Spontaneous combustion of substances and materials. Spontaneous combustion and self-ignition of combustible substances

Parameter name	Meaning
Article subject:	Spontaneous combustion.
Rubric (thematic category)	Education

Spontaneous combustion is a process low temperature oxidation of dispersed materials, ending smoldering or flame burning. The tendency to spontaneous combustion of substances is determined by the complex of their physical and chemical properties : calorific value, heat capacity, thermal conductivity, specific surface area, bulk density and conditions of heat exchange with the external environment.

For process development spontaneous combustion: Of decisive importance is the possibility of accumulation in the material of heat released during oxidation (or the activity of microorganisms). The better the conditions for heat accumulation, the earlier spontaneous combustion begins at a lower temperature.

The processes of spontaneous combustion develop in materials at a fairly low temperature ( up to 250 o C) During a long time. Under such conditions, to maintain the spontaneous combustion process, there is not enough heat released during oxidation by the outer surface. A prerequisite is the involvement in the oxidation or decomposition reaction of the entire mass of the material. And the larger the mass, the easier the processes of self-heating and spontaneous combustion develop in it. Increasing the ambient temperature shortens the time to spontaneous combustion.

Can be distinguished two self-ignition mechanisms:

Thermal spontaneous combustion consists of the following. Many disperse materials interact with atmospheric oxygen already at normal temperature. AT conditions , favoring the accumulation of heat in the mass of the material, there is an increase in temperature. This, in turn, increases the rate of oxidation reactions, while raising the temperature etc. Eventually may happen spontaneous combustion of material .

Thermal spontaneous combustion is a physical and chemical process, the rate of which depends 1 ). on the rate of a chemical reaction, 2 ). supply of oxygen to the reacting surface and from 3 ).intensity of heat exchange of the material with the environment.

When dispersed materials are stored in air, oxygen penetrates into the material between the particles. Getting into the pores, oxygen is adsorbed in the surface layer, which causes an increase in temperature. The presence of a developed surface of a solid material with oxygen adsorbed on it is a necessary condition for the onset of thermal spontaneous combustion.

An essential role in the development of the spontaneous combustion process is played by porosityand adsorption capacity of the material . The more pores, the more developed the contact surface and the adsorption of oxygen on it. For this reason, materials with greater porosity are most prone to spontaneous combustion.

Self-heating of the mass of material is not uniform . Due to different heat dissipation conditions, a). the central zone of the volume heats up faster, than the surface, and at the initial stage of spontaneous combustion, the appearance of the material is preserved, although there is charring inside . Further on the charred surface develop smoldering processes, which can go to fiery burning. Since the intermediate product during spontaneous combustion of most organic substances is coal , then the main role is played by the laws of spontaneous combustion of coal.

It should be noted that a significant role in the spontaneous combustion of coal is played by its ability adsorb water vapor from the surrounding air. It was found that in this case, coal can be heated up to 65-70 about C . For example, when adsorbing 0.01 g H 2 O will stand out 22.6 J thermal energy.

Acceleration of the spontaneous combustion process contributes to A). heat accumulation, b. developed surface, c. easy flammability, i.e. low activation energy, and d. temperature increase. At the same time, spontaneous combustion also develops in the presence of e) in the substance. impurities.

For example, if in ammonium nitrate ( NH4NO3) there are no impurities, then its transportation and storage are safe. The decomposition temperature is within 200 o C. But with small additions organics or metal particles starts autocatalytic decomposition , and saltpeter ignites spontaneously at 110 o C. It is believed that autocatalysis is caused by liberated CO 2 and water vapor. The addition of oils to saltpeter also causes its explosive decomposition (In this regard, it is used to prepare explosives).

big role in spontaneous combustion hazards !!! plays duration of the period before spontaneous combustion . It is different for different substances.

Microbiological spontaneous combustion. To microbiological spontaneous combustion are prone, mainly, materials of vegetable origin. Οʜᴎ serve as a nutrient environment for bacteria and fungi.

Opportunities for the development of the microbiological process limited as the temperature self-heating of the material should not exceed 75 ° C. Because at higher temperatures microorganisms, as a rule, die. Examples microbiological spontaneous combustion can be called charring of wheat in heaps , self-heating of manure, etc. .

AT Auto-ignition of coal can involve adsorption, microorganisms (in the initial stage), and impurities. So, there were theories that the causes of spontaneous combustion of coal are iron sulfides (FeS), iron carbonates Fe (CO) 4, etc.
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Today it is believed that iron impurities mainly affect, regardless of the type of its chemical compounds.

