Structural changes mechanism of lignite during drying: Correlation between macroscopic and microscopic

The structure of lignite is affected by fragmentation and pulverization during drying, which can reduce the safety and continuity of both drying and coal quality improvement processes. In this study, the drying characteristics of a two-dimensional (2D) lignite plate were studied, and both the shrinkage process and crack development were accurately quantified by image-processing techniques. The full-scale pore characteristics of lignite were analyzed by mercury intrusion porosimetry and inert gas adsorption-desorption. Finally, a relationship between the macro volumetric shrinkage and the micropore structure damage was proposed. Moisture diffusion coefficients (Deff) of 1.15198 x 10-9, 1.23095 x 10-8, and 1.70815 x 10-9 m2/s were obtained for normal temperature drying (NTD), hot air drying (HAD), and freeze drying (FD), respectively. The maximum shrinkage occurred in the HAD sample followed by the NTD sample. A shrinkage factor (phi s) of 27.94 was calculated for the former, whereas the FD sample exhibited almost no shrinkage. Dense microcracks formed rapidly on the sample surface under the hot-air condition, whereas a crack grid in which long cracks were present developed slowly under normal-temperature conditions. Both capillary force and high-pressure vapor caused serious damage to macro/mesopores, high-pressure vapor caused the most severe damage to macro/mesopores. The damage factor (phi I) values of macro-pores for the high-pressure water vapor and the capillary force processes were 23.07 and 18.69, respectively, which were significantly larger than those computed for mesopores and micropores. The collapse of macropores was considered the factor that most influenced to the shrinkage in the volume of 2D lignite plates.