NO20150411A1

NO20150411A1 - Method and plant for oxygen generation

Info

Publication number: NO20150411A1
Application number: NO20150411A
Authority: NO
Inventors: Geir Inge Olsen; Tom-Arne Solhaug; Kjell Olav Stinessen
Original assignee: Aker Solutions As
Priority date: 2015-04-09
Filing date: 2015-04-09
Publication date: 2016-10-10
Also published as: WO2016162522A1

Description

Technical Field

[0001] The present invention relates to a method and device for generation of substantially pure oxygen. More specifically, the present invention relates to a method and device for generation of oxygen for in remote locations, such as for offshore subsea combustion where it will be particularly suitable, but also for offshore and onshore combustion for generation of

electric power.

Background Art

[0002] The traditional method for generation of substantially pure oxygen is cryogenic separation of air in an Air Separation Unit (ASU), a method that is well developed and refined over decades. However, a cryogenic ASU is a complex, expansive, energy consuming and expensive process unit. This combined with that ASU units also have large dimensions and are heavy and therefore not suitable neither for offshore installation nor on platforms or subsea.

[0003]

[0004] Additionally, semi permeable membranes are used to generate oxygen-enriched air. Theoretically, semi permeable membranes may produce substantially pure oxygen but in practical use, the membranes do not produce oxygen of sufficient purity for some purposes. The current status of such membranes is that they are not suitable for combustion where large quantities of pure oxygen is required such as for supply of oxygen for oxyfuel combustion to generate electric power.

[0005] An object of the present invention is to provide a method and a plant for local generation and supply of substantially pure oxygen, in situations where it is impossible, impractical, too complex, or too expensive to build a local cryogenic ASU or provide oxygen from remote locations. Examples on such locations are offshore locations, either on a platform or subsea, or on onshore locations. A specific object of the present invention is to provide a method and a plant for providing substantially pure oxygen to a power plant for generation of electrical energy from combustion of a carbonaceous fuel, and allowing CO2generated in the combustion to be captured and safely deposited. Yet a specific object is to provide a method and plant for generation of substantially pure oxygen for a subsea power

plant, or a power plant placed on an offshore platform.

Summary of invention

[0006] According to a first aspect, the present invention relates to a method for generating of oxygen, the method comprising the steps of: a. Decomposition water by electrolysis in an electrolysis unit to generate oxygen and hydrogen in separate fractions; b. Withdrawing oxygen through an oxygen line for export of oxygen; c. Withdrawing hydrogen through a hydrogen line; d. Introducing the hydrogen into a combustion unit;

e. Introducing air into the combustion unit;

Wherein the method also comprises the step of:

f. Introducing air into the combustion unit and allowing oxygen in the air and hydrogen to react to form water in the combustion unit for generation of electrical power and/or heat.

[0007] The method for oxygen generation according to the first aspect is based on electrolysis of water to form hydrogen and oxygen, where oxygen is withdrawn for other purposes and the hydrogen is allowed to react with oxygen from air to produce water. The only energy cost for the oxygen generation is the energy loss in the electrolysis and in generation of electrical power by combustion of hydrogen and oxygen from air. The method also provides for a flexible oxygen production / generation and for oxygen generation adapted to the need of oxygen in difference from traditional ASUs.

[0008] According to one embodiment, the electrical power and/or heat generated in the combustion unit is transferred to the electrolysis unit to be used for electrolysis of water therein.

[0009] According to one embodiment, the exhaust gas from the combustion in step f) is cooled to condensate and separate water formed in the combustion in step f) from the remaining exhaust gas, and recycle the water back to the electrolysis chamber. By separating water from the exhaust gas and recycling of water to the electrolysis unit, hydrogen is recycled between the electrolysis and combustion units, and oxygen is being extracted from air by combustion with hydrogen, and is used for oxygen generation in the electrolysis unit, i.e. the effect of the method is to extract oxygen from air to produce pure oxygen for other uses. This recycling substantially reduces to need to supply specially cleaned water for electrolysis to the plant as the loss of water together with the exhaust gas released into the surroundings, is minimal.

[0010] According to one embodiment, the combustion unit is a fuel cell unit, where the electrical power is generated by direct conversion of the chemical energy of the combustion to electrical energy. Fuel cells units are relatively efficient and maintenance free as long as the air and hydrogen to be combusted are dean. Fuel cells may additionally produce heat energy for heating of the electrolysis unit, and thereby increase the total energy efficiency substantially. It is assumed that fuel cells might be preferred combustion units in the future but that some further development of large-scale fuel cells at affordable cost is necessary.

