Detailed Reaction Scheme Research Articles

Consistent definitions of “Flame Displacement Speed” and “Markstein Length” for premixed flame propagation

The definition of the flame displacement speed (FDS), often used to characterize the dynamical properties of premixed flames, is generally ambiguous because, except for a steadily propagating planar flame, the mass flow rate through the combustion region varies with distance through the flame and one is therefore faced with the difficulty of choosing a proper iso-surface to represent the flame surface. A directly related issue is the determination of the proportionality coefficient in the linear flame speed-flame stretch relation of weakly-stretched flames, known as the Markstein length, which depends strongly on the location inside the flame zone where it is measured or calculated. The objective of the present study is to identify an iso-surface and thereby a definition of the FDS that is well conditioned and less prone to uncertainties, and a consistent and unambiguous expression for the Markstein length. With a selected isotherm to represent the flame surface, the two most common definitions of the FDS are based either on the energy equation with the temperature as the progress variable, or on the kinematic characteristics of the surface (the propagation speed relative to the flow). In this study we examine the spherical flame geometry, a setup that provides an independent determination of the FDS that is not contingent upon an arbitrary selection of the flame surface and thus permits a proper evaluation of the two FDS definitions. A large number of simulations of premixed spherical propane/air flames with equivalence ratio ranging from 0.8 to 1.4 were carried out at various temperatures and pressures using both global single-step and detailed reaction schemes. Outwardly propagating spherical (or cylindrical) flames and inwardly propagating stationary spherical flames were examined. The dependence of the flame speed and flame temperature on stretch, and the corresponding Markstein length were identified for different isotherms selected to represent the flame surface, and the results were carefully compared to the asymptotic theory of weakly-stretched flames. The excellent agreement between theory and simulations provides a clear explanation and quantification of the differences found between the trends in the flame speed-flame stretch relation and the corresponding Markstein lengths, exhibited when the FDS was calculated based on an isotherm in the burned or unburned sides of the flame. We show that the proper isotherm for the evaluation of the FDS which is well-conditioned and properly accounts for the physics must be sufficiently close to the burned side of the flame. This choice is less prone to uncertainty, as the slope of the flame speed-flame stretch relation when reaching the burned side of the flame becomes less dependent on the selected iso-value. On the other hand, the choice of the fresh combustible mixture temperature as a reference location for the calculation of the FDS, and the corresponding unburned Markstein length, is ill conditioned and should be avoided.

A chemical mechanism for low to high temperature oxidation of methylcyclohexane as a component of transportation fuel surrogates

Surrogate fuels consisting of a mixture of well-studied hydrocarbons are often used to model real fuels in typical combustion studies. A major challenge, however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the diverse nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species (Blanquart et al., 2009), as well as several substituted aromatic species and n-dodecane (Narayanaswamy et al., 2010, 2014), well suited as a base to model surrogates, has now been extended to describe the oxidation of methylcyclohexane, a representative of the cyclic alkane class, which is often used in jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of methylcyclohexane is extracted from the detailed scheme of Pitz et al. (2007) and integrated in a systematic way into the previous model. Rate coefficient changes based on recent recommendations from literature, and an additional concerted elimination pathway important at moderate to low temperatures are introduced to the resulting chemical mechanism, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

Effect of chloralkanes on the phenyltrichlorosilane synthesis by gas phase condensation

To enhance the process of phenyltrichlorosilane synthesis using gas phase condensation, a series of chloralkanes were introduced. The influence of temperature and chloralkane amount on the synthesis was studied based on the product distribution from a tubular reactor. The promoting effect of chloralkane addition was mainly caused by the chloralkane radicals generated by the dissociation of C–Cl bond. The promoting effect of the chloromethane with more chlorine atoms was better than those with less chlorine atoms. Intermediates detected from the reactions with isoprene and bromobenzene demonstrated that both trichlorosilyl radical and dichlorosilylene existed in the reaction system in the presence of chloralkanes. A detailed reaction scheme was proposed.

