Conversion of carbon dioxide and water to synthesis gas for producing methanol and hydrocarbon products

Abstract

A method and system for producing a synthesis gas for use in the production of methanol, or a hydrocarbon product such as a synthetic fuel, comprising the steps of: providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2; providing a water feedstock and passing it through an electrolysis unit for producing a feed stream comprising H.sub.2; combining said feed stream comprising CO and CO.sub.2 and said feed stream comprising H.sub.2 into said synthesis gas; and converting said synthesis gas into said methanol or said hydrocarbon product.

Claims

1. A method for producing methanol, comprising the steps of: providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2 providing a water feedstock and passing it through an electrolysis unit for producing a feed stream comprising H.sub.2, combining said feed stream comprising CO and CO.sub.2 and said feed stream comprising H.sub.2 into a synthesis gas, converting said synthesis gas into said methanol, wherein the step of providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2, is conducted as a once-through operation in a solid oxide electrolysis cell unit, and wherein the molar ratio CO/CO.sub.2 in the feed stream comprising CO and CO.sub.2, or the synthesis gas, is in the range 0.2-0.6.

2. Method according to claim 1, wherein the molar ratio CO/CO.sub.2 in the feed stream comprising CO and CO.sub.2, or the synthesis gas, is 0.25, 0.30 or 0.35, 0.40 or 0.45, 0.50 or 0.55.

3. Method according to claim 1, wherein the step of providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2, and the step of providing a water feedstock and passing it through an electrolysis unit for producing a feed stream comprising H.sub.2, are conducted separately.

4. Method according to claim 1, comprising by-passing a portion of said a carbon dioxide-rich stream prior to passing it through said solid oxide electrolysis unit.

5. Method according to claim 1, comprising cooling down said synthesis gas resulting from combining said feed stream comprising CO and CO.sub.2 and said feed stream comprising H.sub.2, suitably cooling down from 800-400C.

6. Method according to claim 1, wherein the step of combining said feed stream comprising CO and CO.sub.2 and said feed stream comprising H.sub.2, is conducted after compressing either stream.

7. Method according to claim 1, wherein the carbon dioxide-rich stream is produced by passing a carbon dioxide-feed stream through a CO.sub.2 cleaning unit for removing impurities, such as Cl, sulfur, Si, As.

8. Method according to claim 1, wherein the electrolysis unit for producing the feed stream comprising H.sub.2 is an alkaline/polymer electrolyte membrane electrolysis unit i.e. alkaline and/or PEM electrolysis unit.

9. Method according to claim 1, wherein the electrolysis unit for producing the feed stream comprising H.sub.2 is a solid oxide electrolysis cell unit.

10. Method according to claim 1, wherein said water feedstock comprises steam such as steam produced from other processes of the method, such as from steam generation or downstream distillation.

11. Method according to claim 1, wherein said carbon dioxide-rich stream comprises carbon dioxide from external sources such as from biogas upgrading or fossil fuel-based syngas plants.

12. The method according to claim 1, wherein the electric power required in the step of electrolysis of the carbon dioxide-rich stream or the water feedstock, is provided at least partly by renewable sources, such as wind and solar energy.

13. Method according to claim 1, wherein the step of converting the synthesis gas into methanol comprises passing the synthesis gas through a methanol synthesis reactor under the presence of a catalyst for producing a raw methanol stream, said step optionally further comprising a distillation step of the raw methanol stream for producing a water stream and a separate methanol stream having at least 98 wt % methanol.

14. A method for producing a hydrocarbon product such as a synthetic fuel, comprising the steps of: providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2, providing a water feedstock and passing it through an electrolysis unit for producing a feed stream comprising H.sub.2, combining said feed stream comprising CO and CO.sub.2 and said feed stream comprising H.sub.2 into a synthesis gas, converting said synthesis gas into said hydrocarbon product, wherein the step of providing a carbon dioxide-rich stream and passing it through an electrolysis unit for producing a feed stream comprising CO and CO.sub.2, is conducted as a once-through operation in a solid oxide electrolysis cell unit, wherein the feed stream comprising CO and CO.sub.2, or the synthesis gas, has a molar ratio CO/CO.sub.2 of 0.8 or higher such as 0.9, and wherein the step of converting the synthesis gas into a hydrocarbon product comprises passing the synthesis gas through a Fischer-Tropsch (FT) synthesis unit.

