Submitted to: Tappsa Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/13/2012
Publication Date: 4/7/2012
Citation: Swart, S.D., Heydenrych, M.D., Boateng, A.A. 2012. Dual fluidized bed design for the fast pyrolysis of biomass. Tappsa Journal. 2:18-25. Interpretive Summary: Through a non-funded cooperative research agreement between ARS and University of Pretoria in South Africa, scientists sought to develop an energy self-sufficient reactor for the production of renewable biocrude. A two-zone fluidized-bed system capable of generating hot material in one zone (combustion zone) for use in enabling the thermal decomposition of the biomass in another zone comprising an inert atmosphere (pyrolysis zone) was designed. A cold-flow model of the design was constructed and successfully tested at ARS laboratory in Wyndmoor, Pennsylvania. No pressure or materials transfer problems were encountered. The proposed dual bed system therefore proved to be a feasible system for an efficient conversion of biomass by the so-called fast pyrolysis of biomass. The design has been the basis for farm-based biofuels production system currently under construction at ARS promised to help farmer meet their energy demand.
Technical Abstract: A mechanism for the transport of solids between fluidised beds in dual fluidised bed systems for the fast pyrolysis of biomass process was selected. This mechanism makes use of an overflow standpipe to transport solids from the fluidised bed used for the combustion reactions to a second fluidised bed, which is used for the endothermic pyrolysis reactions. A screw conveyor is used to transport the solids back to the combustion fluidised bed. Several experiments were performed on a cold model of the system in order to test the performance of the solid transfer mechanism in the dual fluidised bed design. It was found that the pressure drops over the combustion and pyrolysis fluidised beds were unaffected by changes in the speed of the screw conveyor and pyrolysis gas flow rate. The pressure drop over the pyrolysis bed was found to be mostly dependant on the flow of gas in the combustion bed due to its higher flow rate. As the combustion gas flow rate increased, the pressure drop over the combustion bed decreased and the pressure drop over the pyrolysis bed increased. This may be due to the flow of gas from the pyrolysis bed through the standpipe. A change in the amount of solids charged to the system had a negligible effect on the response of the pressure drop over the combustion and pyrolysis fluidised beds and the height of the solids in the pyrolysis bed to changes in the combustion and pyrolysis gas flow rate and the screw conveyor speed. However, an increase in the amount of solids charged did have a dampening effect on the rate of spills of solids into the overflow standpipe. It also stabilised the response of the rate of spills to changes in the combustion gas flow rate. The solid transfer mechanism conformed to the requirements which were identified for the feasibility of the mechanism in the fast pyrolysis of biomass process. The proposed dual fluidised bed system is therefore a feasible system for the fast pyrolysis of biomass.