Initially, we constructed a state-of-the-art solid-state Microwave (MW) plasma reactor capable of powers of circa 1 kW. Next, we established a suitable diagnostic that could measure the conversion output in real-time, a key benefit for efficient experimentation. The reactor exhaust was coupled with a non-dispersive infra-red/ultra-violet absorption diagnostic to study the total NOx formation (i.e. NO and NO2), the primary species of interest. The device was calibrated internally (to remove cross-interference between the detection channels) and externally using pre-mixed gases to ensure accurate measurement of the total NOx produced.
Investigations of the ignition characteristics regarding the plasma were then carried out. A large parameter study of the reactor operation for a range of powers, flow rates, and gas mixtures (i.e. O2/N2) was carried out for both pulsed and continuous powering of the plasma, with the aim of finding the most energy-efficient conditions with the highest (absolute) NOx production. We demonstrated that continuous powering was the most efficient route for plasma-produced NOx under atmospheric pressure at the power range up to 1 kW. Further, we discovered the most energy-efficient conditions reported to date for atmospheric pressure plasma-based Nitrogen fixation (NF). Notably, the record efficiencies of our MW reactor, costing ~2 MJ/mol, allowed absolute NOx production of 0.77 L/min which is a seven-fold increase compared to previous reports based on the gliding arc (GA) design which report production of ~0.11 L/min at their most efficient conditions (~2.5 MJ/mol). In addition, in contrast to GA plasmas, our design, which uses a surface wave MW plasma, does not involve metallic electrodes, a key benefit for scalability and ensuring reactor longevity and safety from metal contamination.
In parallel to the experimental tasks, we developed a quasi-1D chemical kinetics model capable of simulating the full chemistry for an air plasma at atmospheric pressure. In order to link the quasi-1D model with experiments, plasma power measurements were carried out using an Auto-Tuner and power measurement device and combined with camera imaging to determine the plasma volume. This produced an accurate estimate of the power density, which allowed a coupling between the model and experiments. Our modelling activities allowed us to successfully determine the dominant chemical pathways for our most efficient conditions (i.e. plasma power, flow rate, and gas mix) and provided a fundamental analysis of the plasma behaviour. In particular, our model confirmed experimental evidence that the higher flow rates gave better performance by reducing the time available for the back reaction. This seminal work was published in a very high impact journal, with a broad readership, “Joule”.
Throughout the project, we used different dissemination channels of our research. I published 2 peer-reviewed journal papers as the first author. One of these publications was published in the leading journal “Joule” (Cell Press), which has an impact factor of 41.2 ranked first in energy research with a comparable impact factor to Nature (49.9) and Science (41.8). We also succeeded in publishing in a field-specific journal targeting the plasma community, namely, “Plasma Sources Science and Technology” (Institute of Physics), which is the leading and most prestigious plasma physics journal in the world. Our results were also presented to the scientific community in the form of two workshop contributions. This included an invited talk at a European Research Council (ERC) Synergy Grant workshop and a poster presentation at an international workshop organised by the Solvay Institutes in Université Libre de Bruxelles on “Plasma technology and other green methods for nitrogen fixation”.
To increase the impact of our work, a YouTube video for the project was created and published on the UAntwerpen YouTube channel and our research was disseminated in a publication by the Rossel media group in their farming newspaper ‘Landbouwleven’ (Dutch) or ‘Le Sillon Belge’ (French). I also participated in a European Researchers Nights (ERN) event (i.e. the Global Science Show). These efforts were very fruitful in introducing the plasma technology to an audience with different backgrounds in different countries.
