## 2 Biofuels Classification
### Second-Generation Biofuels  
Innovative \"second-generation\" biofuels are obtained from lignocellulosic biomass and represent a promising technological improvement. These biofuels show better characteristics and performance than those of the first generation (Tables 1 and 2) but, at the same time, require more complex and improved technological and processing solutions. * **Biohydrogen** is obtained from the biomass and/or the biodegradable fraction of organic wastes through two different processes: a thermochemical process, which involves gasification followed by a steam-reforming phase to increase the final hydrogen content; and microbial fermentation of sugars in the dark, under anaerobic conditions. Different microorganisms (_Enterobacter_, _Citrobacter_, _Bacillus_, and _Clostridium_) have been reported to produce hydrogen through dark fermentation [16]. This option is similar to AD but is reformulated to produce hydrogen directly instead of methane. While dark fermentation is a major light-independent process, other biological options for biohydrogen production are light-mediated processes, which include direct or indirect biophotolysis and photofermentation. Biophotolysis involves the light-driven decomposition of water in the presence of micro-algae or cyanobacteria, while in photofermentation, photosynthetic microorganisms convert organic acids, intermediate products of the microbial metabolic pathways, into biohydrogen in the presence of solar radiation [16]. * **Bio-oil** is the liquid product of lignocellulosic biomass pyrolysis, with similar characteristics to petroleum. The pyrolysis process is the initial stage of combustion and gasification processes. Pyrolysis is a thermochemical transformation process at a temperature of about 500 \({}^{\circ}\)C, generally carried out in an oxygen-free environment, which yields liquid (bio-oil), solid (biochar), and gas products. Fast pyrolysis is preferred to increase the bio-oil yield, while slow processes are used to obtain biochar [17]. In the fast pyrolysis process, the biomass is rapidly heated to a high temperature in the absence of oxygen, producing about 60%-75% oil products, 15%-25% solid products, and 10%-20% gaseous phase. The characteristics of the fast pyrolysis process are a high rate of heat transfer and heating, a very short residence time of the steam, and rapid cooling of the vapors and aerosol to give a high yield of bio-oil and precision in the control of the reaction temperature. This technology has been widely studied in the last decade because it shows several advantages, like: (i) the production of renewable fuels for boilers, engines, and turbines; (ii) low costs; (iii) low CO\({}_{2}\) production; (iv) the possibility of using second-generation bio-oil and waste materials (forest residues, urban and industrial waste, etc. ); (v) ease of storage and transport of liquid fuels; (vi) high energy density compared to combustible gases; (vii) the possibility of separating the minerals, to be recycled as nutrients for the soil; (viii) the possibility of primary separation of sugars and lignin [17]. Bio-oil is one of the highest-quality combustible hydrocarbons, although it is currently burdened by unacceptable energy and economic costs [18; 19]. * **Biomethanol** is obtained from lignocellulosic biomass, while traditional methanol is obtained by the catalytic conversion of a fossil fuel (usually natural gas). However, the most widespread process involves the gasification of the biomass and the catalytic conversion (with chromium oxide and zinc oxide) of the CO\({}_{2}\) and H\({}_{2}\) present in the syngas obtained into biorethanol. The reaction for the production of biomethanol generally occurs under conditions of high temperature (400 \({}^{\circ}\)C) and pressure (40-80 atm; [11]). The main problem of biomethanol is linked to the safety of the storage, transport, and handling phases, as it burns without a visible flame and is toxic by inhalation, contact, and ingestion.