Nitromethane pyrolysis and oxidation in a jet-stirred reactor: Experimental measurements, kinetic model validation and interpretation

Fuel 263 (2020) 116491

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Nitromethane pyrolysis and oxidation in a jet-stirred reactor: Experimental measurements, kinetic model validation and interpretation

T

Meng Yang, Zhongquan Gao, Chenglong Tang , Zhaohua Xu, Zhenhua Gao, Erjiang Hu, Zuohua Huang ⁎

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

ARTICLE INFO

ABSTRACT

Keywords: Jet-stirred reactor Nitromethane Pyrolysis Oxidation Kinetic model analysis

A jet-stirred reactor was used to investigate the nitromethane pyrolysis and oxidation characteristics over the temperature range of 675–1250 K at atmospheric pressure. Mole fraction profiles of reactants, intermediate hydrocarbon species and products were identified using gas chromatography and gas chromatography-mass spectrometer. Results show that the fuel begins to be destructed at around 725 K and completely vanishes at 900 K for all experimental conditions. Pyrolysis of nitromethane as a function of temperature shows three distinct regions for H2, CO and C2H4 species. In addition, a negative temperature coefficient (NTC) behavior of CO2 species for nitromethane oxidation was observed, which has never been reported previously. The NTC region of CO2 species extends with the increase of equivalence ratio. These measured data were then used to validate several recently developed models of nitromethane. Results show that the model of Mathieu et al. (Mathieu et al. Fuel 182(2016):597) predicts the measured data well at low temperature (below 900 K) for all conditions. However, at intermediate temperature Mathieu model predicted intermediated species and main products deviate noticeably from measured data. Detailed reaction pathway analysis was conducted to direct future model refinement. Results show that the deviation between model predictions and measurements at intermediate temperature may be attributed to the reactions that involve the H and OH radicals. Further comparisons of the sensitive reactions for H and OH formations from different literatures were conducted, and large deviations in rate constants among these reactions suggest the directions of future model refinement.

1. Introduction Nitromethane (NM, CH3NO2) is a liquid compound at room temperature. It is also an energetic material with simple molecular structure and has a variety of industrial applications. Nitromethane has been considered as a model compound to gain fundamental understanding on combustion chemistry of more complex explosives or monopropellants [1], such as hexahydro- 1,3,5- trinitro-1,3,5- triazinane (RDX) and 2,4,6-trinitrotoluene (TNT) [2,3]. Adding moderate amount of nitromethane into internal combustion engines (ICEs) can increase the power output of race cars [4–6]. Combustion chemistry of nitromethane favors the understanding of detonation characteristics of the energetic materials that contain nitro functional groups (-NO2) [7–11]. Accurate combustion chemistry model is also important for high fidelity reactive flow simulation [12]. Extensive investigations have reported the fundamental combustion parameters of nitromethane for developing and validation of the combustion chemistry mechanism, including the pyrolysis species profiles ⁎

[13–19] and the global flame parameters [20–30]. Specifically, the species profiles of CH3NO2 and NO2 were recorded over the temperature range of 900–1400 K by Glänzer and Troe via shock heating facility [14]. A kinetic model for pyrolysis of nitromethane was developed and validated against their measurements. However, only these two species were reported. Hus and Lin [15] measured the real-time production of two important decomposition species NO and CO in a shock tube for the temperature range of 940–1520 K and pressure range of 0.4–1.0 atm. A kinetic model that includes 37 reactions was developed and the simulated formation of NO profiles agrees well with the measurements, while the predicted CO profiles are lower than the measurements. Zhang and Bauer [16] investigated the decomposition of nitromethane in reflected shock waves over the temperature range between 1000 and 1100 K. CH3NO2 and several light hydrocarbons were recorded by gas chromatography (GC). These experimental data were then predicted with fairly good agreement by their developed model that includes 99 reactions. Glarborg et al. [17] developed a detailed model for nitromethane pyrolysis and

Corresponding author. E-mail address: [email protected] (C. Tang).

https://doi.org/10.1016/j.fuel.2019.116491 Received 20 May 2019; Received in revised form 18 October 2019; Accepted 22 October 2019 Available online 23 November 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. The schematic diagram of JSR.

