Optimisation of a Pilot-scale Hydrotreatment Process for Biogenic Pyrolysis Oils

The continuous hydrotreatment of pyrolysis oil generated by Thermo-Catalytic Reforming (TCR) of sewage sludge was investigated in a three-stage fixed-bed reactor cascade at technical scale. The objectives were to (i) assess the impact of key operating parameters on heteroatom removal and product composition, (ii) demonstrate the separation of the hydrotreated oil into fuel-range fractions. Experiments were performed at 60–120 bar and up to 390°C using Ni- and NiMo/Al2O3-based catalysts. Systematic variation of the hydrogen-to-feed ratio (˜2.000–10.000 NL kg-1) and space velocity (cascade LHSV ˜0.05–0.09 h-1) showed that deep hydrotreating required undiluted TCR oil, elevated hydrogen supply and high average bed temperatures. Under optimised conditions (120 bar, 4.000 NL h-1 H2, 400 g h-1 feed), nitrogen and sulfur were reduced from 8.3 wt% and 1.0 wt% in the feed to ˜0.03 wt% N and =40 mg kg-1 S in the product; GC–MS indicated >98 area% hydrocarbons dominated by alkanes and cycloalkanes. Batch distillation of the hydrotreated oil yielded a naphtha cut (42 wt%, S = 6.9 mg kg-1) and a diesel cut (46 wt%, S = 11.8 mg kg-1) with densities and heating values within the ranges specified by EN 228 and EN 590, respectively. The results highlight that the temperature profile, H2/feed ratio, space velocity and product conditioning are all critical design variables for scaling the hydrotreating of complex pyrolysis oils.