Direct numerical simulation of lean premixed CH4/air and H2/air flames at high Karlovitz numbers

Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH₄/air and H₂/air flames at high Karlovitz numbers (Ka ∼ 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (∼500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O₂+M = HO₂ + M in lean H₂/air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH₄/air and H₂/air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H₂/air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection–diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important.     

Mechanisms of NO formation in MILD combustion of CH4/H2 fuel blends

The mechanisms of formation and destruction of NO in MILD combustion of CH₄/H₂ fuels blends are investigated both experimentally and numerically. Experiments are carried out at a lab-scale furnace with the mass fraction of hydrogen in fuel ranging from 0% to 15%; furnace temperature, extracted heat and exhaust NO_x emissions are measured. Detailed chemical kinetics calculations utilizing computational fluid dynamics (CFD) and well-stirred reactor (WSR) are performed to better analyze and isolate the different mechanisms.     

A detailed numerical study of NOx kinetics in low calorific value H2/CO syngas flames

The present study provides an extensive and detailed numerical analysis of NO_x chemical kinetics in low calorific value H₂/CO syngas flames utilizing predictions by five chemical kinetic mechanisms available out of which four deal with H₂/CO while the fifth mechanism (GRI 3.0) additionally accounts for hydrocarbon chemistry. Comparison of predicted axial NO profiles in premixed flat flames with measurements at 1 bar, 3.05 bar and 9.15 bar shows considerably large quantitative differences among the various mechanisms. However, at each pressure, the quantitative reaction path diagrams show similar NO formation pathways for most of the mechanisms. Interestingly, in counterflow diffusion flames, the quantitative reaction path diagrams and sensitivity analyses using the various mechanisms reveal major differences in the NO formation pathways and reaction rates of important reactions. The NNH and N₂O intermediate pathways are found to be the major contributors for NO formation in all the reaction mechanisms except GRI 3.0 in syngas diffusion flames. The GRI 3.0 mechanism is observed to predict prompt NO pathway as the major contributing pathway to NO formation. This is attributed to prediction of a large concentration of CH radical by the GRI 3.0 as opposed to a relatively negligible value predicted by all other mechanisms. Also, the back-conversion of NNH into N₂O at lower pressures (2–4 bar) was uniquely observed for one of the five mechanisms. The net reaction rates and peak flame temperatures are used to correlate and explain the differences observed in the peak [NO] at different pressures. This study identifies key reactions needing assessment and also highlights the need for experimental data in syngas diffusion flames in order to assess and optimize H₂/CO and nitrogen chemistry.     

An investigation on the mechanism of the increased NO2 emissions from H2-diesel dual fuel engine

The addition of hydrogen (H₂) into the intake air of a diesel engine was found to significantly increase the emissions of nitrogen dioxide (NO₂). Previous research demonstrated a strong correlation between the emissions of NO₂ and unburned H₂ in exhaust gas. However, the mechanism whereby H₂ addition in increasing NO₂ formation in a H₂-diesel dual fuel engine. Previously has not been investigated.This research numerically verified the hypothesis that the increased NO₂ emissions observed with the addition of H₂ was formed through the conversion from NO to NO₂ during the post combustion oxidation process of the unburned H₂ when mixed with the hot NO-containing combustion products. A variable volume single zone model with detailed chemistry was applied to simulate post-combustion oxidation process of the unburned H₂ and its effect on NO₂ emissions. The mixing of the unburned H₂ with the NO-containing hot combustion products was found to convert NO to NO₂. Such a conversion is promoted by the hydroperoxyl (HO₂) radical formed during the oxidation process of the H₂. The factors affecting the NO₂ formation and its destruction include the concentration of NO, H₂, O₂, and the temperature of the bulk mixture. When H₂ and hot NO-containing combustion products mixed during the early stage of expansion stroke, the NO₂ formed during H₂ oxidation was later dissociated to NO after the complete consumption of H₂. The complete combustion of H₂ exhausted the source of HO₂ necessary for the conversion from NO to NO₂. The mixing of H₂ with combustion products during the last part of the expansion stroke was not able to convert NO to NO₂ since the temperature was too low for H₂ to oxidize and to provide the HO₂ needed. The bulk mixture temperature range suitable for meaningful conversion from NO to NO₂ aided by HO₂ produced during the oxidation of H₂ was examined and presented.     

