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

Oxyfuel combustion is envisaged as one of the main options for near future CO2 reduction in conventional power production. There are many aspects of oxy-combustion still at the 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 (FeS2) 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 800HT). Metallic surface temperatures were kept at 400, 500, 600 or 700 °C, and after deposition, metallic coupons were aged for long periods (150 and 1500 h) at the selected conditions (O2/N2, CO2/N2). The characterisation of deposits was performed with XRD, SEM-EDX and carburisation tests.The first finding is that the oxidation progression is different when partially transformed pyrite covers metallic surfaces. In that case, no iron oxide (Fe2O3) scale is generated, only the chromium oxide (Cr3O4) grows between the steel and the deposit as a response to oxidation. There is a clear presence of chromium sulphides in competition with the chromium oxide. On the other hand, comparison of scales in CO2 vs. air indicates the 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 oxy-fuel combustion.

► Realistic pyrite slagging in oxy-fuel combustion is simulated by projection gun.
► Pyrrhotite deposits involve a risk of sulphidation but a hindrance to oxidation.
► No relevant differences found in oxy-combustion compared to conventional combustion.