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Developing an Alternative CO Limit for Biomass Stoker Units

Having a CO CEMS

The current biomass stoker subcategory CO limit is 490 ppm at 3% O2 for existing units. As 

discussed in the companion NCASI paper Review of the Biomass Stoker CO Limits and Suggestions 

for Revisions, there are several reasons why this limit needs revision. The key reason is that 

additional Method 10 tests have been located for some of the best performers in this subcategory, 

and these additional tests provide a more complete picture of the variability in CO emissions 

amongst biomass stokers. Still, this is a 3 hour average limit, and it is well documented that 

routine biomass stoker operations are such that frequent spikes in CO concentrations occur. A 

boiler equipped with a CO CEMS would record such elevated emissions, thus providing credible 

evidence of exceedances of the 490 ppm limit. EPA has acknowledged that an alternative

CEMS-based CO limit is a possible solution for this dilemma. This limit would have an

averaging time considerably longer than 3 hours in order to minimize the impact of temporary 

spikes and provide sufficient time for boiler operators to make appropriate adjustments to lower 

CO levels.

To establish a CO limit for a longer period than 3 hr, EPA Method 10 tests are unsuitable and 

CEMS data are needed. EPA strongly favors using CEMS data from the best performing units in 

a subcategory, as identified from Method 10 data, to develop such a limit. Since none of the 

current or prospective best performers in the biomass stoker subcategory have a CO CEMS, this 

leaves only two options, namely (1) install a CO CEMS on one or more of the top performers 

and gather several months of data, or (2) demonstrate that a non-best performer stoker boiler 

with CO CEMS data has CO emissions that are equivalent to or less than those measured during 

Method 10 tests at the best performers. 

Generating New Data 

If it is assumed any biomass stoker boiler with a Method 10 test average of 210 ppm at 3% O2 or 

less (when 90% or more of the heat input during the test came from biomass) will be a best 

performer for CO in the biomass stoker subcategory after EPA completes its review of new 

information submitted by July 15, potential candidate boilers in the forest products industry for 

additional CO sampling using a temporary CEMS are:

• WVWeyerhaeuserBuckhannon

• ARWestFraserHuttig

• IDChilcoLakeSawmill

• IDRileyCreekLumber

• MSShuqualakLumber (uncertain if it belongs in subcategory)

Company representatives for these facilities have been contacted. At least two have expressed 

interest in having a temporary CO CEMS installed for the purpose of gathering additional data. 

AF&PA and AWC would provide funding for testing contractors, and NCASI would be 

responsible for overseeing the effort and analyzing the resulting data.
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If EPA adheres to its timetable for proposing and promulgating changes to the Boiler MACT rule

by April 30, 2012, results from a 4-month sampling effort could be provided to EPA around 

March 1 if there are no unforeseen problems with the testing program. This timing might allow 

EPA to issue a quick Notice of Data Availability or include a reference in the preamble to the 

final rule, noting that the alternative CO standard with a longer averaging time described in the 

proposed rule was still under consideration and development. Four months of CEMS data from 

at least two boilers is the absolute minimum amount of data needed to make a credible case for 

an alternative 24 hr block or 7 day average standard. Obtaining similar CEMS data from 

additional boilers would be highly desirable. However, four months of CEMS data would likely 

be insufficient to support a 30 day average standard – at least 12 months should be obtained from 

a minimum of at least two boilers to represent the full year of variability including winter and 

spring months when biomass fuels are typically wetter and average CO emissions higher than in 

summer and fall. 

Using Existing CEMS Data

There are at least 26 stoker boilers that burn biomass in the forest products industry that have CO 

CEMS. Outside of the forest products industry, there are at least 18 biomass stoker units 

operated by electricity generating firms with CO CEMS, but the majority of these boilers are at 

facilities which are not major sources of hazardous air pollutants, or the boilers burn some 

biomass materials which may be classified as solid wastes. It is understood that EPA will not 

use any data from boilers that will be subject to GACT or CISWI standards to set MACT limits. 

As previously mentioned, none of current or potential best performers have a CEMS. Thus to 

use existing CEMS data to develop an alternative standard with a longer averaging time, it would 

have to be shown that CO emissions from a non-best performer are substantially similar to those 

of best performers. Presumably EPA’s benchmark for comparisons will be Method 10 test 

results from the best performers. 

