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1 MEMORANDUM

SUBJECT: Determination of Representative Detec tion Level (RDL) and 3 X RDL Values for Mercury Measured Using Sorbent Trap Technologies

FROM: Conniesue B. Oldham, Ph.D., Group Leader Measurement Technology Group, AQAD

TO: Bob Schell, Group Leader Measurement Policy Group, SPPD

This memorandum describes the process that was used to develop a representative detection level (RDL) as well as 3 X RDL values for merc ury measured using sorbent trap technology. We based these values on information in the record concerning the best performing electric utility steam generating units (EGUs) designed for coal w ith a heating value greater than or equal to 8,300 BTU/lb. As explained further below, we have developed the RDL and 3XRDL values assuming short sampling periods so that comp liance monitoring methods could provide timely data for operation of mercury control systems.

General Method for Determining RDLs

As explained in the note from Peter Wes tlin and Raymond Merrill to SPPD Management and MACT Rule Writers, we determine the repres entative detection level (RDL) based on the average of the reported pollutant specific me thod detection levels (MDLs) for the best performing units (those in the floor); we consider the resulting average MDL representative or characteristic of acceptable performance of source emissions testing and thus an RDL. 1 We use a multiplication factor of three with the RDL to in crease the RDL pollutant concentration to a level where, when measured by the compliance te st method, the precision of the test method approximates that of other EPA methods as estimated by the ASME study, nominally 10 to 20 percent relative standard deviation. 2,3 This three times the RDL (3 X RDL) value expressed in units of the emission standard is then compared to the MACT floor value to ensure that the resulting emission limit is in a range that can be measured with reas onable precision. In other

1 Note from Peter Westlin and Raymond Merrill to SPPD Management and MACT Rule Writers “Data and procedure for handling below detectio n level data in analyzing various pollutant emissions databases for MACT and RTR emissions limits,” revised April 5, 2012.

2 (ReMAP): PHASE 1, Precision of Manual Stack Emission Measurements; American Society of Mechanical Engineers, Research Committee on I ndustrial and Municipal Waste, February 2001.

3 The factor of three used in the 3XRDL calcu lation is based on a scientifically accepted definition of level of quantitation – simply stat ed, the level where a test method performs with acceptable precision. The level of quantitation ha s been defined as ten times the standard deviation of seven replicate anal yses of a sample at a concen tration level close to the MDL (which translates to approximately three times the MDL which is defined as three times the standard deviation of seven replic ate analyses of a sample at a concentration level close to the MDL (40 CFR Part 136, Appendix B)).

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2 words, if the 3 X RDL value were less than the calculated floor or emissions limit (e.g., calculated from the UPL), we woul d conclude that measurement variability has been adequately addressed; if it were greater than the calculate d floor or emissions limit, we would adjust the emissions limit to comport with the 3 X RD L value to address measurement variability.

Concerns Raised By Petitioners

With regard to the mercury emission limit for ne w EGUs designed for coal with a heating value greater than or equal to 8,300 BTU/lb, some petitioners asserted that the frequency of measurement contemplated for the sorbent trap monitoring system option in the final rule may not provide sufficiently timely feedback for operation of EGUs and mercury control systems.

The petitioners also suggest that the mercury emission limit for new EGUs designed for coal with a heating value greater than or equal to 8,300 BTU/lb be adjusted upward to allow for compliance monitoring using Hg CEMS, which woul d provide the frequency of feedback needed to operate the process and control systems.

We understand the petitioners’ concerns regard ing timely feedback on process and control systems. In response to these concerns, EPA ev aluated the sorbent trap monitoring systems and assessed whether these systems could be utili zed over the shorter tim e periods petitioners maintain are needed to provide more timely data. 4 In promulgating the MATS rule, we thought that most facilities would opt for 1 to 2 w eek sampling periods when operating sorbent trap monitoring systems, but we now understand the n eed for more frequent data necessitating much shorter sampling periods. With shorter sampling periods appro aching the frequency of CEMS, comes a need to examine the MDL/RDL for sorb ent trap-based mercury measurements which was more than adequate for the sampling periods originally contemplated for sorbent trap monitoring systems. Though we didn’t use the sorb ent trap monitoring syst ems to collect data under the ICR, the technology used in the sorbent tr ap monitoring systems is identical to that of the short term method for mercury, Method 30B a nd, thus, it is reasonable to rely on the MDL information provided for the Method 30B testi ng conducted under the ICR to develop an RDL value for the sorbent trap monitoring systems.

