ARTICLE pubs.acs.org/EF
Estimation of Opacity Tendency of Ethanol and BiodieselDiesel Blends by Means of the Smoke Point Technique Octavio Armas,*,† M. Arantzazu Gomez,† Eduardo J. Barrientos,‡ and Andre. L. Boehman‡ †
Departamento de Mecanica Aplicada e Ingeniería de Proyectos, Universidad de Castilla—La Mancha, Edificio Politecnico, Av. Camilo Jose Cela s/n, 13071, Ciudad Real, Spain ‡ EMS Energy Institute, The Pennsylvania State University, 411 Academic Activities Building, University Park, Pennsylvania 16802-2308, United States ABSTRACT: Oxygenated fuels in diesel engines represent an alternative to conventional petroleum-derived fuel to achieve current European emissions standards (EURO 5), especially with regard to particulate matter (PM), where the reduction of the limit in mass concentration is 80% compared to that for the previous EURO 4 standard. Among these oxygenated fuels, biodiesel and ethanoldiesel blends have great potential in reducing smoke opacity and, therefore, the particulate matter emitted. The smoke point is a technique used for determining the sooting tendency of kerosene and aviation fuels and it can be used as an indicator of smoke opacity. The smoke point technique can also be used with an extensive variety of fuels including automotive diesel fuels. This work proposes a new methodology for estimating the decrease in diesel engine exhaust opacity when different ethanol and biodieseldiesel blends are used instead of conventional diesel fuels. Different binary biodieseldiesel and ethanoldiesel blends were tested in a smoke point lamp together with ternary ethanolbiodieseldiesel blends. The results of binary blends were compared to the opacity obtained in a light duty diesel engine operating in a steady state mode confirming that the molecular weight to smoke point ratio accurately reproduces the decreasing trend in opacity, when normalized to a conventional diesel fuel, as a function of the percentage of biodiesel or ethanol in the blend. Moreover, the new proposed methodology showed how not only the oxygen content but also its functional group plays an important role in this decreasing opacity trend. In this way, tests of ternary ethanolbiodieseldiesel blends on a smoke point lamp could predict the trend in smoke opacity when a diesel engine is fuelled with them.
’ INTRODUCTION Nowadays, light and heavy duty diesel engines face more stringent emission regulations due to the hazardous effects of emissions on human health and the environment. In Europe, the Euro 5 standard, effective since September 2009, limits NOx emissions from diesel engines to 180 mg/km, which translates into a 28% reduction compared to the previous Euro 4 standard. Even more dramatic are the PM regulations, which state a reduction from 25 to 5 mg/km (i.e., an 80% reduction). Oxygenated fuels can be a viable alternative to conventional petroleum-based fuels. Among of these, biodiesel and ethanol possess a great potential in diesel engines due to the significant reduction in smoke opacity and PM emissions by the presence of oxygen functional groups even at small concentrations.15 This reduction can be explained by the decrease in probability of soot nuclei formation in locally rich zones due to the increase of conversion of larger fractions of fuel carbon to CO.6 Moreover, the absence of aromatic and sulfur compounds in the biodiesel fuel plays an important role in this reduction.7 Biodiesel consists mainly of alkyl-esters originated from fattyacids of vegetable-oil or animal fat. Biodiesel presents very similar properties to conventional petroleum-based oil, which can be used as substitute in diesel engines, either in pure form or blended in different proportions. On the other hand, ethanol is a petrochemical alcohol produced through the hydration of ethylene. Ethanol can be also produced from renewable raw materials, by fermenting sugary biomass with yeast, or by hydrolysis of starchy compounds and lignocellulosic biomass. r 2011 American Chemical Society
Compared to biodiesel, the ethanol, which is commonly used in spark-ignition engines, presents more challenges when used in conventional diesel engines. Ethanol has a low cetane number, low lubricity, and a limited miscibility with diesel fuel.8 Despite all these disadvantages, the 35% oxygen content due to its molecular composition, 3 times the oxygen content of biodiesel, makes ethanol very attractive as a diesel blending agent. With the aim of increasing the miscibility of ethanol in diesel fuel, additives are used to ensure the stability of blends. Biodiesel molecules have a polar end with an affinity for ethanol, which is also polar in nature, but also a nonpolar carbon chain similar to diesel. Therefore, biodiesel can be used