JP2015155078A - Mercury removal apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress a reduction in the oxidation rate of metal mercury.

SOLUTION: A mercury removal apparatus comprises: a wet desulfurizer 12 provided in a channel for metal mercury-containing exhaust gas discharged from a boiler 11 and removing sulfur oxides in the exhaust gas; and an air preheater 18 provided upstream of this desulfurizer 12, and implementing heat exchange between combustion air supplied to the boiler 11 and the exhaust gas to heat the combustion air to a set temperature, a mercury oxidation catalyst oxidizing the metal mercury in the exhaust gas into mercury oxide being coated only onto a contact surface 24 of this air preheater 18 with which the exhaust gas at 200°C or higher contacts.

COPYRIGHT: (C)2015,JPO&INPIT

Description

  The present invention relates to a mercury removing apparatus for removing metallic mercury from exhaust gas.

  Mercury compounds are contained in trace amounts in exhaust gas discharged from combustion apparatuses such as boilers using oil or coal. Once released into the atmosphere, this mercury compound is known to have a great influence on the human body through the food chain, and in recent years there has been an increase in the regulation of emission of this kind of harmful substances.

In general, mercury compounds are discharged as gaseous metallic mercury having a high vapor pressure from a combustion zone near 1500 ° C. in a combustion apparatus. Metallic mercury discharged from the combustion apparatus reacts with hydrogen chloride (HCl) coexisting in the exhaust gas in a relatively low temperature region of the flue, and is partially oxidized to mercury chloride (HgCl ₂ ). This reaction is promoted on a denitration catalyst of a denitration apparatus installed in the flue. Mercury chloride (hereinafter referred to as mercury oxide) produced on the denitration catalyst has a lower vapor pressure and higher solubility than metallic mercury, so downstream dust removal devices (electric dust collectors, bag filters, etc.) and wet It is removed by a desulfurization device or the like.

  By the way, since the oxidation reaction of metallic mercury on the denitration catalyst generally proceeds more advantageously at lower temperatures, the metallic mercury may not be sufficiently oxidized in the normal denitration temperature range (about 350 to 450 ° C.). In particular, when the temperature is 400 ° C. or higher, the oxidation rate of metallic mercury by the denitration catalyst is remarkably reduced in thermal equilibrium. Further, it is known that the oxidation rate of metallic mercury is remarkably reduced in exhaust gas when burning low-chlorine-containing coal such as PRB (subbituminous coal) or lignite (brown coal). Thus, when the oxidation rate of metallic mercury falls, a part of metallic mercury may be emitted from a chimney without being oxidized.

In order to improve the oxidation rate of metallic mercury, a method of lowering the inlet temperature of the denitration apparatus can be considered. However, in a general denitration method using selective catalytic reduction, ammonium sulfate is generated by a reaction between ammonia added to the exhaust gas and SO ₃ in the exhaust gas. If this ammonium sulfate is deposited on the denitration catalyst, the catalyst pores of the denitration catalyst are clogged, and the denitration rate is liable to decrease. Also, the ash adheres to relatively viscous ammonium sulfate, thereby clogging the exhaust gas flow path. There is a fear.

On the other hand, when the NOx removal device is not provided as in the exhaust gas treatment device of a thermal power plant, for example, it may be possible to install a mercury oxidation catalyst in a low temperature region (about 120 to 180 ° C.) downstream of the dust removal device. Since exhaust gas contains a large amount of SO ₂ and SO ₃ , the catalyst component may be sulfated, or SO ₃ may become sulfuric acid and accumulate in the catalyst pores, resulting in rapid deterioration of the catalyst.

  For example, Patent Document 1 discloses a technique of applying a catalyst for converting metallic mercury to mercury oxide on a contact surface where exhaust gas contacts in each device provided in an exhaust gas flow path between a boiler and a wet desulfurization apparatus. ing. According to this, the mercury metal in the exhaust gas comes into contact with the catalyst and is oxidized into water-soluble mercury oxide, and this mercury oxide is absorbed and removed by the absorption liquid of the downstream wet desulfurization apparatus.

JP 2006-29673 A

However, in Patent Document 1, when a catalyst is applied to a low temperature region (for example, about 120 to 180 ° C.) of the exhaust gas flow path, SO _{3 in} the exhaust gas is condensed into sulfuric acid and accumulated in the catalyst pores. There is a risk of reducing the oxidation rate of metallic mercury. Further, SO ₃ in the exhaust gas reacts with ammonia used for the denitration treatment, so that precipitated ammonium sulfate may block the catalyst pores.

