OCR Text |
Show shift reaction. The reformed gas consists of a mixture of carbon dioxide, hydrogen, excess water vapor, and small quantities of carbon monoxide, methane, and other hydrocarbons. In the pre-combustion carbon dioxide removal concept, the carbon dioxide is separated from the gas mixture and the remaining hydrogen/residual methane fuel gas is combusted with air in place of the original hydrocarbon. Most of the energy consumed during the reforming step (Reaction 1) is recoverable from the combustion of the hydrogen-rich gas which has a heat of combustion that exceeds that of the original methane fuel. By employing this pre-combustion separation concept, atmospheric emission of carbon dioxide may be almost completely eliminated; however, the results of this study suggest that elimination of >90% of the carbon dioxide emissions may impose excessive energy and economic burdens on the process. Separation of carbon dioxide from the reformed gas mixture may be performed by employing technologies developed for application in commercial hydrogen production facilities, such as preferential absorption of carbon dioxide into various solvent solutions, molecular sieving, or pressure-swing adsorption (PSA), followed by liquefaction; or by staged phase transformations of water vapor and carbon dioxide. The liquefied carbon dioxide is injected into the deep ocean where local conditions would prevent its introduction into the atmosphere. Technology and equipment required to facilitate the three major steps that comprise the precombustion carbon dioxide removal concept- (1) methane/steam catalytic reforming, (2) carbon dioxide separation and liquefaction, and (3) liquid carbon dioxide transport and containmentexist; however, additional development is needed to optimize the carbon dioxide separation and liquefaction processes for this application, and the combustion of hydrogen in industrial boilers. Since deep ocean injection of liquid carbon dioxide has not been attempted to date, studies must be perfonned to verify the long-term fate of the discharged carbon dioxide, to evaluate its impact on the ocean ecosystem, and to identify the optimum depth and manner of injection. If these issues can be addressed satisfactorily, adoption of the pre-combustion carbon dioxide removal strategy should be feasible in the near-term. The authors have analyzed a methane-frred Rankine power cycle retrofitted with two possible pre-combustion carbon dioxide removal options. Salient features of the two options, described in detail by Mori, et al. (1991a, 1991b), are discussed below. The three major subsystems that comprise the pre-combustion carbon dioxide removal concept are the fuel reforming subsystem, the carbon dioxide separation and liquefaction subsystem, and the steam/power generation subsystem. In the fuel refonning subsystem, methane is compressed and mixed with water vapor, heated, and then reacted on the surface of a nickel alloy catalyst in a reformer/shift reactor operating at approximately 800° C and 2 MPa. Thermal energy from the hot gas mixture leaving the refonner is recovered at several locations within the fuel reformer itself and elsewhere in the system. The cooled, refonned gas mixture, consisting primarily of hydrogen and carbon dioxide, then enters the carbon di~xide separation and liquefaction subsystem. Two alternative means to separate and liquefy carbon dioxide were investigated by the authors - separation of carbon dioxide by capture in a hot potassium carbonate solvent (Case A), and physical separation, i.e., staged phase transformations of water vapor and carbon dioxide (Case B); these are illustrated in Fig. 2a and Fig. 2b, respectively. In Case A (Fig. 2a), the cooled reformed gas mixture enters a refonned gas cooler (regenerator reboiler) where the gas is further cooled to approximately 1150 C by heat transfer to liquid extracted from the carbon dioxide regenerator, producing stripping steam. This gas then enters the carbon dioxide absorption tower where the carbon dioxide is absorbed into a hot potassium carbonate solution (Kohl and Riesenfeld, 1985). The resultant hydrogen-rich gas from the absorption tower is depressurized in a turbo-expander and then burned in the steam/power 3 |