[0011] According to yet an embodiment, the combustion unit comprises a boiler in which the heat of combustion is used to generate steam in steam coils in the boiler. The generated steam might be used to produce electrical power in a steam turbine. Steam turbine technology is well known, robust and well proven. Additionally, or alternatively, all or parts of the steam might be transferred to the electrolysis unit to heat the electrolysis unit.

[0012] According to one embodiment, the pressure in the electrolysis unit is 40 bar or higher. Compression of gases is energy consuming. If the oxygen is to be used in a high pressure application, such as combustion in a combustion chamber at a high pressure, such as 40 bar or higher, it is preferred to operate the electrolysis unit at the pressure at which the generated oxygen is to be used, as the energy cost for producing oxygen by electrolysis at such a pressure is by far lower than compressing the oxygen from a lower pressure.

[0013] According to one embodiment, the pressure in the electrolysis unit is 65 bar or higher. The preferred pressure in the electrolysis unit is equal to or higher than the pressure needed for the intended use of the oxygen formed.

[0014] According to another aspect, the present invention relates to a method for production of electrical power from carbonaceous fuel, where the carbonaceous fuel is combusted in a combustion chamber in the presence of substantially pure oxygen to produce electrical power and a flue gas, where the combustion is performed at a pressure of 40 to 200 bar, where the flue gas is withdrawn from the combustion chamber and cooled to a temperature that according to figure 3 results in condensation of the flue gas, or conversion of the flue gas to a supercritical fluid håving a density of at least 600 kg/m3, where the liquid or supercritical fluid formed, is safely deposited, where the substantially pure oxygen for the combustion is generated as described above.

[0015] According to one embodiment, the flue gas is cooled to a temperature of 80 °C or colder, such as 60 °C, 40 °C , 30 °C, 20 °C or colder or 10 °C or colder.

[0016] According to a third aspect, the present invention relates to a plant for producing substantially pure oxygen, wherein the plant comprises an electrolysis unit for electrolysis of water into oxygen and hydrogen in separate fractions, an oxygen withdrawal line for withdrawal of oxygen from the electrolysis unit, a hydrogen withdrawal line for withdrawal of hydrogen from the combustion unit and introduction of the hydrogen into a combustion unit, an air line for introduction of air into the combustion unit, the combustion unit being adopted for combustion of hydrogen and oxygen from the air for generation electrical power and an exhaust gas mainly comprising water and nitrogen, means for separation of water from the remaining exhaust gas, a water return line for returning water from the combustion unit to the electrolysis unit, an exhaust gas withdrawal line for releasing exhaust gas into the surroundings, a power line for transferring electrical power generated in the combustion unit to the electrolysis unit, and a power feed line for delivering additional power to the power line . Water present in the exhaust gas is conveniently separated from the remaining exhaust gas e.g. by cooling and condensation to liquid water.

[0017] According to a fourth aspect, the present invention relates to a plant for production of electrical power from carbonaceous fuel, wherein a plant for generation of electrical power and capturing of CO2, the plant comprising a plant for oxygen production as described above, for providing substantially pure oxygen to a boiler and a fuel line for providing carbonaceous fuel to the boiler at a pressure of 40 bar or more to the boiler, in which the fuel is combusted and steam generated, a steam line for withdrawing steam generated in the boiler and introduction of the steam into a steam turbine for generation of electrical power, and a condensate return line for returning condensed steam from the steam turbine and introduction thereof into the boiler, a flue line for withdrawal of flue gas from the boiler and for introduction of the flue gas into a condenser in which the flue gas is cooled for condensing of, or forming a supercritical fluid håving a density of at least 600 kg/m3, of CO2and H2O, present in the flue gas, and a CO2withdrawal line for withdrawal of condensed liquid or supercritical fluid from the condenser.

[0018] According to one embodiment, the condenser comprises two condenser units separated by a liquid/gas separator, to separate condensed water from the exhaust stream in the liquid/gas separator and to further cool and condense the CO2in the second condenser unit. Condensation of the flue gas can either be in one step forming a mixture of liquid or supercritical fluid of CO2and H2O, or in two steps by first cooling to a temperature that mainly condenses out H2O followed by a second step that condenses out CO2. The skilled person will understand that separation of water from the CO2may be required for injection of the captured CO2into oil or gas fields,

if required by the end user.

