Soot Emissions of Various Oxygenated Biofuels in Conventional

Article pubs.acs.org/EF

Soot Emissions of Various Oxygenated Biofuels in Conventional Diesel Combustion and Low-Temperature Combustion Conditions Haifeng Liu,*,† Xiaojie Bi,‡ Ming Huo,§ Chia-fon F. Lee,§,∥ and Mingfa Yao† †

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China § Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ∥ Center for Combustion Energy and State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China ‡

ABSTRACT: To investigate the effects of oxygenated biofuels on soot formation, oxidation, and distribution, a detailed comparative study using the forward illumination light extinction method was conducted in an optical constant volume combustion chamber. Various ambient temperatures (800 and 1000 K) and ambient oxygen concentrations (21, 16, and 10.5%) were investigated to mimic both conventional diesel combustion and low-temperature combustion conditions. Five oxygenated biofuels were used, including neat soybean biodiesel S100, neat butanol B100, and three alcohol−biodiesel blends that contained by volume 20% ethanol E20S80, 20% butanol B20S80, and 50% butanol B50S50. It is found that the composition of the biofuel has a larger effect on soot suppression efficiency at 800 K ambient temperature than that at 1000 K. Soot distribution is observed at larger distances from the injector, and less soot is located near the wall region with the additional oxygen content of biofuels at 21% ambient oxygen concentration. With a declining oxygen concentration, the soot concentration reduces at 800 K but increases at 1000 K. Soot formed in the spray jet region decreases at lower oxygen concentrations, and soot appears mainly near the wall region. Further, the soot distribution is more dispersed over a wider region at lower oxygen concentrations. B100 has shorter ignition delays at 10.5% oxygen concentration than B50S50 and S100 fuels, despite the fact that it has a lower cetane number. Therefore, the conventional correlation between ignition delay and cetane number does not hold for neat butanol at low oxygen concentrations. Soot concentrations are dramatically increased for soybean biodiesel from 800 to 1000 K at 10.5% oxygen, while such increases are not found for B50S50 and B100 fuels, indicating that proper choosing of the fuel will be very important to the high efficiency and clean low-temperature combustion.

1. INTRODUCTION Soot emissions from diesel engines remain a serious environmental concern. To meet the increasingly stringent regulations, a number of soot reduction strategies and techniques have been under development and are starting to penetrate the market. Among these soot reduction strategies, blending oxygenated fuels with diesel is seen as a very effective route for soot reduction.1−7 A wide variety of oxygenated fuels have been tested, and the results demonstrate that soot emissions decline steadily with an increasing oxygen content. In fact, as the oxygen content of the fuel reaches 27−35% by mass, all soot emissions disappear, as shown in Figure 1.7 It is also clear from this figure that, even with the same oxygen mass fraction, different fuels result in various amounts of soot reduction relative to diesel. As such, many studies consistently note that fuel properties other than the oxygen content must play a role in soot reduction. Some studies have reported that the molecular structure of oxygenated fuel affects soot suppression efficiency.1−3,8−13 For example, Westbrook et al.9 and Kohse-Hoeinghaus et al.10 concluded that esters were less effective than alcohols or ethers given the same mass fraction of oxygen. They attribute this difference to both O atoms remaining bound to the single C atom, leading to a considerable fraction of direct CO2 © 2012 American Chemical Society

Figure 1. Reduction of particulate matter (PM), smoke, or integrated jet soot as a function of the weight percent of oxygen in the fuel from numerous experiments reported in the literature7 for the use of B50S50 (diamond) and B100 (star) fuel results in no measurable soot.

formation from the ester. Besides the molecular structure, other fuel properties, such as the cetane number, aromatic content, boiling point, viscosity, etc., can also affect soot Received: November 3, 2011 Revised: February 17, 2012 Published: February 21, 2012 1900

dx.doi.org/10.1021/ef201720d | Energy Fuels 2012, 26, 1900−1911

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emissions.1,3,13−15 Both Donkerbroek et al.1 and Boot et al.3 reported that the cetane number of oxygenated fuels affected soot emissions via its effect on flame lift-off length (FLoL) and ignition delay. Pepiot-Desjardins et al.14 mentioned that replacing some of the aromatics in diesel fuel with oxygenated fuels would decrease soot production. Song and Litzinger15 reported that the reduction of soot formation could be the result of the low boiling point (55%) can reduce the combustion temperature low enough to simultaneously suppress soot and NOx formation in diesel engines.48−52 This concept is referred to as EGR-diluted low-temperature combustion. In the lower temperature regions, soot formation can be suppressed because the reactions that normally form soot particles from polycyclic

