variability of nox emissions from precalciner cement kiln systems

Variability of NOx Emissions from Precalciner Cement Kiln Systems Control # 80 Walter L. Greer Curtis D. Lesslie Trinity Consultants, 25055 West Valley Parkway, Suite 101, Olathe, KS 66061 ABSTRACT In 2003, there are forty-two precalciner kilns out of about 190 total portland cement kiln systems operating in the United States. As a class, precalciner kiln systems have the lowest annual mass emissions of the oxides of nitrogen (NOx) per ton of clinker when compared to wet, long-dry and preheater kiln systems; however, like all cement kilns, precalciner systems experience significant variability in hourly NOx emission rates when compared to other processes such as electric power generating plants. This inherent variability in NOx emissions frequently is not given sufficient consideration by regulators and others involved in the selection of permissible emission rates. An undesirable result can be the selection of inappropriate emission rate limits for compliance periods of less than one year. Representative annual operating data from twelve of the domestic precalciner kiln systems were obtained. These data were normalized and the variability of the NOx emissions has been presented as kiln-specific standard deviations over several of the averaging periods that commonly are used by regulators for short-term NOx emission rate limits. Normalized data for each participating plant including and excluding periods of zero NOx emissions were displayed graphically as a time series plot and as a frequency distribution. These data served to visually demonstrate the representative variability of NOx emissions from precalciner kiln systems, and to provide the basis for developing realistic NOx emission limitations from existing and new precalciner kiln systems. The results of a linear regression analysis of the data for standard deviation versus the rated daily production rate of the respective kiln systems are presented graphically and as tables of correlation coefficients. Retrospective and prospective permitting strategies are proposed.

INTRODUCTION This report presents the results of a statistical study of the variability of the emissions of the oxides of nitrogen (NOx) from representative precalciner cement kiln systems operating in the United States. In 2003, there were forty-two operating precalciner kilns out of about 190 total domestic cement kiln systems. Twenty-five kilns were identified initially as potential study participants because they were equipped with continuous emission monitoring systems (CEMS) and flue gas flow rate monitors, had at least six months of archived hour-by-hour average NOx mass emission rate data, and were willing to provide data for the study. Ultimately, suitable data from twelve precalciner kiln systems (29% of all precalciner kilns and 48% of potential participants) were obtained for use in the study. The identities of the plants participating in the study are not disclosed in this report. For all but two participants that supplied slightly more than the minimum six-months of data, the participants supplied twelve months of consecutive, hourly-

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average mass NOx emission rate data in units of pounds (lbs) per hour. In response to industry confidentiality concerns, the data were normalized to further mask the identity of the participating kilns. Time-series plots of the normalized NOx emissions data were prepared to illustrate the variability of the emissions from each of the participating kilns. Also, plots of frequency distributions of the normalized hourly mass emission rate data were prepared and were subjectively determined to represent approximate normal distributions. The consecutive hourly data were divided into two sets of consecutive block periods of three, eight, twenty-four and 720 hours (30 days) for determination of the standard deviation (σ) of each data set. In the first data set, hourly averages of zero emissions of NOx were included. In the second data set, hourly averages of zero emissions of NOx were excluded. A linear regression analysis was performed to determine the strength of the relationship between σ and the rated daily production rate of each kiln as reported by the Portland Cement Association (PCA).1 The authors have suggested potential retrospective and prospective NOx permitting strategies. PROCESS DESCRIPTION The study that is the subject of this paper concerns itself only with NOx emissions from precalciner kilns systems, the most-modern and most-efficient of the four basic kiln systems (wet, long-dry, preheater and precalciner) that are used to produce portland cement clinker in the United States. Almost all new and replacement kiln systems are expected to be of the precalciner type. All these systems employ countercurrent process flow and a rotary kiln to produce essentially identical clinker in a similar burning zone, i.e., the high-temperature combustion and reaction zone. A preheater kiln system employs a series of cyclone-like vessels arranged vertically in a preheater tower to quickly and efficiently preheat the cement raw materials to or near the calcining temperature of calcium carbonate before they are introduced into the rotary kiln. In the precalciner kiln system, an evolution of the preheater kiln system, half or more of the required fuel is combusted at relatively low temperature in an additional vessel at the bottom of the preheater tower to almost completely calcine the raw materials prior to their introduction into the rotary kiln. There are several proprietary arrangements for the calciner representing vendor preferences and advancements in the technology. A typical arrangement of a precalciner kiln system is shown in Figure 1. Small amounts of fuel can be burned elsewhere in a precalciner kiln system for the purposes of NOx emission reduction and the efficient combustion of waste-derived fuels, e.g., whole scrap tires. SOURCES OF NOX EMISSIONS Nitrogen oxide (NO) comprises 90% or more of the oxides of nitrogen emitted from cement kilns. The balance of the NOx consists of nitrogen dioxide (NO2).2 Thermal NOx The production of portland cement clinker requires the combustion of fuels in the burning zone of the rotary kiln at a flame temperature of approximately 1870°C (3400°F) to heat the reacting mass of materials to approximately 1480°C (2700°F). At this temperature, nitrogen in the combustion air is oxidized to NOx. Because of its high-temperature origin, this combustion product is known as thermal NOx. The calciner of a precalciner kiln system operates near the temperature of 1200°C (2200°F) at which the formation of thermal NOx essentially ceases

