The present invention relates to a process for producing compounds of the CxHyOz type, particularly with x>1; y is between 0 and 2x+2 and z is between 0 and 2×, by reduction of carbon dioxide (CO2) and/or carbon monoxide (CO), particularly from very reactive hydrogen species generated by water electrolysis.
Today ceramic conducting membranes are the subject of much research to increase their performance; these membranes find particularly interesting applications, among others, in the fields of:
Today hydrogen (H2) appears to be a very interesting energy carrier, that is called upon to take on more and more importance to treat, among other substances, petroleum, oils and lubricants, and that may, in the end, be advantageously substituted for petroleum and fossil energies, whose reserves are going to sharply decrease in the decades to come. However, in this perspective, it is necessary to develop effective hydrogen production processes.
Admittedly, many hydrogen production processes have been described from various sources, but a number of these processes have proved to be unsuitable for the massive industrial production of hydrogen.
In this context, one may, for example, cite the synthesis of hydrogen from steam reforming hydrocarbons. One of the main problems with this synthetic pathway is that it produces, as by-products, significant quantities of CO2-type greenhouse gases. In fact, 8 to 10 tons of CO2 are released to produce 1 ton of hydrogen.
Therefore two challenges are presented for future years: To search for a new energy carrier that is usable without danger to our environment such as hydrogen, and to reduce the quantity of carbon dioxide.
The techno-economic valuations of industrial processes now take the latter data into account. However, it is essentially sequestration, in particular underground sequestration in crevices that do not necessarily correspond to old oil deposits, which in the end cannot be without danger.
It would seem wise to recycle this carbon dioxide in the form of compounds usable in the chemistry field or in the energy production field. The energy necessary for this transformation would be electricity, for example of nuclear origin, and in particular that from reactors such as HTR “High Temperature Reactor” type nuclear reactors or EPR (registered trademark) European pressurized water reactors.
A promising path for industrial hydrogen production is the technique known as steam electrolysis, for example at high temperature (EHT), at medium temperature, typically over 200° C., or even at an intermediate temperature of between 200° C. and 1000° C.
Two steam electrolysis production processes are presently known:
According to a first process illustrated in
More precisely,
The application of a potential difference between anode 3 and cathode 4 leads to a reduction in H2O steam from the cathode 4 side. This reduction forms hydrogen H2 and ions O2− (Oox in Kroger-Vink notation) at the surface of the cathode 4 according to the reaction:
2e′+Vo..+H2O→Oox+H2
The O2− ions, more precisely the oxygen vacancies (Vo..), migrate through electrolyte 2 to form oxygen O2 at the surface of anode 3, electrons e being released according to the oxidation reaction:
Thus, the first process enables, from the output of the electrolyzer 1, oxygen—anode compartment—and hydrogen mixed with steam—cathode compartment—to be generated.
According to a second process illustrated in
More precisely, this
The application of a potential difference between anode 12 and cathode 13 leads to oxidation of the H2O steam from the anode side 12. The steam injected in anode 12 is thus oxidized to form oxygen O2 and H+ ions (or OHo.in Kroger-Vink notation), this reaction releasing electrons e—according to the equation:
The H+ ions (or OHo.Kroger-Vink notation) migrate through the electrolyte 11 to form hydrogen H2 at the surface of cathode 13 according to the equation:
2e′+2OHo.→2Oox+H2
Thus, this process provides, from the output of electrolyzer 10, pure hydrogen—cathode compartment—and oxygen mixed with steam—anode compartment.
More precisely, the formation of H2 passes by the formation of intermediate compounds that are hydrogen atoms adsorbed at the surface of the cathode with variable energies and degrees of interaction and/or radical hydrogen atoms H• (or HElectrodex in Kroger-Vink notation). As these species are highly reactive, they usually recombine to form hydrogen H2 according to the equation:
2HElectrodex→H2
The present invention aims to reduce the quantity of existing carbon dioxide, for example by recycling this carbon dioxide in the form of compounds usable in the chemistry field or in the energy production field.
For this purpose, the invention proposes a process for electrolyzing water steam injected under pressure into an anode compartment of an electrolyzer provided with a proton-conducting membrane made of a material enabling protonated species to be incorporated into this membrane under steam, oxidation of the water injected in steam form taking place at the anode so as to generate protonated species in the membrane that migrate within this same membrane and are reduced at the surface of the cathode in the form of reactive hydrogen atoms capable of reducing carbon dioxide CO2 and/or carbon monoxide CO, said process comprising the following steps:
As explained above, reactive hydrogen atoms is understood to refer to hydrogen atoms adsorbed at the surface of the cathode with variable energies and degrees of interaction and/or radical hydrogen atoms H• (or HElectrodex in Kroger-Vink notation).
The invention results from the observation that the second process described above generates highly reactive hydrogen at the electrolyzer cathode (particularly hydrogen atoms adsorbed at the surface of the electrode and/or radical hydrogen atoms).
