Abstract:
Gas containing nitrogen oxides is treated in two separate stages. In the first stage, the nitrogen oxides in the gas are reacted with sulfurous acid. The gaseous product of this reaction is reacted with an alkaline solution, typically caustic alkali. As a result of these treatments, the nitrogen oxides are converted into neutral salts.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the treatment of gases to remove pollutants, and has particular relationship to the removal of nitrogen oxides from gases containing these oxides to prevent the emission of these oxides to the atmosphere. Nitrogen oxides are known atmospheric pollutants and are believed responsible for the production of photochemical smog and &#34;acid rain&#34; phenomena. Although there are many chemically derivable oxides of nitrogen, the principal oxides implicated in atmospheric pollution are mixtures of the stable oxides, nitric oxide, NO, and nitrogen dioxide, NO 2 , and their equilibrium compounds. These nitrogen oxides will be referred to herein as &#34;NO x  &#34;. 
     Very high concentrations of NO x  may be evolved in processes utilizing concentrated nitric acid in the chemical milling and passivation of stainless steels, tungsten, molybdenum, copper and the like, and the dissolution of precious metals. Lower concentrations of NO x  are present in tail-gases from nitric acid manufacture (0.1 to 0.5% by volume) and in combustion flue gases from industrial furnaces or internal combustion engines. Various types of treatment processes have been proposed which involve either catalytic conversion or catalytic reduction of the nitrogen oxides, or wet scrubbing with various solutions of metal complexes or metallo-organic compounds. Catalytic methods of NO x  removal generally require the use of elevated gas temperatures and the addition of a reducing agent such as NH 3 , CO, H 2  or hydrocarbons. Catalytic NO x  reduction methods become very expensive if the NO x  concentrations are higher than normal flue gas NO x  concentrations, or if the bulk gas must be heated from near ambient conditions to the elevated temperatures required for catalytic reduction. Therefore, catalytic NO x  reduction processes are not normally applicable to processes generating high NO x  concentrations in the exhaust gases; i.e., concentrations higher than 0.5% by volume (5,000 parts per million), or where the exhaust gases are at or near atmospheric or ambient temperatures. 
     Wet processes proposed for NO x  absorption include various forms of aqueous alkaline solutions, transition metal complexes, and reductive reagents, such as catalyzed sulfite solutions, or oxidative reagents, such as permanganates. Processes employing transition metal complexes.sup.(1) and/or organic-compound-containing salts or reductive/oxidative chemicals, generate liquid waste byproducts which are themselves biotoxic water pollutants and cannot be sewered or easily disposed of. Wet process treatment with alkaline-earth oxides, hydroxides and carbonates, particularly calcium carbonates, has also been proposed, but these treatments have very unfavorable overall absorption kinetics for NO x  and are ineffective unless very large and uneconomic sizes of equipment are employed. 
    
     It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the prior art and to remove nitrogen oxides from a gas effectively and at rapid, economic absorption rates, without the creation of by-product waste liquor pollution problems. It is an object of this invention to provide a method and apparatus for carrying out such removal. 
     SUMMARY OF THE INVENTION 
     This invention arises from the realization that wet scrubbing processes for NO x  removal involving the use of water or aqueous solutions of salts depend on the absorption from the gas of an oxide of nitrogen as the primary rate-controlling step. The dissolution behavior of the various forms of oxides of nitrogen has been reviewed by Sherwood and Pigford.sup.(2) and Wilke.sup.(3). The principal conclusions of these authors are: 
     1. NO is insoluble and unreactive toward water and aqueous alkaline solutions. 
     2. NO 2  has a very slow rate of solution in water, and the dissolution rate in caustic soda is slower than in water. 
     3. There are only two significant mass transfer processes which cause dissolution of gas-phase NO x  in aqueous solutions: the absorption of nitrogen tetroxide, N 2  O 4 , and the absorption of nitrogen trioxide, N 2  O 3 . 
     4. At 25° C., N 2  O 3  is only 30 percent as soluble in water as N 2  O 4 , but it reacts with water 40 times as fast, so that at equal concentrations of N 2  O 4  and N 2  O 3 , the primary dissolution path is by way of N 2  O 3 . 
     5. Nitrogen tetroxide, N 2  O 4 , the dimer of NO 2 , is formed in small equilibrium concentrations from NO 2 , and the nitrogen trioxide, N 2  O 3 , is also formed in small concentrations from nitrogen dioxide, NO 2 , and nitric oxide, NO: 
     
         NO.sub.2 +NO⃡N.sub.2 O.sub.3                   ( 1) 
    
    
    
     In highly concentrated NO x  gases the efficiency of nitrogen oxides absorption in high-pressure nitric acid absorbers is large, primarily because the reaction involving both diffusion and reaction of N 2  O 4  is favored. For dilute gases or gases at atmospheric pressure however, the partial pressure of N 2  O 4  becomes so small that the second reaction path involving N 2  O 3  produces HNO 3  more efficiently. The basic problem is that the equilibrium concentration of both N 2  O 4  and N 2  O 3  in gases at atmospheric pressure is so small that both efficiency and rates of NO x  absorption in aqueous systems, are low. 
