CHAPTER 3 VEGETABLE OIL, BIODIESEL AND OXYGENATES AN OVERVIEW

Transcription

1 38 CHAPTER 3 VEGETABLE OIL, BIODIESEL AND OXYGENATES AN OVERVIEW 3.1 VEGETABLE OIL AND ITS BLENDS Vegetable fats and oils are lipid materials derived from plants. Physically, oils are liquid at room temperature, and fats are solid. Chemically, both fats and oils are composed of triglycerides, as contrasted with waxes which lack glycerin in their structure. Although many different parts of plants may yield oil, in commercial practice, oil is extracted primarily from seeds. Vegetable fats and oils may be edible or non-edible. Vegetable oils from crops such as soybean, peanut, sunflower, rape, coconut, karanja, neem, cotton, mustard, jatropha, linseed and castor have been used as fuels for compression ignition engines in many parts of the world which lack petroleum reserves. Vegetable oils mostly consist of saturated hydrocarbons. It is nothing but triglycerides consisting of glycerol esters of fatty acids. The fatty acids vary in their carbon chain length and in numbers of double bonds. Table 3.1 summarizes data for some fatty acids that are commonly found in vegetable oils.

2 39 Table 3.1 Common fatty acids present in the vegetable oil Sl. No. Fatty Acid Structure * Chemical Formula 1 Myristic 14:0 C 14 H 28 O 2 2 Palmitic 16:0 C 16 H 32 O 2 3 Stearic 18:0 C 18 H 36 O 2 4 Avachidic 20:0 C 20 H 40 O 2 5 Belenic 22:0 C 22 H 44 O 2 6 Lignoceric 24:0 C 24 H 48 O 2 7 Oleic 18:1 C 18 H 34 O 2 8 Ricinoleic 18:1 C 18 H 34 O 2 9 Erucic 22:1 C 22 H 42 O 2 10 Linileic 18:2 C 18 H 32 O 2 11 Linolenic 18:3 C 18 H 30 O 2 * xx: y indicates xx carbons in the fatty acids chain with y double bonds. Ricinoleic is the only fatty acid, which contains a hydroxyl (OH) group. Palmitic (16:0) and Stearic (18:0) were the two most common saturated fatty acids with every vegetable oil containing at least a small amount of each. 3.2 PROPERTIES OF VEGETABLE OIL Most of the vegetable oils have the following properties: Cetane numbers which are generally in the range or close to that of diesel fuel. Heat contents of various vegetable oils are nearly 90% that of diesel fuel (Kalam and Masjuki 2002).

3 40 Long chain saturated, un-branched hydrocarbons are especially suitable for diesel engine. 3.3 PROBLEMS WITH VEGETABLE OIL The diesel fuel has a chain of carbons and fresh vegetable oil has a chain of 18. To burn in an engine, the chain needs to be broken down to the one similar in length to diesel. Incomplete combustion but characterized by nozzle choking, engine deposits, lube oil dilution, ring sticking, scuffing of the cylinder liners, injection nozzle failure and lubricant failure due to polymerization of the vegetable oil (Singh et al 2007; Sudharsan and Anupama 2006). Both cloud and pour points of esters are significantly higher than those for diesel fuel. These high values may cause problems during cold weather. Severe carbon deposit build up and sticking of the piston rings. The droplet size, low volatility, the long penetration distances as well as chemical properties of vegetable oils cause such problems.

4 MAJOR DIFFERENCES BETWEEN VEGETABLE OILS AND DIESEL FUEL The viscosities of vegetable oils are significantly higher, while the densities are only moderately higher. Vegetable oils have lower heating values. Vegetable oils raise the stoichiometric F/A ratio due to the presence of molecular oxygen. Vegetable oils may experience thermal cracking at the temperatures encountered by the fuel spray in naturally aspirated diesel engines. 3.5 VISCOSITY REDUCTION TECHNIQUES High viscosity of vegetable oils has been reported by almost all researchers as the major bottleneck in their use as fuel. To overcome this problem, various techniques have been successfully tried and the advances in this area are summarized below Preheating High viscosity is a major problem with vegetable oil in using it as engine fuel. One possible solution is to heat the oil in order to reduce its viscosity or to heat the intake air in order to accelerate the evaporation of the vegetable oil in the engine (Deepak Agarwal and Avinash Kumar Agarwal 2007).

5 Blending Vegetable oil can be directly mixed with diesel fuel and may be used for running an engine. The blending of vegetable oil with diesel fuel were experimented by several researchers. It has been proved that the use of 100% vegetable oil is also possible with some minor or without modifications in the fuel system (Pereira et al 2007; Ramadhas et al 2008) Micro-Emulsification The formation of micro-emulsion (co-solvency) is one of the potential solutions in solving the problems of vegetable oils viscosity. Microemulsion is defined as transparent, thermodynamically stable colloidal dispersions. The droplets diameters in micro-emulsions range from nm. A micro-emulsion is made of vegetable oils with ester and dispersant (co-solvent), or of vegetable oils, an alcohol and a surfactant and a cetane improver, with or without diesel fuels Cracking/Pyrolysis Cracking is the process of conversion of one substance into another by means of heat or with the aid of catalyst. It involves heating in the absence of air or oxygen and cleavage of chemical bonds to yield small molecules. The pyrolyzed material can be vegetable oils, animal fats, natural fatty acids and methyl ester of fatty acids. The pyrolyzate has lower viscosity, flash and pour point than diesel fuel but equivalent calorific value. The cetane number of the pyrolyzate is lower Transesterification Transesterification is a most commonly used and an important method to reduce the viscosity of vegetable oils. In this process, triglyceride

