Pore structure characteristics of low-rank coal under various degassing temperatures: A case study of lignite from Xishanyao Formation, Turpan-Hami Basin, Xinjiang

The degassing temperature presents a significant influence on the pore structure of the low-rank coal when the low-temperature nitrogen adsorption measurements are conducted, while the appropriate degassing temperature is still equivocal.Aiming this, the low-rank coal sample collected from Turpan-Hami Basin was used to make low-temperature nitrogen adsorption measurements, and the pore structure characteristics under various degassing temperatures were also investigated.The results show that the coal sample mass decreases continuously with the increased degassing temperatures.When the degassing temperature is lower than 180 ℃,the sample mass loss rate is slow, following that the sample mass loss rate tends to increase, especially when the degassing temperature exceeds 240 ℃,the massive evaporation of the volatiles would lead to the sharp decrease of the sample mass with the increased degassing temperature.When the degassing temperature increases, the hysteresis loops of the coal sample feature continuously decrease, and the knee points of the desorption branch at the relative pressure of 0.5 becomes weaken, indicating the various dynamic change of the pore structure, and the pore structure of the low-rank coal sample varies from complicated to simple.With the increased degassing temperatures, the maximum nitrogen adsorption quantities and the pore volume feature vary in three-step-style, while the pore specific surface area of the low-rank coal decreases, which is dominantly related to the sharply decomposition of the volatile matters in the low-rank coal under high degassing temperature.The continuously decrease of the incremental pore volume and pore specific surface area with the increased degassing temperatures mainly occur at the pores with the aperture below 20 nm, indicating the collapse of these pores contributed dominantly to the decrease of the pore volume and pore specific surface area.Due to the collapse of micropores and partly of mesopores, the pores with larger apertures tend to be concentrated, the average pore apertures present an increasing trend when compared with that of pore specific surface area with the increased degassing temperatures.With higher degassing temperature, due to the massive evaporation of the volatiles and the collapse of the pores, the pore structure has been reformed significantly.When the degassing temperature is lower than 150 ℃,the differences for the cumulative pore volumes and pore specific surface areas are least, indicating the pore structure still maintains.Therefore, the lower degassing temperature(≤150 ℃) would be an appropriate degassing temperature for the pore structure of the low-rank coal when the low-temperature nitrogen adsorption is used.