A process for the removal of NOX and dinitrogen oxide in a sulfur oxides containing off-gas

Abstract

Process for the simultaneous reduction of NOx (NO, NO2) and nitrous oxide (N2O) in an off-gas containing sulfur oxides containing off-gas comprising the steps of (a) increasing or maintaining temperature of the off-gas to above 3000C; (b) adding an amount of a reducing agent into the off-gas; (c) passing the off-gas admixed with the reducing agent through a catalyst comprising an Fe-zeolite; (d) reducing the content of NOx and N2O in the off-gas; and (e)withdrawing a cleaned off-gas containing the N2, H2O and remaining amounts of sulfur oxides.

Description

[0001] Title : A process for the removal of NOx and dinitrogen oxide in a sulfur oxides containing off-gas

[0002] The present invention relates to a process for the combined removal of NOx (NO and NO2 ) and nitrous oxide ( dinitrogen oxide , N2O) in process of f-gas containing sul fur oxides .

[0003] N2O is a potent greenhouse gas with 265 times the ef fect of CO2. N2O makes a considerable contribution to decomposing ozone in the stratosphere and to the greenhouse ef fect . For environmental protection reasons there is therefore an increasing need for technical solutions to the problem of reducing N2O emissions together with NOx emission .

[0004] N20 decomposes at high temperatures , but quite high temperatures are needed, typically above 1000°C to obtain reasonable reduction rates .

[0005] Catalytic N2O and NOx removal proceed at much lower temperatures , thus reducing costs and climate impact from heating .

[0006] N2O and NOx occur in of f-gases inter alia from waste incineration in particular by fluid bed combustion, of f-gases from Caprolactam production and in exhaust gas from engines operated with ammonia using diesel as support fuel .

[0007] A number of of f-gases , like the above-mentioned gases , contain sul fur oxides . The removal of NOx and N2O in gases containing sulfur oxides is problematic, because the N2O reduction catalyst is sensitive to sulfur oxides.

[0008] Currently, N20 reduction catalysts are not employed in streams containing sulfur oxides.

[0009] We have observed that Fe-zeolite catalysts in a sulfur oxides containing gas are active in the reduction of N2O and NOx. The Fe-zeolite catalyst is capable of removing both NOx and N2O using NH3(ammonia) as a reducing agent. The N2O conversion is slightly affected by the sulfur oxides, but not very seriously. The reduction agent, typically ammonia, consumption (for N2O removal) is reduced. This is because part of the SO2 is used as reducing agent for N2O. It is likely that SO3 is formed in this process.

[0010] Pursuant to the above findings and observations, the present invention provides a process for the simultaneous reduction of NOx (NO, NO2) and nitrous oxide (N2O) in an offgas containing additional sulfur oxides containing off-gas comprising the steps of

[0011] (a) increasing or maintaining temperature of the off-gas to above 300°C;

[0012] (b) adding an amount of a reducing agent into the off-gas from step (a) ;

[0013] (c) passing the off-gas admixed with the reducing agent through a catalyst comprising an Fe-zeolite;

[0014] (d) reducing the NOx and N2O in the off-gas to N2 and H2O; and

[0015] ( e ) withdrawing a cleaned off-gas containing the N2, H2O and remaining amounts of sulfur oxides. Advantageously, N2O and NOx can be removed by the process of the invention simultaneously at the same temperature in a gas stream containing sul fur oxides , resulting in process cost savings .

[0016] The term "NOx" as used herein refers to nitrogen oxides other than nitrous oxide .

[0017] The term "reduction of NOx" and "reduction of nitrous oxide (N2O) " should be understood as substantially reducing the amounts of NOx and N2O, even i f minor amounts of NOx and N2O can still be contained in the process of f-gas .

[0018] Preferably, the NOx is removed with at least 95% and the N2O is removed with at least 90% .

[0019] In accordance with the invention, sul fur oxides removal upstream the catalyst to very low levels can be prevented .

[0020] Sul fur sensitive catalysts will require very high degree of sul fur removal upstream to avoid sul fur poisoning . With the present invention such upstream sul fur removal can be avoided .

[0021] Preferably, the reducing agent comprises ammonia or precursors thereof .

[0022] Additionally, it has been observed that the sul fur dioxide acts as a reducing agent for NOx or N2O and less amounts of ammonia or precursors thereof are needed in the process . Depending on the amount of sulfur dioxide, NOx and N20 the amount of ammonia added to the off-gas is NH3:NOx as 1:1 and NH3:N2O as [ 0.6-1.1 ] : 1. The amount of NH3:N2O is adjusted for temperature and concentration of sulfur oxides.

[0023] It is preferred that the dosing of the ammonia reducing agent is adjusted to result in a concentration below 20 vol ppm, such as between 5 and 20 vol ppm, measured downstream the catalyst.

[0024] NH3dosing should be adjusted to avoid excessive NH3slip to reduce the risk of ammonium bisulfate formation in downstream cold spots.

