## 2 General Aspects of Biodiesel
A large amount of vegetable oils and animal fats have been studied for biodiesel production, but the vegetable oils are the main raw material used in the biodiesel industry. Globally, there are more than 350 oil-bearing crops identified as potential sources for biodiesel production [7]. Biofuels can be classified in four groups depending on the feedstock by which it is produced: first, second, third, and fourth generation. First, generation biodiesel is derived from edible oils as coconut, palm, soybean, mustard, sunflower oils, and others. Biodiesel produced from non-edible oils is the second generation of this biofuel. Feedback for second generation of biodiesel are oils obtained from cotton seed, jatropha, jojoba, karanja, linseed, mahua, neem, rubber seed, and tabaco [27]. The third generation of biodiesel is the biofuel derived from algae, waste oils, and animal fats. Genetically modified algae are used for produce the fourth generation biodiesel. Only first and second generation biofuels are commercially produced [28]. Now, biodiesel is facing some important challenges that must be overcome in the future. Some challenges are associated with biodiesel feedstock selection, optimization and production processes, the oil extraction and conversion process, fuel characterization and property analysis, and storage and transport [29]. Moreover, the limitations of the usual catalysts used for the production of biodiesel have created a niche for the investigation of new catalysts as ionic liquid that can be recycled easily [30] or nano-catalysts [31]. The raw material selection and catalyst type play significant roles in the economic viability of biodiesel production [32]. In addition, to improve the biodiesel production, recently new research has been focused on the use of advanced transesterification reactors [24]; the use of novel transesterification by enzyme that has advantages in terms of conversion, yield and reusability conversion, yield and reusability [33], and the production of third- and fourth generation biodiesel [27]. Another challenge that biodiesel faces, and in general all biofuels, is the adequate transition from conventional fossil fuels to biofuel. It implies the establishment of adequate policies to achieve a sustainable energy transition. In this sense, recent studies have focused on this aspects. [PERSON] et al. (2017) identify and recommend the most effective policy combinations to steer a sustainable energy transition under alternative crisis scenarios in the case of biofuels in Italy [34]. [PERSON] et al. (2019) develop a methodological approach to the formulation of an effective policy aimed at supporting the development of an innovation niche (liquid biofuels in Italy) facing adverse conditions [35]. [PERSON] and [PERSON] (2019) give policy recommendations for the conversion of car and urban bus fleets to 100% electric or biofuel (bioethanol and biodiesel) use by 2050 compared to a business as usual (BAU) by development of a life-cycle analysis of the Brazilian urban passenger transport system [36]. ASTM D6751 (USA), EN 14213 (European Union), IS 15607 (India), etc. standards are used to maintain the quality of produced biodiesel for marketing. These standards are used for pure biodiesel (B100), not for biodiesel blends. For biodiesel blends, standards are only given by ASTM [27].