Main indicators, characterizing the danger spontaneous combustion substances are considered by us in topic 4:

· self-heating temperature;

· smoldering temperature;

· conditions of thermal spontaneous combustion;

· the ability to explode and burn on contact with water, atmospheric oxygen and other oxidizing agents .

The latter indicator qualitatively characterizes the special fire hazard of substances, called pyrophoric.

To pyrophoric are substances that have a temperature self-ignition belowambient temperature , unlike most substances, which ignite spontaneously only as a result of external heating. Samoflammable substances are very flammable .

Self-igniting substances can be divided into three groups:

1. Self-igniting on contact with air: phosphorus, sulfurous metals, magnesium powder, coal, soot, etc.
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For example, in tracer bullets, fireworks, spontaneously combustible substances are used.

2. Flammable on contact with water - ϶ᴛᴏ alkali metals, their carbides, etc.
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For example, calcium carbide used in acetylene generators. Quicklime does not burn, but the heat released by its reaction with water can heat the materials to a self-ignition temperature.

3. The third group includes organic compounds that ignite on contact with oxygen and other oxidizing agents. (chlorine, bromine, nitrogen oxides); it's oils. This also includes substances resulting from endothermic reactions, such as acetylene, which, when exposed to heat or shock, decompose with the possible occurrence of an explosion.

Spontaneous combustion. - concept and types. Classification and features of the category "Spontaneous combustion." 2017, 2018.

Among the huge variety of chemical compounds there is a large group of substances that can ignite (explode) and burn when interacting with atmospheric oxygen, water and other substances. Substances and materials with a self-heating temperature below 50 °C are usually considered prone to chemical spontaneous combustion.

Substances that ignite on contact with air

These include:

Alkali metals - potassium, rubidium and cesium.

Carbides and hydrides of alkali metals.

Powdered metals - zinc, aluminum, iron, nickel,

cobalt, titanium, zirconium

Metal sulfides - sulfur pyrite or pyrite FeS 2.

White (yellow) phosphorus.

Phosphines, silanes, arsine, etc.

So, for example, alkali metal hydrides - sodium, potassium, rubidium and cesium intensively interact with air moisture according to the reaction:

MeH + H 2 O ¾® MeOH + H 2.

Among metal sulfides sulfur pyrite or pyrite FeS 2 is a component of fossil coals and ores of ferrous and non-ferrous metals. Other iron sulfides - FeS and Fe 2 S 3 - are formed in technological devices, pipelines and tanks, where sulfur-containing substances (sour oils and oil products, hydrogen sulfide gases, etc.) are processed, transported and stored. At temperatures up to 200°C, organic sulfur is hydrolyzed with the release of hydrogen sulfide, which reacts with iron corrosion products to form sulfide:

2Fe(OH) 2 + 3H 2 S ® Fe 2 S 3 + 6H 2 O .

At temperatures above 200 ° C, organic sulfur is able to stand out in its pure form and react with iron:

Fe+S ® FeS + 100 kJ.

Iron sulfides ignite spontaneously in air, which is a fairly common cause of fires and explosions in the mining and processing industries, as well as in transport. Sulfides of many other metals are also prone to self-heating and spontaneous combustion, especially when ground and in contact with moist air.

Substances that are flammable and cause combustion

when exposed to water

These include:

alkali metals.

Hydrides and carbides of alkali and alkaline earth metals.

organometallic compounds, etc.

alkali metals react with water with the release of hydrogen and a large amount of heat according to the general scheme:

2Me + 2H 2 O ® 2MeOH + H 2 + Q.

Many organometallic compounds extremely sensitive to oxygen - derivatives of alkali and alkaline earth metals, some elements of groups 3 and 5 of the periodic system. Their lower alkyl derivatives (methylates, ethylates, and others) ignite spontaneously in air. Derivatives of alkali and alkaline earth metals, as well as Be, Mo, Zn, Cd, Ga, In react violently with water, and many of them self-ignite the released hydrocarbon.

In addition to those mentioned, there is a large group of flammable substances that interact vigorously with water with the release of self-igniting gases in air. For example, metal silicides(Mg 2 Si, Fe 2 Si, etc.) are decomposed by water to form silane, which ignites spontaneously in air:

Mg 2 Si + 4H 2 O ® 2Mg (OH) 2 + SiH 4 + 646 kJ,

SiH 4 + 2O 2 ® SiO 2 + 2H 2 O + 1517 kJ.

