Pyrolysis characteristics analysis of Chang-7 oil shale using thermal analysis and pyrolysis-gas chromatograph-mass spectrometry

Abstract

There are vast resources of oil shale in the Chang-7 section of the Upper Triasic Yanchang Formation, Ordos Basin, China. They would provide secure access to transportation fuels, if utilized in an effective, economic and environmental manner. A Chang-7 oil shale sample containing pyrite has been pyrolysed in a thermogravimetric analyzer at constant heating rates of 5, 10, 20℃/min up to 1150℃ with nitrogen as purge gas. Meanwhile, the pyrolysis-gas chromatograph-mass spectrometry experiments were performed on parallel samples of Chang-7 oil shale sample at 20℃/min in the four temperature sections, ranging from 100 to 700℃ to understand the decomposition mechanism. The initial decomposition temperature and the final decomposition temperature of hydrocarbonaceous material were about 367℃ and 521℃ (average values), respectively, which raised with the increase of heating rate. The decomposition of pyrite began to take place over 500℃, and the pyrolysates involving sulfur compounds are harmful to the environment and corrode the equipment. Thus, we recommended that the final temperature of Chang-7 oil shale pyrolysis was best not to exceed 550℃ considering the heating rate to avoid environmental pollution and damage of the instruments. Chang-7 oil shale exhibited a single stage decomposition in the range of about 367–521℃, representing the rearrangement of kerogen molecules in the temperature range of 100–350℃. The activation energies of kerogen decomposition were calculated using Coats–Redfern method at different heating rate. We found that both activation energy and pre-exponential factor increased with an increasing heating rate. Finally, the DTA curves of Chang-7 oil shale verified the reliability of the calculated activation energies.