Necessary conditions for the occurrence and propagation of combustion. Conditions for the initiation and termination of combustion

LECTURE 1

SECTION 1. Basic concepts of combustion

The phenomena observed during the burning of a candle are such that there is not a single law of nature that would not be affected in one way or another.

M. Faraday

TOPIC 1. BASICS OF COMBUSTION PROCESSES

Questions:

1. Determination of the combustion process, necessary and sufficient conditions for combustion. Combustion types.

2. The main characteristics of the flame. Flame temperature.

3. Classification of combustible substances, oxidants and ignition sources. Chemical reactions during combustion.

In the middle of the 18th century, M.V. Lomonosov first suggested that the combustion process is a process of interaction flammable substance with atmospheric oxygen, i.e. oxidation. The French scientist A. Lavoisier in 1772-76 experimentally confirmed this. In 1883, French chemists Malyard and Le Chatelier measured the normal speed of flame propagation. Representatives of the Russian and Soviet schools made an outstanding contribution to the creation and development of combustion theory. Our compatriot, physicist and meteorologist V.A. In the early 1900s, Mikhelson established the dependence of the speed of propagation of the flame front on the composition of the combustible mixture, laid the foundations for the thermal theory of explosive combustion, and developed the theory of gas combustion in a Bunsen burner.

The founder of the Soviet school of combustion, Nobel Prize winner, Academician N.N. Semenov developed the theory of branched chain reactions and thermal self-ignition (explosion). Academician Y.B. Zel'dovich and Professor D.A. Frank-Kamenetsky created the theory of flame propagation. The foundational research of our scientists has received worldwide recognition.

Combustion is fast (seconds or fractions of seconds), redox, exothermic,

self-sustaining process, often accompanied by glow and flame formation.

The absence of any of these signs will indicate that the process under consideration does not apply to combustion, for example, metal corrosion, light bulb glow, phosphorescence, etc.

The concept of combustion does not include slow reactions (low-temperature oxidation, biochemical oxidation) and very fast (explosive transformations). Combustion occurs not only due to the formation of oxides, but also due to the formation of fluorides, chlorides and nitrides. It has been established that oxygen-containing anhydrides, salts and acids of variable valence elements (sulfur, nitrogen, chromium, manganese, chlorine, etc.) can act as an oxidizing agent in combustion reactions.

Oxidation reactions are exothermic, therefore, during combustion, a large number of warmth. This is due to the high temperature of combustion processes, for example, wood - 700-800 ° C, oil products - 1300-1500 ° C. According to the Van't Hoff rule, with an increase in temperature for every 10 ° C, the reaction rate increases by 2-4 times, that is, the rate of the oxidation reaction must be high. It follows that the combustion processes are based on high-speed and high-temperature oxidation reactions. During combustion, volatile products heated to a high temperature are formed: C0 2, H 2 0, CO, etc. The density of incandescent combustion products is 3-5 times less than the density of the surrounding air. Therefore they are displaced fresh air up, i.e. above the combustion center there is a continuously rising convective flow hot determination of T c. Beginning with certain limit values, mixtures, both lean and rich, are not flammable. This is confirmed experimentally. For example, the curve of dependence Т с = f (C) for oxide

on the composition of the mixture

4. The combustion reaction rate depends on pressure and catalysts, therefore the autoignition temperature also depends on these factors (Table 1). Table 1 Changes in autoignition temperature depending on pressure

As you know, catalysts are divided into positive (accelerating) and negative (slowing down the reaction). Positive catalysts reduce the autoignition temperature, while negative catalysts increase it.

The walls of the vessel containing the combustible mixture can have catalytic properties. With an increase in the catalytic activity of the material of the vessel wall, Tc decreases.

The autoignition temperature of a mixture of combustible substances usually does not obey the additivity rule. For example, the autoignition temperature of a mixture of methanol and diethyl ether of different composition always lower than calculated according to the additivity rule.

