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What process is called heterogeneous combustion. Types and modes of combustion

The physical phenomena listed in the previous section are observed in a wide variety of processes that differ both in the nature of chemical reactions and in the state of aggregation of the substances involved in combustion.

There are homogeneous, heterogeneous and diffusion combustion.

Chapter 1 combustion theory concepts

Homogeneous combustion includes premixed gases*. Numerous examples of homogeneous combustion are the processes of combustion of gases or vapors in which the oxidizer is atmospheric oxygen: the combustion of mixtures of hydrogen, mixtures of carbon monoxide and hydrocarbons with air. In practically important cases, the condition of complete preliminary mixing is not always satisfied. Therefore, combinations of homogeneous combustion with other types of combustion are always possible.

Homogeneous combustion can be realized in two modes: laminar and turbulent. Turbulence accelerates the combustion process due to the fragmentation of the flame front into separate fragments and, accordingly, an increase in the contact area of the reactants with large-scale turbulence or acceleration of heat and mass transfer processes in the flame front with small-scale turbulence. Turbulent combustion is characterized by self-similarity: turbulent vortices increase the combustion rate, which leads to an increase in turbulence.

All parameters of homogeneous combustion are also manifested in processes in which other gases, rather than oxygen, act as an oxidizing agent. For example, fluorine, chlorine or bromine.

During fires, diffusion combustion processes are the most common. In them, all the reactants are in the gas phase, but are not preliminarily mixed. In the case of combustion of liquids and solids, the process of fuel oxidation in the gas phase occurs simultaneously with the process of liquid evaporation (or decomposition of solid material) and with the mixing process.

The simplest example of diffusion combustion is the combustion of natural gas in a gas burner. On fires, the mode of turbulent diffusion combustion is realized, when the burning rate is determined by the rate of turbulent mixing.

A distinction is made between macromixing and micromixing. The process of turbulent mixing includes successive crushing of gas into smaller and smaller volumes and mixing them together. At the last stage, the final molecular mixing occurs by molecular diffusion, the rate of which increases as the fragmentation scale decreases. Upon completion of macromixing

* Such combustion is often called kinetic.

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The burning rate is determined by the processes of micromixing inside small volumes of fuel and air.

Heterogeneous combustion occurs at the interface. In this case, one of the reacting substances is in a condensed state, the other (usually atmospheric oxygen) enters due to diffusion of the gas phase. A prerequisite for heterogeneous combustion is a very high boiling point (or decomposition) of the condensed phase. If this condition is not met, combustion is preceded by evaporation or decomposition. From the surface, a stream of steam or gaseous decomposition products enters the combustion zone, and combustion occurs in the gas phase. Such combustion can be attributed to diffusion quasi-heterogeneous, but not completely heterogeneous, since the combustion process no longer occurs at the phase boundary. The development of such combustion is carried out due to the heat flow from the flame to the surface of the material, which ensures further evaporation or decomposition and the flow of fuel into the combustion zone. In such situations, a mixed case arises when combustion reactions partially proceed heterogeneously - on the surface of the condensed phase, partially homogeneously - in the volume of the gas mixture.

An example of heterogeneous combustion is the combustion of coal and charcoal. During the combustion of these substances, two kinds of reactions take place. Some grades of coal emit volatile components when heated. The combustion of such coals is preceded by their partial thermal decomposition with the release of gaseous hydrocarbons and hydrogen, which burn in the gas phase. In addition, when pure carbon is burned, carbon monoxide CO can be formed, which burns out in bulk. With a sufficient excess of air and a high temperature of the coal surface, bulk reactions proceed so close to the surface that, in a certain approximation, it gives grounds to consider such a process as heterogeneous.

An example of truly heterogeneous combustion is the combustion of refractory non-volatile metals. These processes can be complicated by the formation of oxides that cover the burning surface and prevent contact with oxygen. With a large difference in the physicochemical properties between the metal and its oxide, the oxide film cracks during combustion, and oxygen access to the combustion zone is ensured.

Chapter 1. Basic concepts of the theory of combustion

Topic 4. TYPES OF COMBUSTION.

According to various signs and features, combustion processes can be divided into the following types:

According to the state of aggregation of the combustible substance:

Burning gases;

Combustion of liquids and melting solids;

Combustion of non-consumable solid dust-like and compact substances.

According to the phase composition of the components:

homogeneous combustion;

heterogeneous combustion;

Burning explosives.

According to the preparedness of the combustible mixture:

Diffusion combustion (fire);

Kinetic combustion (explosion).

According to the dynamics of the flame front:

Stationary;

Non-stationary.

According to the nature of the movement of gases:

laminar;

Turbulent.

According to the degree of combustion of a combustible substance:

Incomplete.

According to the flame propagation speed:

Normal;

deflagration;

Detonation.

Let's take a closer look at these types.

4.1. Combustion of gaseous, liquid and solid substances.

Depending on the state of aggregation of a combustible substance, combustion of gases, liquids, dusty and compact solids is distinguished.

According to GOST 12.1.044-89:

1. Gases are substances whose critical temperature is less than 50 ° C. T cr is the minimum heating temperature of 1 mole of a substance in a closed vessel, at which it completely turns into steam (see § 2.3).

2. Liquids are substances with a melting point (dropping point) of less than 50 ° C (see § 2.5).

3. Solids are substances with a melting point (drop-fall) of more than 50 0 С.

4. Dusts are particulate solids with a particle size of less than 0.85 mm.

The zone in which a chemical reaction takes place in a combustible mixture, i.e. combustion is called the flame front.

Consider the combustion processes in the air on examples.

