Methods for co-processing carbon dioxide and hydrogen sulfide

A method for co-processing H2S and CO2 in an electrolyzer includes feeding a first gas stream having H2S to an anode and feeding a second gas stream having CO2 to a cathode. The H2S is split into hydrogen and elemental sulfur. The hydrogen is transferred from the anode to the cathode, and the CO2 is hydrogenated with the transferred hydrogen. A method for producing electricity in a fuel cell includes feeding a first gas stream having H2S and CO to an anode, and feeding a second gas stream having oxygen to a cathode. The H2S and CO forms hydrogen and carbonyl sulfide. The hydrogen is transferred from the anode to the cathode. The transferred hydrogen is oxidized with the oxygen of the second gas stream, and electricity formed from the oxidation is collected.

BACKGROUND

Field

The present specification generally relates co-processing carbon dioxide (CO2) and hydrogen sulfide (H2S). More particularly, the present specification is directed to electrochemical methods for producing high-value chemicals by co-processing byproduct streams of CO2and H2S.

Technical Background

Hydrogen sulfide and carbon dioxide are two common compounds that are present in some naturally occurring hydrocarbon deposits. These compounds are generally removed from the hydrocarbon and are of low value as extracted. Therefore, H2S and CO2are generally either discarded or further processed into various compounds of more value. Currently, CO2and H2S are separately processed, which leads to higher overhead costs and other inefficiencies.

Accordingly, systems and methods that enable more efficient treatment of H2S and CO2that is removed from hydrocarbon feeds are desired.

SUMMARY

According to some embodiments, a method for co-processing H2S and CO2in an electrolyzer is disclosed. The electrolyzer comprises an anode, a cathode, and an electrolyte positioned between and in electrochemical contact with the anode and the cathode. The method according to embodiments comprises: feeding a first gas stream comprising H2S to the anode of the electrolyzer; feeding a second gas stream comprising CO2to the cathode of the electrolyzer; splitting H2S of the first gas stream into hydrogen and elemental sulfur; transferring the hydrogen split from the H2S of the first gas stream from the anode across the electrolyte to the cathode; and hydrogenating the CO2from the second gas stream with the hydrogen that was transferred from the anode.

According to other embodiments, a method for producing electricity in a fuel cell is disclosed. The fuel cell comprises an anode, a cathode, and an electrolyte positioned between and in electrochemical contact with the anode and cathode. The method according to embodiments comprises: feeding a first gas stream comprising H2S and CO to the anode; feeding a second gas stream comprising oxygen to the cathode; splitting the H2S from the first gas stream into hydrogen and elemental sulfur; forming carbonyl sulfide from the elemental sulfur split from the H2S of the first gas stream and the CO of the first gas stream; transferring the hydrogen split from the H2S of the first gas stream from the anode across the electrolyte to the cathode; oxidizing the hydrogen that is transferred from the anode across the electrolyte to the cathode with the oxygen of the second gas stream; and collecting electricity formed from the oxidizing of the hydrogen that is transferred from the anode across the electrolyte to the cathode.

According to yet other embodiments, another method for co-processing H2S and CO2in an electrolyzer is disclosed. The electrolyzer comprises an anode, a cathode, and an electrolyte positioned between and in electrochemical contact with the anode and cathode. The method according to embodiments comprises: feeding a first gas stream comprising H2S and CO2to the anode; splitting the H2S from the first gas stream into hydrogen and elemental sulfur; forming SOx and CO from the elemental sulfur split from the H2S of the first gas stream and the CO2of the first gas stream; transferring the hydrogen split from the H2S of the first gas stream from the anode across the electrolyte to the cathode; exhausting the hydrogen transferred from the anode across the electrolyte to the cathode from the electrolyzer consuming electricity.

DETAILED DESCRIPTION

Methods for treating H2S and CO2separately are disclosed below. These methods generally require significant amounts of energy and are inefficient compared to embodiments disclosed herein. While the below methods are described in detail and with specific reaction mechanisms, it should be understood that various other reaction mechanisms may occur and fall within the scope of this disclosure.

Methods for processing H2S involve Claus reactions that use high temperatures to oxidize H2S to SO2and then further to elemental sulfur. For example, in a Claus unit gaseous H2S generally undergoes a substoichiometric combustion at temperatures of about 1,000° C. to form gaseous SO2that then reacts with uncombusted gaseous H2S to form elemental sulfur, as shown in reactions (1) and (2) set forth below:

Diatomic S2, as formed in the above reactions, is highly reactive and reacts primarily with other diatomic S2to form an S8allotrope as shown in reaction (3) below:
4S2→S8(3).

As shown in reactions (2) and (3) above, the recovery of elemental sulfur from H2S comprises three sub-steps: heating a mixture of H2S and SO2to a temperature from about 200° C. to about 300° C.; a catalytic reaction; and cooling plus condensation. These three sub-steps are generally repeated up to three times to convert a maximum amount of H2S to elemental sulfur and water. These reactions are exothermic and a portion of the reaction energy may be recovered as low-grade energy, such as by producing steam or the like. However, the energy capture is not very efficient.

In conventional treatments, when CO2is present in the gas feed, the same reaction processes shown in reactions (1)-(3) above may be used. In such a case, the CO2is inert and does not react or combust with the other constituents. Thus, in conventional treatments CO2exits the reaction unit without reacting, and is discarded or further treated. This leads to inefficiencies by requiring additional units and further processing.

In reactions (1)-(3) shown above, the Claus process converts hydrogen atoms of H2S into steam via an oxidation reaction. As shown above, the Claus reactions do not produce very useful products, and the above reactions cannot be used to treat CO2that may be present in the feed stream, or that is exhausted from a CO2capture unit.

