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NOx Adsorbers [subscription]
DieselNet Technology Guide È Diesel Catalysts
NOx Adsorbers
W. Addy Majewski
Abstract: NOx adsorbers (traps) are the newest control technology
being developed for partial lean burn gasoline engines and for diesel
engines. The adsorbers, which are incorporated into the catalyst
washcoat, chemically bind nitrogen oxides during lean engine
operation. After the adsorber capacity is saturated, the system is
regenerated, and released NOx is catalytically reduced, during a
period of rich engine operation.
NOx Adsorber Concept
Performance And Operation
Diesel Engine Systems
NOx Adsorber Concept
Overview
The concept of the NOx adsorber/catalyst has been developed based on
acid-base washcoat chemistry. It involves adsorption and storage of
NOx in the catalyst washcoat during lean driving conditions and
releasing it under rich operation. The released NOx is catalytically
converted to nitrogen just as it happens over the three-way catalyst
(TWC), which is widely used in stoichiometric gasoline engines.
Normally, three-way catalysts are inactive in converting NOx under
lean exhaust conditions, when oxygen is present in the exhaust gas.
Through the lean storage and rich release and conversion, the
applicability of the three-way catalyst is being extended to lean
burn engines, such as gasoline direct injected engines (GDI) and
compression ignition engines. NOx adsorbers, which are being
developed by catalyst manufacturers with notable progress, are
perceived by many as the diesel NOx control technology of the future.
Different authors use different terms when discussing NOx adsorbers,
such as lean NOx traps (LNT), NOx storage/reduction catalysts (NSRC),
or NOx adsorber catalysts (NAC). All these names are synonyms
describing the same emission control technology. The term lean NOx
catalyst, on the other hand, refers to the selective catalytic
reduction of NOx by hydrocarbons-an entirely different technology
which should not be confused with NOx adsorbers.
Adsorption and desorption of NOx in the NOx adsorber is primarily
related to lean and rich cycling during the vehicle operation.
Desorption with accompanying conversion, also referred to as the
regeneration, occurs during rich air-to-fuel mixture excursions.
Quite clearly, this concept is very compatible with "partial lean
burn" GDI engines, where the engine controller can easily provide a
mixture enrichment when needed. With modern, electronically
controlled fuel systems, it is also possible to periodically enrich
the air-to-fuel mixture in the diesel engine in order to facilitate
the regeneration of diesel NOx adsorbers. However, unlike more
conventional diesel catalysts, this technology requires an
unparalleled degree of integration with the engine management system.
Therefore, while NOx adsorbers are a promising technology for new
engine/vehicle applications, they are not particularly suitable for
retrofit emission control systems.
The discussion of adsorber/catalyst systems in this paper is based on
the developments for both lean burn gasoline and diesel engine
applications. While there are many similarities between catalysts
used in these applications, there are also important differences that
affect the catalyst system design, as listed in Table 1 and
illustrated graphically in Figure 1 [Dou 2002a].
Figure 1. Operation of NOx Adsorbers on Gasoline and Diesel Engines
Table 1. NOx Adsorber Catalysts for Gasoline and Diesel Engines
GDI Diesel Comment
A/F ratio Partially lean (lean A/F 20-35) Continuous lean (A/F
20-50) GDI is operated stoichiometric at high speed/load conditions.
Catalyst functionality NOx adsorber + 3-way catalyst NOx adsorber
only Due to the partial stoichiometric operation, gasoline
adsorbers must be also good 3-way catalysts of wide A/F catalyst
window, high HC conversion, etc.
Rich regeneration Easy Difficult Rich operation of
diesel is problematic, may cause high PM emission.
Fuel economy penalty Lower Higher
Operating temperature 250 - 600¡C 150 - 550¡C Diesel
adsorbers must be optimized for low temperature performance, which
may require different formulations and higher adsorber loadings
[Klein 2001].
Reductants CO, HC, H2 HC, CO, H2
Durability required 800 - 850¡C 650 - 700¡C
Depending on the fuel quality, diesel adsorbers may also require
higher sulfur tolerance than gasoline adsorbers. However, considering
the great sensitivity of this technology to sulfur, NOx adsorbers
will likely require ultra low sulfur levels in both gasoline and
diesel.
Chemical Reactions
The NOx adsorption/reduction mechanism is illustrated in Figure 2.
The catalyst washcoat combines three active components: (1) an
oxidation catalyst, for example Pt, (2) an adsorbant, for example
barium oxide (BaO), and (3) a reduction catalyst, for example Rh.
Figure 2. NOx Adsorption and Reduction Mechanism
The adsorption process involves two steps, as represented by Equation
(1) and Equation (2), which occur during lean engine operation. NOx
emissions from the diesel engine are typically composed of 90-95%
nitric oxide, NO. In the first step, the nitric oxide reacts with
oxygen on active oxidation catalyst sites (e.g. platinum, Pt) to form
NO2. The second step involves adsorption of the NO2 by the storage
material in the form of an inorganic nitrate.
(1)NO + 1Ú2O2 = NO2
(2)BaO + NO2 + 1Ú2O2= Ba(NO3)2
When the engine runs under excessive fuel conditions or at elevated
temperatures the nitrate species become thermodynamically unstable
and decompose, producing NO or NO2, according to Equation (3) [Brogan
1998] [Erkfeldt 1999]. Under rich conditions, these nitrogen oxides
are subsequently reduced by carbon monoxide, hydrogen, and
hydrocarbons to N2 over the reduction catalyst, in a conventional
three-way catalyst process. One of the possible reduction paths is
shown by Equation (4).
(3a)Ba(NO3)2 = BaO + 2NO + 11Ú2O2
(3b)Ba(NO3)2 = BaO + 2NO2 + 1Ú2O2
(4)NO + CO = 1Ú2N2 + CO2
This simplified set of reactions allows for an understanding of the
basic NOx adsorber chemistry. A more detailed analysis should also
include other reaction paths and species, for example carbonates
which may be formed in reactions between barium nitrate and carbon
dioxide [Kobayashi 1997].
