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MEMORANDUM

DATE: January 7, 2004

SUBJECT: CO Removal Efficiency as a Surrogate for HAP Removal Efficiency 

 

FROM: Melanie Taylor and Jennifer Snyder

Alpha-Gamma Technologies, Inc.

TO: Sims Roy, EPA OAQPS ESD Combustion Group

The purpose of this memorandum is to establish whether carbon monoxide (CO)

removal efficiency can be used as a surrogate for hazardous air pollutants (HAP)

removal efficiency from engines equipped with HAP control. The purpose of this

memorandum is also to determine if CO concentration can be used as a surrogate for

HAP concentration. It has been suggested that the oxidation catalyst removal

efficiencies of CO and HAP exhibit the same behavior for spark ignition lean burn

engines and compression ignition engines, making CO removal efficiency a possible

surrogate for HAP removal efficiency. It is beneficial for both industry and EPA to

monitor HAP removal through the use of CO as a surrogate, if that is appropriate. It is

much easier to test for one pollutant instead of all HAP. Testing for CO is easier and

more economical than testing for several HAP, and can be done either through source

testing or continuous emission monitoring systems, making it more flexible than testing

for several HAP. 

A previous version of this memorandum based on initial data from the testing at

Colorado State University was developed at the time of proposal and entered into the

docket (Document ID Number OAR-2002-0059-0208 or A-95-35 II-F-4). This

memorandum has been updated to this version and reflects emissions data from final

test reports.

In order to determine if CO is an appropriate surrogate for HAP, emissions data from

EPA’s emissions tests on lean burn natural gas and diesel engines and an industry test

on rich burn natural gas engines were analyzed. To simplify this analysis, CO and

formaldehyde emissions were compared. Formaldehyde is representative of HAP

emissions because it is the hazardous air pollutant emitted in the largest

concentrations. 
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In summary, the emissions data suggest that CO removal efficiency is a reasonable

surrogate for HAP removal efficiency for an oxidation catalyst. The emissions data do

not indicate that CO removal efficiency is an appropriate surrogate for HAP removal

efficiency for non-selective catalytic reduction (NSCR). Emissions data do not indicate

that CO concentration is a surrogate for HAP concentration. A more detailed

discussion is provided below for each engine type. 

2-Stroke Lean Burn Natural Gas Engine

Table 1 presents the CO and formaldehyde emissions data (corrected to 15% O2, dry

basis) for the 2-stroke lean burn engine tested. Figure 1 displays the percent reduction

of CO and formaldehyde versus catalyst temperature for the 2-stroke lean burn engine. 

The catalyst control efficiency increases as the catalyst temperature increases, as

demonstrated by the least squares best-fit line. Higher CO removal results in higher

formaldehyde removal, as shown in Figure 1. Because the control efficiencies of CO

and formaldehyde behave similarly with respect to the catalyst temperature, CO

reduction is considered a surrogate for formaldehyde reduction and other HAP

reduction for 2-stroke lean burn natural gas and other spark ignition lean burn engines. 

 

4-Stroke Lean Burn Natural Gas Engine

Table 2 presents the CO and formaldehyde emissions data (corrected to 15% O2, dry

basis) for the 4-stroke lean burn engine tested. Figure 2 shows the percent reduction

versus catalyst temperature. Catalyst temperature is known to have an effect on the

catalyst removal efficiency. Using least squares analysis, the best-fit lines for CO and

formaldehyde show that the control efficiency for both CO and formaldehyde decreases

slightly with increasing temperature. Both lines have nearly identical slopes. The

figure shows that higher CO removal levels correspond to higher HAP removal levels. 

From this it can be concluded that the control efficiencies of CO and formaldehyde

exhibit similar behavior, and thus CO reduction is a reasonable surrogate for HAP

reduction for 4-stroke lean burn natural gas engines. Natural gas is a clean burn fuel

and represents the majority of engines in the spark ignition engine subcategory. In

addition, natural gas engines share the same design as other clean gaseous engines;

therefore, CO reduction can be considered a surrogate for other 4-stroke spark ignition

lean burn engines burning fuels such as propane, liquid petroleum gas, and process

gas. 

4-Stroke Diesel Engine

The CO and formaldehyde emissions data (corrected to 15% O2, dry basis) for the 4-

stroke diesel engine are shown in Table 3. Figure 3 presents the percent reduction of

CO and formaldehyde versus catalyst temperature for the 4-stroke diesel engine. The

least squares best-fit lines in the figure show that the control efficiencies of CO and

formaldehyde behave similarly. The removal efficiency of formaldehyde is actually

higher than the removal efficiency of CO for this engine. The figure demonstrates that

high CO removal corresponds to high HAP removal for the 4-stroke diesel engine. 
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Because of similarities in engine operating parameters, emissions from 2-stroke diesel

engines are expected to behave similarly to emissions from 4-stroke diesel engines. 

Thus, CO removal efficiency is a surrogate for HAP removal efficiency for diesel

engines, which are compression ignited. The compression ignition engine subcategory

also contains dual fuel engines, which use natural gas and diesel fuel. Carbon

monoxide reduction is found to be a surrogate for diesel engines and for natural gas

engines, hence it is considered a surrogate for dual fuel engines.

