Investigation of anisole combustion in laminar premixed low-pressure flames
Thomas Bierkandt(1,*), Patrick Hemberger(2), Patrick Oßwald(3), Markus Köhler(3), Tina Kasper(1)
(1)
Mass Spectrometry in Reactive Flows – Thermodynamics, University of Duisburg-Essen, Germany
Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institute, Switzerland
(3)
Institute of Combustion Technology, German Aerospace Center (DLR), Germany
(2)
* Corresponding author; email address: [email protected]
Typical biofuels are oxygenates like ethanol or butanol and a significant source of biofuel
production is biomass. Depending on the source of the biomass (e.g. woody materials or
agricultural residues), it mainly consists of cellulose, hemicellulose and lignin in different
amounts. In chemical terms, lignin has a phenolic structure and the simplest compounds
which mirror this structure and are derived from lignin are phenol, anisole and guaiacol.
Therefore, they are important model compounds for lignin to study the phenolic-carbon
structure. Several studies on the pyrolysis and oxidation of anisole already exist but the
combustion of anisole in flames was not yet investigated.
Two fuel-rich laminar premixed low-pressure flames were investigated. Both anisole/O2/Arflames have a cold gas velocity of 65.2 cm/s with 50% argon dilution and a total gas flow of 4
slm at a pressure of 40 mbar. Burner diameter was 6 cm. The stoichiometry of the anisole
flame investigated at the Advanced Light Source (ALS) is 1.2 while it is 1.6 for the flame at
the Swiss Light Source (SLS). In both experiments, molecular-beam mass spectrometry is
used for identification and quantification of combustion intermediates formed during the
combustion of anisole. The instrument at the ALS provides a high-resolution reflectron timeof-flight mass spectrometer (m/Δm>3000). Therefore, it is possible to directly distinguish
between neat hydrocarbons and oxygenated compounds (e.g. propene and ketene on mass
42). At the SLS a photoelectron photoion coincidence (PEPICO) spectrometer is used. This
can be helpful for isomer identification, especially if three or more isomers are present [1].
Transport data were added to the kinetic reaction mechanism of Nowakowska et al. [2] so
that it can be used to simulate burner-stabilized flames. The temperature of the flame from
the ALS was measured by an Al2O3-coated thermocouple. In general, the mole fractions of
many combustion intermediates under these fuel-rich conditions are well predicted by the
model. Species with the highest measured mole fractions (on the order of 10-3-10-2) are CH3,
CH4, C2H2, C2H4, C2H6, CH2O, C5H5 (cyclopentadienyl radical), C5H6 (cyclopentadiene), C6H6
(benzene), phenol, and benzaldehyde. There are two major routes for the fuel
decomposition. The first one is the formation of benzaldehyde (C6H5CHO) starting with the
formation of the anisyl radical (C6H5OCH2) from anisole (C6H5OCH3) by H-abstraction.
Isomerization to C6H5CH2O and β-scission give finally benzaldehyde. The second route yields
phenol (C6H5OH) by unimolecular decomposition of anisole to phenoxy (C6H5O) and CH3
radicals. In the experiment, only the phenoxy radical could be measured while the anisyl
radical was not directly detectable. The phenoxy radical gives access to five-membered ring
species. The cyclopentadienyl radical is formed from phenoxy radicals over a bicyclic
structure and subsequent decarboxylation. A species which is totally underpredicted by the
model (more than one order of magnitude) is C6H8 which can be attributed based on the
PEPICO experiment to three methylcylopentadiene isomers. The presented speciation data
give an insight into the combustion kinetics of anisole under fuel-rich conditions.
[1] D. Felsmann et al., Proc. Combust. Inst. 35 (2015) 779–786.
[2] M. Nowakowska et al., Combust. Flame 161 (2014) 1474–1488.