BioCycle December 2006, Vol. 47, No. 12, p. 44
This special report from a USDA Research Laboratory in South Dakota examines the growing challenge and opportunities in the production of ethanol from corn grain.
Kurt A. Rosentrater
BIOFUELS, renewable sources of energy, can be produced from biomass, including agricultural residues, straw, corn stover, perennial grasses, legumes, and other biological materials. At the moment, the most heavily utilized is corn grain. Industrial ethanol production from corn is readily accomplished at a relatively low cost compared to other biomass sources. In coming years, however, the conversion of lignocellulosic materials is expected to become cost-competitive as this industry matures. Because ethanol is the fastest growing of all the renewable energy sectors, it is imperative to discuss this industry, as well as the by-product/coproduct streams that it generates.
It is an exciting time to be working with biofuels, especially fuel ethanol. On one hand, the U.S. is anticipated to domestically produce only approximately 35 percent of its required oil supply of nearly 15.4 million barrels per day, the balance of which will have to be supplied by foreign imports (www.eia.doe.gov). On the other hand, the corn-based fuel ethanol industry is well poised to help augment the nation’s demand for transportation fuels.
Over the last decade, the industry has witnessed phenomenal innovations, not only in production processes used and final products produced, but also in terms of optimizing resources and energy consumed. Due to many advantages, including lower capital and operating costs (including energy inputs), most new ethanol plants are dry grind facilities (Figure 1), as opposed to the older style, much larger wet mills.
The number of ethanol plants has been rapidly increasing in recent years (Figure 2). As of October of this year, 106 manufacturing plants in the U.S. have an aggregate production capacity of over 5 billion gal/yr. Moreover, 45 plants are currently under construction and seven are undergoing expansion, all of which will contribute an additional 3.5 billion gal/yr (www.ethanolrfa.org). It is projected that the industry will continue to expand at a rate between 10 and 20 percent per year for the foreseeable future. As production volume increases, the processing residues will increase in tandem (Figure 3).
Briefly, dry grind ethanol manufacturing from corn grain typically results in three products: ethanol, the primary end product; residual nonfermentable corn kernel components, which are marketed as “distillers grains”; and carbon dioxide. The typical rule of thumb is that for 1 kg of corn processed, approximately 1/3 kg of each of the constituent product streams will be generated. Another rule of thumb states that each bushel of corn will yield approximately 2.7 gal of ethanol, 18 lb of distillers grains, and 18 lb of carbon dioxide.
The overall production process (Figure 4) consists of several unit operations. Grinding, cooking and liquefying release and converting the starch, so that it can be fermented into ethanol using yeast. After fermentation, the ethanol must be separated (via distillation) from the water and nonfermentable residues. Downstream separations, drying and mixing are then used to remove water from the solid residues and to produce a variety of by-product streams, known as distillers grains – wet or dry, with or without added condensed soluble materials.
Carbon dioxide arises from fermentation, and is generated during the metabolic conversion of sugars into ethanol by yeast. This by-product stream can be captured and sold to compressed gas markets, such as beverage or dry ice manufacturers. Often, however, it is just released to the atmosphere because location and/or logistics make the sales and marketing of this gas economically unfeasible.
OVERALL COSTS AND BENEFITS OF ETHANOL
Concern about resource inputs and outputs, economics, impacts of manufacture, and use of corn-based ethanol has led to many Life Cycle Assessment studies to examine the overall costs and benefits of this biofuel. Some of the more prominent studies include Andress (2002), Kaltschmitt et al. (1997), Kim and Dale (2002, 2004), Lynd and Wang (2004), Pimentel and Patzek (2005), Shapouri et al. (1995, 2002, 2003a, 2003b), Sheehan et al. (2002, 2004). Even though there have been many questions over the years regarding the energy balance for the production and use of ethanol, it appears that this discussion has finally been resolved. Farrell et al. (2006) provide a thorough review and synthesis of this debate.
It is clear that the modern fuel ethanol industry is economically viable, and makes sense. Even so, the coproducts (i.e., nonfermentable residues) from the production process are key to this, and must be dealt with at each facility in order for the entire industry to grow successfully, sustainably, and thus maintain viability. Because the starch is converted during the fermentation process, the nonfermentable materials consist of corn proteins, fibers, oils and minerals. These are typically used to produce various by-product feed materials, the most popular of which is known as Distillers Dried Grains with Solubles (or DDGS – Figure 5). DDGS is dried to approximately 10-12 percent moisture content, to ensure a substantial shelf life, and then sold to local livestock producers or shipped via truck or rail for use in livestock feed rations around the nation. Distillers Wet Grains (DWG – Figure 6) is popular with livestock producers near the ethanol plants. But, because the moisture contents are generally greater than 50 to 60 percent, their shelf life is very limited, and shipping economics are cost prohibitive.
