Investigating the oxidative reactivity and nanostructural characteristics of diffusion flame generated soot using methyl crotonate and methyl butyrate blended diesel fuels

Abstract Combustion-generated particulate matter has a consequential fallout on climate, environment, and human health. Thus, comprehension of soot formation and mitigation processes using biodiesel additives is a focal point of modern combustion research. This study presents an intricate investigation of enhanced oxidative reactivity and its dependence on the soot nanostructural properties induced by distinctive blending of a saturated and an unsaturated biodiesel surrogates namely, methyl butyrate (MB) and methyl crotonate (MC) respectively with pure diesel. 25% MB-75% diesel fuel resulted in the lowest sooting propensity and lowest activation energy (155.4 ± 2 kJ/mol) for soot oxidation in comparison to pure diesel fuel and diesel soot (175.1 ± 2 kJ/mol). The soot was collected using a smoke point apparatus involving a wick-fed laminar diffusion flame at atmospheric pressure after meticulous observation of the smoke points. The detailed nanostructural characterization of the flame generated soots were performed using high-resolution transmission electron microscopy, X-ray diffraction, elemental analysis and Raman spectroscopy. BET surface area analysis, and thermogravimetric analysis were carried out for surface area distinction and activation energy calculations respectively. It revealed that, addition of MB to diesel resulted in improved fuel combustion with reduction in primary soot particle diameter, greater inter-planar separation, increased fringe tortuosity and greater crystal structure disorder resulting in its enhanced reactivity with O2. The 25% MC-75% diesel blend resulted in relatively greater sooting propensity while its soot exhibited lower crystal structure disorder and greater activation energy (165.0 ± 2 kJ/mol) for O2 induced oxidation compared to 25% MB-75% diesel, possibly due to the resonance stabilized radical (RSRs) formation that altered the fuel combustion chemistry. Therefore, this study successfully depicts that the structural differences in the surrogate fuels do influence the soot formation and oxidation kinetics. These structural effects therefore need to be considered when formulating the global multiphase kinetic models for biodiesel-diesel fuel combustion. 1