EPA-R2-73-051a
June 1973
Environmental Protection Technology Series
            !•
            iiiiiiiiiB                      ^lliiiiii
                                                          I


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                                             EPA-R2-73-051a
Development  of Aqueous Processes
            for  Removing NOx
     from Flue  Gases  - Addendum
                          by

                    Gilford A. Chappell

              Esso Research and Engineering Co.
               Government Research Laboratory
                  Linden, New Jersey 07036
                  Contract No. 68-02-0220
                 Program Element No. 1A2014
               EPA Project Officer: D.A.Kemnitz

                 Control Systems Laboratory
             National Environmental Research Center
          Research Triangle Park, North Carolina 27711
                      Prepared for

             OFFICE OF RESEARCH AND MONITORING
           U.S. ENVIRONMENTAL PROTECTION AGENCY
                  WASHINGTON, D.C. 20460

                       June 1973

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This report has been reviewed by the Environmental Protection Agency and




approved for publication.  Approval does not signify that the contents




necessarily reflect the views and policies of the Agency, nor does




mention of  trade names or commercial products constitute endorsement




or recommendation for use.
                                  11

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                                ABSTRACT
           This report summarizes the findings of the laboratory program
 for "Development of Aqueous Processes for Removing NOX and SC>2 from
 Combustion Flue Gases."  This project is the second phase of the flue
 gas scrubbing work sponsored by EPA under Contract No. 68-02-0220.  The
 results of the Phase I program are contained in report EPA-R2-72-051,
 entitled, "Development of the Aqueous Processes for Removing NOX from
 Flue Gases."

           The present report contains discussions of analytical techniques
 and scrubber design in addition to experimental results obtained with a
 vertical spray tower scrubber.  The blended flue gases passed up the
 unpacked glass column countercurrent to the absorbing solution which was
 sprayed down from the top.  The scrubbing experiments showed:

        •  N02 and S02 are effectively absorbed by 1.0 molar
           Na2SO~ solutions.

        9  NC>2 absorption by 1.0 molar NaOH solution is enhanced
           by the presence of S02 in the flue gas.

        «  Neither NO nor NOo is effectively absorbed by 1.0
           molar NaOH solution in the absence of S02> and NO
           absorption is not improved by the presence of SO-.

        »  Increasing the L/G ratio improves N02 and S02
           absorption by 1.0 molar Na^SO.,.

        ©  Under similar scrubbing conditions Mg(OH)2 slurry
           is not as effective as Na?SO  solution for NO
           absorption.

           The data show that sulfite solutions would effectively absorb
 NOX and S02 from flue gases provided the NO  (mostly NO) has been
 oxidized to NO,, upstream from the scrubber.


                               ACKNOWLEDGEMENTS
          This work was conducted by the Government Research Laboratory
of the Esso Research and Engineering Company for the Environmental Protection
Agency under EPA Contract 68-02-0220.  Dr. Gilford A. Chappell, the Principal
Investigator for the work reported herein, was a member of the Air Conservation
Section managed by Mr. Alvin Skopp.

          The invaluable assistance of Mr. William Moss in the laboratory
during the entire project is sincerely appreciated.

          Mr. Douglas Kemnitz, was the EPA Technical Project Officer
during the program.
                                   - iii -

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-   IV   -

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                            TABLE OF CONTENTS
    ABSTRACT	  iii

1.  INTRODUCTION	    1

2 .  LABORATORY STUDIES	    4
    2 .1  Ana lyt ica 1 Techniques	    4

         2.1.1  Analysis of Solutions for Nitrite
                and Nitrate Levels	    4
         2.1.2  Measurement of NO Levels in Flue Gases	   10
    2 .2  Scrubbing System	•	   13

         2.2.1  Flue Gas Blending	   13
         2.2.2  Flue Gas Scrubber	   14

    2.3  Results of Flue Gas Scrubbing Experiments	   16

3 .  CONCLUSIONS AND RECOMMENDATIONS	   20
    3 .1  Cone lus ions	   20
    3.2  Recommendations for Future  Work	   21

    APPENDIX - Measurement  of Molar  Absorptivities
               for Nitrite  and Nitrate	  22
                                  -  v  -

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                             1.  INTRODUCTION

           The oxides of nitrogen (NO, N02) are essential components in
the formation of photochemical smog in addition to being pollutants
in their own right.  Sulfur dioxide (S02) is a major air pollutant.  The
major sources of these oxides is the large fossil fuel fired boilers such
as those found in electric power generating plants.  Nitric oxide  (NO)
is formed in the high temperature zone of the furnace by the reaction
between atmospheric nitrogen and oxygen; as the combustion gases cool,
a small percentage (10%) of the NO is oxidized to N02«  Collectively,
these two oxides are referred to as 'NOx'-  If the fuels contain organically
bound nitrogen, as do coal and oil, part of this nitrogen is converted
to NOX during combustion.  Similarly, sulfur containing species present
in the fuel provide a source for S02-  The composition of different
flue gases is shown in Table 1.

                              TABLE  1

                TYPICAL COMPOSITIONS OF'FLUE  GASES

                                Volume % Combustion Of

            Component

               N2
               co2

               H20

               °2
               so2

              .NO
                 x
            Participates
            grams/ft3
Coal(a) (
76.2
14.2
6.0
3.3
0.2


0.5(e)
Dil(b)
77.0
12.0
8.0
3.0
0.15
0.07(d'
0.01
Gas(c)
72.3
9.1
16.8
1.8
—
)

__
            (a)   Calculated for burning with 20%  excess  air a typical
                 high volatile  bituminous  coal  of the  following  com-
                 position = carbon  -70.1%,  oxygen -6.6%, hydrogen  -4.9%,
                 nitrogen -1.4%,  sulfur -3.0%,  ash -12.7%,  and H20 -1.3%.

            (b)   Calculated a typical  residual  fuel oil  of  the following
                 composition =  86.5%  carbon, 10.3% hydrogen,  2.5%  sulfur,
                 0.7% nitrogen  with 20% excess  air.
            (c)   Calculated for burning natural gas with 10%  excess air.
            (d)   This is  an average value.   Actual values range  from
                 0.01% to 0.15%.

            (e)   Assumes  90% particulates  removal.

