Homogeneous combustion. Theory of heterogeneous combustion Homogeneous combustion

There are homogeneous, heterogeneous and diffusion combustion. Homogeneous combustion refers to the combustion of pre-mixed gases. Examples of homogeneous combustion are the combustion processes of gases or vapors in which the oxidizing agent 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 met. 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 by fragmenting the flame front into separate fragments and, accordingly, increasing the contact area of ​​reacting substances with large-scale turbulence or accelerating 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 speed, which leads to an increase in turbulence.

During fires, the most common processes are diffusion combustion. In them, all reacting substances are in the gas phase, but are not pre-mixed. In the case of combustion of liquids and solids, the process of oxidation of fuel in the gas phase occurs simultaneously with the process of evaporation of the liquid (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. In fires, the regime 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 involves the sequential crushing of gas into smaller and smaller volumes and mixing them with each other.

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 gas phase diffusion. 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. A flow of steam or gaseous decomposition products enters the combustion zone from the surface, 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 occur partially heterogeneously - on the surface of the condensed phase, and partially homogeneously - in the volume of the gas mixture.

An example of heterogeneous combustion is the combustion of coal and charcoal. When these substances burn, two types of reactions occur. Some types of coal release 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, during the combustion of pure carbon, carbon monoxide CO can be formed, which burns out in volume.

Combustion of 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, flames always exist in moving streams.

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

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

visible mixture, equal to the normal flame propagation speed 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 applies only to a flat flame. Generalizing it to the real case 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 having a speed v and cross section, 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 at a speed U.Then the volume of the combustible mixture burned 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 a reference system in which the flame front moves through a stationary gas mixture, relation (1.5) means that the flame propagates relative to the gas at a speed v. Formula (1.5) is a mathematical expression of the law of areas, from which an important conclusion follows: when the flame front is curved, the burning speed increases in proportion to the increase in its surface. Therefore, non-uniform gas movement always intensifies combustion.

From the law of areas it follows that turbulence increases the burning rate. 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, when a homogeneous mixture burns in the turbulent combustion mode, two cases are possible: the occurrence of small-scale and large-scale turbulence. This division is made depending on the ratio of the scale of turbulence and the thickness of the flame front. When the scale of turbulence is smaller than the thickness of the flame front, it is classified as small-scale; when larger -
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. The highest combustion rates are observed during large-scale turbulence. In this case, two mechanisms for accelerating combustion are possible: surface and volumetric.

One type of combustion of gases is deflagration combustion. The composition of flammable mixtures may be different. In general, the content of the flammable 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 concentration range. When mixtures whose composition exceeds these limits are ignited, the combustion reaction initiated by the ignition pulse fades at a short distance from the ignition site. For mixtures of fuel and oxidizer that are in a gaseous state, there are minimum and maximum concentrations of fuel that limit the area of ​​flammable mixtures. These concentrations are called the lower and upper concentration limits of flame propagation, respectively. Outside the limits, flame propagation through this mixture is impossible. Let us consider the reasons that determine the existence of limiting conditions for the propagation of flame through gas mixtures. At the initial moment of combustion initiation (by a spark, a hot body or an open flame), a high-temperature zone appears in the combustible mixture, from which the heat flow will be directed into the surrounding space. Part of the heat enters the fresh (not yet burned) mixture, the other part goes into the combustion products. If the flow of heat into the fresh mixture is not sufficient to initiate a combustion reaction in it, the original source of flame will die out.

Thus, the presence of limits for flame propagation through gas mixtures is explained by heat loss from the reaction zone. Detonation is the process of transformation of a flammable mixture or explosive substance, 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 determined by relatively slow processes of diffusion and thermal conductivity, 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 starting substances into combustion products occurs extremely quickly in a very thin layer directly adjacent to the shock wave front (Fig. 1.2).

Rice. 1.2. Detonation wave diagram

The shock wave compresses and heats the flammable mixture (or explosive), causing a chemical reaction, the products of which expand greatly - an explosion occurs. The energy released as a result of the chemical transformation maintains the existence of the shock wave, preventing it from dying out. The speed of movement of the detonation wave is constant for each combustible mixture and explosive substance 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 flammable mixtures
and explosives

End of table. 1.1

The pressure at the shock wave front during detonation of gas mixtures reaches 1–5 MPa (10–50 atm), and for condensed substances – 10 GPa.
In gaseous flammable mixtures, the propagation of detonation is possible only under conditions where the concentration of flammable gas (or flammable liquid vapor) is within certain limits, depending on the chemical nature of the combustible mixture, pressure and temperature. For example, in a mixture of hydrogen and 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 enriching the combustible mixture with oxygen;

● with very large gas clouds;

● in the presence of combustion turbulators.

