Char structure evolution during molten salt pyrolysis of biomass: Effect of temperature

Pyrolysis of wheat straw in Li2CO3-K2CO3 binary salt was conducted in a fixed-bed reactor, the char structure evolution under different pyrolysis temperatures was analyzed. The results showed that the biochar yield decreased with temperature increasing, the addition of Li2CO3-K2CO3 salt enhanced heat transfer and promoted charring reactions to form more char. At 450-550 degrees C, the changes in biochar structure from molten salt pyrolysis showed similar trend with those from conventional pyrolysis, the C content increased and the H and O contents decreased, the aromatization degree of biochar increased, the surface functionalities on char surface all decreased. At 600-700 degrees C, the effect of temperature on biochar structure from molten salt pyrolysis showed significantly different trend. The increase of temperature further decreased H content, but increased O content and decreased C content, the molar ratio of H/C decreased while that of O/C increased. The etching of K salt consumed more C and retained more O in char structure, thus caused insignificant increase in aromatization level of biochar and retaining more C-O/C-O-C and -OH functionalities. More porous structures were formed due to the enhanced activation effect of molten salt at higher temperature. The BET surface area of 700MBC could achieve 568.20 m2/g. A functionalized mesoporous biochar with enriched O-containing groups was thus obtained from molten salt pyrolysis of biomass.