Radiant coils in steam-cracking furnaces operate under severe reducing and oxidizing conditions at temperatures even beyond 1100 °C. Sulphur is present in feedstocks as natural constituent (naphtha cracking) or as deliberate addition (ethane cracking), and can influence the coke formation and also the stability of tube materials.
The influence of sulfur as H2S-addition on the stability or degradation of tube materials and on the coke formation was studied. The investigated samples were alumina-forming- as well as chromia-forming alloys taken from conventional radiant tubes. In a laboratory-scale reactor comparative cracking-decoking tests were applied to the samples. Testing conditions were relevant to industrial steam cracking. Sulfur was continuously added as H2S to the synthetic feed. Sample temperature could be varied during cracking up to 1100 °C. The influence of continuous H2S addition to the feed on coke formation and on the tube material deterioration was studied. The investigations reflect the impact of sulfur on the rate of coke formation and on the coke morphology. Particular focus is on how sulfur influences the physical and chemical nature of catalytically active sites. Sulphur stimulates a process of carbon-induced corrosion, which particularly can deteriorate chromia-forming alloys, whereas alumina scales are resistant.
Key words: Steam-cracking, sulfur, catalytic coking, high-temperature corrosion, coil material, alumina scale, chromia scale, high-temperature material