Sooting Tendency Research Articles

Small scale pool fires: The case of toluene

Industrial applications adopting toluene as a solvent have been largely extended in recent years, including solutions within the framework of the energy transition and energy storage technologies. The potential use of this flammable compound in a different set of operative conditions and compositions requires a comprehensive and complete knowledge of its fire behaviour and combustion kinetic. To this scope, an innovative experimental procedure applicable to liquid reactive systems was developed for this scope and implemented at different boundary conditions. More specifically, the specimen was exposed to air and heat fluxes between 7 and 50 kW/m2, at a constant sample surface of 0.01 m2, an initial sample thickness of 0.01 m, and a distance between the sample and the horizontally oriented conical-shaped heater of 0.025 m. Measurements were compared with data from the current literature, when available, demonstrating the robustness and validity of the adopted procedure. Although an increase in the external flux leads to growing mass burning rates (i.e., from 0.47 g/s to 0.85 g/s), negligible effects on the ignitability were observed. Conversely, a peak in the heat release rate at 35 kW/m2 was measured. The observed reduction at higher external heat fluxes was attributed to less effective combustion, demonstrating that the maximum expected heat flux cannot be considered as an aprioristic worst-case scenario for the evaluation of pool fires. The collected data were, then, further utilized to obtain insights on the formation of the main products, including soot tendency. Based on the collected data a simplified kinetic model suitable for the computational fluid dynamics was proposed to reproduce the chemistry of the system.

Experimental and numerical study of the decomposition, product spectrum, and sooting properties of adamantane fuels

This work combined experimental measurements with two theoretical approaches, reactive Molecular Dynamics (MD) simulations and Quantum Mechanics (QM) calculations, to investigate the combustion and sooting properties of three adamantane fuels, adamantane (AD), 1,3-dimethyl-adamantane (DMAD), and 1-ethyl-adamantane (EAD). These fuels were selected since the adamantane fuel family can potentially be used as sustainable aviation fuels and comparing these fuels would reveal the effects of side chain on their combustion and sooting properties. We determined the bond dissociation energies of the three test fuels using QM calculations and found that the functionalized side chains have the weakest bonds and their presence only slightly affects the bond strength in the AD multi-cyclic core. We performed pyrolysis simulations for all three fuels using ReaxFF-based MD simulations and found that DMAD and EAD have higher decomposition rates than AD and also generate more high-molecular-weight products. For all three fuels, these large products were observed to contribute significantly to hydrocarbon growth processes, which lead to large soot nucleating species and even soot-like structures. A higher yield of such soot nucleating compounds was found during the pyrolysis of DMAD and EAD than AD since the decomposition products of DMAD and EAD are more branched and those of AD have mostly straight chains. These theoretical analyses were supported by experimental measurements of yield-based sooting tendencies, which suggest that DMAD and EAD are sootier than AD and that all three fuels are sootier than standard alkane jet fuel surrogates but less sooty than jet fuel aromatic content.

Development of two diesel surrogates focusing on the soot emission under engine-like conditions

ABSTRACT Threshold sooting index (TSI) is one of the critical properties related to the soot emission of diesel. Investigating the effects of TSI on the prediction of soot emission and the response of the soot model to TSI may improve our understanding of the soot formation process and model establishment during the combustion of diesel under engine-like conditions. To meet this need, two different d.iesel surrogates, named S1 and S2 were developed. S1 comprised n-dodecane, iso-octane, methylcyclohexane, and 1-methylnaphthalene, while S2 replaced 1-methylnaphthalene with toluene. Both were adjusted for similar cetane numbers, H/C ratios, and heating values, differing in TSI to analyze sooting tendencies to ensure the S1 and S2 had different soot tendencies but close ignition quality. Then, the skeletal mechanism containing the kinetics of the components in S1 and S2 was developed and validated against ignition delay times, speciation, laminar flame speeds and the 3-D spray combustion with results agreeing well with predictions. Finally, the soot performances of S1 and S2 in different soot models were compared in the constant volume combustion chamber (CVCC). The results showed that the comparison of S1 and S2 in different soot models could promote the understanding of soot models.