Another conventional method for treating H2S is the Stenger-Wasas Process (SWAP) developed by SWAPSOL Corp. In SWAP, H2S and CO2are reacted to form water, sulfur, heat, and carsul (a black insoluble material formed by the reaction between carbon and sulfur). The carsul formed by SWAP can then be heated to produce carbon-based monomers and sulfur. SWAP may be used as an alternative of the Claus process described by Reactions (1)-(3) above, and can be applied to flue gasses, sour gasses, landfill gasses, Claus tail gasses, hydrocarbon waste recycling, and the like. A reaction mechanism for SWAP is shown in reaction (4) below:
CO2(g)+H2S(g)→S+H2O+carsul+heat  (4).

Although SWAP does treat a feed comprising both CO2and H2S, it produces intermediary components that are not very useful and should be further processed into usable chemicals.

As an alternative of, or in addition to, the above chemical reactions for processing H2S, electrochemical processes may be used. Electrochemical processes according to embodiments may be carried out using an electrolysis unit, such as the electrolysis unit depicted inFIG. 1. In the electrolysis unit ofFIG. 1a proton conducting electrolyte membrane is used to split H2S into elemental sulfur (Sn, where n is from 2 to 8) at an anode and diatomic hydrogen (H2) at the cathode.

With reference now toFIG. 1, an electrolysis unit100comprises a feed gas channel140where a feed gas110comprising H2S is fed into the electrolysis unit100. The feed gas110is fed into the electrolysis unit100at the anode160side of the electrolysis unit100. The anode160is positively charged and splits the H2S in the feed gas into elemental sulfur and hydrogen atoms. The hydrogen atoms migrate across an electrolyte170, as shown by arrows190, toward the negatively charged cathode180, and gaseous diatomic hydrogen is formed. The gaseous hydrogen enters an exhaust channel150of the electrolysis unit100and is then exhausted from the exhaust channel as shown by arrow130. Elemental sulfur exits the electrolysis unit100at the end of the feed channel140, as shown by arrow120. A reaction mechanism that occurs at the anode160is shown in reaction (5) below, and a reaction mechanism that occurs at the cathode180is shown in reaction (6) below:
H2S(g)→2H+1/nSn+2e−(5);
2H++2e−→H2(g)  (6).

To achieve the above electrolysis of H2S, specific anodes160and electrolytes170that facilitate the treatment of feed gases comprising H2S are used. For example, platinum is a common anode catalyst material because it generally has good catalytic activity (i.e., it has good H2S adsorption). However, platinum degrades over time when exposed to feed gases comprising H2S and, thus, is not a preferable anode material for electrolyzer designed to treat H2S. Likewise, many other metal oxides commonly used as anode materials degrade rapidly when exposed to H2S. Thus, in embodiments, other anode materials are used. For instance, in embodiments the anode may comprise a metal sulfide, such as, for example, Li2S/CoS1.35WS2, NiS, MoS2, CoS, VO5, LiCoO2, Pt/TiO2, Pd, Au, Ag, Ru, Rd, Ir, FeMoS, NiMoS, CoMoS and mixtures thereof. In embodiments, Ni-based compounds may be used as anode materials, such as Nb2O5—Ni, BaO—Ni, Ce0.8Sm0.2O2—Ni. In embodiments, thiospinels may be used as anode materials, such as CuFe2S4, CuNi2S4, CuCoS4, NiCo2S4, NiFe2S4, and mixed metal oxides of La, Sr, Mn, Ti, Cr, Ga, Y, V, Fe, Co, Mo, Ce, Mg, Gd, and Ba (e.g. La0.4Sr0.6BO3, where B=Mn, Ti, Cr). Most of the above-mentioned materials have been reported to exhibit electrochemical activity for H2S utilization in both fuel cells and electrolyzers. According to embodiments, the above materials can be used either as is or in mixture with another oxide in the form of ceramic-metallic (cermet) electrode. In embodiments comprising cermet electrodes, oxides that conduct oxygen or protons can be used, such as YSZ, ScSZ, ScYSZ, GDC, CGO, CeO2, TiO2, Nb2O5, SDC, BCY, CZI, BCN, or the like.

Although many electrolytes that transmit H+may be used as the electrolyte170in the electrolysis unit100, in embodiments the electrolyte170is chosen according to its proclivity to treat H2S, such as a resistance to sulfur degradation. In general, perovskite materials of the general type ABO3and ABMO3exhibit proton conductivity at high temperatures (600-1000° C.). In some embodiments, zirconia-based electrolytes are used in the electrolysis unit, such as SrZrO3and NiO-doped BZY. In other embodiments, ceria-based electrolytes are used in the electrolysis unit, such as BaCeO3, SrCeO3or YDC, SDC, BCY, BCN and CZI. In yet other embodiments, the electrolyte may be a solid acid of the general type MHXO4and M3H(XO4)2, where M can be Cs, NH4, Rb, and X can be S or Se. These materials exhibit protonic conductivity at low temperatures, in the range 25 to 300° C. and are known to undergo a “superprotonic” phase transition.

In addition to the above treatment options for H2S, it can be used in solid oxide fuel cells to produce electric energy. Two types of fuel cells may be used with H2S; fuel cells comprising oxygen-conducting solid electrolytes and fuel cells comprising proton-conducting solid electrolytes.

