Special Fuel Combustion Issues: study guide 9786010437371

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

A.S. Askarova S.A. Bolegenova S.A. Bolegenova

SPECIAL FUEL COMBUSTION ISSUES Study guide

Almaty «Qazaq University» 2019

UDC 539.2/6 LBC 22.36 я 73 A 79 Recommended for publication by the Scientific Council of the Faculty of Physics and Technology and RISO of Al-Farabi Kazakh National University (Protocol №2 dated 20.12.2018)

Reviewers: Doctor of technical sciences, professor A.B. Ustimenko Doctor of physical and mathematical sciences, professor M.E. Abishev

Askarova A.S. A 79 Special Fuel Combustion Issues: study guide / A.S. Askarova, S.A. Bolegenova, S.A. Bolegenova. – Almaty: Qazaq University, 2019. – 123 p. ISBN 978-601-04-3737-1 The study guide discusses proper organization of fuel combustion processes in boiler plants operating on different types of fuel, selection of the burner taking into account the features of the combustion furnace. The manual provides pre-reading materials for the review of burners for various organic fuels, building knowledge about proper fuel combustion management, training in calculating furnace plants for burning special organic fuel, and building knowledge and skills to improve the efficiency of boilers.

UDC 539.2/6 LBC 22.36 я 73 © Askarova A.S., Bolegenova S.A., Bolegenova S.A., 2019 © Al-Farabi KazNU, 2019

ISBN 978-601-04-3737-1

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This manual describes solid and liquid fuels and natural gases used in thermal power plants and industrial boiler plants. To ensure the efficiency and activity of combustion process, it is necessary to know the composition, thermal and thermophysical properties of these types of fuels. Generally, for the proper fuel combustion management at TPPs one must be able to understand the schemes for preparing solid fuels, natural gas and fuel oil for combustion. Future heat and power engineering specialists should be able to distinguish their peculiar characteristics, select the combustion device required for the effective use of boiler plant and have the knowledge necessary for its application. The technology of preparation of solid and liquid fuels for thermal power plants and boilers using solid fuels is given in the textbook called Fuel Technology at Thermal Power Stations and Industrial Enterprises. This manual discusses the final schemes for preparing solid fuels and provides the information necessary for designing and using carbon dust preparation systems. In addition, it describes general possibilities of using various fuels at stations. It provides the information on training of experts in the field of fuel combustion in power boilers of industrial boiler plants, on contemporary methods for highly efficient combustion of gaseous, liquid and solid fuels, on the selection and calculation of fuel combustion means depending on the type and composition of the fuel burnt. The purpose of this manual is to explore the basics of the theory of combustion, the mechanism of burning of all types of fuel in steam generator boilers; to describe special characteristics of burning fuels with low calorific value and high ash content and with the pulverized fuel flame combustion mechanism, and efficient fuel combustion methods.

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In industrial combustion processes, natural and artificial organic substances are used as fuel. Atmospheric air is primarily used as an oxidizing agent. The oxidation rate of fuel can be divided into three processes: slow oxidation, rapid oxidation and ultrafast oxidation (in other words, “explosion”). This paper discusses rapid oxidation processes in the hightemperature region or combustion processes. The authors used the following basic terms: combustible mixture, combustion, combustion kinetics, burner, combustion device, laminar combustion, turbulent combustion. The combustible mixture consists of an admixture of fuel gases or vapors and oxidizer. Combustion is a chain self-accelerating chemical reactions that occur after the appearance of active centers in a combustible mixture. In the process of burning, the components of the combustible mixture are transformed into combustion products, resulting in a large amount of heat and radiation energy. A device that prepares a combustible mixture, supplies it to the furnace and stabilizes the flame front is called a burner. The combustion device is a complex of one or several burners and a combustion chamber. Laminar (sequential) combustion is implemented due to the slow mixing of gaseous fuel and oxidizer streams during their laminar movement due to molecular diffusion. Turbulent (chaotic) burning is the process of bringing gases and oxidants into a chaotic motion and mixing in order to accelerate this mixing. 4

Fuel and its composition. Fuel is combustible substances used to produce heat in large amounts. Fuels are divided into organic and inorganic. Organic fuels contain various plants. Fuels may be solid, liquid and gaseous. Solid fuels include: coal, peat, wood, herbal fuel; liquid fuels are oil, fuel oil, gasoline, diesel fuel, etc.; gaseous fuels are natural, blast-furnace and generator gases. All types of solid and liquid fuels consist of such combustible elements as carbon (C), hydrogen (H) and sulfur (S). Sulfur is found in fuels in three types – organic sulfur S0, pyrite sulfur Sk and sulfur sulfate Sa, which is a part of fuel ash. Organic and pyritic sulfur burns easily, while sulfate turns into ashes. Sulfur burning products are deposited on the boiler walls; these products also exercise a serious adverse effect on the human respiratory tract. Ash is unburned solid residues from burning fuel. Ash limits the burning of fuel, fills the steam boiler paths preventing gas from escaping, thus leading to the deterioration of thermal surfaces and environmental pollution. Fuel types Wood is a solid fuel. The carbon and hydrogen content, in contrast to that of oxygen and nitrogen, constituting the internal ballast, is low (1-2%). The main ballast that affects the quality of wood is moisture. Freshly cut wood contains 50% of moisture. The combustion heat of wood depends on its humidity and ranges 8.4 to12.5 MJ/kg. The amount of dry substances in dry branches, straws and grass make up 65-75% of their weight, and their combustion heat is 14-16.5 MJ/kg. Peat is a type of fuel that is formed as a result of incomplete disintegration of plants and wood residues under moist conditions. Moisture content of fresh, undried peat reaches 90%. For this reason, combustion heat is very low and amounts to 8.4-10.5 MJ/kg. Coal contains moisture in a small amount; combustion heat may be 21-25 MJ/kg. Fuel oil content of moisture is very low, the heat of combustion – 40 MJ/kg. Combustion heat of gaseous fuels, not containing any moisture, reaches 33 MJ/kg. Reference Fuel Combustion heat of the fuel varies over a wide range. For wood and peat Q = 10 mJ/kg, for fuel oil it is 40 kJ/kg. Depending on the 5

fuel ash and moisture content, their combustion heat also changes. To compare differences in fuel consumption, the "reference fuel" concept is introduced. Combustion heat of such a conditional fuel is 29.3 mJ/kg. For recalculation of 1 kg of natural fuel into a reference fuel, K coefficient is applied: K = Q нр /29.3; where Q нр is the lower limit of fuel combustion heat, mJ/kg. Natural fuel: conversion factor of fuel oil into fuel is K = 1,43; peat: K = 0.25÷0.5; Donetsk coal: K = 0.93; wood: K = 0.3; Podmoskovny coal: K = 0.33  0.43; Kostrinsky coal: K = 0.4  0.5. Furnaces for liquid and gaseous fuels Heavy residues of distillation and fuel oil obtained by the method of oil cracking (decomposition) are primarily burned in the boiler units, as a liquid fuel. Liquid fuel, as well as fuel dust, is burned in chamber furnaces. In addition, apart from liquid fuel, fuel dust is also burned in this furnace. In this case, to burn fuel dust, it is necessary to revise furnace design and provide for a funnel for ash and a bottom for liquid slag. If the furnace only works on burning of liquid fuel, a flat horizontal bottom is attached thereto, that eliminates the need to remove slag (solid fuel residues) from the furnace. To burn liquid fuel in the flare process, it must be pre-sprayed, since non-sprayed fuel does not burn. Using a spray gun, liquid fuel is sprayed into the furnace, where it begins to actively evaporate and gets ready for combustion. Currently, various sprayer designs are widespread in heat and power engineering. They are divided into three types: mechanical, steam and air. The principle of operation of mechanical sprays is based on spraying liquid fuel into a cylindrical chamber using centrifugal force. The fuel that is strongly swirling in the chamber exits through the central opening of the spray device. The liquid flowing out of the hole forms a bed with a crust. With further fluid movement, the bed becomes thinner, and then breaks up into individual fluid particles. To achieve this, liquid is squeezed out with the help of a pump until the pressure of 10-20*106 MPa is reached, then it is supplied to the spray device. The enterprises produce mechanical sprays with a capacity of 250-1250 km/h and a pressure of 12*106 MPa. 6

This spray device consists of a body, a circle inserted into it and a spray circle at the end. Fuel oil Fuel oil

or

Figure 1. Fuel oil atomization burners

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Figure 1 shows gas burners, solid fuel dust burners and oil fuel burners. Mechanical pressure sprayer devices include spray devices where fuel is released through a small orifice under high pressure (1-2 MPa). In mechanical rotary spray devices, fuel is supplied to a rapidly rotating glass, where the fuel forms a thin film. At the exit of the glass, the fuel is sucked in using an air vortex. The advantage of mechanical spray devices: steam is not used when spraying fuel. Disadvantages of mechanical spray devices: the fuel exit holes are clogged, accumulating oil products; besides, it is rather difficult to regulate their performance. To ensure normal operation of mechanical spray devices, the assumed viscosity of the fuel to be sprayed should not exceed 6 RV (assumed viscosity). For this purpose, brand 40 fuel oil is heated to 900 °C, brand 100 fuel oil – to 105 °С. Steam and air sprays can be combined into one class – medium dispensers. Steam spray devices use water vapor (p = 0.41.6 MPa). Fuel oil is fed to the spray device at the pressure of 0.30.4 MPa. At high steam flow rates, the fuel leaving the spray device disintegrates into tiny particles. Compared to mechanical spray devices, steam spray device designs are simple. However, due to the large amount of steam used when burning 1 kg of fuel oil (0,30,35 kg), the steam boiler produces loud noise. Therefore, such sprays are only used in steam boilers with the capacity of up to 3.3 kg/s. Special aspects of liquid fuel burning Combustion process consists of the following stages: spraying fuel using a spray device, evaporation, thermal decomposition of liquid fuel, mixing the resulting product with air, the obtained mixture burning (Figure 2). The purpose of pulverization is to increase the contact surface of the liquid with air and gas. However, the area of the surface bed increases several thousand times. When the flame reaches its maximum radiation, droplets begin to rapidly evaporate and enter into the process of thermal decomposition. Figure 2 shows the furnace areas in various processes, such as evaporation, cracking and burning. 8

In the course of combustion liquid fuel undergoes three phases: liquid, solid and gaseous. The burning rate (just as in case with combustible gases burning) depends on the ability to form impurities, the degree of pre-aeration, the temperature in the combustion chambers, the conditions of flame development and the degree of flame turbulence. Due to incandescent carbon particles flame becomes lightcolored.

1 – zone of evaporation; 2 – disintegration zone; 3 – combustion zone; 4 combustion products Figure 2. Black oil flame scheme

Laminar and turbulent combustion In technical devices, there is combustion of gas and air mixtures flow, which is called a flame. Flames can be of a certain geometric shape. The intensity of combustion and its stability are greatly influenced by the type of flow – laminar or turbulent. Combustion of the mixture of gas and air that are not miscible with each other is called laminar combustion. Such burning occurs primarily in low duty atmospheric burners. In laminar flow, mixing of gases in a gas mixture occurs slowly and is accompanied by molecular diffusion. At that time the flame attains a large size. The flame reaches its maximum size when, instead 9

of burning of air and gas flowing in the laminar mode, combustion occurs separately, in flows. At high temperatures gases containing high molecular weight carbon break down into simple compounds, ash carbon, the particle size of which is very small (~0.3*10-3 mm), is released. Such particles, when heated, make the flame bright. Flame brightness of heavy carbons may be reduced by mixing with a small amount of air. Oxygen changes the nature of the organic molecules breakdown – carbon is released not in solid form, but in the form of carbon monoxide burning with a transparent blue flame. In most cases, the burning flame of gaseous fuels spreads in a turbulent flow, the difference from the laminar flow is as follows: at each point of the jet, the speed varies in magnitude and direction, and the individual jets are mixed and are in a random motion. The higher the flame turbulence is, the more unevenly the front of the flame forms, and many small flames occur on its surface bed. Individual streams of flame are located next to colder additional flows and flows of a burnt, highly heated product (Figure 3).

Figure 3. Overall turbulent combustion

The Ut flame turbulent flow propagation velocity is higher than the flame propagation speed of the mixture in the laminar mode and depends on the scale of the turbulence. The higher the flow rate coming out of the atomizer, the higher the Uт velocity is observed: 10

2

W 1  ,  U t  U H 1  B  Ut 

(1)

where W1 is the pulsation velocity depending on the average flow velocity; B – coefficient depending on the properties of gaseous fuel. Under certain conditions, a turbulent flame is shorter than that of a diffusion-burning flame. The length of the flame in the combustion of a given gas depends on the ability of the combustible gas to form a mixture, the degree of preliminary mixing of gases, the outflow velocity, the atomizer diameter, the coefficient of excess air and jet development conditions. Figure 3 shows approximate flame lengths in selected gas combustion areas. The maximum flame length in a diffusion burner (scheme a) is 1 > 20 d, while flame length in a burner with pre-mixed gases is the shortest (diagram d) – 1 = 3 d, such a burner is called a non-flame burner.

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In boiler plants construction, preparation of the fuel used for the methods of fuel combustion is carried out. For example, fluidized bed combustion requires sorting of additional fuel; for cyclone-vortex furnace combustion the fuel must be sustained, and to obtain flame, the preparation is carried out in two stages: first, the fuel is crushed to 15-25 mm in size, then it is crumbled to especially small particles (microns) in the form of dust. Fuel is supplied to thermal stations in various amounts. From fuel depots fuel is fed to grinding machines through special conveyors. At the exit of the conveyor, the fuel is cleansed of impurities using electromagnetic separators and a chip breaker. Here, before grinding, small pieces of fuel are sifted through large sieves, and large pieces are fed to the grinding plant. The grinding plant is preliminarily automatically supplied with suitable sizes of fuel pieces intended for combustion, since the process of preparation of fuel for combustion requires a certain capacity of a power facility and energy consumption. The smaller the size of the fuel pieces is, the more energy and electricity is consumed to prepare the dust. After grinding, the pieces of fuel are sent to the dust preparation system and this is the last stage of the combusted fuel preparation. Here, coal is milled to a powdery state with a particle size of about 0.1 microns. However, it should be noted that the particle size of the fuel may be nonuniform. In this case, we may speak about heterogeneity of the coal fuel. In other words, after dust preparation, fuel is characterized by poly-nonuniformity, where particle sizes vary from 0.1 to 500 microns. Large fuel particles (over 100 microns) in the combustion process cause mechanically indiscriminate fuel 12

combustion. For efficient combustion, fuel particles with the size of 50-100 microns are suitable. Thus, grinding may be called the main characteristic of the process of preparing fuel for combustion. As a rule, such a process is determined by the granularity of the fuel, which is attained by sifting a part of the dust in sieves with certain cell sizes, that may be used to graphically determine the total amount of dust in the sieve and the particle size. Based on the analysis of this dependence, we may speak about the degree of grinding and homogeneity of the fuel. As a rule, to determine the grinding accuracy, coal samples are calculated by the following ratio through the cumulative Rosin, Rammler and Sperling curves [62]: [  ( d )n ] k

R  100  e

,

(2)

where k and n are constants determined from a couple of sizes, and the larger n, the more homogeneous (closer to homogeneous) the degree of particle separation is. For the most rapid assessment of the quality of fuel preparation, residues from two sieves are often used in dust preparation: R90 – the cell size of 90 microns, that determines economic fineness of grinding, and R200 corresponding to the cell size of 200 microns, that in turn determines the share of large particles. For each grade of combustible fuel, there is its own effective grinding method, which ensures reduction in the cost of dust preparation. Typically, this method is determined experiment-tally when tested in the combustion process. The main indicator of the economic fineness of grinding is the yield of volatile substances – the greater the yield of volatile substances in the course of fuel combustion, the larger the grinding is, since the output of a complex of volatile substances reduces the amount of coke residues that ensure early ignition of the fuel. Depending on the yield of volatile substances and the fractional composition of fuel dust, the economic fineness of grinding may be estimated by the following formula: опт R90  4  0,8nV Г ,

13

(3)

where n is the index of polyhomogeneity of the fuel dust, determined by the nature of the grain.

2.1. Solid fuel combustion To oxidize fuel, a large amount of air is required, the mass of which must exceed the mass of the fuel several times. The P force of aerodynamic pressure of the flow when blowing a bed of fuel with air may be more or less than the G mass of the fuel particle. The force of aerodynamic pressure of the outflow is determined as follows: d 2 02 (4) PC , 4 2 * 9.81

where d is the specified particle diameter, m; w – relative gas velocity, m/s; ρ is the gas density, kg/m3; C – coefficient depending on the value of the Reynolds criterion. When dynamic pressure and gravity force of the flow equalize, G=P, the fuel particles in the flow float to the surface, as if they have lost their mass. This is called the critical flow rate (steaming speed). Depending on the aerodynamic conditions, the beds are divided into a dense (fixed) bed G > P and a weightless (floating) bed G > P. In a weightless bed, small fuel particles are carried away by gas flow. There is a bed called "boiling bed", which is observed when the flow velocity passes through a dense bed and when it leaves the stability limits. Boiling depends on the division of fuel particles, the area of its bed increases 1.5-2.5 times. The movement of a fuel particle (fine under normal conditions, 2 to 12 mm) resembles the movement of liquid in boiling. In a “fluidized bed” furnace in a bedded region, a stream of gas air does not enter into circulation, but is directly blown out of the bed. Due to the uniform deceleration of the air flow that pierced the bed, a complex velocity field appears, that is, the particles constantly change their velocity and direction depending on conditions in the 14

flow. Here particles rotate and periodically enter into impact motion, that is, they resemble a boiling liquid. Aerodynamic features of the fluidized bed are described by the following ratio: wc2 wn2 (5) Сf  c  GCf n , 2 * 9..81 2 * 9.81 where wc, wn – the real flow velocity in the bed and above the bed, d 2 – the particle section. (wn 150-200 °C). Among crystalline hydrates, the main ones are silicates (for example, Аl2O3·2SiO2·2H20; Fe2O3·2SiO2·2H2O) and sulfates (СаSO4·2H2O; MgSO4·2H2O). Hydrate moisture accounts for only a few percent of the total moisture in the fuel. 6) Fuel Composition. Composition of solid and liquid fuels is expressed as mass percent, and the composition of the gaseous fuel is expressed as volume percent. Let us consider the distinction between organic, combustible, dry, working and analytical mass of solid fuel. Organic mass is the percentage of C, H, O, S and nitrogen in the organic compounds that make up the fuel. C 0  H 0  O 0  N 0  S 0  N 0  100%

(6)

The moist basis of the fuel is the fuel in the state in which it is received by consumer. C р  H р  O Р  N Р  S Р  A Р  W Р  100%

(7)

Analytical fuel mass is the fuel that is crushed and dried to the moisture content that, when stored in a laboratory, does not spontaneously change. 23

C а  H а  O а  N а  S а  А а  W а  100%.

(8)

Dry mass of the fuel is the totality of all parts of the fuel, except moisture. (9) C с  H с  O с  N с  S с  Ас  100% Ash-free matter is the mass of fuel minus mineral impurities. C з  H з  O з  N з  S з  W з  100%

(10)

Combustible (dry ash-free) matter is ash-free dry mass or ash-free mass without moisture.

C г  H г  О г  N г  S г  100% .