the model shows good predictions with literature data of Glänzer et al. [14] and Zhang et al. [16] which cover the temperature range of 1000–1400 K and pressure range from 0.5 to 6.0 bar. Theoretical calculations for the thermal decomposition of nitromethane [18,19] indicate that the main nitromethane decomposition paths are CH3NO2 = CH3 + NO2 and CH3NO2 = CH3O + NO via the roaming isomerization CH3ONO. Furthermore, Weng et al [31] studied the pyrolysis of nitromethane in a plug flow reactor over the temperature range of 735–1476 K at low pressure (5 Torr). Results show that NO is the major nitrogenous product. However the mole fraction of CO which is the most important carbonaceous product was not reported in their work. It is worth noting that the experimental data of pyrolysis at low to intermediate temperature are still scarcely reported. For the oxidative environment at high temperature, a low pressure laminar premixed CH3NO2/O2/Ar flame has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry at 4.67 kPa and equivalence ratio of 1.39 by Tian et al. [21]. A detailed oxidation model consisting of 69 species and 314 reactions has been developed and shows reasonable agreement with the experimental results. The laminar flame speeds of CH3NO2/Air mixtures at 423 K, pressure range of 0.5–3.0 bar and equivalence ratios of 0.5–1.3 were measured using the spherically expanding flame method by Brequigny et al. [23]. The model from Zhang et al. [22] was updated by modifying rate constants of several reactions, the modified model shows better agreement with their measured flame speeds. In addition, Brackmann et al. [26] investigated the profiles of CH2O, CO and NO species using laser-induced fluorescence and temperature profiles using coherent anti-Stokes Raman spectroscopy. They suggested that further modification of Brequigny model [23] was necessary. Ignition behaviors of CH3NO2/O2/Ar mixtures have been investigated behind reflected shock waves in the temperature range of 875–1595 K, pressure range of 2.0–35 atm and equivalence ratio of 0.5, 1.0 and 2.0 by Mathieu et al. [28]. A detailed kinetic mechanism consisting of 166 species and 1204 reactions was developed, which shows reasonable performance in predicting their measured ignition delay times. Recently, Gao et al. [30] measured the ignition delay times of CH3NO2/O2/Ar mixtures in shock tube at extended range of test conditions. They refined the model from Mathieu et al and predictions agree well with the high temperature ignition delay time data and other literature data. For the low temperature oxidation kinetics of nitromethane, however, very limited works have been reported. Tricot et al. [32] investigated the low temperature oxidation of nitromethane in a static

vessel over temperature range of 700–740 K using GPC techniques associated with an original sampling procedure and spectroscopic measurements. Recently, Weng et al. [31] studied the oxidation of nitromethane in a jet-stirred reactor over the temperature range of 600–875 K at atmospheric pressure and fuel–air equivalence ratios of 0.4 and 2.0. Mole fraction profiles of major products and intermediates were measured with tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry. A detailed mechanism involving 364 species and 2389 reactions was developed based on their previous study for the oxidation of acetylene, a sub-mechanism for nitromethane and some calculated rate constants through ab-initio kinetics. Their model provides a reasonable agreement with their measured data. So far, no other work on the nitromethane about low to intermediate temperature oxidation experiment has been reported. As such, to further validate and understand the existing nitromethane mechanisms over a wider range of conditions, especially at low to intermediate temperature is of merit and experimental data of species concentration profiles on the pyrolysis and oxidation of nitromethane are needed. Our first objective is to provide more gas phase pyrolysis and oxidation data of nitromethane in the low to intermediate temperature rang by using a well validated jet-stirred reactor (JSR). We note that the work by Weng et al. [31] in JSR covers the temperature range of 600–875 K and we will then extent the work to a wider range of temperature but with GC and GC-MS sampling techniques. In addition, their pyrolysis of nitromethane work was reported at low pressure (5 Torr) using a flow reactor, we will also report the species profiles for the pyrolysis but at atmospheric pressure. In addition, since there have been several kinetic models for nitromethane, each of which show reasonable good agreement with their own data, most of which are high temperature data such as laminar flame speed, shock tube ignition delay time, and low pressure premixed flame structure, our second objective is then to validate these reported models against our new JSR pyrolysis and oxidation data. Finally, the detailed kinetic analysis for nitromethane pyrolysis and oxidation will be conducted using the Mathieu model [28]. In addition, the key reactions that control the pyrolysis and oxidation of nitromethane will be scrutinized for future model refinement. 2. Experimental specifications The nitromethane pyrolysis and oxidation experiments were 2

