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Soot Formation Effects of Oxygen Concentration in the Oxidizer Stream of Laminar Coannular Nonpremixed Methane/Air Flames KYEONG-OOK LEE†
Department of Chemical Engineering, The University of Illinois at Chicago, Chicago, IL 60607, USA
CONSTANTINE M. MEGARIDIS*, SERGUEI ZELEPOUGA, ALEXEI V. SAVELIEV, and LAWRENCE A. KENNEDY
Department of Mechanical Engineering, The University of Illinois at Chicago, Chicago, IL 60607, USA
OLIVIER CHARON
Chicago Research Center, American Air Liquide, Countryside, IL 80525, USA and
FOUAD AMMOURI
Centre de Recherche Claude-Delorme, Air Liquide, Jouy-en-Josas 78353, France This experimental investigation analyzes the soot formation effects of oxygen concentration in the oxidizer stream (O2 ⫹ N2) ventilating laminar jet nonpremixed methane flames. The base flame incorporates air as the oxidizer; two additional flames, with respective oxygen concentrations of 50% and 100% in the ventilating coflow, are also examined. The microstructure of soot collected from selected flame locations is determined combining thermophoretic sampling and transmission electron microscopy. A laser-light extinction technique is employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the respective rates in the methane/air base flame. The rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot yields diminish with increasing oxygen concentration, as do luminous flame spatial dimensions. Soot aggregate data on the soot annulus suggest a higher degree of agglomeration under oxygen-enriched conditions. Finally, the fractal dimensions of selected soot aggregate samples are measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame), being similar to previously published values for carbonaceous soot. © 2000 by The Combustion Institute
INTRODUCTION Conventional nonpremixed gaseous combustion has primarily employed air as an oxidizer, mainly because of its relatively low cost and ample supply. As a result, hydrocarbon/air combustion systems frequently emit harmful pollutants, such as CO, HC, NOx, as well as soot particulates. Among these pollutants, NOx is essentially produced due to the high relative concentrations of nitrogen (79% in volumetric proportion) in the high temperature environment. As hydrocarbon-fuel combustion systems are used extensively, concerns for the reduction of pollutant emissions have been increased. A number of investigations have focused on re*Corresponding author: E-mail: [email protected] † Current address: Energy Systems Division, Argonne Nat. Laboratory, Argonne IL 60439 COMBUSTION AND FLAME 121:323–333 (2000) © 2000 by The Combustion Institute Published by Elsevier Science Inc.
ducing harmful pollutant emissions by introducing several effective emission reduction techniques, such as mixing different fuels [1, 2], or using either gaseous [3, 4] or metal additives [5– 8]. Among continuing efforts for the development of low-emission combustion systems, oxygen-enhanced combustion is deemed promising because of several advantages [9, 10]. For example, the use of pure oxygen as an oxidizer in hydrocarbon-fuel combustion reduces the required volume of the oxidizer, increases the maximum flame temperature and thermal efficiency, and improves flame stability. More importantly, when possible, the use of pure oxygen also eliminates the emission of NOx. Compared to hydrocarbon/air combustion, the higher flame temperatures in hydrocarbon/oxygen systems should lead to higher rates of soot inception and growth. On the other hand, the in0010-2180/00/$–see front matter PII S0010-2180(99)00131-5
324 creased presence of oxygen promotes oxidative mechanisms, thus tending to diminish soot concentrations. It is well established that the residence time of soot particles is an important parameter affecting soot formation in nonpremixed flames [11–14]. Due to changes in flame structure, flame residence times decrease when oxygen concentrations are raised in the oxidizer stream. These flame structure changes with varying oxygen concentrations are expected to play an important role in the current study. Selected studies of gaseous hydrocarbon/oxygen premixed combustion have been performed to characterize flame radiation emitted from both luminous and nonluminous flames [15, 16], and calculate the optical properties of soot particles produced therein [17]. Other oxygenenhanced premixed hydrocarbon flame configurations were investigated to determine the characteristics of soot inception/nucleation and growth in terms of soot concentration measurements [18, 19]. The fractal morphology and microstructure of soot aggregates have also been analyzed in premixed methane/oxygen flames [20, 21]. While the above investigations have contributed to better understanding of soot formation mechanisms in premixed hydrocarbon/ oxygen flames, improved understanding of the corresponding characteristics of nonpremixed hydrocarbon/oxygen combustion may allow exploitation of the associated low pollutant emission properties and high energy efficiency. To this end, several soot studies ([22] and references cited therein) have targeted the effect of varying oxygen concentration in the oxidizer flow (called oxygen index) in nonpremixed flames. Vandsburger et al. [22] measured soot production along the stagnation streamline in ethylene and propane stagnation point nonpremixed flames, and paid particular attention to the effect of oxygen index. They reported that soot particle production was increased and the region of particle formation was extended with rising oxygen index. Glassman and Yaccarino [23] investigated the sooting height of an axisymmetric nonpremixed flame as a function of oxygen index. They reported that sooting tendency was minimum at an oxygen index of around 0.24. A recent study of gaseous methane/oxygen and methane/enhanced-oxygen combustion was performed to characterize flame radiation [24]. The current study focuses on well-defined
K.-O. LEE ET AL. laminar jet nonpremixed methane flames featuring increased oxygen content in the oxidizing stream. The work provides comparisons of these flames with their methane/air counterpart. Soot microstructure and fractal morphology are investigated in these flames by combining thermophoretic soot sampling and subsequent transmission electron microscopy (TEM) analysis. A computer-based image processing system is used to calculate soot primary sizes and fractal dimensions of soot aggregates at selected flame locations. Soot volume fraction distributions are also measured by laser light extinction and tomographic reconstruction techniques. EXPERIMENTAL SETUP AND TECHNIQUES The coflow laminar burner employed in this study is identical to those used in other investigations [6, 14, 25]. The methane fuel and oxidizer were supplied through two concentric tubes of 11.1 mm and 101.6 i.d., respectively. Three stable laminar nonpremixed flames featuring different oxygen content in the oxidizer stream were considered: 1) A flame burning methane fuel and bottled dry air flowing at 7.5 cm3/s and 635 cm3/s, respectively. These flow rates are identical to those adopted in the steady methane/air nonpremixed flame studies of [11, 14], and correspond to overall residence times of ⬃100 ms [26]. This flame is referred to as methane/air flame (luminous height ⬃79 mm). 2) A flame burning methane fuel and oxygen (99.8 % pure) flowing at 7.5 cm3/s and 635 cm3/s, respectively. This flame is referred to as methane/oxygen flame (luminous height ⬃12 mm). 3) A flame burning methane fuel and a 50% N2–50% O2 gas mixture flowing at 7.5 cm3/s and 635 cm3/s, respectively. This flame is referred to as a methane/50% oxygen flame (luminous height ⬃25 mm). All three methane jet nonpremixed flames considered in this study burned 99.7% pure fuel and released no soot from their closed tip. In order to observe the morphological changes that the soot particles are subjected to, thermo-
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES phoretic soot sampling [27] was performed at several axial locations in each flame. The residence time of the sampling probe was approximately 35 ms for all tests; in all cases a sufficient number of soot aggregates was collected for subsequent analysis by electron microscopy. TEM analysis of each probe provided the spatial distribution of soot morphology at fixed heights above the burner mouth. A standard laser-light extinction technique [28] was employed to measure the soot volume fraction distributions within the three flames. A 6-mW HeNe laser was used as a light source (632.8 nm). The laser was coupled with an intensity stabilizer to assure long-term stability of the beam intensity; the drift was less than 0.1% in a period of 8 hours. The laser beam was focused on the flame axis (beam waist of ⬃150 m). Beam shift due to thermal lens effects was minimized through the procedure described in [29]. A broad band power/energy meter was employed to determine the preflame and transmitted light intensities (I0 and I, respectively). The power meter was equipped with a laser line filter to avoid interference from light originating from other sources. In order to measure the radial distributions of the transmitted light intensity at fixed heights above the burner mouth, the entire burner assembly was mounted on a two-dimensional step motor-driven system, while the optical setup remained fixed. The projection function P(x), equivalent to the lineof-sight integral of the soot volume fraction fv along a path at a distance x from the axis, was calculated using the following expression P 共 x兲 ⫽ ⫺ ln共I/I 0兲/K ext The corresponding field distribution function, fv, was obtained using a three-point Abel deconvolution algorithm [30]. The soot extinction constant Kext was taken to be 4.9; this value corresponds to an index of refraction m ⫽ 1.57– 0.56i [31]. It is noted that the above mentioned value of the extinction constant is smaller by nearly a factor of 2 compared to the one reported recently by Zhou et al. [32]. Kext affects the corresponding value of fv, but the issues related to the correct value of this quantity are beyond the scope of this work and will not be further addressed herein. A good treatment of this subject has been published by Mulholland and Choi [33].
