Research Of Pulverized Coal Combustion Visualization Measurement Based On High-speed Digital Holography

Coal combustion process involves complex physical and chemical reactions.There are still a lot of problems to be revealed at present,so as to cope with the requirements of combustion efficiency increasement and pollutant emission reduction.Nowadays,the research on the visualization of combustion process often uses high-speed camera,which is limited to the depth of focus and can not measure the combustion particle field systematically.Taking McKenna premixed flat flame burner as the platform,the pulverized coal combustion system based on high-speed digital holography is designed.The high-speed camera records combustion process.Holography realizes in situ three-dimensional multi-parameters measurement which maks up the limit of small depth of focus of high-speed camera.In this paper,the changes of single particle and particle field of pulverized coal are traced,the details of combustion devolatilization and fragmentation process are recorded,and the temporal and spacial changes of three-dimensional particle field are analysed.The main results are as follows:The error of the digital holography used in the measurement of flow field is studied in this paper.The cold state measurement of tungsten wire shows that the size error is less than 5%,and the positioning error is less than 6%.The measurement results of holography and Malvin laser particle sizer have been obtained,and the error of cold hologram measurement is less than 3.14%.In combustion particle field,flame radiation,plasma and temperature gradient will disturb the refractive index of the medium,affect the holographic fringes,and lead to the error of thermal holographic measurement.The error of the hologram in the combustion field is analyzed by the tungsten and quartz particles.It is found that the size identification error of tungsten wire is greatly influenced by the combustion field,and the positioning error of z axis is smaller.The size range of quartz particles is 130-150 μm,and the difference of measurement results of cold and hot states at different heights is less than 5.68%.It is proved that the application of holography to combustion particle field measurement is reliable.Visualization of pulverized coal combustion under different conditions are realized.Based on the test platform,the particle size,oxygen content and coal type are changed to compare the changes of pulverized coal combustion process.By comparing the results of direct photography and holography,the rotation,devolatilization and fragmentation process in the combustion process are observed and analyzed.The devolatilization process of Shanxi bituminous coal begins with a layer of dark materials enveloping the coal particle,which will be elongated by the velocity slip of particle and flow and finally small particles show up.It is probably the process of soot evolution.With the increasement of particle size,more phenomenon is observed.As the oxygen content increases,more phenomenon will also be recorded.Less phenomenon is observed during Ximeng lignite devolatilization process and there is almost no phenomenon of Indonesia lignite devolatilization process.There are two typical fragmentation performance of coal particles:shell exfoliation and center fragmentation.The two patterns can occur at the same time.Ximeng lignite shows more fragmentation phenomenon,while Shanxi bituminous coal shows less.Shanxi coal devolatilization products jet away from the particle surface to reduce the volatile pressure,which make it break not as easily as the Ximeng coal.With the increase of Ximeng lignite particle size,more phenomenon of fragmentation is observed.The statistical results of the particle size change of pulverized coal combustion can correspond to the phenomena in the combustion process.The change of the average particle size of pulverized coal field under the conditions of different initial granularity,coal and oxygen content is observed.With the decrease of the initial particle size of Shanxi coal,the reaction rate increases.In the later stage of combustion,Ximeng coal shows the characteristics of particle size increasing which is related to the high alkali metal and alkaline earth metal elements(AAEM).It causes low ash melting point,melting and slagging easily.With the increase of oxygen content,the reaction is greatly accelerated and the particle size shows a trend of gradual increase.This is related to the particle melting and bonding of small particles at the higher reaction temperature.On the basis of high-speed digital holography,the combustion process of pulverized coal field with different initial particle size,coal sample and oxygen content is studied in this paper.The visualization of pulverized coal devolatilization and the fragmentation process is realized,which lays the foundation for further research on the combustion mechanism of pulverized coal.