Mild hydrotreating of bio-oils with varying oxygen content produced via catalytic fast pyrolysis

Abstract

Several biomass catalytic fast pyrolysis oils produced over HZSM-5 at varying levels of catalyst of deactivation, and therefore exhibiting oxygen contents ranging from 6 wt% to 33 wt% (enabled due to catalyst deactivation) were subjected to mild hydrotreating over Ru/C at 140 °C. The purpose of this study was to evaluate the performance of these catalytic pyrolysis oils in this process, which is often used as a stabilization step prior to exposure of the bio-oil to more severe hydrodeoxygenation conditions in pyrolysis based biorefinery concepts. This will help better define the desired deoxygenation rate during CFP, and inform decisions about yield/quality and cost tradeoffs during CFP. High liquid product recoveries were achieved for the mild hydrotreating in all samples with varying O-contents, meaning the yield of hydrogenated liquids from biomass was nearly exclusively dependent on the catalytic pyrolysis step. However, a trend was observed that the bio-oils were more responsive to the hydrogenation with increasing oxygen content. For example, no aromatic ring hydrogenation was observed for the bio-oils with <20 wt% oxygen content but ring saturation was observed for bio-oils with oxygen contents of 25–30 wt%. The bio-oils with about 25–30% oxygen content had the largest increase in H/C ratio upon hydrogenation. This trend may be attributed in part to a solvent effect, where the more polar bio-oils acting as their own solvents, helped promote hydrogenation. With regard to product stability an increase in the average molecular weight of the bio-oils with >25 wt% oxygen content was observed for exposure to the hydrogenation conditions, suggesting some oligomerization occurred prior to hydrogenation in these cases. Taken together, bio-oils with about 25 wt% oxygen content responded well to the mild hydrotreatment and were stable to the conditions, and offered a 2–3 fold increase in carbon yield from biomass over the bio-oils that were more severely deoxygenated during catalytic pyrolysis. This suggests that severe reduction of oxygen in the CFP step may be not needed to economically produce fungible, stabilized biofuel intermediates in high carbon yield.