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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: D raft T echnology Review for Primary Aluminum Reduction Plants

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. 0 BACKGROUND

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. 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.

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. 2 Description of the Prim ary Aluminum Production Source Category an d 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 (6 2 FR 52407) as 40 CFR part 63, subpart LL (subpart LL)

. The rule was amended at 69 FR 18803

, April 9, 200 4 ; 70 FR 66284

, November 2, 2005 ; and 71 FR 20458

, April 20 , 200 6 . The NESHAP applies to affected sources of HAP
emissions at primary aluminum reductio n plant s. 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 reduct ion cells

.

T here are currently 15 facilit ies 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

an d 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.

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. 3 Current Pract ices

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 ap proximately 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 configuration s were promulgated. Presently

, 6 of these pot configuration s 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 S oderberg 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 furna ce. 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 device s are used and control efficiency of captured emi ssions is similar. In addition, the Soderberg process emits POM at greater rates because POM emissions produced from electrode production in the pot s are higher than those produced from
anode bake furn a ce s at prebake plants (reflecting both the low capture efficiency of the Soderberg cell configuration s and that prebaked anode production emissions are more efficiently captured and controlled from anode bake furnaces). The different variants of pot configuration s in both the prebake and Soderberg processes h ave different TF emission limits reflecting capture efficiency. The different variants of pot configuration s in the Soderberg process have different emissions limits for POM which
reflect both the difference in capture efficiency and the higher POM emissio ns 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 pot line s.

Emissions of POM and TF from potrooms are comprised of emissions from stacks of the pot line control device (typically a dry alumina scrubber followed by a baghouse) , referred to a s 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 s tream 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.3

Carbonyl sulfide (COS), not currently regulated under subpart LL , i s 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 ( SO 2 ). No add

- on control devices

are currently in use by the p rimary 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 pot s ) 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 facilit ies. Pitch storage tanks emit POM from the displacem ent of POM laden gases during filling and from working losses that result from temperature changes. Most pitch storage tanks are uncontrolled. However, f our facilities route pitch storage tank emissions through some type of control device. The most effecti ve control devices are
condensers and activated carbon adsorbers.

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. 0 DEVELOPMENTS IN PRAC TICES, PROCESSES AND CONTROL TECHNOLOGIES
For the purposes of this technology review, a “development” was considered to be a (n):

 A dd

- on control technology that was not identified during the development of the current NESHAP for the source category;

 I mprovement to an existing add

- on control technology that could result in significant additional HAP emissions reductions;

 W ork practice or operational procedure that was not identified during development of the
current NESHAP for the source category; or

 Applicable p rocess change or pollution prevention alternative that was not identified and considered during the development of the current NESHAP for the source category.4

We in vestigated developments in practices, proce sses and control technologies through a literature review and discussions with industry representatives, and included questions in a s ection 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

.

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. 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 practi ces, processes or control technologies that have been implemented in this source category since promulgation of the current NESHAP, i n general, existing technologies can be adapted to achieve lower emissions. As an example, POM and TF c apture can be improv ed 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 concentr ation secondary emissions from the potroom roof vents. Similarly, conversion of existing Soderberg potlines to prebake potlines would
result in decrease d emissions of POM and TF.