Combustion is a chemical reaction. Combustion (reaction)

Almost every day we all have to deal with one or another manifestation of the combustion process. In our article we want to tell in more detail what features this process includes from a scientific point of view.

It is the main component of the fire process. A fire begins with the occurrence of combustion, its development intensity is usually the path traveled by the fire, that is, the burning rate, and extinguishing ends with the cessation of combustion.

Combustion is usually understood as an exothermic reaction between a fuel and an oxidizer, accompanied by at least one of the following three factors: flame, glow, smoke formation. Due to the complexity of the combustion process, this definition is not exhaustive. It does not take into account such the most important features combustion, as the rapid occurrence of the underlying exothermic reaction, its self-sustaining nature and the ability for self-propagation of the process through the combustible mixture.

The difference between a slow exothermic redox reaction (iron corrosion, rotting) and combustion is that the latter occurs so quickly that heat is produced faster than it is dissipated. This leads to an increase in temperature in the reaction zone by hundreds and even thousands of degrees, to a visible glow and the formation of a flame. In essence, this is how flaming combustion is formed. If heat is released but there is no flame, then this process is called smoldering. In both processes, an aerosol of complete or incomplete combustion of substances occurs. It is worth noting that when some substances burn, the flame is not visible, and there is also no smoke emission; such substances include hydrogen. Too fast reactions (explosive transformation) are also not included in the concept of combustion.

A necessary condition for combustion to occur is the presence of a flammable substance, an oxidizer (in a fire, its role is played by oxygen in the air) and an ignition source. Direct combustion requires the presence of critical conditions according to the composition of the combustible mixture, geometry and temperature of the combustible material, pressure, etc. After combustion occurs, the flame itself or the reaction zone acts as the ignition source.

For example, methane can be oxidized by oxygen with the release of heat to methyl alcohol and formic acid at 500-700 K. However, for the reaction to continue, it is necessary to replenish heat due to external heating. This is not combustion. When the reaction mixture is heated to a temperature above 1000 K, the rate of methane oxidation increases so much that the released heat becomes sufficient to further continue the reaction, the need for external heat supply disappears, and combustion begins. Thus, the combustion reaction, once it occurs, is capable of supporting itself. This is the main one distinctive feature combustion process. Another related feature is the ability of a flame, which is a chemical reaction zone, to spontaneously spread through a flammable medium or combustible material at a speed determined by the nature and composition of the reaction mixture, as well as the process conditions. This is the main mechanism of fire development.

A typical combustion model is based on the oxidation reaction of organic substances or carbon with atmospheric oxygen. Many physical and chemical processes accompany combustion. Physics is about the transfer of heat into a system. Oxidation and reduction reactions are a chemical component of the nature of combustion. Hence, from the concept of combustion, a variety of chemical transformations arise, including the decomposition of initial compounds, dissociation and ionization of products.

The combination of a flammable substance or material with an oxidizing agent constitutes a flammable medium. As a result of the decomposition of flammable substances under the influence of an ignition source, a gas-vapor-air reaction mixture is formed. Combustible mixtures, which in composition (ratio of fuel and oxidizer components) correspond to the equation of a chemical reaction, are called mixtures of stoichiometric composition. They are the most dangerous in terms of fire: they ignite more easily, burn more intensely, ensuring complete combustion of the substance, as a result of which they release the maximum amount of heat.

Rice. 1. Shapes of diffusion flames

a – burning of a jet stream, b – burning of a spilled liquid, c – burning of forest litter

Based on the ratio of the amount of combustible material and the volume of oxidizer, lean and rich mixtures are distinguished: poor mixtures contain an abundance of oxidizer, rich mixtures contain combustible material in abundance. The minimum amount of oxidizer required for complete combustion of a unit of mass (volume) of a particular combustible substance is determined by the equation of the chemical reaction. When burning with the participation of oxygen, the required (specific) air flow rate for most combustible substances is in the range of 4-15 m 3 /kg. Combustion of substances and materials is possible only when there is a certain content of their vapors or gaseous products in the air, as well as when the oxygen concentration is not lower than a specified limit.

