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Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures
Fuel 94 (2012) 342–347
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures M.L. Botero a,b, Y. Huang a,⇑, D.L. Zhu a, A. Molina b, C.K. Law a a b
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA School of Processes and Energy, Universidad Nacional de Colombia - Sede Medellín, Colombia
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 25 September 2011 Accepted 20 October 2011 Available online 15 November 2011 Keywords: Castor oil biodiesel Droplet combustion Soot formation Microexplosion Ethanol
a b s t r a c t Freely-falling droplets of ethanol, diesel, (castor oil) biodiesel, and their mixtures were experimentally examined in a high-temperature combustion chamber. The combustion characteristics including the burning rate, microexplosion, and sooting propensity are reported. Results show that adding biodiesel to diesel significantly reduces the extent of soot formation and slightly reduces the burning rate. In addition, higher soot production of methyl stearate than castor oil biodiesel is observed suggesting strong oxidation propensity of the OH function group in castor oil biodiesel. Furthermore, by adding ethanol to diesel and biodiesel, microexplosion is observed, with the biodiesel/ethanol mixtures exhibiting stronger propensity, leading to significantly reduced gasification time and extent of soot formation. The burning of these mixtures occurs in a three-staged, liquid-phase diffusion-limited manner previously observed for distinct bi-component droplets with highly disparate volatilities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Reduction of petroleum reserve and the concomitant increase in greenhouse gas emission from burning fossil fuels have generated strong incentive to totally or partially replace petroleum-based fuels by alternative fuels as the primary energy source, particularly for transportation [1]. Non-fossil oxygenated fuels are considered as promising alternatives given their renewable nature of supply and the potential of achieving minimal lifecycle greenhouse gas emission. Among them biodiesel and the alcohols, especially ethanol, have emerged as strong candidates [2–4], either as total replacement or as blends with petroleum fuels. Furthermore, recognizing that while one of the main concerns with diesel is its propensity to soot, the burning of biodiesel and ethanol is minimally sooting [5–7]. Indeed, fundamental studies on the droplet combustion of mixtures of diesel and methylated ester biodiesel [8,9] have unambiguously demonstrated reduced soot formation. The reduction is primarily due to the lack of aromatics which is a major component of diesel, although the presence of oxygenated functional groups in the molecular structure of the biofuels could also potentially contribute to the oxidation of the soot precursors. In spite of the various merits mentioned above, the use of biodiesel and ethanol also poses substantial concerns [1]. First, the lack of aromatics in the composition could lead to sealing problems. Second, biodiesel usually have high pour points, rendering their use in cold weather difficult, especially as aviation fuels. ⇑ Corresponding author. E-mail address: [email protected] (Y. Huang). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.10.049
Third, biodiesel also have high boiling points, and hence slow gasification rates. This could lead to various deleterious heterogeneous combustion responses such as extensive nonpremixed burning and hence elevated levels of NOx emissions. Concerning ethanol, since the predominant feedstock for its synthesis at present is food based, its large-scale use could compete with food supply. Furthermore, its lower energy density also renders it not suitable as aviation fuels [1]. Recognizing the merits and constraints with each of the above three fuels, namely ethanol, diesel and biodiesel, the primary motivation of the present study is to explore potential synergies in their blending by capitalizing on the difference in their physical and chemical properties. The possibility of such a synergistic formulation can be appreciated by recognizing that mixing biodiesel with diesel can potentially reduce the sooting propensity of diesel while at the same time alleviate the sealing problem as well as moderate the freezing problem for the biodiesel. Furthermore, mixing a moderate amount of ethanol with diesel is expected to reduce soot formation, while mixing it with either diesel and/or biodiesel could increase the volatility of the blend and as such lead to facilitated gasification. The dominant mechanism through which the overall gasification rate can be potentially facilitated is however somewhat subtle and is briefly discussed next. It is well established in studies of multicomponent droplet combustion [10,11] that the droplet tends to violently explode during burning when its components possess vastly different volatilities. This microexplosion event is induced by the superheating of the more volatile components that are diffusionally trapped in the interior of the droplet, whose temperature is controlled by
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M.L. Botero et al. / Fuel 94 (2012) 342–347
the (higher) boiling point of the less volatile component [12,13], as will be further elaborated later. The occurrence of such a microexplosion event constitutes a secondary atomization process, and as such could significantly facilitate the overall droplet burning rate. The instant obliteration of the droplet and dispersion of the fuel mass into the gas medium also improves mixing and thereby substantially reduces the sooting propensity and NOx formation. Recognizing the vast volatility differential between ethanol and the high-boiling-point biodiesel, it is reasonable to expect that the occurrence of microexplosion could be greatly facilitated for their mixtures. A demonstration of its feasibility however is still necessary because previous studies have used distinct bi-component mixtures with sharp volatility differentials, while diesel and biodiesel are blends of many components; a continuous distribution of the fuel volatility could moderate the attainment of the state of superheating in the droplet interior. A secondary motivation of the present study is to explore and characterize the combustion of a specific class of biodiesel, namely castor oil biodiesel. This biodiesel was produced in the Crude Oil and Derivatives Laboratory of the Universidad Nacional de Colombia - Sede Medellín, and its main properties are listed in Table 1, together with those of ethanol and a commercial diesel that were investigated in this study. This castor oil biodiesel is mainly (85%) composed of methyl ricinoleate, (C17H32OH)COOCH3, whose molecular structure is shown in Fig. 1 and is compared with methyl stearate, (C17H35)COOCH3, which is a saturated methyl ester and is a common component of conventional biodiesels, as in the case of the biodiesel used in Ref. [9]. The properties of the present castor oil biodiesel differ from the biodiesel studied previously in two major aspects. First, it has a pour point 45 °C, which is about 10° lower than those of the conventional biodiesels [14]. As such, it is more conducive for cold weather use, especially when mixed with diesel. Second, the castor oil biodiesel has an extra OH function group. Since OH is a strong oxidizing agent, its presence in the molecular structure could potentially facilitate oxidation of the soot precursors. In view of the above considerations, we have performed a systematic experimental investigation of the combustion, microexplosion and sooting propensity of droplets of mixtures of ethanol, diesel and biodiesel, in various combinations. We shall show in due course that the castor oil biodiesel is indeed a very low soot emitter, either burning by itself or blended with diesel, and that ethanol blending with either diesel and/or biodiesel exhibits strong microexplosion events and as such sharply reduces the droplet gasification time and the extent of deleterious effects associated with heterogeneous burning.
