Combustion free-radical mechanism

The combustion of gaseous fuels is believed to proceed via a free radical mechanism (1, 2) A number of propagating and chainbranching reactions are critical for maintaining the combustion process. Some of these reactions are illustrated below ... 

Being organic substances, polymers provide an excellent fuel source for the propagation of fire. The combustion of polymers occurs via a free-radical mechanism the heat from the fire vaporizes and ionizes the constituents of the gas forming a cloud of free radicals. The spread of combustion occurs via the well-known free radical mechanisms of propagation chain branching and termination. There are a number of ways to interrupt the chain of events involved in fire propagation ... 

Polycyclic aromatic hydrocarbons (PAH) are mainly products of incomplete combustion and can be found at high concentrations in PM. PAHs require metabolic activation to electrophiles, catalyzed by various enzymes through free radical mechanisms, to exert their carcinogenic effects. Research found that there are... 

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. 

Combustion does not involve simple singje-step collisions between a fuel molecule and an oxidant. Rather, the reactions that occur during combustion are based on free radicals. In a free-radical mechanism, three generic steps take place ... 

The basis of the free-radical mechanism of oxidation, which includes combustion and explosion, was created by N. Semenov as part of his fundamental work dedicated to the theory of branched chain reactions (Nobel Prize for Chemistry in 1956). Based on this (now common) theory, the oxidation of hydrocarbons takes place according to the mechanism of the free-radical chain reaction with forced branching. Let s look at the main rules of this mechanism, which can take place in the extracellnlar skin matrix. 

Free radicals are molecular fragments having one or more unpaired electrons, usually short-lived (milhseconds) and highly reaclive. They are detectable spectroscopically and some have been isolated. They occur as initiators and intermediates in such basic phenomena as oxidation, combustion, photolysis, and polvmerization. The rate equation of a process in which they are involved is developed on the postulate that each free radical is at equihbrium or its net rate of formation is zero. Several examples of free radical and catalytic mechanisms will be cited, aU possessing nonintegral power law or hyperbohc rate equations. 

Primary chemical processes. The external heat source may supply free radicals which accelerate combustion. The heating material might also be activated by autocatalytic or autoignition mechanisms. 

S.K. Sinha W.D. Patwardhan, Explosiv-stoffe 16 (10), 223-25 (1968) CA 70,49144 (1969) The mechanism causing the plateau effect in the combustion of proplnts with ad-mixt of Pb compds (ie, the independence of pressure of the combustion rate in a certain range) is discussed. This effect is caused by the transport of free Pb alkyl radicals from the foam zone to the fizz zone, which decomn there, causing a more efficient combustion, and increase the temp of this zone by reaction1 with NO. An increase of pressure is assumed to displace the free radicals from this zone because of the increase of the collision rate . this leads... 

Free radicals are short-lived, highly-reactive transient species that have one or more unpaired electrons. Free radicals are common in a wide range of reactive chemical environments, such as combustion, plasmas, atmosphere, and interstellar environment, and they play important roles in these chemistries. For example, complex atmospheric and combustion chemistries are composed of, and governed by, many elementary processes involving free radicals. Studies of these elementary processes are pivotal to assessing reaction mechanisms in atmospheric and combustion chemistry, and to probing potential energy surfaces (PESs) and chemical reactivity. 

Flame retardants impart to the polymers some ability to resist ready combustion. Since fuel, oxygen, and high temperature are essential for the combustion of polymers, the removal of any of these prerequisites retards combustion. Flame retardants act through a variety of mechanisms including char formation and combination with free radical species that promote further combustion, through release of water. 

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. 

AUTOXIDATION. A word used to describe those spontaneous oxidations, which take place with molecular oxygen or air at moderate temperatures (usually below 150°C) without visible combustion. Autoxidation may proceed through an ionic mechanism, although in most cases the reaction follows a free radical-induced chain mechanism. The reaction is usually autocatalytic and may be initiated thermally, photoehemically, or by addition of either free radical generators or metallic catalysts. Being a chain reaction, the rate of autoxidation may be greatly increased of decreased by traces of foreign material. 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

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