Characteristics and Evolution Mechanism of Three Phase Products of Lignite by Hydrothermal Treatment Dewatering

Baiyinhua lignite was treated by hydrothermal treatment dewatering (HTD). The production characteristics of the gas, solid, and liquid were studied. Results show that HTD is an effective means to decrease water content and water-holding capacity. When the treatment temperature was increased to 310 °C, the moisture was reduced from 26.55% to 5.27%, and the dehydration rate reached 80.20%. At the same time, the carbon content and calorific value increased during the HTD process, which increased energy density. The H/C atomic ratio increased first, then decreased with the increasing temperature. The increase in the H/C atomic ratio was due to the breakdown of aromatic ether and formation of phenolic compounds at the low temperature. The phenolic compounds started to break at the high temperature, which resulted in the decrease in the H/C atomic ratio. These results can be proven by ¹³C NMR analysis. Combined with the analyses of calorific value, dehydration ratio, recovery of combustible product, and heat loss, the relative balance dehydration and deoxidation efficiency were evaluated, and 250 °C is a suitable temperature for the HTD process in lignite upgrading. The HTD process promoted the breakage and decomposition of weak chemical bonds in lignite, which resulted in many organic compounds in wastewater after the HTD process. The chemical oxygen demand and biochemical oxygen demand continually increase, and the biodegradability of the wastewater is relatively good. The index of biodegradability for wastewater is greater than 0.3 even at a hydrothermal treatment temperature of 310 °C. This indicates that wastewater can be subjected to biochemical treatment at a low treatment cost. At the same time, the metal ions and nonmetallic ions in wastewater and the gas component were studied. These research results aim to provide theoretical guidance for the industrialization of lignite hydrothermal modification.