Thus, the given data show that the temperature

self-ignition is really not a constant, but depends on

many factors. Its true value at point C in Fig. 2 can be experimentally determined only by direct measurement of the temperature. but modern facilities measurements do not yet allow this to be done with a sufficient degree of accuracy, since it is not known at what point in the volume of the combustible mixture the initial combustion center occurs. Thermal theory self-ignition suggests a way out of this situation. At the point of contact C, on the one hand, there is equality of heat release and heat removal. On the other hand, at point C, each function is tangent to the other, i.e. the derivatives with respect to temperature of q + and q_ must also be equal to each other In mathematical form, this will have the following form:

Qrop - V-k 0 -C r0 p-C 0 K-exp (-E / RT c) = a (T-To) -S (27)

and for derivatives:

Q r0p -V-k o -C r0p -C 0K -exp (-E / RTc) -E / RT c 2 = a-S (28)

Dividing (27) by (28), we get:

RT c 2 / E = T c - T 0. (29)

By simple mathematical transformations from this quadratic equation, you can find an expression for Tc, which will have the form: T c = To + RT c 2 / E. (thirty)

From Fig. 2 it can be seen that during self-ignition, the mixture in the vessel is heated from the temperature T 0 to T c. Calculations show that the difference between them is small. For example, for hydrocarbons it is only 30 ° C.

This circumstance is used in practice: the lowest temperature of the vessel wall at which autoignition occurs is taken as the autoignition temperature.

Since the autoignition temperature depends on the conditions for its determination (on the material of the vessel, its shape, dimensions, etc.), in order to exclude this moment, in our country and abroad, the same test conditions for all laboratories, fixed in GOST, are legally established. 12.1.044. It should be noted that this technique is universal and is used to determine the autoignition temperature of gases, liquids and solid combustible substances. Autoignition temperature is currently defined for many substances and can be found in reference literature. For alkanes, aromatic hydrocarbons and aliphatic alcohols, it can be approximately calculated from the conditional average length of the compound molecule.

Ø presence of a combustible substance,

Ø presence of an oxidizer

Ø presence of an ignition source.

The combustible substance and oxidizer must be heated to a certain temperature by the ignition source. In a steady combustion process, the combustion zone is a constant source of ignition, i.e. the area where the reaction takes place gives off heat and light.

Ignition sources:

Ø open fire,

Ø heat heating elements and devices,

Ø electrical energy,

Ø energy of mechanical sparks,

Ø discharges of static electricity and lightning,

Ø energy of the processes of self-heating of substances and materials (spontaneous combustion), etc.

Combustion of substances can be complete or incomplete. With complete combustion, products are formed that are incapable of further combustion (CO 2, H 2 O, HCl); in case of incomplete combustion, the resulting products are capable of further combustion (C, CO, CH, H 2 S, HCN, NH 3), as a rule, the products of incomplete combustion are toxic. An indication of incomplete combustion is the presence of smoke containing unburned carbon particles (soot). Combustion products are gaseous, liquid and solids, formed as a result of combining a combustible substance with oxygen during combustion. Their composition depends on the composition of the burning substance and the conditions of its combustion. Under fire conditions, organic substances (wood, fabrics, gasoline, plastic, rubber, etc.) most often burn, which mainly consist of carbon, hydrogen, oxygen and nitrogen. Less often, during a fire, inorganic substances burn, such as sulfur, phosphorus, sodium, potassium, aluminum, titanium, magnesium, etc.

With a change in the concentration of oxygen in the air, the intensity of combustion also changes. The combustion of most substances stops when the oxygen content in the air is less than 16%.

When heated, all liquid combustible substances and most solid ones, evaporating or decomposing, turn into gaseous ones, which form a combustible mixture with oxygen or other oxidizing agent. For the combustion of the gas-air mixture to begin, the presence of an external ignition source is not necessary; an increase in temperature to a certain limit is sufficient.

A fire, in addition to burning, includes the phenomena of mass and heat transfer that develop in time and space. These phenomena are interrelated and are characterized by fire parameters: burnout rate, temperature, etc. and are determined by a number of conditions, many of which are random.

