Properties of bioethanol - diesel oil mixtures

Transcription

1 Properties of bioethanol - diesel oil mixtures A. Matuszewska 1, M. Odziemkowska 1 and J. Czarnocka 1 1 AUTOMOTIVE INDUSTRY INSTITUTE, Jagiellonska 55, Warsaw, Poland The authors investigated the effect of bioethanol content in diesel oil on the properties of such biofuel. This paper presents the research results of the homogeneity and physical stability of prepared mixtures as well as their viscosity, lubricity, distillation characteristic, cetane number and anticorrosive properties. The mixtures were prepared under laboratory conditions with different content of the bioethanol. Due to the limited miscibility of bioethanol and diesel oil, the impact of different co-solvents on mixtures stability was also studied. It was found that at low levels of alcohol the mixtures of diesel oil and bioethanol are characterized by similar properties as diesel fuel. However, some parameters require modification by additives. Keyword: bioethanol; diesel oil; mixture properties 1. Introduction The diesel engines are very commonly used due to their high combustion efficiency and reliability. Unfortunately, they contribute significantly to an increase in emissions of pollutants and hazardous substances, including particulate matter and nitrogen oxides [1]. Reduction of exhaust emissions can be achieved for example by changing the fuel composition, by introduction of bioethanol to diesel oil. The bioethanol is the most tested oxygen compound for use as a fuel component e.g. for motor gasoline. This alcohol can be obtained from many materials, for example: sugar beets, sugar cane, corn, waste biomass materials, barley, rye, corn, etc. It could be also used as a feedstock for the production of oxygen compounds intended for fuels [2] as well as a carrier of hydrogen for fuel cells [3]. Attempts are being made in use of bioethanol as a component of diesel fuel (in amounts up to 20% v/v) [4, 5] or as fuel to power the self-ignition engines [6]. The anhydrous bioethanol is characterized by low viscosity, poor lubrication properties, low flash point and very low cetane number approximately 8 [5]. The addition of bioethanol to the diesel oil changes fuel s properties. Alcohol causes a deterioration of: viscosity, flash point, cetane number and lubricity. Appropriate viscosity of fuel is very important parameter due to the prevention or the limitation of the fuel injection system wear. Viscosity determines the lubricity of the fuel [5]. Viscosity also affects parameters of combustion process, such as fuel atomization and characteristics of the fuel injection into the combustion chamber [7]. The cetane number influences engine-start ability, emissions, peak cylinder pressure and combustion noise [5]. However, the flash point has no direct effect on the engine performance, but it should be taken into account during storage, handling, shipping and use of fuel [8]. Bioethanol may contain some amounts of chloride ions, acetic acid and azeotropic water. Alcohol molecules are characterized by polarity. Because of that the alcohol presence in the fuel can have a corrosive effect on the construction materials. Therefore, it is necessary to evaluate in detail the physical and chemical properties of bioethanol and diesel oil mixtures, their compatibility with construction materials [9]. Another disadvantage of bioethanol and diesel oil mixture is its high affinity for water, even a small amount of water can disturb the equilibrium of oil and alcohol and cause separation of the alcohol phase. Thus, it is important to improve the stability of such mixture, for example by co-solvent addition. Co-solvent enables to easily mix the two abovementioned components and provides a mixture homogeneity. As a co-solvent can be used, inter alia: vegetable oil methyl esters, monohydric alcohols, tetrahydrofurane or ethyl acetate [9]. Some properties of blends of diesel oil and bioethanol can be adjusted by adding small amount of fuel additives. The aim of this study was the investigation of properties of diesel oil blends with bioethanol and some additives. 2. Experimental mixtures The examined mixtures were prepared under laboratory conditions. These samples were composed of commercial diesel oil (without fatty acid methyl esters FAME) and with different content of the anhydrous bioethanol (from 5 to 25% v/v). The mentioned components met the quality requirements defined in the standard specifications. The selected parameters of these components are presented in Table FORMATEX 2013

