Method for synthesizing nitrogen oxides and nitric acid in a thermal reactor

Abstract

Providing a gas mixture comprising oxygen and nitrogen, and heating the gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar to produce NO x forming a gas mixture containing nitrogen oxides (NO x A method for the synthesis of HNO3 is also provided.

Description

[Technical Field]

[0001] The present disclosure relates to the use of nitrogen oxides (NO x ), particularly for the production of NO, NO2 and HNO3. [Background technology]

[0002] Nitrogen oxides (NO x Nitric acid is commonly used to produce nitric acid, which is used to produce ammonium nitrate, a key component of fertilizers, explosives, as a solvent, and for other chemical processes, as well as for bleaching and sterilization. Nitrogen is essential for plants, making it one of the most important nutrients in fertilizers. In the last century, nitrogen-containing fertilizers were essentially produced from atmospheric nitrogen by the Haber-Bosch process, which produces ammonia from molecular nitrogen and hydrogen derived from fossil fuel-based steam reforming, and the Ostwald process, which produces nitric acid by subsequent oxidation of ammonia. Nitric acid is then used, for example, as a feedstock for producing nitrate fertilizers.

[0003] Alternatively, nitric acid can be produced by nitrogen fixation via the Birkeland-Eyde process, invented in 1903. With the aid of an electric arc, atmospheric nitrogen is thermally oxidized to NO, which spontaneously converts to NO2 when the gas is cooled in the presence of oxygen. The NO2 is then scrubbed with water, thereby converting it to nitric acid. However, because the thermal oxidation of nitrogen gas is very energy-intensive, the Haber-Bosch and Ostwald processes have become the primary processes commercially used to fix nitrogen from air to produce nitric acid.

[0004] Due to greenhouse gas emissions (CO2, NO, NO) from steam reforming and the Haber-Bosch process in combination with the Ostwald process, thermal oxidation of nitrogen has resurrected as an interesting option. However, the high energy requirements of the Birkeland-Eyde process mean that improvements are needed, especially with regard to energy efficiency. Summary of the Invention [Problem to be solved by the invention]

[0005] One purpose is to x The goal is to make available an essentially carbon-neutral and industrially feasible method that is a more energy-efficient route compared to previous thermal oxidation methods for producing nitric acid (HNO3) and nitric acid (HNO3). [Means for solving the problem]

[0006] Accordingly, there is provided as a first aspect of the present disclosure: Nitrogen oxides (NO X ), comprising the steps of: Follow these steps: - providing a gas mixture comprising oxygen and nitrogen; and - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO X forming a gas mixture comprising A method comprising:

[0007] Also provided as a second aspect of the present disclosure are: A method for synthesizing nitric acid (HNO3), comprising the steps of: Follow these steps: - providing a gas mixture comprising oxygen and nitrogen; - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO x forming a gas mixture comprising: - The above NO Xcooling the gas mixture comprising NO to form a cooled gas mixture comprising NO X a cooling step comprising quenching the gas mixture comprising: - optionally depressurizing the cooled gas mixture comprising NO2; - subjecting the NO2-containing gas mixture to wet scrubbing to form HNO3; and - optionally recycling unreacted exhaust gas from said wet scrubbing to said thermal reactor. A method comprising:

[0008] The combination of temperature and pressure converts NO from oxygen and nitrogen gases x The inventors have demonstrated that by applying pressures of 10-100 bar and temperatures above 2300 K, NO X It was found that this has a significant effect in reducing the energy required for the formation of

[0009] In general, all terms used in the claims should be interpreted according to the ordinary meaning of those terms in the art, unless expressly defined otherwise herein. Any reference to an "element, apparatus, component, means, step, etc." should be interpreted open-endedly as a reference to at least one example of that element, apparatus, component, means, step, etc., unless expressly stated otherwise herein. The steps of methods disclosed herein do not have to be performed in the exact order disclosed, unless expressly stated otherwise. [Brief explanation of the drawings]

[0010] Aspects and embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:

[0011] [Figure 1] FIG. 1 shows a schematic diagram of the synthesis process of nitrogen oxides (NOX).

[0012] [Figure 2] FIG. 2 shows a schematic diagram of the synthesis process of HNO3, including the oxygen sieve / oxygen concentrator, turbine, and scrubber.

[0013] [Figure 3] FIG. 3 shows the results of a simulation of NO formation from N2 and O2 at increasing temperature and pressure.

