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Introduction§

Nitrogen, in the form of nitrogen oxides, is essential to life and many chemical processes. The Birkeland-Eyde (B-E) process is a plasma-based nitrogen fixation process. It was one of the first processes that was used industrially, but it never played a dominant role due to the existence of more efficient processes. The B-E process, in contrast to the Haber-Bosch (H-B) process, necessitates the use of air and high voltages while avoiding high pressures, rendering it more appealing to the agricultural sector and amateurs. This article explores the ecological significance, chemistry, and ease of building such a prototype.

Ecological motivation§

The increased use of nitrogen fertilization has been a crucial component in the increased productivity of the conventional food system. A significant part of nitrogen fertilizer is produced by the Haber-Bosch process, which consumes 3-5% of the world's natural gas production. Eventually, plants grown with these fertilizers may be given to livestock. Where livestock generates 65% of the ammonia pollution. Ammonia pollution may be reduced by exposing livestock waste to nitrogen dioxide \((\text{NO}_2),\) which reacts with the volatile ammonia gas to produce ammonium nitrate \((\text{NH}_{4}\text{NO}_{3}).\) The non-volatile nature of ammonium nitrate makes it possible for the nitrogen content of livestock waste to increase due to the addition of \(\text{NO}_2,\) or the inability to release ammonia \((\text{NH}_{3})\) gas, thereby increasing the nitrogen content. Hence, reducing the energy consumption required to fix nitrogen with the H-B process.

The B-E process is highly energy inefficient; to make the current B-E process ecologically responsible, it should use power from responsible sources. For instance, livestock housing tends to have roofs with a large surface area, which could be covered with photovoltaics, from which the excess power may be used to fix nitrogen.

Birkland-Eyde process§

The B-E process comprises three primary stages. The initial stage involves the ionization of the air to yield nitric oxide \((\text{NO}),\) as described by the chemical equation \(\text{N}_{2} + \text{O}_{2} \to 2 \text{NO}.\) Subsequently, nitric oxide can undergo oxidation to form nitrogen dioxide \(2\text{NO} + \text{O}_{2} \to 2 \text{NO}_{2}.\) Finally, within the absorber, nitrogen dioxide can be subjected to a reaction with the desired reactant; in this prototype, water is used to fix nitrogen in the form of nitric acid, as exemplified by the chemical equation \(3 \text{NO}_{2} + \text{H}_{2}\text{O} \to \text{HNO}_{3} + \text{NO}.\) The primary goal of this prototype is the generation of nitric acid, with exploration of potential areas for optimization.

Prototype§

The schematic of the prototype given below represents the model we have built.

Developing a prototype that was both safe and had observable production was quite a challenge. Problems that had to be dealt with were finding the correct electrode material, a high-voltage transformer, cooling the reactor chamber, and safety measures.

Electrodes§

Selecting the appropriate electrode material presents a significant challenge, contrary to initial expectations. In order to achieve desirable yields of 4-5% in the process, it necessitated the application of exceedingly high temperatures, exceeding \(3000^{\circ}\text{C}.\) The exigency of such extreme temperatures suggests a demand for electrodes characterized by elevated melting points while maintaining cost-effectiveness. Tungsten, known for its exceptionally high melting point of \(3422^{\circ}\text{C},\) exhibits a problem in that it succumbs to oxidation when exposed to red-hot temperatures, forming undesirable tungsten trioxide \((\text{WO}_{3}).\)

An alternative strategy involved the deployment of copper electrodes, engineered to dissipate heat and stave off the onset of melting. However, this approach proved unsuitable for our specific configuration, as the electrodes were sealed using an adhesive lacking thermal resilience.

Ultimately, the resolution to this problem was achieved through the use of steel electrodes arranged in a V-shaped configuration. The introduction of a continuous flow of cold air from below served the dual purpose of cooling the electrodes and guiding the electrical arc on an upward trajectory. As the arc ascended to its extremum, the inter-electrode distance increased beyond the sustainable range, prompting the arc to extinguish and reinitiate at the lowermost point, thus commencing a cyclic process. This deliberate oscillation prevents the formation of localized hotspots by gradually disseminating heat across the entirety of the electrode. While steel is subjected to corrosion by the extreme temperatures, the rate of corrosion is sufficiently low that this electrode may be used.

HV Transformer§

In order to initiate the arc, it is required to apply a sufficiently high voltage. In the initial stages of this project, we employed a flyback transformer in conjunction with a ZVS circuit. This configuration readily facilitated the generation of rectified high-voltage arcs, reaching up to 15 kV. However, it became evident that this transformer exhibited rapid heating and necessitated a cooling mechanism. To address this issue, a solution was implemented involving the introduction of duty cycles and the incorporation of temperature sensors for core temperature monitoring. The duty cycle consisted of approximately one minute of runtime followed by a four-minute cooldown period, which would be automatically adjusted based on temperature conditions. It is important to note that this method entailed a considerable expenditure of energy and time.

Subsequently, an alternative approach was pursued by transitioning to the use of a neon sign transformer, which operates as a constant current source. This transition markedly enhanced the efficiency of the processes involved. Significantly less energy was required to sustain the arc once it was generated, leading to a more resource-efficient system. This progression leads us to the critical consideration of the reactor chamber, which must dissipate the generated heat.

Reactor chamber§

In the initial iteration of the prototype, glass material was chosen due to its advantageous characteristics, including optical transparency, non-conductivity, chemical inertness, and resistance to UV radiation. However, glass exhibits a susceptibility to cracking when subjected to excessive temperature gradients and serves as a thermal insulator, impeding the cooling process within the reactor. Following a series of refinements, a decision was made to transition to steel as the primary material, owing to its superior thermal and electrical conductivity, while the latter property is undesirable and potentially hazardous in this context. This heightened thermal conductivity, in conjunction with augmented airflow, facilitated the establishment of a continuous operational cycle, ensuring that the temperature of the reactor walls remained below \(40^{\circ}\text{C}\) throughout the operation.

