Paper 070RK 070RK 070RK 070RK-0168 0168 0168 0168 Topic: Reaction Kinetics 8 th 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 Studies of pyrolysis and oxidation of methyl formate using molecular beam mass spectrometry Naoki Kurimoto, Xueliang Yang and Yiguang Ju Department of Mechanical and Aerospace Engineering, Princeton University, Princeton NJ-08542, United States Molecular beam mass spectrometry with electron impact ionization coupled with a quartz flow reactor has been employed to study the pyrolysis and the oxidation mechanism of methyl formate (MF) at atmospheric pressure. The measurement was carried out with a mixture of gas phase at 5000 ppm MF in Helium and Argon dilution in the temperature range from 500 K to 1000 K with the reaction residence time of 600 milliseconds. Important stable species such as methanol, carbon monoxide, carbon dioxide, formaldehyde, water as well as radical species such as HCO were quantified in the mass spectrum. Effect of electron impact fragmentation of MF and methanol on species measurements is calibrated and subtracted. Experimental uncertainty is estimated to be approximately 10% for MF at 95% confidence. The experimental results showed that pyrolysis and oxidation take place for the temperature higher than 700 K with the increase of products such as methanol, carbon monoxide and carbon dioxide. Numerical simulations using a Princeton Ester-Mech kinetic model and a LLNL model have been performed. The results show that the model under-predicts the formation of CO for the pyrolysis study and CO 2 for the oxidation study, respectively. The discrepancy between the measured and the predicted profile implies that the reaction pathways for hydrogen abstractions from MF forming CH 2 OCHO and CH 3 OCO are under-predicted in the modeling. 1. Introduction Fuel diversity is essential to a sustainable energy society. Fatty acid methyl esters made from plant oil such as palm oil, soybean oil, rapeseed oil, sunflower oil and Jatropha oil are the major compositions of biodiesels. Production of biodiesels is under way in earnest since the finite nature and the price increase of the conventional fossil fuel is now widely recognized. Nevertheless, to be a major transportation fuel, biodiesels still face several technical challenges. One of the challenge is that biodiesel is much more expensive than conventional fossil fuels. Another challenge is that they require conventional engines to be extensively redesigned to meet emission regulations. In order to overcome these disadvantages and make biodiesels highly competitive in the market, it is necessary to understand their reaction kinetics in detail so that we could actively utilize its chemical properties to improve an engine performance with a resultant economic advantage. Especially, development of a validated methyl ester kinetic model is practically important to facilitate parametric studies with conventional engines (Herbinet et al., 2008; Diévart et al., 2012 and 2013; Dooley et al., 2008). Methyl formate (MF: CH 3 OCHO) is the simplest methyl ester and thus is an ideal molecule to isolate the effect of the ester functionality on its combustion process. Kinetic modeling or experimental studies of the methyl formate pyrolysis and oxidation have been carried out in the past decade, and the kinetic models have reproduced experimental results (Fisher et al., 2000). Especially, Dooley et al. (2010) developed the kinetic model and could reproduce the production of stable intermediate species formed in a flow reactor at a fixed temperature. However, in their experiment, significant amount of major intermediate species were detected downstream a diffuser of the flow reactor due to catalytic reaction, and thus the initial mixture was not well characterized. In addition, since their experiment was carried out at the fixed temperature of 975 K, experimental data across a range of temperature lower than 1000 K is not yet adequate to
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Paper 070RK070RK070RK070RK----0168016801680168 Topic: Reaction Kinetics
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
Studies of pyrolysis and oxidation of methyl formate using
molecular beam mass spectrometry
Naoki Kurimoto, Xueliang Yang and Yiguang Ju
Department of Mechanical and Aerospace Engineering, Princeton University,
Princeton NJ-08542, United States
Molecular beam mass spectrometry with electron impact ionization coupled with a quartz flow reactor has been
employed to study the pyrolysis and the oxidation mechanism of methyl formate (MF) at atmospheric pressure. The
measurement was carried out with a mixture of gas phase at 5000 ppm MF in Helium and Argon dilution in the temperature
range from 500 K to 1000 K with the reaction residence time of 600 milliseconds. Important stable species such as methanol,
carbon monoxide, carbon dioxide, formaldehyde, water as well as radical species such as HCO were quantified in the mass
spectrum. Effect of electron impact fragmentation of MF and methanol on species measurements is calibrated and subtracted.
