Method for producing hydrogen

Abstract

The invention relates to a method and plant for producing hydrogen comprising an electrochlorination unit (10) and a water electrolysis unit (11). In the electrochlorination unit (10), seawater (12) or brine is electrolyzed to produce a liquid hypochlorite stream and an oxygen-polluted hydrogen gas stream. The hydrogen gas is separated from the liquid phase in a degasser vessel (18). High-purity hydrogen produced in the water electrolysis unit (11) from demineralized water (22) is divided into two portions, one portion (37) being mixed with the oxygen-polluted hydrogen in the degasser vessel (18) to form a non-flammable mixed gas, and the other portion (38) being supplied to an ejector (50) for compressing and further concentrating the mixed hydrogen. Optionally, residual oxygen is removed in a DE-OXO unit (54). The invention enables recovery and utilization of hydrogen from electrochlorination processes while improving overall hydrogen yield and safety.

Description

[0001] INDUSTRIE DE NORA S . p . A .

[0002] METHOD FOR PRODUCING HYDROGEN

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method for producing hydrogen from an electrochlorination unit and a water electrolysis unit .

[0005] PRIOR ART

[0006] Water electrolysis is a process whereby water is split into oxygen and hydrogen through the application of electrical current between a cathode and an anode . The reaction can take place under acidic or basic conditions . The overall electrochemical reaction of water electrolysis can be summari zed as follows .

[0007] Electrochlorination is defined as applying an electrical current to an electrolyte compris ing chloride ions , such as seawater, to produce hypochlorite and hydrogen . The overall electrochemical reaction of electrochlorination can be summari zed as follows .

[0008] NaCl + H2O + ENERGY NaOCl + H2

[0009] The concentration of sodium hypochlorite produced in electrochlorination of seawater is around 2 % . Such low concentration is suf ficient to treat big volumes of seawater and deemed safe to impart a biocide ef fect to the chlorinated seawater when used as cooling fluid in industrial applications or feedstock in desalination

[0010] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S.p.A. plants, it is known that hypochlorite concentration in seawater in the range of 0.5-2.0 ppm is enough to destroy eggs, larvae and other microorganisms present in water thus prevent biofouling. Additionally, electrolytic sodium hypochlorite from synthetic brine is utilized as common disinfecting agent for drinkable water treatment worldwide .

[0011] Disinfected water produced from seawater electrochlorination is currently used in many industries such as cooling circuits for coastal power stations, desalination plants, LNG evaporation plants, etc.

[0012] Electrochlorination process produces gaseous hydrogen as a byproduct. Yielded hydrogen is not pure hydrogen but rather combined with oxygen gas at a concentration of about 95-96% hydrogen and about 4-5% oxygen by volume. Hydrogen may be further polluted by air present in the electrolyzers, pipelines or storge units.

[0013] Hydrogen and oxygen form flammable gas mixtures when mixed together at a concentration equal or above 3.8% by volume of oxygen into a hydrogen body and a percentage equal or above than 4% by volume of hydrogen into an oxygen body under atmospheric pressure and room temperature. Thus, hydrogen concentration must be kept out of this range in order to ensure the safety of the electrochlorination plant.

[0014] Currently, the risk of combustion or explosion of oxygen-

[0015] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S . p . A . polluted hydrogen produced in electrochlorination plants is managed by immediately diluting the produced hydrogen with air until it reaches a value well below the lower flammability limit of hydrogen in air of 4 % , generally around 1 % of hydrogen in air stream .

[0016] Diluted hydrogen at the level of around 1 % cannot be feasibly exploited in any application . Therefore , it i s usually vented to atmosphere . Venting hydrogen, which is an indirect greenhouse gas , causes harm to the environment . Moreover, venting hydrogen is considered wasting of a valuable resource .

