surface of this cone, which is called the kinetic combustion front.
The advantages of this method of burning include:
Low probability of chemical underburning;
Short flame length;
Flame high temperature.
The need to stabilize the gas flame is a disadvantage of the kinetic method of gas combustion.
In addition, there is mixed (diffusion-kinetic) combustion. In this case, the gas is pre-mixed with a certain amount of air, then the resulting mixture enters the working volume, where the rest of the air is separately supplied.
In the furnaces of boiler units, kinetic and mixed methods of fuel combustion are mainly used.
Gas burners can be classified according to the following criteria:
a) along the length of the resulting torch into long-flame and short-flame ones;
b) according to the luminosity of the flame on a luminous or weakly luminous torch;
c) according to the calorific value of the burnt gas for burners for high-calorie and low-calorie gases;
d) by pressure in front of the burner for low- and high-pressure;
e) by the number of supply pipelines for one- and two-wire, etc.
One of the essential features is the method of mixing the combusted gas with the air necessary for combustion. On this basis, burners can be divided into the following three types:
1) Burners without pre-mixing gas with air. Gas and air, in the amount necessary for combustion, are supplied separately through the corresponding burner channels. The combustible mixture is formed in the torch in the process of turbulent mixing of gas and air after they exit the burner. An example of this type of burner is a tubular burner for low-calorie gases (Figure 1). The gas enters through the gas manifold and pipes connected to it, and the air through the opposite manifold into the annulus. Mixing occurs in the jet streams at the outlet of the pipes.
Figure 1 - Tubular burners for low-calorie gases
These burners are used for burning low-calorie gases in large quantities and in furnace technology, when it is necessary to have a stretched luminous flame with more uniform heat transfer along the length of the furnace working space.
2) Premix burners. Burners operating on the principle of kinetic combustion are used in cases where it is required to burn gas with a high thermal stress of the chamber volume and cross section of the order of (10-40) 103 kW / m 3 to (50-80) 103 kW / m 2 with minimal chemical underburning and with a short, faint flame. Pre-mixing is carried out in mixers, from which the prepared mixture enters the burner. This type includes tunnel and other types of burners of a homogeneous gas-air mixture obtained by preliminary mixing of gas with air in mixers of various designs.
In the thermal power industry, tunnel-type injection burners (Figure 2) are widely used, which provide automatic control of a constant ratio of gas and air consumption and allow the combustion of dusty gases. The burners are more heat-resistant and have a higher throughput at low resistances.
Figure 2 - Injection burners with ceramic tunnel
a - a single-wire burner with a single-channel tunnel; b - two-wire burner with a multi-channel tunnel
At high pressure of the combusted gas, single-wire burners (Figure 2a) with air ejection from the atmosphere are used, and when low-pressure gas is burned, two-wire burners (Figure 2b) with forced air supply are used. Single-wire injection burners are also widely used, in which the cylindrical mixing chamber ends not with a ceramic channel, but with a metal diffuser-confuser section.
3) Burners with partial mixing. These burners are equipped with short mixers in which partial mixing takes place. Mixing continues and ends in the flame during combustion.
Burners operating on this principle are widely used in the energy sector for burning natural gases.
In burners with partial mixing for low-calorie gases, in particular in the VNIIMT burner for blast-furnace gas (Figure 3), due to the comparable flow rates of gases and air, gases and air are supplied in alternating flat flows through channels into the prechamber, in the channels of which mixing and combustion begin. The mixing and combustion process continues and ends in the outlet channels. The cross section of the burner tunnel is determined by the amount of combustion products and their speed, taken in the range of 30-40 m/s.
Figure 3 - Blast furnace gas burner
In conclusion, a feature of the diffusion type of combustion associated with the presence of chemical incompleteness of combustion should be noted. In a diffusion laminar flame, the temperature reaches its maximum value in the combustion zone. The gas flowing out of the burner, before entering the combustion zone, is heated due to the heat propagating from the flame both by thermal conductivity and by diffusion of hot combustion products. Some gases, such as hydrogen and carbon monoxide, are heat-resistant and, when heated to temperatures of 2500-3000 o K, retain their molecular structure. The combustion of heat-resistant gases occurs in a transparent flame of pale blue color.
Gases containing hydrocarbon compounds are thermally unstable. In the case of combustion of these gases, heating in the reduction zone in the absence of oxygen causes them to decompose with the formation of soot and hydrogen. The decomposition of hydrocarbon-containing gases proceeds the more intensively, the higher the temperature, while at the same time the proportion of formed heavy, complex, difficult-to-burn hydrocarbons increases. For example, the decomposition of methane begins at a temperature of about 680-700°C. When heated without air access to 950°C, 26% of methane decomposes, and when heated to 1150°C, 90%.
The finely dispersed particles of soot and free carbon located in the flame, the dimensions of which are extremely small and amount to tenths of a micron, are heated due to the heat released during combustion, emit more or less bright light, causing the flame to glow.
Diffusion combustion of particles proceeds relatively slowly, as a result of which part of the free carbon and heavy hydrocarbons do not have time to burn out and leave the torch in the form of soot. The presence of carbon according to the equilibrium C+CO 2 ==2CO causes the formation of CO. The amount of carbon, heavy hydrocarbons and CO present in the combustion products determines the amount of chemical underburning.
3.2 Combustion of liquid fuels
The main liquid fuel used in thermal power engineering and industrial heat engineering is fuel oil. In installations of small power, a mixture of technical kerosene with resins is also used.
The method of combustion in the atomized state has received the greatest application. This method makes it possible to significantly accelerate its combustion and obtain high thermal stresses in the volumes of the combustion chambers due to an increase in the surface area of contact between the fuel and the oxidizer.
The combustion process of liquid fuel can be divided into the following stages:
1) heating and evaporation of fuel;
2) formation of a combustible mixture;
3) ignition of a combustible mixture from an external source (spark, hot spiral, etc.);
4) actual combustion of the mixture.
Determination of theoretical and actual air consumption for fuel combustion Combustible substances of fuel interact with air oxygen in a certain quantitative ratio. The oxygen consumption and the amount of resulting combustion products are calculated from the combustion equations, which are recorded for 1 kmole of each combustible component.
On boilers of powerful gas-oil power units without any measures when operating on gas, the concentration of NO x in the combustion products is in the range of 650-1050 mg/m 3 .
Technological methods for suppressing NO x are based on reducing the temperature and oxygen content in the active combustion zone, as well as creating zones with a reducing environment in the combustion chamber, where the products of incomplete combustion, interacting with the resulting nitrogen oxide, lead to the reduction of NO x to molecular nitrogen.
Based on experimental data and available practical experience The following main technological methods for reducing NOx in gas-oil boilers can be recommended for implementation:
Implementation of regimes with small values of α;
With staged combustion - lowered α on the verge of the appearance of chemical incompleteness of combustion;
Recirculation of flue gases through burners mixed with air;
Two-stage fuel combustion, which can be implemented in the design of the burners or in the furnace as a whole;
Three-stage fuel combustion (most appropriate for new boilers);
The use of special burners;
Water injection (reduces NO x by 20-25%, but leads to a decrease in boiler efficiency by approximately 0.8%);
Double-light screens (for new boilers);
Special combustion methods (e.g. fluidized bed);
Reducing the temperature of hot air;
Two-stage gas combustion achieved a 40% reduction in nitrogen oxides;
Simultaneous application of several technological methods makes it possible to reduce NOx emissions by 4-5, and sometimes even more, during gas combustion;
Since the NOx formed during the combustion of gas-oil fuel is mainly thermal nitrogen oxides, then, as a rule, in-furnace measures are aimed at reducing local temperatures and excess air.
Reducing excess air supplied for fuel combustion reduces the formation of both thermal and fuel NOx;
The maximum effect of reducing the NOx output is observed when flue gases are introduced together with air or through separate burner channels.
The most versatile method of NOx suppression for oil-fired boilers is the staged combustion method.
