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TECHNICAL MEMORANDUM 

TO: Tina Ndoh, U.S. EPA/OAQPS/SPPD - Minerals and Manufacturing Group 

FROM: Eastern Research Group, Inc.

DATE: June 9, 2015

SUBJECT: Maximum Achievable Control Technology (MACT) Floor Analysis for 

Phosphate Rock Calciners at Phosphoric Acid Manufacturing Plants – Final Rule

1.0 INTRODUCTION 

This memorandum presents the methodology and results of the maximum achievable 

control technology (MACT) floor determinations for mercury (Hg) and total fluoride for 

phosphate rock calciners in the final rule for phosphoric acid manufacturing plants (40 CFR part 

63, subpart AA). The proposed rule included emissions limits for hydrogen fluoride, instead of 

total fluoride, but these limits were not finalized. Total fluoride emission limits, as a surrogate 

for hydrogen fluoride, were determined; see the preamble for discussion regarding total fluoride 

versus hydrogen fluoride limits for phosphoric acid manufacturing plants. The organization of 

this memo is as follows: 

2.0 Summary of Existing Limits 

3.0 MACT Floor Determination Methodology for Phosphate Rock Calciners 

3.1 Use of Non-Detect Values 

3.2 Phosphate Rock Calciners – Hg Emission Limit 

3.3 Phosphate Rock Calciners – Total Fluoride Emission Limit 

4.0 References 

Appendix A: Hg UPL Calculations 

Appendix B: Total Fluoride UPL Calculations 

2.0 SUMMARY OF EXISTING LIMITS 

The existing National Emissions Standard for Hazardous Air Pollutants (NESHAP) for 

phosphate rock calciners in the phosphoric acid manufacturing plants source category (40 CFR 

Part 63, subpart AA) established emission limits for particulate matter (as a surrogate for 

hazardous air pollutant (HAP) metals). Table 2-1 summarizes this emission limit. 
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Table 2-1. Existing Emissions Limits for Particulate Matter from Phosphoric 

Acid Manufacturing Plants under 40 CFR Part 63, Subpart AA 

Equipment Description Particulate Matter (gr/dscf)

Existing Sources New Sources

Phosphate rock calciner 0.080 0.040

gr/dscf – grains per dry standard cubic foot 

3.0 MACT FLOOR DETERMINATION METHODOLOGY FOR PHOSPHATE 

ROCK CALCINERS 

Section 112(d) of the CAA requires EPA to set emissions standards for HAP emitted by 

major stationary sources based on performance of the MACT. The MACT standards for existing 

sources must be at least as stringent as the average emissions limitation achieved by the best 

performing 12 percent of existing sources (for which the Administrator has emissions 

information) or the five best performing sources for source categories or subcategories with less 

than 30 sources (CAA section 112(d)(3)(A) and (B)). For new sources, MACT standards must be 

at least as stringent as the control level achieved in practice by the best controlled similar source 

(CAA section 112(d)(3)). These requirements constitute the MACT floor for new and existing 

sources. EPA may not consider costs or other impacts in determining the MACT floors. EPA 

must consider cost, non-air quality health and environmental impacts, and energy requirements 

in connection with any standards that are more stringent than the MACT floor (beyond-the-floor 

(BTF) controls). 

In order to determine the MACT floor, the EPA calculates the upper prediction limit 

(UPL). Justification for using the UPL and the UPL calculation procedures are discussed in the 

UPL memorandum1. The UPL represents the value the mean of future test run observations can 

be expected to fall below, based on the results of the independent sample of size (n) from the 

same population. Therefore, should a future test condition be selected randomly from any of 

these sources (e.g., average of three runs), one can be confident (99 percent confident in the case 

of a 99 percent UPL) that the reported level will fall below a MACT floor emission limit 

calculated using a UPL. 

