Process for CO 2 capture using a regenerable magnesium hydroxide sorbent

• US 8,470,276 B1
• Filed: 03/10/2010
• Issued: 06/25/2013
• Est. Priority Date: 03/10/2010
• Status: Active Grant

• Current Assignee: US Department of Energy (Government of the United States of America)
• Original Assignee: US Department of Energy (Government of the United States of America)
• Inventors: Siriwardane, Ranjani V., Stevens, Robert W. Jr.
• Primary Examiner(s): Hendrickson, Stuart
• Assistant Examiner(s): BERNS, DANIEL J
• Application Number:
  US12/720,766
• Time in Patent Office: 1,203 Days
• Field of Search: 423/220, 423/225, 423/230
• US Class Current: 423/225
• CPC Class Codes: B01D 2251/402 of magnesium
• CPC Class Codes: B01D 2257/504 Carbon dioxide
• CPC Class Codes: B01D 53/62 Carbon oxides
• CPC Class Codes: B01D 53/96 Regeneration, reactivation or recycling of reactants
• CPC Class Codes: C01B 2203/043 Regenerative adsorption process in two or more beds, one for adsorption, the other for regeneration
• CPC Class Codes: C01B 2203/0475 the impurity being carbon dioxide
• CPC Class Codes: C01B 3/56 by contacting with solids; Regeneration of used solids C01B3/508 takes precedence
• CPC Class Codes: Y02C 20/40 of CO2
• CPC Class Codes: Y02P 20/151 Reduction of greenhouse gas [GHG] emissions, e.g. CO2
• CPC Class Codes: Y02P 30/00 Technologies relating to oil refining and petrochemical industry

Abstract

A process for CO₂ separation using a regenerable Mg(OH)₂ sorbent. The process absorbs CO₂ through the formation of MgCO₃ and releases water product H₂O. The MgCO₃ is partially regenerated through direct contact with steam, which acts to heat the magnesium carbonate to a higher temperature, provide heat duty required to decompose the magnesium carbonate to yield MgO and CO₂, provide an H₂O environment over the magnesium carbonate thereby shifting the equilibrium and increasing the potential for CO₂ desorption, and supply H₂O for rehydroxylation of a portion of the MgO. The mixture is polished in the absence of CO₂ using water product H₂O produced during the CO₂ absorption to maintain sorbent capture capacity. The sorbent now comprised substantially of Mg(OH)₂ is then available for further CO₂ absorption duty in a cyclic process.

Claims

1. A method of separating CO₂ from a process gas stream comprised of CO₂ and other gaseous constituents in an absorption/desorption cycle using a regenerable magnesium hydroxide sorbent, comprising:

contacting the process gas stream with the regenerable magnesium hydroxide sorbent in an absorption reactor at a first temperature, where the regenerable magnesium hydroxide sorbent is comprised of a first magnesium hydroxide, and where the first temperature is sufficient to cause some portion of the first magnesium hydroxide to react with some portion of the CO₂ in the process gas stream to produce magnesium carbonate and a water product, thereby separating the some portion of the CO₂ from the process gas stream, and transferring heat from the absorption reactor with a first heat transfer to maintain the absorption reactor at the first temperature;

discharging a rehydroxylation stream from the absorption reactor, where the rehydroxylation stream is comprised of some portion of the water product;

transferring the magnesium carbonate to a regeneration reactor and introducing a flow of steam to the regeneration reactor, and contacting some portion of the magnesium carbonate with the flow of steam, where the flow of steam is at a second temperature, where the second temperature is sufficient to decompose the some portion of the magnesium carbonate to produce magnesium oxide and gaseous carbon dioxide, and such that some portion of the magnesium oxide reacts with water in a first portion of the flow of steam to produce a second magnesium hydroxide;

discharging a separation stream from the regeneration reactor, where the separation stream is comprised of the gaseous carbon dioxide and a second portion of the flow of steam;

