Abstract:
An FGD system is provided which can be retrofitted on existing utility coal-fired boilers. The design is based on a horizontal co-current scrubber capable of generating a pressure rise across the absorber. Modifications to existing plant equipment are minimized by the co-current horizontal scrubber design. The system includes features, which eliminate much equipment typically associated with other FGD designs, and reduces the use of support equipment such as tanks, agitators, and pumps. It also minimizes or eliminates the need for new buildings.

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/196,196, filed Oct. 15, 2008. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to flue gas desulfurization (“FGD”) systems, and more specifically relates to a low cost wet lime/limestone/sodium FGD system, to control sulfur dioxide (SO 2 ) and other pollutants (e.g., hydrochloric acid, hydrofluoric acid, sulfuric acid, mercury, selenium, and other trace constituents) emitted from coal-fired boilers. 
     BACKGROUND OF INVENTION 
     Increasing awareness of the undesirable effects of industrially generated and emitted coal combustion products (flue gas), has led to a corresponding need to treat these gases so as to remove the pernicious components or convert them to harmless (and often useful) products. The industrial flue gases with which the present invention is especially concerned are those produced by coal-fired boilers as are commonly employed in electric utility installations. Among the relatively pernicious flue gases of concern produced by these boilers are sulfur dioxide (SO 2 ) and other pollutants (e.g., hydrochloric acid, hydrofluoric acid, sulfuric acid, mercury, selenium, and other trace constituents). These pollutants have for many years been removed from industrial flue gases by “scrubbing” the flue gas with lime/limestone slurries or the like, most commonly in some type of vessel in which the flue gas is contacted with a counter-current flowing stream of the mentioned slurry. Such methodology can and is used in the large newer boiler installations found in many utility operations. However there also exist in the electric utility industry a large number of older, coal-fired boilers, which presently include no flue gas scrubbers, and thus are urgently in need of some instrumentality to remedy their continuing polluting emissions. Typically an overall SO 2  removal efficiency of 80% to 99% is desirable, but the costs of installing or retrofitting equipment capable of such results has in the past been very high, and therefore has tended to discourage the purchase and installation of what otherwise would be most desirable enhancements. The present invention has as one of its key objects to provide a system which will remedy such reluctance by virtue of producing outstanding results at what are comparatively modest costs which may also extend the useful life of the older, boilers rather than retiring them as an alternative to retrofitting more expensive FGD systems. 
     SUMMARY OF INVENTION 
     In accordance with the present invention, an FGD system is provided which can be retrofitted on existing coal-fired boilers. The design is based on a horizontal co-current scrubber capable of generating a pressure rise across the absorber. Modifications to existing plant equipment are minimized by the co-current horizontal scrubber design. The pressure rise created by the co-current design reduces or eliminates the pressure drop introduced by the retrofit FGD system to minimize or eliminate modifications or upgrades to the existing boiler induced draft (“ID”) or booster fans. The low profile of the horizontal scrubber reduces the costs associated with the new inlet duct from the ID fans to the absorbers and from the absorbers to the existing stack breech as well as associated structural steel and platforms. The compact equipment layout for the system allows it to be installed in plants where space is limited. The compact design also reduces the amount of structural steel required for the system and allows the system to be constructed in less time than conventional wet lime/limestone/sodium FGD systems. If necessary, bypass reheat or other modifications to cost-effectively convert the existing chimney to wet operation can be provided to allow the existing chimney to be used with minimal chimney modifications. 
     The FGD system of the invention is capable of achieving an SO 2  removal efficiency of 80-99% (with bypass, up to 92% removal; with reheat or wet stack, up to 99% removal). The system is capable of achieving these removals with any of the following reagents: 1) Inhibited oxidation, magnesium-enhanced lime; 2) lime, or limestone, forced oxidized to produce gypsum; 3) lime or limestone natural oxidation to produce a disposable waste, or 4) sodium-based reagents. The system may include additives to enhance SO 2  removal (e.g., organic acids such as DBA, i.e. di-basic acid, or sodium formate), or to control scale formation (e.g., thiosulfate to inhibit oxidation). 