NOx adsorbers also show some undesired reactivity, primarily in
regards to sulfur compounds which are present in exhaust gases from
both diesel and gasoline engines. Reactions of sulfur are basically
equivalent to the reactions of NOx, Equation (1) - Equation (3), and
can be written as follows:
(5)SO2 + 1Ú2O2 = SO3
(6)BaO + SO3 = BaSO4
First, sulfur dioxide is oxidized to sulfur trioxide over platinum,
Equation (5). Then, the SO3 reacts with BaO to form barium sulfate,
Equation (6). This causes gradual saturation of the barium sites with
sulfur and loss of activity towards the adsorption of NO2. BaSO4 can
be thermally decomposed in a process equivalent to reaction Equation
(3a,b). However, sulfates of barium and other NOx adsorber compounds
are more stable than their nitrates and require much higher
temperatures to desulfate. For this reason, sulfur deactivation is
the major problem in the development of NOx adsorber systems.
There are a number of other possible reactions in NOx adsorbers which
can produce secondary, unregulated emissions. These are mainly
reduction processes, also known to occur in the three-way catalyst,
which involve NO/NO2, as well as SO2, and generate products other
than nitrogen. The main secondary emissions include nitrous oxide
(N2O), and hydrogen sulfide (H2S).
Catalyst Systems
NOx adsorber systems generally consist of a conventional three-way
catalyst which additionally incorporates NOx storage components.
These storage components in NOx adsorbers are typically compounds of
the following elements [Strehlau 1999]:
* Alkaline earths-barium (Ba), calcium (Ca), strontium (Sr), magnesium (Mg)
* Alkali metals-potassium (K), sodium (Na), lithium (Li), cesium (Cs)
* Rare earth metals-lanthanum (La), ytrium (Y)
While most catalysts incorporate adsorbers in the form of oxides, a
number of defined systems, such as perovskites or metal substituted
zeolites, have also been tested [Chaize 1998].
Different adsorbing elements, their chemical compounds and/or
mixtures may yield catalysts of different properties, such as
different NOx storage capacity, thermal stability of the nitrate and
its desorption temperature, susceptibility to sulfur poisoning, or
sulfur desorption (desulfation) temperature. These properties are
important in designing the catalyst system and its regeneration
strategy, as will be discussed later. Compounds of more basic
chemical character can provide higher NOx storage capacity.
Importantly, alkali metals, such as potassium, exhibit superior NOx
adsorption performance at high temperatures relative to barium, as
illustrated in Figure 3 [Hachisuka 2002]. If sodium, potassium,
and/or cesium are incorporated into NOx adsorber washcoat containing
barium, they significantly increase NOx conversion in the temperature
range of about 350-600¡C. Alkali metal oxide adsorbers (up to 50% Na
in alumina washcoat) were also reported to show superior resistance
to sulfur poisoning [Hepburn 1998].
Figure 3. Temperature Activity of Storage Elements
As the research continues, it becomes apparent which catalyst
formulations can provide the most desired combination of properties.
Most of current NOx adsorbers are based on one of the following
systems:
* Alkaline earth, primarily Ba-based adsorbers
* Combined, Ba + alkali metal (K, Na, ...) adsorbers
Ba + alkali metal adsorbers provide superior NOx conversion, both
fresh and aged, at temperatures above 250-300¡C, with only small NOx
conversion penalty at lower temperatures [Dou 2002]. Ba + alkali
metal adsorbers offer also better nitrate stability. Finally, Ba +
alkali metal adsorbers have better sulfur poisoning resistance than
Ba-only adsorbers, with higher NOx conversion after repeated
poisoning and desulfation cycles. Their disadvantage is poor
hydrocarbon conversion, which is lower in comparison to Ba adsorbers
at all temperatures. Among all alkali metals, Cs is most effective in
enhancing NOx conversion, followed by K, Na, and Li, as shown in
Figure 4 [Dou 2002].
Figure 4. NOx Conversion With Different Alkali Metals
5.0 L V8 gasoline engine; Modulation A/F=21.5 for 30 s, A/F=12.5 for
2 s; SV=50,000 1/h; Inlet NOx = 500 ppm; After 50 hrs aging @800¡C
Alkali metal catalysts also have a number of disadvantages, such as a
potential negative impact on the durability of platinum catalysts by
promoting noble metal sintering, or the already mentioned poor HC and
low-temperature performance. Possibly the most serious issue is
related to the high mobility of water-soluble alkali metal compounds,
such as nitrates. In catalysts, alkali metals migrate with time away
from the adsorber washcoat. Due to their strong affinity to silicon
oxide, a component of cordierite, alkali metals accumulate within the
ceramic catalyst substrate, causing loss of its physical strength
[Cutler 1999]. Cs was found to have the highest migration intensity,
followed by K and Na, as illustrated in Figure 5 [Dou 2002]. The
alkali metal maps shown below were obtained by analyzing (scanning
electron microscopy) aged catalyst samples of the same molar amount
of metals. The Cs map shows that Cs migrated throughout both the
cordierite substrate cell corner and cell wall. Lower cesium
concentration was detected on the washcoat after aging. A significant
amount of potassium migrated away from the washcoat and accumulated
at the interface of washcoat and cordierite substrate. Little K was
detected at the center region of the cell corner. In contrast to Cs,
Na is concentrated at the interface of cordierite substrate and
washcoat, with little Na found in middle of the cell wall and the
center of the cell corner. After aging at 900¡C, Cs and K containing
NOx adsorbers showed skin cracking while the Na containing adsorber
was free of any visible cracking.
Figure 5. Maps of Cs, K, and Na in Aged NOx Adsorbers Coated on
Cordierite Substrates
After aging at 900¡C for 16 hrs under air/H2O
(Courtesy of Delphi and Corning)
Since alkali metals that are more mobile have also better NOx
conversion effect, catalyst designers face the difficult task of
optimizing their formulations to achieve the required NOx
performance, while minimizing the chemical attack on the cordierite
substrate. Advanced catalysts and substrates with barrier Si coatings
have been proposed to minimize the impact of alkali metals [Iwachido
2001]. NOx adsorbers coated on metallic substrates have been also
tested [Bergmann 2000][Klein 2001]. However, since material
durability issues were not included in these studies, it is not
certain if metallic substrates would be sufficiently resistant to the
attack from alkali metals in NOx adsorbers.
