Process for Removing No.sub.x and So.sub.x From Exhaust Gas

1. Field of the Invention

This invention relates to a process for separating contaminants from the exhaust gases generated from combustion of fossil fuels, incineration, furnaces or gas streams of chemical processes, and more particularly, to the removal of nitrogen and sulfur oxides from an exhaust gas stream.

2. Description of the Prior Art

Nitrogen oxides (NO.sub.x) and sulfur oxides (SO.sub.x) are the main air pollutants found in the flue gases from combustion sources and chemical plants. The systems for removal of SO.sub.x gases by dry or wet scrubbing have reached an advanced stage of development. Other processes based on regenerative adsorbents are also available. Dry technologies are based on adsorption and wet technologies are based on absorption.

It is well known in the art to remove nitrogen oxides from flue gases by a number of dry and wet processes. Dry processes generally utilize catalytic decomposition or adsorption, while wet processes normally utilize absorption technology. A majority of the dry flue gas treatment (FGT) processes utilize catalytic decomposition or homogeneous decomposition at high temperatures. Dry flue gas treatment processes are normally carried out after the combustion process. Sometimes CaO/CaCO.sub.3 is added in the fluidized bed during the combustion process. The major dry processes for NO.sub.x removal are: selective catalytic reduction (SCR) with NH.sub.3, selective (non-catalytic) reduction with NH.sub.3, non-selective catalytic reduction, and adsorption.

Wet processes are usually added downstream of all equipment prior to entering the stack. The major wet: processes for NO.sub.x removal are: absorption with liquid phase oxidation, absorption with liquid phase reduction, and gas phase oxidation followed by absorption.

U.S. Pat. Nos. 3,473,298; 4,681,744; and 4,799,941 disclose processes and devices where exhaust gases are first chilled with direct sprays and thereafter solid contaminants and water soluble substances are removed from the gases by contacting with the water sprays in a spray chamber. The water combines with water soluble gases, such as SO.sub.x, contained in the gases to form sulfurous and sulfuric acids, which are collected with the water spray in a chamber. U.S. Pat. No. 3,881,004 discloses recovery of nitric acid by scrubbing a tail gas with an acid or an alkaline solution which minimizes the discharge of nitrogen oxides to the atmosphere. The publication entitled "Selection of Reactive Solvent for Pollution Abatement of NO.sub.x " by K. R. Jethani et al., Gas Separation & Purification, vol. 4, March 1990, systematically reviews ten chemical reactive systems for removal of NO.sub.x.

A number of U.S. patents disclose NO.sub.x either oxidized or reduced, preceding or following absorption in a liquid solution along with SO.sub.x. Representative U.S. patents disclosing the state of the art include the following. U.S. Pat. No. 4,011,298 discloses removal of NO.sub.x by first oxidizing with ozone and absorbing in acidic solution containing iron compound. U.S. Pat. Nos. 4,541,999 and 4,564,510 disclose oxidation of NO.sub.x to NO.sub.2 by addition of ozone followed by reaction with ammonia forming nitrite and nitrate on absorption and oxidation with air to form nitrate during desulfurization process. U.S. Pat. No. 4,247,321 discloses oxidation of NO.sub.x with ozone and absorbing in lime or limestone or calcium phosphate solution. U.S. Pat. No. 4,107,271 discloses use of iodide solution and ozone. U.S. Pat. No. 3,957,949 discloses use of ozone to convert NO.sub.x to NO.sub.2 and absorbing in reactive medium like sodium chlorite. U.S. Pat. No. 3,997,415 discloses reduction of NO.sub.x and SO.sub.x by irradiation of gas stream. U.S. Pat. No. 4,971,777 discloses oxidation of NO.sub.x containing gas stream with help of organic compounds in the temperature range 300-900.degree. C. then absorbing with ammonia. U.S. Pat. No. 4,119,702 discloses oxidation of NO with ozone, H.sub.2 O.sub.2, chlorine dioxide and nitrogen dioxide followed by reduction with urea. U.S. Pat. No. 4,035,470 discloses a process wherein NO.sub.x containing gases are first oxidized with ozone and chlorine dioxide and then absorbed by sulfite, sulfide, polysulfide or thiosulfate of alkali or alkaline earth metals. U.S. Pat. Nos. 4,999,167; 5,206,002 and 5,316,737 disclose lowering the flue gas temperature near ambient temperature range, i.e 125.degree. F., before mixing with ozone for NO.sub.x abatement. The oxidized contaminants are absorbed in water, or alkaline medium for removal of NO.sub.x.

