Study On On-line Monitoring Algorithm Of Ultra-low Concentration NO Gas Based On UV-DOAS

Air pollution is one of the most serious environmental pollution,which seriously threatens the survival of mankind.The flue gas emissions from coal-fired power plants are one of the main sources of air pollution.Therefore,monitoring flue gas is the key to controll air pollution.This paper introduces the basic principle and concentration inversion method of ultraviolet differential optical absorption spectroscopy for monitoring ultra-low concentration of NO in flue gas,and proposes an improved UV-DOAS algorithm-area integration method to achieve ultra-low concentration detection with a detection limit of 5ppb,good repeatability and class A uncertainty of0.15%.The absorption spectrum of the mixed gas has the characteristics of additive,and the differential optical density of NO is accurately separated from the mixed gas of NO,NO₂ and SO₂,which improves the anti-interference ability of the system and realizes accurate inversion of NO in the mixed gas.In this paper,the influence of temperature on concentration inversion is also studied.A temperature compensation method suitable for UV-DOAS algorithm is proposed.Experiments show that the temperature compensation method can effectively suppress the influence of gas temperature on the measurement results and effectively improve the equipment pair.Environmental adaptability.The specific research contents mainly include:（1）Theoretical research.The principle of molecular absorption spectrum is introduced.The molecular absorption spectrum is generated by the energy level transition.There are electron energy levels,vibrational energy levels and rotational energy levels in the molecule.In this paper,ultraviolet light irradiation can simultaneously make the transition of these three energy levels.Which forms ultraviolet absorption spectrum.The Lambert Beer’s law is introduced,and it is corrected by considering the extinction effects such as Mie scattering and Rayleigh scattering.The spectral signal preprocessing method was studied,and the spectrum was preprocessed by wavelet filtering in this paper.（2）The concentration inversion algorithm.The inversion band selection of NO,NO₂ and SO₂ concentrations is introduced respectively.The principle of selecting the concentration inversion band is as strong as possible in the case of strong absorption characteristics.NO selects absorption peaks 2（213～216nm band）and peaks 3（224～227nm band）;NO₂ selects 335～370nm band,SO₂ selects 290～320nm band.The method of obtaining differential absorption cross section is introduced.The limitation of Lambert Beer’s law is analyzed,which leads to the nonlinear error of the differential absorption cross section.The obtained differential absorption cross section combined with the least squares inversion result is nonlinear,whose results needed nonlinear correction.Then the traditional UV-DOAS concentration inversion algorithm-least squares method is analyzed,and an improved UV-DOAS concentration inversion algorithm-area integration method is proposed.The area integral method inverts the concentration by constructing a mathematical model of differential optical density and concentration.The mathematical model construction adopts nonlinear fitting,so the inversion result does not require nonlinear correction,and the detection limit is low,up to 5 ppb.（3）The NO feature extraction in the mixed gas was studied.The algorithm for detecting NO under SO₂ interference,NO₂ interference was studied.Under SO₂interference,using SO₂ in the 290～310nm band not affected by NO,calculate the SO₂ concentration,find the SO₂ differential optical density corresponding to the concentration in the database,subtract the SO₂ differential optical density from the mixed gas differential optical density to obtain the NO difference Optical density,and inverse NO concentration.Under NO₂ interference,the effects of different concentrations of NO₂ on NO were studied,and the absolute error of NO was calculated.The absolute error of NO and the concentration of NO₂ were fitted by linear function.When processing the mixed gas of NO and NO₂,the NO concentration in the case of containing NO₂ is first calculated,then the concentration of NO₂ is calculated,and the absolute error at the NO₂ concentration is obtained by the absolute error function.Then,by subtracting the absolute error from the concentration of NO,the true concentration value of NO is obtained.（4）The impact of environmental factors.The effect of NO on different temperatures was studied,and the relationship between inversion concentration and temperature was constructed.The experimental results show that the function relationship can correct the influence of temperature on the inversion concentration and improve the adaptability of equipment to the environment.