1

MEMORANDUM
To: David Putney, U.S. Environmental Protection Ag ency (EPA), OAQPS

From: David Green and Mark Bahner, RTI

Date: November 13, 2014
Subject: Revised Draft Technology Review for the Pr imary Aluminum Production Source

Category

1.0 BACKGROUND

1.1 Requirements of Section 112(d)(6) of the CAA Section 112 of the CAA requires the EPA to establis h technology-based standards for sources of hazardous air pollutants (HAP). These technology-ba sed standards are often referred to as maximum achievable control technology (MACT), standards. Se ction 112 also contains provisions requiring the EPA to periodically revisit these standards. Specif ically, paragraph 112(d)(6) states: (6) REVIEW AND REVISION. – The Administrator shall review, and revise as necessary (taking into account developments in practices, pro cesses, and control technologies), emissions standards promulgated under this section no less of ten than every 8 years.

1.2 Description of the Primary Aluminum Production Sour ce Category and Requirements of the Current NESHAP
The current National Emissions Standards for Hazard ous Air Pollutants (NESHAP) for Primary Aluminum Reduction Plants was promulgated on Octobe r 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. Subpart LL applies to affected sources of HAP emissions at primary aluminum reduction plants. A p rimary aluminum reduction plant is defined as any facility manufacturing aluminum by electrolytic red uction. This electrolytic reduction process takes place in reduction cells, or pots, that are electri cally connected in series to form potlines. Typical ly, a potline will be comprised of 100 or more reduction cells. There are currently 14 facilities that are subject to subpart LL, four of which are presently idle. One of these subject facilities does not manufactur e aluminum but does produce prebaked anodes for use at primary aluminum reduction plants that utili ze the prebake process and contains an affected source. Affected sources include new pitch storage tanks and new and existing paste production plants, anode bake furnaces and potlines. The current subpa rt LL includes emission limits for total fluoride (TF), as a surrogate for hydrogen fluoride (HF), fr om new and existing potlines and anode bake furnace s and for polycyclic organic matter (POM) from new an d existing Soderberg potlines and anode bake furnaces. The current NESHAP also includes work pra ctice requirements to limit POM emissions from new pitch storage tanks and new and existing paste production plants.2

Control devices currently in use to reduce emission s from affected sources subject to subpart LL include dry alumina scrubbers (with baghouses) or w et scrubbers to control TF and POM emissions from potlines, dry alumina scrubbers (with baghouse s) to control TF and POM emissions from anode bake furnaces, dry coke scrubbers (with baghouses) to control POM emissions from paste production plants and carbon adsorbers and condensers to contr ol POM emissions from pitch storage tanks. One facility also operates wet scrubbers to control sec ondary emissions from potlines. 1.3 Additional Requirements of Proposed NESHAP
The proposed NESHAP amendments include emission lim its for carbonyl sulfide (COS) from new and existing potlines; for POM from new and exi sting prebake potlines; for particulate matter (PM, as a surrogate for metal HAP) from new and existing potlines, anode bake furnaces and paste production plants; and for nickel and arsenic from new and exi sting Soderberg potlines and work practice requirements to limit POM emissions from existing p itch storage tanks and from potlines, anode bake furnaces and paste production plants during periods of startup.

1.4 Current Practices

1.4.1 Potrooms At the time of promulgation of subpart LL two main primary aluminum reduction processes (i.e., the Soderberg and prebake processes) were in use in the primary aluminum production source category and several different pot configurations were in us e for each of these processes (i.e., three for Sode rberg processes and four for prebake processes, for a tot al of seven pot configurations). The differences in achievable emission rates for each of the seven dif ferent pot configurations were recognized during development of subpart LL and different emission li mits were promulgated for each pot configuration. With the closure of approximately one half of the p lants in the industry since promulgation of subpart LL, two Soderberg pot configurations are no longer in use. Thus, only five of the original seven pot configurations presently exist in the U.S. No impro ved pot configurations have been developed in the U.S. or in other parts of the world since promulgat ion of subpart LL. The Soderberg process is an older, less efficient, higher emitting process in which the anode paste mixture is baked in place in the reduction ce lls to produce anodes. The prebake process is a new er, lower emitting process in which the anode paste mix ture is baked in a separate anode bake furnace to produce anodes. The difference in the TF emissions between the different pot configurations is due almost entirely to the capture efficiency achievabl e with the different pot configurations. The same t ypes of control devices are used and control efficiencie s of the captured emissions are similar. The differ ent variants of pot configurations in the prebake and t he Soderberg processes have different TF emission limits, reflecting their different capture efficien cies. The current subpart LL rule includes more str ingent TF limits for new potlines (without regard to pot c onfiguration, or process type) recognizing that the best controlled processes are those prebake process es with the best capture efficiency.

