JPS59132922A - Removal of dust and sox in exhaust gas - Google Patents

Abstract

PURPOSE:To enable the sufficient rising in the temp. of exhaust gas after wet desulfurization while avoiding acid smut or the corrosion of an apparatus caused by H2SO4 mist, by fixing and removing SO3 in exhaust gas by spraying an alkali liquid to the exhaust gas after dust removal. CONSTITUTION:Exhaust gas containing dust such as the exhaust gas of a coal firing boiler is guided to a first dust collecting apparatus 4 through a denitration apparatus 2 and an air heater 3 while the removed dust (a) is favorably utilized in cement. In the next step, an alkaline aqueous solution or suspension (b) containing NaOH is sprayed to the exhaust gas to fix SO3 in the gas and, thereafter, the treated exhaust gas is guided to a second dust collecting apparatus 5 to collect and remove solid substance (c) such as Na2SO4. The exhaust gas from which dust and SO3 are removed is lowered in temp. by a gas/gas heater 6 and, after SO2 is removed in a wet desulfurization apparatus 7, the treated gas is exhausted while the temp. thereof is raised by the gas/gas heater 6. By this method, the temp. of the exhaust gas at the outlet of the air heater 3 can be set high and the temp. rising of the exhaust gas after desulfurization is sufficiently performed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to
The present invention relates to a method for treating exhaust gas containing incoming SO2 and 503 (hereinafter both referred to as SOx).

High-temperature exhaust gas, such as coal-fired boiler exhaust gas, containing air pollutants such as dust, nitrogen oxides (NOxl + sulfur oxides (SOx)), is generally treated comprehensively as follows.

First, ammonia (NH3) is added to the exhaust gas discharged from the boiler.
In a denitrification reactor filled with a catalyst, nitrogen oxides are reduced to water and nitrogen and rendered harmless.

Next, set the temperature at which the heat can be economically recovered using an air heater.
Generally, after recovering heat to 150~180c and removing dust with an electric precipitator or cyclone precipitator,
Wet desulfurization treatment.

In wet desulfurization methods such as the wet lime/gypsum method, the gas after treatment becomes a moisture-saturated gas with a temperature of 45 to 70'C, and if it is discharged from the chimney as it is, it mixes with the atmosphere and becomes white smoke, and is difficult to diffuse due to the low temperature. Since this problem arises, a method of reheating the low temperature exhaust gas has been put into practical use.

Reheating methods include indirect heating using a heat medium such as running water, oil, or steam, and direct heating using a heat storage body such as a Ljungström heat exchanger, but the latter has recently become popular from the perspective of energy conservation. It has come to be put into practical use,
Among these, a method called the gas-gas heating method is said to be the most desirable. [Seo et al.

``Man-powered nuclear power generation j Vat, 50, N [L2 +
P 29-55 (+9791. "Mitsubishi Heavy Industries Technical Report" ■
ot, 17. Nagi 2゜P217-222 (19801) This gas-gas heating system uses high-temperature gas (150 to +5OC) at the inlet of the wet desulfurization equipment and low-temperature gas (150 to +5OC) at the outlet.
45 to 70℃) by circulating (rotating) the heat storage body to lower the exhaust gas temperature just before wet treatment.
This is a so-called energy saving method that reduces the amount of water supplied for humidification and cooling to reduce the amount of temperature drop associated with wet exhaust gas treatment, and at the same time raises the temperature of the gas exiting the wet desulfurization equipment.

However, since this method uses the high-temperature gas at the inlet of the desulfurization equipment as a heat source, the amount of heat exchanged depends on the temperature of the high-temperature gas. That is, when the inlet gas temperature is low, the temperature of the exhaust gas after the wet exhaust treatment is insufficient, and the exhaust gas discharged from the chimney does not reach a predetermined temperature, so that the white smoke does not disappear. To prevent this, gas
In addition to the heating method, a method has been proposed in which a steam heater is installed to raise the temperature of the exhaust gas after wet treatment to a predetermined temperature, but this method is not preferred from the viewpoint of energy saving.

It is also possible to raise the exhaust gas temperature at the inlet of the wet desulfurization equipment, that is, the temperature at the outlet of the air heater in the previous stage.
This causes problems in the following points.

In other words, the gas discharged from the furnace is generally sulfur trioxide gas (including SO31, which has recently been produced by oxidizing a portion of the sulfur contained in the fuel during combustion).
When a denitrification device is installed, accompanying the denitrification reaction,
05-4% of sulfur dioxide (SO2) is oxidized to SO
8 is generated and added to S03 generated in the furnace. Therefore, S03 reaching 1 air-peta population will reach 5 to 50 ppm depending on the sulfur content in the fuel.

When the reached S03 passes through the air heater, the exhaust gas temperature decreases and becomes below the dew point, so some of it is converted to H2So4+ (so3+ H2O-U2S○4) and is attached to the entrained dust. When the temperature at the air heater outlet is higher than the equilibrium condition, the condensation reaction [(2So4 is small, so the amount of S0 remaining in the air heater outlet gas is
3 The amount of gas increases.