Some inorganic compounds become very hot when interacting with water, such as calcium oxide CaO (quicklime). When a small amount of water gets on quicklime, it heats up to a bright glow and can set fire to combustible materials in contact with it.

Substances that ignite spontaneously when in contact with each other

These mainly include various oxidizing agents: oxygen, halogens, hydrogen peroxide, nitric acid and its salts, potassium permanganate, chromic anhydride, salts of chlorine acids, etc. They themselves are non-combustible, but upon contact with organic substances cause their chemical ignition.

pure oxygen extremely flammable. Many non-combustible substances in air become combustible in an atmosphere of oxygen (iron). The most dangerous oxygen in a compressed and liquefied state. Thus, mineral oils ignite on contact with compressed oxygen and explode with liquefied oxygen.

Halogens - chlorine, bromine, fluorine, iodine. Chlorine is the most widely used in industry. Mixtures of combustible gases (hydrogen, methane, ethane, ethylene, acetylene, and others) with chlorine self-ignite when exposed to light (i.e., these reactions are photocatalytic). Some of them, for example, with hydrogen, proceed with an explosion.

H 2 + Cl 2 ® 2HCl + Q.

A characteristic feature of the combustion of hydrocarbons in chlorine is the release of significant amounts of pure carbon in the form of soot.

Many metals and non-metals ignite spontaneously in halogen environments. In this case, the corresponding halide is formed and a large amount of heat is released. Some organic halogen derivatives explode on contact with alkali metals, such as tetrachloroethane.

Hydrogen peroxide H 2 O 2- a strong oxidizing agent, usually available as a 30% solution in water (perhydrol). An unstable compound, easily decomposes in the presence of traces of heavy metals (copper, iron, manganese, platinum group metals and others) and their ions with the release of atomic oxygen. At concentrations of 65% and above, hydrogen peroxide causes spontaneous combustion of many combustible substances: paper, sawdust, rags, alcohols, etc.

Nitric acid HNO 3- a strong oxidizing agent. Concentrated nitric acid acts vigorously on many metals and non-metals. Organic substances (straw, paper, sawdust and shavings, coal, oils, turpentine, ethyl alcohol, etc.) are destroyed and ignite when exposed to it.

Salts of nitric acid (nitrates, nitrates) less active than nitric acid. Of these, potassium KNO 3 , ammonia NH 4 NO 3 and sodium NaNO 3 nitrate are most widely used, mainly as mineral fertilizers and components of industrial explosives. Mixtures of saltpeter with many powdered combustible materials (sulphur, charcoal, soot, etc.) explode when heated, from impact and friction, with the release of a large amount of hot gases.

Potassium permanganate KMnO 4 causes spontaneous combustion of polyhydric alcohols (ethylene glycol, glycerin, etc.). When interacting with ammonium nitrate, ammonium permanganate NH 4 MnO 4 is very sensitive to heat, shock and friction.

Chromic anhydride CrO 3 is a very strong oxidizing agent. Upon contact with it, all classes of oxygen-containing organic compounds ignite: alcohols, esters, acids, etc.

Chlorates and Perchlorates - salts of the corresponding chlorine acids: chloric HClO 3 and perchloric HClO 4 . Chlorates and perchlorates (hypochlorites) are among the strongest oxidizing agents; their behavior is similar to saltpeter.

The considered list of substances, of course, is far from complete, their number is immeasurably greater. More complete information about these, as well as about the flammable properties of other substances, is contained in the reference and special literature.

spontaneous combustion chemical

Spontaneous combustion is studied by thermostating the material under study at a given temperature and establishing a relationship between the temperature at which combustion occurs, the size of the sample and the time it is heated in the thermostat.

The phenomena of chemical spontaneous combustion also include the ignition of a number of substances (for example, finely divided A1 and Fe, hydrides of Si, B and certain metals, organometallic compounds - organic aluminum, etc.) upon contact with air in no heating. The ability of in-in to spontaneous combustion in such conditions is called. pyrophoric. The peculiarity of pyrophoric substances is that their self-ignition temperature is below room temperature: - 200 ° C for SiH4, - 80 ° C for A1 (C2H5) 3. To prevent chemical spontaneous combustion, the procedure for joint storage of combustible substances and materials is strictly regulated.

The propensity for microbiological spontaneous combustion is possessed by combustible materials, especially moistened ones, serving as pi-tat. an environment for microorganisms whose vital activity is associated with the release of heat (peat, sawdust, etc.). For this reason, a large number of fires and explosions occur during storage of agricultural products. products (e.g. silage, moistened hay) in the elevators. For microbiological and chemical spontaneous combustion, it is characteristic that the temperature of self-heating does not exceed the usual values ​​of Tocr and m.b. negative. Materials with TSN above room temperature are capable of thermal spontaneous combustion.

