基于不同热解方式下烟煤裂隙结构演化的对比

Comparative study on the evolution of fracture structure of bituminous coal based on different pyrolysis methods

煤炭地下原位热解开采是未来煤炭资源利用的新趋势,探明不同热解方式下煤的孔裂隙结构、空间分布及连通性尤为关键,现有的关于煤热解后孔裂隙结构参数的研究均处于无应力状态且缺乏不同热解方式下孔裂隙结构参数的对比。基于以上不足,利用自制的高温高压热−流体−应力−化学耦合作用的三轴试验机,结合显微CT技术,探究了烟煤在自然、蒸汽原位热解(600℃)条件下的裂隙结构差异,进而就差异性进行了机理探究。结果表明:①自然、蒸汽600℃原位热解后,煤样内部产生的裂隙形态以细长型裂纹及孔洞为主,蒸汽热解后裂隙尺度进一步提升,裂隙网络更为丰富复杂;②在轴压5 MPa、围压3 MPa的应力条件下,烟煤自然热解后裂隙率为原始裂隙率的2.68倍,蒸汽热解后裂隙率为原始裂隙率的3.65倍且每一层位裂隙率更均匀;③蒸汽热解使得烟煤受热均匀,换热面积大,有机质热解更加充分。驱替作用保证热解产物的持续产出;孔隙压力的局部应力作用既扩大了原有的孔裂隙体积又会破坏孔壁弱面,大大增加了孔裂隙连通性;蒸汽剥蚀效应将盲孔及孔裂隙附着的黏稠度较高的焦油“冲刷−运移”至煤体外;富氢环境促进了重质焦油的裂解,增强了油品的流动性;④原位与无应力状态煤样热解裂隙率及裂隙形态存在较大差异,蒸汽热解是煤炭原位注热开采工程实践的最优加热方式。

Underground in-situ pyrolysis of coal is a new trend in the utilization of coal resources in the future.It is especially critical to investigate the coal pore-fissure structure,its spatial distribution and connectivity under different pyrolysis methods,and the existing studies on the pore-fissure structure parameters of coal after pyrolysis are in non-stress condition and lack of the comparison of pore-fissure structure parameters under different pyrolysis methods.Based on the above deficiencies,using the self-made triaxial testing machine of thermo-fluid-mechanical-chemical coupling at high temperature and pressure,combined with the micro-CT technology,the fissure structure differences of bituminous coal under the conditions of natural and steam in-situ pyrolysis(600℃)were investigated,and then the mechanism of the differences was explored. The results show that ① After the natural and steam in-situ pyrolysis at 600 °C, the fissure forms generated inside the coal sample are mainly slender cracks and holes, and the fissure scale is further improved after steam pyrolysis. The fissure network is more abundant and complex. ② Under the stress condition of 5 MPa axial pressure and 3 MPa con-fining pressure, after natural pyrolysis, the fissure rate is 2.68 times of the original fissure rate. However, after steam pyro-lysis, the fissure rate is 3.65 times of the original fissure rate, and the fissure rate of each layer is more uniform. ③ Steam pyrolysis makes the bituminous coal heated evenly, with large heat exchange area and more adequate pyrolysis of organic matter. The displacement effect ensures the continuous production of pyrolysis products. The local stress effect of pore pressure not only expands the original pore-fissure volume, but also destroys the weak surface of pore wall, which greatly increases the pore connectivity. The steam denudation effect will “scour and transport” the tar with high viscosity attached to the blind fissures out of the coal body. Hydrogen-rich environment promotes the cracking of heavy tar and enhances the fluidity of oil. ④ There are large differences in the fissure rate and fissure morphology between in-situ and non-stress of coal pyrolysis. Steam pyrolysis is the optimal heating method for the engineering practice of in-situ thermal injection coal mining.