Article pubs.acs.org/EF
Spray and Combustion Characteristics of Neat Acetone-ButanolEthanol, n‑Butanol, and Diesel in a Constant Volume Chamber Han Wu,†,‡ Karthik Nithyanandan,‡ Timothy H. Lee,‡ Chia-fon F. Lee,*,‡ and Chunhua Zhang† †
School of Automobile, Chang’an University, Xi’an, Shanxi 710064, China Department of Mechanical Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
‡
ABSTRACT: Intermediate product of biobutanol production, acetone-butanol-ethanol (ABE) as an alternative fuel has drawn increasing attention in recent years due to its potential to eliminate various production costs. In this work, neat acetone-butanolethanol (ABE) with a component volumetric ratio of 3:6:1, n-butanol, and neat diesel were studied in a preburn type constant volume chamber with optical access. The ambient temperature and oxygen ranged from 800 to 1200 K and 21% to 11%, respectively, covering both conventional and low temperature combustion (LTC) regimes. Time resolved images of the spray and natural flame luminosity (indicator of soot) were captured by a high speed camera coupled with a copper vapor laser beam. The images show that the flame lift-off length and liquid penetration of n-butanol and ABE are much longer and shorter, respectively, than that of diesel under all tested conditions. This results in a longer “gap” between the liquid spray and the flame in ABE and n-butanol combustion that provides more space and time for the droplets to evaporate and mix with the ambient air, which is expected to decrease the local equivalence ratio at the combustion region. Indeed, the natural flame luminosity of ABE and n-butanol is reduced significantly under all tested conditions compared to that of diesel. For all the tested fuels, especially ABE, the combustion duration decreases with the reduction of ambient temperature due to a stronger premixed combustion, while it increases with the reduction of oxygen concentration due to the dilution effect. Therefore, ABE has a high potential to reduce soot emissions when used in diesel engines, but it would also suffer from combustion phasing retardation like butanol under LTC conditions with high EGR.
1. INTRODUCTION Concerns due to limited fuel resources and environmental preservation have drawn increased global attention to renewable energy in the past few decades. For example, the Energy Independence and Security Act in the U.S. mandated in 2007 that 36 billion gallons of renewable fuel were to be blended into U.S. transportation fuels by 2022.1 European commissions also committed that renewable energy will play a key role in the transition toward a competitive, secure, and sustainable energy system. The share of renewable energy had increased to 13% in 2012 as a proportion of final energy consumed and is expected to rise further to 21% in 2020 and 24% in 2030.2 China has pledged to boost renewable energy to 15% of its total energy consumption. Brazil has been a leader for the past 30 years in the development and commercialization of vehicles powered by ethanol, where the ethanol from sugar cane is widely used as a gasoline extender (up to 25%) or used as a transportation fuel.3,4 Among various renewable fuels, biobutanol has been recognized as a superior next-generation biofuel with commercialization potential due to its excellent fuel properties. Compared to short-chain alcohols such as ethanol and methanol, butanol has fuel properties similar to traditional transportation fuel due to a longer hydrocarbon chain and has demonstrated the capability to work in vehicles designed for traditional fuel without any modifications.5−7 Butanol also has a higher energy density than ethanol, leading to less reduced power and can be used either in its neat form or as a blend with fossil fuel. It has a relatively high cetane number and excellent © 2014 American Chemical Society
