On-Farm Biofuel Production from Sweet Sorghum Juice

View the project final report

Commodities

• Agronomic: sorghum (milo)

Practices

• Crop Production: application rate management
• Education and Training: demonstration, on-farm/ranch research, workshop, technical assistance
• Farm Business Management: new enterprise development, value added
• Sustainable Communities: new business opportunities, sustainability measures

In recent years, most citizens in North Carolina and other states have started to feel the impact of limited petroleum resources in the cost of transportation fuels for their vehicles and indirectly with prices of consumer goods. There is an increased desire in the community to have more economical alternative biofuel sources that can support and strengthen activities of producers and agribusiness. Ethanol is an attractive, sustainable energy source that has uses as a fuel additive and/or alternative transportation fuel that can reduce the negative environmental impacts of current fuel sources. Currently in the U.S., corn is the primary raw material for ethanol production and this crop has proven quite valuable for growers in mid-west states. Starch, which constitutes about 70% of the corn kernel, is broken down with enzymes into glucose and directly fermented by yeast to ethanol. The corn (starch) to ethanol industry is quite mature, yet projected production levels fall short of the nation’s fuel requirements, with limitations in corn yields and committed uses as food and feed. Furthermore, corn cannot be competitively grown in most states including North Carolina; and efforts are being made to identify promising sustainable feedstocks and conversion technologies that will allow other regions to become players in the production of fuel ethanol. Accordingly, plant-derived resources have been studied as alternative raw materials for conversion into ethanol. Feedstocks that have been investigated include corn stover, switchgrass and wood chips. These crops and their associated residuals offer growers an opprotunity to contribute to the growing sustainable energy market. Processing of these lignocellulosic materials requires the liberation of soluble sugars from long chains of cellulose and hemicellulose surrounded by lignin prior to fermentation (Sheehan and Himmel, 1999). The complexity of lignocellulosic feedstocks has limited the development of cost effective conversion processes. In general, there are a number of barriers at various stages in the accepted process for direct fermentation that need to be addressed before an economical technology for lignocellulosics is available (Mielenz, 2001; Eriksson et al., 2002). There is a need to identify more viable crops that will give rise to simplified, timely ethanol processes and provide new opportunities to rural communities. Sweet sorghum is a sugar crop, similar to sugar cane and sugar beets, that may show promise as a source of sugar for ethanol fermentation (Nathan, 1978). It is an annual crop in the grass family. It is noted for its high photosynthetic efficiency, adaptability to temperate regions and drought resistance (Worley et al., 1992; Gnansounou et al., 2005; Martini et al., 2006). The pith or stalks can be mechanically pressed to release a sugar juice (15-22 °Brix) that can be filtered and directly fermented by yeasts. The resulting ethanol can be separated through subsequent centrifugation and distillation processes. The primary advantage of sweet sorghum over starch and lignocellulsic sources is the reduced processing steps and inputs required for complete conversion, which may reveal improved economic benefits over corn feedstocks (Worley et al., 1992). Challenges in using sweet sorghum juice include the harvest time that is limited to 3-4 months per year and maintenance of juice stability. A number of reports suggest that juice extraction should occur soon after harvest and processing needs to take place immediately (Gnansounou et al., 2005; Kundiyana et al., 2006). In effort to obviate some of the issues with juice transportation, storage and stability, this proposal aims at providing growers with the opportunity to generate their own fuel ethanol. Efforts have been made to achieve in-field ethanol production facilities with some success, yet the design has not been fully developed and implemented (Kundiyana et al., 2006). With some technical assistance, NC farmers have a great opportunity to benefit from the growing need for biofuels and other biobased products that can be generated through bioprocessing of farm-produced biomass. Recently, a local grower from Nash County, North Carolina realized this potential and sought our assistance in configuring a production system that would process sweet sorghum into juice and yield ethanol. His vision was to design a process that could assist farmers in raising their own fuel from the farm and support the cultivation of a sustainable, wholly value added crop. Thus, we identified a need to define a working process for on-farm conversion of sweet sorghum at an intermediate level as a step toward enhancing economic development. This proposed work will assist the development of an on-farm sorghum-to-ethanol production system for the purpose of evaluating the techno/economic potential and the level of grower interest in on–farm production of biofuels. Preliminary small-scale field studies were conducted during October 2006 using the sweet sorghum crop that Mr. Gerald Sykes (Nash County) had grown and offered for our use. The whole stalks were pressed with fodder removed to improve juice quality and yield (Worley et al., 1992). The juice was filtered (for removal of bacterial contaminants) and stored at three conditions (fresh, frozen, concentrated syrup ~30o Brix). The completed fermentations indicated that Red Star® yeast can effectively convert non-sterile sweet sorghum juice (~15o Brix, pH 5.2) to ethanol within five days at ambient temperature (13 – 24o C).

