heat-integrated reactor concepts
TRANSCRIPT
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Heat-integrated reactor concepts for catalyticreforming and automotive exhaust purification
Grigorios Kolios 1, Achim Gritsch, Arstides Morillo, Ute Tuttlies,Jens Bernnat, Frank Opferkuch 2, Gerhart Eigenberger *
Institut fur Chemische Verfahrenstechnik, Universitat Stuttgart, Boblingerstr. 72, D 70199 Stuttgart, Germany
Available online 30 June 2006
Abstract
Optimal solutions in environmental catalysis require a well-coordinated development of catalysts and of process design. This contribution is
devoted to energy integrated design concepts for fuel reforming and for automotive exhaust purification. The examples presented demonstrate the
importance of an innovative process design for optimal utilization of existing catalysts and show the potential of future developments.
New concepts for steam reforming through the efficient coupling of the endothermic reforming reaction with an exothermic combustion
reaction are discussed in the first part. These concepts have been implemented for methanol steam reforming in a counter-current reactor with
distributed side feed of burner gas and for methane steam reforming in a modular reactor with a co-current reaction section for the endothermic and
the combustion reaction and attached counter-current heat exchangers. Both applications employ the so-called folded sheet reactor design, which
ensures an excellent heat transfer between the reforming and combustion channels and efficient heat recovery.
A similar design solution is introduced for the apparently different case of automotive exhaust purification. The proposed concept aims at
decoupling exhaust after-treatment from engine control. Its main component is a counter-current heat exchanger with integrated purification stages
for HC-oxidation, NOXstorage and reduction and soot filtering. A small catalytic burner at the hot end of the heat exchanger provides both heat and
oxidizing or reducing agents on demand. A new soot filter design allows for safe soot filter regeneration.
# 2006 Published by Elsevier B.V.
Keywords: Autothermal reactors; Heat-integrated reactors; Process integration; Steam reforming; Automotive exhaust purification; Diesel soot filtering
1. Introduction
Environmental catalysis aims at the efficient conversion of
raw materials into chemical products. This implies high yield of
the desired products, minimal noxious side products and high
energy efficiency. Particularly the last point requires a close
cooperation between catalyst development and process design.
In this contribution, the focus is set on process design concepts
for energy efficient catalytic processes in two rather differentapplication areas, namely for high temperature endothermic
reactions like steam reforming and for automotive exhaust
purification. In both applications, the aim is to provide optimal
temperatures for the required reactions as well as high thermal
efficiency. Several novel multifunctional heat-exchanger
reactor concepts will be discussed both through simulations
and experimental results. They accomplish optimal energy
utilization by recovering the heat from the treated hot stream to
heat up the cold feed.
The respective principle of autothermal reactors is well
established for weakly exothermic reactions like combustion of
pollutants in exhaust air [13]. Fig. 1 (top left) shows thegeneral concept with regenerative heat exchange, leading to the
so-called reverse-flow reactor. The reactants are periodically
fed to the fixed-bed reactor from opposite reactor ends. To start
the reaction, the active catalyst section needs to be heated up
above ignition temperature Tign of the reaction. The end
sections of the reactor can be inert and serve as regenerative
heat exchangers where the respective cold feed is heated up by
the hot packing and the hot gas leaving the reaction section is
cooled down. Fig. 1 (right) shows the typical temperature and
conversion profiles for a weakly exothermic reaction in an
www.elsevier.com/locate/apcatbApplied Catalysis B: Environmental 70 (2007) 1630
* Corresponding author. Tel.: +49 711 685 85257; fax: +49 711 685 85242.
E-mail address: [email protected] (G. Eigenberger).1 Present address: Christ AG, Hauptstr. 192, CH 4147 Aesch, Switzerland.2 Present address: Modine Europe GmbH, Modinestr. 1, D 70794 Filderstadt,
Germany.
