Physical Modelling and Hydrodynamical Characterization of an Internally Circulating Fluidized Bed for Biomass Fast Pyrolysis

Biomass represents a promising renewable source for the substitution of fossil feedstocks in the production of fuels and platform chemicals. Biomass fast pyrolysis offers a direct route to produce liquid fuels and chemicals, with high feedstock flexibility and energy input efficiencies. Fast pyrolysis is characterized by operation at a moderate temperature, short residence time of the vapor and high heating rate. The liquid products offer significant advantages in storage and transport and for this reason they can be used as biofeedstock in a biorefinery scenario.

Critical issues of biomass fast pyrolysis are the heating rate, the raw biomass feeding point, the mixing of the solid fuel with the inert bed material, the contact time between char and vapours. Furthermore, residence times have to be carefully limited below 1s to avoid secondary homogeneous and heterogeneous pyrolysis reactions and the fluid dynamic behaviour of both solid and gaseous phases should be accurately controlled on the basis of the desired reactive pathways.

This study addresses a hydrodynamical characterization of an internally circulating fluidized bed reactor for the fast pyrolysis of biomass. Inert particles are continuously circulated between a fluidized, and a moving bed section, divided by a vertical baffle and interconnected by an opening at the bottom. Particles in the fluidized bed section are transported upwards over the baffle to the moving bed section, where they flow down by gravity and return in the fluidized bed across the opening. The hydrodynamic behaviour of the internally circulating fluidized bed is experimentally investigated in a cold flow model apparatus. Experiments aim at studying the driving force and the mechanism of particle circulation in the system. Experimental tests allow the measurement of solids circulation rate and the assessment of particle residence time distributions under different operating conditions and for different opening ratios.