KR100306292B1 - Incineration flue gas treatment method - Google Patents

Abstract

Flue gas from the waste incinerator 1 is cooled to a temperature range of 120 ° C. to 200 ° C. in the cooling tower 2. Cooled flue gas is introduced into the first dust collector (3) to remove soot and smoke. Then, the slaked lime is injected into the flue gas of the tube 7. Flue gas is guided to the 2nd dust collector 4 with the injected slaked lime, and dust collection is performed. A part of the dust collected in the second dust collector 4 is injected into the flue gas of the tube 7 together with the slaked lime powder, thereby performing recycle injection of slaked lime.

Description

Incineration flue gas treatment method

The present invention relates to a method for treating incineration flue gas, particularly a method for treating flue gas generated during incineration of waste such as municipal waste.

Most of the waste, such as municipal waste, is incinerated. However, flue gases from incinerators contain acid gases such as HCl and SOx and also soot and fumes. So these flues are treated to keep them clean. The treatment method of flue gas is to allow acid gas to be absorbed into the slaked lime powder injected into the flue gas.

In the municipal waste incineration facility, the method of treating flue gas by the dry flue gas treatment described above is carried out conventionally according to the process shown in FIG. According to this process, the flue gas from the incinerator 1 enters the gas cooling device 20 such as a cooling tower for cooling it. Then, the slaked lime powder is injected into the flue gas, where hydrogen chloride and the like in the flue gas react with the slaked lime to decrease. Then, the flue gas into which the slaked lime powder is injected enters the dust collector 21, and the slaked lime which is not reacted with the reaction product such as calcium chloride produced by the injection of slaked lime is collected together with the soot and dust from the waste incinerator. Thus, clean flue gas is discharged to the atmosphere.

On the other hand, the material collected by the dust collector 21 contains an addictive heavy metal. So these substances are classified as general waste under special regulations. Therefore, these substances are discarded after being treated in one of four methods designated by the Minister of Health and Welfare: dissolution coagulation, cement coagulation, chemical stabilization, or acid extraction.

However, when the combustion gas is cleaned with the above-described prior art, the removal of the acid gas is not sufficient. Thus, if a high removal efficiency is desired, a large amount of lime powder must be injected. Therefore, the consumption of slaked lime is increased and the discharge amount from the dust collector is increased. As a result, the costs of stabilizing and detoxing emissions from wastes increase and the costs of cleaning flue gas increase.

Furthermore, flue gas from municipal waste incinerators contain toxic organochlorine compounds such as dioxins. Although these organochlorine compounds in the flue gas are absorbed by soot and flue during flue gas treatment, the content of these toxic compounds in the flue gas gradually decreases before being discharged to the atmosphere. It does not reduce much. Therefore, there is a problem that a considerable amount of the organic chlorine compound remains in the flue gas discharged to the atmosphere.

It is an object of the present invention to provide a flue gas treatment method that can efficiently remove acidic gases and reduce the residual amount of toxic organochlorine compounds.

In order to achieve this object, the present invention comprises the steps of (a) cooling the flue gas; (b) removing soot and fumes from the flue gas; (c) injecting lime powder into the flue gas; (d) collecting dust; (e) providing a flue gas treatment method comprising the step of injecting the dust collected in the flue gas.

In the step (a) of cooling the flue gas, the flue gas from the waste incinerator is cooled to a temperature range of 120 to 200 ° C.

In step (b) of removing soot and fumes, the cooled flue gas is introduced into the first dust collector, where soot and fumes from the waste incinerator are removed. The first dust collector is preferably a bag filter.

In step (c), after removing soot and dust, hydrated lime powder is injected into the flue gas. The injected slaked lime preferably has an equivalent ratio of 1.0 to 1.5. This equivalence ratio is the ratio of the theoretically necessary amount of slaked lime to complete the reaction with acid gases present in the flue gas.

In step (d) of collecting dust, the flue gas injected with slaked lime is introduced into the second dust collector to collect dust. The second dust collector is preferably a bag filter.

In the step (e) of collecting dust, the dust collected by the second dust collector is injected into the flue gas together with the slaked lime in the step (c) after the soot and the dust are removed. The ratio of the amount of dust injected in step (e) to the amount of slaked lime injected in step (c) is preferably in the range of 0.5 to 3.

1 shows a schematic view of an incineration flue gas treatment method according to the present invention.

2 shows the relationship between flue gas temperature and HCl removal efficiency.

3 shows the relationship between the equivalence ratio of slaked lime powder and the HCl removal efficiency.

4 shows the relationship between the flue gas cooling temperature and the residual dioxin concentration in the flue gas.

