Research On Heat And Mass Transfer Characteristics Of Oxygen Carrier Particles Based On Multiphysics Coupling

Chemical looping combustion is a novel combustion technology with internal separation of CO₂ and reduction of NOx emissions.It is of great importance to investigate the ability of the oxygen and energy transfer of the oxygen carrier between the reactors,and the characteristics of its heat and mass transfer in the reactors.The heat and mass transfer for the single particle and the reactor involves multi-physics and multi-scale coupling issues,and further research is required.COMSOL Multiphysics was used to establish the general reaction-diffusion model for the single-particle oxygen carrier and the dual-porosity model for the fixed-bed.To guide the design and optimization of oxygen carrier particle preparations and the design of reactors,the reaction performance,heat and mass transfer characteristics of oxygen carrier particles were analyzed in detail from both particle and reactor scalesThe calculation results of the single particle model show that there is no obvious boundary between the complete reaction zone and the partial reaction zone during the reaction of the oxygen carrier particles,and the reaction proceeds in the partial reaction zone.The reaction of Cu/Fe/Ni-based oxygen carrier with 40wt.%Al₂O₃ as the inert support under reducing atmosphere is finished in 25—40s.The type and content of active metal oxides,kinetic parameters,porosity,particle size,and temperature all have significant effects on the reactions and internal diffusions.The influence of kinetic parameters is most important.The smaller the activation energy,the faster the reaction proceeds.When the chemical reaction rate constant gets smaller,the reaction becomes slower as the active component content increases.The effect of the porosity on internal diffusion and chemical reaction is related with that of the particle size.When the porosity is less than 0.3,the conversion increases with the decrease of the particle size;when the porosity is greater than 0.3,the effect of particle size is not significant.The increase in temperature has a certain promotion effect on gas diffusion and chemical reactions.The calculation results of the heat transfer model of single particle show that the rise of the oxygen carrier temperature depends mainly on the enthalpy of the reactions.The radial temperature difference of the particles is small,and the temperature difference increases first,then decreases,and finally stablizes.Activation energy,porosity,particle size,and velocity may all have affects on the rise of the particle temperature.When the activation energy is less than30k J/mol and the porosity is between 0.1 and 0.5,the effect on the temperature of the oxygen carrier particle is most obvious.As the particle size increases,the temperature rise of the oxygen carrier particles increases significantly,the time to reach the highest temperature rise is postponed,and the temperature drop curve slowes down.The temperature rise of the particle increases with the increase of velocity,but the influence is gradually weakened.The simulation results of the fixed-bed dual-porosity model show that the time for the gas reactant to reach a steady state in the axial direction is lower than that for the gaseous products,and the gas concentration has an inlet effect in the radial direction.Along the height of the reactor,the concentration gradient of the gas reactant inside the oxygen carrier particles on the central axis decreases.The temperature peak of the reactor appeared at the first half of the bed height,and a fixed-bed reactort could be designed as a multi-stage reactor in the order to decrease the temperature rise.The increase of particle size promotes the gas diffusion in the bed and reduces the maximum temperature rise in the bed.When particle size is between 1.0mm and 5.0mm,the temperature rise must not cause the change of the internal structure for Fe₂O₃/Al₂O₃ particles.For the Cu O/Al₂O₃particle sizes of 1.0mm and the Ni O/Al₂O₃ particle sizes of 1.0-4.0 mm,partical structural changes must occur in the high temperature zone in the fixed bed,which cause the inactivation of oxygen carrier particles.