Predicting the steam drying behavior of brown coals and lignites

Our drying model explicitly accounts for the three forms of moisture in low-rank coals -monolayer, multilayer, and bulk -as a basis to predict the distinctive drying behavior and energy requirements of individual low-rank coal samples. The mass loadings of the moisture forms are estimated from a database of equilibrium moisture contents that were correlated to parameters from a coal's proximate and ultimate analyses. The drying analysis is validated with laboratory tests on steam drying in autoclaves and a fluidized bed that cover broad ranges of steam pressures and temperatures for an assortment of Australian brown coals. Then the analysis determines which forms of moisture and operating conditions most affect drying times. For some selections of samples, drying time will be proportional to total moisture, and for others, there will be exceptions. One key aspect is whether or not the target moisture level requires removal of any monolayer moisture at all. Coals with abundant bulk and multilayer moisture can achieve targets on residual moisture by vaporizing only a portion of the multilayer moisture. Consequently, their drying times will be relatively modest. But coals with little bulk moisture and relatively abundant monolayer moisture will require relatively long drying times simply because monolayer moisture is always released much more slowly than multilayer moisture. Hence, the levels of the three forms of moisture in any particular coal sample are determining factors for drying times. (C) 2015 Elsevier Ltd. All rights reserved.