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.
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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
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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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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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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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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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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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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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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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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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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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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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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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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.
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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.
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- 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
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