     In the practice of this invention, the disadvantages and drawbacks of the prior art are overcome by a wet scrubbing process including two separate stages. In the first stage, the gas is treated with an aqueous solution of sulfurous acid. In the second stage, the gas leaving the first stage is treated with an alkaline or basic solution. 
     The two stages are indispensible because the first stage conversion-treatment alone provides for little or no removal of the total NO x  from the gas phase. Omission of the first stage results in non-absorption of NO x  in the basic-solution treatment stage and also causes the formation of HNO 3  and HNO 2  mist in the vapor phase with copious dense white acid mist emissions at high NO x  concentrations. While the chemistry of this treatment has not been positively established, it is believed that the first-stage sulfurous-acid treatment converts the nitrogen oxides to a form that is rapidly and readily absorbed by the basic solution in the second stage. The two-stage system provides for removal of NO x  and a final exhaust gas free of color, fume or acid gas content. 
     It is of interest to note that Collins, in U.S. Pat. No. 3,920,421, calls for the removal of NO x  from gas streams also containing sulfur dioxide by absorption in water of the sulfur dioxide to form sulfurous acid as part of a single-stage reduction process. However, Collins reduces the nitrogen oxides to nitrogen and finds it necessary to add a multivalent metal, such as iron, to the solution to achieve this purpose. Similarly, Kudo and Haguvara, in U.S. Pat. No. 4,288,421, treat an exhaust gas containing both NO x  and SO x  with an absorbing solution containing an iron chelate salt and potassium sulfite in order to form imidodisulfonates. The present invention avoids the use of expensive or water-polluting catalysts and achieves NO x  removal by a unique separation of in situ chemical conversion and absorption steps. 
     Possible chemical reactions involved in the invention will now be discussed. It is emphasized that these reactions are formulated based on hindsight and that there is no certainty that they are the reactions which actually occur. They are presented here with the thought that they provide a basis for understanding of the invention. It is to be understood that this presentation is in no way to be regarded as critical to, or limiting of, this invention and that the existence of other formulations which may logically describe the reactions of this invention in no way reflects negatively on the merit of this invention. 
     The process chemistry of this invention is believed to be essentially and ideally comprised of two separate steps: 
     (a) converting NO x  in the gas under treatment to N 2  O 3 , and 
     (b) reacting the N 2  O 3  with aqueous alkaline solution. 
     The conversion of NO x  to N 2  O 3  in the first-stage contactor is effected by scrubbing with sulfurous acid solution. 
     In the now-obsolete lead chamber process for the production of H 2  SO 4 , nitrogen oxides are used as the oxidant for SO 2 , which is oxidized to SO 3 . The SO 3  then reacts with water and/or is absorbed in H 2  SO 4  in the Gay-Lussac tower where the nitrogen oxides are regenerated for recycle. If the process is now reversed, so that SO 2  or H 2  SO 3  solutions are used to control the formation of soluble forms of gaseous nitrogen oxides, and/or liquid-phase complexes, the &#34;old&#34; chemistry may be utilized to explain the probable reaction sequence of the present invention. 
     The exact nature of the reactions between SO 2  and NO x  in the lead chamber process was the subject of continuous investigations for some 50 years, (1895-1935) but the mechanisms were never fully clarified. The most self-consistent mechanistic explanations have been provided by Berl.sup.(4) in 1935, and earlier by Lunge and Berl.sup.(5) in 1906. Both reaction sequences involve intermediate compounds or complexes of NO, NO 2 , SO 2  and H 2  O. The compound common to both theoretical chain mechanisms is nitrosylsulfuric acid, SO 5  NH, or structurally, as shown by Elliott.sup.(6), (HO.SO 2  ONO). Nitrosylsulfuric acid is stable in concentrated H 2  SO 4 , but undergoes rapid hydrolysis in H 2  O as follows: 
     
         2 SO.sub.5 NH+H.sub.2 O→2H.sub.2 SO.sub.4 +NO+NO.sub.2 ( 2) 
    
    
    
    
     This reaction is the generating reaction for the desired equimolar mixture of NO 2  and NO, or N 2  O 3 . The mechanism of formation of the nitrosylsulfuric acid from SO 2 , NO x  and H 2  O was the subject of experimental and theoretical investigations of Lunge and Berl, who established that a second, transient complex, sulfonitronic acid, (H 2  SO 4 ).NO, also called &#34;violet acid&#34; because of its intense color, was also involved. Although this compound has been synthesized in pure form, it occurs as a transient intermediate in the chain reaction leading (in the case of the chamber process) to the formation of H 2  SO 4 . Oxidation of sulfonitronic acid yields nitrosylsulfuric acid. The chain sequence postulated by Berl and Lunge can be stated as: ##EQU1## 
     While the individual reactions of the chain sequence are of both theoretical and practical interest (for the lead chamber operation) the net overall reaction, Reaction (7) is the statement of the first-stage chemistry of the present invention. Laboratory tests described below are consistent with the reaction sequence as written. 
