Process for catalytic filtration of sulfur-containing gases using selective catalytic reduction - Patents.com

Abstract

A method for catalytic filtration of sulfur-containing gases using a selective catalytic reduction (SCR) bag filter, the SCR bag filter being operated at a temperature below the dew point of ammonium bisulfate, in which the catalyst is periodically regenerated using ozone injection, in this manner effectively removing ammonium bisulfate from the pore structure of the catalyst.

Description

[Technical field]

[0001] The present invention relates to a novel method for catalytic filtration of sulfur-containing gases using selective catalytic reduction (SCR) with catalyst regeneration using ozone injection. [Background technology]

[0002] It is well known that when conventional SCR catalysts are operated in sulfur-containing flue gases at low temperatures, the formation of ammonium bisulfate (NH4HSO4) is a significant design and operational issue.

[0003] Selective catalytic reduction is primarily a means of converting NOx (nitrogen oxides) to N2 and H2O. A gaseous reducing agent, typically anhydrous ammonia, aqueous ammonia or urea, is added to the flue gas or exhaust gas stream and then adsorbed onto a catalyst. When urea is used as the reducing agent, CO2 is the reaction product.

[0004] SCR catalysts are made from various ceramic materials, such as titanium oxide, used as supports, and the active catalyst components are usually either oxides of base metals (e.g. vanadium, optionally reinforced with molybdenum and / or tungsten), zeolites, or various precious metals (such as palladium and platinum). Alternative catalysts based on activated carbon have also been developed that are applicable for the removal of NOx at low temperatures.

[0005] The NOx reduction reaction occurs as the gas passes through the catalytic chamber. Ammonia or other reducing agent (e.g., urea) is injected and mixed with the gas before entering the catalytic chamber. The chemical equation for the stoichiometric reaction using either anhydrous ammonia or aqueous ammonia for the selective catalytic reduction process is shown below: 4NO+4NH3+O2→4N2+6H2O 2NO2+4NH3+O2→3N2+6H2O NO+NO2+2NH3→2N2+3H2O With several secondary reactions: 2SO2+O2→2SO3 2NH3+SO3+H2O→(NH4)2SO4 NH3+SO3+H2O→NH4HSO4 During the combustion process in power plants, sulfur is typically formed as various sulfur oxides (SOx), typically about 97% SO2 and up to 3% SO3, along with CO and hydrocarbons. For example, fuels with higher sulfur content tend to produce greater amounts of SO3.

[0006] As shown above, SO3 can react with ammonia to produce ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4). Certain SCR catalysts, such as vanadium-based catalysts, are particularly susceptible to contamination from ammonium sulfate and especially ammonium bisulfate, which condenses in the catalyst's pore structure at lower temperatures, thereby physically blocking the pores and deactivating the catalyst. On the other hand, operation of SCR at lower temperatures is desirable because it can result in higher efficiencies in terms of power production in the plant.

[0007] One proven way to address this dilemma is to periodically operate the SCR at higher temperatures, which releases the ammonium bisulfate from the catalyst and makes the catalyst pores available for catalytic reactions. In this way, the catalyst is reactivated.

[0008] Catalytic bag filters in SCR systems are often operated at low temperatures. For sulfur-containing gases, the formation of ammonium bisulfate is a major cause for concern and often prevents the use of bag filter SCR. Generally, filter bags are stable up to around 230°C, which makes cyclical heat treatment impossible.

[0009] The problem of the formation of ammonium bisulfate when SCR catalysts are operated in sulfur-containing flue gases at low temperatures is known from the prior art. For example, WO2016 / 028290 (Patent Document 1) describes an exhaust gas aftertreatment system that includes a sulfur trioxide trap configured to selectively capture SO3 from exhaust gas. In this way, the formation of ammonium bisulfate is prevented.

[0010] KR20160102691 (Patent Document 2) describes an apparatus and method for regenerating an SCR system catalyst by a controlled increase in the system temperature to or above a temperature at which ammonium bisulfate deposited within the catalyst pores can be removed.

[0011] Furthermore, various Chinese documents address the problem of ammonium bisulfate removal, for example, CN103953420(A) (Patent Document 3) describes a method and device for cleaning SCR catalyst deposit particles in diesel engine exhaust aftertreatment, and CN103920540(A) (Patent Document 4) describes a method and device for regenerating SCR denitration catalyst applied in diesel engine exhaust aftertreatment.

[0012] The idea underlying the present invention is to operate a catalytic bag filter in an SCR below the dew point of ammonium bisulfate and periodically regenerate the catalyst using ozone injection, thereby removing ammonium bisulfate from the catalyst pore structure.

[0013] From CN102133547(A) (Patent Document 5), it is known to use ozone treatment to regenerate vanadium-titanium-based flue gas denitration catalyst. The treatment method includes: filling a deactivated flue gas denitration catalyst into a catalyst regeneration reaction bed, introducing a mixture of ozone and air into the reaction bed, and completing the catalyst regeneration after oxidation.

