On the relative importance of SO2 oxidation
to high dust SCR DeNOx units
by:
Morten Thellefsen Nielsen
Haldor Topsøe A/S
Lyngby, Denmark.
Presented at:
DOE/NETL 2003 Conference on Selective Catalytic Reduction (SRC)
and Selective Non-Catalytic Reduction (SNCR) for NOx Control
Introduction
In power plants SO3, which is inevitably formed during combustion of sulphur containing fuels,
can cause problems in downstream installations, e.g. corrosion by condensation of sulphuric
acid in air heaters and downstream installations and high stack opacity caused by acid mist.
Due to an undesired side reaction SO2 is further oxidized in SCR DeNOx installations and ammonium bisulphate fouling can occur in the air heater as sulphuric acid reacts with unreacted
NH3 from the denitrification process.
As a rule of thumb the SO3 concentration from a coal fired furnace is 0.7% of the SO2 concentration [1] and the SO2 oxidation across the SCR reactor is usually 0.5-1%.
Due to SO3 scavenging by alkaline fly ash components, such as CaO, MgO, Na2O, K2O and
Fe2O3, the observed SO2 oxidation across an operating SCR is lower than found in laboratory
measurements at dust free conditions.
Based on industrial SO2 oxidation measurements and pilot plant experiments a model for SO3
capture has been developed to predict the SO3 concentration out of the SCR. A quantitative
prediction of the SO3 scavenging will
• provide more accurate estimates of the gas phase SO3 concentration of the SCR in operation
• enable prediction of the effect of fuel switching and retrofitting a power plant with an SCR
• be useful in optimizing the air heater operation
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 2 of 12
Contents:
The SO2 capture model ................................................................................................................. 3
SO3 determination in high dust condition ...................................................................................... 4
SO3 measuring technique ............................................................................................................. 6
The fate of SO3 in coal-fired power plants..................................................................................... 8
Conclusion................................................................................................................................... 11
References .................................................................................................................................. 12
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 3 of 12
The SO2 capture model
By systematically changing experimental conditions in our pilot plant we have found that the
SO3 capture by the fly ash follows a simple first order mechanism:
−rSO3 = k ⋅ C SO3 ⋅ CCaO
Where –rSO3 is the rate of removal of SO3, k is the reaction constant, CSO3 the gas phase SO3
concentration and CCaO is the concentration of available alkaline fly ash sites. The latter depends on the concentration, chemical composition and size distribution of the ash. Due to their
high specific surface area and number concentration the fine ash particles play an important
role in the SO3 capture.
35
30
Bituminous coal ash
25
1.05 s
20
2.10 s
15
10
1.05 s
PRB-ash
5
2.10 s
0
0
2
4
6
8
10
12
Cash [grains/scf]
Figure1: Results from SO3 capture experiments carried out on the pilot plant. The triangular points are experimental values and the dotted lines represent best fits of the
SO3 capture model. The full lines represent the maximum SO3 capture by the fly ash,
i.e. saturation of the ash or infinite residence time. Residence times around 1-2 seconds are commonly found in SCR’s. 32 ppm inlet SO3 concentration. 660°F flue gas
temperature.
At 660-750°F a residence time of 1-2 seconds in the SCR reactor is sufficient for extensive SO3
capture by the ash. This is seen in Figure 1, where the SO3 capture model has been fitted to the
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 4 of 12
experimental data for two different ash types: a highly alkaline PRB-ash and a low sulphur bituminous coal ash. The full lines represent saturation of the ash with SO3, i.e. the maximum capacity of the ash. Due to higher alkaline content and larger fraction of fine ash particles the PRB
ash has a marked higher SO3 capture capacity compared to the bituminous ash. The fitted
curves show significant SO3 capture in the SCR reactor, thereby reducing the observed SO2
oxidation compared to dust free conditions. The capture is rapid and in the case of the PRB ash
mainly limited by the capacity. Compared to the PRB ash, the bituminous ash has a smaller
fraction of fine particles, which is probably why the SO3 capture is a bit slower.
