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  • Amir Muhaisen

Birkeland-Eyde : Case-Study

in this article, non-thermal plasma arc reactors are illustrated with an aim to develop an energy-efficient process for aqueous nitric acid production


Recently, Birkeland-Eyde has been re-commercialized since non-thermal plasma arc proved to be efficient for aqueous nitric acid production compared to 120-year-old thermal arc process which was replaced by present-day ammonia oxidation linked to Haber-Bosch in which concentrated nitric acid is produced


Therefore, Non-Thermal Birkeland-Eyde is modern sustainable alternative in which downstream nitrogen-fertilizers are produced from aqueous nitric acid via air, water, and renewable electricity thus allowing local farms to be fertilized upon an emerging energy-crisis that is expected to hit hard on ammonia value chain in winter



Nitric Acid : Production


Here you will find present-day industrial synthetic routes and simplified flow diagrams to further illustrate upstream ammonia and concentrated nitric acid production



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Market Trend


Commercially, concentrated nitric acid inventories has been considered critical to secure upon seasonal demand for downstream fertilizers ; mainly ammonium nitrate , potassium nitrate , calcium nitrate and to less extent sodium nitrate


According to Chemiprobe, concentrated nitric acid prices in China have been on upward trend in second quarter this year mainly as for seasonal demand for fertilizers combined with tight supply , and concerns upon ports congestion , not to mention that China implemented export limit upon local producers back then to saturate domestic market

In contrast, concentrated nitric acid prices in China have surged in third quarter this year upon upstream linkage ; ammonia , natural gas , and naphtha in which crude oil market dynamics exerted an obvious impact that is clearly observed till present day off-season. Keep in mind that copy-cat to asian market trends has been observed in middle-east


Non-Thermal Birkeland-Eyde


Normally, if electrical insulator is exposed to potential difference beyond what is called breakdown voltage, then an intense electric field forms in which insulator suddenly becomes conductor as current flows through, thus electrons and cations are released and transmitted ; this is called electric breakdown


Therefore, atmospheric air once exposed to potential difference beyond breakdown voltage, then no more an insulator as electric discharge modes are observed upon current increase as illustrated below

Notice that arc discharge mode is defined as prolonged electric discharge caused by electric breakdown of atmospheric air ; mode characteristics are visible-light emission as atmospheric air becomes ionized in form of visible plasma, high current density that leads to voltage reduction because arc discharge relies on therm-ionic emission from electrodes , besides high temperature in which arc discharge mode is divided into multiple phases below :

  • non-thermal phase as therm-ionic emission (non-thermal plasma) does not create sufficient heat beyond electrodes to ionize air molecules through arc discharge >> electrons temperature = cations temperature ≠ molecules temperature


  • thermal phase as therm-ionic emission (thermal plasma) creates sufficient heat to ionize air molecules through arc discharge as heat is caused by electrical resistance of atmospheric air >> electrons temperature = cations temperature = molecules temperature


Basically, Birkeland-Eyde made use of thermal plasma inside reactor as atmospheric air feed was exposed to breakdown voltage (> 5 kV @ 50 Hz AC) in which an arc discharge connected electrodes while electro-magnets or neodymium magnets exerted an externally magnetic field to form thermal plasma into thin-disc shape as illustrated via lab-scale reactor below

Critical to mention here that alternating-current (AC) arc discharge re-strike on each alternation (half-cycle) in contrast to direct-current (DC) arc discharge. That means upon frenquency increase, required breakdown voltage decreases because frequency increase means further charge density on electrodes or in other words ; an increase in magnetic field strength


In reference to chemical perspective, atmospheric air is mainly composed of nitrogen and oxygen (image : volume%). Accordingly, nitrogen reacts with oxygen to form nitric oxide via zeldovich mechanism that is illustrated in reaction (1+2) as atmospheric air is ionized in form of thermal plasma @ 3000 °C in which reactive species of oxygen and nitrogen are formed

