Energy & Fuels, Vol.27, No.11, 6598-6606, 2013
Comparison of Superheated Steam and Air Fluidized-Bed Drying Characteristics of Victorian Brown Coals
Brown coals represent a large resource for Victoria, with reserves of over 500 years at the current rate of consumption. However, its high moisture content (similar to 60%) impedes its utilization in an efficient manner. Information on drying kinetics of these coals is still scarce, affecting the development of economic and efficient drying technology. This experimental and modeling study presents a comparison of the drying kinetics between steam fluidized-bed drying and air fluidized-bed drying of three Victorian brown coals: Loy Yang, Yallourn, and Morwell. The effects of gas temperature (100-200 degrees C), gas velocity (0.32-0.61 m/s), and particle size (0.5-1.7 mm) on the drying kinetics have been examined. Both air and steam fluidized bed drying has been found to result in similar trends, with air drying having shorter drying times compared to steam drying. Increase in temperature and velocity, and a reduced particle size decreases the length of time required for complete drying to occur. Comparing air to steam fluidized bed drying, the relative drying ratios for a similar conditions (such as 130-170 degrees C) remains consistent, with the exception of particle size. The resultant 130 degrees C dried coal was used to analyze the moisture readsorption properties of the coals. The data show that steam fluidized-bed dried coals readsorb less moisture than air fluidized-bed dried coal, regardless of the coal type, with an average difference in moisture of 1.6%. The physical and chemical characterization of dried coal shows that moisture readsorption is a function of the oxygen functional group content. Several drying models available in the literature were compared against the experimental data acquired during this study. The results indicate that the Midilli- Kucuk model accurately describes the drying kinetics of the three investigated Victorian brown coals.