5 is a schematic diagram of a conventional method for treating incinerated flue gas.

* Explanation of symbols for main parts of the drawings

1: waste incinerator 2: cooling tower

3: first dust collector 4: second dust collector

5,6,7: Tube 10: slaked lime tank

11: storage tank

According to the invention, the treatment process comprises the steps of cooling the flue gas from the waste incinerator to a temperature of 120 to 200 ℃; Introducing the cooled flue gas into the first dust collector to remove soot and smoke from the waste incinerator; Injecting hydrated lime powder into the flue gas after removing soot and smoke; Introducing flue gas containing the injected slaked powder into a second dust collector to collect dust; After removing the soot and the dust, a part of the dust collected by the second dust collector is introduced together with the slaked lime powder into the flue gas.

The equivalent ratio of slaked lime injected into the flue gas in the above-described method for treating flue gas is preferably in the range of 1.0 to 1.5.

Acid gas according to the present invention refers to SOx including HCl and SO ₂ and SO ₃ .

The equivalence ratio of the slaked lime powder is the ratio of the theoretically required amount of the slaked lime powder to complete the reaction with the acidic gas present in the flue gas according to the following reaction formulas (1) to (3). This equivalence ratio is calculated based on the equivalent of Ca (OH) ₂ to the acid gases described above.

Scheme 1

2HCl + Ca (OH) ₂ = CaCl ₂ + 2H ₂ O

Scheme 2

SO ₂ + Ca (OH) ₂ = CaSO ₃ + H ₂ O

Scheme 3

SO ₃ + Ca (OH) ₂ = CaSO ₄ + H ₂ O

According to the flue gas treatment method of the present invention, dust collection to the flue gas furnace section is performed in two stages. That is, after completing the first dust collection, hydrated lime is injected into the flue gas to absorb the acid gas. Therefore, the first precipitator collects soot and dust (fly ash) from the incinerator, and the second precipitator collects the reaction product of the injected slaked lime powder and acid gas. In this way, fly ash containing addictive substances and lime reaction products containing no toxic substances which cannot be discarded unless detoxification treatment is applied are collected separately from each other.

The reaction between HCl and SOx in slaked lime and flue gas proceeds according to equation (1-3) to remove HCl and SOx. These reactions are preferably carried out in the appropriate temperature range. According to the invention, the temperature range for carrying out the reaction described above is limited to 120 to 200 ° C. for two reasons given below.

First, the temperature range specified above is given to efficiently carry out the reaction between acid gas and slaked lime. As will be described later, according to the test results carried out by the inventor of the present invention, the increased reaction temperature is likely to reduce the removal efficiency of the acid gas. However, if the flue gas temperature drops below about 120 ° C, the acid gas will reach its dew point and cause corrosion of the plant. Therefore, it is not desirable to reduce the flue gas temperature below 120 ° C. As a result, the lower limit of the flue gas temperature is set at 120 ° C.

Second, the temperature ranges specified above are given to prevent resynthesis of addictive organochlorine compounds such as dioxins. The above-mentioned organochlorine compounds generated in the incinerator are gradually reduced during the treatment process. However, if the flue gas is in the temperature range of 200 to 300 ° C, the reaction to resynthesize the organochlorine compounds described above occurs and the rate of decrease is gradually reduced. Will be lowered. Therefore, the upper limit of the flue gas temperature is set to 200 ° C to avoid the resynthesis reaction of the organochlorine compound described above.

According to the present invention, a part of the dust collected by the second dust collector is injected into the flue gas to return to the upstream side of the processing flow and efficiently perform the reaction between the acid gas and the slaked lime. The reaction does not proceed so fast because the reaction between the acidic gas in the flue gas and the slaked lime powder is not carried out in the wet state and because the reaction begins by contacting the slaked lime particles with the acidic gas. Therefore, the conversion of the injected slaked lime is very slow so that the dust collected by the second dust collector contains a large amount of unreacted slaked lime. In order to cope with this phenomenon, the dust containing unreacted slaked lime is returned to the upstream and recycled to increase the frequency of contact between the slaked lime particles and the acid gas and the reaction efficiency between the slaked lime and the acid gas.

1 is a flowchart of an example of practical application of the method according to the invention. The flowchart shows a waste incinerator 1, a cooling tower 2, a first dust collector 3, a second dust collector 4, and flue gas pipes 5, 6 and 7, which are devices connecting them.

Flue gas from the waste incinerator (1) enters the cooling tower (2). Water is injected into the cooling tower 2 to cool the flue gas to a specified temperature in the range of 120 to 200 ° C. Then, the cooled flue gas is introduced into the first dust collector 3 and subjected to dust collection. The fly ash from the incinerator 1 is collected by 99% or more while the flue gas passes through the first dust collector. The collected fly ash is discharged to a separation process.

