All combustible (combustible) substances contain carbon and hydrogen, the main components of the gas-air mixture involved in the combustion reaction. The ignition temperature of combustible substances and materials is different and does not exceed 300°C for most.
The physical and chemical bases of combustion consist in the thermal decomposition of a substance or material to hydrocarbon vapors and gases, which, under the influence high temperatures enter into chemical action with an oxidizing agent (oxygen in the air), turning into carbon dioxide (carbon dioxide) during combustion, carbon monoxide(carbon monoxide), soot (carbon) and water, and this releases heat and light radiation.
Ignition is the process of flame propagation through a gas-vapor-air mixture. When the speed of the outflow of combustible vapors and gases from the surface of a substance is equal to the speed of flame propagation along them, stable flame combustion is observed. If the flame speed is greater than the speed of the outflow 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 the outflow of gases and the speed of propagation of the flame through them, one can observe:
- combustion on the surface of the material, when the rate of release of a combustible 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 combustible mixture is greater than the rate of propagation of the flame along it.
The combustion of a gas-vapor-air mixture is divided into diffusion or kinetic. The main difference is the content or absence of an oxidizing agent (air oxygen) directly in the combustible vapor-air mixture.
Kinetic combustion is the combustion of pre-mixed combustible gases and an oxidizer (air oxygen). In fires, this type of combustion is extremely rare. However, it is often found in technological processes: in gas welding, cutting, etc.
In diffusion combustion, the oxidizer enters the combustion zone from outside . It comes, as a rule, from the bottom of the flame due to the rarefaction that is created at its base. In the upper part of the flame, which releases heat during combustion, creates pressure. The main combustion reaction (oxidation) occurs at the edge of the flame, since the gas mixtures flowing from the surface of the substance prevent the penetration of the oxidizer 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, betrays the products of incomplete combustion (CO, CH 4, carbon, etc.).
Diffusion combustion, in turn, can be laminar (controversial) and turbulent (uneven in time and space). Laminar combustion is typical when the velocities of the outflow of the combustible mixture from the surface of the material and the speed of the spread of the flux along it are equal. Turbulent combustion occurs when the rate of exit of the combustible mixture significantly exceeds the rate of flame propagation. In this case, the flame boundary becomes unstable due to the large diffusion of air into the combustion zone. The instability first occurs at the top of the flame, and then moves to the bottom. Such combustion occurs in fires during its volumetric development (see below).
Combustion of substances and materials is possible only with a certain quality of oxygen in the air. The oxygen content, which excludes the possibility of burning various substances and materials, is established empirically. So, for cardboard and cotton, self-extinguishing occurs Ori 14% (vol.) oxygen, and polyester wool - at 16% (vol.).
The exclusion of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, the storage of flammable and combustible liquids, calcium carbide, alkali metals, phosphorus should be carried out in tightly closed containers.
1.2.2. Sources of ignition.
Necessary condition ignition of the combustible mixture are sources of ignition. Sources of ignition are divided into open fire, heat heating elements and appliances electrical energy, the energy of mechanical sparks, discharges of static electricity and lightning, the energy of self-heating processes of substances and materials (self-ignition), etc. Particular attention should be paid to the identification of sources of ignition available in the workplace.
The characteristic parameters of ignition sources are taken according to:
The temperature of the lightning channel is 30,000°C at a current strength of 200,000 A and an action time of about 100 μs. The energy of the spark discharge of 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 a high potential is brought into the building through metal communications reaches values of 100 J or more, which is sufficient to ignite all combustible materials.
PVC insulation electric cable(wire) 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 temperature of the drops when cutting metal is 1500°C. The temperature of the arc during welding and cutting reaches 4000°C.
The particle expansion zone 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, °С, on the flask of an electric incandescent bulb depends on the power, W: 25 W - 100°С; 40 W - 150°С; 75 W - 250°С; 100 W - 300°С; 150 W - 340°С; 200 W - 320°С; 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 (smoldering) temperature and burning (smoldering) time, "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 rooms, pipes of steam locomotives and diesel locomotives, as well as other machines, fires, it was found that a spark with a diameter of 2 mm is a fire hazard if it has a temperature of about 1000 ° C, a diameter of 3 mm - 800 ° C, a diameter of 5 mm - 600 ° C.
1.2.3. Spontaneous combustion
Spontaneous combustion is inherent in many combustible substances and materials. This distinguishing feature this group of materials.
Spontaneous combustion is of the following types: thermal, chemical, microbiological.
