JP2012143699A - Exhaust gas treating system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress decrease in desulfurization performance and mercury removal performance of a desulfurization apparatus with a simple constitution while suppressing an equipment cost.SOLUTION: An exhaust gas treating system is equipped with a boiler 1 which burns fuel with combustion gas obtained by mixing oxygen-rich gas and circulation exhaust gas, a dust collector 5 which is disposed at a first flue 13 where the exhaust gas exhausted from the boiler 1 flows, a wet desulfurization apparatus 7 in which the exhaust gas flowing in the flue at a downstream side of the dust collector 5 is guided and is subjected to desulfurization treatment, an exhaust gas recirculation duct 17 which guides a part of the exhaust gas extracted from the flue by branching the flue at a downstream side of the dust collector 5 to the boiler 1, and a COseparation means 9 which separates COby compressing the exhaust gas flowing in the flue at a downstream side of the desulfurization apparatus 7. Water separated at the process for compressing the exhaust gas by the COseparation means 9 is supplied to absorption liquid which is used by being circulated in the desulfurization apparatus 7.

Description

  The present invention relates to an exhaust gas treatment system, and more particularly to an exhaust gas treatment technology for a plant including a boiler that burns fuel using a combustion gas in which an oxygen-rich gas and a circulating exhaust gas are mixed.

In recent years, an oxyfuel combustion system using an oxyfuel boiler (hereinafter abbreviated as “boiler”) has attracted attention as a technique for reducing carbon dioxide (CO ₂ ) emission, which is one of the causes of global warming. . In this system, oxygen rich oxygen as an oxidant, that is, high-concentration oxygen gas (hereinafter abbreviated as oxygen) is supplied to the boiler along with fossil fuels such as coal, so the exhaust gas discharged from the boiler is composed of water and carbon dioxide. Becomes the main component. For this reason, carbon dioxide can be easily separated from the exhaust gas by directly compressing the exhaust gas.

The exhaust gas discharged from the boiler of the oxyfuel combustion system is guided to the dust collector after the gas temperature is lowered by the heat exchanger installed in the flue downstream of the boiler, and the dust component is removed. The exhaust gas that has passed through the dust collector is guided to a desulfurization apparatus to remove SO _2, and then guided to a CO ₂ separation means to be compressed, whereby liquid carbon dioxide is separated from the exhaust gas.

  On the other hand, when the fuel is burned with oxygen in the boiler, since the combustion temperature becomes very high, a circulation line is provided for leading a part of the exhaust gas extracted by branching the flue on the downstream side of the dust collector to the boiler, A method of lowering the combustion temperature by supplying circulating exhaust gas flowing through this circulation line to a boiler together with oxygen is disclosed (for example, see Patent Document 1).

Moreover, as a desulfurization apparatus used in an oxyfuel combustion system, an absorbing liquid is sprayed from a spray nozzle toward the flow of exhaust gas rising in the absorption tower, and SO ₂ and mercury in the exhaust gas are absorbed by the absorbing liquid. A wet desulfurization apparatus that separates from exhaust gas is widely used. In this type of desulfurization apparatus, a circulation system is employed in which the absorption liquid stored in the bottom of the absorption tower is pumped up and sprayed again from the spray nozzle.

JP 2009-270753 A

  In such an oxyfuel combustion system, when exhaust gas is introduced into the desulfurization apparatus, the moisture in the absorption liquid evaporates due to the heat of the exhaust gas (for example, 90 ° C. to 200 ° C.). Is taken away and the temperature is reduced. However, since there is generally no means for escaping heat outside the absorption tower, the temperature of the exhaust gas flowing in the absorption tower varies depending on the moisture concentration of the exhaust gas introduced into the absorption tower. For example, during normal air combustion operation, since the moisture concentration of the exhaust gas introduced into the absorption tower is as low as about 10%, the temperature of the exhaust gas flowing through the absorption tower is reduced to about 55 ° C., whereas oxygen During the combustion operation, the exhaust gas introduced into the absorption tower does not contain nitrogen in the air, and the moisture concentration is as high as 30 to 35%. Therefore, the temperature of the exhaust gas flowing through the absorption tower is reduced only to about 70 ° C. Not.