Combustion of gases in a gas burner. There are 3 flame zones (Fig. 12.):

Rice. 12. Scheme of gas combustion: 1 - transparent cone - this is the initial gas heated (to the temperature of self-ignition); 2 – luminous zone of the flame front; 3 - combustion products (they are almost invisible during the complete combustion of gases and, especially during the combustion of hydrogen, when soot is not formed).

The width of the flame front in gas mixtures is tens of fractions of a millimeter.

Combustion of liquids in an open vessel. When burning in an open vessel, there are 4 zones (Fig. 13):

Rice. 13. Liquid burning: 1 - liquid; 2 - liquid vapor (dark areas); 3 - flame front; 4 - combustion products (smoke).

The width of the flame front in this case is greater; the reaction proceeds more slowly.

Combustion of melting solids. Consider burning a candle. In this case, 6 zones are observed (Fig. 14):

Rice. 14. Candle burning: 1 - hard wax; 2 - melted (liquid) wax; 3 – dark transparent vapor layer; 4 - flame front; 5 - combustion products (smoke); 6 - wick.

The burning wick serves to stabilize combustion. Liquid is absorbed into it, rises along it, evaporates and burns. The width of the flame front increases, which increases the luminosity area, since more complex hydrocarbons are used, which, evaporating, decompose, and then react.

Combustion of non-consumable solids. We will consider this type of combustion using the example of burning a match and a cigarette (Fig. 15 and 16).

There are also 5 plots here:

Rice. 15. Burning a match: 1 - fresh wood; 2 - charred wood; 3 - gases (gasified or evaporated volatile substances) - this is a darkish transparent zone; 4 - flame front; 5 - products of combustion (smoke).

It can be seen that the burnt area of the match is much thinner and has a black color. This means that part of the match was charred, i.e. the non-volatile part remained, and the volatile part evaporated and burned. The burning rate of coal is much slower than gases, so it does not have time to completely burn out.

Fig.16. Cigarette burning: 1 - initial tobacco mixture; 2 - smoldering area without a flame front; 3 - smoke, i.e. product of burnt particles; 4 - smoke drawn into the lungs, which is mainly gasified products; 5 - resin condensed on the filter.

The flameless thermal-oxidative decomposition of a substance is called smoldering. It occurs when there is insufficient diffusion of oxygen into the combustion zone and can occur even with a very small amount of it (1-2%). The smoke is blue, not black. This means that it contains more gasified, rather than burned, substances.

The surface of the ash is almost white. This means that with a sufficient supply of oxygen, complete combustion occurs. But inside and on the border of the burning layer with fresh ones there is a black substance. This indicates incomplete combustion of charred particles. By the way, vapors of volatile resinous substances condense on the filter.

A similar type of combustion is observed during the combustion of coke, i.e. coal, from which volatile substances (gases, resins) have been removed, or graphite.

Thus, the combustion process of gases, liquids and most solids proceeds in gaseous form and is accompanied by a flame. Some solids, including those with a tendency to spontaneous combustion, burn in the form of smoldering on the surface and inside the material.

Combustion of dusty substances. The combustion of the dust layer occurs in the same way as in the compact state, only the combustion rate increases due to the increase in the contact surface with air.

The combustion of dust-like substances in the form of an aero suspension (dust cloud) can proceed in the form of sparks, i.e. combustion of individual particles, in the case of a low content of volatile substances that are not capable of forming a sufficient amount of gases during evaporation for a single flame front.

If a sufficient amount of gasified volatile substances is formed, then flame combustion occurs.

Burning explosives. This type includes the combustion of explosives and gunpowder, the so-called condensed substances, in which the fuel and oxidizer are already chemically or mechanically bound. For example: in trinitrotoluene (TNT) C 7 H 5 O 6 N 3 × C 7 H 5 × 3NO 2, O 2 and NO 2 serve as oxidizing agents; in the composition of gunpowder - sulfur, saltpeter, coal; as part of home-made explosives, aluminum powder and ammonium nitrate, a binder - solar oil.

4.2. Homogeneous and heterogeneous combustion.

Based on the considered examples, depending on the state of aggregation of the mixture of fuel and oxidizer, i.e. from the number of phases in the mixture, they distinguish:

1. Homogeneous combustion gases and vapors of combustible substances in the environment of a gaseous oxidizer. Thus, the combustion reaction proceeds in a system consisting of one phase (aggregate state).

2. Heterogeneous combustion solid combustible substances in a gaseous oxidizer environment. In this case, the reaction proceeds at the interface, while the homogeneous reaction proceeds throughout the volume.

This is the combustion of metals, graphite, i.e. practically non-volatile materials. Many gas reactions are of a homogeneous-heterogeneous nature, when the possibility of a homogeneous reaction occurring is due to the origin of a simultaneously heterogeneous reaction.

The combustion of all liquid and many solid substances, from which vapors or gases (volatile substances) are released, proceeds in the gas phase. The solid and liquid phases play the role of reservoirs for the reacting products.

For example, a heterogeneous reaction of spontaneous combustion of coal passes into a homogeneous phase of combustion of volatile substances. Coke residue burns heterogeneously.

4.3. Diffusion and kinetic combustion.

According to the degree of preparation of the combustible mixture, diffusion and kinetic combustion are distinguished.

The types of combustion considered (except for explosives) are diffusive combustion. Flame, i.e. the combustion zone of a mixture of fuel with air, to ensure stability, must be constantly fed with fuel and oxygen in the air. The flow of combustible gas depends only on the rate of its supply to the combustion zone. The rate of entry of a combustible liquid depends on the intensity of its evaporation, i.e. on the vapor pressure above the surface of the liquid, and, consequently, on the temperature of the liquid. Ignition temperature called the lowest temperature of the liquid at which the flame above its surface does not go out.

