Analytical Techniques Used in CEM System Instrumentation
The analytical techniques used in extractive and in‐situ CEM systems encompass a wide range of chemical and physical methods. These vary from chemical methods using simple electrochemical cells to advanced electro‐optical techniques such as wavelength modulation and Fourier‐transform infrared spectroscopy. Table 1‐1 summarizes the analytical techniques that are used in currently marketed CEM systems for gases. Table 1‐2 gives a summary of analytical techniques used in continuous emission monitoring systems for particulate matter (PM CEMS).
Techniques used for laboratory analysis, as well as techniques applied specifically for emissions monitoring, have been incorporated into commercially marketed systems. New analyzers have been developed using established electro‐optical methods, but are beginning to incorporate new light sources and detectors, such as tunable diode lasers, quantum cascade lasers, and diode arrays and new techniques such as cavity ringdown spectroscopy. The incorporation of microprocessors into today's analyzers has added useful features such as data storage, troubleshooting diagnostics, and external communication.
To lower the cost of CEM systems, CEM system manufacturers are employing multi‐gas techniques to avoid subsystem duplication that occurs when using single‐gas dedicated analyzers. One approach is to use multi‐gas methods such as dispersive, FTIR, or photoacoustic spectrometry. Another approach is to incorporate discrete, multiple, and interchangeable sensors into a single chassis.
TABLE 1‐1 Analytical Techniques Used in Continuous Emission Monitoring Systems for Gases and Volumetric Flow/Velocity
| Gases | Flow/Velocity | |
|---|---|---|
| Extractive | In‐situ | In‐situ |
| Absorption spectroscopy: | Path: | Path: |
| Differential absorption | Differential absorption – IR/UV | Acoustic velocimetry |
| Photoacoustic | Second‐derivative spectroscopy | Time‐of‐flight |
| Gas filter correlation | Wavelength modulation | |
| Fourier transform IR | Gas filter correlation | |
| Luminescence methods: | Point: | Point: |
| Fluorescence (SO2) | Differential absorption – IR/UV | Differential pressure |
| Chemiluminescence (NOx) | Gas filter correlation | Thermal sensing |
| Electroanalytical methods: | ||
| Polarography | ||
| Potentiometry | ||
| Calorimetry | ||
| Electrocatalysis (O2) | ||
| Paramagnetism (O2) | ||
| Methods for HAPS: | ||
| Differential absorption | ||
| Gas chromatography | ||
| Mass spectrometry | ||
| Fourier‐transform IR | ||
| Ion‐mobility spectrometry | ||
| Atomic emission (Metals) | ||
| Atomic absorption (Metals) | ||
| Atomic fluorescence (Metals) |
TABLE 1‐2 Analytical Techniques Used in Particulate Matter Continuous Emission Monitoring Systems (PM CEMS)
| Extractive | In‐Situ | |
|---|---|---|
| Point | Point: | Path |
| Beta radiation attenuation | Light scattering | Transmissometry |
| Light scattering | Contact charge transfer | Light scattering |
| Electrodynamic induction |
Succeeding chapters present details of both extractive and in‐situ systems – their advantages, disadvantages, and limits of application. The sampling interface is of particular importance in extractive system design and is treated separately in Chapter 3. Extractive system analyzers are discussed in Chapter 5. For in‐situ system design, the analyzer type is most important. In‐situ monitors for measuring gases are discussed in Chapter 6 and monitors designed for measuring flue gas flow, opacity and particulate matter in Chapters 7–9.
Mercury monitoring, a field in itself, has advanced significantly, within 15 years of research and development. This topic is treated separately in Chapter 12, to outline how a new generation of mercury monitoring systems has evolved to enable continuous monitoring of stationary source mercury emissions down to less than 1 μg/m3. Monitoring for hazardous air pollutants (HAPs) has developed