Relationship between the soot absorption function, the mobility size distribution, the absorption Angström exponent and the chemical composition of soot issued from diesel B7 combustion in a mini-CAST burner: Effect of catalytic stripping

Abstract

The present study aims to relate the optical properties and chemical composition of the soot particles generated in a mini-CAST burner supplied with diesel B7 over several operating points (OPs) with an overall equivalence ratio in the range 0.10–1.54. The extinction spectra are acquired in a custom 1 m long cell and analyzed using the Rayleigh-Debye Gans for fractal aggregates theory to extract the soot absorption function 𝐸⁡(𝑚), its spectral dependence, the absorption Angström exponent (AAE) and the single scattering albedo (SSA). The chemical composition of soot is determined using a surface diagnostic (time-of-flight secondary ions mass spectrometry) from which the C/H ratio is determined, while the gas phase is analyzed by gas chromatography, yielding the total hydrocarbon content. Analyses performed on untreated soot (as sampled) and soot treated with a catalytic stripper are compared, to reach a detailed understanding of the impact of the stripping process on both the optical properties and the chemical composition of the emissions. Untreated samples show a clear dependence of AAE on the C/H ratio that is suggested to be affected by the post-sampling deposition of high-m/z species on the soot particles. Samples treated with a catalytic stripper are almost completely depleted of high-m/z species: their AAE is significantly lower than untreated samples, and no correlation is found between AAE and the C/H ratio. The AAE decreases as the geometric diameter of the soot increases. The methodology presented in this work can be transposed in principle to any liquid fuel.