[0017] The Applicant has thus faced with the problem of finding a method for hydrogen production which allows the utili zation of hydrogen produced in electrochlorination plants in an economically and energetically feasible way . Additionally, the Applicant has faced with the problem of finding an alternative solution to the ris k of combustion or explosion of the oxygen-polluted hydrogen produced in electrochlorination plants .

[0018] SUMMARY OF THE INVENTION

[0019] The Applicant has now found a method for producing hydrogen through combining the hydrogen produced in an electrochlorination unit with a hydrogen produced in a water electrolysis unit . Speci fically, the Applicant has found that by combining the hydrogen produced in an

[0020] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S . p . A . electrochlorination unit with hydrogen produced in a water electrolysis unit , a safe and ef ficient overall hydrogen generation system can be achieved

[0021] The present invention relates also to a plant according to the attached claims .

[0022] By integrating both hydrogen sources , the present invention allows for the production of hydrogen from two di f ferent units in an economically and energetically feasible way, as well as reducing the amount of hydrogen emitted to the environment . Thus , the present invention provides a solution to the international regulatory requirements which aims at reducing greenhouse gas emissions .

[0023] BRIEF DESCRIPTION OF THE FIGURES

[0024] Fig . 1 shows a representation scheme of the present invention .

[0025] DETAILED DESCRIPTION OF THE INVENTION

[0026] For the purposes of the present invention, water electrolysis unit is a unit compri sing at least one anode , at least one cathode and an aqueous electrolyte , such as water, having a low chloride ion content , typically less than 100 ppm . The water electrolysis unit is adapted, upon applying electrical current , to produce hydrogen gas and oxygen gas .

[0027] For the purposes of the present invention, electrochlorination unit is a unit comprising at least

[0028] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S.p.A. one anode, at least one cathode and an electrolyte comprising a chloride salt, for instance sodium chloride. In a preferred embodiment, the electrolyte in the electrochlorination unit is an aqueous solution such as brine or seawater. The electrochlorination unit is adapted, upon applying electrical current, to produce hydrogen gas and hypochlorite.

[0029] For the purposes of the present invention, the lower flammability limit is defined as the minimum concentration by volume of a combustible substance, in this case hydrogen, that is capable of propagating a flame under specified conditions.

[0030] For the purposes of the present invention, the upper flammability limit is defined as the maximum concentration by volume of a combustible substance, in this case hydrogen, that is capable of continued propagation of a flame under specified conditions.

[0031] It is well known for the person skilled in the art that the flammability limits are not constant values, but rather varying based on different conditions such as pressure, temperature, and the composition of the oxidizing gas, such as air or oxygen.

[0032] For the purposes of the present invention, oxygen- polluted hydrogen gas is preferably a hydrogen gas having a percentage concentration in volume within the flammability limit, more preferably comprised between 90-96% by volume of hydrogen.

[0033] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S.p.A.

[0034] -6-

[0035] In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared.

[0036] For the purposes of the present invention the term "comprise" or "include" also comprises the term "consist in" or "essentially consisting of".

[0037] For the purposes of the present invention, the definitions of the ranges always comprise the extreme values unless otherwise specified.

[0038] A first object of the invention therefore relates to a method for producing hydrogen, which comprises: producing oxygen-polluted hydrogen gas and hypochlorite in an electrochlorination unit, the electrochlorination unit comprising at least one anode, at least one cathode and an electrolyte comprising a chloride salt,

[0039] - producing hydrogen gas in a water electrolysis unit, the water electrolysis unit comprising at least one anode, at least one cathode and an aqueous electrolyte having a chloride ion content of less than 100 ppm,

[0040] - separating the oxygen-polluted hydrogen gas from the hypochlorite,

[0041] - adding the hydrogen gas produced in the water electrolysis unit to the oxygen-polluted hydrogen gas to form a mixed gas. Preferably, the hydrogen gas produced

[0042] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S.p.A. in the water electrolysis unit is added to the oxygen- polluted hydrogen gas to form a mixed gas after the oxygen-polluted hydrogen gas has been separated from the hypochlorite .

