Method for synthesis of nitrogen oxides and nitric acid

Abstract

The invention relates to a method for the synthesis of nitrogen oxides (NO X ) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 2500 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 10 6 K / s and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1.0 x 10 6 K / s, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position at which the quenching is started is higher than the amount of nitrogen oxides with respect to the amount of nitrogen at a thermal equilibrium between oxygen and nitrogen and nitrogen oxides.

Description

[0001] METHOD FOR SYNTHESIS OF NITROGEN OXIDES AND NITRIC ACID

[0002] The present invention relates to a method for the synthesis of nitrogen oxides (NOx) and nitric acid (HNO3).

[0003] Currently, nitrogen process chemistry starts with Haber-Bosch-produced ammonia. The ammonia is partially combusted in case NOx (x=1 ,2) is the targeted intermediate, which implies a detour from an energy point of view.

[0004] Direct NOx production from N2and O2would be preferable, but is thermodynamically limited. ChemSusChem 2017, 10, 2145 - 2157 describes the NOx concentration under equilibrium conditions. This document shows that the maximum NOx concentration of a mixture of N2:02=50:50 molar ratio under equilibrium conditions is around 7mol% at a temperature of around 3500 to 4000K and 1 barA.

[0005] W02020115473 discloses a method of manufacturing nitric oxide and / or nitrogen dioxide comprising forming a plasma from nitrogen and oxygen as the reacting gases which are passed through one or more gas inlets into a first reaction chamber to create a vorticular flow in the reaction chamber, energising said nitrogen and oxygen in a microwave transparent first inner plasma containment cylinder by a microwave source to create a plasma.

[0006] WO2022159018A1 discloses a method for the synthesis of nitrogen oxides (NOx) by providing a gas mixture comprising oxygen and nitrogen; and heating the gas mixture to a temperature of at least 2300 K at a pressure of 10-100 bar in a thermal reactor forming a gas mixture comprising NOx. This gas mixture can be converted to HNO3.

[0007] WO2023137047 discloses a microwave-plasma system for generating fixed-nitrogen products comprising: a microwave generator operably coupled with a gas chamber, the microwave generator providing microwave power to the gas chamber; a source of gas operably coupled with the plasma chamber, the microwave power producing a plasma of the gas; and an absorber unit fluidically connected to the gas chamber to capture product from the plasma in the gas chamber. It is an object of the invention to provide a method for the synthesis of NOx from nitrogen and oxygen with a high yield.

[0008] Accordingly, the present invention provides a method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 105K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1 .0 x 106K / s.

[0009] The method according to the invention surprisingly results in a high yield of NOx.

[0010] In the method according to the invention, a feed gas mixture comprising oxygen and nitrogen is fed to the inlet of a container (which can have the shape of a tube elongated along a central longitudinal axis between an inlet and an outlet) of a plasma reactor and passed through the container towards the outlet of the container. A heated gas mixture comprising a high temperature portion is formed inside the container. The high temperature portion is formed in a first longitudinal section of the container, which first longitudinal section axially extends from a first longitudinal position of the container to a second longitudinal position of the container downstream of the first longitudinal position. The quenching is performed in a second longitudinal section of the container directly downstream of the first longitudinal section.

[0011] The high temperature portion in the container has a temperature gradient in which the temperature increases along the central axis from the first longitudinal position to the second longitudinal position of the container. The temperature of the heated gas mixture is highest at the central axis of the container. The cross section of the high temperature portion becomes larger along the central axis from the first longitudinal position to the second longitudinal position of the container. The first longitudinal position of the container is the longitudinal position at which the temperature at the central axis reaches the temperature required for the high temperature position. According to the invention, it was surprisingly found that a quick heating of the gas to a high temperature results in the concentration of NOx becoming higher than the thermodynamic equilibrium concentration of NOx. Sudden increase in the temperature of a gas mixture comprising molecular oxygen and molecular nitrogen leads to reactions that form nitric oxide (NO) without excessive production of atoms nitrogen and oxygen. The opportunity lies in the fact that NOx dissociation is initially slower than NOx formation. Whilst the oxygen atom concentration has not yet reached its equilibrium value, NOx concentration can overshoot its equilibrium value. It was thus realized that a quick heating to a high temperature results in a longitudinal section in the container in which the concentration of NOx exceeds the thermodynamic equilibrium concentration of NOx.

