Combustion is a complex physical and chemical process. Physico-chemical combustion process. Physico-chemical foundations of combustion and explosion processes. Conditions of occurrence and types of combustion
A fire is an uncontrolled combustion that develops in time and space, dangerous to people and causing material damage.
Fire factors that are dangerous to people include open flames, sparks, elevated temperatures, toxic combustion products, smoke, decreased oxygen content, and collapse of buildings or installations.
Combustion is a rapidly occurring physical and chemical reaction, accompanied by the release of heat and smoke, the appearance of a flame or smoldering. Under normal conditions, combustion is the process of oxidation or combination of a combustible substance with oxygen in the air. However, some substances (for example, compressed acetylene, nitrogen chloride, ozone) can explode without oxygen, producing heat and flame. Consequently, combustion can result from reactions not only of combination, but also of decomposition. It is also known that hydrogen and many metals can burn in an atmosphere of chlorine, copper in sulfur vapor, magnesium in carbon dioxide, etc.
The most dangerous combustion occurs when a flammable substance is oxidized by atmospheric oxygen. In this case, it is necessary to have an ignition source capable of providing the combustible system with the required amount of energy. The most common sources of ignition are: sparks that appear during electrical equipment malfunction, impact of metal bodies, welding, forging; heat resulting from friction; technological heating devices; firing devices; heat of adiabatic compression; spark discharges of static electricity; overheating of electrical contacts; chemical reactions that occur with the release of heat.
The heating temperature of these sources is different. Thus, a spark arising from the impact of metal bodies can have a temperature of up to 1900 ° C, the flame of a match is about. 800°C, the drive drum of the belt conveyor when slipping is up to 600°C, and in the heat of an electric discharge the temperature reaches 10,000°C, and chemical reactions are completed almost instantly.
Combustion can be complete or incomplete. With complete combustion occurring in excess of oxygen, the reaction products are carbon dioxide, water, nitrogen, and sulfur dioxide. Incomplete combustion occurs when there is a lack of oxygen; the products of combustion in this case are toxic and flammable substances - carbon monoxide, alcohols, ketones, aldehydes, etc. For complete combustion of a combustible substance, a certain amount of air is required: 1 kg of wood - 4.18, peat - 5 .8, propane - 23.8 m3.
The combustion process can be imagined as follows. When a thermal pulse is introduced, a cold flammable medium heats up, intense oxidation of the flammable medium with oxygen occurs and additional heat is released. This, in turn, leads to heating of the adjacent layer of flammable substance, in which an intense chemical reaction also occurs. With such layer-by-layer combustion of a combustible substance, the combustion zone moves; the speed of this movement determines the intensity of the combustion process and is its the most important characteristic. The process of layer-by-layer heating, oxidation and combustion continues until the entire volume of the combustible substance is exhausted.
The narrow zone in which the substance is heated and the chemical reaction occurs is called the flame front.
Combustible systems can be chemically homogeneous or heterogeneous. Chemically homogeneous systems are mixtures of flammable gases, vapors or dusts with air, in which the flammable substance and air are evenly mixed. The combustion of such systems is called homogeneous. In chemically heterogeneous systems, the flammable substance and air are not mixed and have an interface. These are most often solid combustible materials and their combustion is called heterogeneous.
The total combustion time of the combustible mixture tg consists of the time required for contact between the combustible substance and oxygen τ to, and the time during which the chemical oxidation reaction itself occurs τ x
Depending on the ratio of these two terms, combustion is distinguished between diffusion and kinetic. When burning solid combustible substances, the time required for the penetration (diffusion) of oxygen to the surface of the substance is much longer than the time of the chemical reaction, therefore the overall combustion rate is completely determined by the rate of diffusion of oxygen to the combustible substance. The combustion of such substances most often occurs in fires and is called diffusion. Combustion, the rate of which is determined by the rate of the chemical reaction, is called kinetic. This type of combustion is typical for homogeneous combustible systems.
There are calorimetric, theoretical and actual combustion temperatures.
The calorimetric combustion temperature is the temperature to which the products of complete combustion are heated, if all the released heat is spent on heating them, the amount of air is equal to the theoretically required one, complete combustion of substances occurs and the initial temperature is 0 ° C. Heat losses are assumed to be zero. If the initial temperature of the combustible substance and air is 0°C, then the calorimetric combustion temperature
where Qn is the lower heat of combustion of a combustible substance, kcal/kg; V - volume of combustion products, m3/kg; с - average volumetric heat capacity of combustion products, kcal/m3 deg.
Consequently, the calorimetric combustion temperature depends only on the properties of the combustible substance and does not depend on its quantity. The theoretical combustion temperature takes into account heat loss during combustion due to dissociation. The calorimetric combustion temperature is the highest for a combustible substance and is used for qualitative assessment. In reality, during combustion there is always heat loss due to radiation, heating of excess air and the environment.
The actual combustion temperature is the fire temperature. There is a distinction between internal and external fire temperatures. The temperature of an external fire is the temperature of the flame, and the temperature of an internal fire is the temperature of the smoke in the room. Actual temperatures developing during a fire due to heat loss in environment, heating combustion products and structures
always less than theoretical by 30...50%. For example, the theoretical combustion temperature of gasoline is 1730°C, and the actual combustion temperature is 1400°C.
A mixture of flammable vapors and gases with an oxidizer is capable of burning only if it contains a certain amount of fuel.