aromatic hydrocarbons do not progress even with the rich conditions.49 The increased ignition delay provides more time for fuel evaporation and reduces inhomogeneities in the reactant mixture, thus reducing NOx formation because of local temperature spikes and soot formation from locally rich mixtures. Non-soot combustion has been demonstrated for low ambient temperature,53 low ambient oxygen concentration,54 or high EGR dilution48 conditions with diesel fuel and n-heptane. However, few comparative studies have been conducted on combustion and emissions of ethanol, butanol, and biodiesel under high EGR rate conditions. EGR via its dilution and thermal effects can produce changes in soot suppression efficiency. With high EGR rates, the impact of the fuel properties on soot reduction may also be altered for different oxygenated fuels. It is commonly believed that high EGR dilution strategies will be essential to the combustion and emission control of diesel engines in the future, and the use of oxygenated biofuels with high EGR represent a potential route to clean diesel engines. For this reason, it is necessary to determine the soot formation and oxidation mechanisms that occur with different EGR rates. On the basis of surveying the literature, it can be found that ethanol, butanol, and biodiesel are all potential biofuel replacements for diesel engines. A detailed comparative study on soot formation, oxidation, and distribution is important for their use in diesel engines. In the current study, different biofuels were tested, including neat soybean biodiesel, ethanol−biodiesel, butanol− biodiesel, and neat butanol. Soybean biodiesel was used as the baseline fuel to eliminate the effect of the aromatic content on soot suppression efficiency and to study the effect of using a pure biofuel. On the basis of engine experiments, it is difficult to isolate the effects of basic parameters (e.g., injection pressure or incylinder gas temperature or pressure) on the soot process. Therefore, a constant volume combustion chamber was used to create well-characterized conditions with various ambient temperatures (800 and 1000 K) and oxygen concentrations (21, 16, and 10.5%). The forward illumination light extinction (FILE) method was used to explore soot formation and oxidation mechanisms.

2. EXPERIMENTAL APPARATUS AND PROCEDURES The study was conducted in a constant volume chamber with a bore of 110 mm and a height of 65 mm, as shown in Figure 2. A Caterpillar

Figure 2. Schematic of the constant volume chamber and experimental setup. hydraulic-actuated electronic-controlled unit injector (HEUI) with 6 holes was mounted in the center of the chamber in the bottom surface. 1901


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The orifice diameter is 0.145 mm. The injection pressure was set to 134 MPa. The injection volume was 120 mm3/injection to simulate a high load fuel mass because the main difficulty in obtaining clean diesel combustion is at high load. The chamber wall was heated to 380 K before the test to mimic the typical wall temperature of a diesel engine. A pressure transducer (Kistler 6121) was embedded in the chamber wall and used in conjunction with a charge amplifier for recoding the in-chamber pressure. The test procedure began by filling the chamber with the premixed combustible mixture, including acetylene, oxygen, and nitrogen. The mixture was then ignited with a spark plug and burned to create a high-temperature, high-pressure environment in the chamber. As the products of combustion cool over a relatively long time (∼2 s) because of heat transfer to the chamber walls, the pressure slowly decreases. When the desired ambient condition was reached, the injector was triggered and the fuel injection, autoignition, and combustion took place sequentially. This method has also been used in other studies.53−55 Diesel combustion with 1000 K ambient temperature, 21% oxygen concentration, and 14.8 kg/m3 ambient density mimics the typical incylinder environment with the piston at top dead center. For the current experiments, two different ambient temperatures (800 and 1000 K) were considered, where the ambient temperature is defined as the in-chamber temperature at the start of injection. Meanwhile, the ambient oxygen concentrations were set to 21, 16, and 10.5% after premixed combustion by mixing different percentages of acetylene, oxygen, and nitrogen in the chamber, representing no EGR, medium EGR (∼40%), and high EGR (∼60%) rates in a real diesel engine. Changes to the soot suppression efficiency were revealed with these various ambient temperatures and ambient oxygen concentrations via different biofuels. The detailed experimental operating conditions are summarized in Table 1.

mention that E20S80 and B50S50 have a similar oxygen content of ∼15 wt %, as shown in Table 2. The neat butanol was also selected in this study considering that there were few studies on combustion and emissions of butanol under diesel-like engine conditions.

3. OPTICAL DIAGNOSTIC PRINCIPLE AND SETUP 3.1. Optical Diagnostic Principle. The FILE method has been developed and verified by Xu and Lee.58 This technique can achieve 2D line-of-sight soot quantitative measurements with only one window. Soot measurement using the light extinction method has been conducted in a number of studies.7,53,59,60 All of these soot extinction measurements demonstrate that the light intensity after passing through the particle can be given by Lambert−Beer’s law. The FILE method shares a similar principle of extinction, but the light passes through the soot cloud twice instead of once, as shown in Figure 3. The change of reflected light intensity only resulted from the presence of soot similar to Lambert−Beer’s law

Table 1. Summary of the Experimental Conditions control parameters

values

ambient temperature, Ta (K) ambient oxygen concentrations (O2 %) ambient density (kg/m3) injection system injection pressure (MPa) orifice diameter (mm) injection duration (ms) fuel volume per injection (mm3)

800 and 1000 21, 16, and 10.5 14.8 HEUI 134 0.145 3.5 120

Figure 3. Schematic of light extinction by the soot cloud.