     Since a new FGD system installation may increase plume opacity in some cases, the system can be provided with features for SO 3  control. Other multi-pollutant controls can be added to the system if desired for specific applications such as Hg removal. In addition to the unique design features of the co-current horizontal scrubber, the system is designed to reduce costs during engineering, procurement, and construction phases of an installation project. 
     Principal unique features of the FGD system of the invention include the use of co-current spray headers to create flue gas pressure rise; high velocity nozzles relative to the flue gas velocity (e.g., double hollow cone nozzles to promote pressure rise and SO 2  removal); flue gas sneakage control; bulk entrainment separation; quench system; flow distribution control; single step dewatering without reclaim tank and bleed pumps; agitation (without a mechanical mixer in lime/limestone forced oxidation systems); reagent preparation with pre-ground limestone(if available), sump and a unique reagent blending system; and a single process island requiring no tanks and associated equipment and controls. The system is based on a modular absorber design, using the absorber as building wall, and integrated buildings, and integrated reheat (if required). Organic acids such as DBA or other additives may be used for enhanced SO 2  removal 
     Standard modular absorber designs (e.g., 150, 250, 350 MW) and layouts can be quickly and easily applied to a wide range of boiler installations. The system eliminates equipment typically associated with other FGD designs by including: Supplying pre-ground limestone if available, hydrated lime or liquid sodium reagents to reduce reagent preparation equipment such as grinding or slaking systems; elimination of slurry storage tanks and pumps; elimination of reaction tank agitators by mixing the slurry with air spargers in lime/limestone forced oxidation systems; and provision of a single dewatering step (if required) to eliminate primary dewatering, filter feed tanks and pumps, reclaim tank and pumps, and associated electrical and instrumentation. The system minimizes byproduct conveyors and reduces the use of support equipment such as tanks, agitators, and pumps. It also minimizes or eliminates the need for new buildings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention is diagrammatically illustrated, by way of example of the most complex of the configurations for a limestone, force-oxidized system which includes all the features of the invention, in the drawings appended hereto, in which: 
         FIG. 1  is a schematic block diagram of an FGD system in accordance with the present invention; 
         FIG. 2  is an external perspective view of the  FIG. 1  system; 
         FIG. 3  is a perspective view taken from a viewpoint toward the upstream side of the FGD system, but with the inlet duct and most other portions outside the absorber removed to more clearly show the absorber details; 
         FIG. 4  is a further perspective view, similar in nature to  FIG. 3 , but taken from a viewpoint toward the downstream side of the absorber; 
         FIG. 5  is a schematic elevational view of the absorber portion of the system; 
         FIG. 6  is a schematic plan view of the absorber portion of the system; 
         FIGS. 7 and 8  are, respectively, schematic plan and elevational views of portions of the spray nozzle assemblages used in the absorber of the FGD system; and 
         FIG. 9  is a perspective view of the structural steel and process island portions of the system, with other parts of the system removed for clarity. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     This description refers to a limestone force-oxidized system, which is the most complex configuration of the invention. Referring to  FIG. 1 , a simplified block diagram appears illustrating an FGD system  15  in accordance with the invention.  FIGS. 2 through 9  can be considered simultaneously with  FIG. 1  for a better understanding of the invention. The system  15  integrates on a single foundation or process island  16 , numerous features to provide one compact, close-coupled, high-performance and cost-efficient design. 
     Raw flue gas  17  from a coal-fired boiler is provided to an inlet duct  1  which, in some applications, runs close-coupled to the absorber  18  ( FIGS. 5 and 6 ) and the absorber outlet duct  8 . Thereby, the inlet duct  1  can serve as the roof of the absorber spray and recycling pumps  10  and the oxidation air compressors  11 , and the cost of the duct itself can be reduced. The inlet duct  1  is also in close proximity to the outlet duct  8 , which facilitates regenerative reheat, if required. An alternative arrangement where a new stack is to be installed along with a new FGD system  15 , is to bring the flue gas  17  directly into the inlet  1  of the absorber  18  and have it exit as treated gas  19  directly into the stack  22  with no turns of direction. By eliminating turns in the flue gas, the overall pressure drop can be kept to a minimum, helping eliminate the need for a new booster fan. Conventional vertical absorbers require two 90 degree turns in the flue gas, one at the inlet to the tower and one at its outlet, which cause increases in pressure drop. These turns are unavoidable, regardless of how the inlet and outlet ductwork are arranged. Gas flow distribution devices  2  such as spreader vanes and/or turner vanes, and for perforated plates are located in the inlet duct  1  to distribute the gas, side-to-side and top-to-bottom, such that the flue gas exhibits an even velocity profile as it enters into the spray zone  4  of absorber  18 . As a result, recirculation zones are reduced and buildup in the absorber inlet duct is avoided. 
