Synthesis, Application, and Carbonation Behavior of Ca<inf>2</inf>Fe<inf>2</inf>O<inf>5</inf> for Chemical Looping H<inf>2</inf> Production

Abstract

Chemical looping hydrogen production uses the oxidation and reduction of metal oxides, typically iron, to produce hydrogen. This work focuses on the modification of iron oxide with calcium oxide to form an oxygen carrier containing dicalcium ferrite (Ca2Fe2O5), which presents favorable thermodynamics for achieving higher conversions of steam to hydrogen, compared to chemically unmodified iron oxide. Different methods of synthesis, viz. mechanochemical synthesis and coprecipitation, were used to produce Ca2Fe2O5, and their resulting performances were compared. Consistent with thermodynamic predictions, it was found that CO2, or steam, was sufficient to fully regenerate the reduced carriers to Ca2Fe2O5. The cyclic stability of the oxygen carriers were studied in fluidized bed reactors and by thermogravimetric analysis (TGA). Good stability of the materials was observed for up to 50 cycles, with no evidence of agglomeration, even up to 950 °C. The rate of deactivation was found to correlate with the purity of Ca2Fe2O5 and the presence of impurity phases such as CaFe2O4, which had a tendency to segregate into its constituent elemental oxides. Carbonation of the oxygen carriers was examined by TGA, and it was found to occur appreciably only for the reduced carrier (a mixture of CaO and Fe) between temperatures of 500-700 °C and 0.1-0.5 atm of CO2, whereas the oxidized carrier (viz. Ca2Fe2O5) did not carbonate. Fresh and cycled materials were characterized by XRD, SEM, and BET analysis. Ca2Fe2O5 is a potentially viable material as an oxygen carrier for hydrogen production; however, because of thermodynamic limitations, it cannot be used for complete fuel oxidation.