Combustion as a chemical reaction. Combustion theory
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MINISTRY OF EDUCATION OF THE RUSSIAN FEDERATION
SAINT PETERSBURG
ENGINEERING AND ECONOMIC ACADEMY
INSTITUTE OF GENERAL MANAGEMENT
ESSAY
BY DISCIPLINE
"SAFETY OF LIVING"
FIRE EXTINGUISHING METHODS AND MEANS
performed:
2nd year student, gr. 1082
V.V. Zatolokin
checked:
St. Petersburg
1999
Introduction
Combustion is a chemical oxidation reaction that produces heat and light. For combustion to occur, three factors are required: a combustible substance, an oxidizer (usually oxygen in the air) and a source of ignition (impulse). The oxidizing agent can be not only oxygen, but also chlorine, fluorine, bromine, iodine, nitrogen oxides, etc.
Combustion is homogeneous or heterogeneous depending on the properties of the combustible mixture. In homogeneous combustion, the starting materials have the same state of aggregation (for example, combustion of gases). Combustion of solid and liquid fuels is heterogeneous.
Combustion is also differentiated by the speed of flame propagation and, depending on this parameter, can be deflagration (about ten meters per second), explosive (about hundreds of meters per second) and detonation (about a thousand meters per second). Deflagration combustion is characteristic of fires.
The combustion process is divided into several types.
Flash - rapid combustion of a combustible mixture, not accompanied by the formation of compressed gases.
Combustion - the occurrence of combustion under the influence of an ignition source.
Ignition - Combustion accompanied by the appearance of a flame.
Spontaneous combustion is a phenomenon of a sharp increase in the rate of exothermic
reactions leading to the combustion of a substance (material, mixture) in the absence of an ignition source.
Spontaneous ignition - spontaneous combustion, accompanied by the appearance of a flame.
An explosion is an extremely fast chemical (explosive) transformation, accompanied by the release of energy and the formation of compressed gases capable of performing mechanical work.
The occurrence of combustion of substances and materials when exposed to heat pulses with a temperature above the ignition temperature is characterized as ignition, and the occurrence of combustion at temperatures below the autoignition temperature refers to the process of spontaneous combustion.
When evaluating fire safety substances and materials, it is necessary to take into account their state of aggregation. Since combustion, as a rule, occurs in a gaseous environment, it is necessary to take into account the conditions under which a sufficient amount of gaseous combustible products is formed for combustion as indicators of fire hazard.
The main indicators fire hazard that determine the critical conditions for the onset and development of the combustion process are the autoignition temperature and the concentration limits of ignition.
Autoignition temperature refers to the minimum temperature of a substance or material. at which there is a sharp increase in the rate of exothermic reactions, resulting in the appearance of flame combustion. The minimum concentration of flammable gases and vapors in the air at which they are able to ignite and spread the flame is called the lower concentration limit ignition; the maximum concentration of flammable gases and vapors, at which the spread of the flame is still possible, is called the upper concentration limit of ignition. The area of compositions and mixtures of combustible gases and vapors with air lying between the lower and upper ignition limits is called the ignition area.
The flammable concentration limits are not constant and depend on a number of factors. Greatest influence the ignition limits are affected by the power of the ignition source, the admixture of inert gases and vapors, the temperature and pressure of the combustible mixture.
The fire hazard of substances is characterized by linear (expressed in cm / s) and mass (g / s) rates of combustion (flame propagation) and burnout (g / m 2 * s), as well as the limiting oxygen content at which combustion is still possible. For conventional combustible substances (hydrocarbons and their derivatives), this limiting oxygen content is 12-14%, for substances with a high value of the upper flammable limit (hydrogen, carbon disulfide, ethylene oxide, etc.), the limiting oxygen content is 5% and lower.
In addition to the above parameters, to assess the fire hazard, it is important to know the degree of flammability (combustibility) of substances. Depending on this characteristic, substances and materials are divided into flammable (combustible), hardly combustible (hardly combustible) and non-combustible (non-combustible).
