Drying of Low-Rank Coals: A Review of Fluidized Bed Technologies

The high moisture content of low-rank coals is a matter of concern in many areas, creating high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in conversion processes. It is important to reduce this moisture content to acceptable limits in order to decrease energy losses, transportation costs, and to increase product quality. Drying characteristics, chemical structure change, and energy consumption of low-rank coals as well as the affecting factors are discussed. This article represents a review of conventional fluidized bed drying technologies and additional field-assisted fluidized bed drying technologies, such as microwave, vibration, agitation, and acoustic field. Relative merits and limitations of the various additional fluidized bed drying technologies and the current state of their development are presented. Some of the unresolved problems are identified and directions for further research are suggested.