Production of nitrogen oxides

Abstract

There is provided a method for nitrogen oxide (NOx) synthesis, the method comprising: - providing a gas mixture comprising oxygen and nitrogen in a plasma reactor chamber; and - exciting the gas mixture into plasma in the plasma reactor chamber, wherein a duration of the excitation is in the range ≥0.15 / P nanoseconds & ≤90 / P nanoseconds, wherein P is pressure of the gas mixture in atm, and wherein the gas mixture is provided at a pressure P of ≥ 1.1 atm.

Description

PRODUCTION OF NITROGEN OXIDESTECHNICAL FIELD

[0001] The present disclosure relates to the field of production of nitrogen oxides(NOx), and in particular to production of nitrogen oxides by plasma technology.BACKGROUND

[0002] Nitrogen oxides (NOX) are regularly used in production of nitric acid, which is used for the production of ammonium nitrate, a major component of fertilizers, for the production of explosives, as solvents and for other chemical processes, as well as for bleaching and sterilization. Nitrogen is a key element for plants and therefore one of the most important nutrients in fertilizers. In the last century, nitrogen-containing fertilizers have been produced from atmospheric nitrogen essentially through the Haber-Bosch process to synthesize ammonia from hydrogen originating from fossil-based steam reforming and molecular nitrogen followed by the Ostwald process to produce nitric acid through oxidation of the ammonia. The nitric acid is then used for example as a source for producing nitrate fertilizers.

[0003] Alternatively, nitric acid can be synthesized through nitrogen fixation through the Birkeland-Eyde process, invented in 1903. By the aid of an electric arc, a thermal oxidation of atmospheric nitrogen into NO is conducted, and the NO spontaneously converts to N02as the gas is cooled in the presence of oxygen. N02is subsequently scrubbed with water and, thereby, converted into nitric acid. However, the thermal oxidation of nitrogen gas is highly energy demanding and therefore the Haber-Bosch and Ostwald processes became the leading processes used commercially to fix nitrogen from the air and to produce nitric acid.

[0004] Due to the emissions of greenhouse gases (C02, NO, N2O) from the steam reforming and Haber-Bosch process combined with the Ostwald process, the direct oxidation of nitrogen has returned as an interesting option. However, due to the high energy demands of the Birkeland-Eyde process, there are improvements to be made especially regarding energy efficiency.SUMMARY

[0005] There is an objective of the present disclosure to provide a method for nitrogen oxide (NOx) synthesis with an improvement in yield compared with previous methods, as well as being industrially feasible and more energy efficient.Accordingly, the present disclosure provides as a first aspect a method for nitrogen oxide (NOx) synthesis, the method comprising: providing a gas mixture comprising oxygen and nitrogen in a plasma reactor chamber; and exciting the gas mixture into plasma in the plasma reactor chamber, wherein a duration of the excitation is in the range SO.15 / P nanoseconds & <9o / P nanoseconds, wherein P is pressure of the gas mixture in atm, and wherein the gas mixture is provided at a pressure P of > 1.1 atm.

[0006] By combination of excitations with the specified durations and provision of the gas mixture at pressure above ambient pressure, the inventors have realized that the yield of NOx molecules is efficiently increased as well as that the energy consumption is beneficially decreased compared with previous plasma processes.

[0007] It is believed that at the specific duration of the excitation, the plasma is efficiently excited but also de-excited, which is advantageous as the number of backward reactions can be kept to a minimum or even avoided. Another advantage is that the temperature that the plasma reactor is exposed to is kept low which allows for less wear as well as less demands on the material for constructing the plasma reactor. Yet another advantage is that wasting of heat after the reaction has occurred can be reduced or even avoided. Longer excitation times will also be significantly more energy consuming.

[0008] As a second aspect of the present disclosure, there is provided a system comprising:- a plasma reactor having an inlet and an outlet configured to conduct the method according to the first aspect of the present disclosure for nitrogen oxide (NOx) synthesis;- a compressor configured for increasing pressure of a gas mixture comprising oxygen and nitrogen to a pressure P of > 1.1 atm;- a conversion arrangement for converting the NOXto nitric acid (HNO3), preferably an absorption column, wherein the conversion arrangement is having an inlet and an outlet; wherein the compressor is fluidically connected to the inlet of the plasma reactor and the inlet of the conversion arrangement for converting the NOXto HNO3is fluidically connected to the outlet of the plasma reactor.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 shows NOx production by a single discharge as a function of pressure.

[0010] Figure 2 shows how much energy is needed per mole of nitrogen oxide produced as a function of pressure.

[0011] Figure 3 schematically shows a system of at least some example embodiments.

[0012] Figure 4 schematically shows a system of an example embodiment.

[0013] Figure 5 schematically shows a system of another example embodiment.

[0014] Figure 6 schematically shows a system of yet another example embodiment.

[0015] Figure 7 schematically shows a system of yet another example embodiment.

[0016] Figure 8 schematically shows a system of yet another example embodiment.

[0017] Figure 9 schematically shows a system of yet another example embodiment.DETAILED DESCRIPTION

[0018] As a first aspect, the present disclosure provides a method for nitrogen oxide (NOX) synthesis, the method comprising:providing a gas mixture comprising oxygen and nitrogen in a plasma reactor chamber; and exciting the gas mixture into plasma in the plasma reactor chamber, wherein a duration of the excitation is in the range SO.15 / P nanoseconds & <9o / P nanoseconds, wherein P is pressure of the gas mixture in atm, and wherein the gas mixture is provided at a pressure P of > 1.1 atm.

