JP2009028655A - Exhaust gas treating method and exhaust gas treatment apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas treating method and an exhaust gas treatment apparatus, which remove mercury with high efficiency and at a low cost regardless of the kind of coal or the property of exhaust gas discharged from a combustion apparatus, when burning coal in the combustion apparatus like a boiler, and easy in management of removed mercury. <P>SOLUTION: This invention relates to the exhaust gas treating method for removing mercury contained in exhaust gas discharged from a coal-burning boiler B, when burning fossil fuel like coal C in the coal-burning boiler B. After feeding crushed coal C and a crushed chlorine compound Cl to the coal-burning boiler B and burning, mercury contained in exhaust gas is captured in sludge and collected by a wet type desulfurization unit 7 disposed in a flue R from the coal-burning boiler B, and the sludge containing mercury is subjected to recovery treatment. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas treatment method and an exhaust gas treatment device used to remove mercury contained in exhaust gas discharged from a combustion device when fossil fuel such as coal is burned by a combustion device such as a boiler. It is.

The above-mentioned fossil fuel combustion apparatus, for example, coal combustion exhaust gas discharged from a coal-fired boiler contains a trace amount of mercury due to coal. This mercury is present in the exhaust gas in three forms: hardly soluble metallic mercury Hg ⁰ , water-soluble divalent mercury Hg ²⁺ (HgCl ₂ ), and particulate mercury Hg ^P adhering to combustion ash. .
As an exhaust gas treatment apparatus that removes mercury from exhaust gas, for example, there is an apparatus in which a denitration unit, a dedusting unit, and a desulfurization unit are sequentially arranged in a flue from a coal-fired boiler to a chimney.

Of the mercury contained in the exhaust gas, most of the particulate mercury Hg ^P is removed by a dust removal part such as an electric dust collector or a bag filter of this exhaust gas treatment device, and the divalent mercury Hg ²⁺ is wet desulfurization. However, most of the metallic mercury Hg ⁰ contained in the exhaust gas is released to the atmosphere without being almost removed by the dedusting part or the desulfurization part.
Metallic mercury Hg ⁰ released into the atmosphere is converted into more harmful organic mercury (methylmercury) in the environment, and this organic mercury is accumulated in edible organisms such as fish and shellfish, which passes through the food chain. There is concern about entering the human body.

In light of this situation, the US Environmental Protection Agency has decided to regulate mercury emissions from coal-fired thermal power plants, reducing mercury emissions by 30% by 2010, and 2018 In the same way, Canada has decided to regulate mercury emissions from coal-fired thermal power plants.
Metallic mercury Hg ⁰ contained in the exhaust gas is converted into denitration catalyst, coal ash, and unburned carbon as shown in the reaction formula (1) by hydrogen chloride (HCl) caused by chlorine contained in coal as well as mercury. Oxidized on the surface.

Hg ⁰ + 2HCl + 1 / 2O ₂ → HgCl ₂ + H ₂ O Reaction formula (1)
However, divalent mercury Hg ²⁺ is HgCl ₂ .
The mercury oxidation efficiency on the denitration catalyst increases as the HCl concentration increases. That is, the higher the HCl concentration, the higher the generation ratio of HgCl _2. As a result, the ratio of mercury collected in the dedusting section and desulfurization section also increases.

Since HgCl ₂ has a stronger adsorptivity than Hg ⁰ , it is adsorbed on the surface of ash in the dust removal part and collected as particulate mercury Hg ^P. At this time, since the proportion of mercury collected in the dust removal part depends on the amount of unburned carbon in the ash, if there is little unburned carbon, the percentage collected by the ash in the dust removal part Will be less.
Thus, if the ratio collected as particulate mercury Hg ^P in the dedusting part is not so high, it is equivalent to metal mercury Hg ⁰ oxidized on the denitration catalyst and divalent mercury Hg ²⁺ (HgCl ₂ ). The amount passes through the dedusting part and is collected in the drainage of the desulfurization part.

