The concept of combustion. Modes of occurrence of combustion. General information about the combustion process, fire and its development
1.1. Brief information about the combustion process and the nature of combustion of the most common combustibles.
Combustion is complex physical chemical process, which is based on rapidly ongoing oxidation reactions, accompanied by the release of heat and, as a rule, light radiation. Combustion occurs and proceeds in the presence of a combustible substance, an oxidizing agent (usually oxygen) and an ignition source.
There are two types of combustion: homogeneous and heterogeneous. Homogeneous combustion occurs when a combustible substance is in a gaseous state. If the reaction takes place between a solid combustible substance and a gaseous oxidizer, then one speaks of heterogeneous combustion.
An external sign of homogeneous combustion is a flame, heterogeneous - glow. The flame is an area where the reaction of the combination of vapors (gases) of a burning substance with oxygen takes place. The flame temperature is also the combustion temperature. In case of fires in residential and administrative buildings it averages 850-900°, in the forest - 500-900°.
The duration and intensity of combustion depend on many factors and, first of all, on the supply of oxygen to the process, on the amount and condition of the material. The burning rate of solid combustible substances largely depends on their specific surface area and degree of humidity. The burning of peat is especially dangerous. Peat has a low self-ignition temperature (225 - 280°C) and high fragmentation, which determines its stable combustion. When there is no wind or light wind, peat burns very slowly. At peat extraction sites, peat combustion begins on the surface of peat extracted from deposits and gradually spreads into the depth of the extracted layer. The ignition of peat can occur during its drying. Roast summer time in high places, peat dries up to such an extent that it can ignite from the slightest spark. The burning of peat is accompanied by an abundant release of thick white smoke. With prolonged burning of peat over large areas, during an increase in wind, huge masses of dry peat and peat dust can rise from the places of extracted peat, which burn with a flame, forming so-called tornadoes. Fire tornadoes can lead to the death of people, as well as to the destruction of nearby settlements.
The combustion of dust (flour, coal, sugar, etc.) occurs at the speed of an explosion, massive pieces of these substances ignite with difficulty. Increasing the amount of moisture in combustible materials reduces the burning rate.
Flammable liquids (FL) and combustible liquids (FL), which include oil and oil products, are of particular danger during combustion. The burning rate of FL and FL is determined by their ability to evaporate. This is due to the fact that it is not the liquid itself that burns, but its vapors. Oil and oil products are usually stored vertically in cylindrical tanks, as well as in small containers (barrels, cans). Combustion in a tank with flammable liquids and combustible liquids begins, as a rule, with an explosion of a vapor-air mixture, accompanied by partial or complete separation of the tank roof and ignition of the liquid over the entire free surface. The burning of oil and oil products on the free surface after the explosion occurs relatively calmly. The temperature of the luminous part of the flame, depending on the type flammable liquid fluctuate within 1000-1300°C. Gasoline and other light oil products burn relatively quietly. The burning rate of dark oil products is very uneven. The burning rate of gaseous substances can change even more sharply. When combustible gases escape under pressure, they burn in the form of a torch, but if the gas accumulates gradually with the formation of a combustible mixture with air, then an explosion occurs.
Oil and fuel oils long burning in the tanks they warm up in depth, so combustion is accompanied by boiling and ejection of the burning liquid. Gasoline and other light petroleum products do not heat up when burned in large tanks.
When burning petroleum products, the smoke is black, from burning wood - grayish-black, phosphorus and magnesium fumes are white.
In the case when the combustion process is under the supervision of a person, this is not dangerous. However, escaping from under his control, the fire turns into a terrible disaster, whose name is fire.
1.2. General concepts about the fire and its development.
A fire is an uncontrolled burning, outside a special focus, accompanied by the destruction of material values and endangering people's lives.
The main parameters characterizing a fire are: the area of the fire seat, the intensity of combustion, the speed of propagation and the duration of the fire.
The seat of fire is understood as the place (area) of the most intense combustion under three main conditions:
continuous supply of an oxidizing agent (air);
continuous supply of fuel (combustible materials);
continuous release of heat necessary to maintain the combustion process.
Three zones are distinguished in the fire seat: the combustion zone, the heat affected zone and the smoke zone.
The combustion zone is a part of the space in which combustible substances are prepared for combustion.
The heat impact zone is a part of the space adjacent to the combustion zone, in which the heat effect makes it impossible for people to stay in it without special thermal protection.
Smoke zone - a part of the space adjacent to the zone of combustion and smoke from flue gases in concentrations that pose a threat to life and health of people or hinder the actions of the rescue unit.
The intensity of fires largely depends on the fire resistance of objects and their components.
All fires can be classified according to external signs of burning, the place of origin of the fire and the time of arrival of the first fire departments.
A) By external signs of combustion fires are divided into external, internal, both external and internal, open and hidden.
to outdoor include fires in which signs of combustion (flame, smoke) can be visually identified. Such fires occur during the burning of buildings and their structures, stacks of sawn timber, coal, peat and other material assets located in open areas. storage areas; when burning oil and oil products in tanks, etc. Outdoor fires are always open.
to internal include fires that occur and develop inside buildings. They can be open and hidden.
Signs of burning open fires can be established by inspections of premises (for example, burning of property in buildings for various purposes; burning of equipment and materials in production shops etc.).
At hidden fires combustion takes place in the voids of building structures, ventilation ducts and mines, inside tonra deposits or peat piles, etc. Signs of burning are detected by smoke escaping through cracks, discoloration of the plaster, etc.
The most difficult fires are both external and internal, open and hidden. As the situation changes, the type of fire changes. So, when a fire develops in a building, latent internal combustion can turn into open internal combustion, and internal combustion into external combustion and vice versa.
B) According to the place of occurrence fires occur in buildings, structures, in open areas of warehouses and in combustible massifs (forest, steppe, peat and grain fields).
C) By the time of arrival of the first fire departments fires are divided into launched and non-started.
To the running include fires that, by the time the first fire departments arrived, had developed significantly according to various reasons(for example, due to late detection of a fire or a message to fire department). As a rule, there are not enough forces and means of the first divisions to extinguish the started fires.
Unstarted fires in most cases are eliminated by the forces and means of the first arriving unit, the population or workers of the facility.
The fire development process can be divided into three phases. In the first phase, the spread of combustion occurs when the fire covers the main part of the combustible materials (at least 80%). In the second phase, after reaching the maximum burnout rate of materials, the fire is characterized by active flame combustion with a constant rate of loss of combustible materials. In the third phase, the burnout rate drops sharply and burning out of smoldering materials and structures occurs.
1.3. Ways to stop burning. Classification of the main fire extinguishing agents, general information about them: types, a brief description of, areas and conditions of application.
Water is the main and most common fire-extinguishing agent for extinguishing forest fires. However, air-mechanical foam is more effective, which, covering the surface of burning wood, protects it from radiant heat, and the wetting agent contained in the foaming agent contributes to better penetration of water into the pores of wood, and, consequently, a faster decrease in temperature.
Depending on the burning materials, there are 3 main types forest fires: grassroots, riding, soil and underground.
A ground fire is a forest fire in which the main combustible material is the ground cover, undergrowth, undergrowth or deadwood.
Riding fires are fires that burn the canopy of a forest stand. These fires arise from grassroots as a further stage of their development.
Forest soil fires are flameless burning of the upper peaty soil layer. Soil fires are observed in areas with peaty soils.
In the first stages of drying, the peat layer burns out only under the trees, which randomly fall, and the forest area, damaged by fire, looks like a pitted one. ground fires behind short term cover large area, and then continue as soil, deepening in separate funnels into peat.
With large peat fires the greatest danger is a sudden change in the wind, an increase in the speed of the spread of fire, the transfer of sparks through the areas where people work, and the formation of new fires in the rear, as a result of which people can become disoriented and find themselves surrounded by fire.
The occurrence and development of a fire in a tank with oil or oil products, as a rule, begins with an explosion of a vapor-air mixture, partial or complete separation (collapse) of the tank roof and ignition of the liquid on the entire free surface.
The complete separation of the roof and its dropping by the force of the explosion to the ground (sometimes it is thrown a few tens of meters) is most favorable for the subsequent fire extinguishing.
The combustion of enriched oil and oil products on the free surface occurs quite calmly.
fighting rescue units for extinguishing a fire in an oil and oil products storage tank are organized depending on the current situation, namely:
carry out fire reconnaissance;
immediately organize the cooling of the burning and neighboring tanks;
organize the preparation of a foam attack using mobile means.
When several tanks are on fire and there is a lack of forces and means to extinguish all tanks at the same time, it is necessary to concentrate all forces and means on extinguishing one tank located on the windward side or the tank whose fire most threatens neighboring non-burning tanks. After the combustion ceases, the supply of foam to the tanks is continued for approximately 3-5 minutes. to prevent re-ignition of the oil product. In this case, it should be that the entire surface of the oil product is covered with foam. Cooling is continued until the tank is completely cool.
At the beginning of the supply of foam to extinguish oil and dark oil products, boiling up of burning liquids and their emissions are possible. In such cases, measures are taken in advance to ensure the safety of people involved in extinguishing, and to protect hose lines located in the zone of active flame exposure with water jets.
Sometimes a burning oil product is ejected to a considerable height and spreads at a distance of 70-120 m from the burning reservoir, posing a threat not only to neighboring reservoirs, but also to individual installations, structures, fire engineering and personnel. To provide personnel and equipment in case of a threat of ejection, fire trucks are installed on the windward side at a distance of at least 100 m.
Fires in storage tanks for liquefied hydrocarbon gases (LPG) and unstable gasoline stored under high pressure can occur when the equipment and communications of the tanks are depressurized, as well as as a result of other emergencies. As a rule, fires start with the flare burning of DGS in places where they are passed or with the explosion and burning of spilled liquids.
In the process of burning liquefied gas almost always there is a danger of rupture of tanks and pipelines as a result of a rapid increase in pressure in them due to heating.
In case of fires at the stages of liquefied gas, it is necessary to take measures to reduce the pressure in tanks and pipelines exposed to the thermal effects of a fire, bleed gas to the torch and pump (pass) gas into free tanks.
The fight against fires of rubber and radio engineering products presents a number of difficulties associated mainly with the physical and technical properties of these substances. As the experience and practice of extinguishing fires have shown, burning rubber and Rubber products can be extinguished with water, although their wettability cannot be considered satisfactory.
Fire containment is an action aimed at limiting the spread of fire. When extinguishing (liquidating) a fire, a complete cessation of combustion is achieved. Typically, localization is integral part, the first stage of measures to extinguish the fire.
The cessation of combustion can be achieved either by separating the reactants or by cooling the burning materials below their ignition temperature. For this purpose, apply various means fire extinguishing. These include fire extinguishing agents and various devices, machines, units.
All fire extinguishing agents, depending on the principle of stopping burning, are divided into types:
cooling the reaction zone or burning substances (water, aqueous solutions of mixtures, etc.);
diluents in the combustion reaction zone (inert gases, steam, water mist and others);
insulating substances from the combustion zone (chemical and air-mechanical foams, fire extinguishing powders, non-combustible bulk substances, sheet materials and others).
All existing fire extinguishing agents have a combined effect on the combustion process of a substance. Water, for example, can cool and isolate (or dilute) the source of combustion; foam products act insulating and cooling; powder formulations isolate and inhibit the combustion reaction; the most effective gas agents act simultaneously as diluents and as inhibitors of the combustion reaction. However, any fire extinguishing agent has one dominant property.
Water is the main fire-extinguishing coolant, the most accessible and versatile. When it comes into contact with a burning substance, water partially evaporates and turns into steam (1 liter of water turns into 1700 liters of steam), due to which air oxygen is displaced from the fire zone by water vapor. The fire-extinguishing efficiency of water depends on the way it is supplied to the fire (solid or sprayed jet). The greatest fire extinguishing effect is achieved when water is supplied in atomized state, because. the area of simultaneous uniform cooling increases. Atomized water quickly heats up and turns into steam, taking away a large amount of heat. Atomized water jets are also used to reduce the temperature in rooms, protect against thermal radiation (water curtains), to cool the heated surfaces of building structures, structures, installations, as well as to deposit smoke.
