Impact of dewatering on pyrolysis oil upgrading: A comparative study of properties and hydrodeoxygenation

Pyrolysis oil derived from residual biomass waste, such as sawdust, holds significant potential as a feedstock for biofuel production. However, due to its inherent properties, it cannot be directly processed in conventional hydrotreating reactors and requires pretreatment. The primary goals of upgrading pyrolysis oil via hydrotreatment are to reduce its oxygen content and facilitate the efficient separation of water. Dewatering pretreatment through azeotropic distillation with mesityl oxide offers a straightforward approach for partial upgrading, making it an attractive method for enhancing pyrolysis oil quality. In this study, pyrolysis oil underwent dewatering using this technique. The stability of the dewatered oil was evaluated through accelerated aging at 80 °C for 2 h and compared with untreated pyrolysis oil. The results indicated that the dewatered pyrolysis oil exhibited greater stability than the untreated sample under the aging treatment. The dewatered oil was subsequently upgraded by hydrodeoxygenation (HDO) using a NiMoS/Al₂O₃ catalyst to assess the impact of dewatering on the HDO process. Compared to untreated pyrolysis oil, the dewatered oil characterized by reduced concentrations of acidic and carbonyl-containing compounds and reduced water content yielded a higher proportion of deoxygenated products. This improvement was accompanied by increased water formation in a separated phase after reaction, attributed to HDO and condensation reactions, as well as a moderate increase in char formation. Notably, when water was reintroduced into the dewatered oil, catalyst inhibition effects reappeared, indicating that it is primarily the presence of water rather than changes in the organic composition of the oil that plays the dominant role in suppressing HDO performance. • Hydrotreatment of untreated, dewatered, and rehydrated pyrolysis oils was compared. • Water addition suppressed HDO via site blocking or competitive adsorption. • Rehydrated oil showed similar HDO behavior to untreated pyrolysis oil. • Dewatered oil showed higher stability during accelerated aging tests. • Catalyst performance was strongly influenced by water content.