The combustion process depends on many conditions, the most important of which are:
· composition of the combustible mixture;
· pressure in the combustion zone;
· reaction temperature;
· geometric dimensions systems;
· state of aggregation of fuel and oxidizer, etc.
Depending on the state of aggregation of fuel and oxidizer, the following types of combustion are distinguished:
· homogeneous;
· heterogeneous;
· combustion of explosives.
Homogeneous combustion occurs in gas or vapor combustible systems (Fig. 1.1) (fuel and oxidizer are evenly mixed with each other).
Since the partial pressure of oxygen in the combustion zone is (equally) close to zero, oxygen penetrates quite freely into the combustion zone (practically it is in it), therefore the combustion rate is determined mainly by the flow rate chemical reaction, increasing with increasing temperature. Such combustion (or combustion of such systems) is called kinetic.
Fig.1.1. Scheme of the combustion process of vapors or gases
The total combustion time in the general case is determined by the formula
t р = t Ф + t Х,
where t Ф is the time of the physical stage of the process (diffusion of O 2 to the source through the layer); t X – time of the chemical stage (reaction).
When burning homogeneous systems (mixtures of vapors, gases with air), the time of the physical stage of the process is disproportionately less than the rate of chemical reactions, therefore t P » t X - the rate is determined by the kinetics of the chemical reaction and combustion is called kinetic.
When burning chemically inhomogeneous systems, the time of penetration of O 2 into the combustible substance through the combustion products (diffusion) is disproportionately longer than the time of the chemical reaction, thus determining the overall rate of the process, i.e. t P » t F. Such combustion is called diffusion.
Examples of diffusion combustion (Fig. 1.2) are the combustion of coal, coke (combustion products prevent the diffusion of oxygen into the combustion zone)
Fig.1.2. Scheme of oxygen diffusion into the combustion zone of a solid substance
(heterogeneous combustion)
The concentration of oxygen in the volume of air C1 is significantly greater than its concentration near the combustion zone C0. In the absence of a sufficient amount of O 2 in the combustion zone, the chemical reaction is inhibited (and is determined by the rate of diffusion).
If the duration of the chemical reaction and the physical stage of the process are comparable, then combustion occurs in the intermediate region (the combustion rate is influenced by both physical and chemical factors).
At low temperatures, the reaction rate depends slightly on temperature (the curve slowly rises upward). At high temperatures ah the reaction rate increases greatly (i.e., the reaction rate in the kinetic region depends mainly on the temperature of the reactants).
The rate of the oxidation (combustion) reaction in the diffusion region is determined by the rate of diffusion and depends very little on temperature. Point A is the transition from the kinetic to the diffusion region (Fig. 1.3).
The combustion process of all substances and materials, regardless of their state of aggregation, occurs, as a rule, in the gas phase (liquid evaporates, solid combustible substances release volatile products). But the combustion of solids has a multi-stage character. Under the influence of heat - heating of the solid phase - decomposition and release of gaseous products (destruction, volatile substances) - combustion - heat heats the surface of the solid - entry of a new portion of flammable gases (destruction products) - combustion.
Rice. 1.3. Dependence of speed V kinetic (1)
and diffusion (2) on temperature. Point A – transition
from the kinetic region to the diffusion region
Many solid combustible substances (wood, cotton, straw, polymers) contain oxygen. Therefore, their combustion requires less oxygen from the air. And the combustion explosive(BB) practically does not require an external oxidizer at all.
Thus, the combustion of an explosive is the self-propagation of an exothermic reaction zone of its decomposition or the interaction of its components by transferring heat from layer to layer.
All flammable (combustible) substances contain carbon and hydrogen - the main components gas-air mixture, participating in the combustion reaction. The ignition temperature of flammable substances and materials varies and does not exceed 300°C for most.
The physicochemical basis of combustion consists in the thermal decomposition of a substance or material into hydrocarbon vapors and gases, which, under the influence of high temperatures, enter into a chemical reaction with an oxidizing agent (oxygen from the air), turning into carbon dioxide(carbon dioxide), carbon monoxide (carbon monoxide), soot (carbon) and water, and produces heat and light.
Ignition is the process of flame propagation through a gas-vapor-air mixture. When the rate of flow of flammable vapors and gases from the surface of a substance is equal to the speed of flame propagation along them, stable flaming combustion is observed. If the flame speed is greater than the flow rate of vapors and gases, then the gas-vapor-air mixture burns out and the flame self-extinguishes, i.e. flash.
Depending on the speed of gas flow and the speed of flame propagation through them, one can observe:
combustion on the surface of a material, when the rate of release of the flammable mixture from the surface of the material is equal to the rate of fire propagation along it;
combustion with separation from the surface of the material, when the rate of release of the flammable mixture is greater than the speed of flame propagation along it.
Combustion of a gas-vapor-air mixture is divided into diffusion or kinetic.