transferring the magnesium oxide and the second magnesium hydroxide to a polishing reactor at a third temperature and introducing the rehydroxylation stream into the polishing reactor, where the third temperature is less than the second temperature, and where the third temperature is sufficient to cause some portion of the magnesium oxide to react with some quantity of the some portion of the water product in the rehydroxylation stream to produce a third magnesium hydroxide, thereby producing a polished magnesium hydroxide sorbent, where the polished magnesium hydroxide sorbent is comprised of the second magnesium hydroxide and the third magnesium hydroxide, and transferring heat from the polishing reactor with a second heat transfer to maintain the polishing reactor at the third temperature; and

transferring the polished magnesium hydroxide sorbent to the absorption reactor and repeating the contacting the process gas stream step using the polished magnesium hydroxide sorbent as the regenerable magnesium hydroxide sorbent,

thereby separating CO₂ from the process gas stream comprised of CO₂ and other gaseous constituents in the absorption/desorption cycle using the regenerable magnesium hydroxide sorbent.

2. The method of claim 1 where the second magnesium hydroxide comprises greater than 30 mole percent of the magnesium oxide and the second magnesium hydroxide transferred to the polishing reactor.

3. The method of claim 1 where the first temperature is between from about 150°

C. to about 315°

C., and where the second temperature is at least 375°

C., and where the third temperature is between from about 150°

C. to about 315°

C., and where pressure in the regeneration reactor is greater than 10 atmospheres.

4. The method of claim 3 where the second temperature is at least 400°

C., and where pressure in the regeneration reactor is at least 20 atmospheres.

5. The method of claim 1 where the process gas stream is a shifted syngas stream from a water-gas-shift reactor.

6. The method of claim 1 where the second temperature is maintained by a third heat transfer from the flow of steam to the regeneration reactor.

7. The method of claim 1 further comprising introducing the separation stream to a separation chamber, where the separation chamber is maintained at a temperature such that a quantity of the second portion of the flow of steam condenses to a liquid water, and including discharging a CO₂ stream from the separation chamber, where the CO₂ stream is the separation stream less the quantity of the second portion of the flow of steam.

8. The method of claim 1 where the process gas stream is further comprised of a first quantity of H₂, where the first quantity of H₂ is at least 10 mole percent of the process gas stream, and where the rehydroxylation stream is further comprised of a second quantity of H₂, where the second quantity of H₂ is comprised of some portion of the first quantity of H₂, and where the second quantity of H₂ is at least 10 mole percent of the rehydroxylation stream, and further comprised of discharging a fuel stream from the polishing reactor, where the fuel stream is comprised of a third quantity of H₂, where the third quantity of H₂ is comprised of some portion of the second quantity of H₂, and where the third quantity of H₂ is at least 10 mole percent of the fuel stream.

9. The method of claim 8 further comprised of producing generated steam from a power island combusting at least a portion of the third quantity of H₂ comprising the fuel stream, and supplying some portion of the generated steam to the regeneration reactor, such that the some portion of the generated steam serves as the flow of steam in the regeneration reactor.

10. The method of claim 8 where the rehydroxylation stream is further comprised of a quantity of CO₂, where the quantity of CO₂ is comprised of the CO₂ comprising the process gas stream less the some portion of the CO₂ separated from the process gas stream in the absorption reactor.

11. A method of separating CO₂ from a process gas stream in an absorption/desorption cycle using a regenerable magnesium hydroxide sorbent, where the process gas stream is comprised of CO₂ and a first quantity of H₂, and where the first quantity of H₂ is at least 10 mole percent of the process gas stream, comprising:

contacting the process gas stream with the regenerable magnesium hydroxide sorbent in an absorption reactor at a first temperature, where the regenerable magnesium hydroxide sorbent is comprised of a first magnesium hydroxide, and where the first temperature is sufficient to cause some portion of the first magnesium hydroxide to react with some portion of the CO₂ in the process gas stream to produce magnesium carbonate and a water product, thereby separating the some portion of the CO₂ from the process gas stream, and transferring heat from the absorption reactor with a first heat transfer to maintain the absorption reactor at the first temperature;

