The Application Of Laser Induced Breakdown Spectroscopy For Diagnosis Of Power Station

In recent years, with the rapid development of electric power production and the major changes of power consumption structure, the boiler will be affected by many adverse factors (e.g. coal changes, complex and diverse quality characteristics of coal, a wide range of load variability, etc.). These adverse factors may lead to a series of problems, such as low combustion efficiency, poor combustion stability, fouling of heating surface, slagging, abrasion, etc. which are not conducive to the stability of the boiler control. There are not only direct impact on the safety of boiler operation and economy, but also seriously affect the power grid security. The lack of online measurement technology for coal-fired power plant boiler units' has become one of bottleneck problems that limite the development of optimal operation of thermal power technology. The purpose of this thesis is to investigate the potential of laser-induced breakdown spectroscopy technology used in power station for boiler diagnosis, of particular interest to the theoretical analysis and experimental research of unburned carbon of fly ash, slagging characteristics of coal and ash content and the failure trend of heat exchange surface.Firstly, the research status of the measurement technology of unburned carbon of fly ash, slagging characteristics of coal and ash content and the failure trend of heat exchange surface were reviewed. The research contents of this thesis were details based on the representation of backgroung and purpose. Moreover, the principles of laser induced breakdown spectroscopy and the structures of the experimental system were detailed. The transformation of trigger signal for achieving simultaneous control of lasers and spectroscopy was also introduced. Then multivariate analysis method was introduced to the qualitative and quantitative analysis of laser-induced breakdown spectroscopy, the mathematical theory and calculation process of these methods were described in detail. At last, the potential of linear regression, principal component regression and partial least squares regression method for simultaneously quantitative analysis of nutrients in fertilizer was demonstrated by experiment.Secondly, the impact of laser energy on the measurement of fly ash carbon content was investigated. The analysis of unburned carbon in coal fly ash using laser-induced breakdown spectroscopy in deep UV was also discussed. Besides, in order to correct the adverse effect of matrix effects, the multiple linear Regression and multi-line intensities as the internal standard method were employed to quantitative analyze the unburned carbon in fly ash. The analysis shows that traditionally used univariate calibration in LIBS does not qualify quantitative analysis of fly ashes from different kinds of coal due to the presence of matrix effects. Instead, multivariate calibration has a better performance as the matrix effects can be taken into account with the influence of the spectroscopic signals of other components in fly ash. It is indicated that laser-induced breakdown spectroscopy is suitable for rapidly detect unburned carbon in fly ash with receptible applicability.Then, the effect law and influence mechanism of the sample morphology and the pulse laser energy on the measurement of the major elements in coal ash were analyzed. On this basis, the quantitative analysis results of SiO₂ and Al₂O₃ in different placement of focus len that was directly related to the application of LIBS in field were compared. The indicator of slagging characteristics (SiO₂/Al₂O₃) calculated by the measurement of laser induced breakdown spectroscopy was compared with that of routine laboratory analysis. Furthermore, the multivariate analysis method was employed to extracte coal ash content from LIBS spectral. These analysis results were compared with that provided by elemental analysis.At last, in order to develop nondestructive examination technology for analysis the failure tendency of boiler heat exchange surface before the appearance of macrography craze, LIBS was employed as a potential analysis technology. On the basis of the detail of characteristics of boiler heat exchange surface, the interaction mechanisms between laser and boiler heat exchange surface materials with different microstructure and pearlite spheroidization were analyzed. The variations of line intensity, plasma temperature and ablated crater of the pearlitic/ferritic and martensitic phases were investigated. Alternatively, principal component analysis method was employed to discriminate the pearlitic/ferritic from martensitic phases. Furthermore, the lines intensities, the intensity ratio of atomic and ionic lines, plasma temperature and electron density of raw material and different pearlite spheroidization samples were also investigated. It should be noted that there are good corresponding relationship between the intensity radio of atomic and ionic lines, electron density and the hardness of these samples.