Ignition Zone Temperatures Preheat Zone Temperatures

Furnace Average

Standard 

Deviation

Percent 

Variation Average

Standard 

Deviation

Percent 

Variation

F F % F F %

11 2377 60 3% 1935 60 3%

12 2372 62 3% 1937 62 3%

6 2299 45 2% 1925 46 2%

NOx Generation in Northshore Mining Furnaces

Because Northshore burns natural gas, which contains little or no nitrogen, the majority of NOx generated 

in the furnaces is thermal NOx. Thermal NOx is generated through fixation of nitrogen in the combustion

air at high temperatures (thermal NOx is generally a significant contributor at combustion temperatures 

above 2000OF). The driver for thermal NOx generation rates in a natural gas furnace is flame 

temperature. Northshore does not directly monitor flame temperature, but does monitor temperature in 

the ignition and preheat zones of the furnace. The stability of temperatures in these zones as evidenced 

in the below table correlates to steady flame temperatures. Consistent temperatures correlate to 

consistent NOx emissions.

Furnace Operating Data Summary: 2000 - 2004

MPCA is asking for NOx CEM’s with the impression that furnaces can be ‘tweaked’ to reduce NOx 

generation. 

Furnace NOx Reduction Options:

 Decrease temperature of preheat air to combustion zone and/or burners: NSM already runs much 

lower preheat air than other facilities due to lower grade materials in furnace that cannot withstand 

higher temperatures (low grade steel, less refractory, no refractory lined ductwork). Burners are fed 

with same lower-temperature preheat air.

 Decrease excess air: Not practical, furnaces must run air-rich to create oxygen rich environment for 

induration to occur. The furnaces also require a large amount of excess air (roof air) as a 

temperature buffer in the furnace to prevent roof meltdown. Manual air control on burners – not 

practical to try and tune 28 burners per furnace to operate with slightly low air. Tuning burners is 

difficult on a manual system because it has to occur burner by burner and changing one burner may 

affect surrounding burners so tuning becomes an iterative process.

Furnace Adjustment Options:

 Gas Flow: Automatically controlled by bank (12 ignition, 10 preheat, 6 downdraft), but control on 

each burner has to be done manually.

 Burner Air: Manual controls for each burner. Real-time emissions data would not help because 

Northshore can’t make real-time adjustments to combustion.

 Pressure in firebox: Automatically controlled to keep firebox just slightly positive and prevent cold air 

from ‘leaking’ into firebox – poor fuel efficiency concerns.

Note that Northshore’s lack of controls on the furnace is probably the reason that our temperature profile 

is so steady. It is difficult to tune combustion so once the ideal setpoints were determined, they are not 

departed from.

NOx Emission rates are already quite low for Northshore Mining Furnaces. Benefits to be gained when 

emissions are this low from ‘tweaking’ burners are questionable.

2000 and 2005 Stack Test Summaries:

Date Stack Run NOx Emissions

ppm lb/hr lb/MMBtu

12/20/2000 12 WG --- 89 37.0 0.56

12/20/2000 F6 HE/WG --- 32 13.3 0.58

12/21/2005 11 WG --- 102 42.7 0.65

12/21/2005 F6 HE/WG --- 26 9.3 0.42
Source Data: Empire Mine: Nov, 

2004 Compliance Stack 

Test

Tilden Mine: Dec, 1999 

Engineering Tests

Northshore: Dec, 2005 

Compliance Stack 

Tests

HibTac: Jul 2004, Jul 

2005 Compliance Stack 

Tests

Inland: 2003, 2005 

Compliance Stack 

Tests

NOx Emission Rate Comparison

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

N. Gas N. Gas N. Gas N. Gas N. Gas

Unit 4 Unit 2 Furnace 11 Lines 1-3 Unit 1

Grate-Kiln Grate-Kiln Straight Grate Straight Grate Straight Grate

Empire Tilden NSM HibTac Inland

Facility / Furnace

NOx Emission Rate (lb/MMBtu)