Recent developments in lignocellulosic biomass pretreatment with a focus on eco-friendly, non-conventional methods

Recalcitrant nature of lignocellulosic biomass is a major hurdle in the holistic utilization of cellulose, hemicellulose and lignin in biorefinery operations. Pretreatment of lignocellulosic biomass is indispensable for the disintegration of native structure and to catalyse separation of all the components for their respective use(s). Currently used conventional methods of pretreatment are incompetent to achieve optimal production benchmarks due to high costs, longer process durations and harmful environmental impacts. Contrary to this, several recently advocated approaches including cold plasma, irradiation, high pressure, ultrasound and microwave have shown encouraging results in the pretreatment of a wide variety of biomasses. This review discusses the modes of action, advantages and limitations of selective non-conventional pretreatment methods. In addition, their current status in lignocellulosic biomass valorization has been explored. From the cited literature, it can be interpreted that the non-conventional methods may not completely replace the need of harsh chemicals in the pretreatment process, however, they may significantly decrease their required quantity and processing time to reduce adverse effects on the environment as well as economy of the overall biomass valorization process. Out of all the methods debated, ultrasound and microwave pretreatment have been studied most extensively and have shown up to 100% lignin removal and >90% reducing sugars yield on laboratory scale. However, merely superficial cost and energy analyses have been reported which are not sufficient to advocate the transition of these processes to industrial scale. Thus, it is hoped that this review will encourage the researchers working on lignocellulosic biomass to further investigate microwave and ultrasound-based pretreatment by specifically focussing on their feasibility and environmental sustainability at industrial scale. Moreover, the other under-researched, non-conventional methods should also be further investigated by more exhaustive optimization of process parameters and system design with simultaneous focus on their economic and environmental feasibility at commercial scale. Overall, the methods explained in this review, if properly investigated, have the potential to significantly contribute in cleaner production of biofuels and other value added products from lignocellulosic biomass.