Temperature-resolved evolution and speciation of sulfur during pyrolysis of a high-sulfur petroleum coke

Knowledge is limited on the thermal behavior of sulfur during petroleum coke (PC) pyrolysis, a critical topic for its any thermal utilization processes. In this work, temperature-resolved evolution and speciation of sulfur in both gas and solid phases during pyrolysis of a high-sulfur petroleum coke were investigated, with simultaneous thermogravimetric and mass spectrometric analysis (TG-MS) and X-ray photoelectron spectroscopic analysis (XPS). The results show that the original PC contains dominantly thiophene and sulfoxide, while mercaptan, sulfate and sulfone are only present in minor amounts. The desulfurization ratio generally increases with increasing temperature or decreasing heating rate. Two slow desulfurization stages (Stage I (<500 °C) and Stage III (900–1100 °C)) and two fast desulfurization stages (Stage II (500–900 °C) and Stage IV (1100–1375 °C)) are identified, which roughly coincide with those of sample mass loss. It is first found that the surface thiophene and sulfoxide increase with increasing temperature at Stages I and III, while they decrease with increasing temperature at Stages II and IV. Five sulfur-containing gases (H2S, CH3SH, SO2, COS and CS2) are detected. Nevertheless, only the evolution of H2S, CH3SH and SO2 is readily distinguishable. Appreciable H2S release only takes place at temperatures below about 750 °C, and is primarily attributed to the transformation of mercaptan. Significant evolution of CH3SH and SO2 occurs at Stages II and IV, and is associated with the cracking and polymerization of thiophene and sulfoxide. Their release patterns show a similar bimodal feature, most likely due to their similar sources.