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diesel fuel blends in a compression ignition engine: Preliminary exergy analysis
Energy Conversion and Management 85 (2014) 227–233
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effect of the use of olive–pomace oil biodiesel/diesel fuel blends in a compression ignition engine: Preliminary exergy analysis I. López a, C.E. Quintana b, J.J. Ruiz c, F. Cruz-Peragón d, M.P. Dorado c,⇑ a
Center of Energy Studies and Environmental Technologies (CEETA), Universidad Central de Las Villas, Villa Clara, Cuba Technological Institute of Santo Domingo, Dominican Republic c Dep. of Physical Chemistry and Applied Thermodynamics, Edificio Leonardo da Vinci, Campus de Rabanales, Universidad de Cordoba, Campus de Excelencia Internacional Agroalimentario, ceiA3, Spain d Dep. of Mechanics and Mining Engineering, EPS, Universidad de Jaen, Campus Las Lagunillas s/n, 23071 Jaen, Spain b
a r t i c l e
i n f o
Article history: Received 21 March 2014 Accepted 23 May 2014 Available online 13 June 2014 Keywords: Diesel engine Engine performance Exergy analysis Biofuel Residue recycling
a b s t r a c t Although biodiesel is among the most studied biofuels for diesel engines, it is usually produced from edible oils, which gives way to controversy between the use of land for fuel and food. For this reason, residues like olive–pomace oil are considered alternative raw materials to produce biodiesel that do not compete with the food industry. To gain knowledge about the implications of its use, olive–pomace oil methyl ester, straight and blended with diesel fuel, was evaluated as fuel in a direct injection diesel engine Perkins AD 3-152 and compared to the use of fossil diesel fuel. Performance curves were analyzed at full load and different speed settings. To perform the exergy balance of the tested fuels, the operating conditions corresponding to maximum engine power values were considered. It was found that the tested fuels offer similar performance parameters. When straight biodiesel was used instead of diesel fuel, maximum engine power decreased to 5.6%, while fuel consumption increased up to 7%. However, taking into consideration the Second Law of the Thermodynamics, the exergy efficiency and unitary exergetic cost reached during the operation of the engine under maximum power condition for the assessed fuels do not display significant differences. Based on the exergy results, it may be concluded that olive– pomace oil biodiesel and its blends with diesel fuel may substitute the use of diesel fuel in compression ignition engines without any exergy cost increment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction There are numerous research works focused on the evaluation of internal combustion engines (ICE) running on different biofuels, many of them limited to the determination of engine operating parameters to establish a comparison between diesel fuel and biodiesel [1–3] and vegetable oils [4], while other authors have assessed the influence of fuel properties on engine behavior [5]. Based on experimental tests, some researchers have performed energy analysis, namely thermal balance, applying the First Law of the Thermodynamics, to study the behavior of the engine running on different fuels [6–9]. But when Second Law of the Thermodynamics is taken into consideration, the conventional energy analysis is enriched with the calculation of the true thermodynamic value of usable energy, as thermodynamic inefficiencies and process losses are stated [10].
⇑ Corresponding author. Tel.: +34 957 218332; fax: +34 957 218417. E-mail address: [email protected] (M.P. Dorado). http://dx.doi.org/10.1016/j.enconman.2014.05.084 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
The concept of exergy is extremely useful for this purpose, as the main advantage of the exergy analysis consists on the possibility of determining the value of the irreversibilities associated to the process. In this sense, this analysis reveals the destruction of exergy in different systems and associated exergy efficiency of the processes [11,12]. An exergy analysis identifies the components of a system that generate major destruction of exergy, besides the processes that cause it. Although this research line has been applied to in-cylinder processes [13–15], nowadays, studies are focused on engine operation under different applications and technical modifications that can alter their operation when different fuels (including those of renewable nature) are used. In this sense, Caton [16] studied the implications of the use of several alcohols, carbon monoxide and hydrogen on a spark-ignition engine. Although the thermal efficiency was alike for all fuels considering the same operating conditions, the Second Law showed different destruction of exergy during the combustion process between the fuels [8,16]. Considering spark ignition engines, Sezer and Bilgin [17] stated that peak values for Second-Law efficiencies are around 0.9
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equivalence ratios and beyond this value there is no further contribution to work output. As destruction of exergy is always present, thermoeconomics quantification is of special interest. Temir and Bilge [18] applied exergy and thermoeconomic analyses to a gas ICE-based tri-generation installation used for the production of electric power, heat and cool. Using this method, system exergies and exergy destruction, in addition to the investment and operational costs allowed the determination of the corresponding thermoeconomics costs. A similar study was developed by Abusoglu and Kanoglu [19] in an ICE-based co-generation system. Other works have been focused on the evaluation of ICE operated with different biofuels and the comparison with fossil fuel at different experimental conditions. Canakci and Hosoz [20] performed energy and exergy analyses of a turbocharged 4-cylinder diesel engine running on two types of biodiesel, diesel fuel and their mixtures. Results under both First and Second Laws of the Thermodynamics points of view showed similar behavior between biodiesel and diesel fuel. Azoumah et al. [21] evaluated the operation of an ICE using straight cotton and palm oils and their blends with diesel fuel. Exergy analysis helped to determine the optimum engine load considering each alternative fuel. Also, fundamental parameters of engine combustion and heat transfer process can be found from the Second Law of the Thermodynamics approach (including losses and exergy destruction). This procedure takes into consideration the exergy of the fuel, based on its elementary composition. In this sense, two fuels depicting same Low Heating Value (LHV), but showing differences in composition, may show variations in their chemical exergy. Moreover, biodiesel properties, i.e. cetane number and LHV fluctuate with the oil fatty acid composition, influencing both combustion process and engine performance. Other properties, like viscosity and density, may modify the fuel–air mixture formation into the cylinder, thus influencing combustion efficiency. In sum, although biodiesel must be forced to meet international quality standards, exergy evaluation of the fuel through the engine is also needed. Exergy analysis provides the tool to evaluate the influence of the fuel. Refined olive oil is dehydrated in special ovens that may be working at temperatures above 1000 °C, which causes the appearance of benzopyrenes in large quantities. This is the case of olive– pomace oil. The International Agency for Research on Cancer (IARC) classifies benzopyrenes like substances probably carcinogenic to humans. To avoid benzopyrenes formation, thus converting olive–pomace oil in edible oil, the process must follow a strict vacuum and use specific absorbent materials, which makes more expensive olive–pomace oil extraction. With the expected reduction of olive oil price, linked to the future reduction of subsidies, it can be economically converted into potential oil for biodiesel production. Besides, it has a high content of oleic acid that presents excellent characteristics with regard to fuel ignition quality and fuel stability, according to the standard EN 14214 [22]. Although, many vegetable oils are used for biodiesel production [23], the use of biodiesel from olive–pomace oil (also known as orujo) as fuel for compression ignition engines from the exergy point of view, has not been studied. In the present work, to gain knowledge about the implications of its use, the behavior of a diesel engine running on olive–pomace oil biodiesel, diesel fuel and their mixtures, has been evaluated from the exergy point of view. ICE constitutes a thermal system in which exergy analysis included in the thermodynamic study of the engine operation may provide interesting information referred to the evaluation of different alternative fuels. The quantification of exergy efficiency for each alternative is considered key parameter for the selection of the most viable fuel [24].
