Experimental investigation on biomass gasification mechanism in supercritical water for poly-generation of hydrogen-rich gas and biochar

Supercritical water gasification (SCWG) technology can realize efficient utilization of biomass. But the gasification mechanism of real biomass is relatively complicated, and complete gasification only can be realized at a relatively high temperature. Identifying reaction mechanism and realize resource utilization of biomass via SCWG technology at a more moderate condition is necessary for industrialization of this technology. In this paper, biomass gasification experiment was conducted, and the gasification mechanism was analyzed. Experiment in quartz tube reactor showed that gasification efficiency and hydrogen yield increased with time and temperature, which reached 90% 14.95 mol/kg at temperature of 700 degrees C, reaction time of 30 min. Carbon distribution study showed that more than 90% carbon was distributed in gas and biochar, proving that SCWG could realize cogeneration of hydrogen and biochar. Thus, experiment was conducted in batch tank reactor to obtain enough liquid and solid products for further study. Reaction mechanism study showed that soybean stem was decomposed to cellulose, hemicellulose, and lignin; then cellulose and hemicellulose were converted to small molecule sugar, which can be further converted to gas; as for lignin, it was decomposed to its monomers first, then it went through de-methoxy reactions and formed gases and phenol; phenol was difficult to be decomposed and could form biochar easily. Study of pore structure evolution of biochar showed that gasification in SCW could produce abundant pore structure, and with gasification proceeding, content of pores increased greatly. Pore size of biochar obtained from gasification of 15 min mainly focused on 6.5 nm and 16 nm. More mesopore were formed at the higher temperature and the diameter of macropore size increased with temperature due to the release of volatiles.