ABSTRACT

A research project was undertaken to determine if coal addition to

fluxed-magnetite pre-fired pellets would reduce nitrogen oxide (NOx)

emissions during pellet induration. Iron oxidation rate tests, coal

oxidation rate tests and calcination rate tests were conducted in a

laboratory furnace to determine the amount of coal necessary to

provide adequate heat for limestone calcination without producing

detrimental effects to magnetite oxidation. Pilot scale pot-grate tests

were then performed with and without coal in the pre-fired pellets to

verify pellet quality parameters and nitrogen oxide emissions. The offgas during each pot-grate test was analyzed for NOx, CO2, CO, SOx,

and O2. There was approximately a ten percent reduction in NOx emissions with coal addition during the normal pot-grate firing cycle.

There was a 75% reduction in NOx emissions during firing cycles that

used 25% to 75% excess air during firing rather than the normal 300%

excess air.

INTRODUCTION

The production of nitrogen oxide during pellet induration is an

environmental concern. Previous work performed by the Coleraine

Minerals Research Laboratory (CMRL) and the United States Bureau

of Mines (USBM), demonstrated that NOx formation during iron ore

pellet induration is directly related to the amount of fuel used (BTU

input) and the type of fuel used. Ninety-nine percent of the NOx emitted from pellet-induration furnaces is NO (Zahl, et al., 1995).

When NO enters the atmosphere it eventually reacts with oxygen to

form ozone and NO2. The NO2 is soluble in water and, therefore,

contributes to the formation of acid rain. Ozone is an air pollutant that

is a major component of smog and poses a human health risk (USEPA, 1998). Natural gas produces more NOx during pellet induration

than coal (Zahl, et al, 1995 and Carter, et al., 1984). Fluxed pellet

production requires more heat input than does acid pellet production

due to the additional heat required to decompose (calcine) the

limestone and dolomite in the fluxed pellets. The heat required for

calcination is added to the induration process through pre-heat

burners that burn only natural gas. It takes approximately 350,000

Btu to indurate one long ton of acid pellets and 500,000 Btu to

indurate one long ton of fluxed pellets. Coal having a heat content of

11,500 Btu per pound would produce about 150,000 Btu per long ton

of pellets if the dry pre-fired pellets contain 0.6% coal. Coal addition

to pre-fired pellets could eliminate the need for pre-heat burners.

Coal addition will increase the pellet temperature during coal

oxidation and thus require less heat from the natural gas flame. This

sounds good in theory, however; the coal competes for oxygen with

the magnetite. The oxidation of magnetite (Fe3O4) to hematite

(Fe2O3) creates a crystalline-structure change that gives the fired

pellet strength. Coal that is heated in a magnetite pellet and cannot

obtain oxygen from the air will obtain oxygen from the magnetite. The

magnetite is reduced rather than oxidized. This means that the

process must be controlled so that both coal oxidation and magnetite

oxidation occur. Correct oxidation control of magnetite and coal

would mean that the addition of coal to pre-fired fluxed pellets would

eliminate the use of natural gas for pre-heating the pellets and thus

decrease the amount of NOx formed during pellet induration.

Pellet temperature and an adequate quantity of air (oxygen) are

very important parameters for creating rapid oxidation of magnetite.

Pellets are heated to a temperature of approximately 2200 F to 2300

F with a natural gas flame that uses 300 percent to 400 percent

excess air during the induration process. The flame temperature in a

natural gas fired induration furnace can be over 3000° F. A natural

gas flame at 3000° F with 400 percent excess air present can produce

more than 4 pounds of NOx (as NO2) per thousand standard cubic

feet (MSCF) of natural gas burned (Zahl, et al., 1995). Figure 1

illustrates NOx production based on flame temperature in an

induration furnace burning natural gas with and without excess air

(Davis, 1995). Firing cycles that contain low quantities of excess air

should produce less NOx than firing cycles with large quantities of

excess air.

A test program was developed to determine if coal addition to prefired fluxed pellets and firing cycles with low quantities of excess air

produce less NOx than normal fluxed pellet induration.

