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Low-temperature combustion chemistry of biofuels: Pathways in the initial low-temperature (550 K-750 K) oxidation chemistry of isopentanol

Physical Chemistry Chemical Physics

Welz, Oliver W.; Zador, Judit Z.; Savee, John D.; Ng, Martin Y.; Meloni, Giovanni; Fernandes, Ravi X.; Sheps, Leonid S.; Simmons, Blake S.; Lee, Taek S.; Osborn, David L.; Taatjes, Craig A.

The branched C 5 alcohol isopentanol (3-methylbutan-1-ol) has shown promise as a potential biofuel both because of new advanced biochemical routes for its production and because of its combustion characteristics, in particular as a fuel for homogeneous-charge compression ignition (HCCI) or related strategies. In the present work, the fundamental autoignition chemistry of isopentanol is investigated by using the technique of pulsed-photolytic Cl-initiated oxidation and by analyzing the reacting mixture by time-resolved tunable synchrotron photoionization mass spectrometry in low-pressure (8 Torr) experiments in the 550-750 K temperature range. The mass-spectrometric experiments reveal a rich chemistry for the initial steps of isopentanol oxidation and give new insight into the low-temperature oxidation mechanism of medium-chain alcohols. Formation of isopentanal (3-methylbutanal) and unsaturated alcohols (including enols) associated with HO 2 production was observed. Cyclic ether channels are not observed, although such channels dominate OH formation in alkane oxidation. Rather, products are observed that correspond to formation of OH via β-C-C bond fission pathways of QOOH species derived from β- and γ-hydroxyisopentylperoxy (RO 2) radicals. In these pathways, internal hydrogen abstraction in the RO 2 QOOH isomerization reaction takes place from either the -OH group or the C-H bond in α-position to the -OH group. These pathways should be broadly characteristic for longer-chain alcohol oxidation. Isomer-resolved branching ratios are deduced, showing evolution of the main products from 550 to 750 K, which can be qualitatively explained by the dominance of RO 2 chemistry at lower temperature and hydroxyisopentyl decomposition at higher temperature. © 2012 The Owner Societies.

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TYPE Journal Article YEAR 2012
ScopusOSTI

Competing channels in the propene+OH reaction: Experiment and validated modeling over a broad temperature and pressure range

Zeitschrift fur Physikalische Chemie

Kappler, Claudia; Zador, Judit Z.; Welz, Oliver W.; Fernandes, Ravi X.; Olzmann, Matthias; Taatjes, Craig A.

Although the propene+OH reaction has been in the center of interest of numerous experimental and theoretical studies, rate coefficients have never been determined experimentally between ∼600 and ∼ 750 K, where the reaction is governed by the complex interaction of addition, back-dissociation and abstraction. In this work OH time-profiles are measured in two independent laboratories over a wide temperature region (200-950 K) and are analyzed incorporating recent theoretical results. The datasets are consistent both with each other and with the calculated rate coefficients. We present a simplified set of reactions validated over a broad temperature and pressure range, that can be used in smaller combustion models for propene+OH. In addition, the experimentally observed kinetic isotope effect for the abstraction is rationalized using ab initio calculations and variational transition-state theory. We recommend the following approximate description of the OH+C 3H6 reaction: C3H6+OH⇄C 3H6OH (R1a,R-1a) C3H6+OH→C 3H5+H2O (R1b) k1a(200K ≤ T ≤ 950 K;1 bar ≤ P) = 1.45×10-11 (T/K)-0.18e 460K/Tcm3 molecule-1s-1 k -1a(200 K ≤ T ≤ 950 K; 1 bar ≤ P) = 5.74×10 12e-12690K/Ts-1 k1b(200 K ≤ T ≤ 950 K) = 1.63×10-18 (T/K)2.36e -725K/T cm3 molecule-1s-1. © by Oldenbourg Wissenschaftsverlag, München.

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TYPE Journal Article YEAR 2011
ScopusOSTI

Branching fractions of the CN + C 3H 6 reaction using synchrotron photoionization mass spectrometry: Evidence for the 3-cyanopropene product

Journal of Physical Chemistry A

Trevitt, Adam J.; Soorkia, Satchin; Savee, John D.; Selby, Talitha S.; Osborn, David L.; Taatjes, Craig A.; Leone, Stephen R.

The gas-phase CN + propene reaction is investigated using synchrotron photoionization mass spectrometry (SPIMS) over the 9.8 - 11.5 eV photon energy range. Experiments are conducted at room temperature in 4 Torr of He buffer gas. The CN + propene addition reaction produces two distinct product mass channels, C 3H 3N and C 4H 5N, corresponding to CH 3 and H elimination, respectively. The CH 3 and H elimination channels are measured to have branching fractions of 0.59 ± 0.15 and 0.41 ± 0.10, respectively. The absolute photoionization cross sections between 9.8 and 11.5 eV are measured for the three considered H-elimination coproducts: 1-, 2-, and 3-cyanopropene. Based on fits using the experimentally measured photoionization spectra for the C 4H 5N mass channel and contrary to the previous study (Int. J. Mass. Spectrom.2009, 280, 113 - 118), where it was concluded that 3-cyanopropene was not a significant product, the new data suggests 3-cyanopropene is produced in significant quantity along with 1-cyanopropene, with isomer branching fractions from this mass channel of 0.50 ± 0.12 and 0.50 ± 0.24, respectively. However, similarities between the 1-, 2-, and 3-cyanopropene photoionization spectra make an unequivocal assignment difficult based solely on photoionization spectra. The CN + CH 2CHCD 3 reaction is studied and shows, in addition to the H-elimination product signal, a D-elimination product channel (m/z 69, consistent with CH 2CHCD 2CN), providing further evidence for the formation of the 3-cyanopropene reaction product. © 2011 American Chemical Society.

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TYPE Journal Article YEAR 2011
ScopusOSTI

Analysis of advanced biofuels

Taatjes, Craig A.; Dec, John E.; Yang, Yi Y.; Welz, Oliver W.

Long chain alcohols possess major advantages over ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. Rapid developments in biofuel technology have made it possible to produce C{sub 4}-C{sub 5} alcohols efficiently. These higher alcohols could significantly expand the biofuel content and potentially replace ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C{sub 5} alcohol, isopentanol, as a fuel for homogeneous-charge compression-ignition (HCCI) engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. The elementary autoignition reactions of isopentanol is investigated by analyzing product formation from laser-photolytic Cl-initiated isopentanol oxidation. Carbon-carbon bond-scission reactions in the low-temperature oxidation chemistry may provide an explanation for the intermediate-temperature heat release observed in the engine experiments. Overall, the results indicate that isopentanol has a good potential as a HCCI fuel, either in neat form or in blend with gasoline.

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TYPE SAND Report YEAR 2010
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Results 151–175 of 222
Results 151–175 of 222