WO2020240549A1

WO2020240549A1 - System and method for catalytic oxidation and wet-scrubbing of simultaneously both nox and sox from a flue gas in ship engines

Info

Publication number: WO2020240549A1
Application number: PCT/IL2020/050577
Authority: WO
Inventors: Haim Cohen; Tomer ZIDKI; Gad A. PINHASI; Alon KHABRA; Haim Sternfine; Mordechai HARKABI
Original assignee: Ariel Scientific Innovations Ltd.
Priority date: 2019-05-30
Filing date: 2020-05-27
Publication date: 2020-12-03

Abstract

The present invention solves the exisiting problem of using very expensive oxidation reagents, such as H₂O₂ and O₃, at temperatures higher than 300°C, in removal of NO_x and SO_x from flue gases in ship engines, by performing simultaneous oxidation of NO_x and SO_x with atmospheric oxygen in the presence of an oxidation catalyst. Because oxygen-enriched air may cause technical issues on ships, activation of the oxidation catalyst at relatively low temperatures of about 50-90°C using small amounts of activation reagents may become necessary. Two major configurations of the oxidation system are disclosed in the present invention. The first configuration operates on oxygen-enriched air to increase efficiency of the oxidation reaction and requires an additional oxygen concentrator unit. The second configuration operates on atmospheric air at ambient conditions and requires an additional catalyst activation unit. In the second configuration, the efficient oxidation process is carried out at low temperatures of 50-90°C in the presence of recovered and re-activated catalyst. This temperature is a result of the exothermic character of the reaction, and therefore, no heating is required in the process.

Description

SYSTEM AND METHOD FOR CATALYTIC OXIDATION AND WET-SCRUBBING OF SIMULTANEOUSLY BOTH NOX AND SOX FROM A FLUE GAS IN SHIP ENGINES

TECHNICAL FIELD

[0001] The present invention relates to a method and system for combined catalytic oxidation of nitrogen oxides (NO_x) and sulphur oxides (SO_x) in flue gases emitted from ship engines. In particular, the present invention relates to use of oxygen as the oxidation reagent in the combined catalytic oxidation and removal process of nitrogen oxides (NO_x) and sulphur oxides (SO_x) from flue gases in ship engines.

BACKGROUND

[0002] Air pollution from ships is on the rise, and global emission standards are getting stringent every year. The International Convention for the Prevention of Pollution from Ships (MARPOL), Annex VI, limits the main air pollutants contained in ships exhaust gas, including sulphur oxides (SO_x) and nitrous oxides (NO_x). The main changes to this convention, highlighted in Annex VI, are a progressive reduction globally in emissions of both SO_x and NO_x and particulate matter and the introduction of particular emission control areas (ECAs) to reduce emissions of those air pollutants further in designated sea areas.

[0003] In general, nitrogen oxides (NO_x) and sulphur oxides (SO_x) are air pollutants emitted in large quantities from nitrogen- and sulphur-contaminated fuel engines, such as ship diesel engines. Two of the most common nitrogen oxides are nitric oxide (NO) and nitrogen dioxide (NO₂). Nitric oxide (NO) is a colourless to brown gas at room temperature with a sharp and sweet smell. Nitrogen dioxide (NO₂) is a reddish-brown gas at temperatures above 20 °C and becomes colourless to brown liquid, with a strong and harsh odour on cooling. It is highly reactive and exists in equilibrium with the colourless gas dinitrogen tetroxide (N₂O₄): 2NO₂ ^ N₂O₄.

[0004] Unlike NO2, dinitrogen tetroxide N2O4 is diamagnetic because it has no unpaired electrons. N2O4 can be crystallised as a white solid having a melting point -11.2 °C. The liquid N2O4 is also colourless but can appear as a brownish yellow liquid due to the presence of NO2 according to the above equilibrium. The equilibrium is exothermic and characterised by DH = -57.23 kJ/mol. Thus, higher temperatures push the equilibrium towards NO2, while at lower temperatures, dinitrogen tetroxide (N2O4) predominates. Inevitably, some N2O4 is a component of NCE-containing smog. Nitrous oxide (N2O) is a well-known greenhouse gas that contributes to climate change. Sulphur dioxide (SO2) is the predominant form of the sulphur oxides found in the lower atmosphere. It is a colourless gas that can be detected by taste and smell in the range of 1,000 to 3,000 micro grams per cubic meter (pg/m³).

[0005] Nitrogen oxides, when combined with volatile organic compounds, form ground-level ozone, or smog. NO_x and SO_x react with oxygen and undergo reactions with water vapours in the atmosphere to yield acid rains. These oxides are the most common pollutants found in the air around the world. Exposure to high levels of NO_x and SO_x can cause collapse, rapid burning and swelling of tissues in the throat and upper respiratory tract, difficult breathing, throat spasms, and fluid build-up in the lungs. It can interfere with the blood's ability to carry oxygen through the body, causing headache, fatigue, dizziness and eventually death. Therefore, in accordance with stringent environmental restrictions regarding pollutant emissions the removal of these pollutants from exhaust gas streams is very important. For example, as per MARPOL Annex VI, the emission limits of NO_x from marine diesel engines are: for engine speed less than 130 rpm, the limit is 3.4 g NO_x per kWh, for engine speed in the range of 130-2000 rpm, the emission limit is 9xS ° ² g NO_x per kWh, while for engine speed above 2000 rpm, the engine limit is 2 g NO_x per kWh, where S is the engine's rated speed (in rpm units). To comply with these requirements, ships must have to be installed or retrofitted with special compact equipment which can effectively reduce NO_x and SO_x below the international standards.

[0006] Currently, in most fuel burning processes, NO_x and SO_x are treated separately. Emissions of SO_x are being strongly reduced and completely removed from flue gases by wet scrubbing technology using a slurry of alkaline sorbent, usually limestone or lime, or seawater to scmb gases. Sulphur dioxide is an acid gas, and, therefore, the typical sorbent slurries or other materials used to remove the SO2 from the flue gases are alkaline. In some designs, the product of the SO2 wet scrubbing, calcium sulphite (CaSO¾), is further oxidised to produce marketable gypsum (CaS0₄-2H20). This technique is also known as forced oxidation, flue gas desulfurization (FGD) or fluidised gypsum desulfurization. It is the most effective technology for SO_x removal.

[0007] Although the flue gas desulfurization process is able to achieve relatively high removal efficiency of SO_x, it is not effective in NO_x removal. This is because nitric oxide gas (NO), which comprises more than 90 % of NO_x in the flue gas, is quite insoluble in water. The oxidation of nitrogen to its higher valence states yields NO_x soluble in water. When this is carried out, a gas absorber becomes effective. In general, an oxidiser must be added to the scmbbing system in order to convert insoluble NO gas (5.6 mg per 100 ml of water at room temperature) to soluble NO2 as a prerequisite step.

[0008] Thus, absorption of NO_x gases is probably the most complex when compared with other absorption operations because of nitric oxide low solubility. Therefore, the control of NO_x is mostly achieved by using chemical reduction technique, for example, selective catalytic reduction (SCR). The yields of nitrogen oxides reduction to nitrogen in the SCR are typically high, but this technique is extremely expensive.

