Catalytic and Engine Exhaust Characterization Utilizing Gas Phase

Application Note
Catalytic and Engine Exhaust Characterization
Utilizing Gas Phase FTIR for Real Time Feedback*
INTRODUCTION
The 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), CH4, CO, CO2 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.
EQUIPMENT
The 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 DISCUSSION
The MKS 1Hz FTIR was run along side a Chemiluminescence
detector (CLD). While the FTIR provides a direct measurement for NO and NO2 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 NO2 to NO for a final
NOx number. The CLD total NOx values can be biased low
due to NO2 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 NH3, 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 NO2 content in the mix of a 100 ppm NOx gas
stream.
*In collaboration with Christine Gierczak from Ford Motor Company, and as presented at the North American Catalysis Society, 22nd North American Meeting, June 2011.
Application Note
Page 2
NO2
NO
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%
Figure 1 - Plot of NOx (dry) values as found by FTIR vs. the difference between FTIR and the CLD values.
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% CO2, and very low levels of NO2. At low NO2 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 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 DISCUSSION
Non-dispersive infrared (NDIR) analyzers are single
component analyzers for CO and CO2 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 CO2 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 CO2 data analysis.
Page 3
Application Note
METHANE, THC AND NMOG —
RESULTS AND DISCUSSION
Figure 3 - Plot of error between the NDIR and the FTIR CO2 (dry) values
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 CO2 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.
This 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 NonMethane Organic Gas (NMOG) emissions in flex fuel exhaust.
Methane
The 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 timeresolved CH4 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 CO2, the 1Hz FTIR appears to have lower
time resolution and does not match the FID emission profile
nearly as well.
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).
Page 4
Application Note
Figure 6 is a bar chart comparing the sums of the FTIR timeresolved CH4 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.
Figure 6 - Comparison of FTIR and bag FID data
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.
Figure 7 - Comparison plot of the THC measured in gasoline exhaust
determined by the FID, the FTIR and the FTIR “FID Equivalent”
method.
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 8 - Comparison plot of the THC in diesel exhaust gas samples
measured by a FID and the FTIR “FID Equivalent” values.
Page 5
Application Note
Non-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, CH4 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.
Figure 9 - Bar chart comparison of the federal test procedure weighted
average NMOG values determined using the FTIR and the
regulation methods.
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 CH4 can provide a less complex means for estimating
NMOG emissions. In addition, FTIR can provide timeresolved 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 10 - Regression equation correlating the NMOG values
determined using the FTIR and the regulation methods.
Application Note
Page 6
SUMMARY
REFERENCES
The 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, CO2, 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.
1. “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.
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 NO2
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.
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).
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, CO2,
NO, NO2, N2O, NH3, H2O, SO2, HCl, HF, speciated hydrocarbons, 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.
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.
App. Note 07/11 - 10/11
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