     Laboratory investigation of the SO 2  --NO x  --H 2  O system, confirmed qualitatively net Reaction (7) but not its stoichiometry. Of basic importance was the determination in the laboratory that less-than-stoichiometric addition of SO 2  with respect to NO x  could be used. Most laboratory runs made were made with a ratio of SO 2  /NO x  of 0.5 or less. Additionally, although SO 2  is converted into H 2  SO 4  in the first-stage, nitrosylsulfuric acid is stable only in concentrated H 2  SO 4 , and the very dilute aqueous solution of H 2  SO 3  would make the presence of this compound somewhat suspect. Nevertheless, comparative laboratory runs with air and nitrogen verified the necessity for oxygen in the reaction sequence. In view of the uncertainty as to the actual formation of N 2  O 3 , the reaction of the first stage is frequently referred to in this application and in the claims as the conversion of the nitrogen oxides in the gas into a form in which they readily react with the alkaline solution. Based on the experience in arriving at this invention, this certainly occurs. 
     It should be noted that the transient complex, sulfonitronic acid, NO.H 2  SO 4 , can be considered as an unstable compound of NO and H 2  SO 4 , indicating that NO can participate in the chain sequence producing N 2  O 3 . Visual evidence of the formation of the sulfonitronic acid complex was obtained in a number of runs in which the amount of sulfurous acid solution was limited, and the H 2  SO 4  was allowed to accumulate until a solution of pH of 0.5 was reached. At these high concentrations of H 2  SO 4 , the solution frequently turned violet on continued addition of nitrogen oxides and SO 2 . Additionally, globs of violet liquid accumulated in the exhaust line from the conversion scrubber well prior to the change in solution color from colorless to violet. However, the change in solution color occurred well after the exhaustion point where the solution had lost its conversion ability. H 2  SO 4  is both a byproduct and reactant in the reduction sequence, so that its formation in solution is not detrimental to the conversion reaction until it reaches a high enough concentration to inhibit the absorption of SO 2  gas (Reaction (3)). The H 2  SO 4  concentration is controlled at the desired level by recycling the first-stage absorption liquor and bleeding off a part of the solution, replacing the solution blowdown with fresh water. The solution absorption capacity for SO 2  may be monitored by means of solution oxidation-reduction potential (ORP) and the solution H 2  SO 4  concentration may be monitored by means of a pH indicator. 
     It is economically desirable to recover the SO 2  content of the recycle solution blowdown from the first stage contactor. Accordingly, it is preferred that the first stage blowdown liquor be stripped of its SO 2  content by air or steam in a conventional desorption device and that the stripper off-gas be fed back to the SO 2  absorber for reabsorption of the SO 2 . 
     Some gas streams, particularly those generated in combustion processes wherein a sulfur-containing fuel oil, or coal, is used, inherently contains significant concentrations of SO 2 . In such cases, it may not be necessary to add incremental SO 2  to either the gas fed to the first-stage scrubber, or to the first-stage recycle liquor. The combustion gas naturally containing both the SO 2  and NO x  can be scrubbed with (initially) water, which upon continued recycle, becomes the sulfurous acid solution required for the operation of this invention. Although combustion gases are hot, the gas will rapidly quench to approximately the wet-bulb temperature upon contact with the first-stage aqueous liquor, or, more preferably, may be precooled prior to entry to the sulfurous acid-conversion scrubber. While the SO 2  solubility in the solution will decrease at increasing absorber temperatures, the decreased solubility is compensated for by the increase in the reaction rates at the higher temperatures. 
     There is a significant heat of reaction generated in the first-stage sulfurous acid conversion scrubber, and the adiabatic temperature rise of the liquid can be considerable at high NO x  conversion rates. A theoretical estimate of solution adiabatic temperature rise may be obtained from net Reaction (7). Assuming a feed rate of one lb. mole of SO 2  per hour, the total heat liberated is comprised of the heat of solution of gaseous SO 2 , plus the heat of reaction as represented by Reaction (7). The heat of solution is--15,410 Btu/lb mole of SO 2 , and the heat of reaction is--136,764 Btu/lb mole of SO 2 , giving a total exothermic quantity of--152,174 Btu/lb mole of SO 2 . At a 1:2 ratio of SO 2  :NO x , this heat production corresponds to the conversion of 2 moles of NO x , or 92 lbs. of NO x  as NO 2 . Assuming this quantity of NO x  is fed to the first-stage conversion scrubber per hour, the temperature rise for once-through water fed to the scrubber at a rate of 10 GPM could be 30.3° F. under adiabatic conditions. Under recycle conditions, it would therefore be theoretically possible for the aqueous recycle solution to go to its boiling point. However, even under adiabatic conditions, the major heat sink is the evaporation of water, and evaporative cooling will generally prevent the recycle liquid from excessive temperature rise. The actual equilibrium temperature of the recycle absorption liquor depends on the NO x  load, the SO 2  :NO x  ratio, the liquid/gas ratio, makeup rate and other factors. When these factors cannot be suitably controlled by design to ensure against excessive liquor temperatures, a heat exchanger may be placed in the recycle liquor loop of the first-stage scrubber to adequately cool the liquor. 