miscibility with diesel, thus making it more suitable to be blended with diesel. Also, n-butanol is much less hygroscopic and corrosive, therefore known as a “drop-in” fuel that would be compatible with the current fuel distribution infrastructure.8 Apart from the aforementioned properties, alcohol-fueled diesel engines have, in general, demonstrated reductions in carbon monoxide (CO), total hydrocarbons (THCs), and particulate matter (PM) compared to conventional diesel engines, mainly due to the extra oxygen content in the fuel, thus enhancing the oxidation of soot, CO, and THC.9−13 The amount of soot reduction varies almost linearly with the amount of added oxygen, but not all oxygenated additives are equally effective in soot reduction. Additives with a single embedded oxygen atom, such as methanol, ethanol, butanol, and dimethyl ether, are among the most effective in reducing soot precursor production.14−16 Zhang et al.17 reported butanol-diesel blends can reduce PM mass concentration and the reduction being more obvious at high engine load with a high proportion of butanol in blends. Michikawauchi et al.18 found that with the butanol blended in diesel, the trade-off between NOX and thermal efficiency was improved and that NOX could be significantly reduced at a constant thermal efficiency at both medium and high loads. Zhang et al.19 tested butanol-diesel blends on a single cylinder diesel engine under low temperature combustion (LTC) conditions. They found Received: June 21, 2014 Revised: September 16, 2014 Published: September 17, 2014 6380
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Table 1. Fuel Properties31−34 properties molecular formula cetane number octane number oxygen content (wt %) density at 15 °C (g/mL) autoignition temperature (°C) flash point at closed cup (°C) lower heating value (MJ/kg) boiling point (°C) stoichiometric ratio latent heat at 25 °C (kJ/kg) flammability limits (vol %) saturation pressure at 38 °C (kPa) viscosity at 40 °C (mm2/s) surface tension cycle energy ratio (%)
diesel C12−C25 40 min
0.82−0.86 230 64 42.7 282−338 14.3 270 0.6−5.6 0.3 1.9−4.1 23.8 100
acetone C3H6O
27.6 0.791 560 17.8 29.6 56.1 9.54 518 2.6−12.8 53.4 0.35 22.6 64.5
butanol
ethanol
C4H9OH 25 96 21.6 0.813 385 35 33.1 117.7 11.21 582 1.4−11.2 2.27 2.63 24.2 74.2
C2H5OH 8 108 34.8 0.795 434 8 26.8 78.4 9.02 904 4.3−19 13.8 1.08 21.8 57.8
ABE(3:6:1)
24.7 0.805
31.4 10.5 595
69.6
study also demonstrated an increase in brake thermal efficiency of up to 8.56% using the ABE-diesel blends. In another work,28 they compared the combustion performance and emissions of ABE-biodiesel-diesel blends. The results showed that the use of ABE-biodiesel-diesel blends could simultaneously reduce both PM and NOX by 4.30−30.7% and 10.9−63.1%, respectively, in comparison with regular diesel. At the same time, ABEbiodiesel-diesel blends have a lower aromatic content, which leads to more complete combustion and results in lower PAH and toxicity equivalency of PAHs emissions of the engine. The combustion characteristics of ABE and diesel blends have also been studied in the authors’ lab.29 Longer flame lift-off length (FLoL) for ABE-diesel blends was reported which allowed more air entrainment upstream of the spray jet thus providing a better air−fuel mixing. At a low ambient temperature of 800 K and ambient oxygen concentration of 11%, ABE20 (20% vol ABE and 80% vol diesel) presented close-to-zero soot luminosity with better combustion efficiency compared to neat diesel. The author’s previous work30 on the impacts of the different ABE component ratios also showed longer soot lift-off length for all the ABE-diesel blends. Previous studies have suggested that ABE is a very promising alternative fuel to be directly used in diesel engines. As the major component of ABE, butanol has already been researched widely and its influences on combustion characteristics of blends have been documented. Hence, as a continuing study, it is necessary to address the differences and similarities between ABE and butanol in spray and combustion under real dieselengine like conditions. Neat ABE (referred to as ABE100), neat n-butanol (referred to as B100), and neat ultralow sulfur diesel (ULSD) (referred to as D100) were studied in a preburn type constant volume combustion chamber with optical access. By adjusting the partial pressure of the premixed burn mixture and the injection timing, the ambient temperature and oxygen content were varied from 1200 to 800 K and 21% to 11%, respectively, covering both conventional and LTC combustion regimes. Time resolved spray and broadband flame luminosity images of the injection events were captured by using a high speed camera coupled with a copper vapor laser beam. Liquid penetration and flame lift-off lengths were also acquired through image postprocessing. On the basis of the results, the detailed spray and combustion characteristics of ABE, nbutanol, and diesel injection were analyzed and compared.
