MEMORANDUM SUBJECT: Comparison of 30-day and 90-day Averag ing 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 aver aging 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 re lationship 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 thei r Part II ICR submittals. These data were received from 99 sources. Review of these data indicate informati on sufficient to conduct an analysis (i.e., unit load, lb/MMBtu emission rate, and data qua lity flags present to exclude calibrations and other suspect data) was provided from 87 of the 99 sources. For this particular an alysis, only sources with sufficient data to calculate emissions for a 90-day averaging period were include d. This further reduced the sources available for analysis to 76 sources. Technical Approach Prior to analysis, the following steps were taken on th e 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 ge nerating rate recorded in the data set. These hours were not included in the calculation of dail y averages. Daily averages were calculated in accordance with Equation 19-19 from Method 19. 3. All 30-day and 90-day averaging periods calcula ted excluded startup and shutdown periods.2 The 90-day and 30-day emissions values were calculated for each source and the 99 th percentile of the 90-day and 30-day emissions value for each source was determined. The averaging period ratio (APR 30/90 ) of the 99th percentile 30-day emissions rate to the 99 th percentile 90-day emissions rate was calculated for each source according to the following equation: Averaging Period Ratio 30/90 (APR 30/90 ) = (99 t h percentile of 30-day rolling average recorded by Hg CEMS) (99 t h percentile of 90-day rolling average recorded by Hg CEMS)

The median of the 76 APR 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 

### APR 30/90 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 (99 th percentile) lb Hg/MMBtu 90 ‐ Day Avg (99 th percentile) Averaging Period Ratio, APR 30/90
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+004 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+005 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+006 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 APR 30/90 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