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investigated in a jet-stirred reactor facility. Fig. 1 shows the sketch of experimental system. It consists of the mixture preparation assembly, the jet-stirred reactor, and the sampling analysis facilities. Liquid nitromethane with 99% purity is used and pumped into the vaporizer by a high pressure infusion pump (AP0010 HPLC, uncertainty of the flow rate control is 0.1%). Ar from one Ar vessel is introduced to the vaporizer to pre-mix nitromethane vapor. The vaporizer temperature and the whole filling assembly are fixed at 408 K, which is higher than the boiling point of nitromethane to avoid fuel condensation. Another Ar line and O2 are introduced to the inlet and the flow rate is controlled by thermal mass-flow controllers (MKS) with a maximum uncertainty of 0.5%. Vaporized nitromethane/Ar, oxygen and Ar are then mixed in inlet before entering into JSR. The jet-stirred reactor consists of a fused silica sphere (volume of 106 cm3) and an injection cross with 4 nozzles of 0.3 mm inner diameter located in the center. One important issue of the jet-stirred reactor is that it resembles a 0-D reactor such that the temperature and concentration of species are assumed homogeneous within the reactor. As such, sufficient stirring is realized by turbulent jets flowing from the carefully arrayed nozzles. The reactor is heated using an electrical furnace (SK-G05123K, ZHONGHUAN) with temperature accuracy of 1.0 K. The reliable temperature range of heating system is (300–1400 K). The temperature of reactor is recorded using a K-type thermocouple (Omega, uncertainty is ± 0.4%), which is installed in the top of reactor. The outlet of JSR is connected to three lines, including the exhaust with after treatment, the sampling by GC (Agilent 7890B) and by GCMS (Agilent 7890B-5977A). The GC is equipped with 3 packed columns (2 Hayesep Q and 1 Molecular Sieve 5A). A thermal conductivity detector (TCD) is used to detect the light species concentration such as CO, CO2, O2 and H2. The GC-MS is fitted with two flame ionization detectors (FID) and two capillary columns (HP-5 and Plot Q) to analyze hydrocarbons and lighter oxygenated compounds, respectively. Verification of JSR is provided in Supplemental material. All experiments were conducted at atmospheric pressure with fixed initial fuel mole fraction of 1%. The residence time is set as 2.0 s. The experiments are conducted in a completely dry environment. The mixture preparation is in a closed mixture formation assembly shown in Fig. 1. There is no chance for nitromethane to be exposed to air. In addition, before the start of each experiment, we use Ar to flush the pipeline that may contain air with water vapor before we open the nitromethane valve for fuel loading. The pyrolysis of nitromethane was conducted over the temperature range of 675–1225 K. Meanwhile, the nitromethane oxidation experiments were conducted over the temperature range of 675–1250 K and the equivalence ratios of 0.5, 1.0 and 2.0. Test conditions are summarized in Table 1. Each experimental data point was repeated at least three times and good repeatability was observed. Species concentration is calibrated using the standard gas mixture purchased from AIR PRODUCTS. The uncertainties of major species and intermediates were about ± 5% and ± 10%.

