Chain and thermal self-ignition
For interaction of substances in chemical reactions including for oxidation, it is necessary that the reacting molecules possessed some stock (surplus) of energy.
This stock is called activation energy. It has to be sufficient that molecules overcame electron-pair bindings and entered the vzayoimodeystviye. Speed of reactions of oxidation at the invariable tempeyoratura of gas mixture is proportional to concentration with, s2... the reacting substances:
- where k0 - the constant of proportionality;
- е = 2,71 - Napierian base;
- Е - the activation energy equal to summaryony energy of atoms or molecules, entering chemical reaction, kcal/mol;
- R = 1,986 kcal / (моль°С) - universal gas constant;
- T - temperature of the reacting substances, °К.
Follows from the equation that the constant to and speeds of reactions of oxidation sharply increase in cases when temperature of T of gas mixture increases or there are atoms and molecules for which interaction small activation energy E is sufficient
Self-ignition is called process spontaneous (without introduction of the flame or the spark) accelerations of reactions of oxidation, terminating in flash (or explosion) and burning. The mechanism of reactions of self-ignition and burning differs from the mechanism of slow oxidation. It can be chain (in gases) and teployovy (in gases, Paro - and pylevozdushny mixes).
Reactions of self-ignition and burning of gases belong to tsepyony. They are followed by intermediate stages with participation of free atoms or unstable connections of so-called active molecules which require small energy of the aktiyovation. If the number of active molecules in the course of interaction increases and they form new chains, then such chain reakyotion are called branched.
Processes of burning of hydrogen, carbon monoxide, hydrocarbons (acetylene, benzene, etc.) are branched chain reactions.
Speed of the reactions proceeding on chains depends on pressure and temperature of gas, on the form, the size and the condition of the internal surface of the vessel in which oxidation, from gas impurity proceeds.
Chains break and consequently, oxidation at collision of active molecules with less active or with vessel stenyoka stops. Therefore for delay of chain self-ignition or for clearing of the flame it is necessary to increase the vessel surface at the invariable volume. For example, in fire prevention devices gas is passed through grids (fig. 1 and), lamellar grids (fig. 1) or through narrow gaps And (fig. 2). The specific poverkhyonost of S/V of flat narrow vessels, slots, gaps and also vessels of small volume is many times more, than the specific surface area of vessels in the form of the cube or the large sizes; chain reactions in them stop; the flame dies away and cannot get into sosedyony space.
Impurity of inert gases - nitrogen, the carbon dioxide gas, haloids (compounds of chlorine, for example four-chloride carbon, CC 4, S02CI2, SiCl4, etc.) - also tear off chain reactions of self-ignition and burning.
Thermal self-ignition happens under the condition if initial flameless reactions of oxidation are followed by heat production and growth of temperature. Preliminary heating of mix is necessary for thermal self-ignition. Further mix will begin to samonagrevatsya due to heat production in process the substance okisleyoniya.
Speed of heat production of QB at self-ignition and the goreyoniya is proportional to the speed of chemical reaction. Follows from the equations that heat release of QB depends on temperature under the exponential law too. Speed of return of heat (kcal/h) through walls of the vessel is proportional to the area of the poverkhyonost of the vessel:
- where and - coefficient of heat emission, kcal / (sq.m - °C);
- S - sq.m vessel surface;
- T and Tst - the average temperature of goryuyochy mix and the surface of the vessel.
In fig. 3 dependences of speeds of allocation (the curve 1) and otyodach of heat (2, 3) to the environment from temperature are shown. At the small initial temperature of mix T when oxidation proceeds slowly, QB heat release slightly exceeds QP heat emission. The point And corresponds to equality of QB=QP. This steady state is characterized by process of slow oxidation of combustible substance. Temperayotura of mix cannot rise higher than A T' as teplovydeyoleny there is less heat emission.
If to increase the initial temperature of mix to the CU, then the curve 3 heat emission of QP becomes tangent to the heat release curve. In the point of B of contact of curve speeds of heat release and the teplootyodacha of QB ~ - Qn. Further acceleration of reaction will lead to bystyory temperature increase of mix and self-ignition as the speed of heat production will exceed the speed of its return:
Self-ignition temperature is called the smallest temyoperatura to which it is necessary to heat previously gas mixture that reactions of oxidation in it began самоускоряться before emergence of the flame. In fig. 6-3 this temperature corresponds to the point of the CU on abscissa axis. TV temperature corresponding to QB ravenyostvo = QP (the point of B) is called ignition temperature. TV ignition temperature many times over exceeds the temperayotura of self-ignition of the CU.
Time from the beginning of self-heating of mix before its ignition nayozyvatsya by the induction period. It characterizes potential of explosion of mix. In air-gas mixes time of induction sostavyolyat fractions of a second, and it can be measured in firm environments for hours.
Self-ignition temperature is not physical konyostanty. It depends on properties and composition of gas mixture and conditions of heat emission, i.e. on the form and the surface of the vessel in which gas mixture nakhoyoditsya.
The self-ignition temperature is higher, the gas mixture is less explosive. Heat emission to the environment is proportional to S surface, therefore, the temperayotura of self-ignition of the gas mixture which is in the volume of V that will be higher, than more specific surface area of S/V. Therefore fire prevention devices in the form of grids with narrow channels cells of small volume and the branched surface (140 cells/cm2) ispolyyozutsya in miner's lamps, in reservoirs with flammable substances, in fuel oil storage tanks and gas pipelines (see fig. 6-1). In narrow channels cells of these grids or plates teployootdacha speed many times over exceeds heatreceipt speed. Therefore does not proyoiskhodit transfer of the flame to the explosive environment.
Fire prevention devices in the form of the slot (see fig. 6-2) use for explosion-proof execution of lamps, electric motors. If there is the explosion in their cover, then the flame cannot get into the environment as it stops in the slot.
By the physical nature the same process, as self-ignition, but the term "self-ignition" apply self-ignition to the substances having self-ignition temperature equal or temperatures of the atmospheric air are lower. Tveryody, liquid and gaseous substances ignite spontaneously. Self-ignition of coals is result of oxidation and absorption (absorption) of air oxygen by them.
Crushing of coals promotes their self-ignition. Peat, especially milling, ignites spontaneously as a result of biologiyochesky processes (development of microorganisms) at the temperature from 10-18 °C to 70 °C. In very dry or very wet peat razviyoty microorganisms it is complicated.
Temperature of self-ignition depends on the specific surface area of heat emission, as well as at self-ignition. Reduction of height of the stack of coal prevents its samovozgorayony as the specific surface area of heat emission increases and by that self-ignition temperature increases. By Smochenyonye rag drying oil, being crumpled, ignite spontaneously at the temperature of 20 °C, the same rags in expanded form cannot ignite spontaneously at 20 °C.
For prevention of self-ignition of coals and peat ogranichiyovat height of stacks, condense and fill up them with the trifle layer in order that air did not get inside, periodically control temperature in the stack. For isolation from air the surface of slopes is covered with the crust from mix of lime, clay, sand.Top