JP2008030017A - Removal apparatus of trace harmful substance in exhaust gas and its operation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a long-term stable and highly reliable removal apparatus of trace harmful substances in exhaust gas, and its operation method. <P>SOLUTION: The removal apparatus of the trace harmful substances in the exhaust gas is constituted by arranging, in a flow channel of the exhaust gas discharged from a combustion facility in order from the upstream side, a denitration device equipped with a denitration catalyst layer having a function of removing nitrogen oxide in the exhaust gas to oxidize metallic mercury, an air preheater for heat-exchanging air for combustion of the combustion facility to the exhaust gas, a dust removal device having a bag filter containing oxidation catalysts for metallic mercury and a desulfurization device for removing sulfur oxide in the exhaust gas. The bag filter may be disposed in front of the desulfurization device. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an apparatus for removing metallic mercury compounds as trace harmful substances contained in combustion exhaust gas of petroleum or coal and its operation method, and in exhaust gas capable of removing metallic mercury compounds stably and efficiently even after long-term use. The present invention relates to an apparatus for removing trace harmful substances and an operation method thereof.

Exhaust gas from combustion facilities such as boilers that use oil or coal is a trace amount of harmful substances in addition to photochemical smog and acid rain causing nitrogen oxides (NOx) and sulfur oxides (SOx). Contains heavy metal compounds such as metallic mercury. As an effective method for removing NOx, a flue gas denitration method by selective catalytic reduction using ammonia (NH ₃ ) or the like as a reducing agent is widely used mainly in thermal power plants. The catalyst mainly uses vanadium (V), molybdenum (Mo) or tungsten (W) as an active component and titanium oxide (TiO ₂ ) as a carrier. In particular, vanadium is used as one of the active components. The inclusions are not only high in activity, but also are less prone to deterioration due to impurities contained in the exhaust gas, and can be used from a lower temperature. etc). Further, the catalyst component is usually used after being formed into a honeycomb-like or plate-like structure, and various production methods for that purpose have been invented.

On the other hand, regarding the removal of SOx in the exhaust gas from the combustion facility, a wet desulfurization apparatus that absorbs and removes SOx in exhaust gas using limestone slurry can achieve high-efficiency desulfurization. It has become. Apart from this, a semi-dry type desulfurization apparatus using lime or magnesium hydroxide (Mg (OH) ₂ ) as an absorbent has been proposed. In the desulfurization method using the semi-dry desulfurization apparatus, an absorbent such as limestone is directly sprayed into the exhaust gas in the exhaust gas flow channel on the upstream side of the dust removal device, and the sprayed exhaust gas flow channel or the dust removal equipment for a predetermined time. This is a method for removing SOx in the exhaust gas by allowing it to stay. Although this method is not suitable for high-efficiency desulfurization, it has an advantage that it is economical with a low equipment cost.

On the other hand, in recent years, movements related to emission reduction of metallic mercury compounds present in petroleum or coal combustion exhaust gas have become active. Metallic mercury compounds are said to have a great influence on the human body through the food chain once discharged into the atmosphere. A trace amount of harmful substances such as metallic mercury brought in by oil or coal is burned, and the vaporized components are transferred into exhaust gas. Normally, metal mercury is said to be discharged as gaseous metal mercury in the combustion zone near 1500 ° C, but metal mercury coexists in a relatively low temperature region (300-450 ° C region) of the exhaust gas flow path (HCl ) Has been confirmed to be partially oxidized to mercury chloride (HgCl ₂ ) as shown in the following formula (1), and this reaction is performed on a denitration catalyst installed in a temperature range of about 300 ° C. to 450 ° C. It is also known to be promoted by. It is known that this reaction proceeds almost completely to the right at a temperature of 300 ° C. or lower, and metal mercury is oxidized to mercury chloride.

Hg + HCl + 1 / 2O ₂ = HgCl ₂ + H ₂ O (1)
Mercury chloride (HgCl ₂ ) generated by the above reaction formula (1) has a lower vapor pressure than metal mercury, and is adsorbed and removed by dust with a dust removing device disposed downstream of the exhaust gas flow path. It is known that it is absorbed by an absorbent such as limestone slurry of a wet desulfurization apparatus or absorbed by a spray absorbent of a semi-dry desulfurization apparatus.

  However, most of the metal mercury that is not oxidized may be discharged from the chimney as it is in the gaseous state (metal mercury vapor).

Therefore, as a technology to reduce metal mercury emissions, activated carbon is sprayed into the exhaust gas flow channel upstream of the dust removal equipment installed in the low temperature range, and metal mercury is effectively removed by the adsorption effect and catalytic effect of the activated carbon. Method (conventional technology 1), or a heat exchanger in an exhaust gas passage in which a denitration catalyst, an air preheater, a dust removal device, and a heat exchanger are sequentially arranged from the upstream side as in the invention described in JP-A-2003-53142 A method in which a solid oxidation catalyst layer is placed in a downstream low temperature region (300 ° C. or lower) to oxidize metallic mercury, and then absorbed and removed by an absorbent using a wet desulfurization apparatus (conventional technology 2), and further included in exhaust gas There has been proposed a method using a bag filter for exhaust gas treatment in which a metal mercury adsorbent is held on a filter cloth for removing harmful substances (Japanese Patent Laid-Open No. 10-66814; Prior Art 3).
JP 2003-53142 A Japanese Patent Laid-Open No. 10-66814

The prior arts (1 to 3) have the following problems.
That is, when the prior art 1 is applied to a combustion facility of a large-scale power generation facility, the amount of processing gas is enormous, so that the amount of activated carbon used is extremely large, and it is not possible to use activated carbon continuously. It is not economical and practical application is difficult. In addition, since a large amount of activated carbon is mixed in the dust collected by the dust remover, there is a problem that dust treatment becomes difficult.

