March 2004

E-Mail This Article Printer-Friendly Version Prof. Schmidt's ethanol reactor, shown here next to an ear of corn to indicate scale.

MINNEAPOLIS/ST. PAUL, MN - Take an ordinary automotive fuel injector, add some appropriate catalyst material, attach to a preheated glass tube, and what do you get? Presto, a powerful chemical reactor the length of an ear of corn that turns alcohol into hydrogen.

That's somewhat of an oversimplification, says Prof. Lanny Schmidt, Regents Professor of Chemical Engineering at the University of Minnesota here and leader of a team that developed the device. But he concedes that's essentially the gist:

A paper published by Schmidt and his three colleagues - G. A. Deluga, J. R. Salge, and X. E. Verykios - in the Feb. 13 issue of "Science" describes the hardware generating hydrogen from ethanol as an automotive fuel injector that converts more than 95% of ethanol with about 100% selectivity via catalytic partial oxidation at relatively low temperatures and high speeds - catalyst residence times of less than 10 milliseconds - into hydrogen, with minimum byproducts such as carbon, acetaldehyde, ethylene and various combustion products.

Two innovations are key: the modified fuel injector and a catalyst based on the metals rhodium and ceria. As described in a university press release, the pre-heated - about 140 deg C - and thereby vaporized fuel mixture is injected together with air via the injector into the glass tube stoppered up with a porous plug made from the catalytic metals. The fuel gas passes through the catalytic plug in less than 50 milliseconds, emerging at about 700 deg. C on the other side of the plug as a mixture of hydrogen, carbon dioxide and other minor products and cooling off as it travels further down the tube.

Schmidt: Coffee Cup-Sized Reactor Good for 500 W

The process hasn't been optimized, but "I like to argue that a coffee cup-sized reactor can produce 500 watt worth of hydrogen" from ethanol, but also ethanol-water mixtures and other biomass materials such as crop wastes, Schmidt told H&FCL.

In fact, reacting both ethanol and water together yields a bonus of extra hydrogen: a university press release says ethanol alone produces three hydrogen molecules from each ethanol molecule. Adding water could ideally yield two extra H₂ molecules; so far Schmidt and his co-workers have achieved four extra hydrogen molecules.

Without added water, ethanol conversion produces 50% hydrogen, plus the other byproducts, making it unsuitable for PEM cells with their intolerance of CO. But it could be used to fuel solid oxide fuel cells, said Schmidt's team in their "Science" paper. They believe, however, "simple changes in experimental conditions and catalysts" could reduce those byproducts by factors of at least 2, but more cleanup via preferential CO oxidation might be required.

One bonus: these autothermal reactors have "extremely short" startup times - less than 5 seconds - and wide flow ranges, making them potentially suitable as small portable, or onboard, reformers.

Higher Energy Content than Methanol

Another advantage of ethanol, typically made from corn, is that it has a higher energy content than methanol - about 75-80% compared to gasoline, while methanol has only about 50%. Also, unlike methanol, ethanol - i.e., alcohol, the stuff of wine, vodka, cognac and other spirits - is not toxic.

Ethanol hasn't attracted all that much attention because of a perception that there isn't enough available to make a major dent in replacing fossil-based fuels. In his paper, Schmidt says ethanol is "readily and increasingly available by fermentation of biomass at costs of about $1/gallon, but "a significant fraction" of that cost is due to the need to remove all water from ethanol when used as an additive to gasoline, a major use.

Schmidt told H&FCL ethanol usage is only about 1 % of petroleum-based fuels today, but it's going up: "In a couple of years, it will be 2%," he predicts. Today, ethanol is made mostly from corn, but "if we get it out of switchgrass or hybrid poplars," two fast-growing biomass materials that have been looked at by renewable fuels advocates, "you will have enormous potential there" - another renewable resource option for an increasingly energy-hungry world.

Contact: Prof. Schmidt, e-mail, schmidt@cems.umn.edu.