I = I0 exp( −

The fuels used in this work are listed in Table 2. Neat soybean biodiesel (S100) was used as the baseline fuel and was purchased from Incobrasa Industries, Ltd., Gilman, IL. Detailed fuel properties, including total glycerin, free glycerin, sulfated ash, etc., can be found in previous studies.56,57 Three alcohol−biodiesel blends, which contained by volume 20% ethanol E20S80, 20% butanol B20S80, and 50% butanol B50S50, were also studied. It is worthwhile to

∫0

2L

K extdx)

(1)

where I or I0 is the reflected light intensity with or without the presence of soot cloud. respectively, Kext is the extinction coefficient, and L is the path length through the soot cloud. Differing from the back illumination method, the light extinction is related to 2L rather than L because of the two passes through the soot cloud.

Table 2. Properties of Tested Oxygenated Biofuels

a

properties

ethanol

butanol

soybean biodiesel

molecular formula cetane number oxygen content (wt %) density (g/mL) at 20 °C autoignition temperature (°C) flash point (°C) at closed cup lower heating value (MJ/kg) boiling point (°C) stoichiometric ratio latent heating (kJ/kg) at 25 °C viscosity (mm2/s) at 40 °C

C2H5OH 8 34.8 0.790 434 8 26.8 78.4 9.02 904 1.08

C4H9OH 25 21.6 0.81 385 35 33.1 117.7 11.21 582 2.63

CH3OOCR 51 (D 613) 10 0.887 (D 1298) at 15 °C 363 173.9 (D 93) 37.53 (D 240) 342a (D 1160) 12.5 200 4.0 (D 445)

E20S80 (ASTM)

B20S80 (ASTM)

B50S50 (ASTM)

14.96 0.869 (D 1298)

12.32 0.871 (D 1298)

15.80 0.850 (D 1298)

35.38

36.64

35.32

11.80 340.8 3.08 (D 445)

12.24 276.4 3.68 (D 445)

11.86 391.0 3.24 (D 445)

The boiling point for soybean biodiesel is the distillation temperature of 90% recovered. 1902


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through the same window. For an uncoated window, the reflectance of each surface will cause roughly 4% intensity loss. The reflectance from the chamber window will attenuate both the incident light and the reflected light for the FILE measurement. However, this attenuation will not affect the soot measurement, because it is attenuation of light with or without soot at the same ratio and, thus, is self-compensating. However, if light reflected from the window is collected by the camera, it will affect the measurement. Especially when the incident light is much stronger than the reflected light, the incident light reflected from the window will not be negligible compared to the reflected light; it will affect the accuracy of the measurement. To solve this problem, a slight tilt of the window toward the axis of the camera and the laser system will help to prevent light reflected from the window from being collected by the camera, such as 3° in the current setup. For diesel combustion, it is difficult to totally eliminate the flame emission even with the narrow band filtering system, because diesel flame luminosity is very strong in most cases. One of the efficient ways to relieve this problem is to increase the monochromatic source light intensity and to use a short camera exposure time when using a pulsed laser, which is very helpful to suppress the flame luminosity, as shown in section 3.2. Generally, less soot concentration can be observed from tested oxygenated biofuels compared to neat diesel in the current work. Thus, the light intensity after the soot attenuation will become much stronger, and flame luminosity effects can be reduced further for these oxygenated biofuels. The soot measurement range and accuracy is also determined by the camera bit resolution and noise level. For the 12-bit camera used in this experiment, the light intensity ratio can reach 4095:1 without noise. However, the full range of the camera cannot be adopted because of the non-uniformity of the reflected light. The noise level will also limit the minimum measurement range. For the current camera, the uncertainty of each pixel is about 10 counts. On the basis of these improvements, the FILE measurement has been successfully applied to the constant volume chamber to study transient combustion and yielded new insights in soot formation for diesel, tripropylene glycol monomethyl ether (TPME), dibutyl maleate (DBM), biodiesel, and other fuels.56,57,66−71

With the Rayleigh approximation, a simple expression for the soot volume fraction can be shown as Cv =

⎛I ⎞ λ ln⎜ 0 ⎟ (2L)Ka ⎝ I ⎠

(2)

where λ is the wavelength of monochromatic light. Ka is the dimensionless absorption constant, which is determined by the soot refractive index m. In this paper, a value of 5.47 is adopted with m = 1.62 + i0.66.61 The images recorded with and without soot clouds are processed pixel by pixel to calculate the soot volume fraction using eq 2. However, for the non-axisymmetric diesel flame, the thickness of the soot cloud is unknown and what is obtained is a line-of-sight measurement CvL. If the area represented by each pixel is known as Δs and the soot density is chosen as ρs = 2.0 g/cm3,62,63 then the soot mass at each pixel can be calculated as eq 3 mi = ρsC vLΔs

