|
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:
 |
(14) |
 |
(15) |
 |
(16) |
 |
(17) |
 |
(18) |
 |
(19) |
 |
(20) |
 |
(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,
 |
(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,
 |
(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 |
