Understanding catalytic hydrotreating of hydrothermal liquefaction nitrogen-rich biocrude in a two-stage continuous hydrotreater

Hydrothermal liquefaction biocrude from non-lignocellulosic feedstocks often shows properties, such as a high presence of inorganics, oxygen and especially a high nitrogen content, which are challenging in view of hydroprocessing for the production of drop-in biofuels. In particular, nitrogen removal requires severe conditions, that might compromise continuous operations and affect the yield of the different hydrocarbon fractions. This work demonstrates the possibility of successful long-time continuous operations for microalgae biocrude, a feed with a high content of metals and nitrogen, thanks to a multi-stage approach with different sulfided catalysts. Whereas hydrodeoxygenation was achieved at relatively mild conditions, nitrogen removal was challenging. By using a number of advanced characterization techniques, among which FTICR-MS, it was possible to unveil how nitrogen removal takes place at different reaction severities, in terms of temperature, WHSV and pressure. High nitrogen removal needs high temperature (∼400 °C) and low space velocities and is associated with a high H₂ consumption. This causes also a remarkable change in the boiling point distribution, which is shifted to products in the gasoline range, while the yields of middle distillates are marginally affected by hydrotreating severity. This seems to be a consequence of the denitrogenation mechanism, promoting the fragmentation of larger molecules in the heavy fractions of biocrude.