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                                   - 2 -
           In order to  remove  NOX  from flue  gases,  Esso  Research and
 Engineering  Company carried out a  flue gas  scrubbing  program which
 was  sponsored by  EPA.   The program consisted  of  screening  various
 aqueous  absorbents for NOX absorption potential.   The results  of the
 screening  program are  contained in the report  EPA-R2-72-051 . '  The
 main conclusions  of the batch screening studies were:

 • The addition of N(>2  to flue gas  to  improve NOX  (mostly NO)  absorption
   does  not  appear promising.  While  the presence  of N0£ does  improve the
   absorption of  NO, the magnitude  of  the increase is insufficient to
   support a viable process.
 • Sulfite solutions and slurries  are  efficient N02-S02 absorbents.
   Soluble sulfites (Na2S03)  are better N02 absorbers than insoluble
   slurries  (CaSOn) because of the  higher level of sulfite ion in solution.

 • Calcium,  magnesium,  and zinc hydroxide slurries are effective N02~S02
   absorbers .  The sulfite formed when S02 is absorbed is necessary for
   efficient N02  scrubbing.

 • Limestone (CaC03) is also  a good N02-S02 absorbent for the  same rea-
   sons  as for Ca(OH)2.
 • N02 scrubbing  is enhanced  by removing oxygen from  the flue  gas or by
   adding  an anti-oxidant such as  hydroquinone to  the scrubbing solution.

 « Sulf ide solutions are excellent  N02 and S02 absorbers but do generate
   a small amount of NO.

 • Part  of the absorbed S02 is oxidized to sulfate.

           These results  led to the  second phase of the program which
was  to design, construct, and test  a  continuous, gas scrubbing system
for  simultaneous  removal of N02 and S02 from blended flue gases.  The
underlying assumption  is that flue  gas  NOX (mostly NO) from a  real plant
could be oxidized  to N02 upstream from the gas scrubber.  This may be
technically  feasible using ozone or a  catalyst.

          The basic approach  is to  use  the sulfite formed during
S02  absorption to  remove the N02 from  the same gas stream.
          S02 + 20H~   - >   S03~  +


          20H~  +  S03~  .+  2N02  - >  S04   + 2N02"  + HO

The resulting nitrite ion (N02~) may subsequently be oxidized to nitrate
ion (N03~) by the molecular oxygen (3%) present in the flue gas.

          In addition to testing sulfite ion scrubbing, certain analytical
techniques needed improvement.  These included the spectrophotometric
procedure  for analyzing solutions for nitrite and nitrate levels,  and the
procedure for measuring nitric oxide (NO) levels in damp flue gas.

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                                 - 3 -
          The objectives of the program were to:

(1)   Design and construct a continuous flue gas scrubber for
     SC>2  absorption.

(2)   Obtain scrubbing data using several absorbents to verify the
     results of the screening study.

(3)   Develop and improve the analytical procedures so that  accurate
     mater ia 1 ba lances may be obtained.

          These will  be discussed in  detail in the following section.

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                                  - 4 -
                         2.   LABORATORY STUDIES
2.1  Analytical Techniques

          This section discusses the procedures for solution analysis
and for gas analysis.                                         ,..•..

     2.1.1  Analysis of Solutions for Nitrite and Nitrate Levels

          When absorbed by aqueous solutions NOX is converted primarily
to nitrite and nitrate ions whose levels must be accurately determined
in order to insure a satisfactory NOx material balance.   J.'H. Wetters
and K. L. Uglum [Analytical Chemistry, 42, 335 (1970]  have described
a spectrophotometric technique capable of the direct,  simultaneous deter-
mination of nitrate and nitrite levels in aqueous solutions.  The pro-
cedure involves taking two ultraviolet absorbance readings (302 nm and
355 nm) on the solution followed by calculation of the levels using
molar absorptivities determined with standard solutions  of nitrite and
nitrate ions.  The authors claim a lower detection limit, using 1.0 cm
cells, of 0.02 mg/ml for nitrite and 0.09 mg/ml for nitrate.  Three
molar absorptivities are required because nitrite ion  absorbs at  302 nm
and 355 nm whereas nitrate ion absorbs only at 302 nm.  Thus, the
nitrite level in an unknown solution may be determined by a single
absorbance reading at 355 nm whereas the nitrate level must be calculated
from- readings taken at both wavelengths.   For example,  the nitrite level
in a solution containing both nitrite and nitrate may  be calculated
from the absorbance reading at 355 nm using Beer's Law.

                    A = ebC

                    A = measured absorbance
                    e = molar absorptivity
                    b = cell path length  (centimeters)        !
                    C = solute concentration (molarity)

The nitrite concentration is calculated from the following quantities.

                    .355    355  ,_
The nitrate determination is slightly more involved because the
measured absorption at 302 nm contains contributions from both species

                     302    302  L      ^  302  ^0
                    A    - £- bC- + £- bC-

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                              - 5 -
However,  the concentration of nitrite is already known from the
reading taken at 355 nm.
                          =
                             355
Substituting into the expression for the total absorption at 302 nm
gives, after cancelling the equal path lengths,  .
                    .302  / N02-\   .355 .  .302
                    A    =1       I   A
                           :NO,
                                             .355
                    302
                    N03
Therefore, the nitrate level may be determined from three molar absorptivities,
two absorbance measurements, and the cell path length.

           In  order  to measure  the molar  absorptivities,  stock  solutions
of NaN02  and  KN03 were prepared  from reagent  grade  chemicals and  dis.tilled
water.. Samples  of  solution were placed  in  4.0  cm qua.rtz cells which
were  inserted into  an Optica. Spectrophotometer  for  absorbance measure-'
ments.  The results, tabulated in the Appendix,  gave  the following
molar  absorptivities and average deviations:
                       302
                              =   9.4  + 0.2
                       355
                              =   24.2  + 0.7
                      ,302
                      "NO,/
7.5 '+ 0.2
These  values were  used  to  test  the  procedure by  analyzing  prepared
solutions  containing  both  NaN02 and KN03.  These 'results are  shown
in Table 2..

