Methane and Methyl Propanoate High-Temperature Kinetics - Energy

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Experimental study of methane and methyl propanoate high-temperature kinetics Shirin Jouzdani, Xuan Zheng, Deshawn M. Coombs, and Benjamin Akih-Kumgeh Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02277 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Experimental study of methane and methyl propanoate high-temperature kinetics Shirin Jouzdani, Xuan Zheng, Deshawn M.Coombs, and Benjamin Akih-Kumgeh∗ Syracuse University, Department of Mechanical and Aerospace Engineering, 263 Link Hall, Syracuse, NY 13244, USA E-mail: [email protected] Phone: +1 (315)443-2335. Fax: +1 (315)443-6999

Abstract The biodiesel surrogate, methyl propanoate (MP), is more reactive than methane. Mixtures of the two can be used to control combustion initiation in various combustion systems. Reported here is a shock tube study of the influence of chemical interactions resulting from mixing the two fuels on observable combustion properties, such as global chemical time scales and species time histories. Experiments are carried out at pressures of about 4, 7.4, and 10 atm covering a temperature window of 1000 K to 1500 K. Using direct laser absorption, CO time histories during MP pyrolysis are obtained. The CO absorbance is further used to determine pyrolysis times by means of which the effect of temperature on MP pyrolysis is probed. Reactivity differences are first examined with the fuel concentration maintained at 3% and then with the oxygen concentration fixed at 10%. The evidence of chemical interactions during ignition is observed through a reduction of methane ignition delay times caused by MP addition. The influence is nonlinear, with the result that ignition delay times of blends of 50%

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of each fuel are much closer to the ignition delay times of MP, the more reactive fuel. This is understood to result from the rapid generation of radicals during MP oxidation which further react with methane in low-activation energy elementary reactions, such as OH which reacts almost barrier-less. With respect to CO formation during MP pyrolysis, the presence of methane is not observed to significantly influence the pyrolysis time, indicating limited radical withdrawal by methane during the propanoate pyrolysis as it is the case during oxidation when the chemical interactions are accentuated by exchange of oxygen-mediated radical formation. The measured data are compared with two model predictions, showing reasonable agreement for the ignition data and discrepancies with respect to the pyrolysis data.

Keywords: methane ignition, methane-biodiesel blends, CO direct absorption, pyrolysis time, ignition enhancement

Introduction Environmental concerns and increased demand for energy lead to a sustained search for alternative fuels that can be burned in various combustion systems. The relative abundance of natural gas and its potential to emit less combustion emissions motivate the development of more combustion systems fueled with natural gas. 1–3 Natural gas is mostly made up of methane, thus it is characterized by a higher H/C ratio than other fossil fuels, so that natural gas combustion generally yields comparable thermal energy with less CO2 emissions than the other fossil fuels characterized by a lower H/C ratio. Although most transportation systems use spark-ignition engines, their efficiencies are limited by the early onset of uncontrolled auto-ignition of most fuels. In order to increase the efficiencies of transportation engines, advanced compression-ignition engines are considered as a viable solution. While most natural gas engines used for transportation are sparkignited, using it in compression-ignition engines will increase the overall efficiency. For effective utilization of various natural gas compositions in compression-ignition en2


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gines, combustion initiation might pose a problem for natural gas mixtures with very little or no higher alkanes, since the main component, methane, is resistant to ignition. 4 In the review by Spadacini and Colkett, 5 it is shown that depending on the origin of natural gas, methane content can be as high as 99.8% such as in the Alaska natural gas considered in the review. The ignition propensity of such predominantly methane-containing natural gas can be boosted by adding a more reactive fuel. The interest in natural gas for combustion engines also supplements existing and growing interest in biofuels for combustion systems. Biofuels are of interest because of the possibility to recycle the CO2 generated during combustion as feedstock for the production of new fuels through photosynthesis. Among biofuels, biodiesel, which can be produced from fat and vegetable oils, 6,7 is of interest. Biodiesel can be used in compression-ignition combustion systems where their higher reactivity proves to be an advantage. Blending biodiesel with various natural gas mixtures can therefore result in a more dependable compression-ignition fuel. Combustion systems designed to use various natural gas mixtures and biodiesel can thus be seen as responsibly making the shift from more pollutant fossil fuels to a cleaner and more sustainable energy economy. Compression-ignition engines using natural gas and diesel have been investigated in several studies. 8–10 The studies show stable combustion compared to pure natural gas and reduced emissions compared to diesel. However, this fossil-fuel based solution to the natural gas ignition problem does not promote energy sustainability. The influence of diesel or biodiesel on natural gas combustion can be investigated directly in engine systems or in fundamental experiments which allow for the development and validation of models of the controlling chemical kinetics. Compression-ignition engine studies of natural gas and biodiesel have been carried out in the past. 1,11–15 They also generally point to more stable ignition compared to pure natural gas and reduced emission levels compared to diesel or biodiesel. There are very limited fundamental experiments aimed at understanding the chemical kinetic interactions involved in natural gas and biodiesel blend ignition. Such fundamental studies would first proceed using surrogates for natural gas and biodiesel.

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Small esters have been used to unravel the kinetics of biodiesel combustion 16,17 and in this work, methyl propanoate (MP), shown to be a very reactive methyl ester 17 has been chosen to represent biodiesel. Although each of the two fuels, methane and MP, has been investigated in many chemical kinetic studies, fundamental studies of their chemical interactions and ignition behavior are not sufficiently addressed in the literature. One of the fundamental combustion properties relevant to compression-ignition engines is the auto-ignition behavior which can be studied in shock tubes. This study is therefore focused on characterization of methane ignition as the main hydrocarbon in natural gas (especially natural gas with almost 100% methane or very limited traces of higher alkanes), and MP as a biodiesel surrogate, including the blends of the two fuels. It further examines the pyrolysis of MP through measurement of CO formation. The study of MP and methane/MP pyrolysis is aimed at establishing to what extent methane addition can slow down the reactivity of MP. To put the study of the surrogates and their blends in context, previous studies can be briefly reviewed. These fuels have each been examined in several experimental and chemical kinetic modeling studies. Using natural gas in spark-ignition engines has been studied, 2,18,18 establishing that with the appropriate logistic modifications, burning natural gas in such engines occurs without problems. In terms of the fundamental combustion properties, such as ignition, the studies of natural gas combustion kinetics has relied on extensive studies of methane. 4,19–27 In the 1970s, several experimental and analytic studies were undertaken in order to understand the ignition differences between methane and ethane as well as the degree to which methane ignition can be modified by ethane addition. 28–32 The choice of ethane was logical because of its appearance in natural gas mixtures and because of the observed trend in the study of C1-C5 alkanes by Burcat et al., 4 where ethane demonstrated the shortest ignition delay time among the alkanes. Thus, in the presence of higher alkanes, ethane can sensitize methane ignition to a greater degree. This fact is established on the basis of an analytical

