Date: September 15, 2011

Subject: Technical Approach for Mineral Wool MACT Floor Calculations

EPA Contract No. EP-D-06-118; EPA Work Assignment No. 4-10

RTI Project No. 0210426.004.010

From: Cindy Hancy

Dave Reeves

To: Susan Fairchild, EPA/OAQPS/SPPD/MMG

Introduction

Section 112 of the Clean Air Act (CAA) requires that the U.S. Environmental Protection 

Agency (EPA) to establish National Emission Standards for Hazardous Air Pollutants

(NESHAP) for the control of the hazardous air pollutants (HAP) emitted from both new and 

existing major sources in a source category. These standards must reflect the maximum degree 

of reduction in the HAP emissions that is achievable. The minimum level of control is referred 

to as the “Maximum Achievable Control Technology (MACT) floor.” The method for 

determining the MACT floor for a NESHAP is defined for both new and existing sources in

CAA section 112(d)(3). For new sources, the MACT floor cannot be less stringent than the 

emission control that is achieved in practice by the best-controlled similar source. For existing 

sources, the MACT floor cannot be less stringent than the average emission limitation achieved 

by the best-performing 12% of existing sources for source categories with 30 or more sources, or 

the best-performing five sources for source categories with fewer than 30 sources.

The purpose of this memorandum is to present the data, the methodology, and the results of the 

MACT floor analysis for the Mineral Wool Production NESHAP source category. This analysis 

is part of EPA's obligation under CAA section 112(f)(2) and 112(d)(6) to conduct a residual risk 

and technology review. The MACT floor analysis is also being performed in response to a 

petition for rulemaking by the Natural Resources Defense Council and Sierra Club that states 

EPA failed to set emission limits for COS and other HAPs emitted by mineral wool facilities and 

challenges HAP surrogacies claimed in the current Mineral Wool NESHAP. This MACT floor 

analysis uses data collected from a nationwide voluntary Information Collection Request (ICR) 

of mineral wool manufacturers conducted by EPA in 2010. Data on process operations, emission 

controls, and air emissions data reported by respondents to the ICR were compiled into a 

Microsoft Access data base that serves as the data set used for this MACT floor analysis 

(referred to in this memorandum as the “ICR data set”).

Background Information

The current Mineral Wool Production NESHAP was promulgated on June 1, 1999 and 

applies to existing, new, and reconstructed cupolas and curing ovens at mineral wool production 
facilities that are major sources of HAP emissions. The 1999 Mineral Wool Production NESHAP 

sets particulate matter (PM) emission limits for new and existing cupolas and carbon monoxide 

(CO) limits for new cupolas. The NESHAP also established emission limits for formaldehyde 

from new and existing curing ovens. Based on the 1999 NESHAP, these regulated pollutants 

currently serve as surrogates for other HAPs as follows: PM serves as a surrogate for HAP 

metals, CO for carbonyl sulfide (COS), and formaldehyde for phenol and methanol. 

The mineral wool production source category currently consists of 7 facilities in the US; 

combined, these 7 facilities operate 11 cupolas and 3 curing ovens. The three facilities that 

operate curing ovens produce bonded mineral wool products. In addition to curing ovens, 

sources of HAP emissions associated with bonded product manufacturing include collection 

operations. There were no MACT emission limits established for collection operations in the 

1999 Mineral Wool NESHAP. Bonded lines use HAP containing binders that result in

formaldehyde, phenol, and methanol emissions from curing ovens and collection sources. The 

stack configurations of curing and collection operations vary at the three facilities operating 

bonded lines. The proposed amendments to the rule as a result of this review will include 

formaldehyde, methanol, and phenol limits for collection operations; however, due to the 

differences in processes, associated exhaust streams, and stack configurations at the three bonded 

lines, EPA is proposing pollutant-specific emission limits for both curing ovens and collection 

operations combined. 

The specific chemicals, compounds, or groups of compounds designated as HAP are 

listed in CAA section 112(b). From this list, the following additional HAPs were identified as

being emitted from cupolas and would be regulated for new and existing sources in the proposed 

rule: carbonyl sulfide (COS), hydrogen fluoride (HF), and hydrogen chloride (HCl). 

MACT Floor Analysis - Subcategories

Under CAA section 112(d)(1), EPA has the discretion to “...distinguish among classes, 

types, and sizes of sources within a category or subcategory in establishing...” standards. When 

separate subcategories are established, a MACT floor is determined separately for each 

subcategory. To determine whether the mineral wool production facilities warrant 

subcategorization for the MACT floor analysis, EPA reviewed unit and process designs, 

operating information, and air emissions data compiled in the ICR data set and other information

collected by the Agency for development of the NESHAP for this source category. Based on 

this review, EPA concluded that there are significant design and operational differences in the 

collection operations at each of the three facilities that operate a bonded line. 

