heterogeneous combustion. Diffusion and kinetic combustion

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 of particle surface per unit of 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 proceeding 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.

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 implemented 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 the oxidizing agent is not oxygen, but other gases. 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.

Korolchenko AND I. combustion and explosion processes

The burning rate is determined by the processes of micromixing within 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 provides 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

Heterogeneous combustion - liquid and solid combustible substances in a gaseous oxidizer. For heterogeneous combustion of liquid substances, their evaporation is of great importance. Heterogeneous combustion of volatile combustible substances practically refers to homogeneous combustion, because such combustibles have time to completely or almost completely evaporate before ignition. Of great importance in technology is the heterogeneous combustion of solid fuels, mainly coals, which also contain a certain amount of organic substances, which, when the fuel is heated, decompose and are released in the form of vapors and gases. The thermally unstable part of the fuel is usually called volatile, and volatile. With slow heating, a clear staging of the beginning of the combustion stage is observed - first the volatile components and their ignition, then the ignition and combustion of the solid, the so-called coke residue, which, in addition to carbon, contains the mineral part of the fuel - ash.
See also:
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Encyclopedic Dictionary of Metallurgy. - M.: Intermet Engineering. Chief editor N.P. Lyakishev. 2000 .

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General information about combustion. Homogeneous and heterogeneous combustion

Combustion is an intense chemical oxidative reaction, which is accompanied by the release of heat and luminescence. Combustion occurs in the presence of a combustible substance, an oxidizing agent and an ignition source. Oxygen, nitric acid, sodium peroxide, Bertolet salt, perchlorates, nitro compounds, etc. can act as oxidizing agents in the combustion process. Many organic compounds, sulfur, hydrogen sulfide, pyrite, most metals in free form, carbon monoxide, hydrogen and etc. Combustion also differs in the speed of flame propagation and, depending on this factor, it can be: - deflationary (flame speed within a few meters per second); explosive (flame speed up to hundreds of meters per second); - detonation (flame speed of the order of thousands of meters per second). Homogeneous combustion. In homogeneous combustion, the initial substances and combustion products are in the same state of aggregation. This type includes the combustion of gas mixtures (natural gas, hydrogen, etc. with an oxidizer - usually air oxygen), the combustion of non-gasifying condensed substances (for example, termites - mixtures of aluminum with oxides of various metals), as well as isothermal combustion - the spread of a chain branched reaction in a gas mixture without significant heating. When burning non-gasifying condensed substances, diffusion usually does not occur and the process of combustion propagation occurs only as a result of heat conduction. In exothermic combustion, on the contrary, diffusion is the main transfer process. heterogeneous combustion. In heterogeneous combustion, the initial substances (for example, solid or liquid fuel and gaseous oxidizer) are in different states of aggregation. The most important technological processes of heterogeneous combustion are the combustion of coal, metals, the combustion of liquid fuels in oil furnaces, internal combustion engines, combustion chambers of rocket engines. The process of heterogeneous combustion is usually very complex. The chemical transformation is accompanied by the crushing of the combustible substance and its transition to the gas phase in the form of droplets and particles, the formation of oxide films on metal particles, the turbulence of the mixture, etc. Homogeneous combustion: the components of the combustible mixture are in a gaseous state. Moreover, if the components are mixed, then combustion is called kinetic. If - not mixed - diffusion combustion. Heterogeneous combustion: characterized by the presence of a phase separation in a combustible mixture (combustion of liquid and solid combustible substances in a gaseous oxidizer medium).

There are homogeneous, heterogeneous and diffusion combustion. Homogeneous combustion includes premixed gases. 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 implemented 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 in the case of large-scale turbulence or the acceleration of heat and mass transfer processes in the flame front in the case of small-scale turbulence. Turbulent combustion is characterized by self-similarity: turbulent vortices increase the combustion rate, which leads to an increase in turbulence.

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 evaporation of the liquid (or decomposition of the 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.

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. 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.

Burning gases

The term commonly used to describe combustion processes is normal flame speed, which characterizes the speed of a conventional flame front in a stationary gas mixture. In real combustion conditions, the flame always exists in moving streams.