[0009] Kasper et al (1996) in“ Control of Nitrogen Oxide Emissions by Hydrogen Peroxide - Enhanced Gas-Phase Oxidation Of Nitric Oxide”, Journal of the Air and Waste Management Association 46(2), pages 127-133, described that the removal of NO_x in wet scrubbers may be greatly enhanced by gas-phase oxidation of water-insoluble NO gas to water-soluble NO2, HNO2, and HNO3 (the acid gases are much more soluble in water than nitric oxide). The gas-phase oxidation may be accomplished by injecting liquid hydrogen peroxide into the flue gas, so that H2O2 vaporises and dissociates into hydroxyl radicals. The oxidised NO_x species may then be easily removed by caustic water scmbbing.

[0010] Oxidants that have been injected into the gas flow are ozone, ionised oxygen or hydrogen peroxide. Non-thermal plasma generates oxygen ions within the air flow to achieve this. Other oxidants have to be injected and mixed in the flow. The kinetic problem of fast oxidation of nitrogen and sulphur oxides, NO to NO2 and SO2 to SO3, during the short residence time in the exhaust stack has been taken care by using very strong oxidants like ozone or hydrogen peroxide. Stamate et al (2013) in“ Investigation of NO_x Reduction by Low Temperature Oxidation Using Ozone Produced by Dielectric Barrier Discharge” , Japanese Journal of Applied Physics 52(5S2), 05EE03, suggested that in order to enhance the wet scrubber operation, ozone may be used for NO_x gases oxidation. Stamate and Stalewski (2012) in“NO_x reduction by ozone injection and direct plasma treatment' , Proceedings of ESCAMPIG XXI, Viana do Castelo, Portugal, July 10-14, 2012, compared the NO_x reduction by ozone injection with direct plasma treatment. Hutson et al. (2008) in“ Simultaneous Removal of Oz, NOx, and Hg from Coal Flue Gas Using a NaC lOz-Enhanced Wet Scrubber” , Industrial and Engineering Chemistry Research (I&EC) 47(16), pages 5825-5831, taught using sodium chlorite NaCICE as an oxidiser for removal of NO_x gases.

[0011] Thus, the aforementioned oxidation techniques intentionally raise the valence of nitrogen in a nitrogen oxide to allow water to absorb the oxidised nitrogen oxide. This is accomplished either by using a catalyst, injecting hydrogen peroxide, creating ozone within the gas flow, or injecting ozone into the gas flow. Non-thermal plasma, when used without a reducing agent, can be used to oxidise NO_x as well. A wet scrubber must be added to the process in order to absorb N2O5 emissions into the atmosphere. Any resultant nitric acid may then be neutralised by a scmbber liquid and then sold (usually as a calcium or ammonia salt to produce fertilisers). Alternatively, it may be collected as nitric acid for sales.

[0012] However, the above processes using hydrogen peroxide and ozone require very expensive and corrosion-resistant systems to be installed in the wet scrubber units, which significantly increase the production costs of both hydrogen peroxide and ozone (which are expensive reagents themselves) used for NO_x and SO_x oxidation of gases. This, in turn, requires the introduction of the expensive production units which also use high voltage and high safety standards. Therefore, the oxidation methods mentioned above for wet scrubbing technology are extremely expensive. In addition, both H2O2 and O3 are reactive and corrosive, which creates several maintenance problems.

[0013] More than 90% removal of NO_x from flue gases in ship engines is achieved today by using the aforementioned selective catalytic reduction (SCR) technology to comply with MARPOL emission standards. In the SCR system, urea or ammonia is injected in the exhaust gas before passing it through a catalytic unit, which consists of special catalyst layer, at a temperature between 300 and 400 °C. Chemical reaction between urea or ammonia and NO_x in exhaust gases reduces NO_x to nitrogen. SCR unit is installed between the exhaust manifold (receiver) and the turbocharger. High efficiency turbocharger is required for this system as there is a pressure drop across the SCR reactor because of high temperature, which makes the SCR system extremely expensive Engine load should be 40% and above because NO_x is reduced to nitrogen within this specific temperature window (300- 400 °C). If temperature is above 400 °C, ammonia will burn rather than reacting with NO_x which will lead the system to be ineffective and dangerous to the environment. If the temperature is below 270 °C, the reaction rate will be low, and the ammonium sulphates formed will destroy the catalyst.

[0014] In WO2019016808 Al, the present inventors described the method and system, which is relatively cheap, compact, safe and easily up-scaled for industrial needs, and which effectively removes simultaneously both NO_x and SO_x from the flue gases in the wide range of industrial applications. However, in those method and systems, ammonia was added in the last stage of the process for the purpose of manufacturing ammonium nitrate NH4NO3 and ammonium sulphate (NH₄)₂S0₄ fertilisers. The process was carried out at pH 4-7, which was maintained with ammonium hydroxide injection in order to keep the reaction going and not to create the alkaline solution, from which ammonia gas might evolve. This process resulting in ammonium-based products is clearly unsuitable for removal of nitrogen oxides (NO_x) and sulphur oxides (SO_x) in flue gases emitted from ship engines because of the very strict environmental regulations prohibiting nitrification of marine aquatic environment (when ammonia and nitrate are disposed into sea water). According to the international environmental regulations, ships are not permitted to discharge any liquids containing nitrites or nitrates within an area from which a water supply is drawn or in any area restricted for the discharge of wastes by any national or local authority.

[0015] Therefore, despite recent developments in this field, there still remains a long-felt need for a system, which would be economical, safe to environment, compact and operated at relatively low temperatures, for removal of NO_x and SO_x from flue gases in ship engines.

SUMMARY

[0016] The aforementioned problems in removal of both NO_x and SO_x simultaneously from flue gases using expensive oxidation reagents, for example hydrogen peroxide (H2O2) or ozone (O3), at elevated temperatures of higher than about 300 °C may be solved by using atmospheric oxygen from oxygen-enriched air instead in the presence of an oxidation catalyst. Because oxygen -enriched air may cause technical issues on ships, activation of the oxidation catalyst at relatively low temperatures of about 50-90 °C using small amounts of activation reagents may become necessary. No heating is required in the process, because the process itself is exothermic. The observed elevated temperature of 50-90 °C originates from the reached thermodynamic equilibrium, and therefore, no additional heat is applied.

[0017] In one embodiment, a system for combined catalytic oxidation and removal of NO_x and SO_x from a flue gas in a ship engine, comprises:

a) An oxidation reactor (1) filled with an oxidation catalyst or with an adsorbing dispersion containing said oxidation catalyst and configured:

^■ to receive either a mixture of oxygen-enriched air streamed from an oxygen concentrator and a flue gas containing NO_x and SO_x, emitted from a ship engine, or a mixture of atmospheric air and the flue gas containing NO_x and SO_x emitted from the ship engine;

^■ to adsorb said gases on the particles of the oxidation catalyst;

^■ to carry out catalytic oxidation of said NO_x and SO_x to yield oxidised NO_x and SO_x, and

^■ to perform wet-scrubbing of said oxidised NO_x and SO_x, thereby yielding nitrous, nitric and sulphuric acids;

b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor

(1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNCh), sodium nitrate (NaNC ) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine; and

c) Said separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaNCh and Na2SCfr), and to control said catalytic oxidation reaction and wet-scmbbing of the gases in the reactor (1),

wherein if the oxidation reactor (1) is configured to receive the mixture of atmospheric air and the flue gas containing NO_x and SO_x emitted from the ship engine, said system further comprises an activation chamber (8) for activating the oxidation catalyst, said activation chamber (8) contains an activation reagent and is in fluid communication with the separating and reactor controlling unit (2), from which it receives the deactivated oxidation catalyst, and with the oxidation reactor (1), to which it feeds the oxidation catalyst after its activation.

