reform thinking
A study for the Department of Energy weighs ideas for feeding fuel cells.
Hydrogen fuel cells hold promise for reducing
tailpipe emissions of automobiles, but as with any new technology, the
idea poses some challenges to designers. One of the key questions is how
to get hydrogen to the fuel cell in the first place. One suggestion: Equip
gasoline stations with equipment to reform hydrogen from natural gas.
In the spring of last year, Directed Technologies Inc., an Arlington,
Va., technical consulting firm, completed a cost comparison for the Department
of Energy to evaluate two natural gas reformer technologies: autothermal
reforming and steam methane reforming. Both expose natural gas to a catalyst,
usually nickel, at high temperature and pressure to extract the hydrogen.
The autothermal reformer burns a portion of the natural gas within the
reforming vessel to provide heat for the reaction. The steam methane reformer
uses hot gases to externally heat tubes containing a mixture of steam
and methane.
"The autothermal reformer was known to be a lower capital-cost system,
but the steam methane reformer was more efficient," said Gregory
Ariff, a senior engineer for Directed Technologies. "The question
was: At a fueling station scale, which technology would produce lower-cost
hydrogen for the consumer?"
The autothermal and steam methane designs use different hardware. For
example, the catalytic reaction in the autothermal reformer takes place
in a single, large vessel. By contrast, the steam reformer chamber consists
of more than a hundred parallel metal tubes running end to end inside
a large heating vessel. Steam and methane flow through the externally
heated tubes, which contain catalyst material.
This
proposed layout shows a 10-atmosphere steam methane reforming system mounted
on an 8 x 13-foot pallet for ease of transportation and installation.
The SMR vessel is the light blue upright vessel near the lower left corner
of the pallet. Mounting components, such as racks, are not shown.
Moreover, the potential materials for the two reformer technologies involved
a wide range of metals, from stainless steel to expensive high-temperature
alloys. Each choice built in different processing expenses for welding
and machining.
To aid in the cost comparison, the company used Design for Manufacture
and Assembly software from Boothroyd Dewhurst Inc. of Wakefield, R.I.
The software helps engineers evaluate product designs for ease of assembly
and manufacturing efficiency.
Chemical engineers created an ideal step-by-step model of the hydrogen
extraction process with chemical simulation software. Next, the team applied
existing technologies to perform each step of the process. The team then
created a rough bill of materials for the initial reformer design and
set a hypothetical production volume of 250 units.
In the case of the steam methane reformer, the shell-and-tube vessel where
the hydrogen extraction takes place accounted for high assembly and manufacturing
costs. "Materials choices build in costs you may not expect,"
Ariff said. "For instance, high-temperature-resistant alloys can
be five to nine times more expensive than stainless steel. They also are
more difficult to work with."
The type and number of assembly tasks also made the vessel expensive.
Each reformer tube was welded at both ends to a tube sheet, and the welding
procedures followed strict guidelines for pressure vessels.
The engineers used the software to price out the different materials by
volume needed, process costs, and assembly times and costs. They were
also able to evaluate alternative processes. As a result, the team discovered
that selecting orbital welding for the tube ends helped reduce design
costs.
The final report from Directed Technologies predicted that the steam methane
reformer could be designed at a cost of $123,545. That was $20,000 more
than a comparable autothermal reformer, but the steam reformer's operating
efficiency makes up the difference.
This article was prepared by staff writers in collaboration
with outside contributors.