Soot reactivity analysis and implications on diesel filter regeneration

Abstract Internal combustion engines are among the main sources of soot particles, especially when non-homogeneous and high fuel-air ratio conditions are achieved in the combustion chamber (both diesel and direct injection spark ignition engines). Environmental regulations for the road transportation limit the number and mass of particles significantly, making the use of diesel and gasoline particle filters -DPF, GPF- essential for trapping and burning off, or regenerating, the soot. The tendency of soot to be burnt is called oxidative reactivity, which depends on the combined effect of all its properties and characteristics. The reactivity of soot particles was considered a detrimental characteristic in the past, prompting atmospheric reactions that form more harmful pollutants or lubricant oil degradation. However, some beneficial effects of this characteristic are now acknowledged, such as reducing the number of active filter regeneration events, and thus fuel consumption, driver annoyance and filter thermal stress. This review summarizes the results of soot characterization by applying different analysis techniques. These techniques have been categorized in structural, chemical and thermal, depending on the type of soot property they look into. Structural techniques, such as Raman spectroscopy and electron energy loss spectroscopy, analyze the defects in the graphitic nanostructure, or describe the bulk patterns of the graphitic nanostructure (X-ray diffraction spectrometry) or the porous microstructure (physisorption or chemisorption). Also, microscopic techniques provide valuable information about the visible nano- and microstructure. Chemical techniques, such as Fourier transform infrared spectroscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, or magnetic nuclear resonance describe the chemical oxidizing state, mainly on the soot surface. Other techniques, such as thermogravimetry and differential calorimetry directly analyze the thermal or mass response of soot under a hot and oxidizing environment. Finally, soot is oxidized in particulate filters during active or passive-active regeneration processes depending on its structural and chemical characteristics. These characteristics have been found to be associated with both engine conditions (mainly engine load) and type of fuel (with preferential effect of oxygenated fuels). However, it has been demonstrated that different soot characteristics may contribute to opposite trends in reactivity, and therefore, a partial analysis of soot characteristics may lead to incorrect conclusions.