FIELD OF THE INVENTION

The present invention is related to a system and method for the removal of carbon dioxide from an atmosphere, more particularly by removing carbon dioxide from an atmosphere using alkaline solutions and water electrolysis, which produces hydrogen.

The system and method are based on improvements related to the electrolyser which is fed by a CO2-rich, post-capture (bi)carbonate solution, wherein said improvements enable isolation of a 85:15 wt.% (or 66:34 vol.%) CO2/O2 gas mixture from the anolyte during operation, with an in line CO2/O2 separation at the anode of the electrolyser.

BACKGROUND TO THE INVENTION

Carbon dioxide continues to build up in the environment. According to the National Oceanic and Atmospheric Administration (NOAA), the growth of carbon dioxide in the earth's atmosphere was 3 parts per million (ppm) per year in 2015 and 2016, and 2 ppm in 2017. Techniques to remove carbon dioxide from the air have become a critical area of research.

CO2-emissions from industry and electricity production consists of large and localized streams (100 kt to 1 Mt per year) and smaller point sources with cumulative emissions in the Mt range. It is recognized that CO2 capture at these point sources is the border stone of emission reduction, and that even in case existing value chains proceed without change, yearly emission reduction by capture remains important. Novel more environmentally friendly production technologies can of course, reduce CO2 capture need, if they could be implemented faster or cheaper.

As mentioned above, it is an object of the present invention to provide a CO2 capture method and system using water electrolysis which produces O2 and hydrogen. The market demand for hydrogen has grown significantly over the recent years with growth rates from 3.5 to 6 %. The chemical industry represents the largest demand, with the refining industry as an important enduser (25 % of total demand). Smaller end users are steel producers apply H2 for direct reduction of iron ore (3 %). This industry plays an important role, as CO2 emitter and depends on carbonbased feedstocks for production of (in)organic chemicals. This industry is energy-intensive and accordingly could also acts as enabler in the energy sector and its renewable transition, with the development of large-scale (Carbon Capture Use (CCU) / Carbon Capture Storage (CCS) Hubs through their industrial clusters and connections.

Current technologies of CCU and CCS are typically based on scrubbing with amine solutions. An alternative is potassium carbonate capture that combines low capture costs with little toxicity, ease of regeneration, low corrosiveness, high stability and favorable absorption capacity. As a result, the process has been applied in more than 700 plants. The process is based on (bi)carbonate cycles, where dissolved K2CO3 captures CO2, resulting in KHCO3, which is precrystallized and dissociated as solid into CO2 and carbonate above 100 °C. However, absorption kinetics are rather slow, which can be remediated by using engineered (thermostable) carbonic anhydrase enzymes, and doesn’t produce Hydrogen often used by the energy-intensive industries mentioned herein before.

There is accordingly a need for a cost-competitive CCU system that combines the supply of low cost CO2 and green H2 at a point sources of CO2 emission. It is an object of the present invention to provide such CCU/CCS system relying on an electrolyser which produces O2 and H2 from water, and that comprises an integrated CO2 capture and release configuration. This system of integrated CO2 capture and Hydrogen production will hereinafter also be referred to as ICO2CH system.

SUMMARY OF THE INVENTION

The present invention provides a method of removing carbon dioxide (CO2) from an atmosphere and generating hydrogen, comprising;

Capturing carbon dioxide from the atmosphere in an aqueous alkaline capture solution; Obtaining a post-capture (bi)carbonate solution, hereinafter also referred to as (bi)carbonate solution;

Feeding the (bi)carbonate solution to the anode cell of a water electrolyser;

OER in the (bi)carbonate solution at the anode with formation of CO2 and O2;

HER at the cathode with formation of H2 and regeneration of the aqueous alkaline capture solution; characterized in that the anode cell of the electrolyser comprises an integrated CO2 / O2 separator.