The lowest concentration of flammable gas at which combustion is already possible is called the lower flammable concentration limit (LCFL). The highest concentration at which combustion is still possible is called the upper flammable concentration limit (UCL). The concentration region lying inside these boundaries is called the ignition region. Ignition is an ignition (the beginning of combustion) accompanied by the appearance of a flame. This is a stable, long-lasting combustion that does not stop even after the ignition source is removed. The values of the lower and upper flammability limits depend on the properties of gas, steam and dust air mixtures, and the content of inert components in the combustible mixture. Adding inert gases to a flammable mixture narrows the area of ignition and ultimately makes it non-flammable. Some impurities that slow down combustion reactions significantly narrow the flammability limits. The most active of them are halogenated hydrocarbons. Both noted properties are used to stop combustion. Lowering the pressure of the mixture below atmospheric also narrows the area of ignition, and at a certain pressure the mixture becomes non-flammable. An increase in the pressure of the combustible mixture expands the ignition area, but, as a rule, insignificantly. An increase in the temperature of the combustible mixture expands the area of ignition. The ignition concentration limits are also affected by the power of the ignition source.
There are not only concentration limits, but also temperature limits of ignition.
Temperature limits for ignition of vapors in air are those temperatures of a flammable substance at which it saturated couples form concentrations corresponding to the lower or upper concentration limit of flammability. The ignition temperature is the lowest temperature at which a substance ignites or begins to smolder and continues to burn or smolder after the source of ignition is removed. The ignition temperature characterizes the ability of a substance to burn independently. If a substance does not have a flammable temperature, then it is classified as low-flammable or non-flammable.
Acceleration of the oxidation reaction under the influence of temperature leads to spontaneous combustion. Unlike the combustion process, in which only a limited part of the volume - the surface - ignites, self-ignition occurs throughout the entire volume of the substance. The auto-ignition temperature is the lowest temperature to which a substance must be heated in order for it to ignite as a result of further auto-oxidation. Self-ignition is possible only if the amount of heat released during the oxidation process exceeds the heat transfer to the environment.
The auto-ignition temperature is not constant for a substance, since it largely depends on the conditions under which it is determined. To obtain comparative data, testing equipment and methods for determining the auto-ignition temperature of gases and vapors have been standardized (GOST 13920-68). The minimum temperature determined by the standard method to which a mixture of gases and vapors with air must be uniformly heated in order for it to ignite without introducing an external ignition source into it is called the standard auto-ignition temperature.
A type of self-ignition is spontaneous combustion, i.e. combustion as a result of self-heating without the influence of an ignition source. The difference between spontaneous combustion and spontaneous combustion is the magnitude of the temperature. Spontaneous combustion occurs at ambient temperature, and for spontaneous combustion it is necessary to heat the substance from the outside.
In simple terms, combustion is understood as a fast-flowing exothermic process of oxidation of substances by atmospheric oxygen with the release of a significant amount of heat and the emission of light.
Combustion is a complex physical chemical process the interaction of a combustible substance and an oxidizer, as well as the decomposition of certain substances, characterized by a self-accelerating transformation with the release of a large amount of heat and the emission of light. Typically, air oxygen with a concentration of 21 is involved as an oxidizing agent in this process. about. %. For the occurrence and development of the combustion process, a combustible substance, an oxidizer and an ignition source are needed, which initiates a certain rate of chemical reaction between the fuel and the oxidizer.
Combustion, as a rule, occurs in the gas phase, therefore flammable substances in a condensed state (liquids and solids) must undergo gasification (evaporation, decomposition) in order to initiate and maintain combustion. Combustion is distinguished by a variety of types and features determined by heat and mass transfer processes, gas dynamic factors, kinetics chemical reactions and other factors, as well as feedback between external conditions and the nature of the development of the process.
2.4.2.1. Classification of combustion processes.
Combustion may be homogeneous And heterogeneous depending on the state of aggregation of combustible substances and the oxidizer.
Homogeneous combustion occurs when the reacting components of the combustible mixture have the same state of aggregation. Homogeneous combustion can be kinetic And diffusion depending on the conditions of mixture formation of flammable components and on the ratio of the rates of chemical reactions and mixture formation. One or another combustion mode is realized, for example, during a fire, depending on which stage of the combustion process is limiting: the rate of mixture formation or the rate of chemical reactions.
Kinetic is the combustion of pre-mixed gas or steam-air mixtures (the limiting stage of the process is the rate of chemical reactions), which often has an explosive nature (if the mixture is formed in a confined space), because The energy released in this case does not have time to be discharged outside this space. Kinetic combustion can also be calm if the combustible mixture is first created in a small, open space with a continuous supply of fuel to the combustion zone.
The diffusion combustion mode is realized when a combustible mixture is created directly in the combustion zone, when the oxidizer enters it due to diffusion processes, for example, when heterogeneous burning.
Heterogeneous combustion occurs under different aggregate states of the combustible substance and the oxidizer. In heterogeneous combustion, an important role is played by the intensity of the flow of vapors formed from condensed combustible substances (liquids, solids) into the reaction zone.
From a gas-dynamic point of view, combustion can be laminar And turbulent.
The laminar combustion process occurs when the components of the combustible mixture enter the reaction zone at low values of the Reynolds criterion (0< R e < 200), т.е. в основном за счёт молекулярной диффузии. Процесс характеризуется малыми скоростями газовыхfuel and oxidizer flows and layer-by-layer propagation of the reaction zone (flame front) in space. The burning rate in this case depends on the rate of formation of the combustible mixture.