0926-3373/$ see front matter # 2006 Published by Elsevier B.V.
doi:10.1016/j.apcatb.2006.01.030
mailto:[email protected]://dx.doi.org/10.1016/j.apcatb.2006.01.030http://dx.doi.org/10.1016/j.apcatb.2006.01.030mailto:[email protected] -
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adiabatic reactor in the ignited periodic steady state for the limit
of fast flow reversal. In case of catalytic combustion, the
reaction zone is usually confined to a small region of the
catalytic section where the reaction runs to completion. From
an overall heat balance for the adiabatic reactor it follows that
the difference between entrance and exit temperature and hence
the driving temperature difference for the counter-current heat
exchange is identical to the adiabatic temperature rise DTad of
the reaction mixture. Regenerative heat exchange allows to
increase the temperature in the center of the reactor by a value
DTmax of 1020 times the adiabatic temperature rise, dependingon the flow rate and the characteristics of the heat exchange.
This corresponds to a heat exchanger efficiency of
hHEX DTmax DTad
DTmax 9095%:
It can be shown [4] that the behavior of the reverse-flow reactor
in the limit of fast cycling is equivalent to a counter-current
fixed-bed reactor with indirect (recuperative) heat exchange
(Fig. 1, left: middle). Here, two gas streams of equal heat
capacity enter the reactor simultaneously at the opposite ends
and are separated through heat transferring walls. The two end
sections serve as counter-current heat exchangers. Since thebehavior in one flow direction is just the mirror image of that in
the other direction, a counter-current reactor for weakly
exothermic reactions can be simplified to a reactor with only
one common feed and effluent side if the gas flow is turned
around in the hot center of the reactor (Fig. 1, left: bottom).
Compared to the reverse flow reactor the counter-current
fixed-bed reactor has the advantage of continuous operation
without flow switches. However, it requires excellent heat
transfer between adjacent channels in order to compete with the
direct heat transfer between gas and packed bed in regenerative
heat exchangers. This requires narrow channels in the order of
1 mm and (preferably) catalytically coated walls for ensuring
direct transfer of the heat of combustion. Simple operation
control of counter-current reactors but a more complex design
make them suited for small to medium scale applications,
whereas reverse-flow reactors can easily be scaled up to large
units. The following discussion is restricted to recuperative
concepts with indirect heat exchange. However, it should be
mentioned that powerful regenerative concepts with similar
functional properties have been developed as well [57].
A suitable recuperative design uses parallel plate channels
with a channel width in the order of millimeters and a length in
therange of half a meter. A foldedsheet reactor designhas provenparticularly useful for this purpose [8]. It consists of a high-
temperature-resistant stainless steel foil of 0.20.4 mm thick-
ness, which is folded to form many parallel flow pockets and
fixed in a housing as shown in Fig. 2 (left). The folded sheet
separates two reaction compartments from each other. Feed/exit
ports at the two ends distribute/collect the gas streams into/from
the respective pockets. The thin folded sheet is supported by
spacers in order to keep the intended channel geometry. Different
spacer designs with improved mixing or heat transfer properties
can be used. Moreover, they canbe catalytically coated(cat 1 and
2). Corrugated spacers as shown in Fig. 2 (left) have a low
pressure drop, serve as (replaceable) catalyst carriers and
improve heat transfer through their close contact with the foldedsheet. Several counter-current reactors with heat exchange areas
up to 10 m2 have been built and successfully tested. One of such
reactors is shown in the middle ofFig. 2. It has an inert entrance
section with the active catalyst starting at 0.4 m. On the right-
hand side, measured and simulated temperature profiles are
shown for air flow with different propene feed concentrations.
Thetemperature drop after themain reaction zone at theentrance
of the active catalyst is due to heat losses.
