JP2010201412A - Method for removing so3 in exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for removing SO<SB>3</SB>in an exhaust gas wherein a suitable amount of NH<SB>3</SB>is introduced and a dust adhered to a discharge electrode can be fallen down even at a discharge-gas temperature at which NH<SB>4</SB>HSO<SB>4</SB>is formed. <P>SOLUTION: There is provided a method in which NH<SB>3</SB>is injected into the exhaust gas containing SO<SB>3</SB>and air is supplied to an electric dust collector 24 to remove SO<SB>3</SB>in the exhaust gas, wherein the adding amount of NH<SB>3</SB>for SO<SB>3</SB>is controlled to remove SO<SB>3</SB>in accordance with the exhaust-gas temperature where NH<SB>4</SB>HSO<SB>4</SB>is generated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates in particular SO ₃ removal method in the exhaust gas to remove SO ₃ by injecting ammonia (NH ₃₎ in exhaust gas containing SO 3 generated by combustion, such as a boiler _(sulfur trioxide).

As a method for treating exhaust gas generated in a boiler, a combustion furnace, etc., there is a method using an electric dust collector. Such exhaust gas contains sulfur trioxide (SO ₃ ). In particular, heavy oil combustion exhaust gas containing a high concentration of sulfur components also has a high SO ₃ content in the exhaust gas. SO ₃ has a problem of being corroded when it is misted at a predetermined temperature and adheres to various devices and pipes of the exhaust gas treatment apparatus.

As a conventional method for removing SO ₃ from exhaust gas, there is a method of injecting ammonia (NH ₃ ) into the exhaust gas and collecting the SO ₃ component as ammonium sulfate ((NH ₄ ) ₂ SO ₄ ) with a subsequent electric dust collector. Are known.

Formula 1 shows the production reaction of ammonium sulfate by the reaction of ammonia and SO ₃ .

As shown in Chemical Formula 1, in order to produce ammonium sulfate, 2 mol of ammonia is required for 1 mol of SO _{3 in} the exhaust gas.
The equilibrium reaction of ammonium sulfate is shown in Chemical Formula 2.

As shown in Chemical Formula 2, if ammonia does not exist excessively, ammonium hydrogen sulfate (NH ₄ HSO ₄ ) is generated. Ammonium hydrogen sulfate is corrosive and has a problem of corroding the discharge electrode of the electrostatic precipitator.

  Ammonium hydrogen sulfate tends to be decomposed when the exhaust gas temperature increases. When ammonium hydrogen sulfate is liquid and adheres to machinery and equipment in the processing apparatus, it is difficult to remove dust, and there is a problem that it takes time for maintenance.

  In order to suppress such generation of ammonium hydrogen sulfate, there is a method of injecting excess ammonia. However, generation of ammonium hydrogen sulfate can be suppressed by excess ammonia, but the following problems arise. That is, denitrification means for treating unreacted ammonia is required, and the equipment cost increases.

In order to reduce the amount of ammonia injected as described above, conventionally, treatment methods disclosed in Patent Documents 1 and 2 have been disclosed.
In the exhaust gas treatment method of Patent Document 1, after adjusting the temperature of exhaust gas containing SO ₃ to about 140 ° C. or less, ammonia is injected to convert SO ₃ into ammonium hydrogen sulfate, and then collected by a dust collector. It is disclosed.

Further, the SO ₃ collection method for exhaust gas disclosed in Patent Document 2 is such that ammonia injected into the exhaust gas has a molar ratio of 2 or more with respect to SO ₃ , and the exhaust gas temperature is 150 ° C. or less. SO ₃ is collected.

JP 7-308540 A JP-A-2-265618

According to the methods of Patent Documents 1 and 2, the amount of ammonia added can be reduced. However, according to the exhaust gas treatment methods as shown in Patent Documents 1 and 2, the exhaust gas temperature must be controlled to 150 ° C. or lower at which the ammonium hydrogen sulfate is not generated by the gas heater provided between the boiler and the electrostatic precipitator. Don't be. At the time of this temperature control, there is a problem that SO ₃ adheres to the preceding gas heater and corrodes. Therefore, the temperature control by the gas heater is desirably a temperature higher than 150 ° C.

