155 South 1452 East Room 380

SO3 Formation During Oxy-Coal Combustion

Salt Lake City, Utah 84112

1-801-585-1233

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

- 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). 

Equilibrium calculations of oxyfiring Illinois 6 coal

0 500 1000 1500 2000 25000.00E+00

5.00E+02

1.00E+03

1.50E+03

2.00E+03

2.50E+03 SR1.2 Oxygen 20%SR1.2 Oxygen 25%SR1.2 Oxygen 30%

Temperature (K)

mole fractio

n of SO3 (ppm

)

0 500 1000 1500 2000 25000

500

1000

1500

2000

2500

Temperature (K)

mol fractio

n of SO2 (ppm

) 

(b) SO2 ppm(a) SO3 ppm

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

CoalLoss on drying Ash C H N S O

Volatile Matter

Fixed Carbon HHV*

[wt. %] [wt. %] [wt. %] [wt. %] [Btu/lb]

PRB 23.69 4.94 53.72 6.22 0.78 0.23 34.11 33.36 38.01 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 Ca Fe Mg Mn P K Si Na S Ti    Al2O3 CaO Fe2O3 MgO MnO P2O5 K2O SiO2 Na2O SO3 TiO2

PRB 14.78 22.2 5.20 5.17 0.01 1.07 0.35 30.46 1.94 8.83 1.30Utah 14.52 6.11 5.09 1.39 0.02 0.59 0.57 60.89 1.41 2.33 0.88Illinois 

617.66 1.87 14.57 0.98 0.02 0.11 2.26 49.28 1.51 2.22 0.85

Experiment Results: SO2

SO2 concentration (ppm) measured in the pilot scale experiments

500 600 700 800 900 1000 11000

500

1000

1500

2000

2500Oxy fuel, Illinois 6Air fuel, Illinois 6Oxy fuel, UtahAir fuel, UtahOxy fuel, PRBAir fuel, PRB

Gas temperature at the point of measurement (K)

SO2  co

ncentration (ppm

)

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.

Experimental Results: SO3

SO3 concentration (ppm) measured in the pilot scale experiments

500 600 700 800 900 1000 11000

1

2

3

4

5

6

7

8

9

10 Oxy fuel, Illinois 6Air fuel, Illinois 6Oxy fuel, UtahAir fuel, UtahAir fuel, PRB

Gas temperature at the point of measurement (K)

SO3 concentration (ppm

)

- For Illinois 6 coal, which had the highest sulfur composition, the     concentration of SO3 at the   optimum sampling      temperature (755.2K/900F)    increased up to 5 times for oxy- fuel combustion compared to air-fuel combustion. - At higher sampling    temperatures, limited     difference was found between oxy- and air-fuel combustion for Illinois 6 coal.

Experimental Results: SO3

800 805 810 815 820 825 830 835 840 845 8500

10

20

30

40

50

60

Oxy fuel, Illinois 6

Gas temperature at the point of measurement (K)

SO3 Concentration (ppm

)

0 500 1000 1500 2000 2500 30000.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

1.00E-02

1.20E-02

0

10

20

30

40

50

60SR1.1, Oxygen 20%SR1.1, Oxygen 25%SR1.1, Oxygen 30%Oxy fuel, Illinois 6

Temperature (K)

mole  fractio

n of  SO3

Measured SO

3 Concentration 

(ppm

)

SO3 concentration (ppm) measured in a small range of temperatures and comparison with the equilibrium calculations

- 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

Experimental Results: SO3

400 500 600 700 800 900 1000 1100 1200 1300 14000

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02 Oxy-fuel, Illinos 6Air-fuel, Illinois 6Oxy fuel, UtahAir fuel, UtahAir fuel, PRB

Gas temperature at the point of measurement (K)

SO3 concentration (lb/MMBtu)

SO3 concentration (lb/MMBtu) from the pilot scale experiments

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.

Experimental Results: SO3

20.00% 25.00% 30.00% 35.00%0

10

20

30

40

50

60

Measured at 800KMeasured at 842K

Overall oxygen concentration (wt%)

SO3 Concentration ( ppm )

20.00% 25.00% 30.00% 35.00%0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Overall oxygen concentration (wt%)

SO3 Concentration    

(lb/M

MBtu)

SO3 concentration of Illinois 6 coal measured in oxy-fuel combustion

- 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.

(a) Molar concentration (ppm)  (b) Mass concentration (lb/MMBtu)

Experimental Results: SO3

SO3 concentrations measured with Illinois #6 coal under staged and unstaged air- and oxy-fired combustion

- 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.-   The figure does not indicate any       significant correlation between       SO3 concentration and staged or        unstaged combustion.0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

0

10

20

30

40

50

60 Oxy fuel at 800KAir fuel at 800K

Burner stoichiometric ratio

SO3 Concentration (ppm

)

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 DE-NT0005015 and DE-NT0005288.

Questions?