Branching-chain reactions combustion

It is more than likely that when sulfur occurs in a crude oil or in coal (other than the pyrites), it is organically bound in one of the three forms listed in Table 3—the thiols, sulfides, or disulfides. The combustion of these compounds is very much different from that of other sulfur compounds in that a laige portion of the fuel element is a pure hydrocarbon fragment. Thus in explosion or flame studies, the branched-chain reactions that determine the overall consumption rate or flame speed would follow those chains characteristic of hydrocarbon combustion rather than the CS, SO, and S radical chains which dominate in H2S, CS2, COS, and Sg combustion. 

The slow combustion of methylene chloride is a degenerately branched chain reaction it proceeds by a mechanism similar to that involved in the pyrolysis of the same compound which takes place at a slightly higher temperature [153]. The primary chains are the same and several of the chlorinated hydrocarbon minor products are identical. Oxygen is only involved in the conversion of the intermediate dichloroethylene to the final products hydrogen chloride and carbon monoxide. 

Straight and branched chain reactions almost invariably have complex rate expressions, as shown by -d[H2]/df = A [H2]° [02]° [N2]° for the H2 + O2 reaction in aged boric-acid-coated vessels at 500 Torr and 773 K [6]. Change of pressure can have striking effects even when achieved by addition of an inert gas such as N2. Hydrocarbon combustion reactions proceed through the formation of many intermediates, both radical and molecular, prior to formation of the final products CO2 and H2O. Another striking feature is the sensitivity of chain reactions to traces of impurities and to changes in surface properties. This is particularly pronounced in the case of some explosion boundaries or limits where parts per million quantities of an impurity may completely subdue the explosion. 

Prior to 1930, attention of fundamental combustion scientists was focused mainly on the morphology of the cool-flame and ignition regions. The acceptance of free radicals, followed by the masterly and elegant Semenov theory (outlined in Chapter 5), which established the principle of branched chain reactions, provided the foundation for modern interpretations of hydrocarbon oxidation. The significant processes are ... 

Free Radical Trap Theories. Combustion vapor-phase reactions have been studied using premixed gas flames such as methane. Considerable information concerning the mechanism of flame propagation has resulted from this work 40, 49, 50). Basically the process occurs predominantly by branching chain reactions among free radicals. The major chain branching reactions are... 

In the case of a rapid combustion at temperatures above 700 K, the reaction mechanism is different. H2O2 is no longer the initial intermediate product, since at these temperatures it starts to be thermally decomposed, yielding two OH radicals. The branched chain reaction becomes more extensive and it is originated via the HO2 radical as follows ... 

The combustion process in the reaction zone is a branched chain reaction, and the individual steps in such a reaction may be classed as ... 

The reaction system H2-O2 has been greatly studied by the works of HINSHELWOOD and SEMENOV in around 1930, and even more with the spatial applications and due to the fact that its mechanism is an integral part of any hydrocarbon oxidation or combustion mechanism. It will therefore be taken as an example to illustrate the theory of branched chain reactions. 

An example of a branched-chain reaction is the combustion of hydrogen initiated by fission of a hydrogen molecule ... 

In Chapter 1 we briefly considered the critical phenomena in branching chain reactions consisting in the qualitative transfer from the slow mode of reaction to the intensive (self-accelerated) one at negligible changes in the parameters of a reaction system. Investigations of critical phenomena are still urgent. Increased interest in this problem is closely associated with the practical tasks of combustion, fire and explosion safety, inhibition of oxidation processes, etc. [1-3, 26-50],... 

As stated before (p. 190), the hydrogen combustion is one of the reactions providing the experimental data underlying the theory of branched chain reactions. However, the basic features of this reaction are also inherent in the combustion of other gases. Consequently, the hydrogen combustion can be taken as a model reaction, to some or other extent representing combustion in general. 

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. 

The calculated boundary of the change in the dominant chemical reaction mechanism is confined inside the shaded area between fines A and B in Fig. 6.4. To the left of the separating zone (sometimes called the forth ignition limit [12]), branching-chain reactions dominate. For the first time, the possibility of the growth of a self-ignition delay with a pressure rise in a combustible mixture was indicated by the improved expression for t, in [17]. 

Hydrogen fluorination was studied in detail. This reaction occurs with self-ignition typical of branched chain reactions, which appears in a certain (pi < p < P2) range of pressures of the H2 + F2 mixture as in the case of hydrogen combustion. In the self-ignition regions, the reaction kinetics is described by the law e. The reaction mechanism includes the following elementary steps ... 

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]) ... 

The strength of the London forces between alkane molecules increases as the molar mass of the molecules increases hydrocarbons with unbranched chains pack together more closely than their branched isomers. Alkanes are not very reactive. but they do undergo oxidation (combustion) and substitution reactions. 

As before, reaction (3.71) is slow. Reactions (3.72) and (3.73) are faster since they involve a radical and one of the initial reactants. The same is true for reactions (3.75M3.77). Reaction (3.75) represents the necessary chain branching step. Reactions (3.74) and (3.78) introduce the formyl radical known to exist in the low-temperature combustion scheme. Carbon monoxide is formed by reaction (3.76), and water by reaction (3.73) and the subsequent decay of the peroxides formed. A conversion step of CO to C02 is not considered because the rate of conversion by reaction (3.44) is too slow at the temperatures of concern here. 

Most combustion reactions involve chain branching reaction steps. Under conditions where these steps are less significant than linear chain reactions, the reaction appears to be stable, but when the chain branching steps dominate, the overall reaction rate can accelerate uncontrollably. 

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