Therefore, in order to solve the above-described problems of the prior art, the present invention can inject dust attached to the electrode of the electrostatic precipitator by injecting an appropriate amount of ammonia even at the exhaust gas temperature at which ammonium hydrogen sulfate is generated. and its object is to provide an SO ₃ removal method in the exhaust gas.

SO ₃ removal method in the exhaust gas of the present invention, ammonia is injected into exhaust gas containing SO _3, in the SO ₃ removal method of the exhaust gas to remove SO ₃ in the exhaust gas is introduced into the electrostatic precipitator, ammonium hydrogen sulfate depending on the higher exhaust gas temperature than the melting point of, by controlling the amount of ammonia to SO ₃ is characterized by the removal of SO _3.

SO ₃ removal method in the exhaust gas of the present invention, ammonia is injected into exhaust gas containing SO _3, in the SO ₃ removal method is introduced into an electrostatic precipitator unit exhaust gas for removing SO ₃ in the exhaust gas, ammonium sulphate And the molar ratio of ammonia to SO ₃ is at least NH ₃ / SO ₃ molar ratio> 2.2 when the exhaust gas temperature is 150 ° C. to 180 ° C. or less, depending on the exhaust gas temperature at which ammonium hydrogen sulfate is generated. When the temperature is from 180 ° C. to 185 ° C., ammonia having an NH ₃ / SO ₃ molar ratio> 2.8 is injected to remove SO ₃ in the exhaust gas.

SO ₃ removal method in the exhaust gas of the present invention, ammonia is injected into exhaust gas containing SO _3, in the SO ₃ removal method is introduced into an electrostatic precipitator unit exhaust gas for removing SO ₃ in the exhaust gas, ammonium sulphate And the molar ratio of ammonia to SO ₃ is at least NH ₃ / SO ₃ molar ratio> 2.8 when the exhaust gas temperature is 150 ° C. to 160 ° C., depending on the exhaust gas temperature at which ammonium hydrogen sulfate is generated. Is 160 ° C. to 180 ° C., NH ₃ / SO ₃ molar ratio> 5.0, and when the exhaust gas temperature is 180 ° C. or higher, NH ₃ / SO ₃ molar ratio> 10.0 is injected. It is characterized by removing SO ₃ .

SO ₃ removal method in the exhaust gas of the present invention, ammonia is injected into exhaust gas containing SO _3, in the SO ₃ removal method is introduced into an electrostatic precipitator unit exhaust gas for removing SO ₃ in the exhaust gas, ammonium sulphate and in response to the temperature of the exhaust gas ammonium bisulfate is produced, the molar ratio of ammonia to SO ₃ is the the exhaust gas temperature is 175 ° _C., poured ammonia as the NH 3 / SO ₃ molar ratio from 2.5 to 3.0 In addition, the water content in the exhaust gas containing SO ₃ is 20% or less, and SO ₃ in the exhaust gas is removed.

According to the SO ₃ removal method from the exhaust gas of the present invention having the above-described configuration, even if the exhaust gas temperature at which ammonium bisulfate is generated, the discharge wire can be controlled by controlling the NH ₃ / SO ₃ molar ratio corresponding to the gas temperature. It is possible to reduce the amount of ammonia to be injected while adhering dust to the surface and suppressing charging failure due to dust adhering to prolong the operating time of the electrostatic precipitator.

  Conventionally, operation and maintenance were performed alternately with two electrostatic precipitators. However, since the frequency of maintenance can be suppressed, the operation time can be extended with a single electrostatic precipitator, reducing equipment costs. Can be achieved.

The schematic configuration of the SO ₃ removing apparatus in the exhaust gas of applying SO ₃ removal method in the exhaust gas of the present invention. FIG. Exhaust gas _{temperature: 150 ℃ ~185 ℃, NH 3} / SO 3 molar ratio: is a diagram showing a fixing state of the dust discharge electrode in the range of 2.2 to 10.0. It is a figure which shows the relationship between an electric current density and elapsed time. It is explanatory drawing which shows the influence of the dust adhering to a discharge wire. NH ₃ / SO ₃ molar ratio a 2.5 to 3.0 is a table showing the fixing state of the discharge line of the dust in water content versus temperature of the exhaust gas.

