Iron-based oxygen carriers (OCs) have been the focus of studies at the Center for Applied Energy Research (CAER), University of Kentucky, for the application of chemical looping combustion (CLC) to solid fuels. Freeze granulation (FG) was chosen as a method to produce Fe2O3 on an alumina (Al2O3) OC of appropriate particle size for separation from ash and reactivity to oxidize a coal char in a bench-scale fluid-bed reactor. The purpose of this study was to gain an understanding of the iron oxide transformations, which would occur under CLC fuel reactor conditions, including gas compositions similar to CLC syngas concentrations. A FG OC composed of 50:50 Fe2O3/Al2O3 and a commercial Fe2O3 powder were tested in a thermal analyzer equipped with a water vapor (WV) furnace and a WV generator and coupled to a mass spectrometer (TGMS). This TGMS system allowed for testing of OCs under simulated CLC reducing reactor conditions containing WV. Two reducing gas concentrations consistent with potential syngas in a CLC reactor and a single oxidation gas mixture were used with and without 10% WV in the balance gas for comparison. The two OCs were also tested in the individual components of the simulated syngas, 10% H2 in Ar and 15% CO in Ar. The reaction temperature was 950 °C, and five redox cycles were completed for each test. Samples from before and after TG testing were analyzed by X-ray diffraction (XRD) to determine the forms of iron. A comparison of weight loss/gain during redox under both reducing gas concentrations, wet and dry, indicated the FG OC had the same oxygen capacity as indicated by the weight loss under both conditions. In contrast, the Fe2O3 powder weight loss (oxygen-transfer capacity) was half the amount under wet gases than dry. Both FG OC and Fe2O3 powder had significantly lower maximum reaction rates of redox in wet gases, and the concentration of reducing gases significantly affected the maximum reduction rates. A total of 10% WV in argon was found to be capable of the oxidation of Fe3O4, FeO, and the reduced FG OC under test conditions, which helped explain the slow reaction rates in wet reduction gas. These reduced materials were fully reoxidized to Fe2O3 with WV alone. The rates of oxidation and their effect on the overall reduction rates in gases containing WV are discussed. XRD results provided insight into the OC reduction and explained the difference in weight loss seen in wet and dry gases. The end product of Fe 2O3 reduction in dry gases was Fe/FeO, and under wet conditions, Fe3O4 was the primary iron oxide identified.
|Number of pages||9|
|Journal||Energy and Fuels|
|State||Published - Oct 20 2011|
ASJC Scopus subject areas
- Chemical Engineering (all)
- Fuel Technology
- Energy Engineering and Power Technology