Here, the ash collected in the dust removal part is effectively used as a cement or concrete material. However, since it contains more mercury than in the past, further consideration must be given to measures such as mercury elution.
On the other hand, mercury that has reached the desulfurization section is collected as part of the sludge in the drainage of the desulfurization section, and can be treated in a state managed as industrial waste.
Therefore, it is considered that mercury oxidized on the denitration catalyst is more preferably collected in the desulfurization part than in the dust removal part.

Conventionally, as a technology for removing mercury in exhaust gas discharged from combustion equipment, mercury adsorbents such as activated carbon are blown upstream of dust removal parts such as electric dust collectors and bag houses that remove soot and dust. There has been proposed a method for removing mercury by adsorbing mercury on the surface of the agent (for example, see Patent Document 1).
However, this removal technique using an adsorbent has the disadvantages that the adsorbent needs to be constantly blown, so that the running cost is high and the efficiency varies depending on the gas properties such as HCl concentration in the exhaust gas.

  In terms of the concentration of HCl in the exhaust gas, the amount of chlorine contained in the coal is originally small, from several ppm to several hundred ppm, and the content varies greatly depending on the type of coal. When converted to HCl concentration in the exhaust gas, the concentration is from less than 1 ppm to several tens of ppm. When the HCl concentration in the exhaust gas is low in this way, depending on the properties of the exhaust gas, it depends on the denitration catalyst and unburned carbon. As a result, the mercury oxidation efficiency decreases, and the mercury collection efficiency in the dedusting section and desulfurization section also decreases.

In order to cope with this, a method of injecting a halogen-containing substance into the flue from the coal-fired boiler to the chimney (see, for example, Patent Document 2), or a chlorine compound is supplied to the combustion device together with the coal and burned. Thereafter, a method is proposed in which the exhaust gas temperature is cooled to 150 ° C. or lower in front of a dust removal unit such as an electric dust collector and mercury is removed in the dust removal unit (see, for example, Patent Document 3).
U.S. Patent No. 6521021 Japanese Patent Laid-Open No. 10-230137 JP 2000-325747 A

However, in the mercury removal method described above, in the former method in which a halogen-containing substance is injected into the flue, compounds such as CaCl ₂ , NaCl, and NH ₄ Cl are exemplified when chlorine compounds are assumed. Although it can be decomposed into HCl by heating to oxidize Hg ⁰ , except for the case of injecting HCl directly into the flue, when a halogen compound is injected, it is heated at a sufficient temperature and decomposed. It is necessary to take sufficient residence time to proceed.

On the other hand, in the latter method of removing mercury in the dust removal part, only a part of the mercury can be removed, and as described above, the ash collected in the dust removal part contains a large amount of mercury. When using it as a concrete material, there is a problem that strict management is required, and it has been a conventional problem to solve these problems.
The present invention has been made to solve the above-described problems, and relates to the type of coal and the properties of exhaust gas discharged from the combustion device when fossil fuel, for example, coal, is burned by a combustion device such as a boiler. In addition, an object of the present invention is to provide an exhaust gas treatment method and an exhaust gas treatment device that can remove mercury at high efficiency and at low cost, and that can easily manage the removed mercury.

  The invention according to claim 1 of the present invention is an exhaust gas treatment method for removing mercury contained in exhaust gas discharged from a combustion device when fossil fuel such as coal is burned in a combustion device such as a boiler, After the pulverized fossil fuel and the pulverized halogen compound are supplied to the combustion device and combusted, the mercury contained in the exhaust gas is taken into the sludge and captured by the wet desulfurization section disposed in the flue from the combustion device. It is characterized by collecting and collecting sludge containing mercury, and the configuration of this exhaust gas treatment method is a means for solving the above-mentioned conventional problems.

