Sulfur
The metal temperatures of evaporator sections of syngas coolers is determined primarily by the temperature and thus by the pressure of the steam side. With typical high — pressure steam conditions of 100 bar, this implies metal temperatures between 300 and 350°C. EPRI and KEMA have conducted corrosion tests specifically to determine corrosion rates by sulfiding in this temperature range, and they found a clear dependency between corrosion rate and partial pressure of H2S (van Liere, Bakker, and Bolt 1993). According to these results, in an atmosphere with 1 mol% H2S at 40 bar and 300°C, a corrosion rate of 1.5 mm/y can be expected with low alloy steels, with which they specifically include alloy T22 (10CrMo910). The investigation was made primarily with coal gasification in mind, so it is unclear from the published results what other trace components were in the gas. In our experience with gasification of heavy petroleum residues at 60 bar where 1 mol% H2S is by no means uncommon, any corrosion rate must be at least an order of magnitude lower than this, since evaporator coil lifetimes of 60,000 hours are common (Higman 1994). It is unclear what protection mechanism exists in the latter case to explain this apparent contradiction.
When superheating is included in the syngas cooler, then much higher metal temperatures must be expected, and they are likely to be in the range 300 to 600°C. Van Liere, Bakker, and Bolt’s results indicate clearly that an austenitic steel is required for this service, typically SS310, which interestingly performs better than alloy 800.
Corrosion during shutdowns, particularly on the ingress of oxygen, requires careful attention. Appropriate operation and maintenance procedures have been published (National Association of Corrosion Engineers 1993).