FLUE GAS TREATMENT SYSTEM AND METHOD

A flue gas treatment system in which the desulfurization of an flue gas is attained by: catalytically converting SO2 contained in an flue gas into SO3; subjecting the SO3-containing flue gas to be heat exchanged with air in a heat exchanger to lower the temperature of the flue gas to a sulfuric acid dew point or lower; making dust adhere to the surfaces of sulfuric acid droplets contained in the flue gas; and collecting the sulfuric acid with dust together with dust by a dust collector.

TECHNICAL FIELD

The present invention relates to a flue gas treatment system and a flue gas treatment method.

BACKGROUND ART

Nitrogen oxides (NOx), sulfur oxides (SOx), dust, or the like, which are generated due to the combustion of fossil fuels in thermoelectric power plants or the like, are contained in flue gas (exhaust gas). Since NOx, SOx, dust, or the like cause atmospheric pollution, thermoelectric power plants or the like having a coal-fired boiler are provided with a flue gas treatment system having a denitrification device for removing NOx and a flue gas desulfurization device for removing SOx.

Here, in the flue gas desulfurization device, a wet type limestone-plaster method is mainly used. In particular, a spray type method in which a limestone slurry is sprayed in a spray form from a nozzle and is brought into gas-liquid contact with the flue gas has high reliability and is primarily adopted as a method of spraying the limestone slurry into a tower (PTL 1).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, although the wet type desulfurization device has excellent desulfurization performance, there is a problem in that the amount of water consumption increases.

Although dry type desulfurization devices other than the wet type methods are proposed for removing SO2, for example using activated coke, there is a problem in that performance is insufficient or energy loss and utilities are great.

In recent years, since environmental regulations around the world have become more rigorous, installation of desulfurization devices is increasingly needed even in areas where there is an insufficient amount of water and where the desulfurization devices are not installed in the related art.

Thus, emergence of a flue gas treatment system that can perform desulfurization processing of a flue gas without using water such as the wet type desulfurization device is desired.

An object of the invention in view of the problems is to provide a flue gas treatment system and a flue gas treatment method that can perform desulfurization processing in a flue gas without using water such as a wet type desulfurization device.

Solution to Problem

In order to solve the above problems, a first invention of the invention is a flue gas treatment system including an SO2.SO2conversion catalyst unit that is provided in a flue through which a flue gas from a boiler is discharged and that has a conversion catalyst which converts SO2in the flue gas into SO2; a heat exchanger that is provided in the flue on a downstream of the SO2.SO2conversion catalyst unit, that conducts heat exchange between the flue gas and air to lower the temperature of the flue gas to a sulfuric acid dew point or lower, and that condenses SO3contained in the flue gas, the temperature of which has been lowered to the sulfuric acid dew point or lower, as sulfuric acid droplets mixed with dust; and a dust collector that collects the sulfuric acid droplets mixed with the dust contained in the flue gas.

A second invention is the flue gas treatment system according to the first invention in which an air preheater is interposed on an upstream side of the SO2.SO3conversion catalyst unit.

A third invention is the flue gas treatment system according to the first or second invention further including an air supply line that supplies preheated air, with which heat exchange is conducted in the heat exchanger, or boiler circulated water to the boiler.

A fourth invention is the flue gas treatment system according to the first or second invention further including powder supply means for supplying powder, between the SO2.SO3conversion catalyst unit and the heat exchanger.

A fifth invention is the flue gas treatment system according to the fourth invention in which the powder is limestone or collected dust powder that is collected in the dust collector.

A sixth invention is a flue gas treatment method including an SO2.SO2conversion catalyst process that is provided in a flue through which a flue gas from a boiler is discharged and that has a conversion catalyst which converts SO2in the flue gas into SO2; a heat exchange process that is provided in the flue on a downstream of the SO2.SO2conversion catalyst process, that conducts heat exchange between the flue gas and air to lower the temperature of the flue gas to a sulfuric acid dew point or lower; and a dust collection process that collects sulfuric acid droplets with dust formed in the heat exchanger in making sulfuric acid droplets contained in the flue gas, the temperature of which has been lowered to the sulfuric acid dew point or lower, mix with dust contained in the flue gas.

A seventh invention is the flue gas treatment method according to the sixth invention further including an air preheating process on an upstream side of the SO2.SO2conversion-catalyst process.

An eighth invention is the flue gas treatment method according to the sixth or seventh invention in which powder is provided between the SO2.SO2conversion catalyst process and the heat exchange process.