(3)

When all pixel values are summed together from eq 3, the total soot mass in the flame can be calculated. A more detailed introduction and discussion for the FILE method can be found in the study by Xu and Lee.58 3.2. Optical Diagnostic Setup. The current optical setup of FILE is shown in Figure 2. The incident light was supplied by a copper vapor laser (Oxford Lasers LS20-50). The copper vapor laser has a pulse repetitive frequency from 4.5 to 20 kHz, and each laser pulse lasts 25−30 ns. The energy for each pulse varies from 4 mJ at 4.5 kHz to 1 mJ at 20 kHz. The copper vapor laser has two color outputs at 511 and 578 nm, with a power ratio of 2:1. To filter out the light at 578 nm for this monochromatic light extinction and suppress the flame emission, two interference filters at 510 and 515 nm with 10 nm full width at half maximum were adopted. The light emitted from the fiber was condensed by an aspheric condenser lens. The laser beam entered the constant volume chamber via a reflecting mirror of 6 mm diameter placed in front of the condenser lens. The high-speed camera (Phantom V7.1) with a 105 mm focal length lens (Nikkor) was located above the chamber. In the current study, the high-speed camera and the laser were synchronized up to 15 037 frames per second to produce timeresolved measurement at a resolution of 256 × 256 pixels. The camera was triggered by the injection signal, and the exposure time for each frame was set to 3 μs. For the natural flame luminosity measurement, the diffuser as well as the interference filters were removed and the light source from the laser was shut down. Images of natural flame luminosity were directly obtained by the high-speed camera with the same time-resolved measurement as FILE. A neutraldensity filter was adopted to fit the light intensity within the camera measurement range. 3.3. Accuracy Analysis. The accuracy of FILE has been proven by the measurement of the laminar diffusion flame of ethylene in the previous study.58 In the laminar diffusion flame, the soot volume fraction measured from the FILE technique has an excellent match with the laser-induced incandescence data from Shaddix and Smyth64 and with the one-dimensional (1D) extinction result by Santoro et al.65 However, it should be noted that the combustion study inside the chamber will involve more factors that may affect the accuracy compared to open fire measurements. For the current combustion chamber setup, because only one window is installed, the incident and reflected light will pass

4. RESULTS AND DISCUSSION 4.1. Total Soot Mass and Soot Distribution at 21% Oxygen Concentration. Figure 4 shows the spatial integrated total soot mass using different oxygenated biofuels at 800 and 1000 K ambient temperatures. The term “total soot mass” in this paper describes the competition results of soot formation and oxidation. As shown in Figure 4, total soot mass increases sharply and reaches a quasi-steady state before it commences a continual decline to zero approaching the end of combustion. With the increase of the oxygen content in tested biofuels, reduced total soot mass, retarded total soot mass emergence time, and shortened total soot mass duration are observed. A higher oxygen content in biofuels compensates for the lack of air entrained in the flame and reduces the fuel−air equivalence ratio. Once the oxygen/fuel mass ratio is sufficiently high, the soot formation disappears for neat butanol with an oxygen/fuel mass ratio of 21.6% under 800 K ambient temperature. It may be argued that the different ignition delays that resulted from various biofuels can play an important role in lowering soot production, because a longer ignition delay 1903


