JPH11319477A - Treatment of combustion waste gas from fossil fuel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently and safely neutralize and remove sulfur oxides (SOx) in combustion waste gas of fossil fuel by adding an aq. soln. of sodium carbonate (including natural soda) to the combustion waste gas of fossil fuel in the flue in the downstream of the combustion device. SOLUTION: As for the position where an aq. soln. of sodium carbonate is added to the combustion waste gas of fossil fuel, the flue in the downstream of the combustion device, namely, a proper position is selected according to the purpose among the positions before the air preheater 4, between the air preheater 4 and electric dust collector 5, between the electric dust collector 5 and desulfurizing device 6, and between the desulfurizing device 6 and a funnel 7. In this method, 0.1 to 20 wt.% sodium carbonate aq. soln. is added by 0.01 to 20 mol to 1 mol of SO3 in the waste gas, preferably 0.01 to 10 mol. Thereby, sulfur oxides (SOx) produced when a fuel containing sulfur is used in a boiler or the like can be efficiently and safely neutralized, and therefore, problems mainly caused by sulfur oxides can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a neutralizing treatment method of the combustion exhaust gas of fossil fuel, and more particularly sulfur oxides generated by the use of fuels containing sulfur in a boiler or the like (SO _X) and efficiently And safely neutralized and removed,
The present invention relates to a method for suppressing a failure mainly caused by a sulfur oxide.

[0002]

2. Description of the Related Art When a fuel containing sulfur is used, SO
_{Since 3} is contained in exhaust gas and greatly affects corrosiveness, the problem of conventional SO ₃ is to add calcium and magnesium oxides or hydroxides to the fuel as an oil slurry and add it to the fuel. Neutralized in the low temperature range. In this method, it is difficult to use a large amount of the additive because a large amount of the additive is deposited on the heat exchange section of the boiler and hinders operation and the like.

[0003] As a method of positively neutralizing in the middle of the flue, powder such as calcium hydroxide, magnesium oxide, magnesium hydroxide or the like, or a water slurry after an air preheater (AH: air heater) is used. The additive was injected into the flue. In the case of powder in this manner, dust is scattered near the storage and injection device, significantly deteriorating the working environment. In addition, due to the use of a screw feeder or the like in the injection method, there are cases where the quantitative effect is poor and a stable effect cannot be obtained. In the case of using a water slurry for a flue, there is a serious problem in that the slurry is deposited and clogged in an injection transfer line and is stably used.

Further, ammonia is gasified and injected into the flue. However, in terms of effect, ammonium ammonium sulfate is generated, which causes a new generation of acid dust.

[0005] In an electric precipitator, ammonia is widely injected to prevent corrosion and improve dust collection efficiency. However, it causes charging failure, and MgO is used as a preventive measure.
Powder and the like have been added. However, injection of ammonia has many restrictions on handling, has a high degree of danger, and is a major problem.

In a wet-type flue gas desulfurization apparatus, for example, an apparatus for spraying an aqueous solution of sodium hydroxide into a flue, SO ₂ and S ₂ are used.
Due to the difference in the reaction rate with O ₃ , SO _{2 in} the flue gas can be removed, but SO ₃ cannot be removed. And S
SO ₃ instead of O ₂ causes white smoke. That is, in a wet type flue gas desulfurization apparatus, SO ₃ is rapidly cooled to generate sulfuric acid mist, and the sulfuric acid mist is observed as white smoke.

[0007] As a countermeasure against white smoke, MgO powder is added before the flue gas desulfurization device to neutralize SO ₃ and sulfuric acid mist, whereby fluttering of white smoke can be reduced.
However, the introduction of the powder has a low quantitative property, and the effect of reducing white smoke is unstable, so that it is not used much.

[0008]

SUMMARY OF THE INVENTION Emissions of fossil fuel combustion exhaust gas, for example, sulfur oxides (SO _x ) generated by the use of a fuel containing sulfur in a boiler or the like, are efficiently and safely neutralized and removed. There is a coveted improvement in the way you do it.

It is an object of the present invention to provide a method for efficiently and safely neutralizing and removing sulfur oxides (SO _x ) in fossil fuel combustion exhaust gas. It is an object of the present invention to suppress a decrease in the operating rate of a boiler or the like and to obtain a stable operation.

Another object of the present invention is to provide a sulfur oxide (S
O _X) is an air preheater, electrostatic precipitator, in the flue side wall or the like, H ₂ SO ₄ reaches the dew point at the point where the exhaust gas is cooled
It is to suppress the corrosion and the obstruction failure generated thereby.