[0014] [Figure 4] FIG. 4 shows the results of a simulation of NO formation from N2 and O2 at 0.05 bar and 50 bar.

[0015] A detailed description of these figures is provided below. DETAILED DESCRIPTION OF THE INVENTION

[0016] As a first aspect of the present disclosure, there is provided a method for producing nitrogen oxides (NO X ) is provided: - providing a gas mixture comprising oxygen and nitrogen; and - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO X forming a gas mixture comprising:

[0017] In the gas mixture containing oxygen and nitrogen, the oxygen is typically oxygen gas (O2) and the nitrogen is typically nitrogen gas (N2).

[0018] Nitrogen oxides (NO X ) refers to compounds produced by the reaction of nitrogen and oxygen. Usually, nitrogen oxides are nitric oxide (NO) and / or nitrogen dioxide (NO2). As a rule, NO is formed by heating a gas mixture containing oxygen and nitrogen in a thermal reactor at a pressure of 10-100 bar to a temperature of at least 3000 K. X The answer is NO.

[0019] The thermodynamic balance for the formation of NO2 from N2 and O2 is:

number

[0020] NO from nitrogen and oxygen X The energy consumption for the formation of NO decreases significantly when applying pressures above 10 bar, as shown in Figures 3-4 and also illustrated in the Examples.

[0021] The yield in the formation of NO, ie the equilibrium concentration of NO in reaction step (1), is typically up to 11%, for example 4-10%.

[0022] The pressure of the gas mixture containing oxygen and nitrogen is 10 to 100 bar, preferably 15 to 70 bar, more preferably 15 to 60 bar, and even more preferably 15 to 50 bar. In some embodiments, the pressure may be greater than 20 bar, for example, 23 to 100 bar, such as 23 to 70 bar. The increased pressure is typically achieved by compressing the gas mixture in a compressor.

[0023] The temperature of the gas mixture comprising oxygen and nitrogen is at least 2300 K, preferably at least 2500 K, more preferably at least 2800 K, and even more preferably at least 3000 K. Furthermore, the temperature is preferably at most 4500 K, more preferably at most 4000 K. Here, the upper limit can be arbitrarily combined with the lower limit so that the temperature of the gas mixture comprising oxygen and nitrogen is preferably 2300 K to 4500 K, more preferably 2300 K to 4000 K, even more preferably 2500 K to 4500 K, even more preferably 2500 K to 4000 K, even more preferably 2800 K to 4500 K, even more preferably 2800 K to 4000 K, even more preferably 3000 K to 4500 K, and even more preferably 3000 K to 4000 K.

[0024] Thermal reactors are typically constructed from materials that are both temperature-resistant and mechanically resistant, and can withstand both temperature and pressure differential loads. Examples of such materials include MgO, which has high oxidation resistance and can withstand temperatures above 2100 K, and the composite HfB2 / 20%SiC. Further examples can be found in the paper "Super-strong materials for temperatures exceeding 2000 °C" (L. Silvestroni et al., Sci Rep. 2017; 7).

[0025] Alternatively, the reactor walls can be cooled to a temperature strong enough to support the pressure difference. Special flow schemes can also be used so that contact between the hot gases from the plasma and the reactor walls can be avoided or minimized. "Reversed vortex flow" is an example of such a gas flow scheme.

[0026] An example of a reactor setup that can be used is the setup described by Hans-Peter Schmidt and Gunter Speckhofer (see Schmidt, H.P. and G. Speckhofer, "Experimental and theoretical investigation of high-pressure arcs. I. The cylindrical arc column (two-dimensional modeling)," IEEE Transactions on Plasma Science 24.4 (1996): 1229-1238). Here, a gas plasma reactor was constructed from bell-shaped Pyrex® glass, reaching argon temperatures of over 20,000 K at a pressure of 100 bar. Another example of a reactor is available from HiiROC Ltd (UK), which is capable of generating plasma at a pressure of 50 bar.

[0027] The thermal reactor is preferably a plasma reactor, by which is meant a reactor in which the temperature applied to the reactor contributes to reaching conditions in which the gas can be ionized and at least partially form a plasma.

