Effect of High Pressure on Nonisothermal Pyrolysis Kinetics of Oil Shale and Product Yield

The current work presents a comprehensive study on nonisothermal high-pressure pyrolysis kinetics of oil shale and the influence of pressure on product yield. High-pressure thermogravimetric analyses of the oil shale samples were carried out from 30 to 900 °C, at heating rates of 5, 10, 15, and 20 °C min–1, under isobaric pressure values of 5, 10, 20, and 30 bar using a well-controlled high-pressure thermogravimetric analyzer (HP-TGA). The organic matter decomposition zone was identified from HP-TGA data and the activation energy values were determined using isoconversional methods. Under the pressurized condition, oil shale exhibited mean apparent activation energy values ranging from 341 to 451 kJ mol–1. The possible decomposition mechanisms were identified as D3 diffusion model, followed by second-order reaction model, and A3 Avrami–Erofeev nucleation model as the decomposition progressed and products formed. Further, the lab-scale pyrolysis experimental matrix was designed based on HP-TGA data, and the products obtained were characterized for compositional analysis. The percentage of permanent gases viz. hydrogen (H2), carbon dioxide (CO2), and carbon monoxide (CO) decreased dramatically with the increase of pyrolysis pressure. The derived pyrolytic oils showed a decrease in specific density (879–831 g cm–3) and carbon number (C38 to C22) with an increase in pressure. Nuclear magnetic resonance (1H NMR) analysis of the pyrolytic oil indicated a reduction in the total aromatic content (from 22 to 9 wt %) due to coking of aromatics, and a subsequent increase in the percentage of aliphatics and alkanes due to cracking of straight-chain moieties. The octane number of obtained pyrolytic oils ranged from 83 to 88. The results obtained suggest that the choice of the pressurized pyrolysis (temperature–pressure combination) system may help in controlling the product distribution and compositions.