Brief description of drawings

[0019]

Fig. 1 is a simplified flow diagram illustrating an oxygen generating unit according to the present invention,

fig. 2 is a simplified flow diagram illustrating a power plant including an oxygen generating unit according to the present invention,

fig. 3 is a plot of fluid density of a flue gas as a function of temperature at

different pressures; and

fig. 4 is a phase diagram for CO2.

Detailed description of the invention

[0020] The present invention relates to a method and a plant for generation of substantially pure oxygen for processes, most specifically combustion processes, requiring substantially pure oxygen.

[0021] The expression "combustion" is in the present description and claims used to encompass the reaction between oxygen and any convenient fuel, such as natural gas, methane hydrate, other carbonaceous fuels, or hydrogen that may find use in connection with the present invention. The term is not limited to oxidation by high temperature and includes both high temperature combustion with and without open flame, catalysed oxidation of any of the mentioned fuels, and the oxidation of a fuel in fuel cells for direct conversion of chemical energy to electrical energy.

[0022] The present method and plant is most specifically developed for generation in remote, such as offshore, locations where a full scale cryogenic ASU is too large and / or too complex to be operated efficiently, or at all. Examples on such locations may be offshore subsea locations, where plants low maintenance demand are preferred due reduced availability of the equipment subsea, and the extreme cost for subsea maintenance.

[0023] Figure 1 illustrates the basic principles of an oxygen generation unit 1 according to the present invention. The oxygen generation unit 1 comprises an electrolysis unit 2 in which water is split to produce hydrogen and oxygen in separate fractions or compartments of the electrolysis unit 2 to avoid mixing of hydrogen and oxygen. Oxygen and hydrogen produced in the electrolysis unit are withdrawn through an oxygen line 4, and a hydrogen line 5, respectively. Electrical power for the electrolysis is introduced into the electrolysis unit 2 via a power line 9.

[0024] The hydrogen withdrawn via the hydrogen line 5 is introduced into a combustion unit 3, where the hydrogen is reacted with oxygen in the air to give water, and the energy of the reaction is converted directly into electrical energy and/or to heat energy. The heat energy may be used to produce electrical power, and/or be used in other processes e.g. as described below.

[0025] The air for combustion in the combustion unit 3 is introduced via an air line 6. The combustion unit 3 may be a fuel cell unit, where electrical power is

generated directly by reacting hydrogen with oxygen. Alternatively, the combustion unit comprises a combustion chamber a boiler where the heat of combustion of hydrogen and oxygen is used to generate steam in boiler tubes arranged in the boiler. The steam generated and heated in the steam tubes may be introduced into a steam turbine for generation of electrical power, or may be used for other purposes as will be further described below. Electrical power generated in the combustion unit is withdrawn in the power line 9.

[0026] The skilled person will understand that the term "boiler" is used herein about a unit comprising a combustion chamber where a fuel is combusted and the hot combustion gases are used to generate and heat steam in steam tubes arranged in the combustion chamber. The skilled person will understand that the term "steam" may include any other convenient vaporized liquid, and that a steam turbine as mentioned above, may conclude a steam turbine using water and steam, or an organic cycle turbine system using an organic fluid as medium.

[0027] The exhaust gas from the combustion unit 3, mainly comprising water resulting from the combustion therein, and nitrogen from the air introduced into combustion unit, is cooled to condensate the water. The water is separated from the rest of the exhaust gas, and withdrawn through a water return line 8 and recycled into the electrolysis unit 2 and added to the water to be electrolysed therein. The remaining exhaust gas, mainly comprising nitrogen, is withdrawn via an exhaust line 7, and released into the surroundings.

[0028] Oxygen and hydrogen is generated in the electrolysis unit according to eq. 1, below, whereas hydrogen is combusted in the combustion unit 3, according to eq. 2 below, whether it is a fuel cell or a boiler for steam generation:

[0029] Nitrogen, and minor constituents present in the air in the combustion in the combustion unit 3 is inert in the combustion and does not participate in the reactions. It is important to note that the only reaction product of the combustion is water, i.e. no pollution products. The nitrogen and other constituents present in the air can therefore be released to the atmosphere or the sea without creating pollution. Water produced by combustion in the combustion unit 3 is preferably recycled back to the electrolysis unit as described above, as this water is pure and may be used directly in the electrolysis unit, which reduces the need to add specially purified water for the electrolysis.