[0043] The electrolyte comprising a chloride salt of the electrochlorination unit has a chloride ion content of more the 1,000 ppm, preferably more than 10,000 ppm.

[0044] In one embodiment, the electrolyte of the water electrolysis unit is obtained from demineralized water. Demineralized water is defined as water in which substantially all minerals and salts have been removed, reducing the total dissolved solids (TDS) concentration to a level less than 10 ppm, preferably between 1 and 10 ppm. The chloride ion content of demineralized water is less than 10 ppm, preferably less than 1 ppm. In certain embodiments, demineralized water is used directly in the water electrolysis unit, for instance in a DEM (Proton Exchange Membrane) water electrolysis unit. In other embodiments, such as an AWE (Alkaline Water Electrolysis) unit, alkali hydroxides such as KOH or NaOH are added to the demineralized water in order to obtain an alkaline aqueous electrolyte. The addition of alkali hydroxides will increase TDS above the threshold mentioned above but chloride ion content will remain low. Water electrolysis unit produces high purity hydrogen from demineralized water.

[0045] Water electrolysis units may be present in vicinity of electrochlorination units as the electrolyte of both can be obtained from seawater.

[0046] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S . p . A .

[0047] Deminerali zed water for the water electrolysis unit can be obtained from seawater through a desalination process , e . g . by reverse osmosis or distillation . Prior to desalination, the seawater can be subj ected to screening, coagulation, and ultrafiltration to remove particulates and organics . After desalination, ion exchange or electro-deioni zation can be applied to further reduce the TDS .

[0048] For electrochlorination, seawater can be used directly . Thus , in one embodiment , the electrolyte of the electrochlorination unit is seawater .

[0049] Preferably, the electrolyte of the water electrolysis unit and the electrolyte of the electrochlorination unit obtained from seawater . The method of the present invention using seawater is more energetically preferred compared to the use of only water electrolysis unit for the production of hydrogen since seawater entering the electrochlorination unit does not need desalination .

[0050] Advantageously, the hydrogen concentration in the mixed gas is adj usted to a value above the upper flammabi lity limit of hydrogen in oxygen-containing gas mixtures , i . e . the mixed gas contains such a high proportion of hydrogen that it lies outside the flammability range . In this condition, the mixed gas does not pose any explosion risk within the system .

[0051] Water electrolysis unit produces high purity hydrogen suitable to be mixed with the hydrogen produced in electrochlorination units without the need of

[0052] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S.p.A.

[0053] -9- additional filtration or concentration steps. Advantageously, the hydrogen gas produced in the water electrolysis unit has a concentration of at least 98% by volume, preferably at least 99% by volume.

[0054] Preferably, hydrogen gas produced in the water electrolysis unit is green hydrogen.

[0055] Green hydrogen is hydrogen produced using renewable energy sources, like wind or solar energy, to power water electrolysis processes. Green hydrogen is the most environmentally benign type of hydrogen.

[0056] Green hydrogen is particularly advantageous for the purpose of the present invention because the overall system, i.e. the electrochlorination unit and the water electrolysis unit, produces hydrogen with less greenhouse emissions.

[0057] In one embodiment, the water electrolysis unit is a PEM water electrolysis unit and its electrolyte is demineralized water. In another embodiment, the water electrolysis unit is an AWE unit and its electrolyte is demineralized water with added alkali hydroxide such as KOH or NaOH. Alkaline water electrolysis (AWE) is considered one of the most industrially promising type of water electrolysis, where electrodes, i.e. a cathode and an anode, operate in alkaline electrolyte such as aqueous potassium hydroxide or aqueous sodium hydroxide. AWE is usually carried out in divided cells, i.e. the electrolyzer cells of the AWE unit further comprise a porous diaphragm placed between the electrodes to

[0058] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S . p . A . separate the produced hydrogen gas and oxygen gas whi le ensuring the ionic conductivity of the system .

