BAGASSE DRYER DESIGN USING MULTI-SCALE COMPARTMENT MODELS

COGENERATION IN the Australian sugar industry has the potential to provide significant environmental and economic benefits. Central to increasing cogenerated energy output is the use of bagasse drying to improve boiler efficiency. In a previous numerical study, pre-drying of bagasse from 50% moisture to 30% moisture using waste flue gas was shown to deliver up to 16% reduction in bagasse usage and up to 24% increase in specific steam production. Across the world both flighted rotary dryers and flash dryers have been used to dry bagasse fibre for enhanced boiler operation. In this paper, the design and preliminary costing of a flighted rotary dryer processing 50 t/h of Burdekin bagasse fibre from 50% moisture to 38% moisture using waste flue gas is described. Operational scenarios, such as the effect of varying flue gas temperature on dryer design, are evaluated. Key design data such as the drying rate and bagasse angle of repose were determined by experiments using a Spencer oven and pilot scale rotary dryer, respectively. The development and use of a multi-scale compartment model for sizing the dryer is described. In this model, the impact of flight and drum geometry on dryer loading and dryer design were directly accounted for. The results of this investigation showed that both energy and material costs of a dryer are highly dependent on the loading state and material properties such as angle of repose and bulk density. A new design methodology which used simulated trails with variable dryer loading states showed a distinct minimum in costs occurring at the point where the particular dryer was operating closest to its design loading. The optimum flighted rotary bagasse dryer design was found by minimising total cost (electrical energy and raw material requirements), and was shown to occur at design loading. Although the results presented in this work relate to a fixed set of operating conditions, it is likely that the methodology used and conclusions drawn can be applied to the general design and operation of flighted rotary dryers.