Decisive for an efficient heat recovery in an autothermal
reactor is a proper design of the heat exchange sections. It
requires counter-current heat exchange with equal heat capacity
fluxes in both directions and low axial heat conductivity. This is
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 17
Fig. 1. Left: reverse flow reactor (top); counter-current fixed-bed reactor, with similar flow and heat transfer pattern as the reverse flow reactor (middle); counter-
current fixed-bed reactor, exploiting profile symmetry(bottom). Right: temperature (T) and conversion profiles (X) of the reverse flow reactor overthe reactor lengthz
in the cyclic steady state for fast flow reversal, similar to counter-current reactor profiles.
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exemplified in Fig. 3. In the middle the temperature profiles of
an optimally designed counter-current folded sheet heat
exchanger are given. A metal foil of 0.18 mm thickness isused; the channels of 1 mm width contain inert spacers. The
resulting axial Peclet number of 125 indicates that convective
heat transfer dominates the behavior of the device. It provides
an excellent heat exchange efficiency of 98%. If the same heat
exchanger is operated with twice the heat capacity flux
downstream as compared to upstream, the profiles of Fig. 3
(left) result. Obviously, the smaller hot stream can heat up the
larger cold stream only to half the total temperature difference,
reducing the heat exchange efficiency from 98 to 67%.
Considering a micro-heat exchanger with 0.1 mm channel
diameter, the same heat transfer area as above requires only one
tenth of the original length but the wall thickness (0.18 mm)will stay the same. Now the axial Peclet number is reduced to 2,
indicating that axial heat conduction through the channel walls
dominates over convection. This changes the shape of the
resulting temperature profiles as shown in Fig. 3 (right) and
reduces the heat recovery to 65%. This shows that micro-
reactors with micro heat exchange sections usually do not
provide sufficient heat recovery, in spite of their excellent heat
transfer properties.
2. Coupling of endothermic and exothermic reactions
Efficient energy integration is even more important for
endothermic high temperature reactions as compared with
weakly exothermic reactions. Here, the additional requirement is
to efficiently supply the heat of reaction, e.g., by an exothermic
reaction (usually a waste-gas combustion), in order to achieve an
overall slightly exothermic process. Then the above-mentioned
autothermal concepts should be in principle applicable.
However, it turns out that major problems may result from axial
separation of the two reactionzones, resultingin excessively high
combustion temperatures. This is already the case in large-scalesteam reformers equipped with packed-bed reformer tubes of
several centimeters internal diameter and natural gas burners at
the side or top walls [9]. Chemical equilibrium requires exit
reformate temperatures closeto 900 8C for sufficient equilibrium
conversion. The burners provide the heat of reaction with flame
temperatures in the order of 2000 8C. This heat is transferred to
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163018
Fig. 2. Counter-current heat exchanger reactor for waste air purification through catalytic combustion. Left: folded sheet reactor model with corrugated spacers as
catalyst carriers (cat 1 and 2). Middle: design sketch of a respective reactor. Right: measured and simulated temperature profiles for different propene feed
concentrations in air.
Fig. 3. Temperature profiles in counter-current heat transfer for a folded sheet heat exchanger with 0.18 mm wall thickness. Left and middle: channel width of 1 mm
and channel length 25 cm, with heat capacity flow ratio of 1 (middle), and 0.5 (left). Right: micro-heat exchanger with 100 mm channel diameter and 2.5 cm channel
length.
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the reforming catalyst through radiation and convection. The
whole process is strongly heat transfer limited, since the
reforming catalyst is very active in the mentioned temperature
range. Only about 50% of the heat of combustion can be
transferred to thereforming reaction.The rest hasto be recovered
through a complex network of heat exchangers. This is the
reason, why such reformers are only effective and profitable, if
they are integrated into the heat transfer network of a large
chemical production site.
Recent developments aim at a higher energy integration and
efficiency. As an example a sketch of the heat-integrated Haldor
Topsoe Convection Reformer is shown in Fig. 4. The tube-in-
tube reformer allows for internal backflow of the produced
syngas using the sensible heat of the process gas to heat up the
cold feed. In addition, the heat of the burner flue gas is taken-up.
