Abstract:
One embodiment of the present invention relates to a process for treating a flue gas stream ( 22 ) from a coal-fired boiler to reduce the amount of mercury contained therein. The process includes subjecting the flue gas stream ( 22 ) to a first wet scrubbing process for reducing the concentration of sulfur oxides therein and thereby producing a water-saturated flue gas stream ( 22 ) and subjecting the water-saturated flue gas stream to a second wet scrubbing process having an aqueous acidic solution comprising hydrogen peroxide to produce a flue gas having a reduced mercury content.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process and system for removing contaminants from a flue gas. More particularly, the present invention relates to a process and system for removing mercury from a flue gas. 
     2. Description of the Related Art 
     The combustion of coal in a coal-fired boiler, such as incident to the production of steam for turbines at a power generating station or process steam at industrial plants, produces a waste gas, referred to as flue gas, containing numerous particulate and gaseous contaminants. Among these contaminants are fly ash, nitrogen oxides, sulfur oxides and the like, whose concentration in the flue gas must be reduced, in compliance with federal, state or municipal laws and ordinances, before the gas may be emitted to the atmosphere. Included among the contaminants is mercury, which is released from the coal as gaseous elemental mercury in the elevated temperature environment of the combustion process and which, as a result of cooling of the flue gas and various contaminant-reduction treatments to which the flue gas is subsequently subjected, may end up as a mix of elemental and oxidized mercury species in the flue gas. 
     In a typical flue gas treatment process, the flue gas from the coal-fired boiler is first subjected to fly ash removal, such as with an electrostatic precipitator or a baghouse, followed by processes for removing sulfur oxides from the flue gas (flue gas desulfurization, or FGD). A common FGD system is a so-called wet flue gas desulfurization (WFGD) method, also referred to simply as wet scrubbing, in which the flue gas enters a large vessel (spray tower or absorber) and is sprayed with a water slurry containing lime or limestone. The calcium in the slurry reacts with the sulfur oxides, most prominently SO 2 , to remove these oxides from the flue gas in the form of calcium sulfite or calcium sulfate. Such process produces a substantially desulfurized water-saturated flue gas for eventual emission into the atmosphere. Optionally, prior to emission into the atmosphere, the flue gas is further treated in various ways. For example, a moisture removal step, nitrogen oxide reduction treatment, or the like. 
     In this typical process, a small portion of the mercury in the combustion flue gas is removed in association with the fly ash during the fly ash collection step, while additional portions of mercury are removed via dissolution in the course of the wet scrubbing process. Nevertheless, the mercury removal efficiency achieved in a typical fly ash pre-removal/WFGD process for a flue gas from a coal-fired boiler is not sufficient for achieving compliance with applicable mercury removal requirements. Improvements have been attempted in which mercury complexing agents are added to the scrubbing slurry so as to promote more effective mercury removal during the WFGD process, but even here the best results obtained are not satisfactory for compliance with applicable air quality regulations. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention relates to a process for treating a flue gas stream produced by combustion of coal, to reduce a content of mercury in the flue gas stream. The process includes: subjecting the flue gas stream to a particulate collector; thereafter subjecting the flue gas stream to a first wet scrubbing process for reducing a concentration of sulfur oxides therein through contact of the flue gas stream with an aqueous calcium-based slurry to form sulfate and sulfite salts and produce a flue gas stream having a reduced sulfur oxide content; thereafter passing the flue gas stream through a mist eliminator to remove moisture from the flue gas stream; and thereafter subjecting the flue gas stream to a second wet scrubbing process having an aqueous acidic scrubbing solution comprising hydrogen peroxide to produce a flue gas having a reduced mercury content. 
     Another aspect of the invention relates to a system for reducing a mercury content of a flue gas stream from a coal-fired boiler. The system includes: a particulate collector for receiving a flue gas stream from a coal-fired boiler; a first wet scrubber apparatus for receiving the flue gas stream from the particulate collector; a mist eliminator operatively connected to the first scrubber apparatus; and a second wet scrubber apparatus for receiving the flue gas stream from the mist eliminator, wherein the second wet scrubber apparatus comprises means for contacting the flue gas stream with an aqueous acidic scrubbing solution comprising hydrogen peroxide to reduce the mercury content of the flue gas stream. 
