Stack Gas Pollutant Detection Using Laser Raman Spectroscopy

Abstract

An experimental system to investigate the feasibility of using laser Raman spectroscopy to detect stack gas pollutants is described. With spontaneous Raman spectroscopy, one unit is capable of measuring all gas species simultaneously and, when applied to a continuous emissions monitoring (CEM) system, could both simplify it and improve reliability. Minimum detectabilities with the use of this weak light scattering process 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 excite the test gas held in a sample cell. A charge-coupled device connected to a spectrometer was used as the detection system, providing complete spectral information. A set of experiments was carried out for different concentrations of the three test gas species. With a 200-s integration 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 N₂ was made to simulate typical stack gas mixtures. Baseline strength and associated shot noise are more severe when gas mixtures are being analyzed. One possible explanation is the broad-band fluorescence of NO₂, perhaps produced photolytically. System modifications to improve minimum detectabilities are discussed.