With a multi-tiered arrangement of burners, an effective means of reducing nitrogen oxide emissions is non-stoichiometric fuel combustion, which is implemented by organizing two combustion zones that differ in the oxidizer excess coefficient and temperature. In the first zone, the decrease in the formation of NOx occurs due to a decrease in the effective oxygen concentration in the combustion zone from α< 1 (α = 0,9÷0,95), а во второй зоне - за счет снижения температуры в ядре факела при сжигании топлива с α >1.0 (α \u003d 1.25 ÷ 1.35) while maintaining the total excess air at the level α "t \u003d 1.05.
When working on gas and the simultaneous use of flue gas recirculation, staged combustion and water injection into the furnace, it was possible to reduce the concentration of NOx in the combustion products from 1.05 to 0.18 g/m 3 (almost 6 times);
With the simultaneous use of staged combustion and redistribution of fuel and air over the tiers of burners, the concentration of NOx was reduced from 0.34 to 0.19 g / m 3 (1.8 times) when operating on gas and from 0.29 to 0.15 g / m 3 (1.9 times) when working on fuel oil;
When using flue gas recirculation in the amount of 20%, the concentration of NO x is reduced from 0.3 to 0.15 g/m 3 (2 times);
With the simultaneous use of staged gas combustion and flue gas recirculation, the concentration of NO x is reduced from 0.26 to 0.085 g/m 3 (3 times);
A positive feature of flameless burners is that the combustion products after them contain significantly less of the most harmful products of underburning - carbon oxides CO and nitrogen NO;
Preheating fuel oil up to 200÷250°C (compared to the normal mode of heating up to 130°C) reduces the output of NOx by 2-3 times.
The data given and the analysis of other materials show that the result achieved depends on the type of boiler, the initial level of NOx concentrations and the applied technological method of suppression. The best results are obtained by the simultaneous use of staged combustion and flue gas recirculation.
3.3 Solid fuel combustion
The combustion process consists of the following stages:
1) drying of the fuel and heating to the temperature of the beginning of the release of volatile substances;
2) ignition of volatile substances and their burnout;
3) heating the coke to ignition;
4) burnout of combustible substances from coke. These stages sometimes partially overlap one another.
Coal preparation and combustion technologies evolved during the 19th and 20th centuries as industrial consumption increased.
To date, many technologies for the preparation and combustion of coal are used. However, technologies that combine both high economic efficiency and high environmental friendliness are of practical interest.
These technologies include:
pseudoflare combustion of a dusty-coal-air mixture;
flaring of coal-water suspension;
combustion of coal in a fluidized bed;
low-temperature vortex combustion method;
Technology of stage-stage combustion of pulverized coal;
The technology of burning solid fuels in a high-temperature
circulating fluidized bed (CFB).
Let's consider these technologies in more detail.
3.3.1 Pseudo-flaring
The preparation of coal for this combustion method consists in dry grinding of the initial fuel with a moisture content of up to 21 percent in centrifugal mills to obtain homogeneous coal particles with an average size (dispersion) of 50-300 microns, forming coal dust.
The prepared dust enters a vibrating prefabricated booker-separator, where coal particles larger than 70 microns are taken back to the mill, and particles with a size of 50-70 microns or less are sucked in by a jet apparatus, pumped through heated (up to a temperature of +300 °C or more) air, while preparing a dry pulverized-coal-air mixture (PUVS).
Further, PUVS is supplied with air to fuel burners with a reduced output of nitrogen oxides.
With the help of burners, the mixture is sprayed into the furnace volume and ignited, forming a torch similar to fuel oil. For primary heating of coal particles and constant maintenance of the combustion process under the root part of the torch,
a small amount of liquid or gaseous fuel is jerkily supplied, forming a backlight.
Pseudo-torch combustion of coal has a homogeneous character, as a result of which the total area of contact between the fuel and the oxidizer is the maximum possible, and the coefficient of excess air for organizing the combustion of this type of fuel is minimal and does not exceed 1.3.
The considered technology of preparation and combustion of coal has shown its high environmental and economic efficiency in high-capacity boilers of UK TPPs, in particular Eggborough and Longannet, and in boiler plants. large thermal power plants France, USA, Canada and Taiwan.
Pseudo-flare coal combustion technology is constantly being improved at the MitsuiBabcock and Ratcliffe experimental centers located in Scotland and England.
3.3.2 Flaring
For the first time this method of coal combustion was proposed, developed and tested in Russia. The preparation of coal for combustion includes grinding the initial fuel in ball or drum mills until homogeneous coal particles no larger than 40-50 microns are obtained. After that, the resulting coal dust is mixed with fresh water and a coarse carbohydrate suspension (HC) is prepared, including 65-70 percent of coal and 30-35 percent water. Further, the hydrocarbon is supplied by screw pumps to the nozzles of the fuel burners, which spray the suspension into the boiler furnace in the form of a torch.
Both steam and air are used as atomizing medium. Ignition of the torch of the carbohydrate suspension is carried out by fuel oil supplied by the kindling nozzle, and upon reaching its stable homogeneous combustion, the fuel oil supply stops and the kindling nozzle is turned off. Subsequent combustion of hydrocarbons proceeds without illumination.
The coefficient of excess air when burning coal in this way is not more than 1.2. The technology of flaring combustion of a hydrocarbon suspension has confirmed its high environmental and economic efficiency in the power boilers of Belovskaya GRES and Novosibirsk CHPP-5 (Russia).
In addition, this coal combustion technology is used in the United States, Canada, Japan, Sweden, China and Italy. At present, China is actively promoting the presented technology of coal preparation and combustion in the global energy market.
3.3.3 Fluidized bed combustion
To implement the method of burning coal in a fluidized bed, the fuel is crushed to obtain particles no larger than 25-30 millimeters in size.
The crushed coal is fed by a conveyor into the bunker, from which it is fed to the region of the first blast zone of the grate with the help of a scraper feeder.
At the same time, part of the air (about 60 percent) heated in the air heater is blown by a blast fan into the blast zones under the grate through the gaps between the grates to form a high-temperature fluidized bed and organize the combustion process of coal.
The remaining air (about 40 percent) is fed into the secondary blast nozzles for afterburning the products of incomplete combustion and creating special aerodynamics in the combustion chamber, as well as for the operation of an air jet apparatus that returns combustible components for afterburning.
In the case of coal combustion in a fluidized bed, combustion is homogeneous-heterogeneous.
The complete release of energy in the fluidized bed is provided by all coal particles burning in it. The coefficient of excess air during combustion in a fluidized bed is 1.3. The highest efficiency of this method of combustion is achieved in boiler plants of medium and low power.
For the practical implementation of this method of coal combustion, it is necessary to equip the boilers with high-temperature fluidized bed furnaces.
3.3.4 Low-temperature swirl combustion
This method of coal combustion was first proposed, developed and implemented by Russian engineers and scientists.
When implementing this method, before being fed to combustion, coal is subjected to in-depth grinding to obtain coal particles with a maximum size of up to 10‑25 millimeters. Primary air is injected into the combustion zone from below along the furnace axis and twisted.
Coal particles are transported to the combustion zone by secondary, air flow, forming a coal-air mixture, which is fed into the vortex flow of primary air by burners located at an angle to the axis of the furnace.
The first ignition of the mixture is carried out with gas, diesel fuel or fuel oil using a kindling nozzle, then the process of combustion of coal particles proceeds in the form of a turbulent flame without illumination. In the boiler furnace, two combustion zones are organized, spaced apart in height: vortex and direct-flow.
The vortex zone is the main one and occupies the lower part of the internal volume of the furnace from the mouth of the cold funnel to the burners. The direct-flow combustion zone is located above the vortex zone.
In the lower volume of the furnace (vortex zone) rotational movement of the gas flow with a horizontal axis of rotation is organized. Burning coal particles and hot flue gases circulate in the vortex zone and are removed from it to the area of the burners, through which a new, fresh portion of the air-fuel mixture is supplied to the furnace.
Mixing with hot particles and gases, a new portion of ground coal quickly warms up and ignites, ensuring stable combustion in the furnace.
