WO2024091136A1

WO2024091136A1 - Nitrogen fertiliser production process and system using plasma technology

Info

Publication number: WO2024091136A1
Application number: PCT/PT2023/050039
Authority: WO
Inventors: Júlio Paulo DOS SANTOS DUARTE VIEIRA HENRIQUES; Júlia Da Natividade FERNANDES BARROSO PEREIRA JORGE
Original assignee: Instituto Superior Técnico
Priority date: 2022-10-27
Filing date: 2023-10-27
Publication date: 2024-05-02
Also published as: PT118293A

Abstract

The present invention relates to a process for producing nitrogen fertilisers using plasma technology comprising the steps of a) producing a swirling flow; b) irradiating said flow from step a) with electromagnetic radiation emitted by a microwave plasma volume produced by surface- waves; c) passing said flow from step b) inside said microwave plasma volume; d) subjecting said flow from step c) to a pressure drop at the exit of said microwave plasma volume; e) moving said flow from step d) through an aerosol volume; and f) collecting the fertiliser formed in the previous step. The invention also relates to a system for implementing the process. The invention finds application in the field of nitrogen fertiliser production.

Description

DESCRIPTION

"NITROGEN FERTILISER PRODUCTION PROCESS AND SYSTEM USING

PLASMA TECHNOLOGY"

Field of invention

The present invention relates to a process and a system for the production of nitrogen fertilisers using plasma technology, more speci fically, microwave plasmas excited by a surface-wave . The invention finds application in the agricultural industry .

State of the art

The United Nations (UN) estimates that the world' s population will continue to grow until it reaches 9 . 7 billion people in 2040 , which will result in an increase in demand for food . However, due to climate change , soil erosion and depletion and the allocation of previously agricultural land to other activities , the amount of arable land is decreasing worldwide . According to the Economic and Social Development Department of the Food and Agriculture Organi zation of the United Nations ( FAO) ( Jelle Bruinsma, "The resource outlook to 2050 : by how much land, water and crop yields need to increase by 2050" 2009 Expert Meeting on How to Feed the World in 2050 ) the amount of arable land available per person is decreasing and the prospects of reversing this process are non-existent . This puts the world' s agricultural production under great pressure , particularly that of crops , and there is a need to increase productivity per unit area . Intensi fying crop production is possible , as has been proven over the years , through the application of fertilisers , especially nitrogen fertilisers .

Atomic nitrogen (N) is considered one of the most important nutrients for plant growth and is one of the main building blocks of li fe , forming an integral part of proteins and other biomolecules . Its availability is one of the main factors limiting agricultural production .

Although the Earth' s atmosphere is primarily composed of molecular nitrogen (N2 ) , approximately 78 % of dry atmospheric air, plants are unable to absorb it in its molecular form due to the di f ficulty they have in breaking the triple covalent bond between its atoms (N=N) .

However, this obstacle has been easily overcome by giving plants synthetic nitrogen in the form of fertilisers , called nitrogen fertilisers . These fertilisers are made up of molecules easily assimilated by plants with a high content of atomic nitrogen . The process of trans forming molecular nitrogen into molecules containing atomic nitrogen that can be easily assimilated by plants is called nitrogen fixation .

Nitrogen fertilisers are mainly applied to agricultural crops such as cereals , fruit and vegetables . Of these crops , the one that particularly stands out is cereals , which is clearly the main application for this type of fertiliser . It has seen growing demand in recent years , especially in Asian markets such as the People ' s Republic of China and the Republic of India . This increase in demand is correlated with the growing demand from the food and animal production industries .

As a result of the increase in food consumption by the world' s growing population, as well as the decline in the amount of arable land, the global market for nitrogen fertilisers is expected to increase in the coming years. In 2019 the world market for this product totalled approximately 65 billion euros and is expected to increase by around 5.1% by 2026.

Currently, most synthetic nitrogen fertilisers are made using the Haber-Bosch synthesis process, which synthesises ammonia (NH3) . This catalytic process is produced at high pressures (around 200 to 300 atmospheres) and requires very high temperatures (400 to 500 °C) . It combines molecular nitrogen (N2) extracted from the air with molecular hydrogen (H2) obtained from natural gas (methane) or liquid hydrocarbons .

At present, around half of the world's population is nourished by agricultural products produced using nitrogen fertilisers based on the Haber-Bosch process. But this process has very high environmental costs. Current production is achieved through the consumption of around 2% of all the world's energy, mainly in the form of natural gas, where around 3% to 5% of the world's production is consumed. Of all the natural gas consumed in the Haber-Bosch process, around 65% is used as a source of hydrogen while the remaining 35% is used to heat the process. Natural gas consumption represents more than 50% of the cost of producing this type of fertiliser using the Haber-Bosch process (European Commission, "Fertilisers in the EU - Prices, trade and use" 2019 EU Agricultural Markets Briefs) .

Currently, there are industrial applications using the Haber-Bosch process that consume less than 30 gigajoules per tonne of atomic nitrogen produced, i.e. 30 GJ/tN, a value very close to the minimum thermodynamic limit calculated for this process, which is approximately 24 GJ/tN (Vaclav Smil, "Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production", MIT Press (2001) Cambridge; V. Smil, "Nitrogen and food production: proteins for human diets", AMBIO (2002) 31 (2) 126-131) .

Moreover, the production of nitrogen fertilisers using the Haber-Bosch process relies on unsafe, expensive and large equipment that uses aggressive chemistry and emits large quantities of greenhouse gases (CO2) into the atmosphere, with high energy production costs. Storage and transport costs to the end customer must also be taken into account.

To sum up, the main problems with the current chemical processes for producing nitrogen fertilisers, including the Haber-Bosch process, are:

- High environmental costs due to the use of aggressive and dangerous chemicals and the emission of large quantities of greenhouse gases into the atmosphere.

- High production costs due to the use of heavy and expensive equipment. The Haber-Bosch process is responsible for consuming around 2% of all the world's energy, which makes increasing nitrogen fertiliser production from this process unsustainable in the future. The process requires a lot of skilled labour, with all the associated costs. It also requires the use of expensive catalysts.

The Haber-Bosch process uses very high working temperatures which can range between 400 °C and 500 °C, and also very high working pressures which can range between 200 and 300 atmospheres. In the Haber-Bosch process, production costs are further increased by the cost of storing and transporting the fertiliser to the end customer, since it is not produced where it is consumed.

An alternative to traditional chemical processes are ones that use the exceptional properties of plasmas to produce nitrogen fertilisers. One of the best-known nitrogen fixation processes that utilises these plasma properties is a natural process that occurs during the formation of lightning .

Lightning is a highly energetic natural electrostatic discharge that ionises the air along its path, producing extremely hot plasma columns (> 30000 Kelvin) with high electron densities (> 10²⁴ nr³) , capable of breaking down, i.e. dissociating, the nitrogen molecules in the atmosphere, producing atomic nitrogen (N) which reacts with oxygen to produce nitrogen oxides. Nitrogen oxides react when they come into contact with rainwater, producing nitrates, which, upon falling into the soil are easily assimilated by plants (J. F. Noxon, "Atmospheric nitrogen fixation by lightning", Geophysical Research Letters (1976) Volume 3, Issue 8 p. 463-465; U. Schumann and H. Huntrieser, "The global lightning-induced nitrogen oxides source", Atmos. Chem. Phys. (2007) 7, 3823-3907) . The production of nitrogen oxides by lightning was first observed by the German chemist Justin von Liebig in 1827, and nearly two centuries later the subject still arouses interest in the scientific community.

Patent application US 4010897A, by Richard W. Treharne and Charlton K. McKibben, entitled "Method and Apparatus for Home Production and Application of Nitrogen Fertilizer", filed on 31 March 1976, discloses the use of plasma technology to produce nitrogen fertilisers . The application discloses a method and apparatus for producing and applying nitrogen fertilisers in the form of nitrogen oxides . The device uses an atmospheric arc discharge to ionise molecular nitrogen and oxygen to form nitrogen oxides . The oxides thus formed are inj ected into water supplied by a garden hose for small home installations such as gardens and backyards .

