JP2000317258A - Flue gas desulfurization equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the product cost of ammonium sulfate by discharging a part of separated liq., lowering its quantity and also improving a recovering efficiency of the ammonium sulfate moreover to improve a product value of the ammonium sulfate by preventing the inclusion of soot and dust and heavy metal to the ammonium sulfate to be recovered. SOLUTION: In this desulfurization equipment, a filter 39 for separating solid matter, a crystallization can 25 for precipitating the ammonium sulfate, a separator 27 for separating the separated ammonium sulfate and a return line 29 for returning the separated liq. separated at the separator to the crystallization can 25 are provided from upstream side at an ammonium sulfate recovering line 38 for discharging an ammonium sulfate soln. 24. Then, the flue gas is discharged to the outside of system so that the conc. of ammonium chloride in the separated liq. is not more than combined solubility by supplying a pH adjusting soln. to the upstream side of the filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a flue gas desulfurization apparatus, and more particularly to a wet ammonia desulfurization apparatus for absorbing sulfur dioxide in flue gas with ammonia and recovering ammonium sulfate (ammonium sulfate) as a by-product.

[0002]

2. Description of the Related Art In a coal-fired boiler, nitrogen and sulfur in fuel are converted into nitrogen oxides and sulfur oxides by combustion and are contained in exhaust gas. These nitrogen oxides NOx and sulfur oxide S
Ox (mostly SO2) causes air pollution,
A flue gas treatment system of a coal-fired boiler plant is provided with a denitration device for removing nitrogen oxides and a desulfurization device for removing sulfur oxide.

Referring to FIG. 2, an outline of a flue gas treatment system of a coal-fired boiler plant will be described.

Nitrogen oxides are removed from the exhaust gas from the coal-fired boiler 1 by a denitration device 2. The exhaust gas that has passed through the denitration device 2 exchanges heat with combustion air in an air preheater 3 to heat the combustion air, and further passes through an electric dust collector 5 through a gas gas heater (heat recovery unit) 4. In the process of passing through the electric precipitator 5, solids such as dust are removed. Exhaust gas discharged from the electrostatic precipitator 5 is sent to the desulfurizer 7 by the induction ventilator 6. The exhaust gas from which sulfur oxide has been removed by the desulfurization device 7 is heated by a gas gas heater (reheater) 8. Exhaust gas from the gas gas heater 8 is pressurized by a ventilator 9,
It is discharged into the atmosphere from zero. In the figure, reference numeral 11 denotes a forced draft fan for blowing combustion air. Note that, depending on the scale of the coal-fired boiler plant, the gas gas heater 4 and the gas gas heater 8 are omitted, and the exhaust gas from the desulfurization device 7 is directly discharged from the chimney 10.

When ammonia is used as the sulfur oxide absorbent in the desulfurizer 7, ammonium sulfate (ammonium sulfate) is produced as a by-product of the reaction.

Referring to FIG. 3, the above-mentioned conventional desulfurizer 7 will be described.

Exhaust gas from the electric precipitator 5 is introduced into the absorption tower 12, and an absorption liquid 15 is provided below the absorption tower 12.
Is stored. The absorbing liquid 15 is sucked up by the absorbing liquid circulating pump 16 and dispersed in the absorption tower 12. An ammonia water supply line 17 joins the circulation of the absorption liquid by the absorption liquid circulation pump 16, and the ammonia concentration of the absorption liquid 15 from the ammonia water supply source 18 through the ammonia water supply line 17 becomes a predetermined value. It is replenished appropriately so as to maintain the range.

When the ammonia water is sprayed in the absorption tower 12, the SO2 gas in the exhaust gas and the ammonia water come into gas-liquid contact, and [2NH4OH + SO2 → (NH4) 2SO4]
[3 + H2 O], the SO2 gas is absorbed from the exhaust gas. (NH4) 2 produced by the desulfurization reaction
When the concentration of SO3 (ammonium sulfite) in the absorbing solution 15 becomes a predetermined value or more, a part of the circulating absorbing solution is supplied to the oxidation tower 20 and stored in the lower part of the oxidation tower 20. Oxidation air 21 is blown into the stored ammonium sulfite concentrate, and [(NH4) 2 SO3 +1/2
O 2 → (NH 4) 2 SO 4], (NH 4
) 2 SO4 (ammonium sulfate) is produced. Excess air after the reaction in the oxidation tower 20 is returned to the absorption tower 12 and discharged from the chimney 10 together with exhaust gas.

