Material selection for the super-heater and re-heater tubes on PFBC, based on the results of laboratory test and ex-serviced materials survey

摘要

All the super-heater and re-heater tube bundles are generally arranged in fluidizing bed in design of Pressurized Fluidized Bed Combustion (PFBC) system. Their design metal temperature (steam parameter) is set from middle to high. For a material selection, it requires following design philosophies; a) Materials should form oxide layer on a tube surface to have good erosion resistance against impact or abrasive action of particles in the bed. b) Materials should have enough strength for the service steam temperature. c) Material costs should be minimized. To study material selection on super-heater and re-heater tube bundles on PFBC, extensive laboratory erosion tests were conducted for the materials of 9Cr steels, 18Cr-8Ni austenitic steels and 25 Cr austenitic steels. The test specimens installed in the water-cooling system are running in the bed at design temperature, while the bed temperature was kept at 800deg.C. The test results was reviewed with 16,136 hours ex-serviced materials removed from the 71MW-Japanese first PFBC demonstration plant in Wakamatsu, operated by Electric Power Development Company. Experimental results correspond to the investigation of ex-serviced materials. It is concluded that A) High Cr ferritic steel such as ASME SA213-T91 and conventional 18Cr-8Ni austenitic steel show excellent erosion resistance in the middle metal temperature range (550-600deg.C). B) 25Cr austenitic steel has excellent resistance to erosion in the high metal temperature (over 600deg.C) but not as well in the middle temperature. C) In the oxide layer formed on the steels, there have been much Al, Si, and Ca oxides. It suggested that Al, Si oxides coming from bed particle constituents might contribute to the erosion resistance of metals. This paper summarizes the study results of PFBC material selection based on the laboratory tests and survey of ex-service materials.