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EXPERIMENTAL RESULTS AND ANALYSIS Luminous Flame Appearance Luminous flame appearance does not define the reaction zone, but it does provide a means of evaluating the spatial extent of the soot zone. Figures 1a– c show luminosity images of the three jet nonpremixed flames studied herein. The methane/air flame of Fig. 1a was verified to be identical to that analyzed in [11, 14]. The flame displays the familiar elongated structure with a closed tip indicative on its nonsootemitting character. Figure 1b corresponds to the methane/50% oxygen flame; it is apparent that the flame luminous length decreases considerably with increased oxygen content in the oxidizer stream. This trend is also verified in Fig. 1c, which displays the corresponding image for the methane/oxygen flame. The luminous heights depicted in Fig. 1 can be compared to the flame lengths obtained using Roper’s approach [34, 35]. According to this model, a jet diffusion flame ends on the symmetry axis at the point where oxygen and fuel meet at the stoichiometric ratio. The flame heights predicted by Roper’s model [36] were 64 mm for the methane/air flame, 30 mm for the methane/50% oxygen flame, and 17 mm for the methane/ oxygen flame. These height estimates for the first two flames are within 20% of the observed values listed in Fig. 1. But the height estimate for the methane/oxygen flame does not closely approximate the actual luminous length (17 mm vs. 12 mm). This is probably due to the relatively larger influence of burner heat losses in the methane/oxygen flame, which is the smallest one of all flames seen in Fig. 1. Despite this disagreement, the predicted flame heights confirm the observed trend of diminishing flame size with rising oxygen content in the ventilating stream. The flame luminosity measurements showed the methane/oxygen flame to be much brighter than the methane/air flame. The methane/50% oxygen flame appeared to be darker than the methane/oxygen flame, but was still brighter than the methane/air flame. These variations in luminous appearance suggest substantial disparity in temperatures among these flames. The
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K.-O. LEE ET AL. into consideration, were 2226 K for the methane/ air flame, 2814 K for the methane/50% oxygen flame, and 3054 K for the methane/oxygen flame. The increasing temperature with rising oxygen content in the oxidizer stream is consistent with the observed flame luminosity differences. Soot Morphology
Fig. 1. Luminosity images of the three jet nonpremixed flames studied herein. Hf denotes the luminous zone height. (a) Methane/Air (Hf⫽79 mm), (b) Methane/50% O2(Hf⫽25 mm), (c) Methane/O2(Hf⫽12 mm).
adiabatic flame temperatures for the three cases were calculated using STANJAN.* These temperatures, which were determined taking dissociation *Reynolds, W. C. The element potential method for chemical equilibrium analysis: Implementation in the interactive program STANJAN, Stanford University, Jan. 1986.
For the methane/50% oxygen flame, thermophoretic sampling tests were performed at several axial locations ranging from 8 mm to 20 mm, interspaced by 4 mm. For the methane/air flame the sampling results published in Ref. 14 were utilized. In that work, the axial locations sampled ranged from 35 mm to 70 mm. Finally, the small volume of the methane/oxygen flame (see Fig. 1c) did not allow extensive thermophoretic soot sampling tests in that flame. Such tests were performed only at an axial height Z ⫽ 6 mm, in order to obtain a representative picture of soot morphology in that flame. An additional difficulty in sampling thermophoretically the methane/oxygen flame was caused by its much higher temperature compared to the other two flames. The high gas temperatures allowed only very rapid insertion tests, which were necessary to avoid deterioration of the 20-nm-thick carbon substrate supported on the copper grid. The results of the thermophoretic sampling experiments are presented below; they have been based on tests that left the carbon substrates intact, thus precluding oxidation of the overlaying soot particles after their extraction from the flame gases. Figures 2a– d show selected TEM photomicrographs of soot particles extracted thermophoretically from the three methane flames investigated in this study. These micrographs were taken at the same magnification (100,000⫻). As established previously for laminar jet nonpremixed flames [27], aggregates collected from the same flame location consist of primary particles that are approximately spherical and have a nearly uniform diameter dp. This parameter changes with flame coordinates. Frames 2a and 2b, respectively, display soot collected from the axis and the annulus of maximum soot volume fraction (called soot annular region [28]) of the methane/50% oxygen flame at an axial height Z ⫽ 12 mm above
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES
327
Fig. 2. TEM photomicrographs of soot particles extracted thermophoretically from the three methane flames investigated in this study. Frames (a) and (b), respectively, correspond to the axis and the soot annulus of the methane/50% oxygen flame at an axial height Z ⫽ 12 mm. Frame (c) corresponds to the annular region of the methane/air flame at Z ⫽ 50 mm. Frame (d) shows soot collected from the annular region of the methane/oxygen flame at Z ⫽ 6 mm.
the burner mouth. The soot sample extracted from the flame axis (Fig. 2a) includes soot precursor particles as well as larger aggregates. The soot precursor particles appear as small diffuse particles in the background, as established in Ref. 37. The presence of the precursor particles there indicates that soot inception occurs at this location. The larger soot aggregates seen in Fig. 2a were probably collected when the probe pierced through the soot annulus during insertion. The particles in Fig. 2b correspond to the soot annulus at the same height Z ⫽ 12 mm; they represent the overall maximum primary size seen in this flame (see below). The apparent similarity of the aggregates seen in Fig. 2b with those in Fig. 2a verifies the earlier statement that the larger aggregates seen near the flame axis were collected when the probe penetrated the soot annulus. Frame 2c displays soot collected from the annular
region of the methane/air flame at a height Z ⫽ 50 mm. These particles also correspond to the overall maximum primary size seen in this flame (see below). Frame 2d shows soot collected from the annular region of the methane/oxygen flame at the only sampled height of Z ⫽ 6 mm. It can be seen that the largest aggregates in the methane/50% oxygen flame (Fig. 2b) are larger than those in the methane/air flame (Fig. 2c), as well as those in the methane/oxygen flame (Fig. 2d). Finally, TEM morphological observations of soot collected from the vicinity of the flame axis at the height Z ⫽ 6 mm of the methane/ oxygen flame also revealed the presence of precursor particles. A large number of TEM photomicrographs was utilized to analyze the microstructure of soot particles for each flame. A population of more than a hundred primary particles was measured to determine the mean soot primary size at the soot annulus of each axial
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Fig. 3. Soot primary particle diameters measured at selected axial locations of the three methane flames investigated herein. These values represent the peak values of soot primary size at each height. The measurement uncertainty is indicated by the vertical error bars; each bar denotes two standard deviations. Each curve is marked to represent the O2 composition of the oxidizing stream (the balance is N2). The height uncertainty is ⬃1 mm.