  Moreover, although the prior art 2 pays attention to the catalytic reaction of the said Formula (1) progressing more effectively at low temperature, it is necessary to add an oxidation catalyst layer newly to the existing exhaust gas treatment apparatus. is there. For this reason, since a space for installing the oxidation catalyst layer is required, it is not only disadvantageous in terms of cost to cope with the existing exhaust gas treatment apparatus, but when a new oxidation catalyst layer is installed, it becomes an exhaust gas flow. Ventilation resistance in the road will be greatly increased, and it will be necessary to install an induction fan in the exhaust gas flow path. Further, when a large amount of SOx is present in the exhaust gas, as shown in FIG. 3, there is a problem that the low-temperature oxidation catalyst is remarkably deteriorated with time and lacks long-term stability.

  Furthermore, the prior art 3 is a method of holding a metal mercury adsorbent on a filter cloth, but when adsorbing metal mercury, the adsorption capacity of the adsorbent decreases with use time and the adsorption amount is saturated. The time required to replace the adsorbent, which is not economical, and the disposal cost of the filter cloth that adsorbs a large amount of the metal mercury compound becomes enormous.

  An object of the present invention is to overcome the above-described problems of the prior art and to provide a device for removing a trace amount of harmful substances in exhaust gas that is stable for a long time and has high reliability, and a method for operating the apparatus.

  The invention according to claim 1 is an air preheater (6) for exchanging heat of combustion air of a combustion facility with exhaust gas in order from an upstream side to an exhaust gas flow path for discharging from a combustion facility for burning oil or coal. It is an apparatus for removing trace harmful substances in exhaust gas in which a dust removing device (7) having a bag filter provided with a filter cloth carrying an oxidation catalyst is arranged.

The metallic mercury component contained in the exhaust gas discharged from the combustion facility (4) such as a boiler using petroleum or coal as the fuel is a metallic mercury compound present in the fuel in the process of burning the fuel at a high temperature around 1500 ° C. Is a metallic mercury produced by decomposition. Most of the metallic mercury in the exhaust gas exists as vapor of metallic mercury, although it depends on the fuel properties. The vapor of metallic mercury discharged from the combustion equipment (4) such as a boiler is reduced in exhaust gas temperature during exhaust gas purification treatment in the exhaust gas flow path, and as shown in the above formula (1) in the course of the temperature decrease. It is oxidized by hydrogen chloride (HCl) coexisting in the exhaust gas and partially converted to mercury chloride (HgCl ₂ ).

This reaction is more likely to proceed to the right in the reaction formula (1) as the temperature is lower due to thermodynamic equilibrium, and the residence time of the vapor of metallic mercury in the atmosphere of 60 to 400 ° C. is greatly affected. In addition, this reaction is promoted by the denitration catalyst of the denitration device (5), which is a component device of the exhaust gas treatment device used under a temperature condition of 300 to 400 ° C., and particularly when the HCl concentration in the exhaust gas is high. I know that. Therefore, the metal mercury compound that has been oxidized by the denitration catalyst and converted to HgCl ₂ or the like is a dust removal device that is a component device of the exhaust gas treatment device disposed on the downstream side of the denitration device (5) in the exhaust gas flow channel due to its characteristics. In (7), it adsorbs to the dust surface and is easily absorbed by an absorbent such as lime slurry by the desulfurization apparatus (8a, 8b).
Therefore, the exhaust gas treatment system that oxidizes metallic mercury by the denitration catalyst in the denitration device (5) and removes the oxidized metal mercury compound obtained by the equipment downstream of the denitration device (5) is the metal in the exhaust gas. It is an effective means for removing mercury compounds.

  However, even if there is no denitration device (5) using a denitration catalyst in the existing equipment, it can be an effective means to install only the bag filter carrying the catalyst in the exhaust gas passage of the combustion equipment.

  In the invention of claim 1, the bag filter itself is provided with an oxidation function of metallic mercury, and the compound in which metallic mercury is oxidized and converted into mercury chloride or the like on the bag filter is easily adsorbed by dust particles, In addition, since the filter cloth uses non-woven fabric, the gas flow is not as simple (laminar) as a normal solid oxidation catalyst, and because it is turbulent, contact with HCl and metal mercury even at low concentrations of HCl and metal mercury Since the efficiency is high, a highly efficient oxidation reaction of metallic mercury can be expected.

  Thus, in the invention according to claim 1, the dust removing device (7) having the bag filter carrying the oxidation catalyst of metallic mercury contacts the exhaust gas which has been subjected to heat exchange by the air preheater (6) and has a relatively low temperature. The structure is suitable for efficiently oxidizing low-concentration metallic mercury.

In the dust removing device (7) provided with the bag filter carrying the oxidation catalyst according to the first aspect of the invention, the denitration device (5) is installed on the upstream side, and the aging deterioration of the denitration device (5), etc. Even if the amount of NH ₃ leakage increases significantly, the dust cake layer formed on the filter cloth surface of the bag filter accelerates the precipitation of pore clogging substances such as acidic ammonium sulfate that precipitates at low temperatures, Stable operation for a long time is possible by suppressing deterioration of the oxidation catalyst of the metal mercury contained (acid ammonium sulfate is deposited in the cake layer of dust formed on the filter cloth surface of the bag filter, and the oxidation catalyst Since ammonium sulfate does not reach the attached filter cloth, the pores of the filter cloth are not blocked by ammonium sulfate).

  In the prior art 3, the metal mercury adsorbed on the filter cloth remains in a metal state, and the filter cloth adsorbing capacity decreases as the use time of the filter cloth increases. Then, the mercury metal is converted into mercury chloride or the like by the oxidation catalyst supported on the bag filter, and the mercury chloride is easily detached from the filter cloth, so that the filter cloth is not blocked.

  Thus, according to the first aspect of the present invention, a trace amount of harmful substances contained in the exhaust gas discharged from the oil or coal-fired combustion facility (4) can be stably oxidatively decomposed for a long time and efficiently removed. .

  According to a second aspect of the present invention, there is provided a denitration apparatus comprising a denitration catalyst layer having a function of removing nitrogen oxides in the exhaust gas and oxidizing metal mercury in the exhaust gas flow path on the upstream side of the air preheater (6). (5) is disposed, and the exhaust gas flow path on the downstream side of the dust removing device (7) is sprayed with an absorbent slurry (using an absorbing liquid such as limestone slurry) to remove sulfur oxides in the exhaust gas. The apparatus for removing trace amounts of harmful substances in exhaust gas according to claim 1, wherein a desulfurization apparatus (8a) is arranged.

