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The concept of chemical looping reactions has been widely applied in chemical industries, for example, the production of hydrogen peroxide (H2O2) from hydrogen and oxygen using 9,10-anthraquinone as the looping intermediate. Fundamental research on chemical looping reactions has also been applied to energy systems, for example, the splitting of water (H2O) to produce oxygen and hydrogen using ZnO as the looping intermediate. Fossil fuel chemical looping applications had been used commercially with the steam-iron process for coal from the 1900s to the 1940s and had been demonstrated at a pilot scale with the carbon dioxide acceptor process in the 1960s and 1970s. There are presently no chemical looping processes using fossil fuels in commercial operation. A key factor that hampered the continued use of these earlier processes for fossil energy operation was the inadequacy of the reactivity and recyclability of the looping particles. This factor led to higher costs for product generation using the chemical looping processes, compared to the other processes that use particularly petroleum or natural gas as feedstock. With CO2 emission control now being considered as a requirement, interest in chemical looping technology has resurfaced. In particular, chemical looping processes are appealing because of their unique ability to generate a sequestration-ready CO2 stream while yielding high energy conversion efficiency. Renewed fundamental and applied research since the early 1980s has emphasized on improvements over earlier shortcomings. New techniques have been developed for direct possessing of coal or other solid carbonaceous feedstock in chemical looping reactors. Significant progress demonstrated by the operation of several small pilot scale units worldwide indicates that the chemical looping technology may become commercially viable in the future for processing carbonaceous fuels. This perspective article describes the fundamental and applied aspects of modern chemical looping technology that utilizes fossil fuel as feedstock. It discusses chemical looping reaction thermodynamics, looping particle selection, reactor design, and process configurations. It highlights both the chemical looping combustion and the chemical looping gasification processes that are at various stages of the development. Opportunities and challenges for chemical looping process commercialization are also illustrated.

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The syngas chemical looping (SCL) process coproduces hydrogen and electricity. The process involves reducing metal oxides with syngas followed by regeneration of reduced metal oxides with steam and air in a cyclic manner. Iron oxide is determined to be a desired oxygen carrier for hydrogen production considering overall properties including oxygen carrying capacity, thermodynamic properties, reaction kinetics, physical strength, melting points, and environmental effects. An iron oxide based particle can maintain good reactivity for more than 100 reduction-oxidation (redox) cycles in a thermogravimetric analyzer (TGA). The particle exhibits a good crushing strength (>20 MPa) and low attrition rate. Fixed bed experiments are carried out which reaffirm its reactivity. More than 99.75% of syngas is converted during the reduction stage. During the regeneration stage, hydrogen with an average purity of 99.8% is produced.

The syngas redox (SGR) process to produce hydrogen from coal derived syngas is described. The process involves reduction of a metal oxide to metallic form with syngas and subsequent regeneration with steam to generate hydrogen in a cyclic operation. Metal oxides of Ni, Cu, Cd, Co, Mn, Sn, and Fe were evaluated for this process based upon thermodynamic equilibrium limitations. It was found that Fe2O3 provided the best conversion of syngas to combustion products CO2 and H2O along with a high conversion of steam to hydrogen. Other oxides provide high conversion of syngas but were found lacking in producing hydrogen from steam. Composite particles with Fe2O3 as the key ingredient were developed that undergo multiple redox cycles without loss of activity. Analysis of process economics with respect to particle recyclability showed that the particles should undergo at least a 100 redox cycles without diminishing its activity. Process modifications to address carbon formation on reaction of syngas with iron oxide are discussed. Detailed process simulation showed that the SGR process is capable of converting 74% of the coal energy into hydrogen energy (higher heating value (HHV) basis) while delivering a pure CO2 stream without the need for costly separation technology.

in Wiley InterScience (www.interscience.wiley.com).The syngas chemical looping process co-produces hydrogen and electricity from syngas through the cyclic reduction and regeneration of an iron oxide based oxygen carrier. In this article, the reducer, which reduces the oxygen carrier with syngas, is investigated through thermodynamic analysis, experiments, and ASPEN Plus V R simulation. The thermodynamic analysis indicates that the countercurrent moving-bed reducer offers better gas and solids conversions when compared to the fluidized-bed reducer. The reducer is continuously operated for 15 h in a bench scale moving-bed reactor. A syngas conversion in excess of 99.5% and an oxygen carrier conversion of nearly 50% are obtained. An ASPEN Plus V R model is developed which simulates the reducer performance. The results of simulation are consistent with those obtained from both the thermodynamic analysis and experiments. Both the experiments and simulation indicate that the proposed SCL reducer concept is feasible. V V C 2009 American Institute of Chemical Engineers AIChE J,

Addition of TiO 2 was found to significantly enhance the ionic diffusivity of O anion within iron and its oxides, thereby changing the dominating ionic transfer mechanism for iron oxidation from ''outward Fe cation diffusion'' (in pure Fe case) to ''inward O anion diffusion'' (in Fe with TiO 2 support case).