TW201838707A - Process for the removal of sulphur oxides and nitrogen oxides contained in off-gas from an industrial plant - Google Patents

Abstract

Process for cleaning an off-gas containing sulphur ox-ides (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.

Description

 Method for removing sulfur oxides and nitrogen oxides contained in exhaust gas from a plant  

The present invention relates to a method for purifying sulfur oxides (SOx) and nitrogen in a single filter unit, in particular in a filter bag house having one or more catalytic fabric filter assemblies, using SOx removal and ammonia-SCR removal. A method of exhaust gas of oxides (NOx).

Selective catalytic reduction (SCR) is mainly a means of converting nitrogen oxides (NOx) into N ₂ and H ₂ O by reacting with anhydrous ammonia or ammonia in the presence of an SCR catalyst.

NO x reduction reaction occurs when the gas contacts the SCR catalyst. Ammonia or a precursor such as urea is injected and mixed with the gas upstream of the SCR catalyst.

The chemical equation for the stoichiometric reaction of a selective catalytic reduction process using anhydrous or aqueous ammonia is as follows: 4NO+4NH ₃ +O ₂ →4N ₂ +6H ₂ O

2NO ₂ +4NH ₃ +O ₂ →3N ₂ +6H ₂ O

NO+NO ₂ +2NH ₃ →2N ₂ +3H ₂ O

Exhaust gases from a variety of industrial processes contain SOx in addition to NOx. A well-known problem is that SCR using ammonia as a NOx reducing agent causes the formation of ammonium bisulphate (ABS) at low temperatures in the sulfur oxide-containing exhaust gas, resulting in deactivation of the SCR catalyst and generation of an ABS bond layer on downstream equipment.

An example of such a process is the carbonization of coal to produce coke for the steel industry. Coke oven gas (COG) is a valuable by-product of coal carbonization. COG is a potential feedstock for separating hydrogen, enriching methane, producing methanol, and producing syngas via partial oxidation of COG. It can also be effectively used to produce electricity and liquefied natural gas.

A typical SOx content in COG is 150 mg/Nm 3-180 mg/Nm3. In order to be used as a valuable exhaust gas, COG and other SOx and NOx containing exhaust gases must be purified by removing such impurities.

As already mentioned above, the problem of a gas having a high SOX (detailed SO ₃ ) content in the ammonia-SCR denitrification process is the formation of ammonium hydrogen sulfate. SO ₃ is reacted with ammonia by the following reaction scheme to form ammonium sulfate ((NH ₄ ) ₂ SO ₄ ) and ammonium hydrogen sulfate (NH ₄ HSO ₄ ): 2SO ₂ + O ₂ → 2SO ₃

2NH ₃ +SO ₃ +H ₂ O→(NH ₄ ) ₂ SO ₄

NH ₃ +SO ₃ +H ₂ O→NH ₄ HSO ₄

The bulk dew point of ammonium hydrogen sulfate at the inlet of the SCR reactor is typically about 290 ° C, but the dew point observed is higher due to the capillary forces in the microporous structure of the SCR catalyst.

Certain SCR catalysts, such as vanadium-based catalysts, are particularly sensitive to the contamination of ammonium sulfate, and especially ammonium hydrogen sulfate, which condenses at lower temperatures in the pore structure of the catalyst, thereby physically plugging the pores and The catalyst is deactivated.

One method of avoiding exhaust gas purification below the ammonium hydrogen sulfate dew point is to perform SCR prior to cooling the gas.

On the other hand, it is desirable to operate the SCR system at a low temperature below the dew point of ammonium hydrogen sulfate, because when the hot exhaust gas from the production plant is cooled before the SCR reaction, the energy demand is reduced and it is possible to separate some of the exhaust gases. Valuable organic compounds (such as benzene, toluene and xylene).

Another method of dealing with this problem is to periodically operate the SCR at elevated temperatures, wherein ammonium bisulfate is released from the catalyst and allows catalyst pores to be used to catalyze the reaction. In this way, the catalyst is reactivated.

When catalytic fabric bag filters are employed in SCR service and particulate matter filtration, the process must be carried out at operating temperatures below the bag breaking temperature. In general, the filter bag can withstand up to about 230 ° C, making it impossible to perform periodic heat treatment at elevated temperatures.

