The influence of gas velocity and fibre density on the drying kinetics of bagasse

THE PRODUCTION EFFICIENCY of steam and electricity from cogeneration boilers can be optimised by employing waste heat (in the form of boiler flue gas) for pre-combustion drying of bagasse fibre. Safety and economic concerns demand accurate drying kinetics to guide dryer design and mitigate risk. This research involved the commissioning of a convective drying apparatus to manipulate and control gas temperature, gas velocity and fibre density. Bagasse moisture content was monitored over time and moisture ratio data were fit to Fick?s Second Law of Diffusion. The experimental design was formulated such that the impact of gas velocities and fibre bundle densities could be statistically analysed using ANOVA procedures. Gas temperature was maintained at 120? C throughout all testing. Building on the work of Slogrove (2016), bundle densities of 110, 130 and 150 kg/m? were examined, and gas velocities were varied to encompass typical industrial dryer gas velocities (0.64?1.92 m/s). Fresh bagasse was sourced from the Burdekin growing region and samples were dried to a moisture content of 10wt%, to expand the model?s region of validity. Fick?s Second Law of Diffusion fitted the data well (R? > 0.92) and effective diffusivities ranged from 1.3 to 17 x 10?7 m?/s across treatment groups, suggesting a 5 to 25-fold increase in drying rate associated with gas driven convection. The ANOVA analysis concluded a significant effect of both velocity and density on the effective diffusivity. Post-hoc testing was used to identify a critical maximum velocity above which drying rates plateaued. Regression analysis was used to develop functions describing the effect of velocity on the diffusion constant (Do).