Bridging the Carbon Gap: Fossil Fuel Use for the 21st Century
in this option would be separated and then sequestered in suitable geologic formations or the deep ocean. 13, 14, 28
Because of the global abundance of coal and lignite, the coal gasification and CO 2 sequestration option could provide as much as a century of lead time for development and commercialization of sustainable and carbon-emission-free energy technologies. However, this option also carries with it many unresolved questions. 28
For example, should the coal gasification facilities be centralized, with development of a costly transmission, distribution, and storage grid for hydrogen, similar to the existing pipeline grid for natural gas? Or should one rely on dispersed sources of power generation, such as the modified IGCC Process?
The IGCC Process.
As noted before, the first step under the modified IGCC Process is pressure gasification of coal in an entrained flow process with oxygen and steam to carbon monoxide, hydrogen, and CO 2. That is followed by catalytic water gas shift of the carbon monoxide with additional steam to hydrogen and CO 2, removal of sulfur compounds (either before or after water gas shift depending on the sulfur resistance of the shift catalyst) and, finally, CO 2 removal and sequestration. 15
There is little doubt that the modified IGCC option is more cost-effective and practical since the construction of a hydrogen grid would be prohibitively capital-intensive, because it has been shown that the conversion of the existing natural gas grid to hydrogen is not feasible. 29, 30 Table 3 summarizes the promising projected economics of the modified IGCC option, which provides for 90 percent CO 2 removal, and compares them with natural gas-fired combined-cycle power generation and supercritical and ultra-supercritical pulverized coal combustion, both with and without 90 percent CO 2 removal. 15
Although individual processing steps for such modified IGCC plants have been practiced commercially, they have not been fully integrated to optimize efficiency and minimize investment cost. The Electric Power Research Institute (EPRI) has estimated that it may take as long as 25 years to fully develop, demonstrate, and commercially deploy the economically optimal configuration of the IGCC process and require the expenditure of several billion dollars for research and development. 31
Carbon Sequestration. Nevertheless, there are many physical, chemical and environmental problems with CO 2 sequestration. 2 In addition, there is the substantial cost of CO 2 transport and disposal.
The issue of the adequacy of the available CO 2 storage capacity is of special importance. For example, the roughly 1,000 gigatonnes of carbon (GtC) contained in the proved global reserves of coal (Table 2) would generate about 70,000 Tcf of CO 2. The ultimate global resource base of natural gas is at most 20,000 Tcf, so that depleted gas reservoirs could not accommodate this amount of CO 2. The storage potential of depleted oil reservoirs and the quantities of CO 2 that can be used for enhanced recovery of petroleum liquids and natural gas are also quite limited.
The ocean, with an inventory of 38,000 to 40,000 GtC (in the form of CO 2), would seem to