FIG. 2Ashows a solid oxide fuel cell200with a proton conducting solid electrolyte250. The fuel cell200includes a first feed channel230and a second feed channel280. Between the first feed channel230and the second feed channel280are an anode240, a cathode260, and the electrolyte250between, and in electrochemical contact with, the anode240and the cathode260. The anode,240, electrolyte250, and cathode260are constructed so that protons can be transferred from the anode240through the electrolyte250and to the cathode260. The fuel cell200also comprises an electrical connection290between the anode240and the cathode260.

In operation, the solid oxide fuel cell depicted inFIG. 2Ais fed a gas stream210comprising H2S at the first feed channel230and air212is fed to the second feed channel280. At the anode240the H2S is anodically oxidized to H+and elemental sulfur Snas shown in Reaction (7) below:
H2S(g)→2H++1/nSn+2e−(7).

The protons (H+) formed in Reaction (7) are transferred from the anode240through the electrolyte250to the cathode260, as shown by arrows292. The electrons formed by Reaction (7) are transferred by the electrical connection290between the anode240and the cathode260, as shown by arrow270. The Elemental sulfur Sn220formed by Reaction (7) exits the fuel cell via the first feed channel230.

At the cathode260the H+ions are oxidized by the oxygen in the fed air212and water is produced, as shown in Reaction (8) below:
2H++½O2+2e−→H2O(g)  (8).

A mixture222of the gaseous H2O produced by Reaction (8) and the unreacted air exit the fuel cell200at the second feed channel280.

FIG. 2Bshows a solid oxide fuel cell200with an oxygen-ion conducting solid electrolyte250. The fuel cell200includes a first feed channel230and a second feed channel280. Between the first feed channel230and the second feed channel280are an anode240, a cathode260, and the electrolyte250between, and in electrochemical communication with, the anode240and the cathode260. The anode,240, electrolyte250, and cathode260are constructed so that oxygen ions can be transferred from the cathode260through the electrolyte250and to the anode240. The fuel cell200also comprises an electrical connection290between the anode240and the cathode260.

In operation, the solid oxide fuel cell depicted inFIG. 2Bis fed a gas stream210comprising H2S at the first feed channel230and air212is fed to the second feed channel280. At the cathode260oxygen from the air is ionized, as shown in Reaction (9) below:
½O2(g)+2e−→O2−(9).

The oxygen ions formed in Reaction (9) transfer from the cathode260through the electrolyte250to the anode240, as shown by arrows294. Unreacted air222exits the fuel cell200at the second feed channel280.

At the anode240the O2−ions, which were transferred from the cathode260and through the electrolyte240, react with the H2S to form elemental sulfur (Sn), SOx, and H2O, as shown in Reactions (10) and (11) below:
H2S(g)+O2-→H2O+⅛S8+2e−(10);
H2S(g)+3O2-→H2O+SO2+6e−(11).

A mixture222of the gaseous H2O, SOx, and elemental sulfur produced by Reactions (10) and (11) exit the fuel cell200at the first feed channel230. The electrons formed by Reactions (10) and (11) are transferred from the anode240to the cathode260via the electrical connection290, as shown by arrow270.

Although fuel cells using a proton transferring solid oxide electrolyte and an oxygen ion transferring solid oxide electrolyte can both be used, the power efficiency per mole of H2S is higher in the oxygen ion transferring electrolyte because both the hydrogen and the sulfur atoms act as fuel and are electrochemically oxidized to produce electricity. Solid metal sulfide-based catalyst electrodes, such as MoS2or WS2, may be used as electrodes, particularly the anode, in the fuel cells disclosed above.

The above processes are conventional treatments for H2S that split the H2S in a feed stream to form elemental sulfur and hydrogen. As disclosed above, splitting of H2S results in products that can either be reused, or in the formation of electricity in the case of a fuel cell. However, as disclosed above, these processes can be inefficient by requiring large amounts of energy.

Like H2S, CO2is present in many hydrocarbon deposits and is receiving attention as a pollutant. It is expected that CO2will be more heavily regulated in the near future. Accordingly, processes that convert CO2into usable chemicals are desired. One such process is CO2hydrogenation, where CO2is hydrogenated to usable chemicals such as, for example, hydrocarbons, monomers or polymers, and oxygenates. Hydrogenation of CO2to hydrocarbons or alcohols is attractive because it is a potential source of renewable fuels while decreasing CO2emissions.

Many CO2hydrogenation processes use metal catalysts, such as, for example, Pt, Rh, Pd, Ru, Cu, Fe, Co, and Ni. The hydrogenation process takes place in fixed bed reactors where the metal catalysts are supported by metal oxide supports, such as, for example, Nb2O3, ZrO2, Al2O3, and SiO2. The catalytic hydrogenation generally operates at high pressure (such as, for example, from about 5 to about 70 atm, or from about 10 to about 60 atm, or even from about 20 to about 50 atm) to increase the thermodynamic equilibrium conversion to light hydrocarbons or alcohols, such as methanol. The two main reactions that take place during the catalytic hydrogenation are shown in reactions (12) and (13) below:

In the above, Reaction (12) takes place at temperatures from about 300° C. to about 1000° C. and is favored at higher temperatures, and Reaction (13) takes place at lower temperatures, such as from about 200° C. to about 800° C., depending on the desired product (CxHyOz). Reviewing the reaction temperature ranges of Reactions (12) and (13) interestingly shows the importance of using intermediate-temperature electrolysis and fuel cell devices, such as devices that operate from about 500° C. to about 700° C.