Catalytic bag filter operating at a dew point as the idea underlying the invention is in the NH ₃ -SCR less than ammonium bisulfate and at the same time by means of the powdered sulfur sorbent to remove sulfur oxides, ammonia reductant then added to the gas in.

Accordingly, the present invention provides a method for removing sulfur oxides and nitrogen oxides contained in exhaust gas from a plant, comprising the steps of: (a) cooling the exhaust gas to a temperature between 240 and 130 ° C (b) adding a powdered sulfur oxide adsorbent to the cooled exhaust gas to obtain the exhaust gas and an exhaust gas solid mixture of the sulfur oxides adsorbed on the powdery sulfur adsorbent; (c) Adding an exhaust gas solid mixture to the ammonia nitrogen oxide reducing agent; and (d) delivering the exhaust gas solid mixture together with the ammonia nitrogen oxide reducing agent to the filter bag house; (e) in step (c) of the filter bag house The solid mixture of exhaust gas containing the ammonia nitrogen reducing agent is passed through one or more fabric filter assemblies and will be adsorbed onto the powdered sulfur oxide adsorbent on the dispersed side of the one or more fabric filter bag assemblies The sulfur oxides are filtered off; and (f) is contacted with an SCR catalyst coated on the permeate side of the one or more filter bag assemblies by selective use of the ammonia nitrogen oxide reducing agent Catalytic reduction to reduce or remove the filtered The content of such nitrogen oxides in the exhaust gas.

The method of the present invention is particularly useful in the main portion ₃ of sulfur oxide SO, SO ₃ with ammonia as quickly reacted to ammonium bisulfate.

Preferably, the exhaust gas is cooled by indirect heat exchange in a heat exchanger, which is typically present in most existing purification systems for removing sulfur compounds. This makes the process of the invention attractive for the finishing of such purification systems.

When operating the process of the invention, sodium bicarbonate powder is preferably blown into the cooled off-gas as a sulfur oxide adsorbent. The sulfur oxides in the exhaust gas will thus be adsorbed onto the adsorbent powder and, optionally, with the dust and particulate matter on the dispersed side of the filter bag assembly.

Typically, the filter baghouse will contain a plurality of fabric filter bag assemblies that are disposed in the house in a conventional manner.

The filter bag assemblies may each consist of a single fabric filter bag with an SCR catalyst coated on the fabric on the permeate side of the bag.

In another embodiment of the invention, the filter bag assemblies each include an outer filter bag and one or more inner filter bags that are separate and concentrically disposed within the outer tubular filter bag.

The term "external filter bag" refers to a filter bag through which the process gas passes first, and the term "internal filter bag" refers to a filter bag through which the process gas passes sequentially after passing through the outer bag.

The latter embodiment has the advantage that different types and/or amounts of catalyst can be applied to different filter bags in each filter bag assembly.

The SCR catalyst applied to the filter bag comprises vanadium pentoxide and titanium oxide, and optionally additional oxides of tungsten and/or molybdenum.

The catalytically active material may further comprise palladium or platinum in a metallic and/or oxidized form.

Such catalyst is removed VOC (if present) and the carbon monoxide, and are active by the reaction with NH SCR ₃ of removing nitrogen oxides (NOx) in the.

Pd/V/Ti catalysts are preferred catalysts because (i) they have dual functionality (removing NOx and removing VOCs); (ii) they are sulfur tolerant; and (iii) are associated with other catalyst constituents It has lower oxidation activity than SO ₂ .

In another preferred embodiment of the invention, the SCR catalyst comprises a mixture of manganese, cerium and iron oxide supported on titanium dioxide. Such SCR catalysts have sufficient catalytic activity at temperatures well below 190 ° C (eg, 130 ° C). Thereby, ammonia slip can be removed or substantially reduced from the SCR catalyst at a lower temperature.

The sulfur oxide sorbent particles present in the process gas will be deposited on the exterior surface, i.e., the fabric filter bag faces the dispersed side of the unpurified exhaust solids mixture.