[0018] In some embodiments, when the oxidation reactor (1) is configured to receive a mixture of oxygen-enriched air and a flue gas (i.e. the system operates on the oxygen-enriched air), the system further comprises said oxygen concentrator designed to concentrate oxygen from ambient air and generate an air stream enriched with oxygen. The oxygen concentrator (not shown in the figures) is in fluid communication with the oxidation reactor (1). The mixture of the oxygen-enriched air and flue gas streamed into the oxidation reactor (1) consists of about 50-80% of the oxygen-enriched air and 20-50% of the flue gas.

[0019] The activation reagent in the activation chamber (8), capable of activating the deactivated oxidation catalyst, is selected from any suitable strong oxidizer, non-limiting examples of which are hydrogen peroxide (H2O2), benzoyl peroxide and the like.

[0020] The oxidation reactor (1) may be dry and packed with inert solids, such as ceramic beads, promoting a better contact between said oxygen stream and said flue gas stream, or wet containing a liquid circulating inside. In a specific embodiment, the oxidation reactor (1) is selected from a bubble column, packed bed and spray tower equipped with spray means. These spray means can spray either:

(i) water or mother liquor, which is recycled after filtration and sprayed on the dry oxidation catalyst particles, thus forming floating drops of the adsorbing dispersion directly inside the spray tower, or

(ii) the adsorbing dispersion prepared in advance and containing the oxidation catalyst. The spray tower may be a wet scmbber comprising an empty cylindrical vessel made of steel or plastic, and inlets for gas streams. The spray means may comprise one or more spray nozzles arrayed within the spray tower along the flue gas flow path and configured to spray said water, mother liquor or adsorbing dispersion into the vessel. These spray nozzles are equipped with a demister (3) for mist removal.

[0021] In a further embodiment, the oxidation catalyst may comprise the mixture of an aqueous solution of a metal salt precursor with silica gel particles and used for catalysing the oxidation reaction of NO_x and SO_x in the flue gas. The metal salt precursor is a water-soluble inorganic salt of a transition metal selected from cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), copper (Cu) and chromium (Cr). In a specific embodiment, the metal salt precursor is cobalt sulphate (C0SO4). The oxidation catalyst may also comprise an aqueous suspension of cobalt oxide/hydroxide particles supported on silica gel particles.

[0022] In yet further embodiment, the separating and reactor-controlling unit (2) for separation and removal of the obtained salts comprises at least one of the following processing units: a crossflow filtration unit (6), a filter, a crossflow separator, a mixer-settler, a decanter, or a tricanter. It may further comprise sensors for measuring and controlling temperature and pH of the processed liquids.

[0023] In some embodiments, the separating and reactor-controlling unit (2) comprises a crossflow filtration unit (6) comprising a cascade (plurality) of crossflow filtration units or separators capable of separating floating solid particles of the catalyst from aqueous solution of the produced salts, based on the particle size of the catalyst and salts.

[0024] In a particular embodiment, the separating and reactor-controlling unit (2) further comprises a salt separation vessel connected to the aforesaid crossflow filtration unit (6) and configured to receive from the crossflow filtration unit (6) an aqueous solution containing the dissolved NaNCb, Na2SC>4 and NaNCh salts and separate them. Such separation of these salts is carried out in order to comply with the strict environmental regulations prohibiting nitrification of aquatic environment (i.e. disposing nitrites and nitrates into sea water).

[0025] In another embodiment, the system of the present invention further comprises a separate oxidation chamber (4). This oxidation chamber (4) is either connected to the oxygen concentrator and configured to receive the mixture of the oxygen-enriched air stream and the stream of the flue gas containing NO_x and SO_x from the ship engine, or connected to a separate vessel (not shown here), which contains the activation reagent, and configured to receive the stream of the flue gas from the ship engine and the activation reagent from said separate vessel. This oxidation chamber (4) is installed in fluid communication with the reactor (1), is pre-filled with the oxidation catalyst and is capable of carrying the catalytic oxidation of NO_x and SO_x in the flue gas. It may be dry and packed with inert solids, such as ceramic beads, promoting a better contact between said air stream and said flue gas stream, or wet containing a liquid circulating inside.

[0026] In a further embodiment, a method for catalytic oxidation and removal of both nitrogen oxides (NO_x) and sulphur oxides (SO_x) simultaneously from a flue gas in a ship engine comprises the steps of:

I. Catalytic oxidation of NO_x and SO_x contained in the flue gas emitted from the ship engine, wherein said catalytic oxidation provides the oxidised NO_x and SO_x, said catalytic oxidation is carried out in the oxidation reactor (1) of the system of the present invention; and

II. Wet-scrubbing of the oxidised NO_x and SO_x with an adsorbing dispersion of an oxidation catalyst suspended in water, wherein said wet-scrubbing yields nitrous, nitric and sulphuric acids dissolved in water.

[0027] The catalytic oxidation in Step I can be carried out either with oxygen-enriched air obtained from atmospheric oxygen concentrated in the oxygen concentrator or with ambient air. The process with using ambient air is possible if an additional step of activating (restoring) the oxidation catalyst deactivated in the process is added to the method of the invention. The oxidized catalyst is then restored using the activation reagents mentioned above and fed back to the catalytic system.

[0028] The above process further comprises the step (III) of simultaneously removing the nitrous, nitric and sulfuric acids from water by contacting them with sodium hydroxide (NaOH) to yield sodium sulphate (Na2SC>4), sodium nitrate (NaNCh) and sodium nitrite (NaNC ). The obtained salts are further subjected to separation and removal from the system.

[0029] In yet further embodiment, the above process comprises the additional steps of:

IV. Separation and either collection or discharging of sodium sulphate (Na2SC>4) into aquatic environment;

V. Optional collection and utilisation of hazardous sodium nitrite (NaN0₂) and sodium nitrate (NaNOs); and

VI. Recylcing of water and recovering or activating catalyst remained in water after filtering out the obtained salts (mother liquor).

[0030] In a specific embodiment, the above process is carried out at the relatively low temperature of 50-90 °C and pH 8-10. This pH is maintained with sodium hydroxide injection in order to convert the dissolved nitrous, nitric and sulphuric acids obtained after wet-scrubbing into their corresponding sodium salts. [0031] Various embodiments of the invention may allow various benefits and may be used in conjunction with various applications. The details of one or more embodiments are set forth in the accompanying figures and the description below. Other features, objects and advantages of the described techniques will be apparent from the description and drawings and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the appended figures. Various exemplary embodiments are well illustrated in the accompanying figures with the intent that these examples not be restrictive. Of the accompanying figures :

Fig. la shows the operational diagram of the system of the present invention for combined catalytic oxidation and wet-scmbbing of nitrogen oxides (NO_x) and sulphur oxides (SO_x) from the flue gas emitted from a ship engine, where the system operates on a mixture of oxygen -enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. lb shows the operational diagram of the system of the present invention for combined catalytic oxidation and wet-scmbbing of nitrogen oxides (NO_x) and sulphur oxides (SO_x) from the flue gas emitted from a ship engine, where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

Fig. 2a schematically shows the one -stage system of the present embodiments, wherein the oxidation reactor (1) is a spray tower, and where the system operates on a mixture of oxygen -enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. 2b schematically shows the one-stage system of the present embodiments, wherein the oxidation reactor (1) is a spray tower, and where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

Figs. 3a-3d show the scanning electron microscope (SEM) images of the obtained silica particles coated with cobalt hydrous oxide. According to the EDS analysis of cobalt, the weight of cobalt is 17.6% (s = 0.8) and the weight of oxygen is 82.4% (s = 0.8) in the catalyst.

