Biomass Torrefaction in a Two-Stage Rotary Reactor: Modeling and Experimental Validation

A mechanistic model of torrefaction was developed for a two-stage rotary reactor, and it was verified with experimental results. Mass and energy balances for each phase are considered in the model. A kinetic model that considers the progressive decomposition of biomass into volatiles and char released simultaneously from the raw biomass was coupled to the balances. Mathematical expressions for residence time, heat transfer coefficient, and bed height inside the kiln were taken from the literature for model calculations. Release of condensable and noncondensable volatiles from biomass during the process was considered in the gas phase, while the solid phase included raw and torrefied biomass. The model can predict different output parameters of torrefaction in a rotary continuous torrefier, such as final amounts of solids, gas yields, and temperatures, for different operational conditions. Properties for torrefied solid, such as high heating value, fixed carbon, and volatile matter, can also be predicted by the model through mathematical correlations obtained in a previous experimental work. The results obtained from the model were compared to experimental data, and good agreement was found.