A new insight into chemical reactions between biomass and alkaline additives during pyrolysis process

Understanding the chemical reactions during biomass and alkaline additives during pyrolysis process is critical to better utilization of biomass resource. In this study, the influence of seven alkaline additives (KOH, K₂CO₃, KHCO₃, CH₃COOK, NaOH, Ca(OH)₂, and Mg(OH)₂) on biomass pyrolysis was investigated in fixed-bed system, as well as the reaction mechanism through combining the evolution properties of biochar, bio-oil, gas products, and alkaline additives. Results showed that alkaline additives promoted the formation of biochar and gas products, while inhibited the generation of bio-oil. The content of phenols (reaching 80% for NaOH, mainly non?methoxy phenols) and hydrocarbons (reaching 38% for Ca(OH)₂, mainly aromatics) increased greatly at the cost of acetic acid and O-species. Carbon content of biochar decreased largely, while oxygen content increased greatly, and the main O-containing species were –OH, -O–CO and -COOH groups. The surface area of biochar also increased greatly (reaching 764 m²/g for KOH). During pyrolysis process, alkaline additives could react with high active O-containing species and carbon fragments to generate lots of vacancies, then some anions of alkaline additive quickly occupied these vacancies to form new O-containing groups in biochar. Alkaline additives also converted into more stable structure (K₂CO₃, Na₂CO₃, CaO, and MgO). The possible reaction pathway between biomass and alkaline additives was first proposed.