EVALUATION OF SO2 AND NOX OFFSET RATIOS TO ACCOUNT FOR SECONDARY PM2.5 FORMATION

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On January 4, 2012, the EPA committed to engage in rulemaking to evaluate updates to the Guideline on Air Quality Models (AppendixWof 40 CFR 51) and, as appropriate, incorporate new analytical techniques or models for secondary PM2.5. As a result, the National Association of Clean Air Agencies (NACAA) developed a screening method involving offset ratios to account for
secondary PM2.5 formation. This method can be used to evaluate total (direct and indirect) PM2.5 impacts for permitting purposes. Therefore, the evaluation of this method is important to determine its viability for widespread use.

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EVALUATION OF SO2 AND NOX OFFSET RATIOS TO ACCOUNT FOR SECONDARY PM2.5 FORMATION

  1. 1. EVALUATION OF SO2 AND NOX OFFSET RATIOS TO ACCOUNT FOR SECONDARY PM2.5 FORMATION Sergio Guerra, Shannon Olsen, Jared Anderson AWMA Specialty Conference March 20, 2013
  2. 2. Background: PM2.5 Secondary Formation • On January 4, 2012, the EPA granted a petition submitted on behalf of the Sierra Club on July 29, 2010. • EPA committed to engage in rulemaking to evaluate updates to the Guideline on Air Quality Models as published as Appendix W to 40 CFR 51, and, as appropriate, incorporate new analytical techniques or models for ozone and secondary PM2.5.
  3. 3. PM2.5 Offset Ratios • EPA’s NSR implementation rule for PM2.5 (73 FR 28321, May 16, 2008). • Ratios first introduced by the EPA for nonattainment areas to offset emissions increases of direct PM2.5 emissions with reductions of PM2.5 precursors and vice versa. • On July, 21 2011, the EPA changed their position and established that these offset ratios were no longer considered presumptively approvable but must be subject to a technical demonstration.
  4. 4. NACAA’s Recommended Approach for Assessing Secondary PM2.5
  5. 5. EPA’s PM2.5 Compliance Demonstration: Assessment Cases
  6. 6. Minnesota-specific Offset Ratios • Developed by MPCA modelers using CAMx. • Secondary PM2.5 emission rate is defined as the sum of the SO2 emission rate divided by 10 and the NOx emission rate divided by 100. • The total equivalent emission rate is to be used in AERMOD modeling demonstrations to show compliance with the PM2.5 NAAQS.
  7. 7. How were they developed • The EPA ratios are based on the 75th percentile distribution for NOx and on the 90th percentile distribution for SO2. • Minnesota’s offset ratios seem to be based on the absolute minimum value.
  8. 8. Box Plots of Concentration Over Distance McCourtney, Margaret. Single Source Secondary PM2.5 Modeling with AERMOD and CAMx; 2012 RSL Modelers’ Workshop; Chicago, IL, 2012. http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2012/presentations/Wed/6-3_RSLWorkshop_PM25_Point_Src_ProjectsMcCourtney_May02_v2anigif.pdf
  9. 9. Box Plots of Concentration Over Distance McCourtney, Margaret. Single Source Secondary PM2.5 Modeling with AERMOD and CAMx; 2012 RSL Modelers’ Workshop; Chicago, IL, 2012. http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2012/presentations/Wed/6-3_RSLWorkshop_PM25_Point_Src_ProjectsMcCourtney_May02_v2anigif.pdf
  10. 10. Input Parameters for four source types Input parameter Case 1 Case 2 Case 3 Case 4 Facility EGU Taconite Mine Food Processing Facility Pulp and Paper Mill Boiler Indurating furnace Boiler Boiler 270 540 200 250 Natural gas / Fuel oil / wood ESP / Cyclone Emission source Capacity (MMBtu/hr) Fuel(s) Coal Natural gas Fuel oil / Propane / Natural gas Controls ESP Baghouse / Cyclone LNB / FGR 60 100 50 75 0.1 40 80 7 4.5 7 0.5 3.5 1.0 2.5 10 150 340 320 427 450 2.5 5 1.2 1.8 22 15 15 13 Stack height (m) Emiss PM2.5 ion NOx rate SO2 (g/s) Exit temperature (degrees K) Diameter (m) Exit velocity (m/s)
  11. 11. Modeling Conditions • AERMOD version 12345 • No terrain • Assessment of building effects (40 meters in x,y,z) • Assumed Minnesota’s Lowest 98th monitored 3-year average concentration of 17 g/m3
  12. 12. Results of primary and total 24-hour PM2.5 concentrations Building Effects Included? Case 1 (EGU) Case 2 (Taconite Mine) Case 3 (Food Proc. Plant) Case 4 (Pulp & Paper Mill) Predicted Impact from Primary Emissions (µg/m3) Predicted Impact from Secondary Emissions (µg/m3) Total Equivalent PM2.5 (µg/m3) Yes No Yes 0.20 0.06 1.75 16.72 4.86 0.19 No 1.75 Yes Background (µg/m3) Total Predicted Impact PM2.5 (µg/m3) Primary PM2.5 (% Total Pred) Secondary PM2.5 (% Total Pred) 16.92 4.92 1.94 17 17 17 33.9 21.9 18.9 0.6% 0.3% 9.3% 49.3% 22.2% 1.0% 0.19 1.94 17 18.9 9.3% 1.0% 6.11 1.65 7.76 17 24.8 24.7% 6.7% No 0.60 0.16 0.76 17 17.8 3.4% 0.9% Yes 2.71 16.38 19.09 17 36.1 7.5% 45.4% No 1.30 7.87 9.18 17 26.2 5.0% 30.1%
  13. 13. Benefits from the Offset-ratio Method • Avoids the use of complex chemistry models (i.e., CAMx, CMAQ). • Simple to use.
  14. 14. Uncertainties of the Offset Ratio Method • Variability of CAMx generated offset ratios. • Distance • Season • Grid resolution • Stack height • Emission rate • Assume primary and secondary emissions occur concurrently in time and space.
  15. 15. Current Sources of Conservatisms • Combining the 98th percentile modeled concentration with the 98th percentile of monitored concentration yields 99.96%: equivalent to one exceedance every 6.8 years. • Assumed that permitted (PTE) emissions are emitted constantly.
  16. 16. Conclusion • Offset ratio method may be viable option for facilities that have low PM2.5 , NOx and SO2 emissions. • Older facilities, or facilities with large emissions of NOx and SO2 may not be able to model compliance with this method.
  17. 17. Sergio A. Guerra Environmental Engineer Phone: (651) 395-5225 sguerra@wenck.com

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