Optimization of torrefied black liquor and its combustion characteristics with pulverized coal

•Proximate analysis (PA), elemental analysis (EA), and Poison index (PI) were employed as the indices to evaluate the performance of combustion.•Various torrefied black liquor (TBL) optimized by the Taguchi method was combusted in the single pellet combustion system.•Higher torrefaction temperatures (300 °C) resulted in lower CO/CO2 ratios during 800 °C char combustion, dominating overall combustion efficiency.•Elevated torrefaction temperatures reduced sodium sulfate content in post-combustion ash, effectively preventing boiler fouling.