location sampled in the three flames. In this process, a Micro-Comp high-resolution image acquisition/analysis system was utilized. Figure 3 displays the average primary diameters measured at each location. These values represent the peak values of soot primary size at each height. The measurement uncertainty is indicated by the vertical error bars in Fig. 3; each bar denotes two standard deviations. As also reported in [14], the experimental data for the methane/air flame clearly show that the soot primary particle size is strongly dependent upon the axial location within the luminous flame. The maximum value of dp for the methane/air flame was approximately 20 nm at Z ⫽ 50 mm. The corresponding data for the methane/50% oxygen flame exhibit a significant change in soot primary size with axial distance within this relatively small flame; luminous flame height was ⬃25 mm. The maximum value of dp for this flame was approximately 22 nm and was seen at Z ⫽ 16 mm. Based on the conducted soot morphological observations, the soot particles in the methane/50% oxygen flame experience the same sequence of events (inception, growth, oxidation) as those in the methane/air flame, but in a much shorter period. Finally, as seen in Fig. 3, the soot primary diameter measured in the soot annulus at Z ⫽ 6 mm of the methane/oxygen flame was about 11 nm. Since this height is midway
K.-O. LEE ET AL. along the luminous zone of the methane/oxygen flame, the overall maximum soot primary size in this flame is expected to be close to 11 nm. The experimental data on dp displayed in Fig. 3, combined with the significantly different residence times in the three flames investigated herein, are worthy of further discussion. The reduced residence times in the oxygen-enriched flames tend to inhibit soot growth; at the same time, the higher temperatures in these flames are expected to enhance soot formation; finally, the ample presence of oxygen in the oxygenenriched flames is expected to promote soot oxidative mechanisms. It has been established [25] that primary particle growth in laminar nonpremixed flames represents soot aggregate growth via surface reactions. Consequently, the similar maximum soot primary sizes observed for the methane/50% oxygen and methane/air flames (Fig. 3) suggest that soot surface growth rates are much stronger under oxygen-enriched conditions. Furthermore, the rapid decrease of soot primary sizes above Z ⫽ 16 mm in the methane/50% oxygen flame indicates stronger soot oxidation rates in this flame, compared to its air counterpart. On the other hand, the substantially lower value of dp in the methane/oxygen flame (see Fig. 3), combined with its low residence time, do not allow any firm conclusions on how soot growth or oxidation rates in this flame compare to the respective rates in the other two flames. Du et al. [4] also used a methane diffusion flame to examine the soot formation effects of oxygen concentrations on the oxidizer side. They concluded that the effect of varying the oxygen concentration between 15% and 21% (mole) is almost entirely thermal. The corresponding dilution and chemical effects were found to be negligible. But it is important to note that the results reported by Du et al. [4] were based on oxygen concentrations that were substantially lower than those considered in the current study. Soot Volume Fraction Distributions Figures 4a– c present the spatial distributions of soot volume fractions for the methane flames operated at three different oxygen concentrations in the ventilating stream: 21% (air), 50%, and 100%, respectively. In the case of the methane/air flame (Fig. 4a), the soot annular
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES
329
maximum soot volume fraction in that flame was ⬃0.36 ppm, and was measured near the symmetry axis at Z ⫽ 60 mm. The soot volume fraction dramatically decreases with axial distance thereon, and eventually vanishes near the tip of the flame. Figure 4b presents the radial distributions of soot volume fractions in the methane/50% oxygen flame at several axial locations. The soot distributions have higher overall values and display a similar structure to those in the methane/air flame. The maximum soot volume fraction in the methane/50% oxygen flame is approximately 0.6 ppm and was measured at a height Z ⫽ 16 mm. As seen in other laminar nonpremixed flames of this type [25], the height at which the maximum soot concentration appears in this flame coincides with the axial location at which the maximum soot primary size was measured (Fig. 3). It is noted that in the methane/air flame, the maximum soot concentration occurs at Z ⫽ 60 mm (Fig. 4a), while maximum primary size attained is at Z ⫽ 50 mm (Fig. 3). Figure 4c shows the radial distributions of soot volume fractions in the methane/oxygen flame at four different axial locations, including Z ⫽ 6 mm where the only soot sampling test was performed. The overall maximum soot volume fraction through the entire flame appears to be approximately 0.15 ppm and was measured at Z ⫽ 6 mm. This value is lower than those in the methane/air and methane/50% oxygen flames by a factor of 2.4 and 4, respectively. The fact that the maximum soot volume fraction appears at Z ⫽ 6 mm in these measurements, validates the choice of this height for thermophoretic soot sampling tests in the methane/oxygen flame (see previous section). Furthermore, since the heights where the maximum soot volume fraction and maximum primary size occur are approximately the same, it can be inferred that the only data point for dp in Fig. 3 probably marks the peak of the dp vs. Z curve for the methane/oxygen flame. Fig. 4. Spatial distributions of soot volume fractions for the methane flames operated at three different oxygen concentrations in the ventilating stream: (a) 21% (air), (b) 50%, and (c) 100%. The balance is nitrogen.
region shifts to the flame axis between the heights of 50 mm and 60 mm. The overall
Soot Aerosol Properties With the available measurements of soot primary size dp and soot volume fraction fv, and under the assumption of point contact among primary particles forming each aggregate, the total number of primary particles per unit volume (Np) and the
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K.-O. LEE ET AL. TABLE 1
Selected Soot Aerosol Properties in the Three Methane Nonpremixed Flames d p,max Z fv Np ST Ms (nm) (mm) (ppm) (#/cm3) (cm⫺1) (g)
Flame
Methane/Air 19.8 Methane/50% O2 21.7 Methane/Oxygen 10.9
50 16 6
0.29 0.7 ⫻ 1011 0.88 0.77 0.60 1.1 ⫻ 1011 1.66 0.31 0.15 2.2 ⫻ 1011 0.83 0.05
total surface area (ST) of particulate phase per unit volume of the flame gas can be estimated using the following expressions [38] Np ⫽
6f v , d p3
ST ⫽
6f v dp
(1)
The number density of primary particles Np is, in general, higher than the corresponding number density of soot aggregates at each flame location. The value of ST calculated from the above expression forms an upper bound of the actual soot surface area, because soot primary particles are usually fused together as a result of surface deposition [38]. Table 1 lists the overall maximum soot primary sizes dp,max, along with the specific axial locations (Z) where these were measured in the three flames. The table also lists the measured values of soot volume fraction fv at the location of dp,max, as well as the calculated values of Np and ST for the three flames. As seen in the table, the soot number density (Np) in the methane/ 50% oxygen flame is ⬃60% higher than that in the methane/air (base) flame, while the Np value in the methane/oxygen flame appears to be higher by a factor of approximately 3 compared to the base flame. It has been reported [38] that, while the primary particle size and the degree of agglomeration are strongly influenced by the location within a steady coannular diffusion flame, Np remains constant along the soot annulus throughout the soot growth period. This implies that primary-particle collisional coalescence does not occur. Thus, the Np data listed in Table 1 suggest that the rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. This trend is in agreement with the results reported in [22]. The data for ST in Table 1 represent that, although the maximum soot primary size in the methane/
oxygen flame is considerably smaller than dp in the other two flames, the soot surface area ST is similar to the base flame. The reason for the higher value of ST in the methane/50% oxygen flame (Table 1) is not known at this time. Soot Yields Based on the measured soot volume fraction distributions fv(Z,r), the instantaneous mass of soot particles (Ms) within the entire flame volume can be calculated using the following expression Hf
R
冕 冕
M s ⫽ s dZ f v共Z,r兲2 r dr 0
(2)
0
where R is the radius of the luminous flame envelope at height Z, Hf is the height of the luminous zone, and s is the density of soot, taken as 1.8 g/cm3 [12, 13]. This value is very similar to 1.84 g/cm3, as measured by Rossman and Smith for acetylene black [39], and slightly larger than the value of 1.74 g/cm3 reported by Choi et al. [40] for soot collected in the postflame region of an acetylene/air premixed flame. The inner integral seen in Eq. 2 represents the area-integrated soot volume fraction at a fixed height Z. For each of the three flames, this quantity was determined at eight equally interspaced axial stations. A linear interpolation scheme was used to evaluate the values of this integral at axial stations where no measurements of fv(Z,r) were conducted. Finally, the calculation of Ms was performed using the measured soot volume fraction distributions up to Z ⫽ 7/8Hf. Above that height, only negligible amounts of soot existed. It is noted that the fuel flow rates in all three flames investigated herein were identical. Thus, comparison of the values of Ms also translates into comparison of soot yields in these flames. As seen in the last column of Table 1, the soot yields for the oxygenenriched flames are lower by approximately a factor of 2.5 (methane/50% oxygen flame) and 15 (methane/oxygen flame), compared to the methane/air flame. The results suggest that the primary cause for this reduction is the decrease of flame dimensions in the oxygen-enriched flames. This is especially obvious for the meth-
