Net Reaction Rate Research Articles

Temperature Effects and Activation Barriers in Aqueous Proton-Uptake Reactions.

Aqueous proton transfer reactions are fundamental in biology and chemistry, yet kinetics and mechanisms of strong base-weak acid reactions are not well understood. In this work, we present a temperature-dependent reaction kinetic study of the water-soluble photobase actinoquinol, in the presence and absence of succinimide, a weak acid reaction partner. We study the temperature dependence of the reaction and connect the observed dynamics to the reaction's thermodynamics. We find that actinoquinol reacts in associated complexes with water/succinimide, creating an intermediate complex that can undergo either dissociation to create products, or reverse proton transfer within the complex to recreate the initial reactants. We find that the intermediates' formation is energetically unfavorable with both reaction partners, which impacts the net reaction rates. We also find that the net reaction rate is additionally strongly influenced by the competition between the dissociation of the intermediates and their reverse reaction.

Numerical Study on Chemical Kinetic Characteristics of Counterflow Diffusion Flame Extinction of Methane/Ammonia/Air Flame under High Pressure or Air Preheating Temperature.

Green ammonia has become an increasingly popular fuel in recent years because of its combustion process without carbon oxide release. Adding ammonia to methane fuel for co-combustion has become one of the important research topics in the current combustion field. In the present study, the CH4/NH3/Air counterflow diffusion flame was taken as the research object, and Chemkin-2019 R3 software was used to explore and analyze the flame extinction limit and chemical kinetics characteristics under different ammonia mixing ratios, initial pressures, and air preheating temperatures. It was obtained that the flame extinction stretch rate was decreased by increasing the NH3 mole fraction in the CH4/NH3 mixed fuel. The increase in pressure or air preheating temperature would accelerate the chemical reaction rate of each component in the combustion process, increase the flame extinction limit, and counteract the "stretching effect" of the flame, thus restraining the flame extinguishing phenomenon. The results of a path analysis show that the formation and consumption of OH had an important influence on flame extinction in the chain reaction. The net reaction rate of OH increases with increasing the initial pressure or air preheating temperature, which leads to an increase in flame intensity, combustion stability, and the extinction limit. Furthermore, the function curve between the reaction influences the RIF factor and the stretch rate of the first-to-ten reactions, affected by the heat release of flame combustion, was drawn and quantitatively analyzed. Eventually, a sensitivity analysis of the flame under different working conditions was completed, which found that promoting the forward reaction R39 H + O2<=>O + OH also promotes the positive combustion as a whole when the flame was near extinction. The sensitivity coefficient of R39 in the CH4/NH3/Air flame increases with the growing initial pressure. The increasing air preheating temperature was capable of switching the reaction of R248 NH2 + OH<=>NH + H2O in the CH4/NH3/Air flame from an inhibiting reaction to a promoting reaction, while decreasing the sensitivity coefficient of inhibiting the forward reaction R10 O + CH3<=>H + CH2O, R88 OH + HO2<=>O2 + H2O, and R271 H + NO + M<=>HNO + M. Thus, the inhibition effect of flame extinction was weakened, and the positive progress of combustion was promoted.

Experimental and numerical study on the influence of N2/H2 on the laminar combustion characteristics of syngas premixed flames

This paper primarily analyzes the influence of N2 and H2 content on the laminar combustion characteristics of low calorific value syngas, using a constant-volume combustion bomb to measure the laminar combustion velocity (SL) of the fuel and compare it with five commonly used mechanisms. The study reveals that higher N2 content significantly reduces the SL of CO/CH4/CO2 mixed fuels, primarily due to a general decrease in the mole fractions of key intermediates and net reaction rates. Increasing H2 content leads to an increase in the SL of CO/CH4/CO2/N2 mixed fuels. The thermal effect becomes increasingly prominent with the elevation of H2 content, while the proportion of thermal effect initially decreases and subsequently increases as the equivalence ratio rises. With the increase in N2 content, the mole fraction of NO experiences a significant decrease, and the rate of decline in thermal NO production is even faster. Conversely, with the increase in H2 content, the rate of rise in non-thermal NO generation is faster, and the mole fraction of NO tends to increase.