[0019] In the gas mixture comprising oxygen and nitrogen, the oxygen is preferably oxygen gas (02) and the nitrogen is preferably nitrogen gas (N2). The gas mixture comprising oxygen and nitrogen may be air.

[0020] By nitrogen oxides (NOX) is meant compounds produced by reacting nitrogen and oxygen. Preferably, the nitrogen oxides are nitric oxide (NO) and / or nitrogen dioxide (N02).

[0021] The thermodynamic balance in formation of N02from N2and 02is:(1) N2+ 022NO(2) 2NO + 02N02

[0022] Gas turns into plasma when sufficient amounts of heat and / or energy is added to it to excite the molecules and atoms to the right level, by temperature or by electromagnetic waves exciting electrons or provoking collisions. As a first step, molecules dissociate which forms free radicals, as a second step, atoms that make up the gas start to lose their electrons and become positively charged ions. The lost electrons are then able to float freely, i.e. they are ionized. Plasma is one of the four common states of matter: solid, liquid, gas, and plasma. Plasma is an electrically charged gas. Because plasma particles have an electrical charge, they are affected by electrical and magnetic fields. The gas mixture is converted into a plasma when sufficiently excited.

[0023] The plasma in the plasma reactor is produced from the excitation of the gas mixture comprising oxygen and nitrogen.

[0024] The plasma chamber is not limited to be of a certain material. The plasma reactor is typically constructed in a material where the demands for resistance against both load from a pressure difference and temperature is not highly demanding, such as steel, polytetrafluoroethylene (PTFE, such as Teflon), plastics orglass. This is since the excitation time is short enough to excite and de-excite the plasma so even though the gas of the plasma is heated to high temperatures, the reactor wall is much cooler. Thereby, the plasma reactor can be made from conventional materials which is also beneficial from a cost perspective. Alternatively, the reactor can be constructed in a material capable of withstanding higher temperatures. Examples of such materials are ceramics such as alumina (AI2O3), zirconium oxide, SiC, MgO or the composite HfB2 / 20% SiC that has a high oxidation resistance and can resist temperatures above 2100 K. In yet another alternative, the reactor walls can be cooled internally by for example an external cooling system or allowing cold gas to enter along the walls inside the plasma reactor. The gas can also be pre-cooled before entering the plasma reactor.

[0025] Examples of reactor set-ups that can be used to provide high pressures include the setup described by Hans-Peter Schmidt and Gunter Speckhofer (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.), where a gas plasma reactor reaching argon temperatures above 20 000 K at a pressure of 100 bar was created in a bell-shaped Pyrex® glass. Another example of a reactor is available from HiiROC Ltd, UK, capable of producing a plasma at a pressure of 50 bar.

[0026] By plasma reactor is meant a reactor where the temperature and / or an electric (or electromagnetic) field applied in the reactor contributes to reaching conditions where the gas can be ionized and, at least partially, form a plasma.

[0027] The excitations are conducted with a certain pulse length, i.e. the time the excitation is actively conducted. It has been realized that the optimum duration of each pulse length is pressure dependent. If the pressure is lower, the optimum duration of the excitation is longer and if the pressure is higher, the optimum duration of the excitation is shorter. Accordingly, the duration of each excitation is in the range SO.15 / P nanoseconds & <9o / P nanoseconds, wherein P is the pressure of the gas mixture in atm. Typically, the duration of each excitation is in the range SO.15 / P & <5o / P nanoseconds, such as range SO.15 / P & <3o / P nanoseconds.

[0028] The plasma may be repeatedly excited so that the duration of each excitation is in the range SO.15 / P nanoseconds & <9O / P nanoseconds. By repetitively exciting the plasma a larger volume of gas can be excited. For the avoidance of doubt,it is the plasma that in such case is repeatedly excited. There may be a scenario where an infinitesimal sub-volume of gas is only excited once in the duration according to the present disclosure during the residence time of said sub-volume in the plasma reactor.

[0029] Typically, the plasma is generated via electrical discharges, radiowaves, microwaves, X-rays, UV-light or laser. In embodiments where the plasma is generated via electrical discharges, the peak current of the electrical discharges is typically between 10 Amperes and 100 kiloamperes.

[0030] The yield, i.e. NOx molecules formed per Joules of energy is significantly improved upon application of a pulse length in the order of nanoseconds compared with microseconds. This is shown in the example.