So, for cardboard and cotton, self-extinguishing occurs already at 14 vol. % oxygen, and polyester wool - at 16 vol. %. In the combustion process, as in other chemical processes, two stages are required: the creation of molecular contact between the reagents and the very interaction of fuel molecules with the oxidizer to form reaction products. If the rate of transformation of the initial reagents is determined by diffusion processes, i.e. transfer rate (vapors of flammable gases and oxygen are transferred to the reaction zone due to a concentration gradient in accordance with Fick's laws of diffusion), then this combustion mode is called diffusion. In Fig. 1 are given various shapes diffusion flames. In the diffusion mode, the combustion zone is blurred and a significant amount of incomplete combustion products is formed in it. If the combustion rate depends only on the rate of the chemical reaction, which is significantly higher than the rate of diffusion, then the combustion mode is called kinetic. He is characterized by more high speeds and completeness of combustion and, as a consequence, high rates of heat release and flame temperature. This regime occurs in pre-mixed mixtures of fuel and oxidizer. Hence, if the reagents in the chemical reaction zone are in the same (usually gas) phase, then such combustion is called homogeneous; when the fuel and oxidizer are in different phases in the reaction zone, it is called heterogeneous. The combustion of not only gases is homogeneous, but also most solids. This is explained by the fact that in the reaction zone it is not the materials themselves that burn, but their vapors and gaseous decomposition products. The presence of a flame is hallmark homogeneous combustion.

Examples of heterogeneous combustion are the combustion of carbon, carbonaceous wood residues, and non-volatile metals, which remain in a solid state even at high temperatures. The chemical combustion reaction in this case will occur at the interface between the phases (solid and gaseous). Note that the end products of combustion can be not only oxides, but also fluorides, chlorides, nitrides, sulfides, carbides, etc.

The characteristics of the combustion process are varied. They can be divided into the following groups: shape, size and structure of the flame; flame temperature, its emissivity; heat release and calorific value; burning rate and concentration limits of sustainable combustion, etc.

Everyone knows that combustion produces a glow that accompanies the combustion product.

Let's consider two systems:

• gaseous system
• condensed system

In the first case, when combustion occurs, the entire process will occur in the flame, while in the second case, part of the reactions will occur in the material itself or its surface. As mentioned above, there are gases that can burn without a flame, but if we consider solids, there are also groups of metals that are also capable of burning without a flame.

Part of the flame with maximum value, where intense transformations occur, is called the flame front.

Heat exchange processes and diffusion of active particles from the combustion zone, which are the key mechanisms for the movement of the flame front through the combustible mixture.

The speed of flame propagation is usually divided into:

• deflagration (normal), occurring at subsonic speeds (0.05-50 m/s)
• detonation, when speeds reach 500-3000 m/s.

Rice. 2. Laminar diffusion flame

Depending on the nature of the speed of the gas flow creating the flame, laminar and turbulent flames are distinguished. In a laminar flame, the movement of gases occurs in different layers, all processes of heat and mass transfer occur through molecular diffusion and convection. In turbulent flames, the processes of heat and mass transfer are carried out mainly due to macroscopic vortex motion. A candle flame is an example of a laminar diffusion flame (Fig. 2). Any flame higher than 30 cm will already have random gas mechanical instability, which is manifested by visible swirls of smoke and flame.

Rice. 3. Transition from laminar to turbulent flow

A very clear example of the transition of a laminar flow to a turbulent one is a stream of cigarette smoke (Fig. 3), which, having risen to a height of about 30 cm, acquires turbulence.

During fires, flames have a diffusion turbulent character. The presence of turbulence in the flame increases heat transfer, and mixing affects chemical processes. In a turbulent flame, the burning speed is also higher. This phenomenon makes it difficult to transfer the behavior of small-scale flames to large-scale flames with greater depth and height.

It has been experimentally proven that the combustion temperature of substances in air is much lower than the combustion temperature in an atmospheric oxygen environment

In air the temperature will fluctuate from 650 to 3100 °C, and in oxygen the temperature will increase by 500-800 °C.

