Biodiesel

Production and Properties

By Amit Sarin

The Royal Society of Chemistry

Copyright © 2012 Amit Sarin
All rights reserved.
ISBN: 978-1-84973-470-7

Contents

List of Abbreviations, xxi,
Chapter 1 Introduction, 1,
Chapter 2 Vegetable Oil as a Fuel: Can it be used Directly?, 5,
Chapter 3 Biodiesel Properties and Specifications, 31,
Chapter 4 Oxidation Stability of Biodiesel, 51,
Chapter 5 Low-Temperature Flow Properties of Biodiesel, 80,
Chapter 6 Dependence of Other Properties of Biodiesel on Fatty Acid Methyl Ester Composition and Other Factors, 107,
Chapter 7 Major Resources for Biodiesel Production, 168,
Chapter 8 Diesel Engine Efficiency and Emissions using Biodiesel and its Blends, 140,
Chapter 9 Present State and Policies of the Biodiesel Industry, 204,
Chapter 10 The Food Versus Fuel Issue: Possible Solutions, 231,
Subject Index, 239,

CHAPTER 1

Introduction

1.1 Energy Demand

Today, more and more countries are prospering through economic reforms and becoming industrially advanced. Energy is a basic requirement for the economic development of every country. Every sector of the economy, including the agriculture, industry, transport, commercial and domestic sectors require energy. According to a report available on the United States Energy Information Administration (EIA) website, total world energy consumption was 406 quadrillion British thermal units (Btu) in 2000 and is projected to increase to 769.8 quadrillion Btu by 2035 (Figure 1.1). This is an increase of approximately 47.25%, and energy consumption will definitely increase further.

Can you imagine a world without the current mainstream fuels such as diesel or petrol used to run your vehicles, to cook your the food, to run every sector of the economy and so many other things where these fuels are required? The obvious answer is no. But the next question that arises here is for how long will this fuel be there to serve the needs of mankind? Are these fuels never going to end? No, the supply of these fuels is decreasing for so many obvious reasons. But what will happen if these fuels are not sufficient to cater for the needs of mankind? What will happen if crude oil prices will reach beyond the range of most people? We all are intentionally, consciously or subconsciously aware that day will come.

Therefore there is a need to explore alternative energy resources and the research community is taking part in this. Mainstream forms of renewable energy include wind power, hydropower, solar energy, biomass, and biofuels. The contribution of all these resources is important because of the aforementioned reasons, and biodiesel could be one of the major contributors.

1.2 Biodiesel: Green Fuel, Fuel of the Future or Magic Fuel?

Diesel fuels play an important role in the industrial development of most countries. These fuels play a major part in the transport sector and their consumption is steadily increasing. The intensity of fuel consumption is directly proportional to the development of the society. Diesel engines have been widely used in engineering machinery, automobiles and shipping equipment because of their excellent drivability and thermal efficiency. Diesel fuels are used in heavy trucks, city transport buses, locomotives, electricity generators, farm equipment, underground mining equipment etc.

The energy generated from the combustion of fossil fuels has indeed enabled many technological advancements and social–economic growth. However, it has simultaneously created many environmental concerns, which can threaten the sustainability of our ecosystem. The high demand for diesel in the industrialized world and pollution problems caused by its widespread use make it necessary to develop renewable energy sources of limitless duration with a smaller environmental impact than these traditional sources.

Therefore, to replace diesel fuel another renewable fuel is required and that could be biodiesel. Biodiesel is defined technically as ‘a fuel comprised of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats, designated B100, and meeting the requirements of the American Society for Testing and Materials D-6751’.

The importance of the use of vegetable oil as a diesel engine fuel was cited by none other than Sir Rudolf Diesel. Speaking to the Engineering Society of St. Louis, Missouri, in 1912 he said, “The use of vegetable oils for engine fuels may seem insignificant today, but such oils may become in the course of time as important as petroleum and the coal tar products of the present times”.

