Process and system for producing hydrogen and / or syngas

Abstract

A process for producing hydrogen and / or syngas comprises a first operation mode, wherein an active metal is reduced electrolytically to at least one first electrode (E1) immersed in an electrolytic solution, generating a solution enriched with negative ions, and the solution enriched with negative ions is used on a second electrode (E2 ) to produce oxygen gas; and a second operation mode, wherein the reduced metal of the first electrode (E1) is thermochemically oxidised in an oxidation reaction with oxidation reagents producing hydrogen or synthesis gas.

Description

[0001] PROCESS AND SYSTEM FOR PRODUCING HYDROGEN AND / OR SYNGAS

[0002] Cross-Reference to Related Applications

[0003] This Patent Appl ication claims priority from Italian Patent Application No . 102024000021744 filed on October 1 , 2024 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical Field

[0005] This invention relates to a process and system for producing hydrogen and / or synthesis gas ( syngas ) from a source of electricity as well as water and / or CO2 and / or a mixture rich in hydrocarbons and CO2 .

[0006] Background

[0007] As is known, hydrogen and synthesis gas ( syngas ) can be produced electrolytically .

[0008] For example , alkaline electrolytic cells , but also PEM ( Proton Exchange Membrane ) or AEM (Anion Exchange Membrane ) cells are known to be used to produce hydrogen, even on an industrial scale .

[0009] For syngas production, SOECs ( Solid Oxide Electrolyzer Cells ) have also been known to be used .

[0010] So-called "membraneless" electrolysis systems have also been proposed, such as the process known as "ETAC" (Electrochemical-Thermally Activated Chemical Process ) , but also systems based on other fluid-dynamic or even magnetic segregation techniques . These systems , although tested in the laboratory, are so complex and costly that there are currently no applications for them on an industrial scale .

[0011] US2021 / 261407A1 describes a process for producing hydrogen through a thermochemical reaction in which a metal is brought into contact with water vapour to produce a metal oxide and / or hydroxide and hydrogen; the metal oxide and / or hydroxide are / is then brought into contact with a basic aqueous solution to produce a solution containing a metal ion; finally, an electrochemical reaction is conducted by applying a voltage between an anode and a cathode , where the cathode comes into contact with the solution containing the metal ion to produce hydrogen, oxygen and the metal . This process involves an electrochemical step in which both hydrogen and oxygen are produced, thus posing the problem of separating the two components . In addition, this process involves the use of a metal ion solution, thus requiring a circulation of metal in powder form ( a complex circulation to achieve by handling a mixture o f solids and liquids that is di f ficult to pump ) and a step for dissolving the metal to form ions . The use of metal ions for the electrolytic step also entails risks of overvoltage and deposition on the electrodes , ruling out the possibility of using electrodes made of standard material and common electrolyte solutions .

[0012] Rodriguez-Santiago et al . "Study o f the electrochemical step of novel active metal alloy thermochemical cycles for hydrogen production" (ECS Transactions , Vol . 11 ) describes an electrochemical cycle based on the K-Bi system, in which electrochemical reactions take place in a liquid phase of the K-Bi alloy with molten potassium hydroxide ; thi s requires both special components and high operating temperatures , and thus high costs . Therefore , the process is complex and expensive .

[0013] CN113278984A describes a system for producing hydrogen through the oxidation of water vapour and oxygen by electrolytic reduction of aluminium oxide . The system involves circulation of metal ( aluminium) and electrolysis at a very high temperature ( over 1000 ° C ) with solids ( alumina, aluminium, cryolite ) in a molten state , thus involving great complexity and high costs .

[0014] The aim of this invention is to provide a process and system for producing hydrogen and / or syngas that increase ef ficiency and reduce overall costs compared to known systems based on electrolysis , both in terms of operating costs and installation costs .

[0015] In addition, another purpose of the invention is to provide a process and system for producing hydrogen and / or synthesis gas that does not require the use of hard-to- find materials and allows for easy scalability in terms of production volumes . Summary

[0016] This invention thus relates to a system and process for producing hydrogen and / or synthesis gas as defined in claims 1 and 13 , respectively .

[0017] Additional features of the invention are defined in the dependent claims .

[0018] According to the invention, a membrane- free electrolysis system operating in an alkaline environment where the production of hydrogen / synthesis gas is decoupled from the production of oxygen, taking place in two di f ferent steps , is thus used .