The combustion of solids differs from the combustion of gases by the presence of a stage of decomposition and gasification, followed by the ignition of volatile pyrolysis products.

Pyrolysis- this is the heating of organic substances to high temperatures without air access. In this case, decomposition, or splitting, of complex compounds into simpler ones occurs (coking of coal, cracking of oil, dry distillation of wood). Therefore, the combustion of a solid combustible substance into the combustion product is not concentrated only in the flame zone, but has a multi-stage character.

Heating of the solid phase causes decomposition and evolution of gases that ignite and burn. The heat from the torch heats the solid phase, causing its gasification and the process is repeated, thus supporting combustion.

The solid combustion model assumes the presence of the following phases (Fig. 17):

Rice. 17. Combustion model

solid.

Heating of the solid phase. For melting substances, melting occurs in this zone. The thickness of the zone depends on the conductivity temperature of the substance;

Pyrolysis, or the reaction zone in the solid phase, in which gaseous combustible substances are formed;

Pre-flame in the gas phase, in which a mixture with an oxidizing agent is formed;

A flame, or reaction zone in the gas phase, in which the conversion of pyrolysis products into gaseous combustion products;

combustion products.

The rate of oxygen supply to the combustion zone depends on its diffusion through the combustion product.

In general, since the rate of a chemical reaction in the combustion zone in the types of combustion under consideration depends on the rate of arrival of the reacting components and the flame surface by molecular or kinetic diffusion, this type of combustion is called diffusion.

The flame structure of diffusion combustion consists of three zones (Fig. 18):

Zone 1 contains gases or vapours. There is no combustion in this zone. The temperature does not exceed 500 0 C. Decomposition, pyrolysis of volatiles and heating to the self-ignition temperature occur.

Rice. 18. The structure of the flame.

In zone 2, a mixture of vapors (gases) with atmospheric oxygen is formed and incomplete combustion occurs to CO with partial reduction to carbon (little oxygen):

C n H m + O 2 → CO + CO 2 + H 2 O;

In the 3rd outer zone, the products of the second zone are completely burned and the maximum flame temperature is observed:

2CO+O 2 \u003d 2CO 2;

The height of the flame is proportional to the diffusion coefficient and the flow rate of the gases and is inversely proportional to the density of the gas.

All types of diffusion combustion are inherent in fires.

Kinetic combustion is the combustion of pre-mixed combustible gas, vapor or dust with an oxidizing agent. In this case, the burning rate depends only on the physicochemical properties of the combustible mixture (thermal conductivity, heat capacity, turbulence, concentration of substances, pressure, etc.). Therefore, the burning rate increases sharply. This type of combustion is inherent in explosions.

In this case, when the combustible mixture is ignited at some point, the flame front moves from the combustion products into the fresh mixture. Thus, the flame during kinetic combustion is most often unsteady (Fig. 19).

Rice. 19. Scheme of flame propagation in a combustible mixture: - ignition source; - direction of motion of the flame front.

Although, if the combustible gas is mixed with air and fed into the burner, then a stationary flame is formed during ignition, provided that the mixture supply rate is equal to the flame propagation speed.

If the gas supply rate is increased, the flame breaks away from the burner and may go out. And if the speed is reduced, then the flame will be drawn into the inside of the burner with a possible explosion.

According to the degree of combustion, i.e. the completeness of the combustion reaction to the end products, combustion happens complete and incomplete.

So in zone 2 (Fig. 18) combustion is incomplete, because insufficient oxygen is supplied, which is partially consumed in zone 3, and intermediate products are formed. The latter burn out in zone 3, where there is more oxygen, until complete combustion. The presence of soot in the smoke indicates incomplete combustion.

Another example: when there is a lack of oxygen, carbon burns to carbon monoxide:

If you add O, then the reaction goes to the end:

2CO + O 2 \u003d 2CO 2.

The burning rate depends on the nature of the movement of gases. Therefore, laminar and turbulent combustion are distinguished.

So, an example of laminar combustion is the flame of a candle in still air. At laminar combustion layers of gases flow in parallel, but without swirling.

Turbulent combustion- vortex motion of gases, in which the burning gases are intensively mixed, and the flame front is washed out. The boundary between these types is the Reynolds criterion, which characterizes the relationship between the forces of inertia and the forces of friction in the flow:

where: u- gas flow rate;

n- kinetic viscosity;

l- characteristic linear size.

The Reynolds number at which the transition of a laminar boundary layer to a turbulent one occurs is called critical Re cr, Re cr ~ 2320.

Turbulence increases the rate of combustion due to more intense heat transfer from the combustion products to the fresh mixture.

4.4. Normal combustion.

Depending on the speed of flame propagation during kinetic combustion, either normal combustion (within a few m / s), or explosive deflagration (tens of m / s), or detonation (thousands of m / s) can be realized. These types of combustion can pass into each other.

Normal burning- this is combustion in which flame propagation occurs in the absence of external disturbances (turbulence or changes in gas pressure). It depends only on the nature of the combustible substance, i.e. thermal effect, coefficients of thermal conductivity and diffusion. Therefore, it is a physical constant of a mixture of a certain composition. In this case, the burning speed is usually 0.3-3.0 m/s. Normal combustion is named because the velocity vector of its propagation is perpendicular to the flame front.

4.5. Deflagration (explosive) combustion.

Normal combustion is unstable and tends to self-accelerate in a closed space. The reason for this is the curvature of the flame front due to friction of the gas against the walls of the vessel and changes in pressure in the mixture.

Consider the process of flame propagation in a pipe (Fig. 20).

Rice. 20. Scheme of the occurrence of explosive combustion.

