Effect of gas-phase reaction on catalytic reaction for H2/O2 mixture in micro combustor

The catalytic reaction characteristics of the hydrogen and oxygen mixture in the catalytic micro-combustor were detected by thermocouple, infrared thermal imager and OH-PLIF. The equivalence ratio of transition stage from coupling reaction (coupled homogeneous-heterogeneous combustion) to pure catalytic reaction (heterogeneous combustion) was obtained. The three-dimensional pure catalytic reaction model with detailed catalytic reaction mechanism was established in a rectangular catalytic micro-combustor. The simulation model was validated with experimental data. The effects of the intermediate and final products from gas-phase reactions on the pure catalytic reaction were discussed as well as the effects of gas-phase reactions on the catalytic reaction in the process of coupling reactions were studied. The intermediate product OH radical can improve the hydrogen conversion of the surface reaction, and the effect of O radical is not obvious. The final product H₂O has an inhibitory effect on the surface reaction. Since the mass fraction of H₂O is much higher than other gas-phase reaction products, the dominant effect of the gas-phase reaction on the catalytic reaction is suppression. In the coupling reactions, the fuel consumed by the gas-phase reaction weakened the catalytic reaction.     

ReaxFF molecular dynamics simulations of n-eicosane reaction mechanisms during pyrolysis and combustion

The pyrolysis and combustion mechanism of the hydrocarbon fuel has important scientific and practical significance. However, it is difficult to detect the whole intermediates and products using traditional methods, which brings trouble to the analysis of the reaction process. In this paper, the microscopic reaction mechanism and the main products of n-eicosane (C₂₀H₄₂) were simulated based on the reactive force field molecular dynamics (ReaxFF-MD). The effects of temperature (2000–3500 K) and oxygen on the initial decomposition, the distribution of main products, and the reactive pathways of C₂₀H₄₂ fuel were studied to determine its reaction mechanism. The initial decomposition of C₂₀H₄₂ was mainly initiated by small alkyl radicals in pyrolysis, and by the oxygen-containing radicals in combustion. The participation of oxygen had a greater effect on accelerating the decomposition reaction. The reactions involving oxygen of C₂₀H₄₂ initial decomposition accounted for 87.5% of the total reactions at 2000 K. Moreover, the detailed distribution and formation pathways of the main products of H₂, C₂H₄, CH₄, H₂O, CO, and CO₂ were depicted to construct the overall reaction mechanism of C₂₀H₄₂. •H radical formed from the composition of C₂H₄ was exactly consistent with the •H radical consumed by the generation of CH₄ and H₂ in the pyrolysis stage. The feasibility of the simulation method was verified by the result of thermal analysis. The results are helpful for further research on the reaction mechanism of hydrocarbon fuels.     

Detailed kinetic modeling of homogeneous HI decomposition for hydrogen production,part I: Effect of H2O on HI decomposition

A new, detailed kinetic model was developed for the homogeneous decomposition of HI–H₂O solutions in vapor phase in the sulfur–iodine cycle. The kinetics of the process was represented by a reaction mechanism involving 32 reactions and 11 species. Comparisons between the kinetic calculations and experimental data showed that this model correctly predicted the hydrogen yield at the 500 °C–1000 °C temperature range under 1 atm. The effects of temperature, reaction time, and HI/H₂O ratio on HI decomposition and hydrogen sensitivity analysis were investigated in the modeling process. The model predicted that the effect of the addition of H₂O changed from inhibiting the decomposition ratio to promoting it with increasing temperature. The sensitivity analysis showed that elementary reactions (1) HI + HI = H₂+I₂, (4) HI + H = H₂ + I, (5) HI + I = H + I₂, and (8) HI + OH = H₂O + I played important roles in hydrogen production. The reaction path of HI decomposition with H₂O was constructed based on detailed kinetic modeling and sensitivity analysis results.     

Supercritical water partial oxidation mechanism of ethanol

In recent years, supercritical water as a green reaction medium to convert organic matter to produce hydrogen has attracted significant attention. At present, the mechanism of the supercritical water partial oxidation (SCWPO) process is still unknown when complete gasification is achieved. In this paper, a detailed mechanism for SCWPO of ethanol was proposed by establishing a novel kinetic model with a wide reaction temperature range. This model described the formation and consumption of gas products (H₂, CO, CH₄, and CO₂) from ethanol by partial oxidation in the supercritical water environment through nine reactions. The results showed that the dehydrogenation and pyrolysis of ethanol were the main ways to generate H₂ in the early stage, and the water-gas shift reaction had the most significant impact on hydrogen production in the later stage. The hydrogenation reaction of the intermediate product ethane was a key step for complete conversion into combustible gas products. The steam reforming reaction of a large amount of CH₄ produced by ethanol and intermediate product will become the rate-determining step for hydrogen production.     

