| As my country’s regulations on air pollutant emission standards become more stringent,coal-fired power plants as the main emission source are one of the important targets for emission control.Among them,NOx is one of the main air pollutants,and the state and local governments continue to raise its emission limit standards.In order to meet the increasingly stringent emission limits,power plants must carry out low-nitrogen combustion retrofits,and use corresponding NOx removal technologies to treat exhaust fumes.The NO concentration in the flue gas at the outlet of the SCR denitration system is an important indicator for evaluating the performance of SCR denitration,and it is also a core parameter to ensure that the NOx emission concentration meets environmental protection requirements and strictly control ammonia escape.Therefore,the demand for high-sensitivity detection of low-concentration NO is extremely prominent.Among many gas detection methods,wavelength modulation technology based on tunable diode laser absorption spectrum has been widely concerned and studied because of its high sensitivity and strong anti-noise interference ability.However,the current NO detection based on TDLAS has problems such as large noise interference,low system detection limit and insufficient detection sensitivity.Research on low-concentration NO high-sensitivity detection technology of infrared TDLAS-WMS.Firstly,this article introduces the basic principles and important parameters of TDLAS technology,and analyzes the measurement mechanisms of direct absorption spectroscopy and wavelength modulation spectroscopy.By analyzing the characteristics of NO absorption peaks in different bands and combining the principle of gas absorption spectrum selection,the strong absorption peak of mid-infrared NO 5.184μm is selected as the measurement spectrum.Based on this,a NO detection system is built,and the key experimental devices are explained.Secondly,the various noises existing in the TDLAS system are explained,several commonly used noise reduction processing methods are briefly introduced,and the empirical mode decomposition algorithm and its noise reduction effect on the second harmonic signal are mainly introduced and analyzed,according to the results,the empirical mode decomposition algorithm for noise reduction,second harmonic signal signal-to-noise ratio than the original second harmonic signal signal-to-noise ratio increased 3.4 times.Then,through experiments,the influence characteristics of different modulation parameters on the second harmonic signal are compared,and the optimal modulation frequency and amplitude of the detection system built in this paper are determined to be 8k Hz and 0.15V respectively.On this basis,through the experiment of low concentration NO gas samples with different ratio,the NO gas concentration inversion model based on the second harmonic signal was established,and the linearity reached 0.99968.Then,the reliability and accuracy of low concentration NO detection were verified by the independent sample test,and the relative error of the measurement results was relatively small.The maximum relative error of the measured values was-2.56%.Finally,aiming at the background signal drift phenomenon in the long-term operation of the detection system,a comprehensive correction method for the background signal drift was proposed.The maximum relative errors of the inversion concentration before and after the correction of the background signal wavelength drift were 6.30%and 3.85%respectively,and the standard deviations were 0.19×10-6 and 0.07×10-6,respectively.The mean square values of relative error of inversion concentration are 24.39%and 9.99%,respectively.Allan analysis of variance was used to evaluate the modified detection sensitivity and detection limit,and the experimental results showed that the detection limit of the system decreased from 653×10-9 before noise reduction to 442×10-9.The detection limit of the system was reduced from 272.7×10-9 to 185.5×10-9 before noise reduction. |
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