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Background | Description | Advantages | Acknowledgement | References 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. BACK TO TOP | BACK TO RESEARCH PROJECTS In this instrument the GC column is eliminated and a micro-sorbent trap (referred to as the microtrap) is used for separation of VOCs from CO2, CH4 and moisture. Several ml (5 to 10) of the air sample is injected onto the microtrap using a gas-sampling valve. The microtrap selectively traps the organics but allows the rest to pass through. Thus, the microtrap serves as a separator as well as an on-line preconcentrator. Then, the microtrap is rapidly heated with a high amperage electric current. The desorption pulse serves as an injection for the NMOC detector. This method has low detection limit because a large volume of air is analyzed. Since the CO2 and moisture pass right through the microtrap unretained, the problem of interference is also eliminated. A schematic diagram of C-NMOC analyzer is shown in Fig. 1. The injection system consists of the pneumatically controlled gas-sampling valve with a large sample loop. The microtrap is put in series with the gas sampling valve. The NMOC detector consisted of an oxidation unit, reduction unit and a FID. Air and H2 were supplied as oxidizing and reducing agents to convert organics to CO2, and to reduce the CO2 to methane. The analysis frequency is anywhere between 30 seconds to 5 minutes. Typical monitoring involved running the air emission continuously through the gas sampling valve and periodically making injection to the C-NMOC analyzer. Corresponding to eac h injection, a NMOC peak is obtained. Results from a recent field test at a coatings facility in North Carolina is presented here. Fig. 2. shows typical output for a series of injections of the stack gas into the NMOC analyzer. The solvents from paints and coatings passed through a methane burner and a catalytic oxidizer before being vented into the stack. Thus the emissions had high concentrations of moisture, CO2 and unburned methane from the burner. Peak 1 in the detector output of Fig. 2 is from CO2, CH4 and CO, while peak 2 is from NMOC. BACK TO TOP | BACK TO RESEARCH PROJECTS
BACK TO TOP | BACK TO RESEARCH PROJECTS 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. BACK TO TOP | BACK TO RESEARCH PROJECTS REFERENCES
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