Numerical investigation and optimization of porous media burner for NH3/O2/H2O combustion

NH3/O2/H2O porous media combustion is a potentially clean and efficient technology for ammonia combustion power generation. To achieve high efficiency with low NOx emissions, the challenge is to inhibit flame extinction under high steam dilution ratios. This study develops a 1D porous media combustion model based on experimental validation to investigate the effects of burner structure on flame propagation, extinction limits, and emission characteristics. The results indicate that the key to stable combustion lies in enhancing volumetric heat exchange in the unburned and post-flame zones, reducing the heat transfer resistance from the post-flame zone to the unburned zone, and suppressing heat loss in the combustion zone. Compared to typical two-layer burners, the three-layer PPI (pores per inch) design more effectively enhances heat transfer and accumulates heat. Using low thermal conductivity materials in the post-flame zone reduces heat loss caused by thermal conduction, and the graded PPI design enhances thermal convection and radiation by reducing the optical depth. According to the flame stabilization mechanisms and optimization criteria of effective ignition temperature, using a ring matrix with low thermal conductivity in the unburned zone and reducing the solid porosity in the post-flame zone are proven to be potential optimization strategies. The optimized burner's fuel-lean and rich limits at ZH2O = 0.45–0.55 are 0.24–0.34 and 1.45–1.76, respectively, and steam dilution limits are 0.56–0.68 at Φ = 0.6–1.4. At Φ = 1.4 and ZH2O = 0.45–0.55, the NOx emissions of the optimized burner are 15∼56 ppmv@15%O2, achieving ultra-low NOx emissions. The maximum solid temperatures of the optimized burner are 1448∼1677 K, not exceeding the thermal tolerance of the materials. At Φ = 1.4, achieving an ignition temperature greater than 1320 °C is required at ZH2O = 0.65, presenting a challenge for burner structure optimization efforts.