4-Stroke Rich Burn Natural Gas Engine

Table 4 presents the CO and formaldehyde emissions data (corrected to 15% O2, dry

basis) for one of the 4-stroke rich burn engines tested (engine 7A). Figure 4 shows the

percent reduction versus catalyst temperature for engine 7A. Engine exhaust

temperatures for the other engine tested (engine 9A) were not recorded during the field

test. Using least squares analysis, the best-fit lines for CO and formaldehyde show

that the control efficiency for CO decreases while the formaldehyde control efficiency

increases with increasing temperature. The figure shows that with increasing

temperature, the control efficiencies for CO and formaldehyde behave differently. From

this it can be concluded that the control efficiencies of CO and formaldehyde do not

exhibit similar behavior, and thus CO reduction is not a reasonable surrogate for HAP

reduction for 4-stroke rich burn natural gas engines. As previously mentioned, natural

gas engines are similar to other clean gaseous fueled engines. For that reason, CO

reduction cannot be considered a surrogate for other 4-stroke rich burn engines

burning fuels such as propane, liquid petroleum gas, and process gas. 

In addition to investigating whether CO reduction can be used as surrogate for HAP

reduction for 4-stroke rich burn engines, the relationship between NOx reduction and

HAP reduction was also examined. It has been suggested that NOx monitoring should

be allowed as a surrogate for NSCR function. The data in Table 4 show the reduction

levels of NOx and formaldehyde with varying temperatures for the 4-stroke rich burn

engine tested where temperature data was obtained. As the data show, there is not a

consistent trend. For example, in one case, as the temperature was increased from

887F to 911F, the NOx reduction increased slightly, while the formaldehyde

reduction decreased. The opposite occurred when the temperature was increased from

911F to 945F. In this case, the NOx reduction decreased while the formaldehyde

reduction increased. Based on this it can be concluded that NOx reduction is not an

appropriate surrogate for HAP reduction for 4-stroke rich burn engines with NSCR.

Conclusion

Figures 1-3 show that CO and formaldehyde reductions achieved with oxidation

catalysts exhibit the same behavior for spark ignition lean burn engines and

compression ignition engines. Therefore, CO removal efficiency is a reasonable

surrogate for HAP removal efficiency for spark ignition lean burn engines and

compression ignition engines. High CO reduction efficiency is a good indicator of high

HAP reduction for oxidation catalysts. This conclusion is advantageous for both

industry and EPA because testing for CO is easier than testing for several HAP. Figure
4

4 shows that CO and formaldehyde reductions achieved with NSCR do not exhibit the

same behavior for spark ignition 4-stroke rich burn engines. Therefore, CO removal

efficiency is not a reasonable surrogate for HAP removal efficiency for spark ignition 4-

stroke rich burn engines. It was also concluded that NOx removal efficiency is not an

appropriate surrogate for HAP removal efficiency for 4-stroke rich burn engines with

NSCR. 

In addition, the emissions data presented in Tables 1-3 were analyzed to determine if

there is a relationship between formaldehyde and CO concentration. As Figures 5-7

indicate, the inlet concentration of formaldehyde and inlet concentration of CO do not

behave similarly with varying engine exhaust temperatures. As Figure 5 shows, the CO

inlet concentration reduced drastically as the formaldehyde inlet concentration

remained fairly consistent, with increasing engine exhaust temperatures. Figures 6 and

7 indicate a different trend; the CO inlet concentration increased slightly as the

formaldehyde concentration remained relatively consistent, with increasing engine

exhaust temperatures. For these reasons, a relationship between formaldehyde and

CO concentration could not be established and CO concentration is not an appropriate

surrogate for HAP concentration.
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Table 1: Emissions Data for 2-Stroke Lean Burn Natural Gas Engine

 (corrected to 15% O2, dry basis)

Catalyst CH2O CH2O CH2O CO CO CO

Temp. inlet outlet Percent inlet outlet Percent

(deg F) (ppm) (ppm) Reduction (ppm) (ppm) Reduction

556 16.7 9.0 46.1 83.0 27.1 67.3

483 24.1 15.9 34.0 258.9 80.1 69.1

451 22.4 17.3 22.8 244.7 91.3 62.7

518 15.3 8.2 46.4 74.9 28.8 61.5

537 17.2 10.5 39.0 115.6 40.1 65.3

567 16.1 7.5 53.4 75.3 26.9 64.3

500 19.0 10.0 47.4 134.5 44.0 67.3

548 16.7 8.7 47.9 77.6 28.1 63.8

558 17.4 8.7 50.0 78.9 27.4 65.3

503 18.0 12.2 32.2 122.6 50.9 58.5

508 18.4 11.9 35.3 119.6 47.9 59.9

569 18.2 9.4 48.4 80.6 29.2 63.8

538 15.8 8.3 47.5 95.0 31.8 66.5

559 18.3 9.5 48.1 91.8 29.9 67.4

557 18.0 9.2 48.9 87.6 29.6 66.2

Table 2: Emissions Data for 4-Stroke Lean Burn Natural Gas Engine

 (corrected to 15% O2, dry basis)

