Key Takeaways & Executive Findings
- •• A critical temperature threshold of 500 °C triggers severe degradation of coal pillar bearing capacity, with strength and elastic modulus reductions up to 45.53% and 61.34%, respectively. • Above 500 °C, coal undergoes significant pyrolysis under N2 and CO2 atmospheres, leading to decreased oxygen-containing functional groups and increased aromaticity and graphitization. • High-temperature exposure induces microstructural changes, including dislocation and slip within coal crystal nuclei, resulting in increased porosity and deformation. • The failure mode of coal transforms from brittle to ductile with rising temperature, impacting the stability of underground coal gasification gasifiers.
Abstract
Coal pillars are critical supporting structures between underground coal gasification gasifiers. Its bearing capacity and structural stability are severely threatened by high-temperature environments. To elucidate the high-temperature deterioration mechanism of coal pillars at multiple scales, coal strength features as a function of temperature were investigated via uniaxial compression and acoustic emission equipment. The pyrolysis reaction process and microstructure evolution were characterized via X-ray diffractometer (XRD), scanning electron microscope (SEM), thermogravimetric (TG), Fourier transform infrared spectroscopy (FTIR), and computed tomography (CT) tests. Experimental results reveal a critical temperature threshold of 500 °C for severe degradation of the coal bearing capacity. Specifically, both the strength and elastic modulus exhibit accelerated degradation above this temperature, with maximum reductions of 45.53% and 61.34%, respectively. Above 500 °C, coal essentially undergoes a pyrolysis reaction under N2 and CO2 atmospheres. High temperatures decrease the quantity of O2-based functional groups, growing aromaticity and the degree of graphitization. These changes induce dislocation and slip inside the coal crystal nucleus and then lead to deformation of the coal molecular structural units and strain energy generation. This process results in a great increase in porosity. Consequently, the stress deformation of coal increases, transforming the type of failure from brittle to ductile failure. These findings are expected to provide scientific support for UCG rock strata control.
1. Introduction
By establishing carbon markets and being active in carbon trading, carbon emission reduction has emerged as a focal point in energy strategy development across various countries [1]. As a new elemental mining method for coal resources, underground coal gasification (UCG) has charmed the interest of nations worldwide because of its low-carbon and environmentally friendly advantages [2]. In the UCG process, maintaining gasifier stability is crucial for ensuring continuous gasification [3]. An intact gasifier can effectively prevent crude gas leakage and heat loss, ensure smooth gasification channels, and enhance the efficiency of the gasification reaction [4].
The isolation coal pillar between multiple gasifiers is the crucial structure supporting the overlying strata [5]. Nonetheless, coal pyrolysis occurs due to elevated temperatures during gasification. The gasification process results in different temperature distributions in the gasifier (Fig. 1). This leads to different degrees of coal pillar pyrolysis around the gasifier and substantial changes in its physical and chemical properties [6]. Hence, to confirm gasifier stability and prevent roof collapse and water permeation accidents, it is fundamental to examine the degradation mechanism of coal pillars within the UCG and further grasp the evolution of bearing characteristics.
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Jian Li, Jinwen Bai, Guorui Feng, Erol Yilmaz, Yanna Han, Zhe Wang, Shanyong Wang (2025). Degradation mechanism of coal pillars in an underground coal gasification environment: Bearing capacity, pyrolysis behaviour and pore structure. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.05.002
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Frequently Asked Questions
What is the critical temperature for severe degradation of coal pillars in UCG?
The critical temperature threshold is 500 °C, above which the bearing capacity of coal pillars degrades significantly, with strength and elastic modulus reductions up to 45.53% and 61.34%, respectively.
How does high temperature affect the chemical structure of coal?
High temperatures decrease the quantity of oxygen-based functional groups, increase aromaticity and the degree of graphitization, leading to dislocation and slip inside the coal crystal nucleus and deformation of molecular structural units.
What is the impact of high temperature on coal porosity?
High temperatures cause a great increase in porosity due to pyrolysis and microstructural changes, which contributes to increased stress deformation and a transition from brittle to ductile failure.
Why is studying coal pillar degradation important for UCG?
Coal pillars are critical supporting structures between gasifiers. Understanding their degradation mechanism under high temperatures is essential to ensure gasifier stability, prevent roof collapse and water permeation accidents, and support rock strata control.
What experimental methods were used to study coal degradation?
The study employed uniaxial compression and acoustic emission equipment for strength testing, and XRD, SEM, TG, FTIR, and CT tests for characterizing pyrolysis and microstructure evolution.
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