| As one of the three types of traditional energy sources,coal takes an important part of energy construction in our country.With the continuous reduction of high quality coal resources and the increasing demanding for coal resources,the low metamorphic coal,such as lignite,is much more important today.However,due to its higher moisture content,higher volatile content,lower ignition temperature and easier to weathering,there are many restrictions for lignite mining and utilization.It is a major problem for us to utilize lignite cleanly and efficiently.Thus,the concept of lignite in-situ thermal injection mining is put forward.Theoretically,lignite in-situ thermal injection mining is an extremely complex process under multi-field coupling function,such as the stress,temperature and fluid flowing.Therefore,it is necessary to study the physical-mechanical properties of lignite under thermal-mechanical coupling in depth and systematically.In this paper,the deformation characteristics,structure evolution and thermodynamic parameters evolution of lignite,which is taken from Xilinhot of Inner Mongolia Province,have been deeply studied by a series of experiments,theoretical analysis and numerical simulation.The effects of steam temperature and pressure on lignite have been also analyzed.The results and conclusions are as follows:(1)The lignite deformation characteristics are studied with temperature increasing from room temperature to 400 ℃by using DIL402 PC thermal expansion instrument.The experiment results indicate that,under thermal effect,the deformation process of lignite can be divided into three stages: slow deformation stage,rapid compression stage and slow compressed deformation stage.The thermal deformation characteristics of lignite are obviously anisotropic.(2)Based on the experiments of thermal-mechanical coupling effect,the lignite deformation in steam atmosphere has been obtained at 300 m depth stress level.The conclusions are as follows: with trixial compression,the deformation of lignite with temperature increasing can be also divided into three stages: slow expansion stage,rapid compression stage and relatively slow compression stage.At the same steam temperature,the total axial thermal strain of lignite decreases with the increase of steam pressure.At the same steam pressure level,the axial thermal strain of lignite increases with temperature increasing.The lignite axial deformation under constant temperature and force is proved to firstly increase then decrease with temperature increasing.And the axial strain is the largest at 200℃.(3)When the steam pressure is lower,the axial strain for heating stage is larger than that for constant temperature and force stage.When the steam pressure is higher,the axial strain for heating stage turns out to be smaller than that for constant temperature and force stage.That is to say the lignite deformation process is delayed by the effect of steam pressure.At the same steam temperature,the axial strains of heating stage and stable stage both decrease with steam pressure increasing.The experiment results indicate that the total axial strain of stable stage is larger than that of heating stage when the temperature is lower than 200℃,while the total axial strain of stable stage is smaller than that of heating stage when the temperature is higher than 200℃.At the same steam pressure level,the total axial strain of lignite for heating stage is proved to increase with temperature rising while the total axial strain of lignite for stable stage firstly increases then decreases with a transition temperature of 200℃.(4)The lignite radial strain and volume strain are both analyzed by the analysis of Micro-CT images.And the results indicate that the radial strain and volume strain increase with temperature rising while decrease with steam pressure rising.(5)Steam temperature has much more effect on lignite pyrolysis than steam pressure.The weight loss rate of lignite is only about 3% at 100 under trixial ℃stress of 7MPa.The weight loss rate of lignite is as much as 25% at 200℃.The weight loss rate of lignite is 37% at 300℃.At 400 level,the weight loss rate ℃is within a certain range of 40% to 45%.The components dissipation and thermal pyrolysis effect are key factors of lignite weight loss under the coupling function of temperature and pressure.(6)The evolution law of pores and fissures,which is larger than 1.1nm in diameter,has been obtained by using Micro-CT,Mercury injection and liquid nitrogen adsorption under the coupling action of stress,steam pressure and temperature.The results indicate that slit pore and blind pore are the main part of nanometer pores in lignite.The mercury injection curve can be divided into three types,which are reserved ‘S’ curve,the upward convex arc curve and convex mansard upward curve.The results of Micro-CT experiments indicate that the lignite structure evolution becomes more seriously with temperature rising from 100℃ to 400℃.The higher the steam temperature is,the more the steam pressure contributes to lignite porosity evolution.At 100℃,the total porosity of lignite changes a little with steam pressure increasing.At 200℃,the total porosity of lignite increasing from 20.5% to 25.4% with steam pressure rising from 0.1MPa to 5MPa.The total porosity of lignite sharply increases from 11.9% to 32% with steam pressure rising at 400℃ temperature.(7)The lignite specific heat capacity has been obtained under different temperatures by using differential scanning calorimeter.The experimental results indicate that specific heat capacity of raw lignite firstly increases then decreases with temperature rising and an obvious peak value is found at 128.2℃.For dried lignite,the specific heat capacity is proved to slowly increase from room temperature to 200℃.When temperature is over 200℃,the specific heat capacity of heated lignite decreases with temperature rising.Especially,when temperature is higher than 300℃,the specific heat capacity sharply decreases.The peak value of heated lignite specific heat capacity is about 0.882J/(g·K).Based on experimental results,the evolution laws of lignite under different situation are analyzed with piecewise fitting and plotted.The components of lignite are the key factors which have effect on the specific heat capacity.Accouding to the temperature range of components removing,the evolution process of lignite specific heat capacity can be divided into four stages with temperature increasing.(8)The thermal diffusivity has been analyzed by using thermal conductivity analyzer.At the same temperature,the thermal diffusivity of lignite on the direction vertical to the bedding plane is smaller than that on the direction parallel to the bedding plane.With temperature increasing,the thermal diffusivity of lignite first increases and then decreases.The maximum thermal diffusivity is found at about 150℃.The relationshiop between thermal diffusivity and the temperature is fitted as an exponential form.(9)The temperature,stress and displacement characteistics of coal seam with high temperature steam injection have been analized by numerical simulation.The result indicates that the farther the distance away from injection well is,the lower would the temperature of coal seam be.And the temperature field of coal seam is proved to be non-uniform distribution.Moreover,the temperature diffusion in fractured coal seam is faster than that of normal coal seam.With the extension of time,the stress state of coal seam would change from compressive stress to tensile stress.The area of tensile stress in coal seam turns out to be larger with time going on.Similarly,the bump height of ground surface between the injection well and production well would be larger with time going on. |
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