Method for generating energy from a gas flow, and system and plant for energy generation from a flue gas

The present invention relates to a method for

generating energy from a gas flow, such as a flue gas, comprising CO ₂ .

Worldwide energy demands are rising. Although the use of renewable energy and/or sustainable energy, like wind energy and solar energy, is growing it is expected that fossil fuels will be the dominant energy source for still some period of time. Plants, such as power plants and process plants, emit still an increasing amount of CO ₂ . To a large extent this CO ₂ emission results from the combustion of fossil fuels. To minimise the effect this CO ₂ emission may have on the environment and the climate this CO ₂ emission is captured and stored. One of the practical obstacles against application of this technology is the required energy input.

The object of the present invention is to provide a method for generating energy from a gas flow comprising CO ₂ that obviates or at least reduces the above stated problems and contributes to an overall efficient energy production and use.

This object is achieved by the method according to the present invention for generating energy from a gas

comprising a gas component, such as CO ₂ , and/or separating the gas component, such as CO ₂ , from a gas flow, the method comprising the steps of:

- providing the gas flow to a flow channel with the gas flow having a relatively high partial pressure of the gas component;

- providing a gas to a compartment that is

separated from the flow channel with a membrane selective for the gas component; and - transfer of the gas component through the membrane from the flow channel to the gas compartment .

In a presently preferred embodiment the gas component is CO ₂ . By separating the gas component from the gas flow a purified gas flow is achieved. Furthermore, by transferring the gas component to the gas compartment the gas pressure in the gas compartment will increase. This increased gas pressure may contribute to energy generation, for example by providing gas from the gas compartment to a turbine.

In an alternative embodiment according to the present invention electrodes are being used for generating energy from a gas flow, more specifically generating energy from a CO ₂ flow. Such method comprises the steps of:

- providing a gas flow to a flow channel;

- production of cations and anions;

- diffusing of the cations towards a cation- selective electrode and of the anions towards an anion-selective electrode;

- adsorbing the cations and anions by the electrodes; and

- transporting of electrons through an electrical circuit to maintain electro-neutrality of the electrodes and generate electrical energy.

Providing at least two electrodes with one being anion- selective and the other being cation-selective that are capable of sorbing, preferably absorbing (including

adsorbing) , anions and cations respectively, energy can be generated through this sorption process. Such sorption process starts with the at least two electrodes not yet saturated with ions.

As an example, in a presently preferred embodiment of the invention the gas in the flow channel, including any type of flow compartment, is a flue-gas with a C0 ₂ -level of say 15%. First, the gaseous CO ₂ absorbs in the water, according to reaction Rl:

K _H

i¾: C0 ₂ (g)^ C0 ₂ (aq)

Next, the absorbed CO ₂ reacts with water to produce carbonic acid (R2) , which can dissociate into a

proton and a bicarbonate ion (R3) . The bicarbonate ion can further dissociate into a carbonate ion and a second proton (.R4) according to reactions R ₂ , 3 and R ₄ :

K ₀

R ₂ : C0 ₂ (aq)+H ₂ 0 ^ H ₂ C0 ₃

R ₃ : H ₂ C0 ₃ =H +HCO ₃

- ^K + 2- 4: HCO3 =H +CO3

When the electrodes are not yet saturated with ions, the protons and bicarbonate ions will spontaneously diffuse towards the electrodes. Carbonate may have little effect as this species is expected to have a very low concentration. The presence of the ion exchange membranes (one allowing transport of cations, one allowing that of anions) leads to the protons and bicarbonate ions being absorbed in different electrodes: the proton will be adsorbed in the cation- selective electrode and the bicarbonate ion in the anion- selective electrode. To maintain electro-neutrality, electrons will be transported through the (external) electrical circuit, from the anion-adsorbing side towards the cation-adsorbing electrode. This selective adsorption process thus induces an electric current. This process will continue until the electrodes are saturated.

Anion-select ive and cation-selective electrodes can be achieved in different ways. For example, the (carbon) electrodes can be chemically modified by filling the interpart icle pores between carbon particles by

polyelectrolyte gel and/or by placing an ion-selective layer in front of the electrode such as a membrane. Such ion- exchange membrane is a thin water-filled porous structure containing a high internal concentration of fixed charge groups (e.g. 5 M per volume of water in membrane) of either positive or negative sign. In the case these groups are positive (e.g., from quaternary amine-groups present in the membrane), the membrane has a high selectivity to allow anions (ions of negative sign) passage, while blocking access to cations (such as protons) . This is called an anion-exchange membrane. The reverse situation is achieved with sulfonate groups, and this is called a cation-exchange membrane. Other options to provide selective electrodes includes the use of chemically selective inorganic

materials .

