Wet flue gas desulfurization process and apparatus

Systems, apparatuses, and processes for controlling free ammonia in wet flue gas desulfurization processes in which an ammonia-containing scrubbing solution is used to produce ammonium sulfate. Such an apparatus includes an absorber having a contactor region through which a flue gas comprising sulfur dioxide is able to flow and a reaction tank containing a scrubbing solution containing ammonium sulfate. The tank has a sidewall and bottom wall that define the perimeter and bottom of the tank. Lance-agitator units are distributed around the perimeter of the tank, each having a lance that injects a mixture of oxygen and a dilute ammonia-containing fluid toward the bottom of the tank and an agitator that agitates the mixture and propels the mixture toward the bottom of the tank. The apparatus includes a source of the mixture of oxygen and dilute ammonia-containing fluid, and recirculates the scrubbing solution from the tank to the contactor region.

BACKGROUND OF THE INVENTION

The present invention generally relates to systems, apparatuses, and processes for removing acidic gases from gas streams, including but not limited to utility and industrial flue gases. The invention particularly relates to the control of free ammonia and ammonium sulfate aerosol in wet flue gas desulfurization processes and apparatuses in which an ammonia-containing scrubbing solution is used to produce ammonium sulfate as a byproduct.

Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. These 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 these gases are removed with a gas-liquid contactor or absorber is commonly referred to as wet flue gas desulfurization (WFGD).

The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs the targeted gas(es) and particulate matter. Wet flue gas desulfurization processes have typically involved the use of an alkaline scrubbing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. While effective, gas-liquid contactors and absorbers utilizing calcium-based slurries produce large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate byproduct that is usable as a fertilizer. In these processes, sulfur dioxide is absorbed from flue gases with an ammonium sulfate solution, after which the sulfur dioxide is reacted with oxygen and anhydrous or aqueous ammonia injected into the solution to form additional ammonium sulfate solution or ammonium sulfate crystals ((NH4)2SO4). Particular examples of such processes are disclosed in U.S. Pat. Nos. 4,690,807, 5,362,458, 6,187,278, 6,277,343, 7,771,685, and 9,327,234. In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution, which becomes more acidic with the absorption of sulfur dioxide.

The use and addition of anhydrous or aqueous ammonia to control sulfur oxide gases can result in undesirable levels of ammonia slip. As used herein, ammonia slip refers to free ammonia (anhydrous ammonia, NH3) entrained in a scrubbed flue gas exiting a gas contactor or absorber. In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gas reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that may be visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite produced by the reaction of sulfur dioxide and ammonia in the absence of sufficient oxygen. High pH values result in high ammonia vapor pressure, which promotes ammonia slip. High levels of unoxidized ammonium sulfite also promote ammonia slip.

FIGS. 1 and 2schematically represent a flue gas scrubbing apparatus10that is disclosed in U.S. Pat. No. 6,187,278 as effective to reduce ammonia slip. As shown, the apparatus10includes an upright absorber12that is supplied with flue gas through an inlet duct14. The apparatus10operates in a manner that causes absorption of sulfur dioxide from the flue gas using a scrubbing liquid. The scrubbed flue gas that leaves the absorber12can be delivered to a stack (not shown) or other suitable equipment through an outlet duct20. The source of the flue gas may be any process involving the combustion of fossil fuels or various industrial operations in which undesirable gases or particulate matter are produced.

U.S. Pat. No. 6,187,278 discloses the apparatus10as utilizing an ammonia-rich scrubbing solution22, such as an aqueous ammonium sulfate solution containing free dissolved ammonia as the reagent for the desulfurization process.FIG. 1shows ammonia (NH3) being delivered from a source32to a reaction tank18via a pump26, conduit28, and injection system30that comprises multiple spargers34that extend across the entire diameter of the tank18. A recirculation pump40serves to recycle the scrubbing solution22from the tank18through a conduit16to a contactor region of the absorber12, where the solution22is introduced through a number of nozzles24or other suitable devices. The scrubbing process involves spraying the scrubbing solution22into the absorber12so as to provide intimate contact between the solution22and the flue gas. As a result, the solution22absorbs sulfur dioxide and other acid gases, such as hydrogen chloride (HCl) and hydrogen fluoride (HF), if they are present in the flue gas. The solution22then falls into the reaction tank18, where the absorbed sulfur dioxide reacts with the ammonia and is oxidized to form ammonium sulfate. Specifically, sulfur dioxide reacts with ammonia to form ammonium sulfite ((NH4)2SO3) and ammonium bisulfite (NH4HSO3), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate and ammonium bisulfate (NH4HSO4), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the scrubbing solution22and/or ammonium sulfate crystals that form in the solution22can then be drawn off to yield the desired byproduct of this reaction. A sufficient amount of ammonium sulfate may be removed from the scrubbing solution22prior to delivery to the absorber12in order to maintain ammonium sulfate at a desired concentration in the solution22.