It is well known that NO is relatively inactive, and at low concentrations, its removal from gas steam is difficult and inefficient. It is also known that NO can be oxidized with many different chemical oxidants to form NO.sub.2. With an oxidant, such as ozone, NO.sub.2 is further oxidized to N.sub.2 O.sub.5, which not only enhances reactivity but also solubility several fold resulting in ease in removal by absorption or adsorption with or without chemical reaction.

A disadvantage experienced with the prior art processes, as disclosed above, is either incomplete oxidation or mixing excessive oxidant in an absorption medium where ozone is wastefully consumed. As disclosed in U.S. Pat. No. 5,206,002, the temperature of flue gas is reduced to near ambient 125.degree. F. before mixing ozone and the required residence time is provided to convert NO.sub.x to N.sub.2 O.sub.5 with stoichiometric quantities of ozone. It is also well known that lowering temperature reduces the rate of oxidation of NO. Further, when ozone and NO.sub.x are both depleted to extinction in the oxidation chamber, the rate of oxidation decreases significantly. In addition, lowering temperature by recovering heat may not be an option when low temperature heat has no gainful use.

Thus, there is need for further improvements in the known oxidative processes to make them commercially viable processes. While oxidation of NO.sub.x to NO.sub.2 is known to improve solubility and reactivity, it is not adequate to remove NO.sub.2 in a cost effective manner. Therefore, there is need for an NO.sub.x removal process by which NO.sub.2 can be further oxidized to a higher state, preferably to NO.sub.3 of at least half of the quantity of NO.sub.x.

Further, there is need to provide a process and an apparatus for removing contaminants, specifically NO.sub.x and SO.sub.x emissions, from exhaust gas that can be applied to any combustion (low or high sulfur coal fired, gas fired or oil fired, incineration, furnaces) or flue gas from chemical process system. NO.sub.x and SO.sub.x must be removed simultaneously but independently of one another. The process must not be dependent upon capital intensive equipment. The process must not cause scaling or lose performance with time. The abatement process must not produce secondary emissions or hazardous products and must be operable in a wide temperature range. The process must also be applicable for varying compositions of NO.sub.x while at the same time reduce the content of the contaminants in the exhaust gases to the required levels as prescribed by air quality regulations. An improved emission system is therefore required that not only brings the content of the contaminants in the exhaust gases into compliance with regulated air quality standards, but also has the capability to meet future standards.

In accordance with the present invention there is provided a process for removing nitrogen oxides and sulfur oxides from an exhaust gas stream that includes the steps of directing an exhaust gas stream containing contaminants including nitrogen oxides and sulfur oxides at an elevated temperature from a process system to an exhaust duct. The exhaust gas stream is conveyed through the exhaust duct. The exhaust gas is mixed with ozone in stoichiometric excess in a molar ratio in the range between about 0.2 to 2.8 moles of ozone to each mole of nitrogen oxide. The exhaust gas is maintained in contact with the ozone for a preselected period of time to convert the nitrogen oxides to equimolar amounts of NO.sub.2 and NO.sub.3 to form N.sub.2 O.sub.5. A reagent liquid is introduced into the exhaust gas stream to transform N.sub.2 O.sub.5 and SO.sub.2 therein to dilute acids of HNO.sub.3, H.sub.2 SO.sub.3, and H.sub.2 SO.sub.4. The dilute acids of HNO.sub.3, H.sub.2 SO.sub.3, and H.sub.2 SO.sub.4 are absorbed into liquid water. The dilute acids are converted to salts, including nitrates, sulfites and sulfates for removal from the exhaust gas stream. Thereafter, the exhaust gas stream is discharged substantially free of nitrogen oxides and sulfur oxides from the exhaust duct.

Further the present invention is directed to a process for removing nitrogen oxides from an exhaust gas stream that includes the steps of directing an exhaust gas stream containing nitrogen oxide contaminants at an elevated temperature from a process system to an exhaust duct. The exhaust gas stream is conveyed through the exhaust duct. Ozone in stoichiometric excess is mixed with the exhaust gas to oxidize the nitrogen oxide contaminants to convert nitrogen oxides in the exhaust gas stream to equimolar ratios of NO.sub.2 and NO.sub.3. NO.sub.2 and NO.sub.3 are transformed in the presence of ozone to N.sub.2 O.sub.5. A reagent liquid is mixed with the exhaust gas stream to convert N.sub.2 O.sub.5, to dilute acids of nitrogen and to neutralize the acids to nitrogen salts. Thereafter, the exhaust gas stream is discharged substantially free of nitrogen oxide contaminants from the exhaust duct.