Sale of distillers grains contributes substantially to the economic viability of ethanol plants, and is thus a vital component to their operations. That is why these process residues are referred to as coproducts, instead of by-products. Because of the dynamics of the free market economy under which this industry operates, the quantity of these processing residues that will be produced as the industry grows could substantially influence the future of the industry. The increased supply of distillers grains will affect sales price and potential feed demand, while carbon dioxide generation may eventually be affected by greenhouse gas emission constraints. Both of these issues could severely impact the production economics of the industry in the near future.
MANAGING COPRODUCT STREAMS
The ethanol industry is very dynamic and is continually evolving. The modern dry grind plant is vastly different from the Gasohol plants of the 1970s. New developments, to name only a few, include better enzymes, higher starch conversion efficiencies, cold cook technologies, decreased energy consumption throughout the plant, and more value streams from both the corn kernel (i.e., upstream fractionation) as well as the resulting distillers grains (i.e., downstream fractionation). Additionally, much research has focused on modifying existing processes in order to reduce the quantity of residuals produced. As these process modifications are validated and commercially implemented, reductions in the generated coproducts will be realized, and unique coproducts (such as high protein, low fat, and/or low fiber distillers grains) will be produced as well.
Currently, the ethanol industry’s only outlet for thenonfermentable residues, which are primarily in the form of DDGS, and to a lesser degree in the form of DDG and DWG, has been livestock feed. As with many food and organic processing waste streams, feeding ethanol coproducts to animals is a viable method for utilizing these materials because they contains high nutrient levels. Over the years, numerous research studies have been conducted to optimize their use in livestock feed rations. Aines et al. (1986) and UMN (2006) provide comprehensive reviews of this research. Even so, much work remains to be investigated in order to improve and maximize the utilization of these coproducts in animal feeds. This approach to utilization is well established, but needs to be augmented if it is to retain its high-value returns as the generated quantities of distillers grains increase over time. Other novel uses, such as human foods and industrial products, are also potential avenues and should be pursued.
Several studies have been conducted that have examined the possibility of utilizing these by-product streams in human food products. Rosentrater and Krishnan (2006) provide a comprehensive overview of these studies. To date, however, no food products are commercially available that incorporate DDGS, primarily because of lack of functionality (due to the starch removal) and residual fermentation odors and flavors, which most consumers find unacceptable. In order for viable food products, or functional adjuncts, to be successfully manufactured, additional research is needed to overcome these challenges.
Very little work has been undertaken to develop other value-added industrial applications for ethanol residue streams. Initial trials have been conducted using these materials as soil amendments and fertilizers (Erdem and Ok, 2002; Ramana et al., 2002a; Ramana et al., 2002b). Initial research has examined incorporating DDGS streams into plastic composites (Julson et al., 2004), and extracting industrial components and chemicals (Kwiat-kowski and Cheryan, 2002; Singh and Cheryan, 1998; Singh et al., 2001; Singh et al., 2002).
It should also be noted that there are currently several key issues associated with the value and utilization of distillers grains, both from the ethanol production standpoint, and from a livestock feeding perspective. Some of the most pressing include the large quantities of energy required to remove water during drying, coupled with the high cost of energy itself; moving DDGS to diverse and distant markets when there can be fluctuations in supply and demand around the nation; how to avoid mycotoxin contamination; variability in nutrient content, quality, and associated quality management programs, all of which ultimately impact the end users – the livestock producers; lack of an industry-wide quality grading system; inconsistent product identity and nomenclature; lack of standardized laboratory testing procedures; lack of education and technical support for the industry; international marketing and export challenges; and lack of a national by-product organization to address these issues and spearhead marketing these coproducts. These are discussed in more depth by Rausch and Belyea (2006), Rosentrater and Giglio (2005), Rosentrater (2006), and UMN (2006). The industry is currently working to address each of these, and in so doing will increase the worth of these coproducts.
Moreover, very little work has been conducted on the value-added utilization of carbon dioxide from ethanol plants (Ginger, 2004; Marland and Turhollow, 1991). As demand for ethanol increases, and more manufacturing plants are constructed and expanded, this issue will become essential for consideration, especially as the issue of greenhouse gas emissions continues to gain importance.
It is true that ethanol may not be the entire solution to our energy needs as a nation, but it will be a key component to the bigger picture of energy independence. Support for ethanol has been growing considerably in recent years, and the industry is rapidly expanding in response to increased demand. This means there will be many opportunities for those involved in process and product development to help add value to the processing residuals, namely the coproduct known as distillers grains.
Kurt Rosentrater is the Lead Scientist with the USDA, ARS, North Central Agricultural Research Laboratory based in Brookings, South Dakota. He e-mail is: firstname.lastname@example.org.
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