WO1996014137A1

WO1996014137A1 - Forced oxidation system for a flue gas scrubbing apparatus

Info

Publication number: WO1996014137A1
Application number: PCT/US1995/013738
Authority: WO
Inventors: Dennis James Laslo
Original assignee: General Electric Company
Priority date: 1994-11-08
Filing date: 1995-10-23
Publication date: 1996-05-17
Also published as: TR199501383A2; IL115846A0; TW276189B

Abstract

A forced oxidation system is provided for a spray tower which employs an alkaline slurry to remove gases and particulate matter from flue gases produced by utility and industrial facilities. The forced oxidation system operates to control the composition of the slurry in order to enhance the efficiency of the scrubbing operation. In the scrubbing operation, the slurry absorbs sulfur dioxide gas which is then reacted within the slurry to form sulfites. The forced oxidation system includes a device for delivering an oxygen-containing gas to the tank for the purpose of oxidizing the sulfites present in the slurry. In addition, the tank is equipped with a device for detecting the degree of oxidation of the slurry within the tank. The detecting device produces an output signal which is relayed to a device for regulating the flow of oxygen-containing gas to the tank, so as to closely regulate the degree to which the sulfites in the slurry are oxidized.

Description

FORCED OXIDATION SYSTEM FOR A FLUE GAS SCRUBBING APPARATUS

This invention generally relates to gas- liquid contactors used in the removal of particulate matter and gases from utility and industrial flue gases. More particularly, this invention is directed to a gas-liquid contactor of the type used in wet flue gas desulfurization processes, wherein a controllable forced oxidation system is employed to achieve a desired level of oxidation of the contact liquid, so as to minimize the operation costs of the oxidation system. Background of the Invention

Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (S0₂) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and their emission into the atmosphere is closely regulated by clean air statutes. The method by which such gases are removed with a spray tower or other type of gas-liquid contactor is known as wet flue gas desulfurization.

The cleansing action produced by a gas- liquid contactor is generally derived from the passage of gas upwardly through a tower countercurrently to a descending liquid which cleans the air. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia- based solutions. As used herein, a slurry is a mixture of solids and liquid in which the solids content can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaC0₃) slurries and hydrated lime (calcium hydroxide; Ca(OH)₂) slurries formed by action of water on lime (calcium oxide; CaO) . Such slurries react with the acidic gases to form precipitates which can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF) , result in the absorption of the gases by the slurry.

Thereafter, the slurry is typically accumulated in a tank, where the sulfur dioxide reacts with water to produce sulfites (S0₃ ) , which further reacts with oxygen present in the slurry to produce sulfates (S0₄ ) . Dissociation of the lime or limestone within the slurry provides calcium ions which react with the sulfates to produce gypsum (CaS0₄*2H₂0) as a saleable byproduct of the scrubbing operation. Forced oxidation of the slurry by aeration is conventionally employed to ensure that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum. A spray tower 10 shown in cross-section in Figure 4 is an example of a known type of gas-liquid contactor. The spray tower 10 generally is an upright structure composed of a tower 14 equipped with an inlet duct 12 through which flue gases enter the tower 14. Immediately above the inlet duct 12 is a lower bank of spray headers 16 which introduce a spray 20 of an alkaline slurry into the tower 14. A second, upper bank of spray headers 18 is typically provided above the lower bank of spray headers 16, with additional banks of spray headers being used as may required for a given application. A pump 26 serves to recycle the alkaline slurry by pumping the slurry from a tank 30 to the banks of spray headers 16 and 18. Intimate contact between the alkaline slurry spray 20 and the flue gases rising through the tower 14 results in a cleansing action, by which the slurry and the entrapped or reacted gases are collected at the bottom of the tower 14 in the tank 30. The cleansed gases which continue to rise through the tower 14 then typically pass through a mist eliminator 22, and thereafter are either heated or passed directly to the atmosphere through a chimney 24.

