Abstract:
The process is by injection of calcium chloride into the combustor and lowering the flue gas temperature in sufficient time to enhance oxidation of mercury and nitrogen oxides into more soluble products prior to their absorption in a wet scrubber. The additive also increases the alkalinity and the pH of the scrubber water, therefore, favorably increasing the absorption of the pollutants. The flue gas is then cooled to ambient temperature or less to enhance the removal of mercury.

Description:
FIELD OF THE INVENTION 
     This invention relates to removal of both mercury (Hg) and nitrogen oxides (NO x ) from exhaust gas generating from combustion of carbonaceous materials and apparatus for effecting such removal. 
     BACKGROUND OF THE INVENTION 
     Among the 189 substances listed as hazardous air pollutants in the Clean Air Act Amendments (CAAA) of 1990, mercury is a metal species of great concern due to its extreme toxicity and the risk that it can cause to humans and animals if released to the environment. In several countries, legislation is being prepared to limit the emission of mercury to the atmosphere. While most of the trace metals can be efficiently removed in today&#39;s air pollution control system, mercury is present mainly in its vapor phase and is difficult and expensive to remove. 
     In the US, the EPA maximum achievable control technology (MACT) will limit mercury emissions to 40-110 μg/dscm at 7%O 2  for hazardous waste incinerator. For municipal sewage sludge combustor, federal regulation (40CFR Part 61, Subpart E) limits mercury emissions to 3,200 grams per 24 hours. 
     Nitrogen oxides (NO x ) are also an environmental problem because they can initiate reactions resulting in the production of ozone and acid rain. These pollutants can harm forests and lakes, damage buildings and cause health problems. Guidelines for controlling NO x  emissions are provided in the 1990 CAAA under the “Nitrogen Oxides Emission Reduction Program” and “Ozone Non-Attainment Program”. For municipal sewage sludge combustors, NO x  is not regulated at the Federal level yet. However, as a result of the 1990 CAAA, Federal regulation on NO x  is anticipated. Consequently, NO x  emission from municipal sewage sludge combustors can be limited to the MACT standard, which is no more than the average emissions achieved by the best performing 12% of all operating incinerators. Most state authorities already regulate NO x  emissions for their municipal sewage sludge combustors and have very stringent limits. 
     Of the different technologies available for reducing NO x  and Hg, most require additional equipment and the use of expensive and/or hazardous chemicals. Therefore, it would be advantageous to develop a method for removing these compounds utilizing only the standard pollution control equipment and cost efficient non-hazardous chemicals, thereby meeting strict emission limits economically. 
     A number of different methods have been proposed to reduce mercury and/or NO x  emissions from combustor exhaust gas. However, the majority of these processes are more sophisticated due to either the extension of additional equipment or the hazardous nature of the additives. Very few of those methods propose simultaneous reduction of both mercury and nitrogen oxides. 
     Mercury Removal: 
     The chemical form of Hg in the gas to be treated is of considerable interest. Ionic mercury is removed with control processes that employ various aqueous scrubbing techniques. Elemental mercury, however, is essentially unaffected by wet scrubbers and requires some type of sorbent or carbon injection process. 
     Mercury typically can be removed from the combustor exhaust gas in two ways, (1) adsorption via sorbent injection into the exhaust gas or via flow through fixed sorbent bed at low temperature upstream of a particulate matter collector, and (2) wet scrubbing with conversion of the elemental mercury into a more soluble species that can be easily absorbed in a scrubber. 
     WO 9,517,240 describes a method for improving mercury removal capability of a flue gas purification system by introducing sulfur vapors into the flue gas stream where admixed flue gases and sulfur vapors contact solid particulate (calcium hydroxide) materials in the flue gas. Calcium hydroxide adsorbs mercury and sulfur vapors and catalyzes reactions forming solid products comprising mercury. The solid products comprising mercury are separated, thereby forming a purified flue gas stream. The solid particulate materials are formed in situ by reaction in a spray dryer between an aqueous dispersion of calcium hydroxide and the acidic materials in the flue gas at a temperature between 70 and 170° C. 
     U.S. Pat. No. 4,889,698 discloses a process in which powdery activated carbon is injected immediately before, during or after an alkali reagent (limestone or sodium carbonate) spray dryer for simultaneous removal of acid gases and trace contaminants such as mercury. The process requires cooling the flue gas by spray drying in the presence of large amounts of alkali sorbent material together with the activated carbon to enhance overall mercury removal. 
     U.S. Pat. No. 5,695,726 discloses a process in which toxic mercury vapor is removed from combustion gas by contact with dry alkaline material and dry activated carbon in a reaction chamber followed by solids separation. The adsorptive capacity of activated carbon decreases with increasing gas temperature. US &#39;726 emphasizes that a minimum level of HCl is necessary and a low temperature of the reaction chamber of from about 175° C. to about 235° C. are important for achieving high removal of the mercury. HCl is needed in the gas phase to react with elemental mercury or mercury oxide to convert them to chlorides. US &#39;726 also teaches that in the combustion of the wastes that are chlorine-deficient, an HCl-generating material such as scrap polyvinyl chloride plastic can be added to the chlorine-deficient waste prior to incineration. However, it is well known that adding chlorine to the waste stream and at the same time lowering the flue gas to the temperature range of 200° C.-350° C. are the two most favorable conditions for the synthesis or reformation of dioxins and furans. 
