Economic Comparison of Incorporation of Sweet Sorghum Juice into the Current Dry-Grind Ethanol Process

Abstract

Sweet sorghum is a promising feedstock for ethanol production. The harvest approach used for sugarcane is currently being applied to sweet sorghum in which the panicle is removed and left on the land as residue. Recovering the panicle will allow for the utilization of sweet sorghum grains in the dry-grind ethanol process. In this paper, an economic comparison between the current dry-grind ethanol process and an alternative process that replaces water with sweet sorghum juice is applied. A literature review was conducted to draw assumptions about the economics and sustainability of incorporating sweet sorghum juice into the ethanol process based on previous related research. Studies show substituting sweet sorghum juice for water in the slurry provides an increase of 28% in ethanol yields and a 30 minute reduction in enzymatic hydrolysis time. This suggests a conventional 40 million gal/year ethanol facility would produce an additional 11.2 million gallons of ethanol annually. Incorporating sweet sorghum juice into the slurry also reduces water usage and thus water costs for ethanol producers. If 47% of the total water requirement is used for ethanol fermentation, a 40 million gal/year plant will save approximately $105,750 in water costs annually when sweet sorghum juice is used in place of water. For a 55 million gal/year ethanol facility, an estimated 50 million gallons of non-recycled water is added to the slurry, suggesting 50 million gallons of sweet sorghum juice would be needed annually. The estimated economic value of sweet sorghum juice is $0.19/gal. Sweet sorghum juice for industrial use is currently more expensive than water. However, the significant increase in ethanol yields may compensate for the cut in profits and government subsidies are available. Conflict regarding water over-consumption is becoming more prominent as the availability of freshwater is plummeting globally. Thus, incorporating sweet sorghum juice into the current dry-grind ethanol process is both an environmentally friendly and sustainable implementation.

Introduction

Sweet Sorghum: A Biofuels Feedstock

Sweet sorghum (Sorghum bicolor (L.) Moench) is an eminent energy crop utilized for the production of bioethanol. The advantages that make sweet sorghum a viable feedstock option are1,2,3:

  • High sugar content
  • Adaptability to diverse environmental conditions (climate, salinity, alkalinity and drought)
  • Short growth period (120-150 days)
  • Low requirement of fertilizers
  • High water-usage efficiency
  • Ability to grow on marginal lands
  • Sweet sorghum provides starch in its grain, sugar in its juice and lignocellulosic biomass, all of which can be utilized in the production of ethanol. The concentration of fermentable carbohydrates within the stalk is 40-50% of the total biomass. This is equivalent to more than 200 bushels/acre of corn yield, making sweet sorghum a promising ethanol feedstock4.

    Description

    Utilization of Sweet Sorghum Grains

    The harvest approach used for sugarcane is currently being applied to sweet sorghum, in which the panicle is removed and left on the land as residue. The developing grain is comprised of 75% starch, which can be easily fermented into ethanol if co-extracted with the soluble sugars. Thus, excluding the grain from the biomass causes a significant loss in the amount of available carbohydrates for fermentation. The residue may also result in volunteer sorghum as a weed, impairing the next growing season. Recovering the panicle will allow for the utilization of sweet sorghum grains in the ethanol production process.

    Incorporation of Sweet Sorghum Juice into the Dry-Grind Process

    In the current dry-grind ethanol process from sorghum, sorghum grain is ground and mixed with water to form mash, which is cooked, liquefied, saccharified, and fermented by yeast to produce ethanol. An alternative method is to substitute sweet sorghum juice for water in the slurry. Previous investigators have reported that ethanol yield from slurry of sweet sorghum juice and sorghum flour showed an increase of 28% over the ethanol yield of conventional ethanol process; they also found that starch enzymatic hydrolysis time could be reduced by 30 minutes5. Sweet sorghum juice is extracted from the stalk by mechanical crushing using a roller mill. The water content of sweet sorghum juice is approximately 80%, and the typical sugar composition is 9-33% glucose, 53-85% sucrose, and 6-21% fructose.6

    Sustainable and Economic Implications

    The incorporation of sweet sorghum juice into the bioconversion process is expected to improve the overall starch-to-ethanol conversion efficiency, resulting in increased ethanol yields. Higher ethanol yields should allow for a more energy-efficient, profitable process7. Currently most ethanol plants require approximately 3 gallons of water per gallon of ethanol produced8. The high water content of the sweet sorghum juice will significantly reduce this water requirement, increasing the water-usage efficiency of the process. This means reduced water costs for ethanol producers, and less conflict regarding water over-consumption. Substituting sugar-rich sweet sorghum juice for water also increases the ethanol yield per ton of biomass and per acre of production.

