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Addressing Variability in MACT Floor Standards: Methods and Recommendations

This document outlines various methods for addressing variability when setting maximum achievable control technology (MACT) floors and standards. It emphasizes a focus on methodologies rather than specific numerical results, recognizing the importance of accounting for variability in analyses. Key approaches discussed include worst-case performance assessments, averaging time strategies, control technology parameters, and statistical methods. The document also invites feedback and further analyses from group members to refine the approach and ensure thorough consideration of variability in emissions and standards development.

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Addressing Variability in MACT Floor Standards: Methods and Recommendations

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  1. Utility MACT Working Group Addressing Variability OAQPS/ESD August 8, 2002

  2. Purpose • To review possible methods for addressing variability in setting a MACT floor and standard • Focus on the methods – not the results • Would prefer not to provide numbers for various scenarios – tendency is to focus on numbers and select method based on what results it will give • Have not done analyses on all available methods

  3. Background • Current data base contains a significant number of emission tests at a variety of plants – but still represents only “snapshots” in time • Variability must be accounted for in floor analyses and in ultimate standard • Presumption is that data are adequate

  4. Approaches to addressing variability • Worst-case performance • Averaging time • Control technology parameters • Format of standard • Correlation of mercury and…something else • Statistical approach(es)

  5. Worst-case performance • Base emission limit on worst-case performance within floor facilities based on analysis of actual performance or good engineering judgment

  6. Averaging time • Establish compliance averaging period to account for variability over time • 12-month rolling average • 365-day rolling average • 30-day rolling average

  7. Control technology parameters • Base emission limit on control technology which employs • Parameters which can be adjusted to deal with variety of input concentrations (e.g., lime feed rate) • Inherent characteristics which inhibit variability or dampen inlet variability (e.g., fabric filters)

  8. Format of standard • Base standard on either-or approach • Percent reduction OR emission rate

  9. Correlation of mercury and… something else • Plot potential mercury emissions based on (good) correlation with another coal constituent • Select emission value based on desired confidence limit • Similar to approach presented by Ralph Roberson at December 2001 Working Group meeting using ICR data and EPRI algorithm • Uses coal mercury vs. chlorine contents to establish mercury emissions estimate

  10. Statistical approach • Uses standard SAS program • Equation MACT floor level = Mean of top 12% + (standard deviation of 3-run average for single plant * t value for “x” percent confidence interval) • Results different from those presented in July • Variability for the MACT from only the data used to obtain the MACT • Variability much less in the best units than in all of the data • Caution: Do not focus on the numbers – they may not be final

  11. Sample results – no subcategorization DRAFT Values in lb/TBtu

  12. Sample results – subcategorization by fuel DRAFT Values in lb/TBtu Waste coals not included

  13. Sample results – subcategorization by fuel, no FBC DRAFT Values in lb/TBtu Waste coals not included

  14. So where does this leave us... • Multiple approaches have been, and can be, used to address variability • More analyses on each potential approach warranted • Approaches may be combined • Advice/recommendations from the members on the approach to be taken are welcomed

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