2. Experimental methodology Experiments were conducted in a well-characterized droplet combustion chamber, schematized in Fig. 2 [15]. The experiment involves the combustion of a freely-falling stream of droplets of controlled size, spacing and velocity, generated by the ink-jet
printing technique. The stream of droplets is adjusted to fall through a small vertical channel connected to a high temperature combustion chamber heated by the products of a flat-flame burner. The chamber temperature typically varies between 980 and 1040 K along the distance over which droplet gasification takes place. The combustion environment is controlled by the composition of the premixed gas consisting of methane, oxygen and air that is fed to the flat-flame burner. In this work, the flow conditions were kept constant and the oxygen molar fraction of the flow was 0.21. The droplet image was photographically determined using stroboscopically back-lighted microphotography synchronized with the droplet generation system, while the images of the flame streaks were recorded through simple photography with a fixed aperture and exposure time. The initial droplet diameter (Do) ranged from 220 to 250 lm, while the droplet spacing was in excess of 100 droplet diameters so that droplet–droplet interaction was negligible. The droplet Reynolds number is estimated to be always less than one. Details of the apparatus and experimental procedure can be found in Refs. [11] and [15].
3. Results and discussion 3.1. Diesel/biodiesel mixtures Fig. 3 shows representative flame streaks of a burning droplet stream of diesel, (castor oil) biodiesel and their mixtures; note that the legends Dxx, Byy and Ezz respectively designate xx% diesel, yy% biodiesel, and zz% ethanol. It is seen that the diesel flame has a strong yellow brightness indicating the presence of soot. As biodiesel is added, not only the yellow luminosity reduces visually but at a certain instant the reduction becomes drastic, yielding a blue flame. This abrupt event indicates the transition from the more volatile, diesel-dominated burning of high sooting propensity to one involving the less-volatile, biodiesel of minimal sooting propensity. As such, neat biodiesel burns in a light blue flame, with practically no soot formation. Fig. 4 then plots the normalized flame diameter (Dfs/Do) and droplet diameter (Ds/Do) at the state of transition of the flame color as a function of the amount of biodiesel in the mixture. It is seen that the droplet diameter at flame transition increases with biodiesel addition, indicating the earlier transition of the dominant gasifying component from the more volatile, and sooty, diesel to the less volatile, less sooty biodiesel. Fig. 5 compares the flame streak of the present castor-oil biodiesel, Fig. 5a, with those of other fuels. Fig. 5b is the biodiesel from [9] whose major component is methyl stearate. It is seen that while some yellow luminosity is still present with methyl stearate, the flame streak is basically blue for the present biodiesel. This therefore supports the notion that the present castor oil biodiesel, with its extra OH functional group, has a stronger tendency to suppress soot than the methylated ester of [9]. We next recognize that the primary reason that diesel is strongly sooting is due to its content of aromatics, typically 20– 30%. In other words, the fact that the biodiesels are much less
Table 1 Comparison of chemical components and properties of fuels. Species
Component (% mol)
Castor oil biodiesel
C17H34O2 C19H38O2 C19H35O2 C19H34O2 C19H32O2 C19H36O3 C11–C19
Diesel Ethanol
1.953 1.004 4.005 5.874 2.836 84.328
Equivalent composition
MW (g/mol)
C18.96H35.75O2.84
270 298 296 294 292 312 140–210
C12H23 C2H6O
ql (g/ml)
Tb (K)
HHV (MJ/kg)
308.8
0.936
571–634 (640 mm Hg)
37.48
167.0 46.1
0.827–0.850 0.789
473–623 351
45.40 23.40
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Fig. 1. Molecular structure of (a) methyl ricinoleate, (C17H32OH)COOCH3 and (b) methyl stearate, (C17H35)COOCH3.
