Reaction Path Analysis Research Articles

Interaction of NH3 and NO under combustion conditions. Experimental flow reactor study and kinetic modeling simulation

The interaction between ammonia and NO under combustion conditions is analyzed in the present work, from both experimental and kinetic modelling points of view. An experimental systematic study of the influence of the main variables for the NH3NO interaction is made using a laboratory tubular flow reactor installation. Experiments are performed at atmospheric pressure and variables analyzed include: temperature in the 700–1500 K range, air stoichiometry, from pyrolysis to very oxidizing conditions, and the NH3/NO ratio, in the 0.7–3.5 range. Nitrogen and argon have been used as diluent gas. A literature reaction mechanism has been used to simulate the present experimental results and discuss the main findings. Reaction path analysis has allowed the identification of the reaction routes under the studied conditions. The simulations reflect the main experimental trends observed. Main results show that NO reduction by NH3 occurs at any conditions studied, but is more intense under oxygen excess conditions. Interactions of NH3 and NO proceeds in a molar basis with optimum conversions of NO of up to almost 100%.

Raman spectroscopy for real-time and in situ monitoring of mechanochemical milling reactions.

Solid-state milling has emerged as an alternative, sustainable approach for preparing virtually all classes of compounds and materials. In situ reaction monitoring is essential to understanding the kinetics and mechanisms of these reactions, but it has proved difficult to use standard analytical techniques to analyze the contents of the closed, rapidly moving reaction chamber (jar). Monitoring by Raman spectroscopy is an attractive choice, because it allows uninterrupted data collection from the outside of a translucent milling jar. It complements the already established in situ monitoring based on powder X-ray diffraction, which has limited accessibility to the wider research community, because it requires a synchrotron X-ray source. The Raman spectroscopy monitoring setup used in this protocol consists of an affordable, small portable spectrometer, a laser source and a Raman probe. Translucent reaction jars, most commonly made from a plastic material, enable interaction of the laser beam with the solid sample residing inside the closed reaction jar and collection of Raman-scattered photons while the ball mill is in operation. Acquired Raman spectra are analyzed using commercial or open-source software for data analysis (e.g., MATLAB, Octave, Python, R). Plotting the Raman spectra versus time enables qualitative analysis of reaction paths. This is demonstrated for an example reaction: the formation in the solid state of a cocrystal between nicotinamide and salicylic acid. A more rigorous data analysis can be achieved using multivariate analysis.

Combustion characteristics change induced by n-decane catalytic reactions and its effects on the coupled combustion occurrence

Flame in homogeneous-heterogeneous coupled combustion can be promoted or restrained, which results from that the upstream catalytic reaction changes the fuel characteristics and gas composition. The fuel gas after n-decane/air catalytic combustion (FGDC) were investigated by experiments and chemical kinetics analysis (τig and SL). Two criteria, t/τig and SL/v, were defined. When coupled combustion occurs, FGDC meets the following limitation: t/τig ≥ αth and SL/v ≥ βth. In view of composition change, catalytic pretreatment slightly increases τig, while greatly reduces SL. τig and SL are mainly affected by the decrease of O2 and C10H22 concentration. H2O significantly shorten τig only at high equivalence ratio. CO2 reduces SL and slightly shortens τig simultaneously. The improvement of ignition performance by adding H2 is mainly reflected in the improvement of SL. The oxidation, dehydrogenation and cracking reactions of large molecules/radicals (C10H22/C10H21) have a greater impact on τig, while SL is mainly affected by the small molecules (C2H3/CH3/HCO). Reaction path and sensitivity analysis found that τig is mainly affected by OH, while H/O/OH have the significant effect on SL. The sensitivity coefficients show that the chain termination reactions generating methane are the main elementary reactions that hinder ignition and combustion. SL presents a linear relationship with (CO + CH + COH)max: SL = 97.1*(CO + CH + COH)max + 0.32.

Study of the oxidation of ammonia in a flow reactor. Experiments and kinetic modeling simulation

The present work is focused on the analysis of the ammonia oxidation process and the formation of main nitrogen oxides (NO, NO2 and N2O) over a wide range of temperatures and O2 reaction environments. Experiments are performed at atmospheric pressure in a laboratory quartz tubular flow reactor, covering the temperature range of 875 to 1450 K and for different air excess ratios (from pyrolysis to very oxidizing conditions). The experimental results are simulated and interpreted in terms of a detailed chemical-kinetic mechanism. Reaction path and sensitivity analyses are used to delineate the NH3 oxidation scheme.

