Simulation of two-step redox recycling of non-stoichiometric ceria with thermochemical dissociation of CO2/H2O in moving bed reactors – Part I: Model development with redox kinetics and sensitivity analysis

Abstract

Chemical looping syngas production is a two-step process that produces CO and H2 from water and CO2 splitting. This is performed by exploiting a metal oxide as oxygen carrier material, which is thermally reduced and releases oxygen in a subsequent step. The core-process layout is composed of two reactors (oxidation reaction and reduction reactor) and oxygen carriers (metal oxides) circulating between the two reactors. A comprehensive moving-bed reactor model is developed and applied to simulate both the syngas production from water and carbon dioxide by ceria oxidation as well as the thermal reduction of metal oxide. An extensive FORTRAN model is developed to appropriately simulate the complexities of ceria reaction kinetics and implemented as subroutine into an ASPEN Plus® reactor model. The kinetics has been validated with the model developed by comparing experimental and simulated data on the reduction reactor. The sensitivity of both the reduction and oxidation reactors have been performed. The reduction reactor temperature and pressure were varied between 1200–1600 °C and 10−3–10−7 bar, respectively. The oxidation reactor was evaluated by varying the inlet temperatures of the reactants as well as the relative gas composition between CO2 and H2O. Results indicate a non-stoichiometry achievable from the reduction of ceria of 0.198 at 1600 °C and 10−7 bar vacuum pressure. In the oxidation reactor, water splitting yields significantly better solid conversion (metal oxide conversion) of 97%, as compared to 91% by CO2 splitting with 5% excess gas flow than the stoichiometric requirements. The metal oxide inlet temperature significantly improves the yield of the oxidation reactor, in contrast to the minimal impact of variation of gas inlet temperature. A selectivity of over 90% can be achieved irrespective of gas composition with over 90% metal oxide conversion in the oxidation reactor.