## 4 Valorization Technologies of Avocado-Derived Biomass through Obtaining Bioenergy and Biofuels
### Bioenergy  
Considering the increasing energy needs, the imminent depletion of fossil resources and the consequent global warming, alternative energy sources have been sought. Because biomass is the only renewable source of carbon, bioenergy would reduce environmental pollution, meet climate targets, and replace fossil energy; it is the best alternative with the potential to reduce greenhouse gas emissions. It has been the fourth largest energy source in the world after coal, oil, and natural gas, accounting for 9.5% of the world primary energy supply and 69.5% of the world renewable energy supply in 2016 [36]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Element (\%)** & **Avocado Peel** & **Avocado Stone** & **Olive Stone** & **Vegetal Coal** \\ \hline C & 49.8 & 42.1–48.0 & 46.6 & 79.3 \\ H & 5.7 & 5.6–5.8 & 6.33 & 2.7 \\ N & 1.0 & 0.5–0.7 & 1.81 & 0.7 \\ O & 42.2 & 43.0–50.8 & 45.20 & 17.0 \\ S & ND & ND–0.1 & 0.11 & 0.3 \\ \hline \hline \end{tabular} ND, not detected.  
\end{table} 
Table 2: Elemental analysis of some avocado residues and other biomasses [12; 29; 34]. Figure 6: Overview of the valorization ways applied to the avocado-based wasteby-products. Bioenergy refers to the use of biological products (or biomass) used specifically for purposes such as the generation of electricity and heat and the conversion of biomass into secondary products, such as biofuels for use in the transport sector. Concerning agri-food residues, there is much work to do. However, research is ongoing to incorporate this biomass into sustainable processes to produce solid, liquid, and/or gas fuels. To determine which type of transformation process is the most suitable for each type of biomass, it must be previously characterized by determining the humidity, size, and shape of the particles, as well as the chemical composition (carbohydrates, lignin, ash, etc.) and calorific value [37]. This can help to exclude agri-food residues with a low energy potential [38; 39]. Several studies have been performed to characterize the residual biomass of avocado and whether there are chemical compounds with industrial value and/or the ability to generate biofuels (Tables 1 and 2). From these studies, it can be concluded that the avocado peel and stone are in the range of desired values for ultimate analysis, but the nitrogen content in the peel is slightly high. [PERSON] and coworkers recommend nitrogen values up to 0.5%, since it can lead to NOx in combustion gases [40]. Concerning calorimetry studies, Table 3 shows the high heating value (HHV) and low heating value (LHV) of some avocado biomasses, along with the volatile matter (VM) and fixed carbon (FC). One of the main drawbacks of avocado-derived biomass is the humidity (Table 1), as commented on before. Despite the percentage of humidity, the avocado stone has advantages in energy terms (a high calorific value, HHV, up to 19.1 MJ/kg) (Table 3) compared to other common biomass sources for combustion in small boilers; e.g., its low sulfur content (0.010-0.032%) minimizes the generation of SO\({}_{\mathrm{x}}\) (acid rain compound), and in addition the amount of chlorine (0.024 and 0.025%) is less than in other biomasses, which helps prevent corrosion in the boilers. Another parameter determined was an LHV of 17.9 MJ/kg [29]. Therefore, as a fuel for domestic or industrial heating, these authors recommend drying and grinding before burning. Concerning other avocado biomasses, the HHV is closer to that of an avocado stone. The VM is in the range volatile matter of 65% to 85%, as recommended for general biomass, and this parameter and the FC values are similar to those of olive stones, a reference biomass. Alternatively, the ash content in these avocado resources is higher than in other biomasses, such as olive stone and almond shell. This implies that more frequent boiler maintenance tasks are required [29]. After combustion, even the ash can be subjected to further valorization following the example of other biomasses [41].