To predict the SO3 concentration at the SCR outlet the SO3 capture mechanism must be coupled with the SO2 oxidation mechanism and the derived equations solved simultaneously. The
SO3 capture capacity of the ash and the concentration of alkaline sites at the SCR inlet must be
estimated. Currently the capacity is measured by saturating the ash with SO3 and the inlet alkaline concentration is estimated from the sulphur content in the ESP hopper ash, but in the future
when more refined models have been developed these parameters can be estimated from the
coal analysis.
SO3 determination in high dust conditions
Where the fly ash SO3 capture is beneficial in power plant operation it should be avoided or at
least minimized in flue gas sampling for SO3 determination. During sampling dust is collected on
a filter and although the residence time in the filter is low, the mass transfer between the gaseous SO3 and the fly ash is high compared to SO3 capture in the duct.
Two tests carried out at the outlet of an SCR installation in Belgium showed that the fly ash
collected on the filter did capture significant amounts of SO3 during sampling, see Figure 2. In
these tests the in-duct quartz wool filter (see Figure 3) was re-used in consecutive samples;
otherwise the sampling was carried out as usual. During probe extraction the low pressure in
the duct sucked out some of the ash collected on the filter. The packing of the quartz wool plug
and the ash distribution on the plug limits reproducibility and quantification, but the qualitative
effect is very clear, i.e. the more ash on the filter the more SO3 capture.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 5 of 12
SO 3 concentration [ppm, dry]
25
20
15
10
5
0
0
4
8
12
16
Accumulated sample gas volume [dscf]
Figure 2: SO3 concentrations measured at the SCR outlet on a coal fired power plant
in Belgium. Bituminous coal was burned and the flue gas temperature was 745°F.
The in-stack quartz wool filter was reused after each sample and approximately 4
dscf flue gas was extracted in each sample. The two curves represent measurements carried out on two different SCR units.
Tests were carried out in our pilot plant to quantify the effect and determine the influence of filter
temperature on SO3 capture during sampling. A 37 mm quartz plane filter was placed in an exduct temperature controlled glass filter house and SO3 measurements with three different sample volumes (1.8, 3.5 and 5.3 dscf) were carried out for each filter temperature. The filters were
changed after each sample and sulphur content and mass was determined.
The pilot plant experiments were able to reproduce the behaviour found in Belgium, i.e. the
measured SO3 gas phase concentration decreases as flue gas volume increases. At constant
dust concentration the SO3 capture by the fly ash (in ppm SO3/ft3 gas sampled) was independent of the temperature in the range 500-750 °F.
For a bituminous fly ash with 2 grains/ft3 filter dust loading and 30 ppm initial SO3 concentration
a value of approximately 1.7 ppm SO3 captured per ft3 gas sampled was found. For a normal
sample run that means a 6 ppm underestimation of the gaseous SO3 concentration.
The mentioned filter dust concentration is a bit higher than expected for perpendicular sampling
in full scale power plants, but based on fly ash surface area the difference is less because the
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 6 of 12
perpendicular sampling excludes large particles (low surface to mass ratio) due to their high
inertia.
Ideally 0 ft3 flue gas should be sampled to determine the true gas phase SO3 concentration, but
in real life a minimum volume must be sampled to exceed the detection limit of the sampling
equipment.
SO3 measuring technique
There is no universally accepted standard for SO3 determination but the Controlled Condensation method is the most frequently one used. The method was developed in the 1960’s and
collects SO3 by selective condensation of sulphuric acid on a 160-195 °F surface, usually the
wall of a glass coil. Since the solubility of SO2 in sulphuric acid is very low there will be no interference from SO2 if the surface temperature is kept above the water dew point.
If the gas-cooling rate is sufficiently high, a fraction of the SO3 will nucleate to form a fine acid
mist, which is only very inefficiently collected in the coil. A medium porosity (G4) glass fritt located downstream the coil can collect the mist.[2]
The fly ash is either collected on in-duct or ex-duct quartz filters (wool plug or plane filter).