Fuels include substances and materials that, when ignited by an extraneous source, continue to burn even after removal. Hard-flammable substances include those that are not able to spread the flame and burn only at the place of the impulse; non-combustible are substances and materials that are not flammable even when exposed to sufficiently powerful impulses.
Fires in areas inhabited by people, at enterprises occur in most cases in connection with a violation of the technological regime. Unfortunately, this is a frequent occurrence and the state provides for special documents describing the basics of fire protection. These are standards: GOST 12.1.004-76 "Fire safety" and GOST 12.1.010-76 "Explosion safety".
Fire prevention measures are divided into organizational, technical, routine and operational.
Organizational measures provide for the correct operation of machines and internal plant transport, the correct maintenance of buildings, territory, firefighting briefing workers and employees, organizing voluntary fire brigades, fire-technical commissions, issuing orders on strengthening fire safety, etc.
TO technical activities include compliance fire regulations, norms for the design of buildings, for the installation of electrical wires and equipment, heating, ventilation, lighting, the correct placement of equipment.
Regime measures are the prohibition of smoking in unidentified places, the production of welding and other hot work in fire-hazardous premises, etc.
Operational measures are timely preventive inspections, repairs and testing of technological equipment.
Fire extinguishing agents and fire extinguishing devices
In the practice of extinguishing fires, the following principles of stopping combustion are most common:
1) isolation of the seat of combustion from air or reduction of oxygen concentration by diluting the air with non-combustible zags to a value at which combustion cannot occur;
2) cooling the combustion center below certain temperatures;
3) intensive inhibition (inhibition) of the rate of a chemical reaction in a flame;
4) mechanical breakdown of the flame as a result of exposure to a strong jet of gas and water;
5) creation of conditions for fire barriers, i.e. such conditions in which the flame spreads through narrow channels.
Water
The fire-extinguishing ability of water is determined by the cooling effect, the dilution of the combustible medium by the vapors formed during evaporation and the mechanical effect on the burning substance, i.e. blowing off the flame. The cooling effect of water is determined by the significant values of its heat capacity and heat of vaporization. The diluting effect, leading to a decrease in the oxygen content in the ambient air, is due to the fact that the volume of steam is 1,700 times the volume of evaporated water.
Along with this, water has properties that limit its scope. So, when extinguishing water, oil products and many other flammable liquids float up and continue to burn on the surface, so water may be ineffective in extinguishing them. The fire extinguishing effect when extinguishing with water in such cases can be increased by supplying it in a sprayed state.
Water containing various salts and supplied with a compact jet, has significant electrical conductivity, and therefore it cannot be used to extinguish fires at objects whose equipment is energized.
Fire extinguishing with water is carried out with water fire extinguishing installations, fire trucks and water barrels (manual and fire monitors). To supply water to these installations, they are used arranged at industrial enterprises and in settlements water pipes.
In case of fire, water is used for external and internal fire extinguishing. Water consumption for outdoor fire extinguishing is taken in accordance with building codes and regulations. Water consumption for fire extinguishing depends on the category of fire hazard of the enterprise, the degree of fire resistance of the building structures, the volume of the production room.
One of the main conditions that external water supply systems must satisfy is to ensure constant pressure in water supply network supported by permanent pumps, a water tower or a pneumatic unit. This pressure is often determined from the operating conditions of internal fire hydrants.
In order to extinguish a fire at the initial stage of its occurrence, in most industrial and public buildings, internal fire hydrants are arranged on the internal water supply network.
According to the method of creating water pressure, fire water pipelines are divided into high and low pressure... High pressure fire water pipelines are arranged in such a way that the pressure in the water supply system is always sufficient to directly supply water from hydrants or stationary fire monitors to the fire site. From low-pressure water pipelines, mobile firefighting pumps or motor pumps take water through fire hydrants and supply it under necessary pressure to the place of the fire.
