Gas phase of combustion

The central thesis of the theory of the non-steady combustion of powders and explosives developed by Ya.B. in this article is the assumption of rapid readjustability of the gas phase of combustion compared to thermal changes in the condensed phase, which allows us to consider the gas phase as quasi-steady. This fundamental property of burning condensed materials allows us not only to significantly simplify the solution of the problem by reducing it to an analysis of the non-steady temperature distribution in the surface layer of the condensed material, but also not to carry out a detailed analysis of the complex structure of the combustion zone above the material (the multi-stage character of the chemical transformation, thermal decomposition, and gasification of the dispersed particles of condensed material and other processes). 

Work in Russia has shown that the use of elemental silicon plus tin chloride can enhance the flame retardancy of both polypropylene and polyamide 66 compounds. Silicon tetrahalides are formed in the gas phase of combustion and inhibit the chain reactions that develop in a fire. These silicon halides do not act like halogen hydrocarbons in the atmosphere so there is no likelihood of contributing to ozone depletion. 

Reactions involving radicals occur widely in the gas phase the combustion of any organic compound is nearly always a radical reaction, and the oxidative breakdown of alkanes in internal combus-... 

The above equations are combined to form the condition for the burning rate at the surface, y = 0. This becomes a boundary condition for the gas phase where combustion is occurring. This is done best by adding Equations (9.87), (9.88), (9.90) and (9.91). In doing so, a grouping of the enthalpies at the vaporization interface arises which represents the enthalpy of vaporization defined in terms of energy per unit mass of the virgin solid (by convention) ... 

Table9.1. Heats of combustion (AHC, kcal mol-1) in the gas phase of n-membered cycloalkanes per methylene group, and enthalpies (AH, kcal mol-1) and entropies (AScy0, cal mol- K- ) forthe cyclization H(CH2) H —> cyclo-(CH2) + H2[42, 43],...

Moreover, the sensitivity of the effect of a flame retardant to the ambient pressure should also be taken into account. Flame retardants that are active only in the gas phase usually fail to affect the composition of the volatile pyrolysis products and the coke yield. In this case whatever the nature of the polymer, the flame retarding element is released into the gas phase during combustion the type of oxidant (O /Nj, N O/N ) strongly affects the flammability. On the other hand the effect of flame retardants active in the solid phase depends on the polymer nature, but is independent of the nature of the oxidant. Variations of the pressure of the oxidative environment affect the rates of gas-phase as well as heterogeneous (interfacial solid-gas) reactions. 

The retarding effect of HX, X, RX introduced into the flame may be of either physical or chemical nature (in either case, the residue R in RX is an extra fuel source). In the first case, the flame retardant reduces the oxygen concentration in the combustible mixture in the flame reaction zone by mere dilution, in the same way as added carbon dioxide or nitrogen. The heat capacity of the resultant mixture determines the amount of heat drained off for its heating. In the second case, the flame retardant directly participates in the flame reactions and affects the complex combustion process kinetics. The HX molecule is the main inhibiting particle. When RX is released into the gas phase of fuel-rich flames, HX is formed predominantly via RX -b H HX + RX, when Xj is released, HX is formed via X + H HX + X. 

Aliphatic compounds (RXn) are more effective than aromatic compounds. There is sufficient evidence that in one form or another the halogen is released into the gas phase during combustion. Halogen-containing compounds behave in an overwhelming majority of cases as gas-phase flame retardants124-126. However, each particular compound may retard flaming combustion of polymers in its own special way. 

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