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MEMORANDUM

To: David Putney, U.S. Environmental Protection Agency (EPA), OAQPS

From: David Green, RTI

Date: October 31, 2011

Subject: Draft MACT Floor Analyses for the Primary Aluminum Production Source Category

This memo describes the data and calculations supporting the proposed MACT floor determinations for 

Primary Aluminum Reduction Plants, beyond the floor analyses, and the estimated impacts of these 

options. MACT floors were established for the following three previously unregulated hazardous air 

pollutant (HAP) sources at primary aluminum plants:

1. Polycyclic organic matter (POM) emissions from prebake potlines

2. Carbonyl sulfide (COS) emissions from prebake and Soderberg potlines

3. POM emissions from existing pitch storage tanks

POM Emissions from Prebake Potlines

MACT Floor Determination

Limited data were available for emissions of POM from prebake potlines because these emissions are 

not regulated under the 1997 NESHAP, and no testing of these emissions has been done in recent years. 

Primary POM emissions are presently controlled with dry alumina scrubber/baghouse systems which 

also control total fluoride (TF). With regard to secondary emissions from vents in the roof of the 

potrooms, typically there are no add-on control devices (with the exception of one facility that operates a 

wet roof scrubber). However these emissions are limited to some degree by minimizing the opening of 

the pots and, thereby, maximizing the potline fume capture efficiency.

MACT floors for existing prebake potlines were determined for emissions of POM from prebake 

potlines using test data cited in the Technical Support Document for the 1997 NESHAP (Primary 

Aluminum Industry: Technical Support Document for Proposed MACT Standards, US EPA, July 1996). 

This report included estimates of POM/TF ratios for the sum of primary and secondary emissions from 

prebake potrooms of various types. The ratio applicable to center-worked prebake one (CWPB1) 

potlines and side-worked prebake (SWPB) potlines is 0.3 whereas the ratio applicable to CWPB2 and 

CWPB3 potlines is 0.5. Using these ratios and TF emissions reported in calendar year 2010, POM 

emissions were estimated for currently operating prebake potlines. These data are shown in Table 1. The 

MACT floor for existing sources is calculated based on the average emissions of the 5 best performing 

facilities plus an amount to account for variability. This accounting for variability is usually 

accomplished by using the 99-percent upper prediction limit (UPL). 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 
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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 dataset.

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

distribution to which it is applied. Attachment B 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, increases1(Durrett, 1996).

The facility averages of estimated POM emissions were used to calculate the 99% UPL for the POM 

MACT emissions limits applicable to prebake potlines. Only 2 facilities have CWPB2 and CWPB3 type 

potlines. Each of those facilities has 4 currently operating potlines. Only 1 facility has SWPB potlines 

and only 2 of those potlines are currently operating. Therefore, the 99-percent UPL for the POM 

emissions from CWPB1 facilities is based on inter-facility variability (i.e., the variability between 

CWPB1 type facilities) whereas the 99-percent UPL for the POM emissions from CWPB2, CWPB3 and 

SWPB facilities is based on intra-facility variability (i.e., the variability between potlines located within 

the same production facility).

Using the methods described above, the calculated POM MACT floor emission limits for existing 

prebake potlines are 0.62 pounds POM per ton of aluminum produced (lb/ton Al), 1.3 (lb/ton Al), 1.26 

(lb/ton Al), and 0.65 (lb/ton Al) for CWPB1, CWPB2, CWPB3 and SWPB potlines, respectively. The 

lowest of these POM standards [i.e., 0.62 (lb/ton Al)] is proposed as the MACT floor emission limit for 

all new prebake potlines.