Kinetic combustion is the combustion of pre-mixed combustible gases and an oxidizer (air oxygen). This type of combustion is extremely rare in fires. However, it is often found in technological processes: V gas welding, cutting, etc.
During diffusion combustion, the oxidizer enters the combustion zone from the outside. It comes, as a rule, from below the flame due to the vacuum that is created at its base. In the upper part of the flame, the heat released during the combustion process creates pressure. The main combustion reaction (oxidation) occurs at the flame boundary, since gas mixtures flowing from the surface of the substance prevent the oxidizer from penetrating deep into the flame (displace air). Most of the combustible mixture in the center of the flame, which has not entered into an oxidation reaction with oxygen, is the products of incomplete combustion (CO, CH4, carbon, etc.).
Diffusion combustion, in turn, can be laminar (calm) and turbulent (uneven in time and space). Laminar combustion is characteristic when the speed of flow of the combustible mixture from the surface of the material is equal to the speed of flame propagation along it. Turbulent combustion occurs when the exhaust speed
flammable mixture significantly exceeds the speed of flame spread. In this case, the flame boundary becomes unstable due to the large diffusion of air into the combustion zone. Instability first occurs at the top of the flame and then moves to the base. Such combustion occurs in fires with their volumetric development (see below).
Combustion of substances and materials is possible only with a certain amount of oxygen in the air. Oxygen content at which the possibility of combustion is excluded various substances and materials, is established experimentally. So, for cardboard and cotton, self-extinguishing occurs at 14% (vol.) oxygen, and for polyester wool - at 16% (vol.).
Elimination of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, storage of flammable and combustible liquids, calcium carbide, alkali metals, phosphorus should be carried out in a tightly closed container.
7.3.2. Ignition sources
A necessary condition for ignition of a combustible mixture is ignition sources. Ignition sources are divided into open fire, heat heating elements and devices, electrical energy, energy of mechanical sparks, discharges of static electricity and lightning, energy of self-heating processes of substances and materials (spontaneous combustion), etc. Particular attention should be paid to identifying ignition sources available in production.
The characteristic parameters of ignition sources are taken according to:
The lightning channel temperature is 30,000°C with a current intensity of 200,000 A and an action time of about 100 μs. The energy of a spark discharge from the secondary impact of lightning exceeds 250 mJ and is sufficient to ignite combustible materials with a minimum ignition energy of up to 0.25 J. The energy of spark discharges when high potential is carried into a building through metal communications reaches values of 100 J or more, which is sufficient to ignite all combustible materials materials.
Polyvinyl chloride insulation electric cable(wires) ignites when the short circuit current ratio is more than 2.5.
The temperature of welding particles and nickel particles of incandescent lamps reaches 2100°C. The droplet temperature when cutting metal is 1500°C. The arc temperature during welding and cutting reaches 4000°C.
The scattering zone of particles during a short circuit at a wire height of 10 m ranges from 5 (probability of hitting 92%) to 9 (probability of hitting 6%) m; when the wire is located at a height of 3 m - from 4 (96%) to 8 m (1%); when located at a height of 1 m - from 3 (99%) to 6 m (6%).
The maximum temperature, °C, on the bulb of an incandescent light bulb depends on the power, W: 25 W - 100 °C; 40 W - 150°C; 75 W - 250°C; 100 W - 300°C; 150 W - 340°C; 200 W - 320°C; 750 W - 370°C.
Sparks of static electricity generated when people work with moving dielectric materials reach values from 2.5 to 7.5 mJ.
Flame temperature (smoldering) and burning time (smoldering), °C (min), of some low-calorie heat sources: smoldering cigarette - 320-410 (2-2.5); smoldering cigarette - 420-460 (26-30); burning match - 620-640 (0.33).
For sparks chimneys, boiler houses, pipes of steam and diesel locomotives, as well as
other machines, fires, it has been established that a spark with a diameter of 2 mm is fire hazardous if it has a temperature of about 1000°C, with a diameter of 3 mm - 800°C, and with a diameter of 5 mm - 600°C.
1.3.3. Spontaneous combustion
Spontaneous combustion is inherent in many flammable substances and materials. This distinctive feature this group of materials.
Spontaneous combustion can be of the following types: thermal, chemical, microbiological.
Thermal spontaneous combustion is expressed in the accumulation of heat by the material, during which self-heating of the material occurs. The self-heating temperature of a substance or material is an indicator of its fire hazard. For most flammable materials, this indicator ranges from 80 to 150°C: paper - 100°C; construction felt - 80°C; leatherette - 40°C; wood: pine - 80, oak - 100, spruce - 120°C; raw cotton - 60°C.
Prolonged smoldering before the start of flaming combustion is distinctive characteristic processes of thermal spontaneous combustion. These processes are detected by the long-lasting and persistent smell of smoldering material.
