Materials Selection for Metallic Heat Exchangers in Advanced Coal-Fired Heaters

The application of advanced coal-fired heaters to heat the working fluid for a closed-cycle gas turbine provides some challenging problems for the selection of metallic heat exchanger materials. The requirements of a working fluid temperature of 1550°F (1116 K) at a pressure of 300–600 psig (2.07–4.14 MPa/m²) necessitate that the alloys used for the hottest part of the heat exchanger possess high-temperature strength in excess of that available in widely used alloys like alloy 800. The maximum-duty alloys must therefore be selected from a group of essentially nickel-base alloys for which there is scant information on long-term strength or corrosion resistance properties. The susceptibility to corrosion of a series of candidate heat exchanger alloys has been examined in a pilot plant size fluidized-bed combustor. The observed corrosion behavior confirmed that at certain locations in a fluidized-bed combustor nickel-base alloys are susceptible in varying degrees to rapid sulfidation attack, and must be protected by coating or cladding.