  Thus, when the exhaust gas temperature in the absorption tower becomes high, the temperature of the desulfurization region where the sprayed absorption liquid and the exhaust gas contact, and the temperature of the absorption liquid rise, and the desulfurization performance and mercury removal characteristics of the desulfurization apparatus. Will drop. In order to avoid such a phenomenon, a method of enlarging the desulfurization device or separately providing a mercury removal device or the like on the downstream side of the desulfurization device can be considered, but there is a problem that the equipment cost becomes high.

On the other hand, as the CO ₂ separation means used in the oxyfuel combustion system, after the exhaust gas is pressurized, the heat in the exhaust gas is removed by removing heat with a heat exchanger, and the exhaust gas from which the moisture is separated is further pressurized. A method of separating liquid carbon dioxide by removing heat is employed. Therefore, a large amount of moisture from the heat exchanger, i.e. drain water is generated, the exhaust gas treated with the desulfurization apparatus, since a small amount of SO ₂ remaining, a small amount of SO ₂ to drain water include. For this reason, since wastewater treatment is required, there is a problem that the system becomes complicated and the equipment cost becomes high.

  The present invention has been made in view of such circumstances, and provides an exhaust gas treatment system that is capable of suppressing a decrease in desulfurization performance and mercury removal characteristics of a desulfurization apparatus with a simple configuration and suppressing facility costs. This is the issue.

FIG. 4 is a diagram showing the relationship between the temperature of the desulfurization region in the absorption tower and the SO ₂ removal rate in the exhaust gas when the exhaust gas is introduced into the desulfurizer from the boiler during the oxyfuel combustion operation. Here, a desulfurization area | region means the area | region where the sprayed absorption liquid and exhaust gas contact. From this figure, it can be seen that the SO ₂ removal rate decreases as the temperature of the desulfurization region increases. This is presumably because the solubility of SO _{2 in} the absorbing solution decreases due to an increase in temperature.

  FIG. 5 shows the results of investigating the relationship between the temperature of the desulfurization region in the absorption tower and the mercury removal rate in the exhaust gas when the exhaust gas is introduced into the desulfurizer from the boiler during the oxyfuel combustion operation. From this figure, it can be seen that the mercury removal rate decreases as the temperature of the desulfurization region increases. This is thought to be because the mercury oxide once absorbed in the absorption liquid in the absorption tower was reduced to metallic mercury in the liquid droplets of the absorption liquid and re-released into the exhaust gas.

Therefore, in the exhaust gas treatment system of the present invention, in order to solve the above problems, a boiler that burns fuel with a combustion gas in which oxygen-rich gas and circulating exhaust gas are mixed, and a flue through which the exhaust gas discharged from the boiler flows. The installed dust removal device, a wet desulfurization device that introduces desulfurization treatment by introducing exhaust gas flowing through the flue downstream of the dust removal device, and a flue downstream from the dust removal device are branched and extracted from the flue. In an exhaust gas treatment system comprising an exhaust gas circulation passage for leading a part of exhaust gas to a boiler, and a CO ₂ separation means for compressing exhaust gas flowing through a flue downstream of a desulfurization apparatus and separating carbon dioxide, CO ₂ separation The water separated in the process of compressing the exhaust gas of the means is supplied to the absorbent used by being circulated in the wet desulfurization apparatus.

