Multiple-stage magnetic regulation of soot characteristics and emission across a wide range of oxygen concentrations

This study explored the performance of soot reduction in inverse diffusion flames (IDF) with various oxygen concentrations by simultaneous applying magnetic fields to multiple soot evolution stages. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were employed to analyze the structural characteristics of soot. Imposing an upward magnetic gradient on soot at their in-flame growth stage effectively promoted soot inception. In contrast, as soot within the flame exhaust traveled through the magnetic field, the magnetic effect significantly reduced soot emissions by accelerating soot oxidation. For flames at oxygen concentrations of 25% and 30%, multiple-stage magnetic field increased the content of disordered carbon inside soot particles. However, for flames with oxygen concentrations of 35% and 40%, multiple-stage magnetic intervention resulted in a more ordered nanostructure within the soot particles. Such oxygen‑dependent discrepancies in magnetic responses originated from the distinct regulatory effects induced by magnetic field loading in different soot evolution stage. Specifically, the imposition of an upward magnetic gradient in the flame zone expanded the high‑temperature region, which extended the residence duration of soot particles within the flame, thereby facilitating the carbonization process. Additionally, magnetic field application could strengthen the oxidation of soot within flame exhaust region by enriching local oxygen concentration. Meanwhile, as soot particles transported across the magnetic field, magnetic effect could promote the transformation from ordered carbon to disordered carbon within soot particles. Under varied oxygen conditions, the relative contributions of these coupled mechanisms differed, thus resulting in an oxygen‑dependent evolution of soot properties under magnetic regulation.