Experimental Study And Kinetic Analysis Of Lignite Drying

The lignite accounts for about 13% of the reserves of coal resources in China. Due to the increasing energy demand and environmental concern, the technology of clean efficient utilization of lignite gains more attention. However, high moisture content in lignite has kept it from being deeply exploited, drying is therefore a key process for the technology of lignite utilization and gains much concern in the whole world. In this paper, the drying characteristics of the lignite are investigated through the drying experiments and drying kinetic analysis.The thermo-gravimetric and physical-chemical analysis is applied to determine the temperature range of drying finding that the lignite can be dried at temperature lower than 180℃. The equilibrium moisture content (EMC) of Ningxia lignite in the condition of temperature from 50℃to 70℃and relative humidity from 10% to 98% is analyzed with static desiccate method. The optimal empirical formula which can be used to predict the value of equilibrium moisture content (EMC) is obtained by fitting the experimental data to five kinds of experimental formulas.Ningxia lignite granules with different particle sizes were dried at 80℃～120℃to investigate the drying characteristics with constant temperature. The results show that there are two stages of lignite drying process-accelerating rate stage and decelerating rate stage, and the drying rate increases with the temperature rising and the particle sizes decreasing. The relationship between temperature and the effective diffusivity coefficient can be expressed by Arrhenius-type equation.In order to investigate the effect of relative humidity on drying rate, drying experiments of lignite were carried out on an experimental system which can keep temperature and relative humidity constant. It is found that when the air temperature rises from 50℃to 90℃and the relative humidity decreases from 30% to 10%, the drying rate increases significantly. Different mathematical models were applied to simulate the lignite drying curves. The results indicate that the Page model can better describe the change of the moisture content with the drying time. The traditional Arrhenius-type equation which only takes the effect of temperature into consideration is modified to accurately express the effect of both temperature and relative humidity on the effective diffusivity coefficient.