Electrochemical process for the production of synthesis gas using atmospheric air and water

Abstract

A process is provided for synthesizing synthesis gas from carbon dioxide obtained from atmospheric air or other available carbon dioxide source and water using a sodium-conducting electrochemical cell. Synthesis gas is also produced by the coelectrolysis of carbon dioxide and steam in a solid oxide fuel cell or solid oxide electrolytic cell. The synthesis gas produced may then be further processed and eventually converted into a liquid fuel suitable for transportation or other applications.

Claims

1. An electrochemical process for producing synthesis gas comprising: decomposing water within an anode chamber comprising an electrochemically active anode according to the following reaction: H.sub.2O.fwdarw.O.sub.2+H.sup.++e.sup.; removing oxygen from the anode chamber; transporting H.sup.+ ions from the anode chamber to an intermediate chamber via a cation exchange membrane; reacting Na.sub.2CO.sub.3 and H.sup.+ ions within the intermediate chamber according to the following reaction: H.sup.++Na.sub.2CO.sub.3.fwdarw.CO.sub.2+H.sub.2O+Na.sup.+; removing CO.sub.2 from the intermediate chamber; transporting Na.sup.+ ions from the intermediate chamber to a cathode chamber; decomposing water within the cathode chamber comprising an electrochemically active cathode according to the following reaction: Na.sup.++H.sub.2O+e.sup..fwdarw.NaOH+H.sub.2; removing H.sub.2 from the cathode chamber; removing NaOH from the cathode chamber; and reacting CO.sub.2 and H.sub.2 to form synthesis gas comprising CO and H.sub.2.

2. The electrochemical process according to claim 1, wherein the NaOH removed from the cathode is reacted with a source of CO.sub.2 to form Na.sub.2CO.sub.3.

3. The electrochemical process according to claim 2, wherein the source of CO.sub.2 is selected from atmospheric air, combustion gases, or aerobic decomposition gases.

4. An electrochemical process for producing synthesis gas comprising: decomposing water within an anode chamber according to the following reaction: H.sub.2.fwdarw.O.sub.2+H.sup.++e.sup.; removing oxygen from the anode chamber; reacting Na.sub.2CO.sub.3 and H.sup.+ ions within anode chamber according to the following reaction: H.sup.++Na.sub.2CO.sub.3.fwdarw.CO.sub.2+H.sub.2O+Na.sup.+ at a location a distance away from the anode but within the anode chamber, wherein the H+ ions react with Na.sub.2CO the after passing through a cation exchange membrane; removing CO.sub.2 from the anode chamber at a location near where it is produced; transporting Na.sup.+ ions from the anode chamber to a cathode chamber; decomposing water within the cathode chamber according to the following reaction: Na.sup.++H.sub.2O+e.sup..fwdarw.NaOH+H.sub.2; removing H.sub.2 from the cathode chamber; removing NaOH from the cathode chamber; and reacting CO.sub.2 and H.sub.2 to form synthesis gas comprising CO and H.sub.2.

5. The electrochemical process according to claim 4, wherein the NaOH removed from the cathode is reacted with a source of CO.sub.2 to form Na.sub.2CO.sub.3.

6. The electrochemical process according to claim 5, wherein the source of CO.sub.2 is selected from atmospheric air, hydrocarbon combustion gases, or aerobic decomposition gases.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

(2) FIG. 1 is a schematic view of a two-compartment electrochemical cell for generating synthesis gas within the scope of the present invention;

(3) FIG. 2 is a schematic view of an electrochemical cell for generating synthesis gas showing recycle of NaOH and regeneration of NaCO.sub.3 and capture and separation of CO.sub.2;

(4) FIG. 3 is a schematic view of an oxygen ion conducting electrolysis cell useful in generating synthesis gas;

(5) FIG. 4 is a schematic view of a three-compartment electrochemical cell for generating synthesis gas within the scope of the present invention;

(6) FIG. 5 is a schematic view of a synthesis gas generating system which produces synthesis gas from the coelectrolysis of steam and carbon dioxide; and

(7) FIG. 6 is a schematic view of another oxygen ion conducting electrolysis cell useful in generating synthesis gas.

DETAILED DESCRIPTION OF THE INVENTION

(8) The presently preferred embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the processes, methods, and systems for the production of synthesis gas for fuel or other applications of the present invention, as represented in FIGS. 1 through 6, is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention.

