Soot is the term given to small particles that are mostly composed of carbon, and that result from incomplete combustion processes. Soot formation plays both positive roles (radiation from hot particles is the dominant mechanism for heat transfer in boilers) and negative (in the same boilers, soot may coat furnace walls and soot particle impingement on the turbine blades of a commercial airline engine leads to wear) in energy generation. Soot is also important in the environment as it increases mortality in urban areas and contributes to climate change.
In the laboratory, what we understand about soot formation comes from measurements near the beginning (through the measurements of concentrations of small precursor molecules) and near the end (through optical measurements in flames and/or microscopic analysis of collected particulate matter) of the process. As a consequence, there are several fundamental questions that have gone unanswered: What is the mechanism for the transition from flat, planar molecules to three-dimensional particles? What is the nature of the carbon bonding in the youngest particles?
The goal of this project is to answer these questions through a study of particles sampled from several types of flames. By understanding the detailed chemistry and physics of soot formation and destruction in flames, we enable combustion system engineering that can enhance the positive effects of soot in flames (heat transfer) and while minimizing deleterious heath effects and environmental impacts.
The project's principal investigator has been a leader in the science of soot formation for more than 20 years. In the 1980s, he proposed that soot particles are composed of aggregates of flat, and extremely stable, molecules known as polynuclear aromatic hydrocarbons (PAH). His calculations suggest that the binding energy that holds clusters of PAH together is strong enough for them to survive even at flame temperatures. To prove this hypotheses, in this project particles are studied using Raman spectroscopy, in which laser light is "scattered" from the carbon particles, providing information about the optical and electronic properties of the particles, which can then be used to infer structure and morphology. Analysis is performed on particles that are rapidly extracted from laboratory flames that mimic a wide range of conditions found in practical combustion devices.
Broader Impacts: In addition to the technical motivation for the project described above, an additional outcome of this award will be the education of a doctoral student in the Chemistry Department of GWU. The GWU Ph.D. program is smaller than most US doctoral chemistry programs. However, its size has been an attribute in attracting top-quality students who are looking for the type of individual attention that is available there. Its small size has catalyzed interdisciplinary research that links the chemistry program with scientists and engineers with mutual interests at GW, at other US academic campuses and laboratories, and at partner institutions throughout the world. This strategy has been a successful recipe in insuring that their graduates find employment in both the public and private sectors. George Washington University is ranked among the top few universities in the country in the population of women in science and engineering graduate programs. In line with this fact, the chemistry department's graduate program and the PI's lab have majority female populations.
With funding provided by the National Science Foundation, the research group of Prof. J. Houston Miller at the George Washington University studied the formation of "soot" particles in laboratory flames. Under some conditions, during the combustion of either fossil or bio-derived fuels some fraction of the carbon atoms are converted into "soot", a form of amorphous carbon. In the study of soot, we traditionally have been limited to diagnostics that measure species near the beginning of the process (specifically, molecular species that can be detected with instruments such as mass spectrometers) or near the end, when particles are large enough to be detected using scattered light from lasers or seen on electron microscope grids. As a consequence, many theories have been proposed for how this transition from planar molecules into complicated "fractal" structures occurs. The purpose of our efforts has been to develop diagnostics that will probe this intermediate size regime and thus unravel some of the mysterious of the transition from molecule to particle. In particular, we have developed two diagnostics for application in this field. The first is Raman scattering in which a color shift of wavelength is observed for light impending on a molecule or particle and that that is emitted. These "Raman shifts" are specific to the type of chemical bonding and can be interpreted to learn about molecular structure. In this case, we use Raman scattering to determine the size of the aromatic molecular regimes within soot and consequently infer how big molecules were when they first started to "stick" together without the benefit of chemical reaction. The second diagnostic uses a special type of laser that has a broad range of colors available to probe molecular structure. With this "supercontinuum" source we get an independent measurement of aromatic size. The results of studies using both types of probes provide consistent results for the size of this transition: a molecule about 4-5 aromatic rings across. Finally, using funding from this award, we used mass spectrometry and tunable diode laser absorption spectroscopy to probe the structure of the flames by determining concentrations of small molecules including fuel, the components of air, intermediates, and products of the combustion process. The impact of this funded program is in both the relevance of the science to national and international interests in energy and the environment, but also in the education and training of future generations of scientists. Hydrocarbon growth processes in flames are important both to the understanding of soot production from combustion systems and for the development of continuous processes for the manufacture of carbon-based materials. On the negative side, as a primary component of urban aerosol, understanding soot formation is important in assessing and mitigating deleterious health effects of sub micron sized particles that we breathe. Finally, soot (known as "black carbon" in climate science research) is now known to be a major manmade emission into the atmosphere that contributes to warming. The bulk of the support from this grant underwrote the education and training of several GWU graduate and undergraduate students. Of particular note is Ms. Erin (Webster) Adkins, currently a fourth year doctoral student in Chemistry at George Washington University. In addition to support provided by NSF, Ms. Adkins was the recipient of the ARCS Endowment Fellowship for the academic year 201415. (The ARCS Foundation advances science and technology in the United States by providing financial awards to academically outstanding U.S. citizens studying to complete degrees in science, engineering, and medical research.) In addition, two undergraduate chemistry majors received support from this award that allowed them to dedicate part of an academic year summer to research. These students contributed to both projects in Raman spectroscopy and mass spectrometry that resulted in published products from the grant.
