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MEMORANDUM

SUBJECT: Assessment of Using Single Point Stack Test Data to Derive 30-Day Rolling Average 

Emissions Limits 

FROM: Stephen Boone, Roy Neulicht, and Jeff Cole, RTI

TO: Bill Maxwell, EPA, SPPD, Energy Strategies Group

DATE: August 18, 2011

Summary of Technical Directive

The WAM requested that RTI further assess using single-point data as the basis for establishing MACT 

limits with 30-day rolling average compliance periods. Specifically ESG asked, “... have we adequately and 

accurately developed an emission limit from manual, single-point-in-time testing that will truly reflect what 

would occur through the use of CEMS and 30-day-day rolling averages (e.g., should “m” in the statistical 

variability analysis be made equal to the “number of future runs” i.e., 1 ... or 30 ...)?”

Problem Statement for Evaluation of Hg Standard for >8,300 Btu/lb coal units

Based on available CEMS data, what is the relationship between a unit’s 30 day average emission rate 

(including startups, shutdowns, and malfunctions) and the emission rates occurring during short-term periods 

at operating conditions equivalent to typical stack testing conditions (i.e., steady stack flow rate and steady 

load at greater than or equal to 90 percent of full load conditions). 

Background

Eight of the sources included in the initial MACT floor pool of 328 sources (in the >8,300 Btu/lb coal-fired 

subcategory), submitted Hg CEMS data in lieu of manual stack test data. However, only seven of these 

CEMS data sets reflected the minimum mercury average of the source when compared to available Part II 

manual stack test data and thus were used in the initial floor ranking. The seven CEMS sources submitted 

30 days of daily Hg emissions averages, however none of the seven CEMS sources fell into the top12 percent of performers in the initial ranking of sources. With the exception of these seven CEMS data sets,

all other data used for the initial floor ranking were comprised of Part II or Part III manual stack test data 

submitted for the ICR.

Prior to proposal, the 30-day CEMs data from the seven units were analyzed to determine the variability.

Using the UPL methodology, the daily values for all the CEMS data from the seven units were averaged to 

find the mean value and the variability of the daily values from the mean was determined. The variability of 

the CEMS data was about 2 times the mean value. The CEMS variability was then compared to the manual 

stack test data variability, and it was found to be about one quarter the variability of the manual stack tests.

Since the CEMS data variability was less than the variability of the data set used to establish the MACT 

floor, EPA determined that the MACT floor emission limit (corrected for variability) was achievable on a 

30-day rolling average basis, as EPA proposed in the rule. 
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Some facilities also provided historical CEMS data with their Part II ICR submittals. These data were 

received from approximately 100 sources. Review of these data, indicated that information sufficient to

conduct an analysis (i.e., unit load, lb/MMBtu emission rate, and data quality flags present to exclude 

calibrations and other suspect data) was provided from about 60 of the 100 sources. This analysis is

described in the following paragraphs. Note that, to date, only historical CEMS data from 39 sources have 

been analyzed.

Technical Approach

In this analysis the historical CEMS data were used to derive a mathematical relationship between the 30-day 

average emission limit that is achievable (i.e., the maximum 30-day average emission rate in the CEMS data 

set) and the minimum six-hour average emission rate that is achievable at stack test conditions (i.e., the 

minimum emission rate recorded by CEMS during periods of steady operations at greater than or equal to 

90 percent of capacity).

The Hg CEMS historical data emissions data covers Hg CEMS operated between 1/1/2005 through 12/31/09 

(with some data sets continuing a month or two into 2010). Only one of the sources included in the Hg 

MACT floor pool of 40 units (determined exclusively based on each unit’s minimum manual stack test 

result), also provided Hg CEMS data under Part II of the ICR. This source (Public Service Company of New 

Mexico’s San Juan Station, Unit 4, ORIS 2451) had the 33rd lowest Hg emissions rate in the MACT floor 

pool. Additionally, a second unit (not included within the top 40 because the source did not provide manual 

stack test data) provided CEMS data demonstrating 30-day averages at or below the median MACT floor 

rate without variability1at a higher frequency than San Juan Unit 4. Performance versus the MACT floor 

(without variability) was based upon analysis of “surrogate emissions tests” as explained in the following 

paragraphs.

