THE BEHAVIOR OF HIGH-ALLOY STEELS USED AS HEAT-EXCHANGE COMPONENTS INCOAL-GASIFICATION PLANTS

The behaviour of high alloy steels, especially ferritic, Al-containing alloys based upon Fecralloy(R), has been studied in simulated coal ga sification atmospheres (CGAs) containing HCl at 450 degrees C. In addi tion, alloy modifications with increased Si and Cr levels were studied . Samples were also exposed in gasification pilot plants to compare wi th the behaviour observed in the laboratory. In CGA tests containing 1 000 ppm HCl, which simulated conditions in an atmospheric gasifier, al l the Fecralloy-based alloys tested exhibited low rates of attack, so these materials would be suitable for long-term exposure in these cond itions. It was shown that, while initial formation of sulfides occurre d, an Al-rich layer was eventually established that was rate controlli ng and prevented further formation of sulfides. Prior oxidation before exposure to the CGA prevented sulfide formation, and the scale compos ition and morphology that were observed in this case correlated well w ith those observed in the pilot plant. At levels of HCl that would cor respond to the operation of a pressurized gasifier, greatly increased rates of attack were observed and a thick sulfide layer was developed which, during the early stages of the reaction, was disrupted by chlor ide formation. The mass change data indicated that rates of attack for all the alloys examined were considered borderline for this applicati on.