Abstract:
The apparatus comprises a NO X  reducing unit, a duct to receive a flue gas stream from said NO X  and reducing unit, and an activation source associated with the duct. In use, the activation source applies energy to the flue gas stream to facilitate the removal of contaminants from the flue gas stream. Further, the method comprises providing an activation source is downstream of a NO X  reducing unit. The activation source is then activated to facilitate the removal of contaminants from the flue gas stream.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 09/371,256 filed Aug. 10, 1999 now U.S. Pat. No. 6,267,940. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates generally to the control of pollutants from combustion processes. More particularly, this invention relates to a technique for enhancing NO X  reducing catalyst activity and thereby efficiently removing NO X  from a combustion process gas stream. 
     BACKGROUND OF THE INVENTION 
     The 1990 Clean Air Act Amendments require major sources of air emissions to limit the discharge of NO X  . NO X  is present in the flue gas emitted from combustion processes. Therefore, cost-effective methods for controlling NO X  are of significant interest. 
     Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR) catalysts for NO X  removal are known in the art. Conventional NO X  SCR catalysts require large amounts of catalyst and the flue gas stream to be at relatively high temperatures (between approximately 300 to 400° C.) in order to have sufficient activity for effective NO X  reduction. In such schemes, ammonia or urea are also typically added as a reducing agents. However, significant problems are associated with the use of reducing agents, including, for example the formation of ammonium compounds from ammonia, referred to as ammonia “slip.” Ammonia “slip” occurs when unreacted ammonia and ammonium compounds pass out of the SCR unit. Such ammonia “slip” can plug downstream air heaters and impact ash use and disposal. SNCR is not as effective as SCR processes and the use of urea in SNCR also produces unwanted ammonia and ammonium compounds. Therefore, it would be desirable to have a process than can supplement NO X  reduction downstream of the SCR/SNCR process, where the process can also reduce ammonia slip. 
     In addition to SCR and SNCR, low NO X  burners (LNB) positioned in the furnace are also commonly used for NO X  reduction. However, the LNB&#39;s are not as effective as SCR/SNCR at removing NO X  from the combustion process. Therefore, it would also be desirable to have a process that can supplement NO X  reduction downstream of LNB burners to meet more stringent NO X  emissions control requirements. 
     There are ongoing efforts to develop low temperature catalysts for applications between approximately 100 to 250° C. Unfortunately, these low temperature catalysts are sensitive to high SO x  concentrations in the flue gas. There are also NO X  SCR catalysts being developed that are reagentless, so, for instance, ammonia is not required as a reducing agent. These catalyst systems can benefit from the imposition of additional and alternative activating agents to the catalyst so that they can operate at lower temperatures, be less sensitive to poisoning agents, and more reactive without the addition of chemical agents. 
     In view of the foregoing, it would be highly desirable to provide an approach to enhance NO X  removal. Ideally, the technique would reduce the amount of catalyst needed, or would operate at relatively low temperatures, or reduce the amount of required chemical reducing agents. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an apparatus for decreasing the concentration of contaminants, such as NO X  or reducing agents, present in a gas stream. The apparatus comprises a NO X  reducing unit, a duct to receive a flue gas stream from said NO X  and reducing unit, and an activation source associated with the duct. In use, the activation source applies energy to the flue gas stream to facilitate the removal of contaminants from the flue gas stream. 
     Further, according to the invention there is provided a method of decreasing the concentration of contaminants within a flue gas stream. An activation source is provided downstream of a NO X  reducing unit. The activation source is associated with a duct configured to convey a flue gas stream. The activation source is then activated to facilitate the removal of contaminants from the flue gas stream. 
     In this way, the inclusion of an activation source downstream of a NO X  reducing unit decreases the concentration of NO X  and reducing agents within the flue gas stream. The activation source of the invention may also be used with a low NO X  burner to reduce NO X  even in the absence or reducing agents that are present in other embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a NO X  reduction apparatus constructed in accordance with a first embodiment of the invention. 
     FIG. 2 illustrates a NO X  reduction apparatus constructed in accordance with a second embodiment of the invention. 
     FIG. 3 illustrates a testing apparatus used to establish the efficacy of the technique of the invention. 
     FIG. 4 illustrates a system for decreasing the concentration of contaminants within a gas stream in accordance with an embodiment of the invention. 
     FIG. 5 illustrates a method for decreasing the concentration of contaminants within a gas stream in accordance with the embodiment of the invention described in relation to FIG.  4 . 
    