[0019] In a first step of a purely electrolytic nature , an active metal ( for example , a transition metal ) is reduced, while in a second step of a thermochemical nature , the reduced metal is oxidised, producing hydrogen or synthesis gas .

[0020] Here and in the following, an "active metal" means a transition metal that selectively assumes an oxidised state ( in which the metal has given up electrons ) , and a reduced state ( in which the metal has received electrons and / or is in its metallic form) . The active metal can thus be either thermodynamically oxidised, resulting in the production of a hydrogen molecule , or electrolytically reduced by switching from an oxide metal state to a metal ( or to a lower oxidation state ) . The active metal that is alternately reduced and oxidised is the one present on the hydrogen evolution electrode and is not in fact dissolved in the electrolyte solution ( except to a very smal l extent ) , but remains on that electrode ; the electrolyte solution, which is enriched with negative ions (particularly OH- ions ) during the reduction of the active metal and is essentially devoid of metal ions , is used on the oxygen evolution electrode to produce oxygen gas ; after removing the electrolyte solution enriched with negative ions from the hydrogen evolution electrode , the reduced metal at this electrode is thermochemically oxidised in an oxidation reaction with oxidation reagents , producing hydrogen and / or synthesis gas . In this way, the production of hydrogen or synthesis gas and the production of oxygen occur separately .

[0021] Both the hydrogen evolution electrode ( that is , the one used first for the reduction of the active metal and then in the oxidation reaction for the hydrogen evolution) and the oxygen evolution electrode are mainly made from non-precious and widely available metals , and possibly with a surface layer of their oxides , without using rare earths .

[0022] In particular, the hydrogen evolution electrode comprises a transition metal as its main component (with a content above 50% ) , especially iron, or steel , and, optionally, a layer of surface coating made of a metal oxide , in particular a transition metal oxide , for example an iron or nickel oxide .

[0023] For example , the hydrogen evolution electrode may be based on ( that is , contain as its main component ) iron or mild steel and have a metal-mesh structure ( for example , with a mild steel mesh possibly covered with iron or a ferrous alloy) .

[0024] However, other structural types of electrode are possible : metal foam or metal foam coated with metal oxides ; metal oxide pellets or random fillings of metal oxides ( or covered with metal oxides ) ; solutions / slurries rich in oxidised metal ( to be recirculated and dried in a dedicated section) .

[0025] In essence , the invention employs , for producing hydrogen or synthesis gas ( as well as oxygen) , an oxidoreductive cycle based on a metal ( in particular a transition metal ) and occurring in two distinct steps : a reduction step based on an electrochemical mechanism (via an electrolytic reaction) , in which oxygen is produced; and an oxidative phase of the metal , in which hydrogen / syngas is produced and based on a thermochemical principle ( oxidation of the metal at medium / high temperature ) .

[0026] In the reduction step, the active metal that is reduced is present on the surface of the hydrogen evolution electrode in an oxidised form; note that according to the invention, an active metal in oxidised form is formed on the surface of the hydrogen evolution electrode during the oxidation step, thus restoring the metal layer in an oxidised form at each step . The formation of the surface oxide layer does not af fect the electrolytic reactions conducted on the electrode , as it only af fects the surface of the electrode , which remains conductive in its underlying, metal part .

[0027] Continuous production of oxygen and hydrogen / syngas is also possible with the introduction of an additional section .

[0028] Speci fically, in the first step, the metal oxide ( for example , iron) is reduced at a first electrode , and oxygen is evolved at the other electrode ( for example based on titanium or nickel or another metal ) . The reaction takes place in an alkaline electrolyte solution, for example KOH or NaOH in the liquid phase (with concentrations between 5% and 90% , but preferably around 50% ) or in another electrolyte solution or even in a salt solution or pure water . The temperature , depending on the operating pressure , is indicatively between 50 ° C and 420 ° C, preferably (but not necessarily) operating under saturated conditions .

[0029] In the second step, after removal of the electrolyte solution (possibly preceded by a dedicated drying step, for example with steam / air, and / or a pre-wash step with clean water to remove solution residues ) the metal / metal oxide (preferably iron / iron oxide ) in a reduced state compared to the previous step is exposed to steam at medium / high temperature (or a stream of CO2 or a mixture of hydrocarbons, steam, CO2 ) generating hydrogen during oxidation of the metal. In this step, one operates for example in a temperature range between 250°C and 980°C, preferably in a mid-temperature range below or equal to 580°C and above or equal to 380°C.