that the ignition delay was prolonged due to the low cetane number of butanol, but the indicated thermal efficiency was increased while keeping the CA50 at the same timing. Yao et al.20,21 reported that the impacts of pilot and post injection on a heavy-duty diesel engine using butanol blends were similar to those of neat diesel. The in-cylinder pressures presented no apparent difference for low n-butanol blending ratios (0−15% vol), whereas higher blending ratios resulted in a higher premixed combustion. In addition, the butanol additive could endure a higher EGR ratio without THC, CO, and soot emissions penalty. Liu et al.10 compared combustion characteristics and soot distribution of butanol and biodiesel in a constant volume chamber. They found the combustion pressure of butanol to be lower than that of soybean biodiesel, but butanol had a higher normalized peak pressure indicating a higher potential thermal efficiency. Meanwhile, they found that the soot formation for butanol was much less than that of soybean biodiesel, at lower ambient temperatures. Biobutanol is typically produced via acetone-butanol-ethanol (ABE) fermentation from renewable feedstock using various strains of Clostridium acetobutylicum or Clostridium beijerinckii in anaerobic conditions.22−25 Also, as it has been reported biobutanol is a suitable alternative fuel for the diesel engine. However, the high costs of ABE components separation from the dilute fermentation broth have so far prohibited industrial scale production of biobutanol. If the ABE mixture could be directly used for clean combustion, the separation costs would be eliminated and a vast amount of time and money would hence be saved in the production chain of biobutanol. It is in this respect that the direct application of ABE has received increased attention recently. Nithyanandan et al.26 investigated the performance of ABE-gasoline blends in a spark-ignited engine and found that the addition of 40% vol ABE or lesser did not change the combustion characteristics significantly. Chang et al.27 carried out a study of ABE-diesel and water-ABE emulsions-diesel blends on a diesel engine generator and diesel engine testbed. They reported that with the use of ABE-diesel blends, both particulate matter (PM) and total toxicity equivalency of polycyclic aromatic hydrocarbons could be reduced significantly, but NOX emission would increase when using a blend of 20% vol ABE and 80% vol diesel. However, the undesired NOX increase could be further eliminated using a solution containing 0.5% vol water. The diesel engine in their 6381
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2. EXPERIMENTAL METHODS 2.1. Fuel Preparation. ULSD was selected as the base fuel in this study. The ABE solution was first prepared at a volume ratio of 3:6:1 (A/B/E); this ratio was used to simulate the composition of the ABE fermentation product. Analytical grade acetone (99.5%), butanol (99.5%), and ethanol (99.8%) were then mixed with baseline diesel using a temperature-controlled magnetic stirrer. The properties of individual fuels are listed in Table 1. The dilute broth produced during ABE fermentation usually contains a certain amount of water, which is removed later in the recovery process. The water could be a major concern when blending with diesel, although previous studies have suggested that diesel-ABE blends may remain stable with a small amount of water even without any surfactant addition.27,28 In this work, we particularly focus on the spray and combustion characteristics of ABE and n-butanol without blending with diesel, and water containing ABE-diesel blends will be addressed in future research. 2.2. Test Chamber. A constant volume chamber with a bore of 110 mm and a height of 65 mm is used in this study. The chamber can mimic the high temperature/pressure conditions of a diesel engine, allowing a maximum operating pressure of 18 MPa. The chamber has an open end on the top with a fused silica window installed opposite to the injector allowing optical access (see Figure 1). The window has
Figure 2. Constant volume chamber: pressure history of an individual injection event. Using the in-chamber pressure data, the apparent heat release can be calculated using the first law of thermodynamics. The start of combustion (SOC) was defined as the timing when 10% of total heat release is reached. Since all the timings recorded in this work were in reference to the start of injection (SOI) signal, ignition delay was then determined as the duration between SOI signal and SOC. Moreover, the combustion duration was defined as the period between 10% and 90% of total heat release. The tested conditions for this study are summarized in Table 2.
