Method and apparatus for removing mercury species from hot flue gas

• US 20030185718A1
• Filed: 03/12/2002
• Published: 10/02/2003
• Est. Priority Date: 03/12/2002
• Status: Active Grant

• Current Assignee: Foster Wheeler Energy Limited (John Wood Group PLC)
• Original Assignee: Foster Wheeler Energy Limited (John Wood Group PLC)
• Inventors: Sellakumar, Kumar M.
• Granted Patent: US 6,960,329 B2
• Time in Patent Office: Days
• US Class Current: 422/171
• CPC Class Codes: B01D 2251/206 Ammonium compounds
• CPC Class Codes: B01D 2251/304 of sodium
• CPC Class Codes: B01D 2251/306 of potassium
• CPC Class Codes: B01D 2251/404 of calcium
• CPC Class Codes: B01D 2251/60 Inorganic bases or salts
• CPC Class Codes: B01D 53/64 Heavy metals or compounds thereof, e.g. mercury
• CPC Class Codes: Y02C 20/10 of nitrous oxide (N2O)

Abstract

A method of removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant and passing through a flue gas duct. The method includes (a) contacting the mercury in the flue gas with a solution containing at least one chloride-containing salt dissolved in a solvent by injecting the solution into the flue gas duct at an injection location, in order to oxidize mercury into HgCl₂, (b) heating the solution prior to or after step (a) to at least about 300° C., and (c) removing oxidized mercury from the flue gas with a device for removing particulate solids from the flue gas.

Claims

1. A method of removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant and passing through a flue gas duct, said method comprising the following steps:

(a) contacting the mercury in the flue gas with a solution containing at least one chloride-containing salt dissolved in a solvent by injecting the solution into the flue gas duct at an injection location, in order to oxidize mercury into HgCl₂;

(b) heating the solution prior to or after step (a) to at least about 300°

C.; and

(c) removing oxidized mercury from the flue gas with means for removing particulate solids from the flue gas.

2. A method according to claim 1, further comprising selecting the injection location go that the flue gas temperature therein is below about 650°

C. and the solution is heated to at least about 300°

C. prior to the injection.

3. A method according to claim 2, further comprising selecting the injection location so that the flue gas temperature therein is from about 370°

C. to about 650°

C. and the solution is heated to at least about 300°

C. prior to the injection.

4. A method according to claim 1, further comprising selecting the injection location so that the flue gas temperature therein is from about 650°

C. to about 980°

C., thereby rapidly heating the solution to at least about 300°

C. in the flue gas duct.

5. A method according to claim 4, further comprising a step of heating the solution to at least about 100°

C. prior to the injection.

6. A method according to claim 1, wherein the solvent is water.

7. A method according to claim 1, wherein the chloride-containing salt is ammonium chloride (NH₄Cl).

8. A method according to claim 7, further comprising selecting the injection location so that the flue gas temperature therein is from about 650°

C. to about 980°

C., thereby rapidly heating the solution to at least about 300°

C. in the flue gas duct to dissociate the NH₄Cl to at least NH₃ and chlorine species, and allowing the formed NH₃ to reduce NO_x levels of the flue gas by a selective non-catalytic reduction process.

9. A method for removing mercury according to claim 7, further comprising heating the solution to at least about 300°

C. prior to the injection, and selecting the injection location upstream of a NO_x catyalyst unit so that the flue gas temperature therein is from about 370°

C. to about 650°

C.

10. A method according to claim 1, further comprising selecting the chloride-containing salt from a group consisting of sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl₂).

11. A method according to claim 1, further comprising recirculating a portion of particulate solids removed from the flue gas back to the flue gas duct.

12. A method according to claim 11, further comprising treating recirculated particulate solids prior to injection back to the flue gas duct by screening out largest particles from the recirculated solids.

13. A method according to claim 11, further comprising treating recirculated particulate solids prior to injection back to the flue gas duct by adding substances that catalyze mercury oxidation.

14. A method according to claim 1, further comprising removing oxidized mercury from the flue gas by a wet scrubber.

15. A method according to claim 1, further comprising removing oxidized mercury from the flue gas by a spray dryer.

16. An apparatus for removing mercury from flue gas containing mercury and particulate solids emanating from a fossil-fuel energy conversion plant, said apparatus comprising:

a flue gas duct for conveying the flue gas containing mercury and particulate solids from the fossil-fuel energy conversion plant;

means for injecting a solution of chloride-containing salt dissolved in a solvent into an upstream portion of the flue gas duct, for oxidizing mercury in the flue gas to HgCl₂; and

means for removing particulate solids and oxidized mercury condensed on the particulate solids from the flue gas.

17. An apparatus according to claim 16, wherein said flue gas duct comprises a first heat exchanger, and said means for injecting the solution into said flue gas duct is disposed so that the solution is injected upstream of said first heat exchanger.

18. An apparatus according to claim 17, wherein the plant comprises a circulating fluidized bed reactor, a separator to separate solid particles entrained with exhaust gases and means to return separated particles back to the reactor, and said means for injecting the solution into the flue gas duct is disposed so that the solution is injected to a channel between the reactor and the separator.

19. An apparatus according to claim 16, further comprising means for recirculating a portion of particulate solids removed from the flue gas back to said flue gas duct.

20. An apparatus according to claim 19, wherein said means for recirculating particulate solids back to said flue gas duct comprises means for treating the solids by screening out largest particles from the recirculated solids.

21. An apparatus according to claim 19, wherein said means for recirculating particulate solids back to said flue gas duct comprises means for treating the solids by adding substances that catalyze mercury oxidation.

22. An apparatus according to claim 16, further comprising a wet scrubber for removing oxidized mercury from the flue gases.

23. An apparatus according to claim 16, further comprising a spray dryer for removing oxidized mercury from the flue gases.

24. An apparatus according to claim 16, wherein the solvent is water.

25. An apparatus for removing mercury from flue gas containing mercury and particulate solids emanating from a fossil energy conversion plant, said apparatus comprising:

a flue gas duct for conveying flue gas containing mercury and particulate solids;

means for heating a solution of chloride-containing salt dissolved in a solvent to at least about 300°

C. and means for injecting the solution into said flue gas duct, for oxidizing mercury in the flue gas to HgCl₂; and

means for removing particulate solids and oxidized mercury condensed on the particulate solids from the flue gas.

26. An apparatus according to claim 25, further comprising an NO_x reducing catyalyst unit for supplying NO_x reducing catalyst into said flue gas duct, wherein said means for injecting the solution into said flue gas duct is disposed so that the solution is injected upstream of said catyalyst unit.

27. An apparatus according to claim 25, further comprising means for recirculating a portion of particulate solids removed from the flue gas back to said flue gas duct.

28. An apparatus according to claim 27, wherein said means for recirculating particulate solids back to said flue gas duct comprises means for treating the solids by screening out largest particles from the recirculated solids.

29. An apparatus according to claim 27, wherein said means for recirculating particulate solids back to said flue gas duct comprises means for treating the solids by adding substances that catalyze mercury oxidation.

30. An apparatus according to claim 25, further comprising a wet scrubber for removing oxidized mercury from the flue gases.

31. An apparatus according to claim 25, further comprising a spray dryer for removing oxidized mercury from the flue gases.

32. An apparatus according to claim 25, wherein the solvent is water.