as a stabilizing agent. In addition, its high cetane number and good lubricating properties offset reductions in those properties due to ethanol.9 The number of studies on emissions in compression ignition engines fuelled with these ternary blends recently is increasing.1012 Smoke point measurements are defined as the height in millimeters of the highest flame produced without smoking when the fuel is burned in a specified test lamp.13 The tendency to smoke of a flame depends on the balance of the required and available oxygen content. The height of the flame in which it begins to smoke is related to the tendency to smoke the burning compound, providing a good indication of the opacity produced during the diffusion combustion of a fuel. Received: April 15, 2011 Revised: June 1, 2011 Published: June 01, 2011 3283
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Energy & Fuels Smoke points have been used for more than 50 years to evaluate the sooting tendency of pure and mixed hydrocarbons, including oxygenated compounds.1417 Further studies by Calcote and Manos18 generated an empirical correlation called the Threshold Sooting Index (TSI), which is directly proportional to the ratio of the molecular weight (MW) to smoke point (SP). These studies determined that the molecular composition of the fuel is one of the most important factors that influence the sooting tendency. Gill and Olson19 proposed TSI mixing rules for fuel blends where the TSI of the mixture can be expressed as the summation of the TSI of the individual components by their respective mole fraction. Yang et al.20 found that the TSI model and the mixture rules defined by Gill and Olson correlate excellently with hydrocarbon compositions over a wide range of fuel samples, even mixtures as complex as jet fuels, which contain hundreds of compounds. Furthermore, Yan et al.21 developed some correlations to predict the TSI of several compounds knowing only the molecular structure of fuels. In 1954, the American Society for Testing and Materials (ASTM) standardized the procedure for determining the smoke point using a lamp with wick, the most recent revision being in 2008.13 The corresponding ISO regulation for smoke points is the 3014:1993(E) Petroleum products Determination of the smoke point of kerosene. Although the standard methods specify the methodology for kerosene and aviation turbine fuels, these methods can be applied to other fuels, including diesel fuels. This work proposes a new methodology focused on the application of the smoke point technique to fuels used in diesel engines whose combustion process is also diffusive. The smoke point of a reference diesel fuel together with different soybean biodiesel diesel, ethanoldiesel, and ethanolbiodieseldiesel blends were determined with a standard smoke point lamp. The molecular weight to smoke point ratio of the binary blends was compared to the opacity obtained from a light duty diesel engine operating in a high load steady state mode without exhaust gas recirculation (EGR). A good agreement between trends was observed. Further studies proved how the decrease in opacity is related not only to an increase of the oxygen content in the blend but also to the oxygen functional group. Ternary blends were used to predict the trend in smoke opacity when a diesel engine is fuelled with them, showing the role of esters and alcohol groups in reducing smoke tendency. Although the smoke point values do not represent a real estimation of the opacity in the engine, which depends not only on the fuel employed but also on the engine conditions, this method can serve as a qualitative comparison between different fuels and their opacity tendency, showing the relative decrease in opacity when a diesel blend is used instead of pure diesel fuel.
’ EXPERIMENTAL SETUP Fuel Samples. Three different groups of fuel blends were tested on the smoke point lamp along with neat diesel fuel, which served as reference. The first tests were performed on blends of soybean biodiesel with diesel. The second group corresponds to blends of ethanol and diesel on different volume fractions. The third group of tests was performed on blends of ethanolbiodieseldiesel. The main properties of pure fuels are presented in Table 1, while Table 2 shows tests schedule. As shown in Table 2, all ternary blends (EBD) have a constant percentage of 7.7% v/v of ethanol. Therefore, the EBD5 corresponds to a fuel blend with 7.7% v/v of ethanol on B5 (instead of pure reference diesel). The EBD15 corresponds to a blend with 7.7% v/v of ethanol in B15 and so on. The 7.7% ethanol content is derived from a previous study about blend stability.8 This ethanol content ensures the stability of the blend in a wide temperature range.