Due to the importance of high temperature performance in spark
ignited engines, the Ba + alkali metal system becomes the NOx
adsorber of choice in gasoline applications [Dou 2002][Hachisuka
2002]. Adsorbers for light-duty diesel engines, which operate at
lower temperatures, will likely avoid the necessity of using alkali
metals. Ba + alkali metal formulations, however, may be needed in
heavy-duty diesel applications, especially those that require
emission certification on hot test cycles such as the US steady-state
Supplemental Emission Test (SET, based on the European ESC) [McDonald
2002].
Most NOx adsorber washcoats are based on g-alumina, Al2O3, which is
also the main washcoat material in gasoline three-way catalysts.
Three-way catalysts for SI engines also include high loadings of
zirconia-stabilized cerium oxide (ceria), which provides the oxygen
storage capacity (OSC). By storing oxygen at lean and releasing it at
rich condition, the OSC component is able to extend the three-way
catalyst window. The OSC function is less important in diesel NOx
adsorbers, but it is important for lean burn gasoline applications
which spend significant operating time at stoichiometry. Another
important function of the ceria is to promote the reaction between
NOx and CO. Because of the high level of instantaneous NOx conversion
which is required for rapid, efficient adsorber regeneration, this
feature becomes extremely important, in both the diesel and gasoline
applications. Unfortunately, the incorporation of ceria also has a
negative effect on fuel economy due to the oxygen storage. Some of
NOx adsorber systems include ceria in the catalyst washcoat, while
others do not.
Other common washcoat materials used in NOx adsorbers include titania
(TiO2) and zirconia (ZrO2). Acidic oxides, such as TiO2, are unable
to store sulfur oxides as effectively as high surface area alumina,
thereby enhancing sulfur resistance of NOx adsorbers. For this
reason, titania was incorporated in NOx adsorbers for the Toyota D-4
GDI engine in Japan in 1996 [Hachisuka 2000]. Further addition of
Rh/ZrO2 to BaO-based adsorbers was found to catalyze the water shift
reaction with an increased generation of hydrogen and, as a result,
improved desorption of sulfur [Hachisuka 2000]. The use of titania
and zirconia was further investigated with NOx adsorbers utilizing Ba
+ alkali metal system (Ba/K) [Hachisuka 2002]. While the acidic TiO2
was confirmed to enhance sulfur resistance and to promote sulfur
desorption at lower temperatures, it was also found to reduce the
high temperature stability of KNO3, thus defying the function of
alkali metals and causing poor NOx performance at higher
temperatures. The more basic oxide ZrO2, on the other hand, enhanced
the thermal stability of KNO3, but its sulfur poisoning resistance
was far lower than that of TiO2. A complex oxide catalyst, designed
to combine the sulfur resistance of titania systems with the alkali
adsorber stability of zirconia systems, was developed and
commercialized in 2001 on the D-4 GDI engine [Hachisuka 2002].
Noble metal systems used in most NOx adsorbers include platinum and
rhodium, the classic three-way catalyst components. Platinum is the
most active catalyst for NO oxidation, Equation (1). Since this step
is critical for the NOx storage efficiency, platinum plays an
important role in adsorber catalysts. Rhodium is used as the NOx
reducing catalyst in most adsorbers. Trimetal formulations
(Pt/Pd/Rh), common in stoichiometric 3-way catalysts, can be also
used in NOx adsorbers, as long as their platinum content remains
relatively high. For instance, Delphi reported two trimetal
formulations with precious metal loadings of 80 g/ft3 (60:15:5) and
100 g/ft3 (70:20:10) [Dou 2002]. The catalysts were designed for GDI
engines, where they may be required to perform as typical 3-way
catalysts (i.e., under stoichiometric conditions) over prolonged
periods of time; it is not clear if palladium would offer a benefit
in diesel adsorbers.
Performance And Operation
Conversion Efficiency
NOx adsorber catalysts exhibit high NOx conversion efficiencies, in
excess of 80-90%, as illustrated by example data in Figure 6 [Brogan
1998]. The activity of NOx adsorbers covers a fairly wide catalyst
temperature window, extending from about 200¡C to 450-500¡C. The
lower end of the temperature window, T1, is determined by the
catalyst activity in regards to the oxidation of NO to NO2, as well
as NOx release and reduction (3-way function). The upper temperature,
T2, is related to the thermodynamic stability of nitrates, which
undergo thermal decomposition at higher exhaust temperatures, even
under lean conditions.
Figure 6. NOx Adsorber Temperature Window
2.47 l adsorber on 1.8 l SI engine cycled 59 s lean and 1 s rich
The NOx adsorber temperature window is in fairly good correlation
with diesel engine loads and exhaust temperatures at which most of
diesel NOx generation occurs. It is this favorable position of the
temperature window and the high NOx conversion efficiency, which
makes NOx adsorbers an extremely attractive diesel NOx control
technology.
In gasoline engine applications, the NOx adsorber performance was
reported to be enhanced by a three-way catalyst positioned upstream,
in a configuration typical for many GDI studies [Brogan 1998] [Asik
2000] [Erkfeldt 1999]. In diesel applications, NOx conversion
efficiency can be improved by positioning an oxidation catalyst
upstream of the adsorber [Dou 2002a].
Regeneration
During the adsorption cycle, the adsorber is gradually converted into
its nitrate form (e.g., barium nitrate) and the adsorption capacity
becomes saturated. At this time the stored NOx needs to be released
and catalytically reduced in a process called the regeneration. At
lean exhaust conditions, barium nitrate decomposes at temperatures
above 450 - 500¡C. The regeneration occurs at much lower temperatures
if a short pulse of fuel rich mixture is provided. NOx adsorbers can
fully regenerate at 250¡C, with the onset of a partial regeneration
at temperatures as low as 150¡C, if the air-to-fuel equivalence ratio
is maintained at l < 1 [Pott 1999]. Therefore, the operation of the
adsorber catalyst system involves continuous cycling through lean and
rich fuel condition.