As is generally known, the overall reaction of water electrolysis can be divided into two half-cell reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). HER is the reaction where water is reduced at the cathode to produce H2, and OER is the reaction where water is oxidized at the anode to produce O2. Using an alkaline capturing solution, the electrolyser used is an alkaline water electrolyser, wherein the alkaline capturing solution, regenerated at the cathode typically comprises hydroxide solutions of alkali metals (e.g., sodium and potassium) and alkaline earth metals (e.g., calcium) with a pH of at least 7. In a particular embodiment the aqueous alkaline capture solution is selected from a KOH or a NaOH solution.

Key to the method according to the invention is the presence of the integrated CO2 / O2 separator in the anode cell of the electrolyser. This integrated configuration allows the ICO2CH system for example to be part of oxy-fuel combustion in a reciprocating engine with CO2 dilution. In this combustion mode, some or all of the incoming air is replaced with a mixture of oxygen and CO2 recirculated from the exhaust of the engine. The quantity of CO2 that can be fed back to the engine intake is dependent on the concentration of oxygen in the fuel/oxidizer mixture. The stream of CO2/O2 produced at the anode of the ICO2CH system, can be directly valorised by a natural gas (NG) fuelled Internal Combustion (IC) engine to realise ultra-low carbon emitting, partial oxy-fuel combustion, i.e. displacement of a portion of the engine air with a stream of CO2/O2 oxidizer. With an engine appropriately sized to a given electrolyser rating, a substantial portion of CO2 remains captive in the system, as the CO2 in engine exhaust is combined with KOH and recirculated to the electrolyser, where CO2 is eventually produced from the anode and cycled back to engine as diluent for the partial oxy-fuel combustion process. It is accordingly an object of the present invention to the use of the ICO2CH system as part oxy-fuel combustion in a reciprocating engine with CO2 dilution.

In another aspect the present invention provides a system, herein also referred to as the ICO2CH system, for removing carbon dioxide (CO2) from an atmosphere and generating hydrogen, comprising;

An aqueous alkaline solution -based CO2 capturing system;

An alkaline water electrolyser, wherein; o the anode cell has an inlet for and is configured for OER in the alkaline capture solution at the anode with formation of CO2 and O2; o the cathode is configured for HER with formation of H2 and regeneration of the aqueous alkaline capture solution; and characterized in that the anode cell of the electrolyser comprises an integrated CO2 / O2 separator.

In one embodiment the integration of the CO2 / O2 separator into the anode cell of the electrolyser is based on performing the OER of the alkaline capture solution at pressures up to 60 bar and at reduced temperatures up to and above cryogenic temperature. Under said circumstances the produced Oxygen will be in the gas phase whilst the produced Carbon Dioxide remains in solution. Such set-up accordingly allows an instant and in-line separation of the Oxygen produced at the anode without the need of an additional CO2 / O2 separator. As such it simplifies the installation, and enables for example, the system according to the invention to be part of oxy-fuel combustion in a reciprocating engine with CO2 dilution.

In the system according to the invention, and relying on an aqueous alkaline solution -based CO2 capturing system, the OER at the anode is expected to yield CO2/O2 mixtures comprising a mole fraction of at least 70% CO2; in particular from about 70:30 CO2:O2 to about 85:15 CO2:O2; more in particular from about 80:20 CO2:O2 to about 85:15 CO2:O2. For such CO2/O2 mixtures the anode should be kept at pressures up to 70 bar and at reduced temperatures up to and above cryogenic temperature; in particular at temperatures from about 0°C to about -40°C and respective pressure from about 50 bar to about 10 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 - Schematic cell set-up of an alkaline water electrolyser involving 1 ) OER and acidification of (bi)carbonate solution to form CO2 and O2 at the anode, 2) Cation (M ⁺ ) transport through the membrane and 3) HER and MOH regeneration, at the cathode.

Figure 2 - Isotherms for CO2-O2 mixtures. Arrows indicate for a CO2:O2 mixture with a O2 mole fraction of 0.2 the minimal pressures at which CO2 is still in the liquid phase at the indicated temperatures of the crossing isotherms. For example for the isotherm of 0°C the pressure equals 48 atm; for the isotherm of -10°C the pressure equals 35 atm; for the isotherm of -20°C the pressure equals 25 atm; for the isotherm of -30°C the pressure equals 18 atm; for the isotherm of -40°C the pressure equals 12 atm. Depending on the temperature at which the anode is kept, a different minimal pressure well be required to keep the CO2 the liquid phase. In an embodiment according to the invention the temperature will be kept at freezer temperatures ranging from about -15°C to about -20°C, whilst keeping a minimal pressure of 30 atm.