The first known report of using esters of vegetable oils as a motor fuel was described in a Belgian patent granted on 31 August 1937. Following this, there was a report on using esters in Brazil. A conference entitled ‘Vegetable Oil Fuels’ was held in Fargo, North Dakota, in August 1982, under the auspices of the American Society of Agricultural Engineers (ASAE; now the American Society of Agricultural and Biological Engineers, ASABE). The conclusion of the conference was that raw vegetable oils can be used as fuels but can lead to problems such as injector coking, polymerization in the piston ring belt area causing stuck or broken piston rings, and a tendency to thicken lubricating oil causing sudden and catastrophic failure of the rod and/or crankshaft bearings. A method for reducing the viscosity of the oil must be developed and the most likely technique for that was transesterification of vegetable oil (discussed in detail in Chapter 2). After that, various researchers worked to develop and search various resources for the production of biodiesel (discussed later in this book).

1.2.1 When and Who Coined the Term ‘Biodiesel’?

While searching for the answer to the question when and who coined the term biodiesel, I found an article entitled ‘Biodiesel: An alternative fuel for compression ignition engines’. In the article, the authors mention that they found a flyer from Bio-Energy (Australia) Pty Ltd that promotes equipment to produce a ‘Low Cost Diesel Fuel’ called ‘Bio-Diesel’, but the flyer was not dated. The flyer was found attached to a letter that uses the word ‘Bio-Diesel’ and the letter was also not dated. The authors had found three other articles dated 1984, one of which stated “… the two fuels, BioDiesel and distillate, are virtually identical …”. Therefore it can be concluded that the term ‘biodiesel’ was coined around 1984, however who actually used the term first is still doubtful.

Can there be any fuel which can act like magic? Can biodiesel be that fuel? Can biodiesel be the fuel of the future? In this book, I have tried to explore everything related to biodiesel: how biodiesel came into the picture; how it is synthesized; what its main properties are; the various sources of biodiesel the current status of the biodiesel industry and solutions to the food versus fuel issue. For me, biodiesel could be a magic fuel or the fuel of the future if explored properly. We shall see what your thoughts are after reading this book.

CHAPTER 2

Vegetable Oil as a Fuel: Can it be used Directly?

2.1 Introduction

The possibility of using vegetable oil as a fuel has been recognized since the beginning of diesel engines. Researchers have searched for alternate fuel sources and concluded that vegetable-oil-based fuels can be used as alternatives. Speaking to the Engineering Society of St Louis, Missouri in 1912, Rudolph Diesel, said, “The use of vegetable oils for engine fuels may seem insignificant today, but such oils may become in the course of time as important as petroleum and the coal tar products of the present times”. During the 1930s and 1940s, particularly during World War II, vegetable oils were used in emergencies as substitutes for diesel.

Vegetable oils have higher viscosities than commercial diesel fuel. The high viscosity of raw vegetable oil reduces fuel atomization and increases penetration, which is partially responsible for engine deposits, piston ring sticking, injector coking and the thickening of oil. Different methods have been developed to reduce the viscosity of vegetable oils such as dilution (blending), micro-emulsification, pyrolysis (thermal cracking), and transesterification.

2.2 Dilution (Blending)

Crude vegetable oils can be blended directly or diluted with diesel fuel to improve their viscosity. Dilution reduces the viscosity and engine performance problems such as injector coking and the creation of carbon deposits. In 1980, Caterpillar Brazil used a 10% mixture of vegetable oil and diesel to maintain total power without any modification or adjustment to the engine. A diesel engine study with a blend of 20% vegetable oil and 80% diesel fuel has also been carried out. Twenty-five percent sunflower oil and 75% diesel were blended as a diesel fuel and the reduced viscosity was 4.88 cSt at 313 K, while the maximum specified American standard test method (ASTM) value is 4.0 cSt at 313 K. This mixture was not suitable for long-term use in a direct-injection engine. The viscosity decreases with increasing percentage of diesel. Further, it was also reported that the viscosity of a 25 : 75 high oleic sunflower oil : diesel fuel blend was 4.92 cSt at 40 °C and that it has passed the 200 h Engine Manufacturers Association (EMA) test. Another study was conducted by using the blending technique on frying oil.