After sampling the condenser parts are rinsed in water with either water or 80 vol% iso-propanol
and the sample is analyzed for SO42- content. Ion Chromatography or titration with NaOH or Basalts is commonly used.
Several variations of the Controlled Condensation methods exist, varying in the means of ash
filtration, SO3 condensation and SO42- determination. The two first issues are crucial since SO3
capture by the ash and insufficient SO3 collection in the condenser will result in underestimation
of the gaseous SO3 concentration. The different methods for SO42- analysis are of minor importance because SO42- standards are used for calibration.
Using the Controlled Condensation method is by no means a simple task. Jaworowski and
Mack [3] reported that different teams sampling the same flue gas with the same type of sampling equipment gave relatively poor precision in the SO3 concentration. Unfamiliarity with the
equipment was mentioned as one of the reasons for the poor precision.
Several authors state that the reproducibility of the Controlled Condensation method is 10% or
less. [4],[5],[7]
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 7 of 12
At Haldor Topsøe we use a modification of the ASTM D-3226-73T standard. A sketch of the
sampling system is shown in Figure 3.
To minimize the dust loading on the in-duct quartz wool filter, perpendicular sampling, which
acts as an inertial filter, is applied. The filter is changed after each measurement.
Via the heated quartz lined probe and Teflon sampling hose the dust free gas reaches the condenser coil, where it is cooled below the sulphuric acid dew point. The condensed sulphuric acid
is seen as a white ‘frosting’ in the first couple of windings. Acid mist formed during rapid cooling
is collected in the G4 glass fritt downstream the coil. The SO3 collection has been tested in our
laboratory and found to be greater than 99%.
Figure 3: Topsøe method 1305 measuring equipment for determination of SO3 by
controlled condensation and SO2 by absorption and oxidation in H2O2. The system is
developed from the ASTM D-3226-73T standard.
All SO2 leaves the condenser and is absorbed and oxidized in H2O2 filled impingers. The gas is
then dried and the sample volume is measured with an integral gas meter.
The SO3 sample is the combined rinses of the probe, sample hose and SO3 condenser with 80
vol% iso-propanol. For quick results the SO42- content in the samples is determined by a (ClO4)2
titration with Thorin as indicator, otherwise Ion Chromatography is used.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 8 of 12
The fate of SO3 in coal-fired power plants
In June 2003 SCR testing was carried out on the two Allegheny Energy coal-fired power plants
Harrison (3x640 MW) and Pleasants (2x625 MW) in West Virginia, USA. The power plants burn
almost similar high sulphur bituminous coal and are equipped with SCR DeNOx, ESP and wet
SO2 scrubbers for flue gas cleaning.
The results from the testing are shown in Table 1 and Table 12.
The SCR catalyst has a high SO3 adsorption capacity, the amount depends on the temperature,
SO3 vapour pressure and vanadium content in the catalyst. In order to ensure that the catalyst is
in equilibrium with the flue gas, stable operation of the boiler is required for 12-24 hours before
sampling.
To determine the fate of SO3 in the power plants, SO2/SO3 measurements were carried out at
four locations i.e. at the inlet and outlet of the SCR, at the ESP outlet and in the stack. The locations, which are shown in Figure 4, possess their own problems regarding correct and representative SO3 determination (see also [8] and [9] for more information).
Figure 4: Sketch of major SO3 affecting installations on a coal fired power plant: furnace, SCR for NOx control, rotary air heater, ESP and wet type scrubbers (FGD) for
SO2 control. The sampling locations for the Harrison and Pleasants measuring campaign are also shown.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 9 of 12
Due to SO3 capture by the ash on the filter there is a risk of underestimating the SO3 concentration in the high dust locations at the SCR inlet and outlet. At the SCR inlet also stratification can
occur due to difference in individual burner operation in the furnace. Static mixers installed to
mix NH3 with the flue gas will reduce that but if temperature and flow gradients exist in the catalyst layers SO3 stratification is generated.