[0028] Thermal reactors are preferably operated by heating a gas mixture containing oxygen and nitrogen with radiofrequency waves or microwaves. The radiofrequency waves or microwaves transfer energy to the gas molecules. In such cases, a radiofrequency wave plasma or microwave plasma can be formed within the thermal reaction zone. When the plasma is formed, the input radiofrequency wave or microwave energy primarily couples with dissociated electrons within the plasma. The radiofrequency waves or microwaves also transfer energy to the molecules. The use of radiofrequency waves or microwaves is advantageous because no electrodes need to be in direct contact with the heated thermal reaction zone, allowing energy to be concentrated in a specific sector of the reactor, or in principle, any sector of the reactor. If electrodes need to be close to the thermal reaction zone, or even in direct contact, they may become too hot and be consumed. An alternative solution is to limit the temperature, but this would reduce the efficiency of the process.

[0029] The gas mixture containing oxygen and nitrogen usually has an oxygen content of 25 to 60% (volume / volume), preferably 25 to 55% (volume / volume). X This reduces the required energy input by providing a more balanced formation of oxygen and nitrogen. Such an oxygen content is typically provided by passing a gas mixture containing oxygen and nitrogen, such as air, through an oxygen sieve or oxygen enrichment device. Molecular sieves for separating / enriching gases are well known to those skilled in the art and are available from commercial suppliers. Alternatively, the gas mixture containing oxygen and nitrogen has an oxygen content of 20-25% (volume / volume), which is advantageous because it avoids the need to enrich the gas mixture with oxygen.

[0030] NO formed X The gas mixture containing NO is typically cooled in a cooling step to form a cooled gas mixture containing NO. X The method includes quenching a gas mixture comprising:

[0031] NO in the cooling gas mixture X Quenching is typically performed until a sufficiently low temperature is achieved so that NO is primarily NO and does not essentially revert to N2 and O2 according to equilibrium (1). Quenching is typically performed immediately downstream of the thermal reaction in a thermal reactor. X The gas mixture comprising is quenched by reducing the temperature to below 2200K, preferably below 2000K, more preferably below 1900K, even more preferably below 1700K.

[0032] In one embodiment, the gas mixture is quenched to a temperature below 750 K, preferably below 373 K, more preferably between 293 and 363 K to promote the formation of NO2 via equilibrium (2).

[0033] In another embodiment, the gas mixture is quenched, followed by a second cooling step in which the gas mixture is cooled to a temperature below 750 K, preferably below 373 K, and more preferably between 293 and 363 K. The second cooling step typically involves cooling at a rate at least 10% lower than the cooling rate of the quench. In such an embodiment, the formation of NO2 by equilibrium (2) is promoted during the second cooling step. A lower cooling rate in the second cooling step is beneficial because it facilitates energy recovery.

[0034] Preferably, quenching is performed by contacting the gas mixture with a quench medium. The quench medium is preferably a gas mixture containing water or oxygen and nitrogen. Typically, the gas mixture contains oxygen, nitrogen, and NO recycled from the cooling step and / or oxygen and nitrogen intentionally bypassed the thermal reactor. If the gas mixture contains oxygen, nitrogen, and NO recycled from the cooling step, the gas is preferably recycled back after quenching and mixed with the gas from the thermal reactor. This is advantageous because it is energy efficient. Alternatively, the gas is recycled back after the complete cooling step. Before being used as a quench medium, i.e., before being mixed with the gas from the reactor, the gas mixture is typically cooled in a heat exchanger capable of recovering energy, for example, by generating steam for use in a steam turbine (e.g., for power generation). The cooled gas mixture is then injected into the quenching step. Such a gaseous quench medium is advantageous because it is more energy efficient than water as a quench medium, since no energy is required to vaporize water. Furthermore, when using a turbine, the risk of water in the turbine is reduced.

[0035] Water as a cooling medium is particularly beneficial when no turbine is used. In this case, NO is used according to equilibrium (1). X Water is added to the gas through a sprinkler in order to maintain the NO equilibrium content in the gas mixture containing

[0036] In the cooling step, the gas mixture is usually depressurized, preferably during a second cooling step following quenching. Alternatively, the pressure is maintained throughout the cooling step. In this case, HNO3 is produced at high pressure and extracted from the process to atmospheric pressure, while the exhaust gas can be recycled to the reactor.

[0037] During the cooling step, energy can be extracted from the gas mixture. At least a portion of the energy extraction is performed using a turbine system, which preferably includes a cooler or condenser. A useful system includes a compressor, which feeds the gas mixture containing oxygen and nitrogen to maintain pressure, and a turbine located downstream after the thermal reactor. It has been estimated that the combined use of the compressor and turbine can recover approximately 40-45% of the energy input to the system, depending on the scale of practical use.