That is, since the exhaust gas generated from the oxyfuel combustion boiler contains more moisture than the exhaust gas generated from the air combustion boiler, more water is recovered by the CO ₂ separation means. Here, the moisture recovered in the CO ₂ separation means is drain water generated by condensation of water vapor in the exhaust gas, and the temperature of this drain water is lower than the temperature of the exhaust gas introduced into the separation means, It is collected at. Therefore, by supplying such drain water to the absorption liquid in the wet desulfurization apparatus, the temperature of the absorption liquid can be lowered and the temperature in the desulfurization apparatus, that is, the temperature of the desulfurization region can be kept low. Thereby, the fall of desulfurization performance and a mercury removal rate can be suppressed, without enlarging a desulfurization apparatus. Further, by supplying the drain water collected by the CO ₂ separation means to the absorbing liquid in the desulfurization apparatus, SO ₂ remaining and dissolved in the drain water can be separated in the desulfurization apparatus. Thereby, since the fall of desulfurization performance is suppressed and the drainage waste water treatment equipment becomes unnecessary, simplification of a system can be achieved and equipment cost can be reduced.

More specifically, the wet desulfurization apparatus includes a spray nozzle that sprays the absorbing liquid toward the exhaust gas rising in the absorption tower, an absorption tower bottom that collects and stores the absorbing liquid sprayed from the spray nozzle, A circulation pump for supplying the absorption liquid stored in the bottom of the absorption tower to the spray nozzle is provided, and the water separated by the CO ₂ separation means is supplied to the bottom of the absorption tower.

Further, instead of supplying the water separated by the CO ₂ separation means to the bottom of the absorption tower, the water is supplied to another spray nozzle disposed above the spray nozzle, and the water is sprayed from the other spray nozzle. You may comprise.

  That is, the re-release of mercury from the absorption liquid occurs in the process of mercury in the absorption liquid flying as a droplet in the absorption tower. For this reason, mercury can be prevented from being re-released from the droplets of the absorbing liquid by injecting moisture separated by the separation means into the exhaust gas from the upper spray nozzle toward the exhaust gas. It is possible to suppress a decrease in the removal rate.

  According to the exhaust gas treatment system of the present invention, even when the amount of moisture in the exhaust gas is high at the time of oxyfuel operation, the desulfurization performance and mercury removal characteristics of the desulfurization apparatus are reduced with a simple configuration and reduced equipment costs. Can be suppressed.

1 is a schematic system diagram of an exhaust gas treatment system to which the present invention is applied. It is a block diagram of the desulfurization tower in the exhaust gas processing system to which this invention is applied. It is a schematic system diagram of the separation means in the exhaust gas treatment system to which the present invention is applied. It is a graph showing the relationship between the temperature and the SO ₂ removal rate in the exhaust gas of the desulfurization region in the absorption tower of the desulfurization apparatus. It is a figure which shows the relationship between the temperature of the desulfurization area | region in the absorption tower of a desulfurization apparatus, and the mercury removal rate in waste gas. It is a schematic system diagram of other embodiments of an exhaust gas treatment system to which the present invention is applied.

  Hereinafter, an embodiment of an exhaust gas treatment system to which the present invention is applied will be described with reference to FIGS. 1 to 3. In addition, although the example which uses pulverized coal as a fossil fuel burned with a boiler is demonstrated in the exhaust gas treatment system of this embodiment, it is not limited to this example.

The exhaust gas treatment system of the present embodiment includes a boiler 1, a heat exchanger 3, a dust collector 5, a desulfurization device 7, and CO ₂ separation means 9. The boiler 1 is provided with a burner (not shown), and a fuel supply path 11 for supplying pulverized coal fuel is connected to the burner. A first flue 13 through which exhaust gas flows is connected to the outlet of the boiler 1, and a heat exchanger 3, a dust collector 5, and a desulfurization device 7 are disposed in the middle of the first flue 13 from the upstream side. , CO ₂ separation means 9 are arranged in order.

  A distributor 15 is provided in the first flue 13 connecting the dust collector 5 and the desulfurizer 7. One end of an exhaust gas recirculation duct 17 for extracting a part of the exhaust gas flowing through the first flue 13 is connected to the distributor 15. The other end of the exhaust gas recirculation duct 17 is connected to the exhaust gas mixer 19 provided in the fuel supply path 11 communicating with the burner via the heat exchanger 3. Between the distributor 15 of the exhaust gas recirculation duct 17 and the heat exchanger 3, a circulation fan 21 for adjusting the flow rate of the circulating exhaust gas flowing through the exhaust gas recirculation duct 17 is provided. By adjusting the number so that the circulation amount of the circulating exhaust gas is 70 to 80% of the entire exhaust gas amount, it is possible to adjust to a gas temperature equivalent to that during air combustion operation. The exhaust gas recirculation duct 17 may be branched and connected to the first flue 13 on the downstream side of the desulfurizer 7.