According to a third aspect of the present invention, there is provided a denitration apparatus comprising a denitration catalyst layer having a function of removing nitrogen oxides in the exhaust gas and oxidizing metallic mercury in the exhaust gas flow path on the upstream side of the air preheater (6). (5) is disposed, and an absorbent slurry (using slaked lime, magnesium hydroxide, or the like) is sprayed into the exhaust gas passage between the air preheater (6) and the dust removing device (7). The apparatus for removing trace amounts of harmful substances in exhaust gas according to claim 1, wherein a semi-dry desulfurization apparatus (8b) for removing sulfur oxides in the exhaust gas is disposed.
In addition, in the trace amount harmful substance removal apparatus according to the third aspect, a wet desulfurization apparatus (8a) may be further installed downstream of the semi-dry desulfurization apparatus (8b).

  By the way, in the United States and the like, two types of coal are currently used. They are EB charcoal (Eastern Bituminous) and PRB (Powder Rover Basin) charcoal. In particular, PRB charcoal has a large reserve and is cheap, so it is expected that the amount used will continue to increase. Both coals have the following characteristics.

  Since EB coal contains high concentrations of sulfur and chlorine, SOx and HCl are also contained at high concentrations in the combustion exhaust gas of the coal. Moreover, although PRB charcoal contains a very low concentration of chlorine, since it has a high ash concentration, there is a feature that it tends to adhere to the boiler wall.

Therefore, the exhaust gas treatment technology generated by the combustion of both the coals is generally performed in the following procedure.
EB charcoal: exhaust gas → denitration → heat exchange → dust removal → wet desulfurization (high efficiency desulfurization) → chimney (a)
PRB charcoal: exhaust gas → denitration → heat exchange → semi-dry desulfurization (simple desulfurization) → dust removal → chimney (b)
In the process (b), a wet desulfurization process may be further incorporated on the downstream side of the dust removal process. The process (a) corresponds to the invention described in claim 2 of the present invention, and the process (b) corresponds to the invention described in claim 3 of the present invention.

  Regarding the removal of mercury metal in the boiler exhaust gas, in the process (a), high concentration of HCl is present in the exhaust gas. Therefore, the oxidation catalyst contained in the denitration catalyst in the denitration process can be converted to mercury chloride relatively easily. Although it can be recovered, in the process (b), since the HCl concentration in the exhaust gas is low, the oxidation reaction by the oxidation catalyst in the denitration process is difficult to occur. In particular, in the process (b), how to effectively remove metallic mercury is the primary technical concern.

In the second and third aspects of the present invention, metallic mercury in the exhaust gas is removed as follows.
Metallic mercury is converted to mercury chloride by its oxidation and removed by adsorbing hydrogen chloride to dust particles. In the process (a) corresponding to claim 2, as described above, Since the HCl concentration is relatively high and the temperature conditions are met, the reaction in the right direction of (1) is promoted in the denitration device (5), and the oxidation product of metallic mercury is compared with the process of (b). Many can be obtained. Unreacted metallic mercury is oxidized by a bag filter carrying a metallic mercury oxidation catalyst, and part of it is adsorbed and removed by the bag filter. Further, oxidation of metallic mercury flowing from the bag filter to the downstream side of the exhaust gas flow path is generated. A thing is removed by the method of making an absorption liquid absorb in a wet desulfurization apparatus (8a).

  That is, in the process (a), since the exhaust gas before passing through the wet desulfurization device (8a) can be treated by the denitration device (5) and the dust removal device (7), the high concentration HCl in the exhaust gas is effectively used. Thus, it can be oxidized to mercury chloride and HCl is not consumed in the wet desulfurization apparatus (8a).

Further, in the process (b) corresponding to claim 3, since the HCl concentration in the exhaust gas is relatively low, oxidation of metallic mercury does not occur in the denitration apparatus (5) as in the process (a). Mercury components, such as mercury chloride, which are oxidized from the metal mercury by the oxidation catalyst on the bag filter of the dust removal device (7) in the low temperature region, can be recovered and removed from the dust removal device (7).
In the process (b), the temperature of the exhaust gas is lowered by the air preheater (6) disposed in the exhaust gas flow path on the downstream side of the denitration device (5), and the vacuum is lower than the reaction temperature in the denitration device (5). Since the reaction of the reaction formula (1) easily proceeds in the right direction by the oxidation catalyst supported on the filter, the metal mercury oxidation reaction can be performed, and the metal mercury is effectively oxidized even with a low concentration of HCl. Cheap.

That is, in the process (b), even if the HCl concentration in the exhaust gas is relatively low, metal mercury is oxidized by oxygen in addition to chlorine by the oxidation catalyst on the bag filter of the dust removal device (7) (Hg + 1 / 2O ₂ = HgO). Even if the denitration catalyst layer of the denitration device (5) does not change to a sufficient mercury chloride (HgCl ₂ ), it can be efficiently converted to mercury chloride by a bag filter carrying an oxidation catalyst in a low temperature range (HgO + 2HCl = HgCl ₂ + H). ₂ O).

  The oxidation product of metallic mercury such as mercury chloride recovered in the processes (a) and (b) is transferred to the water layer for purification.

  The process (b) is a semi-dry desulfurization step in which the slurry absorbent is sprayed into the exhaust gas, dried up and recovered by the dust removing device (7). Since it needs to be removed, the order of “semi-dry desulfurization → dust removal” cannot be changed. In the process (b), even in the semi-dry desulfurization step, it is possible to remove a certain amount of the mercury component that has been oxidized into mercury chloride by oxidation of metallic mercury in the denitration device (5) on the relatively upstream side. The removal is not enough. In addition, a certain amount of HCl in the exhaust gas (which has an effect of promoting oxidation by a metal mercury oxidation catalyst) is also removed in the semi-dry desulfurization step. Therefore, an oxidation catalyst is supported on a bag filter installed in a dust removal step installed in a balanced and preferable low temperature region, and oxidation of metallic mercury and adsorption of mercury chloride to filter cloth collecting dust are performed simultaneously. Furthermore, since the filter cloth uses a non-woven fabric, the gas flow is not as simple as that of a normal solid oxidation catalyst, and the exhaust gas passes through the filter cloth. The efficiency of contact between HCl and metal mercury increases, and a highly efficient metal mercury oxidation reaction occurs.

