Abstract:
A scrubber system is provided which enables a substantially horizontal flow path for the gas which is being subjected to scrubbing. Among other advantages, this permits operation of the absorber with a differential pressure of zero or less. Scrubber composition spray means are positioned in the horizontal gas flow path for spraying an aqueous scrubber composition in a direction which is generally cocurrent with said gas flow. The system is free of means which spray the scrubber composition in directions countercurrent to said gas flow.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority from Applicant&#39;s provisional patent application, filed Nov. 2, 2000 under Serial No. 60/245,305. 
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates generally to apparatus for desulfurization of flue gases, and more specifically relates to an improved scrubber system which enables effective use of a horizontally oriented gas flow path for the gas being treated in the apparatus. The system characteristics are such as to permit operation of the absorber with a differential pressure drop of zero or less.  
         DESCRIPTION OF PRIOR ART  
       BACKGROUND OF THE INVENTION  
         [0003]    Air pollution is a very serious and urgent international problem. The sources of air pollution are primarily the products of combustion and are numerous and widespread. Many of the air pollutants are in the form of sulfur-bearing flue gases discharged by fossil-fuel-burning electrical power generating plants or other industries. While the precise impact of these pollutants on the environment is still a subject of some speculation, evidence continues to mount which demonstrates serious adverse effects. Yet, under foreseeable circumstances, it will be necessary to burn more and more fuel to meet the demands of a rapidly growing population requiring for each person ever more heating comfort and power, and the fuel which will generally be used will not contain much less sulfur, but will likely contain more sulfur.  
           [0004]    Thus, sulfur oxides, principally present as sulfur dioxide, are found in the waste gases discharged from many metal refining and chemical plants, and in the flue gases from power plants generating electricity by the combustion of fossil fuels. In addition, sulfur-containing gases, notably sulfur dioxide, may be formed in the combustion of sulfur-containing fuels, such as coal or petroleum residues. The control of air pollution resulting from the discharge of sulfur dioxide into the atmosphere has thus become increasingly urgent.  
           [0005]    As used herein the term “flue gas” is meant to encompass all of the foregoing gaseous discharges. It should additionally be noted that while sulfurous gases (notably sulfur dioxide) are the principal contaminants of concern, further undesirable components are usually present in the sulfurous flue gases, including acid halogen gases such as hydrogen chloride, as well as carbon dioxide and monoxide. The present invention will be seen to be useful in removing certain of these further gases from the flue gas, i.e., in addition to the sulfurous gases, and thus the term “flue gas desulfurization” as used herein, should not be interpreted to imply that only sulfurous components are removed by the invention.  
           [0006]    The most common flue gas desulferization (FGD) process is known as the “wet process”. In that process the sulfur dioxide-containing flue gas is scrubbed with a slurry containing, e.g., limestone. The scrubbing takes place, for example, in an absorption tower in which the gas flow is countercurrent to and in intimate contact with a stream i.e. a spray of slurry. Most commonly the slurry is made to flow over packing or trays. The spent slurry product of this FGD process contains both calcium sulfite and calcium sulfate. It has been found to be advantageous to convert the calcium sulfite in the product to calcium sulfate by bubbling air or other oxygen-containing gas through the slurry. In addition to calcium based scrubbing compositions, it is well-known to utilize ammonium or sodium based scrubbing reagents. Accordingly as used herein the term “scrubber composition” is intended to encompass all of these conventional scrubber compositions, including clear aqueous liquors of e.g., ammonium sulfate; and aqueous slurries, e.g., of calcium carbonate, calcium sulfate or ammonium sulfate.  
         SUMMARY OF INVENTION  
         [0007]    Briefly, and in accordance with the present invention, a scrubber system is provided which enables a substantially horizontal flow path for the gas which is being subjected to scrubbing. Among other advantages, this permits operation of the absorber with a differential pressure of zero or less.  
           [0008]    Existing cocurrent absorber designs require packing to achieve reasonable SO 2  removal efficiencies. The presence of this packing results in a positive differential pressure inlet-to-outlet (i.e., a net pressure drop) for the treated flue gas across the absorber which requires a booster fan or booster fan modification to overcome. Other absorber designs also result in a significant flue gas pressure drop and thus have the same booster fan requirement. In accordance with the present invention it has been found possible to even achieve a pressure rise in a cocurrent absorber if no packing material is included.  
