Evaluation of the quality of bioethanol produced from various sweet sorghum stalks

Abstract

This study assessed bioethanol production from sweet sorghum stalks in Uganda, aiming to provide knowledge on local sorghum varieties' potential for bioethanol production. The study identified sorghum varieties with potential for bioethanol production by screening them at the National Livestock Resource Research Institute (NaLIRRI). The selected varieties of (P11, P13, B1P9, P12, B7P1, P1), were cut, cleaned, and processed at NaCRRI's Biosciences laboratories. The Response Surface methodology was used to optimize bioethanol production parameters. The reduced quadratic model was found to be most effective for bioethanol concentration and high heating value. The optimal conditions for bioethanol production were found to be 25g yeast loading, 10 days reaction time, and 100rpm agitation rate. For the selected varieties, the protein and carbohydrates content in the stalk’s bagasse ranged from 2.98 %w/w to 12.11 %w/w and 2.77 %w/w to 12.88 %w/w respectively. The concentration of the produced bioethanol from the sweet sorghum stalks juice after a series of successive distillation was found to be between 56 %v/v and 90.20 %v/v while for the bagasse it was between 15 %v/v and 40 %v/v. The HHV was found to range from 12.46 MJ/Kg to 16.79 MJ/Kg respectively. The bomb calorimeter, density pycnometer and the fire and flash point experiment were carried out to test the quality of the produced bioethanol and the result were found to range from 13.06 MJ/Kg to 20.20 MJ/Kg for the HHV, 0.82 g/cm3 to 0.87 g/cm3 juice density, 0.90 g/cm3 to 0.96g/cm3 bagasse density, 17.80 0C to 23.00 0C juice flash point, and 25.50 0C to 45.30 0C bagasse flash point respectively. Results show that for 5 litters of sorghum stalk juice or stalk hydrolysate, the optimum yeast loading rate and reaction time are 25g and 10 days respectively. Yeast loading rate and fermentation time have a high influence on the bioethanol concentration compared to agitation rate. The bioethanol concentration and HHV at this yeast loading and reaction time were 90.20%w/w and 20.20 MJ/kg respectively. The research indicated that local sweet sorghum stalks can generate bioethanol, with a 90% concentration after triplicate distillation. This could be used for domestic cooking at a lower cost than synthetic bioethanol, reducing carbon emissions and meeting Uganda's 2040 vision.