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An Energy and Economic Analysis of Energy Crops Processing into Bioethanol as a Gasoline Substitute
Published: 31 October 2013 by MDPI in The 3rd World Sustainability Forum session Energy Efficiency and Renewable Energy Sources
Abstract: Two crops, energy cane and sweet sorghum which could be cultivated at marginal lands in subtropical climate (southeast U.S.) were analyzed to determine their potential to be processed into bioethanol as a gasoline substitute. A solution of sucrose and reducing sugars (shortly sugars) separated in the form of "juice" by squeezing those crops is a well known semi-product to be converted into bioethanol by a greatly established in Brazil technology (1G-technology). The residue is called bagasse and consisted of fiber and moisture. Fiber (mainly composed of cellulose, hemicellulose, and lignin) could be employed in the process of its lignocellulosic conversion to make a solution of sugars as an intermediate step in their fermentation into bioethanol. This technology (2G-technology) is yet at a development stage. Bagasse, on the other hand, is an energy carrier required to generate electricity and steam essential for both technologies. The analysis was done for the case when all the necessary energy demand is satisfied by the internally generated bagasse what makes the process of bioethanol production fully renewal and self-sufficient (sustainable). Due to a seasonal character of harvesting and continuous bioethanol manufacturing, the calculated energy balance accounts for sugars storage in the form of their concentrated solution (syrup). The economic efficiency for bioethanol production as a gasoline substitute was determined in comparison with the power generation option in dependence on gasoline and electricity prices meaning that both crops could be combusted to generate renewable electricity. As was shown by the analysis, manufacturing of bioethanol from sugars in "juice" compared to the sugars obtained through lignocellulosic conversion is associated with 2.5-4 times higher economic efficiency. In order to stay competitive with renewable electricity, bioethanol from fiber should include both cellulose and hemicellulose conversion into fermentable to bioethanol sugars. In spite of a higher sugars percentage in the sweet sorghum "juice", the relative selling cost of one tonne of energy cane supposed to be higher due to greater (more than three times) fiber content. The analysis showed that sugars in crops have value about three times higher than fiber; therefore, taking into account this proportion, increasing sugars content at the expense of fiber could be the way to improve quality of energy cane varieties. A greater yield of energy cane also favors a better economic land-use efficiency to produce bioethanol as a gasoline substitute.