Thermal-Mechanical-Chemical Coupling Analysis of Coal Rock in Underground Pyrolysis Conversion
Authors: Xue Junjie , Chen Yanpeng , Zhao Yufeng , Dong Zhen , Chen Hao , Chen Shanshan , Zhang Mengyuan 
Published: August 2024

ABSTRACT:
Underground coal pyrolysis (UCP) represents an environmentally friendly approach to exploiting coal resources through in-situ conversion, with its efficacy hinging on the efficient flow of gas through porous media. Consequently, the study of flow behavior, particularly permeability characteristics, holds significant importance for UCP. While the strength characteristics in thermal environments have been extensively researched, the reaction-dependent structural evolution remains relatively unexplored, despite its crucial role in reservoir stability and gas flow optimization. In this study, initially, permeability tests were conducted on coal samples after heating. Subsequently, the microscopic structures of the heated coal samples were observed using a scanning electron microscope (SEM). Lastly, a model incorporating thermal, mechanical, and chemical coupling effects was developed. The key findings reveal that the pyrolysis process can be categorized into three distinct stages based on temperature thresholds, including drying (100-300°C), medium-temperature pyrolysis (300-600°C), and high-temperature pyrolysis (600-1000°C). Additionally, the microstructural alterations that occur during the reaction are highly dependent on temperature, particularly above 350°C. These microstructural changes further influence the permeability characteristics of the samples. After reaching a temperature of 300°C, numerous new pores and microscopic fractures emerge within the sample model. Taking into account the impact of chemical reactions, the evolution of the microscopic structure influences the overall strength of the coal model through processes such as depolymerization and polycondensation during pyrolysis.

1 INTRODCTION
Underground coal gasification is the direct controlled combustion of underground coal, extraction of energy-containing components in coal, ash and residue left underground. Compared with traditional coal mining, coal underground gasification technology is more beneficial to realize the low-carbon utilization of coal. The whole process is the result of multi-field coupling, involving heat transfer, solid deformation and gas flow. The underground coal gasification space is large, the combustion height is large, the temperature is high, and the roof and floor are also affected by high temperature. Underground coal gasification is of great strategic significance to the development of natural gas in China, and it is a new strategic way to effectively supply methane and hydrogen with Chinese characteristics. The physical and mechanical properties of coal rock and roof and floor rock in different blocks are very discrete and closely related to rock facies. In the process of coal gasification, the physical and mechanical properties of coal and rock are strongly affected by temperature. The design and optimization of coal gasification construction parameters involves the numerical simulation of underground coal gasification process, which requires the physical and mechanical property parameters and evolution laws of coal and rock.