Fuel combustion is evenly distributed throughout the entire volume of the furnace and does not depend on changes in the loads on the boiler.
Such combustion of coal reduces the maximum temperature in the core of the torch and equalizes the temperature field throughout the combustion volume.
The coefficient of excess air in the specified technology of coal combustion is not more than 1.3. The technological process of preparation and low-temperature swirl combustion of coal has been used for a long time in power boilers of medium and high power at power facilities in Russia, for example, at Irkutsk CHPP-10 and Ust-Ilimsk CHPP.
The release of volatile substances for various fuels begins at different temperatures: for peat at 550-660 0K, for brown coal at 690-710 0K, for lean coal and anthracite at 1050-1070 0K.
Furnace devices of boilers can be layered - for burning lumpy fuel and chamber - for burning gaseous, liquid and solid pulverized fuels. Some of the options for organizing furnace processes are shown in Figure 4. Layer furnaces come with a dense and fluidized bed, chamber furnaces are divided into flare and cyclone.
Figure 4 - Schemes of organization of furnace processes
When burning in a dense layer, the combustion air passes through the layer without violating its stability, i.e. the force of gravity of the fuel particles is greater than the dynamic pressure of the air.
During combustion in a fluidized bed, due to the increased air velocity, the stability of the particles in the bed is disturbed, they go into the state of "boiling", i.e. go into a suspended state. In this case, intensive mixing of fuel and oxidizer occurs, which contributes to the intensification of the combustion process.
During flaring, the fuel burns in the volume of the combustion chamber, for which solid fuel particles must have a size of up to 100 microns.
During cyclone combustion, fuel particles are thrown onto the walls of the combustion chamber under the influence of centrifugal forces and, being in a swirling flow in the high temperature zone, completely burn out. Larger particle sizes are allowed than in flaring. The mineral component of the fuel in the form of liquid slag is continuously removed from the cyclone furnace.
3.3.5 Technology of stage-stage combustion of pulverized coal
The technology of stage-stage combustion of pulverized coal using low-emission direct-flow burners ensures the achievement of extremely low emissions of nitrogen oxides. This technology and design of the burner are designed and recommended for pulverized combustion of hard and brown coals in boilers of thermal power plants and large boiler houses. The new technology allows:
Reduce NOx emissions to 350-400 mg/Nm3%;
Ensure high efficiency and combustion stability with low CO emission;
Reduce slagging and corrosion of furnace screens.
The three-stage combustion system is one of the directions in the development of low-emission combustion technology. The essence of the system is to organize three zones in the furnace space. In the lower zone, 70..85% of all fuel is burned with an excess of air close to one or less. Above this zone, the remaining part of the fuel (15 ... 30%) is supplied to the furnace with an excess of air significantly below unity. Even higher, in the third zone, the remaining part of the air (15 ... 25%) is supplied to the furnace in order to afterburn the products of chemical and mechanical underburning formed in the previous zones.
The proposed scheme of three-stage combustion with gaseous recovery fuel ensures that NOx emissions are below 300 mg/Nm3, which is 2 times lower than with conventional combustion of the same coals.
Figure 5 - Three-stage combustion system
3.3.6 Technology of combustion of solid fuels in a high-temperature circulating fluidized bed (HTCF).
The only technology for today that allows efficient combustion of low-grade solid fuels is the technology of the so-called. fluidized bed, when the particles of coal are in suspension, which ensures their rapid and complete combustion.
Currently, the main technology for burning low-grade and/or fine-grained coals in steam and hot water boilers of small and medium power (up to 35 MW) in Russian Federation Recognized as one of the most cost-effective fluidized bed technologies - high-temperature circulating fluidized bed technology (HRCF), which reduces to a reasonable minimum the amount of equipment and cost of work, while maintaining all the advantages of the "classic" fluidized bed.
The VCCF technology is one of the modifications of the advanced method of fuel combustion in a fluidized bed and retains all its main advantages, namely:
The ability to burn almost any brand of coal, including screenings and fines;
Low level of harmful emissions;
Significantly higher efficiency in comparison with the actual efficiency. layer boilers on similar fuel;
High maneuverability (30–100% of nominal capacity).
In addition, the VCCF technology, in comparison with the "classical" low-temperature fluidized bed (LTF), has a number of additional advantages, especially in the reconstruction of existing boiler houses that are not very suitable for installing overall equipment - additional systems for supplying and removing inert (sand) from the furnace and not always having the ability to use gas or fuel oil to kindle boilers.
These additional benefits of VCKS include the following factors:
The formation of a fluidized bed does not require a special inert material, the layer is formed from particles of coal, coke and ash;
The absence of an inert backfill can significantly reduce the working height of the layer, so the use of a high-pressure fan is not required;
Instead of the fixed air-distributing grate characteristic of the “classic” fluidized bed, a movable inclined conveyor grate is used, assembled from standard grates, one of the functions of which is to transport slag to the ash-ash removal channel (SHZU);
Most of the fuel ash is discharged from the grate along with the slag due to the effect of ash agglomeration in the VCFG (the so-called Godel effect), which sharply reduces the likelihood of slagging of the boiler heating surfaces and reduces the load on the ash collecting equipment, i.e. provides a sharp reduction in solid emissions into the atmosphere;
Preliminary heating of the bed is not required, ignition of the VCKS boiler with a capacity of up to 35 MW can be carried out without the use of starting gas-oil burners and reserve fuel, i.e. similarly to ignition of a conventional layered boiler - from a fire;
The circulation of the layer material is ensured without the use of large-sized "hot cyclones" with water cooling;
Restrictions on the fractional composition of the fuel are not so high, the presence of pieces up to 30 mm is allowed;
Depending on the layout of the boiler, the VCKS grate can be installed under the boiler both with an inclination towards the front screen (forward running of the grate) and with an inclination towards the rear screen (reverse running of the grate);
The operation and maintenance of VCKS furnaces as a whole is not too different from the operation and maintenance of conventional layer furnaces, which contributes to the rapid mastering of new technology by the boiler house personnel.
When reconstructing the boiler at the VCKS, it is possible to increase its nominal load by 20–40%, depending on the type and quality of the fuel being burned.
K.P.D. of the boiler after reconstruction at the VCKS, as a rule, it increases by 10-15% (up to 85-87%) and more compared to the actual efficiency. boiler before reconstruction, and the level of harmful emissions is reduced at least 1.5 - 2 times.
Figure 6 - Schematic view of a DKVr boiler with a VCKS furnace
The reduction of harmful emissions into the atmosphere was achieved mainly by changing the structure of the fuel burned and the introduction of technological methods and regime measures at thermal power plants.
The amount of solids emitted into the atmosphere is determined by the ash content of the fuel, the completeness of combustion of the combustible mass, and the depth of ash cleaning.
The reduction of SO2 in the flue gases of industrial thermal power is carried out in two ways:
1) preliminary removal of sulfur from the fuel;
2) flue gas cleaning during or after the combustion process.
When burning coal with an excess air ratio of 1.05-1.2, the degree of purification of flue gases from nitrogen oxides reaches 60-70%;
Reducing harmful emissions of sulfur oxides SO 2 in fluidized bed furnaces;
By reducing the excess air ratio from 1.18 to 1.04, NO x can be reduced from 325 mg/m 3 to 190 mg/m 3 ;
The technology of stage-stage combustion of pulverized coal using low-emission direct-flow burners ensures the achievement of extremely low emissions of nitrogen oxides.
The new technology allows:
· reduce NOx emissions to the level of 350-400 mg/Nm3%;
· provide high efficiency and combustion stability with low CO emission;
· to reduce slagging and corrosion of furnace screens.
Three-stage combustion with gaseous recovery fuel ensured NO x emissions below 300 mg/Nm³, which is 2 times lower than conventional combustion of the same coals;
The use of VCCF technology (high-temperature circulating fluidized bed) provides:
· reduction of ash emissions without the use of expensive and cumbersome gas cleaning devices (due to the return of fly ash);
· reduction of NOx emissions due to multi-stage combustion;
· when burning low-sulphurous coals, reduction of sulfur oxide emissions to an acceptable level without the use of special methods of desulphurization;
· when burning high-sulphurous coals, suppression of sulfur oxides in a simple and least costly way - by adding a limestone additive to the fuel.
