Fluidized bed drying of Loy Yang brown coal with variation of temperature, relative humidity, fluidization velocity and formulation of its drying rate

The fluidized bed drying of Loy Yang coal, which is one of Australia's representative Victorian brown coals exhibiting a high moisture content (up to ca. 65 wt.%, as-received base), using air as the bubbling gas was examined by varying temperature, relative humidity, and fluidization velocity at 40-80 °C, 0-40%, and 10.0-35.0 cm/s, respectively. The effects of the three variables on the drying rate and drying time were investigated. Higher temperature, lower relative humidity, and higher fluidization velocity were favorable for drying, i.e., they showed high drying rate. Drying rate was maximum immediately after the set drying temperature was attained; then, it decreased at a roughly constant rate, indicating a falling-rate drying period (constant-decrease drying period) and finally became zero. Drying rate could be described by a simple equation, dw/dt = -k(1-X)ⁿ; therefore, it was a function of the drying rate constant k, drying fraction X, and drying rate order n. The drying rate order n could be taken as 0.25; it is independent of temperature, relative humidity or fluidization velocity. The drying rate constant k could be expressed as a function of temperature, relative humidity, fluidization velocity; within the limits of the experimental error, k was a linear function of each variable, and it increased with increasing temperature, decreasing relative humidity, and increasing fluidization velocity.