Numerical Investigation of Combustion Characteristics in a Binary Fuel Blend of C &lt;sub&gt;8&lt;/sub&gt; H &lt;sub&gt;18&lt;/sub&gt; and H &lt;sub&gt;2&lt;/sub&gt;

&lt;div&gt;The escalating energy demand in today’s world has amplified exhaust emissions, contributing significantly to climate change. One viable solution to mitigate carbon dioxide emissions is the utilization of hydrogen alongside gasoline in internal combustion engines. In pursuit of this objective, combustion characteristics of iso-octane/hydrogen/air mixtures are numerically investigated to determine the impact of hydrogen enrichment. Simulations are conducted at 400 K over a wide range of equivalence ratio 0.7 ≤ Ф ≤ 1.4 and pressure 1–10 atm. Adiabatic flame temperature, thermal diffusivity, laminar burning velocity, and chemical participation are assessed by varying hydrogen concentration from 0 to 90% of fuel molar fraction. As a result of changes in thermal properties and chemical participation, it is noticed that the laminar burning velocity (LBV) increases with higher hydrogen concentration and decreases as pressure increases. Chemical participation and mass diffusion were found to be the main contributors to the LBV increase in binary fuel blends. To circumvent NO&lt;sub&gt;X&lt;/sub&gt; formation, a binary fuel blend at Ф = 0.7 and 80% H&lt;sub&gt;2&lt;/sub&gt; is selected to increase combustion intensity while maintaining a relatively low flame temperature and retaining 85% of energy density by volume. It is noted that the concentration of H, O, and OH radicals increase with hydrogen enrichment. Furthermore, the analysis revealed that the LBV increases linearly with the peak mole fraction of radicals. Key reactions are identified through sensitivity analysis and net reaction rates. A significant increase in net reaction rate is observed for H&lt;sub&gt;2&lt;/sub&gt; + O &amp;lt;=&amp;gt; H + OH and H&lt;sub&gt;2&lt;/sub&gt; + OH &amp;lt;=&amp;gt; H + H&lt;sub&gt;2&lt;/sub&gt;O, which in turn increases the pool of radicals. This is evident by the increase in the net production rate of H, O, and OH radicals.&lt;/div&gt;

Studies on properties of laminar premixed oxygen-enriched NH3/H2/O2/N2 flames at elevated temperature and pressure

In this work, the effects of oxygen enrichment coefficient (Ω = 0.21 − 0.42) and initial temperature (Tu = 298 − 448 K) on the laminar burning properties of NH3/H2/O2/N2 flames are systematically investigated by using the spherical flame propagation method at equivalence ratios (Φ = 0.8 − 1.4) and initial pressure (Pu = 3 atm). LBV is analyzed by obtaining experimental data. In order to analyze flame instability, thermal diffusion and hydrodynamic instability are separated by linear stability theory. The thermal and chemical effects of oxygen enrichment on LBV and its effect on heat release rate are analyzed by numerical simulation. The results show that the laminar burning velocity (LBV) exhibits a positive correlation with both the oxygen enrichment coefficient and the initial temperature. The temperature index (α) exhibits a non-monotonic variation in response to the initial temperature of the NH3/H2/O2/N2 flame. Leeff is insensitive to temperature and positively correlated with the oxygen enrichment coefficient. The NH3/H2/O2/N2 flame is mainly affected by hydrodynamic instability when the oxygen enrichment coefficient is large. The peak NHRR scales positively with the oxygen enrichment coefficient. Oxygen enrichment helps to contribute to the overall net reaction rate.

EFFECT OF MICROSTRUCTURES ON MASS TRANSFER INSIDE A HIERARCHICALLY STRUCTURED POROUS CATALYST

In this paper, the pore network model to investigate the reaction-diffusion process in the hierarchically structured porous catalyst particle is extended to consider the phenomenon of deactivation by coking. In this framework, the interaction of internal particle pore structure and mass transfer under the condition of coke deposition is examined. Then, the effect of structural features, namely macroporosity and pore size ratio, the deactivation properties, the maximum loading of coke, as well as the transport properties, the pore Damk&amp;#246;hler number on the net reaction rate, and deactivation of the particle have been investigated. Three deactivation mechanisms are accounted for, namely, the site coverage, the pore narrowing, and the pore blockage. It is found that the deactivation of the catalyst particle can be divided into two conditions: the kinetic deactivation and the structural deactivation. It is shown that depending on the Damk&amp;#246;hler number, increasing the macroporosity does not necessarily improve the reactivity and deactivation resistance of the catalyst. The key finding of this work is to demonstrate and quantify how changing the typical fresh catalyst microstructure into a hierarchical one influences the reactivity and deactivation.

