Reaction Path Analysis Research Articles

Flame kinetics at scramjet-engine-relevant conditions: Role of prompt dissociation of weakly-bound radicals

Combustion in high-speed ram-based propulsion engines occurs under distinct thermodynamic conditions of high reactant temperatures (greater than 1000 K) and relatively low pressures (<5 atm). There is a lack of fundamental flame measurements at such conditions that result in adiabatic flame temperatures (Tad) exceeding 2500 K. In this work, we have measured laminar flame speeds of oxygen-enriched CH4/oxidizer mixtures at sub-atmospheric conditions to probe kinetics at high Tad using the isobaric spherically expanding flame approach. Simulations with recent kinetic models revealed increasing differences between data and model predictions with increasing Tad, reaching up to 25 %. Kinetic analyses reveal that at the thermodynamic conditions in these O2-enriched flames, i.e., lower pressures and higher Tad, the effects of HCO prompt dissociation are accentuated. In addition to HCO, the prompt dissociations of CH2OH and C2H5 are also considered. The prompt dissociations of all three radicals were evaluated and their effects considered in flame speed simulations. Reaction path analysis for the present flames revealed that approximately half of the reaction flux for HCO formation undergoes prompt dissociation to H + CO. Furthermore, these analyses also revealed that the pathways and sensitive reactions are similar between oxygen-enriched fuel/oxidizer mixtures and preheated fuel/air mixtures, if both have similar Tad. Thus, flames of oxygen-enriched mixtures could be a surrogate to probe the flame chemistry of highly preheated mixtures at relatively low pressures that are often encountered in ram-based propulsion engine combustors.

Assessment on the rings cleavage mechanism of polycyclic aromatic hydrocarbons in supercritical water: A ReaxFF molecular dynamics study

The ring opening reaction of polycyclic aromatic hydrocarbons (PAHs) is one of the rate-determining steps of coal gasification within supercritical water, while the involvement of water remains debated between addition and hydrogen abstraction theories. In this study, reactive molecular dynamics (MD) simulations were performed to explore the ring opening reaction of naphthalene, the simplest PAH, at temperatures of 2500–2700 K. Species and elementary reactions were quantitatively extracted from bond order trajectories using an in-house program. Reaction path analysis shows that ring opening paths of naphthalene mainly include thermolysis, H˙ atom abstraction, and O˙H radical addition. Flux analysis indicates O˙H radical largely from the H˙ + ▪ = O˙H + ▪ reaction, while the water dissociation ▪ = O˙H + H˙ reaction is near equilibrium, seldom contributing to the O˙H source. This atomistic kinetics analysis is intended to help the modeling of supercritical water gasification of coal.

Optimization of the full-load combustion schemes of n-butanol/biodiesel reactivity-controlled compression ignition (RCCI) toward high-efficiency and low-emission engines

Biodiesel and n-butanol, as popular alternative fuels, can be used in advanced reactivity-controlled compression ignition (RCCI) engines for efficient and clean combustion. However, the complex interactions between n-butanol and biodiesel, as well as the best combustion schemes at different loads, remain poorly understood. Given this, the objective of this paper is to identify optimal fuel organizations for n-butanol/biodiesel RCCI engines across different load scenarios by combining engine simulation with a global optimization algorithm, explore the potential synergies of control parameters, and analyze microscopic dual-fuel interactions based on the integrated average reaction path analysis. Results show that the optimal scheme at low load employs almost homogeneous fuel-air mixtures at elevated temperature atmosphere above 390 K, with n-butanol energy share exceeding 96% and optimal biodiesel injection timing between −70 and −50 °CA. Higher n-butanol ratio coupled with increased intake temperature improves engine performance. For mid load, biodiesel injection timing is retarded to −55 to −30 °CA, and biodiesel proportion is increased to introduce reactivity stratification, while maintaining its energy share below 20% to mitigate NOx emissions. Notably, biodiesel density is identified as the primary physical property influencing NOx formation. At high load, a split combustion scheme involving premixing and subsequent diffusion combustion is optimal. The physical effects of n-butanol addition minimally impact the low-temperature ignition of biodiesel, while its chemical effects significantly defer the low-temperature reactivity. The reaction path analysis reveals that n-butanol competes with biodiesel for OH radical consumption, with a large proportion of OH and HO2 consumed in the cyclic reactions of nC4H9OH and nC4H8OH. Adjusting the reactivity stratification affects the reaction state of n-butanol entrained by the biodiesel jets. Higher reactivity gradient intensifies biodiesel reactions to induce the pyrolysis of the involved n-butanol through nC4H8OH =&gt; 2C2H4 + OH, therefore resulting in more staged heat release.

