FIRE PREPAREDNESS
Introduction
Extinguishing fires and rescuing people in danger are the most important tasks of rescue units taking part in extinguishing a fire.
Fire extinguishing tactics are a set of military actions to organize the efforts of rescue units to successfully eliminate the fire in the size that it had assumed by the time the units arrived, and to save people in the event of a threat to their lives. It is based on the theory and practice of fire fighting, the results scientific research and patterns of processes of development and extinguishing of fires, achievements in the field of fire equipment and fire extinguishing agents.
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 occurring oxidation reactions, accompanied by the release of heat and, as a rule, light radiation. Combustion occurs and occurs in the presence of a combustible substance, an oxidizer (usually oxygen) and an ignition source.
There are two types of combustion: homogeneous and heterogeneous. Homogeneous combustion occurs when a flammable substance is in a gaseous state. If the reaction occurs between a solid flammable substance and gaseous oxidizing agent, then they talk about heterogeneous combustion.
External sign homogeneous combustion is a flame, heterogeneous is incandescence. The flame is the area where the reaction between the vapors (gases) of a burning substance and oxygen occurs. 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 quantity and condition of the material. The burning rate of solid combustible substances largely depends on their specific surface area and degree of humidity. Burning 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 mining sites, peat combustion begins on the surface of peat extracted from deposits and gradually spreads deep into the extracted layer. Combustion of peat can occur during the drying process. In the roast summer time in high places the peat dries out so much that it can ignite from the slightest spark. Combustion of peat is accompanied by abundant release of thick white smoke. When peat burns for a long time over large areas and when the wind increases, huge masses of dry peat and peat dust can rise from the sites of extracted peat, which burn with flames, forming so-called tornadoes. Fire tornadoes can lead to the death of people, as well as the destruction of nearby populated areas.
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.
A particular danger during combustion is posed by flammable liquids (FLL) and flammable liquids (FL), which include oil and petroleum products. The burning rate of flammable liquids and flammable liquids is determined by their ability to evaporate. This is due to the fact that it is not the liquid itself that burns, but its vapor. Oil and petroleum products are usually stored vertically in cylindrical tanks, as well as in small containers (barrels, cans). Combustion in a tank with flammable liquids and gases begins, as a rule, with an explosion of the steam-air mixture, accompanied by partial or complete separation of the roof of the tank and ignition of the liquid over the entire free surface. The combustion of oil and petroleum products on a free surface after an explosion occurs relatively calmly. Temperature of the luminous part of the flame depending on the type flammable liquid fluctuates between 1000-1300°C. Gasoline and other light petroleum products burn relatively calmly. The burning rate of dark petroleum products is very uneven. The combustion rate of gases can change even more sharply. different substances. When flammable gases escape under pressure, they burn in the form of a torch, but if the gas accumulates gradually with the formation combustible mixture with air, an explosion occurs.
Oil and fuel oils at long burning in the tanks they are heated deep down, so combustion is accompanied by boiling and the release of burning liquid. Gasoline and other light petroleum products do not heat up when burning in large tanks.
When burning petroleum products, the smoke is black, from burning wood it is grayish-black, phosphorus and magnesium smoke are white.
In the case when the combustion process is under human supervision, it is not dangerous. However, having escaped from his control, the fire turns into a terrible disaster, whose name is fire.
1.2. General concepts about the fire and its development
Fire is an uncontrolled combustion outside a special source, accompanied by destruction material assets and creating a danger to human life.
The main parameters characterizing a fire are: the area of the fire, the intensity of combustion, the speed of spread and the duration of the fire.
The source of a fire is understood as the place (area) of the most intense burning under three main conditions:
continuous supply of oxidizing agent (air);
continuous supply of fuel (combustible materials);
continuous release of heat necessary to maintain the combustion process.
There are three zones in the source of a fire: the combustion zone, the thermal impact zone and the smoke zone.
The combustion zone is the part of the space in which flammable substances are prepared for combustion.
Thermal impact zone is a part of the space adjacent to the combustion zone, in which the thermal impact makes it impossible for people to stay in it without special thermal protection.
Smoke zone is a part of the space adjacent to the zone of combustion and smoke from flue gases in concentrations that pose a threat to the life and health of people or impede 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 by external signs of combustion, the location 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, simultaneously external and internal, open and hidden.
To the outside These include fires in which signs of combustion (flame, smoke) can be identified visually. Such fires occur when buildings and their structures, stacks of lumber, coal, peat and other material assets located in open storage areas burn; when burning oil and petroleum products in tanks, etc. External fires are always open.
To internal include fires that start and develop inside buildings. They can be open or hidden.
Signs of burning when 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.).
In hidden fires combustion occurs in voids building structures, ventilation ducts and mines, inside peat deposits or peat stacks, etc. Signs of combustion are detected by smoke escaping through cracks, changes in the color of the plaster, etc.
The most complex fires are both external and internal, open and hidden. As the situation changes, the type of fire changes. Thus, 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) At the place of origin fires occur in buildings, structures, open areas warehouses and in burning areas (forest, steppe, peat and grain fields).
B) By the time of arrival of the first fire departments fires are divided into neglected and unstarted.
To launched include fires that, by the time the first fire departments arrived, had developed significantly for various reasons (for example, due to late detection of the fire or reporting to fire department). To extinguish neglected fires, as a rule, the forces and means of the first units are not enough.
Unlaunched fires in most cases are extinguished by the forces and means of the first arriving unit, the population or workers of the facility.
The process of fire development can be divided into three phases. In the first phase, combustion spreads when the fire covers the bulk of combustible materials (at least 80%). In the second phase, after reaching the maximum rate of burning of materials, the fire is characterized by active flaming combustion with a constant rate of loss of combustible materials. In the third phase, the burnout rate drops sharply and smoldering materials and structures burn out.
1.3. Methods for stopping combustion. Classification of main fire extinguishing agents, general information about them: types, a brief description of, areas and conditions of application
The main and most common fire extinguishing agent for extinguishing fires in timber warehouses is water. 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 promotes better penetration of water into the pores of the wood, and, consequently, a faster decrease in temperature.
