Insight into soot formed in coal combustion flame: Evolution of physiochemical structure, oxidation reactivity

Abstract

Combustion experiments of a bituminous coal were performed on a flat-flame burner that provided a realistic low-O2 combustion atmosphere. Soot samplings were performed via both thermophoresis and dilution-filtering sampling methods along the flame centerline. The morphology, size, fractional- and nanostructure evolution, and especially the detailed oxidation characteristics of soot of different maturities in flame were comprehensively characterized. Results showed that liquidus tar was formed in the flame bottom which acted as soot precursor, and a direct evidence was obtained that the solid soot particles in the coal flame were mostly derived from the liquidous tar via carbonization. In the nanostructure of the mature soot particles, both of fullerenic, amorphous and diamond phases were detected, indicating a multiple nucleation process during the soot formation in coal flame. Furthermore, it was found that soot in the bright flame core was more difficult to be oxidized than the soot upstream and downstream it. And finally based on the Raman spectrum and particle micromorphology analysis, the non-linear change of the oxidation reactivity of the soot along the flame centerline was revealed to be controlled firstly by the gradual graphitization with increasing of the flame temperature and then by the change of soot aggregate size.