[0038] According to another embodiment, the cooling step is carried out using NO X This is accomplished by directing a gas mixture containing the components of the present invention through a suitably sized heat exchanger. The heat exchanger may be operated to produce steam via direct or secondary circulation. The steam produced may be used for heating purposes or to generate electricity in a steam turbine.

[0039] The gas mixture containing NO2 is wet scrubbed to form HNO3, which is preferably done after a cooling step.

[0040] In one embodiment, a cooling step is performed followed by downstream wet scrubbing without reducing the pressure, for example, with a turbine. This facilitates the recirculation of unreacted exhaust gas to the reactor, since the pressure is not significantly reduced and less energy is required to repressurize the gas to the desired pressure in the thermal reactor. Furthermore, HNO3 can be produced at high pressure, and typically only the HNO3 extracted from the process is reduced to atmospheric pressure. Furthermore, because liquids are generally incompressible, work losses are kept low.

[0041] Typically, unreacted exhaust gas is recycled from the scrubber to the thermal reactor. This can be done both when the gas mixture is maintained at elevated pressure or when it is depressurized. Recycling is beneficial because the recycled gas mixture already has an elevated oxygen content, and only the reacted amounts of N2 and O2 need to be introduced externally. When the gas mixture is maintained at elevated pressure, a further advantage is that the amount of fresh gas supplied is small and the need for a large compressor is reduced because the gas is already pressurized. Similarly, the need for a large turbine can be reduced because there is no need to depressurize the gas. When the gas is depressurized, the gas is recirculated after being depressurized, for example, by a turbine. In such cases, a compressor is applied, but the capacity of the oxygen sieve / oxygen enrichment device may be reduced compared to the case without recirculation. It is estimated that approximately 10% of the capacity is required.

[0042] As a second aspect of the present disclosure, there is provided: A method for synthesizing nitric acid (HNO3), comprising the steps of: Follow these steps: - providing a gas mixture comprising oxygen and nitrogen; - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO x forming a gas mixture comprising: - The above NO X cooling the gas mixture comprising NO to form a cooled gas mixture comprising NO X a cooling step comprising quenching the gas mixture comprising: - optionally depressurizing the cooled gas mixture comprising NO2; - subjecting the NO2-containing gas mixture to wet scrubbing, thereby forming HNO3; and - optionally recycling unreacted exhaust gas from said wet scrubbing to said thermal reactor. A method comprising:

[0043] What has been said above in relation to the first aspect applies mutatis mutandis to the second aspect.

[0044] Aspects of the present disclosure will now be described with reference to the accompanying drawings, in which specific embodiments of the invention are shown.

[0045] However, these aspects may be embodied in many different forms and should not be construed as limiting. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the scope of all aspects of the invention to those skilled in the art. Like numbers refer to like elements throughout the detailed description of the invention.

[0046] FIG. 1 illustrates a method for providing a gas mixture comprising oxygen (O) and nitrogen (N); compressing the gas mixture to a pressure P in a compressor 101; and heating the compressed gas mixture in a thermal reactor 102 to produce NO. X 1 shows a schematic diagram of a process for the synthesis of nitrogen oxides, comprising forming a gas mixture comprising:

[0047] Figure 2 shows a schematic diagram of the synthesis process of nitric acid (HNO3). This process includes the following steps: - providing a gas mixture comprising oxygen (O2) and nitrogen (N2); - increasing the oxygen content in the gas mixture in the oxygen sieve 100; - compressing the gas mixture to a pressure P in a compressor 101; - heating the compressed gas mixture in a thermal reactor 102 to produce NO X forming a gas mixture comprising: - cooling the gas mixture by quenching in a quencher (quencher) 103; - decompressing the gas in a turbine using a generator 104 to form decompressed NO2 while recovering energy 106 in a compressor and / or thermal reactor; and - Wet scrubbing the NO2 in a scrubber 105, thereby forming HNO3. [Example]

[0048] N as temperature and pressure increase 2 and O 2 Simulation of NO formation from Simulations were performed using the MATLAB® HGS chemical equilibrium code, an implementation of the NASA computer program CEA, which minimizes the Gibbs free energy for a specified temperature, pressure, and reactants to reach an equilibrium composition. Details can be found in the following references: S. Gordon, BJ McBride, NASA Reference Publication 1311, 1994; and BJ McBride, S. Gordon, NASA Reference Publication 1311, 1996.