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                                   - 6 -
                                 Table  2
          Spectrophotometric  Analysis  of Nitrite-Nitrate Mixtures
             Actual Solution
                Molarity
    Measured  Solution
        Molarity
Exp #
I
2
3
4
5
6
6a*
N00~
,__ £ .
0.00488
0.00488
0.00975
0.00520
0.00520
0.0104
0.0104
N00"
- J —
0.00908
0.00908
0.00908
0.00908
0.00908
0.00908
0.00908
NO,"
£.
0.00500 .
0.00485
0.00986
.0.00525
0.00526
0.0102
0.0104
NO,,'
j
0.00961
0.00908
0.00958
0.00930
0.00922
0.00958
0.00951
NO '

2.5
0.6
1.1
1.0
1.2
1.5
0
N0_

5.8
0
5.5
2.4
1.5
5.5
•4.7
    Used a 1.0 cm cell instead of the 4.0 cm cell  used  in  all  the  other
    experiments .        .          .                            ''
Although the nitrate measurements tended to be high,  the overall results
were satisfactory.

          Since the bulk of the flue gas scrubbing experiments were to be
carried out in the presence of sulfite ion, it was important to ascertain
the effect of sulfite ion on the analysis for nitrate and nitrite levels.
The Spectrophotometric absorbance of sodium sulfite (Na2SQ3) solutions
exhibited a strong pH dependence as shown in Table 3.
                                 Table 3
           Effect of pH on the Molar Absorptivity of Sulfite Ion
                               Molar Absorptivity
                                   Measured at
                        EiL
                        9.7
                        7.7
302 nm

0.0038
0.131
355 nm

0.0018
0.0035

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                                   - 7 -
 The  natural  pH of  1.0 molar Na2SC>3  is 9.7.  Addition of hydrochloric
 acid, which  does not absorb  high at either wavelength,  was  necessary  to
 lower  the  pH to 7.7.  The  dramatic  increase in molar absorptivity at
 302  nm  was  likely  due to the  formation of  bisulfite ion.


                        S°3~   +   H3°  - *   HS°3~ +  H2°


 This would not seriously affect  the determination of nitrite levels
 at  the   lower pH but would completely negate any attempt to measure
 nitrate levels. Thus,  to  minimize  sulfite  interference requires that
 the  solution be made sufficiently alkaline (pH>-9) prior to analysis.
 In  order to  verify this observation, solutions containing nitrite and
 sulfite were analyzed at both wavelengths.  In the absence of nitrate
 each wavelength gives an independent measurement of the nitrite con-
 centration and any interference  should show up in the reading taken
 at  302  nm.   Duplicate experiments on a solution containing 0.00975
 molar nitrite and  0.90  molar  Na2S03 (pH =  9.8) produced consistent
 results; at  302 nm the  measured  nitrite level was 0.00994 molar
 (27,  error) and at  355 nm the  measured level was 0.0104 molar (57» error).
 Another sample of  the same solution was treated with a  small quantity
 of  concentrated hydrochloric  acid to lower the pH to 7.8. Spectro-
 photometric  measurements then showed the nitrite values to be 0.0131 molar
 at  302  nm and 0.00894 molar at 355  nm.  These values differ by +35%
 and  -87,,  respectively,  from the  original nitrite level.  The +357,
 error  is  consistent with sulfite interference whereas the -87, error is
 less obvious.  The dropwise addition, with stirring, of concentrated
 HCl  produced locally high  acidities in the region of drop impingement.
 This very  low pH  condition lasted momentarily  until the mixing effect
 of  stirring  caused the  pll  to  shift  toward  more alkaline values.   However,
 part of the  nitrite may have  been  lost via  breakdown of the  nitrous
 acid formed  at low p-H .                                             '
           2HN02   - : - >   N2

           N20 (g)  — - — >   N0(g)    +  NO (g)   (escape from solution)

The problem was eliminated by improved stirring and by introducing the
HCl below the surface of the solution.

          Alkaline scrubbing solutions will absorb C02 to form carbonates
which may or may not be soluble depending on the cations present.   Because
soluble carbonate interferes with spectrophotometric determination of
nitrite and nitrate, the species must be removed prior to analysis.  This
is easily accomplished by acidifying the solution which expels the car-
bonate as C02 gas.  Subsequently, the pH must be raised to eliminate
sulfite interference.  In order to check the effect of'pH cycling, several
experiments were made with no carbonate present.  Table 4 shows the
results from two runs using solutions containing nitrite and sulfite
ions .

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                                   - 8 -


                                  Table 4

                  Effect of pH Cycling on Nitrite Readings;
                              	After pH Cycling*	
          Original  Solution    	[N02~]	           % Error
 Exp #  IM>2lLi*  [S03 ]  pH   302 nm   355 nm   _p_H	  302 nm  355 nm

   1C    0.00956   0.88   9.8  0.00924  0.00966  10.1  -3.       1.
   2C    0.00953   0.88   9.8  0.00974  0.00957  11.9  2.      •£ 1.
 *  The pH was lowered to 7.7 with concentrated HC1.  This was followed
    by addition of concentrated NaOH to raise the pH.  The slight  dilution
    effect was taken into account in the calculations.
**  All concentrations. ar,e in units of molarity.
 Since the pH cycling showed no effect on nitrite measurements  in the
 presence of sulflte, several more experiments were made with  solutions
 containing nitrite, nitrate, and sulfites as shown in Table  5.
                                  Table 5

            Effect of pH Cycling on Nitrite  and  Nitrate  Readings


          Original Solution      After pH Cycling*        % Error
 Exp #  IN0^* [N0"]    pH   IN021_  [NO]    pH    [NO^]   [NO
  3C   -0.00520  0.00908  9.5  0.00557   0.00892   11.6  7.1      -1.8

  4C    0.00520  0.00908  9.6  0.00524   0.00936   11.7  1.       3.1

  5C    0.00520  0.00908  9.4  0.00540   0.00918   11.5  4.6      1.7
*   Same as in Table 4.
**  Same as in Table A.   All solutions contained

The effect of pH cycling appears to be more pronounced in Table 5  than
in Table 4 although the  only difference is the presence of nitrate ion.
Because the goal of pH cycling was to eliminate carbonate and  sulfite
spectral interferences,  it was decided to move on to work with solutions
containing all four ions - N03", N02", S03= and C03~.

          Stock solutions containing the four ions  gave totally
unsatisfactory analyses  after pH cycling as shown in Table 6.

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                                   - 9 -
                                   Table 6

                Effect of pH Cycling on Solutions  Containing
                Nitrite,  Nitrate,  Sulfite, and Carbonate  Ions

        	Original Solution	After pH Cycling*        7. Error
Exp #   [NO^'l** fNO-~l     pH
 6C     0.00520  0.00908  11.10.00143  0.0103  .9.5   -73.     13,

 7C     0.00520  0.00908  11.l" 0.00131  0.0109   9.8   -75.     20,

 8C     0.00520  0.00908  11.10.00384  0.0100   9.8   -26.     10,
*   Same as in Table 4.
**  Same as in Table 4.   All solutions contained  0.85  M SO ~ and  0.85M CO  .


