New reactor puts hydrogen from renewable fuels within reach

02/09/04

MINNEAPOLIS / ST. PAUL--The first reactor capable of producing hydrogen from a renewable fuel source--ethanol--efficiently enough to hold economic potential has been invented by University of Minnesota engineers. When coupled with a hydrogen fuel cell, the unit--small enough to hold in your hand--could generate one kilowatt of power, almost enough to supply an average home, the researchers said. The technology is poised to remove the major stumbling block to the "hydrogen economy": no free hydrogen exists, except what is made at high cost from fossil fuels. The work will be published in the Feb. 13 issue of Science.

The researchers see an early use for their invention in remote areas, where the installation of new power lines is not feasible. People could buy ethanol and use it to power small hydrogen fuel cells in their basements. The process could also be extended to biodiesel fuels, the researchers said. Its benefits include reducing dependence on imported fuels, reducing carbon dioxide emissions (because the carbon dioxide produced by the reaction is stored in the next year's corn crop) and boosting rural economies.

Hydrogen is now produced exclusively by a process called steam reforming, which requires very high temperatures and large furnaces--in other words, a huge input of energy. It's unsuitable for any application except large-scale refineries, said Lanny Schmidt, Regents Professor of Chemical Engineering, who led the effort. Working with him were scientist Gregg Deluga, first author of the Science paper, and graduate student James Salge. All three are in the university's department of chemical engineering and materials science.

"The hydrogen economy means cars and electricity powered by hydrogen," said Schmidt. "But hydrogen is hard to come by. You can't pipe it long distances. There are a few hydrogen fueling stations, but they strip hydrogen from methane--natural gas--on site. It's expensive, and because it uses fossil fuels, it increases carbon dioxide emissions, so this is only a short-term solution until renewable hydrogen is available."

Ethanol is easy to transport and relatively nontoxic. It is already being produced from corn and used in car engines. But if it were used instead to produce hydrogen for a fuel cell, the whole process would be nearly three times as efficient. That is, a bushel of corn would yield three times as much power if its energy were channeled into hydrogen fuel cells rather than burned along with gasoline.

"We can potentially capture 50 percent of the energy stored in sugar [in corn], whereas converting the sugar to ethanol and burning the ethanol in a car would harvest only 20 percent of the energy in sugar," said Schmidt. "Ethanol in car engines is burned with 20 percent efficiency, but if you used ethanol to make hydrogen for a fuel cell, you would get 60 percent efficiency."

The difference, Deluga explained, is due in large part to the need to remove all the water from ethanol before it can be put in an automobile gas tank--and the last drops of water are the hardest to remove. But the new process doesn't require pure ethanol; in fact, it strips hydrogen from both ethanol and water, yielding a hydrogen bonus.

The invention rests on two innovations: a catalyst based on the metals rhodium and ceria, and an automotive fuel injector that vaporizes and mixes the ethanol-water fuel. The vaporized fuel mixture is injected into a tube that contains a porous plug made from rhodium and ceria. The fuel mixture passes through the plug and emerges as a mixture of hydrogen, carbon dioxide and minor products. The reaction takes only 50 milliseconds and eliminates the flames and soot that commonly accompany ethanol combustion.

In a typical ethanol-water fuel mixture, one could ideally get five molecules of hydrogen for each molecule of ethanol. Reacting ethanol alone would yield three hydrogen molecules. So far, the Schmidt team has harvested four hydrogen molecules per ethanol molecule.

"We're confident we can improve this technology to increase the yield of hydrogen and use it to power a workable fuel cell," said Salge.

Source: Eurekalert & others

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