[0030] The invention thus achieves the following main advantages with respect to the prior art.

[0031] Compared to known technologies for producing hydrogen / syngas electrolytically, the invention reduces overall costs, both in terms of operating costs, as it is more efficient, and in terms of installation costs, as it requires a simpler configuration and avoids the use of noble metals or rare earths in the electrodes.

[0032] In particular, this invention achieves an overall efficiency of around 85% and even higher.

[0033] In addition, by using more readily available materials, the invention enables production even in countries with shortages of the strategic raw materials normally required by known technologies, as well as easier scalability in terms of production volumes.

[0034] In particular, the electrodes are based on widely available non-precious metals, which therefore do not suffer from the risks of price f luctuations / increases typical of conventional materials such as noble metals and rare earths , and on industry-standard processing techniques and materials , so that they can be easily manufactured and industrialised .

[0035] The electrodes and hal f cells also have a very simpli fied configuration and are therefore easy to produce at a low cost .

[0036] The system of the invention is not af fected by the pressure limitation associated with the mechanical resistance of membrane systems and can therefore also operate at high pressures .

[0037] Since according to the invention there is no production of H2 and 02 in the same step , the risk of mixing is ef fectively eliminated and a better turndown value can be achieved than with standard alkaline cells .

[0038] In certain embodiments , the invention also makes it possible , by decoupling the H2 and 02 production steps , to store excess electricity .

[0039] It is also possible to use the invention to produce a synthesis gas instead of a pure hydrogen stream, using both a 002 stream and a 002 -rich hydrocarbon mixture as raw material .

[0040] Brief Description of the Drawings

[0041] The invention is further described in the following non-limiting embodiments , with reference to the figures in the accompanying drawings in which :

[0042] - Figure 1 is a schematic view of a hydrogen / syngas production system according to the invention, implementing a first operation mode of the process of the invention;

[0043] - Figure 2 is a schematic view of the system in Figure 1 implementing a second operation mode of the process of the invention;

[0044] Figure 3 is a schematic view of a variant of a component of the system of the invention, to be used in a di f ferent embodiment of the process of the invention .

[0045] Description of Embodiments

[0046] With reference to Figures 1 and 2 , a hydrogen and / or synthesis gas ( syngas ) production system 1 comprises : a first vessel VI housing a first electrode El ; a second vessel V2 housing a second electrode E2 ; a third vessel V3 housing a third electrode E3 ; an electrical circuit 2 connected to an electrical source 3 and to the electrodes El , E2 , E3 for supplying power to the electrodes El , E2 , E3 ; a circuit 4 in which an electrolyte solution circulates and which connects the vessels VI , V2 , V3 and a first and a second vessel V4 , V5 for liquid / gas separation; a reagent circuit 5 equipped with a first and a second heat recovery section Rl , R2 .

[0047] The electrodes El , E3 are the hydrogen / syngas evolution electrodes and contain an active metal as defined above . The electrode E2 is the oxygen evolution electrode . The circuit 4 comprises : a supply line 11 that is equipped with a circulation pump 12 and a valve G, to supply solution to an inlet 13 of the f irst vessel VI ; an outlet line 14 connecting an outlet 15 of the first ves sel VI to the fourth vessel V4 , passing through the heat recovery section Rl , and equipped with a valve D placed between the outlet 15 and the heat recovery section Rl ; a connecting line 16 branching of f from the line 14 upstream of the valve D and connected to an inlet 17 o f the second vessel V2 and fitted with a valve F; a second outlet line 18 connecting an outlet 19 of the second vessel V2 to the fifth vessel V5 , passing through the heat recovery section R2 ; a recirculation line 20 connecting the fourth and fi fth vessels V4 , V5 with the supply line 11 ; an additional supply line 21 connecting the supply line 11 with an inlet 22 of the third vessel V3 and equipped with a valve C ; another connecting line 23 connecting an outlet 24 of the third vessel V3 with the inlet 17 of the second vessel V2 and equipped with a valve E ; a connecting line 25 departing from the line 23 , upstream of the valve E , and connecting with the line 14 , downstream of the valve D, and equipped with a valve H .