    
     Like reference numerals refer to corresponding parts throughout the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a pollution removal system  20  for use with a combustion source, such as a fossil-fuel-fired boiler  22 , which receives air through an air inlet duct  24  to combust fuel, such as coal received through a fuel inlet duct  26 . The combustion process within the boiler  22  produces a gas stream in the form of flue gas which exits the boiler  22  through an outlet duct  28 . The flue gas produced within the boiler  22  is comprised of air, products of combustion in the gaseous form, such as water vapor, carbon dioxide, oxides of nitrogen and sulfur, halides, organic compounds, mercury, selenium and other trace metal vapors and particulate matter. A particulate collection device  30  is connected to the outlet duct  28  and removes particulate matter  32  from the flue gas. The particulate collection device outlet duct  34  directs the flue gas to the stack  36  where it is discharged. 
     The power plant components discussed up to this point are conventional. The invention is directed toward supplementing the operation of these components to include a process wherein a NO X  reducing catalyst is used to remove NO X  from flue gas in the outlet duct  28 . The invention utilizes a NO X  reducing catalyst injector  40  to inject a NO X  reducing catalyst powder into the outlet duct  28 . A separate activation source  42  is used to activate the NO X  reducing catalyst while it is in the output duct  28  and/or in the particulate collection device  30 . 
     The combination of the catalyst injection  40  and activation source  42  enhance the activity of the NO X  reducing catalyst and enable significant NO X  reduction in the duct  28  and the particulate collection device  30 . The invention also allows a NO X  reducing catalyst to be used at relatively low temperatures, e.g., between approximately 100 to 250° C. 
     The NO X  reducing catalyst injector  40  may be implemented as any standard particle injector. The activation source  42  is implemented as an energy producing mechanism. The energy created by the energy producing mechanism activates the NO X  reducing catalyst. The energy producing mechanism may be implemented to create an electrical field across the catalyst surface. The catalyst structure may also be irradiated with electromagnetic energy, such as microwave radiation, ultraviolet radiation, or infrared radiation. The activation source  42  may also be implemented to produce a magnetic field. 
     Those skilled in the art will appreciate that the foregoing activation techniques may be used alone or in combination. The underlying principle of the activation technique is to render the NO X  reducing catalyst sufficiently active through supplemental and alternative energy input and excitation energies so that lower amounts of catalyst are needed or the catalyst can operate in a relatively low temperature (between approximately 100 to 250° C.) flue gas stream. 
     In one embodiment of the invention, the catalyst injector  40  injects a fine NO X  catalyst powder into the duct  44  and the output duct  28 . The catalyst powder is irradiated by electromagnetic waves produced by the activation source  42 . This irradiation may occur in the duct  44  and/or the output duct  28 . The fine catalyst powder is then captured in the downstream particle collection device  30 , which may be a baghouse or electrostatic precipitator, where the powder may be further irradiated with electromagnetic waves to continue the reaction with NO X  . In this embodiment, the NO X  in the gas stream reacts with the catalyst suspended in the gas stream as well as when the catalyst is deposited on the surface of the filter bags or precipitator. 