The designer of an adsorber system has to analyze very carefully all
pertinent operation temperatures, including exhaust gas temperatures
during real life duty cycle and during the emission certification
test, the NOx adsorber temperature window, the rich regeneration
temperature, and the lean decomposition temperature. It is important,
that adsorber temperatures during lean operation are below the
thermal decomposition temperature of the stored nitrate (e.g., barium
nitrate). Otherwise, NOx may be released at lean, leading to a
decrease in the average conversion efficiency. While diesel adsorbers
are not likely to see temperatures of 450 - 500¡C, especially if
installed away from the exhaust manifold, this may be a problem with
gasoline engines, which experience higher combustion temperatures.
An example of lean/rich cycling is illustrated in Figure 7, which
shows concentrations of NOx upstream and downstream of the adsorber,
and the exhaust gas temperature [Brogan 1997]. The data was generated
on a light-duty DI diesel engine. Rich spikes were achieved by simple
throttling of the intake air, resulting in an oxygen concentration
decrease to below 0.4%. The 60 s storage/regeneration pattern used
during the test is visible in the peaks in NOx concentration. Engine
out NOx shows minimums, which are caused by lower
pressures/temperatures in the combustion chamber during intake air
throttling. The tailpipe NOx, on the other hand, shows maximums which
represent that portion of released NOx which has not been reduced
over the rhodium catalyst. The engine was tested at a constant speed,
but different, steady state torque levels resulting in stepwise
changes in the exhaust temperature (red line in the graph).
Figure 7. NOx Adsorber Regeneration
1.7 liter adsorber (4.66"´6") on a 1.9 liter DI diesel engine, 1200
rpm/15-50 Nm, SV=20,000-60,000 1/h, fuel sulfur 2 ppm, cycled 58 s
lean/2 s rich
At the highest tested temperature of about 330¡C the adsorber
catalyst achieved 95% NOx conversion efficiency. As the exhaust
temperatures were lowered, the conversion gradually decreased to
reach about 50% at about 200¡C. The declining catalyst efficiency
manifests itself by increasingly higher tailpipe NOx peaks during
regeneration, as can be seen in Figure 7.
The author reported that hydrocarbon break-through was observed
during rich spikes at all temperatures, while no CO break-through was
seen. It is an indication that the NOx reduction mechanism may
involve reactions with CO rather than with HC.
To maximize NOx conversion efficiency, the storage capacity and
frequency of regeneration must be optimized during the design of the
adsorber system. Typical capacity of barium adsorbers in the fresh
state amounts to around 2 g NOx per liter of catalyst volume [Brogan
1998]. Depending on the engine emissions, catalyst size and
condition, and the desired NOx reduction, regeneration must be
performed every 30 - 120 seconds. The duration of NOx adsorber
regeneration is short, between one and a few seconds.
Inhibition by Sulfur
Diesel exhaust contains certain quantities of sulfur, primarily as
sulfur dioxide, derived from diesel fuel and engine lubricating oil.
In the presence of an oxidation catalyst, these compounds form stable
sulfates with the NOx storage materials, Equation (5) and Equation
(6). The adsorption of sulfur is preferential over the adsorption of
NOx. As a result, the catalyst performance gradually declines as
fewer sites are available for NOx adsorption.
Higher levels of sulfur in fuel result in faster and more severe
deactivation, as shown in Figure 8 [Ford 1999]. It should be
realized, however, that even sulfur levels less than 10 ppm
eventually lead to NOx adsorber poisoning [Bailey 2000], not to
mention sulfur contribution from the engine lube oil. Ultra low
sulfur fuels are the necessary condition for implementation of this
technology, but even if such fuels are available NOx adsorbers are
still likely to require some form of desulfation mechanism.
Figure 8. NOx Adsorber Efficiency at Different Fuel Sulfur Levels
Sulfur poisoning begins on the surface of the catalyst inlet and
progresses deeper into the washcoat and in the axial direction [Dou
1998] [Erkfeldt 1999]. Sulfates derived from the known NOx storage
materials are more thermally stable than the corresponding nitrates.
They do not decompose at conditions that are usually encountered
during adsorber operation, including both the adsorption and NOx
regeneration cycles. The problem of sulfur deactivation is equally
affecting the diesel and GDI engine applications.
Desulfation
In general, the sulfur poisoning is reversible and site activity can
be restored by a desulfation process involving decomposition of the
sulfate species. The desulfation of NOx adsorbers requires
temperatures between 500 and 700¡C, accompanied by mixture
enrichment. It was reported that optimum desulfation of barium NOx
adsorbers is achieved at 650¡C and l = 0.98 [Strehlau 1997]. Studies
on gasoline engines have found that adsorber desulfation was most
effective in the presence of H2 and CO, with H2 being more effective.
However, H2 desulfation produced a mixture of H2S and SO2, while only
SO2 was detected among desulfation products when CO was used
[Erkfeldt 1999].
In theory, the desulfation of NOx adsorbers can restore their full
adsorption capacity. In practice, a permanent and irreversible
poisoning of some barium sites has been reported [Dou 1998]. Another
source of permanent performance loss after repeated desulfation is
thermal degradation of washcoat and catalyst materials due to high
temperatures during the desulfation process [Theis 2002]. The
desulfation strategy is a critical function in the NOx adsorber
design, still far from being solved, especially on diesel engines. If
sulfates are left in the catalyst for prolonged periods of time, the
NOx conversion efficiency is compromised. Frequent desulfation, on
the other hand, may involve significant fuel economy penalties and
accelerated thermal deterioration of the catalyst.
Various methods of desulfation through the increase of exhaust gas
temperatures have been under development. For diesel engines, exhaust
temperatures can be increased by post-injections of fuel. Algorithms
have been developed which facilitated catalyst desulfation through
system integration with the engine control unit, the common-rail fuel
injection system, and the on-board diagnostics [Pott 1999a].
Desulfation strategies for gasoline engines can also include a rapid,
large amplitude modulation of the air-to-fuel ratio to create
exothermal reactions increasing the catalyst temperature and
minimizing H2S release [Asik 2000]. A desulfation strategy involving
short A/F ratio pulses has been also proposed for the diesel engine
[Klein 2001]. A desulfation through a series of short rich pulses,
rather than a single, continuous rich period, allowed to minimize
both H2S emission and fuel economy penalty (reported at 1% when using
10 ppm S fuel). The pulses of l=0.95 and 30 s duration were repeated
every 250 s over a nearly 1 hour long desulfation process.
