Radicals in nascent soot from laminar premixed ethylene and ehtylene-benzene flames by electron paramagnetic resonance spectroscopy

Present particle size distribution (PSD), electron paramagnetic resonance (EPR), and Raman spectroscopy measurements of soot particles, collected across the inception region were conducted in two ethylene-air premixed laminar flames and two flames obtained by substituting 30% of the ethylene carbon with benzene. The experimental results were employed to investigate the effect of aromatic fuel on the nature and role of radicals in particle nucleation and growth. For all the particles sampled, both in pure ethylene flames and in the ethylene/benzene ones, an EPR signal typical of persistent carbon-centered aromatic radicals was measured. The results revealed an involvement of ?-radicals during particle nucleation and initial growth process. Under the conditions investigated, results showed that the electron spin density of particles produced by different fuels was mainly related to the amount of H-atoms available in the particles. Also, for all the investigated flames, the spin density presentsed an inverse dependence with the particle size suggesting that particle surface plays an important role in the early steps of the soot formation process.