Changes in Physicochemical Properties and the Release of Inorganic Species during Hydrothermal Dewatering of Lignite

Hydrothermal dewatering (HTD) is a nonevaporative drying method mainly useful for lignite, which in general has high moisture content. The lignite upgraded with HTD is expected as a clean fuel for power station; however, the changes in physicochemical properties of the product still have not been systermetically studied. In this research, hydrothermal treatments of lignite were performed at 200, 250, 300, and 350 degrees C. The effects of HTD on chemical composition and physical structure of lignite were studied. After HTD treatment, about 30 similar to 40% of carboxyl and 30 similar to 80% of hydroxide compounds in lignite were removed. HTD significantly facilitated the development of mesopores in lignite, different from the change of macropores. HTD can effectively reduce the moisture holding ability in whole relative humidity range by removing oxygen-containing functional groups, and large amounts of monolayer water in coal were removed. The release of inorganic species during HTD was also studied. It showed that the HTD can greatly reduce Na, K, Mg, and Ca in lignite but the loss of organics may result in the enrichment of some elements mainly associated with inorganics in product. In case of trace elements, B, Ba, Sr, and As in lignite had the potential to be leached out during HTD.