SULFUR-TRAP R&D AIMS TO SOLVE NOX TRAP ULSD INTOLERANCE

Diesel Fuel News, Sept 17, 2001 by Jack Peckham

Two new on-board vehicle sulfur trap technologies -- one plucking out remaining sulfur molecules in ultra-low-sulfur diesel (ULSD) fuel, the other capturing sulfur compounds in post-combustion diesel exhaust -- are moving ahead with R&D funding support, Diesel Fuel News has learned.

One, $2.4 million project -- solicited through U.S. Department of Energy's National Energy Technology Laboratory (NETL) in Pittsburgh -- aims to develop and demonstrate a fuel filter (initially for heavy-duty) that would employ established reactive chemistries for removing remaining sulfur compounds such as sterically-hindered thiophenes. Those are the sulfur molecules that are hardest (and costliest) to remove at oil refineries.

The other, $1.9 million project -- solicited through DOE's Albuquerque, N.M., laboratory management unit -- involves Atlanta-based technology developer Apyron and engine/truck maker International. They aim to develop exhaust gas sulfur trapping units for light-duty diesels meeting U.S. EPA Tier 2 emissions limits.

This exhaust-gas sulfur trap (tapping Apyron's copper oxide/aluminum oxide "advanced material synthesis technology") would need to handle whatever combustion sulfur products result from up-to-23-ppm sulfur in fuels and lubes, 15 ppm of which would originate from EPA's ULSD fuel, plus an equivalent of up to 8 ppm fuel sulfur contribution derived from (unregulated) lube oils combustion, the DOE project proposal says.

No more than 0.05 ppm of sulfur should ever slip past the proposed exhaust-sulfur trap, according to DOE's R&D solicitation.

While U.S. refiners may succeed in slashing highway diesel sulfur to below 15 ppm (per U.S. EPA requirements in mid-2006), even 3 ppm sulfur is "too much" sulfur for today's ultra-sensitive diesel nitrogen oxides (NOx) traps.

U.S. EPA considers NOx traps as the leading candidate for heavy-duty diesel NOx control, probably in combination with particulate matter (PM) traps, for 2007 limits. But sulfur poisoning means NOx traps might fail to hit EPA's 0.2 grams/bhp-hr NOx emissions and durability limits, partly due to thermal stress from repeated high-temperature desulfurization/regeneration.

Separately, DOE/NETL's fuel sulfur polishing R&D solicitation, now being negotiated with research proponent, Honeywell, would try a variety of chemical fuel-sulfur removal approaches. This effort will tap established techniques from published chemical literature, rather than try to make science breakthroughs, as Honeywell researcher Ron Rohrbach explained to Diesel Fuel News.

"There are lots of established thiophene removal chemistries outside the refining industry, for applications at room temperature and low flow rates," Rohrbach said. Most refiners never would have noticed this, however, since it's not to be found in typical chemical literature for high-volume refining applications.

The key distinction between Honeywell's proposed fuel-sulfur filter and refinery-scale fuel desulfurization is the enormous difference in residence time and flow rate through the "reactors."

Refiners put tens of thousands of barrels per day of sulfur-containing streams through hydrotreaters -- and probably will have similar flow-rates if they can make new, alternate-technology desulfurization units (such as S-Zorb, Unipure, Sulphco) live up to their promise of ability to compete with hydrotreating.

In contrast, Honeywell only needs to de sign for fuel-filter reactor flow rates and residence times that are in the neighborhood of milliliters per second, rather than the high-temperature, high-pressure, huge-quantity, super-fast catalyst/reactant contact times required of gigantic refinery hydrotreating reactors.

* Long Time Scale

"Your time scales can be much longer on a truck," Rohrbach explains, "and remember that part of truck fuel is actually recycled" back to the fuel tank, before it's later injected into cylinders for combustion. So there's comparatively lots of time to treat the fuel before it finally combusts.

"In some cases, you'll have different types of sulfur compounds to remove, so we'll need to short-list the chemistries that look the most promising" for treating the typical variety of diesel fuels produced by refineries.

Marathon-Ashland Petroleum (MAP) and the Shell-Texaco "Equilon" venture have already volunteered to provide test fuels for the Honeywell R&D project.

Exhaust catalyst developers (including Johnson-Matthey), a diesel engine maker (Mack) and a filter recycling specialist (American Waste Industries) are also on Honeywell's R&D project team, according to its proposal to DOE.

Various sulfur-removal techniques could be employed, including a single-pass reaction as fuel makes its way to or from fuel injectors. Various adsorption, polymerization, extraction or oxidation technologies could be employed, perhaps off-line while fuel is sitting in the vehicle tank, awaiting its eventual trip to the engine.

Several organometallic compounds are proven effective for sulfur removal, and some new zeolite catalysts initially developed for other applications also might prove useful in the proposed fuel-sulfur filter, said Rohrbach, who used to work at UOP, a company well-known for refinery/petrochemical catalysis.