OXIDATION CONTROL FOR IMPROVED FLUE GAS DESULFURIZATION PERFORMANCE

Abstract

A system and method for controlling oxidation of sulfites in a slurry. The system includes a tank having an inlet for receiving a slurry used in wet flue gas desulfurization. The tank also includes an inlet for receiving a gas. The inlet for receiving the gas is configured so that at least a portion of the gas received in the tank is dispersed through at least a portion of the slurry received in the tank. A sensor is configured to measure a sulfite concentration S1 of the slurry received in the tank. In some embodiments, the sensor is a sulfite analyzer. In other embodiments, the sensor is a virtual analyzer. The system includes a controller. Software executing on the controller generates a signal indicative of an adjustment of a flow rate of gas into the tank based at least in part on the sulfite concentration S1.

Description

TECHNICAL FIELD

The present disclosure relates to a system and method for controlling a portion of a wet flue gas desulfurization process. More particularly, the present disclosure relates to a system and method for controlling the rate of oxidation of sulfites during a wet flue gas desulfurization process.

BACKGROUND

Gas-liquid contactors, also referred to as absorbers, are capable of removing substances such as gases and particulate matter from flue gases produced by power plants. Sulfur dioxide (SO₂), for example, is produced by the combustion of fossil fuels. Sulfur dioxide emissions are of a particular concern because they are known to be hazardous to the environment. Wet flue gas desulfurization is a process in which sulfur dioxide is removed from flue gas in the most common process, by contact with an alkaline slurry using a spray tower.

Wet flue gas desulfurization typically uses calcium-based slurries, or sodium-based or ammonia based solutions to contact flue gases and remove sulfur dioxide therefrom. A slurry is a mixture of solids and liquid. Examples of calcium-based slurries include limestone (calcium carbonate; CaCO₃) slurries and lime (calcium oxide; CaO) slurries. Such slurries react with acidic gases to form precipitates which can be collected for disposal or recycling. Contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, result in the absorption of at least a portion of the acidic gases by the slurry.

After the desulfurization process, the slurry is typically accumulated in a tank, where the sulfur dioxide reacts with water to produce sulfites (SO₃⁻²). The sulfites further react with oxygen coming from aeration air bubbled into the slurry to produce sulfates (SO₃⁻⁴). This process may be referred to as forced oxidation. Disassociation of the lime or limestone within the slurry provides calcium ions which react with the sulfates to produce gypsum (CaSO₄⁻²H₂O). The gypsum can be resold and used, for example, in the home construction industry. Forced oxidation of the slurry received in the tank by aeration ensures that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum.

A disadvantage of known aeration processes is that is difficult to control oxidation of the sulfite slurry received in the tank since it is difficult to measure or estimate the dissolved sulfite concentration. As a result, it is typical to provide more than a sufficient amount of air into the tank to ensure that generally all of the sulfite is oxidized. Providing the additional pressurized air can reduce the efficiency of the power plant because energy is required to generate the pressurized air. Another disadvantage with known systems is that providing excessive levels of air to the slurry oxidizes essentially all of the sulfites to form sulfates and any benefits arising from sulfites not oxidized is lost.

SUMMARY

According to aspects illustrated herein, a system for controlling oxidation of sulfites in a slurry is disclosed. The system includes a tank having an inlet for receiving a slurry used in wet flue gas desulfurization. The tank also includes an inlet for receiving a gas. The inlet for receiving the gas is configured so that at least a portion of the gas received in the tank is dispersed through at least a portion of the slurry received in the tank. A sensor is configured to measure a sulfite concentration S₁ of the slurry received in the tank. In some embodiments, the sensor is a sulfite analyzer. In other embodiments, the sensor is a virtual analyzer. The system includes a controller. Software executing on the controller generates a signal indicative of an adjustment of a flow rate of gas into the tank based at least in part on the sulfite concentration S₁.

According to other aspects illustrated herein, a method for controlling oxidation of sulfites in a slurry is disclosed. The method includes the steps of receiving a slurry in a tank, the slurry comprising one or more sulfites. A flow of gas is provided into the slurry received in the tank. Next, the sulfite concentration S₁ of the slurry received in the tank is measured. The flow rate of gas being provided to the slurry received in the tank is adjusted based at least in part on S₁.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of spray tower.

FIG. 2 is a cross-sectional view of a forced oxidation system for use with the spray tower shown in FIG. 1.

FIG. 3 is a cross-sectional view of a forced oxidation system for use with the spray tower shown in FIG. 1.