  This invention is made | formed in view of such a problem, and makes it a subject to suppress the fall of the oxidation rate of metallic mercury.

  In order to solve the above problems, a mercury removal apparatus according to the present invention is provided in a flow path of exhaust gas containing metallic mercury discharged from a combustion apparatus, and a wet desulfurization apparatus that removes sulfur oxide in the exhaust gas, and this wet An air preheater that is provided in a flow path on the upstream side of the desulfurization device and heats the combustion air supplied to the combustion device to a set temperature by exchanging heat with the exhaust gas, and the exhaust gas at 200 ° C. or higher in the air preheater As long as the contact surface is in contact with mercury, a mercury oxidation catalyst that oxidizes metallic mercury in the exhaust gas and converts it to mercury oxide is applied.

By limiting the temperature range of the contact surface on which the mercury oxidation catalyst is applied in this way, it is possible to prevent the condensed SO ₃ from adhering to the surface of the mercury oxidation catalyst. As a result, catalyst deterioration due to clogging of catalyst pores and the like can be prevented, so that a reduction in the oxidation rate of metallic mercury can be suppressed.

  In this case, the mercury oxidation catalyst includes at least one first component selected from titanium oxide, silicon dioxide, and aluminum oxide, and at least one second component selected from nickel, manganese, molybdenum, vanadium, tungsten, and copper. It shall comprise.

By using a catalyst having such a composition, it is possible to effectively suppress a decrease in catalytic activity due to the condensation of SO ₃ .

  Further, the mercury oxidation catalyst having such a composition gradually increases in catalytic activity at a temperature lower than 400 ° C., and exhibits the highest catalytic activity in a temperature range of 250 ° C. or higher and 270 ° C. or lower. Therefore, a higher mercury oxidation rate can be obtained by applying this mercury oxidation catalyst to a contact surface where exhaust gas at 250 ° C. or higher and 270 ° C. or lower contacts.

Further, the mercury oxidation catalyst may be one in which the second component is made of a sulfate, or may be made of a phosphate. Further, the mercury oxidation catalyst may include both sulfate and phosphate. According to this, it is possible to more effectively suppress the reduction in catalytic activity due to the condensation of SO _3.

  According to the present invention, a reduction in the oxidation rate of metallic mercury can be suppressed.

It is a schematic block diagram of the mercury removal apparatus to which this invention is applied. It is a schematic block diagram of an air preheater. It is a figure which shows the relationship between the temperature of a mercury oxidation catalyst, and catalytic activity. It is a figure which shows a time-dependent change when the catalyst temperature of a mercury oxidation catalyst differs.

  Hereinafter, an embodiment of a mercury removal apparatus to which the present invention is applied will be described with reference to the drawings. The mercury removal apparatus 10 of the present embodiment removes nitrogen oxide (NOx) and sulfur oxide (SOx) in exhaust gas from the exhaust gas, while converting metallic mercury in the exhaust gas to mercury oxide and removes it from the exhaust gas. To do. Here, metallic mercury is a generic name for water-insoluble gaseous mercury compounds, and mercury oxide is a generic name for water-soluble gaseous mercury compounds (mainly mercury chloride).

  FIG. 1 shows a schematic configuration of a mercury removing apparatus to which the present invention is applied. The mercury removing device 10 of this embodiment is a device that is suitable for removing metallic mercury from the exhaust gas, particularly among trace amounts of harmful substances in the exhaust gas generated when the fuel is burned by the boiler 11. This mercury removing device 10 is configured by sequentially connecting a boiler 11, a denitration device 12, an air preheater 13, a dust collector 14, a desulfurization device 15, and a chimney 16 in series with each other via a flue 17. Composed. The air preheater 13 is supplied with the combustion air blown from the forced air blower 18, and the combustion air preheated to the set temperature is supplied to the boiler 11. Hereinafter, the configuration of each device will be described in order.

  The boiler 11 is a combustion device that generates exhaust gas by burning fuel such as coal together with combustion air. The exhaust gas discharged from the boiler 11 contains nitrogen oxide (NOx), sulfur oxide (SOx), metallic mercury, hydrogen halide (for example, HCl), and the like. The outlet portion of the boiler 11 is connected to the inlet portion of the denitration device 12 via the flue 17.

  The denitration device 12 is formed with a denitration catalyst layer in the housing, and introduces a reducing agent such as ammonia and exhaust gas injected into the upstream flue 17 to the denitration catalyst layer to reduce and decompose nitrogen oxides. It is like that. The outlet portion of the denitration device 12 is connected to the inlet portion of the air preheater 13 via the flue 17.

