Researchers at the Department of Energy's 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 will present their developments at the American Institute for Chemical Engineers spring meeting in New Orleans, on April 27th, 2004.

Fuel cells use hydrogen to produce electricity which runs the vehicle. Fuel cell-powered vehicles 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.

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. 

PNNL is a DOE Office of Science laboratory that solves complex problems in energy, national security, the environment and life sciences by advancing the understanding of physics, chemistry, biology and computation. PNNL employs 3,800, has a $600 million annual budget, and has been managed by Ohio-based Battelle since the lab's inception in 1965. (www.pnl.gov).

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Dec.:2002/Jan. 2003
This component was developed for fuel processing system for fuel cell powered vehicles is actually an integrated combuster and vaporizer. On one side of the unit, simulated anode effluent from a Proton Exchange Membrane (PEM) fuel cell is catalytically combusted by downstream reactors to produce hydrogen gas for the fuel cell. This represents a successful first generation integrated microreactor.
 

Microchannel Reactors for Automotive Fuel Processors

Robert S. Wegeng, Anna Lee Y. Tonkovich, Yong Wang, Sean Fitzgerald, Micheal J. LaMont, David P. VanderWiel and Jennifer L. Zilka
Project Description

Microchannel reactors reduce the size of conventional chemical reactors without lowering the throughput. Heat and mass transport limitations slow the observed reaction rates in conventional reactors, but are minimized in microchannel reactors. The distance between heat generation and removal is reduced from tens of centimeters in conventional reactors to tens of microns in microchannel reactors. As this distance shrinks, the corresponding contribution of slow conduction and diffusion to the heat exchange or catalyst surface is reduced. Fast heat and mass transfer increases the process efficiency, enabling process miniaturization without sacrificing productivity.

A fuel processor is a critical reactor technology for the deployment of PEM-based fuel cells for automotive applications. The fuel processor produces hydrogen rich streams from gasoline or methanol in a multi-step process (fuel vaporizer, primary conversion reactor to produce synthesis gas, water gas shift reactor, and CO clean-up reactor). Conventional fuel processing technology is based on fixed-bed reactors, which do not scale well with the small modular nature of fuel cells. Microchannel reactor-based fuel processors, however, are small, efficient, modular, lightweight and potentially inexpensive. Based upon our results with other component investigations, we project a complete system volume of less than 9 liters to produce hydrogen at a sufficient rate and quality to produce 50-kWe from a PEM fuel cell.

Figure 1. Fuel Processing System for PEM Fuel Cell Power System
Accomplishments

The development and scale-up of a microchannel methanol vaporizer as part of a fuel processor for automotive applications was completed in 10 months. The feasibility of the technology was demonstrated initially with the use of catalyst powders. In subsequent trials, catalyst scale-up issues were addressed and ceramic and metal monoliths were investigated in a single-cell bench-scale vaporizer. Finally, a full-scale methanol vaporizer was built, tested, and demonstrated (patent pending). This device contains four cells per plate and shows the linear scaling laws for microchannel reactors. This individual component occupied a volume less than 0.3 liters to support a 50-kW system. This full-scale vaporizer is 4"x 6"x 1" (roughly the size of a paperback novel) and can process nearly 1000 L/min of gas (at STP) to vaporize the required fuel for an automobile. The total system pressure drop after optimization should be less than several psi with thermal efficiencies approaching 90%. Similar performance is expected for other microchannel reactor components in the complete fuel processor system.

Modeled after the methanol unit, a full-scale 50-kW gasoline vaporizer has also been developed and successfully demonstrated. The dimensions of this unit are 3"x 4"x 1.5"(0.29-L). The gasoline vaporization capacity of the unit was measured to be in excess of 250-mL/min. Finally, two of these units were delivered to D.O.E. clients for further testing.

Another critical component of the system, the water-gas shift (WGS) reactor, has been investigated in catalytic studies. This reactor removes fuel cell damaging carbon monoxide and converts steam to hydrogen gas for utilization in the fuel cell. Both powder and monolithic catalysts studies have been conducted to examine the kinetics of the reaction at fast residence times. The results of these studies show that over 95% conversion of carbon monoxide to carbon dioxide is possible at temperatures as low as 350ºC and residence times as low as 50-milliseconds.
Publications and Presentations

Tonkovich, A.Y., C.J. Call, D.M. Jimenez, R.S. Wegeng, and M.K. Drost, 1996, Microchannel Heat Exchangers for Chemical Reactors, Proceedings of the 1996 National Heat Transfer Conference, Houston, Texas.

Tonkovich, A.Y., D.M. Jimenez, J.L. Zilka, M. LaMont, Y. Wang, and R.S. Wegeng, 1998b, Microchannel Chemical Reactors for Fuel Processing, Proceedings of the Second International Conference of Microreaction Technology, March 1998, New Orleans, Louisiana.

Tonkovich, A.Y., J.L. Zilka, M. LaMont, Y. Wang, and R.S. Wegeng, 1998c, Microchannel Reactors for Fuel Processing Applications. I. Water Gas Shift Reactor, accepted for publication in Chemical Engineering Science.

Tonkovich, A.Y., J.L. Zilka, M.R. Powell, and C. J. Call, 1998a, The Catalytic Partial Oxidation of Methane in a Microchannel Chemical Reactor, Proceedings of the Second International Conference of Microreaction Technology, March 1998, New Orleans, Louisiana.

Tonkovich, A. L., J. L. Zilka, Y. Wang, M. J. LaMont, S. Fitzgerald, D. P. VanderWiel and R. S. Wegeng, Microchannel Reactors for Automotive Fuel Processors, 3rd International Conference on Microreaction Technology, April 18-21, 1999, Frankfurt am Main, Germany.

Wegeng, R.S., and AY. Tonkovich, 1997, Microchannel Fuel Vaporizer for Fuel Cells, Proceedings of the Annual Automotive Technology Development Customers' Coordination Meeting, Dearborne, Michigan, October 1997.