Research On Key Technology Of Gas Detection Based On TDLAS Technology

Tunable diode laser absorption spectroscopy（TDLAS）technology has the advantages of high sensitivity,high precision,fast response,non-invasive,etc.It is suitable for trace gas detection and combustion process diagnosis and other fields.It has been widely used in air pollution gas monitoring,combustion diagnosis of coal-fired boiler and aerospace engine.However,the measured light intensity signal will be distorted due to the influence of strong vibration,strong turbulence and other factors in the complex environment such as aerospace engine and coal-fired power plant.Thus,the measured light intensity signal can not be used to extract useful information of gas concentration.Therefore,it is very important to improve the accuracy of gas parameter measurement.In this paper,the key technology of gas detection based on TDLAS technology is studied.In the complex and harsh environment,strong vibration,strong turbulence,particle interference and other factors will distort the light intensity signal,resulting in the decrease of gas measurement accuracy.The research work in this paper is as follows:1.A gas concentration measurement error analysis model is established to analyze the influence of interference signal.This model can be used to improve the measurement performance of measurement system by selecting the optimal gas parameters.First of all,the TDLAS light intensity signal is interfered by simulated single frequency interference signals with different amplitude and different frequency,which can calculate the concentration results by different interference.Then,the relationship between the amplitude and frequency of interference signal and the concentration error is discovered though analyzing errors of methane concentration.At the same time,the approximate expression between the amplitude and frequency of interference signal and the concentration error is established.Finally,the validity of the model is verified by comparing the experimental methane concentration error with the methane concentration error calculated by the model though collecting the light intensity signal under a single frequency interference under a methane concentration experimental verification system.2.Aiming at the problem that the intensity signal is easily affected by high frequency interference in complex environment,a technology of frequency multiplexing by high-frequency reference signals is proposed.The interference signal can be extracted by digital phase locking and low-pass filtering of the detected light intensity signal at the reference high-frequency.Then,the interference signal can be extracted from the detected light intensity signal.The transmitted light intensity is modified,which can improve the accuracy of gas concentration measurement under strong interference.Firstly,the interference phenomenon is simulated by coupling the interference signal of single frequency and different frequency range with the light intensity signal.Then,the feasibility of the method is verified theoretically by analyzing the calculation results of methane concentration.Finally,a DFB laser with a central wave number of 6046.95 cm^-1（wavelength of 1653.72 nm）is used to verify the feasibility and effectiveness of the method.3.The methane concentration measurement system is optimized to solve the problem that strong interference has great influence on accuracy and precision of gas concentration measurement in the complex measurement environment.This system is combined with software system and hardware modules,including temperature and current control module,gas detection module,signal generation and acquisition module and signal processing module.At the same time,a Lab VIEW software system for gas concentration measurement is developed,which is based on frequency multiplexing by high-frequency reference signals.In addition,the interference phenomenon in the measurement field is simulated by controlling different gas flow rates into gas cell,rising from 20 m L/min to 200 m L/min in an instant.The reliability of hardware and software system is tested.The accuracy of methane concentration is checked.