A reduced chemical kinetic mechanism of a diesel fuel surrogate (n-heptane/toluene) for HCCI combustion modelling

The combination of a CFD (Computational Fluid Dynamic) code and a detailed chemical kinetic mechanism is a very powerful tool for its use in the development of new Internal Combustion Engines (ICE). However, the large computation demands of this combination have encouraged researchers to develop efficient reduction techniques that produce relatively small kinetic mechanisms while keeping the relevant information (reaction paths and compounds) from the detailed reaction schemes. This challenge is especially significant when simulating new combustion concepts such as Low Temperature Combustion (LTC) modes, in which the fuel oxidation chemistry, together with the mixing phenomena and turbulence, play a very important role. In this work, a multi-technique reduction methodology has been used to obtain a reduced kinetic mechanism of a diesel fuel surrogate (consisting of a mixture of 64% n-heptane/36% toluene (% by wt)). Firstly, Directed Relation Graph with Error Propagation (DRGEP) was used since it has been proved in the literature to be a very efficient method to remove species from a large kinetic scheme. Further reduction can be achieved with the use of Reaction Analysis (RA), a methodology capable of detecting unimportant reactions from a reaction mechanism. Finally, a lumping methodology was applied in order to reduce the number of isomers in the kinetic scheme. The methodology has been proved to produce very accurate results when reducing large kinetic reaction schemes such as the diesel fuel surrogate mechanism. The detailed kinetic mechanism, that includes 772 species and 3216 reactions, has been reduced to a scheme consisting of 184 species and 463 reactions which reproduces accurately the behaviour of the diesel surrogate under different engine LTC conditions.

The Importance of Detailed Chemical Mechanisms in Gas Turbine Combustion Simulations

<p>This paper – in memory of Jürgen Warnatz – summarizes selected recent papers of the Chemical Kinetics Group at the German Aerospace Center in Stuttgart. It shows the need for detailed chemical reaction mechanisms to understand practical combustion systems. A comprehensive description of combustion processes based on detailed mechanisms is especially important in the design of new gas turbine combustion chambers and in the optimization of existing ones to improve efficiency and to reduce pollutant emissions, with fuel-flexibility and load-flexibility ever becoming more important. Different aspects of combustion processes where detailed reaction mechanisms provide useful insights will be covered in this paper: Fuels (alternative jet fuels, biomass based fuels), pollutants (soot), diagnostics (chemiluminescence), and thermochemistry. Furthermore, the underlying thermodynamics inevitably connected with detailed reaction schemes will be addressed. Exemplified results will be presented clearly demonstrating the predictive capabilities of detailed reaction mechanisms to be explored in computational fluid dynamic simulations to further optimize technical combustion systems.</p>

Exciplex Formation in Bimolecular Photoinduced Electron-Transfer Investigated by Ultrafast Time-Resolved Infrared Spectroscopy

The dynamics of bimolecular photoinduced electron-transfer reactions has been investigated with three donor/acceptor (D/A) pairs in tetrahydrofuran (THF) and acetonitrile (ACN) using a combination of ultrafast spectroscopic techniques, including time-resolved infrared absorption. For the D/A pairs with the highest driving force of electron transfer, all transient spectroscopic features can be unambiguously assigned to the excited reactant and the ionic products. For the pair with the lowest driving force, three additional transient infrared bands, more intense in THF than in ACN, with a time dependence that differs from those of the other bands are observed. From their frequency and solvent dependence, these bands can be assigned to an exciplex. Moreover, polarization-resolved measurements point to a relatively well-defined mutual orientation of the constituents and to a slower reorientational time compared to those of the individual reactants. Thanks to the minimal overlap of the infrared signature of all transient species in THF, a detailed reaction scheme including the relevant kinetic and thermodynamic parameters could be deduced for this pair. This analysis reveals that the formation and recombination of the ion pair occur almost exclusively via the exciplex.