In general, many have a tendency to all types of spontaneous combustion. solid materials with a developed surface (eg, fibrous), as well as some liquid and melting substances containing unsaturated compounds in their composition, deposited on a developed (including non-combustible) surface. Calculation of critical conditions for chemical, microbiol. and thermal spontaneous combustion is carried out according to equations (1) and (2). Experimental methods. definitions of the self-heating temperature and the auto-ignition temperature of the auto-ignition conditions are set out in spec. standard. Chemical spontaneous combustion is associated with the ability of substances and materials to enter into a chemical reaction with air or other oxidizing agents under normal conditions with the release of heat sufficient to ignite them. The most typical examples are cases of spontaneous combustion of oily rags or phosphorus in air, flammable liquids in contact with potassium permanganate, sawdust with acids, etc. Therefore, we say: “Oxidizers - fight!” - and we mean that the storage of substances and materials must meet the requirements of their compatibility.

Another type of chemical reactions of substances is associated with the interaction of water or moisture. At the same time, a temperature sufficient for spontaneous combustion of substances and materials is also released. Examples are substances such as potassium, sodium, calcium carbide, quicklime, etc. A feature of alkaline earth metals is their ability to burn even without oxygen. They themselves produce the oxygen necessary for the reaction, splitting the moisture of the air into hydrogen and oxygen under the influence of high temperature. That is why extinguishing such substances with water leads to an explosion of the resulting hydrogen.

And, finally, microbiological spontaneous combustion is associated with the activity of the smallest insects. They multiply in unprecedented quantities in compressed materials, eat everything organic and die there, releasing a certain temperature along with their decomposition, which accumulates inside the material. The most characteristic example is the spontaneous combustion of last year's stacks of hay.

After all of the above, it becomes clear that all types of spontaneous combustion have a purely conditional division. For most combustible substances, the process of spontaneous combustion looks like a combination of thermal, chemical and microbiological reactions.

Most often, spontaneous combustion in apartments is associated with improper storage of substances and materials that are stored on balconies (loggias) without protection from sunlight, in loosely closed containers, which ensures their heating and oxidation by atmospheric oxygen. Therefore, the main requirement of fire safety rules is the requirement of strict adherence to the instructions for the storage of substances and materials, which must necessarily be on the container with them or attached in the form of a passport for the material. In apartments and living rooms, it is allowed to store no more than 10 liters of paints, varnishes, gasoline, kerosene and other flammable and combustible liquids and no more than 12 liters of combustible gases. At the same time, storage of these substances is not allowed on balconies and loggias. In all cases, the storage of substances of unknown composition is prohibited.

Combustion is a chemical oxidation reaction, accompanied by the release of a large amount of heat and usually glow. The oxidizing agent in the combustion process can be oxygen, as well as chlorine, bromine and other substances.

In most cases, during a fire, the oxidation of combustible substances occurs with atmospheric oxygen. This type of oxidizing agent is adopted in what follows. Combustion is possible in the presence of a substance capable of burning, oxygen (air) and an ignition source. In this case, it is necessary that the combustible substance and oxygen are in certain quantitative ratios, and the ignition source has the necessary supply of thermal energy.

It is known that air contains about 21% oxygen. The combustion of most substances becomes impossible when the oxygen content in the air drops to 14-18%, and only some combustible substances (hydrogen, ethylene, acetylene, etc.) can burn when the oxygen content in the air is up to 10% or less. With a further decrease in the oxygen content, the combustion of most substances stops.

A combustible substance and oxygen are reacting substances and constitute a combustible system, and an ignition source causes a combustion reaction in it. The source of ignition can be a burning or heated body, as well as an electric discharge that has an energy reserve sufficient to cause combustion, etc.

Combustible systems are divided into homogeneous and heterogeneous. Homogeneous are systems in which the combustible substance and air are evenly mixed with each other (mixtures of combustible gases, vapors with air). The combustion of such systems is called kinetic combustion. Its rate is determined by the rate of a chemical reaction, which is significant at high temperatures. Under certain conditions, such combustion can be in the nature of an explosion or detonation. Heterogeneous are systems in which the combustible substance and air are not mixed with each other and have interfaces (solid combustible materials and non-sprayed liquids). In the process of combustion of inhomogeneous combustible systems, air oxygen penetrates (diffuses) through the combustion products to the combustible substance and reacts with it. Such combustion is called diffusion combustion, since its rate is determined mainly by a relatively slow diffusion process.

