CO.SUB.2 .capture process with electrolytic regeneration

Abstract

A method of scrubbing a gas, such as flue gas or exhaust gas, comprising carbon dioxide to deplete the gas of carbon dioxide (CO.sub.2), the method comprising the steps of: scrubbing the gas in a scrubber (210) with a first alkaline, aqueous scrubbing liquid to dissolve carbon dioxide (CO.sub.2) as hydrogen carbonate (HCO.sub.3.sup.−) and/or as carbonate (CO.sub.3.sup.2−) in the first alkaline, aqueous scrubbing liquid, thereby providing a first spent aqueous scrubbing liquid comprising hydrogen carbonate (HCO.sub.3.sup.−) and/or carbonate (CO.sub.3.sup.2−), the first spent aqueous scrubbing liquid having a pH from about 7 to about 9; feeding the first spent aqueous scrubbing liquid to an anode chamber of an electrolytic cell (310) comprising the anode chamber (313) and a cathode chamber (312) separated by a membrane (311); regenerating the first spent aqueous scrubbing liquid in the electrolytic cell (310) by electrolysis, the electrolysis increasing the pH of the first spent aqueous scrubbing liquid in the cathode chamber (312), the electrolysis further depleting the first spent aqueous scrubbing liquid of hydrogen carbonate (HCO.sub.3.sup.−) and of carbonate (CO.sub.3.sup.2−) in the anode chamber (313) by decreasing the pH, the regeneration further comprising generating gaseous hydrogen in the cathode chamber (312) and a gaseous mixture of oxygen and carbon dioxide (CO.sub.2) in the anode chamber (313) by electrolysis; and withdrawing regenerated alkaline, aqueous scrubbing liquid from the cathode chamber (312) and re-circulating it to the scrubber (210); wherein: the gaseous hydrogen is withdrawn from the cathode chamber (312); and the gaseous mixture of oxygen and carbon dioxide is withdrawn from the anode chamber (313).

Claims

1. A method of scrubbing a gas comprising carbon dioxide to deplete the gas of the carbon dioxide (CO.sub.2), the method comprising: scrubbing the gas in a scrubber with a first alkaline, aqueous scrubbing liquid to dissolve carbon dioxide (CO.sub.2) as hydrogen carbonate (HCO.sub.3.sup.−) and/or as carbonate (CO.sub.3.sup.2−) in the first alkaline, aqueous scrubbing liquid, thereby providing a first spent aqueous scrubbing liquid comprising hydrogen carbonate (HCO.sub.3.sup.−) and/or carbonate (CO.sub.3.sup.2−), the first spent aqueous scrubbing liquid having a pH from about 7 to about 9; feeding the first spent aqueous scrubbing liquid to an anode chamber of an electrolytic cell comprising the anode chamber and a cathode chamber separated by a membrane; regenerating the first spent aqueous scrubbing liquid in the electrolytic cell by electrolysis, the electrolysis increasing the pH of the first spent aqueous scrubbing liquid in the cathode chamber, the electrolysis further depleting the first spent aqueous scrubbing liquid of hydrogen carbonate (HCO.sub.3.sup.−) and of carbonate (CO.sub.3.sup.2−) in the anode chamber by decreasing the pH, the regeneration further comprising generating gaseous hydrogen in the cathode chamber and a gaseous mixture of oxygen and carbon dioxide (CO.sub.2) in the anode chamber by electrolysis; and withdrawing regenerated alkaline, aqueous scrubbing liquid from the cathode chamber and re-circulating it to the scrubber; wherein: the scrubbing of the gas is performed in a first stage and in a second stage, the regenerated alkaline, aqueous scrubbing liquid withdrawn from the cathode chamber, being fed as a second alkaline, aqueous scrubbing liquid to the second stage of scrubbing downstream of the first stage of scrubbing, and wherein a second spent scrubbing liquid, resulting from the second stage of scrubbing, at least partly is fed as the first alkaline, aqueous scrubbing liquid to the first stage of scrubbing upstream of the second stage of scrubbing, the pH of the second alkaline, aqueous scrubbing liquid being higher than the pH of the first alkaline, aqueous scrubbing liquid, the gaseous hydrogen is withdrawn from the cathode chamber; and the gaseous mixture of oxygen and carbon dioxide is withdrawn from the anode chamber.

2. The method according to claim 1, wherein the method further comprises separating the gaseous mixture of oxygen and carbon dioxide into: a first stream rich in oxygen and/or depleted of carbon dioxide; and a second stream rich in carbon dioxide and/or depleted of oxygen.

3. The method according to claim 1, wherein: the regenerated alkaline, aqueous scrubbing liquid withdrawn from the cathode chamber is mixed with a part of the second spent scrubbing liquid to provide the second alkaline, aqueous scrubbing liquid, whereby the pH of the second alkaline, aqueous scrubbing liquid is lower than the pH of the regenerated alkaline, aqueous scrubbing liquid withdrawn from the cathode chamber; and/or part of the second spent scrubbing liquid is mixed with a part of a first spent scrubbing liquid, resulting from the first stage of scrubbing, to provide the first alkaline, aqueous scrubbing liquid, whereby the pH of the first alkaline, aqueous scrubbing liquid being higher than the pH the first spent scrubbing liquid.