     U.S. Pat. No. 5,900,042 describes a process to remove elemental mercury from a gas stream by reacting the gas stream with an oxidizing solution to convert the elemental mercury to soluble mercury compounds. The gas stream is then passed through a wet scrubber to remove the mercuric compounds and oxidized constituents. The oxidizing solutions are solutions of aqueous iodine, aqueous bromine, aqueous chlorine, aqueous chloric acid and alkali metal chlorate and others. 
     U.S. Pat. No. 5,607,496 discloses a removal process, in which the elemental mercury of the combustion gas is first catalytically oxidized to form a mercury compound, and then the mercury compound is either adsorbed on adsorbent particles such as alumina or removed from the gas stream by scrubbing. The catalysts include mostly oxides of existing heavy metals in the combustion gas such as manganese, vanadium, lead, chromium, iron, cobalt, nickel and selenium. 
     UK Patent No. 1,336,084 discloses a process in which mercury vapour in the flue gas is removed by scrubbing the flue gas with a solution of alkaline earth metal hypochlorite containing an alkali metal chloride or alkaline earth metal chloride in excess of the chemical equivalent of the alkaline earth metal hypochlorite at a pH in the range of 8 to 12. 
     Nitrogen Oxides Removal: 
     Nitrogen oxides can be removed from combustor exhaust gas by selective catalytic reduction (SCR), selective non catalytic reduction (SNCR), and wet flue gas denitrification. 
     U.S. Pat. No. 4,220,632 discloses a process in which ammonia is used to reduce nitrogen oxides in combustion exhaust gas in the presence of a catalyst by SCR. High performance can be achieved with this technique, but it requires injection of ammonia into the exhaust gas prior to entering the SCR reactor. Sometimes it is necessary to first pass through a wet removal process to eliminate dust and poisonous chemicals that hinder the SCR process, then reheat the gas for the SCR. This method requires space due to the extent of the treatment equipment and generates a potential hazardous spent catalyst. Therefore, both capital and operating costs are high. 
     U.S. Pat. No. 3,900,554 describes a process called selective non-catalytic reduction (SNCR) in which ammonia is used to reduce nitrogen oxide from combustion effluents. Application of the technique is limited, because excessive unreacted ammonia or ammonia slip can not only add to the pollution, but also cause pluggage of the downstream equipment. 
     U.S. Pat. No. 4,719,092 describes another SNCR process but, instead of ammonia, urea is injected in the post combustion zone at a temperature between 850-950° C. This reductant reagent is oxidized to ammonia, which then reacts with NO x  to produce N 2 , water vapor and CO 2 . The technique claims better control of ammonia slip than the technique using ammonia. Maintaining a close temperature control is critical and difficult under this technique. 
     Since the majority of NO x  in the off-gas is in the form NO, which has a very low solubility in water (k 0   H =0.0019 [mol/kg.bar] @ 298.15° K.), it is difficult to reduce the amount of NO x  in standard wet scrubbing pollution control equipment. However, if the NO can be oxidized to a higher state such as NO 2  or NO 3 , and/or formed into another compound which has a higher solubility (NO 2 : k 0   H =0.01-0.04 [mol/kg.bar] @ 298.15° K.; NO 3 : k 0   H =0.6-12.0 [mol/kg.bar] @ 298.15° K.), then a larger amount of NO x  can be removed. 
     U.S. Pat. No. 4,035,470 describes a process to remove both sulfur oxides and nitrogen oxides from the exhaust gas by adding ozone (O 3 ) or chlorine dioxide (ClO 2 ) to the exhaust gas and by scrubbing the exhaust gas with an aqueous scrubbing solution. O 3  or ClO 2  are good oxidants and are capable to convert NO in the gas phase to more soluble forms such as NO 2  or N 2 O 5 . However, O 3  is expensive and ClO 2  is difficult to store and is hazardous. 
     U.S. Pat. No. 4,294,928 describes a liquid phase process using chlorine as oxidant in the presence of water in the scrubbing system. It has been claimed that the oxidation of nitric oxide to other oxides of nitrogen proceeds over a wide range of temperatures of the aqueous solution. A nitric oxide reduction of over 90% has been achieved at a temperature between 10° C. and 50° C. 
     JP 63-100,918 discloses a method of removing both mercury and nitrogen oxides from exhaust gas by washing the exhaust gas in a washing column using a solution comprising alkali and hypochlorite or chlorite. 
     EP 0 962,247 discloses a process of removing both NO x  and SO x  from a gaseous effluent by passing the gaseous effluent through an aqueous alkaline scrubber. The pH of the scrubber should be between 7 and 14, but is preferably very basic, i.e. between pH 10 and 14. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an economical and simple method to remove both mercury and/or nitrogen oxides from combustion gas. 
     It has been found that calcium chloride added to the feed of the combustor promote the gas phase oxidation of elemental mercury to a more soluble form mercuric chloride, which can then be separated from the flue gas in a typical wet scrubber. 
     It has been found that calcium chloride added to the feed of the combustor promote gas phase and/or liquid phase oxidation of nitrogen monoxide which comprises the majority of flue gas NO x  to a more soluble form of nitrogen oxides (NO 2 , NO 3 , N 2 O 5  or others), which can be more easily scrubbed from the flue gas in a typical wet scrubber. 
     The process according to one aspect of the invention includes the following steps: 
     1. Introducing calcium chloride into the feed about to or undergoing incineration to facilitate formation in situ of hydrochloric acid in the flue gas generated by the sludge incinerator and reacting calcium with water contained in the feed at ambient temperature or at combustor operating temperature and with water in the wet scrubber to produce CaO and/or Ca(OH) 2: 
     