     FIG. 2 illustrates an alternate embodiment of the invention. The apparatus  50  of FIG. 2 corresponds to the apparatus of FIG. 1, except in FIG. 2, a catalyst injector is not used. Instead, a NO X  reducing catalyst structure  52  is placed in the output duct  28 . The structure  52  is activated by the activation source  42 . The structure  52  may be implemented as a honeycomb-shaped structure or as a set of parallel plates. NO X  in the gas stream is reduced to harmless nitrogen (with or without added reagents, such as ammonia, methane, or hydrogen) as the gas stream passes over the catalyst structure. 
     FIG. 3 illustrates a test apparatus  60  used to substantiate the effectiveness of the invention. The test apparatus  60  includes a catalyst  62  positioned between quartz wool barriers  64 . An ultraviolet light source  66  is used to irradiate the catalyst  62  in a heating zone  67 . 
     The test apparatus  60  further includes an inner quartz tube  68  and an outer pyrex tube  70 . A input port  72  receives a controlled flow of gas, which is discharged at output port  73 . A wire  74  delivers power to the ultra violet light source  66 . An air input port  78  receives pumped air, which is discharged at an air output port  80 . 
     In one embodiment, the catalyst  62  was prepared by the decomposition of metal nitrates (Catalyst I) or other water soluble species on a support. An aqueous solution containing amounts of metal nitrates in concentration ratios appropriate to obtain a desired catalyst stoichiometry was used. The solution was added drop-wise to a support (silica gel, large pore, −8 mesh, 300 m 2 /g) until excess of the liquid just appeared (incipient wetness impregnation). In the case of a V 2 O 5 -TiO 2 SCR catalyst (Catalyst II), ammonium metavanadate (NH 4 VO 3 ) was used as the vanadium oxide source. 
     The reactor employed in evaluating these systems is shown in FIG.  3 . The following points are noteworthy: 1) products were monitored with electrochemical NO X  sensors; 2) the flow rates of the gases He, NO, and O 2  through ports  72  and  73  were controlled with precision metering valves and measured with flow meters; 3) the reactor consisted of a straight quartz tube  68  with a 12 mm outside diameter, which housed the UV source  66  and an outer Pyrex tube  70  of inside diameter 15.8 mm in between which the catalyst  62  was packed between two pieces of quartz wool  64 ; and 4) the reactor was heated using a temperature controlled furnace. The catalyst  62  was a powder of weight 1.15 g and a volume of 2.0 cm 3 . Space velocities reported here were based on the reactor volume occupied by the catalyst  62 . A description of the analysis techniques that were utilized is presented in the following paragraphs. 
     NO X  sensors (not shown) were used in connection with the apparatus of FIG.  3 . Both NO and NO 2  were measured electrochemically using conventional sensors. The sensors were operated in parallel using Helium (He) as a carrier gas. The He flow was adjusted by a needle valve to approximately 10 mL/min. Samples were introduced with a syringe through injection ports located in the gas stream immediately before each sensor. The gas flow lines through the sensors were made of polytetrafluroethylene to reduce surface adsorption of NO 2 . 
     Gas chromatography (GC) was used to quantify the products of NO X  decomposition (nitrogen and nitrous oxide). An HP 5890 Series II gas chromatograph equipped with a thermal conductivity detector (TCD) and a CTR I column operating at a temperature of 30° C. and using a helium carrier gas was used. A sampling value equipped with a 2 ml sample loop was employed. 
     Activity data obtained for catalysts evaluated at 350° C. are summarized in Table 1. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Data for UV Activation of NO x  Reducing Catalysts. T = 100° C. λ = 254 nm. 
               
