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| Fig. 1: Process Flow Diagram for Cold Plasma Nitrogen Fixation. (Image source: A. Sazi, after Chen et al. [10]) |
When Fritz Haber developed the laboratory-scale process for ammonia synthesis from hydrogen and nitrogen, and Bosch scaled it to industrial levels, it addressed the critical shortage of fixed nitrogen for tissue construction and maintenance. [1,2] This process, known as the Haber-Bosch (HB) process, proved far more efficient than traditional biofixation (by Rhizobium bacteria symbiotic with legumes and by cyanobacteria), enabling rapid human population growth from 1.6 billion in 1900 to over 7 billion today through the use of modern fertilizers. [2-4]
One issue with the HB process is that, although we are highly dependent on it (HB makes 96% of global ammonia), it largely relies on fossil fuels as an energy source. It accounts for 1.2% of anthropogenic CO2 emissions. [3] Additionally, current production of reactive N (RNS) is 120 terragrams, and 50-70% of it is lost ot the environment, whose accumulation from either fertilizer runoff or N2O and NO3 release into the atmosphere can cause eutrophication and climate change, respectively. [5]
Non-thermal (cold) plasma nitrogen fixation has been proposed by some researchers as an alternative make reactive nitrogen under methane conditions which are close to ambient temperatures (Fig. 1). In this context, "cold plasma," non-thermal plasma, and ambient- temperature plasma refer to the same physical concept, as electrons are highly energetic while the gas remains cool. Scientists propose that energetic electrons can populate N2 that is vibrationally excited and drive reactions which normally require high temperatures, and thus a high amount of heat in the reactor compared to Haber-Bosch temperatures. [5]
For these comparisons between NF pathways, it is essential to note that the lower bound of energy needed to synthesize NH3 from N2 and H2 O is fixed from thermodynamics. However, we must acknowledge the differences in the processes that give rise to certain inefficiencies and side reactions, rather than relying on explicit thermodynamic calculations.
The HB process is the established quantitative benchmark for large-scale nitrogen fixation and relies on specific operating conditions to achieve scalable kinetics and convert dinitrogen and hydrogen into ammonia. [6] This process usually requires temperatures from 400-500 °C. Additionally, the system needs a high pressure of about 30 MPa to account for the equilibrium shift at high temperatures and Le Châtelier’s principle. [6] Iron-based oxide catalysts are used in the synthesis and need fossil fuels to generate hydrogen. [3,6]
This process, however, has been optimized over the last century and is highly efficient, as shown in Table 1 for the best available technique (BAT) for NF from various sources. [3,4]
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| Table 1: Energy consumption and efficiency metrics for best-available HaberBosch nitrogen fixation processes. [3,6] |
According to Table 1, the energy consumption of BAT ranges from 27.4 to 31.8 GJ per metric ton (t) of NH3. To discern a comparable efficiency, we need to convert this to the energy required per mol of ammonia (kJ/mol NH3).
| Lower Bound: | 27.4 × 106 kJ tonne-1 × 17 × 10-6 tonnes mol-1 = 466 kJ mol-1 |
| Upper Bound: | 31.8 × 106 kJ tonne-1 × 17 × 10-6 tonnes mol-1 = 540 kJ mol-1 |
From these calculations, we can determine that the methane-fed HB has an energy requirement of 466 to 541 kJ/mol (i.e., 0.470.54 MJ/mol NH3). This result is consistent with an earlier report, which cites a commercial energy requirement of 0.48 MJ NH3 (480 kJ/mol). [3]
The reported values for cold plasma energy consumption have a wide range of values because different scientific authors don't report the same process boundary of the same product. Some papers report electric energy needed to fix nitrogen into NOx/nitrates (a plasma fixation step), and quote results in MJ per mole of fixed nitrogen atoms (MJ/mol N). Other papers report direct plasma synthesis of ammonia, quoting MJ per mole of ammonia produced (MJ/mol NH3). Thus even when a "per mole" unit looks similar between measurements, these values can represent different measurements and energy accounting methods to account for efficiency of ammonia production. Therefore, values such as 0.7 MJ/mol N and 1.7 MJ/mol NH3 should not be compared without understanding the processes that produced the measurements.
Cold plasma (non-thermal plasma, NTP) nitrogen fixation uses an electrical discharge to create non-thermal plasma (NTP), which allows heating electrons alone to activate N2 rather than relying on the immense heat and pressure that HB does. [6] It can operate under ambient conditions because, while the electrons become extremely hot (10,000 °K), the surrounding gas can remain at a low temperature (400 °K). [5] However, in practice, a significant fraction of the electrical energy supplied to the discharge is lost as heat and other non-productive channels, so the practical energy cost is higher than the thermodynamic minimum would suggest. Table 2 compares the highest achieved efficiencies between the HB and Cold Plasma methods. [4-7] Note that EC is short for "Energy Consumption".
