Drying Characteristics of Wet Brown Coal Particles in a Steam Fluidized Bed

Though the world has abundant resources of brown coal, its moisture content is often high. Thus, a fluidized bed drying system using superheated steam for fluidizing gas has been developed as a high-capacity and low-energy-consumption drying technology. Brown coal contains many small pores and hydrophilic functional groups due to its low coal rank, and the form of water in brown coal is classified to four types: monolayer water, multilayer water, capillary water and free water. Therefore, it is important to understand the drying characteristics of water in brown coal in the design of steam fluidized bed dryers. In this research, experiments were carried out to evaluate the drying characteristics of three kinds of brown coal by using batch-type steam fluidized bed drying test equipment. As a result, a quantitative relationship between moisture content and drying characteristics was obtained. Most water was removed at a constant rate. Critical moisture content was around 35%-dry. The bed temperature for an equilibrium moisture content of 10%-dry was 115 degrees C. A drying model of brown coal in which bed temperature and latent heat were given as functions of moisture content was proposed. The calculated results and the test results agreed well, and the model is expected to be effective in the design of steam fluidized bed dryers for brown coal.