           These results indicate that nitrite is disappearing  during
 pH cycling and is not being totally converted to nitrate.  It  is likely that
 nitrite is being stripped.from the solution by the  effervescence
 produced during HCl addition.   As discussed previously the nitrite
 ion may decompose into the two gases, NO and N02, which may then
 diffuse into a nearby bubble of C02 gas instead of recombining  to reform
 nitrite ion.'
                                          co2
                                                 •> escape from solution
                                         <•   — - - ^ eoudiJc 1. 1. win oi_< icii_ A.^ii
                                         bubbles
                                         mixing eliminates,  nitrite ion
                                   *     -local acidity
 Of the two gases, NO and N02 ,  which may escape,  N02 may react with water
 to form more nitrate.  Nitric  oxide is very unreactive and once entrapped
 in a gas bubble, will be stripped from the solution.  The overall
 sequence shows the  possibility of one nitrate being formed for every
 three nitrites which disappear.
                  ^cid
                                  NO
                                 2HN03.  +  NO
           3HNO  (nitrite)   acld»   HO  +  2ND +  HN03 (nitrate)

 This sequence is approximately consistent with the results of experiments
 6C and 7C and accounts for the high levels of nitrate found after pH
 cycling.  In experiment 8C, however, we can account for only 40% of the
 total excess nitrate error.  Since the cycling in this experiment
 lowered the pH only to 8.3 as compared to 6.5 and 7.7 for 6C and 7C,

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vi'
                                 -  10 -

           I y , more  ca rbona Li- sliould  remain  in solul ion t.o raise the
 302 nm reading  (carbonate, has  Less  influence,  at  355 nin) .  The  fact  thai
 the pll was  only  lowered  to 8.3 also  explains  the  1/3  l.e.ss error in
 nitrite level for experiment 8C .  Since  less  acid was added, leas
 nitrite decomposed.

          If the solutions listed in Table 6 do  not undergo  pH  cycling,  but
are analyzed directly (pH = 11.1) the resulting  nitrite  and  nitrate  values
are in excess by approximately 1270.
      »
          All further attempts at removing  soluble  carbonate by acid
 addition failed  to  yield  satisfactory results.  At  this juncture we
 turned to precipitation  techniques with  the hope of resolving the
 problem. A concentrated  solution of CaClZ was added to the test
 solution containing all  four of  the  important ions.  Unfortunately,
 calcium carbonate and  calcium  hydroxide  precipitates  gelled the entire
 sample. After  breaking  the gel  and  centrifuging the mixture the
 clean solution  was  analyzed spectrophotometrically  for nitrite and
 nitrate. Unfortunately,  the results were highly erratic and totally
 unsat is factory .

          At this point  we, decided  not to pursue this question any
 further but concluded  that a total  nitrogen level would have to suffice
 for those few samples  containing high  levels  of soluble carbonate.  A
 complete NOX material balance  would  still be  possible for these samples;
 only the ratio  of nitrite to nitrate would be unavailable.  A reasonable
 approximation to the  ratio could be  obtained  by extrapolation from
 data  taken  from a similar absorbent  in which  carbonate is insoluble.

          The results  of  this  effort can be briefly summarized:

           • Raising  the  pH to >9 ,  effectively minimizes sulfite
             interference in the spectrophotometric determination
             of  nitrite  and nitrate.

           « High levels  of soluble  carbonate interfere  with
             the nitrite  and nitrate measurements;  no simple
             procedure was found which would  eliminate this
             problem.  Fortunately,  a total NOX materia.l balance
             is  still  possible for  those few  samples exhibiting
             this problem.

      2.1.2  Measurement of NO  Levels in  Flue  Gases

          Flue  gases contain three gaseous pollutants:
               2.  N02

               3.  NO

Two Dupont 400 spectrometers were used for on-line analysis of S02 and
N02 •  These instruments were also employed in the screening study
referred to previously.  The hot (130°F), damp, flue gas flowed through
the heated gas analyzer cells where the appropriate measurements
were made.  No pretreating of flue gas was necessary.  The analyzer
readings were recorded continuously on strip charts.

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                                    - 11 -

           The; detenu i ii;il i on of NO  levels  in the  flue c,,is w.-is more
 comp I i edited.  During I he earlier screening  study a  lUu-kman  NDTR  (non-l)i spers i ve
 Inlr.i-Ke.d) w;is used lor NO mr;i sure.iucnl: .   Hn I or I un,-i I e I v , I he  i ns t runienl
 was sensitive1 l"o water v;ipor which required thai   the w.-il.e.r he  removed
 prior  to ana l.-ys Is .   This w;is not a simple, task because ol  the  complex
 reaction chemistry associated with NO, N(>2, S0;>  and  liquid water. A
 cumbersome yet reliable technique  evolved in which  the.  [hie  gas
 passed through a sequence of calibrated  traps designed  to remove
 N02, S02 and water vapor.  The complexity of the system created
 problems and introduced a delay time  in  the ,NDIR output.               ;

           In order to improve the  analytical procedure  for  NO, we   .
 discussed chemiluminescence techniques with Thermoelectron  Corporation
 which manufactures an instrument for NO  analysis. Their conventional
 instrument was unable to handle damp gases  directly  but it  seemed
 that specially heated inlet lines  could  resolve  that problem.  As
 long as the water vapor did not condense  in the  inlet  system,  no
 difficulties should arise.  Consequently, Thermpelectron Corporation
 modified a standard instrument to  our  specifications and loaned
 it to us for test experiments.

           In the first series of tests we compared  the  response  of
 the NDIR and the chemiluminescence unit  (CML) to a gas  blend consist-
 ing of nitrogen and varying amounts of NO.  Table 7  contains the
 results .