Figure 2 illustrates known techniques by which forced oxidation of the slurry 32 within the tank 30 is achieved. Conventionally, a blower 34 of any suitable type is employed to inject air into the tank 30 through a pipe 44. Other types of known aeration devices include spargers, air lanced agitators and aspirators. The type of aeration device used determines the air bubble size, which effects the oxygen transfer characteristics. Agitation of the slurry 32 is often provided with a fan 36 emersed in the slurry 32, which assists in distributing and dissolving the oxygen in the slurry 32.

While variations of the above are known. Figure 2 is useful in illustrating the fundamental aspects of forced oxidation systems which serve to ensure that essentially all sulfites present in the slurry are converted to sulfates, so as to allow the production of gypsum as a useful byproduct of the scrubbing operation. Such oxidation systems must be configured for maximum operating conditions, such as maximum flue gas flow rates and maximum concentrations of sulfur dioxide in the flue gases. Because the exact amount of air required to convert the sulfites to sulfates is difficult to determine, the air flow rate is typically increased by some factor. While gas-liquid contactors configured as described above generally perform satisfactorily, the efficiency of the scrubbing operation declines if low flue gas flow rates exist or if low sulfur dioxide concentrations are present in the flue gases, because the air flow rate provided will be excessive for such conditions. As a result, power is wasted in operating the oxidation system at an unnecessarily high level.

Another disadvantage with present forced oxidation systems is that providing excessive levels of air to the slurry oxidizes essentially all of the sulfites to form sulfates. However, if some level of sulfites could be maintained in the slurry, the dissolved alkalinity of the slurry would be enhanced, thereby increasing the removal of sulfur dioxide from the slurry by reacting sulfur dioxide with the sulfites to form bisulfite (HS0₃~) . In turn, more efficient removal of sulfur dioxide from the slurry would enhance the efficiency of the scrubbing operation. Any improvement in the efficiency of the scrubbing operation is significant due to the poor solubility of limestone in water, necessitating large amounts of slurry in order to adequately remove sulfur dioxide from the flue gases. Accordingly, it would be desirable if a flue gas scrubbing apparatus were available in which oxidation of the sulfites within the slurry could be precisely controlled in order to advantageously regulate the scrubbing operation, such as by enhancing the efficiency of sulfur dioxide removal from the slurry and/or by minimizing the amount of air delivered to the slurry to not more than that necessitated by an existing operating condition. Summary of the Invention It is an object of this invention to provide a flue gas scrubbing apparatus which utilizes a slurry to remove sulfur dioxide from flue gases produced by utility and industrial facilities.

It is a further object of this invention that such a scrubbing apparatus be equipped with a forced oxidation system for oxidizing sulfites which form in the slurry.

It is another object of this invention that the dissolved sulfite concentration in the slurry be monitored to enable precise control of the oxidation system, so as to promote the efficiency of the oxidation system.

It is still a further object of this invention that the oxidation system be controlled so as to further enhance the efficiency of the scrubbing operation by precisely controlling the oxidation level within the slurry.

The present invention provides a gas-liquid contactor for removing gases and particulate matter from flue gases produced by processing operations of the type carried out in utility and industrial plants. In particular, the gas-liquid contactor utilizes an alkaline slurry to absorb acidic gases and particulate matter in the flue gases, wherein the composition of the slurry is controlled to enhance the efficiency of the scrubbing operation.

The gas-liquid contactor is generally composed of a chamber or tower having a tank located near its lower end. The tank serves as a reservoir for an alkaline slurry which is employed to remove sulfur dioxide and particulate matter from flue gases forced through the tower. The tower is further equipped with an inlet through which the flue gases enter the tower. The slurry is sprayed into the tower such that intimate contact between the slurry and the flue gases results in the absorption of particulate matter and acidic gases, and particularly sulfur dioxide, from the flue gases, so as to yield a cleansed flue gas. The slurry is then returned to the tank, where absorbed sulfur dioxide reacts with water in the slurry to form sulfites.

