JP2005334716A - Method for treating exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating exhaust gas capable of stably maintaining the removal rate of harmful gas even just after dust removal operation with a bag filter. <P>SOLUTION: In this method for treating exhaust gas, a treating agent is injected from a treating tank 36 into exhaust gas containing harmful gas which passes through an inlet flue 12, thereafter, the exhaust gas is made to pass through the bag filter 10 of a poststage and a reactant between harmful gas and the treating agent is collected by the bag filter 10. Therein, the quantity of injection of the treating agent is temporarily increased by being linked with dust removal operation in the bag filter 10. The quantity of injection of the treating agent is controlled by increasing or decreasing the number of rotation of a rotary valve 38 disposed on the bottom part of the treating tank 36 by the use of a controller 34. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas treatment method, and more particularly to an exhaust gas treatment method in which a treatment agent is injected into exhaust gas containing harmful gas, and then the exhaust gas is passed through a bag filter at a subsequent stage.

  Combustion exhaust gas discharged from boilers and municipal waste incinerators that use coal or petroleum as fuel contains acidic gases such as sulfur oxides (SOx) and hydrochloric acid (HCl), dioxins, and the like. As a method for removing these harmful gases from the exhaust gas, a technique is known in which a treatment agent such as slaked lime or activated carbon is injected into the exhaust gas, and then the exhaust gas is passed through a bag filter (see, for example, Patent Document 1 or Patent Document 2). .

The amount of treatment agent injected is usually equal to or greater than the reaction equivalent with respect to the harmful gas that is the removal target. For example, when using slaked lime as a treatment agent, powdery slaked lime with an average particle size of several μm is used as an exhaust gas so that the reaction equivalent is about 2 to 3 times that of harmful gases such as sulfur oxide and hydrochloric acid. inject. A part of the slaked lime injected from the flue of the exhaust gas reacts with harmful gas in the exhaust gas before reaching the subsequent bag filter. The reaction at this time is mainly performed near the surface of the powdery slaked lime. Therefore, the injected powdered slaked lime reaches the bag filter as a reaction product having unreacted slaked lime remaining therein and having a reaction capacity, and is captured by the filter cloth of the bag filter. For this reason, slaked lime with reaction capacity gradually accumulates on the filter cloth of the bag filter, and the reaction with the remaining harmful gas proceeds with the slaked lime deposited on the filter cloth, thereby removing harmful gas. Performance gradually improves over time.
JP-A-7-204432 JP-A-8-117549

  However, in the bag filter, since the ventilation resistance increases as the amount of accumulated dust collected on the filter cloth surface increases, the adhered dust is periodically removed. During the dust removal operation, slaked lime having a reaction capacity is also removed from the filter cloth surface as dust. As a result, immediately after the dust removal operation, it became difficult to expect a reaction between harmful gas and slaked lime on the filter cloth surface, and it was found that the harmful gas concentration of the exhaust gas after passing through the bag filter temporarily increased. . Such a problem is not limited to the case where powdered slaked lime is used as a treatment agent, but is common in treatment methods in which a powdery treatment agent such as powdered activated carbon is injected into exhaust gas.

  The object of the present invention is to improve the above-mentioned problems of the prior art and provide an exhaust gas treatment method capable of stably maintaining the harmful gas removal rate even immediately after the dust dusting operation with the bag filter. There is.

  In order to achieve the above object, the exhaust gas treatment method according to the present invention injects a treatment agent into exhaust gas containing harmful gas, and then passes the exhaust gas through a bag filter at a subsequent stage to react the harmful gas with the treatment agent. In the exhaust gas treatment method in which the bag filter is used to collect gas, the amount of the treatment agent injected is temporarily increased in conjunction with a dust removal operation in the bag filter. In this case, it is preferable that the amount of the treatment agent injected when temporarily increasing is 1.5 to 2.5 times the steady amount. Moreover, it is desirable to set the time zone to be temporarily increased between 1 minute before the start of the dust removal operation and 2 minutes after the start.

  When the treatment agent injection amount is temporarily increased in conjunction with the dust removal operation in the bag filter, the reaction between the harmful gas and the treatment agent before the exhaust gas reaches the bag filter is accelerated during that time period. . For this reason, the deterioration of the harmful gas removal performance caused by the dust removal operation is covered, and the harmful gas removal rate can be stably maintained even immediately after the dust removal operation.