  The fuel supply path 11 is supplied with pulverized coal pulverized to a predetermined particle size by a coal pulverization mill (not shown) along with the carrier gas. An upstream side of the exhaust gas mixer 19 in the fuel supply path 11 is provided with an oxygen mixer 23 to which an oxygen supply path is connected. In the oxygen mixer 23, high-concentration oxygen separated from air is supplied. It is supplied so that pulverized coal fuel and oxygen are mixed. The exhaust gas mixer 19 is supplied with the circulating exhaust gas flowing through the exhaust gas recirculation duct 17 where the pulverized coal fuel, oxygen and the circulating exhaust gas are mixed.

  The heat exchanger 3 exchanges heat between the exhaust gas discharged from the boiler 1 and the circulating exhaust gas flowing through the exhaust gas recirculation duct 17. Thereby, the circulating exhaust gas flowing through the exhaust gas recirculation duct 17 is heated to a predetermined temperature through heat exchange with the exhaust gas discharged from the boiler 1.

  As shown in FIG. 2, the desulfurization apparatus 7 is provided with an exhaust gas introduction port below the absorption tower 25, and the first flue 13 a is connected to the introduction port. A discharge port is provided, and the first flue 13b is connected to the discharge port. In the height region between the introduction port and the discharge port, spray nozzles 27 for spraying the absorbing liquid from above to the exhaust gas rising in the absorption tower 25 are arranged in a plurality of stages. At a position lower than the introduction port, a bottom portion 29 for collecting and storing the absorbent sprayed from the spray nozzle 27 is formed. The absorbing liquid 31 stored in the bottom 29 is pumped up by a circulation pump 33 and supplied to each spray nozzle 27.

The CO ₂ separation means 9 is configured such that the first compressor 35, the heat exchanger 37, and the second compressor 39 are connected in order by a pipe so that moisture and carbon dioxide are sequentially separated in the process of compressing exhaust gas. It has become. The heat exchanger 37 is configured by arranging a heat transfer tube 43 through which cooling water flows in a container 41 into which exhaust gas is introduced. One end of a drainage supply duct 45 for extracting moisture separated from the exhaust gas from the container 41 is connected to the container 41 of the heat exchanger 37, and the other end is connected to the bottom 29 of the desulfurization apparatus 7. In addition, a circulation pump 47 is provided in the drainage supply duct 45.

  Next, the operation of the exhaust gas treatment system configured as described above will be described.

  The boiler 1 is supplied with pulverized coal, oxygen and circulating exhaust gas, and the pulverized coal is combusted, and high-temperature and high-pressure steam is generated from a steam generator (not shown) by this combustion heat. This steam is supplied to a steam turbine power generation facility (not shown) to generate power.

  On the other hand, the exhaust gas discharged from the boiler 1 passes through the first flue 13 and is led to the heat exchanger 3 to be reduced in temperature (for example, about 160 ° C. to 200 ° C.). The exhaust gas exiting the heat exchanger 3 is guided to the dust collector 5 to remove the dust component contained in the exhaust gas.

The exhaust gas exiting the dust collector 5 is introduced into the absorption tower 25 of the desulfurization device 7 and comes into contact with the absorbing liquid sprayed from the spray nozzle 27 when rising in the absorption tower 25. Thereby, SO ₂ and mercury contained in the exhaust gas are stored in the bottom 29 of the absorption tower 25 in a state of being absorbed by the absorption liquid. Here, SO ₂ absorbed in the absorbing solution becomes H ₂ SO ₃ and further reacts with CaCO ₃ in the absorbing solution to become CaSO ₃ . Then, the oxidizing air is supplied into the absorbing liquid 31 stored in the bottom 29, whereby CaSO ₃ is oxidized to CaSO ₄ and separated from the absorbing liquid 31. On the other hand, mercury absorbed in the absorption liquid becomes mercury oxide in the absorption liquid and is separated from the absorption liquid 31 stored in the bottom 29. As a result, the absorption liquid 31 at the bottom 29 where CaSO ₄ and mercury oxide are separated is pumped up by the circulation pump 33, supplied to the spray nozzle 27, and sprayed again into the absorption tower 25.