These three collection subcategories are listed below. 

Vertical Collection: In this design, the molten rock/slag mixture is poured from the

cupola spout onto a group of stainless steel drums spinning in opposite directions. The 

spinning drums form fine fibers of the mineral mixture. High air volume directs the 

fibers off the fiberization spinners, toward a fast-moving porous vertical conveyor belt. A 

strong vacuum is drawn on the opposite side of the belt, causing the fibers to lie against 

the vertical belt as it moves upward. At the top of the conveyance, the belt travels around 

a curve, the vacuum is released, and the fibers are removed on a second belt that conveys

the layer of binder-sprayed mineral wool fibers into the curing oven. 
The vertical conveyer belt requires high volumes of air to be drawn through the belt and 

results in the collection of a thin fiber layer upon the belt. In this design, ‘shot’, BB-sized 

black granules that are high in iron (a result of using slag from the iron and steel 

industry), fall out of the fiber layer. The vertical design is used to produce a specific type 

of mineral wool that is low in ‘shot’ and may be used in the hydroponic gardening 

market, as well as, in a specialized market of insulation products in which shot is 

undesirable. 

Horizontal Collection: This process is similar to vertical collection, except that the

conveyor belt works with gravitational forces and results in a thicker layer of mineral 

wool collected upon the horizontal belt. This method also requires lower air volume 

drawn through the fiber layer than in the vertical design, and the ‘shot’ is not selectively

removed. The air stream is conducive to thermal oxidation at the hottest part of the 

cupola exhaust stack or at the existing thermal oxidizer on the curing oven. 

Drum Collection: In this design, fibers are drawn using a very high volume air flow into 

the center of a rotating drum. The sides of the rotating drum have small holes through 

which the air flow may exit but the fibers are caught. The angle of the drum, the vacuum, 

and centrifugal forces pull the fibers against the inside wall of the drum and out the end. 

The entire drum is enclosed and the air flow may be vented to the hottest part of the 

cupola exhaust stack or to an existing thermal oxidizer on the curing oven. 

MACT Floor Analysis Methodology

Existing Sources 

A MACT floor analysis was completed for each proposed regulated pollutant as 

summarized in Table 1. 

Table 1 – Summary of Proposed Limits

Existing Sources New Sources

Cupolas

PM Current PM limit will remain the same. No 

MACT floor analysis performed.

CO

EPA is proposing to remove the current CO 

limit since it was/is a surrogate for COS in the 

1999 NESHAP

COS X X

HF X X

HCl X X

Curing & Collection (combined)

Formaldehyde X X

Phenol X X

Methanol X X

A MACT floor analysis was not performed for PM since EPA is not proposing any
changes to the current PM limits. The current formaldehyde limit only applies to curing ovens.

As previously mentioned, EPA is now developing new MACT emission limits for collection 

operations. Therefore, a MACT floor analysis that includes both curing and collection was 

performed for new and existing sources. No current emission limits exist for COS, HF, HCl, 

phenol, or methanol; in order to propose limits for each pollutant, a MACT floor analysis was 

performed for each of these pollutants for new and existing sources. 

The first step in the MACT floor analysis for each regulated source and HAP was to rank

each unit (for which emissions data was provided) by emission level (lowest to highest) for each 

pollutant. From this ranking, a MACT floor pool of sources was identified for determining the 

minimum control level allowed for the MACT floor, consistent with the criteria defined for new 

and existing sources by CAA section 112(d)(3). For the new source MACT floors, the bestcontrolled source was identified for which there were individual source test run data in the ICR 

data set. For the existing source MACT floors, selection of the MACT floor pool size (i.e., 

number of emission units to be included in the determination of the average emission limitation 

value) was determined on an individual unit category basis as described below. 

Cupolas. This category includes fewer than 30 sources. Therefore, the existing source 

MACT floors for COS, HF, and HCl emissions were based on the top 5 best performing 

cupolas. For new sources, the MACT floor was based on the best performing cupola for 

each pollutant.

Curing & Vertical Collection. Currently, only one facility operates this type of 

collection. Formaldehyde, phenol, and methanol MACT floors for new and existing 

sources were based on emission test runs for combined curing and collection operations 

from this facility. 

Curing & Horizontal Collection. Currently, only one facility operates this type of 

collection. Formaldehyde, phenol, and methanol MACT floors for new and existing 

sources were based on emission test runs for combined curing and collection operations

from this facility.

Curing & Drum Collection. Currently, only one facility operates this type of collection. 

Formaldehyde, phenol, and methanol MACT floors for new and existing sources were 

based on emission test runs for combined curing and collection operations from this 

facility.