Sooting tendency of substituted aromatic oxygenates: The role of functional groups and positional isomerism in vanillin isomers

Substituted aromatics are commonly observed in lignin-based biofuel; however, their high sooting tendency prevents direct utilization in commercial combustors. Recent studies have revealed that oxygenated functional group substitution could effectively suppress the soot emission from aromatic biofuels. This study aims to enhance the understanding of sooting tendencies in aromatic oxygenates with mono-, di-, and tri-substitutions, focusing on various functional groups and their positional isomerism. We established a yield sooting index (YSI) database of 42 single-ring aromatic compounds, including 30 new measurements from the present study. The constructed database was utilized to develop a multivariate linear regression (MLR) model to predict the YSI of substituted aromatic oxygenates based on their structural features. The fitted coefficients of the MLR model indicate vastly different impacts of hydroxyl, formyl, and methoxy functional group, as well as the importance of positional isomerism. To understand the role of oxygenated functional groups, we used substituted vanillin isomers containing hydroxyl, methoxy, and formyl groups as a model system. Comparing the sooting tendencies of these compounds revealed a high sensitivity of YSI to positional isomerism. A further mechanistic study using quantum-mechanical calculations showed that subtle interactions between three oxygenated functional groups in vanillin isomers can alter their thermal decomposition pathway, affecting the sooting tendencies of these aromatic fuels. The present study provides a novel statistical and theoretical explanation of how oxygenated substitution and its positional isomerism influence sooting behaviors, facilitating the rational design of lignin-based biofuels with minimal soot emission.

Optical diagnostics and chemical kinetic analysis on partially premixed combustion characteristics fueled with methanol and various cetane improvers

The need for decarbonization has inspired the utilization of methanol in transportation industry. Via both experimental and modeling frameworks, this study, for the first time, reveals the ignition, heat release and flame development characteristics of methanol mixed with different cetane improvers, i.e., 2-ethylhexyl nitrate (EHN) and di-tertiary‑butyl peroxide (DTBP), under practical, advanced engine conditions. Particularly, a detailed chemical kinetic model is newly developed for methanol/EHN/DTBP mixtures, incorporating the most updated sub-chemistries for relevant fuel components and critical intermediates, to assist in further analysis on the effects of EHN and DTBP on ignition and emission formation under engine thermodynamic environments derived from recorded in-cylinder pressure histories. Results show that stable combustion of methanol/EHN mixtures can be realized under low direct injection (DI) pressure, high in-cylinder thermodynamic condition, featuring the lowest coefficient of variation of indicated mean effective pressure (COVIMEP) of 1.46 %. Throughout the combustion process, the yellow and white flame dominates. At the same DI timing, methanol/EHN mixtures present shorter ignition delay, faster heat release rate (HRR) and more advanced combustion phasing compared with methanol/DTBP mixtures. At similar combustion phasing, the peak flame/OH* natural luminosity intensity of methanol/EHN mixtures is higher. KL factor, which is utilized to evaluate soot concentration, also shows similar trend, indicating higher sooting tendency of methanol/EHN mixtures. Flux analysis further highlights higher production of soot precursor (i.e., C2H2) with EHN than DTBP, primarily via the pathway of C2H4→ C2H3→C2H2. The active atmosphere created by the decomposition of EHN or DTBP has significant effects on decreasing methanol ignition delay in the early stage of their exothermic process, while the active atmosphere of EHN remains impactful whereas that of DTBP diminishes at later stages. The thermal atmosphere shows greater effects on methanol ignition at later stages of the exothermic process of EHN or DTBP decomposition than the early stages.

