Ammonia synthesis process intensification: Solar energy driven chemical looping air separation for nitrogen production

Date

2023-12-28

Author

Bak, Yaşar Görkem

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Chemical looping technologies using metal oxides are evaluated in this work to utilize and store solar energy. Chemical looping air separation (CLAS) process works on less energy compared to cryogenic air separation units. The advantages of CLAS intensify even more when coupled with solar energy to provide the required heat input to the system which is tested with parabolic dishes to observe the power output that can be obtained. During the selection of metal oxides, promising candidates were examined kinetically and thermodynamically. It was discovered that the feasible chemical looping redox cycles can be conducted between the couples CuO/Cu2O, Co3O4/CoO, and Mn2O3/Mn3O4. Manganese oxides and cobalt oxides are found favorable in terms of operating temperature, reversible interaction with oxygen, and mechanical stability. The capacity tests were performed with thermal gravimetric analysis. Several additional experiments were conducted in TGA to test the reversibility and the effect of water vapor on the reduction. Working temperatures observed in TGA enabled the design of a system to conduct redox cycles isothermally using metal oxide packed bed. Then, a unique experimental setup is designed and established in the laboratory to test the performance of the selected metal oxides. The results indicate that at isothermal operating conditions, changing the inlet gas condition to steam stimulates the Mn2O3 to reduce and air then reoxidizes the reduced form, Mn3O4, in the successive cycle. Another option in the reduction step is to feed methane, which causes further reduction to MnO. A cycle capacity of around 70-80 mL of oxygen and 250-280 mL of nitrogen per gram of metal oxide is obtained in steam reduction. However, the accumulation of water in the packing and quartz wool creates an additional mass transfer resistance and causes a reduction in the oxidation rate. The solution to prevent condensation of steam in the pipeline is left as a future work. This study demonstrates that manganese oxides can perform chemical looping air separation cycles at almost total capacity with oxidant as air and steam as the reducing gas. Thus, producing nitrogen during the oxidation, and oxygen during the reduction of the material. Enabling a process intensification for ammonia synthesis by feeding produced nitrogen into the ammonia reactor.

Subject Keywords

Sustainable Ammonia, Chemical Looping Air Separation, Thermochemical Energy Storage

URI

https://hdl.handle.net/11511/107853

Collections

Graduate School of Natural and Applied Sciences, Thesis