Page 313 - Advanced Thermodynamics for Engineers, Second Edition

302 CHAPTER 13 EFFECT OF DISSOCIATION ON COMBUSTION PARAMETERS

the weak mixture region, but surprisingly there is a lower level of carbon monoxide in the rich region
than when there is no dissociation. The explanation of this is that the dissociation of the water releases
some oxygen that is then taken up by the carbon. The production of carbon monoxide in lean mixtures
is the reason for carbon monoxide in the untreated exhaust gas from internal combustion engines even
when they are operating stoichiometrically or lean.
Considering the carbon dioxide, it can be seen that this peaks at an equivalence ratio of unity
(f ¼ 1) without dissociation. However, it peaks at an equivalence ratio of around f ¼ 0.9–1.0 (i.e.
weak) when there is dissociation, when the peak level of carbon dioxide is only about 75% of that
without dissociation.
The mole fractions of products for the combustion of octane are shown in Fig. 13.9. The trends are
similar to those for the combustion of methane. The carbon dioxide fraction maximises on the lean side
of stoichiometric, at f of about 0.9. It is also noticeable that the mole fractions of water and carbon
dioxide are much closer with octane than methane, which simply reflects the higher carbon/hydrogen
ratio of octane.

13.6 THE EFFECT OF FUEL ON COMPOSITION OF THE PRODUCTS
Twodifferent fuelswereusedinthe previousanalysis of the effect of dissociation on com-
bustion products. Both are paraffinic hydrocarbons, with a generic structure of C n H 2nþ2 :for
methane the value of n ¼ 1, while for octane n ¼ 8. This means that the carbon/hydrogen ratio varies
from0.25to0.44. This hasaneffect on theamountofCO 2 produced for each unit of energy released
(i.e. kg CO 2 /kJ, or more likely kg CO 2 /kWh). The effect of this ratio can be seen in Figs 13.6 and
13.9. The maximum mole fraction of CO 2 released with octane is almost 10%, whilst it peaks at
around 7.5% with methane: the energy released per unit mass of mixture is almost the same for both
fuels, as shown by the similarity of temperatures achieved (see Figs 13.4 and 13.5). Hence, if it is
required to reduce the amount of carbon dioxide released into the atmosphere, it is better to burn
fuels with a low carbon/hydrogen ratio. This explains, in part, why natural gas is a popular fuel at
this time.

13.7 THE FORMATION OF OXIDES OF NITROGEN
It was shown in Chapter 12 that when air is taken up to high temperatures the nitrogen and oxygen will
combine to form oxides of nitrogen, and in particular nitric oxide, NO. It was also stated that it is not
enough to simply add the equation

N 2 þ O 2 52NO (13.7)
but a chain of reactions including 11 species has to be introduced. This is necessary because the
formation of atomic oxygen and nitrogen, and the radical OH play a significant part in the amount of
NO produced in the equilibrium mixture.
These equations representing equilibrium of the 11 species have been used to analyse the equi-
librium constituents of the products of combustion for the cases considered above, i.e. the combustion
of methane and octane in an engine operating on an ‘Otto’ cycle. The results are shown in Figs 13.10
and 13.11. It should be noted that the ordinate axis is in logarithmic form to enable all the important
constituents to be shown on a single graph: the numerical results for carbon monoxide, carbon dioxide