problem. A major difficulty in capturing and sequestering CO2 is the huge volume of the gas stream. Fossil fuels (especially coal) produce a significant amount of carbon per unit energy; thus, a lot of CO2 must be captured. If the CO2 is diluted by nitrogen, the volume may be two or three times as large. Recovering this large volume of gas places a huge burden on the system in terms of the high cost and the large amount of energy consumed in the separation process. The significant energy required for CO2 separation (e.g., steam or refrigeration) greatly reduces the net energy output and efficiency of the power plant. After separation, the CO2 must be compressed, cooled to a liquid, and transported long distances to a storage area underground or in the ocean. Fully one-third of the energy produced might be consumed in the compression process. Overall, recovery of CO2 by coal-fired power plants is technologically complex and lowers the efficiency of coal conversion. In the long run, CO2 capture technology will have to be integrated with thermal physics systems and chemical engineering systems, which will require some technological breakthroughs.
Current clean-coal technologies all offer opportunities for capturing CO2. These include integrated gasification combined-cycle (IGCC) technology, integrated internal and external coal-fired combined-cycle technology, and coal-based polygeneration systems. These three technologies, as well as a coal-based energy network suitable for China, are discussed below.
IGCC, a relatively new technology, has drawn worldwide attention because it increases efficiency, reduces pollution, and requires less water than current systems (DOE, 1996). Dozens of IGCC demonstration plants have been established or are being established in the United States and Europe. China is beginning to carry out research on IGCC techniques, including gasification, gas purification, gas turbines, and heat-recovery systems, and is considering establishing its first 300–400 megawatt (MW) IGCC demonstration plant in Yantai.
The inherent advantages of IGCC in reducing pollution are indisputable. Waste emissions (SO2, NOX, and PM) from IGCC are lower than from a pressurized, fluidized-bed combustor combined-cycle plant or a pulverized coal-fired power plant with flue-gas desulfurization. The most distinctive characteristic of IGCC is the generation of a concentrated stream of CO2 with no dilution by nitrogen; pre-decarbonization is also feasible in the IGCC system.
The two critical technologies for IGCC with CO2 recovery are gasification and water-shift reaction (Figure 1). The synthesis gas (syngas) from the gasifier contains a large portion of CO, which is converted to CO2 and H2 through a water-shift reaction:
CO + H2O → CO2 + H2