Research On Two-dimensional Reconstruction Of Temperature And Gas Concentration Based On Tunable Diode Laser Absorption Spectroscopy

With the continuous development of China’s national economy and society,the demand for energy increases every year.Coal is an important part of China’s energy consumption structure,and coal combustion power generation is one of the main sources of electricity in China.Combustion is a very complicated process,which is the main way of transforming from traditional primary energy to secondary energy.The two-dimensional distribution reconstruction for parameters such as temperature,gas concentration in the combustion field could not only reveal the generation mechanism of the relevant pollutants to achieve pollution source control,but also transfer flow field information to feedback signal to adjust the combustion conditions and then improve the efficiency of energy transformation,realize the efficient utilization of fuel,ensure the safe operation of the equipment.In addition,selective catalytic reduction is the most widely used flue gas denitrification technology in large coal-fired power plants,and the uniformity of the mixture of reducing agent ammonia and flue gas is one of the key parameters that affects the removal efficiency of nitrogen oxides in flue gas.Tunable diode laser absorption spectroscopy,as a non-intrusive laser measuring method,combined with computer tomography is very suitable for the measurements of two-dimensional spatial distribution of flow fields such as combustion field and gas concentration field involved in coal-fired power generation.In this paper,the algorithm and beam distribution of tunable diode laser absorption tomography for two-dimensional distribution reconstruction were studied.Moreover,this technology was applied to the measurements of combustion field and ammonia concentration field,which provided the theoretical and experimental foundation for the practical application of coal combustion field.Firstly,this paper introduced the basic theories of molecular absorption spectroscopy.Some important parameters of tunable diode laser absorption spectroscopy,various absorption lines shapes and lines broadening mechanisms were described in detail.Besides,the principle of measuring the mean value of parameters by direct absorption spectroscopy and two-dimensional measurements by combining direct absorption spectroscopy with algebraic reconstruction technique were introduced in detail.For the combustion field,the suitable absorption lines were selected for simulaneous two-dimensional reconstruction of temperature and H₂O concentration.Then,after being obtained the temperature tomography through adaptive algebraic reconstruction technique,the direct gas concentration reconstruction technique,algebraic gas concentration reconstruction technique and adaptive algebraic gas concentration reconstruction technique for gas concentration tomography were compared by numerical simulations.The results demonstrated that adaptive algebraic gas concentration method had better robustness for different phantoms and stability in the case that measured projections contained different levels of noise.Finally,Experimental measurements in a flat methane/air flame on a Mc Kenna burner under three different equivalence ratios were conducted for simultaneous temperature and H₂O concentration tomographic imaging using adaptive algebraic gas concentration reconstruction technique.The improved projection coefficient matrix algorithm was constructed,which could get the projection coefficient matrix directly.The average calculation time was only 0.0288s.This algorithm was then applied to calculate the projection coefficient matrix composed of fan-shaped beams,and the reconstruction results of different fan-shaped beams were compared.For 8×8 grids,the improvement of reconstruction quality by increasing the number of projected beams was not obvious when the number of projected beams exceeded 70.At the same time,the reconstruction results after introducing the virtual beams into the parallel beams were simulated,and the results showed that the total reconstruction error was slightly higher than that of the real beams projection error.Finally,the concepts of orthogonal correlation degree and orthogonal correlation degree vector were proposed,and the influences of beams on the whole reconstruction areas and reconstruction results were analyzed.The photoelectric detection system with high integration degree and robust signal amplification effect was developed with the spatial resolution reaching 35mm×35mm.The photoelectric detection system is mainly composed of germanium diode without bias and signal amplifier system including transimpedance amplifiers and secondary amplifiers.The signal amplifier system could expand flexibly according to the measurement needs,and it only needed an external power supply,which was suitable for practical measurements.The reliability of the photoelectric detection system was verified by the one-dimensional average temperature in a high temperature tube furnace.At the same time,a two-dimensional system with point-to-point arrangement of parallel beams was built in the space area of 385mm×385mm,and experimental researches on two-dimensional tomography for temperature and H₂O concentration were carried out.The Levenberg-Marquardt iterative method was used for on-line fitting of absorption lines,which could guarantee the fast convergence rate of results.Online calculations of absorbances were realized through original data processing,polynomial fitting,calculation of-ln?(/₀)value,time-frequency conversion,absorption lines fitting,calculation of integral area of absorbance curve,etc.In view of the overlapping phenomenon of NH₃ absorption lines near 6612.7cm^-1,the absorbances were corrected by standard gas measurement experiments.Finally,the influence of different noise conditions on the two-dimensional reconstruction of NH₃ were simulated numerically,and the reconstruction experiments of the two-dimensional NH₃ concentration tomography were carried out.