| The coalfield fires burning around the world are an environmental catastrophe, whichhave a large distribution range and serious degree in China. According to investigation, thereare more than200coalfield fires with the total area of720km2in seven coal-rich provinces orautonomous regions included Xinjiang, Gansu, Qinghai, Ningxia, Shaanxi, Shanxi, and InnerMongolia. This is a complicated kinetics process with multi-field coupling when the largearea coalfield fires are formed by the outcrop coal from self-igniting to fire source burningand developing along its deep part. In this dissertation, the formation and distribution of thefissure field of coal-rock mass in coalfield fires are paid more attention, and its mechanicalcharacteristics of coal-rock mass of coalfield fires with thermo-hydro-mechanical coupling inseepage for fissure field are discussed in order to reveal the reason of coalfield fires developingalong the deep part of the coal seam. The main contents and contributions are summerized asfollows:The coal mass produces a thermal effect by the physics and chemical action of coal andoxygen and forms a thermal storage environment. When the temperature reaches the ignitionone, the coal mass happens burning. If the oxygen is supplied continuously, the burning centerdevelops along the deep part of the coal seam. Simultaneously, the fissure of the coal-rockmass in coalfield fires duo to the thermal destruction is formed by the high temperature, andthe burnt empty area is developed after the coal seam is burning, which can cause overlyingstrata collapse and produce a wide range of fissure field. Under the action of heat-pressure,the fissure field provides the transport channels of supplying oxygen for the new air from the surface to the fire area, which keeps burning coal mass. In the meantime, it also provides theways for emission gases and heat from burning center of coal mass to atmosphericenvironment, accordingly, a circular process is formed. Therefore, this is the development andevolvement process of the coalfield fires, which reveals the rule that the burning center ofcoal mass further develops along its deep part in coalfield fires.Based on the μCT225kvFCB high precision CT system, the experiments of anthraciteand mudstone samples are carried out from the atmospheric temperature to600℃in order toobtain the laws of expansion and distribution of coal and rock mass by thermal destruction.The results indicated that when the temperature at200℃, the fissure in the coal sample isstarted to form and its number is gradually to increase, its width is to grow and expand, itsthreshold of thermal destruction for anthracite sample is about300℃. However, for themudstone sample, the obvious fissures are not found from the atmospheric temperature to600℃, its appearance is pale in color and shape becomes irregularly.The mathematical model with thermo-mechanical coupling for fissure field of coal-rockmass in coalfield fires is established. Based on the Functional Theory, the dimension of thefinite element model is extended from2D to3D, and the control equations are obtained. Therules of rate of thermal destruction of coal and rock mass as rising the temperature areanalyzed by numerical method, and the results indicated that the rates of thermal destructionfor coal sample, rock sample and coal-rock combine sample increase with increasing thetemperatures. At the same temperature, the rate of thermal destruction of coal sample isgreater than that of the rock sample, and the coal-rock combine sample has a phased variationin the interface between the coal and rock. The coal sample takes place the pyrolytic reactionafter300℃and its rate of thermal destruction is23.302%at325℃, however, they are12.992%at320℃and12.6258%at300℃for the rock sample and the coal-rock combine one,respectively.The stress-strain relationships of the coal and rock sample are tested by using the testerMTS815.02under uniaxial, triaxial and temperature conditions, in which the peak stresses aswell as strains in axial and lateral directions are obtained, respectively. The analysis showsthat the peak stresses of coal and rock sample are increased under the action of confiningpressure, however, they are decreased by the effect of thermal stress. On the other hand, thepercolation features of coal sample for all process stress-strain are measured, on which thepermeability of Darcy flow, the permeability of non-Darcy flow, the factor β of non-Darcyand acceleration coefficient are obtained. By comparative analysis, the difference of the actualflow and Darcy is obviously, the permeability of non-Darcy flow is lower than that of the Darcy flow and its pressure gradient is greater, thus, the phenomenon of seepage instabilityoccurs easily. As the burning center of coal-rock mass in coalfield fires goes forward, thenearby coal-rock mass is mostly in the state of post-peak stress by the thermal stress of hightemperature and stress of surrounding rock, however, the larger pressure gradient is formedbetween the coal-rock mass and the burnt empty area because the rock pressure increases dueto coal burning and temperature stress is from the combustion center, which intensifies thepossibility of seepage instability.The mathematical model of seepage in fissure field of coal-rock mass withthermo-hydro-mechanical coupling for the coalfield fires is established. For Wuda coalfieldfire, the process of thermo-hydro-mechanical coupling is numerical simulated on finiteelement method, its the distributions of fields of displacement, the shear, the principal stresses,the principal strain and temperature are obtained when the burning center of coal mass goes todifferent locations along its deep part. While the burning center goes to72-78m, the overlyingstrata fractures, accordingly, a large area collapse fissure field is formed, in which themaximum displacement in vertical direction is0.157m. Additionally, the laws of flow field ofcoal-rock mass are also obtained, which included the oxygen concentration, seepage velocityand pressure fields. |
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