A comprehensive kinetic modeling study of ethylene combustion with data uncertainty analysis

A revision and upgrade of the ethylene (C2H4) oxidation kinetic sub-mechanism were carried as a next step in the optimization of the C<3 chemistry, which is a base for the upcoming PAH sub-model improvement. The main emphasis of the work was focused on the assessment of uncertainties of the thermo-kinetical and experimental data to involve that principally in the methodology of reaction model uncertainty. The principal targets of mechanism extension and update are: inspection of the reaction rate coefficients with accounting recently published pressure-dependent reactions and analysis of reaction paths related to the C2H4 low-temperature oxidation and the formation of aromatic precursors. The experimental data (auto-ignition, premixed laminar flame speeds, and concentration profiles) with evaluated uncertainty and consistency were used for model optimization. The uncertainty bounds of the key reaction rate coefficients were evaluated from the statistical treatment of the published data, which provided constraints in the reaction rate parameters. The rate parameters of 57 reactions of C2H4 and key intermediates were optimized. The revised reaction mechanism demonstrates a good agreement with the majority of the existing experimental data. Results of the sensitivity and rate of production analyses performed for several kinetic mechanisms from the literature were compared to visualize the variations and ambiguity in the importance of reaction paths and highlight the uncertainty problems in mechanism optimization and integration.

Combustion regimes of syngas flame in a micro flow reactor with controlled temperature profile: A numerical study

Flame characteristics of stoichiometric syngas/diluted-air mixture (CO:H2:O2:N2=1:1:1:7) in a micro-flow reactor with controlled wall temperature profile were numerically studied. In addition to single normal flame and single weak flame that respectively occur at high and low inlet velocities, two new combustion regimes, i.e., “coexistence of upstream normal flame and downstream weak flame (UNF-DWF)” and “uninterrupted flame with repetitive extinction and ignition (UFREI)”, were found at moderate inlet velocities. It is revealed that the coexisting downstream weak flame of UNF-DWF is attributed to incomplete consumption of CO by the upstream normal flame. Analysis of reaction path further demonstrates that the downstream weak flame of UNF-DWF is partially sustained by H2O through H2O+O=2OH due to H2O production in the upstream normal flame. To elucidate the dynamic process of UFREI, it is artificially divided into “upstream flame” and “downstream flame (actually an oscillating weak flame)”, and an entire period of UFREI is divided into four sub-regions. The temporal variations of CO mole fraction profile and heat release rate profile along the centerline of each sub-region are exhibited and analyzed. Briefly speaking, the formation of UFREIs is essentially a result of early re-ignition of residual CO from the upstream flame before the occurrence of its extinction. In summary, the present study sheds some light on the complicated combustion characteristics of syngas in the micro flow reactor with a prescribed wall temperature profile.

Reaction Path Analysis of CO2 Reduction to Methanol through Multisite Microkinetic Modelling over Cu/ZnO/Al2O3 Catalysts

The changing hierarchical structure of the applied heterogeneous Cu/ZnO/Al2O3 material during methanol synthesis reactions hinders an efficient engineered process condition optimization, causing sub-optimal functional performance. A robust literature comparison is conducted to determine that activity is tightly coupled with Cu–Zn interactions. In order to investigate this physical behaviour further, characteristic experimental data is acquired through the catalytic reactor tests with an activated commercial catalyst, aged at different input measurements, monitored and characterized by the Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), H2 transient adsorption (TA) and N2O pulsed surface oxidation (PSO) methodologies. It is shown that apparent rate law, exponents and activation energies do not vary significantly by increasing the ZnOX coverage from 7% to 23 %, while not all of ZnOX over-layer is catalytically active. For Cu/ZnO/Al2O3 with ZnOX over 7%, a highly-dispersed Al2O3 decreases the measured intrinsic kinetics of the Cu–Zn site, implying a steric hindrance effect. Finally, building on unveiled chemical relations, a thorough multisite system micro-kinetic model, based on systematic contribution analysis, mechanisms and quantitative density functional theory (DFT) constants is developed. Values were optimized using the sequential screening results for an industrially relevant application (the temperatures of 160–260 °C, 50 bar pressure, 12,000–200,000 h–1 gas hourly space velocity (GHSV) flow and relative feed compositions).Designed mathematical relationships can therefore be utilised to accurately predict the turnover, selectivity and stability/deactivation in correspondence to ZnOX over Cu.

