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Fuel Alcohol Production from Post-Harvest Sugarcane Residue
Sugarcane has been an integral part of the south Louisiana economy and culture for more than 200 years. Sugarcane arrived in Louisiana with the Jesuit priests in 1751 and, in 1795, Etienne de Bore granulated sugar on a commercial scale at Audubon Park in New Orleans. The Louisiana sugar industry is in its third century of existence, having celebrated its 200th year of continuous sugar production in 1995 . Louisiana is the second largest U.S. sugarcane producer, with sugarcane production accounting for 41.4% of the nation’s total.
Post-Harvest Sugarcane Residue
In 2001, sugarcane was grown on 493,773 acres by 773 producers in 24 Louisiana parishes. An estimated 465,000 acres were harvested for sugar, with a total production of 1,554,965 tons of sugar. The total acreage reported in 2001 was about the same as reported in 2000, which had set a new record for the Louisiana sugar industry. The sugarcane plant consists of about 75 to 80% net cane (stalks) from which the juice is extracted and the sugar crystallized. The other 20 to 25% of the plant consists of leafy material, including tops, from which little or no sugar is produced. This leafy material is called trash or post-harvest residue. This residue accounts for 3 to 10 tons per acre. Every year during and after the harvest the residue, mainly leaf litter, is burned by open air burning by the sugarcane farmers. This is done for two reasons: (1) to remove fibrous content which would greatly reduce milling efficiency and decrease profits and (2) at 3 to 10 tons/acre this residue in the field is unmanageable to the farmer for farming practices for subsequent cultivation. The current practice of open air burning of post harvest sugarcane residue not only affects air quality, but increasingly the general public is objectionable to this practice. The sugarcane farmers are looking for alternative to burning of the residue and if the sugarcane residue could be used to produce fuel alcohol via fermentation process, it will be a win – win situation.
The sugarcane residue is available every year. At an average of 5 acres/ton and with 465,000 acres in sugarcane cultivation, the amount of residue available is over 2.3 million tons/year. This is a renewable resource and could be used to produce energy in the form of ethanol.
Bagasse
Sugarcane refining generates a large volume of residue called bagasse. Disposal of bagasse is critical for both agricultural profitability and environmental protection. The sugarcane stalk consists of two parts: an inner pith containing most of the sucrose and an outer rind with lignocellulosic fibers. During refining, the sugarcane stalk is crushed to extract sucrose. This procedure produces a large volume of residue, bagasse, containing both crushed rind and pith fibers. In Louisiana, there are 16 sugar mills located in three regions, namely, Northern region, Teche Region, and River-Bayou Lafourche Region. On a yearly basis, approximately 5 million tons of bagasse is produced in the sugar mills of Louisiana. According to LSU Ag center approximately 75% of bagasse is used as in-house fuel for power generation or as raw material for producing low-value products such as mulch and ceiling tiles. The remaining 25% is waste that goes to a landfill or is allowed to decay and this 25% accounts 1.25 million tons of bagasse, which could be used for ethanol production every year.
Leaf stripping in Sugar Mill
The excess leafs that are brought to the mill along with cane stalk are usually converted to bagasse during the sugar production process. Some mills are separating leafs out of the process by using blowers and collecting these leafs as leaf stripping in the mill. The leaf stripping accounts for 5%/ton of processed canes. Currently, the Enterprise sugar mill in Iberia parish has half of its cane processed through leaf stripping process (diffusion process). This mill processed over 1 million tons of cane through diffusion process this year and this generated 50,000 tons of leaf in the mill. This is another source of raw material for ethanol production.
Ethanol Production
The close physical and chemical association between lignin and plant cell-wall polysaccharides is a major limitation to efficient utilization of sugarcane residue for ethanol production. A variety of pretreatments have been applied to agricultural residues and wood in an effort to increase the susceptibility of cellulose in these materials to enzymatic hydrolysis. Most of these pretreatments do increase the yield of glucose obtainable from lignocellulosic materials, but the yields are well below the theoretical maximum. In nature, lignin is degraded by various organisms, primarily to increase the amount of cellulose available for enzymatic digestion. Although the mechanism of natural lignin degradation is largely unknown, it is thought that oxidants such as hydrogen peroxide may play an important role. Hydrogen peroxide is known to react with lignin under certain conditions and has been widely used for many years to bleach high-lignin wood pulps. Delignification of wheat straw and corncobs by hydrogen peroxide has been reported. In this research, we demonstrate that the hydrogen peroxide treated sugarcane residue can be rapidly fermented to ethanol with greater than 92% overall efficiency. The major objective of this research is to evaluate whether alcohol can be made from fermentation of post-harvest sugarcane residue and to optimize a pre-treatment condition for high-efficiency alcohol production.
Results
Research at Nicholls State University explores the possibilities of making alcohol from the sugarcane residue. A chemical pre-treatment process using alkaline peroxide was applied to remove lignin, which acts as physical barrier to cellulolytic enzymes. Two yeast strains including Saccharomyces cerevisiae ATCC strains 765 and 918 were used in the experiment. The pre-treatment process effectively removed lignin. Alcohol production in the culture sample was monitored using gas chromatography. The results indicate that ethanol can be made from the sugarcane residue. The fermentation system needs to be optimized for evaluating the economics of producing ethanol from the sugarcane residue.
Figure 1. Solubilization of lignin from post-harvest sugarcane leaf treated with various concentrations of hydrogen peroxide at a pH of 11.5. Data represent means of four replications and the error bars indicate the standard deviation.
Figure 2. Solubilization of lignin from post-harvest sugarcane leaf treated with 1% hydrogen peroxide at various pHs. Data represent means of four replications and the error bars indicate the standard deviation.
Figure 3. Ethanol production by Saccahromyces cerevisiae strain 765 from the post-harvest sugarcane leaf pretreated with 1% hydrogen peroxide at pH 11.5. Data represent means of four replications and the error bars indicate the standard deviation.
Figure 4. Ethanol production by Saccahromyces cerevisiae strain 918 from the post-harvest sugarcane leaf pretreated with 1% hydrogen peroxide at pH 11.5. Data represent means of four replications and the error bars indicate the standard deviation.