This may increase the thermal efficiency to 7080%. Never-
theless, the reformer reaction remains strongly heat transfer
limited and large temperature differences are required to transfer
the required heat of combustion to the reforming catalyst pellets.
For optimal heat integration it would be intriguing to couplethe flow of the reforming gas counter-currently with that of the
combustion gas and to adjust the flow rates such that about
equal heat capacity flows run in both directions. Such a design
was studied for the case of methane steam reforming combined
with methane combustion in [10] using the flow scheme as
shown in Fig. 5 (top). The simulation results shown in Fig. 5
represent the behavior of a ceramic monolith reactor of 2 mm
channel diameter where the channels are divided in a chess
board-like pattern to the combustion and the reforming gas. The
bars in the two lower diagrams of Fig. 5 show the extension of
the respective combustion (bottom) and reforming catalysts
(above), which were assumed to be coated at the monolith
walls.
The results represent an optimal configuration with respect
to the flow rates on combustion and reforming side as well as
catalyst distribution. Nevertheless, the maximum temperature
exceeds 1500 8C. This is the result of the axial separation of the
exothermic and endothermic reaction as indicated by the two
methane conversion profiles XCH4 in Fig. 5. A more detailed
analysis [11] shows that under pure counter-current operation it
is indeed impossible to achieve the desired overlapping of the
reaction zones, if full conversion of the reforming reaction is
required.
One way to enforce overlapping is a distribution of one feed
of the combustion reaction (air or fuel) over the length of theactive catalyst zone. This is shown in Fig. 6, again for the case
of methane steam reforming. A uniform burner gas distribution
at sufficient temperatures results in a uniform heat supply,
which nicely corresponds to the heat request Qendo of the
reforming reaction and leads to its almost linear conversion
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 19
Fig. 4. Sketch of the Haldor Topsoe Convection Reformer with improved heat recovery as compared to standard reformer design.
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profile. Details of an appropriate design of the combustion
relative to the reforming zone are given in [11,12].
A counter-current fixed-bed reactor with distributed burner
gas feed has been realized experimentally for methanol steam
reforming with a production capacity of hydrogen with 10 kW
lower heating value (LHV) [13,14]. In addition to the reforming
stage, the reactor contains a section for the evaporation of the
methanol/water feed mixture and a water-gas shift section. The
reactor design, based on the folded sheet concept, is shown in
Fig. 7. On the combustion side air flows from top to bottom. The
anode off-gas from a fuel cell stack or the purge gas of a PSA
hydrogen purification unit is used as a fuel and is distributed
individually to every pocket of the burner side at five levels.
Fig. 7(a) shows the folded sheet concept, Fig. 7(b) the locations
of the combustion catalyst sections, the reforming catalyst and
the water-gas shift catalyst section. Fig. 7(c) shows a picture of
the reactor from the burner gas side, Fig. 7(d) shows one fuel
gas distributor element during a test run. The flamelets illustrate
the achieved uniformity of gas distribution into the combustion
side pockets.
Fig. 8 shows simulated temperature and conversion profiles.
These predictions have been completely confirmed by the
experiments, yielding methanol conversions higher than 90%
and CO concentrations below 1.6%. A remarkable feature ofthe reactor is the fast dynamic response upon load changes as
shown in the bottom graph of Fig. 8.
In spite of its successful implementation for methanol steam
reforming the filigree side feed distributor was considered too
sensitive to be used at the substantially higher temperatures
occurring during methane steam reforming. Instead, a modular
reactor concept was conceived where the heat exchange
between the reforming and the combustion reaction takes place
in co-current mode. As shown in Fig. 9 (top), this concept has
the additional advantage that the reforming product gas
exchanges heat with its feed in the left heat exchange section
while the combustion gas does the same in the right heatexchanger. This allows choosing the throughput on the
reforming and the combustion side independently from each
other, yielding improved design flexibility.
