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 1.1 [15].
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