Burning Droplets Composed of Light Cycle Oil and ... - ACS Publications

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Energy & Fuels 2002, 16, 366-378

Burning Droplets Composed of Light Cycle Oil and Diesel Light Oil Guangwen Xu,* Masiki Ikegami, Senji Honma, Khoji Ikeda, Hiroshi Nagaishi, Daniel L. Dietrich,† and Yasuhiro Takeshita‡ AIST Hokkaido, National Institute of Advanced Industrial Science and Technology (AIST), 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan, NASA Glenn Research Center, Mail Stop 110-3, 21000 Brookpark Road, Cleveland, Ohio 44135, and Japan Space Utilization Promotion Center, 3-30-16 Nishiwaseda, Shinjuku-ku, Tokyo 169-8624, Japan Received May 21, 2001. Revised Manuscript Received October 15, 2001

Burning of single droplets composed of a light cycle oil (LCO) and a diesel light oil (LO) was investigated in normal and micro gravity conditions, with the intention of developing technologies burning these oils in gas turbines. While the normal gravity test used a hot-air chamber to simulate the burning condition that occurs in actual burners, the microgravity test employed an igniter, set beneath the droplet, to make the burning free of extra radiation. The result showed that the burning of the LCO droplet had higher soot yield, while that of the LO droplet was more disruptive. In the conditions tested, coke formation was negligible for both the oils. The LCO droplets exhibited lower ignition delay times, but the burning times between ignition and extinction of equi-sized droplets were slightly larger for LO. A burning rate constant, which was an explicit value when microexplosions were present, could be defined according to the d2-law. The constant, however, was closely related to the initial droplet diameter (d0), causing the relative size d/d0 to be unified (normalized) into a single curve by a relative burning time t/d0n (1.0 < n < 2.0). The droplet temperature histories in microgravity demonstrated that the burning of the LCO and LO droplets proceeded by vaporizing fuel constituents according to their volatility, suggesting a distillation-like mechanism dominating the burning. The test in gravity showed also that the soot particles from the LCO droplets were easier to glow (fire) after flame extinction, while the droplets required as well longer times to burn up the soot particles.

Light cycle oil (LCO) is a typical middle distillate produced in the fluid catalytic cracking (FCC) process. It is poor in quality compared to the straight-run middle distillate derived from atmospheric distillation of crude feedstock.1,2 The oil contains more sulfur and aromatics, and possesses a lower cetane number and a higher density. LCO is commonly blended with straight-run middle distillates and then hydrotreated to increase the production pool of diesel light oil (LO).1,3 The hydrotreating (HDT) process, such as hydrodesulfurization (HDS) and hydrogenation, performs to reduce the contents of sulfur and aromatics in the oil blend.3-5 The test in diesel engines revealed that the higher the sulfur and aromatics contents in LO, the larger the pollutant

emissions tend to be.6-9 Therefore, ongoing tightening of the sulfur and aromatics contents in LO still prevails worldwide.9 This implies rather higher severity for hydrotreating LCO and the oil bends between LCO and the straight-run distillates. Now we are considering using LCO as well as its blend with LO in gas turbine (GT) or partially in the combined cycling gas turbine (CCGT),10 as to avoid the tight standards on oil compositions. This entails an understanding of the droplet evaporation and combustion characteristics of these oils. Although the reports about pollutant emissions when burning LO and LCO in diesel engines are prolific, few works are available for the combustion characteristics of the oils. Sienicki and co-workers12 measured several parameters relevant to the combustion of LO by using a diesel engine. The

* Corresponding author. Tel: 0081-11-857-8439. Fax: 0081-11-8578900. E-mail: [email protected]. † NASA Glenn Research Center. ‡ Japan Space Utilization Promotion Center. (1) Collins, J. M.; Unzelman, G. H. Oil Gas J. 1983, 81 (22), 71-78. (2) Anabtawi, J. A.; Al-Jarallah, A. M.; Aita, A. M. Energy Sources 1992, 14, 155-167. (3) Danaher, W. J.; Palmer, L. D. Fuel 1988, 67, 1441-1445. (4) Kasztelan, S.; Marchal, N.; Kressmann, S.; Billon, A. Production of Environmentally Friendly Middle Distillates by Deep Sulfur and Aromatics Reduction. Proceedings of the 14th World Petroleum Congress; John Wiley & Sons: New York, 1994; pp 19-26. (5) Juarez, J. A.; Rodriguez, E. A.; Sotelo, D. S.; Rivera G. B.; Nuncio, M. L. Appl. Catal., A: General 1999, 180, 195-205.

(6) Ullman, T. L. SAE paper 892072, 1989. (7) Sienicki, E. J.; Jass, R. E.; Slodowske, W. J.; McCarthy C. I.; Krodel, A. L. SAE paper 902172, 1990. (8) Morinaga, M.; Yoshida, H.; Takizawa, H.; Yamada, S.; Iwamiya, Y. Paper Collection of JSAE Spring Congress 1995, 952 (Paper 9535233), 191-194. (9) Inoue S.; Takatsuka, T. Sekiyu Gakkaishi 1999, 42, 365-376. (10) Japan Space Utilization Promotion Center (JSUP). Studies and Developments of Combustion Technologies in Response to FuelDiversification. 1999s Annual Report; JSUP: Tokyo, 2000; pp 11-21. (11) Xu, G.; Ikegami, M.; Honma, S.; Sasaki, M.; Ikeda, K.; Nagaish, H.; Takeshita, Y. Distinctive Burning Rate of Diesel Light Oil and Light Cycle Oil Droplets. Proc. 2001 Int. Conf. on Power Eng.; Tsinghua University Press: Beijing, 2001; pp 243-250..