Advantageous Effects of Invention

According to the invention, the desulfurization of the flue gas can be attained by catalytically converting SO2contained in the flue gas into SO2; subjecting the converted SO2-containing flue gas to be heat exchanged with air in the heat exchanger to lower the temperature of the flue gas to the sulfuric acid dew point or lower; making the dust adhere to the surfaces of the sulfuric acid droplets contained in the flue gas; and collecting the sulfuric acid with dust together with the dust by the dust collector.

DESCRIPTION OF EMBODIMENTS

Preferred examples of the invention will be described below in detail with reference to the accompanying drawings. In addition, the invention is not limited by the examples and includes those configured by combining respective examples when there are a plurality of examples.

FIG. 1is a schematic view of a flue gas treatment system according to Example 1. As illustrated inFIG. 1, a flue gas treatment system10A according to the present example has a denitrification device14that is provided in a flue13through which a flue gas12is discharged from a boiler11and that removes nitrogen oxides from the flue gas12; an SO2.SO2conversion catalyst unit15that is provided in the flue on the downstream of the denitrification device14and that has a conversion catalyst which converts SO2(sulfur dioxide) in the denitrified flue gas12A into SO2(sulfur trioxide); a heat exchanger16that is provided in the flue13on the downstream of the SO2.SO2conversion catalyst unit15and that conducts heat exchange between the flue gas12B, which has passed through the catalyst, and air20to lower the temperature of the flue gas12B to a sulfuric acid dew point or lower; and a dust collector17that collects not only sulfuric acid with dust32formed in making dust31adhere to the surfaces of sulfuric acid droplets30contained in the flue gas, the temperature of which has been lowered to a sulfuric acid dew point or lower in the heat exchanger16, but also the dust31contained in the flue gas12C. In this drawing, F1represents a forced draft fan, and F2represents an induced draft fan.

The denitrified and discharged flue gas12A from the denitrification device14has an flue gas temperature, for example, around 300° C. to 400° C., and is introduced into the SO2.SO2conversion catalyst unit15at this temperature. SO2is oxidized and turned into SO2by the conversion catalyst that converts SO2provided in the SO2.SO2conversion catalyst unit into SO3.

This SO2is still in a gaseous state.

Next, the flue gas12B containing the converted SO2is introduced into the heat exchanger16, and conducts heat exchange with the air20in the heat exchanger, and thereby the temperature of the flue gas12B is dropped down to the sulfuric acid dew point (for example, 100° C.) or lower.

Here, a heat exchanger of a heat exchange type that makes gas and gas come into contact with each other to conduct direct heat exchange, a heat exchanger of an indirect heat exchange type that conducts indirect heat exchange between gas to be cooled and gas to be warmed, using a heat medium, or the like can be used as the heat exchanger16.

As a result of this temperature drop, SO3in the gaseous state is turned into sulfuric acid (H2SO4) droplets containing moisture in the flue gas.

Here, the sulfuric acid droplets contained in the flue gas12B are mixed with a large amount of the dust contained in the flue gas12B, and are turned into the sulfuric acid with dust.

In this way, according to the invention, in the heat exchanger16, the temperature of the flue gas is lowered to the sulfuric acid dew point or lower, and SO3contained in the flue gas, which has been lowered to the sulfuric acid dew point or lower, is condensed as the sulfuric acid droplets mixed with the dust.

As for the sulfuric acid with dust, the surfaces of the sulfuric acid droplets are surrounded by innumerable dust, and the flue gas12C containing the sulfuric acid with dust is introduced to the dust collector17. Then, in the dust collector17, the sulfuric acid is collected together with the dust, desulfurization in the flue gas is completed, and the cleaned flue gas12D is obtained.

Thereafter, the cleaned flue gas12D is sent to a chimney18by an induced draft fan F2.

In the present example, preheated air20A is sent to the boiler11side by an air supply line21, using the air20as the heat exchange medium of the heat exchanger16. However, water may be used as the heat medium so as to be used for preheating of boiler circulated water.

Additionally, when heat exchange is conducted between the air20and the flue gas12B, water may be used as the heat medium.

As a result, the energy efficiency of entire power plants can be improved.

According to the invention, since the air at an atmospheric temperature is used as the heat exchange medium, it is unnecessary to use a large amount of water like the related-art wet-type desulfurization devices.

Moreover, since a mechanism that removes a sulfur component contained in the gas is based on condensation, moisture serving as a counteraction medium becomes unnecessary.

Additionally, recovered heat can be used for preheating of boiler combustion air, and heat exchange efficiency is improved by conducting heat exchange down to 100° C. or lower compared to the related art. That is, in the related art, sulfurous acid or sulfuric acid corrosion occurs during heat exchange in an air preheater if the temperature of heat is lowered to 100° C. or equal to or lower than the sulfuric acid dew point. Thus, the heat exchange is conducted at about 150° C.