[0048] The reagent circuit 5 comprises a water supply line 31 , which connects a water inlet 32 with the heat recovery sections Rl , R2 , and a steam line 33 , which brings steam produced in the heat recovery sections to the vessels VI , V3 . In particular , the water supply line 31 is divided into a first branch 34 , equipped with a valve that feeds a heat exchanger XI and a superheater SI of the heat recovery section Rl , arranged in series ; and a second branch 35 , which feeds a heat exchanger X2 of the heat recovery section R2 and flows into the first branch 34 upstream of the superheater S I . The steam line 33 departs from the superheater S I and is divided into a first branch 36 , which connects to the line 11 and is thus connected to the inlet 13 of the first vessel VI , and a second branch 37 , which connects to the line 21 and is thus connected to the inlet 22 of the third vessel V3 ; the branches 36, 37 are equipped with respective valves A, B .

[0049] The heat recovery sections Rl , R2 may also comprises additional heat exchangers , for example air coolers Cl , C2 traversed by the lines 14 , 18 ,

[0050] Optionally, a supplementary supply line 38 connects a reagent inlet 39 to the line 33 , connecting to the line 33 upstream of the two branches 36 , 37 , to supply additional steam and / or oxidation reagents ( CO2 , CO, H2 , hydrocarbons , etc . ) to the first and third vessels VI , V3 .

[0051] The circuits 4 and 5 define feeding means to selectively feed the electrolyte solution to the first electrode El to reduce the active metal electrolytically and enrich the solution with negative ions and generate oxygen gas at the second electrode E2 ; and the oxidation reagents to thermochemically oxidise the reduced metal at the first electrode El , producing hydrogen or synthesis gas .

[0052] In use , the system 1 operates by implementing the process of the invention as follows .

[0053] The process according to this invention is carried out , for example using the system in Figures 1 and 2 , in two operation modes : in a first operation mode ( Figure 1 ) there is an electrolytic reduction of the active metal at the electrode El of the first vessel VI , filled with electrolyte solution, to generate a solution enriched with OH~ ions , and oxygen is evolved at the electrode E2 of the second vessel V2 using the enriched solution produced in the first vessel VI ; in a second operation mode ( Figure 2 ) , hydrogen is produced in the first vessel VI , previously used for metal reduction, by thermochemically oxidising the metal reduced in the previous mode .

[0054] For there to be a continuous process , the first and third vessels VI and V3 are used alternately for the reduction of the active metal electrolytically and for the thermochemical oxidation of the reduced metal : when the active metal is reduced in the first vessel VI , hydrogen is produced in the third vessel V3 ( Figure 1 ) , and vice versa ( Figure 2 ) . The first and third vessels VI , V3 are then used alternately in an active metal reduction mode and in a reduced metal oxidation mode to produce hydrogen; in the vessel V2 , oxygen is produced using the negative ion-enriched solution coming alternately from the first and third vessels VI , V3 .

[0055] In the first operation mode shown in Figure 1 , the electrical circuit 2 connects the electrodes El , E2 in the first and second vessels VI , V2 (while the electrode E3 is not connected / powered) .

[0056] The first vessel VI is supplied with electrolyte solution via the inlet 13 ; the solution circulates in the line 11 and through the valve G, which is open, driven by the pump 12 .

[0057] The valve C is closed, thus , the third vessel V3 is not supplied with the solution .

[0058] In the first vessel VI , the electrode El metal changes from the oxidised state to the reduced metal state ( so-called "electrosmelting" ) at the electrode El ; no gas is produced in the first vessel VI at this step, and the solution is enriched with OH- ions- .

[0059] The solution enriched with OH- ions- exits the first vessel VI through the outlet 15 and circulates by pressure di f ference in the line 16 through the valve F, which is open, and is fed to the second vessel V2 via the inlet 17 ; the valve D on the line 14 is closed, so that the solution exiting the first vessel VI is sent to the line 16 and not to the other line 14 .

[0060] In the second vessel V2 , gaseous molecular oxygen evolves at the electrode E2 ; the solution leaves through the outlet 19 dragging oxygen with it and circulates in the line 18 to the fi fth vessel V5 , where the oxygen is separated from the solution and sent to an oxygen outlet 42 , which is the pressurised battery limit of the system 1 .

[0061] Upstream of the fi fth vessel V5 , the line 18 runs through the heat recovery section R2 , which is fed through the line 31 with a water stream from the water inlet 32 ; in the heat exchanger X2 of the heat recovery section R2 , the solution gives of f heat and steam is produced, which is sent through the line 33 and branch 37 (valve B being open and valve A closed) to the inlet 22 of the third vessel V3 .

