Method for removing nitrogen oxides from a gas
A method for removing nitrogen oxides NOx from a gaseous current, comprising the steps of: passing the gaseous current through a de-NOx catalytic bed with iron exchanged zeolite as a catalyst with the addition of ammonia as a reducing agent, wherein the molar ratio of NH3 over NOx is greater than 1.33.
1. A method for removing nitrogen oxides NOx from a gaseous current, the method comprising:
passing the gaseous current through a first de-NOx catalytic bed and then into a second de-NOx catalytic bed, the second de-NOx catalytic bed including a catalyst that is an iron exchanged zeolite, with an addition of ammonia as a reducing agent;
wherein a molar ratio of NH3 over NOx in the gas admitted to said second de-NOx catalytic bed is 1.4 to 2;
wherein said second de-NOx catalytic bed is operated at a temperature in a range of 420° C. to 435° C.; and
wherein a space velocity in said second de-NOx catalytic bed is 10000 h−1 to 14000 h−1.
2. The method according to claim 1 , wherein said molar ratio of NH3 over NOx is 1.4 to 1.6.
3. The method according to claim 2 , wherein said molar ratio of NH3 over NOx is 1.5.
4. The method according to claim 1 , wherein, after the passage though said second de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 100 ppm.
5. The method according to claim 1 , wherein, after the passage though said second de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 50 ppm.
6. The method according to claim 1 , wherein, after the passage though said second de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 25 ppm.
7. The method according to claim 1 , wherein the temperature at which said second de-NOx catalytic bed is operated is 430° C.
8. The method according to claim 1 , wherein the iron exchanged zeolite catalyst includes MFI, BEA, FER, MOR, FAU, MEL, or combinations thereof.
9. The method according to claim 1 , wherein the iron exchanged zeolite includes an Fe-ZSM-5 type iron exchanged zeolite.
10. The method according to claim 1 , wherein the space velocity in said second de-NOx catalytic bed is 13000 h−1.
11. The method according to claim 1 , wherein the gas in said second de-NOx catalytic bed exhibits an absolute pressure of greater than 1 bar.
12. The method according to claim 11 , wherein the absolute pressure is 2 bar to 25 bar.
13. The method according to claim 11 , wherein the absolute pressure is 5 bar to 15 bar.
14. The method according to claim 11 , wherein the gaseous current is a flue gas of a combustion process, or a tail gas of a process for making nitric acid withdrawn from an absorption column.
15. The method according to claim 1 wherein the gaseous current is a tail gas of a process for making nitric acid, withdrawn from an absorption column.
16. The method according to claim 1 , further comprising:
adding ammonia as a reducing agent to the effluent of said first de-NOx catalytic bed until the molar ratio of NH3 over NOx in said effluent gas is 1.4 to 2; and
passing the effluent gas and ammonia directly through said de-NOx catalytic bed.
17. The method according to claim 16 wherein adding ammonia as a reducing agent to the effluent of said first de-NOx catalytic bed until the molar ratio of NH3 over NOx in said effluent gas is 1.4 to 2 includes adding ammonia as the reducing agent to the effluent of said first de-NOx catalytic bed until the molar ratio of NH3 over NOx in said effluent gas is 1.4 to 1.6.