Binary and ternary liquid metal-steam cycles for high-efficiency coal power stations

Since in the use of coal the direct recourse to combined cycles is impractical, binary alkali metal steam cycles are recognized as an interesting and feasible option. Past attempts to employ metal vapour conversion cycles for power generation are surveyed. After selecting potassium and cesium as possible candidate fluids, the binary cycle is optimized taking as variables the top temperature, the number of condensation levels of the metal vapour cycle, and the characteristics of the bottoming steam cycle. At vaporization temperatures in the range of 750-850 degrees C, metal vapour cycle efficiencies of about 20-24 per cent and binary cycle efficiencies of 57-61 per cent seem achievable. A survey of available building materials in the steel and in the super-alloy class showed that top temperatures of 800-850 degrees C could be reached with state-of-the-art alloys. Metal vapour turbines are recognized as a key issue of binary plant design in that exhaust volume flows are very large even for a moderate turbine capacity. For a double flow solution, limiting turbine dimensions to those of existing 1500 r/min steam low pressure stages leads to metal vapour turbine capacity of 120 MW for potassium and 170 MW for cesium. Assuming that in the future, better materials will be available allowing alkali metal vaporization temperatures in the range of 1400-1500 degrees C, a ternary solution is proposed which employs lithium, potassium, and steam as working fluids. At 1450 degrees C top temperature, a cycle efficiency in excess of 70 per cent is attained.