Behavior of pressurized tubes of candidate alloys in a simulated low-btu environment

Abstract

Alloy tube specimens prepared from four candidate alloys for coal gasifiers (Alloy 800H, Type 310 stainless steel, HAYNES 188*, and IN-657) were exposed in an environment simulating low-Btu conditions at 815 °C. The test environment selected to represent slagging low-Btu gasifiers was an ambient pressure environment having oxygen and sulfur partial pressures of 1 x 10^-14Pa (1 x 10^-19 atm) and 1.3 x 10^-2 Pa (1.3 x 10^-7 atm), respectively. It was generated from a mixture of Ar, H₂, H₂O, and H₂S. Lifetimes of these alloys under pressurization were compared with those observed in air and in other less sulfidizing simulated gasifier environments. The development of oxide/sulfide scales upon the outer surfaces was evaluated. Sulfidation was more severe with exposure to the low-Btu conditions compared to exposures in the other environments. The formation of external sulfides was stimulated by strain induced cracks. Alloy 800H experienced the greatest amount of sulfidation, which led to hot corrosion, i.e., the formation of molten eutectics. This contributed to a reduction in the endurance of this alloy. Although external sulfides developed on the other alloys, particularly at surface cracks, lifetimes were not noticeably affected for test durations of up to 400 hours. The development of these sulfides, however, reflects possible limitations of long-term performance under these conditions.