TEST PROGRAM

Vertical Tube Furnace Tests: Thirty-six vertical-tube furnace tests

were conducted on pellets produced with Minntac fluxed filter cake,

bentonite, and coal. Pellets were produced using no coal, 0.6% coal,

and 1.0% coal. Tests were run at temperatures of 850 C (1562F),

950 C (1742F), 1050 C (1922F), and 1200 C (2192F). Test duration

was 6, 10 and 14 minutes. The dried pellets were lowered into the hot

1 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Preprint 03-001

NITROGEN OXIDE (NOX) EMISSION REDUCTION DURING PELLET INDURATION BY FUEL-ADDITION TO THE

GREEN-PELLETS AND DECREASED EXCESS AIR

J. Engesser

Minnesota Dept of Ntrl Resrcs

Hibbing, MN
zone of the furnace with an airflow of one liter per minute for the test

duration. At the end of each test the air was turned off and a nitrogen

purge of five liters per minute was started. The pellets were then

lowered into a cooling chamber and purged with nitrogen until cool.

These tests were conducted to study the oxidation rates of coal, the

oxidation rates of iron, and the calcination rates of the fluxstone

(limestone and dolomite). The tests were also used to study how coal

oxidation in a pellet affects magnetite oxidation. Pre-fired dried

pellets and fired pellets were pulverized and analyzed for carbon (C),

carbon dioxide (CO2), total iron (Fe), ferrous iron (Fe++), and

magnetite (Fe3O4).

Pot-Grate Furnace Tests: Twelve pot-grate furnace tests were

performed. Tests were conducted with and without coal in the prefired pellets. A Continuous Emission Monitor (CEM) trailer was

connected to the off-gas stack of the furnace to continuously analyze

the off-gas for oxygen (O2), carbon dioxide (CO2) , carbon monoxide

(CO), nitrogen oxide (NOx) , and sulfur dioxide (SO2) during each

test. Fuel rates and airflow rates were continuously measured during

each test. Pellet bed temperatures, and hood exhaust temperatures

were recorded at 5-second intervals. Three firing cycles were tested

and are described in Table 1. The pre-fired pellets and fired pellets

were analyzed for total Fe, Fe++, CO2, C, S, CaO, and MgO. Pellet

quality tests (LTD, reducibility, pellet compression, before tumble, and

after tumble) were conducted on samples from each pot-grate test.

DISCUSSION OF RESULTS

Vertical Tube Furnace Tests: Vertical tube furnace tests were

performed to study the oxidation rate of magnetite when coal is

present in pre-fired pellets based on time, temperature, and coal

content. Rate equations for magnetite oxidation, coal oxidation and

calcination were calculated. Pre-heat pellet compression strengths

were also determined. Data from the vertical tube furnace tests are

tabulated in Table 2. Calculated rate equations for magnetite

oxidation, coal oxidation, and fluxstone calcination are listed in Table

3. Oxidation and calcination rate equations were calculated using a

reiteration program that maximized the correlation coefficient. The

equations were of the form

%Oxidation = 100 – 100e-kt

%Calcination = 100 – 100e-kt

where k is the rate constant and t is the time in minutes. Figures 2, 3

and 4 are plots of the rate constants versus temperature. A larger rate

constant represents a faster reaction.

Maximum oxidation rates, as illustrated in Figures 2 and 3,

occurred at approximately 1900 F. Magnetite oxidation rates

dramatically slowed when coal addition was increased from 0.6% to

1.0% (Figure 2). Magnetite oxidation rates were higher with 0.6%

coal than with no coal when oxidation temperatures were less than

1922 F. As the temperature increased from 1922 F to 2192 F,

2 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 1 Figure 2

Figure 4

Figure 3
oxidation rates of both magnetite and coal decreased. This was

probably due to rapid oxidation and formation of an outer shell on the

pellet that did not allow oxygen to penetrate into the core of the pellet.