Catalyst CH2O CH2O CH2O CO CO CO

Temp. inlet outlet Percent inlet outlet Percent

(deg F) (ppm) (ppm) Reduction (ppm) (ppm) Reduction

704 34.1 10.9 68.0 329.7 22.0 93.3

676 36.6 9.3 74.6 314.7 14.3 95.5

640 35.0 6.4 81.7 305.0 11.6 96.2

657 33.4 9.8 70.7 314.0 16.0 94.9

685 41.8 14.0 66.5 420.1 30.2 92.8

723 33.8 8.2 75.7 334.9 19.6 94.1

694 37.9 7.5 80.2 322.7 13.2 95.9

641 37.2 12.8 65.6 363.8 20.3 94.4

704 33.8 10.5 68.9 326.1 21.4 93.4

705 34.4 11.0 68.0 332.5 22.2 93.3

701 34.0 10.9 67.9 330.1 22.2 93.3

704 34.9 10.7 69.3 328.6 21.9 93.3

734 36.6 9.4 74.3 348.3 23.7 93.2

685 33.9 11.4 66.4 348.8 22.7 93.5

707 35.1 10.6 69.8 335.3 22.7 93.2 

698 34.8 10.6 69.5 333.9 22.4 93.3
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Table 3: Emissions Data for 4-Stroke Diesel Engine

 (corrected to 15% O2, dry basis)

Catalyst CH2O CH2O CH2O CO CO CO

Temp. inlet outlet Percent inlet outlet Percent

(deg F) (ppm) (ppm) Reduction (ppm) (ppm) Reduction

809 1.5 0.58 61.3 44.1 12.1 72.6

830 0.93 0.16 82.8 44.1 12.4 71.9

837 1.4 0.14 90.0 76.8 13.2 82.8

804 1.6 0.13 91.9 76.2 12.0 84.3

878 1.3 0.16 87.7 39.8 11.4 71.4

792 1.6 0.17 89.4 43.5 12.1 72.2

809 1.4 0.15 89.3 41.6 12.0 71.2

775 1.6 0.15 90.6 42.7 11.8 72.4

813 1.6 0.16 90.0 45.2 11.8 73.9

811 1.4 0.15 89.3 41.6 12.5 70.0

Table 4: Emissions Data for 4-Stroke Rich Burn Engine (7A)

 (corrected to 15% O2, dry basis)

Catalyst CH2O CH2O CH2O CO CO CO NOx NOx NOx

Temp. inlet outlet Percent inlet outlet Percent inlet outlet Percent 

(deg F) (ppm) (ppm) Reduction (ppm) (ppm) Reduction (ppm) (ppm) Reduction

955 1.72 0.34 80.47 915.73 369.25 59.68 645.48 18.23 97.18

945 1.51 0.35 76.47 1059.9 745.10 29.70 615.67 20.02 96.75

887 1.47 0.37 74.52 1215.0 664.90 45.28 588.75 16.34 97.23

911 1.35 0.36 73.11 1535.3 597.42 61.09 524.38 13.41 97.44
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Figure 1: Percent Reduction vs. Catalyst Temperature

2-Stroke Lean Burn

0.0

10.020.030.040.050.060.070.080.090.0

100.0

440 460 480 500 520 540 560 580

Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2) CH2O Pe

CO Perce
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Figure 2: Percent Reduction vs. Catalyst Temperature

4-Stroke Lean Burn

0.0

10.020.030.040.050.060.070.080.090.0

100.0

620 640 660 680 700 720 740

Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2) CH2O Pe

CO Perce
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Figure 3: Percent Reduction vs. Catalyst Temperature

4-Stroke Diesel

0.0

10.020.030.040.050.060.070.080.090.0

100.0

760 780 800 820 840 860 880 900

Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2) CH2O 

CO Pe
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Figure 4: Percent Reduction vs. Catalyst Temperature 

4-Stroke Rich Burn

0.00

10.0020.0030.0040.0050.0060.0070.0080.0090.00

880.00 890.00 900.00 910.00 920.00 930.00 940.00 950.00 960.00

Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2) CH2O Percent 

CO Percent ReLinear (CO PerLinear (CH2O P
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Figure 5: Inlet Concentration vs. Temperature

2-Stroke Lean Burn

0.0

50.0

100.0150.0200.0250.0300.0

440 460 480 500 520 540 560 580

Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2) CH2O Con

CO ConceLinear (CO Linear (CH
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Figure 6: Inlet Concentration vs. Temperature

4-Stroke Lean Burn

0.0

50.0

100.0150.0200.0250.0300.0350.0400.0450.0

620 640 660 680 700 720 740

Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2) CH2O Con

CO ConceLinear (CO Linear (CH
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Figure 7: Inlet Concentration vs. Temperature

4-Stroke Diesel

0.0

10.020.030.040.050.060.070.080.090.0

760 780 800 820 840 860 880 900

Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2) CH2O Con

CO ConceLinear (CO Linear (CH