Continuous Emission Monitoring. James A. Jahnke

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      and therefore

      (4‐11)

      The expression can be written in terms of particulate concentration c, instead of the particle number density n:

      (4‐12)

      where

      This expression merely states that optical density is directly proportional to the particulate matter concentration and also to the pathlength. It is a very useful relation, because if, for example, the pathlength should increase by a factor of 2, the optical density will increase by the same factor. If the particulate matter concentration is decreased by 1/2, D also decreases by 1/2. Applications of this expression are examined in Chapter 8 on opacity monitors.

COMPONENTS OF A SPECTROMETER: BUILDING AN INSTRUMENT

      The types of analytical techniques used in today's commercially available CEM monitoring analyzers are listed in Table 1‐1 of Chapter 1. Of these techniques, absorption spectroscopy has been the most commonly applied technique in continuous monitoring systems. Spectrometers developed for pollutant and diluent gas monitoring typically incorporate four essential components:

      These components differ depending upon the region of the spectrum in which the instrument operates and the analytical technique itself. The following sections give examples of these components.

      Radiation Sources

      Light sources used in CEM system analyzers emit in the ultraviolet, visible, and infrared regions of the spectrum. Advances in semiconductor electronics have led to the application of light emitting diodes (LEDs) and lasers in CEM analyzers. LEDs have largely supplanted the incandescent lamps once used in opacity monitors, and diode and quantum cascade lasers, alternatives to earlier broadband infrared light sources, have led to a new generation of extractive and in‐situ CEM system gas analyzers.

       UV Light Sources.

      For the ultraviolet region of the spectrum, hollow cathode gas discharge tubes, high‐pressure hydrogen or deuterium discharge lamps, xenon arc, and mercury discharge lamps have been used. UV lamps have shorter lifetimes than those operating in the infrared, and it is sometimes difficult to maintain stable UV intensities over the extended periods of time that they operate.

       Visible Light Sources.

      Visible light is used in opacity monitors, where the peak and mean spectral response is required to be between 500 and 600 nm, with less than 10% of the peak response below 400 nm and above 700 nm. This “photopic” region is established so as to be within the visual range of human observing stack exit opacity. Tungsten lamps, green light emitting diodes, and lasers have been used for this application, however green light emitting diodes are the most commonly used today.

       Broadband Infrared Light Sources.

      Heated materials will emit radiation in the infrared region of the spectrum. Among those used are Nernst Globars (fused hollow rods of zirconium and yttrium oxides, heated to about 1500 °C), Globars (heated rods of silicon carbide), carbon rods, and heated nichrome wire. These sources emit light over a range of wavelengths, from which the analyzer selects to make gas concentration measurements.

       Light Emitting Diodes.

Light emitting diodes are semiconductors that emit light when an electric current is applied. Light is emitted from the recombination of the electron–hole pairs of semiconductor materials. The light wavelength emitted is dependent upon the energy bandgap between the valence and conduction bands of the semiconductor and can range from the infrared to the ultraviolet region as the energy of the bandgap increases. LEDs are not lasers. They do emit over a spectrum narrower than incandescent sources, but the light emitted is incoherent.

       Tunable Diode Lasers.

      Tunable diode lasers (TDLs) are now being used as infrared sources by a number of instrument manufacturers in North America and Europe for both extractive and in‐situ system analyzers in a wide variety of applications (Mettler‐Toledo 2017). Although first introduced for source monitoring applications in the СКАЧАТЬ