Thermal spontaneous combustion is expressed in the accumulation of heat by the material, during which the material self-heats. The self-heating temperature of a substance or material is an indicator of its fire hazard. For most combustible materials, this indicator lies in the range from 80 to 150 ° C: paper - 100 ° C; construction felt - 80°С; leatherette - 40°C; wood: pine - 80, oak - 100, spruce - 120°С; raw cotton - 60°C.
Prolonged smoldering before the onset of fiery combustion is hallmark thermal spontaneous combustion processes. These processes are detected by the long-lasting and persistent smell of smoldering material.
Chemical spontaneous combustion immediately manifests itself in fiery combustion. For organic matter this species spontaneous combustion occurs upon contact with acids (nitric, sulfuric), vegetable and industrial oils. Oils and fats, in turn, are capable of spontaneous combustion in an oxygen environment. inorganic substances capable of spontaneous combustion on contact with water (for example, sodium hydrosulfite). Alcohols ignite spontaneously on contact with potassium permanganate. Ammonium nitrate ignites spontaneously upon contact with superphosphate, etc.
Microbiological spontaneous combustion is associated with the release of thermal energy by microorganisms in the process of life in a nutrient medium for them (hay, peat, sawdust, etc.).
In practice, combined spontaneous combustion processes are most often manifested: thermal and chemical.
2. Indicators of fire and explosion hazard.
The study of the fire and explosion hazardous properties of substances and materials circulating in the production process is one of the main tasks of fire prevention aimed at eliminating a combustible environment from the fire system.
In accordance with GOST 12.1.044 According to the state of aggregation, substances and materials are divided into:
GASES - substances whose saturated vapor pressure at a temperature of 25 ° C and a pressure of 101.3 kPa (1 atm) exceeds 101.3 kPa (1 atm).
LIQUIDS - the same, but the pressure is less than 101.3 kPa (1 atm). Liquids also include solid melting substances, the melting or dropping point of which is less than 50 ° C.
SOLID - individual substances and their mixtures with a melting or dropping point above 50 ° C (for example, vaseline - 54 ° C), as well as substances that do not have a melting point (for example, wood, fabrics, etc.).
DUST - dispersed (crushed) solids and materials with a particle size of less than 850 microns (0.85 mm).
The nomenclature of indicators and their applicability for characterizing the fire and explosion hazard of substances and materials are given in Table 1.
The values of these indicators should be included in the standards and specifications on substances, as well as indicated in the passports of products.
Table 1
| Index | gases | Liquids | Solid | Dust |
| Flammability group | + | + | + | + |
| Flash point | - | + | - | - |
| Flash point | - | + | + | + |
| Auto ignition temperature | + | + | + | + |
| Flammable concentration limits | + | + | . - | + |
| Conditions for thermal spontaneous combustion | - | - | + | + |
| Oxygen index | - | - | + | - |
| Smoke generation coefficient | - | - | + | - |
| The ability to explode and burn when interacting with water, atmospheric oxygen and other substances | + | + | + | + |
| The toxicity index of combustion products polymer materials And others | + |
(The sign “+” indicates the applicability, the sign “-” the inapplicability of the indicator)
FLASH point (Tvsp,) - for liquids only - the lowest temperature of a condensed substance at which, under special test conditions, vapors are formed above its surface that can flash in air from an ignition source; stable combustion does not occur in this case.
FLASH POINT (Тв,) - except for gases - the lowest temperature of a substance at which a substance emits flammable vapors and gases at such a rate that ignition is observed when exposed to an ignition source.
AUTO-IGNITION temperature (T St) - the lowest temperature environment, at which self-ignition of the substance is observed.
CONDITIONS OF THERMAL AUTO-IGNITION - only for solids and dusts - an experimentally revealed relationship between the ambient temperature, the amount of a substance (material) and the time until its spontaneous combustion.
SELF-HEATING temperature is the most low temperature substance in which the spontaneous process of its heating does not lead to smoldering or fiery combustion.
A safe temperature for prolonged heating of a substance is considered to be a temperature not exceeding 90% of the self-heating temperature.
THE ABILITY TO EXPLODE AND BURN WHEN INTERACTION WITH WATER, AIR OXYGEN AND OTHER SUBSTANCES (mutual contact of substances) is a qualitative indicator that characterizes a special fire hazard some substances.
SMOKE GENERATION COEFFICIENT - only for solids - an indicator that characterizes the optical density of smoke generated during flame combustion or thermal-oxidative destruction (smoldering) of a certain amount of a solid substance (material) under special test conditions.
There are 3 groups of materials:
Materials with moderate smoke-generating ability have less amount of smoke when a person loses the ability to navigate.
or equal to the amount of combustion products at which fatal poisoning is possible. Therefore, the probability of loss of visibility in the smoke is higher than the probability of poisoning.