An easy but quantitative assessment of soot production rate and its dependence on temperature and pressure

The challenge of soot emission persists in combustion research due to the complexities of tracking the crucial stages of growth from fuel to soot nuclei and ultimately mature particles. Studying soot formation in flames often requires a sophisticated approach, involving detailed measurements of gaseous soot precursors and soot particles using multiple complementary diagnostics. On the other end of the spectrum of studies are simpler methods that capture the sooting tendency using a single index, akin to the cetane number in compression ignition engines and the octane number in spark ignition engines. This article seeks a middle ground, aiming to quantify the soot production rate while maintaining the simplicity of single-index characterizations. The approach involves establishing counterflow diffusion flames, measuring soot volume fraction through pyrometry, and accurately computing velocity and temperature profiles using a commercial code. These data allow for the quantification of the production rate from the soot governing equation. The methodology is applied to counterflow ethylene diffusion flames to examine the temperature dependence of the soot production rate across peak temperatures varying by several hundred degrees and pressures in the 1–32 atm range. The soot production rate per unit flame area falls within the range of 10−7–10−3 g/(cm2s) range and, when normalized with respect to the carbon flux, it ranges between 10−6 and nearly 10−2. On a logarithmic scale, it linearly correlates with the peak temperature at a fixed pressure. Although this study deals only with flames of ethylene, the approach can be generalized to any fuel. The resulting database should be valuable not only for industry practitioners but also to the scientific community for the global validation of detailed soot models.

Simulation of a rapid compression machine for evaluation of ignition chemistry and soot formation using gasoline/ethanol blends

Due to the projected decline of demand for gasoline in light duty engines and the advent of ethanol as a green fuel, the use of gasoline/ethanol blend fuels in heavy duty applications are being investigated as they are projected to have lower cost and lower lifecycle green house gas (GHG) emissions. In heavy duty engines, the primary mode of combustion is mixing controlled combustion where wide range of mixture conditions (equivalence ratio) exist. Soot emissions of these fuels in richer conditions are not well understood. The goal of this research is to evaluate some commercially available soot modeling codes for the particulate matter emissions from gasoline/ethanol fuel blends, especially at fuel rich conditions. A Rapid Compression Machine (RCM) is modeled in a three-dimensional numerical simulation using CONVERGE computational software using a reduced chemical kinetic mechanism with SAGE chemistry solver and a RANS k-ϵ turbulence model with a sector model including the creviced piston. The creviced piston is used in the experimental setup to reduce boundary layer effects and to maintain a homogeneous core in the reaction cylinder. Computational fluid dynamics simulations are conducted for different gasoline-ethanol fuel blends from E10 (10% ethanol v/v) to E100. The fuel blend is modeled as a surrogate mixture of toluene, iso-octane, n-heptane for gasoline content, and ethanol. The computational results were validated against experimental results using pressure measurements and laser extinction diagnostics. Different soot models are investigated to evaluate their capability of predicting the sooting tendencies of fuel blends, especially in richer conditions experienced during mixing-controlled combustion. The experimental combustion characteristics such as the ignition delay of different blends of fuel are reasonably well predicted. The Particulate Size Mimic (PSM) model accurately predicts the soot generation characteristics of the different fuels, but the Hiroyasu-NSC model falls short in this regard. For accurate prediction of soot with the PSM model, the thermodynamic conditions during combustion must be accurately modeled. While the current computational modeling tools can produce accurate results for the prediction of particulate matter emissions, there is much work to be done in improving our understanding of the underlying fundamental processes.