                                  Table 7

                    Comparison of the CML with the NDIR
              NDIR Response
 Series #
CML Response
                  287*
                  198
                  475
                  120
                  360
    287*
    202
    485
    115
    365
                      Comments
                               CML  inlet  system
                               is cold; heater  not
                               on
    2
395*
200
 87
353
480

460*
110
175
440
277

265

250

302
395*
208
 87
360
485

460*
106
185
450
290

280

265

318
                 CML inlet system
                 heater is on
                                                 'Gas  blend  contains
                                                  12%  C02  in Series #
                                                  3  runs  except  for  '
                                                  last  run
                                                  steam  (^57=)  added

                                                  steam  (<57o)+370 00
                                                  No  C0n
*  At the beginning of each  series,  both  instruments  were adjusted to
   give the same correct  reading.

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                                  - 12 -
The two instruments were connected in parallel to the main line carrying
the blended gas.'  The agreement is quite good between the NDIR and the
CML which indicated satisfactory reliability for the CML.

          The CML has two readout modes; one for NO only and the other for
total NOx-  The second series of tests was designed to check the agree-
ment between these two modes and to determine the influence of all
the flue gas constituents on the NO reading under normal operating
conditions.  The NO level was fixed at 600 ppm initially and left '
unchanged during the entire test.  The results are shown in Table 8.

                                 Table 8

               Effect of Flue Gas Components on CML Reading '        •
           NO Reading     NO* Reading
Exp . #        (ppm)         (ppm)	     	\	Comments 	

   Id          588            590          gas contains N2 and NO

  2d          598            598          3% 02 added

  3d          600            600          12% C02 added to the above

  4d          600            600          107» steam added to the  above

  5d          600            600          one hour after steam added

  6d          590            1625        £ 1000 ppm N02 added to  above
  7d      .    580            1500        ^ 3000 ppm S02 added to  above

  8d          580            1500 .        S02 source turned off
  9d          580            1500         S02 source turned back  on

The data in Table 8 show the CML to be reliable,  stable, and not  significantly
affected by the various constituents of flue gases.  The modified instrument
circumvented all the previous problems associated with the NDIR and,  in
addition, provided a check on our Dupont 400 N02  analyzer.  The total NOX
readout on the CML gave the sum of NO plus N02 from which the N02
level could be calculated by subtracting the NO reading.

          The final test was made with the NDIR  again  connected  in parallel
with the CML and both analyzing a hot, blended flue gas containing N2.
37= 02, 127, C02,  '10% H20, and NO.  The NDIR was equipped with the  proper
traps for pretreating the flue gas.   The CML read 116 ppm NO and  the  NDIR
read 111 ppm NO, which is good agreement.  Based  on these results,  we
consider the problem of measuring NO in a damp flue gas containing N02
and S02 to be resolved.

-------
                                 -  13 -

2 .2  Scrubbing System

     2.2.1  FJLue Gas Blending

          The gas blending system was capable of producing synthetic flue
gas consisting of 107, steam, 127,, C02, 37, 02,  3000 ppm S02, 1000 ppm
N02, 1000 ppm NO and N2•   The concentration of any component  could be
varied over a wide range.  In addition,the system was capable of total
flow rates of 80 to 800 SCFH.  Figure 1 depicts the total scrubbing
system and include a schematic of the gas blending system. All of
the gases  were  derived  from  pure  gas  sources  except for oxygen,
whose source was compressed air.  Nitrogen, air, and steam were available
in the laboratory whereas C02> N02> NO,  and S02 were supplied from
cylinders.  In order to minimize gas phase reactions between  S02
and N02» each, was thoroughly diluted before mixing.  The  pure SOo
was blended with air, C02, and NO,  while the pure N02 was dilutee  with
N2 •  After mixing the two diluted streams  the blend passed into a
heated steam box to receive the appropriate steam flow.   The  steam
supplied to the lab was wet so the steam box provided the heat to  dry
it out.  The dewpoint of the blended flue gas exiting from the steam
box was approximately 115°F so that all downstream lines  had  to be
heated .

          The biggest problem encountered involved N02•   This substance
is a liquid (N20^) under normal conditions which boils at 70°F under
a pressure of one atmosphere.  The. vapor pressure rises  to 17 psig at
100°F.  A small heated shed was constructed just outside  the  laboratory
to hold one cylinder, each of pure N02 and pure S02•  It was probably
unnecessary to heat the.S02 because of its high vapor pressure but
since both cylinders had  to be outdoors  in winter, we decided to take
the extra precaution against the possibility of unusually low temperatures.
The shed temperature was  maintained at 100-115°F by a thermostatically
controlled electric heater.' Because of  its low vapor pressure, conventional
corrosive gas regulators  would not regulate N02 flow effectively.   We
tried to control the flow with a metering valve but temperature fluctuations
in the shed generated large uncontrolled changes in gas  flows.  Eventually
we obtained a specially modified corrosive gas pressure  regulator  which
could effectively regulate the N02  pressure.   However, even this controller
failed once because of the highly corrosive nature of pure N02• The
stainless steel lines carrying the pure  NOo were heated  up to the  point
of dilution with nitrogen.  The N02 flowmeter was ineffective because
droplets of liquid.formed inside the glass barrel; we were unable  to
conveniently provide sufficient heat to the flowmeter.  To simplify the
situation, the flowmeter was replaced with a  straight piece of stainless
steel tubing with a metering valve. .The N02  level in the flue gas was
obtained by opening the metering valve until the proper  response occurred
in the Dupont 400 N02 analyzer.  When fixing the various  pollutant con-
centrations, the scrubber column shown in Figure 1 was by-passed and
the flue gas went directly to the gas analyzers.

-------
                                 - 14 -
          All lines were SS 316 stainless steel.  All  lines delivering
 gases from their sources were 1/4" i.d.  except  for  the  nitrogen  and  steam
 lines which were 1/2"  i.d.  Beyond the first level of  blending all
 lines were 1/2" i.d.

          Each flowmeter had a pressure gauge on the downstream side
and each flowmeter was calibrated with a  wet test meter.

          The blending manifold functioned very  well after the problems
discussed above were resolved.

     2.2.2 , Flue Gas Scrubber

          The continuous tower scrubber is shown in  Figure 1.   The
vertical glass column  is constructed of sections of  glass pipe each
of which is 8 inches long and 2 inches inside diameter.   The sections
are coupled with threaded aluminum collars.  The collars and the
glass pipe were obtained from the Fisher-Porter'Company.  The  design
makes it easy to change the height of the column.  Each  section
contains a thermocouple and a port for removing  samples  of scrubbing
fluids.  The top of the column held a demister head  packed with glass
wool for removing entrained droplets from the gas stream.  The entire
column sat on a 50 liter heated flask which served as  the scrubbing
fluid resevoir.  The flask was provided with a stirrer.   Pressure
gauges were located at the top and bottom of the column  and a  water
manometer measured the pressure drop across the  entire column.

