Carbon Mitigation Initiative
CMI

Ninth Year Annual Report:
Carbon Capture: Combustion of Biofuels and Flame Dynamics

Chung K. Law and colleagues have been conducting research on the combustion chemistry of alternate fuels and their combustion characteristics within the high-pressure environment of internal combustion engines.

A New Center for Combustion of Alternative Fuels

In April 2009, the Department of Energy announced that Princeton would serve as the home for a new $20 million energy research center for combustion science led by Chung Law. The fiveyear award for the Combustion Energy Frontier Research Center (EFRC) is part of a federal initiative to spur discoveries that lay the groundwork for an economy based on clean replacements for fossil fuels. The goal of the Combustion EFRC will be to develop new simulation tools for understanding alternative fuel combustion and optimizing the design of next-generation fuels, such as those derived from plants and other renewable sources, in advanced engines for transportation vehicles.

The Princeton center's projects will be led by 14 of the nation's top combustion scientists from seven academic institutions and the Sandia National Laboratories, with expertise in quantum chemistry, chemical kinetics, combustion theory, modeling and diagnostics. In addition to Law, other Princeton participants are faculty members Emily Carter, Frederick Dryer and Yiguang Ju from the Department of Mechanical and Aerospace Engineering.

Reduced Soot Formation in the Burning of Biodiesels from Waste Vegetable Oils

Law and colleagues have collaborated with colleagues at the University of Kentucky in developing an innovative acid-catalyzed process for the synthesis of biodiesel from waste vegetable oils. Biodiesel produced from biomass holds the potential to reduce greenhouse gas emissions and to facilitate energy sustainability and security by providing an alternative to petroleum-based diesel. Furthermore, it is also a renewable energy source because the carbon dioxide released during its combustion can be absorbed by plants which are subsequently harvested to produce the next batch of biofuels.

The synthesized biodiesels were studied for their droplet combustion processes relevant for the burning in diesel engines. Perhaps the most interesting result from the experimental study is that the biodiesel, being a methyl ester, was found to have a much lower sooting propensity than that of the regular diesel fuel. Figure 5 shows the flame streaks of streams of downwardly-moving 100-µm size fuel droplets in a hot combustion environment simulating the interior of internal combustion engines. Various fuels were tested, including: (a) neat diesel, (b, c) diesel with 10 and 20% addition of biodiesel, (d) biodiesel, (e) hexadecane (a reference fuel for diesel combustion). Yellow luminosity indicates soot formation, while blue luminosity its absence. It was then found that while diesel is a heavy soot emitter, the extent of soot formation can be substantially reduced with biodiesel addition. In particular, soot formation can be mostly eliminated for the biodiesel droplets during the later part of their burning. The bottom panel of the figure shows the soot abstracted from the burning droplets and collected on filter papers, which further demonstrates the heavy sooting tendency of diesel and the very low sooting tendency of biodiesel. The reason for the reduced sooting tendency of the biodiesel is the presence of the two oxygen atoms in the molecular structure of these esters, in that they can readily oxidize the soot precursors where they are initially formed in the reaction process.

Figure 5: Time-exposed photographic images
of the flame streaks of burning droplets of (a) diesel, (b) diesel/10% biodiesel, (c) diesel/20% biodiesel (d) biodiesel, and (e)
hexadecane.

Ignition and Flame Propagation of Bio-Butanols

Interest in the use of alcohols as renewable fuel sources has recently shifted from bio-ethanol to bio-butanol because of the latter’s diverse source of supply, higher octane rating, and other auxiliary advantages over ethanol. In order to support such practical developments, there is a corresponding need for studies on the kinetic mechanisms and experiments in well-defined combustion environments. To acquire such kinetics information, in the present investigation the Law Group has performed two well-controlled experiments on three bio-butanol fuels, namely n-butanol, iso-butanol and methyl butanoate. The experiments involve measuring the ignition temperature of a liquid pool by heated oxidizer in a stagnation flow and the laminar flame speeds from outwardly propagating spherical flames. These experimental data, relevant to intermediate-to relatively-high temperature ignition chemistry and high-temperature flame chemistry respectively, are important global combustion parameters in developing and validating the kinetic mechanisms.

Figure 6: Image showing the presence of
target patterns over a flame surface.

In addition to the experimental data, the researchers have further obtained reduced mechanisms of the smallest size possible from the detailed comprehensive ones, with well-defined error control. The reduction of mechanisms is particularly essential for simulations of practical combustion situations, since otherwise the computation would be extremely demanding due to the relatively large size of the detailed mechanisms.

Surface Morphology and Pulsating Instability in Expanding Spherical Flames

In previous studies, Law and colleagues have found that wrinkles representing cellular instability develop over surfaces of lean hydrogen/air and rich hydrocarbon/air flames. In the present study, spiral and target patterns representing pulsating instability develop over rich hydrogen/air and lean hydrocarbon/air flames. Figure 6 shows such target patterns over a flame surface. These patterns appear and disappear rapidly, and could signal the incipient occurrence of flame extinction. As such, they can be used as markers to detect and hence prevent flame extinction.

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Last update: February 17 2011
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