Types of combustion, their characteristics. Physicochemical foundations of combustion and explosion processes. Conditions of occurrence and types of combustion
Safe operation lifting mechanisms.
On lifting mechanisms safety devices must be installed:
1. Load limiters - disable the mechanism for lifting the load or changing the boom when the rated load is exceeded;
2. Travel stops are mounted at the end of the track;
3. Anti-theft devices;
4. Fencing of all types of gears;
5. Braking devices;
3. Grounding devices using from crane tracks, lightning protection tower cranes;
5. Fire-fighting equipment(fire extinguishers, sand boxes).
Undergo a full technical examination once every 3 years and a partial one once a year. With the participation of RosTekhnadzor.
Full technical inspection includes: - inspection (with a check of the safety devices, as well as static and diagnostic tests); with static load, load exceeding the rated load capacity by 25%, lift the load by 10 cm and hold it for 10 minutes. During dynamic tests, the carrying capacity exceeds the nominal by 10%, at least 2 times raised and lowered, in order to check the action of mechanisms or brakes. With a partial survey, inspection + static or dynamic tests can be carried out. In addition, a complete technical examination is subject to auxiliary devices: ropes, lifting devices and devices.
Fire Is an uncontrolled combustion outside a special hearth, causing material damage and posing a danger to human life.
Combustion types:
1) According to the speed of flame propagation, the following types of combustion are distinguished: 1. Normal (the speed of propagation is several m / s); 2. Explosive (speed of hundreds m / s); 3. Detonation (speed of thousands m / s)
2) Depending on the state of aggregation of combustible substances: homogeneous and heterogeneous.
If the components of the combustible mixture are premixed, then kinetic combustion occurs, which is determined by the rate of the chemical reaction. If the components are not mixed, then diffusion combustion occurs, which is determined by the diffusion of oxygen to the combustible substance through the combustion products. Laminar combustion is characterized by layer-by-layer propagation of the flame front over a fresh combustible mixture. Turbulent combustion - characterized by mixing of layers and increased speed burnout.
3) According to the combustion mode: 1. Self-ignition - spontaneous occurrence of flame combustion of a combustible mixture preheated to the boiling point, which is called the self-ignition temperature (); 2. Propagation of a flame front along a fresh combustible mixture when it is locally ignited by an external source. Spontaneous combustion processes are divided into the following types:
Thermal as a result of prolonged action of a heat source;
Microbiological occurs as a result of increased temperature and humidity due to the vital activity of organisms (sawdust, grain, peat);
Chemical occurs when substances interact with each other or with oxygen.
According to the second mode, the following types of combustion are distinguished:
- Flash- rapid combustion of the gas-vapor-air mixture above the surface of the condensed substance, accompanied by a short-term visible glow without the formation of increased gas pressure. It is characterized by a flash point. Flash point is the lowest temperature at which vapors and gases are formed above the surface of the condensed substance, which can flash in the air from the ignition source.
- Combustion- the occurrence of combustion from an ignition source;
- Ignition- fiery combustion of a substance, continuing after the removal of the ignition source. It is characterized by its flash point. The ignition temperature is the lowest temperature at which sustained flame combustion occurs over the surface of the condensed substance.
- Explosion- extremely fast combustion in which energy is released and the formation of compressed gases capable of producing mechanical damage. It is characterized by the maximum explosion pressure.
Combustion can be: 1. Complete - with an excess of oxygen. Combustion products are water vapor and carbon dioxide. 2. Incomplete - there is not enough oxidizing agent, and carbon monoxide is formed.
Dangerous fire factors include: - open fire, - sparks, - high air temperature, - toxic combustion products, - low oxygen concentration.
Basic definitions
Lecture 12
FIRE SAFETY
1. General information about the combustion process
1.1. Basic definitions
1.2. Combustion types
1.3. Combustion process
1.4. The main indicators of the fire hazard of substances
1.5. Classification of substances by fire hazard
2. The main sources of fires at the enterprise, during the transportation and storage of liquefied gases and SEI. Grade fire hazard industrial enterprises.
3. Classification of industries and zones by fire and explosion hazard
4. Activities for fire prevention... P. p. industrial buildings.
1... General information about the combustion process
Fire - uncontrolled combustion outside a special hearth, causing material damage (standard definition).
