Thermal conversion property and NO emission characteristics of oxy-fuel co-combustion of biomass and semicoke

Oxy-fuel co-combustion of biomass and coal/semicoke is recognized as a promising technique for negative CO2 and low NOx emissions. Oxy-fuel co-combustion thermal properties and volatile emission characteristics of pine sawdust (PS) and Shenmu semicoke (SC) blends were investigated using thermogravimetric-mass spectrometry (TG-MS) and an innovatively designed two-tube reactor. PS and SC were mixed in a mass ratio (mass%) of 0:100, 10:90, 20:80, 30:70, 40:60 and 100:0. The effects of combustion temperature, blend ratio and atmosphere on NO emissions were also studied. Results showed that the SC combustion characteristics significantly improved when blended with PS. The thermal conversion properties of PS/SC blends in a CO2/O2 atmosphere were divided into four reaction stages, namely drying, pyrolysis, combustion and mineral decomposition at the temperature of < 200, 200–350, 350–700 and > 700 ºC, respectively. The main contribution to the increase in the NO conversion rate was the synergistic effect between char-N and volatile-N from PS. As the reaction temperature increased from 850 to 1000 ºC, the NO release time for the SC and PS sample reduced from 500 to 300 s and 80 to 50 s, respectively. The NO conversion rate increased initially and then decreased, with the highest rate at 900 ºC. In addition, increasing oxygen concentrations may significantly enhance the combustion rate. Based on the experimental results, the synergistic effect of nitrogen migration during the oxy-fuel combustion of biomass and coal char blends was proposed. The research results can provide scientific guidance for the clean and efficient utilization of SC in pulverized coal boilers of a power station.