## 2 Biofuels Classification
### First generation Biofuels  
* **Biodiesel** consists of a mixture of methyl esters produced by the chemical conversion of animal and/or vegetable oils and fats and is characterized by a high energy density (37 MJ kg\({}^{-1}\); [8]). The materials used for the production of biodiesel are oilseed crops, such as sunflower and rapeseed. Other species, such as soybean or palm oil, are of less interest because their seeds have fairly modest oil contents or they pose problems of environmental and socioeconomic sustainability, respectively [11]. The production process includes the extraction of oils from the seeds, refining, and chemical conversion into biodiesel by transesterification reactions [13]. The process can be considered energetically efficient, with an output/input energy ratio between 1.25-3.67 for soybean (depending on the production conditions) and 2.29 for rapeseed oil [9], greater than 0.84 considered for diesel oil. Biodiesel is used in thermal motors for the direct production of electrical and thermal energy. However, the biodiesel production sector faces strong competition from the oil production sectors for human consumption of the same resources. Due to some of its properties (like a higher cetane number than gas), biodiesel performs better than diesel, as it shows a greater readiness to ignite and its higher flash point value, compared to diesel, ensures greater safety in handling [11]. * **Bioethanol** is the ethyl alcohol produced by the fermentation of the sugars present in biomass in absence of oxygen and is characterized by a high energy content (27 MJ kg\({}^{-1}\)) [14]. The raw materials are divided, according to their carbohydrates content, into saccharides (simple sugars), starch, and lignocellulosic (cellulose and hemicellulose). Traditionally-used crops are sugar beet in Europe and sugar cane in Brazil as sugar crops [8], and maize as a starch crop [14]. The use of lignocellulosic biomass requires different pre-treatments followed by a hydrolysis (acid or enzymatic) step to obtain the monomeric sugars for microbial fermentation [14]. In the case of bioethanol production from starch or sugar crops (wheat grain or sugar beet), hydrolysis, fermentation, and distillation are the most energy demanding steps (64%-74% of total energy input) and involve the greatest GHG emissions, but the use of straw reduces the GHG emissions as this is a waste by-product of grain production [9]. Therefore, bioethanol production from by-products is a more environmentally friendly procedure than the production from grain. In terms of energy efficiency, the energy ratio ranges between 1.2 (for maize) to 2.78 for sugar beet [9]. But the efficiency depends on the conditions of the industrial process. However, clear advantages in terms of GHG emissions have been revealed by [PERSON] et al. [9], with values generally lower that 12 g C eq MJ\({}^{-1}\), in comparison with the range found for fuels from fossil sources (18-36.4 g C eq MJ\({}^{-1}\)). * **Bio-ETBE** (ethyl-tert-butyl-ether) is an organic compound derived from ethyl and isobutyl alcohols that can be used as an anti-detonator to increase the octane level in gasoline [15]. Bio-ETBE is produced from bioethanol and, therefore, shares its raw materials (cereals and sugary raw materials). It is produced through the chemical reaction between isobutanol and bioethanol, with acid catalysis, which takes place on the surface of an ion exchange resin [11]. * **Pure vegetable oils** are obtained by mechanical extraction from oil seeds. Some properties, such as the net calorific value (or lower heating value, LHV), are comparable to those of biodiesel [11]. * **Biogas** is a mixture of the gases carbon dioxide (CO\({}_{2}\)) and methane (CH\({}_{4}\)), produced as a result of the successive biochemical reactions that take place during the AD process--biomethanation (hydrolysis, acidogenesis, acetogenesis, methanogenesis). The presence of methane in the composition of biogas (50%-75%) [5] has a decisive influence on its energy capacity (average of 23 MJ Nm\({}^{-3}\)).