To consider any feedstock as a biodiesel source, the oil percentage and the yield per hectare are the important parameters. The estimated oil content and yields of many biodiesel feedstocks can be found in Refs. [8,19,21,22]. The feedstock for biodiesel production is chosen according to quality, availability in each country, physicochemical properties, and its production cost. Composition of the oil is also one of the important criteria to determine the suitability of oil as a raw material for biodiesel production [8].

Feedstock alone represents 75% of the overall biodiesel production cost [21,23]. Therefore, selecting the cheapest feedstock is vital to ensure low biodiesel production cost. In general, biodiesel feedstock can be divided into four main categories as [5,23,24]:

The use of edible oils raises many concerns, such as food versus fuel crisis, and major environmental problems, such as serious destruction of vital soil resources, deforestation, and usage of much of the available arable land. Moreover, since mid-2000s, the prices of vegetable oil plants have increased dramatically that negatively affect the economic viability of biodiesel [25–27]. Furthermore, their use is not feasible in the long term because of the expected growing gap between supply and demand of such oils in many countries. For instance, dedicating all US soybean to biodiesel production would meet only 6% of diesel demands [28].

Nonedible oils are one of the possible solutions to reduce the utilization of the edible oil for biodiesel production. Nonedible oil resources are gaining much attention because they are easily available in many parts of the world, especially waste lands that are not suitable for food crops, eliminate competition for food, reduce deforestation rate, are more environmentally friendly, produce useful by-products, and are very economical when compared with edible oils. There are many publications that have been published on nonedible oils in the past few years. For further reading, see Refs. [18,26,29–41].

More recently, microalgae have emerged to be the third generation of biodiesel feedstock. Microalgae are photosynthetic microorganisms that convert sunlight, water, and CO₂ to algal biomass, but they do it more efficiently than conventional crop plants. It represents a very promising feedstock because of its high photosynthetic efficiency to produce biomass, higher growth rates, productivity, and high oil content when compared to edible and nonedible feedstocks. Microalgae have the potential to produce an oil yield that is up to 25 times higher than the yield of oil palm and 250 times the amount of soybeans. This is because microalgae can be grown in a farm or a bioreactor. Moreover, they are easier to cultivate than many other plants. It is believed that microalgae can play an important role in solving the problem between the production of food and that of biodiesel in the near future. Moreover, among other generations of biodiesel feedstocks, microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for fuels and can be sustainably developed in the future. The main obstacle for the commercialization of microalgae is its high production cost due to the requirement of high oil–yielding algae strains and effective large-scale bioreactors. Recent studies indicate that algae for biodiesel production can grow on flue gas, giving opportunities in consuming greenhouse gas [23,42–44]. Januan and Ellis [3] and Lin et al. [45] show that genetically engineered plants such as poplar, switchgrass, miscanthus, and big bluestem can be considered new feedstocks for biodiesel production. These feedstocks will create new bioenergy crops that are not associated with food crops. Therefore, they are expected to represent a sustainable biodiesel feedstock in the future. However, precaution on biosafety must be considered for these feedstocks. Fig. 17.1A and B show some plant families contributing to biodiesel production and some important plant species with the highest oil content, respectively, extracted from the aerial parts as well as the roots [46].