Experimental Research on Coal and Gas Delay Outburst and AE Characteristics under Conditions of Geostress and Gas Pressure Disturbance

Adopting yellow mud as barrier layer materials, coal and gas delay outburst experiments under conditions of geostress and gas accumulation disturbance were carried out by using self-developed simulation system, to find out roles of geostress and gas pressure played in the process of the delay outburst and ways to predict it, through analysis of variations of gas pressure, and AE characteristics during the process. The results show that after the geostress increased by 0.11 MPa from 1.80 MPa, an outburst occurs, while in gas accumulation situations, the gas pressure increase of 0.27 MPa from 0.67 MPa induces an outburst; hence, geostress is one of the dominant factors impacting an outburst occurrence. The lasting time of the outburst triggering under geostress disturbance is shorter than that under gas accumulation disturbance, while the duration of the outburst development under gas accumulation conditions is longer than that under geostress conditions. Coal seam breakage by geostress is the precondition for an outburst risk, and gas expansion energy is the dominant parameter influencing the duration of the outburst development. The AE signals show distinctive features in different stages of the outburst under geostress disturbance. At the preparation stage of the outburst, the AE signals increase sharply but have a low intensity and then drop to a lower balance level. At the triggering stage, the AE signals become active and increasing until up to the peak where the outburst occurs, and the intensity is highest.

Dynamic Characterization during Gas Initial Desorption of Coal Particles and Its Influence on the Initiation of Coal and Gas Outbursts

The law of gas initial desorption from coals is greatly important for understanding the occurrence mechanism and predicting coal and gas outburst (hereinafter referred to as ‘outburst’). However, dynamic characterization of gas initial desorption remains to be investigated. In this study, by monitoring the gas pressure and temperature of tectonically deformed (TD) coal and primary-undeformed (PU) coal, we established the evolution laws of gas key parameters during the initial desorption. The results indicate that the gas pressure drop rate, mass flow rate, initial desorption rate, and gas velocity increase with increasing gas pressure, with stronger gas dynamic effect, generating a high pressure gradient on the coal surface. Under the same gas pressure, the pressure gradient formed on the TD coal surface is greater than that formed on the surface of the PU coal, resulting in easily initiating an outburst in the TD coal. Moreover, the increased gas pressure increases temperature change rates (falling rate and rising rate) of coal mass. The minimum and final stable temperatures in the TD coal are generally lower compared to the PU coal. The releasing process of gas expansion energy can be divided into two stages exhibiting two peaks which increase as gas pressure increases. The two peak values for the TD coal both are about 2–3 times of those of the PU coal. In addition, the total gas expansion energy released by TD coal is far greater than that released by PU coal. The two peaks and the total values of gas expansion energy also prove that the damage of gas pressure to coal mass increases with the increased pressure, more likely producing pulverized coals and more prone to initiate an outburst.

Gas Expansion Energy Model and Numerical Simulation of Outburst Coal Seam under Multifield Coupling

Due to the insufficient understanding of the outburst mechanism, the coal and gas outburst disasters in China are more serious. Gas expansion energy is the main source of energy that causes outburst. In order to explore the distribution law of gas expansion energy in outburst coal seams, a gas-solid coupling equation of outburst coal seams was established. The distribution law of coal stress field, deformation field, gas flow field, and gas expansion energy were simulated and analyzed by using COMSOL Multiphysics. The results showed that from the excavation face to the deep part of coal seam, the stress presented unloading zone, stress concentration zone, and original stress zone. The volumetric strain and permeability reached the minimum, while the gas pressure reached the maximum at the peak value of vertical stress. As time goes on, the gas pressure in the fracture near the working face gradually decreased and was less than the pressure in coal matrix. The total gas expansion energy consists of free gas and desorption gas expansion energy. Affected by the excavation, free gas expansion energy maintained a constant value in the original coal seam and gradually decreased in the area close to the working face. The expansion energy provided by desorption gas was zero in the original coal seam. And it first increased and then decreased rapidly near the working face. Compared with stress and coal seam thickness, gas pressure and initial diffusion coefficient had significant influence on gas expansion energy of coal seam. When the diffusion coefficient was greater than 1e-9 m2/s, the gas expansion energy of the coal seam near the working face was significantly higher than that of the original coal seam, which had the risk of inducing outburst.