In this case, assuming a chlorine compound as the halogen compound, compounds such as CaCl ₂ , NaCl, and NH ₄ Cl are listed. In general, CaCl ₂ that is sold becomes powdery as the purity increases, and becomes larger and flakes as the purity decreases. However, flakes with low purity can be pulverized.
Further, in NH ₄ Cl, not only a powdery reagent but also an inexpensive mineral (ammonite chloride) can be directly used.

Further, the size of the halogen compound particles obtained by pulverization is preferably 70 to 80% for the passage ratio of 200 mesh (75 μm), and the passage of 100 mesh (150 μm) in order to promote the decomposition effect. It is desirable that the ratio be around 90%.
Furthermore, the amount of halogen compound is supplied to the combustion apparatus, in order to increase the oxidation efficiency of Hg ^0, fossil fuels, for example, be determined based on the concentration of chlorine contained in coal.

The invention according to claim 2 of the present invention is configured such that a fine powder mixture obtained by pulverizing and mixing the fossil fuel and the halogen compound together is supplied to the combustion device and burned.
According to a third aspect of the present invention, the fossil fuel and the halogen compound are individually pulverized, the pulverized fossil fuel is supplied to the combustion device, and the secondary combustion air used in the combustion device is pulverized as a carrier. The produced halogen compound is supplied to the combustion device and burned.

  On the other hand, the invention according to claim 4 of the present invention is an exhaust gas treatment device for removing mercury contained in exhaust gas discharged from a combustion device when fossil fuel such as coal is burned by a combustion device such as a boiler. And a supply unit for supplying the pulverized fossil fuel and the pulverized halogen compound to the combustion device, and the mercury contained in the exhaust gas discharged from the combustion device in the sludge. It is set as the structure provided with the wet desulfurization part which takes in in and collects.

In the invention according to claim 5 of the present invention, the supply unit includes a mill that pulverizes and mixes the fossil fuel and the halogen compound together and supplies them to the combustion device.
In the invention according to claim 6 of the present invention, the supply unit pulverizes the fossil fuel and supplies it to the combustion device, a compound mill for pulverizing a halogen compound, and a pulverized halogen compound for the pulverized halogen compound. It is configured to have an introduction path that leads from the compound mill to the flow path of the secondary combustion air used in the combustion apparatus.

In the exhaust gas treatment method and the exhaust gas treatment apparatus of the present invention, the flue from the combustion device to the wet desulfurization unit is usually arranged with a denitration unit and a dedusting unit in sequence, and exists in the denitration unit (and in the flue). On the surface of unburned carbon or ash, Hg ⁰ and HCl are reacted to oxidize to HgCl ₂ .
On the other hand, in order to prevent mercury from being adsorbed by the ash in the dust removing portion, although not particularly limited, it is desirable to operate the dust removing portion at a relatively high temperature of 150 ° C. or higher.

  Since the exhaust gas treatment method according to claim 1 and the exhaust gas treatment apparatus according to claim 4 of the present invention have the above-described configuration, removal of mercury when fossil fuel, for example, coal, is burned by a combustion device such as a boiler. In addition, the running cost can be reduced by the amount not using the adsorbent, and the mercury collection efficiency does not change due to the gas properties such as HCl concentration in the exhaust gas.

In other words, it is possible to remove mercury with high efficiency and low cost regardless of the type of coal and the nature of the exhaust gas discharged from the combustion device, and in addition, the sludge containing mercury is recovered and treated. As a result, it is possible to easily manage mercury.
In addition, since the exhaust gas treatment method according to claim 2 and the exhaust gas treatment apparatus according to claim 5 of the present invention are configured as described above, a fine powdery mixture of fossil fuel and halogen compound is supplied to the burner nozzle of the combustion apparatus. In addition, since it can be vaporized and decomposed at a high temperature, it is possible to efficiently generate gaseous HCl.

