Pyrolysis Of Nitromethane Research Articles

Ignition characteristics of nitromethane-doped ethanol in a heated shock tube

Ignition delay times of nitromethane-doped ethanol dual fuels are measured under the micro-combustion engine conditions (8 ​× ​105 ​Pa and 904–1477 ​K). The promotion effect of nitromethane on ethanol ignition is more pronounced at a fuel-lean condition; it first increases and then decreases with increasing blending ratio and temperature. The optimal promotion effect is achieved at 950–1150 ​K with an addition of ~50% nitromethane (mole fraction). A chemical kinetic model is proposed to describe the ignition process of nitromethane-doped ethanol, and can reproduce the measured ignition delay times. Reaction pathway, sensitivity, and rate of production analyses are performed to reveal the promotion mechanisms of nitromethane. The consumption of ethanol via the H-abstraction reactions is enhanced due to the pyrolysis of nitromethane, and the produced ethanol radicals are beneficial to the re-establishment of the radical pool after the first-stage ignition of nitromethane. With decreasing temperature, nitromethane decomposes slowly, while the low-temperature oxidation of ethanol becomes more important. At a moderate temperature, both these two reasons result in a shorter ignition delay time.

Nitromethane pyrolysis in shock tubes and a micro flow reactor with a controlled temperature profile

Nitromethane has many applications, such as in racing, as a gasoline fuel additive, and as a monopropellant. Despite a large number of studies and the small size of the molecule, the combustion chemistry of nitromethane is still not well understood. To improve models, the pyrolysis of nitromethane (CH3NO2) was investigated experimentally in shock tubes and in a micro flow reactor with a controlled temperature profile (MFR), under dilute conditions. Several spectroscopic diagnostics were used in the shock tubes to follow the concentration time histories of CO, H2O (both using IR laser absorption), and CH3NO2 (UV light absorption). A quadrupole mass spectrometer was used to measure CH3NO2, NO2, CH4, C2H4, and C2H2 at various temperatures with the MFR. These unique experimental results were compared to modern, detailed kinetics models from the literature, and no mechanism was able to reproduce these data over the wide range of conditions investigated. Predictions for the CO and H2O levels were generally inaccurate, and the CH4, C2H4, and C2H2 predictions were poor in most cases for the MFR data. Importantly, all models largely differ in their predictions. A numerical analysis was performed to identify ways to improve the next generation of nitromethane models. Results indicate that nitromethane decomposition needs to be improved below 1050 K, and that hydrocarbon-NOx interactions still need to be further investigated.

Shock tube measurement of NO time-histories in nitromethane pyrolysis using a quantum cascade laser at 5.26 µm

The pyrolysis of nitromethane, CH3NO2, was studied at ∼3.5 atm and 1013 K–1418 K in a heated shock tube by measuring the key product nitric oxide (NO) using mid-infrared laser absorption spectroscopy. We used a quantum cascade laser (QCL) at 5.26 µm to exploit the strong NO absorption at 1900.08 cm−1. With the NO absorption cross-section data characterized at 1006 K–1789 K and 2.7 atm–3.5 atm behind reflected shock waves, we measured the NO concentration time-histories during the pyrolysis of nitromethane at two different concentrations (1.05% and 0.6%). The absorption interference from other major products such as CO and H2O was analyzed to be negligible, leading to an interference-free NO diagnostic in nitromethane pyrolysis. A recent kinetic model of Shang et al. (2019) was adopted to interpret the shock tube data. All the NO time-histories measured over the entire temperature range 1013 K–1418 K were well-predicted by this model in terms of the initial NO formation rate and the final plateau level. The rate-of-production, sensitivity, and reaction flux analyses were performed to identify four important reactions (CH3NO2 = CH3 + NO2, CH3NO2 ↔ CH3ONO = CH3O + NO, CH3 + NO2 = CH3O + NO, and NO2 + H = NO + OH) that determine the NO formation during CH3NO2 pyrolysis. The satisfactory agreement between the simulation and shock tube/laser absorption measurement further validated the kinetic mechanism of nitromethane decomposition. The developed mid-infrared NO absorption sensor provides a promising diagnostic tool for studying fuel-nitrogen chemical kinetics in the shock tube experiments.

