Scale formation on ferritic and austenitic steels in high CO2/H2O containing gases simulating oxy-fuel environments

The oxy-fuel process represents a promising technology for CO2 capture from the exhaust gas in coal fired power plants. The principle of this process consists of burning coal with pure oxygen, a recirculation of the CO2 gas produced and finally a separation of the CO2 from the combustion gases, by various methods which are presently being developed. Depending on the actual process parameters used, a number of metallic heat exchanging components will in the oxy-fuel process be subjected to service environments containing high amounts of CO2 and water vapour. In the present study, the oxidation behaviour of selected commercial ferritic and austenitic steels was investigated in simulated oxy-fuel environments at temperatures in the range 550-650 C. Thereby, the behaviour of the materials in the simulated oxy-fuel gas was compared with that in CO2, H2O and CO2-O2. The experiments showed that ferritic steels components exposed to CO2/H2O gas mixtures, may exhibit substantially higher oxidation rates than during air exposure. The oxidation rates are similar to those observed in service environments of conventional power plants; however the steels exhibit a substantially larger tendency to carburization than in conventional service gases. Austenitic steels exposed to the CO2/H2O atmospheres exhibit oxidation and carburization rates which are strongly dependent on the actual Cr content of the steel and also on component surface preparation. The paper will discuss how the individual species present in the mixed gases affect the scale formation mechanisms especially in respect to their positive or adverse effect on formation of protective chromia base oxide scales. (authors)