First, at the open end of the pipe, the flame propagates at normal speed, because combustion products freely expand and come out. The mixture pressure does not change. The duration of the uniform spread of the flame depends on the diameter of the pipe, the type of fuel and its concentration.

As the flame front moves inside the pipe, the reaction products, having a larger volume compared to the initial mixture, do not have time to go outside and their pressure increases. This pressure begins to push in all directions, and therefore, ahead of the flame front, the initial mixture begins to move in the direction of flame propagation. The layers adjacent to the walls are decelerated. The flame has the highest speed in the center of the pipe, and the lowest speed is near the walls (due to heat removal in them). Therefore, the flame front is extended in the direction of flame propagation, and its surface increases. In proportion to this, the amount of the combustible mixture increases per unit time, which entails an increase in pressure, and then, in turn, increases the speed of gas movement, etc. Thus, there is an avalanche-like increase in the speed of flame propagation up to hundreds of meters per second.

The process of flame propagation through a combustible gas mixture, in which a self-accelerating combustion reaction propagates due to heating by heat conduction from an adjacent layer of reaction products, is called deflagration. Usually, the rates of deflagration combustion are subsonic, i.e. less than 333 m/s.

4.6. detonation combustion.

If we consider the combustion of a combustible mixture in layers, then as a result of the thermal expansion of the volume of combustion products, each time a compression wave occurs ahead of the flame front. Each subsequent wave, moving through a denser medium, catches up with the previous one and is superimposed on it. Gradually, these waves merge into one shock wave (Fig. 21).

Rice. 21. Scheme of the formation of a detonation wave: R o< Р 1 < Р 2 < Р 3 < Р 4 < Р 5 < Р 6 < Р 7 ; 1-7 – нарастание давления в слоях с 1-го по 7-ой.

In a shock wave, as a result of adiabatic compression, the density of gases instantly increases and the temperature rises to T 0 of self-ignition. As a result, the combustible mixture is ignited by a shock wave and detonation- propagation of combustion by ignition by a shock wave. The detonation wave does not go out, because powered by shock waves from the flame moving behind it.

A feature of detonation is that it occurs at a supersonic speed of 1000-9000 m/s, determined for each composition of the mixture, therefore it is a physical constant of the mixture. It depends only on the calorific value of the combustible mixture and the heat capacity of the combustion products.

The meeting of a shock wave with an obstacle leads to the formation of a reflected shock wave and even greater pressure.

Detonation is the most dangerous form of flame propagation, because. has a maximum explosion power (N=A/t) and a huge speed. In practice, detonation can be "neutralized" only in the pre-detonation section, i.e. at a distance from the point of ignition to the point of detonation combustion. For gases, the length of this section is from 1 to 10 m.

There are homogeneous, heterogeneous and diffusion combustion.

Chapter 1 combustion theory concepts

All parameters of homogeneous combustion are also manifested in processes in which other gases, rather than oxygen, act as an oxidizing agent. For example, fluorine, chlorine or bromine.

* Such combustion is often called kinetic.

Korolchenko AND I. combustion and explosion processes

the burning rate is determined by the processes of micromixing inside small volumes of fuel and air.

1.3. Combustion in a moving gas

The term “normal flame speed”* is used to describe combustion processes. It characterizes the speed of the flame front in a stationary gas mixture. Such an idealized state can only be created in a laboratory experiment. In real combustion conditions, the flame always exists in moving streams.

The behavior of the flame under such conditions is subject to two laws established by the Russian scientist V. A. Mikhelson.

The first of them establishes that the component of the gas flow velocity v along the normal to the flame front propagating through a stationary mixture is equal to the normal speed of flame propagation and divided by cos

where is the angle between the normal to the flame surface and the direction of the gas flow.

Value v characterizes the amount of gas that burns per unit time in an oblique flame. It is customary to call it the actual burning rate in the stream. The actual speed in all cases is equal to or exceeds the normal speed.

This law only applies to flat flames. Generalizing it to real flames with a curvature of the flame front gives the formulation of the second law - the law of areas.

Let us assume that in a gas flow with a velocity v and the cross section of a stationary curved flame front with a common surface S. At each point of the flame front, the flame propagates along the normal to its surface with a speed and. Then the volume of the combustible mixture burning per unit time will be:

In accordance with the balance of the source gas, the same volume is equal to:

* This term is adequate to the term "normal burning rate".

Equating the left parts of (1.2) and (1.3) we get:

In the frame of reference in which the flame front moves through a stationary gas mixture, relation (1.4) means that the flame propagates relative to the gas at a speed v. Formula (1.4) is a mathematical expression of the area law, from which an important conclusion follows: when the flame front is curved, the burning rate increases in proportion to the increase in its surface. Therefore, the inhomogeneous movement of gas always intensifies combustion.

1.4. Turbulent combustion

From the law of areas it follows that turbulence increases the rate of combustion. In fires, this is expressed by a strong intensification of the flame propagation process.

Distinguish (Fig. 1.2) two types of turbulent combustion: combustion of a homogeneous gas mixture and microdiffusion turbulent combustion.

Rice. 1.2. Classification of turbulent combustion

When a homogeneous mixture burns in the turbulent combustion mode, two cases are possible: the appearance of small-scale and large-scale

Chapter 1. Basic concepts of the theory of combustion

headquarters turbulence. Such a division is made depending on the ratio of the scale of turbulence and the thickness of the flame front. At a scale of turbulence less than the thickness of the flame front, it is referred to as small-scale, and at a larger scale - to large-scale. The mechanism of action of small-scale turbulence is due to the intensification of combustion processes due to the acceleration of heat and mass transfer processes in the flame zone. When describing small-scale turbulence in the formulas for the flame propagation velocity, the diffusion and thermal diffusivity coefficients are replaced by the turbulent exchange coefficient.