In accordance with this invention, the tank is equipped with a forced oxidation system which includes a device for delivering an oxygen-containing gas, such as air, to the tank for the purpose of oxidizing the sulfites present in the slurry. In addition, the tank is equipped with a device for detecting the dissolved sulfite concentration of the slurry within the tank. The detecting device produces an output signal which is relayed to a device for regulating the flow of oxygen-containing gas to the tank.

In accordance with this invention, the interrelationship between the dissolved sulfite concentration of the slurry and the degree of oxidation which occurs in the slurry enables precise control of the device which delivers the oxygen- containing gas to the tank, so as to closely regulate the degree to which the sulfites in the slurry are oxidized.

Therefore, a significant advantage of the present invention is that control of the amount of oxygen-containing gas delivered to the tank enables the scrubbing operation to be carried out with only that amount of oxygen necessary to achieve a desired oxidation level. Accordingly, the forced oxidation system need not be operated at a level in excess of that required for the existing flue gas flow rate or sulfur dioxide concentration present in the flue gases. As a result, the efficiency of the scrubbing operation is enhanced because power is not wasted in operating the oxidation system at an unnecessarily high level.

Another significant advantage of the present invention is that the forced oxidation system can be precisely controlled to allow a limited quantity of sulfites to remain in the slurry. As a result, the dissolved alkalinity of the slurry is increased, such that removal of sulfur dioxide from the slurry is increased through a reaction between sulfur dioxide and the sulfites which remain in the slurry. Consequently, with a lower level of sulfur dioxide within the slurry, the overall efficiency of the scrubbing operation is enhanced because the slurry will more readily absorb _^sulfur dioxide from the flue gases.

Other objects and advantages of this invention will be better appreciated from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become more apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a graphical representation of the relationship between the sulfite oxidation reaction and the oxidation-reduction potential of a calcium-based alkali slurry used in the scrubbing of flue gases;

Figure 2 shows in cross-section a slurry tank for a spray tower of a type known in the prior art;

Figure 3 schematically illustrates a forced oxidation system for a slurry tank in accordance with a preferred embodiment of this invention; and

Figure 4 shows in cross-section a spray tower of a type known in the prior art. Detailed Description of the Invention

As noted previously, Figure 4 illustrates a spray tower 10, which represents one type of gas- liquid contactor known in the art. Omitted from Figure 4 is a forced oxidation system, a conventional configuration of which is shown in Figure 2. In accordance with this invention, an improved forced oxidation system for a gas-liquid contactor, such as the spray tower 10 of Figure 4, is illustrated in Figure 3. While the spray tower 10 of Figure 4 is illustrated as being of a particular construction, those skilled in the art will recognize that the teachings of this invention can be readily applied to various other gas-liquid contactors, scrubbing structures and operations which serve to remove undesirable gases, mist, dust, fumes, smoke and/or particulate matter from a stream of gas.

Figure 3 is an isolated view of the tank 30 of Figure 4, for purposes of more clearly illustrating the features of the present invention. As seen from Figure 4, the tank 30 is located at the lower end of a tower 14 which forms an upright structure of the spray tower 10. An inlet duct 12 forms an opening at the perimeter of the tower 14 and directly above the tank 30 through which flue gases enter the tower 14. The source of the flue gases may be a process involving the combustion of fossil fuels or various industrial operations by which undesirable gases or particulate matter are produced.

As with prior art spray towers of the type illustrated in Figure 4, the tank 30 holds a slurry 32 which is employed to remove acidic gases and particulate matter from flue gases. The slurry 32 is an alkaline slurry, preferably a calcium-based slurry composed of lime or limestone suspended in water, though it is foreseeable that other slurry compositions could be used, including those based on magnesium, ammonia and sodium. As illustrated, the scrubbing process involves spraying the slurry 32 into the tower 14 above the inlet duct so as to provide intimate contact between the alkaline slurry 32 and the flue gas, such that the slurry 32 absorbs acid gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF) , present in the flue gases. Thereafter, the slurry 32 accumulates in the tank 30, where the sulfur dioxide reacts with the water in the slurry 32 to produce sulfites. The amount of oxygen in the slurry 32 controls the degree to which sulfates are produced from oxidation of the sulfites. Calcium ions made available from dissociation of the lime or limestone within the slurry 32 react with the sulfates to produce gypsum, which can be removed as a solid from the slurry 32 as a saleable byproduct of the scrubbing operation.

