JP4853737B2 - Exhaust gas treatment method and apparatus - Google Patents

Description

  TECHNICAL FIELD The present invention relates to an exhaust gas treatment method and apparatus, and in particular, exhaust gas containing exhaust gas containing sulfur oxides discharged from a boiler in the order of a dry electrostatic precipitator, a cooling device, a wet desulfurizer, and a wet electrostatic precipitator. The present invention relates to a processing method and apparatus.

  The exhaust gas discharged from boilers that use heavy oil or coal as fuel contains sulfur oxides generated from the sulfur content in the fuel. Therefore, for this type of exhaust gas discharged from boilers for thermal power plants, etc., first dust is removed by a dry electrostatic precipitator, then desulfurized by a wet desulfurizer, and finally the exhaust gas is led to a wet electrostatic precipitator. Mist is removed and released to the atmosphere.

  Sulfur oxides contained in this type of exhaust gas are mainly sulfur dioxide, but there are several tens of ppm of sulfur trioxide. This sulfur trioxide easily reacts with moisture to become sulfuric acid. When the gas temperature falls below the dew point (acid dew point) of sulfuric acid, it condenses and becomes sulfuric acid mist. Since sulfuric acid mist is highly corrosive, the exhaust gas is maintained at a temperature higher than the acid dew point (for example, around 170 ° C.) before the wet desulfurization apparatus. However, when this exhaust gas is led to a wet desulfurization apparatus and rapidly cooled to around 60 ° C., which is the dew point of moisture, fine sulfuric acid mist is generated. This fine sulfuric acid mist is difficult to remove with a wet desulfurization apparatus, and is removed with a subsequent wet electrostatic precipitator.

  However, in the wet type electrostatic precipitator, the collection performance is greatly influenced by the particle size of the sulfuric acid mist or the like that is the collection target, and the collection efficiency is lowered when the mist diameter is reduced. If the sulfuric acid mist produced by the wet desulfurizer contains a lot of sub-micron level, the collection performance of the wet electrostatic precipitator is significantly reduced due to the space charge effect, and the exhaust gas discharged from the wet electrostatic precipitator. The concentration of sulfuric acid mist in the inside may increase, leading to air pollution. For this reason, the capacity | capacitance of a wet electrostatic precipitator must be enlarged, and there exists a fault that the installation cost and operating cost rise.

Patent Document 1 discloses a method of preliminarily cooling the exhaust gas introduced into the wet desulfurization apparatus in order to improve the disadvantages of this type of exhaust gas treatment method. According to the method disclosed in Patent Document 1, the diameter of sulfuric acid mist in the exhaust gas leaving the wet desulfurization apparatus and introduced into the wet electrostatic precipitator can be increased. Mist collection efficiency can be increased.
JP 2002-45643 A

  However, according to the knowledge of the present inventor, when the method disclosed in Patent Document 1 is adopted, it has been found that the optimum temperature of the exhaust gas cooled in the cooling device varies depending on the concentration of sulfur trioxide in the exhaust gas. did. That is, when the concentration of sulfur trioxide in the exhaust gas is low, if the temperature of the exhaust gas cooled in the cooling device is lowered, the sulfuric acid mist diameter in the exhaust gas at the outlet of the wet desulfurization device increases, and the sulfuric acid mist in the wet electrostatic precipitator The removal rate is improved. On the other hand, when the concentration of sulfur trioxide in the exhaust gas is high, if the temperature of the exhaust gas cooled in the cooling device is increased, the sulfuric acid mist diameter in the exhaust gas at the outlet of the wet desulfurization device increases, and the sulfuric acid mist in the wet electric dust collector The removal rate is improved. Therefore, in order to increase the removal rate of sulfuric acid mist in the wet electrostatic precipitator, the cooling temperature when preliminarily cooling the exhaust gas before introducing it into the wet desulfurizer is based on the concentration of sulfur trioxide in the exhaust gas. It turned out to be most desirable to change. However, there is no practical measuring means that can detect the concentration of sulfur trioxide in the exhaust gas in real time with the current technology. For this reason, it has been difficult to apply the above knowledge obtained by the present inventors to actual exhaust gas treatment.