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

Related information.

Combustible environment

Oxidizing agents

Oxidizing agents are substances whose atoms accept electrons during chemical transformations. Among the simple substances, these include all halogens and oxygen.

The most common oxidizing agent in nature is atmospheric oxygen.

In real fires, combustion mainly occurs in air, but many technological processes use air enriched with oxygen, and even pure oxygen (for example, metallurgical production, gas welding, cutting, etc.). An atmosphere enriched with oxygen can be encountered in underwater and spacecraft, blast furnace processes, etc. Such flammable systems have an increased fire hazard. This must be taken into account when developing fire extinguishing systems, fire prevention measures and during fire-technical examination of fires.

In addition to atmospheric oxygen and halogens, complex substances can also act as oxidizing agents in combustion reactions, for example, salts of oxygen-containing acids - nitrates, chlorates, etc., used in the production of gunpowder, military and industrial explosives and various pyrotechnic compositions.

A mixture of fuel and oxidizer in the same state of aggregation in certain proportions and capable of burning (and combustion is possible only at certain ratios) is called a flammable medium.

There are two types of flammable media: homogeneous and heterogeneous.

Homogeneous flammable medium is called a pre-mixed mixture of fuel and oxidizer, and, accordingly, heterogeneous flammable environment – when the fuel and oxidizer are not mixed.

The influence of a large number of factors on the combustion process determines the variety of types and modes of combustion. Thus, depending on the state of aggregation of the components of a combustible mixture, combustion can be homogeneous and heterogeneous, on the conditions of mixing the components - combustion of a pre-prepared mixture (kinetic) and diffusion, on gas-dynamic conditions - laminar and turbulent, etc.

The main types of combustion are homogeneous and heterogeneous.

Homogeneous combustion - This is the process of interaction between fuel and
oxidizers in the same state of aggregation. Most
Homogeneous combustion of gases and vapors in air is widespread.

Heterogeneous combustion- this is the combustion of solid combustible materials -
als directly on their surface. Characteristic feature
heterogeneous combustion is the absence of flame. Examples of it
are the combustion of anthracite, coke, charcoal, and non-volatile metals.
Flameless combustion is sometimes called smoldering.

As can be seen from the definitions, the fundamental difference between homogeneous combustion and heterogeneous combustion is that in the first case the fuel and the oxidizer are in the same state of aggregation, in the second they are in different states.

It should be noted that the combustion of solids and materials is not always heterogeneous. This is explained by the combustion mechanism of solids.

For example, wood burning in air. In order to light it, you need to bring some kind of heat source, such as a flame from a match or lighter, and wait a while. The question arises: why does it not light up immediately? This is explained by the fact that in the initial period, the ignition source must heat the wood to a certain temperature at which the process of pyrolysis, or in other words thermal decomposition, begins. At the same time, as a result of the decomposition of cellulose and other components, their decomposition products begin to be released - flammable gases - hydrocarbons. Obviously, the greater the heating, the greater the rate of decomposition and, accordingly, the rate of release of flammable gases. And only when the rate of GH release is sufficient to create a certain concentration in the air, i.e. formation of a flammable environment, fire may occur. What does it have to do with burning not of wood, but of its decomposition products - flammable gases. This is why wood combustion, in most cases, is homogeneous combustion, not heterogeneous.

You may object: wood eventually begins to smolder, and smoldering, as mentioned above, is heterogeneous combustion. This is true. The fact is that the end products of wood decomposition are mainly flammable gases and carbonaceous residue, the so-called coke. You have all seen this very carbonaceous residue and even bought it for cooking kebabs. These coals are approximately 98% pure carbon and cannot emit GH. The coals burn in the heterogeneous combustion mode, that is, they smolder.

Thus, the wood burns first in a homogeneous combustion mode, then, at a temperature of approximately 800°C, the flaming combustion turns into smoldering, i.e. becomes heterogeneous. The same thing happens with other solids.

How do liquids burn in air? The mechanism of combustion of liquids is that they evaporate first, and it is the vapors that form a flammable mixture with air. That is, in this case homogeneous combustion also occurs. It is not the liquid phase that burns, but the vapor of the liquid

The mechanism of combustion of metal is the same as that of liquids, except that the metal must first be melted and then heated to a high temperature in order for the evaporation rate to be sufficient to form a flammable medium. Some metals burn on their surface.