DESCRIPTION

In reference to FIG. 1, a spray tower 10 in accordance with the present disclosure is shown. The spray tower 10, also referred to as an absorber, is an example of a gas-liquid contactor. It should be understood that the present disclosure is not limited to a spray tower as other types of gas-liquid contactors can be employed. This disclosure, for example, can readily be applied to scrubbing structures, and operations which serve to remove undesirable gases, mist, dust, fumes, smoke and/or particulate matter from a stream of gas. Although the disclosed system and method are described in relation to a specific embodiment of a spray tower, the disclosed systems and methods are not limited in this regard. For example, the disclosed system and method can be adapted to account for variations in the design of the spray tower 10, the operating conditions of the spray tower, and the properties of the slurry being used in the spray tower, among other variables. Additionally, the disclosed system can be used with other types of absorbers, including, but not limited to, tray towers and fountain spray towers. To the extent specific dimensions, values, or specific operating conditions are included in this description, they are provided to broadly illustrate the system and method and are not intended to limit the scope of this disclosure.

The spray tower 10 is generally an upright structure defining a tower cavity 14. The tower cavity 14 is in fluid communication with an inlet 12 for receiving flue gases into the tower cavity 14. The flue gases may be generated, for example, during the combustion of one or more fossil fuels. The tower cavity 14 includes a first bank of spray headers 16 above the flue gas inlet 12. The first bank of spray headers 16 provides a spray 20 of an alkaline slurry into the tower cavity 14. The tower 10 includes a second bank of spray headers 18 above the first bank of spray headers 16. The second bank of spray headers 18 also provides a spray 22 of an alkaline slurry into the tower cavity 14. Although embodiment shown in FIG. 1 is disclosed as having a first row of spray headers 16 and a second row of spray headers 18, the present disclosure is not limited in this regard, and the number of spray headers, and rows thereof, in the tower 10 may vary for a given application.

During operation of the spray tower 10, alkaline slurry is sprayed from the first row of spray headers 16 and the second row of spray headers 18. At least a portion of that slurry flows downward in the tower cavity 14. It should be understood that the term flow, as used herein, can also refer to a drifting or settling action of an atomized slurry in the spray tower 14. As the slurry flows downward in the tower cavity 14 it contacts flue gas flowing upward in the tower cavity 14. The slurry is received in a tank 30 typically at the base of the spray tower 10. The tank 30 is in fluid communication with the first row of spray headers 16 and second row of spray headers 18 through one or more conduits 17. A pump 26, in fluid communication with the conduit 17 and the spray headers 16, 18 pumps slurry from the tank 30, through the one or more conduits 17, and to the first and second row of spray headers 16, 18. It should be understood that the present disclosure is not limited to this specific configuration, and that different configurations of the conduits 17, pump 26, and spray headers 16, 18 are possible.

During processing in the spray tower 10, sulfur dioxide in the flue gas will react with the calcium carbonate-rich slurry (limestone and water) to form calcium sulfite, thereby removing the sulfur dioxide, or at least a portion thereof, from the flue gas and forming a transformed slurry comprising calcium sulfite. The cleaned flue gas is exhausted from the spray tower 10 through an outlet 13. The resulting transformed slurry is received in the tank 30. The tank 30 and the spray tower 10 typically reside in a single structure, for example as shown in FIG. 1, with no physical separation between them. It should be understood that the present disclosure is not limited in this regard and the tank and the spray tower may reside, for example, in distinct structures.

FIG. 2 is a view of the tank 30 of the spray tower 10 of FIG. 1. The tank 30 receives slurry from the spray tower 10 as the transformed slurry flows downward. The tank 30 includes an inlet 31 for receiving the transformed slurry. In the embodiment shown in FIG. 2, the inlet 31 is defined by the cross-section of the tower cavity 14 at which the transformed slurry is received in the tank 30. It should be understood that the present disclosure is not limited in this regard. For example, in some embodiments the inlet may comprise a conduit that is in fluid communication with the tower cavity 14 and the tank 30.

The tank 30 includes an inlet 60 for receiving a gas. The inlet 60 is in fluid communication with a conduit 62 for providing the gas. In the embodiment shown in FIGS. 1-3, the inlet 60, 160 is located below a static fluid line of the slurry received by the tank 30. This configuration allows at least a portion of the gas received in the tank 30 to be dispersed through at least a portion of the slurry received in the tank. A source of pressurized gas (not shown in the FIGS.) is in fluid communication with the conduit 62. The source of gas may be, for example, a blower capable of providing air at a continuous pressure, or a variable pressure, above atmospheric pressure. The air comprises oxygen for oxidation of the sulfites in the slurry. The tank 30 further includes an agitator 36 for distributing the oxygen received through the inlet 60 and the slurry received in the tank 30. It should be understood that although the embodiments shown in FIGS. 1-3 include an agitator 36, 136, the disclosure is not limited in this regard and the disclosed system or method can be practiced without an agitator.