(9) In a first embodiment of the processes of the present invention, a process is disclosed for producing synthesis gas, also known as syngas, comprising a mixture of CO and H.sub.2. This process 10 is illustrated schematically in FIG. 1. The process 10 utilizes a two-compartment electrochemical cell 12. The electrochemical cell includes an anode compartment or chamber 14 and a cathode compartment or chamber 16. A sodium-conducting membrane 18 separates the anode chamber 14 from the cathode chamber 16. Suitable sodium-conducting membranes 18 include NaSICON (sodium (Na) Super Ionic CONductor) ceramic materials, as well as other ceramic and polymer cation exchange membranes known to persons skilled in the art.

(10) The anode chamber 14 includes an electrochemically active anode 20 and a source of water in which the water (H.sub.2O) is decomposed according to the following reaction:
H.sub.2O.fwdarw.O.sub.2+H.sup.++e.sup.(1)
The O.sub.2 is collected and removed from the anode chamber 14.

(11) In this process 10, sodium carbonate (Na.sub.2CO.sub.3) is decomposed in the anode chamber 20 of the electrochemical cell 12 to produce carbon dioxide (CO.sub.2) according to the following reaction:
H.sup.++Na.sub.2CO.sub.3.fwdarw.CO.sub.2+H.sub.2O+Na.sup.+(2)
The CO.sub.2 is collected and removed from the anode chamber 14. Sodium ions are transferred across the sodium-conducting membrane 18. A source of sodium carbonate is provided sufficiently close to the anode 14 so that the sodium carbonate reacts with H.sup.+ ions in solution. It may be preferred to space the sodium carbonate a sufficient distance away from the anode to facilitate separate removal of the O.sub.2 produced at the anode 14 and the CO.sub.2 produced by the decomposition of sodium carbonate. Sodium carbonate may be directly provided for use in the cell 12, or may alternatively be readily obtained by the reaction of sodium hydroxide with a carbon dioxide source according to the following reaction:
NaOH+CO.sub.2.fwdarw.Na.sub.2CO.sub.3+H.sub.2O(3)
Examples of typical carbon dioxide sources include, but are not limited to, atmospheric air, combustion gases, or aerobic decomposition gases. Aerobic decomposition gases include gases naturally produced upon decomposition of various organic materials, including waste materials.

(12) The cathode chamber 16 includes an electrochemically active cathode 22 and a source of water in which the water is reduced according to the following reaction:
Na.sup.++H.sub.2O+e.sup..fwdarw.NaOH+H.sub.2(4)
The H.sub.2 is collected and removed from the cathode chamber. The NaOH may optionally be collected and removed from the cathode chamber, as shown by NaOH stream 24 in FIG. 2. As mentioned above, sodium hydroxide may be reacted with a source of carbon dioxide 26 to form sodium carbonate which may be recycled to the anode chamber as a sodium carbonate source 28, shown in FIG. 2.

(13) By applying an electrical potential across the two-compartment cell 12, water and sodium carbonate are decomposed and produce a flow 30 of oxygen and a flow 32 of carbon dioxide. The reduction of water at the cathode 22 generates hydroxyl ions and a flow 34 of hydrogen gas. As the sodium ions migrate through the membrane 18 from the anolyte side (anode chamber 14) of the cell 12 to the catholyte side (cathode chamber 16), they will combine with the hydroxyl ions produced by the reduction of water to form sodium hydroxide solution as shown in reaction (4).

(14) During operation of the electrochemical cell 12, oxygen and carbon dioxide may be collected as separate gas flows 30, 32 or they may be mixed gases 36, shown in FIG. 2. If mixed, the oxygen is preferably separated from the carbon dioxide in a carbon dioxide separator 38 so that the carbon dioxide may be used in subsequent process steps. Carbon dioxide separation in carbon dioxide separator 38 may be conducted using various processes known to those of ordinary skill in the art. Such processes include, but are not limited to, use of activated carbon or molecular sieves to absorb CO.sub.2 from the gases removed from the cathode 14. Such techniques often utilize multiple absorption beds which are rotated such that those which are full are rotated out of the gas stream and stripped prior to continued use. Other methods include filtration of the gas stream 36 using a polymeric membrane selectively permeable by either O.sub.2 or CO.sub.2, or using CO.sub.2-absorbent fluids such as amine solutions presented to the gas stream 36 as in a scrubber unit. Oxygen may be separated from carbon dioxide by exposing the mixed CO.sub.2 and O.sub.2 gas stream to an oxygen-conducting membrane such as yttria-stabilized zirconia (or YSZ) to extract the oxygen. Other methods of separating CO.sub.2 and O.sub.2 are well known to those of ordinary skill in the art and may be implemented within the scope of the invention.

