Why is energy measurement essential to the natural gas trade?

Natural gas measurement, as an important basis of natural gas trade and calculation, reflects the fairness and justice of both sides of the transaction, and has attracted wide attention.

As a kind of clean energy, natural gas accounts for an increasing proportion of the world's energy consumption structure. With the continuous expansion of natural gas trade, the requirements for natural gas metering methods are also increasing. Natural gas measurement, as an important basis of natural gas trade and calculation, reflects the fairness and justice of both sides of the transaction, and has attracted wide attention. Natural gas metering includes volume metering, mass metering and energy metering. At present, international natural gas trade and Europe & the United States and other developed countries mostly use energy metering methods. And in the 1990s, a relatively complete natural gas trade measurement laws and regulations, standards and detection methods was established.

Different gas sources have different calorific values

Natural gas is a kind of mixed gas, the composition of natural gas produced by different natural gas sources is different. The main use of natural gas is as fuel. Different components will result in different calorific values.

The necessity of natural gas energy measurement

If a gas company sells the lower calorific value natural gas at the price of the higher calorific value natural gas, it may gain more profits as a result, which is very unfair to the end users. If a gas company sells high calorific value natural gas at the price of low calorific value natural gas , high calorific value natural gas cannot be sold at high price, which is unfair to the gas company. Therefore, necessary measurement and detection of natural gas should be carried out.

At present, natural gas metering mainly includes volume metering, mass metering and energy metering. International natural gas trade and developed countries such as Europe and the United States mostly adopt the energy measurement method, while China still adopts the volume measurement method. The measurement instruments used include orifice flow meter, turbine flow meter, ultrasonic flow meter, roots flow meter, VSF(vortex street flow meter) , HLUX etc.

However, natural gas, as an energy source for combustion, its value lies in the heat it provides. However, natural gas is a kind of multi-component mixed gas. Due to the different origin sources, the components and contents are also different, which makes the same volume and mass of natural gas from different sources generate different energy from combustion. Therefore, from the perspective of scientific fair metering, energy metering is more reasonable than volume metering and quality metering in natural gas metering, which is conducive to accurate metering, fairness, reduction of settlement disputes and healthy development of natural gas industry.

Two common energy measurement techniques for natural gas

The calorific value analysis of gas components is based on the principle of the contribution of each component to the calorific value of the mixed gas.The calorific value can be calculated by weighting the calorific value of gas components with different mole fractions and their corresponding components.The molar calorific value of gas mixture can be calculated by this principle.The volume calorific value of mixed gas needs to consider the P-V-T conversion under real working conditions. At present, gas chromatography GC method and non-spectroscopic infrared NDIR method are widely used in natural gas analyzer in the domestic and overseas.

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1. Calorific value analysis by gas chromatograph GC

GC consists of gas path system, sampling system, chromatographic column, electrical system, detection system, recorder or data processing system.Its working principle is: the mixed gas under test first by inert gas (namely the carrier gas, usually N2 and H2, He, etc.) to the chromatographic column, the column contains a liquid or a solid stationary phase, due to the sample of each component in the boiling point, polarity or adsorption performance is different, each of these components are inclined to in allocation between the mobile phase and stationary phase or adsorption equilibrium.However, since the carrier gas is flowing, such a balance is actually difficult to establish. It is also because of the carrier gas flow that the sample components are repeatedly distributed or adsorbed/desorbed during the movement. As a result, the components with high concentration distributed in the carrier gas first flow out of the chromatographic column, and the components with high concentration distributed in the stationary phase flow out.When the components flow out of the column, they enter the detector immediately. The detector can convert the presence or absence of sample components into electrical signals. The size of electrical signals is proportional to the amount or concentration of the components under test.In the absence of component outflow, the chromatogram record is the background signal of the detector, which is the baseline of the chromatogram.

Thermal conductivity detector (TCD) is usually used for gas chromatographic analysis of fuel gas. Since pure gases have different thermal conductivity, when they flow through the detector they cause a change in resistance, and this signal can be recorded and formed into a chromatogram.

Gas chromatograph requires more for the person to operate the instrument, and requires carrier gas. The operation is complicated. In practical applications, it is necessary to ensure that the hot wire does not burn out. Before the detector is energized, be sure to ensure that the carrier gas has passed through the detector, otherwise, the hot wire may be burned out, resulting in the detector scrapped;Shut down must first turn off the detector power, and then turn off the carrier gas.Turn off the detector power at any time for any operation that may cut off the carrier gas flow through TCD.In addition, when the carrier gas contains oxygen, the life of the hot wire will be shortened, so the carrier gas must be completely de-oxygenated; When hydrogen is used as carrier gas, the gas should be discharged outdoors.

The gas chromatograph can be calibrated using one or more calibration gases to calculate the calibration coefficient.These coefficients can be used to evaluate the molar fraction of each individual component.The advantage of this method is that physical quantities other than the calorific value, such as the standard density, can be calculated.

2.Non-spectroscopic infrared NDIR calorific value analysis

NDIR infrared analysis is generally composed of electrically modulated infrared light source, high-sensitivity filter, micro-infrared sensor and local thermostatic control circuit. It works on the basis that the absorption of infrared light by polar gas molecules conforms to Lambert-Beer law. For mixed gases, in order to analyze specific components, a narrow band filter suitable for analyzing the absorption wavelength of the gas is installed in front of the sensor or infrared light source, so that the signal change of the sensor only reflects the change of the concentration of the gas under test.

Take CH4 analysis as an example, the infrared light sources emit 1-20 microns microns, through a certain length of gas chamber absorption, through a narrow band filter wavelength 3.33 microns, infrared sensors monitor the intensity of infrared light through 3.33 microns microns, to reflect the concentration of CH4 gas.

Generally, there are two filters on the infrared detector. One filter does not attenuate the infrared light signal as a reference channel, and the other filter absorbs the infrared signal band with the largest absorption as a measurement channel signal. After the comparison, they participate in the data calculation, so as to eliminate the drift caused by the change of light source signal to the greatest extent. The structure of the detector is a single light source with double beam, which adopts semiconductor technology. It is characterized by less mutual interference between different gases, high measurement accuracy, and multi-component measurement can be realized by increasing the number of channels of the detector.

In the future, energy metering of natural gas will be widely used in China's natural gas market, which is conducive to docking with international management of natural gas trade. It is conducive to the fair opening of natural gas infrastructure and reflects the fairness and impartiality of gas prices from different sources.We will promote the market-oriented reform of natural gas in China.At the same time, it will also promote the healthy development of the natural gas industry.