# ncasi NATIONAL COUNCIL FOR AIR AND STREAM IMPROVEMENT, INC.
SOUTHERN REGIONAL CENTER Mailing and Street Address:
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...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, es pecially biomass fuel combustion, have been correlated with higher levels of the products of incomplete combusti on (PICs), and therefore gaseous organic HAPs, this correlation breaks do wn at low CO emission levels, especially for

fossil fuel combustion. Data for coal-fired boile rs extracted from the Industrial Boiler MACT database suggest the threshold for such breakdown may be around 200 ppm CO @ 3% O 2

. 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 (WVDuPontWash ingtonWorks) 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 a nd 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 abnorma l boiler operating conditions such as startup and shutdown. The correlation equation intercepts the x-axis at around 200 pp m 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 correlati on 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 co nfirm that for stokers driving CO levels below

a threshold does not result in re ductions in organic HAP emissions. Based on these data and the underlying fundamental reasons why a relations hip 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 li mit no less than 200 ppm @ 3% O 2 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 se t by EPA using a mathematical formula that involved the estimation of an uppe r prediction limit (UPL) based on short duration stack test data considered to represent the “bes t 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 th ere 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 Indu strial Furnaces” (56 Fed. Reg. 7134, February 21, 1991) under the Resource Conservation and Recove ry Act (RCRA). In that rule EPA set a CO standard of 100 ppmv. EPA chose that limit b ecause their research in dicated that while CO was a good surrogate for the destruc tion 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 protec tive 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 monoxi de standard of 100 ppmv and the hydrocarbon standard of 10 ppmv are appropr iate 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 conser vative surrogate for organic HAP. Organic HAP emissions may or may not be substantial at ca rbon monoxide levels great er 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 ppm v. (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 es tablish 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 rep licate their emission levels (and other sources may not be able to duplicate those emission leve ls) using the exact types of good combustion practices they used during the compliance test docu mented in our data base. This is because there are myriad factors that affect combustion e fficiency and, subseque ntly, carbon monoxide emissions. Extremely low carbon monoxide emissi ons cannot be assured by controlling only one or two operating parameters.”3

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

MACT database for the simultaneous measuremen t of THCs and CO on a pulverized coal (PC) and a stoker unit. An analysis was conducted to determine whet her or not the stack gas THC

data correlate with CO in the low CO concentr ation range where the CO standards have been proposed as surrogates for organic HAPs.

Analysis of Boiler MACT CE M 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% O 2 (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 m easured 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 regression 1 (r 2 ) of 0.046, Figure 1 demonstrates the correl ation between THC and CO in this PC unit is extremely low below 123 ppm CO.

Nine of the 605 total data point s representing stack CO < 123 ppm for this PC unit had stack oxygen contents above 16%. Since this is cl early indicative of star tup/shutdown conditions,

these were eliminated and the rest of th e data re-plotted in Figure 2. The new coefficient of regression is seen to be close to zero (r 2 = 2.0E-06) and the slope e quals 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 r 2 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 be tween CO and THCs at higher kevels of CO, all the CEM data with CO exceeding 123 ppm are plo tted versus THC in Figure 3 for this same PC boiler. The plot include s data points where the stack oxygen content was hi gher 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 st ack 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 th e 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 th at 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 W VDuPontWashingtonWorks 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, s o none were eliminated. Unfortunately, all CO emissions fe ll below about 120 ppm, so only a correlation

between THC and CO for CO less th an 120 ppm could be examined. It is seen that for this coalfired stoker there exists no correlation betw een CO and THC at CO levels below 120 ppm (r 2 = 0.003) . This conclusion is similar to the earlier conclusion regarding pu lverized coal-fired boilers. This suggests that the relationship betw een CO and THC is similar for coal-fired stokers and PC boilers and, consequently, the MACT CO st andard for stokers should also be set at the same level (200 ppm) as for pul verized coal-fir ed boilers.6

Further, it should also be noted that while the 10 da y moving average for the WVDuPontWashington Works stoker unit was es timated 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 ha d 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% O 2

. This indicates that most coal stoker units woul d have a very difficult time mimicking the 30-day snap performance of the DuPont unit and a more plau sible limit would be one that is closer to the

short duration limit.