  The air preheater 13 is a rotary regenerative heat exchanger that uses the heat of exhaust gas to preheat combustion air to a set temperature. FIG. 2 shows a schematic configuration of the air preheater 13 of the present embodiment. In the air preheater 13, a disk-shaped element 20 is accommodated in a housing 19, and the element 20 is rotatably supported by the housing 19 around a shaft 21. The element 20 is formed by combining a large number of metal plates, and gas flows in parallel with the shaft 21 along a gap between the metal plates. The housing 19 is connected to a flue 17 connected to the denitration device 12 and an air supply pipe through which combustion air blown from the forced blower 18 flows. The interior of the housing 19 is divided into a heating chamber 22 through which exhaust gas flows and a preheating chamber 23 through which combustion air flows.

  The element 20 is provided so as to be exposed in both the heating chamber 22 and the preheating chamber 23, and accumulates heat of exhaust gas when passing through the heating chamber 22 as it rotates, and burns when passing through the preheating chamber 23. The working air is heated (preheated). The element 20 is coated with a mercury oxidation catalyst that converts metallic mercury in the exhaust gas into mercury oxide. In the air preheater 13, the outlet portion of the heating chamber 22 is connected to the inlet portion of the dust collector 14 via the flue 17.

  As the mercury oxidation catalyst applied to the element 20, at least one of titanium oxide, silicon dioxide, and aluminum oxide is the first component, and at least one of nickel, manganese, molybdenum, vanadium, tungsten, and copper is the second component. The following materials are used. In order to support the mercury oxidation catalyst on the element 20, the solid powder having the above composition is dispersed in a liquid, applied to the element 20, and then dried at a predetermined temperature. Thereby, the mercury oxidation catalyst can be coated on the surface of the element 20 in a layered manner.

  The dust collector 14 is a device that removes particles and the like in the exhaust gas. As the dust collector 14, for example, a well-known electric dust collector is used. The outlet portion of the dust collector 14 is connected to the inlet portion of the desulfurizer 15 via the flue 17.

  The desulfurization device 15 is a wet desulfurization device that removes sulfur oxides and the like in exhaust gas by absorbing them into the exhaust gas. For example, the desulfurization device 15 is configured to absorb and remove sulfur oxides by bringing the absorbent sprayed inside the housing into contact with sulfur oxides in the exhaust gas. The outlet portion of the desulfurization device 15 is connected to the chimney 6 via the flue 17.

  Next, the basic operation of the mercury removal apparatus will be described. The exhaust gas discharged from the boiler 11 is guided to the denitration device 12. In the denitration device 12, nitrogen oxides in the exhaust gas are reduced and decomposed by the reaction between ammonia added to the flue 17 and the denitration catalyst. Further, on the denitration catalyst, part of the metal mercury in the exhaust gas is converted to mercury oxide.

  The exhaust gas exiting the denitration device 12 is introduced into the air preheater 13, and when passing through the element 20, the heat is exchanged with the combustion air introduced from the air supply pipe and the temperature is reduced. Further, the metallic mercury in the exhaust gas reacts with the halogen in the exhaust gas by contacting with the mercury oxidation catalyst when passing through the element 20, and is converted into mercury oxide (for example, mercury chloride). The exhaust gas that has passed through the air preheater 13 is guided to the dust collector 14, and particles in the exhaust gas are removed. Further, since mercury oxide has a low vapor pressure, a part of it is removed from the exhaust gas together with the particles in the exhaust gas. Subsequently, the exhaust gas that has passed through the dust collector 14 is guided to the desulfurization device 15, and the sulfur oxide in the exhaust gas is absorbed and removed by the absorption liquid. At this time, the water-soluble mercury oxide is absorbed and removed by the absorbing solution together with the sulfur oxide.

  Here, the effect | action which converts the metal mercury in waste gas into mercury oxide is demonstrated. FIG. 3 shows the relationship between the temperature and catalytic activity of the mercury oxidation catalyst of the present embodiment. As shown in the figure, the catalytic activity (mercury oxidation rate) of the mercury oxidation catalyst gradually increases as the temperature becomes lower than 400 ° C., and becomes highest at 250 to 270 ° C. For this reason, in order for the mercury oxidation catalyst to obtain a desired mercury oxidation rate, it is necessary to provide a mercury oxidation catalyst at a position where the exhaust gas of less than 400 ° C., preferably 250 to 270 ° C. is in contact.

On the other hand, when low temperature (for example, 150 ° C. or lower) exhaust gas comes into contact with the mercury oxidation catalyst, SO _{3 in} the exhaust gas is condensed and accumulated in the catalyst pores, and the catalyst rapidly deteriorates. Further, since the condensed SO ₃ has a relatively high viscosity, if the condensed SO ₃ adheres to the surface of the mercury oxidation catalyst or the like, the ash in the exhaust gas adheres to this surface, and the air preheater 13 May cause clogging.

