Patent Description:
In particular, the carbon dioxide (CO<NUM>) capture and utilization method comprises a) alkaline solution with metallic iron and/or metallic magnesium to convert carbon dioxide (CO<NUM>) to hydrogen (H<NUM>) or b) alkaline solution with metallic iron and/or metallic magnesium and mix hydrogenotrophic methanogens to convert carbon dioxide (CO<NUM>) to methane (CH<NUM>).

Carbon dioxide (CO<NUM>), as the most significant anthropogenic greenhouse gas (GHG), substantially contributes to global warming and climate change. By the <NUM> Paris Agreement has been firmly established that CO<NUM> is the primary driver of global warming and, for this reason, investor pressure and voluntary responses from many multinational firms and industries have been stimulated, creating significant opportunities for technological advances. In the European Union, the industrial plants are subject to the EU Emissions Trading System and have to reduce their GHG by <NUM>% in <NUM> compared to <NUM>. From <NUM> onwards, if EU countries do not meet their decarbonization path up to <NUM>, they will have to purchase emission permits to make up for the shortfall in emission reduction which will be funded by the public budget.

To mitigate this, researchers and engineers have examined processes for transforming CO<NUM> into commodity chemicals and fuels; the process is also known as carbon dioxide (CO<NUM>) capture and utilization (CCU). However, this is considered a challenge, as CO<NUM> presents kinetic and thermodynamic stability due to its centrosymmetric structure and the use of high-energy substances or processes is required for its reduction.

The most frequent carbon dioxide capture technologies are absorption, adsorption, membrane separation, and cryogenic technology. Among them, the most mature and most widely used technology is absorption, which captures CO<NUM> through chemical reactions.

Monoethanolamine (MEA) process has become the most widely used process for the CO<NUM> capture; however, the main disadvantages of MEA in CO<NUM> absorption are that MEA is volatile and easily degraded, leading to a large amount of solvent loss.

Another practical method to capture CO<NUM> is by absorbing it into alkaline aqueous solutions, like NaOH, and producing bicarbonate. The CO<NUM> absorption capacity of NaOH solution is higher than that of MEA, and NaOH is even more abundant and cheaper than MEA.

For example, the <CIT> discloses a method of carbon dioxide capture, wherein in a step (a) anhydrous sodium carbonate is separated from a first aqueous solution formed by reacting carbon dioxide and an aqueous solution of sodium hydroxide. In step (b) the anhydrous Sodium carbonate is treated by causticization to generate carbon dioxide and sodium hydroxide. The first aqueous solution of step (a) is formed by scrubbing a gas containing carbon dioxide with an aqueous solution of Sodium hydroxide.

For example, the <CIT> is related to a method and related device for capturing carbon dioxide gas by contacting it with a liquid alkaline solution, which react with the carbon dioxide and converts it into (bi)carbonate, and including a further processing step to convert the (bi)carbonate to an economically valuable chemical compound.

For example, the <CIT> discloses an anaerobic process for converting CO<NUM> into methane. In one embodiment is described a module/phase comprising the following stages:.

The problem of capturing carbon dioxide (CO<NUM>) by absorbing it into alkaline aqueous solutions, like NaOH, producing bicarbonate is that bicarbonate has a high solubility, is stable and is hardly decarbonized. Moreover, NaOH, can hardly be regenerate from the bicarbonate.

Several studies have demonstrated the use of bicarbonate solution as a feed for microalgae, while other research have mixed the bicarbonate solution with calcium or magnesium waste to produce CaCO<NUM> and MgCO3, respectively.

For example, the <CIT>, discloses a process for producing CaCO<NUM> or MgCO<NUM> from a feedstock comprising a Ca- or Mg- comprising mixed metal oxide, and to a process for the production of an aqueous solution of Ca(HCO<NUM>)<NUM> or Mg(HCO<NUM>)<NUM>.

As may be understood from the cited examples, so far, a circular process that using alkaline aqueous solutions, like NaOH, can lead to the capture of CO<NUM>, forming bicarbonate solution and producing Hydrogen, regenerating at the same time the alkaline NaOH solution, is not available from the art, and therefore is desirable.