A simple method for the quantitative assessment of soot production rate

Sooting tendency has been characterized with various empirical approaches such as smoke height and Threshold Sooting Index (TSI), that can be regarded as qualitative, as well as Yield Sooting Index (YSI), that is semiquantitative since it relies on the measurements of at least the peak soot volume fraction in the flame. All these techniques have the convenience of being easy to implement and of relying on inexpensive equipment. In the present work we present a comparatively easy but quantitative alternative to determine the soot production rate in counterflow flames. The approach is rooted in the use of: a) soot volume fraction measurements by pyrometry, b) the well established one-dimensional computational modeling of such flames for the determination of temperature and velocity profiles and c) the use of the soot governing equation. The technique is applied to several aliphatics, including methane, propane, ethylene, propene and acetylene. Soot production rate per unit flame area for the tested aliphatics ranges between 10−4 and 10−7 g/(cm2s) and, when normalized with respect to the carbon flux, between 10−5 and 10−2. On a logarithmic scale it correlates linealry with the peak temperature for all fuels. Soot yield scales as alkanes< alkenes < alkynes, with acetylene showing the highest sooting tendency even in flames at relatively low temperatures.

Spray combustion of fast-pyrolysis bio-oils under engine-like conditions

In the EU funded SmartCHP project, biomass derived fast-pyrolysis bio-oil (FPBO) is used to fuel modified stationary diesel engines for combined heat and power applications. In this study, the spray combustion characteristics of fast-pyrolysis bio-oil and FPBO/ethanol blends are experimentally investigated in a combustion research unit. Due to the special physicochemical properties of fast-pyrolysis bio-oil, major modifications are made to the combustion research unit to facilitate testing this fuel, including the installation of a high-temperature chamber and a heavy-fuel oil injection system. Experimental tests are carried out at a chamber wall temperature of 750 °C and an ambient pressure of 50 bar. The pressure-based heat release analysis is used to evaluate fuel ignition and combustion characteristics, while a high-speed imaging technique is adopted to visualize natural luminosity of spray flames through a borescope.Results show that fast-pyrolysis bio-oil has lower sooting tendency than diesel, and it significantly alleviates the fuel dribble phenomenon. At injection pressure of 300 bar, the poor atomization of fast-pyrolysis bio-oil leads to rather grainy-looking spray flame images, and nozzle orifices becoming clogged within couple of injections. The increase of injection pressure to 900 bar results in an obvious improvement of the atomization quality and nozzle durability for fast-pyrolysis bio-oil. Adding ethanol into fast-pyrolysis bio-oil could improve fuel atomization and further reduce the natural luminosity. The ignition delay of fast-pyrolysis bio-oil is slightly retarded by 10% ethanol addition, while it is advanced by 30% ethanol addition.

Effect of preheating air temperature on sooting tendency in laminar co-flow diffusion flame of n-heptane

This work investigates the effect of preheating air temperature on the sooting tendency in the laminar co-flow diffusion flames of n-heptane. A modified co-flow burner was employed to produce steady n-heptane laminar diffusion flames with the preheating temperatures of co-flow air varying from 344 K to 588 K. The planar laser-induced incandescence (LII) calibrated by the line-of-sight attenuation (LOSA) was performed to quantitatively measure the soot volume fraction and the soot yield in flames and an ICCD camera was used to capture the flame luminosity images. The results reveal that the visible flame height is decreased with the preheating co-flow air temperature, and it is hardly affected by the air velocity with a constant air temperature. As the co-flow air temperature increases, the soot inception and oxidation and the peak soot concentration initiate at the lower positions in flames, and the soot loading in flames has a remarkable increasing trend by assessing the evolution of the soot volume fraction and the soot yield. However, the enhancement rate of total soot loading in flames with the co-flow air temperature below the initial fuel temperature is lower than that for the co-flow air temperature above the initial fuel temperature. In addition, the temperature sensitivity of soot loading is assessed by the maximum soot volume fraction, which is in accordance with those of the alkane in previous research. According to the response of the maximum soot yield to the co-flow air temperature and adiabatic flame temperature, the temperature sensitivity of sooting tendency in n-heptane flames is further confirmed.