Kinetic Insights into Methanol Synthesis from CO2 Hydrogenation at Atmospheric Pressure over Intermetallic Pd2Ga Catalyst.

This study presents a single-site microkinetic model for methanol synthesis by CO2 hydrogenation over intermetallic Pd2Ga/SiO2. A reaction path analysis (RPA) combining theoretical results and realistic catalyst surface reaction data is established to elucidate the reaction mechanism and kinetic models of CO2 hydrogenation to methanol and CO. The RPA leads to the derivation of rate expressions for both reactions without presumptions about the most abundant reactive intermediate (MARI) and rate-determining step (rds). The formation of H2COOH* is found to be the rds (step 19) for methanol synthesis via the formate pathway, with CO2 and H-atoms adsorbed on intermetallic sites as the MARIs. The derived kinetic model is corroborated with experimental data acquired under different reaction conditions,using a lab-scale fixed-bed reactor and Pd2Ga/SiO2 nanoparticles prepared by incipient wetness impregnation. The excellent agreement between the experimental data and the kinetic model (R 2 = 0.99) substantiates the proposed mechanism with an activation energy of 61.52 kJ mol-1 for methanol synthesis. The reported catalyst exhibits high selectivity to methanol (96%) at 1bar, 150 °C, and H2/CO2 ratio of 3:1. These findings provide critical insights to optimize catalysts and processes targeting CO2 hydrogenation at atmospheric pressure and low temperatures for on-demand energy production.

Ammonia pyrolysis oxidation excited by nanosecond pulsed discharge: Global/fluid models hybrid solution

The kinetic characteristics of plasma-assisted oxidative pyrolysis of ammonia are studied by using the global/fluid models hybrid solution method. Firstly, the stable products of plasma-assisted oxidative pyrolysis of ammonia are measured. The results show that the consumption of NH3/O2 and the production of N2/H2 change linearly with the increase of voltage, which indicates the decoupling of non-equilibrium molecular excitation and oxidative pyrolysis of ammonia at low temperatures. Secondly, the detailed reaction kinetics mechanism of ammonia oxidative pyrolysis stimulated by a nanosecond pulse voltage at low pressure and room temperature is established. Based on the reaction path analysis, the simplified mechanism is obtained. The detailed and simplified mechanism simulation results are compared with experimental data to verify the accuracy of the simplified mechanism. Finally, based on the simplified mechanism, the fluid model of ammonia oxidative pyrolysis stimulated by the nanosecond pulse plasma is established to study the pre-sheath/sheath behavior and the resultant consumption and formation of key species. The results show that the generation, development, and propagation of the pre-sheath have a great influence on the formation and consumption of species. The consumption of NH3 by the cathode pre-sheath is greater than that by the anode pre-sheath, but the opposite is true for OH and O(1S). However, within the sheath, almost all reactions do not occur. Further, by changing the parameters of nanosecond pulse power supply voltage, it is found that the electron number density, electron current density, and applied peak voltages are not the direct reasons for the structural changes of the sheath and pre-sheath. Furthermore, the discharge interval has little effect on the sheath structure and gas mixture breakdown. The research results of this paper not only help to understand the kinetic promotion of non-equilibrium excitation in the process of oxidative pyrolysis but also help to explore the influence of transport and chemical reaction kinetics on the oxidative pyrolysis of ammonia.