(11)

7) Solid fuel ash. From non-combustible part, furnace slag is formed, which accumulates in different places of the boiler in the gas paths. Depending on the combustion conditions, the slag is called ash or slag. The amount of ash in the working state of solid fuel is referred to as Ap. Slag is the ash portion of fuel that has been subjected to hightemperature heating, as a result of which it is melted or sintered, acquiring considerable strength. Ash is a powdered non-combustible residue. It may be of two types – fly ash and dumping. Fly ash is dust-like fractions of ash carried out by combustion products from the boiler’s furnace or deposited in convective gas ducts. Dumping is large fractions of ash falling out of low-speed flows into the lower part of the furnace. Thus, fuel ash consists of two components. р

р

A р  Aун  Aпр .%

(12)

The main constituents of fuel residues are oxides SiO2, Al2O3, FeO, Fe2O3, CaO, MgO, a small proportion accounts for sulfates such as CaSO4, MgSO4, FeSO4, while phosphates, oxides of alkali metals K2O, Na2O and many other compounds are presented in even lower amounts. 24

In fuel burning, slag is removed from the furnace in solid or liquid form. For the removal of slag in liquid form, it is necessary in the process of combustion to continuously carry away the resulting ash and melted slag from the bottom of the furnace. 8) Solid fuels density. A distinction is made between actual, apparent and bulk density. The actual density of the solid fuel determines the average density of solid coal particles. If the volume of solid particles that make up the fuel is Vт , and g is the test mass taken from solid fuel, then the actual density of solid fuel (g/cm3) is determined as follows:

 д  Vg . т

(13)

Actual density must be known to determine fuel composition and process design of the pneumatic movement of pulverized coal. Bulk density is the density that expresses the total area of pieces of fuel and gaps between them Vв.п and the total area of gaps on its surface V п .  нас 

g . V т  V п  Vв . п

(14)

Bulk fuel density is used to determine the capacity of the fuel bunker, the area of the warehouse and the parameters of transport devices. Apparent fuel density takes into account total area of the solid part of the fuel and cracks on the surface.  каж 

g . Vт  Vп

(15)

The work of all individual links of the fuel and transportation sector and difficulty in transporting fuel depend on fuel flowability. Flowability refers to the ability of pieces of fuel or individual fuel surfaces to move relative to each other under the influence of gravity. 25

Fuel flowability indicators include bulk density, free fall angle and the coefficients of external and internal friction. 9) Coal porosity. On the surface and inside the mound of coal pieces there is a very complex system consisting of pores, apertures and closed cracks, which is the cause of porosity of solid fossil fuels. The size and shape of the pores and holes can vary. The equivalent diameter of apertures in coal is in the range of 50-100 microns. Porosity of solid fuels affects acceleration of chemical reactions and increases their specific surface. The specific surface of a piece of coal is the surface per 1g of coal, consisting of the outer and inner surfaces of the piece. Inner piece surface is the sum of surfaces of pores and holes, the type of which determines the porosity of the coal. 3.2. Solid fuel classification In the CIS countries, coal produced by the standard is divided into three types [4]: anthracite, coal and brown coal. At the country's thermal power plants, coal and products obtained in the course of its processing are used as solid fuels. Anthracite is a type of solid fuel with the amount of volatile matter г V = 2-9%, with the amount of coal in the combustible mass of 90-93 percent and the lower combustion limit of 27.33-34.7 MJ/kg. Table 1 Industrial Classification of Coal Coal grades Long-flame coal Gas Gas fat coal Fat Common bituminous Coking Forge coal Lean Lean low-caking

Designation

V г ,% in

Carbon residue

L (Д) G (Г) GF (ГЖ) F (Ж) CB (КЖ)

combustible matter More than 36 More than 35 ›31 24-37 25-33

Powdered or low-caking Caking Caking Caking Caking

C (К) FC (ОС) L (Т) LLC (СС)

17-33 14-27 9-17 17-37

Caking Caking Powdered or low-caking Powdered or low-caking

26

Ash content of coal can be A р = 5-15%, moisture – W р  5  10% р and the lower limit of combustion – Q н = 23-27.33 MJ/kg. In the CIS countries, coal is classified in compliance with the state standard based on the amount of volatile substances V г and coke residue. Hygroscopic humidity of brown coal is quite high and the amount in the moist basis C р is very low, whereas, on the contrary, O р is high. There is a lot of ballast in the composition, A р = 15-25%, moiр sture W р  20  35% . The lower combustion limit is Q н = 10.5-15.9 MJ/kg. Brown coal is grouped as follows: Table 2 Brown coal classification Coal grades B1 B2 B3

Amount of moisture in moist basis, % More than 40 30-40 Less than 30

For the correct choice of equipment parameters of the fuel and transport sector, ensuring its reliable and efficient operation, it is necessary to know such solid fuel properties as particle size distribution, density, flowability, congelation, heating tendency and others. Granulometric (fractional) composition of fuel is a characteristic of the size of its pieces. The choice of gratings on receiving hoppers of discharge devices, screens, crushers, conveyor belts, etc. depends on it. The size of fuel particles is determined by sieving the sample on standard screens with a cell size of 150, 100, 50, 25, 13, 6, 3 and 0.5 mm. According to the maximum sizes of pieces, black and brown coal are divided into size classes (grades) listed in the table below. Table 3 Solid fuel classification by size Class 1 Slabby Large

Class designation 2 S (П) L (К)

27

Size of pieces, mm 3 100-200(300) 50-100

1 Nut Fine Rice Culm Rice with culm Fine and rice with culm As mined

2 N (О) F (М) R (С) C (Ш) RC (СШ) FRC (МСШ) AM (Р)

3 25-50 13-25 6-13 0-6 0-13 More than 25 0-200 in deep mining, 0-300 in open mining

3.4. Liquid fuel and its main characteristics As a liquid fuel the product resulting from the processing of crude oil – fuel oil is used. The elemental composition of fuel oil consists of five basic elements: C, H, O, N, S, which make up the organic part of fuel oil. The elemental composition of fuel oil is similar to the composition of oil. In comparison with oil, in high-sulfur fuel oil the ratio  H  does not matter much, respectively, Q р of that may be lower. н   C

The  H  value of fuel oil is variable and often changes. The higher C

density of fuel oil and the amount of cracking residues, the lower is  H  , that causes the decrease in Q р . н   C

Fuel oil consists of hydrocarbons and substances similar to asphalt tar. The following main thermal characteristics of fuel oil should be noted: heat of combustion, density, viscosity, surface tension, flash, ignition and solidifying points, coking behavior. The density of fuel oil is the main parameter characterizing its chemical nature, type and quality. Density must be known to determine the storage capacity of fuel oil, as well as the amount of energy expended in transportation of fuel oil. For practical purposes, we use relative density equal to tt12 . In the CIS countries, the temperature of fuel oil t2 is assumed to be 20 °C, and the temperature of distilled water is 4 °C. The relative density of fuel oil is the ratio of the absolute density of fuel oil  tt at 20 °C to the absolute density of water at 4°: 2

1

28

ρt4 = ρ204 – λ*(t – 20),

(16)

where ρ204 is fuel oil density at 20 °С, g/cm3; ρt4 – is the apparent density of fuel oil at the analysis temperature, g/cm3; t is the temperature at which the analysis is performed, °С; λ – is the temperature correction for density. Viscosity is the most important parameter of fuel oil. Viscosity determines the energy costs of fuel oil transportation by pipeline, the time for its transfer, and the effectiveness of the injectors. The coefficient of dynamic viscosity is denoted by  , Па с , and coefficient 2 of kinetic viscosity by  , м , having the form of    . In the use of с 

petroleum products assumed viscosity denoted as E t and measured by the degree of assumed viscosity 0 AV is applied. Dynamic viscosity is determined by the falling ball method: by the time the ball falls from a certain height in a narrow dish with liquid fuel: λ.

  c (  ш   м ) ,

(17)

where с  is a ball constant determined by gradation using reference viscosity fluids. When the dynamic and kinetic viscosity are known, the assumed viscosity may be represented as follows: 

 t  10 2 7,2Et  

6,25  .  Et 

(18)

Assumed viscosity is used to label fuel oil. 3.5. Gaseous fuel and its main characteristics Gaseous fuel may be obtained naturally and artificially. Natural gas, depending on the method of extraction, is divided into pure natural gas, associated petroleum gas and colliery gas. Artificial gases include gases from oil refineries, liquefied gases, semi-coke gases, generator gases, blast-furnace gases, hydrogen gases and gases derived from biological processes. 29

In the energy sector, natural gas is used as the main gaseous fuel. Main thermal characteristics of gas include composition, calorific value, density and concentration, that determine explosiveness. Combustible gases consist of mixtures of combustible and noncombustible gases, which are used in engineering as fuel, and in chemical production – as raw materials. The volume of combustible gases is measured in m3 under normal (0 °C, pressure 760 mm Hg) or standard conditions (20 °C, pressure 700 mm Hg). Some impurities are present in combustible gases – water vapor, tar, dust, etc. The amount of such impurities per 1 m3 of dry gas under normal conditions is expressed in grams. Composition of combustible gases is expressed as percent (%) of the dry part. The main part of natural gas is methane and its homologues. In general, composition of dry natural gas may be expressed as a percentage equation in its composition, which consists of combustible components and non-combustible ballast [5]. СО+H2+CH4+CmHn+H2S+CO2+O2+N2=100%.

(19)

The composition of a wet combustible gas may be expressed as: СО+H2+CH4+CmHn+H2S+CO2+O2+N2+Н2О=100%.

(20)

The range of concentrations that determine the explosiveness of a natural gas mixture with air is 5-15%. If the amount of air contained in gas is within this range, gas may self-ignite from a nearby source of fire. Relative density of gas with the presence of air means the ability of gas accumulation in the upper or lower part of the room or the plant. Density of natural gas under normal conditions is 0.74 kg/m3.

3.6. Combustion heat (upper and lower limits of the moist basis) and reduced fuel characteristics Combustion heat is the amount of heat released in the course of combustion of a mass or bulk unit of fuel. In accordance with the above, it is measured in kJ/kg or kJ/m3. In the literal expression com30

bustion heat is shown by the corresponding superscript, Q р , Qс , Q г , etc. Combustion heat, based on the fuel state, reflects involvement in a particular mass. When adding values of water vapor condensation heat Qk (k – condensation), which is a part of combustion products, and combustion heat, we obtain the highest combustion heat Qh (h – highest) Condensation heat, not combined with combustion heat, is called the lower combustion limit (l – lowest) and takes the following form: Qh = Ql + Qk. In the process of fuel burning, combustion products are released into the air, primarily at the temperature at which water vapor does not condense. In the absence of experimental results, the lower limit of combustion heat of the working state of solid and liquid fuels is determined by the following formula:







Qнр  4,19 81С р  300 Н р  26(О р  S р )  6 9 Н р  W р , кДж . (21) кг

The lower limit of calorific value of fuel oil used for furnaces is Qнр  39,4  40,2 . Combustion heat may be determined by the formula of D.I. Mendeleyev given that the composition of this fuel is known. To calculate combustion heat of solid and liquid fuels ð ( Q í , kJ/kg) the following simple and fairly accurate formula is used: 𝑝

𝑄н = 339C p + 103OH p – 109 (O F – S F) – 25W p.

(22)

For gas fuel, combustion heat of its dry volume in 1m3, MJ/m3, is determined by the following equation of D.I. Mendeleyev: Qнc  0,0110,8H 2  12,65CO  35,85CH 4  63,8C2H6  91,3C3H8  ...  23,4H 2S. (23)

Most solid fuels differ in their heat of combustion. Therefore, it is impossible to assess energy value of the fuel, and even compare it with another fuel based on a separate absolute technical characteristic. In this regard, it is necessary to use the reduced or comparative 31

characteristics of the fuel. The fuel characteristics given are the ratio of the absolute value of this characteristic to the calorific value of the fuel. The following three characteristics of solid fuel are important [6]: р

– reduced moisture content of fuel W рпр  W р ,%  kg / Mj ; Qн – reduced ash content of fuel Aрпр 

Aр ,%  kg / Mj; Qнр

– reduced sulfur content of fuel S рпр 

Sр ,%  kg / Mj. Qнр

Reduced characteristics are used to compare various fuels in the units available for comparison. For example, reduced moisture of the Moscow region brown coal is

W рпр  3,06 %  kg / Mj . Reduced moisture of anthracite culm is

W рпр  0,376 %  kg / Mj. That is, in the units available for comparison, the moisture of the Moscow region brown coal is 8,1 times higher than that of anthracite culm.

32

4.1. Fuel burning. Oxidizing agents. The material balance of combustion process The purpose of burning fossil fuels in furnace units of boiler plants is to obtain heat generated by exothermic chemical reactions in the furnace, as well as hot combustion products. The fuel burning process results in products of complete and incomplete combustion. Complete combustion products include carbon, hydrogen and sulfur, in fuel composition CO2 is found, in water vapor – H2O, and in sulfur dioxide – SO2. Such components as CO, H2, CH4, Cm H n , that indicate incomplete combustion may also be present in flue gases [7]. The presence of incomplete combustion products may occur for various reasons, such as insufficient mixing of fuel with air, due to that not all the volume of oxygen may enter into a chemical reaction with combustible fuel elements; low temperature in the furnace; high heat load of the furnace, etc. Combustion is a chemical process where heat is actively released in the reaction of a substance with an oxidizing agent. This is the difference between burning and slow oxidation at low temperature or decomposition due to the presence of organic substances in the same medium as the oxidizing agent. As an oxidizing agent, it is possible to use a substance that liberates oxygen and is available for widespread use. Metallurgical plants only use oxygen as an oxidizing agent. Industrial boiler units use atmospheric oxygen. Oxidation of combustible fuel elements is accompanied by very complex chemical changes. Chemical reactions of individual combustible elements do not determine the exact oxidation mechanism, but only show its material balance. 33

To display the material balance of combustion process, a general view of the boiler plant is used, as shown in Figure 4. Material balance is the balance of input and output parts. Fuel is supplied to the boiler plant in the amount of B (kg/s) and air in the amount of Lв (kg/s). Also, due to the leakiness of various insulating surfaces, air is drawn into the installation through gas paths in the amount of L1 , L2 , L3 (kg/s). All this constitutes the input part of material balance. In the output part, ash is formed – solid mineral residues Gзл1 , Gзл2 Gзл3 , released together with gaseous combustion products and removed by ash collector [8].

Figure 4. Material balance of the boiler plant

In general terms, the material balance is described by the following equation: B  Lв   L  Lг  Gзл .

(24)

In the use of gaseous fuel, the component of the left side of the equation, which indicates the amount of solid material residues, is removed from the formula.

34

4.2. Heat balance of combustion. Adiabatic and theoretical combustion temperatures In the process of combustion of natural fuel temperature rises:

𝑄𝑔𝑒𝑛 + 𝑄𝑜𝑥𝑖𝑑 + 𝑄𝑒𝑥𝑡 + 𝑄𝑒𝑥𝑜𝑡ℎ = 𝑄𝑛.𝑠. + 𝑄𝑓𝑢𝑟 + 𝑄𝑒𝑛𝑑 , (25) where 𝑄𝑔𝑒𝑛 , 𝑄𝑜𝑥𝑖𝑑 generated by fuel and oxidizing agent in the area of combustion; 𝑄𝑒𝑥𝑡 , 𝑄𝑒𝑥𝑜𝑡ℎ – additional heat introduced into combustion zone by external heat sources and heat generated as a result of exothermic reactions. In the other part of the equation the total heat of combustion products: 𝑄𝑓𝑢𝑟 , 𝑄𝑒𝑛𝑑 – heat carried from combustion area to the furnace walls and heat of endothermic combustion reactions. In case of pre-heating of fuel using other heat source (for example, heating with fuel oil steam), the 𝑄𝑓𝑢𝑟 value is taken into account in the equation of heat. If fuel was not heated, 𝑄𝑓𝑢𝑟 = 𝑐𝑓𝑢𝑟 ∙ 𝑡𝑓𝑢𝑟 is only valid for brown coals, and fuel temperature is assumed to be 20 °С. The heat of the oxidizing agent (air) is taken equal to its energy 0 𝑄𝑜𝑥𝑖𝑑 = 𝐼𝑖𝑛 = 𝛼 ∙ 𝐼𝑖𝑛 = 𝛼 ∙ 𝑉 0 (𝑐𝜗)𝑖𝑛 .

Qexoth– thermal efficiency of exothermic reactions is assumed to be equal to the specific lower limit of combustion heat for the working mass of the combusted fuel, 𝑄𝑒𝑥𝑜𝑡ℎ = 𝑄𝐻𝑃 . Total heat of combustion products is equal to their energy: 𝑄𝑛.𝑐. = 𝐻г0 = 𝑉𝑅𝑂2 (с𝜗)𝑅𝑂2 + 𝑉 0 𝑁2 (с𝜗)𝑁2 + 𝑉 0 𝐻2 𝑂 (с𝜗)𝐻2 𝑂 , if   1 . Or, in case of   1, the energy of the combustion products is assumed to be equal to the energy of gases, H  H 0  (  1) H â0 . 𝑄𝑒𝑥𝑜𝑡ℎ determines thermal effect of endothermic reactions on dissociative reactions of combus-tion products. The heat absorbed as a result of dissociative reactions is determined at temperatures above 2000 °C. Therefore, 𝑄𝑒𝑛𝑑 = 0. 35

If all the heat fed into the furnace device is only spent to heat combustible substances, such combustion is called adiabatic. In such combustion, the furnace chamber walls do not receive heat, 𝑄𝑓𝑢𝑟 = 0. The temperature of flammable substances in adiabatic combustion is called adiabatic temperature. Equation of adiabatic combustion heat looks as follows: (26) 𝑄𝑔𝑒𝑛 + 𝐼𝑖𝑛 + 𝑄𝐻𝑃 = 𝑄𝑛.𝑐. Hereof it follows that adiabatic temperature of combustion products: 𝑃 𝑄тл +𝐼в +𝑄𝐻

𝜗𝑎 = 𝑉

𝑅𝑂2 (𝑐𝑝 )𝑅𝑂

2

+𝑉𝑁02 (𝑐𝑝 )

𝑁2

+𝑉𝐻02 𝑂 (𝑐𝑝 )

.

(27)

𝐻2 𝑂

Adiabatic temperature of stoichiometric combustion, i.e., adiabatic temperature of combustion products in the absence of air and fuel preheating determines heat output of the fuel. 𝑃 𝑄𝐻 . 0 0 𝑅𝑂2 (𝑐𝑝 )𝑅𝑂 +𝑉𝑁2 (𝑐𝑝 )𝑁 +𝑉𝐻2 𝑂 (𝑐𝑝 )𝐻 𝑂 2 2 2

𝑡ℎ = 𝑉

(28)

Taking into account heat used for disassociation of combustion products 𝑄𝑒𝑛𝑑 = 𝑄𝑑 , combustion heat equation may be written as follows: (29) 𝑄𝑔𝑒𝑛 + 𝐼𝑖𝑛 + 𝑄𝐻𝑃 = 𝑄𝑛.𝑐. + 𝑄𝑒𝑛𝑑 . Thus, the value obtained using the formula (3.6) while maintaining adiabatic condition 𝑄𝑒𝑛𝑑 = 0, is called theoretical combustion temperature.