An embodiment of the method for removing SO ₃ in exhaust gas of the present invention will be described in detail below with reference to the accompanying drawings.
FIG. 1 is a diagram showing a schematic configuration of an exhaust gas treatment apparatus to which the SO ₃ removal method in exhaust gas of the present invention is applied. As shown in the figure, the SO ₃ removal device 10 in the exhaust gas includes a boiler 12, a gas heater (GAH) 14, an ammonia injection means 16, a temperature sensor 18, an SO ₃ concentration measurement means 20, a control means 22, The electric dust collector 24 and the chimney 26 are the main components.

The SO ₃ removal device 10 in the exhaust gas is arranged in the order of the gas heater 14 and the electric dust collector 24 in the flue from the boiler 12 to the terminal chimney 26.
The gas heater 14 controls the temperature of the exhaust gas discharged from the boiler. The temperature-controlled exhaust gas is introduced into the electric dust collector 24, where dust in the gas is collected and discharged from the chimney 26 at the end to the atmosphere.

An ammonia injection means 16, a temperature sensor 18 and an SO ₃ concentration measurement means 20 are attached to the flue between the gas heater 14 and the electric dust collector 24.
The ammonia injection means 16 injects gaseous ammonia into the flue gas exhausted from the gas heater. As a result, SO ₃ contained in the exhaust gas is neutralized by ammonia to become ammonium sulfate. The ammonia injection means 16 injects a predetermined ammonia injection amount based on a control signal input from the control means 22 described later.

The temperature sensor 18 measures the temperature of the inlet through which the exhaust gas discharged from the gas heater 14 is introduced into the subsequent electric dust collector 24 and outputs the measured value to the control means 22 described later.
The SO ₃ concentration measuring means 20 measures the concentration of SO ₃ contained in the flue gas exhausted from the gas heater 14. SO ₃ concentration measurement means 20, specifically measured by the general method of SO ₃ contained in exhaust gas from gas heater 14.

The control means 22 is electrically connected to the ammonia injection means 16 and the temperature sensor 18. Also the control unit 22, the measured value of the SO ₃ concentration by SO ₃ concentration measurement means 20 is input. The control means 22 calculates an ammonia injection amount based on the measured values of the temperature sensor 18 and the SO ₃ concentration measurement means 20 and outputs a control signal for a predetermined ammonia injection amount to the ammonia injection means 16.

Next, the SO ₃ removal method in the exhaust gas of the present invention having the above-described configuration will be described.
The exhaust gas containing SO ₃ discharged from the boiler 12 is introduced into the gas heater 14 and the temperature is controlled to a temperature higher than the melting point of ammonium hydrogen sulfate, preferably higher than 150 ° C. The temperature-controlled exhaust gas is introduced into the flue between the gas heater 14 and the electrostatic precipitator 24. At this time, the SO ₃ concentration measuring means 20 measures the SO ₃ concentration in the exhaust gas. A measurement value obtained by the SO ₃ concentration measurement means 20 is output to the control means 22. The temperature sensor 18 measures the exhaust gas temperature at the inlet of the electric dust collector 24 and outputs the measured value to the control means 22.

The control means 22 calculates the ammonia injection amount required for neutralization of SO ₃ based on the input SO ₃ concentration and the measured value of the exhaust gas temperature, and sends a control signal for a predetermined ammonia injection amount to the ammonia injection means. 16 is output.

Here, in the present invention, in calculating the injection amount of ammonia, the following evaluation test by dust adhering to the discharge line was performed.
First, the operating conditions of the SO ₃ removal device in the exhaust gas are as follows: electric field strength: 6 kV / cm, current density: 8.3 mA / m ^{2 to} 12.9 mA / m ² , gas flow rate: 30 m ³ / h, gas temperature: 150 ° C. ˜185 ° C., SO ₃ concentration: 50 ppm, NH ₃ / SO ₃ molar ratio: 2.2-10.

FIG. 2 shows the dust of discharge lines at exhaust gas temperatures of 155 ° C., 175 ° C., and 185 ° C., and NH ₃ / SO ₃ molar ratios: 2.2, 2.8, 3.5, 5.0, and 10.0. It is a table | surface which shows an adhering state. The circles in the figure show the state where dust is not fixed after the discharge line is beaten (the remaining rate of the fixed dust is 10% or less), and the triangle mark shows the amount of dust stuck after the discharge line is beaten. Although it decreases, the remaining rate of the fixed dust is 10% to 50%, and the cross indicates the state of the fixed dust remaining 50% or more after hitting the discharge line. .