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Table 1. Main Properties of the Pure Fuels diesel
soybean biodiesel
ethanol
density at 15 °C (kg/m3)
835
885
792
kinematic viscosity at 40 °C (cSt)
2.72
4.09
1.13
higher heating value (MJ/kg)
45.54
39.42
28.04
lower heating value (MJ/kg)
42.61
36.86
25.18
% C (by weight)
86.13
77.13
52.14
% H (by weight)
13.87
11.94
13.13
% O (by weight)
0
10.93
34.73
% H2O (by weight) % S (by weight)
0.0057 0.0034
0.0272 0.00025
0.2024 0
C/H mass ratio
6.209
6.459
3.971
stoichiometric fuel-air ratio
1/14.67
1/12.50
1/9.00
derived cetane number
54.9
53.7
58
Because compounds with high molecular weights are more viscous and, therefore, more difficult to burn in the diffusion flame of a wick lamp, biodieseldiesel blends with more than 30% v/v of biodiesel could not be tested on the smoke point lamp. Test Facilities and Experimental Procedure. Smoke point testing was performed on an ASTM D1322-97 standard smoke point lamp. A black painted steel frame surrounding the apparatus was used to control air flow during the measurements. The results were calibrated in accordance with ASTM D1322-97 using a standard reference fuel blend of 20% (v/v) of toluene and 80% (v/v) of 2,2,4-trimethylpentane at 1 atm.13 At least three readings of the flame height were taken for each sample and then averaged and corrected by the corresponding standard value. In addition, binary blends were tested on a 4-stroke, direct injection, common rail, diesel engine coupled to an asynchronous brake dynamometer model Schenck Dynas3 LI 250. The main engine characteristics are shown in Table 3. The tests were carried out on a steady state operating mode whose parameters are displayed in Table 3. This particular engine operating mode, translated from the transient certification cycle NEDC, was selected on the basis of the high engine load with no EGR. An AVL 439 opacity meter based on the light extinction theory was used to measure the opacity of the exhaust gases on % Op. Data Analysis Methodology. Calcote and Manos18 demonstrated that all of the literature data on the sooting tendency of hydrocarbons in different burners is consistent with respect to molecular structure and that sooting tendency could be characterized and predicted by an arbitrary scale called the Threshold Sooting Index (TSI). This parameter is defined for diffusion flames according to eq 1, where MW is the molecular weight and SP is the smoke point of the tested fuel. The coefficients a and b are constants that depend on the test lamp used. A TSI value of 0 is assigned to ethane (less sooty element) and a value of 100 to methylnaphthalene, respectively. MW þb ð1Þ TSI ¼ a SP The introduction of the molecular weight into eq 1 is done to compensate for the lower increase in height of the flame with the increasing molecular weight of the sample.20 More oxygen is required to diffuse into the flame when the molecular weight is increased to consume a unit volume of the fuel.18 Gill and Olson19 established a mixing rule for calculating the TSI of fuel blends according to eq 2 where the TSI of the individual compounds are calculated with eq 1 and xi is the mole fraction of each individual compound. TSImix ¼ 3284
∑i xi TSIi
ð2Þ
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Table 2. Test Schedule nomenclature
compound
diesel
D
diesel
biodiesel blends
B5
biodiesel
% v/v 100 5
diesel B15
15
diesel
85
engine
point test
test X
211.7
0
X
215.01
0.61
X
221.36
1.72
X X
biodiesel
30
231.75
3.41
B70
diesel biodiesel
70 70
263.75
7.78
X
diesel
85 292.92
10.92
X
180.78
1.65
X
biodiesel
100
ED5
5 95
ED7.7
ethanol
7.7
167.52
2.55
X
ED10
diesel ethanol
92.3 10
157.64
3.31
X
diesel
90 139.67
4.98
X
133.56
5.65
ED15
ethanol
15
diesel
85
ED17
ethanol
17
EBD5
Tests for Prediction of Sooting Tendency ethanol 7.7 169.44
EBD15
EBD30
biodiesel
4.615
diesel ethanol
87.685 7.7
biodiesel
13.845
diesel
78.455
ethanol
7.7
biodiesel
27.69
diesel
64.61
diesel
83
Table 3. Engine Characteristics and Test Operating Conditions Engine Characteristics engine
NISSAN YD 2.2
type
4 stroke diesel
intake system
turbocharger with intercooler
injection system
common-rail
EGR system
hot EGR externally controlled
no. of cylinders
4
bore (mm)
86.5
stroke (mm)
94
displacement (dm3)
2.2
maximum rated power (kW)
82 (at 4000 min1)
Engine Test Conditions 1853
torque (Nm)
110
EGR rate
0
maximum rated torque (Nm)
248 (at 2000 min1)
X
0
ethanol diesel
engine speed (min1)
smoke
(% m/m)
95
biodiesel
diesel
ethanolbiodieseldiesel blends