Effect of High Pressures on the Formation of Nitric Oxide in Lean, Premixed Flames

Abstract Increasingly stringent regulations are imposed on nitrogen oxides emissions due to their numerous negative impacts on human health and the environment. Accurate, experimentally validated thermochemical models are required for the development of the next generation of combustors. This paper presents a series of experiments performed in lean, premixed, laminar, jet-wall stagnation flames at pressures of 2, 4, 8, and 16 atm. To target postflame temperatures relevant to gas turbine engines, the stoichiometry of the nonpreheated methane–air mixture is adjusted to an equivalence ratio of 0.7. One-dimensional (1D) profiles of temperature and NO mole fraction are measured via laser-induced fluorescence (LIF) thermometry and NO-LIF, respectively, to complement previously published flame speed data (Versailles et al., 2018, “Measurements of the Reactivity of Premixed, Stagnation, Methane-Air Flames at Gas Turbine Relevant Pressures,” ASME. J. Eng. Gas Turbines Power, 141(1), p. 011027). The results reveal that, as the pressure increases, the maximum postflame temperature stays relatively stable, and the concentration of NO produced through the flame front remains constant within uncertainty. Seven thermochemical models, selected for their widespread usage or recent date of publication, are validated against the experimental data. While all mechanisms accurately predict the postflame temperature, thanks to consistent thermodynamic parameters, important disagreements are observed in the NO concentration profiles, which highlights the need to carefully select the models used as design tools. The lack of pressure dependence of NO formation that many models fail to capture is numerically investigated via sensitivity and reaction path analyses applied to the solution of flame simulations. The termolecular reaction H+O2(+M)↔HO2(+M) is shown to hinder the production of atomic oxygen and to consume hydrogen radicals at higher pressures, which inhibits the formation of nitric oxide through the N2O pathway.

Autoignition behavior of methanol/diesel mixtures: Experiments and kinetic modeling

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650−1450 K, pressures of 6−20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (~1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (~940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates •OH, whereas •OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, •OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via •HO2 while CH3OH consumes a large percentage of •OH. Consequently, the competition between methanol and diesel for •OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes.

An experimental and kinetic modeling study on the laminar burning velocity of NH3+N2O+air flames

In the present work, the laminar burning velocities of the NH3+N2O+air flames were measured using the heat flux method at 1 atm and 298 K, with varied equivalence ratios and N2O mixing ratios. For the mixing ratio N2O/(N2O+air) = 0.5, a full range of equivalence ratios was covered. Moreover, at three equivalence ratios an extended range of mixing ratios was investigated. The laminar burning velocity has an approximately linear relationship against the fraction of nitrous oxide in the oxidizer mixture, regardless of the tested equivalence ratios. Several recently published NH3 mechanisms were compared with these new experimental data; among them the models of Nakamura et al. and Stagni et al. show the best performance for NH3+N2O+air flames over the entire range of the mixing ratios. The H/N/O kinetic mechanism of the authors was analyzed and updated focusing on the rate constants of reactions most sensitive in ammonia flame propagation and self-ignition of NH3+O2 and H2+N2O mixtures. The choice of the new rate constants is outlined, however, no modification (adjustment or tuning) of the rate parameters to accommodate experimental results was attempted. The updated mechanism demonstrates significantly improved agreement with all measurements used for the model development and with other experimental data from the literature for ammonia flames and self-ignition. A comparative reaction path analysis for NH3+N2O+air and NH3+air flames revealed that an almost linear increase of the laminar burning velocity with an increased fraction of N2O in the oxidizer originates from the rate controlling reaction N2O+H = N2+OH, which produces OH radicals dominating ammonia oxidation.

