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Title: Evaluation of Sweet Sorghum and Sorghum x Sudangrass Hybrids as Feedstocks for Ethanol Production

Author
item Tew, Thomas
item Cobill, Robert
item Richard Jr, Edward

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2008
Publication Date: 6/25/2008
Citation: Tew, T.L., Cobill, R.M., Richard Jr, E.P. 2008. Evaluation of Sweet Sorghum and Sorghum x Sudangrass Hybrids as Feedstocks for Ethanol Production. BioEnergy Research. 1:147-152.

Interpretive Summary: Currently world ethanol production from agronomic crops comes from either corn or sugarcane. Sugarcane growers and processors would benefit from a complimentary biofuel feedstock that could 1) be planted and harvested on a timely basis in fallow fields, 2) be harvested, transported, and processed with preexisting sugarcane equipment, and 3) extend the period that biofuel feedstocks can be harvested throughout the year. We evaluated five sweet sorghum varieties and two sorghum x sudangrass hybrids for their potential in producing ethanol from their sugar and fiber components. The sorghums were planted in the spring and harvested at 85, 101, 119, and 138 days after planting. Based on our results, higher-yielding sweet sorghums (M81-E, Theis, and Topper) harvested at about 120 days after planting should be able to produce in excess of 9,000 liters ethanol per hectare, about half from sugar and half from fiber. The higher-yielding sorghum x sudangrass hybrid (MMR 333/47 should produce the same amount of ethanol as the higher-yielding sweet sorghums at 120 days, with about two-thirds from fiber and one-third from sugar. By planting sweet sorghum in fallowed sugarcane fields, sugarcane growers in Louisiana could obtain revenue from a short-season rotational crop without restricting their ability to replant these fields to sugarcane. Having a complimentary crop would also benefit the biorefiner by providing an additional crop to process prior to the start of the sugarcane harvesting season.

Technical Abstract: There is much interest in the conversion of sugar from sugarcane and related grasses to ethanol. Field studies were conducted at the USDA-ARS Sugarcane Research Laboratory in southeast Louisiana to evaluate the ethanol yield potential of five sweet sorghums (Dale, M 81-E, Rio, Theis, and Topper) and two non-flowering sorghum x sudangrass forage hybrids (M 333-27 and M 333-47). The sorghums were planted in the spring and harvested at 85, 101, 119, and 138 days after planting (DAP). Theoretical sugar-based ethanol yield increased for the sweet sorghums (except Rio) from 85 through 119 days, but did not significantly increase further at 138 days. The forage sorghums did not show a similar plateauing of sugar-based ethanol yield. Conversely, theoretical fiber-based ethanol yields increased two-fold in the two forage sorghums from 85 to 138 DAP, while a significant increase in fiber-based ethanol yield was not observed in any of the sweet sorghums over the same period. At 138 DAP, sugar-based ethanol yield of Theis (6060 L ha-1), was greater than that of Rio or either of the two forage hybrids; fiber-based ethanol yield of MMR 333/47 (8860 L ha-1) was greater than that of any other variety in the test. Theoretical total ethanol yield from hexose sugar and fiber components, averaged across all sorghums, was 6500, 7720, 9100, and 10810 L ha-1 at the four harvest intervals. As a complementary crop for Louisiana’s sugarcane growers, sorghum would need to be harvested not later than 120 DAP so as to not interfere with the planting of sugarcane in these fields. Both Theis and MMR 333/47 produced greater than 11000 L ha-1 combined theoretical ethanol at 119 DAP, Theis, equally from sugar and fiber, MMR 333/47 about two-thirds from fiber. Choice of sorghum type would depend on the conversion process(s) being used at the biorefinery.