FIG. 5.12.2 Probe-type stack gas analyzer. (Reprinted, with permission, from Lear Siegler Inc.)

FIG. 5.12.3 Standard extractive system.

Backflush Path Calibration Path I 1 Bypass Path

I 1 Sample Path

FIG. 5.12.3 Standard extractive system.

tors, the environmental engineer must measure or calculate the flue-gas moisture to account for the moisture removed before dry-extractive analysis. This additional measurement or calculation adds complexity to the system, which lowers the reliability and accuracy of dry-extractive CEM systems.

Operating an extractive system is complicated by the daily calibration, zero and span checks of the analyzers, and the need to backpurge the sample handling system to clear the probe and filters. These tasks are accomplished with additional lines and connections to the stack probe and control valves. A controller system sequences the operation of all valves and controls gas flows.

The major advantages of extractive systems is their ability to share analyzers and spread the analyzer cost over several measurement points. An extractive system can be used to monitor two or eight points.

Wet-Extraction Systems

Wet-extraction CEM systems are similar to dry-extraction ones except that the sample is maintained hot, and mois ture in the flue gas is retained throughout. Heat tracing is the critical component of a wet-extractive system. Sample temperatures must be maintained between 360° and 480°F to prevent acid gases from condensing within the sampling lines and analyzer.

Since nothing is done to the sample before analysis, wet-extractive CEM has the potential of being the most accurate measurement technique available. However, only a limited number of vendors supply wet-extractive CEM systems. In addition, increased flexibility and accuracy makes the wet-extractive CEM systems more costly than dry-extractive ones.

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