Performance Evaluation of Low-Concentration Methane Combustion in a Four-Layer Gradually-Varied Porous Burner

Porous media combustion (PMC) has made a comeback as a practical technology for the utilization of low-concentration methane (LCM) from coal mining. However, the conventional direct-fired PM burner still suffers from the rampart of flame stability and combustion efficiency not addressing industrial demands. Herein, a four-layer gradually-varied porous burner was innovatively developed for LCM combustion to investigate combustion performance under lean combustion conditions (CH4 volume fraction below 5%). The methane conversion, pollutant emissions, and flue gas temperature were also evaluated in detail. The results indicated that the burner offered a favorable combustion resistance due to the gradually-varied PM arrangement to heighten combustion stability with subtle temperature fluctuation and flame migration. The flammability limit of LCM was extended to the lowest equivalence ratio of 0.43 with stationary combustion at 240 degrees C for 120 min. The energy efficiencies of the LCM combustion under lean combustion conditions were greatly boosted with the highest combustion and thermal efficiencies attained at 99.52% and 70.05%, respectively. The maximum 99.93% CH4 conversion was acquired at an equivalence ratio of 0.45 and a flow rate of 80 L/min. LCM combustion in the burner achieved extremely low pollutant emission levels and the overall CO and NOx emissions were 58.04 ppm and below 23 ppm respectively under the experimental conditions. In addition, the high-quality flue gas with an average temperature of more than 516 degrees C was detected in the operating process, which allowed the available heat utilization at the coal mine scenes or in other industries.