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Show Air Heater Materials Materials are the key enabling technology for successful operation and commercialization of the HIPPS system. The use of high temperature heat exchangers in a coal combustion environment, coupled with the cost constraints, make proper materials selection a considerable challenge. Nonetheless, utilization of state of the art materials and joining methods, as well as advanced oxidation and corrosion resistant coatings, can yield reasonable compromises. The RAH must tolerate running coal slag on it's inner surface, while providing protection and reasonable heat transfer to the working fluid (air) contained near the outer walls of the coal combustor. Phase I efforts have indicated several potential approaches for this component: 1) use of metal tubing with protective coating(s) and refractory ceramic lining(s), 2) use of structural ceramics such as silicon carbide or silicon carbide /alumina particulate composites, with a protective refractory ceramic lining, and 3) use of fusion cast ceramics such as those used for glass furnace tank linings. The use of a metallic based RAH, particularly under moderated temperatures and environments afforded by the 6 5 % coal combustion case offers several advantages: 1) ease of fabrication, (i.e. conventional processes can be used to shape and weld components); 2) existence of a significant supplier base; and 3) high strength under moderate (65% coal) system conditions. These alloys offer superior performance compared to the steels, currently utilized in similar applications such as water wall slagging coal combustors. However, it is clear that, for the radiant air heater section, a system of refractory ceramic linings (with or without supplemental thin coatings on the base alloy) will be required to protect the alloy from the slagging coal ash environment. A diagram of an R A H wall cross-section is shown in Figure 3. The use of structural ceramics and/or fusion cast refractories have some disadvantages. Joining and sealing technologies are not well developed. For structural ceramics, while a supplier base exists for producing relatively large tubes, production costs remain high. Moreover, fusion cast ceramics are not normally produced as hollow shapes. Nonetheless, the allowable use temperatures of these ceramic materials make them the primary alternatives for use in the "all coal" combustion case when the working fluid is heated to 2500°F or higher. The metal based approach to the RAH clearly offers a high probability of success for fabricating and operating a prototype HIPPS system. All of the various material elements (metals, coatings, and ceramics) that make up this approach to the R A H wall will require careful testing to determine survivability in coal combustion environments. In addition to selecting and testing of metals, coatings, and refractory ceramic liners, consideration must be given to both on-line and off-line repair of the materials during use. Techniques for bonding, joining and attaching both the dissimilar materials of construction and individual wall subsections will require development and validation. |