Impacts at MACT Floor Level

Particulate emissions from prebake potlines are currently well controlled via the associated controls 

(e.g., dry alumina scrubbers, wet scrubbers and baghouses). Therefore, we expect that all prebake 

facilities presently in operation would be able to comply with the POM MACT floor emission limits 

without any additional controls. An estimated 50% of the TF emissions from the Primary Aluminum 

Production source category is in particulate form and it is expected that much of the POM from this 

source category is also in particulate form at the exhaust temperatures of control device stacks (typically 

less than 180 degrees Fahrenheit (°F)) and potroom vents (typically less than 100 °F). While no 

additional control costs would be expected to comply with the MACT floor limits, the proposed rule 

 

1 Durrett, R. Probability: Theory and Examples. Cambridge. (1996).
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would require periodic demonstrations of compliance with the appropriate POM emission limit. 

Assuming compliance testing would be required for each potline once every five years and that this 

demonstration can be made either by 1) summing the results of emission tests on the primary control 

system exhaust and the secondary (roof vent) emissions or, 2) summing the results of an emission test on 

the primary control system exhaust and the estimated secondary emissions (based on the secondary 

emissions exhaust volume, the secondary TF emission rate and the ratio of POM to TF present in the 

primary control system exhaust, we estimate that the 12 prebake type facilities would need to conduct 

performance tests for POM emissions from prebake potlines on 45 prebake potlines every 5 years at an 

estimated cost of $10,000 per test, for an average annual cost of $7,500 per prebake facility or $90,000 

total annual cost for the industry.

Beyond the Floor Analysis

It is estimated that more than 2/3 of the potline POM emissions are emitted as secondary emissions (i.e., 

emitted through the roof vents). Since secondary POM emissions are comprised of high-volume, lowconcentration emissions streams, additional controls are estimated to be very expensive. A cost estimate 

was prepared for the installation of wet roof scrubbers on a Soderberg plant, which has considerably 

higher POM emissions. It was estimated that capital costs would be $42 million and total annual costs 

(including capital recovery) would be $13.1 million. POM reductions of 40 tons per year were estimated 

for this system. However, since prebake facilities have much lower POM emissions, emissions 

reductions of no more than about 15 tons per year of POM would be expected for prebake facilities. 

Therefore, we estimate a cost-effectiveness of approximately $800,000 per ton of POM for installation 

of wet roof scrubbers on a prebake plant.

COS Emissions from Potlines

MACT Floor Determination

Carbonyl sulfide (COS) is emitted from potlines at primary aluminum manufacturing facilities. Sulfur 

present as an impurity in anodes is emitted either as SO2 or COS. Industry representatives have reported 

that most (an estimated 90%) of the sulfur is converted to COS in the bath, with most (an estimated 

90%) of the COS oxidizing to SO2 in the pots and associated hooding and ductwork before it is emitted

from the control device stack. COS is not well controlled by the existing control devices used for potline 

emissions (typically dry alumina scrubbers and wet scrubbers). Anodes are produced for prebake 

facilities (which represent all but one of the currently operating primary aluminum facilities) from a 

paste comprised of pitch and coke. Available data2,3and information submitted by industry indicates that 

essentially all of the sulfur in prebake electrodes originates in the coke, and that the sulfur present in the 

pitch is lost in the anode production process. Recent COS emission test data are limited, and inadequate 

for calculating a COS emission limit directly. Primary aluminum production facilities estimate their 

COS emissions based on the sulfur content of anode coke, the coke content of the anodes, the mass of 

anode consumed per ton of aluminum produced, and a fractional sulfur to COS conversion rate (reported 

by industry to range from 4.5 to 10%. The remaining sulfur is converted to SO2.

EPA sent an information collection request (ICR) to the primary aluminum production companies in 

April, 2011 which included requests for data on the typical sulfur content of coke used in anode 

production, the range of anode coke sulfur contents over the last year or the last twelve months, and 

 

2 Hay, S. J. The Formation and Fate of Carbonyl Sulfide (COS) Gas in Aluminum Smelting. Ph. D Thesis. University of 

Aukland. Aukland, New Zealand. (2002)

