Flame soot characterization using combined multi-wavelength thermo-optical analysis and Raman spectroscopy

Multi-wavelength thermo-optical analysis (MW-TOA) and Raman spectroscopy are commonly used methods to study soot sampled on filters, in terms of carbon content, nanostructure, and absorption properties. In this work, these techniques were combined to study how these characteristic properties of soot change in premixed flames, 1) at various flame heights during soot maturation and 2) as a result of potassium addition to the fuel. It was found from MW-TOA that sampled soot showed decreasing organic carbon (OC) fraction and increasing elemental carbon (EC) fraction with flame height. During heat treatment, light absorption increased significantly in the immature sampled soot. Soot from all sampling heights reached similar MAC values at the end of the inert heating stage. In the immature soot, the initially high absorption Ångström exponent (AAE) decreased significantly during heat treatment in MW-TOA due to evaporation of volatile OC and through carbonization, while in the mature soot the initially low AAE changed negligibly during heat treatment. Potassium (K) addition to the fuel/air-mixture resulted in lower soot absorption, lower EC fraction, a strong reduction of the nucleation size mode and a shift of the accumulation size mode toward smaller sizes. Despite the clear changes in soot properties from MW-TOA, Raman spectroscopic analysis showed relatively small differences with K addition.