STACK GAS POLLUTANT DETECTION USING LASER RAMAN-SPECTROSCOPY

An experimental system to investigate the feasibility of using laser R aman spectroscopy to detect stack gas pollutants is described. With sp ontaneous Raman spectroscopy, one unit is capable of measuring all gas species simultaneously and, when applied to a continuous emissions mo nitoring (CEM) system, could both simplify it and improve reliability. Minimum detectabilities with the use of this weak light scattering pr ocess were determined for the three primary pollutants from coal-based power plants: carbon monoxide (CO), sulfur dioxide (SO2), and nitric oxide (NO). A frequency-doubled, pulsed Nd:YAG laser was used to excit e the test gas held in a sample cell. A charge-coupled device connecte d to a spectrometer was used as the detection system, providing comple te spectral information. A set of experiments was carried out for diff erent concentrations of the three test gas species. With a 200-s integ ration time, the following minimum detectability limits were obtained: 100 ppm for SO2, 250 ppm for CO, and 250 ppm for NO. A mixture of all three pollutants in a base of N-2 was made to simulate typical stack gas mixtures. Baseline strength and associated shot noise are more sev ere when gas mixtures are being analyzed. One possible explanation is the broad-band fluorescence of NO2, perhaps produced photolytically. S ystem modifications to improve minimum detectabilities are discussed.