As a fire extinguishing agent, water has disadvantages: it reacts with certain substances and materials, which therefore cannot be extinguished with water; poorly wets solid materials due to high surface stress, which prevents its rapid distribution over the surface, penetration into the depths of burning solid materials and slows down cooling. When extinguishing a fire with water, it must be remembered that it is electrically conductive.
Fire extinguishing agents that have an insulating effect include: foam, fire extinguishing powders, non-combustible bulk substances (sand, earth, graphite and others), sheet materials (felt, asbestos, tarpaulin covers, shields).
Foam is the most effective and widely used insulating fire extinguishing agent - it is a colloidal system of liquid bubbles filled with gas. Foams are divided into air-mechanical and chemical. foam is enough universal remedy and are used to extinguish liquid and solid substances, with the exception of substances interacting with water. Foams are electrically conductive and corrode metals. The most electrically conductive and active chemical foam. Air-mechanical foam is less electrically conductive than chemical, however, more electrically conductive than water, which is part of the foam.
Fire-extinguishing powder compositions (OPS) are increasingly used to extinguish fires. Currently, the industry produces OPS grades PS, PSB-3, SI-2 and P-14.
Fire extinguishing powders are non-toxic, non-conductive and non-irritating. harmful effects on materials, they do not freeze, so they are used at low temperatures.
The fire-extinguishing effect of the OPS consists mainly in isolating the burning surface from the air, and in the case of volumetric extinguishing, in the inhibitory effect of powders associated with the breakage of the combustion reaction chains. A necessary condition for stopping the burning of a surface is to cover it with an OPS layer no more than 2 cm thick.
Diluted fire extinguishing agents lower the concentration of reactants below the limits required for combustion. As a result, the rate of combustion reaction decreases, the rate of heat release decreases, and the combustion temperature decreases. The most common dioxins are carbon, water vapour, nitrogen and water mist.
Dioxin charcoal kind It is used to extinguish fires in warehouses, battery stations, drying ovens, archives, book depositories, as well as electrical equipment and electrical installations.
Nitrogen is used to extinguish fires of sodium, potassium, beryllium and calcium, as well as some technological apparatus and installations.
Water vapor is most effectively used when extinguishing fires in sufficiently sealed rooms up to 500 m 3 (ship holds, drying and painting chambers, pumping stations, oil refineries, etc.).
Send your good work in the knowledge base is simple. Use the form below
Students, graduate students, young scientists who use the knowledge base in their studies and work will be very grateful to you.
Hosted at http://www.allbest.ru/
- ESSAY
- on the topic
The concept of combustion. Combustion occurrence modes
- St. Petersburg, 2012
- CONTENT
Introduction
1. General information about burning
1.1 Heat sources
1.3 Complete and incomplete combustion
1.4 Flame and smoke
Conclusion
Literature
INTRODUCTION
Combustion is usually understood as a set of physical and chemical processes, the basis of which is a rapidly propagating oxidation reaction, accompanied by the release of heat and the emission of light. The area of a gaseous medium in which an intense chemical reaction causes luminescence and heat release is called a flame.
The flame is an external manifestation of intense oxidation reactions of substances. One of the types of combustion of solids is smoldering (flameless combustion).
Two stages are observed in the combustion process: the creation of a molecular contact between the fuel and the oxidizer (physical) and the formation of reaction products (chemical). The excitation of molecules during combustion occurs due to their heating. Thus, three components are necessary for the initiation and development of combustion: a combustible substance, an oxidizing agent, and an ignition source (i.e., a source of heat).
Flame diffusion combustion of all types of combustible materials and substances in the air is possible with an oxygen content in the fire zone of at least 14% by volume, and smoldering of solid combustible materials continues up to 6%.
The ignition source must have sufficient thermal energy to ignite the combustible material. Combustion of any material occurs in the gas or vapor phase. Liquid and solid combustible materials, when heated, turn into vapor or gas, after which they ignite. With steady combustion, the reaction zone acts as an ignition source for the rest of the combustible material.
1. General information about combustion
There are the following types of combustion:
Complete - combustion with a sufficient amount or excess of oxygen;
Incomplete - burning with a lack of oxygen.
With complete combustion, the products of combustion are carbon dioxide (CO 2), water (H 2 O), nitrogen (N), sulfur dioxide (SO 2), phosphorus anhydride. With incomplete combustion, caustic, toxic, combustible and explosive products are usually formed: carbon monoxide, alcohols, acids, aldehydes.
Combustion of substances can proceed not only in an oxygen environment, but also in an environment of certain substances that do not contain oxygen, chlorine, bromine vapor, sulfur, etc.
Combustible substances can be in three states of aggregation: liquid, solid, gaseous. Separate solids melt and evaporate when heated, others decompose and emit gaseous products and a solid residue in the form of coal and slag, others do not decompose and do not melt. Most combustible substances, regardless of the state of aggregation, when heated, form gaseous products, which, when mixed with atmospheric oxygen, form a combustible medium.
According to the state of aggregation of the fuel and oxidizer, they distinguish:
Homogeneous combustion - combustion of gases and combustible vapor-forming substances in a gaseous oxidizer;
Burning explosives and gunpowder;
Heterogeneous combustion - combustion of liquid and solid combustible substances in a gaseous oxidizer;
Combustion in the "liquid combustible mixture - liquid oxidizer" system.
1.1 Heat sources
Most combustible materials under normal conditions, as is known, do not enter into a combustion reaction. It can only start when a certain temperature is reached. This is explained by the fact that air oxygen molecules, having received the necessary supply of thermal energy, acquire the ability to better combine with other substances and oxidize them. In this way, thermal energy stimulates the oxidation reaction. Therefore, as a rule, any cause of a fire is associated with the effect of heat on combustible materials and substances. The complex physicochemical and many other phenomena occurring in fires are also determined primarily by the development of thermal processes.
The processes (impulses) that contribute to the development of heat are divided into three main groups: physical (thermal), chemical and microbiological. Flowing under certain conditions, they can cause heating of combustible materials to a temperature at which combustion of materials occurs.
The first group of impulses that cause ignition should mainly include open flame, heated body - solid, liquid or gaseous, sparks (of various origins), focused Sun rays. These pulses are manifested by the external action of heat on the material and can be otherwise called thermal.
The vast majority of fires that occur from ordinary, that is, the most common causes, are associated with the ignition of substances and materials under the influence of mainly the first three of the noted ignition sources.
Undoubtedly, the indicated division of the impulses of the physical, thermal group is to some extent conditional. Sparks of metal or burning organic materials are also bodies heated to the glow temperature. But from the point of view of assessing them as the cause of fires, sparks of all kinds should be separated into a separate group.
Heating and sparking can be the result of friction, compression, impact, various electrical phenomena, etc.
With the development of chemical or microbiological impulses, the accumulation of heat occurs due to a chemical reaction or the vital activity of microorganisms. Unlike a heat source acting from outside, in this case the process of heat accumulation takes place in the mass of the material itself.
An example of the processes of the second group can be exothermic reactions of the interaction of certain chemicals with moisture or among themselves, oxidation processes vegetable oils, not infrequently causing their spontaneous combustion, etc.
The third type of thermal impulse - microbiological - leads to the accumulation of heat in the material and spontaneous combustion due to a number of successively developing processes. The initial activity may be plant cells in the event that plant products are not completely dried. A certain amount of heat formed in this case, in the presence of conditions for its accumulation, contributes to the development of the vital activity of microorganisms, leading in turn to the further development of heat. Plant cells die at temperatures above 45°C. With an increase in temperature to 70--75 ° C, micro-organisms also die. In this case, porous products (porous yellow coal) are formed that are capable of absorbing (adsorbing) vapors and gases. The absorption of the latter occurs with the release of heat (the heat of adsorption), which can be accompanied by the development of a significant temperature in the presence of conditions favorable for heat accumulation. At a temperature of 150--200 ° C, the oxidation process is activated, which, with its further development, can lead to spontaneous combustion of the material.
In practice, cases of spontaneous combustion of undried hay, animal feed, etc., products of plant origin are well known.
A microbiological process can also occur in plant materials in which cell activity has already ceased. In these cases, moistening of the material can be favorable for the development of such a process, which also contributes to the development of the vital activity of microorganisms.
The listed processes, leading to the development of heat, in some cases exist in close interconnection. The microbiological process is followed by the physicochemical phenomenon of adsorption, the latter giving way to a chemical oxidation reaction with increasing temperature.
1.2 Occurrence of the combustion process
Despite the variety of heat sources capable of causing combustion under certain conditions, the mechanism of the combustion process in most cases is the same. It does not depend on the type of ignition source and combustible substance.
Any combustion is preceded, first of all, by an increase in the temperature of the combustible material under the action of some source of heat. Of course, such an increase in temperature should take place under conditions of access of oxygen (air) to the zone of incipient combustion.
Let us assume that heating occurs under the action of an external heat source, although, as is known, this is not necessary for all cases. Upon reaching a certain temperature, which various substances is not the same, the process of oxidation begins in the material (substance). Since the oxidation reaction proceeds exothermically, i.e., with the release of heat, the material (substance) then continues to heat up not only as a result of the action of an external source of heat, which may stop after some time, but also due to the oxidation process.
A heating substance (solid, liquid or gaseous) has a certain size, volume, surface. Therefore, simultaneously with the accumulation of heat by the mass of this substance, it is dissipated into the environment due to heat transfer.
Further results of the process will depend on the heat balance of the heating material. If the amount of heat that is dissipated exceeds the amount of heat received by the material, the temperature rise will stop and it may drop. Another thing is if the amount of heat received by the material during its oxidation exceeds the amount of dissipated heat. In this case, the temperature of the material will steadily increase, which in turn activates the oxidation reaction, as a result of which the process can go into the combustion stage of the material.
When analyzing the conditions for the occurrence of fires occurring for some reason, the specified mechanism for the start of combustion should be taken into account. Especially it must be taken into account in cases where the possibility of self-ignition or spontaneous combustion is being investigated. The latter can sometimes occur due to prolonged exposure to heat at a relatively low temperature and cause fires, for example, from central heating systems, etc.
Solid and liquid substances, before the process of burning them, decompose under the influence of heat, evaporate, turn into gas and vapor products. Therefore, the combustion of solid and liquid substances, as a rule, proceeds in the form of the release of vapors and gases. Thus, heat not only activates oxygen. Part of the heat released during combustion is spent on preparing the next sections of the combustible substance for combustion, i.e. on their heating, transformation into a liquid, vapor or gaseous state.
When investigating the causes of fires, one often has to deal with cellulosic materials. Products of mechanical and chemical processing of wood, cotton, flax contain cellulose and its derivatives as the main component. When heated, cellulose materials undergo decomposition, the process of which proceeds in two stages. At the first - preparatory - stage, thermal energy is absorbed by the mass of the material.
According to TsNIIPO, cellulosic materials dry out at a temperature of 110°C and begin to release odorous volatile substances. At a temperature of 110--150°C, yellowing of these materials and a stronger release of volatile constituents are observed. The presence of a smell can sometimes be a sign, which, taking into account other circumstances of the case, should be taken into account when establishing the place and time of the fire, as well as when checking versions of the cause of the fire. At a temperature of 150-200°C, cellulosic materials acquire a brown color as a result of charring. At a temperature of 210--230°C, they emit a large amount of gaseous products that ignite spontaneously in air. In this case, the second stage of thermal decomposition of the material begins - its smoldering or fiery combustion. This stage is characterized by the release of thermal energy, i.e., the reaction is exothermic. The release of heat and increase in temperature occurs mainly due to the oxidation of the decomposition products of the burning material.
The combustion of cellulose materials proceeds in two periods. At first, it is mainly the gases and other products that form during the thermal decomposition of the material that burn out. This is the phase of fiery combustion, although the combustion of coal also occurs at it.
