Bioethanol production from agro-wastes: a comprehensive review with a focus on pretreatment, enzymatic hydrolysis, and fermentation

The entire scientific world is currently working on bioethanol production from lignocellulosic biomass as the most abundant renewable resource of biofuel to mitigate the acute shortage of fossil fuels in the near future. Bioethanol is the most alternative to petroleum based fossil fuels and being a renewable and environmental friendly cheap energy source. Bioethanol can be produced using a variety of feedstocks, but second generation biofuel production from agricultural waste feedstocks is a promising step. However, economical bioethanol production is constrained by a number of factors, including the transportation and handling of biomass, the effectiveness of pretreatment for delignification of lignocellulosic substrates, enzymatic hydrolysis, and the availability of suitable fermentative organisms. Nowadays, petroleum and natural gas supply 98% of the gasoline required for road transportation, with biofuels providing the remaining 2%. Biofuels are fuels derived from biomass. In 2018, global biofuel output hit a record 154 billion L, with bioethanol accounting for 110 billion L and biodiesel and hydro treated vegetable oil accounting for the remainder. By 2024, global bioethanol output is expected to rise by 20%, reaching 130 billion L. The present review article critically discusses the various agricultural wastes, different pretreatment techniques used for the delignification of waste biomass, their enzymatic hydrolysis and different fermentation techniques used for the conversion of simple sugar to bioethanol. Further, the application of microbes and integration of various modern techniques have also been highlighted for enhanced bioethanol production from agro waste biomasses.