Page 219 - Advanced Thermodynamics for Engineers, Second Edition

P. 219

CHAPTER

THERMODYNAMICS

OF COMBUSTION 10

Combustion is an oxidation process and is usually exothermic (i.e. releases the chemical (or bond)
energy contained in a fuel as thermal energy). The most common combustion processes encountered in
engineering are those which convert a hydrocarbon fuel (which might range from pure hydrogen to
almost pure carbon e.g. coal) into carbon dioxide and water. This combustion is usually performed
using air because it is freely available although other oxidants can be used in special circumstances e.g.
rocket motors. The theory which will be developed here will be applicable to any mixture of fuel and
oxidant and any ratio of components in the products; however, it will be described in terms of
commonly available hydrocarbon fuels of the type used in combustion engines or boilers.
The simplest description of combustion is of a process that converts the reactants available at the
beginning of combustion into products at the end of the process. This model presupposes that com-
bustion is a process that can take place in only one direction and it ignores the true statistical nature of
chemical change. Combustion is the combination of various atoms and molecules, which takes place
when they are close enough to interact: but there is also the possibility of atoms which have previously
joined together to make a product molecule, separating to form reactants again. The whole mixture is
really taking part in a molecular ‘barn dance’ and the tempo of the dance is controlled by the tem-
perature of the mixture. The process of molecular breakdown is referred to as dissociation; this will be
introduced in Chapter 12. In reality, a true combustion process is even more complex than this because
the actual rate at which the reactions can occur is ﬁnite (even if extremely fast). This rate is the basic
cause of some of the pollutants produced by engines, particularly NO x . In fact, in most combustion
processes the situation is even more complex because there is an additional factor affecting com-
bustion which is related to the rate at which the fuel and air can mix. These ideas will be introduced in
Chapter 15. Hence, this approach to combustion is a simpliﬁed one but, in reality, it gives a reasonable
assessment of what would be expected under good combustion conditions. It cannot really be used to
assess emissions levels but it can be extended to this simply by the introduction of additional equa-
tions: the basic approach is still valid.
The manner in which combustion takes place is governed by the detailed design of the com-
bustion system. The various different types of combustion process are listed in Table 10.1, and some
examples are given of where the processes might be found. There is an interdependence between
thermodynamics and ﬂuid mechanics in combustion, and this interaction is the subject of current
research. This book will concentrate on the thermodynamics of combustion, both in equilibrium and
nonequilibrium states. The ﬁrst part of the treatment of combustion will be based on equilibrium

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