Oxygen consumption as the definitive factor in predicting heat of combustion

An accurate one parameter correlation for estimating higher heating value (HHV) as a function of the mass fraction of oxygen consumed by combustion is presented. In its derivation, a theoretical and quantitative approach based on the reduction/oxidation half-reactions of the combustion reaction is used. The derivation relates HHV to changes in bond dissociation enthalpies with respect to the oxygen species and fuel elements. HHV is found to be a strong proportional function of changes in bond enthalpies with respect to the oxygen species compared to the other fuel elements, and may be described simply as Delta(c)h degrees vertical bar(HHV) =-13.87m(O2) - Delta h(vap) (MJ kg(-1)), where m(O2) is the mass of oxygen required for combustion per 1 kg of fuel. The constant is a simplification of a modifier function mu(DH) =-13.87e(-0.092xO2) (MJ kg(-1)) (being a function of the mass fraction of oxygen consumption only) that is used to reduce the complexity of the theoretical equation describing the heat of combustion. This yields a correlation for the heat of combustion Delta(c)h degrees vertical bar(HHV) = mu(DH) m(O2) - Delta h(vap)(MJ kg(-1)), which may also be expressed as Delta(c)h degrees vertical bar(HHV) = x(O2)/1-x(O2)mu(DH) - Delta h(vap) (MJ kg(-1)). Using 1087 fuel combustion data of wide chemical composition based on the chemical formula of CvcHVHOVNSVSPVP, the resulting correlation is shown to perform well statistically, with R-2 = 0.98, RMSE = 1.5 MI kg(-1), and MBE = 0.0 %. Applying the proposed correlation, HHV was found to be much more sensitive to changes in oxygen content in the fuel than for similar changes in carbon and hydrogen content. Energy quality of fuels, especially in the production of biofuels that make use of highly oxygenated feedstock, should therefore rely on reducing oxygen content in the fuel via deoxygenation pathways such as decarboxylation and decarbonylation while avoiding routes that sacrifice hydrogen (such as hydrodeoxygenation and dehydrogenation).