Intensification of Micro-Plasma Bubble Processing for N-Fixation and its Potential Application
Date
2025
Authors
Zhuang, Changping
Editors
Advisors
Hessel, Volker
Tran, Nam Nghiep
Cullen, Patrick (University of Sydney)
Tran, Nam Nghiep
Cullen, Patrick (University of Sydney)
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
This thesis investigated the intensification of non-thermal micro-plasma bubble processing for nitrogen fixation and reviewed its application both on Earth and in space. The study was carried out by performing five main tasks, these being: (1) air/N2 plasma processing in batch water/recycling water with/without heterogenous catalysts; (2) air/N2 plasma processing determining the influence of the process medium and catalysts as process ingredients; (3) fundamental process systems study, deciphering core chemical-electrical engineering intensification opportunities; (4) air plasma processing in urea/synthetic urine solutions for ammonia recovery as well as synergy of plasma-enzyme catalysis; (5) mini-review of potential plasma treatment that could be applied in human space exploration. Firstly, air plasma is much superior to nitrogen plasma for general nitrogen fixation in both effectiveness and sustainability, as well as demonstrated by the characteristics of key plasma parameters. This basically makes air plasma a promising candidate for decentralised on-site fertiliser production. To improve its potential, system design matters. Lower gas flowrate, higher input voltage, cooler solvent and reduced surface tension are important factors to improve nitrogen species production and nitrogen fixation efficiency. Besides, the proper placement of heterogeneous catalysts and a looping water supply system were shown to substantially boost nitrogen fixation efficiency. When homogeneous catalysis is applied, the presence of metal ions such as copper revealed an important catalytic effect, selectively increasing nitrate yield. This opens up opportunities to explore a wider array of homogeneous catalysts in future research. Parallel to these developments, plasma bubble processing was investigated for the decomposition of urea in synthetic urine - a valuable step toward nitrogen nutrient recovery as fertiliser. Optimal results were achieved under longer reaction times, higher input voltages, and isothermal conditions at room temperature. However, some urea was found to decompose into carbon dioxide and nitrogen, indicating a side reaction pathway that requires further study to improve conversion efficiency. Encouragingly, it was demonstrated that air plasma-based nitrogen fixation can be integrated with urease-driven ammonium formation without performance interference, suggesting that hybrid processing systems are feasible and scalable. Above all, microplasma bubble processing has proven to be a highly promising approach to nitrogen fixation, especially when operated under optimised conditions – lower gas flowrate, cooler solvent temperatures, higher voltages, and reduced surface tension. With the synergy with proper catalysts, higher yield of nitrate and better selectivity of ammonia can be achieved to intensify the process. In addition, turbulence and cavitation caused by plasma can act as process intensification mechanism. Besides, a taller bubble column is not applied in this study but is recommended to achieve more gas absorption of plasma species to obtain high yield in the solvent. Meanwhile, the improvement of decomposition of urea and synthetic urine by plasma bubble processing demonstrated the good potential for nitrogen nutrient recovery, as well as hybrid processing integrated with urease-driven ammonium formation. In fact, a container unit called eNFix is already being piloted in practice, which should be technically supported by plasma bubble processing. While the technology shows great promise, further refinement and system integration are still essential to realise its full potential in practical and broader applications. Beyond Earth-based applications, this study also considered the use of non-thermal plasma processing in space, highlighting the versatility of this technology under diverse environmental conditions.
School/Discipline
School of Chemical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2025
Provenance
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