    Dry-Grind Ethanol Process

    Description

    In the current dry-grind process for ethanol production, the whole cereal is first ground into a fine powder using a roller mill (Figure 1). The biomass should be ground to a particle size in which the starch is exposed, but not so fine that the powder causes balling in the slurry tank or affects the efficiency of the by-product recovery process. The cereal is then mixed with a blend of water and backset stillage. The slurry is treated with the hydrolyzing endoenzyme alpha amylase, which converts the cereal to oligosaccharides, called dextrins. The mash is cooked to release all bound carbohydrates. The alpha amylase breaks down the granular structure of the starch, causing gelatinization. As it is cooked the meal absorbs water and swells, increasing the viscosity of the mixture.

    In the process of liquefaction, the viscosity of the mixture is reduced as the starch is hydrolyzed into dextrins. The exoenzyme glucoamylase is added to further hydrolyze the dextrins into glucose. During saccharification, the individual glucose molecules are released from the liquefied mixture of dextrins, and this mixture is then cooked again to break down the starches. The yeast strain Saccharomyces cerevisiae is added to feed on the free sugars and ethanol is given off as a byproduct.

    Results and Discussion

    Ethanol Plant Expenditures

    Description

    Table 1 illustrates the approximate cost of operation, chemicals, and enzymes for a conventional ethanol facility. The costs are broken down by thousands of dollars spent annually and the expenditure per gallon of ethanol produced. The cost of the feedstock is excluded from the data. These costs are based on the assumption that the plant produces 40 million gallons of ethanol per year.

    Water Usage in Ethanol Production

    Ethanol production requires water for liquefaction, fermentation, separation, cooling, and steam generation10. There are two types of water used to make ethanol: process water and non-process water. Process water is used in the carbon dioxide scrubber and yeast tanks, whereas non-process water is involved in the cooling tower and boiler11. A plant producing 40 million gallons of ethanol annually can use up to 330,000 gallons of water per day or 120 million gallons of water each year12. The cooling tower has the highest water demand, accounting for 53% of the total water requirement for ethanol production13.

    Working with such high volumes of water leads to water losses throughout the ethanol production process. Water losses occur via evaporation, drift and blowdown from the cooling tower and boiler13. An estimated 750,000 gallons of water are evaporated for every 1 million gallons of water added to the ethanol production system. Blowdown is responsible for 250,000 gallons of this evaporation14 and is costly for ethanol producers who must pay to have it treated and discharged from the plant. Despite the great amounts of water loss throughout ethanol production, a significant amount of water can be recovered and reincorporated into the process. Evaporation from the beer distillation can be collected and recycled into the slurry.

    Description

    Incorporating Sweet Sorghum Juice

    Ethanol facilities use a significant amount of resources on water costs. Table 1 highlights the estimated water cost of a plant producing 40 million gallons of ethanol annually. A plant of this stature can expect to pay $225,000/year on process water and $286,000/year on water pre-treatment costs. This equates to roughly $511,000 in total water costs. Incorporating sweet sorghum juice into the ethanol production process will reduce plant water usage, in turn reducing water costs. If 47% of the total water is used for ethanol fermentation the plant can expect to save approximately $105,750 in water costs annually when sweet sorghum juice is used in place of water.

    The 55 million gal/year ethanol facility Kansas Ethanol LLC recycles and reincorporates over 80% of water required for their production process. With a slurry capacity of 250 million gallons, an estimated 50 million gallons of non-recycled water is added to the slurry. The water used to make this slurry can be replaced with sweet sorghum juice. This suggests 50 million gallons of sweet sorghum juice would be needed annually. The average amount of juice that can be extracted from the crop and made available for industrial use ranges from 4435.7 and 5179.0 kg/ha4. This equates to 1171.8-1368.1 gal/ha. The economic value of sweet sorghum juice is $0.19 per gallon, suggesting it would cost ethanol producers $222.6-260/ha for sweet sorghum juice.