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.     

Biogas upgrading for on-board hydrogen production: Reforming process CFD modelling

Hydrogen production through fuel reforming can be used to improve IC (internal combustion) engines combustion characteristics and to lower vehicle emissions. In this study, a computational fluid dynamics (CFD) model based on a detailed kinetic mechanism was developed for exhaust gas reforming of biogas to synthetic gas (H₂ and CO). In agreement with experimental data, the reactor's physical and chemical performance was investigated at various O₂/CH₄ ratios and gas hourly space velocities (GHSV). The numerical results imply that methane reforming reactions are strongly sensitive to O₂/CH₄ ratio and engine exhaust gas temperature. It was also found that increasing GHSV results in lower hydrogen yield; since dry and steam reforming reactions are relatively slow and are both dependent on the flow residence time. Furthermore, the hot spot effect, which is associated to oxidation reforming reactions, was investigated for catalyst activity and durability.     

A wide range modeling study of formation and nitrogen chemistry in hydrogen combustion

The chemistry of nitrogen species and the formation of NO_x in hydrogen combustion are analyzed here on the basis of a large set of experimental measurements.The detailed kinetic scheme of H₂/O₂ combustion was updated and upgraded using new kinetic and thermodynamic measurements, and was validated over a wide range of temperatures, pressures and equivalence ratios. The mechanism's performance at high pressures was greatly improved in particular by adopting higher rate parameters for the H+OH+M=H₂O reaction.The NO_x sub-mechanism was further validated and updated. The kinetic parameters of the NO₂+H₂=HONO+H and N₂H₂+NO=N₂O+NH₂ reactions were updated in order to improve model predictions in specific conditions.Sensitivity analyses were carried out to determine which reactions dominate the H₂/O₂ and H₂/NO_x systems at particular operating conditions.Good overall agreement was observed between the model and the wide range of experiments simulated.     

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C₆H₆ and n-C₆H₇ and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.     

Effects of hydrogen addition on the combustion characteristics of diesel fuel jets under ultra-high injection pressures

Many applications use hydrogen addition and high-pressure fuel injection technology to improve combustion performance. In this study, spray atomization and combustion characteristics of a diesel fuel jet, under the injection pressure of 350 MPa, injecting into a constant volume combustion vessel filled with air-hydrogen mixture at the diesel engine relevant condition are investigated by simulation method. A simplified mechanism of the n-heptane (C₇H₁₆) oxidation chemistry mechanism consisting of 26 reactions and 25 species integrated with the Kéromnès-2013 hydrogen combustion mechanism and EDC combustion model are utilized to predict the diesel fuel spray auto-ignition and combustion. The ambient gas is the mixture of air and hydrogen range in volume fraction from 0% to 10%. The ambient temperature and pressure is set to 1000 K and 3.5 MPa, respectively. The results indicate that as the hydrogen volume fraction is 2%, the minimum overall droplet SMD (Sauter Mean Diameter) is approximately 0.95 μm, which is obviously smaller than that of the case with the conventional high injection pressure. In cases that H₂ v/v% larger than 4%, the maximum gaseous temperature increased significantly up to 2700 K. There are two peaks in the temperature growth rate curves as the hydrogen fraction of 8% and 10%. The high temperature at the outer edge of the spray is clearly seen due to its high value when the hydrogen fraction is larger than 4%. The hot reaction layer is the main location of CO formation. The H, OH radicals are formed at the edge of the spray where the temperature is high. The hydrogen species obviously promotes the oxidation and combustion of diesel fuel.     

Mechanisms and possible applications of the Al–H2O reaction under extreme pH and low hydrothermal temperatures

The reaction of Al and H₂O is a promising method for the renewable production of H₂ (an environmentally friendly fuel whose combustion produces only water), because it does not directly include fossil fuels conversion. This reaction was studied at extreme pH values as low as 1 and as high as 13.5 with HCl, H₂SO₄, and NaOH, and at low hydrothermal temperatures of 40-100 °C. Factors such as pH, temperature, and solution medium influenced H₂ production, which was considerably greater at higher temperatures and more extreme pH (acidic or alkaline). Alkaline conditions consistently favored more rapid H₂ production than acidic conditions. Under the most extreme conditions, the activation energy was ∼60 kJ mol⁻¹ for both acidic and alkaline reactions. A model predicting H₂ production in acidic reactions was derived from the reaction mechanism and kinetics. The model yielded a good fit to on-site measurements at the Tamagawa and Zao hot springs in northeast Japan. This study would aid the development of industrial H₂ production systems using natural acidic hot springs or alkaline industrial wastewater.     