[0014] Ozone is known in the art to be a strong oxidizing agent that can also increase the reactivity of catalysts that convert heavy hydrocarbons at room temperature. Ozone (trioxygen, O3) is known as a strong oxidizing agent for wastewater and drinking water treatment, sterilization, and deodorization. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O2 and decomposes to normal dioxygen in the lower atmosphere. Ozone has many industrial applications related to oxidation, since it is a strong oxidizing agent (significantly more than dioxygen). However, due to the fact that ozone itself is toxic, the residual ozone from these oxidation processes must be removed. Moreover, ozone is a pollutant that cannot be discharged, since it is highly harmful to animal and plant tissues, even at low concentrations of around 100 ppb. For these reasons, much research has been done to find suitable catalysts for oxidation reactions using ozone, and also to find effective methods to remove residual ozone after such oxidation reactions.

[0015] In the presence of ammonia, acid gases in sulfur-containing flue gases form ammonia salts, namely ammonium chloride (NH4Cl), ammonium nitrate ((NH4)2NO3) and ammonium bisulfate (NH4HSO4). In most cases, NH4HSO4 has the highest dew point, but in waste incineration units with HCl concentrations of several hundred ppm NH4Cl, condensation determines the lowest temperature. SCR equipment in coal-fired power plants is usually operated at temperatures between 330-430°C with a typical ammonium bisulfate catalyst having a dew point between 280-320°C. Below the dew point, ammonia and sulfuric acid condense as liquid ammonium bisulfate within the catalyst pore structure, which inhibits catalyst performance. At high enough temperatures, gaseous sulfuric acid is in equilibrium with SO3, and thus the ammonium bisulfate dew point depends on the water content, ammonia content and SO3 concentration.

[0016] Ammonium bisulfate has a melting point of 147 °C. Although the formation of ammonium sulfate ((NH4)2SO4) is thermodynamically more favorable, analysis of the condensed salts indicates that only limited amounts of the sulfate salt are formed due to kinetic limitations.

[0017] The inhibition of ammonium bisulfate is reversible, and ammonium bisulfate is easily evaporated by increasing the catalyst temperature. The bulk dew point at the SCR reactor inlet is typically about 290°C, but the observed dew point is higher due to capillary forces in the micropore structure. As ammonia is consumed in the SCR reaction, the ammonium bisulfate dew point decreases through the SCR reactor. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] WO2016 / 028290 [Patent Document 2] KR20160102691 [Patent Document 3] CN103953420(A) [Patent Document 4] CN103920540(A) [Patent Document 5] CN102133547(A) Summary of the Invention [Problem to be solved by the invention]

[0019] Catalyst activity is directly related to the degree of pore condensation, which means that ammonium bisulfate gradually increases as the temperature decreases towards the bulk dew point. Except for very low SO3 concentrations in low dust SCR installations, operating below the bulk dew point is not an option because ammonium bisulfate condenses not only within the catalyst pores but also on the catalyst surface, thereby creating a sticky surface that can lead to catalyst plugging over time. [Means for solving the problem]

[0020] Accordingly, the present invention provides a method for catalytic filtration of sulfur-containing gases using a selective catalytic reduction (SCR) bag filter comprising a substrate, a support, and one or more catalytic materials, comprising: - the SCR bag filter is operated at a temperature below the dew point of ammonium bisulfate; and - The catalyst is periodically regenerated using ozone injection, This removes the ammonium bisulfate from the pore structure of the catalyst. The present invention relates to the method.

[0021] The selective catalytic reduction (SCR) bag filter used in the method of the present invention is preferably in the form of a filter bag assembly comprising a plurality of fabric filter bags arranged coaxially inside an outer filter bag. Inside said outer filter bag, one or more inner tubular filter bags are independently installed inside the outer filter bag and inside each other for the removal of dust and particulate matter in the process gas. At least one of the inner tubular filter bag and / or the outer tubular is provided with a catalytically active material. The substrate of the catalytic material is a fiber structure, which can be, for example, woven or knitted glass fiber, and the carrier can be titanium dioxide or another suitable compound. Typically, the SCR catalyst consists of a carrier (TiO2) and a catalytic material comprising vanadium pentoxide (V2O5). The catalytic material can also comprise other compounds, for example W and / or Mo oxides.

[0022] Bag filters are well suited for the removal of dust and particulate matter from gas streams. Catalytic bag filters have the dual utility of being able to both remove particulates from a gas stream and catalyze one or more desired reactions in the gas. Catalytic bag filters can contain one single layer of filter fabric, but typically contain two or three layers of filter fabric, each layer containing a tailor-made catalyst optimized for removing a specific type of compound from the gas passing through it. Dust and other particulate matter collect on the surface of the outer bag, from which it can be easily removed. The two- or three-layer construction provides flexibility to tailor different catalytic combinations for different purposes. One of the layers of the catalytic filter bag serves the primary purpose of reducing residual ozone.

[0023] The term "outer bag" refers to the filter bag through which the process gas first passes, and the term "inner bag" refers to the filter bag or bags through which the process gas subsequently passes after passing through the outer bag.

