Study On Characterization And Release Mechanism Of Coal And Gas Outburst Energy

Coal and gas outburst is a kind of underground dynamic disaster driven by the coal deformation energy and the gas expansion energy.Although the comprehensive effect hypothesis of outbursts has been widely recognized,the contribution of coal deformation energy in outbursts is often underestimated compared to gas expansion energy.A thorough characterization of outburst energy,exploration of the intrinsic relationship between outburst energies,and analysis of the release mechanism of outburst energy are of great scientific and engineering significance.This dissertation focuses on the control of deformation energy on coal and gas outbursts,based on theories such as rock mechanics,critical state soil mechanics,rock fragmentation,thermodynamics,gas dynamics,and adsorption science.Through a combination of experimental research and numerical analysis,a deformation energy calculation model that considers the nonlinear characteristics of tectonic coal and a gas expansion energy calculation model based on diffusion theory were constructed and validated.The magnitudes of tectonic coal deformation energy and gas expansion energy were compared.The release mechanism of tectonic coal deformation energy was then studied,and the intrinsic relationship between deformation energy release and gas expansion energy release was further discussed.Finally,taking the driving roadway as the research object,the principle of deformation energy release during the outburst preparing and triggering stages was clarified,and the necessity of pre-releasing deformation energy for the accumulation of gas expansion energy and the triggering of outbursts was demonstrated.The main conclusions are as follows:1)A deformation energy calculation model that considers the non-linear characteristics of tectonic coal was constructed and validated.The model innovatively reflects the power function relationship between coal deformation energy and stress,and determines that the power exponent for tectonic coal is mainly between 1.1 and 1.3,while that of intact coal is mainly between 1.7 and 1.9,further indicating that the properties of tectonic coal are more similar to those of soil,while the properties of intact coal are closer to those of ideal elastic bodies.The experimental results demonstrate that even with deviatoric stress,the energy-stress relationship can still be described by a power function calculation model,but the characteristic stress in the model needs to be changed.Under the same bulk stress conditions,the deformation energy of tectonic coal is always greater than that of intact coal.As stress increases,the deformation energy ratio of tectonic coal to intact coal gradually decreases.2)A gas expansion energy calculation model based on the diffusion kinetics theory was established.The experimental and numerical analysis results show that the gas expansion energy involved in work during outbursts is of a similar order of magnitude to the deformation energy of tectonic coal,ranging from 10~2 to 10~3 k J/t.For the important calculation parameter in the model,the diameter averaged by gas desorption speed,a new definition is given in this dissertation,and a calculation method for the diameter averaged by gas desorption speed is proposed based on the assumption of fractal particle size distribution.Summarizing the outburst coal’s particle size distribution data in existing literature,it is believed that the gas-desorption average diameter for outburst coal is around 1 mm.The inconsistency between the apparent diameter and the gas-desorption average diameter of a single coal particle is an important reason for the deviation of the scale effect of initial gas desorption speed from the theoretical law and the change in diffusion coefficient with particle size.3)The mechanical path,mechanical conditions,and energy conditions for the release of deformation energy of tectonic coal were obtained.During the outburst preparing process,the release of coal deformation energy is caused by mechanical structural damage.The coal undergoes loading and unloading damage processes successively.The mechanical conditions for the release of deformation energy of intact coal are higher than those for tectonic coal.For the experiments in this dissertation,the strength ratio of intact coal to tectonic coal under uniaxial conditions is 11.76.The ratio decreases with the increase of confining pressure.When the confining pressure is 20MPa,the ratio decreases to 2.28.Coal will only release energy after reaching the energy storage limit,and the relationship between the energy storage limit and the linear yield criterion is established.4)The control effect of deformation energy release on gas expansion energy release was revealed.The external release of energy increases the coal porosity and the internal dissipative energy provides conditions for the rapid desorption of gas,both of which control the release of gas expansion energy.The release of deformation energy in tectonic coal has strong dissipative characteristics.The intensity of the acoustic emission signal generated during the full stress-strain process of intact coal is 2 to 4orders of magnitude higher than that of tectonic coal.The cyclic loading and unloading experiment results show that intact coal generates very little dissipative energy during the loading stage before breakage(dissipative energy/input energy≈0.09),while tectonic coal generates dissipative energy as soon as it is loaded(dissipative energy/input energy≈0.84).Through experimental statistical relationships,a model for the evolution of pre-peak/post-peak porosity,permeability,and particle size distribution of coal under stress control was established,quantitatively characterizing the impact of deformation energy release on expansion energy of free gas and potentially desorbed gas.5)The principle of deformation energy release during the preparing-triggering stage of outbursts has been revealed.Based on the mathematical representation model of outburst energy obtained in this dissertation,a calculation method for the distribution of coal deformation energy and gas expansion energy in front of the working face was established,and the distribution characteristics of coal deformation energy and gas expansion energy in front of the working face were obtained.The release amount of coal deformation energy under three different instability conditions during the outburst preparing stage was theoretically analyzed,indicating that the amount of coal deformation energy release when uncovering the coal seam was about 1.56 to 2.22times that of conventional mining,and 1.47 to 3.01 times that of coal creep failure,providing an explanation for the high outburst risk of uncovering the coal seam from the perspective of deformation energy release.6)The necessity of pre-releasing deformation energy for outburst triggering has been demonstrated.During the preparing-triggering stage of outbursts,the pre-release of coal deformation energy is a necessary condition for the accumulation of gas expansion energy.After the release of deformation energy in the tectonic coal for a short period of time,although the porosity increases,the replenishment rate of free gas in the pores is greater than the escape rate,resulting in an abnormal increase in the free gas expansion energy and a significant increase in the potential desorption gas expansion energy due to the reduction of matrix scale,providing necessary energy for outburst triggering.The outburst is jointly controlled by coal deformation energy and gas expansion energy,and they play equal important roles in the coal and gas outburst.This dissertation has 145 figures,43 tables and 311 references.