A method for removing nitrogen oxides from a gas using an iron exchanged zeolite catalyst

Abstract

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 - 11 . (canceled) 
     
     
         12 . A method for removing nitrogen oxides NOx from a gaseous current, the method comprising:
 passing the gaseous current through a 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 de-NOx catalytic bed is 1.4 to 2;   wherein said 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 de-NOx catalytic bed is 10000 h −1  to 14000 h −1  .   
     
     
         13 . The method according to  claim 12 , wherein said molar ratio of NH3 over NOx is 1.4 to 1.6. 
     
     
         14 . The method according to  claim 13 , wherein said molar ratio of NH3 over NOx is 1.5. 
     
     
         15 . The method according to  claim 12 , wherein, after the passage though said de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 100 ppm. 
     
     
         16 . The method according to  claim 12 , wherein, after the passage though said de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 50 ppm. 
     
     
         17 . The method according to  claim 12 , wherein, after the passage though said de-NOx catalytic bed, a residual amount of NOx in the gas is not greater than 25 ppm. 
     
     
         18 . The method according to  claim 12 , wherein the temperature at which said de-NOx catalytic bed is operated is 430° C. 
     
     
         19 . The method according to  claim 12 , wherein the iron exchanged zeolite catalyst includes MFI, BEA, FER, MOR, FAU, MEL, or combinations thereof. 
     
     
         20 . The method according to  claim 12 , wherein the iron exchanged zeolite includes an Fe-ZSM-5 type iron exchanged zeolite. 
     
     
         21 . The method according to  claim 12 , wherein the space velocity in said de-NOx catalytic bed is 13000 h −1 . 
     
     
         22 . The method according to  claim 12 , wherein the gas in said de-NOx catalytic bed exhibits an absolute pressure of greater than 1 bar. 
     
     
         23 . The method according to  claim 22 , wherein the absolute pressure is 2 bar to 25 bar. 
     
     
         24 . The method according to  claim 22 , wherein the absolute pressure is 5 bar to 15 bar. 
     
     
         25 . The method according to  claim 12  wherein the gaseous current is a tail gas of a process for making nitric acid, withdrawn from an absorption column. 
     
     
         26 . The method according to  claim 12 , further comprising:
 passing the NOx-containing gas through a first de-NOx catalytic bed;   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 without a passage through a de-N2O catalytic bed.   
     
     
         27 . The method according to  claim 26  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. 
     
     
         28 . The method according to  claim 12 , 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; and
 the method does not comprise passing the gas through a series of another de-NOx catalytic bed and a subsequent de-N2O catalytic bed, before the passage through said de-NOx catalytic bed.