Structure–activity relationship of a dual-function amine-based electrolyte for integrated CO2 capture and electrochemical conversion

Integrated carbon dioxide (CO2) capture and electrocatalytic reduction processes through dual-function amine-based electrolytes and their integration into a single reactor can lead to significant energy savings, simplified process flow, and reduced transportation risks. This work investigates the structure–activity relationship of dual-functional amine-based electrolytes for CO2 capture and electrocatalytic reduction and develops an efficient dual-functional amine-based electrolyte system. We investigated 10 different amine-based solutions for their CO2 capture and reduction performances. The substituents on the nitrogen atom of the intermediate carbamate and the positions of the hydroxyl and branched groups affect the capture and electrochemical reduction. The 1,3-propanediamine system demonstrated enhanced CO2 solubility uptake (58.1 %), Faradaic efficiency (10.6 %), and current density (52.1 %) compared with the monoethanolamine system, indicating its potential as a dual-function amine-based electrolyte. Furthermore, density functional theory calculations indicated that the charge distribution of carbamate and the dissociation of C–N bonds directly influence the adsorption of carbamates on the electrode surface and the Faradaic efficiency of the reduced products. In addition, reaction path analysis based on the zwitterionic mechanism indicated that intermediates (carbamate R1R2NCOO-) with higher electronegativity are more conducive to adsorb onto the electrode surface and inhibit hydrogen generation from protonated amines (R1R2NHH+). The results suggest that the selection or design of dual-functional amine-based electrolytes should prioritize amines with straight alkanol chains rather than branched ones, which do not form intramolecular hydrogen bonds or heterocyclic structures. This work provides directions and guidance for designing and developing dual-functional electrolytes for integrated carbon capture and electrocatalytic conversion technologies.

High-Temperature Pyrolysis Study of 2-Methyl-2-butanol behind Reflected Shock Wave: Shock Tube and Computational Study.

Pyrolysis of a branched alcohol, 2-methyl-2-butanol (2M2BOH), was carried out behind the reflected shock wave in the temperature range of 1011-1303 K and under pressures varying from 9.3 to 14.6 atm. Qualitative and quantitative analysis of the postshock mixture was performed using gas chromatography-mass spectrometry and gas chromatography with a flame ionization detector, respectively. The rate coefficients for the C-C and C-O bond cleavage reaction pathways were calculated using the variational transition-state theory. The Rice-Ramsperger-Kessel-Marcus/Master equation was employed to calculate the rate coefficients for the H2O-elimination reactions, unimolecular dissociation, and isomerization reaction pathways. The overall decomposition rate coefficient for the 2-methyl 2-butanol (2M2BOH) was estimated to be , where activation energy is given in kcal mol-1. The reaction path analysis was performed, which gives information regarding the contribution of individual intermediate species toward the decomposition of 2M2BOH. A set of reactions was proposed and used to simulate the combustion chemistry of 2M2BOH, which consists of 48 reactions and 39 species. The experimentally measured and simulated mole fractions for the reactant and products showed reasonably good agreement. This work additionally investigates the effect of branching on the decomposition kinetics of long-chain alcohols.

A shock-tube experimental and kinetic simulation study on the autoignition of methane at ultra-lean and lean conditions

Coalbed methane represents an important kind of natural gas resource in many countries. However, the low-concentration property of coalbed methane limits its applications. To gain insight into the combustion kinetics of coalbed methane and facilitate its combustion utilization, this work reports an experimental and kinetic simulation study on the autoignition properties of methane at ultra-lean and lean conditions. A shock-tube (ST) facility is used for ignition delay time (IDT) measurements with equivalence ratios at 0.5, 0.1, and 0.05 with pressure at 2 and 10 bar under the temperature ranging from 1320 to 1850 K. The measured IDTs can be correlated into a general Arrhenius expression, and the equivalence ratio effect on IDTs is then analyzed. Seven detailed chemical kinetic mechanisms are employed to predict the IDTs and statistical error indicators are used to evaluate their performance. Detailed kinetic analysis via sensitivity and reaction path analysis is performed to uncover the kinetic differences among the seven mechanisms. It is shown that some of the reaction paths only exist in the NUIGMech1.3 mechanism, while the other detailed mechanisms do not consider them. Reaction path analysis indicates that the reactions related to O2, OH and O species become more important compared to the reactions involving CH3 and H radicals as the equivalence ratio decreases from lean to ultra-lean conditions. Detailed chemical kinetics analysis is also conducted to demonstrate the uncertainty of key reactions. The present work should be valuable to gain insight into the methane ignition characteristics and to facilitate kinetic mechanism optimization of methane combustion.

