Study On The Mechanism Of Lignite Dewatering By Vibration Mechanical Thermal Expression Process

As the production of lignite in China gradually increased in recent years, lignite became the main source of energy in some areas. But the high moisture content in lignite exerted strong influences on their utilization processes. Effective dewatering technologies are necessary for lignite preparation prior to large scale utilization. In this study, the Vibration Mechanical Thermal Expression(VMTE) dewatering process was developed from Mechanical Thermal Expression(MTE) dewatering process. The dewatering process was accelerated and the efficiency of dewatering was enhanced by vibration. The lignite could be simultaneously dewatered and briquetted by VMTE process under moderate conditions, problems existing in lignite dewatering processing, such as serious dust pollution, reabsorption and spontaneous combustion was avoid.The effect of temperature, mechanical pressure and vibration on Zhaotong, Xiaolongtan and East Mongolia lignite dewatering during VMTE and MTE process was studied and compared, the effect of VMTE and MTE process on coal properties was analyzed. The dewatering process was accelerated, the dewatering rate was enhanced and the time cost for dewatering was reduced by vibration. The lower moisture content in lignite could be reached by VMTE process compare to MTE process with same temperature and mechanical pressure. Under 200 oC, mechanical pressure 10 MPa and vibration pressure 3.14 MPa, 80.0% of moisture in Zhaotong lignite was removed by VMTE process, about 10% higher than MTE process. With decreasing moisture content in lignite during dewatering process, the porosity of lignite gradually decreased, the pore size distribution was significantly changed. The distribution of inter-particles pores and macropores decreased and shift to mesopores. The mean pore size decreased with the decrease of moisture content, but the surface areas remain relatively unaffected. The oxygen functional groups gradually decomposed with the increase of temperature. The hydroxyl and carboxyl decomposed when the temperature reached 150 oC and 200 oC, respectively.The mechanism of VMTE dewatering process affected by temperature, mechanical pressure and vibration was discussed. More moisture could be activated under higher temperature. So, more moisture could cross the energy barrier in unit time, the flow rate of water from lignite increased and the dewatering process was accelerated under higher temperature. The increasing mechanical pressure increased the pressure act on liquid phase in lignite. As the outlet pressure kept constant, the seepage pressure increased with the increasing mechanical pressure which resulted in the increase the flow rate of water from lignite and the accelerating of dewatering process. At the same time, the increasing mechanical pressure increased the pressure act on solid phase in lignite which decreased the porosity of lignite where the moisture exists in. The increase of vibration pressure contributed to the decrease of moisture content in lignite. The variation of vibration pressure in range 1.13 MPa to 3.40 MPa has relative obvious effect on lignite dewatering by VMTE process. The best dewatering performance of VMTE process occurred when the frequency of vibration approached to natural frequency. The higher or lower frequency of vibration than natural frequency acted negative effect on lignite dewatering by VMTE process. The time need for reducing the moisture content in lignite to the same value by VMTE process was significantly lower than that by MTE process. With more initial lignite sample mass, the acceleration of dewatering process by vibration was more obvious.The feasibility of lignite briquetting by VMTE process was proved. Within the range of experimental conditions, the compressive strength of briquette produced by VMTE process from Zhaotong lignite and Xiaolongtan lignite reached 935.3 k Pa and 514 k Pa which are sufficient to ensure the survival of the briquette from handling and transportation. The VMTE conditions and coal properties significantly affected lignite binderless briquetting during VMTE process. The binder materials, such as coal tar pitch, could be softened with increasing temperature until it became a flowable fluid resulted in the enhancement of the performance as binders which increased the compressive strength of briquettes. With high mechanical pressure, the coal particles were forced to contact each other sufficiently which increased the contact area and resulted in high compressive strength. Vibration could accelerate dewatering and make the briquette more compact which has positive effect on briquetting. But too high intensity of vibration has negative effect of the formation of interaction force between coal particles which decreased the compressive strength of briquette. The best briquetting performance of VMTE process occurred when the frequency of vibration approached to natural frequency. The higher or lower frequency of vibration than natural frequency acted negative effect on lignite binderless briquetting by VMTE process. With decrease of moisture content in briquette, the average distance of long distance attraction force was reduced and more hydrogen bond replaced the long distance force between surfaces of coal particles resulting in the increase of compressive strength. Humic acid, carboxyl(-COOH) and hydroxyl(-OH) could form hydrogen bonds which increased the interaction force between surfaces of coal particles and enhance the compressive strength of briquette.The energy consumptions during coal dewatering process were examined. The dewatering process was divided to three stages. In the 1st stage, the energy consumption of dewatering lower than 2300 k J/kg, the water which originally existed in the spaces between coal particles and large pores was removed. In the 2nd stage, the energy consumption of dewatering ranged from 2300 to 3500 k J/kg, and the water in progressively smaller capillaries or pores and water clusters around the functional groups was removed and degraded. In the 3rd stage, the energy consumption of dewatering higher than 3500 k J/kg, the water molecules directly absorbed by polar functional groups on coal surface were removed. Carboxyl concentration on coal surface significantly affected dewatering process. The reduction of moisture contents of coals at each stage were linearly related with carboxyl concentrations. In the 2nd and 3rd stage, the energy consumptions of coals in the same residual moisture content almost linearly increased with the increasing carboxyl concentration. Based on experimental data, the numerical calculation method of energy consumption for coal dewatering as the function of coal properties was obtained.The activated moisture content during dewatering process was calculated based on the activation of water by thermal and the energy consumption during dewatering process. The pore occupied by the activated water in coal is the effective channel for water transfer in the dewatering process was proposed. The effective porosity was defined and applied to the modeling of VMTE process instead of porosity in common sense. The numerical relationship between the mechanical pressure and the pressure act on liquid phase during MTE process was obtained and corrected by semi empirical formula for its application to VMTE process. The model of lignite dewatering by VMTE process was established. The simulation results of the model were in agreement with the experimental results, which proved the model could be applied to predict of lignite dewatering by VMTE process.