2 Table 1 Properties of diesel oil and bioethanol Density, kg/m 3 Viscosity at 40 C, mm 2 /s Boiling point, C Flash point, C Sulphur content, mg/kg Water content, % m/m Diesel oil 833 (at 15 C) Anhydrous 791 bioethanol (at 20 C) < The following compounds were tested as co-solvents of bioethanol in diesel oil mixtures: 2-etylo-1-heksanol, 2- butanol, 1-propanol, 2-propanol and FAME. To improve the physicochemical properties of tested mixtures different fuel additives in quantities recommended by manufacturers were added. During research the following additives were used: - Cetane improver (designated CI) on the base of alkylated nitrates; - Antioxidants (designated AO1, AO2 and AO3) containing active compounds such as alkylated phenols and aromatic amines; - Ashless additive packages with anticorrosion and lubricating action (designated AP1 and AP2) on the base of carboxylic acids, amines and amine salts of carboxylic acids. 3. Methodology Physical stability of prepared samples with and without co-solvent was studied by two methods: visual and with the use of Turbiscan apparatus. In case of visual method, the samples were stored, in tightly closed containers, at 22, 15 and 5 o C for seven days at each temperature. Homogeneity, transparency, presence of impurities and/or precipitates of test samples were periodically assessed. Turbiscan apparatus is an optical device designed for the characterization of liquid emulsion suspensions and solutions. The device has an infrared light source of 880 nm wavelength, and two synchronous detectors that receive respectively light transmitted through sample and light backscattered by the sample. The sample is contained in a cylindrical glass cell and it is examined per 40 µm throughout all its length. The transmission and backscattering light flux profiles in function of sample height were derived from the data obtained during experiment. The test samples were examined at 22, 15 and 5 o C for eighteen hours at each temperature. Physicochemical properties of prepared mixtures were compared with diesel oil properties. The tests were carried out according to methods listed in PN-EN 590 standard [10]. In case of lubricating properties the tests were performed using HFRR (High Frequency Reciprocating Rig) apparatus with some exceptions caused by the specificity of tested mixtures. Bioethanol boiling temperature (78 o C) is lower than for a diesel oil. Performing of the friction test at 60 C (according to the standard) can cause the evaporation of a significant part of alcohol from the mixture and change the properties of the blend. Therefore, the tribological tests were performed at 25 o C. In order to avoid excessive heating of testing fluid due to friction and evaporation of bioethanol, the enclosed research vessel with capacity increased up to 10 cm 3 was applied. The lubricity of investigated mixtures were determined on the basis of the average wear scar diameter, calculated from diameters measured in parallel and perpendicularly to the direction of friction. The influence of bioethanol on the construction materials was estimated by a comparison of properties of the samples before and after treatment with selected mixtures. Prepared samples of materials were exposed to mixtures in closed glass containers for 14 days at 70 C. For plastic materials the changes in volume and hardness, permeability through the walls of the tank and pipes, strength of the fuel pipes (tear resistance, tensile strength) were estimated. For pipes made of PVC the tests simulating the continuous operation were carried out. The blends were pumped through the pipe line by 12 hours per day for 14 days (a total of 168 hours) at 70 C. Before and after the end of the test the pipe samples were weighed and their lengths were measured. Additionally a corrosion action of the tested blends was estimated. The tests were carried out on the base of NACE TM0172 standard [11]. In this test method, a cylindrical steel rod was immersed in a mixture of the test fuel and distilled water at 38 o C for 3.5 hours. After the test the surface of tested rod was evaluated and compared with diesel oil result. 4. Research results 4.1 Physical stability of blends The results of physical stability tests for diesel oil and bioethanol blends are presented in Table 2. In case of diesel oil and ethanol blends, only mixture with 5% v/v of alcohol content remained homogeneous and transparent in the examined temperature range. The mixtures containing more than 10% v/v of bioethanol showed instability at 5 C. FORMATEX

