Combustion reaction products. Combustion (reaction)

Topic 3. CHEMICAL BASES OF COMBUSTION.

3.1. Chemistry of combustion reactions.

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

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

Oxidizers serve: oxygen, ozone, halogens (F, Cl, Br, J), nitrous oxide (NO 2), ammonium nitrate (NH 4 NO 3), etc. In 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, when acetylene breaks down:

C 2 H 2 \u003d 2C + H 2.

exothermic Reactions are reactions that release heat.

Endothermic Reactions are reactions that take place with the absorption of heat.

For example:

2H 2 + O 2 \u003d 2H 2 O + Q - exothermic reaction,

2H 2 O + Q \u003d 2H 2 + O 2 - endothermic reaction,

where: Q - thermal energy.

Thus, endothermic reactions can proceed only 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 is equal to the weight of the substances formed after the reaction. When equalizing the chemical equations, we get stoichiometric formulations.

For example, in the reaction

CH 4 + 2O 2 \u003d 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 a complete reaction of methane, it is necessary to mix 1 mol of CH 4 with 2 moles of O 2, or 1/3 \u003d 33.3% CH 4 and 2 / 3=66.7% O 2 . Such a 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 \u003d CO 2 + 2H 2 O + 2 × 3.76N 2.

1 mol CH 4 +2 mol O 2 +7.52 mol N 2 \u003d 10.52 mol of a 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% \u003d 71.5% N 2.

This means that in the most combustible 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 released.

The same picture will be obtained if we mix arbitrary, non-stoichiometric compositions.

For example, in the reaction 2CH 4 + 2O 2 \u003d CO 2 + 2H 2 O + CH 4 1 mol of CH 4 does not pro-react.

In reaction CH 4 + 4O 2 \u003d 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 for their heating.

Thus, if we compare the combustion reactions of methane in oxygen and air or in excess CH 4 and O 2, then 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 total mixture;

Part of the heat will be spent on heating the ballast: nitrogen, oxygen or methane.

Let's ask questions:

What energy can be released in the reaction?

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

How much heat energy must be added to

endothermic reaction?

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

3.2. Heat content of substances.

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

Here is an example of a simpler reaction:

2H 2 + O 2 \u003d 2H 2 O + Q

This means that the energy level of the stoichiometric mixture of hydrogen and oxygen was higher than that of the H 2 O reaction product, 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, there is a redistribution of atoms in the water molecule with the formation of hydrogen and oxygen. In this case, the heat content of H 2 and O 2 increases.

Thus, each substance during its formation receives or gives away 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.

Reference books give the heats of formation of compounds from simple substances.

For example:

At CO 2 \u003d - 393.5 kJ / mol

U H 2 O gas \u003d - 241.8 kJ / mol

But for substances formed during endothermic processes, for example, acetylene C 2 H 2 \u003d + 226.8 kJ / mol, during the formation of a hydrogen atom H + according to the reaction H 2 \u003d H + + H + \u003d + 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 conditionally taken equal to zero.

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

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

Each atom has energy, part of which is associated with electrons, and part - with the nucleus. Electrons have the kinetic energy of rotation around the nucleus and the potential electrical energy of 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 get the molar heat content of this substance.

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

The 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 of intranuclear particles and the transformation of one chemical element into another during nuclear reactions. For example, the decay reaction of uranium:

or more simply: U+n®Ba+Kr+3n.

where: n ’ o is a neutron particle with mass 1 and zero charge.

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

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

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 DH of single-element gas molecules and atoms of pure substances is taken equal to zero.

The given decay reaction of uranium is classic example chain reaction. We will consider the theory of the chain mechanism of the combustion reaction later. But where does the neutron come from and what makes it react with uranium - this is due 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 serves as an explanation for thermal effects. chemical reactions.

According to Hess' 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 \u003d 2H 2 O ± DH.

; ; .

As a result, the general reaction equation will look like this:

2H 2 + O 2 \u003d 2H 2 O - 582 kJ / mol.

And if DH is signed (-), then the reaction is exothermic.

Consequence 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 reaction of water decomposition will be:

2H 2 O \u003d 2H 2 + O 2 +582 kJ / mol, i.e. this reaction is endothermic.