For ignition, the heat of the ignition source must be sufficient to convert combustible substances into vapors and gases and to heat them to the auto-ignition temperature. According to the ratio of fuel and oxidizer, combustion processes of lean and rich combustible mixtures are distinguished. Lean mixtures contain an excess of an oxidizing agent and have a lack of a combustible component. Rich mixtures, on the contrary, have an excess of a combustible component and a deficiency of an oxidizing agent.

The occurrence of combustion is associated with the obligatory self-acceleration of the reaction in the system. The process of self-acceleration of the oxidation reaction with its transition to combustion is called self-ignition. Self-acceleration of a chemical reaction during combustion is divided into three main types: thermal, chain and combined - chain-thermal. According to the thermal theory, the process of self-ignition is explained by the activation of the oxidation process with an increase in the rate of the chemical reaction. According to the chain theory, the process of self-ignition is explained by the branching of chemical reaction chains. In practice, combustion processes are carried out mainly according to the combined chain-thermal mechanism.

Spontaneous combustion is called a sharp increase in the rate of exothermic reactions that cause self-heating of substances, leading to combustion in the absence of an ignition source.

Depending on the cause of heat release in the initial phase of self-heating of substances and materials, there are thermal, microbiological and chemical spontaneous combustion.

Thermal self-ignition is called spontaneous combustion caused by self-heating, which arose under the influence of external heating of a substance above the self-heating temperature. Many substances and materials are prone to thermal spontaneous combustion, which include oils and fats, coals, etc.

Spontaneous combustion of oils and fats is often the cause of fires. There are three types of oils: mineral, vegetable and animal.

Mineral oils containing saturated hydrocarbons are not capable of spontaneous combustion. Waste mineral oils may contain unsaturated hydrocarbons capable of spontaneous combustion.

Vegetable (linseed, hemp, cottonseed, etc.) and animal (butter) oils are different from mineral oils in their composition. They are a mixture of glycerides of fatty acids: palmitic C 15 H 31 COOH, stearic C 17 H 35 COOH, oleic C 17 H 33 COOH, linoleic C 17 H 31 COOH, linolenic C 17 H 29 COOH, etc. Palmitic and stearic acids are limiting, oleic, linoleic and linolenic - unsaturated. Glycerides of saturated acids, and hence oils containing them in large quantities, are oxidized at temperatures above 150 ° C and are not capable of spontaneous combustion. Oils containing a large amount of glycerides of unsaturated acids are capable of spontaneous combustion.

Oils and fats can ignite spontaneously only under certain conditions:

b) with a large oxidation surface of oils and fats and low heat transfer;

c) if any combustible materials are impregnated with fats and oils;

d) at a certain compaction of the oiled material.

The amount of glycerides of unsaturated acids in oil and fat is judged by the iodine number of the oil, i.e. by the number of grams of iodine absorbed by 100 g of oil. The higher the iodine number of the oil, the more it is capable of spontaneous combustion (linseed oil has an iodine number in the range of 192-197, hemp - 145-167, castor - 82-86). If the iodine number of oils is less than 50, their spontaneous combustion is impossible.

Oils, fats or drying oils stored in barrels, bottles, tanks cannot ignite spontaneously, since the surface area of ​​their contact with air is very small. To create conditions for spontaneous combustion, it is necessary to increase the oxidation surface (moisten fibrous, porous materials). However, spontaneous combustion also requires that the oxidation surface be much larger than the heat transfer surface. Such conditions are created when oiled materials are stacked in heaps, stacks, packages and are close to one another. The ability of oils and fats to ignite spontaneously is greater, the more compacted the oiled material is. With a strong compression of materials, the probability of oxidation decreases due to the deterioration of the conditions for the diffusion of oxygen to the oil. The ability of oiled materials to ignite spontaneously increases in the presence of catalysts (metal salts - manganese, lead, cobalt).

The lowest temperature at which spontaneous combustion of oils and fats was observed in practice was 10-15°C. The induction period for self-ignition of oiled materials can range from several hours to several days.

The main cause of spontaneous combustion of coals is their ability to oxidize and adsorb vapors and gases at low temperatures. The rise in temperature to 60°C in the spontaneous combustion site is very slow and can be stopped by airing the stack. Starting from 60°C, the self-heating rate increases sharply, so this coal temperature is called critical. Spontaneous combustion of coals is facilitated by their grinding and the presence of pyrite and moisture. All fossil coals are divided into two categories according to their ability to spontaneous combustion: category "A" - dangerous (these include brown and hard coals), category "B" - stable (anthracite and black coals of grade T - Kuznetsk, Donetsk, etc. .).

