Two-stage catalytic hydrotreatment of highly nitrogenous biocrude from continuous hydrothermal liquefaction: A rational design of the stabilization stage

Effective catalytic hydrotreatment of highly nitrogenous biocrudes derived from the hydrothermal liquefaction (HTL) of primary sewage sludge and microalga Spirulina biomass was explored. A critical issue is the lack of thermal stability of raw HTL biocrudes at the severe conditions (~400 °C) required for hydrodenitrogenation. This fact suggests the need for a two-stage approach, involving a first low-temperature stabilization stage followed by another one operated at higher temperature. In this study, DSC was successfully used to indicate the thermal stability of both biocrudes. During hydrotreating, it was observed that complete deoxygenation was already achieved in the first stage at 350 °C, with limited coke formation. Moreover, after second stage up to 92% denitrogenation associated with the higher hydrogen consumption (39.9 g kg^-1 for Spirulina and 36.9 g kg^-1 for sewage sludge) was obtained for both biocrudes. Consequently, comparable oil yields but significantly less coke yields were recorded during two-stage upgrading (1.0% for Spirulina and 0.7% for sewage sludge), compared to direct processing at 400 °C (9.1% for Spirulina and 3.4% for sewage sludge). In addition, the properties of the upgraded oils were enhanced by increasing the temperature in the first stage (310 °C, 330 °C and 350 °C respectively). Finally, the results indicated that remarkable drop-in fuel properties were obtained, with respect to heteroatom (O and N) removal, HHV, and H/C ratio during the two-stage hydrotreatment. Two-stage hydrotreating is therefore proposed as a successful approach for the upgrading of HTL biocrudes with high nitrogen content.