oxidation was performed using the Perfectly Stirred Reactor (PSR) code from CHEMKIN-PRO software [33]. We used the kinetic mechanism with 166 species and 1204 reactions developed from Mathieu et al. [28] to conduct the PSR simulation. To further understand the model in detail, we performed flux and sensitivity analyses to track the key reaction pathways governing nitromethane consumption and intermediates production. 4. Results and discussion We have firstly measured species mole fractions for both pyrolysis and oxidation at three equivalence ratios. We have also validated several recently developed models, including Brequigny et al. [23] and Weng et al. [31], against our measured JSR pyrolysis and oxidation data and the performances of different models are presented in Supplemental material. Here in the main text we only report the comparison between measurements and simulations using Mathieu model [28]. 4.1. The pyrolysis of nitromethane Scatters in Fig. 2 are the measured major species and intermediates of nitromethane pyrolysis results at 1% fuel concentration. It can be seen from Fig. 2(a) that the decomposition of nitromethane begins at around 725 K under the conditions used in this work. Then it decomposes sharply as the temperature increases, primarily consumed through the reaction of CH3NO2 = CH3 + NO2. At about 900 K, the nitromethane is consumed completely. With the further increase of temperature, NO, as the major nitrogenous product, is formed through the reactions of CH3 + NO2 = CH3O + NO and CH3NO2 = CH3O + NO [31]. In addition, the measured species concentration profiles for H2, CO and CH4 are shown in Fig. 2(b). For H2 and CO species profiles, there are three distinct regions, in terms of the different CO and H2 mole fraction dependences on temperature: (1) in the low temperature region (725 K < T < 900 K), the concentrations of CO and H2 species increase as the temperature increases. Nitromethane direct decomposition contributes the increases of CO and H2 species, as well as CH4 species; (2) in the intermediate temperature region (900 K < T < 1000 K), the increase of H2 species becomes slow. Because nitromethane is consumed completely for T > 900 K, the radicals from nitromethane pyrolysis reach the maximum. With the further increase of temperature, H2 mole fraction increases through the H-abstraction reactions of CH2O, HNO and C2H6. However, CO mole fraction keeps almost unchanged. For CH4 species, it will keep the same concentration at T > 900 K; (3) in the relatively high temperature region (T > 1000 K), CO concentration continues to increase, while H2 has a faster increase than that in (2). Fig. 2(c)–(d) show typical intermediates such as C2H6, C2H4, C2H2 and CH3OH for nitromethane pyrolysis, as a function of temperature. For C2H4, there are also three distinct regions, similar as that shown in Fig. 2(b) for H2 and CO. In addition, for the C2 species, as the temperature increases, C2H6, C2H4, finally C2H2 is sequentially produced indicating that intermediates with higher unsaturation will be favorably produced at higher temperature. Perfectly stirred reactor simulations for nitromethane pyrolysis at the experimental conditions were also included in Fig. 2 by the lines. Mathieu model [28] can predict nitromethane mole fraction profile well at the temperature range of 675–1225 K, as shown in Fig. 2(a). For the main products as shown in Fig. 2(b), below 900 K Mathieu model [28] predicted mole fractions for H2, CO and CH4 agree well with the experimental data. Above 1000 K, however, the model underestimates the formations of these species. This is because based on reaction pathway analysis and sensitivity analysis of OH radical (at 1100 K) in Sec 4.3, H2, CO and CH4 radicals are consumed mainly through the reactions OH + H2 = H + H2O, CO + OH = CO2 + H and CH4 + OH = CH3 + H2O. At high temperature condition, OH radical formation increases rapidly because of the reaction NO2 + H = NO + OH. As a consequence, OH radicals promote the H-

3. Kinetic modeling The chemical kinetic modeling for nitromethane pyrolysis and Table 1 Detailed experimental conditions and mixture compositions. Mix #

1 2 3 4

Equivalence ratio ϕ

∞ 0.5 1.0 2.0

P = 1 atm

Mole percentage (%mol)

Temperature range (K)

[Fuel]

[O2]

[Ar]

675–1225 675–1025 675–1100 675–1250

1.00 1.00 1.00 1.00

0.00 2.50 1.25 0.63

99.00 96.50 97.75 98.37

3

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Fig. 2. Comparison between measured and predicted species mole fractions for nitromethane pyrolysis, using the model from Mathieu et al. [28].

abstraction from H2, CO and CH4, resulting the underestimated predictions of H2, CO and CH4 mole fractions using Mathieu model. For stable pyrolysis intermediate species such as C2H2, C2H4, C2H6 and methanol as shown in Fig. 2(c) and (d), Mathieu model predicts well the trends of species formations as a function of temperature, though the predicted peak mole fraction of these species occur at lower temperatures. This is because the formations of OH and H radicals increase with the temperature increase in this temperature range, which promotes the H-abstraction of C2H6 and C2H4 in Mathieu model. This accelerates the consumptions of C2H6 and C2H4, causing the peak C2H6 and C2H4 mole fraction temperature to decrease. It then further leads to a rapid increase for the prediction of C2H2 mole fraction due to contentious Habstraction.

temperature coefficient (NTC) behavior. Above 875 K, the CO2 mole fractions begin to decrease for all the three equivalence ratios and the NTC region covers a larger temperature region with the increase of equivalence ratio. We note that this negative temperature coefficient behavior has not been observed previously, since most of previous work [28–30] focused on the high temperature shock tube study. In addition, because the work of Weng et al [31] targets in the low temperature region (T < 875 K), the intermediate temperature range where NTC behavior occurs was then not reported in their work. The reason for observed NTC behavior will be discussed in reaction flux analysis in Section 4.3. In Fig. 4(a) and (b), the measured mole fractions of H2 and CO at ϕ = 0.5, 1.0 and 2.0 were almost the same below 800 K. As temperature increases, the H2 and CO formations are significantly favored with the equivalence ratio increases. This is reasonable because at lean and stoichiometric conditions, sufficient O2 exists in the reactor and the complete combustion products such as carbon dioxide (Fig. 3 (c)) and water are expected, such that H2 and CO formations are minimized for lean mixtures. While at rich condition (ϕ = 2.0), the H2 and CO are the main products. As depicted in Fig. 4(c)–(f), the major hydrocarbon intermediates in nitromethane oxidation were methane, ethyne, ethylene and ethane with maximum mole fractions of 9.34 × 10-4, 1.21 × 10−5, 1.27 × 10−4, and 4.36 × 10−5 under lean condition. For stoichiometric mixture, the peak mole fraction of methane, ethyne, ethylene and ethane were 2.40 × 10−4, 2.19 × 10−7, 1.56 × 10−5 and 2.75 × 10−6. While at rich condition, these species decrease to the mole fraction of 1.19 × 10−5, 0, 5.90 × 10−7 and 0. Among them, the CH4 species is the most abundant hydrocarbon intermediate, which is developed from CH3, after the C-N bond rupture of nitromethane [31]. In addition, for measured main intermediate species, the temperature for peak mole fraction will increase with the equivalence ratio