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spark-ignition (SI) engine and found that burning butanol at high blend ratios (83 vol %) resulted in an increased level of soot precursors, such as propene, 1,3-butadiene, and acetylene emissions, compared to the ethanol additive with the same fuel oxygen content. Therefore, soot emissions were expected to be higher in SI engines by burning gasoline−butanol blends compared to gasoline−ethanol blends. Under the current diesel combustion condition, however, the trend is reversed. It is interesting that the fuel oxygen content and ignition delay are nearly the same for B50S50 and E20S80; thus, other factors are expected to explain the soot reduction with the butanol blends. First of all, the ethanol and butanol have lower boiling points than biodiesel. A larger volumetric (50%) replacement of biodiesel by butanol for B50S50 fuel results in a higher fuel volatility compared to 20% replacement of biodiesel for E20S80, and consequently, the total soot mass is reduced, as studied by Song and Litzinger.15 Another possible explanation rises from the chemical kinetics because a previous study has reported that the overall production rate of OH radicals decreases from pure n-heptane to a n-butanol−heptane blend and further to an ethanol−heptane blend in a homogeneous charge compression ignition (HCCI) combustion condition.39 However, the combustion mode, fuel properties, and fuel blend ratio by Saisirirat et al.39 are different from this current study; therefore, it is hard to clarify the effects of chemical kinetics for various biofuel blends on the soot concentration in current conditions. In fact, there are few reports on the chemical kinetics of these pure biofuels or their blends under conventional diesel combustion or low-temperature combustion conditions. Some previous reports on chemical kinetics of biofuels focused on ideal premixed flames or diffusion flames, which was different from the real diesel engine condition. The study on chemical kinetics of biofuel under diesellike combustion conditions is being conducted in our group to further clarify the soot reduction mechanism. A sequence of soot distributions are demonstrated in Figure 6 at various ambient temperatures. The time on the upper left corner represents the time at which the image was captured. It can be seen from Figure 6a that the soot is first observed near the wall region for S100 and B20S80 fuels at 800 K ambient temperature. In previous studies,56,57 the flame luminosity was first observed for S100 and B20S80 fuels near the wall region because the lower ambient temperature resulted in a longer flame lift-off, and thus, the flame luminosity was far from the injector. Therefore, the soot is seen in this diffused flame near the wall region. Subsequently, more soot is formed near the wall region at 4.85 ms, and the soot is propagating toward the injector with the development of the combustion flame. After 4.85 ms, the soot distribution can be measured at the reacting spray jet and near the wall region. For B50S50 and E20S80, the main soot distribution is located at the middle and down reacting spray jet and little soot is observed near the wall region. It should be pointed out that the original flame luminosity for E20S80 is also observed near the wall region, as shown in Figure 7, where no soot is detected. This demonstrates that the soot formation of E20S80 fuel is suppressed effectively because of its higher fuel oxygen content. Less premixed combustion takes place as the ambient temperature is increased from 800 to 1000 K; therefore, the diffusion flame as well as the soot can first be seen much closer to the injector, as shown in Figure 6b. A rise in the temperature is known to significantly decrease the FLoL,73 resulting that less air entraining into the spray jet. Thus, soot precursors formed from the premixed reaction zone are expected to be closer to

Figure 4. Total soot mass at 21% oxygen concentration with various oxygenated fuels.

allows for more mixing time and initiates combustion regions closer to stoichiometric and, therefore, reduces soot formation. However, the combustion pressures shown in Figure 5

Figure 5. Combustion pressure with various oxygenated biofuels at 1000 K ambient temperature and 21% oxygen concentration.

demonstrate that the gap of the ignition delay is within 0.1 ms among various biofuels at 1000 K ambient temperature, indicating that the ignition delay is not expected to be the major factor in soot reduction. In addition, it is noted that B50S50 fuel has a lower total soot mass compared to E20S80, although the oxygen content in both biofuels is nearly the same (∼15 wt %). Wallner and Frazee72 performed the study on both gasoline−ethanol and gasoline−butanol blends in a direct-injection 1904


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Figure 6. Soot distribution at 21% oxygen concentration with various oxygenated fuels.

At both ambient temperatures, there is a general trend that soot is observed further downstream from the injector with a higher oxygen content in fuels. Moreover, with the increase of the fuel oxygen content, more soot is seen on the reacting spray jet rather than near the wall region. 4.2. Total Soot Mass and Soot Distribution at 16 and 10.5% Oxygen Concentrations. Figure 8 presents the ignition delay of S100, B50S50, and B100 fuels at various ambient oxygen concentrations and temperatures to explore the impact of the ignition delay on soot emissions. An interesting

the injector, and correspondingly, the soot formation is also closer to the injector. After the flame propagation, the soot reaches the chamber wall and a higher soot concentration is observed near the wall region, as shown in the second line in Figure 6b. With the further propagation of the combustion, the soot cloud also displays different distributions. For S100 and B20S80 fuels, the soot is seen at the reacting spray jet and near the wall region, while the soot for the rest of three tested fuels, B50S50, E20S80, and B100, mainly concentrates on the reacting spray jet. 1905


