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45787-G6
The Ignition of the Butanol Isomers

Matthew A. Oehlschlaeger, Rensselaer Polytechnic Institute

Summary

            A new shock tube has been designed, constructed, characterized, and validated for high-temperature combustion chemistry experiments. The first experiments performed in the new shock tube were a comprehensive study of the ignition of the four butanol isomers, possible future high-octane rated additives for gasoline. The difference in reactivity of the four butanol isomers was experimentally characterized and in conjunction with kinetic modeling efforts of a collaborator (F. Battin-Leclerc and co-workers, Nancy University) the importance of the various classes of consumption reactions (dehydration, unimolecular decomposition, and H-atom abstraction) for the four isomers was elucidated. In parallel to the butanol study we have developed a kinetic spectrograph capable of the high-speed acquisition of ultraviolet spectra. In the next year this system will be employed for the originally proposed study of the ultraviolet absorption and oxidation kinetics of the phenyl radical.

New Shock Tube

            A new shock tube has been designed and constructed for the high-temperature investigation of combustion chemistry. The fully instrumented stainless steel shock tube has a 12.3 cm inner diameter, a 7.5 m long driven section, a 3.1 m long driver, and is capable of test pressures of 0.1-10 atm; see Figure 1 for images of the shock tube. The shock tube has been fully characterized and validated. Validation was performed by measuring ignition delay times for propane/oxygen/argon mixtures and comparing to the measurements of Horning et al. (2002), which we have a high level of confidence in; see Figure 2 for a comparison.

Figure 1. A photo of the new RPI shock tube.

Figure 2. Comparison of the previous Horning et al. (2002) propane shock tube ignition measurements to validation measurements made in the new RPI shock tube facility.

Butanol Ignition

Butanol has received recent interest as a possible future high-octane rated additive for gasoline. Therefore, the autoignition of the four isomers of butanol (1-butanol, 2-butanol, iso-butanol, and tert-butanol) has been experimentally studied at high temperatures in a shock tube. Ignition delay times for butanol/oxygen/argon mixtures have been measured behind reflected shock waves using electronically excited OH emission and pressure measurements to determine ignition delay times; see Figure 3 for an example measurement and Figure 4 for example ignition time results. A detailed kinetic mechanism has been developed by our collaborators at Nancy University (F. Battin-Leclerc and co-workers) to describe the oxidation of the butanol isomers and validated by comparison to the shock tube measurements. Reaction flux and sensitivity analysis illustrates the relative importance of the three competing classes of butanol consumption reactions: dehydration, unimolecular decomposition, and H-atom abstraction. 1-butanol and iso-butanol, the most reactive isomers, are consumed primarily by H-atom abstraction resulting in the formation of radicals, the decomposition of which yields highly reactive branching agents, H-atoms and OH radicals. Conversely, the consumption of tert-butanol and 2-butanol, the least reactive isomers, takes place primarily via dehydration, resulting in the formation of alkenes, which lead to resonance stabilized radicals with very low reactivity. To our knowledge, the ignition delay measurements and oxidation mechanism for 2-butanol, iso-butanol, and tert‑butanol are the first of their kind.

Figure 3. Example butanol ignition delay time measurement (pressure and OH* emission).

Figure 4. Ignition time measurements for all four butanol isomers for a mixture composition of 1% butanol / 6% O₂ / Ar (Ф = 1.0) and reflected shock pressures near 1 bar.

Kinetic Spectrograph

            An ultraviolet kinetic spectrograph has been purchased, assembled, and initially tested for the future measurement of ultraviolet absorption spectra (200-400 nm) in shock-heated gases at high speed (100 kHz and faster). The kinetic spectrograph consists of a high-powered fiber-coupled deuterium light source that is transmitted through shock-heated gases of interest; the light is dispersed with a 500 mm spectrograph, and recorded on a CCD camera. In order to record multiple spectra at a high repetition rate, only the top few rows of the CCD array are exposed to the dispersed light. The CCD array is binned in a manner that allows the exposed CCD rows to be recorded and quickly transferred to the unexposed portion of the array. The process is repeated until the entire CCD array is full of spectra, each of which is separated in time by 2-10 μs. A photograph of the kinetic spectrograph is shown in Figure 5 and an example spectral intensity measurement is shown in Figure 6. This diagnostic tool will be used, in the next few months, for the originally proposed study of the ultraviolet absorption and oxidation kinetics of phenyl radicals in shock-heated gases. Phenyl radical oxidation, C₆H₅ + O₂à products, is an important reaction in the oxidation of aromatic hydrocarbon fuels found in gasoline, jet fuel, and diesel and in the description of soot formation from aromatics.

Figure 5. A photo of the kinetic spectrograph.

Figure 6. Example spectral intensity measured with the kinetic spectrograph.

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