[0031] As the duration of the excitation is becoming shorter, ultimately it comes to a point where the duration is so short that the collisions between molecules do not anymore have sufficient time to occur during the active stage of the discharge. Therefore, there is a range of durations that gives optimum energy efficiency, longer durations will increase the energy consumption per mol of NOx generated as compared to the energy consumption in the range and long durations will also give time for undesired backward reactions, while shorter durations will not be sufficient to generate any collisions between molecules during the energy input, thus not forming any NO. In the current experiments, the number of collisions per excitation was from 100 to substantially no collisions at all. With other sources for the plasma, this number is typically higher, but the durations of the excitation will still give a low number of collisions compared with how many collisions that can be provided using longer excitation times, such as long pulses or constant excitation, with the particular technique. The number of collisions of gas molecules during the time of the discharge is kept low when the duration of each excitation is no longer than about 90 nanoseconds at atmospheric pressure. The inventors have realized that the average time between collisions of gas molecules at standard atmospheric conditions is about 0.15 nanoseconds, thereby the duration of the excitation is providing an improved yield when the excitation is at least 0.15 nanoseconds if measured at atmospheric pressure, i.e. about 0.136 nanoseconds at a pressure of 1.1 atm. In the present disclosure, this was shown using electrical discharges. However, the insight is also valid for radio waves, microwaves, X-rays, UV-light or laser as a source for theplasma since the inventors have realized that the yield is increased when the number of collisions is kept low. This is a general insight which is not dependent on a certain technique for exciting the plasma.

[0032] The inventors have further realized that the average time between collisions of gas molecules is pressure dependent according to that an increase of pressure with a factor X, decreases that optimum time of duration of each excitation with a factor of X. For example, at 1 atmosphere the duration of each excitation provides improved yield when >0.15 nanoseconds, whereas at 10 atmospheres the duration of each excitation provides improved yield when >0.015 nanoseconds, and at 100 atmospheres the duration of each excitation provides improved yield when >0.0015 nanoseconds. Typically, the gas mixture is provided at a pressure P of > 1.2 atm, such as > 1.3 atm, such as > 1.5 atm. A typical upper limit is 100 atm, such as 50 atm, such as 25 atm. Typically, the gas mixture is provided at a pressure P of 1.2-100 atm, such as 1.2-50 atm, such as 1.2-25 atm.

[0033] The plasma may be generated by electrical discharges and the plasma chamber is in such case typically equipped with electrodes to generate the electrical discharges. Example of materials for the electrodes are copper, such as copper with a tungsten coating, or steel, such as stainless steel, or alloys, such as Inconel. Alternatively, the plasma is generated by radiowaves, microwaves, X-rays, UV-light or laser and the plasma chamber is equipped with coupling mechanisms to couple the radio waves, microwaves, X-rays, UV-light or laser. Examples of coupling mechanism are antennas and other wave launchers and various light transmission techniques such as optical fibers.

[0034] The plasma reactor is in one embodiment operated by heating the gas mixture comprising oxygen and nitrogen with radiofrequency waves or microwaves. The radiofrequency waves or microwaves transfer energy to the gas molecules. In such a case, a radiofrequency wave plasma or a microwave plasma, can be formed in a plasma reaction zone. When a plasma is formed, the radiofrequency wave or microwave energy input excite the gas, which can be sufficient to generate NOx. When exciting the already free electrons, the plasma becomes more and more thermal as energy is successively transferred to the atoms. The radiofrequency waves or microwaves also transfer energy to the molecules. The use of radio waves or microwaves or lasers is beneficial since it is possible to focus the energy to a sector, inprinciple an arbitrary sector, of the reactor without requiring that any electrodes are in direct contact with the heated thermal reaction zone. If the electrodes are required to be in the vicinity of the thermal reaction zone, or even in direct contact, the electrodes may become so hot that they are consumed. One alternative solution is to limit the temperature, but that can have adverse effects on the efficiency of the process.

[0035] The gas mixture may be pre-ionized prior to the excitation. An advantage of pre-ionization is that a reduced voltage can be applied in the excitation.

[0036] The plasma is typically a non-equilibrium plasma. The non-equilibrium plasma may be a non-thermal plasma. A non-thermal plasma is also known as cold plasma. A non-equilibrium plasma is a plasma in which only electrons are thermalized. Hence, a non-equilibrium plasma is not in thermodynamic equilibrium because the electron temperature is much hotter than the temperature of heavy species, e.g. ions and neutral atoms. This is due to that in a non-equilibrium plasma cooling of the ions and uncharged molecules is a more efficient process than the transfer of energy from the electrons to them, so the gas stays at a relatively low temperature The usage of a non-equilibrium plasma is beneficial with respect to that the lower temperature is more gentle to the reactor walls of the plasma reactor and it beneficial with respect to give lower energy consumption per mol of NOx formed.

[0037] Alternatively, the plasma is a thermal plasma. In such case, also the atoms are heated. The usage of a thermal plasma is beneficial with respect to the yield of NOx.

[0038] The gas mixture comprising oxygen and nitrogen typically has an oxygen content of 15-70 % (vol / vol), such as 25-55 % (vol / vol). The gas mixture may also be air. Oxygen content in the range 22-70 % decreases the energy consumed for the generation of NO due to a more balanced formation of NOXas compared to air. Such oxygen content is typically provided by running a gas mixture comprising oxygen and nitrogen, such as air, through an oxygen sieve or oxygen enriching device. Molecular sieves for separating / enriching gases are well-known to persons skilled in the art, and available from commercial suppliers. Alternatively, the gas mixture comprising oxygen and nitrogen may have an oxygen content of 15-21 % (vol / vol) that is advantageous because it avoids the need for enriching the gas mixture with oxygen from air and it reduces the oxidative oxygen pressure on the reactor walls and theelectrodes. The gas mixture comprising oxygen and nitrogen typically has a nitrogen content of 30-85 % (vol / vol).

