Solid SCR — an alternative for the future? - Emissions Technology - selective catalytic reduction

Diesel Progress North American Edition, Oct, 2002

Impending diesel emissions legislation that focuses on very low [NO.sub.x] and particulate matter (PM) emissions has resulted in the development of several technologies aimed at addressing exhaust after treatment solutions. [NO.sub.x] adsorber catalysts and urea/water solution-based selective catalytic reduction (SCR) systems have demonstrated [NO.sub.x] reduction capabilities of greater than 60 percent over the U.S. and European certification cycles. However, both systems face significant application-dependent challenges.

Although the [NO.sub.x] adsorber catalyst SAC) appears to be the most convenient solution -- since it uses on-board diesel fuel to regenerate -- the real challenge is the management of the associated parameters that control the entire regeneration process of the NAC. In contrast to DI gasoline engines, which have utilized NAC technology in production since 2001, diesel engines need to overcome a much larger gap to run with air/fuel ratios below stoichiometric in order to regenerate the NAC.

Depending upon the particular operating point of the diesel engine, this process can be very challenging, according to Dean Tomazic, department manager for diesel combustion systems at Auburn Hills, Michigan-based FEV Engine Technology Inc. Although the duration of this rich phase may be only a matter of seconds, the associated torque, smoke and noise effects need to be well controlled. Because post injection is required to produce the necessary reductants for the regeneration process, the potential for lube oil dilution represents another challenge.

In addition, fuel economy penalties cannot be avoided. These concerns have resulted in the use of a dual-leg system, featuring two parallel NACs. In this concept, one of the two NACs is regenerated while the other stores [NO.sub.x]. Using diesel fuel injection in the exhaust, the system allows the engine to run lean all the time, while reducing the fuel economy penalty compared to a single-leg system.

However, the packaging and cost of such a system still represent the biggest challenges. Additionally, the NAC is the most sulfur-sensitive of all current emission control systems. It will require a desulfurization process capability to restore the [NO.sub.x] conversion efficiency. This will still be a requirement, even with the 15 ppm low sulfur cap for diesel fuel that is planned for 2006 and beyond.

In contrast, SGR technology presents significant technical advantages over NAC. Typically when injecting a urea/water solution (eutectic point at -53[degrees]F) directly into the exhaust stream, [NO.sub.x] emissions can be reduced significantly over the downstream catalyst system. However, limitations are placed on the injected urea/water solution quantity to avoid excessive ammonia slip. Overall, the calibration effort for such a system is significantly reduced, when compared to a NAC system.

The biggest challenge for the SCR technology is represented by the infrastructure, required for the urea/water solution. The implementation of such an infrastructure will be extremely costly and therefore it seems unlikely that SGR technology will succeed in the U.S. market within the next few years.

For this reason, engineers at FEV have developed and tested a solid SCR system designed to produce and dose the ammonia required for the [NO.sub.x] reduction process. This system represents an alternative [NO.sub.x] reduction system based upon the SCR principle. It can be adapted to heavy-duty engines (on- and off-highway, gen-sets, etc.) as well as light-duty engines and benefit from the associated advantages.

The targeted benefits for the development process of this solid SCR system were as follows:

* High [NO.sub.x] reduction efficiencies (even at low exhaust temperatures).

* Supported by economically feasible infrastructure.

* Low energy consumption.

* Stores only small amounts of ammonia or other toxic compounds.

* Exhibits beneficial dosage dynamics

* Compact (packaging).

* Low cost.

During an initial concept study at FEV, ammonia carbamate was chosen as the prime reductant among the possible options. One major reason for this decision was that it sublimates at 140[degrees]F (60[degrees]C), resulting in relatively high sublimation rates at lower temperatures. This process is also reversible; in a closed system, cooling of the gas phase will lead to the formation of ammonia carbamate. This process ensures that the amount of ammonia that is temporarily stored inside the closed system is very low. Ammonia carbamate is a precursor during the urea production process and can be readily manufactured. However, current production capabilities would have to be increased in order to eventually support a future infrastructure.

Figure 1 briefly summarizes the possibilities for the generation of ammonia, which is the ideal reductant for [NO.sub.x] reduction, due to its high selectivity.

The amount of reductant to be injected into the exhaust gas depends on several factors:

* [NO.sub.x] concentration in the exhaust.