Soot Precursor Species Research Articles

Experimental and numerical investigation of the synergistic effect on soot formation in ethanol/n-heptane laminar diffusion flames at elevated pressures

As a carbon-neutral fuel, ethanol is commonly used as a partial replacement for gasoline, but its impact mechanism on the soot formation of gasoline remains unclear, especially under elevated-pressure conditions. In this work, the soot formation of ethanol/n-heptane was experimentally and numerically investigated in laminar diffusion flames in the pressure range of 1.2–5 bar. For all experimental conditions, the total carbon flow rate in the fuel was maintained at 0.353 mg/s with 0%, 10%, 20%, and 30% ethanol contributions. The soot volume fraction and flame temperature were obtained by laser-induced incandescence coupled with light extinction and color-ratio soot pyrometry technique. In the considered pressure range, the synergistic effect on the soot formation was observed with 10% ethanol addition and became more pronounced at higher pressures. The soot formation was suppressed when the amount of ethanol was increased to 20% or 30%. Numerical simulations are performed using a reduced n-heptane/ethanol chemical mechanism and a soot model based on the bivariate Hybrid Method of Moments. The synergy effect was successfully predicted by the model. The simulation results indicate that the reason for the synergistic effect is the increase in acetylene and methyl caused by ethanol pyrolysis, which increases the mass fraction of soot precursor species and consequently promotes PAH condensation.

Reduced Mechanism for Combustion of Ammonia and Natural Gas Mixtures

A fuel mixture of ammonia and natural gas as a low-carbon alternative for future power generation and transportation is an attractive option. In this work, a 50-species reduced mechanism, NH3NG, suitable for computational fluid dynamics simulations (CFD), is developed for ammonia–natural gas cofiring while addressing important emission issues, such as the formation of nitrogen oxides (NOx), soot, carbon monoxide, and unburnt methane/ammonia. The adoption of reduced mechanisms is imperative not only for saving computer storage and running time but also for numerical convergence for practical applications. The NH3NG reduced mechanism can predict soot emission because it includes soot precursor species. Further, it can handle heavier components in natural gas, such as ethane and propane. The absolute error is 5% for predicting NOx and CO emissions compared to the full Modified Konnov mechanism. Validation with key performance parameters (ignition delay, laminar flame speed, adiabatic temperature, and NOx and CO emissions) indicates that the predictions of the reduced mechanism NH3NG are in good agreement with published experimental data. The average prediction error of 13% for ignition delay is within typical experimental data uncertainties of 10–20%. The predicted adiabatic temperatures are within 1 °C. For laminar flame speed, the R2 between prediction and data is 0.985. NH3NG over-predicts NOx and CO emissions, similar to all other literature methods, but the NOx predictions are closer to the experimental data.

A comparison between fossil and synthetic kerosene flames from the perspective of soot emissions in a swirl spray RQL burner

Swirl-stabilized turbulent spray flames operating at atmospheric pressure, with both a conventional Jet-A kerosene (‘A2’) and a synthetic fuel blend (‘C5’) and radially-injected dilution air were investigated from the perspective of soot emissions in a model Rich-Quench-Lean (RQL) combustor. The primary and downstream dilution air flow split was varied while keeping the global equivalence ratio constant (ϕg=0.4). 2.5 kHz PIV, 5 kHz OH-PLIF, and simultaneous 10 Hz Mie scattering and LII were used. Results revealed a correlation between OH and soot distributions. With no dilution air, soot volume fractions (fv) peaked in a v-shaped cone between the OH-containing regions along the spray cone and those anchored to the bluff body. With dilution air at 20% of total air flow, fv shifted to the central axis as OH distributions widened. At 40% dilution, soot initiation began farther downstream along the spray cone, OH regions merged into one, and fv peaked farther downstream at the edge of the Central Recirculation Zone (CRZ). With no air dilution, peak time-averaged fv was 20% lower in the C5 flame compared to A2, despite C5 having a higher aromatics content. However, at 20% dilution fv in C5 increased by 75% compared to the no dilution case. fv in the C5 flame dropped by 12.5% in the high dilution case compared to 40% reductions in A2. Mie scattering showed that the C5 spray was shorter and more resistant to change from dilution air compared to A2. The different spray behaviours were due to C5’s flat boiling range, lower viscosity and density, leading to higher evaporation rates. Detailed chemistry laminar flamelet simulations of A2 and C5 showed that peak soot precursor species mass fraction values and temperatures decreased as the scalar dissipation increased and a significantly stronger presence of toluene in the C5 flame compared to A2. Toluene’s soot yielding behaviour may cause C5’s increased fv with moderate dilution compared to A2. C5 demonstrates the importance of the fuel’s chemical and physical properties when developing future sustainable aviation fuels for practical applications to maximize their soot mitigation potential.

