ncasi NATIONAL COUNCIL FOR AIR AND STREAM IMPROVEMENT, INC. 

SOUTHERN REGIONAL CENTER 

Mailing and Street Address: 

402 SW 140th Terrace, Newberry, FL 32669 

Phone (352) 331-1745 

FAX (352) 331-1766

...environmental research for the forest products industry since 1943 

June 19, 2012 

RELATIONSHIP BETWEEN CO AND THCS AT LOW 

CO LEVELS FOR COAL-FIRED INDUSTRIAL BOILERS 

Executive Summary 

While high CO emissions from fuel combustion, especially biomass fuel combustion, have been 

correlated with higher levels of the products of incomplete combustion (PICs), and therefore 

gaseous organic HAPs, this correlation breaks down at low CO emission levels, especially for 

fossil fuel combustion. Data for coal-fired boilers extracted from the Industrial Boiler MACT 

database suggest the threshold for such breakdown may be around 200 ppm CO @ 3% O2. 

These data could justify a 200 ppm CO MACT emission standard as a surrogate for minimizing 

PIC emissions from coal-fired industrial boilers. 

EPA’s boiler MACT database includes CEM data (one month duration) for two boilers where 

CO and total hydrocarbons (THCs) were measured simultaneously. The two boilers are 

identified as a stoker coal unit (WVDuPontWashingtonWorks) and a pulverized coal (PC) unit 

(641 hourly averages, VAPhilipMorrisPark500). The data for the VAPhilipMorrisPark500 PC 

boiler show that no correlation exists between stack THCs and CO between 0 and 120 ppm. 

However, there is a strong correlation between CO and THCs at CO levels above 150 ppm, even 

if the high levels were likely caused by abnormal boiler operating conditions such as startup and 

shutdown. The correlation equation intercepts the x-axis at around 200 ppm suggesting that at 

levels above 200 ppm, CO does serve as a useful surrogate for THC emissions. 

The data for the stoker unit suggest no correlation exists between THCs and CO up to about 120 

ppm CO. CO data were not available above 120 ppm to determine if the range for lack of 

correlation could have been higher. These data confirm that for stokers driving CO levels below 

a threshold does not result in reductions in organic HAP emissions. Based on these data and the 

underlying fundamental reasons why a relationship between THCs (and therefore gaseous 

organic HAPs) and CO may not exist at very low stack CO levels in coal-fired industrial boilers, 

one could conclude that a CO limit no less than 200 ppm @ 3% O2 for coal stoker and PC units 

would be appropriate. 
2

Background on Short-Term CO Limits for Industrial Coal-fired Boilers 

The short-term standards for CO on industrial boilers were set by EPA using a mathematical 

formula that involved the estimation of an upper prediction limit (UPL) based on short duration 

stack test data considered to represent the “best performing” 12 percent of units (the “floor”). 

For coal PC and stoker units, this mathematical procedure resulted in quite low short-term 

standards for CO because relatively few emission tests were in the floors. In setting these 

standards, EPA apparently did not consider the possibility that the proposed CO standards were 

in a region where no meaningful correlation exists between the stack gas CO concentration and 

the presence of gaseous organic HAPs, although there is a long history of EPA’s articulation of 

this issue in other emission standards. 

The first EPA regulation with a carbon monoxide (CO) standard for combustion sources was the 

“Burning of Hazardous Waste in Boilers and Industrial Furnaces” (56 Fed. Reg. 7134, February 

21, 1991) under the Resource Conservation and Recovery Act (RCRA). In that rule EPA set a 

CO standard of 100 ppmv. EPA chose that limit because their research indicated that while CO 

was a good surrogate for the destruction of organics, the validity of that surrogacy broke down at 

CO levels of approximately 400 ppmv. Based on EPA’s authority under RCRA to establish 

standards that are protective of human health and the environment, the Agency established a 100 

pmmv standard as protective. 

EPA later justified a 100 ppmv standard in 70 Fed. Reg. 59462 (October 12, 2005) for the 

hazardous waste combustion NESHAP as follows: 

“We explained at proposal why the carbon monoxide standard of 100 ppmv and the hydrocarbon 

standard of 10 ppmv are appropriate floors. See 69 Fed. Reg. at 21282. The floor level for carbon 

monoxide of 100 ppmv is a currently enforceable Federal standard. Although some sources are 

achieving carbon monoxide levels below 100 ppmv, it is not appropriate to establish a lower 

floor level because carbon monoxide is a conservative surrogate for organic HAP. Organic HAP 

emissions may or may not be substantial at carbon monoxide levels greater than 100 ppmv, and 

are extremely low when sources operate under the good combustion conditions required to 

achieve carbon monoxide levels in the range of zero to 100 ppmv. (See also the discussion below 

