| Coal has accounted for about 70% for a long time in China’s energy consumption structure. With the high-quality coal resources dropped significantly, large-scale exploitation of lignite is imperative. Lignite reserve is abundant in China, but high water content, low calorific value, easy to weathering and easy to spontaneous combustion characteristics restrict its utilization fields. Lignite contains a number of polar oxygen containing functional groups, meanwhile its porous structure makes it absorb water again even after drying. Therefore, studying the change of the physical and chemical structure during the drying process is the theoretical basis of cleaning and efficient utilization of lignite that is very important in practical.In this paper, typical Chinese lignite was selected as the research object. To study the decomposition,oxidation and conversion of different group during lignite upgrading process, in situ Fourier infrared spectrometer was used to characterize the changes of different groups on lignite surface in different conditions. Combined with chemical titration and the nitrogen adsorption characterization results, the evolution of physical and chemical structure under different atmosphere and time were examined and its influence on water re-adsorption property is also studied. The main findings include the following aspects:(1) EMC of coal samples upgraded in N2 atmosphere is obviously lower that in air when the drying temperature is 160 oC and 200 oC. When temperature rises to 240 oC, EMC is quite close in air and N2. Equilibrium moisture capacity of upgraded lignite decreases gradually with the increase of drying time and temperature.(2) When temperature higher than 200 oC,CHX decreased obviously especially in air. The order of decomposition activation energy is: CH2> CH > CH3, oxidation activation energy is: CH > CH3> CH2. CH3 in the end of structure chain is unstable and easy to split away from lignite. CH2 has smaller oxidation activity that means it is easy to be oxidized. Carboxyl in lignite has the strongest hydrophilic ability which is about 9 times that of ketone. Aldehyde hydrophilic ability is probably 3 times of ketone. Oxygen containing groups such as quinones, esters and anhydride have no obvious relationship between equilibrium moisture content.(3) Because of the large heat capacity of water, when the temperature is above 160 oC,weight loss of coal samples became bigger after mixing with water vapor in N2 and CO2. Water vapor makes the mesopores fewer and specific surface area smaller which directly led to the EMC of coal samples decreased. When relative humidity is 95%, water molecules filled up with less than 40 nm pore channels and EMC has good linear relationship with pore volume.(4) COOH content of coal sample dried in N2+H2O atmosphere is obviously bigger than in N2, but water vapor mixed in CO2 has a little influence on the decomposition of carboxylic acid. OH and C=O absorption peak area gradually reduced in N2 atmosphere as drying degree deepen. While in N2+H2O atmosphere, C=O reduced firstly and then appeared to increase.(5) When the relative humidity is less than 17%, moisture on lignite surface formed monolayer adsorption which is mainly related to the content of oxygen containing functional groups. When relative humidity less than75%, moisture adsorption increased at a slower pace that implied water formed multilayer on lignite surface. Moisture formed capillary condensation in lignite pore channel when humidity up to a certain degree, and moisture hysteresis phenomenon appeared in the process of moisture desorption. |
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