     In some processes, the gaseous NO x  stream may be admixed with acid mists or with other acid gases, such as HCl. The latter acid gas would be normally present as a contaminant if aqua regia is used in the process which generates the NO x . HCl is preferentially absorbed by water as compared to SO 2 , and HCl and similarly highly-soluble acid gases will either displace SO 2  from a H 2  SO 3  solution, or impede the dissolution of SO 2 . Accordingly, it is desirable to prevent the penetration of such acid gases to the first-stage sulfurous acid scrubber, and an upstream pre-absorber is desirable when such acid gases are present. The pre-absorber may be any conventional contactor such as a spray chamber or packed scrubber, having sufficient absorption capacity for the removal of HCl and similar acid gases. If the concentration of NO x  is very high, it is preferred that the pre-absorber aqueous scrubbing solution be neutral or acidic. The contact of alkaline solutions with high gaseous NO x  concentrations results in the formation of copious amounts of nitric-acid fume and mist in the vapor phase. In the practice of this invention, the NO x  is converted in the first stage scrubber to a form which does not form the nitric-acid fume and mist on subsequent contact with alkaline solutions. 
     Similarly, in applications where there are acid mists contained in the gas to be treated, the presence of such mists in the sulfurous acid scrubber is undesirable because of the possibility of inhibition of SO 2  dissolution. Accordingly, it is preferred that acid mists, including nitric acid mists, be removed upstream of the sulfurous acid scrubber by means of a suitable efficient mist eliminator. 
     The second-stage contactor utilizes an aqueous solution of a basic alkali metal or alkaline-earth or ammonium salt or hydroxide, selected from the group of alkaline salts, or hydroxides and their mixtures, for the absorption of the reactive nitrogen oxides generated in the first-stage. Another function of the second-stage alkaline scrubber is to absorb any excess SO 2  coming over from the first-stage gas-liquid contactor, preventing emission of SO 2  from the combined system. As long as free alkali is present in the second-stage liquor, there is no danger of significant SO 2  emission from the second-stage. Nevertheless, any excess SO 2  coming over from the first-stage represents a potential reagent loss and it is preferred to minimize first-stage SO 2  evolution by avoiding saturation or near-saturation operating conditions for the first-stage scrubbing liquor. However, it was noted during the course of development of this invention that alkaline sulfites, which would be formed by carryover of SO 2  into a second-stage alkaline liquor scrubber, are also fairly effective in the absorption of nitrogen oxides, so that some degree of SO 2  carryover enhances second-stage NO x  removal, rather than inhibiting it, and SO 2  carryover is not wholly detrimental. 
     Even without recovery of SO 2  from first-stage scrubber blowdown, the consumption of reagent for the process is economically acceptable. For a typical application involving concentrations of NO 2  of 33 mg/m 3  and NO of 0.7 mg/m 3  at a total flow of 1.113 kg/hr of NO x , 24 hrs/day, a flow of 0.783 kg/h of SO 2  would be required if the molar input rate of SO 2  was half that of the NO x . This amounts to a total consumption of 18.79 kg/day of SO 2 , or at a unit cost of $0.231/kg, a total cost of $4.34/day for the SO 2  cost. To this cost, the cost of the NaOH, or other alkaline reagent consumed in the second-stage, must be added. However, the NaOH reaction costs will be incurred in any process scheme for removal of NO x  as nitrite or nitrate salt, so that the incremental cost of the two-stage operation is represented by the SO 2  cost of $4.34/day. 
     The reactive forms of NO x  absorbed in the second-stage scrubber rapidly react with the alkaline solution to form soluble nitrites and nitrates. As in the first-stage scrubber, recycle liquor operation is desirable in the second-stage scrubber to conserve reagent chemicals. Assuming the use of NaOH as the alkaline reagent in the second-stage scrubber, the theoretical generation of N 2  O 3  in the H 2  SO 3  scrubber should yield the following second-stage reaction: 
     
         N.sub.2 O.sub.3 +2 NaOH→2NaNO.sub.2 +H.sub.2 O      (8) 
    
     Assuming the dominance of Reaction (8) in the second-stage scrubber, the liquor and the blowdown liquor from the scrubber will contain nitrites and sulfites, which would have a very high chemical oxygen demand (COD) if allowed to go directly to sewer. While some oxidation will be effected by the contact with air or gas phase oxygen in the scrubber itself, it is desirable to reduce the COD of the blowdown from second-stage liquor by oxidation in a separate aeration unit prior to disposal. 
     The oxidized blowdown liquor from the second-stage scrubber contains residual caustic salts plus neutral salts such as sulfates and nitrates. The stripped liquor blowdown from the first-stage scrubber is acidic. One stream may be used to fully or partly neutralize the other so that subsequent sewage treatment loads are minimized. 
     The heat generated in the second-stage scrubber is also significantly exothermic, with an estimated heat of reaction of --18,180 Btu/lb mole of N 2  O 3  from Reaction (8). Additionally, if solid NaOH is used for replenishing the alkaline strength of the second-stage solution, the heat of solution of --18,360 Btu/lb mole of NaOH is additive to the heat of the chemical reactions. Heat exchange in the scrubber recycle liquor loop is desired to control the temperature of the absorber if the evaporative cooling effects are insufficient. 
