Impacts of the caking property of tar-rich coals on their pore structure evolution and seepage during pyrolysis
[Objective]The in-situ pyrolysis of tar-rich coals stands as a technique whereby coal seams are heated under-ground to produce tar and gas products.During the in-situ pyrolysis,the injection of heating media and the production of tar and gas are affected by the pore structure and permeability of coal seams.Since pyrolysis of caking tar-rich coals is accompanied by the formation of colloids,the pore structure and permeability of caking tar-rich coals differ from those of non-caking tar-rich coals.[Methods]Caking tar-rich coals were pyrolyzed at 300,400,500,and 600℃.During the pyrolysis,the pore parameters of the semi-coke were tested using the saturated fluid method and nitrogen adsorption method,and the pore structure of the semi-coke was characterized using micro-computed tomography(micro-CT).Meanwhile,using equivalent pore network models established in this study,the laws of changes in parameters such as the pore number,pore radius,and coordination number were analyzed,and the seepage characteristics of high-temperat-ure nitrogen in the pore network were simulated.[Results and Conclusions]The results revealed small numbers of fis-sures in the coal samples after the pyrolysis at 300℃,with a total porosity maintained at about 5%.After pyrolysis at temperatures ranging from 400℃to 600℃,the total porosity gradually increased to about 50%,while micropores be-came more abundant only after degassing at 600℃.When the pyrolysis temperature rose from 300℃to 400℃,the number of pores and throats increased significantly due to the formation and expansion of colloids,whereas the average pore and pore-throat radii were maintained at about 160 μm and 88 μm,respectively.As the pyrolysis temperature in-creased further from 400℃to 600℃,the precipitation of volatile constituents promoted the pore connectivity.Accord-ingly,pores and throats gradually decreased in number,with equivalent radii trending upward in terms of probability dis-tribution.Specifically,the average pore and pore-throat radii increased to 292.81 μm and 170.60 μm,respectively,and the average coordination number of pores increased from 5.82 to 6.60(500℃)and 6.33(600℃).The increasing poros-ity and coordination number led to a significant increase in the average simulated permeability of the semi-coke from 246.75 μm2 to 1 377.49 μm2.The findings of this study can serve as a reference for the R&D of the in-situ pyrolysis pro-cess of caking tar-rich coals.
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