To prevent spontaneous combustion of coals during storage:

1. limit the height of the stacks;

2. compact coal in stacks to prevent or limit air infiltration.

Iron sulfides FeS, FeS 2, Fe 2 S 3 are also capable of spontaneous combustion. The main reason for the spontaneous combustion of sulfides is their ability to react with atmospheric oxygen at ordinary temperatures with the release of a large amount of heat:

FeS 2 + O 2 = FeS + SO 2 + 222.3 kJ

At temperatures below 310°C, iron sulfides in industrial equipment are formed when hydrogen sulfide acts on iron corrosion products.

Spontaneous combustion of iron sulfides in industrial equipment is prevented by the following methods:

Protection against hydrogen sulfide from the processed or stored product by anti-corrosion coating of the inner surface of the equipment;

Blowing the equipment with steam or combustion products;

Filling the apparatus with water and slowly lowering it, which leads to the oxidation of sulfide without accelerating the reaction.

White (yellow) phosphorus is rapidly oxidized at room temperature. Therefore, it quickly ignites spontaneously with the formation of white smoke:

4P + 5O 2 \u003d 2P 2 O 5 + 3100.6 kJ

Phosphorus should be stored and cut under water, as in air it can ignite from the heat of friction.

Diethyl ether and turpentine are also capable of self-ignition in air. The cause of spontaneous combustion is the ability to oxidize in air at low temperatures.

Chemical spontaneous combustion is called spontaneous combustion, which arose as a result of the chemical interaction of substances. The group of substances that ignite spontaneously upon contact with water includes potassium, sodium, rubidium, cesium, calcium carbide and alkali metal carbides, alkali and alkaline earth metal hydrides, calcium and sodium phosphides, quicklime, sodium hydrosulfide, etc.

Alkali metals - potassium, sodium, rubidium and cesium - interact with water with the release of hydrogen and a significant amount of heat:

2Na + 2H 2 O \u003d 2NaOH + H 2

The released hydrogen spontaneously ignites and burns together with the metal only if the piece of metal is larger than a pea in volume.

Many substances, mostly organic, are capable of spontaneous combustion when mixed or in contact with oxidizing agents. Oxidizing agents that cause spontaneous combustion of such substances include compressed oxygen, halogens, nitric acid, sodium and barium peroxide, bleach, etc.

For example, acetylene, hydrogen, methane, ethylene mixed with chlorine spontaneously ignite in the light or from the light of burning magnesium.

Do not store halogens together with flammable liquids. In contact with nitric acid, turpentine and ethyl alcohol ignite spontaneously.

Microbiological called spontaneous combustion as a result of self-heating, which arose under the influence of the vital activity of microorganisms in the mass of a substance.

Milled peat, hay, clover, silage, leaves, malt, cotton, etc. have the greatest ability to spontaneous combustion. Under-dried materials are especially susceptible to spontaneous combustion. Moisture and heat promote the reproduction of microorganisms. Due to the poor thermal conductivity of plant materials, the heat released during decay is mainly used to heat the materials, the temperature rises and can reach 70 ° C. At this temperature, microorganisms die, but the process of increasing the temperature in plant materials does not end there. Some organic compounds char even at 70°C. The resulting porous coal tends to absorb vapors and gases. Adsorption is accompanied by the release of heat, and in the case of low heat transfer, the coal is heated already before the start of the oxidation process. As a result, the temperature of plant materials rises and reaches 200°C. At 200°C, fiber, which is part of plant materials, begins to decompose, which leads to charring and further intensification of oxidation.

Burnt documents

The remains of burnt papers and other objects made from burnt materials may contain forensically significant information and should be handled with the utmost care. No matter how much the paper is over-charred, if the sheet is preserved, then it is possible to establish the nature of the paper and the text written on it. According to the texture and composition, the expert can determine what kind of paper, plain or banknote, ruble, dollar or other foreign currency. Text on paper can also be restored if it is sufficiently well preserved. Therefore, at the scene of a fire, it is necessary: ​​a) as far as possible, do not touch and save the remnants of papers if a fire occurred in a bank, office, service room of a store, warehouse, etc.;

b) to stop the burning of papers, isolate them from the air supply by covering them with a saucepan, a tank, and similar improvised means. Blowing out or, moreover, water supply will result in the irretrievable loss of paper;

c) if documents or money are in a safe or an iron box (cabinet), do not open it immediately after a fire. The safe must be cool, otherwise the entry of air inside can lead to an outbreak and the rapid destruction of the contents by fire.