2. Materials and methods 2.1. 1. Fuels description Oil was purchased from KOIPE (SOS Cuétara, Madrid, Spain). Properties are depicted in Table 1. Reaction conditions to produce biodiesel were selected from previous kinetic studies [25,26]. Olive–pomace oil transesterification was performed in a stirred tank reactor at 60 °C using a solution of 1.2% KOH and 30% methanol (wt reagent/wt oil), equivalent to 8.6:1 (molar ratio), after 40 min of vigorous stirring. Reaction was then stopped and settled to decant. To remove the alcohol and catalyst residues from biodiesel, the ester phase was washed with the aid of distilled water [27]. Fatty acid methyl ester (FAME) conversion was analyzed using a gas chromatograph equipped with flame ionization detector (GC– FID) model Clarus 500 from Perkin–Elmer (Shelton, Connecticut, USA) and following the UNE EN 14103 standard. A SGE capillary column, 30 m length, 0.32 mm inner diameter and 0.25 lm film, maximum temperature 250 °C was used. Fatty acid composition besides some properties of biodiesel and generic No. 2 diesel fuel are shown in Table 2. Olive pomace oil methyl ester (B100) and its blends with diesel fuel, i.e. 20% biodiesel/80% diesel fuel (B20), 50% biodiesel/50% diesel fuel (B50) and 80% biodiesel/20% diesel fuel (B80) blends, were used to carry out performance tests in a diesel engine. Results were compared to those obtained by the use of straight diesel fuel in the same engine. 2.2. Tests equipment The fuel tests were performed in a 2500 cm3, three cylinder, four-stroke, water-cooled, 18.5:1 compression ratio, direct injection diesel engine Perkins AD 3-152. The maximum torque was 162.8 N m at 1300 min1 and the maximum engine power was 34 kW at 2250 min1 (DIN 6270-A). The engine was reconditioned to original specifications. The injection type was a DPA–CAV distributor and the injection pressure was 18.74 MPa. The engine dynamometer was an electric Froment testing device (model XT200), with maximum engine power of 136 kW and ±1.44 kW of accuracy at 100% of the engine speed (reported by the National Institute of Agricultural Engineering, UK) as described in [28,29]. The fuel was metered by a positive displacement gear type sensor, by means of a Froment Electronic Fuel Flow Monitor (FM502), placed in the fuel line between the tank and the engine fuel filter. The return fuel from the engine was circulated back into the engine supply line, as described in [28,29]. The engine speed was measured by the Froment testing device and monitored electronically to the nearest 5 min1. Atmospheric conditions were collected to correct the brake specific fuel consumption and engine power, following the SAE standard J1349 (revised August 2004). 2.3. Engine performance tests The performance curves were conducted at maximum load and speed settings. A first baseline test was run with straight No. 2
Table 1 Olive–pomace oil properties. Property
Unit
Olive–pomace oil
Density (15 °C) Kinematic viscosity (40 °C) Acid value Iodine number Gross heating value
kg/m3 mm2/s mg KOH/g g I2/100 g MJ/kg
908 46.27 0.6 99.8 40.49
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I. López et al. / Energy Conversion and Management 85 (2014) 227–233 Table 2 Fuel properties of olive–pomace oil biodiesel and diesel fuel. Property
Unit
FAME Monoglyceride content Diglyceride content Triglyceride content Glycerol Density (15 °C) Viscosity (40 °C) Flash point Conradson carbon residue Cetane number Water content Copper strip corrosion Oxidation stability (110 °C) Acid value Iodine number Cold filter plugging point Cold point Pour point Gross heating value
% (w/w) % (w/w) % (w/w) % (w/w) % (w/w) kg/m3 mm2/s °C % (w/w) – mg/kg – h mg KOH/g g I2/100 g °C °C °C MJ/kg
Fatty acid composition Palmitic (C16:0) Palmitoleic (C16:1) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3)
% % % % % %
Diesel fuel standard (En 590)
820–845 2.00–4.50 >55 <0.30 >51.0 <200 Class 1
Diesel fuel
Biodiesel standard EN 14214
Olive–pomace biodiesel
827.1 2.47 70.2 0.04 53.9
>96.5 <0.8 <0.2 <0.2 <0.5 860–900 3.50–5.00 >120 <0.30 >51.0 <500 Class 1 >6.00 <0.50 <120
97.72 0.47 0.16 0.19 0.013 876 4.53 138 1.38 58.7 <50 1a 15.5 0.1 78 9 2 6 39.96
1a
(w/w) (w/w) (w/w) (w/w) (w/w) (w/w)
diesel fuel at the beginning, followed by the rest of the fuels, in order to compare engine performance with the fuels and to determine whether the use of biodiesel and its blends had affected the engine performance. Each test consisted of measurements of speed n (min1), fuel consumption cf (kg/m3), torque M (N m), room temperature Tr (K) and room pressure Pr (kPa). 2.4. Exergy analysis To perform this analysis, the main flows associated to an ICE need to be identified (Fig. 1). The input streams correspond to _ a (kg/s) and fuel, m _ f (kg/s), which were meathe mass flow of air, m sured with a flow meter. The outputs are the engine power developed in the shaft, N _ g (kg/s), the heat trans(kW), the mass flow of exhaust gases, m ferred to the coolant, Q_ ref (kW) and the heat transferred to the environment, Q_ e (kW). For the exergy balance, flow temperature values were taken into consideration.
