A multi-step kinetics study on Chang-7 shale pyrolysis: Impact of shale inherent minerals

Pyrolysis is a significant process for the in-situ conversion and aboveground retorting of oil shale. However, the impact of inherent minerals on shale pyrolysis is still unclear. This study analyzed the effect of different inherent minerals on pyrolysis of Chang-7 oil shale, which is noted for its low carbonate, high silicate, and high pyrite content, through an integrated evaluation of kinetic and thermodynamic parameters. The pyrolysis process, arranged from 350℃ to 600℃, was deconvoluted into three distinct processes—bitumen, kerogen, and pyrite pyrolysis—using the bi-Gaussian method. Thermodynamic results showed that pyrolysis was endothermic and non-spontaneous. Minerals significantly reduced the pyrolysis activation energy. The ratio of pyrolysis activation energies for shale to kerogen increased with the carbonate-to-silicate content ratio. Master plot analysis indicated that, mineral removal shifted the reaction model from the contraction geometry model (Rn) to the diffusion model (Dn). This transition in reaction model was due to the formation of pores from demineralization and organic decomposition, facilitating the diffusion of heat and activated molecules into the interior of particles, which has been confirmed by porosity determination. This work provides an in-depth understanding of the impact of inherent minerals on shale pyrolysis, which is conducive to the efficient development and utilization of oil shale resources.