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

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.

Insights into nitromethane combustion from detailed kinetic modeling – Pyrolysis experiments in jet-stirred and flow reactors

The pyrolysis of nitromethane highly diluted in helium was studied in a plug flow reactor and in a jet-stirred reactor at 1.07 bar and over the temperature range from 500 to 1100 K. Mole fraction profiles of major products and of intermediates were identified with gas chromatography and Fourier transform infrared spectroscopy. Using these experimental data, as well as published ones, we have developed a newly compiled model for the prediction of the pyrolysis and of the oxidation of nitromethane in jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames, as well as in shock-tubes. The experimental results from the present work and from the literature are interpreted with the help of the kinetic model derived here. This study mainly focuses on the analysis of speciation in different reactors. Among the nitrogenous species, NO is found to be a major product for pyrolysis and oxidation. The model suggests that for nitromethane pyrolysis and oxidation the thermal dissociation channel to CH3 and NO2 is the main reaction path for the nitromethane degradation followed by the H-atom abstraction channel. The most sensitive reactions for nitromethane pyrolysis in a flow reactor and during pyrolysis and oxidation in a jet-stirred reactor are found to be CH3NO2(+M) ⇋ CH3 + NO2(+M) and CH3 + NO2 ⇋ CH3O + NO. The reaction CH3 + NO2 ⇋ CH3O + NO is found to be the most important reaction for all conditions studied. In a burner-stabilized premixed flame, as the mixture gets richer, the thermal dissociation channel CH3NO2(+M) ⇋ CH3 + NO2(+M) becomes more important as the contribution of the H-atom abstraction channel is decreased. Furthermore, in the burner-stabilized premixed flames, it was found that NO is mainly formed via NO2: NO2 + H ⇋ NO + OH, NO2 + CH3 ⇋ CH3O + NO. The model provided an overall reasonable agreement with the experimental data. However, for pyrolysis conditions, future work is desirable to improve predictions of intermediate species. This work extends the kinetic database and helps to improve the understanding of nitromethane chemistry. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2 and nitrogen-containing compounds higher than C1 as well as for pure nitrogen compounds.

Thermal Decomposition of Nitromethane and Reaction between CH3 and NO2.

The thermal decomposition of gaseous nitromethane and the subsequent bimolecular reaction between CH3 and NO2 have been experimentally studied using time-resolved cavity-enhanced absorption spectroscopy behind reflected shock waves in the temperature range 1336-1827 K and at a pressure of 100 kPa. Temporal evolution of NO2 was observed following the pyrolysis of nitromethane (diluted to 80-140 ppm in argon) by monitoring the absorption around 400 nm. The primary objectives of the current work were to evaluate the rate constant for the CH3 + NO2 reaction (k2) and to examine the contribution of the roaming isomerization pathway in nitromethane decomposition. The resultant rate constant can be expressed as k2 = (9.3 ± 1.8) × 10-10(T/K)-0.5 cm3 molecule-1 s-1, which is in reasonable agreement with available literature data. The decomposition of nitromethane was found to predominantly proceed with the C-N bond fission process with the branching fraction of 0.97 ± 0.06. Therefore, the upper limit of the branching fraction for the roaming pathway was evaluated to be 0.09.