Experimental uncertainty is estimated to be approximately 10% for MF at 95% confidence. The experimental results showed
that pyrolysis and oxidation take place for the temperature higher than 700 K with the increase of products such as methanol,
carbon monoxide and carbon dioxide. Numerical simulations using a Princeton Ester-Mech kinetic model and a LLNL
model have been performed. The results show that the model under-predicts the formation of CO for the pyrolysis study and
CO2 for the oxidation study, respectively. The discrepancy between the measured and the predicted profile implies that the
reaction pathways for hydrogen abstractions from MF forming CH2OCHO and CH3OCO are under-predicted in the
modeling.
1. Introduction
Fuel diversity is essential to a sustainable energy society. Fatty acid methyl esters made from plant oil such as palm
oil, soybean oil, rapeseed oil, sunflower oil and Jatropha oil are the major compositions of biodiesels. Production of
biodiesels is under way in earnest since the finite nature and the price increase of the conventional fossil fuel is now
widely recognized. Nevertheless, to be a major transportation fuel, biodiesels still face several technical challenges. One
of the challenge is that biodiesel is much more expensive than conventional fossil fuels. Another challenge is that they
require conventional engines to be extensively redesigned to meet emission regulations. In order to overcome these
disadvantages and make biodiesels highly competitive in the market, it is necessary to understand their reaction kinetics
in detail so that we could actively utilize its chemical properties to improve an engine performance with a resultant
economic advantage. Especially, development of a validated methyl ester kinetic model is practically important to
facilitate parametric studies with conventional engines (Herbinet et al., 2008; Diévart et al., 2012 and 2013; Dooley et al.,
2008).
Methyl formate (MF: CH3OCHO) is the simplest methyl ester and thus is an ideal molecule to isolate the effect of
the ester functionality on its combustion process. Kinetic modeling or experimental studies of the methyl formate
pyrolysis and oxidation have been carried out in the past decade, and the kinetic models have reproduced experimental
results (Fisher et al., 2000). Especially, Dooley et al. (2010) developed the kinetic model and could reproduce the
production of stable intermediate species formed in a flow reactor at a fixed temperature. However, in their experiment,
significant amount of major intermediate species were detected downstream a diffuser of the flow reactor due to catalytic
reaction, and thus the initial mixture was not well characterized. In addition, since their experiment was carried out at the
fixed temperature of 975 K, experimental data across a range of temperature lower than 1000 K is not yet adequate to
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validate those kinetic models. Later, Diévart et al.(2013) performed the flame extinction limit studies of one sets of small
methyl esters, and concluded that methyl formate combustion character heavily relies on the reaction kinetics of
methanol which is one of the main consumption pathway for methyl formate. Therefore, a more careful combustion
kinetics model of methanol and methyl formate including theoretical assessment on the relevant reaction rates and more
serious test against the currently available experimental observation has been developed (Diévart et al., 070RK-0276).
The final goal of the present study is to provide reliable experimental data of the major intermediate species such as
methanol, carbon monoxide, carbon dioxide, methane and formaldehyde, and provide constraints on the prediction of
branching reaction pathways of methyl formate.
2. Experimental methods
2.1 Flow facility and Flow reactor
A reactant mixture supplied to an atmospheric flow reactor consists of helium (He), argon (Ar), oxygen (O2) and
methyl formate (MF). The helium, the argon and the oxygen were supplied from industrial-grade compressed gas
cylinders, and each flow rate was regulated by a mass flow controller (MKS instruments, 1179A Mass-Flo). These gases
were mixed to form a career gas. The career gas was preheated up to 306 K in order to promote vaporization of the
methyl formate. The methyl formate (Sigma-Aldrich, 99% purity) was injected into the preheated career gas from a fine
tube with the inner diameter of 0.3 mm. The small inner diameter allows rapid preheating and reduces timescale of
temperature of non-uniformity. The flow rate of the methyl formate was controlled by a syringe pump with a 200 ml
stainless steel syringe (Harvard Apparatus, PHD2000 HPSI). This injection part was placed at 700 mm upstream of a
flow reactor to ensure homogeneous mixing. The mixture flowed to the flow reactor through a stainless tube at a room
temperature, and thus any reaction upstream the flow reactor should be negligible.
A cylindrical quartz tube of 17 mm inner diameter and 355 mm in length was employed as the flow reactor. The
reactor was tightly jacketed within a copper sleeve, and the assembly was placed inside an oven to generate a uniform
temperature profile in the reactor. The reactor temperature was controlled by a PID temperature controller with a K-type
thermocouple installed inside at the streamwise center of the reactor. In order to heat the reactant mixture up to the set
temperature for the flow reactor rapidly, a preheating section with the inner diameter of 2 mm was built at the entrance of
the reactor. The residence time of the mixture in the preheating section is less than 1% of the total residence time.
