Application NoteCatalytic and Engine Exhaust Characterization

Utilizing Gas Phase FTIR for Real Time Feedback*

EQUIPMENTThe MKS MultiGas™ 2030HS (MG2030HS) high speed FTIR with 5Hz acquisition was used to collect the bulk of the data presented but in some cases the MKS 1Hz acquisition FTIR (MG2030) was used for comparison. While the MG2030HS acquisition was performed at 5Hz, the data was averaged to 1Hz for a more direct comparison to the other analyzers. As a note, the sample line material as well as flow rates made a significant difference in the FTIR response rates and were adjusted in order to match the conventional analyzer response.

The bulk of the data was collected at the Ford Vehicle Emissions Research Laboratory. The results from the Horiba 7000 series emission analyzers were compared to the results from the MKS 1Hz and 5Hz FTIR instruments. Other comparisons shown also used standard Non-dispersive Infrared (NDIR), Flame Ionization Detector (FID) and Chemiluminescence (CLD) analyzers.

MKS MG2030 HS / LS FTIR Parameters• 200 ml, 5.11 pathlength Aluminum gas cell• 3/8'' PTFE sample lines (HS) and 1/4'' PTFE sample

lines (LS)• Flow Rate: 15 - 18 L/min (HS) and 5 - 6L/min (LS)• Data Acquisition Rate: 5 scans/sec (5Hz, HS) and

1 scans/sec (1Hz, LS)

Horiba 7000 Series• Flow Rate: ~10 L/min (split to NDIR, FID & CLD

analyzers)• Data Acquisition Rate: 1Hz and 10Hz

NO AND TOTAL NOx — RESULTS AND DISCUSSIONThe MKS 1Hz FTIR was run along side a Chemiluminescence detector (CLD). While the FTIR provides a direct measure-ment for NO and NO

2 present in the gas stream, the CLD

can be operated in the NO or total NOx mode. In the total NOx mode, the CLD converts any NO

2 to NO for a final

NOx number. The CLD total NOx values can be biased low due to NO

2 to NO conversion inefficiency. The presence of

NH3 can also poison the CLD converter and further reduce

conversion efficiencies. To eliminate converter poisoning due to NH

3, acid washed filters are typically used [1] but need to

be replaced often.

A typical CLD conversion efficiency of NO2 to NO is 95%

[2]. Table 1 illustrates the calculated error that can result from the CLD for 95% conversion efficiency as a function of increasing NO

2 content in the mix of a 100 ppm NOx gas

stream.

INTRODUCTIONThe California Air Resources Board (CARB) and the Environmental Protection Agency (EPA) are driving new monitoring levels of light duty vehicles calling for a reduction of the ozone levels caused predominantly by the emissions of nitrogen oxides (NOx) as well as non-methane reactive organic gases (NMOG). Lower reporting levels as well as the demand for exhaust component speciation is challenging for single component emission analyzers.

Fourier Transform Infrared Spectroscopy (FTIR) based instruments have the advantage over many analyzers of a direct measurement that speciates components within the sample matrix without removing water from the gas stream. Testing confirms that the FTIR analyzer correlates very well to conventional emission analyzers used for total Nitrogen Oxides (NOx), CH

4, CO, CO

2 and it also provides direct

measurement of H2O in the gas stream. The FTIR Total

Hydrocarbon (THC) values calculated from the sum of the speciated components compares very well for fuels that have fewer higher hydrocarbon species, such as natural gas. With the use of surrogate compounds for longer chain hydrocarbons and correction factors that compensate for reduced FID responses to oxygenated species, the FTIR can also be used to estimate THC and NMOG for more complex exhaust gas samples.

Another benefit of multi-component FTIR analyzers is the direct time correlation provided between different species. This can not be said of single component analyzers which run at different flow rates and may involve chemical conversions processes that cause variations in analyzer response times.

*In collaboration with Christine Gierczak from Ford Motor Company, and as presented at the North American Catalysis Society, 22nd North American Meeting, June 2011.