       
         CaCl2+H 2 O→2HCl+CaO  (1) 
       
     
     
       
         CaCl2+H 2 O→2HCl+Ca(OH) 2   (2) 
       
     
     2. Converting gaseous Hg and HCl into HgCl 2  by cooling Hg and HCl containing flue gas from typical operating temperature of 850° C. to a temperature of about 450° C. 
     
       
         Hg+2HCl+½O 2 →HgCl 2 +H 2 O  (3) 
       
     
     The mercury speciation of reaction (3) is favored by low temperature and occurs in the gas phase downstream of the combustor when the temperature starts to drop from tie typical 850° C. to about 450° C. A temperature lower than about 450° C. is acceptable but not desirable, to avoid the temperature zone attributed to the formation of dioxins and furans. Furthermore, since reaction (3) is a rate limited reaction, adequate time is provided for speciation to occur within the favorable temperature window of 850° C. and 450° C. 
     3. Oxidizing nitrogen monoxide, which comprises the majority of flue gas NO x  to a more soluble form of nitrogen oxides: 
     
       
         2NO+Ca(OH) 2 +½O 2 →Ca(NO 2 ) 2 +H 2 O  (4) 
       
     
     
       
         4NO 2 +2Ca(OH) 2 →Ca(NO 2 ) 2 +Ca(NO 3 ) 2 +2H 2 O  (5) 
       