[0039] Typically, a time duration between two consecutive excitations is at least 3 times the duration of each excitation, such as at least 5 times the duration of each excitation, such as at least 10 times the duration of each excitation. A suitable upper limit is up to 100 000 times, such as 50 000 times the duration of each excitation, such as up to 5000 times. Examples of time durations between two consecutive excitations is between 5 microseconds and 3 milliseconds, such as between 10 microseconds and 3 milliseconds. A repetition frequency of 300 Hz is equivalent to a time duration between two consecutive excitations of approximately 3 milliseconds. The inventors have realized that the number of NOx molecules created per second is proportional to the repetition frequency. Thus, by increasing the repetition frequency to 100 kHz or even several hundreds of kHz the NOx yield can be increased by about thousand folds. Accordingly, the time duration between two consecutive excitations is typically up to 10 microseconds at a pressure of 1 atm. Preferably, the excitations do not get too close in time so that their effects overlap as this can stimulate backward reactions.

[0040] The plasma chamber typically has a central longitudinal axis around which the chamber may be symmetrical. Alternatively, it is not symmetrical. The cross section of the plasma chamber is typically circular, squared or rectangular. The plasma chamber is equipped with plasma generating means, such as electrodes or coupling mechanisms. The distance between opposing plasma generating means is typically 0.1 mm to 10 cm. In particular, in case the plasma generating means are electrodes.

[0041] The process can be run continuously, and in such case the gas mixture is continuously supplied to the plasma chamber and NOx is continuously extracted from the plasma chamber. A continuous process is advantageous as the NOx is produced continuously, which is efficient. The flow of gas mixture into the plasma chamber is not particularly limited. If a larger plasma chamber is used, a higher flow can be used. The continuous operation can be made in a way that allows exposure of certain gas volumes to excitation without overlap, thus avoiding that the same gas volume is exposed to several excitations, which can risk to raise temperature and give more backward reactions. Alternatively, the process is run batch-wise. In such case, aportion of the gas mixture is added into the plasma chamber, after a certain time t and after a number of discharges exposing the enclosed gas volume, the produced NOx is discharged and a consecutive portion of gas is added into the plasma chamber. Thus, allowing the same gas volume to be excited many times to successively increase the NOx concentration. In an embodiment of the bat ch- wise process, a certain amount of gas can be continuously circulated, for example to cool down the gas outside the reactor before re-entering the plasma reactor. The circulation of the gas can be conducted either inside the plasma chamber or out through an outlet of the plasma chamber and back in again through an inlet of the plasma chamber.

[0042] As a second aspect of the present disclosure, there is provided a system comprising:- a plasma reactor having an inlet and an outlet configured to conduct the method according to the first aspect of the present disclosure for nitrogen oxide (NOx) synthesis;- a compressor configured for increasing pressure of a gas mixture comprising oxygen and nitrogen to a pressure P of > 1.1 atm;- a conversion arrangement for converting the NOXto nitric acid (HNO3), preferably an absorption column, wherein the conversion arrangement is having an inlet and an outlet; wherein the compressor is fluidically connected to the inlet of the plasma reactor and the inlet of the conversion arrangement for converting the NOXto HNO3is fluidically connected to the outlet of the plasma reactor.

[0043] The compressor is fluidically connected to the inlet of the plasma reactor. As understood by the skilled person, the term “fluidically connected to” means that it can be in direct contact, it can also be connected via a conduit or the like, and it can also be connected with one or several arrangement(s) in between as long as a fluid can be directed through the system between the compressor and the plasma reactor forming part of the system.

[0044] As also understood by the skilled person, a “fluid” can be a liquid and / or a gas.

[0045] The inlet of the conversion arrangement for converting the NOXto HNO3is fluidically connected to the outlet of the plasma reactor. As for the compressor, theoutlet of the plasma reactor can be in direct contact with the arrangement for converting the NOXto HNO3, the inlet of the arrangement can also be connected via a conduit or the like, and it can also be connected with one or several arrangement(s) in between as long as a fluid can be directed through the system between the inlet of the conversion arrangement for converting the NOXto HNO3and the outlet of the plasma reactor forming part of the system.

[0046] The arrangement for converting the NOXto HNO3is preferably an absorption column. In an absorption column, the nitrogen oxides (NOx) are absorbed into water, and reacting to form nitric acid (HNO3).

[0047] Typically, the system further comprises a heat-exchanger being fluidically connected to the outlet of the plasma reactor and fluidically connected to the inlet of the conversion arrangement for converting the NOXto HNO3so that the heat exchanger is arranged fluidically between the plasma reactor and the conversion arrangement for converting the NOXto HNO3. In such case, there is typically a conduit connected from the outlet of the plasma reactor to the heat exchanger and a conduit arranged from the heat exchanger to the conversion arrangement for converting the NOXto HNO3. Such heat exchanger is typically used for cooling down the gas before entering the conversion arrangement. The system may further comprise an additional heat-exchanger for energy recovery. Such additional heatexchanger is beneficial for the overall energy efficiency of the system. In case of an additional heat-exchanger, this heat-exchanger is typically arranged fluidically between the plasma reactor and the other heat-exchanger. In such case, there is typically a conduit connected from the outlet of the plasma reactor to the additional heat exchanger for energy recovery and a conduit arranged from the additional heat exchanger to other heat exchanger for cooling down the gas and a conduit arranged from the other heat-exchanger for cooling down the gas to the conversion arrangement for converting the NOXto HNO3. The heat-exchanger for cooling down the gas can also be referred to as a first heat-exchanger and the additional heatexchanger for energy recovery can also be referred to as a second heat-exchanger.

