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.
4.3. Diffusion and kinetic combustion.
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.
heating 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 transformation 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 the chemical reaction in the combustion zone in the types of combustion under consideration depends on the rate of entry of the reacting components and the flame surface through molecular or kinetic diffusion, this type of combustion is called diffusion.
The structure of the diffusion combustion flame 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 the combustion of pre-mixed flammable gas, steam or dust with an oxidizer. In this case, the burning rate depends only on physical and chemical properties 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 first mix the flammable gas with air and feed it into the burner, then when ignited, a stationary flame will form, 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:
,
(4.1)
Where: - gas flow speed;
- 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.
General information about combustion. Homogeneous and heterogeneous combustion
Combustion is an intense chemical oxidation reaction that is accompanied by the release of heat and glow. Combustion occurs in the presence of a flammable substance, an oxidizer, and an ignition source. Oxygen can act as oxidizing agents in the combustion process. Nitric acid, sodium peroxide, Bertholet salt, perchlorates, nitro compounds, etc. Many are used as fuel organic compounds, sulfur, hydrogen sulfide, pyrite, most metals in free form, carbon monoxide, hydrogen, 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. With homogeneous combustion, the starting materials 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, thermites - mixtures of aluminum with oxides various metals), as well as isothermal combustion - the propagation of a branched chain 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 thermal conductivity. In exothermic combustion, on the contrary, the main transfer process is diffusion. Heterogeneous combustion. In heterogeneous combustion, the starting substances (for example, solid or liquid fuel and gaseous oxidizer) are in different states of aggregation. The most important technological processes of heterogeneous burning - burning coal, metals, combustion liquid fuels in oil furnaces, engines internal combustion, combustion chambers of rocket engines. The heterogeneous combustion process is usually very complex. The chemical transformation is accompanied by fragmentation of the flammable substance and its transition into the gas phase in the form of drops and particles, the formation of oxide films on metal particles, turbulization of the mixture, etc. Homogeneous combustion: the components of the combustible mixture are in gaseous state. Moreover, if the components are mixed, then the combustion is called kinetic. If - not mixed - diffusion combustion. Heterogeneous combustion: characterized by the presence of phase separation in a combustible mixture (combustion of liquid and solid combustible substances in a gaseous oxidizer).
Heterogeneous combustion of liquid and solid combustible substances in a gaseous oxidizer. For heterogeneous combustion liquid substances great importance has their evaporation. Heterogeneous combustion of easily evaporating flammable substances practically refers to homogeneous combustion, because Such combustibles have time to evaporate completely or almost completely even before ignition. In technology, heterogeneous combustion of solid fuels, mainly coals, containing a certain amount of organic substances, which, when the fuel is heated, decompose and are released in the form of vapors and gases, is of great importance. The thermally unstable part of the fuel is usually called volatile, and - volatile. With slow heating, a clear staged pattern 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 .
See what “heterogeneous combustion” is in other dictionaries:
heterogeneous combustion- Combustion of liquids and solids. flammable substances in gaseous form. oxidizing agent For the city of liquid things, the process of their evaporation is of great importance. G. g. easily evaporating flammable substances in practice. refers to a homogeneous city, because such flammable things even before... ... Technical Translator's Guide
heterogeneous combustion- heterogeninis degimas statusas T sritis chemija apibrėžtis Skysčio ar kietosios medžiagos degimas. atitikmenys: engl. heterogeneous combustion rus. heterogeneous combustion... Chemijos terminų aiškinamasis žodynas
heterogeneous combustion- heterogeninis degimas statusas T sritis Energetika apibrėžtis Degimas, kai reaguojančiosios medžiagos yra skirtingos agregatinės būsenos ir reakcija vyksta jų skirtingų fazių sąlyčio paviršiuose. atitikmenys: engl. heterogeneous combustion vok.… … Aiškinamasis šiluminės ir branduolinės technikos terminų žodynas
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
A complex, rapid chemical transformation of a substance, such as fuel, accompanied by the release of a significant amount of heat and a bright glow (flame). In most cases, the basis of combustion is exothermic... ...