4. The method according to claim 1, wherein carbon dioxide (CO.sub.2) and/or oxygen (O.sub.2) withdrawn from the anode chamber is compressed into liquid carbon dioxide and/or compressed oxygen (O.sub.2).

5. The method according to claim 1, wherein the gas is flue gas or exhaust gas.

6. The method according to claim 1, wherein the carbon dioxide (CO.sub.2) is dissolved as the hydrogen carbonate (HCO.sub.3.sup.−) and the carbonate (CO.sub.3.sup.2−) in the first alkaline, aqueous scrubbing liquid, thereby providing a first spent aqueous scrubbing liquid comprising the hydrogen carbonate (HCO.sub.3.sup.−) and the carbonate (CO.sub.3.sup.2−).

7. The method according to claim 1, wherein the pH of the second alkaline, aqueous scrubbing liquid is 12 to 14 and the pH of the first alkaline, aqueous scrubbing liquid is 8 to 10.

8. The method according to claim 1, wherein hydrogen withdrawn from the cathode chamber is used as a fuel to provide electricity.

9. The method according to claim 8 wherein the fuel is provided in a fuel cell.

10. The method according to claim 1, wherein the method further comprises: withdrawing an aqueous stream still comprising some hydrogen carbonate (HCO.sub.3.sup.−) from the anode chamber; concentrating the withdrawn aqueous stream comprising some hydrogen carbonate (HCO.sub.3.sup.−) to provide a concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−); and re-circulating the concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−) to the electrolytic cell.

11. The method according to claim 10, wherein the concentrating is achieved by reversed osmosis.

12. The method according to claim 1, wherein the first alkaline, aqueous scrubbing liquid comprises a dissolved metal hydroxide.

13. The method according to claim 12, wherein the dissolved metal hydroxide comprises one or more of dissolved potassium hydroxide (KOH), dissolved sodium hydroxide (NaOH), and dissolved lithium hydroxide (LiOH).

14. The method according to claim 12, wherein the first alkaline, aqueous scrubbing liquid comprises potassium hydroxide (KOH).

15. The method according to claim 1, wherein part of the regenerated alkaline, aqueous scrubbing liquid is re-circulated to the cathode chamber.

16. The method according to claim 15, wherein the regenerated alkaline, aqueous scrubbing liquid is diluted by an aqueous stream before re-circulating it to the cathode chamber.

17. The method according to claim 15, wherein said aqueous stream is provided by withdrawing an aqueous stream still comprising some hydrogen carbonate (HCO.sub.3.sup.−) from the anode chamber and concentrating it to provide an aqueous stream depleted of hydrogen carbonate (HCO.sub.3.sup.−) and a concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−).

18. The method according to claim 17, where the concentrating is achieved by reversed osmosis.

19. A system for scrubbing a gas comprising carbon dioxide to deplete the gas of the carbon dioxide (CO.sub.2), the system comprising: a scrubber configured to scrub the gas with a first alkaline, aqueous scrubbing liquid to dissolve carbon dioxide (CO.sub.2) as hydrogen carbonate (HCO.sub.3.sup.−) and/or as carbonate (CO.sub.3.sup.2−) in the first alkaline, aqueous scrubbing liquid, thereby providing a first spent aqueous scrubbing liquid comprising hydrogen carbonate (HCO.sub.3.sup.−) and/or carbonate (CO.sub.3.sup.2−), the first spent aqueous scrubbing liquid having a pH from about 7 to about 9; an electrolytic cell comprising an anode chamber and a cathode chamber separated by a membrane, the anode chamber being configured to receive the first spent aqueous scrubbing liquid; a regeneration arrangement configured to regenerate the first spent aqueous scrubbing liquid in the electrolytic cell by electrolysis, the electrolysis increasing the pH of the first spent aqueous scrubbing liquid in the cathode chamber, the electrolysis further depleting the first spent aqueous scrubbing liquid of hydrogen carbonate (HCO.sub.3.sup.−) and of carbonate (CO.sub.3.sup.2−) in the anode chamber by decreasing the pH, the regeneration further comprising generating gaseous hydrogen in the cathode chamber and a gaseous mixture of oxygen and carbon dioxide (CO.sub.2) in the anode chamber by electrolysis; wherein, in use: the scrubber is configured to perform scrubbing at least in a first stage and in a second stage, the regenerated alkaline, aqueous scrubbing liquid withdrawn from the cathode chamber, being fed as a second alkaline, aqueous scrubbing liquid to the second stage of scrubbing downstream of the first stage of scrubbing, and wherein a second spent scrubbing liquid, resulting from the second stage of scrubbing, at least partly is fed as the first alkaline, aqueous scrubbing liquid to the first stage of scrubbing upstream of the second stage of scrubbing, the pH of the second alkaline, aqueous scrubbing liquid being higher than the pH of the first alkaline, aqueous scrubbing liquid, the regenerated alkaline, aqueous scrubbing liquid is withdrawn from the cathode chamber and re-circulated to the scrubber; the gaseous hydrogen is withdrawn from the cathode chamber; and the gaseous mixture of oxygen and carbon dioxide is withdrawn from the anode chamber.