In the case when the burner is fed gas only, combustion occurs due to the interaction of gas with oxygen from the surrounding air. Since combustion occurs in the process mutual diffusion fuel and oxidizer, such combustion is called diffusion combustion. WITH burning rate determined by intensity mixing process fuel and oxidizer. Depending on the nature of the mixture, there are laminar And turbulent diffusion combustion.
Laminar diffusion combustion occurs under laminar flow of gas flowing from the burner. Sustainable combustion zone installed on the surface where the fuel and oxidizer are in stoichiometric ratio. The resulting combustion products diffuse both into the surrounding space and inside the torch. The structure of a diffusion laminar torch during hydrogen combustion is shown in Fig. 3.19. The fuel concentration decreases from highest value on the axis of the jet to zero in the flame front, and the oxygen concentration increases from zero in the flame front to its value in the surrounding flow. The concentration of H 2 O products and temperature T are maximum in the flame front.
Figure 3.19 - Structure of a diffusion laminar flame during hydrogen combustion
In a diffusion laminar flame, the temperature reaches its maximum value in the combustion zone. The gas escaping from the burner is heated by heat transferred by conduction and diffusion before entering the combustion zone.
In the case of combustion of hydrocarbons, their heating leads to thermal decomposition with the formation soot And hydrogen. Fine particles of soot and free carbon located in the flame, when heated, cause flame glow. Diffusion combustion of soot particles proceeds relatively slowly, which can result in underburning of fuel.
Laminar flow height diffusion flame can be calculated using the formula
Where W– gas flow rate;
R– radius of the nozzle hole;
D– molecular diffusion coefficient.
The intensity of diffusion combustion depends on the intensity of mixture formation.
For industrial conditions, the method is more important turbulent diffusion combustion, since mass transfer in the flame is more intense. With increasing speed, the size of the torch increases, reaching a maximum. At the same time, the correctness of the outlines and the stability of its top are lost, and flame turbulization, capturing more and more of its length. As the turbulent front approaches the plume root, its height is several is decreasing, remaining constant further. Upon reaching critical speed gas jet, the entire torch becomes turbulent, and subsequently, as the speed increases, the height of the torch does not change. The relative height of the turbulent diffusion plume is calculated by the formula
Where h– torch length;
d– diameter of the burner mouth;
Combustible systems can be chemically homogeneous or heterogeneous. TO chemically homogeneous These include systems in which flammable substances and air are uniformly mixed (mixtures of flammable gases, vapors or dusts with air). TO chemically heterogeneous These include systems in which flammable matter and air are not mixed and have interfaces: solid combustible materials and liquids in the air, jets of flammable gases and vapors entering the air, etc.
An example of the combustion of vapors and gases (homogeneous combustion) is the combustion of vapors rising from the free surface of a liquid, or the combustion of gas leaving a pipe. Since the partial pressure of oxygen in the air is 21.2 kPa, and the pressure in the combustion zone is zero, oxygen from the air diffuses through the layer of combustion products to the combustion zone. Consequently, the rate of combustion reaction depends on the rate of oxygen diffusion.
An example of combustion on the surface of a solid (heterogeneous combustion) is the combustion of anthracite, coke, charcoal. In this case, the diffusion of oxygen to the combustion zone is also hampered by combustion products, as can be seen from the diagram. The oxygen concentration in the air volume (C 1) is significantly greater than its concentration near the combustion zone (Co). In the absence of a sufficient amount of oxygen in the combustion zone, the chemical combustion reaction is inhibited.
Thus, the total combustion time of a chemically inhomogeneous combustible system consists of the time required for the occurrence of physical contact between the combustible substance and air oxygen f, and
time spent on the chemical reaction itself x
In the case of homogeneous combustion, the value φ is called the mixture formation time, and in the case heterogeneous combustion- the time of transport of oxygen from the air to the solid combustion surface.
Depending on the ratio of φ and x, combustion is called diffusion or kinetic. When burning chemically inhomogeneous combustible systems, the time of diffusion of oxygen to the combustible substance is disproportionately longer than the time required for the chemical reaction to occur, i.e. φ >> x, and practically φ x. This means that the combustion rate is determined by the rate of oxygen diffusion to the combustible substance. In this case, the process is said to occur in the diffusion region. This kind of combustion is called diffusion. All fires are diffusion combustion.
If the time of the physical stage of the process turns out to be disproportionately less than the time required for the chemical reaction to occur, i.e.<< х, то можно принять г х. Скорость процесса практически определяется только скоростью химической реакции. Такое горение называется kinetic. This is how chemically homogeneous combustible systems burn, in which oxygen molecules are well mixed with molecules of the combustible substance, and no time is spent on mixture formation. Since the rate of chemical reaction at high temperatures is high, the combustion of such mixtures occurs instantly and has the character explosion.