[0012] If this heated gas mixture having a NOx concentration higher than equilibrium is maintained at high temperature, the NOx concentration will decrease and reach the concentration at thermodynamic equilibrium. It was therefore realized according to the invention that quenching at the longitudinal position at which the NOx concentration is higher than equilibrium allows maintaining the high NOx concentration, resulting in the final gas mixture with a high NOx concentration obtained from the plasma reactor. Accordingly, the quenching is performed at a second section of the container starting from the second longitudinal position of the container, i.e. the longitudinal position at which the quenching is performed marks the end of the high temperature portion.

[0013] The desired heating rate and the desired temperature profile of the high temperature portion formed can be achieved by suitably selecting the feed rate of the feed gas mixture and the power density of the plasma source in relation to the dimension of the container.

[0014] The desired cooling rate of the high temperature portion can be achieved by suitably selecting the quenching method and relevant parameters such as the temperature and the feed rate of the cooling medium depending on the temperature profile of the high temperature portion.

[0015] It was further found that a high nitrogen oxide concentration is achieved by the method according to the invention for a relatively low energy consumption. Accordingly, the present invention further provides a method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 105K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1 .0 x 10® K / s, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position of the container at which the quenching is started is higher than the amount of nitrogen oxides with respect to the amount of nitrogen at thermal equilibrium between oxygen and nitrogen and nitrogen oxides.

[0016] Thus, the feed rate of the feed gas mixture and the power density of the plasma source in relation to the dimension of the container are selected such that the NOx concentration in the high temperature portion becomes higher than the NOx concentration at the thermal equilibrium between oxygen and nitrogen and nitrogen oxides at the temperature and pressure concerned. After the longitudinal position at which the temperature at the central axis of the container reaches 3500 K, the NOx concentration in the cross section of the high temperature portion is higher than the thermal equilibrium. The longitudinal position at which the quenching is started is selected so that this high NOx concentration (higher than equilibrium) achieved in the high temperature portion is maintained.

[0017] The present invention further provides a method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 10® K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1 .0 x 10® K / s, wherein the high temperature portion is formed in a first longitudinal section of the container axially extending from a first longitudinal position of the container to a second longitudinal position of the container downstream of the first longitudinal position, the high temperature portion in the container has a temperature gradient in which the temperature increases along the central axis from the first longitudinal position to the second longitudinal position of the container, the quenching is performed at a second section of the container starting from the second longitudinal position of the container, the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the second longitudinal position of the container is higher than the amount of nitrogen oxides with respect to the amount of nitrogen at thermal equilibrium between oxygen and nitrogen and nitrogen oxides.

[0018] The present invention further provides the present invention further provides a method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 10sK / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1 .0 x 106K / s, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the central axis of the container at the longitudinal position of the container at which the quenching is started is at least 9.0 mol%, preferably at least 10.0 mol%, more preferably at least 11.0 mol%, more preferably at least 12.0 mol%. In some preferred embodiments, the molar ratio between oxygen and nitrogen in the feed gas mixture is 40:60 to 60:40 and the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the central axis of the container at the longitudinal position of the container at which the quenching is started is at least 13.0 mol%, more preferably at least 14.0 mol%, more preferably at least 15.0 mol%, more preferably at least 16.0 mol%, more preferably at least 17.0 mol%, more preferably at least 18.0 mol%, more preferably at least 19.0 mol%, more preferably 20.0 mol%. The present invention further provides a method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 105K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1 .0 x 10® K / s, wherein the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.3 ms*barA, preferably wherein the pressure in the container is 100 mbar to 10 bar, 100 mbar to 9.0 bar or 100 mbar to 5.0 bar or the pressure in the container is less than atmospheric pressure, e.g. 100 mbar to 1000 mbar or 100 mbar to 900 mbar.