An example of a more complex reaction:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O.

Then the reaction will be written as:

CH 4 + 2O 2 \u003d CO 2 + 2H 2 O - 742.3 kJ / mol, which means the reaction is exothermic.

3.4. Kinetic foundations of gas reactions.

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

The rate of a chemical reaction ( υ ) it is customary to consider the amount of a substance reacting per unit 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 the concentration of reactants with time and an increase in the concentration of reaction products, we can write:

, (3.1)

where the minuses at 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 a direct irreversible reaction at T \u003d const proceeds at a rate of:

, (3.2)

where: k is the 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 the reaction components are included in the kinetic equation to a degree equal to the stoichiometric coefficient of this component.

So for the reaction

aA + bB = cC + dD

The kinetic equation has the form:

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

For example, reactions like

A ® bB + cC - I order,

2A \u003d bB + cC - II order,

A + B \u003d cC + dD - III order.

Since the concentrations of all reacting components are interconnected 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 the first order equation of the type

for which . (3.4)

Denote by the concentration of component A before the start of the reaction and, integrating 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.

The kinetic energy of gases explains it this way. According to the Arrhenius hypothesis, the 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 rate 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 \u003d n o * e - E / RT, (3.6)

where: E is the activation energy,

T is the temperature of the gas mixture,

n o - total number molecules.

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

υ p \u003d Z eff \u003d Z 0 * e - E / RT, (3.7)

where: Z 0 is the total number of molecular collisions.

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 amount of energy sufficient to break or weaken interatomic bonds. Activation consists in the transition of molecules to a state in which chemical transformation is possible.

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

Thus, not only for the occurrence of endothermic processes, an external supply of energy is needed, but also for exothermic ones. For an exothermic reaction to occur, some impulse of thermal energy must be imparted 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 the activation energy.

3.5. Reaction activation energy.

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 hill. Therefore, the ball could roll down by itself, if not for the slide. But for a spontaneous descent, it must be raised to the top of the hill. In this case, not only the energy of going up the hill will be released, but also the energy of going down.

Rice. 9. Scheme of activation of the reaction.

Consider two reactions:

1) H 2 + O 2 \u003d H 2 O-

2) H 2 O \u003d 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, the activation energy is the energy necessary for the transformation of reacting substances into the state of an active complex (breaking interatomic bonds, bringing molecules together, accumulating energy in a molecule ...).

With an increase in the temperature of gases, the proportion of active molecules (e - E / RT) sharply increases, and hence the reaction rate according to an exponential dependence. This dependence can be illustrated as follows:

Rice. 10. Dependence of the reaction rate on temperature: 1 - the rate of the 1st reaction, 2 - the rate 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 rate of reaction.

The reason for this is that when E is larger, then 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 less.

It can be seen from the above that the magnitude of the activation energy is the most important characteristic chemical process. It determines the height of the energy barrier, the overcoming of which is a condition for the reaction to proceed. On the other hand, it characterizes the reaction rate as a function of 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, to speed up a chemical reaction, they use catalysts, i.e. substances that are introduced into the reacting mixture, but are not consumed in the reaction, but accelerate it by lowering the activation energy.

The process of increasing the reaction rate with the help of 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, 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 following scheme:

Reaction without a catalyst: A + B = AB.

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

We present a picture similar to that shown in Fig. 9.

Rice. 11. Diagram of the action of the catalyst: E b.cat and E with cat are the activation energies 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 a new lower energy barrier can overcome more 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 direct reaction, i.e. both reactions speed up equally, and catalysts do not initiate the reaction, they only speed up the reaction, which can occur in their absence, but much more slowly.

Intermediate reaction products 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, solids are used as catalysts, which adsorb the molecules of the reactants. 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 a substance from a gaseous medium or solution by a surface layer of another substance - a liquid or solid body.

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

Distinguish between physical and chemical adsorption.

At physical adsorption, the trapped 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 higher (10-100 kcal/mol). Thus, chemical reactions during catalysis can be accelerated.

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

1. The autoignition temperature of a mixture of H 2 + O 2 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 clothes (physical adsorption). And in the presence of a spark or flame, clothes flare up easily.

