A Techno-economic evaluation of integrating first and second generation bioethanol production from sugarcane in Sub-Saharan Africa

[摘要] ENGLISH ABSTRACT: Climate change that results from greenhouse gases (GHG's) released from the burning of fossil fuels,together with the rising price of oil, have sparked interest in renewable biofuels. The production ofbiofuels also presents potential socio-economic benefits.There are two types of technologies for bioethanol production:· First generation bioethanol is produced from food feedstocks such as juice of sugarcane.· Second generation bioethanol is produced from non-food feedstocks (lignocellulosicmaterials).This project is concerned with 1st and 2nd generation bioethanol production from sugarcane juice andbagasse and the integration of these technologies. This project comprises a combination ofexperimental and process modelling work to assess energy efficiencies and the economic viability ofintegrated and stand-alone processes in the sub-Saharan African context.First generation fermentation experiments were conducted and high ethanol concentrations of up to113.7 g/L were obtained. It was concluded that a recombinant yeast strain may be able to replace anatural hexose fermenting yeast for 1st generation fermentations to reduce costs. 2nd generationfermentation experiments were performed and ethanol concentrations of close to 40 g/L wereobtained. Combinations of 1st and 2nd generation fermentation experiments were performed toimprove the 2nd generation fermentation. In one of the experiments it was concluded that thecombination of 1st and 2nd generation fermentations significantly improved the 2nd generationfermentation with an overall ethanol concentration of 57.6 g/L in a shorter time than for the pure 2ndgeneration experiments.It was determined from washing and pressing experiments that pressing the pre-hydrolysate liquorout of the pre-treated bagasse will sufficiently lower the levels of inhibitors in a 2nd generationfermentation when using a hardened yeast.Some of the data from the 1st generation experiments were used along with literature data to modela first generation process in Aspen Plus® which processes 493 tons of cane per hour (tc/hr). Pinchheat integration was used to reduce the utility requirements. The process used the bagasse that wasgenerated to co-produce steam and electricity. The excess electricity was sold for additional revenue.In one scenario the excess bagasse was determined at 57.5%. This bagasse was sold to a stand-alone 2nd generation plant. The first generation process produced 85.5 litres of ethanol per ton of cane(L/tc), the integrated process produced 128 L/tc while the stand-alone 2nd generation processproduced 185 litres of ethanol per ton of bagasse (50% moisture) or 25.5 L/tc. The amount of excesselectricity that was produced ranged from 14.3 to 70.2 kWh/tc.Economic analyses were performed using South African economic parameters to resemble the sub-Saharan African context. Data from the 1st generation process model and literature data forintegrated 1st and 2nd generation and stand-alone 2nd generation processes were used for theanalyses. It was found that the integrated plant is the most economically viable (IRR = 11.66%) whilethe 1st generation process basically broke even (IRR = 1.62%) and the 2nd generation process isunviable. This was as a result of high sugarcane prices and too few incentives for 2nd generationethanol.

[发布机构] Stellenbosch University