15. A system for the production of methanol or a hydrocarbon product such as a synthetic fuel, comprising: a once-through solid oxide electrolysis cell unit, arranged to receive a carbon dioxide-rich stream for producing a feed stream comprising CO and CO.sub.2 and to produce a feed stream comprising CO and CO.sub.2, an electrolysis unit arranged to receive a water feedstock for producing a feed stream comprising H.sub.2, a compressor section arranged to receive the feed stream comprising CO and CO.sub.2 and the feed stream comprising H.sub.2, for compressing and combining said streams into a synthesis gas, a methanol synthesis unit arranged to receive said synthesis gas for producing said methanol, preferably having a concentration, i.e. purity, of at least 98% methanol, wherein said once-through solid oxide electrolysis unit is arranged to produce said feed stream comprising CO and CO.sub.2 or the synthesis gas, with a molar ratio CO/CO.sub.2 of 0.2-0.6; or a hydrocarbon product synthesis unit, preferably a Fischer-Tropsch (FT) synthesis unit, for producing said hydrocarbon product, such as a synthetic fuel, wherein said once-through solid oxide electrolysis unit is arranged to produce said feed stream comprising CO and CO.sub.2, or the synthesis gas, with a molar ratio CO/CO.sub.2 of 0.8 or higher, such as 0.9.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] FIG. 1 shows a schematic method and system (process and plant) for the production of a synthesis gas for further conversion to methanol according to the prior art.

[0074] FIG. 2 shows a schematic method and system for the production of synthesis gas and further conversion to methanol according to an embodiment of the invention.

[0075] With reference to FIG. 1 (prior art), a carbon dioxide-feed stream 1 is passed through a CO.sub.2-cleaning unit 20 for removing impurities and thereby producing a CO.sub.2-rich stream 2. A water feedstock 3 passes through an electrolysis unit 30 such as an alkaline/PEM-electrolysis unit powered by a sustainable source such as wind or solar energy, thereby producing a feed stream 4 comprising H.sub.2, i.e. a H.sub.2-rich stream. Both streams 2 and 4 pass through a compression section 40 whereby they are compressed and combined into a synthesis gas stream 5 having a molar ratio H.sub.2/CO.sub.2 of about 3. While the module M defined previously is used in any gas mixture comprising carbon dioxide and carbon monoxide and hydrogen, the molar ratio of hydrogen to carbon dioxide is only relevant to use for a gas mixture of carbon dioxide and hydrogen. The synthesis gas 5 enters the methanol loop 50 as is well-known in the art, whereby the synthesis gas 5 is converted to a raw methanol stream 6 having a molar ratio CH.sub.3OH/H.sub.2O of about 1. The water in the raw methanol stream 6 is then removed in distillation unit 60, where the raw methanol stream 6 is then purified or enriched in methanol. A methanol product 7 having a concentration of at least 98 wt % is then produced, as well as water stream 8.

[0076] Now with reference to FIG. 2, which is in accordance with an embodiment of the invention, a carbon dioxide-feed stream 1 is passed through a CO.sub.2-cleaning unit 20 for removing impurities and producing a CO.sub.2-rich stream 2, and then through an electrolysis unit 70, here a once-through SOEC-CO.sub.2 unit, which is also powered by a sustainable source such as wind or solar energy, thereby producing a feed stream 2 comprising CO and CO.sub.2 and having a molar ratio CO/CO.sub.2 above 0.2, in particular 0.2-0.6. Separately, the water feedstock 3 also passes through an electrolysis unit 30, such as a PEM-electrolysis unit or SOEC unit also powered by a sustainable source, thereby producing a feed stream 4 comprising H.sub.2. Both streams 2 and 4 pass through a compression section 40 whereby they are compressed and combined into a now more reactive synthesis gas stream 5 having a module M=(H.sub.2CO.sub.2)/(CO+CO.sub.2) which is highly suitable for the downstream conversion into methanol. This synthesis gas 5 enters the methanol loop 50 as is well-known in the art, whereby it is converted to a raw methanol stream 6 now having a molar ratio CH.sub.3OH/H.sub.2O of 1.3 or higher, i.e. at least 30% less water on a molar basis compared to the prior art. The water in the raw methanol stream 6 is then more expediently removed in distillation unit 60, where this stream is purified or enriched in methanol. A methanol product 7 having a concentration of at least 98 wt % is then produced, as well as water stream 8 which may be used as part of the water feedstock 3.