  As shown in FIG. 2B, which is a side view of the bag filter of FIG. 2A and an enlarged view of the surface thereof, the metal mercury in the exhaust gas is oxidized by the protrusion 1 a on the filter cloth surface of the bag filter 1. The metal mercury compound 2 such as particulate mercury chloride, which is oxidized by the catalyst and converted to mercury chloride, is collected together with the dust adsorbed relatively inside the filter cloth, and then removed from the filter cloth. It is clear in the present invention that a sufficient time (residence time) for adsorbing to the dust is extremely effective for removing the metal mercury compound 2 as an oxidation product. It became.

  Therefore, even in the case of the process (b) in which it is necessary to install a semi-dry type desulfurization apparatus (8b) upstream of the bag filter, the effective use of mercury chloride by the oxidation catalyst supported on the bag filter. Removal is possible.

  As described above, in the present invention, both the process (a) and the process (b) can effectively oxidize metallic mercury and recover the resulting mercury chloride with a high removal rate.

  Conventionally, it was necessary to collect and remove mercury oxide in dust with a desulfurizer after oxidizing metal mercury to mercury chloride, etc., so it was necessary to provide a desulfurization step after the oxidation of metal mercury. In the present invention, coal is required to use a semi-dry type desulfurization apparatus (8b) (recovery rate of mercury chloride, which is an oxide of metal mercury, is considerably lower than that of the wet desulfurization apparatus (8a)). Even if it is an exhaust gas treatment device, a metal mercury component is removed from the exhaust gas with a high removal rate by arranging a bag filter carrying a metal mercury oxidation catalyst in the exhaust gas flow path downstream of the semi-dry desulfurization device (8b). be able to.

  Thus, in the invention according to claim 2 (invention comprising the process of (a)), the metal mercury oxide generated by the upstream denitration device (5) and the bag filter is partially a bag filter. It is adsorbed and removed, and further absorbed and removed by the absorbent in the wet desulfurization apparatus (8a) with high efficiency.

  In the invention according to claim 3 (invention comprising the process of (b) above), a certain amount of oxidation products such as mercury chloride in which metallic mercury is oxidized mainly by oxygen in the upstream denitration device (5). Is removed by the semi-dry desulfurization unit (8b), and the metal filter oxidizes the metal mercury and adsorbs the oxidized product to the filter cloth dust by a bag filter containing an oxidation catalyst installed in a low temperature region. Mercury can be removed.

According to a fourth aspect of the present invention, the denitration catalyst of the denitration catalyst layer is composed of two or more compounds selected from titanium oxide (TiO ₂ ), silicon oxide (SiO ₂ ), and aluminum oxide (Al ₂ O ₃ ). The trace amount in exhaust gas in any one of Claim 1 thru | or 3 which uses two or more types of metals selected from molybdenum (Mo), vanadium (V), and tungsten (W) as a component, or its oxide. It is a device for removing harmful substances.

According to a fourth aspect of the present invention, the denitration catalyst of the denitration catalyst layer is selected from Mo, V, and W, with two or more compounds selected from TiO ₂ , SiO ₂ , and Al ₂ O ₃ as the first component. Two or more kinds of metals or oxides thereof are used as the second component. The first component has a function as a catalyst carrier, and the second component has a denitration reaction and an oxidation action of metallic mercury. However, the actions of the first component and the second component are not single, and it is considered that both actions are combined by the first component and the second component.

According to the fourth aspect of the invention, conversion of Hg into HgCl ₂ , HgO _2, etc. is promoted by the denitration catalyst by HCl or oxygen contained in the exhaust gas. Further, the obtained HgCl ₂ or the like is adsorbed on the dust surface by the dust removing device and is easily absorbed by the absorbent such as lime slurry by the desulfurizing device.

By the way, the denitration catalyst layer of the denitration device (5) having a function of oxidizing metal mercury installed for treating exhaust gas from the combustion facility (4) such as a boiler is used to achieve the target denitration rate. In general, it is divided into a plurality of denitration catalyst layers and installed in a plurality of stages in the gas flow direction, and an amount of NH ₃ corresponding to the required denitration performance is supplied to perform denitration of exhaust gas. NH ₃ supplied into the exhaust gas is adsorbed to active sites on the surface of the denitration catalyst, reacts with NOx in the exhaust gas, and is decomposed into harmless nitrogen (N ₂ ). Since the NH ₃ is adsorbed on the denitration catalyst active site to suppress the adsorption of metal mercury (Hg) on the denitration catalyst active site, the oxidation reaction rate of metal mercury is reduced.

That is, in the denitration device (5), a plurality of stages of denitration catalyst layers are provided in the gas flow direction, and in the denitration catalyst layer on the relatively upstream side, NH ₃ supplied in the exhaust gas has a high concentration. Since it exists, it is not comparatively effective for the oxidation reaction of Hg, and finally Hg is effectively oxidized in the region near the denitration catalyst layer on the downstream side where NH ₃ is consumed by the denitration reaction. In addition, when the HCl concentration in the exhaust gas is low, the reaction rate of the Hg oxidation reaction further decreases, so the upstream denitration catalyst layer with a high NH ₃ concentration in the denitration catalyst layer contributes almost to the Hg oxidation reaction. It is better to think not.

Accordingly, there are often cases where sufficient Hg oxidation cannot be expected with the amount of denitration catalyst required to obtain the desired denitration performance. Furthermore, since the combustion exhaust gas of petroleum or coal contains a large amount of alkali, alkaline earth metal, arsenic (As) compound, phosphorus (P) compound, etc. as catalyst poisons, the denitration catalyst deteriorates when used for a long time. In this case, the poisoning component adsorbed on the catalyst surface significantly reduces the amount of NH ₃ adsorbed on the denitration catalyst, and a high concentration of NH ₃ is also supplied to the downstream denitration catalyst layer, resulting in Hg oxidation. The speed is further greatly reduced. When the HCl concentration in the exhaust gas is dilute, the degree of decrease in the Hg oxidation rate becomes significant.