Page 2 Application NoteNO

2NO CLD NOx Error

10 90 99.5 -0.5%

20 80 99 -1.0%

30 70 98.5 -1.5%

40 60 98 -2.0%

50 50 97.5 -2.5%

60 40 97 -3.0%

70 30 96.5 -3.5%

80 20 96 -4.0%

90 10 95.5 -4.5%

100 0 95 -5.0%

Table 1 — Potential error of 100 ppm NOx result from CLD analyzers based upon 95% conversion rate

The MKS MG2030 1Hz FTIR and a CLD analyzer were used to analyze diesel exhaust containing 3-10% moisture and 2-10% CO

2, and very low levels of NO

2. At low NO

2 levels

there should not be large errors due to conversion inefficiency. Figure 1 shows the difference between the FTIR and CLD for this gas stream. Readings are found to be in good agreement (within 2%) for a wide range of NOx levels.

Figure 2 is a plot of the time-resolved NO emissions from an uncatalyzed diesel engine. The Horiba 7000 CLD was run in the NO mode and the CLD dry NO values were converted to wet concentrations for direct comparison to the FTIR NO values. These results indicate that the MG2030 1Hz system is directly comparable to the Horiba 7000 1Hz system while the MG2030HS 5Hz system appears to have better time resolution not only when the Horiba is run at 1Hz but at 10Hz as well.

Figure 1 - Plot of NOx (dry) values as found by FTIR vs. the difference between FTIR and the CLD values.

Figure 2 - Plot of time-resolved NO data comparing the 1Hz MKS FTIR MG2030 (pink), the 5Hz MKS MG2030HS averaged to 1Hz (blue), and the Horiba 7000 CLD Analyzer at 1Hz (green) and at 10Hz (yellow) data acquisition rates.

CO AND CO2 — RESULTS AND DISCUSSIONNon-dispersive infrared (NDIR) analyzers are single component analyzers for CO and CO

2 based on a narrow

band region in the infrared. Depending on the filter used and the sample matrix, these systems can be used as direct analyzers similar to FTIR or may require moisture removal. The MKS MG2030 1Hz FTIR and a NDIR analyzer were used to analyze CO and CO

2 in diesel exhaust containing 3-10%

moisture and 2-10% CO2. In both cases, the match between

the FTIR and NDIR was within 2%. Figure 3 shows the plot of the CO

2 data analysis.

Page 3 Application NoteMETHANE, THC AND NMOG — RESULTS AND DISCUSSIONThis final section examines the ability of the MKS MG2030 FTIR to speciate and detect hydrocarbons present in engine exhaust. For less complex exhaust samples, such as the exhaust from the combustion of natural gas, the sum of the speciated smaller chain HCs can be used as a direct measurement of total hydrocarbon (THC). For more complex exhaust gas samples, the speciation of the smaller chain HCs combined with the use of surrogate compounds for the evaluation of larger chain hydrocarbons can provide a THC response number from the FTIR which correlates closely to conventional THC analyzers. In addition, the ability of the FTIR to speciate smaller chain oxygenated organic compounds, such as formaldehyde, acetaldehyde and ethanol can also provide fairly accurate values for time-resolved Non-Methane Organic Gas (NMOG) emissions in flex fuel exhaust.

MethaneThe FTIR can accurately speciate and measure smaller chain HC species, such as methane, in complex exhaust gas samples because of the distinct infrared absorption characteristics associated with smaller molecules. Figure 5, a plot of the time-resolved CH

4 emissions from an uncatalyzed diesel engine,

shows that the MKS MG2030HS FTIR values for CH4 are in

very good agreement with the 1Hz FID values. But, as was the case for CO and CO

2, the 1Hz FTIR appears to have lower

time resolution and does not match the FID emission profile nearly as well.

Figure 3 - Plot of error between the NDIR and the FTIR CO2

(dry) values

Figure 4 - Plot of time-resolved CO data comparing the 1Hz MKS FTIR MG2030 (pink), the 5Hz MKS MG2030HS averaged to 1Hz (blue), and the Horiba 7000 NDIR Analyzer at 1Hz (green).