The UPL is the appropriate statistical tool to use to account for variability in the 

concentration data from phosphoric acid manufacturing plants. For these emission points, where 

there is a limited sampling of the source category and EPA does not have test data from all of the 

emission points for a pollutant in the subcategory, the predictive aspect of the UPL calculation is 

especially important. 

Only one facility currently operates phosphate rock calciners; PCS Aurora. In response to 

an April 2010 CAA section 114 request, PCS Aurora provided Hg (by US EPA Method 30B) 

and total fluoride (by US EPA Method 13B) emissions testing results for one of the six calciners 

to the EPA. In addition, the facility provided Hg emissions testing results (by US EPA Method 

30B) for another, previously untested calciner in response to a January 2014 CAA section 114 

request. For additional information, refer to the separate Information Collection memorandum2. 

The following sections describe the methodology used to calculate MACT floors for total 

fluoride and Hg for phosphate rock calciners at phosphoric acid manufacturing plants. 
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3.1 Use of Non-Detect Values 

As part of the MACT floor methodology, we compare the UPL to non-detect data, in the 

form of three times the representative detection limit (RDL), an as additional UPL check. At 

very low emission levels where emission tests resulted in ND values, the inherent imprecision in 

the pollutant measurement method has a large influence on the reliability of the data underlying 

the regulatory floor emission limit. Because of sample and emission matrix effects, laboratory 

techniques, sample size and other factors, method detection levels normally vary from test to test 

for any specific test method and pollutant measurement. The confidence level that a value 

measured at the detection level is greater than zero is about 99 percent. The expected 

measurement imprecision for an emission value occurring at or near the method detection level is 

about 40 to 50 percent. Pollutant measurement imprecision decreases to a consistent level of 10 

to 20 percent for values measured at a level about three times the method detection level. 

The procedure for determining a representative detection limit (RDL) starts with 

identifying all of the available reported pollutant specific method detection levels for the bestperforming units regardless of any subcategory (e.g., existing or new, fuel type, etc.). From that 

combined pool of data, the arithmetic mean value is calculated. By limiting the dataset to those 

tests used to establish the floor or emissions limit (i.e., best performers), the result is 

representative of the best-performing testing companies and laboratories using the most sensitive 

analytical procedures. The outcome should minimize the effect of a test(s) with an inordinately 

high method detection level (e.g., the sample volume was too small, the laboratory technique was 

insufficiently sensitive, or the procedure for determining the minimum value for reporting was 

other than the detection level). The resulting mean of the method detection levels is designated as 

the RDL as characteristic of accepted source emissions measurement performance. 

The second step in the process is to calculate three times the RDL to compare with the 

calculated floor or emissions limit. The multiplication factor of three is used to reduce the 

imprecision of the analytical method until the imprecision in the field sampling reflects the 

relative method precision as estimated by the Reference Method Accuracy and Precision 

(ReMAP) study.3 That study indicates that such relative imprecision remains a constant 10 to 20 

percent, over the range of the method. For assessing the calculated floor results relative to 

measurement method capabilities, if three times the RDL were less than the calculated floor or 

emissions limit (e.g., calculated from the UPL), it would be determined that measurement 

variability was adequately addressed. The calculated floor or emissions limit would need no 

adjustment. If, on the other hand, the value equal to three times the RDL were greater than the 

UPL, it would be concluded that the calculated floor or emissions limit does not account entirely 

for measurement variability. Where such was the case, the value equal to three times the RDL 

would be substituted for the calculated floor or emissions limit, which results in a concentration 

where the method would produce measurement accuracy on the order of 10 to 20 percent, which 

is similar to other EPA test methods and the results found in the ASME ReMAP study. 

We used a procedure similar to the one discussed above to calculate the three times RDL 

value for Hg measured by US EPA Method 30B and for total fluoride measured by US EPA 

Method 13B. Facilities submitted US EPA Method 30B and US EPA Method 13B concentration 

data (including non-detect values) in response to the April 13, 2010 section 114 request and PCS 

Aurora submitted additional US EPA Method 30B concentration data in response to the January 
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24, 2014 section 114 request. The concentration data included testing from phosphate rock 

calciners, WPPA, SPA, and phosphate fertilizers process lines. First, we identified the method 

detection levels reporting by the facilities. Second, we then treated the average of the method 

detection levels as the RDL value for Hg and total fluoride. Finally, we calculated three times the 

RDL value and compared to the UPL values. 

