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

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(CO2), 40 (Ar), 32 (CH3OH and O2), 31 (CH3O), 30 (CH2O), 29 (HCO), 28 (CO), 18 (H2O), 15 (CH3), 16 (CH4), 4 (He).

It should be noted that we did not find a distinct peak at m/z = 59 (CH2OCHO and CH3OCO), which are produced by the

hydrogen abstraction from MF. This indicates that the decomposition rate of the CH2OCHO and the CH3OCO radical

into CH2O + CHO, CH3O + CO and CH3 + CO2 is fast enough, and the concentration of these species were too low to be

detected at the sampling point. It should be noted again that each detected peak was not only from a combustion product

but also from a fragment of larger species than the target species. Thus, hereafter, the contributions of the fragment ions

from the MF, the CH3OH, the CO2 and the O2 ions were eliminated from each peak following to Eq. (1).

3.2 Pyrolysis

3.2.1 Measured and predicted species profiles

Figure 3 shows the profiles of the measured and the predicted major species for the pyrolysis conditions of

0.945/0.05/0.005 He/Ar/MF at atmospheric pressure with residence time of 600 milliseconds. As shown in Fig. 3 (a) and

(b), although the concentration of MF was almost constant at the temperature lower than 700 K, a very small amount of

CH3OH and CO formation was detected at 700 K. This is probably due to the surface catalytic reaction observed at the

inlet of a flow reactor in Dooley et al., (2010).

The concentration of MF started to decrease gradually with the increase of the major intermediate species at the

temperature as low as 700 K. The concentration decreased rapidly at T = 900 K - 925 K with the significant increase of

CO concentration, and subsequently decreased gradually again at T = 925K - 1000 K. Both models by Westbrook et al.

(2009) and Diévart et al. (2013) show the similar trend. However, both models predict that the MF concentration starts to

decrease at higher temperature and drops more rapidly with the temperature increase. This indicates that these models

under-predict the reactivity of the fuel consumption at the low temperature range while over-predict the reactivity at the

high temperature range.

The measured concentrations of CH3OH and CO started to increase at the temperature as low as 700 K. It should be

noted that the measured CO concentration was approximately at the same level as the CH3OH concentration at T = 800

K - 850 K, while the CO concentration became more than twice of the CH3OH concentration at the temperature higher

than 900 K where the CH3OH concentration still increases. As described later, this trend implies that, before CH3OH

starts to decompose, the hydrogen abstraction from MF forming the CH2OCHO radical becomes more dominant than the

concerted elimination reaction of MF = CH3OH + CO.

On the other hand, the model prediction shows that the CH3OH and the CO concentration start to increase at 850 K.

This means that the models under-predict the reactivity of the MF decomposition as described above. In addition, the

model of Westbrook et al. (2009) shows a peak of the CH3OH concentration at T = 973 K, where both of the experiment

and the model prediction of Diévart et al. (2013) show that the concentration still increases. This indicates that the model

of Westbrook et al. (2009) over-predicts of the decomposition rate of CH3OH. Moreover, the model prediction of

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Diévart et al. (2013) shows that the CO concentration is at similar level to the CH3OH concentration until the CH3OH

concentration has a peak at T = 1023 K. The model prediction of Westbrook et al. (2009) reproduces the experimental

trend better since it shows that CO concentration is larger than the CH3OH concentration before the CH3OH

concentration has a peak at T = 973 K.

The measured CO2 concentration was as low as 40 ppm - 100 ppm while the CH4 concentration was 30 ppm – 50

ppm over the temperature range examined (Fig. 3c). On the other hand, both models over-predict both of the CO2 and

CH4 concentrations for the temperature higher than 925 K.

In the present measurement, the concentration increase of CH2O was very slightly observed at the temperature

higher than 975 K while that of HCO radical was distinctly observed from the temperature as low as 800 K (Fig. 3d).

The reason that the measured CH2O concentration was much smaller than that of the HCO radical is probably because

most of the CH2O ions decomposed into its fragment ions of HCO radical due to the excessive energy of an electron

impact.