The fire water supply system is used in various combinations: the choice of this or that system depends on the nature of the production, the territory it occupies, etc.
Water fire extinguishing installations include sprinkler and deluge installations. They are a branched, water-filled pipe system equipped with special heads. In the event of a fire, the system reacts (differently, depending on the type) and irrigates the structures of the room and equipment in response to the action of the heads.
Foam
Foams are used to extinguish solid and liquid substances that do not interact with water. Fire extinguishing properties foam is determined by its multiplicity - the ratio of the volume of the foam to the volume of its liquid phase, stability, dispersion and viscosity. These properties of the foam, in addition to its physical and chemical properties, are influenced by the nature of the combustible substance, the conditions of the fire and the supply of foam.
Depending on the method and conditions of production, fire-extinguishing foams are divided into chemical and air-mechanical. Chemical foam is formed by the interaction of solutions of acids and alkalis in the presence of a foaming agent and is a concentrated emulsion of carbon dioxide in an aqueous solution of mineral salts containing a foaming agent.
The use of chemical foam is reduced due to the high cost and complexity of organizing fire extinguishing.
Foam generating equipment includes air-foam barrels for obtaining low-expansion foam, foam generators and foam sprinklers for obtaining medium-expansion foam.
Gases
When extinguishing fires with inert gaseous diluents, carbon dioxide, nitrogen, flue or exhaust gases, steam, as well as argon and other gases are used.The fire-extinguishing effect of these compounds is to dilute the air and reduce the oxygen content in it to a concentration at which combustion stops.The fire-extinguishing effect when diluted with these gases is due to heat loss due to heating of the diluents and a decrease in the heat effect of the reaction.A special place among fire extinguishing compositions is occupied by carbon dioxide (carbon dioxide), which is used to extinguish flammable liquids warehouses, battery stations,
drying ovens, electric motor test stands, etc.
It should be remembered, however, that carbon dioxide should not be used to extinguish substances containing oxygen, alkaline and alkaline earth metals, or smoldering materials.To extinguish these substances, nitrogen or argon is used, and the latter is used in cases where there is a risk of the formation of metal nitrides possessingexplosive properties and impact sensitivity.
Recently, a new method of supplying gases in a liquefied state to the protected volume has been developed, which has significant advantages over the method based on the supply of compressed gases.
With the new method of filing, there is practically no need to limit the sizes allowed for protectionobjects, since the liquid occupies about 500 times less volume than an equal mass of gas,and does not require much effort to submit it. In addition, the evaporation of liquefied gas achievessignificant cooling effect and the limitation associated with the possible destruction of weakened openings disappears,since when liquefied gases are supplied, a soft filling mode is created without a dangerous increase in pressure.
Inhibitors
All of the above fire extinguishing agents have a passive effect on the flame. More promisingfire extinguishing agents that effectively inhibit chemical reactions in a flame, i.e. have an inhibitory effect on them. Most used infire extinguishing found fire extinguishing compositions - inhibitors based on saturated hydrocarbons, in which oneor several hydrogen atoms are replaced by halogen atoms (fluorine, chlorine, bromine).
Halocarbons are poorly soluble in water, but mix well with many organicsubstances. The fire extinguishing properties of halogenated hydrocarbons increase with increasing seathe mass of the halogen contained in them.
Halocarbon compounds have physical properties that are convenient for fire extinguishingproperties. So, high values of density of liquid and vapors determine the possibilitycreating a fire extinguishing jet and penetration of drops into the flame, as well as retention of fire extinguishingvapors near the source of combustion. Low freezing temperatures allow these formulations to be used at subzero temperatures.
V last years powder compositions based on inorganicalkali metal salts. They are distinguished by their high extinguishing efficiency and versatility,those. the ability to extinguish any materials, including those not extinguished by all other means.
Powder formulations are, in particular, the only means of extinguishing alkaline fires.metals, organoaluminum and other organometallic compounds (they are manufactured by the industry on the basis of sodium and potassium carbonates and bicarbonates, phosphorus-ammonium salts, powder based on griffin for extinguishing metals, etc.).