[0049] Simulations were performed using stoichiometric gas mixtures of N2 and O2, i.e., 50% N2 and 50% O2 (vol / vol), and an air-like gas mixture with 80% N2 and 20% O2 (vol / vol). Calculations were performed for each 100 K increase in temperature.

[0050] The results are shown in Figure 3. Lines representing stoichiometric gas mixtures are dashed, while lines representing air-like gas mixtures are solid. Thin lines (pointing upward as pressure increases) indicate the temperature with the lowest energy consumption per kg NO, while thick lines (pointing downward) indicate the optimized energy consumption (kJ / mol) for NO formation (indicated by the normal-thickness line pointing upward) at a given temperature and pressure.

[0051] Nitrogen oxide formation in a plasma is limited by the thermodynamic equilibrium concentration approached in the thermal core of the plasma. The graph in Figure 3 shows how the thermodynamic equilibrium limit improves with pressure. Plasma reactor performance qualitatively follows this trend with pressure.

[0052] In Table 1, the optimized energy consumption data for both gas mixtures are shown at three different pressures: 1 bar, 10 bar, and 50 bar. [Table 1]

[0053] Increasing the pressure from 1 bar to 10 bar reduces energy consumption by about 20%, and increasing the pressure to 50 bar reduces energy consumption by a further 10%. At pressures above 100 bar, the effect is less pronounced and the energy consumption curve levels off.

[0054] Figure 4 shows further results from the simulation: energy cost (energy consumption / energy loss), i.e., energy consumption per unit amount of NO formed from O2 and N2 as a function of temperature at two different pressures (0.05 bar and 50 bar) for two gas mixtures. Lines representing stoichiometric gas mixtures are dashed, while lines representing air-like gas mixtures are solid. Regular-weight lines represent energy consumption at 0.05 bar, while thicker lines represent energy consumption at 50 bar. As shown in Figure 4, NO formation occurs at temperatures lower than those shown in Figure 3, but at higher energy costs per unit amount of NO formed, resulting in lower yields.

[0055] Table 2 shows data on the energy consumption for the formation of NO from both gas mixtures at 2300 K at two different pressures: 0.05 bar and 50 bar. [Table 2]

[0056] Increasing the pressure from 0.05 bar to 50 bar for a stoichiometric gas mixture at both 2300 K and 2500 K reduces energy consumption by approximately 10%. For air-like gas mixtures, energy consumption decreases by approximately 5% at 2300 K and 10% at 2500 K. Thus, energy efficiency decreases significantly at temperatures below the temperature at which energy consumption is optimized.

[0057] Aspects of the present disclosure have been primarily described above with reference to certain embodiments, and examples thereof. However, those skilled in the art will readily appreciate that embodiments other than those disclosed above are equally possible within the scope of the present invention as defined by the appended claims.

[0058] Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and not limitation, with the true scope and spirit being indicated by the following claims. Various aspects or embodiments that can be included in the present invention are summarized as follows. [1] Nitrogen oxides (NO X ), comprising the steps of: Follow these steps: - providing a gas mixture comprising oxygen and nitrogen; and - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO X forming a gas mixture comprising A method comprising: [2] 2. The method according to item 1, wherein the thermal reactor is a plasma reactor. [3] 3. The method according to item 1 or 2 above, wherein the gas mixture containing oxygen and nitrogen has an oxygen content of 25 to 60% (volume / volume). [4] 4. The method according to any one of the above items 1 to 3, wherein the thermal reactor is operated by heating the gas mixture containing oxygen and nitrogen with radio frequency waves or microwaves. [5] Follow these steps: - The above NO X Cooling the gas mixture containing NO 2 forming a cooled gas mixture comprising NO X a cooling step comprising quenching the gas mixture comprising The method according to any one of items 1 to 4 above, further comprising: [6] the quenching is carried out by contacting the gas mixture with a quenching medium which is a gas mixture containing oxygen and nitrogen or water; 6. The method according to item 5, wherein the gas mixture preferably contains oxygen, nitrogen and NO recycled from the cooling step, and / or oxygen and nitrogen intentionally bypassed around the thermal reactor. [7] The above NO 2 7. The method according to claim 5 or 6, further comprising the step of depressurizing the cooled gas mixture comprising: [8] During the cooling step, energy is extracted to generate NO 2 The method according to any one of items 5 to 7, further comprising forming a cooled gas mixture comprising: [9] 9. The method according to item 8, wherein at least a portion of the energy extraction is performed in a turbine system, which preferably includes a cooler or condenser.

