Thermokinetic Analysis of Ethylene Soot and Diesel Engine Soot

Khalid  Al-Qurashi, Penn State Univ.

Soot oxidative reactivity is dependent upon its physico-chemical properties.  These properties can be manipulated by altering the fuel source or soot formation conditions.  Recent studies in our laboratories showed that soot oxidation reactivity can be enhanced by changing the combustion conditions.  We used the exhaust gas recirculation (actual and simulated) to alter the conditions under which soot is formed.  The soot samples are (1) ethylene flame soot generated with and without CO2 addition to the oxidizer stream (2) diesel soot generated from a single cylinder engine with and without CO2 addition to the intake air and (3) diesel soot generated from a 4-cylinder DI common rail turbocharged diesel engine with and without EGR. The kinetic parameters of interest are the activation energy (Ea), the pre-exponential factor (A), and the reaction order with respect to the soot (n).  Knowledge about these parameters is a prerequisite for successive modeling and design of diesel particulate filters (DPF) and the regeneration strategy for the DPF.  These parameters were determined based on multiple nonisothermal TGA experiments.  The results show that the CO2 or EGR addition leads to an increase in the reaction rate of soot oxidation.  These diluents do not affect the activation energy of the soot from the same origin but increase the pre-exponential factor significantly.  It is concluded that the rate-determining step (RDS) in the soot oxidation is the same for all soot samples and that the soot follows the same oxidation mechanism irrespective of its formation history.  The kinetic analysis conforms to the TGA reactivity measurements and suggests that the enhanced oxidation of the soot is ascribed solely to the increase of active sites which are incorporated implicitly in the pre-exponential factor.  Based on these data, a simplified thermokinetic model that predicts soot conversion with temperature was developed.

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