For people in case of fire dangerous factors are:
· Open fire, sparks, high temperature of air and objects;
Radiant streams of energy, an increase in the temperature of the environment, inhalation of hot air, damage and necrosis of the upper respiratory tract
Toxic products of combustion, smoke, oxygen depletion of air
Loss of visibility due to smoke
Collapse of buildings and their elements, installations, equipment
· Explosions.
Toxic substances from a fire are caused by chemical composition combustible substances: hair, skin, fabrics, wool - unpleasant smelling products, cyanide compounds containing soda, aldehydes, ketones, rubber, rubber - isoprene, hydrocarbons, varnishes, products containing neurocelluloid - CO, N 2 O, HCN, Plastics, celluloid - CO, N 2 O, cyanides, formaldehydes, phenol, fluorofosine, ammonia, acetone, styrene, etc. are highly toxic compounds.
Sunburn - burning without causing material damage.
A person who has received II degree burns of more than 30% of the body area has little chance of surviving (without the provision of specialized medical care). Time of getting second degree burns:
1) 26 s at t ° = 71 ° C
2) 15c at t ° = 100 ° С
3) 7s at t = 176 ° C.
Studies carried out in Canada have shown that in a humid environment typical for a fire, grade II burns cause t = 55 ° C when exposed for 28 s and 70 ° C for 1 s.
Thus, in a fire in the department store "Invation" in Brussels, 350 people died and 150 were injured in 10 minutes of the fire. During this time, a large department store, occupying a whole hectare in area, turned into a fire.
Combustion- a rapidly proceeding chemical reaction (most often oxidation), accompanied by the release a large number warmth and usually a bright glow (flame).
Combustion requires 3 factors:
1) oxidizing agent (usually О 2, also Сl, F, Br, I, NOX)
2) flammable substance
3) the source of ignition (i.e. the beginning of the pulse).
Depending on the properties and composition of the combustible substance, they are distinguished:
A. Homogeneous combustion (the same aggregate composition, for example, gases)
B. Heterogeneous combustion(for example, solid and liquid).
Depending on the speed of propagation of the flame, the following are distinguished:
A. Deflagration(typical of fires)
B. Explosive~ h × 100 m / s
B. Detonation~ 1000 m / s ¸ 5000 m / s
Depending on the conditions for the formation of the combustible mixture:
Diffusion combustion- characterized by the fact that the formation of a combustible mixture occurs during combustion as a result of oxygen diffusion into the combustion zone. For example, burning a liquid with open surface or gases escaping through equipment leaks
Deflagration combustion is diffusion combustion.
Kinetic combustion corresponds to explosive combustion. In this case, the combustible substance and oxygen are pre-mixed into the combustion zone. The determining factor is the rate of the chemical oxidation reaction between the oxidizing agent and the combustible substance that occurs in the flame front. If the process of kinetic combustion occurs in a closed volume, then the pressure in this volume increases, the temperature of the combustion products increases.
According to the ratio of fuel and oxidizer, the following are distinguished:
A. Combustion of lean combustible mixtures(in the subject - an oxidizer, combustion is limited by the compound of the combustible component).
B. Combustion of rich combustible mixtures- accordingly, on the contrary - the fuel limits the content of the oxidizer (contains hump above the glass-metric ratio of components).
The onset of combustion is associated with the obligatory self-acceleration of the reaction. There are 3 types of self-acceleration:
1) thermal: under the condition of the accumulation of heat in the system, the temperature rises, which leads to the acceleration of chemical reactions;
2) chain: associated with catalysis of chemical transformations by intermediate reaction products, has a special chemical activity (active centers). (i.e., the chemical process does not occur through direct interaction of the initial molecules, but with the help of fragments formed during the decay of these molecules).
Real combustion processes are usually carried out according to a combined chain-thermal mechanism.
FUNDAMENTALS OF FIRE SAFETY
Fire safety is a state of protection of the individual, property, society and the state from fires ( the federal law No. 69-FZ " About fire safety"). The fire safety system is a combination of forces and means, as well as measures of a legal, organizational, economic, social, scientific and technical nature, aimed at combating fires.
Fire safety solves 4 problems.