11 0.8 3 75.2 7.2 0.5
The associated heat flows transferred to the cooling liquid and exhaust gases have been included in the exergy analysis by many authors, mainly in co-generation applications [21,30]. However, considering small engines, like vehicle engines or tractor engines like Perkins AD 3-152, which operate at different speeds, the possibility of cogeneration is reduced due to the small output power and the non-uniform operating conditions. For this reason, in the experimental conditions of this study, the only valuable output is the mechanical work developed by the engine, therefore the rest of the flows are considered as losses. According to this statement, the mathematical expression of the exergetic balance is as follows (1):
E_ f þ E_ a ¼ N þ E_ l þ E_ d
ð1Þ
where E_ f is the exergy flow of the fuel (kW), E_ a is the exergy flow of the intake air (kW), E_ d is the destroyed exergy flow (kW) and E_ l is the exergy flow associated to the heat transferred to the cooling liquid, exhaust gases and environment, as follows (2):
E_ l þ E_ g þ E_ e þ E_ ref
ð2Þ
where E_ g is the exergy flow of the exhaust gases (kW), E_ e is the exergy flow of the heat transferred to the environment (kW) and E_ ref is the exergy flow of the cooling liquid (kW). The exergy of the fuel (Ef) is the resultant of the sum of its physical (ef) and chemical (ech) components, as shown in expression (3):
Ef ¼ ef þ ech
Fig. 1. Main flows associated to an ICE.
ð3Þ
Each component depends on its thermodynamic state referred to the dead (reference) state. According to this and considering that the fuel is introduced into the engine under both room pressure and room temperature, the physical component of its exergy is not considered. So, only the chemical exergy is taken into consideration. For this propose, High Heating Value (HHV) of each fuel was calculated in an IKA C 200 calorimeter and was used to calculate LHV, taking into account the vaporization heat of the water that appears during the combustion reaction. The fuel chemical exergy is calculated following the expression developed by Szargurt and Styrylska, mentioned in Rodríguez [31]
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for liquid fuels, where H, C, O and S correspond to the mass fraction of each element, as follows in expression (4). The mass fraction of H, C, O and S was determined considering the fatty acid composition of olive–pomace oil methyl ester (Table 2) and assuming a conversion (FAME) value of 100%.
H O _ f LHV 1:0374 þ 0:0159 þ 0:0567 E_ f ¼ m C C S H 1 0:1737 þ0:5965 C C
ð4Þ
The air is introduced in the engine during the admission, at room conditions, thus the thermomechanical and chemical exergies are insignificant with respect to the exergy of the fuels; so the exergy flow is considered to be zero. Considering the power developed by the engine (N) as the sole valuable output of this system, the exergetic efficiency (n) may be calculated by means of the chemical exergy of each fuel (Ef), as follows (5):
n¼
N E_ f
ð5Þ
The inverse of the exergetic efficiency provides interesting information when different fuels are compared. Considering that the unitary exergetic cost (c) represents the amount of minimum exergy resources that the equipment requires to produce one exergy unit of product [32], the unitary exergetic cost of an ICE using different fuels is calculated according to the following expression (6):
C¼
E_ f N
ð6Þ
3. Results and discussion Table 2 shows that biodiesel from olive–pomace oil meets main properties of European biodiesel standard EN 14214, exception made of Conradson carbon residue. This property provides an indication of the relative carbon forming propensity of the fuel. On the contrary, cold weather behavior properties, oxidation stability, viscosity and cetane number values show good results. This is due to its high content on oleic acid and is in agreement with previous results [22]. According to Fig. 2, it may be observed that the engine power (N) exhibits very similar values for all tested fuels. The slight maximum power loss (up to 6%) when biodiesel is used is related to the lower LHV and the higher viscosity of biodiesel, compared to
diesel fuel values, which lead to low released heat and worse combustion, respectively [28]. In any case, the ANOVA test showed that engine power differences between each tested fuel were not significant. Brake-specific fuel consumption (BSFC) shows the opposite trend, as depicted in Fig. 3. The maximum increase in BSFC (16%) when biodiesel and its mixtures are used is related to both the higher biodiesel density and the lower biodiesel LHV compared to diesel fuel values. In other words, considering the same volumetric flow of fuel supplied by the injection pump, the mass flow during the operation of the engine at the same speed is increased. Moreover, the ANOVA test showed that the increase in BSFC was inversely proportional to the heating value. In terms of exergy analysis of the engine, the performance values for the tested fuels at full load under different speed settings have been considered. In addition, the fuel consumption per hour ch (kg/h) allows the calculation of the exergy flow. Fig. 4 exhibits the exergy flow of the fuel during the engine operation at full load and different speed values. As may be seen, although this parameter should depict a similar trend to that of fuel consumption per hour, as it depends on it, when the content of biodiesel in the blend is increased, the higher the fuel consumption the (slightly) lower the exergy flow. This is due to the fuel chemical composition and lower calorific value, according to equation (4). Fig. 5 shows the trend depicted by the exergy efficiency at the same engine operating conditions. This parameter behaves in a similar way for the different tested fuels, maybe due to the lower exergy destruction during engine operation when biodiesel is used. It may be seen that the chemical composition of the studied biodiesel blends leads to an improvement of the exergy efficiency of the engine, despite a slight increase in fuel consumption. The behavior of unitary exergetic cost is depicted in Fig. 6. As expected, it shows an opposite trend to Second Law efficiency. Fig. 7 summarizes the percentage of variation between fuels considering the studied parameters, at full load. As may be seen, the maximum power values vary from 28.1 kW to 26.41 kW when diesel fuel and B100 are used, respectively. Considering BSFC, it can be seen that it varies from 327.4 g/kW h (diesel fuel) to 386.7 g/ kW h (B100). According to biodiesel blends, they follow the same previous trends. In this sense, with the addition of biodiesel to the blend, engine power decreases while BSFC increases. In spite of engine power and fuel consumption differences when biodiesel, biodiesel/diesel fuel blends and straight diesel fuel are used, the ICE exergy efficiency exhibits similar values no matter
Fig. 2. Engine power vs. speed provided by the tested fuels at full load and different speed settings.