Depending on the materials burned, there are 3 main types forest fires: grassroots, upland, soil and underground.
A ground fire is a forest fire in which the main combustible material is ground cover, undergrowth, undergrowth or dead wood.
Raising fires include fires in which the canopy of a tree stand burns. These fires arise from the grassroots, as a further stage of their development.
Forest soil fires are the flameless burning of the top peaty layer of soil. Ground fires are observed in areas with peaty soils.
In the first stages of drying out, the peat layer burns out only under the trees, which randomly fall, and the forest area damaged by the fire looks like it has been dug up. Ground fires cover a large area in a short period of time, and then continue as soil fires, going deeper into the peat in separate funnels.
For large peat fires The greatest danger is posed by unexpected changes in wind, an increase in the speed of fire spread, the transfer of sparks through areas where people are working, and the formation of new fires in the rear, as a result of which people may lose their orientation and find themselves surrounded by fire.
The occurrence and development of a fire in a tank with oil or petroleum products, as a rule, begins with an explosion of the steam-air mixture, partial or complete separation (collapse) of the tank roof and ignition of the liquid on the entire free surface.
The complete tearing off of the roof and throwing it to the ground by the force of the explosion (sometimes it is thrown several tens of meters) is most favorable for subsequent fire extinguishing.
The combustion of enriched oil and petroleum products on a free surface occurs quite calmly.
Combat actions of rescue units to extinguish a fire in an oil and petroleum products storage tank are organized depending on the current situation, namely:
conduct fire reconnaissance;
immediately organize cooling of the burning tank and its adjacent tanks;
organize the preparation of a foam attack using mobile means.
If several tanks are burning and there is insufficient strength 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 stops, the supply of foam to the tanks is continued for approximately 3-5 minutes. to prevent re-ignition of the petroleum product. In this case, the entire surface of the oil product should be covered with foam. Cooling is continued until the tank has completely cooled.
At the beginning of the supply of foam when extinguishing oil and dark oil products, boiling 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 influence of the flame with water jets.
Sometimes a burning oil product is thrown to a considerable height and spreads at a distance of 70-120 m from a burning tank, creating a threat not only to neighboring tanks, but also to individual installations, structures, fire equipment and personnel. To provide personnel and equipment with the threat of a blowout, 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 due to depressurization of equipment and communications of the tanks, as well as as a result of other emergency situations. As a rule, fires begin with the flare burning of liquid gas systems in places where they pass through or with the explosion and combustion of spilled liquids.
During the combustion of liquefied gas, there is almost always a danger of rupture of containers and pipelines as a result of a rapid increase in pressure in them due to heating.
In case of fires at the liquefied gas stages, it is necessary to take measures to reduce the pressure in containers and pipelines exposed to the thermal effects of the fire, venting the gas to a flare and pumping (passing) gas into free containers.
Fighting fires of rubber and radio engineering products presents a number of difficulties associated mainly with the physical and technical properties of these substances. As experience and practice in extinguishing fires have shown, burning rubber and rubber products can be extinguished with water, although their wettability cannot be considered satisfactory.
Fire localization is actions aimed at limiting the spread of fire. When extinguishing (eliminating) a fire, complete cessation of combustion is achieved. Typically, localization is integral part, the first stage of fire suppression measures.
Stopping combustion can be achieved either by separating the reacting substances or by cooling the burning materials below their ignition temperature. For this purpose they use various means extinguishing the fire. These include fire extinguishing agents and various devices, machines, and units.
All fire extinguishing agents, depending on the principle of combustion cessation, are divided into types:
cooling the reaction zone or burning substances (water, aqueous solutions of mixtures and others);
diluents in the combustion reaction zone (inert gases, water vapor, finely sprayed water 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 as an insulating and cooling agent; powder formulations isolate and inhibit the combustion reaction; The most effective gas agents act both as diluents and as inhibitors of the combustion reaction. However, any fire extinguishing agent has one dominant property.
Water is the main fire extinguishing cooling agent, 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 effectiveness of water depends on the method of supplying it to the fire (solid or sprayed stream). The greatest fire extinguishing effect is achieved when water is supplied in a sprayed state, because the area of simultaneous uniform cooling increases. The sprayed water quickly heats up and turns into steam, taking away a large amount of heat. Sprayed water jets are also used to reduce the temperature in rooms, protect against thermal radiation (water curtains), to cool heated surfaces of building structures, structures, installations, and also for smoke deposition.
As a fire extinguishing agent, water has disadvantages: it reacts with some substances and materials that therefore cannot be extinguished with water; poorly wets solid materials due to high surface tension, 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, you must remember that it is electrically conductive.
Fire extinguishing agents that have an insulating effect include: foam, fire extinguishing powders, non-flammable bulk substances (sand, earth, graphite and others), sheet materials (felt, asbestos, tarpaulin blankets, shields).
Foam, the most effective and widely used insulating fire extinguishing agent, is a colloidal system of liquid bubbles filled with gas. Foams are divided into air-mechanical and chemical. Foams are a fairly universal means and are used to extinguish liquid and solids, with the exception of substances that interact with water. Foams are electrically conductive and corrode metals. Chemical foam is the most electrically conductive and active. Air-mechanical foam is less electrically conductive than chemical foam, however, it is more electrically conductive than the water included in the foam.
Fire extinguishing powder compositions (OPS) are increasingly used for extinguishing fires. Currently, the industry produces OPS of the PS, PSB-3, SI-2 and P-14 brands.
Fire extinguishing powders are non-toxic, non-conductive and do not cause harmful effects on materials, they do not freeze, so they are used at low temperatures.
The fire extinguishing effect of the fire extinguishing agent consists mainly in isolating the burning surface from the air, and in case of volumetric extinguishing - in the inhibitory effect of powders associated with breaking the combustion reaction chains. A necessary condition for stopping the burning of a surface is to cover it with a layer of ops not more than 2 cm thick.
Fire extinguishing agents dilute the concentration of reactants below the limits required for combustion. As a result, the rate of combustion reaction, the rate of heat release, and the combustion temperature decrease. The most common dioxins are carbon, water vapor, nitrogen and water mist.