Sooting tendency of isopropanol-butanol-ethanol (IBE)/diesel surrogate blends in laminar diffusion flames

Recent advancements in metabolic engineering have demonstrated the cost-effective production of isopropanol-butanol-ethanol (IBE) from switchgrass biomass. As such, IBE is seen as a viable future fuel to transition the transportation sector towards renewable biofuels. In this study, an atmospheric co-flow diffusion flame burner was used to examine the sooting tendency of IBE when mixed with a diesel surrogate for blend ratios varying from 0% to 100% by volume. Experimental measurements of soot volume fraction were obtained by implementing a color-ratio pyrometry technique and reporting the variation of sooting tendency in terms of the Yield Sooting Index (YSI). Measurements demonstrated soot formation and hence YSI decreased nonlinearly with increasing IBE content. To gain further insight into the kinetics affecting soot formation, numerical modeling studies were carried out using laminarSMOKE++ and a comprehensive kinetic mechanism with soot chemistry. From a reaction path and rate of production analysis, IBE demonstrated that it can perturb OH and HO2 radical pools and enhance CO and CO2 reactive channels from IBE oxygenated intermediates to suppress the formation of soot precursors. Studies were also extended to examine the role of CO2 dilution on soot formation. It was shown that further soot reduction can be attained by lowering the rate of overall soot formation kinetics, perturbing reaction intermediates leading to soot formation by the depletion of H-atom radical pool, while enhancing the oxidation through enhanced OH-radical production.

Experimental measurements of soot formation in fuel-rich homogeneous mixtures using an optical rapid compression machine

Advanced combustion strategies are necessary for the use of more environmentally sustainable fuels than traditional diesel. Alcohol fuels and alcohol/gasoline blends are of particular interest as they are readily available in the marketplace. Heavy-duty engines typically use compression ignited, conventional diesel mixing controlled combustion. Mixing controlled combustion features a non-premixed diffusion flame with a wide range of local equivalence ratios, leading to potentially high rates of soot formation. This work studies the sooting behavior of iso-octane and ethanol as a function of equivalence ratio. Measurements are carried out in a rapid compression machine (RCM) and are reported for pre-ignition conditions of 10–30 bar and temperatures of 650–800 K. Theoretical equilibrium and bulk gas temperatures are calculated for both fuels. These data are used to identify the critical equivalence ratio, the lowest equivalence ratio where soot is detected with a single-pass laser extinction diagnostic. The critical equivalence ratio for iso-octane varies between 1.82 and 1.77 for compressed pressures of 10 and 20 bar, respectively. Ethanol, sometimes considered sootless, had a critical equivalence ratio between 2.37 and 2.12 for compressed pressures of 20 and 30 bar, respectively. When characterizing soot formation by oxygenated equivalence ratio, the critical equivalence ratios for ethanol approach those of iso-octane. This suggests the oxygenated nature of alcohol fuels reduces sooting tendency, but other factors such as fuel molecular structure and morphology may play a role. It was observed that ethanol will form soot at equivalence ratios only slightly higher than iso-octane, which could have implications in mixing controlled combustion. It was seen for both fuels that soot formation is pressure sensitive, with the critical equivalence ratio being inversely proportional to compressed pressure and the rate of soot formation. Future work will investigate the sooting behavior of gasoline/ethanol blends.

Particulate Matter Emissions in Gasoline Direct-Injection Spark-Ignition Engines: Sources, Fuel Dependency, and Quantities