An experimental investigation of NO2 emission characteristics of a heavy-duty H2-diesel dual fuel engine

This paper investigated the nitrogen dioxide (NO₂) emissions of a heavy-duty diesel engine operated in hydrogen (H₂)-diesel dual fuel combustion mode with H₂ supplemented into the intake air. Preliminary measurements using the 13-mode European Stationary Cycle (ESC) demonstrated the significant effect of H₂ addition on the emissions of NO₂. The detailed effects of H₂ addition and engine load on NO₂ emissions were examined at 1200 RPM. The addition of a small amount of H₂ increased substantially the emissions of NO₂ and the NO₂/NO_x ratio, especially at low load. Increasing the engine load was found to inhibit the enhancing effect of H₂ on the conversion of NO to NO₂ with the maximum NO₂/NO_x ratio observed at lower H₂ concentration. The maximum NO₂ emissions of the H₂-diesel dual fuel operation were three (at 70% load) to five (at 10% load) times that of diesel operation. Further increasing the addition of H₂ beyond the point with maximum NO₂ emissions still produced more NO₂ than for diesel-only operation. Based on the experimental data obtained, the engine load and maximum averaged bulk mixture temperature were not the main factors dominating the formation of NO₂ in the H₂-diesel dual fuel engine. A preliminary analysis demonstrated the significant effect of the unburned H₂ on NO₂ emissions. When mixed with the hot combustion product, the unburned H₂ that survived the main combustion process might further oxidize to raise the HO₂ levels and enhance the conversion of NO to NO₂. In comparison, the changes in the combustion process including the start of combustion, combustion duration and maximum heat release rate may not contribute substantially to the increased NO₂ emissions observed.     

Experimental studies on syngas catalytic combustion on Pt/Al2O3 in a microreactor

Synthesis gas (a mixture of CO and H₂) oxidation is studied over a supported Pt/Al₂O₃ catalyst in a novel microreactor fabricated for studying the intrinsic chemical kinetics of highly exothermic reactions. CO was found to significantly inhibit H₂ oxidation. In contrast, H₂ addition promotes CO oxidation at low mole fractions but has a small promoting effect at high hydrogen mole fractions. As a result, the apparent reaction order of H₂ changes from positive to zero. The change in hydrogen reaction order is associated with hysteresis. Possible mechanisms for the observed behavior are discussed.     

Effect of the equivalence ratio on the concentration of CH4/NO2/O2 combustion products

A chemical kinetic model for determining the mole fractions of stable and intermediate species for CH₄/NO₂/O₂ flames is developed. The model involves 30 different species in 101 chemical elementary reactions. The mole fractions of the species are plotted as a function of the distance from the surface of the burner. The effects of the equivalence ratio on the concentrations of CO, CO₂, N₂, NH₂, OH, H₂O, NO and NO₂ for lean CH₄/NO₂/O₂ flames in the post flame zone at 50 Torr are obtained. The flames are flat, laminar, one dimensional and premixed. The calculated concentration profiles as a function of the equivalence ratio and distance from the surface of the burner are compared with the experimental data. The comparison indicates that the kinetics of the flames are reasonably described by the developed model. The mole fraction of N₂, NH₂, OH, H₂O, CO₂ and CO increase while the mole fractions of NO and NO₂ decrease by increasing the equivalence ratio for lean flames. Copyright © 1999 John Wiley & Sons, Ltd.     