Figure 3 - The proportion of combustion CO2 emissions that are recirculated back to a stoichiometric, natural gas reciprocating engine from the electrolyser anode as a function of the volume of intake air replaced with a 34% 02/66% CO2 mixture.

Figure 4 - Accumulated charge (solid line) and corresponding proton concentration (dots) during the chronopotentiometry experiment.

Figure 5 - Gas composition of the head space of the anodic compartment. At time 0 and from bottom to top the graph shows first (1 ) CO2, next (2) O2 and finally (3) N2. To the right N2 is displaced by CO2 with O2 remains more or less stable over time.

Figure 6 - Cell voltage plotted as a function of (A) reaction time and (B) charge passed for experiments at p = 1 , 6 and 12 bar. Figure 6 (C) is a plot of the anolyte conductivity versus reaction time for experiments at p = 1 , 6 and 12 bar. For Figures 6 (A) to (C) curve (1 ) corresponds to i=10.4A: p=1 bar - curve (2) corresponds to i=5.2A: p=1 bar - curve (3) corresponds to i=5.2A: p=6bar - curve (4) corresponds to i=5.2A: p=12bar Figure 6 (D) provides the evolution of the anolyte gas composition during the reaction for the experiment at I = 400 mA/cm2 and p = 1 bar with at 1 h from bottom to top H2, N2, O2, CO2 again showing the theoretical ratio of CO2/O2 of 4 when steady state is reached.

Figure 7 - CO2/O2 ratio during the reaction for experiments at different pressures. At 4h from top to bottom, pressure at 1 bar, 6bar, and 12bar.

Figure 8 - Gas composition during reaction at p = 12 bar with the change in signals due to depressurization encircled. At 4h from top to bottom N2, CO2, O2, and H2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an improved system and method for capturing CO2 from an atmosphere and converting it in an output gas for valorisation such as green H2 use, Carbon Capture Use (CCU), Carbon Capture Storage (CCS) and partial oxy-fuel combustion. The system and method herein disclosed are based on an electrolyser as core technology in combination with CO2 capturing technology relying an alkaline capturing solution. Compared to existing systems and methods the present invention differs at least in the configuration of the reaction environment at the anode of the electrolyser. The oxidation reaction is performed at a reduced temperature up to and above cryogenic temperature and increased pressures up to 60 bar.

A schematic representation of the cell setup of an electrolyser is provided in figure 1 . It shows M ⁺ transport across an ion-permeable membrane held between the two electrodes. As membranes for electrolysis, asbestos, nonwoven fabric, ion exchange membranes, polymer porous membranes, composite membranes of inorganic substances and organic polymers, and the like have been proposed. For example, JP2008-144262 includes an organic fiber cloth incorporated in a mixture of a hydrophilic inorganic material of calcium phosphate compound or calcium fluoride and an organic binder selected from polysulfone, polypropylene, and polyvinylidene fluoride. An ion permeable diaphragm is shown. Further, for example, in JP62- 196390, a granular inorganic hydrophilic material selected from antimony, zirconium oxide and hydroxide, and a fluorocarbon polymer, polysulfone, polypropylene, polyvinyl ch loride, and polyvinyl butyral are selected. An ion permeable membrane comprising stretched organic fiber fabrics is shown in a film-forming mixture consisting of a modified organic binder. The electrolyser used in the context of the invention is in particular a water electrolyser, more in particular an alkaline water electrolyser.