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

Combustion is a chemical reaction of fuel oxidation with oxygen, which proceeds relatively quickly in time with the release of a large amount of heat.

During combustion, the products of combustion are heated to high temperatures.

The general equation for the combustion of any hydrocarbon gas with oxygen is as follows:

where m and n- respectively, the number of carbon and hydrogen atoms in the molecule

Q is the thermal effect of the oxidation reaction.

Table 3.1 shows the combustion reactions of the main combustible gases with oxygen.

Combustion reactions of combustible gases with oxygen

Table 3.1

Table 3.1 shows the oxidation reactions of the most known combustible gases with oxygen. However, in real conditions, the oxidizing agent (oxygen) is supplied to the combustion zone not in its pure form, but as part of the air. It is known that air mainly consists of two parts: oxygen and nitrogen. The composition of the air also includes a small amount of carbon dioxide CO 2, as well as rare gases. Given their small amount in the composition of the air, we neglect them.

Thus, if we take the volume of air as 100%, then the oxygen content will be 21%, and nitrogen 79%. Therefore, on 1 m 3 air oxygen accounted for 79/21 = 3.76 m 3 nitrogen, or 1 m 3 oxygen is contained in 100/21 = 4.76 m 3 air.

Given the above relationships, we can write the general equation for the combustion of hydrocarbons with air:

Table 3.2 shows the equations for the combustion reaction of combustible gases with air.

It should be noted that the equations given in tables 3.1 and 3.2 are stoichiometric, i.e. such a ratio of combustible gas and oxidizer (oxygen, air), in which the combustible gas is supplied theoretically required amount oxidizer. However, in the practice of gas combustion under real conditions, it is necessary to supply a slightly larger amount of oxidizing agent to the zone than follows from the stoichiometric equations. This is mainly due to the imperfection of the quality of the mixing of the combustible gas and the oxidizer.

Equations for combustion reactions of combustible gases with air

Table 3.2

The ratio of the actual flow rate of the oxidizer (oxygen or air) to the theoretically required one is called the excess air coefficient and is denoted α , i.e.:

where V d– actual air consumption;

V t is the theoretically required amount of air.

Table 3.3 shows the values ​​of the theoretically required amount of oxidant (oxygen and air), as well as the volume of combustion products during combustion 1 m 3 gas and excess air ratio equal to 1 ( a = 1).

Theoretically required amount of oxidant and the volume of combustion products during combustion 1 m 3 at α = 1

Table 3.3

In practical calculations, sometimes we do not know chemical composition gases, and only the heat of combustion is known. It is necessary to determine the theoretically required amount of air required for complete combustion 1 m 3 gas.

For this case, there is an empirical formula D.I. Mendeleev:

where Q n- lower calorific value of gas, kJ/m 3 .

The equations for the reactions of combustion of various gases with oxygen and air reflect only the ratio between the fuel and the oxidizer, and do not explain the mechanism of these reactions. In real conditions, the combustion process is much more complicated.

Developed modern theory mechanism of the kinetics of the combustion reaction of gases Soviet scientist, academician N.N. Semenov. According to his theory, chain reactions of combustion of gases occur in the flame of a gas-air mixture. As a result, intermediate unstable products are formed in the form of free radical atoms. In accordance with the theory of N.N. Semenov's combustion reaction of hydrogen with oxygen is not reduced simply to the combination of two hydrogen molecules and one oxygen to form two water molecules. During the interaction of these two gases, the formation of intermediate substances in the form of hydrogen and oxygen atoms first occurs, and the formation of free OH hydroxyl radicals also occurs.

To start the combustion process, it is necessary to somehow activate the combustible mixture. In other words, it is necessary to create conditions under which the reagents will have a large amount of energy. This energy reserve is necessary for the implementation of the combustion process. The above energy reserve can be created by heating the gas-air mixture to its ignition temperature. This energy, called the activation energy, is needed mainly to break the existing intermolecular bonds in the reagents.