[0059] In contrast , electrochlorination is usually carried out in undivided cells , i . e . the cells of the electrochlorination unit have no diaphragm or membrane arranged between the anode and cathode . The goal o f electrochlorination is often to produce a hypochlorite solution directly in si tu, rather than separate chlorine gas and sodium hydroxide ( as in the chlor-al kal i process ) . The mixing in an undivided cell enables the immediate reaction between Cl2produced at the anode and NaOH formed from OH- produced at the cathode to form NaOCl , avoiding the need for additional mixing steps or handling hazardous chlorine gas .

[0060] Advantageously, the cathode and / or the anode of the water electrolysis unit comprises a catalytic coating . The catalytic coating reduces the voltage of the cell , and the energy consumption needed for the production of hydrogen .

[0061] Preferably, the catalytic coating comprises metal s selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal . More preferably, the one or more noble metal is selected from a group consisting of platinum, palladium, ruthenium and combinations thereof , said rare earth metal is praseodymium, even preferably, the noble metal is a mixture of platinum and palladium, the rare earth metal is praseodymium or said noble metal i s ruthenium,

[0062] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S . p . A . the rare earth metal is praseodymium .

[0063] The noble metal allows for ef ficient HER catalysts while the rare earth metal can improve the catalysts robustness and decrease the amount of noble metal required .

[0064] Preferably, the method further comprises a step of further adding of a combustible gas to the mixed gas to form a second mixed gas , wherein the combustible gas i s selected from a group consisting of hydrogen, carbon monoxide , a hydrocarbon, or a combination thereof . The further adding of the combustible gas has the purpose of further reducing the oxygen concentration and optionally the purpose of increasing the pressure of the mixed gas to levels suitable for speci fic applications . Then, the second mixed gas should have oxygen concentration less than the oxygen concentration of the mixed gas .

[0065] In according to particularly preferred embodiment , in case the hydrogen is to be used as fuel , for example for a steam generator, a gas turbine or a burner, a hydrocarbon may be used as a combustible gas to be mixed with the mixed gas .

[0066] Mixtures of hydrogen and hydrocarbons are already used in various industrial processes as a relatively clean fuel .

[0067] Advantageously, the adding of the combustible gas to the mixed gas may take place inside an ej ector adapted to reduce oxygen concentration of the mixed gas and, at the

[0068] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S . p . A .

[0069] - 12 - same time , increase the pressure of the mixed gas . In one embodiment , the mixed gas is compres sed and further concentrated in hydrogen in an ej ector that entrains the mixed gas using high-pressure hydrogen from the water electrolysis unit .

[0070] Preferably, the method further comprises a step of removing oxygen from the mixed gas or the second mixed gas . Oxygen removal from the mixed gas or the second mixed gas may take place in a DE-OXO reactor . Oxygen removal increases the purity of hydrogen gas up to 99 , 999% , which may be required in certain applications .

[0071] A second obj ect of the invention relates to a plant comprising : an electrochlorination unit , the electrochlorination unit producing oxygen-polluted hydrogen gas and hypochlorite , the electrochlorination unit comprising at least one anode , at least one cathode and an electrolyte comprising a chloride salt ,

[0072] - a water electrolysis unit producing hydrogen, the water electrolysis unit comprising at least one anode , at least one cathode and an aqueous electrolyte having a chloride ion content of less than 100 ppm, and a degasser vessel connected with the electrochlorination unit and the water electrolysis unit , the degasser vessel being adapted for separating the oxygen-polluted hydrogen from the hypochlorite and for mixing the hydrogen gas with oxygen-polluted

[0073] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S . p . A . hydrogen gas to form a mixed gas .

[0074] Advantageously, the mixed gas has a percentage concentration in volume higher than the upper flammability limit of the mixed gas .

[0075] Preferably, the plant further comprising an ej ector adapted to reduce oxygen concentration of the mixed gas and increase the pressure of the mixed gas .

