Natural gas composition and calorific value analysis

Natural gas composition and calorific value analysis

Composition and calorific value analysis of natural gas: gas chromatography, infrared gas analysis and Raman spectroscopy natural gas is the general name of hydrocarbon and a small amount of non hydrocarbon mixed gas. Because natural gas from different origins has different components and combustion characteristics, even natural gas with the same volume will produce different energy. At present, natural gas energy measurement and pricing has become the most popular measurement and settlement method of natural gas trade in the world. As the main analysis method of natural gas energy measurement, the calorific value analysis method of natural gas composition can effectively avoid the calorific value deviation caused by different gas sources, accurately measure the calorific value of natural gas, reduce trade settlement disputes, and promote the healthy development of the natural gas industry.
The calorific value analysis method of natural gas composition is based on the principle of the contribution of each component of natural gas to the calorific value. The purpose is to determine the mole fraction of different gas components through appropriate analysis methods. The calorific value can be calculated by weighting the molar calorific values of gas components with different mole fractions and their corresponding component gases. According to this principle, the molar calorific value of natural gas can be calculated. At present, the commonly used technologies for calorific value analysis of natural gas components at home and abroad include gas chromatography (GC), non spectroscopic infrared (NDIR) and laser Raman spectrum natural gas analysis. Their working principles and characteristics are described below.
1. Calorific value analysis by gas chromatography (GC)
GC is composed of gas circuit system, sampling system, chromatographic column, electrical system, detection system, recorder or data processing system. The working principle is that the mixed gas to be measured is first brought into the chromatographic column by inert gas (i.e., carrier gas, generally N2, H2, he, etc.), and the column contains liquid or solid stationary phase. Due to the different boiling points, polarities or adsorption properties of each component in the sample, each component tends to form a distribution or adsorption equilibrium between the mobile phase and the stationary phase. However, since the carrier gas is flowing, it is difficult to establish this equilibrium in practice. It is also because of the flow of the carrier gas that the sample components are repeatedly distributed or adsorbed / desorbed in the movement. As a result, the components with high concentration are distributed in the carrier gas to flow out of the chromatographic column first, and the components with high concentration are distributed in the stationary phase to flow out. When the components flow out of the chromatographic column, they immediately enter the detector. The detector can convert the presence or absence of the sample components into electrical signals, and the size of the electrical signals is proportional to the amount or concentration of the tested components. When these signals are amplified and recorded, a chromatogram can be formed, which contains all the original information of the chromatography. When no component flows out, the record of the chromatogram is the background signal of the detector, that is, the baseline of the chromatogram.

Thermal conductivity detector (TCD) is usually used for gas chromatographic analysis of gas. Because pure gases have different thermal conductivities, when they flow through the detector, they will cause resistance changes. This signal can be recorded and constitute a chromatogram.
The gas chromatograph has high requirements for the personnel who operate the instrument, and requires carrier gas, which makes the operation complicated. In practical application, it is necessary to ensure that the hot wire is not burned. Before the detector is powered on, make sure that the carrier gas has passed through the detector, otherwise, the hot wire may be burned out, resulting in the detector being scrapped; Power off the detector first, and then turn off the carrier gas. Whenever any operation that may cut off the flow of carrier gas through TCD is performed, the detector power shall be turned off. In addition, when the carrier gas contains oxygen, the hot wire life will be shortened, so the carrier gas must be completely deoxidized; When hydrogen is used as carrier gas, the gas shall be discharged outdoors.
The gas chromatograph can be calibrated with one or more calibration gases to calculate the calibration coefficient. The mole fraction of each individual component can be evaluated using these coefficients. The advantage of this method is that it can calculate physical quantities other than calorific value, such as standard density.
2. Calorific value analysis by non spectroscopic infrared (NDIR) method
NDIR infrared analysis method is generally composed of electrically modulated infrared light source, high sensitivity filter, micro infrared sensor and local temperature control circuit. Its working principle is based on that the absorption of infrared light by polar gas molecules conforms to Lambert Beer law. For mixed gas, in order to analyze specific components, a narrow-band filter suitable for the absorption wavelength of the analyzed gas is installed in front of the sensor or infrared light source, so that the signal change of the sensor only reflects the concentration change of the measured gas.

Figure 2. Principle of dual beam infrared analysis
Taking CH4 analysis as an example, the infrared light source emits 1-20 μ M U.M infrared light, after being absorbed by a certain length of air chamber, passes through a 3.33 μ M U.M wavelength narrow-band filter, and the infrared sensor monitors the transmission of 3.33 μ M U.M wavelength red light to reflect the concentration of CH4 gas.
Generally, there are two filters on the infrared detector. One filters the infrared signal without attenuation as the reference channel, and the other filters the infrared signal band with the largest absorption as the measurement channel signal. The two participate in data calculation after comparison, so as to eliminate the drift caused by the change of the light source signal to the maximum extent. The structure of this detector is a single light source and two light beams. It adopts semiconductor technology. It is characterized by less mutual interference between different gases and high measurement accuracy. Multi-component measurement can be realized by increasing the number of channels of the detector.
The infrared detector has high sensitivity and can be used for both constant analysis and micro analysis; It has good stability and can be used for continuous real-time analysis of gas concentration, which is not only suitable for continuous online measurement, but also for portable real-time measurement. At present, this calorific value instrument based on infrared natural gas composition analysis technology is widely used in the fields of LNG, CNG quality control, natural gas combustion equipment quality control and combustion control.
Natural gas is widely used in Foshan
For example, high-grade ceramics, lamps and lighting products have strict range requirements on the temperature of the kiln during the process, and thus indirectly have quantitative requirements on the calorific value of the fuel. Since the gas provided by the natural gas company inevitably has calorific value fluctuations, although the natural gas company has the detection report of the measurement center, as an enterprise, it still needs to periodically compare the detected calorific value with the data of the measurement center to prevent the gas supply unit from forging, which is also a process monitoring means. Therefore, many units have adopted Ruiyi automatic control gasboard-3110p to analyze and detect the composition and calorific value of natural gas.