Reynolds number-based global modification of EDC constants and simulation of a syngas and a piloted CH4/air flames

The Eddy Dissipation Concept (EDC), as a promising flame prediction model, is reviewed comprehensively. Instead of strong modifications of the standard EDC constants to fit experiments, a modification method of the EDC constants based on the fuel jet Reynolds number is presented in this work and used to predict a simple jet nonpremixed syngas Sandia Flame CHN–B and a piloted CH4/air partially premixed Sandia Flame D. Compared with the experiments, the predictions show the two flames are properly reproduced with the modified EDC constants. Some over-predictions might be attributed to the under-estimation of turbulence. The microscale and the macroscale Damköhler numbers are presented showing decreasing trends with the increased Reynolds number. Three clear combustion regions in the two flames are firstly identified from the results of the net reaction rate of OH. The reaction heat in the fuel-rich region is found larger than that in the fuel-lean region for Flame D. The contours of the net reaction rate of OH show a stronger reaction region in the fuel-lean side for Flame CHN–B and in the fuel-rich side for Flame D. The predictions with the modified EDC constants agree with the experiments better than those with the standard EDC constants.

Effect of diluent N2 addition on NH3/H2/air combustion characteristics

As a clean energy source with high-energy hydrogen carriers, ammonia has a promising application prospect in gas turbine power production. The existence of N2 as an ammonia combustion product and as an ammonia decomposition product at high temperatures influences ammonia combustion characteristics. Accordingly, this paper measured the combustion characteristics of the NH3/H2/air flame by the method of a constant-pressure spherical flame under different N2 dilution ratios and different equivalent ratios (0.7–1.5), with initiating temperatures of 303 K and 500 K and an initiating pressure of 1 atm, respectively. The experimental results show that the influence of diluent N2 leads to a downward trend in the laminar burning velocities (LBVs) of the NH3/H2/N2/air mixture, and the dilution effect inhibits the propagation of the NH3/H2/N2/air flame. The laminar flame propagation of the mixture is aided by the initiating temperature increase. The existence of diluent N2 reduces the major branching reactions’ net reaction and net heat production rates while enhancing inhibition of the H/OH radical pool and the rate-limiting effect of the primitive reactions. The initiating temperature reduces the impact of the radical reactions on the flame rate and enhances the chemotaxis of NHi (i = 1, 2). The diluent N2 changed the NH3 consumption and effectively suppressed the NOx production, showing the NO emission potential of NH3/H2/N2/air mixtures combustion under fuel-rich conditions.

Investigating Reforming Kinetics of a Synthetic Biogas Mixture on Ni: Model Development and Experimental Validation

A thermodynamically consistent mechanistic rate model for steam reforming (SRM) and catalytic partial oxidation (CPOx) of a synthetic biogas mixture on a Ni catalyst is derived in this work. A microkinetic model (MKM) consisting of 42 irreversible reactions served as the starting point for the development of the model equations presented in this study. A global sensitivity analysis based on ANOVA using Sobol sampling is conducted on the MKM to identify important reactions that describe SRM, CPOx, and water–gas shift reaction (WGSR), which occur alongside CH4 reforming. The unimportant reactions are eliminated to obtain a reduced elementary reaction model, which is subjected to validation by reproducing the predictions of 42 reactions of the MKM. The rate-determining steps (rds) are identified for SRM, WGSR, and CPOx, from the reduced mechanism by using a net reaction rate and partial equilibrium analysis. Mechanistic rate expressions are derived from the identified rds by adopting the Langmuir–Hinshelwood–Hougen–Watson approach. The parameters of these mechanistic rate models for SRM, WGSR, and CPOx are estimated by using a real coded genetic algorithm. The rate models with the estimated parameters could reproduce the experimental observations with good accuracy. The predictive capability of the derived rate models is established by comparing the model predictions with the experimental measurements of SRM, combined steam and dry reforming, and trireforming. The reforming experiments were performed for a synthetic biogas mixture containing CH4/CO2 = 2, in a packed bed reactor using 10 wt % Ni/γ-Al2O3. Overall the mechanistic models were able to predict the experimental measurements with good accuracy.