Subsequently, the exhaust gas exiting the desulfurization device 7 is guided to the CO ₂ separation means 9. In the separation device 9, first, the exhaust gas is compressed by the first compressor 35 (for example, pressurized at about 1.5 MPa), and the heat generated by the compression is removed by the heat transfer tube 43 of the heat exchanger 37. It is. By this heat removal, the exhaust gas is cooled to about room temperature, and most of the water contained in the exhaust gas is condensed and liquefied to be separated into drain water. The exhaust gas from which moisture has been separated is further compressed by the second compressor 39 to separate liquefied carbon dioxide. The separated liquid carbon dioxide is stored in a carbon dioxide storage (not shown). The exhaust gas that has exited the CO ₂ separation means 9 is released into the atmosphere from a chimney (not shown).

  By the way, the exhaust gas introduced into the absorption tower 25 is deprived of heat to a predetermined temperature because heat is taken away when the moisture of the absorption liquid evaporates by the heat. On the other hand, since there is no means for releasing heat to the outside in the absorption tower 25, the exhaust gas temperature in the absorption tower 25 changes depending on the moisture concentration of the exhaust gas introduced into the absorption tower 25. For example, during normal air combustion operation, the moisture concentration of the exhaust gas discharged from the boiler is as low as about 10%, so the temperature is reduced to about 55 ° C. in the absorption tower 25. On the other hand, during the oxyfuel combustion operation, the exhaust gas discharged from the boiler does not contain nitrogen and has a high water concentration of 30 to 35%, so that the temperature is reduced only to about 70 ° C. in the absorption tower 25. Thus, when the exhaust gas temperature in the absorption tower 25 rises, the temperature of the desulfurization region where the absorption liquid sprayed from the spray nozzle 27 and the exhaust gas contact and the temperature of the absorption liquid rise, and as a result, the desulfurization device 7. There is a risk that the desulfurization performance and mercury removal characteristics of the steel will deteriorate (FIGS. 4 and 5).

On the other hand, in this embodiment, the drain water separated by the CO ₂ separation means 9 is extracted from the heat exchanger 37 by the circulation pump 47 and supplied to the bottom 29 of the absorption tower 25 through the drainage supply duct 45. I have to. Here, since the drain water is recovered in a state of approximately 20 ° C., the drain water at 20 ° C. is circulated in the absorption tower 25 by supplying the absorption liquid 31 stored in the bottom 29 of the absorption tower 25. The temperature of the absorbing liquid can be lowered. And since the exhaust gas which flows through the inside of the absorption tower 25 is temperature-reduced because the temperature of absorption liquid falls, it becomes possible to reduce the temperature of a desulfurization area | region to 50 degreeC, for example. Therefore, according to the present embodiment, the desulfurization performance and the mercury removal performance of the desulfurizer 7 are equivalent to those in the air combustion operation even when the moisture concentration in the exhaust gas is high, such as the exhaust gas generated from the boiler during the oxyfuel combustion operation. Can be maintained to a degree. It should be noted that the concentration of the absorbing solution gradually decreases when drain water is supplied into the absorption tower 25. In this regard, for example, the chemical in the absorbing solution is maintained so as to keep the concentration of the absorbing solution within a predetermined range. By adding periodically, the concentration can be kept constant.

In the present embodiment, the drain water separated by the CO ₂ separation means 9 is supplied to the absorption tower 25, so that SO _{2 and the} like dissolved in the drain water can be separated in the absorption tower 25. This eliminates the need for a condensate wastewater treatment facility, which simplifies the system and reduces facility costs.

