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 ...

A lattice Boltzmann method is used to model gas–solid reactions where the composition of both the gas and solid phase changes with time, while the boundary between phases remains fixed. The flow of the bulk gas phase is treated using a multiple relaxation time MRT D3Q19 model; the dilute reactant is treated as a passive scalar using a single relaxation time BGK D3Q7 model with distinct inter- and intraparticle diffusivities. A first-order reaction is incorporated by modifying the method of Sullivan et al. [13] to include the conversion of a solid reactant. The detailed computational model is able to capture the multiscale physics encountered in reactor systems. Specifically, the model reproduced steady state analytical solutions for the reaction of a porous catalyst sphere (pore scale) and empirical solutions for mass transfer to the surface of a sphere at Re=10 (particle scale). Excellent quantitative agreement between the model and experiments for the transient reduction of a single, porous sphere of Fe2O3 to Fe3O4 in CO at 1023K and 105Pa is demonstrated. Model solutions for the reduction of a packed bed of Fe2O3 (reactor scale) at identical conditions approached those of experiments after 25 s, but required prohibitively long processor times. The presented lattice Boltzmann model resolved successfully mass transport at the pore, particle and reactor scales and highlights the relevance of LB methods for modelling convection, diffusion and reaction physics.

Solid solutions of hematite (alpha-Fe2O3) and corundum (alpha-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 H-2 formed/theoretical amount possible on reduction to Fe and oxidation to Fe3O4) of hydrogen of similar to 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.

Modified iron oxide, Fe2O3, 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 + N-2 or (ii) H-2 + N-2; reoxidation was performed with (iii) steam + N-2 and additionally with (iv) air + N-2 in some cases. Stable yields of H-2 over repeated cycles were observed if Fe2O3 was reduced only to FeO. Decreasing yields of H-2 with an increasing cycle number were observed if Fe2O3 was fully reduced to Fe. Samples of modified Fe2O3 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 Fe2O3 was reduced to Fe. Metal additives which (i) formed an intermediate with a higher melting temperature than the iron species involved, i.e., Fe2O3, Fe3O4, 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 center dot Al2O3 to Fe2O3 and Al2O3.

Composite particles with different mass ratios of Fe2O3 and Al2O3 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 degrees 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 Fe2O3 gave stable conversions over 40 cycles and no Al2O3 support was needed (2) For reduction to the Fe phase over 10 cycles, 10 wt % Al2O3 was sufficient to give stable conversions above 0.9 Over 30-40 cycles, however, the conversion for particles with 10-20 wt % Al2O3 dropped below 0 35. (3) For reduction to the Fe phase over 40 cycles, 40 wt % Al2O3 was replied and gave stable conversions near 0.75. The formation of FeO center dot Al2O3 was confirmed using X-ray diffraction. Steam in N-2, followed by air, is the recommended sequence for oxidizing the composite carriers, since temperature excuisions and agglomeration of particles could be avoided and higher conversions could be achieved

Results are reported for the repeated reduction of iron oxide particles. 300-425 mu m diameter, by a mixture of CO, CO2, and N-2 in a fluidized bed of 20 mm internal diameter. The conclusions were as follows: (1) Reduction of either Fe2O3 to Fe3O4 or of Fe3O4 to Fe0.947O is first-order in CO. (2) With the particle sizes used, the rates of the reduction reactions are controlled by intrinsic chemical kinetics. Activation energies and pre-exponential factors are reported. (3) The first cycle gave anomalous results, but (a) the rate of reduction of Fe2O3 to Fe3O4 remained constant over cycles 2-10; (b) the rate of reduction of Fe3O4 to Fe0.947O declined by 60-85% over cycles 2-10. (4) The rates of reduction declined with solids conversion down to zero at 80% conversion. The rates were incorporated into a conventional model of a fixed bed, which was used to predict, satisfactorily, the reduction behavior of iron oxide. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1016-1029, 2010

A chemical looping process using the redox reactions of iron oxide has been used to produce separate streams of pure H2 and CO2 from a solid fuel. An iron oxide carrier prepared using a mechanical mixing technique and comprised of 100wt.% Fe2O3 was used. It was demonstrated that hydrogen can be produced from three representative coals – a Russian bituminous, a German lignite and a UK sub-bituminous coal. Depending on the fuel, pure H2 with [CO] ≲50vol.ppm can be obtained from the proposed process. The cyclic stability of the iron oxide carrier was not adversely affected by contaminants found in syngas which are gaseous above 273K. Stable quantities of H2 were produced over five cycles for all three coals investigated. Independent of the fuel, SO2 was not formed during the oxidation with steam, i.e. the produced H2 was not contaminated with SO2. Since oxidation with air removes contaminants and generates useful heat and pure N2 for purging, it should be included in the operating cycle. Overall, it was demonstrated that the proposed process may be an attractive approach to upgrade crude syngas produced by the gasification of low-rank coals to pure H2, representing a substantial increase in calorific value, whilst simultaneous capturing CO2, a greenhouse gas.

Two modified Chemical Looping Combustion (CLC) schemes were investigated: (a) CLC with in situ gasification of a solid carbonaceous fuel in the fuel reactor, and (b) CLC for the production of high purity hydrogen from low grade syngas. A comparison between the performance of the two modified cycles using (i) syngas from cylinders and (ii) syngas derived from the gasification of various solid fuels was made. Preliminary results indicate that both processes can be operated with sufficient conversions using low and high-rank coals. However, agglomeration of the oxygen carrier was observed if wood was used in process (a), probably owing to the formation of low-melting eutectics between the oxygen carrier and metals from the wood ash.

In this paper, a chemical looping combustion (CLC) system, using haematite (Fe2O3) as an oxygen carrier, has been simulated in conjunction with a steam–coal gasification process. The analysis has assumed thermodynamic equilibrium throughout. Full heat integration was considered for a range of operating conditions (e.g. by varying oxygen carrier recycle rate). It was found that for low to moderate flows of oxidising steam, it was possible to operate within a regime which could be fully heat-integrated. Furthermore, the size of this operating regime increases with the recycle rate of oxygen carrier. The peak exergetic efficiencies achieved for fully heat-integrated systems were 48.4% and 58.3% at operating pressures of 1 atmosphere and 10 atmospheres respectively, and these were increased respectively to 53.7% and 59.7% when a bottoming steam turbine cycle was included to utilise waste heat. These values compare favourably with those achieved by hydrogen production via steam reformation of methane. The range of suitable operating conditions available at both pressures was encouraging, and showed considerable promise for the successful coupling of a chemical looping system with a gasifier.