Soot Threshold Research Articles

Soot formation of renewable gasoline: From fuel chemistry to particulate emissions from engines

Renewable fuels are essential to meet climate protection goals, especially for decarbonizing the existing vehicle fleet. In addition, optimized and specially designed fuels allow emissions to be avoided before they occur. This paper presents fuel effects on emission characteristics of six chemically complex gasoline blends compliant with the DIN EN 228 standard and one ethanol/gasoline blend, focusing on particle emissions. First, the sooting propensity of the fuels was examined using different soot indices, such as the Particulate Matter Index (PMI), the Oxygen Extended Sooting Index (OESI), the Yield Sooting Index (YSI), and the soot threshold (φST), which is drawn on measurements of the particle number (PN) density in the exhaust gas of a laminar flat flame. Second, PN emissions (particle number concentration) from a production-calibrated turbo gasoline direct injection engine with four cylinders were investigated. Third, soot precursor chemistry was investigated using a laminar flow reactor at atmospheric pressure. Finally, correlations to fuels’ volatility and chemical characteristics were made for the complete data set. Based on the soot precursor chemistry in the homogeneous gas phase reaction, the analysis shows that engine PN emissions are primarily influenced by fuel chemistry for the investigated engine conditions. In particular, the composition of the hydrocarbon fraction is decisive, with the heavy C9+ aromatics and the upper distillate significantly influencing soot formation. In order to reduce PN engine emissions, future fuels should contain fewer soot-increasing hydrocarbon fractions.

Partially Premixed Combustion Application for Diesel Power Improvement

A partially premixed combustion (PPC) approach was applied in a single cylinder diesel research engine in order to characterize engine power improvements. PPC is an alternative advanced combustion approach that generally results in lower engine-out soot and oxides of nitrogen (NOx) emission, with a moderate penalty in engine-out unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. In this study, PPC is accomplished with a minority fraction of jet fuel injected into the intake manifold, while the majority fraction of jet fuel is delivered directly to the combustion chamber near the start of combustion (SOC). Four compression ratios (CR) were studied. Exhaust emissions plus exhaust opacity and particulate measurements were performed during the experiments in addition to fast in-cylinder combustion metrics. It was seen that as CR increased, the soot threshold equivalence ratio decreased for conventional diesel combustion; however, this afforded an increased opportunity for higher levels of port injected fuel leading to power increases from 5% to 23% as CR increased from 14 to 21.5. PPC allowed for these power increases (defined by a threshold opacity level of 3%) due to smaller particles (and lower overall number of particles) in the exhaust that influence measured opacity less significantly than larger and more numerous conventional diesel combustion exhaust particulates. Carbon monoxide levels at the higher PPC-driven power levels were only modestly higher, although NOx was generally lower due to the overall enriched operation.

Coagulation of combustion generated nanoparticles in low and intermediate temperature regimes: An experimental study

Coagulation of combustion-generated particles has been investigated in low and intermediate temperature regimes in a tubular reactor with a residence time of 1.65s. Particles, generated by premixed ethylene/air flames with equivalence ratios above the soot threshold limit, are fed to a tubular reactor, which can be operated at temperatures up to 650K. A wide range of equivalence ratios are used to generate particles with different characteristics. The evolution of the particle size distributions has been evaluated by a differential mobility analyzer with high sensitivity in the 2–100nm size range. The effect of the reactor temperature on coagulation has been systematically studied. Particles exhibit different coagulation efficiencies at the different temperatures. At room temperature, 2–4nm particles fed to the reactor coagulate forming particles as large as 10–20nm, whereas at higher temperatures the size distribution of the particles does not change with respect to that measured at the inlet of the reactor. This behavior suggests a very ineffective coagulation efficiency at higher temperatures for small nanoparticles. Larger particles do not exhibit this high sensitivity to temperature, substantially maintaining very high coagulation efficiencies. These considerations have been confirmed by numerical simulations conducted both with constant and size-dependent coagulation efficiency. The numerical results confirm that also at low and intermediate temperature regimes, the use of a size-dependent coagulation efficiency is mandatory to match the evolution of the particles during coagulation. On the other hand, the simple model of coagulation based on the van der Waals interactions between particles in the framework of gas kinetic collision theory is in slightly disagreement with the experimental results for very small particles, suggesting that more advanced modeling based on quantum mechanism and molecular dynamics are necessary to correctly reproduce the experimental data.

