Trap characteristic and mechanism of volatiles during pyrolysis of tar-rich coal

Tar-rich coal is regarded as a coal-based oil and gas resource. There are abundant resource of tar-rich coal in the western China. The large-scale extraction of oil and gas from tar-rich coal is conducive to improving the ability of energy supply, and optimizing the high-carbon properties of coal. For the extraction of oil and gas, the underground in-situ pyrolysis of tar-rich coal has the advantages of green, low-carbon, and less damage to the geological structure. However, the volatiles such as oil and gas may be blocked and even trapped in the coal matrix during pyrolysis. In this study, low-field nuclear magnetic resonance measurement and thermogravimetric analysis were conducted to confirm the existence of trapped oil and gas in the oil-rich coal after pyrolysis. Then the effect of coal molecular structure on the migration of pyrolytic volatiles and the tap mechanism were discussed based on Fourier infrared spectroscopy. The results show that the low-field nuclear magnetic resonance T₁-T₂ spectrum displays a strong signal of oily small molecule in the tar-rich coal after the pyrolysis at 500 ℃. Moreover, the thermogravimetric analysis was conducted on the tar-rich coal preheated at 500 ℃. Its initial pyrolysis temperature is only 461.5 ℃,which is lower than the preheated temperature. Comparing this pyrolyzed sample with the raw coal about the activation energy ranging from 371 ℃ to 462 ℃,the former is also much lower than the latter. All the characteristics above confirm the existence of oil trapped in the tar-rich coal during pyrolysis at 500 ℃. With the increase of pyrolysis temperature, the oxygen-containing functional groups and aliphatic structure of coal decrease, which result in the improvement of coal-oil interaction and the coal-water interaction weakened. These changes enhance the difficulty of volatiles migration, and improve the content of free water. In addition, the increase of coal aromatic structure weakens the thermal plasticity of the coal that leads the-urface tension increase. It is not conducive to the volatile bubbles to break through the matrix shell. However, the tar-rich coal pyrolysis at 700 ℃ has almost no trapped oil signal. The reason is that the strong secondary pyrolysis reaction occurs with the trapped volatiles at this temperature, which exist mostly in the form of gaseous small molecules. This research is helpful to have a deep understanding on the volatile migration and extraction in tar-rich coal during underground in-situ pyrolysis.