0223.04.020 1

Date:   January 2011

To:  Brian Shrager, US EPA

From:  Amanda Singleton and Bryan Lange, ERG

Subject:   Analysis of Carbon Monoxide Continuous Emissions Monitors (CEMS) Data for the Industrial,

Commercial, and Institutional Boilers and Process Heaters National Emission Standards for Hazardous

Air Pollutants – Major Source

Numerous commenters disagreed with EPA’s statement that CO emissions do not vary

significantly over the operating range of a unit, 75 FR 32029.  These commenters provided data across

the operating range of boilers showing significant variation in CO emissions; in some instances they also

show that CO emissions are higher at low load.  CEMs data was collected at both full and lower load

conditions.

The purpose of this memorandum is to present the available CEMS data and demonstrate that

there is insufficient data to either 1) establish CO limits that rely strictly on a CEMs data or 2) combine

the stack test performance data with CEMs that considers lower loads.  The CEMS data was obtained

from two recently implemented Information Collection Requests (ICRs) approved under OMB Control

No. 2060‐0616: ICR numbers 2286.01 and 2286.03, as well as new data and data corrections received

before, during and after the public comment period on the proposal.1,2,3  The memorandum is organized

as follows:

1.0 Available CEMS data  

2.0 Methodology and Results of a Hypothetical CEMs based MACT floor

3.0 Concerns with Combining Stack Tests and CEMs data  

4.0 Conclusion

5.0 References

Appendix A: CEMS Observations by Subcategory

Appendix B: Load Variability Analysis by Subcategory

1.0 Available CEMS data 

The ICR Phase I survey requested that owners submit CO CEM data for the most recent 30 days

of operation (i.e., thirty 24‐hour averages).  Owners were asked to exclude data if they were from

periods of start‐up, shutdown, or malfunctions (SSM) and submittal was not required if the CEMs

monitor was not installed for regulatory purposes.  Owners were also requested to submit a “typical”

operating rate, which often represented the average rate for the 30‐day period, but it did not request

daily load averages to correspond to the daily average CO CEM emission concentrations.  Phase II of the

ICR requested 30 days of CEMS data, including hourly data and the corresponding hourly heat input and
0223.04.020 2

steam output of the boiler.  For this analysis, data from both ICRs were combined for currently operating

boilers and process heaters.  

To capture unit variability, both short and long‐term CEMs data were analyzed.  Although 30‐

days of operation data was requested in the ICR, some units provided additional CEMs data for longer

periods, while others provided additional CEMs data that flagged SSM periods. The long‐term data

threshold was lowered from 30 to 25‐days because the population of available data points qualifying for

the analysis expanded significantly. It was determined that a 24‐hour period and a 25‐day rolling were

the most robust available averaging periods for each subcategory.

Similar to the stack test based MACT floor analysis, the CEMs data was organized according to

the relevant subcategories outlined in Table 1 below.  The procedures used to assign the data to each

subcategory are discussed in another memorandum.4

  Table 1 below shows the distribution of CEMs

data by subcategory.  As a first step in considering the available CEMs data, EPA examined the available

data for different averaging periods.  Table 1 shows the available data for both 24‐hour and 25‐day

averaging periods, which represent  the shortest period considered by EPA and the longest period for

which data are available for most subcategories. For 24‐hour averages, CO CEMs data are available for

70 units, while 25‐day average CEMs data are available for 47 units.   

Table 1: Number of units per subcategory with CEM data.

Subcategories 

Combustors with 

24-hour average

CEMS data 

Subcategories 

24-hour avg. 

observations

Combustors with 

25-day average 

CEMS data 

Subcategories 

25-day avg. 

observations

Biomass 

 Stoker 19 1,186 18 49

 Dutch Oven/ 

 Suspension Burner 1 30 1 2

 Combined 

 Suspension Grate 1 841 1 33

 Fluidized Bed 3 683 3 27

 Fuel Cell 1 25 1 2

Coal 

 Pulverized Coal 7 222 5 9

 Fluidized Bed 5 605 4 23

 Stoker 11 258 8 12

Liquid 22 336 6 10

 Non-continental Liquid - - - -

Total 70 4,186 47 167

Most units did not report CEMS data through the ICRs or through public comment.  As a result,

CEMS data was not available for boilers/process heaters burning process gas 2 (e.g. Coke Oven Gas) or

non‐continental liquid units.

2.0 Methodology and Results of a Hypothetical CEMs-based MACT floor 

We used a similar approach to the stack test‐based MACT floor analysis described in a separate

memorandum to analyze the available CEMs data.5

  First, we identified the highest CEMS observation

(24‐hour average and 25‐day rolling average) at each boiler/process heater.  Next, we ranked the
0223.04.020 3

highest CEMS observation for each unit in ascending order.  For the 25‐day rolling average many units

only had a single observation.  We then determined how many units with CEMS data in each of the

subcategories would be representative of the top performing units by multiplying the number of

boilers/process heaters with CEMs data by 12 percent.  Next, we rounded up this value to the nearest

integer.  If the number of top‐performers was three, we then identified the units with the three lowest

maximum observations and took the average for the basis of a hypothetical CEMs‐based MACT floor.  