[0062] The solution leaving the fi fth vessel V5 through a liquid outlet 43 circulates in the line 20 and returns by pressure di f ference to the pump 12 , closing the solution path .

[0063] In this operation mode , hydrogen is produced in the vessel V3 .

[0064] The third vessel V3 is supplied with oxidation reagents consisting of steam from the heat recovery sections Rl , R2 via the line 33 , which has the valve B open (while the other valve A is closed) , and optionally additional steam and / or a gaseous mixture rich in CO2 and / or hydrocarbons , supplied via the reagent inlet 39 and the l ine 38 , which connects to the other line 33 .

[0065] As will be explained below, the electrode E3 of the third vessel V3 retains metal in a reduced state , which has remained during the second operation mode : the metal present in the electrode E3 , in a reduced state , is oxidised by the mixture fed into the third vessel V3 , generating a gaseous mixture rich in H2 ( or synthesis gas , that is , a mixture of hydrogen and CO, i f the supply includes CO2 and / or hydrocarbons ) that exits the outlet 24 and circulates by pressure di f ference in the line 25 and through the valve H, which is open, and then into the heat recovery section Rl , where it passes through the superheater S I and the heat exchanger XI in series . The valve E is closed, preventing circulation of solution in the line 23 .

[0066] The heat recovery section Rl is supplied through the line 31 with the water stream coming from the water inlet 32 and which also supplies the heat recovery section R2 ; steam is produced in the heat exchanger XI of the heat recovery section Rl which is superheated, together with the steam produced in the heat exchanger X2 of the heat recovery section R2 , in the superheater S I and sent through line 33 to the inlet 22 of the third vessel V3 .

[0067] The gaseous mixture , having passed through the heat recovery section Rl where it gives of f heat and undergoes partial condensation, is fed to the fourth vessel V4 where the hydrogen or synthesis gas is separated from a liquid phase and exits through a hydrogen outlet 44 and is sent to the battery limit of the system 1 for use .

[0068] The liquid phase separated in the fourth vessel V4 exits through a liquid outlet 45 and is sent into the line 20 to be recirculated, together with the solution exiting the fi fth vessel V5 .

[0069] The hydrogen production reaction that takes place in the third vessel V3 is exothermic, and part of the heat generated evaporates the residual liquid (water ) on the electrode E3 , which will then become part of the reaction itsel f as a reagent . The rest of the heat generated increases the outlet temperature of the third vessel V3 .

[0070] In the second operation mode , the third vessel V3 is used for metal reduction, while the first vessel VI is used for hydrogen production . Before the start of the second operation mode , the liquid contained in the first vessel VI at the end of the first operation mode is drained, while the third vessel V3 is filled with solution . This can be done for example by acting on the valves in the system 1 , in particular by opening the valves G, C (with the valves D, E , F, H closed) , or with a support vessel (not shown) equipped with a dedicated pump to move the liquid .

[0071] In the second operation mode shown in Figure 2 , the electrical circuit connects the electrodes E2 , E3 in the vessels V2 , V3 (while the electrode El is not connected / powered) .

[0072] The solution circulates , again driven by the pump 12 , through the valve C, which is now open while the valve B is closed, in the line 21 and is fed to the third vessel V3 via the inlet 22 .

[0073] In the third vessel V3 , the electrode 3 metal changes from the oxidised state to the reduced metal state ("electrosmelting" ) at the electrode E3 ; no gas is produced in the third vessel V3 at this step, and the solution is enriched with OH- ions- .

[0074] The solution enriched with OH- ions exits the third vessel V3 via the outlet 24 and circulates by pressure di f ference in the line 23 through the valve E , which is open (while the valves F, H are closed) , arriving at the second vessel V2 via the inlet 17 .

[0075] As in the first operation mode , gaseous molecular oxygen evolves at the electrode E2 of the second vessel V2 ; the solution leaves through the outlet 19 dragging oxygen with it and circulates in the line 18 to the fi fth vessel V5 , where the oxygen is separated from the solution and sent to the battery limit ( oxygen outlet ) .

[0076] Upstream of the fi fth vessel V5 , the line 18 runs through the heat recovery section R2 , which like before is fed through the line 31 ; in the heat exchanger X2 of the heat recovery section R2 , the solution gives of f heat and steam is produced, which is sent through the line 33 and branch 36 (valve A being open and valve B, on the branch 37 , being closed) to the inlet 13 of the first vessel VI .

