A two-stage, inclined continuous rotary torrefier with novel flights has been developed in the Biomass Conversion Laboratory at Dalhousie University for improving biomass torrefaction processes. Experimental work on torrefaction of small poplar wood particles (0.5–1.0 mm) in the torrefier was undertaken for a deeper understanding of the working of such torrefiers where the volatile gas released was used as the torrefaction medium instead of nitrogen. The rotary torrefier is operated under different operating conditions by varying its rotational speed, tilt angle and temperature. Measured chemical and physical properties of the torrefied products included ultimate and proximate analysis, structural analysis, energy density, mass yield, energy yield, and bulk density. A novel probe was developed to collect samples of biomass and measure temperature at different interior points along the length of the rotary torrefaction reactor while the biomass was being progressively torrefied in it. Axial temperature distribution of the rotary torrefier showed a parabolic profile but the fixed carbon content, volatile, and energy density of biomass undergoing torrefaction varied linearly along the length of the torrefier. For torrefaction at 300 °C and 5 rpm and 1° of tilt angle the change in heating value was 40%, while the mass yield and energy yield of torrefied biomass were 34% and 48%, respectively. Results showed that temperature is the most important parameter in this torrefaction process.

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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.

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