[0019] The residence time in the high temperature portion is the duration for the feed gas mixture to reach from the first longitudinal position of the container to the second longitudinal position of the container. The residence time can e.g. be determined by including colored particles in the feed gas mixture and determining the duration for the colored particles to reach from the first longitudinal position of the container to the second longitudinal position of the container. In order to maintain the high NOx concentration achieved by the quick heating, the residence time cannot be too long. Such maximum residence time is influenced by the pressure in the container.

[0020] In some embodiments, the high temperature portion has temperatures of at least 4000 K and the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.1 ms*barA.

[0021] In some embodiments, the high temperature portion has temperatures of at least 4500 K and the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.06 ms*barA. In some embodiments, the high temperature portion has temperatures of at least 5000 K and the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.01 ms*barA.

[0022] Preferably, the high temperature portion has temperatures of at least 3750K, more preferably at least 4000 K, more preferably at least 4250 K, more preferably at least 4500 K. Preferably, the high temperature portion has temperatures of at most 8000 K, for example at most 7000 K or at most 6000 K. Preferably, the high temperature portion has temperatures of 4000 K to 6000 K, more preferably 4250 K to 5750 K, more preferably 4500 to 5500 K. Such temperature range was found to allow reaching particularly high NOx concentrations while allowing relatively easy control of the quenching step. The high temperature portion may have temperatures of at least 4600 K, at least 5000 K, at least 5500 K or at least 6000 K.

[0023] At the longitudinal position at which the quenching is to be performed (second longitudinal position), the amount of nitrogen oxides with respect to the amount of nitrogen is higher than that at the thermodynamic equilibrium. Preferably, the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position at which the quenching is to be performed (second longitudinal position) is at least 9.0 mol%, preferably at least 10.0 mol%, more preferably at least 11.0 mol%, more preferably at least 12.0 mol%. In some preferred embodiments, the molar ratio between oxygen and nitrogen in the feed gas mixture is 40:60 to 60:40 and the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position at which the quenching is to be performed (second longitudinal position) is at least 13.0 mol%, more preferably at least 14.0 mol%, more preferably at least 15.0 mol%, more preferably at least 16.0 mol%, more preferably at least 17.0 mol%, more preferably at least 18.0 mol%, more preferably at least 19.0 mol%, more preferably 20.0 mol%.

[0024] The temperature of the heated gas mixture is highest at the central axis of the container (tube). Thus, seen in a cross section, the high temperature portion may be surrounded by a portion having lower temperatures. Preferably, the amount of the high temperature portion with respect to the amount of the heated gas mixture from the first longitudinal position to the second longitudinal position of the container is at least 10 vol%, at least 20 vol%, at least 30 vol%, at least 40 vol%, at least 50 vol%, at least 60 vol%, at least 70 vol%, at least 80 vol%, at least 90 vol%, at least 95 vol% or at least 99 vol%. The heated gas mixture is quenched to obtain a non-reactive gas mixture still having a high NOx concentration. The cooling rate of the high temperature portion down to 2000 K is at least 1.0 x 106K / s. After having cooled to 2000 K, the cooling rate may be lower. For example, the cooling rate from 2000 K to 1000 K may be 1.0 x 105to 106K / s. The quenched gas may finally be cooled to even lower temperatures such as to 373 K or lower, e.g. room temperature. Such further cooling from 1000 K e.g. to room temperature may be even slower.

[0025] Preferably, the molar ratio between oxygen and nitrogen in the feed gas mixture is 15:85 to 60:40, for example 15:85 to 40:60 or 40:60 to 60:40.

