Migration of Alkali and Alkaline earth metallic species and structure analysis of sawdust pyrolysis biochar

In order to resolve the AAEM species migration routes and the interaction relationship between biochar structure and AAEM species during biomass pyrolysis, experiments were performed in an entrained flow reactor with N₂ at 500∼900°C. ICP-AES, XPS and SEM-EDX were used to examine content and distribution of AAEM species and the physicochemical structures of biochar. The results show that at 500∼700°C, the precipitation rate of AAEM species is relatively high. At high temperature (>700°C), the AAEM species continue to migrate from interior to exterior, but little precipitation from biochar surface. And the migration of AAEM species is mainly realized by the C-O bond as the carrier medium. The AAEM species on biochar surface are mainly Na, Mg and Ca (<700°C), while changing to K, Mg and Ca (≥700°C). From 500°C to 900°C, the biochar particle morphology gradually changes from fibers to porous structures, finally to molten particles. At 700∼900°C, Ca element is obviously enriched on the molten edge of the biochar porous structures.