JP2009166013A - Exhaust gas treatment system of coal fired boiler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas treatment system of a coal fired boiler efficiently removing mercury in the exhaust gas from the coal fired boiler. <P>SOLUTION: The exhaust gas treatment system is provided with: a denitration device removing nitrogen oxides in the exhaust gas from the coal fired boiler; an air preheater recovering heat in the gas after nitrogen oxides removal; an electrostatic precipitator removing soot and dust in the gas after heat recovery; a gas-liquid contact type desulfurizer removing sulfur oxides in the gas after dust removal by the lime-gypsum method and mercury oxide; and a chimney discharging the gas which has undergone desulfurization to the outside. Coal fed as fuel having a mole ratio of mercury (Hg)/sulfur (S) of 1.3×10<SP>-6</SP>or less is used for power generation. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas treatment system for a coal fired boiler that removes mercury from the exhaust gas of a boiler.

  Since exhaust gas discharged from boilers, which are combustion devices such as thermal power plants, contains highly toxic mercury, various systems for removing mercury in exhaust gas have been studied.

  Usually, a boiler is provided with a wet desulfurization apparatus for removing sulfur content in exhaust gas. In a flue gas treatment facility in which a desulfurization device is attached to such a boiler as an exhaust gas treatment device, since divalent mercury oxide is soluble in water, mercury may be easily collected by the desulfurization device. Widely known.

  Therefore, in recent years, various devices and methods for treating this metallic mercury have been devised in combination with a denitration device that reduces NOx and a wet desulfurization device that uses an alkaline absorbent as an SOx absorbent.

  As a method for treating metallic mercury in exhaust gas, a removal method using an adsorbent such as activated carbon or a selenium filter is known, but a special adsorption removal means is required, and treatment of large-volume exhaust gas such as power plant exhaust gas Not suitable for.

Here, as a method for treating metal mercury in large-capacity exhaust gas, as a conventional desulfurization method, a gas-liquid contact type desulfurization apparatus was mainly used by a reaction shown in the following formulas (1) and (2). The lime-gypsum method is frequently used.
SO ₂ + CaCO ₃ + 1 / 2H ₂ O → CaSO ₃ · 1 / 2H ₂ O + CO ₂ (absorption) (1)
CaSO ₃ .1 / 2H ₂ O + 3 / 2H ₂ O + 1 / 2O ₂ → CaSO ₄ .2H ₂ O (oxidation) (2)

JP 2007-7612 A

In the gas-liquid contact type desulfurization apparatus, mercury oxide (Hg ²⁺ ) is adsorbed and fixed in gypsum (CaSO ₄ ) slurry to remove mercury. At this time, the mercury (Hg) removal rate generally depends on the production rate of gypsum (CaSO ₄ ).

Therefore, in order to increase the mercury removal rate, it is necessary to increase the production rate of gypsum (CaSO ₄ ), but the ratio of mercury (Hg) and sulfur (S) in the coal depends on the properties of the coal. However, it is difficult to increase only the rate of formation of gypsum.
For this reason, when coal with less sulfur (S) than mercury (Hg) is used, there is a risk that mercury (Hg) removal performance may be insufficient if the amount of gypsum in the slurry containing gypsum-lime is small. .

In addition, the reduction of mercury oxide (Hg ²⁺ ) (Hg ²⁺ → Hg ⁰ ) is prevented by adding the air or oxygen-enriched air to the oxidized state, and zero-valent mercury into the gas phase (Hg ⁰ ) is prevented from re-scattering.

However, when a large amount of reducing substance is present in the exhaust gas, the predetermined oxidation state (the oxidation-reduction potential (ORP) value is +150 mV or more) cannot be maintained, and the zero-valent mercury (Hg ⁰ ) gas. There are cases where re-scattering into the phase cannot be suppressed. For this reason, it is anxious to remove mercury in exhaust gas efficiently by other countermeasures.

  In view of the above problems, an object of the present invention is to provide an exhaust gas treatment system for a coal fired boiler that can efficiently remove mercury in the exhaust gas from the coal fired boiler.

A first invention of the present invention for solving the above-mentioned problems is a denitration device that removes nitrogen oxides in exhaust gas from a coal fired boiler, and an air preheat that recovers heat in the gas after removal of nitrogen oxides A dust collector that removes dust in the gas after heat recovery, a gas-liquid contact type desulfurization device that removes sulfur oxide in the gas after dust removal by the lime / gypsum method, and removes mercury oxide, An exhaust gas treatment system comprising a chimney that discharges gas after desulfurization to the outside, wherein the coal has a molar ratio of mercury (Hg) / sulfur (S) of 1.3 × 10 ⁻⁶ or less. It is in the exhaust gas treatment system of a boiler.

