Aluminum Process 

Heaters

Aka: Annealing and Preheat Furnaces
Agenda

n Aluminum Process Heater Description, Design, 

Operation

n Aluminum versus Other Process Heater

n Consideration of Standard

n Formaldehyde Variability

n Recommendation
Process Description

n Indirect-fired furnaces primarily used to either:

n Pre-heat (soften) metal for further processing , or 

n Heat treat metal to effect physical properties

n Furnaces are essentially large ovens with 

temperature-controlled zones

n All natural gas-fired

n Industry wide – approx. 300 furnaces

n Alcoa – approx. 165 furnaces

n Average 23 per plant (range 2 -56 per plant)
Process Description

n Aluminum process heaters are small sources:

Total Heat Input 

(MM BTU/hr)

Percent age of 

Furnaces

Typical No. of 

Burners

<10 42% 12 to 30

10 to 30 47% 48 to 64

30 to 60 11% 35 to 140
Process Description

n Preheat Furnaces and Reheat Furnaces: 

Heat scalped ingots in preparation for Hot 

Rolling (890° – 1175° F)

n Annealing Furnaces: Heat sheet products 

(coil and flat) to increase ductility (400° -

1000°F)

n Aging Furnaces – Artificially age products to 

increase strength (225° - 400°F)

n Heat Treat Furnaces – Heat and water quench 

products to increase strength (875° - 1050° F)
Furnace Construction

n Furnaces have radiant tube burners.

n Objective is to transfer heat from combustion gases 

to the radiant tube and then radiate the energy to the 

load.

n Used in specific heating processes where 

combustion products cannot come in contact with 

the load or material.

n Radiant tube burners are an indirect heating burner -

- heat is transferred without any direct flame or 

combustion exhaust. 
Furnace Design

n Radiant Tube Burners

n < 1 MMBTU/hr each

n Between 16 And 140 Burners 

per furnace

n Burners controlled by zones

n Between 2 and 14 burners per 

control zone

n Full fire to achieve 

temperature and then cycle 

on-off to maintain

n Exhaust from burners vented 

to a header and then to 

atmosphere by natural draft
Furnace Design
Furnace Design
Furnace Design
Furnace Design

Burners

Common 

Exhaust 

Duct
Furnace Operation

n Furnaces operate in batch mode

n Controlled temperature ramp up from ambient

n Hold at constant temperature (soak)

n Controlled cool down

n Heat and cool pattern may repeat during cycle

n Water quench (heat treat only)

n Cycle time varies from hours to several days
Furnace Operation

n Load temperature critical to quality

n Temperature controls

n Burners modulate between high-fire (100%) and 

low-fire (20%) and pulse between low-fire and off

n By zone

n Fuel usage

n High (near capacity) at startup

n Low remainder of the cycle

n Annual burner capacity factor of 25%
Prior Alcoa/EPA Discussions

n Potential need for separate subcategory

n Alcoa committed to conduct furnace testing

n Data for three furnaces submitted
Aluminum Process Heaters

n Compared our units with ICR

n Significant design and operation differences

n Batch vs. Continuous

n Operating Temperatures 

n Aluminum Preheat vs. Steel vs. Paperboard vs. Hafnium kiln

n Large Number of Small Burners vs. fewer large burners

n Frequent Burner Cycling (low to high fire)

n Testing approach

n Test Time Duration (entire cycle vs. portion of cycle)?

n Alcoa believes it would be unworkable for EPA to set a 

single standard for all process heaters

n Aluminum requires a separate subcategory 
Considerations for Standard

n Basis for standard

n As measured vs. O2

adjustment

n Concerns about O2

adjustment factor

n Furnaces very close to 20.9% O2

n Measurement error becomes significant

n Standard corrected to a % O2

not workable

n Limited number of emission tests

n Does data represent MACT floor?

n Full range of operating conditions not tested

n Relationship between emissions and operating conditions not 

fully understood or characterized
Formaldehyde Variability
Formaldehyde Variability

n Load size, time, temperature, soak cycle
Recommendations

n Not technically feasible to set an emission limit 

for this subcategory

n Emission Control for existing units based on 

burner tuning/maintenance

n Work practice standard based on annual tuning 

only viable option