Soot formation and oxidation are important because of soot's adverse health effects, role in climate forcing, and importance to ambient air quality and visibility. The adverse health effects of soot have been reported by numerous researchers. Soot has been implicated in climate forcing; however, its actual contribution is still somewhat uncertain. Manufacturers of air pollution control equipment, i.e. diesel particulate filters, are also interested in understanding the rates and mechanisms of soot oxidation at lower temperatures.
Previous work has identified the fragmentation of soot particles under various combustion conditions, including high soot burnout (70% carbon burned) and low burnout, close to the burner (about 10% carbon burned) in a high temperature, flame environment. In addition, recent modeling has identified the importance of fragmentation in predicting the particle size distribution and other soot properties. However, little experimental data exist for fully understanding the conditions under which fragmentation potentially occurs and the mechanisms. Others have suggested that oxygen diffuses into the particles which allows for internal burning, breaking apart the soot agglomerate. The proposed work has the following three objectives:
Objective 1. Identify the conditions, i.e. temperatures, equivalence ratios, radical pool, etc. where fragmentation occurs for ethylene flames at both low burnout and high burnout; Objective 2. Determine the mechanisms of fragmentation, such as oxygen diffusion in breaking of bridges and soot particle break down; Objective 3. Perform tests on surrogate liquid fuels to determine the applicability of the mechanisms identified in Objective 2 to these fuels and the role of aromatic versus aliphatic components.
The experiments will be conducted in a two-stage, premixed burner system which can be used for both gaseous and liquid fuels. In this system the first burner generates soot, while the second burner is used to oxidize the soot under various temperatures and equivalence ratios. A Scanning Mobility Particle Sizer (SMPS), with a size range of 3-100 nm, will be used to determine particle size distributions as a function of height above burner. When needed, long-differential mobility analyzer measurements can extend the range to 660 nm. Gas-phase concentrations of key components (i.e. O2, H2, CO, CO2) and temperature profiles will be taken. Objective 1 will use ethylene to gather the data and develop mechanisms, while Objective 3 will explore liquid fuels. Utilizing TEM and HR-TEM and particle size distribution data, we will explore in Objective 2 the oxidation of "bridge" material between particles via oxygen diffusion and also increases in particle porosity which can lead to particle fragmentation. The intellectual merit of the study is in understanding the mechanisms of soot fragmentation during oxidation and the conditions under which it is likely to occur. The novelty of the work is exploring the phenomenon under the two scenarios: high and low soot burnout. Furthermore, soot oxidation has not been studied in depth, and recent advances in particle measurements can add to the existing knowledge. The data can be used to modify oxidation mechanisms which, when used in simulations, will more accurately predict the structure, size distribution and mean properties of soot as it oxidizes. In addition, this project will begin to address the role of different fuel constituents in fragmentation.
The broader impacts are realized when industry can apply these fundamentals to their systems to reduce soot emissions, important to protect human health and to better understand/mitigate climate forcing. On the other hand, for industries producing carbon black, these data will add to the existing knowledge base of soot formation/oxidation.