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES ane/50% oxygen flame, which despite having higher soot volume fractions compared to the methane/air flame, has a lower value of Ms. It is acknowledged, however, that other important factors (such as flow fields or flame structure) may be at play. For example, Sugiyama [41], who investigated the mechanism of soot reduction in laminar coflow nonpremixed flames by fuel dilution and oxygen enrichment of the oxidizer stream, suggested that the observed soot yield reduction can be explained by changes in the velocity field and the flame shape. Du and Axelbaum [42] reported that soot inception can be dramatically affected by flame structure through changes in fuel pyrolysis and oxidation rates adjacent to the pyrolysis zone. Finally, Lin and Faeth [43] demonstrated the significance (on soot production) of flow velocities normal to the flame sheet, in an opposed-jet nonpremixed flame geometry. Aggregate Fractal Properties Monomer– cluster and cluster– cluster collisions are considered as typical soot aggregate growth mechanisms [44]. Such collisions may be classified into one of the following three different regimes: reaction-limited, ballistic, or diffusionlimited. The growth model of soot aggregates for a specific flame can be identified by evaluating the fractal dimension Df of the aggregate population. This quantity is defined by n ⫽ kf
冋 册 Rg dp
Df
(3)
where n is the number of primary particles per aggregate, kf the prefactor, and Rg the radius of gyration of the aggregate. With the primary particle cross section Ap ⫽ dp2/4, n is defined as n⫽
冋 册 A Ap
␣
(4)
where A represents the projected area of the aggregate, and ␣ ⫽ 1.09 [25]. In order to evaluate the fractal dimension, the radius of gyration Rg must be measured. This quantity is defined in terms of the distance ri of the center of each primary particle from the centroid of mass of the aggregate,
R g2 ⫽
1 n
冘r
331
2 i
(5)
i
In order to measure A and Rg of soot aggregates representing specific populations (i.e., collected from specific locations of a flame), a 512 pixel ⫻ 480 pixel resolution data acquisition board was first used to digitize a large number of aggregate images. The digitized image of each aggregate was automatically scanned to determine its projected surface area A and radius of gyration Rg. The value of dp at that location was used along with Eq. 4 to determine the number of primaries n in that aggregate. The process was repeated until all aggregates had been analyzed. Based on these calculations, the average number of soot primaries per aggregate was obtained for the locations where the maximum primary sizes were observed for the methane/air and the methane/ 50% oxygen flame. These values were 16 and 32, respectively, and were based on the analysis of 96 aggregates in the methane/air flame and 159 aggregates in the methane/50% oxygen flame. The increased number of primaries per soot aggregate in the methane/50% oxygen flame is consistent with the higher values of fv and Np in that flame (see Table 1). It is also important to note that the larger aggregates in the methane/50% oxygen flame are formed within shorter residence times compared to the methane/air flame. The higher degree of agglomeration in the oxygen-enriched flame may be a result of its higher temperatures, which cause more rapid and possibly more effective collision processes, thus resulting in higher soot agglomeration rates. Unfortunately, the small spatial extent of the methane/oxygen flame degraded the spatial resolution of thermophoretic sampling and did not allow a similar measurement of aggregate properties in that flame. The calculated values of n and Rg for each soot aggregate of a specific population were substituted into Eq. 3 to evaluate the fractal dimension of soot in each flame. Figures 5a and 5b display a logarithmic plot of n vs. the ratio Rg/dp. Each open circle in the plot corresponds to a measured soot aggregate. As indicated by Eq. 3, the slope of the linear regression line represents the fractal dimension Df. The fractal dimensions of 1.64 and 1.65 were obtained, respectively, for the methane/air and the methane/50% oxygen flames. These val-
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K.-O. LEE ET AL.
Fig. 5. Logarithmic plots of n vs. Rg/dp for (a) the methane/air flame, and (b) the methane/50% oxygen flame. Each open circle represents a measured soot aggregate. The slope of the linear regression line drawn in each plot represents the fractal dimension Df.
ues of fractal dimension are similar to the values obtained for soot by other investigators [20, 25]. The values of Df obtained in this study indicate that soot particles are formed in both methane flames by cluster– cluster aggregation in the diffusion-limited regime, which eventually creates the chainlike soot clusters. CONCLUSIONS Soot aerosol properties and aggregate microstructure have been investigated in laminar jet nonpremixed methane flames, featuring varying oxygen concentrations in the oxidizer stream. The conditions investigated correspond to oxygen-enriched operation (50% and 100% oxygen content in the ventilating O2/N2 coflow). The soot properties in the oxygen-enriched flames were compared to those in a conventional methane/air coannular flame of 79 mm luminous height. Increased concentrations of oxygen produced highly luminous flames with drastically reduced spatial dimensions (25 mm height for the methane/50% oxygen flame; 12 mm height for the methane/oxygen flame). The microstructure of soot collected from selected flame locations was determined by combining thermophoretic sampling, transmission electron microscopy and image analysis techniques. A laser-light extinction technique
was employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot primary sizes in the methane/air and the methane/50% oxygen flames are in the same range, whereas the corresponding primary sizes in the methane/oxygen flame are lower by a factor of 2. As a consequence, soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the methane/air flame. Comparisons of primary particle number densities in the three flames suggest that the rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot surface areas per unit volume of the aerosol did not show any coherent trend. Soot yields, as estimated in terms of instantaneous soot mass in each flame, were drastically diminished with increasing oxygen concentration. Soot aggregate population data on the high-density soot annulus suggested a higher degree of agglomeration in the methane/50% oxygen flame (compared to the base methane/air flame). Finally, the fractal dimensions of selected soot aggregate samples were measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame); these values are similar to fractal dimensions published previously and suggest that soot in methane flames is formed by cluster– cluster aggregation in the diffusion-limited regime.
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES This research was supported by the Air Liquide Corp. under Grant 98-1-374. The authors are indebted to Prof. Mun Y. Choi of the Department of Mechanical Engineering of UIC, who developed the image processing algorithm for the aggregate analysis. K.-O.L. is grateful to Prof. Kenneth Brezinsky of the Department of Chemical Engineering of UIC for his valuable discussions and encouragement. Finally, S.Z. gratefully acknowledges the Graduate Fellowship provided by Air Liquide.
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Gupta, S. B., and Santoro, R. J. (1994). Eastern States Section of The Combustion Institute, pp. 238 –241. Kitano, M., and Kawamura, T., JSME Int. J., Series B 39:822 (1996). Sidebotham, G. W., and Glassman, I., Combust. Sci. Technol. 81:207 (1992). Du, D. X., Axelbaum, R. L., and Law, C. K., TwentyThird Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 1501. Howard, J. B., and Kausch, W. J. Jr., Prog. Energy Combust. Sci. 6:263 (1980). Zhang, J., and Megaridis, C., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, p. 593. Bonczyk, P. A., Combust. Sci. Technol. 59:143 (1988). Haynes, B. S., Jander, H., and Wagner, H. G., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1979, p. 1365. Duboudin, T., Charon, O., and Genies, B. The Advantages of Pure Oxygen Use in High Temperature Processes, IFRF Conference, Loughborough, U.K., 1993, p. 1. Milosavljevic, I., Charon, O., Degrand, M., and de Freitas, J. (1998). Combustion Improvements for the Environment in MSW Incineration. International Conference on Incineration and Thermal Treatment Technologies, Salt Lake City, p. 283. Shaddix, C. R., and Smyth, K. C., Combust. Flame 107:418 (1996). Lee, K. O., and Choi, M. Y., Microgravity Sci. Technol. X/2:86 (1997). Choi, M. Y., and Lee, K. O., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, p. 1243. Zhang, J., and Megaridis, C. M., Combust. Flame 112:473 (1998). Tien, C. L., and Lee, S. C., Prog. Energy Combust. Sci. 8:41 (1982). Baukal, C. E., and Gebhart, B., Int. J. Heat Mass Transfer 40:2539 (1997). D’Alessio, A., Beretta, F., and Venitozzi, C., Combust. Sci. Technol. 5:263 (1972). Wieschnowsky, U., Bockhorn, H., and Fetting, F., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 343.
