Overview of Chemical-Looping Reduction in Fixed Bed and Fluidized Bed Reactors Focused on Oxygen Carrier Utilization and Reactor Efficiency

Abstract

A model-assisted comparison of two types of chemical-looping (CL) reactors (fixed bed and fluidized bed), with the same oxygen carrier loading and fuel capacity, is carried out to examine performance and efficiency of CL Reducers, operating with methane as the feedstock and nickel oxide as the oxygen carrier. The study focuses on the reduction step of chemical-looping combustion (CLC), for which the reactor efficiency and fuel utilization are crucial in terms of economics and carbon capture efficiency. Process models (a three phase dynamic model for bubbling fluidized beds and a two dimensional homogeneous model for fixed beds) and reaction kinetics developed and validated in previous studies are used. A fluidized bed chemical-looping combustion Reducer is compared to a fixed bed equivalent reactor, scaled-up from a smaller experimental reactor, constrained to bed height to reactor diameter ratios that prohibit excessive temperature and pressure drops across the bed. Through a detailed comparison, CLC operated in the fluidized bed reactor is shown to deliver superior performance, i.e., uniform temperature and pressure distribution; high methane conversion (> 95%) and carbon dioxide selectivity (> 95%) sustained for longer reduction periods; negligible carbon formation (< 2 mol% C basis); and better efficiency in oxygen carrier utilization.