             
          
           
               
                   
                   
                   
                   
                   
                 % NO &amp; 
                   
                   
               
               
                   
                 UV 
                 ppm NO 
                 % 
                 Reducing 
                 NO 2   
                 ppm NO 2   
                 Space Velocity 
               
               
                 CATALYST 
                 Activation 
                 &amp; NO 2   
                 O2 
                 Agent 
                 Removed 
                 Produced 
                 (hr −1 ) 
               
               
                   
               
             
          
           
               
                   
                 Off 
                 665 
                 0 
                 0 
                 0.6 
                 0.0 
                 432 
               
               
                   
                 On 
                 738 
                 0 
                 0 
                 28.7 
                 32 
                 432 
               
               
                   
                 Off 
                 596 
                 1.42 
                 0 
                 −2.9 
                 30 
                 522 
               
               
                 Catalyst I 
                 On 
                 576 
                 1.42 
                 0 
                 −6.2 
                 233 
                 522 
               
               
                 (11.1 wt. % 
                 Off 
                 554 
                 0 
                 0 
                 6 
                 −1 
                 4340 
               
               
                 Sr 2 Bi 2 Cu 2 O 2 /silica gel) 
                 On 
                 527 
                 0 
                 0 
                 6.7 
                 15 
                 4340 
               
               
                   
                 Off 
                 593 
                 0 
                 0 
                 5.6 
                 0 
                 5296 
               
               
                   
                 On 
                 558 
                 0 
                 0 
                 11.1 
                 22 
                 5296 
               
               
                   
                 Off 
                 674 
                 2.67 
                 0 
                 2.5 
                 3 
                 5147 
               
               
                   
                 On 
                 678 
                 2.67 
                 0 
                 4.1 
                 71 
                 5147 
               
               
                   
                 Off 
                 3 
                 0 
                 0 
                 50.0 
                 0 
                 580 
               
               
                   
                 Off 
                 2 
                 0 
                 0 
                 −50.0 
                 2 
                 580 
               
               
                   
                 Off 
                 2 
                 2.50 
                 0 
                 0.0 
                 0 
                 536 
               
               
                   
                 On 
                 3 
                 2.50 
                 0 
                 −33.9 
                 2 
                 536 
               
               
                   
                 Off 
                 544 
                 0.0 
                 351 ppm 
                 3.1 
                 2 
                 1077 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 On 
                 509 
                 0.0 
                 351 ppm 
                 1.8 
                 58 
                 1077 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 Off 
                 811 
                 2.50 
                 373 ppm 
                 −2.8 
                 31 
                 1012 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 On 
                 813 
                 2.50 
                 373 ppm 
                 −6.6 
                 233 
                 1012 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 Off 
                 823 
                 3.01 
                 941 ppm 
                 0.0 
                 33 
                 1037 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 On 
                 767 
                 3.01 
                 941 ppm 
                 −11.0 
                 239 
                 1037 
               
               
                   
                   
                   
                   
                 CO 
               
               
                   
                 Off 
                 641 
                 2.68 
                 509 ppm 
                 5.9 
                 3 
                 945 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 636 
                 2.68 
                 509 ppm 
                 38.4 
                 51 
                 945 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 Off 
                 430 
                 1.79 
                 574 ppm 
                 0.0 
                 0 
                 1416 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 439 
                 1.79 
                 574 ppm 
                 37.7 
                 36 
                 1416 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                 Catalyst II 
                 Off 
                 550 
                 0.0 
                 0 
                 1.6 
                 −2 
                 430 
               
               
                 1.2% V 2 O 5  on TiO 2   
                 On 
                 509 
                 0.0 
                 0 
                 8.1 
                 0 
                 494 
               
               
                   
                 Off 
                 491 
                 2.57 
                 0 
                 −5.6 
                 10 
                 589 
               
               
                   
                 On 
                 516 
                 2.57 
                 0 
                 8.9 
                 6 
                 589 
               
               
                   
                 Off 
                 460 
                 2.57 
                 0 
                 0.4 
                 −1 
                 959 
               
               
                   
                 Off 
                 468 
                 0.0 
                 0 
                 5.7. 
                 2 
                 976 
               
               
                   
                 On 
                 445 
                 1.83 
                 0 
                 8.2 
                 4 
                 976 
               
               
                   
