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Figure 1.1 General reaction for transesterification of vegetable oil.
The second usual method of producing BD involves the use of an acid as a substitute of a base catalyst. Any mineral acid can be employed to catalyze the process; the most used acids are sulfuric acid and sulfonic acid. Although yield is high, the acids, being corrosive, may cause damage to the equipment, and the reaction rate is also observed to be relatively low [9, 21]. Oil feedstocks containing more than 4% FFAs must pass through an acid esterification process to increase the BD yield [25]. Such feedstocks are filtered and preprocessed to remove water and contaminants and then fed to the acid esterification process. The catalyst (sulfuric acid) is dissolved in methanol and then mixed with the pretreated oil [26].
The alcohols employed in the transesterification are generally short‐chain alcohols such as methanol, ethanol, propanol, and butanol producing esters named as methyl‐, ethyl‐, propyl‐, and butyl‐esters, respectively [9, 10]. It is reported that when transesterification of soybean oil using methanol, ethanol, and butanol was performed, 96–98% of ester’s yield could be obtained after an hour of reaction [27]. Though utilizing different alcohols presents little differences with regard to the kinetic of reaction, the final yield of esters remains unchanged. Thus, assortment of the alcohol is based on cost and performance consideration. Generally, reaction temperature is set at near the boiling point of the alcohol used [28].
Due to the reality that many vegetable oils, including soybean, canola (rapeseed) oil, and rice bran oil, have a major number of FAs with double bonds, oxidative stability is a problem, particularly when storing BD for longer period of time [29, 30]. This problem becomes severe due to improper storage conditions, which may include exposure to air and/or light, temperatures above ambient, and presence of extraneous materials (contaminants) with catalytic effect on oxidation. Some additives such as antioxidants might control the oxidation.
Characterization of BD fuel properties and evaluation of its quality are the matters of great concern for the successful commercialization of this fuel. A high fuel value with no operational problems is a condition for market acceptance of BD. Accordingly, the analysis of BD and the monitoring of the transesterification reaction have been the subject of numerous publications [31, 32]. The constraints, which are used to define the quality of BD, can be divided in two groups [33]. One of them is also used for mineral diesel, and the second illustrates the composition and purity of fatty esters. The former includes, for example, density, viscosity, flash point, sulfur percentage, carbon residue, sulfated ash percentage, cetane number, and acid number. The latter comprises, for example, methanol, free glycerol, total glycerol, phosphorus contents, water, and esters content. Chromatography and spectroscopy are the mainly used analytical methods for BD analyses, but procedures based on physical properties are also available [34]. Furthermore, it is important to mention that in most chromatographic analyses, mainly gas chromatography (GC) has been applied to methyl and not to ethyl esters [29].
As the demand for vegetable oils for food has increased tremendously in recent years, hence, the contribution of nonedible oils such as jatropha, Moringa oleifera, rice bran oils, etc. can play an important role for BD production. In view of the limited petro‐oil resources and rapidly growing energy demands of the world, there is an extensive need to take immediate initiatives for exploring alternative energy sources to meet the domestic needs and reduce the dependence on imported fossil fuels. In view of the future perspectives of biofuels, the present book chapter was designed with the main purposes to assess the feasibility of BD production from multi‐feedstock vegetable oil sources.
1.2 History of the Use of Vegetable Oil in Biodiesel
The idea to use vegetable oils as fuels for diesel engines dates back to more than one hundred years. Historically, Rudolf Diesel, the inventor of diesel engine, at the Paris Exhibition in 1900, conducted engine tests, for the first time, on peanut oil [22, 35]. At that moment Diesel said, “The use of vegetable oils for engine fuels may seem insignificant today. However, such oils may in course of time be as important as petroleum and the coal tar products of the present time.” Today, over a century later, the scientific community is working to fulfill his dream by considering potential benefits of BD as an alternative fuel to petrodiesel for future uses.
1.3 Feedstocks for Biodiesel Production
All over the world, the usual lipid feedstocks for BD production are refined vegetable oils. In this group, the oil of choice varies with location according to availability; the most abundant lipid is generally the most common feedstock. The bases for this are not only the desire to have an ample supply of product fuel but also because of the inverse relation between supply and cost. Refined oils can be comparatively costly under the best of conditions, compared with petroleum products, and the choice of oil for BD production depends on local availability and corresponding affordability. The four oil crops clearly dominate the feedstock sources used for worldwide BD production. With a share of nearly 85%, rapeseed oil is by far leading the field, followed by sunflower seed oil, soybean oil, and palm oil [36]. Apart from the “great four” – rapeseed oil, sunflower seed oil, soybean oil, and palm oil in BD production – other edible plant oils have also successfully been transesterified to produce BD.
The choice of raw material used for BD production in a specific region mainly depends on the respective climatic conditions. Thus, rapeseed and sunflower oils are mainly used in the European Union [37], palm oil predominates in BD production in tropical countries [38, 39], and soybean oil [40] and animal fats are the major feedstocks in the United States. FA ester production has also been demonstrated from a variety of other feedstocks, including the oils of coconut [41], rice bran [42], Thespesia populnea [43], safflower [44], palm kernel [45], M. oleifera [46], Citrus reticulata (mandarin orange) [47], Jatropha curcas [48], Ethiopian mustard [13], Cynara cardunculus [49], Hibiscus esculentus [50], maize [51], Cyperus esculentus (Barminas et al. [52]), Prunus mahaleb [53], kapok [54], tobacco [55], milkweed [7], Yucca aloifolia [56], Oleum papaveris seminis [57], Pongamia [58], Brassica napus [59], Citrullus colocynthis [53], rubber seed oils [60], palm FA distillate [61], the animal fats, tallow [7, 62], lard [63], and waste oils [64, 65]. As such, any animal or plant lipid should be a ready substrate for the production of BD. Such features as supply, cost, storage properties, and engine performance will determine whether a particular potential feedstock is actually acceptable for commercial fuel production.
One way of reducing the production costs for BD fuels is the use of nonedible oils, which tend to be considerably cheaper than edible vegetable oils [66]. A number of plant oils contain substances that make them unsuitable for human consumption. In some cases, these substances can be removed by refining. For example, gossypol contained in cottonseeds can effectively be eliminated from the oil and the press cake to allow utilization as a cooking oil and animal feed, respectively [55]. Sometimes harmful ingredients can also be eliminated by breeding, as was the case with glucosinolates and erucic acid in rapeseed. In many cases, however, the removal of toxic components from the fatty material has not been accomplished or even attempted yet.
1.3.1 Generations of Biodiesel
BD production has proven to be sustainable because of the wide coverage of raw material availability, estimated at more than 350 types of oilseed crops worldwide. The feedstocks are generally easily accessible but vary depending on geographic location, weather conditions, land type, and agricultural practices in any country. In addition, the BD feedstocks portray 75% of total manufacturing cost; thus it is essential to choose appropriate feedstock to ensure the BD production feasibility. Typically, BD raw materials can be categorized as first‐generation, second‐generation, and third‐generation BD, accordingly to the material used for the BD synthesizing as shown in Table