Examples smoke generating capacity building materials during smoldering (burning), m 3 / kg,:
Wood fiber (birch, aspen) - 62 (20)
Decorative laminate - 75 (6)
Plywood brand FSF - 140 (30)
Fiberboard lined with plastic - 170 (25)
INDEX OF TOXICITY OF COMBUSTION PRODUCTS OF POLYMERIC MATERIALS - the ratio of the amount of material to a unit volume closed space, in which the gaseous products formed during the combustion of the material cause the death of 50% of the experimental animals.
The essence of the method is to burn the test material in a combustion chamber and reveal the dependence of the lethal effect of gaseous combustion products on the mass of the material (in grams) per unit volume (1 m3) of the exposure chamber.
The classification of materials is given in the table:
* For materials extremely dangerous in terms of toxicity, the mass does not exceed 25 grams in order to create a lethal concentration of 1 m 3 in a time of 5 minutes. Accordingly, for a time of 15 minutes - up to 17; 30 min - up to 13; 60 min - up to 10 grams.
For example: Douglas pine - 21; vinyl fabric - 19; polyvinyl chloride - 16; elastic polyurethane foam - 18 (rigid - 14) g/m 3 with an exposure time of 15 minutes.
CONCENTRATION LIMITS OF FLAME PROPAGATION (IGNITION) - except for solids.
Lower (upper) concentration limits of flame propagation (ignition) - the minimum (maximum) content of a combustible substance in a homogeneous mixture with an oxidizing environment, at which flame propagation through the mixture is possible at any distance from the ignition source.
Bottom-top examples concentration limits, %: acetylene - 2.2-81; hydrogen - 3.3-81.5; natural gas - 3.8-24.6; methane - 4.8-16.7; propane - 2-9.5; butane - 1.5-8.5; gasoline vapors - 0.7-6; kerosene vapor - 1-1.3.
Smoldering temperature - for solids and dusts - the temperature of the substance at which sharp increase the rate of exothermic oxidation reactions, ending in the appearance of smoldering.
COMBUSTIBILITY GROUP - a classification characteristic of the ability of any substances and materials to burn.
By combustibility, substances and materials are divided into three groups: non-combustible, slow-burning and combustible.
Non-combustible (non-combustible) - substances and materials that are not capable of burning in air. Non-combustible substances can be fire and explosion hazardous (for example, oxidizing agents or substances that release products when interacting with water, atmospheric oxygen, or with each other).
FIRE-RESISTANT (flammable) - substances and materials that can burn in the air when exposed to an ignition source, but are not able to burn on their own after its removal.
COMBUSTIBLE (combustible) - substances and materials capable of spontaneous combustion, as well as ignite when exposed to an ignition source and burn independently after its removal.
Combustible liquids (GZH) with Tvsp<61°С в закрытом тигле или 66°С в открытом тигле относят к легковоспламеняющимся (ЛВЖ).
Particularly dangerous HZH is called flammable liquid with TVSP< 28°С.
GASES are considered combustible in the presence of concentration flammable limits (KLV); slow-burning - in the absence of CPV and the presence of Tsv; non-combustible - in the absence of CPV and Tsv.
LIQUIDS are considered combustible in the presence of TV; slow-burning - in the absence of TV and the presence of Tsv; non-combustible - in the absence of TV, Tsv, Tvsp, temperature and concentration limits of flame propagation (ignition).
3. Categories of premises for explosion and fire hazard.
According to the provisions of the fire safety standards NPB 105-03, categories of premises and buildings (or parts of buildings between fire walls - fire compartments) are established according to explosion and fire hazards, depending on the amount and fire and explosion hazardous properties of substances and materials located (circulating) in them, taking into account the characteristics technological processes of the productions located in them.
Premises, compartments, parts of a building, buildings of classes are subject to categorization depending on their belonging to a particular class according to functional fire hazard. Buildings and parts of buildings - premises or groups of premises that are functionally interconnected, according to functional fire hazard, are divided into classes depending on the method of their use and on the extent to which the safety of people in them in the event of a fire is endangered, taking into account their age, physical condition, the possibility of being in a state of sleep, the type of the main functional contingent and its quantity.
Premises, parts of buildings, buildings of classes F3.5., F4.3., F5.1., F5.2., F5.3. and workshops in buildings of classes F1, F2, F3 and F4, according to the provisions of clause 5.21* of SNiP 21-01-97*, belong to class F5.
The methodology given in NPB 105-03 should be used in the development of departmental technological design standards relating to the categorization of premises and buildings.