Dust deposited in a flue or the like is discharged as a lump containing a large amount of sulfuric acid having a low pH due to load fluctuation or the like, resulting in acidic fallen dust, leading to environmental degradation. Therefore, an object of the present invention is to suppress the generation of the dust.

Still another object of the present invention is to suppress a decrease in the operation rate and efficiency of a boiler or the like and to enable stable operation for a long period of time.

[0013]

According to the present invention, there is provided a method for treating fossil fuel combustion exhaust gas, comprising adding a sodium carbonate (including natural soda) aqueous solution to a fossil fuel combustion exhaust gas to a flue downstream of the combustion apparatus. And a method for treating exhaust gas to be neutralized.

In the present invention, it is preferable to add 0.1 to 20% by weight of an aqueous solution of sodium carbonate in an amount of 0.01 to 20 mol per mol of SO _{3 in} the exhaust gas.
More preferably, it is added in an amount of 0.01 to 6 mol.
It is even more preferred to add them in molar.

In this specification, the flue means a flow path of exhaust gas from a combustion device such as a boiler, and also includes a flow path inside a chimney.

In the present invention, as shown in FIG. 1, the point where the aqueous solution of sodium carbonate is injected into the flue gas of fossil fuel is located in the flue downstream of the combustion device, that is, the air preheater 4.
Before, between the air preheater 4 and the electric precipitator 5, between the electric precipitator 5 and the desulfurizer 6, the flue between the desulfurizer 6 and the chimney 7, and the smoke inside the chimney 7. Although there is a way, it is appropriately selected according to the purpose.

[0017] Fossil fuels include petroleum, heavy oil, coal and the like. In particular, the present invention can be suitably applied to heavy oil and coal having a relatively high sulfur content.

The concentration of the aqueous sodium carbonate solution is 0.1%
If it is less than the amount used, the tank or transfer pump becomes too large, and at the same time as the limit of practical use, there is evidence that water vapor that cannot be completely evaporated collects at the bottom of the flue at the addition position, and the flue where rust is generated Has been observed. On the other hand, if it exceeds 20%, it cannot be stably dissolved at a low temperature in winter, and crystals may be precipitated.

To prevent clogging and corrosion of the air preheater by SO _3, an aqueous solution of sodium carbonate is used in an amount of 0.01 to equimolar with respect to 1 mol of SO ₃ to the flue between the combustion device and the air preheater. It is preferable to spray.

In order to suppress the acid deposition dust, it is preferable to inject the aqueous solution of sodium carbonate into the flue between the air preheater and the electrostatic precipitator in an amount of 1 to 20% by weight based on the amount of the dust.

In order to suppress the charging failure of the electrostatic precipitator and to reduce the ammonia, the aqueous solution of sodium carbonate is used in an amount of 1-
Preferably, 20% by weight is injected into the flue between the air preheater and the electrostatic precipitator.

In the case where the injection of ammonia into the electrostatic precipitator is replaced with an aqueous sodium carbonate solution, the range of the aqueous sodium carbonate solution is from 1 to 20 mol per mol of SO ₃ to the air preheater and the electric precipitator. Preferably, it is injected into the flue in between, more preferably from 1 to 10 mol is injected into the flue between the air preheater and the electrostatic precipitator.

In order to suppress fluttering of white smoke in a flue gas desulfurization apparatus or the like, 0.1 to 20 mol of an aqueous sodium carbonate solution with respect to a sulfuric acid mist (converted to 1 mol of SO ₃ ) is used by an electric dust collector and a desulfurization apparatus. It is preferable to inject into the flue between
More preferably, 10 to 10 moles are injected into the flue between the electrostatic precipitator and the desulfurizer.

The relative addition amount of the aqueous sodium carbonate solution depends on the flow rate of the smoke in the flue. Generally, when the flow rate is high, the amount of the added sodium carbonate aqueous solution tends to increase. Also, by increasing the amount of the aqueous sodium carbonate solution, the temperature of the exhaust gas can be lowered, and the boiler efficiency is improved.