[0026] In some preferred embodiments, the molar ratio between oxygen and nitrogen in the feed gas mixture is 15:85 to 25:75. It was surprisingly observed that the increase in the concentration of nitrogen oxides achieved by the invention is particularly large in these embodiments. This range of molar ratio is also advantageous in that air can be used as the gas mixture to be fed without having to substantially change its composition. In particularly preferred embodiments, the feed gas mixture is air.

[0027] In some preferred embodiments, the molar ratio between oxygen and nitrogen in the feed gas mixture is 40:60 to 60:40. This results in a particularly high concentration of nitrogen oxides in the gas mixture obtained by the method of the invention.

[0028] The total amount of oxygen and nitrogen in the feed gas mixture may e.g. be at least 95 mol% or 98 mol%.

[0029] The pressure in the container is not particularly limited and may e.g. be 100 mbar to 10 bar, 100 mbar to 9.0 bar or 100 mbar to 5.0 bar. The pressure in the container may be less than atmospheric pressure, e.g. 100 mbar to 1000 mbar or 100 mbar to 900 mbar.

[0030] In some embodiments, the high temperature portion has temperatures of at least 3500 K, the heating rate of the feed gas mixture from 2000 K to a higher temperature is at least 3.0 x 106K / s and the cooling rate of the high temperature portion to 2000 K is at least 7.0 x 106K / s. In some embodiments, the high temperature portion has temperatures of at least 4000 K, the heating rate of the feed gas mixture from 2000 K to a higher temperature is at least 15 x 10sK / s and the cooling rate of the high temperature portion to 2000 K is at least 20 x 106K / s.

[0031] In some embodiments, the high temperature portion has temperatures of at least 5000 K, the heating rate of the feed gas mixture from 2000 K to a higher temperature is at least 150 x 106K / s and the cooling rate of the high temperature portion to 2000 K is at least 150 x 106K / s.

[0032] In some embodiments, the high temperature portion has temperatures of at least 6000 K, the heating rate of the feed gas mixture from 2000 K to a higher temperature is at least 800 x 106K / s and the cooling rate of the high temperature portion to 2000 K is at least 800 x 106K / s.

[0033] The quenching can be performed by feeding cold fluid to the plasma reactor. For example, the quenching can be performed by feeding water or a gas comprising nitrogen and / or oxygen in the second longitudinal section of the container starting from the second longitudinal position of the container. The method of quenching of gas in a plasma reactor is per se known, e.g. as described in WO2022159018, but the quenching is performed according to the present invention such that the cooling rate is very high.

[0034] A gas mixture comprising nitrogen oxides is obtained from the plasma reactor according to the method according to the invention. Preferably, the amount of nitrogen oxides with respect to the amount of nitrogen in the gas mixture obtained from the plasma reactor is at least 9.0 mol%, preferably at least 10.0 mol%, more preferably at least 11.0 mol%, more preferably at least 12.0 mol%.

[0035] In some preferred embodiments, the molar ratio between oxygen and nitrogen in the feed gas mixture is 40:60 to 60:40 and the amount of nitrogen oxides with respect to the amount of nitrogen in the gas mixture obtained from the plasma reactor is at least 13.0 mol%, more preferably at least 14.0 mol%, more preferably at least 15.0 mol%, more preferably at least 16.0 mol%, more preferably at least 17.0 mol%, more preferably at least 18.0 mol%, more preferably at least 19.0 mol%, more preferably 20.0 mol%. The container (tube) in the plasma reactor used in the method of the invention may be made of any suitable material known for a plasma reactor, for example quartz, ceramic, metal or alloy.

[0036] The plasma reactor used in the method of the invention is preferably selected from the group consisting of a microwave plasma reactor, a plasma spark discharge reactor, a (gliding) arc plasma reactor, a radio frequency plasma reactor and a nanosecond pulse plasma reactor.