(15) The cathode chamber 16 of this embodiment of the electrochemical cell 12 contains water and sodium hydroxide as the catholyte. In the cathode chamber 16, water is reduced in the presence of sodium ions to release a flow of hydrogen gas 34 and form sodium hydroxyide. Thus, the reactions conducted in the electrochemical cell 12 may be represented as follows: Anode H.sub.2O.fwdarw.O.sub.2+2 H.sup.++2e.sup. 2H.sup.++Na.sub.2CO.sub.3.fwdarw.CO.sub.2+H.sub.2O+2 Na.sup.+ Na.sub.2CO.sub.3 (aq).fwdarw.CO.sub.2+O.sub.2+2 Na.sup.++2e.sup.(overall) Cathode 2Na.sup.++2H.sub.2O+2 e.sup..fwdarw.2NaOH+H.sub.2 Overall Na.sub.2CO.sub.3+2H.sub.2O.fwdarw.2NaOH+CO.sub.2+H.sub.2+O.sub.2

(16) The CO.sub.2 flow 32 from the anode chamber 14 and the H.sub.2 flow 34 from the cathode chamber 16 may be combined and further processed to convert the CO.sub.2 into CO to form syngas 40. CO.sub.2 and H.sub.2 may react to form the components of the syngas mixture 40 (CO+H.sub.2). Syngas may be obtained from CO.sub.2 and H.sub.2 according to one or more of the following reactions:
CO.sub.2+2e.sup..fwdarw.CO+O.sup.2(5)
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O(6)

(17) It should be noted that such syngas mixtures 40 may also include amounts of CO.sub.2 and H.sub.2O. Means may be provided to facilitate the reaction of CO.sub.2 and H.sub.2 to form synthesis gas. Such means may include a catalytic reactor 42 in which a suitable catalyst is exposed to the mixture of CO.sub.2 and H.sub.2, as shown in FIG. 2 For example, the catalyst may be a watergas shift catalyst, also known as a shift catalyst or reverse shift catalyst. The catalyst may comprise a Fischer-Tropsch catalyst. The CO.sub.2 and H.sub.2 gases may be heated to facilitate a homogenous gas phase equilibrium reaction. Other means to facilitate the reaction of CO.sub.2 and H.sub.2 to form synthesis gas may comprise an oxygen ion conducting electrolysis cell 44 to cause electrolysis of CO.sub.2 and form CO and O.sub.2. The CO is collected, and with H.sub.2, forms synthesis gas.

(18) A schematic representation of an oxygen ion conducting electrolysis cell 44 is shown in FIG. 3. Cell 44 includes an oxygen ion conducting membrane 46. A variety of materials may be used to fabricate the oxygen ion conducting, including but not limited to, yttria-stabilized zirconia (YSZ). The electrochemical device for the electrolysis of carbon dioxide to product carbon monoxide may also include a source of carbon dioxide in contact with a cathode 48. The cathode 48 is attached to a surface of the oxygen ion conducting membrane 46 which is electrochemically active to reduce carbon dioxide to form carbon monoxide and oxygen ions according to reaction (5), above. CO is collected and recovered at the cathode the oxygen ions are conducted through the oxygen ion conducting membrane to the and anode 50. The anode 50 is attached to an opposite surface of the oxygen ion conducting membrane 42 which is electrochemically active for recombination of oxygen ions into oxygen molecules, according to the following reaction:
O.sup.2.fwdarw.O.sub.2+2e.sup.(7)

(19) The cathode 48 and anode 50 should be sufficiently permeable to allow diffusion of carbon dioxide, oxygen, or other gaseous species that may react or be produced at the interface of the oxygen ion conductor 46 and the cathode 48 and the interface of the oxygen ion conductor 46 and the anode 50. By applying an electrical potential across the oxygen ion conducting electrolysis cell 44, carbon dioxide is reduced to form carbon monoxide and oxygen. It produces a flow 52 of carbon monoxide and a flow 54 of oxygen. The reduction of carbon dioxide at the cathode 48 generates oxygen ions and the flow 52 of carbon monoxide. As the oxygen ions migrate through the membrane 46 from the cathode 48 to the anode 50 they will combine to form oxygen as shown in reaction (7), above.

(20) The materials and configurations used for the cathode 48 and anode 50 may be the same or similar to those used in connection with the cathode and anode described below in connection with FIG. 6.