Each electrode consists of a current collector that connects the system to the outer electrical circuit.

The current collector is in direct contact with a conductive material with a high capacitance, which

preferably is a porous carbon electrode. At the high

internal surface area within the porous carbons, ions can be stored next to the electrical charge: a so-called electrical double layer (EDL) is formed. The EDL achieves that at the carbon/water interface electronic charge can only be in the carbon, and ions (ionic charge) can only be in the water. The two charges will be very close, only separated by a few nanometers. In magnitude the two charges cancel one another: thus, overall, the EDL is charge-neutral. When the

electrical charge is of negative sign, the electrode will therefore attract and adsorb cations in the water-filled micro-pores in the carbon (next to the carbon surface) . This electrode is called the cathode. In the opposite electrode all processes are reversed in sign, this is the anode. In a presently preferred embodiment the electrode, preferably a porous carbon electrode, is sealed or separated from the flow channel with either a cation exchange membrane or an anion exchange membrane. The space between the membranes, i.e. the flow compartment, is in a presently preferred embodiment filled with water, through which the flue gas is flowing in the form of bubbles. The water provides for an ionic connection between the two different electrodes .

In alternative embodiments the electrodes are separated by CC> ₂ -selective membranes, or ion-selective membranes without capacitive electrodes. As mentioned earlier, ion- selective electrodes can be applied thereby obviating the need for membranes.

According to the aforementioned operation, at the start there will be a high electrical potential and current, and both will slowly decrease while the electrodes get

saturated. After removing the cations and anions from the electrodes in a regeneration step the electrodes can be used again for generation energy from the gas flow.

One of the advantages of generating energy from a gas flow with a relatively high level of CO ₂ is that such gas flows are commonly available, for example as a flue gas. This renders the energy generation according to the

aforementioned method very cost effective. Although the aforementioned method protons and bicarbonate are mentioned as cations and anions it will be understood that it could be possible to use other cations and anions in combination therewith and/or as an alternative thereto. A specific example of flue gas is the waste flue gas produced by a power plant that is known for huge production of CO ₂ .

In an advantageous embodiment according to the present invention the method further comprises the step of desorbing the protons and ions from the electrodes by providing an acceptor gas to the flow channel.

By replacing the donor gas, preferably providing CO ₂ , by an acceptor gas the electrodes are regenerated as the cations, such as the protons, and the anions desorb to the acceptor gas. This provides an effective cycle of absorbing and desorbing that occur alternately in time.

Such a cycle of absorption and desorption with

regeneration of the electrodes will be referred to as reversible capacitive absorption of CO ₂ . This reversible capacitive absorption is a versatile process making use of the mixing energy that is present in the flue gas.

In an advantageous embodiment according to the present invention the acceptor gas has a relatively low CO ₂

concentration such that CO ₂ is desorbed spontaneously.

When the donor gas is replaced by acceptor gas with a relatively low CO ₂ concentration in the desorption step this will lead to a spontaneous desorption of CO ₂ . This desorption will start from the aqueous phase and consequently by diffusing also from the electrodes. To maintain electro- neutrality of the electrodes an electric current will start flowing in an opposite direction relative to the current that is produced during the CO ₂ absorption step. This means that during this desorption step with regeneration of the electrodes also energy can be generated. When the

regeneration of the electrodes has been completed, the electrodes can be exposed to the donor gas again and the cycle will start again.

In a presently preferred embodiment the acceptor gas is outside air that has a relatively low CO ₂ concentration.

To maintain a relatively low CO ₂ concentration of the acceptor gas this gas should be replenished regularly or even continuously. In a further advantageous embodiment according to the present invention when desorbing cations and anions from the electrodes, the electrodes are provided with electrical energy to force CO ₂ desorption to the acceptor gas to produce a gas with a high CO ₂ concentration.

By providing and/or producing an acceptor gas with a relatively high CO ₂ a CO ₂ flow will be generated. In

combination with the adsorption step this effectively separates CO ₂ from the original incoming gas, for example a flue gas. For the desorption an electric potential has to be provided because the CO ₂ content of the acceptor gas may have a higher CO ₂ content as compared to the donor gas already at the start of the desorption step and certainly at the end of this desorption step when the electrodes have been

regenerated.

