Retrofit Flue Gas Scrubber System

The present invention relates generally to a gas/liquid contacting device. More particularly, this invention concerns an improved wet scrubber module for retrofit in a system having an existing upstream precipitator in order to simultaneously remove both residual fly ash and sulfur dioxide from the flue gas of a coal-fired power plant, particularly one using coal of relatively high sulphur content, before discharge into the atmosphere.

Scrubber systems have been developed for controlling harmful emissions from industrial and utility boilers like those used in power plants. Such scrubber systems are generally of either the dry or wet types. Dry scrubber systems generally include an open chamber in which the flue gas is directed through a liquid spray of lime and fly ash slurry. A reaction occurs with the sulfur dioxide in the gas to form a calcium compound in dry particulate form which can then be collected at the outlet of the chamber, thereby "scrubbing" the flue gas free of sulfur dioxide pollutants.

On the other hand, in the so-called "wet scrubbers", the sulfur dioxide is not collected in dry particulate form, but rather is collected in the form of a slurry in a tank of aqueous absorbent for periodic removal in liquid slurry form. In order to avoid excessive maintenance costs from the build-up of deposits of fly ash, the fly ash must first be removed by means of filtration or electrostatic precipitation before the flue gas can be treated in a wet scrubber. This results in two large, expensive structures. Further, since the fly ash is typically removed separately, the alkalinity which otherwise would have been provided by the presence of fly ash is not present, and thus some of the fly ash which was removed must then be added to the aqueous absorbent, which of course adds further expense and complication.

U.S. Pat. No. 3,836,630 shows a system wherein sulfur dioxide absorption and oxidation is carried out in separate vessels.

U.S. Pat. Nos. 4,156,712 and 4,229,417 disclose a method and apparatus for removing sulfur dioxide from flue gases by simultaneous absorption and oxidation in an aqueous absorbent within a single vessel. As the liquid absorbent in the vessel is agitated, flue gas is sparged into the upper region as oxygen-containing gas is sparged into the lower region of the body of liquid absorbent. Alkaline material is introduced into the body of liquid absorbent that fixes the sulfur dioxide in the form of a sulfate which then precipitates out in solution for removal. These references suggest that gas/liquid contacting and solid (fly ash) precipitation can be carried out in the same device, however, since the flue gas is introduced into the upper region of the liquid absorbent via perforate or notched pipes, at least some prescrubbing of the fly ash particulate therefrom is required to avoid clogging.

My U.S. Pat. No. 4,762,686 is directed to a system for simultaneous fly ash and sulphur dioxide removal, but which does not incorporate an existing baghouse or precipitator upstream from the wet scrubber module.

A need has thus arisen for a retrofit flue gas scrubber system of the wet scrubber type for use with a scrubber system having an existing baghouse or precipitator, so that residual particulate removal and primary sulfur dioxide removal can be accomplished more efficiently.

The present invention comprises an improved flue gas scrubber system which overcomes the foregoing and other difficulties associated with the prior art. In accordance with the invention, there is provided a flue gas scrubber system of the wet scrubber type which is particularly adapted for removing both fly ash particulate by wetting, and sulfur dioxide gas by absorption and oxidation, from the flue gas of a power plant in one step. The lower end of the chamber inside the scrubber module is filled with aqueous absorbent having a controlled pH between about 3.5 and 5.0, while the upper end is divided by a vertical partition into two subchambers. Dirty flue gas from the boiler is received in one subchamber, while scrubbed or clean flue gas leaves the other subchamber on the other side of the partition for exhaust through a stack to the atmosphere. The partition takes the form of a zigzag or corrugated vertical side wall, the upper edges of which are closed by top walls in order to define a series of hollow "fingers". The lower edges of the side walls are continuous and are submerged in the liquid absorbent. As raw flue gas enters the scrubber chamber, it is directed into the ends of the fingers of the partition, downward around their submerged lower edges and through the liquid absorbent. Simultaneously, an oxygen-containing gas such as air is injected into the lower region of the aqueous absorbent beneath the partition, as the absorbent is agitated. The fly ash in the flue gas provides the primary reagent to react with the sulfur dioxide to form a sulfate that precipitates out of solution and can then be removed as fly ash slurry. If desired, additional calcium compound, such as limestone, can be added to the aqueous absorbent for best efficiency.

A better understanding of the invention can be had by reference to the following Detailed Description in conjunction with the accompanying Drawings, wherein:

FIG. 1 is a partial schematic view of a power plant including a boiler and several scrubber modules for removing pollutants from the boiler flue gas before discharge into the atmosphere;

FIG. 2 is a vertical cross-sectional schematic view of one of the improved scrubber modules of the present invention;

FIG. 3 is a partial perspective view of the vertical partition located in the scrubber module;

FIG. 4 is an enlarged partial vertical sectional view of the scrubber module; and

FIG. 5 is a vertical cross-sectional schematic view of a modified scrubber module.