The reduction of CO 2 emissions with the technology of incomplete gasification with the formation of semi-coke is approximately 35% compared to the traditional technology of fuel combustion. This effect is achieved for a set of carbon deposition in semi-coke.
The use of gasification technology allows in some cases to reduce the emission of major pollutants by 96% (for sulfur dioxide - by 96%, for nitrogen oxides - by 84%, for dust - by 83%) and reduce the social damage from their emissions by 96% in total. %.
To reduce emissions of sulfur oxides into the atmosphere during the combustion of low-grade coals, it is recommended to burn coal and biomass together, including in the form of biogranules.
6.1 Influence of fuel composition and combustion conditions on the environmental performance of the boiler plant
Anthropogenic pollution of the atmosphere has become global in recent decades. The sources of air pollution are thermal power engineering, industry, oil and gas processing, transport, Agriculture. Each of these sources, each branch of production is associated with emissions of certain substances. Modern energy is a large highly developed industry, closely related to all sectors of the economy.
The impact of energy on the biosphere is manifested at all stages of energy production: in the extraction and transportation of resources, in the production, transmission and consumption of energy.
For example, the extraction of coal is associated with a change in the landscape, with the formation of mines, quarries, dumps; coal transport - with losses, dispersion of solid particles into the soil and into the atmosphere. When fossil fuels are burned, oxides of carbon, sulfur, nitrogen, lead compounds, soot, hydrocarbons, including carcinogenic ones (for example, benz (a) pyrene C 20 H 12), and other substances in solid, liquid and gaseous states are formed. The transmission of electricity leads to the formation of powerful electromagnetic fields near power lines. The operation of power plants is inevitably associated with emissions of thermal energy.
In addition, large areas of land are withdrawn from use, especially during the construction of hydroelectric power plants.
The environmental impact of TPP thermal power plants depends on the fuel used. When solid fuels are burned, fly ash, particles of unburned fuel, sulfurous and sulfuric anhydrides, nitrogen oxides, and fluorine compounds enter the atmosphere. Ash contains various toxic compounds - arsenic, silicon dioxide, calcium oxide and others. The use of liquid fuels (fuel oil) excludes only ash from production waste. This eliminates the problem of ash dumps, which occupy large areas and are a source of constant air pollution in the area of the station. When burning natural gas, nitrogen oxides are a significant pollutant, but on average they are 20% lower than when burning solid fuels. This is due not only to the properties of the fuel itself, but also to the peculiarities of its combustion. Thus, the environmental damage from the harmful effects of thermal power plants on the environment in the case of using gas will be minimal in comparison with other types of fuel.
Due to the high level of industrial development, 93% of all gas emissions are concentrated in the Northern Hemisphere of the Earth. The main part of the products of combustion of all types of fuel (90%) is emitted over an area of about 3% of the planet's surface - in Europe, Japan and North America. Of the gaseous substances, carbon dioxide and carbon monoxide are emitted in the largest quantities, which are formed during the combustion of fuel (coal, oil, gas, automotive fuel, etc.). The most toxic compounds emitted into the atmosphere are sulfur dioxide and nitrogen oxides.
The annual world emission of these gases is more than 255 million tons. If one of the most toxic oxides - sulfur dioxide - were not processed by higher plants, then in 20 years all higher animals would die. The sources of sulfur dioxide and nitrogen oxides are coal-fired thermal power plants, industrial enterprises, transport. In the air, these gases react with water vapor, forming sulfuric and nitric acid. As a result, precipitation falls in some regions, the acidity of which is 10–1000 times higher than normal. Rain is considered acidic if it has a pH less than 5.6.
Air pollution has serious consequences. There is a threat to human health, the normal functioning of ecosystems. For the normal functioning and sustainability of ecosystems and the biosphere as a whole, certain loads on them should not be exceeded. In this regard, it is necessary to search for the most sensitive links in ecosystems, to find indicators corresponding to the most powerful factors, as well as the sources of such an impact. These activities are included in the system of environmental monitoring, which is understood as single system means and methods of continuous monitoring of the state of the environment and a system for predicting the results of anthropogenic impact on it. The tasks of monitoring include monitoring the state of the biosphere, assessing and forecasting the state of the environment, identifying factors and sources of anthropogenic impact, substantiating decisions on the rational use of natural resources, and regulating the process of nature management. The organization of monitoring should solve both local tasks of monitoring the state of individual ecosystems and tasks of a planetary order, i.e., provide for a system of global monitoring.
The thermal power industry is the leader in terms of total emissions of pollutants into the atmosphere. Its share in the total emissions of industrial pollutants from stationary sources reached 21.7% in 2009. In 2010, pollutant emissions amounted to 5.37 million tons, which is 2.3 million tons lower than the level of 1990. In 2005, pollutant emissions amounted to 3.9 million tons, which is lower than the level of 2004 by 56 thous. tons Maintaining a steady trend of reducing emissions is due to an increase in the share of natural gas in the structure of the fuel and energy balance (FEB) to 64%. In addition, the environmental culture of operating thermal power plants is being improved, technologies are being introduced at thermal power plants aimed at improving the efficiency of existing ash collecting plants. In order to provide a regulatory framework for reducing the impact on the atmosphere from power plants, GOST R 50831–95 “Installations of boiler houses. Technical equipment. General requirements”, which sets standards for specific emissions for newly commissioned boiler plants that meet international standards.
Large sources of environmental pollution are oil and gas fields and main pipelines. Pollution of soil, ground and surface waters with oil and its components, highly mineralized reservoir and waste waters, slags also occurs at the stage of preparation of oil and gas raw materials for processing. At the same time, a significant amount of oil components, oil gas and its combustion products enter the atmosphere.
Gas industry. Volumes of emissions of pollutants into the atmospheric air from stationary sources for 1995–2008. decreased by more than 3 times (excluding methane emissions). It should be noted that, despite the ongoing work to reduce air pollution, emissions of pollutants gas industry in 2007 amounted to more than 590 thousand tons. overhaul. The increase in the burden on the environment is mainly due to the growth of methane emissions, taking into account which emissions of pollutants in 2009 amounted to 1.83 million tons. Methane and carbon dioxide emissions in the gas industry occur at all stages of the technological process. The dominant influence is exerted by the gas transportation system, which accounts for 70% of all emissions.
Coal industry. Emissions of harmful substances into the atmosphere by the coal industry for the period 1995–2009 decreased by 1.5 times. Its share in industrial emissions is 4.8% (2007). In 2009, the total volume of emissions of pollutants into the atmospheric air amounted to 450 thousand tons
The use of coal-bed methane in power plants will reduce the cost of heat supply and improve the environmental situation in residential areas by eliminating coal combustion. Compared to other energy carriers, coal contains the largest amount of sulfur - 0.2-7.0%, fuel oil - 0.5-4.0%, diesel fuel - 0.3-0.9%, natural gas - an insignificant share.
In the context of a growing shortage of natural resources, an increase in the scale and number of man-made accidents and disasters, the most important direction in the development of the fuel and energy complex is to increase the efficiency of the use of fuel and energy resources, reduce the negative impact of the activities of the fuel and energy complex on the environment in order to prevent ecological disaster and creating conditions for the transition to energy saving.