6 43 reacts with three molecules of alcohol in the presence of a catalyst producing a mixture of fatty acids, alkyl ester and glycerol. The process of removal of all the glycerol and the fatty acids from the vegetable oil in the presence of a catalyst is called esterification (Vedararaman et al 2005a). This esterified vegetable oil is called bio-diesel. Biodiesel properties are similar to diesel fuel. It is renewable, non-toxic, biodegradable and environment friendly transportation fuel. After esterification of the vegetable oil, density, viscosity, cetane number, calorific value, atomization and vaporization rate, molecular weight, and fuel spray penetration distance are highly improved. So these improved properties give good performance in CI engine. Physical and chemical properties are improved in esterified vegetable oil because esterified vegetable oil contains more cetane number than straight vegetable oil. These parameters induce good combustion characteristics in vegetable oil esters. So the unburnt hydrocarbon level is decreased in the exhaust. It results in lower generation of hydrocarbon and carbon monoxide in the exhaust than diesel fuel. In addition to the above methods, supercritical method is also found to reduce the viscosity of oil (Balat 2008; Saka and Kusdiana 2001). With this method it is easy to convert vegetable oil into biodiesel. 3.6 CHEMISTRY OF TRANSESTERIFICATION Transesterification is the process of using an alcohol (e.g. methanol or ethanol) in the presence of catalyst, such as sodium hydroxide (NaOH) or sodium methoxide (NaOMe) or potassium hydroxide (KOH), to chemically break the molecule of the raw renewable oil into methyl or ethyl esters with glycerol as a byproduct. The chemical structure of transesterification is as follows.

7 44 CH 2 OOCR CH 2 OH CHOOCR + 3CH 3 OH 3CH 3 OOCR + CHOH (Catalyst NaOH) CH 2 OOCR CH 2 OH Triglyceride Methanol Methyl Esters Glycerol Benefits of Transesterification 1. Reduces viscosity of the oil. 2. Increases the volatility. 3. Improves cetane number. 4. Reduces sulphur and aromatics. 5. Improves emission with oxidation catalysts. 6. Improves oxygen content (11%). 7. Improves lubricity. 8. Improves winter operability (-22 0 C). 3.7 DEFINITION OF BIODIESEL Biodiesel is a domestic, renewable fuel for diesel engines derived from natural oils like soybean oil, and which meets the specifications of ASTM D Biodiesel can be used in any concentration with petroleum based diesel fuel in existing diesel engines with little or no modification. Biodiesel is not the same as raw vegetable oil. It is produced by a chemical process which removes the glycerin from the oil (Banapurmath et al 2008).

8 PROPERTIES OF BIODIESEL Low content of free glycerin. High degree of Transesterification. Low acid number. No polymers, very clean. Comparable density with diesel. Comparable calorific value with diesel. Higher flash and fire points. Oxygen content presence up to 11%. 3.9 BENEFITS OF BIODIESEL Biodiesel has some clear advantages over vegetable oil: it works in any diesel engine, without any conversion or modifications to the engine or the fuel system (Can Hasimoglu et al 2008). It also has better cold-weather properties than vegetable oil but not as good as diesel. It has many advantages over diesel. Biodiesel substantially reduces unburned hydrocarbons, carbon monoxide and particulate matter in exhaust fumes. Sulphur dioxide emissions are eliminated (biodiesel contains no sulphur). Biodiesel is plant-based and adds no CO 2 to the atmosphere. As a sustainable energy source it merely recycles carbon, with the help of the sun and photosynthesis (Dincer 2008).

9 46 The ozone-forming potential of biodiesel emissions is nearly 50% less than conventional diesel fuel. Nitrogen oxide emissions could slightly increase but can be reduced to well below conventional diesel fuel levels by adjusting engine timing and other means. Biodiesel can be used in any diesel engine. Fuel economy is about the same as conventional diesel fuel. Biodiesel has a high cetane rating, which improves engine performance: 20% biodiesel added to conventional diesel fuel improves the cetane rating 3 points, making it a premium fuel. Biodiesel can be mixed with ordinary petroleum diesel fuel in any proportion, with no need for a mixing additive. Even a small amount of biodiesel means cleaner emissions and better engine lubrication. Biodiesel can be produced from any fat or vegetable oil, including waste cooking oil (Utlu 2007; Zafer Utlu and Mevlut Sureyya Kocak 2008). Biodegradable and non toxic. It can be used as fuel for cooking stove (Natarajan et al 2008). Auto ignition, fuel consumption, power output and engine torque are relatively unaffected by biodiesel. Esters have lower viscosities than the straight vegetable oil. 90% reduction in cancer risks.

