Burning Droplets of Heavy Oil Residual Blended with Diesel Light Oil

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Burning Droplets of Heavy Oil Residual Blended with Diesel Light Oil: Analysis of Coke Behaviors Guangwen Xu, Masiki Ikegami,* Senji Honma, Kouji Ikeda, Xiaoxun Mao, and Hiroshi Nagaishi National Institute of Advanced Industrial Science and Technology (AIST), Sapporo 062-8517, Japan Received September 5, 2002. Revised Manuscript Received January 6, 2003

Isolated droplet burning was conducted in a hot-air chamber for blended oils formed from mixing a heavy oil residual (HOR) and a diesel light oil (LO) at different fractions to investigate the burning characteristics of diversified heavy fuel oils (HFOs). Coke behaviors were analyzed based on measures of coke particle size, structure, burnout time, and droplet temperature history diagram during burning. The HOR amount in the droplet was found to be determinative for the coke particle sizes at different chamber temperatures for a given oil, and to the coke burnout times for different oils at a specified chamber temperature. From the same amount of HOR the pure-HOR oil tended to give a larger coke particle compared to the particle sizes of other blend oils with LOs higher than 10 wt %. The coke particle size, however, varied little with the chamber temperature when fuel was specified. The coke burnout time was definitely longer at higher chamber temperatures for equisize coke particles, but the times for different oils at each specified chamber temperature were correlative with the HOR amount in the droplet. The coke particle from pure-HOR oil might be more hollowed out, although its shell appeared to be thinner and less penetrated with holes. A few temperature inflections, which defined several characteristic temperatures of burning, were identified on the droplet temperature history diagram. This further clarified a necessary temperature, which is about 1150 K, for setting fire to the coke particles from HFOs, where the firing refers to the coke smoldering with luminescence or glowing. At rather higher chamber temperatures the coke residues formed during volatile burning should proceed into coke smoldering (oxidation) with a zero delay time after flame extinction. The article analyzed also the characteristic temperatures at flame extinction and ignition, and the peak temperature during coke smoldering.

Introduction 1.1. In Succession to Burning Step and Phase Distinctions. Intercorrelating, even unifying, the burning characteristics of different heavy fuel oils (HFOs) is indispensable to the development of topographical energy systems requiring fuel diversification. The HFO is a typical fuel for topographical energy plants (such as for the electricity-heat cogeneration boilers) operated at relatively small scales. The oil refers usually to the heavy fractions produced from petroleum refineries, but it is also typical and representative of many other fuels, such as waste and prolysis oils.1 The latter species of oils are becoming an increasingly important fuel resource in response to the stringent regulations on waste management and recycling. The burning characteristics of different HFOs has been generally investigated with respect to the initially different oils from different refineries2,3 or by adjusting the major property parameters of the oils.4-7 This was * To whom correspondence should be addressed. Tel: 81-11-8578961. Fax: 81-11-857-8900. E-mail: [email protected]. (1) Wornat, M. J.; Porter, B. G.; Yang, N. Y. C. Energy Fuels 1994, 8, 1131-1142. (2) Goldstein, H. L.; Siegmund, C. W. Environ. Sci. Technol. 1976, 10, 1109-1114.

recently accomplished in a different way by Xu et al.8,9 on the basis of a concern that any HFO can be treated as a blend of a heavy oil residual (HOR) and a diesel light oil (LO) at an appropriate mixing fraction (the bend being called HOR-LO blend hereafter). In fact, the blending can change the oil properties continuously to cater to the diversified HFOs practically encountered. With this, these two previous studies implemented a generalization of burning behaviors for different HFOs, while provided an insight into how the dilution of an HOR with a light oil influences the burning of the residual at the same time. Today, the efficient use of residual fuels is both attractive and challenging as well.10,11 (3) Cunningham, A. T. S.; Gliddon, B. J.; Jackson, P. J.; Jones, A. R.; Lawn, C. J.; Sarjeant, M.; Squires, R. T.; Street, P. J. Proc. R. Soc. London 1989, A423, 233-265. (4) Lightman, P.; Street P. J. J. Inst. Energy 1983, 56, 3-11. (5) Allen, G. C.; Hocking, W. H.; Watson, D. G.; Wild, R. J.; Street, P. J. J. Inst. Energy 1984, 57, 260-265. (6) Marrone, N. J.; Kennedy, I. M.; Dryer, F. L. Combust. Sci. Technol. 1984, 36, 149-170. (7) Villasenor, R.; Garcia, F. Fuel 1999, 78, 933-944. (8) Xu, G.; Ikegami, M.; Honma, S.; Ikeda, K.; Nagaishi, H.; Takeshita, Y. Combust. Sci. Technol. 2002, 174, 115-145. (9) Xu, G.; Ikegami, M.; Honma, S.; Ikeda, K.; Ma, X.; Nagaishi, H. Combust. Sci. Technol. 2003, 175, 1-26.