It is apparent from the vertical-tube-furnace oxidation data that an

optimum oxidation temperature existed whereby the oxidation rates of

both coal and magnetite were maximized. A temperature that was

either too high or too low decreased the oxidation rates of both

magnetite and coal. The optimum oxidation temperature for the filter

cake used in these tests was about 1900 F.

The calcination rates when 0.6% coal was present and when no

coal was present were about equal (Figure 4). Calcination rates were

higher with 1.0% coal than with 0.6% coal (Figure 4).

Pellet compression is an important quality parameter. Figures 5

and 6 demonstrate what occurred to pellet compression strength with

and without coal in pre-fired pellets. Figures 5 and 6 indicate that if

coal is present in the pre-fired pellets, the pre-heat firing temperature

must be held below 2000 F. Pellets that were heated at 1922 F had

about the same compression strength for tests with and without coal.

The compression strength decreased when the pre-heat temperature

increased from 1922 F to 2192 F and coal was present in the pre-fired

pellets. The data illustrated in Figures 5 and 6 demonstrate that when

coal is added to pellets, the pre-heat firing temperature must be

controlled in order to maintain adequate pellet strength. It appears

that the optimum oxidation temperature was equal to the optimum

compression-strength temperature. At 2192 F oxidation of magnetite

in the pellet core was apparently inhibited due to the formation of a

gas impermeable outer pellet shell that formed. The non-oxidized

magnetite inside these pre-heat pellets led to weak pellets.

Pot-Grate Furnace Tests: Eight fluxed-pellet pot-grate tests were

conducted using a normal firing cycle with 300 percent excess air

during the firing cycle. Four fluxed-pellet pot-grate tests were

conducted using 75% and 25% excess air in the firing cycles. The

firing cycles used are described in Table 1. 

NOx Production: There was a decrease in the amount of nitrogen

oxides (NOx) produced with coal present in the pre-fired fluxed-pellets

(Figure 7). 

It should be noted that most of the heat input used in the pot-grate

pellet induration is used to heat the pot-grate apparatus. If only 20

percent of the heat is used to indurate the pellets, then a 10%

reduction in NOx production in the pot-grate test could mean a 50%

NOx reduction in a continuous indurating-furnace. Figure 7 illustrates

the decrease in NOx emitted from fluxed pellets that contained coal

during pot-grate induration tests.

There was about a 75 percent reduction in NOx formation when

the firing cycle included an oxidizing air cycle with no flame and firing

cycles with low amounts of excess air (25% to 75%) supplied during

firing. Figure 8 illustrates the NOx produced with and without coal

using a low amount of excess air. The base case data were run

without coal and with 300% excess air during firing. A summary of the

gas analysis data, airflow data and BTU data for the pot-grate tests

are listed in Table 4. 

Fired Pellet Quality – Normal Cycle (High Excess Air): A summary

of the fired pellet quality data for the pot-grate tests is listed in Table

4. 

Pellet compression strength of the fired pellets decreased when

coal was present in the pre-fired pellets. Pellet compression strength

generally increased with increasing pellet-firing temperatures;

however, when 1% coal was present in the pre-fired pellets, the pellet

compression strength decreased with increased pellet firing

temperature. The addition of 0.6% coal to the pre-fired pellets

decreased the compression strength of the fired pellets by

approximately 100 pounds when compared to pellets that contained

no coal. The addition of 1.0% coal to pre-fired pellets decreased the

compression strength of fired pellets by more than 200 pounds.

Figure 9 illustrates the relationship that exists between pellet

compression strength, coal addition rate, and pellet firing temperature

for fluxed pellets.

