Higher Sooting Tendency Research Articles

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.

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.

Unraveling chemical structure of laminar premixed tetralin flames at low pressure with photoionization mass spectrometry and kinetic modeling

AbstractThis work reports an investigation on laminar premixed flames of tetralin at 30 Torr and equivalence ratios of 0.7 and 1.7. Measurements of the chemical structure including identification and mole fraction measurements of free radicals, isomers, and polycyclic aromatic hydrocarbons (PAHs) were performed using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV‐PIMS). A kinetic model with 296 species and 1 577 reactions was developed and validated against the flame chemical structure data measured in this work. Modeling analysis reveals the key reaction pathways in tetralin decomposition and PAHs formation. The H‐atom abstraction reactions by H, O, and OH are found to control the consumption of tetralin in the lean flame, while those by H play the dominant role in the rich flame. Indene and naphthalene have very high concentration levels in the rich tetralin flame due to the existence of direct formation pathways from the decomposition of tetralin. The two bicyclic PAHs and their radicals play significant roles in the PAHs growth process of tetralin combustion, which results in the high sooting tendency of tetralin compared to those of alkylbenzenes with smaller or same carbon atom numbers.

Experimental flat flame study of monoterpenes: Insights into the combustion kinetics of α-pinene, β-pinene, and myrcene

Pinenes and pinene dimers have similar energy densities to petroleum-based fuels and are regarded as alternative fuels. The pyrolysis of the pinenes is well studied, but information on their combustion kinetics is limited. Three stoichiometric, flat premixed flames of the C10H16 monoterpenes α-pinene, β-pinene, and myrcene were investigated by synchrotron-based photoionization molecular-beam mass spectrometry (PI-MBMS) at the Advanced Light Source (ALS). This technique allows isomer-resolved identification and quantification of the flame species formed during the combustion process. Flame-sampling molecular-beam mass spectrometry even enables the detection of very reactive radical species. Myrcene was chosen because of its known formation during β-pinene pyrolysis. The quantitative speciation data and the discussed decomposition steps of the fuels provide important information for the development of future chemical kinetic reaction mechanisms concerning pinene combustion. The main decomposition of myrcene starts with the unimolecular cleavage of the carbon-carbon single bond between the two allylic carbon atoms. Further decompositions by β-scission to stable combustion intermediates such as isoprene (C5H8), 1,2,3-butatriene (C4H4) or allene (aC3H4) are consistent with the observed species pool. Concentrations of all detected hydrocarbons in the β-pinene flame are closer to the myrcene flame than to the α-pinene flame. These similarities and the direct identification of myrcene by its photoionization efficiency spectrum during β-pinene combustion indicate that β-pinene undergoes isomerization to myrcene under the studied flame conditions. Aromatic species such as phenylacetylene (C8H6), styrene (C8H8), xylenes (C8H10), and indene (C9H8) could be clearly identified and have higher concentrations in the α-pinene flame. Consequently, a higher sooting tendency can generally be expected for this monoterpene. The presented quantitative speciation data of flat premixed flames of the three monoterpenes α-pinene, β-pinene, and myrcene give insights into their combustion kinetics.

ReaxFF-based molecular dynamics study of bio-derived polycyclic alkanes as potential alternative jet fuels

This work investigates the initial stages of the pyrolysis of HtH-1 (C18H32; 2,2,7,7,8a,8b-hexamethyl-dodecahydrobiphenylene) and HtH-2 (C18H34; 1,1′,3,3,3′,3′-hexamethyl-1,1′-bi(cyclohexane)), which are bio-derived polycyclic alkanes and potential jet fuels, using ReaxFF force field based molecular dynamics (MD) simulations. Global Arrhenius parameters, such as activation energies and pre-exponential factors, are calculated and used to analyze the overall decomposition kinetics of the fuels. HtH-1 decomposes faster than HtH-2 at the same temperature and density conditions, and they have a faster decomposition rate compared to some existing jet-fuels, such as JP-10. A systematic reaction analysis framework developed in this work is applied to determine a temperature-dependent decomposition mechanism. At lower temperature, the central C–C bond connecting the two cyclohexane rings is dominantly broken in both HtH-1 and HtH-2. However, C-CH3 bond breaking becomes dominant with increasing temperature due to the large increase in entropy during this reaction. Major products from HtH-1 are C5H8 and C4H8, and those from HtH-2 are C4H8 and C2H4. The major products predict that HtH-1 has a higher sooting tendency than HtH-2, which is consistent with measurements. The impact of HtH-2 on the pyrolysis of HtH-1 is also investigated in their binary mixtures. HtH-1 and HtH-2 decompose by unimolecular reactions, and they rarely interact with each other during the pyrolysis of the mixtures. This work demonstrates that ReaxFF can be used to investigate pyrolysis and combustion chemistry of existing or future fuels and to contribute to the development of their chemical kinetic models without any a priori input and chemical intuition.