I. López et al. / Energy Conversion and Management 85 (2014) 227–233
f
Fig. 3. Brake-specific fuel consumption vs. speed provided by the tested fuels at full load and different speed settings.
(%)
Fig. 4. Exergy flow of the fuel vs. speed provided by the tested fuels at full load and different speed settings.
Fig. 5. Exergy efficiency vs. speed provided by the tested fuels at full load and different speed settings.
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Fig. 6. Unitary exergetic cost vs. speed provided by the tested fuels at full load and different speed settings.
C (%)
BSFC (g/kWh)
N (kW)
Diesel fuel
B20
B50
B80
B100
Fig. 7. Differences between the studied fuels considering main parameters during the engine operation at full load.
which fuel is used. The presence of oxygen in biodiesel, which is taken into consideration during the exergy flow analysis, enhances the formation of the mixture fuel–air inside the cylinder and provides better chemical combustion efficiency [33–35]. Thus, it contributes to similar exergy efficiency for the different fuels under study. In this sense, the unitary exergetic cost depicts a similar trend for the studied fuels, meaning that the production of one unit of exergy of mechanical work requires around four units of exergy resources. Mechanical work is the principal product of this study. This result is in agreement with the similar exergy efficiency value showed by the engine (approximately 25%). The slight differences reported for exergy efficiency and unitary exergetic cost between diesel fuel and the rest of the tested fuels are mainly due to the design of the fuel supply system of the engine and the combustion chamber, that suits the main properties of diesel fuel, i.e. viscosity, density, surface tension and other parameters related to air–fuel mixture formation and combustion process. This design enhances the combustion process while reducing the exergy destruction, when diesel fuel is used.
4. Conclusions When olive–pomace oil biodiesel and its mixtures with diesel fuel are used, a slight fuel consumption increase, besides a non-significant reduction of the engine power during the operation of the ICE were achieved. The differences with diesel fuel become more evident when the amount of biodiesel in the blend increases. Although considering BSFC, statistical tests show no significant differences between the tested fuels. On the other hand, the higher the concentration of biodiesel in the mixture, the lower the exergy flow under maximum engine power. In any case, the reduction of the value of the exergy flow experienced by the tested fuels is minimal. Moreover, the exergy efficiency and unitary exergetic cost reached during the operation of the engine under the condition of maximum power do not display significant differences for the assessed fuels. Based on these results, it can be concluded that olive–pomace oil biodiesel and its blends with diesel fuel may substitute the use of diesel fuel in compression ignition engines providing a renewable fuel with similar exergy efficiency and without any exergetic cost increment.
I. López et al. / Energy Conversion and Management 85 (2014) 227–233
Acknowledgments This research was supported by the Spanish Ministry of Education and Science (ENE2010-15159) and the Andalusian Research, Innovation and Enterprise Council, Spain (TEP-4994). Authors are also grateful for the provision of a research mobility grant to I. López from AECID, program II-A and Escuela Internacional de Doctorado en Agroalimentación eidA3, ceiA3, Spain. References [1] Rahman SMA, Masjuki HH, Kalam MA, Abedin MJ, Sanjid A, Sajjad H. Production of palm and Calophyllum inophyllum based biodiesel and investigation of blend performance and exhaust emission in an unmodified diesel engine at high idling conditions. Energy Convers Manage 2013; 76:362–7. [2] Silitonga AS, Masjuki HH, Mahlia TMI, Ong HC, Chong WT. Experimental study on performance and exhaust emissions of a diesel engine fuelled with Ceiba pentandra biodiesel blends. Energy Convers Manage 2013;76:828–36. [3] Dorado MP, Ballesteros E, Arnal JM, Gomez J, Gimenez FJL. Testing waste olive oil methyl ester as a fuel in a diesel engine. Energy Fuels 2003;17:1560–5. [4] Kruczyn´ski SW. Performance and emission of CI engine fuelled with camelina sativa oil. Energy Convers Manage 2013;65:1–6. [5] Ciubota-Rosie C, Ruiz JR, Ramos MJ, Pérez Á. Biodiesel from Camelina sativa: a comprehensive characterisation. Fuel 2013;105:572–7. [6] Bhattacharyya S. Optimizing an irreversible Diesel cycle – fine tuning of compression ratio and cut-off ratio. Energy Convers Manage 2000;41:847–54. [7] Rakopoulos CD, Mavropoulos GC, Hountalas DT. Measurements and analysis of load and speed effects on the instantaneous wall heat fluxes in a direct injection air-cooled diesel engine. Int J Energy Res 2000;24:587–604. [8] Durgun O, Sßahin Z. Theoretical investigation of heat balance in direct injection (DI) diesel engines for neat diesel fuel and gasoline fumigation. Energy Convers Manage 2009;50:43–51. [9] Ajav EA, Singh B, Bhattacharya TK. Thermal balance of a single cylinder diesel engine operating on alternative fuels. Energy Convers Manage 2000; 41:1533–41. [10] Kotas TJ. The exergy method of thermal plant analysis. Krieger Publishing Company; 1995. [11] Cziesla F, Tsatsaronis G, Gao ZL. Avoidable thermodynamic inefficiencies and costs in an externally fired combined cycle power plant. Energy 2006;31:1472–89. [12] Meyer L, Tsatsaronis G, Buchgeister J, Schebek L. Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems. Energy 2009;34:75–89. [13] Foster D. An overview of zero-dimensional thermodynamic models for IC engine data analysis. SAE paper 852070 Estados Unidos; 1985. [14] Shapiro H, Van Gerpen J. Two zone combustion models for Second Law analysis of internal combustion engines. SAE paper 890823 Estados Unidos; 1989. [15] Anderson M, Assanis D, Filipi Z. First and Second Law analyses of a naturallyaspirated, Miller cycle, SI engine with late intake valve closure. SAE paper 980889 USA; 1998.
