Co-Processing Biomass and Natural Gas into Clean Transportation Fuels at Small Scale: A Techno-Economic Assessment

Concerns over climate risks, price volatility, and energy insecurity associated with petroleum-derived fuels motivate the search for alternatives. Liquid fuels derived from domestic natural gas and sustainable biomass can help mitigate these concerns. Biomass is a dispersed resource best suited for smaller-scale conversion plants. We explore the economic viability of smaller-scale production of synthetic diesel and gasoline from biomass or biomass and natural gas. Our three designs each process 2,000 dry tonnes of biomass per day and incorporate an indirectly-heated gasifier and tar cracker, a microchannel Fischer-Tropsch (FT) reactor for liquid-fuels production, and capture of CO₂ for storage (CCS) by injection into spent shale gas wells. Our base design uses only biomass. In a second design, we take advantage of the synergistic opportunity to blend H₂-poor syngas from biomass gasification with H₂-rich syngas from natural gas reforming to obtain the 2:1 H₂:CO ratio needed for FT synthesis. The feed to this plant is 10% natural gas on an energy basis. In a third design, we increase the natural gas content to 50% of the feed to augment the heat input to the gasifier to increase biomass carbon conversion to fuels. We also explore the impact of post-combustion CO₂ capture in addition to the baseline pre-combustion capture. The biomass-only case provides 80 gallons of energy-equivalent gasoline per dry tonne of biomass (gge/t), with strongly negative greenhouse gas (GHG) emissions. The second design provides 100 gge/t, with neutral to negative GHG emissions; the latter occurs when additional post-combustion capture is utilized. The third design provides 170 gge/t, with positive to neutral GHG emissions. Even at these levels, the emissions do not exceed 50% of the emissions of plants producing equivalent petroleum-derived fuels. The economics of fuels production for the three designs were investigated under different assumed GHG emission prices. Despite its smaller scale, at high GHG emissions prices (> $100/tCO₂), the biomass-only design with additional post-combustion capture provides the lowest production costs that rival production costs of petroleum fuels even at $100 per barrel crude oil prices due to its strongly negative emissions.