The highest burning rates are observed during large-scale turbulence. In this case, two combustion acceleration mechanisms are possible: surface and volumetric.

The surface mechanism consists in the curvature of the flame front by turbulent pulsations. In this case, the burning rate increases in proportion to the increase in the front surface. However, this is true only for conditions where chemical transformations in the flame are completed faster than turbulent mixing has time to occur. In this case, when the turbulent mixing overtakes the chemical reaction, the reaction zone is washed out by turbulent pulsations. Such processes are described by the laws of volumetric turbulent combustion.

The turbulent mixing time is equal to the scale ratio

turbulence to fluctuating speed Therefore, the acceleration

flame due to turbulent pulsations occurs according to the surface mechanism, if the following condition is met:

Korolchenko A.Ya. combustion and explosion processes

where is the time of the chemical reaction at the temperature of the burner

If condition (1.5) is not satisfied, then the mechanism of bulk turbulent combustion takes place.

The time of a chemical reaction can be expressed in terms of macroscopic quantities: normal flame speed and flame front thickness

Then the surface acceleration criterion takes the form:

(1.8)

To estimate the propagation velocity of a turbulent flame under surface acceleration, K. I. Shchelkin proposed the following formula:

where AT - a weakly varying number not exceeding one. In the limit with strong turbulence, the turbulent flame velocity tends to the pulsating velocity, i.e. AT- to the unit.

1.5. Features of burning explosives

Explosives are individual substances or mixtures thereof that are capable, under the influence of some external influence (heating, impact, friction, explosion of another explosive) to a rapid self-propagating chemical transformation with the release of a large amount of heat and the formation of gases.

From ordinary combustible substances, the combustion of which occurs when interacting with oxygen or other external oxidizers, explosives, being in a condensed (solid or liquid) phase, contain all the components involved in combustion. Explosives can be both individual chemical compounds and mechanical mixtures.

Most individual explosives are nitro compounds: trinitrotoluene, tetryl, hexogen, octogen, nitroglycerin.

Chapter 1. Basic concepts of the theory of combustion

cerine, cellulose nitrates, etc. Chlorates, perchlorates, azides, organic peroxides also have explosive properties.

Molecules of organic nitro compounds contain weakly bound oxygen in the form of a nitro group - Thus, one molecule contains both a fuel and an oxidizer. Their combustion due to intramolecular oxidation can begin with minor external influences.

A significant group of explosives are endothermic compounds, the molecules of which do not contain oxygen. In this case, the source of energy is not oxidation, but direct decay. These compounds include azides of lead, silver and other metals. Mechanical mixtures include mixtures of solid fuels and solid oxidizing agents. An example of such a mixture is black powder.

1.6. Thermodynamics of combustion

hydrocarbon-air mixtures

The laws of thermodynamics make it possible to calculate the parameters necessary for describing combustion processes: the coefficient of expansion of combustion products under initial conditions for the ratio of heat capacities at constant pressure and constant volume, both for fresh mixture and for combustion products; maximum explosion pressure p e; adiabatic temperature of combustion products under isobaric and isochoric conditions, composition of combustion products

This section describes the algorithm for calculating the equilibrium state of combustion products of C-H-0-N- containing combustibles in air in a wide range of initial temperatures, pressures and concentrations, developed by prof. V.V. Molkov. The algorithm is based on the generalization and systematization of thermodynamic and mathematical methods using the most accurate data on the thermodynamic properties of individual substances.

To increase the reliability of the results in the calculations, it is necessary to take into account not only the oxygen and nitrogen of the air, but also other gases included in its composition - , H 2 0, C0 2. An increase in the number of components of combustion products to 19 (H 2, H 2 0, CO 2, N 2, Ag, C-gas, H, O, N, CO, CH 4, HCN, 0 2,

And carrying out calculations taking into account the composition of

Korolchenko A.Ya. combustion and explosion processes

spirit of medium humidity

They do not complicate calculations on a computer, the use of which can significantly reduce the time of calculations while increasing their accuracy in comparison with the approximate approach without using a computer.

The overall reaction for the combustion of fuel in air of medium humidity per mole of fresh mixture can be written as

where is the volumetric concentration of fuel in the fresh mixture: -

the number of carbon, hydrogen, oxygen and nitrogen atoms, respectively, in the fuel molecule; is the number of moles of the i-th component of the combustion products;

- th component of combustion products.

The total number of atoms in the system, calculated from the composition of the fresh mixture, is

The ratios of the number of atoms of carbon, hydrogen, nitrogen, and argon, respectively, to the number of oxygen atoms are constants for a particular mixture and do not depend on the thermodynamic state of the closed system:

The number of oxygen atoms in the system.

Chapter 1. Basic concepts of the theory of combustion

For an adiabatic combustion process under isobaric conditions, the law of conservation of energy is equivalent to the law of conservation of the enthalpy of a closed system

Hi = Hj,(1.15)

where H is the enthalpy, and the indices and j denote the parameters of the fresh mixture and combustion products, respectively. Mole enthalpy of fresh mixture

where and are the enthalpy of fuel and air, respectively, when

initial temperature The dependence of the enthalpy of fuel and air on the initial temperature in the range from 250 to 500 K is given by a polynomial of the fourth degree

where(298) is the enthalpy of formation of a substance at a temperature of 298 K;

Enthalpy at temperature T;- numerical coefficients,

determined by solving a system of linear equations, for example, by the Gauss-Jordan elimination method; T 0 - some arbitrary constant temperature value.

Enthalpy of combustion products obtained by burning a mole of fresh mixture

where the sum in parentheses is equal to the number of moles of products during the combustion of one mole of fresh mixture; - molar fraction of the th component of the combustion products; - enthalpy of the th combustion product at temperature

tour T.

