Energy efficient method for concurrent methanisation and carbon dioxide recovery

The present invention relates to a method for storing energy from methanisation of carbon dioxide and hydrogen in an energy efficient manner by combining the methanisation and carbon dioxide recovery.

The present invention relates to a method for storing energy by combining a process for recovering carbon dioxide from a gas stream, such as a biogas, flue gas, fermentation gas or other carbon dioxide containing gases to provide carbon dioxide which is reacted to provide preferably methane in order to store energy for prolonged periods where the energy is in the form of wind, solar or wave energy.

This otherwise expensive conversion of carbon dioxide to methane is rendered profitable by utilizing energy in the form of heat generated from the methanisation reaction in a carbon dioxide recovery process providing the pure carbon dioxide for the reaction or the heat is used in the process generating the carbon dioxide feed stream.

Background

The amount of electricity generated from wind, wave and sun power is increasing, and consequently the requirements for energy storage also increases since electricity demand and actual possible electricity recovery not always align and therefore in periods introduce an excess of produced electricity.

Of the known ways of storing energy are so called power-to-gas or power-to-fuel processes where CO2 is reacted with H2 generated by electrolysis following the reaction 2H2O + e- <-> 2H ₂ + O2 utilizing electricity from the power grid.

Thereby energy is stored chemically as illustrated by the overall reactions below

CO2 may be converted to CH ₄ by reaction with H ₂ , power to gas.

CO2 + 4H ₂ <-> CH ₄ + 2H ₂ 0

Or CO2 may be converted to liquid fuel by reaction with H ₂ , power to fuel

CO2 + H ₂ <-> CO + H ₂ 0

(2n + l)H ₂ + n CO <-> CnH( _2n +2) + n H ₂ 0 EP2873030 describes such a process for preparing methane from car- bon dioxide. However, the formation of hydrogen by electrolysis requires a relatively high energy input which renders the methanisation process less attractive. US 3,766,027 similarly describes a combined carbon diocxide recovery and methanisation proces.

Since, it is desirable to provide more and more energy from alternative sources there is a need for storing the energy when in excess of the demand in order to be able to supply electricity continuously preferably as much as possible from renewable sources. Hence, there is a continued need for finding additional ways of increasing the efficiency of methanisation reactions in order to provide a cost efficient and yet environmental friendly solution.

Summary of the invention

With this background it is an object of the present invention to provide a method for storing energy in the form of a hydrocarbon compound comprising the steps of

a. providing a feed gas (Gl), optionally pressurised, at least comprising carbon dioxide;

b. subjecting the feed gas (Gl) to an absorption step in an absorption column (Al) using an absorbent (LI) to provide a carbon dioxide lean gas (G2) and a carbon dioxide rich liquid (L2);

c. optionally, heat exchanging the carbon dioxide rich liquid (L2) in a first heat exchanger (A3) to provide a heated carbon dioxide rich liquid (L3);

d. desorbing the heated carbon dioxide rich liquid (L3) in a regenerator column (A5) to provide a recovered absorbent (L4) and a carbon dioxide rich gas (G5);

e. feeding the carbon dioxide rich gas (G5) to a methanisation unit

(A15);

f. providing a hydrogen stream (G14), optionally by the step of electrolysing water in a electrolysis unit (A13) providing a hydrogen stream (G14) and an oxygen stream (G15), and feeding the hydrogen stream (G14) to the methanisation unit (A15); g. reacting the carbon dioxide rich gas (G5) with the hydrogen stream (G14) whereby the reaction provides a methane rich stream (G18) and heat;

and whereby the heat provided from the methanisation reaction is utilized in the energy storing method and/or in a process providing the feed gas.

The problem with storing energy originating from renewable energy sources, such as wind, solar and/or waves in the form of methane or liquid fuel is that the energy input to provide hydrogen necessary for e.g . methanisation is too high for the process to be cost efficient.

The invention provides a solution where combining the upgrade of car- bon dioxide and hydrogen to methane/liquid fuel, with a carbon dioxide recovery process in a specific manner, such that the heat generated from the methanisation can conveniently be used elsewhere in the carbon dioxide recovery process or in the feed gas process. More specifically it has turned out that the heat generated may be sufficient to provide e.g. a stripper reboiler with energy. Hence, the energy that would otherwise have to be provided to such a unit in the form of a steam cycle can be omitted. This results in a carbon dioxide recovery process where the need for an external heat source may be omitted. Thereby the combined process both recaptures carbon dioxide, saves and stores energy.