A chemical mechanism for low to high temperature oxidation of n-dodecane as a component of transportation fuel surrogates

Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species [G. Blanquart, P. Pepiot-Desjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607], and several substituted aromatic species [K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898], ideally suited as a base to model surrogates, has now been extended to describe the oxidation of n-dodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of n-dodecane is extracted from the detailed scheme of Sarathy et al. [S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2-methylalkanes from C7 to C20, Combust. Flame 158 (12) (2011) 2338–2357] and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

Kinetics of the Reversible Reaction of CO2(aq) and HCO3– with Sarcosine Salt in Aqueous Solution

Aqueous sarcosine salts are fast carbon dioxide (CO(2)) absorbents suitable for use in postcombustion CO(2) capture in coal-fired power plants. We have developed a detailed reaction scheme including all the reactions in the sarcosine-CO(2)-water system. All unknown rate and equilibrium constants were obtained by global data fitting. We investigated the temperature-dependent rate and equilibrium constants of the reaction between aqueous CO(2) and sarcosine using stopped-flow spectrophotometry, by following the pH changes over the wavelength range 400-700 nm via coupling to pH indicators. The corresponding rate and equilibrium constants ranged from 15.0 to 45.0 °C and were analyzed in terms of Arrhenius, Eyring, and van't Hoff relationships. The rate constant for the reaction between CO(2) and sarcosine to form the carbamate at 25.0 °C is 18.6(6) × 10(3) M(-1) s(-1), which is very high for an acyclic amine; its activation enthalpy is 59(1) kJ mol(-1) and the entropy is 33(4) J mol(-1) K(-1). In addition, we investigated the slow reaction between bicarbonate and sarcosine using (1)H nuclear magnetic resonance spectroscopy and report the corresponding rate and equilibrium constants at 25.0 °C. This rate constant is 5.9 × 10(-3) M(-1) s(-1).

A new kinetic model for pyrolysis of Athabasca bitumen

AbstractPyrolysis of bitumen is a key contributor to gas production in in situ combustion and in situ gasification recovery processes, yet a detailed reaction scheme, that includes a breakdown of products into the most abundant gas components, that is simple enough to be used in detailed thermal‐reactive simulation models, does not exist. Here, we present a novel reaction scheme for pyrolysis of Athabasca bitumen that was developed and calibrated against four experimental data sets (65 data points) over a temperature range from 130 to 422°C. The new model was then verified by comparing its thermogravimetric behaviour against published literature. © 2012 Canadian Society for Chemical Engineering

A Spectroscopic and Computational Study of a Photoinduced Cross‐Dehydrogenative Coupling Reaction of a Stable Semiquinone Radical

Norrish-type-II reaction on a semiquinone radical: stable semiquinone radicals serve as novel molecular platforms on which a Norrish-type-II photoreaction can be initiated. A detailed reaction scheme involving a 1,5-hydrogen transfer followed by a cyclization step that finally leads to a new C-C bond formation could be verified. Transient absorption spectroscopy and DFT calculations trace convincingly the intermediates and transition states along the reaction path.

Experimental and Detailed Kinetic Modeling Study of Ethyl Pentanoate (Ethyl Valerate) Oxidation in a Jet Stirred Reactor and Laminar Burning Velocities in a Spherical Combustion Chamber

To improve our understanding of the combustion characteristics of ethyl pentanoate, a possible second generation biofuel, new experimental data were acquired for its oxidation kinetics in two complementary experiments. In a JSR (jet stirred reactor), concentration profiles of stable species were measured at 10 atm over a range of conditions (equivalence ratios of 0.6, 1, 2, fuel initial concentration of 1000 ppm, and temperatures between 560 to 1160 K). In a spherical combustion chamber, unstretched laminar burning velocities of ethyl pentanoate–air mixtures were measured at different pressures and temperatures and for equivalence ratios in the range 0.7–1.4. The oxidation of ethyl pentanoate was modeled using a new detailed kinetic reaction scheme (2719 reactions, 522 species). The chemical kinetic mechanism proposed here yielded good agreement with the present data. To interpret the results, reactions flux and sensitivity analyses were carried out.