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because of the relatively low temperature. Consequently, thermal NOx predominates NOx formation in the burning zone of the rotary kiln and is a lesser factor in NOx formation in the calciner or elsewhere in a precalciner kiln system where lower-temperature combustion takes place.

Figure 1. Typical Precalciner Kiln System

SecondaryAir

Fuel and Primary Air

Raw Material

Raw Material

t

Alkali Bypass

To PMCD

To PreheaterTower

Raw Material Mix to Preheater

Fu Ananbuoth Fe Thin labprduNO Pr PrhyNO

I.D. Fan
Mix Silo
el NOx

y nitrogen contained in the fuels used in y of the combustion temperatures found inrning zone of the rotary kiln but predominer lower-temperature combustion sites.

ed NOx

ere is the potential for oxidization of nitrothe range of 300-800°C (570-1470°F). Toratory and has not been demonstrated in

ecalciner kiln system is thought to result iring the slower heating of the kiln feed th

x is not likely to be a significant contribu

ompt NOx

ompt NOx potentially results from the reacdrocarbon flame with elemental nitrogen

x is a very minor contributor to NOx emi

Rotary Kiln

precalciner kiln systems ca the pyroprocess. Fuel NOates the generation of NOx

gen in the kiln feed materihis phenomenon has only b the field. Rapid heating on less feed NOx generation at is found in other kiln systor to NOx emissions from

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Clinker Cooler

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Tertiary Air Duc

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3

CAUSES OF THE VARIABILITY OF NOX EMISSONS Process Cement kiln systems are perhaps the most complex of all continuous chemical reactors in the world. To convert the calcareous, siliceous, argillaceous and ferriferous cement raw materials to cement clinker minerals, there are numerous simultaneous and sequential chemical reactions occurring within the mass of the hundreds of tons of material moving through the kiln system in any instant. It is the goal of the kiln operator to achieve and maintain process equilibrium. In practice, this is an elusive goal. Many events and situations can affect the process and its equilibrium. The most obvious of these events are mechanical and electrical failures of the system and its auxiliary components. These minor or catastrophic failures can be planned or unplanned, but will directly alter the process or will require some operator response that will alter the process. Almost any change in the precalciner process will result in an alteration of the NOx emission rate. The results of these changes range from zero emissions of NOx during a complete outage of the system to significantly higher than normal NOx emissions when the fuel input is increased to restore the process to equilibrium and a normal production rate. More subtle changes in the process, e.g., variations in ambient temperature (short-term and seasonal), variations in the chemistry of the raw material mix, variations in fuel type or quality and variations in the feed rates of fuel or raw materials, demand operator responses that also serve to vary NOx emissions. For a variety of factors, the operation of some precalciner kiln systems is more stable than others. Almost invariably, however, precalciner kiln systems exhibit more stable operation than the three other types of kiln systems, and experience the least variability in NOx emissions. Thermal NOx Thermal NOx is directly dependent on the temperature of the flame from which it is generated. There are numerous intentional and unintentional causes of changes in the burning-zone flame temperature that occur during the normal operation of a precalciner kiln system. The generation of thermal NOx also is directly dependent on the available oxygen at the combustion site. The amount of excess air (oxygen) in the combustion zones of the kiln system is typically maintained as low as possible for purposes other than the reduction of NOx generation, e.g., thermal efficiency, but slight changes in excess air that result in variable NOx generation are unavoidable. Because of the relatively low combustion temperature in the calciner, thermal NOx from this combustion source generally is not a factor; but, to the degree that thermal NOx is a factor, variability based on temperature and excess oxygen can be expected. Fuel NOx The variability of fuel NOx is directly dependent on the variability of nitrogen in the fuel and the variability of oxygen at the combustion site.

