The Power To Make It Happen

FOUR CORNERS POWER PLANT P.O. Box 355 FRUITLAND, NEW MEXICO 87416

Page 1 of 4 

### February 18, 2010

Mr. Steven Frey USEPA Region IX

75 Hawthorne Street

Mail Code: AIR-5 San Francisco, CA 94105

Re: Mercury Monitoring and Emissions at Four Corners Po wer Plant

Dear Mr. Frey:

This is in response to your telephone request dated February 4, 2010 for information on mercury emissions from the Four Corners Power Plant -- spec ifically data collected using continuous emissions monitoring systems (CEMs). You also aske d for data on the different chemical forms of mercury emitted from the Plant and for the quali ty control measures used in collecting the mercury data. Given below are the requested data a nd our assessment of mercury emission control resulting from existing pollution control d evices.

Background

The Four Corner Power Plant (Plant) consists of 5 u nits with a combined generating capacity of 2,060 MW. Units 4 & 5 (750 MW, each) are equipped with bag-houses and SO 2 scrubbers. Units 1 and 2 (170 MW, each) and Unit 3 (220 MW) us e venturi scrubbers to remove SO 2 and particulates.

Coal-fired power plants emit small amounts of mercu ry because mercury is a trace element in coal. These mercury emissions occur in three forms: elemental, particulate, and ionized. The relative proportions of these fractions depend on t he chlorine-content of the coal and existing pollution control devices. Ionized mercury is wate r soluble (and is easily removed in SO 2 scrubbers); particulate mercury is also easily capt ured in bag-houses/venturi scrubbers, while gaseous elemental mercury passes through convention al pollution control equipment. Ionized and particulate mercury released from the stack pre cipitate out of the atmosphere within a few hundred miles from the stack via wet and dry deposi tion processes, while elemental mercury travels farther (thousands of miles) before precipi tating out of the atmosphere.

As you are aware, the U.S. Environmental Protection Agency (EPA) finalized the Clean Air Mercury Rule (CAMR) in 2006. It called for annual, national, mercury emissions caps for the electric utility sector. It also provided for a me rcury cap & trade program and a mercury emissions monitoring program, including the use of CEMs. In February 2008, the Court of Appeals for the District of Columbia Circuit vacate d CAMR.The Power To Make It Happen

FOUR CORNERS POWER PLANT P.O. Box 355 FRUITLAND, NEW MEXICO 87416

Page 2 of 4 During 2007-2009, APS began a process to procure, i nstall, and performance test mercury CEMs. APS installed Thermo Fisher Mercury Freedom S ystems mercury CEMs on Units 1-2 and Units 4-5 in 2008. APS completed the initial ce rtification tests of those monitors and collected emissions data from late February 2009 to early September 2009. APS decided not to install mercury CEMs on Unit 3 because CAMR was vac ated in February 2008.

The Mercury Freedom System is a dilution-based CEMs , consisting of an ionic and particulate mercury converter (in the probe) and a cold vapor a tomic fluorescence (CVAF) elemental mercury analyzer. The CEMs were operated in specia tion mode—continuously switching between total mercury and elemental mercury measure ments. Ionic and particulate mercury

were calculated from the difference between the tot al and elemental mercury measurements.

Initial CEMs Certification and Quality-Assurance Te sting

Initial certification testing, per 40 C.F.R. §75 pe rformance specifications, was conducted in late February and early March 2009. The following initi al certification tests were performed:

• Seven-day calibration error test

• Three-level converter efficiency (system integrity) check

• Linearity check

• Relative accuracy test audit (RATA), via reference method 30B, and bias test

• Cycle time test The following quality-assurance tests, also per 40 C.F.R. §75 performance specifications, were conducted during the monitoring period (late Februa ry to early September 2009):

• Daily calibration error tests

• Weekly (168 hours) converter efficiency checks

• Quarterly linearity checks (except for third quarte r 2009)

Hourly average data were validated per 40 C.F.R. §7 5 requirements. However, missing data substitution and percent monitor availability (PMA) were not utilized, due to data acquisition and handling system (DAHS) limitations.

The NIST-traceability of the mercury calibrators an d mercuric chloride generators was never determined. Brief sorbent trap testing on the calib rators showed differences of 0.02-0.85 μ g/scm

from the reference method.