After the fly ash is removed, the flue gas passes through the tube 7 and enters the second dust collector 4. The slaked lime powder is injected into the flue gas passing through the pipe 7, and the injected slaked lime powder and the gas are mixed. The amount of slaked lime corresponding to the analyzed concentration of acidic gases (HCl, SO ₂ , SO ₃ ) in the flue gas is quantitatively discharged from the slaked lime tank 10, transported by air transfer and then injected into the tube 7. . The injected slaked powder is transported aerodynamically during the reaction with acid gases in the flue gas. The slaked lime powder not reacted with the reaction product enters the second dust collector 4 together with the flue gas. The slaked lime powder which is not reacted with the reaction product conveyed as smoke by the flue gas is collected by the second dust collector 4. Cleaned flue gas is sent to the stack. The collected dust is discharged from the second dust collector 4 and accumulated in the reaction product storage tank 11.

Since the dust accumulated in the reaction product tank 11 contains unreacted slaked lime, part of it is quantitatively recovered, transferred to air and injected into the tube 7. The injected dust is mixed with the flue gas together with the individually injected slaked lime powder and then collected by the second dust collector 4. In this way, the slaked lime powder circulates between the tube 7 and the second dust collector 4 and is collected by the second dust collector 4. A part of the collected slaked lime powder corresponding to the injected amount of the slaked lime powder is recovered and processed separately.

The amount of dust circulated from the total amount of dust collected by the second dust collector 4 is preferably in the approximate range of the ratio (recycling ratio) of 0.5 to 3.0 with respect to the amount of the injected slaked lime powder. If the recycle ratio is less than 0.5, the removal of acid gases becomes insufficient and leaves much unreacted limestone powder in the dust, which hinders the use of the dust. However, the excessive recycle ratio reduces the unreacted slaked lime in the flue and does not increase the contact of the unreacted slaked lime and the acid gas in the flue gas. Therefore, the increase in the removal efficiency of the acid gas cannot be expected to a high degree, and the burden on the fourth dust collector 4 increases. Therefore, it is desirable to limit the recycle ratio to about 3.0.

Applicable dust collectors for implementing the method according to the invention are not particularly limited in type and may be bag filters, electroprecipitators, or dust collector (centrifugal) separators. Among these types, a bag filter which is a filter type dust collector is particularly preferable. When the bag filter is used as the first precipitator, high dust collection efficiency is achieved stably, and the collected fly ash forms a dust collection layer on the surface of the palter cloth. This layer is expected to enhance the absorption of the organochlorine compounds described above while the flue gas passes through the dust collection layer. When the bag filter is used as the second dust collector, high dust collection efficiency is achieved, and a dust collection layer including the reaction product and the unreacted slaked lime is formed on the surface of the filter cloth, so that the contact between the unreacted slaked lime and the acid gas is Flue gas is efficiently performed because it passes through the dust collecting layer. This ensures the absorption of acid gases. As described above, according to the present invention, dust collection of the flue gas is performed in two stages. Therefore, most of the fly ash containing the toxic substance is collected by the first dust collector 3 and a small amount thereof is collected by the second dust collector 4. Therefore, only fly ash collected in the first dust collector 3 can be sent to a detoxification treatment such as a dissolution treatment for disposal to discard the discharged material. The fly ash discharged from the first precipitator 3 is collected before the injection of slaked lime, so that the fly ash does not contain a reaction product such as calcium chloride (CaCl ₂ ) salt so that the content of salt does not increase. Therefore, detoxification of fly ash by the dissolution process can be easily applied. In the prior art this dissolution process was inhibited due to the presence of salts in the fly ash. In particular, even when dissolution treatment using electric resistance heating is applied in the furnace, no interference is caused to the effect of the presence of molten salt, thereby ensuring a smooth operation.

The dust collected by the second dust collector 4 contains no toxic substance, contains a very small amount of unreacted slaked lime and shows high purity of calcium chloride. So far more can be used.

[Yes]

[Exam 1]

Flue gas was treated for cleaning using the apparatus shown in FIG. The relationship between flue gas temperature and acid gas removal efficiency was investigated.

Flue gas from the waste incinerator was sent to the cooling tower (2). The flue gas temperature was set to several temperature levels at the exit of the tower, and water was injected into the tower to cool the flue gas. The removal efficiency of acid gas was measured at each set temperature level. The analysis of the flue gas (at the exit of the cooling tower) was HCl in the range from 600 to 800 ppm, and SO _{2 in the} range from 30 to 50 ppm. The operating conditions are as follows. The results of calcined lime injection did not explain the reaction amount of SO ₃ because the SO ₃ content in the flue gas was minimal.