Therefore, this separation method provides an

alternative to existing gas stripping operations. This may enable a reduction of CO ₂ emissions. As in the first

adsorption step energy can be generated the overall energy usage can be kept to a minimum when stripping CO ₂ as compared to conventional techniques involving carbon capture and storage requiring organic solvent, a scrubber and a stripper using steam thereby requiring an amount of heat such that the overall efficiency of this process is rather limited.

In a further advantageous embodiment according to the present invention energy that is generated when desorbing ions to the acceptor gas is provided to a second set of electrodes and a second flow channel to force CO ₂ desorption to a second acceptor gas for CO ₂ separation.

By operating at least two processes in parallel and/or in series, the energy that is generated in the spontaneous desorption step can be provided to the forced desorption step. This means that part of the CO ₂ desorption is used for energy generation thereby enabling a CO2 gas to be produced in the forced desorption process. The forced desorption of the CO2 can be performed even without addition of external energy thereby enabling a CO2 separation process that can be operated energy-neutral. It can be calculated that 70% of the CO2 that is present in a combustion gas or flue gas can be concentrated into a pure CO2 flow in such energy efficient combined process. As described above, this is achieved by using a part of the CO2 in the flue gas for to generate electrical energy with this electrical energy being used to separate and concentrate another part of the CO2 .

As a further advantage it can be calculated that although different temperatures have a substantial effect on the amount of electricity that can be generated, the

temperature effect on the working point of energy neutral separation is rather limited. This further contributes the practical implementation possibilities for such a combined spontaneous and forced desorption process resulting in an energy effective CO2 separation.

Optionally, the method according to the invention may comprise the additional step of transferring the electrodes to another flow, for example from the acceptor gas to the donor gas or vice versa. This means that the electrodes are being switched in stead of the flows. The electrodes can be transferred using a transfer mechanism. The electrodes can be shaped as plates, wires and/or flowable/floating

electrodes. Further details of the transfer mechanism and the different embodiments of the transferrable electrodes will be discussed in relation to the system.

In a preferred embodiment of the invention, the method for generating energy from a gas flow comprising CO2

comprises : — providing a first compartment, a second compartment and a third compartment, wherein the first

compartment is separated from the second compartment by a cation exchange membrane and the second

compartment is separated from the third compartment by an anion exchange membrane;

— providing water in the compartments;

— providing the gas flow to the second compartment for dissolving the CO ₂ in the water in the second compartment;

— production of cations and anions;

— diffusing of the cations towards the first

compartment and of the anions towards the third compartment, thereby creating a potential difference; and

— generate electrical energy.

This method employs a process known as reversed

electrodialysis (RED) . Conventional methods of reversed electrodialysis make use of the difference in sodium

chloride (NaCl) concentration between two water streams. In contrast, the method according to the invention utilizes the concentration difference in dissolved CO ₂ to generate

electrical energy. The bicarbonate and carbonate ions will diffuse through the anion exchange membrane to the third compartment, while the protons (H ⁺ ions) diffuse through the cation exchange membrane to the first compartment. This creates a flow of charged particles and hence a current.

Preferably, a stack of alternating anion membranes and cation membranes is used to increase the potential

difference created due to the diffusion of the anions and cations .

Preferably, an electrolyte is provided to convert the flow of ions in a flow of electrons, e.g. by means of a redox reaction. The electrolyte may be provided in outer compartments in a reversed electrodialysis stack.

Preferably, the method comprises using a CO ₂ adsorbing material, such as active carbon. For example, the first, second and/or third compartment are provided with a CO ₂ adsorbing material.

In a preferred embodiment of the invention, the method for generating energy from a gas flow comprising CO ₂

comprises the step of:

— providing a first compartment and a second compartment, separated by a membrane;

— providing water in the compartments;

— providing the gas flow to the first compartment for

dissolving the CO ₂ in the water,

such that an osmotic pressure between the two compartments forces water from the second compartment to the first compartment, thereby increasing the water level in the first compartment, the method further comprising:

— generating electrical energy by connecting the first compartment to a device for generating electrical energy from the pressure of the water in the first compartment .