[0030] Even though electrolysis of water is a well-established technology, the technology has undergone substantive development the recent years for increasing the thermodynamic efficiency of the electrolysis primarily for production of hydrogen as an energy carrier as a substitute for carbonaceous fuels. The skilled person will be aware of both available technology and technology under development for optimization of the thermodynamic efficiency of the electrolysis unit in the present method and plant. The presently available types of electrolytic cells for electrolysis of water are a) Solid Oxide Electrolysis Cells (SEOC), Proton Exchange Membrane or Polymer Electrolyte Membrane (PEM) (PEM, and Alkaline Electrolysis Cells. For further information see e.g. Badwal, Sukhvinder P. S. ; Giddey, Sarbjit; Munnings, Christopher. " Hydrogen production via solid electrolytic mutes". Wiley Interdisciplinary Reviews: Energy and Environment 2 ( 5) : 473- 487.

[0031] It is presently assumed that the preferred electrolysis cells to be used in connection with the present invention are SEOC, and especially SEOC operated at elevated temperatures, such as between 100 °C to 850 °C. Development of new materials may even allow for operating such cells at temperatures higher than 850 °C. The requirement for electrical power for electrolysis is reduced with increasing temperature, up to the temperature of 2500 °C where water is broken down by thermolysis. The energy efficiency of electrolysis is highly dependent on temperature, as the electrolysis is more efficient at higher temperatures than at lower temperatures. Additionally, at higher temperatures a part of the total energy for is tåken from the surroundings. Accordingly, a substantial part of the total energy needed for the electrolysis may be in the form of heat, which may be less expensive than electrical power.

[0032] The energy efficiency of fuel cells in the combustion unit is higher than for alternative systems for converting the energy of reaction between hydrogen and oxygen to form water, and is about 40 - 60 %. However, the efficiency may be increased to 85 to 90% by a combined heat and power (CHP) system.

[0033] As the electrolysis unit preferably is operated at a high temperature as indicated above need supply of heat energy to maintain the elevated temperature, a part of the heat generated in the combustion unit may be transferred from the combustion unit to the electrolysis unit as heated

steam, or by direct heat transfer between the units.

The net combined effect of this electrolysis and combustion process is that the H2

is continuously re-circulated, while water is regenerated by reaction of the re-circulated H2. New oxygen is brought into this cycle from air supplied to the combustion chamber reacted with hydrogen to form water, which is

separated from the combustion gas and introduced into the electrolysis unit for generation of oxygen and hydrogen, as described above. In other words, the process is an alternative method for separating oxygen from air to produce pure oxygen. The electrical power generated on the combustion unit 3 is introduced into the electrolysis unit and used for electrolysis of water. Due to the energy loss in the electrolysis unit and in the combustion unit, additional energy has to be introduced into power line 9 from another power supply. It is assumed that 50% or more, such as

65% or more of the energy needed for the electrolysis of water may be provided from the combustion unit, as electrical power and /or heat energy.

[0034] The electrolysis unit 2 may be operated under reduced pressure, ambient or atmospheric pressure or under elevated pressure, such as > 40 bar, or > 65 bar, such as > 100 bar, e.g. up to 200 bar or higher. Operation of the fuel cell under elevated pressure may be preferred for some fields of use. Operation under elevated pressure allows for building smaller or more compact units due to the reduced volume of the gases, and thus give lower volume requirement. Additionally, if the generated oxygen is to be used at elevated pressure, the electrolysis unit is preferably operated at elevated pressure to avoid or at least reduce the need for compressors.

[0035] The electrolysis unit, which indicatively can have an efficiency in the range of 60-90 %, may be any state of the art electrolysis unit, such as units used for production of hydrogen by electrolysis for commercial use, such as hydrogen for fuel purposes or the like. The anode and cathode chambers are preferably separated by a semi-permeable membrane to allow flow of electrolytes between the chambers but to avoid mixing of hydrogen and oxygen gas, and collect hydrogen and oxygen as separated gas fractions. The skilled person will understand that also other electrolysis units may be used, such as electrolytic chambers where anode and cathode chambers are separated by wall connected by flow channel(s) allowing flow of electrolytes but not allowing mixing of hydrogen and oxygen.

