| Natural gas is widely used in today’s society due to its advantages of clean and low carbon,high efficiency and abundant reserves.The international standard ISO 13686:2013 Natural gas—Quality designation specifies the composition of natural gas.Therefore,quantitative analysis of natural gas components has become very important.Among many online multi-component gas detection technologies,Raman spectroscopy can be used in the analysis of natural gas components because of its fast analysis speed,no need for sample pretreatment and convenient measurement.This paper studied Raman spectroscopy method for natural gas components.The specific research and development work include:(1)A preprocessing algorithm for natural gas spectrum is proposed to reduce baseline drift and background noise.The above algorithm was tested using 400 continuously measured natural gas spectra,and the pretreatment effect was excellent.(2)An automatic decomposition algorithm for Raman spectral peaks is proposed.It can decompose the Raman spectrum of a mixture sample,which may contain a small amount of unknown components,into the sum of several known pure component spectra and a synthetic unknown component spectrum.The synthetic spectrum of the unknown components is obtained by fitting some Lorentz peaks.The automatic decomposition algorithm not only can accurately decompose natural gas Raman spectra,but also can be applied to the Raman spectral decomposition of other mixture.(3)A Raman quantitative analysis model of natural gas components was established.Natural gas components can be divided into methane,ethane,propane,carbon dioxide,nitrogen,hydrogen,carbon monoxide and unknown alkane components of Ca or more(denoted as C4+).The first seven components are the main components in natural gas.The Raman spectrum of a natural gas sample could be decomposed into the sum of the Raman spectra of pure constituents and several Lorentz peaks by a nonlinear least-square optimization algorithm based on the automatic decomposition algorithm.The concentration of the unknown alkane component C4+was determined according to the area of the C-H deformation vibration peak which was common for most alkane molecules.Some training samples were used to establish the model between Raman characteristic peak areas and corresponding concentrations for each component.The experiments showed that the maximum absolute errors of this algorithm for methane,ethane,propane,carbon dioxide,nitrogen,hydrogen,carbon monoxide,C4+ respectively reached 0.57%,0.37%,0.21%,0.07%,0.18%,0.04%,0.13%,0.26%and the correlation coefficient to gas chromatographic results also reached 0.997,0.986,0.991,0.998,0.993,1.000,0.995,0.982,respectively.(4)An online Raman analysis system for natural gas component was developed.The system consists of two parts:hardware and software.The repeatability and accuracy of the system were examined.For a sample,the analytical repeatability of methane,ethane,propane,carbon dioxide,nitrogen,hydrogen,carbon monoxide and C4+were 0.072%,0.002%,0.00%,0.013%,0.018%,0.029%,0.014%,and 0.037%,respectively.For 26 mixture samples of natural gas,the standard deviation of the online analysis system for the 8 components were 0.234%,0.141%,0.176%,0.068%,0.056%,0.017%,0.015%,0.358%respectively,compared with the analytical values of the gas chromatography.The results prove that the system has good accuracy. |
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