First, the Part II CEMS data were reviewed for completeness. The hourly CEMS data selected for analysis 

were then reduced to 30-day rolling averages. Further analysis was done to identify 6-hour periods of CEMS 

data at operating conditions typical of manual stack tests and to determine the average emission rate during 

such periods (i.e., periods of steady load and stack flow for at least six hours while the source operated at 

90 percent capacity or greater). These periods are referred to in this memorandum as of “surrogate emissions 

tests.” As previously noted, San Juan Unit 4 was the only source that provided CEMS data in the top 40 best 

performing units based on the manual stack tests. Figure 1 compares the manual stack test data used in the 

floor analysis for San Juan Unit 4 versus the 30-day rolling averages measured by CEMS over approximately 

8,000 hours (including the hours of the Part III San Juan Unit 4 manual stack test used in the floor 

calculation). Figure 1a adds to the figure the calculated 6-hr CEMS average “surrogate stack test” data. The 

CEMS data averages included periods of startup, shutdown, and malfunction and were calculated based on 

the lbs Hg/MMBtu compliance option using a diluent cap in accordance with §63.10005(l) of the proposed 

rule (if necessary) during startup and shutdown periods. In other words, the data were screened to identify 

start-up and shut-down periods (based on an estimated startup and shutdown threshold equal to 28 percent of 

full load), and for these periods, if necessary based on the actual diluent concentration, the emissions rates

 

1

 1.97 x 10-8lbs Hg/MMBtu, which is the average of Chambers Cogeneration LP, Unit 1 (ORIS 10566) and Reid 

Gardner, Unit 1 (ORIS 2324)
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were recalculated using the appropriate cap value in accordance with the proposed provisions of

§63.10005(l). Note that based on these 30-day average CEMS data, San Juan Unit 4 complied with the 

proposed MACT limit (1.2 x10-6lb/MMBtu) during all 30 day averages occurring in the submitted CEMS 

data. 

The overall objective of this analysis was to determine the averaging period ratio (APR) or the ratio of the 

maximum average emission limitation achieved on a 30-day rolling average basis divided by the minimum 

surrogate stack test as measured by Hg CEMS. For the purposes of this analysis, the APR was defined as 

follows:

Averaging Period Ratio (APR) =

(maximum 30-day rolling average recorded by Hg CEMS)

(minimum short-term Hg emission rate at stack test conditions*)

*Surrogate stack test conditions are steady flow operations at a generating rate that is > 90% of capacity.

This technical approach provided for a very large data set for each unit with hundreds of surrogate stack tests 

and hundreds of 30-day averages during all operating conditions. Each APR is unit specific and provides an 

estimate of differences between the short-term and the long-term emission averages that are related to factors 

including 

 A particular source’s fuel contracts (i.e., variability of fuel-Hg content),

 Hours since the last major maintenance event (for the larger data sets), and

 Seasonal factors (for the larger data sets) 

 Routine cycling to match system demand

Other analyses such as the UPL method, applied across multiple best performing units, may be necessary to 

account for variability associated with unit age, design, measurement technique, and cycling profiles not 

represented in the CEMS database.

Subsequently, Part II CEMS data files for 39 units (a total of approximately 180,000 hours of Hg CEMS 

data) submitted in response to the ICR were reviewed to evaluate periods of operation consistent with the 

normal procedures for manual stack tests (i.e., surrogate stack tests). Table 1 summarizes these CEMS data.

The companies operating units identified in bold text within Table 1 all provided 900 or more hours of Hg 

CEMS data. These data were utilized to derive an equation for predicting the APR for a unit based strictly on 

short-term stack test data. A threshold of 900 hours of Hg CEMS data was chosen because a data set of this 

size or greater provides at least 180 thirty-day averages for comparison against the minimum short-term 

emission rate. Thirty-six units had 900 or more hours of Hg CEMS data.

The APR, as defined above, was calculated for 35 of the units (all the units other than San Juan Unit 4) 

identified in the bold text, and the ratios were plotted (see Figure 2) versus the minimum “surrogate stack 

test” in the same data set to obtain a relationship between the lowest, achievable, short-term (i.e., 6-hour),

emission rate at stack test conditions and the maximum 30-day rolling average that was achieved. The 

relationship was evaluated by calculating the predicted value for San Juan Unit 4 and comparing the 

predicted value with the actual value recorded by CEMS.
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Summary of Findings

The relationship between the maximum 30-day average emission rate was found to have a good correlation 

to the value of the minimum “surrogate stack test” as defined by Equation 1:

(Equation 1) y = 0.0018x

-0.569

r2 = 0.75

The amount of error in this prediction was determined by applying Equation 1 to the San Juan Unit 4 actual

minimum surrogate stack test (1 x 10-8lbs Hg/MMBtu on a 6-hr average) to predict the APR and 

subsequently the to predict the maximum 30-day average for comparison against the actual maximum 30-day 

average in the CEMS data:

APR predicted for San Juan U4 = 0.0018(1 x 10-8)

-0.569 = 64.2

Therefore, the 30- day average emission rate predicted for San Juan Unit 4 is estimated as follows:

 Predicted maximum 30-day average for San Juan Unit 4 = APR x 1 x 10-8= 6.42 x 10-7lbs Hg/MMBtu

 The actual maximum 30-day average for San Juan Unit 4 was 4.94 x 10-7lbs Hg/MMBtu.

 The percent error of the predicted value versus the actual value for San Juan U4 was 30 percent.

 Therefore, for minimum emission rates about 1 x 10-8lbs Hg/MMBtu, Equation 1 is expected to predict the

APR for a large CEMS data set with an error margin of approximately 30 percent.

As mentioned earlier, the emission rate at the median position in the data set of minimum manual stack test 

averages for the forty sources that composed the MACT floor data set was 1.97 x 10-8lbs Hg/MMBtu. 

Applying Equation 1 to this emission rate yields the following estimate of the 30-day average that could be 

achieved by a unit with a minimum six-hour stack test averaging 1.97 x 10-8lbs Hg/MMBtu:

 APR predicted at median of MACT floor (without variability) = 0.0018(1.97 x 10-8)

-0.569

 = 43

 Predicted maximum 30-day average at median of MACT floor (without variability) = APR x 1.97 x 10-8

 Predicted max. 30-day avg at median of MACT floor (without variability) = 8.52 x 10-7lbs Hg/MMBtu + 30%

Additional work to be conducted

Additional work planned includes:

1. Additional QA of the data set and calculations, and 

2. Variability analysis of the equation

3. Calculate UPL using predicted maximum 30-day averages
Table 1 – Summary of Hg CEMS Data

Figure 1 
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0.00E+00

2.00E-07

4.00E-07

6.00E-07

8.00E-07

1.00E-06

1.20E-06

1.40E-06

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

lbs Hg/MMBtu

Max Stack Test in Floor

San Juan Unit 4 - Facility ID 2451 - Hg Emissions

MACT Floor Stack Tests versus Hg CEMS Data

lbs Hg/MMBtu

Hg CEMS Hrs

Minimum Stack Test in Floor Data

Median Stack Test in Floor Data

Maximum 30-Day Avg in CEMS Data

Median CEMS Surrogate Stack Test 

Maximum CEMS 6-hr Surrogate Stack Test

Figure 1a 
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Figure 2
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Facility_ID Surrogate Hg CEMS Maximum Maximum Median Median Minimum Minimum

& "_" & Stack_ID Capacity 6-hour Stack Tests Hours Surr. Stack Test 30 Day Rolling Avg Surr. Stack Test 30 Day Rolling Avg Surr. Stack Test 30 Day Rolling Avg

Factor (Count) (Count) lb/MMBtu lb/MMBtu lb/MMBtu lb/MMBtu lb/MMBtu lb/MMBtu

8069_Stk2 94% 462 3775 1.14E-06 1.01E-06 3.48E-07 3.66E-07 1.00E-08 2.42E-07

6165_STK3_MDL_applied.xlsx 84% 335 6772 1.07E-06 4.74E-07 1.10E-07 1.88E-07 1.00E-08 3.53E-08

San Juan Unit 4 90% 791 8215 1.21E-06 4.94E-07 2.58E-07 2.58E-07 1.00E-08 1.22E-07

3396_SK001 89% 283 2168 6.57E-07 2.81E-07 2.03E-07 2.22E-07 1.17E-08 1.50E-07

8069_Stk1 95% 482 3610 2.75E-06 1.36E-06 6.51E-07 7.21E-07 1.50E-08 1.89E-07

703_Scrubber Stack (2) 90% 632 4522 3.36E-06 1.68E-06 7.08E-07 5.80E-07 4.00E-08 3.10E-07