On the other hand, in order to oxidize metallic mercury (Hg) in the presence of HCl at a low temperature to produce HgCl ₂ using a denitration catalyst as described above, the concentration of Hg to be oxidized is extremely low, so that ordinary solid denitration is performed. In the case of using a catalyst, it is necessary to devise a technique for increasing the contact efficiency between low concentration HCl and low concentration metal mercury. Furthermore, the reaction of sulfur oxide (SOx) present in the exhaust gas and NH ₃ leaking from the upstream denitration catalyst produces acidic ammonium sulfate (NH ₄ HSO ₄ ), and the denitration catalyst's oxidizing ability is significantly reduced. It is necessary to devise a technique to suppress such precipitation of acidic ammonium sulfate as much as possible.

  In addition, since the exhaust gas from petroleum or coal-fired combustion facilities contains a relatively high concentration of dust, even when a solid denitration catalyst layer is installed in the exhaust gas flow path after dust removal, a certain interval (inter-catalyst opening; pitch) However, if the pitch between the adjacent catalyst layers is increased, the rate of diffusion and adsorption of metallic mercury, which is a trace component, on the catalyst surface may be reduced. Therefore, it is desirable to provide a denitration catalyst layer designed to make the pitch between the catalyst layers as small as possible, but in this case, the gas flow path in the catalyst layer is blocked by dust and an increase in pressure loss that cannot be ignored, The power generation efficiency of the power plant will be impaired. In addition, the concentration of HCl required for the oxidation reaction of metallic mercury may be low depending on the type of fuel used. In this case, the contact efficiency between low-concentration HCl and low-concentration metallic mercury is increased. There is a need.

The invention according to claim 5 of the present invention has been invented in view of such circumstances.
According to the fifth aspect of the present invention, the concentration of the second component of the denitration catalyst in the denitration catalyst layer on the relatively downstream side in the denitration catalyst layer arranged in a plurality of stages in the gas flow direction in the denitration device is relatively The apparatus for removing trace harmful substances in exhaust gas according to any one of claims 1 to 4, wherein the concentration of the second component of the denitration catalyst in the upstream denitration catalyst layer is reduced stepwise.

  In order to maintain the desired denitration performance, the exhaust gas denitration apparatus normally has a plurality of stages of denitration catalyst layers 3 in the gas flow direction (denitration catalyst layers 3a, 3b,... In FIG. 5) as shown in FIG. However, according to the invention described in claim 5, the denitration catalyst layer (for example, the denitration catalyst layer on the relatively upstream side of the denitration catalyst layer provided in a plurality of stages in the gas flow direction). 3a) is characterized by being dispersed and supported at a high concentration. In this case, nitrogen oxide is efficiently denitrated in the relatively upstream denitration catalyst layer in which the second component is supported at a high concentration. As a result, NH3 supplied into the exhaust gas Most of the denitration catalyst layer in the upstream stage is consumed. As a result, the mercury metal in the exhaust gas can be efficiently oxidized in the denitration catalyst layer (for example, the denitration catalyst layer 3b) on the relatively downstream stage side. In this case, the denitration performance of the denitration catalyst in each catalyst layer is planned so that a satisfactory denitration rate can be obtained in the entire denitration catalyst layer 3.

In addition, petroleum or coal combustion exhaust gas contains a large amount of sulfur oxide (SOx) in the exhaust gas, and it is necessary to suppress the oxidation of SO _2. Since the amount of the second component having oxidation activity in the layer is low, it is possible to suppress the SO ₂ oxidation rate of the entire denitration catalyst.

This is because the SO ₂ oxidation is represented by the following chemical formula (2) and is in an equilibrium relationship with the SO ₃ concentration in the exhaust gas. As the SO ₂ oxidation reaction proceeds in the catalyst layer, the SO ₃ concentration in the denitration catalyst layer on the relatively downstream side increases, and the oxidation reaction of SO ₂ hardly occurs from the chemical equilibrium, so that the SO ₂ oxidation of the entire catalyst is suppressed. work.

SO ₂ + 1 / 2O ₂ → SO ₃ (2)
According to the fifth aspect of the present invention, among the denitration catalyst layers installed in a plurality of stages in the gas flow direction, the denitration catalyst layer on the relatively upstream stage side of the plurality of denitration catalyst layers in the gas flow direction. The second component having an oxidation activity dispersed and supported at a relatively high concentration efficiently denitrifies the exhaust gas in the presence of ammonia, and the second component dispersed and supported at a relatively low concentration in the denitration catalyst layer on the relatively downstream stage side. The two components can oxidize metallic mercury in the exhaust gas efficiently in the denitration catalyst layer on the downstream side where the ammonia concentration inhibiting the oxidation reaction is lower than that on the upstream side. Further, since the denitration catalyst layer on the relatively downstream stage side has a low supporting concentration of the second component having oxidation activity, the SO ₂ oxidation rate as a whole of the denitration catalyst can be suppressed, and SO _{3 in} the exhaust gas can be suppressed. It is possible to reduce the concentration.

  According to a sixth aspect of the present invention, in the dust filter (7), the metal mercury oxidation catalyst comprises titanium (Ti), molybdenum (Mo), vanadium (V) metal or an oxide thereof. The apparatus for removing trace amounts of harmful substances in exhaust gas according to any one of the above.

  Even if the denitration catalyst layer of the denitration device (5) is provided with a denitration catalyst containing an oxidation catalyst in a temperature range (300 to 400 ° C.) that is normally installed to convert metallic mercury into mercury chloride, the temperature is high. In particular, it is difficult to oxidize metallic mercury when the HCl concentration in the exhaust gas is low.

  Therefore, the air preheater (6), which is a component device of the exhaust gas purification device, is equipped with a bag filter carrying an oxidation catalyst in the exhaust gas in the low temperature range after the heat exchange so that the exhaust gas temperature is lowered in a state where the exhaust gas temperature is lowered. Metallic mercury can be oxidized.

