Page 227 - Advanced thermodynamics for engineers

214 CHAPTER 10 THERMODYNAMICS OF COMBUSTION

Mechanical engineers are usually concerned with the combustion of hydrocarbon fuels, such as
petrol, diesel oil, or methane. These fuels are commonly used because of their ready availability (at
present) and high-energy density in terms of both mass and volume. The combustion normally
takes place in the presence of air. In some other applications, e.g. space craft, rockets, etc., fuels
which are not hydrocarbons are burned in the presence of other oxidants; these will not be
considered here.
Hydrocarbon fuels are stable compounds of carbon and hydrogen which have been formed through
the decomposition of animal and vegetable matter over many millennia. It is also possible to synthesise
hydrocarbons by a number of processes in which hydrogen is added to a carbon-rich fuel. The South
African Sasol plant uses the Lurgi and Fischer–Tropsch processes to convert coal from a solid fuel to a
liquid one. The chemistry of fuels is considered in Chapter 11.

10.4 APPLICATION OF THE ENERGY EQUATION TO THE COMBUSTION
PROCESS – A MACROSCOPIC APPROACH
Equations (10.3)–(10.6) show that combustion can take place at various air-fuel ratios and it is
necessary to be able to account for the effect of mixture strength on the combustion process, especially
the temperature rise that will be achieved. It is also necessary to be able to account for the different fuel
composition: not all fuels will release the same quantity of energy per unit mass and hence it is
required to characterise fuels by some capacity to release chemical energy in a thermal form. Both of
these effects obey the First Law of Thermodynamics i.e. the energy equation.

10.4.1 INTERNAL ENERGIES AND ENTHALPIES OF IDEAL GASES
It was shown previously (Chapter 7, Section 7.2) that the internal energies and enthalpies of ideal gases
are functions of temperature alone (c p and c v might still be functions of temperature). This means that
the internal energy and enthalpy can be represented on U–T and H–T diagrams. It is then possible to
draw a U–T,or H–T line for both reactants and products (Fig. 10.2). The reactants will be basically
diatomic gases (neglecting the effect of the fuel) whereas the products will be a mixture of diatomic
and triatomic gases – see Eqn (10.3).
The next question which arises is what is the spacing between the reactants and products lines: this
spacing represents the energy that can be released by the fuel.

10.4.2 HEATS OF REACTION AND FORMATION
The energy contained in the fuel can also be assessed by burning it under a specified condition: this
energy is referred to as the heat of reaction of the fuel. The heat of reaction for a fuel is dependent on
the process by which it is measured. If it is measured by a constant volume process in a combustion
bomb then the internal energy of reaction is obtained. If it is measured in a constant pressure device
then the enthalpy of reaction is obtained. It is more normal to measure the enthalpy of reaction because
it is much easier to achieve a constant pressure process. The enthalpy of reaction of a fuel can be
evaluated by burning the fuel in a stream of air, and measuring the quantity of energy that must be
removed to achieve equal reactant and product temperatures, see Fig. 10.3.