     In certain applications of the process of this invention to gas streams containing very high concentrations of NO x , it may be economically desirable to recover HNO 3  in an upstream pre-scrubber. This may be done by countercurrent pre-scrubbing of the gas with water or dliute HNO 3  to effect the partial absorption of NO 2  or N 2  O 4 , and the conversion of the dissolved nitrogen oxides to HNO 3  by hydrolysis in the liquid phase. Although such a pre-scrub operation is effective for only partial absorption of NO x , it does provide for some recovery of reusable HNO 3 , and reduces the downstream process load and reagent consumption. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description, taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a block diagram showing the basic features of the invention; 
     FIGS. 2A and 2B together consitute a diagrammatic view showing apparatus in accordance with this invention and for practicing the method of this invention on a commercial scale; and 
     FIG. 3 is a diagrammatic view of apparatus with which the effectiveness of this invention in removing NO 2  from a gas under treatment is demonstrated. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The apparatus shown in FIG. 1 includes a first scrubber 11 and a second scrubber 13. In the first scrubber 11 the gas to be treated is reacted with H 2  SO 3  and in the second scrubber 13, the gas product of the reaction in the first scrubber is reacted with an alkaline solution, typically NaOH, an aqueous solution, of 2 to 25% by weight, preferably 4-10% by weight. The gas to be treated is injected into the first scrubber at its gas input 15. The first scrubber 11 includes a source 17 of H 2  SO 3  which is supplied to a second input 19. The H 2  SO 3  reacts with the NO x  in scrubber 11. The gaseous product of this reaction is predominately N 2  O 3 . This product is supplied to scrubber 13 through input 21. At another input 23 to scrubber 13, an alkaline solution from a source 25 is supplied. This solution is typically caustic soda, NaOH, or potassium hydroxide. The treated gas free of NO x  is derived from output 27 of scrubber 13. The liquid product of the reaction of the N 2  O 3  and the alkaline solution is derived from output 29 of scrubber 13 and is treated as waste. 
     FIGS. 2A and 2B together show apparatus 31 for treating gas containing NO x . This apparatus includes an upstream water pre-scrubber 33, an H 2  SO 3  scrubber 35 and an alkaline-solution scrubber 37. A blowdown stripper 39 is cooperatively connected to the H 2  SO 3  scrubber 35 and a blowdown oxidizer 41 is cooperatively connected to the alkaline solution scrubber 37. There is also a source of SO 2  which may be one or more bottles or cylinders 43. Ancillary components including pumps, valves and indicators are associated with each of the scrubbers. 
     The upstream water scrubber 33 includes a scrubbing tower 45 and a scrubber recycle tank 49. The tower 45 has packing which may be of suitable type such as Pall rings or the packing disclosed in U.S. Pat. No. 4,238,386 to Bernard J. Lerner. The tower 45 is vertical and has a gas inlet 51 below the packing and an inlet or header or header 53 above the packing. The gas to be treated containing the NO x  flows through inlet 51 and water or recycle acid is supplied to a distributor 55 through inlet 53. The liquor flows through the packing in countercurrent-flow relationship to the gas absorbing a fraction of the NO x  and any HCl from the gas to be treated. The resulting liquid flows into tank 49. In its top, the tower 45 has an outlet 57 for the treated gas. This outlet is connected to duct 58. 
     Water or recycle acid for the distributor 55 is derived from the tank 49. The tank 49 has an outlet 59 in its base which is connected to inlet 53 through a line 61 which includes a valve 63, a pump 65, a temperature indicator 67 and a valve 69. The pump 65 drives the liquor from tank 49 to the distributor 55. 
     The water or dilute HNO 3  flowing from the distributor reacts with the gas in the packing 47 to produce nitric acid. The nitric acid may be removed batchwise or continuously through a branch line 73 including valve 74 when it reaches a predetermined concentration. Pressure of the liquid discharged from the pump is measured in branch line 75 which includes indicator 77 and valve 82. The liquor in tank 49 may be replenished through a makeup water line 79 which includes control valve 72 and 76 and check valve 78. The valve 76 is controlled dependent upon the level of the liquid in tank 49. The valves 63, 69, 71 and 74 may be manually operable by the attendant of the apparatus or all or some of these valves may be responsive automatically to conditions in the system such as the concentration of the HNO 3  in the recycle liquor. 
     The H 2  SO 3  scrubber 35, like the scrubber 33, includes a vertical scrubbing tower 81 and a recycle tank 83. The tower 81 has packing 85 similar to the packing in tower 45, an inlet 87 for the gas from tower 45, an inlet or header 89 for the recycle liquid H 2  SO 3  solution and an outlet 91 for the treated gas. The outlet 91 is connected to duct 93. Through inlet 89, the recycle H 2  SO 3  solution is supplied to the distributor 95. The H 2  SO 3  solution is distributed over the packing 85 and flows in countercurrent relationship to the gas entering the tower from duct 58 through the gas inlet 87, and drains into recycle tank 83. The H 2  SO 3  solution reacts with the NO x  content of the input gas exothermically. 
     The duct 58 is connected to the tower gas inlet 87 through a junction 60 which may be a T-joint and inclues a branch 97 for supplying supplementary air to tower 81 if no oxygen is initially present in the gas in duct 58. In addition, SO 2  is supplied to inlet 87 from tank 43 through a valve 99 and a rotameter 101 which measures the flow rate of the SO 2  into inlet 87. 