Surface enhanced Raman spectroscopy (SERS) of particles produced in premixed flame across soot threshold

Surface enhanced Raman scattering (SERS) has been employed for the first time to characterize particles produced in flames across the soot inception.The strong enhancement of Raman signal produced by SERS enabled the measurement of the Raman spectra of size-selected particles collected by a dilution probe-DMA system and deposited by electrophoresis in very low concentrations on SERS substrates. It also enables investigation of second order 2D Raman bands in soot and soot precursors, providing complementary information to the first order Raman bands.Raman spectra, together with the measurement of particle size distribution by DMA, help to characterize particles produced across the soot threshold limit. Three particle modes have been identified: flames with C/O<0.65 produce only modeI particles with a diameter of d≈2nm, which present the typical bands of amorphous carbon and have a very low, if any, three-dimensional ordering of graphitic planes. Other aromatic macromolecules smaller than the lower detection limit of the DMA (d<1.5nm) are present in the flame, and are collected by thermophoretic sampling. In flames with higher C/O ratio the size distribution becomes bimodal with the formation of modeII particles, d≈4nm. In non-sooting flames, the SERS spectra of modeI and modeII particles are similar except for features indicating the presence of a higher degree of disorder in modeII, likely due to distorted or five order rings.In soot-forming flames, larger modeIII particles collected with a diameter of 40nm present a higher ordering and a reduction of graphitic interplane distance, which, nevertheless, still remains larger than 3.50Ǻ. Two types of particles were observed in smaller modeII particles collected from sooting flame conditions: one with Raman spectra similar to the modeIII particles and the other with spectra similar to particles collected from leaner non-sooting flames.

A reaction pathway for nanoparticle formation in rich premixed flames

Aromatics growth beyond 2-, 3-ring PAH is analyzed through a radical-molecule reaction mechanism which, in combination with a previously developed PAH model, is able to predict the size distribution of aromatic structures formed in rich premixed flames of ethylene at atmospheric pressure with C/O ratios across the soot threshold limit. Modeling results are in good agreement with experimental data and are used to interpret the ultraviolet absorption and the light scattering measured in flames before soot inception. The model shows that the total number concentration of high molecular mass aromatics and the different moments of the size distribution are functions of both the PAH and H-atom concentrations, two quantities which have different trends as functions of the residence time and the C/O ratio. Regimes of nearly stoichiometric or slightly rich premixed combustion are dominated by reactions between aromatics which lead to the formation of particles with sizes of the order of 3 to 4 nm. At higher C/O ratios the formation of nanoparticles is less efficient. Particles with sizes of the order of 2 nm are predicted in flames at the threshold of soot formation, whereas particles with sizes around 1 to 1.5 nm are predicted in fully sooting conditions.

Detailed kinetic modeling of soot formation in ethylene/air mixtures reacting in a perfectly stirred reactor

A series of perfectly stirred reactor (PSR) calculations has been performed that allow the comparison of numerical predictions for sooting trends in PSRs with those observed in experiments. A detailed chemical kinetics model for soot production is employed that has been extensively tested for laminar flames. The chemical kinetics model is used without any adjustments. The PSR code Aurora that is a CHEMKIN III application has been modified considerably to incorporate the method of moments. Gas-phase and surface reactions are rigorously coupled in the model. Predicted sooting characteristics are consistent with behavior observed experimentally for premixed laminar flames and well-stirred reactors. Specifically, soot volume fraction exhibited the same qualitative behavior with equivalence ratio, temperature, and pressure as observed for flames. Concentration profile shapes were consistent with profile shapes measured for flames as well. The model also predicts soot threshold values and the relationship of soot volume fraction with temperature and equivalence ratio that has been observed experimentally for well-stirred reactors. Under the conditions of the present investigation, soot formation dynamics was dominated by nucleation. The temperature dependence of surface reaction rates revealed the importance of the HACA mechanism (H-abstraction-C2H2 addition reaction sequence) in surface growth. Under PSR conditions, coupling the gasphase and surface chemistry is significant with the degree of coupling increasing with pressure.