3 Hay, S. J. The Influence of Dry Scrubber Alumina and Humidity on COS Smelter Emissions. Light Metals, p. 289-296. (2002)
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information on the methods used to estimate COS emissions. Typical anode coke sulfur contents and 

fractional sulfur to COS conversion rates were provided by 9 operating primary aluminum facilities. We 

used an industry average anode coke content of 78% and anode consumption rate of 0.57 tons of green 

anode per ton of aluminum produced. These industry average values were provided by The Aluminum 

Association (a trade group for primary aluminum companies). These data were used to calculate 

estimated facility wide COS emissions. COS emissions were calculated on the basis of the lowest 

(4.5%) and highest (10%) reported sulfur to COS conversion rates. The facilities were ranked in 

ascending order of estimated COS emissions. Data used in the analysis are summarized in Table 2. The 

COS emissions are calculated as a facility wide monthly average since the facilities track their anode 

coke sulfur content levels on a monthly average basis and each aluminum production facility has only 

one paste production facility that supplies the anode paste for all of their potlines.

The 5 facilities with the lowest estimated COS emissions were designated as the best performing 

facilities for existing source COS MACT floor determination. Using the 99% UPL method to account 

for variability, we calculate that the MACT floor limit for existing facilities is 3.9 lb COS per ton of 

aluminum produced. The COS MACT floor for new sources was calculated based on the emissions of 

the top performing facility. Using the 99% UPL method to account for variability, we calculate that the 

MACT floor limit for new facilities is 3.1 lb COS per ton of aluminum produced.

Impacts at MACT Floor Level

It is expected that all primary aluminum production facilities presently in operation would be able to 

comply with the existing source COS MACT floor emission limit without any additional controls. The 

owner or operator will be required to demonstrate compliance on a monthly basis via a mass balance 

equation with 3 variables: anode consumption rate, aluminum production rate, and anode coke sulfur 

content. However, the owner or operators of the affected facilities already monitor all 3 variables. 

Therefore, we estimate that there would be no emission reductions or cost impacts due to the proposed 

COS MACT floor limit. However, industry representatives have indicated that they expect the sulfur 

content of available petroleum coke to increase in the future due to changes in refinery operations. 

Therefore, it is possible that compliance costs will increase. Industry representatives provided the 

following documents to support their position:

1. The Minerals, Metals & Materials Society, 2004. “A Preview of Anode Coke Quality in 2007,” 

Franz Vogt, Robert Tonti, Maia Hunt, Les Edwards. pp. 489 through 493.

2. The Minerals, Metals & Materials Society, 2011. “Analysis on the Market Developing 

Conditions of Prebaked Carbon Anodes for the Aluminum Industries,” Guanghui Lang, Chunyu 

Fu, Ronald Logan, Yan Li. pp. 163 through 166.

3. The Minerals, Metals & Materials Society, 2009. “Use of Shot Coke as an Anode Raw Material” 

Les Edwards, Franz Vogt, Mike Robinette, Ric Love, Anthony Ross, Marilou McClung, RJ 

Roush, William Morgan. pp. 985 through 990.

4. McCloskey 6th Petcoke Conference, May 2008. “Petcoke and its role within the aluminum 

industry,” Calvin Graham. pp. 1 through 99.

Based on the submitted information, it is possible that lower sulfur content anode coke will increase in 

price in the future and that the MACT floor COS emission limits will, therefore, result in future costs to 

the primary aluminum industry.
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Beyond the Floor Analyses

In regard to beyond the floor analyses for COS emissions, we considered COS emission limits of 3.5 lb 

COS per ton of aluminum produced and 3.1 lb COS per ton of aluminum produced. Based on current 

anode coke cost data, we estimate that the limits of 3.5 lb COS per ton of aluminum produced and 3.1 lb 

COS per ton of aluminum produced would impose annual costs of approximately $2 million on two 

facilities and $13 million on 5 facilities, respectively. We predict these options would reduce COS 

emissions by 52 tons per year and 224 tons per year, respectively, and result in cost effectiveness of 

$35,000 per ton and $57,000 per ton, respectively. Again, based on the submitted information referenced 

in the MACT floor discussion above, it is possible that the costs cited in this paragraph for the listed 

beyond the floor options will increase in the future.

