CASTELLINO FRANCESCO (DK)
SAADI SOUHEIL (DK)
JAKOBSSON NIKLAS BENGT (SE)
| WO2006103040A1 | 2006-10-05 | |||
| WO2015140305A1 | 2015-09-24 |
| US4220633A | 1980-09-02 | |||
| EP0268353A1 | 1988-05-25 | |||
| US5540897A | 1996-07-30 | |||
| CN104190193B | 2015-11-18 | |||
| US9168517B2 | 2015-10-27 |
| Claims : 1. Process for the removal sulphur oxides and nitrogen oxides contained in off-gas from an industrial plant com- prising the steps of (a) cooling the off-gas to a temperature of between 240 and 130°C; (b) adding a pulverous sulphur oxide adsorbent to the cooled off-gas to obtain an off-gas solid mixture of the off-gas and the sulphur oxides adsorbed on the pulverous sulphur adsorbent ; (c) adding into the off-gas solid mixture an ammonia nitro¬ gen oxide reducing agent; and (d) passing the off-gas solid mixture together with the am- monia nitrogen oxide reducing agent to a filter bag house; (e) in the filter bag house passing the ammonia nitrogen reducing agent containing off-gas solid mixture obtained in step (c) through one or more fabric filter assemblies and filtering off the sulphur oxides adsorbed on the pulverous sulphur oxide adsorbent on dispersion side of the one or more fabric filter bag assemblies; and (f) reducing or removing content of the nitrogen oxides in the filtered off-gas by selective catalytic reduction with the ammonia nitrogen oxide reducing agent by contact with an SCR catalyst coated on fabric within permeation side of the one or more filter bag assemblies. 2. The process of claim 1, wherein main part of the adsorbed sulphur oxides consists of SO3. 3. The process of claim 1 or 2, wherein the off-gas is cooled by heat exchange. 4. The process of any one of claims 1 to 3, wherein the pulverous sulphur oxide adsorbent comprises sodium bi¬ carbonate . 5. The process of any one of claims 1 to 4, wherein each of the one or more filter bag assemblies consists of a single fabric filter bag. 6. The process of any one of claims 1 to 4, wherein the one or more filter bag assemblies each comprises an outer filter bag and one or more inner filter bags separately and concentrically arranged within the outer filter bag . 7. The process of any one of claim 1 to 6, wherein the SCR active catalyst comprises vanadium pentoxide and tita¬ nium oxide. 8. The process of claim 7, wherein the SCR active cat- alyst further comprises oxides of tungsten and/or molyb¬ denum. 9. The process of claim 7 or 8, wherein the SCR active catalyst further comprises palladium or platinum in metal- lie and/or oxidic form. 10. The process of any one of claims 1 to 6, wherein the SCR active catalyst comprises a mixture of oxides of manganese, iron and cerium supported on titania. 11. The process of any one of claims 1 to 10, wherein the SCR active catalyst comprises zeolitic material pro¬ moted with iron and/or copper. 12. The process of any one of claims 1 to 10, wherein the SCR active catalyst comprises one or more acidic zeo¬ lite or zeotype components selected from the group consist¬ ing of BEA, MFI, FAU, FER, CHA, MOR or mixtures thereof physically admixed with one or more redox active metal com¬ pounds selected from the group consisting of CU/AI2O3, Mn/Al203, Ce02-Zr02, Ce-Mn/Al203 and mixtures thereof. 13. The process of any one of claims 1 to 12, wherein the off-gas originates from a regenerative oxidation pro¬ cess with a sulphur containing fuel. 14. The process of any one of claims 1 to 12, wherein the off-gas originates from production of coke. |
The present invention relates to a process for cleaning an off-gas containing sulphur oxides (SOx)and nitrogen oxides (NOx) employing removal of SOx and ammonia-SCR in a single filtration unit, in particular a filter bag house with one or more catalysed fabric filter assemblies. Selective catalytic reduction (SCR) is primarily a means of converting nitrogen oxides (NOx) into 2 and H 2 0 by reac ¬ tion with anhydrous ammonia or aqueous ammonia in presence of an SCR catalyst. The NOx reduction reaction takes place as the gas contacts an SCR catalyst. Ammonia, or a precursor, such as urea is injected and mixed with the gas upstream the SCR catalyst.
