Evaluation of energy cane and sweet sorghum as feedstocks for conversion into fuels and chemicals *

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1 651 Daira Aragon, Mark Suhr, Vadim Kochergin Evaluation of energy cane and sweet sorghum as feedstocks for conversion into fuels and chemicals * Bewertung von energiereichem Zuckerrohr und Zuckerhirse als Rohstoffe für die Gewinnung von Kraftstoffen und Chemikalien Sweet sorghum and energy cane (high fiber cane) are potential crops for conversion into fuels and chemicals due to their low agricultural input requirements, potentially high fiber content and processing similarities with established sugarcane crops. A conceptual approach to a biorefinery producing fuels and chemicals from sweet sorghum and energy cane is proposed. The front-end of the plant processes 10,000 t/d of feedstock to extract convertible sugars by milling and concentrate them into storable syrups. The latter can be processed into gasoline, jet fuel and isoprene using proprietary technologies. The fiber remaining after extraction, called bagasse, is used in the boilers of the front-end plant to provide steam and power for entire facility and to produce additional second generation sugars by pretreatment and hydrolysis in a lignocellulosic conversion plant. Material and energy balances for the front-end plant were calculated using Sugars TM software. Results show that for the selected variety of energy cane, up to 46% of bagasse is available for further lignocellulosic conversion resulting in production of additional 33.6% of sugars. In this case, however, surplus electricity production is reduced by 86%. Calculations for sweet sorghum follow the same trend. Results show that a 13% reduction in fiber content by processing sweet sorghum instead of energy cane, reduces power export by 71% and second generation sugars by 40%. Key words: sweet sorghum, energy cane, fuel, chemical, biorefinery, bagasse, lignocellulosic conversion, surplus electricity Zuckerhirse und Zuckerrohr mit hohem Fibergehalt sind mögliche Kulturpflanzen für die Gewinnung von Kraftstoffen und Chemikalien. Eine Bioraffinerie zur Produktion von Kraftstoffen und Chemikalien aus Zuckerhirse und Zuckerrohr wird vorgestellt. Im Vorderbetrieb der Anlage werden t/d Rohstoff verarbeitet (Extraktion + Konzentration), um konvertierbare Zucker in Form von lagerfähigen Sirupen gewinnen zu können. Letztere können mit speziellen Technologien in Benzin, Kerosin und Isopren umgewandelt werden. Die nach der Extraktion verbleibende Bagasse wird zur Dampf- und Elektroenergieerzeugung für den gesamten Betrieb genutzt und um Zucker durch Vorbehandlung und Hydrolyse in einer Lignocellulose-Konversionsanlage zu gewinnen. Stoff- und Energiebilanzen für den Vorderbetrieb wurden mit Hilfe der Software Sugars TM erstellt. Die Ergebnisse zeigen, dass für die ausgewählte Zuckerrohrsorte bis zu 46 % der Bagasse für eine weitere Lignocellulosekonversion zur Verfügung steht, was einer zusätzlichen Zuckergewinnung von 33,6 % entspricht. In diesem Fall wird jedoch die Überschuss-Elektroenergieerzeugung um 86 % reduziert. Berechnungen für Zuckerhirse zeigen den gleichen Trend. Die Ergebnisse zeigen, dass eine 13%ige Reduzierung des Fibergehalts durch die Verarbeitung von Zuckerhirse statt energiereichem Rohr die Überschuss-Elektroenergieerzeugung um 71 % und die Gewinnung zusätzlicher Zucker um 40 % verringert. Schlagwörter: Zuckerhirse, Zuckerrohr mit hohem Fibergehalt, Kraftstoff, Chemikalie, Bioraffinerie, Bagasse, Lignocellulosekonversion, Überschuss-Elektroenergieerze 1 Introduction Need for decreasing greenhouse gas emissions, exhaustion of fossil resources and the desire for energy independence, have encouraged worldwide interest in fuels and chemicals obtained from renewable sources. In the U.S., for example, 121 bn L (36 bn gallons) per year of renewable fuels are to be produced by 2022, of which 60.5 bn L (16 bn gallons) must originate from cellulosic material [1]. Biomass is currently the * Paper presented at the 3rd ESST Conference in Warsaw, Poland, May 6 8, only renewable source for production of liquid substitutes of petroleum-based fuels for transportation [2]. However, first experiences with biomass conversion to liquid fuels have not resulted in cost-effective solutions. Alternative products, such as biobased chemicals and power generation must be considered, to make the integrated biorefinery concept more viable. Regional scenarios must be considered that take advantage of specific growth areas, types of feedstock and available infrastructure. A multidisciplinary consortium of agricultural scientists, biotechnologists, technology and engineering providers, economists and educators has been created to facilitate conversion