10.1021/ef020197b CCC: $25.00 © 2003 American Chemical Society Published on Web 05/02/2003

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Isolated oil droplet was burned in an atmospheric hotair chamber (in gravity) in the former works,8,9 noting that HFOs are commonly processed with spray flames. The entire process burning the oil droplet was analyzed in terms of two burning phases and four burning steps. The two burning phases, in succession, refer to a liquid phase for volatile matters and a solid phase for solid residues. Each of the phases was further subdivided into two steps to denote the ignition and burning of the fuel matters, respectively. This resulted in the preheating and flaming steps for the liquid phase and the glowing and smoldering steps for the solid phase (smoldering meaning burning without flame). Corresponding to those burning steps four component burning times were defined. In succession they are the ignition delay td, flame lifetime tf, glowing delay tg, and ember time te. Those reports measured such component times and analyzed their constitutional characteristics in making up the total time8 and phase durations9 for burning out an oil droplet. The analyses were made with respect to oil composition (LO mass fraction), chamber temperature, and initial droplet diameter, which revealed some important results regarding the burning of diversified HFOs. For example, a very high glowing delay was identified at low chamber temperatures,8 irrespective of oils. Applying this to practical burners requires a necessarily higher temperature of burning ambience for quick burnout of coke residues. A particular importance of coke burning to the entire burning process of HFO droplets was also clarified. For the typical HFOs the time duration of coke burning was found to average 1.3 and maximally 2.0 times longer than that of the liquid-phase volatile burning.9 From the viewpoint of fuel utilization and particulate-emission control, the importance of coke burnout has long been recognized.2 While all of these signify the understanding of coke behaviors for HFO burning, we are also wondering how the component burning times tg and te analyzed in refs 8 and 9 for the solid burning phase do correlate with the characteristics of coke formation and oxidation. The present article builds upon the previous reports in further investigating the features of coke formation and oxidation. 1.2. An Extension of Coke Formation and Oxidation Studies. Of the various studies available for coke formation and oxidation about HFO droplet burning, those directly relatated to practical applications may be about the coke formation prediction and coke oxidation conditions. The asphaltene species in HFO are the dominant virgin source for coke formation, and more than 80% of the total asphaltenes may convert to coke residues.3,13 In addition to that, the maltenes in the oil may contribute to coke formation as well,3,4,6 and the contribution was found to be larger when the oil asphaltene content is lower.3,4,6 This feature of multiple fuel sources for coking makes it difficult to correlate the final coke mass with the fuel asphaltene content.13 For that, the use of the so-called coke formation index (CFI) was widely suggested.13,14 The CFI shows the mass percentage of total fuel converted to final coke.6,13,14 Its (10) Legros, R.; Lim, C. J.; Brereton, C. M. H.; Grace, J. R. Fuel 1991, 70, 1465-1471. (11) Guo, D.; Jiang, L. Fuel 1998, 77, 1697-1700. (12) Witzel, L.; Moszkowicz, M.; Claus, G. Fuel 1995, 74, 18811886. (13) Urban, D. L.; Huey, S. P. C.; Dryer, F. L. Proc. Combust. Inst. 1992, 24, 1357-1364.

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values for different HFOs appeared to be little dependent on the droplet diameter but uniquely correlative with the asphaltene amount or Conradson/Ramsbottom carbon residues in the droplet.6,13 Further, the equivalent CFI for maltenes was shown to be 1%-4% percent.3,6,14 Another parameter quantifying the characteristics of coke formation is the coke particle size. It is evident that an initially larger oil droplet would lead to a bigger coke particle,13,15 but the volumetric ratio of coke to initial droplet was verified to be independent of the initial droplet diameter for a given oil by a few researchers.6,14 On the other hand, the coke particle of equisized droplets must be bigger for higher asphaltene oils,4,6 letting the size ratio of coke to droplet linearly correlate with the asphaltene content in the oil.14 Differing from those works,6,14 Masdin and Thring16 linearly correlated the coke size with the initial droplet diameter squared. This indicates a size ratio of coke particle to initial droplet which proportionally varies with the initial droplet diameter. The recent data of Urban et al.13,15 for free droplets revealed some curvatures as well in the plot of coke size versus initial droplet diameter. Therefore, further studies are needed to settle such literature discrepancies. The oxidation process of coke particles is basically subject to coke structure (morphological and chemical) that is determined by fuel properties (including metal additives).5,12,14,18 Meanwhile, the oxidation also closely depends on ambient temperature and oxygen fraction,12,19,20 and on the residence time in burning ambience.2 These show a complexity of the parametric dependencies for coke oxidation, but the typical data used for oxidation characterization are rarely out of three types: the burnout time,3,4,8,9,16 temperature history,3,4,7,19,21,22 and kinetic rate.4,12 Braide et al.21 had measured the history of mass variation during droplet burning but with fewer features demonstrated for coke oxidation. Of the three kinds of data, the burnout time is directly relative to burner/furnace designs, while the kinetic rate is indispensable to making fundamental estimations. The droplet temperature history allowed the identification of characteristic temperatures.3,4,21-23 The well-documented ones for coke oxidation are the temperature at flame extinction (forming the coke) and the peak temperature during oxidation.4,21,22 Those data, however, were never systematically analyzed to clarify the influences of fuels and burning conditions. Northrop et al.19 investigated the effects of ambient temperature and oxygen fraction on temperature history during (14) Kwack E. Y., Shakkottai, P., Massier, P. F., Back, L. H. 1992 Morphlogy of Globules and Cenospheres in Heavy Fuel Oil Burner Experiments. J. Eng. Gas Turbines Power 114, 338-345. (15) Urban, D. L.; Dryer, F. L. Proc. Combust. Inst. 1990, 23, 14371443. (16) Masdin, E. G.; Thring, M. W. J. Inst. Fuel 1962, 35, 251-260. (17) Bomo, N.; Lahaye, J.; Prado, G. Proc. Combust. Inst. 1984, 20, 903-911. (18) Clayton, R. M.; Back, L. H. J. Eng. Gas Turbines Power 1989, 111, 679-684. (19) Northrop, P. S.; Gavalas, G. R.; Levendis, Y. A. Energy Fuels 1991, 5, 587-594. (20) Hampartsoumian, E.; Hannud, B.; Williams, A. J. Inst. Energy 1993, 66, 13-16. (21) Braide, K. M.; Isles, G. L.; Jordan, J. B.; Williams, A. J. Inst. Energy, 1979, 52, 115-124. (22) Michael, M. I.; El-Wakil, M. M. Proc. Combust. Inst. 1967, 11, 1027-1035. (23) Ikegami, M.; Xu, G.; Ikeda, K.; Honma, S.; Nagaish, H.; Dietrich, D. L.; Takeshita, Y. Fuel 2003, 82, 293-304.