Document RU2650545C1 (Mpni-ia ©eoKTixcTOBHa rojiOE.au.Kax et al . "Nitrogen Fertili zer and Method of its Production" , 2018 ) refers to a chemical plasma method for the production of nitrogen fertilisers in aqueous solutions , with the aim of feeding plants hydroponically or in soil growing conditions in open or closed environments . This method involves pumping a mixture of water and air through the noz zle of a low- temperature plasma torch, operating at a frequency of 2 . 45 GHz and a power of 1 . 2 KW, and is capable of producing aqueous solutions of nitrous ( HNO2 ) and nitric (HNO3 ) acids with nitrite ion concentrations (N02~ ) of 20-30 mg per litre .

To make current processes that uses plasma technology competitive with chemical proces ses , particularly with the Haber-Bosch process , they need to become more energy ef ficient , i . e . capable of producing more fertiliser with less energy .

One of the most important challenges facing the industry today is to produce nitrogen fertilisers at low environmental cost and in a sustainable manner .

A new approach needs to be adopted that allows the industry to produce nitrogen fertilisers in both aqueous and solid form, sustainably, with energy ef ficiency equivalent to the best that the industry can currently do with Haber-Bosch processes, using renewable energy sources, and using raw material sources that are abundantly available in nature .

It is also necessary for such an approach to be able to regulate fertiliser inputs according to crop needs, while avoiding pollution problems in rivers and aquifers due to fertiliser leaching, for example.

Consequently, there is a need in the art for a new, non-chemical process for producing nitrogen fertilisers that solves the above-mentioned problems of the prior art.

Brief description of the designs

The following is a detailed description of the invention with reference to the attached drawings, wherein:

Fig. 1 schematically shows a system of the invention in operation, configured to carry out the process of the present invention, where the reference signal (1) represents a surface-wave waveguide; (2) represents a swirling flow; (4) represents a surf atron-type field applicator; (5) represents a microwave plasma volume; (6) represents a surface-wave; (8) represents an aerosol volume, and (14) represents a constriction to produce a pressure drop in the flow direction (from top to bottom, with reference to the position of the system on the sheet) .

Fig. 2 schematically shows another way of implementing the system of the invention in operation, configured to implement the process of the present invention, where in addition to the elements already identified in Fig. 1 are represented the reference signal (3) which represents the power produced by a microwave generator; (7) represents an injector used to create a volume of aerosol; (9) represents a reservoir containing a liquid substance; (10) represents an inlet A, from which the mixture in flow regime will be introduced; (11) represents an exhaust outlet for gases that have not been broken down by the plasma; (12) represents an outlet for the nitrogen fertiliser diluted in the water reservoir; (13) represents an inlet B for a liquid substance; (15) represents a liquid substance.

Figs. 3 and 4 schematically show systems of the invention in operation, where in the case of Fig. 3 a linear tube is represented and in the case of Fig. 4 an inverted U- shaped tube, where four operating zones are defined in a surface-wave waveguide (1) , where the reference signal (Pl) represents a pre-discharge zone; (P2) represents a surfacewave propagation zone where the plasma is produced; (P3) represents a higher pressure post-discharge zone; (P4) represents a lower pressure post-discharge zone.

Fig. 5 schematically shows an alternative embodiment of a system of the invention in operation, the system comprising an inverted U-shaped tube and configured to carry out the process of the present invention.

Fig. 6 schematically shows another alternative embodiment of a linear system in operation, the system configured to carry out the process of the present invention, in this case recirculating the liquid substance (15) , where the reference signal (16) represents a water pump used to recirculate the liquid substance (15) .

Fig. 7 schematically shows a way of embodying a system of the invention in operation, the system comprising an inverted U-shaped tube configured to carry out the process of the present invention, in this case recirculating the liquid substance ( 15 ) .

Fig . 8 schematically shows another way of realising a system of the invention in operation, the system comprising an inverted U-shaped tube and configured to carry out the process of the present invention . The reference signal ( 17 ) represents a drying device .

Detailed description of the invention

The present invention relates to a process and a system for the production of nitrogen fertilisers using plasma technology, more speci fically, microwave plasmas excited by a surface-wave .

In the context of the present invention, the expression " flow" refers to a fluid in motion .

The term "microwave plasma" refers to a mixture of ionised gases created by the electric field of a surfacewave .

The term " surface-wave" refers to a wave that propagates at a plasma-dielectric interface and is launched by a surf atron-type field applicator .

The term "microwave plasma volume" refers to the volume defined inside a hollow-body surface-wave waveguide , which is occupied by the microwave plasma produced there . "Swirling flow" means the rotating movement of a mixture in a flow regime, whose axis of rotation is the axis of a surface-wave waveguide.

An "aerosol" means small liquid or solid particles suspended in a flow regime mixture.

A "fluid communication connection" means a connection between two different elements, the connection of which allows one of the elements to supply fluids to the other or allows fluid supply to be exchanged between the connected elements .

"Out-of-equilibrium microwave plasmas" means plasmas that are not in thermodynamic equilibrium, i.e. plasmas where the temperature of their electrons is much higher than the temperature of the heavy species (atomic and molecular ions, and neutral atomic and molecular species) .

In the context of this description, the term "comprising" and its verbal variations should be understood as "including, among others". This term should not be interpreted as "consisting only of".

In the present invention, "fertiliser production" means the production of fertilisers through a process that uses only renewable energy sources (i.e. solar, hydro, wind, etc.) , and where the raw materials used are non-polluting.

The process of the present invention for nitrogen fertiliser production using plasma technology comprises the following steps: a) producing a swirling flow (2) , such flow consisting of a mixture of nitrogen, oxygen and at least one tertiary component selected from the group consisting of argon, carbon dioxide, carbon monoxide, methane, hydrogen, helium, neon, atmospheric air, and water vapour; b) irradiating said flow from step a) with electromagnetic radiation emitted by a microwave plasma volume (5) produced by surface-waves (6) ; c) passing said flow from step b) inside the microwave plasma volume (5) , moving it from one inlet to one outlet of said microwave plasma volume (5) ; d) subjecting the flow from step c) to a pressure drop at the exit of the microwave plasma volume (5) ; e) moving the flow from step d) through an aerosol volume ( 8 ) ; f) collecting the fertiliser formed in step e) .

The main advantage of this process is its energy efficiency, since it uses out-of-equilibrium microwave plasmas to produce atomic nitrogen (N) , the primary component of this type of fertilisers, with a higher energy efficiency than competing processes that use plasma technology, and equivalent to the Haber-Bosh process, but without its drawbacks .

The aforementioned tertiary component of step a) of the invention is one of the three essential components added to the mixture that forms the flow, like the nitrogen and oxygen, which constitute the other two essential components of the mixture.

The tertiary component is selected from the group consisting of argon, carbon dioxide, carbon monoxide, methane, hydrogen, helium, neon, atmospheric air, and water vapour and combinations thereof.

The choice of tertiary component influences its role in the mixture: argon, neon, helium are used to start the discharge and then become optional; hydrogen, together with methane and water vapour, which are two sources of hydrogen, can be used when introduced into the plasma to produce a different nitrogen fertiliser, "ammoniacal nitrogen", or to increase the efficiency of the process; carbon monoxide (CO) , carbon dioxide (CO2) , and again water vapour are sources of oxygen; atmospheric air composed mostly of nitrogen and oxygen, approximately 78% and 21%, constitutes a free and environmentally friendly source of these two gases.

In one embodiment, in step a) , the said flow is produced with a flow rate of between 8.3xl0~⁶ and 3.3xl0~³ m³/s, preferably between 1.6xl0~⁵ and 1.6xl0~³ m³/s, more preferably between 3.3xl0~⁵ and 8.3xl0~⁴ m³/s .