The present invention employs the forced oxidation system of Figure 3 to precisely aerate the slurry 32. As a result, only the amount of oxygen necessary to achieve a desired oxidation level is supplied to the tank 30, contrary to prior art oxidation systems in which excess oxygen is supplied to ensure that all of the sulfites will be reacted to form sulfates.

In accordance with this invention, by precisely regulating the operation of the forced oxidation system, the overall efficiency of the scrubbing operation is significantly enhanced because power is not wasted in operating the oxidation system at an unnecessarily high level. Furthermore, the forced oxidation system can be precisely controlled to allow a limited quantity of sulfites to remain in the slurry 32, such that the overall efficiency of the scrubbing operation is further enhanced because the slurry 32 is more readily able to absorb sulfur dioxide from the flue gases.

As illustrated in Figure 4, the forced oxidation system of this invention employs a blower 34 of any suitable type which can operate to inject air into the slurry 32 through a pipe 44. Those skilled in the art will recognize that other types of air injection devices could be employed as an alternative to the blower 34, including spargers, air lanced agitators and aspirators, each of which could be readily employed by one skilled in the art to provide for oxidation of the slurry. In addition, the forced oxidation system of this invention preferably employs an agitator 36 which assists in distributing oxygen and suspended solids in the slurry 32. Those skilled in the art will recognize that numerous other types of agitation devices could be employed as an alternative to the agitator 36 shown in Figure 4. The above configuration is generally in accordance with the teachings of the prior art. However, the forced oxidation system of this invention further includes a feedback system which is employed to monitor the dissolved sulfite concentration of the slurry 32, so as to enable the operation of the forced oxidation system to be optimized for the prevailing operating conditions of the spray tower 10. As illustrated, the feedback system includes a probe 38 in contact with the slurry 32, an inlet vane 40 which operates to regulate the amount of air able to enter the blower 34, and an inlet vane control 42 which operates on the basis of a signal from the probe 38 to make adjustments to the inlet vane 40. While the inlet vane 40 is a suitable device for regulating the amount of air delivered to the tank 30, other types of devices and methods could be employed, such as a valve downstream of the blower 34, or by turning the blower 34 on and off as required.

The probe 38 can be of any suitable type which can sense the dissolved sulfite concentration in the slurry 32. Examples of such a device include sulfite titration monitors and dissolved oxygen analyzers. In a preferred embodiment, the probe 38 is employed to sense the oxidation-reduction potential of the slurry 32 which, in accordance with this invention, is indicative of the degree to which oxidation of the sulfites occurs within the slurry 32. Figure 1 illustrates the relationship which was determined to exist between the oxidation reduction potential of a limestone slurry 32 and the degree to which sulfite oxidation occurred in the slurry 32 due to the presence of oxygen. Tests were performed to measure the oxidation-reduction potential and oxygen transfer efficiency as a function of air volume in the tank 30. The slurry 32 employed in the investigation contained about three weight percent limestone.

The graph shows the response of the oxidation-reduction potential as a function of molar sulfite oxidation in the slurry 32. The graph clearly illustrates a large potential difference between about 100 percent oxidation of sulfites (in excess of about 500 millivolts) and about 99 percent oxidation (less than about 300 millivolts) . The results indicated that regulation of the air flow rate into the tank 30 could be employed to precisely control the oxidation level in the tank 30, corresponding to the amount of sulfites oxidized to form sulfates.

Accordingly, if complete oxidation of the sulfites is desired, the amount of air flow to the tank 30 can be controlled to achieve 100 percent oxidation through the feedback signal provided by the probe 38 and its operation on the inlet vane control 42. Using the results of Figure l as an example, regulating the air flow to the tank 30 to maintain an oxidation-reduction potential of about 500 millivolts would achieve 100 percent oxidation without the use of excessive air. Therefore, a more efficient operation of the forced oxidation system is attained because power is not wasted in operating the oxidation system at an unnecessarily high level.