3 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 5 Figure 6

Table 1

Normal (High Excess Air) Firing Cycle

Cycle Time, Temperature Air

Name minutes F Type

Down Draft Drying 6 900 600% Excess

Down Draft Pre-heat 6 1900 to 1950 300% Excess

Down Draft Firing 10 2200 to 2350 300% Excess

Heat Recoup 2 2000 600% Excess

Air Cooling 8 to 10 No Flame Excess

Low Excess Air Firing Cycle

Cycle Time, Temperature Air

Name minutes F Type

Down Draft Drying 2 650 800% Excess

Down Draft Drying 2.5 1350 200% Excess

Down Draft Pre-heat 2.5 1950 150% Excess

Down Draft Firing 2 2100 75% Excess

Air Oxidation 6 No Flame Excess

Down Draft Firing 5 or 15 2200 to 2350 25% Excess

Air Cooling 12 to 14 No Flame Excess
4 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 7

Figure 8

Figure 9 Figure 10

Figure 11 Figure 12

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Average Peak Pellet Temperature Compared to NOx 

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• No coal ao 6'% cto.r --:4,1.0%-C-oiifPeUet Tumble Index and Pellet Reducibility 
Reducibility and Tumble Index correlate very well with pellet firing

temperature. As the pellet firing temperature increased the

reducibility decreased. The reverse is true for the tumble index. As

the pellet firing temperature increased, the tumble index increased.

The addition of coal to the pre-fired pellets did not affect the

reducibility of the pellets. The pellet reducibility is affected more by

pellet firing temperature than by coal addition. However, coal addition

does change the pellet temperature. Figure 10 illustrates the

relationship that exists between pellet firing temperature, coal addition

rate, and pellet reducibility for fluxed pellets. 

The addition of coal to pre-fired pellets decreased the tumble

index by 1 to 4 percent. Figure 11 illustrates the affect that coal

addition and firing temperature have on tumble index for fluxed

pellets.

An inverse correlation exists between tumble index and

reducibility. This means that if a required reducibility (independent

variable) is a production parameter then the tumble index (dependent

variable) will depend on the reducibility. It also means that if the

required production parameter is the tumble index (independent

variable) then the reducibility (dependent variable) will depend on the

tumble index. Figure 12 illustrates the relationship between

reducibility and tumble index. The correlation coefficient squared for

the regression between tumble index and reducibility is 0.884. 

Low Temperature Degradation (LTD) of fluxed pellets decreased

with increasing pellet-firing temperature. Figure 13 illustrates this

fact. Peak pellet temperatures less than 2200 F were not tested in the

pot-grate tests. If temperatures less than 2200 F had been tested,

then at some temperature the LTD index (percent passing 1/4 inch)

would have started to decrease with decreasing temperature. The

maximum LTD index for fluxed pellets occurred at a peak pellettemperature close to 2200 F. The data demonstrate that the LTD

index is sensitive to firing temperature. The addition of coal to prefired fluxed pellets had a slight affect on the LTD index. 

Pellet Chemistry data for the pot-grate tests are listed in Table 6.

The ferrous iron content in fired fluxed pellets did not appear to be

affected by coal addition to pre-fired fluxed pellets in the pot-grate

5 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Table 2. Vertical Tube Furnace Test Results