(21) This syngas mixture 40 can be further processed to produce liquid hydrocarbon fuel using any suitable process available in the art. One such process commonly used to convert syngas 40 to liquid fuel is the Fischer-Tropsch process, in which syngas 40 is reacted in the presence of a catalyst (such as an iron or cobalt catalyst) to produce liquid hydrocarbon fuels. Other suitable processes, including variations on the Fischer-Tropsch process, are known to those of ordinary skill in the art, and could be used with the processes of the present invention. Here the term liquid hydrocarbon fuel also includes lighter hydrocarbons such as methane, ethane, propane, butane, etc. which may be vapors at ambient conditions but which also may be liquefied under pressure or cryogentic conditions. A typical Fischer-Tropsch process produces a wide distribution of hydrocarbon chain length, all having fuel value.

(22) The processes and systems of the present invention further allow for replenishment of the electrochemical cell 12 using generally-readily-available materials. As mentioned above in relation to FIG. 2, the NaOH generated in the cathode chamber 16 may be recycled in NaOH stream 24 and reacted with CO.sub.2 source 26, such as CO.sub.2 in atmospheric air, via a spontaneous reaction (encouraged by heat) to generate the sodium carbonate. The regenerated sodium carbonate or other sodium carbonate source 28 may replenish the sodium carbonate consumed in the anode chamber 14. Various systems for speeding this process are available and known to one of ordinary skill in the art. This allows the anode chamber 14 to be replenished. The cathode chamber 16, on the other hand, may be replenished by adding water 56. Alternatively, the sodium carbonate regeneration step may be skipped and the electrochemical cell may be replenished by direct supply of sodium carbonate. Thus, this embodiment of the processes of the present invention only requires electric power, CO.sub.2 from a carbon source or from atmospheric air, and water as the only inputs needed to generate syngas.

(23) In another embodiment within the scope of the present invention, a three-compartment electrochemical cell 60 is provided. Cell 60, shown in FIG. 4, is similar to cell 12, except that the anode chamber 14 includes a separator 62 which allows transport of H.sup.+ ions. Suitable separators may include a microporous separator, cation exchange membrane, a mesh or a screen. In this embodiment, the separator 62 defines an intermediate chamber 64 disposed between the separator 62 and the sodium-conducting membrane 18. Water is decomposed at the anode 20 according to reaction (1), above. Sodium carbonate is decomposed in the intermediate chamber according to reaction (2), above. The three-compartment electrochemical cell may facilitate collection, separation and removal of the product gases, oxygen, carbon dioxide, and hydrogen. A separate process for separating carbon dioxide from oxygen may not be required. In addition, a plurality of three-compartment electrochemical cells 60, separated by bipolar plates, may be stacked or connected in a way to efficiently generate large quantities of carbon dioxide and hydrogen for use in producing synthesis gas.

(24) In yet another embodiment of the methods of the present invention, syngas 40 may be produced by the coelectrolysis of carbon dioxide and steam. FIG. 5 includes a diagram of this process 100. In this process 100, a flow 102 of carbon dioxide is provided from a carbon dioxide source 104, such as discussed above. The carbon dioxide source may include a gas separator, discussed above, to separate carbon dioxide from atmospheric air. The process 100 also includes a flow of steam 106 provided from a source of steam 108. The carbon dioxide flow 102 and steam flow 106 are sent to a solid oxide electrolysis cell (SOEC) 110. Electricity 120 and any required heat are provided to the cell 110 to drive the following reactions:
CO.sub.2+2e.sup..fwdarw.CO+O.sup.2(5)
H.sub.2O+2e.sup..fwdarw.H.sub.2+O.sup.2(8)
O.sup.2.fwdarw.O.sub.2+2e.sup.(7)
The following additional reaction may also occur in the SOEC 110:
CO.sub.2+H.sub.2CO+H.sub.2O(6)
The net equation of the SOEC is shown as follows:
H.sub.2O+CO.sub.2.fwdarw.H.sub.2+CO+O.sub.2(9)

(25) In the SOEC 110, oxygen is stripped from the incoming CO.sub.2 102 and H.sub.2O 106 (as steam) by an oxygen ion-conducting electrolysis cell 140, similar to the electrolysis cell 44, discussed above and illustrated in FIG. 3. FIG. 6 shows that electrolysis cell 140 includes an oxygen ion conducting membrane 142. A variety of materials may be used to fabricate the oxygen ion conducting, including but not limited to, yttria-stabilized zirconia (YSZ). A cathode 144 is attached to a surface of the oxygen ion conducting membrane 142 which is electrochemically active to reduce carbon dioxide to form carbon monoxide and oxygen ions according to reaction (5), above, and also electrochemically active to reduce steam (water) to form hydrogen gas and oxygen ions according to reaction (8), above. An anode 146 is attached to an opposite surface of the oxygen ion conducting membrane 142 which is electrochemically active for recombination of oxygen ions into oxygen molecules, according to reaction (7), above.

