Sandia CRF team provides experimental confirmation of oxidation scheme of lower emissions diesel alternative DME; new intermediates
17 October 2015
An international team of researchers led by a group from the Combustion Research Facility (CRF) at Sandia National Laboratories recently provided experimental confirmation of the generally accepted low-temperature oxidation scheme of dimethyl ether (DME)—a lower soot and emissions alternative to diesel—at low temperatures (~540 K, 267˚C). Their paper was published in the ACS Journal of Physical Chemistry A.
Especially significant, they said, was detecting and identifying keto-hydroperoxide (hydroperoxymethylformate, HPMF, HOOCH2OCHO)—a previously undiscovered partially oxidized intermediate—thereby providing critical information needed to improve models.
… because innovative engine designs like homogeneous charge compression ignition (HCCI) engines rely on the control of low-temperature ignition timing, it becomes increasingly necessary to explore the low-temperature combustion (LTC) of transportation fuels. DME has been proposed as an alternative and/or additive to conventional diesel fuel, and it is of particular interest because it has a high cetane number (≥55), low soot emission characteristics, and interesting physical–chemical properties (such as a low boiling point, easy liquefaction under pressure, and photochemical inertness). Also, it can be massively produced both from biomass and from fossil fuel re-forming.
DME represents the simplest molecular structure (CH3OCH3) with rich low-temperature chemistry and the recent experiments with advanced diagnostics clearly showed that the description of DME’s low-temperature chemistry has a large uncertainty in the decomposition pathways and/or the branching ratio of the keto-hydroperoxide species.
Given its great application potential and the simplicity in molecular structure, the high-temperature combustion chemistry of DME has also been extensively studied. Because of the importance of low-temperature chemistry in determining fuel effects in autoignition, the present paper focuses on the key reactions and intermediates of the low-temperature regime, and the results of the high-temperature chemistry studies will not be reviewed here.
—Moshammer et al.
Recent experiments has clearly showed that the description of DME’s low-temperature chemistry contained some uncertainty in the decomposition pathways and the branching ratio of the elusive keto-hydroperoxide species.
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| Schematic representation of the experimental setup. Shown in the figure is the jet-stirred reactor that is located within an oven, all surrounded by a water-cooled stainless steel chamber. Molecules are sampled from the reactor through a quartz probe, ionized via single-photon ionization with vacuum-ultraviolet photons, and the respective ions are mass-selected using a reflectron time-of-flight mass spectrometer. Credit: ACS, Moshammer et al. Click to enlarge. |
The detection of this important intermediate in DME’s LTC regime was made possible by coupling two tools:
A continuously jet-stirred tank reactor (JSR) that is well suited for gas-phase kinetic studies and equipped with molecular-beam sampling capabilities, operated at near atmospheric pressure.
A reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation.
The two tools allowed the international research team simultaneously to detect many intermediates, including radicals and other very reactive species, such as peroxides.
Coupling the mass spectrometer to a tunable vacuum-ultraviolet photon source, such as the chemical dynamics beamline at Lawrence Berkeley National Laboratory’s (LBNL) Advanced Light Source, enabled recording of photoionization efficiency (PIE) curves that permit species identification based on their mass-to-charge ratio (m/z) and their ionization thresholds.
The team identified HPMF on the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations.
Interpreting the experimentally observed ionization thresholds required theoretical characterization of multiple HPMF conformeric structures. This important finding indicates that the usual strategy of considering only the lowest-energy conformer might lead to a significantly different prediction for the observable ionization threshold and thus to potentially erroneous isomeric assignments.
The team’s experiments also detected HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid),and HOC(O)OH (carbonic acid). Though all are conceivable HPMF decomposition products, they are not included in many DME oxidation mechanisms.
In future work, attempts will be made to quantify the identified species. Once reliable photoionization cross sections of the detected species become available, absolute mole fractions will be accessible for different temperature and flow conditions. Such data will then provide validation targets for the development of chemical models and will ultimately help to identify the important low-temperature oxidation pathways.
—Moshammer et al.
Resources
Kai Moshammer, Ahren W. Jasper, Denisia M. Popolan-Vaida, Arnas Lucassen, Pascal Diévart, Hatem Selim, Arkke J. Eskola, Craig A. Taatjes, Stephen R. Leone, S. Mani Sarathy, Yiguang Ju, Philippe Dagaut, Katharina Kohse-Höinghaus, and Nils Hansen (2015) “Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether” The Journal of Physical Chemistry A 119 (28), 7361-7374 doi: 10.1021/acs.jpca.5b00101
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