Reignition characteristics of lignite affected by pre-oxidation and liquid nitrogen cold soaking

The propensity of residual oxidized coal to be reignited within enclosed fire areas in coal mines under liquid nitrogen (LN2) cold soaking remains unclear. Therefore, scanning electron microscopy (SEM), low-temperature nitrogen adsorption (LTNA), and thermogravimetric (TG) experiments were conducted to characterize the pore structure and secondary oxidation of oxidized cold-soaked coal. The results indicate that pre-oxidation or cold soaking alone can promote the generation and merging of pores and make the pore structure more complex, especially when pre-oxidized. However, pore evolution is suppressed and then improves when lignite is simultaneously subjected to pre-oxidation and cold soaking. In particular, the macropore volume and specific surface area decrease and then increase. The cryogenic shrinking of the LN2 results in the collapse of the loose coal matrix, which subsequently blocks the macropores. In contrast, vaporization expansion enhances pore connectivity with continuous LN2 injections, transforming mesopores into macropores, especially in the pore ranges of 2–5 nm and >50 nm. Additionally, pre-oxidation or cold soaking alone improves the oxidizing activity and combustion performance of raw coal. However, continuous cold soaking initially increases and subsequently decreases the characteristic temperatures and activation energy of oxidized coal. Oxidization at 200 °C with one cold soaking can further lower reignition propensity. The average activation energy of oxidized coal decreases by 22.9%–29 % when soaked more than three times. The improvement in combustion property is closely related to the increasing number of macropores because a larger pore volume and specific surface area are conducive to oxygen diffusion and adsorption. Therefore, pre-oxidation and cold soaking promote and inhibit the reignition of coal when performed simultaneously. These results provide theoretical guidance for rational liquid-nitrogen injection to control coal fires.