Flat flame burner
Fig. 2. Schematic of the experimental apparatus.
sooting is mainly due to the lack of aromatics in its composition; the presence of the oxygen-containing functional groups likely plays a secondary, supplementary role in soot reduction. To demonstrate the relative roles of the aromatics and oxygenated functional groups, experiments were conducted by adding toluene and tetralin in biodiesel, recognizing that they are representative components of diesel, and that tetralin is sootier than toluene because of its two-ringed structure. Fig. 5c and d shows the flame streaks of castor biodiesel with 30% toluene and 30% tetralin, respectively. It is seen that while their addition significantly increases the flame luminosity and hence the amount of soot formed, both flame streaks appear less sooty than that of the diesel in Fig. 3a, thereby suggesting that the presence of the oxygenated functional groups in the biodiesel indeed reduces the tendency to soot. It is also noted that because of the weaker sooting propensity and lower boiling point of toluene (Tb 384 K) as compared to tetralin (Tb 480 K) the toluene-doped flame streak not only is less luminous, it is also over in a shorter droplet lifetime. The temporal variation of the square of the normalized droplet diameter for diesel, biodiesel and their mixtures is plotted in Fig. 6. It is seen that, while the variations are not linear due to the multicomponent nature of the fuel composition [16], the d2-law is approximately followed after an initial heating period, resembling the case of pure component combustion. This is because the intense heating from the flame drives the droplet surface tempera-
ture close to the boiling point of the surface components, at which the fuel volatility loses its sensitivity on the fuel vapor concentration at the surface and thereby the fuel gasification rate. Fig. 7 shows the burning rates of diesel, biodiesel, and their mixtures. It is clear that the burning rate of biodiesel is slightly lower than that of diesel because of its lower heat content and higher boiling point. The burning rates of the mixtures are close, with a slight decreasing trend as more biodiesel is added; the average burning rate of mixtures with 75% biodiesel is only 2.6% lower than that of 25% biodiesel. 3.2. Ethanol blending To identify the effects of ethanol blending in diesel, biodiesel, and their mixtures, Figs. 8 and 9 respectively show the flame appearance for diesel and biodiesel mixtures with ethanol. It is seen that ethanol (Figs. 8e and 9g) burns in a blue flame, and is almost visually non-perceptible. For diesel, the addition of ethanol delays the onset of the yellow luminosity of the flame because of its volatility and thereby preferential gasification. This indicates a corresponding reduction in soot formation during the early stage of burning, when the flame size and thereby soot content are large. For biodiesel, the transition from ethanol- to biodiesel-dominated burning is exhibited in a more prominent manner, characterized by the presence of a gap in the flame streak, designated by arrows
M.L. Botero et al. / Fuel 94 (2012) 342–347
Fig. 3. Flame streak images of mixtures of diesel and (castor oil) biodiesel: (a) diesel (b) D75B25, (c) D50B50, (d) D25B75 and (e) biodiesel.
345
Fig. 5. Flame streak images for: (a) castor oil biodiesel, (b) biodiesel used in [9], (c) castor oil biodiesel and 30% toluene and (d) castor oil biodiesel and 30% tetralin.
Fig. 6. d2-Law plot for droplets of diesel/biodiesel mixtures.
Fig. 4. Normalized droplet (triangles) and flame (squares) diameter at the state of flame shrinkage.
in Fig. 9. This indicates the occurrence of a period of intermediate droplet heating as the droplet temperature increases from that close to the lower boiling point of ethanol to the higher boiling point of biodiesel. The attendant requirement to heat up the droplet reduces the droplet burning rate and thereby its flame size and temperature. Fig. 10a and b are plots of the normalized droplet diameter, (D/ Do)2 vs. normalized time, t=D2o , for ethanol blended with diesel and biodiesel respectively. These results resemble those of bi-component mixtures with vastly different boiling points, as described in [16]. In particular, for cases where ethanol is present in abundance the intermediate heating event observed in the flame streaks is now represented as three distinct segments: two relatively straight segments corresponding to nearly steady droplet burning separated by a nearly horizontal segment. Mechanistically, the first segment represents the initial period during which ethanol, the more volatile component of the mixture, is preferentially gasified. The droplet temperature is controlled by its boiling point, and hence
Fig. 7. d2-Law burning rate for various composition of diesel/biodiesel mixtures.
is relatively low. When the concentration of ethanol in the surface layer of the droplet is largely depleted, a transient period occurs during which the droplet temperature increases to approach the boiling point of the less volatile diesel or biodiesel, which is now the abundant component at the droplet surface. The heat required
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Fig. 9. Flame streak images of (castor oil) biodiesel/ethanol mixtures: (a) biodiesel (b) E20B80, (c) E40B60, (d) E50B50, (e) E60B40, (f) E80B20 and (g) ethanol.