Since there is no temperature drop in the exhaust gas, most of the air heater outlet SO3 gas reaches the desulfurization device population. S03 gas turns into H2SO and mist due to the temperature drop in the gas-gas heater, but since the generated H and So4 mist are fine particles, a portion passes through the desulfurization equipment and is discharged from the chimney, resulting in asyntosmut. There is a concern that it may cause white smoke, etc. Furthermore, there is a high possibility that the material of the desulfurization equipment containing the gas heater will be corroded by H2SO4.

Conventionally, as a method to prevent these problems, NH3 is injected into the exhaust gas at the outlet of an oil-fired air heater to produce a reaction product of SO3 and NH, such as acidic ammonium sulfate (NH4F(So, 1) or ammonium sulfate ((NH, 1, SO4+)). (hereinafter referred to as M3.-so, reactant) 1 A method of collecting this in a subsequent dust collector has been put into practical use.

“Mitsubishi Heavy Industries Technical Report J voz, + 0, Sui 5.P2+1
~P21B (1975) However, if this method is applied as a measure to remove S03 from core-containing exhaust gas in a large amount of dust, such as in a coal-fired boiler, there are the following disadvantages.

That is, by collecting both the N[(3-So3 reactant and dust) in the dust collector, a large amount of NH3-S
Treatment of o3 reactant-contaminated waste becomes a problem.

Conventionally, dust from coal-fired boilers has been effectively used for frying cement or disposed of in landfills, but the former produces NH3 and odor when mixed with water and reduces strength, while the latter produces NH3 odor and Leakage into groundwater, etc. is a problem.

Conventionally, the amount of dust in heavy oil-fired mills that have been put into practical use is extremely small compared to coal-fired mills, and N
Since the amount of dust mixed with the SO5 reactant to be treated is small, the treatment is relatively easy, but as described above, if the amount of dust is large, a large amount of dust must be treated, which poses a problem.

As described above, a reasonable method for treating exhaust gas containing SOx and SOx has not yet been established.

The present invention is a method discovered as a result of intensive studies to solve these problems. After removing dust from exhaust gas through a first dust collector, the alkaline suspension in the exhaust gas is replaced with an aqueous solution. Fixes SO3 in exhaust gas by spraying,
A method for removing dust and SOx in exhaust gas, which comprises collecting dry solids in a second dust collector, and further heat-exchanging the exhaust gas before guiding it to a wet flue gas desulfurization device to absorb SO2. be.

By adopting the method of the present invention, even if the temperature of the exhaust gas at the outlet of the air heater is made higher than that of the conventional method to improve the heat exchange efficiency between gas and gas heaters, the problems of acid smut and white smoke caused by 2So4 mist will not be solved. Furthermore, material corrosion of the gas heater and desulfurization equipment can be avoided, and the temperature of the exhaust gas after the wet exhaust treatment is sufficiently increased, and the exhaust gas diffusion from the chimney is also improved.

The most characteristic feature of the method of the present invention is after the exhaust gas has been subjected to dust treatment through the first dust collector.

The point is to spray an alkaline suspension or aqueous solution into the exhaust gas.

Until now, a method of exhaust gas desulfurization using a spray dryer is known. For example, as shown in Japanese Patent Publication No. 57-25522, a method is known in which SO2 is absorbed by spray drying a caustic soda aqueous solution.

Therefore, in our research on treating SOx by spray drying this caustic soda aqueous solution, we found that if the exhaust gas temperature after spray drying is in the range of 15 DC or higher, SO
It was confirmed that only SO□ was selectively fixed, and SO□ was not fixed at all.

In other words, when the exhaust gas after spray drying is passed through the second dust collector, the solids collected by the second dust collector do not contain sulfites, which cause COD, and are convenient for solids treatment. It has the advantage of yielding results.

Furthermore, since even a small amount of liquid is sprayed to remove So, it is effective, so the temperature drop due to spray drying is only 10℃.
A fixed effect of SO8 can be obtained within the range of Therefore, there is an advantage that a sufficient thermal effect can be obtained when the exhaust gas is reheated after wet desulfurization by heat exchange with the waste gas heater located on the downstream side of the exhaust gas.

If the method of the present invention, which has these advantages, is adopted in a comprehensive flue gas treatment system that rationally combines denitrification, dust removal, desulfurization, and heat recovery systems for coal-fired boiler flue gas, the temperature of the flue gas at the outlet of the air heater can be lowered. In other words, the exhaust gas temperature at the smoke inlet can be raised to a predetermined temperature without additionally installing a steam gas heater or the like after the gas heater. The dust collected by the first dust collector is then placed on frying cement etc.
The SO2 is absorbed by a wet lime plaster method flue gas desulfurization device and recovered as gypsum, while the S03 is selectively fixed by a spray dryer and collected by a second dust collector.

The sulfite-free solid matter, which becomes the 120D original prisoner, collected by the second dust collector can be easily disposed of. Of course, E
(Trouble caused by 2So4 mist can also be resolved.

