Towards sustainable hydrocarbon fuels with biomass fast pyrolysis oil and electrocatalytic upgrading
The carbon efficiency of bioenergy systems is of critical importance in discussions pertaining to biomass availability for the displacement of petroleum. Classical carbohydrate fermentations to make simple alcohols are carbon inefficient as they discard 1/3 of biomass holocellulose as CO2. Biomass' lignin is typically burned for heat and power instead of liquid fuel, discarding another sizeable fraction of the biomass carbon. Carbon is the backbone element in hydrocarbon fuels and these losses limit full utilization of the carbon captured by photosynthesis. The DOE Billion-ton Study Update optimistically projects enough biomass carbon to cover 2/3 of the estimated fuel usage in the transportation sector by 2030. Fast pyrolysis combined with electrocatalytic energy upgrading using renewable electricity offers a more carbon-retentive pathway for biomass to renewable fuels. This fast pyrolysis-based sequence offers the added benefit of fixing atmospheric carbon in the form of biochar, which provides a mechanism for long-term carbon storage. An associated challenge is that the liquid “bio-oil” from biomass fast pyrolysis contains functional groups like carboxylic acids, carbonyls, and oxygenated aromatics. Their presence hinders the storage and transportation of bio-oil. We propose a potential solution with localized electrocatalytic hydrogenation as an immediate measure to stabilize bio-oil via oxygen removal and carbonyl saturation. Electrocatalytically stabilized bio-oil can be stored and/or transported to centralized refineries for further upgrading. Compared to microbial bioconversion, the strategy proposed here enables significantly higher yields of renewable hydrocarbon fuels and offers a large-scale mechanism for chemical storage of renewable but intermittently generated electrical energy as transportation fuel.