Fig. 8. Flame streak images of: (a) diesel (b) E25D75, (c) E50D50, (d) E75D25 and (e) ethanol.
to effect this intermediate heating process then substantially slows down the droplet gasification rate, as manifested by the near-horizontal segment. After the heating is mostly achieved, the gasification is then sustained by the co-vaporization of both the volatile and nonvolatile components, and is represented by the third, nearly-straight segment. During this process some ethanol remains trapped in the core of the droplet due to liquid-phase diffusional resistance and could become superheated, leading to internal gasification and subsequently violent rupturing of the droplet. The time required to attain this state depends on the concentrations of the components and the boiling point difference between them. Thus when the initial amount of ethanol in the mixture is small or if the volatility differential is relatively small as for the diesel/ethanol mixtures, the first and third segments merge due to the stronger influence of the less volatile compound, leading to heating of the droplet while it gasifies. However, for higher ethanol concentrations and/or larger volatility differentials, as for the biodiesel/ ethanol mixtures, the three-phase gasification mechanism is prominently exhibited.
Figs. 8 and 9 also clearly demonstrate that both diesel and biodiesel mixtures exhibit strong disruptive burning, i.e. microexplosion, that instantaneously ruptures and gasifies the droplet. It is evident that, compared to the diesel mixture, microexplosion for the biodiesel mixture not only occurs earlier but also possesses stronger intensity, characteristics that are due to its higher boiling point. Fig. 11 shows the normalized droplet size at explosion, (Dexplosion/Do), as a function of ethanol addition, demonstrating that the dependence on the concentration is parabola-like with a maximum around equi-volumetric composition; which is in agreement with previous studies [11]. Physically, the onset of microexplosion requires sufficient high-boiling-point, less-volatile component to drive up the droplet temperature, and sufficient low-boiling-point, volatile component for its ease to attain the limit of superheat. The competing requirements then lead to the non-monotonic dependence on the composition, with the optimum being around equivolumetric. Since the biodiesel mixture microexplodes earlier than the diesel mixture for the same volume of ethanol added because of its higher boiling point, it effectively has a higher overall consumption rate. This is demonstrated in Fig. 12, which shows that microexplosion increases the consumption rate, with the effect being maximized at equi-volume concentration, and is stronger for biodiesel.
Fig. 10. d2-Law plots of: (a) diesel/ethanol mixtures and (b) biodiesel/ethanol mixtures.
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components with lower boiling points; 2. an intermediate transient heating period as the dominant surface components transitions from the more volatile to the less volatile ones; 3. almost steady burning supported by the co-gasification of both the more and less volatile components. Diesel and biodiesel mixtures with ethanol blending both microexplode during burning, with the latter exhibiting it earlier in the droplet lifetime and with stronger intensity, and is also promoted for near equi-volume mixtures. Microexplosion reduces the overall burning time and hence increases the overall fuel consumption rate. The addition of ethanol to diesel reduces the yellow luminosity of the flame at the early stage of the droplet lifetime, indicating an overall reduction in the sooting propensity, especially recognizing the relatively large amount of soot formation when the flame size is also larger during that stage. Fig. 11. Normalized droplet diameters at microexplosion as function of ethanol addition.
Acknowledgments This work was supported by the Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001198. MLB was a visiting student at Princeton University, partially supported by the Young Scientist Research Grant from the Colombian Administration Department of Science Technology and Innovation. References
Fig. 12. Overall droplet consumption rate as function of ethanol addition.
4. Conclusions Burning characteristics of diesel/biodiesel, diesel/ethanol and biodiesel/ethanol droplets were experimentally studied. The main results are summarized as follows: Addition of biodiesel to diesel substantially reduces the extent of soot formation, primarily because of reduced aromatics content of the mixture and the oxidizing functionalities of the biodiesel. Castor oil biodiesel appears to be more effective in soot suppression than the much-used methyl esters, ostensibly because of the presence of the extra OH function group in its molecular structure. Castor oil biodiesel has a slightly lower burning rate than that of diesel because of its lower heat content and higher boiling point. The diffusion-limited mechanism for multicomponent droplet burning with highly disparate boiling points was demonstrated for diesel/ethanol and biodiesel/ethanol mixtures. Three phases were observed: 1. almost steady burning of the more volatile