The characteristics of the use of the aqueous sodium carbonate solution will be described below. 1. When an aqueous sodium carbonate solution is spouted into a high-temperature exhaust gas and injected finely, fine particles are obtained as compared with a solid such as a powder. Further, before the water droplet evaporates, the temperature of the water droplet surface is low, and sulfur oxides, especially SO _3, are easily adsorbed and dissolved, so that very quick neutralization can be performed. 2. The use of sodium carbonate as a neutralizing agent allows safe handling because there is no regulation of high-pressure gas and noxious poisons as compared with ammonia. 3. Since it is an aqueous solution, it can be injected more accurately than powder, and stable and accurate neutralization can be performed. 4. Unlike slurry, there is no sedimentation, there is almost no clogging of transfer lines and injection nozzles, and the management of the apparatus is easy and stable operation is possible. 5. Even when the temperature of the exhaust gas is lowered, the flue is not clogged, so that the boiler efficiency is improved.

[0026]

EXAMPLE 1 An aqueous solution of sodium carbonate was injected into a flue between a boiler and an air preheater 4 with compressed air at a pressure of 4 kg / cm ^{2 by} a diaphragm type proportional injection pump. In this case,
A test was performed on the number of times of water washing required for clogging of the air preheater with SO ₃ and the amount of corrosion by a test piece. The test piece used is plain steel and the original weight is 12.5g
Met. The boiler specifications during the test and the exhaust gas composition during the test are as follows.

Boiler specifications during test Model Natural circulation type Evaporation amount 28 8 T / H Steam temperature 540 ° C Steam pressure 120 kg / cm ² Exhaust gas composition at test CO ₂ 11% O ₂ 1.5% SO ₂ 1150 ppm SO ₃ 65 ppm

The boiler and the flue shown in FIG. 1 were used. The boiler has a furnace 1, a heater for heating the furnace 1, and an echo unit 2. A flue is formed between the exhaust gas outlet of the boiler and the chimney 7. In the stack, an air preheater 4, an electric precipitator 5 for removing dust in exhaust gas, and a wet desulfurization device 6 for spraying a sodium aqueous solution are arranged in this order from upstream to downstream.

Since the high-temperature gas is sprayed with water droplets of the aqueous solution of sodium carbonate, SO ₃ is converted to H ₂ from the relation of latent heat of evaporation of water.
Condensation occurs as SO ₄ , and furthermore, SO ₃ is concentrated on the surface of the water droplets, so that it reacts efficiently and becomes harmless Na ₂ SO ₄ .
Table 1 shows the test results.

[0030]

[Table 1]

When an aqueous solution of sodium carbonate equivalent to the amount of SO ₃ is added, there is no clogging during one year of operation, there is no need to wash once, and the corrosion amount by the test piece is 9%.
It is expected to decrease by nearly 3%, improve the operation rate and extend the life of the air preheater. By adding only 0.01 mol of sodium carbonate aqueous solution, water washing was performed about once every three months before the addition, but the number of times of water washing was improved to about once throughout the year, and corrosion was reduced by 7 times. It has decreased by almost 0%.

When the aqueous sodium carbonate solution is added to the flue, the outlet gas temperature of the air preheater is increased from 160 ° C to 12 ° C.
Even when the temperature was lowered to 5 ° C., no increase in differential pressure due to clogging was observed. By lowering the outlet gas temperature in this way, the boiler efficiency can be increased from 91% to 92.5%.
To save energy. In a technical field with a high degree of perfection such as a boiler, the improvement of the thermal efficiency by improving the boiler itself is almost at its limit.
A rise in% is a noticeable effect. In addition, when the amount of the aqueous sodium carbonate solution is large, the temperature of the exhaust gas can be further reduced, which can further save energy.

The concentration of the aqueous sodium carbonate solution is from 0.1 to
Although the amount was changed in the range of 20% by weight, there was basically no difference, and the molar ratio of the aqueous solution of sodium carbonate to SO ₃ had a greater effect.

Example 2 Influence on pH and Acid Deposition Dust of Electrostatic Precipitator Ash when an aqueous sodium carbonate solution was injected into the flue between the air preheater and the electrostatic precipitator as in Example 1. A test was conducted to determine The boiler specifications and the exhaust gas composition during the test are the same as in Example 1. Table 2 shows the test results.

[0035]

[Table 2]

The addition of the aqueous solution of sodium carbonate has a great effect on the pH of the ash and the fall of acid dust, and it is recognized that a sufficient effect is achieved.