     
     Oxidation reactions (4) and (5) or other similar reaction can occur either in the gas phase or in the liquid phase of the wet scrubber. Excess calcium chloride fed to the Fluid Bed Combustor (FBC) or calcium oxide and calcium hydroxide generated from equations (1) and (2) increase the alkalinity and the pH of the wet scrubber water. A high pH of the scrubber water is favorable to the removal of both mercury and nitrogen oxides. 
     4. Quenching the soluble HgCl 2  and NO x  containing flue gas to about 70° C.-90° C. with water. The pollutants will be absorbed in the liquid phase and separated from the flue gas. 
     5. Cooling the flue gas to about 45-50° C. with water to improve the absorption and the separation of the pollutants as described in step #4. 
     6. Further cooling the flue gas to ambient temperature or lower to condense and separate fugitive Hg from the flue gas. 
     In another aspect, the invention relates to a process for removing Hg from combustion flue gas generated by combustion of carbonaceous material. The steps include: introducing into the sludge a chlorine containing substance to facilitate formation of hydrochloric acid in the flue gas generated by the combustor; converting gaseous Hg and HCl into HgCl 2  by cooling from combustor operating temperature Hg and HCl containing flue gas to a temperature of about 450° C.; quenching the HgCl 2  containing flue gas to about 70-90° C.; separating Hg in the form of HgCl 2  from the flue gas; cooling the flue gas in the presence of water to about 45-50° C; removing residual HgCl 2  and condensed water vapor from the flue gas; condensing fugitive elemental Hg by contacting the flue gas with further cooling water to reduce flue gas temperature to substantially ambient or lower temperature; and separating any condensed fugitive elemental Hg from the flue gas. 
     In another aspect, the invention relates to a process for reducing NO x  emissions generated by combustion of carbonaceous material. The steps include: introducing into the carbonaceous material about to or undergoing combustion an alkali earth metal containing substance; reacting alkali earth metal (M) in the alkali earth metal containing substance with water contained in the feed at ambient temperature or at combustor operating temperature and with water in the wet scrubber to produce MO and/or M(OH) 2 ; reacting MO and/or M(OH) 2  with NO x  in the combustor to produce M(NO 2 ) 2  and/or M(NO 3 ) 2;  and separating water and M(NO 2 ) 2  and/or M(NO 3 ) 2  from flue gases generated by the combustor. 
     The invention and the advantages provided thereby will be more fully understood with the reference to the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic of a first portion of a system used in accordance with the invention including a fluid bed reactor and a feed system for introducing sludge therein. 
     FIG. 2 is a schematic of a system used in accordance with the invention that connects to the system in FIG. 1, including two heat exchangers, venturi scrubber, tray cooler and wet electrostatic precipitator. 
     FIG. 3 is a graph of the reduction in Hg emissions as the flue gas temperature decreases. 
     FIG. 4 is a graph of Hg emission and Hg removal efficiency versus HCl contained in incinerator off gas. 
     FIG. 5 is a graph showing NO x  emissions during normal operating conditions. 
     FIG. 6 is a graph similar to FIG. 5 except that calcium chloride has been added to the feed sludge. 
     FIG. 7 is a graph showing NO x  emissions and oxygen content of the flue gas recorded versus time at the injection of CaCl 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is intended to refer to specific embodiments of the invention illustrated in the drawings and is not intended to define or limit the invention, other than in the appended claims. Also, the drawings are not to scale and various dimensions and proportions are contemplated. 
     Referring to the drawings in general and FIGS. 1 and 2 in particular, a preferred apparatus for employing the method of the invention is shown. FIG. 1 contains a portion of the system that can be considered the upstream portion and FIG. 2 contains the portion of apparatus that can be considered the downstream portion. 
     Referring specifically to FIG. 1, there is a high temperature fluid bed combustor (FBC)  10  which receives feed from a feeding device  12  and an associated connection line  14 . The feed can be any waste stream, solid, liquid or sludge known in the art. The FBC  10  is also fueled in part by fuel oil, such as No. 2 fuel oil, by way of injector oil pump  16  and the associated oil feed connection line  18 . Air is provided by way of injection air blower  17  to atomize the fuel oil in the FBC  10 . Of course, other fuel types may be used as desired. Combustion/Fluidization air is introduced through line  27 . Sand is introduced into the FBC  10  by way of a sand silo  20  and associated connection line  22 . Water may be injected into the FBC  10  by way of line  24  for off gas temperature control and/or line  26  for bed temperature control. At start-up, a preheat burner  2  installed in the lower part of the FBC  10  is used to provide heat, using oil or other fuel provided by way of the preheat burner oil pump  3 . Air is provided to this burner  2  through line  6 , by preheat blower  4 , and through line  5 , by fluidizing air blower  1 . 
