A chemical looping process, which uses a packed bed of the various oxides of iron, has been formulated to produce separate, pure streams of H 2 and CO 2 from syngas. The process has the following stages: (1) Reduction of Fe 2 O 3 to Fe 0.947 O in the syngas from gasifying coal or biomass. This stage generates pure CO 2 , once the water has been condensed. (2) Subsequent oxidation of Fe 0.947 O to Fe 3 O 4 using steam, to simultaneously produce H 2 . (3) Further oxidation of Fe 3 O 4 to Fe 2 O 3 using air to return the oxide to step 1. Step 1 was studied here using a mixture of CO + CO 2 + N 2 as the feed to a packed bed of iron oxide particles, while measuring the concentrations of CO and CO 2 in the off-gas; step 2 was investigated by passing steam in N 2 through the packed bed and measuring the quantity of H 2 produced. The third step simply involved passing air through the bed. Reduction to Fe, rather than Fe 0.947 O, in step 1 gave low levels of H 2 in step 2 after 10 cycles of reduction and oxidation and led to the deposition of carbon at lower temperature. Step 3, i.e. reoxidizing the particles in air to Fe 2 O 3 , led to no deterioration of the hydrogen yield in step 2 and benefited the process by (i) increasing the heat produced in each redox cycle and (ii) preventing the slip of CO from the bed in step 1. The proposed process is exothermic overall and very usefully generates separate streams of very pure H 2 and CO 2 without complicated separation units.
Solid solutions of hematite (α-Fe2O3) and corundum (α-Al2O3) have been synthesized by coprecipitation. The resulting particles have been used as oxygen carriers for the production of hydrogen by chemical looping and characterized using X-ray diffraction (XRD), temperature programmed reduction (TPR), specific surface area measurements (BET), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDXS). The particles were repeatedly (i) reduced with, e.g., CO to, nominally, Fe, (ii) then oxidized with steam to Fe3O4 to produce hydrogen, (iii) then returned to Fe2O3 by oxidizing with air. The optimum loading of Al2O3 in the composite particles was found to be 25 wt % for the production of hydrogen over 50 cycles, resulting in an average yield (mole H2 formed/theoretical amount possible on reduction to Fe and oxidation to Fe3O4) of hydrogen of ∼48%. It was found that although Al2O3 is often thought of as inert, it participates in the oxidation and reduction reactions by forming FeAl2O4 and various solid solutions with the iron oxides. This behavior has been explained with the help of phase diagrams, and the applicability of these particles for the production of hydrogen by chemical looping is discussed.
Composite particles with different mass ratios of Fe 2 O 3 and Al 2 O 3 were prepared using a sol-gel method and were examined for use in chemical looping combustion through repeated reduction and oxidation cycles in a packed bed reactor at 850 °C. Unlike traditional chemical looping combustion which reduces an oxygen carrier in methane and oxidizes it in air, the reducing gas here was a mixture of CO and N 2 . Oxidation was performed in a mixture of steam and N 2 to produce H 2 , followed by oxidation in air in some cases. The results were as follows: (1) For reduction to the FeO phase, unsupported Fe 2 O 3 gave stable conversions over 40 cycles and no Al 2 O 3 support was needed. (2) For reduction to the Fe phase over 10 cycles, 10 wt % Al 2 O 3 was sufficient to give stable conversions above 0.9. Over 30-40 cycles, however, the conversion for particles with 10-20 wt % Al 2 O 3 dropped below 0.35. (3) For reduction to the Fe phase over 40 cycles, 40 wt % Al 2 O 3 was required and gave stable conversions near 0.75. The formation of FeO • Al 2 O 3 was confirmed using X-ray diffraction. Steam in N 2 , followed by air, is the recommended sequence for oxidizing the composite carriers, since temperature excursions and agglomeration of particles could be avoided and higher conversions could be achieved.
Modified iron oxide, Fe 2 O 3 , was used to produce pure H 2 using repeated cycles of reduction and oxidation. Reduction was performed in a packed bed at 1123 K with either (i) CO þ N 2 or (ii) H 2 þ N 2 ; reoxidation was performed with (iii) steam þ N 2 and additionally with (iv) air þ N 2 in some cases. Stable yields of H 2 over repeated cycles were observed if Fe 2 O 3 was reduced only to FeO. Decreasing yields of H 2 with an increasing cycle number were observed if Fe 2 O 3 was fully reduced to Fe. Samples of modified Fe 2 O 3 were prepared via wet impregnation with Al, Cr, Mg, and Si to give loadings of 1, 10, and 30 mol % of the metal additive. The addition of a metal additive was shown to improve and sometimes stabilize the quantity of H 2 produced when Fe 2 O 3 was reduced to Fe. Metal additives which (i) formed an intermediate with a higher melting temperature than the iron species involved, i.e., Fe 2 O 3 , Fe 3 O 4 , FeO, and Fe, and (ii) formed an intermediate that decomposed either during reduction or oxidation to release reactive iron, increased the quantity of H 2 produced. Stable H 2 yields over 10 cycles were obtained for the sample with 30 mol % Cr; stable H 2 yields over 10 cycles were obtained for the sample with 10 mol % Al if additional oxidation in air was performed to oxidize FeO 3 Al 2 O 3 to Fe 2 O 3 and Al 2 O 3 .
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