An ammonium sulfate solution 24 in which ammonium sulfate is dissolved is extracted from the oxidation tower 20 by a delivery pump 22 and
Sent to The water in the ammonium sulfate solution 24 evaporates in the crystal can 25 using the steam 26 as a heat source. The concentration of ammonium sulfate in the liquid is increased by evaporation, and ammonium sulfate precipitates. The ammonium sulfate slurry extracted from the crystal can 25 is sent to a separator 27, where solid ammonium sulfate 28 is separated. The separated liquid is returned to the crystal can 25 by a return line 29.

[0010]

The amount of hydrogen chloride (HCl) contained in coal, which is the fuel of the coal-fired boiler 1, is small but small.
And ammonium chloride (ammonium chloride) (NH4 Cl) is generated when sulfur dioxide is absorbed by aqueous ammonia. By precipitating ammonium sulfate from the ammonium sulfate solution 24, the concentration of ammonium salt increases. If ammonium salt precipitates and mixes with ammonium sulfate, the purity as ammonium sulfate decreases, and the commercial value of ammonium sulfate decreases. Further, as described above, the impurities contained in the liquid separated by the separator 27 are concentrated, which adversely affects the crystallization of ammonium sulfate,
There was a problem that good quality ammonium sulfate could not be recovered.

In general, in a wet-type ammonia desulfurization apparatus, dust is small in the target exhaust gas, but may be large depending on the equipment.
In the process of absorption of ammonium sulfate. If the trapped dust is not efficiently removed in advance, it will be mixed into the solid ammonium sulfate 28 and cause a decrease in the purity of ammonium sulfate.

Further, although a trace amount of heavy metal is contained in the fuel, in the above-mentioned conventional example, it was concentrated in the system and mixed with the solid ammonium sulfate 28. Furthermore, maintenance work such as cleaning has been required on a regular basis in order to remove what remains in the system due to scaling or the like.

In view of such circumstances, the present invention discharges a part of the separated liquid to the outside of the system, reduces the amount thereof, and at the same time improves the recovery efficiency of ammonium sulfate, thereby reducing the product cost of ammonium sulfate and further recovering it. The purpose of the present invention is to improve the product value of ammonium sulfate by preventing dust and heavy metals from being mixed into ammonium sulfate.

[0014]

According to the present invention, a filter for separating solids from an upstream side, a crystal can for depositing ammonium sulfate, and a separator for separating precipitated ammonium sulfate are provided in an ammonium sulfate recovery line for extracting a solution of ammonium sulfate. A return line for returning the separated solution separated by the separator to the crystallizer, supplying a pH adjusting solution upstream of the filter, and the concentration of salt and salt in the separated solution being less than the composite solubility. The ammonium sulfate recovery line relates to a flue gas desulfurization unit that extracts ammonium sulfate solution from an oxidation absorption tower where desulfurization and oxidation reactions take place. Pertains to a flue gas desulfurization unit equipped with a stirrer that stirs the absorption liquid stored in the tower.
The present invention relates to a flue gas desulfurization device which is supplied to an ammonium sulfate solution so as to have a pH value at which heavy metals are precipitated, wherein the adjustment liquid is ammonia water.

Since the ammonium salt dissolved in the separated solution is discharged at a concentration before the precipitation, the recovered ammonium sulfate is not mixed with the salt ammonium, the amount of the salt can be reduced to a minimum, and the heavy metal can be further removed. Therefore, the separated solution can be used as a high-purity liquid fertilizer, and the absorbing solution stored in the oxidation absorption tower is stirred by a stirrer, so that the pH of the absorbing solution can be stabilized, and the absorbing solution can be used. This makes it easier to control the supply of sulfur, improves the desulfurization performance, and prevents dust in the exhaust gas from settling in the oxidation absorption tower.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, the same components as those shown in FIG. 3 are denoted by the same reference numerals. The oxidation absorption tower 31 shown in FIG. 1 is an integrated absorption tower and chimney.

The oxidation absorption tower 31 has an exhaust gas introduction line 3
2, supply water supply line 33, oxidizing air supply line 34,
An ammonia water supply line 35 communicates with the exhaust gas introduction line 32, and a ventilator 36 is provided. Further, a stirrer 41 for stirring the absorbing solution 15 is provided in the oxidation absorption tower 31.
Is provided and an absorption liquid circulation line 37 including the absorption liquid circulation pump 16 is provided.
An ammonium sulfate recovery line 38 is provided branching from 7.

A filter 39, a crystal can 25, and a separator 27 are provided in this order from the upstream side of the ammonium sulfate recovery line 38, and the ammonia water branched from the ammonia water supply line 35 is provided upstream of the ammonia water supply line 35. The supply branch line 40 is connected.