Conventionally, it was necessary to add a large amount of magnesium, calcium, etc. to the fuel in order to raise the pH of the acid dust to 4 or more. However, if too much is added, the additive components accumulate in a furnace or the like. In many cases, the heat exchange and the like have deteriorated, and the exhaust gas temperature has increased remarkably, which has sometimes hindered long-term stable operation of the boiler. For this reason, a method of adding MgO 2 or CaO powder or slurry has been adopted. However, in the case of powder, there is a drawback that the quantitative effect is poor and a stable effect cannot be obtained. In the case of powder, the area around the injection device becomes dirty,
The working environment is poor. In the case of slurry, accumulation at the bottom of the storage tank, clogging of the injection line, and the like occurred, and it required a lot of manpower for management, and stable injection was difficult. In this regard, since the present invention uses an aqueous solution, stable operation can be performed without any hindrance, and a sufficient effect can be obtained.

In Example 2, it was confirmed that the addition of the aqueous solution of sodium carbonate had a remarkable effect in preventing charging failure of the electrostatic precipitator. Conventionally, SO ₃ and H ₂
To prevent corrosion by SO ₄ , ammonia is injected into the flue in front of the entrance of the electrostatic precipitator, but some acidic ammonium sulfate is generated and adheres to the discharge electrode. For this reason, the voltage value increases, a hunting phenomenon appears, and a failure of charging failure occurs. Since the charging failure temporarily exceeds the dust regulation value that causes air pollution, the operation may be stopped and the water may be washed. Since sodium carbonate is an alkaline substance, it raises the electrical resistivity of the dust and at the same time raises the pH of the ash of the electrostatic precipitator, thereby converting acidic ammonium sulfate to normal ammonium sulfate. As a result, the hygroscopicity of the electric precipitator ash is reduced, the flowability is improved, and the clogging of the electric precipitator is improved. Since the pH has risen, the injection amount of ammonia can be reduced by 50% or more. And S in the exhaust gas
With respect to 1 mol of O ₃ , the concentration of the aqueous sodium carbonate solution was changed in the range of 0.1 to 20% by weight of sodium carbonate, and in the range of 1 to 20 mol as sodium carbonate,
When added into the flue between the air preheater and the electrostatic precipitator, p
H becomes close to 7 and pH 5
It was proved that it could be used as a substitute for ammonia, because it remained in the range of 6. This phenomenon is due to the fact that sodium salts form sodium sulphate, but little sodium sulphate is produced above pH4. Ammonia injection requires storage and vaporization of liquid ammonia, etc.
There are laws and regulations on high-pressure gas, toxic substances, occupational safety, and flammable gas. Sodium carbonate is cheaper than ammonia, and there are no restrictions on equipment and safety,
With simple equipment, it is easier to handle and store than ammonia. Therefore, the economic improvement is of great value. Table 3 shows the test results of the addition of the aqueous solution of sodium carbonate and the charging status of the electrostatic precipitator (ash).

[0039]

[Table 3]

Further, there is a phenomenon in which white smoke discharged from the wet desulfurization apparatus flutters for a long time, which is a kind of pollution. During the tests in Table 3 above, it was observed that white smoke was shortened. The main cause of the white smoke is that SO ₃ is rapidly cooled in the process of contacting with the desulfurization water in the desulfurization device, thereby producing sulfuric acid mist. The aqueous solution of sodium carbonate of the present invention has an
₃ is desorbed by condensation and concentrated to convert SO ₃ into sodium sulfate to promote the vaporization of white smoke. Table 4 shows the relationship between addition and white smoke when the aqueous sodium carbonate solution was injected into the flue between the electrostatic precipitator and the desulfurizer in the same manner as described in Example 1. The amount of the aqueous sodium carbonate solution in Table 4 is a molar amount based on 1 mol of SO ₃ .

[0041]

[Table 4]

The relationship between the addition of the aqueous sodium carbonate solution and the white smoke is a value obtained by visually observing the fluttering of the white smoke, and it is recognized that the addition of the aqueous sodium carbonate solution slightly shortens the relationship.

[Brief description of the drawings]

FIG. 1 is a process schematic diagram of a method for treating a combustion exhaust gas of a fossil fuel.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Furnace, 2 ... Echo part, 3 ... Heater, 4 ... Air preheater, 5 ... Electric precipitator, 6 ... Desulfurizer, 7 ... Chimney

Claims (2)

[Claims] 1. A method for treating exhaust gas, comprising adding an aqueous solution of sodium carbonate to a flue gas of fossil fuel to a flue downstream of the combustion device and neutralizing the flue gas. 2. An aqueous solution of 0.1 to 20% by weight of sodium carbonate is used in an amount of 0.01 to 1 mol of SO ₃ in exhaust gas.
The method according to claim 1, wherein 20 mol is added.