Future prospects of delignification pretreatments for the lignocellulosic materials to produce second generation bioethanol

Abstract

Summary Production of bioethanol is winning support from masses because it is a workable choice to solve the problems associated with the fluctuating prices of crude petroleum oil, climatic change, and reducing non‐renewable fuel reserves. First‐generation biofuels are produced directly from food crops. The biofuel (bioethanol, biodiesel) is ultimately derived from the starch, sugar, animal fats, and vegetable oil that these crops provide. It is important to note that the structure of the biofuel itself does not change between generations, but rather the source from which the fuel is derived changes. Corn, wheat, and sugar cane are the most commonly used first‐generation bioethanol feed stocks. Lignocellulosic materials are used as a feed stock for the production of second‐generation bioethanol. The major production steps are (1) delignification, (2) depolymerisation, and (3) fermentation. Agricultural residues are waste materials produced through the processing of agricultural crops. The main reason to use of these agricultural residues to produce bioethanol is to convert waste to value added products. The main challenges are the low yield of the cellulosic hydrolysis process due to the presence of lignin and hemicellulose with cellulose. Pretreatments of lignocellulosic materials to remove lignin and hemicellulose are the techniques used to enhance the hydrolysis. Present review article comprehensively discusses the different pretreatment methods of delignification for ethanol production. Published literature on pretreatments from 1982 to 2018 has been studied. Perspectives, gaps in studies, and recommendations are given to fully describe implementation of eight prominent pretreatments (milling, pyrolysis, organic solvents, steam explosion, hot water treatments, ozonolysis, enzymatic delignification, and genetic modification) for future research. The energy and environmental features of lignocellulosic materials are elaborated to show a sustainable aspect of second‐generation biofuel. It was felt necessary to discuss the concept of bio refinery to make biofuel production financially more attractive as well because the future prospects of second‐generation biofuel are promising.

Fungal Treatment for the Valorization of Technical Soda Lignin

“…A current bottleneck in the use of such feedstock in industrial processes is the presence of substantial amounts of recalcitrant lignin (10–30%), resisting a wide range of chemical and biological attack and hindering the access of enzymes to cellulose, the main fermentable sugar stock [ 2 ]. To overcome this impasse, different pre-treatments have been integrated into lignocellulose biorefinery processes, including the delignification pre-treatments that generate technical lignins as by-products [ 3 ].…”

Techno‐economic analysis and a novel assessment technique of paper mill sludge conversion to bioethanol toward sustainable energy production

“…The calculated value was around 13 520 MJ/h, as depicted in Table 3. In the woody biomass process, a separate treatment of the lignocellulosic feedstock is required to render the complex cellulose‐lignin structure 39,40 . Rendering the structure is essential to reduce the crystallinity degree of the cellulose and fractionate the hemicellulose portion 40 .…”

“…In the woody biomass process, a separate treatment of the lignocellulosic feedstock is required to render the complex cellulose-lignin structure. 39,40 Rendering the structure is essential to reduce the crystallinity degree of the cellulose and fractionate the hemicellulose portion. 40 There are several treatment technologies available today, however, they all share a high energy demand that ranges from 2 to 14 MJ/L Ethanol.…”

Co‐torrefaction followed by co‐combustion of intermediate waste epoxy resins and woody biomass in the form of mini‐pellet

“…Different from pyrolysis, the main biofuel formed during torrefaction is biochar rather than bio‐oil, as a consequence of relatively low‐temperature operation (200°C–300°C) of this pretreatment. In addition to increasing biomass calorific value, grindability is also improved after torrefaction which is conducive to industrial applications such as boilers and furnaces. However, the literature review suggests that the application of torrefaction on waste resins is scarce.…”