Deoxygenation of biomass pyrolysis vapors: Improving clarity on the fate of carbon

Deoxygenating biomass pyrolysis vapors prior to condensation would ideally yield chemically stable and petroleum-miscible streams comprised primarily of hydrocarbons that could be integrated into the existing refinery infrastructure. Both in-situ and ex-situ catalytic fast pyrolysis have shown promising results in terms of product quality, i.e., lower acidity, higher heating values, and product stability. However, these improvements come at the expense of carbon loss as CO, CO2, and coke, as well as cracking of larger molecules (potentially more valuable) into smaller molecules. It is thus of utmost importance to track the carbon, both product distribution and carbon yields, in order to obtain the full picture of deoxygenation efficacy. Despite potentially interesting results reported in the literature, it is unfortunately difficult to compare the true ability of various catalyst systems for deoxygenating biomass pyrolysis vapors primarily due to insufficient product characterization and data reporting. In this review paper, it is argued that results ultimately need to be reported in terms of carbon yields of individual products. This would enable an accurate accounting of where the carbon ultimately resides after processing, which is required for a more conclusive comparison between various systems. This information would enable the identification of effective catalytic systems for pyrolysis vapor deoxygenation and accelerate the advancement of catalyst design. Additionally, hydrodeoxygenation is discussed as a promising process in terms of carbon recovery. This article aims to propose a set of standard criteria for future reports on deoxygenation of biomass pyrolysis vapors by scrutinizing the existing literature.