Primary Aluminum Production 

and COS Emissions

Charles Johnson

May 23rd, 2011
2

Aluminum Smelter COS Emissions

• Source of COS Emissions

• Health Effects Data

• COS Emissions from US Smelters

• Current Industry Permit Limits
Source of COS Emissions

• Carbonyl sulfide (COS) is produced during the consumption of carbon anodes

• Process involves electrolytic reduction of alumina to aluminum:

2Al2O3 + 3C -> 4Al + 3CO2

• Large blocks made from petroleum coke are source of carbon

Molten Aluminum

Cathode

Electrolytic bath

and alumina
Source of COS Emissions

• Carbon anodes contain sulfur as thiophene (C4H4S) or 

similar carbon-sulfur species

• Carbon-sulfur species form COS in the gases below the 

anode

• COS oxidizes to SO2 and CO2 as the anode gases pass 

up through the electrolyte bath:

2COS + 3O2 -> 2SO2 and 2CO2

• COS formation in the smelting process is not 

preventable
Health Effects Data

• COS is the most abundant sulfur compound naturally 

occurring in the atmosphere

– Emitted by oceans, soils, volcanoes, salt marshes, etc.

• A listed hazardous air pollutant

• Little toxicological data available for health effects on 

humans and animals

– Concerns about COS exposure due to acute toxicity

– Central nervous system effects in lab animals at very high 

concentrations (>1000 ppm)

– Limited data show no chronic effects on lab animals at low 

concentrations (50 ppm)

• COS concentrations:

– Smelter stack gas concentration ~ 8 ppm

– Ambient air (24-hr average) ~ 8 ppb

– Natural background – 0.5 ppb
COS Emissions in US Smelters

• Limited data available on COS emissions

• Design and operating factors that impact 

COS emissions are not fully understood

– Additional data needed to define appropriate 

COS standards

• Standard based on low coke sulfur content 

not workable for industry long-term

– Availability of low sulfur coke in question

– Cost of Low S Coke affects global 

competitiveness

– Control technology not economically feasible

– Shutdowns possible
Aluminum Smelter COS Emissions

May 23, 2011
8

Aluminum Smelter COS Emissions

Factors Affecting Emissions

COS Emission Controls

Industry Emission Factor Approach

Summary of Alcoa COS Data

Options for COS Regulation

Coke Market and Availability

Suggestions for COS MACT
Factors Affecting Emissions

Formation of COS in the smelting process is not preventable

Total COS emissions depend on:

 Smelting pot design (pot size and anode configuration)

 Anode gas residence time

 Pot operating temperature

 Type and amount of sulfur compounds present in the coke

Impact of design and operating practices on COS have not been 

studied extensively by the aluminum industry

 No demonstrated work practices for the control of COS

“No control floor” concluded as part of original Subpart LL MACT
COS Emission Controls Not Cost-Effective

No add-on controls currently installed specifically to control COS

Alumina dry scrubbers do not reduce COS emissions

 All sulfur in raw materials released as either COS or SO2

Massena Soderberg operates a wet scrubber for SO2 control

 Wet scrubber allowed installation of alumina dry scrubber system

 Sodium bicarbonate reagent

 Limited data suggests 34% COS removal (one test run)

No other smelters operate wet scrubbers for SO2 or COS control

 Wet scrubbing not economically viable for existing smelters

 Capital cost >$200 to $250 million per smelter

 Operating cost > $40 million/year

 Annual cost to control SO2 would be ~$9000/ton SO2 removed

 Annual cost to control COS would be ~$230,000/ton COS removed
Industry Emission Factor Approach: “%S as COS”

Factors based on “% S emitted as COS”

 Provides estimate of COS emissions

 Assume “%S as COS” does not change with anode sulfur content

 Assume average factor applies for different pre-bake smelters 

Alcoa emission tests at several smelters:

 Simultaneous measurement of COS and SO2 at dry scrubber stack

 Extractive FTIR procedure (high level of QA/QC)

 Not an EPA Reference Test Method

Calculate %S emitted as COS: 

Mass balance for potline total sulfur

 Measure baked anode sulfur content

 Track number of anodes consumed
Variation in COS emission across pot types

Large variability in short-term emission rates

% Sulfur as COS

Center-work Prebake Soderberg

Side-work

Center-work Prebake Soderberg

Side-work
COS emissions strongly correlated with smelting pot size

COS emissions do not correlate with coke sulfur level

Emissions based on “lb COS/ton Al” strongly correlated with pot 

design

 Larger pots have lower emission in terms of “lb COS/ton Al”

Coke sulfur level not a good basis for predicting COS emissions

 No clear correlation between COS emissions and coke sulfur levels

• Pot design dominates differences

 No data available to compare COS emission rates from same potline 

with different coke sulfur levels
COS emission rate strongly correlates with smelting pot size

lb COS/ton Al

Increasing Prebake Pot Size 
COS emissions do not correlate with coke sulfur level

lb COS/ton Al

%S in coke (Prebake potlines)
Options for COS Emission Standard

Existing MACT floor for COS is “no control”

 Except possibly wet scrubber at HSS potline

Option 1: Provide aluminum industry time to develop a 

suitable basis for COS standard

 Thorough evaluation of design and operating approaches for 

minimizing COS emissions

 Subcategorize pot types where appropriate

 Set emission standards by subcategory

Option 2 – Set standard based on coke sulfur content

 Simple, pollution prevention limit

 However, such standard also must consider long-term availability 

of anode quality coke

 Standard based on coke sulfur content <3% not workable for 

industry long-term
A 2% sulfur limit in CPC will curtail aluminum 

production due to lack of supply
Anode grade CPC is defined by more than the % sulfur

