Study On The Evolution Deformation And Seepage Characteristics Of Long Flame Coal Coupled By Thermo-mechanical Effect

Long flame coal is a kind of bituminous coal with the lowest metamorphic degree,which is second only to lignite.Long flame coal has high moisture content,high ash content,high volatile content and low calorific value.At present,it is mostly used for thermal power generation and processing chemical products,but the residues of thermal power generation and chemical processing are easy to cause a series of environmental problems such as water pollution and soil pollution.However,China has a large demand for coal and abundant reserves of low metamorphic coal,and the efficient and clean utilization technology of low metamorphic coal is in urgent need of breakthrough.The thermal deformation of coal seams under thermodynamic coupling is of great importance to the evaluation of rock strata movement and deformation in high temperature underground engineering,such as underground gasification,direct liquefaction and in-situ thermal injection of low metamorphic coal.Therefore,the Key Laboratory of In-situ Modified Mining,Ministry of Education,Taiyuan University of Technology,has put forward "In-situ heat injection fluidized clean mining technology for low metamorphic coal".A series of basic studies have been carried out around this topic.Through the experimental study on the mechanical properties and seepage characteristics of Shanxi typical low metamorphic coal--long flame coal under high temperature triaxial stress,the following conclusions have been drawn:(1)Under the hydrostatic stress of 5MPa,7.5MPa and 12.5MPa,the thermal deformation of long-flame coal at 500℃ is generally expansive deformation,and the deformation is-1.131 mm,-0.794 mm and-0.619 mm,respectively.Under the thermodynamic coupling action,the deformation of coal samples is combined by thermal expansion and pyrolysis,which can be divided into three stages according to the dominant factors: coal thermal expansion leading stage(RT-250℃),thermal expansion and pyrolysis coexistence stage(250-450 ℃),and pyrolysis leading stage(450-500 ℃).The thermal expansion coefficient can be divided into three stages with the increase of temperature: initial increase stage,sharp increase stage and decrease stage.The quantitative relationship between temperature and thermal expansion coefficient is established by piecewise polynomial fitting.(2)The characteristics of the full stress-strain curve of long-flame coal under the action of thermodynamic coupling are mainly affected by temperature.When the temperature is 300℃,the long flame coal has gone through four stages: initial stress loading stage,linear elastic deformation stage,plastic deformation stage and failure stage.When the temperature is between 300℃ and 450℃,the total stress-strain curve of long-flame coal can be divided into three stages: initial stress loading stage,plastic deformation stage and plastic failure stage.When the temperature reaches 500℃,the coal sample only goes through the initial stress loading and plastic stage in the full stress-strain process.Under the continuous action of axial stress,the coal sample will not be damaged.In the low temperature stage,the increase of hydrostatic stress makes the bearing capacity and elastic modulus of coal samples increase.With the increase of temperature,the plasticity of coal samples increases and the peak strength increases.(3)Under the action of thermodynamic coupling,the deformation of long-flame coal is characterized by continuous thermal expansion.As a result,the permeability of coal sample decreases with the increase of temperature under the action of temperature.When the temperature reaches 400℃,the permeability of coal sample decreases to 0.Due to the relatively developed pores and cracks in the long-flame coal,its permeability is relatively high at room temperature.With the increase of temperature,the permeability of coal samples decreases rapidly at first and then decreases slowly,showing a negative exponential function relationship between permeability and temperature.With the increase of static water stress,the pore cracks of the coal sample are closed.The higher the stress is,the smaller the permeability is.The permeability of the coal sample has an exponential function relationship with the confining pressure.With the increase of pore pressure,the flow rate of gas through the coal sample increases,but the permeability decreases,and the relationship between permeability and pore is exponential.Based on the above factors,the quantitative equations of temperature,hydrostatic stress,pore pressure and permeability were established by the least square method.The results show that the influence of temperature and porosity on the permeability of coal sample is much greater than that of hydrostatic stress at low temperature stage.In the high temperature stage,the permeability of coal sample is mainly affected by hydrostatic stress,and the permeability of 12.5MPa is one order of magnitude lower than that of 5MPa and 7.5MPa.