FIGURE 2.16 Estimated net life-cycle CO₂-equivalent (CO₂-eq) emissions for production, transportation, and use of alternative liquid transportation fuels. Emissions are shown in units of tonnes of CO₂ equivalent per barrel of gasoline equivalent produced from biomass, coal, or a combination of coal and biomass. For comparison, the CO₂-eq emissions for gasoline are shown on the left. Negative CO₂-eq emissions mean that on a net life-cycle basis, CO₂ is removed from the atmosphere; for example, the negative CO₂ emissions for BTL and cellulosic ethanol result from an estimate that the sequestration of biomass carbon in power-plant char or the buildup of carbon in soil and roots will exceed the emissions of carbon in biofuel production. Growing perennial crops for cellulosic fuels provides CO₂ benefits because these crops store carbon in the root biomass and the associated rhizosphere, thereby increasing soil carbon sequestration. The precise value of CO₂-eq emissions from CBTL depends on the ratio of biomass to coal used. Indirect land-use effects on CO₂ emissions are not included.

Note: BTL = biomass-to-liquid fuel; CBFT = coal-and-biomass-to-liquid fuel, Fischer Tropsch; CBMTG = coal-and-biomass-to-liquid fuel, methanol-to-gasoline; CBTL = coal-and-biomass-to-liquid fuel; CCS = carbon capture and storage; CFT = coal-to-liquid fuel, Fischer-Tropsch; CMTG = coal-to-liquid fuel, methanol-to-gasoline; CTL = coal-to-liquid fuel.

Sources: Data from Chapter 5 in Part 2 of this report and from NAS-NAE-NRC (2009b).