Elucidation of the Pumping Effect during Microwave Drying of Lignite

Lignite could be a potentially valuable energy resource, but high moisture contents in the raw material hinder its use in industrial applications. While various studies have investigated the use of microwave radiation for pretreating and dehydrating lignite, the drying mechanism has still not been clearly elucidated. In this study, a digital video recorder was used to document progressive moisture migration on the surface of subdecimeter-sized lignite particles and track the reduction in moisture contents. The temperature and pressure history inside the samples during microwave drying was also investigated. The results showed that the drying process could be categorized into the following three stages: preheating, constant-rate drying, and falling-rate drying. The microwave pumping effect was observed on the surface of the samples at the constant-rate drying stage, thus indicating that the moisture inside the samples could be partly driven out in liquid form. An interesting observation was that the moisture content at the exterior zone of the sample was even higher than the initial moisture content at the constant-rate drying stage. Meanwhile, the highest temperature at the center core of the particles reached 120 degrees C and the corresponding internal vapor pressure equaled 2 atm, according to the acceptable vapor-liquid equilibrium. This high internal vapor pressure may have been responsible for generation of the jet flow of moisture in the solid matrix, namely, the microwave pumping effect. This knowledge should be very instructive for the development of efficient lignite drying technologies, especially from the perspective of energy conservation.

褐煤可能是一种具有潜在价值的能源资源，但原料中较高的水分含量阻碍了其在工业应用中的使用。虽然各种研究已经对微波辐射用于褐煤的预处理和脱水进行了调查，但干燥机制仍未得到清晰阐释。在这项研究中，使用了一台数字录像机来记录分米级以下褐煤颗粒表面水分的逐渐迁移，并追踪水分含量的降低。还对微波干燥过程中样品内部的温度和压力变化进行了研究。结果表明，干燥过程可分为以下三个阶段：预热、恒速干燥和降速干燥。在恒速干燥阶段，在样品表面观察到了微波泵浦效应，这表明样品内部的水分可以部分以液态形式被驱出。一个有趣的观察结果是，在恒速干燥阶段，样品外部区域的水分含量甚至高于初始水分含量。同时，根据可接受的气液平衡，颗粒中心核心的最高温度达到了120℃，相应的内部蒸汽压等于2个大气压。这种较高的内部蒸汽压可能是固体基质中水分射流产生的原因，即微波泵浦效应。这一认识对于高效褐煤干燥技术的发展，特别是从节能的角度来看，应该是非常有指导意义的。