Incipient Soot Research Articles

A study on the effect of temperature on soot formation in a jet stirred combustor

In an attempt to understand soot formation mechanisms and predict soot yields in practical combustion systems, experiments were conducted in a toluene/air turbulent premixed flame in a jet stirred combustor (JSC). Soot mass concentrations were measured via a laser extinction technique and soot particle diameter and number density were measured via a laser extinction technique and soot particle diameter and number density were measured via a combination laser extinction/scattering technique. A global soot model was utilized to analyze rates of particle nucleation, coagulation, and specific surface growth. In order to study the effect of flame temperature on soot formation, one uncooled and three water cooled JSCs were prepared. Soot mass concentration measurements showed strong dependency of the soot production on stoichiometry, residence time, and flame temperature. It was found that as temperature became lower the incipient soot limit became lower, which indicated a greater sooting tendency at a lower flame temperature. The soot mass concentration was found to maximize at a flame temperature of approximately 1200∼1400°C. It was also found that the soot particle diameter increased with temperature. The measurements and the results of a global soot model calculation showed that particle nucleation decreased and the specific surface growth rate increased with an increase in temperature above the maximum sooting temperature.

Optical detection of large soot precursors

We have recently proposed a model of the soot particle inception process that involves the conversion, largely by coagulation and coalescence, of high-molecular-weight hydrocarbon molecules into incipient soot particles. In this work we make absorption measurements at a series of wavelengths between 488 and 1100 nm that show that in the immediate postinception zone the surface growth kinetics follows the profile of the very-high-molecular weight hydrocarbons (we estimate >500 amu) which then disappear. This result confirms an important prediction of our model. The rate at which these species disappear is consistent with the observed rate of mass increase of very young particles. Surface growth is dominated by acetylene only after the disappearance of these species.

Comments on “Influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames,” by M. M. Harris, G. B. King, and N. M. Laurendeau

The carbon soot has generally negative connotation due to the adverse impact on the human health and ambient environment. Most of the studies involving soot analysis focus on the need of preventing or reducing the sources of soot production. However, if the atmospheric soot aerosols are capable of repelling the ambient water vapor and thus impeding the water film formation on the soot particles’ outer shell, they can be considered as competitive and cost-effective candidates for the development of multifunctional non-wettable coatings. The physicochemistry of the soot, particularly the surface oxidation, the presence of heteroatoms and the degree of surface heterogeneity, regulates its chemical non-polarity and hygroscopicity. Appropriately selecting the types of soot sources facilitates the material’s recycling into a liquid-repellent tool with potential applicability in cryobiology, medicine, water purification devices, desalination systems, passive anti-bioadhesive and icephobic surfaces. This review article summarizes for the first time the main soot aerosol patterns based on their water uptake and provides detailed guidelines for the choice of suitable coating fabrication protocol and successive chemical processing. The latter are determinative factors in designing mechanically robust soot withstanding the impact of gusty winds, water jetting, abrasion or finger wiping. Finally, the state-of-the-art in the practical relevance of the hydrophobic soot is discussed, while the existing limitations and the future trends in the soot coatings technology are outlined.

Response to comments and errata, “influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames”

Response to comments and errata, “influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames”

Influence of temperature and hydroxyl concentration on incipient soot formation in premixed flames

Critical equivalence ratios ( φ c) have been measured as a function of temperature (1600–1880K) for premixed flames at atmospheric pressure. The five fuels studied are methane, ethane, propane, ethylene, and acetylene. The flames were stabilized on a flat flame burner and the temperatures were measured using sodium D-line reversal. A linear relationship is found between ln φ c and 1 T for each fuel. Based on a global kinetic model in which soot precursors are formed by fuel pyrolysis and oxidized by OH, a predictive correlation has been developed which shows the influence of temperature, OH concentration, and C H ratio on sooting tendency. This correlation describes all of the measured φ c versus temperature data, suggesting that the overall mechanism of soot formation is similar among aliphatic fuels.

Effect of molecular structure on incipient soot formation

A rational threshold soot index (TSI) varying from 0 to 100 is defined for evaluating the onset of soot formation in both premixed and diffusion flames. It is shown that all of the data in the literature on either premixed or diffusion flames, taken by many techniques, are consitent with respect to molecular structure for each of the two types of flames. There is also a closer similarity between the effect of molecular structure on soot formation in premixed and diffusion flames than previously thought. The use of TSI permits one to use all of the literature data to interpret molecular structure effects and thus arrive at rules for predicting the effect of molecular structure for compounds which have not yet been measured or to correlate the results from one experimental sytem with another. If a correlation can be demonstrated between the effect of molecular structure on soot formation in laboratory and in practical systems, then TSIs will be useful to the synfuels program for defining the desired fuel components to be prepared from a given feedstock.