As shown in the figure, exhaust gas temperature: 155 ° C., NH ₃ / SO ₃ molar ratio: 2.2, exhaust gas temperature: 175 ° C., NH ₃ / SO ₃ molar ratio: 2.2, exhaust gas temperature: 175 ° C., NH ₃ / SO ₃ molar ratio: 2.8, exhaust gas temperature: 175 ° C., NH ₃ / SO ₃ molar ratio: 3.5, exhaust gas temperature: 185 ° C., NH ₃ / SO ₃ molar ratio: 3.5, exhaust gas temperature: 185 ° C. , NH ₃ / SO ₃ molar ratio: 5.0, dust sticking occurs after hitting the discharge line (the residual rate of dust sticking after hitting is 10% to 50%).

Further, when the exhaust gas temperature is 185 ° C. and the NH ₃ / SO ₃ molar ratio is 2.8 or less, after the discharge wire is beaten, the residual rate of the fixed dust becomes 50% or more.
On the other hand, exhaust gas temperature: 155 ° C., NH ₃ / SO ₃ molar ratio: 2.8 or higher, exhaust gas temperature: 175 ° C., NH ₃ / SO ₃ molar ratio: 5.0 or higher, exhaust gas temperature: 185 ° C., NH ₃ / SO _{At a 3} molar ratio of 10.0, almost no dust adheres after the discharge line is beaten (the residual rate of dust adherence after beating is 10% or less).

The above results show that when the NH ₃ / SO ₃ molar ratio is low, ammonium hydrogen sulfate is produced instead of ammonium sulfate. The melting point of ammonium hydrogen sulfate is about 147 ° C., and it is considered that fixing occurs when dissolved.
On the other hand, when the NH ₃ / SO ₃ molar ratio is high, it is considered that only the ammonium sulfate is generated, so that the dust adhered by the strike can be collected without sticking.

  Further, it was confirmed that triammonium hydrogen sulfate was produced when the exhaust gas temperature was high. Triammonium hydrogen sulfate is produced by reacting 1 mol of ammonium sulfate with 1 mol of ammonium hydrogen sulfate. For this reason, since triammonium hydrogen sulfate passes through ammonium hydrogen sulfate in the production process, it is considered that sticking occurs.

When the NH ₃ / SO ₃ molar ratio is 2.8, the sticking of dust increases as the temperature rises. This is presumably because NH ₃ molecules are easily released from ammonium sulfate due to temperature rise, and are likely to be ammonium hydrogen sulfate or triammonium hydrogen sulfate, so that the amount of fixing is increased. Further, when the temperature is high, the collision / condensation frequency between the floating particles generated by the NH ₃ gas-SO ₃ gas reaction is increased, and it is considered that the fixation becomes intense by enlarging on the discharge line.

Further, when the NH ₃ / SO ₃ molar ratio is increased at an exhaust gas temperature of 185 ° C., the fixation rate is lowered. This is stabilized with ammonium sulfate because NH ₃ molecules are less likely to be released. For this reason, it is considered that sticking hardly occurs under a high molar ratio.

FIG. 3 is a diagram showing the relationship between current density and elapsed time. In the figure, the horizontal axis represents the elapsed time (h), the vertical axis represents the current density (mA / m ² ), the circle represents the exhaust gas temperature 175 ° C., the NH ₃ / SO ₃ molar ratio 2.2, and the square represents the exhaust gas temperature 155 ° C. The NH ₃ / SO ₃ molar ratio is 2.8, and the triangles indicate the exhaust gas temperature 185 ° C. and the NH ₃ / SO ₃ molar ratio 10.0, respectively.

In the case of exhaust gas temperature: 175 ° C. and NH ₃ / SO ₃ molar ratio: 2.2, as shown by the circle in FIG. 5 mA / m ² ). This is because NH ₃ / SO ₃ molar ratio is low, and ammonium hydrogen sulfate is produced, and it is considered that fixation occurs when dissolved. Therefore, it is considered that the dust adhered to the discharge line remains even by the striking and a charging failure occurs.

On the other hand, when the exhaust gas temperature is 155 ° C., the NH ₃ / SO ₃ molar ratio is 2.8, and the exhaust gas temperature is 185 ° C. and the NH ₃ / SO ₃ molar ratio is 10.0, the square marks and triangle marks in FIG. As shown, no decrease in current density was observed over time. In this case, a decrease in current density could be suppressed even with an increase in molar ratio.

