A Multi-Scale Model for Nickel-Based Oxygen Carrier in Chemical-Looping Combustion

Abstract

The selection of oxygen carrier (OC) particles is crucial for the development of chemical-looping combustion (CLC) technology. Common OC particle models often involve first-order chemical reactions with respect to the concentration of fuel gas, which may not be able to account for the complex reaction mechanisms taking place on the contacting surface between gas and solid reactants.

In this work, we apply a multiscale modelling framework on NiO-based OC particle in order to explicitly consider and understand the effect of reaction kinetics. The proposed multiscale model consists of gas diffusion model and surface reaction. Continuum equations are used to describe the gas diffusion inside OC particles, whereas mean-field approximation and kinetic Monte Carlo methods are adopted to simulate the microscale events, such as molecule adsorption and elementary reaction, occurring on the contacting surface. These sub-models communicate through a boundary condition that defines the mass fluxes of both reactant and product gas species. Surface reaction mechanisms and the corresponding reaction rate constants considered in the present work are obtained from a systematic density functional theory (DFT) analysis.

The qualititive comparison with experimental data available in the literature suggests that the kMC-based multiscale model is able to provide better results than the MFA-based counterpart. A sensitivity analysis on the rate constants of key elementary reactions, length of intra-particle pore, and particle porosity was conducted to assess the effect of reaction kinetics and mass transport on the overall reaction process and validate the proposed multiscale model. The simulation results show reasonable tendencies and responses to changes in these modelling parameters, which indicates that the proposed multiscale modelling scheme on OC particle is suitable. To the author's knowledge, this is the first implementation of a multiscale model in CLC technology.