The second period - it is especially indicative for wood - is characterized by predominant smoldering of coal. The intensity and thermal effect of the second stage of wood combustion are related to the extent to which the surface of the coal mass is in contact with atmospheric oxygen, what is its porosity. The latter is largely determined by the combustion conditions in its first phase.
The worse the gas exchange in the combustion zone and the lower the combustion temperature in its flame phase, the slower the combustion process, the more volatile and other products of thermal decomposition (dry distillation) are retained in the mass of coal, filling its pores. This, together with insufficient gas exchange, in turn prevents oxidation, i.e. combustion of coal in the second phase of combustion.
Under such conditions, coarse coal is formed, and overcharging, for example, of a wooden structural element can occur in the entire section of the element without subsequent combustion of the coal mass.
This leads to three conclusions:
1. The burnout rate depends on the conditions in which the combustion process takes place. The conditions of combustion (for example, air access, temperature) in different parts of the fire and even in one place, but at different times are not the same. Therefore, the information found in the literature on the average burning rate of wood, equal to 1 mm / min, cannot be sufficient for conclusions about the duration of burning in specific cases.
2. The degree of burning of wooden structures, i.e., their loss of section due to fire, cannot be established only by the depth of charring, since coal begins to burn out already during the period of fiery combustion of wood. Different degrees of burning, sometimes determined in practice by the thickness of the coal layer, can only relatively characterize the unevenness of fire damage to structures or their elements. The actual section loss will, as a rule, always be greater.
3. Large, low-porosity coal, which is sometimes found when structures are opened, indicates that the combustion process was incomplete and not intense. This sign, taking into account the circumstances of the case, can be taken into account when establishing the source of the fire and the time of the fire, when checking versions of the cause of the fire.
To characterize the initial, preparatory stage of combustion of solid materials, we will use two basic terms - ignition and spontaneous combustion.
The ignition of a solid combustible material occurs under the influence of a thermal pulse with a temperature exceeding the self-ignition temperature of the decomposition products of the material. For the ignition process, the source of ignition is the decisive factor.
Burning of a heating material, such as felt, caused by a flame blowtorch with careless warming water pipes, - one of the cases of ignition of a solid combustible material.
Self-ignition of a solid combustible material occurs in the absence of an external thermal impulse or under conditions of its action at a temperature that is lower than the self-ignition temperature of these products. For the process of spontaneous combustion, the conditions of heat accumulation are decisive.
How better conditions heat accumulation, less its dissipation in the initial stage of the combustion process, the less at lower temperatures environment spontaneous combustion of cellulose materials is possible. Great importance in these cases acquires a duration of heating. There are many known fires that occurred, for example, in wooden structures buildings as a result of exposure to steam pipelines of central heating systems at a coolant temperature of 110--160 ° C, which lasted for a number of months. Such cases are sometimes referred to as thermal spontaneous combustion. Recall that the self-ignition temperature of materials during rapid heating is in the range of 210–280°C. The above feature of these materials must be taken into account when investigating the causes of fires.
The concepts of ignition, self-ignition and smoldering of solid combustible materials are derived from the previous two concepts - ignition and spontaneous combustion.
Ignition is the result of ignition of a material and is manifested by fiery combustion.
Self-ignition is the result of spontaneous combustion of substances and is also manifested by fiery combustion.
Smoldering is flameless combustion and can be the result of both ignition and spontaneous combustion of a material.
In other words, if in our example the felt ignites under the action of the flame of a blowtorch with the formation of a flame, in this case we can say: the felt has ignited. In the absence of necessary conditions for fiery combustion, the ignition of felt may be limited to its smoldering. The same should be noted about the ignition or smoldering of any spontaneously ignited material.
Ignition and spontaneous combustion of solid materials differ in the nature of the thermal impulse that caused them. But each of them, representing a certain type of initial stage of ignition, can lead to both smoldering and ignition of solid combustible materials.
The smoldering process can turn into flame combustion with the activation of the oxidative process due to a further increase in temperature or an increase in the amount of oxygen involved in combustion, i.e., with better air access.
Thus, the occurrence of the combustion process does not depend on only one heat pulse. The action of the latter can cause combustion only if the combination of all conditions necessary for the combustion process is favorable. Therefore, if in one case a large fire impulse may be insufficient, then in another case, combustion will occur as a result of a very weak ignition source.
1.3 Complete and incomplete combustion
The role of the oxidative process during combustion in fires. The role of heat in the development of combustion was noted above. At the same time, the close relationship that exists between thermal and oxidative processes was obvious. However, the latter play a very important role in the combustion of substances and materials.
Oxidation of substances during combustion most often occurs due to oxygen in the air.
For complete combustion of the same amount of different substances, different amounts of air are required. So, for the combustion of 1 kg of wood, 4.6 m 3 of air is needed, 1 kg of peat - 5.8 m 3 of air, 1 kg of gasoline - about 11 m 3 of air, etc.
In practice, however, during combustion, complete absorption of oxygen from the air does not occur, since not all oxygen has time to combine with the fuel. An excess of air is required, which can reach 50% or more in excess of the amount of air theoretically required for combustion. The combustion of most substances becomes impossible if the oxygen content in the air drops to 14-18%, and for liquids - up to 10% by volume.
Gas exchange on fire. The flow of air to the combustion zone is determined by the conditions of gas exchange. Combustion products heated to a significant temperature (of the order of several hundred degrees) and, as a result, having a lower volumetric weight compared to the volumetric weight of the environment, move to the upper layers of space. Less heated air, in turn, enters the combustion zone. The possibility and intensity of such an exchange, of course, depend on the degree of isolation of the combustion zone from the surrounding space.
Under fire conditions, combustion is most often incomplete, especially if it is associated with the development of a fire in a mass of materials or in parts of buildings. Incomplete, delayed combustion is typical for fires that develop, for example, in structures with hollow elements. Unfavourable conditions gas exchange cause insufficient air supply, which hinders the development of a fire. Heat accumulation and mutual heating of burning structural elements do not compensate for the inhibitory effect of reduced gas exchange.
There are cases when, with the termination of the furnace heater, in the chimney of which a crack formed at the level of the ceiling, with the cessation of the temperature effect on the elements of the floor, the combustion "spontaneously" stopped. In this case, the lack of oxygen and the cessation of the additional supply of heat necessary to maintain combustion under these conditions were decisive.
Cases of delayed, incomplete combustion caused by a lack of oxygen, and even spontaneous cessation of combustion can be observed not only in parts of buildings, but also in rooms lacking the necessary air exchange. Such conditions are most typical for basement rooms, pantries, etc., especially tightly closed window and door openings.
This is also facilitated by a large volume of released gaseous products, since they prevent air from entering the combustion zone from the outside. So, when burning 1 kg of wood in a fire, up to 8 m 3 of gaseous products are formed. Although less of them are released during incomplete combustion, however, in this case, the amount of combustion products is calculated in cubic meters from each kilogram of the burnt substance (the theoretical volume of gaseous combustion products is 1 kg of wood, reduced to normal conditions, i.e. at a pressure of 760 mm Hg. article and a temperature of 0 ° C, is about 5 m 3).
This circumstance leads to a noticeable decrease in the intensity of combustion and increases its duration indoors with insufficient air exchange.
The products of incomplete combustion contain substances resulting from the thermal decomposition and oxidation of combustible materials. Among them are carbon monoxide, vapors of acetaldehyde, acetic acid, methyl alcohol, acetone and some other substances that give the place of fire, burnt objects a specific taste and smell, as well as soot.
The products of incomplete combustion are capable of burning, and at certain ratios in a mixture with air, form explosive mixtures. This explains the cases of explosive ignitions that sometimes occur during fires. The reasons for such phenomena are often mysterious. Intense ignition, sometimes very close in its effect to an explosion, occurs in rooms, in conditions in which, it would seem, there should not be any explosives.
The formation of explosive concentrations of products of incomplete combustion (mainly carbon monoxide) and their filling of separate closed volumes of unventilated rooms is possible even in the process of extinguishing a fire. The latter cases, however, are very rare. More often, explosive ignition can be observed at the first stage of extinguishing a fire that has arisen in enclosed spaces with poor gas exchange, when, when opening openings, the concentration of incomplete combustion products can be in explosive limits, if before that it was beyond their upper limit.
Finding out the conditions under which the combustion process took place on a fire, especially before its discovery, is directly related to determining the period of the onset of a fire, and therefore to the study of certain versions about the cause of its occurrence.
The combustion occurring in fires with insufficient gas exchange sometimes closely resembles the process of dry distillation. Such fires, if not detected in a timely manner, can last for hours. As a rule, they occur at night in institutions and facilities in which supervision is weakened during off-hours and at night, and there is no automatic fire alarm.
Sometimes it was possible to observe how, as a result of such fires, the enclosing structures of the premises and objects located in them were covered with a black shiny layer of condensed products of thermal decomposition of smoldering materials.
Incidents of incomplete combustion occurring in small living quarters, for example as a result of careless smoking in bed, are associated with fatal consequences for their perpetrators. The content in the air of 0.15% carbon monoxide by volume is already life-threatening, and the content of 1% carbon monoxide causes death. When investigating such fire cases, therefore, it is necessary to take into account the likelihood of non-violent death, which can occur as a result of an accident from the action of carbon monoxide. The immediate cause of death is established by a forensic medical examination.
Insufficient gas exchange can cause inconspicuous and prolonged smoldering of materials not only at the stage of an incipient fire, but also after extinguishing it, when, for one reason or another, individual small centers remained unliquidated. The next, repeated departure of the fire brigade in these cases is associated with the elimination of the same previously unextinguished fire. Such cases are more probable during the combustion of fibrous and bulk materials, in the mass of which gas exchange is difficult.
1.4 Flame and smoke
The combustion process usually produces flames and smoke, which are usually the first signs of a fire. A flame is a gas volume in which an exothermic reaction of the combination of gaseous decomposition products or vapors of a combustible material with oxygen takes place. Therefore, flames burn those substances that, when heated, are capable of releasing vapors and gases. These include cellulose materials, petroleum products and some other substances.
A luminous flame contains incandescent unburned particles of carbon, which was part of the burning substance. Subsequent cooling of these particles forms soot. The soot deposited on the surface of structures and materials during a fire burns out in areas with a higher temperature and remains where the temperature for soot combustion was insufficient. Therefore, the absence of sooting on separate, sometimes sharply defined sections of enclosing structures, objects, or the presence of traces of soot, taking into account the nature of these signs, is taken into account when establishing the source of the fire.
The temperature of a glowing flame depends not only on the nature and composition of the burning substance, but also on the combustion conditions. So, the flame temperature of wood can be from 600 to 1200 ° C, depending on its species, completeness and combustion rate.
The flame temperature usually corresponds to the practical combustion temperature of the given substance. The latter is determined by the calorific value of the burning material, the completeness and speed of combustion, and excess air. It is the excess air that leads to the fact that the practical combustion temperature is always lower than the theoretical one.
The smoldering of materials, as well as the combustion of those materials that do not emit gaseous combustible thermal decomposition products, are examples of flameless combustion. In particular, coke and charcoal burn without a flame, heating up to a high temperature, while radiating heat and light.
By such an indirect sign as the color of hot steel objects, structures, bricks, stone, and flames, one can sometimes get an approximate idea of the temperature in the combustion zone in a fire.
The colors of heated steel correspond to the following temperature (approximately):
dark red 700°C;
light orange 1200°C
cherry red 900°C;
white 1300°С
bright cherry red 1000°C;
bright white 1400°C
dark orange 1100°C;
dazzling white 1500°С
Smoke accompanies combustion in a fire, sometimes to a greater extent than an open flame, especially in the stages of an incipient fire.
Combustion can still occur in the form of smoldering, but it will already be accompanied by the release of smoke. Therefore, in cases where a fire proceeds without flaming combustion or it occurs hidden in the structures of a building, smoke formation can be one of the first signs of an emerging fire.