           [0009]    With the packing removed, gas flowing through the absorber will have momentum transferred to it by the slurry spray and the gas pressure can actually rise across the absorber (i.e., the absorber will have a negative pressure drop). Thus, it is feasible to install a cocurrent absorber without the addition of a special booster fan or with minimum modification of an existing fan. A design such as this is especially useful for FGD retrofit applications, eliminating the need for expensive fan modifications.  
           [0010]    Another advantage of the cocurrent absorber design in retrofit applications is that cocurrent absorbers can be operated at higher gas velocities than countercurrent absorber designs. This advantage has two related benefits. First, the absorber cross-section can be smaller for cocurrent absorbers than for countercurrent absorbers. Thus, less space is required, which can be especially important in retrofit applications where available space is at a premium. Secondly, the “turn-up” ratio for cocurrent absorbers is better than for countercurrent absorbers. That is to say, the gas flow rate can be increased with less deleterious impact on performance for cocurrent absorbers than for countercurrent absorbers. Thus, it is easier to take a scrubber module “out of service” and treat all of the gas in the remaining on-line absorbers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the drawings:  
         [0012]    [0012]FIG. 1 is a schematic cross-sectional view of a first embodiment of a cocurrent scrubber system in accordance with the invention;  
         [0013]    [0013]FIG. 2 is a schematic cross-sectional view of a second embodiment of a cocurrent scrubber system in accordance with the invention;  
         [0014]    [0014]FIG. 3 is a schematic cross-sectional view of a third embodiment of a cocurrent scrubber system in accordance with the invention; and  
         [0015]    [0015]FIG. 4 is a schematic cross-sectional view of a fourth embodiment of a cocurrent scrubber system in accordance with the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]    The general features of the scrubbing systems of the present invention are illustrated in FIG. 1. The system  10  shown therein is characterized by several new and innovative features. A major such feature is the horizontal gas path which makes the scrubber very compact and provides a very low profile. The gas paths leading into the scrubber and leaving from the scrubber are very simple and can connect to an upstream particulate control device or possible booster fan without complicated duct runs. Similarly, the outlet duct can be connected through a straight duct run with the stack.  
         [0017]    The gas velocity at the inlet  12  to and exit  14  from the scrubber will typically be in the range of 50 to 60 fps. The velocity inside the scrubber will typically be between 20 to 30 fps.  
         [0018]    The scrubber spray zone  16  is integrally connected to the reaction tank  18 . Spray introduced into the scrubber path will fall by gravity into the reaction tank. The reaction tank  18  spans the entire gas path of the scrubber module. A one-stage or two-stage mist eliminator  20  is located at the end of the scrubber gas path to control entrained emissions of liquid droplets. This design is considered to be very cost efficient.  
         [0019]    Reagent  21  and make-up water  22  is added to the reaction tank  18  as needed based on pH and level. The reagent used can e.g., be the aforementioned limestone, in which event the resulting aqueous scrubber composition is a limestone slurry, typically containing e.g., 10-25% solids. The scrubber composition in the reaction tank  18  is put in contact with the gas path by means of multiple recycle pumps  24  that serve individual spray headers  26  located in the gas path. Flue gas sulfur dioxide (as well as the acid halogen gases such as Hcl) is absorbed into the spray and the chemical reactions are allowed to be completed in the reaction tank including dissolution of the reagent, oxidation of the byproduct via oxidation air  30  provided to sparge ring  32  and crystallization of the byproduct, e.g., gypsum which is removed at  28 .  