Critical Role of Oxygen Evolved from Layered Li–Excess Metal Oxides in Lithium Rechargeable Batteries

The high capacity of the layered Li–excess oxide cathode is always accompanied by extraction of a significant amount of oxygen from the structure. The effects of oxygen on the electrochemical cycling are not well understood. Here, the detailed reaction scheme following oxygen evolution was established using real-time gas analysis and ex situ chemical analysis of the surface of the electrodes. A series of electrochemical/chemical reactions involving oxygen radicals constantly produced and decomposed lithium carbonate during cell operation. Moreover, byproducts, including water, affected the cycle life and rate capability: hydrolysis of the electrolyte salt formed hydrofluoric acid that attacked the surface of the electrode. This finding implies that protection of the electrode surface from damage, for example, by a coating or removal of oxygen radicals by scavengers, will be critical to widespread usage of Li–excess transition metal oxides in rechargeable lithium batteries.

Oxidation of dibenzo-p-dioxin: Formation of initial products, 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran

This study combines the results of experimental measurements and theoretical computations to investigate the initial steps in the oxidation of dibenzo-p-dioxin (DD). The aim is to discover new pathways leading to the formation of toxic species in combustion systems, as well as compounds that may act as precursors for polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). The experiments were performed in a tubular reactor equipped with sampling systems for condensable and gaseous products. The reaction conditions were characterised by a residence time of 5s and a fuel equivalence ratio of 0.12. We investigated the effect of temperature (400–800°C) on the type and the yield of the oxidative products, trapped in a cartridge, and gases collected in tedlar bags for subsequent FTIR analyses. The analyses of VOC, performed on a high resolution gas chromatograph–triple quadrupole mass spectrometer (HRGC–QQQMS), identified 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran as the initial products. This has been confirmed by injection of authentic standards and the application of collision induced dissociation (CID) that fragmented the isolated parent ions into specific product ions affording the identification of parent species. The oxidative decomposition of DD initiated at around 450°C, with the evolution of VOC peaking at between 650 and 700°C. In excess of 750°C, all VOC were completely oxidised. The potential energy surface (PES), based on the density functional theory (DFT) of B3LYP, mapped the initial steps involved in the oxidation of DD, and yielded a detailed reaction scheme for the onset of oxidation of DD that results in the formation of 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran. Initial oxidation of DD is most likely to commence via addition of the singlet oxygen molecule and the subsequent facile formation of the propagating OH radical.

Detailed modeling of the atmospheric degradation mechanism of very-short lived brominated species

Detailed chemical reaction schemes for the atmospheric degradations of the very short-lived species (VSLS) bromoform (CHBr3) and dibromomethane (CH2Br2) have been established. These degradation schemes have been implemented in the meteorological/tracer transport model CATT-BRAMS used in the present case as pseudo one-dimensional model with chemistry of CH4, CO, HOx, NOx, NOy and Ox. They include the main possible reactions of the intermediate brominated peroxy radicals RO2 (with R = CH2Br, CHBr2 and CBr3) for which the most likely reaction pathways with HO2 have been found using ab initio computational calculations. The full degradation schemes have been run for two well-defined realistic scenarios, “clean” atmosphere and “moderately” NOy-polluted atmosphere, as representative of a tropical coastal region where these VSLS natural emissions are expected to be important. The Henry’s law constants of the brominated organics products have been estimated by using the Bond Contribution Method (BCM; Meylan and Howard, 1991) or the Molecular Connectivity Index (MCI; Nirmalakhandan and Speece, 1988). Using these constants, the least soluble species formed from the VSLS degradation are found to be CBr2O, CHBrO, CBr3O2NO2, CHBr2O2NO2, BrO, BrONO2 and HOBr, which leads those to be potentially transported into the tropical tropopause layer (TTL) in case of deep convection and contribute to stratospheric bromine additionally to the original substances. For bromoform and dibromomethane degradation, the moderate NOy pollution increases the production of the least soluble species and thus approximately doubles the bromine quantity potentially able to reach the TTL (from 22.5% to 43% for CHBr3 and from 8.8% to 20.2% for CH2Br2). The influence of the reactions of the RO2 radicals with HO2, CH3O2 and NO2 on the nature and abundance of the stable intermediate and end-products has been tested for CHBr3 degradation. As a result, the reactions of the RO2 radicals with NO2 have no impact. Taking into account the reaction between RO2 and CH3O2 and modifying the branching ratios of the reaction between RO2 and HO2 lead to a small impact on the bromoform degradation by slightly decreasing (by 10%) the bromine quantity potentially able to reach the TTL. As a final point, in contrast to CHBr3, CH2Br2 degradation produces negligible quantities of organics species and the effects of pollution increase only the inorganic species production. By taking into account the results of these tests, new simplified degradation schemes for CHBr3 and CH2Br2 are proposed.