A NOx adsorber desulfation strategy was also developed and
demonstrated on a diesel engine by the DECSE program (U.S. DOE)
[DECSE 2000][Tomazic 2001]. The strategy, developed for a single
point on the engine map of exhaust temperature of 400¡C, involved a
common rail post-injection and a close-coupled warm-up catalyst.
Hydrocarbons generated through the post-injection were oxidized in
the warm-up catalyst to create an exotherm increasing the NOx
adsorber inlet temperature to 700¡C (Figure 9).
Figure 9. Schematic of DECSE Desulfation Strategy
Engine: 1.9 l HSDI, 1943 cc, 81 [EMAIL PROTECTED] rpm, turbocharged, common
rail, EGR
Warm-up catalyst: 2.5 l (5.66 in dia. ´ 6 in), 400 cpsi/6.5 mil, Pt, 70 g/ft3
NOx adsorber catalyst: 2.5 l (5.66 in dia. ´ 6 in), 400 cpsi/6.5 mil,
10:3.9:1, 164 g/ft3
DECSE experiments to develop the strategy were conducted by operating
the engine on commercial, 380 ppm S fuel for approximately two hours,
until the initial NOx conversion efficiency of 80% dropped to 60% due
to sulfur poisoning. At that moment, the desulfation event was
triggered. The adsorber inlet temperature of 700¡C was achieved 90 -
180 seconds after initiating the desulfation, depending on the
post-injection quantity and the EGR rate (the quantity of 30
mm3/stroke and the EGR of 26.6% were eventually selected). Restoring
the NOx efficiency to the original 80% required a duration of the
desulfation event of about 5-6 minutes. The above procedure was
applied a number of catalysts of different poisoning histories, all
of which were restored to over 85% NOx reduction efficiency over the
catalyst inlet operating temperature window of 300¡C - 450¡C. This
performance level was achieved while staying within the 4% fuel
economy penalty target defined for the adsorber regeneration.
Variations of the engine torque due to post-injections during the
desulfation event were controlled within 1%.
Sulfur Traps
Obstacles encountered in developing of adsorber desulfation
strategies stimulated the development of alternate methods of
protection from sulfur, leading to the idea of sulfur traps. A sulfur
trap, also called the SOx trap, is another adsorber catalyst,
specifically designed to store sulfur. It is placed in the exhaust
system upstream of the NOx adsorber, to reduce its rate of poisoning
by sulfur. Due to the high temperature requirements for sulfur trap
regeneration, its preferred position is close to the exhaust
manifold, as illustrated in Figure 10. The NOx adsorber, which
operates at lower temperatures, can be positioned away from the
engine, such as in the underfloor position on passenger cars.
Although sulfur traps were developed that can effectively protect NOx
adsorbers, the trap regeneration (desulfation) management remained an
open issue; unless an exhaust gas bypass is used, the sulfur released
from the trap is re-adsorbed while passing through the downstream
adsorber, thus making the application of sulfur traps problematic.
Figure 10. Sulfur Trap Configuration
The sulfur storage mechanism in NOx adsorbers and in many sulfur
traps is similar. Since sulfur is stored in the form of a metal
sulfate, the sulfur dioxide has to be first oxidized to SO3, Equation
(5). Therefore, sulfur traps must include an oxidation catalyst, e.g.
platinum, to facilitate this reaction. Also the storage systems are
similar to those used in NOx adsorbers, but specifically optimized
for the following features:
* High adsorption capacity for sulfur
* Optimal desulfation temperatures for a given system
* Fast and complete release of sulfur (regeneration) after reaching
the desulfation temperature
* Selectivity towards sulfur adsorption
* No secondary emissions
* Thermal durability
Typical sulfur storage systems are based on alkaline earth or alkali
metal oxides and their mixtures on alumina (titania, zirconia)
washcoat. A number of other materials have also been tested,
including zinc, nickel, chromium, copper, and silver. These metals or
their oxides are used either as stand-alone scavengers or as
promoters, to modify trap performance, strengthen desired reactions,
and influence the adsorption/desorption temperatures [Strehlau 1999].
Silver, for example, has been reported to enhance the sulfur trap
selectivity towards adsorption of sulfur oxides over nitrogen oxides
and to lower the regeneration temperature [Nakatsuji 1998].
After its storage capacity is saturated, the sulfur trap has to be
regenerated. Unless a complex piping system involving valves and
bypasses is used [Hiromi 1999], sulfur released from the trap must
pass through the downstream NOx adsorber. Depending on the
temperatures in both devices, a fraction of sulfur released from the
trap will be re-adsorbed in the NOx adsorber. This presents one of
the major obstacles in implementing sulfur traps.
Thermal release of sulfur under lean conditions is possible, but
would require temperatures beyond the thermal stability limits of
existing NOx adsorber designs. On the other hand, materials have been
developed, that allow desulfation of sulfur traps under rich
condition at temperatures as low as 300 - 350¡C [Bailey 2000]
[Nakatsuji 1998]. For this reason, most studies have focused on rich
regeneration in a manner parallel to that of the NOx adsorbers. There
are two possible maintenance strategies for the sulfur trap:
* Continuous maintenance strategy, where sulfur scavengers of low
regeneration temperature are regenerated every time the NOx adsorber
is regenerated
* Periodic regeneration strategy, where the sulfur trap is
regenerated less frequently than the NOx adsorber.
In the periodic strategy the sulfur trap is regenerated at higher
temperatures, thus, minimizing the re-adsorption of sulfur in the NOx
adsorber. A certain fuel economy penalty would be associated with
increasing the temperature of the sulfur trap. The continuous
regeneration approach, while minimizing the fuel economy penalty,
presents a larger material development challenge to develop
scavengers that would regenerate at low temperatures and over short
periods of time. Irrespective of the strategy, periodic desulfation
of NOx adsorbers is likely to be required, even in the presence of
sulfur traps.
