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DEVELOPMENT OF REAL-TIME AIR MONITORING
Background |
Description |
Advantages |
Acknowledgement |
References
BACKGROUND
There is a need for simple, rugged instrumentation that can perform continuous, on-line
monitoring and provide important information such as the performance of an air toxic control
device without necessarily identifying individual components. Direct flame ionization detector
(FID) analysis is one such method (EPA Method 25A). Commercial instruments are available where
the air emissions are continuously fed into the FID. While this method is inexpensive, rugged
and simple, the limitation here is that different compounds have different response factors in
the FID. Consequently there is significant uncertainty in the measurement of total organic
carbon measured by this instrument. The FID also responds to methane, which is neither toxic,
nor an ozone precursor. There is often a high methane background due to natural gas use.
Non-methane organic carbon (NMOC) is a measure of total organic carbon in an air emission except
that from methane. It is a convenient way of expressing total organic emissions in terms of
carbon (e.g. ppmc or ppbc). Since speciation of different components is not required, NMOC is a
fast and relatively inexpensive method. The NMOC measurement also allows different emission
sources to be compared in terms of total carbon irrespective of the specific compounds being
emitted. EPA Standard Method 25 has been used to measure NMOC in air emissions from stationary
sources. In this method, the gas samples are collected using a canister and are sent to the
laboratory for analysis. The NMOC analyzer is designed to produce an equal response for each
carbon atom. An aliquot of the air sample is injected into a GC column which separate the
organics from CO2 and CH4 . After CO2 and CH4 have eluted, the column is backflushed into the
NMOC detector. The principle of NMOC detection is to catalytically oxidize all organic compounds
to CO2 , and then reduce the CO2 to CH4 which is measured by a conventional FID. The reduction
step is necessary because CO2 itself does not respond to FID.
The chromatographic separation is a critical issue in the conventional NMOC analyzer. For
example it can not handle more than 8% CO2 because the resolution between CO2 and the organics
decreases. Consequently, emissions such as those from combustion sources that contain large
amounts of CO2 are prone to interference. The presence of large quantities of moisture also
causes problems in GC separation and produces biased results. Another problem with this method
is that detection limits are fairly high as only a small sample volume (1 cm3) can be injected
into the GC column to obtain reasonable resolution. Moreover, this method is not designed for
continuous, on-line monitoring. Recently we have developed a columnless NMOC analyzer that can
be used for continuous, on-line monitoring. This technique is referred to as the continuous-NMOC
or C-NMOC analysis.
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DESCRIPTION OF C-NMOC
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ADVANTAGES OF C-NMOC
- Detection limits at ppb level.
- Can be used as a Continuous Emission Monitor and also for Ambient Air monitoring.
- Stable over long periods of operation.
- Simple Instrumentation.
- Successfully field tested.
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ACKNOWLEDGMENT AND DISCLAIMER Although this project was funded in parts from a grant from the US EPA Office of Air Quality
and Standards, it has not been subjected to agency review. Therefore, it does not reflect the
views of the agency, and no official endorsement should be inferred. Mention of trade names or
commercial products does not constitute endorsement and recommendation for use.
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