10 47 The higher cetane number of biodiesel compared to diesel indicates potential for higher engine performances. Their higher flash point makes them safer to store. They contain higher amount of oxygen (up to 11%) that ensures more complete combustion of hydrocarbons. Biodiesel promotes rural development (Muralidharan et al 2004). Biofuel plantation reduces soil erosion DRAWBACKS OF BIODIESEL Increase in NO x emissions. Higher viscosity and lower volatility. Decrease in fuel economy on energy basis. Poor cold flow properties. This means that it thickens more than diesel fuel in cold weather. More expensive, it can be reduced only by mass production OXYGENATES The use of oxygenated fuels such as alcohols and non-alcohol oxygenated compounds as alternative fuels or as additive with diesel fuel in diesel engines is beneficial in enhancing energy sources and reducing pollutant emission. It is commonly accepted that with the use of oxygenates, a cleaner combustion can be achieved in the diesel engines. However, they are not suitable for using them alone in diesel engines because of their various

11 48 physical and chemical properties which might lead to different effects on the engine performance and exhaust gas emission. Hence their application to diesel engines has to be in the blended form with diesel engine fuels PROPERTIES OF OXYGENATES Good inter-solubility with diesel/biodiesel. Abundant sources for synthesis. Higher oxygen content and H/C ratio; this may improve the combustion process of the engine. Higher latent heat of vaporization in comparison with biodiesel. Lower calorific value. Comparable density with diesel/biodiesel. Almost all oxygenates have lower kinematic viscosity; hence the break up and spray formation can be improved BENEFITS OF OXYGENATES Many oxygenates can be easily mixed manually with diesel/bio-diesel. Hence, there is no phase separation. The increased oxygen content involved in oxygenated fuel will improve the combustion, and hopefully resolve the tradeoff relation between NO x and smoke in normal diesel engine. The properties of oxygenated fuel are much different from diesel and bio-diesel fuel, such as boiling point, viscosity

12 49 which influences the combustion process, fuel atomization and the spray characteristics. Volatility of biodiesel fuel can be improved with the addition of oxygenated compounds with low boiling temperatures. Addition of oxygenates assures clean and complete combustion. Increases the engine performance. Important factor in soot reduction is related to the chemical bonding of the carbon and oxygen atoms in the additive compound. All carbon atoms bonded to oxygen in the fuel will remain bonded to the oxygen; thus, they will not be incorporated into soot precursors during combustion. Therefore, the addition of oxygenated compounds effectively reduces the amount of carbon in the fuel that can participate in the formation of soot. Almost all oxygenates have physical and chemical properties suitable for application in IC engines. Adiabatic flame temperature may get lowered. Oxygenates have potential to reduce toxic gas emissions DRAWBACKS OF OXYGENATES More expensive. Lower calorific value.

13 50 Slight variation in unburned hydrocarbons and carbon monoxide emission levels. At low engine loads, the oxygenated fuel has only a slight effect on particulate emissions. Deterioration of engine thermal efficiency when oxygen content is above 30% w/w SELECTION OF OXYGENATES Among the various oxygenates, alcohols, DEE, DMC and DGL have been considered based on their safety, toxicity, availability, price and miscibility with biodiesel Benefits of Alcohols Addition of ethanol may change the spray characteristics, combustion, performance and emissions of the engine. Suitable oxygen content. Alcohols except methanol is less corrosive and alcohols are volatile in nature. Among the various alcohols, ethanol can be produced easily and economically and can be extracted from number of raw materials (sugar cane, molasses, cassava, waste biomass materials, sorghum, corn, barley, sugar beets, etc) Decrease in smoke, CO and NO x emissions.

14 51 Blending ethanol with other fuels lowers the volumetric energy density of the fuel. Enhances combustion efficiency Drawbacks of Alcohols Alcohols have an extremely low cetane number which may deteriorate autoignition capability of the fuels. Alcohols except ethanol are expensive Benefits of DEE The addition of DEE with diesel/biodiesel may change the spray characteristics, combustion, performance and emissions of the engine. Oxygen content is 21.6% by weight. High oxygen content causes a complete combustion and consequent lowering of CO and HC emissions. High cetane number. Non-corrosive and volatile in nature Drawbacks of DEE The addition of higher concentration of diethyl ether with diesel/biodiesel can cause vapor lock problems and erratic engine operation (due to the higher volatility).

15 52 NO x emission and smoke has no constant dependency on the diethyl ether percentage Benefits of DMC Non-toxic. Increase in engine thermal efficiency. Simultaneous reduction of NO x and smoke is possible. Decrease of PM and CO emissions. Suitable boiling point. High oxygen content (53.3%). Suitable inter-solubility with diesel/biodiesel fuel Drawbacks of DMC Slight increases in HC emissions. Lower calorific value. Lower cetane number Benefits of DGL Easily soluble. High cetane number. Low soot formation.

16 53 Simultaneous reduction in smoke and NO x emissions. High oxygen content. Suitable physical and chemical properties for application in diesel engines. Lower boiling point enhances the formation of fuel/air mixture. Lower kinematic viscosity improves the breakup and spray formation. Has no sulphur content.