The desulfation of sulfur scavengers involves release of secondary
emissions, including hydrogen sulfide (H2S) and carbonyl sulfide
(COS) [Strehlau 1999]. The ratio of H2S/SO2 released during
regeneration increases with decreasing air-to-fuel ratio. If low
air-to-fuel ratios are used for regeneration, which typically reduce
the duration of regeneration, most of sulfur may be released as H2S.
Even though the tendency to release sulfur as SO2 can be maximized by
the selection of sulfur scavengers [Bailey 2000], the SO2 may be
subsequently reduced to H2S in the downstream NOx adsorber under rich
conditions. H2S production increases with the gas residence time in
the catalyst, i.e., larger sulfur traps and NOx adsorbers produce a
higher proportion of hydrogen sulfide. In the system shown in Figure
11, over 90% of the sulfur is released from the adsorber as H2S
[Strehlau 1999].
Figure 11. SO2 and H2S Fraction During Regeneration
0.8 liter sulfur trap on an SI passenger car engine, regenerated at 640¡C
Considering the obnoxious rotten egg odor of hydrogen sulfide, it is
desirable that sulfur be released from the vehicle tailpipe in the
form of SO2 rather than H2S. If satisfactory H2S-suppressed sulfur
scavengers are not developed, it may be necessary to provide an
additional catalyst, downstream of the NOx adsorber, to store the
hydrogen sulfide during rich regeneration periods and oxidize and
release it as SO2 during lean operation. Suitable H2S scavengers,
based on nickel, manganese, zinc, or iron, have been developed for
the gasoline three-way catalyst [Strehlau 1999] [Kim 1988].
Formation of another secondary emission, ammonia (NH3), was reported
over a silver-based sulfur trap at lean condition with resulting
decrease in NOx adsorber efficiency [Nakatsuji 1998]. An ammonia
decomposition catalyst was installed between the sulfur trap and the
NOx adsorber to alleviate the problem.
Diesel Engine Systems
Mixture Enrichment Strategies
Mixture enrichment (l < 1) on lean burn gasoline engines may require,
in order to minimize torque disturbances, some coordination of spark
advance with fuel injection, but otherwise seems to be fairly
straightforward. Generation of rich air-to-fuel mixtures on the
diesel engine, which normally operates at l > 1.3 and leaner, is
certainly more challenging.
Theoretically, the methods of diesel mixture enrichment can be
grouped into two categories, as follows:
* Exhaust gas enrichment
* In-cylinder enrichment
Exhaust system enrichment may be realized through injection of diesel
fuel upstream of the catalyst, in a similar manner as it was
suggested in active deNOx catalysts which work through selective NOx
reduction by hydrocarbons. However, efficient NOx adsorber
regeneration requires not only the presence of reductants, but also
the absence of oxygen. In the case of exhaust enrichment, the
existing oxygen needs to be combusted to CO2 and H2O before NOx
conversion can proceed. This can be realized in the NOx adsorber
itself, or in an upstream oxidation catalyst. Disadvantages of this
process include exotherms, which can be damaging to the catalyst, and
an additional fuel economy penalty.
Theoretically, reducing gases other than hydrocarbons derived from
the diesel fuel can be also used for the regeneration of NOx
adsorbers. Regeneration of a NOx adsorber through the injection of
syngas (CO+H2) into the exhaust system was demonstrated on a light
duty vehicle [West 2000]. In practical applications, the reductants
would have to be generated on-board by reforming diesel fuel. It was
proposed that hydrogen-rich gas (CO+H2) be produced from diesel fuel
by electric discharge (plasma) continuously applied to flowing
fuel/air mixture in a device termed the "diesel plasmatron reformer"
[Bromberg 2002].
In-cylinder enrichment involves such methods as altering fuel
injection timing and rate, throttling, and using EGR [DECSE 1999a].
Advanced, electronically controlled diesel fuel injection systems,
such as the common rail, can now offer much more flexibility for
practical in-cylinder enrichment strategies. The common rail is the
first diesel injection system in which both the injection rate and
timing can be controlled totally independent of the engine speed and
load condition. In particular, it is possible to provide
post-injections of fuel, as may be required to increase the exhaust
gas temperature.
Common rail injection has already been used for increasing diesel
exhaust gas temperature as a means of assisting the regeneration of a
particulate filter [Salvat 2000]. The task was accomplished with
three injections and required a turbocharger adjustment in order to
maintain torque. The filter application, however, differs from that
of the NOx adsorber in the fact that quantities of oxygen remain in
the exhaust gas. Systems specifically developed for regeneration of
NOx adsorbers are likely to require a combination of methods, e.g.,
implementing intake air throttling in addition to multiple injections
[Terazawa 2000].
A new diesel combustion process, called the "low temperature
combustion", has been developed by Toyota for the regeneration of NOx
adsorbers at low load and speed conditions in light-duty engines
[Sasaki 2000]. The process involves massive EGR, intake air
throttling, and injection timing designed for smokeless combustion
despite rich A/F ratio.
Integration of Engine-Catalyst System
One of the most important aspects of utilizing NOx adsorber
technology is to establish engine operating conditions that would
facilitate a satisfactory level of NOx conversion through proper
adsorber regeneration and desulfation, while minimizing the
associated fuel economy penalty. This optimization is achieved by
defining a lean/rich modulation strategy, while paying close
attention to resulting NOx, CO, and HC concentrations, as well as to
exhaust temperatures. More fuel-rich modulations typically result in
faster and more complete regeneration of the adsorber and, thus, in
higher average NOx conversion efficiencies.
The DECSE program concluded, that an over 80% peak NOx conversion
efficiency can be achieved at a fuel economy penalty of less than 4%
[DECSE 1999a]. While the 4% figure provides a useful reference number
on the anticipated fuel cost of this technology, there is still room
for optimization of the engine-catalyst system. This is best
illustrated in Figure 12, taken from the same DECSE study. Two NOx
conversion curves shown in the graph were generated using two
different engine calibrations. In both cases the fuel penalty was
kept below 4% and identical rich/lean timing was used, but different
NOx conversions were seen. It is clear that the regeneration cycle
modulation has to be very carefully tuned to match the catalyst
requirement; otherwise, quantities of fuel may be wasted through
unproductive mixture enrichment.