Powders have a number of advantages over halocarbons: they and their decomposition products are not hazardous.for human health; as a rule, do not have a corrosive effect on metals; protect peopleextinguishing a fire, from thermal radiation.
Fire extinguishing devices
Fire extinguishing devices are divided into mobile (fire vehicles), stationary installationsand fire extinguishers (manual up to 10 liters and mobile and stationary with a volume of more than 25 liters).
Fire trucks are divided into tank trucks that deliver water and a solution of a foaming agent to a fireand equipped with nozzles for supplying water or air-mechanical foam of various multiplicity, and special,intended for other fire extinguishing media or for specific objects.
Stationary installations are designed to extinguish fires at the initial stage of their occurrencewithout the participation of people. They are installed in buildings and structures, as well as to protect outdoor technologicalinstallations. According to the fire extinguishing agents used, they are divided into water, foam, gas,powder and steam. Stationary installations can be automatic and manual with remotestart-up. As a rule, automatic installations are also equipped with devices for manualstart-up. Installations are water, foam and gas extinguishing installations. The latter are more efficient and less complicated.
and cumbersome than many others.
Fire extinguishers by type of fire extinguishing agent are divided into liquid, carbon dioxide, chemical foam, air-foam, freon, powderand combined. Water with additives is used in liquid fire extinguishers (to improve self-absorption,lowering the freezing point, etc.), in carbon dioxide - liquefied carbon dioxide, in chemical foam - aqueous solutions of acids and alkalis,in freon - freons 114B2, 13B1, in powder - powders PS, PSB-3, PF, etc. Fire extinguishers are markedletters characterizing the type of fire extinguisher by category, and a number indicating its capacity (volume).
Application of fire extinguishers:
1. Carbon dioxide - extinguishing objects under voltage up to 1000V.
2. Khimpenny - extinguishing solid materials and combustible liquids on an area of up to 1 sq.m.
3. Air-foam - extinguishing the ignition of flammable liquids, combustible liquids, solid (and smoldering) materials (except for metal and energized installations).
4. Freon - extinguishing the ignition of flammable liquids, combustible liquids, combustible gases.
5. Powder - extinguishing materials, energized installations; charged MGS, PX - quenching of metals; PSB-3, P-1P - extinguishing flammable liquids, GZh, combustible gases.
Fire alarm
The use of automatic fire detection means is one of the main conditions for ensuringfire safety in mechanical engineering, as it allows you to notify the personnel on duty about the fire and the place of its occurrence.
Fire detectors convert non-electrical physical quantities (radiation of heat and light energy, movement of smoke particles) into electrical ones,which, in the form of a signal of a certain shape, are sent through wires to the receiving station. By conversion methodfire detectors are divided into parametric ones that convert non-electrical quantities into electrical ones using an auxiliarycurrent source, and generator ones in which a change in non-electrical quantity causes the appearance of its own EMF.
Fire detectors are divided into manual devices designed to issue a discrete signal when pressedthe corresponding start button, and automatic action for issuing a discrete signal when a given value of a physical parameter (temperature, spectrum of light radiation, smoke, etc.) is reached.
Depending on which of the parameters of the gas-air environment triggers the fire detector, they are:thermal, light, smoke, cobminated, ultrasonic. By design fire detectorsdivided into normal execution, explosion-proof, intrinsically safe and sealed. According to the principle of action - maximum (they react to the absolute values of the controlled parameter and are triggered at a certain value) and differential (they register only for the rate of change of the controlled parameter and are triggered only at its certain value).
Heat detectors are based on the principle of changing the electrical conductivity of bodies, contact potential difference, ferromagnetic properties of metals, changing linear dimensions solids etc. Heat detectors of maximum performance are triggered at a certain temperature. The disadvantage is the dependence of the sensitivity on the environment. Differential heat detectors have sufficient sensitivity, but are of little use in rooms where temperature fluctuations can occur.