Diffusion flame
The space in which vapors and gases burn is called flame or a torch. The flame can be kinetic or diffusion depending on whether a pre-prepared mixture of vapors or gases with air burns or whether such a mixture is formed in the flame during the combustion process. In fire conditions, gases, liquids and solids burn by diffusion flame.
The structure of the diffusion flame significantly depends on the cross section of the flow of flammable vapors and gases and its speed. Based on the nature of the flow, laminar and turbulent diffusion flames are distinguished. A laminar flame occurs with small sections of the flow of vapors or gases moving at low speed (the flame of a candle, match, gas in a small-diameter burner, etc.). During fires, turbulent flames are formed. It has been less studied, and the principles of the laminar flame theory are used to explain this phenomenon.
The flame consists of a combustion zone and a vapor zone, the latter
occupies almost the entire volume of the flame. A flame of a similar structure is also formed during the combustion of gases and solids, if the speed of movement of gases and vapors corresponds to the laminar regime.
The combustion zone in a diffusion flame is a very thin layer in which the combustion reaction occurs. The transformation of substances and the release of heat in this layer cause the occurrence of molecular diffusion in the adjacent layers of air and fuel. The cause of molecular diffusion is the difference in partial pressures and temperatures of the gases involved in combustion.
The distribution of concentrations of gases and vapors in a laminar diffusion flame and its surrounding environment reflects the diffusion processes occurring in the flame. Emerging
in the combustion zone, combustion products diffuse both into the air and into flammable vapors and gases. In a small-sized flame, combustion products are found throughout the entire volume of the vapor and gas zone, and in a large-sized flame only in the layer adjacent to the combustion zone. The oxygen concentration in the combustion zone is zero, since it completely reacts. Due to
However, oxygen cannot diffuse into the vapor zone, and there is no combustion in it.
A turbulent flame differs from a laminar flame in that it does not have clear outlines and a constant position of the flame front. Its temperature when burning petroleum products is: 1200 °C for gasoline, 1100 °C for tractor kerosene, diesel fuel, crude oil and 1000 °C for fuel oil. When burning wood in stacks, the temperature of the turbulent flame is 1200-1300 °C.
Combustion air consumption
The minimum amount of air required for complete combustion of a unit of mass (kg) or volume (m 3) of a combustible substance is called theoretically necessary and is designated V o c.
Flammable substance -
For such flammable substances, regardless of their state of aggregation, the theoretically required amount of air is determined from the combustion reaction equations. On m kmol of combustible substance accounted for P kmol of oxygen and nitrogen from the combustion reaction equation. Having designated the mass (in kg) of a flammable substance, numerically equal to its molecular mass, through M, make up the proportion
TM kg- P 22.4 m 3
1 kg - V o in m 3,
where 22.4 is the volume of 1 kmol of gases (at O °C and 101325 Pa).
Theoretically, the required volume of air for combustion of 1 kg of substance is equal (from the proportion)
If the volume of air obtained according to formula (1) must be brought to other conditions, then use the formula
Where T- set gas temperature, K;
R- set pressure, Pa.
The theoretically required volume of air for the combustion of 1 m 3 of combustible gases is determined by the formula
Flammable substance -
Such substances are wood, peat, coal, etc. To determine the theoretically required volume of air, you need to know the elemental composition of the combustible substance, expressed in mass percent, i.e., the content of C, H, O, S, N, ash (A), moisture (W). The elemental composition of a substance is determined in an analytical laboratory. To calculate V o in , Let's write down the equation for the combustion reaction of carbon, hydrogen and sulfur and the mass ratio of the reactants
C + O 2 = CO 2 2H 2 + O 2 = 2H 2 O S + O 2 = SO 2
12 + 32 = 44 4 + 32 = 36 32 + 32 = 64
If the combustion of 12 kg of carbon requires 32 kg of oxygen, then for 0.01 kg of carbon, i.e. 1% (wt.) it will require oxygen 0.01 32/12 = 0.01 8/3 kg, for for hydrogen, accordingly, 0.01·32/4 = 0.01·8 kg will be required and for sulfur 0.01·32/32 = 0.01·1 kg of oxygen.
For complete combustion of 1 kg of combustible substance, oxygen is required (in kg)
[C] + 8 0.01 [N] + 0.01 [S] - 0.01 [O]
where [C], [H], [S], [O] is the content of carbon, hydrogen, sulfur and oxygen in a combustible substance, % (mass.).
For the calculated amount of oxygen in the air, there is 77/23 times more nitrogen. The sum of nitrogen and oxygen is the mass of air L o in (in kg) required for the combustion of 1 kg of substance
After transformation we get
L o in = 0.3478 
(4)
To express the amount of air in volumetric units, you need to divide the right side of expression (4) by the mass of 1 m3 of air under normal conditions, i.e. by 1.293 kg/m3. As a result we get
V o v = 0.269 
(5)
Flammable substance - mixture of gases.