Hence, by coupling the provision of hydrogen through electrolysis to a carbon dioxide recovery plant, the overall energy input for the two processes is minimized.

In a preferred embodiment the feed gas (Gl) is a biogas originating from a biogas production process. Other source gases are contemplated, such as fluegas and fermentation gas, but biogas is preferred, since the methane produced in the methanization reaction can be mixed with the methane otherwise produced from the biogas, and furthermore the oxygen also produced in the electrolysis reaction can be used downstream in the carbon dioxide recovery process as will be described below.

In yet an embodiment the absorption of step b. is a physical absorption, the regenerator column (A5) of the desorption step d . is a flash column and the feed gas is pressurised, and wherein the heat generated in the methanisation is utilized in the biogas production process.

Hence, when for example water is used as the absorbent, a more sim- pie absorbent regeneration can be applied since a simple flashing can be used . When using physical absorption the feed gas should preferably be pressurised before entering the absorption column. For efficient physical absorption the pressure should be selected to suit the absorbent - which will typically be in the range of 1 to 40 barg. Physical absorption of carbon dioxide is well known to the skilled person.

In another embodiment the absorption of step b. is a chemical absorption and the regenerator column (A5) is provided with a regenerator reboiler (A6). In a particular embodiment the heat for the regenerator reboiler (A6) is provided from the heat formed in the methanisation reaction of step g . Hence, the carbon dioxide rich stream (L3) leaving the regenerator is heated in the stripper/regenerator reboiler and subsequently separated to provide a stripper gas (G4) and a recovered absorbent (L4), which may be further cooled if necessary.

The chemical absorbent may be any suitable chemical absorbent used in the art such as but not limited to commercially available alkanol amines. The alkanolamine in the absorbing agent is selected from the group consisting of monoethanolamine, diethanolamine, diisopropanolamine, methyldiethanola- mine and triethanolamine. Most often the absorbing agent is an aqueous solution of one of the above-mentioned alkanolamines. However, mixtures com- prising promoters and two or more of the listed alkanolamines in any mixing ratio may also be used in the method according to the present invention. It is within the skills of a practitioner to determine the optimal amount and composition of the absorbing agent in order to achieve a suitable absorption procedure. Chemical absorbents and their use in absorbing carbon dioxide is well known to the skilled person.

With the embodiment above the stripper reboiler may be driven solely by heat generated by the methanization reaction. The possibility of coupling the methanization reaction to the regenerator reboiler further improves the net energy input required by the electrolysis reaction. Hence, in other words in a particular embodiment, the method is a method for providing heat for a regenerator reboiler.

The electrolysis also generates heat which needs to be removed, usually using cooling water, since the electrolysis requires constant temperature. Typically, 20% - 30% of the supplied electrical energy is wasted in the form of heat. In a preferred embodiment this heat is utilized elsewhere in the process, such as in the carbon dioxide recovery process and/or a biogas process, in particular when the carbon dioxide originates from biogas production, and/or transported as heat to e.g . domestic heating.

In a further particular embodiment the heat formed in the methanisa- tion reaction is provided to the regenerator reboiler (A6) in a closed loop cycle, wherein a first cooled heat transfer fluid (L15) is connected to the methanisa- tion unit to provide a warm heat transfer fluid (G19), said warm heat transfer fluid (G19) passes through the first regenerator reboiler (A6) to provide the first cooled heat transfer fluid (L15). In yet a further embodiment the first cooled heat transfer fluid (L15) is further cooled in a sixth heat exchanging unit (A16) to provide a second cooled heat transfer fluid (L15')-

It is also contemplated that the methane rich stream (G18) formed is subjected to the steps of cooling the stream in a seventh heat exchanger (A17) and separating the cooled methane stream in a second separator (S2) to pro- vide an enriched methane stream (G20) and a liquid water recycle stream (L16), and where said enriched methane stream is returned to the absorber (Al) for separation of methane and remaining carbon dioxide. This embodiment is particularly beneficial when the feed gas is a biogas..

