TransForum Vol. 4, No. 1

FLUID DYNAMICS SIMULATIONS HELP SPEED FUEL PROCESSOR DEVELOPMENT

The current excitement over fuel cell-powered passenger cars makes it easy to forget that several crucial components needed for high-volume production of these vehicles do not yet exist. Possibly the most important of these is a production-ready fuel processor that will permit fuel cell-powered cars to run on conventional fuels, rather than on pure hydrogen, making them considerably more attractive to consumers.

Important milestones in fuel processor development have been reached; one of the first came at Argonne with development of the autothermal fuel reforming catalyst, a key component of a fuel reformer that transforms the hydrocarbon molecules in conventional fuels into hydrogen. A recent improvement in the award-winning catalyst allows the reformer to be 25 times smaller than previous models, making it less expensive, less of a drain on fuel economy, and easier to integrate into cars. The technology was transferred to the private sector through a series of cooperative research and development agreements with private companies, including Süd-Chemie Co. and H2fuel, LLC.

Argonne researchers are now working to incorporate the catalytic reformer into an integrated fuel processor that breaks down hydrocarbon fuels into hydrogen and carbon oxides. Undesirable by-products, such as carbon monoxide and sulfur (contained in hydrocarbon fuels), are removed through a series of catalytic reactors, so that the hydrogen stream meets the stringent quality constraints of fuel cells.

The current effort at Argonne is aimed at developing a 5-kW version as a stepping-stone toward a 50-kW model that would be suitable for light-duty vehicles. Design and integration of the fuel processing components are supported by computational fluid dynamics (CFD) simulations that predict three-dimensional fluid flow, mixing, heat transfer, and chemical reactions inside each component of the fuel processor, as well as throughout the entire unit.

Argonne researchers have developed several CFD codes over the years for various applications, including three copyrighted versions for analysis of combustors, multiphase fluid catalytic cracking risers, and glass furnaces integrated with glass melting tanks. To prepare for fuel processor modeling, the researchers modified one of these versions so that simulations could be performed using several computers at the same time. Distributing the workload in a parallel fashion allowed them to greatly increase the number of computational cells in the grids they use in their simulations (from 50,000 to 400,000), providing a much more accurate picture of conditions inside the processor.

Consumers will undoubtedly demand that fuel cell cars be immediately ready to drive when started. This means that the various zones in the fuel processor must be warmed up very rapidly to their appropriate operating temperatures, so that the hydrogen needed to power the fuel cell is available on demand. CFD simulations are being used to determine fuel processor designs that will enable the reformer to deliver 50-100% of its hydrogen capacity in a very short time (DOE's target for 2005 is 30 seconds). Initial work has also focused on analyzing the impact of incomplete mixing of fuel, steam, and air on hydrogen production in the reformer. This research pointed to the need for an extremely homogeneous feed stream and led to development of a patent-pending mixer design.

CFD simulation shows large variations in computed fuel and oxygen concentrations and an uneven temperature distribution at the inlet to the catalyst (top of column) caused by inadequate mixing. These nonuniformities cause hydrogen production to fall far short of the theoretical maximum.

Attention has now shifted to modeling the chemical kinetics of the fuel-reforming step. Eventually, CFD simulations will be used in designing the other components and in integrating all of them in an efficient fuel processor. The latter step could prove tricky because the catalysts for the various components have different operating temperatures and different maximum temperatures they can tolerate. Testing the prototype that results from this research program, however, will be easier, thanks to Argonne's Fuel Cell Test Facility, which was established by DOE's Hydrogen Fuel Cell and Infrastructure Technologies Program to provide independent, standardized testing for all types of fuel cell systems in support of fuel cell research.