H2O and CO2 dilution in mild combustion of simple hydrocarbons

MILD combustion is a very attractive technology in energy production from diluted gas deriving from bio or thermochemical degradation of biomass for its intrinsic features. An effective use of such a technology for diluted fuel needs a thorough analysis of ignition and oxidation behavior to highlight the potential effects of the different fuel components on the basis of temperature and diluent/oxygen/fuel mixture composition. In this work ignition and oxidation of a model gas surrogating the gaseous fraction of biomass pyrolysis products containing C1-C2 species, CO and CO2 were experimentally and numerically studied in a wide range of temperature and overall composition in presence of large amount of CO2 or H2O. Experimental results showed that such species significantly alter the evolution of the ignition process in dependence on temperature range and mixture composition. Several kinetic models were tested to simulate experimental results. Significant discrepancies occur especially in case of steam dilution. Numerical analyses suggest that such diluents mainly act as third body species at low temperatures, conditioning both radicals production pathways and the relative weight of C1 oxidation/recombination routes, while strongly interact with the H2/O2 high temperature branching mechanisms at high temperatures. Further analyses are mandatory to improve the model ability to reduce the discrepancy between numerical and experimental data obtained in non standard conditions.