单颗粒褐煤烟气干燥过程模型研究

Modeling of Single Lignite Particle Drying Process in Flue Gas

褐煤干燥对提高褐煤品质具有重要意义。为了深入研究高温烟气干燥褐煤的物理过程,在一维球坐标系下对单个球形褐煤颗粒的干燥脱水过程建立了数值模型。模型中以褐煤颗粒内的蒸发界面为基础,将褐煤颗粒分为干区和湿区,对湿区求解传热方程,对干区求解烟气传热和传质方程,模拟褐煤干燥中的水分蒸发过程。模型采用Crank-Nicolson隐式差分方法进行离散化,模拟得到单个褐煤颗粒动态蒸发的过程。利用该模型分析不同粒径褐煤干燥过程中的烟气温度、颗粒含水量、颗粒内部温度分布等变化规律。发现颗粒最高温度不超过烟气和颗粒的最终平衡温度。褐煤颗粒粒径和初始烟气温度对褐煤的干燥过程有重要影响,较高的初始烟气温度条件下所需的干燥时间短,干燥时间近似与褐煤颗粒粒径的平方成正比。

Lignite drying is very useful for improving lignite quality. In order to deeply understand lignite drying process in high temperature flue gas, a onedimensional spherical numerical model of a single lignite particle drying process in flue gas was proposed. In this model, lignite particle was divided into dry region and wet region by an evaporation interface. In the wet region, only heat transfer was considered. In the dry region, both heat transfer and mass transfer were involved to describe water evaporation. Crank-Nicolson scheme was adopted to describe the transport equations so that dynamic evaporation process can be correctly solved in a large time step. With the proposed model, gas temperature, moisture content and temperature distribution within the particle varying with the resident time were simulated during drying process. It is found that the maximum particle temperature during drying process is lower than the particleflue gas balance temperature. Particle diameter as well as initial flue gas temperature has an important effect on lignite particle drying process. Particle drying time, td becomes shorter as the initial flue gas temperature increases, td is also proportional to the square of particle diameter.