Nanostructured Oxygen Carriers for Chemical Looping Combustion and Chemical Looping Hydrogen Production

Solunke, Rahul Dushyantrao (2011) Nanostructured Oxygen Carriers for Chemical Looping Combustion and Chemical Looping Hydrogen Production. Doctoral Dissertation, University of Pittsburgh.

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Abstract

Chemical looping combustion (CLC) is an emerging technology for clean energy-production. In CLC, an oxygen carrier is periodically oxidized with air and then reduced in contact with a fuel. CLC is thus a flame-less oxy-fuel combustion without an air separation unit, producing sequestration-ready CO2-streams without significant energy penalty. However, a major hurdle towards technical implementation of CLC is the development of robust oxygen carrier materials. In this thesis, we report on a combined study of theoretical and experimental investigations of oxygen carriers for CLC. A detailed thermodynamic screening of oxygen carriers based on several comparison criteria was carried out to come up with the best candidates for CLC and then effect of sulfur contamination in the fuel stream on the performance of these selected oxygen carriers was studied. In sulfur-free streams the carriers show stable and fast reduction and re-oxidation kinetics. Sulfur contamination results not only in sulfidation of the metal carrier component, but also in partial sulfidation of the support matrix which marginally alters the redox kinetics but does not affect carrier stability. Interestingly, the support sulfidation leads to a significant increase in the oxygen carrying capacity of the carriers. Further investigation of Cu-based carriers showed that efficient desulfurization of the fuel reactor exit stream is achievable with quantitative S-recovery in the air reactor effluent. Beyond combustion, chemical looping can be used to produce hydrogen by replacing air with steam as oxidant in a 'chemical looping steam reforming' process (CLSR). The effluent of the oxidizer is PEMFC-ready hydrogen without further purification steps, resulting in significant process intensification. Challenges in CLSR are slower steam vs air oxidation kinetics, high-temperature carrier stability, and attrition due to large solids transport in a two-bed process. In the final part of the thesis, we report on experimental investigations of Fe-based nanostructured carriers to study their oxidation kinetics and high-temperature stability. Effect of temperature and particle size on hydrogen production and carrier utilization was studied which further demonstrated the importance of nano-sizing of the carrier. Finally, a reactor model was developed demonstrating that a fixed-bed approach is feasible for CLSR.

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