Effects of Hydrogen Addition on Premixed Combustion of Kerosene in SI Engine

Spark ignition (SI) engines fueled with kerosene have broad application prospects in unmanned aviation vehicles. The knock phenomenon of kerosene in SI engines is a huge challenge, leading to a much lower power output than gasoline engines. In this context, the combustion characteristics of kerosene blending with hydrogen are analyzed numerically regarding the working conditions of an SI engine. First, the ignition delay time of a kerosene/hydrogen mixture is estimated for temperatures of 600–1000 K and pressures of 15–35 bar using the Tay mechanism. The effects of hydrogen addition are evaluated with a ratio of 0–0.4. The sensitivities of the main reactions that affect the ignition delay time are discussed. Then, the laminar flame speed is predicted using the HYCHEM-SK mechanism, and the effects of hydrogen addition on the net reaction rates of the main reactions are analyzed. The results indicate that the ignition delay time is shortened and the laminar flame speed is increased as the hydrogen addition ratio rises. Meanwhile, the ignition delay time decreases except for the NTC range, and the laminar flame speed increases evidently as the temperature rises. In addition, the ignition delay time decreases obviously as the pressure increases with a temperature greater than 750 K. However, the laminar flame speed declines at 600 K and 800 K, while an opposite trend exhibits at 1000 K as the pressure rises. The laminar flame speed increases by 23.85–24.82%, while the ignition delay time only decreases by 4.02–3.59% at 1000 K as the hydrogen addition ratio rises from 0 to 0.4, which will be beneficial for knock suppression.

Origin and significance of ultra-slow calcite dissolution rates in deep sea sediments

Prediction of mineral-fluid reaction rates in geologic materials is subject to large uncertainties even though the rates are critical for many applications, from studies of past climate change to the engineering of carbon removal and storage. Deep sea carbonate sediments, which have been extensively cored and characterized at dozens of sites by ocean drilling programs, provide unique opportunities to examine how minerals and fluids react over multi-million-year timescales in natural conditions at low temperature. It has already been established that the calcite-fluid reaction in these systems is extremely slow, but the explanation for the ultra-slow rates, and the fact that non-zero rates persist for tens of millions of years, remains elusive. We extend the analysis of pore fluid Sr and Ca concentration data in the literature from ocean drilling archives to 21 drill sites in sediment sections where the fraction of carbonate is larger than 80% to systematically estimate the rates at which calcite dissolves and precipitates as a function of depth and sediment age. Pore fluid Sr/Ca used in this study, and Sr and Ca isotopes in other published analyses, provide estimates of the gross calcite dissolution rates, which are mostly balanced by secondary calcite precipitation. Pore fluid Ca concentration data provide key, previously underappreciated, information on the net calcite dissolution rates. Using these data we evaluate simple models for calcite reaction kinetics and pore fluid sediment evolution and conclude that (1) reaction rates are extremely slow because the reactive site density of the bulk sediment decreases continuously and systematically approximately as age−1, and (2) reaction persists to hundreds of meters burial depth and tens of millions of years because pore fluids never become closed due to diffusive communication with the oceans, and hence remain slightly undersaturated with respect to calcite. The systematic decrease in bulk reactive site density begins immediately upon deposition, greatly slowing the rate of dissolution and hence affecting models of calcite dissolution on the seafloor. The failure to reach equilibrium after millions of years and hundreds of meters of burial, is a consequence of extremely slow net reaction rates that cannot drive the pore fluids to equilibrium against the diffusive loss of Ca and DIC to the ocean. The existence of two or more calcite phases of different solubility is a likely source of a long-lived, but small, driving force for reaction, but its effect decreases with continued recrystallization and pore fluid evolution.

