### 1 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 su rrogate 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 t hat 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 appropria te surrogate for HAP, emissions data from EPA’s emissions tests on l ean 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 ai r pollutant emitted in the largest concentrations.

### 2 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 fo r 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% O 2 

### , dry basis) for the 2-stroke lean burn engine tested. Figure 1 displays the percent reduction of CO and formaldehyde versus catalyst tem perature for the 2-stroke lean burn engine. The catalyst control efficiency increases as the catalyst temperature increases, as demonstrated by the least squar es 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 fo r 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% O 2 

### , dry basis) for the 4-stroke lean burn engine test ed. Figure 2 shows the percent reduction versus catalyst temperature. Catalyst te mperature 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 effi ciency for both CO and formaldehyde decreases slightly with increasing temperature. Bo th 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 sa me 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% O 2 

### , 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 temperat ure for the 4-stroke diesel engine. The least squares best-fit lines in the figure s how 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.

### 3 Because of similarities in engine operating par ameters, 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 us e 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% O 2 

### , dry basis) for one of the 4-stroke rich burn engi nes tested (engine 7A). Figure 4 shows the percent reduction versus catalyst te mperature 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 decreas es while the formaldehyde control efficiency increases with increasing temperature. The figure shows that with increasing temperature, the control efficiencies fo r CO and formaldehyde behave differently. From this it can be concluded that the contro l efficiencies of CO and formaldehyde do not exhibit similar behavior, and thus CO reduc tion 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 gas eous 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 ca se, as the temperature was increased from 887 F to 911 F, the NOx reduction increased slightly, while the formaldehyde reduction decreased. The opposite occurred when the temperature was increased from 911 F to 945 F. In this case, the NOx reduction decreased while the formaldehyde reduction increased. Based on this it c an 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 formaldehy de 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 t han 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 ri ch 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 effici ency 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 form aldehyde and inlet concentration of CO do not behave similarly with varying engine exhaust te mperatures. As Figure 5 shows, the CO inlet concentration reduced drastically as the formaldehyde inlet concentration remained fairly consistent, with increasi ng engine exhaust temperatures. Figures 6 and 7 indicate a different trend; the CO inle t concentration increased slightly as the formaldehyde concentration remained relative ly consistent, with increasing engine exhaust temperatures. For these reas ons, a relationship between formaldehyde and CO concentration could not be established and CO concentration is not an appropriate surrogate for HAP concentration.

### 5 Table 1: Emissions Data for 2-Stroke Lean Burn Natural Gas Engine (corrected to 15% O 2 , dry basis) Catalyst CH 2 O CH 2 O CH 2 O 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% O 2 , dry basis) Catalyst CH 2 O CH 2 O CH 2 O 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

### 6 Table 3: Emissions Data for 4-Stroke Diesel Engine (corrected to 15% O 2 , dry basis) Catalyst CH 2 O CH 2 O CH 2 O 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% O 2 , dry basis) Catalyst CH 2 O CH 2 O CH 2 O 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 97.447Figure 1: Percent Reduction vs. Catalyst Temperature2-Stroke Lean Burn0.010.020.030.040.050.060.070.080.090.0100.0440 460 480 500 520 540 560 580Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2)CH2O Percent ReductionCO Percent Reduction8Figure 2: Percent Reduction vs. Catalyst Temperature4-Stroke Lean Burn0.010.020.030.040.050.060.070.080.090.0100.0620 640 660 680 700 720 740Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2)CH2O Percent ReductionCO Percent Reduction9Figure 3: Percent Reduction vs. Catalyst Temperature4-Stroke Diesel0.010.020.030.040.050.060.070.080.090.0100.0760 780 800 820 840 860 880 900Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2)CH2O Percent ReductionCO Percent Reduction10Figure 4: Percent Reduction vs. Catalyst Temperature4-Stroke Rich Burn0.0010.0020.0030.0040.0050.0060.0070.0080.0090.00880.00 890.00 900.00 910.00 920.00 930.00 940.00 950.00 960.00Catalyst Temperature (deg F)

Percent Reduction (corrected to 15% O2)CH2O Percent ReductionCO Percent ReductionLinear (CO Percent Reduction)Linear (CH2O Percent Reduction)11Figure 5: Inlet Concentration vs. Temperature2-Stroke Lean Burn0.050.0100.0150.0200.0250.0300.0440 460 480 500 520 540 560 580Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2)CH2O ConcentrationCO ConcentrationLinear (CO Concentration)Linear (CH2O Concentration)12Figure 6: Inlet Concentration vs. Temperature4-Stroke Lean Burn0.050.0100.0150.0200.0250.0300.0350.0400.0450.0620 640 660 680 700 720 740Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2)CH2O ConcentrationCO ConcentrationLinear (CO Concentration)Linear (CH2O Concentration)13Figure 7: Inlet Concentration vs. Temperature4-Stroke Diesel0.010.020.030.040.050.060.070.080.090.0760 780 800 820 840 860 880 900Catalyst Temperature (deg F)

Concentration (ppm) (corrected to 15% O2)CH2O ConcentrationCO ConcentrationLinear (CO Concentration)Linear (CH2O Concentration)