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MEMORANDUM

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

From: David Green and Mark Bahner, RTI

Date: October 17, 2011

Subject: Draft Technology Review for Primary Aluminum Reduction Plants

1.0 BACKGROUND

1.1 Requirements of Section 112(d)(6) of the CAA

Section 112 of the CAA requires the EPA to establish technology-based standards for sources of 

HAP. These technology-based standards are often referred to as maximum achievable control 

technology, or MACT, standards. Section 112 also contains provisions requiring the EPA to periodically 

revisit these standards. Specifically, paragraph 112(d)(6) states:

(6) REVIEW AND REVISION. – The Administrator shall review, and revise as necessary 

(taking into account developments in practices, processes, and control technologies), emissions 

standards promulgated under this section no less often than every 8 years.

1.2 Description of the Primary Aluminum Production Source Category and Requirements of 

the Current NESHAP

The current National Emissions Standards for Hazardous Air Pollutants (NESHAP) for Primary 

Aluminum Reduction Plants was promulgated on October 7, 1997 (62 FR 52407) as 40 CFR part 63, 

subpart LL (subpart LL). The rule was amended at 69 FR 18803, April 9, 2004; 70 FR 66284, 

November 2, 2005; and 71 FR 20458, April 20, 2006. The NESHAP applies to affected sources of HAP 

emissions at primary aluminum reduction plants. A primary aluminum reduction plant is defined as any 

facility manufacturing aluminum by electrolytic reduction. This electrolytic reduction process takes 

place in reduction cells, or pots, that are electrically connected in series to form potlines. Typically, a 

potline will be comprised of 100 or more reduction cells.

There are currently 15 facilities that are subject to subpart LL, four of which are presently idle.

One of these subject facilities does not manufacture aluminum but does produce anodes for primary 

aluminum reduction plants and contains affected sources. Emission limits have been promulgated for 

total fluoride (TF) as a surrogate for hydrogen fluoride (HF) and polycyclic organic matter (POM).

Affected sources include several types of new and existing potlines, all of which are regulated for TF 

and some of which are regulated for POM, new and existing anode bake furnaces which are regulated 

for TF and POM, new and existing paste production plants which are regulated by work practices to 

limit POM, and new pitch storage tanks which are regulated by work practices to limit POM.
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Control devices currently in use to reduce emissions from affected sources subject to subpart LL

include dry alumina scrubbers (with baghouses) and wet scrubbers to control TF and POM from 

potlines, dry alumina scrubbers (with baghouses) to control TF and POM from anode bake furnaces, dry 

coke scrubbers (with baghouses) to control POM emissions from paste production plants, and carbon 

adsorbers and condensers to control POM emissions from pitch storage tanks.

1.3 Current Practices

1.3.1 Potroom Emissions

At the time of promulgation of subpart LL two main reduction processes (Soderberg and 

prebake) were in use in the industry and several different pot configurations were in use for each of 

these processes. With the closure of approximately one third of the plants in the industry since 

promulgation, some configurations are no longer in use. The differences in achievable emission rates 

was recognized in development of subpart LL and emission limits for 7 different pot configurations 

were promulgated. Presently, 6 of these pot configurations exist in the U.S., however only 5 are 

presently in operation. No improved pot configurations have been developed in the U.S. or in other parts 

of the world since promulgation of subpart LL.

The Soderberg process is an older, higher emitting process in which the anode paste mixture is 

baked in place in the reduction cells to produce anodes. The prebake process is a newer lower emitting 

process and uses anodes produced in a separate anode bake furnace. The difference in the TF emissions 

between the different pot configurations is due almost entirely to the capture efficiency achievable with 

the different configurations. The same types of control devices are used and control efficiency of 

captured emissions is similar. In addition, the Soderberg process emits POM at greater rates because 

POM emissions produced from electrode production in the pots are higher than those produced from 

anode bake furnaces at prebake plants (reflecting both the low capture efficiency of the Soderberg cell

configurations and that prebaked anode production emissions are more efficiently captured and 

controlled from anode bake furnaces). The different variants of pot configurations in both the prebake 

and Soderberg processes have different TF emission limits reflecting capture efficiency. The different 

variants of pot configurations in the Soderberg process have different emissions limits for POM which 

reflect both the difference in capture efficiency and the higher POM emissions from Soderberg potlines. 

