Soot emission results from combustion in nearly every propulsion system that uses fossil fuels as a power source. Besides the health hazard of soot emission, soot formation in engines increases heat loading via radiative transport and can cause erosive engine damage. Current predictive models do not agree with experimental results as well as desired. One possible reason is that ion chemistry may play an important role in the nucleation process (i.e., the early stage of soot formation when the growing entities are still molecular). Whether radical or ion chemistry dominates nucleation is a question that has not been conclusively answered despite over 30 years of research.
The objective of this project was to develop a novel ionization and modeling approach to resolve the issue of the relative importance of ion versus radical nucleation mechanisms in soot formation.
The project seeded ions into various hydrocarbon flames and performed diagnostic tests on the resulting soot or soot precursors to determine the influence of ion chemistry on soot formation. An existing, state-of-the-art burner facility, equipped for resonance-enhanced multiple photoionization (REMPI) spectroscopy, mass spectrometry, and laser-induced incandescence, was used for experiments in this study. A pumped dye laser was used to ionize a small portion of the nascent formyl radicals (HCO) within the flame by (2+1) REMPI. This technique increases the formyl cation (HCO+) concentration in the flame by several orders of magnitude, without seriously altering the HCO concentration. If the ion nucleation mechanisms were indeed important, adding a high concentration of HCO+ to the flame would ultimately lead to high concentrations of positively ionized soot precursors and soot. Measurements were made of both the HCO+ and ions formed (if any) via the HCO+ reactions with flame molecules.
Mechanisms for C1 and C2 hydrocarbons were developed, and the ion thermodynamics of over 100 species were revised. The project performed flame-modeling calculations and compared the data to measurements under pyrolytic and oxidative conditions. Ions in various flames were detected by REMPI, and soot particles were monitored by laser-induced incandescence. In the end, the project was unable to make a strong correlation between ion chemistry and soot formation. Two papers and three presentations resulted from this one-year effort. This project was completed in FY 2001.
This project sought to address the long-standing issue concerning the role of ion chemistry during the soot nucleation process. The lack of understanding of the nucleation chemistry may be the fundamental reason why currently available models do not match experimental results as well as desired. Creation of a reliable model for the prediction of soot formation during combustion would result in reduced design costs for diesel and turbine engines used by the Department of Defense.