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Feed NOx To the degree that nitrogen in the raw material mix is converted to NOx in a precalciner kiln system, the most important factor is perhaps the amount of nitrogen in the raw material mix. Laboratory work suggests that feed NOx generation is inversely related to the speed at which the raw material is heated through the critical temperature range.3 Therefore, precalciner kiln systems have the potential to convert the least amount of feed nitrogen into NOx. The variability of the generation of feed NOx is dependent on the variability of the nitrogen in the feed materials and on the stability of the temperature profile of the pyroprocess in the range of 300-800oC (570-1470oF). Prompt NOx Prompt NOx is not thought to be a significant contributor to overall NOx generation in precalciner kiln systems. Consequently, any variability in the generation of prompt NOx is likely to be lost in the measurement or observation of variability of the other sources of NOx emissions. GRAPHICAL PRESENTATION OF ALL NORMALIZED NOX EMISSION DATA FROM THE PARTICIPATING PRECALCINER KILNS Each of the participating plants was assigned an identification number based on the random order in which the data were received. Missing numbers indicate that a plant originally submitted data that subsequently was found to be unusable for the project. The following Figures 2 and 3 respectively present time series and frequency distribution graphs of the normalized data from Plant No. 1 that includes hourly periods of zero emissions of NOx. The kiln at Plant No. 1 is considered to be well operated and efficient, and represents typical kiln operation. As might be inferred from the time series graph by those unfamiliar with cement kilns, operation of the kiln at Plant No. 1 is not out of control. Because the data for all participating plants have been normalized, the mean of the data has no relevance to real-world conditions in the participating plants. Not all respondents identified periods of kiln downtime; therefore, it was impossible to consistently and correctly eliminate kiln downtime from the included data. All submitted data including that representing kiln downtime, i.e., when feed and/or fire to the kiln is terminated, were included in the initial data analysis and corresponding graphs. From a graphical perspective, the result of including all submitted data is an intermittent line on the x-axis of the typical time series graph displayed in Figure 2, and a large, apparently anomalous, data concentration in the first cell of the plot of the typical frequency distribution displayed in Figure 3. Although the entire body of data is obviously skewed toward the lowest value, the data outside the first class has the general appearance of a normal distribution and prompted the use of standard statistical procedures in the study. Space limitations do not allow the presentation of all the time series or frequency distribution graphs for each of the plants. Suffice it to say that each kiln had a unique time series graph depending on factors such as operating history of the kiln during the period of data gathering. The frequency distribution graphs for each kiln system were unique but similar, i.e., except for the concentration of data points near zero; the remaining data points displayed an approximation to a normal distribution. The maximum value in the right hand tail of the “bell curve” was different for each plant. Experience by the authors has shown that this statistically incorrect, but simplistic, approach has adequately described the

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variability of NOx emissions from cement kilns and has resulted in the establishment of operationally satisfactory and environmentally protective short-term permit limits for NOx emissions. To accomplish streamlined compliance recordkeeping, the inclusion of periods of low or zero NOx emissions in the database provides for the most-simple and straightforward system. The recordkeeping becomes complicated if measured NOx emissions during periods of kiln downtime (even if they are zero) must somehow be excluded from compliance values. Of particular significance, one is never quite certain what to do with an hourly average data point in which both periods of kiln operation and kiln downtime are represented during that hour. The most sophisticated electronic recordkeeping systems may be able overcome this problem. Both the National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide and the Prevention of Significant Deterioration (PSD) increment for NOx are annual values during which downtime of any contributing source can be expected; therefore, it may be consistent to include in the calculations leading to a NOx permit limit the occurrence and the absence of emissions of NOx during the entire potential compliance period of interest. If the NOx emission rate limit is established using periods of zero emissions, care must be exercised that the baseline period is representative of normal operations. Otherwise, the NOx emission rate limit will be too high or too low.

Figure 2. Time Series of All Normalized NOx Emission Data from Plant No. 1

900

1100

1300

1500

1700

1900

2100

2300

2500

2700

0 730 1460 2190 2920 3650 4380 5110 5840 6570 7300 8030 8760

Hours

Poun

ds N

Ox

per H

our

6

Figure 3. Frequency Distribution of All Normalized NOx Emission Data from Plant No. 1

0

500

1000

1500

2000

2500

3000

3500

4000

4500

900 - 950

950 - 1000

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1450 - 1500

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1750 - 1800

1800 - 1850

Pounds NOx per Hour

Freq

uenc

y

Figure 4 contains a time series graph of NOx flue gas concentrations from a typical electric power generating plant. Remembering that a cement kiln is a complex chemical processing unit and that a power plant is essentially a relatively-simple, somewhat-uniform combustion process, a visual comparison of Figures 2 and 4 clearly indicates the higher variability of NOx emissions from a precalciner cement kiln as compared to a power plant. Although the data in Figure 4 is in units of volumetric concentration and the data in Figure 2 is in units of a mass emission rate, a qualitative comparison is possible simply to illustrate relative variability.