2.3 Micro-emulsification

Micro-emulsification is another approach to reducing the viscosity of vegetable oils. A micro-emulsion is defined as a colloidal equilibrium dispersion of an optically isotropic fluid microstructure with dimensions generally in the 1–150 nm range formed spontaneously from two normally immiscible liquids and one or more ionic amphiphiles. In other words; micro-emulsions are clear, stable isotropic fluids with three components: an oil phase, an aqueous phase and a surfactant. The aqueous phase may contain salts or other ingredients, and the oil may consist of a complex mixture of different hydrocarbons and olefins. This ternary phase can improve spray characteristics by explosive vaporization of the low-boiling-point constituents in the micelles. All micro-emulsions with butanol, hexanol and octanol meet the maximum viscosity limitation for diesel engines. A micro-emulsion prepared by blending soybean oil, methanol, 2-octanol and a cetane improver in the ratio of 52.7 : 13.3 : 33.3 : 1.0 has passed the 200 h EMA test.

2.4 Pyrolysis (Thermal Cracking)

Pyrolysis is a method of conversion of one substance into another through heating or heating with the aid of a catalyst in the absence of air or oxygen. It involves cleavage of chemical bonds to yield small molecules. The material used for pyrolysis can be vegetable oils, animal fats, natural fatty acids and methyl esters of fatty acids. The liquid fuel produced by this process has an almost identical chemical composition to conventional diesel fuel. Soybean oil has been thermally decomposed in air using the standard ASTM method for distillation. The viscosity of the pyrolyzed soybean oil distillate is 10.2 cSt at 37.8 °C, which is higher than the ASTM specified range for No. 2 diesel fuel but acceptable as it is still well below the viscosity of soybean oil.

2.5 Biodiesel and its Production: Transesterification

Transesterification, also called alcoholysis, is a chemical reaction of an oil or fat with an alcohol in the presence of a catalyst to form esters and glycerol. It involves a sequence of three consecutive reversible reactions where triglycerides (TGs) are converted to diglycerides (DGs) and then DGs are converted to monoglycerides (MGs) followed by the conversion of MGs to glycerol. In each step an ester is produced and thus three ester molecules are produced from one molecule of TG. Among the alcohols that can be used in the transesterification reaction are methanol, ethanol, propanol, butanol and amyl alcohol. Methanol and ethanol are used most frequently. However methanol is preferred because of its low cost. Figure 2.1 shows the transesterification reaction of TGs with alcohol. A catalyst is usually used to improve the reaction rate and yield. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side. It also produces glycerol as a byproduct which has some commercial value. Here R1, R2, R3 are long-chain hydrocarbons (HC), sometimes called fatty acid chains. Normally, there are five main types of chains in vegetable oils and animal oils: palmitic, stearic, oleic, linoleic, and linolenic. When the TG is converted stepwise to a diglyceride, then a monoglyceride, and finally to glycerol, one mole of fatty ester is liberated at each step (Figure 2.2). All of the steps in biodiesel production are shown in Figure 2.3.

Transesterification is the most viable process adopted known so far for the lowering of viscosity and for the production of biodiesel. Thus biodiesel is the alkyl ester of fatty acids, made by the transesterification of oils or fats, from plants or animals, using short-chain alcohols such as methanol and ethanol in the presence of a catalyst. Glycerin is consequently a by-product of biodiesel production (Figure 2.1). Pure biodiesel or 100% biodiesel is referred to as ‘B100’. A biodiesel blend is pure biodiesel blended with petrodiesel. Biodiesel blends are referred to as BXX. The XX indicates the amount of biodiesel in the blend (i.e., a B90 blend is 90% biodiesel and 10% petrodiesel).