Figure 5 - Plot of time-resolved CH4 data comparing the 1Hz MKS

FTIR MG2030 (pink), the 5Hz MKS MG2030HS averaged to 1Hz (blue) and the FID at 1Hz (green).

The time-resolved CO emissions from an uncatalyzed diesel engine are shown in Figure 4. The Horiba 7000 NDIR CO values were measured dry and converted to wet concentrations for direct comparison to the FTIR values. Figure 4 shows very good agreement between the MKS MG2030HS averaged to 1Hz (blue) and the NDIR 1Hz analyzer (green) while the MKS 2030 1Hz (pink) has lower time resolution. The same was true for the analysis performed for CO

2 in the stream.

This may be due to the lower flow rate that was used on the MG2030 1Hz system because of the limitations of the equipment configuration.

Page 4 Application NoteFigure 6 is a bar chart comparing the sums of the FTIR time-resolved CH

4 mass emission data (grams/mile) and the bag

FID mass emission data from vehicles operating on a variety of ethanol fuel blends. Despite the errors that may be introduced by summing time-resolved (modal) mass emission data [3] and the variation in the fuel types, the agreement between the two techniques is very good.

Total Hydrocarbon (THC)Most conventional THC analyzers consist of Flame Ionization Detectors (FID) which generally respond proportionally to the number of carbon atoms in a molecule. Unfortunately, if electron capturing species such as oxygen are present in a molecule, the "per carbon" response of the FID is measurably reduced [4]. Thus, in order to properly correlate the FID and the FTIR responses for THC, correction factors must be applied.

The next few figures show the comparison of the total hydrocarbon results measured by a FID and those estimated from FTIR results by applying FID correction factors to individual species and summing the corrected values to generate a FTIR “FID equivalent” THC value.

The improved agreement between the THC values determined using the FID and the FTIR “FID equivalent” method for uncatalyzed gasoline exhaust, which contains a complex mixture of HCs, is depicted in Figure 7. In the shaded area of the plot, the original FTIR THC values (blue) are approximately 17% lower than those measured by the FID (green), while the FTIR “FID equivalent” THC values (yellow) are approximately 3.5% higher than the FID.

The graph in Figure 8 is a plot consisting of three separate data sets which compare the results from a FID to that of “FID equivalent” FTIR values for diesel exhaust samples.

Figure 6 - Comparison of FTIR and bag FID data

Figure 7 - Comparison plot of the THC measured in gasoline exhaust determined by the FID, the FTIR and the FTIR “FID Equivalent” method.

Figure 8 - Comparison plot of the THC in diesel exhaust gas samples measured by a FID and the FTIR “FID Equivalent” values.

Page 5 Application NoteNon-Methane Organic Gases (NMOG)Current regulatory methods for evaluating NMOG emissions are typically complex, expensive and time consuming. They involve the batch analyses of diluted exhaust samples acquired during specified driving cycles. Samples of diluted exhaust are collected in Tedlar® bags and the THC, CH

4 and

non-methane hydrocarbon (NMHC) content are evaluated using a FID. Alcohol emissions are typically evaluated by bubbling dilute exhaust through impingers filled with water and analyzing the solution using GC separation with FID detection. Aldehyde and ketone emissions are measured by passing the diluted exhaust through cartridges containing solid adsorbents treated with a solution of DNPH, which is a derivatizing agent. The separation of the DNPH derivatives is performed using HPLC with UV detection.

Ethanol, formaldehyde and acetaldehyde are among the organic gases commonly analyzed using FTIR. The evaluation of these species in combination with FTIR analyses of THC and CH

4 can provide a less complex means for estimating

NMOG emissions. In addition, FTIR can provide time-resolved NMOG emissions data not typically available from the regulatory batch analysis method.

