Abstract
The Democratic Republic of the Congo holds important reserves of oil shale which is still under geological status. Herein, the characterization and pyrolysis kinetics of type I kerogen-rich oil shale of the western Central Kongo (CK) were investigated. X-ray diffraction, Fourier-transform infrared spectroscopy and thermal analysis (TG/DTA) showed that CK oil shale exhibits a siliceous mineral matrix with a consistent organic matter rich in aliphatic chains. The pyrolysis behavior of kerogen revealed the presence of a single mass loss between 300 and 550 °C, estimated at 12.5% and attributed to the oil production stage. Non-isothermal kinetics was performed by determining the activation energy using the iterative isoconversional model-free methods and exhibits a constant value with E = 211.5 ± 4.7 kJ mol−1. The most probable kinetic model describing the kerogen pyrolysis mechanism was obtained using the Coats–Redfern and Arrhenius plot methods. The results showed a unique kinetic triplet confirming the nature of kerogen, predominantly type I and reinforcing the previously reported geochemical characteristics of the CK oil shale. Besides, the calculation of thermodynamic parameters (ΔH*, ΔS* and ΔG*) corresponding to the pyrolysis of type I kerogen revealed that the process is non-spontaneous, in agreement with DTA experiments.
Highlights
In order to satisfy the global energy demand and overcome the regression of conventional oil and natural gas reserves, oil shale constitutes a promising alternative resource from which fossil fuels can be produced (Ngo and Natowitz 2016)
The physicochemical characterization of Central Kongo oil shale (CK) was carried out using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and TG/DTA techniques and showed that it mainly consists of quartz and organic matter (OM) with sapropelic chains (Type I kerogen)
The non-isothermal pyrolysis of CK sample was investigated using thermogravimetric analysis, and the kinetic parameters were estimated using the iterative isoconversional IT-KAS and IT-Ozawa– Flynn–Wall (OFW) methods followed by the Coats–Redfern and Arrhenius plot methods
Summary
In order to satisfy the global energy demand and overcome the regression of conventional oil and natural gas reserves, oil shale constitutes a promising alternative resource from which fossil fuels can be produced (Ngo and Natowitz 2016). (Geng et al 2017; Jiang et al 2015; Maaten et al 2018) These factors should be optimized through the understanding of reaction mechanisms involved during pyrolysis and the determination of kinetic parameters describing the process as the activation energy, pre-exponential factor and reaction model (Li and Yue 2004; Wang et al 2013). Several tools have been used to optimize the energy potential of oil shale pyrolysis, including thermogravimetric analysis (TGA), which allows to study in detail the thermal kinetics of different processes (Khraisha and Shabib 2002; Tiwari and Deo 2012a; Wang et al 2014). The main mass losses monitored during non-isothermal oil shale pyrolysis consist of moisture removal at low temperatures, kerogen degradation at around 200 °C–600 °C and clays and carbonates minerals decomposition at above 700 °C (Maaten et al 2016; Strizhakova and Usova 2008)
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