Chapter 16 - Potential of lignin as biofuel substrate

Abstract

The demand for energy, food, feed, and materials is continuously increasing globally. Presently, petro-based/conventional feedstocks such as coal, natural gas, and crude oil are considered as the main raw materials to meet the current energy and fuel demands. However, their availability is more than questionable for long-term use. Thus, nowadays, lignocellulosic biomass is considered as a potent feedstock for the production of green fuels, chemicals, and materials. At present, a significant amount of research has led to the utilization of second-generation biofuels from lignocellulosic biomass. The cellulose and hemicellulose fractions have been commercially exploited for the generation of fermentable sugars and multiple platform molecules. Meanwhile, lignin, which ranks as the second most abundant biopolymer after cellulose, represents 15%–30% of the total land plant biomass and 40% by weight of lignocellulosic biomass. Lignin, despite having promising properties for the production of fuel, such as higher carbon to oxygen ratio (more than 2:1) and greater energy density as compared to cellulose and hemicellulose, is not utilized to its fullest potential. Therefore, less than 2% of the lignin generated from the processes operating in the pulp and paper industry is funneled into the production of high-value commodities at a commercial level. The rest, being considered as a waste, is combusted as boiler fuel for energy recovery. Recalcitrance of the macromolecule toward degradation is one of the major reasons for this. However, lignin can be seen as a potential feedstock for production chemicals, fuels, and energy products. Thus, in this chapter the focus has been made on highlighting the potential of lignin as a potent substrate for the production of biofuels, and thus major emphasis is placed on: (1) the structural complexity of lignin at the molecular level; (2) established routes to extract bulk lignin; (3) characterization of the isolated lignin; and (4) the potential methods to convert lignin into biofuel precursors and their further upgradation. There exist many paths and gaps which needs to be addressed to utilize the full potential of lignin as a biofuel candidate. Moreover, these challenges are addressed in this chapter, and the Conclusion highlights these areas.