  FIG. 1 is a system diagram for explaining an embodiment of an exhaust gas treatment method according to the present invention. The bag filter 10 is divided into a filter cloth chamber 18 and a ceiling chamber 20 with a partition plate 16 as a boundary. A large number of cylindrical filter cloths 22 are suspended from the partition plate 16 in the filter cloth chamber 18. A dust discharger 24 is disposed at the bottom of the filter cloth chamber 18. An exhaust flue inlet flue 12 is connected to the filter cloth chamber 18, and an outlet flue 14 is connected to the ceiling chamber 20. An air blower 26 for backwashing the filter cloth 22 is provided in the ceiling chamber 20 for each filter cloth. These air ejectors 26 are connected to a compressor 28 via a pipe 30 and eject compressed air sent from the compressor 28 toward the inside of the filter cloth 22. An opening / closing valve 32 is provided in the pipe 30 connecting the air ejectors 26, and the opening / closing of the opening / closing valve 32 is controlled by a controller 34.

  A treatment agent tank 36 filled with a treatment agent is connected to the inlet flue 12, and an appropriate amount of the treatment agent is introduced into the inlet flue by controlling the rotational speed of a rotary valve 38 provided at the lower part of the treatment agent tank 36. 12 is injected into the exhaust gas passing through. The rotational speed of the rotary valve 38 is controlled by the controller 34.

  In the above configuration, exhaust gas containing dust and harmful gas flows from the inlet flue 12 toward the bag filter 10. A processing agent is injected into the exhaust gas from the processing agent tank 36. As a result, a part of the harmful gas in the exhaust gas passing through the inlet flue 12 reacts with the treatment agent to generate a reactant before flowing into the bag filter 10. In the bag filter 10, the dust in the exhaust gas that has flowed in and the reactant that has left the reaction capacity are trapped by the filter cloth 22 as dust. The exhaust gas purified by removing the dust passes through the filter cloth 22, reaches the ceiling chamber 20, and is discharged from the outlet flue 14.

  The injection amount of the treatment agent is set to be equal to or greater than the reaction equivalent with respect to the harmful gas that is the removal target. For example, when using slaked lime as a treatment agent, powdery slaked lime with an average particle size of several μm is used as an exhaust gas so that the reaction equivalent is about 2 to 3 times that of harmful gases such as sulfur oxide and hydrochloric acid. inject. The reaction at this time is mainly performed near the surface of the powdery slaked lime. Therefore, the injected powdered slaked lime reaches the bag filter 10 in a state where unreacted slaked lime remains in the interior and has a reaction capacity, and is captured by the filter cloth 22. For this reason, slaked lime having a reaction capacity gradually accumulates on the filter cloth 22, and the reaction with the remaining harmful gas proceeds with the slaked lime deposited on the filter cloth 22, thereby removing harmful gas. Gradually improves over time.

  However, in the bag filter 10, if the amount of dust collected on the surface of the filter cloth 22 increases, the ventilation resistance increases. Therefore, the adhered dust is periodically removed. This dust removal operation will be described below. FIG. 2 is a plan view illustrating an arrangement state of the filter cloths 22 in the ceiling chamber 20. A large number of filter cloths 22 are grouped into A row, B row, and C row. Each corresponding air ejector 26 simultaneously backwashes the filter cloth 22 by simultaneously ejecting compressed air supplied from the pipe 30A. That is, the on-off valve 32A provided in the pipe 30A is opened by a signal from the controller 34, and the compressed air from the compressor 28 is supplied to the plurality of air ejectors 26 belonging to the A row, corresponding to each air ejector 26. The plurality of filter cloths 22 belonging to the row A are backwashed simultaneously. Even during the back washing of the A row, the exhaust gas flows into the bag filter 10 and the filtration operation by the filter cloths 22 in the B row and the C row is continuously performed. The dust removed from the plurality of filter cloths 22 belonging to row A by backwashing falls to the bottom of the filter cloth chamber 18 and is discharged out of the bag filter 10 by the dust discharger 24.