Preferably, said swirling flow (2) , whose axis of rotation is the axis of said surface-wave waveguide (1) mentioned in step b) , is produced with a centrifugal acceleration higher than 4xl0⁴ m/ s².

The said flow (2) mixture is subjected to a swirling motion with very high centrifugal accelerations, preferably exceeding 4xl0⁴ m²/s, which causes a pressure gradient inside the physical element through which it flows, such as a surface-wave waveguide, with a higher pressure near the wall of said surface-wave waveguide. This swirling flow (2) expands along the mentioned surface-wave waveguide, transporting the said flow (2) mixture with a swirling motion along the surface-wave waveguide. The swirling flow (2) gains intensity inside the microwave plasma volume (5) as a result of the increase in temperature and the maintenance of angular momentum.

This exposure results in the partial decomposition of the molecules that make up the said swirling flow (2) mixture, producing new, highly reactive atomic and molecular species different from those originally present in the referred flow mixture.

The surface-waves (6) referred to in step b) have a frequency between 13.65 MHz and 28 GHz, preferably between 300 MHz and 5.8 GHz, more preferably between 300 MHz and 2.45 GHz.

In step b) of irradiating the flow with electromagnetic radiation emitted by a microwave plasma volume (5) , the energy density of said microwave plasma volume (5) is between 0.5 and 200 MW/m³, preferably between 1 and 100 MW/m³, more preferably between 2 and 50 MW/m³.

In step e) of introducing the flow from step d) into an aerosol volume (8) , said aerosol volume (8) has a mass flow rate of between 8.0xl0~² and 3.2X1G¹ gram/s, preferably between 1.6xl0^-1 and 1.6X1G¹ gram/s, more preferably between 3.2xl0^-1 and l.QxlO¹ gram/s.

Regarding the aerosol volume (8) , this is made up of any type of water selected from the group consisting of distilled water, tap water, alkaline water, brackish water, filtered water from rivers, lakes, dams, cisterns, wells, and combinations thereof.

In one embodiment of the process of the invention, the aerosol volume (8) further comprises an alkaline substance selected from the group consisting of potassium phosphate, calcium oxide and combinations thereof. The addition of these alkaline substances to the aerosol volume (8) makes it possible to produce fertilisers not only in aqueous form, but also in solid form.

In the collection step (f) , the fertiliser formed in step (e) can then be collected in aqueous or solid form, given that, to obtain the aqueous form, the mixture of aerosol and nitrogen fertiliser can be introduced into a reservoir (9) containing a liquid substance (15) and, to obtain the solid form, a drying device (17) , or any other device that can perform this function known to the specialist in the art, can be used.

The said liquid substance (15) , into which the mixture of aerosol and nitrogen fertiliser is introduced to obtain an aqueous form of the fertiliser, is composed of any type of water, such as distilled water, tap water, alkaline water, brackish water, filtered water from rivers, lakes, dams, cisterns, wells or combinations thereof.

In another embodiment of the process of the invention, an alkaline substance selected from the group comprising potassium phosphate, calcium oxide and combinations thereof is added to said liquid substance (15) . In an alternative embodiment, the process further comprises introducing said liquid substance (15) as the aerosol of step e) , by means of reinjection. The present invention also relates to a system for producing nitrogen fertilisers. The system of the invention implements the process described above using plasma technology . Referring to Fig. 1, the nitrogen fertiliser production system of the invention comprises a surf atron-type field applicator (4) and a surface-wave waveguide (1) .

Referring now to Figs. 1, 3 and 4, the system of the invention comprises a surf atron-type field applicator (4) and a surface-wave waveguide (1) in which the surface-wave waveguide (1) has a hollow body comprising a pre-discharge part (Pl' ) , a surface-wave propagation part (P2' ) , a constriction part (P3' ) and a post-discharge part (P4' ) , sequentially connected, and in which the said constriction part ( P3 ’ ) has first and second ends, the first end being connected to the surface-wave propagation part (P2' ) , the second end being connected to the post-discharge part (P4' ) and the said first end having a cross-sectional area greater than the cross-sectional area of the said second end.

The pre-discharge part (Pl' ) corresponds to the part where a mixture is introduced in a swirling flow (2) regime, as produced in step a) of the process of the invention, and the irradiation of said mixture with electromagnetic radiation emitted by a microwave plasma volume (5) generated by surface-waves (6) , according to step b) of said process. The surface-wave propagation part (P2' ) , where the said microwave plasma volume (5) is generated, has first and second ends connected, respectively, to the pre-discharge part (Pl' ) and the constriction part (P3' ) , providing fluid communication between these parts (Pl' , P3' ) and it is in this surface-wave propagation part (P2' ) that step c) of the process of the invention takes place.

The constriction part ( P3 ' ) (of higher pressure) implements a reduction in the cross-sectional area of the surface-wave waveguide (1) , producing a pressure differential (14) inside said surface-wave waveguide (1) , has first and second ends connected, respectively, to the surface-wave propagation part (P2' ) and to the postdischarge part (P4' ) (of lower pressure) , providing fluid communication between these parts (P2' , P4' ) . The constriction part (P3' ) is where step d) of the process of the invention is carried out.

The post-discharge part (P4' ) (of lower pressure) , where an aerosol is introduced in the direction of the movement of the said mixture in a swirling flow (2) regime, thus creating an aerosol volume (8) inside the said surfacewave waveguide (1) , inside and through which the flow moves according to step e) of the process of the invention, forming a nitrogen fertiliser.

As can be seen in Figs. 3 and 4, these parts (Pl' , P2' , P3' , P4' ) define, in the body of the surface-wave waveguide (1) , four respective inner operating zones (Pl, P2, P3, P4) , with the first end of the constriction part ( P3 ’ ) (of higher pressure) having a cross-sectional area that is larger than the cross-sectional area of its second end, which connects to the post-discharge part (P4' ) (of lower pressure) . Preferably, the cross-sectional area of the part (P3' ) decreases progressively from its first to its second end.

In a preferred embodiment, the aforementioned parts (Pl' , P2' , P3' , P4' ) are integrally connected to each other to form a single piece.

The said surface-wave waveguide (1) body is formed of a dielectric material selected from the group consisting of quartz, sapphire, alumina and combinations thereof.

In a preferred embodiment, as shown in Figs. 2 and 5 to 8, the nitrogen fertiliser production system of the invention further comprises at least one injector (7) , used to introduce an aerosol in the direction of movement of said mixture in a swirling flow (2) regime, inside said hollowbody surface-wave waveguide (1) , with this aerosol injector (7) being connected in fluid communication with the postdischarge part ( P 4 ’ ) , connected either integrally or by means of connectors.

In another embodiment, the system of the invention also comprises at least one gas injection device with a swirling flow (2) , placed in such a way as to introduce a mixture in a swirling flow (2) regime into the aforementioned predischarge part (Pl' ) of the surface-wave waveguide (1) .

The following is a description of the nitrogen fertiliser production process of the invention carried out by the device of the invention or, in other words, it is explained how the device described above implements the process of the present invention. In brief, the process of the invention for the ecological production of nitrogen fertilisers using plasma technology begins with the injection into a surface-wave waveguide (1) of a mixture in a swirling flow (2) regime formed by a mixture of nitrogen, oxygen and at least one tertiary component selected from the group consisting of argon, carbon dioxide, carbon monoxide, methane, hydrogen, helium, neon, atmospheric air, and water vapour. This swirling flow (2) has, as its axis of rotation, the longitudinal axis of the said surface-wave waveguide (1) and expands along the surface-wave waveguide (1) , transporting the said mixture in a swirling flow (2) regime from a predischarge zone (Pl) of said surface-wave waveguide (1) , to a surface-wave propagation zone (P2) of said surface-wave waveguide (1) and finally to a constriction zone (P3) of said surface-wave waveguide (1) .

