Technology▼Solutions Fueling the future with citrus waste
USDA
has not yet been tested on a commercial scale. At present, corn grains and sugarcane molasses are the two To establish ethanol as an alternaering ethanol and limonene we have main sources of ethanol—corn in tive to petroleum fuel, researchers probably tripled or quadrupled the the U.S. and sugarcane in Brazil and are seeking new sources of candivalue of the waste-stream material,” Colombia. date materials that are cheap, abunsays Widmer. Ethanol is made from citrus dant, and sustainable. In Florida, The USDA process is suitable for waste in four steps: pretreatment, which is one of the world’s largest Florida because citrus waste is plenenzyme hydrolysis, fermentation, producers of citrus products, retiful and corn is not a major crop, and distillation. Hydrolysis consearchers at the U.S. Department of Widmer says. Once it has developed verts the cellulose, which makes Agriculture’s (USDA’s) Citrus and the process for citrus, USDA plans to up ~10–15% of the dry citrus waste, Subtropical Products Research into glucose, a sugar that yeasts Laboratory are looking for ways can ferment into ethanol. Deto use the ~3 million t of wet velopments by companies such pulp and peel waste produced as Novozymes and Genencor every year by citrus-processhave brought down the cost of ing companies. The waste could the enzymes that govern this yield 50 million gal of ethanol process, Widmer says, making fuel annually, says Bill Widmer, the cellulose-to-glucose conthe researcher heading the version economically feasible. project. Apart from the cellulose, about Widmer and his USDA cola third of the citrus waste conleagues have demonstrated the sists of the fermentable sugars concept with a 100-gal ethanolglucose, fructose, and sucrose. production system. They are About 1% of the waste is in the currently working with Delray form of limonene, which can be Beach, Florida-based Renewable removed during hydrolysis. The Spirits, LLC, to conduct fullremaining waste, made of 15% scale tests at a 10,000-gal pilot pectin as well as small amounts facility at the start of the citrus of flavonoids, phenolic comseason this November. Widmer pounds, ash, and protein, can says that until the season starts, be converted to cattle feed. they are running experiments at Currently, citrus-processthe facility. ing companies dry the pulp and The ethanol produced at the peel waste to make cattle feed new facility could replace meth- The millions of tons of citrus waste created in Florida and convert the liquid into moevery year could be a good source of ethanol. yl-tert-butyl ether (MTBE) as lasses, which they sell to the a gasoline additive in central beverage industry. This is not or southern Florida, he says. Some apply the technique to other prodprofitable for the companies because states, including California and ucts, such as apples and sugar beets; of the cost of producing feed pelOhio, have banned MTBE because the process could then find markets lets and exporting them to Europe, of groundwater pollution probelsewhere, he says. Local sources of where, unlike in the U.S., corn is not lems; Florida has not yet banned ethanol would be useful because, abundantly available to make feed. or restricted MTBE use. Since 1992, unlike for gasoline, no nationwide Citrus waste is currently worth 2– MTBE has been used as a gasoline infrastructure is in place to trans4¢/lb, Widmer says. Because ethanol additive in the U.S. to boost oxygen port it. If the technology gets to that is valued at >$2/gal and limonene levels; this helps the gasoline burn point, it may have to compete with sells for at least 50¢/lb, converting more completely and reduces tailcellulosic ethanol from agricultural the waste to ethanol could raise its pipe emissions. Converting citrus waste such as corn stalks and sugvalue to 10–12¢/lb. waste to ethanol also yields the valuarcane bagasse, the fibrous mateThe new process could be even able coproduct limonene, an organrial left over after sugarcane juice more valuable if a strain of genetiic solvent. Although citrus waste is extraction. The technology for procally engineered E. coli bacteria currently being recycled, “by recovducing ethanol from those sources called K011 were used to convert © 2006 American Chemical Society
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pectin into ethanol, an approach that would nearly double the ethanol yield. However, Widmer says, “if you want to take your waste stream after fermentation and utilize it as cattle feed, then there are concerns. It will have dead bodies of the fermentation organism. The cattle feed is exported to Europe, and they don’t want anything to do with GMOs [genetically modified organisms] right now.” Lonnie Ingram, a microbiologist at the University of Florida who developed the E. coli K011 strain, believes that the prospects of making ethanol from citrus waste are excellent and that the value of the end products will make up for the equipment’s capital costs. “This is an opportunity to make alternative products from citrus pulp,” he says. “It’s raising value of a low-value product to a high-value product.” But Robert Braddock, a food science researcher at the University of Florida, is not so sure. He does not think that citrus processors, even without using the GMO bacteria, will
get any more value for their waste by making ethanol than they are earning right now selling it as cattle feed. “There’s enough written about making ethanol from agricultural materials that shows that you have to use more energy to make it than you get from it,” he says, referring to a report by researchers at Cornell University and the University of California, Berkeley. The cost-effectiveness of the process might depend on the cattle feed’s market value, which fluctuates just like that of any other commodity, he adds. Nevertheless, citrus-processing companies are showing interest in implementing the USDA process at their plants because it might give them more value for their waste. Florida’s Natural, the second-largest orange-juice producer in the U.S., is giving serious thought to manufacturing ethanol from its 312,000 t of annual waste, says Don Gray, a senior engineering manager at the company. Cattle feed is selling at $80/dry t, he says, but with energy prices soaring lately, it doesn’t
make financial sense to convert the waste into cattle feed. They plan to analyze the data from the Renewable Spirits pilot facility and then decide whether to install distillation equipment. “There are two different routes that we can take to produce ethanol—from molasses or by using the Renewable Spirits process of converting peel to ethanol,” he says. “The Renewable Spirits process certainly looks promising, but we haven’t come to final conclusions.” Meanwhile, Widmer is trying to make the process even more costeffective. He believes that he could convert the pectin-rich fermentation waste into other products, such as additives for building products or ion-exchange material for wastewater cleanup. “If we can develop other byproducts from the [pectin] waste material, which I’m confident we can do, I think we could probably triple the value yet again,” he says. “I’m envisioning that citrus waste, instead of being worth 2–4¢/lb, would be worth 30–50¢/lb.” —PRACHI PATEL-PREDD
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