[10] 9. The method of claim 8, wherein at least a portion of the energy extraction is performed by directing the gas mixture to a heat exchanger, which preferably produces steam that can be recovered in a steam turbine.

[11] The above NO 2 The gas mixture containing nitric acid (HNO 3 11. The method according to any one of items 1 to 10, further comprising the step of forming a

[12] 12. The method according to any one of the above items 1 to 11, wherein unreacted exhaust gas is recycled from the wet scrubbing to the thermal reactor.

[13] Nitric acid (HNO 3 ), comprising the steps of: Follow these steps: - providing a gas mixture comprising oxygen and nitrogen; - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, and NO x forming a gas mixture comprising: - The above NO X Cooling the gas mixture containing NO 2 forming a cooled gas mixture comprising NO X a cooling step comprising quenching the gas mixture comprising: - Optionally, the above NO 2 depressurizing the cooled gas mixture comprising: - The above NO 2 A gas mixture containing HNO is subjected to wet scrubbing, thereby 3 forming a - optionally recycling unreacted exhaust gas from said wet scrubbing to said thermal reactor. A method comprising:

[14] 14. The method according to any one of the above items 1 to 13, wherein the gas mixture comprising oxygen and nitrogen is heated to a temperature of at least 2300 K at a pressure of 15 to 70 bar.

Claims

1. Nitrogen oxides (NO Ｘ ), comprising the steps of: Steps below: providing a gas mixture comprising oxygen and nitrogen as the plasma source only; and - heating said gas mixture to a temperature of at least 2300 K at a pressure of 10 to 100 bar in a thermal reactor, Ｘ forming a gas mixture comprising: A method comprising:

2. 2. The method of claim 1, wherein the gas mixture comprising oxygen and nitrogen has an oxygen content of 25 to 60% (volume / volume).

3. 3. The method of claim 1 or 2, wherein the thermal reactor is operated by heating the gas mixture containing oxygen and nitrogen with radio frequency waves or microwaves.

4. Steps below: - The above NO Ｘ Cooling a gas mixture containing NO ２ forming a cooled gas mixture comprising NO Ｘ a cooling step comprising quenching the gas mixture comprising The method of any one of claims 1 to 3, further comprising:

5. the quenching is carried out by contacting the gas mixture with a quenching medium which is a gas mixture containing oxygen and nitrogen or water; 5. The method of claim 4, wherein the gas mixture preferably comprises oxygen, nitrogen and NO recycled from the cooling step and / or oxygen and nitrogen intentionally bypassed around the thermal reactor.

6. The above NO ２ 6. The method of claim 4 or 5, further comprising the step of depressurizing the cooled gas mixture comprising:

7. During the cooling step, energy is extracted to ２ 7. The method of any one of claims 4 to 6, further comprising forming a cooled gas mixture comprising:

8. 8. The method of claim 7, wherein at least a portion of the energy extraction is performed in a turbine system, which preferably includes a cooler or condenser.

9. 8. The method of claim 7, wherein at least a portion of said energy extraction is performed by directing said gas mixture through a heat exchanger, which preferably produces steam that can be recovered in a steam turbine.

10. The above NO ２ The gas mixture containing nitric acid (HNO ３ The method of any one of claims 1 to 9, further comprising forming a

11. The method of claim 10 wherein unreacted exhaust gas is recycled from the wet scrubbing to the thermal reactor.

12. Nitric acid (HNO ３ ), comprising the steps of: Steps below: providing a gas mixture comprising oxygen and nitrogen as the only plasma source; - heating said gas mixture in a thermal reactor to a temperature of at least 2300 K at a pressure of 10 to 100 bar, ｘ forming a gas mixture comprising: - The above NO Ｘ Cooling a gas mixture containing NO ２ forming a cooled gas mixture comprising NO Ｘ a cooling step comprising quenching the gas mixture comprising: - optionally, the above NO ２ depressurizing the cooled gas mixture comprising: - The above NO ２ The gas mixture containing HNO is subjected to wet scrubbing, ３ forming - optionally recycling unreacted exhaust gas from said wet scrubbing to said thermal reactor; A method comprising:

13. A method according to any one of the preceding claims, wherein the gas mixture comprising oxygen and nitrogen is heated to a temperature of at least 2300K at a pressure of 15 to 70 bar.

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