Flue gas temperature after cooling 240-120 ℃

Equivalence ratio of slaked lime powder 2.0

Recycle infusion rate 0

Under the conditions given above, the test results are shown in FIG. In Fig. 2, most of the acid gases in the flue gas were HCl, and the removal efficiency of HCl suggests the trend of removal efficiency of total acid gas. Therefore, the removal efficiency of total acid gas is represented by the removal efficiency of HCl.

According to FIG. 2, the removal efficiency of HCl decreases with increasing flue gas temperature. Especially when the flue gas temperature exceeded about 200 ° C, the removal efficiency of HCl showed a sharp drop.

[Exam 2]

The recycling effect of the slaked lime powder was investigated. Flue gas from the waste incinerator was introduced into the cooling tower (2). Water was injected into the tower to cool the flue gas. The flue gas temperature at the tower outlet was set at 180 ° C. The operation was performed by changing the equivalence ratio of the slaked lime powder. The removal efficiency of HCl at each equivalent ratio level of hydrated lime was measured. The flue gas analysis (at the exit of the cooling tower) during the test gave almost the same results as in test 1 for both HCl and SO ₂ . The test conditions were as follows. Also, for comparison, the operation was performed without recycle injection of slaked lime.

Flue gas temperature 180 ℃ after cooling

Equivalent ratio of slaked lime powder 0.5 to 3

Recycle Injection Ratio 1.0

The results are shown in FIG. Curve A is the result of the method according to the invention and curve B is the result when no particulate recycle infusion is applied. As can be seen, curve A, with recycled injection of slaked lime, provides very good results in the removal efficiency of HCl as high as 95% even at about 1.0 equivalent of slaked lime powder. Moreover, even when the equivalent ratio of the slaked lime powder is increased to 1.5 or more, the increase in HCl removal efficiency is very small. So additional injection is necessary. In contrast, in the case of curve B without the recycle of slaked lime, the HCl removal efficiency is considerably lower than that in curve A, and the equivalent ratio of slaked lime needs to be increased to 2.0 or higher to obtain 95% or higher HCl removal efficiency. have.

[Exam 3]

Dioxin concentration was measured in each part of the flue gas passage by introducing flue gas from the waste incinerator into the cooling tower 2 under the condition that the cooled flue gas temperature at the cooling tower outlet was set at 180 ° C. and 240 ° C., respectively.

The results are shown in FIG. The black circle in the figure indicates that the flue gas is set to 180 ° C, and the other circle indicates that the flue gas is set to 240 ° C. As can be seen, most of the dioxins in the flue gas at low flue gas temperatures disappeared while the flue gas passed through the first precipitator. The comparison of the dioxin concentration in the reaction product collected by the second dust collector between the flue gas temperatures of 180 ° C. and 240 ° C. is about one tenth when the dioxin concentration at 180 ° C. is 240 ° C.

According to the present invention, the flue gas is cooled to a temperature in the range of 120 to 200 ° C., followed by recycling of slaked lime simultaneously with injection of slaked lime powder. As a result, high removal efficiency of the flue gas is obtained without injecting a large amount of slaked lime powder.

Furthermore, according to the present invention, the flue gas temperature is limited to the above specified range so that no resynthesis of the addictive organochlorine compound occurs, so that the residual amount of the toxic organochlorine compound in the flue gas is extremely reduced.

Claims (5)

(a) cooling the flue gas from the waste incinerator to a temperature range of 120 to 200 ° C; (b) removing the soot and fumes from the waste incinerator by introducing the cooled flue gas into a first precipitator; (c) injecting hydrated lime powder into the flue gas from which soot and smoke are removed; (d) collecting dust by introducing flue gas injected with slaked lime powder into a second dust collector; (e) injecting the dust collected by the second dust collector together with the slaked lime in step (c) into the flue gas from which the soot and the dust have been removed. According to claim 1, wherein the slaked lime injected into the flue gas in the step (c) has an equivalence ratio of 1.0 to 1.5, the equivalence ratio of the news in order to complete the reaction of HCl and SOx which is an acid gas present in the flue gas Incineration flue gas treatment method characterized in that the equivalence ratio to the required amount. The incineration flue gas treatment method according to claim 1, wherein the ratio of the amount of dust injected in step (e) to the amount of powder of slaked lime injected in step (c) is in the range of 0.5 to 3. The incineration flue gas treatment method according to claim 1, wherein the first dust collector is a bag filter. The incineration flue gas treatment method according to claim 1, wherein the second dust collector is a bag filter.

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