System and method for low NOx emitting regeneration of desiccants

1. A system for purification of a carbon dioxide rich flue gas generated in a boiler combusting a fuel in the presence of a gas containing oxygen, and being contaminated by NOx gases, wherein the system comprising

one or more gas drier(s) comprising desiccants for removal of at least a portion of water content of the further compressed carbon dioxide rich flue gas; and

a closed loop connected to the drier(s) for regeneration of desiccants of the drier(s) wherein the NOx gases are removed substantially separately from the water vapor.

2. A system for purification of flue gas according to claim 1 wherein the closed loop comprises a gas scrubber operating with a washing medium able to absorb the NOx gases removed from the drier during regeneration.

3. A system for purification of carbon dioxide rich flue gas according to claim 1 wherein the water desorbed from the drier will be condensed in the gas scrubber.

4. A system for purification of carbon dioxide rich flue gas according to claim 1 wherein the pH of the washing medium is suitable adjusted to maximise washing of the gas.

5. A system for purification of carbon dioxide rich flue gas according to claim 1 wherein the pH is suitably adjusted by adding a base selected from the group of ammonia and alkali hydroxide.

6. A system for purification of carbon dioxide rich flue gas according to claim 1 wherein the system is working batch-wise.

7. A method for purification of carbon dioxide rich flue gas generated in a boiler combusting a fuel in the presence of a gas containing oxygen, from contaminants, the method comprising:

a) drying a carbon dioxide rich flue gas by adsorption of water to the drier bed under coadsorption of NOx;

b) isolating the drier from the process;

c) pressure equalizing of the regeneration loop by connecting the drier to the closed loop whereby a substantial amount of the NOx gases is desorbed from the desiccant;

d) elevating the temperature of the drier whereby also water vapor is desorbed from the desiccant;

e) washing the regeneration gas loaded with water and NOx in a gas scrubber comprising a washing medium, whereby the NOx gases are removed and water vapor is condensed into the washing medium; and

f) cooling the regenerated drier and venting the humid regeneration gas by refilling the system with CO 2 rich gas from the dried process stream;

g) isolating the drier when the cooling step is finished;

h) repressurization of the drier to the process pressure using the dried process gas; and optionally

i) bringing the regenerated drier on-stream when the other drier bed is loaded.

8. A method according to claim 7 wherein the washing step e) comprises:

ei) venting the surplus of gases to keep the pressure in the heated system;

eii) increasing the pressure of the regeneration gases using a blower or compressor to compensate pressure losses in the system;

eiii) reheating the regeneration gases after water and NOx removal;

eiv) reintroduction of the flue gas, substantially free from water vapor and the contaminating NOx gases, to the drier; and

ev) isolating the scrubber when the desorption step is finished.

9. A method according to claim 7 wherein the closed loop is at a pressure of about 15-25 bar before pressure equalizing in step c).

10. A method according to claim 7 wherein the washing medium is water, optionally having pH adjusted to about 5.

11. A method according to claim 7 wherein the pH is adjusted by adding a base.

12. A method according to claim 7 wherein the desorbed water condenses in the gas scrubber.

13. A method according to claim 7 wherein the pressure is kept above around 2 bar.

14. A method according to claim 7 wherein step c) also comprising heating the regeneration flue gas.

15. A method according to claim 7 wherein the temperature of the flue gas in step d) may be additionally raised in order to remove the water from the drier.

16. A system for purification of carbon dioxide rich flue gas according to claim 5 wherein the alkali hydroxide is sodium hydroxide.

17. A method according to claim 7 comprising:

performing the repressurization of the drier to the process pressure at about 30-40 bar.

18. A method according to claim 11 wherein the base is selected from ammonia or sodium hydroxide.

19. A method according to claim 13 wherein the pressure is kept above around 10 bar.

20. A method according to claim 14 comprising:

heating the regeneration flue gas to a temperature of about 160° C. to 230° C.