Pilot scale conical spouted bed pyrolysis reactor: Draft tube selection and hydrodynamic performance

A study has been conducted on the selection of a draft tube and its performance on a pilot scale conical spouted bed reactor used in the fast pyrolysis of biomass waste. The nonporous draft tube confers high stability to the bed and, furthermore, reduces the fluidizing agent flow rate, which means significant energy saving. The goal of the study is the optimization of the geometrical factors of the internal device in order to achieve suitable operating conditions, given that they have a great influence on bed hydrodynamics and, therefore, on heat transfer characteristics, residence time of the volatiles in the reactor and pyrolysis kinetics.The criteria for the selection are based on a compromise between gas flow rate and solid circulation rate in a bed operating under stable conditions. Moreover, studies have also been carried out on the performance of wood/sand mixtures and the critical elutriation diameter. The results show that a draft tube conical spouted bed operated with a suitable flow rate gives way to very good mixing, elutriation of light particles and short gas residence times.

The optimization of the draft tube performance on a pilot scale conical spouted bed reactor used in the fast pyrolysis of biomass wastes has been carried out. The figure shows the dimensionless diameter vs. u/ums ratio, the different operating regimes of the particles in the bed (mixing, elutriation, entrainment) are shown.Figure optionsDownload as PowerPoint slideHighlights
► Gas and solid flow patterns and geometric factors have been optimized.
► Draft tube greatly affects cycle times but entrainment height has a minor effect.
► The draft tube significantly affects gas distribution into annulus and spout.
► The operating gas velocity determines the size of the particles elutriated.