Fig. 9 shows simulated temperature profiles for the coupling
of methane steam reforming and methane combustion in a
catalytic wall reactor (left) and a packed-bed catalytic reactor
(right). As discussed before (Fig. 2), the temperature profiles in
the counter-current heatexchangesections are linear due to equal
heat capacity fluxes, and are therefore not shown. The discussion
focuses on the reaction section. In both cases the same specific
heatexchange area has beenassumed. Nevertheless, the behavior
of the two configurations is completely different. In the catalytic
wall reactor combustion and reforming catalyst is deposited onopposite sides of the same wall, leading to an excellent heat
transfer with almost the same temperatures for the two catalysts.
This allows for the overlapping of the combustion and reforming
conversion profile and leads to a smooth temperature profile
where the rapid reforming reaction even tends to quench the
combustion. The packed-bed reactor on the other hand features
run-away of the combustion reaction as shown by the severe
temperature excursion at theinlet cross-section. This is dueto the
substantial heat transfer resistance between the catalytic
packings in both channels. Systematic parametric studies with
the above model revealed a considerable sensitivity with
respect to the kinetics of the combustion reaction [24].
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163020
Fig. 5. Temperature (T) and methane conversion profiles XCH4 for methanesteam reforming and methane combustion in a counter-current monolith rector
of 2 mm channel diameter [10]. The location of the respective catalysts is given
by the bars in the conversion profiles.
Fig. 6. Counter-current fixed-bed reactor for methane steam reforming coupled
with methane combustion and evenly distributed methane side feed [12]. The
color bars in thetop drawing mark thelocationof thereforming andcombustion
catalysts. Xrefand Qendo are the methane conversion and the heat requirement of
the reforming reaction. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of the article.)
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In the above simulations simple kinetic expressions have
been adopted for catalytic and homogeneous methane
combustion [15,16]. In reality, the combustion reaction depends
in a complex way on the gas composition and the flow and back
mixing characteristics. To study this behavior in the small
channels considered, a series of combustion experiments have
been performed in a single channel device as shown in Fig. 10.
If premixed fuel gas and air enter the reactor, back-ignition mayoccur up to the mixing point. Therefore, mixing of fuel gas and
air took place at the entrance of the hot combustion channel as
shown in Fig. 10 (top right). Mixing was enhanced by a static
mixer before entering the catalyst section. The photos ofFig. 10
(middle and bottom) show the heat evolution via the color
patterns (brightness) of the glowing metal housing. With
hydrogen in the feed (Fig. 10, middle left) a back-ignition up to
the mixing point with homogeneous combustion always
resulted. Even with pure methane the danger of back-ignition
into homogeneous combustion was big, if the flow velocity was
low and the methane concentration was high (Fig. 10, bottom
left vs. middle right). However, if CO2 or water vapor was
added as a mediator and radical scavenger, an extendedcombustion zone without homogeneous pre-ignition could be
established. It was shown recently [17,18] that a pre-ignition of
hydrogen containing combustion gas could be safely prevented,
if the catalyst coated channel dimensions are in the range of
100 mm. This would, however, lead to a micro-reactor design
with the restrictions mentioned in connection with Fig. 2.
Based upon the above results and additional considerations
and experiments reported in [19,24], the prototype of a methane
steam reformer for a production capacity of 5 m3/h (STP)
hydrogen (14 kW LHV) has been designed and set-up. Fig. 11
shows a sketch of the set-up along with simulations on its
operating behavior. An additional fuel injection port has been
provided in the catalytic section. According to the design
calculations the specific productivity of the reactor stage is
estimated to 7 Nm3 H2/(l h) (STP) and the thermal efficiency of
the system is expected to be as high as 90%. Meanwhile,
experimental results have fully confirmed the design simula-
tions, showing full conversion for a wide load range and a fast
and smooth response to load changes [24].
Summarizing Section 2, it has been shown that close heatintegration between catalytic reforming and catalytic combus-
tion allows for the design of compact, small-scale reformers for
decentralized hydrogen production for fuel cells with excellent
fuel economy and a rapid start-up and load-change behavior.
