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.