The electric field of a surface-wave (6) launched by means of a surf atron-type field applicator (4) produces a microwave plasma volume (5) in the surface-wave propagation zone (P2) of said surface-wave waveguide (1) , thereby exposing said mixture in a swirling flow (2) regime to said microwave plasma volume (5) . Without intending to theorise, it is believed that this exposure results in the partial decomposition of the molecules that make up said mixture in a swirling flow (2) regime, with new highly reactive atomic and molecular species being produced that are different from those that originally composed the said mixture in a flow regime .

A pressure differential (14) is produced inside said surface-wave waveguide (1) , creating a constriction zone (P3) in said surface-wave waveguide (1) and a post-discharge zone (P4) of lower pressure (due to the existence of the upstream constriction) in said surface-wave waveguide (1) . The production of this pressure differential (14) is thought to better structure the swirling flow (2) between the predischarge zone (Pl) and the constriction zone (P3) inside the aforementioned surface-wave waveguide (1) , and also to increase turbulence in the lower pressure post-discharge zone (P4) of the aforementioned surface-wave waveguide (1) .

An aerosol is introduced into the post-discharge zone

(P4) of the aforementioned surface-wave waveguide (1) , in the direction of movement of the aforementioned mixture in a swirling flow (2) regime, thus creating an aerosol volume (8) within the aforementioned surface-wave waveguide (1) . It is within this aerosol volume (8) that the aforementioned highly reactive atomic and molecular species produced in the aforementioned microwave plasma volume (5) will react with the aerosol, in an extremely efficient manner, to form a nitrogen fertiliser. The aforementioned aerosol, including the nitrogen fertiliser, now becomes one of the components of the aforementioned flowing mixture.

Finally, optionally, the flowing mixture leaving the post-discharge zone (P4) of the aforementioned surface-wave waveguide (1) , which already includes the aerosol and the nitrogen fertiliser, can be introduced into a reservoir (9) containing a liquid substance (15) , when producing fertilisers in aqueous phase, or into a drying device (17) , when producing fertilisers in solid phase.

It is thought that the surprising energy efficiency shown by the present invention for the production of nitrogen fertilisers, which is much higher than that of its competitor processes using plasma technology, and equivalent to that of the Haber-Bosh process, but without its environmental costs, is due to the fact that the present invention uses a process with four unique characteristics, which give it a technological advantage. They are: al) The introduction of a swirling flow (2) mixture consisting of a mixture of nitrogen, oxygen and at least one tertiary component selected from the group comprising argon, carbon dioxide, carbon monoxide, methane, hydrogen, helium, neon, atmospheric air, and water vapour, which expands along the said surface-wave waveguide (1) . a2) The production of a microwave plasma volume (5) out-of-equilibrium generated by the electric field of a surface-wave (6) under strong rotation conditions, i.e. swirling flow (2) , capable of harnessing the energy of the electrons generated in the plasma to create new highly reactive atomic and molecular species with great energy efficiency. The use of strong rotation conditions seems to help to reduce losses in the wall of the surface-wave waveguide (1) , thus increasing the overall efficiency of the process. a3) The production of a pressure differential (14) inside the said surface-wave waveguide (1) , which creates a pressure constriction zone (P3) greater than that which occurs in a post-discharge zone (P4) downstream of the zone (P3) with respect to the direction of the flow, in the said surfacewave waveguide (1) . This feature is thought to better structure the swirling flow (2) between the pre-discharge zone (Pl) and the constriction zone (P3) within the aforementioned surface-wave waveguide ( 1 ) . a4) The production of an aerosol volume (8) within which the aforementioned highly reactive atomic and molecular species produced in the plasma will react with the aerosol in an extremely efficient way to form a nitrogen fertiliser.

As seen above, these four characteristics are noted in four different areas of operation of the surface-wave waveguide (1) •

For the sake of clarity, it should be noted that the pre-discharge zone (Pl) of the surface-wave waveguide (1) is characterised by the absence of plasma; it is the zone where the aforementioned swirling flow (2) is formed and is the highest pressure zone of the aforementioned surface-wave waveguide (1) . The surface-wave propagation zone (P2) of the surface-wave waveguide (1) is characterised by being the zone where a microwave plasma volume (5) is generated under strong rotation conditions; the pressure in this zone is slightly lower than in the pre-discharge zone (Pl) and higher than in the constriction zone (P3) . The constriction zone (P3) of the surface-wave waveguide (1) is characterised by having no plasma, because the energy carried by the surfacewave (6) no longer has sufficient intensity to continue producing plasma, the surface-wave (6) thus becomes evanescent and disappears. The energy of the surface-wave (6) is completely absorbed in the microwave plasma volume (5) . The lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) is also characterised by having no plasma; it is the zone where an aerosol is introduced in the direction of movement of the aforementioned mixture in a swirling flow (2) regime creating an aerosol volume (8) .

Examples

The following are several examples of the ecological production of nitrogen fertilisers using plasma technology applying the process according to the present invention.

The examples described below should not be interpreted as limiting the scope of the present invention, which is defined in the independent claims.

Example 1

For the production of nitrogen fertilisers in aqueous phase containing nitrogen oxides in several of their forms, namely nitrates and ammoniacal nitrogen, with a nitrate production rate of more than 3.67 grams/hour and ammoniacal nitrogen production rate of more than 0.25 grams/hour, a surface-wave waveguide (1) consisting of a quartz tube with an internal radius of 7.5 mm is used, which simultaneously functions as a reactor, see figure 2.

In this example, the concentration of nitrates and ammoniacal nitrogen in water can be adjusted as required, keeping the ratio between their concentrations constant. The maximum concentration for nitrates is 0.712 gram/litre and for ammoniacal nitrogen 0.042 gram/litre .

First, a flow is produced consisting of a mixture with a total flow rate of 3.33*10~⁵ m³/s consisting of molecular nitrogen, molecular oxygen, and molecular hydrogen with incorporation rates in the mixture of 2.6xl0~⁵ m³/s, 0.7xl0~ ⁵ m³/s and 0.03xl0~⁵ m³/s respectively. These flow rates are monitored using a controller coupled to three flow meters.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 2 and 3, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) , whose axis of rotation is the axis of the tube, can be easily produced by injecting this mixture tangentially to the inner wall of the quartz tube in a flow regime through a small orifice with a diameter of 0.6 mm. Any other swirling gas injection unit capable of performing this function can be optionally used.

This swirling flow (2) expands along the surface-wave waveguide (1) carrying the said mixture in a flow regime, which, in the pre-discharge zone (Pl) of the surface-wave waveguide (1) , is initially irradiated with electromagnetic radiation emitted by a microwave plasma volume (5) and then, on passing through the surface-wave propagation zone (P2) , is exposed to a microwave plasma volume (5) where the molecules forming the said flow regime mixture are partially decomposed into atoms.

This microwave plasma volume (5) is generated by a surface-wave (6) produced by a surf atron-type field applicator (4) coupled to a surface-wave waveguide system that includes a tuner, directional couplers and an isolator. The system ends with an adjustable component that short- circuits the microwave.

This surf atron-type field applicator (4) is configured in such a way that the surface-wave (6) is launched in the direction of movement of this mixture in a swirling flow (2) regime, see figure 2.

This microwave plasma volume (5) is produced at atmospheric pressure with 300 W of power produced by a microwave generator (3) operating at a frequency of 2.45 GHz.

It is in this microwave plasma volume (5) where highly reactive atomic species are produced, such as the atomic nitrogen radical (N) , atomic oxygen (0) and atomic hydrogen (H) , that various types of nitrogen oxides (N0_x) are formed, mainly the nitric oxide radical (NO) , and also a significant amount of nitrous acid (HNO2) .

