ABSTRACT

A research project was undertaken to determine if coal addition to fluxed-magnetite pre-fired pellets would reduce nitrogen oxide (NO x ) 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 toverify pellet quality parameters and nitrogen oxide emissions. The offgas during each pot-grate test was analyzed for NO x , CO 2 , CO, SO x , and O 2 . There was approximately a ten percent reduction in NO x emissions with coal addition during the normal pot-grate firing cycle. There was a 75% reduction in NO x 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 NO x 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 NO x 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 NO 2 . The NO 2 is soluble in water and, therefore, contributes to the formation of acid rain. Ozone is an air pollutant thatis a major component of smog and poses a human health risk (USEPA, 1998). Natural gas produces more NO x 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 1 1,500 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 (Fe 3 O 4 ) to hematite (Fe 2 O 3 ) 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 NO x 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 NO x (as NO 2 ) per thousand standard cubic feet (MSCF) of natural gas burned (Zahl, et al., 1995). Figure 1 illustrates NO x 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 NO x 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 NO x than normal fluxed pellet induration.

TEST PROGRAM

V

ertical T ube Furnace T ests : 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 (NO X) EMISSION REDUCTION DURING PELLET INDURATION BY FUEL-ADDITION TO THE GREEN-PELLETS AND DECREASED EXCESS AIR

J. Engesser Minnesota Dept of Ntrl Resrcs Hibbing, MNzone 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 (CO 2 ), total iron (Fe), ferrous iron (Fe ++ ), and magnetite (Fe 3 O 4 ). Pot-Grate Furnace T ests : Twelve pot-grate furnace tests wereperformed. 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 (O 2 ), carbon dioxide (CO 2 ) , carbon monoxide (CO), nitrogen oxide (NO x ) , and sulfur dioxide (SO 2 ) 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 ++ , CO 2 , 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

V

ertical T ube Furnace T ests : 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 3oxidation 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 T ests : 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. NO x Production : There was a decrease in the amount of nitrogen oxides (NO x ) 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 NO x production in the pot-grate test could mean a 50% NO x reduction in a continuous indurating-furnace. Figure 7 illustrates the decrease in NO x emitted from fluxed pellets that contained coal during pot-grate induration tests. There was about a 75 percent reduction in NO x 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 NO x 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

T

able 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

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Figure 11 Figure 12



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### PeUet Tumble Index and Pellet ReducibilityReducibility 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. Themaximum LTD index for fluxed pellets occurred at a peak pellettemperature close to 2200 F. The data demonstrate that the LTDindex 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

T

able 2. Vertical Tube Furnace Test Results T

est Temp % Time % % % % %Mag Pellet Comp. % % Fe % C No. C Coal min Fe Fe ++ C CO 2 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

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Figure 18 Avera.gce Fluxed**Pel!etPeak:~Tempuature**

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2220 2240 2260 2280 2300 .23:20tests. 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 NO x 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 toinduration temperatures. This additional heat required by the potgrate increased the total thermal NO x formation. Only a portion of this NO x 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 NO x 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

T

able 3. Calcination Rate Constants – Carbon and Iron Oxidation Rate Constants Calcination Rate constants % Calcination = 100-100e -kt t= time in minutes, k= rate constant T

emperature 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 T

emperature 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 T

emperature 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 0.4737) 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

T

able 4. Pellet Quality, Heat Input, Nitrogen Oxide, and Air Flow Data for Pellet Pot-Grate Tests Av g After Heat Firing % Firing Test NO x T otal Dry Pel. Comp. Tumble Reducib. LTD Input T

est 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

T

able 5. Peak Temperature Data for Pellet Pot-Grate Tests Firing Avg Peak 2" Peak 4" Peak 6" Peak 8" Peak Hood Peak T

est 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

T

able 6. Pot-Grate Pellet Chemistry T

est %Fe %Fe++ %S %CO 2 %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 0.11REFERENCES

1. S. Wood, 1994, “Select the Right NO x Control Technology”, Chemical Engineering Progress , Jan 1994, pp 32-38. 2. Zahl, Hass, Engesser, 1995, “Formation of NO x 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 NO x 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 NO x 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 NO x Emissions from Portland Cement Kilns” Environmental Science and T echnology , Vol 33, No. 5, pp700-704. 7. North American Mfg. Co. 1993,“ NO x Reduction” Handbook Supplement 7-93. 8. A. Garg,1992, “Trimming NO x ”, Chemical Engineering , Nov. 1992, pp122-129. 9. Parkinson, Fouhy, Moore, 1994, “NO x : US Plants Take Aim at a Moving Target”, Chemical Engineering , Jan. 1994, pp28-31. 10. Zahl,1995, “NO x 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

T

able 7. Coal Analysis Analysis %C %H %N %S %O %Ash %H 2 O %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