Figure 1. Fuel Processing System for PEM Fuel Cell Power System
Robert S. Wegeng, Anna Lee Y. Tonkovich, Yong Wang, Sean Fitzgerald, Micheal J. LaMont, David P. VanderWiel and Jennifer L. Zilka
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.
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.
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.
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