Abstract:
Methods for scrubbing gas streams to remove acid gases including sulfur dioxide, mercury-containing substances, and/or nitrogen oxides from the gas stream. The gas stream is contacted with a potassium-based sorbent effective for removing at least a portion of the acid gases. The partially cleaned gas stream is then contacted with an oxidant effective to remove at least a portion of the nitrogen oxides and/or mercury-containing substances after partially removing the acid gas substance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to methods for removing pollutants, such as sulfur dioxide, nitrogen oxides, mercury compounds, and elemental mercury (Hg), from gas streams and, in particular, to the removal using staged gas/liquid contact of mercury-containing substances and nitrogen oxides from gas streams, such as gas streams generated by the combustion of fossil fuels.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the pollution control field, several diverse approaches have been used to remove sulfur oxides (SO x ) and other contaminants from gas produced by the burning of a fossil fuel in order to comply with Federal and State emissions requirements. One conventional approach involves locating and utilizing fossil fuels lower in sulfur content and/or other contaminants. Another conventional approach involves removing or reducing the sulfur content and/or other contaminants in the fuel, before combustion, via mechanical and/or chemical processes. A major disadvantage to this approach is the limited cost effectiveness of the mechanical and/or chemical processing required to achieve the mandated reduction levels of sulfur oxides and/or other contaminants.  
         [0003]     The most prevalent conventional approaches for removing sulfur oxides and/or other contaminants from gas streams involve post-combustion clean up of the gases. Several conventional methods have been developed to remove the sulfur dioxide (SO 2 ) species from gases.  
         [0004]     One conventional approach for removing SO 2  from gas streams involves either mixing dry alkali material with the fuel prior to combustion, or injection of pulverized alkali material directly into the hot combustion gases to remove sulfur oxides and other contaminants via absorption or absorption followed by oxidation. Major disadvantages of this approach include fouling of heat transfer surfaces (which then requires more frequent soot blowing of these heat transfer surfaces), low to moderate removal efficiencies, poor reagent utilization, and increased particulate loading in the combustion gases which may require additional conditioning of the gas, such as humidification or sulfur trioxide injection, if an electrostatic precipitator is used for downstream particulate collection.  
         [0005]     Another conventional approach for removing SO 2  from gas streams, collectively referred to as wet chemical absorption processes and also known as wet scrubbing, involves “washing” the hot gases with an aqueous alkaline solution or slurry in a gas-liquid contact device to remove sulfur oxides and other contaminants. Major disadvantages associated with these wet scrubbing processes include the loss of liquid both to the atmosphere due to, for example, saturation of the gas and mist carry-over, and to the sludge produced in the process, and the economics associated with the construction materials for the absorber module itself and all related auxiliary downstream equipment (i.e., primary/secondary dewatering and waste water treatment subsystems).  
         [0006]     Yet another conventional approach for removing SO 2  from gas streams, collectively referred to as spray drying chemical absorption processes and also known as dry scrubbing, involves spraying an aqueous alkaline solution or slurry, which has been finely atomized via mechanical, dual-fluid or rotary type atomizers, into the hot gases to remove sulfur oxides and other contaminants. Major disadvantages associated with these dry scrubbing processes include moderate to high gas-side pressure drop across the spray dryer gas inlet distribution device and limitations on the spray down temperature (i.e., the approach to gas saturation temperature) required to maintain controlled operations.  
         [0007]     There are several conventional methods for controlling emissions of nitrogen oxides (NO x ), which include nitric oxide (NO), nitrogen dioxide (NO 2 ), and dimers as principle components. Selective catalytic reduction (SCR) is the most common conventional approach. In this process, ammonia is injected and mixed with the gas at low to medium temperatures. The mixture then flows across a catalyst, often vanadium based over a stainless steel substrate, and the NO x  is reduced to elemental nitrogen (N 2 ). Deficiencies of conventional SCR systems include the high initial cost, the high cost of ammonia which is thermally or chemically decomposed, and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip to the atmosphere.  
