Effects of Feedstock-related Properties on Engine Performance of Biodiesel from Canola and Sunflower Oils of South African Origin

Transcription

1 Effects of Feedstock-related Properties on Engine Performance of Biodiesel from Canola and Sunflower Oils of South African Origin Christopher C. Enweremadu, Omodolu T. Mustapha, and Hilary L. Rutto Abstract Sunflower and canola biodiesel were prepared from local South African feedstock. The most important feedstock-related properties were determined and their effects on real engine operation measured. Canola biodiesel exhibited excellent oxidation stability and at the same has very good cold flow properties. Sunflower biodiesel marginally fulfils all but one cold flow properties and it exhibited poor oxidation stability. In terms of engine performance, sunflower biodiesel was marginally better than canola biodiesel on brake power, torque and specific fuel consumption with differences at maximum speed between the fuels samples being 3.22%, 3.197% and 4.453%, respectively. There was no adequate relationship found between the feedstock-related properties of both biodiesel and engine performance as no singular fuel property could be said to have influenced the engine performance of the biodiesel samples studied. Keywords Biodiesel, engine, performance, properties A I. INTRODUCTION NY fuel based on renewable raw materials and it is suitable for use in diesel engines may be regarded as biodiesel. Biodiesel has been considered a very promising fuel for the transportation, construction, mining and agricultural sectors since it possesses similar properties with diesel fuel which is commonly used to power the engines used in these sectors. It is biodegradable, non-hazardous and has low emission profiles when compared with diesel [1]. Given the fact that most diesel engines are designed to run on diesel fuel, the physicochemical properties of biodiesel should be similar to those of diesel fuel. One peculiarity of biodiesel is that it can be produced from a variety of feedstock. The properties of biodiesel are dependent on the properties of the feedstock used for its production and the feedstock properties vary from one region to another. In addition, the alcohol used in the Christopher C. Enweremadu is with the Department of Mechanical & Industrial Engineering, University of South Africa, Florida 1710, South Africa (corresponding author phone: ; fax: ; enwercc@unisa.ac.za). Omodolu T. Mustapha is with the Mechanical Engineering Department, Vaal University of Technology, Vanderbijlpark 1900, South Africa ( omodolum@vut.ac.za). Hilary L.Rutto is with the Chemical Engineering Department, Vaal University of Technology, Vanderbijlpark 1900, South Africa ( hilaryr@vut.ac.za). transesterification process and the exact chemical process followed has effect on the properties of biodiesel produced from a particular feedstock [2]. Local feedstock is most commonly used for biodiesel production and most countries that use biodiesel have adopted their own standards. The current standards for regulating the quality of biodiesel on the market are based on a variety of factors which vary from region to region. Some specifications for biodiesel are feedstock neutral while others have been formulated based on the locally available feedstock [3]. It is necessary to produce biodiesel from local feedstock and evaluate thoroughly the properties and performance characteristics of the fuel in the region/country where it would be mostly used. To this end, South Africa has issued specifications (South African National Standards (SANS, 1935:2004) that should be met by the biodiesels intended for use in compression-ignition engines in the country. The standards for biodiesel in South Africa, as in some countries, are used to describe a product that represents a blending component in diesel fuel. Table I summarizes these acceptable limits in comparison to conventional petroleum diesel. With respect to these specifications, the major technical problems associated with the use of biodiesel in diesel engines are high production cost (largely due to the high cost of the feedstock)[4], its susceptibility to oxidation as well as it poor low-temperature properties[5], requiring various forms of additives in the form of anti-oxidants and cold-flow improvers. The most abundant and affordable seed oils or fats in a particular region are most commonly used as