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Figure 1. Experimental apparatus and characteristic specification.

carbon oxidation. The studied case was for ash-rich unburnt carbons collected from furnace emissions, which is surely different from the coke burning behind flame extinction. Especially, all of the former studies in the literature did not pay attention to the temperature where at the coke oxidation (smoldering) begins. The importance of such a temperature was clearly demonstrated in refs 8 and 9 by the measured coke glowing delay time tg (coke glowing meaning coke oxidation with luminescence). Therefore, there exist some uncertainties and unresolved problems regarding coke formation and oxidation characteristics. The present article wishes to acquire more understanding on such aspects through systematically measuring, and further analyzing the coke size (volume) and the characteristic temperatures of coke burning for different HOR-LO blends at different ambient temperatures. 2. Experimental Section 2.1. Apparatus and Materials. Figure 1 shows a schematic diagram of the experimental apparatus (Figure 1a) and an illustration of the image method (Figure 1b), about which some details are available in refs 8 and 24. Two electric heaters located in the oven’s right and left sides generated an atmospheric hot-air chamber in size of 155 × 155 × 270 mm3. The temperature inside the chamber was measured with a K-type thermocouple, which was in turn sent to a PID controller for control. An air-driven cylinder, moving up-anddown, moved the chamber. Before the experiment was conducted, the chamber was hung on a bolt such that a fibertethered droplet is right beneath a chamber hole with a size of 100 × 100 mm2. By dropping the chamber down to an appropriate position, the oil droplet can be rightly included into the center of the chamber through the chamber hole. The (24) Xu, G.; Ikegami, M.; Honma, S.; Ikeda, K.; Nagaishi, H.; Dietrich, D. L.; Takeshita, Y. Energy Fuels 2002, 16, 366-378.

whole process from chamber drop to burning finish was photographed with an orthogonally located CCD camera (Figure 1b). For that, a glass window was mounted on the front side of the chamber. A glass window was available on the rear side as well, as to backlight the view of the picture. Notwithstanding, in this work only the flame view was taken with a weak backlight to determine the parameters relevant to coke particle. Beside the droplet fiber, another K-type thermocouple was installed to precisely measure the chamber temperature Tc where at the droplet burned. The tested Tc varied from 993 to 1153 K. Dropping down the chamber caused the picture from the camera to change from an initial droplet view to a black view blocked by the chamber wall, as is illustrated in Figure 1b. The entry of the droplet into the chamber denoted the onset of the droplet heating (2nd picture), there had thus t ) 0.0 for time count. Then, a time of 0.15 s was taken before the chamber finally stopped (3rd picture). During this period the droplet heating was surely affected by the temperature gradient near the chamber entrance. Figure 1c shows the spatial temperatures inside the chamber at Tc ) 955 K (setting temperature being 973 K), where the height refers to a value measured from the outside wall of the chamber. Obviously, the axial temperature gradient existed mainly in the section lower than 160 mm. The droplet was suspended at an elevation of 200 mm, and the temperatures at such a height varied little within the left and right 45 mm (Figure 1c). Hence, there was not a serious temperature nonuniformity in the droplet vicinity, implying a negligible influence of the temperature gradient unless the test duration was about 0.15 s. The tested HOR was produced from fractionating an HFO of C class at a temperature of 563 K under a pressure of 35 mmHg, and the employed LO was commercially available (detailed oil properties being in ref 8). The HOR was like a pitch or soft bitumen at room temperature but became flowable at temperatures above 340 K. It had an n-pentane asphaltene of about 17 wt % and a Conradson carbon residue of 18 wt %. Three different hybrid oils (HOR-LO blends) were prepared at LO mass (weight) fractions of 10%, 26%, and 40%, respectively. Measuring T-G diagrams of these oils in a nitrogen

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Figure 2. Typical shapes of coke particles depositing on fiber bead and their corresponding measurability of particle size. ambience demonstrated that the blend oil with 40 wt % LO was very similar to the original C heavy fuel oil (C HFO).8 Further, no residual carbon was identified for LO in the T-G test, showing that HOR is the only source for carbonaceous coke formation. As a consecutive study of reports 8 and 9, this work used also the fiber-tethered droplets. In the Appendix of ref 9 it is shown why this series of works employed the fiber-supported stationary droplet. For coking investigation the fuel-injectiongenerated free droplets were widely adopted.1,6,12-15,17,25 The test conducted in that way, however, can hardly focus the entire trajectory of the moved droplet at acceptably high magnifications, making it difficult to measure the coke particle size between flame extinction and coke ignition. In fact, the coke samples taken at the trajectory’s end may be either partially oxidized or immature.6,14,15,17,25 The use of a fibertethered stationary droplet can avoid such defects, but the presence of a fiber would more or less influence the burning, such as providing nucleation sites,4,21,26 enhancing heat transfer by fiber conduction,6,27 and creating soot lump by soot agglomeration.4,24 These influences may let the burning become more disruptive6,26 and the coke particle more hollowed,4 while varying the droplet burning rate as well.27,28 However, any of the influences should not be so great as to cause the invalidation of the test using the tethered droplet. In fact, the disruptive burning was similarly observed for free HFO droplets,4,26 and the burning rates differed only a few percent between tethered and free droplets for silicon fiber.28 Later, in Figure 4 it will be shown also that the coke particle sizes for our tethered droplets clarified the same result as that of Urban et al.13,15 obtained using free droplets. The fiber used in this work was usually a quartz fiber of 100 µm in diameter, but a thermocouple was used when measuring the droplet temperature history (thermocouple being detailed later). In both the cases a tip bead of 500 µm in size was made to enable the droplets with sizes up to 1.3 mm. 2.1. Measurement Techniques. In all tests the camera data were sent to a digital VCR, and the video data were captured using a frame grabber card and a computer. The acquired computer images (AVI files) were framed at a rate of 60 HZ via software that separates the odd and even fields of the image. All stated sizes of droplet and coke particle are equivalent values determined as the cubic root of the product of the measured width squared and length, i.e., (width)2/3 × (length)1/3. The measures were made on droplet images at the entry of the droplet into the chamber for the initial droplet diameter d0 and soon after flame extinction for the coke particle size Dc. Figure 2 shows the typical shapes of the coke particles observed. Disruptive burning made the shapes various. There were cokes completely entrapped by a soot lump (no. 4) or seriously abnormal (no. 3); they are thus immeasurable for sizes. The sizes Dc measured in this work were only for the coke particles with regular (nos. 1) or slightly abnormal (no. 2) shapes. Further, Figure 2 (no. 1) shows that at higher (25) Hottel, H. C.; Williams, G. C.; Simpson, H. C. Proc. Combust. Inst. 1955, 5, 101-129. (26) Dryer, F. L. Proc. Combust. Inst. 1977, 16, 279-295. (27) Kumagai, S. Proc. Combust. Inst. 1957, 6, 668-674. (28) Kumagai, S.; Sakai, T.; Okajima, S. Proc. Combust. Inst. 1974, 15, 779-785.