  When the back washing of the row A filter cloths 22 is completed, the controller 34 performs the back washing of the B row filter cloths 22 after a predetermined time has elapsed. That is, the on-off valve 32B provided in the pipe 30B is opened by a signal from the controller 34, and the plurality of filter cloths 22 belonging to the B row are backwashed simultaneously. When the back washing of the row B filter cloths 22 is completed, the controller 34 performs the back washing of the row C filter cloths 22 after a predetermined time has elapsed. That is, the on-off valve 32C provided in the pipe 30C is opened by a signal from the controller 34, and the plurality of filter cloths 22 belonging to the C row are backwashed simultaneously. Thereafter, the dust removal operation by backwashing the filter cloths 22 in the A row, the B row, and the C row is repeatedly performed in the same procedure.

  By the way, at the time of the dust removal operation, slaked lime having a reaction capacity is also removed from the surface of the filter cloth 22 as dust. As a result, immediately after the dust removal operation, it becomes difficult to expect a reaction between the harmful gas and the slaked lime on the surface of the filter cloth 22, and the harmful gas concentration of the exhaust gas after passing through the bag filter temporarily increases. For example, immediately after the dust wiping operation in the A row, the ventilation resistance of the filter cloth 22 belonging to the A row becomes the smallest, so that more exhaust gas passes through the filter cloth 22 in the B row and C row. However, if the reaction between the harmful gas and the slaked lime on the surface of the filter cloth 22 belonging to the A row is not promoted, the harmful gas concentration of the exhaust gas that has passed through the filter cloth 22 belonging to the A row increases. As a result, the harmful gas concentration of the exhaust gas in the outlet flue 14 in which the exhaust gas that has passed through the filter cloth 22 in the A row and the exhaust gas that has passed through the filter cloth 22 in the B row and C row is mixed temporarily increases.

  Therefore, in the present embodiment, the injection amount of the processing agent is temporarily increased in conjunction with the dust removal operation in the bag filter 10. That is, the controller 34 controls the rotational speed of the rotary valve 38 provided below the processing agent tank 36 to be temporarily increased in conjunction with the above-described backwash operation. FIG. 3 is a time chart showing the relationship between the backwash operation and the rotational speed of the rotary valve 38. As shown in FIG. 3, the controller 34 increases the rotational speed of the rotary valve simultaneously with the start of backwashing of the A row, the B row, and the C row, and the rotational speed of the rotary valve 38 for a certain time after the backwashing is finished. Continue driving for about 1 minute. The amount of treatment agent injected is substantially proportional to the rotational speed of the rotary valve 38. Therefore, the treatment agent injected into the exhaust gas temporarily increases during the time period when the rotational speed of the rotary valve 38 is increased.

  As the amount of treatment agent injected increases, the chance of contact between the harmful gas in the exhaust gas and the treatment agent increases, and the overall removal rate of the harmful gas before the exhaust gas reaches the filter cloth 22 of the bag filter 10 is improved. To do. For this reason, even if the reaction between the harmful gas and the treatment agent on the surface of the filter cloth 22 immediately after the backwashing does not accelerate as described above, the concentration of the harmful gas in the exhaust gas discharged from the bag filter 10 is prevented from increasing. can do.

  Moreover, since the ventilation resistance of the filter cloth 22 immediately after backwashing becomes small, more exhaust gas passes through this filter cloth 22. As a result, most of the treatment agent having a temporarily increased reaction capacity is easily captured by the filter cloth 22 immediately after backwashing, and the surface of the filter cloth 22 is quickly coated with a treatment agent having a reaction capacity, Harmful gas removal performance is restored.

  In the above description, the case where powdery slaked lime is mainly used as a treating agent for harmful gases has been described. However, the treatment agent according to the present invention is not limited to powdery slaked lime. For example, when powdered slaked lime is mixed with water and part of the slaked lime is dissolved and sprayed into the exhaust gas as a treating agent, or an aqueous solution of sodium hydroxide is used as a treating agent for removing acidic gas The present invention can also be applied when spraying into exhaust gas. Moreover, although powdered activated carbon is used for the removal of dioxin with respect to the exhaust gas containing dioxin like the incineration exhaust gas of municipal waste, the present invention can be applied to such a case. The dust removal operation according to the present invention is not limited to the above-described backwash operation using compressed air, but also includes an operation of applying vibration to the filter cloth to remove dust. In addition, the timing of increasing the amount of treatment agent injected in conjunction with the dust wiping operation may be almost the same as the start of the dust wiping operation as described above, but from about one minute before the start of the dust wiping operation. It is much safer if you do it.