By coupling carbon dioxide recovery with electrolysis and methanisa- tion a further benefit is obtained in an embodiment where the carbon dioxide rich gas (G5) before being provided in step g . in a further step is purified by catalytic oxidation in a catalytic oxidation reactor (A9) by direct combustion with an oxygen gas stream (G8) and wherein said pure oxygen originates from the oxygen stream (G15) obtained in step f, and whereby a purified carbon dioxide gas (G9) is provided. Hence, pure oxygen which would otherwise be an expensive part of the carbon dioxide recovery process can be taken from the electrolysis of the integrated process and thereby further improving the economy in the process without compromising the product quality.

It is also contemplated that the oxygen stream (G15) provided in step f. can be provided to the biogas process, such as for oxidation of hydrogen sulfide.

In a particular embodiment, the carbon dioxide rich gas (G5) before being further purified is subjected to the steps of

i) cooling the carbon dioxide rich gas (G5) in a third heat exchanger (A7) to provide a cooled carbon dioxide rich gas (G5'); ii) separating the cooled carbon dioxide rich gas (G5') in a third separator (S3) to provide a cooled carbon dioxide rich gas (G6) and a l iq uid condensate (L5) ; and

iii) compressing the cooled carbon d ioxide rich gas (G6) to provide a compressed carbon dioxide rich gas (G7) .

Hence, by adding these intermed iate steps, the purity of the carbon dioxide stream is increased and the oxidation reaction req uires less externally supplied oxygen . Hence, it may be possible, when the carbon dioxide stream is purified in this way, to provide oxygen for the oxidation of trace amounts of organic compounds in the carbon dioxide stream, as well as providing oxygen to the biogas process for oxidation of hydrogen sulfide to minimize the amount of sulfur compounds entering the catalytic oxidation and hence the formation of sulphuric acid, when the combustion products come in contact with water. Traces of hydrogen from the methanization process can also contribute to pu- rifying the methane rich gas (G2), when returned with the enriched methane stream (G20), such as for removing oxygen from the methane stream (G20) . This embod iment is particularly preferred when the absorbent is chemical .

In embod iments where the absorption is physical, the carbon dioxide rich gas (G5) is, subseq uent to the flashing from the absorbent, preferably pressurized before being further purified by oxidation to provide a compressed carbon dioxide rich gas (G7) .

It is also contemplated that the purified carbon dioxide stream (G9) regard less of being recovered by physical or chemical absorption is subjected to the steps of

- liquefying the carbon dioxide, such as by distillation or condensation

- storing the liquefied carbon d ioxide

- re-evaporating the liq uefied carbon d ioxide to provide gaseous carbon dioxide to the methanisation reaction .

It is contemplated that the storage may be for a prolonged time, so that the carbon dioxide may be stored and not reacted with the hyd rogen formed by electrolysis until the energy input for the electrolysis comes at a low cost. Hence, the storing method is extremely flexible.

Embodiments of the invention will now be described in greater details with reference to the figures, in which

Figure 1 is a schematic illustration of the concept of the invention . Figure 2 is an illustration of an embodiment of the invention in which the absorption step is chemical.

Figure 3 is an illustration of an embodiment of the invention in which the absorption step is physical.

Detailed description

In the specification, claims and drawings reference is made to the following streams and units:

Streams: feed gas Gl; wet methane rich gas G2 (carbon dioxide lean gas); dry methane rich gas (dry carbon dioxide lean gas) G3; stripper gas G4; carbon dioxide rich gas G5; cooled carbon dioxide rich gas G5'; cooled carbon dioxide rich gas G6; compressed carbon dioxide rich gas G7; oxygen gas stream G8; purified carbon dioxide gas G9; gaseous water containing stream G10; dry high purity carbon dioxide gas Gi l ; purge gas stream G12; clean carbon dioxide stream G13; hydrogen stream G14; oxygen stream G15; mixed gas stream G16; heated mixed gas stream G17; methane rich stream G18; warm heat transfer fluid G19; enriched methane stream G20; absorbent LI; carbon dioxide rich liquid L2; heated carbon dioxide rich liquid L3; carbon dioxide lean liquid L3'; heated carbon dioxide lean liquid L3"; recovered absorbent L4; liquid condensate L5; liquid cooling medium L6; hot liquid cooling medium L7; liquid cooling medium L8; hot liquid cooling medium L9; liquid carbon dioxide stream L10; carbon dioxide feed stock Ll l; remaining liquid carbon dioxide L12; liquid carbon dioxide stream L13; water stream L14; first cooled heat transfer fluid L15; second cooled heat transfer fluid (L15'); water recycle stream L16; El energy.

