Effects of intrinsic and external potassium on biochar structure evolution in volatile-char interactions during biomass pyrolysis

This study explores the effects of intrinsic and external potassium (K) on structural evolution of biochar in volatile-char interactions during wheat straw pyrolysis using fixed-bed reactor. The results showed that the removal of intrinsic K from biochar through acid-washing reduced its reactivity in volatiles cracking, but the exposure of more carbon structures on biochar surface enhanced its self-reforming with volatile compounds (e.g., H 2 O and CO 2 ), resulting in increased weight loss. Intrinsic K exhibited a strong catalytic effect on biochar self-reforming, contributing to significant weight loss, it was not easily volatilized and became enriched during the interactions. External K (from KOH impregnation) predominantly accumulated on biochar surface and was more prone to volatilization during the interactions. At low dosages (ABC-1K), surface coverage by KOH reduced biochar weight loss, but weight loss increased again with higher KOH loading during the interactions. External K in biochar showed a stronger ability to crack volatiles, reducing oxygen-containing functional groups (e.g., -OH, -COO, and phenolic C-OH) and potentially converting small aromatic rings into larger ones in biochar. This study further deepens the fundamental understanding of K in biochar in regulating volatile-char interactions during biomass pyrolysis. ● AAEMs or K removal reduced biochar reactivity and inhibited volatiles cracking. ● Intrinsic K bound to carbon matrix enhanced biochar self-reforming in interactions. ● External K on char surface with higher volatilization promoted volatiles cracking. ● Char structure and reactivity were affected by K existing form during interactions.