155 South 1452 East Room 380 Salt Lake City, Utah 84112
1-801-585-1233
SO3 Formation During OxyCoal Combustion
Jiyoung Ahn1, Dana Overacker1, Ryan Okerlund1, Andrew Fry2 and Eric G.Eddings1
1Dept. of Chemical Engineering , University of Utah
2Reaction Engineering International
Outline
•Background
•Methodology and Equipment
- Controlled Condensation Method
- Pilot-Scale Combustor (L1500)
•Equilibrium Calculations
•Experimental Results
- SO3 Concentration
- Mass of SO3 emitted
- Effect of Temperature
- Effect of Staged Combustion
Background : SO3 in Combustion
- In general, only a small percentage of the sulfur in fuel is
oxidized to sulfur trioxide (SO3).
- Negative effects of SO3 on plant operations:
1) The potential for corrosion of metallic surfaces
2) The increased emission of acid aerosols, which
create visible plumes and cause acid rain.
- The increased amount of O2 in oxy-fuel combustion
has a higher chance of affecting the oxidation of SO2 to SO3.
- Considering the effects of SO3 on the environment, and the
possibility of increasing SO3 emissions in oxy-fuel
combustion, it is important to investigate the behavior of
sulfur compounds in oxy-fuel combustion.
Method of Measurement
- The Controlled Condensation Method
( ASTM D 3226-73T) is used to
measure SO3 and SO2.
- It takes advantage of the difference
between the dew point of water
and acid to selectively collect SO3.
Controlled Condensation Method
- SO3 is condensed into a sulfuric acid mist in the condenser.
Temperature in the condenser was kept between 167F and 185F.
- The first two impingers contain a hydrogen peroxide
solution that captures SO2.
- The heated quartz filter removes particulate matter.
Titration Methodology
- The amount of SO3 and SO2 present
in the condensed acid and hydrogen
peroxide solutions is quantified
through a titration using barium
perchlorate with thorin indicator
(EPA Method 8A).
- Due to subtle color changes during
titrations, various concentrations of
dilute sulfuric acid were used to
make standards for comparison.
Pilot-Scale Combustor
- The 5 million Btu/hr furnace has a 3.2 ft2 internal cross section
and is approximately 46 feet in length.
- Gas was sampled at three different locations to investigate the
effect of temperature on the formation of sulfur oxides during
air- and oxy-fired coal combustion.
Equilibrium Behavior of SO3
Equilibrium calculations of oxyfiring Illinois 6 coal
SR1.2 Oxygen 20%
SR1.2 Oxygen 25%
SR1.2 Oxygen 30%
2.00E+03
2500
mol fraction of SO2 (ppm)
mole fraction of SO3 (ppm)
2.50E+03
1.50E+03
1.00E+03
5.00E+02
0.00E+00
2000
1500
1000
500
0
0
500
1000
1500
Temperature (K)
(a) SO3 ppm
2000
2500
0
500
1000
1500
Temperature (K)
2000
(b) SO2 ppm
- At higher temperatures (>1273K/1832F), equilibrium favors SO2
formation, not SO3.
- The equilibrium is shifted toward the formation of SO3 at lower
temperatures, with a maximum value at around 900 K (1160 F).
2500
SO3 Measurement Challenges
1) If the gas is sampled at too high a temperature
-> It may be prior to the maximum formation of SO3.
2) If the gas is sampled at too low a temperature
-> Some SO3 may have condensed out prior to sampling
It is important, therefore, to sample for SO3 in an optimal
temperature window to account for the formation that
takes place, but to also sample prior to any SO3
condensation.
Experiment Results: Coal Analyses
Ultimate and Proximate analyses of PRB, Utah, and Illinois 6 coal
*HHV=higher heating value
Coal
Loss on
drying Ash
C
H
N
S
O
Volatile
Matter
Fixed
Carbon
HHV*
[wt. %]
[wt. %]
[wt. %]
[wt. %]
PRB
23.69
4.94 53.72 6.22 0.78 0.23 34.11
33.36
38.01
[Btu/l
b]
9078
Utah
3.18
8.83 70.60 5.41 1.42 0.53 13.21
38.60
49.39
12606
Illinois
6
9.65
7.99 64.67 5.59 1.12 3.98 16.65
36.78
45.88
11598
Experiment Result : Coal analyses
Ash Composition from PRB, Utah, and Illinois 6 coal (wt%)
Coal
Al
Al2O3
PRB 14.78
Utah 14.52
Illinois 17.66
6
Ca
Fe
CaO Fe2O3
22.2 5.20
6.11 5.09
1.87 14.57
Mg
MgO
5.17
1.39
0.98
Mn
MnO
0.01
0.02
0.02
P
P2O5
1.07
0.59
0.11
K
Si
Na
S
K2O SiO2 Na2O SO3
0.35 30.46 1.94 8.83
0.57 60.89 1.41 2.33
2.26 49.28 1.51 2.22
Ti
TiO2
1.30
0.88
0.85
Experiment Results: SO2
SO2 concentration (ppm) measured in the pilot scale experiments
20000
Oxy fuel, Illinois 6
Air fuel, Illinois 6
Oxy fuel, Utah
Air fuel, Utah
Oxy fuel, PRB
Air fuel, PRB
18000
SO2 concentration (ppm)
16000
14000
12000
10000
8000
6000
4000
2000
Because of the
recycling of the flue
gas, the amount of SO2
was much higher in
oxy-coal combustion
than in air-fuel
combustion, ranging
from twice as much
(PRB) to almost six
times as much (Illinois
6) at all temperatures.