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study by Westbrook, 32 using a chemical kinetic mechanism. As brought out by the reaction mechanism, the release of reactive radicals from ethane oxidation leads to enhancement of methane ignition. The underlying radical kinetics is best summed up in Warnatz’s treatment of the high-temperature kinetics of alkane combustion 33 and Westbrook’s model analysis. 32 In more recent times, other studies involving methane have examined the effect of composition on natural gas combustion by including other C2-C5 hydrocarbons. The ignition effects of adding ethane and propane to methane have been investigated by Naber et al., 34 demonstrating that the higher hydrocarbons lead to shorter ignition delay times. The authors also proposed a chemical kinetic model. Petersen et al. 21,35 and de Vries et al. 36 also investigated the effect of propane on methane ignition under lean and stoichiometric conditions, establishing the methane ignition enhancement through propane addition. The study by Petersen et al. 35 also included a chemical kinetic model. In subsequent studies by Petersen’s group and collaborators, 37–40 the effects of various C2-C5 alkanes on methane ignition have been investigated, culminating in an extensively validated chemical kinetic model for C1-C5 hydrocarbons. 40 These studies indicate that adding these higher alkanes to methane leads to reduced ignition delay times. The longer alkanes are more effective. The focus, however, is on understanding the effect of compositional variations on natural gas combustion. This concern is of importance to gas turbines using natural gas. The blend effects in internal combustion engines whereby longer-chain and more reactive hydrocarbons are used to induce methane ignition are characterized by much different composition variations and require further studies. Experimental and modeling studies of biodiesel surrogates have been carried out in the last two decades. Coniglio et al. 41 reviewed progress in characterization and development of chemical kinetic models of biodiesel and its surrogates. The modeling of biodiesel has progressed from detailed chemical kinetic modeling of small esters to the large esters characteristic of practical fuels. MP is one of the small methyl esters used to understand biodiesel combustion.

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Experimental studies on the autoignition of methyl propanoate are more limited than those of other methyl esters. Akih-Kumgeh and Bergthorson 17 measured ignition delay times of C1-C4 methyl alkanoates, seeking to establish structure reactivity trends. In another study, Zhang et al. 42 used a shock tube to measure ignition delay times of MP at different conditions. The authors developed a kinetic model, validating it against the ignition delay and the pyrolysis measurements of Zhao et al. 43 Recently, Kumar et al. 44 studied MP ignition using a rapid compression machine at conditions of high pressure and temperatures that cover the low-to-intermediate range. Comparisons of the reactivity of MP to methyl ethanoate and methyl butanoate were made. There are no fundamental studies which explore the chemical influence of MP on the combustion kinetics of natural gas or its main component, methane. One way to examine the mutual kinetic effects of methane/MP blends is to compare the pyrolysis of MP with that of methane/MP blend to see if the presence of methane slows down the kinetics of MP decomposition. Such a study has not been done and studies of pure MP pyrolysis are also limited. Zhao et al. 43 studied MP pyrolysis in a low pressure flow reactor. From this dataset, they developed a kinetic mechanism for the analysis of MP pyrolysis. Oxidative reactions were also included in the model. Farooq et al. 45 obtained species time-histories MP and ethyl propanoate pyrolysis at temperatures of 1250–1750 K and proposed a kinetic mechanism. Ning et al. 46 conducted shock tube pyrolysis of MP at temperatures of 1292–1551 K at an average pressure of 1.6 atm, as well as chemical kinetic modeling, refining the model by Felsmann et al. 47 Although these pyrolysis studies yield species time histories, they do not provide global kinetic parameters that can capture the complex pyrolysis kinetics and their dependence on thermodynamic conditions of the reactor. Some studies of MP kinetics have focused on laminar flames to expand the validation targets of kinetic models. Felsmann et al. 47 reported experimental measurements of rich laminar low-pressure flames used to develop a kinetic model. Further, Wang et al. 48 reported an experimental study of the laminar flames of several small methyl and ethyl esters, including MP. The experimental observations were compared with predictions from several

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kinetic models, which showed that the mechanisms needed further development. Expanding the set of combustion properties for validation is expected to lead to more predictive models. In this paper we investigate the ignition and pyrolysis of MP and methane using a shock tube reactor. The effect of MP addition on the ignition of methane is examined through comparison of ignition delay times. Further, direct laser absorption spectroscopy is used to measure CO time-histories with the aim of discovering the effect of chemical interaction of the two fuels and the ability of selected chemical kinetic models to predict the experimentally observed properties. Experiments cover a temperature window of 1000 K–1500 K at the reactor pressures of 4, 7.4, and 10 atm. The recent chemical kinetic models chosen for validation are the models by Zhao et al. 43 and Zhang et al. 42 Although the model by Zhao et al. 43 was developed in the framework of a pyrolysis study, it includes oxidative kinetics that can be tested against ignition data to establish the accuracy of the postulated oxidative kinetics.