     Flue gases and any suspended fly ash at typical operating temperature of 850° C. exit the FBC  10  through line  30  which connects to a heat exchanger  32  as shown in FIG.  2 . 
     In the embodiment shown in FIG. 2, there are two separate, but connected heat exchangers  32  (primary heat exchanger) and  34  (secondary heat exchanger). The primary heat exchanger  32  is to preheat the combustion/fluidizing air, provided by way of fluidizing air blower  1 , to typically about 650° C. The secondary heat exchanger  34  is to heat the plume suppression air to about 250° C. The exhaust gas temperatures at the primary heat exchanger  32  and the secondary heat exchanger  34  outlets are about 550° C. and about 450° C., respectively. The heat exchanger  32  connects to heat exchanger  34  directly through connection line  35 . Heat exchanger  34  connects to venturi scrubber  36  by way of connection line  38 . Plume suppression air is introduced to the secondary heat exchanger  34  through connection  33 , and sent via connection  39  to mix with clean gas at the wet electrostatic precipitator (WESP)  50  via connection  56 , and exit the stack. 
     Flue gases enter an upper portion of venturi scrubber  36  through connection  38  and quenching water is introduced through line  42  into an upper portion of venturi scrubber  36 . The quenched flue gas temperature at the venturi scrubber  36  outlet is about 70° C.-90° C. An outlet  44  transfers the contents of venturi scrubber  36  into a lower portion of tray cooler  40 . The excess water and fly ash from the venturi scrubber  36  exit the lower portion of the tray scrubber through connection  48 . Flue gases exit an upper portion of tray cooler  40 , in which cooling water is injected through connection  43 . Cooling water condenses the majority of the water vapor in the flue gas and exits the tray cooler  40  through connection  47 . The flue gas exit the tray cooler  40  through connection  46  at typically about 45° C.-50° C. 
     Flue gases pass through connection  46  into wet electrostatic precipitator (WESP)  50 . Cooling water is introduced into a middle portion of the WESP  50  through connection  54  to wet and cool the flue gas further down to ambient temperature or lower. The flue gas passes through an upper portion of the WESP  50  and outwardly thereof through connection  56 , which leads to a stack  58 , in which hot air is added through connection  39  and to the ambient atmosphere. Cooling water and any condensed fugitive Hg pass out of a bottom portion of the WESP  50  through connection  52 . 
     EXAMPLES 
     A preferred method of operation of the preferred apparatus as shown in FIGS. 1 and 2 and as described above is set forth below in connection with a series of tests that describe the present invention. These tests were conducted on a full-scale FBC used for the incineration of municipal sewage sludge at the North West Bergen County Utilities Authority (N.J., USA). The residence time of the flue gas in the two heat exchangers  32  and  34 , including duct  38  from the secondary heat exchanger  34  to the venturi scrubber  36 , was 1 second. The FBC was operated at 1 metric ton of dry solids per hour. The feed sludge was a mixture of 50% primary sludge and 50% waste activated sludge that was dewatered to approximately 20%-22% dry solids. Sludge and auxiliary fuel ultimate analyses and the gas chemical compositions at different test ports were in accordance with accepted EPA Methods. 
     1. Effect of Temperature on Hg Emission: 
     The effect of temperature on mercury emissions was investigated during two series of tests (test #1 and #2). The results are presented in FIG.  3 . During these tests, Hg emissions were measured simultaneously at two locations along the gas stream: inlet  35  to secondary heat exchanger  34 , and outlet  56  of WESP  50 . The duration of each test was one hour. The average temperature at these two locations was 518° C., and 21° C., respectively. The operating conditions recorded during each test are very similar and presented in Table 1. 
     Since the temperature at the inlet  35  to the secondary heat exchanger  34  was higher than the boiling point of Hg (357° C. at 1 atm), the concentration of Hg at this point was assumed to equal the total quantity of Hg entering the FBC. Measurements at the other location provided the amount of Hg removed from off gas due to condensation at that temperature. As shown in FIG. 3, when the flue gas was cooled to 21° C., the concentration of mercury in the flue gas dropped from 116 [μg/dscm] to 85 [μg/dscm]. Approximately 27% of the Hg was removed predominantly by condensation. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Summary of operating conditions and results of Hg tests 
               