Table 2 below shows the results of the analysis and comparison to the stack test‐based MACT

floor limits; details for each subcategory are available in Appendix A.
0223.04.020 4

Table 2: Results of CEMs MACT floor analysis (ppm@3%O2).

Subcategories 

Number of Units in MACT 

Floor CEMs-based MACT Floor Stack Based

With 24-hr 

CEMs 

With 25-day 

CEMs 

Avg. of 

24-hr of top 

performers 

Avg. of 

25-day of top 

performers 

MACT 

Floor for 

existing 

units 

Biomass 

 Stoker 3 3 145 94 490

 Dutch Oven/ 

 Suspension Burner 1 1 9,844 1,324 470

 Combined 

 Suspension Grate 1 1 5,171 1,104 3,500

 Fluidized Bed 1 1 28 18 430

 Fuel Cell 1 1 975 338 690

Coal 

 Pulverized Coal 1 1 44 27 160

 Fluidized Bed 1 1 48 36 82

 Stoker 2 1 73 29 270

Liquid 4.9 5.4 10

 Non-continental Liquid 3 1 - - -

With the exception of the biomass dutch oven/suspension burner CEM‐based floors and the 24‐

hour averages for the biomass the combined suspension grate and fuel cell subcategories, the CEM‐

based MACT floors are comparable or more stringent that the limits computed from the stack test data.  

It should be noted that the CEMs top‐performer population size for most subcategories is a single unit.  

For example, for the coal fired stoker subcategory eight units had CEMs data; therefore a single unit

defines the top‐performers (8*0.12=1).  Given such a limited CO CEMs dataset, EPA determined that it is

not appropriate to base the MACT floors on CEMs data.  

3.0 Concerns with Comparing Stack Tests and CEMs data 

A stack test result compared with CEMS‐generated data under the same reference load

conditions would produce comparable CO concentrations.  Unfortunately, the conditions under which

these data were collected make the two measurements less comparable.  

The CEMs data submitted under the ICR has three fundamental differences: 1) CEMs

measurement precision is dependent on the instruments span; 2) CEMs measure concentrations

continuously and in real time while the duration of a stack test run is typically one hour; and 3) day‐to‐

day unit operations, as observed by CEMs, will not always be at full‐load conditions.  For these reasons

the two measurements are not comparable.  

CEMs Instrument Span

A CEMs span value means the concentration that is used to set the calibration gas

concentration. 6

  For sources operating CEMS to demonstrate compliance with other standards, an
0223.04.020 5

instrument’s span may be specified in the applicable subpart.  For example, NSPS Subpart J for

Petroleum Refineries requires a CO CEMs span of 1,000 ppm.  CEMs must meet precision requirements

of the emission standard, but the expected precision during a CO spike above 1,000 ppm or a very low

CO measurement would be poor.  The expected precision of stack test, conducted under limited

operating conditions, is greater than day‐to‐day CEMs observations.

Averaging period

Only the ICR Phase II testing requested hourly CEMs with corresponding load data.  This phase II

monitoring  was limited to a handful of units.  There is insufficient hourly CEMs data to analyze and

compare one‐hour stack test averages, or blocks of three one‐hour stack test average (consistent with 3

test runs)with CEMs data.  The only available data comparisons are stack test data and 24‐hours to 25‐

days averages.  As shown above in Table 2, the averaging period can have a significant effect on the

reported concentration.  Table 2 shows that as the averaging period increases from 24‐hours to 25‐days

the concentration decreases with only one exception i.e., liquid combustors.  Therefore, one‐hour stack

test averages are not comparable to 24‐hour or 25‐day CEMs averages.  Further, although CEMS data

will provide better estimates of uncertainty, the distribution of emissions statistics based on CEMS is not

the same as that based on stack test data and the Upper Predicted Limit (UPL) based approach used in

the MACT floor analysis cannot be used to combine these two data types.  

Although some commenters suggested that EPA should adopt an approach where an Upper

Predicted Level (UPL) is determined for each unit using all the test data available, and then ranking on

the UPL, we disagree that a UPL should be used to rank the sources.  The UPL is not a measure of

performance of the source but instead a prediction value for the average of test runs from a new

source.  The UPL has incorporated the uncertainty of the future source, and therefore it does not

represent the current sources and should be not used to rank the sources.  

CEMs Concentration vs. Load

It is generally acknowledged, and confirmed through EPA’s data base, that stack tests for CO are

conducted at or near full‐load conditions.  Although the MACT floor limits based on stack test results are

achieved when operating at or near full load, the MACT floor emission levels for CO might not be

achieved when best performing units are operating at lower loads.  For this reason, we looked at the

available hourly data that had corresponding load information to measure load variability for

subcategories where information was available.