Nonthermal plasma removal of CH4-NOx under LNG engine exhaust environment: experiment, mechanism and kinetic analysis

To explore the mechanism underlying the removal of CH4 and NOx, which are typical emissions from LNG engines, through the nonthermal plasma method, a plasma chemistry model of CH4-NOx under the LNG engine exhaust environment was established in this study. Sixty key reactions affecting the conversion of CH4, CO, and NOx were determined through sensitivity analysis, and their pre-exponential factors were optimized using a genetic algorithm. The proposed model shows good performance in predicting the concentrations of CH4, CO, and NOx under the exhaust conditions of LNG engine. Then, reaction path analyses for evolution of CH4, CO, NO, and NO2 were performed under specific conditions. The results showed that CH4 mainly decomposed into CH2 that is subsequently converted into CO and CO2. CH2 is the main source for CO production. The concentration of NOx is determined by the oxidation reactions of N with O2, OH, and HO2. The specific concentration distributions of NO and NO2 were influenced by the oxidation–reduction reactions between them. Analyses of the time scales for the conversion processes of CH4, CO, and NOx were also conducted, and it was found that the reaction time scales of CH4 and CO were approximately 1 × 10−10−1 × 10−5 s, and that was approximately 1 × 10−9−1 × 10−4 s for NOx. Furthermore, during the overlapping period for the conversion of CH4, CO, and NOx, the NOx was dominant in the competition for O. This study provides a basis for the construction of a plasma catalytic chemistry model of CH4-NOx under LNG engine exhaust conditions.

Thermodiffusively unstable laminar hydrogen flame in a sufficiently large 3D computational domain – Part II: NOx formation mechanism and flamelet modeling

In this work, the NOx formation mechanism in three-dimensional (3D) thermodiffusively unstable premixed hydrogen flames is investigated through direct numerical simulations (DNS) and a reaction path analysis based on the DNS dataset. In part I of this study (Wen et al., 2024), the characteristic patterns of the instabilities were studied using the same dataset. Here, first, the effects of the computational setup (2D vs. 3D) and curvature on the flux ratio of nitrogen atom are quantified. In addition, the composition space model (CSM) proposed in previous works is extended to account for the NOx chemical reaction mechanism to predict NOx formation in thermodiffusively unstable premixed hydrogen flames. The flamelet solutions are obtained from the premixed flamelet equations in composition space so that the wide range of curvatures associated with the strongly corrugated flame front can be considered. The performance of the CSM in predicting the NOx species and the important radicals involved in the NOx formation pathways is evaluated through an a priori analysis. To investigate the effects of curvature on the NOx formation pathways and the performance of the composition space model, the positively- and negatively-curved regions in the DNS are studied separately. The results show that different from the 2D simulation, the NNH reaction pathway becomes dominant in the 3D simulation due to the increased range of curvature, which promotes the accumulation of the highly diffusive H radical in the positively-curved regions. The NNH reaction pathway is dominant in the positively-curved regions, while the N2O reaction pathway is more important in the negatively-curved regions. The flamelet model based on the composition space solutions that consider the effects of curvature yields accurate predictions for the radicals that are sensitive to the effects of curvature.Novelty and significance statementThe novelty of the present work includes the following aspects,(i) Analysis of NOx formation mechanism based on the large-scale 3D DNS dataset of a laminar fuel-lean premixed hydrogen flame on a sufficiently large domain;(ii) Quantification of the effects of computational setup (2D and 3D) and curvature on the NOx formation pathways;(iii) Extension of the previous composition space model (CSM) by incorporating the NOx chemical reaction mechanism to consider the effects of curvature on the NOx formation.

