Modeling and Process Features of Plug Flow Reactor with Internal Recirculation for Biomass Pyrolysis

Abstract

A new fluidized bed design is considered for the pyrolysis of biomass, in a plug-flow with internal-recirculation (PFIR) arrangement, where the movement of solids from feed zone to product zone and the circulation within a closed loop are achieved, by the use of directional high-speed tuyeres that induce both the suspension and horizontal movement of the charge. Separation between feed and product zones is provided through one or more underflow weirs. Wet and hard-to-fluidize biomass is targeted, where fluidization and pyrolysis of the feed is achieved by the internal hot sand recirculation. Plug flow can allow for both stages (pyrolysis and heating) to occur inside a single vessel, but at separate zones. Computer simulation of the PFIR reactor is demonstrated to verify that plug-flow with internal- recirculation can be achieved under reasonable operating conditions. A commercial software (CPFD-Barracuda) is used, which shows promising results, including: solids circulation due to directional tuyeres; large solids mass transport rates relative to reactor size; and controllability of the circulation rate via tuyere velocity. It is shown that the underflow weir(s) offer little resistance compared to the frictional losses and, therefore, multiple zones within a single reactor can be achieved with little impact on the solids recirculation rate. The horizontal mass flux is shown to occur predominantly near the bottom Tuyeres (inclined jet zone), hence enabling the underflow weir to effectively separate the feed and product zones, with a small bottom passage for the solids.