An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH4/H2 proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO2 concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions. ► Experimental demonstration of mild flameless combustion of CH4–H2 mixtures. ► Ultra-low NOx emissions achieved for all hydrogen proportion from pure CH4 to pure H2. ► OH* imaging reveals the change in reaction zones topology with H2 content. ► Operation close to a “zero emission furnace” for pure H2 without air preheating.