On the other hand, take into account that air moisture is ionized in form of thermal plasma @ 3000 °C in which reactive species of hydroxyl and hydronium are formed thus additional nitric oxide besides traces of ammonia are produced as illustrated in reaction (3) and reaction (4). Nevertheless, ammonia is formed upon exposure to ultra-violet radiation hence higher frequency component of plasma. Consequently, nitric oxide is further oxidized into nitrogen dioxide as illustrated in reaction (5) then absorbed in water to produce aqueous nitric acid


Furthermore, Non-Thermal Birkeland-Eyde have emerged as energy-efficient alternative in which atmospheric nitrogen reacts with atmospheric oxygen to form nitric oxide via vibrational-excitation mechanism as atmospheric air is ionized in form of non-thermal plasma thus vibrationally-excited reactive nitrgen species are generated by less energy in reference to thermal-plasma


Process Design


In an engineering sense, chemical process parameters are controlled to provide max. conversion -in terms of yield and selectivity- as well as efficient energy consumption. Conceptually, for process used to fix atmospheric nitrogen via electric arc discharge; process parameters needed to achieve appropriate electric breakdown conditions include applied voltage, atmospheric air flow rate, and electrode operability hence material of construction >> reactor design


On the other hand, process parameters needed to achieve appropriate absorption capture conditions include reactor effluent composition , absorption column pH , and column height >> column design. Practically, any process used to fix atmospheric nitrogen via electric arc discharge is mainly composed of reactor section and absorption section


Birkeland-Eyde proved inefficient as for 60 kWh per kilogram nitric acid not to mention 3% yield. Alternatively, Gliding Arc made use of thermal plasma yet more efficienct as for 40 kWh per kilogram nitric acid ; as atmospheric air feed is exposed to breakdown voltage (> 5 kV @ 10 kHz AC) in which arc discharge is generated via diverging electrodes (stainless steel, aluminum) as turbulent air flow extends plume of thermal plasma as illustrated via lab-scale

Critical to mention that upon experimental observation for Gliding Arc reactors, an increase in arc discharge frequency leads to yield increase by 3% yet if arc discharge frequency exceeds 10 kHz then leads to yield decrease. Therefore (5–10 kHz) is an optimum frequency range. In similar pattern, an increase in atmospheric air flow rate leads to yield increase yet if exceeds 2 liters per minute then leads to yield decrease. Therefore (1.2 - 2.0 liter per minute) is optimum


Another observation is that an increase in atmospheric air moisture-content through spray chamber or bubbling column placed before reactor increases nitrate content by 60% because nitrogen dioxide reacts with hydroxide radicals formed upon plasma excitation of water

peroxide content increases in reactor effluent as reaction (7) takes place, what's more, is that an increase in oxygen content to 50% combined with min. flow rate (max. residence time) leads to 70% increase in nitrate content because excess oxygen leads to complete oxidation of nitric oxide

Recently, an enhanced nozzle-structure prototype that focus non-thermal plasma into compact volume to increase conversion have proved efficient as for 29 kWh per kilogram nitric acid ; Rotating Gliding Arc -illustrated below- is designed by means of atmospheric air flow patterns and temperature profile hence to utilize non-thermal plasma in an effort to reduce energy consumption, though commercialization feasibility is promising while further development efforts are focused on milli-scale reactors to be arranged in parallel to maintain max. conversion and efficient energy consumprion

Attention should not be restricted upon reactor parameters only, absorption process is critical as for efficiency because nitrogen dioxide aqueous absorption requires series of packed columns or absorption towers, take into consideration that Birkeland-Eyde process required multiple absorption towers each 18 meter height to produce 40% aqueous nitric acid because 20% of produced nitrogen oxides were not captured.