𝜗𝑡ℎ𝑒𝑜𝑟 = 𝑉

𝑃 𝑄𝐻 +𝑄𝑔𝑒𝑛 +𝐼𝑖𝑛 −𝑄𝑑

𝑅𝑂2 (𝑐𝑝 )𝑅𝑂

2

+𝑉𝑁02 (𝑐𝑝 )

𝑁2

+𝑉𝐻02 𝑂 (𝑐𝑝 )

.

(30)

𝐻2 𝑂

Knowledge of the theoretical combustion temperature is necessary for calculating combustion process and designing furnaces. The values of adiabatic temperature and theoretical combustion temperature are close and only differ by a few degrees, therefore a  theor . 36

4.3. Volumes of combustion and air products required for combustion. Determination of excess air ratio Combustible elements of solid and liquid fuels are carbon, hydrogen and sulfur, and combustion products are carbon dioxide CO2, water vapor H2O and sulfur dioxide SO2. Knowing the oxidation of combustible substances, you can determine the amount of air required for complete combustion, as well as the amount of volatile substances [1]. For the last stoichiometric reaction, the following equation is true: С + О2 = СО2; 12 + 32 = 44 kg. In other words, the complete burning of 1kg of carbon, requires oxygen in the volume of V0 

32 3  1.866 m /kg. Here 1.428 is 12  1.428

oxygen density, kg/m3. For hydrogen and sulfur, the following equations are applied: 2Н2 + О2= 2Н2О; S + О2= SО2; 4+32= 36 kg; 32+32=64 kg. It means that the amount of oxygen required to burn hydrogen and sulfur will be: V0 

32 32  0.7 m3/kg.  5.55 m3/kg; V0  32  1.428 4  1.428

Having determined the amount of oxygen consumption for combustion of 1 kg of fuel and subtracting from it the amount of oxygen in the initial fuel composition, the theoretical amount of oxygen required to burn 1 kg of solid and liquid fuel is found: Vo02  1.866

Cр Hр S р ор  к Oр .  5.55  0.7  100 100 100 100  1.428

37

(31)

Air contains about 21% of oxygen; therefore, if we assume that all oxygen in the air will be engaged in the reaction, the theoretical volume required to burn 1kg of fuel may be determined by the following formula: V0 

V00

2

0.21



1.866C р 5.55H р 0.7 S р ор  к Oр .    0.21  100 0.21  100 0.21  100 0.21  100  1.428

(32)

Or in a short version: р

V 0  0.0889(C Р  .375Sорр  к )  0.265H р  0.0333O .

(33)

Considering the combustion reaction of combustible compounds contained in gaseous fuels, it is possible to determine the air volume spent on the complete combustion of 1m3 of gas: n   V 0  0.04760.5CO  0.5H 2  1.5 H 2 S   (m  )Cm H n  O2  . (34) 4  

To avoid slowing down the combustion of fuel particles that react, it is necessary to supply a sufficient amount of air to the boiler plant. Since it is air, not oxygen supplied to the furnace, combustion reactions may be weak. In addition, the air and the fuel mass that is ready for combustion may be poorly mixed. So, it turns out that it is necessary to supply more air to the combustion chambers than required. The ratio of the actual amount of air supplied for combustion to the theoretically required amount of air is called the excess air coefficient and is determined by the following formula: 

V . V0

(35)

The excess air coefficient depends on many factors. Its value is determined depending on the type, characteristics and method of fuel combustion, and the design of the combustion device.

38

4.4. Volume of combustion products The basis of the material balance of the boiler plant is the amount of oxidizing agent and combustion products corresponding to the number of units of the fuel burned. CO2 is the product of complete carbon combustion, while monoxide CO is the product of incomplete combustion. Hydrogen is a very active element, so it burns completely and forms water vapor H2O. If there is not enough oxygen in the combustion products, hydrogen in free form can generate such compounds as H2 or CH4 as well as other heavy hydrocarbon compounds. Sulfur also very quickly reacts with oxygen; therefore, the oxides in the composition of the combustion products also include sulfur dioxide SO2. If we assume that in a boiler plant air is used as an oxidizing agent supplied to the furnace in excess, then the following formula may be used to determine the volume of combustion products: г

V  VСО2  V

2

 VСО  V Н 2   VС м Н ь  V  2  VО2  V Н 2О .

(36)

Boiler plants are required to fully convert the chemical energy of fuel into heat. Therefore, industrial and power boiler plants take measures to ensure complete combustion. When   1 and in case of complete fuel combustion, there is no oxygen in the composition of combustion products, but there are such elements as CO2, H2, SO2 and H2O. In this case, a certain volume of combustion products is called theoretical. In the analysis of combustion products, the amount of triatomic gases CO2, SO2 is determined, and therefore their total volume is also determined: (37) VRO2  VCO2  VSO2 . In the course of combustion of 1 kg of carbon, 1.866 m3 of CO2 is formed, and in combustion of 1 kg of sulfur – 0.7 m3 of SO2. Thus, for liquid and solid fuels, the volume of tri-atomic gases is determined by the following formula: VRO2  0.01866(C р  0.375S лр ) .

39

(38)

When considering oil shale combustion, it is necessary to add to this volume the product of 0.00509(CO2 ) rk K , where K is the decay rate of carbonates. In case of oil shale combustion using chamber process, K  1 , while when using a bedded process - K  0.7 . (CO2 ) rk value indicates the amount of carbonate carbon dioxide in the fuel. The theoretical volume of nitrogen in the combustion products consists of the sum of nitrogen in the theoretical volume of air required for air combustion and the volume of nitrogen in the fuel composition: V 0 N 2  0.79V 0  0.008N р.

(39)

In general terms, the theoretical volume of water vapor in the combustion products is determined by the following equation: V0

H 2O

 0.111H р  0.0124W р  0.0161V 0  1.24W р,

(40)

where 0.111 H р – the amount of steam formed in the course of combustion of hydrogen present in the composition of the fuel; 0.124 W р is the volume of water vapor formed in the process of moisture release from the fuel; 0.0161 V 0 – the volume of water vapor that has penetrated into the furnace with moist air; 1.24 W р is the volume of steam taken into account only in the case of the use of a fuel oil atomizer operating on water vapor. When burning gaseous fuel, the amount of volatile substances is determined by other special formulas. The theoretical volume of dry gases consists of the volume of triatomic gases and nitrogen. V 0с.г.  VRO2  V 0 N 2 .

(41)

Total theoretical volume of combustion products: 0 0 V 0 г  Vс.г.  VН 2 О .

40

(42)

Then α=1, in real conditions of operation of the boiler plant it is impossible to ensure complete fuel combustion due to improper conditions at the plant. Excess air supply leads to the increase in the volume of water vapor and nitrogen in the composition of combustion products. Excess oxygen is also produced. Taking into account the above, the actual volumes of these gases can be defined as: VN 2  V 0 N 2  0.79(  1)V 0 ,

VH 2O  V 0

H 2O

 0.0161(  1)V 0 ,

VO2  0.21(  1)V 0 .

(43) (44) (45)

In application of the above formulas, the volume of combustion products is used for the selected values of α and values of intake air.

41

5.1. Sustainable fuel use. Heat balance The sustainable use of fuel in the combustion device depends on two main factors: complete fuel combustion and freezing of combustion products. The heat released in the combustion unit in the process of combustion of the fuel is the lower limit of combustion heat of this fuel, since water vapor in its composition leaves the furnace as steam, not having time to condense in the combustion products. This process is characterized by the following data. Partial water vapor pressure in the combustion products for anthracite and coal is about 0.005 MPa, for oil shale and peat – 0.02 MPa; this pressure corresponds to condensation temperature of 30.... 60 °C. The process of cooling of combustion products to such temperatures is technically difficult and economically unprofitable. The heat balance of a boiler plant characterizes the equality of heat entering the plant and the cost of heat released from the plant. In general, the heat balance is expressed in the following form [3]:

𝑄𝐜𝐨𝐦 = 𝑄𝑐𝑜𝑛 .

(46)

When fuel burns in a furnace unit, its chemical energy is converted into physical heat of hot combustion products. Subsequently, this heat may be used for different purposes in accordance with the purpose of the installation. If the installation performs the function of a heat generator, heat is used for technological purposes to dry materials. In this case, heat is considered useful for heating. Heat balance of any device is calculated for 1 kg of solid fuel or 1m3 of gaseous fuel and for normal conditions, where temperature is equal to 0 °С, and the pressure is 101 kPa. 42

All the heat generated by burning 1kg of solid or liquid fuel at the р

entrance to the heat balance is expressed as Q р :

𝑄𝐜𝐨𝐦 = 𝑄𝑝𝑝 = 𝑄𝐻𝑝 +𝑄𝑖𝑛.𝑏ℎ + 𝑖ℎ𝑒 + 𝑄𝑠𝑝 + 𝑄𝑐 ,

(47)

where Qнр is the lower limit of combustion heat of the working mass of solid or liquid fuel, kJ/kg; 𝑄𝑖𝑛.𝑏ℎ – heat introduced by air heated outside the unit, kJ/kg; 𝑖ℎ𝑒 – physical heat of fuel, kJ/kg; 𝑄𝑠𝑝 – heat brought by steam, spraying fuel oil, kJ/kg; 𝑄𝑐 – heat expended on the decay of carbonates in the process of oil shale combustion, kJ/kg. To accelerate combustion process in the use of different types of fuel in the unit, hot air must be supplied to the furnace. For this purpose, the air is heated outside the unit, in air heaters. The heat brought into the installation by such hot air is determined by the following formula: 0 ] 𝑄𝑖𝑛.𝑏ℎ = 𝛽 ′ [𝑖𝑎0 − 𝑖𝑐.𝑎 ,

(48)

where β is the ratio of the amount of air entering the installation to the theoretically required; 0 𝑖𝑎0 , 𝑖𝑐.𝑎 is enthalpy of air entering the plant and cold air. If the unit performs a steam boiler function, the useful heat in the furnace is used to produce heated or saturated steam. In the process of movement of combustion products through gas paths, these scorch the heating surface. As a consequence, the heating surfaces accept heat, and water passing along them reaches boiling point and evaporates, turning into heated steam. And there, heat is useful. The useful heat is designated as Q1 . However, 10–20% of heat generated in the installation is not used to good advantage. Operation of any boiler plant is associated with various heat losses. The division of heat brought into boiler plant into useful heat Q1 and heat loss is expressed as the equation of the plant heat: 43

𝑄𝐜𝐨𝐦 = 𝑄𝑝𝑝 = 𝑄1 +𝑄2 + 𝑄3 + 𝑄4 + 𝑄5 + 𝑄6 ,

(49)

where Q2 is the loss of heat leaving the plant with gases;

Q3 is heat loss due to incomplete chemical combustion of fuel; Q4 is heat loss from incomplete mechanical combustion; Q5 is loss of heat into the environment through the casing shell; Q6  loss of slag with physical heat. Dividing the right and left parts of the heat balance formula (49) 𝑝 by 𝑄𝑝 and multiplying by 100%, we obtain the following heat balance equation: (50) 100  q1  q2  q3  q4  q5  q6 , where q i is the relative heat loss in the installation. Useful heat is in the following relationship with the boiler performance: 𝑝

𝑄1 =

𝑄𝑝 𝜂𝑘 𝐵

=

0 −𝑖 0 ] 𝐷[𝑖𝑎 𝑐.𝑎

𝐵

,

(51)

0 where 𝑖𝑎0 , 𝑖𝑐.𝑎 is enthalpy of heated steam and feedwater, kJ/kg. By numerical value q2 is the largest of heat losses; q2 is the loss of heat from the gases leaving the plant. This value is determined by the following equation:

q2 

I ух   ух I х0.в Q рр

(100  q 4 ) ,

(52)

where I ух , I х0.в is enthalpy of flue gases and theoretical amount of cold air (determined at the temperature of 30°C);  ух – excess air coefficient in exhaust gases. Combustion products may contain gaseous combustible components CO, H2, CH4, burning out of which outside the furnace chamber due to low temperatures and concentrations of such gases and oxygen 44

is almost impossible. Heat loss due to incomplete combustion of combustible substances is called incomplete combustion and is expressed as Q3 , kJ/kg or kJ/m3. The percentage of such heat loss is determined by the following formula:

𝑞3 =

𝑄𝐶𝑂 ∙𝐶𝑂+𝑄𝐻2 ∙𝐻2 +𝑄𝐶𝐻4 ∙𝐶𝐻4 𝑝

𝑄𝑝

𝑉 𝑣.𝑐𝑝 ,

(53)

where QCO , QH 2 , QCH 4 is calorific value of incomplete combustion products; CO, H 2 , CH 4 – the proportion of incomplete combustion gases in dry combustion products, %; 𝑉 𝑣.𝑐𝑝 – volume of dry combustion products, m3/kg. Heat loss from incomplete combustion in the process of combustion of gaseous and liquid fuels = 0 ÷ 0.5%, and in solid fuel during flaring, is assumed to be zero. Heat loss from incomplete combustion is closely related to the excess air coefficient and the furnace load (see Figures 5 and 6). In case α=1, occurrence of incomplete combustion indicates insufficient mixing of fuel with air. In the value of excess air coefficient αкр (curve), there is no incomplete combustion. As a rule, αкр=1.02÷1.03 indicates aerodynamic underdevelopment of the combustion device. Unburned carbon loss in the process of combustion of solid fuels (peat, coal, oil shale) is caused by the formation of coke particles, which for some time were exposed to high-temperature flame, emitting volatile gases, and did not have time to completely burn out. Unburned carbon loss during combustion of gas and fuel oil may also occur in the form of solid particles or soot that appear in the hightemperature zone with lack of oxygen. Under normal operating conditions of the unit, losses due to incomplete mechanical combustion in solid fuel burning are q 4 = 0.5÷2%. When gas and fuel oil are burned, losses due to incomplete mechanical combustion are insignificant ( q 4 =1,0%); therefore, these are considered together with q3 [4]. 45

Air Ratio Figure 5. The dependence of heat loss and efficiency on the excess air coefficient

In case of chamber combustion of solid fuels, heat losses with incomplete combustion q 4 are divided into losses with carry over ун

шл

ун

and slag q 4 , with the predominant part being q 4 . Losses q 4 significantly depend on excess air coefficient. When air ratio excess is below optimal, the q 4 growth is determined by incomplete mixing of fuel with air at the burner exit and the development of zones with a lack of oxygen, despite the relatively high temperature level. With α  αопт, a decrease in temperature in the combustion zone and a slower oxidation reaction are observed. However, the residence time of particles in the high-temperature zone is reduced due to the increase in the volume of combustion products. Incomplete mechanical combustion is determined by the following formula, %:

q4

Г

Г

𝑓𝑎 𝑠𝑙 𝑞4 = (𝑎𝑠𝑙 100−Г + 𝑎𝑓𝑎 100−Г ) 𝑠𝑙

𝑓𝑎

32,7𝐴𝑝 𝑝

𝑄𝑝

,

where 𝑎𝑠𝑙 , 𝑎𝑓𝑎 are proportions of ash in the slag and fly ash; 46

(54)

Г𝑠𝑙 , Г𝑓𝑎 – the content of combustible components in slag and ash, %; 32.7 – combustion heat of coke particles in slag and fly ash, MJ/kg. The value of heat loss into the environment from external cooling q 5 varies 0.2 to 2.5%. Heat loss from physical heat of slag is determined by the following formula:

𝑞6 =

𝐴𝑝 𝑎𝑠𝑙 (𝑐𝑡)𝑠𝑙 𝑝

𝑄𝑝

,

(55)

where (𝑐𝑡)𝑠𝑙 is the product of temperature and heat capacity of the slag. In reduction of heat load of the boiler, there is a slight drop in flue gas temperature, that leads to a decrease in heat loss with flue gases q2 (see Figure 6). Heat losses due to chemical and mechanical incomplete combustion increase with the decrease of heat load due to deterioration of the process of mixing of fuel and air at lower speeds. The specific heat loss through the brickwork also increases, since the absolute value of these heat losses remains almost unchanged, and heat load decreases. Thus, due to the difference in the dependences of heat losses on the load, it turns out that with a reduced load, performance of the boiler unit reaches its maximum value.

Efficiency, %

Heat Losses, %

Efficiency, %

Heat output, % Figure 6. Dependence of heat loss and efficiency on the heat output of the boiler

47

5.2. Efficiency (gross and net) of the boiler plant. Determining the performance of a boiler plant from inverse inequality Net heat ratio used in the boiler plant to the amount of total heat in the boiler determines thermal efficiency of the plant and is called the efficiency factor. There are gross and net efficiency factors of the plant. Gross efficiency does not take into account energy costs (feedwater pump, smoke exhauster, fuel grinding costs, heating surfaces blowing, etc.) for auxiliary needs of a boiler plant, therefore it is determined as follows:  кбр. у  q1 

Q1 (Q рр B)

100 ,

(56)

where В is fuel consumption, kg/s. The efficiency factor, which takes into account the cost of electricity and heat for the own needs of the installation, is called net efficiency.

 кн. у   кбр. у  qс.н. ,

(57)

where qc.н. is the ratio of the sum of energy consumption for the own needs of the boiler plant to the entire volume of heat, %. The efficiency factor of the boiler plant may be determined using direct and inverse equations:

 кбр. у  100  (q 2  q3  q  q5  q6 ) . 4

(58)

Determination of gross efficiency  к.бру from a direct equation р

requires direct measurement of values of heat fed into the boiler Q р

and the net heat used Q1 . However, this approach causes some difficulties and leads to omissions. But heat loss is possible to determine with great accuracy; therefore, the most accurate method for determining the efficiency of a boiler plant is to determine it using the inverse equation. 48

Using equation (58), it is possible to determine consumption of fuel burned in a boiler plant during one hour: B

Q1 (Q рр кбр. у )

100 .

