Assessment of high-Cr alloys for cathode air pre-heaters in solid oxide fuel cells

Zhang, Kun (2021). Assessment of high-Cr alloys for cathode air pre-heaters in solid oxide fuel cells. University of Birmingham. Ph.D.

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Abstract

Efficient heat recovery and management is critical for the economic and environmental performance of a micro-CHP system. The cathode air pre-heater (CAPH) plays a pivotal role in the operation of Solid Oxide Fuel Cell micro-CHP systems as it heats up the inlet air close to the operating temperature (e.g. 750°C) of the SOFC by utilising the recovered thermal energy from the high temperature exhaust gas of the SOFC system. The current CAPH stacks, which are made from high Cr-containing alloys, are still vulnerable to high temperature corrosion and chromium evaporation, which can have a serious detrimental effect on the lifetime of the SOFC stack. For this reason, the aim of this study was to perform extensive corrosion and chromium evaporation measurements on various high temperature alloys and to implement a material progression for CAPH from chromia-forming alloys to alumina-forming alloys.

The effect of exposure temperature and water content on the oxidation and Cr evaporation behaviour of Inconel 625, SS309 and AluChrom 318 was analysed. The findings of this study showed that Cr evaporation and oxidation rates of Inconel 625 and SS309 were dramatically reduced when the temperature was reduced from 850 to 650°C. AluChrom 318 also exhibited a decreased oxidation rate with decreasing temperature but it also demonstrated a reverse trend in Cr evaporation temperature dependence. The main effect of water vapour on the alloys exposed to high temperature was to accelerate Cr evaporation.

The corrosion behaviour and Cr retention capability of an aluminised alloy (aluminised SS309) and an alumina-forming alloy (AluChrom 318) were compared with a conventional chromia-forming alloy (SS309) in simulated SOFC cathode atmosphere. The SS309 samples failed the specifications for application as cathode air preheater (CAPH) material due to a high Cr evaporation rate and massive scale spallation. The surface aluminisation dramatically decreased the Cr evaporation rate of SS309, but not in the long-term due to breakaway oxidation induced alumina scale spallation. The AluChrom 318 formed a γ-Al\(_2\)O\(_3\)/α-Al\(_2\)O\(_3\) layer scale and demonstrated the highest Cr retention ability and oxidation resistance among the tested alloy samples.

Microstructural analysis was conducted on an AluChrom 318 CAPH stack after 25,000 hours of industrial operation in simulated SOFC CAPH environment. The results showed a large amount of Cr\(_2\)O\(_3\) formation around the exhaust outlet (cold zone) and a high Al oxidation rate around the exhaust inlet (hot zone). This implied that the as-received AluChrom 318 CAPH plates did not have sufficient corrosion protection and a good barrier layer to stop Cr evaporation formed at high temperatures. For material optimisation, pre-treatment of AluChrom 318 in a high temperature oxidising atmosphere was then prescribed to achieve a better oxidation resistance and Cr retention capability of CAPH stacks in service. It was found that pre-treatment applied on the AluChrom 318 developed an α-Al\(_2\)O\(_3\) surface scale that reduced the Al oxidation rate and reduced the tendency towards formation of a Cr\(_2\)O\(_3\) layer.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):