TPPs operate on fossil fuels, which are relatively cheap coal and fuel oil. These fuels are irreplaceable natural resources. The main energy resources in the world today are coal (40%), oil (27%), gas (21%). However, these reserves, according to some estimates, will be enough for 270, 50 and 70 years, respectively, and this is provided that humanity will spend them at the same rate as today. Fuel combustion at thermal power plants is associated with the formation of combustion products containing fly ash, particles of unburned pulverized fuel, sulfur dioxide and sulfuric anhydride, nitrogen oxides and gaseous products of incomplete combustion, and when fuel oil is burned, in addition, vanadium compounds, sodium salts, coke and soot particles . The ashes of some fuels contain arsenic, free silicon dioxide, free calcium oxide, etc. The transfer from solid fuel to gas fuel leads to a significant increase in the cost of generated energy, not to mention the shortage of both. In addition, it will not solve the problem of air pollution. The conversion of installations to liquid fuels significantly reduces ash formation, but practically does not affect sulfur oxide emissions, since fuel oils used as fuel contain more than 2% sulfur. When burning gas, smoke emissions also contain sulfur oxide, and the content of nitrogen oxides is not less than when burning coal. Since there is not enough high-quality fuel, TPPs operate on low-grade fuel. During the combustion of such fuels, pollutants are formed, which are released into the atmosphere with smoke and enter the soil with ash. In addition to the fact that these emissions adversely affect the environment, the products of combustion cause acid precipitation and the greenhouse effect, which threatens us with droughts.
One of the influencing factors coal thermal power plants on the environment are emissions from fuel storage systems, its transportation, dust preparation and ash removal. During transportation and storage, not only dust pollution is possible, but also the release of fuel oxidation products. Ash and slag dumps require large areas that have not been used for a long time, and are centers of accumulation of heavy metals and increased radioactivity, which enter the biosphere by air or with water.
In addition, significant thermal pollution of water bodies occurs when warm water is discharged into them, which accompanies natural chain reactions: overgrowing of water bodies with algae, oxygen imbalance, which poses a threat to the life of the inhabitants of rivers and lakes.
Significant areas of land near reservoirs are experiencing flooding as a result of rising groundwater levels. These lands are classified as wetlands. In flat conditions, flooded lands can be 10% or more of the flooded. The destruction of lands and their ecosystems also occurs as a result of their destruction by water (abrasion) during the formation of the coastline. Abrasive cycles usually last for decades, resulting in the processing of large masses of soil, water pollution, siltation of reservoirs.
The main factors of the impact of thermal power plants on the hydrosphere are heat emissions, which can result in: a constant local increase in temperature in a reservoir; temporary rise in temperature; changes in freezing conditions, winter hydrological regime; changing flood conditions; change in the distribution of precipitation, evaporation, fog.
Thermal power plants with cooling water discharge from 4 to 7 kJ of heat for every 1 kWh of electricity generated. By sanitary standards thermal discharges should not increase the own temperature of the reservoir by more than 5° in winter and 3° in summer.
Sources of air pollution are industrial effluents and emissions of combustion products.
TPP wastewater includes the following waters: containing oil products, after washing the heating surfaces of steam boilers, discharged after chemical treatment plants, equipment conservation and washing, as well as hydroash removal systems. The amount of wastewater containing oil products does not depend on the capacity of the station and the type of equipment, although when using liquid fuel it is slightly higher than for solid fuel thermal power plants. At the same time, their quantity mainly depends on the quality of installation and operation of the power plant equipment. Improving the design of equipment, careful observance of the rules of its operation make it possible to reduce to a minimum the amount of wastewater oil products, and the use of various types of traps and settling tanks makes it possible to exclude their release into the environment. Pollutant impurities from power plant emissions affect the biosphere of the area where the enterprise is located, undergo various transformations and interactions, and are also deposited, washed out by atmospheric precipitation, and enter the soil and water bodies. In addition to the main components resulting from the combustion of fossil fuels (carbon dioxide and water), TPP emissions contain dust particles of various compositions, sulfur oxides, nitrogen oxides, fluorine compounds, metal oxides, and gaseous products of incomplete combustion of fuel. Their entry into the air environment causes great damage to both all the main components of the biosphere, and enterprises, urban facilities, transport and urban population. The presence of dust particles, sulfur oxides is due to the content of mineral impurities in the fuel, and the presence of nitrogen oxides is due to the partial oxidation of air nitrogen in a high-temperature flame. Nitrogen dioxide has the highest biological activity, which irritates the respiratory tract and the mucous membrane of the eye. Heavy metals also pose a great environmental hazard to humans. Getting into the body in large quantities, they can cause acute poisoning for a short time, and with chronic exposure to small doses for a long time, the carcinogenic effect of arsenic, chromium, nickel, etc. can manifest itself. In terms of lethal doses, the annual emissions of a TPP with a capacity of 1 million kW contain more than 100 million doses of aluminum and its compounds, 400 million doses of iron, and 1.5 million doses of magnesium. Emissions from coal-fired thermal power plants also contain oxides of silicon and aluminum. These abrasive materials can destroy lung tissue and cause diseases such as silicosis, which miners used to suffer from. Now cases of silicosis are registered in children living near coal thermal power plants. Along with an increase in carbon dioxide, there is a decrease in the proportion of oxygen in the atmosphere, which is consumed for fuel combustion at thermal power plants.
6.2 Maximum permissible concentrations of harmful emissions from boiler houses according to the requirements of SANPIN
The impact on the animal and plant world is exerted by atmospheric pollution with sulfur oxide (), which destroys plant chlorophyll, and can lead to damage to leaves and needles. The effect of carbon monoxide () on humans and animals is that, when combined with blood hemoglobin, it very quickly deprives the body of oxygen and leads to disruption of the nervous system. Nitrogen oxides reduce the transparency of the atmosphere and contribute to the formation of smog. Toxicity is distinguished by vanadium pentoxide (), which is part of the fuel oil ash. This substance causes irritation of the respiratory tract in humans and animals, circulatory and nervous system disorders, and metabolic disorders.
Benz (a) pyrene is a kind of carcinogen that can cause cancer. Therefore, the design and construction of power plants are carried out in compliance with the requirements for maximum permissible concentrations of the main emissions polluting the atmosphere with exhaust gases from enterprises in the atmospheric air at the level of human breathing (Table 2).
Table 2 - The maximum permissible concentration of the main emissions polluting the atmosphere with exhaust gases from thermal power plants in the atmospheric air at the level of human breathing
Given the enormous damage caused to both the environment and humans, sanitary legislation is industrially developed countries maximum allowable concentrations (MACs) of substances polluting the air, water bodies and soil have been established. For each country, MPC levels are different. United international standards have not been worked out to date. However, most countries (such as Germany, Great Britain, Denmark, Holland, Italy, Hungary, Poland, Russia, Norway, Finland, etc.) are striving everywhere to reduce harmful emissions and tighten requirements for enterprises polluting the environment.
MPC is a standard for the concentration of a chemical compound, which, when exposed daily for a long time to the human body, does not lead to any pathological changes in the state of human health, and also does not violate the biological optimum for humans. Thus, a harmful effect is understood to be such an impact that exceeds the MPC, and a harmful emission is the release of a substance in an amount exceeding the MPC. MPCs for harmful substances (i.e. substances that, when in contact with the human body, can lead to work injury, occupational diseases or deviations in the state of health, or a chemical that causes a violation in the growth, development or health of organisms, including in the chain of generations) are installed in the air of the working area, atmospheric air and in the water of water bodies.
MPC РЗ - the maximum permissible concentration of a harmful substance in the air of the working area, mg / m 3.
MPC MR - the maximum one-time concentration of a harmful substance in the air of populated areas, mg / m 3.
MPC SS - average daily maximum allowable concentration (i.e. the concentration of a pollutant in the air that does not have a direct or indirect harmful effect on a person during round-the-clock inhalation), mg/m 3 .
MPC B - the maximum permissible concentration of harmful substances in the water of reservoirs, mg / dm 3.
Most modern power plants are forced to work in conditions of background pollution created both by other enterprises and by the environment of the area of operation itself. At the same time, background air pollution is considered to be pollution without taking into account the emissions of the enterprise in question. Therefore, when studying emissions from a specific source, background pollution for each ingredient should be taken into account.
The maximum permissible concentration is recognized as such, which does not have a direct or indirect harmful and unpleasant effect on a person, does not reduce working capacity, does not affect his well-being or mood. The interaction of emissions with fog leads to the formation of a stable highly polluted fine cloud - smog, which is most dense near the surface of the earth. One of the impacts of thermal power plants on the atmosphere is the ever-increasing consumption of air required for fuel combustion. Some ways of solving the problems of modern energy. It must be said that the impact of thermal power plants on the environment differ significantly by type of fuel.