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Figure 4. Time series of flue gas NOx concentrations from a typical power generating plant4

TABULAR PRESENTAION OF ALL NORMALIZED NOX EMISSION DATA FROM THE PARTICIPATING PRECALCINER KILNS After normalization, the data from each of the participating kilns were divided into consecutive blocks of time representing the common compliance block-averaging periods of one, three, eight, twenty-four and 720 hours (30 days). A 720-hour (30-day) rolling average that was updated daily also was determined. As a measure of variability of NOx emissions, the standard deviation of each set of data was determined. Because kiln downtime represents an expected but somewhat random event during operation of a precalciner kiln system, the data in the initial analysis included all submitted data, e.g., zero emissions during kiln downtime. Although the standard deviation is larger than would be the case if just operating hours were included, this procedure has the effect of anticipating the simplest and most desirable data handling methodology for a plant, i.e., no need to try to extract periods of zero or abnormally low NOx emissions from the compliance database. Table 1 presents the standard deviations of the NOx emissions from the participating plants and includes all data, including periods of zero emissions. The results are generally as might be expected for a highly variable emission source. The largest standard deviations for each source are associated with the shortest, i.e., one-hour, averaging period. Without exception, the standard deviation for each block average for each plant decreases in magnitude as the averaging period increases beyond one hour. The anomalous sharp decrease in standard deviation for Plant Nos. 7, 10 and 12 in the 720-hour block and rolling averages are caused by the frequency and duration of hourly periods of zero or low NOx emissions. If the data sets for Plant Nos. 7, 10 and 12 are not truly representative of normal plant operation, erroneous conclusions could be made

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regarding the standard deviations for the monthly averaging periods. The anomalous increase in standard deviation for the 720-hourly rolling average for Plant Nos. 8 and 9 over the 720-hour block average also is caused by the frequency and duration of hourly periods of zero or low NOx emissions, i.e., an artifact of the calculation methodology. For Plant Nos. 2 and 12, only the submitted data were included in the statistical analysis, i.e., the data for the missing days were disregarded and not included as zeros but the periods of zero emissions during the reporting period were included. Table 1. Standard Deviation of NOx Emissions (pounds per hour) by Plant Number and

Averaging Period Including All NOx Emission Data.

Plant number 1 Hour 3 Hours 8 Hours 24 Hours 720 Hour

Block

720 Hour

Rolling

1 161 152 142 128 76 73

2 86 85 82 76 36 34

5 227 215 200 176 74 73

6 88 85 83 79 63 58

7 241 213 195 161 8 7

8 291 284 274 255 99 126

9 262 253 243 228 127 143

10 221 185 157 119 6 6

11 115 113 109 103 83 71

12 160 117 93 62 7 5

13 217 213 205 191 139 127

14 162 158 153 143 90 90

GRAPHICAL PRESENTATION OF NORMALIZED NOX EMISSION DATA WITHOUT ZEROS FROM THE PARTICIPATING PRECALCINER KILNS Perhaps the next most obvious treatment of the reported data would be to eliminate from the database all hours in which NOx emissions were zero, i.e., the kiln was definitely out of operation. The problem of partial hours of kiln operation is not addressed by this approach but the skewed nature of the frequency distribution would be reduced significantly. To investigate the effect of eliminating hourly periods of zero emissions of NOx from the database, the normalized data were reprocessed using only those hours in which measured emissions of NOx were reported. The frequency distribution graph of these data for Plant No.1 is shown in Figure 5. Although hours of no NOx emissions have been removed from the data, the first cell of the frequency distribution contains low NOx emissions for those hours in which the kiln was being heated prior to full operation and for those hours in which the kiln was only operated for a part of an hour. The approach to a normal distribution of the actual operating data is now more evident. Again, because of space limitations the frequency distribution graphs for the remaining eleven kilns have not been included in his paper.

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Other alternatives for processing the data exist. To provide an opportunity for future study of the data, the PCA has been provided with electronic spreadsheets containing the raw and normalized data used in this study without plant identification. With these data, users approved by the PCA can construct appropriate averaging-time scenarios to meet their individual needs.

