Char formation during pyrolysis of torrefied cellulose: Role of potassium catalysis and torrefaction pretreatment

Torrefaction is an efficient pretreatment technology for large-scale and high-value utilization of biomass fuel. This study aims to enhance the understanding of the thermochemical conversion of torrefied biomass by delineating the role of potassium and torrefaction on primary/secondary charring reactions during pyrolysis of torrefied cellulose. Celluloses with varying potassium loading levels were torrefied at temperatures ranging from 200 to 320 degrees C and subsequently pyrolyzed at 1000 degrees C. Two contrasting pyrolysis conditions, i.e., limiting and enhancing transport resistance, were employed to tune the occurrence of secondary reactions, thereby determining the role of torrefaction and potassium on primary and secondary charring reactions. Potassium content, torrefaction severity, and pyrolysis conditions both profoundly affects char formation during pyrolysis of torrefied cellulose, resulting in a wide range of pyrolysis char yields from 1.95 wt% to 64.35 wt%. The structural changes induced by torrefaction, including the depolymerization of the carbohydrate structure and the formation of crosslinking structures, promote the char formation. Potassium enhances the char formation of torrefied cellulose in two ways: i) by catalyzing the transformation of the carbohydrate structure into the crosslinking structure during torrefaction, and ii) by catalyzing the charring reactions of both the carbohydrate and the crosslinking structures during pyrolysis. Raman spectral analysis of the char carbon structure indicates that increased potassium loading level and torrefaction severity enlarge the aromatic system of the char. The findings highlight the potential of optimizing biomass pyrolysis through controlled torrefaction and potassium, paving the way for more efficient biofuel production and high-quality biochar applications.