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soot are illustrated in Figure 9a. It is observed that the main soot distribution for S100 at 800 K and 16% oxygen is near the wall region before 5.39 ms. After that, the soot can be seen at both near the wall region and the reacting spray jet. For B50S50 fuel, the main soot is near the wall region and little soot can be seen at the reacting spray jet. At 10.5% oxygen, the soot distribution is more dispersed for S100 fuel, although the measured local soot concentration is lower and the negligible soot formation occurs on the spray jet. Further, there is no soot formation for B50S50 at 10.5% oxygen. For B100 fuel, there is no soot formation in both oxygen concentrations at 800 K. Figure 9b demonstrates the soot distributions of S100, B50S50, and B100 fuels at 1000 K. With a higher ambient temperature, the less premixed combustion results in a larger soot concentration in the chamber. Unlike the results of 800 K, the soot is detected at 1000 K for B100 and B50S50 fuels. At 16% oxygen, the soot distribution for S100 fuel can be seen at both near the wall region and the reacting spray jet. With the increase of the oxygen content in fuels, soot near the wall region is reduced and the main soot distribution concentrates on the reacting spray jet. Furthermore, the soot at the spray jet region is also reduced with the increase of the fuel oxygen content. At 10.5% oxygen, a higher local soot concentration is detected within the flame. For S100 fuel, the soot formation is not inhibited at the reacting spray jet at 1000 K and the soot formation is also dramatically increased near the wall region. For B50S50 and B100 fuels, the soot distributions from the spray jet are reduced in a similar fashion to that of the 800 K case and the main soot distribution is near the wall region. Figure 10 illustrates the total soot mass of S100, B50S50, and B100 fuels under various low oxygen concentrations (16 and 10.5%) and ambient temperatures (800 and 1000 K). It is also found that the total soot mass increases with the increase of the ambient temperature. The longer autoignition delay at 800 K allows more time for ambient air to be entrained into the jet to form a leaner premixed zone, even though the ambient air is already diluted. At a low ambient temperature of 800 K, the total soot mass reduces with the decrease of the ambient oxygen concentration for tested fuels. Although the chemical reaction rate slows at a lower ambient oxygen concentration and reduces the soot oxidation rate, the soot formation rate has also been suppressed effectively because of a lower combustion temperature and longer ignition delay. At a high ambient temperature of 1000 K, however, the total soot mass is dramatically increased with the decline of ambient oxygen concentrations, although the ignition delay is longer at lower ambient oxygen concentrations. It is interesting to notice that the total soot mass for B100 is still lower than that for S100 and B50S50 fuels at 10.5% oxygen, even though the B100 fuel has a shorter ignition delay. Therefore, a longer ignition delay does not necessarily correlate to a lower total soot mass because the effects of the ignition delay on soot are dependent upon both ambient temperature and fuel properties. The difference in total soot mass and soot distribution should be attributed by the opposing effects between the amount of air entrained in the fuel jet and the inert gas dilution. On one hand, a longer autoignition delay allows more time for ambient air to be entrained into the jet to form a lower equivalence ratio at the lift-off location and the greater degree of premixing, leading to less soot production. On the other hand, the fuel air mixture in the lift-off location becomes diluted with the entrainment of more inert gas at lower ambient oxygen concentrations. If the oxygen concentration reduction is

Figure 7. Flame luminosity (left) and soot distribution (right) for E20S80 and S100 fuels at 800 K ambient temperature.

phenomenon is noted that the autoignition timing for B100 is barely changed from 16 to 10.5% oxygen concentrations. Thus, B100 presents a shorter autoignition delay at 10.5% oxygen concentration compared to S100 and B50S50, although it has a

Figure 8. Autoignition delay at various operating conditions.

lower cetane number. Meanwhile, the autoignition delay for S100 is shorter than that of B50S50 and B100 under 21 and 16% oxygen concentrations because of its higher cetane number. This seemingly paradoxical trend can possibly be explained by the fact that the chemical reaction rate is reduced at the lower oxygen concentration (10.5%), which weakens the effect of the chemical autoignition delay because of the different cetane numbers, whereas the physical delay because of a higher volatility of butanol fuel plays a major role in the autoignition. The previous study3 showed that ignition delay became rapidly shorter as the cetane number of fuel increased, regardless of the fuel oxygen content (5−15 wt %) or base fuel characteristics. However, some biofuels, such as jatropha oil and to a lesser extent rapeseed methyl ester, were notable exceptions for the correlation between ignition delay and cetane number because the jatropha oil or the rapeseed methyl ester has higher viscosity.2 In the current work, butanol fuel also presents the exception for the correlation between ignition delay and cetane number at a lower oxygen concentration (10.5%) because of the higher volatility of butanol. Therefore, it is concluded that the ignition delay may not necessarily correlate with the cetane number for some pure biofuels at the given operating conditions. Figure 9 demonstrates a sequence of soot distribution evolutions of S100, B50S50, and B100 fuels. The soot concentration under a low ambient temperature is generally low; therefore, only three sets of images with apparently perceivable 1906


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Figure 9. Soot distribution at both 16 and 10.5% oxygen concentrations with various oxygenated fuels.

compensated by more entrained air, the soot concentration will be reduced; otherwise, the soot will be increased on the contrary. Obviously, a higher fuel oxygen content will help to improve the equivalence ratio at the lift-off location and suppress soot formation. At 800 K, the oxygen concentration reduction is compensated by the longer ignition delay with more air entrained; thus, the soot mass is reduced with the increase of inert gas dilution. At 1000 K, the gap of ignition delay in various oxygen concentrations is smaller compared to 800 K; hence, the oxygen concentration reduction cannot be

compensated by any more entrained air. Thus, the soot formation is dramatically increased near the wall region because of the dilution of a low ambient oxygen concentration, which results in a higher local equivalence ratio. In addition, the combustion slows at a lower oxygen concentration, providing soot with more growth time, while the retarded oxidation because of a lower adiabatic flame temperature at a lower ambient oxygen concentration gives total soot mass more time to reach its peak and to be oxidized completely. For example, at 1000 K ambient temperature, the peak value of total soot mass 1907


