Moisture Readsorption Performance of Air-Dried and Hydrothermally Dewatered Lignite

Lignite is a kind of coal that has high moisture content and needs to be dried before being utilized. The dried lignite will readsorb moisture during its storage. In this work, a typical Chinese lignite was dried by air at 120-180 degrees C and hydrothermally dewatered at 200-320 degrees C. The moisture adsorption isotherms of treated coal and the parent coal were obtained at 30 degrees C to investigate the essential moisture adsorption properties. Then the moisture readsorption performance of the studied coal in the real storage was simulated by placing the samples in the atmosphere and monitoring the evolution of the sample masses. The experimental results indicated that the moisture readsorption capacity of the air-dried lignite was lower than the parent coal and decreased with increasing drying temperature. For the hydrothermally dewatered coal, the moisture readsorption capacity increased with increasing dewatering temperature to 240 degrees C and then decreased significantly as dewatering temperature rose to 320 degrees C. The inhibiting effects of both air drying and hydrothermal dewatering at 280 and 320 degrees C on the moisture readsorption performance were more remarkable in winter than in summer due to the lower relative humidity. The evolution of the lignite pore structure and functional groups after the air drying and hydrothermal dewatering were also examined to understand the changes in moisture adsorption properties.