Synergistic effects on soot formation in counterflow diffusion flames of acetylene-based binary mixture fuels

This study reports experimental data regarding the effects of the addition of propene and propane in the fuel stream on soot formation in acetylene counterflow diffusion flames. Compared to the baseline acetylene flame, a continuous reduction of soot volume fraction was observed as more propene was mixed in the fuel. However, the addition of 1% propane resulted in an enhancement of soot formation, exhibiting an interesting fuel synergistic effect. The propane-doped flames were seen to have a higher sooting tendency compared to the propene-doped one when the molar doping ratio was less than 5%. These experimental results were rather unexpected considering that propene has a much higher sooting tendency than propane. Through a kinetic simulation accounting for the formation of molecular soot precursors, it is shown that the previous CH3-dominated reaction routes in forming propargyl radicals, which were typically used to rationalize the synergistic effect of ethylene-based binary fuel mixtures, were insufficient to explain the present observation of propane-doped acetylene flame having high sooting tendency than the propene-doped one at low doping ratios. The role of H radicals in the formation of C4 species (via the paths C2H2 + H → C2H3 and C2H3 + C2H2 → n-C4H5) was found to be critical in understanding the sooting behavior of C2H2 flames.

Development of a Diesel Surrogate Fuel Library

Diesel fuel is composed of a complex mixture of hundreds of hydrocarbons that vary globally depending on crude oil sources, refining processes, legislative requirements and other factors. In order to simplify the study of this fuel, researchers create surrogate fuels to mimic the physical and chemical properties of Diesel fuels. This work employed the commercial software Reaction Workbench – Surrogate Blend Optimizer (SBO) to develop a Surrogate Fuel Library containing 18 fuels. Within the fuel library, the cetane number ranges from 35 to 60 (in increments of 5) at threshold soot index (TSI) levels representative of low, baseline and high sooting tendency fuels (TSI = 17, 31 and 48, respectively). The Surrogate Fuel Library provides the component blend ratios and predicted properties for cetane number, threshold soot index, lower heating value, density, kinematic viscosity, molar hydrogen-to-carbon ratio and distillation curve temperatures from T10 to T90. A market petroleum Diesel fuel with a cetane number of 50 and a threshold soot index of 31 was selected as the Baseline Diesel Fuel. The combustion, physical and chemical properties of the Baseline Diesel Fuel were precisely matched by the Baseline Surrogate Fuel. To validate the SBO predicted fuel properties, a set of five surrogate fuels, deviating in cetane number and threshold soot index, were blended and examined with ASTM tests. Good agreement was obtained between the SBO predicted and ASTM measured fuel properties. To further validate the Surrogate Fuel Library, key properties that were effected by altering the component blend ratios to control cetane number and TSI were compared to a set of five market Diesel fuels with good results. These properties included density, viscosity, energy density and the T10 and T90 distillation temperatures. The Surrogate Fuel Library provided by this work supplies Diesel engine researchers and designers the ability to analytically and experimentally vary fuel cetane number and threshold soot index with fully-representative surrogate fuels. This new capability to independently vary cetane number and threshold soot index provides a means to further enhance the understanding of Diesel combustion and design future combustion systems that improve efficiency and emissions.

