Comparison of Thermoelectric and Permeation Dryers
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Test methods for determining CEM require a low-moisture gas sample because water vapor can influence ultraviolet absorbance (affecting SO2 readings), chemiluminesence (affecting NOx readings), and non-dispersive infrared (impinging on CO results). Since industrial air emissions, particularly combustion process emissions, can have a high-moisture content, sample conditioning is employed for moisture removal prior to instrument measurement. The goal of sample conditioning is selective moisture removal--leaving the concentrations of other species unaltered. Sample conditioning is usually performed by cooling the gas to approximately 35°F (2°C), to condense the water vapor, and subsequently removing the condensate with a peristaltic pump.
Sample conditioning can affect the concentrations of many emission matrix constituents, either by condensation from cooling below the boiling point or by dissolution in the condensed water. Acids such as HCl and HF and polar organic compounds such as methanol are effectively removed from stack gas in a condensation-type conditioning system. Gas conditioning should have little impact on the concentrations of compounds with low boiling points and low water solubility, such as CO and NO. However, gas conditioning is used extensively for measurement of SO2, which has a relatively high boiling point (-10°C) and water solubility. Experiments have demonstrated that a condensation-type gas dryer effectively removes moisture from a hot, wet gas stream, but also reduces the SO2 concentration in the conditioned gas. The permeation dryer, by comparison, is more effective in removing moisture from the gas stream while having a measurably reduced impact on the SO2 concentration. The use of condensation conditioning systems at industrial sources could conceivably result in even higher SO2 losses than indicated by these experiments. Condensation dryers are commonly used in CEM systems and source-emission tests for criteria air pollutants. The results from these experiments indicate that losses of SO2 from sample conditioning with condensation dryers can cause a low bias in emission measurements, particularly at high moisture sources. Industry emission permits and the system of SO2 emission allotments and trading implemented in recent years rely upon accurate SO2 emission measurements. More accurate SO2 measurements can be obtained by using permeation rather than thermoelectric drying systems for gas conditioning. The foregoing article is adapted from "Comparison of Thermoelectric and Permeation Dryers for Sulfur Dioxide Removal During Sample Conditioning of Wet Gas Streams." The paper was a presentation of the Air & Waste Management Association's 90 Annual Meeting & Exhibition, June 8-13, 1997, Toronto, Ont., Canada. The authors were Thomas A. Dunder, Entropy, Inc., Research Division, PO Box 12291, Research Triangle Park, NC 27709-2291; and David A. Leighty, Perma Pure, Inc., 8 Executive Drive, Toms River, NJ 08754 Edited by Paul Hersch |