Tc the measurement had to be made for fired/glowed coke particle. The situation happened when Tc was high enough to let zero coke glowing delay. Coke sample was taken from the apparatus in Figure 1A to observe the coke structure via scanning electron microscope (SEM). Normally, the coke particle burned out if it remained in the chamber sufficiently long. At lower temperatures, such as 1058 K, there is a considerably long delay time between flame extinction and coke firing (see ref 8 and Figures 2 and 8). It offers a possibility to quickly withdraw the formed coke particle from the chamber as a sample. For that, an external force, usually by hand, was added to lift the chamber suddenly at the moment of flame extinction. This indicates then the difficulty to obtain the coke sample from the employed apparatus at higher Tc (>1150 K) because the coke glowing delay is very close to zero in this case.8 This article will examine the coke structure only at Tc ) 1058 K for intercorrelating the structure with oil composition. Many literature studies had worked on the temperature influence on coking.3,16,17,25 The higher the temperature, the harder and more compact the formed coke particle usually tends to be.16,17 Also, the coke particle will likely have a lower total mass3,25 and a higher shell density17 at a higher Tc. Measuring the droplet temperature history used the same thermocouple specified in Ikegami et al.23 It was made of platinum (Pt) and platinum-rhodium (Pt-Rh, 13 wt % Rh) wires with diameters of 0.1 and 0.025 mm, respectively. The joint bead (largely made) suspended the oil droplet, and measured the temperature inside the droplet. The thermocouple data were sampled through a computer at a frequency of 1000 Hz.

3. Primary Observations 3.1. Coke Size. Figure 3 shows the measured equivalent coke particle diameter Dc versus the initial droplet diameter d0 for four different oils at chamber temperatures 993 and 1153 K. These original measures contained the volume of fiber bead. As shown in the figure, Dc increased with decreasing the LO mass fraction at a given diameter d0, irrespective of the chamber temperature Tc. The lower LO mass fraction implies a higher content of HOR that contributes to coke formation. For the equisize droplets the oil with lower LO contains more HOR, which produces thus more carbonaceous residues during burning. Figure 3 shows this as the greater Dc at lower LO mass fraction. With the same reason, the values of Dc for a specified oil gradually increased with increasing d0. Figure 4 displays the ratio of the effective coke particle size to the effective initial droplet diameter at three different Tc’s for two blend oils with zero and 40 wt % LO, respectively. The effective sizes of coke and droplet were estimated by excluding the fiber-bead volume (0.53) from the measured Dc and d0 plotted in Figure 3, giving thus (Dc3 - 0.53)1/3 and (d03 - 0.53)1/3 for them, respectively. For comparison, the figure plot-



Figure 3. Typical diagram of coke particle size versus initial diameter for droplets of different HOR-LO blends.

ted also the data reported by Urban et al.13,15 from a test using fiber-free droplets in a drop-tube furnace (3, 0, 9). In this case the measured Dc and d0 values are just their effective sizes. The droplet diameters tested by Urban et al. were between 150 and 700 µm, and their furnace temperature was controlled using a methane flame. Hence, there were actually varied ambience temperatures during burning, along with the droplet dropping within the furnace. From Figure 4 we may see first that the size ratio varied little with the ambience temperature Tc for a given oil. This is the first observation on such a temperature-independent coke volume, although further conformation may be needed for a wider temperature range. In principle, a higher Tc leads to a higher flame temperature and further a more intensive heating to the droplet. This causes the fuel pyrolysis to occur at a deeper/faster degree and in turn reduces the fuel amount for coking. As a consequence, there has to be a lower coke mass at a higher Tc, as was implied in the data of refs 3 and 25. On the other hand, the burning at higher ambient temperature is more disruptive,24 due to the higher gasification rate of fuel from the droplet and the more viscous droplet surface that results from the quicker fuel gasification. Correspondingly, the formed coke particle, although with a less mass, might have more cavities that enable it to remain in the same size

Figure 4. Size ratio of coke particle to droplet (free of fiber bead) varying with chamber temperature and initial droplet diameter.