These highly reactive hydrogen atoms HElectrodex are formed at the surface of the cathode according to the reaction:
e′+OHo.→Oox−HElectrodex
In fact, in the presence of CO2 and/or CO on the cathode side, the highly reactive hydrogen HElectrodex reacts with the carbon compounds on the electrode to give reduced carbon dioxide and/or carbon monoxide compounds of the CxHyOz type with x>1; y is between 0 and 2x+2 and z is between 0 and 2x.
By way of example, these compounds are paraffins CnH2n+2, olefins C2nH2n, alcohols CnH2n+2OH or CnH2n−1OH, aldehydes and ketones CnH2nO, acids Cn−1H2n+1 with n>1.
The invention thus enables steam to be electrolyzed with the joint carbon dioxide and/or carbon monoxide electroreduction as described subsequently.
The process according to the invention may also present one or more of the characteristics below, considered individually or according to all technically possible combinations:
The invention also relates to a steam electrolysis device for electrolyzing water steam introduced under pressure into an anode compartment of an electrolyzer provided with a proton-conducting membrane, made of a material enabling the injection of protonated species into this membrane under steam after oxidation, comprising:
The device according to the invention may also present one or more of the characteristics below, considered individually or according to all technically possible combinations:
Other characteristics and advantages of the invention will clearly emerge from the description given below, for indicative and in no way limiting purposes, with reference to the attached figures, among which:
This electrolysis device has a structure similar to that of the device from
In conformance with the invention, the device also comprises means 36 enabling the insertion under pressure of gas (pCO2 and/or CO) into the cathode compartment 33.
The injection of steam is done via means 35 at the level of anode 32 while the injection of gas CO2 and/or CO is done via means 36 at the level of cathode 33.
At anode 32, the water is oxidized by releasing electrons while H+ ions (in OHo. form) are generated according to a process that is similar to the process described with the help of
These H+ ions migrate through the electrolyte 31, carbon compounds of the CO2 and/or CO type react at cathode 33 with these H+ ions to form compounds of the CxHyOz type (with x>1; y is between 0 and 2x+2 and z is between 0 and 2x) and water at the cathode.
In particular, the chemical equations of the various reactions may be written as:
(6n+2)HElectrodex+nCO2→CnC2n+22nH2O
6nHElectrodexnCO2→CnH2n+2nH2O
6nHElectrodex+nCO2→CnH2n+2O+(2n−1)H2O
(6n−2)HElectrodex+nCO2H2nO+(2n−1)H2O
As the nature of the compound formed depends on the process conditions, the overall CxHyOz formation reaction may thus be written as:
(4x−2z+y)HElectrode++xCO2+CxHyOz+(2x−z)H2O
The nature of the CxHyOz compounds synthesized at the cathode depends on many process parameters such as, for example, gas pressure, operating temperature T1 and the voltage-current pair applied to the cathode as described below:
Concerning the gas pressure, the relative pressure of CO2 and/or CO is greater than or equal to 1 bar and less than or equal to a burst pressure of the assembly, the latter being greater than or equal to at least 100 bars.
It will be noted that here the term relative pressure designates the pressure of insertion with relation to atmospheric pressure.
It will be noted that it is possible to use either a gas stream containing only steam or a gas stream partially containing steam. Thus, depending on the case, the term “partial pressure” will designate either the total pressure of the gas stream in the case where the latter is only constituted of steam or the partial pressure of the steam in the case where the gas stream comprises gases other than steam.
We add that the total pressure imposed in a cathode or anode compartment may be compensated for in the other compartment so as to have a pressure difference between the two compartments to prevent rupture of the membrane, electrode support assembly if its rupture resistance is too low.
Concerning the operating temperature T1 of the device, the latter depends on the type of material utilized for membrane 31; in any case, this temperature is over 200° C. and is generally under 800° C., or even under 600° C. This operating temperature corresponds to a conduction ensured by W protons.
The operating temperature T1 of the device is also dependent, within the 200 to 800° C. range, according to the nature of the CxHyOz carbon compounds that one wishes to generate.
In fact, a large variety of compounds may be obtained, such as methane, methanol, formaldehyde, carboxylic acids (formic acid, etc.) and other compounds with longer chains, which may go as far as the formation of synthetic fuel.
For example, one may have the following reactions at the cathode:
8HElectrodex+CO2→CH4+2H2O
6HElectrodex+CO2→CH2+2H2O
6HElectrodex+CO2→CH3OH+H2O
4HElectrodex+CO2→CH2O+H2O
HElectrodex+CO2→COOH
Concerning the voltage-current pair applied to the cathode, it should be noted that the nature of the carbon compounds formed also depends on this voltage. In fact, the more reductive the cathode medium (low oxidoreduction potential E), the more the carbon compounds generated are hydrogenated, as diagrammed in the diagram below (R being, for example, an alkyl group).
For the advantageous embodiment of these reactions, it is necessary to have electrodes presenting a large number of triple contact points, i.e., points or contact surfaces between an ionic conductor, an electronic conductor and a gas phase.
For example, the electrodes considered are preferentially ceramic-metal materials formed by a mixture of ion-conducting ceramic and an electron-conducting metal.
However, the utilization of “all-ceramic” electron-conducting electrodes may also be considered instead of a ceramic-metal material.
