Study On The Spatio-temporal Evolution Characteristics Of Laser-induced Plasma For Solid Fuel

Solid fuel plays an important role in the energy production and consumption.It is of great significance to realize the rapid online detection of its composition and characteristics for the efficient and low pollution utilization of solid fuel.As a rapid in-situ detection technology that can realize simultaneous multi-element and characteristic detection,Laser Induced Breakdown Spectroscopy（LIBS）has proved its application value in the field of solid fuel detection,and it is still pursuing higher accuracy and practical adaptability in production.However,the matrix effect caused by the difference of physical and chemical properties between solid fuels is still an important problem to be solved in the further development of this technology.Moreover,nonmetallic elements with a variety of chemical speciation in solid fuels and plasma contained organic matter involve complex excitation,reaction and evolution mechanisms.The atmosphere can also interfere with the identification of plasma emitters.Therefore,this paper aims to clarify the mechanism of the interaction between laser,solid fuel and atmosphere and laser-induced plasma evolution through the temporal and spatial evolution characteristics of plasma and spectra.Firstly,the influence of the coalification degree on LIBS measurement for coal was investigated by employing coals with different volatile matter contents as the research object.Experiment was carried out in argon at atmospheric pressure to avoid the interference of elements in the air on the determination of the origin of the spectral emitter.The temporal and spatial evolution of the representative characteristic spectra of coal（C,H,O,N,Ca,CN and C₂）,plasma morphology,temperature and electron number density were obtained.The results showed that the spectral emissions for the species that abundant in volatile matter were intense at the plasma front.The characteristic spectra of C,the main component of coal,and Ca,the main typical ash forming element,were in a lower position with the increase of volatile matter content.The spectral intensity of molecular carbon（CN and C₂）increased obviously with the increase of volatile content at the earlier delay time.The volatile matter vaporize preferentially as laser irradiating,which resulted in a significant change of plasma composition and structure with coal volatile content.The spatial distribution of atomized species in the plasma,the annihilation and formation of molecular species,and the overall dynamic evolution of the plasma were affected.By analyzing the birth and evolution process of coal plasma,the origin and propagation of volatile influence are presentedSecondly,according to the actual industrial application environment of LIBS,the characteristics of laser-induced plasma of coal in air and argon at atmospheric pressure were analyzed and compared,and discussed the influence mechanism of air.The results showed that the spectral intensity of atomic C,H,O and N emission in air atmosphere was higher than that in argon atmosphere at a shorter delay time,but it was opposite at a longer delay time.By observing the spatial and temporal evolution image of the spectra,it was found that there existed a low-intensity radiation region at the plasma periphery,and the region shrunk and intensity decreased obviously with the increase of delay time.The ambient air mixed and excited during the plasma evolution process and became the second radiation source of the corresponding nonmetallic characteristic spectrum.Nevertheless,the intensity of the spectrum from air source decreased more obviously with the delay time,which was evidently different from the vaporized coal.In the exploration of the influence of volatile matter on the setting of key laser detection parameters:laser energy and pulse frequency,it was found that too high pulse frequency and high volatile matter content would lead to the increase of the residual aerosol particles generation,and the aerosol in the laser incident path would be easily excited,which affected the effective coupling between the subsequent pulse and the specimens.Finally,to reveal the evolution mechanism of different types of solid fuel plasma,the plasma and spectral characteristics of coal and biomass fuel were analyzed and compared.The results also showed that the spectral emissions for the species that abundant in volatile matter were intense at the plasma front.The characteristic spectrum of C,the main component of biomass,was located at a lower position within plasma.The high volatile matter content and water content of biomass made the plasma have higher initial electron density and plasma temperature drop rate,and its lower carbon content led to poor generation of molecular carbon（CN and C₂）.The above research elaborated the evolution dynamic process of solid fuel plasma,realized the effective identification of spectral signals,and laid a theoretical foundation for improving the effective rapid/real-time quantitative detection performance of LIBS for solid fuel characteristics.