Experimental method Experiments are carried out in a shock tube reactor, previously described in. 49 The tube has an internal diameter of 10 cm, with a driven or test section of 6.0 m, and a driver segment of 3.0 m. The combustible mixtures to be tested are prepared in a 150-liter mixing tank using the partial pressure technique. The minimum mixing time is at least 12 hours. Research grade samples of MP (Sigma-Aldrich, 99.9 + %) with methane, oxygen, and argon (Airgas, >99.999 %) are used to prepare the test mixtures. The shock wave is driven using helium (Airgas, >99.999 %). The reactor thermodynamic conditions at the onset of the experiment and the incident shock wave velocity are used in shock equations to determine the thermodynamic conditions in the post-reflected shock region. The shock equations are solved using the CalTech detonation solver 50 and the CANTERA kinetic solver. 51 The reactive gas behind the incident

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shock wave is assumed to be chemically frozen. The post-reflected shock wave conditions therefore serve as the initial conditions of a homogeneous, stationary reactor. The incident shock velocities at three locations that are 40 cm apart over the last 1.5 m of the driven section are determined from measurements of four fast-response PCB piezoelectric pressure transducers. Shock attenuation is taken into consideration by linearly extrapolating the obtained shock velocities to determine the velocity at the test region at about 2 cm from the endwall. The post-reflected shock pressure is determined from the pressure transducer located at 2 cm from the end wall and confirmed to be in line with that calculated from the shock equations. Ignition is detected using CH chemiluminescence as recorded by a photodiode on the side wall that is equipped with a 430±10 nm narrow band filter. To obtain the ignition delay time, the line of maximum slope of the CH emission signal is first determined and extrapolated to intersect with the baseline photodiode signal. Ignition measurements are done for mixtures containing 3% fuel and another set for mixtures containing 10% O2 . Blends containing 50% and 80% methane are investigated with the total fuel concentration maintained at 3% and another blend mixture containing 50% methane with O2 kept at 10%. For pyrolysis studies of fuel in argon bath gas, pure MP at 3% and a 3% blend with equal proportions are considered. To establish the role of methane in MP pyrolysis, a control mixture of 1.5% MP in argon is also investigated. The idea is that if the chemical evolution of the reactor for the blend and that of the 1.5% MP are comparable, then one can conclude that MP decomposition is not significantly retarded by methane addition. Combined with ignition experiments, this can establish whether it is reasonable to attempt to inhibit MP ignition or decomposition through methane addition. The pressure range of 4-10 atm was chosen to bring out pressure effects while limiting the collisional broadening effects during direct absorption measurement of CO time histories. The pressure of 10 atm is reasonably high to reveal trends that would persist at higher pressures and moderate temperature. CO laser absorbance is measured near 4.6 µm using a mid-infrared quantum-cascade laser

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(QCL). This absorbance is used to determine a characteristic pyrolysis time and to obtain CO time histories, in similar manner to our previous study. 49 The pyrolysis time is defined in relation to a concentration correlation function whereby the rate of formation of a major species, j, is used to represent the fuel being depleted:

fcc ∝ Xj

dXj dt

(1)

If our interest is the chemical time associated with pyrolysis, the measured absorbance time history is used to determine the time without converting it to the corresponding CO concentration time history. Further details about the pyrolysis time can be found in our previous work. 49 The pyrolysis time can be determined as illustrated in Fig. 1. The measured absorbance is first captured using a polynomial and its derivative is used to calculate fcc as well as the time to the maximum of fcc . In this work, the pyrolysis time has been obtained for MP and its blend with methane by focusing on absorption measurement of CO, because of its strong mid IR absorption bands

3.5

Measured ACO Polynomial fit to ACO

ACO fit [2.5 × 10−3] dACO dt dACO ACO dt

0.010

3.0 Arbitrary signal [−]

2.5 Absorbance

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2.0 1.5 1.0

tmax,fcc= 0.225 ms

0.006

0.004

0.002

0.5 0 0

0.008

0.2

0.4

0.6 Time [ms]

0.8

0 0

1

0.2

(a)

0.4

0.6 Time [ms]

0.8

1

(b)

Figure 1: Approach to obtain a pyrolysis time from a pyrolysis experiment. The conditions are 3% MP in Ar at 1394 K and 4.1 atm. a) CO Absorbance time-histories (ACO ) with associated polynomial fit b) dAdtCO and ACO dAdtCO time-histories, with associated pyrolysis times.

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without interference from other species. The pyrolysis time allows us to determine the temperature sensitivity of pyrolysis over a wide temperature range. Apart from the pyrolysis times, quantitative CO time histories can be obtained from the time-dependent absorbance using the standard Beer-Lambert’s Law:

− ln

I I0

= αL

(2)

ν

With I and I0 as the transmitted and incident or initial intensities, respectively. Also, ν, is the frequency of radiation, L is the shock tube inner diameter and αL, is known as the absorbance. Here, α, is the absorption coefficient defined as:

α = S (T ) N φν

(3)

With S [cm/molecule] as the line-strength of the transition being considered, N is the CO molar density defined in terms of the mole fraction x, total pressure, p, and temperature, T , as N =

Xp RT

and φν is the line-shape.

The line-strength, S, depends on the temperature, expressed as by: hcE 00 Q (T ) T0 exp − S (T ) = S (T0 ) Q (T0 ) T K

"

1 1 − T T0

# "

−hcν0 1 − exp KT

!# "

!#−1

−hcν0 1 − exp KT0 (4)

Here Q(T ) stands for the temperature-dependent partition function of the target molecule, with E 00 [cm−1 ] being the lower-state energy, ν0 [cm−1 ] being the line-center frequency, and h [J.s] being the Planck’s constant. The line-shape can be calculated using the Voigt function that captures Doppler and collisional broadening of the spectrum, which, for laser absorption at the frequency corresponding to the maximum line strength, takes the form: √ 2 ln2 √ exp a2 erfc (a) φν (ν0 ) = ∆νD π 10


(5)

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where ∆νD is the full-width of the Doppler-broadened spatial frequency spectrum at half of the maximum line strength and a is a non-dimensional parameter defined as: √ a=

ln2

∆νc ∆νD

(6)

where ∆νc is the full-width of the collisional-broadened spatial frequency spectrum at half of the maximum line strength. ∆νD can be approximated by: s

∆νD , cm−1 = ν0

s

8kT ln2 T ≈ 7.1623 × 10−7 ν0 2 mc M

(7)

With c [cm/s] being the speed of light, and k [J/K] being the Boltzmann constant, while M [g/mol] is the molecular weight of the target species. The full-width of the collisional-broadened spatial frequency spectrum at half of the maximum line strength, ∆νc , can be estimated as: ∆νc , cm−1 = p