The phenomena of mass and heat transfer are called common phenomena , i.e. characteristic of any fire, regardless of its size and location. Only the elimination of combustion can lead to their cessation. In a fire, the combustion process is not controlled by a person for a sufficiently long period of time. The consequence of this process is large material losses.

Common phenomena can lead to particular phenomena , i.e. those that may or may not occur on fires. These include: explosions, deformation and collapse of technological devices and installations, building structures, effervescence or release of oil products from tanks and other phenomena. The emergence and course of particular phenomena is possible only when certain favorable conditions are created on fires.

The fire is also accompanied by social phenomena causing society not only material damage. The death of people, thermal injuries and poisoning by toxic combustion products, the occurrence of panic at facilities with mass stay people, etc. - also phenomena occurring on fires. And they are also private, as they are secondary from the general phenomena accompanying the fire. This is a special group of phenomena that cause significant psychological overload and even stressful conditions in people.

Combustion is a chemical oxidation reaction accompanied by the release of heat and emission of light. Grief occurs and proceeds under certain conditions. It requires a combustible substance, oxygen and an ignition source.

For combustion to occur, a combustible substance must be heated to a certain temperature by an ignition source (flame, spark, incandescent body) or by the thermal manifestation of some other type of energy: chemical (exothermic reaction), mechanical (shock, compression, friction), etc. etc.

Vapors and gases released during heating of a combustible substance mix with air and oxidize, forming a combustible mixture. As heat accumulates as a result of the oxidation of gases and vapors, the rate of the chemical reaction increases, as a result of which self-ignition of the combustible mixture occurs and a flame appears.

With the appearance of a flame, combustion sets in, which, under favorable conditions, continues until the substance is completely burned out.

In a steady-state combustion process, a constant source of ignition is the combustion zone, that is, the area where a chemical reaction takes place, heat is released and light is emitted.

For the occurrence and course of combustion, combustible substance and oxygen must be in a certain quantitative ratio. The oxygen content in the air for most combustible substances must be at least 14-18%. "

Many are known different types combustion centers (burning of a candle, a powerful industrial furnace, fire of a building or structure, etc.). All of them differ significantly from each other and differ in the nature of the combustible substance, however, the main phenomena occurring during combustion and in the process of it are the same.

Consider the combustion process of a simple lamp (wax candles, stearic candles, etc.). A lit candle burns steadily in a normal air environment as long as there is enough fuel (wax, stearin, paraffin) for this. The candle will go out due to violation of one of the basic conditions

Combustion mechanism

Combustion is difficult physicochemical process... Most of the engine performance is influenced, however, not by the physicochemical features of the combustion process, but by the patterns of heat release and the changes in pressure and temperature in the cylinder caused by it. They determine energy and economic indicators cycle, static and dynamic loads on parts, estimated by the maximum cycle pressure p z and the rate of pressure rise during combustion (dp / d (j) max(MPa / ° f.c.h.) or (dp / dt) max(MPa / s), thermal stress of parts, estimated by the distribution of temperatures and heat fluxes, intensity of noise emission, to a certain extent mechanical losses in the engine and toxicity of exhaust gases. Favorable performance of the engine is ensured with heat generation starting 5-15 ° before V. m. t., causing a uniform increase in pressure in the range of angles of rotation of the crankshaft 15-30 ° and generally ending in 45-50 °. Heat use in a real cycle with such a character of heat release differs little from that which takes place in a cycle with heat supply at V = const, since the piston at V. m. t. moves at low speeds and therefore passes a short distance during the heat release. So, if the heat release ends 35 ° after V. m.t., then the degree of subsequent expansion of gases differs from the degree of compression by only 11-12%. In fact, gradual heat release is more advantageous than instantaneous heat release due to a decrease in heat loss to the cooling medium and mechanical losses of the engine. Physicochemical features the combustion process have a significant effect on the radiation of the flame, deposits on parts and the toxicity of the exhaust gases.

