R&D Projects

Measurement and Characterization of NO_x Adsorber Regeneration and Desulfation

Principal Investigator:
C. Stuart Daw

Other collaborators (including ORNL personnel):

Funding Source:
U.S. Department of Energy, OFCVT
Program Managers: Gurpreet Singh, Kevin Stork

Objectives:

Characterize candidate NO_x adsorbers for performance and degradation by assessing various in-cylinder regeneration and desulfation strategies

Quantify H₂, CO, and HC generated by the engine and utilized by the NO_x adsorber catalyst

Examine NO_x adsorber materials in the DRIFTS and benchflow reactors

Develop stronger link between bench and full-scale system evaluations in order to be able to evaluate a new formulation on the bench and then predict how it will behave on an engine

Approach:

Establish a relationship between exhaust species and various regeneration strategies on a fully controlled engine

Characterize effectiveness of in-cylinder regeneration strategies

Develop and execute rapid sulfation/desulfation experiments

Develop experiments for bench-scale work to further characterize adsorber monoliths, wafers, and/or powders

Accomplishments:

Engine cell experimental setup complete and experiments underway

Examined three regeneration strategies at 300 deg C NO_x adsorber bed temperature

Measured instantaneous H₂ and CO generation during regeneration sequences

Generated GC/MS traces detailing similarities and differences in HC species formed by various regeneration strategies

Future Directions:

Complete development of regeneration strategies at three NOx adsorber temperatures

Quantify torque, fuel consumption, and PM effects of each regeneration strategy

Speciate HCs at adsorber inlet/outlet for each strategy and various catalyst formulations

In-situ H₂ measurements with H₂-SpaciMS

Characterize catalysts after sulfation and during desulfation

Examine samples in bench-scale reactors

Introduction
As part of the Department of Energy's strategy to reduce imported petroleum and enhance energy security, OFCVT has been researching enabling technologies for more efficient diesel engines. NOx emissions from diesel engines are very problematic and the U.S. Environmental Protection Agency (EPA) emissions regulations require ~90% reduction in NO_x from light- and heavy-duty diesel engines in the 2004-2010 timeframe. An active research and development focus for lean burn NO_x control is in the area of NO_x adsorber catalysts. NO_x adsorber catalysts adsorb NO_x very efficiently in the form of a nitrate during lean operation, but must be regenerated periodically by way of a momentary exposure to a fuel-rich environment. This rich excursion causes the NO_x to desorb and then be converted by more conventional three-way catalysis to N₂. The momentary fuel-rich environment in the exhaust is created by injecting excess fuel into the cylinder or exhaust and/or throttling the intake air. The controls methodology for NO_x adsorbers is very complex, and there is no clear understanding of the regeneration mechanisms. NO_x regeneration is normally a 2-4 second event and must be completed approximately every 30-90 seconds (duration and interval dependent on many factors; e.g., load, speed, and temperature).

While NO_x adsorbers are effective at adsorbing NO_x, they also have a high affinity for sulfur. As such, sulfur from the fuel and possibly engine lubricant (as SO₂) can adsorb to NO_x adsorbent sites (as sulfates). Similar to NO_x regeneration, sulfur removal (desulfation) also requires rich operation, but for several minutes, at much higher temperatures. Desulfation intervals are much longer, on the order of hundreds or thousands of miles, but the conditions are more difficult to achieve and are potentially harmful to the catalyst function. Nonetheless, desulfation must be accomplished periodically to maintain effective NO_x performance. There is much to be learned with regard to NO_x adsorber performance, durability, and sulfur tolerance.

Different strategies for introducing the excess fuel for regeneration can produce a wide variety of hydrocarbon and other species. One focus of this work is to examine the effectiveness of various regeneration strategies in light of the species formed and the adsorber formulation. Another focus is to examine the desulfation process and examine catalyst performance after numerous sulfation/desulfation cycles. Both regeneration and desulfation will be studied using advanced diagnostic tools.

Approach
A 1.7-L Mercedes common rail engine and motoring dynamometer have been dedicated to this activity (Figure 1). The engine is equipped with an electronic engine control system that provides full-bypass of the OEM engine controller. The controller is capable of monitoring and controlling all the electronic devices associated with the engine (i.e., fuel injection timing/duration/number of injections, fuel rail pressure, turbo wastegate, electronic throttle, and electronic EGR).