The behavior of the flame under such conditions is subject to two laws:

– the first of them establishes that the component of the gas flow velocity v along the normal to the flame front propagating along the

viscous mixture is equal to the normal speed of flame propagation and divided by cos :

v = u/ cos φ, (1.2)

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

This law only applies to flat flames. Its generalization to the real case with the 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 cross section, a curved flame front with a common surface is stationary S. At each point of the flame front, the flame propagates along the normal to its surface with a speed U.Then the volume of the combustible mixture burning per unit time will be

ω = U S.(1.3)

On the other hand, in accordance with the balance of the source gas, the same volume is equal to

ω = v ∙ ε.(1.4)

Equating the left-hand sides of (1.2) and (1.3), we obtain

v = U S/ε.(1.5)

In the reference frame in which the flame front moves through a stationary gas mixture, relation (1.5) means that the flame propagates relative to the gas with a velocity v. Formula (1.5) 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.

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. There are two types of turbulent combustion: combustion of a homogeneous gas mixture and microdiffusion turbulent combustion. In turn, during the combustion of a homogeneous mixture in the turbulent combustion mode, two cases are possible: the appearance of small-scale and large-scale 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 smaller than the thickness of the flame front, it is referred to as small-scale, at a larger −
to the 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. The highest burning rates are observed during large-scale turbulence. In this case, two combustion acceleration mechanisms are possible: surface and volumetric.

One of the types of combustion gases is deflagration combustion. The composition of combustible mixtures may be different. In general, the content of the combustible component can range from zero to one hundred percent, however, not all mixtures of fuel and oxidizer are capable of spreading a flame. Distribution is possible only in a certain range of concentrations. When igniting mixtures whose composition goes beyond these limits, the combustion reaction initiated by the ignition pulse dies out at a small distance from the place of ignition. For mixtures of fuel and oxidizer that are in a gaseous state, there are minimum and maximum fuel concentrations that limit the range of combustible mixtures. These concentrations are called the lower and upper concentration limits of flame propagation, respectively. Outside the limits, the spread of flame through this mixture is impossible. Let us consider the reasons for the existence of limiting conditions for flame propagation through gas mixtures. At the initial moment of combustion initiation (by a spark, hot body or open flame), a high temperature zone appears in the combustible mixture, from which the heat flow will be directed to the surrounding space. Part of the heat enters the fresh (not yet burned) mixture, the other part goes to the combustion products. If the heat flux into the fresh mixture is insufficient to initiate a combustion reaction in it, the initial flame center dies out.

Thus, the presence of limits of flame propagation in gas mixtures is explained by heat losses from the reaction zone. Detonation is the process of transformation of a combustible mixture or explosive, accompanied by the release of heat and propagating at a constant speed exceeding the speed of sound in a given mixture or substance.

Unlike deflagration combustion, where flame propagation is due to relatively slow diffusion and heat conduction processes, detonation is a complex of a powerful shock wave and a chemical transformation zone following its front. Due to the sharp increase in temperature and pressure behind the shock wave front, the chemical transformation of the starting materials into combustion products proceeds extremely quickly in a very thin layer immediately adjacent to the shock wave front (Fig. 1.2).

Rice. 1.2. Diagram of a detonation wave

The shock wave compresses and heats the combustible mixture (or explosive), causing a chemical reaction, the products of which greatly expand - an explosion occurs. The energy released as a result of the chemical transformation maintains the existence of the shock wave, preventing it from decaying. The velocity of the detonation wave is constant for each combustible mixture and explosive and reaches
1000–3000 m/s in gas mixtures and 8000–9000 m/s in condensed explosives (Table 1.1).

Table 1.1

Detonation speed of some combustible mixtures
and explosives

The end of the table. 1.1

The pressure in the shock wave front during the detonation of gas mixtures reaches 1–5 MPa (10–50 atm.), and 10 GPa for condensed substances.
In gaseous combustible mixtures, the propagation of detonation is possible only under conditions when the concentration of combustible gas (or vapors of a combustible liquid) is within certain limits, depending on the chemical nature of the combustible mixture, pressure and temperature. For example, in a mixture of hydrogen with oxygen at room temperature and atmospheric pressure, a detonation wave can propagate if the hydrogen concentration is in the range from 20 to 90% vol.

The transition of deflagration combustion to detonation in gas-air mixtures is possible in the following cases:

● when the combustible mixture is enriched with oxygen;

● with very large gas clouds;

● in the presence of combustion turbulators.

In combustible clouds of sufficiently large sizes, the transition from deflagration combustion to detonation is inevitable, while the analytical estimate leads to the following critical cloud sizes, at which the probability of detonation is high: for hydrogen air mixtures - 70 m, for propane-air - 3500 m, for methane-air - 5000 m. Turbulization of the process of combustion of gas mixtures with the help of various obstacles along the path of a propagating flame leads to a significant reduction in the critical dimensions of gas clouds, and the detonation wave that occurs in this case becomes a source of detonation excitation in unlimited space.

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