INTRODUCTION
Conventional steam reforming of methane to synthesis gas (CO and H2) has a conversion efficiency of about 85%. Replacement of metal tubes in the reformer with ceramic tubes offers the potential for operation at temperatures high enough to increase the efficiency to 98 to 99%. However, the two candidate ceramic materials being given strongest consideration, sintered alpha silicon carbide and silicon carbide particulate-strengthened alumina, have been shown to react with components of the reformer environment. The extent of degradation as a function of steam partial pressure and exposure time has been studied, and the results suggest limits under which these structural ceramics can be used in advanced steam-methane reformers.
Studies sponsored by the U.S. Department of Energy (DOE), Office of Industrial Technologies, indicate that some high-temperature processes in chemical, petrochemical, and other industries could achieve significantly greater efficiencies if improved materials permitted operation at higher temperatures, higher pressures, and/or in more corrosive environments. Consequently, the high-pressure heat exchange system (HiPHES) program was established by DOE to demonstrate the advantages of higher temperatures and/or pressures in industrial-sized systems. Two of these projects involve the assessment of materials for heat exchangers in hazardous, industrial waste incinerators. In another HiPHES project, Stone & Webster Engineering Corporation (SWEC) of Boston, Massachusetts, is developing a high-temperature, high-pressure steam-methane reformer. SWEC proposes to use ceramic tubes in the reformer for containment of the reactant and product gases because metallic tubes would not have sufficient strength to contain the pressure differential across the wall at the proposed operating temperature. Metal tubes would, however, be used within the ceramic tubes to provide a parallel counter flow path for the product gas as shown schematically in Figure 1. The proposed commercial-sized reformer would contain more than 600 ceramic tubes that would have an outside diameter of about 8.9 cm (3.5 in.) and a length, based on design constraints, of about 9 m (30 ft). The length of the tube could likely be reduced if a ceramic-to-metal joining technique were identified that would produce a joint capable of operation at higher temperatures and pressures. Although evaluation of joining techniques is an important aspect of this study, it is not included in this paper. Rather, the paper focuses on the effects of steam partial pressure and time on the corrosion of the two prima~ ceramic materials, sintered a-SiC and SiC-strengthened alumina.