NPB 105-03 does not apply to premises and buildings for the production and storage of explosives, means of initiating explosives, buildings and structures designed in accordance with special norms and rules approved in the prescribed manner.
The categories of premises and buildings defined in accordance with PNB 105-03 should be used to establish regulatory requirements for ensuring the explosion and fire safety of these premises and buildings in relation to planning and development, number of floors, areas, placement of premises, design solutions, engineering equipment. Measures to ensure the safety of people should be assigned depending on the fire hazard properties and quantities of substances and materials in accordance with GOST 12.1.004-91 and GOST 12.3.047-98.
Categories of premises and buildings of enterprises and institutions are determined at the stage of designing buildings and structures in accordance with these standards, departmental standards for technological design or special lists approved in the prescribed manner.
According to the explosion and fire hazard, premises and buildings are divided into categories A, B, C1-C4, D and E. The categories of explosion and fire hazard of premises and buildings are determined for the most unfavorable period in relation to fire or explosion, based on the type of apparatus and premises located in combustible substances and materials, their quantity and fire hazardous properties, features of technological processes.
Determination of fire hazardous properties of substances and materials is carried out on the basis of test results or calculations according to standard methods, taking into account state parameters (pressure, temperature, etc.).
It is allowed to use reference data published by leading research organizations in the field of fire safety or standard reference data issued by the State Service. It is allowed to use fire hazard indicators for mixtures of substances and materials according to the most dangerous component.
| K-I | Characteristics of substances and materials, premises located (circulating) in the premises |
| A | Combustible gases (GH), flammable explosive liquids (flammable liquids) with a flash point of not more than 28 ° C in such an amount that they can form explosive vapor, gas-air mixtures, upon ignition of which an estimated overpressure of the explosion in the room develops, exceeding 5 kPa . Substances and materials capable of exploding and burning when interacting with water, atmospheric oxygen or with each other in such an amount that the calculated overpressure of the explosion in the room exceeds 5 kPa |
| B | Combustible dusts or fibers, flammable liquids, explosive with a flash point of more than 28 o C, flammable liquids (FL) in such an amount that they can form explosive dust-air or vapor-air mixtures, upon ignition of which an estimated excess explosion pressure in the room develops in excess of 5 kPa |
| B1-B4 | GZH and slow-burning liquids, solid flammable combustible and slow-burning substances and materials (including dust and fibers), substances and materials that can only burn when interacting with water, atmospheric oxygen or with each other, provided that the premises in which they available or in circulation, not classified as A or B |
| G | Non-combustible substances and materials in a hot, incandescent or molten state, the processing of which is accompanied by the release of radiant heat, sparks and flames; GG, GL and solids that are burned or disposed of as fuel |
| D | Non-flammable substances and materials in a cold state |
| 2 | | |
The structure of a diffusion torch of a flame above the surface of a combustible liquid, the mechanism and speed of its propagation.
The structure of the diffusion torch of the flame above the mirror of the combustible liquid is approximately the same. The only difference is that combustible vapors coming from the surface of the liquid do not have such an initial supply of kinetic energy as a gas jet, and before ignition they mix with the surrounding gas medium not due to the kinetic energy of the incoming gas flow, but more slowly by the mechanism of convective and molecular diffusion . But if an ignition source is brought to the resulting vapor-air mixture, then a flame will appear, which will change the ratio of gas and heat flows above the liquid mirror: hot combustion products, as lighter ones, will rush upwards, and fresh cold air will come in their place from the surrounding space, which will dilute the vapors of the flammable liquid. A radiant flow of thermal energy will come from the flame to the liquid mirror, which will go to heat the surface layers of the liquid and, as they heat up, intensifies the process of its evaporation.
If the liquid before ignition had a temperature significantly higher than the ignition temperature, then the burning of the liquid above the tank or the spilled liquid intensifies, progresses, and the size of the flame will increase. 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 form of a cone, increasing in size. With further combustion, the flame passes into the turbulent combustion regime, and will grow until the regime of thermal and gas-dynamic equilibrium 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 a liquid mirror depends on the rate of formation of a combustible mixture by the mechanism of molecular and convective diffusion. Therefore, for liquids with a temperature below the ignition temperature, this velocity 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 combustible liquid depends mainly on its temperature.
If the temperature of the liquid is equal to or higher than the ignition temperature, combustion may occur. First, a small flame is established above the surface of the liquid, which then rapidly increases in height and after a short period of time reaches its maximum value. This suggests that a certain heat and mass transfer has been established between the combustion zone and the liquid surface. The transfer of heat from the combustion zone to the surface layer of the liquid is carried out by radiation and heat 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 hotter surface. The amount of heat transferred to the liquid from the combustion zone is not constant and depends on the temperature of the flame, the transparency of the flame, its shape, etc.