FIG. 4 shows changes over time in catalytic activity when exhaust gases having different temperatures are brought into contact with the mercury oxidation catalyst. The catalytic activity of the mercury oxidation catalyst in contact with the exhaust gas at 150 ° C decreases greatly in a short time, but the catalytic activity of the mercury oxidation catalyst in contact with the exhaust gas at 350 ° C is smaller than that at 150 ° C. It has become. For this reason, in order to suppress the catalyst deterioration of the mercury oxidation catalyst, it is necessary to provide the mercury oxidation catalyst only at the position where the exhaust gas at 200 ° C. or higher where SO ₃ does not condense comes into contact.

  Therefore, in this embodiment, the mercury oxidation catalyst is applied only to the contact surface 24 that contacts the exhaust gas of 200 ° C. or higher among the elements 20 of the air preheater 13. As shown in FIG. 2, for example, after about 350 to 400 ° C. exhaust gas and about 50 ° C. combustion air are introduced into the air preheater 13, the exhaust gas is reduced to about 150 to 200 ° C. The combustion air is discharged from the air preheater 13 in a state of being preheated to about 200 ° C. For this reason, the element 20 of the air preheater 13 is formed with a region (temperature distribution) in contact with the exhaust gas at about 150 to 400 ° C. along the flow direction of the exhaust gas. Therefore, it is important to apply the mercury oxidation catalyst only to the contact surface 24 that contacts the exhaust gas of 200 ° C. or more and 400 ° C. or less in the region where the exhaust gas of the element 20 contacts.

In the element 20 of the present embodiment, a gas flow path is formed in a gap between a large number of metal plates arranged substantially parallel to the axial direction. For this reason, a mercury oxidation catalyst is apply | coated by making the partial area | region of the axis | shaft 21 direction (up-down direction of FIG. 2) among these metal plates into the contact surface 24. FIG. Thereby, it is possible to suppress the condensation of SO _{3 on} the surface of the mercury oxidation catalyst, and it is possible to suppress a decrease in the catalytic activity, so that a high mercury oxidation rate can be maintained for a long time.

  Here, it is preferable that the contact surface 24 is applied to a region where at least 250 ° C. or more and 270 ° C. or less exhaust gas contacts. Thereby, the catalytic activity of the mercury oxidation catalyst can be maintained higher.

In addition, when PRB dredging with a low halogen content that affects the mercury oxidation rate is performed at power plants, etc., even if a denitration device is installed in the exhaust gas treatment device, the mercury oxidation rate will decrease. It is also conceivable that the amount of catalyst supported must be increased. However, according to the configuration of the present embodiment, by suppressing the condensation of SO _{3 in} the exhaust gas, a high mercury oxidation rate can be maintained, and the life of the catalyst is prolonged. As a result, the catalyst replacement frequency is low. Thus, the consumption of the catalyst can be reduced.

  In addition, ammonia added to the upstream side of the denitration apparatus 12 when denitrating exhaust gas affects mercury oxidation, and the higher the ammonia concentration in the exhaust gas, the lower the mercury oxidation rate. For example, since most of the ammonia is consumed in the denitration device 12, the downstream air preheater 13 is less affected by the mercury oxidation rate due to ammonia. The air preheater 13 introduces exhaust gas containing nitrogen oxides that have not been processed by the denitration device 12 and ammonia that has been consumed at a low concentration but has not been consumed by the denitration device 12, but in the air preheater 13, A denitration reaction between ammonia and nitrogen oxide proceeds on the mercury oxidation catalyst. For this reason, in this embodiment, a high denitration rate can be obtained together with a high mercury oxidation rate.

Further, when the exhaust gas temperature becomes low (for example, 150 ° C. or less), it is considered that SO _{3 in} the exhaust gas reacts with ammonia to generate ammonium sulfate in the air preheater 13, but was not consumed by the denitration device 12. As described above, since ammonia is consumed by the denitration reaction on the mercury oxidation catalyst of the air preheater 13, precipitation of ammonium sulfate can be suppressed. Thereby, the fall of the catalyst activity accompanying the production | generation of ammonium sulfate can be suppressed.

In this embodiment, the above-described catalyst is provided on the contact surface 24 of the air preheater 13 to suppress the condensation of SO _{3 in} the exhaust gas. However, the second component of the catalyst composition described above is sulfate. Or it may be a phosphate and may contain both a sulfate and a phosphate. With such a composition, condensation of SO ₃ can be more effectively suppressed, and a high mercury oxidation rate can be obtained even at low temperatures.