In reference to FIGS. 1-2, the system includes a controller 50. The term controller 50, as used herein, generally refers to one or more devices capable of executing software. As can be appreciated by a person of ordinary skill in the art, many different devices are available for use as the controller 50. For example, one or more processor based computers can be used as a controller 50. The controller 50 includes an interface 54 for inputting information indicative of a desired operating condition of the spray tower 10. For example, a predetermined sulfite content of the slurry in the spray tower 10 can by input in to the controller 50 via the interface 54. The interface 54 is in communication with the controller 50. Although the controller 50 and the interface 54 are disclosed as separate elements in FIGS. 1-2 and an interface is not shown in FIG. 3, the present disclosure is not limited in this regard. As can be appreciated by a person of ordinary skill in the art, the interface 54 may comprise a component of the controller 50. In yet other embodiments, it is not necessary to include an interface 54. In such embodiments, predetermined parameters can be programmed into the controller 50, or can be transmitted to the controller 50 by some other means.

The system further includes at least one sulfite sensor 80 for measuring a sulfite content of the slurry received in the tank 30. The sulfite sensor 80 is positioned in the slurry received in the tank 30. The sulfite sensor 80 is configured to measure the sulfite content of the slurry received in the tank 30. The sulfite sensor 80 transmits a signal indicative of the sulfite content of the slurry to the controller 50. Based on the received signal, software 52 executing on the controller 50 determines the sulfite content of the slurry received in the tank 30. The combination of the sulfite sensor 80 and the controller 50 can be referred to as a sulfite analyzer. In some embodiments, the sulfite analyzer can provide real time measurements of the sulfite content of the slurry received in the tank 30. In further embodiments, the sulfite analyzer can provide real time and continuous measures of the sulfite content of the slurry received in the tank 30.

Although FIGS. 1 and 2 disclose that the sulfite sensor 80 is positioned inside the slurry tank 30, the present disclosure is not limited in this regard. For example, in reference to FIG. 3, the sulfite sensor 180 may be positioned outside of the slurry tank 130. The sulfite sensor 180 is in communication with the tank 130 by a first conduit 182 and a second conduit 184. Slurry is pumped through the first conduit 182, past the sulfite analyzer 180, through the second conduit 184, and back into the tank 130. The slurry tank 130 includes an agitator 136. The embodiment shown in FIG. 3 also includes a controller 150, software 152 executing thereon, an inlet 160 and a conduit 162 for providing pressurized air to the slurry received in the tank 130 and a valve 170 for controlling the flow of air. A sulfite analyzer in accordance with the present invention is disclosed in European application EP 11184432.0 entitled, “Sulphite Sensor and Method for Measuring Sulphite Concentration in a Substance”. That document, and in the information disclosed therein, is incorporated by reference herein in its entirety. In some embodiments, a virtual sulfite analyzer is used to measure the sulfite content of the slurry received in the tank. An example of such a virtual analyzer is disclosed in U.S. Pat. No. 7,113,835 to Boyden et al., incorporated herein by reference.

During operation, the system controls the rate of oxidation of the sulfite in slurry received in the tank 30, 130 by adjusting the valve 70, 170 between the open and closed position to control the flow rate of oxygen into the slurry received in the tank 30, 130. As discussed above, a predetermined sulfite content is received in the controller 50, 150, for example via the interface 54, 154. The sulfite sensor 80, 180 measures a sulfite content S₁ in the slurry received in the tank. Software 52, 152 executing on the controller 50, 152 generates a signal indicative of an adjustment of the flow rate of gas into the tank 30 based in part on the sulfite content S₁. For example, if S₁ is less than the predetermined sulfite content, software 52, 152 executing on the controller generates a signal indicating that the valve should be adjusted open, thus providing more oxygen to the slurry and increasing the rate of oxidation thereof. Conversely, if S₁ is greater than the predetermined sulfite content, software 52, 152 executing on the controller 50, 150 generates a signal indicating that the valve 70, 170 should be adjusted closed, thus providing less oxygen to the slurry and decreasing the rate of oxidation. To the extent that the predetermined sulfite content is equivalent to S₁, software 150, 152 executing on the controller will generate a signal indicating that no adjustment is necessary to the valve 70, 170. After the signal is generated, it is transmitted to the valve 70, 170 which is then adjusted accordingly. In this way the disclosed system and method can precisely control the rate of oxidation of the slurry received in the tank 30, 130.

