Despite performance enhancements in advanced ceramics, a temperature limit of approximately 1090 °C (2000 °F) currently exists for ceramic heat exchanger materials. A report published in the late 1980s (OTA, 1988) noted that federal government support has been necessary to accelerate development of the ceramic materials and system technology for heat exchangers, despite projected economic and performance advantages. Material manufacturers and end users have considered the technical risks too high to invest their own funds in systems development and implementation.

Membranes

Membranes play a key role in the production of fossil-fuel-based products that meet composition standards for engine and combustor performance and provide environmental compliance through the removal of pollutant molecules (NRC, 1993). Possible applications of membranes to coal-based systems include the separation of hydrogen from coal gas streams and of impurities such as hydrogen sulfide (H2S), ammonia (NH3), SO2, NOx, and trace metal compounds from coal conversion (e.g., gasification) and combustion (flue gas) streams. Such separations can account for a major fraction of the investment and operating cost for coal-based systems. A particularly important application for advanced clean/ efficient power systems is the cleanup of coal gasification streams to drive advanced turbines. As discussed above, the ability of hot gas cleanup systems to reduce the contaminants to levels acceptable for high-temperature advanced turbines remains to be demonstrated. Another possible application of membranes is for the separation of methane from very dilute coalbed methane streams (see Chapter 5).

Low-temperature polymer membrane technology is fairly well developed and is useful for liquid-liquid, liquid-gas, and gas-gas separations (DOE, 1992). However, polymer membranes are limited to relatively low temperatures (less than 250 °C [480 °F]) and are subject to chemical and abrasive attack, particularly in the aggressive environments encountered in coal-based systems. Inorganic (ceramic) membranes have the potential to operate at the high temperatures required for advanced power generation systems (e.g., 815 °C [1500 °F] for removal of hot gas particulates from advanced PFBC and IGCC systems) and to provide significantly enhanced corrosion and erosion resistance compared to polymer membranes. Other expected advantages of advanced inorganic membranes include high permeability (1,000 to 10,000 times organic membrane permeability) and high selectivity (DOE, 1993).

In materials terms, refractory behavior and resistance to environmental attack depend on a suitable choice of ceramic material and associated fabrication process. Possible problems can be anticipated in coal-based systems due to reaction of candidate ceramic membrane materials—such as alumina, zirconia, and silica—with gas stream components, notably SO2 and alkali metals, at tempera-



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