[0076] Advantageously, the plant further comprising a DE-OXO reactor adapted to remove oxygen from said mixed gas .

[0077] Examples

[0078] The examples are provided for illustrative purposes only of the present invention and must not be understood as limiting the scope of protection defined by the appended claims .

[0079] Example 1

[0080] In Example 1 , the present invention is described in more detail in connection with a schematic process flow diagram shown in Fig . 1 . Oxygen-polluted hydrogen obtained from an electrochlorination unit 10 is mixed with fresh, high-purity hydrogen coming from a water electrolysis unit 11 . Speci fically :

[0081] Seawater 12 is fed via a seawater feed line 13 into the electrochlorination unit 10 . In this example , the electrochlorination unit 10 is configured as an undivided cell , or as a stack of undivided cells ,

[0082] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S.p.A. exemplified by an undivided cell compartment 14 comprising an anode 15 and cathode 16. Upon applying electrical current, the electrochlorination unit 10 produces a mixture comprising a liquid hypochlorite stream and a gaseous stream of oxygen-polluted hydrogen gas containing typically 96 % H2and 4 % O2by volume. The two-phase mixture is discharged and transferred via discharge line 17 to a degasser vessel 18.

[0083] The degasser vessel 18 comprises a separator compartment 19 where the liquid hypochlorite phase is separated from the gaseous phase. The hypochlorite solution is withdrawn through an outlet line 20 to a hypochlorite outlet 21 where it can be stored or used, for instance as a disinfectant or anti-fouling agent.

[0084] Hydrogen is also produced in the water electrolysis unit 11 which is fed with demineralized water 22 via water feed line 23. As an optional feature, denoted in Fig. 1 by dashed box A, the demineralized water 22 can be produced from seawater 12a which is fed into a desalination unit 24 via feed line 25. The seawater 12 and the seawater 12a may originate from the same source. Demineralized water produced in the desalination unit is then transferred to the water electrolysis unit 11 via a transfer line 26 and the above-mentioned line 23. In the present example, the water electrolysis unit 11 is configured for alkaline water electrolysis (AWE) . Accordingly, demineralized water is not used directly as the electrolyte. Instead, an alkali hydroxide 27, for instance KOH, is added and mixed with the demineralized

[0085] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S.p.A. water in a mixing chamber 28, and the resulting alkaline electrolyte is fed into the water electrolysis unit 11. The water electrolysis unit 11 comprises a divided cell, or a stack of divided cells, exemplified in Fig. 1 by an anodic compartment 29 equipped with an anode 30, and a cathodic compartment 31 equipped with a cathode 32. The anodic and cathodic compartments are separated by a suitable separator 33, for instance a membrane or a diaphragm. Upon applying electrical current, oxygen 34 is produced in the anodic compartment and discharged via a line 35. In the cathodic compartment, high purity hydrogen gas is produced and discharged from the cathodic compartment 31 via a hydrogen discharge line 36 at a concentration of at least 99.5% and a pressure of 24 - 30 barg (bar gauge) .

[0086] Hydrogen produced in the water electrolysis unit is split in two portions 37, 38. The first portion 37 is used in a first concentrating step to increase the concentration of hydrogen in the oxygen-polluted hydrogen stream from the electrochlorination unit to a level higher than the upper flammability limit of hydrogen. To this effect, the degasser vessel 18 further comprises a mixing compartment 39, which receives the oxygen-polluted hydrogen stream after its separation from the liquid hypochlorite in the separator compartment 19. The first portion 37 of the 99.5% hydrogen stream from the water electrolysis unit 11 is fed into the mixing compartment 39 of the degasser vessel 18 via a lamination valve 40 and is mixed directly with the oxygen-polluted hydrogen from electrochlorination unit.

[0087] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S.p.A.