U.S. Pat. No. 6,187,278 teaches that the manner in which ammonia is injected may promote high levels of ammonia slip, such that ammonia and possibly ammonium sulfate aerosol are discharged into the atmosphere with the scrubbed flue gas exiting the absorber12. As a solution to this problem, U.S. Pat. No. 6,187,278 injects ammonia into the scrubbing solution22in the reaction tank18in a dilute form (for example, a dilute aqueous solution) and through the spargers34shown inFIGS. 1 and 2, which uniformly disperse the dilute ammonia in the scrubbing solution22to reduce the likelihood that pockets of high pH and high ammonium sulfite levels will be present in the scrubbing solution22, such that more uniform and desirable pH and ammonium sulfite levels are achieved that promote absorption of ammonia and control ammonia slip in the absorber12. As represented inFIG. 1, the ammonia injected into the scrubbing solution22is diluted with oxygen from a suitable source38, and the resulting mixture is then delivered to the tank18via the spargers34of the injection system30. Circulation of the injected ammonia and oxygen in the reaction tank18is shown inFIG. 1as promoted by a fan42.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides systems, apparatuses, and processes suitable for controlling free ammonia in wet flue gas desulfurization processes and apparatuses in which an ammonia-containing scrubbing solution is used to produce ammonium sulfate.

According to one aspect of the invention, an apparatus for removing sulfur dioxide from a flue gas includes an absorber having a contactor region through which a flue gas comprising sulfur dioxide is able to flow and a reaction tank containing a scrubbing solution comprising ammonium sulfate. The tank has a sidewall that defines a perimeter of the tank and a bottom wall that defines a bottom of the tank. A plurality of lance-agitator units are distributed around the perimeter of the tank. Each lance-agitator unit comprises a lance that injects a mixture of oxygen and a dilute ammonia-containing fluid toward the bottom of the tank and an agitator that agitates the mixture and propels the mixture toward the bottom of the tank. The apparatus further includes a source of the mixture of oxygen and dilute ammonia-containing fluid, and means for recirculating the scrubbing solution from the tank to the contactor region.

According to another aspect of the invention, a process for removing sulfur dioxide from a flue gas includes delivering the flue gas to a contactor region of an absorber, contacting the flue gas within the contactor region with a scrubbing solution that contains ammonium sulfate to absorb sulfur dioxide from the flue gas, and accumulating the scrubbing solution containing the absorbed sulfur dioxide in a tank having a bottom wall and a side wall that defines a perimeter of the tank. A plurality of lance-agitator units is distributed around the perimeter of the tank to introduce a mixture of oxygen and a dilute ammonia-containing fluid into the tank to react with the sulfur dioxide to produce ammonium sulfate. Each lance-agitator unit comprises a lance that injects the mixture toward the bottom of the tank and an agitator that agitates the mixture and propels the mixture toward the bottom of the tank. The scrubbing solution is recirculated from the tank to the contactor region.

Technical aspects of the processes and apparatuses described above preferably include the ability to generate ammonium sulfate while controlling ammonia slip through the combined use of multiple lances and agitators, which have been shown to be effective and offer advantages over prior art sparger systems.

Other aspects and advantages of this invention will be further appreciated from the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to flue gas desulfurization processes and apparatuses suitable for removing sulfur dioxide gas entrained in flue gases to generate ammonium sulfate as a byproduct. While the invention will be described in reference to a desulfurization system that utilizes an absorber, those skilled in the art will recognize that the teachings of this invention can be readily applied to various other desulfurization systems, and the desulfurization process is compatible with various systems capable of removing other undesirable gases, mist, dust, fumes, smoke and/or particulate matter from a stream of gas.

FIG. 3is a schematic view of a flue gas scrubbing apparatus110in accordance with a nonlimiting embodiment of the invention. As shown, the apparatus110includes an upright absorber112that is supplied with a flue gas through an inlet duct114. The apparatus110operates in a manner that causes absorption of sulfur dioxide from the flue gas through the use of a scrubbing solution122. The scrubbed flue gas exits the absorber112through an outlet duct120and from there may be delivered to a stack (not shown) or other suitable equipment. The source of the flue gas may be any process involving the combustion of fossil fuels or various industrial operations by which undesirable gases or particulate matter are produced.

The scrubbing solution122is an ammonia-rich scrubbing solution, and in particular a scrubbing solution containing free dissolved ammonia as the reagent for the desulfurization process. The dissolved ammonia can be either aqueous ammonia (ammonium hydroxide) or anhydrous ammonia (NH3), depending on the composition of the scrubbing solution. As nonlimiting examples, the solution may contain ammonia diluted with air and/or water, with the latter resulting in the presence of aqueous ammonia.FIG. 3shows ammonia being delivered from a source132to a reaction tank118via a pump126and an injection system130. The ammonia, which may be present in an aqueous solution or other suitable solution as noted above, is a primary reactant when producing ammonium sulfate as a byproduct of the desulfurization process, and the scrubbing solution122serves as the vehicle for delivering the ammonia to the absorber112.

