FR2966472A1 - Production of electricity and hydrogen from hydrocarbon fuel e.g. natural gas, comprises producing electricity by combustion of hydrocarbon fuel with an oxidant to produce a carbon dioxide rich stream, and increasing pressure of stream - Google Patents

Abstract

The process comprises producing electricity by combustion of hydrocarbon fuel (1) with an oxidant (2) comprising oxygen produced by an electrolysis unit, where the combustion produces a carbon dioxide (CO 2) rich stream, increasing a pressure of the CO 2rich stream by compression and cooling steps, treating the CO 2rich stream to remove a part of the dust, nitrogen oxides and sulfur oxides, injecting CO 2rich stream into a saline aquifer (6) and evacuating a saline water aquifer (SA), and introducing saline water in the electrolysis unit. The process comprises producing electricity by combustion of hydrocarbon fuel (1) with an oxidant (2) comprising oxygen produced by an electrolysis unit, where the combustion produces a carbon dioxide (CO 2) rich stream, increasing a pressure of the CO 2rich stream by compression and cooling steps, treating the CO 2rich stream to remove a part of the dust, nitrogen oxides and sulfur oxides, injecting CO 2rich stream into a saline aquifer (6) and evacuating a saline water aquifer (SA), and introducing saline water in the electrolysis unit that decomposes saline water into oxygen and hydrogen by an electric current. The electric current used in the electrolysis unit is partly produced in the step of production of electricity. The hydrogen is combusted to produce electricity that is partially implemented in the electrolysis unit. The electrolysis unit is supplied with saline water and an addition of water.

Description

The present invention relates to the field of generating electricity and hydrogen from a hydrocarbon fuel, with integrated management of associated carbon dioxide emissions.

The reserves of hydrocarbon fuel, particularly coal, on a global scale are important. The combustion of coal can produce energy, such as electricity in a thermal power plant. Nevertheless, combustion generates carbon dioxide (CO2), which is one of the greenhouse gases responsible for global warming.

The combustion reaction of a hydrocarbon feedstock is represented by CnHzn + 2 + (3n + 1) O2 -> nCO2 + (n + 1) H20 + 0 by: 2 Different technological solutions are envisaged to reduce carbon dioxide emissions . For example, carbon dioxide can be captured by an aqueous amine solution within the flue gases, to recover a gas stream containing mainly CO2. This is called a "post-combustion" CO2 capture process. One of the major problems associated with "post-combustion" CO2 capture is the energy required for the step of separating CO2 from nitrogen in the flue gases. This energy is high because the partial pressure of CO2 in the fumes is very low, generally between 0.1 and 0.3 bar. An alternative to the "afterburner" is the capture of CO2 in "oxycombustion" which proposes to first carry out a separation of air oxygen (O2) and nitrogen (N2), for example by cryo-distillation, or again by adsorption. Then the combustion is carried out with the separated oxygen, in the absence of nitrogen. The oxy-combustion technique makes it possible to recover at the outlet of the combustion chamber a gaseous flow consisting mainly of CO2. Nevertheless, the cost associated with the separation of oxygen and nitrogen necessary for the oxy-combustion is equivalent to the cost associated with the separation of the CO2 and the nitrogen produced after the combustion. Indeed, as above, the partial pressure of oxygen contained in the air is very low, of the order of 0.2 bar. The separation between nitrogen and oxygen is therefore difficult to do.

Whether post combustion or oxycombustion is considered, the CO2 stream that is recovered must be sequestered. Different storage locations can be imagined, such as old oil or gas fields, but one of the largest storage capacities is saline aquifers. However, the injection of CO2 into these aquifers necessarily implies an evacuation of saline water to avoid overpressure in the aquifer. Saline water must be treated before evacuation into the environment to avoid altering or degrading the natural environment. Saline water from aquifers can be treated in different ways. It is possible to purify this water by reverse osmosis. This method is well known to those skilled in the art, and already used on an industrial scale for the desalination of seawater. Nevertheless, given the salinity of the water contained in saline aquifers, the pressure It would be necessary to exercise to purify this water would be important. As a result, the cost of desalination would be high. Another alternative is to carry out a distillation, to recover on the one hand fresh water, and on the other hand salts. This stage is also very energy intensive and would require an additional supply of energy, which risks losing interest in CO2 capture and storage operations.