2 652 Technology/Technologie of energy cane and sweet sorghum crops into a portfolio of biobased fuels and chemicals for South Eastern region of the USA. The project was funded by National Institute of Food and Agriculture (NIFA) of USDA. Selected crops and improvement in their production are being evaluated through utilization of low-input, sustainable systems to ensure an uninterrupted supply of carbohydrates and fiber to biofuel and biochemical production facilities. (hybrid of commercial sugarcane, Saccharum officinarum, and wild sugarcanes) and sweet sorghum (Sorghum bicolor L. Moench) are considered as potential feedstocks. is expected to provide high biomass yields, and new varieties are expected to tolerate colder climates. Sweet sorghum provides an interesting alternative as it can be grown on marginal land due to its low agricultural input requirements [3 9]. New hybrid varieties of sweet sorghum offer opportunities to grow it as either main or supplementary crop. Production scenarios have to be considered accounting for seasonal production of the crops and continuous operation of biorefineries. An approach is considered that is based on processing similarities for both feedstocks, where juice has to be extracted in a primary processing plant prior to considering lignocellulosic processing. Furthermore, because commercial sugarcane, sweet sorghum, and energy cane share some characteristics, they can be processed in conventional sugar mills [10 13]. The sugar content in all these crops is high enough to justify primary extraction of first generation sugars. The water content of the crops is too high to be processed directly. Material and energy balances of the primary processing plant are calculated to evaluate the availability of sugars and fiber for lignocellulosic conversion after power and steam requirements to run the facility have been satisfied. Sugars TM software was used to simulate the process operating for maximum power generation and for maximum lignocellulosic (LC) sugars conversion. 2 Feedstock evaluation Generating meaningful scenarios for agricultural crops is imperative for evaluation of crop production cost and evaluating the capital and operating cost for conversion facilities (or biorefineries). This information is critical to assess the competitiveness of biomass crops and justify the new crop production for the growers. Understanding the crop composition and its variability during growing season, harvesting and storage are essential elements for evaluation of availability of convertible sugars and fiber. Most information on the composition cane or sorghum available in the literature does not provide sufficient data to complete heat and material balances for a biorefinery. Composition of the crops is typically available in a form similar to data presented in Table 1. The data are generated using quick methods in the field with no detailed analysis of types of sugars or components of fiber. Although the fiber content of energy cane is almost twice as that of commercial sugarcane, the sugar content values suggest that significant amount of sugar is available. Therefore, it is logical to extract readily available sugars prior to considering use of fiber as a potential source of lignocellulosic (LC), or Table 1: Typical data obtained on the field and raw sugar factories Cane variety Cane yield in t/ha Sucrose content in g/100 g juice Fiber content in g/100 g cane Sugarcane [14] (Ho ) [15] (HoCP )[16] Sweet sorghum (Dale, M81-E, Theis) [14] second generation, sugars. Resulting juice can then be concentrated into syrup and stored for production of biofuels and chemicals during the off-season. The remaining fiber portion (bagasse) can either be used to generate power and steam, or as a source of additional convertible sugars. For accurate calculation of mass and energy balances, more detailed compositional analysis is required. Literature data on this subject is scarce or does not exist, considering that energy cane varieties are not grown commercially. Although the potential of sweet sorghum and energy cane as feedstock for a biorefinery has been widely recognized, few studies have dealt with the issues mentioned above. Most of the work available in the literature considers use of conventional sugarcane for production of ethanol. For sweet sorghum and energy cane crops, papers present ethanol yield obtained per unit of feedstock, not considering the area required to feed an entire conversion plant. Additionally, most