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

• Published in: Fuel (Guildford) Vol. 178; pp. 85 - 92
• Main Authors: Liu, Jianzhong, Wu, Junhong, Zhu, Jiefeng, Wang, Zhihua, Zhou, Junhu, Cen, Kefa
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.08.2016
• Subjects: Bond dissociation enthalpy; Density functional theory; Hydrothermal dewatering; Lignite; Oxygen functional groups; Solid-state NMR; Hydrothermal dewatering; Solid-state NMR; Density functional theory; Bond dissociation enthalpy; Lignite; Oxygen functional groups
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2016.03.045

•The oxygen functional groups in lignite were quantified before and after HTD.•The simplified molecular model of lignite was constructed.•The ESP, bond order, BDE of lignite molecule were studied using DFT method.•The effect of oxygen functional groups on the slurryability of lignite was studied. 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 13C 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 phOCH3, followed by the cleavage of CC bonds in carbonyl and carboxyl, and ended with the cleavage of the CO bonds in alcoholic hydroxyl (phCH2OH) and CH3OCH3. All these bonds had BDEs lower than 90kcal/mol. The cleavage of CO bond in phenolic hydroxyl (phOH) was the most difficult because it has the highest BDE value of 113.4kcal/mol. The raw coal had inferior slurryability with a solid concentration of 48.49%. After HTD at 300°C, the surface property of lignite was significantly improved and the solid concentration of upgraded coal significantly increased to 59.14%.