Similar to the apparatus10ofFIG. 1, one or more recirculation pumps140(FIG. 4) may be used to recycle the scrubbing solution122from the tank118through a conduit116to a contactor region of the absorber112, where the solution122is introduced through a number of nozzles124or other suitable devices. The scrubbing process involves spraying the scrubbing solution122into the absorber112so as to provide intimate contact between the solution122and the flue gas. As a result, the solution122absorbs sulfur dioxide and potentially other acid gases, such as hydrogen chloride and hydrogen fluoride, if present in the flue gas. The solution122then falls into the reaction tank118, where the absorbed sulfur dioxide reacts with the ammonia to form ammonium sulfite and ammonium bisulfite, which are then oxidized in the presence of sufficient oxygen to form ammonium sulfate and ammonium bisulfate, the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the scrubbing solution122and/or ammonium sulfate crystals that form in the solution122may be drawn off to yield a desired fertilizer byproduct of this reaction. A sufficient amount of ammonium sulfate is preferably removed from the scrubbing solution122prior to delivery to the absorber112in order to maintain ammonium sulfate at a desired concentration in the solution122, as a nonlimiting example, about 2% up to the saturation level of ammonium sulfate in the solution122.

Sufficient ammonia is preferably delivered to the tank118to control the pH of the scrubbing solution122, for example, within a typical range of about 4 to 6 pH range, such that the solution122is highly reactive for high efficient capture of sulfur oxide gases. The manner in which the ammonia is injected into the solution122can undesirably promote high levels of ammonia slip, such that free ammonia and potentially an ammonium sulfate aerosol escapes the absorber112and is discharged into the atmosphere. Whereas U.S. Pat. No. 6,187,278 seeks to reduce ammonia slip by injecting dilute ammonia into a scrubbing solution with an injection system that comprises multiple spargers that extend in parallel across the entire diameter of the reaction tank (FIGS. 1 and 2), a substantial test program leading up to the present invention indicated that combinations of lances and agitators selectively located around the perimeter of a reaction tank are capable of having a comparable effect with respect to oxygen transfer and uniformly dispersing a dilute mixture of ammonia and oxygen in the reaction tank.

The ammonia injected from the source132in the reaction tank118is preferably in the form of a dilute solution, for example, an aqueous solution.FIGS. 3 and 4represent the diluted ammonia solution as being further diluted with oxygen from a suitable source138prior to being injected into the tank118via lances134of the injection system130. Air is a suitable source for the oxygen, with a preferred ammonia:air weight ratio being about 1 to about 5. As seen inFIG. 3, each lance134is paired with an agitator142that operates in combination with its associated lance134to uniformly disperse the injected mixture of oxygen and dilute ammonia toward the bottom136of the reaction tank118. Each lance134is represented inFIG. 3as comprising a pipe that extends through a sidewall of the tank118, has a section that projects vertically downward toward the tank bottom136, and terminates with an outlet148facing the tank bottom136. Each agitator142is represented inFIG. 3as comprising a propeller or fan mounted on a shaft that extends through the tank sidewall at a negative angle to horizontal and is driven by a motor. Each lance134injects the ammonia toward the bottom136of the tank118, and its paired agitator142assists to propel and disperse the injected ammonia at the tank bottom136. The negative angle of the shaft is preferably sufficient to promote the downward flow of the injected oxygen-ammonia mixture toward the tank bottom136, with suitable angles believed to be at least eight to about twelve degrees from horizontal.

FIG. 4represents the tank118as equipped with four lance/agitator units144, each having a single lance134paired with a single agitator142. As represented inFIG. 4, the units144are generally equi-angularly distributed around the interior perimeter146defined by the sidewall of the tank118. Based on a reaction tank118having a depth of about 48 feet (about 14 meters), the outlet148of each lance134is preferably located about 4 to about 8 feet (about 1.2 to about 2.4 meters) from the bottom136of the tank118, and agitation caused by each agitator142preferably occurs between the lance outlet148and the tank bottom136. This arrangement has been shown to achieve acceptable results in terms of delivering sufficient ammonia and oxygen while simultaneously avoiding ammonia slip. The ammonia and oxygen mixture introduced with the injection system130is forcibly circulated through the reaction tank118by the agitators142, as opposed to relying on natural circulation cause by the solution122being recirculated from the tank118to the absorber112.

A significant advantage of the present invention is the ability to use lances instead of more expensive spargers to reduce ammonia slip in a desulfurization process that uses an ammonia-based scrubbing solution. Other advantages include minimal pluggage potential, fewer penetrations, and fewer obstructions in the reaction tank.

While the invention has been described in terms of a specific or particular embodiment, it should be apparent that alternatives could be adopted by one skilled in the art. For example, the flue gas scrubbing apparatus110and its components could differ in appearance and construction from the embodiment described herein and shown in the drawings, functions of certain components of the apparatus110could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, various process parameters could be employed, and various materials could be used in the fabrication of the apparatus110and/or its components. In addition, the invention encompasses additional or alternative embodiments in which one or more features or aspects of the disclosed embodiment could be eliminated. Accordingly, it should be understood that the invention is not necessarily limited to any embodiment described herein or illustrated in the drawings. It should also be understood that the phraseology and terminology employed above are for the purpose of describing the illustrated embodiment, and do not necessarily serve as limitations to the scope of the invention. Therefore, the scope of the invention is to be limited only by the following claims.