18

Pet coke can have 3 types of structures that dictate use

Shot coke Sponge coke Needle coke 23

2

67

Demand for GPC

Calcinable Electrode Fuel

3 main end use markets for pet coke

Fuel

-Any sulfur level

-No constraints on 

other parameters

-Main consumer of 

shot coke. Can 

consume any 

grade

Calcinable

-Low to mid range 

sulfur

-Specification 

constraints based 

on application

-Main consumer of 

sponge coke. Non 

anode application 

can consume shot 

in the blend

Electrode

-Typical .5% sulfur

-Very low 

impurities 

including metal

-Primarily used for 

EAF electrodes

-High premium 

product produced 

at 5 locations 

globally

20

1 2

Demand for calcinable GPC

Aluminum TiO2 Other

Million metric tons

Source: Team analysis
-Not all the crude drilled 

is refined in a manner 

that produces coke

- The blend of crudes 

selected by a refinery is 

the cheapest usable 

blend available

- The cheapest blend 

today typically include 

more sour crudes such as 

from Canada

-The blend used in a 

refinery determines the 

quality of the pet coke 

waste stream

The availability and quality of petroleum coke is not determined 

by the needs of the aluminum industry

19

EXPLORATION & 

DRILLING OF CRUDE 

OIL

SIMPLE REFINERIES WITHOUT COKERS

Light Products

Gasoline

Diesel..

Fuel Oil/ 

Asphalt

COMPLEX REFINERIES WITH COKERS

Green Pet 

Coke

-~2% of refinery 

revenue

-1.33 MT needed 

per MT CPC

Light Products

Gasoline

Diesel..

73%

16%

Fuel grade Aluminum smelting

Met coal blending Other

Only ~16% of green coke goes to aluminum

Low sulfur 

coke

Pet coke is a waste stream in the production of light distillates and accounts for <2% of the revenues of a coking 

refinery

-The coke that goes to the aluminum industry is composed of a 

large spectrum of sulfur levels 

- The coke consumed for met coal blending is characterized by 

sulfur levels of 2% and below only

The souring of 

crude slates is 

projected to 

increase fuel 

grade material 

at the expense 

of anode 

grades

To Calciner to 

produce anode 

grade CPC

Source: Team analysis
6,323

10,321

4,073

2,164

1,590

2011 GPC demand 179

5-6% S

4-5% S

3-4% S

2-3% S

1-2% S

0-1% S

2011 demand from the global aluminum industry will challenge 

existing supply of anode grade GPC

20 Source: Jacobs Consulting, Team analysis

There is insufficient supply of GPC at the <2% sulfur level to meet the current demand for anode grade material 

2% level

With a forecasted 

degradation of sulfur levels in 

crude blends refined, there 

will be insufficient coke to 

meet a 2% limit to the sulfur 

levels.

Global Supply Demand

Million metric tons
The cost of coke increases significantly as the % sulfur goes 

down

45% increase in raw material cost to move from 2.5% to 1.9% sulfur

31% increase in raw material cost to move from 1.9% to 1.3% sulfur

 The green coke price increase translates to ~$39/ MT increase in the cost of aluminum produced

21 Source: Jacobs Consulting US Gulf Coast blend prices, Team analysis

Smelting aluminum in the United States will materially cost disadvantaged relative to other 

regions if the max allowed sulfur is lowered.

$307

$235

$162

$120

$157

$128

1.3 1.9 2.5 3.3 3.5 4.1

% Sulfur in green coke blend

Q4 2010 Green Coke Pricing, $/DST
Summary

The souring of crude slates and the underlying economics of the petroleum industry is resulting in 

a fall in production of low sulfur green coke.

Since the 2007 review, refining economics has exacerbated the situation for the aluminum 

industry with refineries curtailing the production of anode grade green petroleum coke

 North America has lost ~16% of anode grade green coke production in the last 18 months alone due to 

refinery shut downs or switch to higher sulfur crudes

Aluminum demand is expected to grow at ~5% annually to ~70M MT by 2020 from the current 

level of ~45M 

The cost of green coke and CPC increases disproportionately as the sulfur level is reduced

There is no viable scenario we are aware of that will allow for sufficient growth in the availability of 

CPC at or below a 2% sulfur limit to meet the demand for aluminum 

 China and Brazil are the only regions of the world where there is a growing availability of low sulfur GPC. 

However strong GDP growth in these regions has resulted in self sufficient markets with low volumes 

available for export to other regions

There is insufficient 2% sulfur CPC available to meet current and projected global aluminum 

demand globally and regionally

22
Smelters with coke sulfur limits <3% have long-term issue

• Smelters with coke sulfur limits below 3% have long-term 

issue

– Seek permit revision – SO2 NAAQS may prevent this

– Install wet scrubbers for SO2 – not economically feasible

– Shutdowns possible

• Coke limits below 3% may also effect viability of wet 

scrubbing

– The lower cost of high sulfur coke may offset portion of wet 

scrubber operating costs

– Low sulfur coke limits eliminate this cost offset 

23
Recommendations for COS MACT 

• Provide aluminum industry time to develop a suitable 

basis for COS standards

– Account for differences in pot type or pot design

– Define operating parameters and practices that minimize 

emissions

• Standard for COS based on low sulfur coke not workable 

for industry long-term:

– Level of sulfur in anode quality coke is increasing

– Expect long-term shortage of lower sulfur anode coke

– 3% sulfur in coke recommended as viable long-term limit