Flue gas scrubbing apparatus

A gas-liquid contactor is provided for removing gases and particulate matter from flue gases, such as those which are produced by processing operations of the type carried out in utility and industrial facilities. The gas-liquid contactor includes a tower into which a slurry is introduced for absorbing gases and particulate matter, and is configured so as to eliminate the requirement for a pump to deliver the slurry to the tower. In addition, the tower is configured to accommodate a maximum flue gas flow velocity through the tower while maintaining proper operation of the tower. Liquid particles in which the gases and particulate matter are entrained are collected in a tank, through which the slurry is recycled to the tower. The level of the slurry within the tank is higher than the entry point of the slurry into the tower, such that the slurry returns to the tower under the force of gravity.

This invention generally relates to gas-liquid contactors used in the
 removal of particulate matter and gases, such as from utility and
 industrial flue gases. More particularly, this invention is directed to a
 gas-liquid contactor which is configured so as to eliminate the
 requirement for a pump to deliver a contact liquid to the contact section
 of a gas-liquid contactor, and further configured to have a high velocity
 section which promotes the absorption of gases and matter by the contact
 liquid, such that the efficiency of the gas-liquid contactor is increased
 while simultaneously reducing its operating and maintenance costs.
 BACKGROUND OF THE INVENTION
 Gas-liquid contactors 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
 (SO.sub.2) 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, such that 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 (FGD).
 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; CaCO.sub.3) slurries and hydrated lime (calcium
 hydroxide; Ca(OH).sub.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 accumulated in a tank.
 A known type of gas-liquid contactor is a spray tower 10 shown in
 cross-section in FIG. 1. The spray tower 10 generally is an upright
 structure composed of a tower 14 equipped with an inlet duct 12 through
 which combustion gases enter the tower 14. 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 required for a given
 application. One or more pumps 26 are required to recycle the alkaline
 slurry by pumping the slurry from a tank 30 to the banks of spray headers
 16 and 18. Each bank of spray headers 16 and 18 may be individually
 equipped with a pump 26 for the purpose of promoting the flexibility of
 the pumping and spraying operation to accommodate varying demands by the
 scrubbing operation.
 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.
 Due to the large quantity of slurry which must be pumped to scrub the flue
 gases, a significant cost in the construction, operation and maintenance
 of gas-liquid contactors is attributable to the pumps 26. Yet, the pumps
 26 also constitute a significant limitation to the scrubbing operation, in
 that the quantity of slurry pumped by the pumps 26 cannot be readily
 adjusted to accommodate changes in the scrubbing operation, such as the
 amount of flue gas which must be scrubbed or the amount of contaminants
 present in the flue gases.
 Another limitation of prior art gas-liquid contactors is the relatively low
 solids content permitted when using a slurry as the cleaning liquid.
 Typically, the solids content of such slurries must be limited to about
 ten to about fifteen weight percent. However, higher concentrations would
 allow the use of a smaller tank 30, since its size is generally dictated
 by, among other things, the residence time for crystallization of solids
 within the slurry. Higher solids contents would also eliminate the
 requirement for primary dewatering devices such as thickeners or
 hydrocyclones, which are well known devices employed in the art to remove
 solids and/or byproducts from a slurry. However, high solids contents
 significantly increase erosion within the tower 14, tank 30, fluid
 conduit, spray headers 16 and 18 and pump 26, while also increasing the
 power required to pump the slurry due to the higher specific gravity of
 the slurry.
 Finally, it would be advantageous to maximize the flue gas velocity within
 the tower 14 from the standpoint of improving contact between the slurry
 and the flue gases, so as to enable a reduced slurry flow to the tower 14.
 Higher flue gas velocities would also allow for the use of a tower 14
 having a smaller cross-sectional area, such that the cost of constructing
 the spray tower 10 is reduced. However, conventionally-accepted design
 practices typically limit the flue gas velocity within the tower 14 to
 about ten feet per second (about three meters per second) in order to
 assure the proper operation of the mist eliminator 22. Higher flue gas
 velocities within the tower 14 tend to increase the gas pressure drop
 within the tower 14, and therefore increase the likelihood of liquid
 particles being carried to and flooding the mist eliminator 22.
 Those skilled in the art will appreciate that, in view of the
 considerations noted above, it would be desirable if a flue gas scrubbing
 apparatus were available which overcame the above-noted disadvantages
 associated with the use of slurry pumps, yet could employ slurries having
 higher solids concentrations and higher flue gas velocities.