The most "clean" fuel for thermal power plants is gas, both natural and obtained during oil refining or in the process of methane fermentation of organic substances. The most "dirty" fuel is oil shale, peat, brown coal. When they are burned, most of the dust particles and sulfur oxides are formed. Although at present a significant share of energy is produced by relatively clean fuels (gas, oil), the trend towards a decrease in their share is natural. According to available forecasts, these energy carriers will lose their leading role already in the first quarter of the 21st century. Here it is appropriate to recall the statement of D. I. Mendeleev about the inadmissibility of using oil as a fuel: “Oil is not fuel - you can also heat banknotes.” The possibility of a significant increase in the global energy balance of coal use is not ruled out. According to available calculations, the coal reserves are such that they can meet the world's energy needs for 200-300 years. Possible coal production, taking into account the explored and forecast reserves, is estimated at more than 7 trillion tons. At the same time, more than 1/3 of the world's coal reserves are located in Russia. Therefore, it is reasonable to expect an increase in the share of coals or products of their processing (for example, gas) in energy production, and, consequently, in environmental pollution. Coals contain from 0.2 to tens of percent sulfur mainly in the form of pyrite, ferrous sulfate and gypsum. For sulfur compounds, there are two approaches to solving the problem of minimizing emissions into the atmosphere during the combustion of fossil fuels:
1) purification of sulfur compounds from fuel combustion products (flue gas desulfurization);
2) removal of sulfur from the fuel before its combustion.
To date, certain results have been achieved in both directions. Among the advantages of the first approach, one should mention its absolute efficiency - up to 90-95% of sulfur is removed - the possibility of using it practically regardless of the type of fuel. The disadvantages include large capital investments. Energy losses for thermal power plants associated with desulfurization are approximately 3-7%. The main advantage of the second way is that the cleaning is carried out regardless of the operating modes of the thermal power plant, while flue gas desulfurization plants sharply worsen economic indicators power plants due to the fact that most of the time they are forced to work in off-design mode. Fuel desulfurization plants can always be used in the nominal mode, storing the purified fuel.
The problem of reducing emissions of nitrogen oxides from TPPs has been seriously considered since the late 1960s. At present, some experience has already been accumulated on this issue. The following methods can be mentioned:
1) reduction of the excess air coefficient (this way it is possible to achieve a decrease in the content of nitrogen oxides by 25-30%, reducing the excess air coefficient from 1.15 - 1.20 to 1.03);
2) the destruction of oxides to non-toxic components.
To reduce the concentration of polluting compounds in the surface air layer, TPP boiler houses are equipped with high, up to 100-200 meters or more, chimneys. But this also leads to an increase in the area of their scattering. As a result, large industrial centers form polluted areas with a length of tens, and with a steady wind - hundreds of kilometers.
6.2.1 Impact of air pollution on human health
Impact of atmospheric air pollution on human health
Flue gases are the main source of pollution at TPPs. The content of harmful substances in them determines not only the state of the atmosphere, but in many respects the state of the soil and water basin, affects the life of flora and fauna and, of course, humans. It is through atmospheric emissions around the cities of Achinsk, Nazarovo, Kansk that areas of technogenic environmental change with a diameter of up to 20 ... 30 km have developed, where the structure of soils, vegetation, bio- and microcenoses is greatly disturbed. A particularly difficult situation has developed in the large industrial centers of Siberia. In Achinsk, for example, the alumina refinery alone emits into the atmosphere annually about 160,000 tons of dust, 22,000 tons of sulfur dioxide, and 14,500 tons of nitrogen oxides. The situation is similar in Novokuznetsk, Nazarovo, Prokopievsk, Kemerovo and a number of other cities.
Benz(a)pyrene.
Benz (a) pyrene is a chemical compound, a representative of the family of polycyclic hydrocarbons, a substance of the first hazard class.
It is formed during the combustion of hydrocarbon liquid, solid and gaseous fuels (to a lesser extent during the combustion of gaseous fuels).
In the environment, it accumulates mainly in soil, less in water. From the soil, it enters plant tissues and continues its movement further in the trophic chain, while at each stage the content of BP in natural objects increases by an order of magnitude.
Benz(a)pyrene is the most typical chemical environmental carcinogen, it is dangerous to humans even at low concentrations, since it has the property of bioaccumulation. Being chemically relatively stable, benzo(a)pyrene can migrate from one object to another for a long time. As a result, many environmental objects and processes that themselves do not have the ability to synthesize benzo(a)pyrene become its secondary sources. Benz(a)pyrene also has a mutagenic effect.
An international panel of experts has classified benzo(a)pyrene as one of the agents for which there is limited evidence of carcinogenicity in humans and strong evidence of carcinogenicity in animals. In experimental studies, benzo(a)pyrene has been tested on nine animal species, including monkeys. Benz (a) pyrene can enter the body through the skin, respiratory organs, digestive tract and through the placenta. With all these methods of exposure, it was possible to cause malignant tumors in animals.
If we take the air velocity as the determining parameter w in relation to the speed of movement of fuel particles v t, then according to this parameter, four fuel combustion technologies are distinguished.
1. In a dense filter layer(w in >> v T).
It is used only for lumpy solid fuel, which is distributed on the grate. The fuel layer is blown with air at a speed at which the layer stability is not disturbed and the combustion process has an oxygen and reduction zone.
The apparent thermal stress of the grate is Q R\u003d 1.1 ... 1.8 MW / m 2.
2. in fluidized or fluidized bed(w in > v T).
As the air speed increases, the dynamic head can reach and then exceed the gravitational force of the particles. The stability of the layer will be broken and random movement of particles will begin, which will rise above the grate, and then reciprocate up and down. The flow rate at which the layer stability is violated is called critical.
It can be increased up to the speed of the particles when they are carried out by the gas flow from the layer.
A significant part of the air passes through the fluidized bed in the form of “bubbles” (gas volumes) that strongly mix the fine-grained material of the bed; as a result, the combustion process along the height proceeds at almost a constant temperature, which ensures the completeness of fuel burnout.
The fluidized bed is characterized by an air speed of 0.5…4 m/s, a fuel particle size of 3…10 mm, and a layer height of not more than 0.3…0.5 m. Thermal stress of the furnace volume Q V\u003d 3.0 ... 3.5 MW / m 3.
A non-combustible aggregate is introduced into the fluidized bed: fine quartz sand, fireclay chips, etc.
The fuel concentration in the layer does not exceed 5%, which makes it possible to burn any fuel (solid, liquid, gaseous, including combustible waste). The non-combustible filler in the fluidized bed can be reactive with respect to harmful gases generated during combustion. The introduction of a filler (limestone, lime or dolomite) makes it possible to convert up to 95% of sulfur dioxide into a solid state.
3. In the air flow(w in ≈ v m) or flare once-through process. Fuel particles are suspended in the gas-air flow and begin to move along with it, burning up during movement within the furnace volume. The method is characterized by low intensity, extended combustion zone, sharp non-isothermal; requires a high temperature of the medium in the ignition zone and careful preparation of the fuel (spraying and pre-mixing with air). Thermal stress of the furnace volume Q V≈ 0.5 MW / m 3.
There are three methods of fuel combustion: layered, in which the fuel in the layer is blown with air and burned; flare, when the fuel-air mixture burns in a torch in suspension while moving through the combustion chamber, and vortex (cyclone), in which the fuel-air mixture circulates in a streamlined contour due to centrifugal forces. Torch and vortex methods can be combined into a chamber method.
Process stratified combustion of solid fuels occurs in a fixed or fluidized bed (fluidized). In a fixed layer (Fig. 2.6, A) pieces of fuel do not move relative to the grate, under which the air necessary for combustion is supplied. In a fluidized bed (Fig. 2.6, b) particles of solid fuel under the action of the velocity pressure of air intensively move one relative to the other. The flow velocity at which the stability of the layer is violated and the reciprocating motion of particles over the lattice begins is called critical. The fluidized bed exists within the velocity range from the beginning of fluidization to the mode of pneumatic transport.