Figure 12. Influence of Engine Calibration on NOx Conversion Efficiency
2.5 liter fresh catalyst on a 1.9 liter HSDI engine rated 81 kW @
4200 rpm, 3 ppm S fuel
While most of the early laboratory studies were performed at
steady-state conditions, real-life NOx adsorber systems must provide
efficient NOx reductions at all operating conditions, including
engine transients. The regeneration and desulfation cycles in high
efficiency systems require a closed-loop control, based on the
concentrations of NOx and oxygen, temperature, and other parameters.
The feedback signals must be provided by sensors, including NOx
sensors and A/F ratio sensors. The control system, integrated with
the engine control module, has to determine the regeneration and
desulfation parameters (timing, duration, A/F ratio, ...) and the
enrichment strategy depending on the process variables and the engine
operating conditions. The optimization of the
regeneration/desulfation control and the integration of the
engine-catalyst system remain perhaps the most challenging task on
the road towards commercial introduction of NOx adsorbers.
Diesel Engine System Concepts
Since the late 1990's, a notable progress has been achieved in the
integration of NOx adsorbers with diesel fueled vehicles, especially
in light-duty applications for the European and Japanese markets. As
the research progresses, there are more attempts to demonstrate NOx
adsorber systems on diesel engines, including heavy-duty applications
targeting the US2007 standards. Since NOx adsorbers can only be
effective in controlling NOx, CO, HC, and SOF emissions, particulate
filters will be required for PM reductions needed to meet emission
standards (Euro5, US2007, Tier2). For this reason, any serious
demonstration programs need to integrate NOx adsorbers with diesel
particulate filters (DPF). There are at least two such integrated
emission control systems, one for heavy- and one for light-duty
engines, that deserve more detailed discussion:
* EPA 2007 "Proof-of-Concept" system has been developed as a
laboratory demonstration of the technical feasibility of the US2007
emission standards. While not optimized and far from mature, the
system became a benchmark and a starting point for NOx adsorber-based
emission control system development by U.S. heavy-duty engine
manufacturers.
* Toyota DPNR system is perhaps the most advanced NOx adsorber
system, which is nearing a commercial deployment on diesel cars. The
DPNR features a vary elegant and compact integration of a diesel
particulate filter and the NOx adsorber, with the latter being simply
coated on the filter substrate.
EPA 2007 Proof-of-Concept
One of the first NOx adsorber demonstrations on a heavy-duty engine
was presented by the U.S. EPA as a "proof-of-concept" for the US2007
HD diesel emission standards (NOx = 0.2 g/bhp-hr; PM = 0.01 g/bhp-hr)
[Schenk 2001]. The system, installed on a production Cummins ISB
engine (5.88 liter, 6-cylinder, turbocharged-aftercooled, DI, 4
valves per cylinder, 194 kW/260 hp @2500 rpm), included two catalyzed
DPF - NOx adsorber banks in parallel, as shown in Figure 13 and in
Figure 14. The exhaust flow was periodically switched through a
valving system to direct most of the flow to the adsorber in
operation, and only a small portion of the gas stream to the adsorber
in regeneration. A secondary fuel injection was applied during
regeneration upstream of the DPF. The injected fuel was oxidized over
the DPF catalyst, depleting oxygen and increasing the temperature. An
oxidation catalyst, shared by both DPF/adsorber banks, was included
at the end of the system to control HC emissions that may have broken
through the regenerating adsorber. The system also included a number
of zirconia NOx sensors (before DOC, after adsorbers, in the engine
exhaust manifold) and oxygen sensors (after adsorbers) to provide
feedback for the control system.
Figure 13. NOx Adsorbers in EPA 2007 "Proof-of-Concept" System
The EPA system provided impressively high conversion levels of 98%
reduction in NOx (down to 0.25 g/bhp-hr) and 93% reduction in PM
(0.002 g/bhp-hr), as measured over the FTP transient cycle. The
engine was operated on a 5 ppm S fuel. A fuel economy penalty of 2.3%
was attributed to the regeneration of the NOx adsorber (this figure
did not include any penalties that may have resulted from the system
pressure drop). An oversized catalyst system, including 70 liters of
combined catalyst/DPF volume, was used in the study. No adsorber
desulfation tests were conducted; adsorber deactivation by sulfur
would eventually occur in real life, even when using diesel fuel of
only 5 ppm S.
In the second stage of the study, the engine was modified by adding a
high pressure common-rail injection system with a Bosch/ETAS engine
management system, and a high pressure loop EGR [Schenk 2001a]. In
this configuration, the adsorber system yielded an average NOx
emission of 0.13 g/bhp-hr (down from a 2.67 g/bhp-hr baseline) over a
hot start FTP test at a reductant fuel economy penalty of 1.49%.
Figure 14. 2007 "Proof-of-Concept" System in the EPA Emission Laboratory
The dual adsorber/DPF bank approach allows to minimize the fuel
economy penalty by regenerating the adsorber at a low exhaust gas
flow, however, it results in a very large total catalyst volume.
Reduction in the system size may be realized by decreasing the
catalyst volume, as well as by optimizing the split between the
catalyst volume in the operation and regeneration modes. The system
shown in Figure 14 includes equal volumes in each mode. However, the
regeneration time that is required for the NOx adsorber is shorter
than the time needed to saturate the adsorber with NOx. This results
in a part of the system volume "idling" after the regeneration cycle
is completed. The EPA system size would be further reduced if the
regeneration was conducted using a smaller proportion of the total
catalyst volume. The EPA planned on splitting the system into four
sections, three of which would be in the operation mode, while one
would be regenerated. Exhaust gas flow would be directed to the
proper section(s) using a rotary valve system upstream of the
filter/catalyst body. The standard truck muffler visible on the floor
underneath the system in Figure 14 represents the target volume for
the entire system utilizing that concept.
The EPA system configuration, based on a production diesel engine, it
is an example of a retrofit NOx adsorber system. In future engines,
the adsorber management strategy will be integrated with that of the
engine, thus allowing for the optimization of the regeneration
process and, preferably, allowing for use of single bank
configuration of much smaller catalyst volume.