[0037] Preferably, the plasma reactor is a microwave plasma reactor. A microwave plasma reactor comprises a container (tube), a microwave radiation source and a waveguide for guiding the microwave to the container. The gas mixture is passed through the container while microwave is irradiated on the gas mixture to generate an active plasma zone inside the container. The waveguide surrounds the container over a section of the container extending over the longitudinal axis. Microwave plasma reactors are per se known and is explained in detail e.g. in W02020115473 and WO2023137047, incorporated herein by reference. The microwave plasma reactor used according to the invention may include a microwave generator, a circulator, a 3-stub tuner, a waveguide and a sliding tuner, all of which are well- known and are described in Fig. 2 of WO2023137047.

[0038] In some preferred embodiments, the second section of the container in which the quenching is performed is positioned immediately after the section of the container surrounded by the waveguide.

[0039] Preferably, the microwave radiation is in one of the frequency bands 2.45 GHz or 896 MHz or 900 MHz or 915 MHz power.

[0040] The peak power of the microwave radiation is not particularly limited. Preferably, the microwave radiation is applied at a peak power of 1 kWto 1 MW.

[0041] The invention further provides a method for producing nitric acid, comprising the method according to the invention and further comprising converting nitrogen oxides into nitric acid. Method for conversion of nitrogen oxides into nitric acid is well-known in the art, e.g. reacting with water. It is noted that the invention relates to the subject-matter defined in the independent claims alone or in combination with any possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

[0042] It is further noted that the term ‘comprising’ does not exclude the presence of other elements. However, it is also to be understood that a description on a product / com position comprising certain components also discloses a product / composition consisting of these components. The product / composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product / composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.

[0043] When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

[0044] The invention is now elucidated by way of the following examples, without however being limited thereto.

[0045] Figure 1 shows a graph of results of simulation regarding molar fraction of NOx over time; Figure 2 shows a schematic of a system employed in experiment;

[0046] Figure 3 shows 2D maps of gas temperature (top) and [NO] / [N2] (bottom) at different positions in plasma reactor obtained from experiment;

[0047] Figure 4 shows a graph of results obtained from experiment regarding molar fraction of NOx. Figure 5 shows a graph of simulation results of gas temperature profile over time.

[0048] Figure 6 shows a graph of simulation results of the ratio of NO to N2 concentrations (black full line) and energy cost per mole of NO (grey) as a function of residence time (tres). Simulation

[0049] Simulation was performed to calculate the molar fraction of NOXin the gas mixture over time at temperatures of 3500 K, 4000 K, 5000 K and 6000 K for a gas mixture of N2:O2 at a molar ratio 80:20. The pressure of the gas mixture was 1000 mbar. Results of the simulation are shown in Figure 1.

[0050] Figure 1 shows that a higher NOx concentration is achieved than at thermodynamic equilibrium condition at 3500 K to 6000 K. A higher temperature leads to a higher maximum NOx concentration and a greater difference between the maximum concentration and the concentration at thermodynamic equilibrium condition. A higher temperature requires a faster heating and cooling to achieve the maximum NOx concentration.

[0051] Experiments

[0052] In situ laser Raman scattering measurements in a microwave plasma discharge were used to prove the viability of the invention.

[0053] Figure 2 depicts a schematic of the employed setup. Synthetic air (20% O2, 80% N2) at a flowrate of 10 standard litres per minute and a pressure of 180 mBar was tangentially fed to a microwave plasma reactor, consisting of a 27 mm quartz tube placed in a microwave cavity. Microwave power was supplied at 2.45 GHz by a 3 kW peak power solid-state oscillator-amplifier system (manufactured by PinkRF) and were tuned for maximum power absorption by impedance matching and a sliding short. The input microwave power for the performed measurement was 150 W. Additional aspects of the flow system have been described by Den Harder et al. (Plasma Process Polym. 2017;14:e1600120), for example Figure 15 and the related description.