(59)

It is impossible to say that, as a result of incomplete mechanical combustion, all the fuel supplied to the plant burns down completely. Therefore, the volume of gases emitted in the process of combustion of fuel within an hour is somewhat less, compared to full combustion. Since the volume and enthalpy of combustion products are calculated for 1 kg (1 m3) of fuel, it is considered that the amount of fuel supplied to the boiler is not enough to take into account incomplete mechanical combustion, for this reason, calculated hourly fuel consumption is used in thermal calculations. Bр 

В(100  q 4 ) . 100

49

(60)

6.1. Elements of combustion theory. Kinetics of chemical combustion reactions Combustion is a complex physical-chemical process based on the fuel reaction with an oxidizing agent. In the combustion process, the temperature rises greatly and heat is emitted in large quantities. If fuel and oxidizing agent are in the same phase state, combustion is called homogeneous. If fuel and oxidizing agent are in different phase states, combustion is called heterogeneous. Combustion of gaseous fuel is a homogeneous process, and, for example, combustion of coke particles in the air stream is a heterogeneous process [7]. Combustion process is based on the following basic chemical reactions: C + O2 = CO2; S + O2 = SO2; 2H2 + O2 = 2H2O. In an isolated boiler of power plants, the above reactions occur over a very short time (5-10s) and air and fuel are spent on them in large quantities. For example, a large energy boiler using Berezovsky coal (steam capacity – 2650 t/h, capacity – W  800 MW) requires 128 kg of fuel and air in a volume of V 0  555 m3/kg per second. To start the combustion process, pulverized coal must be supplied to the 50

furnace, when such coal enters hot gases environment, it is fully mixed with air. Dust should be heated up to the temperature close to the burning temperature. To carry out such a process in a particular boiler, turbulent jets fed from the burners are injected into hot gases. Hot combustible substances in the jets are ejected and together with gases they form a combustible mixture, ready for ignition and combustion. Thus, fuel combustion begins not with the contact with oxygen contained in the air, but when the initial mixture is mixed with combustible substances. Chemical reactions occur at all points of the zone of formation of a combustible mixture. However, the rates of these reactions vary. If in the final combustible mixture the concentration of inert combustible substances exceeds the concentration of the initial mixture, the temperature of the mixture increases and the reaction rate rises exponentially. When the temperature of the mixture reaches the combustion temperature, the mixture must be ignited from another source of ignition. In boilers where pulverized coal is burned, fuel oil is first ignited. Dust is continuously introduced into the released combustible substances. Combustion in the furnace is influenced by operational humidity, sulfur content of the fuel, the amount of nitrogen in it and the amount of nitrogen in the air. Chemical reactions do not occur until the complete change of the starting substances (proven by this experiment). Together with the reaction products in the system, initial and intermediate substances are formed, since the reaction takes place in two directions. Chemical reactions in the boiler occur simultaneously in two directions (direct and reverse):

A  B  M  N ,

(61)

where A, B, M , N are chemical symbols of reactants;  ,  ,  ,  – stoichiometric coefficient expressing the amount (number of moles) of substances that react. Kinetics of combustion is the nature of dependence of a chemical reaction on time. 51

6.2. Chemical reactions rate. Mass action law In general, the rate of chemical interaction of reacting substances is determined by the change in their concentration per unit of time [8]. W 

dC . d

(62)

At the initial stage of chemical reactions in the boiler, the amount of substances ( A, B ) gradually decreases as the rate of direct reaction

W1 decreases and the rate of reverse reactions W2 increases due to the increase in the number of products of direct reaction ( M , N ). At some point, the rates of the forward and reverse reactions are aligned, and chemical equilibrium sets in. Thus, the rate and equilibrium of the reaction depends on the chemical nature of substances entering into reaction, the amount, temperature, pressure and volume of the substances in the combustible mixture that are reacting and newly formed. The speed of a homogeneous direct reaction is determined by the following formula: W1  K1  C A  C B , and the rate of the inverse reaction is determined by the formula W2  K 2  C M  C N , where K1 ,

K 2 are the constants of the direct and reverse reaction. Development of the equality of certain reactions at constant temperature and pressure is equal to the product of concentrations of the substances entering into reaction. This dependence is determined by the mass law. This law explains the concept of kinetics of chemical reactions. The law may be formulated as follows: the rate of chemical reactions in a homogeneous medium at a constant temperature is equal to the product of concentrations entering into reaction. For a homogeneous reaction, the mass law may be expressed as a formula. To perform the balance of reactions,   W1  W2 or K1  CA  CB  K 2  CM  C N .

From this, the equilibrium constant is calculated, which determines the mathematical form of the mass law: 52

Kc 

K 2 C A CB .  K1 CM CN

(63)

The oxidation reaction of methane with oxygen is recorded as follows: СН4 + 2О2 = СО2 + Н2О. Combustion intensity is caused by the reaction rate. The rate of a homogeneous reaction is the amount of substance involved in the reaction for the unit of time in the unit of volume. In this case, oxidation rate of methane [kg/(m3·s)] is equal to the change in its concentration dCH4 over a period of time dτ. W СН4= – dCH4/ dτ.

(64)

The (-) sign in formula 5.4 indicates a decrease in the proportion of the initial components of combustible mixture in the course of the reaction. In the formula for calculation of the speed of the final reaction products, the (-) sign is absent due to the increase in their shares. WСО2 = dCО2/ dτ.

(65)

Each component of the combustible mixture has its own interaction rate. For example, in the oxidation of methane with oxygen, the decrease in the share of oxygen occurs twice as fast, compared to methane, that may be expressed by the following stoichiometric reaction equation: WО2 = 2 W СН4.

6.3. Dependence of the chemical reactions rate on temperature. Arrhenius law According to Arrhenius, under general conditions, the constant of the rate of chemical interaction is temperature dependent [5]. K  K 0e 53



E RT

.

(66)

Arrhenius was an outstanding Swedish scientist, physicist-chemist and astrophysicist, the winner of the Nobel Prize in Chemistry in 1903. The values of the mathematical expression of the Arrhenius law are as follows: K0 – coefficient before the exponent; K0 – constant, the unit of measurement of which is determined by the reaction order; Е is the activation energy, which is the energy required to destroy the internal molecular bonds of the substances involved in the reaction. The unit of measurement used is J/mol. Its value depends on the nature of the reaction. For many flammable gases, the activation energy E is 80–120 J/mol. R = 8,314 J/(mol·K) is the universal gas constant. E and K0 are called kinetic constants, which determine the ability of the fuel to enter into reaction and are found out experimentally. The exponential dependence of the burning rate on temperature is only performed in very narrow temperature ranges, since the interaction mechanism is different at different temperatures. Thus, the activation energy is the main factor determining the reaction rate. The lower activation energy E, the faster is the reaction. From the point of view of molecular-kinetic theory, not every collision of molecules leads to chemical interaction. Arrhenius gives the following brief conclusion: K0 displays the total number of collisions, E indicates the level of sufficient energy required for molecules collision. If K is the number of effectively reacting molecules, then the higher the temperature, the greater is the number of molecules with sufficient energy required for the collision. To determine the values of the activation energy E and the coefficient before the exponent K0, make up the dependence ln k  ln k 0  ( E / R)  (1/ T ) shown in Figure 7.1. This figure shows the dependence of the constant reaction K on temperature. Arrhenius formula, which determines the dependence in Figure 7: ln K  

E  ln K 0 . RT

54

(67)

The ln K 0 member of formula (67) is equal to the ordinate in accordance with 1 / T0 . The slope angle  of the dependence is determined from the expression E  tg . R

Figure 7

Figure 8

In Figure 7, you can observe a rapid change in speed depending on temperature. To explain this change, Arrhenius proposed the following hypothesis: for the start of the reaction, it is necessary that molecules the energy of which exceeds the critical value collide with each other. Such molecules are called active. The higher the temperature, the greater is the number of active molecules. Active molecules appear without any chemical differences from ordinary molecules. The change in the energy of an individual molecule is called the activation process, and the energy absorbed in the process of the appearance of an active molecule is called activation heat. The balance between active and normal molecules is observed. The equilibrium constant is directly related to temperature. The K0 value in the formula (63) shows the total number of molecules colliding per unit of time in a unit of volume, and K expresses the number of molecules that entered into reaction. E is the minimum value of energy at which a collision of molecules will be effective. Hence it follows that the effectiveness of collision is determined by the energy of the molecule only. As shown in Figure 8, the source molecule, the energy of which is determined by state A, absorbs energy from neighboring molecules 55

equal to E1 , then it passes into the active state B. Figure 8 shows the change in the energy as the molecules enter into reaction and in the process of molecules transition to the active state. Now the active molecule in state B, interacting with other molecules, generates reaction products, this process being accompanied by the release of energy E2 . In most cases, the amount of heat released in the ВС part of the graph is greater than the amount of heat absorbed in the AB part, and this difference gives a positive thermal efficiency Q  E2  E1 . If E1  E2 , the reaction absorbs heat, then it is considered endothermic. The Arr  E value means the Arrhenius conventioRT

nality and determines the ability of mixture to react. The higher the value of Arr conditionality, the more inert the combustible mixture is and the slower the burning reactions are.

6.4. Dependence of the equilibrium of chemical reactions on temperature and pressure Dependence of equilibrium on temperature in isochoric process is determined by the following formula: d ln K c Q  v2 . dT RT

(68)

Qv  the amount of heat released or absorbed as a result of the reaction. Thus, in the isochoric process V  const. and K c  K c (T ) dependence is called the isochore of a chemical reaction. With a known constant Kc value, it is possible to determine the composition of the mixture in equilibrium at a constant temperature. The dependence of equilibrium on pressure. The equilibrium constant of a chemical reaction may be expressed by the formula using the partial pressures of substances.

56



KP 



PA PB ,   PM PN

(69)

where PA , PB , PM , PN are partial pressures of individual gases that make up the mixture. The dependence of equilibrium on temperature in the isobaric process is expressed by the following formula:

d ln K p dT



Qp RT 2

.

(70)

Q p  is the amount of heat released or absorbed by the reaction.

Therefore, in the isobaric process, P  const , K p  K p (T ) dependence is called isobar chemical reaction. When mixture temperature decreases as a result of the reaction, heat is released, K c , K p values increase, i.e., the reaction equilibrium shifts towards the starting substances. Hence, low temperature is a favorable condition for exothermic reactions. With an increase in the mixture temperature as a result of the reaction, heat is absorbed, the values decrease, that is, the equilibrium of the reaction shifts towards the final substances. From this it follows that high temperature is a favorable condition for endothermic reactions.

57

7.1. Effect of internal reaction and mixing on combustion kinetics. Kinetics of combustion chemical reactions It is known that the combustion process occurs on the basis of various chemical reactions: C + O2 = CO2, S +O2 = SO2. In the combustion chambers of power boilers, fuel and air are consumed in large quantities over a short period of time (5-10s). For example, in the power unit with steam generating capacity of Dn = 2650 t/h, with the total power of W  800 MW, Berezovsky coal is burned with fuel consumption Bp = 128 kg/s. The theoretical volume of air required for fuel combustion is V0 = 555 m2 [8]. In order for such a voluminous process to take place quickly and within a specified time, it is necessary that the fuel quickly mixes with air and the mixture heats to the temperature close to combustion temperature. For some technical reasons, preliminary provision of these conditions seems impossible. To solve this problem, it is necessary to organize combustion process by directing turbulent jets, passing through the burners, through heated combustible substances. In combustion chamber, the jets of combustible mixture draw in heated combustible substances and together with them constitute a highly heated reaction mixture, ready for ignition and combustion. The amount of gas passing through the boiler is equal to the amount of mixture in the jets passing through the burners. Gases tend to exit from the boiler, and in the zone of mixing with jets they enter into a circulation motion. Thus, the combustion of fuel in the boiler does not begin at the time of contact of the fuel with oxygen, but occurs due to mixing of the fuel-air mixture with combustible substances. The concentration of the initial combustible substances in the reaction mixture is several times less as compared to inert combustible substances. Chemical reactions may 58

occur at all points in the zone of formation of a combustible mixture. However, the rates of these reactions vary. If in the final combustible mixture, the concentration of inert combustible substances exceeds the concentration of these in the initial mixture, the temperature of the mixture rises and the reaction rate increases exponentially. If the mixture temperature approaches 𝑡 = 𝑡𝑐𝑜𝑚 , then   max . To combust the mixture, it must first be ignited. The combustion conditions in the combustion chamber are affected by the presence of Wp, Sp, Np in the fuel composition and nitrogen in the air contained in the oxidizing agent. Sulfur anhydride and toxic nitrogen oxides may form in these gases. 7.2. Equilibrium concentrations of flammable substances, dissociation of molecules. Combustion temperature. The effect of dissociation on combustion temperature If heat generated in the process of complete fuel combustion is only consumed for combustible substances heating, the temperature before heating is called adiabatic combustion temperature. In real conditions, most of the heat released in the course of fuel combustion is received by furnace walls, while the rest is consumed in the process of dissociation of combustible substances. Combustion temperature for real conditions may be calculated using the heat equation for the combustion process [9]:

𝑄 + 𝑄𝑐𝑚 = 𝑄𝑐 + 𝑄𝐿 + 𝑄𝐷 ,

(71)

where Q is the reaction heat, kJ/mol; 𝑄𝑐𝑚 is enthalpy of this combustible mixture, kJ/mol; 𝑄𝑐 is enthalpy of combustible substances, kJ/mol; 𝑄𝐿 – the amount of heat transferred to the wall system and lost in the environment, kJ/mol; 𝑄𝐷 is the amount of heat consumed for dissociation, kJ/mol. If we substitute the value of the enthalpies of combustible substances into the equation 𝑄𝑐 = ∑ 𝑛𝑖 𝑐𝑖 𝜗 (71), after several transformations, the combustion temperature is obtained by the following formula: 59

𝜗= where

𝑄+𝑄𝑐𝑚 −𝑄𝐿 −𝑄𝐷 , ∑ 𝑛𝑖 𝑐𝑖

(72)

 n c – is the sum of the derivative number of moles of comi i

bustible substances for molecular heat capacity;  is combustion temperature. In the absence of heat input under adiabatic conditions and taking into account dissociation, combustion temperature is called theoretical combustion temperature. Determination of the theoretical combustion temperature with regard to dissociation is difficult and requires the following conditions: – for the temperature band in the range of determination of the target theoretical burning temperature, the equilibrium composition of combustible substances of this mixture is calculated using all possible reaction constants; – for the equilibrium composition of each used fuel, the enthalpies are calculated; – if physical energy of the mixture is equal to the sum of its chemical energy, the enthalpy of combustible substances in the calculation interval is expressed as the value of the theoretical temperature.

60

8.1. Flame propagation in gas flow and its normal velocity Flame spreading in a gas stream is a continuous process in which chemical combustion reactions take place actively, since when gas passes through a narrow flame zone due to thermal conductivity of combustible substances and heat is transferred to the gas by transferring diffusive heat [10]. Thus, combustion of combustible gas mixtures, which are in a motionless state or in a laminar motion, is characterized by a normal propagation speed of the flame that has arisen U n (the flame is directed along the normal, built on the flame burning front). In order to understand flame propagation process, movement along a tube of a previously prepared gas mixture is considered. After the start of burning, a -thick burning front is formed, which moves in the direction of the mixture. Securing of the flame front is observed in case of equality of the speed of movement of combustible mixture and the speed of movement of the flame front moving in the opposite direction. When the flame spreads, its front divides gas volume into two parts: a weakly heated gas mixture is ahead of the flame front, and heated combustible substances are behind the flame front. The temperature in the combustion zone changes from the initial value of T0 to the value of Tг, and the concentration of combustible substances – from C0 to zero. Here Tг is the temperature of combustible substances and gases. The reaction rate depends on the concentration and temperature of combustible substances; therefore, it reaches maximum in the flame zone (it increases with the increase in temperature and decreases with the decrease in the amount of combustible substances). The higher the reaction rate, the higher the rate of flame propagation and the shorter the time gas remains in the flame zone (see Figure 8.1). 61

Figure 9 shows the change in flame temperature in italics. To do this, we construct a tangent to the temperature curve that intersects with the straight lines of T0 and Tг. The distance between the intersection points  is called thermal thickness of the flame front (see Figure 10).

Feed mixture

Combustibles Combustion reaction zone

Figure 9. Change in temperature, quantity and reaction rate of flammable substances at the flame front

Feed mixture

Combustion reaction zone

Figure 10. Determination of the thickness of the flame front

By the equation of two expressions determining the heat flows q, we obtain the equality connecting normal velocity of propagation and thickness of the flame front: Un 

λ а  , ρс р δ δ

where a is the temperature coefficient of conductivity, m2/s. 62

(73)

As experiments show, in normal flame propagation, chemical reactions take place in a very thin bed separating combustible substances from the non-burning mixture. This thin zone is called a flame. The flame front is very thin. Since gases have low thermal conductivity, normal flame propagation is slow. Even for slowly burning mixtures, the thickness of the flame front does not reach one millimeter. The velocity U n depends on the temperature of the mixture, the type and composition of the combustible gas and oxidizing agent (in the stoichiometric mixture it reaches the maximum value). U n velocity values for combustible mixtures are as follows: CO + air – 0.43 m/s; CH4 + air – 0.38 m/s; H2 + air – 2.65m/s; H2 + O2 – 13 m/s.

8.2. Determining the velocity of the Bunsen burner The velocity of flame propagation is not difficult to determine with a Bunsen burner (see Figure 11). Imagine a vertical tube through which gas-air mixture moves subject to 1.

Figure 11. Determination of the normal velocity of Bunsen burner flame

63

At the tube exit, the mixture begins to burn, and a conical flame appears. A part of the gas, which did not get enough oxidizing agent (air), burns down in atmospheric air supplied by diffusion [11]. For a constant flame burning, the velocity of gas traveling along the normal constructed from each point of the flame front must be equal to normal velocity of flame propagation: Wn  U n , and

Wn  W cos , then U n  W cos  . Knowing the pattern of change in velocity over a radius W = f(r) and  angle, it is possible to determine U n , but this method is quite complicated. Taking into account the fact that the shape of the internal flame front is conical and the amount of the initial mixture fed to the tube is equal to the amount of the mixture burnt in the flame front, we obtain the following equation:

SW  U n F  G ,

(74)

where S is the burner cross-sectional area; F is the flame front area; W  velocity of the mixture motion in relation to average consumption; G – is a mass flow rate of the original gas mixture. We determine the normal propagation rate of the flame as follows: U n  G F . In this equation, we introduce the area of the side surface of the cone being F  R h 2  R 2 . We obtain a simple formula for calculating the normal rate of flame propagation.

Un 

G R R  h 2

,

(75)

2

where h is the height of the flame cone, R is the radius of the burner. Normal flame spread velocity U n is considered to be the main characteristic of a chemical reaction taking place in a flame. In experiments, U n is measured, cm/s. 64

8.3. The velocity of mass propagation of flame As a flame characteristic, one can consider the volume of the mixture, which burns down per unit of time at a unit of area of the flame surface [cm3/(cm2·s)] and the amount of heat released per unit of time from the surface unit [kW/m2]. The product of normal flame velocity U n and mixture density  is called mass propagation rate of combustion.

U m  U n , g/(cm2·s).

(76)

The rate of mass combustion propagation is equal to the amount of mixture burning on the surface area per unit of time. Mass velocities are used to compare normal flame propagation speeds in different mixtures under the effect of various pressures and temperatures.

8.4. The Law of areas As an example, consider the movement of a flame along a horizontal tube. Experiments show that the flame front in a horizontal tube is asymmetrically relative to the tube axis, as shown in Figure 12. The burning zone separates the original clean high density mixture from combustible substances of low density. Due to its high density, the pure mixture is heavy and flows through the lower part of the tube, and light combustible substances move along the upper part.

Figure 12. Spread of flame in a horizontal tube

65

At each point of the curved flame front, the flame moves at a U n velocity perpendicular to the surface F . Therefore, the volume of the mixture that burns out per unit of time, V , cm3/s, is determined by the following formula: (77) V  UnF. Such a volume of the mixture may be determined using a representative flame propagation velocity U and pipe cross-sectional area S.

V  US .

(78)

The representative flame propagation velocity is the movement of the curved flame front in the initial clean mixture, equal to the distance between the edges of the flame front, observed over 1 s. From equations (77) and (78) we obtain the area formula: U  Un

F . S

(79)

The area formula means the following: the amount of the burning out mixture per unit of time at the flame front is equal to the amount of this initial mixture [11]. According to the law of areas, the larger the flame front area of a pipe section, the more representatively the speed of the flame propagation is greater than the normal speed of its propagation. If the normal, built on the flame front, forms an  angle with the direction of flame propagation, then the area of the element on the flame front is: dS . (80) dF  cos Applying the law of areas to the flame front element, we obtain the following equation: Un , (81) U  cos  66

in other words, the speed of flame propagation is inversely proportional to the cosine of angle  . Equation (80) gives the basic equality of the combustion process in a moving gas. The law is established by Michelson and is called the cosine law. This law determines the need to increase the area of the flame front to accelerate combustion with a certain flame propagation velocity. Secondly, by determining the  angle at any point of the flame, it becomes possible to determine the shape of the flame front.