Additionally the present invention is directed to a process for removing contaminants from an exhaust gas stream that includes the steps of directing an exhaust gas stream containing contaminants at an elevated temperature to an exhaust duct. The exhaust gas stream is conveyed through the exhaust duct. The contaminants are mixed with an oxidant in stoichiometric excess to oxidize the contaminants in the exhaust gas. The oxidized contaminants and unreacted oxidant in the exhaust gas are mixed with a liquid reagent. The oxidized contaminants and unreacted oxidant are absorbed into the liquid reagent for removal from the exhaust gas. The exhaust gas is discharged substantially free of the contaminants and oxidant from the exhaust duct.

Additionally the present invention is directed to a process for removing nitrogen and sulfur oxides from an exhaust stream that includes the steps of directing an exhaust gas stream containing contaminants, including nitrogen oxides and sulfur oxides at an elevated temperature from a process stream to an exhaust duct. The exhaust gas stream is conveyed through the exhaust duct. Ozone in stoichiometric excess is mixed with the exhaust gas in a molar ratio in the range between about 0.2 to 2.8 moles of ozone to each mole of nitrogen oxides to convert the nitrogen oxides to eqimolar amounts of NO.sub.2 and NO.sub.3. The contaminants in the exhaust gas stream are oxidized to increase the absorbability of the nitrogen oxides and sulfur oxides in water. The exhaust gas is maintained in contact with an excess amount of ozone for a preselected period of time. The residence time of the exhaust gas in contact with ozone is monitored to ensure substantially complete conversion of the nitrogen oxides in the exhaust gas to either N.sub.2 O.sub.5 or eqimolar proportions of NO.sub.2 and NO.sub.3 with a marginal amount of unreacted ozone remaining in contact with the exhaust gas. The sulfur oxides, excess ozone, oxidized contaminants containing N.sub.2 O.sub.5 and eqimolar quantities of NO.sub.2 and NO.sub.3 are admixed with a reagent spray to convert N.sub.2 O.sub.5, NO.sub.2 and NO.sub.3 and sulfur oxides to dilute acids including HNO.sub.3, H.sub.2 SO.sub.3 and H.sub.2 SO.sub.4. The admixture of the reagent spray containing the dilute acids, including HNO.sub.3, H.sub.2 SO.sub.3, and H.sub.2 SO.sub.4 and excess ozone are absorbed into liquid water. The dilute acids are converted to salts including nitrates, sulfites, and sulfates for removal from the exhaust gas stream. The ozone dissolved in the reagent is converted into oxygen with sulfite. Thereafter, the exhaust gas stream is discharged at about ambient temperature substantially free of the contaminants from the exhaust duct.

Accordingly, a principal object of the present invention is to provide a process for reducing the content of contaminants, such as NO.sub.x and SO.sub.x, from exhaust gases to a level required by air quality standards while at the same time providing the capability of removing substantially all of the contaminants from the gases emitted from a combustion exhaust stream, chemical process, or process stream of manufacturing chemicals in the temperature range of 40 to 325.degree. F. without causing any secondary emissions of oxidant.

Another object of the present invention is to provide a process for treating emissions from a combustion or chemical process that increases the absorption of contaminants, such as NO.sub.x and SO.sub.x emissions, into a solution to remove the contaminants from the exhaust gas or process gas.

A further object of the present invention is to provide a process for converting dilute nitrogen and sulfur acids to selected nitrates, sulfites and sulfates in solution and treating them, if necessary, prior to disposal of the solution.

An additional object of the present invention is to provide an improved process for economically removing nitrogen and sulfur oxides from streams of gaseous combustion products or other gaseous process streams so that the content of the nitrogen and sulfur oxides is reduced to a level acceptable within air and water quality control standards.

These and other objects of the present invention will be more completely disclosed and described in the following specification, accompanying drawings, and appended claims.

FIG. 1 is a system flow diagram of a process for removing contaminants from the exhaust gases of chemical processes, fired heaters or fossil fired boilers, furnaces, and the like.