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Sadakata, M., Motegi, M., Mashio, Y., Ishida, T., Harano, A., and Kim, H. J., Trans. I. Chem. E. 73:142 (1995). Zhang, H. X., Sorensen, C. M., Ramer, E. R., Olivier, B. J., and Merklin, J. F., Langmuir 4:867 (1988). Lu, J. C., and Sorensen, C. M., Langmuir 9:2861 (1993). Vandsburger, U., Kennedy, I., and Glassman, I., Combust. Sci. Technol. 39:263 (1984). Glassman, I., and Yaccarino, P., Combust. Sci. Technol. 24:107 (1980). Zelepouga, S. A., Saveliev, A. V., Kennedy, L. A., and Fridman, A. A., Combust. Flame, submitted. Megaridis, C. M., and Dobbins, R. A., Combust. Sci. Technol. 71:95 (1990). Kaplan, C. R., Shaddix, C. R., and Smyth, K. C., Combust. Flame 106:392 (1996). Dobbins, R. A., and Megaridis, C. M., Langmuir 3:254 (1987). Santoro, R. J., Semerjian, H. G., and Dobbins, R. A., Combust. Flame 51:203 (1983). Shaddix, C. R., Harrington, J. E., and Smyth, K. C., Combust. Flame 99:723 (1994). Dasch, C. J., Appl. Optics 31:1146 (1992). Smyth, K. C., and Shaddix, C. R., Combust. Flame 107:314 (1996). Zhou, Z.-Q., Ahmed, T. U., and Choi, M. Y., Exp. Thermal Fluid Sci. 18:27 (1998). Mulholland, G. W., and Choi, M. Y., Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, p. 1515. Roper, F. G., Combust. Flame 29:219 (1977). Roper, F. G., Smith, C., and Cunningham, A. C., Combust. Flame 29:227 (1977). Turns, S. R., An Introduction to Combustion, McGraw Hill, New York, 1996, pp. 279 –286. Dobbins, R. A., and Subramaniasivam, H., in Soot Formation in Combustion: Mechanisms and Models (H. Bockhorn, Ed.), Springer-Verlag, Berlin, 1994, p. 290. Megaridis, C. M., and Dobbins, R. A., Combust. Sci. Technol. 66:1 (1989). Rossman, R. P., and Smith, W. R., Ind. Eng. Chem. 35:972 (1943). Choi, M. Y., Mulholland, G. W., Hamins, A., and Kashiwagi, T., Combust. Flame 102:161 (1995). Sugiyama, G., Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, p. 601. Du, J., and Axelbaum, R. L., Combust. Flame 100:367 (1995). Lin, K.-C., and Faeth, G. M., AIAA J. Prop. Power 12:691 (1996). Schaefer, D. W., Mater. Res. Soc., MRS Bulletin 12:22, 1988.
Received 14 January 1999; revised 24 August 1999; accepted 30 August 1999
Department of Chemical Engineering, The University of Illinois at Chicago, Chicago, IL 60607, USA
CONSTANTINE M. MEGARIDIS*, SERGUEI ZELEPOUGA, ALEXEI V. SAVELIEV, and LAWRENCE A. KENNEDY
Department of Mechanical Engineering, The University of Illinois at Chicago, Chicago, IL 60607, USA
OLIVIER CHARON
Chicago Research Center, American Air Liquide, Countryside, IL 80525, USA and
FOUAD AMMOURI
Centre de Recherche Claude-Delorme, Air Liquide, Jouy-en-Josas 78353, France This experimental investigation analyzes the soot formation effects of oxygen concentration in the oxidizer stream (O2 ⫹ N2) ventilating laminar jet nonpremixed methane flames. The base flame incorporates air as the oxidizer; two additional flames, with respective oxygen concentrations of 50% and 100% in the ventilating coflow, are also examined. The microstructure of soot collected from selected flame locations is determined combining thermophoretic sampling and transmission electron microscopy. A laser-light extinction technique is employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the respective rates in the methane/air base flame. The rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot yields diminish with increasing oxygen concentration, as do luminous flame spatial dimensions. Soot aggregate data on the soot annulus suggest a higher degree of agglomeration under oxygen-enriched conditions. Finally, the fractal dimensions of selected soot aggregate samples are measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame), being similar to previously published values for carbonaceous soot. © 2000 by The Combustion Institute
INTRODUCTION Conventional nonpremixed gaseous combustion has primarily employed air as an oxidizer, mainly because of its relatively low cost and ample supply. As a result, hydrocarbon/air combustion systems frequently emit harmful pollutants, such as CO, HC, NOx, as well as soot particulates. Among these pollutants, NOx is essentially produced due to the high relative concentrations of nitrogen (79% in volumetric proportion) in the high temperature environment. As hydrocarbon-fuel combustion systems are used extensively, concerns for the reduction of pollutant emissions have been increased. A number of investigations have focused on re*Corresponding author: E-mail: [email protected] † Current address: Energy Systems Division, Argonne Nat. Laboratory, Argonne IL 60439 COMBUSTION AND FLAME 121:323–333 (2000) © 2000 by The Combustion Institute Published by Elsevier Science Inc.
ducing harmful pollutant emissions by introducing several effective emission reduction techniques, such as mixing different fuels [1, 2], or using either gaseous [3, 4] or metal additives [5– 8]. Among continuing efforts for the development of low-emission combustion systems, oxygen-enhanced combustion is deemed promising because of several advantages [9, 10]. For example, the use of pure oxygen as an oxidizer in hydrocarbon-fuel combustion reduces the required volume of the oxidizer, increases the maximum flame temperature and thermal efficiency, and improves flame stability. More importantly, when possible, the use of pure oxygen also eliminates the emission of NOx. Compared to hydrocarbon/air combustion, the higher flame temperatures in hydrocarbon/oxygen systems should lead to higher rates of soot inception and growth. On the other hand, the in0010-2180/00/$–see front matter PII S0010-2180(99)00131-5
324 creased presence of oxygen promotes oxidative mechanisms, thus tending to diminish soot concentrations. It is well established that the residence time of soot particles is an important parameter affecting soot formation in nonpremixed flames [11–14]. Due to changes in flame structure, flame residence times decrease when oxygen concentrations are raised in the oxidizer stream. These flame structure changes with varying oxygen concentrations are expected to play an important role in the current study. Selected studies of gaseous hydrocarbon/oxygen premixed combustion have been performed to characterize flame radiation emitted from both luminous and nonluminous flames [15, 16], and calculate the optical properties of soot particles produced therein [17]. Other oxygenenhanced premixed hydrocarbon flame configurations were investigated to determine the characteristics of soot inception/nucleation and growth in terms of soot concentration measurements [18, 19]. The fractal morphology and microstructure of soot aggregates have also been analyzed in premixed methane/oxygen flames [20, 21]. While the above investigations have contributed to better understanding of soot formation mechanisms in premixed hydrocarbon/ oxygen flames, improved understanding of the corresponding characteristics of nonpremixed hydrocarbon/oxygen combustion may allow exploitation of the associated low pollutant emission properties and high energy efficiency. To this end, several soot studies ([22] and references cited therein) have targeted the effect of varying oxygen concentration in the oxidizer flow (called oxygen index) in nonpremixed flames. Vandsburger et al. [22] measured soot production along the stagnation streamline in ethylene and propane stagnation point nonpremixed flames, and paid particular attention to the effect of oxygen index. They reported that soot particle production was increased and the region of particle formation was extended with rising oxygen index. Glassman and Yaccarino [23] investigated the sooting height of an axisymmetric nonpremixed flame as a function of oxygen index. They reported that sooting tendency was minimum at an oxygen index of around 0.24. A recent study of gaseous methane/oxygen and methane/enhanced-oxygen combustion was performed to characterize flame radiation [24]. The current study focuses on well-defined
