Biomass Transformation: Sustainable Development

The development of new strategies for the thermochemical conversion of biomass into high value-added products, energy, and fuels represents an exciting and challenging topic, as demonstrated by the success of this Special Issue, in terms of the number of published manuscripts and visualizations. The authors’ contributions fully exhibit that biomass transformation following sustainability criteria represents the best choice to be pursued and developed in the immediate future, also considering the profound and changing evolution of the global scenario, which must rely on easily scalable processing technologies. The authors who actively contributed to this Special Issue have amply shown relevant progress over the current state of the art, emphasizing that technological development in the field of biomass thermochemical conversion is possible.

The importance of valorizing waste biomasses according to the principles of sustainability and circular economy has been specifically highlighted in all publications of this Special Issue, which discuss interesting conversion possibilities. Some authors have advanced a broad and diversified discussion, bringing waste biomass to the center of the reader’s attention. In this context, the conversion of biogenic residues from waste, agriculture, and wastewater sectors into energy is attracting increasing attention, as discussed by Pollack et al. [1], who have proposed a systematic approach to support the design of suitable energy systems for German rural areas. The authors underlined that these biogenic residues can be effectively considered a relevant component of the transition toward locally based, diversified, and climate-friendly energy systems. In another study, Zheng et al. [2] discussed the sustainable resource utilization of residual sewage sludge in China, focusing on building material utilization, energy utilization, and phosphorus recovery. Remarkably, each technology shows advantages and limitations, underscoring that the most suitable technology requires considering the characteristics of the starting feedstock and the application context to achieve the integration of pollution control and resource utilization. The exploitation of coconut waste is another noteworthy case study, as discussed by Viera et al. [3]. In their work, the authors evaluated different possibilities, including the use of bio-products such as ethanol, bio-oil, and organic fertilizers in the civil construction field, the environmental field as an adsorbent for pollutants, and in production. Remarkably, coconut waste represents a rich source of phenolic compounds, which can be exploited for high-value applications, such as their antioxidant and anti-inflammatory properties, and as active ingredients in the pharmaceutical industry.

Among the different exploitation possibilities, biofuel production is certainly attractive, as discussed in some noteworthy works in this Special Issue. In particular, Rojas-Flores et al. [4] proposed the use of Andean tuber (Olluco) waste as fuel for microbial fuel cells, demonstrating the achievement of promising performances, in terms of voltage, electrical current, and electrical potential, and generating enough electrical energy to light a LED light bulb. On this basis, Andean tuber would represent an attractive biomass to be grown in Peru, helping to solve the issue of the lack of electrical energy. In the wide field of biofuel production, bioethanol still represents a key topic, highlighting that current research is focused on the intensification of its production, mainly focusing on the work-up operations. In this context, Rola et al. [5] proposed a simulation of bioethanol production through lignocellulosic biomass fermentation, focusing on the distillation approach. Pressure swing distillation was effectively proposed to obtain fuel-grade ethanol (99.6 wt%), advantageously overcoming the issue of the formation of the ethanol–water azeotrope. In another work, Samuel et al. [6] studied biodiesel production from rubber seed oil by comparing the ethyl-based hydrodynamic cavitation reactor with the mechanical stirring one. The authors stressed the economic and environmental advantages of the ethyl-based hydrodynamic cavitation reactor, providing information for quickly scaling up the production of ethanolic biodiesel from non-edible and third-generation feedstocks. Remaining in the field of biodiesel production, Zhang et al. [7] discussed fatty acid methyl ester (FAME) synthesis from refined soybean oil and methanol, where this reaction was carried out in the presence of sodium silicates that modified calcium oxide as the solid base catalyst. If compared with the sole CaO catalyst, the two catalytic phases (Na₂CaSiO₄ and CaO) of the new synthesized catalyst synergistically act towards the improvement of both the activity and stability parameters. Within the biodiesel production chain from triglycerides, glycerol certainly represents the main by-product, requiring considerable attention to enhance its efficient exploitation for maximizing the sustainability of the entire process. In this context, the acid-catalyzed ketalization of glycerol to solketal is certainly an attractive approach, considering that these products are exploitable as green solvents and fuel additives, as well as in the pharmaceutical and food industries. In developing this approach, Rossa et al. [8] studied the ketalization reaction of glycerol with acetone, employing reduced graphene oxide as a new heterogeneous catalyst, whereas homogeneous p-toluenesulfonic acid was considered the reference. Remarkably, the catalytic activity of the synthesized graphene oxides was 30 times more efficient than the homogeneous p-toluenesulfonic acid, revealing their stability even after five consecutive cycles, maintaining the highest activity without any work-up treatment.