This group of substances includes flammable gases, for example natural, blast furnace, coke, etc. All of them contain CO, CH 4, H 2, H 2 S, C 2 H 4, etc. in varying quantities. The composition of combustible gases is usually expressed in volume percent. To derive the formula for calculating V o in, we write the equation
combustion reactions of the most common gases:
CH 4 + 2O 2 = C0 2 + 2H 2 O H 2 S + 1.5O 2 = H 2 O + S0 2
2CO + 0 2 = 2CO 2 2H 2 + O 2 = 2H 2 O
If the combustion of 1 m 3 of methane requires 2 m 3 of oxygen, as can be seen from the equation, then the combustion of 0.01 m 3 of methane, i.e. 1% (vol.), will require 0.01·2 m 3 of oxygen. For the combustion of 1 m 3 of carbon monoxide, 0.01/2 m 3 of oxygen is required, the same amount of oxygen is required for the combustion of 1 m 3 of hydrogen, and for the combustion of hydrogen sulfide, 0.01·1.5 m 3 of oxygen is required.
For complete combustion of 1 m 3 of combustible gas, oxygen is required (in m 3)
0.01 2[CH 4 ] +
where [CH 4 ], [H 2 ], [CO], [H 2 S] and is the content of methane, hydrogen,
carbon monoxide, hydrogen sulfide and oxygen, % (vol.).
In air, this volume of oxygen accounts for 79/21 times more nitrogen. The sum of nitrogen and oxygen is the volume (m3) of air required for the combustion of 1 m3 of gas
After transformation we get
As can be seen from equation (6), the numbers in its numerator are the coefficients for oxygen in the equations of combustion reactions. Therefore, if the gas contains other flammable components, they can be put into equation (6) with coefficients taken from their combustion equations.
In practice, combustion during a fire consumes much more air than is theoretically required. The difference between the amount of air practically consumed for combustion and the theoretically required amount is called excess air. The ratio of the amount of air practically consumed for combustion (V inc) to the theoretically necessary is called the excess air coefficient and is denoted
Considering that the oxygen concentration in the air is 21% (vol.), and the percentage of free oxygen in the combustion products is determined from the analysis, you can easily find the excess coefficient
air
Combustion products. Smoke
Combustion products are gaseous, liquid and solid substances formed as a result of the combination of a combustible substance with oxygen during the combustion process. Their composition depends on the composition of the burning substance and its combustion conditions. When they burn in a sufficient amount of air and at a high temperature, the products of complete combustion are formed: CO 2, H 2 0, N 2. When burning in an insufficient amount of air or at a low temperature, in addition to the products of complete combustion, incomplete combustion products are formed: CO, C (soot).
Combustion products are inorganic substances such as sulfur, phosphorus, sodium, potassium, calcium, aluminum, titanium, magnesium, etc.
in most cases they are solid substances, for example P 2 O 5, Na 2 O 2, CaO, MgO. They are formed in a dispersed state, therefore they rise into the air in the form of dense smoke. The combustion products of aluminum, titanium and other metals are in a molten state during the combustion process.
Smoke is a dispersed system consisting of tiny solid particles suspended in a mixture of combustion products with air. The diameter of smoke particles ranges from 1 to 0.01 microns.
The smoke contains products of thermal-oxidative decomposition of flammable substances. They are formed when heating flammable substances that are not yet burning and are in an environment of air or smoke containing oxygen.
The products of incomplete combustion and thermal-oxidative decomposition are, in most cases, toxic substances, therefore extinguishing fires in premises is carried out only in oxygen insulating gas masks.
Flammable substance - individual chemical compound.
In this case, the calculation is carried out based on the combustion reaction equation. The volume of wet combustion products per unit mass (kg) of a combustible substance under normal conditions is calculated using the formula
V p..s. = 
(9)
where V p..s. - volume of wet combustion products, m 3 /kg; m co2, m n2o, m N 2, m mountains - the number of kilomoles of carbon dioxide, water vapor, nitrogen and combustible substances in the combustion reaction equation; M- mass of flammable substance, numerically equal to molecular weight, kg.
Flammable substance - complex mixture of chemical compounds.
If the elemental composition of a complex combustible substance is known, then the composition and amount of combustion products of 1 kg of substance can be determined from the equation of the combustion reaction of individual elements. For this
compose equations for the combustion reaction of carbon, hydrogen, sulfur and determine the volume of combustion products per 1 kg of combustible substance. The equation for the combustion reaction of carbon has the form
C + O 2 + 3.76 = CO 2 + 3.76 N 2
When 1 kg of carbon is burned, the result is 22.4/12 = 1.86 m3
CO 2 and 22.4 · 3.76/12 = 7.0 m 3 N 2.
When carbon, hydrogen and sulfur burn, oxygen comes from the air. However, the combustible substance may contain oxygen, which also takes part in combustion. In this case, correspondingly less air is consumed for combustion of the substance.