This method employs a process known as pressure

retarded osmosis (PRO) . Conventional methods of pressure retarded osmosis make use of the difference in sodium chloride (NaCl) concentration between two water streams. PRO utilizes a membrane which allows passage of water but blocks the ions in the water. Due to the osmotic pressure, fresh water from a first compartment diffuses through the membrane to salt water in a second compartment, thereby increasing the water level in the second compartment. Electrical energy can be generated using this increased water level, by employing conventional water pressure turbines. In contrast, the invention makes use of the difference in dissolved CO ₂ . The concentration of ions due to dissolved CO ₂ (protons, bicarbonate and carbonate) is higher in the first compartment than in the second compartment. Therefore, an osmotic pressure establishes over the membrane. Water will diffuse through the membrane from the second

compartment, with a low concentration of ions, to the first compartment, with a higher concentration of ions, thereby restoring equilibrium. The water level in the first

compartment will thus increase. The increased water level gives rise to an increased water pressure, which can be utilized to drive a turbine, as in conventional hydropower installations.

Preferably, the method comprises using a CO ₂ adsorbing material, such as active carbon. For example, the first and/or second compartment are provided with a CO ₂ adsorbing material .

The invention further relates to a system for

generating energy and/or separating a gas component from a gas flow, comprising:

- a gas inlet;

- a flow chamber or gas channel for the gas flow with a gas component; and

- a gas compartment separated from the flow

chamber with a gas component selective membrane.

Such system provides the same effects and advantages as mentioned in respect of the method. These advantages include an effective energy generation of electrical energy using the afore mentioned method. In addition, a gas component, such as CO ₂ , can be separated from a flue gas in an energy efficient manner. The invention further relates to a system for energy generation from a flue gas and a plant comprising such system, with the system comprising:

- a gas inlet;

- at least two capacitive electrodes comprising a current collector and a conductive material with a capacitance;

- a flow channel operatively connected to the gas inlet between the at least two electrodes;

- wherein the at least one electrode is separated from the flow channel with an anion exchange membrane and at least one electrode is separated from the flow channel with a cation exchange membrane .

Such system and plant provide the same effects and advantages as mentioned in respect of the method. These advantages include an effective energy generation of

electrical energy using the afore mentioned method. In addition, CO ₂ can be separated from a flue gas in an energy efficient manner. In fact, the system according to the present invention enables a reversible capacitive adsorption of CO ₂ . This enables an efficient energy generation and/or CO ₂ separation.

In an advantageous embodiment according to the present invention the system further comprises a fixed electrolyte structure to minimise gas flow resistance.

In a presently preferred embodiment of the invention the CO ₂ comprising donor gas reacts with water to form carbonic acid that in turn dissociates to produce the ions required for adsorption. This would imply that an

electrolyte layer is provided between the at least two electrodes. In a presently preferred embodiment water is used in the flow channel. To minimise the gas flow resistance a fixed electrolyte structure is provided. This minimises the resistance to gas flow thereby contributing to an effective energy generation and/or CO ₂ separation.

In a presently preferred embodiment the fixed

electrolyte structure can be solid polymer electrolyte, for example a wire mesh.

In a presently preferred embodiment the electrodes comprise a flat plate. This enables providing a relatively large surface for adsorbing the ions. Alternative for such flat plate configuration, or in addition thereto, the electrodes may comprise wire based electrodes. Such wire shaped electrodes enhance mass transfer as compared to a flat plate as the hydrodynamic resistances may be kept to a minimum. A further advantage of such wire shaped electrodes is the relatively easy fabrication process that may make use of extrusion technology that is capable of producing the absorber structure substantially in one step. This may further improve the efficiency of the system according to the present invention.

In an alternative embodiment of the invention the electrodes comprise floating/flowable electrodes. This may be achieved by providing the electrodes as a suspension. This renders transfer of the electrodes to the other flow, i.e. donor flow or acceptor flow, more easy.

According to the present invention the system further comprises a transfer mechanism to transfer the electrodes to another flow channel.

As an alternative to replacing the donor gas with the acceptor gas and vice versa, or in combination therewith, a transporting mechanism may transfer the electrodes from a first gas phase to another gas phase. For example, the electrodes are used in a absorption step. After saturation of the electrodes the electrodes are transferred to another flow channel thereby enabling the desorption step and regeneration of the electrodes. This transfer mechanism may switch the positions of the electrodes physically by lifting the electrode from a first system reactor and transfer them to a second system reactor, for example. Other embodiments of this transfer mechanism can be envisaged. For example, the separated electrode particles can be transported as a suspension by a flowing carrier liquid.