The surface-wave waveguide (1) has a small constriction about 2 cm from the end of the microwave plasma volume (5) that locally reduces the internal radius of the quartz tube to about one third, producing a small pressure differential (14) inside the surface-wave waveguide (1) , which creates a higher pressure post-discharge zone (P3) and a lower pressure post-discharge zone (P4) in the surface-wave waveguide (1) , see figure 2.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , with its chemical composition already altered, which now includes new chemical species formed in the plasma, including various radicals.

The higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) is characterised by having no plasma, because the energy carried by the surface-wave (6) no longer has enough energy to continue producing plasma, the surface-wave (6) therefore becomes evanescent and disappears. The same happens in the lower pressure postdischarge zone (P4) of the surface-wave waveguide (1) .

It is in this lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) that various highly reactive species produced in the plasma will react with an aerosol made up of micro-droplets of water.

The aerosol is introduced into the lower pressure postdischarge zone (P4) of the surface-wave waveguide (1) , about 4 cm from the end of the microwave plasma volume (5) and 2 cm from the pressure differential (14) , along the axis of the surface-wave waveguide (1) , and in the direction of movement of the aforementioned mixture in flow regime, with a mass flow rate of 1.44 grams per second, after which point it becomes one of the parts of the aforementioned mixture in flow regime. In this example, the aerosol is introduced using the injector (7) which creates the aerosol volume (8) inside of which the highly reactive species produced in the plasma will react. The aerosol flow rate is monitored using a controller coupled to a mass flow meter.

Surprisingly, the various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (NOx) , nitrous acid (HNO2) and others, and even vibrationally excited molecular nitrogen, among many others, react with the aerosol extremely efficiently to form an aqueous nitrogen fertiliser consisting mainly of nitrates and ammoniacal nitrogen.

The microdroplets of water already containing nitrates and ammoniacal nitrogen gradually combine throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid along the way.

Finally, the flowing mixture, which already includes the aerosol containing the nitrates, ammoniacal nitrogen and the fraction of gases that have not been broken down by the plasma, is injected into a large-capacity reservoir (9) via an inlet A (10) .

The final concentrations of the fertiliser containing nitrates and ammoniacal nitrogen, which at inlet A (10) of the large-capacity reservoir (9) are 0.712 gram/litre for nitrates and 0.042 gram/litre for ammoniacal nitrogen, can be adjusted by admitting a liquid substance (15) through inlet B (13) , which in this example is water.

The reservoir, with a total capacity of 1000 litres, also has an outlet (11) for gases that have not been broken down by the plasma, and an outlet (12) for the nitrogen fertiliser diluted in the reservoir water, in this case consisting of nitrates and ammoniacal nitrogen. The flow rates of the inlet B (13) of a liquid substance (15) and the outlet (12) of nitrogen fertiliser diluted in a liquid substance (15) are monitored by means of a controller coupled to two flow meters. Any other reservoir capable of performing this function can be used instead.

Example 2

For the production of calcium nitrate Ca(NOs)2 in aqueous phase, with a production rate of more than 5.9 grams/hour, the same set-up as in example 1 is used. In this example, the concentration of calcium nitrate Ca(NOs)2 in water can be adjusted as required, with the highest value being 0.5 grams of Ca(NOs)2 per litre of water

(0.5 grams/litre) .

First, a flow formed by a mixture with a total flow rate of 3.33xl0~⁵ m³/s is produced, consisting of molecular nitrogen, molecular oxygen, and molecular hydrogen with incorporation rates in the mixture of 2.6*10~⁵ m³/s, 0.7*10~ ⁵ m³/s and 0.03xl0~⁵ m³/s, respectively.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , this mixture is introduced in the form of a swirling flow (2) , as described in example 1. This flowing mixture passes through the surface-wave propagation zone (P2 ) and is exposed to a microwave plasma volume (5) .

This microwave plasma volume (5) is generated at atmospheric pressure, as described in example 1, with 300 W of power produced by a microwave generator (3) and a frequency of 2.45 GHz.

The surface-wave waveguide (1) has a small constriction about 2 cm from the end of the microwave plasma volume (5) , which locally reduces the inner radius of the quartz tube to about a third, and has the same function as described in example 1. The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , with its chemical composition already altered, which now includes new chemical species formed in the plasma, including various radicals.

The higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) is characterised by having no plasma, and the same applies to the lower pressure postdischarge zone (P4) of the surface-wave waveguide (1) . It is in this lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) that various highly reactive species produced in the plasma will react with an aerosol made up of microdroplets of water and calcium hydroxide Ca(0H)2, diluted to a concentration of 0.45 grams/litre. This aerosol is introduced using the injector (7) , as described in example 1, with a mass flow rate of 3.4 grams per second.

The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (N0_x) , nitrous acid (HNO2) and even vibrationally excited molecular nitrogen, among many others, react with the aerosol extremely efficiently to form an aqueous nitrogen fertiliser consisting mainly of calcium nitrate Ca(NOs)2 with some traces of nitrates and ammoniacal nitrogen.

The microdroplets of aerosol already containing the calcium nitrate Ca(NOs)2 gradually aggregate throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid along the way.

Finally, the flowing mixture, which already includes the aerosol containing the calcium nitrate Ca(NOs)2 and the fraction of gases that have not been decomposed by the plasma, is injected, as described in example 1, into a large- capacity reservoir (9) .

The final concentration of the calcium nitrate fertiliser Ca(NOs)2, which is 0.5 grams/litre at inlet A (10) of the large-capacity reservoir (9) , can be adjusted by admitting a liquid substance (15) through inlet B (13) , which in this example is water.

The nitrogen fertiliser diluted in the reservoir water, in this case calcium nitrate Ca(NOs)2, can be collected through the outlet (12) .

Example 3

For the production of nitrogen fertilisers in aqueous phase containing nitrogen oxides in various forms, particularly in the predominant form of nitrates, with a production rate of more than 13 grams/hour, the same set-up as in example 1 is used.

In this example, the concentration of nitrates in the water can be adjusted as required. The maximum nitrate concentration is 0.938 grams/litre.

First, a flow is produced consisting of a mixture with a total flow rate of 5.0*10~⁵ m³/s made up of 50% molecular nitrogen and 50% molecular oxygen, with incorporation rates in the mixture of 2.5*10~⁵ m³/s and 2.5*10~⁵ m³/s, respectively. This mixture can easily be obtained using an oxygen concentrator, for example, or some other unit capable of performing this function. Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 2 and 3, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) can be easily produced by injecting this mixture tangentially to the inner wall of the quartz tube through a 0.8 mm diameter orifice.

This flowing mixture is exposed to a microwave plasma volume (5) as it passes through the surface-wave propagation zone ( P2 ) .

This microwave plasma volume (5) is generated at atmospheric pressure, as described in example 1, with 400 W of power produced by a microwave generator (3) and a frequency of 2.45 GHz.

It is in this microwave plasma volume (5) where highly reactive atomic species are produced, such as the atomic nitrogen radical (N) and atomic oxygen (0) , that various types of nitrogen oxides (NO_X) are formed, mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a small constriction about 2 cm from the end of the microwave plasma volume (5) , which locally reduces the inner radius of the quartz tube to about a third, and has the same function as described in example 1.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , with its chemical composition already altered, which now includes new chemical species formed in the plasma. An aerosol consisting of microdroplets of water is then introduced into a lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) , as described in example 1, with a mass flow rate of 3.85 grams per second. The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (N0_x) and even vibrationally excited molecular nitrogen, among many others, react in this zone with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of nitrates.

The microdroplets of water already containing the nitrates combine throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid along the way .

Finally, the flowing mixture, which already includes the aerosol containing the nitrates and the fraction of gases that have not been decomposed by the plasma, is injected into the large-capacity reservoir (9) described in example 1.

The final concentration of the nitrate-containing fertiliser, which at inlet A (10) of the large-capacity reservoir (9) is 0.938 gram/litre, can be adjusted by admitting a liquid substance (15) through inlet B, which in this example consists of water.