  According to the sixth aspect of the present invention, the metal mercury oxidation catalyst used in the bag filter of the dust removing device (7) is composed of Ti, Mo and V metals or oxides thereof, and these are substantially the same as the denitration catalyst component. The same metal or metal oxide can also be used as it is as an oxidation catalyst for metal mercury.

  According to the invention described in claim 6, metal mercury remaining in the exhaust gas is oxidized by the oxidation catalyst in the bag filter of the dust removing device (7), and the resulting mercury oxide such as mercury chloride is collected by the bag filter. can do.

According to the seventh aspect of the present invention, the amount of the metal mercury oxidation catalyst used in the bag filter of the dust removal device (7) supported on the filter cloth is greater than the amount of the oxidation catalyst in the denitration catalyst used in the denitration device (5). , the amount used is the removal device of the exhaust gas in trace toxic substances according to any one of claims 1 is 100 to 500 g / m ² 6 of.

According to the seventh aspect of the present invention, the concentration of the oxidation catalyst in the bag filter of the dust removal device (7) installed in the exhaust gas flow path downstream of the denitration catalyst layer of the denitration device (5) is determined. The concentration of the second component, which is the same component of the NOx removal catalyst layer, can be made higher because the oxidation reaction of SO ₂ to SO ₃ occurring in the oxidation catalyst temperature region in the bag filter is extremely unlikely to occur at low temperatures. ing.

  The bag filter carrying the oxidation catalyst has a cross-sectional structure as shown in FIG. 2, and the exhaust gas can pass through the gap between the fiber of the bag filter 1 and the catalyst carried on the fiber. This structure is a structure that facilitates mass transfer. As described above, the amount of metallic mercury in the exhaust gas is extremely small. Especially when the HCl concentration in the exhaust gas is low, the reaction between metallic mercury and HCl on the oxidation catalyst in the bag filter can be effectively performed. Although it is necessary to increase the contact efficiency between the two, as described above, the non-woven fabric fibers are suitable for increasing the contact efficiency between metallic mercury and HCl because the mass transfer rate is increased by disturbing the gas flow. In addition, the bag filter 1 is usually designed to have a filter cloth passing speed of 1 m / min or less in order to suppress pressure loss, and a sufficient contact time between metallic mercury and HCl is obtained in the bag filter 1. It is done.

The filter cloth used in the bag filter 1 including the oxidation catalyst function may be any material as long as it is applicable to petroleum or coal combustion exhaust gas. Examples of the material include polyimide, polyamide, polyphenylene sulfide, and polytetrafluoroethylene. Alternatively, glass fiber may be used. The amount of the oxidizing catalyst supported-used filter cloth, the filtrate area per 100g / m ² ~500g / m ² of fabric, preferably suitably 200 to 400 g / m ^2. As shown in FIG. 4, if the amount of the oxidation catalyst used is too small, a sufficient catalytic effect cannot be obtained. If it is too large, the pressure loss of the system is increased and dust is sufficiently generated when the filter cloth is regenerated after exhaust gas treatment. This is because the payment cannot be made.

According to the seventh aspect of the present invention, the concentration of the oxidation catalyst of the bag filter carrying the oxidation catalyst is made higher than the concentration of the second component having oxidation activity in the denitration catalyst layer of the denitration device (5), thereby removing the denitration catalyst. In the layer, oxidation of SO _{2 in} the exhaust gas to SO ₃ can be suppressed. On the other hand, on the bag filter, since the oxidation catalyst is in a lower temperature region than the denitration catalyst layer of the denitration device (5), the oxidation reaction from SO ₂ to SO ₃ hardly occurs, so SO ₂ changes to SO ₃ . There is no fear of being oxidized, and even a mercury metal contained in a trace amount can be oxidized by a sufficient amount of the oxidation catalyst.

In addition, by setting the amount of metal mercury oxidation catalyst used in the bag filter of the dust removal device (7) to be supported on the filter cloth of 100 to 500 g / m ² , a sufficient catalytic effect can be obtained. Pressure loss during exhaust gas passage does not increase, and metal mercury can be oxidized without impairing the dust removal effect.

  The invention described in claim 8 is characterized in that the operating temperature of the denitration device (5) is 250 ° C to 450 ° C, and the operating temperature of the bag filter including the metal mercury oxidation catalyst of the dust removing device is 120 ° C to 250 ° C. It is the operating method regarding the trace harmful substance removal apparatus in exhaust gas in any one of 1 thru | or 7.

  According to the eighth aspect of the invention, the denitration reaction is promoted at 250 ° C. to 450 ° C. where the denitration catalyst is active, and metallic mercury can be oxidized at 120 ° C. to 250 ° C. where the oxidation catalyst is active.

  Even if a denitration catalyst carrying an oxidation catalyst is installed in the temperature range (300-400 ° C) where a normal denitration catalyst is installed and metal mercury is converted to mercury chloride, the temperature is high, especially the HCl concentration in the exhaust gas is low. In some cases it is difficult to oxidize metallic mercury.

  Therefore, the air preheater (6), which is a component device of the exhaust gas purifying device, is equipped with a bag filter carrying an oxidation catalyst in the exhaust gas in the low temperature region after heat exchange so that the exhaust gas temperature is lowered in a state where the exhaust gas temperature is lowered. Metallic mercury can be oxidized.

  Thus, according to the eighth aspect of the present invention, exhaust gas denitration and metal mercury oxidation can be performed in the temperature range where the denitration catalyst is active and the temperature where the oxidation catalyst is active.

   The same effect can be obtained even if the denitration catalyst used in the present invention has a plate shape or a honeycomb shape, and the shape is not limited.

Examples of the present invention will be described below.
In the following embodiments of the present invention, boiler exhaust gas treatment is performed according to the flow shown in FIGS. 1 (a) and 1 (b).