This study investigates sources of particulate matter (PM) from a direct-injected spark-ignition (DISI) engine, all of which are likely the result of poor mixture formation, although through varying mechanisms. Nine unique gasoline formulations, including both oxygenated and non-oxygenated fuels, with varying distillation curves and concentrations of different hydrocarbon classes were employed to investigate the effect of fuel on soot formation. The engine experiments were further supported by reacting and non-reacting experiments in a spray-chamber.The metal-engine experimental results indicated that engine-out PM emissions of the nine fuels generally conforms to their sooting tendency, here quantified by Particulate Matter Index (PMI). Discrepancies with the engine-out PM emissions and PMI were observed under transient operating condition, which is not reflective of the test conditions used to formulate the PMI. A diisobutylene surrogate blend and a high-olefin full-boiling range fuel were investigated further in the optically-accessible engine, showing soot generation from different sources. These differences came from the variation in their spray characteristics, as confirmed in the non-reactive spray-chamber experiments.While the earlier tests were conducted with a fouled injector, the injector was later cleaned and fuels were tested in the optical engine. High olefin fuel and diisobutylene blend showed significantly lower soot emissions with clean injector. E30 fuel exhibited spray impingement and subsequent diffusive combustion, known as pool fire, on the piston top characterized by a yellow flame due to soot luminosity. A final round of experiments in the spray-chamber mimic this pool fire mechanism. A stoichiometric flame was initiated after a spray has impinged on a wall and formed a film. Results showed a large time delay between the flame passing over the fuel film and the formation of soot mass. The slow soot formation via pyrolysis in spray-chamber suggest that pyrolysis may not be a dominating soot-production pathway in SI engines, even for conditions with significant wall wetting. The findings of this study highlight the many considerations which influence the effective design of a combustion system when the minimization of PM emissions is a goal, and the ways in which fuel formulation can change soot-production pathways in DISI engines.

Sooting tendencies of ethylene diffusion flame doped by C[formula omitted]-C[formula omitted] alcohols

The sooting propensity of steady laminar coflow ethylene/air flames is modified doping the fuel stream with vapors of alcohol and evaluated as a function of both the position of the alcohol function and the ramification level (linear/branched) of the molecule. To identify general trends, a range of 3 alcohol sizes (C3, C4, and C5) are compared: the 2 propanol isomers (primary and secondary), the 4 butanol isomers (2 primaries with 1 branched, 1 secondary, 1 tertiary), and 7 pentanol isomers (3 primaries with 2 branched, 3 secondary with 1 branched and 1 tertiary). Every alcohol has been injected as a doping vapor in the central tube of a Santoro’s burner. To evaluate the sooting tendencies, a Line-of-Sight Attenuation (LOSA) setup allows the extinction of a collimated laser beam operating at 645 nm to be measured yielding the field of local soot volume fraction fv. Two indicators are used and contrasted, i.e. one using integrated data (raw data from the camera capturing the laser extinction) and one using the local field (deconvoluted data). From these results, several conclusions can be drawn. The two indicators being in good agreement, it seems unnecessary to obtain local data to compare sooting tendencies at different conditions (for a given experimental setup). The noise from the deconvolution procedure can be drastically reduced by a classical image processing described precisely in the paper, i.e. an average filter. These results reveal a good correlation between alkene produced in rich premixed C2H4 flame. The sooting tendencies are consistent with the literature: linear molecule < branched molecule and primary alcohol < secondary alcohol < tertiary alcohol. Moreover, the number of carbon NC, here varied from 3 to 5, leads to a modification of the sooting tendency that is significantly narrower than that associated with the structure of the alcohol at a given NC.

Optical study on multi-time ignition mixed-mode combustion with gasoline and PODE

In this study, multi-time ignition mixed mode combustion (MIMC) strategy is proposed to improve the dual fuel combustion, based on observation of the ignition and flame development in conventional dual fuel combustion. Gasoline and polyoxymethylene dimethyl ethers (PODE) serve as the premixed and pilot fuel through port fuel injection and direct injection (DI). As PODE have good auto-ignition property and extremely low sooting tendency, MIMC strategy can realize multi-time ignition in conditions which should be avoided by conventional dual fuel strategies. Firstly, the combustion of gasoline-PODE in typical single DI mode is investigated under different conditions. It is found that the ignition location in the intermediate region of radius causes the fastest combustion among the test cases. The independent auto-ignition events during the flame development are also discussed. Then, the idea of MIMC mode is proposed, considering both the spatial and temporal aspects of different batches of ignition. Different with the typical reaction-controlled compression ignition (RCCI) strategy, the MIMC strategy can directly controls the ignition time with the in-cylinder DI time, providing more flexibility for the dual fuel combustion. Optical and thermal data indicate that the MIMC strategy can fasten or slowdown the combustion by combining the control of ignition position and ignition time. When the locations of the ignition events are spatially alternated and their heat release are overlapped temporally, the combustion will be the fastest under the same DI mass.

