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Reaction Kinetics & Chemical Reaction Models

General Features of Reaction Mechanisms

Another important consideration is the formation of chain reactions. The basic premise of chain reaction mechanisms is also that free radicals play a leading role in the destruction of reactant molecules. The chain reaction mechanism itself consists of several steps: initiation, propagation, branching (not always present), and termination. This can be illustrated, for certain range of temperature and pressure, by some of the reactions in the following Hydrogen oxidation mechanism:

Reaction Kinetics8.gif (8)
Reaction Kinetics9.gif (9)
Reaction Kinetics10.gif (10)
Reaction Kinetics11.gif (11)
Reaction Kinetics12.gif (12)
Reaction Kinetics13.gif (13)
Reaction Kinetics14.gif

The initiation step is responsible for the initial decomposition of the reactants. Propagation steps involve a radical-molecule reaction with generation of a radical in the product, while branching reactions produce two radicals per single radical in the reactants. For example, the generation of a flame in combustion is due to branching reactions predominating over termination, with a large generation of radicals resulting in the fast decomposition of the fuel.

To illustrate what has been discussed above, in assembling the main reaction paths for the combustion of methane (Miller et al., 1992) consideration is given to the following reactions:

Reaction Kinetics15.gif (14)
Reaction Kinetics16.gif (15)
Reaction Kinetics17.gif (16)
Reaction Kinetics18.gif (17)
Reaction Kinetics19.gif (18)
Reaction Kinetics20.gif (19)
Reaction Kinetics21.gif (20)
Reaction Kinetics22.gif (21)

Where X represents the free radicals H, OH, or O. The relative concentration of the radicals depend on the stoichiometry of combustion for example, in fuel rich flames, the chemistry of H radicals gains much importance. To the above reactions one must add the very important chain branching reaction,

Reaction Kinetics23.gif (10)

In examining this simplified mechanism, the main conclusions are that at the high temperature of combustion the decomposition of methane is initiated by radical attack such as in reaction (14) since the concentration of free radicals is relatively abundant. Reactions (15), (16), (18), (21) propagate the decomposition of methane. Reactions (17), (10) are chain branching reactions. Reactions (19), (20) lead to stable molecules and would be considered as termination reactions. Obviously, the radical pool is initiated by the thermal decomposition of methane in the hot region of the flame or reaction (22) below,

Reaction Kinetics24.gif (22)

with the very important branching reaction (10) adding to the radical pool.

To summarize, reaction mechanisms can be assembled from elementary reactions using free radicals as the means for decomposition of the reactant, and intermediate products. Chain branching reactions, if they occur, take a very important role in the mechanism as they lead to the formation of increasing concentrations of radicals. Reaction time and temperature have a bearing on radical concentration, and the type of reaction initiating the consumption of the reactant.

Estimation of Kinetic Coefficients

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