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Microchannel technology furnishes solutions

Researchers at Pacific Northwest National Laboratory are developing a system to rapidly produce hydrogen from gasoline in your car. “This brings fuel cell-powered cars one step closer to the mass market,” said Larry Pederson, project leader at PNNL. Researchers presented their developments at the American Institute for Chemical Engineers spring meeting in New Orleans, on April 27. Fuel cell-powered vehicles use hydrogen to get about twice the fuel efficiency of today’s cars and significantly reduce emissions. But how do you “gas up” a hydrogen car? Instead of building a new infrastructure of hydrogen fueling stations you can convert or reform gasoline onboard the vehicle.

One approach uses steam reforming, in which hydrocarbon fuel reacts with steam at high temperatures over a catalyst. Hydrogen atoms are stripped from water and hydrocarbon molecules to produce hydrogen gas.

The problem has been that you have to wait about 15 minutes before you can drive. It has taken steam reformer prototypes that long to come up to temperature to begin producing hydrogen to power the vehicle. This delay is unacceptable to drivers.

However, PNNL has demonstrated a very compact steam reformer which can produce large amounts of hydrogen-rich gas from a liquid fuel in only 12 seconds. “This kind of fast start was thought to be impossible until just a couple of years ago,” said Pederson.

Greg A. Whyatt

The Department of Energy recognized that a fast start was vital to the viability of onboard fuel processing and established an ultimate goal of 30 seconds for cold start time with an intermediate target of 60 seconds by 2004. The steam reformer is the highest temperature component within the fuel processor and represents the biggest hurdle to achieving rapid startup. “Hence, the PNNL achievement of a 12 second steam reformer startup is a big step towards a complete fuel processor which can start up in 30 seconds,” said Greg Whyatt, the project’s lead engineer.

PNNL engineers called upon their expertise in microtechnology to develop the reforming reactor. Microchannels, narrower than a paper clip, provide high rates of heat and mass transport within the reactor. This allows significantly faster reactions and dramatically reduces the size of the reactor. A complete microchannel fuel processor for a 50 kilowatt fuel cell is expected to be less than one cubic foot. At this size, the system will readily fit into an automobile.

“The key feature of the new design is that the reforming reactor and water vaporizer are configured as thin panels with the hot gases flowing through the large surface area of the panel,” said Whyatt. This allows high gas flows to be provided with an inexpensive, low-power fan while still providing efficient heat transfer to rapidly heat the steam reformer.

“In addition, the panel configuration allows higher combustion temperatures and flows without risking damage to the metal structure while a low pressure drop reduces the electrical power consumed by the fan during startup and steady operation” said Whyatt.

PNNL researchers are now working to reduce the fuel consumption and air flow required during startup. In addition, integration with other components is needed to demonstrate a complete fuel processor system that can achieve startup in less than 30 seconds. However, PNNL’s fuel reformer technology appears to have overcome a major stumbling block for onboard reformation: the need for speed.