Dioxin coal sort of 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 with a volume of up to 500 m 3 (ship holds, drying and painting chambers, pumping stations, oil refineries, etc.).
For combustion to occur, three components must be present in one place and at one time: a combustible substance, an oxidizer and an ignition source (Fig. 4.14). In addition, it is necessary that the combustible substance be heated to the required temperature and be in the appropriate quantitative ratio with the oxidizer, and the ignition source needs little energy for the initial impulse (ignition). So, you can light a sheet of paper with a match, but it is impossible to light a wooden block. The need for three components to burn simultaneously, the so-called triangle of fire, was discovered back in the 18th century. French scientist Lavoisier.
Rice. 4.14.
After combustion occurs, the more intense the combustion, the larger the specific area of contact of the combustible substance with the oxidizer (paper scraps burn more intensely than reams of paper) and the higher the concentration of the oxidizer, temperature and pressure. During fires, the temperature reaches 1000-1300 ° C, and in some cases, for example, when burning magnesium alloys - 3000 ° C.
Combustible substances are substances that, when exposed to high temperatures, an open flame or other source of ignition, can ignite and subsequently burn, producing heat and usually emitting light. Combustible substances include: wood, paper, fabrics, most plastics, natural gas, gasoline, kerosene and other substances in solid, liquid, and gaseous states. As a rule, the most dangerous in terms of fire are flammable substances in a gaseous state.
The vast majority of flammable substances include carbon (Carbon) and hydrogen (hydrogen), which are the main flammable components of these substances. Therefore, the main products of complete combustion (with a sufficient amount of oxygen) of combustible substances are CO2 and H20. There are also a number of flammable substances that are simple elements, for example, sulfur (Sulfur), phosphorus (Phosphorus), carbon (Carbon).
Combustible substances have different calorific value, therefore, the temperature in fires depends not only on the quantity of the substance burning, but also on its quality ( chemical composition). In table Table 4.4 shows the flame temperature during combustion of certain substances and materials.
Table 4.4.
The oxidizing agent during the combustion of substances is most often air oxygen - O.,. However, with a decrease in the oxygen content in the air, the combustion rate slows down, and when the oxygen content is less than 14% (the norm is 21%), the combustion of most substances becomes impossible. In addition to oxygen, oxidizing agents can be chemical compounds that contain oxygen, for example, saltpeter (KNO3), nitric acid (HNO3), potassium permanganate (KMn2O4), as well as individual chemical elements (fluorine, chlorine, bromine). Some substances contain so much oxygen that it is enough to burn without air (gunpowder, explosives).
The source of ignition, that is, the initiator of a fire can be: open fire, hot objects, electrical charges, thermal processes of chemical, electrical and mechanical origin, sparks from impacts and friction, solar radiation, electromagnetic and other radiation. Ignition sources can be high-, medium- and low-power (Table 4.5):
Table 4.5.
> Types of combustion
The following types of combustion are distinguished: explosion, detonation, flash, combustion, ignition, spontaneous combustion, spontaneous combustion, smoldering.
An explosion is an extremely rapid chemical transformation, accompanied by the release of energy and the formation of compressed gases capable of performing mechanical work. Mainly this mechanical work comes down to the destruction that occurs during an explosion and is caused by the formation of a shock wave - a sudden abrupt increase in pressure. When moving away from the explosion site mechanical impact the shock wave is weakened.
Detonation is a combustion that spreads at a speed of several thousand meters per second. The occurrence of detonation is explained by compression, heating and movement of the unburned mixture in front of the flame front, which leads to an acceleration of flame propagation and the appearance of a shock wave in the mixture. Thus, the presence of a sufficiently powerful shock wave is a necessary condition for detonation, since in this case the transfer of heat in the mixture is carried out not by the slow process of thermal conduction, but by the propagation of a shock wave.
A flash is a short-term intense combustion of a limited volume of a gas-air mixture over the surface of a combustible substance or dust-air mixture, accompanied by short-term visible radiation, but without a shock wave and stable combustion.
Fire - the beginning of combustion under the influence of an ignition source.
Ignition is a fire accompanied by the appearance of a flame.
Smoldering is the flameless combustion of a material (substance) in the solid phase with visible emission of light from the combustion zone.
Spontaneous combustion is the beginning of combustion due to self-initiated exothermic processes.
Spontaneous combustion is spontaneous combustion accompanied by the appearance of a flame.
Spontaneous combustion occurs when, as a result of exothermic processes, the rate of heat release in the mass of a combustible substance exceeds the rate of its dissipation in environment. The following can initiate exothermic processes and then cause spontaneous combustion:
high temperature of a flammable substance due to the action of an external heating source (thermal spontaneous combustion)
The vital activity of microorganisms in a mass of combustible substance, which leads to its self-heating (microbiological spontaneous combustion)
Chemical reactions resulting from exposure of a substance to air, water or chemically active substances (chemical spontaneous combustion).
Thermal spontaneous combustion occurs in a mass of materials that are in an energetically favorable initial state for entering into an exchange reaction with atmospheric oxygen when heated from the outside. Such heating can be carried out in the following ways:
Contact (due to heat exchange upon contact with a heated object)
Radiation (due to radiant heat);
Convective (due to heat transfer by air flow).
The “mechanism” of thermal self-ignition is as follows. During external heating of the material, its temperature gradually increases (phase a, Fig. 4.15). After reaching the self-heating temperature TSN, a sharp intensification of exothermic processes of oxidation and decomposition occurs in the material, which leads to self-heating and an increase in the temperature of the material (phase b). The most intense self-heating occurs in the place where best conditions heat accumulation. Deep places meet these conditions, since it is in them that the worst conditions for heat dissipation into the environment exist. Thus, the center of self-heating of coal piled up is, as a rule, at a depth of 0.5-0.8 m from the surface.
When the self-ignition temperature TSZ is reached, combustion of the material occurs without an ignition source (phase c).
Rice. 4.15.