1. Prevention (prophylaxis) of fires.
2. Localization and reduction of damage from fires that have arisen.
3. Protection of people and property.
4. Fighting fires.
Combustion is one of the most interesting and vital natural phenomena for people. Combustion is beneficial for a person as long as it does not go beyond subjection to his rational will. Otherwise, it may cause a fire. Fire - it is uncontrolled combustion that causes material damage, harm to the life and health of citizens, the interests of society and the state. Knowledge of the combustion process is required to prevent and extinguish a fire.
Combustion Is a chemical oxidation reaction accompanied by the release of heat. Combustion requires the presence of a combustible substance, an oxidizer and an ignition source.
Flammable substance Is any solid, liquid or gaseous substance capable of oxidizing with the release of heat.
Oxidizing agents there can be chlorine, fluorine, bromine, iodine, nitrogen oxides and other substances. In most cases, in a fire, oxidation of combustible substances occurs with atmospheric oxygen.
Ignition source provides an energetic effect on the combustible substance and oxidizer, leading to the occurrence of combustion. Ignition sources are usually divided into open (glowing) ones - lightning, flame, sparks, incandescent objects, light radiation; and latent (non-luminous) - heat of chemical reactions, microbiological processes, adiabatic compression, friction, impacts, etc. They have different temperatures of flame and heating. Every ignition source must have an adequate supply of heat or energy transferred to the reactants. Therefore, the duration of exposure to the ignition source also affects the combustion process. After the beginning of the combustion process, it is supported by thermal radiation from its zone.
The combustible substance and the oxidizing agent form fuel system which can be chemically heterogeneous or homogeneous. In a chemically inhomogeneous system, a combustible substance and an oxidizer are not mixed and have an interface (solid and liquid combustible substances, jets of combustible gases and vapors entering the air). During the combustion of such systems, atmospheric oxygen continuously diffuses through the combustion products to the combustible substance and then enters into a chemical reaction. Such burning is called diffusion... The diffusion combustion rate is low, since it is slowed down by the diffusion process. If a combustible substance in a gaseous, vaporous or dusty state is already mixed with air (before it is ignited), then such a combustible system is homogeneous and the process of its combustion depends only on the rate of the chemical reaction. In this case, combustion proceeds quickly and is called kinetic.
Burning can be complete or incomplete. Complete combustion occurs when sufficient oxygen enters the combustion zone. If there is not enough oxygen to oxidize all the products involved in the reaction, incomplete combustion occurs. The products of complete combustion include carbon dioxide and sulfur dioxide, water vapor, nitrogen, which are not capable of further oxidation and combustion. Incomplete combustion products are carbon monoxide, soot and decomposition products of a substance under the influence of heat. In most cases, combustion is accompanied by the appearance of intense light radiation - a flame.
There are a number of types of combustion: flash, ignition, ignition, spontaneous combustion, spontaneous combustion, explosion.
Flash- This is the rapid combustion of a combustible mixture without the formation of increased gas pressure. The amount of heat that is generated by the flash is not enough to continue burning.
Combustion Is the occurrence of combustion under the influence of an ignition source.
Ignition- ignition accompanied by the appearance of a flame. In this case, the entire remaining mass of the combustible substance remains relatively cold.
Spontaneous combustion- the phenomenon of a sharp increase in the rate of exothermic oxidation reactions in a substance, leading to the occurrence of its combustion in the absence of an external ignition source. Depending on the internal reasons spontaneous combustion processes are divided into chemical, microbiological and thermal. Chemical spontaneous combustion comes from exposure to oxygen of air, water, or from the interaction of substances. Oiled rags, overalls, cotton wool and even metal shavings ignite spontaneously. Spontaneous combustion of oiled fibrous materials is caused by the distribution of fatty substances in a thin layer on their surface and the absorption of oxygen from the air. Oxidation of oil is accompanied by the release of heat. If more heat is generated than the heat loss to the environment, then combustion may occur without any heat input. Some substances ignite spontaneously when exposed to water. These include potassium, sodium, calcium carbide, and carbides alkali metals... Calcium ignites when interacting with hot water... Calcium oxide ( quicklime) when interacting with a small amount of water, it heats up very much and can ignite combustible materials in contact with it (for example, wood). Some substances ignite spontaneously when mixed with others. These include, first of all, strong oxidants (chlorine, bromine, fluorine, iodine), which, in contact with some organic substances, cause their spontaneous combustion. Acetylene, hydrogen, methane, ethylene, turpentine ignite spontaneously under the influence of chlorine. Nitric acid, which is also a strong oxidizing agent, can cause spontaneous combustion of wood chips, straw, and cotton. Microbiological spontaneous combustion lies in the fact that with appropriate humidity and temperature in plant products, peat, the vital activity of microorganisms is intensified. This raises the temperature and a combustion process can occur. Thermal spontaneous combustion occurs as a result of prolonged exposure to a minor heat source. In this case, the substances decompose and, as a result of the intensification of oxidative processes, they self-heat. Semi-drying vegetable oils(sunflower, cotton, etc.), castor varnish, turpentine varnishes, paints and primers, wood and fiberboard, roofing cardboard, nitrolinoleum and some other materials and substances can ignite spontaneously at an ambient temperature of 80 - 100 ºС.