The next step in the MACT floor analysis was to account for data variability in the 

calculations of the applicable MACT floor limits for the subcategories using the data’s 99%

upper prediction limit (UPL). Specifically, the MACT floor limit was determined as the UPL 

calculated with the Student’s t-test using the “TINV” function in Microsoft Excel software. The 

UPL approach has also been used in other EPA rulemakings (e.g., NESHAP for Portland 

Cement, NSPS for Hospital/Medical/Infectious Waste Incinerators, NESHAP for Industrial, 

Commercial, Institutional Boilers and Process Heaters, and NESHAP for Electric Generating 

Units) to account for variability in emissions data for a specified level of confidence. The level 

of confidence represents the level of protection afforded to facilities whose emissions are in line 

with the best performers. For example, a 99% level of confidence means that a facility whose 

emissions are consistent with the best performers has one chance in 100 of exceeding the floor 

limit. A prediction interval for a single future observation (or an average of several test 

observations) is an interval that will, with a specified degree of confidence, contain the next (or 
the average of some other pre-specified number) of randomly selected observation(s) from a 

population. In other words, the upper prediction limit estimates what the upper bound of future 

values will be, based upon present or past background samples taken. The UPL consequently 

represents the value at which we can expect the mean of future observations for the HAP

emissions to fall within a specified level of confidence, based upon the results of an independent 

sample from the same population. This method accounts for the point-to-point variability in the 

data. 

The form of the UPL equation differs somewhat depending upon the number of data 

points and data distribution to which it is applied. Attachment A includes a flow diagram that 

summarizes the UPL approaches used. To this end, the data sets were evaluated for each HAP to 

ascertain whether the data were normally distributed, or fit another type of distribution (e.g., log 

normal distribution). According to the Central Limit Theorem (Durrett, 1996), when a data set

includes 15 or more sources, the UPL is based on the assumption that the data fit a normal 

distribution. The Central Limit Theorem states that regardless of the shape of the original 

distribution, if the distribution has a finite mean (μ) and variance (σ2), the sampling distribution 

of the mean approaches a normal distribution with a mean of (μ) and a variance of σ2/N as N, the 

sample size, increases (Durrett, 1996).

The mineral wool data sets used to calculate MACT floors for each pollutant all 

contained less than 15 sources. When the sample size is smaller than 15 and the distribution of 

the data is unknown, the Central Limit Theorem cannot be used to support the normality

assumption. Statistical test of the kurtosis, skewness, and goodness of fit test are then used to 

evaluate the normality assumption. The skewness statistic (S) characterizes the degree of 

asymmetry of a given data distribution. Normally distributed data have an S value of 0. An S 

value that is greater (less) than 0 indicates that the data are asymmetrically distributed with a 

right (left) tail extending towards positive (negative) values. The standard error of the skewness 

statistic (SES) was also used in determining the normality of the data distribution. The kurtosis 

statistic (K) characterizes the degree of peakedness or flatness of a given data distribution in 

comparison to a normal distribution. Normally distributed data have a K value of 0. A K value 

that is greater (less) than 0 indicates a relatively peaked (flat) distribution. The standard error of 

the kurtosis statistic (SEK) was also used in determining the normality of the data distribution.

For each data set to which the UPL was applied (i.e., the separate data sets for each HAP 

applicable to a source), the S and K values were calculated using the reported test values. If both 

kurtosis and skewness tests indicate the data is normally distributed, the UPL was calculated 

using the UPL pooled variance Equation 1. 

 Equation 1

where:

= the average (mean) of the best performing existing sources;

t(p,df) = the t statistic for a confidence level p, and df degrees of freedom;

s

2

= the pooled variance;

n = the total number of test runs (all sources) used in the analysis; and

m = the number of (future) compliance test runs [for run-by-run data, m=3].
Degrees of freedom calculated by:

1

1

 











 

N

i

df ni

 Equation 1a

Mean calculated by:

 



N

i

n

j

ij

i

x

n

X

1 1

1

 Equation 1b

Pooled variance calculated by:

 









n

i

xi X

n

s

1

2

2

1

1

 Equation 1c

If the kurtosis or skewness tests indicate the data was not normally distributed, the data were logtransformed. Once the logs of all the test runs were calculated, new kurtosis and skewness tests

were performed on the log-transformed data. If both kurtosis and skewness tests indicated the 

log-transformed data were normally distributed, the UPL can be calculated using the Equation 2. 

  

















    

 



2( 1)

ˆ ˆ

1

2 4

.99 2 ˆ ˆ ˆ 2 2 ˆ ˆ 2

ˆ

ˆ 2 2 2

2

n n

me e m e

m

Z

UPL e

      





 Equation 2

where:

μ = the average of the best performing existing sources;

Z.99 = the z statistic for a lognormal distribution at 99 percent;

2 ˆ = the variance;

n = the total number of test runs (all sources) used in the analysis; and

m = the number of (future) compliance test runs [for run-by-run data m=3].