Alternatively, the forced oxidation system of this invention makes possible the precise control of the oxidation reaction within the slurry 32 so as to allow some level of sulfites to be maintained in the slurry 32. As noted previously, retention of some sulfites in the slurry 32 adds to the dissolved alkalinity of the slurry 32, such that enhanced removal of sulfur dioxide from the slurry 32 can be achieved by reacting the sulfur dioxide with the remaining sulfites to form bisulfite. The result is a more efficient removal of sulfur dioxide from the slurry 32, which promotes the efficiency by which the slurry 32 is able to absorb sulfur dioxide from the- flue gases during the scrubbing operation. Such improvements in the efficiency of the scrubbing operation are highly desirable, particularly in view of the poor solubility of limestone in water, which necessitates that large amounts of slurry 32 be sprayed into the tower 14 so as to adequately remove sulfur dioxide and the other acidic gases from the flue gases.

Again with reference to Figure 1, the air flow to the slurry 32 can be precisely controlled to allow as little as about one percent of the sulfites to remain unoxidized within the tank 30. For this purpose, the air flow to the tank 30 could be regulated to maintain an oxidation-reduction potential of about 300 millivolts. It should be noted that excessive dissolved sulfites present in the slurry 32 can lead to limestone binding, in which sulfites precipitate on the surface of the high pH limestone solids, rendering the limestone useless for forming gypsum. Increased levels of dissolved sulfite within the slurry 32 can be permitted without causing limestone binding by adding salts of ammonia, sodium or magnesium to the slurry 32. Such additions could be controlled in a similar manner as that for the forced air through the use of the feedback signal fro the probe 38.

In view of the above, it can be seen that a significant advantage of the present invention is tha control of the amount of air delivered to the tank 30 enables the scrubbing operation within the tower 10 t be carried out with only that amount of air necessary to achieve a desired oxidation level. Accordingly, the forced oxidation system of this invention overcomes the disadvantage of prior art systems, which must be operated at a level to ensure complete oxidation of the sulfites under worse case conditions.

In contrast, the oxidation system of this invention can be precisely operated at the level required by the existing flue gas flow rate or sulfur dioxide concentration present in the flue gases. As a result, the efficiency of the scrubbing operation is significantly enhanced because power is not wasted in operating the oxidation system at an unnecessarily high level.

Another significant advantage of this invention is that, contrary to prior art, the forced oxidation system of this invention can be precisely controlled to prevent a limited amount of sulfites within the slurry from being oxidized. As a result, the dissolved alkalinity of the slurry is increased, and the remaining sulfites are free to react with additional sulfur dioxide to form bisulfite.

While our invention has been described in terms of preferred embodiments, it is apparent that other forms could be adopted by one skilled in the art, such as by incorporating the novel features of this invention within spray towers which structurally differ from that shown in the Figures, by employing slurry compositions other than that noted, and by employing other devices by which the oxidation- reduction potential of the slurry can be detected and employed to control the oxidation of sulfites in the slurry. Accordingly, the scope of our invention is to be limited only by the following claims.

Claims

What is claimed is:

1. An apparatus comprising: a tank for holding a slurry in which sulfur dioxide is absorbed, the slurry reacting at least a portion of the sulfur dioxide to form a sulfite; means for delivering an oxygen-containing gas to the tank so as to oxidize at least a portion o the sulfite in the slurry; means for detecting the degree of oxidation of the slurry within the tank and producing an output signal corresponding thereto; means for regulating flow of the oxygen- containing gas to the delivering means; and feedback means for relaying the output signal of the detecting means to the regulating means so as to regulate the amount of oxygen-containing gas delivered to the tank relative to the output signal.

2. A scrubbing apparatus as recited in claim 1 wherein the slurry contains an alkali.

3. A scrubbing apparatus as recited in claim 1 further comprising means for contacting the slurry with flue gases so as to absorb the sulfur dioxide from the flue gases.