67

9.1. Special characteristics of gas combustion The following types of gas are burned in boiler plants as gaseous fuel: natural gas, blast furnace gas (metallurgical production residues), coke oven gas (rarely burned in heating boilers, mainly used in metallurgy in open-hearth furnaces). Natural gas is considered to be the most valuable fuel. Its combustion heat (lower) is 35000÷35500 kJ/m3. In Kazakhstan, the explored natural gas reserves total 3.7 trillion m3. Location of the natural gas reserves by regions: in the Atyrau region – 43%, in the Mangystau region – 29%, in the West Kazakhstan region – 19%, in the Aktobe region – 5%. Sources of natural petroleum associated gas are located at Tengiz, Kachagan, Korolevskoe mines; gas condensate fields – in Karachiganak, Zhanazhol, Oriktau [12]. In terms of use, gas fuel has several advantages over solid fuel: 1) widespread use of gas contributes to purification of air in cities (there is no fly ash); 2) despite high efficiency of the boiler for burning pulverized coal, after its conversion to gas, the efficiency increases by 4-6%, while energy consumption for own needs decreases by 25-30%; 3) boiler construction costs are reduced, as there is no need to prepare expensive pulverized coal; 4) the size of the boiler building is reduced. For example, let us compare TGM-84 and TP-80 boiler plants of the same capacity (Dп = 420 t/h). The specific volume of the TGM-84 boiler per one ton of steam is less by 60%; 5) ease of automation of a gas boiler. 68

The combustion of gaseous fuel is homogeneous, in other words, there are no such phases that occur during combustion of solid fuels, such as the release of volatile substances, gasification of coke, and formation of slag. Gaseous fuel does not require prior preparation. Combustion of gaseous fuels consists of three stages: mixing, heating and combustion. The mixing method, where gas is mixed with air within the burner, is called kinetic, and gas combustion velocity is determined by the rate of chemical reactions. The mixing method, where gas is mixed with air within the boiler, is called diffusion, and gas combustion rate is determined by air/oxidant reaching gas molecules. Gas is burned in combustion chamber, into which the combustible mixture is fed using burners. As a result of complex physical-chemical processes occurring in combustion space, a constantly burning gas jet, called a flame, appears in the boiler. Depending on the method of supplying air required for fuel burning to the boiler, three methods of gas combustion have been proposed: – burning of homogeneous gas mixture when a pre-made combustible mixture is burned; – diffusion combustion, where gas and air are fed into the boiler separately; – combustion of gas mixture under the conditions of insufficient amount of air when gas is supplied to the boiler together with air, but the amount of air is not enough to burn the mixture completely.

9.2. Laminar combustion of a homogeneous gas mixture The dynamism of combustion of a premixed mixture depends on the rate of chemical combustion reactions. Therefore, this burning is called kinetic. Combustion of a homogeneous gas mixture occurs with the continuous supply of combustible mixture to the furnace and flame spreading in the boiler. Depending on the nature of the movement of combustible mixture, laminar and turbulent combustion is distinguished. Laminar burning occurs as follows. 69

To prevent flame from bending, a homogeneous gas mixture is fed to a vertically located burner. In case of a laminar motion of the mixture, its velocities are directed in the burner along the parabola [13]. ср = 0,5 0 . (82) Here 0 – gas velocity at the center of the section. This propagation of velocity is also maintained at the outlet of the mixture from the burner, where the velocity of the mixture is very low. Closer to the burner axis the velocity reaches the maximum value. When considering combustion of the mixture that has gone beyond the burner, it is possible to observe stable flame burning. The velocity of the released gas flow is equal to the normal flame propagation velocity. At the burner exit, the walls transmit heat, for this reason U n may be lower than the velocity of the outgoing gas flow, and the flame prevents it from re-entering the burner. In a round burner, the mixture burns in the form of a ring. The flame flows from the edges to the burner axis (please see Figure 8.1), therefore a certain distance is passed from the burner edge before the flame reaches the axis. In this area, the flame takes the form of a cone. A very thin burning zone forms the flame front of light blue color, clearly visible in space. The time of movement of the flame from the perimeter of the burner to the jet axis is determined by the following formula:

 

R , Un

(83)

where R – is a circular burner radius. In this period of time, jets that go straight from the center of the burner move with a W speed and travel a distance l . l

WR . Un

70

(84)

The shape and size of the flame depends on the flame propagation speed and gas flow speed W at various points in the jet. As U n value increases and W value decreases, the flame will be short, and, conversely, as U n value decreases and W value increases, the flame will be long. The larger the radius of the burner, the higher is the torch.

mixture Figure 13. Laminar torch of homogeneous gas mixture

Thus, combustion takes place on the lateral surface of the coneshaped flame, the thickness of combustion front is equal to one tenth of a millimeter, and the bulk of the flame is an inert mass.

9.3. Types, parameters and classification of gas burners A gas burner is a device for supplying a certain amount of gas and an oxidizing agent (air or oxygen) into the furnace or ensuring their mixing and transferring the formed mixture to the place of gas combustion. The main operating parameter of the burner is considered to be the exit velocity from the burner orifice W0. The value of W0 velocity should prevent blowback of flame in the burner and flame lift-off from the burner. Stable burning in the burner depends on gas composition, the excess air coefficient α = and the 0,5 diameter of the burner openings at the outlet of gas-air mixture d0 . The higher the speed W0 for large values, the more often flame leaks 71

into the burner. The higher W0 velocity with higher α = and0,5 d0 values, the higher the maximum speed of the flame leakage Wпр is. To prevent blowback of flame in the burner, W0 velocity value must be greater than the maximum travel speed. To prevent flame lift-off from the burner, the diameter of the burner opening for the mixture exit should be kept to minimum. Such a diameter is called critical [14]. At low values of the excess air coefficient the combustible properties of gas-air mixture deteriorate, the flame cannot penetrate into the burner. Table 8.1 shows the dependence of the predicted values of the limiting speed Wпр (minimum speeds in the absence of flame blowback into the burner) on α = and0,5 d0 values for some combustible gases. For natural gas, the coefficient of excess primary air should be taken as α = = 0,6÷0,65. The rate of mixture exit from the 0,5 burner should not be higher than the rate of flame separation from the burner Wпр (0,6 ÷ 0,7). Table 4 Flame velocity value, normalized, m/s Diameter of burner apertures, mm 4 8

gas natural

coke

butane

α = 0.7

α = 0.8

α = 0.7

α = 0.8

α = 0.7

α = 0.8

0.75 1

1 1.6

0.1 0.1

0.25 0.25

0.2 0.2

0.4 0.4

Let us consider the classification of gas burners, based on the principle of mixing gas with air. Mixing is carried out in a natural way due to diffusion or mixing of jets or by forced means under pressure. In this regard, the burners are divided into three types: diffusion, injection and mixing. Depending on air supply method required for gas burning, the burners are grouped into: – diffusion (air comes in naturally from the surrounding atmosphere); – injection (air is blown into the burner); – forced draught (air is supplied to the burner under pressure). 72

Depending on gas pressure, gas burners are divided into: – low pressure, where gas pressure is up to 5 kN/m2; – medium pressure, where gas pressure is in the range of 5÷300 kN/m2; – high pressure, where gas pressure is above 300 kN/m2. By gas combustion heat gas burners are of the following types: – used for gases with low calorific value, Qсн= 8 MJ/m3; – used for gases with medium calorific value, Qсн= 8 ÷20 MJ/m3; – used for gases with high calorific value, Qсн= 20 MJ/m3. Burners, depending on the type of the unit that uses the fuel (rather than on the type of fuel), are divided into boiler, gas turbine and furnace. In the energy sector, burners are divided into four groups: micro burners (for boilers with a steam generating capacity up to 2 t/h); small (for boilers with steam generating capacity up to 25 t/h); medium (for boilers with steam generating capacity up to 160 t/h); large (for boilers with a steam generating capacity higher than 160 t/h).

9.4. Diffusion burners and management of mixing in burners As an example of a diffusion burner you can consider a burner with two rows of orifices located horizontally at the bottom of a small combustion device. The boiler has a grate.

air Figure 14. Homogeneous diffusion burner

Air is supplied to the boiler through the furnace grate. Stably supplied low pressure (3–4 mmHg) creates in the boiler the conditions 73

for air to enter the boiler. Diffusion mixing of gas with air occurs within the boiler. Diffusion mixing limits the working capacity to gas flow rate of 30-50 m3/h. To improve operating capacity of the burner, it is necessary to provide air supply under pressure to the grate. Diffusion gas burner is located in a vertical hole of the boiler, built of fireclay bricks [1]. Figure 15 shows a diffusion burner for mixing air with a combustible gas. air

газ

1 – boiler embrasure; 2 – burner body; 3 – channels between air supply tubes; 4 – gas pipes Figure 15. Diffusion tube burner for external mixing

A bright long flame comes out of the burner. A uniform burning process occurs throughout the entire working compartment. A tubular burner is used to burn blast furnace gas. Gas is supplied from the gas collector through its pipes, and air – from the air collector located opposite, through the channels between its pipes. The gas is mixed with air in the jets at the tubes exit. The advantage of diffusion burners is as follows: burners are not very sensitive to gas pressure drops; wide range of regulation of the burner operating capacity is available. However, combustion process 74

develops throughout the working compartment of the boiler and requires the necessary volume to complete combustion. The soot factor of the flame is very high, the burner also operates at low gas pressures without back-draft. In diffusion burners, gas and air can be preheated to 600ºС. Disadvantages of diffusion burners: in comparison with kinetic burners, they require high value of excess air coefficient ( = 1.1-1.15), the pressure of the boiler volume is low, and at the very end of the flame combustion process is weak.

9.5. Injection Burners Self-sustained mixing of gas with air prior to combustion is ensured using a gas burner with an injector or a jet mixer. An injection burner may be used to burn low and medium pressure gas. Gas passes through a narrow burner channel, called injector, and then mixes with the primary air entering through the tip, and passes into a wide part of the burner, called diffuser. Secondary air required for complete combustion is supplied through the burner separately, since its pressure is 1-2 mm Hg. [13]. Coefficient of primary excess air α1 shows how many times more air is supplied to the burner compared to the preliminary (before combustion) theoretical amount. The coefficient of secondary excess air α2 shows how many times more air is supplied to the combustion zone compared to the theoretical amount. The injection ratio is the main characteristic of the injection burner. V (85) n г , V where V  is the proportion of intake of primary air, Vг is the amount of gas supplied through the burner. In diffusion burners α1 = 0. In injection burners 1  1 or 1  1 . Some injection burners provide premixing with air.

75

Pre-mixing injection burners are called flameless burners, since they do not have a visible flame when burning gas-air mixture. The air is sucked into the burner spontaneously and is mixed with gas inside the burner. The final mixture that has left the burner quickly burns out in a short boiler flame. As an example of an injection burner, Figure 8.4 shows an IGK burner, 1  1 . The burner consists of an atomizer for gas feeding, a mixer, a secondary air flow regulator and a plate stabilizer. The stabilizer protects the burner from high temperatures and does not require to install an additional ceramic tunnel. A short flare appears from the injection burners. Due to the high operating performance, the burner head at the flame outlet must be cooled with water. There is no need to cool the burner where the stabilizer is installled.

Secondary air Gas

1– plate stabilizer; 2 – mixer; 3 – air cap with gasket for noise suppression; 4 – gas atomizer Figure 16. Injection burner with premixing inside the burner

9.6. External mixing burners In external mixing burners, fuel (gas) and oxidizing agent (air) are fed into the boiler compartment in parallel, that is, in the form of jets that do not mix with each other. Fuel and air are mixed in the boiler, forming a mixture. The speed of the flame front propagation is tens of centimeters per second, and gas and air velocities at the burner exit are 76

much higher, therefore the length of the flame coming out of such a burner is large compared to that of other burners with (60÷80) dвых of the output diameter of the burner. As an example of an external mixing burner, consider a pipe-inpipe type burner developed by the Stalproekt Institute for methodological heating furnaces (see Figure 17) [4]. воздух air View А

gas

air gas

a – low performance; б – medium performance; в – high performance Figure 17. Burners with external mixing

The burner brand is indicated with three letters and a number. The number means the diameter of the burner head dвых in millimeters; the first letter (D) means a two-wire type of burner (that is, two tubes for air and gas supply are connected to the burner); the second letter (В or Н) shows gas combustion heat (high – 20÷36 MJ/m3 or low – 4÷10 MJ/m3); the third letter (M – small, C – medium or Б – large) characterizes the working performance of the burner. Burner calculation provides for the determination of areas of sections through which gas or air passes affected by the pressure of the medium surrounding the burner.

77

10.1. Spread of turbulent flame Laminar motion of a homogeneous gas mixture is possible at low values of the conditional Re. When Re > Reкp, the stable gas flow is disturbed, the whole flow is divided into several parts, the movement of individual parts is erratic and undulating. The change in the flame structure can be seen from the operation of the Bunsen burner (see figure 18). In the laminar flow, the flame front is thin and uniform. The height of the torch varies in proportion to the flow rate [7].

Figure 18. Changes in the flame structure in the course of transition from laminar to turbulent motion

1. Surface Combustion Theory. Under the effect of turbulence (irregularity), the flame front surface bends and increases greatly (see Figure 19). The flame moves across the surface at a constant speed Un, the increase in flame propagation rate is proportional to the increase in the burning surface area, U т  U n Fт / Fn . The surface area of turbulent combustion is 78

considered as the cone area and proportionality of the relative increase in areas Re and turbulent propagation speed is determined: U т  U n Re . This theory is feasible with small-scale turbulence.

Combustibles

Incoming Mixture

Figure 19. Flame spread in small-scale turbulence

2. Overall combustion theory. This theory is based on a large-scale turbulence. The elementary volumes of the burning mixture and flammable substances, moving into the fresh mixture, create ignition foci (see Figure 20). A certain amount of incoming mixture (molder), reaching the combustion zone, breaks the combustion front into separate foci. The combustion process takes place according to the laws of the normal spread of the flame. The flame front consists of the sum of individual surfaces of the molder. In this case, U т  U n  Re and, as the experiment shows, the height of the flame does not depend on the flow rate of the mixture. h

WR WR WR      f (W ) . U т U n  Re U nW  2 R 2U n Combustibles

Incoming mixture

Figure 20. Flame Spread with Large-Scale Turbulence

79

(85)

10.2. Turbulent combustion of a homogeneous gas mixture To accelerate gas combustion its speed should be increased and turbulent combustion applied. The use of atmospheric burner and flame ignition in the open air leads to undesirable results, since it is impossible to develop a high gas velocity, and the flame is quickly extinguished. To ensure stable combustion of a turbulent flame, a homogeneous gas mixture must be burned in an environment full of combustible substances. The mixture leaving the burner is considered to be a nonisothermal jet that heats and expands as the flammable substances enter. According to the non-constant jet temperature theory, the heating of the jet occurs in a turbulent boundary bed, and in the core of the initial zone with constant speeds the temperature is stable and equals the temperature of the mixture leaving the burner. The change in concentration C and temperature T is shown in Figure 21. The closer the jet to the outer edge, the higher the temperature and the lower the concentration of the mixture. Unlike concentrations, temperature exerts a great effect on the rate of chemical reactions. Therefore, jet ignition occurs in its outer bed and passes along the conical surface АД, where the flame propagation velocity reaches its maximum value. Only on this surface does the flame become stable. Heat is transferred by means of turbulent heat conduction to other extreme beds closely located to such outer bed, then they light up. Turbulent motion also affects the structure of combustion surface. Under the effect of turbulent pulsation, the flame front bends, breaks and divides into separate combustion centers. But at the same time the flame retains a conical shape, since ignition occurs on the outer jet surface. Consequently, the main part of the turbulent flame is inert and not fully used. Figure 21 shows the gas plume diagram. The length of the ignition zone l з .в . shows the interval reaching the extreme point of ignition in the flame axis. Let us transform formula (8.2), obtained in considering the laminar motion of a homogeneous gas. Instead of the normal flame propagation rate U n , we substitute turbulent combustion speed U т and determine the length of ignition zone: 80

l з .в . 

WR , Uт

(86)

where W – representative velocity of the mixture; R – burner radius; Um – velocity of turbulent motion of the mixture.

1 – incoming mixture; 2 – conical ignition surface; 3 – turbulent ignition front; 4 – visible burning front; 5 – end of burning zone; C – change in the combustible mixture concentration over the torch section; T – temperature change Figure 21. Diagram of turbulent flame of homogeneous mixture

The non-ignited mixture inside the cone bounded by the ignition zone surface is also in motion. Using  т we denote thickness of the front of a turbulent flame. We take the amount of gas in one mole equal to the mixing path and determine the time of burn out of one mole of gas:

 

lт . 2U n

(87)

In this period, being affected by the pulsation velocity, one mole of gas covers the distance of  т : 81

T 

lT W 1 . 2U n

(88)

This distance is considered to be the thickness of the turbulent combustion front. Visible burning front is the flame length of lз.в.  Т . Here is the ignition of the stream and the burning of the ignited mixture. At the exit from this zone, despite the high rates of gas exit from the burner, 90% of the mixture burns. Combustion ends in the part located behind the visible flame front of l Д . This area is called the continuation zone of turbulent combustion. In this area, combustion should be fully completed. Consequently, the value is greater, the lower is the rate of chemical reaction l Д and the higher the rate of release of gas from the burner. 10.3. Gas-oil boilers and gas-oil burners Furnace devices for simultaneous combustion of gas and fuel oil resemble a tube shielded chamber. All walls and the bottom of the chamber are very tightly shielded, that is, they have a system of tubes with feed water in them – condensate of steam that has been used in the turbine, to which chemically purified water is added. Natural circulation boilers have a sloping bottom, while boilers with forced circulation are horizontal. The main elements of the gas-oil boiler are gas burners and fuel oil atomizers. Very often, combined gas-oil burners are used for simultaneous or separate combustion of gas and liquid fuel. GMGtype gas-oil burners, proposed by CKTI (Russian Scientific and Production Association for Research and Design of Power Equipment named after I.I. Polzunov), were first installed on DKVR waterheating boilers. In the GMG burner (see Figure 22), the primary and secondary air are rotated in the same direction using a blade device. Gas is supplied through very small openings in the annular manifold. The diameter of the hole is determined according to gas combustion heat. The thermal power of GMG type burners developed by CKTI is 1.5; 2; 4; 7 Gcal/h. To spray fuel oil with a fire pressure of 2-5atm, steam mechanical atomizers with a pressure of 0.6-2 atm. are used [10]. 82

Primary air Secondary air

1 – gas-air compartment; 2, 5 – swirlers of the secondary and primary air flows; 3 – plate for burner mounting; 4 – ceramic tunnel; 6 – steam mechanical atomizer Figure 22. GMG burner developed by CKTI

For high power boilers (4÷30Gcal/h), the burners for RGMG were designed. Here, instead of the steam mechanical atomizer, a rotary atomizer is installed (see Figure 19). Also, rotary atomizers are installed in water-heating boilers in the absence of water vapor.