         [0008]     Selective non-catalytic reduction (SNCR) methods are also employed for controlling NO x  emissions. In these processes, ammonia or urea is injected into hot gases resulting in a direct reaction forming N 2 . The problems with SNCR systems are the challenges with mixing and maintaining proper residence time and operating conditions for the reactions to take place optimally, sensitivity to changes in operating load, the high cost of ammonia which is thermally or chemically decomposed (even more than SCR&#39;s), and the introduction of ammonia into the gas stream causing problems with the formation of ammonium bisulfate and ammonia slip (as high as 50 ppm or higher) to the atmosphere. Dry injection of sodium bicarbonate (NaHCO 3 ) may also remove NO x .  
         [0009]     Wet chemical NO x  reduction may use oxidants, such as hydrogen peroxide (H 2 O 2 ). Hydrogen peroxide is an oxidizing agent for organic and inorganic chemical processing as well as semiconductor applications, bleach for textiles and pulp, and a treatment for municipal and industrial waste. Hydrogen peroxide is an effective chemical means of scrubbing nitrogen oxides and has been used for many years. The combined use of H 2 O 2  and nitric acid (HNO 3 ) to scrub both NO and NO 2  is an attractive option because the combination handles widely varying rates of NO to NO 2 , adds no contaminants to the scrubbing solution or blow-down/waste stream, and allows a commercial product to be recovered from the process, such as nitric acid or ammonium nitrate.  
         [0010]     Gas scrubbing is another common form of NO x  treatment, with sodium hydroxide being the conventional scrubbing medium. However, the absorbed NO x  is converted to nitrite and nitrate salts that may present wastewater disposal problems. Scrubbing solutions containing hydrogen peroxide are also effective at removing NO x , and can afford benefits not available with sodium hydroxide (NaOH). For example, H 2 O 2  adds no contaminants to the scrubbing solution and so allows commercial products, such as nitric acid, to be recovered from the process. In its simplest application, H 2 O 2  and nitric acid are used to scrub both NO and NO 2  from many utility and industrial sources. In addition to the methods cited above in which NO x  is oxidized to nitric acid or nitrate salts, other conventional approaches reduce NO x  to nitrogen using hydrogen peroxide and ammonia.  
         [0011]     Several other processes use hydrogen peroxide to remove NO x . The Kanto Denka process employs a scrubbing solution containing 0.2% hydrogen peroxide and 10% nitric acid while the Nikon process uses a 10% sodium hydroxide solution containing 3.5% hydrogen peroxide. Yet another process, the Ozawa process, scrubs NO x  by spraying a hydrogen peroxide solution into the exhaust gas stream. The liquid is then separated from the gas stream and the nitric acid formed is neutralized with potassium hydroxide. Excess potassium nitrate is crystallized out and the solution reused after recharging with hydrogen.  
         [0012]     H 2 O 2  is used for the measurement of NO in the Standard Reference Method 7 of the Code of Federal Regulations (CFR) promulgated test methods published in the Federal Register as final rules by the United States Environmental Protection Agency (EPA). In this procedure, an H 2 O 2  solution is used in a flask to effectively capture the NO x .  
         [0013]     There are at least two primary reasons that H 2 O 2  has not gained widespread use as a reagent for removal of NO x  in utility and large industrial applications. One reason is that H 2 O 2  is not a selective oxidant. Most of these sources also contain other species, primarily SO 2 , which are also effectively removed with hydrogen peroxide. Thus, a large quantity of H 2 O 2  would be required compared to the amount of NO x  removal sought. Even after a limestone scrubber, the amount of SO 2  present in gas may be equal to or greater than the amount of NO x . Another reason that H 2 O 2  has not gained widespread use is the cost, especially when much more H 2 O 2  is required due to reactions with SO 2 , for example, which may be better done prior to the H 2 O 2  stage.  
