Effects of Flame Configuration and Soot Aging on Soot Nanostructure and Reactivity in n-Butanol-Doped Ethylene Diffusion Flames

摘要

Soot has received considerable attention since it is a major pollutant in exhaust gas from fossil fuel combustion and it causes adverse climate and health effects. This work focused on soot morphology, nanostructure, and reactivity variations regarding the soot collected in different flame configurations of n-butanol-doped ethylene inverse diffusion flame (IDF) and normal diffusion flame (NDF) at different heights above burner surface (HAB = 20, 30, and 40 mm). The effects of flame configuration and soot aging on structural and reactivity characteristics were analyzed with emphasis on the differences of young and mature soot generated in IDF and NDF without and with n-butanol addition at specific positions, respectively. The effects of fuel-side n-butanol addition on IDF and NDF soot structures and reactivity were also evaluated. The soot obtained using thermophoretic sampling technique was analyzed by transmission electron microscopy (TEM) to investigate soot morphology evolutions along the centerline and the boundary of the flames at different axial locations. Moreover, soot samples collected by quartz plate were characterized by thermogravimetric analyzer (TGA), high-resolution transmission electron spectroscopy (HRTEM), Raman spectroscopy, elemental analyzer, and surface area and porosimetry analyzer. It showed the soot generated in IDF and NDF could have different nanostructure and reactivity relying on the flame configurations and soot aging. The oxidation reactivity for soot in IDF without and with n-butanol addition slightly decreased with the increase of collection height. While as the collection height rose in ethylene and ethylene-butanol NDF, the final mass loss percent of soot and the average oxidation rate increased. The n-butanol addition in IDF and NDF generally enhanced soot oxidation reactivity. The structural analysis via high-resolution transmission electron microscopy (HRTEM) and Raman indicated that soot from IDF with and without n-butanol addition was young, which presented amorphous particles with irregular shapes. Whereas, the ethylene NDF and ethylene-butanol NDF soot were composed of well-defined spherical particles and showed a typical core shell structure. Furthermore, HRTEM photographs displayed an evident discrepancy between the oxidation modes of soot from ethylene and ethylene-butanol IDF and NDF at HAB = 30 mm. High correlations between the nanostructure and reactivity for the soot of ethylene IDF and NDF without and with n-butanol addition were found. With an increase in the degree of crystallization in soot nanostructure, the soot reactivity decreased.