Units: absorption column Al; water removal unit A2; first and second heat exchangers A3 and A4, respectively; regenerator column A5; regenerator reboiler A6; third heat exchanger A7, compressor A8, catalytic oxidation reactor A9, water removal unit A10; cooler Al l; storage tank A12; electrolysis unit A13; fourth heat exchanger A14; fifth heat exchanger A14'; methanisation unit A15; sixth heat exchanger A16; seventh heat exchanger A17; depressurization means A18; first separator SI; second separator S2; third separator S3; fourth separator S4.

Throughout the description and claims unless otherwise indicated "%" is mole-%.

The general concept of the invention is illustrated in figure 1 in which a carbon dioxide recovery process is coupled to electrolysis, methanisation, energy input for the electrolysis and the feed gas generating unit.

The method will now be described in greater detail with reference to a detailed embodiment as illustrated in figure 2 in which the feed gas is a biogas and the absorption is chemical. In the detailed embodiment shown also optional or preferred features of the general concept of the invention are shown. The invention should not be construed as being limited to the detailed embodiments described below.

A feed gas, which is a biogas, Gl, containing about 20-80% CO2 and 80-20 % CH4, and a liquid absorbent, LI, which is monoethanolamine, are fed to an absorption column, Al, to generate a carbon dioxide rich liquid, L2, in which the CO2 in the feed stream, Gl, is absorbed, and a wet methane rich gas G2 having a CO2 concentration typically below app 2 % (mole/mole) CO2. The wet methane rich gas, G2, is in the embodiment shown further processed in a water removal unit, A2, to produce a dry methane stream G3, which is carbon dioxide and water lean and suitable for being fed to a natural gas grid or similar.

While the embodiment is described with a biogas as an example it is also contemplated that the feed gas may be e.g. a flue gas, syngas or a fer- mentation gas. Hence, when the feed gas is other than a biogas, the gas leaving the absorption column (G2) is not a methane gas but a carbon dioxide lean gas (G2). In some embodiments the carbon dioxide lean gas is disposed of or further processed for example in a hydrogen plant.

The pressure in the absorption column is preferably in the range of 1- 20 bara, and normal pressurizing means may be used to obtain the applied pressure. This is within the skill of the art. When the absorbent is a chemical absorbent such as an alkanolamine, the pressure is typically in the range 1 - 3 bara.

The absorbent, LI, is generally a solvent usable for removal of CO2 by means of physical or chemical absorption. In the embodiment of figure 2 the absorption is chemical while the embodiment shown in figure 3 is a physical absorbent, such as water.

The carbon dioxide rich liquid, L2, is preheated in a first heat exchanger, A3, to provide a heated carbon dioxide rich liquid stream, L3, which enters a regenerator column, A5. A pump (not shown) may be applied to feed the liquid to the regenerator depending on the location of the respective units and the pressure in absorber, Al, and regenerator, A5, respectively.

In the regenerator column, A5, the heated carbon dioxide rich liquid stream, L3, is typically fed at the top part of the regenerator column, A5, whereby carbon dioxide is released from the liquid while the liquid is descending down through the column. Stripping steam/stripper gas, G4, is provided at the lower part of the column and rises in countercurrent flow with the carbon dioxide rich liquid. The regeneration provides a carbon dioxide rich gas, G5, and a carbon dioxide lean liquid, L3', the carbon dioxide lean liquid, L3' is heated in the regenerator reboiler, A6, to provide a heated carbon dioxide lean stream L3", which may be a gas/liquid mixture, said mixture is separated in a first separator, SI, to provide recovered absorbent, L4, and the stripping gas, G4. The recovered absorbent is returned to the absorption column, Al, in heat exchange with the carbon dioxide rich liquid, L2, to provide the absorbent, LI. The recovered absorbent is in the embodiment shown cooled twice in the first, A3, and second, A4, heat exchangers, respectively. In this embodiment the second heat exchanger, A4, transfers heat to a liquid cooling medium, L6, to provide a hot liquid cooling medium, L7, which is being further utilized as detailed below.

The carbon dioxide rich gas, G5, stripped from the absorbent, is further cooled in a third heat exchanger, A7, and separated in a third separator, S3, to generate a liquid condensate, L5, and a cooled carbon dioxide rich stream, G6. Liquid condensate, L5, is returned to the regenerator column, A5.