         [0014]     The overall reactions are:  
         [0015]     1) 3H 2 O 2 +2NO→2HNO 3 +2H 2 O  
         [0016]     2) H 2 O 2 +2NO 2 →2HNO 3    
         [0017]     3) H 2 O 2 +SO 2 →H 2 SO 4    
         [0018]     Chlorine oxide (ClO 2 ) supplied at a rate of approximately 1.2 kg ClO 2 /kg NO is effective for rapidly converting over 90% of gas phase NO in the gas stream to NO 2 . This, of course, requires proper mixing conditions. ClO 2  is a significantly stronger oxidizer than hydrogen peroxide, sodium chlorate, or sodium chlorite. Ozone is also a possible oxidizer, but has greater capital costs relative to ClO 2  generators.  
         [0019]     Sulfur dioxide reacts with chlorine dioxide in the gas phase to form sulfuric and hydrochloric acid.  
         [0020]     4) 2ClO 2 +5SO 2 +6H 2 O→5H 2 SO 4 +2HCl  
         [0021]     Assuming SO 2  is the dominant species in the ClO 2  reaction in the presence of SO 2  and NO, excessive amounts of ClO 2  will be required to compensate for consumption by SO 2 . This will reduce the economic feasibility of using ClO 2  for removing NO x .  
         [0022]     None of these conventional approaches for scrubbing gas streams, like gas streams, simultaneously removes mercury, mercury compounds, and NO x , especially elemental mercury (Hg°) removal. Mercury is volatilized and converted to Hg° in the high temperature regions of fossil fuel combustion devices. As the gas cools, HgO is oxidized to Hg +2 . In coal-fired combustors, Hg° may be oxidized to vapor phase mercuric oxide (HgO), mercuric sulfate (HgSO 4 ), mercuric chloride (HgCl 2 ), or some other vapor phase mercury compound.  
         [0023]     Mercury may be captured, to a limited extent, using powdered activated carbon (PAC) sorbent. The activated carbon sorbent is injected into the gas stream, binds with the mercury in the gas, and captured downstream by a particulate matter control device. However, the mercury concentration in the gas stream may exceed the absorption ability of activated carbon sorbents. In addition, the performance of activated carbon sorbents may be adversely affected by low levels of chlorine in the gas. Carbon injection equipment is also relatively expensive.  
         [0024]     Oxidized mercury (Hg +2  such as in the form of HgCl 2 ), which are water-soluble, may be effectively captured in wet scrubbers used for SO 2  control that use an alkali reagent. However, this process also requires supplemental additives, such as sodium hydrogen sulfide (NaHS) or other sulfides, to chemically bind with the mercury and form compounds like mercury sulfide (HgS). However, Hg° is insoluble in water and must be adsorbed onto a sorbent or converted to a soluble form of mercury that can be collected by wet scrubbing.  
         [0025]     For these and other reasons, it is desirable to provide methods for removing nitrogen oxides, sulfur dioxide, and mercury-containing substances, such as mercury and mercury compounds, from gas streams that overcome the various problems associated with conventional methods for scrubbing gas streams.  
       SUMMARY OF THE INVENTION  
       [0026]     The present invention provides a method of scrubbing a gas stream containing at least one acid gas substance and a mercury-containing substance comprises contacting the gas stream with a potassium-based sorbent effective for removing at least a portion of the acid gas substance. The method further comprises contacting the gas stream with an oxidant effective to remove at least a portion of the mercury-containing substance after removing at least the portion of the acid gas substance.  
         [0027]     In another embodiment of the present invention, a method of scrubbing a gas stream containing at least one acid gas substance and nitrogen oxides comprises contacting the gas stream with a potassium-based sorbent effective for removing at least a portion of the acid gas substance. The method further comprises contacting the gas stream with an oxidant effective to remove at least a portion of the nitrogen oxides after removing at least the portion of the acid gas substance.  
         [0028]     One benefit of the present invention is that acid gas substance is removed from the gas stream using a lower cost sorbent. The use of a potassium alkali with oxidation produces a potassium sulfate final product from gas phase reactions. The potassium alkali may be made on site at the location of the scrubber from potassium chloride (KCl or potash). Conveniently, carbon injection equipment to capture mercury containing substances is not required because Hg, and other air toxics, are removed by the staged process steps of the invention.  
