AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
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Emission Factors and Physicochemical Properties of Soot Produced in Low-Temperature Combustion
OMAR EL HAJJ, Khairallah Atwi, Zezhen Cheng, Alanna L. Koritzke, Matthew G. Christianson, Brandon Rotavera, Rawad Saleh, University of Georgia
Abstract Number: 854 Working Group: Combustion
Abstract Low-temperature combustion has emerged as a promising strategy for reducing pollutant formation in vehicle emissions. While unveiling the differences in ignition chemistry between low-temperature and conventional high-temperature combustion is an active area of research, there is limited understanding of how these differences affect soot formation and its physicochemical properties. Here, we conducted combustion experiments controlled at a constant equivalence ratio and at temperatures varied in a stepwise fashion between 500 K and 1200 K. We performed real-time measurements of soot emission factors using a scanning mobility particle sizer, light-absorption properties using a photoacoustic instrument, and volatility using a thermodenuder. We also collected samples for offline chemical analysis. To illustrate the importance of the fuel chemical structure in dictating soot formation in low-temperature combustion, we used two structurally different model fuels, iso-octane and cyclohexane. Soot emission factors in iso-octane combustion dropped rapidly with decreasing temperature, completely disappearing at temperatures lower than 1000 K. On the other hand, while cyclohexane combustion exhibited a soot formation behavior at high temperatures that was similar to iso-octane, it featured a resurgence in the low-temperature regime (~600 K), with emission factors of the same magnitude as at 1200 K. These findings are consistent with ignition-time simulations, which show a steady decrease in iso-octane reactivity with decreasing temperature, while the decrease in cyclohexane reactivity with decreasing temperature is interrupted by a sharp increase at 600 K. Notably, we observed stark differences in physicochemical properties between high-temperature soot (HTS) and low-temperature soot (LTS) in cyclohexane combustion: 1) the molecular sizes of LTS (< 200 Da) were smaller than HTS (300-3000 Da), 2) LTS was an order of magnitude less absorptive than HTS in the visible spectrum, and 3) LTS was more volatile than HTS.