as that at a lower Tc. This indicates then a lower bulk density for the coke residues at a higher Tc. Notwithstanding, the shell of such a coke is inversely harder and more compact with a bigger density.16,17 Hence, for HFO burning the ambient temperature would exclusively affect the coke structure and coke mass, while it acts little on coke size. The second demonstration of Figure 4 is the gradual increase of the size ratio while increasing the initial droplet diameter. This increase is applicable also to the cited literature data for free droplets. Quantitatively it indicates a coke size proportionally varying with d0n at n > 1.0. Then, Figure 4 would be consistent with the results of Masdin and Thring16 who correlated Dc with d02 (n ) 2.0), although it would disagree with the constant size ratio (n ) 1.0) suggested by Marrone et

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al.6 and Kwack et al.14 In essence, the result shows that the fuel amount responsible for coke formation is not determinative to the coke size (volume). For the same fuel amount, the burning of a bigger droplet would lead to a larger volume of coke residues. This larger coke volume may stem from an increased coke mass or an increased coke void fraction or both of them. Urban et al.13 demonstrated that the coke formation index (CFI) is almost independent of the initial droplet diameter (although with variations), thus implying that the coke mass is determined by the fuel amount. Therefore, it ought to be the increased coke voids that cause the coke volume to vary with d0n at n > 1.0. The reason for increasing the coke voids with raising d0 is again relative to the burning disruption or microexplosion. That is, the initially larger droplet has a more disruptive burning,24,29 which creates more voids in the carbonaceous residues. Notwithstanding, for narrow-size droplets, such as those from well-refined atomizers, we may have a specified size ratio independent of d0. The tested case of Kwack et al.14 probably belongs to this case. In Figure 4 the size ratio of coke particle to droplet is larger for HOR than that for the blend oil with 40 wt % LO. It shows then the ratio increased with increasing the HOR content. Apparently, the result is consistent with the literature report on the increased size ratio for higher asphaltene oil.14 Reference 14 suggested that the size ratio linearly varied with the asphaltene mass fraction in the oil. The correlation obviously ignored the influence of droplet diameter d0 mentioned in Figure 4, leaving thus more work to do with it. The last clarification from Figure 4 is the consistence between the literature and this work, which is shown in two aspects. One is the same dependence of size ratio on the droplet diameter d0, as analyzed above, and another is the consistent range of data. The oil EPRI 4011 produced coke particles with sizes close to that from pure HOR, while the EPT A and B performed similarly in coke sizes as the HOR-LO blend with 40 wt % LO did. Although we cannot identify how the oils in each of two such groups are different, the asphaltene as well as carbon residue (CR) contents in Figure 4 indicate that the oils for the former group should belong to highly coking oils, while those for the latter group refer to fuels with moderate coking tendencies (Conradson CR being usually 1-2 wt % higher than Ramsbottom CR for a given oil13). It is then imaginable that each group may have the similar coking characteristics, as in Figure 4. Therefore, the action of droplet fiber encountered here was not so excessive that violates the data and the measure validity. 3.2. Correlation with Oils. Figure 5 replots the data in Figure 3 through correlating the coke particle volume (Dc3 - 0.53) with the HOR amount containing in the droplet. The HOR amount was determined as the product of the fuel volume and the HOR fraction, i.e., (d03 - 0.53) × (HOR fraction). The exact volumetric fraction of HOR is needed for such a computation. Here we used the experimental HOR mass fraction under a hypothesis that the densities of the tested blend oils with HOR fractions between 60 and 100 wt % are not (29) Maeda, K.; Mikami, M.; Kojima, N. Microexplosion Occurrence in Combustion of Ternary-fuel Droplets. In Proceedings of the 38th Symposium (Japanese) on Combustion; The Combustion Society of Japan: Tokyo, 2000; pp 195-196.

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Figure 5. Correlation of coke particle volume with HOR amount in droplets of different HOR-LO blends.

remarkably different. Because Tc varies little the coke size (volume) for a given oil (Figure 4), Figure 5 shows the cases for Tc equal to 993 and 1153 K to typify the results at arbitrary temperatures. Compared to Figure 3, Figure 5 makes clear that the coke particle volumes from different oils are correlative with the HOR amount rather than with the diameter d0 that shows essentially the fuel amount. Especially, the tested three blend oils with LO fractions greater than 10 wt % had actually the same coke particle volume at a specified HOR amount (O, 9 , )). This reveals a determination of the coke particle volume (size) by the HOR amount. Nonetheless, the pure HOR (2) exhibited a slightly larger coke particle volume than that of other oils, which may indicate a morphology difference between the coke particles from HOR and HOR-LO blends. The SEM graphs in Figure 6 are for the coke particles from pure HOR (Figure 6a) and the blend oil with 40 wt % LO (Figure 6b) burned at 1053 K. The coke particle from the high-LO oil seemed to have a thicker and tighter shell, and the shell was penetrated with more cavities. Compared to that, the coke shell from the pure HOR appears thinner and more fragile, with fewer penetration holes as well. These features consist rightly with the reports of Lightman and Street4 and Cunningham et al.3 on coke morphologies for HFOs with differ-



Figure 7. Typical temperature diagrams of HOR-LO blend droplets during burning at different chamber temperatures.

Figure 6. Typical SEM graphs of coke particles from HORLO blend oils with different LO mass fractions.

ent asphaltene contents. In essence the morphology differences are related to the different burning disruptions for different oils. Blending an LO into the HOR increased the fuel vaporization rate inside the droplet: this in turn enhanced the microexplosive burning that might break through the droplet surface via gas-bubble disruptions. The faster vaporization from the high-LO oil droplet would lead to a compact coke shell as well, an effect similarly observed for the quicker fuel gasification at higher chamber temperature or at higher droplet heating rate.16,17 Then, the slightly larger coke particle volume for HOR droplets in Figure 5 may be due to the larger inside cavities (hollows) of their coke particles. This can become true especially when noting the report of Urban et al.,13 who found that the coke formation index (CFI) is essentially a function of the oil asphaltene or carbon resides. On the other hand, the coke particle appearance in Figure 6a may impress us with a larger inner cavity as well, because the thicker and harder shell in Figure 6b is more likely to compress the coke mass into a smaller space.