Table 2. Test Conditions fuel tested injector type orifice diameter (mm) injection pressure (bar) injection duration (ms) injected fuel volume (mm3) ambient temperature (K) ambient oxygen (vol %) ambient species (vol %)
Figure 1. Schematic of the experimental setup.
Oxy 21 Oxy 16 Oxy 11
dimension of 130 mm in diameter and 60 mm in thickness, with a high UV transmittance down to 190 nm. A valve-covered orifice (VCO) type Caterpillar hydraulic-actuated electronic-controlled unit injector (HEUI), with an orifice diameter of 0.145 mm, is mounted at the bottom of the chamber. Injection pressure and duration were kept constant at 1300 bar and 3.5 ms, respectively, throughout the tests. The cylinder wall is heated to 380 K before the experiment to mimic the wall temperature of a diesel engine as well as to prevent water condensation on the optical windows. A quartz pressure transducer (model Kistler 6121) is responsible for recording the in-cylinder pressure. A plasma ignition system was used to ignite the premixed mixture (see details in previous study35). 2.3. Experimental Methodology. The test procedure was started by filling the chamber with the premixed mixture including acetylene (C2H2), oxygen, and nitrogen. The mixture was then ignited by a spark to create a high-temperature/pressure environment inside the chamber; the environment gets slowly cooled down due to the heat loss through the chamber wall. Spray injection was triggered when the pressure reached the desired test condition, as illustrated in Figure 2. Temperature, density, and oxygen concentrations of the gas at the time of injection in the chamber can be controlled as desired by adjusting injection timing and partial pressure. However, the ambient density was kept constant at 14.8 kg/m3 in this study, which is considered as the typical condition in a diesel in-cylinder environment for the piston at top dead center (TDC). The camera and the injector were synchronized to capture the spray and combustion process accurately.
CO2 8.2 8.2 8.2
D100, ABE100, B100 6-hole, valve-covered orifice 0.145 1300 3.5 120 1200, 1000, 800 21, 16, 11 H2O N2 4.1 4.1 4.1
66.7 71.7 76.7
2.4. Laser Diagnostics. The spray and combustion processes were captured through the top window using a high speed camera (Phantom V7.1) with a 105 mm focal length lens (Nikkor) located above the chamber. The camera had a SR-CMOS 12-bit 800 × 600 pixel monochrome sensor with a spectrum response from 400 to 1000 nm. All the images were taken at 9049 frames per second at a resolution of 416 × 560 pixels. A short exposure time, 3 μs, was used to freeze the motion on each image. For the spray, the light source was supplied by a copper vapor laser (Oxford Lasers LS20-50) which was externally controlled to run up to a maximum frequency of 50 kHz with a pulse duration of 25 ns. The high-speed camera and the coppervapor laser were synchronized to the set frequency. The peak wavelength of the copper vapor laser beam is at 510 nm, thus a 510 nm narrow band-pass filter (10 nm full-width half-maximum (fwhm)) was applied in front of the lens to block the signals of other wavelengths from the environment or those emitted by the burning spray. The liquid spray was captured because of the difference in refractive index between the fuel and the ambient gas. The continuouswave laser beam was expanded to completely illuminate the liquid spray. This “volume-illumination” method, rather than a laser sheet, was utilized to ensure that all droplets spreading from the nozzle were illuminated to identify the maximum axial and radial distances of any liquid-phase fuel. The input beam was directed at a slight angle to avoid interference with the camera. 6382
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For the natural flame luminosity imaging, the setup was the same as that used for spray imaging, except the removal of the laser beam and the band-pass filter. Once the luminosity images were acquired, integrating the pixel values over an image provided the value of spatially integrated natural luminosity (SINL). The flame lift-off was defined as the position closest to the injector tip where significant soot luminosity was observed. The analysis was restricted to a cone around the spray axis and the threshold was set to a certain percentage of the maximum pixel value occurring along the flame centerline. In the present paper, the threshold was set at 15% of the maximum. The data points depicted in both SINL and flame lift-off plots in the remainder of the paper represent an average over at least five injection events. The liquid penetration is defined as the length between the injector tip to the furthest point where liquid can be observed, and the threshold was set at 85%.