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Effect of the use of olive–pomace oil biodiesel/diesel fuel blends in a compression ignition engine: Preliminary exergy analysis I. López a, C.E. Quintana b, J.J. Ruiz c, F. Cruz-Peragón d, M.P. Dorado c,⇑ a
Center of Energy Studies and Environmental Technologies (CEETA), Universidad Central de Las Villas, Villa Clara, Cuba Technological Institute of Santo Domingo, Dominican Republic c Dep. of Physical Chemistry and Applied Thermodynamics, Edificio Leonardo da Vinci, Campus de Rabanales, Universidad de Cordoba, Campus de Excelencia Internacional Agroalimentario, ceiA3, Spain d Dep. of Mechanics and Mining Engineering, EPS, Universidad de Jaen, Campus Las Lagunillas s/n, 23071 Jaen, Spain b
a r t i c l e
i n f o
Article history: Received 21 March 2014 Accepted 23 May 2014 Available online 13 June 2014 Keywords: Diesel engine Engine performance Exergy analysis Biofuel Residue recycling
a b s t r a c t Although biodiesel is among the most studied biofuels for diesel engines, it is usually produced from edible oils, which gives way to controversy between the use of land for fuel and food. For this reason, residues like olive–pomace oil are considered alternative raw materials to produce biodiesel that do not compete with the food industry. To gain knowledge about the implications of its use, olive–pomace oil methyl ester, straight and blended with diesel fuel, was evaluated as fuel in a direct injection diesel engine Perkins AD 3-152 and compared to the use of fossil diesel fuel. Performance curves were analyzed at full load and different speed settings. To perform the exergy balance of the tested fuels, the operating conditions corresponding to maximum engine power values were considered. It was found that the tested fuels offer similar performance parameters. When straight biodiesel was used instead of diesel fuel, maximum engine power decreased to 5.6%, while fuel consumption increased up to 7%. However, taking into consideration the Second Law of the Thermodynamics, the exergy efficiency and unitary exergetic cost reached during the operation of the engine under maximum power condition for the assessed fuels do not display significant differences. Based on the exergy results, it may be concluded that olive– pomace oil biodiesel and its blends with diesel fuel may substitute the use of diesel fuel in compression ignition engines without any exergy cost increment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction There are numerous research works focused on the evaluation of internal combustion engines (ICE) running on different biofuels, many of them limited to the determination of engine operating parameters to establish a comparison between diesel fuel and biodiesel [1–3] and vegetable oils [4], while other authors have assessed the influence of fuel properties on engine behavior [5]. Based on experimental tests, some researchers have performed energy analysis, namely thermal balance, applying the First Law of the Thermodynamics, to study the behavior of the engine running on different fuels [6–9]. But when Second Law of the Thermodynamics is taken into consideration, the conventional energy analysis is enriched with the calculation of the true thermodynamic value of usable energy, as thermodynamic inefficiencies and process losses are stated [10].
⇑ Corresponding author. Tel.: +34 957 218332; fax: +34 957 218417. E-mail address: [email protected] (M.P. Dorado). http://dx.doi.org/10.1016/j.enconman.2014.05.084 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
The concept of exergy is extremely useful for this purpose, as the main advantage of the exergy analysis consists on the possibility of determining the value of the irreversibilities associated to the process. In this sense, this analysis reveals the destruction of exergy in different systems and associated exergy efficiency of the processes [11,12]. An exergy analysis identifies the components of a system that generate major destruction of exergy, besides the processes that cause it. Although this research line has been applied to in-cylinder processes [13–15], nowadays, studies are focused on engine operation under different applications and technical modifications that can alter their operation when different fuels (including those of renewable nature) are used. In this sense, Caton [16] studied the implications of the use of several alcohols, carbon monoxide and hydrogen on a spark-ignition engine. Although the thermal efficiency was alike for all fuels considering the same operating conditions, the Second Law showed different destruction of exergy during the combustion process between the fuels [8,16]. Considering spark ignition engines, Sezer and Bilgin [17] stated that peak values for Second-Law efficiencies are around 0.9
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equivalence ratios and beyond this value there is no further contribution to work output. As destruction of exergy is always present, thermoeconomics quantification is of special interest. Temir and Bilge [18] applied exergy and thermoeconomic analyses to a gas ICE-based tri-generation installation used for the production of electric power, heat and cool. Using this method, system exergies and exergy destruction, in addition to the investment and operational costs allowed the determination of the corresponding thermoeconomics costs. A similar study was developed by Abusoglu and Kanoglu [19] in an ICE-based co-generation system. Other works have been focused on the evaluation of ICE operated with different biofuels and the comparison with fossil fuel at different experimental conditions. Canakci and Hosoz [20] performed energy and exergy analyses of a turbocharged 4-cylinder diesel engine running on two types of biodiesel, diesel fuel and their mixtures. Results under both First and Second Laws of the Thermodynamics points of view showed similar behavior between biodiesel and diesel fuel. Azoumah et al. [21] evaluated the operation of an ICE using straight cotton and palm oils and their blends with diesel fuel. Exergy analysis helped to determine the optimum engine load considering each alternative fuel. Also, fundamental parameters of engine combustion and heat transfer process can be found from the Second Law of the Thermodynamics approach (including losses and exergy destruction). This procedure takes into consideration the exergy of the fuel, based on its elementary composition. In this sense, two fuels depicting same Low Heating Value (LHV), but showing differences in composition, may show variations in their chemical exergy. Moreover, biodiesel properties, i.e. cetane number and LHV fluctuate with the oil fatty acid composition, influencing both combustion process and engine performance. Other properties, like viscosity and density, may modify the fuel–air mixture formation into the cylinder, thus influencing combustion efficiency. In sum, although biodiesel must be forced to meet international quality standards, exergy evaluation of the fuel through the engine is also needed. Exergy analysis provides the tool to evaluate the influence of the fuel. Refined olive oil is dehydrated in special ovens that may be working at temperatures above 1000 °C, which causes the appearance of benzopyrenes in large quantities. This is the case of olive– pomace oil. The International Agency for Research on Cancer (IARC) classifies benzopyrenes like substances probably carcinogenic to humans. To avoid benzopyrenes formation, thus converting olive–pomace oil in edible oil, the process must follow a strict vacuum and use specific absorbent materials, which makes more expensive olive–pomace oil extraction. With the expected reduction of olive oil price, linked to the future reduction of subsidies, it can be economically converted into potential oil for biodiesel production. Besides, it has a high content of oleic acid that presents excellent characteristics with regard to fuel ignition quality and fuel stability, according to the standard EN 14214 [22]. Although, many vegetable oils are used for biodiesel production [23], the use of biodiesel from olive–pomace oil (also known as orujo) as fuel for compression ignition engines from the exergy point of view, has not been studied. In the present work, to gain knowledge about the implications of its use, the behavior of a diesel engine running on olive–pomace oil biodiesel, diesel fuel and their mixtures, has been evaluated from the exergy point of view. ICE constitutes a thermal system in which exergy analysis included in the thermodynamic study of the engine operation may provide interesting information referred to the evaluation of different alternative fuels. The quantification of exergy efficiency for each alternative is considered key parameter for the selection of the most viable fuel [24].