Self-ignition- This is spontaneous combustion, accompanied by the appearance of a flame. Solid and liquid substances, vapors, gases and dust mixed with air can ignite spontaneously.
Explosion(explosive combustion) is an extremely fast combustion, which is accompanied by the release of a large amount of energy and the formation of compressed gases capable of causing mechanical damage.
Combustion types are characterized by temperature parameters, the main ones are as follows. Flash point - This is the lowest temperature of a combustible substance at which vapors or gases are formed above its surface, which can briefly flash in the air from an ignition source. However, the rate of formation of vapors or gases is still insufficient to continue combustion. Ignition temperature - this is the lowest temperature of a combustible substance at which it emits combustible vapors or gases at such a rate that after they are ignited from the ignition source, stable combustion occurs. Autoignition temperature - this is the lowest temperature of a substance at which there is a sharp increase in the rate of exothermic reactions, resulting in ignition. The autoignition temperature of the investigated solid combustible materials and substances is 30 - 670 ° C. The most low temperature self-ignition has white phosphorus, the highest is magnesium. For most types of wood, this temperature is 330 - 470 ºС.
Combustion is a complex physical and chemical process, the basis of which is a fast oxidation reaction, which is accompanied by an intense release of energy in the form of heat and light radiation.
Combustion reactions include not only reactions of interaction between combustible substances and oxygen, but also other redox reactions: the interaction of certain substances with halogens, sulfur vapors, decomposition reactions of explosives, some endothermic compounds, for example, acetylene.
H 2 + Cl 2 = 2 HCl + Q
C 3 H 5 (NO 3) 3 = 3CO 2 + 2.5H 2 O + 1.5N 2 + 0.25O2
C 2 H 2 = 2C + H 2 + Q
CLASSIFICATION OF COMBUSTION PROCESSES
Combustion processes are classified according to several criteria:
1. According to the state of aggregation of the components of the combustible mixture in the combustion zone.
As you know, substances can be in three states of aggregation: gaseous, liquid and solid. Depending on the state of aggregation of the components of the combustible mixture in the combustion zone, two types or modes of combustion are distinguished: if both components are in the combustion zone in the same phase (the same state of aggregation), and if the components of the combustible mixture in the combustion zone are in different states of aggregation.
In most cases, combustion is homogeneous. An example of a heterogeneous, flameless combustion is the combustion of anthracite, coke, and under fire conditions, a solid carbon residue that is obtained from the decomposition of solid combustible materials, such as wood. In this case, all volatile pyrolysis products have already burned out and combustion occurs directly on the surface of the material.
2. By the way the combustible mixture is formed.
Depending on the conditions of mixture formation of the components and the ratio of the rate of the chemical reaction of combustion and the rate of mixture formation, two characteristic combustion modes are distinguished: and. In this case, the decisive factor is which of the stages in the total rate of the combustion process is limiting: the rate of mixture formation or the rate of chemical transformation of the components of the mixture into combustion products.
The total combustion time of a chemically inhomogeneous system consists of the time required for the occurrence of physical contact between the combustible substance and atmospheric oxygen, t f and the time of the chemical reaction itself t x:
t mountains = t f + t x.
In a fire, combustion of unmixed gases is mainly encountered. The combustible mixture is obtained in the combustion zone itself. The reaction components enter the interaction zone from different environments, any of which contains only one of the reacting components. In this situation, interaction is possible only due to the transfer of the reacting components due to diffusion across the interface between the two media.