For example, <CIT> provides a synthesis method to produce hydrogen which is more economical and/or less energy consuming. According to the disclosed method, by using a hydrothermal process in the presence of carbonate ions, it is possible to produce hydrogen with high purity and yield. The capture of carbon dioxide is not envisaged in this case, but carbon dioxide performs a catalytic role in the reaction to let the aqueous composition comprising carbonate ions to react with the metallic iron under hydrothermal conditions.

The <CIT> is an example of a system including a CO<NUM> capture device configured to receive a CO2-containing stream and including an aqueous alkaline solution comprising hydroxide and/or carbonate ions. The system also includes an electrolyzer, wherein a carbon-rich solution is received in an incoming stream to generate hydrogen, oxygen, and CO2 gas streams.

Furthermore, large industries, like power stations, cement plants and steel plants emit vast amounts of CO<NUM>. More specifically, iron steel industries and cement industries, apart from the huge amount of CO<NUM>, make use of siderite as a raw material in their process. For example, magnesium carbonate is used in the cement industry.

Hence a CCU process that produces as reaction by-product iron carbonate, like siderite, or magnesium carbonate, is indeed desirable because, they can be directly utilized as a raw material in steel industry.

Another potential field of application lies in the field of biogas upgrading. Biogas is the product of the degradation of organic matter in the absence of oxygen (anaerobically), and consists mainly of methane and carbon dioxide, and biogas upgrade is the process of separating methane from carbon dioxide and other gases from biogas. There are more than <NUM>,<NUM> biogas plants in Europe and therefore CO<NUM> capture and utilization from biogas increase its calorific value as well as its potential use.

There are many Carbon capture and utilization (CCU) and direct air capture (DAC) based companies that use mainly physicochemical processes. Therefore, being able to take advantage of a new low cost / efficient CCU process would be desirable for this type of business.

Finally, another potential application could be found in the generation of energy in remote places, even in spacecraft mission on Mars. For example, Mars mineral composition consists mainly of iron and magnesium minerals and the atmosphere is CO<NUM>, and there is water-ice on its surface. Therefore, a method which exploits the abundance of carbon dioxide and of minerals on the soil of Mars could be a solution for an abundant energy generation directly on planet.

A specific object of the present invention is that of providing a system and method for capturing the carbon dioxide (CO<NUM>) by converting it as soluble bicarbonate HCO<NUM>- to H<NUM> using zero valent metal (Fe(<NUM>) or Mg(<NUM>)) as defined in the appended set of claims.

In accordance with the present invention, there is thus provided a process by which carbon dioxide (CO<NUM>) is absorbed in an alkaline solution, for example an aqueous solution comprising an alkali metal hydroxide, for example sodium hydroxide (NaOH), and soluble bicarbonate (NaHCO<NUM>) is formed. Soluble bicarbonate (NaHCO<NUM>) is first reacted with zero valent metal, consisting of commercial powder iron (Fe) or magnesium (Mg) ribbon and converted to hydrogen gas (H<NUM>), comprising a concentration ><NUM>% H<NUM>.

According to an aspect of the present invention, the capturing of carbon dioxide (CO<NUM>) is used to generate hydrogen gas (H<NUM>) for being directly used for energy purposes. According to the present invention, the reaction shows higher reaction rate for production of H<NUM> with higher bicarbonate concentration, for example <NUM>-<NUM> [gr NaHCO<NUM>/L] compared to the same reaction with oxygen or with a lower bicarbonate concentration.

According to the present method, a circular process is created, and the solution after having reacted is reusable for CO<NUM> capture. In fact, the siderite, which is created on the outer surface of metal iron Fe(<NUM>) after a first reaction, can be separated by the alkaline solution. By exposing it to citric acid (<NUM>-<NUM>) or other weak acids in a second reaction, the siderite in the outer surface of Fe(<NUM>) is going to be removed. After this the citric acid solution is removed, and the remaining Fe(<NUM>) is recyclable in the reaction for the H<NUM> production.

According to another embodiment, the siderite, instead of being removed from the outer surface of Fe(<NUM>), it can be used as is, for example, as raw material for the steel industry or cement industry or as an iron scrap. With the same purpose, Magnesium carbonate can also be used as a raw material in the cement industry.

