## 4 Valorization Technologies of Avocado-Derived Biomass through Obtaining Bioenergy and Biofuels
### Biofuels through Thermochemical Methods: Gasification, Pyrolysis, Torrefaction, and Liquefaction  
In addition to the latter transformation ways, the conversion of biomass into biofuels is also accomplished by means of thermochemical transformations, such as gasification, pyrolysis, torrefaction, and liquefaction. Gasification implies a partial combustion of the biomass to produce bio-char and gas while pyrolysis is a thermal degradation (400-800 \({}^{\circ}\)C) of the biomass in the absence of oxygen to generate bio-char, condensed liquids (bio-oil), and flue gas. Torrefaction is like a pyrolysis but using lower temperatures (200-300 \({}^{\circ}\)C) [42; 55].  
For the valorization of avocado stones, [PERSON] et al. [42] applied combustion/gasification in a porous media reactor, while for torrefaction and pyrolysis a rotary furnace was applied at temperatures between 150 \({}^{\circ}\)C and 900 \({}^{\circ}\)C. The former was used to improve the concentration of synthesis gases (H\({}_{2}\), CO) and CO\({}_{2}\) by incorporating avocado seeds in the reactor that works with methane/air when operating at high gas fuel equivalence ratios. Applying the second strategy, their findings indicated that the large-scale production of biochar, liquid fuel, and gas from avocado stone, which were pre-dried at 40 \({}^{\circ}\)C and milled, is possible. The partial drying and dehydration of the biomass was executed between 150 \({}^{\circ}\)C and 250 \({}^{\circ}\)C. The thermal degradation of the sugar polymers and lignin occurred between 250 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, forming a gaseous and condensable fraction and the solid fraction was reduced from 90% to 25% by weight. Using also torrefaction at 300 \({}^{\circ}\)C on avocado stone (pre-dried at 110 \({}^{\circ}\)C and milled), a bio-char (fuel ratio, 0.66 fixed carbon/volatile matter, HHV, 23.3 MJ/kg; BET surface area, 2.263 m\({}^{2}\)/g char) with similar characteristics to the coal was produced, eliminating fuel segregation and the energy input required to carbonize the biomass [43]. This value is higher than those reported in Table 3. Similarly, in a rotary reactor, [PERSON] et al. [56] found the optimum torrefaction conditions for avocado stone (pre-dried at 40 \({}^{\circ}\)C) at 304 \({}^{\circ}\)C, which yielded a biochar product (50%) with 36.7% FC, HHV of 24.3 MJ/kg, and LHV of 23.2 MJ/kg for an energy yield of 76.7%. Alternatively, the bio-oil produced had a low carbon content (11.0%) and low HHV (2.5 MJ/kg) due to the high water content of the samples.  
New trends include the use of microwave heating instead of conventional heating. Hot spots, which form under microwave irradiation, affect the yield and the characteristics of the pyrolysis products. For example, solids can reach high heating values and specific surface areas with higher gas and solid yields than conventional pyrolysis, but a lower liquid yield is obtained. Furthermore, almost half of the biomass can be converted into a higher energy gas product, due to the high H\({}_{2}\) yields [57]. Another greener possibility is to use solar pyrolysis by using concentrated solar energy as the heating source. However, none of them have been explored using avocado waste and some challenges have to be overcome, such as those about the design and scaling-up [58].  
Another option is liquefaction. In this technology, temperatures between 250 \({}^{\circ}\)C and 500 \({}^{\circ}\)C at high pressure (5-40 MPa) are applied to obtain the bio- oils. In comparison to some of the aforementioned methods, relatively lower temperatures are required but solvents are used. [PERSON] and [PERSON] [55] investigated the effects of the solvent type, temperature, and catalyst type during liquefaction of avocado stone (pre-dried and milled <0.425 mm) for bio-oil production. They reported that acetone as a solvent and zinc chloride (10%) as a catalyst at 290 \({}^{\circ}\)C offer the optimal conditions for liquefaction, achieving a 76.9% conversion (52.4% liquid + 24.5% gaseous products) [55].