                 Off 
                 636 
                 2.38 
                 444 ppm 
                 5.9 
                 −2 
                 867 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 612 
                 2.38 
                 444 ppm 
                 38.7 
                 −4 
                 867 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 Off 
                 444 
                 1.26 
                 742 ppm 
                 0.0 
                 0 
                 1635 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 442 
                 1.26 
                 742 ppm 
                 33.5 
                 −1 
                 1635 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 Off 
                 453 
                 0 
                 0 
                 0.0 
                 0 
                 4938 
               
               
                   
                 On 
                 526 
                 0 
                 0 
                 6.7 
                 −1 
                 4938 
               
               
                   
                 Off 
                 552 
                 2.95 
                 0 
                 0.0 
                 7 
                 5137 
               
               
                   
                 On 
                 559 
                 2.95 
                 0 
                 8.0 
                 4 
                 5137 
               
               
                   
                 Off 
                 633 
                 4.92 
                 496 ppm 
                 8.8 
                 2 
                 1021 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 573 
                 4.92 
                 496 ppm 
                 41.4 
                 −9 
                 1021 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 Off 
                 588 
                 2.11 
                 488 ppm 
                 14.5 
                 −4 
                 849 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
                 On 
                 656 
                 2.11 
                 488 ppm 
                 62.2 
                 20 
                 849 
               
               
                   
                   
                   
                   
                 NH 3   
               
               
                   
               
             
          
         
       
     
     Several general observations are apparent from the above table. First, appreciable differences in activity were observed when the UV source was in the “On” versus the “Off” state. Two, in the absence of oxygen, a net removal of nitrogen oxides (NO+NO 2 ) via UV activation was observed over a space velocity range of 400-5300 h −1 . Three, Catalyst II exhibited less NO 2  production than Catalyst I when O 2  was introduced. Four, apparently negative conversions resulted from the storage of NO 2  in the catalyst. Finally, activity was still evident after over 1000 hours on-line. 
     Some discussion of the nature of the UV activation process follows. Reagentless reactions are considered first. The observations concerning reagentless reactions absent added oxygen are as follows: 1) NO 2  was not produced in great abundance and 2) the order of activity was Sr 2  Bi 2 Cu 2 O 7 /silica gel&gt;V 2 O 5 /TiO 2 . Introducing 1-3% O 2  gave the result that the net conversion of NO+NO 2  displayed the order V 2 O 5 /TiO 2 &gt;Sr 2  Bi 2 Cu 2 O 7 /silica gel. In the case of Catalyst I, NO 2  was produced extensively. CO was found to be an ineffective reagent. In the case of both V 2 O 5 /TiO 2  and Sr 2  Bi 2 Cu 2 O 7 /silica gel, it was found that ammonia was an effective reagent for the UV activation of nitrogen oxides. In the case of V 2 O 5 /TiO 2 , the disappearance of nitrogen oxides was fully accounted for in terms of the products of ammonia reduction. The product distribution was N 2 (95%) and N 2 O (5%). It is not likely that scissioning of the N—O bond by direct interaction with the radiation occurs since the difference in energies between LUMO (Lowest Unoccupied Molecular Orbital) and HOMO (highest Occupied Molecular Orbital) was less than the source energy (4.0 eV) of the 254 nm UV lamp. Rather, substrate excitation or excitation of a substrate-NO complex must occur. Decomposition in the absence of oxygen occurs according to the following mechanism: 
     
       
         2e−+V+NO=N+O 2−   
       
     
     
       
         2N=N 2   
       
     
     In the case of oxidation, UV radiation probably serves to excite hole states, leading to the following important steps: 
     
       
         M—O+hv=2h + =M—O 
       
     
     
       
         2h + +O 2− =O+V 
       
     
     