According to a further embodiment, the siderite can be used for other applications such as Phosphate absorption or heavy metals adsorption from wastewater.

According to another aspect not falling within the subject matter for which protection is sought, hydrogen gas (H<NUM>), instead of being used for energy purposes, it can be used in another reaction for the production of methane (CH<NUM>), which has more potential final use. For example, is known the use of H<NUM> from electrolysis to feed the hydrogenotrophic methanogens. The hydrogenotrophic methanogens convert H<NUM> and soluble CO<NUM> to methane (CH<NUM>). According to an embodiment of the present method, not falling within the subject matter for which protection is sought, the hydrogen (H<NUM>) generated in a first reaction is then directed in another reactor comprising bicarbonate solution and mix hydrogenotrophic methanogens. Experimental results have shown that in <NUM> days higher than <NUM>% of CH<NUM> is obtainable by a system comprising CO<NUM> and Fe(<NUM>) bicarbonate solution and mix hydrogenotrophic methanogens.

According to still another embodiment not falling within the subject matter for which protection is sought, H<NUM> generated by the first reaction can be directed to a bioreactor comprising 'homoacetogenic' bacteria that converts H<NUM> and CO<NUM> to carboxylic acids, mainly acetic acid.

These and other objectives are fully achieved by virtue of the system and method for capturing carbon dioxide (CO<NUM>) according to the present invention having the characteristics defined in independent claims <NUM> and <NUM>.

Preferred embodiments of the invention are specified in the dependent claims, whose subject matter is to be understood as forming an integral or integrating part of the present description.

For a better understanding of the present invention, preferred embodiments, which are intended purely by way of example and are not to be construed as limiting, will now be described with reference to the attached drawings, where:.

The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, without departing from the scope of the present invention as claimed. Thus, the present invention is not intended to be limited to the embodiments described therein, but it has to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the appended claims.

According to <FIG>, a process (<NUM>) of capturing and converting carbon dioxide (CO<NUM>) is described comprising a first step (<NUM>) by which carbon dioxide, CO<NUM> (<NUM>) is absorbed in an alkaline solution (<NUM>), typically an aqueous solution comprising, for example, sodium hydroxide (NaOH), with a concentration comprised between <NUM>,<NUM> to <NUM>,<NUM>, preferably with a concentration of <NUM>, having an initial pH > <NUM>, and as a result soluble bicarbonate, NaHCO<NUM> (<NUM>) , having pH between <NUM>-<NUM>, is formed, according for example to the following reaction formula (<NUM>):.

<NUM>)     NaOH (aq) + CO<NUM> → NaHCO<NUM> (aq).

Bicarbonate (NaHCO<NUM>), as reaction product, is stable and is hardly decarbonized. To overcome this problem, a second step (<NUM>) is used, wherein bicarbonate (<NUM>) is reacted with zero valent metal, for example commercial powder Fe, to produce hydrogen gas, H<NUM> (<NUM>), according for example to the reaction formula (<NUM>):.

<NUM>)     Fe(<NUM>) (s) + HCO<NUM>- (aq) + H<NUM>O → FeCO<NUM>(s) + H<NUM>(g) + OH- (aq).

As shown in <FIG>, in step (<NUM>) the bicarbonate solution reacts with metallic Fe(<NUM>) and H<NUM> (<NUM>) is generated in the headspace. Also, the initial CO<NUM> in the headspace is moved to the liquid solution and reacts with metallic Fe(<NUM>) therefore the headspace contains final ><NUM>% H<NUM>. As shown by the reaction formula (<NUM>), the second reaction step (<NUM>) comprises the production of solid metal carbonate, for example FeCO<NUM>, as by-product increasing alkalinity (OH-).

Instead of commercial powder Fe, a scrap-Fe can be used, which will significantly reduce the process cost; however, the reaction rate is <NUM>-<NUM> times slower but the final H<NUM> concentration is still more than <NUM>%. Furthermore, Magnesium (Mg(<NUM>)) ribbon can be used instead of Fe(<NUM>) at a lower concentration than Fe, according to the reaction formula (<NUM>):.

<NUM>)     Mg(<NUM>) (s) + HCO<NUM>- (aq) + H<NUM>O → MgCO<NUM>(s) + H<NUM>(g) + OH- (aq).