     The recycle tank 83 has an outlet 105 which is connected to the distributor 95 through a line 107 which includes valve 109, pump 111 (FIG. 2A), temperature indicator 113, valve 112, heat exchanger 115, temperature indicator 116, and tower inlet header 89. The pump 111 circulates the liquid from tank 83 to the distributor 95. 
     A branch line 117 including valve 119 is connected to line 107 for removing liquid from tank 83 under predetermined conditions. Pump discharge pressure in line 107 is measured by a pressure indicator 123 (FIG. 2A). The recycle liquid passing through heat exchanger 115 is cooled by coolant which flows through the primary tubing (not shown) of the exchanger through inlet line 127, including valves 129 and 131 and temperature indicator 133 and outlet line 135 including temperature indicator 137 and valves 139 and 141. Valves 129 and 131 may be controlled in accordance with the measurement of indicator 133 and valves 139 and 141 may be controlled in accordance with the indications of 137. 
     At the start of an NO x  removal operation tank 83 contains water. Initially, the valve 99 responding to the oxygen-reduction-potential controller 103 is fully open so that substantial quantities of SO 2  are fed into tower 81. The SO 2  reacts with the recycle water from tank 83 producing H 2  SO 3  in increasing concentraton. As the concentration of H 2  SO 3  increases, the controller 103 causes valve 99 to be throttled reducing the flow of SO 2  appropriately. During operation, the liquid in tank 83 will contain HNO 2 , HNO 3 , H 2  SO 4  and H 2  SO 3 . When the concentrations of these acids reaches limited or predetermined magnitudes, the liquid in tank 83 is drawn off through valve 119 or through branch line 143 to the blowdown stripper 39. Line 143 includes valve 145. The water in tank 83 is replenished through line 147 including control valve 149 and check valve 153. Valve 149 may be controlled in accordance with the level in tank 83. 
     Instead of being fed directly into the scrubber tower 81, the SO 2  may be supplied to tank 83 or to the circulating liquid prior to the operation of scrubber 35. If this procedure is adopted, half the quantity of SO 2  necessary for saturation of the liquid in tank 83 may be supplied to the tank before operation. During operation, the SO 2  may be admitted to the scrubber 81 at a reduced rate. 
     The blowdown stripper 39 includes tank 213. Air is forced through this tank 213 by compressor 215. The air strips the SO 2  from the liquid which flows into the tank through line 143 and feeds it back into input 87 of H 2  SO 3  tower 81 through duct 217. The air supplied by compressor 215 also flows into tower 81. The liquid from the tank 213 also flows to a waste treatement facility (not shown) through line 219. This liquid includes H 2  SO 4  and HNO 3  and flows into a mixing tank 220. 
     The alkaline-solution scrubber 37, like scrubbers 33 and 35, includes a vertical scrubbing tower 155 and a recycle tank 157 into which the liquid from the tower drains. The tower 155 includes packing 159 similar to the towers 45 and 81, an inlet 161 for the gas in duct 93, an inlet or header 163 for alkaline solution and a top outlet 165 for the treated gas. The outlet 165 is connected to the blower 167 which exhausts the treated gas to the atmosphere and maintains a small negative pressure (suction) in the ducts 58 and 93. The gas from duct 93 is supplied to the bottom of packing 159 and the treating alkaline liquid is supplied to a distributor 169 on top of the packing 159. 
     The tank 157 contains alkaline solution typically NaOH or KOH. The concentration of the alkali may be between 1 and 20%, but is typically about 4 or 5%. The alkaline solution absorbs and reacts with the reactive forms of NO x  derived from the first-stage 35. 
     Tank 157 has an outlet 171. Liquor from this outlet is recycled to the distributor 169 through line 173 including valve 175, pump 177, temperature indicator 179, valve 181, heat exchanger 183, temperature indicator 185 and inlet 163. The alkaline solution from distributor 169 flows through the packing 159 in countercurrent flow to the gas from duct 93. The alkaline solution absorbs and reacts with the reactive forms of NO x  derived from the first-stage 35. The product of this reaction may include nitrites, sulfites, nitrates and sulfates. 
     A branch line 187 including valve 189 is connected to line 173 for draining off the liquid in tank 157 as desired. This liquid is also discharged to blowdown oxidizer 41 through branch line 191, which includes valve 193 in the pump discharge branch line. Pressure in line 173 is measured by indicator 195 in branch line 197 which includes valve 199. 
     The water in tank 157 is replenished through line 201 which includes control valve 203 and check valve 207. Alkaline solution is supplied to tank 157 through line 209 which includes valve 211. Typically, the makeup alkaline solution supplied has a concentration of about 50%. Valve 203 is controlled in accordance with the level in tank 157. 
     The primary tubing (not shown) of heat exchanger 183 is supplied with coolant through inflow duct 200 and outflow duct 202. Inflow duct 200 includes valve 204 and temperature indicator 208 and outflow duct 202 includes valve 210 and temperature indicator 214. 