[0077] The solution leaving the vessel V5 through the liquid outlet 43 circulates in the line 20 and returns by pressure di f ference to the pump 12 .

[0078] In this operation mode , hydrogen production takes place in first vessel VI : the first vessel VI is supplied with the solution circulating in the line 11 and with sel f-produced steam within the system 1 , in particular in the heat recovery sections Rl , R2 and sent through the valve A, and possibly with a gaseous mixture rich in CO2 and / or hydrocarbons added via the line 38 .

[0079] In the vessel VI , the liquid from the first operation mode was partly drained / removed, with a residue still present on the electrode El structure (metal mesh or other structure ) . The metal in the electrode El ( in a reduced state from the first operation mode ) is oxidised by the fed mixture , generating a gaseous mixture rich in H2 ( or synthesis gas where CO2 and / or hydrocarbons are fed) , which exits the outlet 15 and circulates by pressure di f ference in the line 14 through the valve D, which is open (while the valve F is closed) , and then into the heat recovery section Rl , where it passes through the superheater S I and heat exchanger XI in series , generating steam and being superheated .

[0080] The gaseous mixture , having passed through the heat recovery section Rl where it undergoes partial condensation, is fed to the ves sel V4 where the hydrogen or synthesis gas is separated from a liquid phase and exits through a hydrogen outlet 44 and is sent to the battery limit of the system 1 for use .

[0081] The liquid phase separated in the fourth vessel V4 is recirculated via the line 20 .

[0082] In this case too , as already described above , the hydrogen production reaction is exothermic, and part of the heat generated evaporates the residual liquid (water ) on the electrode El , which will then become part of the reaction itsel f as a reagent . The rest of the heat will increase the outlet temperature of the first vessel VI .

[0083] In the preferred embodiment j ust described, the process of the invention is conducted in the system 1 comprising three vessels , one of which (vessel V2 ) is equipped with an electrode ( electrode E2 ) for the production of oxygen and two (vessels VI , V3 ) with electrodes ( electrodes El , E3 ) dedicated to the production of hydrogen . In this way, the system 1 can operate continuously, providing continuous production of oxygen and hydrogen ( or syngas ) . In an alternative embodiment , the process of the invention is carried out , discontinuously, in only two vessels VI , V2 ( the third vessel V3 is missing or not used) .

[0084] In this case, the first operation mode is conducted in the first and second vessels VI , V2 as described above .

[0085] Then in the first vessel VI , the metal at the electrode El changes from the oxidised state to the reduced metal state and the solution enriched with OH~ ions leaving the first vessel VI is fed to the second vessel V2 , where oxygen is evolved at the electrode E2 ; the solution is then sent to the heat recovery section and then to the vessel V5 , where the oxygen is separated .

[0086] Since the third vessel V3 is not present or in use , hydrogen is only produced in the second operation mode in the second vessel V2 .

[0087] In the second operation mode , the solution is still fed to the first vessel VI , now together with the steam produced in the heat recovery sections Rl , R2 , and possibly with the gaseous mixture rich in CO2 and / or hydrocarbons .

[0088] As described above , in the first vessel VI , the metal of the electrode El ( in a reduced state from the first operation mode ) is oxidised by the fed mixture , generating a gaseous mixture rich in H2 ( or synthesis gas where CO2 and / or hydrocarbons are fed) , which is sent to the heat recovery section Rl and then to the fourth vessel V4 where the hydrogen or synthesis gas is separated from the liquid phase , which is recirculated .

[0089] In the vessel V2 , oxygen continues to evolve .

[0090] In another, alternative embodiment , only two vessels are used, with the hydrogen evolution electrode (where the reduction of the active metal and the oxidation of the reduced metal to give hydrogen take place alternately) and the oxygen evolution electrode being in the same vessel , and no forced circulation is required .

[0091] Figure 3 shows a vessel V that can be used in place of the first and second vessels VI , V2 described above .

[0092] The vessel V houses two electrodes El , E2 , electrically connected to the electric circuit 2 , and is filled with electrolyte solution .

[0093] In the first operation mode , when metal reduction takes place , the vessel V is supplied with electrolyte solution via the inlet 13 and a voltage is applied between the electrodes El , E2 ; the electrode E l active metal is reduced and oxygen is evolved at the electrode E2 , which is removed via the outlet 15 .