[1] Law CK, Fuel options for next generation chemical propulsion, AIAA Paper No. 2011-1, 49th AIAA aerospace sciences meeting, January 4–7, Orlando, Florida; 2011. [2] Demirbas A. Progress and recent trends in biofuels. Prog Energy Combust Sci 2007;33:1–18. [3] Sharma YC, Singh B, Upadhyay SN. Advancements in development and characterization of biodiesel: a review. Fuel 2008;87:2355–73. [4] Hansen AC, Zhang Q, Lyne PWL. Ethanol–diesel fuel blends – a review. Bioresour Technol 2005;96:2277–85. [5] Lapuerta M, Armas O, RodrÌguez-Fernandez J. Effect of biodiesel fuels on diesel engine emissions. Prog Energy Combust Sci 2008;34:198–223. [6] Shi X, Pang X, Mu Y, He H, Shuai S, Wang J, et al. Emission reduction potential of using ethanol–biodiesel–diesel fuel blend on a heavy-duty diesel engine. Atmos Environ 2006;40:2567–74. [7] He BQ, Shuai SJ, Wang JX, He H. The effect of ethanol blended diesel fuels on emissions from a diesel engine. Atmos Environ 2003;37:4965–71. [8] Pan KL, Li JW, Chen CP, Wang CH. On droplet combustion of biodiesel fuel mixed with diesel/alkanes in microgravity condition. Combust Flame 2009;156:1926–36. [9] Li TX, Zhu DL, Akafuah NK, Saito K, Law CK. Synthesis, droplet combustion, and sooting characteristics of biodiesel produced from waste vegetable oils. Proc Combust Sympos 2011;33:2039–46. [10] Law CK. Recent advances in droplet vaporization and combustion. Prog Energy Combust Sci 1982;8:171–201. [11] Wang CH, Liu XQ, Law CK. Combustion and microexplosion of freely falling multicomponent droplets. Combust Flame 1984;56:175–97. [12] Law CK. Internal boiling and superheating in vaporizing multicomponent droplets. AIChE 1978;24:626–32. [13] Makino A, Law CK. On the controlling parameter in the gasification behavior of multicomponent droplets. Combust Flame 1988;73:331–6. [14] Barajas Forero CL. Biodiesel from castor oil: a promising fuel for cold weather. In: International conference on renewable energy and power quality (ICREPQ04); 2004.. [15] Lee A, Law CK, Randolph AL. Aerothermochemical studies of energetic liquid materials. Combustion and microexplosion of droplets of organic azides. Combust Flame 1988;71:123–36. [16] Law CK. Combustion physics. New York: Cambridge University Press; 2006.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures M.L. Botero a,b, Y. Huang a,⇑, D.L. Zhu a, A. Molina b, C.K. Law a a b
Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA School of Processes and Energy, Universidad Nacional de Colombia - Sede Medellín, Colombia
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 25 September 2011 Accepted 20 October 2011 Available online 15 November 2011 Keywords: Castor oil biodiesel Droplet combustion Soot formation Microexplosion Ethanol
a b s t r a c t Freely-falling droplets of ethanol, diesel, (castor oil) biodiesel, and their mixtures were experimentally examined in a high-temperature combustion chamber. The combustion characteristics including the burning rate, microexplosion, and sooting propensity are reported. Results show that adding biodiesel to diesel significantly reduces the extent of soot formation and slightly reduces the burning rate. In addition, higher soot production of methyl stearate than castor oil biodiesel is observed suggesting strong oxidation propensity of the OH function group in castor oil biodiesel. Furthermore, by adding ethanol to diesel and biodiesel, microexplosion is observed, with the biodiesel/ethanol mixtures exhibiting stronger propensity, leading to significantly reduced gasification time and extent of soot formation. The burning of these mixtures occurs in a three-staged, liquid-phase diffusion-limited manner previously observed for distinct bi-component droplets with highly disparate volatilities. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Reduction of petroleum reserve and the concomitant increase in greenhouse gas emission from burning fossil fuels have generated strong incentive to totally or partially replace petroleum-based fuels by alternative fuels as the primary energy source, particularly for transportation [1]. Non-fossil oxygenated fuels are considered as promising alternatives given their renewable nature of supply and the potential of achieving minimal lifecycle greenhouse gas emission. Among them biodiesel and the alcohols, especially ethanol, have emerged as strong candidates [2–4], either as total replacement or as blends with petroleum fuels. Furthermore, recognizing that while one of the main concerns with diesel is its propensity to soot, the burning of biodiesel and ethanol is minimally sooting [5–7]. Indeed, fundamental studies on the droplet combustion of mixtures of diesel and methylated ester biodiesel [8,9] have unambiguously demonstrated reduced soot formation. The reduction is primarily due to the lack of aromatics which is a major component of diesel, although the presence of oxygenated functional groups in the molecular structure of the biofuels could also potentially contribute to the oxidation of the soot precursors. In spite of the various merits mentioned above, the use of biodiesel and ethanol also poses substantial concerns [1]. First, the lack of aromatics in the composition could lead to sealing problems. Second, biodiesel usually have high pour points, rendering their use in cold weather difficult, especially as aviation fuels. ⇑ Corresponding author. E-mail address: [email protected] (Y. Huang). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.10.049
