Pore structure and fractal analysis of Ximeng lignite under microwave irradiation

• The changes in pore structure during microwave drying were nonlinear.
• The evolution mechanisms of pore structure during microwave drying were obtained.
• The changes in D1 were similar to those in average pore diameter development.
• The changes in D2 were consistent with those in specific surface area development.
• D2 can reflect the volumetric roughness of mesopores in Ximeng lignite.

The pore structure and fractal analysis of Ximeng lignite (XL) under microwave irradiation were investigated. Effects of drying temperature, microwave irradiation time, and microwave power level were studied. The pore structures of XL were obtained by N2 adsorption/desorption at 77 K. Two fractal dimensions, D1 and D2, at relative pressures of 0–0.5 and 0.5–1, respectively, were calculated with the fractal Frenkel–Halsey–Hill model. The specific surface area of the microwave-treated XL differently decreased, whereas its average pore diameter and total pore volume differently increased. However, the evolution mechanisms of pore structures during microwave drying were similar. These mechanisms included the collapse of pore structures caused by shrinkage forces that resulted from the removal of moisture, open and cross-linking of blind and closed pores, and thermal decomposition of organic macromolecular structures under a high temperature. The changes in D1 were similar to those in average pore diameter development, whereas the changes in D2 were consistent with those in specific surface area development. The reliability of the relationship between D2 and mesopores was examined with some scanning electron microscope images and D2 can reflect the volumetric roughness of mesopores.