The liquid receives part 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 the flames wash over the outer wall of the tank. The heat perceived by the liquid is mostly spent on evaporation and heating it, and a certain amount of heat is lost by the liquid to the environment:
Q \u003d 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 to the environment, kJ / (m 2 -s);
q 2 - the amount of heat consumed for the vaporization of the liquid, kJ / (m 2 s);
qz - the amount of heat spent on heating the liquid, kJ / (m 2 -s).
If the reservoir diameter is large enough, then the value of q1 compared to q 2 and q 3 can be neglected:
Q \u003d q 2 + q 3 \u003d rlc + cpc (T-T 0) u.
Where r is the heat of vaporization of the liquid, kJ/kg;
Cp is the heat capacity of the liquid, kJ/(kg K);
p is the density of the liquid, mg / m 3;
T is the temperature on the liquid surface, K;
T 0 is the initial temperature of the liquid K;
u is the growth rate of the heated liquid layer, m/s;
l - linear velocity 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. 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. These mixtures include oil and all petroleum products. During their combustion, the evaporation of light-boiling fractions to a greater extent occurs, as a result of which the liquid phase changes its composition, and with it the vapor pressure, specific gravity, viscosity and other properties. Table 3.1 shows the change in the properties of Karachukhur oil in the surface layer during its combustion in a tank with a diameter of 1.4 m.
Table 1.11.1
Changes in the properties of Karachukhur oil during combustion
|
Physicochemical characteristics |
Trial before experience |
Samples after burning, h |
|||
|
Density three 293 K, kg / m 3 |
|||||
|
Kinematic viscosity at 373. K, m 2 / s |
|||||
|
Flash point according to Brenken, K |
|||||
|
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 happens with viscosity, flash point, tar 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 the 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 oil products, which occurs in the upper layer, gradually leads to a change in the layer in the thickness of the heated oil product.
If we 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.
During the combustion of mixtures of flammable and some combustible 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 combustible component has a boiling point below the boiling point of water (methyl, ethyl alcohols, diethyl ether, acetone, etc.). With prolonged combustion of such liquid mixtures, due to the increase in water in them, there comes a moment when combustion stops, although not the entire mixture has yet burned out.
A mixture of combustible liquids with water, when the boiling point of the liquid is higher than the boiling point of water, behaves somewhat differently during combustion. 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 burning petroleum products, their boiling point (see Table 1.11.1) gradually increases due to the ongoing fractional distillation, and therefore the temperature of the upper layer also rises. 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 combustible mixture is formed, which, when ignited, burns.
The moved flame heats up the next section of the liquid surface, and so on.
The dependence of the speed of flame movement over the liquid surface on temperature is shown in Fig. 1.11.2.
When the liquid temperature is below the flash point, the flame velocity is low.
It increases as the temperature of the liquid rises and becomes the same as the speed of flame propagation through the vapor-air mixture at a liquid temperature above the flash point.
Fig.1.11.2 Change in the speed of movement of the flame on the surface of liquids depending on the temperature: 1-isoamyl alcohol, 2 - butyl alcohol, 3 - ethyl alcohol, 4 - toluene
| Parameter name | Meaning |
| Article subject: | Diffusion and kinetic combustion. |
| Rubric (thematic category) | Education |
Homogeneous and heterogeneous combustion.
Based on the considered examples, based on the state of aggregation of the mixture of fuel and oxidizer, ᴛ.ᴇ. from the number of phases in the mixture, they distinguish:
1. Homogeneous combustion gases and vapors of combustible substances in the environment of a gaseous oxidizer. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the combustion reaction proceeds in a system consisting of one phase (aggregate state).
2. Heterogeneous combustion solid combustible substances in a gaseous oxidizer environment. In this case, the reaction proceeds at the interface, while the homogeneous reaction takes place in the entire volume.
This is the burning of metals, graphite͵ ᴛ.ᴇ. practically non-volatile materials. Many gas reactions are of a homogeneous-heterogeneous nature, when the possibility of a homogeneous reaction occurring is due to the origin of a simultaneously heterogeneous reaction.
The combustion of all liquid and many solid substances, from which vapors or gases (volatile substances) are released, proceeds in the gas phase. The solid and liquid phases play the role of reservoirs for the reacting products.
For example, a heterogeneous reaction of spontaneous combustion of coal passes into a homogeneous phase of combustion of volatile substances. Coke residue burns heterogeneously.
