Energy & Fuels, Vol.34, No.3, 2969-2979, 2020
Thermal Degradations and Processes of Four Kerogens via Thermogravimetric-Fourier-Transform Infrared: Pyrolysis Performances, Products, and Kinetics
The thermal upgrading of kerogen into fuels has scientific and industrial importance in oil shale utilization. In this study, multiple methods were applied to clarify the pyrolysis behaviors of four kerogens from the Huadian (HD) Basin, Fushun (FS) Basin, Luozigou (LZG) Basin, and Dachanggou (DCG) Basin oil shales. Two model-free isoconversional methods and a master plot were utilized to evaluate the kinetic parameters, and the Coats-Redfern method was used to verify the estimated kinetic results. The results indicated that all studied oil shales had very good hydrocarbon generation potential. HD, FS, and LZG oil shales could be described as type I kerogen, whereas DCG oil shale was type H-1 kerogen with high aromatic content. HD kerogen featured a stronger polarity and contained more aliphatic structures compared with the three other samples, which induced the mass loss of its first stage at similar to 38%, even larger than that of its second pyrolysis stage; the activation energies of the main pyrolysis stage of HD kerogen could also be divided clearly into two stages and in accordance with the thermogravimetric results. The thermal cracking processes of these FS, LZG, and DCG kerogens were mainly concentrated in the second pyrolysis stage, and the activation energies of FS and LZG kerogens did not show a considerable variation with conversion degrees. Moreover, DCG kerogen possessed the largest amount of aromatic structures which resulted in high yields of char, and the activation energies of the main pyrolysis stage of DCG kerogen was almost the largest, especially when the conversation rate was above 0.7. Evolution behaviors of oil and gas products at different temperatures during pyrolysis were related to the cleavage of chemical bonds in kerogen. CH4 and light C2+ aliphatic hydrocarbons were the typical gaseous products released from the main reaction stages of all four kerogens, and the evolving behavior of each typical gaseous product of four kerogens featured good similarity. The results showed that aliphatic and aromatic carbon contents in kerogen had a remarkable effect on its thermal characteristic and kinetic parameters, and the saturated aliphatic bonds and oxygen-containing bonds were easy to crack and had potentially low activation energy of reaction compared with those of aromatic bonds. Results of the master plot and Coats-Redfern methods verified that the Avrami-Erofeev kinetic model was the optimal model for the pyrolysis of the studied kerogens, thereby indicating that the four kerogens evolved under a similar genetic condition.