regarding the progression of hydrocarbon oxidation to carbon dioxide and water). As such, 

lowering the carbon monoxide floor below 100 ppmv may not provide significant reductions in 

organic HAP emissions. Moreover, it would be inappropriate to establish the floor blindly using 

a mathematical approach—the average emissions for the best performing sources—because the 

best performing sources may not be able to replicate their emission levels (and other sources may 

not be able to duplicate those emission levels) using the exact types of good combustion 

practices they used during the compliance test documented in our data base. This is because there 

are myriad factors that affect combustion efficiency and, subsequently, carbon monoxide 

emissions. Extremely low carbon monoxide emissions cannot be assured by controlling only one 

or two operating parameters.” 
3

In this document, NCASI has examined the available coal-fired boiler CEM data in EPA’s Boiler 

MACT database for the simultaneous measurement of THCs and CO on a pulverized coal (PC) 

and a stoker unit. An analysis was conducted to determine whether or not the stack gas THC 

data correlate with CO in the low CO concentration range where the CO standards have been 

proposed as surrogates for organic HAPs. 

Analysis of Boiler MACT CEM Data for Coal PC Units 

Figure 1 is a plot of the CEM data for CO versus THCs for the VAPhilipMorrisPark500 PC 

boiler using only data with stack CO levels below 123 ppm @ 3% O2 (605 of 641 hourly 

averages). The 123 ppm cut-off was chosen since there was a significant gap in the CO emisions 

above this level, with the next highest CO measured at 152 ppm, and the 123 ppm value is close 

to the 100 ppm standard for hazardous waste combustors. Based on the extremely low 

coefficient of regression1

 (r2) of 0.046, Figure 1 demonstrates the correlation between THC and 

CO in this PC unit is extremely low below 123 ppm CO. 

Nine of the 605 total data points representing stack CO < 123 ppm for this PC unit had stack 

oxygen contents above 16%. Since this is clearly indicative of startup/shutdown conditions, 

these were eliminated and the rest of the data re-plotted in Figure 2. The new coefficient of 

regression is seen to be close to zero (r2

 = 2.0E-06) and the slope equals 0.0, indicating that 

excluding the small amount of startup/shutdown data, no correlation existed between THC and 

CO in this particular PC unit below 123 ppm CO. 

                                                           

1 A correlation coefficient r2 of 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.
4

To examine whether or not a correlation exists between CO and THCs at higher kevels of CO, all 

the CEM data with CO exceeding 123 ppm are plotted versus THC in Figure 3 for this same PC 

boiler. The plot includes data points where the stack oxygen content was higher than 16% and/or 

the total heat input to the boiler was below about 60 MMBtu/hr (<25% full load). Figure 3 

shows a strong correlation between stack CO and THC contents with an r

2

 value of 0.96 and a 

significant positive slope. This clearly demonstrates that CO is a useful surrogate for THC and 

therefore HAP emissions in coal-fired boilers at the higher levels of CO. It is interesting to note 

that the linear regression line derived in Figure 3 intersects the x-axis at 197 ppm CO 

(155.7/0.806), implying zero THC in the stack gas at that level. This result supports 200 ppm as 

being a suitable threshold below which there is no relationship between CO and organic HAPs. 
5

Analysis of Boiler MACT CEM Data for Coal Stoker Units 

Figure 4 is a plot of the CEM data for CO versus THCs for the WVDuPontWashingtonWorks 

stoker unit, a unit deemed to be the best performer in its subcategory by EPA. For this boiler, all 

data (711 hourly averages) appear to have represented normal operations, so none were 

eliminated. Unfortunately, all CO emissions fell below about 120 ppm, so only a correlation 

between THC and CO for CO less than 120 ppm could be examined. It is seen that for this coalfired stoker there exists no correlation between CO and THC at CO levels below 120 ppm (r2

 = 

0.003). This conclusion is similar to the earlier conclusion regarding pulverized coal-fired 

boilers. This suggests that the relationship between CO and THC is similar for coal-fired stokers 

and PC boilers and, consequently, the MACT CO standard for stokers should also be set at the 

same level (200 ppm) as for pulverized coal-fired boilers. 
6

Further, it should also be noted that while the 10 day moving average for the 

WVDuPontWashington Works stoker unit was estimated by EPA to be only 22.9 ppm, thus 

making it the best performer in its subcategory, the minimum 10 day moving averages for the 

remaining 5 coal stoker units for which EPA had CEM CO data ranged from 178.4 to 4291 ppm. 

The short duration 3-hour average limit for coal stoker units is currently at 220 ppm @3% O2. 

This indicates that most coal stoker units would have a very difficult time mimicking the 30-day 

snap performance of the DuPont unit and a more plausible limit would be one that is closer to the 

short duration limit.