## 2 Biofuels Classification  
The term \"biofuel\" refers to a fuel in the liquid or gaseous state obtained from biomass [7]. Traditionally, the use of biofuels has been linked to the transport sector, for motor vehicles, to replace fossil fuels. However, there has been a recent and rapid expansion of the production and application of biofuels for electrical and thermal energy generation, and in particular, for cogeneration. Therefore, the current concept of biofuel has outgrown the link with the transport sector, and more emphasis has been given to other applications. Based on the state of maturity of the production and use technologies, biofuels can be divided in two categories [8]:  
1. First-generation biofuels: e.g., biodiesel from pure vegetable oils, bioethanol produced from cereals and sugary raw materials, bio-ETBE (ethyl tert-butyl ether) produced from bioethanol, and biogas from anaerobic digestion (AD) systems. Their production and application has already started, while the main margins for improvement being, at the moment, the reduction of production costs, the optimization of the energy balance, the increase in the energy yields of the engines, and the increase in the percentage of use in mixtures with fossil fuels;
2. Second-generation biofuels: e.g., bioethanol produced from lignocellulosic raw materials, biohydrogen, syngas, bio-oil, biomethanol, biodimethylether, bio-MTBE (methyl tert-butyl ether), biobutanol, and synthetic diesel, obtained through the Fischer-Tropsch reaction. Their production has not yet started on a commercial scale and is limited to experimental installations. The second-generation biofuels are linked by the possibility of being produced from lignocellulosic biomass at low or zero cost. This differentiation between first- and second-generation biofuels is important as the former often stand in concurrence with or have undesired economic influences on food production, which implies a series of social/moral and economic consequences that have to be considered [9]. First-generation biofuels normally offer some advantages in terms of energy production potential, but they have some disadvantages such as raw materials supply, impact on biodiversity and land use, and competition with food crops, which provokes the increase in food prices resulting from increased production of these fuels [8]. However, the production of second-generation biofuels can imply benefits in terms of C-mitigation and ecosystem services, with a demonstrated lower impact on greenhouse gas emissions than the traditional fuels from fossil sources [9]. Therefore, second-generation biofuels, produced from plant biomass, one of the most abundant and underused biological resource systems on the planet, can be considered a viable alternative. Biomass, in fact, can be simply burned to produce heat and electricity, or used to produce liquid biofuels. But, as the production of biofuels from agricultural by-products could only meet part of the growing demand for liquid fuels, interest has shifted towards the use of dedicated crops. Second-generation biofuels could, therefore, significantly reduce CO\({}_{2}\) production, not compete with food crops, and in some cases offer better engine performance. If marketed, the cost of second-generation biofuels has the potential to be comparable to standard petrol or diesel and would be the cheapest route to low-carbon renewable energy for transport [8]. Comparing the higher heating value (HHV) and lower heating value (LHV) of biofuels with those of fossil fuels, it can be seen that they are normally lower than those presented, for example, by gasoline (HHV = 46.9 MJ kg\({}^{-1}\), LHV = 44.2 MJ kg\({}^{-1}\)), fuel oil (HHV = 43.3 MJ kg\({}^{-1}\), LHV = 40.0 MJ kg\({}^{-1}\)), and diesel fuel (HHV = 45.6, LHV = 42.6). However, the use of biomass provides many environmental benefits [10]. The production costs of biofuels are, at present, higher than those of fossil substitutes, even using the most advanced technologies.