Pyrolysis and Upgrading of Lignite by Superheated Steam: Microstructure Evolution and Calorific Value Enhancement

Superheated steam in-situ pyrolysis is an innovative and promising technology for the upgrading and efficient utilization of low-rank coal. This work systematically investigated the microstructure evolution of a Chinese lignite before and after superheated steam treatment by employing scanning electron microscopy, low-temperature nitrogen adsorption, Fourier transform infrared spectroscopy, and Raman spectroscopy. The relationships between coal structural parameters, compositional changes, and calorific value were analyzed to elucidate the upgrading mechanism. The results demonstrated that after treatment with superheated steam for 3 h, the moisture content of the coal decreased by more than 79%, while the coal calorific value increased by over 16%. The escape of volatile components from the coal surface, together with the thermal decomposition of the coal matrix, facilitated the formation of tiny blowholes and pyrolytic pores, leading to increased coal pore volume and pore size. These structural changes were beneficial for mass and heat transfer during thermal conversion processes, thereby contributing to the enhancement of coal calorific value. Meanwhile, the aromaticity, maturity and ordered nature of the coal microcrystalline structure increased with steam treatment. The relative enrichment of oxygen-containing functional groups indicated chemical structural reorganization during steam treatment, further influencing the upgrading behavior of lignite. This work provides mechanistic insights into the microstructure evolution and calorific value enhancement of lignite under superheated steam treatment, and offers experimental support for the upgrading and clean utilization of low-rank coal in engineering applications.