4.2. The oxidation of nitromethane Influence of equivalence ratio on nitromethane oxidation species as a function of temperature is shown in Figs. 3 and 4. The mole fractions of nitromethane and oxygen are presented in the Fig. 3(a) and (b), respectively. At different equivalence ratios, the mole fraction profiles of nitromethane were almost the same. It begins to be consumed at about 725 K and completely vanished at 900 K, which is similar as the pyrolysis of nitromethane. Results show that the reactivity of nitromethane is not sensitive to the amount of oxygen, which is consistent with high temperature shock tube ignition delay times [30]. In addition, at three equivalence ratios and for the temperature below 900 K, the mole fractions of O2 decrease as the temperature increases. However, above 900 K the mole fraction profile of O2 has a slight increase. CO2 was detected as the primary carbonaceous product, as shown in Fig. 3(c). As expected, the CO2 mole fractions increase with for lower equivalence ratios. In addition, CO2 mole fractions indicate the classic negative 4

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Fig. 3. Measured mole fraction profiles of major reactants and products at different equivalence ratios in nitromethane oxidations: (a) CH3NO2, (b) O2 and (c) CO2.

increasing. Figs. 5–7 compare the predicted and measured species mole fractions as a function of temperature for three equivalence ratios, using Mathieu model. It can be seen that the mole fractions of reactants (CH3NO2 and O2) are reproduced well by Mathieu model. At the equivalence ratio of 0.5, as shown in Fig. 5, the predicted mole fractions of CO and CO2 agree reasonably well with the measurements. However, H2 mole fraction is underestimated by the model. In addition, it fails to capture the NTC behavior that is reflected by the three distinct regions of the CO2 and CO mole fractions dependence on temperature. At stoichiometric equivalence ratio, as shown in Fig. 6, better performance of the model for predicting measured species mole fraction profiles was observed for temperature lower than 875 K. However, as the temperature is beyond 875 K, the predicted mole fractions of CO and CO2 are respectively lower and higher than the measurements. In addition, the model again fails to capture the three regions in the CO2 and CO mole fraction profiles. For the main hydrocarbons in Fig. 6(c)–(d), there is a reasonable agreement between predicted and measured data. According to reaction pathway analysis, both CH4 and C2H6 radicals are produced mainly through CH3 radical at ϕ = 1.0. At intermediate temperature, because C2H6 radical is over-estimated, CH4 has an underprediction, as shown in Fig. 6 (c). When C2H6 radical is over-estimated, the downstream products: C2H4 and C2H2 also have a higher prediction as shown in Fig. 6(d). For rich mixture as shown in Fig. 7 (ϕ = 2.0), the model shows similar level of agreement, as compared with the stoichiometric condition. However, at this equivalence ratio, the model reflects three regions in the CO2 mole fraction profile as a function of temperature, which qualitatively agrees with the experimental result. The experimental data of nitromethane pyrolysis and oxidation were conducted at low to intermediate temperature. The measured