         [0029]     Further advantages include, but are not limited to, the ability to custom design each add-on stage to meet the pollutant removal characteristics of the constituents removed in each individual stage and the ability to independently control and monitor the chemistry of each add-on stage to optimize the performance. Each add-on stage is isolated to prevent contamination of reagents/solutions and the solutions in each add-on stage are handled separately.  
         [0030]     These and other advantages of the present invention shall become more apparent from the accompanying drawings and description thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0031]     The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serves to explain the principles of the invention.  
         [0032]     The FIGURE is a schematic representation of a scrubber arrangement in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0033]     With reference to the FIGURE, a scrubber arrangement  10  constitutes a one to three stage add-on technology to a conventional scrubber arrangement. The principles of the invention apply to all scrubbing systems for gases that contain sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), and mercury (Hg)-containing substances. The principles of the invention also apply to both new installations or modifications of existing units. Scrubber arrangement  10  is used to remove acid gases including SO 2 , NO x , and Hg from a gas stream using a staged approach in which acid gases are effectively removed by sulfur oxide removal stage  18  from gas stream  20  producing a second gas stream  24  which has essentially all acid gases removed. This clean gas  24  then proceeds to oxidant stage  22  where NO x  and/or Hg are effectively captured producing a clean gas stream  26 . For gases such as flue gases produced from the combustion of fossil fuels such as coal, coke, oil, and the like, this clean gas stream  26  would consist primarily of nitrogen, oxygen, water vapor, carbon dioxide, and other trace inert gases found in air such as argon, but is essentially devoid of pollutant gases. In some embodiments, the gas stream  26  leaving the oxidant stage  22  may contain some byproducts such as chlorine gas and the like that can be washed with water and/or an alkali solution to produce a very clean gas  30 .  
         [0034]     In another embodiment, the scrubber arrangement  10  is coupled with a wet scrubbing system  12  that receives a stream of gas  14  produced by a device combusting a fossil fuel or a gas from a chemical process. The wet scrubbing system  12  scrubs the gas stream  14  by using a conventional technology for accomplishing partial (i.e., &lt;100%) removal of the acid gases, including SO 2  in the gas stream  14 . To that end, the wet scrubbing system  12  contacts the gas in stream  14  with a scrubbing fluid that is typically composed of water and a basic chemical including, but not limited to, lime, calcium carbonate or limestone, soda ash or other sodium based alkalis, magnesium based alkalis, buffered calcium, and other calcium based alkalis, or mixtures of these materials. The scrubbing fluid may also include any of a number of additives intended to enhance removal, control chemistry, and reduce chemical scale. The wet scrubbing system  12  removes a large fraction of the SO 2  present in the gas stream  14 , perhaps 90 to 98%, or even higher, using scrubbing fluids including sodium-based, magnesium-based, or calcium-based alkalis, but does not effectively remove NO x , especially NO, or Hg containing substances, especially elemental Hg.  
         [0035]     An injection scrubber  16  may optionally pre-condition the gas in stream  14  before the gas stream  14  is introduced into the wet scrubber  12 . The injection scrubber  16  injects absorbents, reagents, adsorbents, or sorbents to reduce a portion of the SO 3  in the gas stream  14 . A portion of the SO 2 , HCl, NO x , or other acid gases such as HF and H 2 S, may also be removed by the operation of the injection scrubber  16 . The injection scrubber  16  may use either wet or dry injection with any of multiple different alkali substances at any of several possible and known locations or temperature zones from the source of the gas stream  14  to the inlet of the wet scrubber  12 . The injection is preferably a dry sodium bicarbonate (NaHCO 3 ) injection because NaHCO 3  also efficiently reacts with sulfur trioxide (SO 3 ), NO x , SO 2 , and other acid gases, present in the gas stream  14 . When injection of sorbents is employed in the injection scrubber  16 , the need for a conventional wet electrostatic precipitator is eliminated because sulfuric acid mist is not formed when the SO 3  is effectively removed by injection scrubber  16  upstream of the wet scrubbing system  12 .  