Thermal spontaneous combustion is observed when stored in piles of coal (Ta = 50-60 ° C) and cotton (Ta = 120-125 ° C), as well as in piles of newspaper (wallpaper) paper and corrugated cardboard(Three = 100-110°C).
Prevention of thermal self-ignition - prevention of heating of materials (substances) from external heat sources.
Organic dispersed and fibrous materials, within which vital activity of so-called thermophilic microorganisms are possible, are capable of microbiological spontaneous combustion. It is the vital activity of such microorganisms that leads to the primary self-heating of the mass of material. Particularly susceptible to microbiological spontaneous combustion are undried substances of plant origin, piled up (hay, straw, grain, flax, cotton, peat, etc.). Microbiological spontaneous combustion occurs within 10 to 30 days from the start of the process.
In Fig. Figure 4.16 shows a typical development curve for the process of microbiological spontaneous combustion of undried hay stored for storage.
Rice. 4.16. Typical development curve of the process of microbiological spontaneous combustion of undried hay stored for storage
Chemical spontaneous combustion occurs due to exposure of a flammable substance to air, water or chemically active substances.
Substances that can spontaneously combust due to exposure to atmospheric oxygen include oils, fats and drying oils. However, this requires appropriate conditions. So, when storing these substances in containers, spontaneous combustion does not occur, since the surface of their contact with air is too small. At the same time, fibrous materials impregnated with them have a developed oxidation surface, which significantly increases their ability to spontaneously combust. However, another indispensable condition is to arrange the impregnated materials in a heap, stacks, bags. In this case, the oxidation surface significantly exceeds the heat transfer surface, which leads to self-heating of substances with subsequent spontaneous combustion.
According to experimental data, 50 g of cotton wool soaked in 100 g of linseed oil showed such an increase in temperature (Table 4.6).
Table 4.6.
After 15 hours from the moment the sample leaks, its temperature will reach 170 ° C and it will ignite without an ignition source.
Substances that can spontaneously ignite when exposed to water include potassium, sodium, cesium, calcium and alkali metal carbides, and the like. When these substances interact with water, they release flammable gases, which are heated by the heat of reaction and spontaneously ignite.
Chemically active substances that can cause spontaneous combustion include mainly oxidizing agents: compressed oxygen, nitric acid, potassium permanganate, sodium peroxide, nitrate, bleach, etc.
For example, compressed oxygen causes spontaneous combustion of mineral oils, which do not ignite in air. And plant materials (straw, hay, flax, cotton, sawdust), turpentine, ethyl alcohol spontaneously ignite as a result of contact with nitric acid.
The ability of substances and materials to spontaneously ignite must be taken into account when developing measures fire prevention during their storage, transportation, heat treatment, execution technological operations etc.
3. The concept of fire as a process
3.1. General information about combustion
Combustion is a complex physical and chemical process of interaction between a combustible substance and an oxidizer, characterized by a self-accelerating chemical transformation and accompanied by the release of large amounts of heat and light. Flame combustion can occur either under the influence of an ignition source (ignition), or due to a sharp increase in the rate of exothermic reactions (autoignition).
Self-ignition mode consists in the spontaneous occurrence of flammable combustion of a combustible mixture preheated to a certain critical temperature (the so-called auto-ignition temperature); this mode manifests itself in the form of a flash and is characterized by the simultaneous combustion of the entire combustible mixture. Table 1 shows some flammable substances and their auto-ignition temperatures.
Table 1.
Autoignition temperature of some flammable substances
Substance | Temperature, °C | Substance | Temperature, °C |
Wood | Aviation gasoline | ||
Sunflower oil | |||
Ethanol | |||
Ignition mode represents the propagation of a combustion wave (propagation of a flame front) through a cold mixture when it is locally ignited (ignited) by an external source. A flame is a visible combustion zone in which glow and radiation of heat are observed. The flame resulting from ignition itself becomes a source of heat flow and chemically active particles into the adjacent layers of fresh combustible mixture, thereby ensuring the movement of the flame front.
On spontaneous combustion of plant products. Among plant products, hay, straw, leaves, malt, and hops are prone to spontaneous combustion. Particularly susceptible to spontaneous combustion are under-dried plant products in which vital activity continues. plant cells.
According to the bacterial theory, the presence of moisture and an increase in temperature due to the vital activity of plant cells contributes to the proliferation of microorganisms present in plant products. Due to the poor thermal conductivity of plant products, the released heat gradually accumulates and the temperature in the product mass rises. At elevated temperatures, microorganisms die and turn into porous carbon, which has the property of heating up due to intense oxidation and therefore is the next source of heat generation, after microorganisms. The temperature in plant products rises to 300°C, and they spontaneously combust.
Charcoal, brown and hard coal, peat also ignite spontaneously due to intense oxidation by atmospheric oxygen.
Vegetable and animal fats, if they are applied to crushed or fibrous materials (rags, ropes, tow, matting, wool, sawdust, soot, etc.) have the ability to spontaneously ignite.
When crushed or fibrous materials are wetted with oil, it is distributed over the surface and upon contact with air, it begins to oxidize. Simultaneously with oxidation, the process of polymerization (combination of several molecules into one) occurs in the oil. Both the first and second processes are accompanied by significant heat release. If the heat generated is not dissipated, i.e. accumulates inside a tightly packed bale, the temperature in the oiled material rises and can reach the auto-ignition temperature.
Combustion occurs when three essential components are present: flammable substance, oxidizer and ignition source. Let's look at each of them in more detail.
Under the term flammable substance means a substance that is capable of burning independently after the external source of ignition is removed. A flammable substance can be in a solid, liquid or gaseous state. Combustible substances are most organic substances, a number of gaseous inorganic compounds and substances, many metals, etc. Gases pose the greatest fire and explosion hazard.
Fluid combustion. For a flammable liquid to ignite, a vapor-air mixture must first form above its surface. Combustion of liquids is possible only in the vapor phase, while the surface of the liquid itself remains relatively cold. Among flammable liquids (FL), there is a class of the most dangerous representatives - flammable liquids (FLL). The flammable liquids include gasoline, acetone, benzene, toluene, some alcohols, ethers, etc.