Critical energy for direct initiation of spherical detonations in H2/N2O/Ar mixtures

Although the detonation phenomenon in hydrogen-nitrous oxide mixtures is a significant issue for nuclear waste storage facilities and development of propulsion materials, very limited amount of critical energy data for direct initiation - which provides a direct measure of detonability or sensitivity of an explosive mixture − is available in literature. In this study, the critical energies for direct blast initiation of spherical detonations in hydrogen-nitrous oxide-Ar mixtures obtained from laboratory experiments and theoretical predictions at different initial conditions (i.e., different initial pressure, equivalence ratio and amount of argon dilution) are reported. In the experiments, direct initiation is achieved via a spark discharge from a high voltage and low inductance capacitor and the initiation energy is estimated accordingly from the current output. Characteristic detonation cell sizes of hydrogen-nitrous oxide-Ar mixtures are estimated from chemical kinetics using a recently updated reaction mechanism. A correlation expression is developed as a function of initial pressure, argon dilution and equivalence ratio, which is fitted to provide good estimation of the experimental measured data. The direct link between cell size and critical energy for direct blast initiation is then analyzed. Good agreement is found between experimental results and theoretical predictions, which make use of the cell size estimation correlation and the semi-empirical surface energy model. The effects of the initial pressure, equivalence ratio and the amount of Ar dilution on the critical initiation energy H₂-N₂O-Ar mixtures are investigated. By comparing the critical energies with those of H₂-O₂-Ar mixtures, it is shown that H₂-N₂O mixtures are more detonation sensitive with smaller initiation energies than H₂-O₂ mixtures at the same initial pressure, equivalence ratio and amount of argon dilution, except for higher diluted condition with amount of argon in the mixture above 20%. Attempt is made to explain the critical energy variation and comparison between the two H₂-N₂O-Ar and H₂-O₂-Ar mixtures from the induction length analysis and detonation instability consideration.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

NO emission characteristics in counterflow diffusion flame of blended fuel of H2/CO2/Ar

Flame structure and NO emission characteristics in counterflow diffusion flame of blended fuel of H₂/CO₂/Ar have been numerically simulated with detailed chemistry. The combination of H₂, CO₂ and Ar as fuel is selected to clearly display the contribution of hydrocarbon products to flame structure and NO emission characteristics due to the breakdown of CO₂. A radiative heat loss term is involved to correctly describe the flame dynamics especially at low strain rates. The detailed chemistry adopts the reaction mechanism of GRI 2.11, which consists of 49 species and 279 elementary reactions. All mechanisms including thermal, NO₂, N₂O and Fenimore are taken into account to separately evaluate the effects of CO₂ addition on NO emission characteristics. The increase of added CO₂ quantity causes flame temperature to fall since at high strain rates a diluent effect is prevailing and at low strain rates the breakdown of CO₂ produces relatively populous hydrocarbon products and thus the existence of hydrocarbon products inhibits chain branching. It is also found that the contribution of NO production by N₂O and NO₂ mechanisms are negligible and that thermal mechanism is concentrated on only the reaction zone. As strain rate and CO₂ quantity increase, NO production is remarkably augmented. Copyright © 2002 John Wiley & Sons, Ltd.     

A mechanism for ignition of high-temperature gaseous nitromethane—The key role of the nitro group in chemical explosives

A detailed chemical mechanism describing ignition of high-temperature pure gaseous nitromethane was compiled and tested using shock tube experiments. The temperatures and pressures behind the reflected shock were in the range 1000–1600K and 1–10 atm. Measurements were made of the time evolution of the pressure at the end wall, as well as of the simultaneous pressure and NO absorption at a short, fixed distance from the end wall. Mass and infrared spectroscopy were used to identify the final products. In the reaction mechanism proposed, initiation starts with the CN bond breaking which yields CH₃ and NO₂. Methoxy and CH₂NO₂ radicals then propagate the reaction through two major parallel pathways, both producing CH₂O. Formaldehyde is then reduced to HCO and carries the reaction toward completion. The radical reactions do not release enough energy to compensate for the energy consumed in breaking the CN bond. Although most of the radicals reach their maximum concentration early in the reaction process, ignition does not occur until virtually all of the nitromethane is consumed. The calculations show that the nitro group is the key to explosion: NO₂ produces OH through its reaction with H radicals. Hydroxyl reactions, which are fast and exothermic, lead to an accelerated consumption of the explosive with heat release. Comparison with the experiments shows that the mechanism predicts correct induction times for the pressure and temperature range of the experiments.     

