Intro4u2u

Analytical equipment has progressed over the years from analogue instruments born in the laboratory to new digital analytical equipment designed for specific applications. The analyser list has grown; we can now add flame ionisation detection gas analysers (FID), gas chromatography (GC), and mass spectrometry (MS). While there are other methods in use today and others under development, those mentioned here are the workhorse methods in process analytical chemistry and have been in everyday use, 24 hours per day, 365 days per year for making process analytical measurements. Some gases are difficult to measure just because they are hard to detect, but in general, reactive and condensable gases such as HCl, NH3, HF and formaldehyde, present the greatest measurement challenges. Such gases may react with other components within the stack gas stream; they may condense or be absorbed by liquid condensate within a cold extractive sampling system, they may adsorb onto surfaces, or they may polymerise before reaching the Analyser. Thus, depending upon the components making up the flue gas stream, special sampling equipment may be needed and special operation and maintenance procedures may be required to achieve reliable results. Thus, in addition to the measurement technologies of the instrumentation, one must also consider the technology of sample handling and transport.

A brief overview of these analytical techniques and advances in their technologies follows. Nondispersive infrared gas Analysers (NDIR) utilises several different detection techniques. Opto-pneumatic detectors, commonly known as Luft detectors (from their inventor, Karl Luft), interference filter photometers (IFC), and gas filter correlation (GFC) are the more predominant types. The theory of operation of these infrared methods is similar and is based upon absorption of infrared energy in the 2 to 11 micron wavelength range. Simple molecules with less than 5 or 6 atoms have infrared absorption spectra with fine structure. Gases fitting this description are ideal for Luft and GFC types of Analysers, which correlate the spectra of the sample gas with the spectra of the pure component of interest. Interference filter correlation is capable of measuring gases with either fine structure in the spectra or broadband absorption. Proximately 80 gases have been measured with NDIR. The capability list of all techniques mentioned herein is based on current availability in the marketplace and not theoretical considerations. Of the 80 gases, approximately 50% of the NDIR instruments sold are for measurement of Carbon Monoxide (CO).