     Yet another aspect of the present invention relates to a system for removing contaminants, including mercury, from a flue gas stream. The system includes: a first scrubber for scrubbing the flue gas stream with an aqueous calcium-based slurry to produced a water-saturated flue gas stream of reduced sulfur oxide content; a moisture removing means for removing water droplets and moisture from the water-saturated flue gas stream; and a second scrubber for scrubbing the flue gas stream located downstream of the moisture removing means, wherein the second wet scrubber introduces an aqueous acidic scrubbing solution comprising hydrogen peroxide, thereby removing mercury from the flue gas stream. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  illustrates one embodiment of a system for processing and treating flue gas; 
         FIG. 2  illustrates one embodiment of a system for processing and treating flue gas; 
         FIG. 3  illustrates one embodiment of system of a system for processing and treating flue gas; and 
         FIG. 4  illustrates one embodiment of a method for treating flue gas. 
     
    
    
     DETAILED DESCRIPTION 
     While not wanting to be bound to any theory, it is believed the inefficiency of current WFGD systems known in the art, including those using complexing agents in the calcium-based scrubbing slurry, in reducing the mercury content of the flue gas arises, at least in part, because the massive sulfur oxidation occurring in the WFGD system incident to sulfur oxide removal correspondingly causes the reduction of mercuric species to elemental mercury, which remain as a gaseous species in the flue gas stream rather than precipitating out in ionic form along with the sulfate and sulfite salts. In one embodiment of the current invention, reducing the mercury content of the flue gas stream occurs when the flue gas stream is scrubbed in a second wet scrubber apparatus with an aqueous acidic scrubbing solution at a time when most of the sulfur oxides have been removed from the flue gas stream. 
     In one embodiment of the present invention as depicted in  FIG. 1 , the system for reducing contaminants, such as sulfur oxides and mercury, in a flue gas stream includes a treatment train first having a particulate collector  20 . Particulate collector  20  may be any apparatus known in the art that facilitates the removal of particulates such as fly ash from a flue gas stream  22  of a coal-fired boiler. Examples of particulate collectors include, but are not limited to, electrostatic precipitators (ESPs), baghouses, screens, sieves, and the like. 
     Flue gas stream  22  exits particulate collector  20  and is directed to a first wet scrubber apparatus  24  through suitable flue work. First wet scrubber apparatus  24  may be any suitable apparatus or arrangement that enables intimate contact between the flue gas and an aqueous calcium-based slurry  26  for removing acid gases, such as sulfur oxides, from flue gas stream  22 . Examples of first wet scrubbing apparatus  24  include a spray tower or absorber used in WFGD systems. 
     Calcium-based slurry  26  is stored in tank  28  prior to introduction to first wet scrubber apparatus  24 . 
     Flue gas stream  22  exits first wet scrubber apparatus  24  and may be directed to a moisture removal device  30  for removing slurry droplets and moisture from the flue gas stream. Moisture removal device  30  may be a mist eliminator. As shown in  FIG. 1 , the flue gas stream  22  may be directed to moisture removal device  30  which is directly integrated with first wet scrubbr apparatus  24 . Alternatively, and as shown in  FIG. 2 , moisture removal device  30  can be a stand alone unit to which the scrubbed flue gas stream is directed through flue work. 
     Downstream of first wet scrubber apparatus  24  and moisture removal device  30  is a second wet scrubber apparatus  32  to which flue gas stream  22  is passed. Second wet scrubber apparatus  32  is similar to first wet scrubber apparatus  24  in that it may be a spray tower or absorber used in WFGD systems. 
     Second wet scrubber apparatus  32  is fed from a tank  34 , which holds an aqueous acidic scrubbing solution. The aqueous acidic scrubbing solution contains hydrogen peroxide. The hydrogen peroxide may be present in the aqueous acidic scrubbing solution in a concentration between about 0.1% to about 15% by weight, based on the weight of the aqueous acidic scrubbing solution. Tank  34  is supplied with a hydrogen peroxide solution having a concentration of between about 15% and about 50% and with dilution water so as to provide an aqueous acidic scrubbing solution of appropriate concentration. The aqueous scrubbing solution can be replenished in tank  34  either continuously or batch-wise. 
     Duct work from tank  34  may terminate at second wet scrubber apparatus  32  in at least one device  36  to control the flow of the aqueous acidic scrubbing solution to the second wet scrubber apparatus. Device  36  can be a nozzle, a valve or the like. Additionally, as shown in  FIG. 3 , the duct work may be connected to a monitoring device  38  to monitor an amount of aqueous acidic scrubbing solution that is fed into second wet scrubber apparatus  32 . Monitoring device  38  may be a meter, a gauge, a computer, or any other device that can monitor the amount of aqueous acidic scrubbing solution transported to second wet scrubber apparatus  32 . Monitoring device  38  optionally has a user interface that allows a user to increase, decrease, stop or start the flow of the scrubbing solution to second wet scrubber apparatus  32 . Other monitoring devices may be placed throughout the system at other connections where a user deems monitoring necessary. 