A few petitioners raised an issue with Hg CEMS, suggesting that we adjust the limit to allow the use of Hg CEMS to demonstrate compliance. Ho wever, part III of the ICR required testing for mercury using Method 30B, a sorbent trap-based measurement method and, thus, there were no Hg CEMS data produced as a re sult of the ICR testing require ments. Though some Hg CEMS data for mercury were submitted under the ICR, none of the best performing units in the floor provided us with any Hg CEMS data. This m eant that for the MACT floor, we had no CEMS data that we could use to establish the level at which measurement precision for the Hg CEMS would be in a range similar to th at of other EPA test methods; therefore, we had no way to adjust the new source emissions limit ba sed on the use of Hg CEMS.

4 We note that we continue to believe that the fi nal Hg limit can be measured using a sorbent trap monitoring system, but we acknowledge the monitoring frequency concern raised in reconsideration petitions.

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3 Determining RDLs for Sorbent Traps

We reviewed all available Method 30B sorbent tr ap testing and laboratory information for each EGU contained in the pool of best performing units designed for coal with a heating value greater than or equal to 8,300 BT U/lb and collected the reported method detection level for each EGU. As shown in Table 1 below, 24 out of 43 best performing EGUs reported MDL values. We calculated the arithmetic average of these ava ilable MDL values to yield the representative detection level (RDL).

The calculated average RDL of 1.1 nanograms of mercury was conve rted into the format of a new source emissions limit in pounds per giga watt-hour (lb/GWh) by making appropriate unit conversions, dividing by the sampling rate in lite rs per minute (L/min), dividing by the sampling collection time in minutes (min), multiplying by a representative heat rate (11,180 BTU/KWh), 5 and multiplying by 1.40 (a factor correcting the oxygen concentration to 6 percent). That product, when multiplied by 3 (in order to obta in the 3XRDL value) and rounded up, is 3.0E-03 lb/GWh.

The calculation is as follows:

3ܴܺܮܦ ൌ 3 כ ቈ ቂቀ ଵ.ଵ ௡௚ · ௠௜௡

଴.ହ ௅ ቁ ቀ ଵଵ,ଵ଼଴ ஻்௎

ଷ଴ ୫୧୬ · ௄ௐ௛ ቁ ሺ 1.40 ሻ ቃ ቀ ଺.ଵଵாି଴଻ ௟௕ · ௅ · ௄ௐ௛ ௡௚ · ஻்௎ · ீௐ௛ ቁ ቉

,

and, when rounded, = 3.0ܧ െ 03 ௟௕

ீௐ௛ .

A sampling rate of 0.5 L/min was used in the cal culation, and it is based on an informal survey of vendor information on the normal sampling rate s for commercially available sorbent trap monitoring systems. To provide f acilities with monitoring data as timely as that for Hg CEMS, we first considered use of a 15-minute sampling pe riod with the sorbent trap monitoring systems. However, about 10 minutes are required to change out and leak check each new pair of sorbent tubes, leaving only 5 minutes of actual sample collection time to represent each 15-minute sampling period, so a 15-minute period was deemed inappropriate. We settled on a 30-minute sampling period (comprised of a 10-minute sorb ent tube change out and a 20-minute sample collection period) as a reasonable compromise.

5 This is the average heat rate of the units used to determine the RDL value.

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4 Facility ID EGU MDL, nanograms 3130 Unit 1 0.293 3130 Unit 2 0.293 7213 Unit 1 0.81 10043 Boiler 1 0.81 10143 Boiler 1 0.293 10566 Boiler 1 0.81 10566 Boiler 2 0.81 10603 31 0.172 10671 Unit 1a 2.4 10671 Unit 1b 2.4 10671 Unit 2a 2.4 10671 Unit 2b 2.4 10673 Boiler A 2.4 10673 Boiler B 2.4 10678 Boiler 1 2.4 50776 Boiler 1 0.25 50888 Boiler 1 0.81 50974 Boiler 1 0.174 50974 Boiler 2 0.174 50976 B1 0.062 54081 Unit 1 0.52 54081 Unit 2 0.81 54081 Unit 3 0.81 54081 Unit 4 0.81 Average (or RDL) 1.1

Table 1. Mercury Sorbent Trap Method Det ection Levels for Best Performing EGUs