Improved environmental and socio-economic impacts of ethanol production from rice straw

Abstract

Improved ethanol production from rice straw, as well as its environmental and socio-economic effects, was studied and compared with fossil fuels. Alkaline, i.e., sodium hydroxide (NaOH) and sodium carbonate (Na2CO3), pretreatments improved the ethanol yield at cold (0 °C), mild (25 °C), and hot (93 °C) pretreatment temperatures. Different substrate characteristics, including enzymatic saccharification, accessible surface area, chemical composition, buffering capacity, enzyme adsorption and desorption, and structural changes, were analyzed to determine the most influencing factors on ethanol yield. Both alkaline pretreatments led to the highest ethanol yields using the pretreatments at 93 °C followed by 0 °C. The maximum ethanol yield (85.1%) belonged to the sample pretreated with NaOH at 93 °C, corresponding to 150% increase compared with ethanol yield of the untreated straw (30.9%). Glucose yield from enzymatic hydrolysis and water swelling capacity were the only characteristics that showed similar trends to the obtained ethanol yield and also presented their optimum values, 84.1% and 5.5 g/g, respectively, at the conditions with the highest ethanol yield (NaOH, at 93 °C). Therefore, glucose yield and water swelling capacity were the factors with the closest correlation with ethanol yield. Environmental assessments estimated that the annual bioethanol production from untreated rice straw in Iran was 141 million L that could be increased up to 615 million L after pretreatment with NaOH at 93 °C. This amount can supply ethanol needs in Iran as E5 for 169 days in a year, which in turn reduces 888 kt of GHG emissions. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.