There are a number of substances (gaseous, liquid or solid) that can spontaneously ignite when in contact with air without preheating (with room temperature), such substances are called pyrophoric. These include: hydrogen fluoride, white phosphorus, hydrides and organometallic compounds of light metals, etc.
There's enough large group substances, upon contact with water or water vapor in the air, a chemical reaction begins, releasing a large amount of heat. Under the influence of the released heat, self-ignition of the flammable reaction products and starting materials occurs. This group of substances includes alkali and alkaline earth metals (lithium, sodium, potassium, calcium, strontium, uranium, etc.), hydrides, carbides, phosphides of these metals, low molecular weight organometallic compounds (triethylaluminum, triisobutylaluminum, triethylboron), etc.
Solid combustion occurs according to a more complex mechanism and has several stages. When exposed to an external source, the surface layer of the solid substance is heated, and gaseous volatile products begin to be released from it. This process can be accompanied either by the melting of the surface layer of a solid substance, or by its sublimation (the formation of gases, bypassing the melting stage). When a certain concentration of flammable gases in the air (lower concentration limit), they ignite and, through the heat released, begin to influence the surface layer themselves, causing it to melt and the entry of new portions of flammable gases and solid vapors into the combustion zone.
Let's take wood as an example. When heated to 110°C, the wood dries out and the resin evaporates slightly. Weak decomposition begins at 130°C. More noticeable wood decomposition (color change) occurs at 150°C and above. The decomposition products formed at 150-200°C are mainly water and carbon dioxide, so they cannot burn. At temperatures above 200°C, the main component of wood, fiber, begins to decompose. Gases formed at these temperatures are flammable because they contain significant amounts of carbon monoxide, hydrogen, hydrocarbons and vapors of other organic substances. When the concentration of these products in the air becomes sufficient, under certain conditions they will ignite.
If a flammable substance melts and spreads, it increases the source of combustion (for example, rubber, metals, etc.). In the event that the substance does not melt, oxygen gradually approaches the surface of the fuel and the process takes the form of heterogeneous combustion (the stage of burning out the coke of the carbon fuel). The combustion process of solids is complex and diverse; it depends on many factors (dispersity of the solid material, its humidity, the presence of an oxide film on its surface and its strength, the presence of impurities, etc.).
More intensely (often with an explosion), fine metal powders and dusty combustible materials (for example, wood dust, powdered sugar) ignite.
How oxidizer Most often during a fire, oxygen appears, the content of which in the air is known to be about 21%. Strong oxidizing agents are hydrogen peroxide, nitric and sulfuric acids, fluorine, bromine, chlorine and their gaseous compounds, chromic anhydride, potassium permanganate, chlorates and other compounds.
When interacting with metals, which exhibit very high activity in the molten state, water, carbon dioxide and other oxygen-containing compounds, which are considered inert in normal practice, act as oxidizing agents.
However, just the presence of a mixture of fuel and oxidizer is not enough to start the combustion process. More needed ignition source. In order for a chemical reaction to occur, the appearance of a sufficient number of active molecules, their fragments (radicals) or free atoms (that have not yet had time to combine into molecules) that have excess energy equal to or exceeding the activation energy for a given system is necessary.
The appearance of active atoms and molecules is possible when the entire system is heated, during local contact of gases with a heated surface, when exposed to a flame, an electric discharge (spark or arc), local heating of the vessel wall as a result of friction or when a catalyst is introduced, etc. The source of ignition can also be sudden adiabatic (without heat exchange with the environment) compression of the gas system or the impact of a shock wave on it.
Currently, scientists have established that the mechanism of occurrence and development of real fires and explosions is characterized by a combined chain-thermal process. Having started in a chain way, the oxidation reaction, due to its exothermicity, continues to be accelerated by heat. Ultimately, the critical (limiting) conditions for the occurrence and development of combustion will be determined by the heat release and conditions of heat and mass transfer of the reacting system with the environment.
3.2. Combustion termination mechanism
The mechanism of combustion cessation is understood as a system of factors leading to the end of the combustion process (reaction).
The combustion cessation mechanism can be naturally determined when it is implemented without human participation (self-destruction of combustion, for example, in nature). At the same time, knowledge of the essence of the combustion termination mechanism allows one to purposefully use its factors both when eliminating small fires and when extinguishing fires.
To stop burning, at least one of the following conditions must be met:
stop the entry of new portions of fuel vapor into the combustion zone;
stop the flow of oxidizing agent (air oxygen);
reduce the heat flow from the flame;
reduce the concentration of active particles (radicals) in the combustion zone.
Based on this, one of the possible principles (methods) of extinguishing a fire could be:
reducing the temperature of the combustion source below the self-ignition temperature or flash point of the fuel by introducing into the flame substances that, as a result of evaporation, sublimation or decomposition, take on a certain amount of heat (a classic substance is water);
reducing the amount of fuel vapor entering the combustion zone by isolating the flammable substance from the effects of the combustion torch (for example, using a thick blanket);
reducing the oxygen concentration in a gaseous environment by diluting the environment with non-flammable additives (for example, nitrogen, carbon dioxide);
reducing the rate of the chemical oxidation reaction due to the binding of active radicals and interrupting the combustion chain reaction occurring in the flame by introducing special chemically active substances (inhibitors);
creating conditions for extinguishing the flame as it passes through narrow channels between particles fire extinguishing agent(fire blocking effect);
flame failure as a result of the dynamic impact of a jet of extinguishing agent on the fire.
As a rule, when a fire extinguishing agent is exposed to a fire, no single mechanism of action occurs in its pure form; the extinguishing process is of a combined nature. Thus, foam has an insulating and cooling effect, while powder compositions have an inhibitory, fire-retarding and dynamic effect.
3.3 Classification of fires
All fires, depending on the state of aggregation of the flammable substances involved in the combustion process, are divided into several classes and are designated in capital Latin letters A, B, C, D, E. Characteristics of fire classes and pictograms used for them designations are given in the appendix.