Experimental and modeling study on pyrolysis of n-decane initiated by nitromethane

Initiator could accelerate the rate of hydrocarbon pyrolysis and reduce the required material temperatures for a hypersonic aircraft heat exchanger/reactor. Nitroalkanes were proposed as the effective initiator because of the lower CN bond dissociation energy. In order to investigate the initiation mechanism of nitroalkanes on hydrocarbon pyrolysis, the pyrolysis of n-decane, nitromethane and their binary mixture were carried out at 30, 150 and 760 Torr in a flow reactor with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The identified and quantified pyrolysis species include C1C2 alkanes, C2C10 alkenes, C3C6 dialkenes, C2C3 alkynes, nitrogen oxides such as NO and NO2, benzene, and radicals including CH3, C3H3, and C3H5, which shed light on the mechanism of n-decane and nitromethane pyrolysis, as well as the interactions of these two fuels. The experimental results indicate that the addition of nitromethane decreases the initial decomposition temperature of n-decane, and a stronger promotion effect could be obtained as the experimental pressure increases. The distributions of alkanes, alkenes, dialkenes, alkynes and benzene are also influenced by the addition of nitromethane. A detailed kinetic model with 266 species and 1648 reactions was developed and validated against the mole fraction profiles of reactants, major products and important intermediates during the pyrolysis of each fuel and their binary mixture. The satisfactory model prediction to the experimental measurements permits the analysis of the kinetic effect of nitromethane initiation on the pyrolysis of n-decane. So that, the increase of the conversion rate at a lower temperature, the selectivity of decomposition products, and reduction of benzene formation are better understood.

Reactive Molecular Dynamics Simulation of Solid Nitromethane Impact on (010) Surfaces Induced and Nonimpact Thermal Decomposition

Which is the first step in the decomposition process of nitromethane is a controversial issue, proton dissociation or C-N bond scission. We applied reactive force field (ReaxFF) molecular dynamics to probe the initial decomposition mechanisms of nitromethane. By comparing the impact on (010) surfaces and without impact (only heating) for nitromethane simulations, we found that proton dissociation is the first step of the pyrolysis of nitromethane, and the C-N bond decomposes in the same time scale as in impact simulations, but in the nonimpact simulation, C-N bond dissociation takes place at a later time. At the end of these simulations, a large number of clusters are formed. By analyzing the trajectories, we discussed the role of the hydrogen bond in the initial process of nitromethane decompositions, the intermediates observed in the early time of the simulations, and the formation of clusters that consisted of C-N-C-N chain/ring structures.

Gas-phase decomposition mechanisms of C-NO2, N-NO2 energetic materials: Reevaluations

The thermal decomposition mechanisms of gaseous nitromethane, methyl nitrite, dimethylnitramine and 1,3,3,-trinitroazetidine (TNAZ) have been reanalyzed using sensitivity and principal component analyses. For each system an adequate, much simplified mechanism was developed, and the fragmentation/reaction sequence leading to the final products was made transparent. The critical roles of free radicals (CH3, H, OH, HCO, HNO, CH3O, etc.) at different stages of these pyrolyses were then identified. The predicted product distributions for these compounds were calculated to facilitate the assessment of their explosive performance parameters. Comparison of the products generated by nitromethane with those produced by methyl nitrite showed that the contribution of a nitro-nitrite isomerization reaction is negligible in the nitromethane pyrolysis. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 655–673, 1999

Evaluated kinetic and photochemical data for atmospheric chemistry: Supplement IV: IUPAC subcommittee on gas kinetic data evaluation for atmospheric chemistry

The pyrolysis of nitromethane highly diluted in helium was studied in a plug flow reactor and in a jet-stirred reactor at 1.07 bar and over the temperature range from 500 to 1100 K. Mole fraction profiles of major products and of intermediates were identified with gas chromatography and Fourier transform infrared spectroscopy. Using these experimental data, as well as published ones, we have developed a newly compiled model for the prediction of the pyrolysis and of the oxidation of nitromethane in jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames, as well as in shock-tubes. The experimental results from the present work and from the literature are interpreted with the help of the kinetic model derived here. This study mainly focuses on the analysis of speciation in different reactors. Among the nitrogenous species, NO is found to be a major product for pyrolysis and oxidation. The model suggests that for nitromethane pyrolysis and oxidation the thermal dissociation channel to CH3 and NO2 is the main reaction path for the nitromethane degradation followed by the H-atom abstraction channel. The most sensitive reactions for nitromethane pyrolysis in a flow reactor and during pyrolysis and oxidation in a jet-stirred reactor are found to be CH3NO2(+M) ⇋ CH3 + NO2(+M) and CH3 + NO2 ⇋ CH3O + NO. The reaction CH3 + NO2 ⇋ CH3O + NO is found to be the most important reaction for all conditions studied. In a burner-stabilized premixed flame, as the mixture gets richer, the thermal dissociation channel CH3NO2(+M) ⇋ CH3 + NO2(+M) becomes more important as the contribution of the H-atom abstraction channel is decreased. Furthermore, in the burner-stabilized premixed flames, it was found that NO is mainly formed via NO2: NO2 + H ⇋ NO + OH, NO2 + CH3 ⇋ CH3O + NO. The model provided an overall reasonable agreement with the experimental data. However, for pyrolysis conditions, future work is desirable to improve predictions of intermediate species. This work extends the kinetic database and helps to improve the understanding of nitromethane chemistry. The kinetic model presented in this work can serve as a base model for hydrocarbons and oxygenated fuels higher than C2 and nitrogen-containing compounds higher than C1 as well as for pure nitrogen compounds.

