中低阶煤纳米孔隙结构的小角X射线散射研究

Small Angle X-ray Scattering Study on Nano-pore Structure of Medium and Low Rank Coal

以7种煤样(镜质组最大反射率R_(o,max)范围为0.30%~2.23%)为研究对象,对其进行低温氮吸附和小角X射线散射(SAXS)测试,分析了散射强度校正后煤样的q-I(q)曲线及Guinier曲线特征,并对得到的最可几孔径、比表面积及表面分形维数(D_(S))的变化规律进行研究。结果表明:伊敏16号煤30 nm以上的孔隙数量最多,平朔9号煤4 nm~30 nm孔隙最发育,其余5种煤样微孔的发育程度随煤阶的增高而增大;随着变质程度增高,SAXS法和低温氮吸附法得到的最可几孔径均呈下降趋势;平朔9号煤因微孔大量发育,其最可几孔径最小;低温氮吸附得到的平均孔径、孔体积与最可几孔径的变化趋势一致,反映了随着煤化程度的提高,煤的物理结构逐渐趋于紧密化;随着变质程度增高,SAXS法所得煤样的比表面积呈先急剧减小后缓慢增大的趋势,褐煤和气煤比表面积较大,焦煤比表面积较小;当R_(o,max)<1.5%时,低温液氮吸附法所得比表面积的变化趋势与SAXS法相近,当R_(o,max)>1.5%时,呈相反趋势,这与封闭孔含量增多有关;随变质程度增高,两种测试手段所得表面分形维数均呈“N”字型变化趋势。

The low-temperature nitrogen adsorption and small angle X-ray scattering(SAXS)tests were carried out on seven coal samples(maximum reflectance of vitrinite R_(o,max) range is 0.30%-2.23%).The characteristics of q-I(q)curve and Guinier curve of coal samples after scattering intensity correction were analyzed.The variation law of some parameters,including most-probable pore diameter,specific surface area,and fractal dimension(D_(S))of the samples were also discussed.The results show that the Yimin No.16 coal is dominantly characterized by the pore size above 30 nm,and the Pingshuo No.9 has a higher content of pores between 4 nm and 30 nm.While for the remaining five coal samples,the degree of micropores development increases as the coal rank increases.With the increase of metamorphism degree,the most-probable pore diameter obtained by SAXS and low-temperature nitrogen adsorption shows a decreasing trend.PingshuoNo.9 coal has the smallest most-probable pore diameter due to the large number of micropores in it.The variation trend of average pore size and pore volume obtained by low-temperature nitrogen adsorption is consistent with the most-probable pore diameter,which reflects that the physical structure of coal tends to become compact with increase of coalification degree.As the metamorphism degree increases,the specific surface area of coal samples obtained by SAXS decreases sharply at first and then increases slowly.Besides,the specific surface areas of lignite and gas coal are larger,while that of coking coal is smaller.When R_(o,max) is less than 1.5%,the variation trend of specific surface area obtained by the low-temperature nitrogen adsorption is similar to that of SAXS.However,when R_(o,max) is more than 1.5%,the trend is opposite,which can be attributed to the increase of pore contents.With the increase of metamorphism degree,the surface fractal dimension obtained by the two test methods shows a“N”shape trend.