MEMORANDUM 

SUBJECT: Comparison of 30-day and 90-day Averaging Periods for CEMS Data Submitted Under 

Part II of the 2010 MACT ICR for EGUs 

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

TO: Bill Maxwell, EPA, SPPD, Energy Strategies Group (ESG) 

DATE: December 15, 2011 

Summary of Technical Directive

EPA requested RTI evaluate the impact of a 90-day averaging period on the mercury (Hg) emissions limit for 

application to emissions averaging for multiple units. In other words, for the Hg emission limit of 1.2 x 10-6

lbs Hg/MMBtu on a 30-day averaging period basis, what is the equivalent emissions limit on a 90-day 

averaging period basis? 

Result

We used available ICR Part II Hg CEMS data to evaluate the relationship between 30-day and 90-day 

emissions levels and applied this relationship to the 30-day emissions limit to derive the equivalent 90-day 

emissions limit. The result is an Hg emission limit of 1.0 x 10-6 lbs Hg/MMBtu for a 90-day averaging 

period. 

Background

Some facilities provided historical CEMS data with their Part II ICR submittals. These data were received 

from 99 sources. Review of these data indicate 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 87 of the 99 sources. For this particular analysis, only sources with sufficient data to calculate 

emissions for a 90-day averaging period were included. This further reduced the sources available for 

analysis to 76 sources. 

Technical Approach

Prior to analysis, the following steps were taken on the data set for each unit to prepare the data set for 

analysis:

1. For each unit, each hour of data was evaluated to determine the operating load (gross MW) and to 

determine if zero emissions were reported for the hour. Hours of zero reported emissions were 

excluded from the calculation of daily averages. 

2. Hours were classified as startup or shutdown periods if the gross megawatts recorded for the hour 

were less than 5 percent of the maximum hourly generating rate recorded in the data set. These 

hours were not included in the calculation of daily averages. Daily averages were calculated in 

accordance with Equation 19-19 from Method 19.

3. All 30-day and 90-day averaging periods calculated excluded startup and shutdown periods. 
2

The 90-day and 30-day emissions values were calculated for each source and the 99th percentile of the 90-day 

and 30-day emissions value for each source was determined. The averaging period ratio (APR30/90) of the 

99th percentile 30-day emissions rate to the 99th percentile 90-day emissions rate was calculated for each 

source according to the following equation: 

Averaging Period Ratio30/90 (APR30/90) = (99th

 percentile of 30-day rolling average recorded by Hg CEMS) 

(99th

 percentile of 90-day rolling average recorded by Hg CEMS) 

The median of the 76APR values in the data set was calculated to be 1.15. 

The Hg emissions limit of 1.14 x 10-6 lbs Hg/MMBtu (before rounding to 1.2 x 10-6 lbs Hg/MMBtu) on a 30-

day averaging period basis was divided by the median APR30/90 to determine the emissions limit on a 90-day 

averaging period basis. The resulting value is 1.0 x 10-6 lbs Hg/MMBtu for a 90-day averaging period. 

Table 1 presents a summary of the data. 
3

TABLE 1. Summary of Maximum 30-day and 90-day CEMS data by Facility Unit 

Facility

(fac_stack)

Max_

MW

CEMS Hr

Count for

Unit

lb Hg/MMBtu

30‐Day Avg

(99th percentile)

lb Hg/MMBtu

90‐Day Avg

(99th percentile)

Averaging

Period Ratio,

APR30/90

1 130_C1

            

643  

                   

25,439   9.34E‐07 6.36E‐07 1.47E+00

2 6165_STK3

            

495  

                     

6,772   5.78E‐07 4.04E‐07 1.43E+00

3 8069_Stk2

            

474  

                     

4,837   1.05E‐06 6.20E‐07 1.70E+00

4 2451_Unit 2 Stack

            

377  

                     

3,615   7.44E‐07 5.22E‐07 1.43E+00

5 6076_1STACK

            

352  

                     

6,770   4.04E‐06 3.53E‐06 1.15E+00

6 8069_Stk1

            

472  

                     

3,759   1.35E‐06 9.67E‐07 1.39E+00

7 3396_SK001

            

906  

                     

2,259   2.79E‐07 2.22E‐07 1.26E+00

8 2451_Unit 4 Stack

            

548  

                     

8,697   4.78E‐07 3.59E‐07 1.33E+00

9 56224_STK1

            

241  

                   

11,691   1.85E‐06 1.57E‐06 1.18E+00

10 2451_Unit 3 Stack

            