Hereinafter, one embodiment of the method of the present invention will be explained based on the diagram. In FIG. 1, exhaust gas discharged from a coal-fired boiler 1 is subjected to NOx removal in a denitrification device 2, and is then supplied to an air heater 5. be done. In the denitration equipment 2, a part of S02 is oxidized and converted to 5o3vc, so the air peak 5
The amount of S03 reaching the population is relatively large. For example, 5
02a degree 1000r, pm, boiler 1 person denitration equipment 2
Assuming that the oxidation rates are 1% and 2, respectively, the SO3 concentration in the 2-construction Arheater 5 population will be 50 ppm.

The exhaust gas temperature at the outlet of the air heater 5 is generally 150 to 18
0C, but the second one showing the relationship between sulfuric acid dew point and SO3 concentration
As is clear from the figure, when the gas temperature is high, SO3
Even if the concentration is high, the dew point temperature is not reached, and therefore most of the S03 gas remains at the outlet of the air heater 5.

Next, the exhaust gas at the outlet of the air heater 5 is first supplied to the first dust collector 4, where most of the dust is removed. Although this dust has a small amount of NH3 discharged from the denitrification device 2 attached to it, the amount is very small, so it cannot be taken out from line a as before and used effectively for cement or dumped. It is possible. Next, NaO is introduced from line b into the exhaust gas from which most of the dust has been removed.
After spraying the H aqueous solution to fix S03 in the gas, the exhaust gas is supplied to the second dust collector 5. Na0
E (the amount of injection is usually more than one times (molar ratio) the amount of SO8). Reacts with the injected NaOHij SO3 to generate Na2804.

Further, the exhaust gas temperature drop due to spray drying is within 100%.

The second dust collector 5 collects Na2804, which is discharged through line C.

The exhaust gas from which dust and S○ have been removed is further supplied to the gas heater 6 (untreated side), and the exhaust gas temperature is lowered to 7
After lowering the temperature to 0 to 90C, it is supplied to the desulfurizer 7. The gas supplied to the gas heater 6 (untreated side) does not contain SO8, so the gas
(H2SO4 mistake due to exhaust gas temperature drop in heater 6)
Therefore, white smoke is not discharged from the chimney 8, and corrosion of the materials of the gas heater 6 and deflow device 7 by sulfuric acid can be avoided.

Furthermore, the exhaust gas from which SO□ has been removed by the desulfurization device 7 is heated again by the gas heater 6 (processing side) and is heated to the chimney 8.
However, according to the method of the present invention, the temperature of the exhaust gas supplied to the gas heater 6 (untreated side) can be set to a maximum of about 180C as described above. 1. Because the temperature of the exhaust gas supplied from the desulfurization device 7 to the gas heater 6 (processing side) can be sufficiently increased. In particular, there is no need to install additional equipment such as a steam gas heater.

As described above, by applying the method of the present invention, SO3 in exhaust gas can be removed at a relatively high temperature, while avoiding material snacking caused by 52so and mist in gas/gas heater pumping desulfurization equipment. , the chimney population temperature can be raised to a predetermined temperature.

Example 4,000 m3N/H of coal-fired boiler exhaust gas was supplied to the flue gas treatment test equipment consisting of the flow sheet shown in Figure 1, and the gas properties before and after each equipment were measured, and the results shown in Table 1 were obtained. Ta.

In addition, 0.74 mol is added to the second electrostatic precipitator 5-man robot.
/l of NaOH aqueous solution from a spray nozzle at 21t/h.
This Na0E (aqueous solution was prepared by dissolving NaOH in the waste water from the wet lime plaster method flue gas desulfurization equipment 7) was used. It was done.

In addition, the amount of corrosion estimated from the weight loss of the 5S-41 test piece installed at the outlet flue of gas heater 6 (untreated side) was less than 0.0107 years, and almost no corrosion tendency was observed from the visual results. I couldn't.

Comparative Example When the injection of NaOH aqueous solution into the second dust collector 5 was stopped and the other conditions were the same as in the example, each device @
The subsequent gas properties were measured and the results shown in Table 2 were obtained.

In addition, the amount of corrosion estimated from the weight loss of the 5S-41 test piece installed at the outlet flue of gas heater 6 (untreated side) was 1.6 yar/year, and corrosion was also confirmed by visual observation. .

From the above examples and comparative examples, N to the second dust collector 5 population
The injection effect of aOH aqueous solution was observed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is an equilibrium diagram showing the dew point of H2SO2 and the SO2 concentration.

Claims (1)

[Claims] (11) In an exhaust gas treatment method for purifying combustion exhaust gas containing dust-containing SOX, after dust is removed from the exhaust gas through a first dust collector, an alkaline suspension or After fixing S03 in the exhaust gas by spraying an aqueous solution and collecting dry solids in a second dust collector, the exhaust gas is guided to a wet flue gas desulfurization device to absorb S02 in the exhaust gas. (2) A method for removing SOx contained in dust. (2) A method according to claim (1), characterized in that wastewater discharged from a wet flue gas desulfurization device is used as an alkaline suspension or aqueous solution to be sprayed into the exhaust gas. the method of.