Depending on the type of charged substance, fire extinguishers can be used to extinguish one or more classes of fire:
class A fire of solid flammable substances
class B fire of liquid flammable substances
class C ignition of gaseous flammable substances
class D fire of metals and metal-containing substances
class E fire of electrical installations under voltage.
It should be noted that the above classification almost coincides with that approved by the international standard ISO 3941. The international standard has no subclasses, and there is no class “E”, but there is a class “F”, which denotes fires that can occur in food preparation areas of facilities nutrition. It must be borne in mind that the national classification in some countries differs from the international one. So in the USA, the letter “A” denotes fires of solid flammable substances, the letter “B” denotes fires of liquid and gaseous substances, but the letter “C” denotes fires of energized electrical equipment, and the letter “D” denotes fires of metals and metal-containing substances. Therefore, when you pick up a fire extinguisher, be sure to look at its label, consider the pictograms of the classes of fires that this fire extinguisher is intended to extinguish.
Pictograms of fire classes for which a fire extinguisher cannot be used are either crossed out with a diagonal stripe or not shown at all.
3.4. Fire hazards
In accordance with GOST 12.01.004-85 “Fire Safety”, dangerous factors of fire are: flames and sparks, elevated ambient temperatures, toxic products of combustion and thermal decomposition, smoke, reduced oxygen concentration.
Flame
The combustion of all liquid, gaseous and most solid combustible substances, which, when decomposing or evaporating, release gaseous products, is accompanied by the formation of a flame. Thus, a flame is a gas volume in which the process of combustion of vapors and gases occurs.
Solid substances burn without flame: graphite, anthracite, coke, soot, charcoal. These substances do not decompose and do not form gases when heated, or they form them in quantities insufficient for combustion.
The glow of a flame when burning organic substances depends on the presence of hot solid carbon particles in it, which have time to burn. A non-luminous (blue) flame usually occurs during the combustion of gaseous products: carbon monoxide, hydrogen, methane, ammonia, hydrogen sulfide.
The flame temperature when burning in air of some flammable substances is: wood - 850-1400°C, petroleum products in a tank - 1100-1300°C, carbon disulfide - 2195°C, stearin - 640-940°C.
Open fire is very dangerous for humans, because... exposure to flames on the body causes burns. An even greater danger is the thermal radiation of the fire, which can cause burns to the body, eyes, etc.
Temperature
Inhalation of heated air leads to damage and necrosis of the upper respiratory tract, suffocation and death of a person. When exposed to temperatures above 100°C, a person loses consciousness and dies within a few minutes.
Skin burns are dangerous for humans. Despite the great successes of medicine in their treatment, a victim who received second-degree burns on 30% of the body surface has little chance of survival. The time it takes for a person to receive second-degree burns is short: at an ambient temperature of 71°C - 26 seconds, at 100°C - 15 seconds. Research has established that in a humid atmosphere, typical of a fire, a second-degree burn is caused by a temperature significantly lower than specified. Thus, an ambient temperature of 60-70°C is dangerous for human life, not only in the burning room, but also in adjacent rooms into which combustion products and heated air have entered.
Reduced oxygen concentration
Most often, people in fires die not from fire and high temperature, but due to a decrease in the concentration of oxygen in the air and poisoning by toxic combustion products.
The first symptoms of oxygen deficiency (increased breathing volume, decreased attention, impaired muscle coordination) are observed in people when the oxygen content in the inhaled gas mixture is 16-17%. A decrease in O 2 concentration to 12-15% causes shortness of breath, increased heart rate, deterioration of mental activity, dizziness, and fatigue. In cases where the O2 concentration decreases to 10-12%, consciousness remains, but nausea, severe fatigue appear, and breathing becomes intermittent. At a concentration of 8%, loss of consciousness quickly occurs, and below 6%, death occurs within 6-8 minutes.
Toxic combustion products
This topic will be discussed more fully by specialists (Chemist, Toxicologist).
How dangerous toxic combustion products are is clearly shown by the example of a fire that occurred in a clothing store in Tokyo (Japan). A fire broke out on the 3rd floor, and in a bar located on the 7th floor of the same building, 118 people died, 96 of them from poisoning by toxic combustion products, 22 people jumped out of the windows. Many people lost consciousness within the first 2-3 minutes; their death occurred within 4-5 minutes. after loss of consciousness.
Smoke
Smoke is dangerous not only because of the toxic substances it contains, but also because of its reduced visibility. This makes it difficult, and sometimes almost impossible, to evacuate people from a dangerous area. To quickly get to safety, people must see clearly emergency exits or their pointers.
When visibility is lost, organized movement (especially in an unfamiliar building, in objects with large numbers of people) is disrupted, becomes chaotic, everyone moves in an arbitrarily chosen direction. Panic sets in. People are overcome by fear, suppressing consciousness and will. In this state, a person loses the ability to navigate and correctly assess the situation.
Explosion
One type of instantaneous combustion is the explosion of special explosives, as well as a mixture of flammable gases, vapors or dust with air. These are chemical explosions.
Explosions of a physical nature are ruptures of various containers and apparatus (boilers, reservoirs, cylinders, etc.) that occur as a result of the development of excessive pressure by gases or vapors, exceeding the pressure that the walls of the containers and apparatus can withstand.
At the moment of a chemical explosion, the substance burns at high speed, and the resulting gases and vapors expand greatly and create great pressure on the environment. This explains the enormous force of destruction caused by the explosion. An explosion usually produces a flame, which can ignite nearby flammable substances.
It is known that for combustion to occur, the presence of:
1. Flammable substance
2. Oxidizing agent
3. Ignition source (energy pulse)
These three components are often called the fire triangle. If one of them is excluded, then combustion cannot occur. This most important property of the triangle is used in practice to prevent and extinguish fires.
Air and combustible matter constitute a system capable of burning, and temperature conditions determine the possibility of spontaneous ignition and combustion of the system.
The highest combustion rate is obtained when the substance burns in pure oxygen, the lowest (combustion stops) when the substance contains 14–15% oxygen.
Combustion of substances can occur due to oxygen contained in other substances that can easily release it. Such substances are called oxidizing agents. Here are the most well-known oxidizing agents.