[0025] Preferably, the dosing of the reducing agent is adjusted using a feed-back control with the measured outlet concentration of reducing agent. Thereby any SO2 in the off-gas functioning as reducing agent for the N2O, will result in less reducing agent consumption as the dosing is controlled by the outlet concentration of reducing agent.

[0026] In order to obtain low emission of N2O and a low slip of reducing agent, highly effective mixing of the reducing agent in the gas is required.

[0027] When using ammonia as reducing agent, then in order for the N2O decomposition reaction to be effective and result in a low slip, the ammonia dosing must be controlled. Preferably ammonia dosing is controlled to typically have an ammonia slip below 20 ppm or lower. It is better for the environment with a low slip, and it reduces the potential formation of ammonium sulfates downstream the catalyst. Keeping the temperature above 300°C means that the iron zeolite is active for N2O reduction, but also that ammonium bisul fate cannot form on the catalyst and cause it to deactivate . It is an important feature to keep the temperature higher than 300°C so the catalyst is not deactivated by ammonium bisul fate blocking the actives sites and the pores of the catalyst .

[0028] As mentioned hereinabove , the catalyst active in selective catalytic reduction of NOx, is also active in removal of nitrous oxide using the same reducing agent .

[0029] Preferably, the metal exchanged zeolite is selected from the group consisting of MFI , BEA, FER, MOR, FAU, CHA, AEI , ERI and / or LTA.

[0030] The most preferred metal exchanged zeolite is Fe-BEA.

[0031] The term "monolithic shaped catalyst" should be understood as a monolithic or honeycomb shape containing or coated with catalytic active material .

[0032] The monolithic shaped catalyst is preferably arranged orderly layered in one or more layers inside reactor ( s ) .

[0033] The monolithic shaped catalysts enable an axial flow reactor design, while at the same time providing a low pressure drop, compared to the radial flow reactor design with pellet catalysts . In another preferred embodiment, the monolithic shaped catalyst is arranged inside the reactor in more than one stacked layer.

[0034] When SO2 acts as a reducing agent for the N20 reactions some SO3 is formed. SO3 can react with water and form sulfuric acid. The acid dewpoint depends on the SO3 and H2O concentrations as well as the pressure. Typical operating conditions results in an acid dewpoint temperature lower than 180°C. Therefore, maintaining a high temperature downstream can be advantageous to avoid acid condensation and corrosion. The sulfur oxides in the gas can be fully or partially removed downstream to reduce the issues with acid condensation or for environmental protection. This can for instance be done in a wet or a dry scrubber.

[0035] In summary, the preferred features of the invention are:

[0036] 1. Process for the simultaneous reduction of NOx (NO,

[0037] N02 ) and nitrous oxide (N2O) in an off-gas containing sulfur oxides comprising the steps of

[0038] (a) increasing or maintaining temperature of the off-gas to above 300°C;

[0039] (b) adding an amount of a reducing agent into the off-gas;

[0040] (c) passing the off-gas admixed with the reducing agent through a catalyst comprising an Fe-zeolite;

[0041] (d) reducing NOx and N2O in the off-gas to N2 and H2O; and

[0042] ( e ) withdrawing a cleaned off-gas containing the N2, H2O and remaining amounts of sulfur oxides. 2. Process of feature 1, wherein the reducing agent comprises ammonia or precursors thereof.

[0043] 3. Process of features 1 or 2, wherein the sulfur oxides are sulfur dioxide.

[0044] 4. Process of feature 3, wherein the sulfur dioxide is used as a reducing agent for NOx or N2O.

[0045] 5. Process of any one of features 1 to 4,

[0046] Process of any one of claims 1 to 4, wherein the Fe-zeolite is MFI, BEA, FER, MOR, FAU, CHA, AEI, ERI and / or LTA.

[0047] 6. Process of any one feature 1 to 5, wherein the Fe- zeolite is Fe-BEA.

[0048] 7. Process of any one of features 1 to 6, wherein the catalyst comprising an Fe-zeolite is monolithic shaped.

[0049] 8. Process of feature 7, wherein the monolithic shaped Fe-zeolite catalyst is arranged in more than one stacked layer .

[0050] 9. Process according to any one of the preceding features, wherein sulfur oxides are removed in a downstream step .

[0051] 10. Process of feature 9, wherein the downstream step comprises a wet or dry gas scrubbing.

[0052] 11. Process of any one of the preceding features, wherein the temperature downstream the catalytic removal of NOx and N2O is maintained above the sulfuric acid dewpoint of the off-gas until the off-gas is emitted or enters a downstream sulfur oxide removal.

[0053] 12. Process of any one of the preceding features, wherein the NOx is removed with at least 95% and the N2O is removed with at least 90%.

[0054] 13. Process of any one of the preceding features , wherein the off-gas contains O2 in the range of 5-15 vol% and H2O in the range of 5-15 vol%.