During the combustion process, new bonds are constantly formed along with the destruction of old ones. When new bonds are formed, a significant release of energy occurs, while the breaking of old bonds is always accompanied by energy costs. Due to the fact that during the combustion process, the energy that is released during the formation of new bonds has great importance, compared with the energy spent on breaking old bonds, the total thermal effect remains positive.

The reaction of hydrogen with oxygen is the simplest and most studied. Therefore, consider this branched reaction with an example.

In accordance with the theory of N.N. Semenov at the initial moment of the reaction, as a result of the activation energy and collision of hydrogen and oxygen molecules, two OH hydroxyl radicals are formed:

. (3.5)

The free hydrogen atom H, in turn, reacts with an oxygen molecule. As a result, a OH hydroxyl radical and a free oxygen atom are formed, i.e.:

. (3.7)

The radical can again enter into a chemical reaction with hydrogen and again, as a result of the reaction, form water and free hydrogen, and the oxygen atom, in turn, can react with a hydrogen molecule, which will lead to the formation of another OH radical and a hydrogen atom H , i.e.:

. (3.8)

The above mechanism of the chain reaction of combustion of hydrogen with oxygen shows the possibility of multiple interactions of one OH radical with hydrogen atoms. As a result of this interaction, water molecules are formed.

Therefore, free atoms and radicals are active centers in creating a chain reaction.

The combustion reaction of hydrogen with oxygen, which explains the chain reaction mechanism, can be written as follows:

H 2 O O + (H 2) ...

OH + (H 2) ® H + (O 2) ® OH + (H 2) ...

O + (H 2) ® OH + (H 2) ® H 2 O

H + (O 2) ® OH + H 2 ...

The combustion mechanism of carbon monoxide with oxygen is more complex. According to scientists from the Institute of Chemical Physics of the USSR Academy of Sciences, carbon monoxide does not react with dry oxygen. They also found that the addition of a small amount of hydrogen or moisture to the mixture leads to the onset of the oxidation reaction. As a result, the following sequence of chemical reactions occurs:

H 2 O ® OH + H; (3.10)

OH + CO ® CO 2 + H; (3.11)

H + O 2 ® OH + O; (3.12)

CO + OH ® CO 2 + H; (3.13)

CO + O ® CO 2 ; (3.14)

H + O 2 \u003d OH + O (3.15)

As follows from the above chemical reactions, the presence of a small amount of moisture leads to the formation of hydroxyls and free atoms in the combustion zone. As noted earlier, both hydroxyl radicals and free atoms are the initiators of the creation and carriers of the chain reaction.

Even more complex mechanism hydrocarbon oxidation. Along with some similarities with the mechanism of combustion of hydrogen and carbon monoxide, the mechanism of combustion of hydrocarbons has a number of significant differences. Analyzing the combustion products, it was found that they contain aldehydes and mainly formaldehyde (HCHO).

Consider the mechanism of hydrocarbon oxidation using the simplest of them, methane, as an example. The mechanism of methane oxidation goes through four stages, in each of which the following chemical reactions occur:

At the first stage:

H + O 2 ® OH + O; (3.16)

CH 4 + OH ® CH 3 + H 2 O; (3.17)

CH 4 + O ® CH 2 + H 2 O. (3.18)

At the second stage:

CH 3 + O 2 ® HCHO + OH; (3.19)

CH 2 + O 2 ® HCHO + O; (3.20)

At the third stage:

HCHO + OH ® HCO + H 2 O (3.21)

HCHO + O ®CO + H 2 O; (3.22)

HCO + O 2 ® CO + O + OH (3.23).

At the fourth stage:

CO + O ® CO 2 (3.24)

Acts of chemical transformation occur with direct contact of the reacting components (molecules, atoms, radicals), but only in those cases when their energy exceeds a certain energy limit, called the activation energy E a. Let us graphically depict the change in the energy of the reacting components (fuel and oxidizer) and reaction products during combustion (Fig. 1.)

Let us graphically depict the change in the energy of the reacting components (fuel and oxidizer) and reaction products during combustion (Fig. 1.)

Fig 1. Energy change of reacting substances and reaction products during combustion

The abscissa shows the path of the combustion reaction, the ordinate shows the energy.
is the average initial energy of the reacting components,
- average energy of combustion products.

