Continuous Emissions Monitoring Systems (CEMS) are confronted with a challenging task: particulates, water, and coalescable liquids must be removed from gas samples prior to analysis without loss of components being monitored. Particulates can be eliminated by filtration, but water and coalescable liquids are difficult to remove without an attendant loss of water-soluble analytes.

Temperature control is a key issue in gas sample conditioning for CEMS. Several chemical reactions occur at elevated temperatures that convert various molecular species of environmental importance from one form to another, so it is desirable to lower the sample temperature as quickly as possible.

Unfortunately, unless water and other coalescable liquids are removed before the temperature falls below their dew points, condensation will occur in the sample lines. Acid gases in the sample have a substantial impact on the effective dew point, so sample composition must be considered in order to select optimal operating temperatures for a gas sampling system.

The preferred cleanup method has been condensation followed by collection of the water and coalescable liquids. Unfortunately, condensation systems suffer from several deficiencies:

The gas sample is exposed to liquid water, however briefly, resulting in the loss of highly water-soluble analytes such as SO2 and HCl.

Their performance is often degraded by: 1) corrosive samples 2) high sample flow rates 3) samples with high water content.

Their operating range is limited to temperatures above freezing. This constraint restricts the sample dew point to a level above 3-4 degrees C, which translates to 0.6% water in the sample.

When SO2 levels of 100 ppm or less must be monitored, SO2 losses into the condensate are significant, and 0.6% water in the sample can cause substantial interference, depending upon the method of analysis. Each 1000 ppm of water contributes a spectral overlap interference to infrared measurements equal to an SO2 level of 1 ppm, so 0.6% water appears as an SO2 level of 6 ppm.

As monitoring requirements become more stringent, condensation systems become more problematic. For example, when monitoring SO2 levels exceeding 1000 ppm, SO2 losses into the water condensate are often considered insignificant, and a 0.6% water residue in the sample causes minimal analytical interference.

Nafion is a form of tetrafluoroethylene (Teflon(R)) chemically modified to selectively absorb water vapor from gases. Since water is removed from the sample stream in the vapor phase, there is no loss of water-soluble gases; the sample is not exposed to condensate that can dissolve analytes.

Like Teflon, Nafion is highly resistant to chemical attack, so it withstands corrosive environments such as those encountered in CEMS applications. Nafion tubing will tolerate operating temperatures of up to 150 degrees C. (For optimal results, however, the temperature of the sample should be maintained only slightly above its dew point, since undesirable chemical reactions can occur at elevated temperatures.)

Nafion dryers remove water to dew points as low as -30 degrees C, in some cases lower, which corresponds to only a few hundred ppm of water in the sample. At these low water levels, not only are water condensation problems eliminated, but interferences are greatly reduced.

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