MODELLING AND EXPERIMENTAL DETERMINATION OF THE DRYING KINETICS OF BAGASSE FIBRE

BAGASSE’S VIABILITY AS a boiler fuel for electricity cogeneration is well-established, with a variety of positive outcomes successfully demonstrated. To further optimise electricity and steam production in cogeneration, waste heat (i.e. boiler flue gases) can be used to pre-dry the entering bagasse. Accurate dryer design depends on reliable drying rate data, but key influences on drying kinetics such as fibre bundle density and gas temperatures are not well-characterised. This research determined the rate of removal of bagasse moisture under varying fibre bundle density and gas temperature values, chosen to fit within the bounds dictated by typical industry processes. Theoretical drying models were fit to the rate data. The drying temperatures were set to 120, 140 and 160 °C to mimic flue gas conditions. As the density of bagasse is quite low when suspended in flash or rotary dryers, density levels of 90, 110, 130 and 150 kg/m3 were examined. Bagasse sourced from the Burdekin growing region (Pioneer and Kalamia Mills), with initial moisture contents varying between 43.7 and 47 wt% (wet basis), was dried to a moisture content of 30 wt% (wet basis). Fick’s second law of diffusion was used to model the data and fit very well (R2=0.9913). Theoretical models such as Fick’s second law are considered to be more reliable than empirical and semi-empirical models for extrapolating between treatment groups. ANOVA proved that both bulk density and temperature had a statistically significant influence on the drying rate. An Arrhenius expression for diffusivity was determined (R2=0.9861) and enabled the effects of temperature and density to be decoupled, as well as providing a high degree of confidence in the theoretical mechanism used to represent bagasse fibre bundle drying.