Role of Calcium Oxide in Directional Regulation Microstructure of Coke during High Sulfur Coal Pyrolysis

To explore the influence of calcium addition on microstructure of coke during high sulfur coal pyrolysis, different amounts of CaO were added to investigate their effect on sulfur migration and transformation during coal pyrolysis, along with the microcrystalline structure, optical texture and microstrength of coke derived from coal pyrolysis were also analyzed to insight the role of CaO in directional regulation microstructure of coke. The results showed that when the sulfur content in Changhong coal (CH) reached 2.79%, increasing the CaO addition from 1 to 3 wt % led to a rise in the total sulfur content in the coke derived from high sulfur coal pyrolysis, from 2.2 to 2.5%. Concurrently, the rate of sulfur fixation during pyrolysis exhibited improvement, escalating from 59.6% to 67.4%. Moreover, upon addition of 3 wt % CaO, the relative proportion of sulfate sulfur in coke attained a peak value of 56.4%, accompanied by reductions in the relative abundance of thiophene sulfur, sulfoxide sulfur, and sulfone sulfur to 21.1, 10.3, and 6.9%, respectively. The migration and transformation of organic sulfur to inorganic sulfur became more pronounced, corresponding to an increase in the Mf + Mm + Mc + Fi structures in the coke from 70.57 to 76.75%. At 2 wt % CaO addition to coke featuring a 2.79% sulfur content, the occurrence of Mf + Mm + Mc + Fi structures in coke peaks at 78.7%, while isotropic structure diminished to a nadir of 10.1%. Concurrently, the optical texture index (OTI) and microstrength index (MSI) for the coke increased to 157.65 and 45.5%, respectively, showing a slight improvement in microstrength compared to the original coke sample. The content of Mf + Mm + Mc and I in the optical texture of coke was correlated with the migration and transformation of sulfate sulfur, thiophene sulfur, and other sulfur species during coal pyrolysis under the addition of calcium.