Effects of devolatilization temperature on chemical structure and oxidation reactivity of soot sampled from a coflow diffusion ethylene flame

Abstract

Devolatilization is a prerequisite for the analysis of chemical structure and oxidation reactivity of soot sampled from engine combustion or lab-scaled flames, while the effects of devolatilization temperature are not sufficiently understood. This work explores the effects of devolatilization temperature on soot chemical structure and oxidation reactivity. Virgin soot samples were collected from five positions along axis of a coflow diffusion ethylene flame using a recently developed capillary-nozzle-hybrid sampling method. Three temperatures (250, 450 and 650 °C) were selected to devolatilize virgin soot samples for further thermogravimetric (TG), Fourier transform infrared and Raman spectroscopic analyses. The results indicate that devolatilization can remove aliphatic and oxygenated groups away from soot surface, mainly below 450 °C. ID1/IG derived from deconvolution of Raman spectra presents positive correlation with nanocrystallite width, however, less sensitive to devolatilization temperature. These changes in chemical structure pose adverse impacts on soot oxidation reactivity, especially for early-stage soot. With increased degree of maturity, soot gradually shows almost overlapped TG curves under all investigated cases. The distributed activation energy model (DAEM) is used to analyze soot oxidation reactivity by decoupling two interrelated processes, i.e. low-temperature conversion and carbonaceous substances oxidation. After devolatilization, the resulted soot samples have narrow reaction temperature ranges in TG curves and higher activation energies. Among the three selected devolatilization temperatures, 650 °C can effectively remove VOF and retain reactivity features of soot sampled at different sampling positions or residence times.