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

- The effects of two drying methods on self-heating of lignite during oxidation were investigated.
- ESR techniques were used to study free radical characteristics of dried lignite.
- The relationship of critical moisture content and heat release intensity were investigated.
- The oxygen functional groups in dried lignite were analyzed and contrasted by chemical and 13C NMR methods.

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 N2 and air were explored in a simulating coal oxidation device. Electron Spin Resonance (ESR) and carbon-13 nuclear magnetic resonance (13C 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 N2 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 N2 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 N2 occurred further shrinkage and collapse to form more micropore. When the coal seam temperature reached 140 °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 13C NMR spectra show that the distinct structural feature of lignite dried in vacuum is larger than that of in N2 due to the greater drying intensity.