Study On Low Concentration Detection Algorithm Of SO₂ And NO With Differential Optical Absorption Spectroscopy

As China’s environmental protection increasing day and day,the pollution gas emission standards have become higher and stricter.Therefore,real-time monitoring and accurate measurement of the concentration of pollutant gases generated in smoke emissions has become an important part of promoting national environmental protection.Base on this requirement,this paper makes an in-depth study on the low-concentration SO₂and NO algorithms and finishes the structural design and hardware selection of the portable UV analyzers by Differential Optical Absorption Spectroscopy（DOAS）,The main contents are as follows:This paper designs the structure of the low-concentration flue gas analyzer based on the principle of DOAS and chooses a suitable light source and a spectrometer.By eliminating the fluctuation interference of the spectrometer to make sure it can be used.Paper also designs a long-path gas pool.This paper studies low concentration measurement algorithm of SO₂ and proposes a low-concentration SO₂ algorithm by the superposition state decomposition principle.After obtaining the differential absorption spectrum,because of the periodicity characteristics of the SO₂ differential absorption spectrum,we can use the trigonometric function as the signal to decompose the original spectrum.It means we can use fast Fourier transform（FFT）to obtain the information of frequency domain.After doing this,we can use the Principal Component Analysis（PCA）to choose the frequency domain which has obvious variety to get the information of absorbance and measure the concentration.By the adjustment of wave band to remove the interference of NO we can measure concentration difference of 1ppm SO₂.It also measure accurately when SO₂and NO are mixed.After 7 days,it can be sure the error doesn’t exceed 2%FS.By studying the low concentration measurement algorithm of NO,paper proposes a low-concentration NO algorithm by wavelet filtering.Because NO differential spectrum exists noise,paper use the suitable wavelet to filter noise.After filtering noise,the differential absorption spectrum possesses periodic characteristics.After using FFT to decompose the spectrum,we can get the frequency domain which is interrelated about concentration.It also verifies that the wavelet filter still can filter SO₂as noise.After seven days,it can be sure the error doesn’t exceed 2%FS.