Detailed Modeling of Carbon Oxidation

The kinetics of low-temperature oxidation of a synthetic carbon black material, i.e. Printex- U, by gaseous oxygen is modeled via conventional mean-field treatment of various detailed reaction schemes as well as lumped models. The significance of different elementary steps is elucidated by comparing predicted CO and CO2 formation during temperature programmed and isothermal carbon oxidation with the corresponding experimental data. In specific the connection between oxidation rate and surface coverages of reaction intermediates is demonstrated. The results suggest that activation of the carbon surface by oxygen adsorption is a key-step for further carbon oxidation and that gas phase oxygen plays a direct role in the formation of CO and CO2. Furthermore, the results suggest that Printex-U is preferably modeled as spherical shaped particles, especially at high conversion stages.

Hierarchical simulation of microcrystalline PECVD silicon film growth and structure

We have designed and implemented a hierarchical simulation methodology capable of addressing the growth rate and microstructural features of thin silicon films deposited through PECVD (Plasma Enhanced Chemical Vapor Deposition). Our main objective is to elucidate the microscopic mechanisms as well as the interplay between atomic level and macroscopic design parameters associated with the development of nano- or micro-scale crystalline regions in the grown film. The ultimate goal is to use multi-scale modeling as a design tool for tackling the issue of local crystallization and its dependence on operating variables. At the heart of our simulation approach is a very efficient, large-scale kinetic Monte Carlo (kMC) algorithm which allows generating samples of representative Si films based on a validated chemistry model. In a second step, the generated film is subjected to an atomistic simulation study which restores the molecular details lost or ignored in the kMC model. The atomistic simulations are computationally very demanding; they are, however, an important ingredient of our work: we use it to back-map the coarse grained model employed in the kMC simulations to an all-atom model which is further relaxed through detailed NPT molecular dynamics (MD) or Monte Carlo simulations. This tunes local structure thus also important morphological details associated with the presence of crystalline and amorphous regions (and the intervening interfacial domains) in the grown film.The kMC algorithm is based on a carefully chosen set of reacting or active radicals (species) in the gas phase impinging the film and a detailed set of surface reactions. Inputs for species fluxes are taken from a well-tuned plasma fluid model that includes a detailed gas phase chemistry reaction scheme. The growth mechanism consists of various surface kinetic events including radical-surface and adsorbed radical-radical interactions, radical-surface diffusion, and surface dissociation reactions. The very fast surface diffusion is decoupled from the rest of the kMC events and is treated deterministically in our work. For a three-dimensional Si(001)-(2x1):H crystalline lattice, our kMC algorithm allows us to simulate film growth over several seconds, resulting in thickness on the order of tens of nanometers. In the following pages we provide more details about the implementation of our kMC algorithm along with validation results.