Only active particles of the fuel and oxidizer will enter into the combustion reaction, which will have the energy necessary for entering into interaction, i.e. able to overcome the energy barrier
. Excess energy of active particles in comparison with average energy
, is called the activation energy . Because combustion reactions are exothermic
. The energy difference between the resulting combustion products and the initial substances (fuel and oxidizer) determines the thermal effect of the reaction:

D The field of active molecules increases with increasing temperature of the combustible mixture.

In Fig.2. shows the distribution of energies between molecules at a temperature If we mark along the energy axis a value equal to the activation energy , then we obtain the fraction of active molecules in the mixture at a given temperature . If, under the action of a heat source, the temperature of the mixture rises to a value , then the proportion of active molecules will also increase, and, consequently, the rate of the combustion reaction.

However, there are chemical reactions that do not require significant preheating for their development. These are chain reactions.

The basis of the theory of chain reactions is the assumption that the starting substances are not converted into the final product immediately, but with the formation of active intermediate products

The product of a primary chemical reaction has a large amount of energy, which can be dissipated in the surrounding space when molecules of the reaction products collide or due to radiation, or can be transferred to the molecules of the reacting components, turning them into an active state. These active molecules (atoms, radicals) of reacting substances give rise to a chain of reactions where energy is transferred from one molecule to another. Therefore, such reactions are called chain reactions.

Chemically active molecules, atoms, radicals formed at the elementary stages of a chain reaction - chain links - are called active centers. Most of the active centers are atoms and radicals, which are the most reactive. But as a result, they are also unstable, because can enter into recombination reactions with the formation of inactive products.

The length of the chain formed by one initial active center can reach several hundred thousand links. The kinetic regularities of chain reactions essentially depend on how many active centers are formed in one link of the chain. If, with the participation of the original active center, only one active center is formed as a result, then such a chain reaction is called unbranched, but if two or more active centers are formed in one link of the chain, then such a chain reaction is called branched. The rate of branched chain reactions increases like an avalanche, which is the reason for the self-acceleration of chemical oxidation reactions during combustion, since most of them are characterized by the mechanism of branched chain reactions.

Almost any combustion reaction can simultaneously have signs of both a thermal and a chain reaction mechanism. The nucleation of the first active centers can be of a thermal nature, and the reaction of active particles by a chain mechanism leads to heat release, heating of the combustible mixture and thermal generation of new active centers.

Any chain reaction is made up of the elementary stages of chain initiation, continuation, and chain termination.

The origin of the chain is an endothermic reaction. The formation of free radicals (i.e. atoms or groups of atoms having free valences, for example,
) from the molecules of the initial substances, possibly as a result of monomolecular or bimolecular interaction, as well as as a result of any extraneous effects on the combustible mixture - initiation.

Initiation can be carried out by adding special substances - initiators, easily forming free radicals (for example, peroxides, reactive gases
), under the action of ionizing radiation, under the action of light - photochemical initiation. For example, the interaction of hydrogen with chlorine

under normal conditions proceeds extremely slowly, and in strong light ( sunlight, burning magnesium) proceeds with an explosion.

To reactions chain continuation include the elementary stages of a chain reaction that proceed with the preservation of free valency and lead to the consumption of starting materials and the formation of reaction products.

chain origin:

chain branching:

circuit break:

homogeneous

heterogeneous

With the development of the chain, when the concentration of active centers becomes large enough, it is possible to form such a link in which the active center will react without generating a new active center. This phenomenon is called an open circuit.

chain break may be homogeneous or heterogeneous.

Homogeneous chain termination is possible either when radicals or atoms interact with each other to form stable products, or when the active center reacts with a molecule foreign to the main process without generating new active centers.

Heterogeneous chain termination occurs on the walls of the vessel where the combustion reaction takes place or on the surface of solid microparticles present in the gas phase, sometimes specially introduced (for example, as in extinguishing with powders). The mechanism of heterogeneous chain termination is associated with the adsorption of active centers on the surface of solid particles or materials. The rate of heterogeneous chain termination strongly depends on the ratio of the surface area of ​​the walls to the volume of the vessel where combustion occurs. Thus, a decrease in the diameter of the vessel significantly reduces the rate of the combustion reaction, up to its complete cessation. The creation of fire arresters is based on this.

