Effect of reservoir moisture on underground in-situ pyrolysis of tar-rich coal: Experimental investigation and ReaxFF MD simulation

Different moisture content is an important aspect of underground coal seam heterogeneity, which would affect the underground in-situ pyrolysis of tar-rich coal. The pyrolysis characteristics and product distribution of water-bearing tar-rich coal were investigated by the fixed bed pyrolysis experiments. Based on the ReaxFF MD simulation, the pyrolysis characteristics and reaction mechanism were explored from a microscopic perspective. The results showed that the pyrolysis characteristic index (D_i) continued to decrease with increasing moisture content, indicating that H₂O could increase the release difficulty of pyrolysis volatiles. As the moisture content increased from 1 wt% to 15 wt%, the tar yield and lighter tar proportion increased from 4.39 wt% and 49.75 % to 11.95 wt% and 61.25 %. Therefore, the two obtained with higher moisture content would be also higher. The above results showed that H₂O could promote the pyrolysis of coal. The tar yield and lighter tar proportion obtained under the CO₂ atmosphere were further improved. The results of the ReaxFF MD simulation showed that the changing trends of pyrolysis product distribution were consistent with experimental results. The statistical results of chemical bonds showed that H₂O could promote the cracking of C-C and C-O bonds within the pyrolysis system. By analyzing the typical pyrolysis reaction mechanism, the results of pyrolysis experiment and ReaxFF MD simulation were further confirmed. Based on the comprehensive analysis of the above results, it could be concluded that the most suitable reaction conditions for water-bearing tar-rich coal pyrolysis were moisture content: 5 wt%∼10 wt%, N₂ atmosphere.