The so-called folded sheet reactor concept proved to be both
sufficiently simple and efficient for this purpose.
3. Automotive exhaust purification
The problems connected with automotive exhaust purifica-
tion seem to be substantially different from the problems and
solutions discussed previously. However, it will be shown that
several of the above concepts may be helpful in futureautomotive exhaust purification systems as their requirements
are dictated by more and more stringent emission regulations.
First, the present state and problems of automotive exhaust
purification for passenger cars with combustion engines will be
briefly reviewed. Table 1 gives a respective summary. Only the
three main types of combustion engines are considered here.
The three-way catalyst is a well-established standard for l-
controlled Otto-engines. The task of NOX-reduction and soot
filtering in diesel cars is much more challenging.
It is obvious from Table 1 that the different purification steps
require different operation temperatures and exhaust composi-
tions. The present dogma of automotive exhaust purification is
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 21
Fig. 7. A 10 kW methanol steam reformer, integrating feed evaporation (Evap.), reforming (Ref.) and water-gas shift reaction (WGS) on side 2 with distributed
hydrogen combustion on side 1 in a folded sheet counter-current reactor design [13,14]. (a) Folded sheet concept and (b) burner gas feed and distribution ofcombustioncatalyst (left) with respect to thesections forevaporation, reformingand water-gasshift. (c) Burner gassideof thereactor and(d) test runof one of thefive
gas distributors to verify uniform burner gas distribution into all pockets of the burner gas side.
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to provide these conditions through elaborate computer control
of the engine combustion conditions. But it becomes more and
more obvious, that this tends to pose insurmountable difficulties
particularly in view of the more stringent future emission limits.
3.1. Three-way catalyst with l-control
This well established technology is based upon the
observation that all three main noxious exhaust components
of Otto-engines, CO, unburned hydrocarbons (HC) and nitrous
oxides (NOX) can be simultaneously converted to CO2, water
and N2 on a noble metal catalyst, if the oxygen content of the
exhaust gas (the l-value) is rigorously controlled to
stoichiometric conditions (l = 1). Since, in practice, the l-
value oscillates around l = 1 with a considerable amplitude, an
oxygen storage component is an essential ingredient of current
three-way exhaust catalysts.
One yet unsolved problem concerns the cold start emissions,
since the catalyst has to be heated up to its operating
temperature through the engine exhaust. Cold start emissions
are currently minimized by placing a pre-catalyst as close as
possible to the engine and the main catalyst further down-
stream. The fuel-rich hot exhaust gases enable a rapid ignition
of the pre-catalyst and in sequence of the main catalyst.
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163022
Fig. 8. Simulated profiles of the 10 kW methanol reformer under design conditions over the length of the reforming section for the reforming side (left) and the
combustion side(right). qloc gives the local heat input through the combustion reaction (positive values) and the heat demand of the reformingreaction (negative). The
bottom graph shows experimental results of the reformate flow produced (also given in kW thermal power) upon step changes of the liquid feed.
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A second problem is connected with engine operation under
full power. Then, the exhaust temperature may exceed 1000 8C
which thermally deactivates the catalyst. This is why the main
catalyst is placed further downstream and occasionally
provided with optional cooling of the exhaust line. A third
problem may results from HC peak emissions, causing
temporary temperature excursions at the catalyst entrance.
Summarizing, three-way catalysts are well established but
require a rigorous design and an efficient operation manage-
ment through engine control.
3.2. NOX storage catalysts
Lean-burn Otto-engines or diesel engines require a NOXreduction technology, dedicated for operating under oxygen
excess. In the following, only the so-called NOX storage
catalyst will be considered, since it is the presently favored
solution for passenger cars. In lean-burn Otto-engines the
storage catalyst is usually positioned downstream a conven-
tional three-way catalyst, since in a number of situations (like
cold start and full load) these engines also run under
stoichiometric conditions (l = 1). Under fuel lean conditions
the exhaust-NOX is incorporated into the storage material in
form of nitrates until the storage capacity is exhausted. In
periodic intervals a short regeneration step under fuel rich
conditions is applied, in which NOX is released and reduced to
nitrogen and CO2 or water.