Combustion is one of the most interesting and vital natural phenomena for people. Combustion is beneficial for a person as long as it does not go beyond the control of his rational will. Otherwise, it may cause a fire. Fire - This is an uncontrolled burning that causes material damage, harm to the life and health of citizens, and the interests of society and the state. To prevent fire and eliminate it, knowledge about the combustion process is necessary.

Combustion - This chemical reaction oxidation, accompanied by the release of heat. For combustion to occur, a combustible substance, an oxidizer, and an ignition source must be present.

Flammable substance is any solid, liquid or gaseous substance that can oxidize and release heat.

Oxidizing agents may contain chlorine, fluorine, bromine, iodine, nitrogen oxides and other substances. In most cases, during a fire, oxidation of combustible substances occurs with atmospheric oxygen.

Ignition source provides an energetic effect on the combustible substance and oxidizer, leading to combustion. Ignition sources are usually divided into open (luminous) - lightning, flame, sparks, incandescent objects, light radiation; and hidden (non-luminous) - heat of chemical reactions, microbiological processes, adiabatic compression, friction, impacts, etc. They have different flame and heating temperatures. Any ignition source must have a sufficient supply of heat or energy transferred to the reacting substances. Therefore, the duration of exposure to the ignition source also influences the combustion process. After the combustion process begins, it is supported by thermal radiation from its zone.

The combustible substance and the oxidizer form fuel system, which can be chemically heterogeneous or homogeneous. In a chemically heterogeneous system, the combustible substance and the oxidizer are not mixed and have an interface (solid and liquid flammable substances, jets of flammable gases and vapors entering the air). When such systems burn, atmospheric oxygen continuously diffuses through the combustion products to the combustible substance and then enters into a chemical reaction. This kind of combustion is called diffusion. The rate of diffusion combustion is low, since it is slowed down by the diffusion process. If a flammable substance in a gaseous, vaporous or dusty state is already mixed with air (before it is ignited), then such a combustible system is homogeneous and its combustion process depends only on the speed of the chemical reaction. In this case, combustion occurs quickly and is called kinetic.

Combustion can be complete or incomplete. Complete combustion occurs when oxygen enters the combustion zone in sufficient quantity. If there is not enough oxygen to oxidize all the products involved in the reaction, incomplete combustion occurs. The products of complete combustion include carbon dioxide and sulfur dioxide, water vapor, and nitrogen, which are not capable of further oxidation and combustion. The products of incomplete combustion are carbon monoxide, soot and products of decomposition of matter under the influence of heat. In most cases, combustion is accompanied by the appearance of intense light radiation - a flame.

There are a number of types of combustion: flash, ignition, ignition, spontaneous combustion, spontaneous ignition, explosion.

Flash – this is the rapid combustion of a combustible mixture without the formation of increased gas pressure. The amount of heat generated during a flash is not enough to continue combustion.

Fire - This is the occurrence of combustion under the influence of an ignition source.

Ignition – a fire accompanied by the appearance of a flame. At the same time, the rest of the mass of the combustible substance remains relatively cold.

Spontaneous combustion – the phenomenon of a sharp increase in the rate of exothermic oxidation reactions in a substance, leading to its combustion in the absence of an external ignition source. Depending on the internal causes, spontaneous combustion processes are divided into chemical, microbiological and thermal. Chemical spontaneous combustion occurs from exposure of substances to oxygen in air, water, or from the interaction of substances. Oily rags, overalls, cotton wool and even metal shavings ignite spontaneously. The reason for spontaneous combustion of oiled fibrous materials is the distribution of fatty substances in a thin layer on their surface and the absorption of oxygen from the air. Oil oxidation is accompanied by the release of heat. If more heat is generated than heat loss to the environment, then combustion may occur without any heat supply. Some substances spontaneously ignite when interacting with water. These include potassium, sodium, calcium carbide and alkali metal carbides. Calcium ignites when it interacts with hot water. Calcium oxide (quicklime), when interacting with a small amount of water, becomes very hot and can ignite flammable materials in contact with it (for example, wood). Some substances spontaneously combust when mixed with others. These primarily include strong oxidizing agents (chlorine, bromine, fluorine, iodine), which, when in contact with certain organic substances, cause their spontaneous combustion. Acetylene, hydrogen, methane, ethylene, and turpentine spontaneously ignite in light when exposed to chlorine. Nitric acid, also being a strong oxidizing agent, can cause spontaneous combustion of wood shavings, straw, and cotton. Microbiological spontaneous combustion lies in the fact that with appropriate humidity and temperature in plant products and peat, the vital activity of microorganisms is intensified. At the same time, the temperature rises and a combustion process may occur. Thermal spontaneous combustion occurs as a result of prolonged exposure to a small heat source. In this case, the substances decompose and, as a result of increased oxidative processes, self-heat. Semi-drying vegetable oils(sunflower, cotton, etc.), castor drying oil, turpentine varnishes, paints and primers, wood and fiberboard, roofing cardboard, nitrolinoleum and some other materials and substances can spontaneously ignite at temperatures environment 80 - 100? C.