Development and validation of a new n-dodecane-n-butanol-PAH reduced mechanism under diesel engine-relevant conditions

In this work, a new n-dodecane-n-butanol-polycyclic aromatic hydrocarbon (PAH) reduced mechanism that consists of 105 species and 584 reactions was developed to model the combustion and soot characteristics of diesel, n-butanol and its blends. The current mechanism was formulated by merging a n-dodecane and PAH reduced mechanism to a n-butanol reduced mechanism that was developed through the direct relation graph with error propagation (DRGEP), isomer lumping and reaction pathway analysis (RPA) methods. Good predictions were obtained when the mechanism was validated against the ignition delay (ID) times, species concentrations in jet-stirred reactor (JSR) and premixed flame and laminar flame speed. Moreover, the non-reacting liquid and vapour penetration lengths of Bu0, Bu20 and Bu40 (0 vol%, 20 vol% and 40 vol% n-butanol blended in n-dodecane) at ambient temperatures of 800 K, 850 K and 900 K were well predicted with a maximum deviation of 4%. The ID and flame lift-off length (FLOL) deviated with a maximum value of 20% and 12%, respectively. As compared to the measurements, the peak soot volume fraction (SVF) of Bu0 fuel at ambient temperatures of 850 K and 900 K were reasonably predicted with maximum deviations of 1.0 ppm and 0.3 ppm, respectively. At higher n-butanol blending ratios and at lower ambient temperatures, soot was reduced significantly due to lower equivalence ratio, lower flame temperature and the high oxygen content which suppressed the formation of soot precursor species.

Modelling of laminar diffusion flames with biodiesel blends and soot formation

Modelling of soot formation and oxidation of biodiesel fuels is a challenge, due to the complexity of the chemical reactions and soot formation pathways. In this paper, the discretised population balance approach and a PAH-HACA based detailed soot model have been coupled with an in-house CFD code to simulate a laminar diffusion flame with blends of different components of oxygenated biodiesel fuel, and employed to predict combustion performance and soot formation. The simulation aims to reproduce a target experiment which investigated the effects of dibutyl ether (DBE) addition to the biodiesel surrogate (methyl decanoate, MD) by increasing the mole fraction of DBE from 0 to 40%. A combined and reduced MD–DBE–PAH mechanism developed from three sub-mechanism branches has been employed in the simulation. The simulation results show that temperature rises as the DBE percentage increases to 40%. The swallow-tail shape of the soot occurrence zone and the quantity of soot production are correctly predicted. Regarding the effect of soot suppression, the model has basically captured the reducing tendency of soot formation in the measurements as the DBE addition increases from 0% to 40%. Concentrations of PAHs and C3 species contributing to the formation of aromatic rings are slightly reduced due to the addition of DBE, which is a leading cause of soot suppression. However, on the whole, the numerical solution featured much smaller differences than those observed in the experiment among laminar flames with different MD/DBE ratios, because the combined MD–DBE–PAH mechanism only present a slight difference of concentrations of soot precursor species.

Effects of ambient CO2 and H2O on soot processes in n-dodecane spray combustion using large eddy simulation

In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO2) and water (H2O) additions on the soot formation and oxidation processes in an n-dodecane spray flame. In the soot model, acetylene (C2H2) is soot precursor and surface growth species, while hydroxyl radical (OH) and oxygen (O2) are soot oxidizers. The effect of ambient CO2 and H2O additions on soot formation/oxidation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO2 and H2O additions increase C2H2 but reduce OH formation by lowering the flame temperature. This leads to a higher soot mass formed. On the contrary to the thermal effects, the ambient CO2 and H2O additions reduce the soot formation due to their chemical effects. The reaction CH2∗+CO2↔CH2O+CO is found to be responsible for reducing C2H2 formation. The ambient H2O addition results in a higher OH but lower the C2H2 mass formed owing to the reverse reactions H2+OH↔H2O+H and OH+OH↔H2O+O. Furthermore, the chemical effects is more significant than the thermal effects under the tested conditions. This leads to a lower soot mass formed when adding ambient CO2 and H2O.