The smoke contains products of complete and incomplete combustion, decomposition of the burning material, nitrogen and partially oxygen in the air (depending on its excess during combustion), as well as soot and ash formed during the combustion of the material.
Thus, smoke is a mixture of combustible and non-combustible vapors and gases, solid organic and mineral particles, water vapor.
The composition and characteristics of burning materials, as well as the conditions of combustion, determine the composition, and, consequently, smell, taste, and others. external signs smoke generated during combustion. Sometimes such eyewitness data from an incipient fire makes it easier to determine the source of the fire and its cause, if the location of certain materials and substances in the fire zone is known. It should be noted, however, that during the joint combustion of different substances, especially in the conditions of a developed fire, the characteristic features of each of them may not be noticeable. In such cases, it is far from always possible to conclude from the smoke about the nature of the burning substance.
2. Transfer of heat and features of the spread of combustion in fires
With the beginning of the combustion process, the spread of heat begins, which can occur by heat conduction, radiation and convection. Heat transfer also occurs and combustion spreads in fires.
Heat transfer by thermal conduction takes place at different temperatures of different parts of a body (material, structure) or different bodies that are in contact with each other. Therefore, this method of heat transfer is also called contact. Heat is directly transferred from more heated parts of the body to less heated, less heated bodies by more heated bodies.
An electric iron left energized on a combustible base, burning coals or parts of structures that have fallen on combustible materials during a fire are examples of the initiation or spread of fires due to contact heat transfer.
When analyzing the causes of fires, sometimes it is necessary to take into account the thermal conductivity of materials, which may be associated with certain versions of the cause of the fire or the conditions for its development.
Thermal conductivity various materials varies and is usually in direct relation to their bulk density. Metals have the highest thermal conductivity. Fibrous and porous materials have low thermal conductivity, and gases, in particular air, have very low thermal conductivity. With an increase in temperature or humidity, the thermal conductivity of materials and substances increases somewhat.
Materials with low thermal conductivity, especially under conditions of insufficient gas exchange, even with prolonged combustion, are able to burn out in relatively small, sometimes strictly limited areas. Such materials include wood, cotton, paper, textile materials and others with a massive section or with dense packing.
Along with this, cases of heat transfer by metal elements passing through fireproof parts of buildings - ceilings, walls, coatings, etc. are well known in practice.
Sometimes this was the cause of fires, in some cases it contributed to their further development with the formation of secondary isolated combustion centers.
The transfer of heat by radiation by the surfaces of heated solid or liquid bodies, as well as gases (radiation) occurs in all fires. But depending on the conditions, the action of radiant heat manifests itself in varying degrees. The source of the strongest radiation in such cases is the flame, less heated bodies and smoke. Important feature This method of heat transfer lies in the fact that the radiation does not depend on the direction of the movement of the environment, for example, from convection or wind.
thermal convection burning fire
3. Convection. The main regularity of the spread of combustion in fires
Heat transfer by convection in fires is the most common.
Convection - the movement of more heated particles - occurs in gases and liquids. It is formed due to the difference in volumetric weights with a change in temperature by separate sections liquid or gas.
The volumes of such a medium heated for any reason move upward (if there are no currents or obstacles deflecting convection), giving way to less heated and therefore heavier parts of the medium.
Convection occurs as soon as the temperature rises with the development of the combustion process. The action of convection stimulates gas exchange, promotes the development of an incipient fire.
In fire conditions, the main masses of heat are transferred by convection.
In the case of a fire that occurred in one of the stores and described earlier, a significant extent of convection currents should have been attributed to the number of characteristic phenomena. Their path is from the source of the fire to the ceiling of the trading floor, under the ceiling to the opening in the floor at the stairs and through this opening to the second floor (about 20 m in total). The charring of the interior decoration and the deformation of plafonds decorated with the use of organic glass made it possible to trace the path of convection and judge the significant temperature of these flows.
Convection currents with a temperature of several hundred degrees, washing structures and materials on their way, heat them up, which can cause ignition of materials, deformation and destruction of non-combustible elements and parts of the building.
Thus, convection, regardless of its scale, in each individual case determines one of the main laws of the spread of combustion in fires. Whether combustion occurs in the volume of a building or a separate room, whether it develops, for example, in furniture, equipment, etc., in all cases, convection has an upward character. This trend in the spread of fire must be taken into account when investigating fires.
Often, during the preliminary investigation or at the trial, one can hear the statements of fire eyewitnesses that the fire was first seen in the upper part of the building. However, this does not mean that the source of the fire is located where the occurrence of fire is detected. The source of fire may be at the base of the structure, but burning, following the indicated pattern, can first of all spread upwards, for example, along the hollow structural elements and there take on an open character.
The presence of openings and holes, including random and small in size, leaks and cracks, the local absence of a protective layer (for example, plaster) or its weakening during a fire contribute to the upward development of combustion. Therefore, we can say that the scheme of propagation of combustion in fires in its general form is directly opposite to the free movement of liquid. The latter always tends to flow down, sometimes seeping into the smallest holes, leaks. The convection of heated combustion products and its associated propagation, as we have noted, have an ascending character.
Sometimes convection causes the transfer of burning objects: smoldering paper, coals, on open fires - smut ("jackdaws") and even burning timber, logs. Combustion in such cases acquires a vortex character. In the fire area, wind arises as a result of a gigantic gas exchange caused by a fire of a spontaneous nature. The removal of such smoldering and burning objects by convection can form new combustion centers.
In passing, we note that wind can lead to similar results in the development of an open fire. The role of wind in development open fires well enough known.
The direction of convection during a fire, both in its individual sections and in the main one, can change. This happens as a result of a violation of window glazing, the formation of burnouts and leaks, the destruction of structures, and also as a result of a special opening by fire departments.
Convection on fires forms signs by which it is possible to establish the direction and ways of development of combustion, and, consequently, the source of the fire. This is due to the fact that more intensive destruction of structures and materials occurs in the convection flow. Especially characteristic in this regard is the movement of convection currents in holes and openings.
Speaking about the role of naturally occurring convection in fires, it is also necessary to note the influence of air movement that is not associated with a fire on the spread of combustion. Air currents can be before the fire in the building structures or in the room, as well as in the atmosphere surrounding the object on which the fire broke out.
The temperature difference in different parts of the building, the connection between them, allowing circulation, the direction and strength of the wind will determine the local conditions for the movement of the air environment as well as influence the occurrence of a fire and the features of its development.
The possibility of the existence of air currents has to be taken into account when investigating the specific circumstances of fire cases. It is this condition that sometimes explains the absence of the first signs of a fire that has begun in one place or their detection in another, the direction of the development of combustion in structures (mainly in the horizontal direction), the speed of the spread of the fire, its scale when the fire took on an open character.
4. Factors determining the nature of combustion in fires and its results
Above, we briefly considered separately the conditions necessary for combustion and the methods of heat transfer. The influence of these factors on the processes of combustion propagation during fires was noted. However, it should be emphasized that, in the overwhelming majority of cases, a combination of these factors or their various combinations take place in fires.
The complex and diverse conditions in which the combustion process takes place in fires lead to the fact that the burning of structures and materials occurs unevenly. The unevenness, in particular, consists in the fact that the speed of fire spread and the area covered by burning do not increase in proportion to the burning time, but progressively, i.e., the time required for the development of fire in a particular area is not directly dependent on her sizes. This is explained by the fact that with an increase in the burning area and its intensity, thermal and other factors that affect the development of a fire progressively increase.
5. Thermal processes occurring during combustion in the fire seat and their influence on the formation of focal signs
As a result of burning occurring in a fire, materials, structures, equipment and individual items that are in the high temperature zone undergo various destruction, deformation or are completely destroyed. As a rule, the most severe burns and destruction occur in the place where the fire started. In other areas of the fire on structures, equipment and materials, as a result of thermal exposure, characteristic signs are formed that indicate the direction of combustion. The reason for the formation of focal signs are naturally occurring thermal processes during combustion in the fire. The main regularities of thermal processes in the fire seat include:
longer burning time in the hearth compared to other areas of the fire;
elevated temperature regime;
heat transfer by ascending convective flow.
Duration of thermal processes in the fire seat
The duration of combustion during a fire in a room is determined by many factors, among which the most important are the value of the combustible load of the room, the rate of burning out of materials and gas exchange conditions.
The results of the study of fires indicate that the duration of burning in the fire seat, as a rule, exceeds the duration of burning in other areas of the fire, and the difference can be significant.
This is explained by the nature of the combustion development process, which can be divided into three successive periods (Fig. 1).
The first period (OA) corresponds to the development of combustion from a small hearth to general ignition in the volume of the room. During this period, the fire develops under non-stationary conditions, when the burn-out rate and gas exchange conditions change with time. In the final stage of this period, the burning area sharply increases, the average volumetric temperature in the room rises rapidly, as a result of almost simultaneous (within 30-60s) ignition of the main part of the combustible material.
Rice. 1. "Temperature-time" curve characterizing the periods of fire development
The time of the first period varies widely and can reach several hours under limited gas exchange conditions. For medium-sized premises (administrative, residential, etc.) with insufficient gas exchange, the time of the first period is 30-40 minutes, and with optimal gas exchange and non-combustible wall cladding - 15-28 minutes.
Significant changes relative to the second period of fire development are also observed in the nature of heat transfer. In the first period, the spread of fire occurs mainly due to heat transfer by convection and thermal conductivity. At the same time, the temperatures in different areas of the room differ markedly from each other.
In the second (main) period of fire development (curve AB), the main part of the combustible material (up to 80% of the total load) burns out at an almost constant rate. In this case, the average volume temperature rises to a maximum value. During this period, heat transfer occurs mainly by radiation.
The third period corresponds to the period of attenuation of the fire, during which the coal residue slowly burns out, and the temperature in the room decreases.
Thus, the duration of burning in the fire seat exceeds similar values in other areas of the fire during the first period of fire development.
Temperature regime in the fire seat
The formation of a higher temperature regime in the fire seat compared to other fire zones is caused by the following factors:
high heat release in the fire seat compared to other fire zones,
the nature of the distribution of the temperature field during a fire in the room;
physical laws of temperature field formation in convective flows.
The heat released during combustion is the main cause of the development of a fire and the occurrence of its accompanying phenomena. The release of heat does not occur in the entire volume of the combustion zone, but only in the luminous layer, where a chemical reaction takes place. The distribution of heat in the fire zone is constantly changing over time and depends on a large number factors. The released heat is perceived by the combustion products, which transfer heat by convection, thermal conductivity and radiation, both to the combustion zone and to the heat affected zone, where they mix with air and heat it. The mixing process occurs along the entire path of movement of combustion products, so the temperature in the heat affected zone decreases as it moves away from the combustion zone. In the initial stage of fire development, the heat consumption for heating air, building structures, equipment and materials is the highest. The warmth perceived building structures, causes their heating, which leads to deformation, collapse and ignition of combustible materials.
The duration of burning in the fire seat exceeds similar values in other areas of the fire during the first period of development. This causes a greater release of heat and causes an increased temperature in the hearth compared to other areas of the fire.
The nature of the distribution of the temperature field during a fire in a room also predetermines the formation of the highest temperature in the source in the initial period of fire development. The maximum temperature, which is usually higher than the average volume, occurs in the combustion zone (seat of fire), and as it moves away from it, the temperature of the gases decreases due to the dilution of combustion products with air and other heat losses to the environment.
The higher temperature in the fire seat is also due to the nature of the formation of the temperature field in cross section convective jet.
Convective currents are formed wherever there are heat sources and space for their development. The emergence of convective flows is due to the following reasons. During combustion, air enters the combustion zone, part of it participates in the combustion reaction, and part is heated. The gas layer formed at the source has a density less than the density of the environment, as a result of which it is subjected to the action of a lifting (Archimedean) force and rushes upward. The vacated place is occupied by dense unheated air, which, participating in the combustion reaction and heating up, also rushes up. Thus, there is a regular ascending convective flow of heated gas from the combustion zone. The gas medium, rising above the combustion zone, entrains air from the environment, as a result of which a temperature field is formed in its cross section. The temperature field in the cross section of ascending convective flows is distributed symmetrically about the vertical axis with a maximum along the jet axis. With distance from the axis, the temperatures decrease to the ambient temperatures at the jet boundary.
