The Art of Smooth Continuous Hydroprocessing of Biocrudes Obtained from Hydrothermal Liquefaction: Hydrodemetallization and Propensity for Coke Formation

Our contribution demonstrates that the continuous hydroprocessing of biocrudes derived from hydrothermal liquefaction (HTL) becomes much simpler and significantly more efficient by contemplating the reactivity of organic species (organometallics and heteroatom O and N) under different hydrotreating temperatures. This knowledge was utilized to optimize the temperature profile within a graded catalyst bed reactor to contain the metal contaminants in a guard bed catalyst. This ensured long-term (335 h time-on-stream) continuous hydroprocessing with no pressure drop and no reactor plugging; the run was stopped because the Spirulina biocrude feed (8 kg) was completely exhausted. During very mild continuous hydrotreatment at 200 °C, we found that organometallics are the most reactive organic constituents along with the O-containing compounds, attaining 45% demetallization and 9.2% deoxygenation of Spirulina biocrude. Herein it was observed that the order of reactivity of different organic contaminants is as follows: HDM > HDO > HDS > HDN. Through subsequent batch experiments, it was also revealed that a certain extent of biocrude stabilization is required prior to high-temperature processing. Our results show that the propensity for coke formation at high temperature (400 °C) already decreases tremendously (from 10.2 to 3.1%) after very mild preprocessing at 200 °C and thereafter becomes negligible (0.03%) when the biocrude is hydrotreated at 340 °C. The finding that the organometallics along with O-containing compounds are the main precursors of coke formation, which can be overcome by using optimized operating conditions and robust hydrotreating catalysts, is a key understanding that will pave the way toward the smooth and efficient continuous hydroprocessing of HTL biocrudes.