         [0020]    The spray headers  26  are located perpendicular to the gas path and span the entire cross section of the gas path. Four rows of such headers are representatively shown; the number can vary depending upon system requirements. Spray nozzles  27  distributed across the spray headers  26  are used to atomize the recycle solution. Since the headers extend into the plane of FIG. 1, spray nozzles  27  actually form a series of matrices. The spray nozzles  27  introduce the recycle solution in a direction which is generally cocurrent with the gas flow and are specifically designed to generate a draft through the scrubber such that the pressure drop through the scrubber is eliminated, or there may actually be a pressure rise. This is achieved by the cocurrent spray, the nozzle spray angle, distribution of spray nozzles  27 , pressure drop across the spray nozzles  27 , and the spray pattern from the spray nozzles. The spray angle is typically 40 degrees but can vary from 20 to 120 degrees. The nozzle distribution is such as to provide an even distribution of recycle solution. Typically 200 gpm nozzles are used but the nozzle size can vary from 100 gpm to 400 gpm. The pressure drop across the spray nozzles is typically 20 psi but can vary from 8 psi to 40 psi. The spray pattern is typically full cone but semi full cones from spiral nozzles are also acceptable. It is important that the nozzles  27  be efficient in converting the fluid pressure to velocity  
         [0021]    The draft generated in the scrubber eliminates the need for a booster fan, simplifying and reducing the cost of retrofitting scrubbers to existing boilers. Many existing boilers typically do not have enough fan capacity to accommodate the pressure drop associated with a scrubber retrofit.  
         [0022]    Sparge ring  32  is designed to introduce oxidation air and to provide agitation of the reaction tank composition. The sparge ring  32  is basically a ring header submersed in the reaction tank  18  and located between 1 and 2 feet of the bottom of the reaction tank. The ring header can be circular, square, or otherwise, depending upon the geometry of the tank. The sparge ring header has a multitude of penetration points which ejects compressed air into the slurry. The main purpose of the ring header is to provide oxidation air for oxidation of liquid phase sulfite ions to sulfate ions. A secondary but very important function of the sparge ring  32  is to agitate the slurry in the reaction tank so that no or very limited buildup of solids occur on the reaction tank floor. This avoids the need for separate agitators and corresponding equipment installation and maintenance.  
         [0023]    [0023]FIG. 2 illustrates an embodiment of the invention, which is particularly applicable to double loop operation. Components of the system  40  corresponding to those in system  10  are identified by the same reference numerals.  
         [0024]    This alternative is particularly designed to provide a byproduct which is very pure and a chemistry in the main spray zone which is free from chlorides (and fluorides) and hence very reactive and efficient in removing flue gas sulfur dioxide.  
         [0025]    The spray zone is divided into a primary gas zone  42  and a secondary gas zone  44 , separated by a one-stage mist eliminator  46 . Similarly, the reaction tank  18  is separated by vertical partition  19  into a primary reaction section  48  and a secondary reaction section  50 . Recycle solution entering the primary gas zone is prevented from entering into the secondary gas zone by the mist eliminator  46  and liquid captured by the mist eliminator is returned to the secondary reaction section  50 . The primary gas zone  42  primarily captures flue gas hydrochloric acid (hydrogen chloride gas) and (if present) flue gas fly ash. No reagent is directly added to the secondary reaction section  50 . The only reagent entering the secondary reaction tank comes with the byproduct bleed from primary reaction section  48  which proceed at  51  via bleed pump  53 . The secondary reaction section  50  operates at a lower pH compared to the primary reaction tank, providing an environment for quick dissolution of residual reagent and hence production of a pure byproduct  55  (e.g. gypsum). The chlorides and/or fluorides exit as well at  55 , and are subsequently washed from the filter cake (e.g., of gypsum).  
         [0026]    Partition of the gas path is very easy and cost effective in a horizontal tower as compared to a vertical tower, which requires considerably more structural components to achieve the same task.  
         [0027]    The reagent  20  (typically limestone) is added to the primary reaction section  48  and the aqueous reagent laden recycle slurry is introduced as a spray into the secondary gas zone by the recycle pumps  24  and the nozzles  44  connected to these pumps. The pH in the recycle slurry can be fairly high as no chlorides are present and therefor the slurry can be very efficient in absorbing flue gas sulfur dioxide. In the design phase, this provides an opportunity to reduce the capacity and cost of the recycle pumps to achieve the required efficiency of the scrubber.  
         [0028]    This embodiment also generates a positive draft and a booster fan is not required to push the flue gas through the scrubber.  