    Description

    Sweet sorghum juice (18% concentrate)

    Use of High Solid Content Fermentation

    With the incorporation of sweet sorghum juice, it can be assumed that all expenses remain the same as a conventional ethanol plant. However, with the use of high solid content fermentation the waste water pretreatment costs could be significantly reduced from the $286,000 conventional ethanol plants pay (Table 1). Energy expenditure in the cooking process could also be significantly reduced with high solid content fermentation by reducing enzymatic hydrolysis time. Typically 60 minutes are required for enzymatic hydrolysis in the lab, but a recent study showed this time could be reduced to 30 minutes with the reduction of solid content. This is true at both lab and industrial scale, even with high temperature jet cookers5. High solid content fermentation allows ethanol producers to get higher ethanol yields with only minor changes within their facilities.

    Benefits of Sweet Sorghum Juice Incorporation

    Based on previous related research, a 28% increase in ethanol yield can be expected with the incorporation of sweet sorghum juice in the current dry-grind process5. A 28% ethanol yield increase for a 40 million gal/year plant suggests the plant would produce an additional 11.2 million gallons of ethanol annually. Replacing water with sweet sorghum juice comes with other beneficial implications. Because sweet sorghum juice retains more moisture in solid than water, separation and drying costs of the co-product distiller's dried grains with solubles (DDGS) may be reduced, in turn conserving energy and reducing utility costs for the plant. Utilizing the juice from the sorghum stalk also suggests a significant amount of water can be conserved throughout the ethanol production process. Water conservation leads to reduction in water costs for ethanol plants and results in less controversy regarding water over-consumption. Major global changes such as land use, climate change, population growth, urbanization, etc. are putting stress on the availability of fresh water. While the human population has tripled within the last century, the demand for water has increased six fold15. Because of this stress, as well as the maximization of profitability, many ethanol producers have committed to reducing water usage within their plants.

    Description

    DDGS

    Conclusions

    With the incorporation of sweet sorghum juice in the current dry-grind ethanol process, conventional ethanol facilities can expect an annual savings of $105,750 in water costs. The estimated economic value of sweet sorghum juice is $0.19/gal. For a 55 million gallon/year ethanol facility, an estimated 50 million gallons of non-recycled water is added to the slurry, suggesting 50 million gallons of sweet sorghum juice would be needed annually. Although the cost of sweet sorghum juice appears to be several magnitudes higher than the water savings for the plant, other factors must be accounted for. Separation and drying costs may be reduced for DDGS, an economically valuable co-product of ethanol production. Sweet sorghum grains are starch-rich, thus ethanol yields from slurry of sweet sorghum juice and sorghum flour showed an increase of 28% over the ethanol yields of conventional ethanol process. The same study also showed energy could be conserved by reducing starch enzymatic hydrolysis time by 30 minutes, reducing utility costs for the plant5. A 28% ethanol yield increase for a 40 million gal/year plant suggests the plant would produce an additional 11.2 million gallons of ethanol annually. Incorporating sweet sorghum juice in the dry-grind process reduces water usage, thus water costs, for the plant.

    The water conservation and increased ethanol production efficiency with the utilization of sweet sorghum juice indicates that this is a sustainable alternative to the current dry-grind ethanol process. Sweet sorghum juice for industrial use is currently more expensive than water. However, the significant increase in ethanol yield may compensate for the cut in profits. Government subsidies are available for ethanol facilities implementing green initiatives. Further research on this topic must be conducted in order to derive more conclusive results.

    Acknowledgements

    This material is based upon work supported by National Science Foundation Grant: REU Site: Summer Academy in Sustainable Bioenergy; NSF Award No.: SMA-1062895, awarded to Kansas State University.

    Description

    Special thanks to:

  • Nana Baah Appiah-Nkansah
  • Dr. Donghai Wang
  • Dr. Mary Rezac
  • Keith Rutlin
  • References

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    Keri N. Brown

      Description

      I am from a small town in Northern Kentucky called Ludlow. I currently am a senior biology and chemistry major at Eastern Kentucky University. I will be graduating with my Bachelor's degree in May, 2015 with the hopes of returning to Kansas State University for graduate school the following fall. I plan to obtain a PhD in bioseparations within the Biological and Agricultural Engineering department and then enter academia at a university.

      Being involved in the Sustainable Bioenergy REU has provided me with a world of opportunity and a summer full of great memories. I got to fall in love with the beautiful Flint Hills and make fantastic new friends!

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