In the present paper the theoretical study of the un-stretched laminar premixed flames of hydrogen-methane mixtures is carried out by using the detailed reaction mechanism GRI-Mech 3.0 implemented in the CHEMKIN software to find out the effect of hydrogen addition on the hybrid fuel burning velocity.
The model results show that the laminar burning velocity of the hydrogen-methane mixtures is not the linear regression of those of the pure fuels since it results substantially less than the proportional averaging of the values for the fuel constituents. Moreover, the effect of hydrogen addition in terms of enhancement of the mixture laminar burning velocity with respect to the methane is relevant only at very high values of the hydrogen content in the hybrid mixtures (> 70 % mol.).
The performed sensitivity analysis shows that these results can be attributed to kinetics and in particular to the concentration of H radicals: depending on the hydrogen content in the fuels mixture, the production of the H radicals can affect the limiting reaction step for methane combustion. Two regimes are identified in the hydrogen-methane combustion. The first regime is controlled by the methane reactivity, the hydrogen being not able to significantly affect the laminar burning velocity (< 70 % mol.). In the second regime the hydrogen combustion has a relevant role as its high content in the hybrid fuel leads to a significant H radicals pool, thus enhancing the reaction rate of the more slowly combusting methane.