Although not shown in FIG.
A detector is provided, and the ammonia water supply line 35,
Each of the ammonia water supply branch lines 40 is provided with a flow control valve (not shown).

The operation will be described below.

Exhaust gas is led from the exhaust gas introduction line 32 into the oxidation absorption tower 31, and comes into gas-liquid contact with the absorption liquid 15 circulated through the absorption liquid circulation line 37 in the oxidation absorption tower 31. The ammonia water in the absorbing solution 15 and the sulfur dioxide in the exhaust gas undergo a desulfurization reaction, and the sulfur dioxide is absorbed into the absorbing solution 15 from the exhaust gas.

The absorption liquid 15 is stored at the bottom of the oxidation absorption tower 31 and further circulated through the absorption liquid circulation line 37. Water is extracted by the ammonium sulfate recovery line 38 and supplied by the makeup water supply line 33 so that the absorption liquid 15 in the oxidation absorption tower 31 is maintained at a required level.
Further, the pH of the absorbing solution 15 is detected by a pH detector, and the amount of ammonia water supplied from the absorbing solution circulating line 37 is controlled so that the detected pH becomes a predetermined value.

Oxidizing air is blown into the absorbing liquid 15 from the oxidizing air supply line 34, and the absorbing liquid 15 is stirred by the stirrer 41. Oxidizing air is the absorption liquid 1
As a result, the ammonium sulfite generated in the desulfurization reaction is further oxidized to produce ammonium sulfate. By blowing the oxidizing air into the absorbing solution 15, sulfuric acid may be locally generated depending on conditions. However, the stored absorbing solution 15 is stirred by the stirrer 41, so that the oxidation is made uniform. In addition, the production of sulfuric acid is suppressed. Therefore, p
H is stabilized and the pH is controlled, that is, the ammonia water supply line 3
The control of the supply amount of ammonia water from Step 5 becomes easy. The pH of the stored absorbent 15 is maintained at 5 to 6.

By providing the stirrer 41 as described above, a fluctuation in pH can be suppressed, and a decrease and instability of desulfurization performance can be avoided.

The dust contained in the exhaust gas is
5 is contained in the stored absorption liquid 15 and is contained in the stored absorption liquid 15, but the absorption liquid 15 is stirred by the stirrer 41, thereby preventing precipitation at the bottom of the oxidation absorption tower 31.

A part (ammonium sulfate solution 24) of the absorbent 15 circulated from the absorbent circulation line 37 is withdrawn through the ammonium sulfate recovery line 38. Ammonia water is supplied to the ammonium sulfate recovery line 38 from the ammonia water supply branch line 40, and the pH of the extracted absorbent 15 is adjusted. The pH to be adjusted is controlled so that the pH becomes 7 or more, for example, pH 7 to 8 upstream of the filter 39. When the pH rises, heavy metals are precipitated.

The ammonium sulfate solution 24 whose pH has been adjusted passes through the filter 39 to remove dust and the precipitated heavy metal. The ammonium sulfate solution 24 from which dust and heavy metals have been removed is stored in a crystal can 25 and is heated and evaporated by a heat source of steam 26, so that the ammonium sulfate concentration of the ammonium sulfate solution 24 increases and ammonium sulfate precipitates. The precipitated ammonium sulfate 28 is separated by the separator 27 and taken out of the system. The separated liquid from which ammonium sulfate has been separated is returned to the crystal can 25 via a return line 29.

As described above, the ammonium sulfate solution 24 contains a small amount of ammonium salt formed during the desulfurization process. The concentration of salt and salt is gradually concentrated in the process of heating and evaporating in the crystal can 25, separating ammonium sulfate in the separator 27, and returning to the crystal can 25 via the return line 29.

Here, the solubility of ammonium sulfate and the salt solution is shown as follows.

Temperature 0 ° C. 20 ° C. 60 ° C. 100 ° C. (NH 4) 2 SO 4 41.35% 42.85% 46.6% 50.4% NH 4 Cl 22.7% 27.1% 35.6% 43.6%

The solubility of ammonium sulfate and ammonium salt is 46.6% at 60 ° C.
And 35.6%, but the composite solubility in a state where both are dissolved at the same time is not the sum of the solubility of both. Therefore, it is necessary to operate in a region where the salt concentration is as high as possible, based on the range of ammonium sulfate concentration at which ammonium sulfate precipitates even when salt salt is present but the salt salt concentration does not precipitate. The amount of extraction will be minimized.