The effect of temperature on soot inception in premixed ethylene flames

The temperature dependence of soot and related hydrocarbon formation was studied in fuel-rich, premixed C 2 H 4 /O 2 flames by changing the flame temperature but keeping a constant C/O ratio. Three flames with temperature levels between the low-and high-temperature soot threshold of the bell-shaped domain of soot formation were analyzed. In addition to the formation of pyrolytic species such as acetylene, benzene, polycyclin aromatic compounds (PAHs), and soot, the large formation of a tarlike material was revealed in the soot preinception region of the flames. The chemical analysis and spectroscopic characterization of these species showed that tar is made up of high-molecular-weight compounds in which aliphatic and aromatic functionalities coexist. The decrease in the concentration and the aromatic character of tar was found to correspond with soot inception, indicating that it could be responsible for soot nucleation. PAH molecules indirectly participate in soot inception, providing the building blocks of tar and/or participating in further soot surface growth. The influence of temperature on soot formation was found to be different at the extremes of the bell-shaped domain of soot. In the flame near the low-temperature soot threshold, benzene, PAHs and pyrolytic species are formed in larger amounts, but low temperatures hinder the reactive coagulation of PAH molecules and aliphatic radicals into tarlike material, hindering, in turn, soot production. The buildup of tarlike material and the process of aromatization occur too slowly to allow the massive soot inception if the flame temperature is not high enough. At higher temperatures, oxidation is very active in the secondary oxidation zone of the flame, thus destroying PAHs that are not then available for formation of tarlike species. Consequently, soot inception is reduced. In the intermediate-temperature flame, where maximum soot formation was found, the temperature level is ideal for the formation of both PAHs and tar and their transformation to soot particles.

The optical characterization of high molecular mass carbonaceous structures produced in premixed laminar flames across the soot threshold limit

The consistence of the dark material in interstellar space is still under discussion. Reasonable candidates are structures containing silicon or carbon. In that light it is interesting to know the physical and chemical properties of carbonaceous structures produced in flames and to compare them with the spectroscopical properties of interstellar dust. Light scattering, fluorescence induced by UV laser and light absorption have been employed to analyse phenomenological aspects of formation; destruction and chemical transformation of carbonaceous structures formed in rich premixed Laminar methane-oxygen and ethylene-oxygen flames at atmospheric pressure with different C/O ratio, at and above the soot threshold limit. Light scattering measurements also show that in non-sooting conditions, i.e. in the non-sooting zone of sooting flames or in flames below the soot threshold limit, high molecular mass structures are formed. Absorption spectra measured in non-sooting conditions show a continuous decay from 200 to 300 Dill, while in fully sooting conditions an additional peak at around 235 nm appears. Further information is obtained by laser-induced fluorescence investigations. Here. a broad-band peak with a maximum at around 320 nm is always measurable. Approaching the soot nucleation zone a further broad-band fluorescence emission in the visible. in addition to that in the UV, is observed. All the experimental results produce evidence that the early formation of high molecular mass carbonaceous molecules containing aromatic functionalities with not more than two rings occurs. Their role as soot precursors seems to be connected to their change into higher polycondensed aromatic structures. Our results give a hint that these soot precursors and the interstellar dust are members of the same family of carbonaceous structures.

Optical and Spectroscopic Characterization of Rich Premixed Flames across the Soot Formation Threshold

ABSTRACT Phenomenological aspects of formation, destruction, and coagulation of high molecular mass structures formed in the main oxidation zone of rich premixed flames and in rich flames well below the soot threshold limit have been examined. High molecular mass structures transparent to the visible radiation, previously detected in the preinception region of soot forming flames, are also present in flames below the soot formation limit. The onset of ultra-violet fluorescence within the main oxidation zone implies that the formation of these species is a very fast process and can be considered as a “polymerization” of small aromatic groups activated by the presence of oxidizing agents. The final concentration of this material falls down as the C/O ratio is decreased and below C/O = 0.35 it is not anymore present. It appears that rich premixed flames present two critical C/O ratios: a first one for soot formation and a lower second one for high molecular mass structure formation. Ultra-violet scattering/e...