3. RESULTS AND DISCUSSION 3.1. Spray Characteristics. The macroscopic characteristics of the burning spray under various conditions were obtained through the high-speed images. Figures 3−5 illustrate Figure 4. Mie scattering images of the spray at ambient temperature of 800 K with an oxygen concentration of 21%.
Figure 3. Mie scattering images of the spray at an ambient temperature of 1200 K with an oxygen concentration of 21%.
the spray evolution from three individual injections for the fuels under different ambient temperatures and different oxygen concentrations. Because of the limitation of the field of view, only one spray plume was fully captured through the entire process. The time stamp on the images represents the time after the injection signal. There was a short delay between the injection signal and the actual injection event (first appearance of liquid on the image) for this specific HEUI injector. The spray images were “reversed” for the purpose of better presentation since the spray jet body was illuminated with a black background on the original unprocessed images. It can be noted that even though the band-pass filter was applied, signal contributed by the natural luminosity may still be captured downstream of the spray, especially at high ambient temperature. When compared to the images under lower ambient temperature in Figures 4 and 5, the first important observation in Figure 3 is that the spray size under high ambient temperature is much smaller than that seen under lower
Figure 5. Mie scattering images of the spray at ambient temperature of 800 K with an oxygen concentration of 11%.
ambient temperature. In addition, the spray for different fuels at high ambient temperature is very similar with respect to spray width as well as liquid penetration. Almost all the physical properties change with temperature, an increase in ambient temperature causes viscosity and surface tension to decrease and vapor pressure to increase; these changes significantly accelerate the atomization and evaporation of the liquid spray. Hence, the sprays for all tested fuels are much smaller under high ambient temperature than those under low ambient temperature. Another observation is that the spray penetration increase quickly up to a peak value and then gradually reduces after that; however, tip fluctuation can still be observed from the individual snapshots. A quasi-steady-state (QSS) on the spray penetration curve under evaporating/nonevaporating conditions has been previously reported;36 nevertheless, the results from the current study suggest a decreasing trend of the 6383
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Figure 6. Liquid penetration for tested fuels.
liquid penetration in a burning spray regardless of the fuel. This is due to the enormous heat release from the reaction zone just downstream of the spray tip that accelerates the fuel evaporation. A similar trend was also reported in a previous study.37 Figures 4 and 5 present the spray evolution at a lower ambient temperature of 800 K and different oxygen concentrations. Compared to the high ambient temperature conditions, it is apparent that the liquid spray exhibited a much longer penetration and a larger jet body for all tested fuels due to the decreased evaporation rate at low ambient temperature. Although the spray size was increased significantly for all the tested fuels, the increment of both spray penetration and spray size is much more pronounced for diesel. As mentioned in a number of studies, the liquid penetration under evaporating/ burning conditions is significantly affected by the fuel properties.38−41 As observed from Table 1, both ABE100 and B100 feature a much lower boiling point, higher vapor pressure, and lower viscosity than D100, which leads to enhanced vaporization and atomization and thus shorter penetration and a narrower jet body. The reason why this behavior is only obvious under lower ambient temperature is because the differences in physical properties of the fuels are amplified under low temperature relative to those at high temperature. With the decrease of ambient temperature, the vaporization rates of all the components become lower, and the volatility difference among these fuels also becomes more obvious. Overall, the sprays of ABE100 and B100 behave similarly, except in very few cases where ABE100 displays an even narrower jet body which would be due to the higher volatility of acetone and ethanol in ABE. Moreover, there is no apparent decreasing trend of the penetration length for ABE100 and B100 on these snapshots, which can be explained by the longer ignition delay. The ignition delays of the two fuels, which will
