Paper # 070LT-0076
Topic: Laminar Flames
8th U. S. National Combustion Meeting
Organized by the Western States Section of the Combustion Institute
and hosted by the University of Utah
May 19-22, 2013
Detailed Analysis of iso-Pentanol Combustion Chemistry
in Flames
Arnas Lucassen1, Julia Warkentin1, Nils Hansen1, Sungwoo Park2, S. Mani Sarathy2
1
Sandia National Laboratories, Combustion Research Facility, 7011 East Avenue, Livermore, CA,
94550 USA
2
King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi
Arabia
Abstract
In this study, two flames of iso-pentanol were stabilized on a 60-mm flat flame burner at a low pressure of
15 Torr and analyzed by a flame-sampling molecular-beam setup coupled to a mass spectrometer (MBMS). Singlephoton ionization by synchrotron-generated vacuum-UV radiation with high energy resolution (E/∆E ∼0.04 eV) and/or
electron ionization was combined with a custom-built reflectron time-of-flight spectrometer providing high mass
resolution (m/∆m = 3000). Mole fraction profiles for more than 40 flame species and the temperature profile were
determined experimentally. The flame temperatures were measured using OH laser induced fluorescence and used as
input parameters for the model calculations. The experimental dataset was used to guide the development of a
combustion chemistry model for the high-temperature oxidation chemistry of iso-pentanol. The chemical kinetic model
is herein validated for the first time against detailed speciation profiles of combustion intermediates and product species
including C5 branched aldehydes, enols, and alkenes. In a separated study, the model was validated against a number of
different datasets including low and high temperature ignition delay in rapid compression machines and shock tubes, jet
stirred reactor speciation data, premixed laminar flame speed, and opposed-flow diffusion flame strained extinction.
1. Introduction
Different biofuels are discussed to reduce usage of fossil fuels in order to prevent climate forcing by increase of carbon
dioxide levels in the atmosphere. Especially conversion of lignocellulosic biomass via biotechnological routes is of
interest.(Peralta-Yahya & Keasling, 2010; Weber et al., 2010) Upon the many aspects to be taken into account, like land
and water use in competition to food production, pollution prevention is a major concern. Biomass contains chemical
structures not present in fossil fuels and thus gives rise to different combustion chemistry. Previous analyses have shown
that the pollution potential of a fuel strongly depends on its molecular structure. Fuel-bound oxygen for instance may
give rise to an increase in harmful pollutants such as aldehydes.(Kohse-Höinghaus et al., 2010)
One potential model compound for the next-generation biofuels is iso-pentanol. This fuel is currently of interest, because
this branched alcohol can be produced from biomass via advanced biochemical routes using fungi.(Connor, Cann, &
Liao, 2010; Singh et al., 2011; G. Strobel et al., 2011; G. A. Strobel et al., 2008) Furthermore, it shows promising
combustion characteristics for controlling the operability of homogeneous-charge compression ignition (HCCI) engines.
There exist some data on the low temperature oxidation chemistry (Welz et al., 2012), low and high temperature ignition
delay in rapid compression machines and shock tubes as well as jet stirred reactor speciation data, premixed laminar
flame speed, and opposed-flow diffusion flame strained extinction. No comprehensive experimental and modeling study
of iso-pentanol flames exists, that featured detailed speciation profiles.
2.
Methods
Two laminar premixed flames of iso-pentanol were stabilized on a 60-mm flat flame burner. One stoichiometric and
the other slightly fuel rich (φ=1.2). Both flames had a total flow of four standard liters per minute and were diluted by
50% Argon. The flames, which were stabilized at a pressure of 15 Torr, were sampled by molecular-beam setup coupled
to a mass spectrometer (MBMS). The principle setup is shown in Figure 1 and basic procedures are described in (Cool
et al., 2003; Cool et al., 2005). The mass spectrometer therein was updated with an orthogonal TOF (Coles & Guilhaus,
1993) manufactured by Kaesdorf. This new mass spectrometer is capable of providing high mass resolution (m/∆m =
3000) like described in (Lucassen et al., 2012) at continuous ionization. Single-photon synchrotron-generated vacuumUV radiation with high energy resolution (E/∆E ∼0.04 eV) or high kinetic electrons were used for ionization. There are
two basic modes of operation “burner scans” where the sampling position is altered relative to the burner surface, thus
deriving the mole fraction profiles of the flame species over the reaction process, and “energy scans” were at one
position the photon energy is increase gradually thus allowing identification and separation of species based on their
ionization threshold. More than 40 flame species could be identified and their quantitative mole fraction profiles were
determined. The flame temperatures were measured using OH laser induced fluorescence and used as input parameters
for the model calculations. The experimental dataset was used to guide the development of a combustion chemistry
model for the high-temperature oxidation chemistry of iso-pentanol.