       
         NO+O=NO 2   
       
     
     In the case of the silica supported catalysts, the ammonia reagent probably works by its reaction with NO 2  formed by UV excitation, since the dioxide species is formed over both of the catalysts in the presence of oxygen when the catalyst is irradiated with ultraviolet light. The mechanism in the case of V 2 O 5 /TiO 2  is clear, since UV does not cause NO 2  to be generated. 
     The foregoing results demonstrate conversion via a nonoxidative pathway has been obtained using ultraviolet activation of nitrogen oxides. Although the conversion may be low relative to current NO X  catalysts, it must be remembered that the catalysts employed here were not optimal: strongly basic catalysts are not expected to be effective at promoting SCR. For example, it is well known that bases including alkali and alkaline earths poison SCR catalysts. In contrast, catalysts found to display significant and even potentially useful activity absent reagents, demonstrated a preferred oxidative pathway under UV irradiation when oxygen was introduced into the system. This is in sharp distinction to the behavior exhibited by the catalysts in thermal catalytic activation of NO 2 . In that case, the catalysts removed NO essentially completely through a nonoxidative pathway, with a nitrogen product. 
     In sum, the foregoing data demonstrates UV activation as a tool for the nonoxidative removal of NO from an exhaust stream containing up to 5% O 2 . Reagentless decomposition was demonstrated in the absence of oxygen. However, there was a marked tendency for the reagentless catalysts to produce NO 2  in the presence of O 2 . In the presence of oxygen, ammonia present in a 0.7 to 1.5:1 ratio with respect to NO+NO 2  resulted in the essentially complete reduction of NO. One of these basic catalysts were found to be more active than an SCR catalyst. 
     The foregoing results indicate that UV radiation is effective for the nonoxidative activation of nitrogen oxides at low temperature (e.g., 100° C.). In particular, the use of a reagent (ammonia) was found to be effective in the UV activation of NO X  . The results point to certain steps that can be taken to optimize performance of the process. For example, increasing wavelength may reduce the tendency to oxidize NO and allow for possible use in reagentless or reduced-reagent catalysis. Catalyst optimization may also be considered to improve performance with specific wavelengths or wavelength ranges. Catalyst distribution may also be considered to improve exposure to radiation. 
     Those skilled in the art will appreciate that the invention provides a technique for promoting catalytic activity in various NO X  SCR catalysts so that they can be operated and installed under conditions where they would not normally be active and may therefore provide more cost-effective options for NO X  reduction in gases. The invention facilitates the use of catalysts at lower temperatures. The invention renders the catalysts less sensitive to poisoning agents, and otherwise more reactive, even without the addition of chemical agents. The NO X  reduction technique of the invention is cost-effective, thereby minimizing the overall cost of generating electricity in fossil-fired power plants. 
     For ease of explanation, the abovementioned apparatus, which uses a catalyst structure and an activation source, will hereafter be referred to as an Advanced Energy Activation (AEA) unit. As mentioned previously, an AEA unit activation source may include an ultraviolet (UV) energy source, an electromagnetic energy source, a microwave energy source, an electric field energy source, an electric current energy source, a magnetic field energy source, and an infrared energy source. Some of these AEA activation sources also act to heat the catalyst to an elevated and more reactive temperature. The AEA unit activation sources, therefore, render an AEA unit catalyst sufficiently active through supplemental and alternative energy input and excitation energies so that an AEA unit can be operated and installed under conditions where it would not normally be active. This provides a more cost-effective reduction of NO X  flue gases. 
     During laboratory and field studies of AEA units, it was found that using a supplemental activation source downstream of existing NO X  reduction units, such as SNCR, SCR, LNB, or AEA units, reduces any NO X  present in the system, and also reduces “slip” caused by reducing agents, such as ammonia or urea, by promoting reactions between NO X  and reducing agents. Therefore, a supplemental activation source is preferably used downstream of an SNCR, SCR, or LNB unit as a NO X  polishing step. 
