Numerical investigation of bed hydrodynamics for biomass-coal blending in a dual fluidized bed system

Abstract

Dual fluidized bed gasification (DFBG) technology yields superior syngas from a variety of feedstock. The performance of a DFBG system is greatly influenced by the hydrodynamics, heat and mass transfer within the solid-gas flow system. The hydrodynamic characteristics become more complex with the use of blended feedstock of varying physio-chemical properties owing to their non-linearity and transient nature. Therefore, it is crucial to understand hydrodynamic characteristics of such a blended gas-solid system. The present study uses Multiphase Flow with Interphase eXchanges (MFiX) simulation to examine the effects of blending biomass and coal with sand on hydrodynamics of a DFBG system. The simulation of the gasifier and riser has been performed using two-fluid model (TFM) considering constant superficial air velocities for six alternative biomass-coal blending proportions, such as 0:5, 1:4, 2:3, 3:2, 4:1, and 5:0. The numerical analysis has revealed that in biomass-coal blending, increasing the biomass fraction reduces static pressure as well as bed pressure drop, while axial voidage rises in the gasifier as well as riser. Similar to the pressure profile, the suspension density of the gasifier declines as biomass content in the blending increases. Nonetheless, voidage and solid velocity profiles in the radial direction of the gasifier and riser reveal an inverted U-shape, with biomass having the highest voidage and velocity, whereas sand possesses the lowest voidage and velocity. This study establishes the foundation for experimental investigation of the DFBG system's performance using blended feedstock.