Study On On-line System For Monitoring Concentrations Of SO₂and NO In Flue Gas With Differential Optical Absorption Spectroscopy Technique

Among global environmental problems, the deterioration of atmosphericenvironment has drawn enough attention by worldwide countries. Great contributionshave been made to air pollution by flue gases such as SO2and NOxfrom thermal powerplant, metallurgical plant, chemical factory and cement plant in our country. Thusmonitoring the emission of flue gases has become the key point of environmentalregulation. Under this background, this paper is devoted to applying a burgeoningcontinuous and on-line optical monitoring technology—Differential Optical AbsorptionSpectroscopy (short as DOAS) to Continuous Emission Monitoring System (short asCEMS). And a systematic study has been carried out including the theory of DOAS,experiment and experimental setup in this paper. The mainly work of this paper is listedas follows.Firstly, in consideration of the features of DOAS and the properties of gases, asingle-side plug-in measurement probe has been developed. Then the relevant opticalpath and optical sensor module have been designed. The absorption cross-sections ofSO2and NO have been obtained by means of polynomial fit technique and Fouriertransform based on the measurement system built up in the laboratory.Secondly, experimental studies of pure SO2, pure NO and the mixed gases havebeen conducted using the measurement system. The concentrations of various gaseshave been calculated by means of least squares method in certain wave bands. Both ofthe relative errors of pure SO2and pure NO gas are lower than5percent. Besides theresult of mixed gases provides two conclusions, one of which is that the gas whichabsorbs most strongly should be calculated first at same wave band, while the rest canbe done in the same manner. Another conclusion is that the measurement of the resultsof the mixed gases used with different wave band is better than in the same wave band.Finally, a continuous emission monitoring system applied to the thermal powerplant has been developed and corresponding analysis software has been programmed.One domestic power plant is chosen to run a test. The on-site result of the experimentshows that the monitoring system suitable to industrial environment works stability.However, the relative error of first experiment is too large. After updating theabsorption cross-sections, the relative error of the next experiment reduces to lower than8percent. In addition, purge system has been added to the measurement probe to reduce the influence of smoke dust. Wavelet transform has been put forward to remove thenoises from on-site absorption spectrum effectively. Five-point moving average methodhas been used to optimize the display of concentration in the monitoring software.