Test Temp % Time % % % % %Mag Pellet Comp. % % Fe % C

No. C Coal min Fe Fe++ C CO2 Fe Four Pellets, lb Calcin. Oxid. Oxid

Head 0.6 0 61.9 19.92 1.44 4.03 59.48 0.0 0.0 0.0

1 850 0.6 6 63.2 11.98 0.78 2.22 37.32 8, 7, 9, 7 44.9 39.9 45.8

2 850 0.6 10 62.9 5.99 0.61 2.05 17.87 7, 4, 2, 6 49.1 69.9 57.6

3 850 0.6 14 62.9 2.86 0.53 1.92 8.00 13, 18, 18, 18 52.4 85.6 63.2

4 950 0.6 6 63.6 6.99 0.52 1.72 19.83 14, 21, 21, 22 57.3 64.9 63.9

5 950 0.6 10 62.6 1.92 0.40 1.73 4.87 32, 35, 35, 32 57.1 90.4 72.2

6 950 0.6 14 63.2 0.34 0.36 1.34 0.50 49, 57, 38, 45 66.7 98.3 75.0

7 1050 0.6 6 64.1 6.67 0.18 0.83 14.04 95, 76, 47, 59 79.4 66.5 87.5

8 1050 0.6 10 63.8 0.37 0.20 0.82 0.74 104, 138, 91, 98 79.7 98.1 86.1

9 1050 0.6 14 63.4 0.30 0.22 0.81 0.67 113, 98, 185, 198 79.9 98.5 84.7

10 1200 0.6 6 64.7 11.16 0.05 0.26 28.42 62, 46, 58, 44 93.5 44.0 96.5

11 1200 0.6 10 64.6 8.29 0.03 0.13 23.40 89, 75, 101, 102 96.8 58.4 97.9

12 1200 0.6 14 64.0 5.54 0.02 0.23 15.33 125, 66, 101, 77 94.3 72.2 98.6

Head 1.0 0 61.9 19.79 1.74 4.06 59.25 0.0 0.0 0.0

13 850 1.0 6 63.7 15.81 1.05 2.39 47.72 8, 6, 8, 8 41.1 20.1 39.7

14 850 1.0 10 62.9 10.68 0.84 2.17 32.61 4, 6, 5, 4 46.6 46.0 51.7

15 850 1.0 14 63.0 6.69 0.68 2.22 20.11 9, 8, 7, 4 45.3 66.2 60.9

16 950 1.0 6 64.0 16.10 0.82 1.99 46.79 9, 6, 9, 10 51.0 18.6 52.9

17 950 1.0 10 63.4 10.53 0.46 1.44 28.34 24, 31, 29, 34 64.5 46.8 73.6

18 950 1.0 14 63.3 4.79 0.30 1.26 11.42 47, 56, 51, 59 69.0 75.8 82.8

19 1050 1.0 6 64.6 18.27 0.31 0.82 38.65 57, 68, 75, 55 79.8 7.7 82.2

20 1050 1.0 10 64.2 9.53 0.13 0.53 16.26 113, 109, 91, 107 86.9 51.8 92.5

21 1050 1.0 14 63.2 4.22 0.08 0.46 7.21 195, 188, 166, 196 88.7 78.7 95.4

22 1200 1.0 6 66.5 21.91 0.04 0.07 41.07 27, 30, 34, 36 98.3 -10.7 97.7

23 1200 1.0 10 64.9 14.20 0.02 0.02 32.25 52, 53, 55, 54 99.5 28.2 98.9

24 1200 1.0 14 64.7 12.23 0.02 0.08 27.47 79, 72, 73, 80 98.0 38.2 98.9

Head 0.0 0 62.2 19.97 1.00 4.17 60.40 0.0 0.0 0.0

25 850 0.0 6 63.0 10.04 0.58 2.28 30.36 16, 14, 15, 14 45.3 49.7 42.0

26 850 0.0 10 62.5 6.19 0.53 2.25 18.33 18, 17, 18, 21 46.0 69.0 47.0

27 850 0.0 14 62.3 3.32 0.50 2.01 9.35 22, 21, 19, 24 51.8 83.4 50.0

28 950 0.0 6 62.6 9.99 0.47 2.11 29.90 5,,10, 17, 8 49.4 50.0 53.0

29 950 0.0 10 62.3 3.00 0.35 1.48 8.79 70, 70, 81, 62 64.5 85.0 65.0

30 950 0.0 14 62.2 0.53 0.33 1.38 1.21 101, 121, 147, 94 66.9 97.3 67.0

31 1050 0.0 6 63.1 6.50 0.25 1.07 19.77 62, 79, 88, 73 74.3 67.5 75.0

32 1050 0.0 10 62.7 0.78 0.13 0.81 2.44 155, 139, 122, 85 80.6 96.1 87.0

33 1050 0.0 14 62.9 0.18 0.13 0.79 0.29 144, 153, 197, 158 81.1 99.1 87.0

34 1200 0.0 6 63.9 7.21 0.02 0.21 25.09 156, 142, 163, 151 95.0 63.9 98.0

35 1200 0.0 10 63.8 4.76 0.00 0.15 16.90 191, 176, 253, 189 96.4 76.2 100.0

36 1200 0.0 14 63.5 3.22 0.00 0.20 11.29 200, 335, 305, 149 95.2 83.9 100.0
6 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Avera.gce Fluxed Pel!etPeak:~Tempuature 

versus Low Temperature Deg:radatlon "' ,--~---------,~---,----, 

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L TO Index of Pellets Produced Using a Low 