Figure 5. Frequency Distribution of Normalized NOx Emission Data Without Zeros from Plant No. 1

0

500

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3000

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4000

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1800 - 1850

Pounds NOx per Hour

Freq

uenc

y

TABULAR PRESENTAION OF NORMALIZED NOX EMISSION DATA WITHOUT ZEROS FROM THE PARTICIPATING PRECALCINER KILNS The standard deviations for all plants and all averaging periods using only hourly periods of measured, detectable NOx emissions, i.e., without zeros, are shown in Table 2.

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Table 2. Standard Deviation of NOx Emissions (lbs per hour) by Plant Number and Averaging Period Including NOx Emission Data Without Zeros.

Plant number 1 Hour 3 Hour 8 Hour 24 Hour 720 Hour

Block

720 Hour

Rolling

1 121 118 109 97 48 47

2 86 85 82 77 35 36

5 147 134 115 93 49 45

6 71 68 64 57 41 37

7 182 144 118 70 8 7

8 218 214 208 205 60 73

9 182 168 148 123 65 64

10 142 121 94 70 11 11

11 115 113 109 103 83 71

12 122 99 79 62 8 10

13 173 172 166 153 135 120

14 104 99 93 83 31 25 RETROSPECTIVE PERMITTING STRATEGY In permitting NOx emissions from any cement plant, the area of interest is only maximum emissions. Minimum emissions are not a concern. If there is a theoretical normal distribution of the hourly average mass NOx emission rates, then half of the values comprising the population of data points will be above the mean or average emission rate, i.e., the mean and median of the data set will be the same value. A graph of the frequency distribution of the emission values will appear as the familiar “bell curve”. Even though the mean and median of a data set in the real world are seldom the same value, one strategy for retrospective permitting of NOx emissions from existing cement kilns with a CEMS employs the idea that the value of the historical mean emission rate plus three standard deviations theoretically equals or exceeds 49.865% of the values of NOx emissions above the mean. For most permitting purposes, this value should be sufficient for a permit limit with only a few potential measured exceedances during a year. In a complete year of 8760 hours, exceedances theoretically would be observed in only about twelve hours of kiln operation. This procedure assumes no effort on the part of the operator to reduce peak emissions of NOx during the gathering of baseline emissions data or, subsequently, during the compliance period. In practice, most operators will respond appropriately to situations in which an emission rate limit is about to be exceeded and the likelihood of any measured exceedance is reduced further. Hence, there can be a high level of confidence that a permit limit will be established that will not result in exceedances during normal operation of the kiln. Often, in operating permits requiring CEMS to demonstrate compliance with NOx emission rate limit, up to 5% of the operating hours are allowed to exceed a nominal short-term, e.g., hourly, emission rate limit. This period of allowed exceedances recognizes the accuracy and performance limitations of the CEMS and the flow rate monitor, and the short-term variability of the process. This exclusion would represent 438 hours for a complete year of operation.

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Because cement kilns usually operate only 90-95% of the time, the actual exclusion period would be even smaller for the typical kiln. The relatively small number of anticipated exceedances resulting from the procedure to establish permits limits that is described in the previous paragraph are well within the typical exceedance allowance and should not affect any compliance demonstration. In addition to satisfying the statistical derivation methodology, a NOx permit limit also must meet the appropriate requirements of a NAAQS and/or the PSD increment. For NOx, these requirements are on an annual basis; therefore, a short-term NOx emission rate limit, e.g., hourly emissions, necessarily is not appropriate. However, it may be prudent or necessary to demonstrate compliance more often than annually. Consequently, an annual emission rate limit with a short-term component, e.g., a rolling annual average updated monthly, often will meet the regulatory criterion of environmental protection, and the operating criteria of flexibility and simplified recordkeeping. Using All Reported Data Including Periods of Zero NOx Emissions To test the hypothesis that the mean plus three standard deviations will provide an adequate maximum NOx emission rate limit, the following tables were constructed to compare the difference between the maximum normalized reported value and the normalized mean value, and the value of three times the standard deviation for each plant in each typical compliance period. In this comparison series, all the reported data, including zero emissions were included. Shaded numbers are those in which the value of three times the standard deviation exceeds the extreme value of the observed maximum minus the mean. Rather than being the result of normal operation, this value could represent an outlier in the data set. However, since these are real data from real kilns, this value would have to be included in any system of compliance demonstration and are, therefore, included in this analysis.