Self-ignition - This is spontaneous combustion accompanied by the appearance of a flame. Solid and liquid substances, vapors, gases and dusts mixed with air can spontaneously ignite.

Explosion (explosive combustion) is an extremely fast combustion, which is accompanied by the release of a large amount of energy and the formation of compressed gases capable of causing mechanical destruction.

Types of combustion are characterized temperature parameters, the main ones are the following. Flash point - this is the lowest temperature of a flammable substance at which vapors or gases are formed above its surface that can briefly flare up in the air from an ignition source. However, the rate of formation of vapors or gases is still insufficient to continue combustion. Flash point - this is the lowest temperature of a flammable substance at which it emits flammable vapors or gases at such a speed that, after ignition from an ignition source, stable combustion occurs. Auto-ignition temperature - this is the lowest temperature of a substance at which a sharp increase in the rate of exothermic reactions occurs, ending in ignition. The auto-ignition temperature of the studied solid combustible materials and substances is 30 – 670 °C. Most low temperature has self-ignition white phosphorus, the highest is magnesium. For most wood species, this temperature is 330 - 470? C.

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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 state of aggregation substance (material) and 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 processes production. 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 during 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 physico-chemical process in which flammable substances and materials, under the influence of high temperatures, enter into a chemical interaction 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

The direct reaction CO + O 2 -> CO 2 is unlikely, since real dry mixtures of CO and O 2 are extremely characterized low speeds burning 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, chemical bonds are broken 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:

Pyrolysis of coal and peat occurs similarly to wood. However, the release of volatiles is observed at other temperatures. Coal consists of more solid, heat-resistant carbon-containing components, and its decomposition proceeds less intensively 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 dense white 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 is a dispersed 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.

Aerosol having a very fine particles, when ignited, it quickly burns out 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 in front of the flame front due to the decomposition of particles of solid material is longer than the time of existence of the flame front, then combustion will not occur.

Factors influencing the speed of flame propagation through dust-air mixtures:

1. dust concentration ( maximum speed flame propagation 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 packing 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 number of poisonous gases that included nitrogen.

Topic 3. CHEMICAL BASES OF COMBUSTION.

3.1. Chemistry of combustion reactions.

As you already understood, combustion is a fast-flowing chemical reaction accompanied by the release of heat and glow (flame). Typically, this is an exothermic oxidative reaction of a flammable substance combining with an oxidizing agent - atmospheric oxygen.

Flammable substances there can be gases, liquids, and solids. These are H 2, CO, sulfur, phosphorus, metals, C m H n (hydrocarbons in the form of gases, liquids and solids, i.e. organic matter. Natural hydrocarbons, for example, are natural gas, oil, coal). In principle, all substances capable of oxidation can be flammable.

Oxidizing agents serve: oxygen, ozone, halogens (F, Cl, Br, J), nitrous oxide (NO 2), ammonium nitrate (NH 4 NO 3), etc. For metals, CO 2, H 2 O, N 2 can also be oxidizing agents .

In some cases, combustion occurs during decomposition reactions of substances obtained in endothermic processes. For example, during the decomposition of acetylene:

C 2 H 2 = 2 C + H 2.

Exothermic reactions are reactions that involve the release of heat.