         [0036]     Following the optional injection step using the injection scrubber  16 , wet scrubbing system  12 , as described above, is used to remove SO 2  and acidic NO x  compounds, such as NO 2 , N 2 O 3  and N 2 O 5  and their associated dimers (e.g., N 2 O 4 ). Conversion of NO to NO 2  by sodium bicarbonate injection was considered undesirable because the NO 2  was a brown gas that was not captured by the downstream equipment. In this case, however, the wet scrubbing system  12  can effectively capture some of the NO 2 , N 2 O 5 , etc. when a sodium-based alkali is used. Some of the NO is captured directly by the sodium bicarbonate. However, NO is not effectively captured with sorbents such as lime, limestone or other calcium-based alkalis, magnesium-based alkalis, or sodium-based alkalis.  
         [0037]     In accordance with the principles of the present invention and with continued reference to the FIGURE, the scrubber arrangement  10  includes an SO 2  removal stage  18  that removes SO 2  in a gas stream  20  supplied from the wet scrubbing system  12 . Preferably, the SO 2  removal stage  18  is a polishing step that effectively removes all, or substantially all, of SO 2  in gas stream  20  remaining after treatment in the wet scrubbing system  12 . The SO 2  removal stage  18  will include appropriate mass transfer surfaces, including but not limited to any conventional combination of sprays, packing, bubble cap trays, etc., or is housed in a separate vessel, to isolate the SO 2  reagent stream from the lower stage acid gas absorber stage supplied by the wet scrubbing system  12 .  
         [0038]     In one embodiment of the present invention, the SO 2  removal stage  18  is a reaction zone that uses a potassium alkali, preferably potassium hydroxide (KOH), as a reagent or reactant. If KOH is used as the reactant, potassium sulfate (K 2 SO 4 ) is produced by the following overall reaction:  
         [0039]     5) 2KOH+SO 2 +½O 2 →K 2 SO 4 +H 2 O  
         [0040]     The KOH used in the SO 2  removal stage  18  may be purchased or, alternatively, may be produced from potash (KCL) on site using conventional methods such as electrochemical methods as understood by persons of ordinary skill in the art. This constitutes one advantage over the use of sodium-based sorbents for SO 2  removal. If produced by electrochemical methods, KCl is split to produce KOH and HCl. The KOH is used in the SO 2  removal stage  18  to produce K 2 SO 4  and the byproduct HCl may be sold or used elsewhere.  
         [0041]     Preferably, a significant portion or, most preferably, substantially all of SO 2  in the gas stream  14  is removed before the oxidant stage  22 . If SO 3  is not present in gas stream  14 , the injection scrubber  16  may be eliminated.  
         [0042]     The oxidant stage  22  removes at least a portion of the NO x , primarily in the form of NO, NO 2 , or other dimers, and mercury, either in an elemental form or oxidized form from a gas stream  24  supplied from the upstream SO 2  removal stage  18  and discharges a gas stream  26  that is highly depleted of these substances. Preferably, the oxidant stage  22  removes a significant portion or, most preferably, substantially all of the Hg and NO x  from gas stream  24 . The oxidant stage  22  may use a tray, like a bubble cap tray, or a separate vessel to hold the reagent, in this case an oxidant stream, separate from the lower stages so as to not interfere with the operation of the injection scrubber  16 , the wet scrubber  12 , and the SO 2  removal stage  18 . Mass transfer surfaces such as additional trays, sprays or packing may be added to the oxidant stage  22 , as required. In one embodiment, the oxidant stage  22  is an integral reaction zone that recirculates an aqueous solution of oxidant and reaction products to effectively and simultaneously remove all of the NO x  and a significant fraction of the mercury.  
         [0043]     The gas steam  26  exiting oxidant stage  22  is free or substantially free of SO x , which is effectively removed upstream of the oxidant stage  22  by the wet scrubber  12 , the injection scrubber  16 , and the SO 2  removal stage  18 . Moreover, the gas steam  26  is depleted of up to 90% to 99% or more of the initial mercury and NO x  in the gas stream  14 . Hence, the scrubber arrangement  10  is capable of eliminating a significant portion, if not substantially all, of the Hg, SO x , and NO x  contamination from gas stream  26 .  