FIG. 4 is an explanatory diagram showing the influence of discharge lines and dust adhesion. In the figure, the horizontal axis represents temperature (° C.), and the vertical axis represents the NH ₃ / SO ₃ molar ratio. Moreover, the broken line in a figure shows the whole solidification of the dust adhering to a discharge line, and the continuous line has shown no solidification of the dust adhering to a discharge line.

As shown in the drawing, in the present invention, at least the molar ratio of ammonia to SO ₃ corresponding to the gas temperature of the exhaust gas is NH ₃ / SO ₃ molar ratio> 2.2 at an exhaust gas temperature of 150 to 180 ° C. When the exhaust gas temperature is 180 ° C. to 185 ° C., the NH ₃ / SO ₃ molar ratio can be set to> 2.8.

  As a result, the dust adhering to the discharge line is in a state where the residual rate is 10% to 50% by the strike of the discharge line. In the region of the molar ratio, a decrease in current density occurs as shown in FIG. 3, but the decrease in current density can be delayed while reducing the amount of ammonia injected.

More preferably, the NH ₃ / SO ₃ molar ratio is such that the NH ₃ / SO ₃ molar ratio is higher than the melting point of ammonia hydrogen sulfate, specifically at a temperature higher than 150 ° C., and at an exhaust gas temperature of 150 ° C. to 160 ° C., NH ₃ / SO ₃ molar ratio> 2.8, exhaust gas temperature 160 ° C. to 180 ° C., NH ₃ / SO ₃ molar ratio> 5.0, and exhaust gas temperature 180 ° C. or higher, NH ₃ / SO ₃ molar ratio> 10.0 , And can be set.

  As a result, the dust adhering to the discharge line is solidified as a whole, and the residual rate is reduced to 10% or less by striking the discharge line. In the region of the above molar ratio, as shown in FIG. 3, the current density does not decrease with time, and the current density can be suppressed while reducing the ammonia injection amount.

FIG. 5 is a table showing NH ₃ / SO ₃ molar ratio in the range of 2.5 to 3.0, and the state of dust adhering to the discharge line in the relationship between the moisture content of exhaust gas and the temperature. The circle mark, triangle mark, and cross mark in the figure are the same as the circle mark, triangle mark, and cross mark in FIG. 2 described above. (The remaining rate is 10% or less), and the triangular mark shows the state where the dust adhesion amount decreases after the discharge wire is beaten, but the residual rate is 10% to 50% of the fixed dust. After the beating, the fixed state of the dust with the remaining rate of 50% or more of the fixed dust is shown.

  As shown in the figure, when the exhaust gas temperature is 155 ° C. and the moisture content in the exhaust gas is 0, dust sticking after the discharge wire is beaten (the residual rate of dust sticking after beating is 10% or less) is almost generated. Absent. The same result was obtained even when the amount of water in the exhaust gas was increased to 20, 40, 60.

  In addition, when the exhaust gas temperature is 165 ° C. and the moisture content in the exhaust gas is 0, dust sticking occurs after the discharge wire is beaten (the residual rate of dust sticking after beating is 10% to 50%). The same result was obtained even when the amount of water in the exhaust gas was increased to 20, 40, 60.

When the exhaust gas temperature is 175 ° C. and the moisture content in the exhaust gas is 0.20, dust sticking occurs after the discharge wire is beaten (the residual rate of dust sticking after beating is 10% to 50%). On the other hand, when the moisture content is 40, 60, after the discharge line is beaten, the residual rate of the fixed dust becomes 50% or more.
When the exhaust gas temperature is 185 ° C. and 195 ° C. and the water content is 0, after the discharge wire is beaten, the remaining rate of the fixed dust becomes 50% or more. The same result was obtained even when the water content was increased to 20, 40, 60.

