New Approach to Materials Behaviour Studies in High-Speed Flue Gas from Oxy-Steam Combustion

Some new proposals are being considered aiming at improving the efficiency and operability of oxy-combustion power plants, such as the so-called oxy-steam combustion. This process consists in replacing the carbon dioxide by steam in the firing atmosphere. One of the main uncertainties for the feasibility of the oxy-steam technology is the materials resistance to steam corrosion at high temperature, mainly due to chromium vaporization. The novel approach to the issue presented in this work is the generation of realistic flue gas velocity (20 ms(-1)) in flames produced with a thermal spray gun using H-2 as fuel. Gas composition was selected to match expected final H2O content after combustion: 40 and 60% H2O in CO2 with excess of O-2. A set of metallic steels and alloys (SS304, SS310, I800HT, Kanthal and IN617) was preoxidised in air to generate external oxide scales. After preoxidation, the samples were treated under steam flames at 700 degrees C, placed in a position either parallel or perpendicular to the flame axis. Surface oxides composition was compared to that obtained in a quasi-static furnace, where metallic samples were exposed to steam atmosphere at 700 degrees C for 20 h (50% steam, 45% CO2 and 5% O-2). The characterisation of the surfaces was performed using low-angle XRD and SEM-EDX for a precise measuring of the variation of oxide amount and chromium content. The analyses showed that mixed Cr-Mn and Cr-Fe external oxides formed during the preoxidation stage suffered the depletion of the surface chromium when they were exposed parallel to the steam flame axis, as a simulation of the lateral flue gas pass in the heat-exchanger tubing. Cr depletion was also observed in the windside of tubes (perpendicular to flame axis) but in minor extent. The reduction in the Cr amount of the samples when tested in the vapour furnace was negligible compared to the Cr loss found after treatment under high velocity flames. The presence of external (Mn,Cr)(3)O-4 in preoxidised SS310 and SS304 did not provide an effective protection against Cr volatilization in the steam flame treatment, whereas surface alumina in Kanthal seemed to prevent Cr volatilization.