 SUMMARY OF THE INVENTION
 It is an object of this invention to provide a flue gas scrubbing apparatus
 for the removal of particulate matter, sulfur dioxide and other acidic
 gases, such as from flue gases produced by utility and industrial
 facilities.
 It is a further object of this invention that such a scrubbing apparatus
 eliminate the requirement for a device to pump a contact liquid which
 serves to remove gases and particulate matter from the flue gases, and
 thereby enable the use of high concentration levels of solids within the
 contact liquid.
 It is still a further object of this invention that such a scrubbing
 apparatus be constructed and configured so as to maximize the velocity of
 the flue gases while in contact with the contact liquid.
 It is another object of this invention that such a scrubbing apparatus
 operate in a manner which does not adversely effect the operation of
 devices used to remove liquid particles from the flue gases.
 Lastly, it is yet another object of this invention that such a scrubbing
 apparatus be configured such that its construction, operation and
 maintenance costs are minimized.
 The present invention provides a gas-liquid contactor of the type suitable
 for removing gases and particulate matter from flue gases produced by
 utility and industrial plants. The gas-liquid contactor is generally
 composed of a passage having a lower end and an upper end. The passage may
 be formed by a tower equipped with an inlet adjacent its lower end through
 which flue gases are introduced into the tower. The gas-liquid contactor
 further includes a device which sprays or otherwise introduces a cleansing
 liquid into the tower above the inlet. The cleansing liquid serves to
 absorb gases and particulate matter from the flue gases, so as to yield a
 cleansed flue gas in which liquid particles are suspended. For the removal
 of sulfur dioxide from flue gases, the cleansing liquid is preferably an
 alkali slurry characterized by the presence of solids in the cleansing
 liquid. As a result of the contact between the cleansing liquid and the
 flue gases, gases and particulate matter are absorbed in the liquid
 particles.
 In accordance with this invention, the tower is sized such that the
 velocity of the flue gases within the tower is sufficient to carry the
 liquid particles to a disengagement section located at the upper end of
 the tower. The disengagement section is adapted to separate the liquid
 particles from the cleansed flue gas, such that the liquid particles fall
 out of the air stream and accumulate in a scrubbing tank or other suitable
 reservoir for containing the cleansing liquid. Importantly, the level of
 the cleansing liquid within the scrubbing tank is maintained at a level
 above that of the device which introduces the cleansing liquid into the
 tower, such that the cleansing liquid is gravity fed to the device without
 the use of a pump. The gas-liquid contactor preferably includes a mist
 eliminating device downstream from the disengagement section for removing
 any remaining liquid particles from the cleansed flue gas. An outlet is
 disposed further downstream through which the cleansed flue gas escapes
 the gas-liquid contactor.
 A significant advantage of the present invention is that a pump is not
 required to deliver the cleansing liquid to the device which delivers the
 cleansing liquid to the tower, because the level of the cleansing liquid
 is above that of the device. Besides eliminating the added capital,
 operational and maintenance costs attributable to such pumps, an
 additional benefit is that the cleansing liquid can have a high solids
 content without concern for eroding a slurry pump. The higher solids
 content permitted by this invention allows the size of the scrubbing tank
 to be less than that typically required by gas-liquid contactors of the
 prior art by providing equivalent solids residence time through higher
 alkali densities. Use of the higher concentrations also eliminates the
 requirement for primary dewatering devices, because the cleansing liquid
 is already sufficiently concentrated for secondary dewatering devices such
 as filters and centrifuges.
 Another advantage of the present invention is that the velocity of the air
 stream through the tower can be significantly increased over that
 practicable with prior art gas-liquid contactors. Because of the
 relatively high velocities within the tower, improved contact between the
 cleansing liquid and the flue gases results, such that a reduced slurry
 flow to the tower can be employed while maintaining a proper cleansing
 effect. The higher flue gas velocities also allow the tower to have a
 reduced cross-sectional area, resulting in reduced cost to construct the
 gas-liquid contactor.
 Other objects and advantages of this invention will be better appreciated
 from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 2 illustrates a flue gas scrubber in the form of a spray tower 110
 configured in accordance with the teachings of the present invention. As
 illustrated, the spray tower 110 shares some structural similarities with
 that of the prior art tower 10 shown in FIG. 1. However, in accordance
 with this invention, the spray tower 110 is configured to eliminate the
 requirement for a pump to deliver a cleansing liquid to the spray tower
 110 for the primary scrubbing operation, and to enable higher flue gas
 velocities within the spray tower 110.
