From: Douglas Hawkins

To: Aagenes, Jason D.

Subject: Re: Possible Statistical Analysis Project

Date: Thursday, July 05, 2007 12:03:14 PM

Attachments: Writeup.pdf

Hi, Jason,

I attach a draft writeup of the data set. There are indeed relationships between the plant parameters

and the NOx emissions.

If we want to use these relationships to reduce NOx, we will need to talk some more about what can

and can not be varied independently, but that is perhaps a question for another day.

Let me know of any changes, additions or clarifications needed, and I'll update and get a clean copy

over to you.

Best,

Doug

----- Original Message -----

From: Aagenes, Jason D.

To: Douglas Hawkins (rr)

Sent: Monday, June 25, 2007 11:55 AM

Subject: RE: Possible Statistical Analysis Project

Doug,

Thanks for your reply. I will wait to receive your proposal. There is some urgency to this project as

the Minnesota Pollution Control Agency is expecting a response from us in the next couple of

weeks. Please let me know if you would be able to support the project in that time period.

Thanks again,

Jason Aagenes

From: Douglas Hawkins (rr) [mailto:dhawkins@douglashawkins.com] 

Sent: Friday, June 22, 2007 5:37 PM

To: Aagenes, Jason D.

Cc: Douglas Hawkins LLC

Subject: Re: Possible Statistical Analysis Project

Hi, Jason,

Just picked up this e-mail.

Yes, I would think I can add useful insights to the project. I will take a look at your attachment and

get back to you.

The e-mail address you used is technically non-existent, and will become genuinely so not too far

into the future. Can you change your address book entry for me to

dhawkins@douglashawkins.com

(and see if you can get Dave's address book updated also.)

Best,
4713 Hibiscus Avenue, Edina, MN 55435-4001 

cell: (612) 718-5033 e-mail:dhawkins@mn.rr.com 

fax: (952) 926-8117 web: www.douglashawkins.com 

Memorandum 

To: Jason Aagenes 

Subject: Analysis of NOx Emission Study Data 

Date: July 4, 2007 

Executive Summary 

Three of the plant parameters have a clear relationship to NOx emissions – the kiln off-gas 

temperature and burn zone temperature have a negative relationship with NOx emissions, and 

heat input a positive relationship. The impact of line 1 feed rate is less clear, and pre-heat 

temperature does not appear to have any relationship with NOx emissions. 

Background

You provided hourly data measured over four days. The variables were: 

• Average NOx emissions (in ppm) over the hour, 

• Average line 1 feed rate (in tph), 

• Pre-heat temperature (in degrees F), 

• Kiln off-gas temperature (in degrees F), 

• Heat input (in MMBtu/hour), 

• Burn zone temperature (in degrees F). 

The object of the study was to determine whether any of the 5 plant operating parameters affects 

NOx emissions, and if so, what the relationship is. 
Relationship of NOx emissions to plant parameters 2 

Preliminaries

The first step was a calculation of the averages, standard deviation, maximum, and minimum of 

the six measured quantities. With some self-explanatory abbreviations for the variable names 

and rounding all values to the nearest whole number, this gave: 

Variable N Mean SD Minimum Maximum 

Avg_NOx 84 300 32 209 389 

L_1_feed 95 234 19 155 272 

Preht_tem 95 2160 29 2117 2289 

Off_gas_t 95 1949 17 1895 2004 

Heat_inpu 95 152 11 124 206 

Burn_temp 94 2473 15 2434 2500 

All variables were fairly tightly controlled, with standard deviations of the order of 10% of the 

mean for the line 1 feed and heat input, and 1% for the three temperatures. The average NOx 

emission had a standard deviation of 10% of its mean, which is also quite low. 

Pairwise Graphics 

Next, the average NOx emission was plotted against each of the five plant parameters. The 

resulting plots are shown as Figures 1 – 5 in the Appendix. Each plot also shows the linear 

regression line that might be used to predict average NOx using that parameter alone as a 

predictor. Main features of the graphs are: 

• Visually, average NOx seems to increase with line 1 feed rate and heat input, and to 

decrease with off-gas temperature. Neither pre-heat temperature nor burn zone 

temperature seems to have much connection with the NOx emission. 

• There is a visual outlier – an hour whose average NOx emission of 209 ppm is far below 

that of any other hour. This observation does not however correspond to an unusual 

setting in any of the plant parameters. 

• The straight regression lines shown on the plots do quite well in capturing the broad 

sweep of the relationships with the plant parameters. 

Correlation Matrix

The next step was to compute the correlation of each pair of the six variables. This gave: 

 Avg_NOx L_1_feed Preht_tem Off_gas_t Heat_inpu 

L_1_feed 0.4391 

Preht_tem -0.0521 -0.0995 

Off_gas_t -0.3960 -0.5749 0.3854 

Heat_inpu 0.4318 0.3552 0.2138 0.1575 

Burn_temp 0.0102 -0.2142 0.2026 0.4835 0.6905 

 

Correlations above 0.2 are statistically significant at the 5% level, indicating a possible ‘real’ 

relationship. 

The significant correlation of average NOx with line 1 feed rate, off-gas temperature and heat 

input quantify the impression of the plots of apparent relationships among these variables. The 

non-significance of pre-heat temperature and burn temperature also agrees with the plots. 
Relationship of NOx emissions to plant parameters 3 

The rest of the table shows correlations among the plant parameters. These correlations 

presumably result from operator management – perhaps high feed rate causes the operator to 

increase the heat input – and from the laws of physics – a higher heat input will lead to a higher 

burn zone temperature. These correlations and their source are not central to this statistical 

analysis, but will become important in any efforts to modify NOx emissions. 