As part of the system any currently known CO2 Capturing technologies using an aqueous alkaline solution to absorb the CO2 can be used. Various scrubbing processes have been proposed to remove CO2 from the air, or from flue gases, but for use in the ICO2CH system according to the invention, the scrubbing process is preferably based on a potassium or sodium hydroxide solution. The CO2 is absorbed in the solution to produce the corresponding (bi)carbonate. The CO2 is subsequently released by applying a nominal voltage across the (bi)carbonate solution in the electrolyser. In general, the alkaline capturing solution is selected from hydroxide solutions of alkali metals (e.g„ sodium and potassium) and alkaline earth metals (e.g., calcium) with a pH of at least 7.

It is a first objective of the present invention to provide a method of removing carbon dioxide (CO2) from an atmosphere and generating hydrogen, comprising;

Capturing carbon dioxide from the atmosphere in an aqueous alkaline capture solution; Feeding the thus obtained aqueous (bi)carbonate solution to the anode cell of a water electrolyser;

Performing the oxygen evolution reaction (OER) in the aqueous (bi)carbonate solution at the anode with formation of CO2 and O2;

Performing the hydrogen evolution reaction (HER) at the cathode with formation of H2 and regeneration of the aqueous alkaline capture solution; characterized in that the anode cell of the electrolyser comprises an integrated CO2 / O2 separator.

It is a second objective of the invention to provide a system, herein also referred to as the ICO2CH system, for removing carbon dioxide (CO2) from an atmosphere and generating hydrogen, comprising ;

An aqueous alkaline solution -based CO2 capturing system;

A water electrolyser, wherein; the anode cell has an inlet for and is configured for OER in an aqueous (bi)carbonate solution coming from the aqueous alkaline solution -based CO2 capturing system at the anode with formation of CO2 and O2; the cathode is configured for HER with formation of H2 and has an outlet for a regenerated aqueous alkaline capture solution to the aqueous alkaline solution -based CO2 capturing system ; and characterized in that the anode cell of the electrolyser comprises an integrated CO2 / O2 separator.

As mentioned hereinbefore, the method and system rely on the combination of an aqueous alkaline solution -based CO2 capturing system; and a water electrolyser, wherein the latter is characterised in that the anode cell of the electrolyser comprises an integrated CO2 / O2 separator.

Said integration simplifies the present setup in that no further CO2 / O2 separator downstream of the electrolyser is needed. In one embodiment the integrated CO2102 separator implies that the anode cell is configured to perform the OER of the (bi)carbonate solution at pressures up to 70 bar and at reduced temperatures up to and above cryogenic temperature. In another embodiment the anode cell is configured to perform the OER of the (bi)carbonate solution at pressures from about and between 10 bar to up to 50 bar and at reduced temperatures from about and between 0°C to -40 °C.

In an even further embodiment not only the anode cell is configured to perform the OER at pressures up to 70 bar, but also the cathode cell of the electrolyser is configured to be operated at said pressures. By maintaining a common pressure at both the anode and the cathode cell of the electrolyser, a pressure difference across the ion-permeable membrane held between the two electrodes can be prevented, simplifying the design of the electrolyser.

In an embodiment according to the invention the methods or systems as herein provided are further characterized in that the cathode is configured to perform the HER with formation of H2 and regeneration of the aqueous alkaline capture solution at pressures up to 70 bar; In particular at pressures from about and between 10 bar to up to 50 bar.

In another embodiment according to the invention the methods or systems as herein provided are further characterized in that the HER at the cathode and the OER at the anode are performed at the same pressure, in particular at the same pressure from about and between 10 bar to up to 70 bar; more in particular at the same pressure from about and between 10 bar to up to 50 bar.

The methods and systems of the present invention produce high purity CO2 and O2 streams which make it suitable in oxy-fuel combustion. Thus in a further aspect the present invention provides the use of the ICO2CH system as defined in any of the foregoing embodiments as part oxy-fuel combustion in a reciprocating engine with CO2 dilution.