     After the aqueous acidic scrubbing solution is introduced to second wet scrubber apparatus  32 , it contacts the flue gas stream  22  present in the second wet scrubber. The hydrogen peroxide present in the aqueous acidic scrubbing solution reacts with mercury present in flue gas stream  22  and removes it therefrom. Typically, elemental mercury is present in flue gas stream  22 , which is in second wet scrubber apparatus  32 . The hydrogen peroxide oxidizes the elemental mercury which is then transformed into a soluble mercury compound, such as mercury oxide. The soluble mercury is then easily removed from flue gas stream  22 . 
     Flue gas stream  22  emanates from second wet scrubber apparatus  32  and is passed to a stack  40  by pipes or ducts which allow the flue gas stream to pass and ultimately be released out into the atmosphere. 
     Referring now to the process sequence shown in  FIG. 4 , in step  42 , flue gas stream  22 , from which all or a substantial portion of the fly ash has been removed, is introduced to first wet scrubber apparatus  24 . First wet scrubber apparatus  24  typically removes contaminants through a first wet scrubbing process that uses a calcium-based slurry sprayed into the first scrubbing apparatus. Typically, such process is for the principal purpose of reducing the concentration of sulfur oxides present in the flue gas stream. 
     In a WFGD system, for example, a calcium-based slurry is sprayed into the scrubbing apparatus for contact with the flue gas, whereby SO 2  present in the flue gas stream reacts with the alkaline slurry components to form calcium sulfate or calcium sulfite. A substantially water-saturated flue gas stream is produced by first wet scrubber apparatus  24  after flue gas stream  22  has been subjected to the scrubbing process. 
     After such treatment, in step  44 , flue gas stream  22  may be introduced to moisture removal device  30 , which is used to remove moisture from the flue gas stream. As shown in  FIGS. 1-3 , moisture removal device  30  is operatively connected to first wet scrubber apparatus  24  to collect slurry droplets and remove moisture from flue gas stream  22 . 
     Once flue gas stream  22  has passed through first wet scrubber apparatus  24  and moisture removal device  30 , in step  46 , the flue gas stream is then introduced to second wet scrubber apparatus  32  located downstream of the moisture removal device. At this stage, flue gas stream  22  contains less than about 100 ppm of SO 2 , however the flue gas stream still contains other contaminants, such as mercury. 
     In step  48 , second wet scrubber apparatus  32  removes contaminants, including mercury, through a second wet scrubbing process that uses an aqueous acidic scrubbing solution containing hydrogen peroxide, sprayed into the second wet scrubber apparatus. From second wet scrubber apparatus  32 , flue gas stream  22  may be subjected to further downstream flue gas treatment, or be released into the atmosphere through stack  40 . 
     Second wet scrubber apparatus  32  is fed by tank  34  which holds the aqueous acidic scrubbing solution that facilitates the removal of mercury from flue gas stream  22 . The aqueous acidic scrubbing solution contains hydrogen peroxide at a concentration of between about 0.1 to about 15% by weight based on the weight of the aqueous acidic scrubbing solution. The hydrogen peroxide content of the solution is replenished through use of commercially available hydrogen peroxide solutions containing from about 15% to about 50% by weight of hydrogen peroxide, based on the weight of the hydrogen peroxide solution, with a separate feed of dilution water to produce the requisite hydrogen peroxide concentration in the scrubbing solution. The aqueous acidic scrubbing solution has a pH less than about 1. 
     As shown in step  50 , the aqueous acidic scrubbing solution containing hydrogen peroxide is effective to remove at least a portion of the mercury from flue gas stream  22  through formation of mercury oxides (HgO x ). The formed mercury oxides are dissolved or otherwise captured by the aqueous acidic scrubbing solution and thus removed from flue gas stream  22 . 
     The aqueous acidic scrubbing solution containing hydrogen peroxide is introduced to second wet scrubber apparatus  32  for contact with flue gas stream  22  by spraying or misting the aqueous acidic scrubbing solution into the scrubbing apparatus. Alternatively, flue gas stream  22  may pass over a collection or pool of the aqueous acidic scrubbing solution located within second wet scrubber apparatus  32 . 
     After being scrubbed with the aqueous acidic solution, flue gas stream  22  is received by a stack  40  and released into the atmosphere. The aqueous acidic scrubbing solution containing the contaminants removed from flue gas stream  22  is discharged from second wet scrubber apparatus  32 . The discharged aqueous acidic solution typically contains sulfuric acid and mercury and can be subjected to further treatment, for example neutralization by the addition of lime or limestone, or any other neutralizing compound. Alternatively, the discharged aqueous acidic scrubbing solution can be sent to a waste water treatment plant for disposal. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.