Multiple Regression Analysis 

The final analysis is a multiple regression. In this, the NOx output is predicted using all five of 

the plant parameters together. This analysis gave rise to the following output: 

Unweighted Least Squares Linear Regression of Avg_NOx 

Predictor 

Variables Coefficient Std Error T P VIF 

Constant 3873.17 880.636 4.40 0.0000 

L_1_feed -0.40003 0.25410 -1.57 0.1195 2.9 

Preht_tem 0.00144 0.09916 0.01 0.9885 1.2 

Off_gas_t -0.91634 0.24708 -3.71 0.0004 2.2 

Heat_inpu 3.30005 0.64446 5.12 0.0000 4.1 

Burn_temp -0.88628 0.33976 -2.61 0.0109 3.6 

R-Squared 0.4565 Resid. Mean Square (MSE) 575.406 

Adjusted R-Squared 0.4212 Standard Deviation 23.9876 

Looking at each of the lines in turn, if all other plant parameters remain the same, 

• a 1 tph increase in line 1 feed rate is associated with a 0.40 ppm decrease in NOx; 

• a 1oF increase in preheat temperature is associated with a 0.001 ppm increase in NOx; 

Neither of these coefficients is statistically significant. Thus the data do not reliably indicate that 

either of these plant parameters has any impact on NOx emissions when the other plant 

parameters are taken into account. Turning to the next three lines: 

• a 1o

F increase in kiln off-gas temperature is associated with a 0.92 ppm decrease in NOx; 

• a 1 MMBtu/hr increase in the heat input, all other plant parameters remaining the same, 

is associated with a 3.3 ppm increase in average NOx. 

• a 1 degree increase in the burn zone temperature is associated with a reduction of 0.89 

ppm in average NOx. 

Interpretation of the relationships should concentrate on these three significant plant parameters. 

Explaining the Discrepancies between the Correlation and the Regression Fits.

There are two discrepancies to notice. The original correlation analysis saw a significant 

association between line 1 feed rate and NOx, but no discernible relationship between burn zone 

temperature and NOx emissions. The multiple regression reached the opposite conclusion p—

that line 1 feed rate did not have a well-quantified relationship with NOx emissions, but that burn 

temperature did. The reason for this difference lies in the relationship among the plant 

parameters due to operator action and the laws of physics. For example, it is likely that, in 
Relationship of NOx emissions to plant parameters 4 

response to increases or decreases in the line 1 feed rate, the operators make adjustments to other 

controllable parameters, and it is the effect of these downstream change that leads to the apparent 

correlation between feed rate and NOx emissions. 

And on the other side, it appears that the burn zone temperature has an effect on NOx emissions 

(as evidenced by the multiple regression) but that the burn zone temperature is so much a 

resultant of other changing plant parameters that the association is masked in the overall 

relationship. 

Quality of the Regression Predictions

Figure 6 is a plot of the actual NOx values against the values predicted by the regression model. 

This graph is a check on the quality of the regression. It shows that the measured NOx values 

follow the line predicted by the model as these predictions range from 240 to 340 ppm. The 

individual hourly averages differ from this line with a standard deviation of 24 ppm. 
Relationship of NOx emissions to plant parameters 5 

Appendix – Graphs 

170 190 210 230 250 270

200

250

300

350

400

Avg_NOx vs L_1_feed

Avg_NOx

L_1_feed

Figure 1 

2110 2170 2230 2290

200

250

300

350

400

Avg_NOx vs Pre-heat Temp

Avg_NOx

Preht_tem

Figure 2 
Relationship of NOx emissions to plant parameters 6 

1920 1950 1980 2010

200

250

300

350

400

Avg_NOx vs Kiln Off-Gas Temp

Avg_NOx

Off_gas_t

Figure 3 

120 140 160 180

200

250

300

350

400

Avg_NOx vs Heat input

Avg_NOx

Heat_inpu

Figure 4 
Relationship of NOx emissions to plant parameters 7 

2430 2450 2470 2490 2510

200

250

300

350

400

Avg_NOx vs Burn Zone Temp

Avg_NOx

Burn_temp

Figure 5 

230 270 310 350

200

250

300

350

400

Actual vs Predicted NOx emissions

Avg_NOx

Predicted

Figure 6 
Doug Hawkins

----- Original Message -----

From: Aagenes, Jason D.

To: dhawkins@mn.rr.com

Sent: Friday, June 15, 2007 1:53 PM

Subject: Possible Statistical Analysis Project

Doug,

Dave Skolasinski of Cleveland-Cliffs referred me to you as a potential resource for

completing a statistical analysis on data that we’ve collected. We are trying to

determine if there is any correlation between the amount of NOx that is emitted from

our furnace stacks and various operating parameters (fuel input rate, production

throughput, operating temperatures, etc.). We believe that a correlation exists and that

we could use some of the operating data to predict and quantify what the resulting NOx

emission rate is.

Would you be able to help with this project? If so, please develop a proposal and

submit to me. I’ve included a sample of the data set with hourly data averages (minute

by minute data is also available). Please review and let me know if you would be

interested in helping with this project.

Regards,

Jason Aagenes

Section Manager - Environmental Affairs

Cleveland-Cliffs United Taconite LLC

P.O. Box 180

Eveleth, MN 55734

Office (218) 744-7803

Mobile (218) 290 -5936

Fax (218) 744-7635

jdaagenes@cleveland-cliffs.com

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