          A circulating pump withdrew fluid from the resevoir  and
pumped it up to the top to be sprayed down the column, countercurrent
to the gas flow.  The  rough pumping rate  was controlled  at the pump
with final adjustment being made at the flowmeter downstream from
the pump.  A pH electrode was situated in the pump intake for  resevoir
pH determinations.

          The temperature of the scrubbing fluid was usually kept
at 125°F.  The gas and liquid flow rates  were sufficiently high to
maintain the glass column temperature at  125°F without additional
heating or insulation.  Also,it was unnecessary  to heat  or insulate
the pump lines .

          All thermocouples on the scrubbing tower and on the  heated
lines were connected to temperature recorders .  Temperature controllers
were used wherever heat was required.

          When making a scrubbing  experiment the following sequence
was followed:

          •  Fix the flue gas composition and flow rate  while
             by-passing the scrubber.

-------
                                                Figure  1
                                      Flow Schematic of  Scrubbing Unit
     heated
     steam
     box
               L
                        -=3-
                    Steam
t   r?4i  t
0
n
               Air
                       c\
                      O
                          o
                                  A
                                  +
                                  A
                                                                                                           -> Vent
                                                Electrical
                                                 Heater
                                                                         NO-NOX
                                                                         Chemil--
                                                                         urainescenjce
                                                                                                Dupont
                                                                                                  400
                                                                                                         •F
                                                                                                           —>   Vent
                                                                                  Recorder
                                                                                and Control
                                                                                  Console

-------
                                  - 16 -
          •  Turn on the circulating pump and set rate to
             predetermined value.

          •  Wait 5 to 10 minutes for column temperature to
             stabilize.

          •  Switch flue gas through the scrubber.

          •  Monitor pH and temperatures.

          •  Periodically withdraw liquid samples.

          e  Periodically by-pass the scrubber to check the
             composition of the flue gas.

The system performed well; the next section discusses the results of
our scrubbing experiments.

2.3  Results of Flue Gas Scrubbing Experiments

          The construction, testing and trouble-shooting of the
flue gas scrubbing system took much more time than anticipated.  Con-
sequently, only a small fraction of the scheduled scrubbing experiments
were completed.

          Despite these shortcomings, however, an important series.of
scrubbing experiments was completed which verified the capability of
sulfite ion to effectively absorb N02 from flue gas.  The results are
shown in Table 9 and were obtained under identical conditions of
temperature and flow rates.  These results allow several conclusions.
The first is that sulfite ion is a much better absorber of N02 than
hydroxide ion.  Unfortunately neither is effective for NO absorption
which also verifies the results obtained in our previous screening
study.  Also the data show that the presence of S02 in the flue gas
enhances N02 absorption by alkaline solutions initially containing
no sulfite (see runs Cl and Dl).  When absorbed, S02 produces sulfite
ion which subsequently reacts with N02, thereby removing it from the
gas stream.  A similar effect is not observed for NO; the presence
of S02 is not significantly beneficial for NO absorption (see runs
Al and Bl).  Since S02 absorption is primarily dependent on pH, both
solutions'were effective for removing essentially all of the S02•

          These experiments were made with an open column which does
not provide good contacting between gas and liquid, although it does
give minimum pressure drop across the column. A small amount of
column packing or even some liquid distribution plates would enhance
contacting and, no doubt, improve the N02 absorption efficiency.
However, the Na2S03 solution does quite well considering the poor con-
tacting between the two phases.

-------
                                  - 17 -
                                  Table 9
             Flue Gas Scrubbing with NaOH and Na?SCL Solutions
Exp #
Al
A2
Bl
B2
Cl
C2
Dl
D2
Input
NO
660
640
680
680
--
--
Levels
N02
--
680
690
690
690
(ppm)
SO?
2450
2500
—
2700
2700
%
NO
12.
0
19.
6.
—
--,
Absorption
N02
--
12.
83.
48.
83.

S02
98.
98.
—
99.
99. .
Scrubbing
Solution
L.OM NaOH
l.OM Na2SO
l.OM NaOH
l.OM Na2S03
l.OM NaOH
l.OM Na2S03
l.OM NaOH
l.OM Na0S00
' NOTES:  .

 (1)  The bulk gas  composition was  10% H20, 3% 02, 12% C02, N2.
 (2)  Scrubbing solution  temperature = 128°F.
 (3) 'Gas flow rate = 5.  SCFM (4.5  ft/sec in column)
 (4)  L/G = 20.
 (5)  Liquid  flow rate =  4.  liters  per minute.
 (6)  Pressure inside column - 3. psig.
 (7)  Five.glass pipe sections in column; pressure drop across entire
      column  = 3" H20.
 (8)  Each run lasted 15  minutes; took about five minutes for system to
      stabilize after going  on line.
 (9)  Resevoir initially  contained  15.. liters of solution.
(10)  Solution pH was constant during brief runs: l.OM NaOH, pH = 13.;
      l.OM Na2S03,  pH = 9.                       ;
(11)  No solution analyses were made.

-------
                                  -  18  -
           Increasing the weight  ratio of  liquid  flow  rate  to' gas  flow
 rate improves the scrubbing efficiency for  N02 and  SC>2  as  shown
 in Figure 2.

                               Figure 2

             Effect of L/G on NO   and SO.  Scrubbing  with Na SO
       Percent
      Absorption
                  100 —
                          10     20    30    40    50
 NOTES:  (1)  Initial N02 level = 620 ppm
         (2)  Initial S02 level = 1800 ppm
         (3)  Resevoir contained 13 liters of l.OM Na2S03
         (4)  Total gas Flow rate = 5. SCFM
         (5)  Scrubbing temperature = 125°F

 Under these conditions the N02 absorption appears to be limited to
 9070 whereas the S02 removal,  as usual,  is better.
         In another scrubbing experiment, using the same column con-
figuration as before, a magnesium hydroxide slurry was employed as the
absorbent for N02 and S02•   Table 10 contains the results.

-------
                                   -  1.9 -

                                 Table LO

                Flue Gas Scrubbing Using Magnesia Slurry
 Run time  (min)  -   0      10

% N02 Absorbed     0       26.

% S02 Absorbed     0       95.