The difference in POM emissions between the differe nt Soderberg pot configurations are mainly due to differences in capture efficiency achievable between the different pot configurations whereas t he3

difference in POM emissions between Soderberg proce sses and prebake processes are due both to differences in capture efficiency achievable betwee n the different pot configurations and because POM emissions resulting from baking anodes in the pots (as in the case of Soderberg processes) are higher than those resulting from baking anodes in bake fur naces (as in the case of prebake processes). These higher POM emissions from the Soderberg processes r eflect both the lower capture efficiencies of the Soderberg cell configurations and that prebaked ano de production POM emissions are more efficiently captured and controlled from anode bake furnaces. T he different pot configurations of the Soderberg process have different POM emission limits for exis ting potlines in the current subpart LL rule, reflecting their different capture efficiencies, an d more stringent POM limits for new Soderberg potli nes, without regard to pot configuration, recognizing th at the best controlled processes are those pot configurations with the best capture efficiency. Emissions of POM and TF from potlines consist of em issions from stacks of the potline control device (typically a dry alumina scrubber followed b y a baghouse), referred to as primary emissions, an d emissions from roof vents which extend over the len gth (typically, about 1,000 feet) of the potroom, referred to as secondary emissions. The secondary e mission stream is much higher in volumetric air flow than the primary emission stream. The secondar y emission stream is controlled by wet scrubbers at one facility and is uncontrolled at each of the rem aining 11 facilities that are equipped with potline s. COS emissions, which are not currently regulated un der subpart LL, are produced as the anodes are consumed in the reduction cells and the carbon, sulfur (found as an impurity in the anodes) and alumina react. Approximately 5 to 10 percent of the sulfur present in the anodes is emitted as COS, wi th the balance being converted to sulfur dioxide (SO 2 ). No add-on control devices are currently in use b y the primary aluminum reduction industry to reduce C OS emissions. Theoretically, COS emissions from primary aluminum reduction facilities can be decrea sed by using coke with a lower sulfur content for anode preparation. However, petroleum coke is the s ource of anode grade coke used in the primary aluminum industry and the availability of low sulfu r content petroleum coke is limited and insufficien t to meet the current needs of the primary aluminum i ndustry. PM emissions, which are not currently regulated und er subpart LL, consist of condensed emissions of HAP metals, metal oxides and fluoride salts volatilized from the pots, as well as suspend ed alumina. Capture systems and control devices presen tly in place at all potlines to control emissions o f TF and POM are also effective for control of PM emi ssions. As with TF and POM, secondary emissions of PM make up the majority of potline PM emissions and are minimized by effective capture of emissions in the hooding and routing them through t he primary emissions control device.

1.4.2 Anode Bake Furnaces Most prebake facilities, as well as one facility th at does not produce aluminum but produces prebaked anodes for use off-site, are equipped with , and operate, anode bake furnaces. Anode bake furnaces emit POM which is released when the anode paste (consisting of a mixture of about 78 percent coke and about 22 percent pitch) is baked to produc e anodes and TF, which results from recycling of th e butt ends of previously used anodes (which have bee n exposed to fluoride in the pots) in the anode4

paste. Anode bake furnaces also emit PM which resul ts from condensed organics volatilized from the green anodes, fluoride salts and dust associated wi th the placement and removal of packing coke (used as an insulator). These PM emissions are known to c ontain HAP metals. POM and TF emissions from all anode bake furnaces are controlled by dry alumi na scrubbers followed by baghouses. These controls are also effective for control of PM.

1.4.3 Paste Production Plants Most prebake and Soderberg facilities are equipped with, and operate, paste production plants. Paste production plants emit POM which is released during the mixing of heated pitch with coke to produce anode paste. Emissions at all paste product ion plants are controlled with dry coke scrubbers followed by baghouses. Paste production plants also emit PM generated from the mixing of petroleum coke and pitch, as well as condensed pitch vapors. These PM emissions are known to contain HAP metals. The systems used to control POM emissions f rom the paste production plants are also effective for control of PM emissions from these sources. 1.4.4 Pitch Storage Tanks Most prebake and Soderberg facilities are equipped with, and operate, pitch storage tanks. Pitch storage tanks emit POM from the displacement of POM laden gases during filling and from working losses that result from temperature changes. Most o f the existing pitch storage tanks at these facilit ies are uncontrolled. However, three facilities do have control devices for pitch storage tank emissions. The most effective control devices are condensers and a ctivated carbon adsorbers.