The combustible substance may contain nitrogen and moisture, which during the combustion process become combustion products. To account for them, it is necessary to know the volume of 1 kg of nitrogen and water vapor under normal conditions. The volume of 1 kg of nitrogen is 0.8 m3, and the volume of water vapor is 1.24 m3.
In air at 0°C and a pressure of 101325 Pa, per 1 kg of oxygen there is 3.76 · 22.4/32 = 2.63 m 3 of nitrogen.
Based on the given data, determine the composition and volume of combustion products of 1 kg of combustible substance
Combustible substance - a mixture of gases.
The amount and composition of combustion products for a mixture of gases is determined by the equation of the combustion reaction of the components that make up
mixture. Then the composition and quantity of combustion products of the gas mixture are determined.
Analysis of combustion products taken from fires in various rooms shows that they always contain a significant amount of oxygen. If a fire occurs in a room with closed window, door or other openings, then the fire in the presence of fuel can continue until the oxygen content in the mixture of air with combustion products in the room decreases to 14-16% (vol.). Consequently, during fires in enclosed spaces, the oxygen content in combustion products can range from 21 to 14% (vol.).
Heat of combustion
Reactions accompanied by the absorption of heat, as well as the compounds formed during this process, are called endothermic. Without external heating, the endothermic reaction stops.
Reactions accompanied by the release of heat, as well as the compounds formed during this process, are called exothermic. All combustion reactions are exothermic. Due to the release of heat, they, having arisen at one point, are able to spread to the entire mass of reacting substances.
Hess's law is as follows: the thermal effect of a chemical transformation does not depend on the path along which the reaction proceeds, but depends only on the initial and final states of the system, provided that the temperature and pressure (or volume) at the beginning and end of the reaction are the same.
Methane can be produced from 1 mole of carbon and 2 moles of hydrogen. When methane is burned, it produces 2 moles of water and 1 mole of carbon dioxide.
C + 2H 2 = CH 4 + 74.8 kJ (Q)
CH 4 + 2O 2 = CO 2 + 2H 2 O + Q mountains
The same products are formed by the combustion of hydrogen and carbon. During these reactions, the total amount of heat released is 963.5 kJ.
2H 2 + O 2 = 2H 2 O + 570.6 kJ
C+ O 2 = CO 2 + 392.9 kJ
963.5 kJ (Q)
Since the initial and final products are the same in both cases, their total thermal effects must be equal according to Hess's law, i.e.
Q 1 + Q mountains = Q
Q mountains = Q - Q 1
therefore, the heat of combustion of methane will be equal to
Q mountains = 963.5 - 74.8 = 888.7 kJ/mol
Thus, the heat of combustion of a chemical compound (or their mixture) is equal to the difference between the sum of the heats of formation of combustion products and the heat of formation of the burned chemical compound (or substances that make up the combustible mixture). Therefore, to determine the heat of combustion of chemical compounds, it is necessary to know the heat of their formation and the heat of formation of the products obtained after combustion.
The heat of combustion is experimentally determined in a bomb calorimeter and a gas calorimeter. There are higher and lower calorific values. Higher calorific value Q in is the amount of heat released during the complete combustion of 1 kg or 1 m 3 of a combustible substance, provided that the hydrogen contained in it burns to form liquid water.
Lower calorific value Qn is the amount of heat released during the complete combustion of 1 kg or 1 m 3 of a combustible substance, provided that hydrogen is burned until water vapor is formed and the moisture of the combustible substance is evaporated.
The higher and lower heats of combustion of solid and liquid combustible substances can be determined using the formulas of D. I. Mendeleev
Q in = 339.4 + 1257 - 108.9 (12)
Q n = 339.4 + 1257 - 108.9 - 25.1(9 + W), (13)
where Q in, Q n - higher and lower calorific value, kJ/kg; [S], [N],
[O], [S], W - content of carbon, hydrogen,
oxygen, combustible sulfur and moisture, %.
There is a lower limit of calorific value, below which substances become incapable of combustion in the air atmosphere. Experiments show that substances are non-flammable unless they are classified as
explosive and if their lower calorific value in air does not exceed 2100 kJ/kg. Consequently, the heat of combustion can serve as an approximate estimate of the flammability of substances. However, it should be noted that the flammability of solids and materials largely depends on their condition. Thus, a sheet of paper, easily ignited by the flame of a match, when applied to the smooth surface of a metal plate or concrete wall, becomes difficult to combust. Consequently, the flammability of substances also depends on the rate of heat removal from the combustion zone.
If soot is formed during combustion, then, consequently, the combustible substance emits less heat than the amount indicated in the tables. For substances rich in carbon, the underburning coefficient is 0.8 -0.9. Consequently, in fires when burning 1 kg of rubber, not 33520 kJ can be released, but only 33520 0.8 = 26816 kJ.
