Stabilization of Softwood-Derived Pyrolysis Oils for Continuous Bio-oil Hydroprocessing

Abstract

The use of fast pyrolysis oil as a potential renewable liquid transportation fuel alternative to crude oil depends on successful catalytic upgrading to produce a refinery-ready product with oxygen content and qualities (i.e., specific functional group or compound content) compatible with the product’s proposed refinery insertion point. Similar to crude oil hydrotreating, catalytic upgrading of bio-oil requires high temperature and pressure. However, processing thermally unstable pyrolysis oil is not straightforward. For years, a two-temperature, downflow trickle bed reactor using sulfided catalysts was the state-of-the art for continuous operation. However, pressure excursion due to plug formation still occurred, typically at the high-temperature transition zone, and led to a process shutdown within 140 h. A plug typically consists of polymerized bio-oil and inorganic constituents that bind catalysts at specific portions preventing liquid and gas flow through the bed, resulting to a potential pressure incursion. Recently, two factors were found to enable continuous operation by preventing reactor shutdown due to plug formation: (1) a bio-oil pretreatment process prior to the two-temperature reactor, and (2) a robust commercial catalyst for the high temperature zone reactor. Here, we report the use and characterization of bio-oil that was pre-treated at 413 K and 8.4 MPa under flowing H₂ (500 L H₂/L bio-oil, 0.5 L bio-oil/L catalyst bed) to enable the long-term (cumulative 1440-h) bio-oil hydroprocessing.