Stability and emissions of hydrogen-enriched methane flames on metal fiber surface burners

Metal fiber surface combustion, with potential advantages of low NOx emission and flashback prevention, is a promising technology to burn hydrogen-enriched methane that is regarded as a low-carbon fuel when seeking the carbon-peaking and carbon neutrality target. In this piece of work, we studied two combustion modes, namely the infrared radiation mode (flame submerging in the metal fibers and the radiation of hot metal fiber dominating the heat transfer) and the blue flame mode (flame roots attached to the metal fibers forming visible blueish flame), of premixed hydrogen-enriched methane/air mixtures on two different metal fiber surface burners in terms of their blowout limit, high-temperature corrosion of the metal fibers, and pollutant emissions. The morphology of the premixed flames on the metal fiber surface burners was found to be essentially a group of tiny premixed jet flames. Higher excess air ratio under the fuel-lean condition, lower hydrogen volumetric fraction in the fuel, and lower porosity of the metal fiber favored the blue flame mode, accompanied by the decreased NOx and increased CO. The detailed characterization of the metal fibers confirmed that the infrared radiation mode may be accompanied by the high-temperature corrosion of the metal fibers. To obtain the optimized operation conditions for hydrogen-enriched methane flames on metal fiber surface burners, we proposed and experimentally verified a dimensionless semi-empirical criterion K* for high-temperature corrosion and flame blowout by considering the ratio of overall flow velocity to laminar flame speed, the ratio of characteristic jet diameter to laminar flame thickness, the porosity of the metal fiber, as well as the preferential diffusion effect induced by hydrogen. The results showed that there existed a high-temperature corrosion risk when K* <1.4, and a flame blowout risk when K* >4. Combining the criterion and NOx emission characteristics, combustion mode phase diagrams were also demonstrated as well as the optimized operation conditions.