be discussed later, were more than 4 ms under an ambient temperature of 800 K. Therefore, the snapshots of the spray in Figures 4 and 5 were well before the SOC, which confirmed the existence of a QSS under evaporating conditions. The occurrence of a blurred area just downstream of the spray tip is also noted for ABE100 and B100 after 1.61 ms, which also indicates that a more violent breakup was taking place at these spray tips. By comparing Figures 4 and 5, the difference in the macro spray features of each corresponding fuel is almost negligible, indicating that the ambient oxygen concentration has minimal effect on spray characteristics. However, the ambient oxygen plays a significant role in the combustion characteristics, as will be discussed later. Figure 6 shows the quantitative comparison of the liquid penetration length of the tested fuels under various conditions. D100 features a much longer peak liquid penetration followed by a decreasing period, which has been shown in Figures 3−5. The difference between ABE100 and B100 in terms of the spray penetration was almost negligible until the ambient temperature dropped down to 800 K where ABE100 exhibited relatively shorter penetration length, which can be attributed to the multicomponent properties of ABE. The low boiling point, high vapor pressure, and low viscosity of both acetone and ethanol in ABE enhanced the spray break up early for ABE100 relative to B100 under low ambient temperature. From the figures, it also can be seen that the ambient oxygen concentration has little impact on liquid penetration while the ambient temperature influence the liquid penetration significantly. 3.2. Combustion Characteristics. Before investigating the combustion characteristics of the tested fuels, two points are to be noted. First of all, most of the previous studies were focused on n-butanol as an additive to diesel while neat butanol studies were found much less in engine tests mostly due to the lubricity 6384
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Figure 7. Combustion pressure and heat release rate for tested fuels.
conditions suggests that combustion is dominated by the air− fuel mixing process. At the same time, the figure also shows that at high temperature conditions, the heat release rate for D100 is higher than those of ABE100 and B100. This is because of the energy difference between the tested fuels; the cycle energy for ABE100 and B100 is lower than D100 due to their lower density and lower heating value, which was further confirmed by their lower spray combustion pressure curves. Usually, a higher heat release rate peak is expected for alcohol blended fuels due to their longer ignition delay resulting from a high latent heat value. Nevertheless, in this study, we found a similar (short) ignition delay for all tested fuels. The ignition delay was so short that the beginning of combustion can be clearly noticed in the second image (1.16 ms after SOI) in the spray evolution sequence for D100 in Figure 4, which illustrates that the premixed combustion is limited for all tested fuels under high temperatures. At low ambient temperature, 800 K, with a high oxygen concentration of 21%, an appreciable premixed burn phase is observed for all three fuels. This is likely due to the fact that the low ambient temperature lowered the fuel evaporation rate of the oxygenated fuels and resulted in a more prepared mixture for the premixed burn. It should be noted that even though all the fuels manifest a stronger premixed combustion, the premixed AHHR spike for ABE100 and B100 is still lower than that of D100. Two possible reasons could have contributed to this phenomenon. On the one hand, the cycle energy difference between the fuels cannot be ignored, because the cycle input energy of ABE100 and B100 is about 69.6 and 74.2%, respectively, of that of D100. On the other hand, significantly higher latent heat value for ABE100 and B100, more than two times than D100, would reduce the local temperature in the combustion region, further reducing the combustion chemical reaction rate. This phenomenon also has been reported by Saisirirat et al.46 on a homogenous charge compression ignition (HCCI) combustion engine, with an
issue occurring in the high pressure fuel pump of the common rail injection system. Such concerns lead to a relatively low butanol blending ratio (typically