Numerical Investigation on the Effect of the Oxymethylene Ether-3 (OME3) Blending Ratio in Premixed Sooting Ethylene Flames

Synthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one promising candidate is oxymethylene ether-3 (OME3). In this work, the sooting propensity of ethylene (C2H4) blended with OME3 is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME3, from pure ethylene to pure OME3. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME3 blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME3 blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME3 is found to be different for soot nanoparticles and larger aggregates. While OME3 blending significantly reduces the amount of larger aggregates, only large amounts of OME3, close to pure OME3, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME3 content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition.

Combustion kinetics of alternative jet fuels, Part-I: Experimental flow reactor study

A comprehensive collection of technical aviation fuels enabled an experimental and numerical study on detailed combustion chemistry and pollutant formation presented in a series of 3 interlinking parts. Part-I: Experimental Flow Reactor Study focuses on the characterization of 42 technical jet fuels and provides experimental speciation data for model development presented in Part-II: Model and Surrogate Strategy. Model validation based on the presented technical fuels here is presented in Part-III: Model Application on Technical Jet Fuels.The fuels investigated in this study cover a broad range of approved SAFs (Sustainable Aviation Fuels), candidates for approval, and technical products outside the present ASTM-D7566 specification and is completed by reference fuels (ASTM-D1655). This includes SAF components such as HEFA (Hydroprocessed Esters and Fatty Acids), ATJ (Alcohol-To-Jet), SIP (Synthesized Iso-Paraffins), and Fischer-Tropsch-products as well as their blends.A systematic investigation of the soot precursor chemistry by analyzing the influence of the complex chemical fuel composition on the intermediate species pool is presented. The experimental set-up consists of an atmospheric flow reactor with coupled molecular-beam mass spectrometer (MBMS). Quantitative evolution of combustion reaction intermediates is recorded for fuel-rich (Φ = 1.2) and fuel-lean (Φ = 0.8) conditions at intermediate temperatures up to 1200 K including small intermediate species (e.g. ethylene, butene) and soot precursor species (e.g. benzene, naphthalene, phenanthrene).A general systematic dependency of the soot precursor concentration on the degree of unsaturation (Index of Hydrogen Deficiency) or the hydrogen content, respectively, is demonstrated. Furthermore, larger soot precursor concentrations depend on the naphthalene content of the fuel.

Understanding the role of soot oxidation in gasoline combustion: A numerical study on the effects of oxygen enrichment on particulate mass and number emissions in a spark-ignition engine

The production of increasingly clean engines has become imperative. More stringent regulations for internal combustion engines are constantly proposed, and recent number-based regulations have become a new challenge, since historically only a mass-based regulation needed to be met. It is known that soot particles detectable at the exhaust of an engine are the results of the competition between the formation of soot precursor species and their oxidation. However, the attention is mainly focused on inhibiting soot precursors formation, and much less research is dedicated at elucidating the benefits achievable from enhancing soot oxidation rates. Soot oxidation can be enhanced by increasing the in-cylinder oxygen content. Oxygenated fuels, which are often added to gasoline in order to achieve more efficient combustion, can represent a possible way in the pursuit of this goal. However, chemical mechanisms are still uncertain for practical fuels, and ambiguous results can be produced when the effect of oxygenated fuels on gasoline engine combustion and soot emissions is considered. In the present study, 3-D Computational Fluid Dynamics simulations were performed and the numerical results were compared with existing experimental data, in which load increases were achieved by pure oxygen addition within the intake manifold of a single-cylinder Spark-Ignition (SI) engine. Studying the effects that an addition of 5% and 10% by volume (with respect to air) of additional oxygen produces on the combustion process, allowed to provide basic additional information on soot formation and oxidation, avoiding the uncertainties associated with chemistry models. A semi-detailed soot model and a chemical kinetic model, including poly-aromatic hydrocarbon formation, were coupled with the G-equation flame propagation model for the SI engine simulations and for predicting soot mass and particulate number density. Improvements in the modeling of gasoline premixed combustion were achieved, as well. Specifically, different approaches in the evaluation of the laminar flame speed of gasoline (which is a key factor for obtaining reliable SI engine simulations) were critically compared and analyzed. The numerical results showed aspects that were not possible to appreciate by only referring to the experimental results on which this work was based. It was possible to observe that the higher soot concentrations were located in regions characterized by lower temperatures and lower OH concentrations. Oxygen addition favored a faster burning velocity and produced higher in-cylinder temperatures. However, the production rates of both OH radicals and soot precursor species resulted enhanced. The analysis of these concurrent phenomena allowed to explain why in the experiments the soot mass per kg of fuel was lower for the oxygenated combustion cases.