Experimental and kinetic modeling study on auto-ignition of ammonia/n-heptane mixtures at intermediate temperatures

Ignition delay times (IDTs) were measured in a shock tube facility for NH3/n-heptane mixtures with NH3 concentrations in the blending fuel ranging from 0.3 to 0.95 by molar fraction. The measurements were conducted under low pressure of 2 atm and intermediate temperatures of 1350–1500 K at equivalence ratios of 0.5, 1, and 2. With the increase of n-heptane content or equivalence ratio, there is a decrease in the IDTs of NH3/n-heptane mixtures at intermediate temperatures. A detailed mechanism was updated in this study based on the mechanism of Dong et al. Subsequently, the proposed mechanism was compared to existing blending mechanisms of ammonia and n-heptane in terms of laminar burning velocities (LBVs), IDTs, and species profiles reported in literature. The present model improved the predictions in reproducing the performed experimental measurements compared to previous mechanisms. Finally, rate of production (ROP), sensitivity analysis, and instantaneous and cumulative reaction path analysis were performed to interpret the experiment observations and deepen the understanding of auto-ignition kinetics of ammonia and n-heptane. The results indicate that intermediate species, such as C2H4 and C3H6, characterized by long lifespans and high concentrations during n-heptane decomposition, play a crucial role at elevated temperatures, while the significance of n-heptane dehydrogenation by NH2 diminishes with increasing temperature.

On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.

Air humidity influence on combustion of R-1234yf (CF3CFCH2), R-1234ze(E) (trans-CF3CHCHF) and R-134a (CH2FCF3) refrigerants

The influence of air humidity on flame propagation in mixtures of hydrofluorocarbons (HFCs) with air was studied through numerical simulations and comparison with measurements from the literature. Water vapor added to the air in mixtures of fluorine rich hydrofluorocarbons (F/H ≥ 1) can be considered as a fuel additive that increases the production of radicals (H, O, OH) and increases the overall reaction rate. The hydrofluorocarbon flame is typically a two-stage reaction proceeding with a relatively fast reaction in the first stage transitioning to a very slow reaction in the second stage which leads to the combustion equilibrium products. The transition to the second stage is determined by the consumption of hydrogen-containing species and formation of HF. Despite a relatively small effect of water on the adiabatic combustion temperature, its influence is significant on the reaction rate and on the temperature increase in the first stage of the combustion leading to the increase in burning velocity. The main reaction for converting H2O to hydrogen-containing radicals and promoting combustion is H2O + F = HF + OH, as demonstrated by reaction path analyses for the fluorine rich hydrofluorocarbons R-1234yf, R-1234ze(E), and R-134a (F/H = 2). The calculated burning velocity dependence on the equivalence ratio ϕ agrees reasonably well with available experimental measurements for R1234yf and R-1234ze(E) with and without the addition of water vapor. In agreement with experimental data, with water vapor, the maximum of burning velocity over ϕ is shifted to the lean mixtures (near ϕ = 0.8).

A novel method for quantifying variations in NO formation pathways using a sub-mechanism accounting and reaction tracing algorithm