[0094] Once the reduction is complete , the vessel V is emptied of the solution and then moves to the second operation mode , in which the oxidation reagents , that is superheated steam or a gaseous mixture with steam, CO2 and hydrocarbons ( or other ) for the production of hydrogen or synthesis gas , are inj ected into the vessel V, in a manner similar to that described above .

[0095] The electrode El for the hydrogen evolution and the electrode E2 for the oxygen evolution are thus arranged in the same common vessel V, and the reduction of the active metal and the production of hydrogen or synthesis gas and the generation of oxygen occur alternately in the vessel V . Again, therefore , the production of hydrogen or synthesis gas and the generation of oxygen occur separately from each other .

[0096] By using two vessels V, it is possible to conduct the process continuously, as described above .

[0097] Also in this embodiment , similarly to the previous process , a heat recovery series can be added for steam production .

[0098] Lastly, it is clear that modifications may be made to the system and process described and illustrated herein, and variations produced thereto , without departing from the scope of this invention, as set forth in the claims .

Claims

CLAIMS1 . A process for producing hydrogen and / or syngas , comprising a first operation mode and a second operation mode , executed alternatively and repeatedly one after the other ; in the first operation mode , a first electrode (El ) containing an active metal in oxidised form on a surface of said first electrode (El ) is immersed in an electrolyte solution and the active metal in oxidised form of said first electrode (El ) is subj ect to electrolytic reduction, generating a solution enriched in negative ions and a layer of reduced metal on the surface of said first electrode (El ) , and the solution enriched in negative ions is used on a second electrode (E2 ) to produce gaseous oxygen; in the second operation mode , after removing the solution enriched with negative ions by the first electrode (El ) , the reduced metal of the first electrode (El ) is thermochemically oxidised in an oxidation reaction with oxidation reagents producing hydrogen or synthesis gas and forming an active metal in oxidised form on the surface of said first electrode(El ) , so that the production of hydrogen or synthesis gas and the production of oxygen occur separately .2 . The process according to claim 1 , wherein the first electrode (El ) for hydrogen evolution and the second electrode (E2 ) for oxygen evolution are arranged in respective vessel s (VI , V2 ) , connected by a connecting line( 16 ) equipped with a valve ( F) , and reduction of the active metal and production of hydrogen or synthesis gas and, respectively, generation of oxygen are carried out in said respective vessels (VI , V2 ) .

3. The process according to claim 1 , wherein the first electrode (El ) for hydrogen evolution and the second electrode (E2 ) for oxygen evolution are arranged in a common vessel (V) , and reduction of the active metal and production of hydrogen or synthesis gas and the generation of oxygen are alternatively carried out in said common vessel (V) .4 . The process according to any one of the preceding claims , wherein the reduction by electrolysis of the active metal and the thermochemical oxidation of the reduced metal are carried out alternately in at least one and the same first vessel (VI ) housing said first electrode (El ) ; and wherein in the first operation mode said first vessel (VI ) is fed with the electrolytic solution and said first electrode (El ) is electrically supplied; and in the second operation mode said first vessel (VI ) is fed, after having at least partially removed the electrolytic solution, with the oxidation reagents , and said first electrode (El ) is not electrically supplied .5 . The process according to any one of the preceding claims , wherein the oxidation reagents comprise water, preferably steam, optionally superheated, and / or mixturescontaining CO2 and / or hydrocarbons .

6. The process according to any one of the preceding claims , wherein the reduction by electrolysis of the active metal and the thermochemical oxidation of the reduced metal are carried out alternately in a pair of first vessels (VI , V3 ) equipped with respective first electrodes (El , E3 ) and used alternately for the reduction of the active metal and the oxidation of the reduced metal to produce hydrogen .7 . The process according to any one of the preceding claims , in which a gaseous mixture containing hydrogen or synthesis gas is obtained from the second operation mode and is sent to a thermal recovery step, in which said gaseous mixture releases heat and undergoes partial condensation, and to a subsequent separation step, in which the hydrogen or synthesis gas is separated from a liquid phase , which is recirculated to the first operation mode .8 . The process according to claim 7 , wherein in the thermal recovery step said gaseous mixture releases heat to a water stream and produces steam, optionally superheated, which is used as a reagent in the oxidation reaction .