Combustion and emission performance of CO2/CH4/biodiesel and CO2/CH4/diesel blends in a Swirl Burner Generator

Renewable biomass derived fuels are of increasing attention for industrial and aerospace applications due to worldwide depletion of fossil fuels and stricter environmental legislations. These facts have prompted continuous development for clean, sustainable and alternative fuels that produce low emissions. Even more, fuel flexibility is a required feature to meet all the former characteristics while reducing operating cost in gas turbines. Thus, some alternative fuels such as syngas or biodiesel can be used for gas turbines as these can comply with these requirements while being obtained from various processes, making them potential candidates for sustainable power generation. On the other hand in many combustion applications, the fuel is originally present as either liquid or solid. To assist mixing and the overall burning rate, the fuel is frequently first atomised and then sprayed into the combustion chamber. Most of the existing approaches dealing with combustion flows are limited to single-phase injection. To remove this limit, a new model for multiphase combustion has been developed. Therefore, this experimental work investigated the performance of a swirl burner using various mixtures of CO2/CH4 blends with either diesel or biodiesel derived from cooking oil. A 20 kW swirl burner was employed to analyse gas turbine combustion features under atmospheric conditions to quantify flame stability and emissions by using these fuels. A TESTO 350XL gas analyser was used to determine NOx and CO emission trends. Comparison between the blends was carried out at different equivalence ratios. CH* chemiluminescence diagnostics was also used and linked with the levels of emissions created through the trials. The results revealed that the use of biodiesel and CO2/CH4 blends mixtures resulted in lower CO production, i.e. 87% lower for the case at 10% CO2. Results showed that a notable reduction of ~50% in NOx was obtained at all conditions for the biodiesel /CO2/CH4 blends. Diesel based flames showed high CH* intensity at the axial profile compared to the biodiesel blends due to their high sooting tendency.

On the instability of a modified cup-burner flame in the infrared spectral region

Summary This study describes the modification of a standardised cup-burner apparatus. The replacement of the original glass chimney is performed by shielding a nitrogen co-flow enabled measurement at a wavelength of 3.9 μm. This modification, together with a special arrangement of the measuring system (spectral filtering, data acquisition and post-processing), permitted the observation of various types of hydrodynamic instabilities, including transition states. The advantages of our arrangement are demonstrated with an ethylene non-premixed flame with high sooting tendency. Two known modes of hydrodynamic instability (varicose and sinuous) that occur in buoyant flames were studied and described quantitatively. Based on the intensity of the infrared emissions, we identified and qualitatively described the modes of periodic hydrodynamic instability that are accompanied by flame tip opening, which has not been observed for this type of flame.

Experimental and kinetic modeling study of styrene combustion

The aim of this work is to report new experimental data on styrene combustion and develop a kinetic model for styrene combustion. Three laminar premixed flames of styrene with equivalence ratios of 0.75, 1.00 and 1.70 were studied at low pressure (0.0395atm) using synchrotron vacuum ultraviolet photoionization mass spectrometry. The JSR oxidation of styrene/benzene mixtures with three equivalence ratios of 0.50, 1.00, and 1.50 was studied at atmospheric pressure using gas chromatography. The mole fraction profiles of flame species and oxidation species were measured. A kinetic model of styrene combustion with 290 species and 1786 reactions was developed from our recently reported toluene model, and was validated on the new experimental data from this work and previous ignition delay time data. The O-atom attack reactions, H-atom abstraction reactions and other H-atom attack reactions were found to dominate the consumption of styrene. In particular, the O-atom attack reactions on the double bond of styrene play an important role in the JSR oxidation and the lean and stoichiometric flames, while the pyrolytic reactions have increasing contributions as the equivalence ratio increases. The phenyl radical plays a crucial role in the decomposition of styrene since it affords most of the reaction fluxes from styrene. In the aromatics growth process, phenylacetylene, the 1-phenylvinyl radical, the phenyl radical, and styrene serve as major precursors of polycyclic aromatic hydrocarbons (PAHs), leading to the high sooting tendency of styrene compared to toluene and ethylbenzene.