These regularities take place in the first period of development, i.e. when burning in a fire. During this period, the burning area is insignificant and the convective jet propagates according to the laws of the upward flow in unlimited space, and the maximum temperatures will be formed in the center above the fire.
In the future, when the fire area increases sharply, the nature of temperature formation in convective flows will change. Under such conditions, the convective jet propagates in a limited space, which changes the picture of the temperature field in the jet. However, the general law of temperature distribution from the maximum on the axis to the ambient temperature at the jet boundary is preserved.
Thus, all three of these factors cause an increased temperature in the fire seat compared to other zones, and this circumstance is a characteristic feature of thermal processes in the fire seat.
The nature of heat transfer from the fire
The regularities of thermal processes in the fire seat also include the expanding nature of the propagation of convective flows from the fire seat and, as a result, a peculiar damage to structures due to the heat contained in the mass of the convective jet.
During combustion, the movement of the convective jet over the fire seat has a turbulent character. The vortex masses, when moving transversely outside the jet, entrain the layers of the immobile medium. During mixing, heat exchange occurs between the jet and the stationary medium. As a result, the mass of the jet grows, its width increases, and the shape of the convective jet takes on an expanding character as it moves upward. The degree of initial turbulence of the convective jet predetermines the angle of its opening. The higher the degree of jet turbulence, the more intensively the environment is mixed with it and the greater the angle of its initial expansion.
Thus, the physical laws of heat exchange and motion predetermine the expanding nature of the propagation of ascending convective flows, and the heat exchange that occurs in this case is characteristic of thermal processes in the fire seat.
The considered basic regularities of thermal processes (longer duration of their course, increased temperature regime in relation to other areas of combustion and the nature of heat transfer by convective flows) are inherent only in combustion in the fire seat. Knowledge of the nature of physical phenomena underlying the formation of thermal processes allows a more reasonable approach to the issue of establishing a fire source.
The indicated patterns of thermal processes in the fire seat are more pronounced in the initial period of fire development or during the elimination of combustion at the beginning of the second period. With the elimination of combustion at a later date, there is a gradual smoothing of the differences between the thermal processes in the source and in other areas of the fire, which naturally affects the nature of damage to structures, materials and equipment. This circumstance must be taken into account when determining the source of the fire.
CONCLUSION
Combustion is chemical reaction accompanied by the release of heat and light. It is possible under a combination of the following three conditions:
Presence of combustible material;
The presence of heat sufficient to ignite combustible material and maintain the combustion process;
The presence of oxygen (air) in the quantities necessary for combustion.
With the beginning of the combustion process, the spread of heat begins, which can occur by heat conduction, radiation and convection.
The duration of combustion in a fire is determined by many factors, among which the most important are the value of the combustible load, the rate of burning out of materials and the conditions of gas exchange. The rate of burnout depends on the conditions under which the combustion process takes place. The conditions of combustion (for example, air access, temperature) in different parts of the fire and even in one place, but at different times are not the same.
After the onset of combustion, a constant source of ignition is the combustion zone. The occurrence and continuation of combustion is possible at a certain quantitative ratio of combustible substance and oxygen, as well as at certain temperatures and the reserve of thermal energy of the ignition source. The highest rate of stationary combustion is observed in pure oxygen, the lowest - when the content of air is 14-15% oxygen. With a lower oxygen content in the air, the combustion of most substances stops.
LITERATURE
Megorsky B.V. Methodology for establishing the causes of fires, - M .: Stroyizdat, 1966.
Zel'dovich Ya.B., Mathematical theory of combustion and explosion. - M.: Nauka, 2000.
Williams F.A., Theory of combustion. - M.: Nauka, 2001.
Fire investigation. Textbook. / Ed. G.N. Kirillova, M.A. Galisheva, S.A. Kondratiev. - St. Petersburg: St. Petersburg University of the State Fire Service of the Ministry of Emergency Situations of Russia, 2007 - 544 p.
Fedotov A.Zh. etc. Fire-technical expertise, - M., 1986.
Investigation of fires, - M .: VNIIPO MVD RF, 1993.
Cheshko I.D. Examination of fires, - St. Petersburg; SPb IPB MIA of Russia, 1997.
V.G. Dontsov, V.I. Putilin. Manual “Inquiry and examination of fires”, Higher School of the Ministry of Internal Affairs of the USSR, Volgograd.
Cheshko I.D. Technical Basics fire investigations, - M., 2002
S.I. Taubkin. Fundamentals of fire protection of cellulose materials. Ed. MKH RSFSR, 1960.
Reference manual for fire and technical experts, - L., 1982
S.I. Zernov. Initial actions on the fact of the fire, M., 2005
Cheshko I.D. Inspection of the fire site, M., 2004
Hosted on Allbest.ru
Similar Documents
Physical and chemical bases of combustion and explosion. Thermal, chain and diffuse theories of combustion of substances, explosives. Properties solid fuels and combustion products, thermodynamic properties of combustion products. Types of flame and the speed of its propagation.
course of lectures, added 01/05/2013
combustion kinetics. Influence of humidity on the combustion of a drop of hydrocarbon fuels. Critical condition for droplet ignition and its dependence. Zeldovich's method. combustion hysteresis. Breakdown of the flame. Combustion in a stream of air. Natural and forced convection.
term paper, added 03/28/2008
Fundamentals of the theory of diffusion and kinetic combustion. Analysis of innovative developments in the field of combustion. Calculation of the combustion temperature of gases. Limits of ignition and pressure during the explosion of gases. Problems of combustion stability of gases and methods for their solution.
term paper, added 12/08/2014
Patterns of the influence of external electric fields on the macroscopic characteristics of the combustion of organic fuels. Schemes of applying an external electric field to the flame. Influence of organized external fields on the combustion process of hydrocarbon fuels.
term paper, added 03/14/2008
Scheme of the device of the boiler of pulsating combustion. General form combustion chambers. Specifications boilers. Promising developments of NPP "Ecoenergomash". Pulsating combustion steam generator with an intermediate heat carrier with a steam capacity of 200 kg.
presentation, added 12/25/2013
Method for calculating fuel combustion in air: determination of the amount of oxygen in the air, combustion products, fuel calorific value, calorimetric and actual combustion temperature. Combustion of fuel in air enriched with oxygen.
term paper, added 12/08/2011
Determination of the calorific value for gaseous fuels as the sum of the products of the thermal effects of the components of combustible gases by their quantity. Theoretically required combustion air flow natural gas. Determination of the volume of combustion products.
test, added 11/17/2010
Useful heat load of the furnace. Calculation of the fuel combustion process in the furnace. Excess air ratio. Construction of a diagram of combustion products. Thermal balance of the combustion process. Selection of waste heat boiler. Calculation of the evaporation surface, economizer.
term paper, added 12/03/2012
Physical and chemical bases of combustion, its main types. Description of explosions as the release of a large amount of energy in a limited volume in a short period of time, its types and causes. Energy sources of chemical, nuclear and thermal explosions.
control work, added 06/12/2010
Determination of air flow and the amount of combustion products. Calculation of the composition of coal dust and the coefficient of excess air during sintering of bauxite in rotary kilns. Using the semi-empirical formula of Mendeleev to calculate the heat of combustion of fuel.
Combustion is a physical and chemical process of interaction of a combustible substance and an oxidizing agent, accompanied by the release of heat and the emission of light. Under normal conditions, this is the process of oxidation or combined; a combustible substance with oxygen in a free state in air or chemical compounds in a bound state.
Some substances can burn in an atmosphere of chlorine (hydrogen), in sulfur vapor (copper) or explode without oxygen (acetylene, nitrogen chloride, etc.).
For food enterprises, the most typical combustion occurs when combustible substances are oxidized by atmospheric oxygen and occurs in the presence of an ignition source with a combustion temperature sufficient for ignition. Combustion stops in the absence of one of these conditions. It should be borne in mind that food enterprises are characterized by all types of combustion, including those that occur without an external source of heat: flash, ignition, self-ignition and spontaneous combustion.
Flash - the process of rapid combustion of a mixture of gases or vapors of a combustible substance with air from an external source of heat without transition to combustion.
Ignition - ignition of gases or vapors of a combustible substance from contact with a heat source with the further development of the combustion process.
Self-ignition - ignition without an external source of heat, arising from the self-decomposition of a combustible substance with the formation of vapors and gases that combine with atmospheric oxygen.
Spontaneous combustion - ignition of a substance as a result of self-heating under the influence of internal biological, chemical or physical processes (wet and raw grain, oil seeds, etc.).
There are two main types of combustion: complete and incomplete. Complete occurs with a sufficient or excess amount of oxygen and is mainly accompanied by the formation of water vapor and carbon dioxide. Incomplete occurs when it is deficient and is more dangerous, as it produces toxic carbon monoxide and other gases.
Rice. 54. Diffusion flame
If oxygen penetrates into the combustion zone due to diffusion, the resulting flame is called diffusion, and it has 3 zones (Fig. 54). The gases or vapors located in zone 1 do not burn (the temperature does not exceed 500 ° C), in zone 2 they burn partially, in zone 5 completely, and the flame temperature is the highest here.
Combustion is homogeneous and heterogeneous. In homogeneous combustion, all reactants have the same state of aggregation, for example, gaseous. When they are in different states of aggregation and there is a phase boundary in the combustible system, combustion is heterogeneous. Heterogeneous combustion, associated with the formation of a flow of combustible gaseous substances, is simultaneously diffusion.
Depending on the speed of flame propagation, combustion can occur in the form of deflagration combustion: explosion and detonation. In the first case, the normal burning rate, representing the speed of flame movement at the boundary between the burnt and unburned parts of the mixture, varies from several centimeters to several meters per second. So, for example, the burning rate of 10.5% mixture of methane with air is 37 cm/s.
The slow uniform propagation of combustion is stable only if it is not accompanied by an increase in pressure. If it occurs in a closed space or when the exit of gas is difficult, the reaction products not only heat the layer of mottled gas adjacent to the flame front by thermal conduction, but also, expanding due to high temperature, set the unburned gas in motion. The disordered movement of gas volumes in the burning mixture causes a significant increase in the surface of the flame front, which leads to an explosion. An explosion is a rapid transformation of matter, accompanied by the release of energy and the formation of compressed gases capable of doing work. The speed of flame propagation during the explosion reaches hundreds of meters per second.
With further acceleration of the flame propagation, the compression of the unburned gas in front of the flame front increases. It propagates through the unburned gas in the form of successive shock waves, which, at some distance ahead of the flame front, merge into one powerful shock wave of highly compressed and heated gas. As a result, a stable mode of propagation of the reaction occurs, called detonation, i.e., a type of combustion that propagates at a speed exceeding the speed of sound. Detonation is characterized by a sharp pressure jump at the site of explosive transformation, which has a large destructive effect.
Combustion is a physical and chemical process characterized by the following features: chemical transformations, release of heat and light. In order for stable combustion to occur, the presence of three factors is necessary: a combustible substance (material, mixture), an oxidizing agent and an ignition source.
A chemical combustion reaction, proceeding with the release of a significant amount of heat, is almost always accompanied by various kinds of physical phenomena. So, in the process of combustion, the heat of the reacting substances and combustion products is transferred from one place to another. All processes occurring in the combustion reaction zone are interconnected - the rate of chemical reactions is determined by the level of heat transfer and the rate of diffusion of the substance, and, conversely, the physical parameters (temperature, pressure, rate of substance transfer) depend on the rate of the chemical reaction.
combustible substance. All substances and materials circulating in production, used as raw materials, semi-finished products, building structural elements, are divided into three groups: non-combustible, slow-burning and combustible.
