Study On The Evolution Law Of Pore And Fissure Structure Of Long Flame Coal After Superheated Steam Pyrolysis

With the continuous increase in energy demand,the shortage of conventional energy is increasing day by day,and the energy structure is gradually shifting to low-carbon.China has rich reserves of low-metamorphic coal seams.Aiming at the difficulty of mining low-metamorphic coal seams and low utilization value,the team of Academician Zhao Yangsheng of Taiyuan University of Technology proposed the theory of in-situ thermal steam injection mining,based on the background of energy transition in Shanxi and based on the theory of in-situ modification In this paper,the long-flame coal in low-metamorphic coal is the research object,and the pore and fissure structure of the long-flame coal in the pyrolysis of long-flame coal with superheated water at different temperatures is analyzed through low-temperature liquid nitrogen experiment,high-pressure mercury injection experiment,fine micro-CT scanning and numerical simulation methods.After the research,the following main conclusions are obtained:(1)According to the low-temperature liquid nitrogen test and the highpressure mercury intrusion test,pyrolysis can be divided into three stages.The first stage is the temperature range of 25?-300?.In this stage,as the pyrolysis temperature increases,more pores appear In the semi-closed state,insufficient pyrolysis results in poor connectivity between the pores;the second stage is the temperature range of 300°C-400°C,at this stage the organic matter inside the long flame coal sample is gradually pyrolyzed,and the pores in the semi-closed state Gradually transformed into semi-open or open pores,the increase of the hysteresis loop in the test curve indicates the increase in the degree of openness of the pores;the third stage is the temperature range of 500?-550?,and the hysteresis loop in the test curve at this stage is the most Obviously,open pore types dominate,and the existence of open pores proves that the higher the pyrolysis temperature,the better the development of pore channels.(2)Comprehensive analysis of the pore size distribution law under different test methods.When the pyrolysis temperature is lower than 300?,there is no obvious change in the pore specific surface area,pore volume and other parameters,and the pore size distribution is relatively uniform with little difference;When the solution temperature reaches 400?,the irregularity of the pore size distribution is obvious,the specific surface area and pore volume parameters of micropores to small pores decrease,and the volume of mesopores and macropores increase,and the difference between the pore diameters is significant,indicating the effect of pyrolysis.The lower part of the microporessmall pores transforms into mesopores-macropores;when the pyrolysis temperature is higher than 500?,this phenomenon becomes more obvious.Hightemperature pyrolysis is conducive to the transformation of micropores to macropores and cracks.(3)Calculate the fractal dimension of the long flame coal samples after different pyrolysis temperatures.It is found that the pores in the range of 0-10000 nm have obvious fractal characteristics.The fractal calculation results show that the higher the pyrolysis temperature,the more complete the pore development.At the same time as the gas adsorption capacity is reduced,the flow capacity is enhanced,and the expansion of the pyrolysis channel not only increases the pyrolysis area,but also provides convenient conditions for the migration of pyrolysis products and steam.(4)The 300 ? pyrolysis temperature is defined as the pyrolysis threshold temperature point of long flame coal.When the pyrolysis temperature is lower than 300 ?,the free gas and moisture loss inside the sample and the insufficient pyrolysis of organic matter are micropores— The development of small pores provides conditions and increases the gas adsorption capacity inside the pores;when the pyrolysis temperature is higher than 300°C,the full pyrolysis of organic matter increases the development of pores,and the pores with poor connectivity are gradually opened and penetrated.(5)The results of the fine micro-CT scan show that when the pyrolysis temperature is higher than 400?,the two-dimensional CT image and the threedimensional spatial structure of the pores will have cracks that penetrate the horizontal bedding.The horizontal bedding of the long flame coal sample The directional and vertical layering direction fluid migration channel simulation results show that when the pyrolysis temperature is 400 ?,the threshold temperature of the crack penetration in the horizontal layering direction is obtained.When the pyrolysis temperature is higher than 400?,the horizontal layering direction will produce a through crack network.,To form a flow channel,when the pyrolysis temperature is higher than 500 ?,the vertical layering direction will form a crack channel penetrating the sample volume to form a stable and continuous flow channel...