Simplified correlation models for CO/H2 chemical reaction times

This paper provides simplified correlation models for CO/H₂ chemical reaction times. The procedure used for the CO/H₂ simplified modeling utilized the full chemical kinetics mechanism run over a range of temperatures from 700 to 1800 K, pressures from 0.5 to 50 atm, mixtures from 0% to 95% CO, and equivalence ratios from 0.2 to 2.0 to determine ignition (or reaction) time. The correlations for ignition times are given in formulas as functions of equivalence ratio, temperature, and pressure. Two different forms of correlations were obtained, one being a single, overall correlation and the other a two-stage correlation representing regions of high and low temperatures. These correlations are shown to work well over a range of chemical time scales spanning ten orders of magnitude. The correlations are also compared with measured data from the literature.     

DSMC Simulation of Non-Premixed Combustion of H2/O2 in a Y-shaped Microchannel

Combustion at the microscale shows an immeasurable potential in the fields of energy utilization and microelectromechanical systems (MEMS) due to its novel combustion characteristics. Studying the mechanism of this kind of combustion is of great significance for deepening the understanding of microcombustion phenomena and designing related devices. In this article, the non-premixed combustion of H₂/O₂ in a two-dimensional Y-shaped microchannel with a height of 10 μm was numerically studied using the direct simulation Monte Carlo (DSMC) method. The total collision energy (TCE) model and a kinetic mechanism including six species and seven reversible reactions were employed. Predicted distributions of velocity, temperature, heat flux, and components inside the microchannel are presented and analyzed. Influences of the Knudsen number and wall surface conditions on the combustion characteristics are discussed. The results show that the exothermic reaction mainly takes place in the junction area of the branch channels and in the first half of the main channel, and the wall heat flux at the microscale is much higher than that at the conventional scale. This is helpful to effectively heat and ignite the gaseous H₂/O₂ mixture. Moreover, the conversions and distributions of individual components in the flow field are mainly controlled by the chemical kinetics; the Kn number and different wall conditions have a sophisticated influence on the combustion process.     

Dilution effects analysis on NOx emissions of opposed-jet H2/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.     

Effect of H2 addition on combustion and exhaust emissions in a heavy-duty diesel engine with EGR

The effect of the addition of hydrogen (H₂) on the combustion process and nitric oxide (NO) formation in a H₂-diesel dual fuel engine was numerically investigated. The model developed using AVL FIRE as a platform was validated against the cylinder pressure and heat release rate measured with the addition of up to 6% (vol.) H₂ into the intake mixture of a heavy-duty diesel engine with exhaust gas recirculation (EGR). The validated model was applied to further explore the effect of the addition of 6%–18% (vol.) H₂ on the combustion process and formation of NO in H₂-diesel dual fuel engines. When the engine was at N = 1200 rpm and 70% load, the simulation results showed that the addition of H₂ prolonged ignition delay, enhanced premixed combustion, and promoted diffusion combustion of the diesel fuel. The maximum peak cylinder pressure was observed with addition of 12% (vol.) H₂. In comparison, the maximum peak heat release rate was observed with the addition of 16% (vol.) H₂. The addition of H₂ was a crucial factor dominating the increased NO emissions. Meanwhile, the addition of H₂ reduced soot emissions substantially, which may be due to the reduced diesel fuel burned each cycle. Furthermore, proper combination of adding H₂ with EGR can improve combustion performance and reduce NO emissions.     