FIG. 2 is a graph quantitatively illustrating NO.sub.x removal from exhaust gas emitted from a gas fired boiler in accordance with the present invention.

FIG. 3 is a graph quantitatively illustrating NO.sub.x removal from exhaust gas emitted from a coal fired utility boiler of a power plant in accordance with the present invention.

Referring to FIG. 1, there is illustrated NO.sub.x and SO.sub.x removal apparatus generally designated by the numeral 10 that is utilized with fired process heaters or fossil fueled boilers, such as a packaged firetube or water-tube boiler. The boiler may be of the type associated with utility power plants or those designed to generate as little as 2 million BTU/hr. fuel input energy to the boiler.

In one embodiment, the apparatus 10 is used as a polishing system to remove NO.sub.x and SO.sub.x contaminants from exhaust streams treated by other methods which are unable to achieve removal levels that are attainable by the present invention. Overall, NO.sub.x and SO.sub.x removal costs are optimized by incorporating the apparatus 10 downstream of a selected pollution abatement process.

The present invention can be combined with conventional dry and wet abatement processes. Example dry flue gas treatment processes adaptable for use with the present invention include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Example wet processes include absorption with liquid phase oxidation, absorption with liquid phase reduction, and gas phase oxidation followed by absorption. With both wet and dry processes, the apparatus 10 is added downstream of all equipment prior to entering the stack. In this manner, the present invention is used to supplement conventional abatement equipment to achieve lower ppm levels of NO.sub.x and NO.sub.x in exhaust gas emissions not otherwise attainable by conventional equipment.

As shown in FIG. 1, a flue gas stream from multiple utility boilers is directed at a flow rate of 6.2 million lbs/hr through a stack or exhaust duct 12. The duct 12 is connected to a supply duct 14 which includes a fan 16 that diverts the combustion exhaust gases from the stack 12 into the supply duct 14. The supply duct 14 is provided with a damper 18 positioned upstream of the fan 16 to seal off the duct 14 and permit the exhaust gases to be emitted from the duct 12, if desired. The fan 16 diverts the exhaust gases from the duct 14 at an elevated temperature through a duct system 20 to a static mixer 22.

It should be understood that the present invention is adaptable for use with a wide variety of boilers or chemical processes including gas fired boilers, major process boilers, liquor recovery boilers, as well as smaller fired process heaters, process steam boilers, furnaces, gaseous process streams and nitric acid plants. In the case of coal fired utility boilers or gas fired boilers, the temperature of the boiler exhaust gases flowing through the exhaust duct 12 is in the range between about 220.degree. F. to 325.degree. F.

The fan 16 conveys the exhaust gas stream from the exhaust duct 12 and supply duct 14 through duct system 20 to static mixer 22 at a temperature range of approximately 40.degree. F. to 325.degree. F. The static mixer 22 has flue gas inlets 24 and 26. The inlet 24 is connected to duct system 20, and inlet 26 is connected to an ozone generator 28 via duct 30. The fan 16 is also operable to effect mixing of ozone. Ozone generator 28 receives through conduit 32 a gas feed stream which can be dried air, oxygen or mixture of air and oxygen. The fluid stream supplied by the generator 28 to the static mixer 22 is 1% to 20% by weight ozone.

In the static mixer 22, the flue gas is mixed with ozone (O.sub.3) and conveyed through conduit 34 to a reaction chamber 36 where the exhaust gas contaminants are oxidized. In the reaction chamber 36, NO.sub.x, SO.sub.x, CO and other contaminants are oxidized by the presence of ozone. The exhaust gas conveyed to chamber 36 comprises approximately 95% NO and 5% NO.sub.2. The reactions that take place in chamber 36 to transform NO to NO.sub.2, NO.sub.3 and N.sub.2 O.sub.5 include the following: ##STR1##

Reaction (1) involving oxidation of NO to NO.sub.2 is almost instantaneous. Between reactions (2) and (3), reaction (2) is slower than reaction (3). The rate of oxidation of NO.sub.2 to NO.sub.3 increases with increase in the temperature.

As disclosed, in CRC handbook (1980-81) N.sub.2 O.sub.5 decomposes at a temperature over 120.degree. F. In one approach to this condition, U.S. Pat. No. 5,206,002, discloses lowering the temperature to 125.degree. F., prior to admixing the ozone and converting NO.sub.x to N.sub.2 O.sub.5. U.S. Pat. Nos. 4,247,321; 4,541,999 and 4,564,510 provide for mixing ozone at higher temperatures but without providing enough residence time for oxidation of NO.sub.x in the gas phase. Also, ozone is directly introduced into the scrubber, which leads to formation of nitrite in the scrubber. In contrast, N.sub.2 O.sub.5 absorption results in selective formation of nitrate. Presence of nitrite in nitrate is undesirable for use in fertilizer.

