Linda F. Benedict, Yang, Ying, Moon, Young Hwan
Young Hwan Moon and Ying Yang
Butanol as a biofuel has many advantages over ethanol as a fuel, including higher energy content, usability in existing pipelines and ease of blending with gasoline. Butanol can be produced from the sugars in biomass. Unlike ethanol, butanol does not absorb water, allowing it to be stored and distributed using the existing petrochemical infrastructure and avoiding the blending limitations of ethanol. Butanol is compatible with gasoline and diesel at any ratio and can be used directly as a fuel in existing vehicles. Butanol has been tested as a jet fuel at a 50-50 blend with current jet fuel. Because of its higher energy content, its fuel economy in miles per gallon is better than that of ethanol. Butanolgasoline blends have lower vapor pressure than ethanol-gasoline blends, which is important in reducing evaporative hydrocarbon emissions. This is valuable to fuel refiners and blenders because a lower vapor pressure allows the light fractions derived from oil refining to be used in the higher- value gasoline.
A number of Clostridium strains, including Clostridium acetobutylicum, Clostridium butylicum and Clostridium beijerinckii, are butanol-producing organisms. Acetone-butanol-ethanol (ABE) fermentation using these strains is one of the oldest known industrial fermentations. It is ranked second only to ethanol fermentation by yeast in its scale of production. This process produces large quantities of hydrogen, ethanol, acetone, acetic acid and butyric acid, limiting the yield of butanol to around 0.35 grams of butanol per gram of glucose consumed.
In typical ABE fermentations, Clostridium strains produce acetone, butanol and ethanol in a ratio of 3:6:1, with the concentration of total solvents of up to 2.4 percent. Numerous attempts have been made to manipulate the genetics of the organism to produce higher yields of butanol and to reduce acetone production to simplify downstream processing. However, attempts to block acetone production to increase selectivity for butanol production result in loss of butanol because acetone and butanol share a common intermediate.
Clostridium beijerinckii optinoii is a butanol-producing bacterium that was isolated by a microbiology team at the Audubon Sugar Institute. Clostridium beijerinckii optinoii produces butanol, isopropanol and very little ethanol or acetone in a ratio of 6.0:3.8:0.2, unlike other ABE organisms where the ratio is 6:3:1. Acetone is not considered to be favorable for downstream processing because of its corrosiveness and volatility. The butanol-isopropanol mix produced by Clostridium beijerinckii optinoii is expected to be more practical for butanol production than the butanol-acetone mix from the ABE fermentations. Because of low productivity for butanol in batch fermentation, a system has been developed to improve butanol productivity. The productivity of butanol only in batch fermentations with Clostridium beijerinckii optinoii is around 0.12 grams per liter-hour, whereas the productivity of butanol under continuous immobilized fermentation is up to 0.65 grams per liter-hour in a glucose medium – a five-fold increase in productivity.
Under an LSU AgCenter program funded by the U.S. Department of Agriculture, sweet sorghum and energycane are being studied as biofuel feedstocks. Using these crops, researchers are producing butanol from the crops’ component sugars through microbial fermentation with the Clostridium bacterium. Sugarcane molasses normally is used as the feedstock for butanol fermentations. Sugarcane molasses contains sucrose, fructose and glucose. The test crops will produce syrups containing the same sugars. The biomass can be converted to glucose and xylose, a sugar also used by this organism. These biomass crops may offer a new way for producing butanol as a biofuel.
Young Hwan Moon is an assistant professor, and Ying Yang is a postdoctoral researcher at the Louisiana State University AgCenter Audubon Sugar Institute in St. Gabriel, Louisiana.
(This article was published in the spring 2015 issue of Louisiana Agriculture.)