Automated chemical kinetic mechanism simplification with minimal user expertise

An improved algorithm to generate skeletal mechanisms from the original detailed chemical kinetic mechanisms is proposed. The new algorithm builds on the computational singular perturbation (CSP) framework, by adding an additional layer of automation based on the tangential stretching rate (TSR) and the species’ participation index to TSR. The main advantage of the new approach is that it does not require the specification of a set of target species. Instead, the target species set is dynamic and automatically identified through the simplification algorithm, which defines the system’s state variables that the skeletal mechanism is required to accurately predict. In this way, the new procedure pursues an optimum set of target species that leads the algorithm to include a minimal number of species/reactions that ensures the replication of global observables, such as ignition delay time, without any a-priori knowledge of the chemical pathways. The capabilities and performance of the new simplification algorithm are demonstrated in a test problem employing the 397-species detailed mechanism for C0-C2 species, augmented with polycyclic aromatic hydrocarbon (PAH) soot precursor species. The results are compared with those obtained from the standard CSP-based algorithm in terms of the ignition delay times, main species evolution and their equilibrium state. Subsequent examples also demonstrate the capabilities of the TSR-based algorithm to generate additional sub-mechanisms that, combined with the core mechanism, allow to extend its range of operating conditions and target variables.

Critical Assessment of Photoionization Efficiency Measurements for Characterization of Soot-Precursor Species.

We present a critical evaluation of photoionization efficiency (PIE) measurements coupled with aerosol mass spectrometry for the identification of condensed soot-precursor species extracted from a premixed atmospheric-pressure ethylene/oxygen/nitrogen flame. Definitive identification of isomers by any means is complicated by the large number of potential isomers at masses likely to comprise particles at flame temperatures. This problem is compounded using PIE measurements by the similarity in ionization energies and PIE-curve shapes among many of these isomers. Nevertheless, PIE analysis can provide important chemical information. For example, our PIE curves show that neither pyrene nor fluoranthene alone can describe the signal from C16H10 isomers and that coronene alone cannot describe the PIE signal from C24H12 species. A linear combination of the reference PIE curves for pyrene and fluoranthene yields good agreement with flame-PIE curves measured at 202 u, which is consistent with pyrene and fluoranthene being the two major C16H10 isomers in the flame samples, but does not provide definite proof. The suggested ratio between fluoranthene and pyrene depends on the sampling conditions. We calculated the values of the adiabatic-ionization energy (AIE) of 24 C16H10 isomers. Despite the small number of isomers considered, the calculations show that the differences in AIEs between several of the isomers can be smaller than the average thermal energy at room temperature. The calculations also show that PIE analysis can sometimes be used to separate hydrocarbon species into those that contain mainly aromatic rings and those that contain significant aliphatic content for species sizes investigated in this study. Our calculations suggest an inverse relationship between AIE and the number of aromatic rings. We have demonstrated that further characterization of precursors can be facilitated by measurements that test species volatility.