Rules for the removal of burnt papers are not considered here; it's best to have an expert do it, and it's the fireman's job to store these leftovers until he arrives.

The same applies to the burnt remains of some other organic materials. Modern expert capabilities make it possible, for example, by analyzing the ashes from a cigarette (by electron microscopy), to determine whether it was pure tobacco or with marijuana and other drugs.

Spontaneous combustion is the process of combustion in the absence of an external source of ignition. This happens with a sharp increase in the rate of exothermic reactions in a certain volume of matter, when the rate of heat release exceeds the rate of heat removal to the environment. Spontaneously combustible substances are substances whose self-heating temperature is lower than the auto-ignition temperature.

The main thing that needs to be done during the inspection of the fire site in the event of a version of spontaneous combustion is to establish:

The nature of the material or materials (substances, mixtures of substances) that were in the source zone at the time of the fire,

Volumes (geometric dimensions) and quantities of stored substance (material);

Storage conditions (ambient temperature, packaging, ventilation, etc.);

The history of the storage object (when it was stored, were there any signs of self-heating (smoke, smell), etc.)

Depending on the primary impulse that triggers the self-heating mechanism of the material, the following types of spontaneous combustion are distinguished:

thermal;

Chemical;

Microbiological.

Thermal spontaneous combustion

The exothermic process of material oxidation with atmospheric oxygen can be initiated by preheating this material to a certain temperature. This can happen upon contact with heated surfaces or a gaseous medium during the manufacture of the material, its storage or operation.

If thermal spontaneous combustion is suspected, in addition to the above information, it is necessary to find out:

Were there sources of additional heating of the material (furnaces, heaters, heating pipes, other heated surfaces);

What are the temperatures of these sources, mass, heating surface, duration of action, distance to the material;

Were there conditions for heat accumulation.

For example, sawdust, shavings, jute fiber, paper in bales, fine organic materials (flour, peat, oil shale concentrate, technological soot), some types of mineral wool and other heaters, etc. are prone to thermal spontaneous combustion.

Self-heating of wood begins at a temperature of 130-150 °C, however, with prolonged (for many years!) Heating, wood can go into the so-called "pyrophoric" state and catch fire at a temperature of 90-110 °C.

It is possible that a spontaneously combustible material, after heating during the production process (for example, during drying), is stored or transported uncooled, as a result of which spontaneous combustion occurs. A characteristic sign of spontaneous combustion in this case is the location of the focus in volume(in the depth of the material), and not on its surface. This circumstance, if it is revealed, must be reflected in the inspection report.

The location of the focus in the bulk of the material, closer to the center of the array, where the best conditions for heat accumulation and the lowest heat loss, is an important qualification feature of the spontaneous combustion process, not only thermal, but also microbiological.

Paint deposits in spray booths and their ventilation systems ignite spontaneously.

Spontaneous combustion of coal in heaps and stacks is possible. If you suspect this kind of reason, you need to find out:

Mark of stored coal;

Heap or stack dimensions;

Possible humidification before the fire;

The degree of grinding (lumpy, dust).

The tendency of a particular substance (material) to thermal spontaneous combustion can be established from reference data. If there is an unknown substance (material) or there is no reference data for it, it is necessary to take an unburned sample of this substance for experimental determination of the self-heating temperature and conditions of thermal spontaneous combustion in accordance with GOST 12.1.044-89. The requirements for the sample to be taken are specified in Appendix 4. With the known dimensions of the stored material, the tests will determine the minimum temperature of the medium and the duration of heating at which spontaneous combustion of this material can occur. These results can be compared with the actual data on the fire under investigation.

Chemical spontaneous combustion

Chemical spontaneous combustion is the result of the interaction of two substances with each other or with the environment (water, oxygen in the air), occurring with the release of a sufficient amount of heat.

It makes sense to consider this version if it is established that in the room where the fire occurred, there were substances prone to exothermic reaction with water, air or with each other. The presence in the zone of the focus of the destroyed container, as well as the remains of at least one of the substances, is also significant.

In air, for example, yellow and white phosphorus, alkali metals (lithium, potassium, sodium), alkali metal carbides ignite spontaneously (decompose in humid air with the release of acetylene). Due to oxidation in air, metal powders and powders (aluminum, zinc, cobalt, etc.) ignite spontaneously.