Optionally, the system according to the invention comprises one or more buffers. These buffers may comprise ammonia (NH ₃ ) and/or an amine, such as monoethanolamine (C ₂ H ₇ NO) , for example. Preferably, the buffer is provided in the flow channel, for example by addition of a buffer solution to water in the flow channel.

The use of a buffer, or a buffer solution, increased the conductivity and the pH difference. This reduces the internal resistance. The increase in pH difference between the air and the C02 saturated solution provides a higher power density. This renders the system according to the invention more effective.

In a further preferred embodiment according to the invention, the system comprises a reversed electrodialysis stack. Such a stack comprises alternating anion and cation exchange membranes, which define chambers which alternating hold water comprising a high concentration of dissolved CO ₂ and water comprising a low concentration of dissolved CO ₂ .

In a preferred embodiment according to the invention, the system for energy generation from a flue gas comprises:

— a first compartment and a second compartment for

holding water, separated by a membrane for allowing passage of water but blocking ions; — a gas inlet connected to the first compartment for dissolving the flue gas in water in the first

compartment; and

— a device for generating electrical energy from water pressure connected to the first compartment for

generating electrical energy from pressure of the water in the first compartment.

The features described for the method can also be applied to the system and vice versa.

Further advantages, features and details of the

invention are elucidated on the basis of preferred

embodiments thereof, wherein reference is made to the accompanying drawings in which:

- Figure 1 illustrates a system for performing the method according to the present invention;

- Figure 2 illustrates a schematic overview of the method according to the present invention;

- Figure 3 illustrates some experimental results with the system of figure 1;

- Figure 4 illustrates an alternative embodiment of the system of figure 1;

- Figure 5 illustrates a further embodiment of a system according to the present invention;

- Figure 6 illustrates experimental results with the embodiment of figure 5;

- Figure 7 illustrates a system for performing a second embodiment of the method according to the invention;

- Figure 8 illustrates a system for performing a third embodiment of the method according to the invention;

- Figure 9 illustrates a system according to the invention for generating energy and/or separating a gas component from a gas flow without electrodes.

System 2 (figure 1) comprises flow channel 4,

first electrode 6 and a second electrode 8. Electrodes 6, 8 comprise a conductive material 10 with a high capacitance. In the illustrated embodiment conductive material 10

comprises porous carbon. Current collectors 12 of electrodes 6, 8 are in contact with conductive material 10. At the relatively large internal surface area within the porous carbon 10 ions can be stored or adsorbed. Current collectors 12 are connected by electrical circuit 14. In the

illustrated embodiment the ions are stored next to the electrical charge, thereby forming a so-called electrical double layer as mentioned earlier wherein at the

carbon/water interface the electronic charge can only be in the carbon and ions (ionic charge) can only be in the water. With an electrical charge of a negative sign the electrode will attract and adsorb cations in the water-filled

micropores in the carbon. This electrode behaves as a cathode. In the opposite electrode the processes are

reversed and this electrode behaves as an anode. Electrode 6 is separated or sealed from flow channel 4 with cation exchange membrane 16. Electrode 8 is separated and/or sealed from flow channel 4 with anion exchange membrane 18. In the illustrated embodiment flow channel 4 is filled with a liquid 20, in the illustrated embodiment water. Gas, such as flue gas, flows through liquid 20 in the form of bubbles 22. Liquid 20 provides an ionic connection between the at least two electrodes 6, 8.

The method 24 according to the present invention

(figure 2) starts with providing gas at inlet 26. In flow channel 4 the reactions R1-R4 described earlier take place in reaction step 28. The cations and anions diffuse towards the electrodes 6, 8 passing the selective membranes 16, 18 in diffusion step 30. The ions are adsorbed and electric energy is generated in electrical circuit 14 in adsorption step 32.

When electrodes 6, 8 are saturated, system 2 is

switched from the adsorption state to the desorption state in switching step 34. Next, desorption will take place in desorption step 36. From the adsorption step 32 energy 38 is generated. From desorption step 36 an amount of energy 40 is generated and/or an amount of separated CO ₂ in flow 42 is being generated with optionally an amount of energy 44 being provided to enable CO ₂ separation. The amount of energy 44 can be provided by the generated energy 38, 40 in adsorption step 32 and/or desorption step 36. Alternatively, energy 44 is provided by an external source.