     The absorber  18  is a vessel, which includes reaction tank  7 , which is integral to the absorber, and as readily seen in  FIG. 5 , is defined at the bottom of absorber  18 , directly beneath spray zone  4  and mist eliminator space  21 . The reaction tank  18  is filled with slurry, typically about 15 percent solids (more or less) in scrubber liquor. The slurry is used as a carrier for dry reagent to eliminate feed equipment. In the case of a limestone, force-oxidized system, the solids are predominantly gypsum with traces of limestone. The slurry is injected into the upper part of spray zone  4  by recycle pumps  10  that take suction from the reaction tank and inject the slurry into the upper part through the spray headers  26  and  27 , via branches  27 ,  28 , and nozzles  30 . The number of spray headers can vary from two or more depending on the specific requirements of an installation to optimize SO 2  removal and pressure rise in the system. In lime/limestone forced oxidation systems, typically the recycle slurry (the upper level  34  of which is seen in  FIG. 5 ) is agitated by the oxidation air from compressor  11  so that mechanical agitators are not required. Side mounted agitators may be used in lime or sodium-based systems or for limestone forced oxidation systems in lower sulfur applications, if there is insufficient oxidation air to allow for good agitation. Typically the reagent is lime, limestone, or sodium and forced oxidation to produce gypsum is employed for the limestone system. Additives such as adipic acid, DBA (from DBA system  25 ), and sodium formate can be used to enhance SO 2  removal performance. SO 2  removal can be optimized by using various reagents and additives known to be useful for this purpose. 
     A single-stage quench header  3  is located ahead of the first main spray header  26  ( FIGS. 3 and 4 ). The function of quench header  3  is to keep the spray header clean and reduce buildup of solids on the walls of the absorber at the wet/dry interface. The quench header  3  can operate with either reclaim water, service water, or a blend of reclaim and service water and can operate continuously or intermittently. It allows flexibility in selection of slurry header materials to reduce cost (e.g., lower alloy and/or FRP headers). The spray zone  4  is provided slurry from main spray headers  26  and  27 . Additional spray headers can be added as necessary, based on performance requirements. These in turn feed vertical feed branches  28  and thence the multiple horizontal branches  29 , which carry the nozzles  30  that introduce the recycle slurry in a co-current fashion (FIGS.  3 , 4 , 5 , and  6 ). The number of headers, feed branches, and nozzles required depend on the size of the module and the performance required of the design for a specific installation. 
     Such a co-current arrangement generates an acceptable gas distribution profile in a horizontal arrangement throughout the absorber  18 . The slurry is injected as a droplet spray at a velocity that is higher than the bulk flue gas velocity. In this way, a favorable environment for mass transfer and momentum transfer is created. A pressure rise is generated as opposed to a pressure drop typical of conventional scrubbers. The higher the required SO 2  removal efficiency of the design, the higher the pressure rise will be due to the need to introduce greater slurry flows into absorber  8  to boost SO 2  capture rates. In many cases, the scrubber system  15  will not require a booster fan to operate as the net pressure drop will be close to zero inches of water gauge. The nozzle design can use either full-cone or hollow-cone spray patterns but typically uses a spray angle between 60 and 120 degrees to maximize momentum transfer, with a typical angle being about 75 degrees. The preferred orientation for the nozzles is to have the axis of the conical spray approximately parallel to the gas flow. However, the spray angle is required to cover the entire cross sectional area of the absorber with slurry. Operating pressure of the nozzles is typically around 20 psig but can vary from 10 psig to 45 psig or more as required. The droplet exit velocity from the spray nozzles  30  is typically from 15 to 45 feet per second, or higher if required of the design for a specific installation. 
