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1.3.2 First‐Generation Biodiesel
First‐generation BD may be defined as edible oils from agricultural products such as palm oil, olive, sunflower, coconut, canola, rapeseed soybeans, etc. [71, 72]. Today, derived BD from edible oils has reached approximately more than 95% and has raised many issues, especially the competition between food supply and oil demand crisis, deforestation, and soil destruction for feedstock plantation purposes [73]. In the past decade, the price of edible oils has escalated, while the production demand for BD conversion is continuously increasing, resulting in edible‐based BD being less economically feasible [74]. Given these circumstances, the exploitation of first‐generation BD as a replacement for diesel fuel has put the world's stock of edibles in jeopardy.
Table 1.1 Main feedstocks of biodiesel.
Source: Adapted from [67–70].
| First‐generation oil | Second‐generation oil | Third‐generation oil |
|---|---|---|
| Soybean Canola Palm Rapeseed Coconut Olive Sunflower Peanut Sesame Mahua Barley Wheat | Rubber seed Cotton seed Tobacco seed Karanja Jojoba oil Neem Moringa Jatropha Coffee ground Used cooking oil Tallow Fish oil Chicken fat Bitter almond oil | Nannochloropsis oculata Chlamydomonas pitschmannii Isochrysis sp. Chlorella vulgaris Monoraphidium sp. |
1.3.3 Second‐Generation Biodiesel
Currently, BD derived from inedible oil has sparked scientists' interest in replacing reliance on edible oil‐based diesel. The inedible oil crops can be planted in wasteland or fallow areas without intensive agriculture, which can produce high oil yields [39]. Besides, waste oils and animal fats can be categorized as second‐generation BD. The use of waste as a BD feedstock may reduce the problem of waste disposal and the cost of BD production [75]. Both waste oils and animal fats are most likely to contain water and a slightly higher FFA value compared with virgin oils, which result in lower oil quality. Different types of raw materials will produce different quantities of yield and characteristics of the oil, so the selection of raw materials is very important because the cost of producing BD is very expensive.
1.3.4 Third‐Generation Biodiesel
Recently, the use of microalgae‐based BD has gained immense awareness and prospects for meeting the growing supply of BD feedstocks. The microalgae‐based BD has the advantage of growing at a faster rate under photoautotrophic condition and is able to produce high yield of oil than edible and nonedible crop oil [76]. Also in the future, microalgae may make a significant contribution to addressing the issue of food production versus BD production and reducing competition for farmland [77]. Moreover, a study discovered that the algae‐based BD has lower carbon footprint, which is beneficial to the ecosystem [67]. Nevertheless, it is important to study the production cost and energy output of algae‐based BD so that it is much more feasible and cost‐effective for mass production as an alternative to fossil fuel sources.
1.4 Basics of the Transesterification Reaction
The direct use of a vegetable oil in diesel engines is problematic because of its high viscosity (about 11–17 times higher than petrodiesel fuel), which reduces the fuel atomization leading to high engine deposits, thickening of lubricating oil, and lower volatilities that cause the formation of deposits in engines due to incomplete combustion [78, 79]. The extremely high flash points of vegetable oils and their tendency for thermal or oxidative polymerization aggravate the situation, leading to the formation of deposits on the injector nozzles, a gradual dilution and degradation of the lubricating oil, and the sticking of piston rings. As a consequence, long‐term operation on neat plant oils or on mixtures of plant oils and fossil diesel fuel inevitably results in engine breakdown [80].
Chemically, the vegetable oils/animal fats consist of triglyceride molecules of three long‐chain FAs that are ester bonded to a single glycerol molecule. These FAs differ by the length of carbon chains and by the number, orientation, and position of double bonds in these chains. Thus, BD refers to alkyl esters of long‐chain FAs, which are synthesized either by transesterification with alcohols or by esterification of FAs. The latter strategy aimed at modifying plant oils by various technologies to produce fuels that approximate the properties and performance of fossil diesel. Four methods to decrease the high viscosity of vegetable oils to enable their use in common diesel engines without operational problems such as engine deposits have been investigated: blending with petrodiesel, pyrolysis, microemulsification, and transesterification [81]. Transesterification is by far the most common method that leads to the products commonly known as BD, which are alkyl esters of vegetable oils or animal fats.
Pyrolysis denotes thermal decomposition reactions, usually brought about in the absence of oxygen. Pyrolysis of vegetable and fish oils, optionally in the presence of metallic salts as catalysts, was conducted as a means of producing emergency fuels during the Second World War, as various Chinese, Japanese, or Brazilian publications show [13]. In addition, the technology has occasionally found entry into the more recent literature as well [82, 83]. This treatment results in a mixture of alkanes, alkenes, alkadienes, aromatics, and carboxylic acids, which are similar to hydrocarbon‐based diesel fuels in many respects. The cetane number of plant oils is increased by pyrolysis, and the concentrations of sulfur, water, and sediment for the resulting products are acceptable. However, according to modern standards, the viscosity of the fuels is considered as too high, ash and carbon residue far exceed the values for fossil diesel, and the cold flow properties of pyrolyzed vegetable oils are poor [81]. Moreover, it is argued that the removal of oxygen during thermal decomposition eliminates one of the main ecological benefits of oxygenated fuels, namely, more complete combustion due to higher oxygen availability in the combustion chamber [22].
Microemulsification is the formation of thermodynamically stable dispersions of two usually not miscible liquids, brought about by one or more surfactants. Drop diameters in microemulsions typically range from 100 to 1000 Å [84]. Various investigators have studied the microemulsification of vegetable oils with methanol, ethanol, or 1‐butanol [85, 86]. They arrived at the conclusion that microemulsions of vegetable oils and alcohols cannot be recommended for long‐term use in diesel engines for similar reasons as applicable to neat vegetable oils. Moreover, microemulsions display considerably lower volumetric heating values as compared to hydrocarbon‐based diesel fuel due to their high alcohol contents [84], and these have also been assessed insufficient in terms of cetane number and cold temperature behavior [87].
Transesterification with lower alcohols, however, has emerged to be an ideal modification, so that the term “biodiesel” is now only used to denote products obtained by this process. The reaction between triglycerides and lower alcohols, yielding free glycerol and the FA esters of the respective alcohol, was first described in 1852 [88]. In the 1930s and 1940s, this reaction was frequently applied in the fat and soap industry. The Belgian patent on the production of palm oil ethyl esters by acid‐catalyzed transesterification describes the first use of a fuel, which would now be referred to as “biodiesel” [7].
Transesterification is one of the reversible reactions and proceeds essentially by mixing the animal fat/vegetable oil with an alcohol (usually methanol). However, the presence of a catalyst (usually a base) accelerates the conversion to produce the corresponding alkyl esters (or for methanol, the methyl esters) of the FA mixture that is found in the parent vegetable oil or animal fat [79]. The general scheme of the transesterification reaction is given in Figure 1.1.
The