The chemical equation for a stoichiometric reaction using either anhydrous or aqueous ammonia for a selective cata ¬ lytic reduction process is as shown below:
4NO + 4NH 3 + 0 2 → 4N 2 + 6H 2 0
2N0 2 + 4NH 3 + 0 2 → 3N 2 + 6H 2 0
NO + N0 2 + 2NH 3 → 2N 2 + 3H 2 0
Off-gases from a number industrial processes contain beside NOx also SOx. It is a well-known problem that SCR employing ammonia as NOx reductant in sulphur oxides containing off- gas at low temperatures leads to formation of ammonium bi ¬ sulphate (ABS) resulting in deactivation of the SCR cata ¬ lyst and sticky layers of ABS on downstream equipment. An example of such processes is coal carbonization to pro ¬ duce coke for the steel industry . Coke oven gas (COG) is a valuable by-product of coal carbonization. COG is a poten ¬ tial feedstock for hydrogen separation, methane enrichment , methanol production and synthesis gas production through partial oxidation of COG . It can also be effectively uti ¬ lized to produce electricity and liquefied natural gas .
The typical SOx level in COG is 150 mg/Nm3 - 180 mg/Nm3. To be suitable as a valuable off-gas, COG and other SOx and NOx containing off-gases must be cleaned by removal of these impurities.
As already mentioned above, a problem in the ammonia-SCR denitrification process of gases with a high content of SOX, in particular S O 3 , is formation of ammonium bisulphate. S O 3 reacts with ammonia to produce ammonium sulphate (( H 4 ) 2 S0 4 ) and ammonium bisulphate ( NH 4 HS O 4 ) by the follow ¬ ing reaction scheme: 2S0 2 + 0 2 → 2S0 3
2NH 3 + S0 3 + H 2 0 → (NH 4 ) 2 S0 4
NH 3 + S0 3 + H 2 0 → NH4HSO4
The bulk dew point of ammonium bisulphate at the SCR reac- tor inlet is typically around 290°C, but the observed dew point is higher due to capillary forces in the micropore structure of the SCR catalyst. Certain SCR catalysts, such as the vanadium-based cata ¬ lysts, are particularly sensitive to contamination from ammonium sulphate and especially ammonium bisulphate, which is condensed in the pore structure of the catalyst at lower temperatures, thereby physically blocking the pores and de ¬ activating the catalyst.
One way to avoid the off-gas cleaning below the dew point of ammonium bisulphate is to perform the SCR prior to cool- ing the gas.
On the other hand, operation of the SCR at a low tempera ¬ ture below the dew point of ammonium bisulphate is desira ¬ ble because of a reduced energy demand and the possibility of separation of valuable organic compounds like benzene, toluene and xylene contained in certain off-gases, when cooling the hot off-gas from production plant prior to the SCR reaction. Another way to handle this problem is to periodically oper ¬ ate the SCR at a high temperature, where the ammonium bi ¬ sulphate is released from the catalyst and the catalyst pores are made available for the catalytic reaction. In this way, the catalyst is reactivated.
When employing catalysed fabric bag filters in SCR service and particulate matter filtration, the process must be per ¬ formed at operation temperatures below the destruction temperature of the bags. Generally, filter bags are durable up to around 230°C which makes periodic heat treatment at high temperatures impossible. The idea underlying the present invention is to operate a catalysed bag filter in NH 3 -SCR below the dew point of am ¬ monium bisulphate and simultaneously remove sulphur oxides by means of a pulverous sulphur adsorbent prior to addition of ammonia reductant into the gas.
