Smoke Point Measurements Research Articles

Study of Sooting Propensity and Related Indices Based on Tests With Isolated Droplets

Abstract This work compares two approaches to characterize the sooting propensity of liquid fuels, namely, the Threshold Soot Index (TSI) obtained from smoke point measurements and the Isolated Droplet Soot Yield (IDSY), an original metric expressing the mass of soot collected from single droplets vaporizing under a hot atmosphere representative of conventional flames. The TSI of different fuels was experimentally obtained by means of a computer-vision method, whereas their IDSY was extracted using a free-falling droplet combustion facility. The results clearly pointed to a linear dependency of the IDSY with mass fraction, allowing to propose a mixture rule for this index for a significantly broader measuring range than that allowed by the smoke point lamp. A nonlinear correlation was found between IDSY and TSI, with relevant deviations for some compounds. These deviations could advise to consider which of the available indices (TSI, IDSY, others) would be more adequate to emulate the sooting behavior.

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.

Sooting characteristics of ethanol-ethylene blends in laminar coflow diffusion flames up to 10 bar

Ethanol, being a renewable product, has been perceived for a long time as a fuel that burns clean and more complete than conventional hydrocarbons as well as being thought as a soot suppressant in blends with gasoline and diesel fuel. Some of the results from a wide-range of studies investigating soot-suppression potential of ethanol at atmospheric pressure, including Smoke Point measurements, have yielded occasionally contradictory conclusions. Further, scaling of soot studies conducted under atmospheric conditions to engine-relevant pressures has always been problematic because of the non-monotonic response of combustion events to pressure. Ethanol’s soot inhibiting characteristics at elevated pressures under tractable conditions have not been previously documented and may not be inferred easily from atmospheric pressure measurements. Information from benchmark investigations with ethanol-hydrocarbon blends at elevated pressures could help in understanding the soot aerosol emissions from practical engines fueled with ethanol blends. Soot production in ethanol/ethylene laminar diffusion flames at elevated pressures was investigated on a 3 mm diameter coflow burner mounted in a high-pressure combustion vessel. Ethanol was added to ethylene, which is diluted with nitrogen, in various quantities from 0% to 40% in terms of total carbon of the fuel stream keeping the carbon flow rate constant at 0.41 mg/s. Experiments were conducted at pressures from 3 to 10 bar, and in addition to keeping the mass flow of carbon constant, the ratio of mass flow of nitrogen to mass flow of carbon was also kept fixed at 6 at the fuel nozzle. The soot spectral emission spectroscopy technique was used to obtain line-of-sight intensities of soot radiation which were then converted to radial soot temperature and volume fraction distributions at a given axial height within the flame. Soot production increased by a small but measurable amount with the fuel mixture containing 10% carbon from ethanol at 5 and 8 bar pressure flames indicating synergistic interactions; while at 3 and 10 bar, no measurable change was observed. When the ethanol’s carbon contribution was increased to 30% and 40%, soot formation was suppressed and the soot yields decreased below the soot level of neat nitrogen-diluted ethylene at 3 to 10 bar pressures. Sensitivity of soot production to pressure in ethylene/ethanol flames is found to be similar to that of ethylene up to 8 bar.

A novel device for automated determination of the smoke point with non-invasive adaptation of ASTM D1322 normalized lamps

Particulate matter emission from the combustion of fossil fuels is a major concern due to its harmful effects on human health and impact on engine performance. Measurement of the smoke point of these fuels is a key issue in order to assess the tendency of fuels to generate particulate matter. Although certain commercial devices for measuring the smoke point are available, they are very expensive and their precision can still be improved. This paper proposes a novel low-cost device to achieve precise and repetitive smoke point measurements. It is based in an improved image processing algorithm that reduces the measurement error compared to previously developed methods. The device design allows easy adaptation to the smoke point lamp normalized in the American Society for Testing and Materials (ASTM) D1322, without any modification to the lamp.