  The present invention was devised based on the above technical background, and the object of the present invention is to perform preliminary measurement before introducing exhaust gas into a wet desulfurization apparatus without detecting the concentration of sulfur trioxide in the exhaust gas. It is an object of the present invention to provide an exhaust gas treatment method and apparatus that can optimally control the cooling temperature of exhaust gas during cooling to increase the removal rate of sulfuric acid mist in a wet electrostatic precipitator.

  In order to achieve the above object, an exhaust gas treatment method according to the present invention is a dry electric dust collector, a cooling device, a wet desulfurization device, and a wet electric dust collector in order of exhaust gas containing sulfur oxides discharged from a boiler. In the exhaust gas treatment method for guiding and treating, the cooling device uses a heat exchanger that indirectly cools the exhaust gas with cooling air as the cooling device, so that the current density in the wet electrostatic precipitator is maximized. The supply amount of the cooling air in is controlled. In this case, it is desirable to use the cooling air supplied to the cooling device having a temperature of 100 ° C. or higher.

  Further, the exhaust gas treatment apparatus according to the present invention is an exhaust gas treatment for treating exhaust gas containing sulfur oxides discharged from a boiler in the order of a dry electric dust collector, a cooling device, a wet desulfurization device, and a wet electric dust collector. In the apparatus, the cooling device is a heat exchanger that indirectly cools the exhaust gas, and the cooling amount in the cooling device is the maximum value indicated by a charge ammeter provided in the high-voltage power supply of the wet electrostatic precipitator. It is characterized by being controlled by a controller.

  In the exhaust gas treatment apparatus having the above-described configuration, a gas cooler that indirectly cools the exhaust gas with cooling air is disposed in front of the dry electrostatic precipitator, and the wet electrostatic precipitator is disposed in the subsequent stage of the wet electrostatic precipitator. It is desirable that a heater for indirectly heating the exhaust gas discharged from the apparatus with heated air is provided. Then, the cooling air heated by the heat exchange in the gas cooler is used as the heated air supplied to the heater, and the heated air cooled by the heat exchange in the heater is used to indirectly cool the exhaust gas in the cooling device. It is desirable that it can be used as a cold heat source.

  According to the exhaust gas treatment method and apparatus of the present invention, even when the concentration of sulfur trioxide in the exhaust gas changes, the current density in the electric field space through which the exhaust gas passes can be always kept at the maximum in the wet electrostatic precipitator. . Therefore, without detecting the concentration of sulfur trioxide in the exhaust gas, the cooling temperature of the exhaust gas when the exhaust gas is preliminarily cooled by the cooling device before being introduced into the wet desulfurization device is optimally controlled, so The removal rate of sulfuric acid mist in the dust device can be increased.

  In addition, if a heat exchanger that indirectly cools the exhaust gas with cooling air is used as a cooling device, the moisture in the exhaust gas does not change during the cooling process in the cooling device, and local exhaust gas quenching phenomenon does not easily occur. Control can be performed stably and generation of fine sulfuric acid mist can be minimized. In addition, if the cooling air supplied to the cooling device has a temperature of 100 ° C. or higher, the temperature difference between the exhaust gas supplied to the cooling device and the cooling air is small, so that the local exhaust gas quenching phenomenon further occurs. Slow cooling near the acid dew point of the exhaust gas greatly contributes to the coarsening of sulfuric acid mist.

  FIG. 1 is a system diagram showing an embodiment of an exhaust gas treatment method and apparatus according to the present invention. First, the exhaust gas 12 of the boiler 10 using heavy oil, coal, or the like as fuel is sent to the denitration device 14, and nitrogen oxides in the exhaust gas 12 are removed. The exhaust gas 16 that has passed through the denitration device 14 is sent to an air heater 18 to exchange heat with the combustion air of the boiler 10. The exhaust gas 20 that has passed through the air heater 18 is sent to a gas cooler 22. Outside air is supplied to the gas cooler 22 as cooling air 26 by a fan 24, and the cooling air 26 indirectly exchanges heat with the exhaust gas 20. The exhaust gas 28 cooled to about 170 ° C. to 180 ° C. by heat exchange is sent to the dry electrostatic precipitator 30, and the dust in the exhaust gas 28 is removed.