This work presents a new implementation of reaction path analysis that traces molar fluxes attributable to various sub-mechanisms from inception through to their final product. The post-processing procedure involves creating a network of these fluxes and quantifying the contributions made by the specified formation and destruction mechanisms through an iterative process starting from the individual initiation reactions. A key distinction of this approach is that the reaction fluxes are directly correlated to their initiation reaction, which is then followed throughout the entire reaction network. This allows for the distinction of contribution to the formation of a target species from an arbitrary number of sources. The effectiveness of this approach is demonstrated by examining the formation of NO in three different case studies: (1) the impact of equivalence ratio on methane/air premixed counterflow flames, (2) the effect of hydrogen addition on ultra-lean methane/air premixed counter flames, and (3) the influence of pressure elevation in partially premixed methane/air flames. The results are presented in emission indexes, NO production rates throughout the flame, and multidigraphs illustrating the allocated flows between the species. Compared to replicated analysis methods previously used for these case studies, this new approach provides a more comprehensive and detailed analysis for several reasons. Firstly, the method does not require duplicated simulations with modified kinetic mechanisms, but is applied purely post-processing. This avoids the previous problems of needing to re-simulate the flame multiple times and allows for the fully coupled chemistry set. Secondly, the method allows for an arbitrarily defined set of initiation pathways which propagate through all possible routes within the network. This allows for the identification of spatial/temporal variability in the sub-mechanisms and thereby yields more information than the integral values of previous methods.Novelty and significance statementUnderstanding reaction pathways that lead to the formation of pollutants such as NO is an essential aspect in the development of new sustainable and low-emission combustion concepts. However, evaluating the different underlying sub-mechanisms is difficult due to the high degree of interconnectedness and spatial/temporal overlap of the reaction pathways. This work, therefore, presents a new algorithm that can be used to investigate the contributions of an arbitrary number of sub-mechanisms to the formation or decomposition of a species. The results can be expressed not only as integral values, but also as spatially and/or temporally resolved production rates and reaction path figures and graphs, leading to a more comprehensive and detailed analysis. The approach is also applied as a purely post-processing technique, which does not require multiple simulations with different chemical kinetic mechanisms as has previously be applied.

Experimental and kinetic modeling of soot formation in counterflow non-premixed flames of surrogate fuel components: n-dodecane and iso-dodecane

Normal dodecane (n-C12) and iso-dodecane (i-C12) are often used as components for surrogate mixtures of aviation and diesel fuels. Although studies have been performed to understand the combustion and spray behavior of dodecane isomers, the soot formation from the combustion of n- and i-C12 in non-premixed flames is not well studied. In this work, soot volume fraction (SVF) profiles of n- and i-C12 were measured across a wide range of strain rates and mixture conditions in a counterflow burner facility at the University of Connecticut. Neat and binary mixtures of n- and i-C12 were investigated to study the influence of alkane branching on soot formation. A soot model was developed and validated in this study by extending the detailed kinetic model developed at the Lawrence Livermore National Laboratory (LLNL) for the formation of polycyclic aromatic hydrocarbons (PAHs) to simulate soot formation and growth based on the discrete sectional method. Additional gas phase reactions forming pyrene and ring enlargement reactions were added to the existing PAH model. The LLNL kinetic model was validated with SVF data obtained in this work for n-C12 and i-C12 flames along with data available in literature for ethylene, iso-butene, n-heptane, and iso-octane. The soot precursor reactions added in this work were found to play a critical role in simulating the experimentally observed non-linear trend of the peak SVF with increased branched alkanes. Reaction path analysis was conducted to illustrate the fuel structure effects on soot formation pathways in n- and i-C12 mixtures. In view of the satisfactory agreement between modeling results and experimental data, as well as capturing the non-linear variation in peak SVF with the alkane branching for the first time, further investigation within the framework of the soot model is discussed and critical insights are provided into the reaction pathways which require further attention.

Structure and nitric oxide formation in laminar diffusion flames of ammonia–hydrogen and air