Any other reservoir capable of performing the function of the large-capacity reservoir (9) described in example 1 can be used instead. Example 4

A surface-wave waveguide (1) consisting of a quartz tube with an internal radius of 20.0 mm in the shape of an inverted U, which simultaneously functions as a reactor, is used to produce nitrogen fertilisers in the aqueous phase containing nitrogen oxides in various forms, particularly in the predominant form of nitrates and ammoniacal nitrogen, with a nitrate production rate of more than 10.2 grams/hour and ammoniacal nitrogen production rate of more than

0.7 grams/hour.

First, a flow formed by a mixture with a total flow rate of 8.33xl0~⁵ m³/s is produced, consisting of molecular nitrogen, molecular oxygen and molecular hydrogen with incorporation rates in the mixture of 6.5*1CF⁵ m³/s, 1.75*1CF ⁵ m³/s and 0.08xl0~⁵ m³/s, respectively.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 4 and 5, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) , whose axis of rotation is the axis of the tube, can be easily produced by injecting this mixture tangentially to the inner wall of the quartz tube through a 1.0 mm diameter orifice.

The aforementioned swirling flow (2) expands along the surface-wave waveguide (1) carrying the said mixture in a flow regime, which in the pre-discharge zone (Pl) of the surface-wave waveguide (1) is initially irradiated with electromagnetic radiation emitted by a microwave plasma volume (5) and then, when passing through the surface-wave propagation zone (P2) , is exposed to a microwave plasma volume (5) . This microwave plasma volume (5) is generated by a surface-wave (6) produced by a surf atron-type field applicator (4) coupled to a surface-wave waveguide system, as described in example 1.

This surf atron-type field applicator (4) is configured in such a way that the surface-wave (6) is launched in the direction of movement of this mixture in a swirling flow (2) regime, see figure 5.

This microwave plasma volume (5) is produced at atmospheric pressure with 600 W of power produced by a microwave generator (3) operating at a frequency of 2.45 GHz.

It is in this microwave plasma volume (5) where various highly reactive atomic species are produced, see example 1, that various types of nitrogen oxides (N0_x) are formed, mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a small constriction about 8 cm from the end of the microwave plasma volume (5) which locally reduces the internal radius of the quartz tube to about one fifth, producing a small pressure differential (14) inside the surface-wave waveguide (1) which creates a higher pressure post-discharge zone (P3) and a lower pressure post-discharge zone (P4) in the surface-wave waveguide (1) , see figure 5.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , already with its chemical composition altered, which now includes new chemical species formed in the plasma, including various radicals. An aerosol consisting of microdroplets of water is then introduced into a lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) . The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (N0_x) , nitrous acid (HNO2) and others, and vibrationally excited molecular nitrogen, among many others, react in this zone with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of nitrates and ammoniacal nitrogen.

The aerosol is introduced into the lower pressure postdischarge zone (P4) of the surface-wave waveguide (1) , approximately 12 cm from the end of the microwave plasma volume (5) and 4 cm from the pressure differential (14) , on the axis of the surface-wave waveguide (1) , and in the direction of movement of the aforementioned flow mixture, with a mass flow rate of 4 grams per second, after which point it becomes one of the parts of the aforementioned flow mixture .

The microdroplets of water already containing nitrates and ammoniacal nitrogen gradually combine throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid.

Finally, the flowing mixture, which already includes the aerosol containing the nitrates, ammoniacal nitrogen and the fraction of gases that have not been broken down by the plasma, is injected into a large-capacity reservoir (9) via an inlet A (10) . The final concentrations of the fertiliser containing nitrates and ammoniacal nitrogen, which at inlet A (10) of the large-capacity reservoir (9) are 0.712 gram/litre for nitrates and 0.042 gram/litre for ammoniacal nitrogen, can be adjusted by admitting a liquid substance (15) through inlet B (13) , which in this example is water.

The reservoir, with a total capacity of 1000 litres, also has an outlet (11) for gases that have not been broken down by the plasma and an outlet (12) for nitrogen fertiliser diluted in the reservoir water, in this case consisting of nitrates and ammoniacal nitrogen. The flow rates of the inlet B (13) of a liquid substance (15) and the outlet (12) of nitrogen fertiliser diluted in a liquid substance (15) are monitored by means of a controller coupled to two flow meters. Any other reservoir capable of performing this function can be used instead.

Example 5

For the production of nitrogen fertilisers in the aqueous phase containing nitrogen oxides in various forms, particularly in the predominant form of nitrates, with a production rate of over 47 grams/hour, a surface-wave waveguide (1) is used, consisting of a quartz tube with an internal radius of 20.0 mm, which simultaneously functions as a reactor, see figure 6.

In this example, the concentration of nitrates in the water can be adjusted as required. Nitrate concentrations of over 1 gram/litre can be achieved through recirculation.

First, a flow formed by a mixture with a total flow rate of 1.66xl0~⁴ m³/s made up of 50% molecular nitrogen and 50% molecular oxygen, with incorporation rates in the mixture of 8.3x10 ⁵ m³/s and 8.3x10 ⁵ m³/s, respectively, is produced.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 3 and 6, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) , whose axis of rotation is the axis of the tube, can be easily produced by injecting the mixture tangentially to the inner wall of the quartz tube through a 1.3 mm diameter hole.

The aforementioned swirling flow (2) expands along the surface-wave waveguide (1) carrying the said mixture in a flow regime, which in the pre-discharge zone (Pl) of the surface-wave waveguide (1) is initially irradiated with electromagnetic radiation emitted by a microwave plasma volume (5) and then, when passing through the surface-wave propagation zone (P2) , is exposed to a microwave plasma volume (5) .

This microwave plasma volume (5) is generated by a surface-wave (6) produced by a surf atron-type field applicator (4) coupled to a surface-wave waveguide system, described in example 1.

This surf atron-type field applicator (4) is configured in such a way that the surface-wave (6) is launched in the direction of movement of this mixture in a swirling flow (2) regime, see figure 6.

This microwave plasma volume (5) is produced at atmospheric pressure with 1300 W of power produced by a microwave generator (3) operating at a frequency of 2.45 GHz. It is in this microwave plasma volume (5) where highly reactive atomic species are produced, such as the atomic nitrogen radical (N) and atomic oxygen (0) , that various types of nitrogen oxides (N0_x) are formed, mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a small constriction about 4 cm from the end of the microwave plasma volume (5) which locally reduces the internal radius of the quartz tube to about one fifth, producing a small pressure differential (14) inside the surface-wave waveguide (1) which creates a higher pressure post-discharge zone (P3) and a lower pressure post-discharge zone (P4) in the surface-wave waveguide (1) , see figure 6.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , with its chemical composition already altered, which now includes new chemical species formed in the plasma.

An aerosol is then introduced into the lower pressure post-discharge area (P4) of the surface-wave waveguide (1) , approximately 6 cm from the end of the microwave plasma volume (5) and 2 cm from the pressure differential (14) , on the axis of the surface-wave waveguide (1) , and in the direction of movement of the aforementioned flow mixture, with a mass flow rate of 14 grams per second, from which point it becomes one of the parts making up the aforementioned flow mixture.

In this example, the aforementioned aerosol is introduced using the injector (7) which creates the aerosol volume (8) inside of which the aforementioned highly reactive species produced in the plasma will react. The injector (7) is fed by the water pump (16) which is connected to a large capacity reservoir (9) , in this case 1000 litres. It contains a liquid substance (15) consisting mainly of water and the nitrate fertiliser produced in the process. This is because the liquid substance (15) is constantly being reinjected into the system via the injector (7) .

The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (NO_X) and even vibrationally excited molecular nitrogen, among many others, react with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of nitrates.

The microdroplets of water already containing the nitrates gradually combine throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid .

Finally, the flowing mixture, which already includes the aerosol containing the nitrates and the fraction of gases that have not been decomposed by the plasma, is injected into a large capacity reservoir (9) via an inlet A (10) .

