A mathematical model of gas flow during coal outburst initiation
|نوع نگارش مقاله||
scopus – master journals – JCR
۴٫۲۷۶ در سال ۲۰۲۰
۲۶ در سال ۲۰۲۱
۰٫۹۰۱ در سال ۲۰۲۰
|شاخص Quartile (چارک)||
Q1 در سال ۲۰۲۰
خرید محصول توسط کلیه کارت های شتاب امکان پذیر است و بلافاصله پس از خرید، لینک دانلود محصول در اختیار شما قرار خواهد گرفت و هر گونه فروش در سایت های دیگر قابل پیگیری خواهد بود.
فهرست مطالب مقاله:
A mathematical model of gas flow during coal outburst initiation
A proposed concept of outburst initiation examines the release of a large amount of gas from coal seams resulted from disintegrating thermodynamically unstable coal organic matter (COM). A coal microstruc- ture is assumed to getting unstable due to shear component appearance triggered by mining operations and tectonic activities considered as the primary factor while COM disintegration under the impact of weak electric fields can be defined as a secondary one. The energy of elastic deformations stored in the coal microstructure activates chemical reactions to tilt the energy balance in a ‘‘coal–gas” system. Based on this concept a mathematical model of a gas flow in the coal where porosity and permeability are changed due to chemical reactions has been developed. Using this model we calculated gas pressure changes in the pores initiated by gas release near the working face till satisfying force and energy criteria of outburst. The simulation results demonstrated forming overpressure zone in the area of intensive gas release with enhanced porosity and permeability. The calculated outburst parameters are well combined with those evaluated by field measurements.
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The development of coal deposits is associated with deepening the underground excavations where numerous coal and gas out- bursts occur thus causing miners’ injuries and deaths accompanied by significant economic damage as well [۱–۳].
The outburst concept prevailing in mining sciences for the last six decades regards rapid changes of stresses near the boundary of a gas-bearing rock as the main cause of this gas dynamic phe- nomenon. The best known theory of V. Khodot describes the out- burst mechanism as a mutual interaction of numerous factors including rock pressure, cracking, gas desorption, increasing coal permeability, changing physical and mechanical coal properties, emerging zones with a high gas pressure gradient, and converting the elastic energy of coal to the dynamic energy of moving parti- cles and fragments [4,5]. Later updates and modifications of this theory were presented, particularly, in studies by Petrosyan et al. [6–۸].
Recent relevant studies have employed more sophisticated models. The concept, outlined in study by Jin et al., is based on a set of nonlinear partial differential equations with the structure forms, material properties, load patterns, and abutment pressure as the factors having much impact on outburst initiation . The
model, presented in study by Chen et al., describes the outburst mechanism according to the mass and momentum conservation principles of the breakdown section in the ‘‘coal–rock” system; and it proposes necessary conditions for extrusion and dump of coal and gas from the frontage tunneling work .
Last years a series of numerical models coupling stresses and gas flow have been developed. The scale and computation stability of the dynamic system ‘‘coal–gas” were investigated by Fan et al. . The stress simulation program FLAC3D and the gas simulation program SIMED II were employed in study by Li and Saghafi to cal- culate a gas flow under typical stress regimes in gassy mines in Australia and China . A theoretical gas-solid coupled model developed in COMSOL Multiphysics software took into account the influence of ground stress, gas pressure, and mining depth . A model, presented by Xue et al., described outburst initiation in roadway excavation coupling coal deformation, pore pressure, principal stress vector redistribution, and yield and tensile failure zones . Three stages of outburst evolution discriminated in study by Choi and Wold M can be listed asfollows: pre-initiation with quasi-static deformation behavior, initiation as the moment of sudden conversion of this behavior into dynamic, and post- initiation with intensive gas and coal fragment ejection . This stage of outburst was simulated by using Ansys Flotran program based on the relations of fluid and gas mechanics applied to mov- ing gas-solid mixture . The role of gas desorption triggered by effective stresses ahead of the working face during the process of initiating outburst and changing porosity and permeability was examined in details in study by Zhi and Elsworth .
Statistical analysis as the alternative approach is applied for deriving the criteria for evaluation to assess outburst hazard under various mining and geological conditions. Based on the analysis of 90 outburst occurrences in Donbas and Vorkuta coal basins (former USSR) the criterion proposed by Feit related the change of gas internal energy with the stress, bulk density, compressive strength, and seam inclination . The latest studies have employed vari- ous methods including cluster and time series analysis to identify the trends and anomalies in outbursts occurred in Hexi colliery (China), a pattern recognition to evaluate the risk class based on the set of attributes characterizing safety conditions at the mine, clustering theory and multi-objective classification to optimize outburst prevention measures at Jinzhushan Tuzhu Mine (China), ANOVA and contingency table analysis of the major factors causing outbursts, coupled sampling with computer modeling, and the geophysical approach combining the relations of geomechanics with the method of acoustic emissions [19–۲۴].
The experimental studies revealed the critical importance to enhancing porosity and permeability in outburst initiation depending on stress, desorption, and coal destruction. Particularly, the results of desorption tests on coal specimens from West Cliff colliery (Australia) showed much lower permeability values in comparison with the values obtained while testing permeability on the same samples due to the impact of gas pressure and rock stress . The study carried out on the samples from Songzao coalfield (China) has identified slow flow rates in the intact coal and the increase of the gas flow rate in the damaged coal propor- tional to the square of a pressure gradient under a low effective stress .
The review of the models and methods to predict outburst allows drawing the following conclusions.
The vast majority of the models applying a ‘‘mechanistic” approach identified mechanically induced desorption as the only reason to cause the gas release and proper attention was not paid to physicochemical transformations in coal under various impacts resulted in critical changes of pressure, porosity, and permeability. Formerly, this approach played a positive role in understanding the driven forces of outbursts and preventing the accidents in mines. Nevertheless, a large number of outbursts occurred in the coal seams considered not to be hazardous by ‘‘mechanistic” models.
To overcome insufficient reliability of these models in terms of outburst predictions various statistical methods were employed and put into mining practice thus giving possibility to establish correlations between outburst hazard and major mining and geo- logical factors. However, the applications of statistical methods are site-limited and do not allow studying the mechanism of out- burst initiation and evolution.
Therefore, the existing theories and models should be updated and the description of physicochemical factors having influence on critical coal properties is recommended to be added. This study aims to develop a numerical model of a gas flow in the coal during outburst initiation where physical-chemical transformations in the ‘‘coal–gas” system are taken into proper account.
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