3 Whereas, the blends with 20% and higher ethyl alcohol content were turbid and separated onto two phases at room temperature. Table 2 Physical stability of diesel oil and ethanol mixtures Composition, % v/v Appearance diesel oil bioethanol t=22 C t=15 C t=5 C Note: + homogenous, transparent, without impurities, precipitates; - hazy, phase separation The data presented in Table 3 show that addition of 5% of 2-ethyl-1-hexanol as well as 2-butanol improves the stability of the examined blends significantly. These mixtures were instable only at 5 o C for the 25% and 30% alcohol content. In case of 5% concentration of other co-solvents some blends were divided into two phases even at 15 o C. 2-propanol showed the worst properties as a co-solvent. Table 3 Physical stability of diesel oil and bioethanol mixtures Appearance at the temperatures in Celsius degrees Composition, % v/v 2-ethyl- FAME 1-hexanol 1-propanol 2-propanol 2-butanol diesel oil bioethanol co-solvent Note: + homogenous, transparent, without impurities, precipitates; - hazy, phase separation The visual assessments of the physical stability of diesel oil, bioethanol and co-solvents mixtures were verified by tests with the use of Turbiscan apparatus. In most cases, the results obtained with both methods were consistent with each other. It was stated that the Turbiscan apparatus is able to record changes in sample stability, which are too small to be seen with the naked eye. Such situation had place for example for mixtures: of diesel oil with 15% v/v of bioethanol at 22 o C; of diesel oil with 15% v/v of bioethanol and 5% v/v of 1-propanol at 5 o C; of diesel oil with 20% v/v of bioethanol and 5% v/v of 2-butanol at 5 o C. Figure 1 shows the examples of transmission profiles for a) instable mixture of diesel oil with 15% of bioethanol at 22 o C, and b) stable mixture of diesel oil with 20% v/v of bioethanol with 5% v/v of 2-ethyl-1-hexanol at 5ºC. 354 FORMATEX 2013

4 a) b) Fig. 1 Transmission profile of blends: a) 75% v/v of diesel oil with 20% v/v of bioethanol and 5% v/v of 2-ethyl-1-hexanol at 5ºC, b) 85% v/v of diesel oil and 15% v/v of bioethanol at 22 o C. 4.2 Cetane number Anhydrous bioethanol has very low cetane number so its addition into diesel fuel lowers the value of this parameter for the mixture. For example, addition of 10% v/v of alcohol to diesel oil decreases this parameter from 53.9 to 44.6 (see Table 4). Co-solvents can also change the cetane number of the mixture so the blends containing 10% v/v of particular compounds were prepared and tested. The results are shown in Table 4. Presented data indicate that only 2-ethyl-1- hexanol and FAME improve the cetane number. Table 4 Cetane number of tested mixtures Mixture Cetane number Diesel oil 53.9 Diesel oil + 10% v/v of bioethanol 44.6 Diesel oil + 15 v/v of bioethanol 40.1 Diesel oil + 20% v/v of bioethanol 36.0 Diesel oil + 10% v/v of FAME 57.1 Diesel oil + 10% v/v of 2-ethyl-1-hexanol 56.4 Diesel oil + 10% v/v of 1-propanol 51.1 Diesel oil + 10% v/v of 2-propanol 53.0 Diesel oil + 10% v/v of 2-butanol 49.5 The blends of diesel oil with 10% v/v of bio-ethanol and 5% v/v of particular co-solvent were also investigated. The mixture containing 2-ethyl-1-hexanol has the highest value of the cetane number (49.7). That co-solvent provided good mixture stability. Therefore the possibility of the cetane number increase by CI additive for mixtures with higher bioethanol content was examined. The results showed that for mixture of diesel fuel with 15% of bioethanol and 5% of 2-ethylhexanol it was necessary to add 1% v/v of CI additive to increase the cetane number to a standard level (51). In pure diesel oil the CI additive increases the cetane rating by 3 to 5 numbers with a typical treatment of 0.1%. Due to the bioethanol content the examined mixtures needed higher dosage of that additive. 4.3 Lubricating properties Ones of the important parameters of diesel fuel are its lubricity and kinematic viscosity. The introduction of any component to the fuel may improve or deteriorate them. Ethyl alcohol has significantly poorer lubricity in comparison with commercial diesel fuel. The average wear scar diameter on the ball lubricated with absolute bioethanol was 472 μm, while on the ball lubricated with commercial diesel oil μm. Therefore, it was expected that alcohol deteriorates the lubrication properties of the mixture. This assumption was confirmed by the results of friction tests. The increase in bioethanol content caused the increase in wear scare diameter. Test results of physicochemical properties of blends showed that the increase in alcohol concentration results in a decrease in viscosity. Therefore, one of the reasons for lubricity deterioration may be the decrease in mixture viscosity. Apart from alcohol content, the kind of co-solvent used in the mixture had an influence on the size of wear scar. Figure 2 shows the results of tribological tests of diesel fuel blends with 10% v/v and 15% v/v of anhydrous bioethanol and 5% v/v of different co-solvents. FORMATEX