Reduced combustion mechanism for C1–C4 hydrocarbons and its application in computational fluid dynamics flare modeling

ABSTRACTEmissions from flares constitute unburned hydrocarbons, carbon monoxide (CO), soot, and other partially burned and altered hydrocarbons along with carbon dioxide (CO2) and water. Soot or visible smoke is of particular concern for flare operators/regulatory agencies. The goal of the study is to develop a computational fluid dynamics (CFD) model capable of predicting flare combustion efficiency (CE) and soot emission. Since detailed combustion mechanisms are too complicated for (CFD) application, a 50-species reduced mechanism, LU 3.0.1, was developed. LU 3.0.1 is capable of handling C4 hydrocarbons and soot precursor species (C2H2, C2H4, C6H6). The new reduced mechanism LU 3.0.1 was first validated against experimental performance indicators: laminar flame speed, adiabatic flame temperature, and ignition delay. Further, CFD simulations using LU 3.0.1 were run to predict soot emission and CE of air-assisted flare tests conducted in 2010 in Tulsa, Oklahoma, using ANSYS Fluent software. Results of non-premixed probability density function (PDF) model and eddy dissipation concept (EDC) model are discussed. It is also noteworthy that when used in conjunction with the EDC turbulence-chemistry model, LU 3.0.1 can reasonably predict volatile organic compound (VOC) emissions as well.Implications: A reduced combustion mechanism containing 50 C1–C4 species and soot precursors has been developed and validated against experimental data. The combustion mechanism is then employed in the computational fluid dynamics (CFD) of modeling of soot emission and combustion efficiency (CE) of controlled flares for which experimental soot and CE data are available. The validated CFD modeling tools are useful for oil, gas, and chemical industries to comply with U.S. Environmental Protection Agency’s (EPA) mandate to achieve smokeless flaring with a high CE.

Radical–radical reactions, pyrene nucleation, and incipient soot formation in combustion

We present a combined experimental and probabilistic simulation study of soot-precursor. The experiments were conducted using aerosol mass spectrometry coupled with tunable vacuum ultraviolet radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Mass spectra and photoionization efficiency (PIE) curves of soot precursor species were measured at different heights in a premixed flat flame and in a counter-flow diffusion flame fueled by ethylene and oxygen. The PIE curves at the pyrene mass from these flames were compared with reference PIE scans recorded for pyrene. The results demonstrate that other C16H10 isomers than pyrene are major components among species condensed onto incipient soot in this study, which is in agreement with the simulations. Species with mass 202u only have a high prevalence in incipient soot particles drawn from the premixed flame, but hydrocarbon species with sizes in the range 200–400u are important to incipient-soot formation in both flames. The simulations predict that some species form through combination reactions involving relatively large radicals and bypass traditional molecular-growth pathways through addition of small hydrocarbon species. The experimental results support this prediction; they demonstrate that these species have higher relative abundances in particles formed close to the fuel outlet than smaller, lighter molecular species and indicate that these species are important to early formation of incipient-soot precursors. The results also imply that a leading role in incipient-soot precursor formation is played by species with lower thermal stability than the even-carbon numbered, unsubstituted polycyclic aromatic hydrocarbons known as “stabilomers”.

The effect of reaction mechanisms on the formation of soot precursors in flames

Nanoparticles formed in gas-phase combustion environments are ubiquitous in modern society as hazardous pollutants. Due to the deleterious health effects of carbonaceous nanoparticles it is important to gain a better understanding of the fundamental processes that lead to their formation and growth. We have utilised our Stochastic NAnoParticle Simulator software to model the growth of soot precursor species in a laminar premixed benzene–air flame. We implemented a new set of pathways containing oxygenation reactions and assessed the effects on the growth mechanisms. The inclusion of oxygenation growth pathways with the previously established hydrocarbon growth pathways creates a much larger set of potential configurations for the particle to explore and thus the rate of growth is faster. Hydrogen abstractions and acetylene additions are still the most common events the particle experiences during its evolution trajectory; however, a significant amount of ethers and oxygen-containing rings are formed. Though the role of soot precursor oxygenation is not completely solved, it certainly adds layers of complexity to the nanoparticles’ formation process and likely influences the formation of soot particles.

