Mechanism Of Moisture Transport In The Pores Of Lignite During Drying Process

There are more than 130 billion tons of lignite resources in China, approximately 13 % of the total coal reserve. The moisture content is up to 30 %-65 % on the wet base, leading to 20 % of the heat was consumed on its own drying in the traditional coal-fired power plants. Therefore, drying is a necessary precondition before the utilization of lignite. Different types of water in lignite was determined to analyze the distribution of water at pore-level in this paper, and the water desorption/adsorption on lignite was also studied to obtain the thermodynamic properties of water in lignite. After that, the interaction between water and lignite molecules was explored based on molecular simulation. The change of the surface properties, pore structure and the water distribution of dried lignite was analyzed, and the machnism of moisture transport at mesoscopic scale was explored by multi-physics coupling methods.In this paper, different types of water in lignite and its thermodynamic properties were explored at first. Specifically, the 1HNMR was used to analyze the forms of water, and the results showed that the SL lignite with the porosity of 41.4%, mainly contained bound water distributing in the pore diameter of 0.003-0.3μm. The porosity of ZT lignite was 73.9% and the range of moisture pore size was slightly larger(0.01-1μm). Through the exploring of the desorption thermodynamic behavior, it was found that there were only a little effective hydrophilic sites, and the water clusters contain 6-7 water molecules. The filled saturation concentration of microporous was 33.7mmol/g, which was much larger than 12.5mmol/g and determined the high MHC of LY lignite, as the main target of dewatering process.Then, the interaction of water and lignite molecules was studied by molecular simulation. Gaussian was used to explore the formation process of the water bunch near the hydroxyl, carboxyl and oxygen functional groups in lignite molecules. It tended to form a cyclic structure by 3-4 water molecules in the water cluster. The interaction between OH1 and water molecules was weaker than the interaction between water molecules. When the water cluster was near the O-H2、COOH and oxygen functional groups, the adsorption energy of coal-water effect was higher than water-water effect and formed the strong water sites. The farthest distance between hydroxyl and the water molecule in the cluster was 4.76 ?, and the pore-filling water out of this range was less affected by the functional groups on the surface.The exploring of the kinetics of drying process showed that the temperatures made significant differences on the drying process. For SL-2mm and ZT-2mm, the drying rate at 150 ℃ was 4.4 times and 13.6 times of the rate at 300 ℃. The influence of temperature was more prominent to the high-water-content sample. And high energy input would reduce the effects of the particle size. Water dynamics predictive equation was constructed by Logarithmic model, which was associated with the temperature and drying time. The volatiles, surface oxygen-containing functional groups and the contact angle of SL had a small change during drying process. While the volatile of ZT decreased 5%, and carboxyl group was significantly reduced, the contact angle was increased and close to or even greater than SL.However, the pores are not only the storage structure of water, but also its migration pathways during the drying process. Thus, the variation of macropores, mesopores and micropores of lignite in drying process were studied. Reduction of macropores mainly occurred in the pore diameter range above 1 μm, which might affect the moisture flow in the drying process. The collapse of the macropore got more serious if higher temperature was used in drying process. The main changes of the mesopores were in the range below 20 nm, and at high temperature, mesoporous volume first decreased, and then increased. CO2 adsorption was used for the micropore analysis, and its adsorption amount was more than N2. The adsorption capacity of CO2 on lignite decreased until last stage of the drying process, so to get higher SA products, the drying time should be controlled in 5-10 min with the removal of 50-70 % moisture, and then turned to slow drying. During the drying process, water of different relaxation time decreased rapidly, which in larger pores was completely discharged and in smaller pores was partly removed. In the first several minutes, stronger interaction between coal and water was selected, which means some water migrated to the smaller pores maybe due to uneven forces in the tapered capillary ends. Differences between wet and dry samples from N2 adsorption was used to demonstrate the water distribution and saturation in mesopores, and found that most of the water was concentrated in the pores of 26-100 nm. During drying process, the water in pores >20 nm first removed. After 5 min, the removal of water expanded to pores >10 nm, and after 10 min, the removal of water expanded to >5 nm. Water saturation decreased first and then increased.Finally, moisture transport mechanisms on pore scale in lignite were explored. SEM images of lignite were obtained first, and then edge extraction was used to get the two-dimensional model for the heat and mass transfer processes under COMSOL simulation. Diffusion process in dry zone was directly related to the concentration gradient, as well as the path lengths of the mass transfer, and was less relevant with pore throat size in the path. While in the wet zone, two-phase flow was not only directly related to the pore size in the path, but also affected by the pore throat size, and maintaining the large pore size was beneficial for mass transfer process. In the initial drying process, the moisture content in the lignite particles was high, and most area of the particle was in wet zone. If more energy was input, there would be higher transient vapor pressure to keep the pore volume in the particles, which was conducive to maintaining the pore volume within lignite particles, and promoted rapid drying process. In the post-drying process, higher vapor concentration gradient between the inside the particles and the outside world and shorter "redundant" diffusion paths are conducive to the diffusion process.In summary, this paper focused on the lignite evaporative drying process, and explored the response of chemistry structure, pore structure and moisture content of lignite to the energy input in drying process. And the moisture transport process on microscopic and mesoscopic scale on surface and in the pores of lignite was explained.