Both the reactions (<NUM>) and (<NUM>) occur in anaerobic carbonate ambient conditions using metallic Fe (powder) or scrap Fe or Magnesium (Mg) ribbon.

According to an aspect of the present invention, the reaction become faster (higher reaction rate) in the production of H<NUM> in the presence of high concentration of bicarbonate, for example (<NUM>-100gr NaHCO<NUM>/L). Final gas composition comprises a Hydrogen (H<NUM>) concentration > <NUM>% after a reaction time ranging from <NUM> to <NUM> hours.

The rate and the yield of reaction depend on several conditions, comprising:.

According to an embodiment of the present method, the reaction is more thermodynamically favorable (compared to the same conditions with oxygen or with low bicarbonate) and increases the reaction rate. Specifically using Fe(<NUM>) powder, the reaction rate (H<NUM>mmol/Fe(kg)·(h)) of H<NUM> is <NUM> - <NUM> higher at higher bicarbonate concentration compared to the condition without bicarbonate or to the use of only CO<NUM>. On the other hand, the reaction rate (H<NUM>mmol/Fe(kg)·(h)) of high bicarbonate is <NUM> - <NUM> higher compared with the Fe(<NUM>) powder exposed to water and nitrogen (N<NUM>). For example, the reaction (<NUM>) is very slow when N<NUM> is used instead of CO<NUM>. However, bicarbonate solution and CO<NUM> in the gas phase (at initial conditions) result in a significantly higher H<NUM> production rate than using water and CO<NUM> in the gas phase. In this process according to reaction (<NUM>), no external energy is needed for the reaction to occur.

At the end of the reactions (<NUM>) or (<NUM>), pH is alkaline (pH <NUM> - <NUM>), therefore once the alkaline solution is separated from Fe or Mg, it can be recycled to be used again for more CO<NUM> absorption as in reaction (<NUM>) and then the final pH will be drop to <NUM> - <NUM>.

According to a first aspect of the present invention, the hydrogen gas, H<NUM> (<NUM>) generated according to the reaction formula (<NUM>) can be directly utilized for energy purpose.

According to an embodiment of the present method the metal carbonate (115b) obtained in the reaction step (<NUM>) is separated obtaining an aqueous alkaline solution, comprising for example NaOH (115a), wherein said aqueous alkaline solution is recycled in the reaction step (<NUM>). According to an embodiment of the present method, a solid metal carbonate (115b), for example the siderite (FeCO<NUM>), is created in the outer surface of Fe<NUM>.

The solid metal carbonate (115b) can be removed, by a further a step (<NUM>), by which weak acid (<NUM>), comprising citric acid or oxalic acid, is used with concentration in the range <NUM> - <NUM>, so that the remaining zero valent metal (<NUM>) can be recycled in the reaction step (<NUM>) and the reaction according to the formula (<NUM>) can be initiated again. The generation of carbonate (115b) on the surface of Fe or Mg is indicated by the reduction of the concentration of H<NUM> from the gas phase to lower than <NUM>% (after several batch cycles). According to a preferred embodiment, after removal of carbonate the remaining solid (FeCO<NUM> or MgCO<NUM>) is exposed to citric acid or weak acid for several hours so the external FeCO<NUM> is removed and the inner Fe can react again in step (<NUM>).

According to another embodiment of the present method, the solid FeCO<NUM> (s) or MgCO<NUM> (s) instead of being directed to step (<NUM>) to be removed from the outer surface, it is used as a raw material in steel industry or cement industry or they are commercialized as iron scrap.

According to a further embodiment of the present method, the siderite is used in other applications, such as Phosphate absorption or heavy metals adsorption from wastewater.

According to a second aspect not falling within the subject matter for which protection is sought, the hydrogen gas (H<NUM>) generated according to the reaction formula (<NUM>) or (<NUM>), instead to be used for energy purposes, is directed to a bioreactor comprising Hydrogenotrophic methanogens for methane (CH<NUM>) production or Homoacetogenic bacteria for acetic acid (CH<NUM>COOH) production. The bioreactor should also contain sodium bicarbonate (NaHCO<NUM>) solution from reaction (<NUM>).