Reaction (12) above is a redox reaction that constitutes a reverse water-to-gas shift reaction. Reaction (13) above is a synthesis reaction that leads to the formation of hydrocarbons (such as, for example, methane, ethane, propane, etc.), alcohols (such as methanol, ethanol, propanol, etc.), or both. For example, in Reaction (13), methane is formed when x=1, y=4, and z=0, but methanol is formed when x=1, y=4, and z=1. An advantage of using gaseous catalytic hydrogenation reactions, such as those shown in Reactions (12) and (13), over liquid-based hydrogenation reactions is that they have relatively high reaction rates that are comparable with other industrial processes (i.e., the hydrogenation reactions consistently produce product). Thus, such gaseous hydrogenation reactions can reliably be used to hydrogenate CO2into more useful chemicals, such as hydrocarbons, polymers, and alcohols.

In addition to the chemical hydrogenation of CO2shown above, electrochemical processes can be used in a reduction-conversion reaction with CO2. Such processes can generally convert liquid phase-dissolved CO2into more usable chemical products. In these electrochemical processes CO2is dissolved into solvent, such as water or another primarily aqueous solvent, and is electrochemically reduced at a cathode. Suitable cathode materials include Cu, Ag, Pd, or Rh. However, electrochemical hydrogenation of gaseous CO2may also be used, but has previously been limited to co-electrolysis of CO2and H2O to syngas (CO and H2). Reactions (14) and (15) below show gaseous hydrogenation:
CO2(g)+2e−→CO(g)+O2(14);
H2O(g)+2e−→H2(g)+O2-(15).

In addition to the above electrochemical reduction-conversion reaction, electrochemical processes can be used to promote the catalytic hydrogenation of CO2, which is shown in Reactions (12) and (13). In these processes, a constant current or potential is provided between a working electrode, which may also be acting as a catalyst, and a counter or reference electrode. Such a current or potential causes a migration of promoting species (ionic species accompanied by their mirror charge in the catalyst) from an electrolyte support to a catalyst/gas phase interface. These promoting species promote the catalytic gas phase reaction. For example, in Reactions (12) and (13), the electrochemically produced proton species can promote the catalytic reaction in a reversible and controllable way by promoting the catalytic activity of the catalyst electrode for the hydrogenation reaction.

As discussed above, there are several differing methodologies for converting H2S and CO2into more useful products. However, these methodologies are time and energy intensive and are generally carried out in separate equipment and/or at separate operating conditions. However, it has been found that a common condition may be used to unite the two methodologies in a more efficient and less energy intensive way. As discussed above, CO2conversion generally requires H2to hydrogenate the CO2thereby forming more useful products. The hydrogen source for hydrogenating CO2is generally obtained from splitting H2O. As discussed in embodiments below, combining the H2S treatment—where H2and elemental sulfur are produced with only 17% of the energy needed to split H2O—and the CO2hydrogenation, where H2is required, creates efficiencies for both processes while reducing the total amount of energy and reaction units required to treat H2S and CO2.

The embodiments disclosed below can be used in any industry where treatment of H2S and CO2is desired. For example, in processes where CO2and H2S are produced separately, the two streams can be combined and treated in the various embodiments disclosed below, or the two streams may be fed to different portion of an electrolyzer, as disclosed in other embodiments below. However, embodiments are particularly useful in the oil and gas processing industries where high levels of CO2and H2S are produced, and where sour gas, which naturally comprises both CO2and H2S, is refined. For instance, an oil refinery may produce about 700 Mt/yr CO2and 70 Mt/yr H2S, which are currently converted at about 4%. Embodiments disclosed herein can increase that conversion percentage and undertake the conversions using less energy and reaction units.

Various embodiments disclosed herein comprise electrochemically splitting H2S in high temperature proton conducting solid oxide electrolyzers for in situ, parallel conversion of CO2over a catalytic cathode. Further details of embodiments will be disclosed with reference to the figures below.

With reference toFIG. 3, embodiments include an electrolyzer300comprising a housing301. The housing comprises inlets350and360and outlets355and365. In embodiments, the electrolyzer300comprises an anode320, a proton-conducting electrolyte340, and a cathode330encased within the housing301. Electric current may be applied to the electrolyzer300by a current source310via an electrical connection315.

In the embodiment shown inFIG. 3, a gaseous feed stream comprising H2S and gaseous feed stream comprising CO2are fed to the electrolyzer300in separate feed streams. The feed stream comprising gaseous H2S is fed to the anode320of the electrolyzer300through the inlet350in the housing301. The gaseous H2S is split at the anode320into elemental sulfur (also referred to herein as Sn, where n=1, 2, 6, or 8) and H+, as shown in Reaction (5) above. The elemental sulfur exits the electrolyzer300as an exhaust gas stream at outlet355in the housing301. Simultaneously, the feed stream comprising gaseous CO2is fed to the cathode330of the electrolyzer300through the inlet360in the housing301. The H+ions, which are generated at the anode320, are transferred through the proton-conducting electrolyte340, as indicated by arrow370, to the cathode's330three phase boundary (tpb) comprising the cathode330, the proton-conducting electrolyte340, and the gaseous phase (CO2and corresponding reactants). The H+ions react at the three phase boundary with CO2adsorbates present at the proximity of the three phase boundary to form various chemicals (such as methane and methanol), as shown in Reaction (16) below:

The CxHyOzcomponent then exits the electrolyzer300from the outlet365as an exhaust gas stream at outlet365in the housing301, where it can be collected for further use. The above Reaction (16) can take place at temperatures from about 200° C. to about 800° C. depending on the desired product. However, in embodiments, temperatures below 700° C. are preferable. The process pressure of Reaction (16) can vary from about 1 atm to about 70 atm, which is similar to the process pressure in methanol synthesis and Fischer-Tropsch reactors. In embodiments, equal pressure is applied to the anode side, and the feed flow is adjusted according to the activity of the catalyst given the desired conversion and reactor size. However, in embodiments, space velocities in the range from about 500 h−1to about 30,000 h−1can be used.

