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WEST Associates’ Assessment of Hg MACT Floor Variability. CAAAC Mercury MACT Working Group Washington, DC March 4, 2003. Who Is WEST Associates?. AZ Arizona Electric Power Cooperative Pinnacle West Capital Corp. Salt River Project Tucson Electric Power Co.

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west associates assessment of hg mact floor variability

WEST Associates’ Assessment of Hg MACT Floor Variability

CAAAC Mercury MACT

Working Group

Washington, DC

March 4, 2003

who is west associates
Who Is WEST Associates?

AZ Arizona Electric Power Cooperative

Pinnacle West Capital Corp.

Salt River Project

Tucson Electric Power Co.

CA Glendale Public Service Dept.

Los Angeles Dept. of Water & Power

Southern California Edison

OR PacifiCorp

ID Idaho Power Company

ND Basin Electric Power

NM Public Service Co of NM,

Xcel Energy

Tri-State G & T

NV Nevada Power Co/ Sierra Pacific Power Co.

CO Colorado Springs Utilities

Xcel Energy

Platte River Power Authority

Tri-State G & T

UT PacifiCorp/Utah Power and Light

WY Basin Electric,

PacifiCorp,

Xcel Energy

Tri-State G & T

west associates efforts to date
WEST Associates efforts to-date
  • September 2002:
    • Mercury Emissions from Western Coal-fired Power Plants: Nature, Extent, and Fate
    • Unique Western Concerns Related to the Role of Chlorine contents of Coal on Hg Emissions
    • Recommended that MACT standard reflect these issues.
  • Statistical Analysis to address coal chemistry issues
west s hg mact data analysis goals
WEST’s Hg MACTData Analysis Goals
  • Determine from 80 unit source test ICR III database, statistically robust datasets for potential MACT subcategories (ENSR ANOVA Study)
    • Coal rank (bit., sub-bit., & lignite)
    • Coal Hg content
    • Hg/Cl ratio
  • Develop a statistically valid approach to integrate operational variability using the ICR II fuel chemistry database to calculate Hg MACT floors
west s hg mact data analysis goals5
WEST’s Hg MACTData Analysis Goals
  • This study used the regulatory framework:
    • “…the average emission limitation achieved by the best performing twelve percent of existing sources” and
    • “… is achievable under the most adverse circumstances which can reasonably be expected to recur.”
variability in mact floor determinations conceptual illustration
Variability in MACT Floor Determinations(Conceptual Illustration)

(Hg, Cl, & Btu Content)

(Soot blowing, load following)t

80 unit source test icr iii does not capture variability
80 Unit Source Test ICR III Does Not Capture Variability
  • Three 1-hr source tests are only a “snapshot” in time taken under steady state operations
  • Three 1-hr source tests do not represent actual emissions over any longer operating time
  • Based only on limited coal chemistry, and operational variability occurring during the 3 tests
  • ICR III source tests represent only a fraction of total variability
west s multi variable method
WEST’s Multi-variable Method
  • Uses ICR III source test and control effectiveness data from 12% best performing units by coal rank, plus annual coal chemistry data from ICR II
  • Integrates key drivers of variability: Coal Hg, Cl, & Btu content (annual variability)
  • Multi-variable Method is based on a 5 step statistical analytical process
multi variable method 5 step analytical process
Multi-variable Method:5 Step Analytical Process
  • STEP 1
    • 80 source test units sorted by coal rank
    • FBC units; petroleum coke units; combination fuel units removed (15 total)
    • Leaves 29 bit. Units; 26 subbit. units; 10 lignite units
  • STEP 2
    • Within each coal rank, units sorted in ascending order of stack tested Hg emissions (# Hg/TBtu)
    • Best performing 12% of units = the 5 units with lowest emissions in each coal rank

Note: Significant differences occur in averages of Cl (ppm) and Hg (#/TBtu) between coal ranks.

multi variable method 5 step analytical process10
Multi-variable Method:5 Step Analytical Process
  • STEP 3
    • To account for “intra-unit” variability, correlation equations were developed to relate Hg emissions to coal chlorine content
    • For each control configuration (e.g., FF/SDA, etc.) determined relationship between Hg removal % and coal Cl concentration using ICR III stack test database for all tested units (not only the best performing units)
multi variable method 5 step analytical process12
Multi-variable Method:5 Step Analytical Process
  • STEP 4:
    • For each best performing unit, “controlled” Hg emissions calculated by multiplying “uncontrolled” Hg emissions by (1-Hg removal fraction)
    • ICR II test data (Btu and Hg content) used to calculate uncontrolled emissions
    • Hg removal fraction derived in one of two ways:
      • If good correlation (from step 3), correlation equation used to calculate Hg removal fraction
      • If poor correlation, ICR III source test Hg removal fraction used
    • Process repeated for each set of measured coal composition data from ICR II database (I.e., Hg, Btu and Cl measurements) yielding a range of Hg emissions for each unit over time
multi variable method 5 step analytical process14
Multi-variable Method:5 Step Analytical Process
  • STEP 5:
    • For each best performing unit, calculated mercury emissions sorted from smallest to largest to obtain a frequency distribution
    • 95% value of this distribution assumed to represent the operation of the unit under the most adverse circumstances reasonably expected to recur for each unit
    • The 95% upper confidence limit (UCL) of the mean of these adverse-case emissions is reported as the Hg MACT floor
elements of variability not captured by this method
Elements of VariabilityNot Captured by this Method
  • Analysis of fuel variability accounts for some, but not all, of the variability in the stack testing of each unit in ICR III
  • Stack test measurement error (+/- 20-25%)
  • Intermittent maintenance events (e.g., operation of air heater soot blowers) affect Hg emission rates
  • Source tests conducted at static load; load following can change results
results of multi variable hg mact floor method
Results of Multi-variable Hg MACT Floor Method
  • Potential national Hg reduction: 15 t/yr; 31%
statistical rationale for alternate mact floors
Statistical Rationale for Alternate MACT Floors
  • Could replace Coronado with Comanche in list of top 5 best performing sub bituminous plants
    • Hg rate for Coronado is only 6% less than Comanche
    • Measured Hg removal data for Comanche show much less scatter than data for Coronado
    • The % removal for Coronado was found to be negative for all 3 source tests
  • Could use simple average of top 5 best performing lignite units (5 out of 10 units).
    • Need for 95% UCL for inter-unit variability among 10 units is less
alternate hg mact floors for subbituminous lignite
Alternate Hg MACT Floorsfor Subbituminous & Lignite
  • Potential national Hg reduction: 17 t/yr; 36%
conclusions of multi variable method hg mact floor study
Conclusions of Multi-variable Method Hg MACT Floor Study
  • Multi-variable method uses the maximum amount of information from both ICR II and ICR III databases in the determination of variability in a MACT floor
  • First known study to comprehensively bridge between the ICR III source test, and ICR II annual coal chemistry data
  • Our MACT Floor levels represent statistically robust estimates of the variability of Hg emissions as a result of annual variability of coal chemistry
  • Variability of coal chemistry accounts for only one driver of variability. The MACT Floor results likely underestimate most adverse circumstances which can reasonably be expected to recur at a unit meeting a mercury MACT limit.
  • This technical analysis conforms to regulatory requirements
supplemental slides

Supplemental Slides

Additional

Correlation Equations