     The blowdown oxidizer includes tank 221. Tank 221 is supplied with air by compressor 223. The air oxidizes the sulfites and nitrites which flow into tank 191 from tank 157. Where the alkaline solution is NaOH, the nitrites and sulfites are predominately NaNO 2  and Na 2  SO 3 . These are reducing agents whih have a high chemical oxygen demand (COD) injurious to marine life. They are converted in the tank 221 to harmless or beneficial NaNO 3  and Na 2  SO 4 . The output of tank 221 flows into mixing tank 220. The liquid from tank 157 includes alkaline solution, typically NaOH. This solution reacts with the H 2  SO 4  and HNO 3  in mixing tank 220. The output of tank 221 flows into mixing tank 220 which also receives the output of tank 213. The outputs of tanks 213 and 220 react to neutralize each other partially or wholly. The output of mixing tank 220 flows into the waste treatment facility. 
     Tests to evaluate the efficacy of the invention were conducted with the apparatus shown in FIG. 3. This apparatus includes a plurality of bubbler flasks 241, 243 and 245. Each flask is provided with a stopper 247. A long inlet tube 249, 251, and 253 extends into each flask through its stopper 247. Each inlet tube terminates near the bottom of its flask. A short outlet tube 255, 257 and 259 extends into each flask through its stopper. Each outlet tube terminates near the top of its flask. The apparatus shown includes a compressed air line 261, an NO 2  cylinder 263 and an SO 2  cylinder 265. The compressed air line 261, the NO 2  cylinder 263, and the NO 2  cylinder 265 are connected in parallel to inlet tube 249. The compressed air line is connected to inlet tube 249 through a valve 267 and a rotameter 269; the NO 2  cylinder 263 is connected to the tube through a valve 271, an infra-red heater 273 and a rotameter 275; the SO 2  cylinder 265 is likewise connected through valve 277, heater 279 and rotameter 281. The heaters 273 and 279 serve to prevent the formation of SO 2  or NO 2  liquid when the valves 271 and 277 are opened. The short outlet tube 255 of flask 241 is connected to long tube 251 of flask 243 and the short outlet tube 257 of flask 243 is connected to long inlet tube 253 of flask 245. The treated gas is emitted from short tube 259 in flask 245 and is observed against the background of a white paper 283. The gas may also be observed in flask 245. 
     Prior to a test, water is poured into flask 241. The level of the water is near the top of the flask but below the end of outlet tube 255. Also, the alkaline solution to be investigated is poured into flask 243 to a level near the top of the flask, but below the end of outlet tube 257. Flask 245 is empty. The flow of compressed air is then started. If flask 241 is to contain H 2  SO 3 , the appropriate quantity of SO 2  is bled into flask 241 through inlet tube 249 under the action of the compressed air. 
     During the test, NO 2  is fed into flask 241 through inlet tube 249 under the action of the compressed air. The emission into and through flask 245 is then observed. When the NO 2  breaks through, a reddish-brown color is seen against the white paper 283. The time which elapses between the start of the injection of the NO 2  and the appearance of the reddish-brown color is measured. A short interval indicates that the H 2  SO 3  and/or the solution in flask 243 are ineffective in absorbing the NO 2  ; a long interval indicates that the H 2  SO 3  and the alkaline solution are effective in removing NO 2 . 
     The results of a series of tests are shown in the following Table III: 
     
                                           TABLE III__________________________________________________________________________    Flask 241            Flask 243                  Air                     NO.sub.2                        SO.sub.2Run   Break Thru    500 Grms.            500 Grams.                  cc cc ccNo.   Time (min)    Solution            Solution                  min.                     min.                        min.                            Comments__________________________________________________________________________ 93   33.8  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.2    White mist as usual    500 ppm                 Colors as usual    PPD 94   19.2  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    100 ppm    PPD 95   23.1  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    200 ppm    PPD 96   26.7  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    5% Na.sub.2 SO.sub.3    200 ppm    PPD 97   14.3  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    5% Na.sub.2 SO.sub.3 98   18.3  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    5% Na.sub.2 SO.sub.3 99   16.5  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    5% Na.sub.2 SO.sub.3100   21.65 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Sol. beige before air    500 ppm                 Sol. dark cream w/air    TBHQ                    No white smoke during run                            During run sol. gets lighter                            Color - Light yellow101   13.7  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Sol. yellow before air    500 ppm                 Wine red after    Propyl Gal.             During run sol. color lightens102   6.1   5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3103   15.25 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Sol. amber - deepens w/NO.sub.2    200 ppm                 Much white smoke    PPD104   16.33 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Much less amber color before run    200 ppm    PPD105   17.45 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    White smoke-very heavy (obscures end    400 ppm                 point)    PPD106   17.7  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Light white mist (smoke) (acid)    50 ppm    PPD107   15.45 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Acid Mist as before    50 ppm                  Solution amber    PPD108   14.75 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Sol. begins w/slight amber cast    50 ppm                  Deep amber w/air - clears w/NO.sub.2    TBHQ                    White acid mist w/50 ppm PPD109   13.25 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Colors and mist same as #108    50 ppm    TBHQ110   10.00 5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3111   13.1  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    20 ppm    PPD112   7.6   5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3113   11.6  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    100 ppm    TBHQ114   12.9  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    100 ppm TBHQ    5 ppm PPD115   10.9  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Small amount of white mist    200 ppm    Triethanol