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Biomass torrefaction in a pulsed fluidized bed reactor Wang, Ruixu


Torrefaction at 200–350 °C in an inert or oxygen-deficient environment is a promising pre-treatment to improve the properties of biomass for energy utilization. Fluidized beds with enhanced heat and mass transfer could be a candidate for biomass thermal treatment. Gas pulsation has been proven effective in fluidizing biomass particles with unconventional natures that caused problems in fluidization. A novel pulsed fluidized bed (PFB) reactor for continuous biomass torrefaction was designed and commissioned in this work. The bed has a length-to-width ratio of 10:3 to limit the solids residence time distribution (RTD), leading to uniform solids products. Biomass particles transport and backmixing were studied by solids RTD measurement. A 2D axial dispersion model with an exchange flow between the active and stagnant zones was proposed to quantify solids backmixing by fitting the model to RTD curves. Horizontal dispersion coefficient representing the backmixing degree were slightly higher in the deeper bed, increased with increasing particle velocity in the shallow bed, and increased greatly with increased gas velocity. Less backmixing was obtained when the bed was operated under a gas pulsation frequency close to the bed natural frequency. Horizontal solids dispersion coefficients obtained in horizontal PFB were significantly smaller than values calculated from literature. A correlation of horizontal dispersion coefficients for biomass particles in the horizontal PFB of shallow beds was established. Continuous biomass torrefaction was successfully performed in the PFB unit with a feed rate of up to 2 kg/h. Maximum weight loss was identified at the frequency range of 2–4 Hz for feed rates of 1 kg/h and 1.5 kg/h. The temperature near the reactor exit exhibited the most significant influence on the biomass weight loss and the product properties, i.e., higher heating value (HHV), contents of proximate analysis components, and elemental carbon content, whereas the feed rate had a slight effect. Torrefied biomass obtained in fluidizing gas with 3–6 vol.% oxygen concentrations showed similar HHV with that of non-oxidative torrefaction. Compared to other continuous torrefaction technologies, the PFB torrefier utilized small particles with low residence time.

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