In the concentration profile, the HCO radical concentration, which also represents the CH2O concentration,

increased from T = 800 K to 850 K and subsequently became almost constant. This indicates that the HCO radical is

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kept accumulated before the production rate of the HCO radical became in balance with its decomposition rate into CO

and H. On the other hand, it is found that both model predictions of Westbrook et al. (2009) and Diévart et al. (2013)

over-predict the CH2O concentration at its peak approximately by a factor of two and four, respectively. Since the

CH2OCHO radical, which decomposes into CO through CH2O and HCO, should not be over-predicted in the modeling

as describe above, the reason that the models over-predict the peak concentration of CH2O is probably because they

under-predict the CH2O consumption.

3.2.2 Reaction path analysis

Figure 4 shows the diagram of the reaction rate of progress calculated with the model of Diévart et al. (2013). The

temperature is set at 973 K, since a significant discrepancy between the experimental results and the model predictions

was observed in the CH3OH and the CO formation. The model shows that MF is consumed almost exclusively by

concerted elimination reaction forming CH3OH + CO (88.6%), CH4 + CO2 (7.6%) and CH2O + CH2O (3.4%).

Contributions of hydrogen abstraction from MF forming CH2OCHO and CH3OCO, which produce CO and CO2 through

the decomposition, are as small as 0.3% and 0.1%, respectively. In the experiment, the CO concentration (2527 ppm)

was more than twice of the CH3OH concentration (1052 ppm) at T = 975 K. The fact the measured CO concentration

was higher than its counterpart species of the concerted elimination reaction implies that the production of the

CH2OCHO radical is underestimated in the model. This conjecture is supported by the measured HCO and CH2O

concentration, which is about four times larger than the model prediction at T = 973 K. Therefore, it is concluded that the

reaction pathway for the hydrogen abstraction from MF on the methyl site is significantly underestimated in the

modeling.

3.3 Oxidation

3.3.1 Measured and predicted species profiles

Figure 5 shows the profiles of the measured and the predicted major species for the oxidation conditions of

0.940/0.05/0.005/0.005 He/Ar/O2/MF at atmospheric pressure with residence time of 600 milliseconds. As shown in Fig.

5 (a) and (b), the concentration of MF were almost constant with a little increase of CH3OH and CO for the temperature

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lower than 700 K, and started to decrease with the increase of the CH3OH and CO concentrations at the temperature

higher than 700 K. This result is in the same trend as the pyrolysis study, indicating that there is no low temperature

chemistry for MF. However, in the MF oxidation process, the MF concentration decreased more rapidly when the

temperature was increased to 925 K - 1000 K. This result is reasonable because the oxygen addition increases hydrogen

abstraction reaction from MF, and the resulting abundant radicals such as OH and HO2 should further promote the MF

consumption. The model predictions show a similar trend, but once again they over-predict the temperature where the

MF consumption starts and also over-predict the sensitivity of the MF consumption to the temperature increase.

As shown in Figure 5b, the measured CH3OH concentration started to increase at the temperature as low as 700 K

just like the pyrolysis study. However, unlike pyrolysis, the CH3OH concentration had a local peak around T = 925 K,

and subsequently decreased at higher temperature. This indicates that CH3OH oxidation starts at this temperature.

Moreover, the CO concentration was at a similar level to the CH3OH concentration until the CH3OH concentration had a

peak around T = 925 K.

Both models of Westbrook et al. (2009) and Diévart et al. (2013) reproduce the experimental trend that the CH3OH

concentration has a local peak, although they over-predict the peak location and slightly under-predict the peak value. It

should also be noted that, unlike the pyrolysis study, the model of Westbrook et al. (2009) over-predict the CO

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concentration until the CH3OH concentration has a peak at T = 943 K. On the other hand, the model of Diévart et al.

(2013) predicts the experimental trend better in CO and CH3OH. The CO concentration is at a similar level to the

CH3OH concentration until the CH3OH concentration has a peak at T = 963 K.

