These plants were designed to harness the fermenting talents of microbes to produce acetone from feedstocks such as molasses and corn starch. The acetone was used to make a smokeless gun powder and a propellant for rockets. Interestingly, acetone was not the only product of this fermentation. Ethanol was produced in small amounts but the major product of the fermentation was butanol.
Starting in the 1960s, the growth of the petroleum industry and the cheaper cost of producing butanol from petroleum products rather than renewable feedstocks made the biobased butanol plant obsolete. The last significant vestige of the industry—a facility in South Africa—ceased its operations in the early 1980s. But rising oil prices and concerns surrounding climate change and national security have rejuvenated interest, research and development into biobutanol. Although the primary use for the alcohol is as an industrial solvent, it offers several advantages over ethanol as a transportation fuel. Since the molecule contains four carbons compared with the two of ethanol, those extra chemical bonds release more energy when burned. In addition, butanol is less volatile than ethanol, it can be used at a 100 percent blend in internal combustion engines without any modifications, it doesn’t attract water like ethanol so it can be transported in existing pipelines and it is less sensitive to colder temperatures. “Butanol is an excellent fuel,” says Nasib Qureshi, a chemical engineer with the USDA Agricultural Research Service in Peoria, Ill. “As a result of gas prices going up it is looking more effective than ethanol and more effective than gasoline.”
Article Continues After Advertisement
Some big names in the energy business seem to agree. In 2006, BP and DuPont announced a joint venture to deliver advanced biofuels, initially targetting biobutanol. This past spring, the companies announced results from fuel testing including: that a 16 percent biobutanol blend performs similarly to a 10 percent ethanol blend and higher biobutanol blends also produce favorable results; that the energy density of biobutanol is closer to unleaded gasoline; and that biobutanol does not phase separate in the presence of water. “Biobutanol addresses market demand for fuels that can be produced from domestic renewable resources in high volume and at a reasonable cost; fuels that can be used in existing vehicles and existing infrastructure; fuels that offer good value to consumers; and fuels that meet the evolving demands of vehicles,” says Frank Gerry, BP Biofuels program manager.
Earlier this year, the companies announced that the partnership was developing biocatalysts for the production of 1-butanol as well as 2-butanol. (The latter is called an isomer of butanol because although it contains four carbons, the atoms of the alcohol are arranged differently). The goal of the partnership is to deliver a biobutanol production process with economics equal to ethanol production by 2010. Currently, the two companies have applied for more than 60 patents in the areas of biology, fermentation processing, chemistry and end uses for biobutanol.
The challenge to improve the process technology and the microbes that carry out the fermentation drives academic and governmental researchers as well. Qureshi, for instance, has been studying biobutanol production for more than 20 years. He came to the United States from New Zealand to develop a membrane process for more effectively recovering butanol from fermentation broth. He’s also worked to develop efficient butanol bioreactors. In the past few years, however, his research has taken a different direction, one that focuses on optimizing the process for more economical substrates such as wheat straw, barley straw, switchgrass and corn stover. “We need to move toward more economical substrates,” Qureshi says. “But it’s not as simple as it looks.”
First of all, there’s an inherent paradox in the microbial fermentation of butanol: although butanol-producing bacteria produce the enzymes that convert simple sugars into the alcohol, butanol itself is toxic to those same bugs. This butanol inhibition results in a lower alcohol concentration in the fermentation broth, which leads to lower yields of butanol and higher recovery costs. These are the challenges that surface when highly pure feedstocks are used. When a cheaper, biomass substrate is used, additional microbial inhibitors are generated during the pretreatment process.
Strategies for reducing butanol toxicity and improving yield, including integrating several steps in the process and manipulating the microbial cultures, are advancing. “We’ve made good progress with raw materials, removing inhibitors and product separation,” Qureshi says. The overall process that Qureshi’s team has developed for the production of butanol from agricultural residues involves four steps: pretreatment, which opens the cell wall structure and removes lignin; hydrolysis of hemicellulose and cellulose into simple hexose and pentose sugars using enzymes; fermentation of simple sugars into butanol using a pure culture of Clostridium beijerinckii P206, an anaerobic bacterium; and recovery of the butanol. The unique aspect of the process is that the last three steps are combined and performed in a single reactor. “We’ve integrated the process and it appears to be very effective economically,” Qureshi says. His team is currently in the process of filing a patent on the process.
Sidebar Articles
| 1 2 | Next Page --> | |
| View Entire Article | ||