Effects of hydrogen addition on methane combustion in a porous medium burner

The effects of hydrogen addition on premixed combustion of methane in porous medium burners are investigated numerically. The numerical model considers solid and gas phase energy equations separately, and includes nongray radiation transport for the solid phase. GRI-Mech 2.11 is used for the detailed chemical kinetic model. The lean limit of pure methane can be extended from φ = 0.52 for free flame to φ = 0.33 for porous medium flame. Adding hydrogen in the fuel, the lean limit can be further reduced to φ = 0.26. The flame speeds of porous ceramic burner flames are several times as that of free flames. For φ = 0.8, the free flame speed of methane is 29 cm/s whereas that of porous burner flame is 72 cm/s approximately. Adding hydrogen to the fuel, the flame speed is increased to 86 cm/s when the fraction of hydrogen in the fuel, g, equals to 0.3. Increasing g to 0.6 raises the flame speed to 118 cm/s. The maximum energy release rate of the porous medium burner flame is much larger than that of a free flame. For φ = 0.5 and g = 0.6, it can be as high as 24 times. The porous medium burner flame is found to have thinner flame thickness than free flame. Increasing the hydrogen fraction in the fuel further reduce the flame thickness. The CO emission increases slightly as the hydrogen fraction in the fuel is raised. The specific NO_x emission for porous medium burner is higher than that for free flame, indicating the non-equilibrium mechanism plays an important role in the porous medium burner.