Following the above concern, the correlation (unification) of the coke particle volume with the HOR amount for the other HOR-LO blend oils in Figure 5 should indicate little difference in coke morphologies, especially in coke inner-hollows, for those oils. Nevertheless, the belief warrants further corroboration via coke-structure measurement. The article did not do that due to the difficulty in coke sampling from the apparatus of Figure 1. Summarizing Figures 4 and 5, we can thus suggest that the coke particle size (volume) of HFO droplet burning is generally a function of the HOR amount, or of the asphaltene or carbon residue amount. The peculiarity for pure HOR in Figure 5 should seldom happen in practice since the actual HFOs are more like the blend oils tested here. 3.3. Temperature History. Figure 7 shows the typical droplet temperature histories during burning for the oil containing 26 wt % LO at three different chamber temperatures. The general feature of the histories is essentially the same for all temperatures. Since the entry of the droplet into the chamber (point A), the measured temperature gradually increased until reached a peak value at point F. The temperature then rapidly decreased to and remained at the value of the chamber temperature Tc. Between points A and F, there were several temperature inflections, such as points B-E. The upside figure replotted the data in a smaller scale of abscissa to show clearly the inflections. On whole we can see that all the inflections are easier for judgment at lower Tc. This should be attributed to the slower

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Figure 8. Event illustrations of coke oxidation at different chamber temperatures [Oil: 26 LO+74 HOR (wt %); C, D, and E refer to the points in Figure 7: td, ignition delay; tf, flame lifetime; tg, glowing delay].

burning at a lower Tc, which causes slower change in the droplet temperature. Important burning events are annotated by those temperature inflections. Figure 8 correlates several inflections with their burning events shown by image data, where the left- and right-side images are for droplets burnt at Tc equal to 1053 and 1153 K, respectively. The time in the figure was counted from flame extinction, and the parenthesized parameters are used to denote the times from droplet heating. The flame extinction (no. 1) and coke glowing (no. 3) caused the inflections C and E. Because there is a substantially long delay at Tc ) 1053 K for coke glowing after flame extinction, Figure 7 exhibits an obvious transition of temperature increase first quickly from C to D and then slowly from D to E. The former quick increase refers to the approach of the coke temperature to the chamber temperature Tc, under the controls of both radiation and heat conduction. Later, both the heat conduction and radiation adsorption become slight, while the quick heat release from coke oxidation does not begin yet. These lead to the slow-temperature rise from D to E. Nonetheless, there should be slow coke oxidation (without luminance) between D and E. Otherwise, the droplet temperature at point E can never become higher than Tc. At higher Tc the coke particle raises its temperature more quickly after flame extinction (see Figure 7). This should be related not only to the action of the higher Tc (mainly for C to D) but also to the fast heat release from the enhanced slow coke oxidation between D and E. With raising Tc the inflection E becomes gradually obscure in Figure 7, which implies actually a gradually diminished difference of coke oxidation before and after

E. Figure 8 verifies this by an immediate coke fire after the flame extinction at Tc ) 1153 K (right inset). The other temperature inflections in Figure 7 respectively indicate the ignition by B and the peak temperature during coke oxidation by F. Between B and C there existed inflections as well (see data from microgravity burning in ref 23), but the data presented here for normal-gravity burning are less able to reflect those inflections. The inflections B, C, and E demarcated actually the burning steps of preheating, flaming, and coke glowing defined in refs 8 and 9. Many literature studies had measured the droplet temperature histories,3,4,7,19,21,22,30 but few of them correlated the temperature with burning steps. Moreover, Figure 8 shows that the soot particles were not fired at Tc ) 1053 K (left), while they simultaneously smoldered with coke particle at Tc ) 1153 K (right). This verifies again the results ref 9 that indicate a higher chamber temperature is needed for starting a fire on soot than on coke, although the soot burnout is quicker (no. 4). Figure 9 displays the droplet temperature diagrams for different oils at Tc ) 1123 K. It makes clear that the droplets of all the tested oils, including an HFO of C class (dotted curve), exhibited the similar temperature history diagrams during burning. Meanwhile, the figure shows that at Tc ) 1123 K all the infections from A to F can be definitely distinguished, although the distinction may become difficult at a rather higher Tc (especially for E). 4. Relating to Burning Steps 4.1. Coke Smoldering. The section (via Figures 1012) tries to elucidate how the coke ember (burnout) time (30) Milk, R. Fuel 1989, 68, 371-374.



Figure 9. Droplet temperature diagrams during burning for a few different oils at the same chamber temperature. Figure 11. Coke ember (burnout) time versus coke particle size for different HOR-LO blend oils.

