孔隙结构对褐煤干燥动力学的影响

Effects of pore structure on drying kinetics of lignite

研究了胜利和昭通褐煤在低温(60～140℃)干燥过程中孔隙结构的演变,计算了2种不同结构褐煤脱水过程的有效水分扩散系数(D_(eff)),讨论了褐煤孔隙结构参数和水分扩散特性参数的关系。结果表明,胜利褐煤的孔隙结构以开放的圆柱形孔隙、平行壁状狭缝孔和尖劈型毛细孔为主;而昭通褐煤以一端封闭的不透气性孔为主。随着干燥时间增加,2种褐煤比表面积均减小,平均孔径变化趋势与孔容变化相反。随着干燥温度升高,2种褐煤比表面积、孔容、平均孔径均增大,平均孔径变化趋势与孔容变化相同。关联动力学参数表明,煤样的孔容比及比表面积都与D_(eff)有较好关联度,2种褐煤的孔容比与比表面积呈正相关;水分扩散系数更是与比表面积呈线性关系。胜利褐煤在孔径小于7 nm时孔容比随温度的升高而增大;而孔径区间介于7～10 nm的孔容比随温度的升高而减小。昭通褐煤在孔径小于10 nm的孔容比随温度的升高而增大;而孔径区间介于10～20 nm的孔容比随温度的升高而减小。2种褐煤的D_(eff)均受微孔及较小孔径的中孔控制。同时,计算得出低温干燥过程胜利褐煤的活化能(16.95 kJ/mol)低于昭通褐煤(21.84 kJ/mol),这说明相同条件下胜利褐煤脱水所克服的能垒更低。

The pore structure variation of Shengli and Zhaotong lignite were investigated in the low temperature drying process(60-140 ℃).Meanwhile,the effective moisture diffusivities( D eff )of the two types of lignite were calculated,from which the relationship between pore structure and effective moisture diffusivities was discussed.The results indicate that pore structure of Shengli lignite is dominated by open cylindrical pores and cracks pores,whilst,it is mainly dead end pores closed at one end for Zhaotong lignite.Furthermore,by increasing the drying time,the specific surface area of two lignite decreases and the variation trend of pore volume is opposite to that of average pore diameter.While the specific surface area,pore volume and average pore size increase with the improvement of drying temperature and the variation trend of average pore size is the same as that of pore volume.The correlation dynamics parameters demonstrate that the pore volume fraction and specific surface area of coal samples have good correlation with D eff .The pore volume fraction of lignite is positively related to the specific surface area,and the D eff is linear with the specific surface area.With the increase of the temperature,the pore volume fraction of the Shengli lignite is increased when the pore diameter is less than 7 nm and decreased at 7-10 nm,but difference is that the pore volume fraction of Zhaotong lignite is increased when the pore diameter is less than 10 nm,and decreased at 10-20 nm. D eff is controlled by micropores and mesoporous pores.Meanwhile,the data manifest that Shengli lignite (16.95 kJ/mol) possesses lower activation energy than Zhaotong lignite (21.84 kJ/mol) in the low temperature drying process,implying that the actual energy required for the drying of Shengli lignite is relatively low.