Method for the simultaneous removal of carbon monoxide and nitrogen oxides from flue or exhaust gas

Abstract

A method in which flue gas or exhaust gas containing harmful carbon monoxide, organic compounds (VOC) and NOx is contacted with a layered catalyst. A first layer of the catalyst comprises an oxidation catalyst. An underlying layer of the catalyst comprises a NH3-SCR catalyst for the simultaneous removal of the carbon monoxide and NOx.

Claims

1. Method for the reduction of amounts carbon monoxide, volatile organic compounds and nitrogen oxides in flue or exhaust gas comprising the steps of: introducing ammonia and/or a precursor thereof into the flue or exhaust gas; converting the precursor, if any, into ammonia; contacting the gas and the ammonia at a temperature of up to 350 C. with a layered catalyst comprising an upper first catalyst layer with an oxidation catalyst and an underlying second catalyst layer with an NH3-SCR catalyst supporting completely the first layer, and oxidizing at least part of the amounts of carbon monoxide and the volatile organic compounds in the upper first layer without affecting the ammonia further contained in the gas, and reducing the amounts of nitrogen oxides in the underlying second catalyst layer by reaction with the ammonia.

2. The method of claim 1, wherein the oxidation catalyst in the first layer consists of palladium, vanadium oxide and titanium oxide.

3. The method of claim 2, wherein the oxidation catalyst consists of 0.45 wt %, palladium, 4.5 wt % vanadium pentoxide and supported on titanium oxide.

4. The method of claim 1, wherein the first catalyst layer has a layer thickness of between 10 and 200 micron.

5. The method of claim 1, wherein the first catalyst layer has a layer thickness of between 10 and 50 micron.

6. The method of claim 1, wherein the NH3-SCR catalyst in the second catalyst layer comprises oxides of tungsten, molybdenum, vanadium and titanium.

7. The method of claim 1, wherein the flue or exhaust gas is additionally treated with a conventional not layered SCR catalyst either upstream or downstream the layered catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a drawing showing the reactions as the gas contacts the layered catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) Referring to FIG. 1, at gas temperatures up to 350 C., CO and VOC will be oxidized to CO2 in the first oxidation layer 2 of layered catalyst 1, while all injected NH3 for the NOx abatement will simultaneously diffuse through oxidation layer 2 and react instead on the underlying SCR catalyst layer 3.

(3) By enhancing the pore structure and thickness of the first catalyst layer, both NOx and NH3 will easily access the underlying SCR catalyst and very limited SCR activity will be lost due to the diffusion rate of reagents across the oxidation catalyst layer.

(4) Thus, in a further embodiment of the invention the first catalyst layer has a layer thickness of between 10 and 200 micron, preferably of between 10 and 50 micron.

(5) In the method according to invention as described above, the flue or exhaust gas can additionally be treated with a conventional not layered SCR catalyst either up or downstream the layered catalyst.

(6) The invention provides additionally a catalyst for simultaneous oxidation of carbon monoxide and volatile organic compounds and selective reduction of nitrogen oxides by reaction with ammonia, the catalyst comprises a first layer of an oxidation catalyst and a second layer of an NH3-SCR catalyst supporting completely the first layer.

(7) Preferably, the oxidation catalyst consists of palladium, oxides of vanadium and oxides of titanium.

(8) The preferred oxidation catalyst according to an embodiment of the invention also has some SCR activity due to the presence of both TiO2 and vanadium oxides. Full SCR activity is thus preserved without the need of increasing the addition of ammonia. In this way, the desired removal of both CO and NOx can be accomplished with a significantly reduced catalyst volume.

(9) In an embodiment of the invention the first catalyst layer has a layer thickness of between 10 and 200 micron, preferably of between 10 and 50 micron.

(10) When structuring the layered catalyst in monolithic form, the resulting monolith catalyst will have a uniform catalyst composition across the monolith length. CO, VOC and NOx removal proceeds simultaneously along the whole length of the monolith.

EXAMPLE

(11) A V/Ti based commercial SCR catalyst has been coated with a catalyst consisting of 0.45 wt % Pd, 4.5 wt % V2O5 on TiO2. The NOx removal efficiency has been measured and compared to the NOx removal efficiency of the same SCR catalyst not coated with the oxidation catalyst. The results and conditions of the test are shown in Table 1 below:

(12) TABLE-US-00001 TABLE 1 DeNOx, % DeCO, % NH3 slip, ppmv SCR 93.5 0 5.7 OXIDATION + SCR 92.9 97.8 4.4

(13) Test Conditions.

(14) Gas inlet composition: 50 ppmv NOx, 55 ppmv NH3, 100 ppmv CO, 15% vol O2, 10% vol H2O, N2 balance. Gas space velocity, NHSV=27 000 h-1. Temperature: 350 C.

(15) As apparent from Table 1, the same (within experimental uncertainty) NOx removal efficiency has been obtained in both tests.