1. Introduction

10.1021/ef010112r CCC: $22.00 © 2002 American Chemical Society Published on Web 01/05/2002

Burning Droplets of Light Cycle Oil and Diesel Light Oil

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Figure 1. Experimental apparatus and illustration of time count for normal gravity test in a hot-air chamber.

results, however, cannot be applied to the situation of GT and CCGT. A GT system burns the oil in a boiler that is equipped with either the spray or the premixed pre-vaporized burners. In this case, the data about droplet combustion and vaporization are basic for burner designs and operations. Recently, we carried out a preliminary study to compare the droplet behaviors of LCO and LO burning in a hot-air chamber preset at a given temperature.11,12 The present work will extend the test to different chamber temperatures and to the case of droplet vaporization. Furthermore, microgravity condition will be employed to demonstrate the evaporation mechanism that controls the fuel vaporization from the droplet during burning. 2. Experimental Section 2.1. Normal Gravity Test. Figure 1 shows a schematic diagram of the experimental apparatus for the normal gravity test (a) and an illustration of the time count used in the test (b). The hot chamber, in size of 155 × 155 × 270 mm, was generated with an electric oven. The oven had two heaters, which were located in the right and left sides of the chamber. The temperature inside the chamber was measured using a thermocouple and was monitored with a PID controller. An air cylinder was adapted to move the chamber up-and-down , and a hole of 100 × 100 mm in size was opened on the lower side of the chamber. A 110-µm quartz fiber, with a 500-µm tip bead, suspended the oil droplet. Before the experiment, the chamber was hung on a bolt such that the fiber is right beneath the chamber hole. By dropping the chamber down to an appropriate position, the fiber-supported oil droplet can be rightly included into the center of the chamber by passing through the chamber hole. The whole process from chamber drop to burning finish was pictured with an orthogonally located CCD camera. For this picturing, a glass window was mounted on the front side of the chamber. A glass window was also available on the rear side of the chamber, as to backlight (12) Xu, G.; Ikegami, M.; Honma, S.; Sasaki, M.; Ikeda, K.; Nagaish, H.; Takeshita, Y. Combustion Characteristics of Droplets Comprised of Light Cycle Oil and Diesel Light Oil in a Hot-Air Chamber. Fuel 2002, in press.

the view of the picture. The flame view without or with weak backlight enabled the determination of various times relevant to burning (such as ignition delay), while the backlight view provided a measure of the droplet size in case the view was not obscured by excessive soot. Beside the fiber bead, a thermocouple was installed to precisely measure the chamber temperature Tc whereat the droplet burned. When the chamber moved down, the pictures taken by the camera changed from an initial droplet view to a black view blocked by the chamber wall, as is illustrated in Figure 1b. The time count was started just at this view change, i.e., from the second picture shown in Figure 1b. It corresponds to the onset of droplet heating by the hot chamber. There was probably a temperature gradient near the chamber hole, Figure 1b (3rd picture) shows that the chamber can finally be braked within 0.15 s. Therefore, the influence of the temperature gradient is usually negligible except that the measured time is near 0.15 s. In the work, the chamber temperature Tc was changed between 823 and 1173 K, and the fuel droplet was manually formed using a syringe. Attention was paid to ensure the fuel attached only on the fiber bead so that ignition delay of the droplet can be correctly detected. 2.2. Microgravity Test. The microgravity test was conducted in Japan Microgravity Center (JAMIC). The drop shaft of JAMIC can generate microgravity conditions better than 10-5g (g ) 9.81 m/s2) with a duration of 10 s.13 Figure 2 shows a schematic of the experimental apparatus. Thermocouples made of platinum (Pt) and platinum-rhodium (Pt-Rh, 13 wt % Rh) wires with diameters of 0.1 and 0.025 mm, respectively, supported the droplets and also measured the temperature inside the droplet (Figure 2a). A 1000 Hz data logger acquired the temperature data from the thermocouple and then transferred the data to a computer (after experiment). Image data were taken using two orthogonally located CCD cameras and digital video recorders (Figure 2b). One view was backlit to image the droplet and the other was a direct image of the flame. The igniter was an aluminum-kanthal alloy wire with a resistance of approximately 1.2 ohms. The fuel droplet was created using a DC motor-activated syringe that dispensed a measured amount of fuel into a fuel (13) Kitano, K.; Honma, S.; Martynowicz, M. J.; Sakuraya, T. Space Forum 1998, 4, 99-119.

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Figure 2. Experimental apparatus and measurement system for microgravity test using a down-laid igniter. Table 1. Properties of Used Oils Diesel Light Oil (LO, commercially sold) aromatics 28.1 wt % C 86.4 wt % H 11.28 wt % S 0.05 wt % IBP 418 K T10 466 K T50 563 K T90 645 K flash point 336 K cetane index 52 viscosity (303 K) 2.59 mm2/s density 0.84 g/cm3 Light Cycle Oil (LCO, from IHI, Ltd.) aromatics 60-70 wt %a C H 11.16 wt % S IBP 424 K T10 T50 540 K T90 flash point cetane index viscosity (303 K) 2.861 mm2/sa density