Ignition delay times of NH3 /DME blends at high pressure and low DME fraction: RCM experiments and simulations

Autoignition delay times of ammonia/dimethyl ether (NH3/DME) mixtures were measured in a rapid compression machine with DME fractions of 0, 2 and 5 and 100% in the fuel. The measurements were performed at equivalence ratios φ=0.5, 1.0 and 2.0 and pressures in the range 10–70 bar; depending on the fuel composition, the temperatures after compression varied from 610 K to 1180 K. Admixture of DME is seen to have a dramatic effect on the ignition delay time, effectively shifting the curves of ignition delay vs. temperature to lower temperatures, up to ~250 K compared to pure ammonia. Two-stage ignition is observed at φ=1.0 and 2.0 with 2% and 5% DME in the fuel, despite the pressure being higher than that at which pure DME shows two-stage ignition. At φ=0.5, a reproducible pre-ignition pressure rise is observed for both DME fractions, which is not observed in the pure fuel components. A novel NH3/DME mechanism was developed, including modifications in the NH3 subset and addition of the NH2+CH3OCH3 reaction, with rate coefficients calculated from ab initio theory. Simulations faithfully reproduce the observed pre-ignition pressure rise. While the mechanism also exhibits two-stage ignition for NH3/DME mixtures, both qualitative and quantitative improvement is recommended. The overall ignition delay times for ammonia/DME mixtures are predicted well, generally being within 50% of the experimental values, although reduced performance is observed for pure ammonia at φ=2.0. Simulating the ignition process, we observe that the DME is oxidized much more rapidly than ammonia. Analysis of the mechanism indicates that this ‘early DME oxidation’ generates reactive species that initiate the oxidation of ammonia, which in turn begins heat release that raises the temperature and accelerates the oxidation process towards ignition. The reaction path analysis shows that the low-temperature chain-branching reactions of DME are important in the early oxidation of the fuel, while the sensitivity analysis indicates that several reactions in the oxidation of DME, including cross reactions between DME and NH3 species presented here, are critical to ignition, even at fractions of 2% DME in the fuel.

Computational investigations of selected enzymes from two iron and α-ketoglutarate-dependent families.

DNA alkylation is used as the key epigenetic mark in eukaryotes, however, most alkylation in DNA can result in deleterious effects. Therefore, this process needs to be tightly regulated. The enzymes of the AlkB and Ten-Eleven Translocation (TET) families are members of the Fe and alpha-ketoglutarate-dependent superfamily of enzymes that are tasked with dealkylating DNA and RNA in cells. Members of these families span all species and are an integral part of transcriptional regulation. While both families catalyze oxidative dealkylation of various bases, each has specific preference for alkylated base type as well as distinct catalytic mechanisms. This perspective aims to provide an overview of computational work carried out to investigate several members of these enzyme families including AlkB, ALKB Homolog 2, ALKB Homolog 3 and Ten-Eleven Translocate 2. Insights into structural details, mutagenesis studies, reaction path analysis, electronic structure features in the active site, and substrate preferences are presented and discussed.

Single Cr atom supported on boron nitride nanotubes for the reaction of N2O reduction by CO: A density functional theory study

The removal of harmful N2O and CO in one step has attracted extensive research interest. Here, we studied the feasibility of N2O + CO reaction on single atom catalysts (SACs) supported on defective boron nitride nanotube (BNNT) by means of density functional theory (DFT) calculations. The Cr single atom catalyst which can avoid catalyst poisoning was screened from five low-price transition metal atoms (Ti, Cr, Mn, Fe, and Co) based on the adsorption strength of reactant and product on catalyst. The stepwise mechanism was considered which reveals the reaction path involves N2O decomposition, CO oxidation and CO2 desorption. The rate-limiting step is CO2 desorption with the desorption barrier of 0.42 eV. Along the reaction path, optimized structures and electronic property analyses indicate Cr atom acts as bridge to transfer electron due to its 3d orbital, which plays an important role in activation of N2O and CO molecules. Meanwhile, BNNT support with high redox stability acts as electron reservoir, withdrawing or donating electron, to facilitate the whole reaction. Therefore, Cr/BNNT is proposed to be a promising and highly efficient catalyst for eliminating environmentally unfriendly N2O and CO gases simultaneously.

