Effect of low-temperature pre-oxidation on microstructure and reignition characteristic of water-immersion lignite

The residual coal is prone to oxidation in different degrees after being immersed-dried in waterlogged goafs, but the reignition characteristics of water-immersion coal by low-temperature pre-oxidation remain unclear. Herein, lignite was pre-oxidized to 40 °C, 80 °C, 120 °C, 160 °C, 200 °C, and 230 °C after 60 days of immersion. Moreover, the microstructure changes of water-immersed lignite after oxidized were investigated using low-temperature nitrogen adsorption and infrared spectroscopy, and its spontaneous combustion characteristics were assessed employing thermogravimetry. The results displayed that the pore volume, specific surface area, and pore diameter increased and then decreased as the pre-oxidation temperature rose. Specifically, lower temperatures (below 80 °C) promoted the formation of micropores and mesopores, as well as the merger and transformation of pores into macropores, which facilitates the diffusion and adsorption of oxygen. However, higher temperature oxidation (> 200 °C) reduced both the number of pores and the content of reducing functional groups (–OH, –CH₃, and –CH₂). Comparatively, the lignite oxidized at 80 °C maintained a strong chemical reactivity owing to the high relative content of -CH₂ and C = O. Furthermore, the pre-oxidation treatment mainly affected the thermal decomposition and gas-phase combustion of the water-immersed coals. Moreover, as the degree of oxidation rose, the characteristic temperatures (T₂–T₄) and the activation energy first decreased and then increased, while the comprehensive combustion index exhibited an opposite trend. Notably, the critical oxidation temperature of 80 °C is a turning point for the above change. Thus, the pre-oxidation at 80 °C heightens the risk of reignition in water-immersion coal owing to its developed pore structure and abundant active groups. It is imperative to avoid the critical temperature during low-temperature oxidation of water-immersed coals.