Experimental evaluation on co-combustion characteristics of semi-coke and coal under enhanced high-temperature and strong-reducing atmosphere

Co-combustion of coal pyrolysis semi-coke and coal can be a promising approach for large-scale utilization of the high-grade carbon-based product with ultra-low volatiles content. In this paper, experimental evaluation on co-combustion characteristics of semi-coke and coal was conducted with a focus on combustion and NOx formation characteristics using a high-temperature (up to 1873 K) vertical tandem-type drop-tube furnace. A novel technology of enhanced high temperature and strong reducing atmosphere (HT&SRA) was proposed, and the effects of fuel type, blending ratio, blending method were evaluated. More NOx and unburned carbon were produced during combustion of semi-coke than bituminous coal. The combustion characteristics of semi-coke were improved by the addition of more bituminous coal due to interaction. The in-furnace blending method had advantage over out-blending method on the improvement of combustion and NOx reduction. Both NOx formation and unburned carbon first declined and then increased with the delayed injection of bituminous coal. When the stoichiometric ratio in primary zone (SR1) is lower than 1.0, the higher temperature (T-1) led to more NOx formation, which was contrary in the oxidizing atmosphere. The parameters of T-1 = 1873 K and 0.6 <= SR1 <= 0.8 were recommended as HT&SRA for the co-combustion. The enhanced HT&SRA was obtained by the approach of increasing oxygen fraction of secondary air (O-sa) with the reasonable SR1, and the NOx formation was fortunately further reduced. The parameters of SR1 = 0.8 and O-sa = 84% were suggested for the co-combustion. The approach of increasing oxygen faction of over-fire air is an alternative to further improve combustion characteristic due to the acceptable increase of NOx formation. The oxygen fraction of primary air should be limited to 42% to avoid excess NOx formation.