Abstract

The so-called oxyfuel process is frequently considered as a promising technology for CO2 capture from the exhaust gas in fossil fuel fired power plants. In the present paper, the oxidation behavior of potentially suitable construction materials for heat exchanging components in coal fired power plants was studied at 650°C. The selected materials (martensitic steels, austenitic steels and a Ni-base alloy) were exposed in a simulated atmosphere typical for oxyfuel combustion and the results were compared with the behavior in a test gas simulating air-firing flue gas. Additionally a set of corrosion tests was performed in the simulated oxyfuel gas with addition of CO to simulate locally occurring reducing operating conditions.

The oxidation/corrosion behavior was studied by gravimetry in combination with a number of characterization methods such as optical microscopy, scanning electron microscopy with energy dispersive x-ray analysis (SEM/EDS) and (for selected specimens) glow discharge optical emission spectroscopy (GDOES), x-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The obtained results are interpreted on the basis of thermodynamic considerations comparing equilibrium activities of the main species in the gas atmospheres with the thermodynamic stabilities of various possibly forming solid and volatile corrosion products.

Introduction

The so-called oxyfuel technology provides a promising option for significantly reducing CO2 emissions in future modern coal fired power plants.1 The oxyfuel process is based on pulverised coal combustion using pure oxygen instead of air.2 The main advantage of the oxyfuel technology is the composition of the flue gas (containing mostly CO2 and H2O), making CO2 separation much easier compared to conventional power plants.3

In an oxyfuel plant, the heat exchanging metallic components are exposed to a flue gas that contains much higher CO2 and H2O contents compared to conventional flue gases.4 Depending on the actual operation temperature of the power plant ferritic/martensitic steels, austenitic steels and Ni-base alloys are being considered as potentially suitable construction materials for such components.5

In the literature several studies concerning the behavior of low alloy steels, martensitic 9-12% chromium steels as well austenitic steels in CO2- and/or water vapor rich gases in the temperature range 500-650°C are available.6-19 It was frequently found 6-8 that in H2O and/or CO2-rich gases, the ferritic/martensitic steels tend to form Fe-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. Austenitic steels with Cr concentrations around 18% are reported to form multi-layered, Fe-rich oxide scales when exposed in H2O (CO2)-rich gases above 600°C.8 For 25% Cr austenitic steels and NiCr-base alloys much lower oxidation rates were observed, however, the presence of water vapor in combination with intentionally added oxygen in the test atmosphere resulted in the formation of volatile chromium species.5, 8

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