Impact of Biomass-Coal Blending on Flow Dynamics in a Dual Fluidized Bed Gasification System

The versatility in the selection of feedstock for the generation of high quality syngas has emerged dual fluidized bed gasification (DFBG) as a very promising technology. The principle of DFBG is heavily dependent on the hydrodynamics and heat transfer characteristics of the gas–solid system. Furthermore, the complexity of the hydrodynamics increases with more number of fuels of different physico-chemical characteristics due to its nonlinearity and transience. It is, therefore very essential to study the hydrodynamics of the multi-phase system. The present work focuses on two fluid model simulations of a dual fluidized bed gasifier to study the impact of biomass-coal blending with silica sand as the bed material. The simulation has been performed using the Multiphase Flow with Interphase eXchanges (MFiX) simulation platform. Six different biomass-coal (BM:C) blending proportions such as 0:0, 1:5, 2:5, 3:5, 4:5 and 5:5 are considered for the 2-D two fluid model (TFM) simulation of the gasifier. An optimum superficial air velocity of 0.2 m/s is considered for the entire simulation for a grid of 6000 computational cells. The impact of biomass-coal blending proportions on the static pressure and pressure drop, axial and radial voidage, suspension density, radial solid velocity profile and granular temperature are analysed using the Paraview software. The numerical investigation has revealed that the static pressure decreases with an increase in biomass-coal blending proportion for a fixed height within the gasifier. The axial bed voidage has also dropped with a surge in biomass-coal blending proportion up to 0.6 m height from the bottom and then has started to rise until it becomes unity at a height of about 0.9 m. Moreover, suspension density has enhanced with a rise in blending proportion due to the density difference between biomass and coal. The difference in density and particle size of biomass and coal has also contributed towards a decrease in the radial voidage and solid velocity with increased blending proportion. An escalation in granular temperature has been observed with the decline in solid volume fraction for all the blending proportions. However, the granular temperature has dropped with an increase in blending proportion. This numerical study will act as a platform for the experimental investigation of the biomass-coal blending performance of the DFBG system.