Fig. 4a schematically shows the separating and reactor-controlling unit (2) of the system of the present invention, with the separating unit (6) and neutralisation reactor (7) of the present embodiments, where the system operates on a mixture of oxygen-enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. 4b schematically shows the separating and reactor-controlling unit (2) of the system of the present invention, with the separating unit (6) and neutralisation reactor (7) of the present embodiments, where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

Fig. 5a schematically shows the expanded crossflow filtration unit (6) of the separating and reactor controlling unit (2) of the present embodiments, where the system operates on a mixture of oxygen- enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. 5b schematically shows the expanded crossflow filtration unit (6) of the separating and reactor controlling unit (2) of the present embodiments, where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

Fig. 6a shows the operational diagram of the industrial two-stage system of the present invention with the separate oxidation chamber (4) containing the oxidation catalyst, where the system operates on a mixture of oxygen-enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. 6b shows the operational diagram of the industrial two-stage system of the present invention with the separate oxidation chamber (4) containing the oxidation catalyst, where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

Fig. 7a schematically shows the two-stage system of the present invention, wherein the wet scmbber (1) is a spray tower that is capable of spraying the adsorbing dispersion, where the system operates on a mixture of oxygen-enriched air, streamed from an oxygen concentrator, and the flue gas emitted from a ship engine.

Fig. 7b schematically shows the two-stage system of the present invention, wherein the wet scmbber (1) is a spray tower that is capable of spraying the adsorbing dispersion, where the system operates on a mixture of ambient (atmospheric) air and the flue gas emitted from a ship engine.

DETAILED DESCRIPTION

[0033] In the following description, various aspects of the invention will be described. For purposes of explanation, specific aspects and details are set forth in order to provide a thorough understanding of the invention. However, it will also be apparent to one skilled in the art that the invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the invention.

[0034] The terminology used herein is for describing particular embodiments only and is not intended to be limiting of the invention. The terms "comprising" and "comprises", used in the claims, should not be interpreted as being restricted to the means listed thereafter; they do not exclude other elements or steps. They need to be interpreted as specifying the presence of the stated features, integers, steps and/or components as referred to, but does not preclude the presence and/or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising x and z" should not be limited to devices consisting only of components x and z. Also, the scope of the expression "a method comprising the steps x and z" should not be limited to methods consisting only of these steps.

[0035] Unless specifically stated, as used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within two standard deviations of the mean. In one embodiment, the term "about" means within 10% of the reported numerical value of the number with which it is being used, preferably within 5% of the reported numerical value. For example, the term "about" can be immediately understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01 % of the stated value. In other embodiments, the term "about" can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. As an illustration, a numerical range of "about 1 to about 5" should be interpreted to include not only the explicitly recited values of about 1 to about 5 , but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3, and 4 and sub-ranges, for example from 1-3, from 2-4, and from 3-5, as well as 1, 2, 3, 4, 5, or 6, individually. This same principle applies to ranges reciting only one numerical value as a minimum or a maximum. Unless otherwise clear from context, all numerical values provided herein are modified by the term "about". Other similar terms, such as "substantially", "generally", "up to" and the like are to be constmed as modifying a term or value such that it is not an absolute. Such terms will be defined by the circumstances and the terms that they modify as those terms are understood by those of skilled in the art. This includes, at very least, the degree of expected experimental error, technical error and instrumental error for a given experiment, technique or an instmment used to measure a value.

[0036] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constmctions may not be described in detail for brevity and/or clarity. [0037] It will be understood that when an element is referred to as being "on", "attached to", "connected to", "coupled with", "contacting", etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, "directly on", "directly attached to", "directly connected to", "directly coupled" with or "directly contacting" another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature may have portions that overlap or underlie the adjacent feature. In addition, the term "in fluid communication with" means a fluid (liquid or gas) passage between interiors of a particular unit or between interiors of two or more units. The kind of means for this attachment, connection, coupling or fluid communication is not essential for the problem to be solved.

[0038] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealised or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity.

[0039] The deficiencies of the prior art as discussed above are alleviated by the systems and processes described in the present application, wherein harmful nitrogen oxides (NO_x) and sulphur oxides (SO_x) of the flue gas emitted from a ship engine are simultaneously oxidised by oxygen in the presence of a catalyst followed by wet scmbbing and neutralisation with sodium hydroxide to yield the corresponding sadium salts which are separated and discharged or utilised.

[0040] As mentioned above, the main problem associated with the existing methods for removal of NO_x and SO_x is the very low solubility of the nitric oxide gas in water. The oxidation of nitrogen to its higher valence states yields NO_x soluble in water. Therefore, the removal of NO_x in wet scrubbers may be greatly enhanced by gas-phase oxidation of water-insoluble NO gas to water-soluble NO₂, HNO₂, and HNO₃ (the acid gases are much more soluble in water than nitric oxide). The gas- phase oxidation may be accomplished by injecting liquid hydrogen peroxide (H₂O₂) into the flue gas, so that hydrogen peroxide vaporises and dissociates into hydroxyl radicals. Ozone (O₃) can also be used for the oxidation purposes. The oxidised NO_x species may then be easily removed by caustic water scmbbing. [0041] However, the use of either O3 or H2O2 as oxidising reagents for NO_x and SO_x creates a series of safety and maintenance problems, because these oxidising reagents are relatively expensive, very reactive and corrosive. As a result, their use in wet scrubbing of NO_x and SO_x significantly increases the operational and maintaining costs of the process and system, particularly on ships.

[0042] Instead of using these strong oxidizing agents O3 or H2O2 for oxidation at high temperature (usually higher than 300 °C), the present inventors suggested using atmospheric oxygen (O2), which is abundant in ambient air, considerably reduces both the operational and maintenance cost of the wet scrubber and produces safer and less hazardous working environment. The use of oxygen obviates the need for using the cumbersome and expensive H2O2/O3 oxidation systems and allows combining the catalytic oxidation and wet-scrubbing of the flue gas in a one-step process.

[0043] Moreover, the present invention discloses two configurations of the oxidation system. The first configuration operates on oxygen-enriched air to increase efficiency of the oxidation reaction and requires an additional air concentrator unit. The second configuration operates on atmospheric air at ambient conditions and requires an additional catalyst activation unit. In the second configuration, the efficient oxidation process is carried out at low temperatures of 50-90 °C in the presence of recovered and re-activated catalyst. This temperature is a result of the exothermic character of the reaction, and therefore, no heating is required in the process.

[0044] Reference is now made to Fig. la showing the operational diagram of the system of the present invention for combined catalytic oxidation and wet-scmbbing of simultaneously both nitrogen oxides (NO_x) and sulphur oxides (SO_x) in their gas mixture from the flue gas emitted from a sip engine. This is essentially a one-stage system of the present invention comprising the following components:

^■ to receive a mixture of oxygen-enriched air streamed from an oxygen concentrator and a flue gas containing NO_x and SO_x, emitted from a ship engine;

^■ to adsorb said gases on the particles of the oxidation catalyst;

b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor (1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNCh), sodium nitrate (NaNC ) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine; and

c) The separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaNCh and Na2SCfr), and to control said catalytic oxidation reaction and wet-scmbbing of the gases in the reactor (1).