Endothermic reactions are reactions that involve the absorption of heat.

For example:

2H 2 + O 2 = 2H 2 O + Q – exothermic reaction,

2H 2 O + Q = 2H 2 + O 2 – endothermic reaction,

where: Q – thermal energy.

Thus, endothermic reactions can only occur with the introduction of external thermal energy, i.e. when heated.

In chemical reactions, according to the law of conservation of mass, the weight of substances before the reaction equal to weight substances formed after the reaction. When balancing chemical equations, we get stoichiometric compositions.

For example, in the reaction

CH 4 + 2O 2 = CO 2 + 2H 2 O

we have 1 mol CH 4 + 2 mol O 2 = 1 mol CO 2 + 2 mol H 2 O.

The number of moles in front of the formulas of substances is called stoichiometric coefficients.

Taking into account the concepts of “molar volume”, “molar concentration”, “partial pressure”, we find that for the complete reaction of methane it is necessary to mix 1 mole of CH 4 with 2 moles of O 2, or 1/3 = 33.3% CH 4 and 2/ 3=66.7% O 2. This composition is called stoichiometric.

If we consider the combustion of CH 4 in air, i.e. in a mixture of 21% O 2 +79% N 2 or O 2 +79/21N 2 or O 2 +3.76N 2, then the reaction will be written as follows:

CH 4 +2O 2 +2×3.76N 2 =CO 2 +2H 2 O+2×3.76N 2.

1 mol CH 4 +2 mol O 2 +7.52 mol N 2 = 10.52 mol mixture of O 2, N 2 and CH 4.

Then the stoichiometric composition of the mixture will be:

(1/10.52)*100%=9.5% CH 4; (2/10.52)*100%=19.0% O 2 ;

(7.52/10.52)*100%=71.5% N 2.

This means that in the most flammable mixture, instead of 100% (CH 4 + O 2) in the reaction with oxygen there will be 24% (CH 4 + O 2) in the reaction with air, i.e. Much less heat will be generated.

The same picture will be obtained if arbitrary, non-stoichiometric compositions are mixed.

For example, in the reaction 2CH 4 +2O 2 =CO 2 +2H 2 O+CH 4 1 mole of CH 4 does not react.

In reaction CH 4 +4O 2 =CO 2 +2H 2 O+2O 2 2 moles of O 2 do not participate in the reaction, but play the role of ballast, requiring some amount of heat to heat up.

Thus, if we compare the combustion reactions of methane in oxygen and air or in excess CH 4 and O 2, it is clear that the amount of heat released in the first reaction will be greater than in the others, since in them:

Less concentrations of reactants in the overall mixture;

Part of the heat will go to heating the ballast: nitrogen, oxygen or methane.

Let's ask ourselves questions:

What energy can be released during the reaction?

What determines the amount of heat, i.e. thermal effect re-

How much thermal energy must be added to make it flow?

endothermic reaction?

For this purpose, the concept of heat content of a substance was introduced.

3.2. Heat content of substances.

Where does the heat come from in the methane combustion reaction? This means that it was hidden in the CH 4 and O 2 molecules, and now it has been released.

Here's an example of a simpler reaction:

2H 2 +O 2 =2H 2 O+Q

This means that the energy level of the stoichiometric mixture of hydrogen and oxygen was higher than that of the reaction product H 2 O and the “extra” energy was released from the substance.

In the reverse reaction of water electrolysis, i.e. decomposition of water with the help of electrical energy, a redistribution of atoms in a water molecule occurs with the formation of hydrogen and oxygen. At the same time, the heat content of H 2 and O 2 increases.

Thus, each substance, during its formation, receives or will give up a certain energy, and the measure of thermal energy accumulated by a substance during its formation is called heat content, or enthalpy.

Unlike chemistry, in chemical thermodynamics the heat of formation of a substance is denoted not by the symbol Q, but by the symbol DH with a sign (+) if the heat is absorbed by a chemical compound, and with a sign (-) if the heat is released during the reaction, that is, it “leaves” from systems.

The standard heat of formation of 1 mole of a substance at a pressure of 101.3 kPa and a temperature of 298 K is denoted.