In the first experiment, exhaust gas was treated using an experimental apparatus similar to the apparatus shown in FIG. The exhaust gas was combustion exhaust gas from a boiler fueled with petroleum, the SO ₂ concentration was about 400 ppm, and the gas temperature was about 180 ° C. Powdered slaked lime was used as the treating agent, and the amount of the treating agent injected was three times the reaction equivalent at the steady state with respect to the SO ₂ concentration of the exhaust gas. The frequency of dust removal operation is once every 10 minutes, and as an embodiment of the present invention, the amount of treatment agent injected is twice as much as that in the steady state for 2 minutes in total, 1 minute before and after the start of the dust removal operation (that is, , 6 times the reaction equivalent). For comparison, an operation was performed in the same manner as in a steady state without increasing the amount of treatment agent injected in the dust removal operation.

The experimental results are shown in FIG. The horizontal axis in FIG. 4 is the elapsed time, and the vertical axis is the SO ₂ concentration of the processing gas. In the figure, the solid line is an example of the present invention, and the dotted line is a comparative example. The SO ₂ concentration of the processing gas at regular time was about 80 ppm. In the comparative example, the SO ₂ concentration of the processing gas increased immediately after the start of the dust removal operation, and rapidly increased to about 120 ppm at the maximum. On the other hand, in the example of the present invention, the SO ₂ concentration of the treatment gas once decreased before the dust removal operation and increased after the removal, but it could be maintained at about 80 ppm, which is the same level as in the steady state.

In the second experiment, the amount of treatment agent injected at the time of dust removal was changed. The other conditions were the same as in the first experiment, and the maximum value of the SO ₂ concentration of the processing gas was examined. The results are shown in Table 1. From Table 1, it can be seen that there is an appropriate amount of treatment agent injected at the time of dust removal, and that there is no significant change in the harmful gas removal performance even if the amount of injection is further increased.

In the third experiment, the treatment equivalent injection ratio was increased 6 times from 1 minute before the start of dust removal, and the injection amount of the treatment agent was increased, and the increase time of the treatment agent injection amount was changed. The other conditions were the same as in the first experiment, and the maximum value of the SO ₂ concentration of the processing gas was examined. The results are shown in Table 2. From Table 2, it can be seen that there is an appropriate value for the increase time of the treatment agent injection amount at the time of dust removal, and there is no significant change in the harmful gas removal performance even if the increase time is further increased.

In the fourth experiment, the reaction equivalent ratio at the time when the treatment agent injection amount is increased is set to 12 times, and the treatment agent injection amount increase time after the dust removal start is set to 1 minute, and the time before the dust removal start is started. Changed. The other conditions were the same as in the first experiment, and the maximum value of the SO ₂ concentration of the processing gas was examined. The results are shown in Table 3. From Table 3, it can be seen that the treatment agent injection amount increase time before and after the start of dust removal can be shortened by further increasing the treatment agent injection amount.

It is an apparatus distribution diagram for explaining an embodiment of a processing method of exhaust gas concerning the present invention. It is the top view which illustrated the arrangement state of the filter cloth in a bag filter ceiling room. It is a time chart which shows the relationship between backwashing operation and the rotation speed of a rotary valve. It is a graph which shows the experimental result of exhaust gas treatment experiment.

Explanation of symbols

10 ......... Bug filter, 12 ......... Inlet flue, 14 ......... Exit flue, 16 ......... Partition plate, 18 ......... Filter room, 20 ......... Ceiling room, 22 ......... Filter Cloth, 24 ......... Dust ejector, 26 ......... Air ejector, 28 ......... Compressor, 30 ......... Piping, 32 ......... Open / close valve, 34 ......... Controller, 36 ......... Processing agent Tank, 38 ... Rotary valve.

Claims (2)

  In the exhaust gas treatment method, after injecting the treatment agent into the exhaust gas containing harmful gas, the exhaust gas is passed through a bag filter at a later stage, and the reaction product of the harmful gas and the treatment agent is collected by the bag filter. An exhaust gas treatment method characterized by temporarily increasing an injection amount of the treatment agent in conjunction with a dust removal operation in the bag filter. A method for treating exhaust gas, characterized in that the amount of the treatment agent to be temporarily increased is 1.5 to 2.5 times that in a steady state.

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