[0045] The oxygen concentrator, which is not shown in this figure, is capable of concentrating oxygen from ambient air by selectively removing nitrogen from the air, thereby producing the air stream enriched with the atmospheric oxygen for oxidation of the NO_x and SO_x gases. In general, any air containing more than 21% is considered an oxygen -enriched air. Concentration of oxygen in the oxygen-enriched air strongly varies with the choice of equipment used for enriching air with oxygen. This equipment is not a part of the present invention, it is commercially available and is not essential for the problem to be solved. Dependent on the brand/model of the external air generator used for preparing the oxygen -enriched air, the level of oxygen in air supplied to the system of the present invention can strongly vary. Moreover, this parameter is not related to the process of the present invention and is not controlled in the process.

[0046] As noted above, practically any commercially available oxygen concentrator can be used in the system of the embodiment. Most of them are based on fractional distillation. However, cryogenic oxygen distillators or oxygen concentrators based on membrane separation of oxygen, pressure swing adsorption and vacuum pressure swing adsorption can also be used to produce the air stream of enriched atmospheric oxygen from ambient air.

[0047] In the second configuration, shown in Fig. lb, the system for combined catalytic oxidation and removal of NO_x and SO_x from a flue gas in a ship engine, comprises:

^■ to receive a mixture of ambient air and the flue gas containing NO_x and SO_x emitted from the ship engine;

^■ to adsorb said gases on the particles of the oxidation catalyst;

^■ to perform wet-scrubbing of said oxidised NO_x and SO_x, thereby yielding nitrous, nitric and sulphuric acids; b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor (1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNCh), sodium nitrate (NaNCh) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine; and

c) The separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaNCh and Na2SCh), and to control said catalytic oxidation reaction and wet-scmbbing of the gases in the reactor (1), and d) An activation chamber (8) for activating the oxidation catalyst, said activation chamber (8) contains an activation reagent and is in fluid communication with the separating and reactor controlling unit (2), from which it receives the deactivated oxidation catalyst, and with the oxidation reactor (1), to which it feeds the oxidation catalyst after its activation.

[0048] The activation reagent in the activation chamber (8), capable of activating the deactivated oxidation catalyst, is selected from any suitable strong oxidizing reagent, non-limiting examples of which are hydrogen peroxide (H2O2), benzoyl peroxide and the like.

[0049] The oxidation catalyst used in the system of the present invention for facilitating the oxidation reaction of NO_x and SO_x is in the form of solid catalyst particles suspended in water. The term "adsorbing dispersion" used herein below thus defines the aqueous suspension of the oxidation catalyst particles suspended in water.

[0050] The system of the present invention can be designed as a one-stage or two-stage system as will be described below. These configurations may contain various types of the oxidation reactor (1), which is selected from a bubble column, packed bed and spray tower. Figs. 2a and 2b show the one- stage system of the present invention in the two configurations defined above, wherein the oxidation reactor (1) constitutes a spray tower. The spray tower spraying the adsorbing dispersion containing the oxidation catalyst is a type of a wet scrubber used to achieve mass and heat transfer between a continuous gas phase and a dispersed liquid phase. The spray tower may consist essentially of an empty cylindrical vessel made of steel or plastic and nozzles that spray the liquid into this vessel. The spray means may include one or more spray nozzles arrayed within the spray tower along the flue gas flow path. The spray nozzles may be equipped with a demister (3) for mist removal. In addition, a bottom tray (5) is used for forming a uniform gas flow in the tower cross section. [0051] The oxidation catalyst particles insoluble in water are produced by mixing the aqueous solution of a metal salt precursor with silica particles, such as nanosilica or silica gel. The metal salt precursor of the embodiment may be any available water-soluble inorganic salt of a transition metal, such as cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), copper (Cu) or chromium (Cr). In a specific embodiment, the metal salt precursor used for the preparation of the oxidation catalyst is cobalt sulphate (C0SO4). For example, the oxidation catalyst may be prepared by mixing 600 mM of the C0SO4 aqueous solution (cobalt precursor) with silica gel powder or silica particles suspension. This process involves adsorption of Co(II) on silica particles with the formation of the 1-2 nm Co(II) hydroxide clusters.

[0052] The obtained oxidation catalyst particles may be stabilised by the negative charges of about 260 -Si-O- surface groups per particle with sodium ions Na⁺ as the counter ions. The pH of the obtained suspension may be adjusted with a basic solution, for example, sodium hydroxide solution, to pH 8 or higher in order to hydrolyse cobalt on the silica particles (so called, "pH-jump"). The hydrolysis is carried out under vigorous stirring at room temperature. A Y-mixer with 20 mL/s flow rate may be employed to provide uniform conditions for cobalt hydrolysis and adsorption on silica. The resultant suspension has a blue colour.

[0053] The size of the silica particles selected for the preparation of the catalyst of the present invention is dependent on the size of soot particles produced when the ship engine is working. Since the soot size ranges between 5 to 600 nm, the smaller or larger silica particles are needed. Based on extensive experimentation, the present inventors found that the large silica particles (more than 600 nm) constituting the oxidation catalyst of the present invention provide relatively low catalytic activity. The strongest catalytic activity was achieved using Cab-O-Sil M5 silica powder having 200- 300 nm long chains of 10 nm spheres (see for example, http://www.C3botcoTp.cotrv'so]utions/p >ducts- plus/fumed-metal-oxides/hydrophiiic). The oxidation catalyst based on these silica particles allowed to remove 88% NO_x from synthetic air-NO_x mixture.

[0054] Another silica used in the experiments performed by the present inventors comprised 10- nm silica spheres, which is commercially available as 30% silica in water from Alfa Aesar (see for example, https://www.aifa.com/en/eatalog/0431 1 1 /), in a form of a colloidal dispersion. This silica is on the lower edge of the soot's size and hence, active too.

[0055] As noted above, the catalyst using these types of silica was prepared by mixing cobalt sulphate (C0SO4) with the silica followed by addition of sodium hydroxide until the obtained suspension reaches pH 10 and maintains this pH at a constant value. At such high pH, the formed cobalt hydroxide is not dissolved in water, which prevents leaching of the metal catalyst from the silica particles into water, thereby preserving its catalytic activity.

[0056] Figs. 3a-3d show the scanning electron microscope (SEM) images of the obtained silica particles coated with cobalt hydrous oxide. According to the EDS analysis of cobalt atoms, the relative weight of cobalt is 17.6% (s = 0.8) and the weight of oxygen is 82.4% (s = 0.8) in the catalyst. As seen from these SEM images, the particles are composed of essentially spheres, cobalt coats the particles and is not floating in solution, since there are no visible areas which contain only cobalt, and in the areas without silica, there is no visible cobalt.

[0057] The obtained Co(OH)₂/Si0₂ catalyst is a high surface-area, heterogeneous, yet suspendable in water catalyst that demonstrates both high selectivity and catalytic activity in the oxidation of the NO_x and SO_x gases by oxygen. The catalyst also shows high stability as no deactivation or precipitation of cobalt is observed upon multiple cycling of cobalt ions through their higher oxidation state that must be involved in the oxidation process.