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Figure 10. Total soot mass at 16 and 10.5% oxygen concentrations with various oxygenated fuels.

for S100 fuel is 190 μg at 16% oxygen concentration and the peak value is 180 μg for B50S50 fuel at 10.5% oxygen concentration, as shown in Figure 10. With nearly the same peak value, S100 fuel spends about 2.5 ms on soot oxidation from the peak value to the complete oxidation, while B50S50 fuel spends about 4 ms. Although the higher fuel oxygen content for B50S50 should have a faster soot oxidation rate, the lower ambient oxygen concentration slows soot oxidation, leading to a longer oxidation time. 4.3. Normalized Time Integrated Soot Mass (NTISM). Figure 11 shows the NTISM of various oxygenated biofuels at different ambient oxygen concentrations and ambient temperatures. Time integrated total soot mass is introduced because it considers not only the soot amount but also the soot existing time within the flame. Further, the effects of input energy derived from the heating value and density in various biofuels have been eliminated before further comparing the NTISM; thus, the NTISM is derived from

Figure 11. NTISM at various ambient temperatures and oxygen concentrations.

A E NTISM = f b Ab E f

previous results, where all soot emission depletions are only achieve at the oxygen content of the fuel reaching 27−35% by mass. This suggests that the use of oxygenated biofuels with high EGR dilution low-temperature combustion represents a potential route to a clean diesel engine. In addition, with the similar fuel oxygen content (∼15 wt %), E20S80 fuel has a higher NTISM value than that of B50S50 fuel without the input energy effects. The value of NTISM for E20S80 fuel is increased approximately 2.92 and 1.35 times at 800 and 1000 K, respectively, compared to B50S50 fuel. An obvious trend illustrated in Figure 11a is that increasing the ambient temperature from 800 to 1000 K leads to an increase in the value of NTISM for a given biofuel. In terms of the effects of ambient oxygen concentrations, as shown in Figure 11b, the NTISM value reduces with the decrease of the oxygen concentration at 800 K, while on the contrary, the NTISM value is increased at 1000 K. These trends at various ambient temperatures and oxygen concentrations with the identical energy input are the same as the results of total soot mass, and the reasons have been discussed in the above sections. In addition, Table 3 demonstrates that the composition of biofuels has a larger effect on soot suppression efficiency at 800 K than that at 1000 K. For example, the NTISM value for B50S50 fuel accounts for 40.3% of S100 fuel at 1000 K, while it only accounts for 9.2% of S100 fuel at 800 K. Moreover, a rapid increase in soot emission for S100 is observed with increasing

where Af is the time integrated total soot mass of tested fuels, which is equal to the inside the loop of total soot mass, as shown in Figure 4a for the hatched area of B50S50 fuel, Ab is the time integrated total soot mass of B100 combustion at 1000 K and 21% oxygen, and Eb and Ef represent the energy input of B100 and tested fuels, respectively, which is calculated by E = ρVQLHV, where ρ, V, and QLHV represent the density, volume, and low heating value for different fuels, respectively. This single value of NTISM demonstrates the soot tendency to potentially appear in the exhaust pipe in a real diesel engine. A higher NTISM value will result in larger soot emission. The specific value of NTISM is illustrated in Table 3, and this value is the average of over 5 injection cycles. It can be seen that the value of NTISM is reduced with the increase of the oxygen content in the tested biofuels at both 800 and 1000 K ambient temperatures. As the oxygen/fuel mass ratio reaches sufficiently high, the soot formation disappears for neat butanol with an oxygen/fuel mass ratio of 21.6% at 800 K, as shown in Figure 1 with the star sign. Furthermore, there is no soot for B50S50 fuel with 15.8% fuel oxygen content at 10.5% ambient oxygen concentration and 800 K ambient temperature, as shown in Figure 1 with the diamond sign. Thus, non-soot combustion can be achieved at a lower fuel oxygen content by carefully controlling the ambient temperature and oxygen concentration in comparison to the 1908


Energy & Fuels

Article

Table 3. Values of NTISM at Various Ambient Temperatures and Oxygen Concentrationsa

a

fuels

Ta = 800 K, O2 = 21%

S100 B20S80 B50S50 E20S80 B100

9.9 (100%) 6.78 (68.5%) 0.91 (9.2%) 2.66 (26.9%) 0 (≤0%)

Ta = 800 K, O2 = 16% Ta = 800 K, O2 = 10.5% 6.63

4.29

0.93

0

0

0

Ta = 1000 K, O2 = 21%

Ta = 1000 K, O2 = 16%

Ta = 1000 K, O2 = 10.5%

19.15 (100%) 17.04 (89%) 7.71 (40.3%) 10.42 (54.4%) 1 (5.2%)

28.84

79.73

9.12

27.81

1.78

3.81

The percentages in parentheses are the ratios that NTISM of various fuels account for the value of biodiesel.