Smoke detectors - there are photoelectric (they work on the principle of scattering thermal radiation by smoke particles) and ionization (using the effect of weakening the ionization of the air interelectrode gap with smoke.
Ultrasonic detectors - designed for spatial detection of the source of ignition and signaling an alarm. Ultrasonic waves are emitted into the controlled area. In the same room, there are receiving transducers, which, acting like a conventional microphone, convert the ultrasonic vibrations of the air into an electrical signal. If there is no oscillating flame in the controlled room, then the frequency of the signal coming from the receiving transducer will correspond to the radiated frequency. If there are moving objects in the room, the ultrasonic vibrations reflected from them will have a frequency different from the emitted one (Doppler effect). The advantage is inertialessness, large controlled area. The disadvantage is false positives.
Fire prevention
Fire breaks
To prevent the spread of fire from one building to another, fire breaks are arranged between them. Atdetermination of fire breaks proceeds from the fact that the greatest danger in relation to possible ignition of neighboringbuildings and structures represents thermal radiation from the fire source. The amount ofthe heat of a building adjacent to a burning object depends on the properties of combustible materials and the temperature of the flame,the size of the emitting surface, the area of the light openings,flammability groups of enclosing structures, presencefire barriers, the relative position of buildings, meteorological conditions, etc.
Fire protection barriers
These include walls, partitions, ceilings, doors, gates, hatches, vestibules, and windows. Fire walls should bemade of non-combustible materials, have a fire resistance limit of at least 2.5 hours and rely on foundations. Fireproofwalls are designed for stability, taking into account the possibility of one-sided collapse of ceilings and other structures in the event of a fire.
Fireproof doors, windows and gates in fireproof walls must have a fire resistance limit of at least 1.2 hours, and fireproof ceilingsat least 1 hour. Such ceilings should not have openings and openings through which combustion products can penetrate during a fire.
Escape routes
When designing buildings, it is necessary to provide for the safe evacuation of people in the event of a fire. When a fire breaks outpeople must leave the building within a minimum time, which is determined by the shortest distance from their location to going outside.
Number emergency exits from buildings, premises and from each floor of buildings is determined by calculation, but must be at least two. Evacuationexits should be dispersed. In this case, elevators and other mechanical means of transporting people are not taken into account in the calculations.The width of the sections of the escape routes must be at least 1 m, and the doors on the escape routes must be at least 0.8 m. External door widthstaircases should be at least the width of the staircase, the height of the passage on the escape routes - at least 2 m.buildings and structures for the evacuation of people should provide for the following typesstaircases and stairs: smoke-free staircases (communicating with the outdoor airzone or equipped with technical devices for pressurizing air); closed cells with naturallighting through windows in the outer walls; closed stairwells without natural light; internal openstairs (without railings interior walls); outdoor open stairs. For buildings with elevation differences, you shouldprovide for fire escapes.
List of used literature:
1. "Labor protection", G.F. Denisenko, Moscow, 1985
2. "Labor protection in mechanical engineering", under. ed. E. Ya. Yudina, Moscow, 1983
3. "Fundamentals of life safety", Luzhkin I.P., St. Petersburg, 1995
1. Physicochemical basics burning
2. Types of explosions
Bibliography
1. Physicochemical basics of combustion
Combustion is a chemical oxidation reaction accompanied by the release of a large number heat and glow.
Depending on the speed of the process, combustion can occur in the form of actual combustion and explosion.
For the combustion process it is necessary:
1) the presence of a combustible medium consisting of a combustible substance and an oxidizer; 2) source of ignition.
In order for the combustion process to occur, the combustible medium must be heated to a certain temperature using an ignition source (flame, spark of electrical or mechanical origin, incandescent bodies, thermal manifestation of chemical, electrical or mechanical energies).