Vegetable and animal oils, turpentine and some other substances containing chemically active unsaturated C-C bonds are prone to spontaneous combustion. Natural drying oil, which is made from linseed oil, is even more prone to spontaneous combustion than linseed oil, because. desiccants are introduced into it, accelerating the oxidation and polymerization of the oil, which leads to its drying.

Mineral (petroleum) oils are prone to spontaneous combustion only when contaminated.

It must be borne in mind that spontaneous combustion of oils and other liquids is impossible in a vessel or if they are spilled in the form of a puddle or film on any surface. Only rags impregnated with liquid, cotton wool, wool, sawdust and other porous materials ignite spontaneously, on the developed surface of which good contact of oil with atmospheric oxygen is possible. Spontaneous combustion requires an optimal amount of oil on the surface of a porous material (not a lot, but not a little) and conditions for heat accumulation. They accelerate the spontaneous combustion of salts of cobalt, manganese, lead, and some other metals.

The lowest temperature at which spontaneous combustion of this kind was observed is 10-15 °C. The induction period is from several hours to several days.

If spontaneous combustion of oil and similar materials is suspected, it is necessary to find out:

Type, type of oil, fat;

What could have been impregnated, in what quantity, how long had it lain before the fire;

Presence of conditions for heat accumulation.

Chemical spontaneous combustion is also possible upon contact of a pair of substances (materials), one of which is a strong oxidizing agent, the other is an easily oxidized substance.

The former include salts of nitric acid (nitrate), potassium and sodium permanganate, chlorates, perchlorates, bichromates, chromic anhydride, concentrated sulfuric (more than 95%) and nitric acids, hydrogen peroxide, organic peroxides, etc.

The second - liquid organic substances (diatomic and trihydric alcohols, some hydrocarbons) and finely dispersed solid organic substances (for example, sawdust, granulated sugar and powder, etc.), the above metal powders.

If chemical spontaneous combustion associated with the exothermic interaction of two substances is suspected, it is mandatory to request information about the substances that could be (stored, transported) at the facility where the fire occurred.

When inspecting a fire site, you must:

a) examine the surrounding structures and objects to identify the zone of long-term low-temperature pyrolysis. As a rule, during spontaneous combustion (chemical, in particular), the released heat is not enough to ensure the instantaneous development of flame combustion. The process usually proceeds at the initial stage in the form of smoldering, in zones where there are conditions for heat accumulation, and only after some time passes into flame combustion. Therefore, we must try to identify and fix such smoldering zones;

b) take samples of coal in order to establish the temperature and duration of pyrolysis (see Chapter 5). This is necessary, in particular, to confirm the combustion regime in the area under study (smoldering or flaming combustion);

c) take samples for subsequent instrumental studies in order to detect residues of substances that reacted with each other in the focal zone.

Microbiological spontaneous combustion

It is typical for organic dispersed and fibrous materials, inside which the vital activity of microorganisms (hay, straw, vegetables, grain, milled peat, etc.) is possible.

When working out the version of microbiological spontaneous combustion, if possible, it is necessary to obtain the following data:

a) humidity of hay at the time of the fire (it is known that for microbiological spontaneous combustion, the humidity must be at least 16%);

b) the time elapsed after laying (the risk of spontaneous combustion persists up to 3-4 months; most likely it is within 10-30 days);

c) the dimensions of the haystack (according to the theoretical thermophysical assessment, they should be at least 2 × 2 × 2 m; with smaller dimensions, the haystack is not able to ignite, since heat losses to the environment are too high).

It is also important to find out the conditions for storing and drying hay. Perhaps a focal (so-called "nesting") occurrence of the process as a result of more moistened hay getting into the stack or moistening of individual sections through the leaky roof of the hay storage. "Plastic" self-ignition can start when moisture migrates in the mass of hay due to temperature differences, for example, during uneven heating or cooling - in this case, condensation moisture forms in the peripheral layers, near the surface.

Qualification signs of microbiological spontaneous combustion, detected during the inspection of the fire site:

1. The hearth is located in the center of a stack or array of another material prone to microbiological spontaneous combustion, and not outside. If a haystack has surface charring (burning), and there are no traces of burning inside, then this is not spontaneous combustion, but combustion that arose from an external source of open fire, sparks, etc.

2. The presence of undeveloped foci, including in separate bales. They are local hay agglomerates of varying degrees of thermal degradation (see Fig. 6.4).

Rice. 6.4. Zones that occur in hay during microbiological spontaneous combustion