Enthalpy values

are determined from the dependence of the reduced Gibbs energy on the temperature Ф(Т) in the temperature range from 500 to 6000 K. It is known that

Korolchenko A.Ya. combustion and explosion processes

where T e - equilibrium temperature of combustion products in the bomb.

The explosion pressure of a gas mixture in a closed bomb is determined by the ratio of the equations of state of an ideal gas for combustion products and a fresh mixture

To find the equilibrium composition of combustion products, it is necessary to solve a system that includes 5 linear (mass conservation equations) and 14 nonlinear (chemical equilibrium equations) algebraic equations.

For an isobaric process, it is advisable to write the mass conservation equations in terms of the mole fractions of combustion products

Chapter 1. Basic concepts of the theory of combustion

Korolchenko A.Ya. combustion and explosion processes

(1.34) (1.35) (1.36) (1.37) (1.38) (1.39) (1.40) (1.41) (1.42) (1.43)

where R is the pressure at which the reaction proceeds, atm. The dependence of chemical equilibrium constants on temperature is taken from reference data for dissociation reactions

which is the equilibrium constant of the dissociation reaction (1.43 a)

at temperature - the reduced Gibbs energies correspond to

of the reactants - thermal effect of the reaction (1.44)

at absolute zero temperature.

The adiabatic parameters for the fresh mixture and combustion products are determined using the Mayer equation by the formula

For a fresh mixture, the values are determined by differentiating expression (1.17) for the enthalpy of gases of the initial mixture (fuel and air) with respect to temperature; for combustion products, by the expressions obtained as a result of differentiating equation (1.19) with respect to temperature T.

When calculating combustion processes in a constant volume, the equilibrium constant, which for an ideal gas depends only on temperature,

at which the equilibrium is calculated, and does not depend on pressure, it is advisable to write it not in terms of mole fractions, as was done when calculating combustion under isobaric conditions in equations (1.30) - (1.43), but in terms of the number of moles P,. Then, for example, for reaction (1.31) we have

where Г is the temperature at which the equilibrium constant is calculated; R, and Г, are the initial values of the pressure and temperature of the fresh mixture. When pe-

Korolchenko A.Ya. combustion and explosion processes

in the transition from mole fractions to the number of moles in the isochoric process in the mass conservation equations (15)-(18), it is necessary to replace the values with the corresponding ones. Equation (19) will then be written as

After multiplying both parts of equation (1.28) by one can calculate the amount needed to calculate the explosion pressure of a gas mixture in a bomb of constant volume according to equation (1.22).

Let us describe a method for solving the system of equations (1.15), (1.23)-(1.43), containing 21 unknown quantities: 19 mole fractions of combustion products, the total number of moles of products during the combustion of a mole of fresh mixture and the enthalpy of combustion products. The mole fractions of hydrogen, water, carbon dioxide, nitrogen, and argon are chosen as independent variables.

the shares of the remaining 14 combustion products are expressed in terms of equilibrium constants and selected independent variables from equations (1.29)-(1.43). Next, we rewrite equations (1.23)-(1.26) and (1.28), respectively, in the form

F(A, B, C, D, E) = 0,

G (A, B, C, D, E) = 0,

H(A,B,C,D,.E) = 0, (1.49)

J (A, B, C, D, E) = 0,

I (A, B, C, D, E) = 0.

Having linearized the system of equations (1.49) by expanding in a Taylor series up to terms containing first derivatives, we obtain

where, etc. (index 0 denotes the use of

input values). The system of equations (1.50) contains five unknowns - which are increments to the original

Chapter 1. Basic concepts of the theory of combustion

known - which are increments to the original

mole fractions A, B, C, D, E. The system can be solved by various methods, for example, by calculating and dividing by each other the determinants of the corresponding matrices of the system of equations (1.50) or by using the Gauss-Jordan elimination method.

At the assumed value of the equilibrium temperature of the combustion products T calculate the values of the equilibrium constants .. Then determine

are based on the initial values of the independent variables A, B, C, D, E the values of the remaining mole fractions of combustion products, and hence the coefficients of the system of equations (1.50). Then, by solving this system of equations, new values are found

The iterative process is repeated until the absolute values of the ratios become less than a certain value, equal, for example, to (at , the calculation results practically do not change). Thus, the equilibrium composition of the combustion products is determined at the expected temperature T. According to the equilibrium composition of the products, according to equation (1.27), the value of £u, - is found, which makes it possible to calculate the values of the enthalpy hj combustion products according to formula (1.18).

For combustion under isochoric conditions, the order of calculations is similar to that described above. The difference, as already noted, is that the calculation is carried out not for mole fractions, but for the number of moles, and instead of enthalpies, the internal energy of the fresh mixture and combustion products is calculated.

In table. Table 1.1 shows the calculated thermodynamic parameters for stoichiometric mixtures of methane, propane, hexane, heptane, acetone, isopropyl alcohol and benzene with air.