X

xi 2γi

(8)

i

With p as the total pressure, xi being the mole fraction of the ith bath gas collision partner and γ being the collision broadening coefficient of the ith bath gas colliding with the absorbing species. The collisional broadening coefficient depends on temperature as defined by: T0 γi (T ) = γi (T0 ) T

ni

(9)

with T0 as the reference temperature (296 K) while ni is the temperature coefficient for collisions of the ith bath gas with the target species. With these equations, the line shape function can be evaluated and together with the line strength at the central frequency, the measured absorbance can be converted into CO concentration. The rovibrational band used for the CO measurement is the R9 transition, 11


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with a central spatial frequency of 2179.8 cm−1 . The online HITRAN resources, 52 can be used to obtain the line strength at this frequency and a given reactor temperature. For the collisional broadening, self broadening and air broadening coefficients are also taken from online HITRAN resources 52 while collisional broadening parameters for CO–Ar taken from Thibault et al. 53 and used to determine the collisional broadening at given reactor pressure and temperature. The Doppler broadening FWHM is determined for the given reactor temperature. The required line shape is then determined from equation (5). Simulations of the homogeneous reactor employ the CANTERA software package, 51 approximating the chemical system behind reflected shock waves as a constant volume reactor. Chemical kinetic models are used to calculate ignition delay times, pyrolysis times, and CO time histories, which are then compared with the measured quantities.

Results and discussion First presented are ignition delay time measurements and comparison to model predictions, followed by reaction pathway and sensitivity analyses. Pyrolysis times and comparison to model predictions are then presented. In addition, sample CO time histories from pyrolysis of MP and its blend with methane are compared with model predictions.

Ignition delay times Ignition delay times for mixtures of fuel/O2 /Ar are reported are plotted against inverse temperatures for given nominal pressures, equivalence ratios, and fuel concentrations. Statistical propagation of the relevant errors is used to estimate the uncertainties in measured delay times. The major contributors to the uncertainty in measured ignition delay times include: temperature uncertainties, estimated to be 1.0 - 1.5%; with pressure uncertainties in the range 1.0-1.5%; and fits and ignition delay measurement uncertainties are approximately 1%. Since ignition delay times depend exponentially on temperature, the resulting

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uncertainties in ignition delay times are mainly caused by the temperature uncertainties. If we consider these, then estimated ignition uncertainties lie between 10% to 20% over the temperature range studied. The differences in ignition propensity of 3% MP and 3% CH4 are first established as shown in Fig. 2. The higher reactivity of MP is unmistakable since the ignition delay times of MP are significantly shorter than those of CH4 , by close to two orders of magnitude. It can therefore be expected that adding MP to CH4 can enhance the ignition of the blend. The effect of adding 20% and 50% MP to CH4 is then investigated and the results are shown in Fig. 3. One observes the disproportionate modification of CH4 ignition by MP since ignition delay times of the blends are more in line with those of MP, despite the high amount of CH4 . For instance, the ignition delay times of CH4 are as much as about 8 times longer than of those of the blend with 20% MP at 1377 K while they are about 18 times longer than those of the blend with 50% MP at 1360 K. The observed reactivity trends can be understood as resulting from the very strong C–H bonds in CH4 which are resistant to initiation and propagation of chain reactions that ultimately lead to ignition. Much higher

3% MP 3% CH4

4

10 Ignition delay time [µs]

Ignition delay time [µs]

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3

10

2

10

3% MP 3% CH4 1.5% MP& 1.5% CH4 0.6 % MP& 2.4% CH4

3

10

2

10

φ=1 ; p= 7.4 atm

φ=1 ; p= 7.4 atm 0.55

0.61

0.67

0.73 0.79 1000/T [1/K]

0.85

0.91

0.55

Figure 2: Ignition delay times at φ = 1.0 for individual fuel, oxygen, and argon mixtures at an average pressure of 7.4 atm. Solid lines are Arrhenius fits to the measured data.

0.61

0.67

0.73 0.79 1000/T [1/K]

0.85

0.91

Figure 3: Measured and predicted ignition delay times of MP, CH4 , and blend of both at scaled pressure of 7.4 atm. Model predictions are represented by lines. Solid lines: Zhang et al. 42 Dashed lines: Zhao et al. 43

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Energy & Fuels

temperatures are needed to induce the controlling chain reactions. The highly reactive MP can be explained by its weaker C–H bonds and the more readily generated reactive radicals that promote chain propagation reactions once the fuel is attached. With the addition of a small amount of MP, many radicals are generated. Their reactions with CH4 are marked by lower activation barriers compared to such high barrier reactions such as the decomposition of CH4 to CH3 and H. The observed ignition delay times are also compared with simulated delay times using chemical kinetic models proposed by Zhang et al. 42 and Zhao et al. 43 as shown in Fig. 3. We observe the closer agreement between predictions of the model by Zhang et al. 42 and the measured ignition delay times, compared to the predictions of the model by Zhao et al. 43 The two models capture the significant reactivity differences, thus, reflecting a proper account of MP and CH4 oxidation kinetics in the models. In Fig. 4, shown are ignition delay times of 3% MP, 3% CH4 and blends of equal molar volumes of the two fuels at a lower average pressure of 4 atm. Similar to the observations at 7.4 atm, adding MP to methane significantly reduces methane ignition delay times, bringing them closer to MP ignition delay times. The model by Zhang el al. 42 consistently shows good accordance with the measurements of MP and the blend whereas the model by Zhao

3% MP 3% CH4 1.5% MP& 1.5% CH4

4

10


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3

10

2

10

φ=1 ; p= 4 atm 0.55

0.61

0.67

0.73 0.79 1000/T [1/K]

0.85

0.91

Figure 4: Measured and predicted ignition delay times of MP, CH4 and blend of both at scaled pressure of 4 atm. Lines represent model simulation results. Solid: Zhang et al. 42 Dashed: Zhao et al. 43 14