K.-O. LEE ET AL. laminar jet nonpremixed methane flames featuring increased oxygen content in the oxidizing stream. The work provides comparisons of these flames with their methane/air counterpart. Soot microstructure and fractal morphology are investigated in these flames by combining thermophoretic soot sampling and subsequent transmission electron microscopy (TEM) analysis. A computer-based image processing system is used to calculate soot primary sizes and fractal dimensions of soot aggregates at selected flame locations. Soot volume fraction distributions are also measured by laser light extinction and tomographic reconstruction techniques. EXPERIMENTAL SETUP AND TECHNIQUES The coflow laminar burner employed in this study is identical to those used in other investigations [6, 14, 25]. The methane fuel and oxidizer were supplied through two concentric tubes of 11.1 mm and 101.6 i.d., respectively. Three stable laminar nonpremixed flames featuring different oxygen content in the oxidizer stream were considered: 1) A flame burning methane fuel and bottled dry air flowing at 7.5 cm3/s and 635 cm3/s, respectively. These flow rates are identical to those adopted in the steady methane/air nonpremixed flame studies of [11, 14], and correspond to overall residence times of ⬃100 ms [26]. This flame is referred to as methane/air flame (luminous height ⬃79 mm). 2) A flame burning methane fuel and oxygen (99.8 % pure) flowing at 7.5 cm3/s and 635 cm3/s, respectively. This flame is referred to as methane/oxygen flame (luminous height ⬃12 mm). 3) A flame burning methane fuel and a 50% N2–50% O2 gas mixture flowing at 7.5 cm3/s and 635 cm3/s, respectively. This flame is referred to as a methane/50% oxygen flame (luminous height ⬃25 mm). All three methane jet nonpremixed flames considered in this study burned 99.7% pure fuel and released no soot from their closed tip. In order to observe the morphological changes that the soot particles are subjected to, thermo-
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES phoretic soot sampling [27] was performed at several axial locations in each flame. The residence time of the sampling probe was approximately 35 ms for all tests; in all cases a sufficient number of soot aggregates was collected for subsequent analysis by electron microscopy. TEM analysis of each probe provided the spatial distribution of soot morphology at fixed heights above the burner mouth. A standard laser-light extinction technique [28] was employed to measure the soot volume fraction distributions within the three flames. A 6-mW HeNe laser was used as a light source (632.8 nm). The laser was coupled with an intensity stabilizer to assure long-term stability of the beam intensity; the drift was less than 0.1% in a period of 8 hours. The laser beam was focused on the flame axis (beam waist of ⬃150 m). Beam shift due to thermal lens effects was minimized through the procedure described in [29]. A broad band power/energy meter was employed to determine the preflame and transmitted light intensities (I0 and I, respectively). The power meter was equipped with a laser line filter to avoid interference from light originating from other sources. In order to measure the radial distributions of the transmitted light intensity at fixed heights above the burner mouth, the entire burner assembly was mounted on a two-dimensional step motor-driven system, while the optical setup remained fixed. The projection function P(x), equivalent to the lineof-sight integral of the soot volume fraction fv along a path at a distance x from the axis, was calculated using the following expression P 共 x兲 ⫽ ⫺ ln共I/I 0兲/K ext The corresponding field distribution function, fv, was obtained using a three-point Abel deconvolution algorithm [30]. The soot extinction constant Kext was taken to be 4.9; this value corresponds to an index of refraction m ⫽ 1.57– 0.56i [31]. It is noted that the above mentioned value of the extinction constant is smaller by nearly a factor of 2 compared to the one reported recently by Zhou et al. [32]. Kext affects the corresponding value of fv, but the issues related to the correct value of this quantity are beyond the scope of this work and will not be further addressed herein. A good treatment of this subject has been published by Mulholland and Choi [33].
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EXPERIMENTAL RESULTS AND ANALYSIS Luminous Flame Appearance Luminous flame appearance does not define the reaction zone, but it does provide a means of evaluating the spatial extent of the soot zone. Figures 1a– c show luminosity images of the three jet nonpremixed flames studied herein. The methane/air flame of Fig. 1a was verified to be identical to that analyzed in [11, 14]. The flame displays the familiar elongated structure with a closed tip indicative on its nonsootemitting character. Figure 1b corresponds to the methane/50% oxygen flame; it is apparent that the flame luminous length decreases considerably with increased oxygen content in the oxidizer stream. This trend is also verified in Fig. 1c, which displays the corresponding image for the methane/oxygen flame. The luminous heights depicted in Fig. 1 can be compared to the flame lengths obtained using Roper’s approach [34, 35]. According to this model, a jet diffusion flame ends on the symmetry axis at the point where oxygen and fuel meet at the stoichiometric ratio. The flame heights predicted by Roper’s model [36] were 64 mm for the methane/air flame, 30 mm for the methane/50% oxygen flame, and 17 mm for the methane/ oxygen flame. These height estimates for the first two flames are within 20% of the observed values listed in Fig. 1. But the height estimate for the methane/oxygen flame does not closely approximate the actual luminous length (17 mm vs. 12 mm). This is probably due to the relatively larger influence of burner heat losses in the methane/oxygen flame, which is the smallest one of all flames seen in Fig. 1. Despite this disagreement, the predicted flame heights confirm the observed trend of diminishing flame size with rising oxygen content in the ventilating stream. The flame luminosity measurements showed the methane/oxygen flame to be much brighter than the methane/air flame. The methane/50% oxygen flame appeared to be darker than the methane/oxygen flame, but was still brighter than the methane/air flame. These variations in luminous appearance suggest substantial disparity in temperatures among these flames. The
326
K.-O. LEE ET AL. into consideration, were 2226 K for the methane/ air flame, 2814 K for the methane/50% oxygen flame, and 3054 K for the methane/oxygen flame. The increasing temperature with rising oxygen content in the oxidizer stream is consistent with the observed flame luminosity differences. Soot Morphology
Fig. 1. Luminosity images of the three jet nonpremixed flames studied herein. Hf denotes the luminous zone height. (a) Methane/Air (Hf⫽79 mm), (b) Methane/50% O2(Hf⫽25 mm), (c) Methane/O2(Hf⫽12 mm).
adiabatic flame temperatures for the three cases were calculated using STANJAN.* These temperatures, which were determined taking dissociation *Reynolds, W. C. The element potential method for chemical equilibrium analysis: Implementation in the interactive program STANJAN, Stanford University, Jan. 1986.
For the methane/50% oxygen flame, thermophoretic sampling tests were performed at several axial locations ranging from 8 mm to 20 mm, interspaced by 4 mm. For the methane/air flame the sampling results published in Ref. 14 were utilized. In that work, the axial locations sampled ranged from 35 mm to 70 mm. Finally, the small volume of the methane/oxygen flame (see Fig. 1c) did not allow extensive thermophoretic soot sampling tests in that flame. Such tests were performed only at an axial height Z ⫽ 6 mm, in order to obtain a representative picture of soot morphology in that flame. An additional difficulty in sampling thermophoretically the methane/oxygen flame was caused by its much higher temperature compared to the other two flames. The high gas temperatures allowed only very rapid insertion tests, which were necessary to avoid deterioration of the 20-nm-thick carbon substrate supported on the copper grid. The results of the thermophoretic sampling experiments are presented below; they have been based on tests that left the carbon substrates intact, thus precluding oxidation of the overlaying soot particles after their extraction from the flame gases. Figures 2a– d show selected TEM photomicrographs of soot particles extracted thermophoretically from the three methane flames investigated in this study. These micrographs were taken at the same magnification (100,000⫻). As established previously for laminar jet nonpremixed flames [27], aggregates collected from the same flame location consist of primary particles that are approximately spherical and have a nearly uniform diameter dp. This parameter changes with flame coordinates. Frames 2a and 2b, respectively, display soot collected from the axis and the annulus of maximum soot volume fraction (called soot annular region [28]) of the methane/50% oxygen flame at an axial height Z ⫽ 12 mm above
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES
327
Fig. 2. TEM photomicrographs of soot particles extracted thermophoretically from the three methane flames investigated in this study. Frames (a) and (b), respectively, correspond to the axis and the soot annulus of the methane/50% oxygen flame at an axial height Z ⫽ 12 mm. Frame (c) corresponds to the annular region of the methane/air flame at Z ⫽ 50 mm. Frame (d) shows soot collected from the annular region of the methane/oxygen flame at Z ⫽ 6 mm.