A laminar flame investigation of 2-butanone, and the combustion-related intermediates formed through its oxidation

2-butanone (methyl ethyl ketone) is a recently identified high-octane, next-generation biofuel candidate, synthesized through microbiological pathways from biomass. Its high octane number (RON = 117), low boiling point (80°C), and relatively high vapor pressure (10.8kPa at 20°C) make 2-butanone a potentially good candidate for use in spark-ignition engines. The flame structure and species formed in 2-butanone combustion are of interest when further considering this compound for use as a fuel. To this end, the present study has, for the first time, quantitatively measured species profiles within a fuel-rich (ϕ = 1.6) laminar premixed flat flame of 2-butanone. Two experiments which used different facilities and measurement techniques were combined to provide a cross comparison of the measured species. One experiment was performed using electron ionization (EI) molecular-beam mass spectrometry (MBMS), and the second relied on synchrotron-generated vacuum ultraviolet (VUV) photoionization (PI) MBMS. Quantitative mole fraction profiles for about 45 species were obtained from the two independent experiments that are found to be in good agreement. The experiments have identified the formation of a number of toxic oxygenated intermediates, such as methyl vinyl ketone (MVK), acetaldehyde, and formaldehyde. In addition, the measurements have also provided quantitative species profiles for a number of soot precursor species, the concentrations of which are notably low.These measurements provide useful validation targets for the further development of detailed chemical kinetic models. As a first test, an existing 2-butanone model was examined against the experimental data with the main aim to identify elements of the model that may need improvement.

The effects of hydrogen addition on soot particle size distribution functions in laminar premixed flame

The effects of hydrogen addition on soot formation in C2H2/air premixed flame was investigated by numerical simulation. In order to clarify the influence of hydrogen addition on soot formation and oxidation, three different contents (0%, 20% and 40%) of hydrogen addition were performed respectively. A Monte-Carlo stochastic method was used to solve the particles dynamical model, which including particle inception, coagulation and surface reaction. Particle size distribution functions (PSDF) in different height above the burner (HAB) were obtained with different amounts of hydrogen addition into C2H2/air flame. The results showed that mole concentrations of important soot precursor species, such as C2H2 and PAHs, are inhibited by the addition of hydrogen. Thereby, the soot volume fraction was reduced. Meanwhile, the number of maximum particles was reduced when hydrogen added to the flame. Hydrogen is an effectively additive in reducing soot formation.

Experimental and kinetic modeling study of the shock tube ignition of a large oxygenated fuel: Tri-propylene glycol mono-methyl ether

Tri-propylene glycol monomethyl ether (TPGME) is an important oxygenated fuel additive that can be used to reduce soot in diesel engines. However, a validated chemical kinetic model that incorporates the low- to high-temperature chemistry, needed to simulate ignition in a diesel engine is not available for TPGME. In addition, no fundamental experimental data are available that can be used to validate a TPGME mechanism. In this study, a surrogate chemical kinetic model for TPGME that includes low- and high-temperature chemistry has been developed, and shock tube ignition delay time data has been acquired for its validation at 0.25% TPGME for temperatures in the range of 980–1545K, at pressures of 10 and 20atm, and at equivalence ratios of φ=0.5, 1.0 and 2.0. The predictions from the model have been compared to the experimental measurements with good agreement. Under the experimental conditions investigated in the shock tube, TPGME was found to be consumed by molecular elimination reactions and also H-atom abstraction by Ḣ atoms and by ȮH and HȮ2 radicals. In performing sensitivity analyses it was found that the ignition of TPGME is most sensitive to reactions involving propene. Considering how the sensitivity analyses change with pressure, the most sensitive reactions involved Ḣ atoms at 10atm and HȮ2 radicals at 20atm. With respect to the effect of equivalence ratio, reactions involving Ḣ atoms are relatively more sensitive under fuel-rich conditions while those involving HȮ2 radicals are relatively more sensitive under fuel-lean conditions. Further experimental work is needed to enable validation of the model under low-temperature conditions. TPGME was compared to n-heptane which has similar ignition properties based on Cetane Number. Predictions showed that TPGME has a higher overall reactivity compared to n-heptane. In addition, TPGME is shown to produce significantly less soot precursor species when TPGME predictions are compared to n-heptane.