The reference books give the heat of formation of compounds from simple substances.

For example:

Y CO 2 = - 393.5 kJ/mol

U H 2 O gas = - 241.8 kJ/mol

But for substances formed during endothermic processes, for example, acetylene C 2 H 2 = +226.8 kJ/mol, when the hydrogen atom H + is formed according to the reaction H 2 = H + + H + = +217.9 kJ/mol.

For pure substances consisting of one chemical element in a stable form (H 2, O 2, C, Na, etc.) DH is conventionally assumed to be zero.

However, if we discuss the macroscopic properties of substances, we distinguish several forms of energy: kinetic, potential, chemical, electrical, thermal, nuclear energy and mechanical work. And if we consider the issue at the molecular level, then these forms of energy can be explained based on only two forms - the kinetic energy of movement and the potential rest energy of atoms and molecules.

In chemical reactions, only the molecules change. The atoms remain unchanged. Molecule energy is the binding energy of its atoms accumulated in a molecule. It is determined by the forces of attraction of atoms to each other. In addition, there is potential energy attraction of molecules to each other. It is small in gases, larger in liquids, and even larger in solids.

Each atom has energy, part of which is associated with electrons, and part with the nucleus. Electrons have kinetic energy of rotation around the nucleus and potential electrical energy attraction to each other and repulsion from each other.

The sum of these forms of molecular energy is the heat content of the molecule.

If we sum up the heat content of 6.02 × 10 23 molecules of a substance, we obtain the molar heat content of this substance.

Why the heat content of single-element substances (molecules of one element) is taken to be zero can be explained as follows.

DH of a chemical element, that is, the energy of its formation, is associated with intranuclear processes. Nuclear energy is associated with the forces of interaction between intranuclear particles and the transformation of one chemical element into another during nuclear reactions. For example, the decay reaction of uranium:

or simpler: U+n®Ba+Kr+3n.

Where: n ’ o– a neutron particle with mass 1 and zero charge.

Uranium captures a neutron, as a result of which it splits (disintegrates) into two new elements - barium and krypton - with the formation of 3 neutrons, and nuclear energy is released.

It should be said that with nuclear reactions millions of times greater energy changes are involved than in chemical reactions. Thus, the decay energy of uranium is 4.5 × 10 9 kcal/mol × uranium. This is 10 million times more than the combustion of one mole of coal.

In chemical reactions, atoms do not change, but molecules do. Therefore, the energy of formation of atoms by chemists is not taken into account, and DN of single-element gas molecules and atoms of pure substances is taken equal to zero.

The above decay reaction of uranium is a classic example chain reaction. Theory chain mechanism We will consider combustion reactions later. But where the neutron comes from and what makes it react with uranium is related to the so-called activation energy, which we will consider a little later.

3.3. Thermal effect of the reaction.

The fact that each individual substance contains a certain amount of energy explains the thermal effects of chemical reactions.

According to Hess's law: The thermal effect of a chemical reaction depends only on the nature of the initial and final products and does not depend on the number of intermediate reactions of transition from one state to another.

Corollary 1 of this law: The thermal effect of a chemical reaction is equal to the difference between the sum of the heats of formation of the final products and the sum of the heats of formation of the starting substances, taking into account the coefficients in the formulas of these substances in the reaction equation.

For example, in the reaction 2H 2 + O 2 = 2H 2 O ± DH.

; ; .

Eventually general equation the reaction will look like this:

2H 2 + O 2 = 2H 2 O – 582 kJ/mol.

And if DH has a (-) sign, then the reaction is exothermic.

Corollary 2. According to the Lavoisier-Laplace law, the thermal effect of the decomposition of a chemical compound is equal and opposite in sign to the thermal effect of its formation.

Then the decomposition reaction of water will be:

2H 2 O=2H 2 +O 2 +582 kJ/mol, i.e. this reaction is endothermic.

An example of a more complex reaction:

CH 4 +2O 2 =CO 2 +2H 2 O.

Then the reaction will be written like this:

CH 4 + 2O 2 = CO 2 + 2H 2 O – 742.3 kJ/mol, which means the reaction is exothermic.