The application of portable infrared natural gas calorific value analyzer gasboard-3110p in the industrial field has been tested in two ceramic enterprises, DP ceramics and HL ceramics, and the quality inspection report of the gas supply unit using GC is compared with the detection data of gasboard-3110p. The test results show that the high and low calorific value data are almost the same as the data measured by GC, and the measurement data are stable and accurate for many times. It takes several hours for GC to get results from sampling to analysis. Gasboard-3110p can realize on-site real-time testing and has obvious advantages in test speed.
3. Natural gas analysis by laser Raman spectroscopy
Raman spectrum is a scattering spectrum. Raman spectroscopy is an analytical method based on the Raman scattering effect, which analyzes the scattering spectrum with different frequency from the incident light to obtain the information of molecular vibration and rotation, and applies it to the study of molecular structure. Raman spectroscopy is very important in many industrial applications, such as calorific value measurement of natural gas, because of its multi-component gas component detection ability, especially its ability to accurately measure diatomic molecules and trace water in natural gas.
Raman scattering is directly proportional to the intensity of the laser and the density (pressure) of the sample. However, since the Raman signal from the gas is very weak, it will require a very high power laser (> 4W) or a very high pressure (> 50 bar) to achieve the sensitivity required for industrial applications using conventional Raman technology. At present, the feasibility study of Raman gas analysis system using high-power laser under pipeline pressure has been reported in the literature. The experimental schematic diagram of the laser Raman gas analysis system is shown in Fig. 5.
Fig. 6. Experimental schematic diagram of Raman analyzer system
A 532nm CW laser was used as the Raman spectrum excitation light source with a total output power of 200 MW. Aluminum mirrors M1, M2 and M3 with a diameter of 1 inch and lenticular lenses L1 and L2 are used to guide the laser light to a Herriott gas absorption cell, and the Raman cell is used to enhance the effective path length of the 532 nm laser beam. The Raman cell is composed of two concave mirrors (Hm1 and hm2), each with a focal length of 100 mm and a diameter of 2 inches. By adjusting the distance between the mirrors and the laser emission angle, the number of times the light beam passes through the Herriott gas absorption cell can be changed.
This system makes 532nm continuous laser beam (200MW) with relatively low power pass through the central area between the mirrors for multiple reflections, thereby enhancing the effective path length of the laser beam and increasing the gas density through the high-pressure chamber. The combined effect of multiple passes and high operating pressure leads to multiple enhancement of Raman signal, and then the volume concentration of the gas to be measured is obtained by cooling the signal measured by a charge coupled device (CCD) grating spectrometer (30s exposure). Finally, a comparison was made with the measurement based on GC (gas chromatograph), and it was found that the measurement reported by the Raman analyzer system was consistent with the GC measurement.
Figure 7. Measurement results of natural gas mixture analyzed by Raman analyzer system
(a) Typical Raman spectra of natural gas (b) extraction of methane from different natural gas mixtures for repeat spectral scanning (c) extraction of ethane from different natural gas mixtures for repeat
Spectral scanning (d) repeated spectral scanning of propane extracted from different natural gas mixtures
Based on the above principles, in view of the low density of the gas to be measured and the small Raman signal of the gas, the self-control laser Raman spectrum gas analyzer lrga-6000 (a national major scientific instrument and equipment development project) can realize the technological innovation and upgrading of low-density Low Raman signal natural gas gas
Accurate measurement of composition and calorific value.
Figure 8. Laser Raman spectrum gas analyzer lrga-6000
In addition, for the problem of gas interference, lrga-6000 combines lasers such as automatic deduction based on AR substrate and automatic correction based on calibration gas interference
Raman's unique software algorithm eliminates the influence of ambient temperature, pressure and interfering gas on the measured gas.
Fig. 9. Application of laser Raman spectrum gas analyzer lrga-6000 in natural gas monitoring field: calorific value is one of the most important technical indicators of commercial natural gas, and its accuracy has a great impact on the whole natural gas industry chain. Accurate measurement of calorific value of natural gas has great economic value in the world. At present, the mainstream calorific value analysis methods of natural gas components at home and abroad include gas chromatography (GC), non spectroscopic infrared (NDIR) and laser Raman spectroscopy. Among them, the non spectroscopic infrared NDIR method and the laser Raman spectrum natural gas analysis method compare the gas chromatography GC technology, which does not need carrier gas and consumables, has fast response speed and simple operation, while the use of GC requires carrier gas and chromatographic column, with long response time and complex operation. Therefore, the non spectroscopic infrared NDIR method and the laser Raman spectrum natural gas analysis method can be used as the preferred methods for calorific value measurement of natural gas.