[0036] Oxygen produced in the electrolysis unit 2 is withdrawn for further use through an oxygen line 4, as will be described below with reference to a specific use. Produced hydrogen is withdrawn through a hydrogen line 5, and introduced into the el-generation unit 3, in which unit the hydrogen is oxidized, or combusted, by reaction with oxygen from air according to eq. 1 above. The presently preferred el-generation unit is steam turbine-generator, but in the future, the preferred solution can be a fuel cell where electrical power is produced directly from the chemical reaction therein. Fuel cells have high efficiency, indicatively in the range of 60-80 %, converting the chemical energy of the reaction of oxygen and hydrogen to electrical power, and have low maintenance cost. At the current state of technology the el-production unit may be gas or steam turbine unit where the hydrogen is combusted for production of electrical power.

[0037] Air for the combustion in the el-production unit is introduced through an air feed line 6. Combustion of hydrogen and air, either in a fuel cell or a combustion chamber in a gas or steam turbine results in an exhaust gas comprising water vapour, and nitrogen from the air, in addition to minor amount of other air gases. Water is condensed in a not illustrated condenser by cooling, and separated from the remaining exhaust gas mainly comprising nitrogen. The exhaust gas is then released to the surroundings through an exhaust gas line 7.

[0038] The water separated from the exhaust gas is led back to the electrolysis unit 2 through a water line 8 to replace water being split up by electrolysis to make up a water circuit. Water being lost, e.g. as non-condensed water vapour in the exhaust gas, is replaced with fresh water via a not shown make-up water line.

[0039] The power line 9 is arranged from the combustion unit to the electrolysis unit to provide power for the electrolysis. Due to the efficiency and loss in energy conversions in the electrolysis unit and the el-generation unit, the electrical power generated in the el-generation unit is less than the power demand for the electrolysis. A power feed line 10 is connected to the power line 9 to provide additional electrical power for the electrolysis. Tåken into account the high efficiency of high temperature electrolysis and high temperature fuel cells, this additional electrical power need can be very low or even eliminated because the combined electrolyses-fuel cell system can have an electrical efficiency of 100% or even higher provided that heat from other sources outside of the oxygen production unit are available for of supply of additional heat. The total efficiency, electricity and heat, must of course be less than 100%.

[0040] Due to the high efficiency of electrolysis of water and of fuel cells in combination with high-pressure electrolysis that saves compression work, this is a preferred, advantageous method. Power consumption by production of oxygen by electrolysis can typically be 7500 kW pertonne. But if we assume that 70 % of this in the future can be regenerated in fuel cells, the net consumption will be 2250 kWh/tonnes or even lower as described above. Together with the other advantages of this way of oxygen production compared to an ASU, this will make it attractive.

[0041] Figure 2 is an illustration of the oxygen generation unit 1 integrated with a power plant 20 where a carbonaceous fuel, introduced via a fuel gas line 21 is combusted in an elevated pressure combustion boiler 22 using pure

oxygen as oxidant for generation of steam for powering a steam turbine

24. Preferably, the pressure in the combustion chamber is higher than 40

bar, such as higher than 60 or even higher than 70 bar, such as up to 200 bar.

[0042] The term carbonaceous fuel as used in the present description and claims, relate to coal and hydrocarbons, such as natural gas, condensate and higher hydrocarbons. Coal may be used in as coal powder. Natural gas as used in the present description and claims, is a hydrocarbon gas mainly comprising methane, in addition to minor amounts of ethane, propane, butane and C5+hydrocarbons. The fuel may also comprise an unprocessed or substantially unprocessed well stream from an oil and /or gas production well. Such a well stream may comprise both natural gas, condensates and higher hydrocarbons in a mixture, in addition to some CO2and water. By substantially unprocessed well stream is meant a well stream that is unprocessed, or where all or a substantial part of produced water is separated and removed from the gaseous and liquid hydrocarbon stream. A requirement for use of an unprocessed or substantially unprocessed well stream as fuel is that the power plant is that the power plant 20 is arranged close to the wellhead for oil, condensate and/or oil production, to avoid problems with multiphase flow, and other problems associated with transport of an unprocessed or substantially unprocessed well stream. A Flow Conditioning Unit (FCU) upstream the oxyfuel burner can be favourable to even out the mixture of gas and condensate/oil and in this way achieve a fairly constant composition and combustion value of the mixture introduced to the burner. Removal of particles upstream the oxyfuel burner may also be advantageous.