According to the degree of preparation of the combustible mixture, diffusion and kinetic combustion are distinguished.
The types of combustion considered (except for explosives) are diffusive combustion. Flame, ᴛ.ᴇ. the combustion zone of a mixture of fuel with air, to ensure stability, must be constantly fed with fuel and oxygen in the air. The flow of combustible gas depends only on the rate of its supply to the combustion zone. The rate of entry of a combustible liquid depends on the intensity of its evaporation, ᴛ.ᴇ. on the vapor pressure above the surface of the liquid, and, consequently, on the temperature of the liquid. Ignition temperature It is customary to call the lowest temperature of a liquid at which the flame above its surface does not go out.
The combustion of solids differs from the combustion of gases by the presence of a stage of decomposition and gasification, followed by the ignition of volatile pyrolysis products.
Pyrolysis- ϶ᴛᴏ heating of organic substances to high temperatures without air access. In this case, decomposition, or splitting, of complex compounds into simpler ones occurs (coking of coal, cracking of oil, dry distillation of wood). For this reason, the combustion of a solid combustible substance into the combustion product is not concentrated only in the flame zone, but has a multi-stage character.
Heating of the solid phase causes decomposition and evolution of gases, which ignite and burn. The heat from the torch heats the solid phase, causing its gasification and the process is repeated, thus supporting combustion.
The solid combustion model assumes the presence of the following phases (Fig. 17):
Rice. 17. Combustion model
solid.
Heating of the solid phase. For melting substances, melting occurs in this zone. The thickness of the zone depends on the conductivity temperature of the substance;
Pyrolysis, or the reaction zone in the solid phase, in which gaseous combustible substances are formed;
Pre-flame in the gas phase, in which a mixture with an oxidizing agent is formed;
A flame, or reaction zone in the gas phase, in which the conversion of pyrolysis products into gaseous combustion products;
combustion products.
The rate of oxygen supply to the combustion zone depends on its diffusion through the combustion product.
In general, since the rate of a chemical reaction in the combustion zone in the types of combustion under consideration depends on the rate of arrival of the reacting components and the flame surface by molecular or kinetic diffusion, this type of combustion is called diffusion.
The flame structure of diffusion combustion consists of three zones (Fig. 18):
Zone 1 contains gases or vapours. There is no combustion in this zone. The temperature does not exceed 500 0 C. Decomposition, pyrolysis of volatiles and heating to the self-ignition temperature occur.
Rice. 18. The structure of the flame.
In zone 2, a mixture of vapors (gases) with atmospheric oxygen is formed and incomplete combustion occurs to CO with partial reduction to carbon (little oxygen):
C n H m + O 2 → CO + CO 2 + H 2 O;
In the 3rd outer zone, the products of the second zone are completely burned and the maximum flame temperature is observed:
2CO+O 2 \u003d 2CO 2;
The height of the flame is proportional to the diffusion coefficient and the flow rate of the gases and is inversely proportional to the density of the gas.
All types of diffusion combustion are inherent in fires.
Kinetic Combustion is usually called the combustion of pre-mixed combustible gas, vapor or dust with an oxidizing agent. In this case, the burning rate depends only on the physicochemical properties of the combustible mixture (thermal conductivity, heat capacity, turbulence, concentration of substances, pressure, etc.). For this reason, the burning rate increases sharply. This type of combustion is inherent in explosions.
In this case, when the combustible mixture is ignited at some point, the flame front moves from the combustion products into the fresh mixture. Thus, the flame during kinetic combustion is most often non-stationary (Fig. 19).
Rice. 19. Scheme of flame propagation in a combustible mixture: - ignition source; - direction of motion of the flame front.
Although, if the combustible gas is mixed with air and fed into the burner, then a stationary flame is formed during ignition, provided that the mixture supply rate is equal to the flame propagation speed.
If the gas supply rate is increased, the flame breaks away from the burner and may go out. And if the speed is reduced, then the flame will be drawn into the inside of the burner with a possible explosion.
According to the degree of combustion, ᴛ.ᴇ. the completeness of the combustion reaction to the end products, combustion happens complete and incomplete.
So in zone 2 (Fig. 18) combustion is incomplete, because insufficient oxygen is supplied, which is partially consumed in zone 3, and intermediate products are formed. The latter burn out in zone 3, where there is more oxygen, until complete combustion. The presence of soot in the smoke indicates incomplete combustion.
Another example: when there is a lack of oxygen, carbon burns to carbon monoxide:
If you add O, then the reaction goes to the end:
2CO + O 2 \u003d 2CO 2.
