Quantitative Analysis On The Main Composition Of Natural Gas Based On Infrared Spectroscopy

Nowadays, energy measurement of natural gas is an international,fashionable, fair and effective measurement, but the traditional way which uses gaschromatography to analyze the composition of natural gas and calculate the calorificvalues is time consuming, expensive, complicated to operate and not well adapted toon-line measurement. Infrared spectroscopy has the advantages of indirectmeasurement, moreover it is fast, simple, accurate and easy to realize onlinemeasurement of multi-component gases. So it has become one of the best methods inthe field of gas detection. Our group innovatively puts forward an analysis system forcalculating the calorific values of natural gas based on infrared spectroscopy, whichhas broad application prospects. That the study on the quantitative analyses of thecomposition of natural gas by Fourier transform infrared spectroscopy （FTIR） is paidclose attention to by the paper, and the effect of the in-situ temperature and pressureof gases on infrared spectrum is preliminarily studied. The main contents andconclusions are as follows:（1） The experimental system based on the infrared spectroscopy for analysisingthe composition of natural gas is established, and the detailedly experimentalprocedures are developed to collect the infrared spectra.（2） The quantitative analysis model of methane is built. By comparison, meancentering is chosen without other spectral data preprocessing methods, and3220.593-2591.902cm^-1 and1403.947-1184.098cm^-1 are selected as the spectralanalysis regions which are the area of the main and minor absorption peaks ofmethane, and the result of the PLS prediction model established is best. Its correlationcoefficient （R） is0.99915, RMSEC and RMSEP are6.16and11.3respectively, andthe maximum relative error is4.85%.（3） PLS prediction models for three composition of natural gas aresimultaneously established in the selected spectrum range without using other spectraldata preprocessing methods. The correlation coefficient （R） of methane, ethane andpropane are0.99811,0.99552and0.99878respectively, RMSEC are20.8,25.9and 13.3respectively, and RMSEP are20.6,27.6and9.02respectively, indicating that theprecision and stability of the models have reached the expected requirements, and areable to meet the needs of online measurement of gas composition.（4） Because the temperature and pressure of the natural gas in pipeline isconstantly changing, the experimental research on the effects of the temperature andpressure on the infrared spectrum is conducted. It is found that there is a very goodlinear relationship between temperature and pressure with the spectral data. Within acertain range, the band absorption intensity and peak width of methane of a certainconcentration will increase as the total pressure and temperature increases, and theabsorption peak area increases proportionally with pressure and temperatureapproximately. The conclusion can also be applied to ethane. The research has animportant significance and engineering application prospects in improving theaccuracy of spectrum quantitative and qualitative analysis.