POM Emissions from Existing Pitch Storage Tanks

MACT Floor Determination

The MACT floor for control of POM emissions from existing pitch storage tanks was determined based 

on the responses to the April, 2011 ICR sent to primary aluminum production companies. When an ICR 

response was not available, the type of control equipment was determined from a review of the facility 

Title V operating permit. The reported control equipment and estimated efficiencies are summarized in 

Table 3. Of the 12 facilities operating existing pitch storage tanks, four had some type of controls in 

place and two additional facilities may have had some type of controls in place. Of the five best 

controlled facilities’ tanks, the median of the estimated control efficiencies is 95%, represented by a 

fume capture and return system accompanied by a charcoal scrubber. The MACT floor for existing pitch 

storage tanks was proposed on this basis. This is equivalent to the MACT floor for new pitch storage 

tanks in the 1997 NESHAP.

Impacts at MACT Floor Level

We estimate that eight facilities would be affected by this standard and would need to add controls, at an

annual cost of approximately $20,979 per facility or $167,836 for the industry. There are no available 

data on pitch storage tank emissions. Potential emissions were estimated on the basis of vapor 

displacement during loading. These calculations assumed anode paste was comprised of 22% pitch (the 

industry average value provided by The Aluminum Association), a pitch specific gravity of 1.25 and 

vapor pressure equal to that of oxidized asphalt at 300 °F (estimates based on available data) and 35% 

HAP content of methylene chloride extractable matter and an average HAP molecular weight of 250

(based on testing conducted at the Kaiser-Tacoma Soderberg facility). Estimated emission reductions by 

facility are summarized in Table 4.

Beyond the Floor Analysis

The cost effectiveness of control at the MACT floor level is approximately $108,000 per ton of HAP. A 

non-contact, single stage, refrigerated, water cooled condenser system was considered as a beyond the 

floor option for POM emissions from existing pitch storage tanks. The cost of such a system was 

estimated at $184,000 per facility, per year, and would affect an additional 2 facilities. The associated 

incremental reduction of POM emissions from the industry was estimated at 0.1 tons per year, resulting 

in an estimated cost effectiveness of $26 million per ton of HAP.
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Attachment A: Tables

Table 1: Estimated POM emissions from operating prebake potline facilities

Facility Name Potline Type Reported facility average 

TF emissions (lb/ton Al)

Estimated facility average

POM emissions (lb/ton Al)

Alcoa – Wenatchee Works CWPB1 1.76 0.53

Alcoa – Warrick Operations CWPB1 1.46 0.44 a

Noranda Aluminum CWPB1 1.38 0.41 a

Alcoa – Massena Operations 

(aka Alcoa – Massena West) CWPB1 0.97 0.29 a

Alumax of South Carolina

(aka Alcoa – Mt. Holly) CWPB1 0.77 0.23 a

Rio Tinto Alcan CWPB1 0.56 0.17 a

Ormet Primary Aluminum CWPB2 2.5 1.3 b

Century Aluminum of 

Kentucky CWPB3 1.83 0.91 c

Intalco Aluminum SWPB 1.34 0.40 d

a These are the top 5 performers used to establish the existing source POM MACT floor limit for CWPB1 potlines.

b The variability applied in the development of the existing source POM MACT floor limit for CWPB2 potlines was 

obtained from estimated POM emissions from this facility of 1.3, 1.1, 1.5 and 1.1 lb POM/ton of aluminum.

c The variability applied in the development of the existing source POM MACT floor limit for CWPB3 potlines was 

obtained from estimated POM emissions from this facility of 1.01, 0.86, 0.81 and 0.99 lb POM/ton of aluminum.

d The variability applied in the development of the existing source POM MACT floor limit for CWPB3 potlines was 

obtained from estimated POM emissions from this facility of 1.01, 0.86, 0.81 and 0.99 lb POM/ton of aluminum.
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Table 2: Estimated COS and SO2 emissions

Facility name

Plant capacity

(ton Al/year)