Oxidation chamber§

The oxidation chamber fulfills multiple crucial roles within the system. Firstly, it serves as a cooling unit for reducing the temperature of the reactor's output. Secondly, it functions as an oxidation chamber, intensifying the conversion of nitrogen monoxide into nitrogen dioxide by oxidation. Additionally, the oxidation chamber acts as a protective barrier against suck-back. This chemical transformation substantially augments the overall system's efficiency.

To accomplish these functions, additional airflow is introduced into the oxidation chamber, elevating the oxygen concentration within it and thereby facilitating the desired oxidation reactions. The reduction in the temperature of the reactor output not only aids in cooling but also enhances the absorption rate of nitric oxides into water, thereby further contributing to system efficiency.

Absorbers§

The absorber plays a dual role: first, it transforms into a desired salt of acid while simultaneously preventing its release into the environment. In the prototype setup, the initial absorber column is packed with glass shards to achieve two objectives. First, it reduces the absorber's volume, and secondly, it minimizes bubble size. Smaller bubbles have a larger surface area, resulting in enhanced absorption. If the aim is to produce nitric acid efficiently, the first absorber should be filled with hydrogen peroxide. This is because nitric oxide gets oxidized by hydrogen peroxide and is subsequently absorbed into the absorber. It's important to note the distinction between hydrogen peroxide and water:

\[3\text{NO}_2 + \text{H}_2\text{O} \to 2\text{HNO}_3 + \text{NO} \] \[2\text{NO}_2 + \text{H}_2\text{O}_2 \to 2\text{HNO}_3.\] The second absorber's primary function is to further minimize environmental impact. Since the absorption capacity of the first absorber will not suffice, more absorbers should be utilized. This secondary absorber is filled with a basic water solution known for its higher absorption rate compared to the first absorber. In the context of agriculture, calcium hydroxide or potassium hydroxide can be particularly advantageous. When nitrogen dioxide is absorbed by this secondary absorber, it results in the production of either potassium nitrate or calcium nitrate, both of which can serve as valuable fertilizers for plants.

However, it's important to note that the second absorber also captures nitrogen monoxide, leading to the formation of the nitrite ion in the solution. Nitrite is highly toxic to both humans and the environment and must be handled with care. One effective method for its disposal involves reacting it with percarbonate, which converts the nitrite into nitrate.

Results§

After continuously running for about 8 hours, the process successfully produced 200 mL of nitric acid at a concentration of 0.8%. While this may seem like a relatively modest output, it's essential to highlight that the process is capable of running continuously. If we extrapolate this data to the extreme, we would achieve the legal upper limit of 4% in 40 hours.

However, we must acknowledge the process's inherent energy inefficiency and the potential for leaks. Considering the current cost of electricity, which is approximately 0.80 €/kWh, the production of this acid becomes prohibitively expensive. Moreover, concerns regarding prolonged exposure have led me to make the decision to temporarily halt the project.

These factors collectively emphasize the need for further optimization and safety enhancements before resuming the project.

Discussion§

Enhancements and additional research efforts should be directed towards both the prototype's development and the chemical component. Firstly, to enhance the efficiency of this prototype, numerous improvements can be considered. Some potential avenues for enhancement include the incorporation of a vacuum ejector within the final absorption chamber. This addition could potentially reduce the amount of nitric oxides escaping from the system. Additionally, a preliminary step of drying the air before it enters the reactor may be beneficial. This can help mitigate the production of unwanted side products that could otherwise interfere with the primary reaction. Please inform me of further improvements to both safety and efficiency.

Secondly, when the urea in urine undergoes a chemical reaction with nitric acid, uronium nitrate is produced, as described by the following reaction: \((\text{NH}_{2})_{2}\text{CO} + \text{HNO}_{3} \to (\text{NH}_2)\text{COHNO}_3.\) The ecological consequences of which remain poorly understood. Existing research indicates that uronium nitrate may exhibit inferior fertilization properties when compared to ammonium nitrate in the context of grass and barley growth. However, the limited scope of studies on uronium nitrate has hindered a comprehensive understanding of its environmental impact. Consequently, further investigations are imperative to assess its suitability for potential future applications in agricultural practices.

References§

• Beusen, A. H. W., Bouwman, A. F., Heuberger, P. S. C., Van Drecht, G., & Van Der Hoek, K. W. (2008). Bottom-up uncertainty estimates of global ammonia emissions from Global Agricultural Production Systems. Atmospheric Environment, 42(24), 6067–6077.
• Graves, D. B., Bakken, L. B., Jensen, M. B., & Ingels, R. (2018). Plasma activated organic fertilizer. Plasma Chemistry and Plasma Processing, 39(1), 1–19.
• Leigh, G. J. (2008). Nitrogen fixation at the millennium. Elsevier.
• Parkes, G. D., & Mellor, J. W. (1967). Modern Inorganic Chemistry.
• Webb, H. W. (1923). Absorption of nitrous gases. E. Arnold.
• Gasser, J., & Penny, A. (1967). The value of urea nitrate and urea phosphate as nitrogen fertilizers for grass and barley. The Journal of Agricultural Science, 69(1), 139-148.
• Li, S., Medrano Jimenez, J., Hessel, V., & Gallucci, F. (2018). Recent Progress of Plasma-Assisted Nitrogen Fixation Research: A Review. Processes, 6(12), 248.