Table 3. Comparison of the Difference Between the Values of the Maximum Observed Value Minus the Mean and Three Times the Standard Deviation for 1-Hour Block Averages

All Data.

Plant Maximum Minus Mean 3σ 1 696 482 2 170 259 5 1099 681 6 179 264 7 754 723 8 1139 873 9 1345 787 10 1271 664 11 404 345 12 648 481 13 885 651 14 447 485

12


All Data.



All Data.


13


All Data.


Table 7. Comparison of the Difference Between the Values of the Maximum Observed Value

Minus the Mean and Three Times the Standard Deviation for 30-Day Block Averages All Data.

Plant Maximum Minus Mean 3σ

1 127 228 2 53 111 5 90 221 6 44 190 7 13 22 8 150 230 9 225 382 10 8 17 11 82 248 12 7 13 13 195 418 14 98 269

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Table 8. Comparison of the Difference Between the Values of the Maximum Observed Value Minus the Mean and Three Times the Standard Deviation for 30-Day Rolling Averages

All Data.


In can be seen from the above tables that although all but three of the differences between the maximum observed value and the mean for 1-hour block averages exceeds three times the standard deviation, by the time the block average has increased to twenty-four elapsed hours and beyond, all the differences are less than three times the standard deviation. The skewed distribution of data caused by the inclusion of periods of zero emissions of NOx probably accounts for the different results in the shorter averaging periods. In the twenty-four hour averaging period and beyond, one of the important theorems of statistics probably is starting to affect the results of the analysis. In essence, this theorem states that the distribution of the means of random samples drawn from any population approaches a normal distribution as the sample size approaches thirty. Even though the twenty-four, 1-hour values comprising the 365 (nominal), 24-hour block average samples are not random but consecutive, it is likely that the distribution of the 24-hour block averages is close enough to a normal distribution that standard statistical procedures for a normal distribution will suffice for purposes of determining a NOx emission rate limit. Using Measured Data Excluding Periods of Zero Emissions of NOx Using the data set that excludes all 1-hour periods in which NOx emissions are zero, the following tables were constructed using the same procedures as before. The same trends can be seen in these tables except they are not as strong. The elimination of the periods of zero emissions of NOx reduces the value of the standard deviation and decreases the value of the maximum minus the mean because the value of the mean is increased. The apparently anomalous situation for the rolling 30-day average for Plant No. 12 appears to be the well-known exception to every rule. Generalities about highly variable processes always may not be true and should be applied with an appropriate degree of caution.

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Data Without Zeros.



Minus the Mean and Three Times the Standard Deviation for 3-Hour Block Averages Data Without Zeros.


1 545 353 2 146 254 5 798 403 6 107 202 7 524 431 8 1032 852 9 1243 505 10 574 362 11 376 339 12 588 298 13 520 515 14 283 296

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Data Without Zeros.



Minus the Mean and Three Times the Standard Deviation for 24-Hour Block Averages Data Without Zeros.


1 369 291 2 121 231 5 287 279 6 51 172 7 136 211 8 469 765 9 550 369 10 154 210 11 298 308 12 136 186 13 419 460 14 131 248

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Table 13. Comparison of the Difference Between the Values of the Maximum Observed Value Minus the Mean and Three Times the Standard Deviation for 30-Day Block Averages

Data Without Zeros.



Minus the Mean and Three Times the Standard Deviation for 30-Day Rolling Averages Data Without Zeros.


1 94 140 2 57 107 5 82 137 6 30 112 7 21 21 8 230 378 9 144 191 10 22 31 11 86 211 12 51 27 13 201 365 14 58 77

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RELATIONSHIP OF RATED KILN CAPACITY TO THE STANDARD DEVIATION OF NOX EMISSION DATA Statistical calculations for all data sets were performed to include a calculation of the correlation coefficient for the standard deviations in each data averaging period, both with and without the inclusion of zeros, and the rated daily production rate for each of the kilns as reported by the kiln operators to the PCA1. The respective correlation coefficients are presented in Table 15. The correlation coefficient represents the extent to which pairs of numbers lie on a straight line.5 The interpretation of the correlation coefficient is somewhat intuitive. A perfect correlation between two variables would result in a correlation coefficient of ±1.0 depending on the slope of the line. A correlation coefficient at or near zero (<0.10) indicates no relationship at all between the two variables. In practice, a correlation coefficient of 0.90 indicates a strong correlation between the two variables. The correlation coefficients presented in Table 15 indicate a positive correlation, i.e., as the rated kiln capacity increases the variability of NOx emissions as expressed by the standard deviation tends to increase. Unfortunately, none of the correlations appear to be very strong. Perhaps if the actual kiln production rate during the NOx data gathering period had been provided by the study participants, a stronger correlation coefficient could have been observed. For a variety of reasons, the rated kiln capacities supplied to the PCA by kiln operators do not always reflect the current production rate of the respective kilns and may be the root cause of the low correlation coefficients.