EXAMPLE

[0077] The results of below Table 1 correspond to a plant for producing methanol for 100 kmol/h CO.sub.2 with water (steam) electrolysis (SOEC) only for producing H.sub.2 (prior art) in accordance with the reaction: 3 H.sub.2+CO.sub.2?CH.sub.3OH+H.sub.2O; and with water (steam) electrolysis (SOEC) for producing H.sub.2 and CO.sub.2 electrolysis (SOEC-CO.sub.2) for producing CO (invention) in accordance with the reaction: CO+2 H.sub.2?CH.sub.3OH:

TABLE-US-00001 TABLE 1 Invention Prior art H.sub.2O + H.sub.2O 100% CO.sub.2 electrolysis electrolysis, Improve- only, MW MW ment Comments H.sub.2O electrolysis 23.85 15.90 100% (SOEC) efficiency CO.sub.2 electrolysis 7.86 100% (SOEC) efficiency Sum electrolysis 23.85 23.76 0.4% Compression to 2.68 2.18 18.7% 14% work 90 bar g loss Duty for steam 1.45 2.46 ?69.4% 0% heat generation loss/25 C. temp. approach in heat exch. Duty for air and 2.13 1.08 49.4% water coolers

[0078] Thus, there is 19% saving for the compressor power due to lower gas volume and density; 70% more duty for steam generationand corresponding 50% less heat lost in coolers. Thus, with the same efficiency by using SOEC for both H.sub.2O-electrolysis and CO.sub.2-electrolysis, there will be no significant savings in electrolysis power. However, by operating SOEC for both H.sub.2O-electrolysis and CO.sub.2-electrolysis in accordance with the invention enables operating with a common system for the cooling of streams thereof, as both SOEC units operate in the same temperature range of about 700-800C, and thus better integration of process units. Further, as SOEC utilizes steam, the energy for distillation of H.sub.2O out of the produced methanol is saved.

[0079] Table 2 below compares now the prior art with water (liquid) electrolysis only (alkaline/PEM electrolysis) for producing H.sub.2 in accordance with the reaction: 3 H.sub.2+CO.sub.2?CH.sub.3OH+H.sub.2O; and an embodiment of the invention with water (liquid) electrolysis (alkaline/PEM electrolysis) for producing H.sub.2 as well as CO.sub.2 electrolysis (SOEC-CO.sub.2) for producing CO in accordance with the reaction: CO+2 H.sub.2?CH.sub.3OH:

TABLE-US-00002 TABLE 2 Invention Prior art H.sub.2O + H.sub.2O 100% CO.sub.2 electrolysis electrolysis, Improve- only, MW MW ment Comments H.sub.2O electrolysis 29.81 19.87 80% (alkaline/PEM) efficiency CO.sub.2 electrolysis 7.86 100% (SOEC) efficiency Sum electrolysis 29.81 27.74 7.0%

[0080] Thus, when using alkaline/PEM for H.sub.2O-electrolysis and SOEC for CO.sub.2-electrolysis in accordance to an embodiment of the invention, there is 7% reduction (improvement) in power consumption with respect to only using alkaline/PEM for producing H.sub.2. The invention according to this embodiment enables therefore not only the formation of a more reactive synthesis gas, but also a reduction in electrolysis power consumption.