According to a second invention, in the first invention, when the molar ratio of mercury (Hg) / sulfur (S) of coal is 1.3 × 10 ⁻⁶ or more, the molar ratio is 1.3 × 10 ^−6. The present invention is an exhaust gas treatment system for a coal fired boiler characterized by co-firing coal having a high sulfur content.

A third invention is characterized in that, in the first invention, when the molar ratio of mercury (Hg) / sulfur (S) of coal is 1.3 × 10 ⁻⁶ or more, a sulfur compound is co-fired. It is in the exhaust gas treatment system of coal fired boiler.

In a fourth aspect of the present invention, when the molar ratio of mercury (Hg) / sulfur (S) in the coal is 1.3 × 10 ⁻⁶ or more in the first aspect, desulfurization is performed while mixing sulfuric acid gas with the exhaust gas. It is in the exhaust gas treatment system of a coal fired boiler characterized by doing.

  A fifth invention is an exhaust gas treatment system for a coal fired boiler according to any one of the first to fourth inventions, wherein an oxidizing agent is added to a slurry containing limestone-gypsum.

  A sixth invention is an exhaust gas treatment system for a coal fired boiler according to any one of the first to fifth inventions, wherein a sulfuric acid compound is added to a slurry containing limestone-gypsum.

  According to the present invention, re-scattering of mercury from a gas-liquid contact slurry absorbing liquid is eliminated, the contact efficiency between mercury and gypsum in exhaust gas can be increased, and mercury adsorption / immobilization can be promoted. .

  Hereinafter, the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. In addition, constituent elements in the following embodiments include those that can be easily assumed by those skilled in the art or those that are substantially the same.

An exhaust gas treatment system for a coal fired boiler according to a first embodiment of the present invention will be described with reference to the drawings.
In this embodiment, a denitration device that removes nitrogen oxides in exhaust gas from a coal fired boiler, an air preheater that recovers heat in the gas after nitrogen oxide removal, and dust in the gas after heat recovery A dust collector to be removed, a gas-liquid contact type desulfurization device that removes mercury oxide while removing sulfur oxide in the gas after dust removal by the lime / gypsum method, and a chimney that discharges the gas after desulfurization to the outside In the exhaust gas treatment system to be provided, power generation is performed using a mercury (Hg) / sulfur (S) molar ratio of 1.3 × 10 ⁻⁶ or less of coal supplied as fuel.

This is usually the case when coal having a low mercury content has a “mercury (Hg) / sulfur (S) molar ratio (referred to as Hg / S molar ratio)” of 1.3 × 10 ⁻⁶ or less. This is because mercury in the exhaust gas can be removed only by controlling the ORP meter.

Next, the reason why the Hg / S molar ratio is defined as 1.3 × 10 ⁻⁶ will be described.
First, from the relationship of ΔHg / ΔCaSO ₄ = Δ mercury removal amount / Δ gypsum generation amount, ΔHg / ΔCaSO ₄ and mercury removal performance (gas mercury concentration at the gas-liquid contact tower outlet) in the gas-liquid contact tower as a desulfurization apparatus The relationship is shown in FIG.
From this graph, it is found that mercury removal performance can be maintained when ΔHg / ΔCaSO ₄ is 2 mgHg / kgCaSO ₄ or less.
Sulfur (S) contained in mercury (Hg) and gypsum (CaSO ₄ ) is attributed to the most upstream coal.

Here, ΔHg / ΔCaSO ₄ = 2 mgHg / kgCaSO ₄ is converted as follows.
Hgmol / Smol = 2 mgHg / kgCaSO ₄ × [(1 / 200.59) × 10 ⁻³ molHg / mgHg] / (1 / 136.144) × 10 ³ molS / kgCaSO ₄ ]
= 1.357 × 10 ⁻⁶ molHg / molS
The molecular weight of mercury (Hg) is 200.59, the molecular weight of sulfur (S) is 32.066, and the molecular weight of gypsum (CaSO ₄ ) is 136.144.
Therefore, ΔHg / ΔCaSO _{4 2} mgHg / kgCaSO ₄ corresponds to 1.36 × 10 ⁻⁶ molHg / molS.
As a result, the threshold value of the Hg / S molar ratio is defined as 1.3 × 10 ^−6, and if this is exceeded, mercury removal will be reduced, so it is necessary to take measures to remove mercury. .

Further, FIG. 2 and FIG. 3 show the relationship between the coal content ratio (Hg / S) and the frequency of bituminous coal and PRB coal.
FIG. 2 and FIG. 3 summarize the Hg / S ratio of 30 samples of bituminous coal and PRB coal of US coal, and the ratio of 1.36 × 10 ⁻⁶ molHg / molS or less is about 70% for bituminous coal. It was about 27% for PRB charcoal.