5 Fig. 2 The results of tribological tests of diesel oil mixtures with anhydrous bioethanol (10 and 15% v/v) and different co-solvents (5% v/v). The presented data (Fig. 2) indicate that, the worst lubricating properties had the blend containing 2-propanol and the best the mixture with 2-ethyl-1-hexanol. It was found that the viscosity of diesel oil mixtures with particular co-solvents was similar, except blends with 2-ethyl-1-hexanol and with FAME which had a slightly higher viscosity. The deterioration of tested fuels lubricity is probably connected with an interaction between the mixtures and the tribological couple. This assumption is reflected in changes in the friction coefficient. Figure 3 presents the exemplary curves of friction coefficient vs. time registered for tribological couple lubricated with diesel fuel, diesel and bioethanol blends of diesel and bioethanol with 2-ethyl-1-hexanol and 2-propanol. The presence of bioethanol in diesel oil caused increase in friction coefficient value and destabilization of the curve run. Similar action showed 2-propanol, which when added to the blend of diesel oil and bioethanol caused additional increase in friction coefficient. Whereas the 2-ethyl-1-hexanol reduced the wear by lowering and stabilizing of friction coefficient during the test. Fig. 3 Friction coefficient curves registered for tribological couple lubricated with diesel fuel, diesel and bioethanol blends of diesel and bio-ethanol with 2-ethyl-1-hexanol and 2-propanol. There were the attempts made to improve the lubricity of selected mixture (80% v/v of diesel oil, 15% v/v of bioethanol, 5% v/v of 2-ethyl-1-hexanol) with AP1 and AP2 additives which were dosed in an amounts of 20 mg/l and 9 mg/l respectively. AP2 additive did not influence the size of wear scar. AP1 resulted in a slight improvement in lubricating properties of tested mixture. About 18% reduction of the wear scare diameter, in comparison with blend without additive, was observed. It should be taken into account that the test conditions were not standardized because there is no research methodology for this type of fuel blends. 4.4 Distillation Another important parameter of diesel oil is a distillation characteristics. Evaporation temperature of the light fraction of diesel oil is responsible for correct cold engine start. The bioethanol may change not only the density or kinematic viscosity of a fuel, but also the distillation curve. Its admixture increases the volume of light fuel fraction. On Figure 4 the examples of distillation curves of tested mixtures with different volumetric proportion of bioethanol (Fig. 4a) and with different co-solvents (Fig. 4b) are presented. 356 FORMATEX 2013

6 o Distillation temperature, C ethanol 5% ethanol 10% ethanol 15% diesel oil Recovered, %(v/v) Recovered, %(v/v) a) b) Fig. 4 Comparison of distillation curves for: a) diesel oil without bioethanol, diesel oil with bioethanol, b) diesel oil with 10% v/v of bioethanol and 5% v/v of co-solvents. Initial boiling point (IBP) of a commercial diesel oil is o C and the final boiling point (FBP) is about 350 o C. The boiling point of the bioethanol is 78 o C, therefore it evaporates as the first component of fuel mixtures. This temperature is much lower (about 100 o C) than IBP of diesel oil. Distillation curves presented on Figure 4 have two characteristic inflexions. The first one is a result of the evaporation of alcohol while the second reflects the evaporation of petroleum fractions. Deformation of the distillation curves is due to the formation a positive azeotropes consisting of ethanol and hydrocarbons from diesel oil [8]. Co-solvents such as FAME, 1-propanol, 2-propanol, n-butanol and 2-ethylo-1-hexanol added to diesel oil and bioethanol blend do not change the run of distillation curves. 4.5 Materials compatibility Influence of bioethanol presence in the mixture on construction materials was investigated for selected fuel blends and for the diesel oil as reference fuel (designated as R). The tested mixtures composed of: blend I diesel oil (80% v/v), bioethanol (15% v/v), 2-ethyl-1-hexanol (5% v/v); blend II - diesel oil (75% v/v), FAME (5% v/v), bioethanol (15% v/v), 2-ethyl-1-hexanol (5% v/v); blend III anhydrous bioethanol (100%). The tests were carried out for elements used in automotive fuel systems made of plastics (pipe made of two-layer rubber, pipe made of PVC, fuel tank made of HDPE plastic) and metals (fuel pipe made of steel, fuel tank made of steel, piston and cylinder from diesel engine injection pump). As a result of research on interaction of laboratory biofuels with the construction materials it was found that they had various influence on the tested parameters. Changes in hardness and in the volume of rubber fuel pipes, monthly mass loss of fuel mixture during tests of permeability through the fuel tank walls were similar as these founded for reference fuel. The greatest negative impact on construction materials had a pure bioethanol (blend III). Alcohol deteriorated mechanical properties of the fuel pipes made of rubber and polyvinyl chloride to a greater extent than other mixtures and diesel oil. Blends I and II interacted with tested samples similarly as reference fuel. Exemplary results of tensile strength tests for fuel pipes made of PVC are presented in Table 5. Table 5 Average tensile strength of the fuel pipes made of PVC before and after exposure to mixtures In the state of delivery, N Blend I, N Blend II, N Blend III, N Blend R, N The