The detailed analysis and appropriate modeling of NOXstorage catalysts is a topic of ongoing research [20], which shall
not be discussed here. It is clear that operation of the NOX-
storage catalyst requires additional sensors and a complex
control strategy which is presently accomplished entirely
through engine control. Accordingly, engine design and control
are more and more adjusted to emission regulations rather than
with respect to optimal performance and efficiency.
3.3. Diesel soot filtering
In addition to NOX removal, diesel engines will soon require
soot filtering, since improvements in the engine combustion
process will no longer suffice to meet the upcoming fine particle
emission limits. Presently, several filter techniques are under
development including fiber and sinter-metal filters. The most
commonly applied soot filters, however, consist of ceramic
(cordierite or SiC) monoliths, where every second channel is
closed on either side. Fig. 12 shows the basic design. Soot filters
are regenerated periodically by burning-off the soot deposited.
The major problem in particulate filter regeneration is the
mismatch between the exhaust gas temperature and the required
regeneration temperature. Diesel exhaust is seldom hotter than
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 23
Fig. 9. Co-/counter-current reactor concept for methane steam reforming (top). Co-current reaction section profiles for (left) a catalytic wall reactor with equilibrium
controlled reforming reaction and right for a fixed-bed reactor with reasonable values for the reforming kinetics; both cases with equal specific heat transfer area.
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350 8C, in city stop-and-go traffic often below 200 8C.
Generally, diesel soot requires ignition temperatures above
600 8C in air. Therefore, several concepts have been proposed
for reducing the soot ignition temperature. If the exhaust
contains reasonable amounts of NO2, the ignition temperatureis reduced to about 350400 8C. This fact is exploited in the so-
called CRT (continuous regeneration trap) design, where the
engine is tuned for producing large amounts of NO, which is
oxidized to NO2 over a noble metal catalyst prior to or at the
soot filter. It has to be noted, however, that during CRT soot
combustion NO2 is only reduced to NO and a subsequent De-NOX stage is still required.
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163024
Fig. 10. Burner channel experiments. Top: sketch of the experimental set-up. Middle and bottom: photos of the glowing patterns at the conditions indicated.
Fig. 11. Designand simulation results of a co-/counter-current methane steam reformer forthe production of 5 m3 STP hydrogen/h. Left: flowconfiguration (top) and
simulated temperature and conversion profiles. Right: design sketch and flow structure of the assembly.
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Using catalytically impregnated soot filters for reducing the
ignition temperature is not very effective due to the poor contact
between the soot and the catalyst particles. More efficient is the
addition of a catalyst precursor to the diesel fuel which ensuresthat the catalyst is finely dispersed in the soot particles (PSA
system). This reduces the soot ignition temperature to about
450 8C but the resulting catalyst dust gradually blocks the filter
and requires its early replacement.
In any case, soot ignition requires increased fuel consump-
tion to produce a sufficiently hot exhaust gas for an extended
period of time. In addition, soot combustion may either die out
after ignition, leading to incomplete soot removal, or run away
into excessive combustion temperatures, which may destroy the
filter and occasionally burn down the whole car. These are
reasons why the car industry is still somewhat reluctant to
introduce soot filters as equipment standard.The problematic of filter regeneration shall be discussed
with a simplified model case. Fig. 13 shows simulation results
of the regeneration behavior of a uniformly loaded soot filter.
The soot combustion kinetics has been obtained with old
almost graphitized soot [21]. This is the reason why the ignition
temperature in the simulation is about 100200 K higher than in
reality. Soot removal is initiated through increasing the feed
temperature. At 700 8C feed temperature the soot combustion
takes place uniformly over the entire filter volume causing a
moderate temporary temperature rise towards its rear end.