(26) The cathode 144 and anode 146 should be sufficiently permeable to allow diffusion of carbon dioxide, steam, oxygen, or other gaseous species that may react or be produced at the interface of the oxygen ion conductor 142 and the cathode 144 and the interface of the oxygen ion conductor 142 and the anode 146.

(27) By applying an electrical potential across the oxygen ion conducting electrolysis cell 140, carbon dioxide and water are reduced to form carbon monoxide, hydrogen, and oxygen. It produces a flow 150 of carbon monoxide, a flow 152 of hydrogen, and a flow 154 of oxygen. The reduction of carbon dioxide and steam at the cathode 144 generates oxygen ions and the flow 150 of carbon monoxide and flow 152 of hydrogen. As the oxygen ions migrate through the membrane 142 from the cathode 144 to the anode 146 they will combine to form oxygen as shown in reaction (7), above.

(28) This syngas mixture 40 can be further processed to produce liquid hydrocarbon fuel using any suitable process available in the art.

(29) The carbon dioxide and steam are reduced at the cathode side 144 of the oxygen-ion conducting membrane. Oxygen ions are transported through the oxygen ion conducting membrane 142 and evolved at the anode 146 as oxygen gas. The SOEC 110 provides an outflow 122 of O.sub.2. As with the process 10 of the invention, this process 100 results in an outflow 122 of O.sub.2 and a separate stream of syngas 124 comprising CO, and H.sub.2. The oxygen may be produced at high purity without any subsequent separation required. As also discussed above, the syngas 124 may be further processed to produce a liquid hydrocarbon fuel suitable for use with current equipment using known methods.

(30) The anode may be a mixture of perovskite and an oxide. In one embodiment, the anode consists of a perovskite and an oxide mixed in the volume ratio of oxide, 0V.sub.oxide70%. The anode perovskite may be (Pr.sub.1-xLa.sub.x).sub.z-yA.sub.yBO.sub.3- where A is an alkaline earth in particular Sr or Ca, and B is a transition metal in particular Mn, Co and Fe and mixtures thereof, with 0x1.0, 0y0.5, 0.8z1.1. is the oxygen non-stoichiometry determined by the crystalline chemistry and electro-neutrality conditions. The anode oxide may be zirconia doped with one or more of yttrium oxide, ytterbium oxide, calcium oxide, magnesium oxide, scandium oxide, and cerium oxide. The anode oxide could also be ceria doped with one or more of yttrium oxide, ytterbium oxide, calcium oxide, and magnesium oxide, scandium oxide, and zirconium oxide. In one embodiment, the oxide doping in zirconia and ceria ranges from about 2 to about 15 mole %.

(31) The cathode may be a mixture of nickel oxide and an oxide. In one embodiment, the nickel oxide is a solid solution with magnesium oxide. The cathode solid solution may contain between about 1 to about 25 mole % magnesium oxide relative to the nickel oxide. The nickel oxide in the solid solution is reduced to nickel during cell operation. The oxide in the cathode may include one of the group of zirconia, ceria, and mixtures thereof. The zirconia in the cathode may be doped with one or more of yttrium oxide, ytterbium oxide, calcium oxide, magnesium oxide, and scandium oxide. Ceria in the cathode may be doped with one or more of samarium oxide, gadolinium oxide, yttrium oxide, ytterbium oxide, calcium oxide, magnesium oxide, and scandium oxide.

(32) The anode and cathode may be infiltrated with a catalyst material. The catalyst may be a surface dispersed catalyst selected from one of the group of Pr, Co, Ce, Eu, other rare earth elements, and combinations thereof. The catalyst in addition may contain one or more of Sr and Ca. The catalyst may be infiltrated as a salt soluble in water or an organic solvent. The catalyst may also be infiltrated as oxide particles.

(33) A source of steam and carbon dioxide in contact with the cathode 144 under conditions which cause the following reactions to occur: H.sub.2O+2e.sup..fwdarw.H.sub.2+O.sup.2, CO.sub.2+2e.sup..fwdarw.CO+O.sup.2 and CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O, wherein synthesis gas comprising CO and H.sub.2 is collected and recovered at the cathode and wherein oxygen ions are conducted through the oxygen ion conducting membrane to the anode where they are recombined to form O.sub.2, which is collected and recovered.

(34) While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and are included within its scope.