The turbulent mode of the process is realized when the components of the combustible mixture enter the reaction zone at high values of the Reynolds criterion (230< R e< 10000). Combustion in this mode occurs with increasing gas velocity streams when the laminarity of their movement is disrupted. In a turbulent combustion mode, the vortex of gas jets improves the mixing of the reacting components, while the surface area through which molecular diffusion occurs increases, resulting in an increase in the speed of flame propagation in space.
According to the speed of flame propagation in space, combustion is divided into:
– deflagration(flame propagation speed is several m/s);
– explosive(flame propagation speed is tens and hundreds m/s, but not more than the speed of sound propagation in air (344 m/s));
– detonation(the speed of flame propagation is greater than the speed of sound in air).
Depending on the depth of chemical reactions, combustion can be complete And incomplete.
With complete combustion, the reaction proceeds to completion, i.e. until the formation of substances that are unable to further interact with each other, with the fuel and the oxidizer (the initial ratio of the combustible substance and the oxidizer is called stoichiometric). As an example, consider the complete combustion of methane proceeding according to the reaction
CH 4 + 2O 2 = CO 2 + 2H 2 O+ Q
Where Q – heat released as a result of an exothermic reaction, J.
When hydrocarbons burn completely, the reaction products are carbon dioxide and water, i.e. non-toxic and non-flammable substances. Complete combustion can occur both with a stoichiometric ratio of fuel and oxidizer, and with an excess of oxidizer relative to its stoichiometric content in the combustible mixture.
Incomplete combustion is characterized by the incompleteness of the chemical reaction, i.e. The reaction products, in the presence of an oxidizing agent, can further interact with it. Incomplete combustion occurs when the oxidizer content in the combustible mixture is insufficient (compared to the stoichiometric one). As a result of incomplete combustion, for example, of hydrocarbons, toxic and flammable components are formed, such as CO, H 2, benzopyrene, WITH(soot), organic resins, etc., a total of about 300 chemical compounds and elements.
All other things being equal, with complete combustion, higher temperatures develop than with incomplete combustion.
2.4.2.2. Basic mechanisms of combustion processes.
Combustion is accompanied by the release of heat and the emission of light and occurs under conditions of progressive self-acceleration of the process associated with the accumulation of heat in the system ( thermal combustion) or catalyzing active reaction intermediates ( chain combustion).
Thermal combustion is possible during an exothermic reaction, the rate of which rapidly increases under the influence of heat accumulating in the system, leading to an increase in temperature. When the temperature is reached at which the heat gain from the reaction exceeds the heat loss to the environment, self-heating of the system occurs, ending with self-ignition of the combustible mixture. Under these conditions, spontaneous development of the reaction is observed, accompanied by heating of the resulting products to a temperature at which they begin to emit light (more than 900 °C). Thermal combustion includes processes both with and without the participation of atmospheric oxygen (decomposition explosives, ozone, acetylene, peroxides (for example, N 2 ABOUT 2), interaction of some metals with halogens, sulfur, etc.).
Chain combustion is possible only in reactions for which the basis of ignition or explosion is a chain process. The latter is accompanied by the formation of unstable intermediate reaction products that regenerate active centers (atoms and molecules with free chemical bonds), which accelerate the process. The accumulation of a sufficient number of active centers contributes to the transition of the chain process to a thermal one and an increase in the temperature of the mixture to the point of its self-ignition. Such active centers arise as a result of an increase in the rate of thermal oscillatory motion molecules, and multiply due to chain branching. At the initial stages of reactions proceeding by a chain mechanism, the chemical energy of the reacting substances is transferred mainly into the formation of new active centers. The process of changing the concentration of active centers is described by the equation:
Where n – number of active centers in the reaction zone;
τ - time;
w 0 – rate of nucleation of active centers;
φ – constant characterizing the difference in the rates of branching and chain termination.
From the standpoint of the molecular kinetic theory (MKT) of the structure of matter, chemical combustion reactions occur as a result of the interaction of fuel and oxidizer molecules. The forces of molecular interaction between two components of a combustible mixture appear at a very short distance, and with increasing distance they sharply decrease. Therefore, interaction between fuel and oxidizer molecules is possible only when they are completely brought together, which can be considered as a collision. Consequently, the chemical reaction between the fuel and the oxidizer must be preceded by the mixing of components and the physical act of elastic collision of molecules.
The number of collisions of gas molecules per unit volume is easily calculated. So, for example, for a stoichiometric mixture of hydrogen and oxygen (2 N 2 + ABOUT 2) at a temperature of 288 TO and atmospheric pressure (~ 101325 Pa) number of collisions in 1 With in 1 cm 3 reaches 8.3·10 28. If all these collisions resulted in a chemical reaction, then the entire mixture would react very quickly. Practice shows that under these conditions the combustion reaction does not occur at all, because all these collisions do not lead to chemical interaction.
In order for a chemical reaction to occur, the reacting molecules must be in an excited state. Such excitation can be chemical when the atoms of the molecules have one or two free valences (such molecules are called radicals and are designated, for example, CH 3 , HE , CH 2, etc.) and physical when, as a result of slow heating, molecules acquire kinetic energy above the critical value.
Molecules that have the necessary energy reserve to break or weaken existing bonds are called active centers of a chemical reaction.
The difference between the average energy levels of molecules in the active state and those in the normal state, i.e. inactive, unexcited state, is called activation energy ( E A). The higher the numerical value of the activation energy, the more difficult it is to force a given pair of reagents to enter into a chemical reaction and vice versa. Therefore, activation energy is an indirect indicator of the degree of fire hazard of combustible substances.
