Effects of the evolutions of coal properties during nitrogen and MTE drying processes on the spontaneous combustion behavior of Zhaotong lignite

Compared to nitrogen drying process, the mechanical thermal expression (MTE) drying process, a typical non-evaporation drying process with great potential in large-scale application, results in significantly different changes in characteristics of lignite such as pore structures, mineral matters contents and particle size. In this study, the effects of the evolutions of coal properties during nitrogen and MTE drying process on the spontaneous combustion behavior of Zhaotong lignite were systematically investigated. The samples were treated by nitrogen and MTE drying process to desired residual moisture contents, and the changes in pore structures, mineral matters and particle size of samples were analyzed. The test of spontaneous combustion behavior of the sample was carried out by wire basket method. It was found that the removal of water from lignite resulted in the decrease of crossing point temperature (CPT) indicating the promotion of spontaneous combustion susceptibility. The stabilization of pore structures by MTE process which inhibited the transform of macropores into smaller pores, resulted in a lower CPT compared to nitrogen dried samples with the same residual moisture content. The removal of mineral matters from lignite by MTE process also contributed to the decrease of CPT. The effect of particle size in a wide range (from <0.2 mm to 10-20 mm) on CPT was also studied. The CPT increased with the enlargement of particle size above the critical diameter, due to the less exposition of internal surface area and inhibited the oxygen diffusion to internal particle surface which inhibited the oxidation reactions. The inhibition on the spontaneous combustion of dried lignite by the MTE process was deprived from the large size of its products.