The burning rate depends on the nature of the movement of gases. For this reason, laminar and turbulent combustion are distinguished.
So, an example of laminar combustion is the flame of a candle in still air. At laminar combustion layers of gases flow in parallel, but without swirling.
Turbulent combustion- vortex motion of gases, in which the burning gases are intensively mixed, and the flame front is washed out. The boundary between these types is the Reynolds criterion, which characterizes the relationship between the forces of inertia and the forces of friction in the flow:
Where: u- gas flow rate;
n- kinetic viscosity;
l- characteristic linear size.
The Reynolds number at which the transition of a laminar boundary layer to a turbulent one occurs is usually called critical Re cr, Re cr ~ 2320.
Turbulence increases the rate of combustion due to more intense heat transfer from the combustion products to the fresh mixture.
Diffusion and kinetic combustion. - concept and types. Classification and features of the category "Diffusion and kinetic combustion." 2017, 2018.
Kinetic combustion is the combustion of a premixed mixture of fuel and oxidizer.
In this case, the flame through the combustible mixture will spread in all directions. The volume engulfed in flames will increase. The flame always spreads towards the unburned mixture.
Rice. 7.1. Scheme of flame propagation through a pre-mixed homogeneous mixture: 1 - initial combustible mixture; 2 - flame front; 3 - combustion products; d f.p. is the thickness of the flame front
The narrow strip between the initial mixture (1) and combustion products (PG) (3) is the flame (2). For most hydrocarbon mixtures with air, the thickness of this strip is 0.1-1.0 mm. This is the combustion zone or flame front. A chemical reaction takes place in it and all the heat is released. The luminescence is the result of the presence of radicals CH, HCO, C 2, etc. in it.
Thus, the flame front is a narrow luminous zone separating the SG and the initial combustible mixture.
In the flame front, as a result of a chemical combustion reaction, the concentration of the initial components sharply decreases to zero, and the temperature reaches its maximum value. Due to molecular heat conduction, the temperature in front of the reaction zone rises monotonously from the initial temperature of the combustible mixture to a temperature close to the combustion temperature, forming a zone of physical heating.
Since the thickness of the flame zone, as a rule, does not exceed fractions of a mm, the flame front is conventionally considered a plane.
If the flame front is moving, then the flame is called non-stationary, if not moving - stationary.
The main characteristics are:
The normal speed of flame propagation is the speed of movement of the flame front relative to unburned gas in a direction perpendicular to its surface. The normal rate is a function of a number of physicochemical properties of the mixture and the rate of the chemical reaction at the combustion temperature.
This is one of the characteristics of the fire hazard of gaseous substances. Since it is determined by the physicochemical properties of the combustible mixture, it is also called fundamental.
Bulk burnout rate. This is the mass of a substance that burns per unit time per unit area of the surface of the flame front.
There are two theories explaining the nature of flame propagation through a combustible mixture.
According to the diffusion theory, the flame front moves due to the diffusion of active particles - radicals - formed in the combustion zone into a fresh mixture, where they initiate a chemical reaction.
According to the thermal theory, the movement of the flame front is carried out due to the transfer of heat by thermal conduction to the fresh mixture, due to which the latter is heated to the self-ignition temperature, followed by a chemical reaction.
In fact, there are elements of both theories, because the process is very complex.
Factors affecting normal speed:
The concentration and composition of the combustible mixture.
Theoretically, u n should be maximum at j st. Almost the maximum falls on a mixture containing more fuel than the stoichiometric ratio (a in< 1 – богатая смесь). u н для различных газов составляет ~ 0,3 – 1,6 м/с. Она редко превышает значение 2,5 м/с, а для углеводородно-воздушных смесей находится в пределах 0,4 – 0,8 м/с. Смеси, имеющие u н < 0,04 м/с, не способны к распространению пламени.
The presence of phlegmatizers (N 2, CO 2, H 2 O (steam), Ar, etc.).
A dilution effect is observed, which entails a decrease in the reaction rate, heat release, and u n. The effectiveness of phlegmatizing gases is determined by their thermophysical properties.
The temperature (initial) of the combustible mixture. With an increase in T o, the temperature of the combustible mixture increases: T g \u003d T o + Q n / (ås p i V PG i)
All combustible (combustible) substances contain carbon and hydrogen - the main components of the gas-air mixture involved in the combustion reaction. The ignition temperature of combustible substances and materials is different and does not exceed 300°C for most.