[0054] The 532 nm output of a nanosecond pulsed Nd:YAG laser (Amplitude Powerlite DLS9030) was passed through the reactor and focused at different radial positions. The Raman scattered light was imaged (without magnification) onto a linear fiber array by two 100 mm lenses. Stray and Rayleigh scattered light was rejected by a sharp-edge longpass filter (Semrock Razoredge LP03-532RE-25), which transmits all scattered light with A > 534 nm. The fiber array relays the scattered light into a commercial Czerny-Turner spectrometer (Andor Kymera 328i), equipped with a fast gated intensified CCD camera (Andor iStar 334T). The linear fiber array comprises 19 fibers that are imaged on the camera, translating into images covering a spatial range of about 4 mm. Measurements have been taken at different axial and radial positions to cover the entire region of interest.

[0055] Figure 3 shows 2D maps of gas temperature (top) and [NO] / [N2] (bottom) at different positions in plasma reactor, made from the results of these measurements. It is noted that the 0 mm axial position in the map corresponds to 13.5 mm from inside edges of the waveguide. The air flow direction is from left to right.

[0056] From these maps, it is possible to see that the portion having a high NOx concentration (amount of nitrogen oxides with respect to amount of nitrogen in the gas mixture) increases from axial position 0 mm to around 6 mm. At axial position of around 6 mm, a large portion of the cross section has a NOx concentration of 12 mol%, which is much higher than the maximum equilibrium NOx concentration (~5 mol%). The NOx concentration at the axis starts to decrease after this position. The radially central portion in which the NOx concentration decreases with the axial position becomes larger. Accordingly, it can be understood that quenching the heated gas mixture at this axial position of around 6 mm would allow having the highest NOx concentration in the final gas mixture.

[0057] Analyzing the spatially resolved measurements in terms of NOx molar fraction in dependence of gas temperature allows a comparison with the thermal equilibrium concentration. This is shown in Figure 4 which shows the NOx molar fraction over temperature. It demonstrates that a factor of two and half enhancement in NOx concentration was achieved.

[0058] Figure 5 shows a graph of simulation results of gas temperature profile over time. In this graph, after being quickly heated to Tg,max, the gas stays at constant temperature for a limited time (residence time tres), and then quickly quenched. When a gas mixture of 02 and N2 experiences such temperature profile, a high NOx concentration is achieved.

[0059] Figure 6 shows simulation results of the ratio of NO to N2 concentrations (black full line) and energy cost per mole of NO (grey) as a function of residence time (tres). The simulation results are obtained at 1barA of pressure, 4500 K of temperature, and with a feed gas composition of 80% N2 and 20% O2. It can be understood that for 4500 K, the [NO] / [N2] ratio is the highest (around 0.14) when residence time is about 0.05 second. The same simulations were performed at different pressures and temperatures for different compositions as shown in Table 1. Table 1 shows the [NO] and [NO] / [N2] achieved at respective pressure, temperature and residence time. It can be understood that a high nitrogen oxide concentration is achieved for relatively low energy consumption by selecting a high temperature and low residence time without having to apply a high pressure.

[0060] Table 1

Claims

CLAIMS1. A method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 106K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1.0 x 106K / s, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position of the container at which the quenching is started is higher than the amount of nitrogen oxides with respect to the amount of nitrogen at thermal equilibrium between oxygen and nitrogen and nitrogen oxides.

2. The method according to claim 1 , wherein the high temperature portion is formed in a first longitudinal section of the container axially extending from a first longitudinal position of the container to a second longitudinal position of the container downstream of the first longitudinal position, the high temperature portion in the container has a temperature gradient in which the temperature increases along the central axis from the first longitudinal position to the second longitudinal position of the container, the quenching is performed at a second section of the container starting from the second longitudinal position of the container, the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the second longitudinal position of the container is higher than the amount of nitrogen oxides with respect to the amount of nitrogen at thermal equilibrium between oxygen and nitrogen and nitrogen oxides, preferably wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the central axis of the container at the second longitudinal position is at least 9.0 mol%, preferably at least 10.0 mol%, more preferably at least 11.0 mol%, more preferably at least 12.0 mol%.