The simultaneously SO2/SO3 measurements at the inlet and outlet of an SCR unit lasts one full
day and the difference between A and B units, which receive flue gas from the same furnace, is
very likely caused by day to day variations in the coals fired. For each SCR unit there is very
good agreement between the inlet and outlet SO2 concentration. In contrast to Pleasants, static
mixer are installed upstream of the sampling ports at the SCR inlet at Harrison. That is the reason why the standard deviations of the inlet concentrations are lower at Harrison than at Pleasants.
The SO3 concentration is approximately 1% of the SO2 concentration and compares well with
the US-EPA rule of thumb. The SCR’s increase the SO3 concentration by 8-27 ppm, depending
on the operating temperature, residence time and flue gas composition. The SO2 oxidation is
between 0.3 % and 0.9 % and well below the guaranteed values.
At the ESP outlet SO3 stratification can be severe, which is caused by both air heater leakage
and local H2SO4 condensation on the colder parts of the rotating air heater. The dust interference is low.
At Harrison there is a marked decrease in the SO3 concentration in the air heater and ESP, very
likely due to sulphuric acid condensation on cold parts of the air heater. At Pleasants the decrease is smaller and the explanation is probably that the ESP operates at a higher temperature
(340 °F) compared to Harrison. Another thing to notice is the severe stratification. At Harrison
two sample ports were used and showed 1 ppm deviation between double determinations (8/8
and 17/18 ppm SO3)
In the wet scrubbers the flue gas is quenched and a sulphuric acid mist is formed that is only
inefficiently captured in the scrubber. In the stack care should therefore be taken to capture the
mist e.g. by isokinetic sampling.
Double determination in a single point in the stack agreed within 1-2 ppm SO3. Due to the severe stratification at the outlet of the ESP’s it is difficult to quantify the exact SO3 removal in the
FGD’s but it seems that Pleasants has a removal of approximately 50%. At Harrison very little
SO3 is removed.
In the literature SO3 removals between 15% [6] and 80 % [1] across SO2 scrubbers are cited,
depending on the design and operation of the FGD.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Sampling port
Inlet SCR
Outlet SCR
Temperature
SO2 oxidation
Outlet ESP*
Stack*
SO2
SO3
n
SO2
SO3
n
°F
%
SO2
SO3
SO2
SO3
Unit 2 A
2323 ± 71
28 ± 2
5
2371 ± 67
39 ± 6
6
640
0.49 ± 0.09
2313 ± 171
13 ± 6
312
10
Unit 2 B
2469 ± 129
30 ± 2
6
2463 ± 90
38 ± 3
6
630
0.29 ± 0.04
Unit 3 A
2431 ± 67
28 ± 3
6
2661 ± 116
50 ± 7
5
678
0.81 ± 0.15
Page 10 of 12
Unit 3 B
2692 ± 106
26 ± 4
6
2632 ± 204
41 ± 2
6
678
0.54 ± 0.09
Table 1: SO2/ SO3 results from the June 2003 performance test at Harrison power
plant, units 2A, 2B, 3A and 3B. All concentrations are given on a dry basis and at actual O2 concentration. *Due to plant operational problems the ESP outlet and stack
testing was carried out on unit 1 A
Sampling port
Inlet SCR
Outlet SCR
Temperature
SO2 oxidation
Outlet ESP
Stack
SO2
SO3
n
SO2
SO3
n
°F
%
SO2
SO3
SO2
SO3
Unit 1 A
3180 ± 231
20 ± 5
6
3037 ± 98
47 ± 6
6
658
0.85 ± 0.25
Unit 1 B
2740 ± 154
37 ± 2
6
2765 ± 68
62 ± 5
6
673
0.92 ± 0.11
2370 ± 323
40 ± 3
468
22
Unit 2 A
3024 ± 179
27 ± 8
6
2891 ± 151
45 ± 7
6
696
0.59 ± 0.11
Unit 2 B
3254 ± 47
19 ± 8
6
3240 ± 58
32 ± 1
6
662
0.42 ± 0.17
Table 2: Table 1. SO2/ SO3 results from the June/July 2003 performance test at
Pleasants power plant, units 1A, 1B, 2A and 2B. All concentrations are given on a dry
basis and at actual O2 concentration.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 11 of 12
Conclusion
Acid mist emissions from power plants depend not only on the formation of SO3 in furnace and
SCR but also the operation of downstream equipment, especially the air preheater and ESP
and to lesser extent the FGD.