The activation energy can be estimated using the formula:
Where E A– activation energy, J;
k – Boltzmann constant, equal to 1.38·10 –23 J/C;
T– absolute temperature, TO.
The nature of the main chemical combustion process depends on a number of physical processes:
– movement of reacting substances and reaction products (diffusion processes);
– release and distribution of heat (heat transfer processes);
– aero- and hydrodynamic conditions that ensure the transfer of heat and matter (convection processes).
The need to take these factors into account greatly complicates the study and theoretical description of combustion processes.
The combustion of solids that do not form a gas (vapor) phase when heated is heterogeneous and occurs at the phase interface, therefore, along with the factors discussed above that influence the nature of the process, the size and nature of the surface of the solid phase play an extremely important role (this is especially important for aerosols).
2.4.2.3. Ignition impulses.
For combustion to occur, in addition to the combustible substance and the oxidizer, an initial energy impulse (most often with the release of heat) is required, which causes the ignition of a small volume of the combustible mixture, after which the combustion spreads throughout the entire space in which it is distributed.
An ignition pulse can occur when physical, chemical and microbiological processes occur that contribute to the generation of heat. Depending on the nature of these processes, impulses are accordingly divided into physical, chemical, And microbiological
Since when a physical impulse acts on a system, heat is released that is not the result of a chemical process, this impulse is considered as a thermal one. The action of a thermal impulse causing heating of the system can be:
– contact– heat transfer is carried out due to the contact of the combustible mixture with its source;
– radiation– heat transfer to the combustible mixture occurs by electromagnetic radiation from the heating source;
– convection– heat transfer to the combustible system occurs by a substance (air or other gas in motion);
– hydraulic(dynamic) - heat generation due to a rapid decrease in the volume of the gas mixture, accompanied by an increase in the pressure of the latter.
The main sources of heat impulse are:
– open flame (temperature ~ 1500 °C);
– heated surfaces (temperature > 900 °C);
– mechanical sparks (temperature ~ 1200 °C)
– electric sparks (temperature up to 6000 °C).
With chemical and microbiological impulses, heat accumulation in the system occurs due to a chemical reaction, a physico-chemical process (for example, adsorption) and the activity of microorganisms for which the combustible substance is food.
2.4.2.4. The rate of combustion reactions.
The speed of the combustion process in general view determined by the equation:
Where A ,b – concentration of reacting components;
τ - time,
Where m, n – concentration of combustion products.
An increase in the combustion rate is accompanied by an increase in the amount of heat entering the system per unit time, and, as a consequence, an increase in the combustion temperature.
2.4.2.5. Combustion temperature.
During combustion, not all of the heat released is spent on increasing the temperature of the reaction mixture, since part of it is spent in the form of losses on:
– chemical and physical underburning, taken into account by the underburning coefficient ( β );
– electromagnetic radiation of a flame, depending on the temperature of the emitting body, its state of aggregation and chemical nature. This dependence is determined by the emissivity coefficient of the radiating body( ε ) and wavelength electromagnetic radiation;
– conductive-convective losses.
Based on this, there are 3 main types of temperatures in combustion processes:
– calorimetric;
– theoretical (calculated);
– factual.
The calorimetric temperature is achieved when all the heat released during the combustion process is spent on heating the combustion products, for example, during the combustion of benzene - 2533 TO, gasoline – 2315 TO, hydrogen – 2503 TO, natural gas – 2293 TO.
The theoretical (calculated) temperature is determined taking into account heat losses due to the dissociation of combustion products. Significant dissociation of combustion products of hydrocarbon flammable substances begins at temperatures > 2000 TO. Such high temperatures during fires in industrial conditions practically do not occur, so heat losses due to dissociation in these cases are, as a rule, not taken into account.
The actual combustion temperature is determined taking into account heat loss to the environment and for almost all combustible substances it is ~ 1300 – 1700 TO.
Combustion- a chemical process of combining substances with oxygen, accompanied by the release of heat and light. For combustion to occur, contact of the flammable substance with the oxidizing agent (oxygen, fluorine, chlorine, ozone) and with the ignition source is necessary, capable of transmitting the necessary energy impulse to the combustible system. Substances burn most rapidly in pure oxygen. As its concentration decreases, combustion slows down. Most substances stop burning when the oxygen concentration in the air decreases to 12...14%, and smoldering stops at 7...8% (hydrogen, carbon disulfide, ethylene oxide and some other substances can burn in the air at 5% oxygen).
The temperature at which a substance ignites and begins to burn is called ignition temperature. This temperature is not the same for different substances and depends on the nature of the substance, atmospheric pressure, oxygen concentration and other factors.
Self-ignition - a combustion process caused by an external heat source and heating of a substance without contact with an open flame.
Self-ignition temperature - the lowest temperature of a combustible substance at which a sharp increase in the rate of exothermic reactions occurs, ending in the formation of a flame. The auto-ignition temperature depends on pressure, the composition of volatile substances, and the degree of grinding of the solid.
The following types of combustion processes are distinguished: flash, combustion, ignition, spontaneous combustion.
Flash- rapid combustion of the combustible mixture, not accompanied by the formation of compressed gases.
Flash point- the lowest temperature of a combustible substance at which vapors or gases are formed above its surface that can flare up from an ignition source, but the rate of their formation is still insufficient for subsequent combustion.
Fire- the occurrence of combustion under the influence of an ignition source.
Ignition- fire accompanied by the appearance of a flame.
Flash point- the lowest temperature of a substance at which, under special test conditions, the substance emits flammable vapors and gases at such a rate that, after their ignition, a stable flaming combustion occurs. The ignition temperature is always slightly higher than the flash point.