Rice. 2.6. Fuel combustion schemes: A– in a fixed layer; b– in a fluidized bed; V– flare once-through process; G– vortex process; d– the structure of the fixed layer during fuel combustion and the change a, O 2 , SO, SO 2 and t according to the layer thickness: 1 - lattice; 2 - slag; 3 – burning coke;
4 - fuel; 5 - superficial flame
On fig. 2.6, d the structure of the fixed layer is shown. Fuel 4 poured onto the burning coke is heated. The released volatiles burn out, forming a superficial flame 5. The maximum temperature (1300 - 1500 °C) is observed in the area of combustion of coke particles 3. Two zones can be distinguished in the layer: oxidative, a > 1; restorative, a< 1.
In the oxidizing zone, the reaction products of fuel and oxidizer are both SO 2 and SO. As air is used, the formation rate SO 2 slows down, its maximum value is reached with an excess of air a = 1. In the reduction zone, due to an insufficient amount of oxygen (a< 1) начинается реакция между SO 2 and burning coke (carbon) to form SO. Concentration SO in combustion products increases, and SO 2 decreases. The length of the zones depending on the average size d to fuel particles is as follows: L 1 = (2 – 4) d to; L 2 = (4 – 6) d to. For zone lengths L 1 and L 2 (in the direction of their decrease) are affected by an increase in the content of volatile combustibles, a decrease in ash content A r, rising air temperature.
Since in zone 2, except for SO contained H 2 and CH 4 , the appearance of which is associated with the release of volatiles, then for their afterburning, part of the air is supplied through blow nozzles located above the layer.
In a fluidized bed, large fuel fractions are in suspension. The fluidized bed can be high temperature and low temperature. Low-temperature (800 - 900 °C) fuel combustion is achieved by placing the heating surface of the boiler in a fluidized bed. In contrast to the fixed bed, where the particle size of the fuel reaches 100 mm, the fluidized bed burns crushed coal with d to£25mm.
The layer contains 5 - 7% of the fuel (by volume). The heat transfer coefficient to the surfaces located in the layer is quite high and reaches 850 kJ/(m2×h×K). When burning low-ash fuels, to increase heat transfer, fillers are introduced into the layer in the form of inert granular materials: slag, sand, dolomite. Dolomite binds sulfur oxides
(up to 90%), which reduces the likelihood of low-temperature corrosion. The lower temperature level of gases in the fluidized bed helps to reduce the formation of nitrogen oxides during combustion, the release of which into the atmosphere pollutes the environment. In addition, slagging of the screens, i.e., sticking of the mineral part of the fuel on them, is excluded.
A characteristic feature of the circulating fluidized bed is the approach to the operation of the bed in the mode of pneumatic transport.
Chamber method of solid fuel combustion carried out mainly in powerful boilers. In chamber combustion, ground to a pulverized state and pre-dried solid fuel is fed with part of the air (primary) through the burners into the furnace. The rest of the air (secondary) is introduced into the combustion zone most often through the same burners or through special nozzles to ensure complete combustion of the fuel. In the furnace, pulverized fuel burns in suspension in a system of interacting gas-air flows moving in its volume. With greater grinding of the fuel, the area of the reacting surface increases significantly, and, consequently, the chemical reactions of combustion.
A characteristic of the grinding of solid fuel is the specific area F pl dust surface or the total surface area of dust particles weighing 1 kg (m 2 /kg). For spherical particles of the same (monodispersed) size, the value F pl is inversely proportional to the particle diameter.
In fact, the dust obtained during grinding has a polydisperse composition and a complex shape. To characterize the quality of grinding polydisperse dust, along with the specific surface area of the dust, the results of its sifting on sieves of various sizes are used. According to the sifting data, a grain (or grinding) characteristic of dust is built in the form of a dependence of the residues on the sieve on the size of the sieve meshes. R 90 and R 200 . Preliminary preparation of fuel and heating of air provide burnout of solid fuel in the furnace for a relatively short period of time (several seconds) of dusty air flows (torches) in its volume.
Technological methods of organizing combustion are characterized by a certain input of fuel and air into the furnace. In most pulverizing systems, the transport of fuel to the furnace is carried out by primary air, which is only a part of the total amount of air required for the combustion process. The supply of secondary air to the furnace and the organization of its interaction with the primary are carried out in the burner.
The chamber method, unlike the layer method, is also used for burning gaseous and liquid fuels. Gaseous fuel enters the combustion chamber through the burner, and liquid fuel enters the combustion chamber in pulverized form through nozzles.
Layer fireboxes
The fixed bed firebox can be manual, semi-mechanical or mechanical with a chain grate. Mechanical firebox called a layered furnace device in which all operations (fuel supply, slag removal) are performed by mechanisms. When servicing semi-mechanical furnaces, manual labor is used along with mechanisms. There are furnaces with a straight line (Fig. 2.7, A) and reverse (Fig. 2.7, b) the course of gratings 1, driven by sprockets 2. The fuel consumption supplied from the hopper 3 is regulated by the installation height of the gate 4 (see Fig. 2.7, A) or the speed of movement of dispensers 7 (Fig. 2.7, b). In gratings with a reverse stroke, fuel is supplied to the web by mechanical casters 8 (Fig. 2.7, b, c) or pneumatic (Fig. 2.7, G) type. Small fractions of fuel burn in suspension, and large fractions - in a layer on the grate, under which air is supplied 9. Heating, ignition and combustion of the fuel occur due to the heat transferred by radiation from the combustion products. Slag 6 with the help of a slag remover 5 (Fig. 2.7, A) or under the action of its own weight (Fig. 2.7, b) enters the slag bunker.
The structure of the burning layer is shown in fig. 2.7, A. Region III burning coke after zone II heating of incoming fuel (zone I) is located in the central part of the lattice. There is also a recovery area. IV. The uneven degree of fuel combustion along the length of the grate leads to the need for a sectional air supply. Most of the oxidant must be fed into the zone III, a smaller amount - to the end of the coke reaction zone and a very small amount - to the zone II preparation of fuel for combustion and zone V slag burning. This condition is met by a stepped distribution of excess air a 1 along the length of the grate. The supply of the same amount of air to all sections could lead to increased excess air at the end of the grate web, as a result of which it would not be enough to burn coke (curve a 1) in the zone III.
The main disadvantage of furnaces with chain grates is the increased heat loss from incomplete combustion of the fuel. The scope of such gratings is limited to boilers with steam output D= 10 kg/s and fuels with volatile \u003d 20% and reduced humidity.
Fluidized bed furnaces are characterized by reduced emissions of such harmful compounds as NO x, SO 2, a low probability of screen slagging, the possibility (due to the low temperature of the gases) of saturation of the furnace volume with heating surfaces. Their disadvantages are increased incompleteness of fuel combustion, high aerodynamic resistance of the grate and layer, and a narrow range of regulation of the steam output of the boiler.
Rice. 2.7. Schemes of operation of chain grates and types of fuel dispensers: A, b- furnaces with forward and reverse gratings, respectively; V, G– mechanical and pneumatic casters;
1 - lattice; 2 - stars; 3 - bunker; 4 - gate; 5 - slag remover; 6 - slag; 7 - fuel dispenser; 8 - caster; 9 - air supply; I – zone of fresh fuel; II – fuel heating zone;
III - area of combustion (oxidation) of coke; IV - recovery zone; V - fuel burning zone
The layered method of fuel combustion is characterized by relatively low rates of the combustion process, its reduced efficiency and reliability. Therefore, he did not find application in boilers of high productivity.
Depending on the method of formation of the gas-air mixture, gas combustion methods are divided (Figure below):
- on diffusion;
- mixed;
- kinetic.