An example of a branched chain reaction is the combustion of hydrogen in oxygen.

chain origin:

chain branching:

circuit break:

homogeneous

Combustion is an oxidation reaction that occurs at a high rate, which is accompanied by the release of heat in large quantities and, as a rule, a bright glow, which we call a flame. The combustion process is studied physical chemistry, in which it is customary to attribute to combustion all exothermic processes that have a self-accelerating reaction. Such self-acceleration can occur due to an increase in temperature (i.e., have a thermal mechanism) or the accumulation of active particles (i.e., have a diffusion nature).

The combustion reaction has a visual feature - the presence of a high-temperature region (flame), spatially limited, where it occurs most of transformation of the initial substances (fuel) into This process is accompanied by the release of a large amount. To start the reaction (appearance of a flame), it is required to spend a certain amount of energy on ignition, then the process proceeds spontaneously. Its rate depends on the chemical properties of the substances involved in the reaction, as well as on the gas-dynamic processes during combustion. The combustion reaction has certain characteristics, the most important of which are the calorific value of the mixture and the temperature (called adiabatic) that theoretically could be achieved with complete combustion without taking into account heat losses.

homogeneous combustion is the simplest, constant speed, depending on the composition and molecular thermal conductivity of the mixture, temperature and pressure.

Heterogeneous combustion is most common both in nature and in artificial conditions. Its speed depends on the specific conditions of the combustion process and on physical characteristics ingredients. For liquid combustibles, the combustion rate big influence has a rate of evaporation, for solids - the rate of gasification. For example, when burning coal, the process forms two stages. On the first of them (in the case of relatively slow heating), the volatile components of the substance (coal) are released, on the second, the coke residue burns out.

The combustion of gases (for example, the combustion of ethane) has its own characteristics. AT gaseous environment the flame can spread over a wide distance. It can move through the gas at subsonic speed, and given property inherent not only in the gaseous medium, but also in a finely dispersed mixture of liquid and solid combustible particles mixed with an oxidizing agent. To ensure stable combustion in such cases, a special design of the furnace device is required.

The consequences that a combustion reaction causes in a gaseous medium are of two types. The first is the turbulence of the gas flow, leading to a sharp increase in the speed of the process. Acoustic perturbations of the flow arising in this case can lead to the next stage - the generation of a mixture leading to detonation. The transition from combustion to the detonation stage depends not only on the intrinsic properties of the gas, but also on the dimensions of the system and propagation parameters.

Fuel combustion is used in engineering and industry. The main task in this case is to achieve the maximum completeness of combustion (i.e., optimization of heat release) for a given period. Combustion is used, for example, in mining - the methods for developing various minerals are based on the use of a combustible process. But under certain natural and geological conditions, the phenomenon of combustion can become a factor that carries a serious danger. A real danger, for example, is the process of spontaneous combustion of peat, leading to the occurrence of endogenous fires.

original document?

PHYSICO-CHEMICAL BASES OF COMBUSTION PROCESSES

Chemical processes during combustion. The nature of combustibles. 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 flow conditions, may be a fire or an explosion.

Fire and explosion hazardsubstances and materials is characterized by indicators, the choice of which depends on the state of aggregation of the substance (material) and the conditions of its use.

When determining fires and explosion hazards substances and materials, the following states of aggregation are distinguished:

gases - substances whose saturated vapor pressure 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 dropping melting 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 a particle size of less than 850 microns.

Combustion as a chemical reaction of the oxidation of substances with the participation of oxygen

Combustion - one of the first complex physical and chemical processes that man met at the dawn of his development. The process, having mastered which, he received a huge superiority over the surrounding living beings and the forces of nature.

Combustion - one of the forms of obtaining and converting energy, the basis of many technological processes production. Therefore, a person constantly studies and learns the processes of combustion.

