Enhancing the Efficiency of Fluidized Bed Reactors for Biomass Torrefaction

This paper examines the study of an advanced design for a continuously operating fluidized bed reactor applied to sunflower husk torrefaction in a superheated steam environment, which is simulated in a cold model of the reactor. The simulated reactor has a diameter of 0.3 m and contains six vertical baffles installed along the reactor walls, providing loop-like movement of crushed husk particles from the reactor loading point to the reactor biomass unloading point. The residence time of crushed sunflower husk biomass particles in the reactor was studied by introducing colored biomass particles into the bed; these particles had the same mass and size as the undyed material. According to modeling results, replacing superheated steam with room-temperature air in the “cold” model may not lead to significant changes in the hydrodynamics of the fluidized bed or their effect on particle mixing. Experiments were conducted at an air velocity of 0.6 m/s relative to the cross-section of the empty apparatus, which is 3.5-fold greater than the minimum fluidization velocity. Samples were collected at the reactor outlet and dissolved in distilled water. The transparency of the resulting solution was measured using a KFK-3 photometer, and the amount of colored substance in each sample was determined accordingly. The most probable average residence time of biomass particles in the cold model amounted to 6–8 min at a Peclet number of 47. To ensure full operation of the torrefaction reactor under ideal plug-flow conditions, the reactor must be equipped with 24 baffles. The residence time of biomass particles required for optimal operation of the reactor was estimated to 24–32 min, which may be sufficient to produce biochar with a high calorific value, suitable for co-firing with coal.