The kinetic modeling for iso-pentanol low-pressure
premixed flame was performed using the CHEMKIN
PRO premix flame code (PREMIX). The detailed
chemical kinetic mechanism for iso-pentanol is based on
previously proposed mechanism for the combustion of
the C4-alcohol (S. M. Sarathy et al., 2012) and extended
by adding 27 species and 136 reactions to represent the
oxidation of iso-pentanol and various intermediate
species. Thermodynamic data were calculated using the
THERM program of (Ritter & Bozzelli, 1991). The
correlations developed by (Tee, Gotoh, & Stewart, 1966)
were used for transport properties to calculate the LJ
collision diameter and potential well depth. The
formulation of the proposed chemical kinetic model and
its validation against the other datasets including ignition
delay in shock tubes and rapid compression machines, jet
stirred reactor speciation profiles, and flame propagation
and extinction is presented in (S.M. Sarathy & et al, Figure 1 Schematic drawing of the setup
2013) at this meeting.
3. Results and Discussion
Following the experience that fuel decomposition is usually initiated by H-atom abstraction from a C-H bond followed
by β-scission or oxidation of the fuel-radical, the likely destruction pathways of iso-pentanol are shown in Figure 2. isopentanol has four principle H-Abstraction sites with different abundance. There is only one possibility of abstraction to
form iso-pentanol-3-yl (1). Via β−scission or oxidation reaction, the following products can be formed: iso-butene (9)
and hydroxymethylene (10), which then gives formaldehyde (11), 3-methylbut-2-en-1-ol (6), and 2-methylbut-3-en-1-ol
(5). The latter one is also formed from iso-pentanol-4-yl (2). This radical itself is formed by abstraction of any H-atom at
any of the two methyl groups, which bear a total six identical H-atoms. Another β-scission reaction of 2 forms propene
(12) and 2-hydroxyethyl (13) potentially reacting to form ethenol (18). The third reaction of 2 leads to methyl (14) and
but-3-en-1-ol (15) which also can be produced starting from iso-pentanol-2-yl (3). 3 also reacts to 3-methylbut-2-en-1-ol
(6), 3-methylbut-1-en (16) and 3-methylbut-1-en-1-ol (7). The latter is also a β-scission and oxidation product of the last
initial isomer iso-pentanol-1-yl (4). 4 reacts also to iso-pentanal (8) as well as iso-propyl (17) and ethenol (18). The isopropyl subsequently forms propene (12).
2
Figure 2 Initial destruction pathways of iso-pentanol
Based on the exact masses provided by the new high resolution mass spectrometer and ionization thresholds obtain from
energy scans many flame species were identified. Especially for some high-mass values many isomers exist and no
attempt was made to separate their contributions experimentally. For instance, the isomers of the iso-pentanolyl radicals
(1-4) could not be separated, because of unknown and very similar ionization energies; furthermore some of the isomers
are expected to have maximum concentrations well below the detection limit. The isomer distribution for the initial
radicals in the modeling results is shown in Figure 3.