  In the flow shown in FIG. 1 (a), a function of removing nitrogen oxides in the exhaust gas in the presence of ammonia in order from the upstream side in the exhaust gas flow path discharged from the boiler 4 for burning oil or coal, and oxidizing metal mercury. Denitration device 5 having a denitration catalyst layer having air, air preheater 6 for exchanging the combustion air of the combustion facility with exhaust gas, dust removal device 7 having a bag filter containing an oxidation catalyst for metallic mercury, and sulfur oxides in the exhaust gas A desulfurization device 8 for removing the waste gas with an absorbent such as limestone slurry and a chimney 9 for discharging the purified exhaust gas into the atmosphere are arranged.

  Further, the flow shown in FIG. 1 (b) is a combustion with a denitration device 5 having a denitration catalyst layer having a function of removing nitrogen oxides in exhaust gas in the presence of ammonia and oxidizing metal mercury in order from the upstream side. A bug that includes an air preheater 6 for exchanging heat of combustion air for the equipment with exhaust gas, a semi-dry desulfurization device 8b for removing SOx in the exhaust gas by directly spraying an absorbent such as limestone into the exhaust gas, and a metal mercury oxidation catalyst A dust removing device 7 having a filter and a chimney 9 for discharging the purified exhaust gas into the atmosphere are arranged.

85 kg of titanic acid powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less) as an oxidation catalyst for metallic mercury supported on the bag filter, ammonium molybdate ((NH ₄ ) ₆ · Mo ₇ O ₂₄ · 4H ₂ O) and 10.7 kg, of ammonium metavanadate (NH ₄ VO ₃₎ 9.9kg and oxalic acid 12.8kg was added kneaded by adjusting the water content, granulation, drying, firing sequentially performed, resulting The powder was pulverized to an appropriate particle size to obtain catalyst raw material powder. Moisture was added to this to obtain a catalyst slurry. A filter cloth made of tephia was immersed in this catalyst slurry, and after supporting the catalyst component, it was dried at 150 ° C. to obtain a bag filter supporting a metal mercury oxidation catalyst. The catalyst loading of the bag filter is 350 g / m ² .

As the first component of the denitration catalyst, 70 kg of titanium oxide powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less), aluminum compound powder (Al ₂ O ₃ ) and 70 kg of silica sol (SiO ₂ ) are used. The catalyst paste obtained by adding 7 kg of molybdenum trioxide (MoO ₃ ) and 1.6 kg of ammonium metavanadate (NH ₄ VO ₃ ) as the second component, adding alumina / silicate fiber, adjusting the moisture and kneading. Was applied to a metal expanded metal and pressed into a predetermined shape to obtain a plate-shaped denitration catalyst. The plate catalyst was calcined at 500 ° C.

  A catalyst layer containing the above denitration catalyst was installed in the coal fired boiler test facility, and an air preheater for exhaust gas temperature reduction and a bag filter carrying an oxidation catalyst were sequentially installed downstream of the catalyst layer. Further downstream, a wet desulfurization apparatus (limestone-gypsum method) was arranged.

A bag filter carrying a metal mercury oxidation catalyst was prepared in the same manner as in Example 1, and titanium oxide powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less) as a first component of the denitration catalyst was 70 kg. , an aluminum compound powder (Al ₂ O ₃₎ 0.9g, molybdenum trioxide as the second component (MoO ₃₎ 7 kg, ammonium metavanadate (NH ₄ VO ₃₎ 1.6kg were added, the addition of alumina / silicate fibers Thereafter, a catalyst paste obtained by adjusting and kneading moisture was applied to a metal expanded metal and pressed into a predetermined shape to obtain a plate-shaped denitration catalyst. The plate catalyst was calcined at 500 ° C.

  A catalyst layer containing the above denitration catalyst was installed in the coal fired boiler test facility, and an air preheater and a bag filter carrying an oxidation catalyst were sequentially installed on the downstream side. Further, a wet desulfurization apparatus (limestone-gypsum method) was disposed on the downstream side.

A bag filter carrying a metal mercury oxidation catalyst was prepared in the same manner as in Example 1, and titanium oxide powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less) as a first component of the denitration catalyst was 70 kg. 0.9 kg of aluminum compound powder (Al ₂ O ₃ ), 20 kg of ammonium metatungstate (NH ₄ ) ₆ [H ₂ W ₁₂ O ₄₀ ] as the second component, ammonium metavanadate (NH ₄ VO ₃ ) After adding 6 kg and adding alumina / silicate fiber, the catalyst paste obtained by adjusting and kneading the moisture was applied to a metal expanded metal and pressed into a predetermined shape to obtain a plate-like denitration catalyst. The plate catalyst was calcined at 500 ° C.

  A catalyst layer containing the above denitration catalyst was installed in the coal fired boiler test facility, and an air preheater and a bag filter carrying an oxidation catalyst were sequentially installed on the downstream side. Further, a wet desulfurization apparatus (limestone-gypsum method) was disposed on the downstream side.

A bag filter carrying a denitration catalyst and an oxidation catalyst was prepared in the same manner as in Example 1. The amount of the catalyst carried by the bag filter carrying the oxidation catalyst was 76 g / m ² . The arrangement of the test equipment is the same as in Example 1.

70 kg of titanic acid powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less) as the first component of the denitration catalyst, 7 kg of molybdenum trioxide (MoO ₃ ) as the second component, ammonium metavanadate ( NH ₄ VO ₃ ) (3.2 kg) and aluminum compound powder (Al ₂ O ₃ ) (0.9 kg), alumina / silicate fiber and silica sol (SiO ₂ ) (14 kg) were added, and the moisture was adjusted and kneaded into the catalyst paste. Was applied to a metal expanded metal and pressed into a predetermined shape to obtain a plate-shaped denitration catalyst. The plate catalyst was calcined at 500 ° C.

  This catalyst is used in a relatively downstream stage denitration catalyst layer, and the denitration catalyst prepared in Example 1 is installed in a relatively upstream stage denitration catalyst layer to form a denitration apparatus. Installed. The bag filter carried in this case is the same as that of the first embodiment.

  Using the bag filter carrying the denitration catalyst and oxidation catalyst used in Example 1, a denitration catalyst and an air preheater were installed in the same manner in the same test equipment, and a semi-dry desulfurization device was installed downstream of the same. After that, a bag filter carrying the catalyst was installed.