Excess Air Firing Cycle 

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Average Fluxed Pellet Peak-Temperature and 

ferrous Iron In Pot--Grata Pellets 

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After Tumble Index of Pellets Produced Using a 

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Averagm PHak Pttl!e-t T empernture, F 

Reducibility ol Pellets Produced Using a Low 

Excess Air Filing Cycle 

Average PeakPeHetTemperature, F o.w 1-----I---------------'---J 

2220 2240 2260 2280 2300 .23:20 
tests. This was not true for the vertical-tube furnace tests. The

ferrous iron content of the pot-grate fired fluxed pellets was affected

by firing temperature. The pot-grate test data and the vertical tube

furnace data demonstrate that there was an optimum pre-heat

temperature that produced a minimum ferrous iron content in fired

pellets. Once this temperature was exceeded, the ferrous iron

content of fluxed pellets increased. It appears that the optimum peak

pellet-temperature for fluxed pellet induration in a pot-grate was about

2200 F. Figure 14 illustrates the relationship that exists between

ferrous iron content, temperature and coal content in the fired pellets.

The Peak Pellet-Temperature increased when coal was added to

the pre-fired pellets. A comparison of the peak-hood temperature and

the average peak-pellet temperature indicate that the increase in the

fluxed pellet temperature during induration with 0.6% coal was 20o to

50° F and with 1.0 % coal the increase in temperature was 30° to 60°

F. Figure 15 illustrates the increase in temperature with coal addition.

Fired Pellet Quality – Low Excess Air Flame: The quality of the

pellets that were fired using oxidizing air followed by heating with a

near stoichiometric flame were similar to the quality of the pellets that

were produced using a normal firing cycle. Figures 16, 17, 18, and 19

illustrate the tumble index, the LTD index, the reducibility and the

compression strength for the pellets fired with a near stoichiometric air

to gas ratio.

COMMENTS AND RECOMENDATIONS

There was a reduction in the quantity of NOx formed during

induration with coal addition to pre-fired fluxed pellets. The pot-grate

test apparatus can be used to study fired pellet quality. However,

because of the heat losses and the amount of heat necessary to heat

the apparatus, an emission study was difficult to perform. A furnace

that is already hot, such as a furnace in a pellet production plant, does

not require the additional heat needed to bring the furnace to

induration temperatures. This additional heat required by the potgrate increased the total thermal NOx formation. Only a portion of this

NOx formed during a pot-grate test was due to the energy needed for

pellet induration. Normal fluxed pellet production in a production

furnace requires 500,000 to 600,000 BTU per Long Ton of pellets

produced. This would mean a pot-grate with 200 pounds of pellets

should require no more than 50,000 to 60,000 BTU of gas to indurate

the pellets. The pot-grate tests performed for this study required

250,000 to 300,000 BTU. This means that 75 to 80 percent of the

heat-input for a pot-grate test was required for heat loss and

apparatus heating. A 10 percent reduction in nitrogen oxide

formation in the pot-grate could translate to a 50 percent reduction in

a continuous furnace operation. Tests need to be conducted using a

continuous kiln operation or a continuous straight grate operation to

determine the actual reduction in nitrogen oxide formation with coal

addition to pre-fired fluxed pellets. 

A firing cycle that used a low quantity of excess air reduced

nitrogen oxide formation by approximately 75 percent. Application of

this technology would require costly redesign of existing furnace

operations and possible reduced production rates. However, the

reduction of nitrogen oxide was so dramatic when this firing cycle was

used that more research should be performed using firing cycles with

low amounts of excess air.