         [0044]     The oxidant stage  22  is selected contingent upon the desired level of removal of NO x  and/or Hg containing-substances. Candidate oxidants that are useful for capture of NO x  and/or Hg or Hg compounds include, but are not limited to, the following substances:  
         [0045]     1) Hydrogen Peroxide  
         [0046]     2) Hydrogen Peroxide/Nitric Acid Solution (H 2 O 2 /HNO 3 )  
         [0047]     3) Hydrogen Peroxide/Nitric Acid/Hydrochloric Acid Solution (H 2 O 2 /HNO 3 /HCl)  
         [0048]     4) Sodium Chlorate Solution (NaClO 3 )  
         [0049]     5) Sodium Chlorite Solution (NaClO 2 )  
         [0050]     6) Sodium Hypochlorite Solution (NaClO)  
         [0051]     7) Sodium Perchlorite Solution (NaClO 4 )  
         [0052]     8) Chloric Acid Solution (HClO 3 )  
         [0053]     9) Oxone Solution (2KHSO 5 -KHSO 4 -K 2 SO 4  Triple Salt)  
         [0054]     10) Potassium Chlorate Solution (KClO 3 )  
         [0055]     11) Potassium Chlorite Solution (KClO2)  
         [0056]     12) Potassium Hypochlorite Solution (KClO)  
         [0057]     13) Potassium Perchlorite Solution (KClO 4 )  
         [0058]     14) Potassium Permanganate (KMnO 4 )  
         [0059]     15) Potassium Permanganate/Sodium Hydroxide Solution  
         [0060]     Other oxidants, or combinations of oxidants, may be used in the oxidant stage  22 . Further, sodium carbonate and sodium bicarbonate, or other alkalis, may be substituted for the sodium hydroxide solutions used for pH adjustment and to provide the ions for complete reactions. Oxidants may be selected to remove only NO x , to exclusively remove elemental Hg and mercury compounds, or to simultaneously remove NO x , elemental Hg, and mercury compounds. Metal ions that promote oxidation, including but not limited to iron, cobalt, and manganese, may be added to the oxidant used in the oxidant stage  22 .  
         [0061]     With regard to the use of sodium hypochlorite (NaClO) in the oxidant stage  22 , potential chemical reactions between NaOCl and NO x  and Hg include:  
         [0062]     6) 2NO+3NaClO+2NaOH→2NaNO 3 +3NaCl+H 2 O  
         [0063]     7) 2NO+3NaClO+Na 2 CO 3 →2NaNO 3 +3NaCl+CO 2↑   
         [0064]     8) 2NO+3NaClO+2NaHCO 3 →2NaNO 3 +3NaCl+2CO 2↑ +H 2 O  
         [0065]     9) 2NO 2 +NaClO+2NaOH→2NaNO 3 +NaCl+H 2 O  
         [0066]     10) 2NO 2 +NaClO+Na 2 CO 3 →2NaNO 3 +NaCl+CO 2↑   
         [0067]     11) 2NO 2 +NaClO+2NaHCO 3 →2NaNO 3 +NaCl+2CO 2↑ +H 2 O  
         [0068]     12) 2Hg+4NaClO+2H 2 O→2HgCl 2 +4NaOH+O 2    
         [0069]     In these chemical reactions, an additional source of sodium, such as bicarbonate, carbonate or hydroxide, may be provided to balance the reaction and to limit the potentially deleterious reaction of liberating Cl 2 , ClO 2 , or other undesirable gases. The addition of the sodium source would eliminate the need for a final wash stage  28 .  