· Berthollet salt (KClO 3).
· Potassium nitrate (KNO 3).
· Sodium nitrate (NaNO 3).
Oxidizing agents contain oxygen, which can be released by decomposition of the salt, for example:
2 KClO 3 = 2KCl + 3 O 2
The decomposition of oxidizing agents occurs when heated, and some of them even under the influence of a strong shock.
2. Combustion products. Complete and incomplete combustion. Ecological aspects of combustion processes.
During the combustion process, combustion products are formed. The composition depends on the burning substance and combustion conditions. Combustion products, with the exception of carbon monoxide, are not capable of burning.
The smoke generated by the combustion of organic substances contains solid particles and gaseous products (carbon dioxide, carbon monoxide, nitrogen, sulfur dioxide and others). Depending on the composition of the substances and the conditions of their combustion, the smoke produced varies in content. The fumes produced by the combustion of different substances differ not only in composition, but also in color and smell. The color of the smoke can be used to determine what substance is burning, although the color of the smoke varies depending on the friction conditions. When wood burns, the smoke has a grayish-black appearance; paper, hay, straw - whitish-yellow; fabric and cotton - brown; petroleum products - black, etc.
Combustion products are gaseous, liquid or solid substances formed during the combustion process. The composition of combustion products depends on the composition of the burning substance and on the conditions of its combustion. Organic and inorganic combustible substances consist mainly of carbon, oxygen, hydrogen, sulfur, phosphorus and nitrogen. Of these, carbon, hydrogen, sulfur and phosphorus are capable of oxidizing at combustion temperatures and forming combustion products: CO, CO 2, SO 2, P 2 O 5. Nitrogen does not oxidize at combustion temperature and is released in a free state, and oxygen is spent on the oxidation of the combustible elements of the substance. All of these combustion products (with the exception of carbon monoxide CO) are no longer capable of burning in the future. They are formed during complete combustion, that is, during combustion that occurs with the access of a sufficient amount of air and at high temperature.
Carbon dioxide or carbon dioxide (CO 2) - the product of complete combustion of carbon. It is odorless and colorless. Magnesium combustion, for example, occurs in the atmosphere carbon dioxide according to the equation:
CO 2 +2 Mg = C + 2 MgO .
When the concentration of carbon dioxide in the air exceeds 3-4.5%, staying indoors and inhaling the gas for half an hour is life-threatening.
Carbon monoxide or carbon monoxide (CO) - a product of incomplete combustion of carbon. This gas is odorless and colorless, making it particularly dangerous.
Sulphur dioxide(SO 2) is a product of combustion of sulfur and sulfur compounds. Colorless gas with a characteristic pungent odor.
Smoke When many substances burn, in addition to the combustion products discussed above, smoke is released - a dispersed system consisting of tiny solid particles suspended in a gas.
With incomplete combustion of organic substances under conditions of low temperatures and lack of air, more diverse products are formed - carbon monoxide, alcohols, ketones, aldehydes, acids and other complex chemical compounds. They are obtained by partial oxidation of both the fuel itself and the products of its dry distillation (pyrolysis). These products produce acrid and poisonous smoke. In addition, the products of incomplete combustion themselves are capable of burning and forming explosive mixtures with air. Such explosions occur when extinguishing fires in basements, dryers and in enclosed spaces with a large amount of flammable material. Let us briefly consider the properties of the main combustion products.
Ecological aspects of combustion processes. Application natural gas reduces air pollution from sulfur oxides, particulate matter and carbon monoxide, but releases large amounts of nitrogen oxides, carbon monoxide and carcinogens (3,4-benzo(o)perene) into the atmosphere. Proper organization combustion, the choice of rational combustion methods allows to minimize the formation harmful substances and releasing them into the air pool. The use of natural gas makes it possible to wage not only a passive, but also an active fight for air purity: the use of afterburning units, the use of exhaust gases to supply gas burner instead of the corresponding amount of air.
Ecological problems combustion. The goal is to do no harm when burning fuels. Negative manifestations:
Technogenic heat release is commensurate with the components of the atmospheric heat balance;
Acoustic noise turbulent flames during the operation of aircraft and rocket engines - an environmental pollutant.
Blowout harmful products combustion – nitrogen oxides, metal oxides, carbon monoxide(at high Tg), sulfur oxides, carcinogenic substances - products of incomplete pyrolysis of organic combustibles, soot, carbon dioxide (at low Tg) - is the cause of: changes in the optical properties of the atmosphere and a decrease in the flux of solar radiation, the occurrence of acid rain, an increase in the “greenhouse effect” , destruction of the Earth's ozone layer, negative impact on flora and fauna, buildings and structures. Overall result: global warming, climate disasters (cyclones, snowstorms, tornadoes, tsunamis, floods, droughts, avalanches, mudslides)..
3. Equations for the combustion of substances in oxygen and air, methods for compiling them. Thermodynamics of combustion processes. Thermal effects combustion reactions.
The general equation for the combustion reaction of any hydrocarbon
C m H n + (m + n/4) O 2 = mCO 2 + (n/2) H 2 O + Q (8.1)
where m, n is the number of carbon and hydrogen atoms in the molecule; Q is the thermal effect of the reaction, or heat of combustion.
Thermal effect (heat of combustion) Q is the amount of heat released during complete combustion of 1 kmol, 1 kg or 1 m 3 of gas under normal physical conditions. A distinction is made between higher Q in and lower Q n heat of combustion: the higher heat of combustion includes the heat of condensation of water vapor during the combustion process (in reality, when burning gas, water vapor is not condensed, but is removed along with other combustion products). Typically, technical calculations are usually carried out based on the lower calorific value, without taking into account the heat of condensation of water vapor (about 2400 kJ/kg).
The efficiency calculated based on the lower calorific value is formally higher, but the heat of condensation of water vapor is quite high, and its use is more than advisable. This is confirmed by the active use of contact heat exchangers in heating technology, which are very diverse in design.