oxygen content
(g/mol)
B30
B100 ethanoldiesel blends
molecular weight
X
X
3.08
X
173.37
4.12
X
179.53
5.67
X
They measured smoke points for ten previously studied pure hydrocarbons. Comparing these to Olson et al.22 data and applying a least-squares analysis, they obtained calibration constants with values a = 3.32 (mol mm/g) and b = 1.47. Using eq 2 and the coefficient values of Gill and Olson,19 Pepiot-Desjardins et al.23 calculated the TSI of mixtures of a given fuel with several oxygenated groups in different concentrations. Using the same procedure by Gill and Olson19 on different blends of toluene with n-heptane doped with two- and three-ring aromatics and their saturated counterparts, Barrientos and Boehman24 found constants a = 3.45 (mol mm/g) and b = 2.55. Likewise, Yang et al.20 obtained constants a = 3.52 (mol mm/g) and b = 7.42 on 12 prototype coal-based jet fuels (JP-900). All these results suggest that constant a is more important than constant b in reference to eq 1 because the former represents the slope that determines how the TSI changes with respect to the fuel property MW/SP, while the intercept b only determines the line’s position relative to the origin, which is related to the arbitrary selection of the reference compound defined to have TSI = 0.20 On the basis of smoke point data of several authors and by using a similar procedure as Gill and Olson, Yang et al. obtained a new correlation of a = 3.18 (mol mm/g) and b = 0. 3285
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Figure 1. (a) Variation of opacity and MW/SP ratio with the % v/v of biodiesel. (b) Normalized variation with respect to reference diesel fuel. Based on the coefficient values obtained by Yang et al., the TSI is only directly proportional to the MW/SP ratio. Therefore, taking diesel as reference base fuel, the TSI value is given by eq 3: MW ð3Þ TSID ¼ 3:18 SP D Considering that the TSI of any other given fuel (F) would be expressed in the same way, the combination of both values will be given by eq 4. Thus, the ratio of a particular compound TSI to the one by the reference fuel (diesel) will equal the MW/SP ratios of both compounds, which will be the parameter employed in this study to present the objective results. MW TSIF SP F ¼ ð4Þ MW TSID SP D
’ RESULTS AND DISCUSSION BiodieselDiesel Blends. Lapuerta et al.25 reviewed the
effect of biodiesel on diesel engine emissions. According to this work, even though some authors report an increase in particle emissions when biodiesel is used, mainly due to an increase of hydrocarbons, the general agreement in the literature is that PM and opacity decrease when biodiesel is used instead of conventional diesel fuel. The Environmental Protection Agency (EPA) correlated PM emissions as a function of biodiesel percentage with data from several works. This correlation is presented in eq 5. PM ¼ e0:006384 3 %B PMD
ð5Þ
Figure 1a shows the opacity and MW/SP ratio as a function of percentage of biodiesel (% v/v) in samples tested. The results follow an exponentially decreasing trend according to eqs 6 and 7. OpB ¼ 4:4242 3 e0:0069 3 %B
MW SP
B
¼ 12:4153 3 e0:0072 3 %B
R 2 ¼ 0:9896 R 2 ¼ 0:9148
ð6Þ
expected. The opacity registered for the engine fuelled with conventional diesel was 4.46%, and the MW/SP ratio determined with the smoke point lamp was 12.75 g/(mol 3 mm). Normalizing the values of opacity and MW/SP ratio with respect to the reference diesel fuel, as in the EPA correlation, the following equations (8) and (9) were obtained, which are plotted in Figure 1b. OpB ¼ e0:0071 3 %B OpD MW SP B ¼ e0:0084 3 %B MW SP D
R 2 ¼ 0:9892
R 2 ¼ 0:8712
ð8Þ
ð9Þ
A good correlation between the exponential coefficients can be observed again. Therefore, some conclusions can be obtained: (1) particulate matter emissions closely follow the trend observed in opacity when the engine is fuelled with biodieseldiesel blends; (2) the normalized MW/SP ratios can reproduce quite accurately the exhaust gas opacity trends, especially with relative low (no more than 30% v/v) biodiesel content in the blend. A deviation of only 3.82% between opacity and MW/SP ratio was registered for a B30 blend; meanwhile, a maximum deviation of 12.19% was registered for pure biodiesel. In the following sections only these normalized parameters referring to reference diesel fuel will be presented. EthanolDiesel Blends. Hansen et al.5 in a review about the effect of ethanoldiesel blends on compression ignition engine performance, durability, and emissions showed an agreement in the literature about the decrease in PM emissions when these blends are used as fuel. Following the same procedure as in the previous section, Figure 2 shows the values of opacity and MW/SP ratio normalized to the reference diesel fuel. In this case the results were depicted as a function of percentage of ethanol (%v/v) in the blend. The normalized correlations are shown in eqs 10 and 11. OpE ¼ e0:0421 3 %E OpD