As an example of the potential CO2 reduction, when using the ICO2CH system as part of an oxy-fuel combustion, consider a single 500 kW natural gas-fired reciprocating engine that is used to produce electricity and operates for the equivalent of 300 days in a year. This engine operates at stoichiometry such that all the oxygen is used to oxidize the fuel. Also, take the anode product stream from the electrolyser to have a nominal composition of 34 % O2 and 66 % CO2 by volume. As the incoming air to the engine is replaced by the recirculated anode gas mixture, a portion of the combustion CO2 emissions are retained in the ICO2CH system, therefore decreasing the carbon-intensity of the combustion process. Figure 3 illustrates this concept for a range of volumetric air replacement by the electrolyser anode products. As shown, replacing only 25 % of the incoming air to the engine with the O2-CO2 mixture results in a system retention of 50 % of the exhaust CO2 emissions per unit of fuel combusted. At this level of air replacement (and assuming the engine is able to operate robustly in a partial oxyfuel combustion mode), a 500 kW, state-of-the-art natural gas generator set running for 300 days per year at a fuel conversion efficiency of 30 % (including both the efficiency of the engine and generator) would avoid nearly 1 100 metric tons of CO2. This figure discounts the possibility for increasing engine efficiency by operating with excess oxidizer relative to the fuel (lean-burn) or other advanced combustion strategies, and therefore this reduction is considered conservative. In short, the integrated electrolyser and reciprocating engine system within ICO2CH has a significant potential to abate carbon emissions related to heat and power generation.

A set of electrolysis experiments were performed to underscore the conceptual workability of the method and systems of the invention. In a first experiment (Example 1 ) it is demonstrated that an ICO2CH system, for removing carbon dioxide (CO2) from an atmosphere and generating hydrogen, in agreement with Figure 1 will actually work in a water electrolyser with the formation of H2 and regeneration of the aqueous alkaline capture solution, with the release of CO2 at the anode. In a second example (Example 2) it is further demonstrated that an in situ separation of CO2 and O2 at the anode can be further controlled without affecting the electrochemical cell performance (cell voltage / current / conductivity).

Example 1

Electrolysis experiments have been carried out in a two-compartment cell (H-cell) under the following conditions:

Electrolyte (anolyte and catholyte): 0.5M KHCO3

Working Electrode (WE) = DSA (anode)

Counter Electrode (CE) = carbon felt

Anion exchange membrane

Room temperature

Ambient pressure

Chronopotentiometry (CP) at 4 different currents (25, 50, 100, 200 mA) was carried out each for 2 hours. Gas analysis of the headspace of the anolyte is performed with Differential Electrochemical Mass Spectrometry (DEMS). Moreover, the pH is monitored as a function of the time.

A schematic of the experimental setup is depicted in Figure 1

Results & Discussion In Figure 4, the accumulated charge for the chronopotentiometry at 25, 50, 100, 200 mA is shown. Based on the measured pH, the increase of H ⁺ concentration in the anolyte is plotted on the right y-axis. The sudden decrease in [H ⁺ ] around t = 400 minutes is presumably due to gas bubbles that block the pH sensor. The protons are produced by the oxygen evolution reaction (OER) given in Eq. 1 a (acidic media) and Eq. 1 b (alkaline media).

4 OH ^ O ₂ + 2 H ₂ O + 4e’ Eq. lb

Simultaneously, hydrogen evolution reaction (HER) occurs at the cathode leading to the formation of H2 (not measured). It is clearly seen that the trend of [H ⁺ ] follows the accumulated charge vs. time.

Moreover, the gases in the headspace were continuously measured with Differential Electrochemical Mass Spectrometry (DEMS). In Figure 5, the evolution of the gas composition of the headspace of the anodic compartment is shown. N2, which is present from the start of the experiments is decreasing during the experiments. The trends of the gases in time can be explained by CO2 production which displaces N2 and O2 in the headspace.

In Figure 4, the ratio of the CO2/O2 signals are plotted together with the [H+] vs. time. The increase of CO2 in the headspace is proportional with the charge (H ⁺ formation rate), since the protons that are formed from Eq. 1 lead to protonation of HCOs' present in the anolyte (Eq. 2) which results in liberation of CO2 according to the following acid-base reaction.

H ⁺ + HCO ₃ ’ -» H2O + CO2 Eq. 2

Since the OER produces 4 protons per O2 molecule, and 1 proton is consumed for the formation of CO2, the theoretical ratio of CO2/O2 should be 4. This theoretical value is also observed after steady state is reached for the last CP of 200 mA in Figure 5.

