The Circulating Fluidized Bed reactor (CFB) for the thermo-chemical treatment of biomass
Chan, C.W., Seville, J.P.K. and Baeyens, J., University of Warwick, UK
(free)
Biomass is a cleaner alternative to fossil fuel and is considered as CO2-neutral having captured the amount of CO2 to be released during its combustion. The cheapest use of biomass comes from waste by-products such as wood bark, sawmill dust, refuse-derived-fuel and wastewater treatment sludge.
Biomass can be combusted directly to produce heat and power, or pyrolysed/gasified to produce a liquid, gaseous and solid (char) fraction. The pyrolysis of biomass is more complicated than combustion. For a CFB application, it requires small biomass particles. The reaction is endothermic but generally very fast.
The CFB is most appropriate for these reactions as it combines a very good temperature control; a good scale-up potential; easy to start up or shut down and the possibility of adding catalysts to the bed to promote tar cracking.
The optimum design of CFB is governed by the solid circulation flux expressed in kg/m².s, and the air flow rate through the riser. Additional design aspects involve the cyclone, the downcomer and the recycle L-valve.
The paper will review the fundamental kinetics of the various thermo-chemical processes and will present recent experimental results obtained by Positron Emission Particle Tracking of a radio-actively traced particle as it moves throughout the various parts of a CFB reactor (riser, cyclone, downcomer, L-valve). These results will be applied to the design of the CFB reactor for the specific applications, with special focus upon (i) the most appropriate operating conditions in the riser; (ii) the operation of the cyclone at high solids loading; (iii) the design of the standpipe and its associated non-mechanical recycle valve (or L-valve).