Fire size is usually characterized by the area of the fire. The amount of heat released per unit area of a fire per unit time is called the heat of the fire Q p
where v m is the mass burnout rate, kg/(m 2 s).
Combustion temperature
The temperature to which combustion products are heated during the combustion process is called combustion temperature. There are calorimetric, theoretical and actual combustion temperatures. The actual combustion temperature for fire conditions is called fire temperature.
The calorimetric combustion temperature is understood as the temperature to which the products of complete combustion are heated under the following conditions:
I) all the heat released during combustion is spent on heating
reduction of combustion products (heat loss is zero);
2) initial temperatures of air and combustible substance
equal to 0°C;
3) the amount of air is equal to the theoretically required ( =1);
4) complete combustion occurs.
The calorimetric combustion temperature depends only on the composition of the combustible substance and does not depend on its quantity.
To assess fire conditions, only calorimetric temperature is used.
combustion temperature and fire temperature. There is a distinction between internal and external fire temperatures.
The internal fire temperature is the average temperature of the smoke in the room where the fire occurs.
External fire temperature - flame temperature.
When calculating the calorimetric combustion temperature and the temperature of the internal fire, it is assumed that the lower heat of combustion Qn of the combustible substance is equal to the energy qg required to heat the combustion products from 0°C to the calorimetric combustion temperature
The quantity q g will be called conventionally the heat content of combustion products
q g = С´ pm ·t g
where V p.s. - volume of combustion products, m 3 /kg; С´ pm - average volume -
heat capacity of combustion products, kJ/(m 3 K); t g - combustion temperature, °C.
Since combustion products consist of several gaseous substances, the heat capacity of which is different, their total heat content can be expressed as follows:
q g =q RO2 + q H2O + q N2 = V RO2 C´ CO2 t g + V H2O C´ H2O t g + V N2 C´ N2 t g
where V RO2, V H2O, V N2 are the volumes of combustion product components
(RO 2 =CO 2 +S0 2); С´СО2, С´Н2О, С´N2 - heat capacity of the components of combustion products (heat capacity of CO 2 is taken for the mixture CO 2
and S0 2).
To determine t g, the heat content of combustion products is calculated at several temperatures and two values are selected, between which the value of the lower heat of combustion of the substance is located. The desired temperature is then determined by interpolation.
In order to judge the nature of temperature changes during a fire depending on various combustion conditions, the concept of average non-volumetric fire temperature was introduced, which is understood as the average value of the value
temperatures measured by thermocouples at various points within an internal fire.
The structure of a diffusion flame above the surface of a flammable liquid, the mechanism and speed of its propagation.
The structure of the diffusion flame above the flammable liquid mirror is approximately the same. The only difference is that the flammable vapors coming from the surface of the liquid do not have such an initial reserve of kinetic energy as a gas stream, and before ignition they mix with the surrounding gaseous medium not due to the kinetic energy of the incoming gas flow, but more slowly through the mechanism of convective and molecular diffusion . But if an ignition source is connected to the resulting steam-air mixture, a flame torch will appear, which will change the ratio of gas and heat flows above the liquid surface: hot combustion products, as lighter ones, will rush upward, and in their place fresh cold air will come from the surrounding space, which will lead to dilution of flammable liquid vapors. A radiant flow of thermal energy will flow from the flame to the liquid mirror, which will go to heat the surface layers of the liquid and, as they warm up, intensify the process of its evaporation.
If the liquid before ignition had a temperature significantly higher than the ignition temperature, then the combustion of the liquid above the tank or spilled liquid will intensify and progress, and the size of the flame will grow. Accordingly, the intensity of the radiant heat flow to the surface of the liquid increases, the evaporation process intensifies, the intensity of the convective gas flow around the flame increases, it will be more strongly pressed from the sides, taking the shape of a cone, increasing in size. With further combustion, the flame enters a turbulent combustion mode and will grow until a thermal and gas-dynamic equilibrium regime is established. The maximum temperature of the turbulent diffusion flame of most flammable liquids does not exceed 1250-1350°C.
The propagation of combustion over the surface of the liquid surface depends on the rate of formation of the combustible mixture through the mechanisms of molecular and convective diffusion. Therefore, for liquids with a temperature below the ignition temperature, this speed is less than 0.05 m/s, and for liquids heated above the ignition temperature it reaches 0.5 m/s or more.
Thus, the speed of flame propagation over the surface of a flammable liquid depends mainly on its temperature.
If the liquid temperature is equal to or higher than the ignition temperature, combustion may occur. Initially, a small flame is established above the surface of the liquid, which then quickly increases in height and after a short period of time reaches its maximum value. This suggests that a certain heat and mass transfer has established between the combustion zone and the surface of the liquid. Heat is transferred from the combustion zone to the surface layer of liquid by radiation and thermal conduction through the walls of the container. There is no convective flow, since the vapor flow in the plume is directed upward, i.e. from a less heated surface to a more heated surface. The amount of heat transferred to the liquid from the combustion zone is not constant and depends on the temperature of the torch, the transparency of the flame, its shape, etc.