biodiesel feedstock. This is so because the physicochemical properties of biodiesel are strongly influenced by the nature and composition of the feedstock used in their production. It is therefore not surprising that there are some significant differences among regional standards, a universal quality specification of biodiesel is, and will be impossible. Also weather conditions, reflected in the regulations of properties describing performances of biodiesel at low temperatures is a serious impediment for both biodiesel imports and exports among different regions of the world, as well as automotive manufacturers, who must adapt their engines to the quality of biodiesel in the region where the vehicle will be used [6]. 24

2 Some reviews are available in literature which summarized the factors of effect on biodiesel engine power and economy to include the effect of feedstock and its properties [7], [8]. Sunflower and soybeans are the major oilseeds produced in South Africa, while canola plays an important role as a rotational crop in the winter rainfall production region. Sunflower is the cheapest and most abundant of the three oilseeds [9]. This study focused on comparing the most important (feedstock-related) properties of locally-produced biodiesel from sunflower due to its abundance and low cost, and canola oilseed due to its winter availability with respect to real engine operation and the possible inefficiencies they may induce. TABLE I REQUIREMENTS FOR AUTOMOTIVE BIODIESEL IN SOUTH AFRICA, SANS 1935:2004 Property Limits min max Ester content, %(m/m) Density at 15 C, kg/m Kinematic viscosity at 40 C, mm 2 /s Flash point, C Sulfur content, mg/kg Carbon residue (on 10 % distillation residue), %(m/m) Cetane number Sulfated ash content, %(m/m) Water content, %(m/m) Total contamination, mg/kg - 24 Copper strip corrosion (3 h at 50 C), rating - Class 1 Oxidation stability (at 110 C), hours 6 - Acid value, mg/kg Iodine value, gi/100g Linolenic acid methyl ester, %(m/m) - 12 Polyunsaturated (> = 4 double bonds) methyl - 1 esters, %(m/m) Methanol content, %(m/m) Monoglyceride content, %(m/m) Diglyceride content, %(m/m) Triglyceride content, %(m/m) Free glycerol, %(m/m) Total glycerol, %(m/m) Group I metals (total of Na & K), mg/kg Group II metals (total of Ca & Mg), mg/kg Phosphorus content, mg/kg Cold Filter Plugging Point (CFPP) Winter/ Summer, C - -4/+3 II. MATERIALS AND METHODS A. Biodiesel production and property testing The vegetable oil samples were obtained from local supply stores in South Africa. To produce biodiesel on laboratory scale, oil to methanol molar ratio of 6:1 was used. To achieve this, a 70/30 mass percentage ratio of oil to methanol was used. 70 grams of oil was measured in a beaker. 30 grams of methanol was measured in another beaker. 1 gram of KOH was dissolved in the 30grams of methanol. The beaker containing oil was put on a heating mantle and kept constant at 55 0 C while stirring at 300 rpm. The methanol and KOH mixture was then added. The whole mixture was allowed to heat and stir for 1hour. A mixture of glycerol and biodiesel was formed. The mixture was put in a separating funnel and left overnight to settle. The glycerol was drained of the bottom of the separating funnel leaving biodiesel. The biodiesel was washed with water and boiled to remove the water left in it. A 27/3 test was performed to confirm biodiesel was produced. The 27/3 test involves putting 27ml of methanol in a cylinder and adding 3ml of biodiesel to it. After shaking, biodiesel dissolves in the methanol while oil will not dissolve. The laboratory specimen was up-scaled using a 50-litre capacity biodiesel pilot plant (see Fig. 1). The biodiesel was dry- washed using Amberlite BD10DRY (dry resin) powder. The physical and chemical properties of the test fuel samples were determined using Omega Version 1.5 software (Petroprogram Company Finland, 2010) and/or tested experimentally according to ASTM procedure. The fuel properties are kinematic viscosity, density, flash point, cloud point and pour point, cold filter plugging point, oxidation stability, cetane number, heat of combustion, acid value, iodine value and distillation temperature. Fig. 1 Schematic and photographic images of the biodiesel plant B. Engine test setup The engine used for the performance tests is a Mercedes Benz OM 364A, four-cylinder, four-stroke direct-ignition turbocharged industrial diesel engine. The engine specifications are shown in Table II. TABLE II SPECIFICATIONS OF MERCEDES BENZ OM 364A ENGINE Engine specifications Model Vertical, in-line with exhaust gas turbocharger Number of cylinders 4 Cylinder bore 97.5 mm Piston stroke 133 mm Connecting rod length 230 mm Total piston displacement 3972 cm 2 Compression (dead) space per 64 cm 2 cylinder Compression ratio 16.5:1 Cut-off ratio 3.75 Crank angle for fuel injection Automotive rating 87 kw at 2600 rpm Engine gross approximate weight 415 kg 25