Combustion (reaction)- (a. combustion, burning; n. Brennen, Verbrennung; f. combustion; i. combustion) a rapidly occurring oxidation reaction, accompanied by the release of means. amount of heat; usually accompanied by a bright glow (flame). In most cases… … Geological encyclopedia
Combustion- an exothermic reaction of oxidation of a flammable substance, usually accompanied by visible electromagnetic radiation and the release of smoke. G. is based on the interaction of a flammable substance with an oxidizing agent, most often atmospheric oxygen. Distinguish... ... Russian encyclopedia on labor protection
COMBUSTION- complex chemistry a reaction occurring under conditions of progressive self-acceleration associated with the accumulation of heat or catalyzing reaction products in the system. With G., high temperatures (up to several thousand K) can be achieved, and often occurs... ... Physical encyclopedia
COMBUSTION- complex, fast-flowing chemical process. transformation accompanied by the release of heat. Typically occurs in systems containing fuel (e.g. coal, natural gas) and an oxidizer (oxygen, air, etc.). Can be homogeneous (in advance... ... Big Encyclopedic Polytechnic Dictionary
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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 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 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
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, 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. Disorganized combustion is characterized by diffusion combustion mode, most flammable 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 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?
Topic 4. TYPES OF COMBUSTION.
According to various characteristics and characteristics, combustion processes can be divided into the following types:
According to the state of aggregation of a flammable substance:
Combustion of gases;
Combustion of liquids and melting solids;
Combustion of non-melting solid dust-like and compact substances.
According to the phase composition of the components:
Homogeneous combustion;
Heterogeneous combustion;
Combustion of explosives.
According to the preparedness of the combustible mixture:
Diffusion combustion (fire);
Kinetic combustion (explosion).
According to the dynamics of the flame front:
Stationary;
Unsteady.
According to the nature of gas movement:
Laminar;
Turbulent.
According to the degree of combustion of the flammable substance:
Incomplete.
According to the speed of flame spread:
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 the 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 o C. Tcr is minimum temperature heating 1 mole of a substance in a closed vessel, in which it completely turns into steam (see § 2.3).
2. Liquids are substances with a melting point (dropping point) of less than 50 o C (see § 2.5).
3. Solids are substances with a melting point (dropping point) of more than 50 0 C.
4. Dusts are crushed solids with a particle size of less than 0.85 mm.
The area in which a chemical reaction occurs in a flammable mixture, i.e. combustion is called a flame front.
Let's look at combustion processes in air using examples.
Combustion of gases in a gas burner. There are 3 flame zones observed here (Fig. 12):
Rice. 12. Scheme of gas combustion: 1 – transparent cone – this is the initial gas that is heated (to the auto-ignition temperature); 2 – luminous zone of the flame front; 3 – combustion products (they are almost invisible during 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 combustion: 1 – liquid; 2 – liquid vapors (dark areas); 3 – flame front; 4 – combustion products (smoke).
The width of the flame front in this case is larger, i.e. the reaction proceeds more slowly.
Combustion of melting solids. Consider the burning of a candle. In this case, 6 zones are observed (Fig. 14):
Rice. 14. Burning a candle: 1 – hard wax; 2 – molten (liquid) wax; 3 – dark transparent vapor layer; 4 – flame front; 5 – combustion products (smoke); 6 – wick.
The burning wick serves to stabilize the combustion. Liquid is absorbed into it, rises through it, evaporates and burns. The width of the flame front increases, which increases the luminosity area, since more complex hydrocarbons are used, which, when evaporated, disintegrate and then react.
Combustion of non-melting solids. We will consider this type of combustion using the example of the combustion of a match and a cigarette (Fig. 15 and 16).
There are also 5 sections 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 – combustion products (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 has become charred, i.e. the non-volatile part remained, and the volatile part evaporated and burned. The burning rate of coal is much slower than that of gases, so it does not have time to burn out completely.
