## 2 Structural Complexity and Composition of Native Lignin
### Lignin in Agricultural and Industrial Waste  
The valorization of lignin from agricultural waste not only addresses the challenges of waste management but also provides a renewable alternative to fossil-based materials, aligning with the principles of the circular bioeconomy. Among the most prominent agricultural residues, sugarcane bagasse stands out due to its high lignin content (20-25%) and widespread availability, particularly in countries with extensive sugarcane cultivation such as Brazil and India [25]. Sugarcane bagasse, a fibrous byproduct of sugar production, is generated in large quantities and represents a readily available feedstock for lignin extraction and utilization [26]. Similarly, oilseed residues, including sunflower stalks and rapeseed straw, contribute significantly to the global lignin supply, with an annual production of approximately 182 million tons. These residues contain lignin concentrations of 20-25%, making them highly suitable for biorefinery processes aimed at producing  
Figure 2: The three main precursors of lignin (monolignols) and their corresponding structures in lignin polymers. biofuels, biochemicals, and bioplastics [27]. In addition to crop residues, the processing of nuts such as peanuts, almonds, and walnuts generates approximately 10 million tons of biomass waste annually, primarily in the form of shells. These agricultural byproducts are characterized by exceptionally high lignin content, typically ranging from 25% to 35%, which makes them ideal candidates for conversion into advanced materials such as carbon fibers, biochar, and activated carbon [28]. The high lignin content in nut shells not only enhances their thermal stability and mechanical properties but also provides a sustainable feedstock for the production of bio-based adhesives and composites [29]. The vaporization of lignin from agricultural waste offers numerous environmental and economic benefits. By converting lignin-rich residues into high-value products, this approach reduces the environmental impact of agricultural waste disposal, such as open-field burning, which contributes to air pollution and greenhouse gas emissions. Furthermore, lignin vaporization supports the development of a circular bioeconomy by transforming waste into valuable resources, thereby reducing reliance on finite fossil fuels and promoting sustainable industrial practices. However, the efficient extraction and utilization of lignin from agricultural waste face several challenges. These include the development of cost-effective and scalable extraction methods, the optimization of lignin properties for specific applications, and the establishment of robust supply chains and markets for lignin-derived products. Advances in extraction technologies, such as organosolv processes, enzymatic hydrolysis, and microwave-assisted extraction, are critical to overcoming these challenges and unlocking the full potential of lignin as a sustainable raw material [30]. Lignin in agricultural waste represents a vast and underutilized resource with significant potential for sustainable applications. By valorizing lignin from residues such as sugarcane bagasse, oilseed stalks, and nut shells, we can address global challenges related to waste management, resource efficiency, and climate change. Lignin derived from industrial waste streams represents a significant and underutilized resource with a high potential for economic and environmental sustainability. Industrial sectors such as pulp and paper production, bioethanol manufacturing, wood processing, and textile industries generate vast quantities of lignin-containing residues, allowing for an important increase in lignin-based companies worldwide [31]. Through emerging lignin extraction and refinement technologies, these waste streams can be transformed into high-value bio-based products, contributing to the circular bioeconomy and reducing dependence on fossil-derived materials [32].