3 The maximum power output of the engine is 87 KW at 2600 rev/min. A 265 KW output Froude eddy current dynamometer was used. The experimental test setup is shown in Figure 2. HEAT EXCHANGER COOLING SYSTEM ENGINE FUEL TANK PUMP TOTALIZER FILTER AVL (DYNAMIC FUEL BALANCE) THROTTLE POSITIONER JUNCTION BOX DRIVE SHAFT THERMOCOUPLES AND PRESSURE TRANSDUCERS TEMPERATURE CONTROLLER M LOAD CELL DYNAMOMETER CONTROLLER PLC JUNCTION BOX COMPUTER DYNAMOMETER Fig. 2 Schematic and photographic images of the test cell setup III. RESULTS AND DISCUSSION A. Properties of the biodiesel fuel samples The chemical composition of biodiesel is dependent upon the length and degree of unsaturation of the fatty acid alky chains. Sunflower seems prone to variation in its composition (sunflower has been the subject of research in many countries worldwide; hence the disparity in its oil composition mirrors different soils, cultivations and growth conditions). Studies have shown that while canola has a high percentage of monosaturated fatty acids (60-62%), sunflower has a high percentage of poly-unsaturated fatty acids (68%). While sunflower is primarily rich in linoleic (18:2) acid, canola to a lesser extent is rich in linolenic (18:3) acid [10]. Table III summarizes the properties of the biodiesel fuel samples produced from sunflower and canola oils. Iodine value The degree of unsaturation biodiesel (feedstock) measured as iodine value has a very important effect. Iodine value (IV) evaluates the stability to oxidation. The South African specifications require that biodiesels used in compression ignition engines have a maximum value of 140. Biodiesel with high IV is easily oxidized on contact with air and has the propensity to polymerize resulting in formation of deposit on injector nozzles, piston rings and piston ring grooves. This means that both canola (IV of 108) and sunflower (IV of 126) meet the specification limits. TABLE III PROPERTIES OF THE BIODIESEL FUEL SAMPLES Property Biodiesel Fuel Sample Canola Sunflower Density at 15 C, kg/m Kinematic viscosity at 40 C, mm 2 /s Flash point, C Oxidation stability at 110 C, hr Acid value, mgkoh Iodine value, gi/100g Cold Filter Plugging Point, C -9-7 Cloud point, C Pour point, C -9-6 Cetane number Lower heating value, kj/kg T 90, C Acid value Acid value determination is used to quantify the presence of acid moieties in a biodiesel sample. This parameter is a direct measure of the content of free fatty acids (FFA), thus the corrosiveness of the fuel, of filter clogging and the presence of water in the biodiesel. A too high amount of free glycerin can cause functioning problems at reduced temperatures and fuel filter clogging. Acid value measures the freshness of biodiesel. The South African standard requires that biodiesels have a maximum acid value of 0.5 mg KOH/kg. The two biodiesels investigated have acid values of 0.19 for canola and 0.28 for sunflower. Hence both biodiesels satisfy the standard. Density Fuel density directly affects fuel performance, as some fuel properties, such as cetane number, heating value and viscosity are strongly related to density [6], [7]. The density of the fuel also affects the quality of atomization and combustion. Biodiesel fuels have a higher density when compared to petro-diesel fuels and is dependent on fatty acid composition and purity [6]. Hence, volumetrically-operating fuel pumps will inject greater mass of biodiesel than petroleum diesel fuel. In South Africa, the acceptable density range is quite wide ( kg/m 3 ), and this is met by the biodiesels investigated. Kinematic viscosity Viscosity influences the ease of starting an engine, the spray quality, the particle size, the penetration of the injected jet and the quality of the fuel-air mixture combustion. Fuel with too low a viscosity provides a very fine spray. However, this leads to insufficient penetration and its attendant operational problems. Similarly, too high viscosity causes operational problems at low temperatures. Therefore, the South African standard specifies an acceptable biodiesel viscosity range between 3.5 and 5.0mm 2 /s. The canola and sunflower biodiesel met the specifications. 26