  Further, in the exhaust gas treatment method according to claim 3 of the present invention and the exhaust gas treatment apparatus according to claim 6, the halogen compound solid is pulverized by a mill and finely powdered halogen is obtained using the two-stage combustion air used in the combustion device. Since the compound is introduced into the combustion apparatus, an effect that the addition of a mechanism for supplying the halogen compound can be reduced can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 show an exhaust gas treatment apparatus according to an embodiment of the present invention. In this embodiment, the exhaust gas treatment method and the exhaust gas treatment apparatus of the present invention are exhausted from a coal-fired boiler (combustion apparatus). A case where this is applied to the process will be described as an example.
As shown in FIG. 1, the exhaust gas treatment apparatus 1 includes a denitration unit 3, an air heater 4, a dedusting unit 5, a heat exchanger 6, a desulfurization unit, which are sequentially arranged in a flue R from a coal burning boiler B to a chimney 2. 2 and a heat exchanger 8 as well as a supply unit for supplying coal C as a fossil fuel and a chlorine compound Cl as a halogen compound to a coal-fired boiler B. This supply unit is shown in FIG. As shown, a mill 9 is provided in which coal C and a chlorine compound Cl are pulverized and mixed together and supplied to a coal-fired boiler B.

In this case, by adding ammonia to the upstream side of the denitration unit 3 in the flue R, so that converted by reducing NO _X contained in the exhaust gas to nitrogen and water.
Further, in the denitration unit 3 and in the flue R, metal mercury Hg ⁰ and HCl are reacted with each other on the surface of unburned carbon and ash present to form water-soluble divalent mercury Hg ²⁺ (HgCl ₂ ). At this time, in order to prevent divalent mercury Hg ²⁺ from being adsorbed by the ash in the dust removing part 5, the air heater 4 is adjusted to operate the dust removing part 5 at a temperature of 150 ° C. or higher. Like to do.

Then, the wet desulfurization unit 7, divalent mercury Hg 2+ which has passed through the dedusted section 5 ^(HgCl ₂₎ and a liquid phase absorption, so that collecting incorporate in the sludge.
In this exhaust gas treatment apparatus 1, coal C and chlorine compound Cl are pulverized and mixed together in advance by a mill 9, and the mixture is supplied to a coal-fired boiler B to start combustion. When performing the processing of the exhaust gas discharged from a coal-fired boiler B, first, by adding ammonia to the upstream side of the denitration unit 3 in the flue R, and nitrogen by reducing NO _X contained in the exhaust gas Convert to water.

Of the mercury contained in the exhaust gas discharged from the coal-fired boiler B, metallic mercury Hg ⁰ reacts with HCl on the surface of unburned carbon and ash present in the denitration section 3 and in the flue R. Thus, it is converted into water-soluble divalent mercury Hg ²⁺ (HgCl ₂ ).
At this time, since the dust removing unit 5 is operated at a temperature of 150 ° C. or more by adjusting the air heater 4, the adsorption of divalent mercury Hg ²⁺ to the ash of the dust removing unit 5 is avoided.

In the wet desulfurization unit 7, the divalent mercury Hg ²⁺ (HgCl ₂ ) that has passed through the dedusting unit 5 is absorbed in the liquid phase and collected in the sludge, and then the mercury-containing sludge is recovered. Like to do.
As described above, in the exhaust gas treatment method and the exhaust gas treatment apparatus 1 of this embodiment, when the coal C is burned by the coal-fired boiler B, the adsorbent is not used for removing mercury. In addition, the mercury collection efficiency does not change due to gas properties such as HCl concentration in the exhaust gas.

Therefore, regardless of the type of coal C and the nature of the exhaust gas, mercury can be removed with high efficiency and low cost, and the sludge containing mercury is recovered and treated, so that the management of the removed mercury becomes easy. .
In the exhaust gas treatment method and exhaust gas treatment apparatus 1 of this embodiment, since a fine powder mixture of coal C and chlorine compound Cl is supplied to the burner nozzle of the coal-fired boiler B, vaporization and decomposition at a high temperature at a stretch. It is possible to efficiently produce gaseous HCl.