     Double hollow cone or full-cone nozzles work well. Such nozzles are available commercially from numerous sources such as Bete, Spraying Systems, Lechler, and others. The first main spray header  26  and its branches is made of alloy material when an intermittent quench is selected, and the subsequent main spray header(s) and its or their branches is or can be made of fiberglass reinforced plastic (“FRP”) to further reduce system cost. If a continuous quench is used, all slurry headers and branches can be made of lower cost FRP if desired. Requirements for intermittent and continuous quench depend on water balance based on factors such as sulfur or chlorine content of coal. The spray nozzles  30  are typically made of abrasion resistant materials such as silicon carbide, but in clear liquor applications such as for sodium scrubbers, may be made of alloy. The ultimate design of the absorber spray zone  4  is based on SO 2  removal requirements and pressure drop requirements to avoid fan upgrades. Design parameters to optimize SO 2  removal and pressure drop include: (1) flue gas velocity in the spray zone  4 ; (2) spray nozzle design parameters such as nozzle pressure, spray angle, nozzle type (full cone, hollow cone, double down hollow cone), droplet velocity and droplet size; and (3) slurry pump design such as discharge pressure and flow rate to produce the optimum liquid-to-gas ratio (L/G) for SO 2  removal and pressure rise. Spray zone design and performance can be optimized by use of techniques such as CFD modeling and FGDPRISM (simulation program) modeling calibrated based on test results, operating data, and experience. 
     A bulk entrainment separator or roughing mist eliminator (“ME”)  5  is present in the ME space  21  adjoining absorber  18  spray zone  4 , typically around 10 ft (more or less) ahead of the conventional ME  6 . The roughing ME  5  design is based on CFD modeling to reduce liquid loading and improve gas velocity profile at the ME face. The design is optimized based on CFD modeling and is designed to minimize pressure drop while achieving necessary removal of entrained liquid and straightening flow upstream of the conventional ME. The roughing ME may typically be fabricated from nominal 8 inch CPVC pipe cut in half lengthwise or other suitable devices. The device such as half pipes typically stretches from the reaction tank  7  liquid level  34  ( FIG. 5 ) to or near the absorber roof. A second stage immediately behind the first stage and offset can be used to maximize efficiency, if required. Pressure drop is typically around a ¼ inch and typically the liquid loading at the face of conventional ME  6  is reduced by around 50 percent or more as needed. The roughing ME  5  also improves the gas velocity profile ahead of the conventional ME  6 . The ME design is based on (1) CFD modeling and/or (2) physical modeling to optimize performance to minimize pressure drop and slurry carryover. If necessary a two stage mist eliminator  6  can successfully operate at velocities up to 20 feet per second provided that the drain boxes are located such that flooding of the vanes is avoided and that the mist eliminator vanes may typically employ a hook if needed and avoid connection hardware on the trailing edge that generate mist carryover. 
     If desired, the outlet duct  8  can be placed in close proximity to the absorber inlet duct  1 , facilitating use of regenerative reheat, if needed. The outlet duct  8  is also at the same elevation as the inlet duct  1 , making possible a very short duct run to the stack  22  leading to a lower installed cost. The outlet duct length is minimized compared to the inlet duct since the materials for the outlet duct are more expensive. In many cases the existing stack  22  can be used when the system  15  is installed to avoid the cost of a new stack. Several options are available to reuse the existing stack  22 . One option is to reheat flue gas, which can be accomplished at reheat  9  with a small bypass of flue gas around the absorber, regenerative close-coupled reheat, or steam/hot water indirect reheat. The existing stack  22  can in many uses be operated wet by lining the stack flue to create a smooth surface and protect the integrity of the stack. Due to the high performance of the system  15 , SO 2  removal efficiency above 90 percent is still possible with 5 to 6 percent flue gas bypass 
     The spray and recycle pumps  10  ( FIG. 4 ) can be dedicated to a single spray header or manifolded together to serve several spray headers. Using an elevated nozzle pressure (e.g., 20 psig or more) drop allows for recycle pump turndown in a manifold arrangement without jeopardizing the integrity of the nozzle spray pattern and provides good transfer of momentum from the slurry to the flue gas to reduce pressure drop. The elevation of the spray header nozzles  30  relative to the liquid height  34  in the reaction tank  7  is very low and the fact that the main slurry recycle stream does not have to be lifted above the inlet duct typical of conventional scrubbers allows the higher pressure drop at the nozzles  30  to be achieved without a resultant increase in pump and operating costs compared to conventional scrubbers. 
