Corrosion and life assessment of IntrexTM superheater tubes in a CFB oil shale boiler

Heating surfaces in oil shale boilers are subject to intensive high temperature corrosion (HTC). The protective oxide scale formed on austenitic steels SA213-TP347HFG, X8CrNiMoNb16-16 and X8CrNiMoVNb16-13 has been investigated and analysed (1) under both controlled laboratory conditions and (2) in the IntrexTM superheater of the circulating fluidized bed (CFB) boiler operating at the Narva Thermal Power Plant. Results show that the associated protective oxide scale tends to spall from tube surfaces. A long term investigation of HTC on the oil shale boiler tubes has demonstrated the applicability of these materials for such use and a comparison of HTC tubes manufactured from differing steel grades allows a selection of the most suitable grade. Remaining life assessments for particular tubes have also been conducted on the basis of this long term study of HTC on CFB boiler heating surfaces. This prognosis is vitally important to warranty safe and reliable further operation of thermal equipment.