Potential Ethanol, Butanol Production from Sweet Sorghum, Energycane

Daira Aragon, Ehrenhauser, Franz, Kimbeng, Collins A., Viator, Sonny  |  9/20/2017 7:50:18 PM

Daira Aragon, Shyue Lue, Sonny Viator, Collins Kimbeng and Franz Ehrenhauser

The need for decreasing greenhouse gas emissions, exhaustion of fossil fuel resources and the desire for energy independence have encouraged worldwide interest in fuels and chemicals derived from renewable resources, especially those that do not compete with food crops. The economics of the renewable fuel market requires regional production of feedstocks and use of available infrastructure. In the case of Louisiana, the well-established sugarcane industry provides the perfect logistics and infrastructure to produce and process cane-type material. Energycane and sweet sorghum are top candidates because of their physical and compositional similarities with sugarcane. Researchers have reported that sweet sorghum can be harvested in Louisiana during the months of August and September, with possibilities of harvesting as early as July. Energycane can be harvested from October through March. These harvesting seasons would complement commercial sugarcane operations, potentially extending the grinding season of October through January for an additional four to six months. Fuels can be produced from the sugar in the juice and the sugars in the bagasse through fermentation. The juice does not need extensive conditioning before fermentation. The bagasse must be pretreated and hydrolyzed to release simpler sugars (lignocellulosic sugars) that can be used by the yeast or bacteria.

Determining product yields that can be obtained from sweet sorghum and energycane is one of the first steps in evaluating the economics of adopting these crops in Louisiana. This requires simulation to predict fuel production based on measured values such as sugar content (sucrose, glucose, fructose, xylose), fiber content and bagasse composition (cellulose, hemicellulose). Simulation of a facility producing fermentable sugar syrups was performed using the software SUGARS to determine the potential production of ethanol and butanol from sweet sorghum and energycane. The first simulation was as a sugar factory, where the juice is extracted from the crops and evaporated into syrup. The second section was a lignocellulosic conversion plant, where the fiber (bagasse) remaining after juice extraction is pretreated and hydrolyzed to release glucose and xylose. Finally, the simulation model included high-pressure boilers to produce power for export by burning bagasse when it is not used for sugars. To evaluate the full potential of these crops, all the bagasse was used for fuel production or power generation for export. Data for whole-stalk samples of sweet sorghum and energycane used in the simulation are in Table 1.

Fermentable sugars from juice and fiber were used to calculate ethanol and butanol production using current attainable conversions reported in literature. Figure 1 shows annual fuel production for sweet sorghum and energycane harvested for 60 days and 120 days, respectively, at a grinding rate of 10,000 tons per day. A biorefinery based on sweet sorghum and energycane potentially could produce more than 57 million gallons of ethanol or more than 41 million gallons of butanol per year using existing technologies. Nearly 52,000 acres would be necessary to reach these production capacities using juice and bagasse. If all the bagasse is burned in high-pressure boilers, the potential power generated would be enough to power about 30,000 homes in Louisiana.

Certain considerations need to be addressed because of the seasonal character of the energycane and the sweet sorghum crops to ensure that supply will not be halted. This constant supply can be accomplished by storing bagasse or juice as concentrated syrup during the three to five months off-season. Simulation shows that in order to have a constant fuel supply during the year, it would be necessary to store up to 483,000 tons of bagasse or 55 million gallons of syrup per processing plant. Syrup storage has advantages over storing bagasse because of easier handling, reduced fire hazard and reduced processing cost.

Louisiana’s sugar industry has the potential of becoming a front-end plant for future biorefineries or biopower producers, taking advantage of the knowledge and infrastructure already in place. Up to 100,000 acres could be available to plant sweet sorghum and energycane during the sugarcane fallow period. To process this acreage at a rate of 10,000 tons per day during 180 days, only two of Louisiana’s 11 sugar factories would be necessary. The potential annual production in the state could reach 228 million gallons of ethanol or 164 million gallons of butanol. Energycane and sweet sorghum are considered low-input crops that could be grown in colder climates than sugarcane. Research shows the crops can be grown as far north as Winnsboro and on land not suitable for food-producing crops such as corn and soybeans.

Ultimately, the adoption of energycane and sweet sorghum will be subjected to the successful development of crop production systems and processing technologies that guarantee competitiveness with petroleum-based fuels. Current government incentives are helping researchers and companies move forward.

Daira Aragon is an assistant professor at the Audubon Sugar Institute, St. Gabriel. Her co-authors are Shyue Lu, research assistant, Audubon Sugar Institute; Sonny Viator, professor and research coordinator, Iberia Research Station, Jeanerette; Collins Kimbeng, associate professor, Sugar Research Station, St. Gabriel; and Franz Ehrenhauser, assistant professor, Audubon Sugar Institute.

(This article was published in the spring 2015 issue of Louisiana Agriculture)

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Energycane. Photo by A. Denise Attaway

Figure 1.

Figure 1. Annual fuel production for sweet sorghum and energycane harvested for 60 days and 120 days, respectively, at a grinding rate of 10,000 tons per day (million gallons per year).

Table 1.
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