The influence mechanism of lignite thermal upgrading on moisture re-adsorption

The influence mechanism of lignite thermal upgrading on moisture re-adsorption was investigated using analytical techniques such as differential thermal analysis–thermogravimetry, Fourier transform infrared spectroscopy and scanning electron microscopy. The results showed that the moisture content of lignite decreased from 44.78 to 25.07% through the drying process. After the moisture in the lignite and the air had reached a dynamic equilibrium, the moisture content of the lignite was 25.75 wt.%. The characteristic functional groups of lignite include hydroxyl, methylene, methyl, carbonyl, carboxyl and ether oxygen. The adsorption distances of the five adsorption sites of carboxyl, phenolic hydroxyl, alcoholic hydroxyl, carbonyl and ether bond to water molecules were 2.081, 2.933, 1.905, 1.860 and 1.881 Å, respectively. The order of the adsorption strength of water molecules on the characteristic functional groups was carbonyl > phenolic hydroxyl > ether oxygen > alcoholic hydroxyl > carboxyl. During the upgrading by drying process, the characteristic functional groups that were prone to hydrogen bonds of water molecules partially decomposed. The reduction of pores and the partial decomposition of hydrophilic functional groups after upgrading in the lignite structure weakened the adsorption capacity of lignite to water molecules. Therefore, the re-absorption of water molecules by lignite was reduced.