Distribution And Evolution Of Closed And Open Pores In Coal

Regarding the connectivity of pores,pores in coal can be divided into open pores,through which gases and fluids can move freely,or closed pores,which are independent pores that isolated from the coal external surface.In this paper,we take different coal ranks and different genetic types of coal(humic type lignite and humosapropelic type jet)as the research objects to discuss the distribution and evolution mechanism of closed and open pores in coal,based on the liquid nitrogen adsorption method with different particle sizes and small angle X-ray scattering method,and the following results are obtained.(1)The types of closed and open pores in coal are revealed.Combined with optical and scanning electron microscope,it is found that the closed pores in coal are mainly isolated pores;the open pores are mainly composed of cellular pores,intergranular pores,mineral pores and fractures which can be connected to coal external surface.(2)The distribution and evolution characteristics of closed and open pores in different coal ranks were analyzed.With the increase of coal rank,the pore volume and specific surface area of closed and open pores in coal decrease continuously.However,the proportion of closed pore volume and specific surface area in total pores increased significantly,up to 84.6% and 84.8%.In addition,with the increase of coal rank,the pore volume and specific surface area of open mesopores-macropores and closed mesopores continually diminishing,but the pore volume and specific surface area of closed mesopores do not change significantly.(3)The difference of distribution of closed and open pores in different genetic types of coals are revealed.The total pore volume of lignite is 1.59-2.27 times of jet.The pore volume and specific surface area of the four pieces of jet are mainly closed pores,and the proportion of pore volume and specific surface area to the total pores is96.79%-98.73% and 98.82%-99.33%,and the mesopores account for a large proportion in the closed pores of the jet.The pore volume and specific surface area of the two pieces of lignite account for 27.13%-36.45% and 13.69%-17.84% of the total pores.The open pore volume and specific surface area of lignite are 29.11 and 24.24 times larger than those of jet,respectively.(4)The evolution mechanism of closed and open pore in coal is clarified.With the increase of coal rank,the active functional groups of coal macromolecule structure continuously fall off,accompanied by the generation and accumulation of a large number of gas,forming a large number of isolated closed pores,resulting in the proportion of closed pores increasing with the increase of coal rank.The structure of lignite is loose,microfractures and macropores are developed,while the structure of jet is compact.Therefore,in the process of coalification,the gas generated by jet cannot escape and accumulate in the coal body to form gas pores,resulting in the extremely development of closed pores of jet.There are 107 figures,32 tables,and 115 references in this paper.