 

Mean is calculated by:

n

x

n

i

 i

 1 

 Equation 2a

Variance is calculated by:

 

1

ˆ

1

2

2









n

x x

n

i

i



 Equation 2b
If the raw data and the log-transformed data were not normally distributed and n ≥13, the UPL

was calculated using the UPL pooled variance with skewness adjustment in Equation 3. Note 

this adjustment cannot be used if the number of individual runs is less than 13. 













 















 



n m

t s

n

Skew s

X df p

1 1

6

2

,

2

 Equation 3

where:

= the average of the best performing existing sources;

t(p,df) = the t statistic for a confidence level p, and df degrees of freedom;

s

2

= the pooled variance;

n = the total number of test runs (all sources) used in the analysis;

m = the number of (future) compliance test runs [for run-by-run data m=3]; and

Skew = the skewness of the test runs used in the analysis.

If the raw data and the log-transformed data were not normally distributed and n was less than 13 

the UPL was calculated using the UPL pooled variance in Equation 1, but instead of using the 

Excel TINV formula to find the t statistic t(p,df), a trial and error method was used to find the t 

static that gives a 99 percent confidence level by correcting the probability values (p) using 

Equation 4

Corrected Probability = P0(t) + Skewness x Pλ3(t) – Kurtosis x Pλ4(t) + λ3

2

xP λ3/2(t) Equation 4

Adjustment for Below Detection Level Emissions Data

Prior to calculating the UPLs, test runs that were below detection limit (BDL) were 

identified. If a dataset contained any BDL values, the UPL was calculated using BDL values as 

reported in the test report. After the UPL was completed for a particular source and pollutant, a 

comparison of the UPL and the representative method detection level (RDL) was performed. 

The first step in the comparison was to, for a particular emissions unit category, find the mean of 

the emissions data set, which was calculated during the UPL calculations. The second step was

to identify the largest BDL value of the dataset that was no greater than the mean of the entire

dataset. This value was the RDL for the pollutant data set of the emission unit category. The 

RDL was then multiplied by 3 and the product is compared to the UPL. If the 3x RDL value was 

less than or equal to the calculated UPL value, the UPL value was used as the proposed MACT 

Floor Limit. If the 3x RDL value was greater than the calculated UPL value, the 3x RDL value 

was used as the proposed MACT Floor Limit. If no BDL values were included in the pollutant 

data set for a particular unit type, these steps were not performed. 

MACT Floor Analysis Results

The MACT floors for curing and collection subcategories for formaldehyde, phenol, and 

methanol were the same for existing and new sources since there is only one source in each 

subcategory. If the new source MACT floor was less stringent than the MACT floor calculated 

for existing sources, the existing MACT floor was used for both new and existing sources. Table 
2 presents the new source and existing source MACT floor values determined for the Mineral 

Wool Production source category. 

Table 2. Summary of Proposed MACT floors for the Mineral Wool Source Category

Total 

Sources in 

Industry

Mean of 

Sources 

Used in 

UPL

Analysis

99% UPL

Caclulated 

for 

Existing 

Sources

99% UPL

Caclulated 

for New 

Sources

Cupolas 11

Carbonyl Sulfide 1.07 3.30 0.017

Hydrogen Flouride 0.0089 0.014 0.014

Hydrogen Chloride 0.0066 0.0096 0.0096

Curing & Drum Collection 1

Formaldehyde 0.045 0.067 0.067

Phenol 0.00039 0.0023 0.0023

Methanol 0.00011 0.00077 0.00077

Curing & Vertical 

Collection

1

Formaldehyde 0.45 0.46 0.46

Phenol 0.50 0.52 0.52

Methanol 0.62 0.63 0.63

Curing & Horizontal 

Collection

1

Formaldehyde 0.054 0.054 0.054

Phenol 0.013 0.15 0.15

Methanol 0.0081 0.022 0.022
9

Attachment A
10

Determine number of 

sources in MACT floor

Are 

there at least 15 

sources in MACT 

floor?

Assume normal 

distribution

Calculate MACT floor 

using UPL Pooled 

Variance Equation 1

Test for normal 

distribution using 

kurtosis and skewness

Does sample have 

normal distribution?

Calculate MACT floor 

using UPL pooled 

Variance with skewness 

adjustment Equation 3

Use UPL as MACT floor

Are any data 

BDL or DLL?

Log transform data

Test data for lognormal 

distribution using 

kurtosis and skewness

Does sample 

have lognormal 

distribution?

Calculate MACT floor 

using UPL Lognormal 

Equation 2

Determine RDL

Compare UPL to 3 x 

RDL

Use greater of UPL and 

3 x RDL as MACT floor

Yes

No

Yes

No

No

Yes

Yes

MACT Floor Flow Chart

Does the dataset 

contain 13 or more data 

points (n≥13)?

Calculate UPL by 

adjusting T statistic 

values using probability 

trial and error. 

No

No

Yes