For a mixture of combustible gases, the higher (and lower) heat of combustion of gases is determined by the relation
Q = r 1 Q 1 + r 2 Q 2 + ... + r n Q n (8.2)
where r 1, r 2, …, r n are the volume (molar, mass) fractions of the components included in the mixture; Q 1, Q 2, …, Q n - heat of combustion of components.
The combustion process is much more complicated than according to formula (8.1), since along with branching of chains, they are broken due to the formation of intermediate stable compounds, which undergo further transformations at high temperatures. With a sufficient oxygen concentration, the final products are formed: water vapor H 2 O and carbon dioxide CO 2. If there is a lack of oxidizing agent, as well as when the reaction zone is cooled, intermediate compounds can stabilize and enter the environment.
High-temperature combustion of hydrocarbons is very complex and is associated with the formation of active particles in the form of atoms and radicals, as well as intermediate molecular compounds. As an example, the combustion reactions of the simplest hydrocarbon - methane are given:
1.
H + O 2 -› OH + O
CH 4 + OH -› CH 3 + H 2 O
CH 4 + O -› CH 2 + H 2 O
2.
CH 3 + O 2 -› HCHO + OH
CH 2 + O 2 -› HCHO + O
3.
HCHO + OH -› HCO + H 2 O
HCNO + O -› CO + H 2 O
HCO + O 2 -› CO + O + OH
4.
CO + O -› CO 2
CO + OH -› CO 2 + H
Summary of a single cycle:
2CH 4 + 4O 2 -› 2CO 2 + 4H 2 O
Thermodynamics of combustion
The initial composition of the combustible mixture is characterized by the molar or mass fractions of the components and the initial pressure and temperature. If the composition of the mixture is selected so that during its combustion both the fuel and the oxidizer are completely converted into reaction products, then such a mixture is called stoichiometric. Mixtures with excess fuel are called rich, and with a lack of fuel - poor. The degree of deviation of the mixture composition from stoichiometric is characterized by the fuel excess coefficient (eng. equivalence ratio) :
Where Y F And YO- mass fractions of fuel and oxidizer, respectively, and (Y F/Y O) st- their ratio in a stoichiometric mixture. In Russian-language literature, the coefficient of excess oxidizer (or air) is also used, inverse of the coefficient excess fuel.
Adiabatic combustion temperature of CH 4 mixtures with air depending on the fuel excess ratio. P = 1 bar, T 0 = 298.15 K.
If combustion occurs adiabatically at a constant volume, then the total internal energy of the system is conserved, but if at constant pressure is the enthalpy of the system. In practice, the conditions of adiabatic combustion are approximately realized in a freely propagating flame (without taking into account heat loss by radiation) and in other cases when heat loss from the reaction zone can be neglected, for example, in the combustion chambers of powerful gas turbine units or rocket engines.
Adiabatic combustion temperature is the temperature of the products reached when the chemical reactions and establishment of thermodynamic equilibrium. For thermodynamic calculations, tables of thermodynamic functions of all components of the initial mixture and products are used. Methods of chemical thermodynamics make it possible to calculate the composition of products, final pressure and temperature under given combustion conditions. There are currently many programs available that can perform these calculations.
Heat of combustion is the amount of heat released during complete combustion of the starting components, that is, up to CO 2 and H 2 O for hydrocarbon fuels. In practice, part of the released energy is spent on the dissociation of products, so the adiabatic combustion temperature without taking into account dissociation turns out to be noticeably higher than what is observed in practice.
Thermodynamic calculation allows one to determine the equilibrium composition and temperature of the products, but does not provide any information about the speed at which the system approaches the equilibrium state. Full description combustion requires knowledge of the mechanism and kinetics of reactions and the conditions of heat and mass exchange with the environment.
4. Types of flame and burning rate. Combustion theories: thermal, chain, diffusion.
IN general case the burning rate depends on the rate of mixing of the initial components in the heating zone and the reaction zone (for heterogeneous systems), on the rate of chemical reactions between the components, on the rate of transfer of heat and active particles from the reaction zone to the initial system. The normal combustion rate (and even more so the shape of the combustion front) depends on the flow conditions of the fresh mixture and combustion products (especially during combustion in engines).
Therefore, combustion theory considers several main types of flames. They are not the same in their scientific and practical significance and degree of knowledge. The parameters of greatest interest for a given type of flame are not the same. The approach to the theoretical consideration of each type of flame is significantly different. There are also some differences in experimental methods.
We list the most important types of flames for combustion theory:
1) laminar flame in a homogeneous gas mixture. The same type includes flames when burning volatile explosives;
2) laminar diffusion flame during the combustion of a jet of combustible gas in an oxidizing atmosphere. This type is associated with a flame during diffusion combustion of liquid fuel poured into a cylindrical vessel, etc.;
3) flame when a drop of liquid fuel or a particle of solid fuel burns in an oxidizing atmosphere;
4) turbulent flames in homogeneous or pre-mixed gas mixtures;
5) flame during the combustion of non-volatile explosives, gunpowders, etc. in cases where the reaction in the condensed phase plays a significant role.
Let us briefly consider some characteristics of the main types of flames to the extent that this is useful for understanding the laws of combustion of condensed mixtures.
First we need to consider the definition burning rate . In laminar combustion of gas mixtures and homogeneous condensed systems, the concept of normal combustion rate ( un). By definition, un equal to the speed of movement of the flame relative to the fresh mixture in the direction perpendicular to the surface of the flame at a given point. Dimension un in the SI system - m/sec, however, for burning speed this unit is still rarely used and only for gas systems. Usually the size un for gas systems they are expressed in cm/sec, and for condensed systems in mm/sec (if you express the combustion rate of condensed systems in m/sec, then in the usual pressure range you get very small fractional numbers).
For homogeneous condensed systems, the burning rate of cylindrical charges burning from the end is most often measured, and the combustion front is assumed to be flat (experience shows that in most cases, in the presence of a proper shell, this assumption is valid, and distortions are observed only at the edges of the charge). In addition, for solid substances (and fairly viscous liquid substances), the original (solid or liquid) substance is motionless during combustion. Therefore, in this case, the normal burning rate is simply equal to the apparent flame speed (in the laboratory coordinate system) and is constant at different points of the charge.