20. The system according to claim 19, further comprising a separator to separate the gaseous mixture of oxygen and carbon dioxide into: a first stream rich in oxygen and/or depleted of carbon dioxide; and a second stream rich in carbon dioxide and/or depleted of oxygen.

21. The system according to claim 19, wherein the system further comprises: an aqueous stream still comprising some hydrogen carbonate (HCO.sub.3.sup.−) is withdrawn from the anode chamber; wherein: the withdrawn aqueous stream comprising some hydrogen carbonate (HCO.sub.3.sup.−) is concentrated to provide a concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−); and the concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−) is re-circulated to the electrolytic cell.

22. The system according to claim 19, wherein the pH of the second alkaline, aqueous scrubbing liquid is 12 to 14 and the pH of the first alkaline, aqueous scrubbing liquid is 8 to 10.

23. The system according to claim 19, wherein the first alkaline, aqueous scrubbing liquid comprises a dissolved metal hydroxide.

24. The system according to claim 23, wherein the dissolved metal hydroxide comprises one or more of dissolved potassium hydroxide (KOH), dissolved sodium hydroxide (NaOH), and dissolved lithium hydroxide (LiOH).

25. The system according to claim 23, wherein the first alkaline, aqueous scrubbing liquid comprises potassium hydroxide (KOH).

26. The system according to claim 19, wherein part of the regenerated alkaline, aqueous scrubbing liquid is re-circulated to the cathode chamber.

27. The system according to claim 26, wherein the regenerated alkaline, aqueous scrubbing liquid is diluted by an aqueous stream before re-circulating it to the cathode chamber.

28. The system according to claim 26, wherein said aqueous stream is provided by withdrawing an aqueous stream still comprising some hydrogen carbonate (HCO.sub.3.sup.−) from the anode chamber and concentrating it to provide an aqueous stream depleted of hydrogen carbonate (HCO.sub.3.sup.−) and a concentrated stream comprising hydrogen carbonate (HCO.sub.3.sup.−).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which:

(2) FIG. 1 shows a flow path between a scrubber and an electrolytic cell;

(3) FIG. 2 shows a process scheme of a system for scrubbing flue gas;

(4) FIG. 3 shows a scrubber arrangement of the process scheme of FIG. 2;

(5) FIG. 4 shows a regeneration arrangement of the process scheme of FIG. 2;

(6) FIG. 5 shows test results of the overall generation of gas flow in the electrolytic cell and the pH-value over time;

(7) FIG. 6 shows test results of the gas flow of CO.sub.2 and O.sub.2 in relation to the pH-value over time;

(8) FIG. 7 shows test results of the gas production of CO.sub.2 and O.sub.2 in relation to the energy consumption over time; and

(9) FIG. 8 shows test results of the relationship between the production of gases leaving the electrolytic cell over time.

DETAILED DESCRIPTION

(10) With reference to FIG. 1, a system 100 according to an embodiment is shown having a scrubber arrangement 200 and a regeneration arrangement 300. Here, a method of scrubbing a gas, such as flue gas or an exhaustive gas, comprising carbon dioxide CO.sub.2, is illustrated. The gas enters the scrubber through the scrubber inlet 213. To deplete the flue gas from carbon dioxide CO.sub.2, the scrubbing method can be described as follows. The gas is scrubbed in the scrubber 210 in a counter flow manner with a first alkaline, aqueous scrubbing liquid to dissolve carbon dioxide CO.sub.2 as hydrogen carbonate HCO.sub.3.sup.− and/or as carbonate CO.sub.3.sup.2− in the first alkaline, aqueous scrubbing liquid. A first spent aqueous scrubbing liquid comprising dissolved hydrogen carbonate HCO.sub.3.sup.− and/or carbonate CO.sub.3.sup.2− results. The first spent aqueous scrubbing liquid has a pH from about 7 to about 9 when it leaves at the outlet 211″ for withdrawing spent aqueous scrubbing liquid of the scrubber 210. The first spent aqueous scrubbing liquid is then fed to an anode chamber 313 of an electrolytic cell 310 via an anode inlet 313′. The electrolytic cell 310 has apart from the anode chamber 313 also a cathode chamber 312. The anode chamber 313 and the cathode chamber 312 are separated by a membrane 311. This membrane 311 may be a semi-permeable membrane, being permeable to cations, but essentially impermeable to anions. Thus, the membrane cation-exchange membrane. The electrolysis increases the pH of the first spent aqueous scrubbing liquid in the cathode chamber 312. In the anode chamber 313, the electrolysis further depletes the first spent aqueous scrubbing liquid of hydrogen carbonate HCO.sub.3.sup.− and of carbonate CO.sub.3.sup.2− by decreasing the pH-value to release gaseous carbon dioxide. The outlet 211″ for spent aqueous scrubbing liquid of the scrubber 210 is in flow communication with the inlet 313′ for the spent aqueous scrubbing liquid of the anode chamber 313. Moreover, the outlet 312″ for regenerated aqueous scrubbing liquid of the cathode chamber 312 is flow communication with the inlet 212′ for the alkaline, aqueous scrubbing liquid of the scrubber 210.

