TW202108230A - A method for ozone-assisted dioxin removal - Google Patents

Abstract

In a method for removing dioxins from a gas in a gas treatment plant comprising a catalyst unit for emissions abatement, ozone is injected into the gas at a point upstream the catalyst unit, and the catalyst unit includes a dust filter. The gas can be a flue gas and the catalytic unit can be a catalytic dust filter unit, especially a bag filter unit.

Description

Ozone-assisted dioxin removal method

The present invention relates to a method for removing dioxin from gas (especially flue gas) using ozone (three oxygen, O _{3) and a catalytic dust filter.}

Selective catalytic reduction (SCR) is a method by which nitrogen oxides (also known as NOx) can be converted into diatomic nitrogen (N ₂ ) and water (H ₂ O) with the help of a catalyst . A gaseous reducing agent (which is typically anhydrous ammonia, ammonia or urea) is added to the stream of flue gas or exhaust gas and adsorbed on the catalyst. Carbon dioxide (CO ₂ ) is the reaction product when urea is used as a reducing agent.

It is known that if ozone is added to the gas, the SCR catalyst, dual-functional SCR catalyst (V ₂ O ₅ + Pd), noble metal catalyst and Cu/Mn catalyst can oxidize volatile organic compounds (VOC) at ambient temperature. In the field of catalytic filtration, and especially in the fields of waste incineration, metal sintering plants and cement plants (if waste is used as fuel), it is necessary to remove dioxin, which is a challenge for many plants.

Ozone is known as a strong oxidant used for waste and drinking water treatment, sterilization, and deodorization. It is an allotrope of oxygen, and its stability is much lower than that of the two-atom allotrope O ₂ because it decomposes in the lower atmosphere to produce normal dioxygen. As mentioned, ozone is a strong oxidant (much greater than dioxygen), so it has many industrial applications related to oxidation. Since ozone has a considerable oxidizing power and molecular oxygen is formed as a by-product, ozone is often selected for oxidation. In fact, the use of ozone for oxidation has at least the following advantages over chemical alternatives:-Ozone can be generated on-site,-Ozone quickly decomposes into oxygen,-The reaction does not produce toxic halogenated compounds, and-Compared with other common oxidants, ozone is more Works faster and more thoroughly.

However, due to the fact that ozone itself is toxic, residual ozone must be removed from this oxidation process. In addition, even if the concentration of ozone is as low as about 100 ppb, it is very harmful to animal and plant tissues and is a pollutant that cannot be emitted. For these reasons, many researches are devoted to finding suitable catalysts for oxidation reactions using ozone and finding effective ways to remove residual ozone after such oxidation reactions.

Dioxin and dioxin-like compounds are compounds characterized by persistent organic pollutants (POP) that are highly toxic to the environment. It is mainly a by-product of various industrial processes, or, in the case of polychlorinated or polybrominated biphenyls similar to dioxin, it is part of an intentionally produced mixture. Substances called dioxin are classified into compounds based on dioxin, such as polychlorinated dibenzo-p-dioxins (PCDD), two benzenes containing two substituted chlorine atoms The ring is connected via two oxygen atoms; and furan-based compounds such as polychlorinated dibenzofuran (PCDF), which are connected via an oxygen atom; and the aforementioned polychlorinated and polybrominated biphenyls (PCB and PBB) . Dioxin includes its isomers, depending on the position and number of the substituted chlorine atoms. Dioxin-based compounds include up to 75 types and furan-based compounds include up to 135 types. In other words, a total of 210 dioxin compounds are known to exist.

Nowadays, most factories use low temperature (below 180°C) to inject activated carbon, capture dioxin by adsorption, or in some cases, use catalytic units at higher temperatures (about 180-240°C), through catalytic Oxidize to destroy dioxin. However, in many cases, activated carbon is expensive and cannot meet the demand for dioxin removal, and increasing the temperature in order to promote catalytic dioxin oxidation is expensive for OPEX and may also require large-scale construction works.

The present invention can remove dioxin from gas (especially flue gas) through low-temperature destruction promoted by ozone injection and the use of a catalytic dust filter.