the burner mouth. The soot sample extracted from the flame axis (Fig. 2a) includes soot precursor particles as well as larger aggregates. The soot precursor particles appear as small diffuse particles in the background, as established in Ref. 37. The presence of the precursor particles there indicates that soot inception occurs at this location. The larger soot aggregates seen in Fig. 2a were probably collected when the probe pierced through the soot annulus during insertion. The particles in Fig. 2b correspond to the soot annulus at the same height Z ⫽ 12 mm; they represent the overall maximum primary size seen in this flame (see below). The apparent similarity of the aggregates seen in Fig. 2b with those in Fig. 2a verifies the earlier statement that the larger aggregates seen near the flame axis were collected when the probe penetrated the soot annulus. Frame 2c displays soot collected from the annular
region of the methane/air flame at a height Z ⫽ 50 mm. These particles also correspond to the overall maximum primary size seen in this flame (see below). Frame 2d shows soot collected from the annular region of the methane/oxygen flame at the only sampled height of Z ⫽ 6 mm. It can be seen that the largest aggregates in the methane/50% oxygen flame (Fig. 2b) are larger than those in the methane/air flame (Fig. 2c), as well as those in the methane/oxygen flame (Fig. 2d). Finally, TEM morphological observations of soot collected from the vicinity of the flame axis at the height Z ⫽ 6 mm of the methane/ oxygen flame also revealed the presence of precursor particles. A large number of TEM photomicrographs was utilized to analyze the microstructure of soot particles for each flame. A population of more than a hundred primary particles was measured to determine the mean soot primary size at the soot annulus of each axial
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Fig. 3. Soot primary particle diameters measured at selected axial locations of the three methane flames investigated herein. These values represent the peak values of soot primary size at each height. The measurement uncertainty is indicated by the vertical error bars; each bar denotes two standard deviations. Each curve is marked to represent the O2 composition of the oxidizing stream (the balance is N2). The height uncertainty is ⬃1 mm.
location sampled in the three flames. In this process, a Micro-Comp high-resolution image acquisition/analysis system was utilized. Figure 3 displays the average primary diameters measured at each location. These values represent the peak values of soot primary size at each height. The measurement uncertainty is indicated by the vertical error bars in Fig. 3; each bar denotes two standard deviations. As also reported in [14], the experimental data for the methane/air flame clearly show that the soot primary particle size is strongly dependent upon the axial location within the luminous flame. The maximum value of dp for the methane/air flame was approximately 20 nm at Z ⫽ 50 mm. The corresponding data for the methane/50% oxygen flame exhibit a significant change in soot primary size with axial distance within this relatively small flame; luminous flame height was ⬃25 mm. The maximum value of dp for this flame was approximately 22 nm and was seen at Z ⫽ 16 mm. Based on the conducted soot morphological observations, the soot particles in the methane/50% oxygen flame experience the same sequence of events (inception, growth, oxidation) as those in the methane/air flame, but in a much shorter period. Finally, as seen in Fig. 3, the soot primary diameter measured in the soot annulus at Z ⫽ 6 mm of the methane/oxygen flame was about 11 nm. Since this height is midway
K.-O. LEE ET AL. along the luminous zone of the methane/oxygen flame, the overall maximum soot primary size in this flame is expected to be close to 11 nm. The experimental data on dp displayed in Fig. 3, combined with the significantly different residence times in the three flames investigated herein, are worthy of further discussion. The reduced residence times in the oxygen-enriched flames tend to inhibit soot growth; at the same time, the higher temperatures in these flames are expected to enhance soot formation; finally, the ample presence of oxygen in the oxygenenriched flames is expected to promote soot oxidative mechanisms. It has been established [25] that primary particle growth in laminar nonpremixed flames represents soot aggregate growth via surface reactions. Consequently, the similar maximum soot primary sizes observed for the methane/50% oxygen and methane/air flames (Fig. 3) suggest that soot surface growth rates are much stronger under oxygen-enriched conditions. Furthermore, the rapid decrease of soot primary sizes above Z ⫽ 16 mm in the methane/50% oxygen flame indicates stronger soot oxidation rates in this flame, compared to its air counterpart. On the other hand, the substantially lower value of dp in the methane/oxygen flame (see Fig. 3), combined with its low residence time, do not allow any firm conclusions on how soot growth or oxidation rates in this flame compare to the respective rates in the other two flames. Du et al. [4] also used a methane diffusion flame to examine the soot formation effects of oxygen concentrations on the oxidizer side. They concluded that the effect of varying the oxygen concentration between 15% and 21% (mole) is almost entirely thermal. The corresponding dilution and chemical effects were found to be negligible. But it is important to note that the results reported by Du et al. [4] were based on oxygen concentrations that were substantially lower than those considered in the current study. Soot Volume Fraction Distributions Figures 4a– c present the spatial distributions of soot volume fractions for the methane flames operated at three different oxygen concentrations in the ventilating stream: 21% (air), 50%, and 100%, respectively. In the case of the methane/air flame (Fig. 4a), the soot annular
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES
329
maximum soot volume fraction in that flame was ⬃0.36 ppm, and was measured near the symmetry axis at Z ⫽ 60 mm. The soot volume fraction dramatically decreases with axial distance thereon, and eventually vanishes near the tip of the flame. Figure 4b presents the radial distributions of soot volume fractions in the methane/50% oxygen flame at several axial locations. The soot distributions have higher overall values and display a similar structure to those in the methane/air flame. The maximum soot volume fraction in the methane/50% oxygen flame is approximately 0.6 ppm and was measured at a height Z ⫽ 16 mm. As seen in other laminar nonpremixed flames of this type [25], the height at which the maximum soot concentration appears in this flame coincides with the axial location at which the maximum soot primary size was measured (Fig. 3). It is noted that in the methane/air flame, the maximum soot concentration occurs at Z ⫽ 60 mm (Fig. 4a), while maximum primary size attained is at Z ⫽ 50 mm (Fig. 3). Figure 4c shows the radial distributions of soot volume fractions in the methane/oxygen flame at four different axial locations, including Z ⫽ 6 mm where the only soot sampling test was performed. The overall maximum soot volume fraction through the entire flame appears to be approximately 0.15 ppm and was measured at Z ⫽ 6 mm. This value is lower than those in the methane/air and methane/50% oxygen flames by a factor of 2.4 and 4, respectively. The fact that the maximum soot volume fraction appears at Z ⫽ 6 mm in these measurements, validates the choice of this height for thermophoretic soot sampling tests in the methane/oxygen flame (see previous section). Furthermore, since the heights where the maximum soot volume fraction and maximum primary size occur are approximately the same, it can be inferred that the only data point for dp in Fig. 3 probably marks the peak of the dp vs. Z curve for the methane/oxygen flame. Fig. 4. Spatial distributions of soot volume fractions for the methane flames operated at three different oxygen concentrations in the ventilating stream: (a) 21% (air), (b) 50%, and (c) 100%. The balance is nitrogen.