To assess the ability of FTIR to evaluate NMOG emissions, numerous tests were conducted on flex fuel vehicles operating on several ethanol/gasoline fuel blends. The vehicles were tested using the Federal Test Procedure and the weighted average NMOG mass emissions were determined using the time-resolved FTIR method and the regulation batch method. A bar chart comparing the results of this study is presented in Figure 9. The regression equation correlating the NMOG values obtained using the FTIR and the regulation method is presented in Figure 10.

Considering the numerous sources for error associated with the regulatory batch method and the time-resolved FTIR method, agreement between the two techniques is fairly good for all fuels tested.

Figure 9 - Bar chart comparison of the federal test procedure weighted average NMOG values determined using the FTIR and the regulation methods.

Figure 10 - Regression equation correlating the NMOG values determined using the FTIR and the regulation methods.

Page 6 Application Note

MKS Global Headquarters2 Tech Drive, Suite 201Andover, MA 01810978.645.5500800.227.8766 (within USA)www.mksinst.com

App. Note 07/11 - 10/11© 2011 MKS Instruments, Inc.All rights reserved.

For further information, call your local MKS Sales Engineer or contact the MKS Applications Engineering Group at 800-227-8766.MultiGas® is a trademark of MKS Instruments, Inc., Andover, MA. Tedlar® is a registered trademark of E. I. du Pont de Nemours and Company, Wilmington, DE.

SUMMARYThe data above strongly supports that the MKS MG2030HS (5Hz averaged to 1Hz) FTIR provides comparable results to the Horiba 7000 (1Hz) for the analysis of CO, CO

2, and

CH4. For this particular set up, the MKS MG2030 (1Hz)

was not able to operate at sufficiently high flow rates to allow for capture of peaks and valleys as well as Horiba 7000 (1Hz) or the MKS MG2030HS, further equipment evaluation is needed.

For NO, the MKS MG2030 (1Hz) system was comparable to Horiba 7000 (1Hz), while the MKS MG2030HS (5Hz) clearly showed better time resolution than the Horiba 7000 CLD analyzer when operated in both the 1Hz and 10Hz modes. For total NOx, other CLD comparisons to the MKS MG2030 (1Hz) showed very good agreement when the NO

2

content in the sample was low.

For exhaust samples which contain predominately smaller chain HCs, such as natural gas, FTIR THC values compare very well to conventional FID analyzers. Increases in the concentrations of longer chain hydrocarbons and oxygenated components requires the use of surrogate compounds and correction factors to provide better agreement between the FTIR and the FID for THCs from a variety of fuels and for NMOG from ethanol/gasoline fuel blends.

One major benefit of using the FTIR as opposed to single component analyzers, is the fact that it provides real time responses for all of the main process components (CO, CO

2,

NO, NO2, N

2O, NH

3, H

2O, SO

2, HCl, HF, speciated hydro-

carbons, aromatic components and others). The values can be easily converted to a form acceptable to the industry, greatly reducing both time and cost by removing the need for many separate analyzers which provide data that may need to be converted to achieve similar response times.

REFERENCES1. “NOx Measurement Errors in Ammonia-Containing Exhaust”, John Hoard, Rachel Snow, Lifeng Xu, Christine Gierczak, Robert Hammerle, Cliff Montreuil and S. Iskander Farooq, SAE Technical Paper Series, Paper No. 2007-01-0330, 2007.

2. “Evaluation of NOx flue gas analyzers for accuracy and their applicability for low concentration measurements”, Steven Gluck, Chuck Glenn, Tim Logan, Bac Vu, Mike Walsh and Pat Williams, Air & Waste Manage. Assoc. 53: 749-758

3. “Laboratory Evaluation of the SEMTECH-G® Portable Emissions Measurement System (PEMS) For Gasoline- Fueled Vehicles”, Christine A. Gierczak, Thomas J. Korniski, Timothy J. Wallington, and Carl D. Ensfield, SAE Technical Paper Series, Paper No. 2007-01-1329, 2007.

4. Reschke, G.D. Society of Automotive Engineers (SAE) paper No. 770141, (1978).