 While the spray tower 110 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 structures and
 operations which serve as gas-liquid contactors, such as to remove
 undesirable gases, mist, dust, fumes, smoke and/or particulate matter from
 a stream of gas. In addition, the teachings of this invention can be
 extended to devices which introduce a substance to a gas, such as
 humidifiers or strippers.
 The spray tower 110 shown in FIG. 2 generally has an upright structure
 composed of a tower 114. The lower section of the tower 114 is equipped
 with an inlet duct 112 which forms an opening at the perimeter of the
 tower 114 through which flue gases enter the tower 114. 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 FIG. 1, a
 reservoir or tank 130 is formed at the lower end of the tower 114 in which
 a liquid is held. A pump 148 is fluidically interconnected with the tank
 130 for the purpose of delivering the liquid from the tank 130 to a bank
 of spray headers 146 located in the inlet duct 112. The liquid is sprayed
 into the intersection of the inlet duct 112 with the tower 114, which
 generally defines a presaturation section of the spray tower 110. As is
 known by those skilled in the art, the liquid discharged into the
 presaturation section serves to quench hot flue gases, and may remove a
 portion of the particulate matter and some of the gases, primarily
 hydrogen chloride and hydrogen fluoride, entrained in the flue gases.
 While spray headers 146 are shown, atomizers of a type known in the art
 could alternatively be employed in place of the spray headers 146 to
 deliver an atomized mist into the presaturation section of the spray tower
 114.
 Though the preferred embodiment of this invention utilizes a presaturation
 section, this section, including the tank 130, pump 148 and spray headers
 146, need not be present or employed in order to realize the operational
 improvements made possible by this invention. However, the use of a
 presaturation section is often highly desirable, given the relatively low
 operating and maintenance costs of the presaturation section and the known
 operational advantages achieved by quenching the flue gases.
 The liquid contained in the tank 130 is not intended to perform the primary
 cleansing operation. As such, the liquid can be water or another suitable
 quenching solution, and need not be an alkaline slurry. However, it is
 foreseeable that a relatively low concentration of alkali could be present
 in the liquid. After contacting the flue gases, the liquid drains back
 into the tank 130, where it is recycled by the pump 148 to the spray
 headers 146. Because the liquid contains at most a small amount of alkali,
 minimal erosion occurs as the liquid passes through the pump 148 and spray
 headers 146.
 Above the presaturation section and within the tower 114, there is provided
 at least a second bank of spray headers 116, and foreseeably more banks of
 spray headers if required or preferred. As shown in FIG. 2, these spray
 headers 116 are fed with a water-based slurry contained in a second tank
 132. As before, atomizers of a type known in the art could alternatively
 be employed in place of the spray headers 116 to deliver an atomized mist
 into the spray tower 114. Numerous other types of devices which are
 capable of introducing a liquid into a gas could also foreseeably be used
 for this purpose.
 The slurry discharged by the second bank of spray headers 116 serves as the
 primary cleansing medium for the spray tower 110. Accordingly, this slurry
 is preferably water with a relatively high concentration of alkali,
 foreseeably in amounts well above the stoichiometric amount for the
 particular alkali used. For purposes of removing acidic gases and
 particulate matter from flue gases, the slurry may be composed of lime or
 limestone suspended in water, though it is foreseeable that other slurry
 compositions could be used. In addition, the solids content of the slurry
 can be well in excess of the conventional ten to fifteen weight percent
 limit imposed by prior art spray towers, with slurries in the form of a
 moist solid being foreseeable with the spray tower 110 of this invention.
 Under such conditions, devices other than the spray headers 116 would be
 used to deliver the slurry to the tower 114.
 The slurry is preferably sprayed into the tower 114 so as to provide for
 intimate contact between the slurry spray 120 and the flue gases rising
 through the tower 114. Additional alkali in the form of a powder or slurry
 can be introduced directly into the tower 114 through a conduit 156 or in
 any other suitable manner so as to replenish the alkali, as may be
 necessary. The interaction between the slurry and the flue gases yields a
 cleansed flue gas in which liquid particles are suspended. Absorbed in the
 liquid particles are substantially the remainder of the particulate matter
 and gases, such as sulfur dioxide, hydrogen chloride and hydrogen
 fluoride, entrained in the flue gases.
 As an important feature of this invention, the slurry does not flow
 countercurrently to the flue gas flow, as required by prior art gas-liquid
 contactors, but instead travels in the same direction as the flue gases
 within the tower 114. Specifically, the velocity of the flue gases within
 the tower 114 is sufficiently high so as to carry the liquid particles to
 a disengagement section 150 located at the upper end of the tower 114, and
 inhibit the liquid particles from draining down into the first tank 130.