2. Materials and methods 2.1. 1. Fuels description Oil was purchased from KOIPE (SOS Cuétara, Madrid, Spain). Properties are depicted in Table 1. Reaction conditions to produce biodiesel were selected from previous kinetic studies [25,26]. Olive–pomace oil transesterification was performed in a stirred tank reactor at 60 °C using a solution of 1.2% KOH and 30% methanol (wt reagent/wt oil), equivalent to 8.6:1 (molar ratio), after 40 min of vigorous stirring. Reaction was then stopped and settled to decant. To remove the alcohol and catalyst residues from biodiesel, the ester phase was washed with the aid of distilled water [27]. Fatty acid methyl ester (FAME) conversion was analyzed using a gas chromatograph equipped with flame ionization detector (GC– FID) model Clarus 500 from Perkin–Elmer (Shelton, Connecticut, USA) and following the UNE EN 14103 standard. A SGE capillary column, 30 m length, 0.32 mm inner diameter and 0.25 lm film, maximum temperature 250 °C was used. Fatty acid composition besides some properties of biodiesel and generic No. 2 diesel fuel are shown in Table 2. Olive pomace oil methyl ester (B100) and its blends with diesel fuel, i.e. 20% biodiesel/80% diesel fuel (B20), 50% biodiesel/50% diesel fuel (B50) and 80% biodiesel/20% diesel fuel (B80) blends, were used to carry out performance tests in a diesel engine. Results were compared to those obtained by the use of straight diesel fuel in the same engine. 2.2. Tests equipment The fuel tests were performed in a 2500 cm3, three cylinder, four-stroke, water-cooled, 18.5:1 compression ratio, direct injection diesel engine Perkins AD 3-152. The maximum torque was 162.8 N m at 1300 min1 and the maximum engine power was 34 kW at 2250 min1 (DIN 6270-A). The engine was reconditioned to original specifications. The injection type was a DPA–CAV distributor and the injection pressure was 18.74 MPa. The engine dynamometer was an electric Froment testing device (model XT200), with maximum engine power of 136 kW and ±1.44 kW of accuracy at 100% of the engine speed (reported by the National Institute of Agricultural Engineering, UK) as described in [28,29]. The fuel was metered by a positive displacement gear type sensor, by means of a Froment Electronic Fuel Flow Monitor (FM502), placed in the fuel line between the tank and the engine fuel filter. The return fuel from the engine was circulated back into the engine supply line, as described in [28,29]. The engine speed was measured by the Froment testing device and monitored electronically to the nearest 5 min1. Atmospheric conditions were collected to correct the brake specific fuel consumption and engine power, following the SAE standard J1349 (revised August 2004). 2.3. Engine performance tests The performance curves were conducted at maximum load and speed settings. A first baseline test was run with straight No. 2
Table 1 Olive–pomace oil properties. Property
Unit
Olive–pomace oil
Density (15 °C) Kinematic viscosity (40 °C) Acid value Iodine number Gross heating value
kg/m3 mm2/s mg KOH/g g I2/100 g MJ/kg
908 46.27 0.6 99.8 40.49
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Unit
FAME Monoglyceride content Diglyceride content Triglyceride content Glycerol Density (15 °C) Viscosity (40 °C) Flash point Conradson carbon residue Cetane number Water content Copper strip corrosion Oxidation stability (110 °C) Acid value Iodine number Cold filter plugging point Cold point Pour point Gross heating value
% (w/w) % (w/w) % (w/w) % (w/w) % (w/w) kg/m3 mm2/s °C % (w/w) – mg/kg – h mg KOH/g g I2/100 g °C °C °C MJ/kg
Fatty acid composition Palmitic (C16:0) Palmitoleic (C16:1) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3)
% % % % % %
Diesel fuel standard (En 590)
820–845 2.00–4.50 >55 <0.30 >51.0 <200 Class 1
Diesel fuel
Biodiesel standard EN 14214
Olive–pomace biodiesel
827.1 2.47 70.2 0.04 53.9
>96.5 <0.8 <0.2 <0.2 <0.5 860–900 3.50–5.00 >120 <0.30 >51.0 <500 Class 1 >6.00 <0.50 <120
97.72 0.47 0.16 0.19 0.013 876 4.53 138 1.38 58.7 <50 1a 15.5 0.1 78 9 2 6 39.96
1a
(w/w) (w/w) (w/w) (w/w) (w/w) (w/w)
diesel fuel at the beginning, followed by the rest of the fuels, in order to compare engine performance with the fuels and to determine whether the use of biodiesel and its blends had affected the engine performance. Each test consisted of measurements of speed n (min1), fuel consumption cf (kg/m3), torque M (N m), room temperature Tr (K) and room pressure Pr (kPa). 2.4. Exergy analysis To perform this analysis, the main flows associated to an ICE need to be identified (Fig. 1). The input streams correspond to _ a (kg/s) and fuel, m _ f (kg/s), which were meathe mass flow of air, m sured with a flow meter. The outputs are the engine power developed in the shaft, N _ g (kg/s), the heat trans(kW), the mass flow of exhaust gases, m ferred to the coolant, Q_ ref (kW) and the heat transferred to the environment, Q_ e (kW). For the exergy balance, flow temperature values were taken into consideration.