Non-combustible are substances and materials that are not capable of burning in air of normal composition. Non-combustible substances and materials constitute a significant group. These include all natural and artificial inorganic substances and materials used in construction, metals, as well as gypsum or gypsum-fiber boards with an organic mass content of up to 8%, mineral wool boards on a synthetic, starch or bitumen binder with its content by weight up to 6%.
Slow-burning substances (materials) are called those that can ignite under the action of an ignition source, but are not able to burn on their own after its removal. These include substances and materials consisting of non-combustible and combustible components, for example: asphalt concrete, gypsum and concrete materials containing more than 8% by weight of organic aggregate; mineral-cotton slabs on a bitumen binder with its content from 7 to 15%; clay-straw materials with a bulk density of at least 900 kg/m 3 ; felt impregnated with clay mortar; wood subjected to deep impregnation with flame retardants; cement fiberboard; certain types of engineering plastics, etc.
Combustible are substances (materials, mixtures) capable of self-combustion in air of normal composition. These include all substances and materials that do not meet the requirements for non-combustible and slow-burning substances and materials, for example: aviation fuels, alcohols, organic and inorganic oils, decorative and finishing materials based on plastics, textile materials, magnesium, sodium, sulfur and other materials and chemicals.
In turn, all combustible substances and materials are divided into three subgroups: flammable, medium flammable, and difficult to ignite.
Flammable are substances (materials, mixtures) that can ignite from short-term exposure to a match flame, spark, incandescent electric wire and similar low energy ignition sources.
Substances (materials, mixtures) that can ignite from prolonged exposure to a low-energy ignition source have medium flammability.
Flammable substances are substances (materials, mixtures) that can ignite only under the influence of a powerful ignition source, which heats a significant part of the substance to the ignition temperature.
The subgroup of flammable substances and materials primarily includes gases and flammable liquids.
Of all the liquids circulating in production, flammable liquids (flammable liquids) include flammable liquids with a flash point not exceeding + 61 ° C in a closed crucible. They are divided into three categories:
I - especially dangerous flammable liquids with a flash point up to - 18 ° C;
II - permanently hazardous flammable liquids with a flash point from - 18 to 23 ° C;
III - PLHIV, dangerous at elevated air or liquid temperatures with a flash point of 23 ° to 61 ° C.
The flash point is the lowest (under special test conditions) temperature of a combustible substance at which vapors or gases are formed above its surface that can flare up in air from an ignition source, but the rate of their formation is still insufficient for stable combustion. For flammable liquids, the flash point is 1 -5 ° C lower than the ignition temperature.
The ignition temperature is the temperature of a combustible substance at which it releases flammable vapors and gases at such a rate that, after igniting them from an ignition source, stable combustion occurs.
Almost all combustible and slow-burning substances and materials burn in the vapor or gas phase, with the exception of titanium, aluminum, anthracite and a number of others. Combustible substances and materials may differ in chemical composition, state of aggregation, and other properties, on the basis of which the processes of preparing them for combustion proceed differently. Gases enter into the combustion reaction If practically without any changes, since their mixing with the oxidizing agent (air oxygen) occurs at any ambient temperature and does not require significant additional energy costs f. Liquids must first evaporate and go into a vapor state, for which a certain amount of thermal energy is spent, and only in the vapor phase they mix with an oxidizing agent and burn. Solids and materials in their preparation for combustion require much more energy, since they must first either melt or decompose. Molten or decomposed substances and materials must evaporate and mix with the oxidizing agent, after which, under the influence of an ignition source, a combustion process occurs. Rubber, rubber and other plastic materials, as well as magnesium and its alloys, melt and evaporate before ignition (while the plastics decompose). Materials such as paper, wood, cotton fabrics and certain types of engineering plastics decompose when heated to form gaseous products and a solid residue (usually coal).
Oxidizer. The oxidizing agent is usually atmospheric oxygen. Air in its composition is a mixture of many gases, the main of which are: nitrogen (N 2) - 78.2% by volume and 75.5% by weight; oxygen (O 2) - 20.9% by volume and 23.2% by weight; inert gases (He, Ne, Ar, Kg) - 0.9% by volume and 1.3% by weight. In addition to these gases, an insignificant amount of carbon dioxide, water vapor and dust is always present in the air volume. All these components of air, except for oxygen, practically do not enter into a combustion reaction during the combustion of organic substances and materials. Oxygen, nitrogen and inert gases are considered constant constituents of air. The content of carbon dioxide, water vapor and dust is not constant and can change depending on the conditions in which a particular combustion process takes place.
Ignition source. It can be a burning or incandescent body, as well as an electric discharge, which have a supply of energy and a temperature sufficient to cause combustion of other substances.
In practice, there are or occur various phenomena that increase the temperature of substances and materials in production or storage, which in most cases leads to the occurrence of a combustion process both locally and in the entire volume of a combustible substance or material. Sources of ignition include: sparks generated when metal strikes metal or other solid materials; sparks and drops of molten metal during short circuits in electrical equipment and in the production of welding and other hot work; heating of electrical wires during overloads of electrical networks; mechanical heating of rubbing machine parts, biological heating during the oxidation of vegetable oils and rags moistened with these oils; burning speech, cigarette butts, etc. The nature of the impact of these ignition sources is not the same. Thus, sparks generated by impacts of metal objects as an ignition source have a very low power and are capable of igniting only gas-vapor-air mixtures: methane-air, acetylene-air, carbon disulfide-air, etc. Sparks that occur during short circuits in electrical equipment or during electric welding have a powerful flammability and can cause combustion of almost all combustible substances and materials, regardless of their state of aggregation.
combustible environment. When the combustion process occurs and proceeds, the combustible substance and the oxidizer are reacting substances and are a combustible medium, and the ignition source is the starter of the combustion process. In steady-state combustion, the source of ignition of not yet burning substances and materials is the heat released from the combustion reaction zone.
Combustible media can be physically homogeneous (homogeneous) and inhomogeneous (heterogeneous). The former include media in which the combustible substance and the oxidizing agent (air) are evenly mixed: mixtures of combustible gases, vapors and dusts with air. Examples of combustion of a homogeneous medium are: combustion of vapors rising from the free surface of a liquid (spilled aviation fuel TS-1 during an aircraft accident); combustion of gas flowing from a damaged cylinder or pipeline; explosions of gas, steam and dust-air mixtures. The heterogeneous media are those in which the combustible substance (material) and the oxidizing agent are not mixed and have an interface: solid combustible substances and materials, jets of combustible gases and liquids entering the air under high pressure, etc. An example of the combustion of an inhomogeneous medium is combustion of titanium, aluminium, anthracite or oil and gas fountains, when oil and gas enter the combustion zone under high pressure and have very significant exhaust velocities.
Flame. The space in which vapors, gases and suspensions burn is called a flame. The flame can be kinetic or diffusion, depending on whether a pre-prepared mixture of vapors, gases or dust with air burns or such a mixture is formed directly in the flame zone during combustion. The processes occurring in a kinetic flame are characterized by high combustion reaction rates (the linear velocity of flame propagation can exceed 1000 m/s) and, as a rule, represent an explosion of a combustible medium, accompanied by a high level of heat release and a sharp increase in pressure in the combustion zone.
Under fire conditions, almost all gases, vapors, liquids and solids and materials burn with a diffusion flame. The structure of this flame essentially depends on the cross section of the flow of combustible vapors or gases and its speed. According to the nature of this flow, laminar and turbulent diffusion flames are distinguished. The first occurs at small cross sections of the flow of combustible vapors or gases moving with low speed (flame of a candle, match, gas in the burner of a home stove, etc.). On fires, during the combustion of various substances and materials, a turbulent diffusion flame is formed, a minar and turbulent flame is a combustion reaction zone that surrounds a zone of vapors or gases, the latter practically occupies the entire volume of the combustion zone. The combustion reaction zone of an infusion flame is a very thin (only a few micrometers) layer in which heat is released and a light turbulent flame, in contrast to a laminar one, is characterized by I, which does not have clear outlines, constant sections and positions of the flame front.
The temperature in the vapor zone is much lower than in the reaction zone.
In the flame of aviation fuels, the temperature of the vapor flow near the surface of the liquid approaches its boiling point (for aviation fuel TS-1, this temperature lies in the range of 150 - 280 ° C). As the vapor flow moves to the reaction zone, their temperature rises first due to the thermal radiation of the flame, and then - diffusion from the reaction zone of heated combustion products. As a result of heating, thermal decomposition (dissociation) of vaporous substances occurs, and the resulting free atoms and radicals, together with the combustion products, enter directly into the reaction zone, i.e., into the flame. Carbon atoms, entering the combustion reaction zone, heat up and begin to glow, forming the so-called luminous flame. The temperature of the combustion reaction zone varies with the height of the flame. In the lower part of the flame, the temperature decreases due to the consumption of a significant amount of heat to heat the mass of cold air entering the combustion zone, and is minimal for each type of combustion. The highest temperature develops in the middle part of the flame, since in the upper part the reaction rate decreases due to a drop in the concentration of the reacting components (burnout), in connection with which the level of heat release decreases and the temperature decreases.
The partial pressure of air oxygen under normal conditions is 228.72 kPa, and in the combustion reaction zone it is 0, therefore, as a result of the difference in partial pressures, oxygen from the ambient air diffuses (filters, seeps) through the layer of combustion products to the reaction zone. The entry of combustible components into the combustion reaction zone is practically not limited by anything. Thus, the rate of the combustion reaction in the developed process depends mainly only on the amount of oxygen entering the reaction zone, i.e., on the rate of its diffusion. In the case of combustion of an inhomogeneous medium, the penetration of oxygen into the reaction zone is also prevented by combustion products released into the space adjacent to the reaction zone.
The absence of a sufficient amount of oxygen in the combustion reaction zone slows down the rate of its progress. If this inhibition did not occur, then all combustion reactions occurring in the atmosphere would proceed at a constantly increasing rate and end with an explosion of reacting substances. Combustion processes, like all chemical processes, proceed at different rates, depending on the conditions in which they occur, on the nature of the reacting substances, and on their state of aggregation. For example, explosives decompose in thousandths of a second, while chemical processes in the earth's crust last hundreds and thousands of years. The interaction of substances in the gas and vapor phases proceed much faster than in the liquid, and even more so in the solid state. Thus, spilled aviation fuel TS-1 burns relatively slowly, forming a smoky flame (incomplete combustion), and the prepared vapor-air mixture of this fuel with air burns with an explosion. The rate of interaction of solids and materials with an oxidizing agent changes dramatically depending on the degree of their crushing. For example, aluminum and titanium, slowly burning in ingots, under special conditions, can form explosive dust-air mixtures in the dust state, developing explosion pressures of 0.62 and 0.49 MPa, respectively, during combustion.
Combustion as a chemical process in all cases is the same. However, as a physical process, it differs in the nature of the combustion reaction, so the combustion processes in the initial stage are divided into the following types: spontaneous combustion, ignition and self-ignition.
Spontaneous combustion. Separate substances (materials, mixtures) during storage and during the operation of technological equipment are capable of spontaneous combustion. Spontaneous combustion is a phenomenon of a sharp increase in the rate of exothermic reactions, leading to the combustion of a substance in the absence of an ignition source. Substances capable of spontaneous combustion include vegetable and fatty oils, rags and rags moistened with vegetable oils, iron sulfides and other individual chemicals. Vegetable and fatty oils (sunflower, linseed, hemp, corn, animal fats, etc.) belong to the class of fats and are a mixture of glycerides of high molecular weight fatty acids. The molecules of these acids have unsaturated (double) bonds, which under certain conditions contribute to the spontaneous combustion of these substances. According to the peroxide theory of A. N. Bach, oxidation can occur due to the addition of oxygen to the methylene group, which is in position with respect to the double bond, with the formation of hydroperoxide. As you know, all peroxides and hydroperoxides are unstable chemical compounds. As they decompose, free radicals are formed that polymerize into larger organic molecules. During polymerization, a certain amount of heat is always released, which in the end can lead to spontaneous combustion of the oxidizing organic matter. Spontaneous combustion of organic substances occurs under certain conditions. These include: the content of glycerides of high molecular weight carboxylic acids in oil or fat is not lower than a certain minimum amount; the presence of a large surface of contact with the oxidizer and low heat transfer; a certain ratio of fats and oils and porous or fibrous material impregnated with them.