3.4. Kinetic principles of gas reactions.

According to the law of mass action, the reaction rate at a constant temperature is proportional to the concentration of the reacting substances or, as they say, “acting masses”.

The rate of chemical reaction ( υ ) it is customary to consider the amount of substance reacting per unit of time ( dt) per unit volume ( dV).

Consider the reaction proceeding according to the equation:

A + B = C + D.

Since the reaction rate characterizes a decrease in time in the concentration of reactants and an increase in the concentration of reaction products, we can write:

, (3.1)

where the minuses of the derivatives indicate the direction of change in the concentration of the components, and the concentrations of the components are indicated in square brackets.

Then the direct irreversible reaction at T = const proceeds at a speed:

, (3.2)

Where: k – rate constant of a chemical reaction. It does not depend on the concentration of the components, but changes only with temperature.

According to the law of mass action, the concentrations of reaction components are included in the kinetic equation to a degree equal to the stoichiometric coefficient of this component.

Yes, for reaction

aA + bB = cC + dD

The kinetic equation has the form:

The exponents a, b, c, d are usually called the reaction orders for components A, B, C, D, and the sum of the exponents is in general reactions.

For example, reactions like

A ® bB + cC – 1st order,

2A = bB + cC – 2nd order,

A + B = cC + dD – III order.

Since the concentrations of all reacting components are related to each other by stoichiometric equations, the simplest kinetic equations of the first order are differential equations of the first order with one independent variable - concentration - and can be integrated.

The simplest kinetic equation is a first order equation of type

for which . (3.4)

Let us denote by the concentration of component A before the start of the reaction and, having integrated the equation under the boundary condition t = 0, [A] = [A 0 ], we obtain:

Or [A]=×e - kt . (3.5)

Thus, the dependence of the reaction rate on the concentration of substances is exponential.

Kinetic energy gases explains it this way. According to the Arrhenius hypothesis, a reaction between molecules takes place only if they are active, i.e. have excess energy sufficient to break interatomic bonds, the so-called activation energy E A.

Those. the speed of a chemical reaction does not depend on the number of collisions of all molecules, but only of activated ones.

According to Boltzmann's law, the number of active molecules

n A = n o * e - E / RT , (3.6)

where: E – activation energy,

T – temperature of the gas mixture,

n o – total number molecules.

Then the number of effective collisions, which coincides with the reaction rate, is equal to:

υ р = Z eff = Z 0 * e - E / RT , (3.7)

where: Z 0 – total number of collisions of molecules.

1) the reaction rate is proportional to the concentration of active molecules, the number of which depends on the temperature and pressure in the mixture, since pressure is the number of molecules colliding with any surface;

2) a reaction is possible only if the interacting molecules receive a certain supply of energy sufficient to break or weaken interatomic bonds. Activation involves the transition of molecules to a state in which a chemical transformation is possible.

Most often, the activation process occurs through the formation of intermediate unstable but highly active atomic compounds.

Thus, not only endothermic processes require an external supply of energy, but also exothermic ones. In order for an exothermic reaction to occur, it is necessary to impart some impulse of thermal energy to it. For example, for a combustion reaction to occur in a mixture of hydrogen and oxygen, it must be ignited.

The minimum amount of thermal energy required to “start” a chemical reaction is called activation energy.

3.5. Activation energy of a reaction.

To explain this phenomenon, the following example is often used (Fig. 9):

There is a ball on the platform. The site is located in front of the slide. Therefore, the ball could have rolled down on its own if not for the slide. But for a spontaneous descent, it must be lifted to the top of the slide. This will release not only the energy of going up the hill, but also the energy of going down.

Rice. 9. Reaction activation scheme.

Consider two reactions:

1) H 2 + O 2 = H 2 O-

2) H 2 O = H 2 + O 2 +

As can be seen from the figure, E 2 =+E 1;

In general, for any reaction

.

And the sign of the thermal effect depends on the difference between E 1 and E 2, which are always positive.

Thus, activation energy is the energy required to transform reactants into the state active complex(breaking of interatomic bonds, bringing molecules closer together, accumulation of energy in a molecule...).