The time of the physical process of oxygen diffusion to the combustible substance is incomparably longer than the time required for the chemical combustion reaction to proceed. In this case
t diff >> t x,
t mountains >> t diff.
If the rate of transfer of a substance is less than the rate of reaction, then the rate of combustion is determined only by the rate of mass transfer (the rate of diffusion of oxygen to the combustible substance):
w mountains >> w f,
w f = gj c.
where j in is the concentration of the oxidizer in the volume,
g - mass transfer coefficient.
In this case, it is customary to say that the combustion reaction takes place in the diffusion region, and the combustion itself is called diffusion.
If there is already ready mix, which consists of a combustible gas and an oxidizing agent, then combustion is classified as kinetic.
The term "kinetic combustion" was introduced due to the fact that the rate of the combustion process depends mainly on the rate of the chemical reaction between the fuel and the oxidizing agent, that is, on the kinetics of the corresponding combustion reaction. In this case, the total rate of the combustion process is limited only by the rate (kinetics) of the chemical reaction.
w mountains >> w chemical r.
The total combustion time of a chemically homogeneous system is approximately equal to the time spent on the course of the chemical reaction itself.
t mountains >> t chemical r.
Kinetic combustion most often occurs at the initial stage of a fire.
If the combustion of such a gas-air mixture occurs in a closed or limited space, it takes on the character of an explosion. An explosive character is observed when the energy released during the combustion of the mixture does not have time to be diverted outside the given volume, while the pressure increases, which often leads to the destruction of structures.
3. By the mechanism of combustion propagation.
After the onset of combustion, the flame front, or chemical reaction zone, begins to spread through the combustible mixture. Depending on the mechanism of combustion propagation through the combustible mixture, two characteristic combustion modes are distinguished: deflagration and detonation.
At relatively low speeds (relatively slow propagation of the zone of chemical reactions, with the speed of the flame front along the combustible mixture from 0.5 to 50 m / s), layer-by-layer ignition of a cold gas mixture due to its heating from the combustion zone. This heat is what keeps the combustion process going. The transfer of a heat impulse from the combustion zone to the cold combustible mixture occurs due to the process of thermal conductivity. This mechanism is called deflagration.
There is another mechanism for the propagation of combustion - due to the rapid adiabatic compression of the combustible mixture. This combustion mechanism is called detonation.
Detonation can occur in an explosive environment in the case of its preliminary rapid compression by a strong shock wave. Such a wave can be created by an external impulse (for example, the combustion of an explosive). Characteristic feature shock compression is a strong heating of the gas (up to 1500-1700K). A combustible mixture heated by a strong shock wave to such a temperature flares up. Spontaneous occurrence of detonation in the burning gas is possible with a sufficient combustion propagation velocity - greater than 500 m / s, while the normal flame propagation velocity does not exceed 10 m / s.
4. According to the gas-dynamic combustion regime.
An important characteristic of homogeneous combustion is the gas-dynamic state of the components of the combustible mixture in the reaction zone, which depends on the intensity of the inflow of the components of the combustible mixture into the reaction zone.
If the components of the combustible mixture enter the reaction zone slowly, according to the laws of molecular or weak convection diffusion, then the combustion process is laminar... In laminar combustion, the speed and direction separate parts the gas flow is practically the same.
If the flows of combustible gas and oxidizer or ready-made combustible mixture flow intensively, then the combustion mode will be turbulent, that is, with intense vortices, mixing of combustion products with the initial mixture, separation of the combustion zone from the main flame.
The parameter that characterizes the gas-dynamic combustion mode is the Reynolds criterion - Re... So, in the case of a mixture burning in a pipe, if Re < 2300, то пламя относится к ламинарному, если 2300 < Re < 10 000 пламя переходное, при Re> 10,000 - turbulent.
1) Definition of the concept of "combustion". Combustion conditions.
By burning is called a rapidly proceeding chemical reaction, accompanied by the release of a large amount of heat and usually glow. Depending on the speed of the process, combustion can occur in the form of actual persecution, explosion and detonation.
The condition for the occurrence of combustion is the excess of the rate of heat release chemical reaction over the rate of heat removal into the environment.
2) Classification building materials and structures for combustion.
