Changes of oxygen-containing groups during thermal treatment and their influences on moisture readsorption of lignite

• Published in: Drying technology 2016-04, Vol.34 (6), p.729-739
• Main Authors: Jing, Xiaoxia, Li, Zhiqiang, Zhang, Yanli, Chang, Liping
• Format: Article
• Language: English
• Subjects: Adsorption; Carbon dioxide; Coal; Decomposition; Heat treatment; Lignite; Moisture; moisture readsorption; Oxygen; oxygen-containing groups; Reduction; Temperature; thermal treatment
• ISSN: 1532-2300; 0737-3937; 1532-2300
• DOI: 10.1080/07373937.2015.1072548

The changes in oxygen-containing groups in the thermal treatment process of lignite were analyzed and the moisture readsorption amounts of upgraded coals were related in this study. A chemical titration method was applied to determine the types and contents of oxygen-containing groups in raw lignite and upgraded coals obtained in the temperature range of 353–633 K under atmospheres of argon, air, and carbon dioxide. Experimental results showed that the oxygen-containing groups mainly involved carbonyl, phenolic hydroxyl, and carboxylic groups (including carboxyl and carboxylate), among which phenolic groups were dominant. These different oxygen-containing groups decomposed discriminatingly with temperature according to thermal stability. The removal of oxygen-containing groups proceeded more easily in a CO ₂ atmosphere compared to in Ar. The oxidation reaction occurring on the surface of lignite exposed to air led to an increase in oxygen-containing groups. The different types of oxygen-containing groups possessed various combining abilities with water, among which carboxyl was considered the most important group for the moisture readsorption. Decomposition of oxygen-containing groups in the thermal treatment process of lignite mainly caused a reduction in monolayer water adsorption and further decreased the multilayer water.