Low-rank coal drying behaviors under negative pressure: Thermal fragmentation, volume shrinkage and changes in pore structure

Abstract

Negative Pressure Drying (NPD) is new alternative technique for Low Rank Coal (LRC) drying. In this work, the dehydration, thermal fragmentation and volumetric shrinkage behaviors of LRC under a sustained environment of 423 K and 20 kPa were studied. The correlations of these behaviors were analysed, and the mechanism of shrinkage-fragmentation of coal was discussed based on a spherical model for coal drying. Besides, the pore structures of LRC before and after drying were compared. Results showed that shrinkage and fragmentation mainly occur at the removal stage of internal moisture, and they almost change synchronously. Moreover, when the moisture content is 4.74%, the maximum values of fragmentation rate and shrinkage rate appear. The possible mechanism of shrinkage-fragmentation is summarized as follows. At the same time of internal water removal, coal volume is reduced, new cracks are generated, the inherent cracks expand, and the volume loss of coal is considered to be equal to the newly formed volume of cracks. With the rapid reduction of the wet core radius, the cracks extend towards spherical core rapidly, and some of which grow into large cracks to promote fragmentation. Besides, the decrease of wet core radius makes the cross-linking between cracks more frequent, and further accelerate the fragmentation. However, when the moisture content is less than 4.74%, the volume shrinkage is mainly caused by thermal stress, and fragmentation rate gradually decreases due to the lack of newly formed large cracks. After NPD, the number of small pores in coal (<52 nm) increased, whereas the number of large pores (>260 nm) decreased. The specific surface area and the fractal dimension of the pore structure increased, which are beneficial to the subsequent utilization of LRC.