Spontaneous combustion - the process of self-heating and subsequent combustion of certain substances without exposure to an open ignition source.
Chemical spontaneous combustion is the result of the interaction of substances with oxygen in air, water, or between the substances themselves. Vegetable oils, animal fats and rags, rags, and cotton wool soaked in them are prone to spontaneous combustion. Heating of these substances occurs due to oxidation and polymerization reactions, which can begin when normal temperatures(10...30 °C). Acetylene, hydrogen, methane mixed with chlorine spontaneously ignite in daylight; compressed oxygen causes spontaneous combustion of mineral oils; nitric acid - wood shavings, straw, cotton.
TO microbiological spontaneous combustion Many crop products are prone to this - raw grain, hay, etc., in which, at a certain humidity and temperature, the vital activity of microorganisms intensifies and a cobwebby gley (fungus) is formed. This causes an increase in the temperature of substances to critical values, after which self-acceleration of exothermic reactions occurs.
Thermal spontaneous combustion occurs during the initial external heating of a substance to a certain temperature. Semi-drying vegetable oils (sunflower, cottonseed, etc.), turpentine varnishes and paints can spontaneously ignite at temperatures of 80...100 °C, sawdust, linoleum - at 100 °C. The lower the spontaneous combustion temperature, the more fire hazardous the substance is.
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PHYSICAL AND CHEMICAL BASICS OF COMBUSTION PROCESSES
Chemical processes during combustion. The nature of flammable substances. Lecture 3
Fire and explosion hazard substances and materials- this is a set of properties that characterize their ability to initiate and spread combustion.
The consequence of combustion, depending on its speed and conditions of occurrence, can be a fire or explosion.
Fire and explosion hazardsubstances and materials are characterized by indicators, the choice of which depends on the aggregate state of the substance (material) and the conditions of its use.
When determining fire and explosion hazard Substances and materials are distinguished into the following states of aggregation:
gases - substances, pressure saturated vapors which under normal conditions (25°C and 101325 Pa) exceeds 101325 Pa;
liquids - substances whose saturated vapor pressure under normal conditions (25°C and 101325 Pa) is less than 101325 Pa. Liquids also include solid melting substances whose melting or dropping point is below 50°C;
solids and materials- individual substances and their mixed compositions with a melting point and drop point above 50°C, as well as substances that do not have a melting point (for example, wood, fabrics, peat;
dust - dispersed substances and materials with particle size less than 850 microns.
Combustion as a chemical reaction of oxidation of substances involving oxygen
Combustion - one of the first complex physical and chemical processes that man encountered at the dawn of his development. A process, having mastered which, he gained enormous superiority over the living beings and forces of nature around him.
Combustion - one of the forms of obtaining and converting energy, the basis of many technological production processes. Therefore, a person constantly studies and learns about combustion processes.
The history of combustion science begins with the discovery of M.V. Lomonosov: “Combustion is the combination of matter with air.” This discovery served as the basis for the discovery of the law of conservation of mass of substances in both their physical and chemical transformations. Lavoisier clarified the definition of the combustion process: “Combustion is the combination of a substance not with air, but with oxygen in the air.”
Subsequently, Soviet and Russian scientists A.V. made a significant contribution to the study and development of combustion science. Mikhelson, N.N. Semenov, Ya.V. Zeldovia, Yu.B. Khariton, I.V. Blinov and others.
The combustion process is based on exothermic redox reactions, which obey the laws of chemical kinetics, chemical thermodynamics and other fundamental laws (law of conservation of mass, energy, etc.).
Burning is a complex physical and chemical process in which flammable substances and materials, under the influence of high temperatures, enter into chemical reaction with an oxidizing agent (air oxygen), turning into combustion products, and which is accompanied by intense heat release and light glow.
The combustion process is based on a chemical oxidation reaction, i.e. compounds of initial combustible substances with oxygen. In the equations of chemical combustion reactions, nitrogen, which is contained in the air, is also taken into account, although it does not participate in combustion reactions. The air composition is conventionally assumed to be constant, containing 21% by volume oxygen and 79% nitrogen (by weight, respectively, 23% and 77% nitrogen), i.e. For every 1 volume of oxygen there are 3.76 volumes of nitrogen. Or for 1 mole of oxygen there are 3.76 moles of nitrogen. Then, for example, the reaction of methane combustion in air can be written as follows:
CH 4 + 2О 2 + 2´ 3.76 N 2 = CO 2 + 2H 2 O + 2 ´ 3.76 N 2
Nitrogen must be taken into account in the equations of chemical reactions because it absorbs part of the heat released as a result of combustion reactions and is part of the combustion products- flue gases.
Let's consider oxidation processes.
Hydrogen oxidation carried out by the reaction:
N 2 + 0.5O 2 = H 2 O.
Experimental data on the reaction between hydrogen and oxygen are numerous and varied. In any real (high-temperature) flame in a mixture of hydrogen and oxygen, the formation of the radical * OH or hydrogen atoms H and oxygen O is possible, which initiate the oxidation of hydrogen to water vapor.
Combustion carbon . The carbon produced in flames can be gaseous, liquid or solid. Its oxidation, regardless of its state of aggregation, occurs due to interaction with oxygen. Combustion can be complete or incomplete, which is determined by the oxygen content:
C + O 2 = CO 2(full) 2C + O 2 = 2CO (incomplete)
The homogeneous mechanism has not been studied (carbon in gaseous state). The interaction of carbon in the solid state is the most studied. This process can be schematically represented in the following stages:
1. delivery of the oxidizing agent (O 2 ) to the interface by molecular and convective diffusion;
2. physical adsorption of oxidizing molecules;
3. interaction of the adsorbed oxidizing agent with surface carbon atoms and the formation of reaction products;
4.desorption of reaction products into the gas phase.