87.8 wt % 0.10 wt % 464 K 619 K 20-30a 0.90 g/cm3a

a

These data are reference values derived from literature report.8

nozzle that in turn sent the fuel onto the thermocouple bead (Figure 2a). The fuel nozzle was fixed on an arm that could be rotated under the controls of a DC rotator. Before the test, the fuel nozzle was definitely away from the thermocouple bead. At about 30 s before microgravity started (dropping the capsule), the nozzle rotated down to a position required for fuel transition from the nozzle to the thermocouple. The DC motor of the syringe then activated to inject fuel, causing a fuel drop to form between the nozzle and the thermocouple. Rotating back of the nozzle then led the droplet to be completely suspended by the thermocouple bead and to be ready for burning. The entire process of droplet formation took usually 20 s. The data logger started acquiring the thermocouple data at the onset of microgravity (later, the time being adopted as the zero moment for data presentation). One second after the capsule entered microgravity, the igniter moved just beneath the droplet (approximately 1-2 mm) and turned on for a preset period of time, typically 1.6 s. The actual time, however, varied from test to test to ensure that the igniter withdrawal was shortly after ignition. The current passing through the igniter was approximately 3.5 amperes. 2.3. Oil Properties. The oils used were an LO commercially sold and an LCO provided by IHI, Ltd. Table 1 summarizes the major properties of the oils, showing that LCO has lower T10, T50, T90, and cetane number but higher aromatics content. This indicates that LCO has higher volatility than LO, although IBPs in Table 1 are nearly the same for both the oils. In all tests, the cameras were connected to digital VCRs, and the video data were transferred to a computer using a frame grabber card. The resultant AVI files were then framed at a rate of 60 Hz by using a software to separate the odd and even fields of the images. This gave an error less than 0.017

s for the measurement of various burning times, such as ignition delay. The droplet was not an exact sphere, but more elliptical in shape in both normal and micro gravity conditions. The stated droplet size is thus an equivalent value determined as the cubic root of the product of the droplet radius squared and the droplet length, i.e., (radius)2/3 × (length)1/3. The initial droplet diameter d0 was measured right before the entry of the droplet into the hot chamber in the normal gravity test and just before the onset of microgravity in the microgravity test. The tested d0 varied from 0.6 to 1.7 mm in the work.

3. General Characteristics The time sequences of video images for the flame and droplet views reported in Xu et al.11,12 demonstrated that the burning of the LCO droplet had a higher soot yield, whereas that of the LO droplet was more apt to microexplode. In both cases, as will also show in Figure 11, soot particles deposited themselves on the fiber filament and formed a soot lump. When burning the LCO droplets, however, the soot lump tended to contract the flame inward (No. 2 in Figure 11). This suppressed the flame front of the LCO droplet on the one hand, and caused the soot lump to be enwrapped by the flame on the other hand. Although microexplosion was evident, no obvious coke formation was identified under all of the tested conditions. The image data showed also that the disruptive burning was more obvious for larger droplets and at higher chamber temperatures. Figures 5 to 7 will further show this as the greater fluctuations in droplet sizes when d0 and Tc are higher. Figures 3 and 4 show ignition delay (td), flame lifetime (tf), and droplet lifetime (td + tf) for the LCO and LO droplets, where the flame lifetime tf refers to the time duration between ignition and flame extinction. From the figures we can get the following characteristics about these burning times. 3.1. Oil Dependencies. Figure 3 compares the times for the LCO and LO droplets under three different temperatures. While each inset of the figure (from a to c) plots the same parameter of time, all the times in each row (from 1 to 3) refer to the values measured at a given Tc. As expected, tf in Figure 3b and (td + tf) in Figure 3c gradually increased with increasing the initial droplet diameter. At Tc ) 988 K, these two times of the LCO droplets were obviously lower than that of the LO



Figure 3. Ignition delay, flame lifetime, and droplet lifetime under different temperatures for LCO and LO droplets.

droplets, just corresponding to the fuel property that LCO had higher volatility than LO (Table 1). The differences of the times, however, decreased as Tc increased, which led the droplets of both the oils to have almost the same tf and (td + tf) at Tc ) 1173 K when d0 was less than 1.1 mm (3rd row). It should be the fact that the difference of volatility between LCO and LO decreases as the temperature increases, since both the oils can vaporize rapidly at higher Tc. When the droplet size was rather larger, Figure 3b (b2 and b3) shows that the flame lifetime of the LO droplets appeared smaller than that of the LCO droplets. This is considered to be due to the fuel loss as well as the speedup of fuel vaporization caused by the microexplosive burning. Microexplosion can enhance fuel inter-circulation inside the droplet, leading thus to rapid mass and heat transfer that in turn result in rapid fuel evaporation from the droplet. On the other hand, violent microexplosion might occur with the formation of fuel satellite (isolated fuel particle), which caused definite liquid fuel to escape from the droplet without vaporization. Then, there was a decrease of fuel for normal droplet evaporation, lowering consequently the time of flaming Obviously, the time decrease owing to the microexplosioninduced fuel loss and vaporization speedup would occur at higher Tc and for larger droplets where the droplets

were easy to microexplode. Because the LO droplets possessed higher aptitude for microexplosion, the time tf exhibited larger decreases in Figure 3b2 and Figure 3b3 when d0 got greater than a certain value, such as between 1.1 and 1.3 mm. At the lower temperature of Tc ) 988 K where there was not pronounced microexplosion (Figure 3b1), tf of the LO droplets was constantly larger than that of the LCO droplets in the whole range of tested d0. Essentially, the droplet lifetime in Figure 3c should be subject to the similar change as the flame lifetime unless the proportion of ignition delay td in (td + tf) is large enough to shield the variational features of tf. Figure 3c3 (1173 K) corresponds to the former case where the droplet lifetime is mainly composed of tf, leading (td + tf) to exhibit the similar lower values as in Figure 3b3 for larger LO droplets. However, this effect was not seen in Figure 3c2 (1053 K) because the microexplosion-induced decrease in tf for larger LO droplets was actually compensated by their corresponding higher td. The measured values of td in Figure 3a are rather scattered, but it can be seen that td more or less increased with increasing d0 unless d0 was too small. For small droplets, the ignition delay time td increased with decreasing d0 at Tc ) 988 K or little varied with d0 at other higher Tc (1053 and 1173 K). According to

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Figure 4. Influences of chamber temperature on ignition delay, flame lifetime, and droplet lifetime of LCO and LO droplets (gravity).