Optical studies of soot particle growth in hydrocarbon oxygen flames

Number densities, particle sizes and flame temperatures were measured in flat, premixed, laminar hydrocarbon oxygen flames at low pressure. Methods used were light scattering, extinction and sodium-D-line reversal. Soot particles in sooting hydrocarbon-oxygen flames are comparable to particles of a coagulating aerosol. This is indicated by testing the long time particle dynamics for particle number densities in the measured flames and by solving numerically the particle balance using a model based on free molecular coagulation which comprises surface growth and particle inception. The computations yield particle size distributions tending to a constant relative standard deviation after short reaction times though particle inception and surface growth should counteract the broadening of the size distributions. Changes of soot loading appear as changes of the soot mass increase by surface reactions because about 90% of the soot is built by surface reactions. Surface growth of soot particles does not follow simple pyrolytic mechanism involving thecomponents of a hydrocarbon pool. This can be concluded from the concentration profiles of those components in the soot forming region of the flames and the surface growth rates. Particle number densities and particle sizes are governed by particle inception over a small interval at incipient soot formation meaning that once solid particles are formed the intermediates of the hydrocarbon pool preferably react with the surfaces of the soot particles than with themselves to form larger intermediates or new particles.

Observation of the behavior of coal particles during thermal decomposition

This paper reports on the observed behavior of coal particles during thermal decomposition. The data presented are the first from a study initiated to address both the physical and chemical behavior of pulverized coal from the time of initial heating through the evolution of soot particulate. In the present study, pulverized (high volatile bituminous) coal was injected through a slit centered in a methaneair flat flame burner, and high resolution holography was employed to record the evolution of volatiles and the structure of soot particulate. Volatiles were observed to evolve in a variety of shapes that range from individual jets to uniformly distributed clouds. Coal particle fragments and/or incipient soot nodules (∼ 3 micron) were found to be present within the volatile gases during the evolution. Further from the burner, stringlike soot particles approaching 1600 microns in length were observed.

Dependence of Soot Production on Fuel Blend Characteristics and Combustion Conditions

Liquid synthetic fuels derived from nonpetroleum resources will play a major role in meeting future national energy demands. In the case of gas turbine applications, it is known that the different properties of these fuels can result in substantially altered combustion performance. Most importantly, decreased fuel hydrogen content resulting from an increased aromatic content has been observed to result in increased exhaust smoke and particulates as well as greater flame luminosity. This paper contributes empirical information and insight which allows the greater soot formation tendencies of low hydrogen content fuels to be better understood. A small scale laboratory device which simulates the strongly backmixed conditions present in the primary zone of a gas turbine combustor is utilized. The Jet Stirred Combustor provides for very rapid mixing between a premixture of vaporized fuel and air and the combustion products within a 5.08 cm dia hemispherical reactor. Results to be presented are gaseous combustion product distributions, incipient soot limits, and soot production (mg/l) for a variety of fuels. The influences of combustor inlet temperature and reactor mass loading have been evaluated and the sooting characteristics of fuel blends have been studied. These results have been analyzed to develop useful correlations which are in general agreement with existing mechanistic concepts of the soot formation process.

Dependence of Soot Production on Fuel Structure in Backmixed Combustion

Abstract Experimental results are described which identify the dependence of soot production on fuel molecular structure in strongly backmixed combustion. This type of aerodynamic process simulates that occurring within the primary zone of many continuous combustion devices (e.g. the gas turbine) where fuel-air ratio conditions are sufficiently rich to form appreciable quantities of soot. Results to be presented are gaseous combustion product distributions, incipient soot limits, and soot production. These parameters have been developed for a variety of hydrocarbons which represent constituents of practical fuels—normal and cyclic paraffins, olefins, single-ring aromatics, and double-ring aromatics. Substantial differences in sooting behavior between the aromatic and aliphatic hydrocarbons have been found and important observations regarding the presence of unburned hydrocarbons at and near the soot limit have been made.

The effect of metal additives on the formation of soot in premixed flames

The influence of low concentrations of alkali and alkaline-earth metals on soot particle size and number density in atmospheric-pressure premixed flat flames is investigated using laser light scattering and extinction measurements. Additions of the alkali metals (Na, K, Cs) only weakly suppress the total amount of soot formed, but they do result in a greater number (up 25 times) of particles than in the unseeded flame. The number of soot particles present correlates with the observed ability of the additive to promote ionization (Cs>K>Na) in the early stages of soot formation. It is proposed that incipient soot particles are charged earlier in the presence of these metals—once charged, these particles resist further collisional growth, so that their number density remains high and their size small. Of the alkaline-earth metlas (Ca, Sr, Ba), only Ba produces enough ions to suppress coagulation in the manner of the alkali metals. However, these metals, Ba included, do reduce the amount of soot formation. These results provide evidence that the influence of metallic additives involves both electrical and chemical factors.

Shock-Tube Measurements of Carbon to Oxygen Atom Ratios for Incipient Soot Formation with C2H2, C2H4and C4H6Fuels

Critical atomic carbon to oxygen ratio measurement for incipient soot formation in shock heated acetylene, ethylene and ethane/oxygen/argon mixtures

Carbon formation in flames

The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.

Carbon formation in the combustion wave

The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.

On the source of light in luminous flames

The route by which gas-phase molecules in hydrocarbon flames form condensed-phase carbonaceous nanoparticles (incipient soot) is reviewed. These products of incomplete combustion are introduced as particulates and materials revealing both their useful applications and unwanted impacts as pollutants. Significant advances in experimental techniques in the last decade have allowed the gas phase precursors and the transformation from molecules to nanoparticles to be directly observed. These measurements combined with computational techniques allow for various mechanisms known to date to be compared and explored. Questions remain surrounding the various mechanisms that lead to nanoparticle formation. Mechanisms combining physical and chemical routes, so-called physically stabilised soot inception, are highlighted as a possible “middle way”.