Removal of oxygen functional groups in lignite by hydrothermal dewatering: An experimental and DFT study

Hydrothermal dewatering (HTD) is a promising method for in-depth upgrading of lignite because of its comprehensive modifications, including dehydration, deoxygenation, carbonization, and surface modification. The oxygen functional groups in XiMeng lignite before and after HTD was quantitatively determined using the C-13 direct polarization/magic angle spinning technique. After HTD, the amounts of carboxyl, alcoholic hydroxyl, ether, and carbonyl groups decreased, whereas those of phenolic hydroxyl were unchanged. The simplified molecular model of lignite was constructed and the electrostatic potential (ESP), bond order, and bond dissociation enthalpy (BDE) of the lignite molecule were investigated using density functional theory. The oxygen functional groups contributed to regions with a large absolute ESP value. These regions also exhibited strong hydrophilicity because of the formation of hydrogen bonds with water. Bond order and BDE analyses are consistent with the experimental results. The hydrothermal treatment began with the cleavage of phOACH(3), followed by the cleavage of CAC bonds in carbonyl and carboxyl, and ended with the cleavage of the CAO bonds in alcoholic hydroxyl (phCH(2)AOH) and CH(3)OACH(3). All these bonds had BDEs lower than 90 kcal/mol. The cleavage of CAO bond in phenolic hydroxyl (phAOH) was the most difficult because it has the highest BDE value of 113.4 kcal/mol. The raw coal had inferior slurryability with a solid concentration of 48.49%. After HTD at 300 degrees C, the surface property of lignite was significantly improved and the solid concentration of upgraded coal significantly increased to 59.14%. (C) 2016 Elsevier Ltd. All rights reserved.