         [0070]     One reaction product of the NO x  reactions with NaOCl, and with other candidate oxidants, is sodium nitrate. This NaNO 3  may be converted to ammonium nitrate, a high value fertilizer product, by reaction with ammonia and carbon dioxide or ammonium bicarbonate, as indicated diagrammatically by reference numeral  23  in the FIGURE. The value of the fertilizer product may produce a revenue stream that offsets a portion of the cost of the equipment and consumables used in the scrubber arrangement  10 . This conversion reaction will also produce sodium bicarbonate. The mercury, in the form of mercury chloride, may be separated from the oxidant solution using mercury specific ion exchange resins, as diagrammatically shown in the FIGURE with reference numeral  25 , and the nitrogenous product converted to fertilizer in block  23 . Of course, mercury separation in block  25  is optional if the gas stream  24  treated by oxidant stage  22  does not contain mercury-containing substances or if the oxidant used in oxidant stage  22  does not remove mercury-containing substances from gas stream  2 . 4   
         [0071]     Gaseous oxidants such as ozone (O 3 ) or chlorine dioxide (ClO 2 ) may be injected into or produced by reaction in the gas stream  24  supplied to the oxidant stage  22  with, preferably, all or substantially all of the SO 2  originally in gas stream  14  removed upstream of oxidant stage  22 . With proper mixing and sufficient residence, such gaseous oxidants are capable of oxidizing NO or Hg in the gas phase. Such gaseous oxidants may be capable of oxidizing NO not only to NO 2  but also to N 2 O 5 , which rapidly reacts with water or alkaline solutions to form nitric acid or nitrates.  
         [0072]     The scrubber arrangement  10  may further include the optional final wash stage  28  to treat gas stream  26 . The wash stage  28 , if present, washes the gas in gas stream  26  to ensure that any byproducts from the oxidant stage  22 , like chlorine gas, NO 2 , etc., are removed. To that end, the final wash stage  28 , if required, contacts the gas stream  26  with water or an appropriate solution effective to remove these byproducts, if present.  
         [0073]     A gas stream  30  ultimately discharged from the scrubber arrangement  10  is advantageously depleted of, preferably, all or substantially all SO x , NO x , Hg, and Hg compounds. This represents a principle benefit of the scrubber arrangement  10  of the present invention.  
         [0074]     Further details and embodiments of the invention will be described in the following example.  
       EXAMPLE  
       [0075]     Bench-scale screening of potential solutions for capturing NO x  and elemental mercury (Hg°) was performed using a simple gaseous mixture (Hg°+NO+NO 2 +CO 2 +H 2 O+N 2 +O 2 ) and an impinger sampling train similar to that described in the American Society of Testing and Materials Method D6784-02 (Ontario Hydro method). Testing identified solutions that effectively removed both NO x  and Hg°. The results are shown in the following table:  
                             TABLE 1                           BENCH SCALE TEST RESULTS                NO x  Removal or NO   Hg Removal       Solution   Conversion to NO 2     (Hg Total and Hg ° )               Hydrogen Peroxide   Low   Low       Nitric Acid (40%) +   30-40%   30-40%        Hydrogen Peroxide       Acidified Potassium   30-40%   ˜100%       Permanganate       Chloric Acid   Low   30-40%        0.1 M NaClO pH adjusted    ˜100%   ˜100%       to 3.74 using       0.25 mole/L KMnO 4  +    ˜98%   ˜100%       2.5 mole/L NaOH   (about 4 ppm       (pH of 11.3)   passed through)       0.1 M NaClO, pH adjusted   75-95%   ˜100%       to 6 using HCl       NaClO pH adjusted to 5    ˜70%   ˜100%       using HCl                  
 
         [0076]     The results in Table 1 indicate that there are several possible candidate solutions from which to choose. Even the situations that show medium removal ranges such as (nitric acid (40%)+hydrogen peroxide) or acidified potassium permanganate will remove at higher rates with an appropriate modification to the mass transfer means. The oxidant selected, will then be based on economics, availability, desired level of capture, and/or desired end product. The results in Table 1 also indicate the relative ineffectiveness of H 2 O 2  alone for NO x  removal and Hg removal.  
         [0077]     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe the best mode of practicing the invention, it is not the intention of applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein we claim: