Hydrotreating of bio-crude obtained from hydrothermal liquefaction of biopulp: effects of aqueous phase recirculation on the hydrotreated oil

Abstract

Aqueous phase recirculation (APR) during the hydrothermal liquefaction (HTL) process is a means to enhance HTL performance and lower the need for intensive residual water treatment. However, the obtained HTL bio-crude cannot be considered a drop-in biofuel partially due to its significant heteroatom content. Thus, catalytic hydrotreating is typically practised to upgrade HTL bio-crude to drop-in biofuel/biofuel precursors. This study establishes a holistic overview of the influence of APR on hydrotreated bio-crude. The employed strategy integrates a four-step APR process to a batch catalytic hydrotreating process operating at mild (350 °C) or severe conditions (400 °C). APR revealed promising results in terms of bio-crude yield and energy recovery. However, the heteroatom content of the resulting bio-crude was noticeably elevated. Regardless of the operational conditions, hydrotreating experiments disclosed a higher oil yield while treating the first cycle's bio-crude along with limited coke formation. Although the added oxygen content by APR was offset through hydrotreating, the nitrogen content of the hydrotreated bio-crude in the consecutive cycles significantly increased. The elemental distribution results revealed that APR increased the nitrogen distribution in the hydrotreated bio-crude. Hence, higher quantities of hydrogen and severe hydrotreating conditions were required to obtain a suitable drop-in quality of the biofuel.