Effect of natural gas composition on the laminar burning velocities at elevated temperatures

Laminar burning velocities of fuel mixtures of methane/ethane/propane with the compositions 100/0/0, 80/20/0, 80/0/20 and 80/10/10 vol% burning with air were determined experimentally using the heat flux method at 1 atm and initial gas temperatures 298, 318 and 338 K. The mixtures were selected as surrogates for natural gas, with the aim to investigate the effect of heavier hydrocarbons on the laminar burning velocity of the main component, methane. It was found, in agreement with the literature data, that the heavier hydrocarbons increase laminar burning velocity compared to that of methane + air flames. A common correlation for the temperature dependence of the burning velocity S-L = S-L0(T/T-0)(alpha), where T-0 is a reference temperature and SL0 is the laminar burning velocity at this temperature, was used to interpret new measurements. The power exponents, alpha, were derived from the experimental data for methane and three surrogates for natural gas. It was found that the temperature dependence of the burning velocities is practically identical for all mixtures studied. The measurements have been compared with the modelling using two kinetic schemes: recent version of the Aramco mech 2.0 and an updated version of a model developed by the authors. Both kinetic mechanisms show systematic trends in slight over- and under-prediction of the burning velocities, respectively, for all fuel blends. However, the temperature dependence of the burning velocities is accurately reproduced by these two models. Further analysis indicates that even though rate constants of the reactions determining flame propagation are somewhat different for the two mechanisms, the power exponents alpha are not sensitive to the differences. This indicates that, detailed kinetic schemes capable in predicting burning velocities at a specific initial mixture temperature are able to operate at higher temperatures as well, at least from lean to moderately rich natural gas mixtures.