Insight into KOH activation mechanism during biomass pyrolysis: Chemical reactions between O-containing groups and KOH

Understanding the specific chemical activation mechanism during biomass pyrolysis is critical for the more efficient use of biomass and biochar. In this study, the effects of KOH/biomass ratios (1:8 to 1:1) and temperatures (400–800 °C) on biomass pyrolysis were investigated. The KOH chemical activation mechanism was explored by revealing the evolution mechanisms of the gaseous product, bio-oil, biochar, and KOH, based on experiments and quantum calculations. Results showed that KOH can react with active O-containing species in biomass, which was the main reaction at lower ratios (1:8–1:2) or lower temperatures (400–600 °C). Here, KOH was completely transformed to K2CO3, leading to the formation of large amounts of gaseous products and phenols (reaching 75%). The reaction between KOH and more stable carbon fragments, however, was enhanced at higher ratios (>1:2) or higher temperatures (700–800 °C), such that it became the main reaction. With a significant decrease in the phenols and O-species, the hydrocarbons became the dominant species (reaching a content of 57.43%). For biochar, the reactions among KOH, O-containing species, and carbon fragments generated an abundance of vacancies in the biochar. The OH– from KOH rapidly entered these vacancies, forming a large amount of new O-containing groups (i.e., CO, OH, CO, OCO, and COOH groups). This also caused an increase in oxygen content (reaching 23.68 wt%) in biochar. At higher temperatures, the reactions between KOH and biomass were significantly enhanced, along with a sharp increase in the specific surface area (reaching 1351.13 m2/g). O-containing groups further transformed to more stable OH, CO, and COOH groups. Based on the evolution mechanism of pyrolytic products and KOH, the KOH chemical activation mechanism during biomass pyrolysis was revealed, allowing us, for the first time, to propose a possible chemical reaction pathway between KOH and O-containing groups.