Guest editorial-special issue on the 4th international symposium on hydrogen energy and energy technologies

In this study, the catalytic hydrothermal liquefaction technology was adopted to convert the Chinese herb residue (CHR) into liquid bio-oil. The effect of the reaction conditions was investigated including temperature, water/biomass mass ratio (W/B) and time. Different combinations of Fe, Ni and ZSM-5 molecular sieve catalyst were evaluated to improve the bio-oil quality. The optimized liquefaction parameters were determined through experiments, which were the reaction temperature of 320 °C, W/B of 15:1, and reaction time of 10 min; the yield and high heat value (HHV) of bio-oil were 24.7 wt% and 29.39 MJ/kg, respectively. There was a synergy between Fe and ZSM-5, and Fe/ZSM-5 can achieve in situ hydrodeoxygenation in the liquefaction process, increasing the HHV of bio-oil to 31.74 M J/kg at 10 wt% loading with the yield of 21.3 wt%. The enhancement of bio-oil quality by Ni/ZSM-5 was weak due to the cleavage of C–C bond by Ni, resulting in the poor HHV and a low yield; the catalytic performance of Fe–Ni/ZSM-5 was between the former two catalysts. Elemental analysis and GC-MS showed that Fe/ZSM-5 increased H/C and decreased O/C in bio-oil, and the content of aliphatic compounds in bio-oil increased to 72.96%.