Chain reactions during combustion

Halogenated flame-retardants are the most widely used agents. They act by inhibiting the chain reactions of combustion. Hydrogen halides (HX) produced from them react with OH radicals formed during the decomposition of polymers ... 

Due to the release of halogen acid during decomposition, halogen-containing compounds interrupt the chain reaction of combustion by replacing the highly reactive HO and H radicals by the less reactive X for example ... 

To set an example, we will calculate the value of the different measures related to a branching chain reaction during hydrogen combustion. To do so, we will consider a simplified diagram inspired by James et al. for the inhibition of hydrogen combustion by methyl-chloride (reaction steps [12.R21] to [12.R27]) ... 

Because propellants are constantly subjected to abnormally high temperatures in various parts of the propulsion system during operation, high thermal stability is desirable. Decomposition of the propellant at temperatures experienced in the combustion chamber cooling jacket, the injector, and/or the gas film on the combustion chamber wall, can cause undesirable product deposition (resulting in local "hot spots and burnout), explosion in the cooling jacket and/or injector, undesired reaction chains in the combustion chamber, etc. 

Emission spectroscopy and, to a lesser degree, absorption spectroscopy have provided considerable information on and insight into the chemistry occurring during the process of combustion. In particular, many of the transient free-radical molecules important in the chain reactions were identified and characterized through their emission spectra in flames. Now, new laser spectroscopic techniques offer the promise of obtaining more detailed and precise information, especially for the ground electronic states of many of the molecules involved in combustion. 

The idea that the processes, which occur during the combustion of a fuel in an engine cylinder, take place by a chain reaction mechanism comparable to that associated with the liquid phase autoxidation of benzal-dehyde, has led to the proposal that knock suppressors act by destroying the chains and reducing the rate at which the flame front accelerates. It is known that surfaces or solid bodies suppress chain reactions, in fact one of the criteria for a chain reaction in gaseous combustion processes is the decrease in rate of reaction caused by the increase in surface exposed to the gases. However, the mode by which the chains are broken or... 

The radical chain reaction is interrupted when the highly reactive HO radical, which plays a key role in the combustion process, is replaced by the less reactive Br- radical (Reaction 1.113). While Equation 1.112 and Equation 1.113 show reactions with HBr (which is released during combustion), experiments have shown that halogen-containing organic compounds may also react as undecomposed molecules. 

Chain growth during addition polymerization has the three steps of classical chain reactions (other examples are combustion processes or nuclear explosions) ... 

Halogen flame retardants are very effective because they interfere with the radical chain mechanism in the gas phase of the combustion process. We have previously seen that during the combustion of polymers, radical chain reactions involving the OH and H radicals occur, as shown in Fig. 4.26. 

Incorporation in the foam of additives able to stop the free radical chain reactions which occur during combustion. 

Free-radical chain reactions also occur during the chlorination of methane (Chapter 10) and of the methyl group of methylbenzene. Ozone depletion by chlorofluorocarbons (CFCs), acid rain formation and formation of photochemical smog (Chapter 25 on the accompanying website) also involve free-radical reactions. (Free-radical reactions are also operating in unpolluted atmospheres and play an important role in all chemical reactions that occur in the gas phase.) The combustion of hydrocarbons, such as petrol, also proceeds via a free-radical mechanism, which has important consequences for the smooth running and performance of combustion engines. Chain reactions may also have ions as intermediates, as opposed to free radicals. 

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