[0043] Methane may also be found as methane hydrate, a solid complex of methane and water, at the seabed or in oil and gas wells. Methane may be liberated from the methane hydrates by heating the methane hydrate, to give a methane gas stream. Waste heat from a power plant as described herein, i.e. heat that is of too low temperature to be used for generation of electrical power in a steam power plant, may be used heating of methane hydrate, as the methane can be liberated at a temperature of 10 to 20 °C or higher dependant of the pressure.

[0044] The fuel stream may be introduced into the boiler 22 at the production pressure of the well stream, which typically is higher than 40 bar, such as

> 60 bar or >75 bar. If the production pressure is too high to be used directly at its production pressure, i.e. if the production pressure is higher than about 200 bar, the well stream is partly expanded to the pressure of the combustion chamber which normally is operated at a pressure lower than 200 bar, such as lower than 75 bar, but higher than 40 bar.

[0045] The heat of combustion in the boiler 22 is used for generating and heating steam which is introduced into a steam power plant 24 via a steam line 23. Condensed steam is returned from the steam power plant 24 to the combustion chamber though a condensate line 23'. The skilled person will understand that other heat media, such as organic compounds may substitute water as a heat medium transferring heat from the combustion chamber to the steam power plant (organic cycle).

[0046] The skilled person will also understand that the steam power plant 24 may receive steam both from the boiler 22 and from the combustion unit 3, so that the combustion unit 3 does not have a steam turbine of its own, if deemed to be advantageous over håving separate systems.

[0047] Instead of natural gas other carbonaceous fuels such as oil, coal powder and hydrates can be used, provided that the combustion is performed at elevated pressures which typically is higher than 40 bar, such as > 60 bar or >75 bar.

[0048] A power line 25 is connected to the steam power plant 24 for exporting power for the intended use. The power feed line 10 is connected to the power line 25 for providing electrical power to the electrolysis unit for substituting the energy lost in the oxygen production unit as described above. A rectifier 26 is arranged between the power line 25 and the power feed line, if necessary.

[0049] The exhaust gas from the boiler 22 is withdrawn through an exhaust-gas withdrawal line 30 and introduced into a CO2condenser 31. As the carbonaceous fuel is combusted with pure oxygen, the exhaust gas from the power plant mainly comprises water vapour and CO2, in addition to minor amounts of other gases introduced into plant together with the fuel. The carbon-dioxide and water can be condensed together forming a liquid that can either be injected for storage, or preferably be injected for EOR (Enhanced Oil Recovery). Optionally, the water vapour can be separated from the CO2in a first step in the CO2liquefaction unit by cooling the exhaust gas to condensate the vapour. The condensed water vapour is separated from the gaseous phase and released into the surroundings, or used in other processes in the plant. For injection for EOR or into an aquifer for safe deposit, it is preferred that the CO2and water vapour from the combustion are condensed together.

[0050] The further treatment of the thus dried CO2gas depends on the pressure in the combustion chamber 22, and thus the pressure of the exhaust gas withdrawn in an exhaust-gas withdrawal line 30. If the operating pressure in the combustion chamber is 40 bar or higher, such as 50, 60, 70 bar or higher, the CO2gas is only cooled to a temperature causing the CO2to liquefy or to form a supercritical fluid that may be pumped like a liquid. If the pressure is lower, the CO2is compressed and cooled to a pressure and temperature resulting in liquid or supercritical CO2. The phase diagram for CO2found in figure 3, illustrates the conditions, i.e. pressure and temperature, at which CO2is a liquid or a supercritical fluid.

[0051] The liquid or supercritical CO2or the C02/water mixture is withdrawn via a liquid withdrawal line 32 and introduced into an injection unit for injection2into the ground, either in an aquifer, an empty oil or gas well, or other suitable sub terrain storage, or into an oil or gas field as pressure support or EOR.

[0052] The total power output of the above-described power plant is not calculated. However, CO2as a fluid or supercritical fluid has a high value for pressure support, or EOR. By providing this kind of plant close to a wellhead and using a unprocessed or substantially unprocessed well-stream, or a source of low value as the carbonaceous fuel, the value of the CO2reduces the demand for energy efficiency for such a plant substantially, compared to traditional gas power plants especially if CO2capture of CO2in the exhaust gas is required.