Combustion carbon monoxide . The total combustion reaction of carbon monoxide will be written CO + 0.5O 2 = CO 2, although the oxidation of carbon monoxide has a more complex mechanism. The main principles of the combustion of carbon monoxide can be explained on the basis of the combustion mechanism of hydrogen, including the reaction of interaction of carbon monoxide with the hydroxide formed in the system and atomic oxygen, i.e. This is a multi-stage process:
* OH + CO = CO 2 + H;O + CO = CO 2
A direct reaction CO + O 2 -> CO 2 is unlikely, since real dry mixtures of CO and O 2 are characterized by extremely low combustion rates or cannot ignite at all.
Oxidation of protozoa hydrocarbon V.Methane burns to form carbon dioxide and water vapor:
CH 4 + O 2 = CO 2 + 2H 2 O.
But this process actually includes a whole series of reactions in which molecular particles with high chemical activity (atoms and free radicals) participate: * CH 3, * H, * OH. Although these atoms and radicals exist in the flame for a short time, they ensure rapid consumption of fuel. During the combustion of natural gas, complexes of carbon, hydrogen and oxygen arise, as well as complexes of carbon and oxygen, the destruction of which produces CO, CO 2, H 2 O. Presumably, the combustion scheme of methane can be written as follows:
CH 4 → C 2 H 4 → C 2 H 2 → carbon products + O 2 →C x U y O z → CO, CO 2, H 2 O.
Thermal decomposition, pyrolysis of solids
When the temperature of a solid combustible material increases, rupture occurs chemical bonds with the formation of simpler components (solid, liquid, gaseous). This process is called thermal decomposition or pyrolysis . Thermal decomposition of molecules organic compounds occurs in a flame, i.e. at elevated temperatures near the combustion surface. The patterns of decomposition depend not only on the fuel, but also on the pyrolysis temperature, the rate of its change, the size of the sample, its shape, the degree of decomposition, etc.
Let's consider the pyrolysis process using the example of the most common solid combustible material- wood
Wood is a mixture of a large number of substances of different structures and properties. Its main components are hemicellulose (25%), cellulose (50%), lignin (25%). Hemicellulose consists of a mixture of pentazanes (C 5 H 8 O 4), hexazans (C 6 H 10 O 5), polyuronides. Lignin It is aromatic in nature and contains carbohydrates associated with aromatic rings. On average, wood contains 50% C, 6% H, 44% O. It is a porous material, the pore volume in which reaches 50- 75%. The least heat-resistant component of wood is hemicellulose (220- 250°C), the most heat-resistant component- lignin (its intensive decomposition is observed at a temperature of 350- 450°C). So, wood decomposition consists of the following processes:
| № pp | Temperature, °C | Process characteristics |
| up to 120 - 150 | drying, physical removal bound water |
|
| 150 - 180 | Decomposition of the least stable components (luminic acids) with the release of CO 2, H 2 O |
|
| 250 - 300 | pyrolysis of wood with the release of CO, CH 4, H 2, CO 2, H 2 O, etc.; the resulting mixture is capable of ignition from an ignition source |
| 350 - 450 | Intensive pyrolysis with the release of the bulk of flammable substances (up to 40% of the total mass); the gaseous mixture consists of 25% H2 and 40% saturated and unsaturated hydrocarbons; maximum supply of volatile components to the flame zone is ensured; the process at this stage is exothermic; the amount of heat released reaches 5- 6% of lower calorific value Q ≈ 15000 kJ/kg |
|
| 500 - 550 | The rate of thermal decomposition decreases sharply; the release of volatile components stops (the end of pyrolysis); at 600 °C the evolution of gaseous products stops |
Pyrolysis of coal and peat occurs similarly to wood. However, the release of volatiles is observed at other temperatures. Coal consists of harder, heat-resistant carbon-containing components, and its decomposition occurs less intensely and at higher temperatures (Fig. 1).
Combustion of metals
According to the nature of combustion, metals are divided into two groups: volatile and non-volatile. Volatile metals have T pl.< 1000 K and T kip.< 1500 K . These include alkali metals(lithium, sodium, potassium) and alkaline earth (magnesium, calcium). Metal combustion is carried out as follows: 4 Li + O 2 = 2 Li2O . Non-volatile metals have T pl. > 1000 K and T kip. > 2500 K.
The combustion mechanism is largely determined by the properties of the metal oxide. The temperature of volatile metals is lower than the melting point of their oxides. Moreover, the latter are quite porous formations. When an ignition spark is brought to the surface of a metal, it evaporates and oxidizes.
When the vapor concentration reaches the lower flammable concentration limit, it ignites. The diffusion combustion zone is established at the surface, a large proportion of the heat is transferred to the metal, and it is heated to the boiling point.
The resulting vapors, freely diffusing through the porous oxide film, enter the combustion zone. Boiling of the metal causes periodic destruction of the oxide film, which intensifies combustion. Combustion products (metal oxides) diffuse not only to the metal surface, promoting the formation of a metal oxide crust, but also into the surrounding space, where they condense and form solid particles in the form of white smoke. The formation of white dense smoke is a visual sign of burning of volatile metals.