[0024] The catalytically active material in at least one of the one or more tubular inner filter bags and / or the outer tubular filter bag is a catalytic composition comprising vanadium oxide and titania.

[0025] The term "vanadium oxide" refers to: Vanadium(II) oxide (vanadium monoxide), VO; Vanadium(III) oxide (vanadium sesquioxide or vanadium trioxide), V2O3; Vanadium(IV) oxide (vanadium dioxide), VO2; or Vanadium(V) oxide (vanadium pentoxide), V2O5.

[0026] The preferred vanadium oxide for use in the process of the present invention is vanadium pentoxide (V2O5).

[0027] The term "titania" refers to titanium dioxide (TiO2).

[0028] The catalytically active material may further comprise palladium or platinum in metallic and / or oxide form.

[0029] These catalysts are active in both the removal of VOCs and carbon monoxide, as well as the removal of nitrogen oxides (NOx) through the SCR reaction with NH3.

[0030] The Pd / V / Ti catalyst is the preferred catalyst because (i) it has dual functionality (NOx removal and VOC removal), (ii) it is sulfur tolerant, and (iii) it has lower SO2 oxidation activity compared to other catalyst compositions.

[0031] It is known that the oxidation activity of Pd-containing catalysts decreases in the presence of a few ppm of NH3. In addition to being an active catalyst in NH3-SCR of NOx, vanadium oxide is also known to be an active oxidation catalyst. Compared to precious metal catalysts such as Pd catalysts, vanadium oxide catalysts are less selective in the formation of CO2, and some CO is produced during the oxidation reaction. CO cannot be oxidized to CO2 at a reasonable reaction rate by contact with vanadium oxide catalysts, but requires the presence of a precious metal catalyst, e.g. Pd. Therefore, when NH3 is present in the gas, it is preferable to catalyze the outer filter bag with a V / Ti catalyst and at least one of the inner filter bags with a Pd / V / Ti catalyst (if CO is present in the process gas or is formed by catalytic oxidation of VOCs in the outer filter bag). In this way, by supporting the Pd / V / Ti catalyst on the inner filter bag, CO and remaining VOCs are effectively oxidized to CO2. In this way, the loading of expensive precious metals on the inner filter bag can be limited to a minimum.

[0032] The outer tubular filter bag and one or more inner tubular filter bags can be installed within a tubesheet to form a tubular filter bag assembly, where a first inner tubular bag having a smaller diameter than the outer bag is independently positioned inside the outer bag forming an effective seal between the filtration side and the clean side of the filter.

[0033] A next inner tubular bag having a diameter smaller than that of the outer bag and the first inner bag is then independently placed inside the previous inner bag. Optionally, three or more inner bags can be placed in the filter bag assembly.

[0034] Dust and particles present in the process gas are deposited on the outer surface of the outer tubular filter bag facing the gas. Thus, the catalyst supported on the outer bag and / or the inner bag(s) is effectively protected from potential catalyst poisons present in the particles contained in the process gas.

[0035] In the process of the present invention, the catalyst is periodically regenerated with ozone, thereby removing ammonium bisulfate from the pore structure of the catalyst.

[0036] When the catalyst is regenerated with ozone, regeneration is conducted in counter-flow through the catalyst filter bag, ie, from the center of the bag through the bag and into the bag house or chamber.

[0037] The injected ozone can come from a variety of sources, such as ozone generators, which are available in a wide variety of industrial sizes and are widely used in industries such as the pulp and paper industry.

[0038] The operating temperature of the process of the present invention is above the freezing temperature of ammonium bisulfate, i.e., above 147°C.

Claims

1. 1. A method for catalytic filtration of sulfur-containing gases using a selective catalytic reduction (SCR) bag filter comprising a substrate, a support, and one or more catalytic materials, comprising: - the SCR bag filter is operated at a temperature below the dew point of ammonium bisulfate; and - The catalyst is periodically regenerated using ozone injection; This removes the ammonium bisulfate from the pore structure of the catalyst. The method.

2. 2. The method of claim 1, wherein the SCR bag filter is an assembly consisting of an outer tubular filter bag and one or more inner tubular filter bags installed within a tubesheet.

3. The method of claim 1 or 2, wherein the catalyst comprises a support and one or more catalytic materials.

4. The support is TiO ２ and the catalytic material is vanadium oxide VO, V ２ O ３ , V.O. ２ and V ２ O ５ The method according to any one of claims 1 to 3, wherein the

5. 5. The method of claim 3 or 4, wherein the catalyst further comprises Pd in ​​metallic or oxidized form.

6. 10. The method of claim 1, wherein one of said SCR bag filters serves the primary purpose of reducing residual ozone.

7. 10. The method of claim 1 , wherein the operating temperature is greater than the freezing temperature of ammonium bisulfate.

8. 10. The method of claim 1, wherein the catalyst is periodically regenerated with ozone, thereby removing ammonium bisulfate from the pore structure of the catalyst.

9. 9. The method of claim 8, wherein ozone regeneration is accomplished by flowing ozone through the SCR bag filter from a clean side of the SCR bag filter.

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