Degradation of β-carotene during fruit and vegetable processing or storage: reaction mechanisms and kinetic aspects: a review

Introduction. Food processing significantly lowers the quality of fruits and vegetables, which is a major concern for the food industry. Micronutrients are particularly affected, and among them β -carotene, which exhibits very interesting sensory, nutritional and biological properties. The literature concerning β -carotene degradation is extensive, but the conclusions are very different as a function of the biological, chemical and food transformation points of view. This paper proposes a synthesis of complementary approaches in the study of β -carotene during food transformation and storage. Degradation reactions. Degradation compounds are numerous, including isomers, epoxides, apocarotenones, apocarotenals and short-chain cleavage products, among them some flavour compounds. A detailed reaction scheme of isomerisation and autoxidation of β -carotene could be deduced from the literature data. The main pathways are well documented, but the global reaction scheme is still incomplete. Furthermore, most of the mechanistic studies are carried out in model systems, thus data may misrepresent β -carotene behaviour in real food products. Kinetics during processing and storage. The determination of degradation kinetics permits the identification of the fastest reactions, i.e. , generally those with the greatest impact, and also the quantification of the effect of the factors which can lower β -carotene content. Temperature, occurrence of oxygen, food composition and food structure are shown to affect the β -carotene loss rate significantly. However, the methodologies used to obtain the kinetic parameters are of major importance, and finally, most of the results found in the literature are specific to a study and difficult to generalise. Discussion and conclusion. Mechanistic and kinetic approaches each provide interesting data to improve understanding and monitoring of β -carotene. The combination of all this data, together with thermodynamic and analytical considerations, permits the building of observable reaction schemes which can further be transcribed through mathematical models. By this multidisciplinary approach, scarcely used for the time being, knowledge could be capitalised and useful tools could be developed to improve β -carotene retention during food processing and storage. (Resume d'auteur)

Theoretical investigation of thermally and photochemically induced haptotropic rearrangements of chromium ligands on naphthalene systems

The description of chemical reactions by means of quantum mechanical methods is an important task and gets even more challenging if excited states have to be considered. This work focuses on the haptotropic rearrangements of chromium atoms bearing three coligands which migrate on a naphthalene-like system. The reactions are either thermally or photochemically controllable and thus the systems are candidates for molecular switches. We propose a detailed reaction scheme for the investigated system. Furthermore, we provide a detailed analysis of the important steps of the reaction cycle. In comparison to previous publications, the scope of this work also involves the quantum mechanical treatment of excited states in order to describe occurring photon absorption processes in a proper way. Linear response time-dependent density functional theory calculations were carried out to describe the molecules’ responses to the external electromagnetic perturbations.

Explicit modelling of SOA formation from α-pinene photooxidation: sensitivity to vapour pressure estimation

Abstract. The sensitivity of the formation of secondary organic aerosol (SOA) to the estimated vapour pressures of the condensable oxidation products is explored. A highly detailed reaction scheme was generated for α-pinene photooxidation using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). Vapour pressures (Pvap) were estimated with three commonly used structure activity relationships. The values of Pvap were compared for the set of secondary species generated by GECKO-A to describe α-pinene oxidation. Discrepancies in the predicted vapour pressures were found to increase with the number of functional groups borne by the species. For semi-volatile organic compounds (i.e. organic species of interest for SOA formation), differences in the predicted Pvap range between a factor of 5 to 200 on average. The simulated SOA concentrations were compared to SOA observations in the Caltech chamber during three experiments performed under a range of NOx conditions. While the model captures the qualitative features of SOA formation for the chamber experiments, SOA concentrations are systematically overestimated. For the conditions simulated, the modelled SOA speciation appears to be rather insensitive to the Pvap estimation method.

Validation of flow simulation and gas combustion sub-models for the CFD-based prediction of NOx formation in biomass grate furnaces

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence–chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NOx formation. Coupling detailed NOx reaction kinetics with advanced turbulence–chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k–ϵ model for turbulence, Eddy Dissipation Concept for turbulence–chemistry interaction and the HK97 reaction mechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NOx formation into account, the accuracy of turbulence modelling and turbulence–chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H2/N2 Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier–Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence–chemistry interaction models and combustion kinetics. Issues related to the NOx formation with a focus on fuel bound nitrogen sources were investigated on a NH3-doped syngas flame. The experimentally observed trend in NOx yield from NH3 was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NOx formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field.