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et al. 43 yields longer ignition delay times than measured. Ignition differences among the two fuels and their blend are further explored under lean conditions (φ = 0.5) as shown in Fig. 5. Similar to the case of the stoichiometric mixtures, it is observed that in contrast to CH4 , MP ignition delay times are over an order of magnitude shorter at comparable temperatures. As already established, the radicals generated by the more readily ignitable MP greatly enhance the ignition kinetics of the blend of MP and CH4 of equal proportions, such that the ignition delay times of the blend are also closer to those of MP under these lean conditions. With respect to the performance of the two models considered, the model by Zhang et al. 42 more closely predicts the experimental data while the predictions of the model by Zhao et al. 43 are under measured CH4 delay times and above those of MP and the blend. Comparing lean and stoichiometric ignition delay times for these mixtures of fixed fuel percentage (3%), Fig. 6 shows that lean mixtures of both fuels ignite more readily than the stoichiometric mixtures on account of the higher oxygen concentration that is conducive to rapid radical generation. The reactivity trends above have been established by fixing the fuel concentration. An-

10 Ignition delay time [µs]

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10

10

4

3% MP 3% CH4 1.5% MP& 1.5% CH4

3

2

φ=0.5 ; p= 4 atm 0.58

0.64

0.70

0.76 0.82 1000/T [1/K]

0.88

0.94

Figure 5: Experimental and model predictions of ignition delay times of MP, CH4 and blend of both for mixtures with 3% fuel and φ = 0.5 at a nominal pressure of 4 atm. Solid lines: model by Zhang et al. 42 Dashed lines: model by Zhao et al. 43

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10

10

10

3% MP, φ=1 3% MP, φ=0.5 3% CH4, φ=1 3% CH4, φ=0.5

4

3

2

p= 4 atm 0.52

0.59

0.66

0.73 0.80 1000/T [1/K]

0.87

0.94

Figure 6: Measured ignition delay times of stoichiometric and lean mixtures of MP and CH4 with fuel maintained at 3% at an average reactor pressure of 4 atm. Lines represent Arrhenius fits to a given series of data. other approach is to fix the oxygen concentration, making the situation closer to the technical case where fuel reactivity trends are often judged based on fixed diluent to oxygen ratios as in air and equivalence ratios. Figure 7 shows the ignition delay times for stoichiometric mixtures of CH4 , MP, and a blend of the two fuels at a nominal reactor pressure of 10 atm, whereby O2 is fixed at 10%. Under these conditions, CH4 ignition delay times are also longer

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

10

10

MP CH4 50% MP& 50% CH4

4

3

2

φ=1 ; 10% O2 ; p= 10 atm 0.62

0.67

0.72

0.77 0.82 1000/T [1/K]

0.87

0.92

Figure 7: Measured and predicted ignition delay times of MP, CH4 and blend of both fuels with O2 maintained at 10% and at 10 atm. Model simulation results are represented by lines. Solid: Zhang et al. 42 Dashed: Zhao et al. 43

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Energy & Fuels

than the delay times of MP by over an order of magnitude. The ignition delay times of the blend is similarly closer to those of MP. While both models accurately predict the CH4 delay times, only the model by Zhang et al. 42 properly captures the delay times of MP and the blend, while the model by Zhao et al. 43 predicts longer delay times of MP and the blend. The observed kinetic effects of methane and MP blends, whereby the ignition delay times of methane are much shortened by adding MP, are similar to previous kinetic analysis of methane and ethane blends by Westbrook. 32 As established by Burcat et al., 4 ethane is the easiest to ignite among C1-C5 alkanes. Westbrook 32 used a kinetic model to show that adding ethane to methane leads to significant enhancement of the ignition of methane. This shows that the radical kinetics underpinning this enhancement of methane by ethane or MP should be comparable. During oxidation of methane in the presence of MP or ethane, radicals are rapidly generated on account of the weaker C–H and C–C bonds in these added fuels. These radicals then attack methane and lead to rapid fuel consumption than would be expected from kinetics involving strong methane C-H bond breaking reactions. The comparisons above have established the pronounced differences between CH4 and MP ignition delay times, and have further shown the enhancement of CH4 by MP toward the delay times of MP in blends of the two fuels. To gain further insight into the chemical influence of MP on the blend ignition, sensitivity and reaction pathway analyses can be carried out. Analysis of ignition delay time sensitivity to elementary reaction rates is carried out based on the model by Zhang et al., 42 to identify controlling reactions. Specifically, the reaction rate is multiplied by 10 to assess the impact on the ignition kinetic time. Three ignition events are considered to cover methane, MP, and a blend of the two with 10% MP. Because of large differences in their ignition delay times, the selected conditions are different. The temperatures are chosen such that the associated ignition delay time is around 1 ms. The identified reactions are considered to be general enough for ignition temperatures above 1000 K. Figure 8 shows the 25 most important reactions during the ignition of 90% CH4

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Page 18 of 32

& 10% MP at 74 atm. Also plotted are sensitivities of corresponding reactions taken from ignition of the 3% MP and the 3% CH4 combustible mixtures. In the case of the blend, the most important reaction is CH3 + HO2 * ) CH4 + O2, where increasing the reaction rate leads to increased ignition delay time, since it acts as a radical sink that regenerates stable molecules. Equally important for the blend ignition is the chain-branching reaction H + O2 * ) O + OH , which is also important during the ignition of the pure fuels. For the blend and MP ignition, the decomposition of MP by C–C bond breaking prove to be important as sources of methyl radicals. For the ignition of pure CH4 , reactions of methyl radicals with molecular oxygen and other radicals also prove to be important. In general, it can be noted that many of these key reactions involve oxygenated radicals, pointing to the role of oxygen in the chain-branching and propagating reactions.