(11) One can say that the first spent aqueous scrubbing liquid is regenerated by generating gaseous hydrogen H.sub.2 and dissolved hydroxide ions OH.sup.− in the cathode chamber 312 and a gaseous mixture of oxygen O.sub.2 and carbon dioxide CO.sub.2 in the anode chamber 313 by electrolysis. This is indicated by the upwards pointing arrows from the cathode outlet 312″ and the anode outlet 313″ in FIG. 1, respectively. The gaseous hydrogen H.sub.2 and dissolved hydroxide ions OH.sup.− is withdrawn from the cathode chamber 312 and the gaseous mixture of oxygen O.sub.2 and carbon dioxide CO.sub.2 is withdrawn from the anode chamber 313. For instance, the hydrogen H.sub.2 may be used in downstream processes (not shown) such as in fuel or methanol production. The regenerated alkaline, aqueous scrubbing liquid from the cathode chamber 312 is then recirculated via the inlet 212′ for receiving the alkaline, aqueous scrubbing liquid to the scrubber 210. Gas depleted of carbon dioxide CO.sub.2 then exits the scrubber 210 via the scrubber outlet 214.

(12) In FIG. 2, a detailed view of the system 100 descried in relation to FIG. 1 is shown. The system is separated into two parts; the scrubber arrangement 200 and the regeneration arrangement 300. These two arrangements are also shown separately in FIGS. 3 and 4, respectively. For increased understanding, the scrubber arrangement 200 and the regeneration arrangement 300 will now be described separately. The dotted arrows in FIGS. 3 and 4 indicate where the scrubber arrangement 200 and the regeneration arrangement 300 may meet in FIG. 2 to form the system 100 in its entirety. Alternatively, the scrubber arrangement 200 and the regeneration arrangement 300, may be operated independently.

(13) With reference to FIG. 3, the scrubber arrangement 200 has a first buffer tank 230 and a second buffer tank 240 for alkaline, aqueous scrubbing liquid. The scrubber 210 has a first absorber 211 and a second absorber 212 for scrubbing the gas of carbon dioxide. Scrubbing liquid may be re-circulated to the first absorber 211 via the first buffer tank 230. Further, scrubbing liquid may be re-circulated to the second absorber 212 via the second buffer tank 240. The first absorber 211 thus has an inlet 211′ for receiving alkaline, aqueous scrubbing liquid from the first buffer tank 230, and an outlet 211″ for withdrawing spent aqueous scrubbing liquid and feeding it to the inlet 313′ for the spent aqueous scrubbing liquid of the anode chamber 313 directly (as shown in FIG. 1) or via the first buffer tank 230, as shown in FIGS. 2 and 3. The second absorber 212 has an inlet 212′ for receiving alkaline, aqueous scrubbing liquid from the second buffer tank 240 and an outlet 212″ for withdrawing spent aqueous scrubbing liquid and feeding it to the inlet 211′ of the first absorber and/or to the second buffer tank 240. The outlet 212″ of the second absorber is in flow communication with the second buffer tank 240 and the inlet 211′ of the first absorber 211. Thus, regenerated alkaline, aqueous scrubbing liquid may be mixed with spent aqueous scrubbing liquid from the second absorber 212 before being fed as alkaline, aqueous scrubbing liquid to the second absorber 212. Further, spent aqueous scrubbing liquid from the second absorber 212 may be mixed with spent aqueous scrubbing liquid from the first absorber 211 before being fed as alkaline, aqueous scrubbing liquid to the first absorber 211. By controlling the flow rates in the scrubber arrangement 200, as well as the mixing ratios, not only the scrubbing efficiency, but also the pH of the spent aqueous scrubbing liquid to be regenerated may be controlled.

(14) In accordance with the description of FIG. 1, flue gas enters the scrubber 210 via the scrubber inlet 213. The scrubbing is then performed in two stages. The first stage takes place in the first absorber 211 and the second stage takes place in the second absorber 212. The scrubbing is performed in a counter flow manner, with the second absorber 212 arranged downstream of the first absorber 211.

