Research On Gas Detection And 2D Distribution Reconstruction By Tunable Diode Laser Absorption Spectroscopy Technique

As much more attention has been paid on the environment protection and demand to improve the safety and efficiency of industry, the optical techniques for gas sensing are developing very rapidly. Tunable diode laser absorption spectroscopy (TDLAS) is the one of the representative measurement method, with features of fast response, high precision, and simultaneous measurement of multi-parameter. The purpose of this paper is to develop the method of on-line remote sensing for the gases in the flue, and do research on the gas temperature and distribution reconstruction with computed tomography (CT) for further investigation in combustion.NH₃ sensing is carried out with tunable diode laser working at the wavelength of 1.53nm for detection in the process of decreasing NOx by SCR in the flue gas. The concentration can be obtained accurately by scanning the spectral line at 1527nm, with minimum detectable concentration of about 10 ppm-m at normal temperature and pressure. By injecting the gaseous NH₃ into the flue gas after combustion, the real time and on line measurement is completed at the small-scale experimental setup.The VCSEL diode laser at 1580 nm is utilized to measure the gas concentration of CO₂ and CO in the range of over-tone by direct absorption and wavelength modulation strategy separately, yielding to a minimum detectable concentration of 0.14%-m and 400ppm-m. The effect of line width variety on the second-harmonic signal casued by pressure or temperature is discussed, and corresponding correction method by introducing the ratio of peak to valley from the second-harmonic signal is accepted to improve the wavelength modulation measurement. The apparatus designed for convenient installation and continuous running is applied to the oil-burning boiler for CO₂ detection continuously.Based on the high-resolution transmission molecular absorption database, two spectral lines of CO₂ near 1.58μm are selected, and the ratio of the linestrengths is utilized to infer the uniform temperature in the optical path. The experiment is carried out during the range from 373K to 773K by direct absorption method, leading to the mean square error of 17K. The light-of-sight gas temperature distribution is also studied to obtain the temperature distribution at non-uniform path. Discretization model is established for the tempmerature reconstruction, and solved by constrained linear least-square fitting method. The simulation and experiement are completed for the two-temperature distribution, and the relative factors are discussed here.For the purpose of on-line monitoring of O₂ in the coke oven gas to ensure the industry safety, the diode laser working at 763 nm is utilized in the lab to detect the O₂ concentration and validate the precision in the conditions of different gas components, pressure and temperature.Theory analysis about the effect of particles in the gas on the second-harmonic signal is presented, together with the experiment on gas detection in the condition of mixture by gas component and quartz sands with different size. Combined with the extinction methods based on Mie theory, the particles concentration can be also obtained in the TDLAS system. The simultaneous measurement of gas and particle concentration is proved in the small-scale system with gas-solid two-phase flow.A multi-source optical computed tomography system for gas distribution based on the tunable diode laser absorption spectroscopy technique is established here, with tomography scanning time about 100ms to cover the region of interest by projections from four fast rotation platforms. The 2D reconstruction image of NH3 concentration distribution is calculated by algebraic reconstruction technique (ART) method using the projections from gas absorption. The pilot research on simultaneous reconstruction of both concentration and temperature distribution with a pair of spectral lines is continued with optimization strategy of cyclic variable method. The research can be useful for the investigation of chemical distribution in the combustion further.