For example, the complex solubility of salt and salt at 60 ° C. is 15
%, And assuming that the solubility of ammonium salt in the ammonium sulfate solution 24 extracted from the oxidation absorption tower 31 is 0.5%, the separated solution until the combined solubility of ammonium salt reaches 15%. Is concentrated in the return line 29.

Thus, a part of the separated liquid separated by the separator 27 is discharged out of the system at a value not exceeding the complex solubility of salt and salt. At this time, the solubility of salt and salt is, for example, 5% or more and 10% or less in the example described above. Therefore, in the present embodiment, when the concentration of salt ammonium in the discharged separated liquid is 10%, the discharged amount of the separated liquid is 1/20 of the absorption liquid 15 extracted from the oxidation absorption tower 31, which is an extremely small amount. Further, assuming that the amount of exhaust gas to be treated by the desulfurization unit 7 is 50,000 cubic meters / h and the sulfur dioxide contained is 2,000 ppm, it is reduced to approximately 30 l / h, though it depends on the content of hydrogen chloride contained. I do. In other flue gas desulfurization treatment methods, for example, the magnesium hydroxide method, the amount of the discharged liquid is 5000 l / h discharged under the same conditions, and it can be seen that the amount of the discharged liquid in the present embodiment is extremely small.

Further, in this embodiment, since the heavy metal of the absorbing solution 15 is removed by the filter 39 in the ammonium sulfate recovery line 38, the return line 29 hardly contains heavy metal, so that a higher concentration of ammonium sulfate can be obtained. , Contains salt and salt. Salt ammonium is also a fertilizer like ammonium sulfate, and the separated liquid itself discharged is a liquid fertilizer. Therefore, if the separated liquid is used as fertilizer for afforestation, the present flue gas desulfurization apparatus can completely eliminate the discharged liquid.

Although the above embodiment has been described with reference to an absorption tower integrated with a chimney, the present invention can be applied to a flue gas desulfurization apparatus provided with a separate chimney. Although the ammonium sulfate recovery line 38 is branched from the absorption liquid circulation line 37, it may be provided so that the absorption liquid 15 is directly drawn from the oxidation absorption tower 31. In addition, although the ammonia water is supplied to the upstream of the filter 39 by the ammonia water supply branch line 40 for pH adjustment, the ammonia water may be supplied independently of the ammonia water supply line 35. .

[0037]

As described above, according to the present invention, since the ammonium chloride dissolved in the separation liquid is discharged at the concentration before the precipitation, the ammonium chloride is not mixed into the recovered ammonium sulfate, and the ammonium sulfate is minimized. Emissions and the removal of heavy metals,
The separated liquid can be used as a liquid fertilizer, and the absorbing liquid stored in the oxidation absorption tower is stirred by a stirrer, so that the pH of the absorbing liquid can be stabilized, and the supply control of the absorbing liquid is facilitated. The desulfurization performance can be improved, and the dust in the exhaust gas captured by the absorbing solution can be prevented from settling in the oxidation absorption tower.The dust can be removed by the filter. And excellent effects such as high quality ammonium sulfate can be obtained because no salt ammonium is mixed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a flue gas treatment system of a coal-fired boiler plant including a flue gas desulfurization device.

FIG. 3 is a schematic diagram showing a configuration of a conventional flue gas desulfurization device.

[Explanation of symbols]

 7 Desulfurizer 24 Ammonium sulfate solution 25 Crystal can 26 Superheated steam 27 Separator 29 Return line 35 Ammonia water supply line 38 Ammonium sulfate recovery line 39 Filter 40 Ammonia water supply branch line 41 Stirrer

Claims (4)

[Claims] 1. An ammonium sulfate recovery line for extracting a solution of ammonium sulfate,
A filter for separating solids from the upstream side, a crystal can for depositing ammonium sulfate, a separator for separating the precipitated ammonium sulfate, a return line for returning the separated liquid separated by the separator to the crystal can, and Supply the pH adjustment liquid upstream of
As the concentration of salt ammonium in the separated solution is less than the complex solubility,
A flue gas desulfurization device characterized in that it is configured to discharge to the outside of the system. 2. The flue gas desulfurization apparatus according to claim 1, wherein the ammonium sulfate recovery line draws the ammonium sulfate solution from an oxidation absorption tower where desulfurization and oxidation reactions are performed. 3. The flue gas desulfurization apparatus according to claim 2, further comprising a stirrer for stirring the absorption liquid stored in the oxidation absorption tower. 4. The flue gas desulfurization apparatus according to claim 1, wherein the pH adjusting solution is aqueous ammonia and supplied to the ammonium sulfate solution so as to have a pH value at which heavy metals are precipitated.