(15) As mentioned, the scrubber arrangement 200 also has a third buffer tank 220 for regenerated aqueous scrubbing liquid. This third buffer tank 220 is in flow communication with the outlet 312″ for regenerated aqueous scrubbing liquid of the cathode chamber 312 and with the inlet 212′ of the second absorber 212. The scrubber arrangement 200 also has a fourth buffer tank 250 for spent aqueous scrubbing liquid. This fourth buffer tank 250 is in flow communication with the first buffer tank 230 and with the inlet 313′ for the spent aqueous scrubbing liquid of the anode chamber 313. The fourth buffer tank 250 also has an outlet for withdrawing spent aqueous scrubbing liquid and delivering to the electrolytic cell 310 via the anode inlet 313′.

(16) If the scrubber arrangement 200 is arranged separately from the regeneration arrangement 300, such as in a vehicle, also the regeneration arrangement 300, such as a charging station or vehicle depot, may comprise a third buffer tank 220 and a fourth buffer tank 250 as shown in FIG. 4.

(17) Now turning to the regeneration arrangement 300 in FIG. 4. Other than the electrolytic cell 310 and its components, which have been previously described in relation to FIG. 1, the regeneration arrangement 300 further comprises a first compressor unit 320 for compressing hydrogen gas withdrawn from the cathode chamber 312. Moreover, the regeneration arrangement 300 further comprises a second compressor unit 330 for compressing oxygen and carbon dioxide withdrawn from the anode chamber. Furthermore, the regeneration arrangement 300 also comprises a first gas separator 340 for separating oxygen O.sub.2 and carbon dioxide CO.sub.2 withdrawn from the anode chamber 313 from liquid. The first gas separator 340 is arranged upstream the second compressor unit 330. The regeneration arrangement 300 also comprises a second gas separator 380 for separating gaseous hydrogen H.sub.2 and, for instance, liquid aqueous potassium hydroxide KOH, withdrawn from the cathode chamber 312. When the system 100 is operating, the oxygen O.sub.2 and carbon dioxide CO.sub.2 leaving the first gas separator 340 is compressed. The composition is then typically around 75% CO.sub.2 and 25% O.sub.2. They may for instance be compressed at about 50 bar at 10° C. where carbon dioxide CO.sub.2 is liquefied and oxygen O.sub.2 still is in gaseous phase. An advantage of keeping CO.sub.2 in a liquid phase is that it is practical during transportation. For instance, a third separator 390 is present downstream of the compressor to separate the oxygen from the liquefied carbon dioxide.

(18) Furthermore, the regeneration arrangement 300 has a separator 350, such as a filter. For instance, the filter may be a reversed osmosis filter. This concentrator 350 is arranged downstream of the first gas separator 340. After passing the first gas separator, the fluid contains an aqueous stream comprising some hydrogen carbonate HCO.sub.3.sup.− withdrawn from the anode chamber 313. The concentrator is configured to provide, such as by filtration, a concentrated stream of hydrogen carbonate HCO.sub.3.sup.− and an aqueous stream depleted of hydrogen carbonate HCO.sub.3.sup.−. The concentrator 350 is in flow communication with the electrolytic cell 310 such that the aqueous stream comprising some hydrogen carbonate HCO.sub.3.sup.− is withdrawn from the anode chamber 313 and fed to the concentrator 350. The concentrated stream of hydrogen carbonate HCO.sub.3.sup.− may be withdrawn from the concentrator 350 and fed to the anode chamber 313. Further, the aqueous stream depleted of hydrogen carbonate HCO.sub.3.sup.− may be withdrawn from the concentrator 350 and fed to the cathode chamber 312.

(19) The regeneration arrangement 300 also has a first balance tank 360 for regenerated alkaline, aqueous scrubbing liquid. The first balance tank 360 has a first inlet 361 for receiving regenerated alkaline, aqueous scrubbing liquid from the second gas separator 380. The first balance tank 360 also has a second inlet 362 for receiving the aqueous stream depleted of hydrogen carbonate HCO.sub.3.sup.− from a third balance tank 351, withdrawn from the concentrator 350. Further, it has an outlet 363 for feeding diluted regenerated alkaline, aqueous scrubbing liquid to the cathode chamber 312 of the electrolytic cell 310.

(20) The regeneration arrangement 300 may also have a second balance tank 370 for spent aqueous scrubbing liquid. The second balance tank 370 has a first inlet 371 for receiving spent aqueous scrubbing liquid from the scrubber arrangement 200, in particular via the fourth buffer tank 250. Further, it has a second inlet 372 for receiving the concentrated stream comprising hydrogen carbonate HCO.sub.3.sup.− from the concentrator 350. It also has an outlet 373 for feeding spent aqueous scrubbing liquid to the anode chamber 313 of the electrolytic cell 310.

(21) To further describe the relationship between the scrubber arrangement 200 and the regeneration arrangement 300, the following description is provided. The regenerated alkaline, aqueous scrubbing liquid withdrawn from the cathode chamber 312 of the electrolytic cell 310 is fed as a second alkaline, aqueous scrubbing liquid to the second stage of scrubbing downstream of the first stage of scrubbing. The second stage of scrubbing may be defined as the process of the second absorber 212. A second spent scrubbing liquid, resulting from the second stage of scrubbing in the second absorber 212, is at least partly fed as the first alkaline, aqueous scrubbing liquid to the first stage of scrubbing upstream of the second stage of scrubbing, i.e. in the first absorber 211.