permeability through the walls of the fuel pipes made of rubber and PVC was determined. All test mixtures interacted adversely with the pipes. They caused the increase in permeability of alcohol fuels to a greater extent than reference oil. This interaction was stronger for pipes made of PVC than rubber. The highest rate of loss in fuel volume was observed for blend II, irrespective of kind of material. The exemplary results are shown in Table 6. Table 6 The mixture permeability through the walls of the fuel pipes Average volume of loss, cm 3 Rate of loss, Fuel Material 24h 48h 72h 96h 120h cm³/m² h R PVC II R rubber II o Distillation temperature, C propanol 5% 1-propanol 5% 2-butanol 5% 2-ethylo-1-hexanol 5% diesel oil FAME 5% FORMATEX

7 The influence of prolonged contact of tested blends with the fuel pipes during pumping operation was examined. It was observed that the bioethanol caused the weight loss and length reduction of the pipes. This effect was opposite to that observed for other mixtures where the elongation of the pipes occurred. This phenomenon could be explained by the rinsing out of the material components by alcohol. Metal samples, as plastics ones, were exposed to mixtures in closed glass containers for 14 days at 70 C. Microscopic observations of material samples of fuel tanks, fuel pipes and fuel injection pump parts (cylinders and pistons), did not show the presence of diffusion layers on contact surfaces. Alcohol in the mixtures also did not influence the hardness of the metal samples. The tested mixtures had influence on the surface roughness. The most susceptible to mixtures action were the piston and cylinder samples made of high-carbon steel - spheroidal cementite in the martensitic matrix. This samples also changed their roughness under the action of diesel oil. The strongest changes of above-mentioned parameter were observed for the materials exposed to the action of blend II. The most resistant elements were fuel pipes made of lowcarbon steel (ferrite with a small amount of perlite). Due to the hygroscopic properties of bioethanol the corrosion tests were performed. It was found that the content of bioethanol in the fuel mixture and the kind of co-solvent had influence on the corrosion properties of blends. The higher alcohol concentration, the stronger effect on the contact surface. From the midst of tested co-solvents, the 2-ethyl-1- hexanol showed the weakest corrosion aggressiveness. Admixture of appropriate additive improved these properties to the level determined for diesel oil. 5. Summary The addition of bioethanol to diesel oil changes the physicochemical properties of the blends. With the increase in bioethanol content the stability, cetane number, lubricity, kinematic viscosity, corrosion resistance decrease. This alcohol changes distillation curve course, increasing the volume of light fuel fraction. It should be taken into account during storage, handling, shipping and use of fuel with ethyl alcohol. This disadvantages connected with the use bioethanol in biofuel can be overcome by adding of fuel additives. Diesel oil and bioethanol are mixing in a limited scale over a range of temperatures from 5 to 20 o C. Even 10% v/v of ethanol concentration can cause phase separation of the mixture. Addition of co-solvent improves blend stability and allows to increase in bioethanol content. The 2-ethyl-1-hexanol as well as the 2-butanol in amount of 5% improves the stability of the examined blends significantly. However the disadvantage of 2-butanol is the reduction in cetane number and corrosion resistance in comparison with the 2-ethyl-1-hexanol. The presence of 2-ethyl-1-hexanol has a positive impact not only on physical stability but also on other tested properties, for example: cetane number, lubricity, corrosion resistance. The presence of bioethanol in the blends can cause problems with durability of system fuel components, especially those made of plastics. It increases permeability of fuel mixtures through the wall of fuel pipes. The use of such biofuels requires the appropriate selection of construction materials. Acknowledgements This study was financially supported by The National Centre for Research and Development of Poland in the project Development of technology of bioethanol biofuels producing and using for combustion engines under contract N R /2009. References [1] Satgé de Caro P., Mouloungui Z., Vaitilingom G., Berge J.Ch. Interest of combining an additive with diesel- ethanol blends for use in diesel engines. 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8 [9] Hansen A.C., Zhang Q., Lyne P.W.L. Ethanol-diesel fuel blends-a review. Bioresource technology. 2005;96: [10] PN-EN 590:2010 Automotive fuels - Diesel - Requirements and test methods. [11] The National Association of Corrosion Engineers (NACE) Standard - TM Determining Corrosive Properties of Cargoes in Petroleum Product Pipelines. FORMATEX