In accordance with the present invention ozone is mixed at temperatures between about 40.degree. F. to 325.degree. F. for a preselected residence time in the reaction chamber 36 to convert the nitrogen oxides in the exhaust gas stream to equimolar amounts of NO.sub.2 and NO.sub.3 to form N.sub.2 O.sub.5 in the reaction chamber 36. In the reaction chamber 36 ozone is mixed in stoichiometric excess with the exhaust gas in a molar ratio in the range between about 0.2 to 2.8 moles of ozone to each mole of nitrogen oxide.

In the reaction (4) above, ozone decomposes at a temperature in the range between about 220.degree. F. to 325.degree. F. Therefore, the dimensions of the reaction chamber 36 are selected to allow the NO.sub.x in the gaseous exhaust stream to occupy a preselected residence time which is sufficiently long to oxidize NO.sub.x to higher order oxides and to consume substantially all of the ozone with minimum decomposition.

It is conventional practice in design of a chemical system to add one chemical in stoichiometric excess over the other (limiting reactant) to deplete the concentration of the limiting reactant to extinction at a reasonably fast rate. However, this leads to an unreacted amount of the excess reactant at the exit of the system. In the present invention, operation of the reaction chamber 36 at temperatures in excess of 125.degree. F. with a marginal increase of ozone requirements significantly reduces the size required for the oxidation chamber 36 in order to attain the desired levels of NO.sub.x reduction.

In one example referring to FIG. 2, there is graphically illustrated data representing the quantity of NO.sub.x removed from the exhaust gas emitted from a gas fired boiler. Flue gas at a temperature in the range of 125.degree. F. to 130.degree. F. required approximately 20 seconds after mixing with ozone to reduce NO.sub.x levels to 2 ppm in the treated exhaust gas stream. The second set of data points on the graph illustrates NO.sub.x removal at approximately 245.degree. F. with an increase in ozone feed by 7 percent to achieve the preferred level of oxidation within 3 to 4 seconds of residence time in the reaction chamber 36.

Further in accordance with the present invention, in addition to NO.sub.x, carbon monoxide and SO.sub.2 are partially oxidized to carbon dioxide and sulfur trioxide reaction in the chamber 36, a small quantity of ozone also decomposes forming oxygen at the temperature range between about 220.degree. F. to 325.degree. F. However, in most cases, the required molar ratio mainly depends on the ratio of NO to NO.sub.2, concentration of NO.sub.x, and the extent of NO.sub.x removal. With the present invention, the ozone to NO.sub.x molar ratio is preferably in the range between about 0.2 to 2.8.

From the reaction chamber 36, the treated contaminants are introduced into a combination spray/absorption chamber generally designated by the numeral 38. However in one embodiment of the present invention before the oxidized exhaust gas contaminants are introduced into the absorption chamber 38, the temperature of the treated gas stream is lowered. This is accomplished as illustrated in FIG. 1 by diverting the treated exhaust gas from duct 40 to conduit 44 by closing valve 41 in duct 40 for flow to a heat exchanger or economizer 42. The treated exhaust gas enters the economizer 42 at a temperature of up to about 325.degree. F. The economizer 42 includes a cooling tube 46 that extends in a serpentine path within the economizer. The tube 46 is equipped with cooling fins, as well known in the art.

Heat is transferred in the economizer 42 to water in the tube 46. The water is heated in the tube and is conveyed from the economizer 42 to a tank (not shown). As a result, both latent and sensible heat is recovered from the exhaust gas passing through the economizer 42.

The economizer 42 also serves to partially scrub or absorb contaminants contained in the treated exhaust gas. This is accomplished by moisture in the exhaust gas condensing on the surface of the tube 46 and also on the fins mounted on the tube 46. Condensation of moisture on the tube 46 and the tube fins progressively lowers the temperature of the exhaust gas below the dew point of the exhaust gas. The condensate passes from the economizer 42 through an outlet 48 to a condensate collector 50 for subsequent holding and treatment in tank 51.