  The exhaust gas 32 that has passed through the dry electrostatic precipitator 30 has a temperature of about 170 ° C. and contains sulfur oxides. The exhaust gas 32 is first sent to a cooling device 34 and is preliminarily cooled within a range of 120 to 170 ° C. by indirectly exchanging heat with the cooling air 35. The exhaust gas 36 that has passed through the cooling device 34 is sent to a wet desulfurization device 38, and mainly sulfur dioxide in the exhaust gas 36 is removed. The exhaust gas 40 that has passed through the wet desulfurization apparatus 38 is in a saturated state at a temperature of about 60 ° C., and contains a large number of fine mists. These fine mists contain many sulfuric acid mists. Therefore, the exhaust gas 40 containing such sulfuric acid mist is guided to the wet electrostatic precipitator 42, and mist such as sulfuric acid mist and remaining dust are removed by the wet electrostatic precipitator 42. The exhaust gas 44 that has passed through the wet electrostatic precipitator 42 is indirectly heat-exchanged with the heated air 48 by a heater 46 for the purpose of preventing white smoke, heated to 100 ° C. or higher, and then released from a chimney (not shown) to the atmosphere. .

  In addition, as the heating air 48 supplied to the heater 46, it is desirable to use the cooling air 26 that has been heated by heat exchange in the gas cooler 22. Further, a part of the heated air 46 cooled to about 100 to 120 ° C. by heat exchange in the heater 46 is used as the cooling air 35 supplied to the cooling device 34, and the remaining part is discharged as the exhaust 50 into the atmosphere.

  The wet electrostatic precipitator 42 is equipped with a high voltage power source 52, and the high voltage power source 52 maintains a high electric field in the space through which the exhaust gas 40 to be treated passes. As the current density in the electric field space increases, the dust collection performance (mist collection performance) improves. The current density in the electric field space can be converted by a charge ammeter 54 attached to the high voltage power source 52. Therefore, in this embodiment, the instruction value of the charge ammeter 54 is transmitted to the controller 56, and the controller 56 supplies the cooling air supplied to the cooling device 34 so that the instruction value of the transmitted charge ammeter 54 is maximized. The flow rate of 35 is controlled. That is, a flow rate adjustment valve 58 is provided in the line of the cooling air 35, and the controller 56 controls the flow rate of the cooling air 35 by adjusting the opening degree of the flow rate adjustment valve 58. Then, the current density of the electric field space through which the exhaust gas 40 passes in the wet electrostatic precipitator 42 is maximized, and the dust collecting performance (mist collecting performance) of the wet electrostatic precipitator 42 is improved as described above.

  The temperature of the exhaust gas 36 supplied to the wet desulfurization device 38 is detected by a thermometer 60, and the flow rate of the cooling air 35 supplied to the cooling device 34 is detected by a flow meter 62. The detected temperature of the exhaust gas 36 and the flow rate of the cooling air 35 are transmitted to the controller 56, and the controller 56 uses these detected values as back data for verifying whether the control is normally executed. .

A treatment experiment was conducted on the exhaust gas 32 which passed through the dry electrostatic precipitator 30 of the exhaust gas processing apparatus having the above configuration. The temperature of the exhaust gas 32 used in the experiment was 170 ° C., the dust concentration was 6 mg / m ³ N, and the sulfur trioxide concentration was 12 ppm. 2 to 4 are graphs showing experimental results. The desulfurization inlet temperature indicated on the horizontal axis in FIGS. 2 to 4 indicates the temperature of the exhaust gas 36 at the inlet of the wet desulfurization apparatus 38 (that is, the outlet of the cooling apparatus 34) in FIG. 32 is changed to 112 ° C., 135 ° C., and 152 ° C. by changing the cooling conditions of the cooling device 34. The mist particle size on the vertical axis in FIG. 2 indicates the average particle size of mist contained in the exhaust gas 44 at the outlet of the wet desulfurization apparatus 38 (that is, the inlet of the wet electrostatic precipitator 42). The current density on the vertical axis in FIG. 3 means the current density in the wet electrostatic precipitator 42. The sulfur trioxide removal rate on the vertical axis in FIG. 4 means the sulfur trioxide removal rate of the wet electrostatic precipitator 42 when the exhaust gas 40 and the exhaust gas 44 are compared in FIG.