The SO3 capture by fly ash plays an important role in the SO3 concentration development from
furnace to stack and at Haldor Topsøe A/S we have developed an SO3 capture model that coupled with the SO2 oxidation mechanism in the SCR catalyst predicts the SO3 concentration at
the outlet of the SCR DeNOx installation.
Fly ash also plays an important role in SO3 sampling as SO3 capture by fly ash collected on the
filter during sampling will underestimate the gaseous SO3 concentration in the flue gas.
Even in an ash free gas the determination of SO3 is not simple, as SO3 not only condenses on
the colder surfaces but if the gas cooling rate is sufficiently high also forms an acid mist in the
sampling equipment that is hard to collect without a filter device.
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.
On the relative importance of SO2 oxidation to high dust SCR DeNOx units
Page 12 of 12
References
[1] Blythe, G., Galloway, C., Rhudy, R. ‘Flue gas Sulfuric Acid Measurement Method Improvements’, The U.S. EPA/DOE/EPRI Mega Symposium, August 20-23, 2001, Chicago Il, 20
pages.
[2] Goksøyr, H., Ross, K., ‘The determination of sulfur trioxide in flue gases’, Thornton Research Centre, Shell Research Limited, Report M.211, October 1961
[3] Jaworowski, R.J., Mack, S.S. ‘Evaluation of Methods for Measurements of SO3/H2SO4 in
Flue Gas’, Journal of the Air Pollution Control Association, Vol. 29, No 1, January 1979, pp
43-46
[4] Maddalone, R.F., Newton, S.F., Rhudy, R.G., Statnick, R.M. ‘Laboratory and Field Evaluation of the Controlled Condensation System for SO3 Measurements in Flue Gas Streams’,
Journal of the Air Pollution Control Association, Vol. 29, No 6, June 1979, pp 626-631
[5] Berger, A.W., Driscoll, J.N., Morgenstern, P. ‘Review and Statistical Analysis of Stack Sampling Procedures for the Sulfur and Nitrogen Oxides in Fossil Fuel Combustion’, American
Industrial Hygiene Association Journal, June 1972, pp 397-405
[6] Szalach, P.J., ‘New York State Electric & Gas Corporation’s Utility Experience with Sulfur
Trioxide and Sulfuric Acid Mist’, 1998 FETC Conference proceedings from ‘Formation, distribution, impact and fate of SO3 in flue gas streams conference’, March 30th –31st (1998)
[7] Oda, R.S., DeVito, M.S., ‘Controlled Condensation Method for Flue gas SO3 measurements’, 1998 FETC Conference proceedings from ‘Formation, distribution, impact and fate
of SO3 in flue gas streams conference’, March 30th –31st (1998)
[8] DeVito, M.S., Oda, R.L., ‘Flue gas SO3 stratification at ESP inlets’, 1998 FETC Conference
proceedings from ‘Formation, distribution, impact and fate of SO3 in flue gas streams conference’, March 30th –31st (1998)
[9] Bionda, J., ‘Flue gas SO3 determination – Importance of Accurate Measurements in Light of
Recent SCR Market Growth’, 2002 Conference on SCR and SNCR Reduction NOx Control,
Pittsburgh, PA, May 15th –16th (2002)
Org.1996
Information contained herein is confidential; it may not be used for any
purpose other than for which it has been issued, and may not be used by
or disclosed to third parties without written approval of Haldor Topsøe A/S.