Gas flaring methods
a - diffusion; b - mixed; c - kinetic; 1 - inner cone; 2 - primary combustion zone; 3 - zone of main combustion; 4 - combustion products; 5 - primary air; 6 - secondary air
In the diffusion method of combustion, gas is supplied to the combustion front under pressure, and the air necessary for combustion is supplied from the surrounding space due to molecular or turbulent diffusion. The mixture formation here proceeds simultaneously with the combustion process, so the rate of the combustion process is mainly determined by the rate of mixture formation.
The combustion process begins after contact between gas and air and the formation of a gas-air mixture of the required composition. Air diffuses to the gas jet, and gas diffuses from the gas jet into air. Thus, a gas-air mixture is created near the gas jet, as a result of which combustion a primary gas combustion zone 2 is formed. The combustion of the main part of the gas occurs in zone 3, and combustion products move in zone 4.
The emitted combustion products complicate the mutual diffusion of gas and air, as a result of which combustion proceeds slowly, with the formation of soot particles. This explains why diffusion combustion is characterized by a significant length and luminosity of the flame.
The advantage of the diffusion method of gas combustion is the ability to control the combustion process in a wide range. The mixture formation process is easily controlled using various adjusting elements. The area and length of the torch can be adjusted by splitting the gas jet into separate torches, changing the diameter of the burner nozzle, adjusting the gas pressure, etc.
The advantages of the diffusion method of combustion include: high stability of the flame when changing heat loads, no flashover, temperature uniformity along the length of the flame.
The disadvantages of this method are: the probability of thermal decomposition of hydrocarbons, low intensity combustion, the probability of incomplete combustion of the gas.
With a mixed combustion method, the burner provides a preliminary mixture of gas with only part of the air necessary for complete combustion of the gas, the rest of the air comes from the environment directly to the torch. In this case, at first only a part of the gas mixed with primary air burns out, and the remaining part of the gas, diluted with combustion products, burns out after the addition of oxygen from the secondary air. As a result, the flame is shorter and less luminous than in diffusion combustion.
With the kinetic method of combustion, a gas-air mixture is supplied to the place of combustion, completely prepared inside the burner. The gas-air mixture burns in a short flame. The advantage of this combustion method is the low probability of chemical underburning, the short flame length, and the high heat output of the burners. The disadvantage is the need to stabilize the gas flame.
The combustion device, or furnace, being the main element of the boiler unit, is designed to burn fuel in order to release the heat contained in it and obtain combustion products with the highest possible temperature. At the same time, the furnace serves as a heat exchange device in which heat is transferred by radiation from the combustion zone to the colder surrounding heating surfaces of the boiler, as well as a device for trapping and removing some of the focal residues during the combustion of solid fuel.
According to the method of fuel combustion, furnace devices are divided into layered and chamber. In layer furnaces, solid lump fuel is burned in a layer, in chamber furnaces - gaseous, liquid and pulverized fuel in a suspended state.
Modern boilers Usually, three main methods of solid fuel combustion are used: stratified, torch, vortex.
Layer fireboxes. Furnaces in which stratified combustion of lumpy solid fuel is carried out are called stratified. This furnace consists of a grate supporting a layer of lumpy fuel and a furnace space in which combustible volatile substances are burned. Each furnace is designed to burn a specific type of fuel. The designs of furnaces are diverse, and each of them corresponds to a certain method of combustion. The performance and efficiency of the boiler plant depend on the size and design of the furnace.
Layered furnaces for burning various types of solid fuels are divided into internal and external, with horizontal and inclined grates.
Furnaces located inside the boiler lining are called internal, and those located outside the lining and additionally attached to the boiler are called remote.
Depending on the method of fuel supply and the organization of maintenance, layered furnaces are divided into manual, semi-mechanical and mechanized.
Manual furnaces are those in which all three operations - fuel supply to the furnace, its skimming and removal of slag (focal residues) from the furnace - are performed by the driver manually. These fireboxes have a horizontal grate.
Semi-mechanical furnaces are those in which one or two operations are mechanized. These include mines with inclined grates, in which the fuel loaded into the furnace manually, as the lower layers burn out, moves along the inclined grates under the action of its own weight.
Mechanized furnaces are those in which the supply of fuel in a sense, its shoving and removal of focal residues from the furnace are carried out by a mechanical drive without manual intervention by the driver. Fuel enters the furnace in a continuous stream.
Layer furnaces for burning solid fuels are divided into three classes:
- furnaces with a fixed grate and a layer of fuel fixed on it, which include a furnace with a manual horizontal grate. All types of solid fuels can be burned on this grate, but due to manual maintenance, it is used under boilers with a steam capacity of up to 1-2 t / h. Furnaces with casters, into which fresh fuel is continuously mechanically loaded and scattered over the surface of the grate, are installed under boilers with a steam output of up to 6.5-10 t / h;
- furnaces with a fixed grate and a layer of fuel moving along it, which include furnaces with a screwing bar and furnaces with an inclined grate. In furnaces with a screwing bar, the fuel moves along a fixed horizontal grate with a special bar of a special shape, which reciprocates along the grate. They are used for burning brown coal under boilers with a steam capacity of up to 6.5 t / h; in furnaces with an inclined grate, fresh fuel loaded into the furnace from above, as it burns under the action of gravity, slides into the lower part of the furnace. Such furnaces are used for burning wood waste and peat under boilers with a steam output of up to 2.5 t / h; high-speed mine furnaces of the V. V. Pomerantsev system are used for burning lumpy peat under boilers with a steam output of up to 6.5 t / h; for burning wood waste under boilers with a steam output of 20 t / h;
- furnaces with moving mechanical chain grates of two types: forward and reverse. The forward running chain grate moves from the front wall towards the rear wall of the furnace. Fuel flows to the grate by gravity. The reverse chain grate moves from the rear to the front wall of the firebox. Fuel is supplied to the grate by a thrower. Furnaces with chain grates are used for burning hard, brown coal and anthracites under boilers with a steam output of 10 to 35 t/h.
Chamber (torch) furnaces. Chamber furnaces are used for burning solid, liquid and gaseous fuels. In this case, solid fuel must be pre-ground into a fine powder in special pulverizing installations - coal-pulverizing mills, and liquid fuel must be sprayed into very small drops in oil nozzles. Gaseous fuel does not require pretreatment.
The flare method makes it possible to burn a wide variety of low-grade fuels with high reliability and efficiency. Solid fuels in a pulverized state are burned under boilers with a steam capacity of 35 t/h and above, and liquid and gaseous fuels are burned under boilers of any steam capacity.
Chamber (torch) furnaces are rectangular prismatic chambers made of refractory bricks or refractory concrete. The walls of the combustion chamber are covered from the inside with a system of boiler pipes - furnace water screens. They represent an effective heating surface of the boiler, absorbing a large amount of heat emitted by the torch, at the same time they protect the lining of the combustion chamber from wear and destruction under the action of high temperature torch and molten slag.
According to the method of slag removal, flare furnaces for pulverized fuel are divided into two classes: with solid and liquid ash removal.
The furnace chamber with solid slag removal from below has a funnel-shaped shape, called a cold funnel. Drops of slag falling from the torch fall into this funnel, solidify due to the lower temperature in the funnel, granulate into individual grains and enter the slag receiver through the neck. The furnace chamber b with liquid slag removal is made with a horizontal or slightly inclined hearth, which has thermal insulation in the lower part of the furnace screens to maintain a temperature exceeding the ash melting point. The molten slag that has fallen from the torch to the hearth remains in a molten state and flows out of the furnace through the tap-hole into a slag-receiving bath filled with water, hardens and cracks into small particles.
Furnaces with liquid slag removal are divided into single-chamber and two-chamber.
In a two-chamber furnace, it is divided into a fuel combustion chamber and a combustion products cooling chamber. The combustion chamber is reliably covered with thermal insulation to create a maximum temperature in order to reliably obtain liquid slag. Flare furnaces for liquid and gaseous fuels are sometimes made with a horizontal or slightly inclined hearth, which is sometimes not shielded. The location of the burners in the combustion chamber is done on the front and side walls, as well as in its corners. Burners are direct-flow and swirling.
The method of fuel combustion is selected depending on the type and type of fuel, as well as the steam output of the boiler unit.
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