In a first experiment the system 2 is used in an experiment providing CC> ₂ -saturated water in flow channel 4. In the experiment an electrical potential will develop using a constant external resistance such that the potential is proportional with the generated current. The experiment results are shown in figure 3 for rise and decrease of electrical voltage (left axis) and power produced (right axis) during adsorption of CO ₂ in a capacitive electrode cell based on system 2 of figure 1.

In a further experiment it is shown that with an increasing replenishment of the acceptor gas relative to the flue gas the amount of available extractable energy

increases. This amount of energy strongly depends on the gas temperature. For example, at a temperature of 150 °C the flue gas undergoes only limit treatment, while 50°C is a

characteristic temperature for system with wet scrubbing and 20°C is a representative ambient temperature. It is

estimated that when system 2 is applied to an average power plant the amount of energy that can be harvested is

equivalent of up to 10% of the electricity produced in such average power plant.

In a further experiment, in the desorption step 36 a combination is made of spontaneous desorption resulting in a net production of energy 40 and a forced desorption

resulting in a production of CO2 flow 42. Although the temperature has a certain effect on the electricity that can be generated, the equilibrium between energy consumption for the forced desorption process as compared to the energy generation of the spontaneous energy production in the desorption step surprisingly remains about the same such that at this equilibrium about 70% of the CO2 can be

separated in a reversible capacitive adsorption process according to the present invention. Therefore the reversible capacitive absorption of CO2 provides a versatile process that be used for energy generation and CO2 separation that can be applied to power plants and also to refineries, gas and oil exploration, steel production, green houses, etc.

As an alternative to system 2 making use of a bubbling flat plate reactor type, an alternative system 44 (figure 4) can be provided. System 44 comprises a cathode 46 and an anode 48 that are connected through electrical circuits 49. The wire based electrodes 46, 48 further comprise the membranes 50 and a solid polymer electrolyte 52 around which flow 54 comprising CO2 can be provided. In a further

alternative configuration (not shown) the electrode can be in the form of a flowing suspension that is pumped slowly around in a closed circuit thereby transporting the

electrodes to a second channel 4. In the illustrated

embodiment, transfer mechanism 56 that is schematically illustrated transfers electrodes 46, 48 from a first flow chamber 4 to a second flow chamber 4. In an experiment, cell 58 (figure 5) comprises aluminum end plate 60, hollowed poly-methyl methacrylate plastic plate 62 provided with graphite electrode 64, silicon gasket 66 with graphite foil current collector with an activated carbon coating 68, anion exchange membrane 70 from Fumatech, Teflon gasket 72, polymer spacer 74, cation exchange

membrane 76 from Fumatech, graphite foil current collector with an activated carbon coating 78, silicon gasket 80, hollowed poly-methyl methacrylate plastic plate 84 provided with graphite electrode 82, aluminum end plate 86. Cell 58 is connected to circuit 88. Flow 90 enters cell 58 at plate 60, passes through the space provided with spacer 74 and leaves cell 58 at plate 86.

Anion exchange membrane 70 was pre-conditioned in a 0.25M KHCO ₃ solution and refreshed two times (once after 2.5 days and one after one extra days) . Cation exchange membrane 76 was pre-conditioned in a 0.25M HC1 solution and refreshed once after 2.5 days. Both electrodes 64, 82 are Norit super 30 based (casted at 500 microns) with 10% pvdf and were soaked in an initially CO2 saturated demi-water solution

(sparkling demi-water that progressively degassed), wherein electrodes 64, 82 stayed in this solution for 3.5 days. The CO2 saturated solution was obtained after bubbling CO2 in demi-water. CO2 was bubbling at least lh30 before starting the measurements. The air saturated solution was obtained by bubbling compressed air from the standard building line.

Internal resistance was measured between two electrodes 64, 82 and was 83Ω in the air saturated solution and 13.5 Ω in the CO2 saturated solution. The pH was measured to be 5.53 in the air saturated solution and 3.96 in the CO2 saturated solution. The cell potential (figure 6) is measured in mV. In CO2 the potential returns to zero due to the saturation effect and flow 90 through cell 58 is switched to air. The potential switches sign and slightly reduces in time. These results illustrate the operation of cell 58 in an embodiment according to the invention.

Further experiments with this experimental setup were performed. Switching the flow of CC> ₂ -rich and CC> ₂ -poor gas about every 400 seconds shows a measured open cell Voltage of about -15 mV to about +40 mV at a temperature of about 20°C and a partial pressure of 1 bar.