2D thermal modeling of a solid oxide electrolyzer cell (SOEC) for syngas production by H2O/CO2 co-electrolysis

Solid oxide fuel cells (SOFCs) can be operated in a reversed mode as electrolyzer cells for electrolysis of H₂O and CO₂. In this paper, a 2D thermal model is developed to study the heat/mass transfer and chemical/electrochemical reactions in a solid oxide electrolyzer cell (SOEC) for H₂O/CO₂ co-electrolysis. The model is based on 3 sub-models: a computational fluid dynamics (CFD) model describing the fluid flow and heat/mass transfer; an electrochemical model relating the current density and operating potential; and a chemical model describing the reversible water gas shift reaction (WGSR) and reversible methanation reaction. It is found that reversible methanation and reforming reactions are not favored in H₂O/CO₂ co-electrolysis. For comparison, the reversible WGSR can significantly influence the co-electrolysis behavior. The effects of inlet temperature and inlet gas composition on H₂O/CO₂ co-electrolysis are simulated and discussed.     

Thermodynamic characteristics of reactants and energy conversion in steam reforming of calcium carbide furnace off-gas to produce hydrogen

In this study, to reveal the thermodynamic reaction mechanism and the conversion characteristics of material and energy for the steam reforming of multi-component materials, a chemical equilibrium model was constructed, and the reaction mechanism of the steam reforming of calcium carbide furnace off-gas (CCFG) was also identified. The conversion characteristics of material and energy were ascertained by assessing four independent reactions and their reverse reactions. And the effects of temperature, pressure and steam-to-gas ratio on the conversion ratios of CO and CH₄, the yield of H₂ and the reaction heat were investigated. In addition, a method for determining the process parameters of steam reforming based on the follow-up utilization scheme of CCFG was proposed; the method involves using a four-panel diagram that plots parameters of the steam reforming process. Thus, the range of H₂/CO mole ratio that could be obtained from the equilibrium system of steam reforming of CCFG was determined.     

Ignition characteristics of heptane-hydrogen and heptane-methane fuel blends at elevated pressures

There is significant interest in using hydrogen and natural gas for enhancing the performance of diesel engines. We report herein a numerical investigation on the ignition of n-C₇H₁₆/H₂ and n-C₇H₁₆/CH₄ fuel blends. The CHEMKIN 4.1 software is used to model ignition in a closed homogenous reactor under conditions relevant to diesel/HCCI engines. Three reaction mechanisms used are (i) NIST mechanism involving 203 species and 1463 reactions, (ii) Dryer mechanism with 116 species and 754 reactions, and (iii) a reduced mechanism (Chalmers) with 42 species and 168 reactions. The parameters include pressures of 30 atm and 55 atm, equivalence ratios of ? = 0.5, 1.0 and 2.0, temperature range of 800-1400 K, and mole fractions of H₂ or CH₄ in the blend between 0 and 100%. For n-C₇H₁₆/air mixtures, the Chalmers mechanism not only provides closer agreement with measurements compared to the other two mechanisms, but also reproduces the negative temperature coefficient regime. Consequently, this mechanism is used to characterize the effects of H₂ or CH₄ on the ignition of n-C₇H₁₆. Results indicate that H₂ or CH₄ addition has a relatively small effect on the ignition of n-C₇H₁₆/air mixtures, while the n-C₇H₁₆ addition even in small amount modifies the ignition of H₂/air and CH₄/air mixtures significantly. The n-C₇H₁₆ addition decreases and increases the ignition delays of H₂/air mixtures at low and high temperatures, respectively, while its addition to CH₄/air mixtures decreases ignition delays at all temperatures. The sensitivity analysis indicates that ignition characteristics of these fuel blends are dominated by the pyrolysis/oxidation chemistry of n-heptane, with heptyl (C₇H₁₆-2) and hydoxyl (OH) radicals being the two most important species.     

A reduced mechanism for ethylene/methane mixtures with excessive NO enrichment

A detailed mechanism for methane–ethylene mixtures enriched with excessive amount of NO was systematically reduced for efficient numerical simulations of flames in arc-heated co-flowing air. Methane and ethylene were selected as the surrogate fuel in the present study due to their drastically different features of ignition and extinction properties and flame propagation speeds, such that the mixtures of them may be utilized to mimic practical hydrocarbon fuels with various kinetic properties in experiments. The recently released USC Mech-II for C1–C4 was grafted with the NO_x sub-mechanism in GRI-Mech 3.0 with updated reaction parameters for prompt NO formation. The resulting detailed mechanism with 129 species and 900 reactions was first validated against experiments involving NO_x enrichment and reasonably good agreements were observed. The detailed mechanism was then employed as the starting mechanism for the reduction. A skeletal mechanism with 44 species and 269 reactions was derived using the methods of directed relation graph (DRG) and DRG-aided sensitivity analysis (DRGASA); a 39-species reduced mechanism with 35 semi-global reaction steps was further obtained using the linearized quasi steady state approximations (LQSSA). Five species related to prompt NO were retained in the reduced mechanism because of their significant impacts on the fuel oxidation. The reduced mechanism closely agrees with the detailed mechanism for ignition and extinction of homogenous mixtures, as well as selected 1-D flames over a wide range of parameters with NO concentrations between 0% and 3%. The observed worst-case relative error of the reduction is approximately 20%. The reduced mechanism was further validated with experiments involving excessive NO_x enrichment.     