CH3 + HO2 CH4 + O2 CH4 + H CH3 + H2 H + O2 O + OH CH3 + CH3 (+ M) C2H6 (+ M) CH3 + H (+ M) CH4 (+ M) CH3 + HO2 CH3O + OH CH3 + O2 CH2O + OH CH4 + OH CH3 + H2O MP + H => H2 + MP2J C2H4 + OH C2H3 + H2O C2H4 + CH3 C2H3 + CH4 HO2 + OH H2O + O2 MP ME2J + CH3 CH3 + O2 CH3O + O H2 + O2 H + HO2 CH2O + H HCO + H2 OH + H2 H + H2O CH2O + HO2 HCO + H2O2 CH2O + CH3 HCO + CH4 CH4 + HO2 CH3 + H2O2 MP + H => H2 + MP3J CH2O + O2 HCO + HO2 HO2 + H OH + OH CH3OH (+ M) CH3 + OH (+ M) MP PAOJ + CH3

10% MP & 90% CH4, 1329 K 3% MP, 1143 K 3% CH4, 1511 K −0.1

−0.05

0 0.05 0.1 0.15 L.S. = (∆τ/τo )/(∆kj /kj )

0.2

0.25

Figure 8: Sensitivity analysis for 90% CH4 & 10% MP, T=1329 K using models by Zhang et al. 42 with sensitivities of corresponding reactions taken from 3% MP, T=1143 K and 3% CH4 , T=1511 K at the conditions: φ= 1, p = 7.4 atm.

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Energy & Fuels

H

3% CH4, 1500 K, φ=1, p=7.4 atm 600 µs, τ=1250 µs

H

H

C H

53.2%

+H

11.3%

+O

3.3%

HO2

CH3

+C2H6 2.63%

C2H5

+OH

16.5%

HCO

+H2O 4.7%

CH3O

+CH2O 7.12% +O2 11.6%

+CH3 12.6%

CH2O

C2H6

Figure 9: Reaction pathway for 3% CH4 (φ= 1, T=1500 K, p = 7.4 atm) using models by Zhang et al. 42 The numbers next to the listed chemical species indicates the percentage of the main reactant species which is thereby transformed into the product through reaction. Reaction pathway analysis of typical oxidation processes is performed based on the model by Zhang el al. 42 because of its better agreement with measurements. Figure 9 shows the results during ignition of 3% methane at 1500 K, 7.4 atm, φ = 1.0 and at 600 µs when ignition delay time is predicted to be 1250 µs. It is observed that the fuel is mostly attacked by radicals, which are generated from further reaction of products from chain initiation by strong C–H bond breaking. Then, the radical pool slowly builds up and leads to further reactions of methyl radicals to products. It is noteworthy that high temperatures are needed to realize a methane ignition event within 2 ms. For 3% MP system, a reaction pathway analysis is also conducted at 1287 K, 7.4 atm, φ = 1.0, and at the 50 µs instance, for which the predicted ignition delay time is 145 µs. As shown in Fig. 10, MP is mainly consumed through H abstraction reactions by H, OH, O and CH3 . H-abstraction from the α-C is dominant and produces CH3 CHCOOCH3 (MP2J). The other important path is H abstraction from the CH3 O group which leads to production of CH3 CH2 COOCH2 (MPMJ). Finally, the less favored H abstraction site produces CH2 CH2 COOCH3 (MP3J). These primary MP radicals further react by beta-scission

19


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Page 20 of 32

to yield radicals and smaller stable oxygenated compounds or ethylene. The faster kinetics of MP is mainly related to the ease with which the primary MP radicals are formed and their subsequent beta-scission reactions. To examine the sensitive kinetic effect of MP on methane ignition, further reaction pathway analysis is carried out for a mixture of 90% CH4 and 10% MP at 1311 K, 7.4 atm, φ = 1.0, and at the 620 µs instance, for which conditions the ignition delay time is predicted to be 1243 µs. The results are shown in Fig. 11. This blend ignition delay time is quite close to the delay time of at the condition chosen to analyze the reaction pathway of methane but the blend mixture is at the lower temperature of 1311 K as compared to 1500 K in the case of pure CH4 . The influence of MP on CH4 comes through the pool of radicals which abstract H atoms from CH4 , such as OH, H, HO2 , O that are generated from MP oxidation. The chain branching reaction, H + O2 * ) O + OH, is influenced by more readily liberated H atoms from MP. Moreover, in the presence of MP, very low barrier reactions, such as CH4 + OH, are greatly enhanced, with OH coming from the enhanced chain branching reaction. The influence of these radicals generated from MP oxidation on the oxidation of methane 3% MP, 1287 K, φ=1, p=7.4 atm 50 µs, τ=145 +H

+H

+OH

19.58% 13% 6.58%

+OH

+O

+CH3

22.6% 9.78% 2.71% 2.18%

33.4%

37.27%

+H 17.3% +OH 16.1%

MP3J

MP2J

33.5% 32% 15.4%

17.7%

MPMJ

+ CH3OCO

CH3CHCO + CH3O

49.9% +H +H

+ C2H5CO

MP2D MP2D

Figure 10: Reaction pathway for 3% MP (φ= 1, T=1287 K, p = 7.4 atm) using models by Zhang et al. 42 The number in bold font represents the percentage of the main species that is transformed into the target product through the indicated channel.

20


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Energy & Fuels

are in line with earlier work on radical kinetics during alkane combustion by Warnatz. 33,54 Thus, we see that the ease with which radicals are initially generated in MP and further decomposition of fuel radicals yield a greater pool of radicals which promote the evolution of the MP reactor toward ignition. In the case of blends, radicals from MP can also attack CH4 , with enhanced reaction rates for reactions with low activation barriers. The case of CH4 resistance to ignition is mostly explained by the strong C-H bonds that greatly lower the rate of initial radical generation.

Pyrolysis time scales The pyrolysis of MP and its blend with CH4 is investigated to establish the influence of CH4 on the decomposition rate of MP and to be able to contrast the time scales of pyrolysis with those of ignition, indicating temperature ranges where one expects non-oxidative fuel decomposition processes to play a greater role during ignition. As previously discussed, the global kinetics of the pyrolysis of MP and CH4 can be assessed using the pyrolysis time, 49 where the absorbance of CO is used for the correlation. As shown in Fig. 12, CO is formed from further reactions of the initial fuel radicals through various channels. 10% MP& 90% CH4, 1311 K, φ=1, p=7.4 atm 620 µs, τ=1243 µs

H H

H

C H

49.5%

+OH

15.7%

+H

9.02%

+O

2.1%

+HO2

+HO2

CH3

+CH3

3.52%

CH4

6.7%

+HO2

+O2 3.81%

9.65%

CH3O

+CH3

12.1%

HCO

CH2O

C2H6

Figure 11: Reaction pathway for 90% CH4 and 10% MP (φ= 1, T=1311 K, p = 7.4 atm) using models by Zhang et al. 42 .