Third, biodiesel also have high boiling points, and hence slow gasification rates. This could lead to various deleterious heterogeneous combustion responses such as extensive nonpremixed burning and hence elevated levels of NOx emissions. Concerning ethanol, since the predominant feedstock for its synthesis at present is food based, its large-scale use could compete with food supply. Furthermore, its lower energy density also renders it not suitable as aviation fuels [1]. Recognizing the merits and constraints with each of the above three fuels, namely ethanol, diesel and biodiesel, the primary motivation of the present study is to explore potential synergies in their blending by capitalizing on the difference in their physical and chemical properties. The possibility of such a synergistic formulation can be appreciated by recognizing that mixing biodiesel with diesel can potentially reduce the sooting propensity of diesel while at the same time alleviate the sealing problem as well as moderate the freezing problem for the biodiesel. Furthermore, mixing a moderate amount of ethanol with diesel is expected to reduce soot formation, while mixing it with either diesel and/or biodiesel could increase the volatility of the blend and as such lead to facilitated gasification. The dominant mechanism through which the overall gasification rate can be potentially facilitated is however somewhat subtle and is briefly discussed next. It is well established in studies of multicomponent droplet combustion [10,11] that the droplet tends to violently explode during burning when its components possess vastly different volatilities. This microexplosion event is induced by the superheating of the more volatile components that are diffusionally trapped in the interior of the droplet, whose temperature is controlled by
343
M.L. Botero et al. / Fuel 94 (2012) 342–347
the (higher) boiling point of the less volatile component [12,13], as will be further elaborated later. The occurrence of such a microexplosion event constitutes a secondary atomization process, and as such could significantly facilitate the overall droplet burning rate. The instant obliteration of the droplet and dispersion of the fuel mass into the gas medium also improves mixing and thereby substantially reduces the sooting propensity and NOx formation. Recognizing the vast volatility differential between ethanol and the high-boiling-point biodiesel, it is reasonable to expect that the occurrence of microexplosion could be greatly facilitated for their mixtures. A demonstration of its feasibility however is still necessary because previous studies have used distinct bi-component mixtures with sharp volatility differentials, while diesel and biodiesel are blends of many components; a continuous distribution of the fuel volatility could moderate the attainment of the state of superheating in the droplet interior. A secondary motivation of the present study is to explore and characterize the combustion of a specific class of biodiesel, namely castor oil biodiesel. This biodiesel was produced in the Crude Oil and Derivatives Laboratory of the Universidad Nacional de Colombia - Sede Medellín, and its main properties are listed in Table 1, together with those of ethanol and a commercial diesel that were investigated in this study. This castor oil biodiesel is mainly (85%) composed of methyl ricinoleate, (C17H32OH)COOCH3, whose molecular structure is shown in Fig. 1 and is compared with methyl stearate, (C17H35)COOCH3, which is a saturated methyl ester and is a common component of conventional biodiesels, as in the case of the biodiesel used in Ref. [9]. The properties of the present castor oil biodiesel differ from the biodiesel studied previously in two major aspects. First, it has a pour point 45 °C, which is about 10° lower than those of the conventional biodiesels [14]. As such, it is more conducive for cold weather use, especially when mixed with diesel. Second, the castor oil biodiesel has an extra OH function group. Since OH is a strong oxidizing agent, its presence in the molecular structure could potentially facilitate oxidation of the soot precursors. In view of the above considerations, we have performed a systematic experimental investigation of the combustion, microexplosion and sooting propensity of droplets of mixtures of ethanol, diesel and biodiesel, in various combinations. We shall show in due course that the castor oil biodiesel is indeed a very low soot emitter, either burning by itself or blended with diesel, and that ethanol blending with either diesel and/or biodiesel exhibits strong microexplosion events and as such sharply reduces the droplet gasification time and the extent of deleterious effects associated with heterogeneous burning.
2. Experimental methodology Experiments were conducted in a well-characterized droplet combustion chamber, schematized in Fig. 2 [15]. The experiment involves the combustion of a freely-falling stream of droplets of controlled size, spacing and velocity, generated by the ink-jet
printing technique. The stream of droplets is adjusted to fall through a small vertical channel connected to a high temperature combustion chamber heated by the products of a flat-flame burner. The chamber temperature typically varies between 980 and 1040 K along the distance over which droplet gasification takes place. The combustion environment is controlled by the composition of the premixed gas consisting of methane, oxygen and air that is fed to the flat-flame burner. In this work, the flow conditions were kept constant and the oxygen molar fraction of the flow was 0.21. The droplet image was photographically determined using stroboscopically back-lighted microphotography synchronized with the droplet generation system, while the images of the flame streaks were recorded through simple photography with a fixed aperture and exposure time. The initial droplet diameter (Do) ranged from 220 to 250 lm, while the droplet spacing was in excess of 100 droplet diameters so that droplet–droplet interaction was negligible. The droplet Reynolds number is estimated to be always less than one. Details of the apparatus and experimental procedure can be found in Refs. [11] and [15].