CONCLUSIONS

1) Coal addition to pre-fired fluxed pellets reduced nitrogen oxide

formation during pellet induration by 5% to 10%.

2) A decrease of 100°F in the average pellet temperature reduced

NOx emissions by 15%.

3) A near stoichiometric air to gas ratio during pellet induration

resulted in a 75 percent decrease in nitrogen oxide formation as

compared to an excess air to gas ratios.

4) A coal addition rate of 0.6% to pre-fired fluxed pellets had very

little affect on the rate of magnetite oxidation during pellet induration.

5) Coal addition to pre-fired fluxed pellets had very little affect on

the tumble index, reducibility, and LTD index of the fired pellets.

6) A coal addition rate of 0.6% to pre-fired fluxed pellets

decreased fired pellet compression strength by about 100 pounds.

7 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Figure 19

Table 3. Calcination Rate Constants – Carbon and Iron Oxidation

Rate Constants

Calcination Rate constants

% Calcination = 100-100e-kt

t= time in minutes, k= rate constant

Temperature 0% Coal 0.6% Coal 1.0% Coal

Degrees F k k k

1562 0.058 0.06 0.051

1742 0.089 0.085 0.092

1922 0.152 0.156 0.21

2192 0.361 0.341 0.429

Iron Oxidation Rate Constants

% Fe Oxidation = 100-100e-kt

t=time in minutes, k=rate constant

Temperature 0% Coal 0.6% Coal 1.0% Coal

Degrees F k k k

1562 0.123 0.134 0.078

1742 0.213 0.235 0.1

1922 0.277 0.289 0.126

2192 0.139 0.089 0.044

Carbon Oxidation Rate Constants

% C Oxidation = 100-100e-kt

t=time in minutes, k=rate constant

Temperature 0.6% Coal 1.0% Coal

Degrees F k k

1562 0.178 0.097

1742 0.249 0.261

1922 0.546 2.692

2192 0.316 0.473
7) Coal addition to pre-fired fluxed pellets had a minor affect on

ferrous iron content of fired-fluxed pellets that were indurated in the

pot-grate.

8) Temperature control was the most important parameter in

determining pellet oxidation and pellet quality parameters. An

optimum temperature existed at which maximum magnetite oxidation

occurred and maximum pellet quality parameters resulted. 

9) Coal addition to pre-fired pellets had a negligible affect on silica

content and the sulfur content of the fired pellets.

ACKNOWLEDGEMENTS

We would like to extend our thanks to USX Minntac for supplying

the filter cake and bentonite that was used as pellet feed in this

project.

8 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Table 4. Pellet Quality, Heat Input, Nitrogen Oxide, and Air Flow Data for Pellet Pot-Grate Tests

