Reducing steam transport pipe temperatures in power plants

A cycle analysis has been applied to a model of a advanced ultra-supercritical steam plant with novel steam pipes. The transfer pipes proposed incorporate internal thermal coatings and are externally jacketed to enable cooling. This enables higher temperature working steam, while keeping the pipe wall temperature below the acceptable limit for more conventional steel alloys and avoiding the need to use higher cost austenitic stainless steels and nickel base alloys. The baseline design had a superheat temperature of 700C and a reheat temperature of 720C. A thermal coating thickness of 2.8 mm is sufficient to keep the wall temperatures of the steam transfer pipe after the supercritical boiler below 600C. For the transfer pipe located after the reheater a thicker coating or less ambitious reheat temperature is required to achieve acceptable pipe wall temperatures. Whereas subcritical plant has a calculated cycle efficiency of 42.1%, the elevated temperature and pressure in a customary ultra-supercritical steam boost cycle efficiency to 52.2%. Modifying this design with a thermal barrier lowers the cycle efficiency to 51.4%, still appreciably better than for subcritical plant. Alternative plant cooling arrangements might improve pipe temperatures but have minimal impact on overall cycle efficiency.