Influences of Coal Type and Particle Size on Soot Measurement by Laser-Induced Incandescence and Soot Formation Characteristics in Laminar Pulverized Coal Flames

Abstract

This paper discussed the limitation of the laser-induced incandescence (LII) method for the measurement of soot in different laminar pulverized coal flames and explored the soot formation characteristic. Multiple methods, including digital photography, thermophoretic sampling, elastic laser scattering (ELS), and LII methods, were adopted to laboratory-scale laminar pulverized coal flames with different coal types (anthracite, lignite, and bituminous coal) and coal particle sizes (61−90, 90−100, and 100−154 μm). The flame structure, the morphology of dominant particles in different combustion regions, and the spatial distribution of soot and micron-sized carbonaceous particles were obtained and analyzed. The optical measurement and sampling results showed that the soot yield was so low that the LII signal from soot did not dominate in the detection area for anthracite, lignite, and the larger size fraction bituminous coal flames. In particular, a comparison of LII and scattering signals indicated that the vast majority of the LII signal was excited from char and unburned coal for anthracite flame, and char combustion was predominant during the combustion process. Moreover, the coal particles were more dispersed for the larger particle size fraction bituminous coal flames, which favored soot oxidation but did not favor local fuel-rich combustion and the formation of soot. Therefore, in the present study, the soot yield was high enough only in 61−90 μm bituminous coal flame, and the interference LII signal from char or unburned coal particles could be neglected by carefully controlling the laser fluence. It was also found that the LII signal intensity increased as the particle size of the pulverized coal jet decreased in the noncombustion case, which could be explained by the smaller total emission surface area with larger particle size. In addition, the temperature of heated coal particles was also lower as the coal particle size increased, which resulted in the weaker LII signal from coal particles. On the basis of this, a new phenomenon of primary fragmentation of coal particles in pulverized coal flames was observed for the first time by LII measurement, which could provide a potential method for the study of primary fragmentation for coal particles.