The history of combustion science begins with the discovery of M.V. Lomonosov: "Combustion is the combination of matter with air." This discovery served as the basis for the discovery of the law of conservation of mass of substances in 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, a significant contribution to the study and development of the science of combustion was made by Soviet and Russian scientists A.V. Michelson, 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 (the law of conservation of mass, energy, etc.).

burning is a complex physico-chemical process in which combustible substances and materials, under the influence of high temperatures, enter into 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 reactions of combustion, nitrogen is also taken into account, which is contained in the air, although it does not participate in combustion reactions. The composition of the air is conditionally assumed to be constant, containing 21% by volume of oxygen and 79% of nitrogen (in weight, respectively, 23% and 77% of nitrogen), i.e. 1 volume of oxygen accounts for 3.76 volumes of nitrogen. Or 1 mole of oxygen accounts for 3.76 moles of nitrogen. Then, for example, the combustion reaction of methane in air can be written as follows:

CH 4 + 2O 2 + 2´ 3.76 N 2 \u003d CO 2 + 2H 2 O + 2 ´ 3.76N2

Nitrogen in the equations of chemical reactions must be taken into account because it absorbs part of the heat released as a result of combustion reactions and is part of the combustion products- flue gases.

Consider the processes of oxidation.

Hydrogen oxidation carried out according to the reaction:

H 2 + 0.5O 2 \u003d 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 a 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 the 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 \u003d 2CO (incomplete)

The homogeneous mechanism has not been studied (carbon in the gaseous state). The interaction of carbon in the solid state is the most studied. This process can be schematically represented by the following steps:

1. delivery of an oxidizing agent (O 2 ) to the interface by molecular and convective diffusion;

2. physical adsorption of oxidant molecules;

3. interaction of the adsorbed oxidant 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. oxygen, i.e. This process is multi-stage:

* OH + CO = CO 2 + H; O + CO \u003d CO 2

The direct reaction CO + O 2 -> CO 2 is unlikely, since real dry mixtures of CO and O 2 are extremely low speeds combustion or cannot ignite at all.

Oxidation of protozoa hydrocarbon in.Methane burns to form carbon dioxide and water vapor:

CH 4 + O 2 \u003d 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 a short time, they provide fast fuel consumption. In the process of burning natural gas complexes of carbon, hydrogen and oxygen arise, as well as complexes of carbon and oxygen, upon destruction of which CO, CO 2, H 2 O are formed. 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 Oz → CO, CO 2, H 2 O.

Thermal decomposition, pyrolysis of solids

When the temperature of a solid combustible material rises, a rupture occurs chemical bonds with the formation of simpler components (solid, liquid, gaseous). This process is called thermal decomposition or pyrolysis . Thermal decomposition of molecules of organic compounds occurs in a flame, i.e. at elevated temperatures close to the burning 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.

Consider the process of pyrolysis on the example of the most common solid combustible material- wood.

Wood is a mixture of many substances different structure and properties. Its main components are hemicellulose (25%), cellulose (50%), lignin (25%). Hemicellulose consists of a mixture of pentazans (C 5 H 8 O 4), hexazanes (C 6 H 10 O 5), polyuronides. lignin is aromatic in nature and contains carbohydrates associated with aromatic rings. On average, wood contains 50% C, 6% H, 44% O. This 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, the decomposition of wood consists of the following processes:

№ pp	Temperature, ° С	Process characteristics
	up to 120 - 150	drying, physical removal bound water
	150 - 180	Decomposition of the least stable components (luminic acids) with the release of CO 2, H 2 O
	250 - 300	pyrolysis of wood with the release of CO, CH 4 , H 2 , CO 2 , H 2 O, etc.; the resulting mixture is capable of being ignited by an ignition source

	350 - 450	Intensive pyrolysis with the release of the bulk of combustible substances (up to 40% of the total mass); the gaseous mixture consists of 25% H 2 and 40% saturated and unsaturated hydrocarbons; the maximum supply of volatile components to the flame zone is ensured; the process at this stage is exothermic; the amount of heat that is released reaches 5- 6% of net calorific value Q ≈ 15000 kJ/kg
	500 - 550	The rate of thermal decomposition is sharply reduced; the release of volatile components stops (end of pyrolysis); at 600 °C, the evolution of gaseous products stops

Pyrolysis of coal and peat proceeds similarly to wood. However, their volatile yield 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).

