Molecular Dynamics Simulation of Coal Structure Damage Under the Action of Surfactant

An anionic surfactant, sodium dodecyl sulfate (SDS) was used to modify the coal structure. This was done to improve the compactness of the coal structure, promote the damage of coal structure, improve the efficiency of gas drainage, and prevent shock pressure disasters. The mercury intrusion experiment and uniaxial compression experiments were used to determine the changes in the pore structure and mechanical properties of coal after modified by surfactant. This work established six groups of water / surfactant / coal simulation systems with different concentrations. Based on the energy behavior and dynamics characteristics ( interaction energy, relative concentration distribution, radial distribution function, mean square displacement) of each system, the effects of surfactants with different concentrations on the structural damage of coal were analyzed by molecular dynamics simulation, and the mechanism of coal structural damage was revealed. The results show that the SDS solution can significantly reduce the mechanical strength of the coal. When the solution concentration is 0.6%, the degree of damage to the coal structure is the maximum. SDS molecules can be detected at the water / coal interface. SDS molecules are adsorbed to the coal surface through intermolecular interactions, and -SO3 groups are preferentially adsorbed to the oxygen-containing functional groups on the coal surface. The difference in SDS adsorption on the coal surface is caused by the difference in the number and spatial distribution of alkyl chains in the SDS molecule. The main modification mechanisms of surfactants on coal are that when SDS is adsorbed on the coal surface, a large number of secondary pores and cracks are generated on the surface and inside of the coal, and cracks are formed under the action of tensile stress. The cracks continue to expand, extend, which ultimately promotes damage to the coal structure. The results are expected to provide a theoretical basis for the structure damage of coal modified by surfactant, and provide a new method for the prevention of rockburst disasters and gas outburst control.

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.

Experimental study on variation law of electrical parameters and temperature rise effect of coal under DC electric field

Joule heats which are generated by coals in an applied electric field are directly correlated with variation resistivity of electrical parameters of coals. Moreover, the joule heating effect is closely related with microstructural changes and relevant products of coal surface. In the present study, a self-developed applied direct current (DC) field was applied onto an experimental system of coals to investigate variation resistivity of electrical parameters of highly, moderately and lowly metamorphic coal samples. Moreover, breakdown voltages and breakdown field intensities of above three coal samples with different metamorphic grades were tested and calculated. Variation resistivity of electrical parameters of these three coal samples in 2 kV and 4 kV DC fields were analyzed. Results show that internal current of all coal samples increases continuously and tends to be stable gradually after reaching the “inflection point” at peak. The relationship between temperature rise effect on anthracite coal surface in an applied DC field and electrical parameters was discussed. The temperature rise process on anthracite coal surface is composed of three stages, namely, slowly warming, rapid warming and slow cooling to stabilize. The temperature rise effect on anthracite coal surface lags behind changes of currents which run through coal samples. There’s uneven temperature distribution on anthracite coal surface, which is attributed to the heterogeneity of coal samples. In the experiment, the highest temperature on anthracite coal surface 65.8 ℃ is far belower than the lowest temperature for pyrolysis-induced gas production of coals 200 ℃. This study lays foundations to study microstructural changes and relevant products on coal surface in an applied DC field.