It is known in the prior art that dioxin needs to be removed from flue gas. Therefore, EP 1 940 546 B1 discloses a use of ammonia reducing agent and natural manganese ore (NMO)-containing V/TiO ₂ catalyst from the flue in a wide operating temperature range from low temperature (250°C or lower) to high temperature of 300-400°C The method of removing nitrogen oxides and dioxin from the air. The V/TiO ₂ catalyst is mixed with NMO to enhance the nitrogen oxide removal activity in the low temperature range.

In EP 1 214 971 B1, in order to prepare a high-efficiency catalyst for the removal of dioxin, the spent catalyst is recycled. The spent catalyst contains an alumina support with a large specific surface area and impregnated with more than 5% vanadium.

US 6.077.431 A describes a method for decomposing and removing dioxin contained in sludge, which includes making the sludge to be treated into a slurry, and applying ultrasonic radiation to the slurry in the reaction field developed by ultrasonic radiation to make Dioxin contained in the sludge is decomposed and the pollutants are transferred to the liquid phase of the slurry, and then the slurry is divided into a liquid phase containing pollutants and a solid phase free of dioxin.

JP H0775720 A describes a method for removing dioxin from the gas by injecting ozone into the gas at a point upstream of the catalytic unit in a gas processing plant containing a catalytic unit, the catalytic unit including a dust trap Bag filter. Therefore, the gas is first passed through a bag filter and then through a catalytic unit containing a catalyst containing 72 wt% TiO ₂ , 7 wt% V ₂ O ₅ and 5 wt% WO ₃ . In addition, Pt or Pd may be present in an amount of 0.01 to 3 wt%. The gas is reheated before passing through the catalytic unit, making it somewhat different in nature.

Finally, US 2014/0170046 A1 discloses a method and equipment for reducing organic compounds and other emissions in a factory that uses cement or mineral kilns that emit high levels of organic compounds. The equipment consists of a filter that controls particulate emissions, and the filter has been catalytically treated to catalytically destroy gaseous emissions when the process gas passes through the porous medium of the filter. Both dioxin and ozone are mentioned, but ozone is used for hydrocarbon reduction, and the leap towards ozone-assisted dioxin removal is not obvious. In addition, there is no mention of ozone decomposition catalysts.

The standard solution for removing dioxin from the flue gas consists in spraying activated carbon into the gas, for example as described in EP 0 930 091 B1. According to the method of the present invention, the removal of dioxin from the flue gas is achieved through low-temperature destruction promoted by ozone injection and the use of a catalytic dust filter.

Therefore, the present invention relates to a method for removing dioxin from gas (especially flue gas) in a gas processing plant containing a filter unit, wherein ozone is injected into the flue gas at a point upstream of the filter unit , And the filter unit includes a catalytic dust filter.

More specifically, the present invention relates to a method for removing dioxin from gas in a gas treatment plant containing a catalytic unit for reducing emissions, wherein -Inject ozone into the gas at a point upstream of the catalytic unit, and -The catalytic unit includes a dust filter.

Preferably, the gas is flue gas, and the catalytic unit is a catalytic dust filter unit.

The catalytic dust filter is preferably a bag filter.

Bag filters are very suitable for removing dust and particulate matter from air streams. The catalytic bag filter has the dual effect of being able to remove one or more of the required reactions of particulates and catalytic gas from the gas flow. Catalytic bag filters typically contain two or three layers of filter fabric. Each layer can contain different catalysts that are optimized to remove specific types of compounds from the gas passing through them. Dust and other particulate matter will settle on the surface of the outer bag so that it can be easily removed from it. The two-layer or three-layer structure provides flexibility to protect the catalyst impregnated fabric and customize different catalytic combinations for different purposes. The catalytic bag filter may also contain a fabric layer that has not been impregnated with a catalyst, for example an outer fabric layer impregnated with a catalyst for removing dust, and then one or more catalyst-impregnated fabric layers are positioned inside the outer fabric layer. Depending on the application, the catalyst impregnated fabric may contain the same catalyst or different types of catalysts.

Preferably, a known ozone decomposition catalyst (such as manganese or its oxide) is added to the filter to remove any residual ozone. This known ozonolysis catalyst is preferably added downstream of the filter unit.