Fireproof are materials and structures that, under the influence of fire or high temperature, do not ignite, smolder and do not char. These include all natural and artificial inorganic materials that do not burn in the event of a fire.
Hard to burn materials and structures under the influence of fire or high temperature ignite, smolder or char and continue to burn, smolder and char in the presence of an ignition source, and after its removal, these processes stop. These include materials consisting of non-combustible and combustible components containing more than 8% by mass of organic fillers, as well as combustible materials protected by non-combustible materials.
Combustible materials and structures under the influence of fire or high temperature ignite, smolder or char, and these processes continue after the removal of the ignition source. These include all organic materials that do not meet the requirements for non-combustible and hardly combustible materials.
3) Types of combustion, their qualitative and quantitative characteristics.
Combustion types:
Flash- rapid combustion of the combustible mixture without the formation of increased gas pressure. rapid, but, in comparison with an explosion, short temporary combustion of a mixture of vapors of a combustible substance with air or oxygen, arising from a local increase in temperature, which may be caused by an electric spark or Combustion- the occurrence of combustion from an ignition source. Ignition- ignition accompanied by the appearance of a flame. persistent ignition of a mixture of vapors and gases of a combustible substance from a local increase in temperature, which can be caused by touching a flame or an incandescent body. Ignition can last until the entire supply of combustible material has been consumed, and vaporization occurs due to the heat released during combustion. Spontaneous combustion- combustion that occurs in the absence of an external ignition source. Physical or chemical processes occurring inside a substance are associated with the formation of heat, which accelerates the oxidation process, which turns into burning with an open fire. Self-ignition- spontaneous combustion, accompanied by the appearance of a flame, arising from external heating of the substance to a certain temperature without direct contact of the combustible substance with the flame of an external combustion source. Explosion- extremely fast combustion, in which energy is released and the formation of compressed gases capable of causing mechanical damage. instant combustion or decomposition of a substance, accompanied by the release of a huge amount of gases, which instantly expand and cause a sharp increase in pressure in the environment. On contact with air: the gaseous decomposition products of certain substances have the "ability to ignite, which not only leads to destruction from the action of the blast wave, but also causes large fires.
4) Definition of the concept of "fire". Damaging fire factors.
Fire - this is uncontrolled combustion outside a special hearth, causing material damage.
The main damaging factors fire: direct exposure to fire (combustion); high temperature and heat radiation; gas environment; smoke and gas contamination of premises and territories with toxic combustion products.
Open fire very dangerous, but cases of its direct impact on people are rare. More often they suffer from the radiant streams emitted by the flame. It was found that at fire in the stage box of a spectacular enterprise, the radiant streams are dangerous for the spectators of the first rows of the stalls already half a minute after the fire.
Medium temperature ... The greatest danger to humans is the inhalation of heated air, leading to damage to the upper respiratory tract, suffocation and death. So, exposure to temperatures above 100 ° C leads to loss of consciousness and death in a few minutes. Skin burns are also dangerous.
Toxic combustion products. In case of fires in modern buildings built with the use of polymeric and synthetic materials, toxic combustion products can affect a person. The most dangerous of these is carbon monoxide. It reacts 200-300 times better with blood hemoglobin than oxygen, as a result of which a person suffers from oxygen starvation. He becomes indifferent and indifferent to danger, he becomes numb, dizzy, depressed, coordination of movements is disturbed, and then respiratory arrest and death occur.
Loss of visibility due to smoke ... The success of evacuating people in case of fire can be ensured only if they move unhindered in the right direction. The evacuees must clearly see emergency exits or exit pointers. When visibility is lost, the movement of people becomes chaotic, each person moves in an arbitrarily chosen direction. As a result, the evacuation process becomes more difficult and then can become unmanageable.
Decreased oxygen concentration. Under fire conditions, during the combustion of substances and materials, the oxygen concentration in the air decreases. Meanwhile, a decrease in it even by 3% causes a deterioration in the motor functions of the body. An oxygen concentration of less than 14% is considered dangerous: it disrupts brain activity and coordination of movements.
5) Definition of the concept of "fire / explosive object"
Fire / explosive facility - A facility that produces, uses, processes, stores or transports flammable and fire-explosive substances that pose a real threat of a man-made emergency.