Moreover, in this embodiment, since it is only necessary to add the drainage supply duct 45 that supplies the drain water separated by the CO ₂ separation means 9 to the bottom 29 of the absorption tower 25 in the existing exhaust gas treatment system, Expansion costs can be reduced.

In the present embodiment, the drain water separated by the CO ₂ separation means 9 is supplied to the bottom 29 of the absorption tower 25, but the supply destination of the drain water is not limited to this example. As shown in FIG. 6, you may make it supply to the spray nozzle 49 only for the drain water installed above the spray nozzle 27 in the absorption tower 25. As shown in FIG.

  The desulfurization apparatus 7 in FIG. 6 has a spray nozzle 27 that sprays an absorbing liquid from above on exhaust gas rising in the absorption tower 25 in a height region between an inlet and an outlet, and above the spray nozzle 27. The spray nozzle 49 which sprays drain water from the top with respect to exhaust gas is provided. Only the drain water is supplied to the spray nozzle 49. The spray nozzles 27 and 49 may be provided in a plurality of stages, but the spray nozzle 49 is always disposed above the spray nozzle 27.

  Even in this configuration, the temperature of the exhaust gas flowing in the absorption tower 25 is reduced by the drain water injected from the spray nozzle 49 and the temperature of the desulfurization region is reduced to, for example, 50 ° C. The desulfurization performance in can be equal to or higher than that in air combustion operation. Further, since drain water not containing mercury is injected from the spray nozzle 49, the region where the liquid droplet of the absorbing liquid injected from the spray nozzle 27 floats can be surrounded by the drain water droplet. Thereby, it is possible to suppress the re-release of mercury from the droplets of the absorbing liquid containing mercury, so that the mercury removal rate can be equal to or higher than that in the air combustion operation.

1 boiler 3 heat exchanger 5 dust collector 7 desulfurizer 9 CO ₂ separator 13 the first flue 17 exhaust gas recirculation duct 21 circulation fan 25 absorption towers 27,49 spray nozzle 29 bottom 31 absorption liquid 33, 47 circulating pump 35 First compressor 37 Heat exchanger 39 Second compressor 45 Drain supply duct

Claims (3)

A boiler for burning fuel with a combustion gas in which oxygen-rich gas and circulating exhaust gas are mixed; a dust removing device disposed in a flue through which the exhaust gas discharged from the boiler flows; and a flue downstream of the dust removing device A wet desulfurization device that introduces exhaust gas flowing through the exhaust gas, desulfurization treatment, a flue gas on the downstream side of the dust removal device and a part of the exhaust gas extracted from the flue to the boiler, the exhaust gas circulation passage, in the exhaust gas treatment system having an exhaust gas that flows on the downstream side of the flue of the desulfurizer compresses the CO ₂ separation means for separating carbon dioxide, water separated in the process of compressing the exhaust gas of the CO ₂ separation means Is supplied to an absorption liquid that is circulated and used in the wet desulfurization apparatus. The wet desulfurization apparatus includes a spray nozzle for spraying an absorbing liquid toward exhaust gas rising in the absorption tower, an absorption tower bottom for collecting and storing the absorbing liquid sprayed from the spray nozzle, and the absorption tower bottom A circulation pump for supplying the absorption liquid stored in the spray nozzle to the spray nozzle,
The exhaust gas treatment system according to claim 1, wherein the water separated by the CO ₂ separation means is configured to be supplied to the bottom of the absorption tower. The wet desulfurization apparatus includes a spray nozzle for spraying an absorbing liquid toward exhaust gas rising in the absorption tower, an absorption tower bottom for collecting and storing the absorbing liquid sprayed from the spray nozzle, and the absorption tower bottom A circulation pump for supplying the absorption liquid stored in the spray nozzle to the spray nozzle,
The exhaust gas treatment system according to claim 1, wherein the moisture separated by the CO ₂ separation means is sprayed from another spray nozzle disposed above the spray nozzle.

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