Pyrolysis characteristics and mechanism of oil shale in the Songliao Basin of China

The in-situ conversion of oil shale involves a complex pyrolysis process, which is characterized by the coupling of multiple reactions. Therefore, methods capable of effectively decoupling these multi-step reactions are essential for investigating the specific pyrolysis mechanisms of oil shale. Taking the oil shale in the Nenjiang Formation of the Songliao Basin (OSNS) as an example, this study proposes a novel strategy to elucidate the multi-step reaction mechanisms, offering a comprehensive understanding of the pyrolysis process of OSNS. Based on the thermogravimetric analysis (TGA) of OSNS, the existing deconvolution method was improved by introducing the uniform activation energy distribution criterion. This improved method was applied to separate overlapping weight loss peaks in the derivative thermogravimetry (DTG) curves. This method segmented the pyrolysis process of OSNS into water evaporation and six distinct reactions, identifying the temperature range and peak temperature for each stage. Using the peak temperatures of each reaction step as reference points, staged isothermal pyrolysis experiments were then performed using a fixed-bed reactor. Characterization of pyrolysis products obtained at each stage revealed the specific pathways of OSNS pyrolysis: water evaporation below 200 degrees C; kerogen conversion to thermal bitumen in 200-410 degrees C, involving cleavage of weak bonds and carbon chains related to cycloalkanes and aromatic carbons; primary cracking of thermal bitumen in 330-460 degrees C, including the dehydrogenation and cleavage of aliphatic compounds and breakdown of hydroxyl groups and adjacent carbon chains; secondary cracking of thermal bitumen in 420-520 degrees C, involving the loss of ester groups and other oxygen-containing functional groups near aromatic rings, along with esterification reactions; mineral reactions in 440-680 degrees C, including decomposition of siderite, clay mineral dehydration, and decomposition of calcite, ankerite, and pyrite. The key stages for oil production were kerogen conversion to thermal bitumen and primary thermal bitumen cracking. Compared to existing methods, the strategy of combining TGA, the improved deconvolution method, and staged isothermal experiments allows for a more accurate segmentation of the pyrolysis process of oil shale and provides a precise understanding of the reaction pathways. The derived pyrolysis mechanism and product distribution provide critical insights for developing in-situ conversion techniques for oil shale extraction.