Fueling the future with citrus waste
USDA researchers are developing cost-effective ways to convert tons of citrus waste in Florida into ethanol and other useful products.
To establish ethanol as an alternative to petroleum fuel, researchers are seeking new sources of candidate materials that are cheap, abundant, and sustainable. In Florida, which is one of the world’s largest producers of citrus products, researchers at the U.S. Department of Agriculture’s (USDA’s) Citrus and Subtropical Products Research Laboratory are looking for ways to use the ~3 million t of wet pulp and peel waste produced every year by citrus-processing companies. The waste could yield 50 million gal of ethanol fuel annually, says Bill Widmer, the researcher heading the project.
Widmer and his USDA colleagues have demonstrated the concept with a 100-gal ethanol-production system. They are currently working with Delray Beach, Florida-based Renewable Spirits, LLC, to conduct full-scale tests at a 10,000-gal pilot facility at the start of the citrus season this November. Widmer says that until the season starts, they are running experiments at the facility.
The ethanol produced at the new facility could replace methyl-tert-butyl ether (MTBE) as a gasoline additive in central or southern Florida, he says. Some states, including California and Ohio, have banned MTBE because of groundwater pollution problems; Florida has not yet banned or restricted MTBE use. Since 1992, MTBE has been used as a gasoline additive in the U.S. to boost oxygen levels; this helps the gasoline burn more completely and reduces tailpipe emissions. Converting citrus waste to ethanol also yields the valuable coproduct limonene, an organic solvent. Although citrus waste is currently being recycled, “by recovering ethanol and limonene we have probably tripled or quadrupled the value of the waste-stream material,” says Widmer.
The USDA process is suitable for Florida because citrus waste is plentiful and corn is not a major crop, Widmer says. Once it has developed the process for citrus, USDA plans to apply the technique to other products, such as apples and sugar beet; the process could then find markets elsewhere, he says. Local sources of ethanol would be useful because unlike for gasoline, no nationwide infrastructure is in place to transport it. If the technology gets to that point, it may have to compete with cellulosic ethanol from agricultural waste such as corn stalks and sugarcane bagasse, the fibrous material left over after sugarcane juice extraction. The technology for producing ethanol from those sources has not yet been tested on a commercial scale. At present, corn grains and sugarcane molasses are the two main sources of ethanol— corn in the U.S. and sugarcane in Brazil and Colombia
Ethanol is made from citrus waste in four steps: pretreatment, enzyme hydrolysis, fermentation, and distillation. Hydrolysis converts the cellulose, which comprises ~10–15% of the dry citrus waste, into glucose, a sugar that yeasts can ferment into ethanol. Developments by companies such as Novozymes and Genencor have brought down the cost of the enzymes that govern this process, Widmer says, making the cellulose-to-glucose conversion economically feasible. Apart from the cellulose, about a third of the citrus waste consists of the fermentable sugars glucose, fructose, and sucrose. About 1% of the waste is in the form of limonene, which can be removed during hydrolysis. The remaining waste, made of 15% pectin as well as small amounts of flavonoids, phenolic compounds, ash, and protein, can be converted to cattle feed.
Currently, citrus processing companies dry the pulp and peel waste to make cattle feed and convert the liquid into molasses, which they sell to the beverage industry. This is not profitable for the companies because of the cost of producing feed pellets and exporting them to Europe, where, unlike in the U.S., corn is not abundantly available to make feed. Citrus waste is currently worth 2–4¢/lb, Widmer says. Because ethanol is valued at >$2/gal and limonene sells for at least 50¢/lb, converting the waste to ethanol could raise its value to 10–12¢/lb.
The new process could be even more valuable if a strain of genetically engineered E. coli bacteria called K011 was used to convert 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 [116KB PDF] 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, the company has been incurring losses. 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 cost-effective for citrus processors. He believes that he could convert the pectin-rich fermentation waste into other useful 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.”