4 Cetane number One of the most influential properties of diesel fuel is the cetane number (CN), which characterizes the ignition quality of fuels for compression ignition engines, particularly during cold starting conditions. Cetane number is a primary indicator of fuel quality as it describes the ease of selfignition. Low cetane numbers lead to long ignition delay and make starting the engine difficult, especially at low temperatures. On the other hand, higher cetane numbers promote faster auto-ignition of the fuel. According to Barabás and Todoruţ [6], cetane number affects specific fuel consumption and optimal range is between 41 and 56, but must not be higher than 65. The cetane number of canola and sunflower are 55 and 52 respectively. The South African specifications dictate a cetane number for biodiesel fuel of at least 51, which means that while canola is within the acceptable limit, sunflower is marginally accepted in pure form. Heating value The heating value is a measure of a fuel s heat of combustion or its ability to release energy for producing work. The presence of oxygen (10-13% w/w) in the ester molecules decreases the heating value of biodiesel by 10-13% [6], [10] compared to conventional diesel fuel. This leads lower energy density, thus more biodiesel fuel needs to be injected in order to achieve the same energy power output with conventional diesel. There is no specification as regards the biodiesel heating in South Africa or any other country. According to values presented in Table III, canola and sunflower exhibit net heating value of and MJ/kg respectively. Flash point The flash point is a measure of the temperature to which a fuel must be heated such that the mixture of vapour and air above the fuel can be ignited. The flash point of biodiesel fuel is higher than that of petroleum diesel, hence the storage of neat biodiesel much safer than diesel. The South African standard requires biodiesel fuel to have at least 120 C flash point. The specification is meant to determine a lower limit of purity in the final fatty acid methyl ester, and is easily met by the biodiesel feedstock studied. Cold flow properties Generally, all fuels for compression ignition engines may cause starting problems at low temperatures, due to worsening of the fuel s flow properties at those temperatures. The key flow properties for winter fuel specification are cloud and pour point. The cloud point is the temperature at which wax crystals first start to form in the fuel. The cloud point of biodiesel depends on the nature of the feedstock it was obtained from [11]. The pour point is the temperature at which the fuel contains so many agglomerated crystals to form a gel and will no longer flow. The cold filter plugging point is the lowest temperature at which 20 ml of fuel passes through a filter within 60 seconds by applying a vacuum of 2 kpa. While there are no South African specifications for cloud and pour points, the standard requires the determination of cold filter pluging point (CFPP) as its value is regulated depending on the climatic conditions of each region or country. The value of the cloud point (CP), pour point (PP) and CFPP of biodiesels produced from the two feedstock studied showed that canola is the best having the lowest values. Distillation temperature Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling mixture. Most biodiesels boil at roughly the same temperature (approximately C). The distillation temperature specific has been incorporated for petrodiesel to ensure that fuels have not been contaminated with high boiling materials such as used motor oil. Hence, the distillation of biodiesel fuel is a method that provides a demonstration of the methyl ester s quality [12]. It has been reported that unusually low distillation temperatures are accompanied by low cetane number and flash point, which is indicative of residual amounts of methanol and/or glycerol content. The most commonly reported is the final distillation temperature, T 90. There are no specifications in South Africa for biodiesel distillation. However, the simulated results gave C and C for canola and sunflower