It should be understood that the provision of the economizer 42 to lower the temperature of the treated exhaust gases and to partially scrub and aid in the absorption of contaminants remaining in the treated exhaust gases is selective. In this respect, the economizer 42 in another embodiment is bypassed by opening valve 41 so that the treated exhaust gas from the reaction chamber 36 is fed directly to the absorption chamber 38.

One advantage in utilizing the economizer 42 to lower the exhaust gas temperature and condense the water vapor in the exhaust gas is to achieve maximum efficiency in the separation of the contaminants from the exhaust gas stream in the absorption chamber. By reducing the temperature of the treated exhaust gas to about ambient temperature, the absorbability of the contaminants in the exhaust gas with a reagent solution is greatly enhanced, particularly in the absorption of nitrogen, sulfur and carbon dioxides. The treated exhaust gas, which may be cooled to about ambient temperature as above described, is conveyed through a duct system 52 to the absorption chamber 38. Preferably, the chamber 38 includes two sections or subchambers. In a first chamber generally designated by the numeral 54 the exhaust gas stream is treated with a reagent liquid, preferably by spray. In a second chamber generally designated by the numeral 56 the exhaust gas stream is scrubbed.

Chamber 54 includes an array of spray nozzles 58 connected to a spray header 60. Spray header 60 receives a reagent solution, such as water, from a tank 62 through a pump 64. The exhaust gas stream is quenched and cooled by the reagent or water sprays. The NO.sub.x and SO.sub.x are absorbed into the sprayed liquid to form nitrogen and sulfur oxy acids. The dilute acids in situ are converted into nitrates, sulfites and sulfates in presence of caustic soda in accordance with the following reactions. In one embodiment lime or lime stone slurry is used, and calcium salts are formed. ##STR2##

Concentrated reagents are supplied from a chemical metering and storage tank 66 into recirculation tank 62. Make-up water through valve 68 maintains a constant resettable level of solution in tank 62 Suitable reagents, in liquid and spray form, for use with the presient invention include water, an aqueous solution of alkali or alkaline earth metal hydroxides and carbonates, an aqueous solution of alkali or alkaline earth metal sulfites, and aqueous ozone scavenger. These reagents may be used individually or in selected combination and an excess of solution is diverted to the holding and treatment tank 70 through valve 72. The reagent solution is maintained at a suitable concentration by controlling the rate of spent solution diverted to the holding and treatment tank 70.

In an alternative embodiment of the present invention, instead of using spray chamber 54 for removal of NO.sub.x, a packed scrubber, or a venturi scrubber, or a plate column with or without ultra sonics, or lime injected dry scrubber is used as an absorption chamber. In operations that require that the exhaust gas be substantially reduced in temperature, a heat exchanger is incorporated in the absorption chamber 54 to bring down the temperature of the gas and used as an absorption chamber. Provisions are also made to recover heat from the chamber 54 when operated with a heat exchanger.

The exhaust gas stream entering the absorption chamber 54, also contains unreacted ozone. In the absorption chamber 54, ozone is absorbed in circulating liquid. Solubility of ozone in water is very low limiting removal by absorption. However, when SO.sub.2 is present in the gas phase, sulfite is formed in situ in the circulating liquid. The sulfite readily reacts with ozone forming sulfate as indicated by reaction (14) below, enhancing the absorption of ozone several fold. Accordingly, the present invention advantageously incorporates the phenomena in depleting unreacted ozone from the exhaust gas stream and the use of scrubbing chamber 56. ##STR3##

When the exhaust gas stream emitted from the boiler in the exhaust duct 12 does not contain SO.sub.2, the treated contaminants from the chamber 58, are introduced through connecting duct 74 into a second or spray/absorption chamber 56 having spray nozzles or a liquid distributor system 76 connected to a spray header 78. Spray header 78 receives a reagent solution, (ozone scavenger), such as aqueous sodium sulfite solution, from a tank 80 through a pump 82. The gas phase of the exhaust gas stream in chamber 56 is brought in intimate contact with the liquid phase generated by the distributor system 76 until almost all of the oxidant is absorbed by the reagent solution.

Treated gas free from contaminants and oxidant is exhausted from the chamber 56 through stack 84. The reagent solution is replenished periodically or continuously with fresh charge of ozone scavenger. In another embodiment of the present invention, the absorption chamber 56 for destroying ozone is replaced with a catalyst chamber (not shown) that uses a catalyst such as MnO.sub.2. Further ozone slip is eliminated by extracting from the ducts before mixing with ozone a small amount of untreated exhaust gas containing NO.sub.x and introducing it into the scrubber chamber 56. The NO.sub.x in the untreated gas promptly reduces any ozone present in the treated gas.