species mole fractions were used to validate the model developed by Mathieu et al. [28]. Results show that Mathieu model can predict the data well at low temperature (below 875 K) for all experimental conditions. However, at intermediate temperature there has a considerable deviation. To further understand this model at low and intermediate temperature chemistry, chemical kinetic analysis would be conducted in the following, especially for H and OH radicals. 4.3. Chemical kinetic analysis 4.3.1. Reaction pathway analysis The major reaction pathways of nitromethane pyrolysis (83.3% fuel conversion) and oxidations at ϕ = 0.5 (87.4% fuel conversion), ϕ = 1.0 (88.3% fuel conversion) and ϕ = 2.0 (88.5% fuel conversion) at 850 K and 1.0 atm are presented in Fig. 8. The residence time is set to be 2.0 s. It is seen that nitromethane pyrolysis and oxidation exhibit similar pathways, though there are some quantitative difference. The nitromethane fuel is primarily consumed by the unimolecular decomand position reactions such as CH3NO2 = CH3 + NO2 CH3NO2 = CH3O + NO. However, the ratios of some important reactions have considerable differences. For the nitromethane pyrolysis, 85.9% fuel is consumed by direct decompositions. Due to the absence of O2, H-abstraction reactions of nitromethane contribute weakly (4%) for nitromethane consumption through H attaching to form CH2NO2 and H2. In addition, CH3O2 are not formed through CH3 + O2 = CH3O2, while the formation of CH4 through the H-abstraction reactions between CH3 and CH2O/HNO becomes more pronounced. The primary consumption path for CH3 radical is CH3 + NO2 = CH3O + NO (56.3%). The formed CH3O radical then goes through: CH3O → CH2O → HCO and finally forms CO. Besides the formations of CH4 and CH3O from CH3 radical, C2H6 is also 5

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Fig. 4. Measured mole fraction profiles of major intermediate species at different equivalence ratios in nitromethane oxidations: (a) H2, (b) CO, (c) CH4, (d) C2H2, (e) C2H4 and (f) C2H6.

produced from recombination of two CH3 radicals. However, this reaction is absent in the oxidation of nitromethane. Subsequently, C2H5 radical is formed by the dehydrogenation of C2H6. C2H5 radicals produce C2H4 through pyrolysis and reacting with O2. In the temperature range here, stable C2H4 is difficult to generate the C2H3 radical through H-abstraction. For the oxidation cases, however, the H-abstraction reactions become significant for the consumption of nitromethane. For the lean mixture, the H-abstraction reaction CH3NO2 + OH = CH2NO2 + H2O accounts for 11.3% of nitromethane consumption. With the increase of equivalence ratio, the reactions CH3NO2 = CH3 + NO2 and CH3NO2 + OH = CH2NO2 + H2O become less important, while the contributions from CH3NO2 + H = CH3 + HNO2 and CH3NO2 + H = CH2NO2 + H2 become progressively important. CH3

radical is the main secondary product of nitromethane. At ϕ = 0.5, 1.0 and 2.0, 76.3%, 70.3% and 65.1% of CH3 radicals are converted to CH3O. In addition, > 20% of CH3 radicals are firstly oxidized to CH3O2, and then react with NO to form CH3O through CH3O2 + NO = CH3O + NO2. The generated CH3O radicals then form CH2O. Subsequently, through the reaction with OH, H and NO2 radicals, CH2O produces HCO, which yields CO via reactions with NO and O2. Finally, CO2 is formed by CO oxidation through CO + OH = CO2 + H. For the equivalence ratio of 2.0, the CH3 radicals react to form certain amounts of CH4 and C2H6, though these species are not observed at ϕ = 0.5 and 1.0. We note that Mathieu model predicts the measured species mole fractions of nitromethane pyrolysis and oxidation well at 850 K. At intermediate temperature, however, some predicted species mole 6

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Fig. 5. Comparison between measured and predicted species mole fractions for nitromethane oxidation, using the model from Mathieu et al. [28] at ϕ = 0.5.

fractions show moderate deviation with the experimental data. In order to access the possible elementary reactions that may aid future mechanism refinement, the reaction pathways at higher temperature are also analyzed, as shown in Fig. 9. At 1100 K, nitromethane pyrolysis primarily happens through the cleavages of C-N bond and O-NO bond, respectively accounting for 89.6% and 10%. In this case, CH4 is formed through CH3 radical reacting with H and HNO radicals. This is consistent with the comparison between measured and predicted species mole fraction of CH4 shown in Fig. 2(b). The H-abstraction reaction CH4 + OH = CH3 + H2O is overestimated in Mathieu model, as a consequence, the simulated CH4 mole fraction is lower than the measured one for temperature higher than 1100 K. At the lean conditions, Mathieu model can't predict the