2.0 DEVELOPMENTS IN PRACTICES, PROCESSES AND CONTROL TE CHNOLOGIES For the purposes of this technology review, a “deve lopment” was considered to be a (n):

• Add-on control technology that was not identified d uring the development of the current NESHAP for this source category;

• Improvement to an existing add-on control technolog y 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 this source category; or

• Applicable process change or pollution prevention a lternative that was not identified and considered during the development of the current NE SHAP for this source category. We investigated developments in practices, processe s and control technologies through a literature review and discussions with industry rep resentatives, and included questions in an informat ion request that was sent to the companies subject to s ubpart LL in 2011.
2.1 Potline, Anode Bake Furnace and Paste Productio n Plant Startup Procedures The MACT control technology typically used for potl ines and anode bake furnaces is a dry alumina scrubber followed by a baghouse. The MACT c ontrol technology typically used for paste5

production plants is a dry coke scrubber followed b y a baghouse. Facilities further decrease emissions from these sources by implementation of certain pra ctices during periods of startup. For example, for anode bake furnaces and paste production plants the se practices include the development of startup schedules and operation of the associated control s ystems within normal parametric limits prior to startup. Some examples of practices used during sta rtup of potlines include the development of startup schedules, minimizing the time that hooding is remo ved during anode changes, reducing cell temperatures to as low as practicable, and resealin g pot crusts that have been broken as often and as soon as practicable. These startup work practices w ould be required under the proposed rule.

2.2 Substitution of Coal Based Coke in Anodes We investigated the feasibility of substituting met allurgical coke for the petroleum coke used by all facilities in the production of anodes. Metallu rgical coke contains lower sulfur and would thus le ad to lower emissions of COS. In addition, metallurgical coke has a lower content of some HAP metals, particularly nickel. Testing concluded that unproce ssed coke produced from coal does not have properties suitable for the production of carbon an odes. Coal-based coke has a high ash content and th is leads to unacceptable properties, including poor el ectrical conductivity, high carbon consumption and unacceptable aluminum purity. 1 Coal or coal-based coke could be processed (for exa mple by solvent extraction) to remove the impurities at a considerable cost. However, anodes produced with 100 percent processed coal-based coke have similar drawbacks, such as brittleness an d high carbon consumption. 2 While it may be technically feasible to produce acceptable anodes b y blending petroleum based coke and processed coalbased coke, the need to blend with petroleum coke t o produce usable or acceptable anodes defeats the initial benefits of removing the impurities. In any case, suitable processed coal-based coke is not commercially available today. 2.3 Improved Potroom Capture Efficiency A process has been developed 3 to improve capture of emissions from prebake pots by detecting the drop in temperature in the air space above the pot crust, but within the hooding, associated with the temporary hooding removal for anode changes and usi ng a control system to alter the air flow on a potby-pot basis. When hoods are open, pressurized air is supplied to the inside of the hood and increased suction is supplied through a second duct to direct emissions to the dry alumina scrubber. As describe d, this system should improve capture efficiency and d ecrease secondary emissions. However, emission standards are not being proposed based on the insta llation of this process because: 1) It is not currently in use in any primary alumin um production facility in the U.S.; 2) It is not known to be suitable for retrofit to e xisting potlines; 3) No data are available regarding the actual emiss ions of TF and POM from a potroom equipped with this system; 4) No information is available about the cost of re trofit or the incremental costs associated with implementing this system in a new plant; and6

5) While the patent description is publically avail able, details necessary for successful implementation are proprietary.

3.0 CONCLUSIONS Other than the startup procedures for potlines, ano de bake furnaces and paste production plants, this review did not identify any developments in pr actices, processes or control technologies that hav e been implemented in this source category since prom ulgation of the current NESHAP that indicate any proposed amendments to NESHAP would be appropriate under the Section 112(d)(6) technology review. 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 primary aluminum production industry has contracted from 23 facilities to 14 facilities (10 of which are presently operating) in the time since promulgation of the current NESHAP, some of the hig her emitting potline subcategories have ceased operation. Proposed new source MACT standards are c onsistent with emissions from the lower emitting potline subcategories. Although no new developments in practices, processe s or control technologies have been implemented in this source category since promulgat ion of the current NESHAP (again, other than startup procedures), existing technologies could be adapted to achieve lower emissions. As an example, POM and TF capture can be improved by increasing th e flow through the dry alumina scrubber. However, such an increase in flow would require a r edesign of the scrubber/baghouse system and the data needed to estimate the associated reduction in emissions are not available. Overall POM and TF emissions could also be reduced by retrofit of a co ntrol device to treat the high volume, low concentration secondary emissions from the potroom roof vents. However, it is not known whether the current buildings could accommodate such control de vices (e.g., it is not known whether the necessary space is available or whether the building structur e could support the additional weight). Therefore, the EPA is not proposing any amendments to the NESHAP b ased on these control measures. Similarly, conversion of the one remaining existing idle Soderberg facility (i.e., Columbia Falls) to prebake potlines would result in decreased emiss ions of POM, TF and PM. However, the costs associated with such a conversion would likely be p rohibitive. Therefore, EPA is not proposing standards based on conversion to a prebake potline configuration.

1 Wells, C. Personal communication to David Putney, May 30, 2014.
2 Hamaguchi, M., et al. Prebaked Anode from Coal - U tilization of Coal Extract as a Coke Feedstock. Light Metals. 2010. p. 923-928.
3 Girault, G, and P. Cantin. Patent PCT/IB2007/00161 2 (US200090159434 A1). System and Process for Collecting Effluents from an Electr olytic Cell. 2009.