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CHAPTER
14
CHEMICAL KINETICS
14.1 INTRODUCTION
Up till now it has been assumed that chemical reactions take place very rapidly, and that the equilibrium
conditions are reached instantaneously. While most combustion processes are extremely fast, often their
speed is not such that combustion can be considered to be instantaneous compared to the physical
processes surrounding them. For example, the combustion in a reciprocating engine running at 6000 rev/
min has to be completed in about 5 ms if the engine is to be efficient. Likewise combustion in the
combustion chamber of a gas turbine has to be rapid enough to be completed before the gas leaves the
chamber. Such short times mean that it is not possible for all the gases in the combustion chamber to
achieve equilibrium – they will be governed by the chemical kinetics of the reactions.
Chemical kinetics play a major role in the formation of pollutants from combustion processes. For
example, oxygen and nitrogen will coexist in a stable state at atmospheric conditions and the level of
oxides of nitrogen (NO x ) will be negligible. However, if the oxygen and nitrogen are involved in a
combustion process then they will join together at the high temperature to form NO x which might well
be frozen into the products as the temperature drops. This NO x is a pollutant which is limited by
legislation because of its irritant effects. NO x is formed in all combustion processes, including boilers,
gas turbines, diesel and petrol engines: it can be removed in some cases by the use of catalytic
converters.
It was shown in Chapters 12 and 13 that significant dissociation of the normal products of com-
bustion, carbon dioxide and water, can occur at high temperature. The values shown were based on the
equilibrium amounts of the substances, and would only be achieved after infinite time; however, the
rates at which chemical reactions occur are usually fast and hence some reactions get close to equi-
librium even in the short time the gases are in the combustion zone. An analysis of the kinetics of
reactions will now be presented.
The effect of chemical, or rate, kinetics can be assessed by considering a simple example. Imagine
a spark-ignition engine in which the mixture is compressed prior to ignition by a spark. If the
compression temperature is not too high it can be assumed that the reactants do not react before
ignition. After ignition the reactants burn to form products at a high temperature, and these are initially
compressed further (as the piston continues to rise to top dead centre, and the combustion process
continues) before being expanded when the piston moves down and extracts work from the gases. As
the pressure increases the temperature of the gas also increases and the products tend to dissociate:
they attempt to achieve an equilibrium composition but the speed of the engine is too rapid for this. The
effect can be seen in Fig. 14.1, which depicts the way in which the actual level of pollutants attempts to
follow the equilibrium level, but lags the equilibrium values. This lag means that the maximum level of
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