Relative to using CO CEMS data for revising the current biomass stoker subcategory CO limit or 

establishing an alternative limit with a longer averaging time for those units having a CEMS, 

EPA suggested CEMS data from non-best performers might be considered if (1) the CEMS for 

CO and O2 met certain QA/QC criteria, e.g. PS-4 and PS-3 provisions and annual RATAs and

periodic calibration and drift tests, (2) at least 90% of the boiler heat input came from biomass 

during each 3-hr block average, and (3) a significant fraction of the 3-hr block averages were at 

or below the maximum 3-hr average Method 10 concentrations measured in tests conducted at 

the best performers. Regarding the last condition, 210 ppm at 3% O2 is currently the cutoff test 

average value for the best performing units in the biomass stoker subcategory.

NCASI has gathered and continues to gather more information on the installed monitors and 

boiler fuel use in biomass stoker units to see what data meeting EPA’s criteria can be obtained.

Thus far, NCASI has obtained hourly average CO CEMS data for sixteen biomass stokers (13 in 

the forest products industry and 3 at electricity generating facilities) for periods ranging from 6 

to 18 months. After screening the data to remove hourly averages associated with 

startup/shutdown conditions and monitoring equipment failure or calibrations, 3-hour block 

averages were calculated and averages of all such blocks were estimated for the 16 boilers. 
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Table 1 summarizes the 3 hr block averages for these 16 units along with the duration of the data 

and the fuel mix. 

Five of the 16 boilers had a long-term 3 hr block average of less than 210 ppm (Boiler Nos. 1, 3, 

8, 12 and 14). However, Boiler Nos. 13 and 14 burned less than 30% biomass on an annual 

average basis, the rest being coal, oil and gas. Boiler No. 8 burned 81% biomass and 19% gas. 

However, besides clean wood residues, the biomass material included sludge and paper 

materials, and may have included chipped railroad ties and industrial wood waste. Thus it is 

uncertain whether this boiler would be subject to CISWI or Boiler MACT standards. Two of the 

16 boilers (Boiler Nos. 1 and 3) had a mean 3 hour average CO value of 208 and 202 ppm @ 3%

O2, respectively. Boiler No. 1 (Boise Paper, Wallula, WA) burned hog fuel and natural gas (78% 

biomass annual average). Boiler No. 3 (Genessee Power, Flint, MI) burned 100% biomass

which included small amounts of animal bedding (wood shavings and manure). For Boiler No. 

1, nearly 3.0% of the hourly data were eliminated before computing three-hour block averages

(periods of startup/shutdown, no steam production, and CEMS malfunctions/calibrations). Of 

the remaining 97.0%, 7.7% corresponded to periods when biomass firing exceeded 90% of total 

heat input (rest gas), and 65.2% of these were less than 210 ppm @3% O2. For Boiler No. 3 that 

burned 100% biomass, 9.8% of the data were eliminated (8.3% for periods of shutdown with no 

steam production, and the remainder for startup/shutdown conditions or CEMS malfunctions/

calibrations) before computing three-hour block averages. Of the remaining 90.2%, nearly 65% 

corresponded to periods when CO emissions were less than 210 ppm @3% O2. Thus, these two 

boilers equipped with CO CEMS should be representative of the “best performer” boilers in the 

biomass stoker subcategory and their data should be appropriate for determining a CEM-based 

CO limit for biomass stokers.

Table 1. Long-Term CO CEMS 3-Hr Averages For 16 Biomass Stoker Units

No. Fuel Mix Data Duration

3 Hr-Avg CO, 

ppm @ 3% O2

1 Wood Residues (78%), Gas (22%) 1-1-10 to 1-1-11 207.8

2 Wood Residues (100%) 12-1-99 to 12-31-00 335.3

3 Wood Residues (100%) - EGU 6-1-10 to 5-31-11 201.5

4 Wood Residues & Sludge (97.5%), Gas (2.5%) 1-1-10 to 1-1-11 605.0

5 Wood Residues (100%) - EGU 9-1-07 to 12-18-08 482.9

6 Wood Residues (100%) - EGU 6-1-09 to 11-4-09 995.8

7 Wood Residues (91.2%), Sludge (8.8%) 1-1-08 to 10-31-08 334.9

8 Wood Residues (81%), Gas (19%) 1-11-08 to 1-10-09 43.4

9 Wood Residues (75%), Gas (18%), Oil (7%) 1-1-09 to 6-11-10 703.7

10 Wood Residues (73%), Gas (27%) 1-1-00 to 12-31-00 211.5

11 Wood Residues, Gas 1-1-09 to 12-31-09 387.7

12 Wood Residues, Coal, Gas 1-5-09 to 12-21-09 1389.9

13 Wood Residues (28%), Coal (49%), Oil (12%), Gas (12%) 1-1-09 to 6-11-10 155.1

14 Wood Residues (29%), Coal (44%), Oil (13%), Gas (14%) 1-1-09 to 6-11-10 143.6

15 Wood Residues (100%) 1-1-08 to 12-31-08 302.1

16 Wood Residues (100%) 1-1-01 to 5-31-01 311.7
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Table 2 summarizes the 99.0 and 99.9 percentile value of CO emissions using the combined 

hourly average CO CEMS data for the above two “best-performing” biomass stokers. The 99.0

and 99.9%ile values are shown for 3-hr, daily, weekly and monthly averages. Also shown are a 

“never-to-exceed” value which simply corresponds to the maximum value in the data set.

