## 2 Biorefinery Concept
### Lignocellulosic Biomass as Raw Material  
Lignocellulosic forestry, agricultural, and agro-industrial wastes accumulate every year in large quantities, often feeding forest fires and causing serious environmental problems when deposited on soil or landfill. Instead, it can be utilized for the production of several value-added products [14]. Lignocellulosic wastes are an abundant, non-edible, and low-cost raw material [15; 16]. The use of these renewable resources as a carbon platform reduces the production costs of bioprocesses and has a positive environmental impact, supporting sustainability ideals. Lignocellulosic biomass (LB) comprises different fractions that can be converted or extracted, through the implementation of the biorefinery concept, to obtain value-added products such as fuels, chemicals, and biomaterials [17; 18; 19; 20]. Besides wood, LB includes corn stover, straw, wheat stover, grass, and others. The primary components of lignocellulosic-based wastes are cellulose, hemicellulose, and lignin (structural components), together with small amounts of other constituents (nonstructural components) such as extractives, ash, and proteins [20]. Depending on the type of LB, the structural components are organized into complex nonuniform three-dimensional structures to different degrees and varying relative compositions [14]. For LB valorization, [PERSON] et al. (2020) highlighted polysaccharides, lignin, and extractives as the most important fractions [20]. Lignin is considered the main renewable source of aromatics. A recent work described benzene production using lignin as feedstock through chemical catalysis, claiming that a further scale-up of the process can be a sustainable alternative to the current production from fossil resources [21]. Besides carbohydrates and lignin, [PERSON] et al. (2020) provided an extensive and detailed description on chemical and biological properties, sources, methods of extraction, and applications of the most important extractable compounds (fatty acids, lipids, proteins, terpen, terpenoids, steroids, and polysaccharides) with a particular focus on phenolic compounds [20]. Xylochemicals (a designation for wood-derived building blocks) are in the forefront of replacing oil-based precursors for polymer high-scale production, although research is still needed to design the steps that make them suitable for specific polymerization-type reactions [18; 19]. The production and characterization of biobased polyesters from bisguaiacol (a bisphenol analogue) derived from lignin and the synthesis of a verbanone-based lactone from (\(-\))-\(\alpha\)-pinene by a chemoenzymatic route are recent examples of this trend [18; 19]. The conversion of organic solid matter through lignocellulosic gasification into a gaseous mixture of H\({}_{2}\), CO, CO\({}_{2}\), and CH\({}_{4}\) is another approach for LB valorization. The obtained synthetic--\"syngas\"--can be directly used as combustible for energy generation or as feedstock for chemical or biological processes [22]. Strict anaerbes, mostly acetogens, are used for syngas conversion to organic acids, alcohols, and other chemicals by \"indirect fermentation\"(as in this process, the biomass is previously converted to syngas and only then fed to microorganisms) [22; 23]. Raw LB is considered unfermentable because most microbes cannot degrade it. In fact, in LB, the sugars are locked in a recalcitrant structure, and the degradation of carbohydrate polymers into fermentable sugars often involves two steps: a pre-treatment, after which most of the hemicellulose are degraded into mono- and oligosaccharides, followed by hydrolysis of the cellulose and the remaining oligosaccharides, either by weak acids or enzymatic catalysis [24]. Nevertheless, LB raw materials are still most probably the most feasible alternative to biorefieries as a carbon source thanks to their unique eco-friendly nature, since they are biosynthesized from available atmospheric CO\({}_{2}\), water, and sunlight through biological photosynthesis [14].