(22) The pH-value of the second alkaline, aqueous scrubbing liquid entering the second absorber 212 is rather high. Preferably, the pH of the second alkaline, aqueous scrubbing liquid is about 12 to 14. This is higher than the pH of the first alkaline, aqueous scrubbing liquid leaving the first absorber 211 on its way to the electrolytic cell 310. Preferably, the pH of the first alkaline, aqueous scrubbing liquid is about 8 to 10.

(23) It is to be noted that the electrolytic cell may be sensitive to impurities in the fluid flowing through the anode and cathode chambers. Hence, there may also be a separate cleaning unit (not shown), which serves to remove impurities such as for instance nitrogen oxides NOx and sulfur oxides SOx from the spent aqueous scrubbing liquid before it enters the electrolytic cell 310. As an example, the cleaning unit may include a filter to remove particulate matter.

(24) It should be noted that in all FIGS. 1-4, the direction of the arrows corresponds to the direction of flow of the fluids circulating in the system 100. Further, the lines of the arrows indicate a fluid or flow communication between the elements of the system.

(25) Chemical Processes

(26) The chemical processes occurring in the system 100 may be divided into two different parts, namely carbon capture and electrochemical regeneration, respectively. The overall reaction electrochemically splits water to oxygen and hydrogen according to the following formula:
2×H.sub.2O.fwdarw.O.sub.2+2×H.sub.2

(27) The chemical reactions have been balanced for the overall process of capturing 4×CO.sub.2 molecules.

(28) The Scrubber

(29) In short, carbon is captured in the scrubber 210 by dissolving carbon dioxide in the alkaline, aqueous scrubbing liquid. This reaction takes place automatically in accordance with the following formula:
4×OH.sup.−+4×CO.sub.2.fwdarw.4HCO.sub.3.sup.−

(30) The solvent is then regenerated in the regeneration arrangement 300 using electrochemistry. In general, the electrochemical reaction can be split into two parts; the anode reaction and the cathode reaction. These reactions will be described below.

(31) The Anode

(32) In the anode chamber 313, O.sub.2 and CO.sub.2 is generated in two different steps. First, O.sub.2 is generated at the anode together with 4 H.sup.+. Then, the H.sup.+ decreases the pH-value of the solvent and releases CO.sub.2. Simultaneously, O.sub.2 is generated at the anode and the two gases are mixed in a ratio of 4:1, CO.sub.2 to O.sub.2. The overall reaction at the anode chamber 313 is:
4×HCO.sub.3.sup.−.fwdarw.O.sub.2+4×CO.sub.2+2×H.sub.2O+4e.sup.−

(33) The reaction at the anode is:
2×H.sub.2O.fwdarw.O.sub.2+4×H.sup.++4e.sup.−

(34) This reaction decreases the pH-value locally. This decrease in pH-value pushes the HCO.sub.3−/CO.sub.2 equilibrium to the right, such that:
4×H.sup.++4×HCO.sub.3.sup.−.fwdarw.4×CO.sub.2+4×H.sub.2O
which results in the release of gaseous CO.sub.2 from the solvent.

(35) The Cathode

(36) At the cathode, H.sub.2 is produced together with OH.sup.−. This reaction both generates valuable H.sub.2 for downstream applications and regenerates the alkaline solvent comprising hydroxide ions (OH.sup.−) for the carbon capture process. The cathode chamber reaction 312 is:
4×H.sub.2O.sup.+4.sup.−.fwdarw.2×H.sub.2+4×OH.sup.−.

(37) Scrubbing Liquid

(38) Ethanolamine (MEA) is an amine used for carbon capture used in conventional scrubbers. It has been suggested that MEA may act as a promoter for the process of scrubbing for instance flue gas. MEA indeed is known to capture CO.sub.2 faster than a hydroxide solution. An envisaged idea has therefore been to combine the carbon capture capabilities of MEA with the electrochemical properties of the hydroxide solution. However, experimental tests have indicated that MEA unfortunately behaves undesirably in an electrochemical cell. MEA appears to be reduced at the cathode, which would reduce the carbon capturing capabilities of the solvent, which is highly unwanted. Thus, using a metal hydroxide, e.g. potassium or sodium hydroxide, is preferred in the system 100 disclosed herein.

(39) Power Need

(40) The electrochemical reaction in the electrolytic cell 310 requires electrical power. The actual power consumption will depend on the technical implementation of the process of the system 100. Assuming 100% efficiency, the minimum current required for the process can be calculated using Faraday's law of thermodynamics: I=mFz/tM. With the parameters as listed in Table 1 below, the current can be calculated.