Takei et al.14 and Spaldling,15 these results are related to the competitions among the factors of fuel vaporization from the droplet, fuel vapor diffusion around the droplet, and chemical reactions for ignition. The ignition at higher Tc for smaller droplets is rather controlled by the ignition reactions and fuel vapor diffusion, while that at higher Tc for larger droplets is mainly subject to the fuel vaporization. Before ignition, the temperature rise on the droplet surface dominates the fuel vaporization from the droplet. Therefore, it is the lower rate of ignition reactions and faster fuel vapor diffusion around the droplet that causes very smaller droplets to have higher td even fail to be ignited at lower Tc.14 When d0 is definitely larger where the fuel evaporation controls ignition, the ignition delay then turns to increase with increasing d0 because a larger droplet requires a longer time to heat the droplet surface to a temperature that is close to the fuel boiling point. Figure 3a shows that the LCO droplet had relatively lower ignition delay on average. This may be related to the lower boiling point and accordingly the higher volatility of LCO. Our knowledge about LCO and LO anticipates that the lower cetane number of LCO would make the oil more difficult to be ignited. Longer ignition delay times for lower cetane number oils were also identified by Sienicki et al.7 through burning different LOs in a diesel engine. These are obviously contrary to the data shown in Figure 3a. The different methods (14) Takei, M.; Tsukamoto, T.; Niioka, Combust. Flame 1993, 93, 149-156. (15) Spaldling, D. B. Combustion of Liquid Fuels. 4th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1953; pp 847-864.

burning the oils are considered responsible for the converse results. For the case of this work (in a hot-air chamber), the ignition might be subject more to the fuel volatility. The higher content of volatile matters in LCO provides gaseous fuel around the droplet faster than LO, leading consequently the LCO droplet to ignite earlier. In diesel engines, the ignition occurs rather to premixed gases of fuel vapor and oxygen,16,17 it is thus little relative to the vaporization rates of fuels. Instead, the higher fractions of aromatic species in LCO would make the ignition slower, just responsible for the lower cetane number of the oil. 3.2. Temperature Influences. Figure 4 plots the ignition delays, flame lifetimes, and droplet lifetimes under different chamber temperatures. The vertical insets are arranged in the same way as in Figure 3, while the times for the LCO and LO droplets are recounted in the 1st and 2nd rows, respectively. Generally, we can say that the similar influences of temperature Tc on the times td, tf, and (td + tf) are demonstrated in Figure 4 for the LCO and LO droplets. The ignition delays in Figure 4a gradually decreased with increasing the chamber temperature Tc, but the magnitude of decrease was specially larger at the temperature rise from 988 to 1053 K. This indicates probably a transition of ignition controlling factors between such two temperatures. At higher Tc (>1053 K), the ignition is rather dominated by the fuel vaporization from the droplet, while at lower Tc (988 K) it is mostly controlled by the (16) Dec, J. E. SAE Paper 970873, 1997. (17) Flynn, P. F.; Durrett, R. P.; Hunter, G. L.; Loye, A. O.; Akinyemi, O. C.; Dec, J. E.; Westbrook, C. K. SAE Paper 1999-01-0509, 1999.



ignition reactions. Nonetheless, more studies are needed to ascertain this transition of ignition control mechanism. Figure 4b shows that the flame lifetime tf at Tc ) 988 K was evidently lower than those at other higher Tc. The much longer ignition delay in the former case caused a considerable amount of fuel to vaporize from the droplet before ignition. This much reduced the fuel for normal flaming, lowering consequently the flame lifetime. Compared to LO, LCO contained much more fuel species that were easier to vaporize before ignition. Figure 4b shows consequently that the decrease of tf induced by the pre-ignition evaporation was larger in Figure 4b1 (LCO) than in Figure 4b2 (LO). When the droplet was smaller (d0 < 1.1 mm approximately), the values of tf at other higher temperatures exhibited little difference for both the LCO and LO droplets. This was related to the fewer microexplosions for those smaller droplets and the slight difference of ignition delays at such higher temperatures (>1053 K), both of which made the fuel amount for flaming not greatly vary with Tc. On the other hand, the temperature of the flame, other than Tc, dominates the fuel vaporization during burning. The droplets of a given oil had certainly similar temperatures of flame, whereby causing the droplets to have nearly the same vaporization rate during burning and accordingly nearly the same flame lifetime tf, even though Tc was considerably changed. For rather larger droplets (>1.2 mm), the influence of microexplosioninduced fuel loss and vaporization speedup came into effect, especially when Tc was high (>1053 K). The higher the temperature, the larger the influence would be. As a consequence, the flame lifetime tf decreased with increasing Tc when the droplet diameter was larger than a definite value (such as 1.2 mm). For very larger droplets, the flame lifetimes at Tc ) 1173 K even became smaller than the corresponding values at Tc ) 988 K. This led also the largest tf to occur at Tc ) 1053 K where the flame lifetime was little affected by both the preignition fuel evaporation (ignition delay being not extremely long) and the microexplosion-induced fuel loss and vaporization speedup (disruptive burning being not serious). Figure 4c shows that the droplet lifetime generally decreased as the chamber temperature increased. Although the flame lifetime tf had lower values at Tc ) 988 K, the much longer ignition delay td at this temperature made the time (td + tf) still higher than the other droplet lifetimes at higher Tc. For smaller droplets at higher temperatures (>1053 K), the slight decrease of (td + tf) with Tc should be mainly attributed to the gradual decreasing of td with increasing Tc, since tf in this case was nearly independent of Tc (Figure 4b). When the droplet was rather larger, the microexplosioninduced decrease in tf gave also lower (tf + td), peculiarly for the cases at higher Tc. Comparing Figures 3 and 4, one may feel hard to judge if the times tf and (td + tf) are subject to linear relationships with d0 or d02, although the linear relationships with d015,18 and d02 19,20 were both reported in