Avg After Heat

Firing % Firing Test NOx Total Dry Pel. Comp. Tumble Reducib. LTD Input

Test Coal Cycle Time,min SCF Flow, SCF pounds % -1/4 in. dR/dT % -1/4 in. BTU

1 0.0 Normal 35.13 5.72 7469 603 91.5 1.11 96.6 259060

2 0.0 Normal 34.17 6.85 8077 725 96.0 0.88 94.0 292350

3 0.6 Normal 33.68 5.98 6934 504 93.0 1.03 94.8 269350

4 0.6 Normal 33.78 5.60 7148 441 89.4 1.21 96.2 266928

5 1.0 Normal 33.47 5.69 7626 447 93.3 1.03 91.7 273587

6 1.0 Normal 32.95 6.52 8002 420 96.5 0.86 93.2 292956

7 0.6 Normal 33.5 6.88 7485 533 96.0 0.84 90.2 289929

8 0.0 Low Air 35.03 0.99 4546 558 93.4 1.08 95.5 226375

9 0.0 Low Air 46.88 1.62 5737 732 96.2 0.80 90.3 328667

10 1.0 Low Air 36.35 1.03 4659 406 94.3 0.86 94.0 197927

11 1.0 Low Air 48.38 1.68 6372 401 96.5 0.69 87.7 343799

12 0.6 Normal 33.78 5.69 7488 538 90.6 1.18 96.6 276764

Table 5. Peak Temperature Data for Pellet Pot-Grate Tests

Firing Avg Peak 2" Peak 4" Peak 6" Peak 8" Peak Hood Peak

Test Temp,F Temp, F Temp, F Temp, F Temp, F Temp,F

1 2204 2221 2221 ND 2169 2202

2 2311 2301 2324 2307 ND 2301

3 2250 2264 2279 2250 2205 2209

4 2209 2228 2244 2199 2164 2148

5 2265 2282 2301 2248 2227 2200

6 2339 2342 2356 2330 2327 2306

7 2326 2325 2339 2320 2320 2308

8 2251 2154 2242 2281 2325 2348

9 2337 2293 2317 2356 2380 2361

10 2236 2132 2266 2332 2214 2302

11 2361 2296 2367 2373 2406 2371

12 2225 2261 2254 2210 2174 2154

Table 6. Pot-Grate Pellet Chemistry

Test %Fe %Fe++ %S %CO2 %C % CaO % MgO

Greenball 62.9 21.12 0.015 3.88 3.41 1.12

1 64.0 0.08 0.005 0.22 3.44 1.09

2 63.8 0.20 0.005 0.21

Greenball 62.4 20.86 3.92 1.54

3 63.7 0.22 0.006 0.12

4 63.5 0.09 0.007 0.43 3.40 1.09

7 63.4 0.73 0.005 0.15

12 63.7 2.66 0.002 0.18

Greenball 62.4 20.88 3.95 1.72

5 63.8 0.32 0.004 0.13 3.47 1.08

6 63.7 1.66 0.003 0.12

Greenball 62.9 21.12 0.015 3.88

8 63.2 0.16 0.004 0.21 3.46 1.10

9 63.5 0.34 0.002 0.17

Greenball 62.4 20.88 3.95 1.72

10 63.2 1.75 0.004 0.15

11 63.9 2.10 0.005 0.11
REFERENCES

1. S. Wood, 1994, “Select the Right NOx Control Technology”,

Chemical Engineering Progress, Jan 1994, pp 32-38.

2. Zahl, Hass, Engesser, 1995, “Formation of NOx in Iron Oxide

Pelletizing Furnaces”, Minnesota Department of Natural Resources –

Division of Minerals, Sept 25, 1995.

3. Carter, Benson, 1984 “Combustion Modification Tests on a

Subscale Cement Kiln for NOx Reduction.” EPA-600/S7-84-075.

4. US-EPA, 1998, “The Regional Transport of Ozone” EPA-456/F

98-006, Sept 1998.

5. Davis, R., 1995 “Thermal NOx Formation in a Taconite Pellet

Rotary Kiln” , Minnesota Department of Natural Resources – Division

of Minerals, March 1995.

6. Walters, et. al.,1999, “Time-Variability of NOx Emissions from

Portland Cement Kilns” Environmental Science and Technology, Vol

33, No. 5, pp700-704.

7. North American Mfg. Co. 1993,“ NOx Reduction” Handbook

Supplement 7-93.

8. A. Garg,1992, “Trimming NOx”, Chemical Engineering, Nov.

1992, pp122-129.

9. Parkinson, Fouhy, Moore, 1994, “NOx: US Plants Take Aim at

a Moving Target”, Chemical Engineering, Jan. 1994, pp28-31.

10. Zahl,1995, “NOx Formation in Methane Combustion with

High Temperature Air”, Minnesota Department of Natural Resources

– Division of Minerals. Nov. 1995. 

9 Copyright © 2003 by SME

2003 SME Annual Meeting

Feb. 24-26, Cincinnati, Ohio

Table 7. Coal Analysis

Analysis %C %H %N %S %O %Ash %H2O %VM

Ultimate Dry 85.51 2.94 1.2 0.74 1.34 8.25

Ultimate As is 85.08 3 1.19 0.74 1.78 8.21 0.5

Proximate Dry 75.04 8.33 16.63