               
                 All concentrations are based on dry gas, corrected to 7% O 2 . 
               
             
          
           
               
                   
                   
                   
                   
                 Hg 
                   
                   
               
               
                   
                   
                   
                   
                 Concentrations 
                   
                   
               
               
                   
                 Temperatures [° C.] 
                   
                 Calculated 
                 [μg/dscm] 
                   
                   
               
             
          
           
               
                   
                   
                 Inlet 
                   
                 CaCl 2 .2H 2 O 
                 HCl in 
                 Inlet 
                   
                 Hg 
               
               
                   
                 Free 
                 2 nd   
                 Outlet 
                 Feed 
                 fluegas 
                 2 nd   
                 Outlet 
                 Removal 
               
               
                 Test # 
                 board 
                 H. Ex. 
                 WESP 
                 [kg/h] 
                 [dppmv] 
                 H. Ex. 
                 WESP 
                 [%] 
               
               
                   
               
               
                 1 
                 861 
                 514 
                 21 
                 0 
                  16 
                 116.23 
                 88.57 
                 23.80% 
               
               
                 2 
                 859 
                 521 
                 21 
                 0 
                  58 
                 116.91 
                 81.93 
                 29.92% 
               
               
                 3 
                 829 
                 518 
                 20 
                 26  
                 321 
                 116.43 
                 58.79 
                 49.51% 
               
               
                 4 
                 837 
                 529 
                 20 
                 9 
                 126 
                  93.62 
                 43.13 
                 53.93 
               
               
                 5 
                 834 
                 529 
                 20 
                 42  
                 524 
                  92.77 
                 45.09 
                 51.40 
               
               
                   
               
             
          
         
       
     
     2. Effect of Calcium Chloride on Hg Emission: 
     The effect of calcium chloride addition is shown in FIG. 4, in which Hg emissions and the removal efficiency are plotted versus the concentration of HCl in the flue gas. Three different flow rates of CaCl 2 .2H 2 O (9 kg/hr, 26 kg/hr and 42 kg/hr) were added directly to the sludge hopper  12  by using a calibrated auger. The calcium chloride used was in the form of white flakes, and is a safe commercially available product, typically used for road de-icing. To ensure that the system was stabilized and that FBC  10  was actually receiving the correct flow of calcium chloride, measurements were started at least 30 minutes after any change in feed rate. Again, Hg emissions were measured at the inlet 35 to secondary heat exchanger  34  and the outlet  56  of WESP  50 , substantially simultaneously, and the length of each test was one hour. The concentration of HCl in the flue gas are calculated values based on the added CaCl 2  feed rate, and the chlorine naturally present in the sludge fed to the FBC. A summary of the operating conditions and the results are also presented in Table 1 (test #3, #4 and #5). 
     The removal of Hg increased from 27% to approximately 52% (50-54%) or an average of 52 [μg/dscm], with the addition of calcium chloride, as shown in FIG.  4 . This increase in Hg removal demonstrates the positive and unexpected effect of calcium chloride on removing Hg. We accordingly believe, but do not wish to be bound by a particular theory, that reactions  1 ,  2  and  3  set forth in the summary of the invention occur at this point. 
     3. Effect of Calcium Chloride on NO x  Emissions: 
     Under normal operating conditions the NO x  emissions for this system ranged from 30 to 75 [ppmv NO x ; dry gas @ 7% O 2 ]. FIG. 5 shows data from two different days of normal operation. Each point represents an average of one hour of stack gas emission data measured at the flue gas outlet  56  of the WESP  50 . 
     During the above mentioned tests when calcium chloride was added to the feed sludge, the NO x  emissions were reduced to a range of 25 to 35 [ppmv NO x ; dry gas @ 7% O 2 ]. FIG. 6 shows data during the tests with calcium chloride addition; again each point represents an average of one hour of stack gas data, measured at the flue gas outlet of the WESP  56 . 
     FIG. 7 shows NO x  and O 2  emission data from before and after the start of calcium chloride addition. On this Figure each point represents a one-minute average, measured at the flue gas outlet  56  of the WESP  50 . The NO x  concentration follows the same trends as the O 2 %, up until 9:15 am, which demonstrates the influence of excess air (here measured as O 2 %) on NO x . The peak in O 2 % (from 8:15 to 8:35 am) was due to a sudden and temporary drop in oil feed. As the excess air increases the formation of NO x  also increases; this is a well-documented relationship. However, after the addition of CaCl 2  there was a deviation in the relation between the NO x  and O 2 % (seen after 9:15 am). Even though O 2 % increased, the NO x  decreased. The NO x  concentration decreased from an average value of 48 ppmv (prior to 8:00 am @O 2 % of 9.0%) to 28 ppmv (after 9:30 am @ O 2 % of 9.0%). 
     As mentioned previously, NO x  reduction in wet scrubbing is limited without first oxidizing NO to another form. Once oxidized to calcium nitrite and calcium nitrate the products are very soluble in water and can, therefore, be removed in the water in the venturi scrubber  36 , tray scrubber  40  or WESP  50 . The addition of calcium chloride to the reactor also increases the pH of the venturi scrubber water, which enhances the solubility of NO 2  and higher oxidized forms of nitrogen oxides. 
     Although this invention has been described with reference to specific forms of apparatus and method steps, it will be apparent to one of ordinary skill in the art that various equivalents may be substituted, the sequence of steps may be varied, and certain steps may be used independently of others, all without departing from the spirit and scope of the invention defined in the appended claims.