for a lower fuel oxygen content (S100 and B20S80). Further, the soot distribution with a higher fuel oxygen content is primarily concentrated on the reacting spray jet, and there is less soot near the wall region compared to a low fuel oxygen content. (3) With the decline of the ambient oxygen concentration, the total soot mass is reduced at 800 K, while it was increased at 1000 K. Soot derived from spray jet is reduced, and the soot distribution is more dispersed over a wider region at lower oxygen concentrations. A longer ignition delay resulting from a lower ambient oxygen concentration does not necessarily correlate to a lower soot concentration, and the effects of the ignition delay on soot are dependent upon the ambient temperature and fuel property. (4) The ambient oxygen concentration has little effect on the ignition delay of butanol between 16 and 10.5% oxygen. B100 presents a shorter ignition delay at 10.5% oxygen than that of B50S50 and S100 fuels in both 800 and 1000 K ambient temperatures, although B100 has a lower cetane number. Therefore, the conventional correlation between ignition delay and cetane number does not hold for neat butanol at a low oxygen concentration. (5) The value of NTISM indicates much higher soot emission at a low oxygen concentration with a high ambient temperature. Thus, a low ambient temperature control is favored to reduce soot emissions in diesel combustion conditions. The effects of ambient oxygen concentrations and temperatures on total soot mass are not sensitive to butanol fuel, indicating that proper choosing of the fuel will be very important to the high efficiency and clean lowtemperature combustion.

the ambient temperature from 800 to 1000 K at 10.5% oxygen concentration, as illustrated in Figure 11b, and the value of NTISM is increased approximately 16 times. Therefore, a low ambient temperature control is very important and favored to reduce the soot concentration in diesel combustion conditions. In this viewpoint, some factors, such as the lack of EGR cooling, pre-injection, and a high compression ratio, can all contribute to the increased charge-gas temperature and, accordingly, increase the soot emission in a diesel engine. Among these factors, EGR cooling becomes more challenging as the amount of EGR increases. These results however emphasize the importance of EGR cooling and its potential benefit on mitigating soot formation. Meanwhile, a pre-injection can lead to a pre-burn, which will decrease the charge-gas oxygen concentration and also increase the in-cylinder temperature. If followed with a main injection into this high-temperature, low oxygen environment, one may expect similar difficulties in inhibiting soot formation. These two factors have been discussed in a previous study.54 In addition, the compression ratio can be lowered moderately to reduce the ambient temperature as well as to increase the premixed time before autoignition. The Miller cycle with variable valve timing might also be considered for the diesel lowtemperature combustion process, or the engine designer can reduce the geometric compression ratio modestly. In comparison to S100 fuel, the increase in the value of NTISM for B50S50 and B100 is not so remarkable at 10.5% oxygen concentration. Therefore, the effects of ambient oxygen concentrations and temperatures on the soot concentration are not sensitive to butanol fuel. That is to say, proper choosing of the fuel should be very important to the high efficiency and clean low-temperature combustion in diesel engines in the future.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-22-27406842, ext. 8007. Fax: 86-22-27383362. E-mail: [email protected]. Notes

5. CONCLUSION To investigate the effects of oxygenated biofuels on soot formation, oxidation, and distribution, a detailed comparative study using the FILE method was conducted in an optical constant volume combustion chamber under various ambient temperatures (800 and 1000 K) and oxygen concentrations (21, 16, and 10.5%). Five oxygenated biofuels were tested, including neat soybean biodiesel (S100), neat butanol (B100), and the alcohol-biodiesel blends that contained by volume 20% ethanol E20S80, 20% butanol B20S80, and 50% butanol B50S50. The main conclusions are as follows: (1) The composition of the biofuel has a larger effect on soot suppression efficiency at 800 K ambient temperature than that at 1000 K. With the increase of the oxygen content in biofuels, reduced total soot mass, retarded total soot mass emergence time, and shortened total soot mass duration are observed. B50S50 has a lower total soot mass compared to E20S80, although the oxygen content is similar in both biofuels. (2) At 21% ambient oxygen concentration, soot distribution is observed further downstream from the injector for a higher oxygen content in biofuels (E20S80, B50S50, and B100) than that

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the help of Dr. Nick Killingsworth of Lawrence Livermore National Laboratory with the final editing of this paper. This work was performed using the equipment at the University of Illinois at Urbana−Champaign, and the support of the university staffs is greatly thanked. This work was supported in part by the U.S. Department of Energy (DOE) under Grant DE-FC26-05NT42634, the DOE Graduate Automotive Technology Education (GATE) Centers of Excellence under Grant DE-FG26-05NT42622, the National Natural Science Foundation of China (50976078), and the International Science & Technology Cooperation Program of China (2010DFA74530).

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Soot Emissions of Various Oxygenated Biofuels in Conventional

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