Iron sulfides FeS, Fe 2 S 3 can be formed in the technological equipment of the warehouses of the fuel and lubricants service of aviation enterprises. They are capable of spontaneous combustion in air, especially in the presence of combustible vapors and gases. Consider the mechanism of the combination of iron sulfides with atmospheric oxygen using the example of the oxidation reaction of the natural pyrite compound FeS2:
FeS 2 + 2O 2 \u003d FeS + 2SO 2 + 222.3 kJ.
In addition to iron sulfides, such material can ignite spontaneously s, as brown coal, peat, vegetable products: hay, straw, silage, etc.
The most dangerous is the spontaneous combustion of individual chemicals in case of improper storage, since this process can lead to a fire at the facility where these substances are stored. These substances, according to their chemical properties, are divided into three groups: spontaneously igniting upon contact with air, with water, and with each other. friend.
We do not consider substances belonging to the first group, since they are practically not found in the technology of aviation enterprises.
The second group includes a number of substances, of which calcium carbide CaC2 and calcium oxide CaO are of greatest interest. When interacting with water, calcium carbide releases acetylene, which is a combustible gas, and a significant amount of heat. With a relatively small amount of water, the calcium carbide - water system can flare up to 920 K, which can cause an explosion of the acetylene-air mixture:
CaC 2 + 2H 2 O \u003d C 2 H 2 + Ca (OH) 2 + 127 kJ.
In addition to calcium carbide, calcium oxide CaO has the ability to heat up to the glow temperature when small amounts of water get on it, which can also lead to fire of containers and combustible structural elements of the warehouse:
CaO + H 2 O \u003d Ca (OH) 2 + 64.5 kJ.
The third group includes strong oxidizing agents, individual chemicals, and organic substances and materials. For example, substances such as potassium permanganate and glycerin cannot be stored together; concentrated nitric acid with turpentine, ethyl alcohol and hydrogen sulfide; halogens with combustible gases and flammable liquids; sulfuric acid with nitrates, chlorates, perchlorates, since in this case a chemical reaction is possible between them, which proceeds with the release of a large amount of heat.
Ignition. In addition to spontaneous combustion, simple ignition is possible, that is, the occurrence of combustion under the influence of an ignition source. Ignition, accompanied by the appearance of a flame, is called ignition. In this case, the volume adjacent to the point of thermal action is heated. As a result of the increase in temperature in the specified volume, heat spreads to the areas (volumes) of the combustible medium adjacent to it. The more combustible substance (material, mixture) is involved in the combustion process, the more heat is released into the surrounding space. Thus, the combustion process develops spontaneously. The ignition source in this case initially heats only a small volume of the combustible mixture, while the temperature of the entire volume of the combustible medium can remain unchanged.
The ignition process differs in nature depending on the type of combustible mixture. The most dangerous are gas-air mixtures. However, even for them, the minimum energy of the ignition source depends on many parameters, the main of which are the percentage composition of the mixture, the type of combustible substance, the pressure of the mixture, since the ignition temperature, the normal flame propagation speed and the combustion temperature depend on these quantities. In addition, the minimum temperature of the ignition source is influenced by the duration of its contact with the combustible medium.
Ignition of liquids is possible only if the temperature of the environment or the liquid itself is sufficient to evaporate such an amount of vapor that is necessary for the occurrence of stable combustion. For various combustible liquids, this temperature is not the same. At temperatures below the ignition temperature, combustion is impossible, since the rate of evaporation of a particular liquid in this case is too low. With an increase in the temperature of the outside air or the combustible liquid itself, all other things being equal, the volatility of liquids increases and the amount of vapor becomes sufficient for the occurrence of stable combustion.
Self-ignition. It is called spontaneous combustion, accompanied by the appearance of a flame. In addition to the processes of spontaneous combustion and ignition, the process of self-ignition of various combustible media is also encountered in practice. By their chemical nature, all these three processes do not differ from each other. The difference between them lies in the physical essence of the combustion process, since, unlike the processes of spontaneous combustion and ignition, the process of self-ignition occurs immediately in the entire volume of the reacting combustible medium. From the point of view of physics, this is a kinetic process of combustion of an already mixed and prepared mixture, proceeding with high flame propagation speeds. When burning steam, dust, and gas-air mixtures, these are, as a rule, explosion velocities. For the self-ignition process to occur, it is necessary that the entire volume of the combustible mixture has the self-ignition temperature of this mixture. The self-ignition temperature is understood as the lowest temperature of a substance (material, mixture) at which there is a sharp increase in the rate of exothermic reactions, ending in the occurrence of fiery combustion. The auto-ignition temperature of a combustible substance is not a constant value. It depends on the rates of heat release and heat removal, which in turn depend on the volume of the mixture, concentration, pressure and other factors. The self-ignition temperature of mixtures of combustible vapors and gases with air varies depending on their percentage composition. The lowest auto-ignition temperature of a stoichiometric mixture or mixtures close to it in terms of concentrations of reactants. The auto-ignition temperature of solids or materials is inversely related to their degree of grinding: the higher the degree of grinding of a substance, the lower its auto-ignition temperature. This is due to the fact that with the grinding of substances and materials, the contact surface area of these combustible components and the oxidizer sharply increases.
Combustion is a physical and chemical process characterized by the following features: chemical transformations, release of heat and light. In order for stable combustion to occur, the presence of three factors is necessary: a combustible substance (material, mixture), an oxidizing agent and an ignition source.
A chemical combustion reaction, proceeding with the release of a significant amount of heat, is almost always accompanied by various kinds of physical phenomena. So, in the process of combustion, the heat of the reacting substances and combustion products is transferred from one place to another. All processes occurring in the combustion reaction zone are interconnected - the rate of chemical reactions is determined by the level of heat transfer and the rate of diffusion of the substance, and, conversely, the physical parameters (temperature, pressure, rate of substance transfer) depend on the rate of the chemical reaction.
combustible substance. All substances and materials circulating in production, used as raw materials, semi-finished products, building structural elements, are divided into three groups: non-combustible, slow-burning and combustible.
Non-combustible are substances and materials that are not capable of burning in air of normal composition. Non-combustible substances and materials constitute a significant group. These include all natural and artificial inorganic substances and materials used in construction, metals, as well as gypsum or gypsum-fiber boards with an organic mass content of up to 8%, mineral wool boards on a synthetic, starch or bitumen binder with its content by weight up to 6%.
Slow-burning substances (materials) are called those that can ignite under the action of an ignition source, but are not able to burn on their own after its removal. These include substances and materials consisting of non-combustible and combustible components, for example: asphalt concrete, gypsum and concrete materials containing more than 8% by weight of organic aggregate; mineral-cotton slabs on a bitumen binder with its content from 7 to 15%; clay-straw materials with a bulk density of at least 900 kg/m 3 ; felt impregnated with clay mortar; wood subjected to deep impregnation with flame retardants; cement fiberboard; certain types of engineering plastics, etc.
Combustible are substances (materials, mixtures) capable of self-combustion in air of normal composition. These include all substances and materials that do not meet the requirements for non-combustible and slow-burning substances and materials, for example: aviation fuels, alcohols, organic and inorganic oils, decorative and finishing materials based on plastics, textile materials, magnesium, sodium, sulfur and other materials and chemicals.
In turn, all combustible substances and materials are divided into three subgroups: flammable, medium flammable, and difficult to ignite.
Flammable are substances (materials, mixtures) that can ignite from short-term exposure to a match flame, spark, heated electrical wire, and similar low-energy ignition sources.
Substances (materials, mixtures) that can ignite from prolonged exposure to a low-energy ignition source have medium flammability.
Flammable substances are substances (materials, mixtures) that can ignite only under the influence of a powerful ignition source, which heats a significant part of the substance to the ignition temperature.
The subgroup of flammable substances and materials primarily includes gases and flammable liquids.
Of all the liquids circulating in production, flammable liquids (flammable liquids) include flammable liquids with a flash point not exceeding + 61 ° C in a closed crucible. They are divided into three categories:
I - especially dangerous flammable liquids with a flash point up to - 18 ° C;
II - permanently hazardous flammable liquids with a flash point from - 18 to 23 ° C;
III - PLHIV, dangerous at elevated air or liquid temperatures with a flash point of 23 ° to 61 ° C.
The flash point is the lowest (under special test conditions) temperature of a combustible substance at which vapors or gases are formed above its surface that can flare up in air from an ignition source, but the rate of their formation is still insufficient for stable combustion. For flammable liquids, the flash point is 1 -5 ° C lower than the ignition temperature.
The ignition temperature is the temperature of a combustible substance at which it releases flammable vapors and gases at such a rate that, after igniting them from an ignition source, stable combustion occurs.
Almost all combustible and slow-burning substances and materials burn in the vapor or gas phase, with the exception of titanium, aluminum, anthracite and a number of others. Combustible substances and materials may differ in chemical composition, state of aggregation, and other properties, on the basis of which the processes of preparing them for combustion proceed differently. Gases enter into the combustion reaction If practically without any changes, since their mixing with the oxidizing agent (air oxygen) occurs at any ambient temperature and does not require significant additional energy costs f. Liquids must first evaporate and go into a vapor state, for which a certain amount of thermal energy is spent, and only in the vapor phase they mix with an oxidizing agent and burn. Solids and materials in their preparation for combustion require much more energy, since they must first either melt or decompose. Molten or decomposed substances and materials must evaporate and mix with the oxidizing agent, after which, under the influence of an ignition source, a combustion process occurs. Rubber, rubber and other plastic materials, as well as magnesium and its alloys, melt and evaporate before ignition (while the plastics decompose). Materials such as paper, wood, cotton fabrics and certain types of engineering plastics decompose when heated to form gaseous products and a solid residue (usually coal).
Oxidizer. The oxidizing agent is usually atmospheric oxygen. Air in its composition is a mixture of many gases, the main of which are: nitrogen (N 2) - 78.2% by volume and 75.5% by weight; oxygen (O 2) - 20.9% by volume and 23.2% by weight; inert gases (He, Ne, Ar, Kg) - 0.9% by volume and 1.3% by weight. In addition to these gases, an insignificant amount of carbon dioxide, water vapor and dust is always present in the air volume. All these components of air, except for oxygen, practically do not enter into a combustion reaction during the combustion of organic substances and materials. Oxygen, nitrogen and inert gases are considered constant constituents of air. The content of carbon dioxide, water vapor and dust is not constant and can change depending on the conditions in which a particular combustion process takes place.
Ignition source. It can be a burning or incandescent body, as well as an electric discharge, which have a supply of energy and a temperature sufficient to cause combustion of other substances.
In practice, there are or occur various phenomena that increase the temperature of substances and materials in production or storage, which in most cases leads to the occurrence of a combustion process both locally and in the entire volume of a combustible substance or material. Sources of ignition include: sparks generated when metal strikes metal or other solid materials; sparks and drops of molten metal during short circuits in electrical equipment and in the production of welding and other hot work; heating of electrical wires during overloads of electrical networks; mechanical heating of rubbing machine parts, biological heating during the oxidation of vegetable oils and rags moistened with these oils; burning speech, cigarette butts, etc. The nature of the impact of these ignition sources is not the same. Thus, sparks generated by impacts of metal objects as an ignition source have a very low power and are capable of igniting only gas-vapor-air mixtures: methane-air, acetylene-air, carbon disulfide-air, etc. Sparks that occur during short circuits in electrical equipment or during electric welding have a powerful flammability and can cause combustion of almost all combustible substances and materials, regardless of their state of aggregation.
combustible environment. When the combustion process occurs and proceeds, the combustible substance and the oxidizer are reacting substances and are a combustible medium, and the ignition source is the starter of the combustion process. In steady-state combustion, the source of ignition of not yet burning substances and materials is the heat released from the combustion reaction zone.
