Homogeneous and heterogeneous combustion.
Based on the examples considered, depending on the state of aggregation of the mixture of fuel and oxidizer, i.e. depending on the number of phases in the mixture, there are:
1. Homogeneous combustion gases and vapors of flammable substances in a gaseous oxidizer environment. Thus, the combustion reaction occurs in a system consisting of one phase (aggregate state).
2. Heterogeneous combustion solid flammable substances in a gaseous oxidizer environment. In this case, the reaction occurs at the interface, while a homogeneous reaction occurs 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, occurs in the gas phase. Solid and liquid phases play the role of reservoirs of reacting products.
For example, the heterogeneous reaction of spontaneous combustion of coal passes into the homogeneous phase of combustion of volatile substances. The coke residue burns heterogeneously.
Based on the degree of preparation of the combustible mixture, diffusion and kinetic combustion are distinguished.
The types of combustion considered (except for explosives) relate to diffusion combustion. Flame, i.e. The combustion zone of a mixture of fuel and air must be constantly fed with fuel and oxygen in order to ensure stability. The supply of combustible gas depends only on the speed of its supply to the combustion zone. Arrival rate flammable 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 is the lowest temperature of a liquid at which the flame above its surface will not go out.
The combustion of solids differs from the combustion of gases by the presence of a stage of decomposition and gasification with subsequent ignition of volatile pyrolysis products.
Pyrolysis- This is the heating of organic substances to high temperatures without air access. In this case, the 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 a combustion product is not concentrated only in the flame zone, but has a multi-stage character.
Heating the solid phase causes decomposition and the release of gases, which ignite and burn. The heat from the torch heats the solid phase, causing it to gasify and the process repeats, thus maintaining combustion.
The solid combustion model assumes the presence of the following phases (Fig. 17):
Rice. 17. Combustion model
solid matter.
Warming up 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 reaction zone in the solid phase, in which gaseous flammable substances are formed;
Pre-flame in the gas phase, in which a mixture with an oxidizer is formed;
Flame, or reaction zone in the gas phase, in which the products of pyrolysis are converted 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 speed chemical reaction in the combustion zone in the types of combustion under consideration, depending on the rate of entry of the reacting components and the flame surface by molecular or kinetic diffusion, this type of combustion is called diffusion.
Flame structure diffusion combustion consists of three zones (Fig. 18):
Zone 1 contains gases or vapors. There is no combustion in this zone. The temperature does not exceed 500 0 C. Decomposition, pyrolysis of volatiles and heating to the auto-ignition temperature occurs.
Rice. 18. Flame structure.
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 external zone, complete combustion of the products of the second zone occurs and the maximum flame temperature is observed:
2CO+O 2 =2CO 2 ;
The flame height is proportional to the diffusion coefficient and gas flow rate and inversely proportional to the gas density.
All types of diffusion combustion are inherent in fires.
Kinetic combustion is called combustion in advance
mixed flammable gas, steam or dust with an oxidizer. 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 in this case When a combustible mixture is ignited at any 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 movement of the flame front.
Although, if you mix it first flammable gas with air and feed it into the burner, then upon ignition a stationary flame is formed, provided that the flow rate of the mixture is equal to the speed of flame propagation.
If the gas supply speed is increased, the flame breaks away from the burner and may go out. And if the speed is reduced, the flame will be drawn into the burner with a possible explosion.
According to combustion degree, i.e. completeness of the combustion reaction to the final products, combustion occurs complete and incomplete.
So in zone 2 (Fig. 18) combustion is incomplete, because There is insufficient oxygen supply, 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 completion:
2СО+O 2 =2СО 2.
The burning rate depends on the nature of the movement of gases. Therefore, a distinction is made between laminar and turbulent combustion.
Thus, an example of laminar combustion is a candle flame in still air. At laminar combustion layers of gases flow in parallel, without swirling.
Turbulent combustion– vortex movement of gases, in which combustion gases are intensively mixed and the flame front is blurred. The boundary between these types is the Reynolds criterion, which characterizes the relationship between inertial forces and friction forces in the flow:
Where: u- gas flow speed;
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 combustion rate due to more intense heat transfer from combustion products to the fresh mixture.
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 the air, but in many technological processes oxygen-enriched air and even pure oxygen are used (for example, metallurgical industries, gas welding, cutting, etc.). An atmosphere enriched with oxygen can be encountered in underwater and spacecraft, blast furnace processes, etc. Such combustible systems have increased fire danger. 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 equal amounts state of aggregation V 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 medium – when the fuel and oxidizer are not mixed.
Influence on the combustion process large number factors determine 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
widespread homogeneous combustion gases and vapors in the air.
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, 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 - in different ones.
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 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, combustion may occur. What does it have to do with combustion is not wood, but the products of its decomposition - 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 carbon 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 be formed during a real fire, but their formation rather precedes the fire or ensures initial stage 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 for 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.
During diffusion combustion, in case of lack of oxygen in the air, for example during fires in indoors, 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 a 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?
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. Required condition 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 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. With a big difference in 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.
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. Chief Editor N.P. Lyakishev. 2000 .
See what “homogeneous combustion” is in other dictionaries:
homogeneous combustion- Combustion of gases and vaporous flammable substances in a gaseous form. oxidizing agent For the beginning combustion needs to start. energetic pulse. Distinguish between self and forced. ignition or ignition; normal spreading combustion or deflagration (leading process of transmission... ...
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Combustion- a complex, rapidly occurring chemical transformation, accompanied by the release of a significant amount of heat and usually a bright glow (flame). In most cases, gas is based on exothermic oxidative reactions of a substance... Great Soviet Encyclopedia
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When burning solid fuel The chemical reaction itself is preceded by the process of supplying an oxidizing agent 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's consider general theory heterogeneous combustion using the example of combustion of a spherical carbon particle, taking 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 secondary reactions There are no particles in the volume or on the surface.
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 fuel particle sizes, i.e., with such an improvement in the conditions for the supply of acid water, when the latter can be delivered to a significantly more"compared to the requirement of a 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 an increase in the gas flow rate and a decrease in particle size, i.e., with the acceleration of 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 rate 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.