The final concentration of the nitrate-containing fertiliser can be adjusted by recirculating the mixture in a flow regime that already includes the nitrate-containing aerosol and by admitting a liquid substance (15) through inlet B (13) , in this case consisting of water.

The reservoir also has an outlet (11) for gases that have not been decomposed by the plasma and an outlet (12) for nitrate fertiliser diluted in the reservoir water. The flow rates of the inlet B (13) of a liquid substance (15) and the outlet (12) of nitrogen fertiliser diluted in a liquid substance (15) are monitored using a controller coupled to two flow meters.

Example 6

For the production of calcium nitrate Ca(NOs)2 in aqueous phase, with a production rate of more than 75.5 grams/hour, the same set-up as in example 5 is used. In this example, the concentration of calcium nitrate Ca(NOs)2 in water can be adjusted as required. Through recirculation, it is possible to obtain calcium nitrate Ca(NOs)2 concentrations of more than 0.5 grams/litre.

First, a flow formed by a mixture with a total flow rate of 1.66xl0~⁴ m³/s made up of 50% molecular nitrogen and 50% molecular oxygen, with incorporation rates in the mixture of 8.3*10~⁵ m³/s and 8.3*10~⁵ m³/s, respectively, is produced.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , this mixture is introduced in the form of a swirling flow (2) , as described in example 5.

When passing through the surface-wave propagation zone (P2) , this flowing mixture is exposed to a microwave plasma volume (5) . This microwave plasma volume (5) is generated at atmospheric pressure by a surface-wave (6) produced by means of a field applicator (4) , as described in example 5. The said microwave plasma volume (5) is produced with 1300 W of power produced by a microwave generator (3) and a frequency of 2.45 GHz.

It is in this microwave plasma volume (5) where highly reactive atomic species are produced, such as the atomic nitrogen radical (N) and atomic oxygen (0) , that various types of nitrogen oxides (N0_x) are formed, mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a small constriction about 4 cm from the end of the microwave plasma volume (5) , which locally reduces the inner radius of the quartz tube to about one fifth, and has the same function as described in example 5.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , already with its chemical composition altered, which now includes new chemical species formed in the plasma.

An aerosol consisting of microdroplets of water and calcium hydroxide Ca(0H)2 diluted with a concentration of 0.45 grams/litre is then introduced into the lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) . The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (N0_x) and even vibrationally excited molecular nitrogen, among many others, react in this area with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of calcium nitrate Ca(NOs)2 with some traces of nitrates and ammoniacal nitrogen. This aerosol is introduced using the injector (7) , as described in example 5, with a mass flow rate of 44 grams per second.

The injector (7) is fed by the water pump (16) which is connected to a large-capacity reservoir (9) containing a liquid substance (15) consisting mainly of water, calcium hydroxide Ca(0H)2 and calcium nitrate fertiliser Ca(N0s)2 which is being produced in the process. This is because the liquid substance (15) is constantly being reinjected into the system via the injector (7) .

The microdroplets of aerosol already containing the calcium nitrate Ca(NOs)2 gradually combine throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid.

Finally, the flowing mixture, which already includes the aerosol containing the calcium nitrate Ca(NOs)2 and the fraction of gases that have not been decomposed by the plasma, is injected through an inlet A (10) into a large- capacity reservoir (9) as described in example 5.

The final concentration of the calcium nitrate fertiliser Ca(NOs)2 can be adjusted by recirculating the mixture in a flow regime that already includes the aerosol containing the nitrogen fertiliser and by admitting a liquid substance (15) , in this case consisting of water, via inlet B (13) .

The reservoir has an outlet (12) for the calcium nitrate Ca(NOs)2 diluted in the reservoir water, and the flow rate is controlled as described in example 5.

Example 7

A surface-wave waveguide (1) consisting of a quartz tube with an internal radius of 20.0 mm in the shape of an inverted U, which simultaneously functions as a reactor, is used to produce nitrogen fertilisers in the aqueous phase containing nitrogen oxides in various forms, particularly nitrates and ammoniacal nitrogen, with a nitrate production rate of more than 47 grams/hour and ammoniacal nitrogen production rate of more than 3 grams/hour, see figure 7.

In this example, the concentration of nitrates and ammoniacal nitrogen in the water can be adjusted as required. Through recirculation, nitrate concentrations of more than 0.7 grams/litre and ammoniacal nitrogen concentrations of more than 0.04 grams/litre can be achieved.

First, a flow consisting of humid air with 2% water vapour is produced with a total flow rate of 4.0xl0~⁴ m³/s consisting of a mixture of molecular nitrogen, molecular oxygen, water vapour and a small percentage of other gases.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 4 and 7, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) , whose axis of rotation is the axis of the tube, can be easily produced by injecting this mixture tangentially to the inner wall of the quartz tube through a 1.5 mm diameter hole.

This swirling flow (2) expands along the surface-wave waveguide (1) , transporting the said mixture in a flow regime, which when passing through the surface-wave propagation zone (P2) is exposed to a microwave plasma volume (5) .

This microwave plasma volume (5) is generated by a surface-wave (6) produced by a surf atron-type field applicator (4) coupled to a surface-wave waveguide system described in example 1. This microwave plasma volume (5) is produced at atmospheric pressure with 1500 W of power produced by a microwave generator (3) operating at a frequency of 2.45 GHz.

It is in this microwave plasma volume (5) where highly reactive atomic species are produced, see example 1, that various types of nitrogen oxides (NO_X) are formed, mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a constriction about 8 cm long about 4 cm from the end of the microwave plasma volume (5) which locally reduces the internal radius of the quartz tube to about one fifth, producing a small pressure differential (14) inside the surface-wave waveguide (1) which creates a higher pressure post-discharge zone (P3) and a lower pressure post-discharge zone (P4) in the surfacewave waveguide (1) , see figure 7.

The flowing mixture then enters the higher pressure post-discharge zone (P3) of the surface-wave waveguide (1) , with its chemical composition already altered.

An aerosol consisting of microdroplets of water is then introduced into a lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) . The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (NO_X) , nitrous acid (HNO2) among others and vibrationally excited molecular nitrogen, among many others, react in this zone with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of nitrates and ammoniacal nitrogen. The aerosol is introduced into the lower pressure postdischarge zone (P4) of the surface-wave waveguide (1) , approximately 14 cm from the end of the microwave plasma volume (5) and 2 cm from the end of the pressure differential (14) , on the axis of the surface-wave waveguide (1) , and in the direction of movement of the aforementioned flowing mixture, with a mass flow rate of 18.7 grams per second, from which point it becomes one of the parts that make up the aforementioned flowing mixture. In this example, the aerosol is introduced using the injector (7) , which creates the aerosol volume (8) inside of which the highly reactive species produced in the plasma will react. The injector (7) is fed by the water pump (16) which is connected to a large capacity reservoir (9) , in this case 1000 litres. It contains a liquid substance (15) consisting mainly of water and the fertiliser consisting of nitrates and ammoniacal nitrogen, which are produced in the process, this is due to the fact that said liquid substance (15) is constantly being reinjected into the system via the injector (7) .

The microdroplets of water already containing nitrates and ammoniacal nitrogen gradually aggregate throughout the aerosol volume (8) , forming larger and larger droplets that turn into a liquid.

Finally, the flowing mixture, which already includes the aerosol containing the nitrates, ammoniacal nitrogen and the fraction of gases that have not been broken down by the plasma, is injected through an inlet A (10) into a large- capacity reservoir (9) like the one described in example 5.

The final concentration of the fertiliser containing nitrates and ammoniacal nitrogen can be adjusted by recirculating the mixture in a flow regime that already includes the nitrate-containing aerosol and by admitting a liquid substance (15) , in this case consisting of water, via inlet B ( 13 ) .