of these studies treat sweet sorghum primarily as a LC source [12]. None of them addressed the price that needs to be paid to a feedstock producer. One of the challenges of an integrated biorefinery is evaluating bagasse utilization. The option of steam and power generation is used routinely in many countries. Production of second generation (2G) sugars using one of the available pretreatment methods has not been implemented commercially. The product yields, and consequently, the economic feasibility of the plant, differ depending on the processing route selected. The biorefinery s product yields have to be calculated using material and energy balances for each processing scenario. Detailed data on juice and fiber composition is necessary as the total amount of sugars, and not only sucrose, are of interest when producing biofuels and chemicals. Most existing models have been developed for sugar production, therefore sucrose inversion, for example, was considered detrimental. Similarly, cellulose, hemicelluloses and lignin must also be considered in the feedstock compositional analysis, when production of 2G sugars from the bagasse is planned. Important figures to obtain from material and energy balances include the amount of electricity and steam necessary to operate the plant, the amount of bagasse necessary if it is burned, amount of bagasse left for LC pretreatment after burning, use of lignin, and amount of biofuel and chemicals obtained. If information on costs and sale prices of sugars and energy are available, full economical evaluation can be performed. Several authors have presented economical evaluation of different production scenarios for 1G, 2G and 1G/2G ethanol production for sugarcane and sweet sorghum. Dias et al. Sugar Industry 138 (2013) No

3 653 (2011) [17 19] simulated first and second generation ethanol production from sugarcane in Brazil, assuming that 50% of leaves and trash is recovered and used in the plant. The amount of LC material available for hydrolysis was calculated for different pretreatments, considering that energy is supplied by burning part of the LC material as fuel in the boilers. Similar studies have performed economical analysis for the production of 1G and 2G ethanol from sugarcane, including calculation of capital cost of the plant, and prices of ethanol to make the process feasible [20]. These studies, however, are based on production of ethanol only during the grinding season, so no bagasse allocation is made for storage. The economic feasibility of cane trash collection and processing remain unknown due to low density and additional cost of transportation and processing. 3 Conceptual approach Fig. 1: Biorefinery for sweet sorghum and energy cane The approach proposed in this work assumes that energy cane and sweet sorghum can be processed either separately or together in a facility similar to a raw sugar factory. Composition of different varieties of energy cane and sweet Table 2: Composition of sugarcane, energy cane and sweet sorghum, adapted from [14] Component Sugarcane (From several Louisiana mills) Sweet sorghum (Dale, M81- E, Theis, Topper) Energy cane (L (L)) Water content in % Sucrose content in % Glucose and fructose content in % Total sugars content in % Fiber content in % Cellulose in g/100 g fiber Hemicellulose in g/100 g fiber Lignin in g/100 g fiber) Ash in g/100 g fiber Yield in t/(ha year) sorghum were measured at Audubon Sugar Institute (ASI), St. Gabriel, Louisiana, USA. Table 2 presents more detailed analysis of energy cane and sweet sorghum. These values underline the idea that both crops are similar to sugarcane, and therefore, readily-available sugars have to be extracted prior to latter handling, while taking advantage of the high fiber content of energy cane for steam, power and possibly LC conversion. Figure 1 describes a general concept of the proposed biorefinery. Separate interconnected plants are considered: a primary processing plant, where juice is extracted from the feedstock and it is concentrated into storable syrup; a lignocellulosic plant, where excess bagasse is pretreated to obtain additional sugars, and finally, a chemical conversion plant (biorefinery) where the syrup is processed by different technologies into butanol, isoprene or gasoline additives. The latter technologies are proprietary to the development partners. Because of the seasonal character of the harvesting, the syrup produced in the primary plant must be stored to provide continuous feed to the conversion plant. All steam and electricity are supplied by a power plant that utilizes bagasse as a fuel.