We used a hierarchical approach to select the data used as inputs into a ratio or a load

variability factor (LVF).  First we looked for hourly CEMS data from units that were the basis of the new

source floor; next, we looked for data from any unit that was in the top 12 percent and used as the basis

of the existing source floor; third, we looked for CEMS data where at least one of the hourly CEMS data

points was below the new source floor for a particular category. Using this hierarchy we could develop a

LVF for the following five subcategories:
0223.04.020 6

Table 3: Units with both CEMs and Corresponding Load Information Organized by

Subcategory.

Subcategory Data used to compute a LVF ratio (a) Units 

Biomass – Stoker

No unit in the top 12% had hourly CEMs data. A

unit had both CO CEMs data and corresponding

load information was used. This unit had at least

one data point (e.g., one hourly average) where

the CEMs data was meeting the new source floor

level.

1

Biomass ‐ Dutch Oven/ 

 Suspension Burner

The boiler used as the basis of the top performing

unit had hourly CEMs data and load information. 1

Biomass – Combined  

     Suspension Grate

A unit that was in the top 12% for existing units

had both CO CEMs data and corresponding load

information and was used.

1

Coal/Solid Fossil – Pulverized 

Coal 

No unit in the top 12% had hourly CEMs data. A

unit had both CO CEMs data and corresponding

load information was used. This unit had at least

one data point (e.g., one hourly average) where

the CEMs data was meeting the new source floor

level.

1

Coal/Solid Fossil – Stoker

A unit that was in the top 12% for existing units

had both CO CEMs data and corresponding load

information and was used.

1

Total units with both CEMS and Load data 5

(a)  No new floor units report CEMs data.

An LVF ratio was computed by dividing CO emissions for loads less 50% by emissions where the

load was greater than 50%.  Three LVF computation methods were used: 1) the range of CO emissions,

2) the average of CO emissions, and 3) the max of CO emissions.  Table 4 below shows the results of the

analysis.  

In total, fifteen LVF were calculated; two‐thirds (10 of the 15) of the LVF resulted in factors

greater than 1 where the CO concentrations were higher at lower loads, but one‐third of the factors

were less than 1.  Given that so few units had both load and CEMs information, no conclusions could be

drawn from this analysis.  The details of the LVF analysis are available in Appendix B.

Table 4: Results of load variability factor analysis.

Subcategories Computation 

Method 1 

Computation 

Method 2 

Computation 

Method 3 

Biomass 

 Stoker 0.69 1.27 0.67 

 Dutch Oven/ 

 Suspension Burner 

2.10 5.57 2.13 
0223.04.020 7

Subcategories Computation 

Method 1 

Computation 

Method 2 

Computation 

Method 3 

 Combined 

 Suspension Grate 

3.77 1.46 3.77 

Coal 

 Pulverized Coal 2.77 7.46 2.92 

 Stoker 0.43 0.76 0.51 

4.0 Conclusion 

Given the limited CEMs data and its poor distribution across subcategories it can be determined

that there is insufficient data to establish a set of CO limits that relies strictly on a CEMs data.  It has also

been determined that the differences between the measurement collection of stack test performance

data with CEMs are significant.  Therefore, it is not appropriate to combine the stack test performance

data with CEMs data to establish a CO limit.  In the final rule EPA determined that the CO MACT floor

would remain calculated based on solely emission stack test data, and it adjusted the compliance

mechanism in the final rule to remove CO CEMS because the limited CO CEMS data were not sufficient

to develop emission limits.  Further, removal of the CO CEMS requirement made the basis of the

emission standard setting and compliance mechanism consistent.

5.0 References 

1. Phase I ICR 2008 Combustion Survey.  [ICR No. 2286.01]. OMB Control No. 2060‐0616 ‐ Information

Collection Effort for Facilities with Combustion Units. Approved by OMB on August 1, 2008.

2. Phase II ICR Test Plan Requesting Emission Testing from Boilers, Process Heaters, and CISWI. [ICR No.

2286.03]. OMB Control No. 2060‐0616 ‐ Information Collection Effort for Facilities with Combustion

Units. Approved by OMB on May 21, 2009.

3. Gibson, G. ERG. Handling and Processing of Corrections and New Data in the EPA ICR Databases.

January 2011.

4. Gibson, G. and A. Singleton, ERG. Revised Development of Baseline Emission Factors for Boilers and

Process Heaters at Commercial, Industrial and Institutional Facilities. January 2011.

5. A. Singleton, ERG. Revised MACT Floor Analysis (2011) for the Industrial, Commercial, and Institutional

Boilers and Process Heaters National Emission Standards for Hazardous Air Pollutants – Major Source.

January 2011.

6. Environmental Protection Agency. Emission Measurement Center. “Performance Specification 2‐

Specifications and Test Procedures for SO2 and NOX Continuous Emission Monitoring Systems in

Stationary Sources.” March 2009. <http://www.epa.gov/ttn/emc/perfspec/ps‐2.pdf>