18 TAPPSA JOURNAL | VOLUME 2 2012 Dual luidised bed design for the fast pyrolysis of biomass S.D. Swart 1 , M.D. Heydenrych 1 , A.A. Boateng 2 1 Department of Chemical Engineering, University of Pretoria, South Africa 2 Agricultural Research Service, USDA, USA ABSTRACT A mechanism for the transport of solids between luidised beds in dual luidised bed systems for the fast pyrolysis of biomass process was selected. This mechanism makes use of an overlow standpipe to transport solids from the luidised bed used for the combustion reactions to a second luidised bed, which is used for the endothermic pyrolysis reactions. A screw conveyor is used to transport the solids back to the combustion luidised 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 luidised bed design. It was found that the pressure drops over the combustion and pyrolysis luidised beds were unaffected by changes in the speed of the screw conveyor and pyrolysis gas low rate. The pressure drop over the pyrolysis bed was found to be mostly dependant on the low of gas in the combustion bed due to its higher low rate. As the combustion gas low 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 low 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 luidised beds and the height of the solids in the pyrolysis bed to changes in the combustion and pyrolysis gas low 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 overlow standpipe. It also stabilised the response of the rate of spills to changes in the combustion gas low rate. The solid transfer mechanism conformed to the requirements which were identiied for the feasibility of the mechanism in the fast pyrolysis of biomass process. The proposed dual luidised bed system is therefore a feasible system for the fast pyrolysis of biomass. Keywords: standpipe, endothermic, cold model, themochemical, solids transport, screw conveyor, L-valve INTRODUCTION Dual luidised bed systems have been used extensively in the ield of pyrolysis for the fast pyrolysis of biomass. Pyrolysis is an endothermic, thermochemical process in which the long polymers present in biomass are cracked in the absence of oxygen to form shorter polymers. The heat required for these endothermic reactions is usually provided by combustion reactions. In the dual luidised bed system, the combustion reactions take place in a luidised bed, which increases the temperature of solids. These hot solids are then fed to a second luidised bed in order to provide the heat required for the endothermic pyrolysis reactions. In this way, the oxygen required for the combustion reactions does not interfere with the pyrolysis reactions. Although many dual luidised bed designs have been developed for the fast pyrolysis of biomass, little attention has been given to mechanisms for the transfer of solids between the two luidised beds. The objectives of the current investigation was the selection of a solid transfer mechanism for dual luidised bed systems, the experimental evaluation of its performance and the assessment of the solid transfer mechanism with respect to its applicability to the fast pyrolysis of biomass process. The following properties of the solids transfer mechanism were required in order for it to be suitable for the fast pyrolysis of biomass process: • The mechanism should not allow for the transport of gas from the combustion luidised bed to the pyrolysis luidised bed. • The mechanism should allow for the easy PAMSA RESEARCH