Secondary air Primary air 1– fire kindling device; 2 – gas inlet; 3 – primary air inlet; 4 – gas collector; 5 – blade apparatus; 6 – gas release openings; 7 – rotary atomizer Figure 23. RGMG Gas-oil burner

83

10.4. Combination gas oil burners and boilers The burners may be located on the front wall or in several rows. To speed up the burning process, the burners are placed opposite each other on the side walls of the boiler or on the front and rear walls opposite each other. This arrangement mainly takes place in directflow steam boilers. For example, in boilers designed by VTI, this combination enhances flame turbulence. When two opposing jets collide in the boiler, mixing improves and the residence time of the combustible mixture in high temperature zone increases, that makes it possible to reduce by 1.01-1.03 the value of the coefficient of excess air in the process of gas and fuel oil combustion. In this case, the rate of the flow coming out of the burner at the designed boiler load may reach 70 m/s. The distance between two opposite walls should be 5-10 calibers. Caliber is a section equal to the diameter of the burner embrasure. In a boiler with angular tangential burners, mutual accidental flame ignition takes place. Combustion occurs in a vortex flow, covering the belt where all the burners are located. The overall temperature level is not very high, but combustion spreads over a large zone, and thermal radiation fluxes are evenly distributed to the walls around the boiler perimeter. According to the Central Institute for Boiler Equipment Design, the air supply to the burner at an angle (45-50o) in the form of a vortex contributes to the stabilization of ignition and accelerates combustion. Figure 18 shows the TKZ gas-oil burner for supplying air through the tangential register and gas from the medium. The air supplied through register 2, when passing the blades, located at the angle of 40-50°, twists and passes into the embrasure. Gas passes through the circular duct 3 and is fed through a radial hole located at the beginning of the duct. The fuel oil atomizer is equipped with an igniter and remote control. The operating capacity of a gas-oil burner for fuel oil varies 1 to 16 t/h and for gas this capacity reaches 10000 m3/h.

84

11.1. Properties and special characteristics of liquid fuel burning In the process of heat treatment of natural liquid fuels (oil), light volatile fractions (gasoline, naphtha, light oils) are emitted from these. As a result of this oil processing, a residue is obtained – fuel oil, which is used as a fuel for transportation means (marine fuel oil of MF1, MF2 and MF3 brands), in metallurgical industry (MP furnace oil) and as a power fuel (M40, M100, M200). It should be noted that today oil is a scarce commodity and in the face of rising oil prices, the use of fuel oil in the power sector is limited. It is possible to use fuel oil only as an emergency fuel [8]. The following basic properties of fuel oil, not specified above, may be mentioned: surface tension, flash points, ignition and hardening temperatures, coking behavior. Such properties relate to the thermal characteristics of fuel oil. The surface tension of fuel oil depends on its viscosity – the higher the viscosity of fuel oil, the higher the surface tension is. This characteristic has a great effect on fuel oil spraying by injectors. To arrange security measures in fuel oil storing in a warehouse, it is necessary to know its characteristics, such as flash point and ignition temperature. The flash point is the temperature at which, in case of certain strict conditions, the heated liquid fuel or other types of oil products evaporate in sufficient amount, and, as the mixture of steam with ambient air approaches the fire source, a flash appears that is not yet burning the fuel. 85

If vapor flash time when this mixture approaches the fire source exceeds 5 seconds, the fuel starts to burn, and the temperature that is fixed at this moment is called the “ignition temperature”. Sometimes this temperature is called the “upper flash limit” and the temperature at the time of the flash is called the “lower explosive limit”, since the flash is an aggregate of small explosions. For any petroleum product, the difference between the two indicated temperatures is rather low and, as a rule, does not exceed 70 °C. As well as the ignition temperature, combustion temperature is an important indicator of the composition and quality of oil products. For power plants, flashpoints determine the fire safety of liquid fuel in the process of storage, the maximum safe temperature for liquid fuel heating in the environment that is not isolated from ambient air. Fuel oil solidification temperature is the temperature at which the level of viscous fuel oil poured into the tube with a 45° incline, remains stationary for 1 minute. Solidification temperature of energy fuel oils ranges from 5 °C to 36 °C. Solidification temperature, along with the viscosity, determines the possibility of the outflow of fuel oil out of the pipe.

11.2. Combustion of liquid fuel from the free surface Liquid fuel combustion begins not on its free surface, but in the vapor phase, located above the liquid surface. The flame appears at a very small distance (0.5-1 mm) from the liquid level. Constant heating of the liquid makes its surface bed evaporate, and directs the steam flow upwards. Oxygen constantly penetrates this flow from the environment due to diffusion, as a result of which a combustible mixture is formed. When the mixture ignites, a diffusion flame appears. The flame beams falling on the liquid surface accelerate the heating of the liquid to the boiling point and its evaporation. The evaporated surface of the liquid is called evaporation surface. The heat balance of evaporation mirror with the area of 1 m2 is recorded as follows [5]: q Л = Wг[ ccр (tк-t0) + λп], кВт/м2 , 86

(89)

where qЛ is the heat transferred to the liquid surface by radiation, kW/m2; WГ is the burning rate with respect to the area of a unit of evaporation surface, kg/(m2s); П is the heat of liquid fuel evaporation, kJ/kg. Each 1 m2 of evaporation surface is used to heat this fluid from the initial temperature t0 to the boiling point tk, as well as on evaporation. Combustion from the free surface of the liquid is classified as diffusion combustion.

11.3. Fuel oil droplet combustion Burning of liquid fuel begins when it evaporates. Therefore, burning droplets may be represented as follows: a droplet of liquid fuel is surrounded by the atmosphere and is saturated with its vapors. There is a combustion zone developed around a droplet with a dг diameter. The vapors separated by a droplet immediately enter into a chemical reaction with oxygen coming in from the atmosphere; therefore, combustion zone is very thin. Duration of the droplet burning is determined by its evaporation rate. There are vapors of liquid fuel and combustible substances in the space between the droplet and the burning zone. Consequently, a stoichiometric bond is established at a rстех distance from the center of the droplet between combustible gases and oxygen. Here, the burning front of fuel vapor is formed. This front takes the form of a sphere surrounding a droplet. The space behind the burning zone contains air and combustion products. Droplet vapors penetrate into combustion zone from inside, while oxygen comes in from outside. The heat from the combustion zone is transferred to the drop, and combustion products move both inward the zone and outward. As the droplet burns, its surface decreases, evaporation weakens, combustion zone decreases and gradually disappears. The droplet burnout time may be determined using the following heat equation: qFdτ = –ρ[ccp(tk – t0) + λn]dV, 87

(90)

where q is the amount of heat transferred per unit of time from the combustion zone to a unit of the area of the drop, kW/m2; F – is the total droplet surface area, m2; dV  Fdr – reduction of the droplet area over a period of time d . The time of complete burnout of the droplet, which received heat from the environment is directly proportional to the radius of the droplet,  (t  t )c   к 0 cp r qл

Fuel Vapors

droplet

Reaction zone

Combustion Products

Oxygen

Figure 24. Liquid fuel droplet burning

Figure 24 shows the pattern of burning of a droplet of liquid fuel. The diameter of the combustion zone is 3-5 times the diameter of the droplet.

88

12.1. Flame combustion of liquid fuel Figure 25 shows the flame structure arising from the burning of liquid fuel, and shows the burning pattern in the liquid fuel droplets flame coming from direct-flow burners. The main part of hydrocarbons in the form of vapor burns in the zone of ignition constituting the outer bed of the flame. Ignition zone 1 divides the entire combustion space into two regions: the inner region 2, where the evaporation process takes place and a combustible mixture is generated; and region 3 – the outer area. OUTER BOUNDARY

1 – ignition zone; 2 – evaporation of the droplet and the formation of a combustible mixture; 3 – zone where combustion of hydrocarbons continues; lз.в – the length of the ignition zone; l Ä – the length of the zone where combustion continues; lф – flame length Figure 25. Type of liquid fuel flame

89

The air required for fuel burning is supplied from the burner nipple, and, covering the liquid fuel, extracts it in the form of a jet dipped into the boiler. The jet enters the heated combustible substances in the boiler and heats up. Due to the jet heat exchange, the droplets in the jet heat up and evaporate. Mixed with air, vapors ignite, resulting in a flame. Thus, jet combustion of liquid fuel comprises the following stages: spraying of liquid fuel; evaporation and thermal decomposition of droplets; formation of gas-air mixture; ignition and combustion of the mixture; burning vapors, coke and ash particles surrounding the droplet. When the jet reaches the combustion axis, the flame travels from the burner nipple to the site l з .в . . This is called the ignition zone. The ignition zone resembles an oblong cone. The main part of hydrocarbons in the form of vapor burns in the combustion zone (2), which covers the outer part of the flame of a small thickness. Combustion of high-molecular hydrocarbons, free carbon, soot and non-evaporated droplets occurs in the zone lд.

12.2. Atomizers for liquid fuel Spray devices that supply heated fuel oil to the furnace compartment are called fuel oil atomizers. They are divided into two groups: mechanical atomizers and atomizers using a sprayed medium (vapor or air). The atomizer diagrams are shown in Figure 26 [6]. To provide a liquid breaking up into droplets in vapor atomizers, the kinetic energy of the vapor supplied from the atomizer is used. A moving droplet falls under the pressure of the gas medium, which tends to stretch and separate it. The force generating pressure is the friction force arising when gaseous medium affects the front section of the droplet. Pressure generated by the friction force is determined by the following formula: Р1 = W2, where ζ is the friction coefficient of the gas medium (vapor) (as a rule, at Re = 103-105, ζ = 0.2);  is the density of the medium, kg/m3; W is relative velocity of the droplet, m/s. Under the effect of surface tension, the droplet acquires a spherical shape. The pressure generated 90

by the surface tension for ces – Р2 = 2 σ/r, where σ is the surface tension coefficient of the liquid, N/m; r is the droplet radius, m. Low-pressure air, 100% vapor (compressed air) air fuel

air

air

air fuel

fuel fuel

Spinning bowl

a) – atomizers using a sprayed medium (high or low pressure); b) mechanical atomizers: 1 – straight-through; 2 – centrifugal; 3 – spray bowl Figure 26. Fuel oil atomizers diagrams

Disintegration of liquid fuel into the smallest droplets occurs under the condition that Р1 > Р2 or W2  2 σ/r. Hence the maximum value of liquid fuel is determined: r  2 W 2  . Separation into small drops depends on the surface tension of the liquid; density of the medium and the relative velocity of the drop and gaseous medium. The higher the temperature of the liquid (when fuel oil is preheated), the lower is the surface tension. The second type of atomizers is mechanical. In such atomizers, under the influence of centrifugal force, the entire flow of the medium (liquid) is disrupted. Further, the process of disintegration into droplets depends on the pressure of the medium. Spraying consists of complex physicochemical processes. Fuel oil is sprayed using steam or air. Depending on the spraying method, atomizers are divided into the following four groups: 91

a) mechanical; b) rotational; c) high pressure (pneumatic, steam or air); d) low pressure, air (ventilating).

12.3. Mechanical atomizers The spray part of the atomizer consists of a chamber that includes several channels. The operating performance of the atomizer depends on the size of the atomizer, pressure and viscosity of fuel oil.

1 – trunk; 2 – coupling nut; 3 – distribution washer; 4 – swirl chamber disk; 5 – atomizers disk Figure 27. Mechanical atomizer head

Effective operation of mechanical atomizer is due to the fuel oil pressure. As a rule, fuel oil is fed into the atomizers under the pressure of 2.5-3.5 MPa. Apparent viscosity (AV) of the fuel oil before entering the atomizer should not exceed 3.5 AV. Atomizers on the device and model parameters should be normalized. With the exception of spraying elements, the rest of the atomizer parts are the same. In small and medium capacity steam generators, the OH-547-01 atomizer is used. With a pressure of 1.96 MPa and the atomizer opening diameter of 2.5-7 mm, the operating capacity is 0.122-0.514 kg/s (0.4-2 t/h). Advantages of the mechanical atomizer are as follows: – due to high atomizing qualities, the atomizer ensures prudent burning of fuel oil, low energy costs for pre-compression of fuel oil before entering the atomizer (for example, if the fuel oil pressure is 3.5÷4 MPa, energy consumption volume of the pump is less than 0.1% of steam generator capacity); – the atomizer is noise-free and easy to use by maintenance staff. 92

Disadvantages of the atomizer: – rapid clogging of the spraying disc holes; – few opportunities to regulate the operating capacity; – since fuel oil is supplied to the atomizer using a pump, the pressure in the fuel oil pipe of the fuel oil handling equipment should be sufficiently high. Mechanical atomizers are used in medium and high power steam generators. The operating performance of the mechanical atomizer can be adjusted. To do this, you need to make changes to the initial pressure of the fuel oil or spray gun. The first method is inefficient, as when the pressure drops, the atomizer spraying quality also deteriorates, while the minimum fuel oil pressure before entering the atomizer should also not be lower than 1 MPa. The range of changes in operating performance by changing the pressure does not exceed 30%. The second method, changing the parameters for fuel oil supplying, complicates the atomizer design. If several atomizers are installed in the steam boiler, this will not change the initial pressure of the fuel oil and may cause several atomizers to switch off. In particularly powerful boilers such adjustment leads to the destruction of the temperature field inside the boiler.

12.4. Rotary atomizers Figure 11.2 below shows a spraying device of a rotary atomizer. Fuel oil pressure should be slightly higher than the atmospheric one (0.12÷0.13 MPa). Fuel oil is transmitted through the constantly rotating hollow shaft 1 to the fuel oil distributor 2. Next, through a multitude of tiny holes on the surface of the distribution disc, fuel oil is fed into the spray bowl 3. The bowl is put on the shaft head, as a result it rotates at the speed of 600-700 revolutions per minute; from the bowl fuel oil is continuously released as a film. The air compressed in the compressor mounted on the shaft leaves the narrow passage between the atomizer body and the bowl at high speed. In the bowl, particles of fuel oil are subjected simultaneously to the friction force of the walls and centrifugal force, that causes them to move along a spiral-shaped trajectory. At the outlet of the bowl, the centrifugal forces disappear in the medium, and the particles begin to move along 93

an imaginary tangent attached to the spiral. Consequently, the droplets of fuel oil come out of the atomizer in the form of a cone, tapering closer to the boiler. The cone is outside with the surrounding air, which prevents it from bursting. As the cone continues to move, the film of the cone becomes thinner and then breaks up into the smallest droplets. The atomizer spray quality does not depend on fuel oil viscosity and is considered satisfactory if VPB = 13ºAV. Such atomizers are used in industrial and marine boilers, as well as high power power-plant boilers.

Figure 28. Dispersing head of rotating atomizer

12.5. Vapor (pneumatic) atomizers For fuel spraying in atomizers with the sprayed medium, the atomizer energy is used – vapor or air moving at high speed. When using atomizers (see Figure 28), steam under pressure of 0.5-2.5 MPa moves along the inner tube and goes to the head with an expanded atomizer. Where fuel oil reaches the annular duct and steam, a jet of steam at a speed of 1000 m/s captures fuel oil. Next, a mixture of fuel oil and steam passes through the diffuser and into the furnace. At the end of the furnace, ending with a diffuser, an atomizer which increases the spraying angle of fuel oil coming out of the furnace along the cone is placed. Thus, the first disintegration of fuel oil into droplets occurs due to kinetic steam energy. The second time fuel is decayed into droplets with the help of air supplied to the boiler. Small droplets are formed in steam atomizing. 94

fuel oil Пар steam fuel oil

1 – steam conducting atomizer; 2 – diffuser; 3 – atomizer. Figure 29. Steam atomizer head

The advantage of steam atomizing is that the quality of spraying with steam atomizers is very high; the channel of such atomizers is blown through with steam, that prevents contamination. The interval of regulation of the atomizer operation is 20–200% (see table 5) [7]. Table 5 Comparative characteristics of fuel oil atomizers Atomizer type

Mechanical Pneumatic, high-pressure Pneumatic, low-pressure

Average droplet diameter, microns 2 40

Energy spent on spraying, %

Adjustment interval, %

Less than 1 2

70-100 20-100

100

5

20-100

Disadvantages of steam atomizing: steam condensate is exhausted; the amount of water vapor in the composition of flammable substances increases; increased consumption of heat carried by gases. Some amount of steam is spent on atomizing. In addition, steam injectors make a lot of noise. Also, the use of steam reduces the temperature of the torch by 150-200 °С. Steam atomizers are used in industrial steam generators for fuel oil burning, as well as in power plants (only for ignition of fuel oil). When steam atomizer is turned on, water flows into it first, and then fuel oil enters. When atomizer is off, the fuel oil supply stops first, then steam supply ceases. The viscosity value of fuel oil, ensuring normal operation of the atomizer is 6-7 °AV. 95

12.6. Low-pressure air-atomizing burners

fuel oil

In low-pressure atomizers 50-100% of the air required for combustion is supplied under the pressure of 5-10 kPa, for this reason such atomizers are large in size. Fuel oil is fed to the atomizer under the pressure of 0.03-0.214 MPa. This type of atomizers in boilers is used very rarely; they have found their application primarily in industrial furnaces. The channels, through which the fuel oil and air pass, end with a narrow atomizer. Such burners are used in thermal and heating forging furnaces. Thus, to ensure effective spraying, the following factors should be taken into consideration: – when using a mechanical atomizer, it is necessary to increase the pressure in front of it; – if air or steam is used for atomization, they should move at the highest possible speed; – preheating of fuel oil reduces its viscosity and surface tension; – the larger the opening angle of the flame, the more effective is fuel oil atomization.

air

Figure 30. Low pressure air atomizer (Rockwell type)

96

13.1. The mechanism of carbon burning in solid fuel Solid fuel containing carbon, moisture, hydrocarbon compounds and ash, when injected into combustion chamber, heats up and releases moisture and volatile compounds. The result is a solid residue consisting of carbon and ash. The release of volatile compounds affects the ignition of fuel, as this is a fairly fast process. In this process, only about 10% of heat is released. The main heat release is due to the coke residue carbon. Therefore, carbon combustion laws determine the burnout of coke particles [9]. Carbon burning is a heterogeneous process, determined both by the kinetics of chemical reactions and by diffusion transfer of oxygen and combustion products in the boundary bed at the surface of the particle. In addition, oxygen adsorption occurs on the surface of the carbon particle with the formation of unstable chemical compounds (chemical absorption). Subsequently, these compounds disintegrate with the release of CO and CO2 into gaseous medium. At low-temperature process (600800°C) the main role is played by the processes of chemical absorption, and the process of fuel oxidation with a low reaction rate occurs. In the range of furnace temperatures (1400-1800°C), the sorption processes occur almost instantaneously and it may be assumed that the amount of oxidizer absorbed by the fuel corresponds to the stoichiometry of oxidation reactions. In this case, the rate of carbon burnout according to experimental data may be represented as follows: W  kC, 97

(91)

where С – is the concentration of the oxidizing agent at the surface of carbon particle; k – is an experimental heterogeneous reaction constant Regardless of the mechanism of carbon combustion, the primary reactions of interaction between the fuel and the oxidizing agent are: С + О2 = СО2, 2С +О2 = 2СО.