Fig. 16. Cigarette burning: 1 – initial tobacco mixture; 2 – smoldering section 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 oxygen (1-2%). The smoke is bluish, not black. This means there are more gasified rather than burnt substances in it.
The surface of the ash is almost white. This means that with a sufficient supply of oxygen, complete combustion occurs. But inside and at the border of the burning layer with fresh layers there is a black substance. This indicates incomplete combustion of charred particles. By the way, vapors of evaporated resinous substances condense on the filter.
A similar type of combustion is observed when burning 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 occurs in gaseous form and is accompanied by a flame. Some solid substances, including those with a tendency to spontaneous combustion, burn as smoldering on the surface and inside the material.
Combustion of dusty substances. The dust layer burns in the same way as in a compact state, only the burning rate increases due to an increase in the surface of contact with air.
The combustion of dusty substances in the form of an air suspension (dust cloud) can occur 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, flaming combustion occurs.
Combustion of explosives. TO this species This includes the combustion of explosives and gunpowder, the so-called condensed substances, which already contain chemically or mechanically bound fuel and oxidizer. For example: in trinitrotoluene (TNT) C 7 H 5 O 6 N 3 × C 7 H 5 × 3NO 2 the oxidizing agents are O 2 and NO 2; gunpowder contains sulfur, saltpeter, coal; The homemade explosive consists of aluminum powder and ammonium nitrate, and the binder is solar oil.
4.2. 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.
4.3. Diffusion and kinetic combustion.
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. The rate of entry of 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 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.
The structure of the diffusion combustion flame 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 the combustion of pre-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 this case, when the 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.
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 occur. These types of combustion can transform into each other.
Normal combustion– this is combustion in which the spread of flame occurs in the absence of external disturbances (turbulence or changes in gas pressure). It depends only on the nature of the flammable substance, i.e. thermal effect, thermal conductivity and diffusion coefficients. 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. Combustion is called normal because the velocity vector of its propagation is perpendicular to the flame front.
4.5. Deflagration (explosive) combustion.
Normal combustion is unstable and closed space tends to self-accelerate. 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.
Let's consider the process of flame propagation in a pipe (Fig. 20).
Rice. 20. Scheme of the occurrence of explosive combustion.
At first, at the open end of the pipe, the flame spreads at normal speed, because combustion products expand freely and come out. The pressure of the mixture does not change. The duration of uniform flame propagation 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 escape 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 towards the spread of the flame. The layers adjacent to the walls are inhibited. The flame has the highest speed in the center of the pipe, and the slowest speed is near the walls (due to heat removal in them). Therefore, the flame front extends in the direction of flame propagation, and its surface increases. In proportion to this, the amount of combustible mixture per unit time increases, which entails an increase in pressure, and this, in turn, increases the speed of gas movement, etc. Thus, there is an avalanche-like increase in the speed of flame propagation to hundreds of meters per second.
The process of flame propagation through a combustible gas mixture, in which the self-accelerating combustion reaction spreads due to heating by thermal conduction from the adjacent layer of reaction products, is called deflagration. Typically, deflagration combustion rates are subsonic, i.e. less than 333 m/s.
4.6. Detonation combustion.
If we consider the combustion of a combustible mixture layer by layer, then as a result of thermal expansion of the volume of combustion products, each time a compression wave appears 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 combine into one shock wave (Fig. 21).
Rice. 21. Scheme of 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 for 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 fueled by shock waves from the flame moving behind it.
The peculiarity of detonation is that it occurs at a supersonic speed of 1000-9000 m/s, determined for each mixture composition, and therefore is a physical constant of the mixture. It depends only on the caloric content 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 look flame spread, because has maximum explosion power (N=A/t) and enormous speed. In practice, detonation can be “neutralized” only in the pre-detonation section, i.e. at a distance from the ignition point to the point of detonation combustion. For gases, the length of this section is from 1 to 10 m.