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Page 22 of 32

The uncertainty in the resulting pyrolysis time comes from several sources including temperature uncertainties estimated at 1.0%, pressure uncertainties of 1.0%, and uncertainties in fit parameters of the CO absorbance (10%). From these uncertainties and assuming an Arrhenius dependence of the pyrolysis time on temperature, the resulting uncertainty of the measured pyrolysis time is estimated to be 22-25%.

22.8%

29.1%

36.6%

21.8%

26.5%

32.4%

MP3J

MP2J

33.6%

15.6%

33.6%

15.7%

MPMJ

32.4%

17.5%

36.6%

17.3%

50%

+ CH3OCO

CH3CHCO + CH3O

50% +H

+H + C2H5CO

MP2D

MP2D

39.7% 39.7%

19.7%

8.97% 50%

HCO

8.19%

21.3%

50%

49% 49%

CO

Figure 12: CO production pathways during the pyrolysis of MP and its blend with CH4 , obtained using the model by Zhang et al. 42 Conditions: 3% MP (top, in regular font), 1.5% MP & 1.5% CH4 (bottom, in italic font) at T=1462 K and p= 4 atm. The effect of fuel percentage on the pyrolysis times of MP and its blend with an equal proportion of CH4 can be seen in Fig. 13a. The pyrolysis time is observed to be relatively insensitive to MP percentage or the presence of methane. The methane addition seems to lengthen the pyrolysis time at low temperatures but not significantly, compared to the case of 1.5% MP. There could be a very weak effect of fuel percentage that cannot be clearly discerned in the present parameter variation. Figure 13a also shows temperature sensitivity of MP and CH4 pyrolysis times. The kinetic time shows Arrhenius behavior with a global activation energy that lies between 50 to 60 kcal/mol. This activation energy range reflects the predominant role of C–C bond 22


Page 23 of 32

3% MP 1.5% MP& 1.5% CH4 1.5% MP

3% MP 1.5% MP& 1.5% CH4 3

Pyrolysis time, tmax,fcc [µs]

Pyrolysis time, tmax,fcc [µs]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3

10

2

10

10

2

10

Fuel in Ar ; p= 4 atm

Fuel in Ar ; p= 4 atm

1

10 0.68

0.70

0.72 0.74 1000/T [1/K]

0.76

0.78

0.65

0.67

(a)

0.69

0.71 0.73 1000/T [1/K]

0.75

0.77

(b)

Figure 13: a) Pyrolysis times of MP and its equal blend with CH4 . b) Comparison of pyrolysis times with model predictions (lines; Solid : Zhang et al. 42 Dashed: Zhao et al. 43 ). breaking and beta scission reactions during pyrolysis. This is quite different from ignition global activation energies which turn to be about 30-40 kcal/mol (about 40 kcal/mol in this study). The observed pyrolysis times are also compared with calculated pyrolysis times in Fig. 13b. Shown are results for mixtures of 3% MP and a mixture of MP and CH4 in argon at an average pressure 4 atm. We see that model by Zhao et al. 43 more closely captures the pyrolysis times at 3% MP while showing greater deviations for the blend mixture. The predictions of the model by Zhang et al. 42 are much shorter than measured pyrolysis times. This deviation contrasts with the agreement observed for ignition, where the model by Zhang et al. 42 is consistently agrees more closely with the measured delay times for all mixtures considered. Sensitivity analysis during pyrolysis is also carried out at a reactor pressure of 4 atm using the model by Zhang et al. 42 to identify controlling reactions. Figure 14 shows the 25 most important reactions for the pyrolysis of 90% CH4 & 10% MP, where sensitivities of corresponding reactions during pyrolysis of 3% MP are also added. Unlike during ignition where oxygenated radicals play a dominant role, it is mostly H atoms and CH3 radicals which dominate in the pyrolysis case. During the pyrolysis of MP, it is mostly H-abstraction 23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CH4 + H CH3 + H2 MP + CH3 => CH4 + MP2J MP + H => H2 + MPMJ MP ME2J + CH3 MP + H => H2 + MP2J C3H8 (+ M) CH3 + C2H5 (+ M) MP PAOJ + CH3 MP + CH3 => CH4 + MPMJ CH3 + CO2 CH3OCO CH3O + CO CH3OCO CH3 + H (+ M) CH4 (+ M) CH2O + H HCO + H2 CH2O + CH3 HCO + CH4 MP CH3OCO + C2H5 MP2D + H => MP2J MP + H => H2 + MP3J CH3 + CH3 H + C2H5 MP3J => MP2D + H MP + CH3 => CH4 + MP3J MP (+ M) CH3O + C2H5CO (+ M) CH3CHCO + H C2H5 + CO C2H4 + H (+ M) C2H5 (+ M) MP2D + H => MP3J MP2J CH3CHCO + CH3O MP2J => MP2D + H −0.1

Page 24 of 32

10% MP & 90% CH4, 1400 K 3% MP, 1340 K −0.05

0 0.05 0.1 0.15 0.2 L.S. = (∆tmax,fcc /tmax,fcc )/(∆kj /kj )

0.25

Figure 14: Sensitivity analysis during pyrolysis of 90% CH4 & 10% MP, T=1400 K using models by Zhang et al. 42 and sensitivity of corresponding reactions taken from 3% MP, T=1340 K at p = 4 atm. from MP by H atoms and CH3 radicals as well as the unimolecular decomposition of MP by direct C–C bond breaking that feature prominently. In the presence of 90% CH4 , the pyrolysis process is sensitive to the MP reactions mentioned and to the H-abstraction from CH4 to form methyl radicals. Since the pyrolysis time is judged based on CO formation from MP, it is reasonable that the competition for H-abstraction by H atom would still favor MP channels. Comparing the ignition and pyrolysis times, we can establish distinctive differences in global kinetics between oxidation and pyrolysis. Figure 15 is a plot of pyrolysis times of mixture of 1.5% MP and 1.5% CH4 and ignition delay times of a mixture with 1.5% MP and 1.5% CH4 at 4 atm and φ = 1.0. We observe from the different gradients that pyrolysis is more sensitive to temperature than ignition as previously mentioned. At lower temperatures,

