An Experimental Study On The Existing Form Of Water In Lignite

Coal is an important energy source for sustainable economic growth indeveloping countries such as China. Judging from the energy distribution in China,lignite reserves are abundant. However, the higher water content greatly limits itsexploitation and utilization and the key to solve the efficient use of lignite isdehydration upgrade. Understanding of fundamental aspects of water in coal and itseffects on physical and chemical characteristics of coal and coal-water interaction arevery important in order to develop new technologies to remove coal water higheffectively and maintain the competitiveness of lignite. The behaviour andcharacteristic of water in coals in various forms is very complex, and this requires usto analyze and study them from different levels containing heating and cooling. Theexisting forms of water in lignite have been widely studied globally.This paper firstly provides a comprehensive overview on the fundamentalunderstanding of water in lignite, including water classification, research methods,physical and chemical structure, and the new research aspect. And then water sorbedin HL and YN lignite was analyzed by differential scanning calorimetry（DSC）over atemperature range from25℃down to-150℃again up to300℃. Based oncongelation characteristics, water was classified into three types, i.e. free water, boundwater and non-freezable water. These two types of freezable water accounted for18.22-78.25%of total water in coal which confirmed the presence of the third type ofwater（non-freezable water）. Heat treatment and alkali washing altered the physicaland chemical structures of lignite resulting in reduced reabsorption of bound water.BET and SEM analyses showed shrinkage and decrease in porosity of coal samples asa result of drying. In order to further investigate and verify the freezing characteristicsof water, a low temperature XRD apparatus was used to observe the crystal structureof ice at-80°C. When coal samples with14.75%water contents were used, water infrozen state was detected by XRD, contradicting the DSC results. Amount of heatgenerated by phase transition of the third type of water for coal samples with watercontents less than18%was very small and therefore was not detected by DSCapparatus.In addition，the weight lose of HL and YN lignite with different particle sizeswere determined at50,80and110℃by thermal gravimetric analysis. The resultsshow that the weight lose of coal samples almost not further increased after a periodof dehydrate at each drying temperature, and the total weight loss increased with the drying temperature increasing, indicating that the bonding strength between the coalsurface and the water is different. The energy consumption for evaporation of waterand the interaction including the hydrogen bonds and the binding force of microsporesfor water between coal and water were increased with the water content decreasing.The TGA results of HL lignite with particle diameter60-100mesh at50℃show thatafter drying the water content was reduced to about7%in the form of the molecularlayer water, and with temperature further increasing to110℃the water was notobviously decreased. The chemical structure of raw coal and water washed coal wasalso studied using FT-IR.