In non-volatile metals with high phase transition temperatures, when burned, a very dense oxide film is formed on the surface, which adheres well to the metal surface. As a result of this, the rate of diffusion of metal vapor through the film is sharply reduced and large particles, for example, aluminum or beryllium, are not able to burn. As a rule, fires of such metals occur when they are introduced in the form of chips, powders, and aerosols. They burn without producing dense smoke. The formation of a dense oxide film on the metal surface leads to the explosion of the particle. This phenomenon, especially often observed when particles move in a high-temperature oxidizing environment, is associated with the accumulation of metal vapors under the oxide film, followed by its sudden explosion. This naturally leads to a sharp intensification of combustion.
Dust burning
Dust - This disperse system, consisting of a gaseous dispersed medium (air) and a solid phase (flour, sugar, wood, coal, etc.).
The spread of flame through dust occurs due to the heating of the cold mixture by the radiant flow from the flame front. Solid particles, absorbing heat from the radiant flow, heat up and decompose, releasing flammable products that form flammable mixtures with air.
The aerosol, which has very small particles, quickly burns when ignited in the area affected by the ignition source. However, the thickness of the flame zone is so small that the intensity of its radiation is insufficient for the decomposition of particles, and stationary propagation of the flame over such particles does not occur.
An aerosol containing large particles is also incapable of stationary combustion. As the particle size increases, the specific heat exchange surface area decreases and the time it takes to warm them up to the decomposition temperature increases.
If the time of formation of a flammable vapor-air mixture before the flame front due to the decomposition of particles hard material longer than the duration of the flame front, combustion will not occur.
Factors influencing the speed of flame propagation through dust-air mixtures:
1. dust concentration (the maximum flame propagation speed occurs for mixtures slightly higher than the stoichiometric composition, for example, for peat dust at a concentration of 1- 1.5 kg/m3);
2. ash content (with an increase in ash content, the concentration of the flammable component decreases and the speed of flame propagation decreases);
Classification of dust according to explosion hazard:
I class - the most explosive dust (concentration up to 15 g/m 3);
II class - explosive up to 15-65 g/m 3
III class - the most fire hazardous > 65 g/m 3 T St ≤ 250°C;
IV class - fire hazardous > 65 g/m 3 T St > 250°C.
Oxygen-free combustion
There are a number of substances that, when their temperature rises above a certain level, undergo chemical decomposition, leading to a gas glow that is barely distinguishable from a flame. Gunpowder and some synthetic materials can burn without air or in a neutral environment (pure nitrogen).
Cellulose combustion (link - C 6 H 7 O 2 (OH) 3 - ) can be represented as an internal redox reaction in a molecule containing oxygen atoms that can react with the carbon and hydrogen of the cellulose unit.
Fire involved ammonium nitrate, can be maintained without oxygen supply. These fires are likely when there is a high content of ammonium nitrate (about 2000 tons) in the presence of organic matter, in particular paper bags or packaging bags.
An example is the accident in 1947. The ship “Grandcamp“settled in the port of Texas City with a cargo of about 2800 tons of ammonium nitrate. The fire started in a cargo compartment containing ammonium nitrate packed in paper bags. The captain of the ship decided not to extinguish the fire with water, so as not to spoil the cargo, and tried to extinguish the fire by battening down the deck hatches and letting steam into the cargo compartment. Such measures contribute to the worsening of the situation, intensifying the fire without access to air, as ammonium nitrate is heated. The fire started at 8 o'clock in the morning, and at 9 o'clock. At 15 minutes an explosion occurred. As a result, more than 200 people who crowded the port and watched the fire died, including the ship's crew and the crew of two four-man aircraft that circled the ship.
At 13:10 the next day, an explosion also occurred on another ship transporting ammonium nitrate and sulfur, which caught fire from the first ship the day before.
Marshall describes a fire that broke out near Frankfurt in 1961. Spontaneous thermal decomposition caused by a conveyor belt ignited 8.. tons of fertilizer, a third of which was ammonium nitrate and the rest- inert substances used as fertilizers. The fire lasted 12 hours. As a result of the fire, a large amount of toxic gases, which included nitrogen, were released.
Let's consider the physical and chemical foundations of the combustion process. Combustion is a complex, fast-flowing physical and chemical transformation of substances, accompanied by the release of heat and light.
Thus, for the combustion process to occur, the presence of three factors is required: a combustible substance, an oxidizer and an ignition source (pulse). Most often, the oxidizing agent is atmospheric oxygen, but some other substances can also play its role: chlorine, fluorine, bromine, iodine, nitrogen oxides, etc. Some substances (for example, compressed acetylene, nitrogen chloride, ozone) can explode producing heat and flame . The combustion of most substances stops when the oxygen concentration drops from 21 to 14-18%. Some substances, for example, hydrogen, ethylene, acetylene, can burn when the oxygen content of the air is up to 10% or less.
Random sparks can serve as ignition sources of various origins(electrical, resulting from the accumulation of static electricity, sparks from gas and electric welding, etc.), heated bodies, overheating of electrical contacts, etc.
A distinction is made between complete and incomplete combustion. Complete combustion processes occur with an excess of oxygen, and the reaction products are water, sulfur and carbon dioxides, i.e. substances that are not capable of further oxidation.
Depending on the properties of the combustible mixture, combustion can be homogeneous or heterogeneous. With homogeneous combustion, the combustible substance and the oxidizer have the same state of aggregation (for example, a mixture of combustible gas and air), and with heterogeneous combustion, the combustion substances have an interface (for example, the combustion of solid or liquid substances in contact with air).