(92)

Along with these reactions, carbon monoxide may react with diffusing oxygen at the surface of the carbon particle: 2СО + О2 = 2СО2

(93)

and on the surface of the particle carbon dioxide reduction due to carbon may be described by the following reaction: С + СО2 = 2СО.

(94)

In the presence of water vapor in the furnace volume, carbon oxidation may occur in the form of the following reactions: С + Н2О = СО + Н2,С + 2Н2О = СО2 + 2Н2, С + 2Н2 = СН4

(95)

and at the surface of the particle the following reactions occur: 2Н2 + О2 = 2Н2О ,СН4 + 2О2 = СО2 + 2Н2О, СО + Н2О = СО2 + Н2.

(96)

These reactions are called secondary reactions. Each of these reactions has its own heat effect and activation energy. Depending on conditions, both primary and secondary reactions may exert a decisive effect on the burnout process. Moreover, in case of "wet" gasification (the presence of water vapor), reactions may occur with the participation of active centers, that reduces activation energy and increases the burn out rate of the coke particle. We will consider “dry” gasification of a coke particle, when there is no H2O in the furnace space. Then the set of reactions determining the burnout process of the particle will be: 98

1) С + О2 = СО 2 + 394 [MJ/mol]; 2) 2С +О2 = 2СО + 219[MJ/mol]; 3) С + СО2= 2СО -186[MJ/mol]; 4) 2СО + О2 = 2СО2 + 570[MJ/mol].

13.2. Combustion of solid fuel in the bed. Distribution patterns of combustible substances in the bed of fuel, measures for the effective combustion of fuel in the bed In industrial boilers, solid fuels are burned in beds located in the combustion device or in flame beds. In other words, bedded combustion of solid fuels is the main method of burning fuels in low and medium capacity boilers. This method may be carried out in furnaces with the following types of gas and air flow, as well as fuel and slag: reverse, parallel, horizontal, and mixed. The main element of the grate-fired furnace is the grate. The furnace device with a chainlike grate is shown in Figure 31.

air

1 – grate surface; 2 – star gears; 3 – fuel dumping pocket; 4 – fuel thickness regulator; 5 – slag discharge; 6 – slag bunker. Figure 31. Overall view of the grate-fired furnace

99

Grate surface consists of individual grates, hinges attached to the chain and put on two star gears. Grate speed is 2-16 m/h. In a boiler equipped with a forward running grate, fuel is fed to the grate from the hopper. The height of the fuel bed in the bunker is regulated by the gate. In a boiler equipped with a backstop grill, fuel is fed from the bunker to the grate surface using pneumatic-mechanical spreader (see Figure 32). The air required for combustion is supplied from under the grate and enters the bed through the gaps between the grate bars. As the grate moves, the fuel on its surface has time to burn out. The slag formed is thrown away from the grate with the help of a slag cleaner. To reduce heat loss from incomplete chemical combustion, sharp blast air, which consumes 10-15% of the total used amount is used. Common disadvantages of forward and reverse grates, that lead to rapid wear of these, are periodic overheating of the grate in the combustion zone of the fuel, and, on the contrary, cooling of the space under the fire box due to cold air. In addition, with grate firing, the level of mechanical incomplete combustion of fuel increases, and heat losses Q4 increase.

1 – front part of the boiler; 2 – coal bin; 3 – fuel spreader; 4 – front engine shaft; 5 – stroke surface; 6 – zoned air supply; 7 – grate frame; 8 – supporting roll table; 9-, 10 – rear shaft and sealing; 11 – slag bunker. Figure 32. PMZ-LCR flyback return device equipped with a chain grid

100

Therefore, this method cannot be used for fuels with a low yield of volatile substances (anthracite and lean coal). Combustion of solid fuel in the bed occurs in the following stages: heating of the bed, drying of the fuel in the bed, release of volatile substances and formation of coke, burning of volatile substances and coke and burning out of slag. Solid fuel is delivered to the grate from above and is placed on the grate without movement. Burning of solid fuel takes place as follows. In the upper part of the bed (1) clean fuel is placed, then it begins to heat up and dry up; (2) burning coke is placed under it, slag is placed below on the surface of the grate; (3) (see Figure 12.1). These beds do not have clearly delineated surfaces, and often mix with each other. However, until complete combustion, solid fuel passes through all these beds. Figure 33 shows the temperature change throughout the bed height. Maximum temperatures are inherent in the combustion zone, from which the largest amount of heat is released. Slag formed in the process of combustion drops out of the heated coke spark. When the drops of slag flow, they react with the air flow from under the grate, cool and freeze, thus accumulating on the grid. This bed of slag protects the grid from very high temperatures. The air that has passed through the slag bed is heated and evenly distributed throughout the beds. Volatiles combustion

1 – clean fuel bed; 2 – burning coke; 3 – slag bed Figure 33. Fuel burning in the bed and changes in the temperature in the bed

101

With such an organization of the combustion process, the ignition of fuel begins from the bottom of the bed. In other words, there are endless possibilities for ignition and combustion of fuel. Here, gas-air flows and the bed of fuel moves in the opposite direction. The velocity of gas-air mixture in a separate bed should have such a value that would not violate the stability of the bed. That is, the mass of the fuel bed must be higher than the dynamic pressure caused by the gas-air flow. In other words, the aerodynamic characteristic of fuel combustion in a bed is considered to be the following inequality: Gч  cF

 2c 2

п ,

(97)

where Gч – mass of fuel particle, kg;

c – coefficient of resistance determined by the Reynolds criterion; F – cross sectional area of the fuel particle, m2;  п – density of the gas flow passing through a separate bed, kg/m3. Chemical reactions occur around coke sparks between the fuel and the oxidant. Depending on the chemical reactions, the entire combustion process may be divided into two zones: the oxygen or oxidation zone and the formation zone. Two carbon oxides, CO2 and CO, are simultaneously formed in the oxidation zone. At the edge of oxidation zone, the O2 concentration decreases, and the CO2 concentration and temperature increase. In the formation zone, carbon reacts with carbon dioxide: СО2 + С = 2СО – Q.

(98)

As a result of the reaction, the proportion of CO increases at the entire height of the formation zone, while proportion of carbon dioxide decreases. The reaction takes place with the absorption of heat, so the temperature in the formation zone drops. In the presence of water vapor in flammable substances, an endothermic reaction takes place, in the course of which decomposition of water vapor occurs: Н2О + С = CО + Н2 – Q. (99) 102

The presence of oxidation and formation zones in the fuel layer is a phenomenon inherent not only in the burning of carbon particles, but also in other types of fossil fuels burning. For natural fuels, the thickness of an individual layer is determined by the ability of fuel to react, and as the level of ash decreases, the thickness of the layer decreases as well. Depending on the method of combustion process management, inert or combustible gases may be obtained from combustion bed. If it is planned to convert fuel combustion heat into physical heat of combustion products, then the process should be carried out in a thin bed and with the addition of a large amount of oxidizing agent. If in the process of combustion it is necessary to obtain combustible gases, combustion should be carried out in a very thick bed and under the conditions of oxidizer deficiency. In the first case, the fuel burns, in the second – it is carbonated. The fuel bed thickness depends on the size of the pieces and its humidity. For example, when burning particles of lignite and coal are less than 20 mm in size, the bed thickness should be 50 mm, and in case of the particle size of 50 mm, the bed thickness must be increased to 200 mm. The moister is the fuel, the thicker the fuel bed should be. In this case, conditions are created for quick preparation of fuel for combustion. As a result, both fuel combustion and burning will be sustainable.

13.3. Methods for burning solid fuel in a chamber, vortex and cyclone furnace. Thermal performance of combustion chamber In combustion chamber of a power-plant boiler, solid fuel is burned as dust, and fuel pellets are burned in swirling-type furnace and fluidized-bed furnace. The method of flame (flaring) combustion of pulverized coal is used when gas flow rate exceeds the rate of evaporation of dust particles, i.e., when the dust soars in the gas flow. Particles of dust contained in gases have time to burn before gases come out of the furnace. Such burning is called flare burning of dust (see Figure 34 a). In order for dust to burn in a flame, the size of its particles should not 103

exceed 100-200 microns. Burning fuel dust increases the surface of its contact with air and facilitates transfer to the furnace [6]. Pulverized coal is transferred to the furnace by air, the excess air coefficient will be: αт = 1.2-1.25. In particularly large boilers, the levels of fuel and air loss are high. For example, in a large 300 MW block, the loss of fuel, that is, anthracite slab, is 115 tons per hour, and air loss is 885000 m3. Fuel air air

fuel air air Rich slag

a – flaring (flame) combustion method; b – method of combustion in a cyclone-swirling chamber; с – fluidized-bed combustion method; 1 – combustion chamber; 2 – grate; 3 – heat exchange surface; wn – gas flow rate; wc – gas flow rate in the bed. Figure 34. Solid fuel in the form of dust combustion patterns

In the method of fuel combustion in a swirling-type furnace (see Figure 34 b), as well as in the flaring method, fuel particles are transferred by a gas-air flow. The difference from flame combustion is that the fuel particles move in a circular motion along a specific contour and remain in such a motion until they burn out. Circular motion is achieved due to centrifugal forces pushing particles toward the walls of the cyclone chamber. Compared to flare combustion, the residence time of particles in the furnace is longer in the cyclone process, and the blowing out activity by gas-air flow is higher, therefore large fuel particles of 2-5 mm in size may be burned in a cyclone chamber. Due to the possibility to burn large fuel particles in cyclone furnaces, energy consumption for fuel grinding by mill is reduced. 104

In the cyclone chamber, air enters the furnace under a tangent angle or the burners are placed at an angle in the furnace, therefore dust-air flow enters the furnace in the form of a vortex, resulting in a rapid whirling flame. In cyclone chamber, the temperatures reach 1700... 1900 °C, and thermal voltage of the furnace volume is 2 ... 4 MW/m3. In fuel combustion method in a fluidized bed (see figure 34 c), fuel particles of 1-6 mm in size are delivered to grate 2 of combustion chamber 1. Air is blown out from under the grate into the furnace at high speed, that leads to a disruption of stability of the fuel bed on the grate and "boiling" of the bed. In other words, the majority of particles located in a bed pass into a rapid translational motion state. In this case, the rate of blowing by the gas-air stream of the layer is higher than the stability limit, but lower than evaporation rate of the particles that make up the bulk of the bed. Combustion of fuel in a fluidized bed facilitates the solution of some problems: it reduces the yield of nitrogen oxides and sulfur oxides, generates opportunities for ensuring stable combustion of fuel and burning of industrial wastes and lowgrade fuel. For the design, evaluation and comparison of combustion devices for the combustion of pulverized coal the following thermal characteristics of the furnace are used: 1. Heat output of the furnace indicates the amount of heat released per unit of time within the furnace: Qт  B  Qнр , MW,

(100)

р

where B is fuel consumption, kg/s; 𝑄н – low heat value, MJ/kg. 2. Furnace heat liberation: qv 

Qт B  Qнр , MW/m3,  Vт Vт

(101)

where Vт – is the furnace volume, m3. 3. Thermal stress of the cross section of the furnace in the area with a large heat release (at the place of location of the burners): 105

qf 

Qт B  Qнр , MW/m2,  fт fт

(102)

where fm= a·b – cross-sectional area of the furnace with the highest heat release, m2; a and b – width and depth of the furnace, m. 4. Efficiency of the furnace determines the efficiency of fuel combustion in the furnace:

 ò  1   qò  1  (q3  q4  q6 ) ,

(103)

where ( q3  q4 ) is the heat loss of the furnace from chemical and mechanical incomplete combustion of fuel; q6 – loss of heat carried by the slag. The amount of air required for fuel combustion is determined by the following formula:

Gв  BV 0  fwну ,

(104)

where wну – is the blowing speed, reduced to normal conditions;  – is a coefficient showing the amount of air exceeding theoretical. From formula (12.8), we calculate the B F  wну ( V 0 ) ratio and plug it into formula (101) to determine thermal stress of the furnace cross section: qf 

Qнр wну . V 0

(105)

Thus, we calculate thermal stress of cross section through the blow-off speed. For most types of fuel, the ratio of combustion heat of fossil fuels to the theoretical amount of air required for fuel combustion is constant and equals 3.8 MJ/m3. Therefore, thermal stress of cross section of the boiler or combustion surface may be represented as follows: 106

q f  3,8

wну , MW/m2. 

(106)

To increase thermal stress of combustion surface, it is necessary to minimize the amount of excess air required for fuel combustion and use the maximum possible value of air flow rate in accordance with this technology (furnace combustion method). The guaranteed thermal stress values in relation to the furnace volume and combustion surface are given in table 6. Table 6 Guaranteed values of thermal stress Furnaces Grate-fired Chamber Cyclone

qf, MW/m2 2 3.5-5 12-14

qV, MW/m 3 0.2-0.4 0.1-0.2 0.6-1.1

Wну, m/s 0.5 1 3.5

To accelerate ignition in the chamber furnace, a large amount of air is fed into the furnace not immediately, but gradually, to ensure the reaction of oxygen with the fuel. Therefore, air is divided into primary and secondary flows. Primary air must dry the fuel and deliver pulverized coal to the furnace. Secondary air may be supplied through the main or auxiliary burner. Dust and air mixture, passing through the burner, is a system of turbulent non-isothermal jets, spreading among hot combustible substances. Hot gases are sucked into dusty jets, and the resulting hot combustible mixture becomes ready for ignition. With steady ignition, a cone-shaped flame from the burner is directed to the furnace.

13.4. Burners for combustion of pulverized coal: straight-flow, swirl burners. Burner classification Burners for combustion of pulverized coal are used to regulate the supply of dust and air into the furnace. By selecting and using suitable burners for heating, you can properly organize the following processes 107

in the furnace: stable flame burning, mixing, active dust burning, operation of the surfaces of steam generator without slagging. Two main types of burners are used to burn pulverized coal: swirl and straight-flow burners. 13.4.1 Swirl burners. There are the following types of swirl burners: 1. Two-scroll burners with the swirling of the air mixture and secondary air in a scroll unit and swirl flow into the furnace. There is one or two scrolls in such swirl or turbulent burners.

1 – scroll for supply of dust and air mixture; 2 – secondary air scroll; 3, 4 – channels through which the dust-air mixture and secondary air moves; 5 – fuel oil atomizer; 6 – furnace wall; 7 – circular channel for natural gas supply; 9 – igniter. A and B – the beginning and the end of the ignition zone; B – direction of flue gases movement Figure 35. Two-scroll swirl burners

Figure 35 shows a two-scroll swirl burner. Into a small scroll (1) dust-air mixture is fed, and into the large one (2) – secondary air. These flows swirl through the circular channels (3) and (4) separately enter the furnace. The burner has a central tube (5) for fuel oil used to ignite the fuel when the load of the boiler unit is reduced. 2. In the scroll and blade burner, air mixture is fed through a straight-flow channel, and secondary air enters in two flows and swirls as it passes through axially located blades. In the central part of this burner there is an atomizer for spraying fuel oil. 108

view A

Screen axis

1– atomizer air box; 2 – air mixture scroll; 3 – double-flow air box; 4 – pipe with the installed electric igniter; 5 – fuel oil atomizer pipe; 6 – inner tube; 7 – safety box; 8 – dust air pipe; 9 – separation pipe; 10 – fixing flange; 11, 12 – registers. Figure 36. Scroll and blade swirl burner

Thus, at first, fuel oil is sprayed by the burner and pulverized coal in the furnace ignites. 3. In a straight-flow scroll burner, air mixture is fed through the straight-flow channel and distributed by the distributors, while secondary air is swirled in the scroll and passes into the furnace as a vortex.

1 – distributor; 2 – expansion pipe; 3 – air mixture (fuel-air mixture) supply pipe; 4 – scroll; 5 – nozzle; 6 – valve gate with a rotary mechanism; 7 – flange; 8 – hole for installation of the igniter Figure 37. Straight flow and scroll swirl burner

Thermal power of the swirl furnaces is in the range of 25 .... 100 MW. 109

The most frequently used types are two-scroll and scroll-blade burners. The latter burner type provides power of 75 ... 100 MW. The swirl burners quickly eject flue gases into dust-air mixture passing through them, so that the mixture heats up in a short time and reaches the ignition temperature. For complete fuel combustion it is necessary to control the speed of the secondary air and the mixture blown into the furnace. The increase in speed increases turbulent mixing of the flows, but at very high speeds the flame may come off. For good mixing of pulverized coal with hot air, it is necessary to ensure different flow rates. For example, the velocity of air mixture at the exit of the burner should be w1 = 14 ... 25 m/s, while the velocity of the secondary air – w2 = (1,2 ... 1,4) w1. Swirl burners are universal and are used for any solid fuel, but, mainly they are widely used for burning solid fuels with a small amount of volatile substances. Swirl burners give short flames, but with a large expansion angle. In the flame, flows are actively mixed, ensuring deep fuel combustion, due to which 90... 95% of the fuel burns in the flame.

13.5. Straight-flow burners Straight-flow burners are required to burn anthracite, dust, lean coal and other types of coal. Figure 12.7a shows a straight-flow burner with a round nozzle. Natural gas may also be fed to a furnace through such burners. In the figure, dust-air mixture is supplied through a round nozzle 3, and secondary air is fed through a round nozzle 4. Narrow channels located next to the latter nozzle are used for natural gas. Nozzle 1 is designed to supply natural gas. Number 2 shows fuel particles that did not have time to burn in the central part of the dust-air mixture. Figure 38 shows a straight-through burner with three vertical holes. Secondary air enters the furnace through the central opening in such a burner, and primary air and a dust-air mixture are supplied through the two extreme openings. To prevent dust-air mixture from spraying down before entering the furnace, and to increase the mixing 110

efficiency, secondary air may be supplied through the lower parts of the two outer apertures.

Figure 38. Operation and layout of straight-flow burners in the furnace

In the burners designed for pulverized coal, such coal is blown into the furnace using primary air, which participated in the grinding and drying of the original fuel, at the temperature of 70-130 °C. Secondary air with a temperature in the range of 250-420 °С passes through the burner. Mixing of these two flows and formation of a combustible mixture is carried out in the combustion chamber. The location of the burners on certain walls of the furnace chamber is shown in Figure 39.

a)

b)

c)

d)

e)

f)

a – frontal arrangement; b – opposite walls arrangement; c – lateral arrangement; d – angular arrangement; e – arrangement with a tangent angle; f – ceiling arrangement Figure 39

With axial or angular (along furnace axis) arrangement of the burners, air flows intersect right in the center of the combustion chamber, as a result some part of the dust that starts to burn goes up 111

and the rest one goes down, then it goes up again and passes through the entrances 40 a and c into the furnace). In the tangent arrangement of the burners (In Figure 40 – b and d), secondary air moves along the tangent to the mentally drawn circle in the center of the combustion chamber and there it generates a vortex motion of dust particles.

c)

a)

b)

d)

Figure 40. Diagrams of the movement of gases entering furnace from direct-flow burners with apertures

112

14.1. Types and classification of pulverized coal combustion furnaces A dust-burning furnace is a device for burning powders of solid fuels (coal, peat, or slate). Pulverized coal combustion furnace

Ash fusion furnaces

Dry-bottom furnaces

Direct-blow furnaces with open embrasure

Industrial feeder furnaces With frontal burners arrangement

With distributor in embrasure

With opposite burner arrangement

With ejection embrasure

With counter offset jets

With flat parallel jets

With angular burner arrangement

open single chamber with straight-through flame

With combustor

With intersecting jets

Cyclone with primary furnace

With mill fan

Swirl

With high-pressure burners

Figure 41. Pulverized-fuel burners classification

113

Modern power-plant combustion devices differ in the principle of their operation and design. Figure 13.1 shows classification of furnaces for pulverized coal combustion, proposed by well-known scientists, D.M. Khzmalyan and Ya.A. Kagan. The classification covers all furnaces currently in use. In accordance therewith, all furnaces are divided into two main types: dry-bottom and ash fusion furnaces [14]. In flame combustion, there is no need to form an excess fuel reserve in the combustion chamber. The amount of dust simultaneously supplied to furnaces of large and powerful steam generators of 4-8 thousand m3 does not exceed 10kg. For continuous steam generation, it is necessary to ensure a continuous flow of dust and air into the furnace. In addition, you have to constantly remove combustion products – slag and ash.