Figure 1 - Experimental setup including engine, control system, motoring dyno, and exhaust system

Various regeneration strategies are being developed with the goal of introducing a broad range of species to the NO_x adsorber catalysts. Advanced tools such as H₂-SpaciMS and GC/MS are being used to characterize the species produced in the engine or in upstream catalysts. The H₂-SpaciMS will be used for both in-pipe and in-situ measurements within the catalyst monoliths. In addition, catalysts and exhaust species will be characterized after rapid sulfation and during desulfation. Some NO_x adsorber catalysts will be provided by Ford under a CRADA, while others will be provided by some MECA members. “Model” catalysts will also be characterized. Catalysts are being studied under quasi-steady conditions, that is steady load and speed but with periodic regeneration, as shown in Figure 2.

Figure 2 - Quasi Steady State NO_x adsorber regeneration trace

Finally, bench-scale work will be used to further characterize adsorber monoliths, wafers, and/or powders using our bench-scale reactor and the DRIFTS reactor. Results and characteristics of the engine experiments will be used to help define more meaningful bench scale studies. In some cases, the exact same catalyst formulation we are characterizing on the engine stand will also be examined in the bench studies.

Results
We have measured H₂, CO and speciated HC compounds for several regeneration strategies. Previous work has shown that H2 and CO are excellent reductants for NO_x adsorber regeneration. Using ORNL's H₂-SpaciMS, we have quantified the hydrogen in the exhaust with both time and space resolved measurements. Also, GC/MS determines the HC species that are generated in the engine or consumed in the catalysts. Using these unique instruments in conjunction with conventional gas analysis, we are developing a portfolio of regeneration strategies that will provide a wide range of NO_x adsorber inlet species to help understand the catalyst mechanisms.

The three strategies developed thus far include “Delayed and Extended Main” (DEM), “Post”, and “Extended Main” (EM). All three strategies use 15%-20% EGR during lean operation and intake throttling during the rich excursion to reduce airflow.

Figure 3 shows an oscilloscope trace of the DEM strategy. The main pulse width is increased to transition from lean to rich, and the start of injection is retarded or delayed to maintain the torque level. (Increasing fuel would tend to increase torque, while retarding the injection timing would decrease torque.) EGR is cycled off during DEM regenerations to avoid intake fouling. The EM strategy uses a combination of larger main injection with increased EGR to transition from lean to rich.

Figure 3 - Oscilloscope trace of "Delayed and Extended Main" regeneration strategy

The Post strategy is shown in Figure 4. An additional late cycle (post) injection is added to transition from lean to rich, and the main injection is modified to maintain torque. EGR is cycled off during Post regenerations to avoid intake fouling.

Figure 4 - Oscilloscope trace of "Extended Main" regeneration strategy

Figure 5 shows chromatograms for engine-out species and raw fuel species. The GC/MS separates HC species by molecular weight. The x-axis is time, and each peak on the chart is an individual compound. Note that there are several light HCs being produced during combustion that are not in the raw fuel. The extent of this fuel cracking in-cylinder can be tailored to some degree by the regeneration strategy.

Figure 5 - GC/MS trace showing light hydrocarbons produced in cylinder

H₂ and CO data collected during the Post injection timing sweep are shown in Figure 6. The average peak concentration for several regenerations is plotted as a function of injection angle. Because H2 and CO are excellent reductants, there is a need to understand how to control the amount produced during regeneration, as well as balancing production with PM formation and fuel penalty. We have shown that H₂ is produced in-cylinder, and we can quantify the amount of H₂ and CO produced by different strategies. Additionally, the H₂-SpaciMS allows us to measure intra-channel H₂, permitting determination of how the hydrogen is utilized, and in conjunction with the GC/MS information, what HC species are the best hydrogen precursors. It is also interesting to note that with comparable H₂ and CO levels, the Post, LFM, and FM fuel penalties were 5.5%, 4%, and 2%, respectively.

Figure 6 - H₂ and CO produced in cylinder for post timing sweep

Conclusions

Regeneration strategies can be tuned to produce different HC pools and amounts of H₂ and CO

The CO to H₂ ratio is consistently near 2:1 for each regeneration strategy

Considerable fuel cracking occurs during in-cylinder combustion and can be quantified with GC/MS

Tradeoffs must be considered between a regeneration strategy's effectiveness and its impact on fuel consumption and PM

Acronyms

Publications

Measurement and Characterization of NO_x Adsorber Regeneration and Desulfation
Authors - Shean Huff, Stuart Daw, John Storey, Brian West, Bill Partridge, Sam Lewis, Dean Edwards, Katey Lenox, Jae-Soon Choi, Todd Toops
Publication - 9th Diesel Engine Emissions Reduction Conference - 2003