Once a combustion occurs, the combustion zone is a permanent source of ignition. The emergence and continuation of combustion is possible at a certain quantitative ratio of combustible substance and oxygen, as well as at certain temperatures and the supply of thermal energy of the ignition source. The highest rate of stationary combustion is observed in pure oxygen, the lowest - when the air contains 14-15% oxygen. With a lower oxygen content in the air, the combustion of most of the substances stops.
There are the following types of combustion:
Complete - combustion with a sufficient amount or excess of oxygen;
Incomplete - combustion with a lack of oxygen.
With complete combustion, the products of combustion are carbon dioxide (CO 2), water (H 2 O), nitrogen (N), sulfur dioxide (SO 2), phosphoric anhydride. With incomplete combustion, caustic, poisonous flammable and explosive products are usually formed: carbon monoxide, alcohols, acids, aldehydes.
Combustion of substances can take place not only in an oxygen environment,
but also in the environment of certain substances that do not contain oxygen, chlorine,
vapors of bromine, sulfur, etc.
Combustible substances can be in three states of aggregation:
liquid, solid, gaseous. Individual solids melt and evaporate when heated, others decompose and release gaseous products and a solid residue in the form of coal and slag, and still others do not decompose and do not melt. Most combustible substances, regardless of their state of aggregation, when heated, form gaseous products, which, when mixed with atmospheric oxygen, form a combustible medium.
According to the aggregate state of the fuel and oxidizer, they are distinguished:
Homogeneous combustion - combustion of gases and combustible vapor-forming substances in a gaseous oxidizer;
Combustion of explosives and propellants;
Heterogeneous combustion - combustion of liquid and solid combustible substances in a gaseous oxidizer;
Combustion in the "liquid combustible mixture - liquid oxidizer" system.
The most important issue in the theory of combustion is flame propagation (zones of a sharp increase in temperature and intense reaction). The following modes of flame propagation (combustion) are distinguished:
Normal combustion;
Deflux combustion;
Detonation.
a) Normal combustion is observed with a quiet heterogeneous two-phase diffusion combustion. The combustion rate will be determined by the rate of diffusion of oxygen to the combustible substance into the combustion zone. Flame propagation occurs from each point of the flame front along the normal to its surface. Such combustion and the speed of flame propagation along a stationary mixture along the normal to its surface is called normal (laminar).
Normal burning rates are low. In this case, the pressure increase and the formation of a shock wave do not occur.
b) In real conditions due to the flow internal processes and with external complicating factors, the flame front is curved, which leads to an increase in the combustion rate. When the speed of flame propagation reaches tens and hundreds of meters per second, but does not exceed the speed of sound in a given environment (300 - 320 m / s), explosive (deflation) combustion occurs.
During explosive combustion, the combustion products are heated to 1.5-3.0 thousand ° C, and the pressure in closed systems increases to 0.b-0.9MPa.
The duration of the combustion reaction before the explosive mode is ~ 0.1 sec for gases, ~ 0.2 - 0.3 sec for vapors, and ~ 0.5 sec for dust.
As applied to accidental industrial explosions, deflebration is usually understood as the combustion of a cloud with an apparent speed of the order of 100 - 300 m / s, at which shock waves with a maximum pressure of 20 - 100 kPa are generated.
c) Under certain conditions, explosive combustion can turn into a detonation process, in which the speed of flame propagation exceeds the speed of sound propagation and reaches 1 - 5 km / sec. This occurs with strong turbulization of material flows, causing a significant curvature of the flame front, a large increase in its surface.
In this case, a shock wave arises, in the front of which the density, pressure and temperature of the mixture sharply increase. With an increase in these parameters of the mixture until the spontaneous ignition of hot substances, a detonation wave arises, which is the result of the addition of a shock wave and the resulting zone of a compressed, rapidly reacting (self-igniting) mixture.
The excess pressure within the detonating mixture cloud can reach 2 MPa.
The process of chemical transformation of combustible substances, which is introduced by a shock wave and is accompanied by a rapid release of energy, is called detonation.