Figure 3 Isomer distribution in the kinetic model for a) the fuel-H radicals and b) ethenol and acetaldehyde
It can be seen that the statistically favored primary radical iso-pentanol-4-yl (2) is formed and consumed fast whereas
iso-pentanol-3-yl (1) shows the highest amounts and reaches its peak position at a position farther from the burner,
because of the stabilization by the inductive effect of the methyl groups. iso-pentanol-2-yl (3) lies in between those two,
while the other two radicals are much less stable. However, we successfully determined the isomer-resolved
contributions of the iso-pentanol-2H isomers (5, 6, 7, 8) shown in Figure 4 to be mainly iso-pentanal (8) with some 3methylbut-3-en-1-ol (5). In the simulation iso-pentanal and 3-methylbut-3-en-1-ol are also by far the most abundant
species.
3
Figure 4 a) Energy scan of C5H10O compared to the scaled cross section (XS) curves of the isomers b) isomere distribution in
the kinetic model for the stoichiometric flame
Of the smaller species present in the destruction scheme the isomers of C4H8 shows a small contribution of 2-buten
besides a big contribution of iso-buten. Ketene and propene, for which the ionization thresholds only differ by 0.1 eV,
can now unambiguously separated as shown in Figure 5.
Figure 5 Separation of ketene and propene a) High resolution mass spectrum b) Intensity as function of photon energy c)
integrate profiles as function of sampling position
All in all the above described destruction pathway is reflected very well in the experimental results and the model. For
example Figure 3 shows the distribution between ethenol and acetaldehyde in the modeling results with the much less
stable ethenol showing higher abundance than acetaldehyde.
Figure 6 Comparison of experimental mole fractions (symbols) with simulation results (lines) for the stoichiometric flame a)
Main species b) Ethylene
4
First quantitative experimental and model results are described here only briefly. The main species shown profiles in
Figure 6 are matched very well both in position and magnitude. For the minor species the position in the model results is
generally closer to the burner surface by about 1.5 mm as can be seen from Figure 6b. The magnitude and profile shape
however agree reasonably well.
4.
Conclusions
The species identified in those flames show several pollutants (formaldehyde, acetaldehyde, as well as higher keto
compounds are formed). There is also a sooting potential as benzene and its precursor propargyl are present in
considerable amounts. The model describes the general flame behavior already very well. The detailed destruction
pathways proposed here based on the experimental found species and profiles for isopentanol will help to further
improve the already mature model.
Disclaimer
This paper describes work in progress and is not meant as an archival publication.
Acknowledgements
This research was funded, sponsored and supported by the Energy Frontier Research Center for Combustion Science
(Grant No. DE-SC0001198), Sandia Corporation (Contract DE-AC04-94-AL85000) and the Advanced Light Source at
Lawrence Berkeley Laboratories (Contract No. DE-AC02-05CH11231). We also thank the Foreign Internship Support
Grand of Bielefeld University. Coauthors from King Abdullah University of Science and Technology acknowledge
funding from the Clean Combustion Research Center (CCRC). The measurements are performed within the “Flame
Team” collaboration at the Advanced Light Source. We thank the students and postdocs for the help with the data
acquisition. The experiments have profited from the expert technical assistance of Paul Fugazzi.
References
Coles, J., & Guilhaus, M. (1993). ORTHOGONAL ACCELERATION - A NEW DIRECTION FOR TIME-OF-FLIGHT
MASS-SPECTROMETRY - FAST, SENSITIVE MASS ANALYSIS FOR CONTINUOUS ION SOURCES.
Trac-Trends in Analytical Chemistry, 12(5), 203-213. doi: 10.1016/0165-9936(93)80021-b
Connor, M. R., Cann, A. F., & Liao, J. C. (2010). 3-Methyl-1-butanol production in Escherichia coli: random
mutagenesis and two-phase fermentation. Applied Microbiology and Biotechnology, 86(4), 1155-1164. doi:
10.1007/s00253-009-2401-1
Cool, T. A., Nakajima, K., Mostefaoui, T. A., Qi, F., McIlroy, A., Westmoreland, P. R., . . . Ahmed, M. (2003). Selective
detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry.