The reservoir has an outlet (12) for the nitrate fertiliser diluted in the reservoir water, and the flow rate is controlled as described in example 5.

Example 8

For the production of calcium nitrate Ca(NOs)2 in solid phase, with a production rate of more than 16.52 grams/hour, a surface-wave waveguide (1) formed by a quartz tube with an internal radius of 20.0 mm, in the shape of an inverted U, which simultaneously functions as a reactor, is used.

Firstly, a flow formed by a mixture with a total flow rate of 8.3xl0~⁵ m³/s is produced, consisting of molecular nitrogen, molecular oxygen and molecular hydrogen with incorporation rates in the mixture of 6.5*10~⁵ m³/s, 1.75*10~ ⁵ m³/s and 8.0xl0~⁷ m³/s respectively.

Then, in a pre-discharge zone (Pl) of the surface-wave waveguide (1) , see figures 4 and 8, this mixture is introduced in the form of a swirling flow (2) . The swirling flow (2) can be easily produced by injecting this mixture tangentially to the inner wall of the quartz tube through a 1.0 mm diameter orifice.

This swirling flow (2) expands along the surface-wave waveguide (1) transporting the said mixture in a flow regime, which when passing through the surface-wave propagation zone (P2) is exposed to a microwave plasma volume (5) . This microwave plasma volume (5) is generated by a surface-wave (6) , at atmospheric pressure, with a power of 600 W and a frequency of 2.45 GHz. It is in this microwave plasma volume (5) where highly reactive atomic species are produced, see example 1, that various types of nitrogen oxides are formed (NO_X) , mainly the nitric oxide radical (NO) .

The surface-wave waveguide (1) has a constriction about 8 cm long, about 4 cm from the end of the microwave plasma volume (5) which locally reduces the internal radius of the quartz tube to about one fifth, thus producing a small pressure differential (14) inside the surface-wave waveguide ( 1 ) , see figure 8.

An aerosol consisting of microdroplets of water and calcium hydroxide Ca(OH)2 diluted with a concentration of 0.45 grams/litre is then introduced into a lower pressure post-discharge zone (P4) of the surface-wave waveguide (1) . The various highly reactive species containing nitrogen in its atomic form produced in the plasma, i.e. atomic nitrogen (N) , various types of nitrogen oxides (NO_X) , nitrous acid (HNO2) and others, and also vibrationally excited molecular nitrogen, among many others, react in this zone with the aerosol to form an aqueous nitrogen fertiliser consisting mainly of calcium nitrate Ca(NOs)2 with some traces of nitrates and ammoniacal nitrogen.

The aerosol is introduced into the lower pressure postdischarge area (P4) of the surface-wave waveguide (1) , approximately 14 cm from the end of the microwave plasma volume (5) and 2 cm from the end of the pressure differential (14) , on the axis of the surface-wave waveguide (1) , and in the direction of movement of the aforementioned mixture in the flow regime, with a mass flow rate of 9.6 grams per second, after which point it becomes one of the parts of the aforementioned mixture in the flow regime. In this example, the aerosol is introduced using the injector (7) which creates the aerosol volume (8) inside of which the highly reactive species produced in the plasma will react.

The microdroplets of water already containing the calcium nitrate Ca(NOs)2 combine over the aerosol volume (8) to form larger and larger droplets that turn into a liquid.

Finally, the flowing mixture, which already includes the aerosol containing the calcium nitrate Ca(NOs)2 and the fraction of gases that have not been decomposed by the plasma, is injected into a drying device (17) via an inlet A (10) . The liquid component of this flowing mixture is evaporated in the drying device (17) , leaving the calcium nitrate fertiliser Ca(NOs)2 in solid phase. Any other drying system capable of performing this function can be used instead .

Claims

CLAIMS Nitrogen fertiliser production process, characterised by comprising the following steps: a) producing a swirling flow (2) , this flow being formed by a mixture of nitrogen, oxygen and at least one tertiary component selected from the group consisting of argon, carbon dioxide, carbon monoxide, methane, hydrogen, helium, neon, atmospheric air, and water vapour; b) irradiating the aforementioned flow from step a) with electromagnetic radiation emitted by a microwave plasma volume (5) produced by surface-waves (6) ; c) passing said flow from step b) inside the microwave plasma volume (5) , moving it from an inlet to an outlet of said microwave plasma volume (5) ; d) subjecting the flow from step c) to a pressure drop at the exit of the microwave plasma volume (5) ; e) moving the flow from step d) through an aerosol volume ( 8 ) ; f) collecting the fertiliser formed in step e) . Nitrogen fertiliser production process according to claim 1, characterised in that it further comprises a reinjection step of an aqueous liquid substance (15) in step e) . Nitrogen fertiliser production process according to claim 1, characterised in that, in step a) , said flow is produced at a flow rate between 8.3xl0~⁶ and 3.3*10- ³ m³/s, preferably between 1.6xl0~⁵ and 1.6xl0~³ m³/s, more preferably between 3.3xl0~⁵ and 8.3xl0~⁴ m³/s . Nitrogen fertiliser production process according to claim 1, characterised in that, in step a) , said swirling flow (2) is produced with a centrifugal acceleration higher than 4xl0⁴ m/s². Nitrogen fertiliser production process according to claim 1, characterised in that, in step b) , said surface-waves (6) have a frequency between 13.65 MHz and 28 GHz, preferably between 300 MHz and 5.8 GHz, more preferably between 300 MHz and 2.45 GHz. Nitrogen fertiliser production process according to claim 1, characterised in that, in step b) , said microwave plasma volume (5) has an energy density of between 0.5 and 200 MW/m³, preferably between 1 and 100 MW/m³, more preferably between 2 and 50 MW/m³. Nitrogen fertiliser production process according to claim 1, characterised in that, in step e) , said aerosol volume (8) has a mass flow rate of between 8.0xl0~² and 3.2X1G¹ gram/s, preferably between 1.6xl0^-1 and 1.6X1G¹ gram/s, more preferably between 3.2xl0^-1 and 1. OxlO¹ gram/ s . Nitrogen fertiliser production process according to claim 1, characterised in that, in step e) , said aerosol volume (8) comprises water selected from the group consisting of distilled water, tap water, alkaline water, brackish water, filtered water from rivers, lakes, dams, cisterns, wells, and combinations thereof. Nitrogen fertiliser production process according to claim 1, characterised in that, in step e) , said aerosol volume (8) further comprises an alkaline substance selected from the group consisting of potassium phosphate, calcium oxide, and their combination. Nitrogen fertiliser production system, comprising a surf atron-type field applicator (4) and a surface-wave waveguide (1) , in which the surface-wave waveguide (1) has a hollow body comprising a pre-discharge part (Pl' ) , a surface-wave propagation part (P2' ) , a constriction part (P3' ) and a post-discharge part (P4' ) , sequentially connected, and in which the said constriction part ( P3 ’ ) has first and second ends, the first end being connected to the surface-wave propagation part (P2' ) , the second end being connected to the post-discharge part (P4' ) and the said first end having a cross-sectional area greater than the cross-sectional area of the said second end. Nitrogen fertiliser production system according to claim 10, characterised in that said cross-sectional area of the constriction part ( P3 ’ ) progressively decreases from the first to the second end of the constriction part (P3' ) . Nitrogen fertiliser production system according to claim 10 or 11, characterised in that said parts (Pl' , P2' , P3' , P4' ) are integrally connected to each other, forming a single piece. Nitrogen fertiliser production system according to any one of claims 10 to 12, characterised in that said hollow body of the surface-wave waveguide (1) is formed of a dielectric material selected from the group consisting of quartz, sapphire, alumina and combinations thereof. Nitrogen fertiliser production system according to any one of claims 10 to 13, characterised in that it further comprises a swirling gas injection device. Nitrogen fertiliser production system according to any one of claims 10 to 14, characterised in that it further comprises an aerosol injector (7) connected in fluid communication to the post-discharge part (P4' ) .