Additional experiments were provided wherein electrodes 6, 8 were provided in a buffer solution. The buffer solution that was used in the experiments was ethanolamine . Results are presented in Table 1 for several pH differences.

Table 1: Power density (mW/m ² ) versus pH difference pH difference Total energy (mJ) Power density (mW/m ² )

0,74 0,0246107 0,0040679

0,79 0,0421384 0,0069881

0,85 0,0672965 0,0111974

0,91 0,1078457 0,0178552

0,98 0,1702072 0,0282736

1,05 0,2461349 0,0409542

1,09 0,3553666 0,0591292

1,15 0,4911211 0,0818535 Results show a higher power density with a larger pH

difference thereby showing the effect of providing a buffer.

System 92 (figure 7) comprises a first compartment 94 and a second compartment 96, separated by a membrane 98 of the type that allows passage of water, but is impermeable to ions. Both compartments 94, 96 are provided with water.

Second compartment 96 comprises a gas inlet 100 for feeding a CO ₂ comprising gas in compartment 96. The CO ₂ dissolves in the water, leading to an increased concentration of protons, carbonate and bicarbonate ions in the water in compartment 96. In contrast, the water in compartment 94 has a

relatively low ion concentration. This creates an osmotic pressure between the compartments 94, 96. This forces water through membrane 98 according to arrow 102. The water level of the water in compartment 96 rises as a consequence, as indicated by arrow 104. This increase water level gives rise to an increased water pressure, which is utilized in a hydropower turbine 106.

The compartments 94, 96 can be connected to inlets and/or outlets for continuous and/or batch-wise operation.

System 108 (figure 8) comprises a first inlet 110 for CO2 comprising water. Alternatively, separate inlets are provided for CO2 and water, and a mixing chamber is provided to dissolve the CO2 in the water.

System 108 further comprises a water inlet 112, connected to compartments 114, 116, 118. The CO2 comprising water from inlet 110 is fed to adjacent compartments 120, 122, 124.

The outside compartments 126, 128 comprise an

electrolyte. These compartments are connected to each other by line 130.

The compartments 114, 116, 118 are separated from compartments 120, 122, 124 by means of cation exchange membranes 132 and anion exchange membranes 134.

Electrolyte compartments 126, 128 are provided with electrodes 136, 138.

Compartments 114, 116, 118, 120, 122, 124 are provided with outlets. System 108 can be operated in continuous or in batchwise operation.

The protons (H ⁺ ) diffuse through the kation exchange membranes 132 from the CO2 rich water to the CO2 poor water, while the carbonate and bicarbonate ions diffuse through the anion exchange membranes. The resulting flow of ions is converted to a flow of electrons by means of the electrolyte in outer compartments 126, 128 and/or the electrodes 136, 138. For example, the electrolyte comprises iron ions (Fe ²⁺ as reductor and/or Fe ³⁺ as oxidator) . The reduction and oxidation reaction is as following:

Fe ³⁺ + e- → Fe ²⁺

Fe ²⁺ → Fe ³⁺ + e ^" .

When the electrodes 136, 138 are connected to an electrical circuit, a current results.

System 202 (figure 9) comprises a gas flow channel 204 and a gas compartment 206 that are separated by a membrane 207. In the illustrated embodiment the membrane is C02 selective. Such membrane is known (e.g. see "Future

Directions of Membrane Gas Separation Technology", Richard W. Baker, Industrial & Engineering Chemistry Research 2002 41 (6), 1393-1411)) . The gas flow through channel 204 has a high CO ₂ concentration, at least significantly higher as compared to the concentration in compartment 206. For example, in flue gases the CO ₂ concentration is about 10% and the partial pressure in such case would be about 0,1 bar. In the illustrated embodiment the gas pressure in channel 204 is about 1 bar. Pump 208 provides a gas, such as outside air with CO ₂ content of about 390 ppm with a pressure of about 50 bar, for example. This results in a partial pressure for the CO ₂ in compartment 206 of about 1950 Pa and CO ₂ will transfer from channel 204 to compartment 206. This increases the pressure in compartment 206. The outflow of compartment 206 is fed to turbine 210 to generate energy.

It will be understood that the features of the

different embodiments that are illustrated and/or described can be combined. For example, the transfer mechanism 56 illustrated for the wire-based type system 44 can also be applied to the flat plate reactor system 2.

The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged. It is thus possible according to the invention to make a combination of the described embodiments, features and measures.