Friday, June 9, 2006
Link: Sulfur .
Robert Service reports in Science on a new sulfur scrubbing technology that may make fuel cells a practical reality and clean up coal gasification plants:
Fuel cells and coal-burning plants may seem worlds apart technologically, but they share a common enemy: sulfur. Even a trace of it in the hydrogen gas that feeds fuel cells will poison the catalysts that convert hydrogen into electricity. Next-generation coal plants that will convert coal into a hydrogen-rich gas must also remove sulfur before the gas can be transformed into liquid fuels or used in fuel cells. Current technologies for capturing sulfur have made some progress, but often at a high cost. Now, new work with compounds called rare earth oxides could shift sulfur removal--and energy-generating technologies potentially stymied by sulfur--into high gear...Chemical engineer Maria Flytzani-Stephanopoulos and colleagues at Tufts University in Medford, Massachusetts, report turning a type of ceramic powder into a chemical sponge that quickly sops up sulfur and then can be "wrung out" and reused over and over...The need for a cheap way to remove sulfur from fuel gases has spurred engineers for decades. In many countries, coal-fired electric plants are required to install smokestack scrubbers to reduce emissions of sulfur dioxide, a chief component of acid rain. And many developers would like to be able to use a wide range of hydrocarbon fuels as a feedstock for generating the molecular hydrogen that powers most fuel cells. But even the trace amounts of sulfur that remain create havoc...One option for removing sulfur has been using another spongelike ceramic called zinc oxide, which readily grabs on to sulfur, converting the zinc oxide to zinc sulfide. But it's far from a perfect solution. Once the outer surface becomes coated with zinc sulfide, the interior of the ceramic has trouble grabbing more sulfur. And zinc sulfide is not easily converted back to zinc oxide. So zinc oxide-based filters must be replaced regularly.
Researchers have explored using lanthanum and other rare earth oxides for years. Like zinc oxide, these ceramics also readily grab sulfur, but unlike zinc oxide they can later release it, making them reusable. In previous studies, researchers have exposed the ceramics to sulfur for long periods, allowing gases to percolate completely through the crystalline structure of the material. But such heavily saturated ceramics give up their sulfur too slowly to be practical for real-world use. The Tuft s researchers tried exposing their rare earth oxides to sulfur-bearing gases for relatively brief periods, so they became coated with sulfur only on their surface. They found that lanthanum-based oxides, in particular, both grabbed and released a full surface complement of sulfur in just minutes. Moreover, they could reduce the sulfur content in fuel streams to the parts-per-billion range--good enough to protect even the most sensitive fuel-cell catalysts. When the researchers ran their materials through about 100 such charging and discharging cycles, they found little change.
An industrial plant could use multiple filters, switching back and forth so some sop up sulfur while others discharge it. In their paper, the Tufts researchers outline such a system for use with solid oxide fuel cells, which are being developed as backup power sources for hospitals and other industrial users. If such a design can keep fuel-cell catalysts working, it could go a long way toward making such fuel cells reliable enough to succeed in the real world.