Process and Apparatus for Eliminating NOx and N2O

Abstract

An apparatus is provided for carrying out a process for removing N 2 O and NO x from offgases by catalytic decomposition of N 2 O by means of iron-containing zeolite catalysts and catalytic reduction of the NO x by means of reducing agents, the deNO x stage connected downstream of the deN 2 O stage being operated at inlet temperatures of T<=400° C., and the inlet gas for the deN 2 O stage comprising water and having a selected N 2 O/NO x ratio, and the operating parameters of temperature, pressure and space velocity of the deN 2 O stage being selected so as to result in an N 2 O degradation of 80 to 98%. Under these conditions, a degree of NO x oxidation of 30-70% is established at the outlet of the deN 2 O stage, which is defined as the ratio of the molar amounts of NO 2 to the total molar amount of NO x . The result of this is that the downstream deNO x stage can be operated under optimal conditions.

1 . An apparatus for reducing the NO x  and N 2 O contents in gases comprising NO x , N 2 O and water, comprising the elements:
 A) an apparatus ( 2 ) for adjusting the water content of the gas ( 1 ) comprising NO x  and N 2 O,   B) a deN 2 O stage ( 3 ) arranged downstream of apparatus ( 2 ) for reducing the N 2 O content of the gas stream comprising an iron-laden zeolite catalyst,   C) a cooling apparatus ( 4 ) arranged downstream of the deN 2 O stage ( 3 ) for cooling the gas stream ( 5 ) leaving the deN 2 O stage,   D) a deNO x  stage ( 6 ) arranged downstream of the cooling apparatus ( 4 ) and comprising a deNO x  catalyst for reducing the NO x  content of the gas stream, and   E) a feed line ( 7 ) for introducing reducing agent for NO x  into the gas stream ( 5 ) leaving the deN 2 O stage, which is arranged between the outlet of the deN 2 O stage ( 3 ) and upstream of the deNO x  catalyst of the deNO x  stage ( 6 ).   
     
     
         2 . The apparatus as claimed in  claim 1 , wherein the cooling apparatus ( 4 ) for the gas stream ( 5 ) leaving the deN 2 O stage is thermally coupled to a heating apparatus ( 8 ) for the gas ( 1 ) comprising N 2 O, NO x  and water, for the purpose of heating this gas ( 1 ) before it enters the deN 2 O stage ( 3 ) and for the purpose of cooling the gas stream ( 5 ) leaving the deN 2 O stage before it enters the deNO x  stage ( 6 ). 
     
     
         3 . The apparatus as claimed in  claim 2 , wherein the heating apparatus is a heat exchanger. 
     
     
         4 . The apparatus as claimed in  claim 3 , wherein the heating apparatus is a recuperator. 
     
     
         5 . The apparatus as claimed in  claim 1 , wherein the apparatus A) is a burner ( 9 ) in which combustion gas ( 10 ) comprising hydrogen and/or hydrogen-containing compounds is incinerated together with oxygen-containing gas ( 11 ) and the offgas ( 15 ) thereof is introduced into the gas ( 1 ) comprising N 2 O, NO x  and water. 
     
     
         6 . The apparatus as claimed in  claim 5 , wherein the oxygen-containing gas is air. 
     
     
         7 . The apparatus as claimed in  claim 2 , wherein the entire gas stream ( 1 ) or a portion of the gas stream ( 1 ) can bypass the heating apparatus ( 8 ) via a bypass ( 13 ) provided upstream of the heating apparatus ( 8 ) in the feed line for the gas stream ( 1 ) comprising N 2 O, NO x  and water, and this bypass ( 13 ) is provided with a valve ( 14 ) with which the portion of the gas stream ( 1 ) which bypasses the heating apparatus ( 8 ) can be controlled. 
     
     
         8 . The apparatus as claimed in  claim 1 , wherein a feed line ( 7 ) for introduction of reducing agent for NO x  into the gas stream ( 5 ) leaving the deN 2 O stage ( 3 ) is provided upstream of the catalyst of the deNO x  stage ( 6 ). 
     
     
         9 . The apparatus as claimed in  claim 8 , wherein a mixer is provided downstream of the feed line ( 7 ) in the feed line of the gas stream into the deNO x  stage. 
     
     
         10 . The apparatus as claimed in  claim 1 , wherein a feed line ( 17 ) for introduction of reducing agent for NO x  into the gas stream ( 1 ) comprising N 2 O, NO x  and water is provided upstream of the catalyst of the deN 2 O stage ( 3 ). 
     
     
         11 . The apparatus as claimed in  claim 10 , wherein a mixer is provided downstream of the feed line ( 17 ) in the feed line of the gas stream into the deN 2 O stage. 
     
     
         12 . The apparatus as claimed in  claim 1 , wherein the catalyst in the deN 2 O stage ( 3 ) is selected from the group consisting of the iron-laden zeolites of the MFI, BEA, FER, MOR, and MEL type and mixtures thereof. 
     
     
         13 . The apparatus as claimed in  claim 12 , wherein the catalyst in the deN2O stage is an Fe-ZSM-5. 
     
     
         14 . The apparatus as claimed in  claim 1 , wherein a guard bed consisting of a structured packing or bed of alumina-comprising shaped bodies is provided upstream of the iron-laden zeolite catalyst of the deN 2 O stage ( 3 ). 
     
     
         15 . The apparatus as claimed in  claim 1 , wherein the deNO x  catalyst in the deNO x  stage ( 6 ) is a transition metal-comprising SCR catalyst or an iron- or copper-laden zeolite. 
     
     
         16 . The apparatus as claimed in  claim 15 , wherein the deNOx catalyst is an SCR catalyst based on V 2 O 5 —TiO 2 . 
     
     
         17 . The apparatus as claimed in  claim 1 , wherein different catalysts are provided in the deN 2 O stage ( 3 ) and in the deNO x  stage ( 6 ).