Combustion Theory... According to the concepts of kinetics chemical reactions, the act of reaction occurs when molecules collide, the energy of which exceeds a certain value for each of the reactions, sufficient to destroy existing intramolecular bonds and replace them with new ones. This critical energy value is called the activation energy, and the molecules themselves that enter into the reaction are called thermally active. The number of collisions per unit time of thermally active molecules increases significantly with temperature. It also depends on the nature of the reagents, their ratio in the mixture and pressure. With increasing pressure, the collision frequency increases due to an increase in the number of molecules of each of the reagents per unit volume, and to the greater extent than more molecules n m participates in an elementary act of reaction. The rate of chemical reactions, measured by the amount of a substance that has reacted in a unit of volume per unit of time [kg / (s m 3) or kmol / (s m 3)],

Here WITH- the concentration of the reagent; t- time; NS- collision constant, depending on the nature and ratio of reagents in the mixture; R- pressure ; n m- the order of the chemical reaction; Q a- activation energy, depending on the nature of the reagents, the reaction mechanism and state parameters; T- mixture temperature, mR is a universal gas constant.

The given dependence is valid for the case when the concentration of reagents is maintained unchanged. In reality, it changes. Therefore, in the course of the reaction, its speed reaches a maximum, and then decreases to zero.

The previously stated concepts of chemical reactions occurring as a result of the collision of thermally active molecules of the initial substances turned out to be insufficient to explain a number of observations, since: 1) the experimentally obtained dependences of the reaction rate on pressure often have a fractional positive exponent, although it is obvious that the reaction does not fractional number of molecules can participate; 2) the addition of certain substances, the so-called additives, to fuels significantly affects the combustion process, despite very low concentrations; 3) the dependence of the rates of pre-flame reactions on the state parameters deviates noticeably from that determined by (2.17) to the extent that, in a certain range, an increase in temperature is accompanied by a decrease in the reaction rate (negative temperature dependence); 4) a number of reactions occur at high rates without increasing the temperature of the mixture.

These and many other phenomena were explained on the basis of the theory of chain reactions, in the development of which an outstanding role belongs to the school of Soviet scientists headed by Acad. N.N.Semenov. In accordance with the concepts of this theory, the overwhelming majority of chemical reactions proceed according to a chain mechanism, that is, the initial substances pass into the final ones through a more or less long chain of separate reactions with the formation of a number of intermediate, often extremely unstable, compounds. The leading role in the development of the chain reaction is played by chemically active particles with free valences, which easily form a compound with the initial or intermediate products without thermal activation. As a result of these reactions, final products are obtained and at the same time a certain amount of the same or other active particles is formed again, which again enter into reactions, renewing the chain of transformations.

If, as a result of an elementary act of a chemically active particle with any molecule, only one active part is recreated, then there is a simple continuation of the reaction and it is unbranched. The rate of an unbranched chain reaction is determined by the number of active particles arising per unit time, and medium length chains. Chemically active particles are formed as a result of collisions or spontaneous decay of thermally active molecules. Therefore dependence w = f (p, T) for an unbranched chain reaction is similar to (2.17). In this case, some effective activation energy is considered, which characterizes the final dependence of the process rate on temperature. If, as a result of an elementary reaction with the participation of one active particle, two or more new active particles appear, then what is called chain branching takes place. The rate of this reaction increases very rapidly over time, even in the absence of an increase in temperature. The chain termination occurs when chemically active particles collide with each other and as a result of adsorption by their walls surrounding the reacting mixture. Therefore, an increase in the concentration of chemically active particles is accompanied by an increase in the number of chain breaks and, as a consequence, the rate of the branched chain reaction is stabilized and then decreases as a result of the burnout of the initial substances.

In accordance with the theory of chain reactions, the fractional order of the reaction is the result of a complex mechanism of the course of the reaction, which includes a number of elementary stages, each of which has its own order. Depending on the significance of each of the intermediate stages, one or another value of the exponent is obtained at R in (2.17). The fact that each reactive particle is the source of a whole series of transformations makes it possible to explain the accelerating or retarding effect of small amounts of fuel additives. Negative temperature dependence w is explained by the fact that an increase in temperature leads to an increase in the concentration of the intermediate reaction product, which inhibits the formation of final products.

