Pyrolysis Behavior, Char Structure Evolution, and Kinetics Characteristics of Oil Shale Under N2 and CO2 Atmospheres

This study investigated the effect of replacing N2 with CO2 on the pyrolysis behavior, char structure evolution, and kinetics of Fushun (FS) and Changji (CJ) oil shales. Non-isothermal thermogravimetric experiments were conducted at 5, 10, 20, and 40 °C·min−1 under N2 and CO2 atmospheres, and the resulting chars were characterized by FTIR, XPS, BET, and SEM. Kinetic parameters were evaluated using Friedman, FWO, KAS, Starink, and Vyazovkin iso-conversional methods. Both oil shales underwent three stages: moisture release, main organic-matter pyrolysis, and high-temperature mineral decomposition. Increasing the heating rate shifted Ts and Tmax to higher temperatures and intensified volatile release. At 40 °C·min−1, replacing N2 with CO2 increased Ts from 322.6 to 399.3 °C for FS and from 368.1 to 377.3 °C for CJ, while reducing the maximum mass-loss rates to 6.81 and 8.66%·min−1, respectively. N2 favored pore development, increasing the specific surface areas of FS and CJ chars to 14.1402 and 6.1464 m2·g−1, whereas CO2 caused pore blockage in FS char and reduced its surface area to 2.7783 m2·g−1. XPS showed that CO2 promoted the formation or preservation of oxygen-containing surface carbon, especially C=O and O–C=O groups. The Eα values first decreased and then increased with conversion and were generally lower in CO2 than in N2. The average activation-energy differences between the two atmospheres were 23.5 and 43.2 kJ·mol−1 for FS and CJ, respectively. These results provide experimental and kinetic data for modeling primary oil shale pyrolysis and subsequent char combustion and gasification under CO2-rich conditions.