On the stabilisation of ultra-lean methane and propane flames in porous media

An experimental porous burner was developed and instrumented for measuring temperature and pollutants emission. The burner consists of an insulated cylindrical alumina casing filled with two layers of alumina-based beads of different porosity and diameters. The first layer is 75mm high and consists of 2 to 3mm-diameter ceramic-based beads. This layer acts as a flame arrestor. The second layer is 350mm high and consists of 6mm-diameter ceramic beads. A mixture of fuel and air is introduced into the burner through the first layer. Measurements were performed for premixed flames of methane/air and propane/air mixtures over a range of flow velocities (10-30 cm/sec) and equivalence ratios (0.45-0.35). These conditions represent the limiting boundaries for stable combustion in this burner. A maximum gas temperature of ~1500K was recorded for all flames. This temperature is at least 250 degrees higher than the corresponding equilibrium temperature. The difference is attributed to the heat transfer between the gas and the beads. The location of the peak temperature however, depends on total flow rate and equivalence ratio. It is found that for similar operating conditions, propane flames exhibit higher sensitivity (in terms of flame stability and location) to variations in flow conditions than methane flames. This sensitivity becomes more pronounced for leaner mixtures or at higher flow rates. Measurements of pollutants at the exit plane of the burner showed CO and NOX concentrations of less than 6ppm and 4ppm, respectively.