the literature. According to the d2-law,21 tf would linearly vary with d02, but the law tackles only the period of quasi-steady burning. On the other hand, the burning rate during the quasi-steady burning period is also size-dependent, as will be shown later in the succeeding section. Therefore, the actual situation may be that the time tf as well as (td + tf) is subject to a linear relationship with respect to d0n, where n is an index depending on the burning conditions (fuel species, temperature, etc.) and it would vary between 1.0 and 2.0. Figures 3 and 4 identified this consideration, but the microexplosion-induced influences at higher Tc made tf and (td + tf) both subject more to a linear relationship with d0.

(18) Suekane, T.; Yasutomi, K.; Hirai, S. Study on Combustion Behavior of Single Droplet in Free Fall Generated by Using Ultrasonic Levitation Technique. Proceedings of the 38th Symposium on Combustion (Japan); The Combustion Society of Japan: Tokyo, 2000; pp 191192.

4. Burning Rate Figure 5 shows the time-series droplet sizes during burning at Tc ) 1053 K for the droplets of LCO, LO, and an oil mixture that blended LO and LCO at volume fractions of 50%. The blending of LO into LCO can reduce the soot yield, and accordingly facilitate the measurement of droplet size during burning. Figure 5a1 mentions that we had actually failed to get the sizes of several burning periods when burning the droplet of LCO only. The droplet sizes in Figure 5 exhibited few fluctuations, indicating that there were not intensive microexplosions during burning at the tested temperature (1053 K). Figure 5a shows that all droplets turned to shrink gradually after a heat-expansion period.19,21 The size regression with burning time t can be well correlated using the d2-law, allowing a burning rate constant (k) to be defined, although LCO and LO are both multicomponent in nature.22,23 We wished that this burning rate constant would be unique for a given fuel and be independent of the initial droplet diameter. Then, there have k ) dd2/dt ) d(d/d0)2/d(t/d02), where d denotes differential calculation. Unfortunately, this relation was not always ensured when we re-plot the data in Figure 5a into a curve of (d/d0)2 vs t/d02 in Figure 5b. Figure 5b shows clearly that the value of d(d/d0)2/d(t/d02) is different from droplet to droplet. In general, it increases with increasing the initial droplet diameter, implying a larger burning rate constant k for a larger droplet under the tested conditions. Kumagai24 and Suekane et al.18 similarly reported that the burning rate constant more or less increases when increasing the droplet diameter d0. The result, however, was not commonly accepted, while it is yet unclear how the initial droplet size influences the droplet burning and droplet evaporation. Figure 5b reveals that the relative size (d/d0)2 yet cannot be unified (normalized) with the relative burning (19) Kobayashi, K. An Experimental Study on the Combustion of a Fuel Droplet. 5th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1955; pp 141-148. (20) Agoston, G. A.; Wise, H.; Rosser, W. A. Dynamic Factors Affecting the Combustion of Liquid Spheres, 6th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1957; pp 708-717. (21) Godsave, G. A. E. Studies of the Combustion of Drops in a Fuel Spray-The Burning of Single Drops of Fuel. 4th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1953; pp 818-830. (22) Law, C. K.; Law, H. K. AIAA J. 1982, 20, 522-527. (23) Law, C. K. AIChE J. 1978, 24, 626-632. (24) Kumagai, S. Combustion; Iwanami Shoten Publisher: Tokyo, 1976; pp 165-212.

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Figure 5. Size variations when burning the droplets of LCO, LO, and an oil blend between LCO and LO at Tc ) 1053 K.

time t/d02. Instead, a larger droplet exhibits a smaller (d/d0)2 for a given t/d02. Figure 5c re-plots (d/d0)2 vs t/d0n (n ) 1.30), demonstrating that the size variation for a given fuel is in fact subject to a single curve of (d/d0)2 vs t/d0n. It means that differently sized droplets correspond to an identical value of (d/d0)2 at a give time of t/d0n. In this case, there is k )dd/d0)2/d(t/d02) ) d02-n d(d/d0)2/d(t/d0n), indicative of the dependence of the burning rate constant k on the initial droplet diameter d0, that is, k is proportional to d02-n. Shown in Figure 6 are the droplet size variations for the burning at Tc ) 1173 K. Although the more disruptive burning at this higher temperature made the sizes more dispersive, the same result as in Figure 5b and Figure 5c can be seen from the figure. That is, the relation of k )dd2/dt ) d(d/d0)2/d(t/d02) cannot be ensured (6a), and the size (d/d0)2 can be unified into a single curve by the time t/d0n (n ) 1.07 in 6b1, and n ) 1.10 in 6b2 and 6b3). Here, the burning rate constant k ) dd2/dt and the unified curve (d/d0)2 vs t/d0n are both determined by ignoring the dispersed sizes of swelled and contracted droplets.11,12 Nonetheless, Figure 6 shows that the disruptive burning seriously affected the size variational curves. For large droplets, such as d0 > 1.18 mm in Figure 6b2 and d0 ) 1.32 mm in Figure 6b3, the curves of (d/d0)2 vs t/d0n are lower-left located comparing to that of the other smaller droplets. The violent microexplosions of these droplets speeded up the depletion of the liquid fuel droplet, as early clarified for Figures 3 and 4. The LCO droplets in Figure 6a1