Toyota DPNR System
A very elegant and compact system, combining a NOx adsorber and a DPF
on one substrate was developed by Toyota and termed Diesel
Particulate-NOx Reduction system, or DPNR [Yamaguchi 2001][Tanaka
2001][Nakatani 2002]. The DPNR utilizes a cordierite wall-flow
particulate filter substrate, with channels are alternatively plugged
at the ends to force the gas through the porous walls, where the
particulates are mechanically trapped. The NOx adsorber catalyst is
coated on the filter substrate, over the channel walls and inside the
pores. The system adsorbs and reduces NOx over a lean-rich modulation
cycle typical for NOx adsorbers. Particulate matter collected on the
filter is continuously regenerated over the Pt-containing catalyst,
just as is the case in catalyzed particulate filters. Interestingly,
a synergistic effect was found between the NOx and PM control
functions in the DPNR, with the rich pulsing enhancing the filter
regeneration process.
A schematic of the DPNR system integrated with a prototype diesel
engine is shown in Figure 15. The engine (in-line 4 cylinder, 2.0
liter, water-cooled, TDI) is equipped with a common-rail fuel system
and cooled, high-pressure loop EGR. The NOx adsorber/DPF unit is
installed downstream of the turbocharger. The reported volume of the
DPNR substrate varies between 1 - 1.5 times the engine displacement;
it was 2.8 liter in the system in Figure 15 [Fujimura 2002]. A
substrate geometry of 300 cpsi, 0.3 mm wall thickness, 55% porosity,
and a relatively large mean pore size of 25 mm to enable catalyst
coating was reported [Nakatani 2002]. The DPNR substrate is followed
by an oxidation catalyst, which removes HC emissions that may pass
through the NOx adsorber during regeneration. The engine is also
equipped with a fuel injector in the exhaust manifold. A number of
sensors are needed to provide feedback to the engine controller,
including gas temperature, DPNR pressure drop, intake air flow, and
A/F ratio sensors, as shown in the schematic.
Figure 15. Schematic of DPNR System
Parameters of the NOx adsorber operation varied in particular tests,
but have been generally reported around 60 s lean operation, followed
by 1 - 3 s rich pulse at A/F = 11.5 - 12.5 [Nakatani 2002]. Most
testing has been performed with fuel of 30 ppm sulfur. Adsorber
desulfation is performed at around stoichiometric condition over a
period of about 50 s and catalyst bed temperature of about 600¡C. H2S
emissions during desulfation are minimized by lean-rich switching.
The PM regeneration in the filter is controlled through the A/F ratio
which impacts the catalyst temperature through both the exhaust gas
temperature and the temperature increase in the catalyst due to HC
oxidation. The control of the A/F ratio involves three mechanisms, as
follows:
* "Low temperature combustion" (high EGR + injection timing + intake
air throttle)
* Post-injection of fuel to the engine cylinders
* Exhaust manifold injection of fuel
The combination of control mechanisms depends on the engine operating
condition, as well as on the state of the DPNR system. For instance,
increased temperatures are needed to force the PM oxidation if the
filter becomes overloaded with soot. The DPNR control strategies are
illustrated in Figure 16 [Fujimura 2002].
Figure 16. DPNR System Control Strategies
EI - exhaust manifold injection; PI - common rail post injection; LTC
- low temperature combustion
It was found that the rich pulsing enhances the oxidation of PM, thus
making it possible to reliably control the filter regeneration at
widely ranging operating conditions. A switch-flow version, in which
the direction of flow through the filter could be reversed by a
valve, was considered during early development but was later
abandoned. The PM regeneration enhancement relative to conventional
catalyzed filters is not entirely understood, but it may be
attributed to a combination of several mechanisms, as follows:
* Nitrogen dioxide, which is an intermediate product in the process
of lean NOx adsorption, Equation (1), is well known to enhance DPF
regeneration [Cooper 1989].
* Toyota researchers found that active oxygen species which are
formed during both NOx adsorption and rich regeneration are very
effective in soot oxidation [Nakatani 2002].
* The PM oxidation rates were found to be higher with "fresh" soot,
as it is the case with the continuously regenerating DPNR. Soot that
may accumulate to higher loadings in conventional filters exhibits
particle growth and collapsing of its micropore structure, both
effects leading to slower oxidation rates. Maintaining low soot
loadings also improves the contact between carbon particles and the
catalyst [Nakatani 2002].
According to Toyota, a fresh DPNR catalyst can reduce both NOx and PM
emissions by over 80%. An aged catalyst achieved 0.13 g/km NOx and
0.005 g/km PM over the ECE+EUDC cycle on a 1400 kg car [Fujimura
2002]. Introduction of the DPNR system on production vehicles is
expected in the near future.
Issues
Before adsorber-catalysts can become a wide-spread commercial NOx
control technology, more progress has to be made in the NOx/SOx
adsorber technology itself and, very importantly, in many areas
relevant to the system integration with the diesel engine. The
following is a summary of issues which need to be addressed:
* Methods of mixture enrichment need to be developed. Potential
problems include torque/drivability issues and high PM and HC
emissions during the enrichment.
* NOx adsorber regeneration and desulfation strategies have to be
developed. They will require a closed loop control in tight
integration with the engine management and other vehicle systems,
such as the on-board diagnostics.
* Optimized NOx adsorbers in terms of adsorption capacity, rich
regeneration temperatures, lean thermal stability, etc., are
necessary for efficient application of the technology.
* All of the above issues affect fuel economy penalty associated with
adsorber regeneration, which needs to be minimized
* Secondary unregulated emissions, including H2S and N2O, from
adsorber systems need to be minimized. Depending on the
regeneration/desulfation strategies, adsorber systems may also
produce off-cycle emissions of regulated pollutants.
* Thermal durability of catalysts and resistance to poisons has to be
demonstrated. The impact of phosphorus and zinc, known poisons of the
3-way catalyst, on NOx adsorbers is still unknown.
* NOx adsorbers will increase the cost of diesel emission control
systems, especially in the view of the rising cost of noble metals.
Diesel systems, especially heavy-duty, are quite large, which will
create additional usage demand and potentially a further increase in
cost.
Furthermore, the commercialization of this technology is only
possible in conjunction with legislation and widespread availability
of ultra low sulfur diesel fuels.
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