In some embodiments of the present disclosure, it is preferred that the predetermined oxidation rate in the tank 30, 130 is less than 100% of the sulfite contained in the slurry. It has been found that the presence of at least some sulfites in the slurry enhances the absorption of sulfur dioxide in the spray tower 10. Maintaining some level of sulfites in the slurry, the dissolved alkalinity of the slurry is enhanced, thereby increasing the removal of sulfur dioxide from the slurry by reacting sulfur dioxide with the sulfites to form bisulfite (HSO₃⁻). In turn, a more efficient removal of sulfur dioxide from the slurry is achieved. Such is likewise true to control manganese levels. Manganese present in the slurry is often introduced therein as a minor component of limestone supplied to the process as the main reagent for the slurry. Manganese may also be introduced into the slurry through a make-up water source. In such cases, manganese present in the slurry often precipitates and forms a scale on the spray tower 10 walls adding to the corrosion potential thereof. Further, manganese present in the slurry can precipitate as a dark solid species in the slurry solids. By controlling the predetermined oxidation rate in the tank 30, 130 to less than 100% of the sulfite contained in the slurry, any manganese present in the slurry is more favored to remain in the liquid phase as a dissolved species rather than in the solid phase as a dark solid species. Controlling the predetermined oxidation rate to maintain manganese in the liquid phase eliminates or diminishes manganese scaling on the spray tower 10 walls and reduces the related corrosion potential thereof. Also, by controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the slurry, manganese is maintained in the liquid phase thereby preventing dark manganese precipitate formation. Further, by preventing dark manganese precipitate formation, collection thereof along with the collection of the gypsum byproduct is prevented. Hence, controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the slurry prevents dark manganese precipitate formation thereby enhancing the whiteness/color of the gypsum byproduct, which is a desired and sometimes required attribute of the byproduct gypsum. Still another advantage of controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the slurry, is that under such process conditions selenium compounds are more favored in the selenite form or even as elemental selenium, rather than in the selenate form. Selenium compounds removed from the flue gas are largely found in the liquid phase of the slurry. As such, the selenium is present in the liquid phase of the slurry in selenate or selenite forms. Purge water from the spray tower 10 is sent to a waste water treatment system 33. The selenate form is more difficult to remove from the purge water with waste water treatment and often requires a costly biological treatment step, such as ABMet® treatment process commercially available from General Electric Company, Fairfield, Conn., USA, in order to meet required discharge limits. By controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the slurry, the selenite form is favored in the liquid phase of the slurry, which is relatively easy to remove from the spray tower 10 purge water in waste water treatment system 33. Hence, controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the slurry eliminates the need for a biological step during waste water treatment of the purge water and/or reduces the size of the biological treatment step and/or the remaining waste water treatment system needed to treat the purge water, thereby reducing the cost thereof. By maintaining a certain positive sulfite level in the spray tower 10 slurry, the speciation of the selenium present in the liquid phase favors the selenite form. In some cases, sulfite control can lower the oxidation reduction potential (ORP) level of the slurry enough to drive selenium present therein into its elemental form, which is a solid readily removed by waste water treatment. As a result, the waste water treatment system 33 used to treat the purge water from the spray tower 10 becomes less expensive in terms of both capital costs and operating costs. For this purpose, desirable sulfite levels in the slurry can be controlled and maintained using a sulfite sensor 80 and an analyzer, i.e., sulfite sensor 80 and controller 50. The concept of controlling the predetermined oxidation rate to less than 100% of the sulfite contained in the process slurry can be applied to spray towers 10 with varying chemistries including those using limestone, lime, sodium hydroxide, sodium carbonate, ammonia and seawater reagents.

In some embodiments, it is possible to increase dissolved sulfite concentrations in the slurry by addition of a cation, such as magnesium. Typically, the cation is added in an amount in excess of the inert anions in the slurry, such as chlorides, which accumulate with the cation instead of the sulfites. Increased levels of dissolved sulfite within the slurry can be permitted without causing limestone binding by adding salts of ammonia, sodium or magnesium to the slurry.

The present disclosure further relates to a method of controlling the rate of oxidation of sulfites in a slurry. The method includes the steps of receiving a slurry in a tank 30, the slurry comprising one or more sulfites. A gas flow is provided to the tank 30. Next, the sulfite concentration S₁ is measured in the slurry received in the tank 30. Is some embodiments, this step is performed using sulfite analyzer 80,50. In other embodiments, this step is performed using a virtual analyzer. Next, the gas flow into the tank 30 is adjusted based on S₁.