The course of chemical reactions in piston engines is influenced by both thermal and chemical activation of particles. For different conditions one of the activation methods may be predominant. In most cases, however, the decisive influence is exerted by the thermal self-acceleration of reactions. The exception is the self-ignition process.

By burning - they call the physicochemical process, which is characterized by three features: chemical transformation, heat release, light emission

The basis of combustion is the redox reaction of a combustible substance with an oxidizing agent. Chlorine, bromine, sulfur, oxygen, oxygenates and other substances can be oxidizing agents.

However, most often it is necessary to deal with combustion in an atmosphere of air, while the oxidizing agent is oxygen in the air.

For combustion to occur, it is necessary to have:

flammable substance;

oxidizing agent;

source of ignition.

But even in this case, combustion will be possible if the combustible substance and oxygen or other oxidizer are in a certain quantitative ratio, and the thermal impulse has a supply of heat sufficient to heat the substances to the temperature of its ignition.

If there is little combustible substance mixed with air or little oxygen (less 14-16% ), the combustion process does not start.

Combustion can be caused by direct action on the combustible substance of an open flame or incandescent heat, weak but continuous and prolonged heating of the combustible substance, spontaneous combustion, chemical energy, mechanical energy (friction, impact, pressure), radiant heat energy heated to high temperatures air, etc.

Therefore, it is necessary to distinguish between the conditions necessary for the occurrence of combustion and the conditions necessary for the combustion process to proceed.

Combustion conditions:

1. The amount of oxygen in the air entering the combustion zone will be at least 14–16% , i.e. the substance and the oxidizing agent are in a certain quantitative ratio.

The temperature of the combustion zone, which is a constant source of ignition and a source of heating of the upper layer of a combustible substance, is higher than its ignition temperature.

3. The rate of diffusion of combustible gases and vapors (decomposition products of the substance) into the combustion zone will be slightly higher than the rate of combustion.

4. The amount of heat emitted by the combustion zone during the combustion of the substance will be sufficient to heat the surface layer to the temperature of its ignition.

If one of these conditions is absent, then the combustion process will not take place.

Fire hazard is the possibility of the occurrence or development of a fire, enclosed in any substance, condition or process.

From this definition, we can conclude that fire hazard represent substances and materials if they, by virtue of their properties, favor the initiation or development of a fire. Such substances and materials are classified as fire hazardous.

Classification of flammable substances

Flammable substances, according to their ability to burn, are divided into:

Flame retardant;

Non-flammable.

Combustible substances that can burn independently after removing the ignition source are called. Combustible substances, in turn, are divided into flammable and hardly flammable.

Flammable substance is a combustible substance that can ignite from short-term exposure to a match flame, spark, and similar low-energy ignition sources.

These include:

Flammable liquids(ГЖ):

Aniline GZh;

ethylene glycol GZh;

motor and transformer oils GZh;

acetone for flammable liquids;

flammable gasoline;

benzene flammable;

diethyl ether, etc.

GZh is a liquid capable of burning independently after removing the ignition source and having a flash point higher 66 0 WITH.

Flammable liquids - flammable liquids having a flash point not higher than 66 0 WITH.

Flammable gases(YY) :

propane, etc.

GG - a gas capable of forming flammable and explosive mixtures with air at temperatures not higher 55 0 WITH.

Combustible substances:

celluloid;

polystyrene;

naphthalene;

wood shavings;

paper, etc.

Flammable substances are called flammable substances that can ignite only under the influence of a powerful ignition source.

These include:

getinax;

PVC tiles;

wood.

Difficult flammable- are called substances that can burn under the influence of an ignition source, but are not capable of self-combustion after removing it.

These include:

sodium trichloroacetate ( Na (CH 3 СОО) Сl 3 );

aqueous solutions of alcohol;

ammonia water, etc.

Non-flammable are called substances that are incapable of combustion in an atmosphere of air of ordinary composition. These include brick, concrete, marble, and plaster. Among non-flammable substances, there are many highly flammable substances that give off flammable products or heat when interacting with water or with each other.