In the detonation mode of combustion of a GW cloud, most of the explosion energy is converted into an air shock wave; in reflux combustion with a flame propagation speed of ~ 200 m / s, the energy transfer into a wave is from 30 to 40%.
2. Types of explosions
An explosion is the release of a large amount of energy in a limited amount in a short period of time.
The explosion leads to the formation of a highly heated gas (plasma) with a very high pressure, which, with instant expansion, has a shock mechanical effect (pressure, destruction) on the surrounding bodies.
An explosion in a solid medium is accompanied by its destruction and crushing, in air or water - it causes the formation of air or hydraulic shock waves, which have a destructive effect on the objects placed in them.
In activities not associated with deliberate explosions in conditions industrial production, an explosion should be understood as a rapid, uncontrollable release of energy, which causes a shock wave moving at some distance from the source.
As a result of the explosion, the substance filling the volume in which the energy is released turns into a highly heated gas (plasma) with a very high pressure (up to several hundred thousand atmospheres). This gas, instantly expanding, has a shock mechanical effect on environment causing her to move. An explosion in a solid medium causes its crushing and destruction in a hydraulic and air medium - it causes the formation of a hydraulic and air shock (blast) wave.
A blast wave is a movement of the medium generated by an explosion, in which there is a sharp increase in the pressure, density and temperature of the medium.
The front (front boundary) of the blast wave propagates through the medium at a high speed, as a result of which the area covered by the movement rapidly expands.
By means of a blast wave (or scattering explosion products - in a vacuum), the explosion produces a mechanical effect on objects located at different distances from the explosion site. As the distance from the explosion site increases, the mechanical effect of the blast wave weakens. Thus, the explosion carries the potential danger of injury to people and has a destructive capacity.
An explosion can be caused by:
Detonation of condensed explosives (HE);
Rapid combustion of a flammable cloud of gas or dust;
Sudden destruction of the vessel with compressed gas or with superheated liquid;
By mixing superheated solids(melt) with cold liquids, etc.
Depending on the type of energy carriers and the conditions of energy release, both chemical and physical processes can be sources of energy during an explosion.
A source of energy chemical explosions are fast self-accelerating exothermic reactions of interaction of combustible substances with oxidants or reactions of thermal decomposition of unstable compounds.
Energy sources of compressed gases (vapors) in closed volumes of equipment (equipment) can be both external (energy used to compress cans, pumping liquids; heat carriers that provide heating of liquid and gases in a confined space) and internal (exothermic physical and chemical processes and heat and mass transfer processes in a closed volume), leading to intense evaporation of liquids or gas formation, an increase in temperature and pressure without internal explosive phenomena.
A source of energy nuclear explosions are fast chain nuclear reactions of fusion of light nuclei of isotopes of hydrogen (deuterium and tritium) or fission of heavy nuclei of isotopes of uranium and plutonium. Physical explosions occur when hot and cold liquids are displaced, when the temperature of one of them is significantly higher than the boiling point of the other. Evaporation in this case proceeds in an explosive manner. The resulting physical detonation is accompanied by the appearance of a shock wave with overpressure, reaching in some cases hundreds of MPa.
Energy carriers of chemical explosions can be solid, liquid, gaseous combustible substances, as well as air suspension of combustible substances (liquid and solid) in an oxidizing environment, incl. and in the air.
Thus, two types of explosions are distinguished. The first type includes explosions caused by the release of chemical or nuclear energy of a substance, for example, explosions of chemical explosives, mixtures of gases, dust and / or vapors, as well as nuclear and thermonuclear explosions. In explosions of the second type, the energy received by the substance from an external source is released. Examples of such explosions are a powerful electrical discharge in the environment (in nature - lightning during a thunderstorm); evaporation of a metal conductor under the influence of a high current; explosion when a substance is exposed to certain radiation of high energy density, for example. focused laser radiation; sudden collapse of the shell with compressed gas.
