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.
Black liquor is a waste stream from the pulping process in papermaking containing the unused portion of the wood and the inorganic compounds after reaction between the pulping chemicals and the wood. Traditionally, a recovery boiler is used to generate heat and steam and regenerate the pulping chemicals (e.g. NaOH and Na2S). Black liquor gasification is a potentially higher efficiency, lower cost and lower emission alternative to recovery boilers. One method of gasification for steam reforming in a fluidized bed uses relatively low temperatures (=605°C) to keep the alkali salts (or smelt) below their melting points. This process was demonstrated at a North Carolina paper mill in 1995, and full scale facilities have begun operation at mills in Virginia and Ontario. Both plants use the semi-chemical sodium carbonate process, which has essentially no sulfur compared to the more widely used kraft process with its more corrosive environment.
In the process described above, heat is transferred to the bed through modules, each of which contains hundreds of tubes that carry hot combustion gases. The hot gas is produced in a refractory-lined combustion chamber burning product gas or auxiliary fuel in a pulsed mode producing temperatures of at least 1300°. To prevent the bed tubes from exceeding 605°C, a short length of shield tube is used inside each bed tube. The shield tube material needs to have sufficient corrosion resistance at 1000°- 1100°C in the combustion gas containing O2, H2O, COx, uncombusted fuel and possible contaminants from the fuel.
The first alloys selected for this application were super austenitic alloys 330 and 800H. However, with a desired component life of 40,000h, this application likely requires more corrosion resistant alloys ? either a chromia-forming alloy with a reactive element addition or an alumina-forming alloy. Alumina-forming alloys have much slower reaction rates at these temperatures, and also are superior in combustion environments where water vapor in the combustion gas causes accelerated attack of chromia-forming alloys due to the formation of volatile oxy-hydroxides. However, while widely studied in the laboratory, wrought, powder metallurgy (PM) or oxide dispersion strengthened alumina-forming alloys are not preferred for industrial applications due to real or perceived cost, fabrication and mechanical property issues. The extreme conditions of this environment represent an opportunity for demonstrating the viability of some of the newest corrosion-resistant commercial alloys.
As a first step in the alloy selection process, various commercial alloy coupons were exposed in laboratory air at 900°-1