(660a) Soot Oxidation Kinetics: Pressure and Fuel Comparisons

Soot emission is the result of the competition between soot formation and oxidation. While there are extensive ongoing studies on the difficult problem of soot formation, few studies have focused their attention on soot oxidation which is considered one method of controlling soot emission. The main objective of this work is to develop kinetic models for soot oxidation to incorporate them into predictive, multi-scale, combustion models to optimize the design and operation of evolving fuels in advanced engines for transportation applications.

Thermogravimetic analysis (TGA) is a widely-used technique to determine oxidation kinetics under well-defined conditions such as temperature and oxygen concentration. The TGA technique offers the advantage that only a small amount of sample is needed, and the kinetic parameters (activation energy and pre-exponential factor) are determined by a limited number of experiments. However, under certain operating conditions, the soot oxidation inside the TGA furnace could be a partially diffusion–controlled reaction. In this case, the kinetic parameters cannot be directly obtained from TGA plots, and a combination of diffusive and kinetic models is necessary

TGA studies were performed with soot samples obtained from different liquid fuels. Soot was generated in a flat-flame, premixed burner under heavily-sooting conditions and captured on a water-cooled stabilization plate which was located 5 cm above the burner surface. The collected soot was crushed into a powder and oxidized using a Cahn TherMax 500 high pressure TGA. An inert material was used in all the tests to minimize thermal and mass transfer effects by decreasing the stagnant region between the top surface of the soot and the entrance of the crucible. Experiments were performed with 10 mg of sample and a volumetric flow rate of 1 l/min. Two total pressures of the system were evaluated (1 and 10 atm) and the O₂concentrations varied between 10 and 21%.

Mass transfer corrections were obtained with simplified model based on effectiveness factors, accounting for internal and external diffusion limitations.  Higher pressures yielded lower effectiveness factors.

The estimated activation energies ranged from 140 to 170 kJ/mol and there was not a significant difference in the activation energies obtained for different soot samples at the pressures evaluated in this study. These values were on the same order of magnitude as other published values.

Future work will focus on investigating the soot nanostructure by using high resolution transmission electron microscopy (HTEM) and study the structural differences of soot and its effects on oxidation reactivity. Oxygenated fuels will be also evaluated in the TGA system under different pressures with the subsequent nanostructure study.