## 2 Biofuels Classification
### Second-Generation Biofuels
Moreover, its low volatility (boiling temperature of 78.4 \({}^{\circ}\)C, higher than that of petrol, normally around 30-35 \({}^{\circ}\)C) leads to problems and higher costs associated with the distribution network [20]. * **Biodimethyl ether** (bio-DME) is dimethyl ether obtained from biomass. The production process is based on the gasification of lignocellulosic biomass to bimethanol and its subsequent conversion to bio-DME. The bio-DME is gaseous at room temperature and liquid at pressures above 5 bar, or if the temperature is below \(-25\)\({}^{\circ}\)C. In general, it can be used in the liquid state, operating at pressure values in the order of 5-10 bar [11]. DME and BioDME are most commonly used as substitutes for propane in liquid petroleum gas (LPG), especially in Asia, but can also be used as substitutes for diesel fuel in transportation. Besides being able to be produced from a number of renewable and sustainable resources, bio-DME also has the advantage of having a higher cetane number than traditional diesel and, therefore, better combustion quality than diesel fuel during compression ignition. As a result, an engine tailored to work with DME can achieve higher efficiencies, better mileage, and emissions reductions [20]. * **Bio-methyl tert-butyl ether** (bio-MTBE) is produced from biomethanol and has the effect of raising the octane number in gasoline, without reducing its energy density or increasing its volatility. Since isobutene from oil also participates in the synthesis reaction, bio-MTBE is considered a biofuel to the extent that biomethanol is present in its composition (36%). With the gradual elimination of lead, since the mid-1980s it has become one of the most used components for the formulation of gasoline. The lower cost and toxicity of bio-MTBE relative to tetraethyl lead and benzene have increased its use as an anti-knocking agent in all green gasolines. Nowadays, bio-MTBE is used in percentages ranging from 7% to 12% in volume [21]. * **Biobutanol** is a liquid biofuel produced, through the fermentation of sugars by the microorganism _Clostridium acetobutylcium_, from the same raw materials as bioethanol [22]. Biobutanol has some positive characteristics compared to bioethanol: it is less corrosive, its mixture with fossil fuels is more convenient because the mixtures do not undergo phase separation, and the storage and distribution of biobutanol are easier. Although biobutanol has a higher energy density than bioethanol, it has a lower octane number and, therefore, better performance [22]. * **Fischer-Tropsch diesel** (FT-diesel, FT-liquid, or synthetic biofuels) consists of synthetic hydrocarbons or mixtures of synthetic hydrocarbons derived from biomass. The best-known process for the conversion of energy from lignocellulosic biomass into liquid biofuels (biomass to liquid, BTL) is Fischer-Tropsch synthesis, which was used on a large scale in Germany during World War II [8]. The so-called Fischer-Tropsch process consists of the gasification of the lignocellulosic biomass, the purification and conditioning of the synthesis gas produced (a mixture of carbon monoxide (CO) and hydrogen (H\({}_{2}\))), and its subsequent conversion to liquid biofuels (FT-liquids). The liquid products consist of straight-chain hydrocarbons, do not contain sulfur compounds (which are eliminated in the purification process), and can be converted into fuels for automotive use [8]. FT-diesel has a behavior similar to that of fossil fuels, in terms of lower calorific value, density, and viscosity, but also a higher cetane number and lower aromatic content, which results in lower emissions of particulate matter and nitrogen oxides. The two fuels can be mixed in any proportion, without the need to make changes in the engine and the distribution infrastructures [11].