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Show atmosphere); and (4) removal of carbon dioxide from systems that combust fossil fuels and containment of the carbon dioxide. This paper identifies a strategy which falls in the last category. In the past, proposals to remove and contain carbon dioxide emissions from industrial combustion systems focused on post-flame (flue) gases, since such strategies generally require the least modification to existing fuel delivery equipment, combustors, and boilers. Steinberg, et al. (1984), Marchetti (1979), Mustacchi, et al. (1979), and Baes, et al. (1980) have proposed preferential absorption of carbon dioxide in flue gases into a recyclable solvent, stripping of the captured carbon dioxide from the solvent, and liquefaction of the carbon dioxide. The liquid carbon dioxide is contained by discharging it into the deep ocean or by injection into spent oil and gas wells, or excavated salt caverns. Recently, however, Golomb, et al. (1989) concluded that separating the carbon dioxide from the flue gas is impractical due to the presence of large volumes of diluent nitrogen. In their comparison of five different techniques to reduce carbon dioxide emissions from coal-fired power stations, they concluded that post-flame separation of carbon dioxide would result in plant thennal efficiency decreasing from 35% to ~18%. The same investigators recommended an alternative technique, proposed earlier by Marchetti (1979) and Baes, et al. (1980), which calls for the separation of nitrogen from air to generate a concentrated oxidant to burn the coal and produce a flue gas comprised primarily of carbon dioxide and water vapor. The water vapor subsequently is condensed out and the remaining gas mixture, containing approximately 90% carbon dioxide, is compressed and liquefied. The extracted carbon dioxide would again be discharged into the ocean, in this case at depths between 700 m and 1000 m. Calculations indicate that this approach would reduce plant efficiency from 35% to about 25%, and that power generation costs would increase by -100%. Advances in fuel reforming technology provide a means to circumvent the extraction of carbon dioxide from post-flame gases diluted with large volumes of nitrogen and make unnecessary the separation of nitrogen from air. This paper describes an approach in which fuel-bound carbon is separated as carbon dioxide prior to combustion via oxidation on a catalyst using high temperature steam. Subsequent disposal of this carbon dioxide is effected by liquefaction and injection into the deep ocean. The following sections provide details of the energy requirements, technical feasibility, and economics of this pre-combustion carbon dioxide removal concept Additional details of the process are described elsewhere by Mori (1990, 1992) and Mori, et al. (1991a, 1991b). PRE-COMBUSTION REMOV AL OF CARBON DIOXIDE A simplified block diagram outlining the major processes in the pre-combustion carbon dioxide removal concept (Mori, 1990; Mori, 1991 b) is presented in Fig. 1. Methane was selected as the model fuel in the evaluation of this concept since: (1) steam reforming of methane is an established technology; (2) methane is the primary component in natural gas which is burned in approximately 16% of electrical power generation plants in the U.S.; and (3) methane is a constituent produced in the gasification of coal or liquid fossil fuels. It should be noted, however, that this concept may be applied to fuelstocks other than methane, such as coal, fuel oil, or biofuels, with minor modifications. In the proposed concept, superheated steam reacts with pressurized, gaseous methane on a catalyst to yield a mixture of hydrogen and carbon dioxide via the overall reaction (1) Reaction (1) is endothermic in the forward direction with a heat of reaction of approximately 193 MJ/kmol of methane convened. Although the stoichiometric ratio of water molecules to fuelbound carbon atoms required for reforming is 2, this analysis follows the common practice of utilizing approximately 100% excess steam (Le., using an H20:C ratio of 4) to facilitate the water- 2 |