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

Abstract

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 °C. The effects of HTD on chemical composition and physical structure of lignite were studied. After HTD treatment, about 30∼40% of carboxyl and 30∼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.

Hydrothermal dewatering of low-grade coal: Product evaluation and primary component analysis of ash deposition

“…Some researchers reported that the HTD process can remove alkali metals, such as potassium, sodium, titanium, iron, calcium, and magnesium, from coal. These metals significantly affect ash deposition during coal combustion [ [18] , [19] , [20] ].…”

Combustion Kinetics of Coal Treated by Microwave Irradiation Combined with Hydrochloric Acid Pickling

“…Mesroghli et al’s study showed that microwave treatment led to degradation of the organic matrix in coal and changes in coal physical properties . MI has a heating effect, which can effectively change the chemical composition and physical structure of coal and improve the quality of coal, and the structure change can be analyzed by the infrared spectrum of coal …”

“…26 MI has a heating effect, which can effectively change the chemical composition and physical structure of coal and improve the quality of coal, and the structure change can be analyzed by the infrared spectrum of coal. 27 The above studies confirmed that MI-combined assisted desulfurization has different effects on the organic structure and sulfur type of coal samples. However, more attention should be paid to the study of coal combustion characteristics after MI combined with hydrochloric acid pickling, which directly affects the further utilization of coal.…”

Study on Chemical Bond Dissociation and the Removal of Oxygen-Containing Functional Groups of Low-Rank Coal during Hydrothermal Carbonization: DFT Calculations

“…The results showed that the ratios of C−O bond to aromatic carbon were 0.229, 0.167, and 0.048 with increasing temperature, and the ratios of carboxyl to aromatic carbon were 0.597, 0.520, and 0.454 with increasing temperature, indicating that the content of oxygen-containing functional groups and C−O chemical bonds showed a declining trend in HTC. Wan 12 performed HTC on three lignites of Shengli (SL), Canada lignite (CAN), and Xiaolongtan (XLT) at 300 °C, and found that the reduction of −COOH and −OH was the largest. By analyzing the ratio of carboxyl to aromatic carbon and the ratio of hydroxyl to aromatic carbon, the results show that the decreases in the ratios of carboxyl to aromatic carbon are 0.505, 0.216, and 0.335, respectively.…”