1 Hydrodynamics of Dual Fluidized Beds M.K. Karmakar and P.K. Chatterjee CSIR-Central Mechanical Engineering Research Institute, Durgapur, West Bengal, India 1. Introduction A dual fluidized beds system essentially comprises of two fluidized bed reactors coupled together with a provision of gas or materials transfer in between. It may be a combination of circulating-circulating or bubbling-circulating or bubbling-bubbling type systems. A noble application is in a gasification process for coal or biomass where nitrogen of air is not allowed to dilute the product gas. In a dual fluidized bed gasifier of bubbling-circulating type system, a bubbling fluidized bed (BFB) reactor acts as the gasifier where steam is used as gasifying medium to get medium heating value syngas and the circulating fluidized bed is a combustor using air as fluidizing medium. The energy demand for the endothermic gasification reaction is met by the combustion of residual char in the fast bed combustor. The circulating bed materials act as heat carrier between the two fluidized beds and maintain the required temperature in gasifier. Therefore, the hydrodynamics of such a dual fluidized beds system needs to be thoroughly understood for successful design and operation of the system for industrial application. In literature, the studies on hydrodynamics of bubbling fluidized bed and circulating fluidized bed systems are available separately. However, the studies on hydrodynamics of combined system of these, a dual fluidized beds system loop predictions are scanty. The flow structure of gas-solids mixture is very complex in CFB system. Variety of models of fluidized bed system have been classified into three broad groups: (i) models predicting solids suspension density in axial variation, but not in radial direction, (ii) models predicting axial and radial variations by assuming two or more regions, such as core- annulus or clustering annulus flow models and (iii) models which employ the fundamental equations of fluid dynamics to predict the two phase gas-solids flow (Harris & Davidson, 1994). Of the three classifications, the type (iii) seems to be most rigorous, but the mathematical complexity of solving the equations limits its usefulness from practical design perspective. According to literature (Pugsley & Berruti, 1996a, 1996b), it is suggested that type (i) and type (ii) models are the best suited as a design tool for CFB to investigate the effects of operating conditions and riser dimensions on the flow structure. A dual fluidized beds system was also investigated (Bai et al., 1997) with two risers, two downcomers and two valves. The model shows how the solids circulation fluxes are affected by the operating conditions such as superficial gas velocities, particle diameter, density, solids inventory and fractional opening of solids flow control valves as well as by geometry. The hydrodynamics of a dual fluidized beds system were also studied (Loffler et al. 2003; Kaiser et al., 2003) which consisted of a fast bed riser with the downcomer, a