Model-based analysis of bench-scale fixed-bed units for chemical-looping combustion

Abstract

The objective of this work is to compare reaction schemes and kinetic mechanisms, and simulate the dynamic behavior of chemical-looping Reducers operating with methane as the feed and nickel oxide as the oxygen carrier. A dynamic model for the reduction step of chemical-looping combustion in fixed-bed reactors is developed, on the basis of the same kinetic network to predict published experimental data of different fixed-bed reactors and data of a chemical-looping fixed-bed reactor operating at the University of Connecticut. Steam reforming, water gas shift, dry reforming, methane decomposition, and carbon gasification by carbon dioxide and steam are considered as reactions catalyzed by the reduced oxygen carrier. Heterogeneous reactions of the oxygen carrier encompass reactions with methane, carbon monoxide and hydrogen. Kinetic expressions reported in the literature are compared and their parameters are estimated on the basis of experimental data of nickel-based oxygen carriers for chemical-looping combustion of methane. Particle shrinking, molar expansion and surface area changes are accounted for in the transient plug flow reactor model. The applicability of the shrinking core and modified volumetric models, which are typically used for the description of gas–solid reactions is verified and compared against various experimental data, showing the superiority of the latter. A global mechanism and kinetic parameters that can be used to simulate CLC Reducers operating with NiO as the oxygen carrier and CH4 as the fuel are proposed. Finally, discrepancies between published data and the model accuracy for different experimental setups are discussed.