[0048] The system may further comprise a heat-exchanger fluidically connected to the inlet of the reactor. Such heat-exchanger can be used to either pre-heat or precool the gas entering the reactor.

[0049] The system may also further comprise a device for pre-ionization of the gas mixture being in fluidically connected to the inlet of the plasma reactor. Such device can be used to pre-ionize the gas mixture so that less energy needs to be added for excitation of the gas in the plasma reactor.

[0050] The plasma reactor may further comprise a quenching arrangement for quenching of the gas to conserve the beneficial equilibrium conditions with a high NOx content. In particular in case the intended use of the plasma reactor involves repetitive excitations on the same volume of gas so that more heat is generated a quenching arrangement could be beneficial. The quenching arrangement can cool the formed NOx forming a cooled gas mixture. The quenching is typically conducted until obtaining a temperature sufficiently low so that the NO essentially does not reverse into N2and 02. The quenching is conducted by bringing the gas mixture in contact with a quenching medium or quenching device, for example an additional heat exchanger and / or a cold surface. The quenching medium is preferably water or a gas mixture comprising oxygen and nitrogen. Typically, the gas mixture comprises oxygen, nitrogen NO and / or N02.

[0051] The system may further comprise an oxygen concentration arrangement fluidically connected to the compressor. An example of such arrangement is an oxgen sieve which adsorbs nitrogen from incoming air to increase the concentration of oxygen in a gas mixture based on air.

[0052] Typically, the outlet of the arrangement for converting the NOXto HN03is fluidically connected to the inlet of the plasma reactor and / or the compressor. By such arrangement unreacted exhaust gas can be recirculated from the arrangement for converting the NOXto HN03to the plasma reactor and / or the compressor. If the exhaust gas is brought to a pressure equal to or above the pressure of the gas mixture comprising oxygen and nitrogen provided from the compressor to the plasma reactor, the exhaust gas from the arrangement for converting the NOXto HN03is typically recirculated to the plasma reactor. Alternatively, in case the exhaust gas has been depressurized, the exhaust gas from the arrangement for converting the NOXto HN03is typically recirculated to the compressor. Such arrangements for recirculation is beneficial for the overall energy efficiency of the system.

[0053] The system may further comprise an arrangement for energy recovery, wherein the arrangement of energy recovery is fluidically connected to the outlet ofthe plasma reactor and to the inlet of the conversion arrangement for converting the NOXto HNO3so that the arrangement for energy recovery is arranged fluidically between the plasma reactor and the conversion arrangement for converting the NOXto HNO3. The arrangement for energy recovery may be fluidically connected to the outlet of the plasma reactor and to the inlet of the heat exchanger. Alternatively, the arrangement for energy recovery is fluidically connected to the outlet of the plasma reactor and to the inlet of the arrangement for converting the NOXto HNO3. The arrangement for energy recovery recovers energy. The arrangement for energy recovery may include a device turning pressure into electricity, such as a piston- driven generator, such as a turbine system. In such case the arrangement typically produces electricity. The turbine system may comprise a cooler and / or condenser. Alternatively, the arrangement for energy recovery includes a heat-exchanger. In such case, the heat recovered can be used for external uses or for downstream utility systems, or to power heat pumps to make steam. An arrangement for energy recovery is beneficial for the overall energy efficiency of the system as it can be used for downstream utility purposes or for preheating gas entering the plasma reactor.

[0054] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. As understood by the skilled person, downstream in the system is to the right in the figures and upstream in the system is to the left in the figures. Accordingly, a component of the system arranged to the right of another component in the system is arranged downstream of the other component. Even though not shown, the system can be numbered 101, and conduits can be numbered starting from no and counting upwards further downstream in the system.

[0055] Figure 3 show an example of a system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. The plasma reactor 1 is configured to conduct the method according to the first aspect of the present disclosure. The compressor 2 is configured for increasingpressure of a gas mixture comprising oxygen and nitrogen to a pressure P of > 1.1 atm. The conversion arrangement for converting the NOXto nitric acid (HNO3), is having an inlet and an outlet (not shown). The compressor 2 is via a conduit in fluidical connection to the inlet of the plasma reactor 1 and the inlet of the conversion arrangement for converting the NOXto HNO33 is via a conduit in fluidical connection to the outlet of the plasma reactor 1.

[0056] Figure 4 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. In this example the system further comprises a heat exchanger 4. The heat exchanger 4 is fluidically connected to the outlet of the plasma reactor 1 and fluidically connected to the inlet of the conversion arrangement for converting the NOXto HNO33 so that the heat exchanger 4 is arranged fluidically between the plasma reactor and the conversion arrangement for converting the NOXto HNO3. The conversion arrangement for converting the NOXto HNO33 is in fluidical connection with the plasma reactor 1 via a conduit from the outlet of the plasma reactor, the heat exchanger 4, and a conduit from the heat exchanger 4 to the conversion arrangement 3.