(41) TABLE-US-00001 TABLE 1 parameters for calculating the minimum current required for the process Symbol Quantity Value m Mass of O.sub.2 182 kg F Faraday's constant 96485 C/mol z Valency number of electrons 2 t Time 1 s M Molar mass of O.sub.2 32 g/mol

(42) The current can thus be calculated to l=1.09×10.sup.9 A. With a minimum voltage of 2 V assumed, the theoretical minimum power consumption for 1 ton of CO.sub.2 will be:
P.sub.min=2V×1.09×10.sup.9 A=2.18×109 J=2.18 GJ.

(43) For real chemical reactions, a higher energy consumption is expected. As suggested by a model based on experiments the ultimate power consumption for the capture of CO.sub.2 and regeneration of the solvent is predicted to 5.88 GJ per 1 ton of CO.sub.2. This process regenerates the solvent, produces H.sub.2 at the cathode 312 and a mixture of CO.sub.2 and O.sub.2 at the anode.

(44) Further energy is required for the separation of the CO.sub.2 and O.sub.2 from the first gas separator 340. This separation may for instance be done cryogenically. Energy consumption for CO.sub.2 cryogenically separated from CH.sub.4 has been studied in the literature for biogas purposes. For cryogenic separation, CH.sub.4 and O.sub.2 have similar physio-chemical properties, as the CO.sub.2 is removed by cooling. The energy consumption for separation of O.sub.2 and CO.sub.2 in the first gas separator 340 is expected to be 1.4±0.4 MJ per kg CO.sub.2.

(45) CO.sub.2 and H.sub.2 is typically produced in a ratio of 2:1. If the downstream application is methanol production, the suitable stoichiometric ratio is 1:3 and additional H.sub.2 is required for this process. Commercial electrolysis equipment produces H.sub.2 with an energy consumption of 55 kWh/kg. For 1 ton of CO.sub.2, the H.sub.2 requirements are therefore (m.sub.CO2M.sub.CO2)×3=68182 mol, which equals: 68182 mol×2 g/mol×55 kWh/kg=7500 kWh=26.98 GJ. The carbon capture regeneration process produced H.sub.2 corresponding to 4.5 GJ, and the remaining energy requirements for H.sub.2 production is therefore: 26.98 GJ-4.50 GJ=22.48 GJ.

(46) The power consumption of the carbon capture process is determined primarily by the electrochemical cell. The purification of CO.sub.2 requires additional energy. However, the substantially largest energy consumption comes from the H.sub.2 production. The carbon capture and purification alone is expected to cost in the order of 7.28 GJ per ton of CO.sub.2. This includes the production of H.sub.2 corresponding to 4.5 GJ as already mentioned. Commercial values for CO.sub.2 capture with amine scrubbers are currently 3.7 GJ per ton of CO.sub.2. This is without the generation of H.sub.2 and the CO.sub.2 purification. Hence, if the CO.sub.2 is used downstream of the electrolytic cell 310 together with H.sub.2 to make for instance methanol, the process disclosed herein will be beneficial.

(47) Using the values from an experimental model, the methanol produced from 1000 kg of CO.sub.2 would costs about 2.5 kr/I, see Table 2 below. This price (Danish krone) is calculated based only on energy consumption and does not take material and personal into account.

(48) TABLE-US-00002 TABLE 2 Energy consumption of producing methanol from 1000 kg of CO.sub.2 Process Energy consumption Case: 1000 kg CO.sub.2 Carbon capture and regeneration 5.88 GJ CO.sub.2 purification 1.4 GJ H.sub.2 production 22.48 GJ Overall 29.76 GJ = 8267 kWh Production 728 kg = 919 I methanol Electricity price 0.3 kr/kWh Carbon tax (saved) 182 kr Price 2.5 kr/I

(49) Currently, methanol made from non-renewable sources is sold at the price of 1.66 kr/I. The price of 2.5 kr/I is higher than 1.66 kr/I. However, currently, it is expected that green methanol would have a higher price than black methanol. Furthermore, the price is highly linked to the electricity price. However, as the carbon tax is expected to increase, as well as the cost for emission rights, the need and request for methanol production will increase in the industry and the system 100 provided herein will be beneficial to meet this increased demand.

EXPERIMENTAL SECTION

Example 1

(50) In the following, a CO.sub.2 capture from a power plant generating 10 MW heat and power from biomass is presented in relation to three process steps; “scrubber”, “regeneration” and “separation”, see Table 3. Overall, the process requires a large amount of electrical energy. This is positive, as electrification of the carbon capture process is highly wanted and completely new. Some of the energy may be recovered as heat for district heating.

(51) TABLE-US-00003 TABLE 3 10 MW power plant CO2 capture Process Scrubber Regeneration Separation In Gas with ~10% Saturated liquid Gas: 80% CO.sub.2 CO.sub.2 2 ton CO.sub.2/h 400 m.sup.3/h and 20% O.sub.2 Lean liquid Power 2 ton/h CO.sub.2 400 m.sup.3/h 3.2 MW Power 0.76 MW Out Gas without CO.sub.2 Lean liquid 400 Gas: pure CO.sub.2 Saturated liquid m.sup.3/h 2 ton/h CO.sub.2 400 m.sup.3/h Gas: H.sub.2 45 kg/h Gas: 80% CO.sub.2 and 20% O.sub.2 2 ton/h CO.sub.2 Operation Automatic process Uses power Uses power

Example 2

(52) To verify the applicability of the process using the system 100 as described herein, laboratory tests have been performed.

