Periodic Reporting for period 1 - PREPARE (Plasma Reactors for Efficient Fertilizer Production Applied in a Real Environment)
Berichtszeitraum: 2022-11-01 bis 2024-04-30
The aim of this project was to build new plasma reactors for sustainable and energy-efficient nitrate (NOx) production from air, as basis for sustainable fertilizer production. Plasma is a partially ionized gas, consisting of neutral gas molecules, but also electrons, various ions, radicals and excited molecules. Hence, it is a reactive cocktail, allowing “difficult” reactions (like the dissociation of N2 molecules in air, necessary for NOx production) to proceed at mild conditions, because the N2 molecules are “activated” by the electrons in the plasma. Moreover, plasma is created by applying electricity, it is quickly switched on/off and has no economy of scale, so it is ideal in combination with renewable (intermittent) electricity, and thus, of interest for electrification of fertilizer production. Traditionally, fertilizer production is based on the Haber-Bosch process for NH3 production, followed by the Ostwald process, which are large CO2 emitters (mainly because of the H2 production needed for the Haber-Bosch process). This project thus offers a sustainable alternative. It originates from my ERC Synergy Grant “SCOPE”, where we obtained record values in plasma-based NOx production and energy cost. In the ERC Proof of Concept project, we made a step further, to bring this into real application through optimization of the reactor design and its performance.
We tested various plasma reactor designs in terms of conversion and energy cost (EC), and we conclude that our current design is already quite optimized. Further improvements are however still expected from the post-plasma region (i.e. quenching to avoid recombination of the products).
Therefore, we combined experiments with computational fluid dynamics and chemical kinetics modelling to comprehensively analyse the recombination reactions post-plasma. We were able to perform 2D spatial mapping of gas product compositions and temperatures, which allowed us for the first time to experimentally confirm the substantial limiting effect of recombination reactions in the post-plasma (afterglow) region. With our model we could predict the reaction rates responsible for these recombinations, and thus propose an afterglow quenching strategy for performance enhancement, which was also demonstrated in practice.
In the next stage, we looked for upscaling options, to bring the plasma reactors to larger application. We pursued two different ways of upscaling. The first way is upon parallelization of reactors within a single housing. Special attention was paid to safety and control features, a key for integration into industrial systems. We tested the performance of our so-called multi-reactor gliding arc plasmatron (MRGAP), focusing on the influence of flow rate and the number of active reactors. We obtained the best performance with the most active reactors (five) at the highest flow rate (80 L/min). Hence, the concept of plasma reactor parallelization within a single housing is a suitable method for scaling up from lab-scale to prototype, and our performance analysis demonstrates that increasing the power (through adding more reactors) and total flow rate result in higher conversion rate without sacrificing absolute conversion or energy efficiency.
The second way of upscaling is by increasing the reactor size. We compared two plasma reactors with the same design (pin-to-pin configuration), i.e. a small one (operating at flow rate of 5 – 20 L/min and current of 200 – 500 mA) and a large one (operating at higher flow rate of 100 – 300 L/min and current of 400 – 1000 mA). In the small reactor, we achieved the lowest EC of 2.8 MJ/mol, for a NOx concentration of 1.7%, yielding a production rate (PR) of 33 g/h. In the large reactor, the higher flow rates reduce the NOx concentration formed, while maintaining a similar EC. Our modeling insights show that the plasma arc is concentrated in the reactor center, limiting the fraction of gas flow passing through the plasma, which is the limiting factor for upscaling. By changing the reactor configuration into a torch design, we could enhance the amount of gas treated by the plasma, reaching much higher NOx concentrations and a PR of 80 g/h. This illustrates the importance of reactor design in upscaling.
Finally, we recently also developed a new modelling approach, to solve the complete set of relevant equations, including gas flow, heat balance, and species transport and chemistry, in a coupled manner. This new, new self-consistent model can capture the main physics and chemistry occurring in plasma reactors, which is an important step towards predictive modelling for plasma-based gas conversion, like for fertilizer applications.
Therefore, we combined experiments with computational fluid dynamics and chemical kinetics modelling to comprehensively analyse the recombination reactions post-plasma. We were able to perform 2D spatial mapping of gas product compositions and temperatures, which allowed us for the first time to experimentally confirm the substantial limiting effect of recombination reactions in the post-plasma (afterglow) region. With our model we could predict the reaction rates responsible for these recombinations, and thus propose an afterglow quenching strategy for performance enhancement, which was also demonstrated in practice.
In the next stage, we looked for upscaling options, to bring the plasma reactors to larger application. We pursued two different ways of upscaling. The first way is upon parallelization of reactors within a single housing. Special attention was paid to safety and control features, a key for integration into industrial systems. We tested the performance of our so-called multi-reactor gliding arc plasmatron (MRGAP), focusing on the influence of flow rate and the number of active reactors. We obtained the best performance with the most active reactors (five) at the highest flow rate (80 L/min). Hence, the concept of plasma reactor parallelization within a single housing is a suitable method for scaling up from lab-scale to prototype, and our performance analysis demonstrates that increasing the power (through adding more reactors) and total flow rate result in higher conversion rate without sacrificing absolute conversion or energy efficiency.