11 0.8 3 75.2 7.2 0.5
The associated heat flows transferred to the cooling liquid and exhaust gases have been included in the exergy analysis by many authors, mainly in co-generation applications [21,30]. However, considering small engines, like vehicle engines or tractor engines like Perkins AD 3-152, which operate at different speeds, the possibility of cogeneration is reduced due to the small output power and the non-uniform operating conditions. For this reason, in the experimental conditions of this study, the only valuable output is the mechanical work developed by the engine, therefore the rest of the flows are considered as losses. According to this statement, the mathematical expression of the exergetic balance is as follows (1):
E_ f þ E_ a ¼ N þ E_ l þ E_ d
ð1Þ
where E_ f is the exergy flow of the fuel (kW), E_ a is the exergy flow of the intake air (kW), E_ d is the destroyed exergy flow (kW) and E_ l is the exergy flow associated to the heat transferred to the cooling liquid, exhaust gases and environment, as follows (2):
E_ l þ E_ g þ E_ e þ E_ ref
ð2Þ
where E_ g is the exergy flow of the exhaust gases (kW), E_ e is the exergy flow of the heat transferred to the environment (kW) and E_ ref is the exergy flow of the cooling liquid (kW). The exergy of the fuel (Ef) is the resultant of the sum of its physical (ef) and chemical (ech) components, as shown in expression (3):
Ef ¼ ef þ ech
Fig. 1. Main flows associated to an ICE.
ð3Þ
Each component depends on its thermodynamic state referred to the dead (reference) state. According to this and considering that the fuel is introduced into the engine under both room pressure and room temperature, the physical component of its exergy is not considered. So, only the chemical exergy is taken into consideration. For this propose, High Heating Value (HHV) of each fuel was calculated in an IKA C 200 calorimeter and was used to calculate LHV, taking into account the vaporization heat of the water that appears during the combustion reaction. The fuel chemical exergy is calculated following the expression developed by Szargurt and Styrylska, mentioned in Rodríguez [31]
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for liquid fuels, where H, C, O and S correspond to the mass fraction of each element, as follows in expression (4). The mass fraction of H, C, O and S was determined considering the fatty acid composition of olive–pomace oil methyl ester (Table 2) and assuming a conversion (FAME) value of 100%.
H O _ f LHV 1:0374 þ 0:0159 þ 0:0567 E_ f ¼ m C C S H 1 0:1737 þ0:5965 C C
ð4Þ
The air is introduced in the engine during the admission, at room conditions, thus the thermomechanical and chemical exergies are insignificant with respect to the exergy of the fuels; so the exergy flow is considered to be zero. Considering the power developed by the engine (N) as the sole valuable output of this system, the exergetic efficiency (n) may be calculated by means of the chemical exergy of each fuel (Ef), as follows (5):
n¼
N E_ f
ð5Þ
The inverse of the exergetic efficiency provides interesting information when different fuels are compared. Considering that the unitary exergetic cost (c) represents the amount of minimum exergy resources that the equipment requires to produce one exergy unit of product [32], the unitary exergetic cost of an ICE using different fuels is calculated according to the following expression (6):
C¼
E_ f N
ð6Þ
3. Results and discussion Table 2 shows that biodiesel from olive–pomace oil meets main properties of European biodiesel standard EN 14214, exception made of Conradson carbon residue. This property provides an indication of the relative carbon forming propensity of the fuel. On the contrary, cold weather behavior properties, oxidation stability, viscosity and cetane number values show good results. This is due to its high content on oleic acid and is in agreement with previous results [22]. According to Fig. 2, it may be observed that the engine power (N) exhibits very similar values for all tested fuels. The slight maximum power loss (up to 6%) when biodiesel is used is related to the lower LHV and the higher viscosity of biodiesel, compared to
diesel fuel values, which lead to low released heat and worse combustion, respectively [28]. In any case, the ANOVA test showed that engine power differences between each tested fuel were not significant. Brake-specific fuel consumption (BSFC) shows the opposite trend, as depicted in Fig. 3. The maximum increase in BSFC (16%) when biodiesel and its mixtures are used is related to both the higher biodiesel density and the lower biodiesel LHV compared to diesel fuel values. In other words, considering the same volumetric flow of fuel supplied by the injection pump, the mass flow during the operation of the engine at the same speed is increased. Moreover, the ANOVA test showed that the increase in BSFC was inversely proportional to the heating value. In terms of exergy analysis of the engine, the performance values for the tested fuels at full load under different speed settings have been considered. In addition, the fuel consumption per hour ch (kg/h) allows the calculation of the exergy flow. Fig. 4 exhibits the exergy flow of the fuel during the engine operation at full load and different speed values. As may be seen, although this parameter should depict a similar trend to that of fuel consumption per hour, as it depends on it, when the content of biodiesel in the blend is increased, the higher the fuel consumption the (slightly) lower the exergy flow. This is due to the fuel chemical composition and lower calorific value, according to equation (4). Fig. 5 shows the trend depicted by the exergy efficiency at the same engine operating conditions. This parameter behaves in a similar way for the different tested fuels, maybe due to the lower exergy destruction during engine operation when biodiesel is used. It may be seen that the chemical composition of the studied biodiesel blends leads to an improvement of the exergy efficiency of the engine, despite a slight increase in fuel consumption. The behavior of unitary exergetic cost is depicted in Fig. 6. As expected, it shows an opposite trend to Second Law efficiency. Fig. 7 summarizes the percentage of variation between fuels considering the studied parameters, at full load. As may be seen, the maximum power values vary from 28.1 kW to 26.41 kW when diesel fuel and B100 are used, respectively. Considering BSFC, it can be seen that it varies from 327.4 g/kW h (diesel fuel) to 386.7 g/ kW h (B100). According to biodiesel blends, they follow the same previous trends. In this sense, with the addition of biodiesel to the blend, engine power decreases while BSFC increases. In spite of engine power and fuel consumption differences when biodiesel, biodiesel/diesel fuel blends and straight diesel fuel are used, the ICE exergy efficiency exhibits similar values no matter
Fig. 2. Engine power vs. speed provided by the tested fuels at full load and different speed settings.
