Research On Deep-sea Dissolved CO₂ Concentration And Carbon Isotope Measurement Based On Mid-infrared TDLAS Technology

With the development of science and technology,mankind has gradually unveiled the mysterious veil of the ocean.The huge resources in the deep sea make it the last resource treasure house and strategic resource base of mankind.Natural gas hydrate is commonly known as combustible ice,which is clean and pollution-free,and is a kind of green and clean energy.The survey found that there are a large amount of natural gas hydrate resources in the near seabed surface in the northern part of the South China Sea.If they are reasonably exploited and applied,they will definitely be able to reduce the consumption of traditional fossil energy and alleviate the current energy crisis.At present,China has made some progress in the exploration of natural gas hydrate in the South China Sea,which has reached the stage of intensive investigation and detailed investigation.The detection of gas dissolved in the sea water is one of the most effective exploration methods.Therefore,it is an urgent task to develop a high-precision deep-sea in-situ gas detection system suitable for marine environment and realize the accurate positioning of natural gas hydrate.At present,leading research institutions abroad have developed a variety of commercial instruments that can effectively obtain dissolved gas concentration or isotope abundance information in water.However,the distribution of natural gas hydrates near the seafloor is irregular and discontinuous.therefore,underwater detectors need to have low detection limits and wide-range detection capabilities.However,the research in my country started relatively late,and the existing research results are basically prototypes that can only be used in the laboratory,and it is currently not possible to work stably underwater for a long time.Therefore,in order to meet the actual application requirements,this article starts from the research of mid-infrared sensing technology and develops a deep-sea mid-infrared CO₂ detection device suitable for marine working environment,thereby realizing high-precision detection of dissolved CO₂ gas in seawater.(1)Based on the developed mid-infrared CO₂ sensing system,the joint multiplexing technology of multi absorption spectral lines is proposed.In the mid-infrared region near 4319nm,CO₂ absorption spectrum clusters are ladder like distribution,and different absorption intensities represent different detection ranges.In view of this feature,three ¹²CO₂ absorption lines(located at 2315.10cm^-1,2315.19cm^-1and 2315.36cm^-1)in the limit range that a single ICL laser can scan are studied.Firstly,the relationship between CO₂ detection performance and temperature and pressure is obtained through simulation and theoretical analysis.The optimal detection background temperature of 25? and the optimal background pressure of 20 Torr are determined.In addition,based on the TDLAS-WMS technology,the distribution characteristics of the second harmonic corresponding to the absorption spectrum line are analyzed.Starting from the research of the modulation depth of the absorption spectrum line and considering the possible overlapping interference between the second harmonic,the actual optimal modulation depth of the three absorption lines is obtained through theoretical analysis and experimental results.By using the 2f/1f detection technology,the interference of light source fluctuation is reduced,and the detection accuracy and stability of the sensing system are improved.For the CO₂ absorption line with the highest sensitivity at 2315.19cm^-1,the detection limit of the system reaches the optimal 11 ppbv when the integration time is 145s.In addition,the detection range of three absorption spectral lines is determined according to the CO₂ concentration calibration experiment.In view of the overlapping detection range of the three absorption spectral lines,the joint multiplexing technology of multi spectral lines is studied based on the concentration threshold setting method,which realizes the function of automatic conversion of different ranges and the wide dynamic range detection of CO₂ concentration from 11 ppbv to 1000 ppmv.(2)A high-precision temperature control system was developed,and the carbon isotope abundance of CO₂ was detected with high accuracy and precision.Based on the theoretical analysis and spectral simulation results,it is found that the detection stability of isotope abundance is proportional to the ground state energy level difference between the absorption lines and the ambient temperature fluctuation.Therefore,in order to reduce the environmental temperature fluctuation,based on the integral separation PID control method combined with flexible PCB film,the high-precision temperature control system is studied.After Ziegier-Nichols engineering setting method,the three parameters P,I and D are determined.Finally,the characteristics of no overshoot and fast response time are realized,and the temperature fluctuation can be stable within 100mk.The ¹²CO₂ and ¹³CO₂ absorption lines(ground state energy level difference?E=4.76×10-3cm^-1)located at 2315.10cm^-1 and 2315.36cm^-1were selected.Based on the developed temperature control system and TDLAS-WMS-2f/1f detection technology,the detection limit of the system reached the optimal 0.083‰when the integration time was 121s.(3)A"linear"optical structure based on free space light transmission is proposed to solve the problem of complex and poor stability of traditional mid-infrared optical structure.In this paper,from the perspective of realizing the"linear"optical path transmission mode,the traditional mid-infrared"foldable"optical structure is improved.Only the optical core devices in the optical structure are kept and arranged in a"linear"arrangement,so as to ensure the laser source to transmit in the most simple and efficient way.In order to meet the requirements of optical alignment,the optical alignment scheme is designed based on the reserved optical adjustment space,and the feasibility of the scheme is verified by experiments.In addition,the optical system based on the"linear"optical structure is integrated with the circuit module and gas path components to study the optimal arrangement of the modules,reduce the crosstalk between each other,and design and process the outer cavity structure,and finally develop a set of mid-infrared CO₂ sensing system prototype.(4)In view of the unstable detection environment in the deep sea,a pulse type gas distribution scheme is proposed.The detection process is divided into four stages,i.e."purge","exhaust","intake"and"detection",and the working modes of different stages are controlled based on the joint use of two sets of solenoid valves and proportional valves.In order to study the optimal working state of the pulse valve train scheme,the overall pulse valve characteristics are analyzed in detail,and the specific implementation scheme is designed based on the experimental results.Finally,the developed mid-infrared CO₂ sensing system is integrated with the gas-liquid separation device to form a set of deep-sea mid-infrared CO₂ detection device.5cm thick titanium alloy is used to machine the outer cavity of the machine,and the structure of the outer cavity is designed as a barrel,which can be uniformly stressed under the sea.Through the simulation experiment,it is verified that the detection device has strong corrosion resistance and can withstand the hydraulic pressure of more than 20MPa.In November 2019,the deep-sea mid-infrared CO₂ detection device developed by the Haiyang No.4 scientific research ship carried out a sea trial test in Shenhu sea area of South China Sea,and finally succeeded in obtaining the data of dissolved CO₂gas concentration and isotope abundance.The measured vertical CO₂ concentration ranges from 739 ppmv to 239 ppmv,and the isotope abundance ranges from 1.8‰to6.9‰.This is the first time in the world that mid-infrared spectral sensing technology has been used to detect the concentration and carbon isotope abundance of dissolved CO₂ in the deep sea,and it is also a major breakthrough made by China in the field of deep-sea geochemical equipment.