A statistical approach to develop a detailed soot growth model using PAH characteristics

A detailed PAH growth model is developed, which is solved using a kinetic Monte Carlo algorithm. The model describes the structure and growth of planar PAH molecules, and is referred to as the kinetic Monte Carlo-aromatic site (KMC-ARS) model. A detailed PAH growth mechanism based on reactions at radical sites available in the literature, and additional reactions obtained from quantum chemistry calculations are used to model the PAH growth processes. New rates for the reactions involved in the cyclodehydrogenation process for the formation of 6-member rings on PAHs are calculated in this work based on density functional theory simulations. The KMC-ARS model is validated by comparing experimentally observed ensembles on PAHs with the computed ensembles for a C₂H₂ and a C₆H₆ flame at different heights above the burner. The motivation for this model is the development of a detailed soot particle population balance model which describes the evolution of an ensemble of soot particles based on their PAH structure. However, at present incorporating such a detailed model into a population balance is computationally unfeasible. Therefore, a simpler model referred to as the site-counting model has been developed, which replaces the structural information of the PAH molecules by their functional groups augmented with statistical closure expressions. This closure is obtained from the KMC-ARS model, which is used to develop correlations and statistics in different flame environments which describe such PAH structural information. These correlations and statistics are implemented in the site-counting model, and results from the site-counting model and the KMC-ARS model are in good agreement. Additionally the effect of steric hindrance in large PAH structures is investigated and correlations for sites unavailable for reaction are presented.     

Thermal field under the effect of the chemical reaction of a direct internal reforming solid oxide fuel cell DIR-SOFC

The effects of direct internal reforming in a fuel cell solid oxide (SOFC) on thermal fields are studied by mathematical modeling. This study presents the thermal fields of a standard fuel cell (Ni-YSZ/YSZ/LSM) anode supported. This study is also made in the perpendicular plane at the flow of gases. The fuel cell is powered by air and fuel, CH₄, H₂, CO₂, CO and H₂O hence the birth of the phenomenon of direct internal reforming (DIR-SOFC). It is based on reforming chemical reactions, steam reforming reaction and water–gas shift reaction. The main purpose of this work is the visualization of temperature fields under the influence of global chemical reactions and the confirmation of the thermal behavior of this chemical reaction. The thermal fields are obtained by a computer program (FORTRAN).     

An experimental and numerical study on characteristics of laminar premixed H2/CO/CH4/air flames

The effects of variations in the fuel composition on the characteristics of H₂/CO/CH₄/air flames of gasified biomass are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position and temperature are performed in the premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames with various H₂ and CO contents in the fuel. The adiabatic flame temperatures and laminar burning velocities are calculated using the EQUIL and PREMIX codes of Chemkin collection 3.5, respectively. Whereas the flame structures of the laminar premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames are simulated using the OPPDIF package with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position and temperature of the stoichiometric H₂/CO/CH₄/air opposed-jet flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that the reaction rate of reactions (R38), (R46), and (R84) increase with increasing H₂ content in the fuel mixture. It is also found that the increase in the laminar flame speed with H₂ addition is most likely due to an increase in active radicals during combustion (chemical effect), rather than from changes in the adiabatic flame temperature (thermal effect). Chemical kinetic structure and sensitivity analyses indicate that for the stoichiometric H₂/CO/CH₄/air flames with fixed H₂ concentration in the fuel mixture, the reactions (R99) and (R46) play a dominant role in affecting the laminar burning velocity as the CO content in the fuel is increased.     

A decade of ceria based solar thermochemical H2O/CO2 splitting cycle

Since 2006, ceria is used as a redox reactive material for production of H₂, CO, and syngas via a two-step solar driven thermochemical H₂O/CO₂ splitting cycle. Different forms of phase pure ceria were studied over a wide range of temperatures and oxygen partial pressures. To increase the redox reactivity and long-term stability, the effects of incorporation of different dopants in to the ceria fluorite structure (in varying proportions) were studied in detail. A variety of solar reactors, loaded with ceria based ceramics, were designed and developed to investigate the performance of these materials towards thermal reduction and H₂O/CO₂ splitting reactions. The thermodynamics and reaction kinetics of ceria based solar thermochemical H₂O/CO₂ splitting cycles were also explored heavily. This paper presents a detailed chronological insight into the development of ceria-based oxides as reactive materials for solar fuel production via thermochemical redox H₂O/CO₂ splitting cycles.