    L/G             10.     10.

    pH              9.4     9.0
20
35.
95.
10.
8.8
30
35.
95.
10.
8.7
. 40
34.
99.
20.
7.5
50
38.
99.
20.
6.7
60
30.
90.
20.
5.6
NOTES:
(1)   Initial NOo  and  S02  levels  = 920 ppm and 2600 ppm, respectively
(2)   Resevoir contained 13. liters of slurry; 10. g Mg(OH)2 per
      liter.
(3)   Total pressure drop across column =3.5" H20
(4)   Scrubbing temperature = 125°F.
(5)   Total gas flow rate - 5. SCFM.
(6)   At L/G = 10, liquid flow rate was 2. liters/min.
(7)   At 40 minutes on line, reset input levels to 710 ppm N02 and
      1920 ppm S02-
(8)   At 50 minutes, added 2. grams of hydroquinone to scrubbing fluid.

These results show that about 1/3 of the N02 is absorbed by the slurry.
Doubling the L/G did not have any effect.  In the screening program a magnesia
slurry (7.4 g/1) absorbed 5870 of the N02 from a flue gas containing
830 ppm N02 and 2460 ppm S02•  The poor removal in the present experi-
ment  implies poor contacting.  The screening studies also showed a
marked absorption improvement with hydroquinone  addition  to the  slurry. This
was not observed in the present case, although the amount of the
antioxidant added may have been too small to be effective or else
the run did not last  long enough for the influence to be demonstrated.
In any case more data is required before any conclusions may be drawn
concerning the effect of hydroquinone.

          These scrubbing  results verify  that sulfite is a good NO-
absorbent but a poor  NO absorbent. - Also,  the effect  of  L/G is  quantified
for Na2S03 solutions.  However,  these results strongly.suggest that
more  information is required on these and other absorbing systems
before engineering and economic evaluations 'can be initiated.  The
bulk  of this information was to be obtained in the third and final
phase of the project.

-------
                                 - 20 -


                   3.  CONCLUSIONS AND RECOMMENDATIONS

3 .1  Conclusions

          The second phase of the flue gas scrubbing program was designed
to realistically apply the results of the Phase I screening study to
the removal of N02 and S02 from flue gases.  The primary goals  of the
project were to design, construct and test a continuous flue gas scrubbing
system which would employ sulfite species as reactive absorbents.  The
program required that suitable analytical procdures be developed so
that accurate NOX material balances may be obtained.  The conclusions
are:


(1)  Analytical

     (a)  Sulfite ion interferes with the spectrophotometric determina-
          tion of nitrite and nitrate levels in scrubbing solutions.
          This interference may be minimized by raising the solution
          pH above 9 prior to analysis.

     (b)  High levels of soluble carbonate ion will also interfere
          with the nitrite and nitrate analysis. '  This problem  was
          not resolved.  Fortunately, few samples will'exhibit  this
          condition.

     (c)  Difficulties were associated with using a Non Dispersive
          Infra-Red instrument for analyzing NO levels in a gas
          stream containing NO, N02»  S02 and H20.   The problems
          disappeared when we changed to a chemiluminescence instru-
          ment modified with a heated inlet system.


(2)  Results with continuous scrubber

     (a)  The removal of N02 from flue gas is enhanced by the presence
          of sulfite in the absorbent or S02 in the flue gas.  This
          verifies the results of the Phase I screening study.

     (b)  NO absorption is not affected by sulfite or S02•

     (c)  S02 absorption is excellent in alkaline media.

     (d)  Increasing the L/G ratio improves the N02 absorption
          in l.OM Na2S03-

     (e)  Only 1/3 of the N02 was scrubbed out by a magnesia slurry.

The results are essentially in agreement with the  earlier screening
study.   However,  much more data is required before we can effectively
evaluate the overall process.

-------
                                   - 21 -

3.2  Recommendations for Future Work

     Our studies have shown that flue gas scrubbing in  3  continuous,
open column unit has promise.  The following recommendations  list  the
work which remains to be done.

     •  Obtain more scrubbing data using different  absorbents  such
        as lime slurry, limestone slurry, and ammonium  hydroxide.

     •  Determine the effect of varying  process  parameters such
        as column height, superficial gas velocity, and column
        packing.

     e  Obtain "complete material balances for NOX and S02 •

     •  Investigate solution regeneration techniques.

     e  The oxidation of NOX to NQ2 upstream from the scrubber
        is vital.  Studies should be instituted  to  optimize  this
        reaction under flue gas conditions.

     The final goal is to successfully combine NOX  oxidation,  gas
scrubbing, and solution regeneration into an integrated process which
cleans the flue gas and minimizes the impact of  scrubbing products
upon the environment.

-------
                         - 22 -


                         APPENDIX

                         TABLE A1

Measurement of Molar Absorptivities  for Nitrite and Nitrate

Solution Molarity
0.00975
0.00975
0.01040
0.01040
0.00975
0.00975
•0.00975
0.01040
0.00908
0.00520
0.00520
0.00488
0.00488
0.00908
0.00908
0.01816
0.02270
0.01816
NOTES :
(1) Used standard
and KN03 .
(2) Used an Optica
N02~
€302
9.87
9.74
9.52
9.38
9.02
9.25
9.28
9.38
•
9.14
9.00
9.64
9.48
--
--
--
--
"* ™

solutions of


e355
25.4
25.3
23.7
23.4
24.1
24.7
24.7
25.0
--
23.2
23.9
23.6
23.7
--
--
--
--
— mt

reagent

N03"
e302
_ _
-- .