3. Results and discussion 3.1. Diesel/biodiesel mixtures Fig. 3 shows representative flame streaks of a burning droplet stream of diesel, (castor oil) biodiesel and their mixtures; note that the legends Dxx, Byy and Ezz respectively designate xx% diesel, yy% biodiesel, and zz% ethanol. It is seen that the diesel flame has a strong yellow brightness indicating the presence of soot. As biodiesel is added, not only the yellow luminosity reduces visually but at a certain instant the reduction becomes drastic, yielding a blue flame. This abrupt event indicates the transition from the more volatile, diesel-dominated burning of high sooting propensity to one involving the less-volatile, biodiesel of minimal sooting propensity. As such, neat biodiesel burns in a light blue flame, with practically no soot formation. Fig. 4 then plots the normalized flame diameter (Dfs/Do) and droplet diameter (Ds/Do) at the state of transition of the flame color as a function of the amount of biodiesel in the mixture. It is seen that the droplet diameter at flame transition increases with biodiesel addition, indicating the earlier transition of the dominant gasifying component from the more volatile, and sooty, diesel to the less volatile, less sooty biodiesel. Fig. 5 compares the flame streak of the present castor-oil biodiesel, Fig. 5a, with those of other fuels. Fig. 5b is the biodiesel from [9] whose major component is methyl stearate. It is seen that while some yellow luminosity is still present with methyl stearate, the flame streak is basically blue for the present biodiesel. This therefore supports the notion that the present castor oil biodiesel, with its extra OH functional group, has a stronger tendency to suppress soot than the methylated ester of [9]. We next recognize that the primary reason that diesel is strongly sooting is due to its content of aromatics, typically 20– 30%. In other words, the fact that the biodiesels are much less
Table 1 Comparison of chemical components and properties of fuels. Species
Component (% mol)
Castor oil biodiesel
C17H34O2 C19H38O2 C19H35O2 C19H34O2 C19H32O2 C19H36O3 C11–C19
Diesel Ethanol
1.953 1.004 4.005 5.874 2.836 84.328
Equivalent composition
MW (g/mol)
C18.96H35.75O2.84
270 298 296 294 292 312 140–210
C12H23 C2H6O
ql (g/ml)
Tb (K)
HHV (MJ/kg)
308.8
0.936
571–634 (640 mm Hg)
37.48
167.0 46.1
0.827–0.850 0.789
473–623 351
45.40 23.40
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Fig. 1. Molecular structure of (a) methyl ricinoleate, (C17H32OH)COOCH3 and (b) methyl stearate, (C17H35)COOCH3.
Flat flame burner
Fig. 2. Schematic of the experimental apparatus.
sooting is mainly due to the lack of aromatics in its composition; the presence of the oxygen-containing functional groups likely plays a secondary, supplementary role in soot reduction. To demonstrate the relative roles of the aromatics and oxygenated functional groups, experiments were conducted by adding toluene and tetralin in biodiesel, recognizing that they are representative components of diesel, and that tetralin is sootier than toluene because of its two-ringed structure. Fig. 5c and d shows the flame streaks of castor biodiesel with 30% toluene and 30% tetralin, respectively. It is seen that while their addition significantly increases the flame luminosity and hence the amount of soot formed, both flame streaks appear less sooty than that of the diesel in Fig. 3a, thereby suggesting that the presence of the oxygenated functional groups in the biodiesel indeed reduces the tendency to soot. It is also noted that because of the weaker sooting propensity and lower boiling point of toluene (Tb 384 K) as compared to tetralin (Tb 480 K) the toluene-doped flame streak not only is less luminous, it is also over in a shorter droplet lifetime. The temporal variation of the square of the normalized droplet diameter for diesel, biodiesel and their mixtures is plotted in Fig. 6. It is seen that, while the variations are not linear due to the multicomponent nature of the fuel composition [16], the d2-law is approximately followed after an initial heating period, resembling the case of pure component combustion. This is because the intense heating from the flame drives the droplet surface tempera-
ture close to the boiling point of the surface components, at which the fuel volatility loses its sensitivity on the fuel vapor concentration at the surface and thereby the fuel gasification rate. Fig. 7 shows the burning rates of diesel, biodiesel, and their mixtures. It is clear that the burning rate of biodiesel is slightly lower than that of diesel because of its lower heat content and higher boiling point. The burning rates of the mixtures are close, with a slight decreasing trend as more biodiesel is added; the average burning rate of mixtures with 75% biodiesel is only 2.6% lower than that of 25% biodiesel. 3.2. Ethanol blending To identify the effects of ethanol blending in diesel, biodiesel, and their mixtures, Figs. 8 and 9 respectively show the flame appearance for diesel and biodiesel mixtures with ethanol. It is seen that ethanol (Figs. 8e and 9g) burns in a blue flame, and is almost visually non-perceptible. For diesel, the addition of ethanol delays the onset of the yellow luminosity of the flame because of its volatility and thereby preferential gasification. This indicates a corresponding reduction in soot formation during the early stage of burning, when the flame size and thereby soot content are large. For biodiesel, the transition from ethanol- to biodiesel-dominated burning is exhibited in a more prominent manner, characterized by the presence of a gap in the flame streak, designated by arrows
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Fig. 3. Flame streak images of mixtures of diesel and (castor oil) biodiesel: (a) diesel (b) D75B25, (c) D50B50, (d) D25B75 and (e) biodiesel.
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Fig. 5. Flame streak images for: (a) castor oil biodiesel, (b) biodiesel used in [9], (c) castor oil biodiesel and 30% toluene and (d) castor oil biodiesel and 30% tetralin.
Fig. 6. d2-Law plot for droplets of diesel/biodiesel mixtures.
