Second-generation bioethanol production from corncob – A comprehensive review on pretreatment and bioconversion strategies, including techno-economic and lifecycle perspective

Bioethanol is highly produced and most used biofuel, with lignocellulosic biomass as an ideal choice of feedstock. This study particularly highlights various strategies of second-generation (2 G)-bioethanol production from corncobs. A detailed account of the effects of different pretreatment methods, detoxification methods and fermentation approaches on ethanol yield obtained from corncobs is given to make the reader understand the possibilities of further research improvements in this field. About 31 % of the works, reported dilute sulphuric acid pretreatment. H2SO4, NaOH and their combination pretreatments collectively accounted for 50 % of the total reports. Most other pretreatments were either less reported or completely missing. A combination of acid and alkali pretreatments along with a proper detoxification step is proven to achieve ethanol yield as high as 100 % of the theoretical yield. Techno-economic analysis (TEA), established that the overall cost of operation is essentially comprised of chemical cost and energy consumption. Hence pretreatment and detoxification are key steps in determining process economics. Genetic engineering to construct inhibitor tolerant and consolidated bioprocessing (CBP) compatible microbes for ethanol fermentation is another way to achieve an economical process. Techno economic analysis (TEA) and life cycle assessment (LCA) studies suggested that the key to achieving an overall sustainable corncob-biorefinery is to simultaneously valorize xylan and lignin along with glucan. Based on the market value of the final products, xylooligosaccharides (XOS) are much more beneficial than xylose based ethanol. Hence an ideal corncob-biorefinery would involve the production of high valued end-products from lignocellulose components. However, methods for uniform research data representation, greener pretreatment technologies, and integrated approaches to put together TEA and LCA studies are yet to be developed to assess the corncob-based 2 G bioethanol technologies.