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 Power 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 -- specifically data collected using continuous 

emissions monitoring systems (CEMs). You also asked for data on the different chemical forms 

of mercury emitted from the Plant and for the quality control measures used in collecting the 

mercury data. Given below are the requested data and our assessment of mercury emission 

control resulting from existing pollution control devices. 

Background 

The Four Corner Power Plant (Plant) consists of 5 units with a combined generating capacity of 

2,060 MW. Units 4 & 5 (750 MW, each) are equipped with bag-houses and SO2 scrubbers. 

Units 1 and 2 (170 MW, each) and Unit 3 (220 MW) use venturi scrubbers to remove SO2 and 

particulates. 

Coal-fired power plants emit small amounts of mercury 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 the chlorine-content of the coal and existing 

pollution control devices. Ionized mercury is water soluble (and is easily removed in SO2

scrubbers); particulate mercury is also easily captured in bag-houses/venturi scrubbers, while 

gaseous elemental mercury passes through conventional pollution control equipment. Ionized 

and particulate mercury released from the stack precipitate out of the atmosphere within a few 

hundred miles from the stack via wet and dry deposition processes, while elemental mercury 

travels farther (thousands of miles) before precipitating 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 mercury 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 vacated 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, install, and performance test mercury 

CEMs. APS installed Thermo Fisher Mercury Freedom Systems mercury CEMs on Units 1-2 

and Units 4-5 in 2008. APS completed the initial certification 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 vacated 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 atomic fluorescence (CVAF) elemental 

mercury analyzer. The CEMs were operated in speciation mode—continuously switching 

between total mercury and elemental mercury measurements. Ionic and particulate mercury 

were calculated from the difference between the total and elemental mercury measurements. 

Initial CEMs Certification and Quality-Assurance Testing 

Initial certification testing, per 40 C.F.R. §75 performance specifications, was conducted in late 

February and early March 2009. The following initial 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 February to early September 2009): 

• Daily calibration error tests 

• Weekly (168 hours) converter efficiency checks 

• Quarterly linearity checks (except for third quarter 2009) 

Hourly average data were validated per 40 C.F.R. §75 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 and mercuric chloride generators was never 

determined. Brief sorbent trap testing on the calibrators showed differences of 0.02-0.85 μg/scm 

from the reference method. 

The converter efficiency test performance specifications in 40 C.F.R. §75 were ambiguous. Four 

Corners followed the interpretations of Thermo Fisher 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 filters 
The 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 that. 

Emissions Data 

The average hourly mercury emission rates (in lbs/TBtu) 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 standard 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 from the entire period of CEMs operation. Different 

standard deviations would likely result if shorter periods (e.g. monthly) of mercury CEMs data were chosen for analysis. 

Estimated percentages of ionic mercury to elemental mercury (Hg2+/Hg0) 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 ratio (%) 

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 heat 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 emissions from Unit 3 during 2009 to be 
The 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 mercury emissions in 2009 are estimated to be 

517.18 lbs. 

Mercury Removal Rate 

Mercury removal rates were estimated via a mass-balance equation, using CEMs and coal 

analysis data. The average daily mercury removal rates 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 Hg2+/Hg0 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 removal 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 consistent. Data from Units 1-2 were more 

variable. This could be due to the difficult application 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 before 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