Experimental Study On Solar Drying Of Lignite

Lignite has low price and worldwide abundant minable reserves. However, due to the high moisture content and low heat value, lignite is not widely used. Dewatering and improving efficiency of lignite when lignite is transported to be utilized is an effective way of reducing costs and enhancing the market competitiveness.Solar drying of lignite is originally proposed here based on high absorptivity of lignite due to its black brown color and rich, clean and renewable solar energy in lignite producing areas. A series of work were performed as follows:Using an oven, the drying characteristics of lignite under isothermal conditions was investigated. It can be concluded that drying process occurred mainly in falling rate drying period when isothermal drying of lignite.Two sets of solar drying systems were designed and built:direct-mode type solar drying systems and mixed type solar drying systems. In the former one, feasibility and preliminary study on solar drying of lignite was conducted. It concluded that the temperature of lignite was much higher than water and air under the same experimental conditions. Therefore, the technology of solar drying is good for lignite. Lignite has different drying characteristics in solar drying preheating and without preheating. Moreover, influencing factors in solar drying of lignite, including the effect of solar radiation intensity, the particle size of lignite and the thickness of lignite layer, were investigated. The results indicated that strong solar radiation, small lignite particles and thin lignite layer are good for the increase of the performance of solar drying of lignite.Thin layer drying characteristics of lignite was studied in the mixed type solar drying systems. It concluded that thin layer solar drying lignite mainly occurred in the falling rate drying period. Besides drying curve and drying rate curves expressed as exponential function.Characteristics of Solar drying lignite were simulated using origin software. Six mathematical models of thin layer drying are selected in order to estimate the suitable model for describing the drying curves. Logarithmic model gives the best prediction of the drying curves. Experimental and computational results show that Logarithmic model can fit the data of indoor experiment under artificial solar systems and outdoor experiment under real sunlight very well with a correlation coefficient R of about 0.99.Therefore logarithmic model can be considered as model of solar drying of ignites.The efficiency of two kinds of drying systems was calculated at last. Both of them have higher efficiency in the earlier period of drying than efficiency in the later period. As a comparison, the efficiency of mixed type solar drying systems is 19.7%, while 14.5% for direct-mode type solar drying systems in the earlier period of drying. the efficiency of the two in whole Stage has little gap, were around 11%.