  2 to 4, it can be seen that when the desulfurization inlet temperature is around 135 ° C., all of the mist particle size, current density, and sulfur trioxide removal rate are maximized. This temperature of 135 ° C. is slightly lower than the acid dew point (139 ° C.) of the exhaust gas 32 (sulfur trioxide concentration is 12 ppm) used in this experiment. FIG. 5 is a graph showing the relationship between the sulfur trioxide concentration in the air and the acid dew point. The higher the sulfur trioxide concentration in the air and the moisture, the higher the acid dew point.

  Based on such knowledge, the same experiment was conducted for the exhaust gas 32 having different sulfur trioxide concentrations. When the desulfurization inlet temperature was set to a temperature slightly lower than the acid dew point, the mist particle size, current density, sulfur trioxide It was found that both removal rates were maximized. As a result, the desulfurization inlet temperature, mist particle size, current density, and sulfur trioxide removal rate have a close correlation, and if the cooling conditions of the cooling device are controlled so that the current density is maximized, the desulfurization inlet temperature is reduced. It has been found that an optimum treatment can be realized that is automatically adjusted to a temperature slightly lower than the acid dew point corresponding to the sulfur trioxide concentration and moisture in the exhaust gas, and that maximizes the mist particle size and sulfur trioxide removal rate.

  In the present embodiment, based on such knowledge, the controller 56 adjusts the opening degree of the flow rate adjustment valve 58 so as to maximize the indicated value of the charge ammeter 54 transmitted to the cooling device 34. The flow rate of the cooling air 35 is controlled. As a result, even when the concentration of sulfur trioxide in the exhaust gas 32 changes, the current density in the wet electrostatic precipitator 42 is always maintained at the maximum, and the optimum treatment with the maximum sulfur trioxide removal rate is realized. it can.

  FIG. 6 is a time chart illustrating a control method for maximizing the indicated value of the charge ammeter 54. In FIG. 6, the horizontal axis indicates time, and the vertical axis indicates the instruction value of the charge ammeter 54. Starting from time a, the controller 56 increases the flow rate of the cooling air 35 to control the temperature of the exhaust gas 36 to be lowered, and continues this cooling condition until the next time b. As a result, when the current instruction value at time b is lower than time a, it is determined that the increase in the flow rate of the cooling air 35 at time a is reverse control, and the controller 56 The flow rate is decreased and control is performed to raise the temperature of the exhaust gas 36, and this cooling condition is continued until the next time c. As a result, the current instruction value at time c is restored. The controller 56 further reduces the flow rate of the cooling air 35 to control the temperature of the exhaust gas 36 so that the cooling condition is continued until the next time d.

  Hereinafter, when the control is performed so that the temperature of the exhaust gas 36 is continuously increased in the order of d, e, f, g, and h by the same control, the current instruction value at each time also gradually increases. However, the current instruction value cannot increase permanently, and for example, it is assumed that the current instruction value is lower than that at time h at time i. Then, the controller 56 increases the flow rate of the cooling air 35 and returns it to the same level as the time g. As a result, the current instruction value is recovered at time j. Therefore, the controller 56 determines that the flow rate of the cooling air 35 in the vicinity maximizes the current instruction value, and performs control to maintain the flow rate of the cooling air 35 set at the time g and the time i after the time j. As a result, the operation in which the current instruction value is maximized is performed until time k, and the temperature of the exhaust gas 36 is automatically adjusted to the optimum value corresponding to the concentration of sulfur trioxide. The optimum treatment that maximizes the current density and sulfur trioxide removal rate can be continued.

  When the concentration of sulfur trioxide in the exhaust gas 32 changes due to changes in the operating conditions of the boiler 10 and the like, the current indication value tends to increase or decrease after time k. When such a tendency appears, the controller 56 searches for a cooling condition that maximizes the current indication value by repeating the operation of increasing or decreasing the flow rate of the cooling air 35 in the same way as described above, and controls to execute it. That's fine.