The microbial inoculum can be a mixed culture such as anaerobic granular sludge or anaerobic sludge from the digester. The inoculum can be pre-exposed to favorable conditions to enrich the hydrogenotrophic methanogens or homoacetogens, depending on the final product.

According to an embodiment of the present method (200a) not falling within the subject matter for which protection is sought, the hydrogen gas (H<NUM>) generated according to the reaction formula (<NUM>) or (<NUM>), is directed to a bioreactor (104a) comprising hydrogenotrophic methanogens, as shown in <FIG>, wherein the reagents H<NUM> and soluble CO<NUM> are converted to CH<NUM>, according to a reaction formula (<NUM>):.

<NUM>)     <NUM><NUM> +HCO<NUM>- → CH<NUM> + <NUM><NUM>O.

Methane (CH<NUM>) has more potential final uses than H<NUM>, said uses comprising energy, fuel vehicle, storage in natural gas grid.

Conventionally, in the process of using hydrogenotrophic methanogens to convert CO<NUM> to CH<NUM>, the external H<NUM> gas is added from a separate electrochemical process unit (electrolysis), which splits water into H<NUM> gas and O<NUM> by utilizing surplus renewable electric power. One of the current drawbacks to use this process is the relatively high cost of electrolysis and the integration of various systems such as electrolysis process and the H<NUM> addition to anaerobic digester/bioreactor. In other studies, microbial electrosynthesis has been used for biogas upgrading; however, the main challenges in this case are the cost reduction and the absence of pilot-scale and full-scale demonstration for the process.

According to an aspect not falling within the subject matter for which protection is sought, in the process of using hydrogenotrophic methanogens to convert CO<NUM> to CH<NUM>, the use of H<NUM> as generated from the reaction (<NUM>) and/or (<NUM>) is proposed. By experimental results, it has been shown that in <NUM> days higher than <NUM>% of CH<NUM> can be obtained starting by a system comprising CO<NUM> and Fe(o).

Alternatively, according to another embodiment of the present method (200b) not falling within the subject matter for which protection is sought, the gaseous hydrogen (H<NUM>) can be directed to a bioreactor (104b, <FIG>) comprising homoacetogenic bacteria to convert H<NUM> and CO<NUM> to carboxylic acids, for example acetic acid (CH<NUM>COOH) according to a reaction formula (<NUM>):.

<NUM>)     2HCO<NUM>- + <NUM><NUM> → CH<NUM>COO- + <NUM><NUM>O.

At the end of reaction (<NUM>) or reaction (<NUM>) pH is alkaline and the solution can be recycled for CO<NUM> absorption (as previously descripted).

According to another embodiment (<NUM>) of the present method not falling within the subject matter for which protection is sought, the reactions (<NUM>) or (<NUM>) can take place "in-situ" with the reaction (<NUM>) in one and the same reactor (<NUM>, <FIG>), reacting them with hydrogenotrophic methanogens in the same reactor, in such a way to produce a final gas with CH<NUM> > <NUM>%, according to the reaction formula (<NUM>) or (<NUM>):.

<NUM>)     4Fe(<NUM>) + 4HCO<NUM>- + <NUM>+ + CO<NUM> → 4FeCO<NUM> + CH<NUM> + <NUM><NUM>O;.

<NUM>)     Mg(<NUM>) + 4HCO<NUM>- + <NUM>+ + CO<NUM> → 4MgCO<NUM> + CH<NUM> + <NUM><NUM>O.

The limitation of using this "in-situ" process is the inhibition of sodium to the microorganisms; therefore, the sodium bicarbonate should be less than 30gr NaHCO3/L. For higher concentrations of sodium bicarbonate, halophilic microorganisms has to be used.

The FeCO<NUM> formation can be removed with citric acid as already shown in the foregoing. The reaction (<NUM>) or (<NUM>) will require about <NUM>-<NUM> days to be completed (CH4 > <NUM>%).