Burning 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 bale .< 1500 K . These include alkali metals (lithium, sodium, potassium) and alkaline earth (magnesium, calcium). The combustion of metals is carried out as follows: 4 Li + O 2 = 2 Li2O . Non-volatile metals have T pl . > 1000 K and T bale . > 2500 K.

The combustion mechanism is largely determined by the properties of the metal oxide. The temperature of volatile metals is below the melting point of their oxides. In this case, the latter are rather porous formations. When an ignition spark is brought to the surface of a metal, it evaporates and oxidizes.

Upon reaching the vapor concentration equal to the lower concentration limit ignition, they ignite. Zone diffusion combustion is installed near 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, contributing to the formation of a metal oxide crust, but also to the surrounding space, where, condensing, they form solid particles in the form of white smoke. The formation of white dense smoke is a visual sign of burning volatile metals.

For non-volatile metals with high phase transition temperatures, during combustion, a very dense oxide film is formed on the surface, which adheres well to the metal surface. As a result, the rate of diffusion of metal vapor through the film is sharply reduced and large particles, such as aluminum or beryllium, are not able to burn. As a rule, fires of such metals take place when they are introduced in the form of chips, powders, aerosols. Their combustion occurs without the formation of dense smoke. The formation of a dense oxide film on the metal surface leads to particle explosion. This phenomenon, which is especially often observed when a particle moves in a high-temperature oxidizing medium, is associated with the accumulation of metal vapors under the oxide film with its subsequent sudden explosion. This naturally leads to a sharp intensification of combustion.

burning dust

Dust - it is a dispersed system consisting of a gaseous dispersed medium (air) and a solid phase (flour, sugar, wood, coal, etc.).

The spread of the flame over the 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, decompose with the release of combustible products that form combustible mixtures with air.

An aerosol having a very small particles, when ignited, quickly burns out in the zone of influence of 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 through such particles does not occur.

An aerosol containing large particles is also incapable of stationary combustion. With an increase in the particle size, the specific heat exchange surface decreases, and the time of their heating to the decomposition temperature increases.

If the time of formation of a combustible vapor-air mixture before the flame front due to the decomposition of particles solid material longer than the time of existence of the flame front, then combustion will not occur.

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

1. dust concentration (the maximum flame propagation speed occurs for mixtures slightly higher than the stoichiometric composition, for example, for peat dust at a concentration of 1- 1.5 kg / m 3);

2. ash content (with an increase in ash content, the concentration of the combustible component decreases and the flame propagation speed decreases);

Classification of dust by explosion hazard:

I class - the most explosive dust (concentration up to 15 g/m3);

II class - explosive up to 15-65 g/m 3

III class - the most flammable > 65 g/m 3 T sv ≤ 250°C;

IV class - fire hazardous > 65 g/m 3 T St > 250°С.

anoxic combustion

There are a number of substances that, when their temperature rises above a certain level, undergo chemical decomposition, leading to a glow of gas, hardly distinguishable from a flame. Gunpowder and some synthetic materials can burn without air or in a neutral environment (in pure nitrogen).

burning cellulose (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.

The fire involved ammonium nitrate, can be maintained without oxygen supply. These fires are likely to occur with 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” was in the port of Texas City with a cargo of about 2800 tons of ammonium nitrate. The fire started in the cargo hold with 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 put out the fire by battening down the deck hatches and letting steam into the cargo hold. Such measures contribute to the deterioration of the situation, intensifying the fire without access to air, since the ammonium nitrate is heated. The fire started at 8 o'clock in the morning, and at 9 o'clock. 15 minutes later there was an explosion. As a result, more than 200 people who crowded in the port and watched the fire died, including the ship's crew and the crew of two aircraft of 4 people flying around the ship.

At 13:10 next day another ship transporting ammonium nitrate and sulfur, which caught fire from the first ship the day before, also exploded.

Marshall describes a fire that broke out near Frankfurt in 1961. Spontaneous thermal decomposition caused by a conveyor belt led to the fire of 8 .. tons of fertilizer, a third of this amount 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 containing nitrogen.