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Analyzers used to monitor stack gases must be incorporated into a system – one that can protect the analyzer from the plant environment and provide a representative stack gas sample for analysis. Either in‐situ or extractive systems can be designed to perform this. Extractive systems were the systems first developed for continuous emission monitoring and are discussed in this chapter; in‐situ systems are discussed in Chapter 6.

Delivering and conditioning a hot, moisture‐saturated, particle‐laden flue gas to an analyzer can be a difficult process. Because the pollutant gases may be lost in this process, it is critical that the extractive system is properly designed and manufactured. Extractive system designs are associated with specific CEM system vendors, having been developed through years of industrial applications experience. Although it is possible and relatively easy to design and construct an extractive system from basic components, most systems purchased today are provided on a turn‐key basis; that is, the system is provided from start to finish. The two basic types of extractive systems that vendors offer are (i) fully extractive, source‐level systems and (ii) dilution systems. Each system has its own advantages and disadvantages in any given application.

This chapter describes both source‐level and dilution extractive systems and the components that are used in their design. Knowledge of these extractive techniques will assist in evaluating proposed system designs or systems that have already been installed.