[0055] 14. Process of any one of the preceding features, wherein the dosing of the reducing agent is adjusted using a feed-back control with the measured outlet concentration of reducing agent.

[0056] 15. Process of any one of the preceding features, wherein the dosing of the reducing agent is adjusted to between 5 and 20 vol ppm as measured downstream the catalyst.

[0057] 16. Process of any one of the preceding features, wherein the off-gas further contains water and oxygen.

[0058] The invention is further discussed in the following detailed description of a speci fic embodiment thereof .

[0059] Temperatures are typically in the range of 300-550 ° C . Pressure is typically near atmospheric but can be both higher and lower . A higher pressure increases activity of NOx and N2O conversion .

[0060] Ammonia is inj ected and mixed into the of f-gas . The of f-gas admixed with the ammonia enters a reactor containing a catalyst comprising Fe-BEA zeolite . In this reactor, NOx reacts with the ammonia according to the well-known SCR reactions . But the iron zeolite catalyst is also active for decomposing N2O using NH3, according to the reaction :

[0061] 3N2O + 2NH34N2+ 3H2O

[0062] This reaction is slower than the SCR reactions removing the NOx . But it means that more NH3can be dosed than what is needed for the NOx reactions and that this excess NH3 will then be used to decompose N2O . The catalyst volume and the amount of ammonia dosing is adj usted such that the gas coming from the catalyst is essentially free from NOx and with a low NH3slip, below 20 ppm or 10 ppm or 5 ppm by volume in the ef fluent gas from the reactor . The optimal choice of catalyst volume and reducing agent addition is governed by the initial concentration of NOx, N2O and Sulfur oxides, the gas temperature and pressure, the injection system for reducing agent and the required conversions of NOx and N2O. Water (H2O) and oxygen (O2) concentration will also affect the optimal choice as the different reactions has different sensitivity towards H2O and 02.

[0063] Balance to 100% by N2. Test 1 and lb: low dosing of NH3. Similar N30 conversion and slightly improved NOx conversion when sulfur oxides are present. Less reducing agent is used to convert similar N2O and more NOx. Test 2 and 2b: High dosing of NH3. N2O performance is reduced, and so is reducing agent consumption when sulfur oxides are present. Less reducing agent consumption per mole of N2O removed.

[0064] Test 3 and 3b: High dosing of NH3, lower temperature. N2O and NOx performance similar. Lower reducing agent consumption per mole of N2O removed.

Claims

Claims :

1. Process for the simultaneous reduction of NOx (NO,N02 ) and nitrous oxide (N2O) in an off-gas containing sulfur oxides containing off-gas comprising the steps of(a) increasing or maintaining temperature of the off-gas to above 300°C;(b) adding an amount of a reducing agent into the off-gas;(c) passing the off-gas admixed with the reducing agent through a catalyst comprising an Fe-zeolite;(d) reducing the content of NOx and N2O in the off-gas; and( e ) withdrawing a cleaned off-gas containing the N2, H2O and remaining amounts of sulfur oxides.

2. Process of claim 1, wherein the reducing agent comprises ammonia or precursors thereof.

3. Process of claim 1 or 2, wherein the sulfur oxides are sulfur dioxide.

4. Process of claim 3, wherein the sulfur dioxide is used as a reducing agent for NOx or N2O.

5. Process of any one of claims 1 to 4, wherein the Fe-zeolite is MFI, BEA, FER, MOR, FAU, CHA, AEI, ERI and / or LTA.

6. Process of any one of claims 1 to 4, wherein theFe-zeolite is Fe-BEA.

7. Process of any one of claims 1 to 6, wherein the catalyst comprising an Fe-zeolite is monolithic shaped.

8. Process of claim 7, wherein the monolithic shapedFe-zeolite catalyst is arranged in more than one stacked layer .

9. Process according to any one of the preceding claims, wherein sulfur oxides are removed in a downstream step .

10. Process of claim 9, wherein the downstream step comprises a wet or dry gas scrubber.

11. Process of any one of the preceding claims, wherein the temperature downstream the catalytic removal of NOx and N2O is maintained above the sulfuric acid dewpoint of the off-gas until the off-gas is emitted or enters the downstream sulfur oxide removal.

12. Process of any one of the preceding claims, wherein the NOx is removed with at least 95% and the N2O is removed with at least 90%.

13. Process of any one of the preceding claims, wherein the off-gas contains O2 in the range of 5 to 15 vol% and H2O in the range of 5 to 15 vol%.

14. Process of any one of the preceding claims, wherein the dosing of the reducing agent is adjusted using a feed-back control with the measured outlet concentration of reducing agent.

15. Process of claim 14, wherein the dosing of the reducing agent is adjusted to below 10 ppm as measured downstream the catalyst.

16. Process according to any one of the preceding claims, wherein the off-gas further contains water and oxygen .

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