might also greatly swell, but there were fewer fuel satellites that occurred with the droplet contractions. The unified curve of droplet sizes, as shown in Figure 6b1, thus suffered little from the disruptive burning. In addition, Figure 6 reveals that the size fluctuations are obviously highly frequent for LO (6a2) and the oil blend (6a3), although large swelling is similarly possible for LCO (6a1) and LO (6a2). The size variational curves are also smoother for smaller droplets in the figure. All of these are evidence again that the disruptive burning is easier for LO and when the droplet is larger. The unification of (d/d0)2 by t/d0n shown in Figures 5c and 6b allows the simple calculation of the timeseries droplet sizes and lifetimes of arbitrarily sized droplets. It can also facilitate the theoretical analysis of droplet combustion because the droplet sizes practically encountered are usually various and widely distributed. Further, this unification may offer a simple method of data conversion between combustion researches and practical applications, since the combustion researches are generally conducted with larger fuel droplets. In light of these significances, we therefore further tested this finding by using different fuels under different conditions.12,25 Shown in Figure 7 are the data for the droplet evaporation of LCO and LO in the hotair chamber illustrated in Figure 1. Obviously, the (25) Ikegami, M.; Xu, G.; Honma, S.; Ikeda, K.; Nagaishi, H.; Dietrich, D. L.; Struk, P. M. Combustion of droplets composed of diversified fuels under different burning conditions. 2000’s Research Report; JUSP: Tokyo, 2001.



Figure 6. Size variations when burning the droplets of LCO, LO, and an oil blend between LCO and LO at Tc ) 1173 K.

unification of (d/d0)2 via t/d0n (n ) 1.36 for LCO and 1.38 for LO) was similarly ensured. In fact, we are sure that the unification would be universal to any liquid fuel and to both droplet burning and evaporation, if treating the index n as a function of operating parameters.25 According to the relationship of k ) d02-n d(d/d0)2/d(t/d0n), a larger n implies a weak dependence on the initial droplet diameter d0, and n ) 2.0 indicates the idealized situation that the rate of droplet burning and vaporization is independent of d0. Figures 5 to 7 revealed that n increased as the chamber temperature decreased, suggesting that the higher circumstance temperature caused the burning (or evaporation) to depend more on the initial droplet diameter. Under the same temperature Tc, the tested LCO and LO had very similar index n, but we felt strongly that it is necessary to further identify whether n is truly independent of fuels. 5. Evaporation Mechanism The heat and mass transfer inside the droplet governs the fuel evaporation process. For multicomponent fuels,

there exist two principal theories about the fuel evaporation behaviors.26 The so-called batch distillation27,28 assumes that the composition and temperature are spatially uniform but time-dependent. The various fuel fractions tend to vaporize in the order of their volatility, and the droplet temperature is likely to increase gradually with burning. Another mechanism is the liquidphase diffusion theory.23 It predicts that the mass transfer inside the droplet is slow relative to droplet surface regression, and thus there is always a concentration gradient inside the droplet. The magnitude of the gradient is a function of the liquid Peclet number that indicates the surface regression rate relative to the liquid-phase mass diffusion rate.29 If the Peclet number is much larger than unity, the fuel evaporation then should be dominated by the liquid-phase diffusion. In (26) Mawid, M.; Aggarwal, S. K. Combust. Flame 1991, 84, 197209. (27) Wood, B. J.; Wise, H.; Inami, S. H. Combust. Flame 1960, 4, 235-242. (28) Law, C. K. Combust. Flame 1976, 26, 219-233. (29) Makino, A.; Law, C. K. Combust. Flame 1988, 73, 331-336.

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Figure 7. Size variations when vaporizing the droplets of LCO and LO at Tc ) 823 K.

contrast, a Peclet number that is much smaller than unity implies the distillation-like vaporization. The liquid-phase diffusion mechanism usually supposes that the concentration profiles of various constituents change little during the droplet lifetime. More recent studies showed that the actual vaporization of most multicomponent fuels may be between the distillation-like and liquid-phase diffusion limits, depending on the volatility differentials of the constituents.22,30 Following these issues, the feature of droplet temperature during burning would be indicative of the droplet evaporation mechanism.31 We therefore measured the droplet temperature histories in microgravity conditions (JAMIC) using the facility illustrated in Figure 2 where a down-laid heater was adopted as the igniter. In this case, the fuel vaporization from the droplet should be exclusively controlled by the fuel properties and the heat and mass transfer within the droplet, due to the removals of the gravity-induced convection and the extra radiation from a hot circumstance that surrounds the droplet. Figures 8 to 10 show the results obtained, giving the following analyses about the fuel evaporation mechanism that dominates the burning of the LCO and LO droplets. 5.1. Basic Analysis. The time-temperature diagrams in Figure 8 demonstrate that the droplet temperature gradually increased before ignition, and the (30) Lerner, S. L.; Homan, H. S.; Sirignano, W. A. Multicomponent Droplet Vaporization at High Reynolds Numbers: Sizes, Composition, and Trajectory Histories. 23rd AIChE Annual Meeting; AIChE: New York, 1980. (31) Ikegami, M.; Ikeda, K.; Honma, S.; Xu, G.; Dietrich D. L.; Takeshita, Y. Influence of Light Oil on Droplet Combustion of Heavy Oil-Tracing the Combustion Process Under Microgravity. Proceedings of the 4th JSME-KSME Thermal Engineering Conference; JSME and KSME: Tokyo, 2000; Vol. 1, pp 121-126.