[0088] After this first concentrating step, the resulting oxygen-polluted hydrogen has a hydrogen concentration of about 97% at an atmospheric pressure, i.e., above the upper flammability limit, thereby eliminating explosion or combustion risks within the system. The hydrogen stream is then transferred via a line 41 to a scrubbing tower 42. Raw water 43, supplied to the scrubbing tower 42 via line 44, serves as a scrubbing medium for removing residual corrosive or oxidizing impurities that may still be entrained in the gas stream, such as hypochlorite droplets, chlorine traces or dissolved salts. The scrubbing agent together with the removed contaminants is discharged as drain water 45 via a drain line 46.

[0089] The scrubbed oxygen-polluted hydrogen gas stream (still having 97% hydrogen concentration at atmospheric pressure) is transferred to a demister 47 via a line 48 to remove any remaining hypochlorite mist to limit the quantity of droplets before the gas stream is transferred via a line 49 into the Venturi profile of an ejector 50. The liquid removed from the gas stream is discarded as drain water 51 via a line 52.

[0090] The second portion 38 of high-pressure, high purity hydrogen from the water electrolysis unit 11 is also delivered to the ejector 50. The ejector 50 draws in the mixed gas from line 49 to conduct a second concentrating step of hydrogen, while simultaneously compressing the mixed hydrogen from atmospheric pressure to 6-8 barg. Hydrogen leaving the ejector 50 via line 53 has a

[0091] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S.p.A.

[0092] - 17- hydrogen concentration of at least 98% by volume and a pressure of around 8 barg.

[0093] As a further optional feature denoted by dashed box B, a final DE-OXO unit 54 may be installed in case high purity hydrogen (>99.9% by volume) is required. In the DE-OXO unit 54, traces of oxygen in the mixed gas are catalytically reacted with hydrogen to produce water, which is continuously removed through a drain line 55. The resulting purified hydrogen stream, now essentially free of oxygen, exits the DE-OXO unit via a line 56 and is suitable for direct industrial use or storage.

[0094] The recovery of hydrogen portion from electrochlorination unit reduces the Total Cost of hydrogen produced by the water electrolysis unit.

[0095] M / 66044-PCT (404B PCT) INDUSTRIE DE NORA S . p . A .

[0096] List of reference signs

[0097] 10 Electrochlonnation unit

[0098] 11 Water electrolysis unit

[0099] 12 Seawater feed

[0100] 12a Seawater for desalination

[0101] 13 Seawater feed line

[0102] 14 Undivided cell compartment

[0103] 15 Anode of electrochlorination unit

[0104] 16 Cathode of electrochlorination unit

[0105] 17 Discharge line

[0106] 18 Degasser vessel

[0107] 19 Separator compartment of degasser vessel

[0108] 20 Hypochlorite outlet line

[0109] 21 Hypochlorite outlet

[0110] 22 Deminerali zed water

[0111] 23 Water feed line to water electrolysis unit

[0112] 24 Desalination unit

[0113] 25 Seawater feed line

[0114] 26 Trans fer line

[0115] 27 Alkali hydroxide

[0116] 28 Mixing chamber

[0117] 29 Anodic compartment of water electrolysis unit

[0118] 30 Anode of water electrolysis unit

[0119] 31 Cathodic compartment of water electrolysis unit

[0120] 32 Cathode of water electrolysis unit

[0121] M / 66044-PCT ( 404B PCT ) INDUSTRIE DE NORA S . p . A .