It should be noted that a given electrolyte may be an O2− proton or ion conductor according to the temperature and the pressure of the applied steam.
But the utilization of proton-conducting membranes generates hydrogen (in hydrogen atom form more or less adsorbed at the surface of the cathode) that is much more reactive than H2 hydrogen (or dihydrogen), thus enabling better hydrogenation of the CO2 and CO compared to a conventional hydrogenation process (in the presence of H2).
Moreover, the utilization of H+ ion conducting membranes operating at moderate temperature enables the synthesis of CxHyOz type complex compounds (with x, y and z greater than 1) while the utilization of O2− conducting membranes, operating at a much higher temperature, preferentially generates CO, a product that is stable at high temperature.
The objective of studies implemented is to obtain the maximum yield for the production of hydrogen and/or the hydrogenation of CO2 and/or CO. To do this, most of the current utilized must intervene in the faraday process, i.e., be utilized for the reduction of water and consequently the production of highly reactive hydrogen.
Thus the voltage utilized for polarization must be affected at least by
In this context, the present invention proposes the utilization of proton-conducting electrolyte under steam pressure for the electrolysis of water at high temperature for hydrogen production as well as for electroreduction of CO2 and/or CO at the cathode.
Thus, the process comprises the following steps:
Thanks to the gas stream containing steam, protonation of the membrane is promoted by the steam under pressure and this pressure is advantageously utilized to obtain the desired conductivity at a given temperature. Such a process is described, for example, in the French patent application filed under No. 07/55418 on Jun. 1, 2007.
As indicated in this application, the applicant observed that the increase in the relative partial pressure of steam leads to an increase in the ionic conductivity of the membrane.
This correlation between the increase in relative partial pressure with the increase in conductivity enables suitable materials to be worked at lower temperatures. In other words, the lowering in conductivity driven by operating at lower temperatures is compensated for by the increase in relative partial pressure of steam.
In conformance with the invention, “highly reactive” hydrogen is produced at the cathode that may generate hydrogen (H2), in the absence of a reducible compound, or CxHyOz type compounds, in the presence of CO2 and/or CO with x>1; y is between 0 and 2x+2 and z is between 0 and 2x.
The proton-conducting membrane is made from a material promoting the insertion of water such as a doped perovskite material of general formula AB1-xD2O3-x/2. The materials utilized for the anode and the cathode are preferentially ceramic-metal materials (mixture of metal with the perovskite material utilized for the electrolyte). The membrane is preferably impermeable to O2 and H2 gases.
In general, the membrane may be of the type: perovskite vacancies, non-stoichiometric perovskites and/or perovskites doped with general formula ABO3, of fluorite, pyrochlore A2B2X7, apatite Me10(XO4)6Y2, oxyapatite Me10(XO4)6O2 structure, of hydroxylapatite Me10(XO4)6(OH)2 structure, silicate structures, aluminosilicates (phyllosilicate or zeolite), silicates grafted with oxyacids or silicates grafted with phosphates.
More generally, the electrolytes may advantageously be all of the compounds utilized as high temperature or intermediate temperature proton conductors either by virtue of their tunnel or sheet structure and/or by the presence of vacancies capable of inserting protonated species whose molecular size is small.
The present invention is capable of many variations. In particular, the material enabling the incorporation of protonated species may be impermeable to O2 and H2 gases and/or may enable the incorporation of protonated species at a densification level of over 88%, preferably equal at least to 94%.
In fact, a good compromise between the densification level that must be the highest possible (particularly for the mechanical strength of electrolytes and gas permeation) and the capacity of the material to enable incorporation of protonated species must be found. Increasing the partial steam pressure that forces the incorporation of protonated species into the membrane compensates for the densification level increase.
According to a variation, the material enabling the injection of water is an oxygen atom-defective oxide such as an oxygen-defective perovskite acting as a proton conductor. In addition, the oxygen atom-defective oxide may present stoichiometric intervals and/or may be doped.
In fact, non-stoichiometry and/or doping allow the creation of oxygen atom vacancies. Thus, in the case of proton conduction, the exposure under pressure of a perovskite presenting stoichiometric intervals and/or being doped (and therefore being deficient in oxygen) to steam induces the incorporation of protonated species into the structure. The molecules of water fill the oxygen vacancies and dissociate into 2 hydroxyl groups (or H+ proton on an oxide site) according to the reaction:
Oox+Vo..+H2O 2OHo.
It will be noted that materials other than non-stoichiometric and/or doped perovskites may be utilized as materials promoting the incorporation of water and its dissociation in the form of protonated species and/or hydroxides.
For example, crystallographic structures, such as fluorite structures, pyrochlore A2B2X7 structures, apatite Me10(XO4)6Y2 structures, oxyapatite Me10 (XO4)6O2 structures, hydroxylapatite Me10(XO4)6 (OH)2 structures, silicates, aluminosilicates, phyllosilicates, or phosphates may be cited.
These structures may possibly be grafted by oxyacid groups.
In fact, all structures having a high affinity with water and/or protons may be contemplated.
Number | Date | Country | Kind |
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0853161 | May 2008 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FR2009/050909 | 5/15/2009 | WO | 00 | 2/4/2011 |