3. The method according to any one of the preceding claims, wherein the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.3 ms*barA.

4. The method according to any one of the preceding claims, wherein the high temperature portion has temperatures of at least 4600 K, at least 5000 K, at least 5500 K or at least 6000 K.

5. The method according to any one of the preceding claims, wherein the high temperature portion has temperatures of at least 4000 K, the heating rate of the feed gas mixture from 2000 K to a higher temperature is at least 15 x 106K / s and the cooling rate of the high temperature portion to 2000 K is at least 20 x 106K / s.

6. The method according to any one of the preceding claims, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the high temperature portion at the longitudinal position at which the quenching is performed is at least 9.0 mol%, preferably at least 10.0 mol%, more preferably at least 11.0 mol%, more preferably at least 12.0 mol%.

7. The method according to any one of the preceding claims, wherein the amount of nitrogen oxides with respect to the amount of nitrogen in the gas mixture from the plasma reactor is at least 9.0 wt%, preferably at least 10.0 wt%, more preferably at least 11.0 wt%, more preferably at least 12.0 wt%.

8. The method according to any one of the preceding claims, wherein the molar ratio between oxygen and nitrogen in the gas mixture fed to the container is 15:85 to 60:40.

9. The method according to claim 8, wherein the molar ratio between oxygen and nitrogen in the feed gas mixture is 40:60 to 60:40.

10. The method according to claim 8, wherein the molar ratio between oxygen and nitrogen in the feed gas mixture is 15:85 to 25:75.

11. The method according to claim 8, wherein the feed gas mixture is air.

12. The method according to any one of the preceding claims, wherein the high temperature portion is formed in a first longitudinal section of the container axially extending from a first longitudinal position of the container to a second longitudinal position of the container downstream of the first longitudinal position, wherein the amount of the high temperature portion with respect to the amount of the heated gas mixture in the first longitudinal section is at least 10 vol%, at least 20 vol%, at least 30 vol%, at least 40 vol%, at least 50 vol%, at least 60 vol%, at least 70 vol%, at least 80 vol%, at least 90 vol%, at least 95 vol% or at least 99 vol%.

13. The method according to any one of the preceding claims, wherein the plasma reactor is selected from the group consisting of a microwave plasma reactor, a plasma spark discharge reactor, a (gliding) arc plasma reactor, a radio frequency plasma reactor and a nanosecond pulse plasma reactor.

14. The method according to any one of the preceding claims, wherein the plasma reactor is a microwave plasma reactor.

15. The method according to claim 14, wherein microwave radiation is in one of the frequency bands 2.45 GHz or 896 MHz or 900 MHz or 915 MHz power.

16. The method according to any one of the preceding claims, wherein the pressure in the container is less than atmospheric pressure, e.g. 100 mbar to 1000 mbar or 100 mbar to 900 mbar.

17. A method for the synthesis of nitrogen oxides (NOx) in a plasma reactor comprising a container such as a tube, comprising, a) passing a feed gas mixture comprising oxygen and nitrogen through the container to obtain a heated gas mixture comprising a high temperature portion inside the container, wherein the high temperature portion has temperatures of at least 3500 K, wherein the high temperature portion is obtained by heating the feed gas mixture such that the heating rate from 2000 K is at least 1.0 x 106K / s, and b) quenching the heated gas mixture such that the cooling rate of the high temperature portion down to 2000 K is at least 1.0 x 106K / s,wherein the product of residence time of the feed gas mixture in the high temperature portion and the pressure in the container is at most 0.3 ms*barA.

18. A method for producing nitric acid, comprising the method according to any one of the preceding claims and further comprising converting nitrogen oxides into nitric acid.