 For this purpose, a minimum velocity of at least about twenty to
 twenty-five feet per second (about six to about eight meters per second)
 is preferred, though it is foreseeable that much higher velocities could
 be employed. Such velocities can be achieved by appropriately sizing the
 cross-sectional area of the tower 114 to the quantity of flue gases to be
 treated, though it is foreseeable that various devices could be employed
 to increase the velocity of the flue gases within the tower 114.
 Shelves 118 attached to the interior walls of the tower 114 are preferably
 provided in order to further inhibit the liquid particles from
 agglomerating and draining down along the walls of the tower 114 and into
 the first tank 130. Detaining the liquid with the shelves 118 allows the
 flue gases to eventually suspend and transport the liquid to the
 disengagement section 150, particularly if the tower 114 has a relatively
 small diameter such that wall effects are significant.
 In addition, packing, plates or other structures known in the art can be
 provided within the tower 114 to promote gas-liquid contact. The use of
 such structures has the advantageous effect of reducing the overall height
 of the tower 114 by decreasing the required height of the region within
 the tower 114 in which absorption of the gases occurs.
 The disengagement section 150 is preferably configured such that the
 velocity within the disengagement section 150 will be approximately the
 same as in the tower 114. As with phase separation devices known in the
 art, the disengagement section 150 serves to separate the liquid particles
 from the cleansed flue gas, and thereafter accumulate the liquid particles
 in the second tank 132 located below the disengagement section 150. As
 shown, the disengagement section 150 is configured to cause the liquid
 particles to impact and flow along the interior surface of the
 disengagement section 150 toward a trough 152, from which the liquid, now
 as the slurry, returns to the tank 132 through a pipe 154. Notably,
 numerous types of separation devices are known which could be employed in
 place of the structure shown in FIG. 2, such as a hydrocyclone.
 As noted previously, the second tank 132 contains the slurry which serves
 as the primary cleansing medium for the spray tower 110. Within the second
 tank 132, the sulfur dioxide in the slurry reacts with water to form
 sulfites (SO.sub.3.sup.--) and bisulf ites (HSO.sub.3.sup.-). Importantly,
 and as illustrated in FIG. 2, the level of the slurry within the second
 tank 132 is maintained at a level above that of the second bank of spray
 headers 116. As a result, the slurry can be fed by gravity through a
 conduit 126 to the second bank of spray headers 116, without the use of a
 pump. The second tank 132 can also be employed to segregate the slurry,
 such that the slurry near the top of the tank 132 will be less dense than
 the slurry which settles closer to the bottom of the tank 132. If desired,
 the less dense slurry near the top of the tank 132 can be drawn and
 delivered to the second bank of spray headers 116, while the denser slurry
 at the bottom of the tank 132 can be used as a filter feed.
 While the tank 132 is shown, those skilled in the art will recognize that
 various other structures could be employed to receive the liquid particles
 from the disengagement section 150. For example, a crystallizer of a type
 known in the art could be substituted for the tank 132 so as to control
 the crystal size of the precipitates which form in the slurry. In
 addition, a conventional thickening device or dewatering device could be
 used in place of the tank 132 or, under appropriate circumstances, a
 simple pipe could be used. In summary, the structure which receives the
 liquid particles from the disengagement section 150 need not be a
 reservoir, but can be any structure which can enable the slurry to be
 appropriately managed and returned to the second bank of spray headers
 116.
 Branching off from the conduit 126 is a bypass pipe 138 which is adapted to
 deliver a portion of the slurry to a dewatering device 140, if required
 due to the type of alkali used. The dewatering device 140 can be of any
 suitable type known in the art, and is employed to remove excess water
 from the slurry for the purpose of extracting some of the solids from the
 slurry. For example, gypsum (CaSO.sub.4.multidot.2H.sub.2 O) can be
 produced as a product of the reaction between sulfates and a calcium-based
 alkali (e.g., lime or limestone) in the slurry. The slurry can be fed
 directly to the dewatering device 140 if it contains a sufficiently high
 solids concentration. The gypsum cake 142 produced by the dewatering
 device 140 can be reused or otherwise disposed of properly.
 A flow control valve 128 is preferably located in the conduit 126 upstream
 of the second bank of spray headers 116. Advantageously, the flow control
 valve 128 can be manually or automatically adjusted to regulate the flow
 of slurry to the second bank of spray headers 116, such that only the
 amount of slurry necessary to suitably scrub the flue gases need be
 delivered to the tower 114.