Liquid cooling medium, L8, used for the third heat exchanger A7 is heated to generate a hot liquid cooling medium, L9, which can be used for heating purpose or heat recovery. In particular, in embodiments where the feed gas source is a biogas source, hot liquid media, L9 and L7, may be used for preheating and pasteurisation purposes of biological waste thereby minimizing the overall heat demand for running the biogas process.

The cooled carbon dioxide rich gas, G6, is subsequently compressed in a compressor, A8, to generate a compressed carbon dioxide rich gas, G7, which is further purified in a catalytic oxidation reactor, A9, by direct combustion with pure oxygen or air supplied, with an oxygen gas stream, G8, to generate a purified carbon dioxide gas, G9, containing mainly CO2, and small amounts of N2, O2, H2O as well as trace amounts of residual CH ₄ not converted in the oxidation reactor, A9. Purified carbon dioxide gas, G9, may be cooled to quench out water before removing water in a suitable water removal unit, A10, to provide a gaseous water containing stream, G10, and a dry high purity carbon dioxide gas, Gi l .

High purity carbon dioxide gas, Gi l, is in the embodiment shown further cooled and liquefied in a cooler, All, followed by separation in a fourth separator, S4, to provide a liquid carbon dioxide stream, LIO, and a purge gas stream, G12, containing CO2 and non-condensable gases, mainly methane.

Depending on the purification requirements of the carbon dioxide product, LIO, the cooling step in cooler, Al l, and separation in a fourth separator, S4, into liquid carbon dioxide stream, LIO, and purge gas stream, G12, may additionally or alternatively include a distillation step.

To improve the methane recovery from the purge gas stream, G12, the stream may be returned to absorber, Al . Liquid carbon dioxide stream, LIO, is pumped or pressed to a storage tank, A12, and stored at a pressure of typically 16 to 21 bara.

According to the invention the carbon dioxide produced and stored in storage tank, A12, is taken as a carbon dioxide feed stock, Ll l, and is separated for methanisation or liquid fuel production (not shown), from liquid car- bon dioxide stream, L13. Any remaining liquid carbon dioxide, L12, may be utilized in a conventional manner. It is also contemplated that the carbon dioxide is taken as a gas directly for methanisation if suitable, i.e as the gas G9.

The example illustrated embodies a power to gas conversion in which liquid carbon dioxide stream, L13, is re-evaporated in a fourth heat exchanger, A14, to give a clean carbon dioxide stream, G13, the cold created by the re- evaporation may be utilized for energy savings in the process e.g . transferred from A14 to a cooler Al l . Water stream, L14, is fed to an electrolysis unit, A13, in which water is electrolyzed into a hydrogen stream, G14, and oxygen stream, G15, by means of addition of electricity, El . Providing energy to the electrolysis of water to hydrogen requires a high energy input and is often a limiting factor for the feasibility of generating methane from carbon dioxide.

The oxygen stream, G15, is a valuable product that may be used internally e.g. as oxygen gas stream, G8, or can be exported.

The hydrogen stream, G14, and clean carbon dioxide stream, G13, are mixed to provide a mixed gas stream G16. The mixed gas stream is preheated in a fifth heat exchanger, A14', to yield a heated mixed gas stream, G17. The heated mixed gas stream, G17, is reacted in methanisation reactor, A15, to give a methane rich stream, G18. Methanisation is a generally known reaction that can be chemical or biological . Preferably it is a chemical reaction using a catalyst for example obtainable from Haldor Topsoe.

The methanisation reaction is exothermic and will release heat. This heat is recovered through heating of a first cooled heat transfer fluid, L15, in a sixth heat exchanger, A16, to yield a warm heat transfer fluid, G19. In the embodiment shown, the heat transferred from the methanisation reaction to the warm heat transfer fluid, G19, is used as a heat source for operating regenerator reboiler A6. The heat supplied from the methanisation may be the sole heat source for the regenerator reboiler, A6, or a supplement.

As an alternative, L7 and L9 may be used for heat recovery. After heat recovery in A14', A16 and A17 the cooled methane gas is separated in a second separator to provide an enriched methane stream, G20, and a liquid water recycle stream, L16. The enriched methane stream G20 is returned to the absorber Al, to upgrade the methane generated . The stream may be mixed with the feed gas as illustrated in the embodiment. It is also contemplated that the enriched methane stream is fed directly to the absorber. This is advantageous when the enriched stream has a higher or lower purity than the feed gas, since it may then be fed at a different position of the column accordingly.