Particulate matter indices using fuel smoke point for vehicle emissions with gasoline, ethanol blends, and butanol blends

Recent studies have led to significant improvements in the prediction of particulate matter (PM) emissions from gasoline vehicles based on differences in fuel composition. This is an issue that has taken on increased importance recently given the introduction of gasoline direct injection (GDI) technology to improve fuel economy and the challenges it has meeting new stringent PM emissions standards. One productive effort in this area is the PM Index (PMI) developed by Aikawa and collaborators. This approach relates the effects of fuel composition on its PM forming potential. The present work explores the use of experimental smoke point measurements and various fuel volatility metrics to better characterize these effects. Smoke point (SP) values as scaled by the Oxygen Extended Sooting Index (OESI) incorporate all fuel molecular structural effects known to have an impact on soot formation, including branching, degree of saturation, carbon chain length, and oxygenate functional group effects. A strong correlation is found for vehicle-level particle number (PN) and PM emissions with SP, OESI and PMI. Fuel volatility effects are considered, including characteristics derived from molecular weight, distillation, vapor pressure, and heat of vaporization. For hydrocarbon blends, no volatility factor significantly improved the correlation as compared to PMI, SP, or OESI alone. However, inclusion of a heat of vaporization term with OESI better matched the emissions trend for ethanol–gasoline blends. Sooting tendencies for n-butanol- and isobutanol-gasoline blends exhibited trends similar to those with ethanol.

Smoke Point Measurements of Diesel-Range Hydrocarbon–Oxygenate Blends Using a Novel Approach for Fuel Blend Selection

The use of oxygenated fuels decreases particulate matter (PM) emissions from diesel engines. Studies using engines, experimental flames, and modeling have shown that the decrease in soot emissions ...

Oxygen Extended Sooting Index of FAME Blends with Aviation Kerosene

The use of biofuels in the aviation sector has economic and environmental benefits. Among the options for the production of renewable jet fuels, hydroprocessed esters and fatty acids (HEFA) have received predominant attention in comparison with fatty acid methyl esters (FAME), which are not approved as additives for jet fuels. However, the presence of oxygen in methyl esters tends to reduce soot emissions and therefore particulate matter emissions. This sooting tendency is quantified in this work with an oxygen-extended sooting index, based on smoke point measurements. Results have shown considerable reduction in the sooting tendency for all biokerosenes (produced by transesterification and eventually distillation) with respect to fossil kerosenes. Among the tested biokerosenes, that made from palm kernel oil was the most effective one, and nondistilled methyl esters (from camelina and linseed oils) showed lower effectiveness than distilled biokerosenes to reduce the sooting tendency. These results may co...

An Improved Method of Smoke Point Normalization

The most widely used method of normalizing laminar smoke points from different studies has been the threshold sooting index (TSI). TSI has several drawbacks, particularly a nonphysical dependence on fuel molar mass. An improved normalization method, called normalized smoke point (NSP), is introduced here. For each fuel, NSP is a weighted average of the smoke point lengths measured by different studies. NSP eliminates the major TSI drawbacks. NSP values were determined for 112 hydrocarbons by averaging 256 smoke point measurements from 12 past studies. This allows for the most extensive evaluation to date of the effects of fuel type on the smoke points of hydrocarbon diffusion flames. Hydrocarbon sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio. For n-alkanes, sooting propensity increases with carbon number, however it generally decreases with carbon number for alkenes and 1-alkynes. Sooting propensity generally increases according to alkanes < alkenes < 1-alkynes < aromatics.

Effect of Dilution, Pressure, and Velocity on Smoke Point in Laminar Jet Flames

Smoke point measurements of diluted methane and ethylene flames were made in a co-flowing laminar jet diffusion flame at pressures up to 8 atm. The smoke point corresponds to the fuel flow rate where the soot production is exactly offset by the soot oxidation, and as such is sensitive to changes in rates of production or oxidation. Flame height in these flames was measured as a function of pressure, diluent, and dilution level as well as both fuel exit velocity profile (i.e., plug or parabolic) and fuel/air velocity ratio. As pressure increases, the smoke point became less sensitive to diluent or dilution level. In addition to heat capacity and thermal diffusivity differences between CO2 and He for example, the large differences in kinematic viscosity was shown to play an important role in the diluent's ability to suppress the fuel's propensity to form soot.

