On-Farm Biofuel Production from Sweet Sorghum Juice

Sweet sorghum varieties were grown at two locations in the eastern North Carolina and processed to determine their value as a sustainable bioenergy feedstock. Juice collected from the crushed sorghum stalks (1500 gal/ac yield) was successfully converted to ethanol using a 40 gallon mobile processing unit. Data collected during this project showed sweet sorghum is a promising source of cheap sugar for biofuel production, if additional mechanization is developed to offset high harvest costs.

Introduction

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 residuals offer growers an opprotunity to contribute to the growing sustainable energy market. However, 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 on-farm. 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).