NOx formation in post-flame gases from syngas/air combustion at atmospheric pressure

Species concentration measurements specifically those associated with nitrogen oxides (NO_x) can act as important validation targets for developing kinetic models to predict NO_x emissions under syngas combustion accurately. In the present study, premixed combustion of syngas/air mixtures, with equivalence ratio (Φ) from 0.5 to 1.0 and H₂/CO ratio from 0.25 to 1.0 was conducted in a McKenna burner operating at atmospheric pressure. Temperature and NO_x concentrations were measured in the post-combustion zone. For a given H₂/CO ratio, increasing the equivalence ratio from lean to stoichiometric resulted in an increase in NO and decrease in NO₂ concentration near the flame. Increasing the H₂/CO ratio led to a decrease in the temperature as well as the NO concentration near the flame. Based on the axial profiles above the burner, NO concentration increases right above the flame while NO₂ concentration decreases through NO₂-NO conversion reactions according to the path flux analysis. In addition, the present experiments were operated in the laminar region where multidimensional transport effects play significant roles. In order to account for the radial and axial diffusive and convective coupling to chemical kinetics in laminar flow, a multidimensional model was developed to simulate the post-combustion species and temperature distribution. The measurements were compared against both multidimensional computational fluid dynamics (CFD) simulations and one-dimensional burner-stabilized flame simulations. The multidimensional model predictions resulted in a better agreement with the measurements, clearly highlighting the effect of multidimensional transport.     

Study of the hydrogen production step of the Mn2O3/MnO thermochemical cycle

In this work, a complete study of the second step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. It includes a complete thermodynamic calculation of the equilibrium phases between MnO, NaOH and H₂, which shows that the reaction takes place theoretically at temperatures above 75 °C. However, the experimental results demonstrate that it is necessary at least 450 °C to achieve a satisfactory reaction rate. It indicates a dramatic influence of chemical kinetics and diffusion process, displacing the reaction to higher temperatures than those predicted by thermodynamics. The resultant solid of the reaction exhibits a phases distribution highly dependent on the temperature and the NaOH:MnO ratio and this is of great influence in the overall rate of the process. The kinetic study shows that the overall process involves not only the chemical reaction between MnO and NaOH, but also a number of physical processes (heat and mass transfer) and solid phase transformations. The apparent activation energy calculated is a composite value determined by the activation energies of those elementary processes.     

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H₂ addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH₂ and H₂NO are most important nitrogen-containing species, and R49 (NH₂+NO<=>N₂+H₂O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H₂ content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas.     

Numerical study on flame structure and NO formation in CH4–O2–N2 counterflow diffusion flame diluted with H2O

Numerical study on flame structure and NO emission behaviour has been conducted to grasp chemical effects of added H₂O on either fuel‐ or oxidizer‐side in CH₄–O₂–N₂ counterflow diffusion flames. An artificial species, which has the same thermodynamic, transport, and radiation properties of added H₂O, is introduced to feasibly isolate the chemical effects. Special concern is focused on the important role of remarkably produced OH radicals due to chemical effects of added H₂O on flame structure and NO emission. The reason why the difference of behaviours between the principal chain branching reaction rate and flame temperature appear is attributed to the drastic change of reaction step (R120) from the production to the consumption of OH. It is also, however, seen that the most important contribution of produced OH due to chemical effects of added H₂O is through reaction step (R127). The importantly contributing reaction steps to NO production are also examined. The production rates of thermal NO and prompt NO are suppressed by chemical effects of added H₂O. The contribution of the reaction steps related to HNO intermediate species to the production of prompt NO is also stressed. Copyright © 2004 John Wiley & Sons, Ltd.     

Chemical storage of solar energy—Kinetics of heterogeneous NH3 and H2SO4 reactions I. Analysis of experimental reaction and mass transfer data

The exothermic heterogeneous reaction of ammonia vapor and liquid sulfuric acid (H₂O + SO₃) to produce ammonium hydrogen sulfate and ammonium sulfate is important for the recovery of stored energy in the small molecules, H₂O, SO₃ and NH₃. As the first part of a chemical reaction engineering analysis, literature data on the reaction and mass transfer were analyzed. The reaction rate constant as a function of temperature and the mass transfer coefficient as a function of the dynamical variables in a flow system were determined. The reaction mechanism in concentrated liquid phase is pseudo-second order and the rate is extremely fast. However, for the heterogeneous system with ammonia transferred from the vapor phase the mass transfer is rate controlling. A broad range of mass transfer data, covering packed columns and dispersed phase droplets, was correlated over 15 orders of magnitude by the j_D-factor correlation method.