ð7Þ
MW SP E ¼ e0:0412 3 %E MW SP D
R 2 ¼ 0:9452
ð10Þ
The exponential coefficients of these three equations are quite similar. Only the prefactors of eqs 6 and 7 are different, which practically correspond to the values registered for pure diesel, as 3286
R 2 ¼ 0:9873
ð11Þ
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Energy & Fuels As in the previous section, the normalized opacity measured on the engine in the selected operating mode follows an exponential decrease with the percentage of ethanol in the fuel blend. Also MW/SP ratio correlates well with the reduction in opacity. In this case, the maximum deviation obtained is 9.42% for pure ethanol compared to 12.19% for pure biodiesel, as reported in the previous section. The deviation for ED17, the blend with the highest ethanol percentage tested in the engine, was only 1.54%. Therefore, the normalized values of MW/SP provide again a good estimation of the opacity tendency presented by these types of oxygenated compounds. Moreover, Figure 2 shows a sharper reduction (about 5 times) in both parameters when the percentage of ethanol is increased compared to that of biodiesel blends presented in Figure 1b. This result can be associated with the higher oxygen content on the molecular structure of ethanol compared to biodiesel, the lower C/H mass ratio of ethanoldiesel blends,26 and the lower cetane number of ethanol compared to biodiesel, which leads to a longer ignition delay. Therefore, the fuel burned in the premixed combustion phase increases, decreasing the fuel burned during the diffusive phase. As a consequence, soot formation and emission is reduced.27 Also, another reason can justify this reduction. Westbrook et al.28 developed a chemical-kinetic model to study the effects of oxygenated fuels on soot emissions. To develop the model, they selected different fuels (ethers, esters, and alcohols) blended with nheptane. Their results proved that esters, the principal oxygenated compound of biodiesel, are less effective in suppressing soot formation than ethers and alcohols. In esters, one carbon is bonded to two atoms of oxygen, one with a double bond (higher bond energy) and another with a single bond. If the two atoms of oxygen are kept bonded to the carbon, they form CO2, and they will not be
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available for hydrocarbon oxidation reactions, which are precursors in the formation of soot nuclei. Therefore, these oxygen atoms are less effective in suppressing soot formation than the other groups of samples tested. Similar results were obtained by Pepiot-Desjardins et al.,23 using the TSI technique with different blends of oxygenated groups with a reference fuel of n-heptane and toluene. The results revealed that aldehydes and ketones, along with alcohols are more effective in soot suppression than esters and ethers. Effect of Oxygen Functional Groups. To confirm that the reduction in opacity due to an oxygenated fuel depends not only on the oxygen content but also on the functional group present in the fuel, Figure 3a shows the results of normalized opacity obtained during engine tests for biodiesel and ethanol blends as a function of the oxygen percentage in the blend. Equations 12 and 13 show the normalized correlations obtained in this work. OpB ¼ e0:0642 3 %O OpD
R 2 ¼ 0:9887
ð12Þ
OpE ¼ e0:1268 3 %O OpD
R 2 ¼ 0:9464
ð13Þ
As Figure 3a shows, for the same oxygen content, the exhaust opacity of a diesel engine is greater when fuelled with a biodiesel diesel blend instead of an ethanoldiesel blend. Combining eqs 12 and 13 expresses this difference by eq 14: OpB ¼ e0:0626 3 %O OpE
The same trend is obtained using the normalized MW/SP ratio, as shown in Figure 3b and setting eqs 14 and 15, whose exponential coefficients are very similar to those obtained for opacity (see eqs 12 and 13), especially coefficients for ethanoldiesel blends. MW SP B ¼ e0:0739 3 %O R 2 ¼ 0:8822 ð14Þ MW SP D MW SP E ¼ e0:1243 3 %O MW SP D
Figure 2. Normalized variation (with respect to reference diesel fuel) of opacity and MW/SP ratio with the % v/v of ethanol.
