Study on pore structure change during different coal grade pyrolysis

In order to study the pore variation of different coal-grade samples during pyrolysis, this paper used a self-made coal thermal simulation device to carry out thermal simulation tests on Inner Mongolia brown coal, Xinjiang long flame coal and Hancheng bituminous coal, and used ASAP2020M automatic surface area analyzer. The pore structure changes of coal char samples during pyrolysis were analyzed. N2 adsorption desorption experiments showed that：there was no or small loop between the adsorption and desorption curves of three kinds of coal samples, and the pore morphology are was relatively simple. It is mainly composed of semi-openings with poor connectivity (wedge-shaped, cylindrical and slit-shaped holes enclosed unilaterally). The pore volume distribution corresponding to pore volume had a multi-peak distribution. The pore volume of coal char in Inner Mongolia mainly came from the contribution of transition pore; Xinjiang coal char pore volume mainly came from the contribution of transition pore and micropore; Hancheng coal char pore volume mainly comes from the contribution of micropore. The specific surface area is mainly contributed by the pore diameter of 2～3 nm. With the increase of coalification, the adsorption capacity, specific surface area and total pore volume decreased，and the pore volume is positively correlated with the specific surface area. The high degree of coalification is conducive to the development of micropores, while low degree of coalification is conducive to the development of meso-pores. As the pyrolysis temperature increases, the volatile matter of the coal species is violently precipitated, more pores are open, and the adsorption amount of coal char is increased. When the temperature was too high, the coal coke skeleton would collapse, and a fraction of micropores would be transformed into transition pores and mesopores, which would decrease the adsorption capacity. The adsorption amount of coal char in Inner Mongolia showed an increasing trend. The adsorption amount of coal char in Xinjiang first increased and then decreased. The adsorption amount of coal char in Hancheng showed a decreasing trend. By comparing the pore sizes of different coal grade samples with pyrolysis temperature, it can be observed that Inner Mongolia lignite is more conducive to the transport of gasification agents and gaseous products in coal.