[0122] - 19 -

[0123] Separator

[0124] Oxygen stream from anodic compartment

[0125] Oxygen discharge line

[0126] Hydrogen discharge line

[0127] First portion of hydrogen

[0128] Second portion of hydrogen

[0129] Mixing compartment of degasser vessel

[0130] Lamination valve for feeding first hydrogen portion

[0131] Line from degasser vessel to scrubbing tower

[0132] Scrubbing tower

[0133] Raw water ( scrubbing agent )

[0134] Raw water feed line to scrubbing tower

[0135] Drain water from scrubbing tower

[0136] Drain line from scrubbing tower

[0137] Demister

[0138] Line from scrubbing tower to demister

[0139] Line from demister to ej ector

[0140] E ector

[0141] Drain water from demister

[0142] Drain line from demister

[0143] Line from ej ector to DE-OXO unit

[0144] DE-OXO unit ( oxygen removal reactor )

[0145] Drain line from DE-OXO unit

[0146] Line for puri fied hydrogen outlet from DE-OXO unit

[0147] M / 66044-PCT ( 404B PCT )

Claims

INDUSTRIE DE NORA S.p.A.-20-CLAIMS1. Method for producing hydrogen, which comprises : producing oxygen-polluted hydrogen gas and hypochlorite in an electrochlorination unit, the electrochlorination unit comprising at least one anode, at least one cathode and an electrolyte comprising a chloride salt,- producing hydrogen gas in a water electrolysis unit, the water electrolysis unit comprising at least one anode, at least one cathode and an aqueous electrolyte having a chloride ion content of less than 100 ppm,- separating the oxygen-polluted hydrogen gas from the hypochlorite,- adding the hydrogen gas produced in the water electrolysis unit to the oxygen-polluted hydrogen gas to form a mixed gas .

2. Method according to claim 1, wherein the electrolyte of the water electrolysis unit is obtained from demineralized water.

3. Method according to claim 2, wherein the demineralized water is obtained from seawater.

4. Method according to any one of claims 1 to 3, wherein the electrolyte of the electrochlorination unit is seawater.M / 66044-PCT (404B PCT)INDUSTRIE DE NORA S.p.A.

5. Method according to any one of the preceding claims, wherein the hydrogen concentration in the mixed gas is adjusted to a value above the upper flammability limit of hydrogen in oxygen-containing gas mixtures.

6. Method according to any one of the preceding claims, wherein the hydrogen gas produced in the water electrolysis unit has a concentration of at least 98% by volume, preferably at least 99% by volume.

7. Method according to any one of the preceding claims, wherein the water electrolysis unit is alkaline water electrolysis unit.

8. Method according to any one of the preceding claims, wherein the water electrolysis unit comprises a cathode and an anode, wherein the cathode and / or the anode comprises a catalytic coating, preferably the catalytic coating comprises metals selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal.

9. Method according to any one of the preceding claims, further comprising the step of:- adding a combustible gas to the mixed gas forming a second mixed gas, the second mixed gas having oxygen concentration less than the oxygen concentration of the mixed gas, wherein the combustible gas is selected from a group consisting of hydrogen, carbon monoxide, a hydrocarbon, or a combination thereof.

10. Method according to any one of the precedingM / 66044-PCT (404B PCT)INDUSTRIE DE NORA S.p.A. claims, further comprising the step of: removing oxygen from the mixed gas or the second mixed gas .

11. Plant comprising: an electrochlorination unit producing oxygen- polluted hydrogen gas and hypochlorite, the electrochlorination unit comprising at least one anode, at least one cathode and an electrolyte comprising a chloride salt,- a water electrolysis unit producing hydrogen gas, the water electrolysis unit comprising at least one anode, at least one cathode and an aqueous electrolyte having a chloride ion content of less than 100 ppm, and a degasser vessel connected with the electrochlorination unit and the water electrolysis unit, the degasser vessel being adapted for separating the oxygen-polluted hydrogen from the hypochlorite, and for mixing the hydrogen gas with oxygen-polluted hydrogen gas to form a mixed gas .

12. Plant according to claim 11, wherein the hydrogen concentration in the mixed gas is adjusted to a value above the upper flammability limit of hydrogen in oxygen-containing gas mixtures.

13. Plant according to any one of claims 11 or 12, wherein the plant further comprises a DE-OXO reactor adapted to remove oxygen from the mixed gas.M / 66044-PCT (404B PCT)

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