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 °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 °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 CO/COC 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.