Page 225 - Advanced Thermodynamics for Engineers, Second Edition

10.3 HEATS OF FORMATION AND HEATS OF REACTION 213

10.2.3 COMBUSTION WITH RICH MIXTURES
A rich mixture occurs when the quantity of air available is less than the stoichiometric quantity;
this means that there is not sufficient air to burn the fuel. In this simplified approach it is assumed
that the hydrogen combines preferentially with the oxygen and the carbon does not have sufficient
oxygen to be completely burned to carbon dioxide: this results in partial oxidation of part of the
carbon to carbon monoxide. It will be shown in Chapter 12 that the equilibrium equations, which
control the way in which the hydrocarbon fuel oxidizes, govern the proportions of oxygen taken by
the carbon and hydrogen of the fuel and that the approximation of preferential combination of
oxygen and hydrogen is a reasonable one. In this case, to define a rich mixture, f is greater than
unity. Then

2 4 3f 4ðf 1Þ 7:52
O 2 þ 3:76N 2 / N 2 : (10.7)
CH 4 þ CO 2 þ 2H 2 O þ CO
f f f f
If the equivalence ratio is 1.2, then Eqn (10.7) is
CH 4 þ 1:667ðO 2 þ 3:76N 2 Þ/0:333CO 2 þ 2H 2 O þ 0:667CO 6:267N 2 : (10.8)

It is quite obvious that operating the combustion on rich mixtures results in the production of
carbon monoxide (CO), an extremely toxic gas. For this reason it is now not acceptable to operate
combustion systems with rich mixtures. Note that Eqn (10.7) cannot be used with values of f > 4/3,
otherwise the amount of CO 2 becomes negative. At this stage it must be assumed that the carbon is
converted to carbon monoxide and carbon. The resulting equation is

2 4 4 7:52
N 2 : (10.9)
CH 4 þ O 2 þ 3:76N 2 /2H 2 O þ 2 CO þ 3 C þ
f f f f
Equation (10.9) is a very hypothetical one because during combustion extensive dissociation oc-
curs and this liberates oxygen by breaking down the water molecules: this oxygen is then available to
create carbon monoxide and carbon dioxide rather than carbon molecules.
In reality it is also possible to produce pollutants even when the mixture is weaker than stoi-
chiometric simply due to poor mixing of fuel and air, quenching of flames on cold cylinder or boiler
walls, trapping of mixture in crevices (fluid mechanics effects) and also due to thermodynamic lim-
itations in the process.

10.3 HEATS OF FORMATION AND HEATS OF REACTION
Combustion of fuels takes place either in a closed system or an open system. The relevant property of
the fuel to be considered is the internal energy or enthalpy, respectively, of formation or reaction. In a
naive manner it is often considered that combustion is a process of energy addition to the system. This
is not true because the energy released during a combustion process is already contained in the re-
actants, in the form of the chemical energy of the fuel (see Chapter 11). Hence it is possible to talk of
adiabatic combustion as a process in which no energy (heat) is transferred to, or from, the system – the
temperature of the system increases because of a rearrangement of the chemical bonds in the fuel and
oxidant.