(53) In the laboratory tests, a standard electrolysis cell from EC Electrocell, model Electro MP Cell was used. The electrolysis cell was provided with a Nafion 117 membrane. In operating the cell, a 1.5 M KHCO.sub.3 solution was circulated over the anode side from a combined degassing/circulation tank. The liquid was circulated at 1.5 L/min. Similarly, a 1.5 M KOH solution was circulated over the cathode side from a combined degassing/circulation tank. The liquid was circulated at 1.5 L/min. Standard flowmeters and lab pumps were used. Gas flow from the degassing tanks were measured by an Aalborg GFM gas flow meter. CO.sub.2 content were measured using a Guardian NG from Edinburgh Sensors. A standard heat plate was used to keep a constant temperature of the liquid at 40 degrees Celsius during the experiments. The pH and temperature were measured in the circulation tanks using standard online pH and temperature meters. The current density applied to the electrolyzer were varied between 1-4 kA/m.sup.2, using a standard power converter.

(54) The results are presented in FIGS. 5-8. It is clear that the pH-value of the scrubbing liquid entering the electrolytic cell 310 plays an important part in the production of gases.

(55) FIG. 5 shows the overall gas generation in the electrolytic cell and the pH-value over time. As is shown, the gas flow on the anode side of the electrolytic cell is fairly low at the beginning of the trial. Here, the pH-value of the scrubbing liquid is around 9.5. As the pH-value drops, the gas flow increases until it reaches its maximum level at a pH of about 8.5. Thereafter, the gas flow stabilizes at about 290 ml gas/min and the pH-value at about 8.

(56) In FIG. 6, the gas flow of CO.sub.2 and O.sub.2 is illustrated in relation to the pH-value over time. As can be seen, in the beginning, only O.sub.2 is produced and there is no release of CO.sub.2. However, as soon as the pH-value begins to drop, the process starts to produce CO.sub.2 gas and the collective rise in gas flow is substantially due to the rise in CO.sub.2. The gas then has the composition of approximately 25% O.sub.2 and 75% CO.sub.2. Thus, it is shown once again that the pH-value plays an important role in the gas composition and in the production of CO.sub.2 from the scrubbing liquid and hence also in the regeneration of scrubbing fluid. Preferably, the process is operated to provide the spent aqueous scrubbing liquid with a pH-value of about 8.5. This is where an optimal gas production and generation of CO.sub.2 occurs.

(57) Hence, the scrubber 210 may be seen not only as a scrubber, but importantly also as a pH-regulator. The scrubber 210 is preferably divided into several steps or absorbers, such as the first and second absorbers 211, 212 depicted with dashed lines in FIG. 2. The scrubber fluid entering the second absorber 212 has a higher pH-value than the fluid exiting the first absorber 211 to the anode chamber 313 of the electrolytic cell 310. This is also clear from FIG. 7 where the gas production of CO.sub.2 and O.sub.2 is shown in relation to the energy consumption. Here, the current has been decreased in three steps, marked by the blue line which has a step-like shape.

(58) FIG. 8 shows the relationship between the production of gases leaving the electrolytic cell per hour as measured over time. It is clear that the production of O.sub.2, H.sub.2 and KOH starts immediately, whereas the CO.sub.2 production begins as the pH-value is decreased to an appropriate level as previously described. The production of H.sub.2 follows the production of O.sub.2 in a ratio of 2 parts H.sub.2 per one part O.sub.2. Moreover, the production of KOH follows the production of H.sub.2 in a ratio of 2 parts KOH per part H.sub.2.

(59) From Table 4 it is clear that essentially no additional energy is required for generating CO.sub.2 and producing KOH for the regeneration of the scrubbing fluid, when compared to conventional H.sub.2 electrolysis. This way, the H.sub.2 production actually can compensate substantially for the energy required for the CO.sub.2 capture.

(60) TABLE-US-00004 TABLE 4 Power consumption in the production of CO.sub.2. CO.sub.2-production: 0.59 mol/h (aim in the trials) Power consumption 40 Watt (aim and assumed from industrial standards) Power consumption pr. 67.8 Watt/mol calculated mol: Power consumption pr. ton 1541 kW calculated CO.sub.2: Total production of H.sub.2 26.7 kg/ton CO.sub.2 calculated Power consumption pr. kg 57.5 kW calculated H.sub.2

(61) In summary, the disclosure intends to describe a system 100 which reduces the cost (per ton of CO.sub.2) for capturing CO.sub.2 as compared to existing technologies, where the costs are associated with green electrical power, CO.sub.2 quota or tax costs as well as hydrogen sales price.