 155 South 1452 East Room 380 SO 3 Formation During Oxy- Coal Combustion  Salt Lake City, Utah  Jiyoung Ahn 1, Dana Overacker 1, Ryan Okerlund 1, Andrew Fry 2 and Eric G.Eddings 1 1 Dept. of Chemical Engineering, University of Utah 2 Reaction Engineering International

Outline Background Methodology and Equipment - Controlled Condensation Method - Pilot-Scale Combustor (L1500) Equilibrium Calculations Experimental Results - SO 3 Concentration - Mass of SO 3 emitted - Effect of Temperature - Effect of Staged Combustion

Background : SO 3 in Combustion - In general, only a small percentage of the sulfur in fuel is oxidized to sulfur trioxide (SO 3 ). - Negative effects of SO 3 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 O 2 in oxy-fuel combustion has a higher chance of affecting the oxidation of SO 2 to SO 3. - Considering the effects of SO 3 on the environment, and the possibility of increasing SO 3 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 T) is used to measure SO 3 and SO 2. - It takes advantage of the difference between the dew point of water and acid to selectively collect SO 3.

Controlled Condensation Method - SO 3 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 SO 2. - The heated quartz filter removes particulate matter.

Titration Methodology - The amount of SO 3 and SO 2 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 ft 2 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 SO 3 - At higher temperatures (>1273K/1832F), equilibrium favors SO 2 formation, not SO 3. - The equilibrium is shifted toward the formation of SO 3 at lower temperatures, with a maximum value at around 900 K (1160 F). Equilibrium calculations of oxyfiring Illinois 6 coal (b) SO 2 ppm (a) SO 3 ppm

SO 3 Measurement Challenges 1) If the gas is sampled at too high a temperature -> It may be prior to the maximum formation of SO 3. 2) If the gas is sampled at too low a temperature -> Some SO 3 may have condensed out prior to sampling It is important, therefore, to sample for SO 3 in an optimal temperature window to account for the formation that takes place, but to also sample prior to any SO 3 condensation.

Experiment Results: Coal Analyses Ultimate and Proximate analyses of PRB, Utah, and Illinois 6 coal *HHV=higher heating value Coal Loss on dryingAshCHNSO Volatile Matter Fixed CarbonHHV* [wt. %] [Btu/l b] PRB Utah Illinois

Experiment Result : Coal analyses Ash Composition from PRB, Utah, and Illinois 6 coal (wt%) CoalAlCaFeMgMnPKSiNaSTi Al 2 O 3 CaOFe 2 O 3 MgOMnOP2O5P2O5 K2OK2OSiO 2 Na 2 OSO 3 TiO 2 PRB Utah Illinois

Experiment Results: SO 2 SO 2 concentration (ppm) measured in the pilot scale experiments Because of the recycling of the flue gas, the amount of SO 2 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: SO 3 SO 3 concentration (ppm) measured in the pilot scale experiments - For Illinois 6 coal, which had the highest sulfur composition, the concentration of SO 3 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: SO 3 SO 3 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 SO 3 concentration by 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 SO 3 concentration as greatly

Experimental Results: SO 3 SO 3 concentration (lb/MMBtu) from the pilot scale experiments When actual furnace exhaust emissions are computed on a mass basis, mass of SO 3 per million Btu is lower for oxy-fuel than air-fuel fired conditions, due to the reduced volume of flue gas.

Experimental Results: SO 3 SO 3 concentration of Illinois 6 coal measured in oxy-fuel combustion - An increase in temperature of 40K in the critical sampling zone can decrease the SO 3 concentration by ppm, and by lb/MBtu. - SO 3 concentration shows an inverse relationship with overall oxygen concentration in oxy-fuel combustion. (a) Molar concentration (ppm)(b) Mass concentration (lb/MMBtu)

Experimental Results: SO 3 SO 3 concentrations measured with Illinois #6 coal under staged and unstaged air- and oxy-fired combustion - Because SO 3 formation is favored at lower temperatures, it is anticipated that SO 3 is formed primarily downstream of the burner. → It is unlikely that the concentration of SO 3 would be affected by staged combustion. - The figure does not indicate any significant correlation between SO 3 concentration and staged or unstaged combustion.

Conclusions - - Temperature at the point of - measurement has a strong impact on - the amount of SO 3 captured in the - sample. - Measurements of SO 3 taken around 800 K (980.6F) during combustion of a high-sulfur coal showed that the SO 3 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 SO 3 was measured in both air- and oxy-fired combustion.

Future Work - More detailed investigation of the effects of O 2 and CO 2 concentration on the amount of SO 3 formed for both air- and oxy-fired combustion - Development of fundamental understanding of the chemistry of N 2 and CO 2 and associated effects on the formation of SO 3 - Investigating the influences of limestone (CaCO 3 ) on SO 3 formation

Acknowledgments This material is based upon work supported by the U.S. Department of Energy under Award Numbers DE- NT and DE-NT

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