discharging a rehydroxylation stream from the absorption reactor, where the rehydroxylation stream is comprised of some portion of the water product, and where the rehydroxylation stream is comprised of a second quantity of H₂, where the second quantity of H₂ is comprised of some portion of the first quantity of H₂, and where the second quantity of H₂ is at least 10 mole percent of the rehydroxylation stream;

transferring the magnesium carbonate to a regeneration reactor and introducing a flow of steam to the regeneration reactor, and contacting some portion of the magnesium carbonate with the flow of steam, where the flow of steam is at a second temperature, where the second temperature is sufficient to decompose the some portion of the magnesium carbonate to produce magnesium oxide and gaseous carbon dioxide, and such that some portion of the magnesium oxide reacts with water in a first portion of the flow of steam to produce a second magnesium hydroxide;

discharging a separation stream from the regeneration reactor, where the separation stream is comprised of the gaseous carbon dioxide and a second portion of the flow of steam;

transferring the magnesium oxide and the second magnesium hydroxide to a polishing reactor at a third temperature and introducing the rehydroxylation stream into the polishing reactor, where the third temperature is less than the second temperature, and where the third temperature is sufficient to cause some portion of the magnesium oxide to react with some quantity of the some portion of the water product in the rehydroxylation stream to produce a third magnesium hydroxide, thereby producing a polished magnesium hydroxide sorbent, where the polished magnesium hydroxide sorbent is comprised of the second magnesium hydroxide and the third magnesium hydroxide, and transferring heat from the polishing reactor with a second heat transfer to maintain the polishing reactor at the third temperature;

discharging a fuel stream from the polishing reactor, where the fuel stream is comprised of a third quantity of H₂, where the third quantity of H₂ is comprised of some portion of the second quantity of H₂, and where the third quantity of H₂ is at least 10 mole percent of the fuel stream; and

transferring the polished magnesium hydroxide sorbent to the absorption reactor and repeating the contacting the process gas stream step using the polished magnesium hydroxide sorbent as the regenerable magnesium hydroxide sorbent,

thereby separating CO₂ from the process gas stream comprised of CO₂ and other gaseous constituents in the absorption/desorption cycle using the regenerable magnesium hydroxide sorbent.

12. The method of claim 11 where the first temperature is between from about 150°

C. to about 315°

C., and where the second temperature is at least 375°

C., and where the third temperature is between from about 150°

C. to about 315°

C., and where pressure in the regeneration reactor is greater than 10 atmospheres.

13. The method of claim 12 where the second temperature is at least 400°

C., and where pressure in the regeneration reactor is at least 20 atmospheres.

14. The method of claim 12 including producing generated steam from a combined cycle combusting at least a portion of the third quantity of H₂ comprising the fuel stream, and supplying some portion of the generated steam to the regeneration reactor, such that the some portion of the generated steam serves as the flow of steam in the regeneration reactor.

15. The method of claim 14 where the process gas stream is a shifted syngas stream.

16. The method of claim 11 where the process gas stream is a shifted syngas stream from a water-gas-shift reactor.

17. The method of claim 11 further comprising introducing the separation stream to a separation chamber, where the separation chamber is maintained at a temperature such that a quantity of the second portion of the flow of steam condenses to a liquid water, and including discharging a CO₂ stream from the separation chamber, where the CO₂ stream is the separation stream less the quantity of the second portion of the flow of steam.

18. The method of claim 11 where the second magnesium hydroxide comprises greater than 30 mole percent of the magnesium oxide and the second magnesium hydroxide transferred to the polishing reactor.

19. The method of claim 11 where the rehydroxylation stream is further comprised of a quantity of CO₂, where the quantity of CO₂ is comprised of the CO₂ comprising the process gas stream less the some portion of the CO₂ separated from the process gas stream in the absorption reactor.