Continuous slurry hydrotreating of sewage sludge-derived hydrothermal liquefaction biocrude on pilot-scale: Comparison with fixed-bed reactor operation

In this study, we demonstrate the continuous catalytic hydrotreating of sewage sludge-derived hydrothermal liquefaction oil on a versatile, pilot-scale testing unit, equipped with both a slurry and a fixed-bed reactor. Comparison of the two reactors shows that slurry hydrocracking is consistently more efficient in both heteroatom removal and cracking performance compared to the fixed-bed operation. The upgraded HTL oil from the slurry reactor contains 35% less nitrogen that the equivalent oil produced from the fixed-bed reactor at 350 °C and is lighter, consisting of 84 wt% molecules in the gasoline and diesel range, compared to 63 wt% in its counterpart. This is tentatively ascribed to the higher residence time and the lower mass-transfer limitations in the slurry reactor that enhance the hydrogenation and cracking reactions. Upgrading the HTL oil in a two-stage configuration improves only the nitrogen removal, which increases from 40‐55% in the one-stage process to 83%. Overall, slurry hydrocracking appears to be a promising strategy for the upgrading of bio-oils from renewable feedstocks, such as waste and biomass. Further research is required to study operability and stability issues for longer time-on-stream and investigate the process in the presence of dispersed liquid catalysts.