Combustion of methane/air mixtures in a two-layer porous burner: A comparison of alumina foams, beads, and honeycombs

This study investigates the premixed combustion of methane/air mixtures in different alumina (Al₂O₃) packings (foams, beads, or honeycombs) based on the almost identical pressure drops for cold flow. A burner was packed with alumina beads with diameter of 3mm in the preheating zone and 10ppi (pore per inch) alumina foams, alumina beads with diameter of 13mm, or 200cpsi (channel per square inch) alumina honeycombs in the combustion zone. The 10ppi foams, 13mm diameter beads, or 200cpsi honeycombs had porosities of 82%, 52% and 80%, respectively. The flame stability limits, flame temperature profiles, flame temperature, pressure drop, and pollution emissions for carbon monoxide (CO), hydrocarbon (HC), and nitric oxide (NO_x) for the studied structures are discussed. The flame stability limits are decreased in the order of 10ppi foams, 13mm diameter beads, and 200cpsi honeycombs. The flame temperature was significantly affected by heat release at lower flame speed and by heat loss at higher flame velocity under various flame speeds. At the same flame speed, the flame temperature of the foams was significantly lower compared with those of the packed beads and honeycombs because of the significant radiative heat transfer characteristics of the foams. The pressure drop of the reaction flow was significantly higher than that of the corresponding cold flow because of the significant density change. The CO emission was mainly determined by the flame temperature, whereas the HC emission was mainly controlled by the mixing uniformity of fuel/air. The NO_x was very low (below 4ppm) in the three structures because the flame temperature was relatively low (below 1250°C).