Oxidation mechanisms of materials for heat exchanging components in CO 2 /H 2 O-containing gases relevant to oxy-fuel

In the context of CO2 emission reduction, the oxy-fuel technology provides a promising option applicable in centralized energy production. This technology is based on pulverized coal combustion with pure oxygen instead of air. Different from the conventional systems, metallic heat exchanging components in the oxy-fuel plants will be subjected to service environments containing high amounts of CO2 and H2O. In the present study the oxidation behaviour of selected ferritic/martensitic and austenitic steels as well as Ni-base alloys, which are candidate materials for heat exchanging components, was investigated in model gas mixtures containing high amounts of CO2 and/or H2O at temperatures in the range of 550 to 700°C and times ranging from a few up to 1000 hours. The results obtained after oxidation in the simulated oxy-fuel environments were compared with the behaviour in air, Ar/CO2 and Ar/H2O gases. For studying the effect of oxygen present in the real oxy-fuel atmosphere, Ar/CO2 gas was mixed additionally with different amounts of O2. It was found that in the CO2 and/or H2O-rich gases, the ferritic/martensitic steels tended to form Fe-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The Fe-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide formation in the CO2-rich gases. It was observed, however, that martensitic steels with higher initial Cr concentration had a stronger tendency to form protective Cr-base oxide scales when 1 or 3% of oxygen was added to the Ar/CO2 gas mixture. The oxide scale formation was affected by minor alloying additions, especially silicon. The poorly protective Fe-base oxide scales formed during exposure of the ferritic/martensitic steels to simulated oxy-fuel environments appeared to be permeable to CO2 molecules resulting in carburization of the steels whereby the extent was reduced by increasing water vapour content in the gas mixture. Carburization of 9-12% Cr martensitic steels was also found to be significantly reduced when 0.5 vol.% of SO2 were added to oxidizing CO2-rich environments. The oxidation behaviour of the austenitic steels strongly depended on the detailed alloy composition. At 550°C all austenitic steels exhibited very slow scale growth rates, however, at and above 600°C steels with lower Cr content (17-20wt.%) started to form multi-layered, Fe-rich oxide scales whereby the outer oxide layer was prone to spalling upon thermal cycling. For the 25% Cr austenitic steel and the Ni-base alloys much lower oxidation rates were observed, however, presence of water vapour in combination with intentionally added oxygen in the test atmosphere resulted in formation of volatile chromium species. The effect of surface modification was studied in the case of the 9-12% Cr and the austenitic steels with lower Cr content. At higher temperatures (650-700°C) a significant improvement in oxidation resistance was observed for austenitic steels when cold work was applied to the surface prior to exposure in CO2/H2O-rich atmospheres