The physical and chemical foundations of combustion consist in the thermal decomposition of a substance or material to hydrocarbon vapors and gases, which, under the influence of high temperatures, enter into chemical action with an oxidizing agent (air oxygen), turning into carbon dioxide (carbon dioxide), carbon monoxide (carbon monoxide) during combustion. carbon), soot (carbon) and water, and this releases heat and light radiation.
Ignition is the process of flame propagation through a gas-vapor-air mixture. When the speed of the outflow of combustible vapors and gases from the surface of a substance is equal to the speed of flame propagation along them, stable flame combustion is observed. If the flame speed is greater than the speed of the outflow 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 the outflow of gases and the speed of propagation of the flame through them, one can observe:
combustion on the surface of the material, when the rate of release of a combustible 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 combustible mixture is greater than the rate of propagation of the flame along it.
The 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). In fires, this type of combustion is extremely rare. However, it is often found in technological processes: in gas welding, cutting, etc.
In diffusion combustion, the oxidant enters the combustion zone from the outside. It comes, as a rule, from the bottom of the flame due to the rarefaction that is created at its base. At the top of the flame, the heat released during combustion creates pressure. The main reaction of combustion (oxidation) occurs at the edge of the flame, since the gas mixtures flowing from the surface of the substance prevent the penetration of the oxidizer into the depths of the flame (displace air). Most of the combustible mixture in the center of the flame, which did not enter 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 typical when the velocities of the outflow of the combustible mixture from the surface of the material and the speed of flame propagation along it are equal. Turbulent combustion occurs when the exit velocity is
combustible mixture significantly exceeds the speed of flame propagation. In this case, the flame boundary becomes unstable due to the large diffusion of air into the combustion zone. The instability first occurs at the top of the flame, and then moves to the bottom. Such combustion occurs in fires during its volumetric development (see below).
Combustion of substances and materials is possible only with a certain amount of oxygen in the air. The oxygen content, which excludes the possibility of burning various substances and materials, is established empirically. So, for cardboard and cotton, self-extinguishing occurs at 14% (vol.) oxygen, and polyester wadding - at 16% (vol.).
The exclusion of the oxidizing agent (air oxygen) is one of the fire prevention measures. Therefore, the storage of flammable and combustible liquids, calcium carbide, alkali metals, phosphorus should be carried out in tightly closed containers.
7.3.2. Sources of ignition
A necessary condition for the ignition of a combustible mixture are ignition sources. Ignition sources are divided into open fire, heat of heating elements and devices, electrical energy, energy of mechanical sparks, static electricity and lightning discharges, energy of self-heating processes of substances and materials (spontaneous combustion), etc. Particular attention should be paid to the identification of sources of ignition available in the workplace.
The characteristic parameters of ignition sources are taken according to:
The temperature of the lightning channel is 30000°C with a current strength of 200000 A and an action time of about 100 μs. The energy of the spark discharge of 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 a high potential is brought into the building through metal communications reaches values of 100 J or more, which is sufficient to ignite all combustible materials.
Polyvinipchloride insulation of an electric cable (wire) ignites at a short-circuit current ratio of more than 2.5.
The temperature of welding particles and nickel particles of incandescent lamps reaches 2100°C. The temperature of the drops when cutting metal is 1500°C. The temperature of the arc during welding and cutting reaches 4000°C.
The particle expansion zone 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, °С, on the flask of an electric incandescent bulb depends on the power, W: 25 W - 100°С; 40 W - 150°С; 75 W - 250°С; 100 W - 300°С; 150 W - 340°С; 200 W - 320°С; 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.
The temperature of the flame (smoldering) and the time of burning (smoldering), ° C (min), 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 from chimneys, boiler rooms, pipes of steam locomotives and diesel locomotives, as well as
other machines, fires, it was found that a spark with a diameter of 2 mm is a fire hazard if it has a temperature of about 1000 ° C, a diameter of 3 mm - 800 ° C, a diameter of 5 mm - 600 ° C.
1.3.3. Spontaneous combustion
Spontaneous combustion is inherent in many combustible substances and materials. This is a distinctive feature of this group of materials.
Spontaneous combustion is of the following types: thermal, chemical, microbiological.
Thermal spontaneous combustion is expressed in the accumulation of heat by the material, during which the material self-heats. The self-heating temperature of a substance or material is an indicator of its fire hazard. For most combustible materials, this indicator lies in the range from 80 to 150 ° C: paper - 100 ° C; construction felt - 80°С; leatherette - 40°C; wood: pine - 80, oak - 100, spruce - 120°С; raw cotton - 60°C.
Prolonged smoldering before the onset of fiery combustion is a distinctive characteristic of thermal spontaneous combustion processes. These processes are detected by the long-lasting and persistent smell of smoldering material.
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