Thus, the catalyst loaded on the outer bag and/or the inner bag effectively prevents potential catalyst poisons, in particular sulfur oxides present in the exhaust gas.

This makes it possible to use iron and/or copper promoted zeolitic materials as effective SCR catalysts, especially in the low temperature range of the process, including, for example, Cu-SAPO-34 and Cu-SSZ-13.

Other SCR compositions suitable for use in the process of the invention include compositions comprising one or more acidic zeolite or zeolite-like components selected from the group consisting of BEA, MFI, FAU, FER, CHA, MOR, or mixtures thereof, and one or a plurality of physical mixtures of redox active metal compounds selected from the group consisting of Cu/Al ₂ O ₃ , Mn/Al ₂ O ₃ , CeO ₂ -ZrO ₂ , Ce-Mn/Al ₂ O ₃ and mixtures thereof, such as US patents Said in 9,168,517.

As already mentioned above, the process of the invention is very suitable for removing sulfur compounds from exhaust gases and preventing the formation of ammonium hydrogen sulfate from coke production and regenerative oxidation processes using sulfur-containing fuels.

Claims (14)

 A method for removing sulfur oxides and nitrogen oxides contained in exhaust gas from a plant, comprising the steps of: (a) cooling the exhaust gas to a temperature between 240 ° C and 130 ° C; (b) Adding a powdered sulfur oxide adsorbent to the cooled exhaust gas to obtain an exhaust gas solid mixture of the exhaust gas and the sulfur oxide adsorbed on the powdery sulfur adsorbent; (c) adding the exhaust gas solid mixture And (d) transporting the exhaust gas solid mixture together with the ammonia nitrogen oxide reducing agent to the filter bag house; (e) obtaining the content obtained in the step (c) in the filter bag house An exhaust solid mixture of ammonia nitrogen reducing agent passes through one or more fabric filter assemblies and will adsorb the sulfur on the powdered sulfur oxide adsorbent on the dispersed side of the one or more fabric filter bag assemblies The oxide is filtered off; and (f) is reduced by selective catalytic reduction with the ammonia nitrogen reducing agent by contact with an SCR catalyst coated on the permeate side fabric of the one or more filter bag assemblies Or removing the nitrogen in the filtered exhaust gas The content of compound.   The method of claim 1, wherein a major portion of the adsorbed sulfur oxides consists of SO ₃ .  The method of claim 1 or 2, wherein the exhaust gas is cooled by heat exchange.    The method of any one of claims 1 to 3, wherein the powdered sulfur oxide adsorbent comprises sodium hydrogencarbonate.    The method of any of claims 1 to 4, wherein each of the one or more filter bag assemblies is comprised of a single fabric filter bag.    The method of any one of claims 1 to 4, wherein the one or more filter bag assemblies each comprise an outer filter bag and one or more inner filter bags disposed separately and concentrically within the outer filter bag .    The method of any one of claims 1 to 6, wherein the SCR active catalyst comprises vanadium pentoxide and titanium oxide.    The method of claim 7, wherein the SCR active catalyst further comprises an oxide of tungsten and/or molybdenum.    The method of claim 7 or 8, wherein the SCR active catalyst further comprises palladium or platinum in a metal and/or oxidized form.    The method of any one of claims 1 to 6, wherein the SCR active catalyst comprises a mixture of manganese, iron and cerium oxide supported on titanium dioxide.    The method of any one of claims 1 to 10, wherein the SCR active catalyst comprises a zeolitic material promoted with iron and/or copper.   The method of any one of claims 1 to 10, wherein the SCR active catalyst comprises one or more acidic zeolites or zeolite-like groups selected from the group consisting of BEA, MFI, FAU, FER, CHA, MOR or mixtures thereof. a physical mixture of one or more redox active metal compounds selected from the group consisting of Cu/Al ₂ O ₃ , Mn/Al ₂ O ₃ , CeO ₂ -ZrO ₂ , Ce-Mn/Al ₂ O ₃ and mixtures thereof .  The method of any one of claims 1 to 12, wherein the exhaust gas is derived from a regenerative oxidation process using a sulfur-containing fuel.    The method of any of claims 1 to 12, wherein the off-gas is derived from coke production.