biodiesel respectively. Oxidation stability Poor oxidation stability is one of the major issues that limit the use of biodiesel as a fuel in compression ignition engines. Biodiesel composition greatly affects its stability in contact with air. Unsaturated fatty acids especially the polyunsaturated ones such as C18:2 and C18:3 have a high tendency to oxidation. The oxidation of biodiesel is influenced by its composition; oxidation increases with the level of unsaturation of fatty acids in its composition, i.e. the feedstock [10]. Results from the biodiesels investigation show that while canola (oxidation stability index (OSI) of 6.4h) fulfils the South African specification, sunflower with OSI of 0.914h failed to meet the South African National Standards (SANS, 1935:2004). B. Engine performance Properties of biodiesel, especially heating value, density, viscosity and lubricity, have an important effect on engine power and fuel economy [13]-[16]. As shown in Fig. 3, diesel has a higher brake power than sunflower and canola biodiesel all through the varied engine speeds. Both biodiesel fuels behave similarly except at engine speeds of more than 2800 rev/min where sunflower biodiesel shows a marginal increase in brake power compared to canola. Since both biodiesel fuel samples have comparable calorific values then the higher viscosity of sunflower biodiesel might have resulted in the slight power loss of 27

5 canola. However, there might be no significant effect of biodiesel feedstock on engine power as the differences in power at maximum speed between the biodiesel fuels were only 3.222%. Fig. 4 Torque performance against engine speed Fig. 3 Brake power performance against engine speed As with brake power, Fig. 4 shows a higher performance in terms of torque for diesel when compared with sunflower and canola biodiesel. The torque performance pattern of both biodiesel fuels are similar except at engine speeds of more than 2800 rev/min with sunflower biodiesel displaying a marginal increase compared to canola. Both biodiesel fuels have comparable calorific values with sunflower having higher viscosity. This might have resulted in the slight reduction in torque noticed for canola biodiesel. However, there might be no significant effect of biodiesel feedstock on the torque as the difference at maximum speed between the biodiesel fuel samples was only 3.197%. Lower heating value, higher density, higher viscosity and cetane number play primary role in engine fuel consumption for biodiesel. Fig. 5 shows that sunflower biodiesel has lower specific fuel consumption than canola biodiesel at higher engine speeds. This may be due to a slight difference in cetane number between both fuels which may affect the combustion timing, increasing the specific fuel consumption [11]. Since both fuels have approximately the same density and heating value, the slight difference in consumption for both fuels may be due to higher viscosity of canola biodiesel. However, the slight increase in fuel consumption of canola biodiesel may be attributed to a combination of and interaction of properties such as lower heating value, higher density, higher viscosity, production process and quality of canola biodiesel. This is in agreement with the results obtained by [17], [18]. Fig. 5 Specific fuel consumption against engine speed IV. CONCLUSION The most important feedstock-related properties of canola and sunflower biodiesel fuels were measured with respect to the effects on real engine operation and the possible inefficiencies they may cause. Canola biodiesel, which is mono-saturated exhibited excellent oxidation stability and at the same has very good cold flow properties and fulfils three major fuel specifications namely, cetane number, viscosity and density. Sunflower, a poly-saturated biodiesel, on the other hand marginally fulfils all but one cold flow properties, but has poor oxidation stability. In terms of engine performance, sunflower biodiesel was marginally better than canola biodiesel on brake power, torque and specific fuel consumption. No singular fuel property could be said to have influenced the engine performance of the biodiesel samples studied. Rather the slight differences in the engine performance between the two biodiesel fuel samples may be attributed to a combination of interaction between the properties of the fuels. 28

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