Figure 8. Temperature-time diagrams during burning in microgravity for LCO and LO droplets.

ignition of the LCO droplets, as expected, occurred earlier at lower temperatures. After ignition, there is a period raising the droplet temperature quickly. This revealed the lower droplet temperature at ignition and also the enhanced heating by the flame.31,32 As Figure 8 shows, the droplet temperatures at ignition were lower than the IBPs of LCO and LO (about 420 K). Although the droplet surface temperature at ignition might be close to the boiling points of the volatile matters in the oil, the temperatures inside the droplet were still low. This allowed the droplet temperature to rise rapidly (32) Ikegami, M.; Xu, G.; Ikeda, K.; Honma, S.; Nagaish, H.; Dietrich, D. L.; Struk, P. M.; Takeshita, Y. Combustion Stages of Single Heavy Oil Droplet under Microgravity. 2001 Sixth International Microgravity Combustion Workshop; NASA: Cleveland, 2001; pp 261264.


when ignition caused a flame to encircle the droplet. The quick temperature increase ended in an inflection that turned the droplet temperature to vary with fuel composition. Thus, the burning between ignition and the inflection was rather transient, which performs to raise the droplet temperature and to make the burning gradually steady (quasi-steady). Under the control fuel vaporization according to volatility, the temperature rise speed greatly slowed after the inflection. In comparison, the LO droplet increased the temperature at a relatively constant rate, while the temperature increasing rate of the LCO droplet slightly increased lately during burning. This indicates an even distribution of fuel constituents for LO within the corresponding range of temperatures, and the fact that LCO contained more volatilematters and fewer high-boiling point fractions. Near flame extinction, the thermocouple bead might be exposed to flame and the droplet temperature was no longer controlled by the fuel vaporization. It caused the temperature to sharply increase and to quickly reach a peak value at the extinction. Then, the droplet temperature rapidly dropped to the environment temperature, indicating that there was not coke and soot smoldering after extinction. Coke formation was also negligible in microgravity, as was similarly observed in normal gravity test. The droplet temperature and its change with burning are also fuel-dependent. In general, the temperature during burning was higher for the LO droplets, leading the droplets to have higher temperatures at the inflection point as well. Figure 8 shows that the inflection occurred between 470 and 500 K for the LCO droplets and between 510 and 540 K for the LO droplets. The LO droplets had also higher temperatures when the sharp temperature increase started near extinction, that is, the sharp increase occurred at about 600 and 630 K for the LCO and LO droplets, respectively. Comparison with Table 1 demonstrates further that those inflection temperatures are lower than T50, while such onset temperatures of sharp temperature increase are close to T90. These facts identified again that the burning proceeded by vaporizing fuel components according to their volatility. LCO contained more volatile-matters, having thus lower droplet temperatures in burning. The higher volatility of LCO gave also higher vaporization rate, which caused the droplets to have shorter lifetimes. In addition, Figure 8 mentions that the general characteristics of temperature increase during burning was independent of d0, although the temperature values for the same burning events, such as the inflection and sharp temperature increase, depended more or less on this diameter. In light of the preceding features of droplet temperature histories, we can then believe that the burning of the LCO and LO droplets is not controlled by the liquidphase diffusion mechanism. By further noting that T90 just coincides the onset temperature of sharp temperature increase near extinction, we can conclude that the fuel evaporation during burning is rather distillationlike. This, however, is inconsistent with the Pelect numbers of the droplets. From Figure 8, one can see that the droplet surface regression rate between ignition and extinction was about 1.5 mm2/s. On the other hand, the diffusion coefficient for LO and LCO should be in


Figure 9. Typical views of flame and droplet of an LCO droplet burning in microgravity (d0 ) 1.30 mm).

the order of 10-8 m2/s, even at higher temperatures.33 These result in the Pelect numbers that are larger than 10, indicating an evaporation mechanism of liquid-phase diffusion, or at least a mechanism between batchdistillation and liquid-phase diffusion. Therefore, the actual fuel diffusion inside the droplet should be greatly enhanced by the fuel inter-circulation, wherewith causing the fuel evaporation from the droplet more distillation-like.28 The fuel inter-circulation is related to the fuel vapor nucleation within the droplet, and it usually causes disruptive burning. The disruptive burnings of the LCO and LO droplets are obviously shown in Figures 5 and 6 for the normal gravity test. In the case of microgravity, the image data revealed that the burning involved with droplet swellings and contractions as well, although they occurred in lower frequency and appeared weaker in intensity. 5.2. Further Evidence. Shown in Figure 9 are the typical views of flame and droplet for an LCO droplet burning in microgravity (d0 ) 1.30 mm). The flame was essentially spherical, allowing thus the measurement of flame sizes. Due to the suppression of the gravityinduced gas convection, the soot yield is higher and soot movement (drift) is slower under the microgravity conditions. These caused a thick soot cloud to be present around the droplet, even early during the burning, such as before the igniter withdrawal shown in Figure 9. The soot cloud seriously obscured the droplet view, which made us fail when trying to get the droplet sizes during burning and to determine the burning rates of the LCO and LO droplets in microgravity. Figure 10 shows the time-series sizes of flame measured using the equivalent size of (radius)2/3 × (length)1/3. The lower ignition delays and longer droplet lifetimes of the LCO droplets are obviously demonstrated in the figure. The flame sizes gradually increased after ignition and gradually decreased lately during burning. This led to a maximal flame size between. For the equi-sized droplets, the LO droplet tended to have a longer period of size decrease, while the LCO droplet appeared to have (33) Shaw, B. D. Combust. Flame 1990, 81, 277-288.