On the sudden reversal of soot formation by oxygen addition in DME flames

Dimethyl ether (DME) is a non-toxic and renewable fuel known for its soot emissions reduction tendencies. In laminar co-flow DME diffusion flames, adding oxygen to the fuel stream increases the sooting tendency until a critical point is reached, at which point the trend suddenly reverses. This work unravels the mechanisms behind this reversal process, and characterizes their contribution to controlling soot production. A series of experimental measurements using diffuse-light line-of-sight attenuation and two-colour pyrometry were performed to measure soot volume fraction and soot temperature considering a fixed mass flow rate of DME and variable addition of oxygen. Soot volume fraction increases from 0.095 ppm in the pure DME flame to 0.32 ppm when the added oxygen concentration reaches 33%. When the oxygen concentration is slightly increased to 35%, soot volume fraction is reduced by 60%. To explain the reasons behind the reversal, a series of numerical simulations were performed, which successfully demonstrated the same trend. Results show that the chemical effects of adding oxygen to the fuel stream are exceedingly more important than the thermal and dilution effects. It was found that the reversal occurred when nearly all DME disassociated before exiting the fuel tube, indicating a sudden transition from a partially premixed DME flame, to one which primarily burns C1 fuel fragments. An analysis of soot formation and oxidation rates showed that near the reversal, soot inception is the least affected process; furthermore, soot precursor availability is not significantly affected in magnitude, rather they appear further upstream. It is concluded that the favourable conditions for rapid DME decomposition into soot precursors enhances soot inception while depleting the necessary species for further soot mass growth, dramatically reducing soot concentration.

Sensitivity of soot formation to strain rates in counterflow diffusion flames of various C3-C5 alkanes and alcohols

In this work, the sensitivities of soot formation to the variations of strain rates have been studied both experimentally and numerically in counterflow diffusion flames of fourteen C3-C5 alkanes and alcohols. The results showed that an increase in strain rate would lead to a decrease of soot volume fraction in all the tested flames. Except for tert-butanol and tert-pentanol, the sensitivity of soot volume fraction to strain rate was observed to be lower for fuels with a higher sooting tendency. When the soot loadings in flames of the different fuels were matched at a reference strain rate by adjusting oxygen mole fraction, their dependence on strain rate became comparable, except that there was a more substantial reduction in soot volume fraction for tert-butanol and tert-pentanol flames. Kinetic pathway analysis found that the main reason for the faster decline of soot loading in tert-butanol flame has to with the unique molecular structure of tert-alcohols which led to a unique pathway for the formation of molecular soot precursors. The present work served as a first test in alcoholic fuels of the general trend that was previously observed in experiments with aliphatic fuels regarding the sensitivities of soot formation to strain rate. The present data identified tert-alcohols as outliers; corresponding numerical simulation helped to explain the observed peculiarities in flames of tert-alcohols.

Sooting transition diagnostics in counter-flow flames of C4 isomer fuels

Macroscopic transition processes of soot formation for 1-butene, isobutene, n-butane and isobutane counterflow diffusion flames from sooting-free to heavy-sooting were studied by varying fuel concentration (XF) in the fuel stream. A novel optical diagnostic algorithm was proposed to classify different sooting transition stages in flames. Effects of saturated/unsaturated bonds and linear/branched structures of fuel molecules on the sooting transition were investigated in details. Flame species were detected and quantified by on-line gas chromatography and also kinetic analysis was performed. KL factor was obtained by optical diagnostics to qualitatively compare soot concentrations. Results showed that XF required for 1-butene flame to reach the critical state of macroscopic sooting transition was the lowest, followed by isobutene, isobutane and n-butane flames. Fuels with higher sooting tendencies could achieve entire sooting transition process with a narrower range of XF. Linear butane was more difficult to achieve the sooting transition than branched butane, while linear 1-butene was easier to form soot than branched butene, reflecting the difference between isomeric structures acting on C4 alkenes and alkanes. These might be largely due to the close association of allyl (C3H5-A) with 1-butene. Higher C3 species content in isobutane flame made it have higher sooting tendency than n-butane.