  In this way, at temperatures of 155 degrees, 165 degrees, 185 degrees, and 195 degrees, the fixed state of dust shows a constant state regardless of the increase in the amount of water. On the other hand, when the exhaust gas temperature is 175 ° C., and the moisture content in the exhaust gas is more than 20%, the residual rate of dust fixation after striking is 50% or more, which is likely to cause charging failure. It was found that if the amount of water was 0 or 20% or less, the residual rate of dust fixation after striking was 10% to 50%, and dust fixation could be reduced. This is because ammonium bisulfate and triammonium hydrogen sulfate produced by the reaction are highly hygroscopic and water-soluble. For this reason, when the moisture content is more than 20%, the particles absorb moisture in the exhaust gas, and the particles are bound to each other, so that the fixation can easily proceed.

In addition, when the exhaust gas temperature is a high temperature of 185 ° C. or higher under the NH ₃ / SO ₃ molar ratio of 2.5 to 3.0, the discharge line is beaten by reducing the exhaust gas temperature to 155 ° C. After that, the adhesion of dust can be reduced. Therefore, when the exhaust gas temperature is high, the exhaust gas temperature is preferably lowered to 155 degrees in order to reduce dust adhesion. The exhaust gas temperature can be controlled by the gas heater 14.

As described above, in this embodiment, by controlling the molar ratio according to the gas temperature at which ammonia hydrogensulfate is generated, it is possible to perform a treatment operation that does not cause defective charging while reducing excessive ammonia injection. Further, the removal method of the present invention makes it possible to reduce the ammonia injection amount and the maintenance frequency, and to more efficiently treat SO ₃ in the exhaust gas.

SO ₃ removing apparatus 10 ......... flue gas, 12 ......... boiler, 14 ......... gas heater (GHA), 16 ......... ammonia injection unit, 18 ......... temperature sensor, 20 ......... SO ₃ concentration measurement Means 22 ... Control means 24 ... Electric dust collector 26 ... Chimney.

Claims (4)

In the exhaust gas SO ₃ removal method of injecting ammonia into the exhaust gas containing SO ₃ and introducing it into an electric dust collecting means to remove SO ₃ in the exhaust gas,
A method for removing SO ₃ in exhaust gas, wherein SO ₃ is removed by controlling the amount of ammonia added to SO ₃ in accordance with an exhaust gas temperature higher than the melting point of ammonium hydrogen sulfate. In the exhaust gas SO ₃ removal method of injecting ammonia into the exhaust gas containing SO ₃ and introducing it into an electric dust collecting means to remove SO ₃ in the exhaust gas,
Depending on the exhaust gas temperature at which ammonium sulfate and ammonium hydrogen sulfate are generated, the molar ratio of ammonia to SO ₃ is at least:
When the exhaust gas temperature is 150 ° C. to 180 ° C., NH ₃ / SO ₃ molar ratio> 2.2,
When the exhaust gas temperature is from 180 ° C. to 185 ° C., NH ₃ / SO ₃ molar ratio> 2.8,
Become ammonia was injected, SO ₃ removal method in the exhaust gas, characterized in that the removal of SO ₃ in the exhaust gas. In the exhaust gas SO ₃ removal method of injecting ammonia into the exhaust gas containing SO ₃ and introducing it into an electric dust collecting means to remove SO ₃ in the exhaust gas,
Depending on the exhaust gas temperature at which ammonium sulfate and ammonium hydrogen sulfate are generated, the molar ratio of ammonia to SO ₃ is at least:
When the exhaust gas temperature is 150 ° C. to 160 ° C., NH ₃ / SO ₃ molar ratio> 2.8,
When the exhaust gas temperature is 160 ° C. to 180 ° C., NH ₃ / SO ₃ molar ratio> 5.0,
When the exhaust gas temperature is 180 ° C. or higher, NH ₃ / SO ₃ molar ratio> 10.0,
Become ammonia was injected, SO ₃ removal method in the exhaust gas, characterized in that the removal of SO ₃ in the exhaust gas. In the exhaust gas SO ₃ removal method of injecting ammonia into the exhaust gas containing SO ₃ and introducing it into an electric dust collecting means to remove SO ₃ in the exhaust gas,
Depending on the exhaust gas temperature at which ammonium sulfate and ammonium hydrogen sulfate are generated, the molar ratio of ammonia to SO ₃ is
When the exhaust gas temperature is 175 ° C., ammonia with an NH ₃ / SO ₃ molar ratio of 2.5 to 3.0 is injected,
The moisture content in exhaust gas containing SO ₃ is less than 20%, SO ₃ removal method in the exhaust gas, which comprises removing the SO ₃ in the exhaust gas.

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