been closed down. In the U.S.S.R. a 25-MW natural-gas-fired system (U-25) provided electricity to the Moscow power grid for several years. The DOE concentrated on smaller-scale coal-fired systems, which included tests of over 2,000 hours on the lower-temperature heat recovery systems. The proof-of-concept high-temperature combustion and MHD generator sections were run for only about 400 hours. While most of the test performance goals were reached, the long-term high-temperature component durability required for utility applications is still in question. A proposal to DOE's CCT program to scale up to a combined MHD-steam plant of about 75 MW with an efficiency of approximately 31 percent was not selected for funding. Work on a tall-loop liquid metal MHD cycle has been concentrated in Israel, where test loops have been built to prove out the concept. These systems have the potential of 45 percent efficiency, but funding for further development is uncertain.

The open-cycle combustion gas systems as tested would extrapolate to a 500-MW coal-fired MHD plant meeting the federal NSPS for SO₂ and NO_x with an efficiency of up to 45 percent (Lohrasbi et al., 1991). This efficiency potential would drop considerably for smaller power plants. A claimed potential efficiency of 60 percent could only be met with the development of a high-temperature heat exchanger to preheat the combustion air to over 1370 °C (2500 °F), but little development work has been done on this exchanger. Furthermore, economic operation would depend on low-cost seed recovery, but only preliminary work has been accomplished in this area. The durability of the high-temperature MHD channel has not yet been demonstrated, and an integrated plant has not operated at any scale. Relative to other advanced technologies now under development, MHD systems pose much greater technological challenges because of the aggressive thermal environment and system complexity. At the same time, the thermal efficiency advantage of MHD systems has been eroded by more recent developments in other coal-based systems employing advanced gas turbines, fuel cells, and gasifiers.

DOE funding of the MHD proof-of-concept facility ended in FY 1993. FY 1994 funding and that requested for FY 1995 are only for site restoration, and no large-scale follow-on work is planned.

While a number of important technical goals were met by the DOE MHD program, significant issues were left unresolved, notably operational reliability.