Effect of Sulphur on the Oxidation Behaviour of Possible Construction Materials for Heat Exchangers in Oxyfuel Plants in the Temperature Range 550-700 °C

During oxyfuel combustion metallic heat exchangers are subjected to service environments which substantially differ from those prevailing during the conventional air firing process. In the present study the behaviour of three selected construction materials (P92, super S304HCu and alloy 617) during exposure in simulated oxyfuel gas with and without addition of SO2 at temperatures between 550 and 700 degrees C has been investigated. The alloy microstructure and the corrosion products formed during exposures up to 1000 h were studied by SEM/EDX and correlated with gravimetric data collected during the discontinuous exposures. It was found that the behaviour of the martensitic steel was hardly affected by the presence of SO2; however, in the case of the austenitic steel S304HCu the SO2 suppressed internal oxidation occurring at 650 degrees C in the SO2-free gas, thus promoting formation of a protective chromium-rich oxide. In the case of the nickel base alloy 617 the SO2 addition increased the corrosion rates at 550 and 650 degrees C due to replacement of the external chromia scale by a multiphase scale with sulphur-containing surface nodules. At 700 degrees C the alloy formed a chromia base surface scale and SO2 addition suppressed the formation of volatile Cr species. The results are explained using classical oxidation theory related to conditions for external scale formation in combination with thermodynamic considerations of phase stability as well as relative rates of adsorption of various gas species.