Thus, this invention provides a Process for the removal sulphur oxides and nitrogen oxides contained in off-gas from an industrial plant comprising the steps of
(a) cooling the off-gas to a temperature of between 240 and 130°C;
(b) adding a pulverous sulphur oxide adsorbent to the cooled off-gas to obtain an off-gas solid mixture of the off-gas and the sulphur oxides adsorbed on the pulverous sulphur adsorbent;
(c) adding into the off-gas solid mixture an ammonia nitro ¬ gen oxide reducing agent; and
(d) passing the off-gas solid mixture together with the am ¬ monia nitrogen oxide reducing agent to a filter bag house; (e) in the filter bag house passing the ammonia nitrogen reducing agent containing off-gas solid mixture obtained in step (c) through one or more fabric filter assemblies and filtering off the sulphur oxides adsorbed on the pulverous sulphur oxide adsorbent on dispersion side of the one or more fabric filter bag assemblies; and
(f) reducing or removing content of the nitrogen oxides in the filtered off-gas by selective catalytic reduction with the ammonia nitrogen oxide reducing agent by contact with an SCR catalyst coated on fabric within permeation side of the one or more filter bag assemblies. The process of the invention is in particular useful when main part of the sulphur oxides is SO 3 , as SO 3 reacts very fast with ammonia to ammonium bisulphate. Preferably, the off-gas is cooled by means of indirect heat-exchange in a heat exchanger, which typically is pre ¬ sent in most of the existing cleaning systems for the re ¬ moval of sulphur compounds. This makes the process accord ¬ ing to the invention attractive for retrofit of these cleaning systems.
When operating the process according to the invention, it is preferred to blow powder of sodium bicarbonate as sul ¬ phur oxide adsorbent into the cooled off-gas. Sulphur ox- ides in the off-gas will thereby be adsorbed on the adsor ¬ bent powder and can filtered off optionally together with dust and particulate matter on dispersion side of the fil ¬ ter bags assemblies. Typically, the filter bag house will contain a plurality of fabric filter bags assemblies arranged in usual manner in the house.
The filter bag assemblies may each consist of a single fab- ric filter bag with an SCR catalyst coated on fabric in the permeation side of the bag.
In another embodiment of the invention, the filter bag assemblies each comprises an outer filter bag and one or more inner filter bags separately and concentrically arranged within the outer tubular filter bag. The term "outer filter bag" refers to the filter bag through which the process gas passes first, and the term "inner filter bag" refers to the filter bag(s) through which the process gas passes successively after having passed through the outer bag.
The later embodiment has the advantage that either differ ¬ ent types and/or amounts of catalysts can be coated on dif ¬ ferent filter bags in each filter bag assembly.
The SCR catalyst applied on the filter bags comprises vana ¬ dium pentoxide and titanium oxide and optionally addition ¬ ally oxides of tungsten and/or molybdenum. The catalytically active material can further comprise pal ¬ ladium or platinum in metallic and/or oxidic form.
These catalysts are active both in the removal of VOCs if present and carbon monoxide and in the removal of nitrogen oxides (NOx) by the SCR reaction with NH 3 .
The Pd/V/Ti catalyst is a preferred catalyst because (i) it has a dual functionality (removal of NOx and removal of VOCs) , (ii) it is sulphur-tolerant, and (iii) it has a lower SO 2 oxidation activity compared to other catalyst compositions .
In further a preferred embodiment of the invention the SCR catalyst comprises a mixture of oxides of manganese, cerium and iron supported on titania. Such an SCR catalyst has a sufficient catalytic activity at temperatures well below 190°C, e.g. 130°C. Thereby, it is possible to remove or sufficiently reduce ammonia slip from the SCR catalyst at lower temperatures.
Sulphur oxide adsorbent particles present in the process gas will be deposited on the outer surface, i.e. the dis ¬ persion side of the fabric filter bag facing the uncleaned off-gas solid mixture.
Thus, the catalysts loaded onto the outer bag and/or the inner bag(s) are effectively protected against potential catalyst poisons in particular sulphur oxides present in the off-gas.
This makes it possible to employ zeolitic material promoted with iron and/or copper as effective SCR catalysts, espe ¬ cially in the low temperature range of the process, includ ¬ ing e.g. Cu-SAPO-34 and Cu-SSZ-13.
Further SCR composition useful in the process according to the invention include compositions comprising one or more acidic zeolite or zeotype components selected from the group consisting of BEA, MFI, FAU, FER, CHA, MOR or mixtures thereof physically admixed with one or more redox ac ¬ tive metal compounds selected from the group consisting of CU/AI 2 0 3 , Mn/Al 2 0 3 , Ce0 2 -Zr0 2 , Ce-Mn/Al 2 0 3 and mixtures thereof, as described in US patent 9,168,517.
As already mentioned above, the process according to the invention is well suited for the removal of sulphur com- pounds and prevents formation of ammonium bisulphate in off-gas from coke production and from regenerative oxida ¬ tion processes with a sulphur containing fuel.