A mini-chemical scheme with net reactions for 3D general circulation models

Context. The chemical inventory of hot Jupiter (HJ) exoplanet atmospheres continues to be observed by various ground- and space-based instruments in increasing detail and precision. It is expected for some HJs to exhibit strong non-equilibrium chemistry characteristics in their atmospheres, which might be inferred from spectral observations. Aims. We aim to model the 3D thermochemical non-equilibrium chemistry in the atmospheres of the HJs WASP-39b and HD 189733b. Methods. We coupled a lightweight, reduced chemical network ‘mini-chem’ that utilises net reaction rate tables to the Exo-FMS general circulation model (GCM). We performed GCM models of the exoplanets WASP-39b and HD 189733b as case studies of the coupled mini-chem scheme. The GCM results were then post-processed using the 3D radiative-transfer model gCMCRT to produce transmission and emission spectra to assess the impact of non-equilibrium chemistry on their observable properties. Results. Both simulations show significant departures from chemical equilibrium (CE) due to the dynamical motions of the atmosphere. The spacial distribution of species generally closely follows the dynamical features of the atmosphere rather than the temperature field. Each molecular species exhibits a different quench level in the simulations, which is also dependent on the latitude of the planet. Major differences are seen in the transmission and emission spectral features between the CE and kinetic models. Conclusions. Our simulations indicate that considering the 3D kinetic chemical structures of HJ atmospheres has an important impact on the physical interpretation of observational data. Drawing bulk atmospheric parameters from fitting feature strengths may lead to an inaccurate interpretation of chemical conditions in the atmosphere of HJs. Our open source mini-chem module is simple to couple with contemporary HJ GCM models without substantially increasing required computational resources.

Concepts Relevant for the Kinetic Analysis of Reversible Reaction Systems.

The net rate of a reversible chemical reaction is the difference between unidirectional rates of traversal along forward and reverse reaction paths. In a multistep reaction sequence, the forward and reverse trajectories, in general, are not the microscopic reverse of one another; rather, each unidirectional route is comprised of distinct rate-controlling steps, intermediates, and transition states. Consequently, traditional descriptors of rate (e.g., reaction orders) do not reflect intrinsic kinetic information but instead conflate unidirectional contributions determined by (i) the microscopic occurrence of forward/reverse reactions (i.e., unidirectional kinetics) and (ii) the reversibility of reaction (i.e., nonequilibrium thermodynamics). This review aims to provide a comprehensive resource of analytical and conceptual tools which deconvolute the contributions of reaction kinetics and thermodynamics to disambiguate unidirectional reaction trajectories and precisely identify rate- and reversibility-controlling molecular species and steps in reversible reaction systems. The extrication of mechanistic and kinetic information from bidirectional reactions is accomplished through equation-based formalisms (e.g., De Donder relations) grounded in principles of thermodynamics and interpreted in the context of theories of chemical kinetics developed in the past 25 years. The aggregate of mathematical formalisms detailed herein is general to thermochemical and electrochemical reactions and encapsulates a diverse body of scientific literature encompassing chemical physics, thermodynamics, chemical kinetics, catalysis, and kinetic modeling.

The effects of steam dilution on flame structure and stability for a H2/air micromix burner

The present work is dedicated to understand the effects of steam dilution on the structure and stability of a micromix H2/air swirl flame. The flame structure and stability were obtained through numerical simulations using Large Eddy Simulation (LES) with the Eddy-Dissipation-Concept (EDC) combustion model. In addition, OH-PLIF images and temperature distribution were used to verity the computed results. The experimental and simulated OH distributions show that great part of OH distributes on midstream and downstream of the flame. From the numerical results, it can be seen that the thermal-dilution effect of steam has the biggest impact on flame temperature. Detailed analysis show that the thermal-dilution effect of steam lowers the OH mass fraction, which is the same as third-body effect, however, the OH mass fraction is increased by the direct chemical effect due to decrease the net reaction rate of R4 (H2 + OH = H2O + H). The stability of the micromix hydrogen flame could be reduced due to the thermal-dilution effect, but the direct chemical effect could slightly improve the stability of the micromix hydrogen flame which is the same as third-body effect.

Mechanistic insight into the effect of active site motif structures on direct oxidation of methane to methanol over Cu-ZSM-5.