Explosions of the first type can be carried out in a chain or thermal way. A chain explosion occurs under conditions when active particles (atoms and radicals in chemical systems, neutrons in nuclear systems) appear in a system in high concentrations, capable of causing a branched chain of transformations of inactive molecules or nuclei. In fact, not all active particles cause a reaction, some of them go beyond the volume of the substance. Since the number of active particles escaping from the volume is proportional to the surface, for a chain explosion there is a so-called critical mass, at which the number of newly formed active particles still exceeds the number of escaping ones. The development of a chain explosion is facilitated by the compression of the substance, since this reduces the surface. Usually chain explosion gas mixtures They are implemented by a rapid increase in the critical mass with an increase in the volume of the vessel or an increase in the pressure of the mixture, and the explosion of nuclear materials is realized by the rapid combination of several masses, each of which is less than the critical one, into one mass greater than the critical one.
A thermal explosion occurs when the release of heat as a result of a chemical reaction in a given volume of a substance exceeds the amount of heat removed through the outer surface that limits this volume to the environment by means of thermal conduction. This leads to self-heating of the substance up to its spontaneous combustion and explosion.
In explosions of any type, a sharp increase in the pressure of the substance occurs, the medium surrounding the explosion center undergoes strong compression and begins to move, which is transmitted from layer to layer - a blast wave arises. An abrupt change in the state of matter (pressure, density, speed of movement) at the front of the blast wave, propagating at a speed exceeding the speed of sound in the medium, is a shock wave. The laws of conservation of mass and momentum relate the speed of the wave front, the speed of motion of the substance behind the front, the compressibility and pressure of the substance.
Bibliography
1. Zel'dovich Ya.B., Mathematical theory of combustion and explosion. - M .: Nauka, 2000 .-- 478 p.
2. Williams FA, Theory of combustion. - M .: Nauka, 2001 .-- 615 p.
3. Khitrin LN, Physics of combustion and explosion. - M.: INFRA-M, 2007 .-- 428 p.
Combustion. The release of light and heat is a symptom of many chemical phenomena. Reactions with such signs are collectively called combustion. Combustion is a widespread chemical phenomenon; people have long used it for their own benefit (Fig. 40).
Combustion Is a chemical phenomenon characterized by the release of light and heat.
Combustion conditions. Combustion of substances in oxygen, which is part of the air, is widespread. Each substance is characterized by a specific ignition temperature. This is the name of the temperature at which combustion begins. To ignite methane in gas stove, even a spark or a lighted match is enough. And in order to reach the ignition temperature of coal, it needs to be heated much longer.
For the combustion process, two conditions are necessary: the creation of a temperature above the ignition temperature of the substance and free access of air.
Let's carry out the experiment. Let's light two identical stearin candles (stearin is an organic substance). Cover one with a glass cover or a large beaker. Let's leave the second open. The candle under the glass will burn out for a while and go out, while the second one continues to burn.
With this experiment, we tested both conditions of burning. The access of oxygen was not limited to the second candle, while the access of air, and hence oxygen, was blocked for the first glass.
While the candle was burning under the glass, light spread from it in all directions. Touching the glass with our hand, we will feel the warmth.
Now that we have figured out the combustion conditions, it is easy to decide on another question - how to stop burning. Of course, one should remember these conditions, only act the other way around. It is necessary to stop air access and create a temperature lower than the ignition temperature.
Combustion in the service of man. For the first time, a person got acquainted with combustion in natural conditions... In those distant times, man both feared him and expected him. I was afraid, because the lightning caused the heat, but I expected, because the fire gave warmth and light, it was possible to cook food, the fire scared away predators. Material from the site
It took a long time before a person learned not only to maintain fire, but also to produce it himself. That is, he learned not to depend on nature, but to independently carry out the chemical phenomenon of combustion.
Now this phenomenon is of great benefit to man. Thanks to combustion, they generate electricity, prepare food, light and heat homes, drive cars, extract metals, and make glass.
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