[0057] Figure 5 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. In this example, the outlet of the arrangement for converting the NOXto HNO33 is fluidically connected to the inlet of the plasma reactor 1 and / or the compressor 2.

[0058] Figure 6 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. In this example, the system further comprises quenching arrangement 5 for quenching NOx. In the figure, there are two quenching arrangements 5, but as understood by the skilled person, there can also be one quenching arrangement or more than two quenching arrangements.

[0059] Figure 7 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. In this example, the system further comprises an oxygen concentration arrangement 6 fluidically connected to the compressor 2.

[0060] Figure 8 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3. In this example the system further comprises an arrangement for energy recovery 7, wherein the arrangement of energy recovery is fluidically connected to the outlet of the plasma reactor 1 and to the inlet of the conversion arrangement for converting the NOXto HNO33 so that the arrangement for energy recovery is arranged fluidically between the plasma reactor 1 and the conversion arrangement for converting the NOXto HNO33.

[0061] Figure 9 show another example of the system comprising a plasma reactor 1, a compressor 2 and a conversion arrangement for converting the NOXto nitric acid (HNO3) 3 and arrangement for energy recovery 7. In this example, the system further comprises the heat exchanger 4. An outlet of the heat exchanger 4 is fluidically connected to the inlet of the conversion arrangement for converting the NOXto HNO33 and the heat-exchanger 4 is in connection with the arrangement for energy recovery 7-

[0062] The examples and embodiments discussed above in connection to the first aspect apply to the second aspect mutatis mutandis.EXAMPLESProduction of NOx using pulsed plasma

[0063] The following experimental procedure was used during the experiment. A spark was created inside a chamber filled with air at atmospheric pressure. The plasma was produced by electrical discharges. The spark of the electrical discharge took place between two electrodes, one of which was connected to the output of an impulse current generator, and the other was connected to ground. In a cylindrical plasma chamber, the plasma was produced. The grounded electrode was connected to the common ground of the high voltage hall in which the whole experimental set-up was located.

[0064] Air, i.e. a gas mixture of about 21% oxygen (vol / vol) and about 78 % (vol / vol) nitrogen, entered the plasma chamber continuously, and thereby brought into plasma state, through a needle valve and was evacuated from the chamber using an 8 m long polytetrafluoroethylene (PTFE) “Teflon” tube with 4 mm inner diameter, filtered, using a 5-micron particle filter, and passed through a calibrated NOxanalyser (model 9841B, Monitor Labs). The NOX analyser was situated in the shielded control room away from the spark. The gas flow rate was 0.60-0.64 dm3 / min. The analyser measured the concentrations of NO and NOx using a chemiluminescence method, with the lowest detectable concentrations being about 1 ppbv (parts per billion by volume). The NO and NOx concentrations measured as a function of time can be extracted as voltage signals proportional to the respective concentrations. These voltage signals were recorded by an oscilloscope (model: LeCroy 9310L). The inaccuracy in the NOx concentration measurements was estimated to be about 5-10%. The number of NOx molecules, NNOX , was calculated by integrating the entire measured concentration curve, after the background level had been subtracted, using the following equation:

[0065] where C(t) is the NOx concentration in parts per million by volume (ppmv) measured as a function of time, B is the background concentration of NOx measured in ppmv, At is the sampling interval of the concentration measurements in minutes, F is the sample air flow rate in dm3 / min, NA is the Avogadro constant in molecules / mol, and V is the ideal gas molar volume in dm3 / mol at the measured temperature and pressure. In general, the background concentration, B, varied only slightly during the measurements, so the mean value of the background NOx concentration was utilised in the above equation. Prior to each measurement, the chamber was flushed fully so that the background level of NOx inside the chamber was identical to the normal background NOx level in the experimental hall.Production of NOx using pulsed plasma with long pulse length

[0066] Production using pulsed plasma with a long pulse length has been previously published in Rahman, Mahbubur, et al. "NOX production by impulse sparks in air." Journal of Electrostatics 69.6 (2011): 494-500. An extract from the experimental work conducted therein is laid out below serves as a reference in the present disclosure.

[0067] In a series of measurement, a standard lightning impulse voltage with a rise time of 1.2 ps, and with the time taken to reach the half-value of 50 ps (henceforth called a 1.2 / 50 impulse) was used to create a 12.8 cm long spark in air, where the measured discharge current had a mean rise time of 0.3 ps and a meantime to half-value of 10 ILLS.. It was estimated that the excitation time was about 100 IL S. Accordingly, as seen from the units, the order of magnitude of the pulses are in the microsecond range, and are thereby referred to as long pulses in the present disclosure. Excitation time and pulse length means the same.

[0068] The total number of sparks created in the first series of measurements was 35. The charging voltage of the generator (Marx impulse voltage generator, model: Haefely SGSA 1000050, maximum charging voltage: 1 MV and energy at maximum charging voltage: 50 kJ) was varied from 0.16 up to 1 MV which gave peak discharge currents ranging from about 0.522 to 2.3 kA through a 12.8 cm long air gap. As the charging voltage increased, the breakdown voltage of the gap (taken as the starting point of the rapid decrease in the voltage waveform) varied from 149 to 383 kV and time to breakdown from the onset of the voltage varied from 0.35 to 5.3 ms.