Experimental and kinetic modeling study of 2,5-dimethylfuran pyrolysis at various pressures

The pyrolysis of 2,5-dimethylfuran (DMF) in a flow reactor was investigated at various pressures (30, 150 and 760Torr) by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Dozens of pyrolysis products, especially a series of radicals and aromatics, were identified from the measurement of photoionization efficiency spectra; and their mole fraction profiles were measured at 780–1470K. Phenol, 1,3-cyclopentadiene, 2-methylfuran, vinylacetylene and 1,3-butadiene were observed with high concentrations in the decomposition of DMF. The pressure-dependent rate constants of the major unimolecular decomposition reactions of DMF were theoretically calculated, and was adopted in the pyrolysis model of DMF with 285 species and 1173 reactions developed in the present work. The model was validated against the species profiles measured in both the present work and the previous pyrolysis studies of DMF. Based on the rate of production and sensitivity analyses, main pathways in the decomposition of DMF and the growth of aromatics were determined. The unimolecular decomposition to produce CH3CHCCH and acetyl radicals, H-atom abstraction to produce 5-methyl-2-furanylmethyl radical, ipso substitution by H-atom to produce 2-methylfuran and H-atom attack to produce 1,3-butadiene and acetyl radical were concluded to dominate the primary decomposition of DMF. Further decomposition of 5-methyl-2-furanylmethyl radical leads to great production of phenol and 1,3-cyclopentadiene which can be readily converted to precursors of large aromatics such as cyclopentadienyl radical, phenyl radical and benzene. As a result, the formation of aromatics in the pyrolysis of DMF is promoted compared with the pyrolysis of cyclohexane and methylcyclohexane under very close conditions. This observation implies the potentially high sooting tendency of DMF, and emphasizes the necessity to investigate the sooting behavior and soot formation mechanism in DMF combustion for the potential application of DMF as an alternative engine fuel.

Experimental and kinetic modeling study of tetralin pyrolysis at low pressure

The pyrolysis of tetralin was studied from 850 to 1500K in an electrically heated laminar flow reactor at 30Torr. Synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry was used for isomeric identification and mole fraction measurements of pyrolysis products, especially free radicals. A kinetic model with 149 species and 554 reactions was developed in this work and validated by measured mole fraction profiles of pyrolysis species. Rate of production (ROP) analysis and sensitivity analysis were performed for mechanistic analysis of tetralin decomposition and aromatic growth processes. Contributions of four overall decomposition pathways of tetralin proposed in previous high-pressure pyrolysis studies were evaluated at the low-pressure condition based on both experimental observations and ROP analysis. It is concluded that tetralin mainly decomposes to dihydronaphthalenes, naphthalene, indene, indenyl radical and styrene via unimolecular decomposition reactions and H-abstraction reactions in low-pressure pyrolysis. Special modeling efforts on the formation pathways of indene were made to explain its high concentration in tetralin pyrolysis. Because of the high concentration levels in the pyrolysis of tetralin, indenyl radical and naphthalene play significant roles in the formation of large polycyclic aromatic hydrocarbons (PAHs), which explains the high sooting tendency of tetralin compared with alkylbenzenes.

Sooting of Propylene-Hydrogen Mixture Diffusion Flames with Diluents Near Smoke Point

Diluents such as steam and inert gases are used to suppress soot formation in hydrocarbon diffusion flames. The curve of the variation of the flow rate of diluent required to eliminate smoke emission from these flames with the fuel flow rate exhibits a skewed bell shape with two distinct regions. In the first region, the diluent flow rate increases with the fuel flow rate, and in the second region the trend is reversed. In a previous study these two regions were deemed to be controlled by chemical kinetics and jet momentum respectively. This study was performed to understand the dominant factors that control soot concentration in different parts of the flame in these two regions. Propylene was used as the fuel because of its high sooting tendency. The flames were always attached to the burner to avoid the complexities caused by flame liftoff. Oxygen concentration was measured by direct in-flame gas sampling. Soot and hydroxyl radical concentrations were measured using laser-inducedincandescence and laser-induced-fluorescence techniques, respectively. Results show that hydroxyl radicals play a stronger role than oxygen in both near-burner and far-burner regions of the flame in chemical-controlled region. The influence of hydroxyl radicals is significant in the near-burner region of the flame in momentum controlled region; however, the effect of oxygen is dominant in the far-burner region.