In some embodiments, in addition to, or as an alternative of, Reaction (16), the H+ions react with one another to form gaseous H2that may participate in the catalytic hydrogenation of CO2, which is shown in Reactions (12) and (13).

The ratio of catalytic hydrogenation by gaseous H2(as shown in Reactions (12) and (13)) and electrocatalytic hydrogenation by H+(as shown in Reaction (16)) is a function of process parameters, such as pressure, temperature, feed flow, etc. For example, H2evolution at the cathode is suppressed at high pressures and low temperatures and thus atomic hydrogen coverage of the cathode catalyst electrode is higher, which can result in higher CO2hydrogenation rates. However, the ratio of catalytic/electrocatalytic hydrogenation may also be affected by the catalyst electrode properties, such as CO2surface dissociation/activation ability. Accordingly, in embodiments, selecting the appropriate cathode material can be important. In some embodiments, typical cathode materials, such as Rh, Ru, Cu, Fe, Co, Pd, Pt, Ni can be used either as metal porous electrodes or as cermet electrodes when mixed with a ceramic electrolyte support (oxygen or proton conductor), like YSZ, ScSZ, ScYSZ, GDC, CGO, CeO2, TiO2, Nb2O5, SDC, BCY, BZY, CZI, BCN, etc, due to their well known activity in CO2hydrogenation.

Water present at the cathode330, such as the H2O formed by Reactions (12), (13), and (16), will facilitate proton transfer370across the electrolyte340. In some embodiments the generation of water will not be sufficient to facilitate proton transfer370across the electrolyte340. In such cases, humidified CO2may be fed to the electrolyzer300. Using water from humidified CO2to facilitate transfer of protons370across the electrolyte340may increase the efficiency of the process because an in-line dehumidifier will not be required. In embodiments, the feed stream comprising CO2may comprise from about 2% to about 15% gaseous H2O, such as from about 3% to about 10% gaseous H2O, or even from about 5% to about 8% gaseous H2O. The exact amount of H2O in the humidified CO2feed depends on the electrolyte material340and the current applied to the anode320and the cathode330. Like the anode320, in embodiments, the electrolyte also must be able to tolerate sulfur exposure without significant degradation. Accordingly, in embodiments, the electrolyte340may include perovskite materials of the general type ABO3and ABMO3that exhibit proton conductivity at high temperatures (600-1000° C.), zirconia- and ceria-based proton conducting electrolytes, like SrZrO3, NiO-doped BZY, BaCeO3, SrCeO3or others like YDC, SDC, BCY, BCN and CZI. In yet other embodiments, the electrolyte may be a solid acid of the general type MHXO4and M3H(XO4)2, where M can be Cs, NH4, Rb, and X can be S or Se. These materials exhibit protonic conductivity at low temperatures, in the range 25 to 300° C. and are known to undergo a “superprotonic” phase transition.

In some embodiments, the electrolyte340may not require H2O to facilitate the transfer of protons370across the electrolyte340. Like the anode320, in embodiments, the electrolyte also must be able to tolerate sulfur exposure without significant degradation. For example, Ni-doped BZY does not require H2O at all, and the above categories of proton conducting electrolytes exhibit an adequate sulfur tolerance.

In some embodiments, the electrolyzer300is used downstream of a CO2capture unit where high purity CO2is available to feed to the cathode330of the electrolyzer300. Although the embodiment shown inFIG. 3is discussed above as a solid oxide electrolyte electrolysis unit, it should be understood that in other embodiments, other proton conducting membrane electrolyzers may be used (such as CsHSO4) as long as the anode and electrolyte have sufficient sulfur tolerance, and the cathode is capable of CO2activation.

FIG. 4AandFIG. 4Bschematically depict embodiments where H2S and CO2are co-fed to an electrolyzer and a fuel cell respectively. In the embodiments disclosed inFIG. 4AandFIG. 4B, CO assists in removing the hydrogen from H2S. InFIG. 4A, the electrolyzer400comprises an anode411, a cathode413, and an electrolyte between, and in electrochemical communication with, the anode411and the cathode413. Electrical current is provided by an electrical source420and fed to the electrolyzer, such as fed to the anode, via an electrical connection421.

A feed stream comprising H2S440is mixed with a stream comprising CO447and the mixture is fed to the anode411of the electrolyzer400. In some embodiments, the anode411may comprise a metal sulfide catalyst. In embodiments, the metal sulfide catalyst-anode may be of a group of metal sulfides exhibiting high activity for the Reaction (17) and also low overpotential for hydrogen oxidation (reverse Reaction (6)), like Co9S8, NiS, FeS, MnS, Cr2S3, ZnS, MoS2, Cu2S, V3S4, Ti5S4, WS2, or thiospinels like: CuFe2S4, CuNi2S4, CuCoS4, NiCo2S4, NiFe2S4or mixtures thereof. These materials can be used either as is or in mixture with another oxide in the form of ceramic-metallic (cermet) electrode. Oxides that conduct oxygen or protons can be used such as YSZ, ScSZ, ScYSZ, GDC, CGO, CeO2, TiO2, Nb2O5, SDC, BCY, CZI, BCN, etc. At the anode, H2S in the mixture reacts with CO in the mixture to form carbonyl sulfide (COS) and hydrogen, as shown in Reaction (17) below:
H2S+CO→H2+COS  (17).