Ignition delay times of nitromethaneoxygenargon mixtures behind reflected shock

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.

Vibrational energy transfer and pyrolysis of nitromethane by the variable-encounter method

Abstract : The pyrolysis of nitromethane has been studied by the Variable Encounter Method (VEM) at temperatures from 816 K to 1092 K with two reactors of differing geometry having fused silica surfaces. The probability of reaction per collision with the reactor surface was measured. The down energy transition jump size, delta E', was determined. It decreased with increasing wall temperature. A comparison is made of delta E' with previous results reported to date for other substrate molecules. Nitromethane is one of the more efficient energy transfer agents. However, in addition to the size (vibrational eigenstate density) and the polarity of the molecules, the nature of the hot surface seems also to play a role. (Author)

Reactions of nitroalkanes in the gas phase. Part 1.—Pyrolysis of nitromethane

The pyrolysis of nitromethane has been studied between 305 and 440°C. The reaction is pressure dependent. Above 150 mm, it is approximately first order, k= 1014.1 exp (–55,200/RT) sec–1. The principal products are methane, carbon monoxide, nitrogen, nitric oxide, hydrogen cyanide and water. The results are explained by considering the reactions of methyl: CH3·+ CH3NO2→ CH4+·CH2NO2, CH3·+ NO2→ CH3NO2, CH3·+ NO2→ CH3O·+ NO, CH3·+ NO → CH3NO The product distribution depends on the relative concentrations of nitric oxide and nitrogen dioxide in the system, and this is confirmed by studying the ratio of products formed on addition of excess formaldehyde and nitrogen dioxide.

The photolysis and pyrolysis of nitromethane and methyl nitrite

The photolysis and pyrolysis of nitromethane and methyl nitrite have been studied using the techniques of flash photolysis and kinetic spectroscopy. The results show that photolysis of nitromethane yields methyl radicals and nitrogen dioxide, and that these fragments undergo recombination and disproportionation reactions to form methyl nitrite, methoxyl, and nitric oxide. In the presence of added nitric oxide, the methyl radicals react principally with nitric oxide to form nitrosomethane, which subsequently dimerizes and also reacts further with nitric oxide to yield nitrogen dioxide. The evidence also suggests that nitrosomethane is removed by a relatively efficient reaction with nitrogen dioxide at elevated temperatures to produce nitromethane and nitric oxide. In the case of methyl nitrite, light absorption results not only in photolysis, but also in the formation of an isomer of the nitrite which then reverts slowly to the stable form. The nature of this isomer is not known, but possibilities are suggested and discussed. It is concluded that the decomposition (photolytic or pyrolytic) of methyl nitrite occurs by the rupture of the O—N bond, and that the methoxyl radicals formed disproportionate to yield methanol and form aldehyde. Nitroxyl is also formed but only as a minor product, and the marked increase in intensity of its spectrum in the presence of added nitric oxide shows that it is not formed by a molecular elimination reaction, but probably by CH 3 O + NO → CH 2 O + HNO.