[0058] Thus, the oxidation catalyst provides a system which does not necessitate the utilisation of the expensive O3 and H2O2 that are usually used. The Co(II) oxidation state changes to Co(III) after its oxidation with oxygen. This oxidised cobalt is capable of oxidising NO and SO2 in the flue gas, thereby being reduced back to Co(II). The oxidation column is therefore filled with an aqueous suspension of cobalt oxide/hydroxide particles supported on silica gel particles, thereby catalysing the oxidation of NO_x and SO_x. When NO or SO2, which is contained in the flue gas, is being absorbed on these catalyst particles, the irreversible oxidation reaction occurs at a finite but high speed according to the following equations:

Oxidation

catalyst

2NO + 0₂ - 2N0₂

(from flue gas) (from air) (aqueous)

Oxidation

catalyst

2S0₂ + 0₂ - 2SO3

(from flue gas) (from air) (aqueous)

The above oxidation reactions are considered to be globally of second order with respect to the reactants. Quite obviously, an increase in the oxygen concentration (enrichment of the air stream with oxygen) enhances the rate of these oxidation reactions

[0059] As shown in Figs. 2a-2b, either the oxygen -enriched air stream and the flue gas, or the flue gas with ambient air enter the spray tower from the bottom and flow counter current to the adsorbing dispersion, which is introduced at the top of the spray tower, sprayed downward the tower and adsorbs the oxidised NO_x and SO_x gases. The spray tower is often packed with some inert solids, such as ceramic beads, in order to promote better contact between the two streams (oxygen and flue gas).

[0060] Separation of the oxidised NO_x and SO_x, for instance, NO₂ and SO₃, from the reactant NO and SO₂ contained in the flue gas is achieved simply because of the solubility of the former in water. The pH of the resulted suspension decreases due to the formation of nitrous, nitric and sulfuric acids via the following reactions:

2N0₂ + H2O HNO3 + HNO2

SO3 + H2O H2SO4

[0061] As mentioned in the background, NO₂ exists in equilibrium with the colourless gas dinitrogen tetroxide (N₂O₄): 2NO₂ ^ N₂O₄. Also, the relatively unstable dinitrogen trioxide (N₂O₃) may be formed according to the following equilibrium: NO + NO₂ ^ N₂O₃. In the presence of water, NO and NO₂ may also exist in the equilibrium with nitrous acid: NO + NO₂ + H₂O ^ 2HNO₂.

[0062] The concentrations of the various nitrogen oxides species present during the entire adsorption-oxidation process (NO, NO2, N2O3, N2O4) are not independent though, which complicates the whole process. When absorbed into water NO₂, N₂O₃ and N₂O₄ undergo relatively fast hydrolysis, thereby producing nitric acid (HNO₃) and nitrous acid (HNO₂), the latter being decomposed in nitrous oxide NO, which desorbs to the gas phase according to the following reaction: 3HNO₂ ^ HNO₃ + H₂O + 2NO. The suspension colour may change from pink (acidic) to blue (alkaline) depending on the pH of the solution. The operating temperature is 50-90°C due to the hot flue gas coming from the ship engine.

[0063] The term "mother liquor" used herein below defines the liquid portion of the circulating adsorbing dispersion that contains almost no suspended or dissolved oxidation catalyst or crystallisation product. It is either recycled into the spray tower together with the oxidation catalyst particles in a form of the aqueous suspension, or sprayed from the nozzles on the dry oxidation catalyst particles floating in the spray tower, thereby forming the aqueous suspension directly inside the reactor. The mother liquor is also the liquid left over after filtering off the obtained salts (NaNC>₂, NaNCb and Na2SC>4) in the separating and reactor-controlling unit.

[0064] As schematically shown in Figs.2a and 2b, the adsorbing dispersion is filtered and recycled continuously in the system. The spray tower is connected to a vessel (not shown in the figure) containing sodium hydroxide solution streamed into the separating and reactor-controlling unit (2) in order to react with the oxidised NO_x and SO_x species and neutralise the obtained acids, according to the following equation:

HNO3 (aq) + H2SO4 (aq) + 3NaOH NaNOs (s) + Na₂S0₄ (s) + 3H₂0

[0065] The obtained sodium salts are then separated from the liquids and either discharged or collected. The system of the present invention may further comprise sensors for measuring and controlling pH and temperature of the liquid. Monitoring the presence of nitrites and nitrates, as well as pH of the solution, is of particular importance since part of the liquids are discharged into aquatic environment.

[0066] The mother liquor may be further recycled by transferring it for feeding a new portion of the suspension in the spray tower. The adsorbing dispersion sprayed in the spray tower may contain the filtered aqueous solution that is recycled from the dry oxidation chamber and contains the dissolved nitrates and sulphates.

[0067] Reference is now made to Figs. 4a-4b schematically showing the separating and reactor controlling unit (2) for the system of the present invention in the two configurations defined above. As shown in these two figures, neutralisation reactor (7) is fed with the acidic adsorbing dispersion containing the suspended catalyst particles (after oxidation reaction). Sodium hydroxide solution is also fed into the reactor (7). The neutralisation reaction of the adsorbing dispersion occurs in the reactor (7) according to the above equation.

[0068] In general, the separating and reactor-controlling unit (2) is used for handling liquid streams transferred from the adsorption and oxidation reactor (1), including a full stream or portion of it, which is called "bleed stream". In the unit (2), the streamed adsorbing dispersion with the dissolved and oxidised NO_x and SO_x is allowed to contact with the injected stream of sodium hydroxide to yield sodium salts. This reaction is carried out in the reactor (7), followed by filtering off and separation of the obtained salts from the circulating stream. The unit (2) therefore comprises at least one of the following processing sub-units: a crossflow filtration unit (6), a filter, a crossflow separator, a mixer- settler, a decanter, or a tricanter.

[0069] Reference is now made to Figs. 5a and 5b schematically showing the crossflow filtration unit (6) of the separating and reactor-controlling unit (2) of the present embodiments. Thus, in some embodiments, the separating and reactor-controlling unit (2) comprises this crossflow filtration unit (6) comprising a cascade (plurality) of crossflow filtration units or separators capable of separating floating solid particles of the catalyst from aqueous solution of the produced salts, based on the particle size of the catalyst and salts. In some embodiment, the crossflow filtration unit (6) further comprises a salt separation vessel connected to the crossflow filters and configured to receive from these crossflow filters a filtered aqueous solution containing the dissolved NaNCh, NaNCb and Na₂SC>₄ salts and separate them. Such separation of the salts (nitrites and nitrates from sulphates) is carried out in order to comply with the strict environmental regulations prohibiting nitrification of aquatic environment (i.e. disposing nitrites and nitrates into sea water). Only separated sulphates can be disposed into sea water, while nitrites and nitrates should be collected and removed from the system or optionally utilised, for example as fertilisers.

[0070] There are actually two options of dealing with nitrites and nitrates in the system of the present invention. First, as mentioned above, it is possible to completely remove nitrate and nitrites from the system without discharging them into aquatic environment. However, there is also another option of diluting the aqueous solution of the salts and disposing the diluted solution in ports. For this, the nitrate/nitrite sensors are the must to control the presence, in general, and their concentration, in particular.

[0071] The separating and reactor-controlling unit (2) may further comprise sensors for measuring and controlling temperature and pH of the processed liquids. The flow, temperature and pH feedback control is performed by measuring the present values and relating them to the reference values using various actuators, such as an electric heater (for temperature control), sodium hydroxide dosing pump (for pH control), and controlled main pump (for flow control). Separation of the suspended catalyst particles from the aqueous solution or mother liquor can also be carried out by the membrane filtration. As an example, Figs. 5a and 5b show Crossflow Filter 2 used for this purpose.