Another topic of great interest is related to biochar production and exploitation. Biochar is obtainable from thermochemical treatments of biomasses, often in high yields after the pyrolysis route, thus largely justifying the interest of the current research in improving its production and identifying new valorization strategies. In this Special Issue, Zhang et al. [9] investigated the pyrolysis of coffee husks into high-performance biochar-based fertilizers, which is a desirable approach for achieving more sustainable resource management and pollution control, at the same time fostering environmentally responsible agricultural practices. Remarkably, the appropriate optimization of the pyrolysis conditions allows for the optimal regulation of nutrient release within the pant-soil system. Further expanding upon this topic, in the review of Filho et al. [10], the synthesis and exploitation of the biochar derived from the pyrolysis of olive mill by-products (e.g., olive pomace and olive stone) were specifically discussed. For this purpose, the agronomic properties of biochars were considered, fully demonstrating their usefulness as plant nutrient reservoirs, good amendments to improve soil properties, and in long-term carbon sequestration. In another work, Meng et al. [11] proposed the co-pyrolysis of mushroom residue with pine sawdust or wheat straw, examining thermal characteristics, kinetic/thermodynamic aspects, and the structural evolution of chars during thermal treatment. The authors provided valuable insights for progressing the co-pyrolysis technology of edible fungi residue with wood/straw biomass. The application of biochar derived from Agave angustifolia bagasse as an adsorbent for ibuprofen removal from aqueous solutions was proposed and optimized by Ruiz-Velducea et al. [12], highlighting the significant potential of biochars in this application.

To develop the fractionation/conversion of the main biomass components into high added-value products, some authors have proposed particularly challenging approaches. Lozano-Pérez et al. [13] investigated liquid hot water (LHV) extraction and the hydrothermal carbonization (HTC) of coffee cherry for the production of valuable compounds, such as carbohydrates (e.g., glucose and xylose), furans (e.g., 5-hydroxymethylfurfural and furfural), and carboxylic acids (e.g., levulinic acid and formic acid). Both thermal approaches were optimized, by also considering the addition of appropriate acid or alkaline catalysts to improve their optimal formation. The authors demonstrated that the proposed hydrothermal treatments of coffee cherry may represent a significant opportunity for sustainable resource management and the development of a circular bioeconomy, also considering that the final solid (hydro)char can be immediately exploited as fuel and/or exploited for other applications, including those previously discussed in this editorial. Carboxylic acids are certainly high value-added biochemicals, as discussed by Aboudi et al. [14], who obtained short-chain fatty acids via an anaerobic fermentation path, employing sugar beet molasses as the starting feedstock and the semi-continuous feeding mode in completely stirred tank reactors. The hydraulic retention time was specifically considered by the authors, who pointed out that longer ones were more suitable for obtaining long carbon chain carboxylates, thus better exploiting the chain elongation of shorter carboxylic acids. Remaining in the context of the mild thermal processing of biomass, Kim et al. [15] claimed a simple two-stage extraction and recovery method for macromolecules from microalgae biomass (Chlorella sp. ABC-001), involving (1) acid pretreatment and (2) high-shear-assisted lipid extraction. After the optimization of the entire process, the highly efficient recovery of esterifiable lipids and glucose in the liquid phase was achieved, at the same time selectively keeping the proteins in the solid one. Remarkably, the proposed approach is attractive for the integrated production of biofuel, bioethanol, and animal feed. Another noteworthy example of biomass fractionation was provided by Mullen et al. [16], who developed the one-pot fractionation and depolymerization of the lignin fraction of biomass to obtain an oil rich in phenolic compounds and a cellulosic pulp. For this purpose, the lignin fraction of oak and switchgrass was fractionated and depolymerized in a solvent phase reaction, and the corresponding cellulosic pulps were tested for conversion to levoglucosan and BTEX via fast and catalytic pyrolysis over HZSM-5, respectively. The authors highlighted the advantages of using reductive conditions for improving levoglucosan yield via fast pyrolysis. Remarkably, within the catalytic pyrolysis over HZSM-5, the yield of BTEX decreased, in comparison with the raw biomass, demonstrating that the solvent phase fractionation of lignin is effective for the selective production of oxygenated compounds (e.g., levoglucosan). Interestingly, Du et al. [17] synthesized a sulfonyl-chloride-modified, lignin-based, porous carbon-supported metal phthalocyanine catalyst, which was proposed to replace the traditional Fenton’s reagent for lignin degradation to small phenols. By employing the synthesized catalyst, the authors optimized H₂O₂ oxidation, thus maximizing the liquid product yield and selectivity to simple phenols. Remarkably, the corresponding char showed good energetic properties and a halved molecular weight compared to that of the pristine lignin. Lastly, the fractionation of high value-added components is also applicable to non-conventional biomasses, as discussed in the work of Shkuratov et al. [18], who isolated chitin nanowhiskers from crustacean biomass, carried out in the presence of sustainable and inexpensive ionic liquids, such as [HN₂₂₂][HSO₄]. The use of these extracting agents shows remarkable advantages over traditional chitin extraction processes, that is, reducing the overall production costs by simplifying the process, eliminating the need for initial purification steps, and reducing the consumption of chemicals and energy.

In summary, these eighteen papers detail innovative aspects in the field of biomass conversion strategies, with a noteworthy preference toward the conversion of waste, in agreement with the criteria of environmental, economic, and social sustainability. We would like to thank all the authors of our Special Issue for their valuable contributions, as well as all the reviewers and the entire Editorial Office of MDPI for having assisted and helped us during the peer-review process.