 The second tank 132 also preferably includes, though does not necessarily
 require, an oxidation system for converting the sulfites in the slurry to
 sulfates (SO.sub.4.sup.--), thereby promoting the recovery of gypsum as a
 saleable by-product of the scrubbing operation. The oxidation system may
 include a blower 134 which injects air into the second tank 132 through a
 pipe 144. In addition, aerators 136 can be employed which assist in
 distributing and dissolving the oxygen in the slurry.
 Finally, located downstream from the disengagement section 150 is a mist
 eliminator 122 of any suitable type known in the art. The mist eliminator
 122 serves to remove any remaining liquid particles from the cleansed flue
 gas. Thereafter, the cleansed flue gases pass through a chimney 124, at
 which point the gases may be heated or exhausted directly into the
 atmosphere.
 In view of the above, it can be seen that a significant advantage of the
 present invention is that a pump is not required to deliver the slurry to
 the second bank of spray headers 116 because the level of the slurry
 within the second tank 132 is above that of the second bank of spray
 headers 116. As a result, the construction, operation and maintenance
 costs of the spray tower 110 are significantly less than that for prior
 art spray towers 110. Furthermore, the elimination of pumps permits the
 use of the flow control valve 128 so as to tailor the amount of slurry
 delivered to the tower 114 in accordance with the operating conditions of
 the spray tower 114.
 An additional benefit is that solids contents well in excess of fifteen
 weight percent and alkali concentrations in excess of the stoichiometric
 amount can be employed for the slurry. Because of the higher solids
 content of the slurry, the size of the second tank 132 can be less than
 that typically required by spray towers of the prior art. The higher
 solids content within the slurry made possible by this invention also
 eliminates the requirement for primary dewatering devices that extract
 byproducts, such as gypsum, from the slurry.
 Another significant advantage of this invention is that the velocity of the
 air stream through the tower 114 is significantly higher than that
 possible with prior art spray towers. In addition, because of the high
 velocities within the tower 114, improved contact between the slurry and
 the flue gases results, such that a reduced flow rate of the slurry to the
 tower 114 can be employed while maintaining an appropriate cleansing
 effect. The higher flue gas velocities also allow the tower 114 to have a
 reduced cross-sectional area, resulting in reduced costs to construct and
 maintain the spray tower 110.
 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
 gas-liquid contactors which differ structurally and functionally from that
 shown in the Figures.
 For example, the teachings of this invention could be employed in a
 gas-liquid contactor which does not employ a presaturation section, mist
 eliminator, forced oxidation system or agitator. Furthermore, a gas-liquid
 contactor incorporating the teachings of this invention could employ
 multiple points of entry for the cleansing liquid into the tower 114. If
 desired, such a contactor could draw the slurry from different levels
 within the tank 132, such that slurries having different chemistries and
 solids contents could be selectively introduced at different locations
 within the tower 114.
 Another foreseeable variation would be to employ a hydrocyclone to deliver
 the slurry from the tank 132 to the tower 114. Advantageously, such an
 approach would enable a first slurry composition having a relatively high
 solids content to be delivered near the lower end of the tower, while a
 second slurry composition having a lower solids content could be
 introduced at a higher point in the tower, resulting in lower operational
 costs while efficiently coordinating the introduction of slurry
 compositions having different solids contents and, therefore, different
 reaction times and characteristics.
 Yet another variation could be to bypass a portion of the liquid from the
 trough 152 directly to the conduit 126 which delivers the slurry to the
 tower 114. For this purpose, a bypass pipe 158 (shown in FIG. 2) could be
 used to divert flow from the pipe 154 to the conduit 126. The advantage
 here would be the intermixing of the slurry with the liquid, which is very
 low in pH and high in dissolved bisulfite. In bypassing the tank 132, the
 liquid increases the dissolved alkalinity of the slurry prior to its
 entering the tower 114. Control of the flow of liquid through the bypass
 pipe 158 could foreseeably be achieved in a variety of ways, and would
 potentially lessen the extent to which the degree of oxidation must be
 controlled in the tank 132. Such an approach would be difficult to employ
 in the conventional spray tower 10 of FIG. 1 due to the almost complete
 mixing of the slurry spray 20 with the slurry in the tank 30.
 In addition, the above-described invention could be employed as a single
 gas contactor stage of an apparatus employing two or more gas contactor
 stages in parallel or in series. Additional stages can be in accordance
 with this invention, or can be prior art gas-liquid contactors, or both.
 Accordingly, the scope of our invention is to be limited only by the
 following claims.