Performance of alternate superheater materials in a potassium-rich recovery boiler environment

One method to significantly improve the efficiency of biomass-fired boilers is to increase the temperature and pressure of the steam generated. However, this requires operating the superheater tubes at considerably higher temperatures than can be tolerated by conventional structural materials. The limiting temperature for conventional materials is primarily determined by corrosion of the superheater tubes that is promoted by interaction with the relatively low melting point deposits that accumulate on the tubes. An air-cooled deposit probe was used to collect samples of the deposits that accumulated on tubes in the superheater area of a recovery boiler in a mill processing primarily hardwood. These deposits were found to be enriched in potassium. Subsequently, a corrosion probe containing multiple samples of nine different alloys was exposed for 2000 h in the same location of the superheater area of the same recovery boiler. The temperature of samples in the probe ranged from a low of about 400°C (752°F) to temperatures above 620°C (1148°F), compared to the boiler's estimated maximum tube temperature of 470°C (878°F). Following exposure, sections were taken from each of the 30 samples and examined using light microscopy and scanning electron microscopy. Results of the examination of these samples showed significant differences among the corrosion rates at temperatures 100°C above the current upper superheater tube temperature. A brief comparison is made to data from a parallel laboratory study. Application: This paper provides guidance on the selection of an alternate material for the highest temperature portion of the superheater tubes.