Based on the speed of flame propagation, the following types of combustion are distinguished: deflagration (flame propagation speed is tens of meters per second), explosive (hundreds of meters per second) and detonation (thousands of meters per second). Fires are characterized by deflagration combustion.
It is customary to distinguish between lean and rich combustible mixtures depending on the ratio of fuel and oxidizer. Lean mixtures contain an excess of oxidizing agent, while rich mixtures contain fuel.
The combustion processes are as follows:
- - Flash - rapid combustion of a combustible mixture, not accompanied by the formation of compressed gases;
- - Ignition - the occurrence of combustion under the influence of an ignition source;
- - Ignition - ignition accompanied by the appearance of a flame;
- - 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;
- - Spontaneous combustion - spontaneous combustion accompanied by the appearance of a flame.
An explosion is an extremely rapid chemical (explosive) transformation, accompanied by the release of energy and the formation of compressed gases capable of producing mechanical work.
During a fire, people are exposed to the following dangerous factors: increased air temperature or individual items, open fire and sparks, toxic combustion products (for example, carbon monoxide), smoke, low oxygen content in the air, explosions, etc.
Let's evaluate the fire hazard (fire hazard) of various substances and materials, taking into account their state of aggregation (solid, liquid or gaseous). The main indicators of fire hazard are the auto-ignition temperature and ignition concentration limits.
Autoignition temperature is the minimum temperature of a substance or material at which a sharp increase in the rate of exothermic reactions occurs, ending in flaming combustion. The difference between this process and the combustion process is that during the latter process only the surface of the substance or material ignites, while during self-ignition combustion occurs throughout the entire volume. The process of self-ignition occurs only if the amount of heat released during the oxidation process exceeds its release into the environment.
Mixtures of flammable gases, vapors and dust with an oxidizer can only burn if there is a certain ratio of combustible substance in them. The minimum concentration of a flammable substance at which it is capable of igniting and spreading a flame is called the lower flammable concentration limit. The highest concentration at which combustion is still possible is called the upper flammability limit. The region of concentration between these limits represents the ignition region.
The values of the lower and upper flammability limits are not constant, but depend on the power of the ignition source, the content of inert components in the combustible mixture, the temperature and pressure of the combustible mixture. In addition to concentration limits, there are also temperature limits (lower and upper) of ignition, which are understood as those temperatures of a substance or material at which its saturated flammable vapors form concentrations in the oxidizing environment equal to the lower and upper concentration limits of flame propagation, respectively.
The flash point is the minimum temperature of a substance or material at which it emits flammable vapors and gases at such a rate that, in the presence of an ignition source, sustained combustion occurs. After this source is removed, the substance continues to burn. Thus, the ignition temperature characterizes the ability of a substance to burn independently.
Flash point (tfsp) is the minimum temperature of a flammable substance at which vapors or gases are formed above its surface that can flare up from a source. The rate of formation of flammable gases during a flash is not yet sufficient to cause a flame to occur.
Flash point is used to characterize all flammable liquids in terms of fire hazard. According to this indicator, all flammable liquids are divided into two classes: flammable (flammable liquids), which include liquids with a flash point of up to 61 ° C (gasoline, acetone, ethyl alcohol, etc.) and flammable (FL) with a flash point above 61 ° C (oil, fuel oil, formaldehyde, etc.).
Ignition temperature, flash point, and ignition temperature limits are indicators of fire hazard. Table 1.1 presents these indicators for some technical products.
Dusts of many solid flammable substances suspended in the air form flammable mixtures with it. The minimum concentration of dust in the air at which it ignites is called the lower concentration limit of dust ignition. The concept of an upper flammable concentration limit for dust is not applied, since it is impossible to create very large concentrations of dust in suspension.
GOST 12.1.004-76 “SSBT. Fire safety. General requirements» provides for the following classification of substances:
NG is a non-flammable substance, i.e. a substance incapable of combustion in an air atmosphere of normal composition;
TG is a low-flammability substance, i.e. a substance capable of burning under the influence of an ignition source, but not capable of spontaneous combustion after its removal;
GV is a flammable substance, i.e. a substance capable of burning independently after removing the ignition source;
GZ - flammable liquid, i.e. a liquid capable of burning independently after removing the ignition source and having a flash point above 61 (in a closed crucible) or 66 ° C (in an open crucible);
flammable liquid - a flammable liquid, i.e. a liquid that can burn independently after removing the ignition source and having a flash point not higher than 61 (in a closed crucible) or 66 ° C (in an open crucible);
GG - flammable gas, i.e. gas capable of forming flammable and explosive mixtures with air at temperatures not exceeding 55 ° C;
Explosive is an explosive substance, i.e. a substance capable of explosion or detonation without the participation of atmospheric oxygen.
In addition to the considered fire hazard characteristics of substances and materials, the concept of flammability of a substance or material is used, i.e., their ability to burn. Based on this criterion, all substances are divided into flammable (combustible), slow-burning (difficult to burn) and non-flammable (non-combustible).
Combustible substances and materials are those that continue to burn even after the ignition source is removed. Substances that are difficult to combust can ignite in air from an ignition source, but after its removal they cannot burn on their own. Non-flammable substances and materials cannot burn in air. For quantitative characteristics flammability of substances and materials use flammability index B, see formula 2.1
where Q u is the amount of heat received from the ignition source;
Q 0 is the amount of heat released by the sample during combustion during the test.
If the value of B is more than 0.5, then the materials are classified as combustible; for difficult-to-burn materials, B = 0.1-0.5, and for non-combustible materials, B is less than 0.1.