Effects of O2 Concentration on Oxy-Fuel Combustion Characteristics and Kinetics of Changji and Fushun Oil Shales

This study investigates how O2 concentration affects the combustion performance and kinetic response of Changji and Fushun oil shales under CO2-based oxy-fuel atmospheres. Coupled TGA–DSC–MS analysis was performed to characterize thermal decomposition, heat release and absorption, gaseous product evolution, and apparent kinetic parameters. The results show that raising the O2 concentration facilitates oil shale combustion. The TG–DTG and heat flow profiles move to lower-temperature regions as O2 concentration increases. At 20 °C·min−1, increasing the O2 concentration from 35% to 100% reduced Tp1 and Tp2 from 357.3 and 519.7 °C to 331.2 and 491.5 °C for CJ oil shale, and from 352.3 and 484.0 °C to 326.6 and 429.7 °C for FS oil shale, respectively. These shifts were accompanied by decreases in ignition and burnout temperatures and an increase in the comprehensive combustion index. Fushun oil shale shows a more concentrated main mass-loss and heat-release region than Changji oil shale. It also exhibits lower ignition and burnout temperatures, indicating stronger overall combustion reactivity. By contrast, Changji oil shale displays more evident mass loss and thermal responses at high temperatures, suggesting a greater contribution from carbonate mineral decomposition in the later reaction stage. MS results further show that CO2, H2O, SO2, and NO2 release mainly occurs within 300–600 °C. Their release peaks shift toward lower temperatures as the O2 concentration increases, indicating that oxygen-enriched atmospheres promote the oxidative conversion of organic carbon, hydrogen-containing structures, and S- and N-containing functional groups. The Vyazovkin nonlinear iso-conversional analysis provides conversion-dependent apparent activation energies rather than a single global kinetic parameter. The substantial variation in Eα with conversion highlights the overlapping and multi-stage nature of oil shale combustion. When the O2 concentration is raised from 21% to 75%, Eα generally follows an upward trend; under pure O2, however, it drops sharply. This non-monotonic variation suggests that O2 concentration changes not only the combustion rate but also the dominant reaction routes at different conversion stages. These findings provide experimental support for selecting suitable oxy-fuel combustion conditions and improving the clean and efficient utilization of oil shale.