9. The process according to any one of the preceding claims , wherein the gaseous oxygen generated at said second electrode (E2 ) is drawn in liquid solution to a thermal recovery step, in which said liquid solution releases heat to a water stream to produce steam that is used as a reagentin the oxidation reaction, and to a subsequent separation step, in which the oxygen is separated from the liquid solution, which is recirculated to the first operation mode .10 . The process according to any one of the preceding claims , wherein said first electrode (El ) contains a transition metal as its main component , especially iron, or steel , and, optionally, a layer of surface coating made of a metal oxide , in particular an oxide of a transition metal , for example an iron or nickel oxide .11 . The process according to any one of the preceding claims , wherein said first electrode (El ) has a metal-mesh structure , for example with a mild steel mesh optionally coated with iron or a ferrous alloy and, optionally, a metal oxide ; or a metal foam structure , optionally coated with metal oxides ; or a metal oxide pellet structure ; or a structure having a random filling of metal oxides or metal oxide-coated elements ; or a structure containing an oxidised metal solution or slurry .12 . The process according to any one of the preceding claims , wherein the electrolyte solution is an alkaline solution, for example an aqueous solution of KOH or NaOH, for example with concentrations between 5% and 90% , preferably between 40% and 60% ; or a salt solution; or pure water .

13. A system for producing hydrogen and / or synthesisgas, comprising at least one first electrode (El) for hydrogen evolution and a second electrode (E2) for oxygen evolution, electrically connected to each other and housed in at least one first vessel (VI) and one second vessel (V2) , respectively, connected by a connecting line (16) , or in a single common vessel (V) ; wherein the first electrode (El) contains an active metal in oxidised form on a surface of said first electrode (El) and is electrically connected to the second electrode (E2) ; and the system also comprises feeding means (4, 5) to selectively feed an electrolyte solution to the first electrode (El) , so as to reduce the active metal in oxidised form on the surface of said first electrode (El) electrolytically and enrich the solution with negative ions and generate oxygen gas at the second electrode (E2) ; and oxidation reagents, so as to thermochemically oxidise the reduced metal of the first electrode (El) producing hydrogen or synthesis gas and forming an active metal in oxidised form on the surface of said first electrode (El) .

14. The system according to claim 13, wherein the first electrode (El) for hydrogen evolution and the second electrode (E2) for oxygen evolution are arranged in a first vessel (VI) and a second vessel (V2) , respectively, connected by a connecting line (16) equipped with a valve (F) .

15. The system according to claim 13, wherein thefirst electrode (El) for hydrogen evolution and the second electrode (E2) for oxygen evolution are arranged in a common vessel (V) .

16. The system according to claim 14 or 15, comprising a pair of first vessels (VI, V3) equipped with respective first electrodes (El, E3) and connected by respective connecting lines (16, 23) equipped with respective valves (F, E) , said first vessels (VI, V3) being alternately fed with the electrolyte solution and the oxidation reagents.

17. The system according to any of claims 13 to 16, wherein the feeding means (4, 5) comprise a circuit (4) in which the electrolytic solution circulates; and a reagent circuit (5) , provided with at least one heat recovery section (Rl, R2 ) and in which water and steam circulate upstream and downstream of the heat recovery section (Rl, R2 ) , respectively; and wherein said circuit (4) passes through the heat recovery section (Rl, R2 ) to transfer heat from the solution circulating in the circuit (4) to the water circulating in the reagent circuit (5) .

18. The system according to any one of claims 13 to 17, comprising at least one liquid / gas separation vessel (V4, V5) connected to said first vessel (VI, V3) for separating hydrogen or synthesis gas from a liquid phase, which is recirculated to the first vessel (VI, V3) via arecirculation line (20) .

19. The system according to any one of claims 13 to 18, wherein said first electrode (El) contains a transition metal as its main component, in particular iron or steel, and, optionally, a layer of surface coating made of a metal oxide, in particular an oxide of a transition metal, for example an iron oxide or nickel oxide.

20. The system according to one of claims 13 to 19, wherein said first electrode (El) has a metal-mesh structure, for example with a mild steel mesh optionally coated with iron or a ferrous alloy and, optionally, a metal oxide; or a metal foam structure, optionally coated with metal oxides; or a metal oxide pellet structure; or a structure having a random filling of metal oxides or metal oxide-coated elements; or a structure containing an oxidised metal solution or slurry.

21. The system according to any of claims 13 to 20, wherein the electrolyte solution is an alkaline solution, for example an aqueous solution of KOH or NaOH, for example with concentrations between 5% and 90%, preferably between40% and 60%; or a salt solution; or pure water.

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