4 654 Technology/Technologie Primary plant processing capacity is assumed at 10,000 t/d. Feedstock is collected from a total of 20,000 ha at 90 t/ha. Sugars are extracted in the primary processing plant, operating during a harvesting season of 180 days. This assumption may require processing of supplemental products and evaluation of crop composition variability during the harvest. A portion of the juice is concentrated into syrup and processed immediately in a biorefinery into fuels and chemicals. The remaining portion of juice is evaporated into more concentrated syrup (70 75% dissolved solids) and sent to storage for processing dur- Table 3: Modeling results: Scenario A: Operation for maximum power generation; Scenario B: Operation for maximum LC sugars conversion Sweet sorghum Description A B A B Bagasse for power and steam generation in % ing off-season. The high-fiber by-product, bagasse, is at 50% water content and is burnt to generate steam and power for the entire facility. Excess bagasse, if any, is pretreated in the lignocellulosic conversion plant to obtain additional sugars. 4 Modeling approach The primary processing plant was modeled similar to a raw sugar factory with a six roll-mill tandem. The LC conversion plant was modeled as a simple conversion unit, calculating the yield of glucose from biomass. The biorefinery itself was not modeled due the proprietary nature of the conversion process, but the estimated energy consumption was included. The model of the primary plant was developed and simulated using Sugars TM software. The model comprises complete process starting from cane loading to the evaporation of juice into syrups. A basic model of the LC conversion plant was also developed in Sugars TM program. 5 Results and discussion Two scenarios were simulated in Sugars TM using the feedstock composition presented in Table 2: (A) Operation for maximum power generation and (B) Operation for maximum LC sugars conversion. In both cases, internal steam and power requirements are met for on- and off-season operation, before exporting electrical power and/or converting fiber into sugars. Steam and power consumed in the primary plant is dependent upon the mass of cane being crushed and the quantity of juice sent to evaporation. In this work, a very efficient milling plant was considered, consuming between 20 and 32 kwh per t of feedstock. On the other hand, steam and energy consumption in the LC conversion plant depends on the amount of bagasse pretreated. Consumption of 162 kwh/(t dry biomass) [21] and 1.7 kg steam/(kg dry biomass) were assumed for the LC plant, with a yield of 37 g of glucose/100 g dry biomass [22]. Also, it was assumed that all heat requirements for the biorefinery are met by internal sources, so steam from primary plant is not required. Additionally, the calculations assume that electricity is bought (traded for exported electricity) during off-season to operate the biorefinery. Primary extraction plant LC conversion plant and biorefinery Bagasse used for production of LC sugars in % Total in % Sugars produced by primary extraction plant in dry kg/t biomass LC sugars produced from fiber in dry kg/t biomass Total sugars produced in dry kg/t biomass Sugars produced from fiber in % total Surplus electrical power in kwh/t biomass Table 3 indicates that sugars obtained by primary extraction from both energy cane and sweet sorghum are significant. Using composition of generic commercial sugarcane shown in Table 2, dry kg of sugars/t biomass are extracted in the primary plant. When energy cane is processed, close to half of the bagasse is available for LC conversion, producing additionally 33.6% of the total sugars. In the case of sweet sorghum, 28.6% of bagasse is available for lignocellulosic conversion producing additional 9.3% of the sugars. It is evident that most sugar is produced in case of primary extraction, and smaller portion can be potentially recovered from fiber. Cost of production of additional sugars will naturally be higher. Additionally, there is a significant loss of electrical power export capability when fiber is used as a source of sugars; 86% and 85% for energy cane and sweet sorghum, respectively. The availability of bagasse for power generation or sugar production is directly dependent on the fiber and sugar content of the feedstock. The decrease of near 13 points on fiber content, when processing sweet sorghum instead of energy cane, causes a reduction in power export capacity by 71% if there is no conversion of bagasse into sugars. In simulations performed by Dias et al. [17] for sugarcane with 14% fiber, the inclusion of LC conversion allowed producing up to 24% more ethanol with loss of power not higher than 66%. However, these results are based on the assumption that 50% of the trash is recovered and used in the boilers for steam and power generation. Cane trash collection and transportation is still questionable due to low density and low value of trash, high dirt content that is detrimental to boiler performance, etc. This indicates that for low fiber feedstocks, higher sugar yields are possible only if other sources of power are employed. Utilization of diffusion process instead of milling for primary processing will allow having more excess bagasse available for either power generation or conversion. In a comprehensive comparison of milling and diffusion processes performed by Mullapudi [23], a diffuser consumed 20% less power and exported 22% more power than a milling tandem. These and other considerations about operability and sugar recovery, present the diffuser as a better alternative for sugar extraction. Further economic analysis is required based on specific conversion technology with certain assumptions on relative cost of power and sugars to determine the feasibility of lignocellulosic conversion from sugarcane bagasse. Sugar Industry 138 (2013) No