Therefore, when using PRB charcoal, a means for measuring the ratio of mercury (Hg) / sulfur (S) of the coal is provided. When the molar ratio is 1.36 × 10 ⁻⁶ molHg / molS, Hg) It is determined that the gypsum (CaSO ₄ ) production rate is insufficient with respect to the removal rate.
For this reason, in the case of coal having a molar ratio of 1.36 × 10 ⁻⁶ molHg / molS or more, a sulfur (S) source or the like is added to make the molar ratio 1.36 × 10 ⁻⁶ molHg / molS or less. .

A method for setting the specific molar ratio to 1.36 × 10 ⁻⁶ molHg / molS or less will be described below.
(1) High S charcoal (1.36 × 10 ⁻⁶ molHg / molS or less) is co-fired.
(2) A sulfur-based compound is co-fired. Here, examples of the sulfur-based compound include sulfur, hydrogen sulfide, metal sulfide (such as pyrite steel FeS ₂ ), sulfate (CaSO ₄ , MgSO ₄ , FeSO _4, etc.), sulfur dioxide (SO ₂ ), and the like. it can.
(3) Sulfurous acid gas or sulfuric acid gas is mixed with the exhaust gas.
Therefore, when using PRB charcoal, mercury can be efficiently removed by using a mixture of charcoal of 1.36 × 10 ⁻⁶ molHg / molS or less such as bituminous coal.

An exhaust gas treatment system for a coal fired boiler according to a second embodiment of the present invention will be described with reference to FIG.
In this example, means for measuring the mercury (Hg) / sulfur (S) molar ratio of coal is provided, and when the molar ratio is 1.36 × 10 ⁻⁶ molHg / molS or less, ORP control and slurry absorption are performed. An oxidant is added to the liquid.
FIG. 4 shows an example of the exhaust gas treatment system.
As shown in FIG. 4, the exhaust gas treatment system includes a denitration device 13 that removes nitrogen oxides by adding ammonia 12 to the exhaust gas from a coal fired boiler 11 that uses coal as fuel F, and after nitrogen oxide removal. An air preheater 14 that recovers heat in the gas, an electric dust collector 15 that removes dust in the gas after heat recovery, and sulfur oxides in the gas after dust removal by the lime / gypsum method, Limestone extracted inside or outside the desulfurization device 16 in an exhaust gas treatment system comprising a gas-liquid contact type desulfurization device 16 for removing mercury oxide and a chimney 17 for discharging the purified gas after desulfurization and mercury removal to the outside. -An oxidizing agent is added to the slurry absorbent 21 containing gypsum.
In the figure, reference numeral 18 denotes air, 19 denotes an oxidation-reduction potential measuring meter (ORP meter), 22 denotes a solid-liquid separator that separates gypsum 24, and 23 denotes supernatant water from which gypsum has been removed.

  Here, the place where the oxidizing agent is added may be any place in the desulfurizer 16 (30A), the upstream side (30B) or the downstream side (30C) of the solid-liquid separator 22.

Moreover, it is preferable that the oxidation-reduction potential of the slurry absorbent in the desulfurization apparatus is set to 150 mV or more by supplying the oxidizing agent.
This is because the redox potential is 150 mV or higher, preferably 175 mV or higher, as shown in the graph of the relationship between “mercury re-scattering rate (%) and ORP redox potential (mV)” shown in FIG. This is because when the voltage is 200 mV or more, the re-scattering rate of mercury can be significantly reduced.
Here, the mercury re-scattering rate (%) is obtained by the following equation.
Mercury re-scattering rate (%) = (Hg ⁰ outlet−Hg ⁰ inlet) / (Hg ²⁺ inlet) × 100

The oxidizing agent is preferably an oxidizing agent having a stronger oxidizing power than oxygen (air) used for general ORP control. For example, ozone (O ₃ ), hydrogen peroxide (H ₂ O ₂ ), permanganic acid Examples include potassium (KMnO ₄ ) and chlorine-based compounds (for example, sodium hypochlorite (NaClO)), but the present invention is not limited thereto.

Further, a manganese compound (KMnO ₄ , MnCl ₂ ) may be added as a catalyst for promoting redox.

As described above, according to this example, the potential of the ORP meter is set to 150 mV or more, so that reduction of mercury oxide (Hg ²⁺ ) (Hg ²⁺ → Hg ⁰ ) is prevented and zero valence to the gas phase is achieved. Of mercury (Hg ⁰ ) can be suppressed, and the removal rate of mercury in exhaust gas can be increased.
Thereby, it is possible to prevent mercury from the slurry absorbing liquid from being released again into the gas.