[0069] In addition to the NOx concentration, the current through the gap was measured at the ground end using either a Rogowski coil (Pearson model 411, maximum peak current 5 kA, rise time 20 ns, bandwidth 20 MHz) or a Pearson current transformer (model 1080, 200 kA, 0.25 ps, 3 Hz to 1.5 MHz) connected to a PC via a data acquisition card (Dias 733 from Haefely). In the first series of experiments, in addition to the NOx produced and the current through the gap, the voltage across the gap was measured using a damped capacitive impulse voltage divider (Haefely, type CS 1000-670). No voltage measurements were carried out in the second and third series of measurements directly in connection with the NOX measurements.Production of NOx using pulsed plasma with short pulse length

[0070] Repetitive electrical discharges containing sub-microsecond duration currents were created inside a reactor.

[0071] The discharges inside the reactor were generated by an induction coil activated by repetitive current pulses. Each current pulse gave rise to a discharge inside the reactor due to the back-emf caused by the rate of charge of the magnetic field inside the coil. The repetition frequency of the discharges generated in the gap was 225 Hz, i.e. a time duration between two consecutive excitations of about 4.4 milliseconds.

[0072] The voltage generated by the induction coil was applied to one of the electrodes (other electrode was grounded) to create electrical discharges inside the reactor.

[0073] In the experiment, the length of the discharge was maintained either at0.01 m or 0.005 m. The duration of the current pulse associated with each discharge was about 20 nanoseconds and the peak current varied from about 10 Ampere to 40 Ampere.

[0074] During the experiment repetitive electrical discharges are created inside the electrode gap which is placed inside a reactor chamber. The chamber was connected to a NOx analyser which circulated ambient air through the reactor at a flow rate of 0.6 litres / minute. The NOx generated inside the reactor by repetitive electrical discharges, activated for a duration of 15 seconds, was measured by the NOx analyser.

[0075] In the experiment, the energy released by the electrical discharges was obtained by measuring the voltage and the current flowing across the electrical discharge.

[0076] In analysing the NOx production by hot electrical discharges it was assumed in the analysis that the energy input into the discharge will heat the surrounding air which is at atmospheric pressure and the NOx generation takes place as this volume of air heated to at least 3000 K is cooled rapidly which is the scenario when the excitations are not exciting the same volume of gas mixture more than once.

[0077] In a separate experiment the discharges with short pulse lengths were produced at pressures above 1 atm done by provision of the gas mixture at pressures of about 1.4, 1.6 and 1.8 atm. A reference experiment at 1 bar (0.986923 atm) was also conducted. The results are presented in Figure 1.

[0078] In both the plasma produced by long pulses and the plasma produced by short pulses the gas temperature is believed to be in the range of 5000-10 000 K. But in the short pulsed plasma there is not sufficient time for reaching thermal equilibrium during the pulse.Results of NOx production with short or long pulse length

[0079] The NOx yield as a function of the calculated spark energy was estimated for the sparks created by the 1.2 / 50 voltage impulse, i.e. the long pulse lengths havinga pulse length of about 100 ps. The energy of the spark was obtained by integrating the product of the measured voltage and current. The variation of the NOx yield as a function of energy was realized to be almost linear for both long and short pulse lengths.

[0080] In Table 1, the comparative data between the two experimental series is presented. The data is NOx molecules generated per Joule of energy and efficiency of the NOx production given as moles per Mega Joule of energy. The Joules of energy reflects the energy needed for the whole circuit including the electronics to generate the excitations.Table 1. Data of NOx production using a pulse lengths of either about 100 ps or 20 nanoseconds.

[0081] For a pulse length of 20 nanoseconds, the NOx molecules per Joule obtained were about 7.5 times larger than the values obtained when using discharges containing the longer 100 ps pulse lenghts. This clearly shows that the shorter pulse length discharges investigated here are more efficient in NOx production than the electrical discharges with longer duration currents. The energy efficiency for NOx production obtained in the short pulses of 20 nanoseconds discharges was 1.3 - 1.5 moles / MJ. This is equivalent on average to 14.2 MWh / metric tonne of elementary nitrogen fixed. However, it should be noted that this calculation did not include the energy consumption of the electric circuits.

[0082] The effective percentage of NOx frozen out from air heated to 3000 K was 13.9 % with the short pulse length.

[0083] The repetition frequency of the discharges was 225 Hz in the experiment. However, the inventors have realized that the number of NOx molecules created persecond is proportional to the frequency. Thus, by increasing the frequency to several hundreds of kHz the NOx yield can be increased by about thousand folds.Results of NOx production at increased pressure

[0084] In Figure 1 the results of the measurements conducted at increased pressure (pressure above 1 atm) are presented. Therein, on the y-axis the number of NOx molecules produced per spark are presented. On the x-axis the pressure of the gas mixture in amospheres are presented. There were in total 4 measurements made and to the data a trendline has been added for visual guidance.

[0085] Starting at about 0.98 atm the number of NOx molecules produced per spark were about 1.65 x IO^, where at 1.38 atm the number increased to about 2.1 x 1015. An increase of almost 30%. As also seen in the figure, there is a substantial effect in increasing the pressure to 1.1 atm, potentially providing an improved yield by about 10%.

