June 7, 2012

1

HAP Emissions and Relation to CO Below 100 ppm for Coal and Oil-Fired Industrial Boilers1

Carbon monoxide (CO) results from incomplete oxidation of the carbonaceous fractions in fossil and 

non-fossil fuels. High levels of CO are generally associated with poor combustion. Solid fuels first 

undergo pyrolysis followed by oxidation of the pyrolysis products. Organic hazardous air pollutants 

(HAPs) are products of incomplete oxidation of the products of pyrolysis. Such incomplete oxidation has 

been historically associated with high levels of CO in the stack. As a result, CO is very often used as a 

surrogate for controlling HAPs and total hydrocarbon (THC) emissions from fuel combustion. However, 

while the evidence for CO as a HAP surrogate during biomass combustion may be considered to be 

reasonably sound, such evidence in the case of fossil fuel combustion is generally less forthcoming 

(except, of course, at very high stack CO levels). 

Coal, oil and natural gas combustion in boilers generally result in much lower CO emissions than biomass 

combustion. Thus, while in general the observation that high levels of CO (say >300 ppm) are indicative 

of higher levels of HAP emissions holds, impirical evaluation of CO data from the Industrial Boiler rule 

record, as shown in this paper, indicates that the correlation appears to breakdown in the case of very 

low CO emissions from fossil fuel combustion (below about 100 ppm). In conversations between the 

AF&PA and the EPA BMACT team in February 2012, a consultant (Carl Bozzuto) with more than 40 years 

of boiler combustion design expertise offered an explanation of a physical/chemical model within the 

combustion chamber of industrial boilers to explain why CO concentrations below about 100 ppm (or 

perhaps as high as the 200 to 300 ppm level traditionally indicative of “good combustion practice”)

would not be expected to correlate with combustion efficiency, nor be a controllable parameter 

indicative of increasing combustion efficiency in industrial boilers. In short, CO at these very low levels is

largely a function of combustion zone inertial physics and boiler “wall CO” and not the efficiency of 

combustion itself.

2

The following plots for CO versus THC emissions from coal and oil combustion, generated from data 

reported to EPA and residing in EPA’s Boiler MACT database, demonstrate this point. Figure 1 is a plot 

of all data for CO versus THCs currently in EPA’s Boiler MACT database when essentially 100% coal was 

 

1

Prepared by the National Council for Air and Stream Improvement

2

At low levels, “wall CO” is not so much a function of combustion zone dynamics or efficiency since it occurs 

primarily outside the combustion zone. During pyrolysis of a solid fuel particle (and atomized liquid fuel droplet), a 

small fraction of the pyrolysis products at the burner exit (where fuel temperature begins to rise) that are first 

oxidized to CO gets caught up in the eddy formed at the burner tip, and is directed towards the boiler wall. From 

here this “wall CO” drifts outside the strong gas circulation in the combustion zone and rises along the boiler wall 

to the flue. Because the boiler wall temperatures (~500 degrees F) are much less than the ignition temperature for

CO (~1200 degrees F or higher), this “wall CO” never has an opportunity to oxidize to CO2. Due to the inertia of the 

fule particles, other than the small volatile fraction subject to the eddy phenomenon, the rest of the pyrolysis 

products enter the combustion zone and are completely oxidized, first to CO and then to CO2. The inference is that 

when stack CO is at high levels, this would be a result of less complete combustion in the combustion zone. At low 

levels of CO, time, temperature and turbulence in the combustion zone have much less influence, and CO 

concentrations are much more transient. Of course, each boiler’s fuel and load characteristics, combined with its 

physical arrangement and fluid dynamics, make it essentially unique. Individual boiler operation therefore is tuned 

or optimized to these unique characteristics. The transient load characteristics of industrial boilers further 

constrains the combustion dynamics. These design constraints (and others not brought into this very brief 

explanation) contribute to varying levels of wall CO, negating the basis for a basic correlation between CO 

emissions and boiler combustion efficiencies, especially at low stack CO levels. AF&PA and CIBO can make 

available the boiler combustion expertise needed to more fully explain these phycial/chemical phenomena.
June 7, 2012

2

combusted. These pairs of tests (THC vs CO) corresponded to typically 3 short duration tests (29 

boilers). Only data corresponding to CO emissions of <100 ppm are shown, although this corresponded 

to the bulk of the data (70 of a total of 84 data points). Based on the extremely low coefficient of 

regression3, r

2

, it is clearly seen that below about 100 ppm CO, there is simply no correlation between 

THC and CO.

 

Seven of the 29 boilers for which THC vs CO data are shown in Figure 1 represented units in the “best 

performing” floor (AZCatalystPaperSnowflake, IAADMCornProcessingCR, IAUofNorthernIowa, 

INNotreDame, OHMortonSaltRittman, OKGPMuskogeeMill and TNEastman_NO_CBIDATA). Figure 2 is a 

plot of just the data for these 7 boilers. 