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| Table 2: Comparison of efficiencies between HB and cold plasma. [4,6,7] |
Using the lowest value in the reported overall plasma pathway range (2.1 MJ/mol NH3 in Table 2) and comparing it to a representative conventional HaberBosch value (0.7 MJ/mol NH3, shown for comparison), we obtain
Current cold plasma technology consumes three times as much energy per mol N as the conventional HB process. Furthermore, extremely low reported energy values (e.g., 0.42 MJ mol⁻ N) usually reflect narrowly defined experiments rather than complete ammonia production. These values may omit downstream synthesis, separation, or operate at low conversion, and therefore do not represent the true energy cost of producing deliverable NH3.,/p.
The biological nitrogenase is a useful benchmark for assessing the energetic efficiency of NF. However, due to difficulties in measuring energy losses from cell maintenance and side reactions, this is difficult to calculate. The commonly quoted requirement of 16 ATP per N2 reduced (8 ATP per NH3) arises from reaction stoichiometry rather than from calorimetric measurements. Using a representative biochemical free energy of ~96 kJ/mol ATP, this corresponds to a bookkeeping value of approximately 0.77 MJ/mol NH3. This conversion does not represent the actual thermodynamic energy cost of biological nitrogen fixation, which has energy losses associated with ATP synthesis, cellular maintenance, and side reactions. The overall mechanistic scheme of biological nitrogen fixation is widely accepted to follow the reaction:
The hydrolysis of adenosine triphosphate (ATP) molecules transfers electrons to the dinitrogenase protein to stepwise reduce the dinitrogen molecule. The core requirement for this process is that two ATP molecules are needed for every electron transfer. The following equation converts the into an equivalent energy scale using the previously cited figures for free energy per ATP molecule. Because mechanistic analyses show 8 ATP are required to make 1 mol NH3, we can multiply that by the amount of estimated free energy per ATP to give a biochemical bookkeeping estimate to indicate the scale of ATP involvement (not to give any sort of calorimetric estimate).
The value of 0.77 MJ/mol provides only a bookkeeping estimate based on ATP coupling and does not constitute a meaningful benchmark. In fact, the efficiency of nitrogenase is much lower due to energy losses from cellular maintenance and side reactions. Cold-plasma-based fixation requires approximately 2.1 MJ/mol. Although the efficiency of biological nitrogen fixation cannot be quantitatively compared with industrial pathways, its ATP-coupling scale provides qualitative context. [3,7-9]
Because HB systems have had a century to reduce energy consumption, one must think about how to improve plasma-catalysis. One way scientists can do this is by maximizing vibrational excitation of N2 and adopting pulsed discharge systems to prevent the decomposition of the NOx and ammonia products. [4,5] Additionally, despite HB's efficiency superiority, cold plasma NF can be small-scale, decentralized, and modular, relying solely on water, electricity, and air. In contrast, HB relies on fossil fuels and iron catalysts (which generate CO2). [4] To be competitive with HB efficiency, scientists must focus on controlling gas temperature to prevent decomposition and enhancing plasma catalysis to guide specific reaction pathways. [4]
© Arshia Sazi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] L. Jiminez, Haber-Bosch Process, Physics 240, Stanford University, Fall 2022
[2] V. Smil, "Detonator of the Population Explosion," Nature 400, 415 (1999).
[3] C. Smith, A.K. Hill, and L. Torrente-Murciano, "Current and Future Role of Haber-Bosch Ammonia in a Carbon-Free Energy Landscape," Energy Environ. Sci. 13, 331 (2020).
[4] N. Cherkasov, A. O. Ibhadon, and P. Fitzpatrick, "A Review of the Existing and Alternative Methods For Greener Nitrogen Fixation," Chem. Eng. Process. Process Intensif. 90, 24 (2015).
[5] D. Aceto, P. F. Ambrico, and F. Esposito, "Air Cold Plasmas as a New Tool For Nitrogen Fixation in Agriculture: Underlying Mechanisms and Current Experimental Insights," Front. Phys. 12, 1455481 (2024).
[6] D. Panchal et al., "Advanced Cold Plasma-Assisted Technology For Green and Sustainable Ammonia Synthesis," Chem. Eng. J. 498, 154920 (2024).
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[8] Y.-H. P. Zhang et al., "High-Yield Hydrogen Production from Starch and Water by a Synthetic Enzymatic Pathway," PLOS ONE 2, e456 (2007).
[9] P. R. Rich, "The Molecular Machinery of Keilin's Respiratory Chain," Biochem. Soc. Trans. 31, 1095 (2003).
[10] H. Chen et al., "Review of Low-Temperature Plasma Nitrogen Fixation Technology," Waste Dispos. Sustain. Energy 3, 201 (2021).