Differences In Pore Structure Of Different Lithotype Of Coal And Their Control Over Methane Desorption Characteristics

The coal seams in the Fengfeng mining area are mined at great depths,and different lithotype of coal have significant differences in their desorption and diffusion characteristics due to differences in composition,structure and physical properties,which have a significant impact on CBM extraction.By building a methane diffusion simulation experiment,combining particle size distribution,industrial analysis,microcomponent quantification and pore structure testing,the pore structure of different lithotype of coal was analysed and the methane desorption and diffusion characteristics of different lithotype of coal were investigated.The effects of factors such as particle size(degree of coal fragmentation),coal rock type,temperature and pore structure on methane desorption and diffusion were discussed in combination.The following insights were mainly obtained:Firstly,the grain size distribution of different lithotype of coal,the grain size selectivity of coal rocks and kerogen,and the pore structure characteristics of the Dashe mine in the Fengfeng mining area were revealed.The content of mirror group and fixed carbon of different lithotype of coal decreased from bright coal,semi-bright coal,semidark coal and dull coal in order,and the content of inert group,ash yield and volatile yield increased in order,while the content of crustal group was all less.Liquid nitrogen adsorption experiments showed that the pores of the bright,semi-bright and semi-dark coals were dominated by slit pores and parallel plate types,and the pores were mainly ink bottle pores.As the particle size decreased,the cumulative pore volume of liquid nitrogen adsorption increased the most for semi-bright coals,and decreased for dull,light and semi-dark coals in that order.The pore volume of the closed micropores of semi-bright coal and semi-dark coal was more obviously modified by crushing,accounting for 56.86% and 63.35% of the total closed pore volume;the pore volume of semi-bright coal was increased for the whole pore section,and the percentage of closed micropores and closed small pores were not much different,accounting for 36.06% and23.77% respectively;the pore volume of closed small pores of dull coal increased more obviously,accounting for 52.88% of the total closed pore volume.Next,the methane desorption diffusion pattern was elucidated under different particle sizes,temperatures and lithotype of coal using the constructed methane diffusion simulation platform.Methane adsorption gradually increased with increasing temperature,and the trend of methane adsorption did not change much with particle size.The desorption rate of bright coal increases with increasing temperature and decreasing particle size,while the desorption rate of other lithotype of coal does not vary significantly with particle size.The methane diffusion coefficients were obtained from the high temperature and high pressure methane diffusion model,and the effective diffusion coefficients were calculated by combining the particle size tests: the initial diffusion coefficient increased with increasing particle size,and gradually decreased from bright coal to dark coal;the initial effective diffusion coefficient of bright coal decreased and then increased significantly with decreasing particle size,while that of semi-bright coal showed a fluctuating increase.The initial effective diffusion coefficient of semi-dark coals decreases and then increases steadily,while the initial effective diffusion coefficient of dark coals has little effect with temperature and grain size.Finally,the influence of different lithotype of coal on methane desorption capacity was investigated,and the factors and modes of action of methane diffusion influence of lithotype of coal were explored.The correlation between small pore pore volume and effective diffusion coefficient was highest for bright coals,followed by semi-bright coals,semi-dark coals and dull coals.This indicates that the methane desorption and diffusion characteristics are mainly controlled by the small pore volume,while the desorption and diffusion of dull coal is mainly controlled by the micro-pore volume.The correlation between pore volume and effective methane diffusion coefficient increases with temperature and decreases with time.The increase in pore volume of different grain size closed pores does not necessarily lead entirely to an increase in the initial effective diffusion coefficient,but is also related to factors such as the sorting of microscopic components and industrial component enrichment of different lithotype of coal at different stages of fragmentation,where an increase in temperature can contribute to an increase in the incremental effective diffusion coefficient.