Experimental and numerical study on soot formation and oxidation by using diesel fuel in constant volume chamber with various ambient oxygen concentrations

The study on effects of exhaust gas recirculation (EGR) on soot behavior is very important to reduce soot emissions and control the low temperature combustion process in diesel engines. In this work, high time-resolved quantitative soot measurements were experimented on a constant volume chamber by using European low-sulfur diesel fuel at three ambient oxygen concentrations (21%, 18%, 15%). Meanwhile, an improved semi-empirical soot model was coupled into computational fluid dynamics (KIVA-3V Release 2) code for in-depth understanding the soot formation and oxidation processes. Results demonstrated that numerical results of the improved semi-empirical soot model showed good agreement with experimental data in the whole processes of soot formation/oxidation and soot distribution under different oxygen concentrations. The mass concentration of acetylene, soot precursor species and soot mass initially increased with decreasing ambient oxygen concentrations from 21% to 15% and then began to decrease at 12% oxygen, while OH radicals reduced monotonically from 21% oxygen to 12%. At 12% oxygen, the concentrations of local rich sooty zone, acetylene and soot precursor species were as high as those under higher oxygen concentrations, but the total area of soot zone shrank remarkably at 12% oxygen. Compared to 21% ambient oxygen concentration, both soot formation and oxidation rates were increased under 18% oxygen, while the higher soot mass under 18% oxygen was the result of stronger soot formation mechanism. At 15% oxygen, both soot formation and oxidation rates were reduced and the maximum peak of soot mass concentration was caused by the suppressed oxidation mechanism. At 12% oxygen, reaction rates of soot formation and oxidation dropped to the very low level and the reduction in soot mass concentration and number density were achieved. Finally, it could be concluded that the retarded and reduced soot mass concentration at low ambient oxygen was the result of suppressed soot inception reaction from precursor species.

Effect of diluents on soot precursor formation and temperature in ethylene laminar diffusion flames

Soot precursor species concentrations and flame temperature were measured in a diluted laminar co-flow jet diffusion flame at pressures up to eight atmospheres while varying diluent type. The objective of this study was to gain a better understanding of soot production and oxidation mechanisms, which could potentially lead to a reduction in soot emissions from practical combustion devices. Gaseous samples were extracted from the centerline of an ethylene–air laminar diffusion flame, which was diluted individually with four diluents (argon, helium, nitrogen, and carbon dioxide) to manipulate flame temperature and transport properties. The diluted fuel and co-flow exit velocities (top-hat profiles) were matched at all pressures to minimize shear-layer effects, and the mass fluxes were fixed over the pressure range to maintain constant Reynolds number. The flame temperature was measured using a fine gauge R-type thermocouple at pressures up to four atmospheres. Centerline concentration profiles of major non-fuel hydrocarbons collected via extractive sampling with a quartz microprobe and quantification using GC/MS+FID are reported within. The measured hydrocarbon species concentrations are vary dramatically with pressure and diluent, with the helium and carbon dioxide diluted flames yielding the largest and smallest concentrations of soot precursors, respectively. In the case of C2H2 and C6H6, two key soot precursors, helium diluted flames had concentrations more than three times higher compared with the carbon dioxide diluted flame. The peak flame temperature vary with diluents tested, as expected, with carbon dioxide diluted flame being the coolest, with a peak temperature of 1760K at 1atm, and the helium diluted flame being the hottest, with a peak temperature of 2140K. At four atmospheres, the helium diluted flame increased to 2240K, but the CO2 flame temperature increased more, decreasing the difference to approximately 250K.

Influence of gas-phase reactions on catalytic reforming of isooctane

The significance of gas-phase reactions in catalytic partial oxidation (CPOX) of isooctane at short contact times and high temperatures is studied experimentally and numerically to gain further understanding of hydrogen production by CPOX of logistic fuels for on-board applications. Special attention is given to the formation of coke precursors. CPOX of isooctane over a rhodium coated monolith with a molar inlet C/O ratio of 1.1 is used as reference case for a two-dimensional flow field description coupled with detailed surface and gas-phase reaction mechanisms. The results reveal catalyst coking and formation of coke precursors in the oxygen-free catalyst zone. Taking the product composition of the rich operated CPOX reactors (C/O = 1.0−1.6) as inlet composition, homogeneous conversion in the gas-phase is studied in the temperature range from 873 to 1173 K in a plug flow reactor. Conversion in the gas-phase is modeled by two detailed reaction mechanisms. Results show that most of the by-products and soot precursor species arise from unconverted fuel and not from additionally added hydrocarbons like ethylene. Both mechanisms well-predict all experimentally observed trends in gas-phase composition, both in axial reactor profiles and for different inlet compositions. The amount of soot precursors raises with increasing fuel feed corresponding to an increasing C/O ratio in CPOX experiments.