Evolution characteristics and mechanism of products from large-particle biomass pyrolysis in molten salt media

Molten salt pyrolysis of biomass is a promising approach for producing clean and renewable energy by combining solar-driven molten salt heat storage with thermochemical conversion of biomass. Pyrolysis of large-particle biomass offers many advantages in practical applications. However, during pyrolysis, heat and mass transfer behavior inside the particles cannot be ignored. This study investigated the product composition and product distribution characteristics during the pyrolysis process of large-particle biomass in molten salt. The influence of molten salt on the physical and chemical structure of biochar from the core to the outer layer was analyzed layer-by-layer by micro computed tomography and temperature programmed oxidation techniques. The evolution mechanism of products during the molten salt pyrolysis of large-particle biomass was also analyzed. The results indicate that the excellent heat transfer efficiency of molten salt promoted the pyrolysis of large biomass particles and increased the aromatic condensation degree of biochar. Compared with traditional pyrolysis, molten salt pyrolysis afforded higher yields of biochar and pyrolysis gas, as well as lower yields of bio-oil. In particular, the catalytic reforming effect of molten salt on volatile matter further reduced the yield of bio-oil and increased the yield of combustible gases such as H2 and CO. Moreover, the etching effect of the molten salt on the biochar resulted in a more porous structure, and led to a 24 % increase in the total pore count across various regions of the biomass particles during molten salt-assisted pyrolysis. There was a 70 % reduction in pores with a volume exceeding 10 mm3, and a 46 % increase in pores with a volume of less than 2.2 mm3. This study can promote the development of pyrolysis technology and help to promote biomass energy utilization technology. •The characteristics of molten salt pyrolysis of large-particle biomass were studied.•The structural evolution of biochar from the core to the outer layer was analyzed.•Molten salt shows good heat transfer ability and promotes the pyrolysis reaction.•Molten salt can catalyze the reforming of volatiles to produce more pyrolysis gas.