Steam Oxidation and Chromia Evaporation in Ultra-Supercritical Steam Boilers and Turbines

Abstract

U.S. Department of Energy's goals include power generation from coal at 60% efficiency, which requires steam conditions of up to 760 {degree sign}C and 340 atm, so called ultra-supercritical (USC) conditions. Evaporation of protective chromia scales is expected to be a primary corrosion mechanism. A methodology to calculate Cr evaporation rates from chromia scales was developed and combined with Cr diffusion calculations within the alloy (with a constant flux of Cr leaving the alloy from evaporation) to predict Cr concentration profiles and to predict the time until breakaway oxidation. At the highest temperatures and pressures, the time until breakaway oxidation was predicted to be quite short for the turbine blade, and of concern within the steam pipe and the higher temperature portions of the superheater tube. Alloy additions such as Ti may allow for a reduction in evaporation rate with time, mitigating the deleterious effects of chromia evaporation.