Referring to <FIG>, an apparatus (<NUM>) for carrying out the method of capturing and converting carbon dioxide (CO<NUM>) as described in the foregoing comprises at least a first reactor (<NUM>) and a second reactor (<NUM>). The first reactor (<NUM>) comprises a first inlet module (<NUM>) for the gaseous carbon dioxide (CO<NUM>), a second inlet module (<NUM>) for an aqueous alkaline solution comprising an alkali metal hydroxide, at least one outlet module (<NUM>), and is configured to absorb the gaseous carbon dioxide in the aqueous alkaline solution comprising an alkali metal hydroxide such that the carbon dioxide reacts with the alkali-metal hydroxide to form a bicarbonate. The second reactor (<NUM>) comprises an inlet module (<NUM>) for the bicarbonate solution, at least one outlet module (<NUM>), and is operatively connected to said first reactor (<NUM>) and is configured to bring in contact the bicarbonate solution with a zero valent metal species to form a reaction mixture of H<NUM>, solid metal carbonate and aqueous alkaline solution.

According to a preferred embodiment of said apparatus (<NUM>), said second reactor (<NUM>) comprises a liquid/solid separation module (<NUM>) configured to separate the reaction mixture into a regenerated aqueous alkaline solution and a solid metal carbonate, wherein said regenerated aqueous alkaline solution is recycled in the first reactor (<NUM>).

According to an embodiment, said apparatus comprises a third reactor (<NUM>), having an inlet module (<NUM>) and at least one outlet module (<NUM>), and is configured to receive Fe carbonate from the second reactor (<NUM>), bring in contact said Fe carbonate with either citric acid or oxalic acid to form zero valent Fe, wherein said zero valent Fe is recycled in the second reactor (<NUM>).

<NUM>) Examined parameters: N<NUM>, CO<NUM> and 10gr NaHCO<NUM>/L. with CO<NUM> at 50gr Fe(<NUM>)/L. Experimental condition: Fe(<NUM>) (<NUM>/L), serum bottles (<NUM>), working volume <NUM> of water, headspace <NUM>, temperature <NUM>, initial pH <NUM>, agitation <NUM> rpm. As can be seen from the chart in <FIG>, representing the H<NUM> mol generation Vs time, when the system was flushed with N<NUM> (for <NUM>), it resulted in negligible H<NUM> production after <NUM> hours. On the contrary, when the system was a) flushed with CO<NUM> or b) with CO<NUM> plus 10gr NaHCO<NUM>/L in the solution, it resulted in a dramatically higher H<NUM> (mmol) generated over time. The presence of 10gr NaHCO<NUM>/L in addition to being flushed with CO<NUM> resulted in double H<NUM> mmol/Fe(kg) than in the samples flushed with CO<NUM> without extra 10gr NaHCO3/L. <NUM>) Examined parameters: N<NUM>, CO<NUM>, 25gr NaHCO<NUM>/L with CO<NUM> and 50gr NaHCO<NUM>/L with CO<NUM> at <NUM> gr Fe(<NUM>)/L. Experimental condition: Fe(<NUM>) (25gr/L), serum bottles (<NUM>), working volume <NUM> of water, headspace <NUM>, temperature <NUM>, initial pH <NUM>, agitation <NUM> rpm. <NUM>) The chart in <FIG> shows that the higher the bicarbonate concentration, the higher the H<NUM>(mmol)/Fe(kg) generated over time. Again, the use of N<NUM> instead of CO<NUM> results in negligible H<NUM> production <FIG> and <FIG>. The samples with bicarbonate solution (<NUM> and 50gr NaHCO<NUM>/L) produced higher than <NUM>% H<NUM> in <NUM> hours, whereas the samples flushed with CO<NUM> without the addition of NaHCO<NUM> generated <NUM>% H<NUM>. Examined parameters: N<NUM>, CO<NUM>, 25gr NaHCO<NUM>/L with CO<NUM> and 50gr NaHCO<NUM>/L with CO<NUM>, 75gr NaHCO<NUM>/L with CO<NUM>, 100gr NaHCO<NUM>/L with CO<NUM> at 25gFe(<NUM>)/L. Experimental conditions: Fe(<NUM>) (<NUM>/L), serum bottles (<NUM>), working volume <NUM> of water, headspace <NUM>, temperature <NUM>, initial pH <NUM>-<NUM>, agitation <NUM> rpm.