[0072] In a further embodiment, the combined system of the present invention either comprises a separate oxidation chamber connected to the oxygen concentrator or receives atmospheric air, and consequently configured to receive either a mixture of oxygen-enriched air and flue gas emitted from a ship engine, or a mixture of ambient air and the flue gas emitted from the ship engine, respectively. Figs. 6a and 6b show the operational diagram of the industrial two-stage system of the present invention in the two configurations defined above, having a separate oxidation chamber (4) containing the oxidation catalyst. This oxidation chamber (4) is configured to either receive an air stream enriched with atmospheric oxygen and a flue gas stream containing NO_x and SO_x (see Fig. 6a) or to receive the flue gas with ambient air (see Fig. 6b), and to carry out the oxidation reaction of these gases with said oxidation catalyst. The wet scrubber (1) in this case may be essentially the same oxidation reactor (1) shown in Figs, la and lb for the one-stage system of the present invention. [0073] The wet scrubber (1) contains an adsorbing dispersion, receives the streams of the air and flue gas containing the oxidised NO_x and SO_x, adsorbs the streamed gases onto the catalyst particles of the adsorbing dispersion and then carries out the wet scmbbing of said gases. The adsorbing dispersion in this case is recycled in the system the same way as explained above for the one-stage configuration and therefore contains the oxidation catalyst capable of completing the oxidation of NO_x and SO_x partially pre-oxidised in the oxidation chamber (4), if necessary.

[0074] Figs. 7a and 7b schematically show the two-stage system of the present embodiment in the two configurations defined above, respectively, wherein the wet scrubber (1) is a spray tower that is capable of spraying the adsorbing dispersion. In the first configuration, shown in Fig.7a, the oxidation chamber (4) is connected to the oxygen concentrator (not shown here) to receive the air stream enriched with atmospheric oxygen and the stream of a flue gas containing NO_x and SO_x. In the second configuration, shown in Fig. 7b, the oxidation chamber (4) receives the flue gas with ambient air.

[0075] The oxidation chamber (4) shown in Figs .7a-7b may be either dry, packed with inert solids, such as ceramic beads, promoting a better contact between said air stream and said flue gas stream, or wet with a liquid circulating inside. Consequently, the oxidation chamber (4) is filled with either dry catalyst particles or wet catalyst particles and capable of carrying out the catalytic oxidation of the flue gases by the supplied oxygen-enriched air. The oxidation chamber (4) may then be refilled with fresh water or with water recycled from the mother liquor left after filtering off the produced salts.

[0076] As shown in Figs. 7a-7b, the gas stream containing oxidised NO_x and SO_x enters from the dry oxidation chamber (4) at the bottom of the spray tower and moves (flows) upward counter current to the adsorbing dispersion, which is sprayed downward from one or more nozzles. Thus, the two- stage system shown in Figs. 6a-6b and 7a-7b may actually be transformed into a one-stage system, when the oxidation chamber (4) and spray tower (1) are combined together or the oxidation chamber is incorporated into the spray tower.

[0077] In another embodiment, a method for catalytic oxidation and removal of both nitrogen oxides (NO_x) and sulphur oxides (SO_x) simultaneously from a flue gas in a ship engine comprises the steps of:

I. Catalytic oxidation of NO_x and SO_x contained in the flue gas emitted from the ship engine, wherein said catalytic oxidation provides the oxidised NO_x and SO_x, said catalytic oxidation is carried out in the oxidation reactor (1) of the system of the present invention; and II. Wet-scrubbing of the oxidised NO_x and SO_x with an adsorbing dispersion of an oxidation catalyst suspended in water, wherein said wet-scrubbing yields nitrous, nitric and sulphuric acids dissolved in water.

[0078] The catalytic oxidation in Step I can be carried out either with oxygen-enriched air obtained from atmospheric oxygen concentrated in the oxygen concentrator or with ambient air. The process with using ambient air is possible if an additional step of activating the oxidation catalyst deactivated in the process is added to the method of the invention. The oxidized catalyst is then restored using the activation reagents mentioned above and fed back to the catalytic system.

[0079] The above process further comprises the step (III) of simultaneously removing the nitrous, nitric and sulfuric acids from water by contacting them with sodium hydroxide (NaOH) to yield sodium sulphate (Na2S04), sodium nitrate (NaNCh) and sodium nitrite (NaNCte). The obtained salts are further subjected to separation and removal from the system.

[0080] In yet further embodiment, the above process comprises at least one of the following additional steps:

IV. Separation and either collection or discharging of sodium sulphate (Na₂S0₄) into aquatic environment;

[0081] In a specific embodiment, the above process is carried out at the relatively low temperature of 50-90 °C and pH 8-10. This pH is maintained with sodium hydroxide injection in order to convert the dissolved nitrous, nitric and sulphuric acids obtained after wet-scmbbing into their corresponding sodium salts.

[0082] The combined system of the embodiment is thus built to replace the existing bulky and expensive wet-scrubbing systems operating currently in ships at relatively high temperatures (more than 300 °C) with much smaller, simpler and cheaper combination systems operating at 50-90 °C. This heat originates from thermodynamic equilibrium in the process, and no additional heat is required and applied. The system of the present invention is available in two configurations described above (with and without enriching air with oxygen). It should be emphasised that the system of the present invention is suitable for ships as it is small, eliminates both pollutants and does not require heating as mentioned above. Also, the possibility of the combined treatment for different types of pollutants (NO_x and SO_x) at the same ship engine is of great operational and economic advantage (at present, only NO_x is treated in the ship engines).

[0083] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

1. A system for combined catalytic oxidation and removal of NO_x and SO_x from a flue gas in a ship engine, comprising:

^■ to receive either (i) a mixture of oxygen-enriched air streamed from an oxygen concentrator and a flue gas containing NO_x and SO_x, emitted from a ship engine, or (ii) a mixture of ambient air and the flue gas containing NO_x and SO_x emitted from the ship engine;

^■ to adsorb said gases on the particles of the oxidation catalyst;

^■ to perform wet-scmbbing of said oxidised NO_x and SO_x, thereby yielding nitrous, nitric and sulphuric acids ;

b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor (1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNC ), sodium nitrate (NaNCh) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine; and

c) The separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaNCh and Na2SCh), and to control said catalytic oxidation reaction and wet-scrubbing of the gases in the reactor (1), wherein if the oxidation reactor (1) is configured to receive the mixture of atmospheric air and the flue gas containing NO_x and SO_x emitted from the ship engine, said system further comprises an activation chamber (8) for activating the oxidation catalyst, said activation chamber (8) contains an activation reagent and is in fluid communication with the separating and reactor-controlling unit (2), from which it receives the deactivated oxidation catalyst, and with the oxidation reactor (1), to which it feeds the oxidation catalyst after its activation.

2. The system of claim 1 , comprising: a) An oxidation reactor (1) filled with an oxidation catalyst or with an adsorbing dispersion containing said oxidation catalyst and configured:

^■ to receive a mixture of oxygen -enriched air streamed from an oxygen concentrator and a flue gas containing NO_x and SO_x, emitted from a ship engine;

^■ to adsorb said gases on the particles of the oxidation catalyst;

b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor (1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNCh), sodium nitrate (NaNCh) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine; and

c) The separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaNCh and Na2SCh), and to control said catalytic oxidation reaction and wet-scrubbing of the gases in the reactor (1).