Direct oxidation of methane to methanol (DMTM), a highly challenging reaction in C1 chemistry, has attracted lots of attention. Herein, we investigate the continuous H2O-mediated N2O-DMTM over a series of Cu-ZSM-5-n zeolites prepared by a solid-state ion-exchange method. Excellent CH3OH productivity (194.8 μmol gcat-1 h-1) and selectivity (67.1%) can be achieved over Cu-ZSM-5-0.3%, which surpasses most recently reported zeolite catalysts. The effect of the active site motif structure on the reaction was systematically investigated by the combined experimental and theoretical studies. It has been revealed that both the monomeric [Cu]+ and binuclear [Cu]+-[Cu]+ sites function to produce CH3OH, following the radical rebound mechanism, wherein the latter one plays a dominant role due to the synergistic effect of neighboring [Cu]+ that can efficiently reduce the N2O dissociation barrier to generate active oxygen for CH4 oxidation. Microkinetic modeling results further show that the dicopper site possesses a much higher net reaction rate (1.23 × 105 s-1) than the monomeric Cu site (0.962 s-1); moreover, H2O can shift the rate determining step from the CH3OH desorption step to the N2O dissociation step over the dicopper site, thereby efficiently favoring CH3OH production and resisting carbon deposition. Generally, the study in the present work would substantially favor other highly efficient catalyst designs.

Influence of operating pressure on combustion and heat transfer in pressurized oxy-fuel combustion evaluated by numerical modeling

Pressurized oxy-fuel combustion (POFC) is an advanced-generation energy plant system that is used to capture and store carbon and can realize a high-efficiency generation cycle with increased heat transfer and minimized efficiency penalty. In this study, we conducted a numerical investigation to characterize the heat transfer, combustion reaction, and NOx emissions of a 0.1 MWth POFC system. In addition, the weighted-sum-of-gray-gases (WSGG) model was applied in the POFC conditions to precisely evaluate the radiative heat transfer based on a comparative analysis of various WSGG models. Both the measured and calculated results revealed that the temperature, including its peak value, decreased with the increasing pressure. Moreover, the average total heat recovery rate increased by approximately 0.2% upon increasing the pressure by 2 bar, and the proportion of the radiative heat transfer rate increased. Furthermore, the combustion reactions containing the O, H, and OH radicals were promoted with the increasing operating pressure, exhibiting a high peak value in a wide area. Consequently, the net reaction rates of the reactions involving HO2 and H2O2 increased with the pressure. Overall, NO was produced in the initial section of the oxy-flame, beyond which the oxidation reaction of NO with HO2 occurred to yield NO2, and the increasing pressure further promoted these reactions.

Electrochemical promotion of catalysis

The Electrochemical Promotion of Catalysis (EPOC) or Non-Faradaic Electrochemical Promotion of Catalysis (NEMCA effect) is a phenomenon observed as a reversible change in catalytic rate (i.e. no net charge transfer rate) of a chemical reaction occurring on a catalyst film (or supported dispersed catalyst) deposited on an ionically conducting or mixed electronically-ionically conducting solid electrolyte support upon the application of an electrical potential between the catalyst and a second conductive film deposited on the solid electrolyte support.

Experimental and numerical study of the effects of N2 dilution and CH4 addition on the laminar burning characteristics of syngas

The study of combustion characteristics of complex biomass syngas under changing conditions is helpful for its practical utilization. In this paper, the effects of N2 dilution (dilution rates from 0% to 60%) and CH4 addition (mixing ratios from 0% to 20%) on the laminar flame properties of H2/CO/N2 syngas were researched. The laminar burning velocities (LBVs) were obtained by constant volume combustion chamber system. Five common mechanisms were used for numerical simulation using the CHEMKIN software package and the results were compared with the measurement data. The Li mechanism that was well matched with the experimental data was selected for kinetic analysis. The results indicate that the LBVs of H2/CO/N2/air mixtures decrease due to the dilution of N2. The addition of CH4 decreases the LBVs of H2/CO/N2/CH4/air mixtures and the LBVs of fuel-rich mixtures decrease faster. Sensitivity analysis illustrates that R29 is the most important branching reaction affecting LBV in H2/CO/N2/air premixed flame, the net reaction rate of all branched reactions and the mole fraction of H and OH in the mixture decrease due to the increase of N2 content. After adding CH4, the dominant branching reaction becomes R1. Due to the increase of CH4 addition ratio, the concentrations of H and O decrease, but the concentrations of OH and CH3 increase slightly. The chemical effect of adding CH4 increases LBV, but the physical effect decreases LBV. The reduction in the proportion of chemical effect is the reason for the faster reduction of LBV in the fuel-rich region. Finally, the formation of NO was analyzed in detail using the GRI 3.0 mechanism. The results show that N2 dilution reduces the mole fraction of NO, but CH4 addition increases the mole fraction of NO. Pathway analysis shows that N2 dilution and CH4 addition decrease and increase the effect of thermal mechanism on NO generation, respectively.