3.2 Phosphate Rock Calciners – Hg Emission Limit 

For Hg, we calculated MACT floors for phosphate rock calciners after evaluating the 

variability in the concentration data reported by PCS Aurora. Hg emissions data were available 

for two phosphate rock calciners, see Table 3-1 for the average Hg concentration for each. 

Calciner Hg emissions are the result of Hg contained in the fuel and raw materials. Because the 

six phosphate rock calciners are designed to be identical and use the same raw materials and 

fuels, Hg emissions from the six calciners are expected to be identical. Therefore, all Hg data 

from the calciners were combined into one dataset to determine both new and existing MACT 

floors. 

Table 3-1. Average Concentration of Mercury from Phosphate Rock Calciners 

(as Measured by US EPA Method 30B) 

Facility and Process Line Average Hg Concentration 

(mg/dscm @ 3% O2) 

PCS Aurora (202) 9.87E-02

PCS Aurora (203) 9.92E-02

mg/dscm @ 3% O2 – milligrams per dry standard cubic meter 

corrected to 3 percent oxygen 

We performed skewness and kurtosis tests, and determined the Hg data have a normal 

distribution. The normal distribution UPL equation was thus applied; see the UPL memorandum1

for additional background. Results of the UPL analysis of Hg emissions data from phosphate 

rock calciners, including the final existing source limit, are presented in Table 3-2. The complete 

UPL analysis for Hg is presented in Appendix A. As previously discussed, the data used to 

determine the MACT floor for existing and new sources are the same. 

Table 3-2. MACT Floor Analysis and Existing Source Limit for Mercury 

(by US EPA Method 30B) – Phosphate Rock Calciners 

Parameter Value

Distribution Normal

99.0% UPL (mg/dscm @ 3% O2) 1.40E-01

3x RDL (mg/dscm @ 3% O2) 5.71E-03

Existing Source MACT Floor Emission Limit for 

Hg[1] (mg/dscm @ 3% O2) 1.40E-01

[1] - MACT floor limits were rounded up to two significant figures (e.g., 100.1 

would be rounded to 110). 

mg/dscm @ 3% O2 – milligrams per dry standard cubic meter corrected to 3 

percent oxygen 

RDL – representative detection limit 

At proposal, Hg emission limits for existing and new sources were set equal to a more 

stringent BTF limit. After additional consideration, we are not finalizing the BTF limit for 
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existing phosphate rock calciners, but instead finalizing floor level limits based on feedback 

received during on the proposed rule. The MACT floor calculated limit for Hg emissions from 

existing phosphate rock calciners is shown in Table 3-2. We are, however, finalizing the BTF 

limit for new phosphate rock calciners. See the preamble and a separate BTF Analysis 

memorandum4 for additional discussion regarding the BTF limits. The BTF new source limit was 

determined by estimating the emissions which would be achievable using a Hg control device 

(e.g., activated carbon injection) at phosphate rock calciners. Based on the available data, it was 

determined that 90 percent Hg reduction is achievable. Therefore, the BTF limit for Hg, as seen 

in Table 3-3, is 10 percent of the calculated MACT limit. 