Sooting tendencies of terpenes and hydrogenated terpenes as sustainable transportation biofuels

Terpenes are a diverse group of molecules that are synthesized by plants and microorganisms through combining units of isoprene (2-methyl-1,3-butadiene). They typically contain rings and methyl branches, which gives them high energy densities and low freezing points and makes them appealing candidates for sustainable transportation biofuels. Between the original biosynthesis and upgrading options such as hydrogenation, they have a large degree of freedom of structures, e.g., different carbon skeletons, positions of double bonds, and functional groups. Therefore, structure-property data is needed to downselect potential fuel candidates. Here, we measured the sooting tendencies of 17 C10 monoterpenes and 7 of their hydrogenated analogues. The hydrogenated compounds were custom synthesized, so the quantities were too small for conventional smoke point measurements. Thus, the sooting tendencies were quantified with yield sooting index (YSI), which is based on the soot yield in a fuel-doped non-premixed methane flame. Derived smoke points (DSPs) were estimated from a correlation between YSI and smoke point for other hydrocarbons. The YSI of terpenes and their derivatives varies widely from 85.6 to 248.5. The YSI follows the trend: terpenes > dihydroterpenes > tetrahydroterpenes. The DSPs of all the tetrahydroterpenes and some dihydroterpenes are higher than that of a Jet-A fuel sample, suggesting that they offer soot reduction benefits. The YSIs depend strongly on molecular structure; for example, α-pinene and β-pinene have identical carbon skeletons and differ only in the position of one carbon-carbon double bond, but the YSI of α-pinene is 34% higher than that of β-pinene. Detailed decomposition analysis via density functional theory (DFT) suggests that compared with β-pinene, α-pinene requires fewer steps to form the first aromatic ring and the process is more thermodynamically favorable. The YSI difference between the pinenes is mainly affected by the identity of the products from the dominant decomposition pathways.

A comparative study on soot and PAH formation of C10 naphthenic ring-containing species in laminar coflow diffusion flames

C10 naphthenic ring-containing species are important components in transportation fuels and have been widely used for surrogate formulation of diesel and jet fuels to represent cycloalkanes or aromatic fractions. In this study, experimental and modeling studies were carried out to compare the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with n-butylcyclohexane, decalin and tetralin. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were applied to measure 2D maps of soot volume fractions and relative PAH concentrations, respectively. Yield sooting index (YSI) was obtained experimentally to indicate the fuel sooting tendency. It can be observed that the soot volume fractions, sooting tendencies and pyrene concentrations of C10 naphthenic ring-containing species follow the order of tetralin > decalin > n-butylcyclohexane. The chemical kinetic simulation reveals that decalin decomposes through two main pathways, including the H-atom abstraction route and ring opening route. Especially, the pathway of ring opening reactions coupled with dehydrogenation results in the strongest benzene formation process among the test components. In the reacting system of tetralin flames, ring opening reactions are suppressed, while the dominant decomposition way of tetralin is via H-atom abstraction reactions, which offer a more direct pathway to form naphthalene and indene without benzene formation, leading to the highest naphthalene concentration of tetralin flame and the strongest soot formation process among all the test components. This fundamental investigation aims to provide valuable data on sooting behaviors and PAH formation characteristics of typical C10 naphthenic ring-containing species and get insight into the kinetic pathways from fuels to soot precursors. Useful information has been obtained for soot reduction from the perspective of the fuel design and surrogate formulation in terms of the sooting tendency.