In addition, hydrogen is considered as the energy carrier of tomorrow. In fact, the hydrogen reacts with the oxygen contained in the air to form a water molecule (non-polluting) and energy according to the reaction described below: H2 +1/202 -> H2O + It is envisaged to use hydrogen in vehicles, but also in thermal power plants for the production of electricity. Unfortunately, hydrogen is not a natural resource, as can coal, oil or gas. Hydrogen is mainly produced from methane (which has a higher ratio between hydrogen atom and carbon atom than oil or coal), by carrying out a partial oxidation (formation of CO and H20), then realizing a Water Gas Shift operation well known to those skilled in the art, to obtain hydrogen and CO2. These reactions are diagrammed below: CH4 + 3/2 02 -> CO + 2H20 CO + H2O -> CO2 + H2 A separation between hydrogen and CO2 is again necessary to obtain a hydrogen free of CO2, and therefore non polluting. An alternative to producing hydrogen by methane reforming is to perform electrolysis of water, at low or high temperature. At low temperature, by the supply of electrical energy, it is possible to produce from water containing electrolytes, hydrogen at the cathode, and oxygen at the anode according to the following reaction: H 2 O + 2e - -> H2 +1/202 Electrolysis can also be carried out at high temperature, with water in vapor form, to obtain better performance.

Because of the principle of electrolysis, hydrogen and oxygen are produced in different places, and therefore does not require a separation between hydrogen and oxygen.

The present invention proposes to integrate the operation of production of electricity by oxy-combustion with capture and storage of CO2 and the operation of production of hydrogen by hydrolysis of water in order to treat the salt water by the hydrolysis operation, to produce oxygen necessary for oxy-combustion and simultaneously to produce hydrogen.

In general, the invention proposes a method for producing electricity and hydrogen comprising the following steps: a) electricity is produced by combustion of a hydrocarbon fuel with an oxidant comprising oxygen produced by a electrolysis unit implemented in step c), the combustion producing a flow rich in CO2, b) the CO2-rich stream is injected into a saline aquifer and saline water is removed from said aquifer, c) introduces saline water into said electrolysis unit which decomposes salt water into oxygen and hydrogen by means of an electric current.

According to the invention, the electric current can be implemented in the electrolysis unit in step c) is at least partly produced in step a). The hydrogen produced in step c) can be burned to produce electricity which is at least partly used in the electrolysis unit in step c). Before step b), the pressure of the CO2-rich stream can be increased by a succession of compression and cooling steps. Prior to step b), the CO2 rich stream can be treated to remove at least a portion of the dusts, nitrogen oxides and sulfur oxides. The electrolysis unit can be supplied with saline water and, in addition, with a make-up of water. The hydrocarbon fuel can be natural gas, liquid fuel oil, coal, petroleum coke, wood. The present invention overcomes one or more disadvantages of the prior art. In fact, better use of natural resources and better management of associated waste is allowed.

Other characteristics and advantages of the invention will be better understood and will become clear from reading the description given below with reference to FIG. 1 which schematizes the method according to the invention.

In FIG. 1, the unit OC implements an oxy-combustion step. The OC unit comprises an oxy-combustion chamber which is fueled with fuel 1 and oxidant 2. The fuel is a hydrocarbon fuel of fossil origin or composed of biomass. For example, you can use natural gas, liquid fuel oil, coal, petroleum coke, or a mixture of these fossil fuels, wood. According to the invention, the oxidant 2 comprises oxygen produced by electrolysis of the water in the unit E. Part of the oxygen may also come from a unit for producing oxygen by cryo-distillation of the oxygen. air or adsorption.

In general, the fuel and oxidant flows flowing in the ducts 1 and 2 are controlled and adjusted to achieve combustion with an excess of oxygen. The combustion chamber in the OC unit consists of a metal casing, commonly called "casing", this envelope being lined with heat-resistant material. The combustion chamber is equipped with one or more burners. In addition, the combustion chamber may optionally comprise tubes in which circulates a fluid to be heated. This fluid can be boiler water that turns into steam to generate electricity. Alternatively, the pressurized fumes produced in the combustion chamber are expanded in a turbine to produce electricity. The fumes 4 produced by the combustion carried out in the OC unit are rich in CO2. A portion of stream 4 may be withdrawn through line 12 to be mixed with the oxygen 2 produced by the electrolysis unit to reconstitute an equivalent air oxidant by diluting oxygen in the CO2.

The flow 4 of CO2 may undergo, in the unit T, dust removal and removal of nitrogen oxides and / or sulfur. For dedusting, electrostatic filters are preferably used. Some treatments can be performed in the OC combustion chamber. For example, in the case of using high sulfur fuel, such as coal, the injection of limestone into the chamber OC allows a first reduction of the sulfur oxide content which is generally followed by a second desulfurization step on the fumes exiting the combustion chamber.

Then, the CO2 flow from the unit T is introduced into the compression unit C to undergo a succession of compression and cooling steps until a pressure compatible with the CO2 storage conditions is reached. that is, a pressure that may be greater than 100 bar. The water collected during successive cooling after each compression step can be discharged through line 3 of unit C. According to the invention, the flow of compressed CO2 from C is injected into a salt aquifer SA. The CO2 injection is carried out in a well that is drilled from the surface to the SA aquifer. An aquifer is any permeable geological formation that contains water. The most superficial aquifers contain fresh water used for the supply of drinking water. The deeper aquifers contain salt water unfit for human consumption, but which can store CO2 especially when the salt aquifer is covered with a layer of watertight formations such as clay or salt above the site. in order to avoid any CO2 rise on the surface. The saline water produced from the saline aquifer generally comprises between 100 and 150 g / l of salts. In most cases, the CO2 injection has the effect of increasing the pressure in the SA salt aquifer. To inject a large quantity of CO2, it is necessary to evacuate salt water from the SA aquifer, for example via a production well. In general, the production well is distinct from the injection well. In FIG. 1, the salt water generated is evacuated from SA schematically by the duct 6.