  In addition, the oxidation state can be maintained by adding an oxidizing agent and, if necessary, a manganese compound, and an effect of maintaining the above-described ORP meter potential at a predetermined level or more can be expected.

An exhaust gas treatment system for a coal fired boiler according to a third embodiment of the present invention will be described with reference to FIG.
FIG. 6 shows an example of the exhaust gas treatment system.
As shown in FIG. 6, a sulfuric acid compound such as a sulfuric acid compound (SO ₄ ²⁻ ) or a sulfite compound (SO ₃ ²⁻ ) is added to the absorption liquid (limestone-gypsum slurry) 21 in the desulfurization device 16 of the gas-liquid contact tower. It may be added to promote the formation of gypsum in the slurry and increase the gypsum concentration.
Reaction formulas for producing gypsum to improve the gypsum concentration are shown in the following (3) to (5).
SO ₃ ^2- + Ca ²⁺ → CaSO ₃ (3)
CaSO ₃ + 1 / 2O ₂ → CaSO ₄ (gypsum formation by oxidation) (4)
SO ₄ ^2- + Ca ²⁺ → CaSO ₄ (gypsum formation) (5)

The sulfuric acid compound may be added at any location in the desulfurization device 16 (40A), on the upstream side (40B) or on the downstream side (40C) of the absorbent gypsum separation device 31. In FIG. 6, reference numeral 32 is gypsum, and 33 is supernatant water.
Thus, according to the present invention, it is possible to promote mercury adsorption and immobilization by increasing the contact probability between mercury and gypsum.
Therefore, according to the present embodiment, by separately adding a sulfate compound, the amount of gypsum produced in the slurry can be increased, and the adsorption and immobilization of mercury can be actively promoted.

  Furthermore, the above-described Embodiments 1 to 3 may be appropriately combined, and the combination may further promote the adsorption / fixation of mercury.

  As described above, according to the exhaust gas treatment system and method according to the present invention, mercury re-scattering is reduced, so that the mercury removal efficiency can be improved and the amount of mercury discharged in the exhaust gas is regulated. Suitable for exhaust gas treatment.

FIG. 4 is a relationship diagram of gypsum concentration in slurry and mercury adsorption amount. It is an aggregation figure of Hg / S ratio of 30 samples of bituminous coal of US coal. It is an aggregation figure of Hg / S ratio of 30 specimens of PRB coal of US coal. 6 is a schematic diagram of an exhaust gas treatment system according to Embodiment 2. FIG. It is a graph which shows the relationship between mercury re-scattering rate (%) and ORP oxidation-reduction potential (mV). 6 is a schematic diagram of an exhaust gas treatment system according to Embodiment 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Exhaust gas treatment system 11 Coal fired boiler 12 Ammonia 13 Denitration device 14 Air preheater 15 Dust collector 16 Desulfurization device 17 Chimney 21 Slurry containing limestone-gypsum 22 Solid-liquid separator 23 Supernatant water 24 Gypsum

Claims (6)

A denitration device for removing nitrogen oxides in exhaust gas from a coal fired boiler;
An air preheater for recovering heat in the gas after removal of nitrogen oxides;
A dust collector to remove the dust in the gas after heat recovery;
A gas-liquid contact type desulfurization device that removes sulfur oxide in the gas after dust removal by the lime / gypsum method and removes mercury oxide,
In an exhaust gas treatment system comprising a chimney that exhausts gas after desulfurization to the outside,
An exhaust gas treatment system for a coal fired boiler, wherein the molar ratio of mercury (Hg) / sulfur (S) of coal is 1.3 × 10 ⁻⁶ or less. In claim 1,
When the molar ratio of mercury (Hg) / sulfur (S) in coal is 1.3 × 10 ⁻⁶ or more, co-firing coal with a high sulfur content with a molar ratio of 1.3 × 10 ⁻⁶ or less An exhaust gas treatment system for coal fired boilers. In claim 1,
An exhaust gas treatment system for a coal fired boiler, wherein a sulfur compound is co-fired when the molar ratio of mercury (Hg) / sulfur (S) in coal is 1.3 × 10 ⁻⁶ or more. In claim 1,
When the molar ratio of mercury (Hg) / sulfur (S) in coal is 1.3 × 10 ⁻⁶ or more, the exhaust gas treatment of a coal fired boiler is characterized by desulfurization while mixing sulfuric acid gas with the exhaust gas system. In any one of Claims 1 thru | or 4,
An exhaust gas treatment system for a coal fired boiler characterized by adding an oxidizing agent to a slurry containing limestone-gypsum. In any one of Claims 1 thru | or 5,
An exhaust gas treatment system for a coal fired boiler, characterized in that a sulfuric acid compound is added to a slurry containing limestone-gypsum.

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