Inhibition effect of methanol addition on carbon deposition in propane pyrolysis

Methanol-mixed fuels consisting of methanol, propane, and a small amount of auxiliary solvent are widely used as inexpensive industrial fuels, but the carbon deposition in the pyrolysis of this fuel seriously restrains its popularization. This paper studied the characteristics of gaseous products and carbon deposition in the pure propane and methanol-propane mixture pyrolysis experimentally, analyzed the effect of methanol addition on the reaction path of propane pyrolysis and production rate of key species by using detailed reaction mechanism. Results show that methanol addition can restrain C2H4 converting to C2H2 in the later stage of pyrolysis. The carbon deposition rate and particle diameter increase exponentially with the increase of reaction temperature in both pure propane and methanol-propane blend pyrolysis. Methanol addition can significantly reduce the carbon deposition rate and particle diameter. The reaction path analysis indicates that the main reaction paths forming benzene are C3H3 → A1 and C4H4 → I-C6H6 → A1, and C3H3 is a key specie to form benzene. The pyrolysis of methanol-propane mixture generates more H2 and H than that of pure propane pyrolysis, which makes most C3H6 convert to C2H4, reduces the amount of C3H3 and suppresses the conversion of C2H4 to C2H2. And then the amount of benzene is reduced significantly and the generation of subsequent PAHs and carbon deposition is inhibited. Lots of H2 also restrains the hydrogen abstraction from benzene resulting in the delay of the formation of PAHs and carbon deposition. This study is of great significance to research the soot formation in hydrocarbon combustion with alcohol fuel.

Hydrogen effects on ignition delay time of methyl butanoate in a rapid compression machine

SummaryThe effect of hydrogen addition on the ignition delay time of methyl butanoate was studied using a rapid compression machine experiment and ANSYS CHEMKIN‐PRO 19.0 numerical analysis. The ignition delay was measured under engine‐relevant conditions by varying equivalence ratios (0.5 and 1.0), compression pressures (15 and 30 bar), compression temperatures (860‐1060 K), and hydrogen mole fractions in the methyl butanoate/hydrogen mixture (0%, 25%, 50%, and 75%). The kinetic mechanism was modified by adding the methyl butanoate mechanism to the NUIG Aramco 3.0 mechanism and modifying the rate parameters of the H‐atom abstraction reactions of methyl butanoate. The numerical results obtained using the modified mechanism were in good agreement with the experimental results. The methyl butanoate/hydrogen mixture did not show a negative temperature coefficient characteristic. With hydrogen addition, the ignition delay increased at low temperatures but decreased at high temperatures. The results of the chemical and dilution effect analysis using the imaginary inert H2 showed that the chemical effects differ depending on the temperature. Most added hydrogen molecules participate in the chain propagation reactions to produce H radicals and consume OH by the reaction H2 + OH → H + H2O. This, in turn, changes the reactions of other species, such as HO2 and H2O2. The results of the reaction path analysis for the related species showed that the addition of hydrogen reduced radical reactions at low temperatures, while increasing them at high temperatures.

Mechanism and Reaction Energy Landscape for Apiose Cross-Linking by Boric Acid in Rhamnogalacturonan II

Rhamnogalacturonan II (RG-II)-the most complex polysaccharide known in nature-exists as a borate cross-linked dimer in the plant primary cell wall. Boric acid facilitates the formation of this cross-link on the apiosyl residues of RG-II's side chain A. Here, we detail the reaction mechanism for the cross-linking process with ab initio calculations coupled with transition state theory. We determine the formation of the first ester linkage to be the rate-limiting step of the mechanism. Our findings demonstrate that the regio- and stereospecific nature of subsequent steps in the reaction itinerary presents four distinct energetically plausible reaction pathways. This has significant implications for the overall structure of the cross-linked RG-II dimer assembly. Our transition state and reaction path analyses reveal key geometric insights that corroborate previous experimental hypotheses on borate ester formation reactions.