The Decoupling Cosmology Theory

By putting forward the basic hypothesis "Energy Possesses No Gravitation", the energy equation and the motion equation of each stage of the development and evolution of the cosmos are obtained by solving the equivalence principle integrally in the flat space or inertial coordinate system. By comprehensive studying the energy equation and motion equation, the expressions of the cosmic critical scale and the initial cluster of nebulae critical scale (namely the galaxies critical scale), were found and given; During the process of matter and energy decoupling, about 3.3 billion initial cluster of nebulae of the critical scale were generated, and the cosmic scale was at least "Expand Expansion (2E)" at 1,300 times long, and the isotropic homogenization was basically realized above the critical scale; by assuming that the cosmos was born in the "Resonance" of the quantum fluctuation of static photons, deriving the early cosmos grew up at the speed of light and produced elementary matter particles at the speed of light, giving the early cosmos "Light-speed Growth Theory (LGT)" without the Big Bang, inflation and singularity theory; It was found that the rotation during the expansion of the gravitational field of the gas matter was the origin of rotational motion and the de-homogenization of anisotropy in galaxies; It was found that the plasma gas photon decoupling energy was the dark energy of accelerated expansion of the cosmos, the missing mass is the illusion of photon decoupling energy beyond the critical scale, the expressions of the photon decoupling energy and the ultimate expansion velocity of the cosmos are derived, and the exact values which are in good agreement with the Hubble constant were given; proposed and proved through multiple evidences that isotropic homogeneous matter field possesses no gravitation; the Hubble constant and cosmic age expression were derived, and the main parameters of the cosmos, such as the critical scale, were given according to the estimation of the relevant motion equation; By macro energy conservation of the cosmos, it was concluded that the gravitational potential energy was the contraction potential formed after the cosmic thermal expansion energy was converted into inertial rotation energy, the thermal expansion energy, the inertial rotation energy and the gravitational potential energy were transformed in turn and equivalent, and predicts that neutrino thermal expansion energy is the main energy of the formation of gravitational potential energy. Finally, by comparing Friedman-Lemaitre solution equal to the energy equations of the various periods of the cosmos, the expressions of curvature constant K and cosmic factor Λ were given, and the "Phenomenon of Gravitational Lens" is explained by the experiment of "Pseudo-gravitational Lens Effect".The core of this theory is through the basic assumption of "Energy Possesses No Gravitation", to separate matter and energy from their motion in coupled study of gravitational field, so it can be named "The Decoupling Cosmology Theory (DCT)". This theory can be supported by Hubble constant and Cosmic Microwave Background (CMB), and can also be verified by the nuances of the isotropic in different directions of the cosmos, the dark matter and critical scales in the galaxies, and the experiments of "Pseudo-gravitational Lens Effect" etc.

Influence of Combined Action of Steel Fiber and MgO on Chloride Diffusion Resistance of Concrete

To improve the chloride diffusion resistance and durability of concrete, a new kind of steel fiber reinforced MgO concrete (SFRMC) was made by adding steel fiber and MgO to concrete simultaneously. With steel fiber for load bearing and expansion limiting, MgO as the expander, SFRMC has both the advantages of fiber reinforced concrete and expansion concrete. The influence of steel fiber and MgO on the strength and chloride diffusion resistance of concrete was evaluated by splitting tensile test and chloride diffusion test. Mercury intrusion porosimeter (MIP) and scanning electron microscopy (SEM) were used to study the microstructure of SFRMC. The results showed that the combined action of steel fiber and MgO reduced the porosity of concrete and the chloride diffusion coefficient (CDC), which could not be achieved by steel fiber and MgO separately. In the free state, the expansion energy produced by the hydration of MgO made the concrete expand outwards. However, under the constraint of steel fiber, the expansion energy was used to tension the fiber, resulting in self-stress. In this way, compared to reference concrete RC, the tensile strength of SFRMC-1, SFRMC-2, and SFRMC-3 increased by 3.1%, 61.3%, and 64.5%, CDC decreased by 8.8%, 36.7%, and 33.1%, and the porosity decreased by 6.2%, 18.4%, and 20.6%, respectively. In addition, the SEM observations demonstrated that the interfacial transition zone (ITZ) between fiber and matrix was denser in SFRMC, which contributed to reduce the diffusion of chloride ions in the concrete.