[0086] In Figure 2, the energy consumed per mole of nitrogen produced is plotted against the pressure. Therein, on the y-axis the energy in MJ per mole of NOx produced per spark are presented. On the x-axis the pressure of the gas mixture in atm are presented. There were in total 4 measurements made and to the data a trendline has been added for visual guidance.

[0087] Starting at about 0.98 atm, the energy in MJ per mole of NOx produced was about 1.15 MJ, where at 1.38 atm the energy demand decreased to about 0.8 MJ, which is a substantial decrease of about 30%. As also seen in the figure, there is a substantial effect in increasing the pressure to 1.1 atm, potentially providing a decreased energy consumption of about 15%.

[0088] Accordingly, combining the findings of Figures 1-2, it has been shown that increasing the pressure of the gas mixture improves both yield and energy efficiency.

Claims

CLAIMS1. A method for nitrogen oxide (NOX) synthesis, the method comprising: providing a gas mixture comprising oxygen and nitrogen in a plasma reactor chamber; and exciting the gas mixture into plasma in the plasma reactor chamber, wherein a duration of the excitation is in the range SO.15 / P nanoseconds & <9o / P nanoseconds, wherein P is pressure of the gas mixture in atm, and wherein the gas mixture is provided at a pressure P of > 1.1 atm.

2. The method of claim 1, wherein the plasma is excited via electrical discharges, radiowaves, microwaves, X-rays, UV-light or laser.

3. The method of claim 1 or 2, wherein the duration of each excitation is in the range SO.15 / P & <so / P nanoseconds, such as range SO.15 / P & <3o / P nanoseconds.

4. The method of any one of the preceding claims, wherein the plasma is generated by electrical discharges, and wherein peak current of the electrical discharges is between 10 Amperes and 100 kiloamperes.

5. The method of any one of the preceding claims, wherein the gas mixture is provided at a pressure P of > 1.2 atm, such as > 1.3 atm, such as > 1.5 atm, such as 1.2-100 atm, such as 1.2-50 atm, such as 1.2-25 atm.

6. The method of any one of the preceding claims, wherein the gas mixture has an oxygen content of 15-70 % (vol / vol).

7. The method of any one of the preceding claims, wherein the gas mixture has a nitrogen content of 30-85 % (vol / vol).

8. The method of any one of the preceding claims, wherein a time duration between two consecutive excitations is at least 3 times the duration of each excitation, such as at least 5 times the duration of each excitation, such as at least 10 times the duration of each excitation.

9. The method of any one of the preceding claims, wherein the plasma chamber has a central longitudinal axis around which the chamber is symmetrical.

10. The method of any claim 9, wherein the distance between opposing plasma generating means typically 0.1 mm to 10 cm.

11. The method of any one of the preceding claims, wherein the plasma is generated by electrical discharges and the plasma chamber is equipped with electrodes to generate the electrical discharges.

12. The method of any one of the preceding claims, wherein the plasma is generated by radiowaves, microwaves, X-rays, UV-light or laser and the plasma reactor is equipped with coupling mechanisms to couple the radiowaves, microwaves, X-rays, UV-light or laser.

13. The method of any one of the preceding claims, wherein the plasma is a nonequilibrium plasma.

14. The method of any one of the preceding claims, wherein the plasma is repeatedly excited so that the duration of each excitation is in the range SO.15 / P nanoseconds & <9o / P nanoseconds.

15. A system comprising:- a plasma reactor having an inlet and an outlet configured to conduct the method according to any one of the preceding claims for nitrogen oxide (NOX) synthesis;- a compressor configured for increasing pressure of a gas mixture comprising oxygen and nitrogen to a pressure P of > 1.1 atm;- a conversion arrangement for converting the NOXto nitric acid (HN03), preferably an absorption column, wherein the conversion arrangement is having an inlet and an outlet; wherein the compressor is fluidically connected to the inlet of the plasma reactor and the inlet of the conversion arrangement for converting the NOXto HN03is fluidically connected to the outlet of the plasma reactor.

16. The system of claim 15, wherein the system further comprises a heat-exchanger being fluidically connected to the outlet of the plasma reactor and fluidically connected to the inlet of the conversion arrangement for converting the NOXto HN03so that the heat exchanger is arranged fluidically between the plasma reactor and the conversion arrangement for converting the NOXto HN03.17- The system of claim 15 or 16, wherein the plasma reactor further comprises a quenching arrangement for quenching NOx.

18. The system of any one of the claims 15-17, wherein the outlet of the arrangement for converting the NOXto HNO3is fluidically connected to the inlet of the plasma reactor and / or the compressor.

19. The system of any one of the claims 15-18, further comprising an oxygen concentration arrangement fluidically connected to the compressor.

20. The system of any one of the claims 15-19, wherein the system further comprises an arrangement for energy recovery, wherein the arrangement of energy recovery is fluidically connected to the outlet of the plasma reactor and to the inlet of the conversion arrangement for converting the NOXto HNO3so that the arrangement for energy recovery is arranged fluidically between the plasma reactor and the conversion arrangement for converting the NOXto HNO3.

21. The system of claim 20, wherein the arrangement for energy recovery comprises a device turning pressure into electricity, such as a piston-driven generator, such as a turbine.

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