Structural group analysis for soot reduction tendency of oxygenated fuels

Oxygenated additives are known to reduce soot formation in diesel engines. Numerous studies, both experimental and numerical, have reported that the reduction of particulate emissions depends on the molecular structure of the additives. In this paper, a structural group contribution approach is proposed to interpret experimental observations on the effect of oxygenated additives on the sooting propensities of hydrocarbon fuels. The statistically based method makes it possible to distinguish between chemical effects caused by the presence of oxygenated groups in the fuel mixture and mere dilution of the original fuel by the additive. The analysis was carried out on several experimental databases encompassing both premixed and nonpremixed configurations that include a new extensive set of smoke point measurements for mixtures of a given fuel with several oxygenated molecules. The current approach unifies the conclusions on the relative efficiency of the various oxygenated functionalities such as alcohols, esters, ethers, and carbonyl groups and provides a potential explanation for the seemingly contradictory trends exhibited by some raw experimental data.

Determination of Soot Formation Rate from Laminar Smoke Point Measurements

In order to develop soot formation models for fuels in fires, a global Arrhenius equation for soot formation rate has been studied in terms of the mixture fraction, the fuel concentration in the fuel supply stream, the local gas density and the gas temperature. A procedure relying on modelling and experimental data determines the parameters for the soot formation rate by exploiting the physics and results in laminar smoke point flames corresponding to a given fuel. The main advantages of such an approach are its simplicity and applicability to fuels (solids or liquids) in fires as long as their smoke point height is known, whereas most of the previous studies focused on pure hydrocarbons.

Soot Formation and Oxidation in Fires from Laminar Smoke Point Measurements

A soot formation and oxidation model appropriate for fires is first developed for laminar axisymmetric nonpremixed flames. This model relies on flow similarity relationships to describe the structure of the flame and relate it to soot processes in laminar smoke point flames. The selection of the parameters is focused on the stoichiometric and visible heights. The evaluation of this model is achieved by means of comparisons of numerical results with experimental data and observations for an axisymmetric coflow laminar ethylene/air nonpremixed flame. The model gives satisfactory results and a good phenomenological explanation of the influence of soot processes on the structure of the flame. This model will be applied next to turbulent flames and fires.

Model fuels to reproduce the sooting behaviour of aviation kerosene

Measurements of spatially resolved soot volume fraction and temperature are reported in diffusion flames fuelled by prevaporised aviation kerosene and model fuels, comprising blends of n-decane and alkyl-substituted aromatics, selected to mimic its sooting behaviour. Although standard smoke point measurements suggest that the specific aromatic does significantly influence the sooting tendency of the blend, measurements in laminar flames exhibit little such sensitivity and a mixture of 30% mesitylene (or propylbenzene)/70% n-decane satisfactorily reproduces the sooting behaviour observed with kerosene in both laminar and turbulent flames. Such comparatively simple blends lend themselves to detailed chemical representation in CFD-based numerical simulation.

Soot formation and inert addition in diffusion flames

An experimental study has been conducted in coflow diffusion flames in order to identify the relative importance of fuel concentration dilution and flame temperature reduction on soot formation when inert is added to fuel. Two different methodologies were used to isolate dilution and temperature effects, both involving substitution of inerts with different specific heats. To quantify the extent of soot reduction, laser-light extinction as well as smoke point measurements were made. The results are in agreement with previous studies in counterflow flames and show that soot formation rates in the coflow flame behave nearly linearly with fuel concentration. Furthermore, white temperature exerts a strong influence on soot formation, dilution can also affect formation rates and smoke points when inerts are added to fuels. It is found that the relative importance of dilution and temperature depends on the extent of addition. When moderate amounts of inert are added, the temperature reduction is typically very small so that the effect of dilution can be considerably greater than that of temperature. When large amounts of inert are added, temperature effects may dominate those of dilution although, in an absolute sense, dilution effects could still be important because fuel concentrations are low.

A quantitative relationship between soot yield and smoke point measurements

A model of laminar round diffusion flames is developed which quantitatively relates the maximum soot volume fraction and soot yield in the flame to the smoke point fuel flow rate. The model is based on the similarity behavior of soot volume fraction profiles, soot temperatures, and flame length and diameter. An energy balance relates radiation heat loss by soot to the smoke point flame length where all fuels have a common soot temperature. The model is tested against optically measured soot volume fractions and soot yields over a wide range of fuels. Predictions are generally satisfactory except for some heavily sooting fuels with very short smoke point lengths.