Meitan xuebao (Mar 2024)
Deformation energy of tectonic coal and its role in coal and gas outbursts
Abstract
Coal and gas outburst is an underground dynamic disaster driven by coal deformation energy and gas expansion energy. Although the comprehensive effect hypothesis of outbursts has been widely recognized, the role of coal deformation energy in outbursts has often been underestimated compared to that of gas expansion energy. To determine whether coal deformation energy can be neglected in outbursts, a systematic review and discussion on the work by Hodot and Zheng (representative achievements in the field of outburst energy) was conducted. It was found that Hodot’s energy criterion for outburst triggering is based on coal deformation energy, and Zheng’s magnitude comparison results cannot be considered as evidence for neglecting coal deformation energy. Most coal and gas outburst accidents occur in tectonic coal seams. In order to reveal the specific contribution of the tectonic coal’s deformation energy in outbursts, various cyclic loading tests on coal samples and triaxial failure tests with synchronized acoustic emission monitoring were conducted. The experimental results showed that unlike the linear and small deformation characteristics of intact coal, the stress-strain curve of tectonic coal exhibited a nonlinear and large deformation behavior. The relationship between coal deformation energy and stress no longer followed a square law. Based on the critical state model in soil mechanics, a theoretical calculation model of deformation energy suitable for the tectonic coal with nonlinear characteristics was developed. This model reflected the power function relationship between coal deformation energy and stress. It was determined that the power exponent of tectonic coal was mainly between 1.1 and 1.3, while the power exponent of intact coal was mainly between 1.7 and 1.9. This further indicated that the properties of tectonic coal were more similar to soil, while the properties of intact coal were closer to those of ideal elastic material. Although tectonic coal exhibited greater deformation energy under the same stress, it released very little external energy after instability, as evidenced by the absence of significant acoustic emission during the process of damage and fragmentation. The release of deformation energy in tectonic coal increased the coal porosity, provided conditions for rapid desorption of gas, and controlled the release of gas expansion energy. The study of gas expansion energy should focus on the gas performing work during outbursts, which refers to the gas released from the coal fragments in a short period of time. Further experimental and numerical analysis results showed that the gas expansion energy involved in outbursts was comparable in magnitude to the deformation energy of tectonic coal, ranging from 102 to 103 kJ/t. The release of coal deformation energy and gas expansion energy exhibited temporal and causal relationships. During the preparation and triggering stages of outbursts, the pre-release of coal deformation energy was a necessary condition for the accumulation of gas expansion energy. Outbursts are controlled by both coal deformation energy and gas expansion energy, and they have equal importance in the outburst process.
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