Thermochemical equilibrium in hydrocarbon-oxygen reactions involving polyatomic forms of carbon

Abstract

Equilibrium composition and flame temperatures have been calculated for the reaction of acetylene and ethylene with oxygen over a range of mixture ratios. Emphasis has been placed on the determination of conditions necessary for the formation of solid carbon in the products. The influence of the presence of long-chain carbon molecules on the mixture ratio and temperature required for carbon deposition has been investigated. Premixed flames of acetylene and ethylene with oxygen have been studied experimentally to determine the correlation between theoretical calculations and actual combustion reactions. Calculations indicate that, for acetylene, carbon begins to appear in the condensed form at an equivalence ratio (actual fuel oxygen ratio divided by the stoichiometry ratio) of approximately 2·7 when the only forms of carbon vapour considered are monatomic and diatomic. The equilibrium flame temperature is 3 275°K. If longer chain forms of carbon vapour are considered (consistent with a triple point pressure of about 100 atm 4 000°K) then carbon formation begins at an equivalence ratio of 3·37 and the flame temperature would be 3095°K. This difference is not so pronounced with the other hydrocarbons considered. Experimentally, conclusive evidence of carbon formation was apparent at an equivalence ratio of 2·9. Based upon the property data used, the experimental evidence would indicate the presence of carbon molecules containing up to five atoms of carbon. Of these forms the odd number species probably redominate.