Effect of additives on physical characteristics, combustion behavior, and pollutant emission of co-pellets produced from torrefied-biomass and semi-dried sewage sludge

Co-pelletizing torrefied Camellia oleifera shell (TCOS) with semi-dried sewage sludge (SS) can improve pellet quality but may create new challenges in pollutant emissions and ash slagging. To address these challenges, single and blended CaO/kaolin additives were evaluated for their effects on the physical properties, combustion behavior, emissions, and ash fusion characteristics of the co-pellets. Additives increased compaction energy by 4.09% – 17.43% but had little effect on extrusion energy or pellet density. The single addition of kaolin reduced cumulative SO 2 from 1.17 to 0.97 mg and PM0.1 by 29.72%, while raising ash fusion temperatures (AFT) and lowering the slagging index. The single addition of CaO advanced the NO 2 emission peak, but reduced AFT by up to 118℃, increasing the slagging tendency. For blended additions, TCS-Ca3K1 gave the lowest cumulative NO 2 (3.31 mg), while TCS-Ca1K3 achieved the greatest PM 0.1 reduction (28.75%). XRD confirmed CaSO4 in CaO-added pellets and KAlSiO 4 in kaolin-containing pellets. These results indicate that CaO and kaolin target different pollutants and ash risks, and their ratio can be tailored to balance emission control and slagging mitigation.