  The denitration catalyst layer used in Example 1 was not installed, and after the temperature was reduced by an air preheater, a bag filter carrying an oxidation catalyst was installed, and a wet desulfurization apparatus was installed downstream thereof.

Comparative Example 1

  An experiment was conducted with the same test equipment arrangement as in Example 1, using the same oxidation catalyst as that supported on the bag filter of Example 1 except that the content of the vanadium component having high oxidation activity was reduced.

That is, as an oxidation catalyst for metallic mercury supported on the bag filter, 85 kg of titanic acid powder (TiO ₂ content: 90 wt% or more, SO ₄ content: 3 wt% or less) is added to ammonium molybdate ((NH ₄ ) ₆ .Mo ₇ O ₂₄ · 4H ₂ O) 10.1 kg, ammonium metavanadate (NH ₄ VO ₃ ) 2.0 kg and oxalic acid 2.6 kg are added to adjust the moisture, kneading, granulating, drying and firing in order. The obtained powder was pulverized to an appropriate particle size to obtain catalyst raw material powder. Moisture was added to this to obtain a catalyst slurry. A filter fabric made of tefay was immersed in this catalyst slurry, and after supporting the catalyst component, it was dried at 150 ° C. to obtain a bag filter supporting a metal mercury oxidation catalyst. The catalyst loading of the bag filter is 350 g / m ² .

Comparative Example 2

  The denitration catalyst layer of Example 1 was disposed, a normal bag filter having no oxidation catalyst function was disposed, and a wet desulfurization apparatus was disposed downstream thereof. The arrangement of the test equipment is the same as in Example 1.

  In addition, the normal bag filter which does not have an oxidation catalyst function used here uses glass fiber as a filter cloth material, and the flow velocity of the pulse jet is in the range of 0.8 to 1.3 m / min. This type is backwashed by the method.

The Hg removal performance in the test facility simulating the whole exhaust gas purification system was compared under the conditions shown in Table 1 with the configurations of Examples 1 to 7 and Comparative Examples 1 and 2. The results obtained are summarized in Table 2. As can be seen from Table 2, when the system is constructed with the catalyst configuration of the present invention, the overall Hg removal performance is excellent.

  INDUSTRIAL APPLICABILITY The present invention can be used for an apparatus for removing a metal mercury compound as a trace harmful substance contained in a combustion exhaust gas of petroleum or coal and a boiler exhaust gas treatment as an operation method thereof.

The flowchart of the waste gas processing system of the Example of this invention is shown. The principal part structure figure of the bag filter which carry | supported the oxidation catalyst is shown. The denitration catalyst function change in the presence of SOx at low temperature is shown. The correlation between the denitration catalyst loading and the catalyst function is shown. The arrangement structure of the catalyst layer in the denitration apparatus is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Bag filter 2 Particulate mercury chloride 3 Denitration catalyst layer 4 Boiler 5 Denitration device 6 Air preheater 7 Bag filter with catalyst function 8 Desulfurization device 9 Chimney

Claims (8)

  Bugs equipped with an air preheater that exchanges heat from the combustion facility's combustion air with the exhaust gas in order from the upstream side, and a filter cloth carrying a metal mercury oxidation catalyst in the exhaust gas flow path that discharges from the combustion facility that burns oil or coal An apparatus for removing trace harmful substances in exhaust gas, wherein a dust removing apparatus having a filter is arranged.   A denitration device having a denitration catalyst layer having a function of removing nitrogen oxides in exhaust gas and oxidizing metal mercury is disposed in the exhaust gas flow path on the upstream side of the air preheater, and the downstream side of the dust removal device The exhaust gas passage according to claim 1, wherein a wet desulfurization device for spraying an absorbent slurry (using an absorbing liquid such as limestone slurry) to remove sulfur oxides in the exhaust gas is disposed in the exhaust gas flow path. Equipment for removing trace amounts of harmful substances.   A denitration device having a denitration catalyst layer having a function of removing nitrogen oxides in the exhaust gas and oxidizing metal mercury is disposed in the exhaust gas flow path on the upstream side of the air preheater, and the air preheater and the dust removal device A semi-dry type desulfurization device that removes sulfur oxide in exhaust gas by spraying absorbent slurry (using slaked lime, magnesium hydroxide, etc.) into the exhaust gas channel in the exhaust gas channel between The apparatus for removing trace amounts of harmful substances in exhaust gas according to claim 1. The denitration catalyst of the denitration catalyst layer is composed of two or more compounds selected from titanium oxide (TiO ₂ ), silicon oxide (SiO ₂ ), and aluminum oxide (Al ₂ O ₃ ) as a first component, molybdenum (Mo), The trace amount of harmful substances in exhaust gas according to any one of claims 1 to 3, wherein the second component is two or more kinds of metals selected from vanadium (V) and tungsten (W) or oxides thereof. Removal device.   The concentration of the second component of the denitration catalyst in the denitration catalyst layer on the relatively downstream side in the denitration catalyst layer arranged in a plurality of stages in the gas flow direction in the denitration device is determined by denitration in the denitration catalyst layer on the relatively upstream side. The apparatus for removing trace amounts of harmful substances in exhaust gas according to any one of claims 1 to 4, wherein the concentration is reduced stepwise from the concentration of the second component of the catalyst.   6. The metal mercury oxidation catalyst in the dust filter bag filter is made of titanium (Ti), molybdenum (Mo), vanadium (V) metal or an oxide thereof. For removing trace amounts of toxic substances from exhaust gas. The amount of metal mercury oxidation catalyst used in the dust filter bug filter supported on the filter cloth is equal to or greater than the amount of oxidation catalyst in the denitration catalyst used in the denitration device, and the amount used is 100 to 500 g / m ² . The device for removing trace harmful substances in exhaust gas according to any one of claims 1 to 6.   8. The operation temperature of the denitration device is 250 ° C. to 450 ° C., and the operation temperature of the bag filter including the metal mercury oxidation catalyst of the dust removal device is 120 ° C. to 250 ° C. The operation method of the removal apparatus of the trace amount harmful substance in waste gas described in 2.

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