In this work, we present an investigation of the structure and formation of nitrogen oxides (NOx) in an axisymmetric laminar diffusion flame in which an ammonia–hydrogen fuel stream is surrounded by co-flowing air. Green ammonia, produced using renewable energy, is a promising carbon-free fuel for replacing hydrocarbon fuels over a diverse range of combustion applications. A key challenge for ammonia combustion is to understand the kinetics of nitrogen oxide formation for designing low-NOx combustion systems. In this experimental study, ammonia fuel is blended with hydrogen to increase fuel reactivity, with a fuel composition range of 15%–100% hydrogen. NOx emissions and fuel slip are measured by downstream sampling of NO, NO2, N2O and NH3 with a Fourier Transform Infrared (FTIR) gas analyzer. Flame structure is visualized by planar laser-induced fluorescence (LIF) for NO, NH, and OH radicals, as well as OH* by filtered chemiluminescence. Species concentration profiles are compared to predictions from 2-dimensional axisymmetric numerical models of the laminar flame using recent kinetic mechanisms developed for ammonia combustion. No measurable ammonia slip was recorded for any fuel composition, in agreement with model predictions. Good agreement between predictions and measurements was obtained for exhaust nitrogen oxide concentrations and for in-flame radical profiles, although larger variation and deviations are observed for predictions of NO2 and N2O than for NO. Numerical predictions for spatial distribution of flame radicals and flame liftoff height also showed good agreement with LIF and chemiluminescence measurements. For high fuel hydrogen content, the measured NH and NO profiles are shifted inward toward the fuel outlet, away from the peak temperature contour. Reaction path analysis is used to illustrate the important contributions to NO creation and destruction, including the differing role of key reaction pathways with varying fuel hydrogen content.

Effect of Equivalence Ratio on Pollutant Formation in CH4O/H2/NH3 Blend Combustion.

This paper investigates the effect of equivalence ratio on pollutant formation characteristics of CH4O/H2/NH3 ternary fuel combustion and analyzes the pollutant formation mechanisms of CO, CO2, and NOX at the molecular level. It was found that lowering the equivalence ratio accelerates the decomposition of CH4O, H2, and NH3 in general. The fastest rate of consumption of each fuel was found at φ = 0.33, while the rates of CH4O and NH3 decomposition were similar for the φ = 0.66 and φ = 0.4. CO shows an inverted U-shaped trend with time, and peaks at φ = 0.5. The rate and amount of CO2 formation are inversely proportional to the equivalence ratio. The effect of equivalence ratio on CO2 is obvious when φ > 0.5. NO2 is the main component of NOX. When φ < 0.66, NOX shows a continuous increasing trend, while when φ ≥ 0.66, NOX shows an increasing and then stabilizing trend. Reaction path analysis showed that intermediates such as CH3 and CH4 were added to the CH4O to CH2O conversion stage as the equivalence ratio decreased with φ ≥ 0.5. New pathways, CH4O→CH3→CH2O and CH4O→CH3→CH4→CH2O, were added. At φ ≤ 0.5, new intermediates CHO2 and CH2O2 were added to the CH2O to CO2 conversion stage, and new pathways are added: CH2O→CO→CHO2→CO2, CH2O→CO→CO2, CH2O→CHO→CO→CHO2→CO2, and CH2O→CH2O2→CO2. The reduction in the number of radical reactions required for the conversion of NH3 to NO from five to two directly contributes to the large amount of NOX formation. Equivalent ratios from 1 to 0.33 corresponded to 12%, 21.4%, 34%, 46.95%, and 48.86% of NO2 remaining, respectively. This is due to the fact that as the equivalence ratio decreases, more O2 collides to form OH and some of the O2 is directly involved in the reaction forming NO2.

Investigating auto-ignition characteristics and kinetic modeling of NH3/CH4 mixtures using an RCM

Ammonia (NH3) has shown promise as a carbon-neutral fuel, and blending NH3 with methane (CH4) can improve its combustion characteristics. This study investigated the auto-ignition characteristics and kinetic modeling of NH3/CH4 mixtures using a rapid compression machine (RCM). Ignition delay times were measured under various conditions, including three different NH3/CH4 ratios, pressures of 15 and 25 bar, temperatures ranging from 910 to 1272 K, and equivalence ratios of 0.5 and 1.0. The results showed the addition of CH4 significantly promoted NH3 ignition. An intriguing phenomenon of pre-ignition pressure rise was noted in blends containing 25 % CH4, which cannot be replicated by previous models. Time-resolved species measurements were conducted using a fast sampling system coupled with gas chromatography. The measurements revealed distinct early consumption of CH4 compared to NH3 in blends exhibiting pre-ignition pressure rise. A kinetic model was developed for NH3/CH4 combustion with emphasis on key “CN” reaction pathways constrained by experimental results. Sensitivity and reaction path analyses highlighted the significance of these “CN” reactions in determining the varying ignition characteristics. Additional simulations systematically assessed factors influencing auto-ignition behavior in NH3/CH4 mixtures. Notably, CH4 concentration emerged as the primary parameter affecting the ignition behavior. Lower CH4 concentrations led to NH3 chemistry primarily controlling the overall ignition process, which could cause early CH4 oxidation and potential gradual pre-ignition pressure rise. Conversely, higher CH4 content facilitated its oxidation to trigger the oxidation of NH3, thereby narrowing the time gap between the initiation of consumption for the two fuels.