The water recycle stream, L16, is rich in water and is advantageously used as the source for about 50% of the water stream, L14, needed to produce hydrogen in the electrolysis reactor, A13.

Hydrogen not converted in methanisation reactor, A15, may also advantageously be used as a means of catalytically removing oxygen that may be present in the wet methane rich gas, G2.

The present invention provides a new way towards neutralizing the energy input required for providing hydrogen from water. The energy input is necessary for methane formation from carbon dioxide and hydrogen . By combining the methane upgrade with a carbon dioxide recovery process in a manner where the energy input for the electrolysis is being outweighed by the energy saved in the carbon dioxide recovery process the overall process becomes more energy efficient.

The process may become even more economical since liquid carbon dioxide can be stored until the electrical grid has an overcapacity of power, for example from sources such as wind power, solar power etc for which storage is limited . In these periods the electricity comes at lower cost and the plant can produce methane and thereby store the green energy as methane during times where the price for electrical energy is at its lowest.

Since hydrogen generation through electrolysis requires a large amount of energy the overall efficiency is determined by the availability of excess electricity. In cases where it is not economically feasible to run the methanisation process due to high cost of electricity the regenerator reboiler, A6, can use an external source of heat to be able to continuously upgrade methane produced, when the carbon dioxide feed source is a biogas plant. When the methanisation is on hold, the carbon dioxide purified may be stored in the storage tank, A12, and used when electricity prices drop or the carbon dioxide may be sold as a valuable product.

Referring to figure 3 an embodiment is illustrated in which the absorbent is physical, e.g . water. All streams and units are as described for figure 2 above with a few modifications. The feed stream, Gl, is a pressurized stream (means for pressurizing not shown) . It is well known in the art that physical absorption is most efficient at higher pressures than chemical absorption, op- erating at pressures, such as in the range of 1 to 40 bara, in order to efficiently absorb carbon dioxide. The pressurized feed stream is absorbed to provide the carbon dioxide rich liquid, L2. The carbon dioxide rich liquid, L2, is depressur- ized before regenerating the carbon dioxide from the absorbent. The depres- surization means, A18, may be any suitable means such as a pressure reducing valve. The depressurized carbon dioxide rich liquid, L2', is then flashed in a flash column or a drum (A5) to provide the carbon dioxide rich gas, G5, and the recovered absorbent, L4. The carbon dioxide rich gas, G5, is, as illustrated, pressurized directly to provide the pressurized carbon dioxide rich gas, G7. The heat provided to the regenerator reboiler, A6, in the embodiment illustrated in figure 2, is in the embodiment of figure 3, provided elsewhere needed, such as in the biogas production process or if waste for biogas production is to be heated for pasteurization .

Example 1 Example 1 illustrates an embodiment as illustrated in figure 2 operating at feed gas rate of 36.69 kmole/hour.

According to the embodiment, using parameters as indicated in table 1, the operating parameters and results of the process illustrated can be cal- culated using commercial available programs such as Chemcad, Aspen Plus or HYSYS.

The energy required for the electrolysis is 4,765 kWh, while the methanisation reaction provides heat corresponding to 921 kWh heat, which again is used for operation of the regenerator reboiler, A6, at 3 bara steam.

In addition, 3,716 kWh of energy based on the gross heating value is recovered from the methane rich stream, G18, in the form of the methane generated in the methanisation reaction.

Hence, the process uses 4,765 kWh for the electrolysis. This required energy input is converted to 3,716 kWh energy stored as methane, and with the heat for the regenerator (921 kWh) the process energy balance becomes neutral/positive, whereby the storing of preferably renewable energy as methane becomes cost efficient.

In addition, energy can be extracted from the system from the hot liquid cooling media L7 and L9, corresponding to 702 kWh and 183 kWh, re- spectively.

Finally, water used for and in the electrolysis can be recovered in the carbon dioxide recovery process, i.e. in the example 537 kg H2O to be used for electrolysis is recovered in water recycle stream L16, which corresponds to 50% of the total water demand. Hence with the invention is proposed a solution where the efficiency of the combined plants is increased whereby the costs of installation become more attractive and at the same time the plant comes with an environmental benefit.