NTC behavior of CO2. That leads to CO2 species be over-predicted and CO species be under-predicted. Through reaction path analysis of CO2, it can be found that the reaction CO + OH = CO2 + H formed 80% CO2 species, and the only consumed reaction is CH2 (S) + CO2 = CH2O + CO. The over/under predictions of CO2/CO species were also observed in the stoichiometric and rich conditions. At intermediate temperature, the mole fraction of H radical is over estimated by Mathieu model, while OH radical is under-predicted. The NTC behavior of CO2 can be captured using Mathieu model for large equivalence ratio case. It is well known that the primary CO2 formation channel is CO + OH = CO2 + H, which will be favored with the increase of temperature. Fig. 4 shows that at intermediate temperature, the stable species (CH4, H2, C2H6 and C2H4) are accumulated

Fig. 6. Comparison between measured and predicted species mole fractions for nitromethane oxidation, using the model from Mathieu et al. [28] at ϕ = 1.0. 7

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Fig. 7. Comparison between measured and predicted species mole fractions for nitromethane oxidation, using the model from Mathieu et al. [28] at ϕ = 2.0.

to sufficiently high concentration. These species will undergo H abstraction reactions by OH attach, which compete with the primary CO2 formation channel CO + OH = CO2. As a consequence, the CO2 mole fraction decreases. At high temperature, those stable species have significantly lower concentration, and the primary CO2 formation channel is not affected. This is the main reason for the experimentally observed NTC behavior represented by the CO2 mole fraction variation with temperature. Additionally, we note that the reaction flux in Figs. 8 and 9 show that the formation of CH3O2 radical by O2 addition to CH3 contributes significantly to the consumption of CH3 radical. The formation of CH3O2 is accelerated as temperature increases. The formed CH3O2 radical then reacts with NO to form the CH3O radical, which subsequently form CO through CH2O → HCO destruction channel. However, NO formation is only activated at high temperature, the reaction CH3O2 + NO to form CH3O is relatively slow at low to intermediate temperature since NO formation is unfavored, as such it slows down the CO and CO2. At high temperature, NO is formed in large quantities. It allows CH3O2 to generate CH3O, which then produces CO and CO2 through CH2O → HCO destruction channel. This also contributes the observed NTC behavior of CO2 mole fraction.

between measurements and predictions for temperatures higher than 875 K. We also note that the reaction path flux shown in Figs. 8 and 9 indicates that H and OH radicals play important roles in kinetic scheme of both nitromethane pyrolysis and oxidation. As such, the most sensitive reactions for H and OH prediction during pyrolysis and oxidation in a Perfectly Stirred Reactor are then scrutinized using Mathieu mechanism [28]. Sensitive reactions for the formation of H radicals during nitromethane pyrolysis at 1100 K and oxidations for three equivalence ratios at 950 K are presented in Fig. 10. It is seen that sensitive reactions for H species formation (or consumption) are the same for pyrolysis and oxidations at different equivalence ratios. Reaction NO2 + H = NO + OH has the highest negative sensitivity value, indicating that the simulated H radical mole fraction at this conditions strongly depends on the rate constant of this reaction. In addition, increasing the rate constant of this reaction is expected to reduce the mole fraction of H radical. The reactions HCO + M = H + CO + M, CH3 + NO2 = CH3O + NO, CH3NO2 = CH3O + NO and CH3O (+M) = CH2O + H (+M) have high sensitivity coefficients for all experimental conditions. In Mathieu model, rate constant of the reaction HCO + M = H + CO + M is adapted from Li et al. [34]. We have compared this rate constant with that reported by Brequigny et al. [23] which is taken from Yang et al. [35], as shown in Fig. 11. In addition, the reaction of methyl radical recombination CH3 + CH3 (+M) = C2H6 is important for hydrocarbon products for both pyrolysis and oxidation condition because the amount of C2H6 species directly determines the content of subsequent intermediates and final products. Furthermore, the reaction HNO + O2 = HO2 + NO shows high sensitivity for the oxidation cases analyzed here, and it has a negative sensitivity coefficient at ϕ = 0.5 and for stoichiometric and rich mixture it has a positive value. The rate constants of the above three reactions in Mathieu model

4.3.2. Sensitivity analysis on H and OH radicals formation Since for temperatures higher than 875 K, the predicted CO and CO2 are respectively lower and higher than the measurements at all the three equivalence ratio cases, as shown in Figs. 5–7. In addition, it is well known that CO consumption and CO2 production are primarily contributed through the main heat release reaction CO + OH = CO2 + H. The rate constant of this reaction in Mathieu model is consistent with all the well validated small molecules mechanism such as GRI 3.0. It is then speculated that reactions that involve formation of OH and H radicals may contribute to the deviation 8