  FIG. 3 shows an exhaust gas treatment apparatus according to another embodiment of the present invention. The difference between the exhaust gas treatment apparatus 11 of this embodiment and the previous exhaust gas treatment apparatus 1 is that the supply unit pulverizes coal C. Fuel mill 19A for supplying to coal-fired boiler B, compound mill 19B for pulverizing chlorine compound Cl, and flow of secondary combustion air A for using pulverized chlorine compound Cl from compound mill 19B to coal-fired boiler B In the exhaust gas treatment device 11 of this embodiment, the coal C and the chlorine compound Cl are individually pulverized, and the pulverized coal C is supplied to the coal-fired boiler B. The chlorine compound Cl pulverized using the secondary combustion air A used in the coal-fired boiler B as a carrier is supplied to the coal-fired boiler B for combustion.

In this exhaust gas treatment device 11 as well, mercury can be removed at high efficiency and at low cost regardless of the type of coal C and the nature of the exhaust gas, and the sludge containing mercury is recovered and treated. Becomes easy.
Further, in the exhaust gas treatment apparatus 11, since the finely divided chlorine compound Cl is introduced into the coal-fired boiler B using the two-stage combustion air A used in the coal-fired boiler B, the chlorine compound Cl is supplied. There is little additional mechanism for

BRIEF DESCRIPTION OF THE DRAWINGS It is schematic structure explanatory drawing which shows the waste gas processing apparatus by one Embodiment of this invention. It is a partial expansion explanatory drawing which expands and shows the supply part of the exhaust gas processing apparatus of FIG. It is a partial expanded explanatory view which expands and shows the supply part of the exhaust gas processing apparatus by other embodiment of this invention.

Explanation of symbols

1,11 Exhaust gas treatment equipment 3 Denitration part 5 Dust removal part 7 Desulfurization part 9 Mil (supply part)
19A Fuel Mill (Supply Unit)
19B Compound Mill (Supplying Section)
20 Introduction route (supply section)
A Two-stage combustion air B Coal-fired boiler (combustion device)
C Coal (fossil fuel)
Cl chlorine compound (halogen compound)
R Flue

Claims (6)

An exhaust gas treatment method for removing mercury contained in exhaust gas discharged from a combustion device when fossil fuel such as coal is burned in a combustion device such as a boiler,
After the pulverized fossil fuel and the pulverized halogen compound are supplied to the combustion device and combusted, the mercury contained in the exhaust gas is taken into the sludge and captured by the wet desulfurization section disposed in the flue from the combustion device. An exhaust gas treatment method characterized by collecting and recovering sludge containing mercury.   The exhaust gas treatment method according to claim 1, wherein a mixture obtained by pulverizing and mixing the fossil fuel and the halogen compound together is supplied to the combustion device and combusted.   The fossil fuel and the halogen compound are individually pulverized, the pulverized fossil fuel is supplied to the combustion device, and the halogen compound pulverized using the secondary combustion air used in the combustion device as a carrier is supplied to the combustion device. The exhaust gas treatment method according to claim 1, wherein combustion is performed. An exhaust gas treatment device for removing mercury contained in exhaust gas discharged from a combustion device when fossil fuel such as coal is burned in a combustion device such as a boiler,
A supply unit for supplying pulverized fossil fuel and pulverized halogen compound to the combustion device;
An exhaust gas treatment apparatus, comprising a wet desulfurization unit that is disposed in a flue from the combustion device and takes in and collects mercury contained in the exhaust gas discharged from the combustion device into sludge.   The exhaust gas treatment apparatus according to claim 4, wherein the supply unit includes a mill that pulverizes and mixes the fossil fuel and the halogen compound together and supplies the pulverized fuel and the halogen compound to the combustion apparatus.   The supply unit pulverizes the fossil fuel and supplies the combustor with a fuel mill, a compound mill for pulverizing a halogen compound, and a pulverized halogen compound from the compound mill for use in the combustor. The exhaust gas treatment apparatus according to claim 4, further comprising an introduction path leading to a combustion air flow path.

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