In summary, the subject disclosure provides a system for controlling oxidation of sulfites in a slurry, comprising a tank having an inlet for receiving a slurry and an inlet for receiving a gas, the inlet for receiving the gas being configured so that at least a portion of the gas received in the tank is dispersed through at least a portion of the slurry received in the tank, a sensor configured to measure a sulfite concentration S₁ of the slurry received in the tank, a controller, software executing on the controller for generating a signal indicative of an adjustment of a flow rate of gas into the tank based at least in part on the sulfite concentration S₁ to adjust oxidation rate to less than 100% of the sulfite contained in the slurry effective to reduce manganese precipitate formation and/or effective to reduce selenate selenium speciation therein. The signal of this system is further based on a predetermined sulfite concentration of the slurry. The gas in the system comprises oxygen. The slurry is used in a spray tower configured to perform wet flue gas desulfurization. The system sensor comprises a sulfite analyzer or a virtual analyzer. The tank further comprises an inlet for receiving magnesium.

In summary, the subject disclosure provides a method of controlling the rate of oxidation of sulfites in a slurry comprising the steps of receiving a slurry in a tank, the slurry comprising one or more sulfites, providing a flow of a gas into the tank, measuring a sulfite concentration S₁ of the slurry received in the tank, adjusting a flow rate of the gas into the tank based on S₁ to adjust oxidation rate to less than 100% of the sulfite contained in the slurry effective to reduce manganese precipitate formation and/or effective to reduce selenate selenium speciation therein. The adjustment of the flow rate of the method is further based on a predetermined sulfite concentration of the slurry. A sulfite analyzer is used in the method to measure the sulfite concentration S₁. The oxidation of sulfites in the slurry received in the tank is less than 100%, and effective in preventing manganese scaling. The method further comprises the step of adding a cation to the slurry received in the tank to increase the concentration of dissolved sulfites in the slurry received in the tank. The cation in the method comprises magnesium. The gas in the method comprises oxygen and the sensor comprises a virtual analyzer.

Although the present disclosure has been described with reference to certain embodiments thereof, it should be noted that other variations and modifications may be made, and it is intended that the following claims cover the variations and modifications within the true scope of the disclosure.

Claims

1. A system for controlling oxidation of sulfites in a slurry, comprising: a tank having an inlet for receiving a slurry and an inlet for receiving a gas, the inlet for receiving the gas being configured so that at least a portion of the gas received in the tank is dispersed through at least a portion of the slurry received in the tank;a sensor configured to measure a sulfite concentration S1 of the slurry received in the tank;a controller;software executing on the controller for generating a signal indicative of an adjustment of a flow rate of gas into the tank based at least in part on the sulfite concentration S1 to adjust oxidation rate to less than 100% of the sulfite contained in the slurry effective to reduce manganese precipitate formation and/or effective to reduce selenate selenium speciation therein.

2. The system of claim 1, wherein the signal is further based on a predetermined sulfite concentration of the slurry.

3. The system of claim 2, wherein the gas comprises oxygen.

4. The system of claim 3, wherein the slurry is received from a spray tower configured to perform wet flue gas desulfurization.

5. The system of claim 4, wherein the sensor comprises a sulfite analyzer.

6. The system of claim 4, wherein the sensor comprises a virtual analyzer.

7. The system of claim 1, wherein the tank further comprising an inlet for receiving magnesium.

8. A method of controlling the rate of oxidation of sulfites in a slurry comprising the steps of: receiving a slurry in a tank, the slurry comprising one or more sulfites;providing a flow of a gas into the tank;measuring a sulfite concentration S1 of the slurry received in the tank;adjusting a flow rate of the gas into the tank based on S1 to adjust oxidation rate to less than 100% of the sulfite contained in the slurry effective to reduce manganese precipitate formation and/or effective to reduce selenate selenium speciation therein.

9. The method of claim 8, wherein the adjustment of the flow rate of the gas is further based on a predetermined sulfite concentration of the slurry.

10. The method of claim 9, wherein a sulfite analyzer is used to measure the sulfite concentration S1.

11. The method of claim 10, wherein the oxidation of sulfites in the slurry received in the tank is less than 100%, and effective in preventing manganese scaling.

12. The method of claim 11, further comprising the steps of: adding a cation to the slurry received in the tank,wherein the added cation increases the concentration of dissolved sulfites in the slurry received in the tank.

13. The method of claim 12, wherein the cation comprises magnesium.

14. The method of claim 13, wherein the gas comprises oxygen.

15. The method of claim 8, wherein the sensor comprises a virtual analyzer.