Pyrrhotite deposition through thermal projection to simulate iron sulphide slagging in oxyfuel combustion

8 Figures, 4 Tables.-- Work presented at the 8th European Conference on Coal Research and Its Applications, Leeds (UK) from 6-8th September 2010.
Oxyfuel combustion is envisaged as one of the main options for power production from fossil fuels in a carbon constrained scenery. There are still certain aspects of oxycombustion still on research stage, one of those is the issue of boiler materials resistance to corrosion due to solid deposits formed as a consequence of slagging in CO2 rich flue gases. The novel approach to the issue is the simulation of realistic slagging by pyrite projection through an oxyacetylene spray gun, flying along a controlled flame and impacting onto metallic surfaces of selected composition for fireside waterwall construction (F22, P91, 409, 347, 304H and I800HT). Metallic surface temperature was kept at 400, 500, 600 and 700ºC, and after deposition, metallic coupons were aged for long periods (150 and 1500 hours) at the selected conditions (O2/N2, CO2/N2). The characterization of deposits was performed with XRD, SEMEDX and carburization tests. The first finding is that the oxidation scale progression is different when partially transformed pyrite covers metallic surfaces: chromium oxide grows as a response to oxidation between the steel and the deposit, less dense and partially interrupted, and no iron oxide scale is generated. There is a clear presence of chromium sulphides in competence with the chromium oxide. On the other hand, comparison of scales in CO2 vs. air indicates same chemical composition but different morphology: in air combustion, corrosion layers are thicker and cracked. These results can improve the prediction of operational problems in coal oxyfuel combustion.