  As the control interval (time t shown in FIG. 6) in the controller 56, a long interval of about 1 to 10 minutes is sufficient. That is, since the residence time of several seconds to several tens of seconds is required until the exhaust gas 32 flows into the wet electrostatic precipitator 42 through the cooling device 34 and the wet desulfurization device 38, there is no control interval below this residence time. Meaning and confusion arises in control. When the operating conditions of the boiler 10 are stable, the control interval can be set to about 1 hour depending on the situation.

  FIG. 7 is an explanatory view showing a model of changes in daily temperature of the exhaust gas 36. In FIG. 7, a broken line A indicates that the temperature of the exhaust gas 32 at the inlet of the cooling device 34 is 170 ° C. Further, a solid line B indicates a change in temperature with time of the exhaust gas 36 at the outlet of the cooling device 34 (inlet of the wet desulfurization device 38). By the control by the controller 56 described above, the exhaust gas 36 is automatically adjusted to about 130 ° C. during the period x, about 140 ° C. during the period y, and about 150 ° C. during the period z. In this case, the concentration of sulfur trioxide in the exhaust gas 32 gradually increases in the order of the period x, the period y, and the period z, and the acid dew point also increases according to the concentration of sulfur trioxide. Can be assumed to be automatically adjusted to the optimum value.

  As shown in FIG. 5, the acid dew point is greatly influenced not only by the concentration of sulfur trioxide in the air but also by moisture. For this reason, for example, when the exhaust gas 32 is cooled by spraying moisture directly, the moisture in the exhaust gas 32 increases, so that the acid dew point increases and the control may become unstable. In addition, innumerable local exhaust gas quenching occurs due to the spraying of water, and a large amount of fine sulfuric acid mist is generated. In the exhaust gas treatment method and apparatus of this embodiment, a heat exchanger that indirectly cools the exhaust gas 32 with the cooling air 35 is used as the cooling device 34. For this reason, the moisture in the exhaust gas 32 does not change during the cooling process in the cooling device 34, and the local exhaust gas quenching phenomenon hardly occurs, so that the control can be performed stably and the generation of fine sulfuric acid mist is minimized. To the limit.

  In the exhaust gas treatment method and apparatus of this embodiment, the cooling air heated by heat exchange in the gas cooler 22 is used as the heated air 48 supplied to the heater 46, so that effective heat recovery is achieved. be able to. Further, the heated air cooled to about 100 to 120 ° C. by heat exchange in the heater 46 is used as the cooling air 35 for indirectly cooling the exhaust gas 32 in the cooling device 34. For this reason, the temperature difference between the exhaust gas 32 supplied to the cooling device 34 and the cooling air 35 is reduced, the local exhaust gas quenching phenomenon is further less likely to occur, and the slow cooling of the exhaust gas proceeds stably. It is considered that the slow cooling action near the acid dew point of the exhaust gas greatly contributes to the coarsening of sulfuric acid mist.

  As described above, according to the exhaust gas treatment method and apparatus of the present embodiment, the controller 56 cools the indicated value of the charge ammeter 54 provided in the high-voltage power source 52 of the wet electrostatic precipitator 42 so as to be maximum. The flow rate of the air 35 was adjusted to control the temperature of the exhaust gas 36 to be optimum. For this reason, even if the concentration of sulfur trioxide in the exhaust gas 32 changes, the current density in the electric field space through which the exhaust gas 40 passes in the wet electrostatic precipitator 42 can always be maintained at the maximum. Therefore, the cooling temperature of the exhaust gas 36 when the exhaust gas 32 is preliminarily cooled by the cooling device 34 before being introduced into the wet desulfurization device 38 is optimally controlled without detecting the concentration of sulfur trioxide in the exhaust gas 32. Therefore, the removal rate of sulfuric acid mist in the wet electrostatic precipitator 42 can be increased.

  In addition, a heat exchanger that indirectly cools the exhaust gas 32 with the cooling air 35 is used as the cooling device 34. For this reason, the moisture in the exhaust gas 32 does not change during the cooling process in the cooling device 34, and the local exhaust gas quenching phenomenon hardly occurs, so that the control can be performed stably and the generation of fine sulfuric acid mist is minimized. To the limit. Furthermore, the heated air cooled to about 100 to 120 ° C. by heat exchange in the heater 46 is used as the cooling air 35 for the cooling device 34. For this reason, since the temperature difference between the exhaust gas 32 supplied to the cooling device 34 and the cooling air 35 becomes smaller, the local exhaust gas quenching phenomenon becomes even less likely to occur, and the slow cooling in the vicinity of the acid dew point of the exhaust gas becomes sulfuric acid. It greatly contributes to the coarsening of mist.

