Sulfur removal from petroleum coke during high-temperature pyrolysis. Analysis from TG-MS data and ReaxFF simulations

Petroleum coke (petcoke) contains high carbon and low ash qualities, but often with an undesirably high sulfur content. High-temperature (>1000 K) calcination produces a coke, suitable for many industrial uses, with an acceptable S content. Here the sulfur removal behavior during high-temperature pyrolysis was evaluated by combining high-temperature thermogravimetric analysis with product gas mass spectrometry (TG-MS), and reactive molecular dynamics (ReaxFF) simulations. From the TG-MS data the pyrolysis temperature of >1000 K significantly affected the S rejection. Three petcokes under 1273–1773 K in six different particle sizes (≤6 mm) were pyrolyzed to determine the desulfurization initiation temperature and desulfurization extent. A non-uniform behavior across the particle size ranges was obtained. Six Qingdao petcoke samples with cut sizes of <0.038, 0.07–0.05, 0.11–0.09, 0.25–0.15, 1.18–0.88, and 5.00–6.00 mm all achieved a similar desulfurization extent (∼80%) at >1673 K. However, considerable variability was shown in larger particles (1.18–0.88 and 5.00–6.00 mm) for Qingdao, Zhenhai, and Qilu petcoke between 1473 and 1773 K. The products included water (presumably from coke quench, 350–410 K), volatiles (430–550 K), CO2 and H2 (>800 K, mainly), CO and SO2 (>1200 K, mainly), and trace quantities of CS2 (>1400 K). The stable sulfur-containing products of this petcoke during high-temperature pyrolysis were SO2 and trace amounts of CS2. However, COS and H2S pyrolysis products were absent or below the detection limits. The pyrolysis behavior was explored using ReaxFF on a macromolecular petcoke structure with the S atoms in thiophene-like functional groups. The mechanism of S loss, under the simulation conditions, involved molecular rearrangement and thermolysis into intermediate states (C2S and CNS) and COS. These were explored for 250 ps for 3000, 3500, and 4000 K with the constant volume/temperature (NVT) ensemble. The sulfur removal transformation during pyrolysis is generally followed: thiophene sulfur → COS, C2S, or CNS → HS → SO2 or CS2.