Experimental and kinetic modeling study of tetralin: A naphtheno-aromatic fuel for gasoline, jet and diesel surrogates

Distillate fuels contain significant proportions of naphtheno-aromatic components and tetralin is a suitable surrogate component to represent this molecular moiety. The presence of aromatic and naphthyl rings makes kinetic modeling of tetralin very challenging. Primary radicals formed during the oxidation of tetralin can be aryl, benzylic or paraffinic in nature. Using available information on reaction paths and rate constants of naphthenes and alkyl-aromatics, a kinetic model of tetralin has been developed in the current study with emphasis on low-temperature chemistry and high-pressure conditions. Due to the lack of high-level quantum chemical calculations on reaction pathways of tetralin, analogous rates from ab-initio studies on benzylic and paraffinic radicals have been adopted here. Some modifications to the reaction rate rules are incorporated to account for the unique characteristics of tetralin's molecular structure. Important reaction channels have been identified using reaction path and brute force sensitivity analyses. In order to investigate the model performance at low temperatures, new experiments are carried out in a rapid compression machine on blends of tetralin and 3-methylpentane. Blending of low-reactivity tetralin with a high-reactivity alkane allowed the investigation of tetralin ignition at very low temperatures (665 – 856 K). The kinetic model developed in the current study is found to predict the current experiments and literature data adequately. The new model will aid in high-fidelity surrogate predictions at engine-relevant conditions.

Detailed analysis of early-stage NOx formation in turbulent pulverized coal combustion with fuel-bound nitrogen

A carrier-phase direct numerical simulation (CP-DNS) of pulverized coal combustion in a mixing layer is performed, considering three NOx formation mechanisms (fuel-NOx, thermal-NOx and prompt-NOx). Detailed analyses, including reaction path analysis, chemical timescale analysis, and a priori and budget analyses are conducted to investigate the NOx production mechanisms and the performance of the flamelet model. Considering the high computational cost of CP-DNS, this work focuses on the early phase governed by devolatilization, where char reactions are less important. The reaction path analyses show that the principal thermal-NO reaction contributes to the net consumption of NO in fuel-bound nitrogen pulverized coal flames, which is essentially different from fuel-nitrogen-free flames. The chemical timescale analyses show that the production rates of NOx species are faster than those of major species, which confirms the suitability of the flamelet tables. The a priori analyses show that the gas temperature and major/intermediate species can be predicted well by the flamelet model, while the NOx species show significant discrepancies in certain regions. Finally, the budget analyses explain why the flamelet model performs differently for major/intermediate and NOx species.

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH3 in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH3 dissociation was varied from 0 to 25% (0 to 20%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH3 in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH3 dissociation. At 25% NH3 dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH3 in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH3. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and thus the conversion of NH to NO and then NO to N2 near LFL and the conversion of NH2 and NO to N2 near UFL leading to wider flammability limits.

Laminar burning velocities of 2-methyltetrahydrofuran at elevated pressures

The laminar burning velocities (LBVs) and cellular instability of 2-methyltetrahydrofuran (2-MTHF) were investigated at the unburned temperature of 423 K and pressures from 1 to 10 atm in a cylindrical constant-volume vessel. The LBVs of 2-MTHF/air flame exhibit a notably dropping with increasing pressure. The cellular instability analysis indicates that the critical flame radius of 2-MTHF/air mixture monotonically increases with increasing pressure and the flame surface suffers more badly cellularity under higher pressures. The critical flame radius exhibits non-monotonic variation versus ϕ and the most unstable flames appear at ϕ ≈ 1.3. It is observed that the measured Markstein length of 2-MTHF/air mixture decreases with increasing ϕ and Pu, leading to an earlier formation of wrinkling and cracks with respect to preferential-diffusional instability. Further investigation found that by using a mixture of 14.2% oxygen with 85.8% helium in place of air as bath gas at 10 atm can effectively suppress the cellular instability. Two recently developed models were used to simulate the experimental results and explore the chemical kinetic effects on LBV. Reaction path analysis reveals that the most consumption of 2-MTHF/air at stoichiometric conditions is through the abstraction of H-atom to form radical C5H9O-5. While the competitiveness of decomposition by CC scission yielding CH3 and tetrahydrofuran radical is relatively weak. Sensitivity analysis illustrates that small-species reactions show a controlling effect on LBV. The increasing pressure leads to an evident increase in the sensitivity coefficient of the recombination reaction H + O2 (+M)=HO2 (+M). The reduction of H atom concentration will cause competition to the initiation reaction H + O2 = O+OH. This could lower the overall oxidation rate and reduce the burning velocity.