--
--
--
--
--
7.35
--
-- •
--
--
7.43
7.43
7.74
7.65
7.71

grade NaN02

Spectrophotometer .
(3) All measurements made with
4.0 cm
(4) Used Beer's Law to calculate molar
quartz cell.
absorptivity
             A = ebc

             A = measured absorbance
             e = molar absorptivity
             b = cell path length  in  centimeters
             c = solute concentration in molarity

-------
      Unclassified
        Security Classification
                               DOCUMENT CONTROL DATA - R 8, D
       (Security clalslflcatlon ol title, body ol abstract and Indexing annotation mutt be entered when the overall report Is clamlllfd)
 l.-ORIGINATING ACTIVITY (Corporate author)
      Esso Research and Engineering Company
      P.O. Box 8      :
      Linden, New Jersey   07036  .
                                      la. REPORT SECURITY CLASSIFICATION
                                             Unclassified
                                      26. CROUP
                                             Air Conservation
 3. REPORT TITLE
      Development of Aqueous Processes  for Removing NOx from Flue
      Gases . .  . Addendum
 4. DESCRIPTIVE NOTES (Type ol report and Inclusive dates)
      Final Report, July 1;  1972  -  June  1,  1973
 5. AUTHOR(S) (Flrit name, middle Initial, lamt nama)
      Gilford A.. Chappell
«. REPORT DATE
June 1973
ta. CONTRACT OR GRANT NO.
EPA 68-02-0220
b. PROJECT NO.
C. , ^' •'
d.
Ta. TOTAL NO. OP PACES 7b. NO. OF KEFS
25 2
»B. ORIGINATOR'S REPORT NUM"BER<3)
GRU.2PJAA.73
ab. OTHER REPORT NO(S) (Any other number* that may be maalgnad
this report)
 10. DISTRIBUTION STATEMENT
      Unclassified - Distribution Unlimited
 II. SUPPLEMENTARY NOTES
                                              12. SPONSORING MILITARY ACTIVITY
                                                 Office of Research and Monitoring
                                                 U.S. Environmental Protection  Agency
 13. ABSTRACT
       This report summarizes the  findings  of the laboratory program for  "Development
   of  Aqueous Processes for Removing  NOX  and S02 from Combustion Flue Gases."   This
   project is the second phase of  the flue  gas scrubbing work sponsored by  EPA  under
   Contract No. 68-02-0220.  The results  of the Phase I program are contained  in
   report EPA-R2-72-051, entitled,  "Development of the Aqueous Processes  for Removing
   NO^ from Flue Gases."
       The present report contains'discussions of analytical techniques and scrubber
   design in addition to experimental results obtained with a vertical spray tower
   scrubber.  The blended flue gases  passed up the unpacked glass column  countercurrent
  ,-tb  the absorbing solution which was  sprayed down from the top.  The scrubbing
   experiments showed:
   o N02  and S02 are effectively absorbed by 1.0 molar Na2S03 solutions.
   o N02  absorption by 1.0 molar NaOH solution is enhanced by the presence  of  S02 in
    the  flue gas.
   • Neither NO nor N02 is effectively  absorbed by 1.0 molar NaOH solution  in  the
    absence of S02, and NO absorption  is not improved by the presence of S02 •
   • Increasing the L/G ratio improves  N02  and S02 absorption by 1.0 molar
   0 Under, similar scrubbing conditions Mg(OH)2 slurry is not as effective  as
    solution for N02 absorption.
       The data show that sulfite  solutions would effectively absorb NOx  and S02 from
   flue gases provided the NOx (mostly  NO)  has been oxidized to N02 upstream from the
   scrubber.
DD /.T..1473
MKPLACKB DO FORM 147*. I.JAN O4. WHICH ID
OOBOLKTB PON AMMY U»B.
                                          Unclassified
                                                               Security Cloooiflcotion

-------
Unclassified
Security Classification
14.
K«V WORD*
Absorption
Air Conservation
Air Pollution
Chemical Analysis
Flue Gases
Flue Gas Scrubbing
Gas Scrubbing
Nitrogen Oxides
NO
. x
S00
2
Sulfites
LINK A
MOLB













WT













LINK •
KOLB:













•FT













LINK C '
MOLC













»T


,









i
                                                                    Security Claaelficotlon

-------
 BIBLIOGRAPHIC DATA
 SHEET
1. Report Nb.
  EPA-R2-73-051a
3. Recipient's Accession No
4. Title and Subtitle
 Development of Aqueous Processes for Removing NOx
  from Flue Gases  -- Addendum
                                           5- Report Date
                                              June 1973
                                           6.
7. Author(s)
          Gilford A. Chappell
                                           8- Performing Organization Kept.
                                             No.
9. Performing Organization Name and Address
                                            10. Project/Task/Work Unit Mr
 EPA, Office of Research and Monitoring
 NERC/RTP, Control Systems Laboratory
 Research Triangle Park, North Carolina 27711
                                            11. Contract/Grant No.

                                             68-02-0220
12. Sponsoring Organization Name and Address
 Esso Research and Engineering Co.
 Government Research Laboratory
 Linden,  New Jersey  07036
                                            13. Type of Report & Period
                                              Covered
                                            14.
 15. Supplementary Notes
16. Abstracts rp^g repOrt summarizes the findings of a laboratory program for developing
 aqueous processes for removing NOx and SO2 from combustion flue gases. It
 discusses analytical techniques  and scrubber design,  as well as results obtained
 experimentally with a vertical spray tower scrubber:  blended flue gases passed up
 an unpacked glass column, countercurrent to the absorbing solution which was
 sprayed down from the top.  The experiments showed that:  NO2 and SO2 are  effect-
 ively absorbed by 1.0  molar Na2SO3 solutions; NO2 absorption by 1. 0 molar NaOH
 solution is enhanced by SO2 in the flue gas; neither NO nor NO2 is effectively
 absorbed by 1. 0 molar NaOH solution in the absence of SO2 (NO absorption is not
 improved by SO2); increasing the L/G ratio   improves NO2 and SO2 absorption by
 1.0 molar Na2SO3; and under similar conditions,  Mg(OH)2 slurry is not as
 effective as Na2SQ3 solution for NO2 absorption.	
17. Key Words and Document Analy
 Air pollution
 Nitrogen oxides
 Sulfur  dioxide
 Combustion
 Flue, gases
 Chemical analysis
 Design
 Washing
 Spraying
17b. Identifiers/Open-Ended Terms
 Air pollution control
 Stationary sources
 Aqueous processes
 Scrubbers
 Liquid/gas ratio
    sis.  17o. Descriptors
     Sodium sulfites
     Absorbers (materials)
     Sodium hydroxide
     Nitrogen oxide
     Nitrogen dioxide
     Magnesium hydroxides
     Sulfites
     Slurries
17c. COSATI Field/Group
                    13B, 7A
18. Availability Statement
                Unlimited
                                 19. Security Class (This
                                   R c port)
                                     UNCLASSIFIED
                                                    20. Security Class (This
                                                      Page
                                                         UNCLASSIFIED
         21- No. of Pages
              25
                                                     22. Price
FORM NTIS-35 (REV. 3-72)
                                                                        USCOMM-DC

-------