Bridging the Carbon Gap: Fossil Fuel Use for the 21st Century
be the ideal sink for disposing of at most 5,000 GtC of fossil fuel carbon, but this option also faces the greatest uncertainties. 2 Deep aquifers are a promising on-shore option that has been used for natural gas storage, but the potential capacity, security of storage, and environmental impacts for CO 2 storage have not been assessed. The International Energy Agency (IEA) estimated the global storage potential in the ocean by dissolution, dispersion (towed pipe or dry ice) or isolation (CO 2 lake at ocean bottom) to be as high as 27,000 GtC, and in deep aquifers as high as 2,700 GtC. 2
Fortunately, we probably have 20 years of lead time before we must resolve these questions, if we optimize the recovery and utilization of global hydrocarbon resources. But the use of the IGCC/carbon sequestration option for central power generation could become the key element in extending the lead time for total phase-out of fossil fuels, especially if the investment cost and efficiency of IGCC plants could be further improved.
Why Not Biomass?
Biomass, or energy crops in general, cannot meet global energy needs because of prohibitive land and large parasitic energy requirements, high labor intensity, and unacceptable environmental impacts. 2
First, reliance on biomass to replace a significant portion of fossil fuel use would disturb the land carbon cycle. Second, reliance on a fuel such as corn ethanol would force the world to give over huge areas of land to energy crops.
Disrupting the Carbon Cycle. If a substantial portion of mature forest lands were converted to fast-growing energy crops, it would reduce the total inventory of biomass active in the 11-year land carbon cycle of revegetation, and would disrupt the cycle. At present the Earth has an inventory of about 550 gigatonnes of carbon (GtC) in the terrestrial biota. Some 50 GtC of this total is lost each year to soil and detritus, but is replaced by 50 GtC of net photosynthesis (100 GtC of photosynthesis less 50 GtC of plant respiration annually). 6 (The annual loss of about 2 GtC to deforestation mostly in the tropics is offset by a roughly equal amount of afforestation, mostly in the Northern Hemisphere.)
Acres of Corn. In 1997, the Illinois Institute of Technology Energy + Power Center evaluated this option both in terms of feasibility of replacing U.S. gasoline or total U.S. petroleum liquids consumption with corn ethanol, and the economics of this option. 11 It was found that just to replace 1996 U.S. gasoline consumption would have required more than all available cropland. To replace total 1996 U.S. petroleum consumption would have required 60 percent of the entire U.S. land area (if this were feasible) and substantially more than the combined area of U.S. cropland, pastures, and meadows.
Vehicle of Consensus
Nearly all major manufacturers of automobiles, trucks, and buses are near commercialization of PEM fuel cell vehicles using hydrogen to provide very efficient electromotive propulsion.
Of course, there is still an ongoing debate over how this hydrogen would be delivered-by on-board pressurized or liquefied storage or on-board reforming of