CFD simulation of hydrodynamics and heat transfer characteristics in gas-solid circulating fluidized bed riser under fast pyrolysis flow condition

Abstract

Pyrolysis process through circulating fluidized bed (CFB) is a promising technology to produce synthetic fuel and other products from biomass feedstocks. Computational fluid dynamics (CFD) computing means provide valuable insights to better understand gas-solid flow hydrodynamics, troubleshoot performance issues and optimize reactor operations. In this study, gas-solid flow hydrodynamics and heat transfer characteristics of a CFB riser for fast pyrolysis are investigated using a three-dimensional (3D) Eulerian-Eulerian CFD model. The main observations are discussed to provide insights on the factors affecting CFB riser performance. The model parameters, specularity and particle-particle restitution coefficients, were considered and tuned to accurately predict of gas-solid flow hydrodynamics and heat distribution with respect to different gas velocities and solid circulation rates. The results have shown that the CFD model predicted well the flow hydrodynamics and both specularity and particle-particle restitution coefficients are critical parameters as they affect particle behavior and temperature distribution fields. The lower specularity coefficient (phi -0.00001) was fairly able to predict the axial solid holdup profile into CFB riser. However, the lower value for particle-particle restitution coefficient (e(ss)- 0.8) significantly overpredict the bottom dense region. The results shows that the increase of operating velocity promotes the mixing behaviors and heat transfer performance. In this work the suitable gas and solid circulation flow rates are U-g = 4.5 m/s and G(s) = 81.23 kg/m(2)s.