5 655 6 Conclusions Most of the sugars that can be recovered from high fiber cane (energy cane) or sweet sorghum can be extracted using technology applied in conventional sugar mills and used for fabrication of biobased fuels and chemicals. Relatively small fraction of additional sugars can be obtained from fiber at higher cost compared with primary extraction. Both sweet sorghum and energy cane provide enough fiber to meet steam and power requirements and produce excess bagasse to export electricity and/or support second generation sugars. Significant power export loss is expected if second generations sugars are produced. Existing data on the composition of crops is not sufficient to complete material and energy balances for engineering studies. More detailed compositional data for energy cane and sweet sorghum varieties are required to evaluate these crops as biorefinery s feedstock. Acknowledgments This work is supported by the United States Department of Agriculture, National Institute of Food and Agriculture, Agricultural and Food Research Initiative, AFRI grant # References 1 Energy, U.S.D.o. (2012): Integrated biorefineries. [cited 2012 June 26]; Available from: 2 U.S. Congress (2005): Energy policy act 3 Gravois, K.A.; Bischoff, K.P. (2008): High-fiber sugarcane varieties: Good choice for alternative energy. Louisiana Agriculture 51(2), 11 4 Bennett, A.S.; Anex, R.P. (2009): Production, transportation and milling costs of sweet sorghum as a feedstock for centralized bioethanol production in the upper Midwest. Bioresource Technology 100(4), Ratnavathi, C., et al. (2011): Sweet Sorghum as Feedstock for Biofuel Production: A Review. Sugar Tech. 13(4), Whitfield, M.B.; Chinn, M.S.; Veal, M.W. (2012): Processing of materials derived from sweet sorghum for biobased products. Industrial Crops and Products. 37(1), Yu, J., et al. (2012): Biorefinery of sweet sorghum stem. Biotechnology Advances. 30(4), Viator, H.P., et al. (2009): Sweet sorghum for biofuel production in Louisiana. Louisiana Agriculture. 53(4), Lingle, S.E. (2010): Opportunities and Challenges of Sweet Sorghum as a Feedstock for Biofuel; in: Eggleston, G. (Editor): Sustainability of the Sugar and Sugar Ethanol Industries. American Chemical Society, Washington, DC 10 F.C. Schaffer & Associates, I. (1986): 1986 Sweet Sorghum Pilot Program Report. Louisiana State University Agricultural Center, Baton Rouge, LA 11 Webster, A.J., et al. (2004): Observations of the harvesting, transporting and trial crushing of sweet sorghum in a sugar mill. Proc. Austr. Soc. Sugar Cane Technol. 26, Polack, J.A.; Day, D.F. (1982): Ethanol from Sweet sorghum. Journal American Society of Sugar Cane Technologists 1, Bradford, V.E. (2008): An Advanced Feedstock for Ethanol: Sweet Sorghum is Crop to Fuel the Future. Sugar Journal, Kim, M.S.; Day, D.F. (2011): Composition of sugar cane, energy cane, and sweet sorghum suitable for ethanol production at Louisiana sugar mills. Journal of Industrial Microbiology & Biotechnology 38(7), USDA-ARS, L. AgCenter, and A.S.c. league (2010): Candidate for energy cane release Ho Gravois, K., et al. (2011): Yield and fiber content of high fiber sugarcane clones. 17 Dias, M., et al. (2011): Simulation of integrated first and second generation bioethanol production from sugarcane: comparison between different biomass pretreatment methods. Journal of Industrial Microbiology & Biotechnology 38(8), Dias, M.O.S., et al. (2012): Integrated versus stand-alone second generation ethanol production from sugarcane bagasse and trash. Bioresource Technology 103(1), Dias, M.O.S., et al. (2011): Second generation ethanol in Brazil: Can it compete with electricity production? Bioresource Technology 102(19), Macrelli, S.; Mogensen, J.; Zacchi, G. (2012): Techno-economic evaluation of 2nd generation bioethanol production from sugar cane bagasse and leaves integrated with the sugar-based ethanol process. Biotechnology for Biofuels 5(22), Kabir Kazi, F.; Fortman, J.; Anex, R.; Kothandaraman, G.; Hsu, D.; Aden, A.; Dutta, A. (2010): Technoeconomic analysis of biochemical scenarios for production of cellulosic ethanol. NREL/TP Technical report 6A Aita, G.A.; Salvi, D.A.; Walker, M.S. (2011): Enzyme hydrolysis and ethanol fermentation of dilute ammonia pretreated energy cane. Bioresource Technology 102(6), Mullapudi, N. (2010): Comparison of diffusion and milling at a cane sugar plant. XXVII ISSCT congress. Veracruz, Mexico, March 7 10 Authors addresses: Daira Aragon, Audubon Sugar Institute, 3845 Highway 75, St. Gabriel, LA 70776, USA; Mark Suhr, MS Processes Intl., LLC, Highway 7 East, Hutchinson, MN 55350, USA; Vadim Kochergin, Amalgamated Research LLC, 2531 Orchard Drive East, Twin Falls, Idaho, USA; DAragon@agcenter.lsu.edu