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December 2007 News

On August 6, Jonathan R. Mielenz, Biosciences Division, Oak Ridge National Laboratory, spoke to the TSK on the hydrogen economy based on polysaccharides. Polysaccharides are formed by linking together monosaccharide molecules under loss of water. Examples of monosaccharides are grape sugar and beet sugar. The best known polysaccharides are starch and cellulose. The general formula of polysaccharides is (C6 H10 O5)n, with n being the number of molecules. Starch consists of glucose (grape sugar) molecules. The research Dr. Mielenz talked about intends to use starch as a hydrogen source for fuel cells. The hydrogen economy is the goal of many different approaches. At this time it is not known which one will be most successful. The U.S. Department of Energy’s 2006 Advance Energy Initiative calls for competitive ethanol from plant sources by 2012 and a good selection of hydrogen-powered fuel cell vehicles by 2020. Fuel cells powered by hydrogen produce electricity with only water as by-product.

Researchers at Virginia Tech, Oak Ridge National Laboratory (ORNL) and the University of Georgia pursue the use of polysaccharides, or sugary carbohydrates, from biomass to directly produce low-cost hydrogen as transportation fuel. According to the DOE, to make hydrogen fuel an economical reality for transportation, advances are needed in four areas: production, storage, distribution, and fuel cells. Most industrial hydrogen currently comes

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from natural gas, a non-renewable, limited and expensive resource . Storing and moving hydrogen gas from some central source is costly and cumbersome, and even dangerous. There is little infrastructure for refueling a vehicle with hydrogen.

Using synthetic biology approaches, Y. H. Percival Zhang, assistant professor of biological systems engineering at Virginia Tech in collaboration with Barbara R. Evans and Jonathan R. Mielenz of ORNL, and Robert C. Hopkins and Michael W.W. Adams of the University of Georgia is using a combination of 13 enzymes never found together in nature to completely convert polysaccharides and water into hydrogen when and where that form of energy resource is needed. Polysaccharides are very stable until exposed to enzymes. Just add enzymes to a mixture of starch and water and, according to Zhang, the enzymes use the energy in the starch to break it up into carbon dioxide and hydrogen. A membrane bleeds off the carbon dioxide and the hydrogen is used by the fuel cell to create electricity and water. The water is recycled for the starch-water reactor. Laboratory tests confirm that it all takes place at low temperature (about 30 C or 86 F), and atmospheric pressure.

The idea is to make this process happen in the fuel tank of the car. A car with an approximately 12-gallon tank could hold 27 kg of starch, which is the equivalent of 4 kg of hydrogen. Zhang estimates that 1 kg starch will produce the same energy output as 1.12 kg (0.38 gallons) gasoline. Since hydrogen is gaseous, hydrogen storage is the largest obstacle to large-scale use of hydrogen fuel from external sources. The Department of Energy’s long-term goal for hydrogen storage was 0.12 kg of hydrogen per kg of container or storage material (12 percent by mass). Using polysaccharides as the hydrogen storage carrier, Zhang’s research team achieved hydrogen storage capacity as high as 14.8 %. “In nature, most hydrogen is produced from anaerobic fermentation. The hydrogen is produced as a co-product and the hydrogen yield is pretty low, only four molecules per molecule of glucose. In the fuel process, hydrogen is the main product and hydrogen yields are three-times higher, and the likely production costs are low, about $2/kg of hydrogen. The same way we now buy our food and the food for domestic animals from the grocery store, we may also buy our cars’ food there in the form of a bag of starch.

Zhang points out that starch is environmentally friendly, energy efficient, requires no special infrastructure, and is extremely safe. The starch based

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hydrogen would be produced by a mild reaction and at low cost. No special infrastructure for hydrogen fuel would be needed. An important aspect of the approach is that hydrogen is produced

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at the consumption site eliminating difficult and dangerous storage and transportation problems. But this utopic sounding approach to solving the transportation fuel problem requires some doing. According to Zhang, research is needed to increase reaction rates and reduce enzyme costs. Those seem to be the central problem areas. A hydrogen fuel cell uses hydrogen and O2 as input and produces electricity and water as output. According to Zhang, the energy conversion efficiency from the sugar-hydrogen-fuel cell system is extremely high, more than three times higher than a sugar-ethanol-internal combustion engine system. In other words, if about 30 percent of transportation fuel can be replaced by ethanol from biomass, as the DOE proposes, the same amount of biomass will be sufficient to provide 100 percent of vehicle transportation fuel through the biomass- carbohydrate technology. Another advantage of this technology is that the transportation fuel will produce zero net carbon dioxide emission, just like the synfuel process described in the November issue of the Soupcon.

There seems to be a myriad of proposals out there that address energy and transportation fuel production alternatives. Many of them should receive the funding necessary to advance them to pilot plant scale and determine their large-scale technical feasibility. Only by such research, which may be time consuming, can the technology be developed that leads to a reliable industrial process.

Contact: Dr. Zhang at ypzhang@vt.edu. http://filebox.vt.edu/users/ypzhang/research.htm.Additional contacts:

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Jonathan Mielenz at ORNL. Biosciences Division Oak Ridge National Laboratory; tel: 865-576-8522; email: mielenzjr@ornl.gov. Susan Trulove; (540) 231-5646; strulove@vt.edu

 
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