 

3

A correlation coefficient r2of between 0.0 and 0.04 implies slight to almost negligible correlation; from 0.04 to 

0.16 it implies low, quite small correlation; from 0.16 to 0.50 it implies moderate correlation; from 0.50 to 0.80 it 

implies high correlation; and from 0.80 to 1.00 it implies very high correlation.

y = 0.0045x + 1.7512

R2 = 0.0029

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100

Stack

T

H

C, p

p

m

@

3

%

O

2

Stack CO, ppm @ 3% O2

Figure 1. CO Vs Total Hydrocarbons - Boiler MACT Short Duration Test 

Data for 100% Coal-Fired Industrial Boilers - CO < 100 ppm
June 7, 2012

3

Figure 3 is a similar plot of all data for CO versus THCs currently in EPA’s Boiler MACT database when 

essentially 100% No. 6 fuel oil was combusted. Once again, only data corresponding to CO emissions of 

<100 ppm are shown, although this corresponded to the bulk of the data (23 of a total of 27 data 

points). Again, THC emissions show no correlation with CO at CO emission levels of < 100 ppm.

y = -0.0283x + 1.8194

R2 = 0.046

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35

Stack

T

H

C, p

p

m

@

3

%

O

2

Stack CO, ppm @ 3% O2

Figure 2. CO Vs THCs - Boiler MACT Short Duration Test Data for Seven 

"Best Performer" 100% Coal-Fired Industrial Boilers - CO < 100 ppm

y = -0.0089x + 1.8538

R2 = 0.0357

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

Stack

T

H

C, p

p

m

@

3

%

O

2

Stack CO, ppm @ 3% O2

Figure 3. CO Vs THC & HCHO - Boiler MACT Short Duration Test 

Data for 100% No. 6 Oil-Fired Industrial Boilers - CO < 100 ppm
Figures 4 and 5 are plots of CEMS-based 

Washington Works coal-fired stoker unit 

Boiler MACT database. Both plots demonstrate how THC 

are a very poor function of CO at CO 

for coal or oil-fired units will not achieve the desired end result of controlling HAPs.

Figure 4. CO Vs THC Emissions For a Best

Figure 5. Formaldehyde Vs THC Emissions For

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 10 20

THC, ppm

WWks Boiler 5 THC vs CO

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20

HCOH, ppm

WWks Boiler 5 HCOH vs CO

4

based CO versus THC and formaldehyde data for the Dupont 

stoker unit which currently is a best performer in its category 

s demonstrate how THC and HCHO emissions, and therefore other PICs, 

are a very poor function of CO at CO levels below about 100 ppm and thus setting limits of <100 ppm CO 

fired units will not achieve the desired end result of controlling HAPs.

CO Vs THC Emissions For a Best-Performing Coal-Fired Unit - Boiler MACT Data

Formaldehyde Vs THC Emissions For a Best-Performing Coal-Fired Unit - Boiler MACT Data

y = 0.0013x + 0.1864

R2 = 0.0035

30 40 50 60 70

CO, ppm

WWks Boiler 5 THC vs CO

y = 0.0008x + 0.5007

R2 = 0.0009

30 40 50 60 70

CO, ppm

WWks Boiler 5 HCOH vs CO

June 7, 2012

for the Dupont 

best performer in its category within EPA’s 

emissions, and therefore other PICs, 

limits of <100 ppm CO 

Boiler MACT Data

Boiler MACT Data

y = 0.0013x + 0.1864

R2 = 0.0035

80 90

y = 0.0008x + 0.5007

R2 = 0.0009

80 90
June 7, 2012

5

Analysis of Paired CEMS Data Available in the EPA Boiler MACT Database as of May 30, 2012

A significant amount of paired CO and THC CEMS data for two coal-fired boilers were posted on the EPA 

website on May 30, 2012. The two boilers included a PC unit (VAPhilipMorrisPark500) and a stoker unit 

(WVDuPontWashingtonWorks). A total of 13144data points for simultaneous CO and THC 

measurements when burning 100% coal in these two boilers were available, of which 38 data points 

with stack oxygen content above 16% were eliminated for this exercise since they most likely 

represented boiler startup/shutdown conditions. Figure 6 looks at all the data where stack CO was < 

100 ppm (bulk of the data - 1292 of the 1314 data points). Once again, it is clearly seen from this figure 

that there exists no correlation between THC and CO in industrial coal-fired boilers for CO levels below 

100 ppm. 

 

4

38 data points with stack oxygen content above 16% were eliminated for this exercise since they most likely 

represented boiler startup/shutdown conditions

y = 0.0069x + 1.3074

R2 = 0.0016

0

5

10

15

20

25

0 20 40 60 80 100 120

Stack

T

H

C

@

3

%

O

2

Stack CO @3%O2

Figure 6. THC Vs CO for Two Coal-Fired Boilers - Boiler MACT 

CEMS-Based Data - May 30, 2012 - CO < 100 ppm (Stack O2 < 16%)