ð13Þ
R 2 ¼ 0:9868
ð15Þ
According to these results, for a given oxygen percentage, a ternary mixture of ethanolbiodieseldiesel should present a
Figure 3. (a) Normalized variation (with respect to reference diesel fuel) of opacity with the oxygen content in the blend. (b) Normalized variation of MW/SP ratio. 3287
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Energy & Fuels normalized MW/SP ratio between those obtained for the binary biodieseldiesel and ethanoldiesel blends. Equation 16, also depicted in Figure 3b, shows how the coefficient of the exponential fit is between those obtained with binary mixtures. Although exhaust opacity tests were not carried out with biodieseldiesel blends, as suggested by results shown in this work, it is possible to predict that the normalized opacity produced by them should be intermediate between binary blends. MW SP EBD ¼ e0:1016 3 %O R 2 ¼ 0:9705 ð16Þ MW SP D
’ CONCLUSIONS The smoke point is a parameter that indicates the sooting tendency of fuels and also provides a good indication of the smoke opacity produced by combustion of diffusion flames. This work has analyzed how this parameter, in particular the molecular weight to smoke point ratio, is also applicable for estimating the opacity of biodiesel and ethanoldiesel blends in a reference diesel fuel. Results showed that the opacity and MW/SP ratio follow an exponential decrease with the percentage of biodiesel or ethanol in the blend due to an increase in the oxygen content. Moreover, a sharper decrease was observed when ethanoldiesel blends were used as fuel. Work also confirmed how not only the increase in the oxygen content in the fuel blend but also the oxygen functional group influence the decrease in exhaust opacity and MW/SP ratio, with the alcohol group of ethanol being more effective than the ester of biodiesel. Therefore, the smoke point technique results in a very useful tool for, on one hand, comparing different fuels and, on the other hand, evaluating the reduction in diesel exhaust opacity due exclusively to the fuel used in the engine. ’ ACKNOWLEDGMENT We acknowledge the University of Castilla—La Mancha for the grant provided to O.A. and M.A.G. for staying at the EMS Energy Institute of Pennsylvania State University. Dr. García-Contreras is also acknowledged for his support on this work. ’ REFERENCES (1) Liotta, F. J.; Montalvo, D. M. The Effect of Oxygenated Fuels on Emissions from Modern Heavy-Duty Diesel Engine. In: Diesel fuels for the nineties : Composition and Additives to Meet Emissions and Performance Needs. doi: 10.4271/932734. Fuels and Lubricants Meeting and Exposition, October 1821,1993, Philadelphia, Pennsylvania. (2) Miyamoto, N.; Ogawa, H.; Nurm, N. M.; Obata, K.; Arima, T. Smokeless, low NOx, high thermal efficiency, and low noise diesel combustion with oxygenated agents as main fuel. Combustion Processes in Diesel Engines, SP-1328, doi: 10.4271/980506. International Congress & Exposition, February 1998, Detroit, MI, Session: Diesel Engine Combustion Processes. (3) Tree, D. R.; Svensson, K. I. Soot processes in compression ignition engines. Prog. Energy Combust. Sci. 2007, 33, 272–309. (4) Agarwal, A. K. Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines. Prog. Energy Combust. Sci. 2007, 33, 233–271. (5) Hansen, A. C.; Zhang, Q.; Lyne, P. W. L. Ethanoldiesel fuel blendsa review. Bioresour. Technol. 2005, 96, 277–285.
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