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Fig. 8. Reaction paths of the nitromethane pyrolysis (black) and oxidations at the equivalence ratio of 0.5 (red), 1.0 (blue) and 2.0 (magenta) corresponding to 850 K and 83.3% fuel conversion, 850 K and 87.4% fuel conversion, 850 K and 88.3% fuel conversion, 850 K and 88.5% fuel conversion. Residence time = 2.0 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Reaction paths of the nitromethane pyrolysis (black) and oxidations at the equivalence ratio of 0.5 (red), 1.0 (blue) and 2.0 (magenta) corresponding to 1100 K and 100% fuel conversion, 950 K and 100% fuel conversion, 950 K and 100% fuel conversion, 950 K and 100% fuel conversion. Residence time = 2.0 s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 12. Sensitivity coefficients for OH radical formation during nitromethane pyrolysis at 1100 K (gray) and oxidations at the equivalence ratio of 0.5 (olive), 1.0 (blue) and 2.0 (orange) at 950 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10. Sensitivity coefficients for H radical formation during nitromethane pyrolysis at 1100 K (gray) and oxidations at the equivalence ratio of 0.5 (olive), 1.0 (blue) and 2.0 (orange) at 950 K. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. The sensitive reactions rate constants of OH radical used in Mathieu model [28] and comparison with those from Brequigny model [23].

Fig. 11. The sensitive reactions rate constants of H radical used in Mathieu model [28] and comparison with those from literature results.

and they show good agreement. Fig. 13 compares the rate constants of the most sensitive reactions in Mathieu model with that are reported in Brequigny model. It can be seen that the rate constant of Mathieu et al. [28] is higher than that from Brequigny et al. [23] for reaction HONO + OH = NO2 + H2O. The deviation becomes more significant at higher temperature. For the sensitive reaction of OH + H2 = H + H2O, the difference between Mathieu et al. [28] and Brequigny et al. [23] exists in low or high temperatures. The reactions of HNO + O2 = HO2 + NO and HCO + O2 = CO + HO2 are sensitive for OH radical at nitromethane oxidation show relatively large difference between in Mathieu model and Brequigny model.

as well as those reported in literatures are compared in Fig. 11. It can be seen that these sensitive reactions still have rate constant uncertainty of an order of magnitude and further refinement of the rate constant of these reactions are needed to better predict the species mole fractions during the pyrolysis or oxidation of nitromethane. Similarly, we also presented the sensitivity coefficients for OH radical in Fig. 12. Unlike the H radical sensitivity analysis in Fig. 10 that shows the same sensitive reactions between the pyrolysis and oxidation cases, the most sensitive reactions for OH radical mole fraction prediction for pyrolysis are quite different from the oxidation cases at different equivalence ratios. Sensitive reactions for OH radical at different conditions depend strongly on the amount of O2 in the reactor. When O2 is sufficient (ϕ = 0.5 and 1.0), the reactions HONO + OH = NO2 + H2O and H + O2 = O + OH have high negative sensitivity coefficient. On the contrary (pyrolysis and rich condition ϕ = 2.0), the reactions CH4 + OH = CH3 + H2O and CH2O + OH = HCO + H2O will play important roles for OH radical formation or consumption. The uncertainty of the rate constant of H + O2 = O + OH is not discussed here. Since Alekseev et al. [36] has validated the rate constant of this reaction against their measured data

5. Conclusion The pyrolysis and oxidation of nitromethane were conducted at 1 atm, for the low to intermediate temperature range in a jet-stirred reactor. New experimental data on the species mole fraction profiles using GC and GC-MS were provided. Measurements show that the nitromethane fuel begins to be destructed at around 725 K and completely vanishes at 900 K for all experimental conditions. With the 10

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increase of temperature, three distinct regions were observed in terms of the different CO and H2 mole fraction dependences on temperature. A NTC behavior of CO2 was observed for all the three equivalence ratios, which has never been reported previously. These jet-stirred reactor measurements were then used to validate several recently proposed mechanisms of nitromethane. Results show that the model of Mathieu et al. [28] can predict the measured data well at low temperature (below 900 K) for all conditions. However, at intermediate temperature the predicted species mole fraction from Mathieu model has a considerable deviation compared with our measured data. To further understand nitromethane chemistry at low and intermediate temperature in Mathieu model, the reaction pathway at typical conditions was analyzed. Results show that the deviation at intermediate temperature is likely to be caused by the reactions involving H and OH radicals. Most sensitive reactions for H and OH radical mole fractions predictions are further scrutinized and the rate constants of these reactions from different literatures are compared, and noticeable uncertainties of these rate constants suggest future model refinement direction.

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