The liquid receives some of the heat from the tank wall. This portion of the heat can be significant when the liquid level in the tank is low and also when flames are flowing around the outer wall of the tank. The heat perceived by the liquid is mostly spent on evaporation and heating it, and some heat is lost by the liquid to the environment:
Q = q 1 + q 2 + q 3
where Q is the amount of heat received by the liquid from the flame, kJ/ (m 2 -s);
q 1 - the amount of heat lost by the liquid into the environment, kJ/ (m 2 -s);
q 2 - amount of heat spent on liquid vaporization, kJ/ (m 2 s);
qз - amount of heat spent on heating the liquid, kJ/ (m 2 -s).
If the diameter of the tank is large enough, then the value of q1 compared to q 2 and q 3 can be neglected:
Q = q 2 + q 3 = rlс + cpс (T-T 0) u.
Where r is the heat of evaporation of liquid, kJ/kg;
Ср - heat capacity of liquid, kJ/ (kg K);
p - liquid density, mg/m3;
T is the temperature at the surface of the liquid, K;
T 0 - initial liquid temperature K;
u is the growth rate of the heated liquid layer, m/s;
l - linear speed of liquid burnout, m/s.
If an individual liquid burns, then the composition of its vapor phase does not differ from the composition of the liquid phase. If a liquid of complex composition (mixture) burns, then fractional distillation occurs in its upper layer and the composition of the spherical phase differs from the composition of the liquid phase. Such mixtures include oil and all petroleum products. When they burn, mostly low-boiling fractions evaporate, as a result of which the liquid phase changes its composition, and at the same time vapor pressure, specific gravity, viscosity and other properties. Table 3.1 shows the change in the properties of Karachukhur oil in the surface layer when it burns in a reservoir with a diameter of 1.4 m.
Table 1.11.1
Changes in the properties of Karachukhur oil during combustion
|
Physicochemical characteristics |
Sample before experiment |
Samples after combustion, h |
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|
Density three 293 K, kg/m 3 |
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|
Kinematic viscosity at 373. K, m 2 / s |
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|
Flash point according to Brenken, K |
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|
Start of boiling, K |
According to Table 1.11.1, due to the burnout of low-boiling fractions, the density of the remaining product increases. The same thing happens with viscosity, flash point, resin content and boiling point. Only the moisture content decreases as the oil burns out. The intensity of changes in these properties during combustion in tanks of different diameters is not the same. In large-diameter tanks, due to an increase in convection and the thickness of the liquid layer involved in mixing, the rate of change in these properties decreases. The change in the fractional composition of petroleum products that occurs in the upper layer gradually leads to a change in the layer in the thickness of the heated petroleum product.
If you use the first law of D.P. Konovalov, the conclusion about the combustion of mixtures can be formulated as follows: a mixture of two liquids is enriched during combustion with those components, the addition of which to the liquid lowers the vapor pressure above it (or increases the boiling point). This conclusion is also valid for mixtures in which the number of components is more than two.
When burning mixtures of flammable and some flammable liquids with water as a result of fractional distillation, the percentage of water in the liquid phase increases all the time, which leads to an increase in the specific gravity of the burning mixture. This phenomenon is typical for mixtures in which the flammable component has a boiling point below the boiling point of water (methyl, ethyl alcohol, diethyl ether, acetone, etc.). When such liquid mixtures burn for a long time, due to the increase in water in them, a moment comes when the combustion stops, although not all of the mixture has yet burned out.
A mixture of flammable liquids with water, when the boiling point of the liquid is higher than the boiling point of water, behaves somewhat differently during the combustion process. The percentage of water in the liquid phase does not increase, but decreases. As a result, the mixture burns out completely. This is how a mixture of acetic acid and water burns.
When petroleum products burn, their boiling point (see Table 1.11.1) gradually increases due to the fractional distillation that occurs, and therefore the temperature of the upper layer also increases. Figure 1.11.1 shows the change in temperature on the surface
Fig.1.11.1
At low liquid temperatures, heat transfer from the flame to the liquid plays a significant role in flame propagation. The flame heats the surface of the liquid adjacent to it, the vapor pressure above it increases, a flammable mixture is formed, which burns when ignited.
The moving flame heats the next section of the liquid surface, and so on.
The dependence of the speed of flame movement on the surface of the liquid on temperature is shown in Fig. 1.11.2.
When the liquid temperature is below the flash point, the flame travel speed is low.
It increases as the temperature of the liquid increases and becomes the same as the speed of flame propagation through the steam-air mixture at a liquid temperature above the flash point.
Fig.1.11.2 Change in the speed of flame movement along the surface of liquids depending on temperature: 1-isoamyl alcohol, 2 - butyl alcohol, 3 - ethyl alcohol, 4 - toluene
