Analysis Of Lignite Heat And Mass Transfer On Micro-scale During The Drying Process

As a combustible fossil fuel, lignite which has been proven to reserve of 130 billion tons in China has good prospects for the development and utilization. The high moisture content, ash and volatiles greatly affect the use of lignite. It is necessary to dry to improve the use of lignite. At present, the study of lignite drying mainly focuses on the drying characteristics. On the other hand, the analysis of heat and mass transfer during the drying process focuses on the theoretical formula derivation and calculations and ignores the impact of lignite internal pore structure on the heat and mass transfer.In this paper, the theory of discrete wavelet transform are applied to transform the 2000X SEM image of lignite to get the microscopic pore structure of the two-dimensional network model. Compared the theoretical with actual values of the average pore diameter and porosity, the error is smaller, which verifies the accuracy of the geometric model. This method provides a basis research on the microstructure of lignite and has a better understanding on the distribution of internal pores.The hydrodynamic of porous media is applied to the heat and mass transfer during the drying process. As a saturation region, the internal drying conditions at low temperatures are analyzed to study the effects of different wind speeds on the distribution of internal flow. The results show that the wind has little effect on the drying rate and the best speed of drying wind is 0.9m/s-1.00m/s. By simplifying the geometric model to simulate the process of water seepage in the skeleton, the largest flow velocity (1.55×10-7m/s) in the pores is much smaller than the inner velocity (5.51×10-5m/s). Therefore the impact of the seepage velocity can be ignored. The theory of heat transfer in porous media is used to analysis the effect of different outlet temperature on distribution in the flow field under the influence of pressure and temperature coupling conditions. The results show that as the temperature increases the maximum velocity internal increases linearly. However, the temperature has little effect on the overall distribution of the internal fluid and the best speed of drying temperature is 90℃. With the temperature inside of the particle rising, the fluid velocity increases further.In order to simulate the volume shrinkage of the lignite particles during the drying process due to thermal deformation or collapse, the pore size and distribution are changed to study the influence of the shrink pores and connectivity on the internal flow distribution. After narrowing the pore, the velocity inside the pores is faster than before. As part of the pore connecting, the internal flow rate increased slightly but less than the pore fluid velocity before. When the shrinkage occurs, it can be appropriate to extend the drying time to achieve better drying effect.Through the above simulation analysis of lignite under low-temperature drying conditions, the research obtains the optimal values of wind speed and temperature and the effect of volume shrinkage on drying efficiency, which plays a guiding role on actual drying experiment.