The second way of upscaling is by increasing the reactor size. We compared two plasma reactors with the same design (pin-to-pin configuration), i.e. a small one (operating at flow rate of 5 – 20 L/min and current of 200 – 500 mA) and a large one (operating at higher flow rate of 100 – 300 L/min and current of 400 – 1000 mA). In the small reactor, we achieved the lowest EC of 2.8 MJ/mol, for a NOx concentration of 1.7%, yielding a production rate (PR) of 33 g/h. In the large reactor, the higher flow rates reduce the NOx concentration formed, while maintaining a similar EC. Our modeling insights show that the plasma arc is concentrated in the reactor center, limiting the fraction of gas flow passing through the plasma, which is the limiting factor for upscaling. By changing the reactor configuration into a torch design, we could enhance the amount of gas treated by the plasma, reaching much higher NOx concentrations and a PR of 80 g/h. This illustrates the importance of reactor design in upscaling.
Finally, we recently also developed a new modelling approach, to solve the complete set of relevant equations, including gas flow, heat balance, and species transport and chemistry, in a coupled manner. This new, new self-consistent model can capture the main physics and chemistry occurring in plasma reactors, which is an important step towards predictive modelling for plasma-based gas conversion, like for fertilizer applications.
A detailed techno-economic analysis (TEA), which we performed together with colleagues from University of Twente, predicts that plasma-based NOx synthesis, using our best results, is almost competitive with the Haber–Bosch process combined with the Ostwald process. Indeed, the energy cost (EC) target value to become competitive is defined as 1.0–1.5 MJ/mol, and we already reached an EC of 2 MJ/mol at 3.8% NOx concentration in microwave plasma, and an EC of 1.8 MJ/mol in a gliding arc plasma at a pressure of 3 barg, at nearly 5% NOx concentration. We believe we can further improve these numbers, towards the target, via smart reactor design, tuning the chemistry and vibrational kinetics, avoiding back-reactions, or combination with catalysts.
Our research attracted a lot of interest from companies. We had a bilateral project with N2 Applied (Norwegian SME, Dec ’22 – Dec ’23) on experiments and modeling for plasma-based NOx production, where we investigated the performance of an upscaled plasma reactor. They are currently seeking investments to start a follow-up bilateral project.
Furthermore, in collaboration with KULeuven (J. Martens, L. Hollevoet), we had a new project directly funded by the Flemish Ministry of Farming, to demonstrate the proof-of-concept of air washer technology with a large plasma reactor, to combine the plasma-produced NOx with NH3 emissions from farms, which will allow farmers making their own NH4NO3 fertilizer, and simultaneously solve the problem of NH3 emission (as described in our ERC PoC project). We also collaborate in this project with CB Group (Belgian company building air washers) and VitalFluid (Dutch company making larger plasma reactors, who also showed interest to further collaborate, and we are seeking for possibilities). The new air washer with plasma reactor is now operational at TRANSfarm (demo-farm in Leuven), and the demo attracted already a lot of interest. We can test here the plasma reactor in real world context, i.e. air with varying humidity and exploring the reactor robustness for both continuous long-term and intermittent operation, as written in our PoC proposal. We are working towards the creation of a spinoff company next year, together with KULeuven. We also have 2 patent applications together, that will form the basis of the spinoff.
Finally, we will start a bilateral project with another company (still confidential), on plasma-based NOx production for fertilizer applications. The large pilot reactor for this is currently being built, and will be operational by end of 2024, in the pilot hall of BlueApp (pre-incubator of UAntwerp, for demo-purposes, as also described in our proposal), where we will evaluate the performance at large scale, in a wide range of conditions.
Our research attracted a lot of interest from companies. We had a bilateral project with N2 Applied (Norwegian SME, Dec ’22 – Dec ’23) on experiments and modeling for plasma-based NOx production, where we investigated the performance of an upscaled plasma reactor. They are currently seeking investments to start a follow-up bilateral project.
Furthermore, in collaboration with KULeuven (J. Martens, L. Hollevoet), we had a new project directly funded by the Flemish Ministry of Farming, to demonstrate the proof-of-concept of air washer technology with a large plasma reactor, to combine the plasma-produced NOx with NH3 emissions from farms, which will allow farmers making their own NH4NO3 fertilizer, and simultaneously solve the problem of NH3 emission (as described in our ERC PoC project). We also collaborate in this project with CB Group (Belgian company building air washers) and VitalFluid (Dutch company making larger plasma reactors, who also showed interest to further collaborate, and we are seeking for possibilities). The new air washer with plasma reactor is now operational at TRANSfarm (demo-farm in Leuven), and the demo attracted already a lot of interest. We can test here the plasma reactor in real world context, i.e. air with varying humidity and exploring the reactor robustness for both continuous long-term and intermittent operation, as written in our PoC proposal. We are working towards the creation of a spinoff company next year, together with KULeuven. We also have 2 patent applications together, that will form the basis of the spinoff.
Finally, we will start a bilateral project with another company (still confidential), on plasma-based NOx production for fertilizer applications. The large pilot reactor for this is currently being built, and will be operational by end of 2024, in the pilot hall of BlueApp (pre-incubator of UAntwerp, for demo-purposes, as also described in our proposal), where we will evaluate the performance at large scale, in a wide range of conditions.