Mechanistic insights into the divergent pathways of Bayer and Sintered red mud additives in synergistic NO/SO2 conversion during coal co-combustion

As a large amount of solid waste residue generated from alumina industry, the resource utilization of red mud (RM) in fluidized bed is of great significance for achieving pollutant reduction. In this paper, the synergistic conversion mechanisms of NO and SO2 during coal co-combustion with Bayer red mud (BRM) and Sintering red mud (SRM) were investigated. The results showed that BRM formed a sodium aluminosilicate melt at above 850 ℃ and iron oxides were dispersed onto the coke surface. Under oxidizing conditions, the char-N was catalytically converted to NO and the total N conversion rate exceeded 60%, while the effect was opposite under reducing conditions. SRM formed a dense calcium silicate network with iron being fixed in the crystal lattice, resulting in a total N conversion rate consistently below 45%. The sodium melts in BRM served as a homogeneous reaction medium, CaSO4 and sodium-based double sulfate salts were uniformly generated. Due to the dense network of SRM, SO2 was restricted from diffusing and CaSO4 was only formed on the surface layer with sulfur enriched at particle boundaries. Eventually, suggestions regarding temperature control, blended ratios, air distribution and equipment adaptability were provided based on the actual operating characteristics of fluidized beds.