region shifts to the flame axis between the heights of 50 mm and 60 mm. The overall
Soot Aerosol Properties With the available measurements of soot primary size dp and soot volume fraction fv, and under the assumption of point contact among primary particles forming each aggregate, the total number of primary particles per unit volume (Np) and the
330
K.-O. LEE ET AL. TABLE 1
Selected Soot Aerosol Properties in the Three Methane Nonpremixed Flames d p,max Z fv Np ST Ms (nm) (mm) (ppm) (#/cm3) (cm⫺1) (g)
Flame
Methane/Air 19.8 Methane/50% O2 21.7 Methane/Oxygen 10.9
50 16 6
0.29 0.7 ⫻ 1011 0.88 0.77 0.60 1.1 ⫻ 1011 1.66 0.31 0.15 2.2 ⫻ 1011 0.83 0.05
total surface area (ST) of particulate phase per unit volume of the flame gas can be estimated using the following expressions [38] Np ⫽
6f v , d p3
ST ⫽
6f v dp
(1)
The number density of primary particles Np is, in general, higher than the corresponding number density of soot aggregates at each flame location. The value of ST calculated from the above expression forms an upper bound of the actual soot surface area, because soot primary particles are usually fused together as a result of surface deposition [38]. Table 1 lists the overall maximum soot primary sizes dp,max, along with the specific axial locations (Z) where these were measured in the three flames. The table also lists the measured values of soot volume fraction fv at the location of dp,max, as well as the calculated values of Np and ST for the three flames. As seen in the table, the soot number density (Np) in the methane/ 50% oxygen flame is ⬃60% higher than that in the methane/air (base) flame, while the Np value in the methane/oxygen flame appears to be higher by a factor of approximately 3 compared to the base flame. It has been reported [38] that, while the primary particle size and the degree of agglomeration are strongly influenced by the location within a steady coannular diffusion flame, Np remains constant along the soot annulus throughout the soot growth period. This implies that primary-particle collisional coalescence does not occur. Thus, the Np data listed in Table 1 suggest that the rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. This trend is in agreement with the results reported in [22]. The data for ST in Table 1 represent that, although the maximum soot primary size in the methane/
oxygen flame is considerably smaller than dp in the other two flames, the soot surface area ST is similar to the base flame. The reason for the higher value of ST in the methane/50% oxygen flame (Table 1) is not known at this time. Soot Yields Based on the measured soot volume fraction distributions fv(Z,r), the instantaneous mass of soot particles (Ms) within the entire flame volume can be calculated using the following expression Hf
R
冕 冕
M s ⫽ s dZ f v共Z,r兲2 r dr 0
(2)
0
where R is the radius of the luminous flame envelope at height Z, Hf is the height of the luminous zone, and s is the density of soot, taken as 1.8 g/cm3 [12, 13]. This value is very similar to 1.84 g/cm3, as measured by Rossman and Smith for acetylene black [39], and slightly larger than the value of 1.74 g/cm3 reported by Choi et al. [40] for soot collected in the postflame region of an acetylene/air premixed flame. The inner integral seen in Eq. 2 represents the area-integrated soot volume fraction at a fixed height Z. For each of the three flames, this quantity was determined at eight equally interspaced axial stations. A linear interpolation scheme was used to evaluate the values of this integral at axial stations where no measurements of fv(Z,r) were conducted. Finally, the calculation of Ms was performed using the measured soot volume fraction distributions up to Z ⫽ 7/8Hf. Above that height, only negligible amounts of soot existed. It is noted that the fuel flow rates in all three flames investigated herein were identical. Thus, comparison of the values of Ms also translates into comparison of soot yields in these flames. As seen in the last column of Table 1, the soot yields for the oxygenenriched flames are lower by approximately a factor of 2.5 (methane/50% oxygen flame) and 15 (methane/oxygen flame), compared to the methane/air flame. The results suggest that the primary cause for this reduction is the decrease of flame dimensions in the oxygen-enriched flames. This is especially obvious for the meth-
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES ane/50% oxygen flame, which despite having higher soot volume fractions compared to the methane/air flame, has a lower value of Ms. It is acknowledged, however, that other important factors (such as flow fields or flame structure) may be at play. For example, Sugiyama [41], who investigated the mechanism of soot reduction in laminar coflow nonpremixed flames by fuel dilution and oxygen enrichment of the oxidizer stream, suggested that the observed soot yield reduction can be explained by changes in the velocity field and the flame shape. Du and Axelbaum [42] reported that soot inception can be dramatically affected by flame structure through changes in fuel pyrolysis and oxidation rates adjacent to the pyrolysis zone. Finally, Lin and Faeth [43] demonstrated the significance (on soot production) of flow velocities normal to the flame sheet, in an opposed-jet nonpremixed flame geometry. Aggregate Fractal Properties Monomer– cluster and cluster– cluster collisions are considered as typical soot aggregate growth mechanisms [44]. Such collisions may be classified into one of the following three different regimes: reaction-limited, ballistic, or diffusionlimited. The growth model of soot aggregates for a specific flame can be identified by evaluating the fractal dimension Df of the aggregate population. This quantity is defined by n ⫽ kf
冋 册 Rg dp
Df
(3)
where n is the number of primary particles per aggregate, kf the prefactor, and Rg the radius of gyration of the aggregate. With the primary particle cross section Ap ⫽ dp2/4, n is defined as n⫽
冋 册 A Ap
␣
(4)
where A represents the projected area of the aggregate, and ␣ ⫽ 1.09 [25]. In order to evaluate the fractal dimension, the radius of gyration Rg must be measured. This quantity is defined in terms of the distance ri of the center of each primary particle from the centroid of mass of the aggregate,
R g2 ⫽
1 n
冘r
331
2 i
(5)
i
In order to measure A and Rg of soot aggregates representing specific populations (i.e., collected from specific locations of a flame), a 512 pixel ⫻ 480 pixel resolution data acquisition board was first used to digitize a large number of aggregate images. The digitized image of each aggregate was automatically scanned to determine its projected surface area A and radius of gyration Rg. The value of dp at that location was used along with Eq. 4 to determine the number of primaries n in that aggregate. The process was repeated until all aggregates had been analyzed. Based on these calculations, the average number of soot primaries per aggregate was obtained for the locations where the maximum primary sizes were observed for the methane/air and the methane/ 50% oxygen flame. These values were 16 and 32, respectively, and were based on the analysis of 96 aggregates in the methane/air flame and 159 aggregates in the methane/50% oxygen flame. The increased number of primaries per soot aggregate in the methane/50% oxygen flame is consistent with the higher values of fv and Np in that flame (see Table 1). It is also important to note that the larger aggregates in the methane/50% oxygen flame are formed within shorter residence times compared to the methane/air flame. The higher degree of agglomeration in the oxygen-enriched flame may be a result of its higher temperatures, which cause more rapid and possibly more effective collision processes, thus resulting in higher soot agglomeration rates. Unfortunately, the small spatial extent of the methane/oxygen flame degraded the spatial resolution of thermophoretic sampling and did not allow a similar measurement of aggregate properties in that flame. The calculated values of n and Rg for each soot aggregate of a specific population were substituted into Eq. 3 to evaluate the fractal dimension of soot in each flame. Figures 5a and 5b display a logarithmic plot of n vs. the ratio Rg/dp. Each open circle in the plot corresponds to a measured soot aggregate. As indicated by Eq. 3, the slope of the linear regression line represents the fractal dimension Df. The fractal dimensions of 1.64 and 1.65 were obtained, respectively, for the methane/air and the methane/50% oxygen flames. These val-
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Fig. 5. Logarithmic plots of n vs. Rg/dp for (a) the methane/air flame, and (b) the methane/50% oxygen flame. Each open circle represents a measured soot aggregate. The slope of the linear regression line drawn in each plot represents the fractal dimension Df.
ues of fractal dimension are similar to the values obtained for soot by other investigators [20, 25]. The values of Df obtained in this study indicate that soot particles are formed in both methane flames by cluster– cluster aggregation in the diffusion-limited regime, which eventually creates the chainlike soot clusters. CONCLUSIONS Soot aerosol properties and aggregate microstructure have been investigated in laminar jet nonpremixed methane flames, featuring varying oxygen concentrations in the oxidizer stream. The conditions investigated correspond to oxygen-enriched operation (50% and 100% oxygen content in the ventilating O2/N2 coflow). The soot properties in the oxygen-enriched flames were compared to those in a conventional methane/air coannular flame of 79 mm luminous height. Increased concentrations of oxygen produced highly luminous flames with drastically reduced spatial dimensions (25 mm height for the methane/50% oxygen flame; 12 mm height for the methane/oxygen flame). The microstructure of soot collected from selected flame locations was determined by combining thermophoretic sampling, transmission electron microscopy and image analysis techniques. A laser-light extinction technique
was employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot primary sizes in the methane/air and the methane/50% oxygen flames are in the same range, whereas the corresponding primary sizes in the methane/oxygen flame are lower by a factor of 2. As a consequence, soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the methane/air flame. Comparisons of primary particle number densities in the three flames suggest that the rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot surface areas per unit volume of the aerosol did not show any coherent trend. Soot yields, as estimated in terms of instantaneous soot mass in each flame, were drastically diminished with increasing oxygen concentration. Soot aggregate population data on the high-density soot annulus suggested a higher degree of agglomeration in the methane/50% oxygen flame (compared to the base methane/air flame). Finally, the fractal dimensions of selected soot aggregate samples were measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame); these values are similar to fractal dimensions published previously and suggest that soot in methane flames is formed by cluster– cluster aggregation in the diffusion-limited regime.
SOOT PROCESSES IN OXYGEN-ENRICHED FLAMES This research was supported by the Air Liquide Corp. under Grant 98-1-374. The authors are indebted to Prof. Mun Y. Choi of the Department of Mechanical Engineering of UIC, who developed the image processing algorithm for the aggregate analysis. K.-O.L. is grateful to Prof. Kenneth Brezinsky of the Department of Chemical Engineering of UIC for his valuable discussions and encouragement. Finally, S.Z. gratefully acknowledges the Graduate Fellowship provided by Air Liquide.
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Received 14 January 1999; revised 24 August 1999; accepted 30 August 1999