In homogeneous combustion, two modes are distinguished: kinetic and diffusion combustion.

Kinetic combustion– this is the combustion of a pre-mixed combustible mixture, i.e. homogeneous mixture. The burning rate is determined only by the kinetics of the redox reaction.

Diffusion combustion– this is the combustion of a heterogeneous mixture, when the fuel and oxidizer are not pre-mixed, i.e. heterogeneous. In this case, mixing of fuel and oxidizer occurs in the flame front due to diffusion. Unorganized combustion is characterized by a diffusion combustion mode; most combustible materials in a fire can only burn in this mode. Homogeneous mixtures, of course, can also form during a real fire, but their formation rather precedes the fire or provides the initial stage of development.

The fundamental difference between these types of combustion is that in a homogeneous mixture the molecules of the fuel and oxidizer are already in close proximity and are ready to enter into chemical interaction, while with diffusion combustion these molecules must first approach each other due to diffusion, and only then enter into interaction.

This determines the difference in the rate of combustion process.

Total burning time t g, consists of the duration of physical
ski and chemical processes:

t g = t f + t x.

Kinetic combustion mode characterized by the duration of only chemical processes, i.e. t g » t x, since in this case no physical preparation processes (mixing) are required, i.e. t f » 0 .

Diffusion combustion mode, on the contrary, it depends mainly on
the speed of preparation of a homogeneous combustible mixture (roughly speaking, the bringing together of molecules), In this case t f >> t x, and therefore the latter can be neglected, i.e. its duration is determined mainly by the speed of physical processes.

If t f » t x, i.e. they are commensurate, then combustion proceeds in the following way
called the intermediate region.

For example, imagine two gas burners (Fig. 1.1): in one of them there are holes in the nozzle for air access (a), in the other there are none (b). In the first case, air will be sucked in by injection into the nozzle, where it is mixed with flammable gas, thus forming a homogeneous combustible mixture, which burns at the exit of the nozzle in kinetic mode . In the second case (b), air is mixed with combustible gas during the combustion process due to diffusion, in this case - diffusion combustion .

Rice. 1.1Example of kinetic (a) and diffusion (b) combustion

Another example: there is a gas leak in the room. The gas gradually mixes with air, forming a homogeneous combustible mixture. And if an ignition source appears after this, an explosion occurs. This is combustion in the kinetic mode.

The same applies to the combustion of liquids, such as gasoline. If it is poured into an open container and set on fire, diffusion combustion will occur. If you place this container in a closed room and wait some time, the gasoline will partially evaporate, mix with air and thereby form a homogeneous combustible mixture. When you introduce an ignition source, as you know, an explosion will occur; this is kinetic combustion.

In what mode does combustion occur in real fires? Of course, mainly in diffusion. In some cases, a fire may begin with kinetic combustion, as in the examples given, but after the homogeneous mixture burns out, which happens very quickly, combustion will continue in the diffusion mode.

With diffusion combustion, in the event of a lack of oxygen in the air, for example during fires in enclosed spaces, incomplete combustion of fuel is possible with the formation of incomplete combustion products such as CO - carbon monoxide. All products of incomplete combustion are very toxic and pose a great danger in a fire. In most cases, they are the ones responsible for the death of people.

So, the main types of combustion are homogeneous and heterogeneous. The visual difference between these modes is the presence of flame.

Homogeneous combustion can occur in two modes: diffusion and kinetic. Visually, their difference lies in the burning rate.

It should be noted that there is another type of combustion - the combustion of explosives. Explosives include fuel and an oxidizer in the solid phase. Since both the fuel and the oxidizer are in the same state of aggregation, such combustion is homogeneous.

In real fires, mostly flaming combustion occurs. Flame, as is known, is identified as one of the dangerous factors of fire. What is a flame and what processes take place in it?

When burning a 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 physicochemical process, consisting of two stages: the supply of oxygen to the surface of the fuel by turbulent and molecular diffusion and a chemical reaction on it.

Let us consider the general theory of heterogeneous combustion using the example of combustion of a spherical carbon particle, accepting 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 oxygen concentration at the surface, i.e., a first-order reaction takes place, which is most likely for heterogeneous processes; the reaction occurs 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 situation, the rate of carbon combustion 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 delivered by diffusion and consumed for the chemical reaction at a certain value of its concentration on the carbon surface.