Investigation of soot inception limits and chemiluminescence characteristics of laminar coflow diffusion flames in C/O ratio space

This study investigates soot inception limits of laminar coflow diffusion flames in the carbon-to-oxygen atom (C/O) ratio space, as well as the mechanism of flame structure affecting soot inception. And chemiluminescence characteristics in flames are evaluated. Oxy-ethylene flames with the same adiabatic flame temperature (2500 K) and different stoichiometric mixture fractions (Zst, 0.3–0.6) are investigated by relocating CO2 from oxidant to fuel. The position of soot inception and the chemiluminescence of CH*, C2*, and CO2* are measured by a hyperspectral imager. A numerical model with a detailed kinetic mechanism is used to obtain the temperature and gas-phase species concentration. The results prove that the critical C/O ratio favorable for soot inception is 0.53, and the critical temperatures on the flame centerline and sheet are 1322 K and 1632 K, respectively. The comparatively low temperature on the flame centerline can cause soot inception, as the polycyclic aromatic hydrocarbons (PAHs) concentration and residence time can guarantee the collision and cluster formation of PAHs. Although fuel pyrolysis is promoted on the flame sheet, oxygen-containing species dilutes the concentrations of fuel pyrolysis products and PAHs to limit soot inception, necessitating high temperatures for chemical bond formation. Furthermore, the mixture fraction, local C/O ratio, and net reaction and production rate are calculated, and the key reaction paths are also identified. The results demonstrate that the increase of Zst achieves the suppression of soot inception by lowering the formation of benzene and PAHs in the high C/O ratio (>0.5) zone, improving the oxygenation in the low C/O ratio (<0.5) zone, and compressing the C/O ratio space favorable for soot formation. Meanwhile, the (CH* and C2*) chemiluminescence zone with a C/O ratio of roughly 0.5 exists in the flame. With Zst increases, the CH* and C2* chemiluminescence diminishes at the flame root and enhances downstream, whereas CO2* chemiluminescence remains unaltered.

Reactive fluid flow guided by grain-scale equilibrium reactions during eclogitization of dry crustal rocks

Fluid flow in crystalline rocks in the absence of fractures or ductile shear zones dominantly occurs by grain boundary diffusion, as it is faster than volume diffusion. It is, however, unclear how reactive fluid flow is guided through such pathways. We present a microstructural, mineral chemical, and thermodynamic analysis of a static fluid-driven reaction from dry granulite to ‘wet’ eclogite. Fluid infiltration resulted in re-equilibration at eclogite-facies conditions, indicating that the granulitic protolith was out of equilibrium, but unable to adjust to changing P–T conditions. The transformation occurred in three steps: (1) initial hydration along plagioclase grain boundaries, (2) complete breakdown of plagioclase and hydration along phase boundaries between plagioclase and garnet/clinopyroxene, and (3) re-equilibration of the rock to an eclogite-facies mineral assemblage. Thermodynamic modelling of local compositions reveals that this reaction sequence is proportional to the local decrease of the Gibbs free energy calculated for ‘dry’ and ‘wet’ cases. These energy differences result in increased net reaction rates and the reactions that result in the largest decrease of the Gibbs free energy occur first. In addition, these reactions result in a local volume decrease leading to porosity formation; i.e., pathways for new fluid to enter the reaction site thus controlling net fluid flow. Element transport to and from the reaction sites only occurs if it is energetically beneficial, and enough transport agent is available. Reactive fluid flow during static re-equilibration of nominally impermeable rocks is thus guided by differences in the energy budget of the local equilibrium domains.