  In the present embodiment, the case where the air preheater 13 includes one element 20 has been described as an example. However, a case where a plurality of (for example, two to three stages) elements 20 are included in the axial direction is also conceivable. In this case, since the temperature of the exhaust gas passing through each stage is different from each other, the mercury oxidation catalyst is applied only to the element that contacts the exhaust gas of 200 ° C. or more and 400 ° C. or less, preferably 250 ° C. or more and 270 ° C. or less. To do. By doing so, it becomes possible to divide the area where the mercury oxidation catalyst is applied, and the work can be easily performed when the catalyst is replaced after a lapse of time.

Further, in the present embodiment, the mercury removal apparatus 10 provided with the denitration apparatus 12 has been described as an example. it is possible to obtain the effect of suppressing the condensation of SO ₃ in.

  Conventionally, in an exhaust gas treatment apparatus such as a power plant that is not provided with a denitration apparatus, there has been a problem in cost such as a large facility for providing a mercury oxidation catalyst, but according to this embodiment, Since mercury removal can be performed simply by applying and coating a mercury oxidation catalyst to the element 20 of the existing air preheater 13, the construction method is simple and the equipment becomes inexpensive, so that the economy can be improved. it can.

As described above, according to the present embodiment, since the mercury oxidation catalyst is applied only to the element 20 of the air preheater 13 with which the exhaust gas at 200 ° C. or more contacts, the denitration apparatus is installed. Regardless of the presence or absence, the condensation of SO _{3 in} the exhaust gas can be suppressed. Thereby, a high mercury oxidation rate can be maintained and clogging of the air preheater 13 due to adhesion of ash or the like can be prevented. In addition, in the exhaust gas treatment apparatus in which the denitration apparatus is set, by performing the denitration reaction with the air preheater 13, it is possible to consume ammonia that has not been consumed in the denitration apparatus and suppress the precipitation of ammonium sulfate.

On the other hand, the technique of patent document 1 applies a mercury oxidation catalyst to the component of each apparatus upstream from a desulfurization apparatus, and the temperature of the exhaust gas which contacts this catalyst is not considered. For this reason, the time-dependent deterioration of the mercury oxidation catalyst proceeds rapidly, and the frequency of catalyst replacement increases. Further, the condensation of SO ₃ increases the possibility that ash adheres to the element 20 and the air preheater 13 is clogged. For this reason, the work burden and expense at the time of a maintenance become large. In this regard, according to the present embodiment, since the position where the mercury oxidation catalyst is provided is limited to the set temperature range (200 ° C. or higher) of the air preheater 13, these problems can be solved.

  As mentioned above, although embodiment of this invention has been explained in full detail with drawing, said embodiment is only an illustration of this invention and this invention is not limited only to the structure of said embodiment. . Needless to say, even if there is a design change within a range not departing from the gist of the present invention, it is included in the present invention.

DESCRIPTION OF SYMBOLS 11 Boiler 12 Denitration apparatus 13 Air preheater 14 Dust collector 15 Desulfurization apparatus 17 Flue 18 Pushing fan 20 Element 21 Shaft 24 Contact surface

Claims (6)

  A wet desulfurization device for removing sulfur oxides in the exhaust gas, provided in a flow channel for exhaust gas containing metallic mercury discharged from the combustion device, and a flow channel on the upstream side of the wet desulfurization device, An air preheater that heats the supplied combustion air with the exhaust gas and heats it to a set temperature, and the metal mercury in the exhaust gas is limited to the contact surface of the air preheater with which the exhaust gas at 200 ° C. or higher contacts. A mercury removal device that is coated with a mercury oxidation catalyst that oxidizes and converts it to mercury oxide.   The mercury oxidation catalyst comprises at least one first component selected from titanium oxide, silicon dioxide, and aluminum oxide, and at least one second component selected from nickel, manganese, molybdenum, vanadium, tungsten, and copper. The mercury removing apparatus according to claim 1.   The mercury removal apparatus according to claim 2, wherein the mercury oxidation catalyst is applied to the contact surface with which the exhaust gas at 250 ° C or higher and 270 ° C or lower contacts.   The mercury processing apparatus according to claim 2, wherein the second component of the mercury oxidation catalyst is a sulfate.   The mercury processing apparatus according to claim 2, wherein the second component of the mercury oxidation catalyst is a phosphate.   The mercury processing apparatus according to claim 2 or 3, wherein the second component of the mercury oxidation catalyst includes both a sulfate and a phosphate.

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