These include:

Calcium carbide ( CaC 2 );

Quicklime ( CaCO 3 );

Diluted acids with metals (sulfuric, hydrochloric);

Oxidants KMpO 4 , Ca 2 O 2 , O 2 , H 2 O 2 , BUT 3 , compressed and liquid oxygen.

Occupational safety and health and safety

Chemical process burning. Combustion factors. For the combustion process to proceed, three factors are required: the combustible substance of the oxidizer and the ignition source. Complete with an excess of oxygen, the combustion products are not capable of further oxidation.

74. Combustion chemical process. Combustion factors. Basic principles of extinguishing fires.

Combustion- this is a complex, rapidly proceeding physicochemical transformation of substances, accompanied by the release of heat and light. For the combustion process to proceed, three factors are required: a combustible substance, an oxidizer and an ignition source.

Oxidizing agent - air oxygen or some other substances: chlorine, fluorine, bromine, nitrogen oxide.

Ignition source- random sparks of various origins (electrical, static, etc.)

Distinguish between complete and incomplete combustion. Complete - with an excess of oxygen, the combustion products are not capable of further oxidation. Incomplete - occurs when there is a lack of oxygen and toxic and flammable products are formed.

According to the speed of flame propagation, they are distinguished: deflagration combustion - the speed of propagation is tens of m / s; explosive - hundreds of meters per second; detonation (thousands of meters per second)

Depending on the combustible mixture, combustion is: homogeneous (one state of aggregation at the oxidizer); heterogeneous.

Combustion processes:

Flash- rapid combustion of a combustible mixture, not accompanied by the formation of compressed gases.

Combustion - the occurrence of combustion under the influence of an ignition source.

Ignition - ignition accompanied by the appearance of a flame.

Spontaneous combustion- the phenomenon of a sharp increase in the rate of exothermic reactions, leading to the occurrence of combustion of a substance in the absence of an ignition source.

- self-ignition- spontaneous combustion, accompanied by the appearance of a flame.

One of effective means extinguishing fires are extinguishers. Currently, the manual fire extinguisher ОХП-10, air-foam ОВП-10 (Figure 10), carbon dioxide ОУ-2, ОУ-5, ОУ-8, mobile carbon dioxide fire extinguisher УП-2М and powder fire extinguishers-OP-1, OPS-6, OPS-10 (Figure 11).

Chemical foam manual fire extinguisher OHP-10 is designed to extinguish fires in the initial stage of their occurrence.

To activate the fire extinguisher, it is necessary to take it by the side and the bottom handles, turn the fire extinguisher cover down, and the handlerotate 180 °. In this case, the valve of the acid glass opens, the acidic part of the charge flows out of the glass and mixes with the alkaline part. Foam forms and pressure builds up in the extinguisher housing. Pressurized foam through showersthrown out. The duration of the action of the fire extinguisher is about 1 min, the length of the jet is 6-8 m, the productivity is 90 liters of foam.

Air-foam fire extinguishers are used to extinguish fires of various substances and materials, except alkali metals, electrical installations under voltage, and substances burning without air access.

To activate the fire extinguisher, press the trigger. In this case, carbon dioxide compressed in the cylinder through the socketthrows out the foaming agent solution. The fire extinguisher operates for 20 s, the jet length is 4.5 m.

except foam fire extinguishers use carbon dioxide fire extinguishers OU-2, OU-5 and OU-8

To extinguish fires, variousextinguishing agents... The most common are water. In addition to it, sand and other types of soil, various foams and powders are used.

Water cannot be used to extinguish oil products, fire in live electrical equipment, sodium, calcium and potassium carbides. Oil products and other substances, the density of which is less than water, float above it and spill over a large area, which can cause the fire to intensify. Water is a conductor of electric current; therefore, do not direct the jet of water at electrical equipment, as an electric shock may occur. Water reacts with alkali metal carbides to form flammable and explosive substances.

Sand and all other types of soil - universal remedy extinguishing small fires. It is thrown onto the fire with shovels, shovels or buckets so as to first localize the fire and then fill it up.