     FIG. 4 illustrates a system  82  for decreasing the concentration of contaminants, such as NO X  or reducing agents, present in a flue gas stream in accordance with an embodiment of the invention. In one embodiment, a supplemental activation source  86  is placed outside of, or within, a duct leading from the boiler to the stack  36 . The supplemental activation source is preferably situated downstream of a NO X  reducing unit, such as a SNCR, SCR, or AEA unit  88 , or a LNB unit  89 . Alternatively, the supplemental activation source may be situated anywhere downstream of the NO X  reducing units  88  or  89 , including just downstream of boiler  22 , downstream of SNCR, SCR, or AEA unit  88 , or between the particulate collection device  30  and the stack  36 . The supplemental activation source  86  may also be placed in the particulate collection device  30 . 
     The energy provided by the supplemental activation source  86  serves to reduce “slip” caused by reducing agents, such as ammonia or urea, by promoting reactions between NO X  and the reducing agent. The reducing agent may be suspended in the flue stream or it may be on flyash surfaces. This not only reduces ammonia “slip”, but also reduces NO X  present in the flue gas by promoting a reaction between the NO X  and the reducing agents. 
     It has also been demonstrated that NO X  in the flue gas can be further reduced by the supplemental activation source  86  even in the absence of reducing agents, such as ammonia or urea. Furthermore, should any catalyst powder be present in the flue gas at the supplemental activation source  86 , such as if injected at a NO X  SNCR, SCR, or AEA unit  88 , any NO X  present in the flue gas will react with the catalyst to further reduce any NO X  present in the flue gas. 
     In another embodiment, a supplemental catalyst  84  is used in conjunction with the supplemental activation source  86 . The supplemental catalyst  84  may either be placed in a stationary position adjacent the supplemental activation source  86  (stationary configuration), similar to that shown in FIG. 2, or a catalyst powder may be injected anywhere upstream of the supplemental activation source  86  (catalyst injection configuration), similar to that shown in FIG.  1 . In the stationary configuration, the catalyst  84  is preferably disposed in a stationary position outside of, or within, outlet duct  34 . In the catalyst injection configuration, catalyst powder may be injected at the supplemental activation source  86  or at SNCR, SCR, or AEA unit  88 . 
     FIG. 5 illustrates a method  90  for decreasing the concentration of contaminants, such as NO X  or reducing agents, present in a gas stream in accordance with the embodiment of the invention described in relation to FIG. 4. A supplemental activation source  86  (FIG. 4) is provided (step  92 ) downstream of a NO X  reducing unit  88  or  89  (FIG.  4 ). Although not an essential step, a catalyst  84  (FIG. 4) may be placed into the system (step  94 ). The supplemental activation source is then activated (step  96 ) to both further reduce any NO X  present in the flue gas stream and to reduce any reducing agents in the flue gas stream, such as ammonia or urea, by promoting reactions between NO X  and the reducing agent on the flyash surface. In the embodiment where a catalyst is placed into the system (step  94 ), any NO X  present in the flue gas further reacts with the catalyst to reduce NO X  concentration. 
     The placing (step  94 ) of the supplemental activation source  86  (FIG. 4) can include either positioning (step  98 ) a stationary supplemental catalyst structure within the flue gas stream (stationary configuration), similar to that described in relation to FIG. 2, or injecting (step  102 ) a powder catalyst into the flue gas stream (catalyst injection configuration), similar to that described in relation to FIG.  1 . The activation step (step  96 ) comprises applying energy to the supplemental catalyst utilizing one or more supplemental activating sources, such as an ultraviolet (UV) energy source, an electromagnetic energy source, a microwave energy source, an electric field energy source, an electric current energy source, a magnetic field energy source, an infrared energy source, or the like. 
     In this way, the inclusion of a supplemental activation source downstream of a NO X  reducing unit, decreases the concentration of reducing agents within a flue gas stream while further decreasing any NO X  present in the flue gas. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.