Ignition Zone Tem peratures Preheat Zone Tem peratures

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 2000 O 

### F). The driver for thermal NOx generation rates in a natural gas furnace is flame temperature. Northshore does not directly monitor flame tem perature, 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 consist ent 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 sa me 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 f urnace to prevent roof meltdown. Manual air control on burners

– not practical to tr y and tune 28 burners per furnace to operate with slightly low air. Tuning burners is difficult on a manual s ys tem because it has to occur burner by burner and changing o ne burner may affect surrounding burners so tuning becomes an iterative process. **Furnace A djustment 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

- tim e emissions data would not help because Northshore can’t make real

- tim e 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 determine d, they are not departed from. NOx Emission rates are already quite low for Northshore Mining Furnaces. Benefits to be gained when em issions 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 W G

--- 89 37.0 0.56 12/20/2000 F6 HE/W G

--- 32 13.3 0.58 12/21/2005 11 W G

--- 102 42.7 0.65 12/21/2005 F6 HE/W G

--- 26 9.3 0.42

### Source Data:
E mpire 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 

### NO **x Em**  **is si**  **on Rat**  **e Com**  **pari son** 

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

N. G as N. G as N. G as N. G as N. G as

Un it 4 Un it 2 Fu rn ac e 11 Lines 1- 3 Un it 1 Gr ate- Kiln Gr ate- Kiln St ra ight Gr ate St ra ig ht G ra te St ra ight G ra te Emp ir e Tild en NS M Hi bT ac In land 

### Facility **/ Furnace N** **O x** **E m** **i s** **s i** **o n** **R a** **t e** **( l** **b /** **M M** **B t** **u )**