The current rule includes more stringent TF limits for new potlines (without regard to pot configuration) 

recognizing that the best controlled processes are those prebake processes with the best capture 

efficiency. The current rule also includes more stringent POM limits for new Soderberg potlines. The 

current rule does not include POM limits for existing or new prebake potlines.

Emissions of POM and TF from potrooms are comprised of emissions from stacks of the potline 

control device (typically a dry alumina scrubber followed by a baghouse), referred to as the primary 

emissions, and emissions from roof vents which extend over the length of the potroom, referred to as the 

secondary emissions. The secondary emission stream is much higher volume than the primary emission 

stream and is controlled by wet scrubbers at one currently operating facility and is uncontrolled at each 

of the remaining facilities.
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Carbonyl sulfide (COS), not currently regulated under subpart LL, is produced from the sulfur 

found as an impurity in the anodes. Approximately 5 to 10% of the sulfur present in the anodes is 

converted to COS, with the balance being converted to sulfur dioxide (SO2). No add-on control devices 

are currently in use by the primary aluminum production industry to reduce COS emissions. However, 

COS emissions from primary aluminum production facilities can be decreased by using coke with a 

lower sulfur content for anode preparation.

1.3.2 Anode Bake Furnaces

Anode bake furnaces are operated in all or nearly all prebake facilities, as well as one facility that 

does not produce aluminum but produces prebake anodes for use off-site. Anode bake furnaces emit 

POM which is released when the anode paste (consisting of a mixture of about 78% coke and about 22% 

pitch) is baked to produce anodes. Anode bake furnaces also emit TF, which results from recycling of 

the butt ends of previously used anodes (which have been exposed to fluoride in the pots) in the anode 

paste. All anode bake furnaces are controlled by dry alumina scrubbers followed by baghouses.

1.3.3 Paste Production Plants

Paste production plants are operated at all or nearly all facilities. Paste production plants emit 

POM which is released during the mixing of heated pitch with coke to produce anode paste. All paste 

plants are controlled with dry coke scrubbers followed by baghouses.

1.3.4 Pitch Storage Tanks

Pitch storage tanks are in operation at approximately 12 facilities. Pitch storage tanks emit POM 

from the displacement of POM laden gases during filling and from working losses that result from 

temperature changes. Most pitch storage tanks are uncontrolled. However, four facilities route pitch 

storage tank emissions through some type of control device. The most effective control devices are 

condensers and activated carbon adsorbers.

2.0 DEVELOPMENTS IN PRACTICES, PROCESSES AND CONTROL TECHNOLOGIES

For the purposes of this technology review, a “development” was considered to be a (n):

 Add-on control technology that was not identified during the development of the current 

NESHAP for the source category;

 Improvement to an existing add-on control technology that could result in significant 

additional HAP emissions reductions;

 Work practice or operational procedure that was not identified during development of the 

current NESHAP for the source category; or

 Applicable process change or pollution prevention alternative that was not identified and 

considered during the development of the current NESHAP for the source category.
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We investigated developments in practices, processes and control technologies through a 

literature review and discussions with industry representatives, and included questions in a section 114 

information collection request (ICR) that was sent to the companies subject to the NESHAP. No new 

practices, processes or control technologies to limit HAP emissions at primary aluminum plants have 

been identified that have been implemented since the promulgation of subpart LL.

3.0 CONCLUSIONS

This review did not identify any new developments in practices, processes or control 

technologies that have been implemented in this source category since promulgation of the current 

NESHAP. At the time that subpart LL was developed, the potline configurations in use differed 

considerably in their potential HAP emission rates, therefore seven separate subcategories of potlines 

were established, each with different MACT floors. As the industry has contracted from 23 facilities to 

15 facilities (11 of which are presently operating), some of the less well controlled potlines have ceased 

operation. New source MACT standards promulgated under the current NESHAP are consistent with 

emissions from the lower emitting potline subcategories.

Although no new developments in practices, processes or control technologies that have been 

implemented in this source category since promulgation of the current NESHAP, in general, existing 

technologies can be adapted to achieve lower emissions. As an example, POM and TF capture can be 

improved by increasing the flow through the dry alumina scrubber. Such an increase would require a 

redesign of the scrubber/baghouse system. Overall POM and TF emissions could also be reduced by 

retrofit of a control device to treat the high volume, low concentration secondary emissions from the 

potroom roof vents. Similarly, conversion of existing Soderberg potlines to prebake potlines would 

result in decreased emissions of POM and TF.