Claims

1. A method for generating of oxygen, the method comprising the steps of: a. Decomposition of water by electrolysis in an electrolysis unit to generate oxygen and hydrogen in separate fractions; b. Withdrawing oxygen through an oxygen line for export of oxygen, c. Withdrawing hydrogen through a hydrogen line; d. Introducing the hydrogen into a combustion unit; e. Introducing air into the combustion unit; Characterized in that the method also comprises the step of f. Introducing air into the combustion unit and allowing oxygen in the air and hydrogen to react to form water in the combustion unit for generation of electrical power and/or heat.

2. The method of claim 1, wherein the electrical power and / or heat generated in the combustion unit is transferred to the electrolysis unit to be used for electrolysis of water therein.

3. The method of claim 1 or 2, wherein the exhaust gas from the combustion in step f) is cooled to condensate and separate water formed in the combustion in step f) from the remaining exhaust gas, and recycle the water back to the electrolysis chamber.

4. The method according to any of the preceding claims, wherein the combustion unit is a fuel cell unit.

5. The method of any of the claims 1 to 3, wherein the combustion unit comprises a boiler in which the heat of combustion is used to generate steam in steam coils in the boiler

6. The method according to any of the preceding claims, wherein the pressure in the electrolysis unit is 40 bar or higher.

7. The method according to claim 6, wherein the pressure in the electrolysis unit is 65 bar or higher.

8. A method for production of electrical power from carbonaceous fuel, where the carbonaceous fuel is combusted in a combustion chamber in the presence of substantially pure oxygen to produce electrical power and a flue gas, where the combustion is performed at a pressure of 40 to 200 bar, where the flue gas is withdrawn from the combustion chamber and cooled to a temperature that according to figure 3 results in condensation of the flue gas, or conversion of the flue gas to a supercritical fluid håving a density of at least 600 kg/m<3>, where the liquid or supercritical fluid formed, is safely deposited, where the substantially pure oxygen for the combustion is generated according to any of the claims 1 to 7.

9. The method according to claim 8, wherein the flue gas is cooled to a temperature of 80 °C or colder, such as 40 °C, 30 °C, 20 °C or colder or 10 °C or colder.

10. A plant for producing substantially pure oxygen, wherein the plant comprises an electrolysis unit (2) for electrolysis of water into oxygen and hydrogen in separate fractions, an oxygen withdrawal line (4) for withdrawal of oxygen from the electrolysis unit (2), a hydrogen withdrawal line (5) for withdrawal of hydrogen from the combustion unit and introduction of the hydrogen into a combustion unit (3), an air line (6) for introduction of air into the combustion unit (3), the combustion unit (3) being adopted for combustion of hydrogen and oxygen from the air for generation of electrical power and an exhaust gas mainly comprising water and nitrogen, means for separation of water from the remaining exhaust gas, an water return line (8) for returning water from the combustion unit to the electrolysis unit (2), an exhaust gas withdrawal line (7) for releasing exhaust gas into the surroundings, a power line (9) for transferring electrical power generated in the combustion unit (3) to the electrolysis unit (2), and a power feed line (10) for delivering additional power to the power line (9).

11. A plant for production of electrical power from carbonaceous fuel, wherein a plant for generation of electrical power and capturing of CO2, the plant comprising a plant (1) according to claim 10, for providing substantially pure oxygen to a boiler (22) and a fuel line (21) for providing carbonaceous fuel to the boiler at a pressure of 40 bar or more to the boiler, in which the fuel is combusted and steam generated, a steam line (23) for withdrawing steam generated in the boiler and introduction of the steam into a steam turbine (24) for generation of electrical power, and a condensate return line (23') for returning condensed steam from the steam turbine (24) and introduction thereof into the boiler (22), a flue line (30) for withdrawal of flue gas from the boiler (2) and for introduction of the flue gas into a condenser (31) in which the flue gas is cooled for condensing of, or forming a supercritical fluid håving a density of at least 600 kg/m<3>, of CO2and H2O, present in the flue gas, and a liquid withdrawal line (32) for withdrawal of condensed liquid or supercritical fluid from the condenser (31).

12. A plant according to claim 11, wherein the condenser (31) comprises two condenser units separated by a liquid/gas separator, to separate condensed water from the exhaust stream in the liquid/gas separator and to further cool and condense the CO2in the second condenser unit.