In the embodiment shown inFIG. 4A, the COS exits the anode411of the electrolyzer as exhaust gas stream445. The COS is then fed to a decomposer410where the sulfur is separated from the CO in the COS at high temperatures in the range from about 600° C. to about 1000° C. and pressure from about 1 atm to about 50 atm. The decomposition reaction can be easily integrated thermally with the electrochemical device (electrolyzer or fuel cell) since they operate at a similar temperature range. Sulfur is released as elemental sulfur (Sii)450from the decomposer410and CO exits the decomposer as feed stream447and is recycled by being combined with the H2S feed440. The decomposition of COS into CO and Snproceeds by Reaction (18) below:
COS→CO+1/nSn(18).

By supplying an electrical current to the electrolyzer400, H2will be oxidized at the anode411and transferred as protons (H+) across the electrolyte412to the cathode413where molecular or gaseous hydrogen will be produced. In some embodiments, there is no additional feed to the electrolyzer (i.e., feed430is not present) and the molecular or gaseous hydrogen will be released from the electrolyzer400as outlet feed435.

In other embodiments, CO2may be fed to the cathode413as feed stream430. In these embodiments, the H+that is formed at the anode411and transferred across the electrolyte412will react at the triple phase boundary of the gaseous phase, the cathode413, and the electrolyte, as shown in Reaction (16) above. Through Reaction (16), CxHyOzis produced at the cathode and exits the electrolyzer as exhaust stream435.

In the embodiment shown inFIG. 4B, electricity is produced by feeding oxygen or air as feed stream430to the electrode416. In such embodiments, the hydrogen that is formed at the electrode415and transferred across the electrolyte412undergoes electrochemical reduction at the electrode416. Although electrode415and electrode416may have the same compositional makeup as anode411and cathode413of the embodiments shown inFIG. 4A, because hydrogen is reduced at electrode416, in the embodiment shown electrode416is referred as an cathode and electrode415is referred to as a anode when referring to the embodiment shown inFIG. 4B.

InFIG. 4Aof these embodiments, a feed stream comprising H2S440is mixed with a feed stream comprising CO447and the mixture is fed to the anode415of the electrolyzer400. In some embodiments, the anode411may comprise a metal sulfide catalyst. In embodiments, the metal sulfide catalyst may be of a group of metal sulfides exhibiting high activity for the Reaction (17) and also low overpotential for hydrogen oxidation (reverse Reaction (6)), like Co9S8, NiS, FeS, MnS, Cr2S3, ZnS, MoS2, Cu2S, V3S4, Ti5S4, WS2or thiospinels like: CuFe2S4, CuNi2S4, CuCoS4, NiCo2S4, NiFe2S4or mixtures thereof. In some embodiments, the cathode413comprises a catalyst, such as Pt, Pd, Ru, Rh, Ni, Cu, Fe, Co or other metals well known for hydrogen evolution in electrolyzers literature either as metal porous electrodes or as cermet electrodes when mixed with a ceramic electrolyte support (oxygen or proton conductor), like YSZ, ScSZ, ScYSZ, GDC, CGO, CeO2, TiO2, Nb2O5, SDC, BCY, BZY, CZI, BCN, etc, or even a perovskite electrode. At the anode411, H2S in the feed stream reacts with CO in the feed stream to form carbonyl sulfide (COS) and hydrogen, as shown in Reaction (17) above. In embodiments, the COS exits the anode411of the electrolyzer as outlet stream445. The COS is then fed to a decomposer410where the elemental sulfur is separated from the CO in the COS at elevated temperatures. The elemental sulfur is released as elemental sulfur (Sn)450from the decomposer410and CO exits the decomposer as outlet stream447and is recycled by being combined with the feed stream comprising H2S440. The decomposition of COS into CO and Snproceeds by reaction (18) above.

In embodiments, the hydrogen that is formed at the anode415and transferred across the electrolyte412is oxidized by the oxygen or air that is introduced to the cathode416as feed stream430, and H2O is released as exhaust stream435. In such embodiments, the electrolyzer400operates as a fuel cell and produces electricity by oxidizing the hydrogen is produced from the H2S—CO cycle present at the anode. The electricity generated by oxidizing the hydrogen exits the fuel cell401via electrical connection421and is sent to an electrical device420.

In either of the embodiments shown inFIG. 4AorFIG. 4B, removal of H2from the electrolyzer400or the fuel cell401during the H2S—CO cycle has a synergistic effect on system performance. Particularly, removing the H2improves the extent of the H2S—CO reaction toward higher conversions (i.e., higher H2production) and it will prevent the H2from reacting with the CO in outlet stream447to form methane and water, which if formed can poison the catalyst present at the anodes411or415. The embodiments shown inFIG. 4AandFIG. 4Bmay also avoid having elemental sulfur deposit on the anodes411or415, which is costly to remove.