The converter efficiency test performance specifica tions in 40 C.F.R. §75 were ambiguous. Four Corners followed the interpretations of Thermo Fish er regarding this issue.

The monitoring systems were plagued by a variety of problems, which may have impacted data quality and/or quantity. Such problems include the following:

• Analyzer lamp instability, failures, and week-long “burn-in” times

• Poor (low) converter efficiency check results

• Plugged and/or contaminated probe filtersThe Power To Make It Happen

FOUR CORNERS POWER PLANT P.O. Box 355 FRUITLAND, NEW MEXICO 87416

Page 3 of 4 • Excessive calibration drift

These problems may have contributed to the scatter in the data included below. However, we have not attempted to analyze the data to verify th at.

Emissions Data

The average hourly mercury emission rates (in lbs/T Btu) are shown in Table 1, based on qualityassured hourly average mercury and heat input data collected from late February to early September 2009.

Table 1: Total mercury mass emission rate ± 1 stand ard deviation (lbs/TBtu)

Unit 1 Unit 2 Unit 4 Unit 5

9.173 ± 2.621 * 7.829 ± 2.612 * 1.149 ± 0.501 * 0.763 ± 0.417 *

• The standard deviation was calculated using data f rom the entire period of CEMs operation. Different standard deviations would likely result if shorter periods (e.g. monthly) of mercury CEMs data were ch osen for analysis.

Estimated percentages of ionic mercury to elemental mercury (Hg 2+ /Hg 0 ) are shown in Table 2. The percentages are based on averages of one-minute total, ionic, and elemental mercury data collected from late July to early September 2009.

Table 2: Estimate ionic mercury/elemental mercury r atio (%)

Unit 1 Unit 2 Unit 4 Unit 5

8.3/91.7 1.7/98.3 29.0/71.0 21.0/79.0

Based on data shown in Tables 1 and 2 and total hea t input for 2009, we calculated the elemental, ionic, and total mercury emissions from the four Units as shown in Table 3 below.

Table 3: Estimated mercury mass emissions (lbs) in 2009

Unit 1 2 4 5

Ionic Hg 10.77 1.98 17.73 8.78

Elemental 119.00 114.28 43.41 33.02

Total Hg 129.77 116.26 61.14 41.80

Existing pollution control technologies for Units 1 , 2, and 3 are the same. Therefore, we used the average of the measured values for Units 1 and 2 to estimate the emission rate for Unit 3 (8.501 lbs/TBtu). Similarly, we estimated mercury e missions from Unit 3 during 2009 to beThe Power To Make It Happen

FOUR CORNERS POWER PLANT P.O. Box 355 FRUITLAND, NEW MEXICO 87416

Page 4 of 4 168.21 lbs. Accordingly, the plant-wide total merc ury emissions in 2009 are estimated to be

517.18 lbs.

Mercury Removal Rate

Mercury removal rates were estimated via a mass-bal ance equation, using CEMs and coal analysis data. The average daily mercury removal r ates are shown in Table 4, based on 11 to 19 days.

Table 4: Average daily mercury removal rate (%)

Unit 1 Unit 2 Unit 3 Unit 4 Unit 5

20.3 28.8 24.5 89.2 93.6

Of the 517.18 lbs of mercury, 90.8% (469.51 lbs) is estimated to be in the elemental form, based on data in Table 3 and an estimated Hg 2+ /Hg 0 ratio of 5.0%/95.0% (average of U1&2 from Table 2) for Unit 3.

Conclusions

Based on the data presented here, APS estimates the plant-wide mercury emissions for 2009 to

be about 517 lbs. The data also show that over 90% of those emissions were in the form of elemental mercury.

The bag-houses and scrubbers on Units 4-5 have a re moval efficiency of approximately 90% based on mercury measured in the coal. Unit 1-2-3 have a removal rate of around 25% based on mercury measured in the coal.

The data collected on Units 4-5 were relatively con sistent. Data from Units 1-2 were more variable. This could be due to the difficult appli cation of trace mercury CEMs on wet stacks. It could also be due to actual data variability.

While we stand by the data presented above, it must be noted that this is a trace analysis and inherently such monitoring has a potentially large margin of error. More accurate monitoring technology and trending data will be required befor e we can make a commitment that the removal capacity stated herein is sustainable from either a regulatory or permitting perspective.

If you have any questions, please contact me.

Sincerely,

Richard Grimes