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f
Fig. 3. Brake-specific fuel consumption vs. speed provided by the tested fuels at full load and different speed settings.
(%)
Fig. 4. Exergy flow of the fuel vs. speed provided by the tested fuels at full load and different speed settings.
Fig. 5. Exergy efficiency vs. speed provided by the tested fuels at full load and different speed settings.
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Fig. 6. Unitary exergetic cost vs. speed provided by the tested fuels at full load and different speed settings.
C (%)
BSFC (g/kWh)
N (kW)
Diesel fuel
B20
B50
B80
B100
Fig. 7. Differences between the studied fuels considering main parameters during the engine operation at full load.
which fuel is used. The presence of oxygen in biodiesel, which is taken into consideration during the exergy flow analysis, enhances the formation of the mixture fuel–air inside the cylinder and provides better chemical combustion efficiency [33–35]. Thus, it contributes to similar exergy efficiency for the different fuels under study. In this sense, the unitary exergetic cost depicts a similar trend for the studied fuels, meaning that the production of one unit of exergy of mechanical work requires around four units of exergy resources. Mechanical work is the principal product of this study. This result is in agreement with the similar exergy efficiency value showed by the engine (approximately 25%). The slight differences reported for exergy efficiency and unitary exergetic cost between diesel fuel and the rest of the tested fuels are mainly due to the design of the fuel supply system of the engine and the combustion chamber, that suits the main properties of diesel fuel, i.e. viscosity, density, surface tension and other parameters related to air–fuel mixture formation and combustion process. This design enhances the combustion process while reducing the exergy destruction, when diesel fuel is used.
4. Conclusions When olive–pomace oil biodiesel and its mixtures with diesel fuel are used, a slight fuel consumption increase, besides a non-significant reduction of the engine power during the operation of the ICE were achieved. The differences with diesel fuel become more evident when the amount of biodiesel in the blend increases. Although considering BSFC, statistical tests show no significant differences between the tested fuels. On the other hand, the higher the concentration of biodiesel in the mixture, the lower the exergy flow under maximum engine power. In any case, the reduction of the value of the exergy flow experienced by the tested fuels is minimal. Moreover, the exergy efficiency and unitary exergetic cost reached during the operation of the engine under the condition of maximum power do not display significant differences for the assessed fuels. Based on these results, it can be concluded that olive–pomace oil biodiesel and its blends with diesel fuel may substitute the use of diesel fuel in compression ignition engines providing a renewable fuel with similar exergy efficiency and without any exergetic cost increment.
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Acknowledgments This research was supported by the Spanish Ministry of Education and Science (ENE2010-15159) and the Andalusian Research, Innovation and Enterprise Council, Spain (TEP-4994). Authors are also grateful for the provision of a research mobility grant to I. López from AECID, program II-A and Escuela Internacional de Doctorado en Agroalimentación eidA3, ceiA3, Spain. References [1] Rahman SMA, Masjuki HH, Kalam MA, Abedin MJ, Sanjid A, Sajjad H. Production of palm and Calophyllum inophyllum based biodiesel and investigation of blend performance and exhaust emission in an unmodified diesel engine at high idling conditions. Energy Convers Manage 2013; 76:362–7. [2] Silitonga AS, Masjuki HH, Mahlia TMI, Ong HC, Chong WT. Experimental study on performance and exhaust emissions of a diesel engine fuelled with Ceiba pentandra biodiesel blends. Energy Convers Manage 2013;76:828–36. [3] Dorado MP, Ballesteros E, Arnal JM, Gomez J, Gimenez FJL. Testing waste olive oil methyl ester as a fuel in a diesel engine. Energy Fuels 2003;17:1560–5. [4] Kruczyn´ski SW. Performance and emission of CI engine fuelled with camelina sativa oil. Energy Convers Manage 2013;65:1–6. [5] Ciubota-Rosie C, Ruiz JR, Ramos MJ, Pérez Á. Biodiesel from Camelina sativa: a comprehensive characterisation. Fuel 2013;105:572–7. [6] Bhattacharyya S. Optimizing an irreversible Diesel cycle – fine tuning of compression ratio and cut-off ratio. Energy Convers Manage 2000;41:847–54. [7] Rakopoulos CD, Mavropoulos GC, Hountalas DT. Measurements and analysis of load and speed effects on the instantaneous wall heat fluxes in a direct injection air-cooled diesel engine. Int J Energy Res 2000;24:587–604. [8] Durgun O, Sßahin Z. Theoretical investigation of heat balance in direct injection (DI) diesel engines for neat diesel fuel and gasoline fumigation. Energy Convers Manage 2009;50:43–51. [9] Ajav EA, Singh B, Bhattacharya TK. Thermal balance of a single cylinder diesel engine operating on alternative fuels. Energy Convers Manage 2000; 41:1533–41. [10] Kotas TJ. The exergy method of thermal plant analysis. Krieger Publishing Company; 1995. [11] Cziesla F, Tsatsaronis G, Gao ZL. Avoidable thermodynamic inefficiencies and costs in an externally fired combined cycle power plant. Energy 2006;31:1472–89. [12] Meyer L, Tsatsaronis G, Buchgeister J, Schebek L. Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems. Energy 2009;34:75–89. [13] Foster D. An overview of zero-dimensional thermodynamic models for IC engine data analysis. SAE paper 852070 Estados Unidos; 1985. [14] Shapiro H, Van Gerpen J. Two zone combustion models for Second Law analysis of internal combustion engines. SAE paper 890823 Estados Unidos; 1989. [15] Anderson M, Assanis D, Filipi Z. First and Second Law analyses of a naturallyaspirated, Miller cycle, SI engine with late intake valve closure. SAE paper 980889 USA; 1998.
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