Fig. 4. Normalized droplet (triangles) and flame (squares) diameter at the state of flame shrinkage.
in Fig. 9. This indicates the occurrence of a period of intermediate droplet heating as the droplet temperature increases from that close to the lower boiling point of ethanol to the higher boiling point of biodiesel. The attendant requirement to heat up the droplet reduces the droplet burning rate and thereby its flame size and temperature. Fig. 10a and b are plots of the normalized droplet diameter, (D/ Do)2 vs. normalized time, t=D2o , for ethanol blended with diesel and biodiesel respectively. These results resemble those of bi-component mixtures with vastly different boiling points, as described in [16]. In particular, for cases where ethanol is present in abundance the intermediate heating event observed in the flame streaks is now represented as three distinct segments: two relatively straight segments corresponding to nearly steady droplet burning separated by a nearly horizontal segment. Mechanistically, the first segment represents the initial period during which ethanol, the more volatile component of the mixture, is preferentially gasified. The droplet temperature is controlled by its boiling point, and hence
Fig. 7. d2-Law burning rate for various composition of diesel/biodiesel mixtures.
is relatively low. When the concentration of ethanol in the surface layer of the droplet is largely depleted, a transient period occurs during which the droplet temperature increases to approach the boiling point of the less volatile diesel or biodiesel, which is now the abundant component at the droplet surface. The heat required
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Fig. 9. Flame streak images of (castor oil) biodiesel/ethanol mixtures: (a) biodiesel (b) E20B80, (c) E40B60, (d) E50B50, (e) E60B40, (f) E80B20 and (g) ethanol.
Fig. 8. Flame streak images of: (a) diesel (b) E25D75, (c) E50D50, (d) E75D25 and (e) ethanol.
to effect this intermediate heating process then substantially slows down the droplet gasification rate, as manifested by the near-horizontal segment. After the heating is mostly achieved, the gasification is then sustained by the co-vaporization of both the volatile and nonvolatile components, and is represented by the third, nearly-straight segment. During this process some ethanol remains trapped in the core of the droplet due to liquid-phase diffusional resistance and could become superheated, leading to internal gasification and subsequently violent rupturing of the droplet. The time required to attain this state depends on the concentrations of the components and the boiling point difference between them. Thus when the initial amount of ethanol in the mixture is small or if the volatility differential is relatively small as for the diesel/ethanol mixtures, the first and third segments merge due to the stronger influence of the less volatile compound, leading to heating of the droplet while it gasifies. However, for higher ethanol concentrations and/or larger volatility differentials, as for the biodiesel/ ethanol mixtures, the three-phase gasification mechanism is prominently exhibited.
Figs. 8 and 9 also clearly demonstrate that both diesel and biodiesel mixtures exhibit strong disruptive burning, i.e. microexplosion, that instantaneously ruptures and gasifies the droplet. It is evident that, compared to the diesel mixture, microexplosion for the biodiesel mixture not only occurs earlier but also possesses stronger intensity, characteristics that are due to its higher boiling point. Fig. 11 shows the normalized droplet size at explosion, (Dexplosion/Do), as a function of ethanol addition, demonstrating that the dependence on the concentration is parabola-like with a maximum around equi-volumetric composition; which is in agreement with previous studies [11]. Physically, the onset of microexplosion requires sufficient high-boiling-point, less-volatile component to drive up the droplet temperature, and sufficient low-boiling-point, volatile component for its ease to attain the limit of superheat. The competing requirements then lead to the non-monotonic dependence on the composition, with the optimum being around equivolumetric. Since the biodiesel mixture microexplodes earlier than the diesel mixture for the same volume of ethanol added because of its higher boiling point, it effectively has a higher overall consumption rate. This is demonstrated in Fig. 12, which shows that microexplosion increases the consumption rate, with the effect being maximized at equi-volume concentration, and is stronger for biodiesel.
Fig. 10. d2-Law plots of: (a) diesel/ethanol mixtures and (b) biodiesel/ethanol mixtures.
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components with lower boiling points; 2. an intermediate transient heating period as the dominant surface components transitions from the more volatile to the less volatile ones; 3. almost steady burning supported by the co-gasification of both the more and less volatile components. Diesel and biodiesel mixtures with ethanol blending both microexplode during burning, with the latter exhibiting it earlier in the droplet lifetime and with stronger intensity, and is also promoted for near equi-volume mixtures. Microexplosion reduces the overall burning time and hence increases the overall fuel consumption rate. The addition of ethanol to diesel reduces the yellow luminosity of the flame at the early stage of the droplet lifetime, indicating an overall reduction in the sooting propensity, especially recognizing the relatively large amount of soot formation when the flame size is also larger during that stage. Fig. 11. Normalized droplet diameters at microexplosion as function of ethanol addition.
Acknowledgments This work was supported by the Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001198. MLB was a visiting student at Princeton University, partially supported by the Young Scientist Research Grant from the Colombian Administration Department of Science Technology and Innovation. References
Fig. 12. Overall droplet consumption rate as function of ethanol addition.
4. Conclusions Burning characteristics of diesel/biodiesel, diesel/ethanol and biodiesel/ethanol droplets were experimentally studied. The main results are summarized as follows: Addition of biodiesel to diesel substantially reduces the extent of soot formation, primarily because of reduced aromatics content of the mixture and the oxidizing functionalities of the biodiesel. Castor oil biodiesel appears to be more effective in soot suppression than the much-used methyl esters, ostensibly because of the presence of the extra OH function group in its molecular structure. Castor oil biodiesel has a slightly lower burning rate than that of diesel because of its lower heat content and higher boiling point. The diffusion-limited mechanism for multicomponent droplet burning with highly disparate boiling points was demonstrated for diesel/ethanol and biodiesel/ethanol mixtures. Three phases were observed: 1. almost steady burning of the more volatile
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