Effect of microwave-assisted cyclic oxidation on the coal internal and surface structure based on NMR and AFM

Exploring new methods for comprehensive mining of coalbed methane (CBM) is a research hotspot. Pore connectivity has a significant impact on the permeability of coal, and as connectivity increases, it has a positive impact on the desorption and seepage of CBM. To quantify and visualize the pore development of coal under three cyclic treatments, T2 and T1-T2 spectra obtained by nuclear magnetic resonance (NMR) and Surface roughness obtained by atomic force microscope (AFM) were selected for characterization. The NMR results indicate that microwave-assisted cyclic oxidation can stimulate the formation of pores and pore throats and effectively dissolve the coal matrix, thereby changing the structure of the coal, manifested by a growth rate of −5.84 % for irreducible porosity and 31.5 % for producible porosity. Cyclic oxidation and microwave have a certain impact on the conversion of micropores to mesopores, but have a relatively less impact on the evolution of new pores. The maximum Rq and Ra obtained by AFM of microwave-assisted cyclic oxidation are 84.7 nm and 70.3 nm, respectively. Moreover, its Rsk and Rku are less than zero and three, respectively. This indicates that microwave-assisted cyclic oxidation has the best effect on changing the internal and surface morphology of coal.