Oxidation Mechanisms of Materials for Heat Exchanging Components in CO2/H2O-containing Gases Relevant to Oxy-fuel Environments
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Beschreibung
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
Autor*in
Tomasz Olszewski