Comparing Climate-Change Mitigating Potentials of Alternative Synthetic Liquid Fuel Technologies Using Biomass and Coal

The climate-change mitigation potentials of alternative options for making synthetic liquid fuel from coal and biomass without and with CO2 capture and storage are explored. The emphasis is on making Fischer-Tropsch liquids, with comparisons to cellulosic ethanol. Particular attention is given to exploitation of the negative CO2 emissions potential of CO2 capture and storage for bioenergy systems. One Fischer-Tropsch option involves coprocessing biomass and coal. All liquid fuel production options involve production of electricity as a net coproduct. Both CO2 aquifer storage and CO2 enhanced oil recovery options are analyzed. The metrics by which the alternatives are compared are: (i) the net greenhouse gas emission rate associated with liquid fuel production and use, (ii) the specific capital cost of liquid fuel production, (iii) the lifecycle cost calculated for a fixed capital charge rate, and (iv) the internal rate of return on equity as a function of both the crude oil price and the price of greenhouse gas emissions. In addition, for options that involve biomass, liquid fuel yields per tonne of biomass are compared. And for enhanced oil recovery applications of the captured CO2, the relative profitability of using CO2 from synfuel plants and integrated gasifier combined cycle power plants is explored. Introduction Synthetic fuels derived from secure domestic resources have attracted wide interest recently in light of high oil prices and oil supply insecurity concerns. Here carbon management options are investigated for the production and use of synthetic liquid fuels derived from coal and biomass. The emphasis is on making synthetic diesel and gasoline via the Fischer-Tropsch (F-T) process. Commercially established F-T technology offers synfuels that are inherently cleaner than the diesel or gasoline fuels derived from crude oil that they would displace and with which they can be blended and thus introduced into transportation fuel markets with no change in infrastructure. Comparisons to cellulosic ethanol derived from biomass are made. The attractions of coal are its abundant availability from secure sources and its low and stable prices. The major concerns about a large coal synfuels program are its potential adverse impacts on climate, and coal mining health and safety and environmental risks associated with expanded coal production. The attractions of biomass are: (i) security of supply (as for coal), (ii) the potential for phasing out subsidies now paid to farmers for producing less food to the extent that they might otherwise earn a livelihood growing biomass for energy, (iii) its renewability as an energy resource, and, perhaps most importantly, (iv) its climate-change mitigation benefits. Because the CO2 emitted from bioenergy combustion is of recent photosynthetic origin, there is no net buildup of atmospheric CO2 if the biomass is produced on a sustainable basis. Moreover, by capturing and storing below ground some CO2 from biomass during its conversion to fuel or electricity, biomass becomes a negative CO2-emitting energy supply; such negative CO2 emissions can be used to offset GHG emissions from difficult-to-decarbonize fossil energy supplies, such as liquid fuels used in transportation. The major concerns about biomass are the typical high cost of biomass compared to coal, and the land intensity of biomass production (reflecting the low efficiency of photosynthesis) and resulting competition with alternative land uses.