Flameless combustion of CH4/CO/H2 fuel blends

Flameless (also called 'diluted') oxidation is an efficient technology successfully applied in industrial furnaces to get higher process thermal efficiency while keeping NOx emission at very low level. It is achieved thanks to specific design of air and fuel nozzles to get aerodynamic dilution of reactants with a substantial amount of inert (flue) gases, resulting in significant decrease of the hot spots and therefore reducing the thermal NOX formation. If information and design guidelines are now available in literature to get flameless oxidation with standard fuels such as methane, propane or natural gas, there remains a lack of data concerning flameless oxidation conditions for alternative fuels. Furthermore, an increasing demand is coming now from industry for new combustion techniques able to safely use gasified waste or by-product gases as alternative fuels in manufacturing processes. Mainly composed of CH4, CO and H2 in variable proportions, these gas blends present significant variations of their heating value that can compromise safe flame stabilization on a standard burner. In this case, flameless oxidation could be a smart way to recover more easily and safely waste gas in manufacturing industry.The purpose of this work is therefore to collect experimental data on a 3kW laboratory bench to characterize combustion of various CH4/H2/CO blends in diluted oxidation conditions. Flameless combustion regime is identified considering NOx and CO content in flue gas, flue gas temperature, and intensity of chemiluminescence emission of OH in the reaction zone. Influence of reactants temperature, composition and inlet velocities on combustion regime is assessed in the experimental study. On the other hand, numerical simulation with ANSYS/FLUENT and COSILAB codes are performed. Measured data are then used in the numerical study to validate the selected combustion model and kinetic mechanism, assessing their ability to reproduce the key features of flameless oxidation.