In this study, a He/Ar/O2/MF mixture of 0.945/0.05/0.0/0.005 and 0.94/0.5/0.05/0.05 mole fraction was employed
for the pyrolysis experiment and the oxidation experiment, respectively. Experiments were carried out at atmospheric
pressure for the temperature ranging from 500 K to 1000 K with the variation by less than +-3 K. The reaction residence
time was set to be 600 milliseconds and the corresponding flow rate was adjusted in order to keep the constant residence
time when the set reactor temperature was varied.
2.2 Concentration measurements through EI-MBMS
Electron-Ionization Molecular Beam Mass Spectrometry (EI-MBMS) was employed to quantify the concentration of
reactants, intermediate species, and products. Figure 1 shows the schematic of the molecular beam mass spectrometry.
The EI-MBMS consists of a sampling-chamber with a quartz sampling nozzle and a skimmer, an ionization-chamber, an
electron gun (e-gun) and a time-of-flight mass spectrometer. Detailed descriptions of the instrument are given elsewhere
(Guo et al., 2013). The pressures in the sampling-chamber, the ionization-chamber and the time-of-flight mass
spectrometer reached 3×10-4 Torr, 5×10
-6 Torr and 5×10
-7 Torr, respectively. In order to suppress a fragmentation of
target species, ionization electron energy of the e-gun was set to be as small as 12 eV with the emission current of 0.1
mA. Moreover, voltage parameters of the time-of-flight mass spectrometer, such as a pulsed ion-extraction field, were
optimized in such a way that each detected peak in a spectrum became a Gaussian profile that provides the minimum
mass resolution among all parameter combinations examined. The nominal mass resolutions, m/∆m, based on full width
at half maximum were thus 490, 800, 710, and 830 for He, Ar, O2 and MF peaks, respectively. The maximum
background random noise was as small as 2 counts, and the resulting signal-to-noise ratio was ensured larger than 20 for
all detected peaks.
In the present study, since the mass resolution was approximately 710 at the mass-to-charge ratio (m/z) of 32, a
methanol peak at m/z = 32.026, which is one of the most important species in MF pyrolysis and oxidation, somewhat
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overlapped an oxygen peak at m/z = 31.989. In order to separate the methanol and the oxygen peaks, two Gaussian
functions were employed to fit the spectrum around m/z = 32. The signal of each species was defined as the integral
from positive infinity to negative infinity of the Gaussian function. The uncertainty due to the fitting itself was
statistically calculated to be 6%.
The concentration calibration of the EI-MBMS was conducted by flowing mixtures with known compositions.
Detailed descriptions of the calibration process are given elsewhere (Guo et al., 2013). In the present study, the
calibrations were directly carried out for MF, O2, CH3OH, CH4, CO and CO2, while the calibration using the
experimentally determined mass discrimination factors and the ionization cross sections obtained from the database of
National Institute of Standard and Technology (NIST) were carried out for CH2O, HCO and H2O. For the reference gas,
Ar 5.0% was employed, and all ion signals were normalized by the intensity of argon ion signal in order to correct the
signal drop due to the decrease of the amount of the sampling gas when the temperature is increased.
EI fragmentations from MF and CH3OH were not negligible in the spectrum analysis. That is, MF fragmented into
CH3OH, CH3O, CH2O, HCO, CO and CH3 by the excessive energy due to an electron impact, while CH3OH fragmented
into CH3O, CH2O, HCO and CH3. Thus, in the present study, the effect of the fragment ions from the MF and the
CH3OH ions on the target species was eliminated following to the equation,
� � ���∙ � (1)
where X, C and S denote a concentration vector, a calibration matrix, and a signal vector, respectively. Each column of
the matrix C is the fragment spectrum experimentally obtained in the calibration.
Measurement uncertainty was systematically analyzed following to the guideline provided by NIST (Taylor and
Kuyatt, 1994). The uncertainty budget showed that the random effect of the signal intensity normalized by Ar signal is a
dominant source in the total uncertainty. Thus, in the present study, a total of three experiments were carried out to
calculate the average, and then the total uncertainty for MF was estimated to be 10% at 95% confidence.
2.3 Simulation
Two kinetic models for MF, the Princeton Ester-Mech (Diévart et al. 2013) and the LLNL model (Westbrook et al.
2009) were employed. Model predictions were obtained by using the software package Cantera (Goodwin, 2001-2005).
A reactor model was solved under a constant temperature condition for the reaction residence time of 600 milliseconds,
assuming that the temperature profile is uniform along the axis of the flow reactor.
3. Results and Discussion
3.1 Mass spectrum
Figure 2 shows a typical mass spectrum for He/Ar/O2/MF (0.94/0.05/0.005/0.005) mixture at T=1000 K with the
residence time of 600 milliseconds. Distinct mass peaks were detected at the mass charges of 60 (MF: CH3OCHO), 44