Theoretical Analysis And Experimental Study On Differential Optical Absorption Spectroscopy (DOAS) For Flue Gas Monitoring

The measurements of flue gas are almost not on-line, and also exist low stability and accuracy at present. The differential optical absorption spectroscopy (DOAS) is introduced to realize on-line measurement of the flue gas.First, based on the measurements of absorption cross-section, the absorption characters of the main polluted gases in flue gas were deeply studied, and found that SO₂, NO, NO₂, NH₃ had a character of approximate periods absorption at some wave band, which is a good chance of a new measurement introduced. Secondly, the factors about absorption cross-section of the gases measured, which affect on the measurement precision of DOAS, is studied deeply. The influence of pressure and temperature on absorption cross-section was given special research, and the compensated measures were given to the gases based on the character of the gases. Thirdly, in practical measurement, it must have been satisfied that the monochromator was consisted with the reference spectrum, which was based on absorption cross-section. In order to satisfy the consistency, the technique of reversion in the image was introduced, the reversions of spectrums were realized with wiener filter and noise elimination based on wavelet transform. Then the spectrums reverted were convoluted with the instrument function of the monochromator to realize the consistency. Finally, based on the theory of DOAS, the least-squares methods-DOAS was expatiated detailedly. There were two conclusions, the first was that the calculation of the concentrations of the mixed gases should base on absorption character in term of high to low at same wave band; the second was that the measurement of the concentrations of the mixed gases used with different wave band was better than the same wave band, which was explained with experiment. Based on the approximate period absorption of the gases, the fourier transform was introduced and a method of fourier transform-DOAS was put forward, which was expatiated detailedly in the thesis. The methods turned out to exhibit high sensitivity, good separation capacity, good accuracy, which was also testified with experiment.It is shown in the research and experiments that the DOAS has wide application prospects. The studying of this paper provided the theoretic and experimental bases of the DOAS applied successfully in flue gas monitoring, and provided the technical support to improve flue gas monitoring level.