Figure 10. Coke ember (burnout) time versus coke particle size at different chamber temperatures.

te relates to the coke size characteristics clarified above. Figures 10 and 11 correlate the time te with the effective coke particle diameter (Dc3 - 0.53)1/3 at different chamber temperatures Tc (Figure 10. O, 0: 993 K. 9, b : 1153 K) for different oil blends (Figure 11). The oils in Figure 10 are the pure HOR (O, b) and blend oil containing 26 wt % LO (0, 9), and the chamber temperature in Figure 11 is 1153 K. The tested droplets for both the figures are the same as those shown in Figures 3-5. For a given oil Figure 10 reveals a higher ember time te at a higher Tc regarding the same coke diameter (Dc3 - 0.53)1/3. Corresponding to this, Figures 10 and 11 clarify a lower te for the pure HOR compared to the other blend oils. Notwithstanding, a very slight difference of te is shown in Figure 11 for equisized coke particles from all the tested blend oils. A few larger values of te, such as the ones encircled in Figure 11, were associated with the blend oil with 10 wt % LO when

the coke particle was larger. They are considered to result from the less fragmented coke smoldering. Statistically, this refers to a particular case because coke smoldering usually proceeds with coke fragmentations (see Figure 8). The above relations of te with (Dc3 - 0.53)1/3 essentially indicate that the coke size is less determinative to the time required for coke burnout. Figure 12 correlates further the coke ember time with HOR amount present in the droplet. Excluding the encircled times in Figure 12b (due to fewer fragmentations), we can see that the time te is generally correlative with the HOR amount at every specified Tc. Combining this with Figures 3 and 5 it shows thus the HOR or asphaltene amount is determinative to the coke particle sizes at different Tc for a given oil, and to the coke ember times for arbitrary oils at a specified Tc. Thus, at different chamber temperatures the coke particles from a given oil may have the nearly same size but different ember (burnout) times. In turn, for different oils the coke particles may have different sizes but the nearly same ember time. The preceding result implies that the coke smoldering, i.e., coke oxidation, is subject more to the coke (shell) structure. Considering the cokes at different chamber temperatures, the equisize coke particle (Figure 4) had a longer time te at a higher Tc (Figure 10). It indicates a lower coke oxidation rate at a higher Tc, just responding to the harder, thicker, and more densified coke shell for the temperature.16,17 At a specified Tc, the coke particle from HOR has a larger size (Figure 5) but the same ember time as those from other blend oils (Figure 12). This corresponds to the looser and thinner shell of the HOR coke particle (Figure 6a), which leads to a rapider burning and thus the same burnout time as for the smaller coke particles from other blend oils. When the LO mass fraction is greater than 10 wt %, both coke size (Figure 5) and ember time (Figure 12) differed little at the same HOR amount. It may indicate a little difference in coke structure, including the shell morphology and the cavity fraction inside the coke. However, from

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Figure 13. Droplet temperature at the onset of coke smoldering for various oils at different chamber temperatures.

Figure 12. Determination of coke ember (burnout) time by the HOR amount contained in the droplet.

the article we are yet unknown how the coke particle mass (not measured here) relates to the coke ember time. A constant particle size was widely suggested during coke smoldering based on image observation4,8 or in conceptual modeling.19 By actually measuring the luminous coke ember diameter, it was revealed that the coke luminous diameter first slightly increased with time and then remained in a nearly constant value until the coke fragmentation finally occurred to shrink the ember (data not presented here). Nonetheless, the largest coke ember diameter is hardly over 1.2 times of the original coke particle size. 4.2. Characteristic Temperatures. The method determining the characteristic temperatures of the various inflections from the droplet temperature history diagram is illustrated in the upside figure of Figure 7 by taking the inflection point E as an example. Two lines, L1 and L2, were drawn to correlate the curves before and after the inflection, respectively. The temperature on the diagram, which possesses the same time as the intersection of such two lines, was then accepted as the characteristic temperature of the inflection referred to. The determined characteristic temperatures were plotted in Figures 13-15. In addition to HOR-LO blend oils, the measures were also made for a heavy oil of C

class (denoted by “HFO”) whose temperature diagram was illustrated in Figure 9 (denoted by “C HFO”). While most tests were at temperatures of 1053 and 1153 K, Figure 13 adopted a slightly lower high-value Tc of 1123 K to ensure a definable inflection E. In addition, all the characteristic temperatures relative to coke residues were plotted versus HOR amount in Figures 13 and 14, while the ignition temperature relative to volatile burning was correlated with d0 (fuel amount) in Figure 15. The oxidation temperature Tx at the inflection E shows the temperature that initializes quick coke oxidation (Figure 13). A bigger Tc led to a higher Tx, and Tx varied little with fuel and the HOR amount. It is thus possible to define a temperature Tx for each specified Tc without recourse to the effects of droplet size and fuel. From Figure 13 we have Tx equal to 1150 and 1205 K on average for Tc ) 1053 K and Tc ) 1123 K, respectively. Noting that there existed a distinct period without luminous coke fire (glowing) at Tc ) 1053 K (Figure 8), the Tx ) 1150 K can then be adopted as the necessary temperature for starting the luminous oxidation of HFO coke residues. Just because of this the coke particle in Figure 8 remained in its fire state at Tc)1153 K after flame extinction. In this case a zero delay time tg is judged for the coke glowing.8 The higher Tx at higher Tc, which is higher than Tc as well, thus must stem from the enhanced slow coke oxidation before the inflection E, as early explained in the section 3.3. Shown in Figure 14 are the extinction temperature Te at the inflection C (Figure 14a) and the maximal temperature Tm during coke smoldering at the inflection F (Figure 14b). The temperatures were higher at higher Tc but little depended on oils. However, the extinction temperature Te is surely lower than the chamber temperature Tc, while it slightly increased with increasing the HOR amount. According to Ikegami et al.,23 the droplet temperature near flame extinction is little



Figure 15. Droplet temperature at ignition for various oils at different chamber temperatures.

