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What are the developments of non-methane total hydrocarbons in monitoring technology?

 Last Update: 2022-02-04
 Source: Internet
 Author: User

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[ Focus on Chemical Machinery and Equipment Network ] Atmospheric pollution control has always been the focus of China's environmental protection work.
Among them, PM2.
5 and ozone pollution are undoubtedly the focus of attention.
In recent years, my country is gradually increasing the control of air pollution, and non-methane total hydrocarbons (NMHC), as an important precursor of PM2.
5 and ozone, has also received more and more attention, and has begun to become an important content of environmental monitoring and evaluation.

Chemical machinery and equipment network hotspots pay attention to chemical machinery and equipment

On the afternoon of December 9, 2020, at the special report meeting of the "Thirteenth China Online Analytical Instrument Application and Development Forum and Exhibition", Yang Yong, an expert from the Shanghai Environmental Monitoring Center, brought "Ambient Air Non-Methane The special report on "Research and Progress in Total Hydrocarbon Continuous Monitoring Technology" serves as the opening of the "Special Discussion on Atmospheric Online Monitoring" and introduced the basic situation and development of China's non-methane total hydrocarbon continuous monitoring technology at this stage.

Analytical Instruments

Non-methane total hydrocarbons (NMHC) is a general term for a class of volatile organic compounds.
my country's definition of non-methane total hydrocarbons is continuously developed in accordance with changes in environmental standards, but it can generally be considered to refer to all volatile carbons except methane.
Hydrogen compounds and their derivatives (mainly C2-C8).

Non-methane total hydrocarbons have strong toxicity and are harmful to the environment and human health.
Therefore, they have become one of the basic indicators of urban environmental monitoring at prefecture-level and above across the country.
At the same time, my country also encourages qualified cities to carry out automatic monitoring of non-methane total hydrocarbons.
At present, more than 70 cities in my country have carried out the automatic monitoring of non-methane total hydrocarbons.
However, due to the complex sources of non-methane total hydrocarbons, large emission differences, difficulty in pollution control, and the lack of relevant methods and technical standards, non-methane total hydrocarbons There are still many difficulties in automatic monitoring.

　　The automatic monitoring methods of non-methane total hydrocarbons in ambient air mainly include gas chromatography-hydrogen flame ionization detection method, catalytic oxidation-hydrogen flame ionization detection method and gas chromatography-photoionization detection method.
Among them, more than 90% of the monitoring methods on the market use gas chromatography-hydrogen flame ionization detection method.
This method can be divided into direct method and indirect method.
The direct method uses a chromatographic column to separate methane and non-methane total hydrocarbons, and then uses FID to directly determine the two.
The indirect method measures the amount of total hydrocarbons and methane, and then calculates the amount of non-methane total hydrocarbons by subtracting the amount of methane from the total hydrocarbons.
After HJ604 was formulated, the indirect method equipment gradually increased, and it has occupied about 80% of the market.

　　However, in low-concentration measurement, the accuracy and repeatability of the indirect method have large deviations.
First of all, the indirect method is obtained by subtracting the total hydrocarbon content and the methane content.
Due to the principle of "large number minus large number", calculation problems sometimes occur.
Secondly, when the chromatographic column separates the air, oxygen and methane are separated.
After the oxygen molecules enter the detector, the original balance of the FID flame will be destroyed and interference peaks will be generated.
In addition, humidity will also affect the test results.
For example, an increase in humidity will increase the nitrogen blank of the indirect method instrument, affect the test results of the calibration gas, and moisture will change the efficiency of the column and the peak shape of other peaks.

　　In contrast, the direct method has better accuracy and precision when measuring low concentrations.
The detection limit of the direct method is low, and it is usually not interfered by the oxygen peak.
Regarding the influence of humidity, the humidification blank of most direct method instruments is basically 0, which can be ignored.
Humidity also has little effect on the calibration gas test results of direct method instruments.
Therefore, although the current market share of direct method instruments is small, in the automatic monitoring of ambient air non-methane total hydrocarbons, as the monitoring requirements become higher and higher, direct method instruments have a larger application space.

　　Original title: What are the developments of non-methane total hydrocarbons in monitoring technology?

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