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Show generation and fuel reforming subsystems. After being depressurized to about 0.14 MPa, the carbon dioxide laden solvent flows into the regeneration tower where gaseous carbon dioxide is released. Desorption is driven by the combined effects of reduced pressure and the addition of thermal energy. The liberated carbon dioxide gas is cooled by ambient air drawn into the combustor and then enters the liquefaction stage. There, it is compressed using energy generated by the hydrogen turbo-expander and condensed by heat transfer to a refrigerant. Liquid carbon dioxide leaves the carbon dioxide separation and liquefaction subsystem at approximately 1.0 MPa and-45°C. In Case B (Fig. 2b), separation and liquefaction of carbon dioxide are accomplished by a combination of compression and refrigeration. The energy for compression is again provided by expanding the high-pressure, hydrogen-rich fuel gas through a turbo-expander to the combustor pressure. Part of the refrigeration load is satisfied by using the low-temperature expanded fuel gas as a thermal sink. The reformed gas is cooled to about 5°C, thus removing essentially all of the water vapor. To facilitate the carbon dioxide phase change and subsequent transport of liquid carbon dioxide down the discharge pipeline, the reformed gas mixture is pressurized to about 5 MPa (the exact value depends on the desired extent of carbon dioxide removal). Since compression increases the temperature of the mixture, additional cooling is needed; this is accomplished by precooling the reformed gas upstream of the carbon dioxide separator with the cold hydrogen leaving the separator. In the separator, gaseous carbon dioxide undergoes a series of phase changes induced by heat transfer to a -82° C liquid coolant provided by a conventional commercial refrigeration unit (Mori, 1991 b). In both Case A and Case B, liquid carbon dioxide is transported to the deep ocean through a pipeline. The operating pressure of the liquefaction process is sufficiently high so that no additional power is required to sustain the pressure drop in the long pipeline and to drive the discharge of carbon dioxide through the injection orifices. In the steam/power generation subsystem, the chemical energy released in the combustion of hydrogen (and residual methane) is transferred to the working fluid of the Rankine cycle by ~ convective and radiative heat transfer. Radiative transfer of energy would be less than that whic would occur with the original methane fuel; however, calculations indicate that a modest increase in convective heat transfer area in the downstream sections of the boiler and ducting would compensate for the decreased radiation. Alternatively, to compensate for the reduction in direct radiative heat transfer in the boiler as well as in the methane reformer, a radiation-enhancement technique proposed by Mori, et al. (1976) and Mori (1980) could be employed. Here, thin, flat ceramic plates, made of a thermal-resistive superalloy, are installed in the boiler and in the reformer. Forced convection from the flame gases heats the plates which, in turn, radiate to the boiler and reformer tubes. The effective emissivity of such plates is expected to lie between 0.5 and 0.7. Extensive reliance on heat recovery minimizes the expenditure of energy in the pre-combustion carbon dioxide removal concept. Principal energy expenditures include: (1) power consumed to pressurize the gaseous methane and (2) power expended for refrigeration during liquefaction. A thermodynamic analysis of a 500 MW power plant retrofitted with the proposed Case B precombustion carbon dioxide removal system indicates that 4% greater heat input would be needed to sustain the reforming reaction. In Case A, the additional heat input would increase to 28%, due largely to the energy requirements of the carbon dioxide gas regeneration step. The incremental parasitic power required to separate and dispose of roughly 90% of the carbon dioxide would be 7% and 14% of the gross power output of the plant for Case A and Case B, respectively (Table 1). Results of the systems studies appear to affinn the technical viability of the pre-combustion carbon dioxide removal concept for industrial combustors that burn natural gas. To increase the scope of application, other fossil fuels need to be investigated. To this end, a study has been initiated by the authors to extend the pre-combustion removal concept to coal- and oil-frred power 4 |