Figure 14. Droplet temperature at flame extinction and its peak value during coke smoldering for various oils at different chamber temperatures.

subject to the oil boiling point but to the carbonization reactions that lead to carbonaceous residues. During carbonization reactions the droplet is not liquid but is more like a soft solid ball. This permits the droplet temperature to increase with increasing the chamber temperature Tc and to vary with the flame heating that is stronger at higher Tc (due to the quicker burning). More HOR means massive carbonization reactions and a larger solid ball. They may retain the carbonization period for longer time, and induces in turn a larger increase in droplet temperature. Nonetheless, the extinction temperature is generally between 930 and 1000 K. The bigger Tm at higher Tc is subject to the heat balance between the heat release from coke oxidation and the heat loss to chamber ambience. For a given amount of HOR, the coke mass may be slightly lower at a higher Tc,3,25 but it burns up in the nearly identical time as at other lower Tc (based on Figure 12). This implies a less heat release from the coke oxidation at the higher Tc. Despite this, the higher Tc reduces considerably the heat loss to chamber ambience. The

overall result becomes thus the increases in Tc raised the particle temperature during coke smoldering. Our image data showed actually that at higher Tc the coke embers were hotter in temperature and whiter in appearance. However, the temperature Tm varied little with oil types and the HOR amount (Figure 14b). This shows essentially that the coke structure and size weakly acted on the coke temperature during burning. Figures 3-6 clarified that the oil property and HOR amount are determinative to the coke morphology and size, respectively. Hence, the temperature during coke smoldering should be a function of the heat balance between coke and chamber ambience exclusively, rendering it sensitive to the chamber temperature and insensitive to the coke structure/mass. One may note that the coke burning temperatures in Figures 7, 9, and 14b are lower than the literature temperatures measured using infrared pyrometers in hot chambers3,4,19 and larger than those using thermocouples for hot-gas flow22 or for radiation-lamp heating.7 The pyrometer measured the surface temperature of fired coke particle, which is thus higher than the inside temperature measured here (according to Wilk30). Therefore, the other lower literature data7,22 should be due to their colder ambiences compared to ours. Figure 14b clarifies that the inside coke temperature may reach 1650 K during coke smoldering when the ambient temperature is around 1200 K. Figure 15 shows the volatile ignition temperature Ti corresponding to the inflection B, clarifying that Ti decreases with increasing LO mass fraction, chamber temperature Tc, and droplet diameter d0. In contrast to temperatures in Figures 13 and 14, Ti is controlled by the droplet surface temperature that is subject to the boiling point of fuels vaporized from the surface. At ignition there must be more LO content in the droplet for higher-LO oil, causing thus a lower surface temper-

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ature and accordingly the lower Ti. The ignition is a function of both reaction kinetics (chemical) and fuel vapor accumulation around the droplet (physical). A higher Tc allows the ignition to occur at a less fuel vapor accumulation, and in turn at a shorter delay time td. The less fuel vaporization implies certainly a lower surface temperature because it retains more volatiles in the droplet. Further, there has a larger gradient of temperature between droplet surface and droplet core at higher Tc, as a result of the shorter but faster heating of the droplet. Because of these two factors, Ti thus has to be lower at higher Tc. The initially larger droplet has a lower Ti (Figure 15) but a longer ignition delay time td (see ref 8). It is the colder droplet core that causes the lower Ti for larger d0. Changing the droplet diameter d0 varies the fuel amount according to d03. To heat this droplet to the same temperature requires thus a heat input and a heating time that are proportional to d03. The actual increase of ignition delay time, however, is much lower than such an expected value.8 The decrease of Ti with increasing d0 reveals also that the fuel amount controls the droplet preheating process. For fiber-tethered droplets we may suspect if the heat capacity of fiber is larger enough to dominate the heating process. Figure 15 disallows such a suspicion, showing actually a finite influence from the fiber. In addition to Figure 4, this further ratifies the test and measures of this article using fiber-tethered droplets. 5. Conclusions (1) Overall. By burning droplets of blend oils consisting of a heavy oil residual (HOR) and a diesel light oil in a hot-air chamber, this work clarified that the HOR (or asphaltene) amount determines the coke ember (burnout) time for different oils at a given chamber temperature and the coke particle sizes (volumes) of a given oil at different chamber temperatures. Regarding different chamber temperatures, the coke particles from a given oil tended to have the nearly same size but different ember times. In turn, for different oils the coke particles likely had different sizes but nearly the same ember time. Measuring the droplet temperature history diagram during burning revealed further a necessary temperature of about 1150 K for setting the coke particle from heavy fuel oil (HFO) onto obvious oxidation (smoldering) with luminous glowing. It means that if the chamber temperature is above such a value the coke residues formed during volatile burning can soon proceed to coke smoldering (oxidation) with a zero delay time after the flame extinction. (2) Coke Size. The HOR amount, which shows also the amount of asphaltene, determined the coke particle size in most cases. It caused the size to increase with raising the initial droplet diameter and the HOR mass fraction in the oil. The chamber temperature little influenced the coke particle size for a given oil but led

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to different coke structures. The size ratio of coke particle to initial droplet increased with increasing the initial droplet diameter and the HOR mass fraction. For different oils, the coke size appeared correlative with the HOR amount in the droplet for blend oils with LO mass fraction greater than 10 wt % but exhibited a slightly higher value for pure HOR. This bigger coke particle of pure HOR was suggested to result from a more hollowed structure inside the coke particle, although its shell appeared thinner, fragile, and penetrated with fewer holes (by SEM graphs). (3) Coke Smoldering. The coke particle size could not determine the coke smoldering (oxidation) features analyzed in terms of coke ember (burnout) time. The equisized coke particle manifested higher ember time at higher chamber temperature for oil with higher LO mass fraction. At the same chamber temperature, however, the ember times of equisized coke particles from different oils, including those from the pure HOR, were likely correlative with the HOR amount in the droplet. On the basis of these results, this work concluded that the coke particle shell structure is the dominant factor influencing the coke smoldering rate, although further corroboration may be required for this conclusion. The coke particle smoldering usually caused a luminous ember that was slightly larger (but

Burning Droplets of Heavy Oil Residual Blended with Diesel Light Oil

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