Table 1.1. Maximum adiabatic explosion pressure in a closed vessel, temperature of combustion products, adiabatic indices of fresh mixture and combustion products Ei for stoichiometric hydrocarbon mixtures at

initial temperature = 298.15 K

Korolchenko A.Ya. combustion and explosion processes

	0,06 0,04	5,188 3,439	2539,6 2521,9		1,247 1,248	2192,7 2183,2	7,412 7.385
3,964	0,10 0,08 0,06 0,04	9,228 7,358 5,494 3,640	2604,4 2594,1 2580,5 2561,2	1,365	1,247 1,248 1,248 1,249	2245,2 2239,4 2231,7 2220,7	7,897 7,880 7,857 7,825
2,126	0,10 0,08 0,06 0,04	9,378 7,478 5,583 3,699	2611,6 2601,2 2587,3 2567,8	1,360	1,248 1,248 1,249 1,249	2251,7 2245,8 2237,9 2226,7	8,025 8,008 7,984 7,951
1,842	0,10 0,08 0,06 0,04	9,403 7,498 5,598 3,708	2613,0 2602,6 2588,7 2569,1	1,359	1,248 1,248 1,249 1,249	2253,0 2247,1 2239,1 2227,9	8,047 8,029 8,005 7,972
4,907	0,10 0,08 0,06 0,04	9,282 7,401 5,527 3,661	2594,2 2583,7 2570,4 2550,9	1,357	1,245 1,245 1,246 1,246	2242,1 2236,2 2228,2 2216,9	7,962 7,944 7,921 7,888
4,386	0,10 0,08 0,06 0,04	9,344 7,451 5,565 3,688	2574 3 2564,4 2551,8 2533,2	1,361	1,244 1,245 1,245 1,246	2219,7 2214,3 2206,9 2196,5	7,999 7,983 7,961 7,929
2,679	0,10 0,08 0,06 0,04	9,299 7,411 5,532 3,662	2678,2 2666,0 2650,6 2628,2	1,377	1,251 1,251 1,252 1,252	2321,1 2313,7 2304,2 2290,4	7,990 7,969 7,942 7,902

The stoichiometric concentration of fuel during combustion in air of medium humidity and in dry air is determined, respectively, by the formulas:

where is the stoichiometric coefficient of oxygen, equal to the number of moles of oxygen per 1 mole of a combustible substance during its complete combustion.

Chapter 1. Basic concepts of the theory of combustion

On the rice. 1.3 as an example, the calculated change in the combustion temperature and mole fractions of the main components of combustion products depending on the volume concentration of fuel for a hexane-air mixture is shown.

Rice. 1.3. Dependence of composition and temperature of combustion products

hexane-air mixture at a pressure of 0.101 MPa and initial temperature

298.15 K from hexane concentration

Homogeneous and heterogeneous combustion.

Based on the considered examples, depending on the state of aggregation of the mixture of fuel and oxidizer, i.e. from the number of phases in the mixture, they distinguish:

For example, a heterogeneous reaction of spontaneous combustion of coal passes into a homogeneous phase of combustion of volatile substances. Coke residue burns heterogeneously.

According to the degree of preparation of the combustible mixture, diffusion and kinetic combustion are distinguished.

The combustion of solids differs from the combustion of gases by the presence of a stage of decomposition and gasification, followed by the ignition of volatile pyrolysis products.

The solid combustion model assumes the presence of the following phases (Fig. 17):

Rice. 17. Combustion model

solid.

Heating of the solid phase. For melting substances, melting occurs in this zone. The thickness of the zone depends on the conductivity temperature of the substance;

Pyrolysis, or the reaction zone in the solid phase, in which gaseous combustible substances are formed;

Pre-flame in the gas phase, in which a mixture with an oxidizing agent is formed;

A flame, or reaction zone in the gas phase, in which the conversion of pyrolysis products into gaseous combustion products;

combustion products.

The rate of oxygen supply to the combustion zone depends on its diffusion through the combustion product.

The flame structure of diffusion combustion consists of three zones (Fig. 18):

Rice. 18. The structure of the flame.

In zone 2, a mixture of vapors (gases) with atmospheric oxygen is formed and incomplete combustion occurs to CO with partial reduction to carbon (little oxygen):

C n H m + O 2 → CO + CO 2 + H 2 O;

In the 3rd outer zone, the products of the second zone are completely burned and the maximum flame temperature is observed:

2CO+O 2 \u003d 2CO 2;

The height of the flame is proportional to the diffusion coefficient and the flow rate of the gases and is inversely proportional to the density of the gas.

All types of diffusion combustion are inherent in fires.

Kinetic combustion is called combustion in advance

mixed combustible gas, vapor or dust with an oxidizing agent. In this case, the burning rate depends only on the physicochemical properties of the combustible mixture (thermal conductivity, heat capacity, turbulence, concentration of substances, pressure, etc.). Therefore, the burning rate increases sharply. This type of combustion is inherent in explosions.

Rice. 19. Scheme of flame propagation in a combustible mixture: - ignition source; - direction of motion of the flame front.

According to the degree of combustion, i.e. the completeness of the combustion reaction to the end products, combustion happens complete and incomplete.

Another example: when there is a lack of oxygen, carbon burns to carbon monoxide:

If you add O, then the reaction goes to the end:

2CO + O 2 \u003d 2CO 2.

The burning rate depends on the nature of the movement of gases. Therefore, laminar and turbulent combustion are distinguished.

So, an example of laminar combustion is the flame of a candle in still air. At laminar combustion layers of gases flow in parallel, but without swirling.

where: u- gas flow rate;

n- kinetic viscosity;

l- characteristic linear size.

The Reynolds number at which the transition of a laminar boundary layer to a turbulent one occurs is called critical Re cr, Re cr ~ 2320.

Turbulence increases the rate of combustion due to more intense heat transfer from the combustion products to the fresh mixture.

When burning solid fuel, the chemical reaction itself is preceded by the process of supplying an oxidizer to the reacting surface. Consequently, the combustion process of solid fuel is a complex heterogeneous physical and chemical process consisting of two stages: oxygen supply to the fuel surface by turbulent and molecular diffusion and chemical reaction on it.

Let us consider the general theory of heterogeneous combustion using the combustion of a spherical carbon particle as an example, assuming the following conditions. The oxygen concentration over the entire surface of the particle is the same; the rate of reaction of oxygen with carbon is proportional to the concentration of oxygen near the surface, i.e., a first-order reaction takes place, which is most likely for heterogeneous processes; the reaction proceeds on the surface of the particle with the formation of final combustion products, and there are no secondary reactions in the volume, as well as on the surface of the particle.