Table 15. Correlation Coefficients and Variance Explanation for All Data Sets

Correlation Coefficient (r) Variance Explained (r2 x 100%)

NOx Data Averaging Period

With Zeros Without Zeros With Zeros Without Zeros

1-Hour 0.35 0.48 12% 23% 3-Hour 0.26 0.51 7% 26%

8-Hour 0.36 0.52 13% 27%

24-Hour 0.35 0.52 12% 27% 30-Day Block 0.35 0.71 12% 50%

30-Day Rolling 0.30 0.65 9% 42%

Squaring of the correlation coefficient and multiplying it by 100% results in another, perhaps more understandable, statistic known as the variance explained.6 In the case of the standard deviation of NOx emissions versus the rated kiln production capacity, these statistics also are presented in Table 15. It can be said that the variance of the standard deviation of NOx emissions in any averaging period is explained or predicted by the rated kiln capacity by the percentage shown in the relevant cell in Table 15. Clearly, the data set from which zeros have been excluded produces a weak, but relatively better, explanation of the variance of the standard deviation of NOx emissions when compared to rated kiln capacity than does the data set that includes zeros. Assuming that the inaccuracies in the reported kiln production rate do not account for the unexplained variance, other factors in the design and/or operation of the

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precalciner kilns, e.g., changes in fuel or the type of clinker produced, must explain the variability of NOx emissions. The determination of these factors is outside the scope of this report. In Appendix 1, the standard deviations of NOx emissions for each participating plant and for each averaging period, with and without zeros, have been plotted against the reported rated kiln capacity. A simple linear regression using the method of least squares was performed and the resulting regression line is shown on each plot. The plots are useful in visualizing the relationship between the kiln production rate and the standard deviation of NOx emissions. PROSPECTIVE PERMITTING STRATEGY In a typical situation involving a new or modified precalciner kiln system, the system vendor will provide the owner/operator with a guaranteed emission factor of pounds of NOx per ton of clinker on an annual basis. Seldom, if ever, do vendors provide an appropriate value for the mass emission rate variability that could be used in developing permissible mass emission rate limits. To provide for mass emission rate limits for averaging periods shorter than a year, the guaranteed emission factor must be converted to a mean mass emission rate (usually using the rated or design kiln capacity) and some estimated value of variability added to the mean mass emission rate. Although the correlation between standard deviation and reported rated kiln capacity is not very strong, the charts in Appendix 3 can be used as a starting point for negotiations about the estimated value of variability to be included in permitted NOx emission rate limits for new or modified precalciner kiln systems. For example, if the vendor projects an annual NOx emission factor for a 3000 ton per day precalciner kiln of 2.8 pounds of NOx per ton of clinker, the average hourly NOx emission rate will be [(3000/24) 2.8] or 350 pounds per hour of NOx. If the desired short-term permit limit will be for 24-hours and the compliance data base will not include periods when the NOx emissions are zero, then the appropriate chart in Appendix 3 yields an approximate standard deviation of 100 pounds per hour of NOx. The appropriate average hourly limit for NOx emissions over a 24-hour period can be determined by adding three times the estimated standard deviation to the hourly average emission rate guaranteed by the vendor, i.e., [(3)(100) + 350] or 650 pounds per hour of NOx. Under the same criteria, except that the compliance period is now a 30-day block period, the estimated standard deviation is reduced to approximately 45 pounds per hour. The appropriate average hourly limit for NOx emissions over a 30-day block period can be estimated at [(3)(45) + 350] or 485 pounds per hour. It should be recognized that projected permit limits that are determined by this procedure might have to be revisited when the kiln has completed its shakedown period and has gathered NOx emission rate data during “normal” operation for at least a year. CONCLUSIONS Emissions of NOx from precalciner cement kilns are highly variable when compared to other combustion sources. Individual precalciner kilns have been shown to have unique NOx emission rate variability characteristics as expressed by the standard deviation statistic. There is no apparent universal value for this statistic that can be applied to all precalciner kilns. There appears to be a direct, but weak, relationship between the rated kiln production rate and the standard deviation of NOx emission rates. While there may be a high level of confidence in the

