|Citrus waste can be converted into ethanol feedstock. Source: Bill Widmer, USDA ARS|
Scientists at the US Department of Agriculture’s Agricultural Research Service (ARS) are developing a process to derive sugars from citrus waste (i.e., the peels) which will then serve as the feedstock for ethanol production.
In Florida alone, citrus processing yields about 5 million tons of wet waste annually, or about 1.2 million tons of dry waste, most of which is currently marketed as low-value feed for cattle. The citrus-to-ethanol process could, given further research and development, yield up to 80 million gallons of ethanol per year just from Florida citrus waste.
Citrus waste is rich in pectin, cellulose and hemicellusic polysaccharides, which can be hydrolyzed into sugars and fermented into ethanol.
Work on the producing ethanol from peels began in 1992, but was shelved due to the low cost of gasoline and the use of other oxygenates (such as MTBE) within gasoline. In 2004, however, Bill Widmer, an ARS chemist at Winter Haven, modified the process to reduce substantially the amount of enzyme required to convert citrus-waste carbohydrates to the sugars which serve as the fermentation feedstock for ethanol.
The research team the scaled the process up to where it shows some promise for large-volume production. While the original processes were 1-quart and 1-gallon sized, Widmer and his team achieved 10-, 100- and 1,000 gallon batch levels. A 10,000-gallon pilot facility is currently under construction, and should be finished by sometime this year.
The new process shows economic promise for large-volume production. His project was partly funded by Renewable Spirits LLC of Delray Beach.
The process uses the recovery of limonene and use of the residue (higher in protein than the original citrus waste) as byproducts generating revenue streams to offset the production cost.
Although the original work began with orange peels, the current research team has is studying the use of other types of citrus waste, such as that from grapefruit.
According to the ARS, the citrus-conversion process can double the value of byproducts obtained from citrus waste. With more research, material from the residue left after limonene removal and ethanol production could be turned into other industrial products, such as building-material additives and ion-exchange resins for wastewater cleanup, thereby doubling or tripling the value of the waste stream yet again.
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