Automated Generation of a Compact Chemical Kinetic Model for n-Pentane Combustion.

We have employed automated mechanism generation tools to construct a detailed chemical kinetic model for combustion of n-pentane, as a step toward the generation of compact kinetic models for larger alkanes. Pentane is of particular interest as a prototype for combustion of alkanes and as the smallest paraffin employed as a hybrid rocket fuel, albeit at cryogenic conditions. A reaction mechanism for pentane combustion thus provides a foundation for modeling combustion of extra-large alkanes (paraffins) that are of more practical interest as hybrid rocket fuels, for which manual construction becomes infeasible. Here, an n-pentane combustion kinetic model is developed using the open-source software package Reaction Mechanism Generator (RMG). The model was generated and tested across a range of temperatures (650 to 1350 K) and equivalence ratios (0.5, 1.0, 2.0) at pressures of 1 and 10 atm. Available thermodynamic and kinetic databases were incorporated wherever possible. Predictions using the mechanism were validated against the published laminar burning velocities (Su) and ignition delay times (IDT) of n-pentane. To improve the model performance, a comprehensive analysis, including reaction path and sensitivity analyses of n-pentane oxidation, was performed, leading us to modify the thermochemistry and rate parameters for a few key species and reactions. These were combined as a separate data set, an RMG library, that was then used during mechanism generation. The resulting model predicted IDTs as accurately as the best manually constructed mechanisms, while remaining much more compact. It predicted flame speeds to within 10% of published experimental results. The degree of success of automated mechanism generation for this case suggests that it can be extended to construct reliable and compact models for combustion of larger n-alkanes, particularly when using this mechanism as a seed submodel.

Experimental and mechanism study on NO formation characteristics and N chemical reaction mechanism in ammonia-coal co-firing

Carbon-free ammonia can significantly reduce CO2 emissions from coal-fired power generation. However, the high nitrogen content of ammonia can lead to high NOx emissions when it is co-fired with coal. Therefore, it is necessary to explore the formation mechanism of NOx in ammonia-coal co-firing to reduce the emission of nitrogen oxides. In this study, the high-temperature furnace experimental platform and CHEMKIN software were combined to investigate the NO formation characteristics and the pathway analyses of the ammonia-N/coal-N reactions during ammonia-coal co-firing under a broad temperature range and variable ammonia blending ratios. The experimental results showed that when the temperature was below 1200 ℃, the amount of NO formed increased with the increase in the ammonia blending ratio. At and above 1300 ℃, the amount of NO formed first increased and then decreased with the increase in the ammonia blending ratio. The blending of ammonia reduced the fuel-N to NO conversion rate. The CHEMKIN simulation results showed that when the temperature was below 1200 ℃, the growth rate of ROP for each elementary reaction of NO formation was higher than that for each elementary reaction of NO reduction. Above 1300 ℃, the growth rate of ROP for each elementary reaction of NO reduction increased significantly, exhibiting a decrease in the NO concentration during ammonia-coal co-firing. The simulation results were in good agreement with the experimental results. Through ROP analysis, sensitivity analysis, and reaction path analysis, it was concluded that HNO was the main intermediate product of NO formation, NH and NH2 free radicals were the main groups of NO reduction, and OH free radicals had a significant influence on NO formation.