Table 2. 99.0%ile and 99.9%ile Values For Two “Best Performing” Biomass Stokers

With Long-Term CO CEMS Data Using 3 Hr, Daily, Weekly and Monthly Averages

3 Hr-Avg Daily Average Weekly Average Monthly Average

99.9 

%ile1

99.0 

%ile1

99.9 

%ile1

99.0 

%ile1

99.9 

%ile1

99.0 

%ile1

99.9 

%ile1

99.0 

%ile1

ppm CO @ 3% O2 990 680 650 550 470 440 420 420

Never-to-Exceed

ppm CO @ 3% O2 1540 680 470 420

1

99.9 or 99.0%ile value from rank ordering the data points

Illustration of How Limits Shown in Table 2 Compare With Available Emission Profiles for Two

Biomass Stokers

In this section, the 3-hour, daily, weekly and monthly average CO emission profiles are 

compared for two of the biomass stokers listed in Table 1 with the limits derived in Table 2.

Boiler No. 2 (12-1-99 to 12-31-00)

Although this boiler had a 3 hr annual average CO emission concentration of 335 ppm, as seen 

from the profile in Figure 1A, stack CO levels were often much higher. Thus, even though the 

99%ile, 99.9%ile or “not-to-exceed” values for the 3-hr averages in Table 2 are significantly 

higher than those for the daily, weekly or monthly averages, on multiple occasions these 3-hr 

values would have been exceeded. Attempting to adjust boiler operating conditions to 

sufficiently reduce CO levels to meet a 3-hr limit is generally not possible; the timeframe is too 

short.

Figure 1A. 3-Hr Average CO Emissions For Boiler No. 2

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

12/01/99 01/19/00 03/07/00 04/21/00 06/07/00 07/31/00 10/25/00 12/12/00

CO (ppm @ 3% O2)

3 Hr Averages Limit Not To Exceed = 1540 ppm 99.9 & 99%ile = 990 & 680 ppm
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The daily average CO profile shown in Figure 1B also shows how the 99%ile, 99.9%ile or “notto-exceed” 24-hr values in Table 2 were often exceeded. However, 24 hours would provide 

more time for modifying boiler operations to lower CO levels. 

Figure 1B. Daily Average CO Emissions For Boiler No. 2

The weekly average CO profile shown in Figure 1C shows how the 99%ile, 99.9%ile or “not-toexceed” 7-day average values in Table 2 would have been exceeded four times (8%) during the 

entire year. More importantly, sufficient time would have been available for modifications to be 

made to boiler operations to lower CO levels. 

Figure 1C. Weekly Average CO Emissions For Boiler No. 2

Finally, the monthly average CO profile shown in Figure 1D demonstrates the 99%ile, 99.9%ile 

or “not-to-exceed” 30-day average values in Table 2 would have been exceeded once during the 

entire year. The 30-day averaging time provides more flexibility for adjusting boiler operating 

conditions to minimize CO emissions. 

0.0

200.0

400.0

600.0

800.0

1000.0

12/02/99 03/04/00 06/02/00 10/15/00

CO (ppm @ 3% O2)

Limit Not To Exceed = 680 ppm Daily Average 99.9 & 99%ile = 650 & 550 ppm

0.0

200.0

400.0

600.0

800.0

1000.0

12/08/99 02/16/00 04/26/00 07/05/00 10/11/00 12/20/00

CO (ppm @ 3% O2)

Limit Not To Exceed = 470 ppm Weekly Average 99.9 & 99%ile = 470 & 440 ppm
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Figure 1D. Monthly Average CO Emissions For Boiler No. 2

Boiler No. 13 (1-1-09 to 6-11-10) 

This boiler had a 3 hr annual average CO emission concentration of only 155 ppm. Yet Figure 

2A shows how the stack CO levels were sometimes 5 to 10 times higher. Thus, even though the 

“not-to-exceed” 3-hr average value of 1540 ppm is exceeded just once, the 99.0 and 99.9%ile 

values are exceeded multiple times. Insufficient time would have been available to adjust boiler 

operating conditions to lower CO emissions to these values. 