With increasing gas temperature, the proportion of active molecules (e -E/ RT) sharply increases, and therefore the reaction rate increases exponentially. This relationship can be illustrated as follows:

Rice. 10. Dependence of the reaction rate on temperature: 1 – speed of the 1st reaction, 2 – speed of the 2nd reaction.

As can be seen from Figure 10, the rate of the first reaction is less than the rate of the second reaction, and the activation energy of the 1st reaction is greater than E of the second. And at the same temperature T 2 υ 2 > υ 1 . The higher the activation energy, the higher the temperature required to achieve a given reaction rate.

The reason for this is that when E is larger, the existing interatomic bonds in the molecules of the reacting components are stronger, and more energy is needed to overcome these forces. In this case, the proportion of active molecules is correspondingly smaller.

From the above it is clear that the value of activation energy is the most important characteristic of a chemical process. It determines the height of the energy barrier, the overcoming of which is a condition for the reaction to occur. On the other hand, it characterizes the reaction rate depending on temperature, i.e. the higher the activation energy, the higher the temperature to achieve a given reaction.

3.6. Catalysis.

In addition to increasing the temperature and concentration of substances, they use catalysts, i.e. substances that are introduced into a reacting mixture, but are not consumed during the reaction, but accelerate it by reducing the activation energy.

The process of increasing the reaction rate using catalysts is called catalysis.

Catalysts participate in intermediate reactions to create an activated complex by weakening the bonds in the molecules of the starting substances, their decomposition, the adsorption of molecules on the surface of the catalyst, or the introduction of active catalyst particles.

The nature of the participation of the catalyst can be explained by the following diagram:

Reaction without catalyst: A + B = AB.

With catalyst X: A + X = AX ® AX + B = AB + X.

Let us present a picture similar to that shown in Fig. 9.

Rice. 11. Catalyst operation diagram: E b.cat And E with cat– activation energy of the reaction without a catalyst and with a catalyst, respectively.

When a catalyst is introduced (Fig. 11), the reaction can proceed along a different path with a lower energy barrier. This pathway corresponds to a new reaction mechanism through the formation of another activated complex. And the new lower energy barrier can be overcome larger number particles, which leads to an increase in the reaction rate.

It should be noted that the activation energy of the reverse reaction decreases by the same amount as the activation energy of the forward reaction, i.e. both reactions are accelerated equally, and catalysts do not initiate the reaction, they will only speed up the reaction, which could occur in their absence, but much more slowly.

Intermediate products of the reaction can become catalysts, then this reaction is called autocatalytic. So, if the rate of ordinary reactions decreases as the reactants are consumed, then the combustion reaction, due to autocatalysis, self-accelerates and is autocatalytic.

Most often, solid substances that adsorb molecules of reacting substances are used as catalysts. During adsorption, the bonds in the reacting molecules are weakened, and thus the reaction between them is facilitated.

What is adsorption?

3.7. Adsorption.

Adsorption- surface absorption of any substance from a gaseous medium or solution by the surface layer of another substance - liquid or solid.

For example, the adsorption of toxic gases on the surface of activated carbon used in gas masks.

A distinction is made between physical and chemical adsorption.

At physical adsorption, the captured particles retain their properties, and when chemical– chemical compounds of the adsorbate with the adsorbent are formed.

The adsorption process is accompanied by the release of heat. For physical adsorption it is insignificant (1-5 kcal/mol), for chemical adsorption it is much greater (10-100 kcal/mol). This can speed up chemical reactions during catalysis.

For combustion and explosion processes, the following examples can be given:

1. The auto-ignition temperature of the H 2 + O 2 mixture is 500 0 C. In the presence of a palladium catalyst, it decreases to 100 0 C.

2. The processes of spontaneous combustion of coal begin with the chemical adsorption of oxygen on the surface of coal particles.

3. When working with pure oxygen, oxygen is well adsorbed on clothing (physical adsorption). And in the presence of a spark or flame, clothing easily catches fire.

4. Oxygen is well adsorbed and absorbed by technical oils to form an explosive mixture. The mixture explodes spontaneously, without an ignition source (chemical absorption).