In the invention, when air is preheated, the temperature of the air can be lowered down to 100° C. or equal to or lower than the sulfuric acid dew point. Thus, energy exchange efficiency is improved by that equivalent to 50° C. compared to the related art.

As a result, it is possible to raise the energy efficiency of the power plants by 0.5% or higher.

Additionally, the power of the induced draft fan F2that induces the flue gas can be reduced by cooling the flue gas12down to 100° C. or lower.

Additionally, in the invention, the amount of the SO2gas, which passes through the heat exchanger and is emitted into the atmospheric air, can be suppressed to be low by using the conversion catalyst that converts SO2into SO3.

Next, a flue gas treatment system according to Example 2 of the invention will be described. In addition, the same constituent members as those of Example 1 will be designated by the same reference numerals, and duplicate description will be omitted.

FIG. 2is a schematic view of the flue gas treatment system according to Example 2 of the invention.

As illustrated inFIG. 2, a flue gas treatment system10B of the present example is provided with powder supply means42that introduces limestone41, which is in a powder form, into the flue13between the SO2.SO3conversion catalyst unit15and the heat exchanger16.

When coal is used as fuel F for the boiler11, there are no problems because the amount of the dust contained in the flue gas12is huge. However, for example, when heavy oil, coal with little ash, or the like is used as the fuel F, the amount of the dust contained in the flue gas12may be small depending on the type of the heavy oil.

In this case, covering of the sulfuric acid droplets30with the dust31in the heat exchanger16may not be precise.

For this reason, the sulfuric acid droplets30are sent to and collected by the dust collector17in a dry state by introducing the limestone41and covering the sulfuric acid droplets30with the dust31and the limestone41.

Here, since the amount of the dust sufficient to mix the sulfuric acid with is determined, the amount of the dust is measured in advance when heavy oil or coal with little ash is used as the fuel F, and the required amount of powder, for example, the limestone41is introduced when the amount of the dust is insufficient.

Additionally, collected dust powder17a,such as limestone, which becomes a surplus and is recovered by the dust collector17, may be partially recycled via a recycling line17band may be supplied as powder to an upstream side of the heat exchanger16.

Next, a flue gas treatment system according to Example 3 of the invention will be described. In addition, the same constituent members as those of Example 1 will be designated by the same reference numerals, and duplicate description will be omitted.

FIG. 3is a schematic view of the flue gas treatment system according to Example 3 of the invention.

As illustrated inFIG. 3, in a flue gas treatment system10C of the present example, an air preheater19is further interposed between the denitrification device14and the SO2.SO2conversion catalyst unit15in Example 1.

Heat exchange is conducted between a high-temperature flue gas12A1and the air20using the air preheater19, and the preheated air20A after the heat exchange is introduced into the boiler11.

In the present example, since the high-temperature flue gas12A1is turned into a low-temperature flue gas12A2of 150° C. by the air preheater19, the temperature of the flue gas introduced into the SO2.SO2conversion catalyst unit15becomes lower than that in Example 1. Hence, in the present example, a catalyst with high catalytic activity at a low temperature (150° C.) is used as the SO2.SO2conversion catalyst to be used in the SO2.SO2conversion catalyst unit15.

Additionally, since the flue gas12B of the SO2.SO2conversion catalyst unit15also has a low temperature, the amount of the air20preheated in the heat exchanger becomes small. However, the preheated air20A is introduced into the boiler11after the air supply line21is passed into the air preheater19and preheated in the air preheater before the preheated air is introduced into the boiler11.

Additionally, in this example, in a flue gas treatment system equipped with a related-art wet-type desulfurization device in which the air preheater19is installed, the SO2.SO3conversion catalyst unit15, the heat exchanger16, and the dust collector17are used for a line after the air preheater19, so that type change can be made from the wet type to a desulfurization type using a heat exchanger in which water is unnecessary. Even in plants in which the wet-type desulfurization device is not installed, the desulfurization type using a heat exchanger can be additionally provided by being bypassed from a duct on the downstream of an existing air preheater.

Hence, even in existing flue gas treatment equipment, type change can be made from the wet type to the desulfurization type using a heat exchanger according to the invention in which water is unnecessary, and the heat of the flue gas can be effectively used. Thus, it is possible to raise the energy efficiency of the power plant by 0.5% or higher.

REFERENCE SIGNS LIST

10A to10C: FLUE GAS TREATMENT SYSTEM

32: SULFURIC ACID WITH DUST