Table 3-3. New Source BTF Mercury Limit – Phosphate Rock Calciners 

Parameter Value 

New Source BTF Limit for Hg 

(mg/dscm @ 3% O2)

1.40E-02

mg/dscm @ 3% O2 – milligrams per dry standard cubic meter 

corrected to 3 percent oxygen 

3.3 Phosphate Rock Calciners – Total Fluoride Emission Limit 

Total fluoride data were available for one phosphate rock calciner at PCS Aurora, with 

three test runs. Therefore, we have a limited dataset for the pollutant total fluoride from 

phosphate rock calciners. The first step in the UPL methodology involves determining the 

distribution. Because there are only three test runs, the kurtosis function in Excel does not work; 

it requires at least 4 values in order to avoid dividing by zero. Therefore, a separate kurtosis 

estimator and distribution methodology is used. This methodology compares the ratio of 

skewness to the standard error of the skewness for both the raw and log transformed data. The 

lower of the two ratios identifies the data distribution that best represents the sample set. After 

applying these equations, we determined the total fluoride data are normally distributed. 

We evaluated this specific dataset to determine whether it is appropriate to make other 

modifications to the UPL approach used to calculate the MACT floor as a result of the limited 

data. For the phosphate rock calciner dataset, we performed the following steps: we selected the 

data distribution that best represents the dataset; ensured that the correct equation for the 

distribution was then applied to the data; and compared individual components of the limited 

dataset to determine if the total fluoride standard based on the limited dataset reasonably 

represents the performance of the units included in the dataset. The results of this analysis are 

presented below. 

The MACT floor dataset for total fluoride from new and existing phosphate rock 

calciners includes three test runs from one phosphate rock calciner. After determining that the 

dataset is best represented by a normal distribution and ensuring that we used the correct 

equation for the distribution, we considered the selection of a lower confidence level for 

determining the emission limit by evaluating whether the calculated limit reasonably represents 

the performance of the unit upon which it is based. In this case, the calculated emission limit is 

about twice the short-term average emissions from the best performing source, indicating that the 

emission limit is not unreasonable compared to the actual performance of the unit upon which 

the limit is based and is within the range that we see when we evaluate larger datasets using our 
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MACT floor calculation procedures. Therefore, we determined that no changes to our standard 

UPL floor calculation procedure are warranted for this pollutant and subcategory. Additional 

details and background on the MACT floor calculation and limited datasets are provided in 

separate memorandums.1, 5 Results of the UPL analysis of total fluoride emissions data from 

phosphate rock calciners, including the final limits, are presented in Table 3-4. The complete 

UPL analysis for total fluoride is presented in Appendix B. 

Table 3-3. MACT Floor Analysis and Limit for Total Fluoride (by US EPA Method 13B) – 

Phosphate Rock Calciners 

Parameter Value

Distribution Normal

99.0% UPL (lb TF/ton phosphate ore feed) 8.93E-04

3x RDL (lb TF/ton phosphate ore feed) 5.47E-04

MACT Floor Emission Limit for total 

fluoride[1] 

9.0E-04

[1] - MACT floor limits were rounded up to two significant figures (e.g., 

100.1 would be rounded to 110). 

RDL - representative detection limit 

4.0 REFERENCES 

1. Ndoh, Tina. EPA. “Use of the Upper Prediction Limit for Calculating MACT Floors.” 

Docket ID: EPA-HQ-OAR-2012-0522. July 30, 2014.

2. Eastern Research Group, Inc. “Information Collection and Additional Data Received for 

the Phosphoric Acid and Phosphate Fertilizer Production Source Categories.” Docket ID: 

EPA-HQ-OAR-2012-0522. May 2014. 

3. “Reference Method Accuracy and Precision (ReMAP): Phase 1, Precision of Manual 

Stack Emission Measurements” American Society of Mechanical Engineers, Research 

Committee on Industrial and Municipal Waste. February 2001. 

4. Eastern Research Group, Inc. “Beyond-the-Floor Analysis for Phosphate Rock Calciners 

at Phosphoric Acid Manufacturing Plants.” Docket ID: EPA-HQ-OAR-2012-0522. June 

8, 2015. 

5. Ndoh, Tina. EPA. “Approach for Applying the Upper Prediction Limit to Limited 

Datasets.” Docket ID: EPA-HQ-OAR-2012-0522. June 8, 2015.