Influence of critical moisture content in lignite dried by two methods on its physicochemical properties during oxidation at low temperature

Different drying methods resulted in the pore structure and free radical distribution changing in lignite, and significantly influences its chemical properties and potential utilizations. In this study, the influence of moisture content on characteristic temperature of lignite dried in N-2 and air were explored in a simulating coal oxidation device. Electron Spin Resonance (ESR) and carbon-13 nuclear magnetic resonance (C-13 NMR) were applied to investigate the free radical parameters and the functional groups concentrations in dried lignite. The initial oxidation of lignite occurs at above separation point temperature (SPT), and make pre-dried lignite achieve self-heating. Compared to vacuum drying, the lignite dried in N-2 with critical moisture content of about 15% release more heat during oxidation. Focus was directed on comparisons of free radical characteristics of lignite dried in N-2 at different oxidation temperature, indicating that dried lignite with higher oxidation temperature have a greater free radical concentration and line-width. Mesopore formed by the shrinkage of macropore in lignite dried in N-2 occurred further shrinkage and collapse to form more micropore. When the coal seam temperature reached 140 degrees C and close to SPT, the yields of gas products such as CO2 and CO increase rapidly. The values of oxygen-containing functional group measured in FTIR and C-13 NMR spectra show that the distinct structural feature of lignite dried in vacuum is larger than that of in N-2 due to the greater drying intensity.