Experimental Study On The Forms And Mechanism Of Moisture Readsorption Combined With Physicochemical Structure Of Upgraded Lignite

Lignite has relatively low market prices and abundant reserves while high moisture content and low calorific value greatly restrict the utilization of these reserves.Lignite dewatering and upgrading can reduce its moisture content and improve heat value,however,the upgraded lignite will adsorb water again if exposed to a damp environment.Moisture readsorption can lower the upgrading effect.This study focus on moisture readsorption of upgraded lignite so as to provide theoretical support for clean utilization and upgrading of the lignite.Hydrothermal and thermal upgrading techniques are promising coal drying methods that enable the utilization of low rank coals.In this study,Zhaotong(ZT)lignite from the Yunnan province of China was hydrothermally upgraded at 150–300 ~oC and thermally upgraded at 200–500 ~oC;then,the moisture readsorption experiments of raw and upgraded lignites were performed on different humidity at 30 ~oC.The changes of physicochemical structures in the process of different upgrading treatment were investigated,and the correlation between the changes and moisture readsorption characteristics were discussed.In addition,the water sorption isotherms of upgraded lignites were described and the forms of water in upgraded lignite after moisture readsorption were identified.The results indicate that the main hydrophilic oxygen-containing groups,including hydroxyl and carboxyl,were effectively removed as the upgrading temperature increased.In addition,the removal of–OH was more remarkable during hydrothermal upgrading.Irreversible changes occurred in the pore structure of the treated lignites;the specific surface area(SBET)and total pore volume(Vtotal)first increased and then decreased with hydrothermal upgrading,whereas the SBET and Vtotal continually increased with thermal upgrading as the temperature increased.The gel-like structure of the lignite experienced violent shrinkages and collapses.The moisture readsorption performance was effectively inhibited under the synergy effects of the physicochemical structure such that the moisture readsorption ratio(MRR)and moisture holding capacity(MHC)decreased continuously with the increasing upgrading temperature.The impact of each factor on the MHC was[OH+COOH]>SBET>Vtotal based on a comparison of the standardized coefficients,and the decrease of main oxygen-containing functional groups(–OH and–COOH)and Vtotal and the increase of SBET could effectively weaken the moisture readsorption ability.During water isothermal adsorption,the content of primary adsorption and secondary adsorption water continuously decreased with the upgrading temperature and increased with the humidity.In addition,the decrease in primary adsorption water of the lignite samples treated by hydrothermal upgrading was more obvious than those treated by thermal upgrading,but the latter was more effective to lower the secondary adsorption water at upgrading temperatures up to 500 ~oC.The optimal upgrading temperature for hydrothermal upgrading and thermal upgrading is 300 ~oC and 500 ~oC,respectively.Additionally,hydrothermal upgrading is more competitive than thermal upgrading in inhibiting the moisture readsorption performance of the upgraded lignites.The forms of water in raw and upgraded lignite mainly consist of free water,capillary water and molecular water.The free water first increased slightly and then decreased,whereas the capillary water and molecular water continually increased with hydrothermal and thermal upgrading as the temperature increased.Hydrothermal upgrading is more effective in restraining the formation of capillary water and molecular water.After hydrothermal upgrading,the control action in moisture readsorption of upgraded lignite had a gradual transition to physical adsorption,however the proportion almost maintained constant between physical and chemical adsorption during thermal upgrading,which implied that the moisture readsorption of thermally upgraded lignite is more difficult to remove.