As shown in Figure 5c, the measured CO2 concentration started to increase at the same temperature as the CO

concentration increases. The measured CO2 concentration at T = 925 K is 646 ppm, which is much higher than that of

CH4. Two reasons are conjectured for the CO2 increase. One is that it comes from the decomposition of the CH3OCO

radical since the increase of the CH4 concentration, which is the counterpart species of the concerted elimination reaction

forming CH4 + CO2, starts at the higher temperature than that of CO2. The other is that it comes from the oxidation of

CO, which is produced through the decomposition of CH2OCHO radicals. On the other hand, in the model predictions,

the CO2 concentration is under-predicted before the CH3OCO starts to be oxidized while over-predicted after that. The

CH4 concentration is reproduced well in the model predictions.

The measured H2O concentration increased with the temperature increase unlike the pyrolysis study, implying that

there were abundant OH radicals. This supports the above-mentioned conjecture that the CO oxidation was promoted in

the oxidation. Like the CO2 concentration, the models also under-predicts the H2O concentration before the CH3OCO

starts to be oxidized while over-predicted after that.

The measured CH2O and the HCO concentration show the same trend as the pyrolysis study except that the CH2O

concentration is higher in the oxidation than in pyrolysis (Fig. 5d). On the other hand, the models predict the peak value

better in the oxidation study than in the pyrolysis study, implying that they predict the CH2O consumption better.

3.3.2 Reaction path analysis

Figure 6 shows the diagram of the reaction rate of progress calculated with the model of Diévart et al. (2013) for the

oxidation. The temperature is set at 923 K, at which the predicted CH3OH concentration becomes half of its peak value,

since a significant discrepancy between the experimental results and the model predictions was observed in the CO2

concentration. The model shows that MF is consumed by concerted elimination reaction forming CH3OH + CO (88.5%),

CH4 + CO2 (7.6%) and CH2O + CH2O (3.4%). Contributions of hydrogen abstraction from MF forming CH2OCHO and

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CH3OCO are as small as 0.3% and 0.2% respectively, which is very similar to the pyrolysis study although OH radical

plays a more role in the hydrogen abstraction. Then, the CO concentration is only 1.5 % larger than the CH3OH

concentration.

On the other hand, the measured CO2 concentration were much higher than the CH4 concentrations at T = 850 K, at

which the measured CH3OH concentration becomes half of its peak value. Moreover, the measured CO2 concentration in

the oxidation study is much higher than in the pyrolysis study, while the CO concentration is at similar level to that of

the pyrolysis. The fact that the CO2 concentration was much higher than its counterpart species of the concerted

elimination reaction implies that the production of the CH3OCO radicals is underestimated in the model. Therefore, it is

concluded that the reaction pathway for the hydrogen abstraction from MF on the ester site should be more intensified in

the kinetic models for the oxidation study.

4. Conclusions

The pyrolysis and the oxidation of methyl formate (MF) have been studied in a quartz flow reactor at atmospheric

pressure over the temperature range of 500 K to 1000 K with the aid of molecular beam mass spectrometry.

For the pyrolysis study, a discrepancy between the experimental results and the model predictions was observed

especially in the CH3OH/CO formation before the CH3OH starts to decompose: The measured CO concentration is more

than twice of the CH3OH concentration. The fact that the measured CO concentrations was larger than its counterpart

species of the concerted elimination reaction forming CH3OH + CO leads us to the conclusion that the reaction pathway

for the hydrogen abstraction from MF on the methyl site is underestimated in the modeling.

For the oxidation study, a discrepancy between the experimental results and the model predictions was observed

especially in the CO2 formation before the CH3OH starts to be oxidized: The measured CO2 concentration was much

higher than the CH4 concentration. Moreover, the measured CO2 concentration in the oxidation was also much larger

than in the pyrolysis, while the measured CO concentration was at a similar level to that in MF pyrolysis. From the

experimental measurements, it can be concluded that, especially for the oxidation study, the reaction rates for the

hydrogen abstraction on the ester site should be higher than those in the kinetic models.

Acknowledgements This work is supported as part of the Combustion Energy Frontier Research Center, funded by the United States

Department of Energy, office of Science, Office of Basic Energy Sciences under Award Number De-SC0001198. This

work is also supported by DENSO CORPORATION.

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