Am.116   8.3   5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Small amount of white mist    200 ppm    Benzyl Alchl.117   6.25  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Moderate amount of mist colors    200 ppm                 immediately    Sugar118   9.0   5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Moderate white mist colors within    200 ppm                 first 2 min. of run    Sugar119   12.7  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    Much white mist    200 ppm    Mond-Ethanol    Amine120   16.5  5% NaOH       500                     300    Air 1600 cc/min. 20 min. before    2% Na.sub.2 SO.sub.3    100 ppm    mixed inh.    1:10:100 PPD    TBHQ: MEA121   0.6   H.sub.2 O Sat.                  500                     300    SO.sub.2 in AM. Run 2 pm    w/SO.sub.2122   4.25  H.sub.2 O Sat.                  500                     300    Run immediately    w/SO.sub.2              Lots of moisture123   2.30  H.sub.2 O     500                     300124   11.3  H.sub.2 O Sat.                  500                     300    A lot of moisture. Sol. turns violet    w/SO.sub.2              late in run.                            Sol. gets very warm during run.                            Gas evolves after run.125   17.3  H.sub.2 O Sat.                  500                     300    Sat. and allow to sit 1 hr. Brown    w/SO.sub.2              cloud immediately, disappears in few                            seconds. Blue liquid forms in outlet                            line from flask.126   48.5  H.sub.2 O Sat.            5% NaOH                  500                     300    Brown cloud forms in 241 immediately,    SO.sub.2                which disappears in few seconds. Froth-                            ing blue liquor in line between #1 and                            #2. Sol. #1 slightly warm. Sol. #2                            very warm.127   8.5*  H.sub.2 O Sat.                  500                     300                        100 White solid forms in outlet line, In-   *step 1    w/SO.sub.2              creasing SO.sub.2 rate reverses                            end-point.128   66.3  H.sub.2 O            5% NaOH                  500                     300                        300 White solid forms in inlet 249 to flask                            241 frothing blue liquor forms in line                            between 241 and 243. Brown gas in line                            between 241 and 243. End point is very                            sharp. 241 sol. slight warm. 243 sol.                            very warm. White solids finally plug                            line.129   50.3  H.sub.2 O Sat.            5% NaOH                  500                     300                        trickle                            Sat. w/SO.sub.2 before run. Reintroduce                            SO.sub.2    w/SO.sub.2              5 min. into run. No white solids build                            up. Solutions not so warm as before.                            At end, increase SO.sub.2 to 300                            cc/min.                            56 min. 243 sol. turned violet. 59.6                            min. second breakthru.130   50.4  H.sub.2 O            5% NaOH                  500                     300                        300 Duplicate #128131   77.8  H.sub.2 O            5% NaOH                  500                     300                        150 SO.sub.2 start first. Some clogging                            from                            white solids near end of run. 243                            sol. turned violet just before break                            thru.132   60.7  H.sub.2 O            5% NaOH                  500                     300                        150 Forced to terminate because of plug-                            ging with white solids.133   See   5% NaOH       500                     300    Small amount brown gas immediately.   comments                      2.5 min. copious brown gas. 7.5 min.                            increase SO.sub.2 700 cc/min. for 1                            min.-                            gas reduced. 9.5 min. copious brown                            gas as before. Sol. very hot.134   Immed.    5% NaOH       500                     300    SO.sub.2 700 cc/min. 2 min. before -                            brown                            gas immediately. Solution__________________________________________________________________________                            hot. 
    
     In Table III, the numbers in the left-hand column identify the run or test, the NO 2  breakthrough time in minutes is in the second column from the left, the content of alkaline solution in flask 241 is in the third column, the content of H 2  SO 3  in flask 243 is in the fourth column, the flow of compressed air in cubic centimeters per minute is in the fifth column, the flow of NO 2  in cc per minute in the sixth column, and the flow of additional SO 2  in cc per minute is in the seventh column. The eighth column describes other parameters of the tests and unusual observations. 
     The initial portion of the program, represented by runs 93 through 120 in Table III, was an investigation of the possibility of inhibiting the oxidation of single-stage sodium sulfite or caustic/sulfite solutions when used to absorb NO x  from air. For the usual case where NO x  is admixed with air or oxygen, the sulfite solution is rapidly oxidized to sulfate, rendering the solution ineffective. The uninhibited-solution runs 97-99 show an average of 16.3 minutes for NO x  color breakthrough time. The various oxidation inhibitors tried included paraphenylene diamine (PPD), tributylhydroquinone (TBHQ), propyl gallol, triethanolamine, benzyl alcohol, and sugar, none of which significantly improved the breakthrough time, and most of which depressed the breakthrough time. Experiments omitting the sulfite and caustic, and using a single-stage sulfurous acid scrubbing solution, Runs 121-122 and 124-125, showed some interesting solution color change behavior, but no improvement on NO x  breakthrough time. The &#34;discovery&#34; run, Run 126, used the two stages of scrubbing in series H 2  SO 3  followed by 5% NaOH, and gave a breakthrough time of 48.5 minutes. The blue liquid observed in the line between the two bubbler flasks is suspected of being N 2  O 3 , which is a blue liquor, but no positive identification of this material could be obtained because of its unstable nature. 
     The results of tests 126 and 128 through 132 demonstrate the efficacy of the invention in removing NO 2 . 
     While preferred embodiments of this invention have been disclosed herein, many modifications thereof are feasible. This invention is not to be restricted except insofar as is necessitated by the spirit of the prior art.