Optical characterization of rich premixed CH4/O2 flamesacross the soot formation threshold

Light scattering, fluorescence, and C2 emission induced by an ultraviolet laser source and uv-visibleabsorption have been used to study the phenomenological aspects of formation, destruction, and chemical transformation of high molecular mass structures formed in rich premixed methane flames at atmospheric pressure. High molecular mass structures are formed in CH4/O2 flames with C/O ratios down to 0.35. This value, corresponding to their formation threshold, is noticeably lower than the soot threshold limit (C/O=0.45). Different flames have been analyzed across the soot threshold limit, varying the C/O ratio and the maximum flame temperature, and no dramatic changes in the properties of the high molecular mass structures have been found in the passage from nonsooting to sooting conditions. Their role as soot precursors seems to be connected to a kinetically controlled passage from aromatic functionalities with one or two rings to higher polycondensed aromatic functionalities more than to a carbonization process of these structures. High molecular mass structures are easily photofragmented by an ultraviolet laser source with high-energypulses, and the consequent C2 emission has been attributed to these structures more than to soot particles. From a practical point of view, the spatial distribution of these potential pollutants, whose toxic properties are largely unknown, can be easily followed in practical diffusion flames by monitoring the 2D fluorescence due to C2 photofragments employing an ultraviolet laser source.

34 O 04 Two classes of aerosols detected in rich methane/oxygen premixed flames above and below the soot threshold limit

34 O 04 Two classes of aerosols detected in rich methane/oxygen premixed flames above and below the soot threshold limit

The influence of pressure and temperature on soot formation in premixed flames

The formation of soot has been investigated in flat C2H4-air and C6H6-air flames, with special reference to low flame temperatures. Measurements of the threshold for soot formation show that as well as the “high temperature threshold” (slightly temperature dependent) there exists a low temperature threshold. For a given C/O ratio the mass of soot formed passes through a maximum between low and high temperature threshold at a temperature near 1600 to 1650 K. These soot yield curves are similar to those obtained in shock wave pyrolysis of benzene etc. by Graham, Frenklach et al. With increasing pressure the soot thresholds widen slightly. Their minima shift towards lower C/O ratio and the maxima of the soot yield curves rise. For the pressure dependence of the soot volume fraction finally formed, a P2 dependence has been found at constant flame temperature for flame temperatures above 1650 K and C/O ratios from 0.65 to 0.75. For benzene-air flames the pressure dependence is almost the same as for C2H4-air flames. The dependence of soot yield on the C/O ratio at constant flame temperature is a little weaker than the data obtained for constant flow velocity. Measurements of light absorption and light scattering along the flame axis show that coagulation and mass growth of the soot particles in the low temperature flames proceed in a similar way to that in flames at temperatures above 1600 K.

Integral Carbon Mass Balance in Premixed Sooting Flames

AbstractIn sooting C2H4‐air, C6H6‐air and C2H2‐air flames the burned gases have been analyzed for different fuel‐air ratios and the fractions of carbon which end up as CO and CO2, as soot and as hydrocarbons have been determined. The amount of soot formed increases strongly with increasing fuel content beyond the threshold of soot formation, following approximately the same formal relation for the different fuels. At a given fuel concentration above the soot threshold the soot mass per cm3 rises from C2H2 to C2H4 and to C6H6‐air flames. The concentrations of hydrocarbons in the burned gases increase also with increasing fuel concentrations but in different ways for the different fuels. For benzene‐air flames the carbon present as soot exceeds that present as hydrocarbon already at fuel concentrations slightly above the soot threshold. The same happens in C2H4‐air flames only at C/O ratios close to one and not at all for the C2H2‐air flames.—There is no uniquely simple correlation between the soot mass per cm3 formed in these flames of various fuels and the concentrations of hydrocarbon in their burned gases.

The effects of molecular structure on soot formation II. Diffusion flames

Soot thresholds, in the form of flame heights and fuel mass consumption rates at the smoke points, have been measured in atmospheric pressure, laminar diffusion flames of 42 pure hydrocarbons using a wick-fed burner. The smoke point fuel consumption rates were converted into threshold soot indices, TSIs, and compared with fuel structural parameters and with previous data from the literature. Averaged TSI values are given for 103 fuels. Soot particle emission temperatures and line-of-sight averaged soot volume fractions were measured at half the total smoke point flame heights, the location at which the soot concentration maximizes. All of the soot emission temperatures were between 1450 and 1550K, with aromatic fuels exhibiting the highest temperatures. Maximum soot volume fractions ranged from 2 to 11 × 10 −6, with aromatic fuels producing the highest total soot concentrations.