In other embodiments, the electrolyzer shown inFIG. 4Acan be used to generate high pressure hydrogen. When the anode411potential is higher than the minimum required potential, hydrogen is not only pumped from the anode411, but it can also be generated at high pressures at the cathode413according to the Nernst equation, which is E=E°+(RT/nF)ln(PH2,cathode/PH2,anode), where E is the applied potential, E° is the standard cell potential (E°=0V in his case), R is the universal gas constant, T is the absolute temperature, F is the Faraday constant, n is the number of electrons transferred in the cell half-reaction, PH2,cathodeis the partial pressure of hydrogen at the cathode, and PH2,anodeis the partial pressure of hydrogen at the anode. In embodiments where hydrogen is generated at the cathode413, no feed430to the cathode is required. The concentration of hydrogen at the anode411may be controlled by the equilibrium of the reaction between H2S and CO. However, the partial pressure of hydrogen at the cathode is dependent on the applied electrode potential. The higher of the applied electrode potential, the higher the pressure of hydrogen at the cathode is. High pressure hydrogen is desirable for use in further processes. Potential values up to the reduction potential of the used electrolyte (Ered=2.3V for YSZ) at the operation temperature can be applied regardless the system pressure, while the generated hydrogen partial pressure is limited to the operation pressure of the system. For example, in embodiments, at 900° K and atmospheric pressure operation and 20% conversion of H2S, 20 kPa hydrogen exists at the anode. The generation of 20 kPa at the cathode is spontaneous and thus no potential difference is needed. However, the generation of 100 kPa hydrogen at the cathode would theoretically require the application of 0.062V potential difference at the cell according to Nernst equation.

Embodiments may also include introducing a hydrogen source into an electrolyzer at the cathode. Referring now to the embodiment shown inFIG. 5, an electrolyzer500includes a housing501having inlet channels560and570and outlet channels565and575. Positioned within the housing are an anode520, a cathode530, and an electrolyte540positioned between, and in electrochemical contact with, the anode520and the cathode530. In the embodiments shown inFIG. 5, a feed stream comprising H2S is fed to the anode520of the electrolyzer500via inlet channel560in the housing501. The H2S is split into elemental sulfur and hydrogen as shown in Reaction (5) above. The elemental sulfur exits the electrolyzer500at outlet channel565in the housing501. An electrical current is provided by current source510to the anode by electrical connection515. The electrical current is provided to the anode520and disassociates hydrogen into H+ions that are transferred across the electrolyte540to the cathode530, as shown by arrow550. Minimum potential required for H2S electrolysis is 0.2 V. A feed stream comprising a mixture of CO2and H2is fed to the cathode530of the electrolyzer500via inlet channel570providing an additional hydrogen source for the hydrogenation of CO2. The H+ions formed at the anode520and transferred across the electrolyte540form promoting species on the surface of cathode530that promote hydrogenation of CO2with the hydrogen supplied with the feed at inlet channel570. By forming the promoting species, the CO2hydrogenation rate can be significantly improved. For example, it has been found that faradaic efficiency values of about 900 times higher than the value without promoting species, such as about 950 times higher than the value without promoting species, or even about 1000 times higher than the value without promoting species have been realized using the promoting species. Hydrogen can also be fed to the anode in the embodiments shown inFIG. 4A, which is described in more detail above. In this instance, the minimum applied potential in the electrolyzer is 0 V.

FIG. 7Ais an embodiment showing an electrolyzer where CO2is used to aid in the sulfur removal. In the embodiment ofFIG. 7A, an electrolyzer700is shown including a housing701having inlet channels760and770and outlet channels765and775. Within the housing are an anode720, a cathode730, and an electrolyte740positioned between, and in electrical contact with, the anode720and the cathode730. As discussed in further detail below, a feed stream comprising CO2is introduced at inlet channel760to aid in the removal of sulfur.

When oxygen-ion solid electrolyte membranes are used in H2S-powered fuel cells, two mechanisms for sulfur removal from the anode720surface are used: 1) the sulfur electrochemical oxidation to gaseous SOx; and 2) the formation of elemental sulfur. The formation of elemental sulfur is common in electrochemical cells equipped with proton (H+) conducting membranes, and SOxis not formed. However, under high current densities and H2S concentrations, a significant amount of elemental sulfur (Sn) is produced at the anode, which creates a situation where sulfur removal is desired. In embodiments, sulfur may be removed by introducing a feed stream comprising CO2to function as an oxidant at the anode720, which facilitates the removal of sulfur species by forming SO2, as shown in Reaction (19) below:
2CO2(g)+S(g)→SO2(g)+2CO(g)  (19).

As shown inFIG. 7A, a feed stream comprising a mixture of CO2and H2S is fed to the anode720of the electrolyzer700via inlet channel760. H2S is electrolyzed to hydrogen and sulfur, while the gaseous CO2and sulfur react according to Reaction (19) to form CO and SO2, which exit the electrolyzer700at outlet channel765. As shown inFIG. 8AandFIG. 8B, where the enthalpy and Gibbs free energy of Reaction (19) are shown as a function of temperature, Reaction (19) appears to be spontaneous (as indicated by Gibbs free energy below 0) and slightly exothermic (as shown by enthalpy below 0). Thus, it is very likely that at high temperatures, CO2will dissociate to gaseous CO and atomic oxygen adsorbed on the anode surface that will oxidize sulfur to gaseous SO2. In embodiments, the temperatures for dissociating CO2are from about 250 to about 1300° K, such as from about 500 to about 1000° K, or even from about 600 to about 800° K.

In further embodiments, the hydrogen H2dissociated from the H2S will be disassociated into H+ions when an electrical current is applied to the anode720. The electrical current may be provided via an electrical device710and an electrical connection715. The H+ions will transfer across the electrolyte740, as indicated by arrow750, to the cathode730. In the embodiments shown inFIG. 7A, a feed stream comprising CO2is fed to the cathode730of the electrolyzer700via inlet channel770. The CO2reacts with the hydrogen according to Reaction (16) at the three phase boundary to form CxHyOzthat exits the electrolyzer700at outlet channel775.

