The Capture Group’s Eric Larson has been part of an initiative to assess the long term potential of biomass-based energy in the United States, particularly for meeting transportation needs. In the “Role of Biomass in America’s Energy Future” (RBAEF) project, researchers from ten U.S. institutions have been analyzing the economics and environmental impacts of large biomass facilities that would use thousands of tons of biomass per day to meet a substantial portion of United States’ transportation energy needs.
Biomass produced on a sustainable basis is essentially a carbon-neutral energy source because the CO2 emitted was absorbed from the atmosphere during photosynthesis and will subsequently be re-absorbed by future biomass growth. Larson’s research carried out under the RBAEF project explored the production of both electricity and liquid fuels from biomass via gasification (e.g., Fischer-Tropsch diesel and gasoline) in plants that would consume 5000 tons per day of plantation-grown switchgrass (a fast-growing species native to the Great Plains) using gasification technology that is near commercially ready.
The results show that liquid fuels and electricity produced in these plants could lead to substantial reductions in GHG emissions per unit of biomass consumed (see B-FT-Vent and B-IGCC-Vent options in Figure 3, middle). However, in the absence of a climate mitigation policy, the economic prospects are poor—even with a $50 a barrel price (see B-FT-Vent and B-IGCC-Vent options in Figure 3, bottom). In contrast, if CO2 emissions are valued at $100/tC, these options are attractive economically (see B-FT-Vent and B-IGCC-Vent options in Figure 3, bottom), but the GHG emissions mitigation potential per unit of biomass consumed is much diminished (see B-FT-Vent and B-IGCC-Vent options in Figure 3, middle). The lower potential for GHG emissions mitigation at $100/tC arises because the electricity that would be displaced (assumed to be the least costly electricity generation option at that carbon price) would have a much lower GHG emission rate than the electricity displaced at $0/tC.
In an effort to see how higher GHG emission rate reductions might be realized for biomass energy under a climate mitigation policy, this RBAEF activity was followed up with analyses exploring the prospects for CCS at bioenergy conversion plants. With CCS, systems that convert biomass to fuel and/or electricity are net negative CO2-emitting energy supplies, because photosynthetic CO2 is being stored underground. In an activity led this year by Larson in collaboration with Haiming Jin of Dartmouth College, the group carried out detailed process design, simulation, and cost estimation for large dedicated facilities that would produce electricity and/or fuels from switchgrass, with and without CCS. Synthetic fuels considered in their analyses are Fischer-Tropsch diesel and gasoline blendstocks, dimethyl ether, and hydrogen. Williams then joined Larson and Jin in evaluating the strategic implications of CCS for bioenergy systems, the results of which are summarized in Figure 3.
Including CCS for bioenergy has a dramatic impact on the GHG emissions reduction benefit per unit of biomass consumed at a $100/tC carbon price—up 11.5X for power generation (compare B-IGCC-CCS and B-IGCC-Vent in Figure 3, middle) and up 2.5X for the coproduction of F-T liquids and electricity (compare B-FT-CCS and B-FT-Vent in Figure 3, middle). It is assumed in these calculations that the negative emissions arising from underground storage of bio-CO2 offset emissions from crude oil derived energy products. This implies that the effective amount of low GHG emitting liquid fuel provided per unit of biomass is much larger than for the cases with CO2 vented, and that the maximum rate of providing low-GHG-emitting liquid fuels is realized for the option that generates no biofuels whatsoever—the B-IGCC-CCS option (see Figure 3, top).