Numerical study of gas-solid flow behaviors in the air reactor of coal-direct chemical looping combustion with Geldart D particles

Chemical Looping Combustion (CLC) is a promising clean combustion technology alternative to conventional oxy-combustion because of its technical feasibility to achieve intrinsic separation of CO2. In a typical Coal Direct Chemical Looping (CDCL) process, iron-based oxygen carriers belonging to Geldart D particles are used. Such system has been seldom studied. In this work, a two-fluid model (TFM) is adopted for describing the gas-solid flows in a bottom-enlarged air reactor of CDCL process. The model is qualitatively and quantitatively validated. The simulation results indicate that the oxygen carrier particles show a slugging flow pattern in the bottom section of the air reactor, an anti-core-annulus structure in the riser and a periodic fluctuation in the whole air reactor. Besides, the effects of several variables on the gas-solid flow in CDCL air reactor are also studied including gas velocity, specularity coefficient and different drag models. The simulation results indicate that the higher gas inlet velocity can increase the pressure drop over the air reactor and show a significant influence on solid allocation in the bottom section and the riser. specularity coefficient of zero and Syamlal-O'Brien drag model give the closest mass flow rate by comparing with experimental values, indicating that they are suitable for the three-dimensional air reactor modelling. This model provides a cost-effective tool for better understanding of gassolid flows in the air reactor of CDCL systems. (C) 2019 Elsevier B.V. All rights reserved.