14 - 14 C hemical K inetics 14.1 Introduction Up until now...

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14 Chemical Kinetics 14.1 Introduction Up until 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 with 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 plays 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,) 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, which might well be frozen into the products as the temperature drops. This NO, is a pollutant which is limited by legislation because of its initant effects. NO, 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 combustion, 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 equilibrium even in the short time the gases are in the combustion zone. An analysis of the kinetics of reactions will now be presented. 14.2 Reaction rates Reaction rates are governed by the movement and breakdown of the atoms or molecules in the gas mixture: reactions will occur if the participating ‘particles’ collide. The number of
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Reaction rates 277 collisions occurring in a mixture will be closely related to the number densities (number per unit volume) of the 'particles'. The number density can be defined by the molar concentration, c, which is the amount of substance per unit volume. This is obviously a measure of the number of particles per unit volume since each amount of substance is proportional to the number of molecules; this is illustrated in Fig 14.1. The molar concentration will be denoted by enclosing the reactant or product symbol in [ 1. ..
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This note was uploaded on 03/09/2010 for the course MECHANICAL ME9802701 taught by Professor Prof.william during the Spring '10 term at Institut Teknologi Bandung.

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14 - 14 C hemical K inetics 14.1 Introduction Up until now...

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