Similar trends are observed in case of a nation membrane or different electrolytes such as KOH, NaOH or K2CO3 (data not shown). This experiment accordingly demonstrates the working principle of the ICO2CH concept as brought forward in the instant application.

Example 2

In this example, it is demonstrated that the CO2 release at the anode can be controlled without affecting the electrochemical cell performance. Thereto the eventual effect of pressure on the ICO2CH system has been studied. Experiments are carried out employing a 13 cm ² round cell using a Nation membrane, a Ni foam (1.6mm 95% porosity) as well as a wide-meshed Ni mesh in between Ni foam and the current collector to achieve a zero-gap configuration. During the experiment, a constant temperature is desired, so the cryostat is operated at 25°C during the whole experiment. Prior to each experiment, the catholyte and anolyte vessels are purged with nitrogen and filled with respectively 800 mL of 1 M KOH and 800 mL of 1 M KHCOa.

Gas analysis of the headspace of the anolyte is performed with Differential Electrochemical Mass Spectrometry (DEMS). The release of gases and mass balance of the electrolyte are studied during water electrolysis (HER at the cathode and OER at the anode) at (1 ) atmospheric pressure, (2) p = 6 bar and (3) p = 12 bar. Pressurization of the cell is realized by purging nitrogen gas. The experiments run at least until steady state is reached in the gas phase (i.e. at least till the expected CO2/O2 ratio of 4 is reached, similar to previous experiments in the H-cell), ideally longer, or until the cell voltage or ohmic heating becomes critical.

A current density of 800 mA/cm ^z or 400 mA/cm ² is applied (galvanostatic operation), corresponding to 10.4 A or 5.2 A total current. The electrolyte is recirculated with a flow rate is 500 mL/min.

Results & Discussion

Looking at the cell voltage for the different experiments, it can be seen from figures 6(A) and 6(B) that for the experiment at I = 800 mA/cm ² (i=10.4A; p=1 bar - Top graph both in 6(A) and 6(B)) this increase starts earlier, while for I = 400 mA/cm ² (i=5.2A) the onset is later. It can be seen that for each experiment, the cell voltage starts to increase after a certain amount of time, when ca. 60-70 kC have passed (curves for I = 400 mA/cm ² ). Foutl Verwijzingsbron niet gevonden.C, shows that the initial conductivity is lower for I = 400 mA/cm ² at 6 bar (in figure 6(C) at 4h from bottom to top curves for I = 400 mA/cm ² at respectively 6bar, 1 bar and 12bar), which may explain a slightly earlier onset of the cell voltage increase. There is no qualitative difference observed for the different pressures.

The increase in cell voltage when ca. 60-70 kC have passed is related to the decrease in conductivity of the anolyte (as shown in Fout! Verwijzingsbron niet gevonden.C. This is caused by the transport of K ⁺ ions from the anolyte to the catholyte, which is dependent on the current (charge). From Foutl Verwijzingsbron niet gevonden.D, it can be seen that the increase in cell voltage after ca. 4 hours of operation also induces a change in gas composition. It is believed that the membrane is damaged (due to high cell voltage stemming from ion depletion on one side of the membrane), leading to cross over of gases as the CO2 concentration starts to decrease while the H2 concentration starts to increase. For all pressures at I = 400 mA/cm ² , the liberation of CO2 could be observed (data not shown) which means that the ICO2CH principle at elevated pressure is proven. However, and as expected the CO2 flow rate decreases with increasing pressure. In other words, the CO2/O2 ratio measured in these experiments is pressure dependent as shown in Fout! Verwijzingsbron niet gevonden.. The reason for this is the increasing CO2 solubility with pressure. This is also visible from the strong increase in CO2 concentration during depressurization of the system from p = 12 bar as shown by the dashed circle in Figure 8.

For the ICO2CH process at elevated pressure, it can be concluded that the CO2 release can be controlled through dissolution in the anolyte, and the contribution of which increases with the pressure.