The catalytic oxidation of dioxin compounds is extremely sensitive to temperature. It is thus known that when the temperature decreases, the catalytic activity of the known catalyst decreases significantly. In addition, it is known that ozone is unstable at a temperature higher than 250°C. Most industrial flue gas filtration units operate at temperatures below 200°C, and most operate at about 140-160°C. At these temperatures, dioxin is adsorbed to a certain extent on activated carbon, which is usually injected into the filter housing. However, the degree of dioxin removal is limited, and the use of activated carbon alone cannot meet stricter emission control requirements. In addition, the use of activated carbon failed to destroy the dioxin component. Dioxin is only adsorbed on carbon, which leads to the fact that all waste activated carbon and the rest of the dust from the process removed together with it must be disposed of as hazardous waste. In order to enhance the removal of dioxin, a catalytic filter can be used, and it can replace the standard filter in the filter housing, but in order for the catalyst to become effective, the temperature must be increased. This may require modification of the factory and replacement of some building materials. In addition, at lower temperatures, _{the removal efficiency of acid gases (HCl, SO 2} etc.) is higher. By introducing ozone injection into the flue gas before the filter unit and the catalytic dust filter, the need for renovation, construction and safety will be eliminated, thereby ensuring a higher dioxin destruction efficiency.

The catalytically active material in the catalytic unit is preferably composed of: one or more metal oxides, where the metal is selected from the group consisting of V, W, Mn, Cu, Ce, Mo, Fe, Ca and Mg; and one or more selected A porous carrier composed of Al ₂ O ₃ , SiO ₂ , SiC, and TiO ₂ may contain other elements with a concentration of less than 1 wt% as the case may be. The catalytically active material preferably contains 1 wt%, 2 wt%, or 3 wt% to 4 wt%, 5 wt%, 10 wt%, 25 wt%, or 50 wt% V ₂ O ₅ . Other preferred catalytically active materials include 1 wt%, 2 wt%, or 3 wt% to 4 wt%, 5 wt%, 10 wt%, 25 wt%, or 50 wt% MnO ₂ .

Preferably, the porous support comprising TiO _2, especially in anatase form of TiO _2.

The catalytically active material preferably further includes 0.01 wt%, 0.02 wt%, or 0.05 wt% to 1 wt% of noble metal, preferably Pd or Pt.

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Claims (10)

A method for removing dioxin from gas in a gas treatment plant containing a catalytic unit for reducing emissions, wherein Inject ozone into the gas at a point upstream of the catalytic unit, and The catalytic unit includes a dust filter. The method of claim 1, wherein the gas is flue gas and the catalytic unit is a catalytic dust filter unit. Such as the method of claim 2, wherein the catalytic dust filter is a bag filter. The method of any one of claims 1 to 3, wherein a known ozone decomposition catalyst, such as manganese or its oxide, is added to the filter to remove any residual ozone. The method of claim 4, wherein the known ozonolysis catalyst is added downstream of the filter unit. The method of any one of the preceding claims, wherein the catalytically active material in the catalytic unit is composed of: one or more metal oxides, wherein the metal is selected from the group consisting of V, W, Mn, Cu, Ce, Mo, Fe, The group consisting of Ca and Mg; and one or more _{porous supports selected from the group consisting of Al 2} O ₃ , SiO ₂ , SiC and TiO ₂ , and other elements with a concentration of less than 1 wt% may be present as appropriate. The method of claim 6, wherein the catalytically active material contains 1 wt%, 2 wt%, or 3 wt% to 4 wt%, 5 wt%, 10 wt%, 25 wt%, or 50 wt% V ₂ O ₅ . The method of claim 6, wherein the catalytically active material comprises 1 wt%, 2 wt%, or 3 wt% to 4 wt%, 5 wt%, 10 wt%, 25 wt%, or 50 wt% of MnO ₂ . The method of claim 6, wherein the porous support comprises TiO ₂ preferably in the form of anatase. The method of claim 6, wherein the catalytically active material further comprises 0.01 wt%, 0.02 wt%, or 0.05 wt% to 1 wt% of noble metal, preferably Pd or Pt.