GEORGIA POWER – Addendum to Supplemental Mercury CEMS Data Submittal

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Electric Utility Steam Generating Unit (EGU) Hazardous Air Pollutant Emissions 

(HAPs) Information Collection Request (ICR), OMB Control No. 2060-0631

May 3, 2010

In the information collection request (“ICR”) sent to Southern Company by the 

Environmental Protection Agency (“EPA”) on December 24, 2009, EPA requested in 

Footnote 46 to Part II, Question 11 that facilities submit available mercury continuous 

emission monitoring system (“CEMS”) data that follow recognized QA/QC procedures. 

While EPA did not make the submittal mandatory, Georgia Power wishes to provide 

available mercury CEMS data to assist the rulemaking process. Georgia Power has 

installed and operated monitors on certain units that, as closely as possible, follow the 

QA/QC procedures as written under the vacated Clean Air Mercury Rule (“CAMR”)

through 40 CFR Part 75. However, Georgia Power does not regularly submit reports to a 

regulatory agency on an ongoing basis. Thus, we required additional time to review and 

prepare the data for submittal to EPA. 

For units with wet flue gas desulfurization (FGD) control technology installed, flue gas 

may exist through either a wet or a dry stack, depending on whether the FGD is in 

operation. In these cases, the mercury CEMS is installed on the wet stack only. 2009 

Mercury CEMS data are available for the following units and stacks:

Bowen Unit 2 FGD Stack Liner

Bowen Unit 3 FGD Stack Liner

Branch Units 3/4 Common Stack Liner

Hammond Units 1-4 Common FGD Stack

Scherer Unit 3 Stack Liner

Wansley Unit 2 FGD Stack Liner

The mercury CEMS data show that measured mercury emissions are highly variable, 

even on units operating emission controls. Furthermore, our experience with the mercury 

monitors has shown that monitor reliability and performance can also vary greatly unit to 

unit or over time. The supplemental mercury monitoring data, along with description of 

concerns and issues we have experienced with the monitors are included in this submittal.

Mercury CEMS Data Variability

The mercury CEMS data provided show that measured mercury emissions exhibit great 

variability over time. For example, even after removal of invalid data, the hourly CEMS 

data from Branch Units 3/4, which operates a cold-side electrostatic precipitator (ESP),

ranges from around 1 lb/TBtu to over 17 lbs/TBtu. Units that are further controlled 

exhibit wide variability as well. For example, the hourly CEMS data from Bowen Unit 3, 

which operates an FGD and SCR as well as the ESP, ranges from close to zero to about 9

lb/TBtu. For a given unit, a particular mercury level may be “achieved” at some point, 

but may not be “achievable” on a consistent basis. Emission standards set through the 

rulemaking process must account for this inherent variability of mercury emissions. 
GEORGIA POWER – Addendum to Supplemental Mercury CEMS Data Submittal

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Mercury CEMS Monitor Reliability

Based on our experience, mercury monitors installed on different units may behave very 

differently in terms of reliability. Even monitors installed on sister units in terms of 

boiler configuration and controls have not always performed consistently. At this point 

in time, we are not able to pinpoint the reasons for these differences. We believe further 

development of the technology is needed to provide for long term consistent and reliable

performance. Monitor availability for the six systems being reported ranged from 74.5% 

to 90.9% (with an average of 83.4%) for 2009. 

One of the factors affecting monitor availability is the recovery time for the mercury 

monitoring system following certain tests, QA procedures, and maintenance. There is 

often an extended period where readings are obviously abnormal and inaccurate and must 

be marked invalid. The recovery time before readings return to normal is often 12 hours

or longer. We also experience variable performance and life from lamps and calibrators. 

CAMR Part 75 QA may reasonably assure consistency of monitored values, but there is 

still no assurance of the accuracy of monitored values. Part 75 QA procedures were 

based on the capabilities of current monitoring systems and established for known 

achievable levels of mercury for use in a trading program. Therefore, the allowed 

calibration tolerance of Part 75 at very low levels of mercury emissions could result in 

significant discrepancies, including negative values or measured emissions that are more 

than double actual emissions. However, tightening this tolerance with current monitoring 

systems would be require more frequent maintenance and calibration which would lead to 

more monitor downtime. Running in speciated mode causes additional problems 

including contamination of the analyzer and umbilicals and unreliable mercury readings. 

Thus, monitors should measure total mercury only for best results.

Mercury CEMS Data Assumptions and Procedures

The mercury CEMS data are provided to demonstrate mercury variability. The QA 

procedures as proposed in 40 CFR Part 75 were followed with one exception. We are not

currently following the Interim Protocol for NIST traceablity for the monitoring system 

calibrators. Many of these calibrators were included in the NIST traceability study used 

to develop the Interim Protocol, and during the study, they were found to be generally 

accurate and stable. Since that time, however, we have had to repair several calibrators; 

thus measured values may have significant error. We have, however, successfully passed

annual RATAs using Method 30B on all of the units.

The submitted data includes valid data only for total mercury. Out-of-control and 

substituted data are not included, except in cases where values of 5% are substituted 

where measured CO2 values were below 5%, in accordance with Part 75 procedures. 

Negative emission rates are considered invalid data and are not included. Only hourly 

data in units of lbs/TBtu are provided. Since the spreadsheet template provided by EPA 

only includes the option for units of lbs/MMBtu, we have inserted an additional column 

with the lbs/TBtu data.