         [0029]    The primary and secondary reaction tanks are again equipped with the aforementioned sparge ring  32 . Separate oxidation in both tanks is required to control the scaling potential in the secondary reaction tank.  
         [0030]    [0030]FIG. 3 illustrates an embodiment of the invention which is particularly applicable to remove residual fly ash in the flue gas ahead of the main scrubbing step to avoid costly upgrades of the station&#39;s existing particulate control devices and to produce a byproduct which is very pure. Components of the system  60  corresponding to those in system  10  are identified by the same reference numerals.  
         [0031]    The spray zone is divided into a primary gas zone  62  and a secondary gas zone  64  separated by a one-stage mist eliminator  66 . Similarly, the reaction tank  18  is separated by vertical partition  67  into a primary reaction section  68  and a secondary reaction section  70 . The secondary reaction section  70  is provided with a side mounted agitator  73 . The scrubbing compositions of the primary reaction section  68  and the secondary reaction section  70  are kept completely separate. Recycle solution entering the primary gas zone  62  is prevented from entering into the secondary gas zone by the mist eliminator  66  and liquid captured by the mist eliminator is returned to the secondary reaction section  70 . Oxidation air  69  is provided to sparging ring  32  which is present only in primary reaction section  68 . Oxidation air is not provided to secondary reaction section  70  as oxidation is not required there. Essentially only water  71  is provided to section  70 . The secondary reaction section  70  solution is introduced into the primary gas zone  62  primarily to remove flue gas fly ash  72  and flue gas hydrochloric (or other halogen) acid. The primary reaction section composition again is typically an aqueous slurry of water  73  and as a reagent  75 , limestone. This slurry is introduced in the secondary gas zone primarily to remove flue gas sulfur dioxide.  
         [0032]    In a vertical tower arrangement, two separate scrubbing towers and associated equipment and duct work would be required to achieve the same result. The horizontal tower configuration is very simple and eliminates costly equipment. This design also generates a positive draft and a booster fan is not required to push the flue gas through the scrubber.  
         [0033]    The embodiment of the invention shown in FIG. 4 is particularly designed to use flue gas heat to evaporate water from a clear liquor scrubbing solution such as ammonium sulfate while maintaining a clear liquor operation in the main scrubbing zone. This offers three distinct advantages, (1) flue gas heat can be used to evaporate water eliminating the use of other costly energy sources, (2) clear liquor operation in the main scrubbing zone eliminates the potential for plugging and scaling as well as physical wear and tear on rotating equipment, and (3) the need for separate crystallization equipment is obviated. Components of the system  80  corresponding to those in system  10  are identified by the same reference numerals.  
         [0034]    The spray zone in system  80  is again divided into a primary gas zone  82  and a secondary gas zone  84  separated by a one-stage mist eliminator  86 . Similarly, the reaction tank is separated by vertical partition  83  into a primary reaction section  87  and a secondary reaction section  88 . As in FIG. 3, the sparger ring  32  provides oxidation air to primary reaction section  87 , but no oxidation air is provided to secondary section  88 . Recycle solution entering the primary gas zone  82  is prevented from entering into the secondary gas zone  84  by the mist eliminator  86  and liquid captured by the mist eliminator  86  is returned to the secondary reaction section  88 . The secondary gas zone and the primary reaction section  87  operate with a clear liquor solution, e.g. around 30 percent ammonium sulfate. Complete oxidation is achieved at the primary reaction section  68  via air from sparge ring  32 .  
         [0035]    Byproduct solution is bled from the primary reaction section  87  to the secondary reaction section  88  by means of washing the intermediate mist eliminator  86  with solution from the primary reaction section  87 . The solution in the secondary reaction section  88  is allowed to operate above the saturation point by evaporating water from the solution using the sensible heat of the flue gas. The crystals generated in the secondary reaction section  88  can be recovered by passing a bleed stream  89  from the secondary reaction section  88  through a hydrocyclone and returning the clear overflow to the secondary reaction section  88 . The agitator  73  serves to inhibit settling of solids in secondary reaction section  88 . This design also generates a positive draft and a booster fan is not required to push the flue gas through the scrubber.  
         [0036]    While the present invention has been described in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.