24


Page 25 of 32

p=4 atm

p=4 atm

3

10

Characteristic time [µs]

Characteristic time [µs]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Ea=63 Kcal/mole Ea=40.5 Kcal/mole 2

10

10

0.61

0.67

0.73 0.79 1000/T [1/K]

0.85

Ea=53 Kcal/mole Ea=39.7 Kcal/mole 2

10

1

1.5% MP& 1.5% CH4 Oxidation 1.5% MP& 1.5% CH4 Pyrolysis

1

3

10

3% MP Oxidation 3% MP Pyrolysis

10

0.91

0.61

Figure 15: Ignition delay times (φ= 1) compared with pyrolysis times of 1.5% MP with 1.5% CH4 .

0.67

0.73

0.79 0.85 1000/T [1/K]

0.91

0.97

Figure 16: Ignition delay times (φ= 1) compared with pyrolysis times of 3% MP.

pyrolysis times are longer than ignition delay times, but at higher temperatures, the opposite trend occurs. For the mixture with 3% MP, the difference in the temperature sensitivity is weaker and the trend reversal is not observed in the measured temperature window as shown in Fig. 16. The difference means that for ignition events, radical reactions involving oxygenated species, such as OH are crucial to the ignition process. As the temperature increases, more fuel-related radicals and reactions of direct C–C bond breaking do participate in the depletion of the fuel, with the ignition process resulting from the further oxidation of the fuel breakdown products.

Selected CO time histories during fuel pyrolysis CO absorbance has been used to determine pyrolysis time but the absorbance time history can also be converted into CO time history for a given experimental realization. Here, selected CO profiles are compared with model predictions to provide a broader kinetic perspective. We expect the CO profiles for the 1.5% MP mixture and the blend of 1.5% MP and 1.5 % CH4 to temporally evolve in a similar manner. The CO time histories are determined in accordance with the Beer-Lambert’s law with the line strength and line shape determined as discussed above. The question arises as to the thermodynamic conditions of the reactor 25


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during the pyrolysis process. Adopting a constant volume reactor model, simulations show temperature and pressure drop of up to 8% as the CO approaches its equilibrium state. This is considered in assessing the absorption cross-section uncertainties. Regarding the accuracy of the CO time histories thus obtained, there are several sources of uncertainties, including: the reactor temperature uncertainties of about 1.0%, reactor pressure uncertainties of about 1.0%, absorption cross-section uncertainties of approximately 8%, absorption path length uncertainties of 0.2%, and absorbance measurement uncertainty of 1.0%. The resulting uncertainty of the measured CO mole fraction is about 13-14%, determined by propagating the contributory uncertainties. Three different experimental realizations under comparable thermodynamic conditions are shown in Fig. 17. CO production is faster for 3% MP and this is followed by the 1.5% MP mixture before the blend of 1.5% MP with 1.5% CH4. Looking at the profiles, it is observed that CO formation is initially very rapid, but after a short while the CO production becomes very slow as it slowly approaches equilibrium. The later time CO concentrations for 1.5% MP and the blend of 1.5% MP with 1.5% CH4 are about the same, indicating limited effect of the added methane. The measured CO profiles are further compared with model simulations in Fig. 18. It is found that both models under-predict the initial rate of CO production but at later times, the model by Zhang et al. 42 captures the CO time histories. The steep rise of the CO profiles at the beginning in the case of the model by Zhang et al. 42 is responsible for the shorter pyrolysis time predictions observed above.

Conclusion There is considerable interest in using natural gas in combustion systems because of its availability, lower costs, and lower emissions. In CI engines, ignition resistance poses a problem which can be solved through addition of biodiesel.

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Page 26 of 32

0.027

0.045

0.024

0.040

0.021

0.035 CO mole fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

CO mole fraction

Page 27 of 32

0.018 0.015 0.012 0.009

0 0

0.025 0.020 0.015 0.010

0.006 0.003

0.030

Exp., 1462 K, 3.86 atm, 3% MP Sim., by Zhao et al. 1462 K Sim., by Zhang et al. 1462 K Exp., 1461 K, 4.0 atm, 1.5% MP & 1.5% CH4 Sim., by Zhao et al. 1461 K Sim., by Zhang et al. 1461 K

Exp., 1460 K, 4.1 atm, 1.5% MP Exp., 1462 K, 3.86 atm, 3% MP Exp., 1461 K, 4.0 atm, 1.5% MP& 1.5% CH4

0.5

1.0

0.005 0 0

1.5

Time [ms]

0.5

1.0

1.5

Time [ms]

Figure 17: CO time history during pyrolysis of two different MP concentration and its equal blend with CH4 .

Figure 18: Measured and simulated CO histories during pyrolysis of 3% MP and its equal blend with CH4 , where the models by the Zhao et al. 43 and Zhang et al. 42 are used.

This work bridges the gap in our understanding of natural gas and biodiesel surrogates ignition by examining the shock tube ignition and pyrolysis of blends of methane as a natural gas surrogate and MP as a biodiesel surrogate. The measurements include shock tube ignition delay times, pyrolysis times, and CO time histories. The results show that MP has shorter ignition delay times than methane (almost two orders of magnitude differences at given conditions of pressure, temperature and fuel percentage). The study brings out the disproportionate enhancement of methane ignition through MP addition, such that a blend of equal proportions yields ignition delay times that are closer in magnitude to those of MP. With regards to MP pyrolysis, the presence of methane is observed not to significantly influence the pyrolysis time. The evolution of CO during pyrolysis of pure or blended MP shows a rapid initial rise and gradual approach to equilibrium. Comparison of experimental results with model predictions show that current models can reasonably well predict ignition delay times albeit with discrepancies in the case of predicting pyrolysis times and CO time histories.

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Acknowledgement Support is acknowledged from the Syracuse University’s College of Engineering and Computer Science and from the Syracuse Center of Excellence for Environmental and Energy Systems.

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