14.2. Dry-bottom furnaces The pulverized coal furnace resembles a square vessel (see figure 42). The flame arising from the combustion of fuel inside the furnace extends to its space, and pulverized coal supplied by each burner burns down in the flame. According the method of slag and ash removal used, pulverized fuel burning furnaces are divided into drybottom and ash fusion furnaces. The temperature range in the dry-bottom furnaces is characterized by the isotherms shown in Figure 13.1. The highest temperature is observed in the flame heart in the central part of the furnace, at the level of the burners. At a slightly remote distances from this level, low temperature isotherms are located, since heat is continuously supplied to combustion chambers. A special characteristic of such furnaces is the presence of a cold funnel at the bottom. To install the funnel, the front and rear walls are tilted by (50 ... 60°) and approximate the distance to b'= 1.0 ... 1.2 m. In dry bottom furnaces, large particles of slag fall onto the bottom (funnel) of the furnace and, moving along its downward and cooled sharp ends, are dumped into the slag pit. Here these particles are cooled by the jets of cold water. Drops of molten slag float in water jets and turn solid. 114

1 – cooled bottom of the furnace (funnel); 2 – slag bath with water; 3 – hydro channel for ash catching; 4 – pulverized burner; 5 – wall screens; 6 – flame heart; 7 – screw-type slag removal mechanism; в – electric engine Figure 42. Dry-bottom furnace

With proper organization of the flue processes, the slag particles, following the “route”, are cooled and leave the furnace as granules. The main part of the ash is carried away by gases leaving the furnace at the speed of 5-10 m/s. The amount of ash carried away by flue gases is 85-90% of the total amount of ash. And the number of ash pieces going out through the chilled furnace bottom is small and makes up 10-15% of the total amount of ash.

14.3. Ash fusion furnaces In ash fusion furnaces, the flame heart is located very close to the furnace bottom. The slag falls onto the furnace bottom in a molten form. For many types of fuel, slag melting point is 1200-1500 °С. Due to thermal insulation of the lower part of the furnace, the flame temperature may be raised up to 1600-1800 °С. For thermal insulation, the lower parts of the walls are made of high-refractory materials. At 115

the furnace bottom, a bath for liquid slag is installed. Into this bath droplets of slag are collected, being accumulated on the walls. From the bath liquid slag is discharged through a slag funnel. In such furnaces, 20 ... 30% of the slag is removed through a funnel, and in furnaces with special designs for liquid slag dumping, 80-90% of the slag is removed using such a slag-removing funnel. Slag flowing through the funnel is collected in a cold water bath box. In Figure 43 you can see a clamping of side surfaces of the furnace. Above the clamping a part of the furnace, called cooling chamber is seen. Due to the clamping, the temperature in this point is prevented from rising. The clamp also reduces heat transfer to the upper part of the furnace with open screens through radiation. Therefore, the temperature of open screens is low and they are not slagged.

1 – combustion chamber; 2 – bottom of combustion chamber; 3 – slag launch; 4 – cooling chamber; 5 – pipe; 6 – spikes preliminary coated with plaster, welded to the pipe; 7 – refractory lining on spikes Figure 43. Ash fusion furnace: a – general view of the furnace; b – view of a lined screen

In order to avoid slagging of the walls of the pulverized coal furnace, it is necessary to pay attention to the aerodynamics in the furnace. Gas temperature near wall screens should not exceed ash temperature of tA. With ash temperature being tA, the particles become tacky, stick to the walls and slag them. 116

To protect the walls, spikes are welded to the screen tubes (10 mm in diameter and 15 to 18 mm long) on the side of the furnace and covered with an insulating coating. A wall screen with such protection is called lined wall screen.

14.4. Formation of nitric oxide and sulfur anhydride by burning pulverized coal and measures aimed at reducing oxides emissions into the atmosphere When burning solid fuels, prior to complete formation of the flame, molecules of volatile substances containing nitrogen are subjected to thermal decomposition. At this time, the following radicals appear: CN, HCN (hydrocyanic acid), NH, NH2, etc. The radicals enter into oxidation reactions faster than nitrogen in the composition of the fuel. NO is formed as the final reaction product. NO oxides, which caused the formation of radicals in nitrogen oxides that appeared in the combustion process, are called “fuel oxides”. Nitrogen oxides may also be formed from the air fed into the furnace. “Fuel” nitrogen oxides are also called thermal because they occur at very high temperatures of t≥1700 °C. Nitrogen dioxide NO2, released into the atmosphere along with combustion products, is a harmful substance that causes inflammation of respiratory organs of living organisms. The TPPs using fossil fuels account for 40% of nitrogen oxides emitted into the atmosphere. Nitric oxide is only formed in the furnace of steam generators (85-99%). Nitrogen oxides, leaving chimney, are cooled, throttle down and form nitrogen dioxide, which is harmful to the environment: a) it accelerates the appearance of smoke fog NO2 in the atmosphere; b) nitric acid is formed in the environment with the participation of NO2: 4NO2 + О2 + 2Н2О = 4HNO3. Having been formed, nitric acid, interacting with salts in the soil, produces nitrates; 117

c) in the process of decomposition of organic substances in certain natural cycles, molecules containing nitrogen form ammonia: NO2 → NO3. There are several practical ways to reduce emissions of nitrogen oxides into the atmosphere: 1) recirculate combustion products formed in the combustion zone; 2) use two-stage fuel combustion. At the first stage, a part of the secondary air is supplied to the main burners; the other part is fed to auxiliary burners located above the main ones. Thus, the combustion zone is stretched, the maximum combustion temperature decreases, and the amount of oxygen in the mixture drops; 3) spray the combustion zone with water with the maximum temperature. All these measures only contribute to reducing emissions of "air" nitrogen and have a small effect on the release of "fuel" nitrogen. Recently, this problem has been actively investigated, but the results obtained are not satisfactory. All these measures reduce operating capacity of the plants, i.e., gross efficiency of the boiler plant decreases, and the losses from chemical and mechanical incomplete combustion increase. Reduction in emissions of fuel nitric oxide increases emissions of cyanic acid CH, HCN. In the process of burning, other harmful substances are emitted, such as sulfur oxides, benzopyrene, carbon radicals, etc. Unburned hydrocarbons and their main combustion product – CO2 – are considered to be a factor in global warming, that may cause natural disasters. There are sulfur compounds in the composition of solid and liquid fuels. In some types of fossil coal, the amount of sulfur is about 5%. When sulfur compounds interact with oxygen, sulfur oxides are formed: sulfur trioxide SO3 is formed as a result of sulfur dioxide SO2 oxidation. There is always water vapor present in combustion products, it interacts with sulfur trioxide, forming sulfuric acid H2SO4. Sulfuric acid abruptly increases saturation temperature (dew point) of water vapor and promotes to the development of thin sulfuric acid foam in the air heater tubes, resulting in corrosion of metal pipes. To remove sulfur compounds from the boiler, it is necessary to keep the temperature of outgoing gases above the dew point temperature. 118

1. Z. Khzmalyan D.Ya., Kagan Y.N., Theory of Combustion and Furnace Devices. – M.: Energy, 1978. – P. 264. 4. Khzmalyan D.Ya., Furnace Processes Theory. – M.: Energoatomizdat, 1980. – P. 351. 5. Fundamentals of the practical combustion theory / Ed. Pomerantsev V.V. L.: Energy, 1973. – P. 264. 6. Blinov E.A. Fuel and combustion theory. Tutorial. – SPb.: Publishing house of SZTU, 2007. – P. 119. 7. Munts V.A., Pavlyuk E.Yu. Fundamentals of the fuels combustion theory. Study Guide-Ekaterinburg: GOU VPO USTU-UPI, 2005. – P. 102. 8. Akmen R.G. Fuel: Basics of combustion theory, combustion devices / Lecture notes. – Kharkov.: NTU "KPI" 2005. – P. 68. 9. Nureken E. Kazandyk kondyrghylar men bu буndіrgіshterdіn Isteu Kakidasi, Rylymasy zhnelyu esteteu. Oku қurali. – Almaty: AEBBI, 2001. – P. 78. 10. Belsky A.P., Lakomkin V.Yu. Special issues of heat and mass transfer in the energy and heat-engineering processes and installations. Electronic textbook, St. Petersburg, 2011. – P. 66. 11. Lebedev B.V., Karyakin S.K. Technology of organic fuels combustion. Publishing house of Tomsk Polytechnic University, 2012. – P. 148. 12. Buznikov E.F., Roddatis K.F., Berzinsh E.Y. Industrial and heating boilers. – M.: Energoatomizdat, 1984. – P. 248. 13. Chokin Sh.Ch., Sartayev T.St., Shkret A.F. Energy and electrification of Kazakhstan. – Almaty, 1990. – P. 336. 14. Beloselsky B.S., Solyakov V.K. Power-plant fuel. – M.: Energy, 1980.

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INTRODUCTION ................................................................................... 3 1. THE ROLE OF ORGANIC FUEL IN THE FUEL BALANCE OF ENERGY SOURCES OF THE REPUBLIC OF KAZAKHSTAN ................................................................................ 4 2. PREPARATION OF SOLID COAL FUEL FOR COMBUSTION .............................................................................. 12 2.1. Solid fuel combustion ......................................................................... 14 2.2. Main Types of Burners ...................................................................... 15 2.3. Impact of the fuel preparation process on the management of the process of obtaining fossil fuels, saving fuel resources and reducing greenhouse gas emissions ..................................................... 18 2.4. Energy saving measures when using fuel resources. Existing Energy Saving Resources ............................................................ 20 3. TYPES AND COMPOSITION OF FOSSIL FUEL ......................... 22 3.1. Solid fuel and its main thermo-technical characteristics ..................... 22 3.2. Solid fuel classification ...................................................................... 26 3.4. Liquid fuel and its main characteristics ............................................... 28 3.5. Gaseous fuel and its main characteristics ............................................ 29 3.6. Combustion heat (upper and lower limits of the moist basis) and reduced fuel characteristics ................................................................. 30 4. SPECIAL ISSUES OF COMBUSTION THEORY ......................... 33 4.1. Fuel burning. Oxidizing agents. The material balance of combustion process ............................................................................... 33 4.2. Heat balance of combustion. Adiabatic and theoretical combustion temperatures............................................................................................... 35 4.3. Volumes of combustion and air products required for combustion. Determination of excess air ratio ............................................................... 37 4.4. Volume of combustion products ......................................................... 39 5. HEAT BALANCE OF BOILER UNIT .............................................. 42 5.1. Sustainable fuel use. Heat balance ...................................................... 42 5.2. Efficiency (gross and net) of the boiler plant. Determining the performance of a boiler plant from inverse inequality ......................... 48

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6. ELEMENTS OF COMBUSTION THEORY AND MANAGEMENT OF FUEL BURNING ...................................... 50 6.1. Elements of combustion theory. Kinetics of chemical combustion reactions .................................................................................................... 50 6.2. Chemical reactions rate. Mass action law .......................................... 52 6.3. Dependence of the chemical reactions rate on temperature. Arrhenius law............................................................................................. 53 6.4. Dependence of the equilibrium of chemical reactions on temperature and pressure ........................................................................... 56 7. COMBUSTION KINETICS .............................................................. 58 7.1. Effect of internal reaction and mixing on the combustion kinetics. Kinetics of combustion chemical reactions ............................................... 58 7.2. Equilibrium concentrations of flammable substances, dissociation of molecules. Combustion temperature. The effect of dissociation on combustion temperature ................................................ 59 8. FLAME PROPAGATION IN GAS MEDIA ..................................... 61 8.1. Flame propagation in a gas flow and its normal velocity. ................... 61 8.2. Determination of the velocity of the Bunsen burner .......................... 63 8.3. The velocity of mass propagation of flame ......................................... 65 8.4. The Law of areas ................................................................................ 65 9. SPECIAL CHARACTERISTICS OF GASEOUS FUEL COMBUSTION ....................................................................................... 68 9.1. Special characteristics of gas combustion .......................................... 68 9.2. Laminar combustion of a homogeneous gas mixture .......................... 69 9.3. Types, parameters and classification of gas burners ........................... 71 9.4. Diffusion burners and management of mixing in burners ................... 73 9.5. Injection burners ................................................................................. 75 9.6. External mixing burners ..................................................................... 76 10. MAIN WAYS TO ACCELERATE GAS COMBUSTION ............. 78 10.1. Spread of turbulent flame ................................................................. 78 10.2. Turbulent combustion of a homogeneous gas mixture ...................... 80 10.3. Gas-oil boilers and gas-oil burners.................................................... 82 10.4. Combination gas oil burners and boilers ........................................... 84 11. PROPERTIES AND SPECIAL CHARACTERISTICS OF LIQUID FUEL BURNING ............................................................... 85 11.1. Properties and special characteristics of liquid fuel burning ............ 85 11.2. Combustion of liquid fuel on the free surface ................................... 86 11.3. Fuel oil droplet combustion ............................................................. 87 12. SPECIAL CHARACTERISTICS OF LIQUID FUEL BURNING (CONTINUED). ATOMIZERS FOR LIQUID FUEL BURNING ...... 89

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12.1. Flame combustion of liquid fuel ....................................................... 89 12.2. Atomizers for liquid fuel .................................................................. 90 12.3. Mechanical atomizers ...................................................................... 92 12.4. Rotary atomizers ............................................................................... 93 12.5. Vapor (pneumatic) atomizers ............................................................ 94 12.6. Low-pressure air-atomizing burners ................................................. 96 13. SPECIAL CHARACTERISTICS OF SOLID FUEL COMBUSTION........................................................................................ 97 13.1. The mechanism of carbon burning in solid fuel ................................ 97 13.2. Combustion of solid fuel in the bed. Distribution patterns of combustible substances in the bed of fuel, measures for the effective combustion of fuel in the bed ..................................................................... 99 13.3. Methods for burning solid fuel in a chamber, vortex and cyclone furnace. Thermal performance of combustion chamber ............................. 103 13.4. Burners for combustion of coal dust: straight-flow, swirl burners. Burner classification .................................................................................. 107 13.5. Straight-flow burners ....................................................................... 110 14. PULVERIZED COAL COMBUSTION FURNACES ................... 113 14.1. Types and classification of pulverized coal combustion furnaces .... 113 14.2. Dry-bottom furnaces ........................................................................ 114 14.3. Ash fusion furnaces .......................................................................... 115 14.4. Formation of nitric oxide and sulfur anhydride by burning pulverized coal and measures aimed at reducing oxides emissions into the atmosphere .................................................................................... 117 REFERENCES ....................................................................................... 119

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Еducational issue

Askarova Aliya Sandybaevna Bolegenova Saltanat Alikhanovna Bolegenova Symbat Alikhanovna SPECIAL FUEL COMBUSTION ISSUES Study guide

Editor V. Pоpova Typesetting and cover design G. Kaliyeva Cover design used photos from sites www.background-2672597_960_720.com

IB №12516 Signed for publishing 11.01.2019. Format 60x84 1/16. Offset paper. Digital printing. Volume 7,68 printer’s sheet. 100 copies. Order №27. Publishing house «Qazaq University» Al-Farabi Kazakh National University KazNU, 71 Al-Farabi, 050040, Almaty Printed in the printing office of the «Qazaq University» publishing house.

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Новые книги издательского дома «ҚАЗАҚ УНИВЕРСИТЕТІ» Ландау Л.Д. Баршаға арналған физика. Физикалық дене: оқу құралы / Л.Д. Ландау, А.И. Китайгородский; орыс тілінен ауд.: А.К. Саймбетов, Б.Қ. Мухаметқали, Б.С. Кусманова [және т.б.]. – Aлмaты: Қaзaқ университеті, 2018. – 198 б. ISBN 978-601-04-3223-9 Бұл кітаптың негізгі мақсаты – оқырмандар үшін заманауи физиканың жетістіктері және негізгі идеяларын нақты түрде түсіндіру. Бақылаушыға байланысты физикалық дене екі жағдайда қарастырылады: инерциалды және инерциалды емес координаттық жүйелерде. Оқу құралының оқырмандар үшін физика ғылымына кірісуде маңызы зор. Сaймбетов A.К. Өлшеуіш техникaның негіздері: оқу құрaлы / A.К. Сaймбетов, М.М. Ғылымжaновa, Н.Б. Құттыбaй. – Aлмaты: Қaзaқ университеті, 2018. – 216 б. ISBN 978-601-04-3224-6 Оқу құрaлы электронды техникaның жұмыс істеу принципін, өлшеу қaтеліктері мен негізгі қaғидaлaрын қамтиды. Бөлімдер өлшеу жүргізу жүйелерінің құрылымдық бөлігінің қызмет aтқaру ерекшеліктерінен және олaрдың тұрғызылуының бaсты қaғидaлaрынaн тұрaды. 5В071900 – «Рaдиотехникa, электроникa және телекоммуникaциялaр» мaмaндығының студенттері үшін aрнaлып жaзылғaн. Саймбетов А.К. Талшықты-оптикалық байланыс желісі: оқу құралы / А.К. Саймбетов, А.А. Толегенова, Н.Б. Құттыбай. – Алматы: Қазақ университеті, 2018. – 194 б. ISBN 978-601-04-3136-2 Қазіргі уақытта байланыс қызметін ұсынушылар әр жыл сайын талшықтыоптикалық кабельдің мыңдаған километрін жер астымен, теңіз, көлдердің түбімен, жерасты жолдары арқылы тартады. Талшықтыоптикалық технологиялар аймағында белсенді зерттеулер жүргізіліп отырады. Оптикалық кабель (ОК) жарық толқынының сәйкесінше белгіленген өлшеміндегі оптикалық диапазонында электромагниттік тербелістерді тасымалдай отырып, ақпараттарды жеткізеді. Оптикалық талшықтарда ұзындықтары 16,6 мкм-ді құрайтын инфрақызыл толқындар қолданылады. Келешекте толқындардың жұмыс аралығы инфрақызыл толқындар тәрізді 5-тен 10 мкм-ге дейін толқын ұзаруы мүмкін. Ұсынылып отырған оқу құралында талшықты оптика элементтері және құрылғыларының жұмыс істеу принциптері, негізгі сипаттамалары, ақпаратты жіберу жолдары қарастырылады. Оқу құралы 5В071900 – «Радиотехника, электроника және телекоммуникациялар» мамандығының студенттеріне арналады.

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