546  

                     

7,127   1.28E‐06 9.97E‐07 1.29E+00

11 130_C2

            

621  

                   

25,742   1.45E‐06 1.25E‐06 1.16E+00

12 2451_Unit 1 Stack

            

377  

                     

4,159   7.96E‐07 7.25E‐07 1.10E+00

13 3644_Stck 2

            

113  

                     

4,873   1.08E‐06 1.01E‐06 1.07E+00

14 1554_Stack 2

            

139  

                     

6,226   1.16E‐06 8.29E‐07 1.40E+00

15 703_Scrubber Stack (2)

            

773  

                     

4,522   1.67E‐06 1.16E‐06 1.44E+00

16 3644_Stck 1

               

75  

                     

4,745   6.09E‐07 5.64E‐07 1.08E+00

17 1378_SK003

         

1,089  

                     

925   5.32E‐07

18 4162_STK 1

            

174  

                     

3,791   1.34E‐06 1.18E‐06 1.13E+00

19 1552_Stack 1

            

204  

                     

4,833   2.87E‐06 1.90E‐06 1.52E+00

20 1552_Stack 2

            

208  

                     

4,222   1.88E‐06 1.13E‐06 1.66E+00

21 3_ESP (4A) Exit

            

377  

                     

3,090   4.93E‐06 2.40E‐06 2.05E+00
4

22 4162_STK 3

            

356  

                     

2,267   2.27E‐06

23 6257_Stack 3

            

909  

                     

7,487   4.62E‐06 3.29E‐06 1.40E+00

24 1378_SK001

            

662  

                     

2,021   1.80E‐06 9.87E‐07 1.83E+00

25 6165_STK2

            

461  

                     

6,305   2.03E‐06 1.90E‐06 1.07E+00

26 3406_SK001

         

1,098  

                     

2,509   2.60E‐06 1.62E‐06 1.60E+00

27 3403_SK001

            

489  

                     

8,048   6.01E‐06 5.42E‐06 1.11E+00

28 6165_STK1

            

460  

                     

8,273   1.82E‐06 1.50E‐06 1.21E+00

29 47_SK001

            

736  

                     

2,002   1.26E‐06 1.01E‐06 1.24E+00

30 703_Scrubber Stack (3)

            

949  

                     

6,354   1.46E‐06 1.24E‐06 1.17E+00

31 4162_STK 2

            

231  

                     

3,558   2.21E‐06 1.69E‐06 1.31E+00

32 6193_Stack 3

            

380  

                     

1,884   6.04E‐06

33 6194_2St

            

570  

                     

5,733   8.88E‐06 7.59E‐06 1.17E+00

34 1570_9B

               

28  

                     

709   2.47E‐06

35 1606_STK1

            

159  

                   

13,401   2.62E‐06 2.35E‐06 1.12E+00

36 3399_SK001

         

1,309  

                     

1,436   1.22E‐06

37 3403_SK002

            

556  

                     

5,427   3.43E‐06 3.12E‐06 1.10E+00

38 26_FGD Stack 5

            

905  

                     

5,700   3.85E‐06 3.45E‐06 1.12E+00

39 4041_OCPP‐S13

            

529  

                     

9,481   4.88E‐06 4.20E‐06 1.16E+00

40 50_SK001

            

344  

                     

779   4.44E‐07

41 3845_FGDSSTACK2

            

726  

                     

4,961   4.83E‐06 4.32E‐06 1.12E+00

42 2187_Stack1

            

166  

                     

7,451   6.48E‐06 5.87E‐06 1.10E+00

43 1554_Stack 3

            

319  

                     

8,760   5.50E‐06 5.16E‐06 1.07E+00

44 6076_2STACK

            

338  

                     

8,144   3.32E‐06 2.93E‐06 1.13E+00
5

45 6170_PPPPS2

            

650  

                     

9,504   3.61E‐06 3.43E‐06 1.05E+00

46 4158_STK41

            

113  

                     

5,698   4.06E‐06 3.32E‐06 1.22E+00

47 50_SK003

            

473  

                     

769   1.16E‐06

48 1570_9A

               

28  

                     

803   2.39E‐06

49 6193_Stack 2

            

373  

                     

7,240   2.46E‐05 1.08E‐05 2.28E+00

50 6170_PPPPS1

            

651  

                     

9,504   5.47E‐06 4.63E‐06 1.18E+00

51 4050_ESP Outlet 4

            

307  

                     