The effect of temperature on soot formation in premixed flames

Soot production in atmospheric pressure, laminar premixed flames of toluene and decalin was studied by varying the composition of the fuel/O 2/N 2 mixture. As the O 2 (N 2 + O 2) ratio was increased, the onset of sooting occurred at higher fuel O 2 ratios and at higher calculated adiabatic flame temperatures. Temperatures measured using two-wavelength emission pyrometry, however, showed that the flame temperatures at the soot thresholds were constant for each fuel (1750K for toluene and 1720K for decalin). Soot volume fractions were also measured, using a multiwavelength laser extinction technique, as functions of equivalence ratio, O 2 (N 2 + O 2) ratio, and experimental flame temperature. The plateau soot volume fractions for each fuel were found, at any given value of the measured flame temperature, to be independent of the fuel O 2 and the O 2 (N 2 + O 2) ratios, over the range of variation of these parameters studies; they were dependent only on the flame temperature.

Optical investigations of low-pressure acetylenefluorine flames, 1. Emissions, temperatures, and equilibrium composition of nonsooting and sooting flames

Emission profiles and spectra in the visible, uv, and ir have been measured in quasi-premixed, flat C 2H 2F 2 flames at 7.0 mbar. The bands observed were those of C 2, CH, and C 3. In addition, three distinct continua have been observed which can be attributed to CF 2, C 3, and to polymeric species. The origin of the chemiluminescence emitters is discussed in connection with previously measured concentration profiles. In contrast to alkane or H 2F 2-flames highly vibrationally excited HF was not found. The HF rotational temperature was determined as a function of height and C F ratio. The maximum temperature was 2650 ± 70K at a C F = 0.19 . The soot threshold lies at a C F = 0.198 , as compared to the equilibrium adiabatic value of 0.178.

Ionization and soot formation in premixed flames

Total ion concentration profiles were measured in laminar, premixed, low pressure, 2.7 kPa, acetylene-oxygen flames at equivalence ratios, , from 1.5 to 4.0. Peak total ion concentrations decrease with increasingup to and beyond the soot threshold (c=2.4). Near the soot threshold, a second peak appears in the ion profiles. This second peak contains some of the same ions, e.g., C13H9+, C15H11+, C18H11+, and C19H11+, as the first peak which is ascribed to chemi-ionization. The second peak reaches a minimum, at =2.9, well on the fuel rich side of the soot threshold, and then rises again with increase in equivalence ratio. These results refute the arguments against an ionic mechanism based on reported observations that the total ion concentration increases at the soot threshold. In a sooting, =3.0, flame the ion concentration profiles measured are in excellent agreement with measurements by others using molecular beam sampling techniques. The decay in the total ion concentration profile occurs simultaneously with an increase in soot concentration, consistent with the viewpoint that neutral soot particles are produced by ion recombination. The results support the chemi-ionization, ion-molecule mechanism of soot formation in flames.

Prediction of Fuel Mixture Soot Thresholds in Premixed Flames

Abstract Soot production in premixed flames of multicomponent fuels is modeled by combining (1) the experimental observation that for pure fuels the soot volume fraction varies exponentially with equivalence ratio, and (2) the assumption that soot production by each fuel component if independent of the presence of the other components, except in the sharing of the available oxidizer. Soot thresholds calculated from this model are in excellent agreement with available experimental data.

Estimation of Soot Thresholds for Fuel Mixtures

Abstract Experimental soot threshold results obtained using binary and ternary hydrocarbon mixtures were analyzed to obtain two empirical rules for estimating the onset of soot formation for fuel blends in atmospheric pressure, laminar premixed and diffusion flames. The results suggest that the onset of soot formation for a fuel mixture of known composition can be accurately calculated from soot threshold data for the pure compounds.

The effects of molecular structure on soot formation, I. Soot thresholds in premixed flames

Soot thresholds have been measured in atmospheric pressure, laminar premixed flames of 55 pure hydrocarbons. Soot threshold data as a function of fuel structure parameters are presented in four forms; threshold soot indices, TSIs; equivalence ratios assuming combustion to CO 2 and H 2O, f c; equivalence ratios assuming combustion to CO and H 2O, ψ c; and carbon to oxygen atom ratios (C/O) c. The relative TSI scale allows comparison with previous measurements and removes apparatus-dependent variations in the data. Data were obtained for more than 40 previously unstudied fuels, including 15 alkanes, 14 alkenes, 7 alkynes, and 15 aromatics. Comparison with similar data in the literature gave good agreement. Suggested TSI values are given for all fuels which have been reported.