Referring now to the Drawings, wherein like reference numerals designate like or corresponding elements throughout the views, and particularly referring to FIG. 1, there is shown a part of a power plant 10 of the type employed by utility companies for generating electricity. The power plant 10 includes a boiler 12 which generates steam for driving a turbine that turns a generator to generate electricity. The boiler 12 is fired by coal, the particular sulfur content of which depends upon the origin of the coal. Coal of low sulfur content is preferred because of the lesser difficulty in meeting pollution control standards, although coal of relatively higher sulfur content is more typical. The flue gas from boiler 12 is directed through duct work 14 to several wet scrubber modules 16, only three of which are shown. In practice, twelve scrubber modules 16 are provided for reserve capacity during peak periods of power generation so that some of scrubber modules can be taken off line for repair and maintenance without affecting the overall performance of the power plant 10. After passing through the scrubber modules 16, the scrubbed flue gas passes through duct work 18 containing induced draft fans (not shown) for exhaust through a stack to the atmosphere. As will be explained more fully hereinafter, the scrubber modules 16 are of an improved construction so as to provide for simultaneous fly ash and sulfur dioxide removal with improved efficiency and reduced cost.

Referring now to FIGS. 2 and 3, there is shown the internal construction of one of the scrubber modules 16. The scrubber module 16 includes a vessel or tank 20 which is partially filled with aqueous absorbent 22. The aqueous absorbent 22 basically comprises water and the fly ash particulate from the incoming flue gas, although a suitable amount of crushed limestone can be added as desired to obtain the desired pH, as will be explained more fully hereinafter. The upper end of the tank 20 is connected to the duck work 14 leading from boiler 12, as well as to a spray tower 24 and duct work 18 leading to the exhaust stack (not shown).

Referring now to FIG. 3 in conjunction with FIG. 2, a vertical partition 26 is secured across the upper end of tank 20 between the flue gas inlet 28 and the outlet 30. The partition 26 comprises a plurality of generally parallel fingers 32 of hollow construction arranged in parallel, spaced-apart relationship across the upper end of the tank 20. The fingers 32 can be rectangular, but are preferably each of generally right triangular configuration to reduce material cost. Each finger 32 is preferably constructed of stainless steel and includes a pair of side walls 34 and 36 and an inclined top wall 38. End walls 40 are connected between adjacent edges of the open divergent ends of the fingers 32 so that all of the flue gas entering inlet 28 is directed into the hollow fingers 32 of the partition 26. It will thus be appreciated that the partition 26 forms a zigzag partition comprised of a number of vertical side walls 34 and 36 and connecting top walls 38 and end walls 40 extending across the upper end of tank 20. The zigzag continuous lower edge of the partition 26 is submerged into the surface of the aqueous absorbent 22.

The flue gas from boiler 12 entering inlet 28 via duct work 14 is at about 300.degree. F. This dirty flue gas enters the tank 20 and is directed into the fingers 32 of the partition 26. The flue gas then passes around the zigzag lower edge of the partition 26 and bubbles upwardly through the aqueous absorbent 22 in the upper region of tank 20 and out onto the opposite side of the partition. Simultaneously, an oxygen-containing gas, such as air, is introduced into the lower region of the aqueous absorbent 22 through a pipe 42. The pipe 42 preferably includes a downwardly-turned end 44 to prevent back flow of aqueous absorbent 22 therein. An agitator 46 is also provided in the tank 20 for circulation. The oxygen containing gas from pipe 42 bubbles upwardly through the aqueous absorbent 22 in tank 20 until it joins with the flue gas bubbling underneath the partition 26, whereupon the fly ash and sulfur dioxide in the flue gas reacts with the calcium and oxygen in the aqueous absorbent 22 to form calcium sulfate, which then precipitates out of solution and collects within the tank 20 in accordance with well-known chemical reactions. The agitator 46 helps to keep the calcium sulfate suspended in solution in the aqueous absorbent 22, which in effect becomes a slurry. The slurry can be selectively withdrawn through a screened outlet 48 via pump 50.

The scrubbed or clean flue gas enters outlet 30 at about 130.degree. F., passing upwardly in tower 24 first through a conventional bulk entrainment separator 52 and then through a conventional mist eliminator 54 to remove excess moisture.

In the preferred embodiment, the scrubber module 16 includes a reheater coil 56 in the upper end of tower 24 after the mist eliminator 54 for heating the scrubbed flue gas to a temperature of about 160.degree. F. in order to increase gas buoyancy and reduce condensation and thus corrosion in the duct work 18. The reheater coil 56 can be connected to the de-aerator (not shown) or any other suitable source of heat.