FIG. 7Bshows an embodiment comprising a fuel cell using CO2to aid removal of sulfur. In the embodiment ofFIG. 7A, an electrolyzer700is shown including a housing701having inlet channels760and770and outlet channel775. Within the housing are an anode720, a cathode730, and an electrolyte740positioned between, and in electrochemical communication with, the anode720and the cathode730. As discussed above with reference toFIG. 7A, a feed stream comprising CO2is fed to the anode720of the fuel cell700via inlet channel760to aid in the removal of sulfur.

In the embodiment shown inFIG. 7B, a feed stream comprising a mixture of CO2and H2S is fed to the anode720of the electrolyzer700via inlet channel760. The gaseous CO2and sulfur react according to Reaction (19) to form CO and SO2, which exit the electrolyzer700at outlet channel765.

In embodiments, the hydrogen dissociated from the H2S will transfer across the electrolyte740, as indicated by arrow750, to the cathode730. In the embodiments shown inFIG. 7B, O2or air is fed via inlet channel770that reacts with H+at the cathode and water is formed which exits the electrolyzer700at outlet channel775, and electricity is generated in the fuel cell from the transfer of protons across the electrolyte and sent from the fuel cell to an electrical device710via an electrical connection715.

EXAMPLES

Embodiments will be further clarified by the following example and comparative example.

Example

In this example the electrical power required to hydrogenate a maximum possible amount of CO2to CH4is calculated. For this example, an electrochemical reactor as shown inFIG. 5is integrated into a refinery or other gas plant that has the following specifications: H2S mass flow rate of 1 ton/h; CO2mass flow rate of 1 ton/h; and the required H2S conversion is 100%. From the above, it is estimated (from stoichiometric calculations) that from each ton of H2S, 0.94 ton/h of elemental sulfur and 0.059 ton/h of H2can be generated by electrochemical splitting by aiming for 100% H2S elimination.

The above requires 2×0.059=0.118 ton H+/h or 16.3 mol H+/s through the proton conducting electrolyte membrane, which is equivalent to a 1573 kA current and 3.3 MW power, assuming that the electrolysis unit operates at about 2.1 V (this assumption is based on about 0.2 V anode overpotential and 1.9 V cathode overpotential (according to H2S electrochemical oxidation and CO2electrochemical reduction studies), and the Faradaic efficiency reaches 100%).

Using the above numbers and assuming that all H+are used to hydrogenate CO2and not to form H2gas, 0.117 ton/h CH4and 0.53 ton/h H2O are produced, which corresponds to about 32% conversion of CO2in the total CO2fed to the reactor. If the desired product is CH3OH rather than CH4, the same amount of power (or H+species) produces 0.31 ton/h CH3OH and 0.18 ton/h H2O, which corresponds to CO2conversion of about 43%.

This example shows an estimation of power consumption for the electrochemical system described above and described with respect toFIG. 5. The example also shows the importance of product selection as it is estimated that over 10% more CO2is converted when methanol is the desired product as opposed to methane. This is believed to be because CH3OH formation requires less H+than CH4formation, which combined with the higher heating value of CH3OH than CH4(4 kJ/mol versus 0.75 kJ/mol, respectively) can significantly benefit the economics of the process.

Comparative Example

This Comparative Example shows electrolysis using H2O in place of H2S. With reference now toFIG. 6, an electrolyzer600includes a housing601having inlet channels660and670and outlet channels665and675. Positioned within the housing are an anode620, an electrolyte640, and a cathode630. In the embodiments shown inFIG. 6, H2O is fed to the anode620of the electrolyzer600via inlet channel660in the housing601. The H2O is split into elemental oxygen and hydrogen as shown in Reaction (20) below:
H2O(g)→2H++½O2(g)+2e−(20).

The elemental oxygen exits the electrolyzer600at outlet channel665in the housing601. An electrical current is provided by current source610to the anode by electrical connection615. The electrical current is provided to the anode620and disassociates water into H+ ions that are transferred across the electrolyte640to the cathode630, as shown by arrow650. CO2is fed to the cathode630of the electrolyzer600via inlet channel670. At the cathode, CO2hydrogenation can occur electrochemically, as shown in Reaction (16) or catalytically, as shown in Reactions (9) and (10). The method of this comparative example produces pure O2as reaction product, which can be used in many different oxidation reactions. However, power demand for H2O electrolysis is estimated to be about 1.94 MW (i.e., about 6 times higher than that of H2S electrolysis), since the water reduction potential is 1.23 V vs. 0.2 V for H2S at ambient conditions.

Thus, various embodiments of methods for co-processing CO2and H2S have been described. In the methods, a feed stream comprising H2S is fed to an anode side of an electrolyzer so that the H2S is split into hydrogen and elemental sulfur. The hydrogen may then be transferred across an electrolyte to a cathode. A gas stream comprising CO2is fed at a cathode side of the electrolyzer. The CO2is hydrogenated by the hydrogen that is transferred across the electrolyte and is hydrogenated into a more useful chemical product, such as methane or methanol. In various embodiments, additional components may be fed to the electrolyzer with the feed streams comprising H2S and CO2. For example, in some embodiments CO may be fed to the anode side of the electrolyzer with the gas stream comprising H2S so that the H2S reacts with the CO at the anode side of the electrolyzer to form COS and H2. In some embodiments, an additional hydrogen source may be fed to the cathode side of the electrolyzer with the CO2to aid in the hydrogenation of the CO2. Unlike with conventional processes that split H2S and hydrogenate CO2separately, co-processing H2S and CO2creates efficiencies, such as reducing the number of units required to process H2S and CO2, by removing the adsorbed sulfur on the anode to the gas phase as SOx, and by using less energy than processes that treat H2S and CO2separately.