The rate H<NUM> (mmol)/Fe(kg). h of the reaction for Eq 2a for Fe(<NUM>) was measured after <NUM>. The higher the bicarbonate concentration, the higher the rate for higher production, as shown in Table <NUM>. In the samples flushed with CO<NUM>, the presence of <NUM> gr NaHCO<NUM>/L almost double the reaction rate compared with no extra NaHCO<NUM> added in the solution. <NUM>) Examined parameters: Concentrations of Fe(<NUM>) (<NUM>, <NUM>, <NUM>, <NUM> gr/L ) under 10gr NaHCO<NUM> and CO<NUM>. Experimental conditions: Fe(<NUM>) (<NUM>/L), serum bottles (<NUM>), working volume <NUM> of water, headspace <NUM>, temperature <NUM>, initial pH <NUM>, agitation <NUM> rpm. Results from section <FIG> show that the higher concentrations of powder Fe resulted in higher production of H<NUM>. At 50gr Fe/L with 10gr NaHCO<NUM>/L, the H<NUM> is higher than <NUM>% after <NUM> hours, whereas at this time point for 25gr Fe/L, the H<NUM> concentration was <NUM>%. However, as shown in <FIG>, the samples with 25gr Fe/L and 25gr NaHCO<NUM>/L generated higher than <NUM>% H<NUM> in <NUM>. With <NUM>/L Fe(<NUM>) and 75gr/L NaHCO<NUM> higher than <NUM>% and <NUM>% H<NUM> in <NUM> and <NUM> hours, are respectively produced. Therefore, both the Fe(<NUM>) and NaHCO<NUM> positively contribute to H<NUM> production in the reaction, although NaHCO<NUM> concentrations had a more profound positive effect than Fe. <NUM>) Examined parameters: exposure to Magnesium ribbon in air, in CO<NUM> ambient and CO<NUM> + NaHCO<NUM> ambient.

<NUM>) Examined parameters: exposure to Magnesium ribbon to CO<NUM>, CO<NUM> + <NUM> NaHCO<NUM>/L, CO<NUM> + <NUM> NaHCO<NUM>/L and CO<NUM> + 40gNaHCO3/L.

<NUM>) Examined parameters: temperature (T) = <NUM>, <NUM>, <NUM>.

<NUM>) Examined parameters: Concentrations of Fe(<NUM>) (<NUM> gr/L), NaHCO<NUM> formed from initial <NUM> NaOH continuously flushed with CO<NUM>.

In <FIG>, the Gibbs free energy change ΔG (KJ/mol) for metallic iron reaction (formula <NUM>) with H<NUM>O, bicarbonate and CO<NUM> at varied pH <NUM>-<NUM> (temperature <NUM><NUM>C, all concentrations <NUM> and all partial pressures <NUM> atm) is also shown. In the presence of bicarbonate the reaction (formula <NUM>) takes place even at alkaline pH.

Claim 1:
A process (<NUM>) for capturing and converting carbon dioxide (CO<NUM>) comprising:
- a first reaction step (<NUM>) wherein a flux of gaseous carbon dioxide (CO<NUM>) is absorbed in an aqueous alkaline solution comprising an alkali metal hydroxide, wherein the carbon dioxide (CO<NUM>) reacts with the alkali metal hydroxide to form a bicarbonate, obtaining a bicarbonate solution,
- a second reaction step (<NUM>) wherein said bicarbonate solution is converted into hydrogen (H<NUM>) characterized in that
in the second reaction step (<NUM>) said bicarbonate in the bicarbonate solution reacts with a zero valent metal species under anaerobic conditions to produce said hydrogen gas H<NUM> and an exhaust alkaline solution,
and in that said second reaction step (<NUM>) comprises the production of solid metal carbonate on the surface of zero valent metal as by-product, wherein said zero valent metal species is selected from the group comprising metallic Fe, scrap Fe, metallic Mg, Mg ribbon, and said solid metal carbonate is Fe carbonate or Mg carbonate,
said metal carbonate being separated (<NUM>) obtaining an aqueous alkaline solution, said aqueous alkaline solution being recycled in the reaction step (<NUM>) of absorbing the flux of gaseous carbon dioxide in the aqueous alkaline solution.