Research On Carbon Emission Detection Technology Based On Cavity Enhanced Absorption Spectroscopy

With the rapid development of the world economy,the global greenhouse effect is increasing,and the concentration of greenhouse gases in the atmosphere continues to increase,resulting in the continuous deterioration of the global environment.Carbon dioxide（CO₂）is an important greenhouse gas.If its content is too high,it will endanger the ecological balance of the earth.Methane（CH₄）,the main component of combustion gases such as natural gas,biogas,and pit gas,is also an important greenhouse gas.When its concentration is too high,it will seriously endanger human health and the quality of the air environment,and its emission standards will become more and more stringent.The generation of a large number of greenhouse gases has led to frequent occurrence of abnormal climate phenomena,making the serious consequences of global climate deterioration more and more perceived by people,and has become the focus of attention of people all over the world.Therefore,high-sensitivity detection of CH₄ and CO₂ gases,controlling greenhouse gas emissions and reducing climate and environmental problems caused by the greenhouse effect are important issues facing the world.At present,there are many methods for detecting trace gases.The earlier developed methods include chemical analysis,electrochemical analysis,and gas chromatography.These methods have relatively low detection sensitivity.The technologies developed in recent years,such as direct absorption spectroscopy,photoacoustic spectroscopy,wavelength modulation and frequency modulation spectroscopy,and Raman spectroscopy,can also measure various gas parameters and are widely used in biomedicine,human respiratory analysis,combustion diagnosis,and environment Monitoring and other aspects,but usually need to be combined with other sensitive detection methods to obtain better detection performance.In order to realize the system’s high-sensitivity detection of gases,increasing the effective absorption optical path is the simplest and direct way,usually cavity enhanced absorption spectroscopy（CEAS）technology can be used.CEAS technology is a technology for intensity domain measurement.Its optical integrating cavity is composed of two lenses with high reflectivity,which can achieve a long absorption path in a limited space,thereby greatly improving the measurement sensitivity of the spectroscopy technology.Therefore,CEAS technology is especially suitable for trace gas sensors with small size,field application and reasonable cost.When the CEAS technology with sealed optical cavity is used for measurement,some reactive gases will be lost to the inner wall of the tube and the sample pool during the extraction sampling process.In order to ensure the accuracy of sample gas concentration,an open path system can be used to avoid.In this paper,a compact open path CEAS system is developed,and the performance of the system is verified by CO₂ detection using a 2.0μm DFB laser as the light source.Firstly,the reflectivity of the cavity mirror is identified through an innovative method,and the effective path of absorption is calculated at 99.8%and 305.53 m.To study the detection sensitivity of the system,a series measurements of CO₂concentration were measured with the spectrum of 5001.49 cm^-1.Additionally,according to the time series measurement of environmental CO₂,the sensitivity of the system is7.819 ppm,and two 24-hour continuous field measurements are carried out at the same time,which further proves the stability of the system.When using the sealed optical cavity CEAS technology for measurement,some reactive gas will be lost to the inner wall of the tube and the sample cell during the extraction and sampling of the sample gas.To ensure the accuracy of the sample gas concentration,an open path system can be used to avoid it.A compact open path CEAS system is developed here,and the 2.0μm DFB laser is used as the light source,and the performance of the system is verified by CO₂ detection.First,a simple and effective method is used to determine the reflectivity of the cavity mirror and the effective absorption path is 99.8%and 305.53 m.In order to confirm the performance of the system,a series of concentration measurements were performed on the CO₂ spectrum at 5001.49 cm^-1.In addition,according to the environmental CO₂ time series measurement,the sensitivity of the system is 7.819 ppm,and two 24-hour on-site continuous measurements have been carried out at the same time,which further proves the stability of the system.In order to further improve the detection limit of the system,the wavelength modulation technology is applied to the CEAS technology,and the CH₄ spectral line at4300.365 cm^-1 is measured and studied with a 2.3μm DFB laser.On the basis of the effective absorption path of 147.15 m,the second harmonic signal of the actual atmospheric CH₄ gas is measured and the detection limit is analyzed,and the minimum detectable concentration of the system for CH₄ is 20 ppb.In order to improve the utilization efficiency of the integrating cavity,a pioneering multi-laser bidirectional detection structure CEAS technology is proposed.This structure can not only apply multiple lasers to the same integrating cavity,but the added mirror can also be used as an enhanced mirror.The signal-to-noise ratio of the system.The performance of the system is demonstrated by the simultaneous measurement of actual atmospheric CH₄ and CO₂ based on 2.0μm and 2.3μm DFB lasers.According to the principle of direct absorption spectroscopy,the absorption paths on both sides（100.46 m and 112.32 m）and the actual atmospheric CH₄ and CO₂ concentrations（3.8 ppm and 370 ppm）are obtained.In addition,this paper cooperates with wavelength modulation technology to detect CH₄ and CO₂ respectively.The limit was raised to 0.078 ppm and 0.966 ppm.Finally,the actual atmospheric CH₄ and CO₂ were measured continuously for 48 hours,and the results obtained were consistent with the actual atmospheric concentration changes.