4. A scrubbing apparatus as recited in claim 1 wherein the regulating means regulates the amount of oxygen-containing gas delivered to the tank so as to react substantially all of the sulfites in the slurry to form sulfates.

5. A scrubbing apparatus as recited in claim 4 wherein the oxidation-reduction potential of the slurry is indicative of the presence of the sulfites in the slurry.

6. A scrubbing apparatus as recited in claim 1 further comprising means for agitating the slurry so as to promote aeration of the slurry.

7. A scrubbing apparatus for removing gases and particulate matter in flue gases, the scrubbing apparatus comprising: a tower having a lower end; a tank located at the lower end of the tower; an inlet located above the first tank through which flue gases are introduced into the tower; means for spraying a slurry into the tower so as absorb sulfur dioxide gas in the flue gases and thereby yield a cleansed flue gas, the slurry thereafter accumulating in the tank, the slurry reacting at least a portion of the sulfur dioxide gas to form a sulfite; means for delivering an oxygen-containing gas to the tank so as to oxidize at least a portion of the sulfites in the slurry; means for detecting the oxidation-reduction potential of the slurry within the tank, the detecting means producing an output signal corresponding to the oxidation-reduction potential of the slurry; means for regulating flow of the oxygen- containing gas to the delivering means; and feedback means for relaying the output signal of the detecting means to the regulating means so as to regulate the amount of oxygen-containing gas delivered to the tank relative to the output signal.

8. A scrubbing apparatus as recited in claim 7 wherein the slurry contains a calcium-based alkaline substance.

9. A scrubbing apparatus as recited in claim 7 wherein the regulating means regulates the amount of oxygen-containing gas delivered to the tank so as to react substantially all of the sulfites in the slurry to form sulfates.

10. A scrubbing apparatus as recited in claim 7 wherein the oxidation-reduction potential of the slurry is indicative of the presence of sulfur trioxide in the slurry.

11. A scrubbing apparatus as recited in claim 7 further comprising means for agitating the slurry so as to promote aeration of the slurry.

12. A method for removing gases and particulate matter in flue gases, the method comprising the steps of: providing a tank for holding a slurry employed to remove gases and particulate matter from flue gases; contacting the flue gases with the slurry such that the slurry absorbs sulfur dioxide gas in the flue gases; accumulating the slurry in the tank, wherein at least a portion of the sulfur dioxide gas is reacted to form sulfites; delivering an oxygen-containing gas to the tank so as to oxidize at least a portion of the sulfites in the slurry; detecting the oxidation-reduction potential of the slurry within the tank, the detecting means producing an output signal corresponding to the oxidation-reduction potential of the slurry; regulating flow of the oxygen-containing gas to the delivering means; and relaying the output signal of the detecting means to the regulating means so as to regulate the amount of oxygen-containing gas delivered to the tank relative to the output signal.

13. A method as recited in claim 12 further comprising the step of removing solids from the slurry.

14. A method as recited in claim 12 further comprising the step of agitating the slurry so as to promote aeration of the slurry.

15. A method as recited in claim 12 wherein the slurry contains an alkali.

16. A method as recited in claim 12 wherein the oxidation-reduction potential of the slurry is indicative of the presence of sulfites in the slurry.

17. A method as recited in claim 12 further comprising the step of maintaining the oxidation of the slurry at a level below 100 percent, such that a portion of the sulfites are not oxidized by the oxygen-containing gas.

18. A method as recited in claim 12 further comprising the step of maintaining the oxidation of the slurry at a level below 100 percent, such that a portion of the sulfites are not oxidized by the oxygen-containing gas.

19. A method as recited in claim 12 further comprising the step of adding to the slurry one or more substances selected from the group consisting of ammonia salts, sodium salts and magnesium salts.

20. A method as recited in claim 12 further comprising the step of adding to the slurry one or more substances selected from the group consisting of sulfite ions and oxygen.