0
500
600
700
800
900
1000
Gas temperature at the point of measurement (K)
1100
Experimental Results: SO3
SO3 concentration (ppm) measured in the pilot scale experiments
60
Oxy fuel, Illinois 6
- For Illinois 6 coal, which had the
Air fuel, Illinois 6
highest sulfur composition, the
Oxy fuel, Utah
Air fuel, Utah
concentration of SO3 at the
Air fuel, PRB
50
SO3 concentration (ppm)
optimum sampling
temperature (755.2K/900F)
increased up to 5 times for oxyfuel combustion compared to
air-fuel combustion.
- At higher sampling
temperatures, limited
difference was found between
oxy- and air-fuel combustion for
Illinois 6 coal.
40
30
20
10
0
500
700
900
1100
1300
Gas temperature at the point of measurement (K)
1500
Experimental Results: SO3
mole fraction of SO3
SO3 Concentration (ppm)
50
40
30
20
10
SR1.1, Oxygen 20% 60
SR1.1, Oxygen 25%
SR1.1, Oxygen 30% 50
Oxy fuel, Illinois 6
1.20E-02
60
1.00E-02
8.00E-03
40
6.00E-03
30
4.00E-03
20
2.00E-03
10
Oxy fuel, Illinois 6
0.00E+00
0
800
810
820
830
840
850
Gas temperature at the point of measurement
(K)
0
500
1000
1500 2000 2500
Temperature (K)
0
3000
- An increase in temperature of 40K in the critical sampling zone can
decrease the SO3 concentration by 10-15 ppm
- The data from Illinois 6 is in the region of very steep gradients in the
equilibrium predictions. However, for Utah coal, small changes in the
same temperatures didn’t affect SO3 concentration as greatly
Measured SO3 Concentration
(ppm)
SO3 concentration (ppm) measured in a small range of temperatures
and comparison with the equilibrium calculations
Experimental Results: SO3
SO3 concentration (lb/MMBtu) from the pilot scale experiments
0.045
Oxy-fuel, Illinos 6
Air-fuel, Illinois 6
Oxy fuel, Utah
Air fuel, Utah
Air fuel, PRB
0.04
SO3 concentration (lb/MMBtu)
0.035
0.03
0.025
0.02
0.015
0.01
When actual furnace
exhaust emissions are
computed on a mass
basis, mass of SO3 per
million Btu is lower for
oxy-fuel than air-fuel
fired conditions, due to
the reduced volume of
flue gas.
0.005
0
400
600
800
1000
1200
Gas temperature at the point of measurement (K)
1400
Experimental Results: SO3
SO3 concentration of Illinois 6 coal measured in oxy-fuel combustion
0.035
SO3 Concentration ( ppm )
60
0.03
SO3 Concentration
(lb/MMBtu)
50
0.025
40
0.02
30
0.015
20
10
0.01
Measured at 800K
Measured at 842K
0
20.00%
25.00%
30.00%
35.00%
Overall oxygen concentration (wt%)
(a) Molar concentration (ppm)
0.005
0
20.00%
25.00%
30.00%
35.00%
Overall oxygen concentration (wt%)
(b) Mass concentration (lb/MMBtu)
- An increase in temperature of 40K in the critical sampling zone
can decrease the SO3 concentration by 10-15 ppm, and by 0.01
0.015 lb/MBtu.
- SO3 concentration shows an inverse relationship with overall
oxygen concentration in oxy-fuel combustion.
Experimental Results: SO3
SO3 concentrations measured with Illinois #6 coal under staged
and unstaged air- and oxy-fired combustion
60
Oxy fuel at 800K
Air fuel at 800K
SO3 Concentration (ppm)
50
40
- Because SO3 formation is favored
at lower temperatures, it is
anticipated that SO3 is formed
primarily downstream of the
burner.
→ It is unlikely that the
concentration of SO3 would
be affected by staged
combustion.
30
20
10
0
0.5
0.7
0.9
1.1
Burner stoichiometric ratio
- The figure does not indicate any
significant correlation between
SO3 concentration and staged or
unstaged combustion.
1.3
Conclusions
-
- Temperature at the point of
measurement has a strong impact on
the amount of SO3 captured in the
sample.
- Measurements of SO3 taken around 800 K (980.6F) during
combustion of a high-sulfur coal showed that the
SO3 concentration was three to five times higher during oxy-coal
combustion as compared to air-fired conditions, but the
difference was strongly coal- or S-content-dependent.
- At higher sampling temperatures (922K/1200F), roughly the
same amount of SO3 was measured in both air- and oxy-fired
combustion.
Future Work
- More detailed investigation of the effects of O2and
CO2 concentration on the amount of SO3 formed for both air- and
oxy-fired combustion
- Development of fundamental understanding of the chemistry of
N2 and CO2 and associated effects on the formation of SO3
- Investigating the influences of limestone (CaCO3) on SO3
formation
Acknowledgments
This material is based upon work
supported by the U.S. Department of
Energy under Award Numbers DENT0005015 and DE-NT0005288.
Questions?