Development Of CH₄ Gas Sensing System With Micro Chamber

Gas hydrate,also known as combustible ice,is an environmentally friendly energy source with the advantages of green and pollution-free.As the global energy crisis worsens,the focus of energy research is shifting from land to sea.The reserves of combustible ice are very large.If Marine combustible ice can be properly developed and utilized,the demand for traditional energy will be reduced and the environmental problems caused by traditional energy will be solved.However,since most gas hydrates are stored in harsh environments such as low temperature and high pressure near the seabed surface,it is important to achieve accurate location of gas hydrates in water to avoid unnecessary consumption.In order to exploit combustible ice,it is first necessary to locate its mineral points.The positioning method is to detect methane dissolved in seawater,because the main component of combustible ice is methane.According to the detection and concentration analysis of methane,the location of combustible ice mineral points can be realized.Therefore,it is very important and urgent to develop a high-precision CH₄gas detection system suitable for Marine environment,which is an important means to detect and exploit combustible ice.At present,leading research institutions abroad are developing various commercial equipment to effectively obtain information about CH₄concentration dissolved in water.However,due to the irregular distribution of gas hydrate in seawater,the detection limit of underwater detection equipment must be low.Therefore,in order to meet the requirements of practical application,this paper starts with the research of Tunable Diode Laser Absorption Spectroscop（TDLAS）technology,and develops a CH₄detection device based on micro chamber（MPGC）,which is suitable for Marine working environment and can achieve high precision for the detection of CH₄gas concentration in seawater.The important research contents of this paper are as follows:（1）Air chamber is the core of TDLAS system.The design of air chamber is the key to improve the system detection performance.The DE optimization algorithm is used for intelligent optimization design of air chamber.The principle of differential evolution（DE）algorithm is introduced,and the fitness function of DE algorithm is constructed.Based on DE algorithm,the DE process is compiled by python software,and the optimal parameters of the designed air chamber are obtained by intelligent optimization algorithm.Then the physical structure of the gas chamber is introduced,the laser spot is simulated,and the light spot results of the gas chamber designed by DE algorithm are compared with those of the commonly used seven ring gas chamber.The results show that the air chamber design based on DE algorithm has more compact structure and better performance.In addition,the effective optical path of the gas chamber is verified.The volume of the gas chamber designed based on DE algorithm is small enough to reduce the volume requirement of the gas cell and meet the real-time detection requirements of the system.In addition,the long optical path of the gas chamber can reduce the detection limit of the system.It can be seen that the small volume and long optical path gas chamber designed based on DE algorithm can meet the practical application requirements of the system,and has a good application prospect.（2）The research and development process of seawater CH₄detection system is introduced,and related design is analyzed in detail.According to the different functions realized,the system is divided into three parts:optical circuit,circuit and gas circuit.The optimization design of optical path structure and the research of valve distribution method are the key to realize the stable and accurate detection of the system.Aiming at the optimal design of optical path structure,a compact optical structure is designed,which not only improves the transmission efficiency of optical path,but also enhances the stability of the system structure,which lays a foundation for realizing the accurate detection of the system.The circuit part introduces the composition of electrical system and the design of temperature control circuit.The gas-liquid separation technology is first introduced in the gas-path part to separate slightly water-soluble gas from water,and then the detection is carried out to avoid errors caused by impurities in seawater,improve the accuracy of detection results of dissolved CH₄gas in seawater,and achieve accurate detection of CH₄gas concentration in seawater.Subsequently,in order to study the gas distribution method suitable for the system,a pulse type gas distribution method was proposed to solve the problem of unstable pressure caused by unstable degasser efficiency.Combined with continuous air intake scheme,it was used to cope with the field seawater development environment.Finally,the CH₄sensing system is integrated to form a seawater CH₄detection system with underwater detection capability.（3）Verify the performance of the developed CH₄sensor system.Firstly,CH₄absorption lines were selected to optimize the parameters.Then,the dynamic performance verification and detection limit analysis verification of the system are carried out.The verification results show that the system has different responses to different concentrations of CH₄,which confirms that the designed CH₄sensor system has good dynamic response and practical application ability.The CH₄sensor system based on the micro-chamber design was used to measure the CH₄concentration under the background of 1000ppmv CH₄for a long time.The maximum CH₄sensitivity was0.76ppmv and the average time was 82s,which confirmed the stability of the designed CH₄sensor system.（4）An overview of the Marine trials and integration of the final CH₄sensing system for seawater detection is presented.Based on the CH₄sensing system applicable to seawater environment,the concentration of dissolved CH₄at fixed vertically distributed monitoring points in seawater was detected,and the detection data was obtained successfully.Based on the temperature control system developed and the selection of CH₄absorption line,the high precision detection of CH₄was researched and realized,and a series of work including detection limit,accuracy,stability test and analysis of test results were completed.