Study Of Lignite Pore Characteristics And Moisture Migration In Dewatering Process

The storage capacity of Lignite is very rich. However, due to the low degree of coalification, resulting in the high water content, and therefore it’s not utilized normally by people. There is large amounts of water in lignite, the drying process is a complex transfer process, which is coupled with heat and mass, including physical and chemical changes, and it’s extremely important for the building of three-dimensional model for lignite. It’s important for improving the efficiency of dehydration and energy optimizing to build the physical model of the lignite, which can accurately describe the characteristic of the lignite and the moisture migration.Firstly, the three kinds of lignite are analyzed by elemental analysis, industry analysis, FTIR, TG and vacuum drying curve to explore the basic physical and chemical properties. Then we used low-temperature nitrogen adsorption, CO2 adsorption, mercury porosimetry and SEM to analyze the pore structure and specific surface of the Shengli, Zhaotong, Xiaolongtan lignite, the results showed that a lot of holes and cracks were dispersed inside and on the surface of the Shengli lignite, which causes a better connection. There are a large amount of holes and cracks between 0.5-0.6nm, 0.8-0.9nm, 0-100 nm and 10-370μm. Much more macroporous, mesoporous and ultrafine pores are slit-shaped holes and the sharp of the microporous is ink bottle type and blind holes. The surface and inside of Zhaotong lignite are denser than Shengli lignite, layered structure can be seen obviously. There are a large amount of holes and cracks between 0.8-1nm, 0-200 nm,50-300μm. Much more macroporous, mesoporous and miscroporous are slit-shaped holes. The sharp of the ultrafine pores is ink bottle type. The surface of Xiaolongtan lignite is similar with Zhaotong lignite. And there are a large amount of holes and cracks between 0.6nm-0.9nm,0-100 nm and 100-300μm. Much more macroporous and mesoporous are slit-shaped holes. The sharp of most of the microporous is ink bottle holes and blind holes. The ultrafine pores are almost ink bottle holes.We select the parameters of the macroporous, mesoporous and microporous generated by low-temperature nitrogen adsorption, CO2 adsorption and mercury porosimetry. Based on these parameters, the two-dimensional physical model unit, three-dimensional model unit and overall three-dimensional model of lignite particle is established by Visual Basic and Matlab, which can characterize the pore structure, space coordinates and connectivity status. And the moisture migration process was been simulated based on the parameters and the state of the path. The length of path and curvature was got in this way. The result shows that the length of moisture migration path in the same positions of,three kinds of lignite is similar, the curvature of the Zhaotong lignite is smallest in these three kinds of lignite, which shows that the moisture migration of Zhaotong lignite is fastest. And by drying experiments and correlation analysis, it was proved that model water migration modeling of three-dimensional has a strong correlation with experimental results.At last the model was tested by the drying experiment base on the experimental platform built by us. The result shows tha the t temperature can significantly enhance the rate of dewatering, under the same conditions of nitrogen flow rate(1.2L/min), the drying rate of Shengli lignite reaches the maximum at 300 ℃, 2-3min and the dewatering rate reached more than 30% in less than 5min, the drying rate of Zhaotong lignite reaches the maximum at 300 ℃, 3min and the dewatering rate reached more than 60% in less than 4min, Xiaolongtan lignite reaches the maximum at 250-300 ℃, 2min and the dewatering rate reached more than 30% in less than 5min. The nitrogen flow rate has a weak influence in the rate of dehydration. Under the same conditions of temperature(200℃), the drying rate of Shengli lignite reaches the maximum at 1.4-1.6L/min,2-3min and the dewatering rate reached more than 30% in less than 5min, the drying rate of Zhaotong lignite reaches the maximum at 1.4-1.6L/min, 3min and the dewatering rate reached more than 55% in less than 10 min, the drying rate of Xiaolongtan lignite reaches the maximum at 1.2-1.6L/min,4min and the dewatering rate reached more than 30% in less than 7min.At last, the moisture migration process was been simulated by using capillary with different path state and aperture size, which is similar with the structural properties of the lignite and produced by ourselves. The result shows that the vaporization occurs only in the path which contains the gas-liquid interface. For drying the fracture of semi-closed type and communicating type, the vaporization occurs at the open end of the fracture, and the surface of liquid was moving toward the interior and the internal gas-liquid interface simultaneously vaporized and pushed the water outward. Water is reduced until it disappears in the process of moving based on the synergies of both internal and external liquid interface. When heating process becomes unevenly, the water flow to the surface and the enrichment occurs. For the drying of macroporous, the liquid interface only move inwardly during the drying process, water flowing is difficult, the drying efficiency and thermal efficiency is low, which suggests that a partial of low temperature was generated and slow down the drying efficiency.