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.