Note that nitrogen oxides undergo multiple reactions when come into contact with water ; nitrogen dioxide hydrolysis takes place upon absorption as illustrated in reaction (8).In addition, reactor effluent contains nitrogen dioxide equilibrium species such as traces of di-nitrogen tetroxide that undergo hydrolysis upon absorption as illustrated in reaction (9). Nevertheless, temperature increase is proportional to equilibrium species shift toward nitrogen dioxide, yet temperature increase has insignificant effect upon aqueous solubility of nitrogen dioxide within atmospheric conditions, while excess heat in reactor effluent serves as reaction promoter because hydrolysis is an endothermic reaction

In contrast, aqueous nitrite and peroxynitrite are consumed as acidity of aqueous absorption column increases as illustrated in reaction (10), because nitrite is an intermediate that forms upon nitrite contact with aqueous traces of hydrogen peroxide as illsutrated in reaction (11), this reactive intermediate decomposes rapidly @ ↓pH to produce reactive radicals to be further hydrolyzed into aqueous nitric acid. That's why using alkalines to enhance aqueous absorption is excluded, meanwhile absorption column effluent contains unreacted nitric oxide in which recycle to pre-reactor oxidation chamber is considered upon recent efforts to develop this process


Chemiprobe started “probing” for technical insights about atmospheric nitrogen fixation since november, and continuously networking with professionals. Therefore, Chemiprobe team is currently on a mission to develop much efficient process via experimental approach plus modelling thus illustrated below is our preliminary process development approach

Photovoltaics


Birkeland-Eyde prototypical industrial-scale reactor that was in service 120 years ago, had made use of nearby hydro-electric power plant in Norway. In present-day, photovoltaics are ideal for modern process as approx. 0.13 kW generated per square meter of panel arrays, hence 100 kW farm is more than sufficient for nitrogen fixation pilot-plant, especially in case of multiple milli-scale reactors to achieve max. energy utilization



Market Players


Conventional nitric acid business requires higher capital cost relative to atmospheric nitrogen fixation business, because it requires larger economy of scale upon value chain hence upstreams such as naphtha, natural gas, and ammonia. On the other hand, atmospheric nitrogen fixation business is promising especially in United States upon an emerging energy-crisis, in which an emerging venture-funded start-up provides on-site aqueous nitrate fertilizer (1% potassium or calcium nitrate) embeded within irrigation system


In reference to process, nitricity pilot plant is built inside iso-container powered by photovoltaic 16-panel array that supplies 2.4 kW, in which efficient small-scale Pulsed-Gliding-Arc reactors and multiple absorption columns are used to produce (1% nitric acid) that is treated with potassium hydroxide or calcium carbonate inside 800 liters tank. Until now, no middle-eastern market for atmsopheric nitrogen fixation. However, It is expected to emerge upon an energy crisis that could hit hard on international scale



Special Thanks to Christophe Pochari for technical data provided by Pochari Technologies, California, US



Birkeland-Eyde : process used to fix atmospheric nitrogen to produce aqueous nitric acid besides nitrate fertilizers via thermal plasma arc ; an inefficient process in terms of energy consumprion


Haber-Bosch : process used to produce ammonia upon catalytic refurtheraction of hydrogen and nitrogen over heterogeneous catalyst composed of iron oxides in which mixed feed is heated inside reformer packed tubes ; an industrial-scale efficient process yet energy-intensive


current transmission through conductors under influence of electric field

electrons path : anode (-) → cathode (+)

cations path : cathode (+) → anode (-)


Nitricity limited -formerly pacific nitrogen company- is an american agro-chemical company headquartered and registered in San Francisco


References


1. Chen, H. et al. (2021) “review of low-temperature plasma nitrogen fixation technology” Waste Disposal Sustainable Energy, 3(3), pp. 201–217


2. Lee, D.H. et al. (2021) “novel energy efficient path for nitrogen fixation using a non-thermal arc” RSC Advances, 11(21), pp. 12729–12738


3. Yang, J. et al. (2016) “nitrogen fixation in water using air phase gliding arc plasma” Journal of The Electrochemical Society, 163(10)


4. Patil, B.S. et al. (2015) “plasma nitrogen oxides synthesis in a milli-scale gliding arc reactor: Investigating the electrical and process parameters” Plasma Chemistry and Plasma Processing, 36, pp. 241–257


5. Burlica, R. et al. (2006) “formation of reactive species in gliding arc discharges with liquid water” Journal of Electrostatics, 64(1), pp. 35–43


6. Leigh, G.J. (2002) nitrogen fixation at the millennium. elsevier science


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