Alloy selection for high temperature heat exchangers

Various commercial alloys are being investigated for use as shield tubes in a high temperature heat exchanger for a black liquor gasification system at a paper mill. As a first step in the selection process, laboratory air exposures are being conducted at 900°-1100°C for times >36,000h to model lifetime as a function of tube wall thickness. The results at the highest temperature illustrate the performance difference between alumina- and chromia-forming alloys. Alumina-formers of sufficient thickness can be expected to meet the 40,000h lifetime goal at 1100°C. Among the chromia-forming alloys, those with a reactive element addition showed better performance at 1100°C. Variations in specimen thickness for one of the most promising chromia-forming alloys showed no change in the oxidation rate, and little increase in the time to breakaway oxidation, with increasing thickness. Characterization of the reaction zone after oxidation showed significant internal oxidation for most of the chromia-forming alloys at these temperatures.