In such a simplified setting, the carbon burning rate can be represented as depending on the rate of its two main stages, namely, on the rate of oxygen supply to the interfacial surface and on the rate of the chemical reaction itself occurring on the surface of the particle. As a result of the interaction of these processes, a dynamic, equilibrium state occurs between the amount of oxygen supplied by diffusion and consumed for the chemical reaction of oxygen at a certain value of its concentration on the carbon surface.

Chemical reaction rate /(°2 g oxygen/(cm2-s) determined by

As the amount of oxygen consumed by a unit of the reaction surface per unit of time, it can be expressed in the following form:

In the equation:

K is the rate constant of a chemical reaction;

Cv is the oxygen concentration at the surface of the particle.

C. on the other hand, the burning rate is equal to the specific flux ki

Oxygen to the reacting surface, delivered by diffusion:

K °" \u003d ad (C, - C5). (15-2)

In the equation:

Ad - coefficient of diffusion exchange;

Co is the oxygen concentration in the flow in which the carbon particle burns.

Substituting the value of Cv found from equation (15-1) into equation (15-2), we obtain the following expression for the rate of heterogeneous combustion in terms of the amount of oxygen consumed by a unit surface of a particle per unit time:

". С°, ■' (15-3)

Denoting through

Kkazh - - C -, (15-4)

Expression (15-3) can be represented as

/<°’ = /СкажС„. (15-5)

In its structure, expression (15-5) is similar to the kinetic equation (15-1) of a first-order reaction. In it, the reaction rate constant "£" is replaced by the coefficient Kkazh, which depends both on the reaction properties of the fuel and on the laws of transfer and is therefore called the apparent rate constant of burning solid carbon.

The rate of combustion chemical reactions depends on the nature of the fuel and physical conditions: the concentration of the reacting gas on the surface, temperature and pressure. The temperature dependence of the chemical reaction rate is the strongest. At low temperatures, the chemical reaction rate is low and, in terms of oxygen consumption, many times less than the rate at which oxygen can be delivered by diffusion. The combustion process is limited by the rate of the chemical reaction itself and does not depend on supply conditions oxygen, i.e., air flow velocity, particle size, etc. Therefore, this region of heterogeneous combustion is called kinetic.

In the kinetic region of combustion, ad>-£, therefore, in formula (15-3), the value of 1 / ad can be neglected compared to 1 / & and then we get:

K°32 = kC0. (15-6)

The equilibrium between the amount of oxygen delivered by diffusion and consumed in the reaction is established at a small gradient of its concentration, due to which the value of the oxygen concentration on the reaction surface differs little from its value in the flow. At high temperatures, kinetic combustion can occur at high air flow rates and small particle sizes of the fuel, i.e., with such an improvement in the conditions for supplying oxygen, when the latter can be delivered in a much larger amount "compared to the need for a chemical reaction.

Various areas of heterogeneous combustion are graphically shown in Fig. 15-1. Kinetic region I is characterized by curve 1, which shows that with increasing temperature, the combustion rate increases sharply according to the Arrhenius law.

At a certain temperature, the rate of a chemical reaction becomes commensurate with the rate of oxygen delivery to the reaction surface, and then the combustion rate becomes dependent not only on the rate of the chemical reaction, but also on the rate of oxygen delivery. In this region, called the intermediate region (Fig. 15-1, region II, curve 1-2), the rates of these two stages are comparable, none of them can be neglected, and therefore the rate of the combustion process is determined by formula (15-3). With an increase in temperature, the burning rate increases, but to a lesser extent than in the kinetic region, and its growth gradually slows down and, finally, reaches its maximum upon transition to the diffuse region (Fig. 15-1, region III, curve 2-3), remaining independent of temperature. At higher temperatures in this region, the rate of the chemical reaction increases so much that the oxygen delivered by diffusion instantly enters into a chemical reaction, as a result of which the oxygen concentration on the surface becomes almost zero. In formula (15-3), we can neglect the value of 1/& compared to 1/ad, then we get that the combustion rate is determined by the rate of oxygen diffusion to the reaction surface, i.e.

And therefore this region of combustion is called diffusion. In the diffusion region, the burning rate is practically independent of the fuel properties and temperature. The influence of temperature affects only the change in physical constants. In this region, the combustion rate is strongly affected by the conditions of oxygen delivery, namely, hydrodynamic factors: the relative velocity of the gas flow and the particle size of the fuel. With an increase in the gas flow rate and a decrease in the particle size, i.e., with an acceleration in the delivery of oxygen, the rate of diffusion combustion increases.

During combustion, a dynamic equilibrium is established between the chemical process of oxygen consumption and the diffusion process of its delivery at a certain value of oxygen concentration at the reaction surface. The oxygen concentration at the particle surface depends on the ratio of the rates of these two processes; if the diffusion rate prevails, it will approach the concentration in the flow, while an increase in the chemical reaction rate causes it to decrease.

The combustion process occurring in the diffusion region can go into the intermediate region (curve 1"-2") or even into the kinetic region with increased diffusion, for example, with an increase in the flow rate or a decrease in particle size.

Thus, with an increase in the gas flow rate and with the transition to small particles, the process shifts towards kinetic combustion. An increase in temperature shifts the process towards diffusion combustion (Fig. 15-1, curve 2"-3").

The course of heterogeneous combustion in one region or another for any particular case depends on these specific conditions. The main task of studying the process of heterogeneous combustion is to establish the areas of combustion and to identify quantitative patterns for each area.

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