Referring again to FIG. 2 in conjunction with FIG. 4, the scrubber module 16 of the invention preferably includes a level control 58 for maintaining a proper level of aqueous absorbent 22 in the tank 20. The level control 58 comprises an outer tube 60 having a closed upper end and an open bottom end. A plate 62 is suspended in spaced-apart relationship from the open bottom end of the outer tube 60 to prevent bubbles from pipe 42 from entering. An inner tube 64 is positioned inside the outer tube 60. The inner tube 64 includes an open top end and a closed bottom end. In this manner excess aqueous absorbent 22 fills the outer tube 60 and spills over into the inner tube 64 for withdrawal by means of overflow pipe 66 which is connected to the inner tube and extends in sealing engagement through the outer tube and on out through the tank 20.

Additional make-up water can be added to tank 20 through pipe 68.

It will be noted that the water level on opposite sides of partition 26 is different. The pressure drop across partition 26 is about 10-15 inches of water, depending upon the scrubbing efficiency desired.

Referring again to FIG. 2, the scrubber module 16 preferably includes a line 70 leading from pump 50 to nozzles 72, only one of which is shown, positioned to discharge slurry from tank 20 onto the inside of fingers 32. Nozzles 72 can be actuated periodically for purposes of cleaning partition 26 and quenching the incoming flue gas to a temperature of about 140.degree.-150.degree. F. for more effective operation.

As indicated above, the pH of the aqueous absorbent 22 ranges between about 3 and 6, and preferably between 3.5 and 5.0. the aqueous absorbent contains both the fly ash and the sulfur dioxide removed from the flue gas, and their chemical combinations with water, oxygen, fly ash constituents, and any additional alkaline material added as necessary to control pH. For example, a slurry of crushed limestone can be added to the aqueous absorbent for pH control. It has been found experimentally that 70% sulfur dioxide removal could be achieved from flue gas containing 600 ppm fly ash particulate with aqueous absorbent of 3.5 pH, without any alkaline additives using only the available calcium in the fly ash for sulfur fixation as calcium sulfate. Alkaline material is added to the aqueous absorbent to control pH as necessary in accordance with the calcium content of the fly ash, the sulfur dioxide content of the flue gas, and the required percentage removal of sulfur dioxide from the flue gas.

Although the scrubber modules 16 are particularly adopted for simultaneous fly ash and sulfur dioxide removal without pre-removal of the fly ash, it will be understood that the scrubber system herein can also be retrofitted for use in a power plant already having a baghouse or precipitator for pre-removal of fly ash. For example, referring back to FIG. 1, there is shown in dashed lines an optional baghouse or precipitator 76 connected in the duct work upstream of the scrubber modules, typical of many power plants. The baghouse or precipitator 76 typically includes a hopper bottom through which the collected particulate; i.e., fly ash, passes into pneumatic conveyor lines 78 leading to a remote disposal site. Such precipitators are available from various commercial sources and effectively remove up to 99.5% or substantially all of the fly ash particulate from the incoming flue gas. Without the calcium from the fly ash, addition of relatively more crushed limestone would be required to maintain the pH of the aqueous absorbent 22 in the range of 3 to 6, and preferably between 3.5 and 5.0. Otherwise, such a retrofitted flue gas scrubber system would be constructed and operated substantially the same.

FIG. 5 shows a modification to the spray tower 24 of the scrubber module 16. An electrostatic precipitator 80 is provided between mist eliminator 54 and the reheater coil 56. The electrostatic precipitator 74 is of conventional construction and can be obtained from various manufacturers, such as the Western Precipitator Division of Joy Technologies, Inc., for example. In addition the water pipe 68 has been positioned above the bulk entrainment separator 52, and nozzles have been provided thereon in order to wash the mist eliminator 54. If desired, another water pipe 82 with downwardly directed nozzles can be added for washing the electrostatic precipitator 80, followed by another mist eliminator 84.

From the foregoing, it will thus be apparent that the present invention comprises an improved flue gas scrubber system having several advantages over the prior art. One important advantage involves the fact that both fly ash removal and sulfur dioxide removal take place simultaneously in a single vessel, whereas in the past is was necessary to remove the fly ash before removing the sulfur dioxide. The present invention avoids the additional equipment, cost, and maintenance heretofore associated with preliminary fly ash removal. Other advantages will be evident to those skilled in the art.

Although particular embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited only to the embodiments disclosed, but is intended to embrace any alternatives, equivalents, modifications, and/or rearrangement of elements falling within the scope of the invention as defined by the following claims.