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sizes during the period of size increase. Although the LCO droplets had larger flame sizes when the flame begun to shrink, they showed smaller flame sizes later during burning due to their faster rate of flame shrink. Comparing to Figure 8, one can find that the inflection for temperature rise took place earlier than the maximal flame size, implying that the temperature inflection occurred at a time burning the volatile matters. Therefore, the flame size diagrams in Figure 10 show also that the burning of the LCO and LO droplets proceeded by vaporizing fuel constituents according to their volatility. This gives evidence again that the evaporation mechanism during burning is rather distillation-like. 6. Soot Burn-up Figure 10. Flame size variations when burning LCO and LO droplets in microgravity.

a longer duration of size increase. The LCO droplets showed also a later onset and a higher speed for the flame size decreasing. All of these indicate that the burning during the size increase period was mainly with respect to volatile matters, while that in the size decrease period was about the other heavier fuel components exclusively. The higher content of volatile constituents in LCO led the oil to have a longer duration of size increase, while the more heavy components in LO caused it to manifest a slower rate of size decrease. The more volatile matters in LCO gave also larger flame

The burning of the LCO and LO droplets, such as in spray burners, requires the burn-up of soot particles as well, from the viewpoints of both environment protection and fuel utilization. Figure 11 shows two patterns of soot particle glowing and smoldering after flame extinction identified in the normal gravity test. The tested situation may be different from that in actual spray burners; a good comparison, however, can be made for the soot burn-ups of the LCO and LO droplets under the same conditions. As illustrated in Figure 11, soot particles (lump) started to glow at a certain time after flame extinction, which made the soot lump fired-like in appearance (No. 4). As for pattern 1, the fired

Figure 11. Two different patterns of soot glowing and smoldering in a hot-air chamber (Tc ) 1173 K).


Figure 12. Glowing delay and ember time of soot particles in a hot-air chamber.

appearance of soot lump constantly remained, leading therefore the soot particles to continuously smolder and to form an ember until the burnout of all particles (Nos. 4-7). This pattern usually occurred when the droplet was relatively larger. Otherwise, the soot glowing and smoldering would take place in accordance with pattern 2. In this case, the fired appearance of the soot lump disappeared shortly after the glowing (No. 5), making thus the glowing sparklike in whole. There were probably several, typically 3, times of this kind of sparklike soot glowing and firing. After the last glowing (No. 6), there still remained definite amount of soot particles on the fiber filament (No. 7). Therefore, Figure 11 defines only the soot glowing delay tsg for pattern 2 and both tsg and the soot ember time tse for pattern 1. Obviously, the times tsg and tse are two of the major parameters that characterize the burn-up of soot particles. Figure 12 shows the measured tsg (12a) and tse (12b) for the droplets of LCO, LO, and the same oil blend tested in Figures 5 and 6. At the same temperature of


Tc ) 1173 K, Figure 12a reveals that the soot glowing delay was larger for the LO droplets. This made the time tsg of the blend oil droplets be intermediate in comparison with that of the LCO and LO droplets. For the LCO droplets, the contraction of soot particles inward the flame (No. 1) caused the soot lump to have probably a higher temperature at flame extinction. This higher temperature might facilitate the soot glowing, but the glowing is rather controlled by the chamber temperature than by the soot temperature at flame extinction. Because of this, the soot glowing (firing) did not occur immediately after extinction, and there was not soot glowing at lower Tc, such as 1053 K. In light of these, Figure 12a shows in fact that the soot particles from the LCO droplets are easier to fire (glow) than that from the LO droplets, denoting some differences of properties between the soot particles from burning the LCO and LO droplets. As a matter of fact, LCO contains more aromatics and is more apt to pyrolyze during burning. It is thus not surprising that the properties of soot particles are different between the LCO and LO droplets. The soot ember times in Figure 12b demonstrate that longer times were required for burning up the soot particles from the equi-sized LCO droplets, in comparison with that from the LO droplets. The blend oil droplets had ember times between the LCO and LO droplets, but they were closer to the times of the LCO droplets. Therefore, the longer tse of the LCO droplets are related not only to the higher soot yield from the droplets but as well to the property differences of soot particles between the LCO and LO droplets. It seems that the soot particles from the LCO droplets smoldered in a lower rate, although Figure 12a showed that they were easier to glow and fire. The sum of the soot glowing delay and ember time was plotted in Figure 12c, demonstrating that the LCO droplets had higher values than the LO droplets. This implies that the LCO droplets required longer times after flame extinction to burn up the soot particles. Figure 12a plots also the soot glowing delay times of the LCO droplets at a lower temperature of Tc ) 1110 K. Obviously, tsg increased when Tc decreased. Image data showed further that the soot lumps did not glow at rather lower temperatures (

Burning Droplets Composed of Light Cycle Oil and ... - ACS Publications

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