Steam-side Oxidation of Materials for Supercritical Boilers

Abstract

In this work the steam-side oxidation resistance of various austenitic boiler steels was evaluated. The aim was to evaluate applicability of tested steels to over 600 °C live steam temperatures. Experimental work has been performed by exposing various austenitic boiler steels to supercritical steam in autoclave. The mass gains per unit area and oxide scale thicknesses are determined from sample coupons. Compositions of oxide scales are analyzed with energy-dispersive X-ray spectroscopy (EDS) and glow discharge optical emission spectrometry (GDOES). The efficiency of steam power plant can be increased by increasing the temperature and pressure of live steam. Today s advanced power plants have supercritical steam parameters with live steam temperature approximately 600 °C, and there are several research efforts to increase the temperature even further. High operating temperature sets very demanding requirements to the materials used in hottest components. The basics of steam boiler technology are introduced in the first part of this thesis. Brief introduction to fluidized bed (FB) combustion is also made. Typically used boiler materials and material degradation mechanisms that are typically occurring in fluidized bed type boilers are also briefly introduced. The main focus is on the steam-side oxidation of boiler steels. Thermodynamic and kinetic considerations affecting the oxidation process are discussed. The effect of alloy composition, grain size and degree of cold-working in surface layer on steam-side oxidation resistance was recognized from the available literature. Boiler steel must have sufficient chromium content so that protective Cr2O3 scale is formed on the surface. Grain size and surface finish affect the diffusivity of Cr in the metal lattice by introducing more diffusion paths for Cr. The effect of cold-working (machining) was observed in the experimental work of this study. However, the detailed analysis of oxide scales revealed that the long term performance of machined alloy may be inadequate. Besides chromium, also other alloying elements were detected from thin oxide scales. This suggests that also other alloying elements (Mn, Si, Al and Ti) have a certain role in the oxidation behaviour of a stainless steel alloy. The experimental work suggests that advanced austenitic alloys with over 22,5 wt-% Cr will be applicable in final superheaters of advanced supercritical power plants with over 600 ºC live steam temperature. More testing in different temperatures is required in order to determine the actual temperature limits. Shot-peened tube pieces should also be tested in autoclave and compared to the already tested machined sample coupons. /Kir11