Passive Lean NOx Trap Technology for Mobile Diesel Engines

Heavy-duty highway diesel engine NOx emission standards will drop from the current 4.0 g/bhp-hr level to 0.2 g/bhp-hr in 2007. No technology, or combination of technologies, has yet demonstrated the ability to achieve such a NOx reduction on the road. During the last year, Sorbent Technologies Corporation has been working on a Phase II research project to develop a new NOx control concept for stationary diesel engines. The large stationary application of that project is very different than the mobile case, but recent advances made in that technology now appear to enable its application to the more difficult situation presented by heavy-duty vehicles. The opportunity presented in this Phase I research project is for a novel, inexpensive, sulfur-tolerant ¿passive lean NOx trap¿ technology for heavy-duty vehicles that does not require tight integration with the diesel engine. Placed literally at the end of the pipe and with very high NOx capacity, it can function passively, adsorbing the transient operation and highly varying NOx emissions of the vehicle. Three recent innovations now make mobile use of this technology possible. First, a new monolithic form of the sorbent has been developed, making the NOx adsorption step more convenient. Second, when the trap becomes largely saturated, an inexpensive device that is smaller than an apple was found to quickly desorb the trap into a concentrated NOx stream. Finally, a simple technique for processing the desorbed NOx has been perfected, promising 95 to 99 percent average NOx ¿destruction.¿ The process uses no consumables, and the total fuel penalty appears to be less than 1 percent. An overall NOx removal efficiency of 80 percent is expected in mobile applications, and the system components do not appear to be especially expensive. The environmental and economic benefits of such a passive lean NOx trap technology for diesel vehicles could be very large. The new technology can be applied as a primary aftertreatment method for 2007 and later heavy-duty vehicles, as a polishing treatment, or as a retrofit technology for the worst vehicles. This last possibility is particularly attractive. A technology such as this may be the only high-NOx removal route that is realistically capable of being inexpensively retrofitted onto existing diesels in non-attainment areas, where its cost-effectiveness would be especially high. The tasks of this project develop those elements of the technology that are unique to applying it to mobile vehicles. A subcontract with Pennsylvania State University¿s Diesel Combustion and Emission Laboratory brings its expertise and facilities to the effort.