6,518   4.27E‐06 3.68E‐06 1.16E+00

52 4158_STK43

            

246  

                     

5,562   6.89E‐06 6.25E‐06 1.10E+00

53 4050_Stack ‐ 5

            

429  

                     

7,313   7.28E‐06 6.70E‐06 1.09E+00

54 6076_3STACK

            

812  

                     

6,974   5.50E‐06 5.23E‐06 1.05E+00

55 4158_STK42

            

115  

                     

5,727   3.29E‐06 2.96E‐06 1.11E+00

56 3845_FGDSSTACK1

            

732  

                     

4,219   3.98E‐06 3.58E‐06 1.11E+00

57 6761_S101

            

308  

                     

8,111   3.98E‐06 3.54E‐06 1.13E+00

58 4158_STK44

            

368  

                     

1,675   5.61E‐06

59 6002_Stack 4

            

725  

                     

8,265   6.73E‐06 5.51E‐06 1.22E+00

60 3_FGD Stack 5

            

773  

                     

2,025   4.55E‐06

61 6101_STK1

            

387  

                     

8,740   1.53E‐05 9.29E‐06 1.65E+00

62 8066_STK4

            

577  

                     

6,935   5.47E‐06 4.96E‐06 1.10E+00

63 6002_Stack 2

            

730  

                     

9,040   6.80E‐06 6.29E‐06 1.08E+00

64 1048_Unit 2 Stack

            

224  

                     

6,245   7.78E‐06 6.77E‐06 1.15E+00

65 6076_4STACK

            

819  

                     

2,579   5.49E‐06 4.60E‐06 1.19E+00

66 8_Shared Stack 8 9 10

         

1,078  

                     

8,393   6.36E‐06 3.35E‐06 1.90E+00

67 8066_STK2

            

571  

                     

6,779   4.95E‐06 4.42E‐06 1.12E+00
6

68 3181_Stack 4

            

307  

                   

10,945   1.18E‐05 6.94E‐06 1.69E+00

69 8066_STK3

            

570  

                     

6,749   4.98E‐06 4.61E‐06 1.08E+00

70 6002_Stack 3

            

726  

                     

8,919   6.73E‐06 5.98E‐06 1.13E+00

71 3_Shared Stack 1 2 3

            

548  

                     

8,728   7.89E‐06 7.14E‐06 1.11E+00

72 6194_1St

            

565  

                     

6,128   6.89E‐06 6.05E‐06 1.14E+00

73 4054_U1U2 CS Stack

            

232  

                     

7,588   4.11E‐06 3.50E‐06 1.17E+00

74 8066_STK1

            

570  

                     

6,419   6.22E‐06 5.87E‐06 1.06E+00

75 6002_Stack 1

            

720  

                     

9,096   7.06E‐06 6.30E‐06 1.12E+00

76 709_Shared Stack 3 4

         

1,027  

                     

7,794   7.59E‐06 7.16E‐06 1.06E+00

77 1104_Stack ‐ 1

            

216  

                     

7,021   7.16E‐06 6.69E‐06 1.07E+00

78 26_Shared Stack 1 2

            

543  

                   

12,081   2.14E‐05 1.78E‐05 1.21E+00

79 6254_Stack‐1

            

771  

                     

7,021   7.78E‐06 7.55E‐06 1.03E+00

80 10_Shared Stack 1 2

            

534  

                     

7,267   2.54E‐05 2.39E‐05 1.06E+00

81 8023_Unit 2 Stack

            

596  

                     

6,835   9.32E‐06 8.95E‐06 1.04E+00

82 1047_Unit 4 Stack

            

261  

                     

6,228   1.06E‐05 9.48E‐06 1.11E+00

83 8023_Unit 1 Stack

            

595  

                     

6,979   1.01E‐05 9.70E‐06 1.04E+00

84 8_Shared Stack 6 7

            

215  

                     

751   2.09E‐05

85 3178_Stack_2

            

189  

                     

1,717   9.20E‐06 7.78E‐06 1.18E+00

86 7_Shared Stack 1 2

            

106  

                     

9,427   2.68E‐05 2.35E‐05 1.14E+00

87 26_Shared Stack 3 4

            

540  

                     

5,108   6.04E‐05 3.58E‐05 1.69E+00

Median APR30/90 1.15E+00

Hg limit, 30‐day (before rounding) 1.14E‐06

Hg limit, 90‐day   9.93E‐07

Hg limit, 90‐day (rounded) 1.00E‐06