Anode coke 

sulfur content

COS emissions 

(lb COS/ton Al)

SO2 emissions 

(lb SO2/ton Al)

Alcoa – Warrick Operations 340,600 1.94% 2.59 a31.74

Noranda Aluminum 325,000 2.00% 2.67 a32.72

Alumax of South Carolina

(aka Alcoa – Mt. Holly) 246,900 2.13% 2.84 a

34.85

Century Aluminum of KY 269,000 2.20% 2.93 a35.99

Alcoa – Massena Operations

(aka Alcoa – Massena West) 137,788 2.41% 3.21 a

39.43

Reynolds Metal Company

(aka Alcoa – Massena East) 143,299 2.45% 3.27 40.09

Alcoa – Wenatchee Works 202,823 2.60% 3.47 42.54

Intalco Aluminum 306,439 2.80% 3.73 45.81

Rio Tinto Alcan 216,051 2.80% 3.73 45.81

a

These are the top 5 performers used to establish the existing source COS MACT floor limit.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Note: %S to COS ranged from 4.5% to 10% and averaged 8%

%S to SO2 ranged from 90% to 95.5% and averaged 92%
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Table 3: MACT floor determination for existing pitch storage tanks

Facility Name Equipment

Estimated 

control 

efficiency

Notes

Lake Charles Carbon

(Alcoa anode plant) Water cooled condenser 99% Condition in Title V permit

Intalco Aluminum Fume capture and return system, 

condensing tank, carbon bed 95%

Alcoa - Wenatchee Works Fume capture and return system, 

scrubber, carbon filter 95%

Alumax of South Carolina

(aka Alcoa - Mt. Holly) TIGG adsorber (carbon filter) 95% No estimate of efficiency in ICR response or Title V permit

Rio Tinto Alcan Two tanks, one empty, not 

necessarily connected NS No estimate of efficiency in ICR response or Title V permit

Alcoa - South Plant Three tanks, one empty, not 

necessarily connected NS No estimate of efficiency in ICR response or Title V permit

Alcoa - Warrick Operations Two tanks, no controls 0%

Alcoa - Massena Operations

(aka Alcoa - Massena West) Two tanks, no controls 0%

Century Aluminum of KY Three tanks, no controls 0% BACT analysis estimated costs of 

control at $18k - $22k/ton POM

Noranda Aluminum Two tanks, no controls 0%

Columbia Falls Aluminum Two tanks, no controls 0% Based on permit conditions

Century Aluminum of WV No controls 0% Based on permit conditions 

(installation in 1997)

Ormet Primary Aluminum Not operating any pitch storage 

tanks, no controls on tanks NA Possibly have empty tanks, using dry pitch, or buying anodes

Reynolds Metal Company

(aka Alcoa - Massena East) No active tanks on site NA Possibly have empty tanks

Alcoa - Rockdale Operations No tanks NA Plant uses dry pencil pitch

Notes: The estimate of control efficiency for the Alumax of South Carolina facility was obtained from the 

document EPA/625/6-91/014 "Handbook: Control Technologies for Hazardous Air Pollutants."

NS = Not specified. There may be some control achieved - but no estimate of the efficiency was made.

NA = Not applicable (i.e., since there are no pitch storage tanks on site).
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Table 4: Estimated emissions reductions from control of HAP emissions from pitch storage tanks

Facility Name Plant Capacity

(ton Al/year)

HAP Controlled

(ton HAP/year)

Rio Tinto Alcan 216,051 0.177

Alcoa – South Plant 237,000 0.194

Alcoa – Warrick Operations 340,600 0.278

Alcoa – Massena Operations

(aka Alcoa – Massena West) 137,788 0.113

Noranda Aluminum 325,000 0.266

Century Aluminum of Kentucky 269,000 0.220

Century Aluminum of West Virginia 187,400 0.153

Columbia Falls Aluminum 185,200 0.151

Total: 1,898,039 1.552

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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Attachment B: MACT Floor Flow Chart