Investigations On Measurement Of Low Concentration And Multi-component Gas And Reconstruction Of 2D Temperature And Concentration Distribution Using Laser Absorption Spectroscopy

In-situ sensors based on absorption spectroscopy are widely used for combustion diagnosis in combustion field and monitoring species concentration and temperature in environmental area,because of the non-intrusive,anti-interference ability,fast and other characteristics.The on-line monitoring of 2D distributions of temperature and gases concentration can ensure the safe operation and improve the combustion efficiency,so as to achieve the objective of energy saving and emission reduction.The main purpose of this paper is to measure and analysis the concentration of various gases in the field of energy combustion and pyrolysis,at the same time study the iterative algorithm and analysis the experimental measurement for 2D distributions of temperature and gas concentration.The main contents and conclusions are as follows:Firstly,the absorption spectrum theory and the important parameters were expounded in details,the fundamental principle,usable range,advantages and disadvantages of different measurement techniques were also introduced,such as direct absorption technique and wavelength modulation technique.Secondly,to measure lower gas concentration and trace gas concentration,two main methods can be used:improving the absorption intensity and reducing the system noise in the measurement system.The absorption intensity can be greatly improved by choosing more strong absorption lines,increasing the optical path and using wavelength modulation spectroscopy.The system noise can be effectively reduced by adopting suitable noise reduction technology.To further reduce the measurement lower limit of gas concentration,a group of NH3 transitions near 2.25 μm though theoretic analysis and calculation were selected to measure lower NH3 concentration in the experiment.The measurement lower limits of NH3 transitions near 2.25μm at atmospheric pressure and different temperature were determined using absorption spectroscopy techniqures,the wavelet transform and cross-correlation technique were used to reduce the noise and control signal drift in the measurement process,then the measurement lower limit was decreased to be ppb range,so as to satisfy the requirement of the power plant atmospheric trace gases（lower than lppm）.In order to solve the disadvantage of the laser can only measuring one gas and the system complexity,the optical switch technology was used combing with DA and WMS to realize multicomponent concentration measurement.The transitions near 1.65μm and 1.579μm were selected to measure CH4,CO and CO2 concentration by using two diode lasers based on photo switch,the concentration of CO、CO2 and CH4 are measured in the process of biomass pyrolysis,and the effect of pyrolysis temperature and gas flow rate of carrier gas on pyrolysis products was also analyzed.Several common kinds of iterative algorithm were summarized and analyzed in the combustion reconstruction.The principle theory of 2D temperature and concentration is introduced.Four different common iterative algorithms,including algorithm reconstruction technique（ART）,simultaneous iteration reconstruction algorithm（SIRT）,multiplicative algebraic reconstruction technique（MART）and adaptive algebraic reconstruction technique（AART）,were used to calculate the 2D temperature and concentration with different relaxation parameter,the reconstruction results,included reconstruction errors,correlations and iteration time,were compared with each other with the best relaxation parameter.And the AART algorithm was considered to be more suitable solving the complex reconstruction problem in flames than the other three algorithms,considering both reconstruction error and computing time,the reconstructed results showed that AART algorithm had a relatively stability when the environmental noise is small,but the anti-interference ability is poor when the noise level is relatively high.The AART algorithm was used to reconstruct the 2D H₂O concentration and temperature distributions for non-uniform combustion flame based on TDLAS,before the experimental measurement,numerical simulation was used to calculate the effect of different laser beams on the 2D distributions of temperature and concentration,Tikhonov algorithm was used to directly calculate the concentration distributions after getting the 2D temperature distributions,and much better results can be obtained compared with traditional algorithm.In the experimental measurement,the LabVIEW program was used to realize the in-situ measurement,on-line calculation for the multi-channel absorption signals,the McKenna burner was used to reconstruct the 2D temperature and H₂O concentration with different equivalence ratio through moving and rotating the combustion apparatus,the reconstructed results of temperature were compared with the results measured by thermocouple,because of the size of lasers and detectors,this measurement can greatly improve the space utilization and spatial resolution of the system,comparing with the finite beam layout.Considering the smoothness of the temperature and concentration,the spline interpolation technique was used to further refine the mesh of 2D temperature and H₂O concentration.