  In addition, the wet electrostatic precipitator 42 includes a plurality of high-voltage power supplies along the exhaust gas flow path, and for example, charge control may be performed separately for the upstream, intermediate, and downstream high-voltage power supplies. When the present invention is applied to an exhaust gas treatment apparatus equipped with such a type of wet electrostatic precipitator 42, various combinations can be selected as the indicated value of the charge ammeter to be controlled. First, the present invention is applied by selecting one of the indicated values of the charge ammeters provided for each of the plurality of high-voltage power supplies as a representative. Secondly, the indication values of the charge ammeters respectively provided in a plurality of high-voltage power supplies are added together, and the present invention is applied based on the added values. In the summation, all indicated values may be summed, or the indicated values of the charging ammeters of only the selected high-voltage power supply may be summed.

1 is a system diagram showing an embodiment of an exhaust gas treatment method and apparatus according to the present invention. It is a graph which shows the relationship between desulfurization inlet temperature and a mist particle size. It is a graph which shows the relationship between desulfurization inlet temperature and a current density. It is a graph which shows the relationship between desulfurization inlet temperature and sulfur trioxide removal rate. It is a graph which shows the relationship between the sulfur trioxide density | concentration in air, and an acid dew point. 6 is a time chart illustrating a control method for maximizing the indicated value of the charge ammeter 54. It is explanatory drawing which modeled and showed the daily temperature change of the waste gas.

Explanation of symbols

  10 ......... Boiler, 12, 16, 20, 28, 32, 36, 40, 44, 46 ......... Exhaust gas, 14 ......... Denitration device, 18 ......... Air heater, 22 ......... Gas cooler, 24 ... ... Fan, 26 ......... Cooling air, 30 ......... Dry electrostatic precipitator, 34 ......... Cooling apparatus, 35 ......... Cooling air, 38 ......... Wet desulfurizer, 42 ......... Wet electric dust collector Equipment, 46 ......... heater, 48 ......... heated air, 50 ...... exhaust, 52 ......... high voltage power supply, 54 ......... charged ammeter, 56 ......... controller, 58 ......... flow control Valve, 60 ......... thermometer, 62 ......... flow meter.

Claims (5)

  In an exhaust gas treatment method for treating exhaust gas containing sulfur oxide discharged from a boiler in the order of a dry electrostatic precipitator, a cooling device, a wet desulfurization device, and a wet electrostatic precipitator, the exhaust gas is cooled as the cooling device. An exhaust gas treatment method using a heat exchanger that is indirectly cooled by air and controlling a supply amount of cooling air in the cooling device so that a current density in the wet electrostatic precipitator is maximized .   The exhaust gas treatment method according to claim 1, wherein the cooling air has a temperature of 100 ° C or higher.   In an exhaust gas treatment apparatus that treats exhaust gas containing sulfur oxide discharged from a boiler in the order of a dry electrostatic precipitator, a cooling device, a wet desulfurization device, and a wet electrostatic precipitator, the cooling device indirectly processes the exhaust gas. The amount of cooling in the cooling device is controlled by the controller so that the indicated value of the charging ammeter provided in the high-voltage power supply of the wet electrostatic precipitator is maximized. A featured exhaust gas treatment device.   A gas cooler that indirectly cools the exhaust gas with cooling air is disposed at the front stage of the dry electrostatic precipitator, and the exhaust gas discharged from the wet electrostatic precipitator is heated at the rear stage of the wet electrostatic precipitator. The exhaust gas treatment apparatus according to claim 3, wherein a heater for indirectly heating with air is disposed.   The cooling air heated by the heat exchange in the gas cooler is used as the heated air supplied to the heater, and the heated air cooled by the heat exchange in the heater is used to indirectly cool the exhaust gas in the cooling device. The exhaust gas treatment apparatus according to claim 4, wherein the exhaust gas treatment apparatus can be used as a cold heat source.

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