2.6 Use of Catalysts in Transesterification

The catalysts used in the transesterification are broadly divided into two types:

(a) Base catalysts and (b) acid catalysts.

Base catalysts are preferred over acid catalysts, due to their capability of completion of reaction at higher speed, their requirement for lower reaction temperatures, and their higher conversion efficiency as compared to acid catalysts. Researchers have suggested that base catalysis is successful only when the free fatty acid (FFA) is less than one percentage, but it was also observed that base catalysts can be used in cases where the FFA content is greater than one, but more catalyst is needed. Later on, it was advocated that base catalysts exhibit excellent results when the FFA content of the oil is below two. It has also been reported that the rate of the transesterification reaction becomes a thousand times faster when a base catalyst is used instead of an acid catalyst. But these are not successful for oils having FFA content greater than three. However, it has also been reported that base catalysts could be effectively used for feedstock having FFA contents up to five.

The base catalysts cause saponification when they react with FFAs present in the vegetable oils or triglyceride, particularly when the acid value of the feedstock is high. In such cases, acid catalysts are used. The acid value of edible oils is normally low compared to non-edible oils. However, the acid value of edible oils also increases when they are used for frying purpose for long periods. In such cases the use of acid catalysts shows better results.

2.6.1 Base Catalysts

The reaction mechanism for base-catalyzed transesterification is shown in Figure 2.4. The reaction mechanism has three steps. The first step is the reaction of the carbonyl carbon atom with the anion of the alcohol, forming a tetrahedral intermediate, from which the alkyl ester and corresponding anion of the DG are formed. Another catalytic cycle is started when the catalyst reacts with a second molecule of alcohol. From there, DGs and MGs are converted into alkyl esters and glycerol.

2.6.1.1 Homogeneous Base Catalysts

If the catalyst used in the reaction remains in the same (liquid) phase to that of the reactants during transesterification, it is homogeneous catalytic transesterification. Various types of homogeneous base catalysts are used for transesterification. The most common among these are sodium hydroxide, sodium methoxide, potassium hydroxide, and potassium methoxide.

Sodium hydroxide (NaOH). The use of NaOH as a catalyst is preferred over potassium hydroxide because it causes less emulsification, eases the separation of glycerol, and is of lower cost. There are various reports of use of NaOH as a catalyst for biodiesel production.

Potassium hydroxide (KOH). KOH is a base catalyst which is widely used in the transesterification process. The performance of KOH was better than that of NaOH and the separation of biodiesel and glycerol was easier when KOH was used as a catalyst; hence it was preferred over NaOH.

Sodium methoxide (NaOCH3). NaOCH3 is more effective than sodium hydroxide as a catalyst because it disintegrates into CH3O- and Na+ and does not form water in contrast to NaOH/KOH. Moreover only 50% of it is required compared to NaOH. But the catalyst is less common due to its higher cost. It was found that 0.5% sodium methoxide and 1% sodium hydroxide exhibited similar results with methanol-to-oil molar ratios of 6. The use of NaOCH3 is reported in more of the literature.

Potassium methoxide (CH3OK). CH3OK is a base catalyst which can also be used for transesterification. Although it has been tested by many researchers, very few have recommended using it on regular basis.

2.6.1.2 Heterogeneous Base Catalysts

If the catalyst remains in a different phase (i.e., solid, immiscible liquid or gaseous) to that of the reactants the process is called heterogeneous catalytic transesterification. The heterogeneous catalytic transesterification is included under the term ‘Green Technology’ for the following reasons: (1) the catalyst can be recycled, (2) there is no or very little waste water produced during the process, and (3) separation of the biodiesel from the glycerol is much easier.

(Continues…)Excerpted from Biodiesel by Amit Sarin. Copyright © 2012 Amit Sarin. Excerpted by permission of The Royal Society of Chemistry.
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