     In limestone forced oxidation systems, the oxidation air compressors  11  (one operating is shown, one spare is otherwise provided) serve dual purpose. First, oxidation air is introduced into the reaction tank  7  to convert the absorbed SO 2  to sulfate and thus produce a gypsum byproduct  23  (shown in  FIG. 2  accumulating at gypsum stack  24 ), which also helps to minimize the chemical scaling inside the absorber. Second, the oxidation air is introduced through a sparge header  32  such that no additional agitation is required, thereby eliminating the need for dedicated agitators. The design and arrangement of the sparge header may vary for the invention depending on performance requirements for the system design. 
     Preground limestone, hydrated lime or sodium-based reagent can be used as reagent and stored in a silo  12  which may be close coupled to the absorber reaction tank  7  depending on site specific equipment arrangements. Two feed systems (one operating, one spare) feed the reagent to the sump  13 . The structural steel  31  ( FIGS. 1 and 9 ) used to support the absorber is used to support the reagent storage silo  12  if required and also provides supports for inlet duct  1 , outlet duct  8 , dewatering station  14 , and structure to enclose equipment if needed in colder climates. The structural steel extends from the absorber out to the two bays  41  and  43 , one on either side of the absorber. This external structural steel can effectively be used to absorb the lateral forces on the absorber walls. Having remote steel in this fashion is much more effective than reinforcing the absorber walls only and results in a reduction in steel quantity. By serving a dual purpose, the installed cost of the absorber is kept to a minimum. 
     The sump  13  is an underground pit typically made of concrete and lined e.g. with ceramic tile. Connected to the pit are trenches  36  that collect any liquid spills in the absorber area and funnel the liquid to the sump. The single absorber area sump  13  serves multiple purposes. First, the sump  13  is used to mix the reagent with recycle slurry from the reaction tank  7 . A constant feed rate of recycle slurry is drained into the sump from the reaction tank (no pump required) and mixed with reagent. The feed rate of reagent is controlled by the reaction tank pH. The reagent is returned to the reaction tank with sump pumps. Second, the sump collects any water or slurry collected in the trench system surrounding the absorber area. This unique design allows reagent feed and storage equipment to be minimized or eliminated. It also is possible to use the sump as the limestone feed tank. 
     Slurry from the reaction tank  7  is bled directly to a hydrocyclone system  38  at dewatering station  14  from the slurry recycle header without the use of bleed pumps. The hydrocyclone overflow is sent into the reaction tank  7  directly without the use of pumps or an intermediary storage vessel. The hydrocyclone underflow is directed to a single belt filer  40 . The belt filter  40  solids content may be adjusted to avoid any blowdown stream of fines and/or chlorides. The close coupling of the hydrocyclones, belt filter and recycle headers makes it possible to accomplish the dewatering step without any tanks, agitators, and pumps. The belt filter  40  vacuum system returns the reclaim water  42  directly to the reaction tank  7 . If the system operator uses a waste pond to store and/or dewater by-product, the dewatering equipment can be eliminated entirely. 
     The entire system  15  is closed coupled with only one process island  16  encompassing all process equipment. The structural steel  31  ( FIG. 9 ) used for the support of the inlet duct, the reagent silo  12 , and the dewatering station  14 , is an integral part of the structural support of the absorber vessel. The structural steel  31  for the absorber is designed to also accommodate structure to enclose equipment if needed in colder climates (e.g., slurry pumps, oxidation air compressors, vacuum pumps, filtrate tank and pumps, etc.). A spreader footer may be used as a low cost foundation as the height and the aspect ratio (low profile) support such a design. No process tanks (and associated foundations, pumps, agitators, control and electrical systems) are required. The structural steel as well as the absorber uses a modular design allowing for reduced construction duration and cost. 
     While the present invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations on the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. For example, while the system described above is designed to efficiently remove sulfur dioxide, halogens, and oxidized mercury, the design will also efficiently remove selenium as well as particulates and arsenic. Accordingly, the invention is to be broadly construed, and limited only by the scope and spirit of the disclosure and of the claims now appended hereto.