Figure 2A. 3-Hr Average CO Emissions For Boiler No. 13

The daily average CO profile shown in Figure 2B also shows how the 99%ile, 99.9%ile or “notto-exceed” 24-hr average values in Table 2 would have often been exceeded. 

0.0

200.0

400.0

600.0

800.0

01/01/00 03/01/00 05/01/00 07/01/00 09/01/00 11/01/00

CO (ppm @ 3% O2)

Limit Not To Exceed = 420 ppm Monthly Average 99.9 & 99%ile also 420 ppm

0.0

200.0

400.0

600.0

800.0

1000.0

1200.0

1400.0

1600.0

12/8/2008 3/8/2009 6/6/2009 9/4/2009 12/3/2009 3/3/2010 6/1/2010

CO (ppm @ 3% O2)

Limit Not To Exceed = 1540 ppm 3 Hr Averages 99.9 & 99%ile = 990 & 680 ppm
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Figure 2B. Daily Average CO Emissions For Boiler No. 13

The weekly average CO profile shown in Figure 2C shows how the 99%ile, 99.9%ile or “not-toexceed” 7-day average values in Table 2 would have been exceeded only three times (6%) 

during the entire year. More importantly, sufficient time would have been available for 

modifications to be made to boiler operations to sufficiently lower CO concentrations. 

Figure 2C. Weekly Average CO Emissions For Boiler No. 13

Finally, the monthly average CO profile shown in Figure 2D demonstrates the 99%ile, 99.9%ile 

or “not-to-exceed” 30-day average values in Table 2 would have been exceeded once during the 

entire year. Ample time would be available for adjusting boiler operations to achieve these 

values. 

0

200

400

600

800

1000

12/8/2008 3/8/2009 6/6/2009 9/4/2009 12/3/2009 3/3/2010 6/1/2010

CO (ppm @ 3% O2)

Limit Not To Exceed = 680 ppm Daily Average 99.9 & 99%ile = 650 & 550 ppm

0

200

400

600

800

12/8 3/8 6/6 9/4 12/3 3/3 6/1

CO (ppm @ 3% O2)

Limit Not To Exceed = 470 ppm Weekly Average 99.9 & 99%ile = 470 & 440 ppm
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Figure 2C. Monthly Average CO Emissions For Boiler No. 13

Conclusion

EPA technical staff believe using CO CEMS data obtained on the best performing units already 

used to calculate the current short-term limit would be the most legally defensible approach for 

developing an alternative CO CEMS-based limit. If temporary CEMS were installed on some of 

these units to obtain just four months of data, the data could not be provided to EPA until shortly 

before EPA’s anticipated date for promulgating changes to the Boiler MACT rule. Thus the 

collection of additional CEMS data would only be worthwhile if EPA commits to waiting for the 

new data before making final decisions on an alternative CEMS-based CO limit for the biomass 

stoker subcategory.

Because of the timing issue, EPA encouraged the forest products industry to investigate how 

existing CO CEMS data could be used to develop an alternative limit. NCASI has done that. 

CEMS data from two biomass stoker boilers substantially similar to those in MACT floor for this 

subcategory were analyzed. Based on 12 months of data for each of these boilers, the analysis 

would support the following numerical limits for a CEMS-based longer-term alternative 

standard:

1. A daily average of 650 ppm @ 3% O2 to be exceeded only once per calendar year (based 

on 99.9%ile value.) 

2. A daily average of 550 ppm @ 3% O2 to be exceeded only four times per calendar year 

(based on 99.0%ile value.)

3. A daily average of 1540 ppm @ 3% O2 never-to-be-exceeded in a calendar year (based 

on the “never-to-exceed” value.) 

4. A weekly average of 470 ppm @ 3% O2 to be exceeded only once per calendar year 

(based on 99.9%ile and “never-to-exceed” value.) 

5. A weekly average of 440 ppm @ 3% O2 to be exceeded only four times per calendar year 

(based on 99.0%ile value.) 

6. A monthly average of 420 ppm @ 3% O2 never to be exceeded in a calendar year (based 

on the 99.0%ile, 99.9%ile and/or “never-to-exceed” value.) 

0.0

100.0

200.0

300.0

400.0

500.0

12/8 3/8 6/6 9/4 12/3 3/3 6/1

CO (ppm @ 3% O2)

Limit Not To Exceed = 420 ppm Monthly Average 99.9 & 99%ile also 420 ppm
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There is no way to predict if limits based on data that would be obtained in a four month study 

on boilers currently in the MACT floor would be the same as these numerical values. 

Nevertheless the available CEMS data show an alternative to the current short-term standard for 

biomass stoker boilers equipped with a CEMS is essential.