Compact Fuel Processors for Automotive Fuel Cells

PNNL researchers are developing a compact, chemical process system for converting liquid hydrocarbons to hydrogen for on-board automotive fuel cells. This system, or fuel processor, is unique in that the size and weight are dramatically reduced compared to conventional approaches, thus truly enabling fuel processing "under the hood."

The Fuel Cell Advantage

The automobile industry is under a great deal of pressure to produce and market low emission vehicles. Beginning in 2003, 10 percent of vehicles offered for sale in California will be required to be zero-emission vehicles. Other states are also moving toward this objective. Fuel cells show great promise as a replacement to internal combustion engines in automobiles due to their high efficiency, low or zero emissions, and quiet, continuous operation. The Proton Exchange Membrane (PEM) fuel cell has additional advantages because of its low operating temperature, high power density, and advanced stage of technical development. The PEM fuel cell uses hydrogen, which is not easily transported or stored. For these reasons, and in order to take advantage of the existing fuel infrastructure, the PEM fuel cell needs to be integrated with a fuel processor that can use gasoline or other liquid hydrocarbons that can be conveniently distributed. Figure 1 illustrates the chemical unit operations necessary to process fuel and how the system is integrated with a PEM fuel cell.

Figure 1. Fuel Processor/Fuel Cell System

A New Architecture

The chemical system miniaturization technology that makes a compact fuel processor possible is based on the enhanced heat and mass transfer exhibited when fluids flow in and around microstructures. These microstructures may consist of machined microchannels up to 500 microns wide or other special structures engineered to enhance chemical reactions or separations. Using many microstructures in parallel, chemical systems can be deployed with radical reductions in size and weight compared to conventional systems.

The process unit operations required for the fuel processor are embodied in parallel sheets that are machined with many parallel micro-scale features. Combinations of reactor, heat exchange, and control sheets are stacked together to form an integrated system that performs needed operations such as steam reforming and/or partial oxidation, water-gas shift reaction, carbon monoxide removal, heat exchange, and sulfur sorption.

Benefits

Dramatically reduced fuel processor size
- More than 10 times size reduction was achieved for the full scale fuel vaporizer.
High thermal efficiency of components
High catalyst activity
Small pressure drops

Figure 2. Each parallel sheet in the sheet architecture may perform one or more chemical process operations.

Figure 3. Full Scale Fuel Vaporizer

Technology Status

The first component of the fuel processor, the vaporizer, has been demonstrated at the full scale required for an automobile, using methanol as the liquid hydrocarbon fuel. A device with dimensions of 7 x 10 x 2.5cm vaporized methanol at a rate of 208 mL/min, which is sufficient for a 25-kW fuel cell. Heat was provided by catalytic combustion of a dilute hydrogen stream that would be supplied as the exhaust from the fuel cell anode. The same miniaturization techniques are being tested at the bench scale for additional system components: steam reforming, partial oxidation, water-gas shift, and preferential oxidation reactors.

Intellectual Property

The technology is protected by United States Patent 5,611,214 ("Microcomponent Sheet Architecture," issued March 18, 1997). Foreign patent applications based on this case and additional United States patent applications are pending.