Referring to FIG. 3, there is graphically illustrated data from a slip stream test conducted on an exhaust gas stream emitted from a coal fired boiler of a power plant. The temperature of the gas stream was in the range between about 240-250.degree. F. The level of NO.sub.x was in the range between about 250-300 ppm. One set of data points represents the condition where the ozone is introduced in the oxidation or reaction chamber 36. A second set of data points represents the condition where ozone is introduced in the duct system 52 downstream of the oxidation or reaction chamber 36 before the exhaust gas stream enters the absorption chamber 38. In the case when oxidation time was inadequate, less NO.sub.x removal was observed for identical amounts of ozone added.

The optimum residence time of the exhaust gas in the reaction chamber 36 is dependent upon the initial concentration of contaminants, the temperature, and the molar ratios of O.sub.3 /NO.sub.x. When the level of NO.sub.x in the treated gas is desired to be less than 10 ppm, ozone is required in excess of the stoichiometric amount required to oxidize the NO and NO.sub.2. The presence of excess ozone in the exhaust gas stream may result in some unreacted ozone entering the absorption chamber. Ozone solubility in an aqueous medium is limited. When SO.sub.2 is present in the gas phase, sulfite or sulfurous acid formed in the absorption chamber 54 may aid in destroying ozone. The provision of the scrubber chamber 56 serves to prevent emission of ozone to the atmosphere.

In one example of the present invention, an exhaust gas flow with 300 ppm by volume of NO.sub.x at 245.degree. F. requires up to 2 to 3 seconds residence time in the reaction chamber 36 at an O.sub.3 /NO.sub.x ratio of 1.68 in presence of 600-700 ppm of SO.sub.2 and 100-200 ppm of CO.

One of the advantages of optimized residence time in reaction chamber 36 is that only a marginal excess amount of ozone enters scrubbing section 56. Wet ozone is very corrosive. Ozone in the scrubber converts sulfite to sulfate. Thus introduction of excessive amount of ozone in an aqueous media to convert sulfite to sulfate is wasteful. Ozone also undergoes decomposition in an aqueous medium when pH is alkaline. Furthermore, when lime or lime stone slurry is used, SO.sub.2 absorption results in formation of calcium sulfite. Ozone further converts to calcium sulfate. Calcium sulfate is relatively more difficult to handle in wet scrubbing as it forms abrasive scales.

The removal of SO.sub.x is highly dependent upon the pH of the reagent. The pH of the reagent has very little effect on NO.sub.x removal. With the present invention, SO.sub.x removal is decreased with lowering the reagent pH below 8.5 while NO.sub.x removal is not affected for a reagent pH in the range of 3-12. In some cases at higher pH, along with high SO.sub.x removal, CO.sub.2 is also absorbed in alkali or alkaline earth metal solutions forming precipitates of carbonate and other co-salts. Also use of reagent for CO.sub.2 removal is wasteful and undesired. These precipitates have tendency to form scales. NO.sub.x removal in accordance with the present invention when conducted at a pH below 7 resulted in the descaling of the scrubbing chamber 56.

With the above described arrangement the exhaust gas is emitted from the stack 84 into the atmosphere at a ppm level that meets the air quality standards established by such public agencies as the E.P.A. or California's South Coast Air Quality Management District. For example, with the present invention the emissions from the stack 84 contain less than 2 ppm of NO.sub.x contaminants. The exhaust gas is emitted at a relatively low temperature due to the wet scrubbing.

As an additional example, the present invention removes nitrogen oxides from a steel strip pickling line using nitric acid as the pickling agent. The NO/NO.sub.2 mixture to be removed consists of 3 moles of NO.sub.2 to every mole of NO with the exhaust gas stream maintained at 180.degree. F. The NO.sub.x is converted to equimolar amounts of NO.sub.2 and NO.sub.3. With a molar ratio of 0.75 to 1 of ozone to NO.sub.x, 95% of the NO.sub.x is removed from the exhaust gas stream exiting the scrubbing chamber from the stack 84.

According to the provisions of the patent statutes, we have explained the principle, preferred construction, and mode of operation of our invention and have illustrated and described what we now consider to represent its best embodiments. However, it should be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically illustrated and described.