According to the invention, a part of the electricity produced by the oxy-combustion step is used to electrolyze the saline water 6 generated during the storage of CO2 in the SA salt aquifer. The electrolysis is carried out in unit E which comprises an electrolytic cell comprising two electrodes, usually of inert metal (in the potential and pH zone considered) such as platinum, immersed in salt water 6 from the aquifer HER. The two electrodes are connected to the opposite poles of a current source, for example produced by virtue of the energy obtained in the oxy-combustion unit OC, or produced in unit D described below. The electrolysis carried out in unit E makes it possible to produce the oxygen discharged through line 2, the hydrogen discharged through line 8 and salts, evacuated via line 7, which constitute ultimate waste. The oxygen 2 produced at the anode is used in the OC unit to achieve oxycombustion. Hydrogen 8 produced at the cathode is used as fuel in unit D, or stored, then transported for the purpose of being used as a clean energy carrier. The operation of electrolysis of the saline water generated during the storage of the CO2 makes it possible, simultaneously in one operation, to treat the salt water, to produce the oxygen necessary for the oxy-combustion and to produce hydrogen . In the cases where the water produced by the flow 6 is not sufficient to produce all the oxygen necessary for the oxycombustion in the OC unit, an additional water can be made by the conduit 11 feeding unit E.

Some or all of the hydrogen 8 produced at the cathode of the electrolysis unit E can be introduced into the unit D to produce electricity. Hydrogen 8, preferably under pressure, is mixed with air, preferably under pressure, arriving via line 9 into a second combustion chamber. The exothermic combustion reaction of hydrogen produces combustion gases which are expanded through a turbine. The relaxation energy recovered by the turbine is converted into electricity by a generator. The expanded gases from the turbine are cooled by heat exchange with a fluid, for example water. The heated and vaporized water is expanded through a second turbine. The relaxation energy recovered by the second turbine is converted into electricity by a generator. The cooled gases that are evacuated through line 10 mainly comprise water and nitrogen. Part of the electricity produced by unit D may possibly be used to carry out the electrolysis of salt water in unit E.

The example below describes a case of operation of the process sequence according to the invention, shown schematically in FIG. 1. The hypotheses are described below: fuel: Coal having a PCI of 25,000 kJ / kg and containing 65% Carbon Wt., 1400 MWth boiler with 50% net electrical efficiency, oxidizer: Oxygen with 5% excess, Production water: Production equivalent to CO2 injected by weight - salinity 120 g / L

The following table shows the material balance of the scheme of Figure 1 proposed with oxy-combustion of coal, sequestration of 100% of the CO2 emitted compressed and dried, and electrolysis of the salt water produced during the injection. Flux 1 2 5 6 7 8 11 Description Coal 02 CO2 dry Salt water Brine H2 Water supply Flow (kg / s) 56 101.9 133.5 133.5 18.84 12.74 0 The above example shows that water produced during the injection of CO2 is sufficient for the production of all the oxygen necessary for oxy-combustion.

The brine produced in unit E and discharged through line 7 contains 850 g / l of salts. 10 15 20

Claims (7)

CLAIMS1) A method of producing electricity and hydrogen comprising the following steps: a) electricity is produced by combustion of a hydrocarbon fuel with an oxidant comprising oxygen produced by an electrolysis unit used in step c), the combustion produces a flow rich in CO2, b) the CO2-rich stream is injected into a saline aquifer and saline water is discharged from said aquifer, c) the saline water is introduced into said electrolysis unit which breaks down saline water into oxygen and hydrogen by means of an electric current. 2) Process according to claim 1, wherein the electric current used in the electrolysis unit in step c) is at least partly produced in step a). 3) Method according to one of claims 1 and 2, wherein the hydrogen produced in step c) is burned to produce electricity which is at least partly implemented in the electrolysis unit to step c). 4) Method according to one of the preceding claims wherein before step b), the pressure of the CO2-rich stream is increased by a succession of compression and cooling step. 5) Method according to one of the preceding claims, wherein before step b), the CO2-rich stream is treated to remove at least a portion of the dusts, oxides of nitrogen and sulfur oxides. 6) Method according to one of the preceding claims, wherein the electrolysis unit is supplied with saline water and, in addition, with a makeup of water. 7) Method according to one of the preceding claims, wherein the hydrocarbon fuel is selected from natural gas, liquid fuel, coal, petroleum coke, wood.