The overall goal of the proposed work is to establish an on farm system capable of processing sweet sorghum stalks into a stable juice that can be subsequently fermented to ethanol and concentrated for use as an alternative fuel. Specific objectives are to 1) design and construct fermentation units to maintain the necessary growth conditions with minimal user input 2) design and construct a cost effective apparatus to separate and purify ethanol produced through fermentation and 3) test system function, efficiency, and economic feasability. The system will be used to examine optimal parameters necessary for ethanol production from sorghum juice, serve as a demonstration unit for helping establish similar systems on North Carolina farms and offer promising technology to initiate biofuels extension efforts in rural communities.
Objective One: Design and construct fermentation units to maintain the necessary growth conditions with minimal user input. Vessels (200 L) will be designed using stainless steel steam kettles (provided by our primary cooperator, Mr. and Mrs. Sykes) to support juice fermentation in either batch or continuous operation, monitor and control temperature and pH, allow for yeast and chemical additions and sampling, provide gas exchange/pressure relief and agitation.
Objective Two: Design and construct a cost effective apparatus to separate and purify ethanol produced through fermentation. A gas fired vat-type cooker (of the type use for molasses by our farmer cooperator) will be fitted with a distillation column for producing 95% ethanol. Methods for improving efficiency of the distillation will be evaluated as funds permit. One option will be continuous fermentation and ethanol concentration using a pervaporation membrane module. This would allow the distillation step to start with 40-60% ethanol rather than the typical 4-6% normally in the fermentation broth. Storage of the low proof ethanol would allow further pervaporation concentration “off-line” after all the crop’s juice production is fermented. This may allow elimination of the energy intensive distillation step giving greater energy efficiency.
Objective Three: Test system function, efficiency, and economic feasibility. The system will be constructed and operated either in the workshop space of Mr. and Mrs. Sykes or Mr. Clay Strickland. It will be readied for on-farm evaluation and demonstration in the fall or late summer of 2007 depending on the appropriate harvest date for next year’s sorghum. The crop will be grown in two locations to assist with outreach activities: a cooperator farm in Nash County and NC State University’s Cunningham Research Station (Kniston, NC). Objective 3a. Preliminary Lab Scale Experiments—Fermentation of the juice is considered to be well understood, yet the key processing parameters for the identified sweet sorghum variety and yeast culture need to be defined (de Mancilha et al., 1984). Fresh juice (100 ml, 250 ml flasks) will be fermented in the lab under controlled conditions to re-evaluate optimal pH, nitrogen and phosphorous supplements, initial yeast concentrations and theoretical ethanol yields. Objective 3b. Testing—Juice will be used to run trials in the final system to evaluate process operation and function. Areas for improvement will be identified. The system will be tested at capacity using juice (fresh and possible other feasible forms) from sweet sorghum grown in year 2007 in both batch and continuous operation. The juice yields, sugar contents, ethanol production capacity and system efficiency will be determined. Objective 3c. Feasibility Study—An economic analysis based on cost inputs derived from feedstock collection, system fabrication and operation, enzymes, and supplemental nutrient requirements by yeasts will be conducted and a model developed to determine the cost of ethanol production from sweet sorghum juice. Objective 3d. System Demonstration—Extension field days/workshops to show and promote the concept will be organized during next summer/fall in the Nash County area and at the Cunningham Research Station with the help of our extension cooperators. While the location for the on-farm activity is eastern NC, an extension publication based on this work will allow the dissemination of the project results statewide and regionally. Efforts will also be made to coordinate related outreach activities in the western part of the state.