Quantitative Analysis of Main Components of Natural Gas Based on Raman Spectroscopy

After desulfurization and dehydration treatment, natural gas is composed of methane, ethane, propane, carbon dioxide, nitrogen, hydrogen, carbon monoxide and unknown alkane components of C-4 or more ( denoted as C-4(+)). The sum of the content of first seven components is more than 90% of natural gas. When the existing Raman spectral analysis methods are applied to analyze the natural gas composition, a small amount of unknown alkane components C-4(+) will have a greater impact on the analysis precision. On the basis of this, a novel Raman analysis method which consists of a spectral automatic decomposition algorithm and a quantitative analysis model has been developed. Based on a linear additivity of Raman spectra, the Raman spectrum of a natural gas sample can be decomposed into the sum of the Raman spectra of pure constituents and several Lorentz peaks by a nonlinear least-square optimization algorithm. The content of the unknown alkane component C-4(+) can be described as the area of C-H deformation vibration peak common for most alkane molecules. Samples of the training set are used to establish the model between Raman characteristic peak area and corresponding concentration for each component relative to methane. Compared with the existing Raman analysis methods, the new method solves the issue of analyzing natural gas containing unknown alkane components and has good stability and accuracy. Experiments show that the maximum absolute errors of this algorithm for methane, ethane, propane, carbon dioxide, nitrogen, hydrogen, and carbon monoxide respectively reach 0. 57%, 0. 37%, 0. 21%, 0. 07%, 0. 18%, 0. 04%, 0. 13%, and the correlation coefficient of gas chromatographic results also reaches 0.997, 0.986, 0.991, 0. 998, 0. 993, 1. 000, 0. 995, 0. 982, respectively.