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projected annual average hourly NOx emission rate for a new precalciner kiln, the short-term variability of NOx emissions from a proposed kiln system are difficult to predict. Care must be exercised in establishing NOx emission rate limits for these kiln systems to avoid limits that are too restrictive to allow normal and efficient operation. If available, historical NOx emission rate data can be used to establish appropriate emission rate limits for any existing kiln system. However, without these data, the process of establishing appropriate NOx emission rate limits for existing kiln systems is beset by the same problems as for a new system. Based on the results of this study, a compliance value for any averaging period of less than 24 hours is highly subject to exceedances and, therefore, is not recommended. Furthermore, there is little need from an environmental perspective to establish a compliance limit based on a 1, 3, or 8-hour block averaging value when the NAAQS and the PSD increment are based on an annual average concentration of NOx. While it may be desirable from the regulatory agency’s perspective to include short-term averages as permit limits, short-term averages impose significant compliance risks to the facilities and the risk of non-compliance increases as the averaging period decreases. A short-term component can be introduced into long-term NOx emission rate limits through several mechanisms that conform to ambient air standards. Among them is a rolling annual average NOx emission rate limit for which compliance is demonstrated on a monthly basis. The concern about NOx emission rate variability is essentially removed through this approach. Another approach to more frequent compliance demonstrations could involve a 30-day rolling average NOx emission rate limit that is updated daily. However, this study has shown that there must be an allowance for variability in establishing any short-term NOx emission rate limit including a 30-day rolling average. ACKNOWLEDGEMENTS Trinity Consultants conducted the research reported in this paper with the sponsorship Pyroprocessing Subcommittee of the Manufacturing Process Committee of the Portland Cement Association. The authors wish to thank the following cement companies that provided data for the study: Ash Grove Cement Company; Capitol Aggregates, Ltd., Capitol Cement Division; Cemex, Inc.; Holcim (US) Inc.; RMC Pacific Materials; Texas Industries Inc.; and Texas-Lehigh Cement Company. The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data presented.

REFERENCES 1. Portland Cement Association, U.S. and Canadian Portland Cement Industry: Plant Information Summary, Portland Cement Association, Skokie, Illinois, USA, December 2001. 2. Penta Engineering Corporation, Report on NOX Formation and Variability in Portland Cement Kiln Systems, Potential Control Techniques, and Their Feasibility and Cost Effectiveness, Portland Cement Association, Skokie, Illinois, USA, R&D Serial No. 2227, 1999. 133 pages. 3. Gartner, E.M., “Nitrogenous Emissions from Cement Kiln Feeds,” Interim Report on Project HM7140-4330; For Presentation at Rule 1112 Ad Hoc Committee Meeting South Coast Air Quality Management District, El Monte, CA, Portland Cement Association, Skokie, Illinois, USA, June 7, 1983.

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4. Shigahara, R., DRAFT Proposed Missing NOX and Flow Rate Data Calculation Routine for Portland Cement Kilns, Portland Cement Association, Skokie, Illinois, USA, R&D Serial No. 2495, 1999, 12 pages. 5. Easton, V.J. and McCall, J.H., Statistics Glossary, http://www.stats.gla.ac.uk/steps/glossary/paired_data.html, December 15, 2003. 6. Hopkins, W. G., 2000, A New View of Statistics, Summarizing Data: Effect Statistics, Correlation Coefficient, www.sportsci.org/resource/stats/correl.html, December 15, 2003.

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http://www.stats.gla.ac.uk/steps/glossary/paired_data.html

http://www.sportsci.org/resource/stats/correl.html

APPENDIX 1

Figure A-1. Relationship of Rated Kiln Production Rate to 1-Hour Standard Deviation All Data

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Figure A-3. Relationship of Rated Kiln Production Rate to 8-Hour Standard Deviation

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Figure A-5. Relationship of Rated Kiln Production Rate to 30-Day Block Standard Deviation All Data

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Figure A-6. Relationship of Rated Kiln Production Rate to 30-Day Rolling Standard Deviation

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Figure A-7. Relationship of Rated Kiln Production Rate to 1-Hour Standard Deviation Without Zeros

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Figure A-9. Relationship of Rated Kiln Production Rate to 8-Hour Standard Deviation

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Figure A-11. Relationship of Rated Kiln Production Rate to 30-Day Block Standard Deviation Without Zeros

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Figure A-12. Relationship of Rated Kiln Production Rate to 30-Day Rolling Standard Deviation

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variability of nox emissions from precalciner cement kiln systems

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