3. The system of claim 1 , comprising:

^■ to adsorb said gases on the particles of the oxidation catalyst;

b) A vessel filled with sodium hydroxide (NaOH) solution and connected to the oxidation reactor (1) or to a separating and reactor-controlling unit (2), said vessel having an inlet configured to stream said sodium hydroxide solution into the oxidation reactor (1) or into the separating and reactor-controlling unit (2) for reacting with the obtained nitrous, nitric and sulphuric acids and to yield sodium nitrite (NaNCh), sodium nitrate (NaNCh) and/or sodium sulphate (Na₂S0₄), thus removing NO_x and SO_x from the flue gas emitted from the ship engine;

c) The separating and reactor-controlling unit (2) connected to said oxidation reactor (1) and configured to separate and remove the obtained salts (NaNCh, NaN(¾ and Na2SC>4), and to control said catalytic oxidation reaction and wet-scmbbing of the gases in the reactor (1), and d) An activation chamber (8) for activating the oxidation catalyst, said activation chamber (8) contains an activation reagent and is in fluid communication with the separating and reactor controlling unit (2), from which it receives the deactivated oxidation catalyst, and with the oxidation reactor (1), to which it feeds the oxidation catalyst after its activation.

4. The system of any one of claims 1 to 3, wherein said oxidation reactor (1) is dry and packed with inert solids promoting a better contact between said gases.

5. The system of claim 4, wherein said inert solids are ceramic beads.

6. The system of any one of claims 1 to 3, wherein said oxidation reactor (1) is wet containing a liquid circulating inside.

7. The system of any one of claims 1 to 3, wherein said oxidation reactor (1) is selected from a bubble column, packed bed and spray tower.

8. The system of claim 7, wherein said oxidation reactor (1) is a spray tower equipped with spray means capable of spraying either (i) water or a mother liquor onto the dry oxidation catalyst particles inside said spray tower, thereby forming floating drops of the adsorbing dispersion directly inside the spray tower, or (ii) the adsorbing dispersion prepared in advance and comprising the oxidation catalyst particles.

9. The system of claim 8, wherein said spray tower is a wet scmbber comprising an empty cylindrical vessel made of steel or plastic, and inlets for gas streams.

10. The system of claim 8, wherein said spray means comprise one or more spray nozzles arrayed within the spray tower along the flue gas flow path and configured to spray said water, mother liquor or adsorbing dispersion into the vessel.

11. The system of claim 10, wherein said spray nozzles are equipped with a demister (3) for mist removal.

12. The system of claim 8, wherein said spray tower is dry and packed with inert solids promoting a better contact between said gases.

13. The system of claim 12, wherein said inert solids are ceramic beads.

14. The system of claim 8, wherein said spray tower is wet containing a liquid circulating inside.

15. The system of any one of claims 1 to 3, wherein said adsorbing dispersion is an aqueous suspension of the oxidation catalyst particles.

16. The system of claim 15, wherein said oxidation catalyst comprises the mixture of an aqueous solution of a metal salt precursor with silica particles and used for catalysing the oxidation reaction of NO_x and SO_x in the flue gas.

17. The system of claim 16, wherein said silica particles are in a form of nanosilica or silica gel.

18. The system of claim 16, wherein the metal salt precursor is a water-soluble inorganic salt of a transition metal selected from cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), copper (Cu) and chromium (Cr).

19. The system of claim 18, wherein the metal salt precursor is cobalt chloride (C0CI2) or cobalt sulphate (C0SO4).

20. The system of claim 19, wherein the oxidation catalyst comprises an aqueous suspension of cobalt oxide/hydroxide particles supported on silica gel particles.

21. The system of any one of claims 1 to 3, wherein said separating and reactor-controlling unit (2) comprises at least one of the following processing units: a crossflow filtration unit (6), a filter, a crossflow separator, a mixer-settler, a decanter, or a tricanter.

22. The system of claim 21 , wherein said crossflow filtration unit (6) comprises a cascade (plurality) of crossflow filtration units or separators capable of separating floating solid particles of the catalyst from aqueous solution of the produced salts, based on the particle size of the catalyst and obtained salts.

23. The system of any one of claims 1 to 3, wherein said separating and reactor-controlling unit (2) further comprises a salt separation vessel configured to receive an aqueous solution containing the dissolved NaNCh, NaNCb and Na₂SC>₄ salts and separate them.

24. The system of any one of claims 1 to 3, wherein said separating and reactor-controlling unit (2) further comprises sensors for measuring and controlling temperature and pH of the processed liquids.

25. The system of claim 2, further comprising a separate oxidation chamber (4) connected to the oxygen concentrator and configured to receive the air stream enriched with atmospheric oxygen from the oxygen concentrator and a stream of the flue gas containing NO_x and SO_x from the ship engine, said oxidation chamber (4) is filled with the oxidation catalyst capable of catalysing oxidation of NO_x and SO_x in the flue gas.

26. The system of claim 3, further comprising a separate oxidation chamber (4) configured to receive the flue gas containing NO_x and SO_x from the ship engine and ambient air, said oxidation chamber (4) is filled with the oxidation catalyst capable of catalysing oxidation of NO_x and SO_x in the flue gas.

27. The system of claim 25 or claim 26, wherein said oxidation chamber (4) is dry and packed with inert solids promoting a better contact between said gases.

28. The system of claim 27, wherein said inert solids are ceramic beads.

29. The system of claim 25 or claim 26, wherein said oxidation chamber (4) is wet containing a liquid circulating inside.

30. The system of claim 3, wherein the activation reagent is selected from any suitable strong oxidizing agent, such as hydrogen peroxide (H2O2) or benzoyl peroxide.

31. A method for catalytic oxidation and removal of nitrogen oxides (NO_x) and sulphur oxides (SO_x) simultaneously from a flue gas in a ship engine, comprising:

I. Catalytic oxidation of NO_x and SO_x contained in the flue gas emitted from the ship engine with oxygen, wherein said catalytic oxidation yields the oxidised NO_x and SO_x, said catalytic oxidation is carried out in the oxidation reactor (1) of the system of any one of claims 1 to 30; and

II. Wet-scmbbing of the oxidised NO_x and SO_x with an adsorbing dispersion of an oxidation catalyst suspended in water, wherein said wet-scrubbing yields nitrous, nitric and sulphuric acids dissolved in water.

32. The method of claim 31 , further comprising Step (III) of simultaneously removing the nitrous, nitric and sulfuric acids from water by contacting them with sodium hydroxide to yield sodium nitrite (NaNC ), sodium nitrate (NaNC ) and sodium sulphate (Na₂S0₄) in aqueous solution.

33. The method of claim 32, further comprising:

IV. Separation and either collection or discharging of sodium sulphate into aquatic environment;

VI. Recylcing of water and catalyst remained in the aqueous solution after filtering off the obtained salts.

34. The method of any one of claims 31 to 33, wherein said method is carried out at the temperature of 50-90 °C.

35. The method of any one of claims 31 to 34, wherein said method is carried out at pH 8-10, said pH range is maintained with sodium hydroxide injection in order to convert the dissolved nitrous, nitric and sulphuric acids obtained after wet-scmbbing into their corresponding sodium salts.