Journal of Chemical Physics, 119(16), 8356-8365. doi: 10.1063/1.1611173
Cool, T. A., Nakajima, K., Taatjes, C. A., McIlroy, A., Westmoreland, P. R., Law, M. E., & Morel, A. (2005). Studies of
a fuel-rich propane flame with photoionization mass spectrometry. Proceedings of the Combustion Institute, 30,
1681-1688. doi: DOI 10.1016/j.proci.2004.08.103
Kohse-Höinghaus, K., Osswald, P., Cool, T. A., Kasper, T., Hansen, N., Qi, F., . . . Westmoreland, P. R. (2010). Biofuel
Combustion Chemistry: From Ethanol to Biodiesel. Angewandte Chemie-International Edition, 49(21), 35723597. doi: 10.1002/anie.200905335
Lucassen, A., Zhang, K. W., Warkentin, J., Moshammer, K., Glarborg, P., Marshall, P., & Kohse-Hoinghaus, K. (2012).
Fuel-nitrogen conversion in the combustion of small amines using dimethylamine and ethylamine as biomassrelated model fuels. Combustion and Flame, 159(7), 2254-2279. doi: DOI 10.1016/j.combustflame.2012.02.024
Peralta-Yahya, P. P., & Keasling, J. D. (2010). Advanced biofuel production in microbes. Biotechnology Journal, 5(2),
147-162. doi: 10.1002/biot.200900220
Ritter, E. R., & Bozzelli, J. W. (1991). Therm - Thermodynamic Property Estimation for Gas-Phase Radicals and
Molecules. International Journal of Chemical Kinetics, 23(9), 767-778. doi: DOI 10.1002/kin.550230903
Sarathy, S. M., & et al. (2013). A comprehensive experimental and modeling study of iso-pentanol combustion. 8th
Annual US Combustion Meeting, 2013, Salt Lake City, Utah.
Sarathy, S. M., Vranckx, S., Yasunaga, K., Mehl, M., Osswald, P., Metcalfe, W. K., . . . Curran, H. J. (2012). A
comprehensive chemical kinetic combustion model for the four butanol isomers. Combustion and Flame,
159(6), 2028-2055. doi: DOI 10.1016/j.combustflame.2011.12.017
Singh, S. K., Strobel, G. A., Knighton, B., Geary, B., Sears, J., & Ezra, D. (2011). An Endophytic Phomopsis sp
Possessing Bioactivity and Fuel Potential with its Volatile Organic Compounds. Microbial Ecology, 61(4), 729739. doi: 10.1007/s00248-011-9818-7
5
Strobel, G., Singh, S. K., Riyaz-Ul-Hassan, S., Mitchell, A. M., Geary, B., & Sears, J. (2011). An endophytic/pathogenic
Phoma sp from creosote bush producing biologically active volatile compounds having fuel potential. Fems
Microbiology Letters, 320(2), 87-94. doi: 10.1111/j.1574-6968.2011.02297.x
Strobel, G. A., Knighton, B., Kluck, K., Ren, Y., Livinghouse, T., Griffin, M., . . . Sears, J. (2008). The production of
myco-diesel hydrocarbons and their derivatives by the endophytic fungus Gliocladium roseum (NRRL 50072).
Microbiology-Sgm, 154, 3319-3328. doi: 10.1099/mic.0.2008/022186-0
Tee, L. S., Gotoh, S., & Stewart, W. E. (1966). Molecular Parameters for Normal Fluids - Lennard-Jones 12-6 Potential.
Industrial & Engineering Chemistry Fundamentals, 5(3), 356-&. doi: Doi 10.1021/I160019a011
Weber, C., Farwick, A., Benisch, F., Brat, D., Dietz, H., Subtil, T., & Boles, E. (2010). Trends and challenges in the
microbial production of lignocellulosic bioalcohol fuels. Applied Microbiology and Biotechnology, 87(4), 13031315. doi: 10.1007/s00253-010-2707-z
Welz, O., Zador, J., Savee, J. D., Ng, M. Y., Meloni, G., Fernandes, R. X., . . . Taatjes, C. A. (2012). Low-temperature
combustion chemistry of biofuels: pathways in the initial low-temperature (550 K-750 K) oxidation chemistry
of isopentanol. Physical Chemistry Chemical Physics, 14(9), 3112-3127. doi: Doi 10.1039/C2cp23248k
6