Research on multi-scale fractal characteristics of pore structure in tectonic coal and analysis of its influence factors

The pore structure of tectonic coal has fractal characteristics，such as inhomogeneity,self-similarity and scale invariance.Therefore,it is difficult to quantitatively describe the pore features by traditional European geometry.In order to study the fractal characteristics and characterization methods of different scales pore structure of tectonic coals,the microporous,mesoporous and macroporous pore structures of four test samples(primary tectonic coal,fragmented coal,granulated coal and mylonitized coal) were tested by low temperature CO2 adsorption,low temperature N2 adsorption and mercury intrusion method,respectively.The method of calculating the fractal dimension of pores of different scales in tectonic coal were analyzed,the comprehensive characterization method for multi-scale fractal features of tectonic coal pore structures was discussed,and the influence factors of tectonic coal pore fractal dimension were studied by gray correlation method.The study results show that:the microporous filling model based on CO2 adsorption data,FHH model based on N2 adsorption data and thermodynamic model based on mercury injection data can effectively characterize the fractal characteristics of tectonic coal microporous,mesopore and macroporous,respectively.The fractal dimension of different scales pores vary according to the type of tectonic coal,in which the fractal dimensions of micropores and 2～6 nm pore diameter segment in mesopores increase with the destruction degree of tectonic coal,while the fractal dimension of pores at other scales has no obvious rule.The multi-scale comprehensive fractal dimension of tectonic coal is calculated by weighting the pore fractal dimensions of different scales by the proportion of the stage pore volume,which can unify the fractal characteristics of different scales pores according to a certain trend.And the trend shows that the multi-scale comprehensive fractal dimension increases with the degree of tectonic coal deformation.According to the order of gray correlation,factors such as median pore size,microporous pore capacity,total specific surface area,and microporous specific surface area have the greatest influence on the fractal dimension,followed by factors,such as most available pore size,total pore capacity,mesopore specific surface area,and proportion of microporous specific surface area,and factors such as ashyield,volatile matter,mesopore pore capacity,and proportion of mesopore specific surface area have relatively little influence on the fractal dimension.