This study was performed by the Department of Engine and Emissions Research under an SwRI® Internal Research and Development Program. The objective of the study was to evaluate the effectiveness of a system design that was an advancement over SwRI's patented Protection of Aftertreatment Systems from Sulfur (PASS™) technology.[1, 2]
A Lean NOx Trap (LNT) was employed as the sulfur-sensitive emissions reduction device. Lean Sulfur Traps (LST) and Rich Sulfur Traps (RST) were formulated to provide the sulfur protection. Testing was performed to evaluate the efficiency of the LNT, the sulfur poisoning of the LNT, the efficiency of the LST, and the regeneration and protection characteristics of the PASS-2™ system.
The program successfully demonstrated that an LST upstream of an LNT does provide protection for the LNT from the adverse effects of fuel-borne sulfur. The work also demonstrated that the LST could be desulfurized, with all of the product sulfur species passing through the LNT, without adversely affecting its performance. These two results validated the PASS-2™ advanced system design.
The data also suggested that an LST could be effective at storing sulfur from 8 ppm sulfur diesel exhaust at very high efficiency for a minimum of 50,000 miles. A single desulfurization of the LST could release most of the stored sulfur in less than five minutes, with only a minor impact on fuel economy. A temporary deactivation of the LNT was reversed within five minutes, so the NOx emission impact would also be very minor. The lack of high temperature required to regenerate the LNT would result in significantly increased durability of the LNT. In essence, the PASS-2™ system could be used to maintain emissions efficiency over 435,000 miles for heavy-duty applications, with an estimated nine desulfurizations of the LST during that period, when the system is fully developed.