The rate of chemical reaction /(°2 g oxygen/(cm2-s), determined

How the amount of oxygen consumed by a unit of reaction surface per unit of time can be expressed as follows:

In the equation:

K is the rate constant of the chemical reaction;

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

On the other hand, the burning rate is equal to the specific flux

Sweat to the reacting surface, delivered by diffusion:

K°" = ad(C, - C5). (15-2)

In the equation:

Ad - diffusion exchange coefficient;

Co is the concentration of oxygen in the stream in which the carbon particle burns.

Substituting the value of St, 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 per unit surface of a particle per unit time:

". С°, ■’ (15-3)

Denoting by

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 Kkaz, which depends both on the reaction properties of the fuel and on the transfer patterns and is therefore called the apparent combustion rate constant of solid carbon.

The rate of chemical combustion 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 rate of the chemical reaction is the strongest. In the region of low temperatures, the rate of the chemical reaction is low and the oxygen consumption is 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 the supply conditions oxygen, i.e. air flow speed, 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 1/ad can be neglected in comparison with 1/& and then we obtain:

K°32 = kC0. (15-6)

Equilibrium between the amount of oxygen delivered by diffusion and consumed for 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 velocities and small particle sizes of fuel, i.e., with such an improvement in the conditions for supplying oxygen, when the latter can be delivered in significantly greater quantities compared to the requirement of the chemical reaction.

Various regions of heterogeneous combustion are graphically depicted 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 the 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 intermediate (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 increasing temperature, the combustion 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 chemical reaction increases so much that the oxygen supplied by diffusion instantly enters into a chemical reaction, as a result of which the oxygen concentration on the surface becomes almost equal to zero. In formula (15-3), we can neglect the value of 1/& compared to 1/ad, then we find that the combustion rate is determined by the rate of oxygen diffusion to the reaction surface, i.e.

And therefore this combustion region 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 changes in physical constants. In this region, the combustion rate is strongly influenced by the conditions of oxygen delivery, namely hydrodynamic factors: the relative speed of the gas flow and the size of the fuel particles. With increasing gas flow velocity and decreasing particle size, i.e., with accelerating oxygen delivery, the rate of diffusion combustion increases.

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

The combustion process occurring in the diffusion region can move into the intermediate (curve 1"-2") or even into the kinetic region when diffusion increases, for example, when the flow rate increases or the particle size decreases.

Thus, with an increase in the gas flow velocity and 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 occurrence of heterogeneous combustion in a particular area for any particular case depends on these specific conditions. The main task of studying the process of heterogeneous combustion is to establish areas of combustion and identify quantitative patterns for each area.

Gases and vaporous flammable substances in a gaseous oxidizer. An initial energy impulse is required to start combustion. A distinction is made between self- and forced ignition or ignition; normally propagating combustion or deflagration (the leading process is heat transfer by thermal conductivity) and detonation (with ignition by a shock wave). Normal combustion is divided into laminar (stream) and turbulent (vortex). A distinction is made between combustion with the flow of pre-mixed gas and combustion with separate flow of combustible gas and oxidizer, when it is determined by mixing (diffusion) of two streams.
See also:
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Encyclopedic dictionary of metallurgy. - M.: Intermet Engineering. Editor-in-Chief N.P. Lyakishev. 2000 .

See what “homogeneous combustion” is in other dictionaries:

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combustion oxygen explosion

Homogeneous combustion refers to the combustion of pre-mixed gases. Numerous examples of homogeneous combustion are the combustion processes of gases or vapors in which the oxidizing agent is oxygen in the air: 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 met. 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 by fragmenting the flame front into separate fragments and, accordingly, increasing the contact area of ​​reacting substances in large-scale turbulence or accelerating heat and mass transfer processes in the flame front in small-scale turbulence. Turbulent combustion is characterized by self-similarity: turbulent vortices increase the combustion speed, which leads to an increase in turbulence.

All parameters of homogeneous combustion also appear in processes in which the oxidizing agent is not oxygen, but other gases. For example, fluorine, chlorine or bromine.

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 gas phase diffusion. 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. A flow of steam or gaseous decomposition products enters the combustion zone from the surface, and combustion occurs in the gas phase. Such combustion can be classified as 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 occur partially heterogeneously - on the surface of the condensed phase, and partially homogeneously - in the volume of the gas mixture.

An example of heterogeneous combustion is the combustion of coal and charcoal. When these substances burn, two types of reactions occur. Some types of coal release 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, during the combustion of pure carbon, carbon monoxide CO can be formed, which burns out in volume. With a sufficient excess of air and a high temperature of the coal surface, volumetric reactions occur so close to the surface that, to a certain approximation, this gives reason to consider such a process heterogeneous.

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