Development of freeze granulated oxygen carriers for chemical looping combustion of solid fuels

Chemical looping combustion (CLC) is proving to be a promising technology for the combustion of carboneous fuels producing electricity and a sequestration-ready concentrated CO2 stream. Recently, more attention is being given to its application to solid fuels. The University of Kentucky, Center for Applied Energy Research (CAER) is presently developing an integrated process to apply CLC to coal combustion. A first step in this work is the development of suitable oxygen carriers (OCs) specifically for the application of CLC to solid fuels. The materials investigated in this study included Fe₂O ₃, NiO and CuO on a support matrix, α-aluminum oxide. The unsupported forms of the metal oxides were examined as well for comparison. All tested OCs were produced by freeze granulation (FG) with subsequent freeze drying and high temperature calcination. After calcination at 1400°C for 6 hours, the particle size distributions were evaluated. Reaction rates were examined by a thermal gravimetric analyzer- mass spectrometer (TG-MS). A mixture of H₂/CO/CO₂/Ar was used as the reducing gas and O ₂/Ar was the oxidizing gas mixture. Oxidation/reduction cycles of the OCs were carried out in wet gases to simulate actual conditions in a CLC reactor while experiments from dry gases were also conducted for comparison. Reaction rates, the extent of oxidation/reduction, recycle stability, and the effect of water vapor will be discussed. The data show that the supported OCs on α-alumina all have fewer tendencies to agglomerate after multiple oxidation-reduction cycles than unsupported OCs.