Combustible media can be physically homogeneous (homogeneous) and inhomogeneous (heterogeneous). The former include media in which the combustible substance and the oxidizing agent (air) are evenly mixed: mixtures of combustible gases, vapors and dusts with air. Examples of combustion of a homogeneous medium are: combustion of vapors rising from the free surface of a liquid (spilled aviation fuel TS-1 during an aircraft accident); combustion of gas flowing from a damaged cylinder or pipeline; explosions of gas, steam and dust-air mixtures. The heterogeneous media are those in which the combustible substance (material) and the oxidizing agent are not mixed and have an interface: solid combustible substances and materials, jets of combustible gases and liquids entering the air under high pressure, etc. An example of the combustion of an inhomogeneous medium is combustion of titanium, aluminium, anthracite or oil and gas fountains, when oil and gas enter the combustion zone under high pressure and have very significant exhaust velocities.
Flame. The space in which vapors, gases and suspensions burn is called a flame. The flame can be kinetic or diffusion, depending on whether a pre-prepared mixture of vapors, gases or dust with air burns or such a mixture is formed directly in the flame zone during combustion. The processes occurring in a kinetic flame are characterized by high combustion reaction rates (the linear velocity of flame propagation can exceed 1000 m/s) and, as a rule, represent an explosion of a combustible medium, accompanied by a high level of heat release and a sharp increase in pressure in the combustion zone.
Under fire conditions, almost all gases, vapors, liquids and solids and materials burn with a diffusion flame. The structure of this flame essentially depends on the cross section of the flow of combustible vapors or gases and its speed. According to the nature of this flow, laminar and turbulent diffusion flames are distinguished. The first occurs at small cross sections of the flow of combustible vapors or gases moving with low speed (flame of a candle, match, gas in the burner of a home stove, etc.). On fires, during the combustion of various substances and materials, a turbulent diffusion flame is formed, a minar and turbulent flame is a combustion reaction zone that surrounds a zone of vapors or gases, the latter practically occupies the entire volume of the combustion zone. The combustion reaction zone of an infusion flame is a very thin (only a few micrometers) layer in which heat is released and a light turbulent flame, in contrast to a laminar one, is characterized by I, which does not have clear outlines, constant sections and positions of the flame front.
The temperature in the vapor zone is much lower than in the reaction zone.
In the flame of aviation fuels, the temperature of the vapor flow near the surface of the liquid approaches its boiling point (for aviation fuel TS-1, this temperature lies in the range of 150 - 280 ° C). As the vapor flow moves to the reaction zone, their temperature rises first due to the thermal radiation of the flame, and then - diffusion from the reaction zone of heated combustion products. As a result of heating, thermal decomposition (dissociation) of vaporous substances occurs, and the resulting free atoms and radicals, together with the combustion products, enter directly into the reaction zone, i.e., into the flame. Carbon atoms, entering the combustion reaction zone, heat up and begin to glow, forming the so-called luminous flame. The temperature of the combustion reaction zone varies with the height of the flame. In the lower part of the flame, the temperature decreases due to the consumption of a significant amount of heat to heat the mass of cold air entering the combustion zone, and is minimal for each type of combustion. The highest temperature develops in the middle part of the flame, since in the upper part the reaction rate decreases due to a drop in the concentration of the reacting components (burnout), in connection with which the level of heat release decreases and the temperature decreases.
The partial pressure of air oxygen under normal conditions is 228.72 kPa, and in the combustion reaction zone it is 0, therefore, as a result of the difference in partial pressures, oxygen from the ambient air diffuses (filters, seeps) through the layer of combustion products to the reaction zone. The entry of combustible components into the combustion reaction zone is practically not limited by anything. Thus, the rate of the combustion reaction in the developed process depends mainly only on the amount of oxygen entering the reaction zone, i.e., on the rate of its diffusion. In the case of combustion of an inhomogeneous medium, the penetration of oxygen into the reaction zone is also prevented by combustion products released into the space adjacent to the reaction zone.
The absence of a sufficient amount of oxygen in the combustion reaction zone slows down the rate of its progress. If this inhibition did not occur, then all combustion reactions occurring in the atmosphere would proceed at a constantly increasing rate and end with an explosion of reacting substances. Combustion processes, like all chemical processes, proceed at different rates, depending on the conditions in which they occur, on the nature of the reacting substances, and on their state of aggregation. For example, explosives decompose in thousandths of a second, while chemical processes in the earth's crust last hundreds and thousands of years. The interaction of substances in the gas and vapor phases proceed much faster than in the liquid, and even more so in the solid state. Thus, spilled aviation fuel TS-1 burns relatively slowly, forming a smoky flame (incomplete combustion), and the prepared vapor-air mixture of this fuel with air burns with an explosion. The rate of interaction of solids and materials with an oxidizing agent changes dramatically depending on the degree of their crushing. For example, aluminum and titanium, slowly burning in ingots, under special conditions, can form explosive dust-air mixtures in the dust state, developing explosion pressures of 0.62 and 0.49 MPa, respectively, during combustion.
Combustion as a chemical process in all cases is the same. However, as a physical process, it differs in the nature of the combustion reaction, so the combustion processes in the initial stage are divided into the following types: spontaneous combustion, ignition and self-ignition.
Spontaneous combustion. Separate substances (materials, mixtures) during storage and during the operation of technological equipment are capable of spontaneous combustion. Spontaneous combustion is a phenomenon of a sharp increase in the rate of exothermic reactions, leading to the combustion of a substance in the absence of an ignition source. Substances capable of spontaneous combustion include vegetable and fatty oils, rags and rags moistened with vegetable oils, iron sulfides and other individual chemicals. Vegetable and fatty oils (sunflower, linseed, hemp, corn, animal fats, etc.) belong to the class of fats and are a mixture of glycerides of high molecular weight fatty acids. The molecules of these acids have unsaturated (double) bonds, which under certain conditions contribute to the spontaneous combustion of these substances. According to the peroxide theory of A. N. Bach, oxidation can occur due to the addition of oxygen to the methylene group, which is in position with respect to the double bond, with the formation of hydroperoxide. As you know, all peroxides and hydroperoxides are unstable chemical compounds. As they decompose, free radicals are formed that polymerize into larger organic molecules. During polymerization, a certain amount of heat is always released, which in the end can lead to spontaneous combustion of the oxidizing organic matter. Spontaneous combustion of organic substances occurs under certain conditions. These include: the content of glycerides of high molecular weight carboxylic acids in oil or fat is not lower than a certain minimum amount; the presence of a large surface of contact with the oxidizer and low heat transfer; a certain ratio of fats and oils and porous or fibrous material impregnated with them.
Iron sulfides FeS, Fe 2 S 3 can be formed in the technological equipment of the warehouses of the fuel and lubricants service of aviation enterprises. They are capable of spontaneous combustion in air, especially in the presence of combustible vapors and gases. Consider the mechanism of the combination of iron sulfides with atmospheric oxygen using the example of the oxidation reaction of the natural pyrite compound FeS2:
FeS 2 + 2O 2 \u003d FeS + 2SO 2 + 222.3 kJ.
In addition to iron sulfides, such material can ignite spontaneously s, as brown coal, peat, vegetable products: hay, straw, silage, etc.
The most dangerous is the spontaneous combustion of individual chemicals in case of improper storage, since this process can lead to a fire at the facility where these substances are stored. These substances, according to their chemical properties, are divided into three groups: spontaneously igniting upon contact with air, with water, and with each other. friend.
We do not consider substances belonging to the first group, since they are practically not found in the technology of aviation enterprises.
The second group includes a number of substances, of which calcium carbide CaC2 and calcium oxide CaO are of greatest interest. When interacting with water, calcium carbide releases acetylene, which is a combustible gas, and a significant amount of heat. With a relatively small amount of water, the calcium carbide - water system can flare up to 920 K, which can cause an explosion of the acetylene-air mixture:
CaC 2 + 2H 2 O \u003d C 2 H 2 + Ca (OH) 2 + 127 kJ.
In addition to calcium carbide, calcium oxide CaO has the ability to heat up to the glow temperature when small amounts of water get on it, which can also lead to fire of containers and combustible structural elements of the warehouse:
CaO + H 2 O \u003d Ca (OH) 2 + 64.5 kJ.
The third group includes strong oxidizing agents, individual chemicals, and organic substances and materials. For example, substances such as potassium permanganate and glycerin cannot be stored together; concentrated nitric acid with turpentine, ethyl alcohol and hydrogen sulfide; halogens with combustible gases and flammable liquids; sulfuric acid with nitrates, chlorates, perchlorates, since in this case a chemical reaction is possible between them, which proceeds with the release of a large amount of heat.
Ignition. In addition to spontaneous combustion, simple ignition is possible, that is, the occurrence of combustion under the influence of an ignition source. Ignition, accompanied by the appearance of a flame, is called ignition. In this case, the volume adjacent to the point of thermal action is heated. As a result of the increase in temperature in the specified volume, heat spreads to the areas (volumes) of the combustible medium adjacent to it. The more combustible substance (material, mixture) is involved in the combustion process, the more heat is released into the surrounding space. Thus, the combustion process develops spontaneously. The ignition source in this case initially heats only a small volume of the combustible mixture, while the temperature of the entire volume of the combustible medium can remain unchanged.
The ignition process differs in nature depending on the type of combustible mixture. The most dangerous are gas-air mixtures. However, even for them, the minimum energy of the ignition source depends on many parameters, the main of which are the percentage composition of the mixture, the type of combustible substance, the pressure of the mixture, since the ignition temperature, the normal flame propagation speed and the combustion temperature depend on these quantities. In addition, the minimum temperature of the ignition source is influenced by the duration of its contact with the combustible medium.
Ignition of liquids is possible only if the temperature of the environment or the liquid itself is sufficient to evaporate such an amount of vapor that is necessary for the occurrence of stable combustion. For various combustible liquids, this temperature is not the same. At temperatures below the ignition temperature, combustion is impossible, since the rate of evaporation of a particular liquid in this case is too low. With an increase in the temperature of the outside air or the combustible liquid itself, all other things being equal, the volatility of liquids increases and the amount of vapor becomes sufficient for the occurrence of stable combustion.
Self-ignition. It is called spontaneous combustion, accompanied by the appearance of a flame. In addition to the processes of spontaneous combustion and ignition, the process of self-ignition of various combustible media is also encountered in practice. By their chemical nature, all these three processes do not differ from each other. The difference between them lies in the physical essence of the combustion process, since, unlike the processes of spontaneous combustion and ignition, the process of self-ignition occurs immediately in the entire volume of the reacting combustible medium. From the point of view of physics, this is a kinetic process of combustion of an already mixed and prepared mixture, proceeding with high flame propagation speeds. When burning steam, dust, and gas-air mixtures, these are, as a rule, explosion velocities. For the self-ignition process to occur, it is necessary that the entire volume of the combustible mixture has the self-ignition temperature of this mixture. The self-ignition temperature is understood as the lowest temperature of a substance (material, mixture) at which there is a sharp increase in the rate of exothermic reactions, ending in the occurrence of fiery combustion. The auto-ignition temperature of a combustible substance is not a constant value. It depends on the rates of heat release and heat removal, which in turn depend on the volume of the mixture, concentration, pressure and other factors. The self-ignition temperature of mixtures of combustible vapors and gases with air varies depending on their percentage composition. The lowest auto-ignition temperature of a stoichiometric mixture or mixtures close to it in terms of concentrations of reactants. The auto-ignition temperature of solids or materials is inversely related to their degree of grinding: the higher the degree of grinding of a substance, the lower its auto-ignition temperature. This is due to the fact that with the grinding of substances and materials, the contact surface area of these combustible components and the oxidizer sharply increases.