1378_SK003 90% 86 900 5.25E-06 5.33E-07 4.72E-07 5.11E-07 4.00E-08 4.72E-07

6165_STK1 91% 769 8128 7.96E-06 1.84E-06 1.15E-06 1.32E-06 4.00E-08 6.93E-07

6257_Stack 3 89% 750 7469 1.16E-05 4.78E-06 8.29E-07 1.29E-06 6.00E-08 3.67E-07

4162_STK 1 89% 363 3700 1.66E-05 1.35E-06 8.28E-07 9.91E-07 8.00E-08 4.34E-07

3644_Stck 1 90% 475 4119 5.91E-06 6.17E-07 3.82E-07 5.24E-07 9.00E-08 3.29E-07

6165_STK2 90% 550 6153 2.88E-06 2.03E-06 1.70E-06 1.60E-06 9.67E-08 8.04E-07

4162_STK 3 87% 173 1756 3.85E-06 2.24E-06 2.23E-06 1.99E-06 1.00E-07 1.39E-06

3644_Stck 2 84% 183 4274 1.87E-06 1.12E-06 4.82E-07 5.50E-07 1.05E-07 3.93E-07

703_Scrubber Stack (3) 87% 632 6341 7.72E-06 1.48E-06 9.34E-07 9.96E-07 1.27E-07 3.52E-07

1378_SK001 89% 219 2020 1.68E-05 1.92E-06 6.30E-07 6.22E-07 1.28E-07 4.00E-07

3403_SK001 79% 531 8023 8.33E-06 6.13E-06 4.92E-06 4.97E-06 1.28E-07 3.92E-06

3406_SK001 60% 20 2492 3.48E-06 2.61E-06 1.26E-06 1.13E-06 1.70E-07 3.16E-07

4162_STK 2 87% 252 3231 6.47E-06 2.25E-06 1.40E-06 1.43E-06 1.72E-07 1.28E-06

47_SK001 51% 32 2001 1.22E-06 1.01E-06 6.12E-07 8.31E-07 2.02E-07 5.93E-07

4158_STK41 89% 470 5673 7.12E-06 4.07E-06 2.72E-06 2.76E-06 3.62E-07 1.88E-06

3403_SK002 78% 280 5427 4.77E-06 3.53E-06 2.61E-06 2.88E-06 4.55E-07 1.80E-06

3399_SK001 90% 128 1426 4.12E-06 1.23E-06 8.91E-07 8.60E-07 5.08E-07 7.66E-07

6170_PPPPS2 63% 408 9365 7.36E-06 3.57E-06 3.73E-06 2.41E-06 5.39E-07 0.00E+00

50_SK001 26% 1 757 6.22E-07 2.31E-07 6.22E-07 1.61E-07 6.22E-07 1.43E-07

4041_OCPP-S13 48% 17 8513 2.72E-06 4.47E-06 1.01E-06 3.06E-06 6.43E-07 1.23E-06

3399_SK002 89% 40 543 1.64E-06 1.03E-06 9.37E-07 9.68E-07 6.92E-07 9.15E-07

6170_PPPPS1 79% 512 8495 7.35E-06 5.26E-06 5.26E-06 3.67E-06 7.08E-07 1.03E-06

50_SK003 80% 29 769 3.53E-06 1.16E-06 1.12E-06 1.14E-06 7.65E-07 1.04E-06

4158_STK43 88% 409 5560 9.70E-06 6.93E-06 5.29E-06 5.31E-06 9.07E-07 3.67E-06

4158_STK42 91% 638 5723 6.12E-06 3.31E-06 2.50E-06 2.79E-06 9.58E-07 2.08E-06

6101_STK1 94% 1135 8647 5.13E-05 1.23E-05 3.94E-06 5.45E-06 1.00E-06 2.49E-06

6761_S101 95% 1130 8111 7.45E-06 4.02E-06 2.78E-06 2.99E-06 1.09E-06 1.93E-06

8066_STK4 89% 759 6868 1.45E-05 5.49E-06 3.69E-06 4.03E-06 1.25E-06 2.28E-06

4158_STK44 84% 103 1637 8.20E-06 5.57E-06 6.06E-06 5.10E-06 1.29E-06 4.74E-06

8066_STK2 91% 770 6486 1.05E-05 4.95E-06 4.12E-06 4.04E-06 1.49E-06 2.86E-06

8066_STK3 89% 750 6732 4.21E-05 4.90E-06 3.99E-06 4.18E-06 1.81E-06 2.97E-06

8066_STK1 90% 688 6365 1.11E-05 6.10E-06 4.72E-06 4.90E-06 2.26E-06 3.63E-06

709_Shared Stack 3 4 50% 171 7794 1.41E-05 7.68E-06 7.21E-06 6.78E-06 3.21E-06 5.28E-06

Table 1 – Summary of Hg CEMS Data