Approximately 10 min are necessary for complete soot
removal. Increasing the feed temperature to 900 8C, the
ignition behavior changes substantially. Now a thermalcombustion front develops where the peak temperature
increases exponentially and most of the soot is burnt off
within 30 s. The filter peak temperature would locally exceed
1300 8C, which is near the melting temperature of cordierite.
The damage potential of runaway of gassolid combustion
reactions has been confirmed experimentally both in engine andcar tests. Typical damages result in destruction of soot filters
due to thermal stress (breakage) or even melting of the filter.
Fig. 13 (right) shows a comparable damage of a partially molten
diesel oxi-cat made of cordierite. It is normally placed in front
of the soot filter and creates the required ignition temperature
through combustion of fuel, injected in the exhaust pipe.
The reason for thermal runaway during soot filter
regeneration can be explained based on a simplified wave
model [22,23]. The velocity WT of a pure thermal wave in a
packed bed is described by the well-known relation:
WT
VZerGcPG
erGcPG 1 erScS ;
e being the void fraction of the bed, rGcPG and rScS the heat
capacities of gas and packing, respectively, and VZ the gas
interstitial velocity.
The propagation velocity of a combustion zone is given from
an integral mass balance assuming total combustion:
WR VZecO2qB
;
cO2 being the oxygen feed concentration and qB is the initial
soot loading of the packed-bed (in mol C/m3). At low oxygen
concentration or high soot loading the combustion zone will lagbehind the temperature wave. Hence, the heat of combustion
will be carried downstream, limiting the maximum temperature
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 25
Table 1
Types of exhaust pollutants (potential) solutions and required purification temperature range for different types of passenger car engines
Passenger car engines and exhaust temperatures Types of pollutants and (potential) solutions Required purification
temperatures (8C)
Conventional Otto engines: air/fuel
ratio l around 1 (3001000 8C)
Three types of pollutants (CO, HC, NOX)
l engine control and three-way catalyst 300700
Lean Otto engines: l) 1 at low power (2004508
C),l 1 at max. power (3001100 8C) Pollutants: NOX, CO, HCNOX storage catalyst 250450
Three-way catalyst 300700
NOX storage catalyst, sulphur regeneration 750
Diesel engines: l) 1 (150350 8C)(up to 700 8C at high load)
Main pollutants: soot, NOX, HC
Soot trap regeneration 450900
NOX storage catalyst 250450
NOX storage catalyst sulphur regeneration 750
Diesel oxidation catalyst 250700
Fig. 12. Ceramic soot filter monolith. Left: photo, right: sketch of the flow through the channel walls.
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of the packing. If, conversely, the oxygen concentration is high
or the soot loading is low, the combustion zone will rash in front
of the thermal wave. Again, the heat of combustion will be
distributed over a larger portion of the packed-bed. However, in
the limiting case of equal propagation velocities of combustion
and thermal front, the heat of reaction accumulates within the
front, leading to excessive temperatures.
The above considerations can be summarized in two simplerelations for the maximum temperature increase in the
combustion front:
DTF WR
WR WTqBDhR
rscsforWR >WT and
DTF WT
WT WRcO2DhR
erGcpGforWT >WR;
where DhR is the enthalpy of the combustion reaction. Fig. 14
shows the graph ofDTF for a given value ofWT. For WR = WT
the relation features a singularity, where the simplified model
predicts an infinite front temperature. Moderate front tempera-
tures are only possible for WR) WT, implying low soot load-ings and high oxygen concentration (the limit is the adiabatic
temperature rise with respect to the soot loading, DTSad qBDhR=rScS or for W
R $ 0, implying low oxygen con-centration and high soot loadings (with the limit of adiabatic
temperature rise with respect to the gas concentration
DTGad cO2DhR=erGcpG.Interestingly, the model implies that lower front tempera-
tures would occur for negative values of WR. Accordingly,
initiating soot regeneration at the rear end of the filter (back-
ignition) and propagation of the regeneration zone in counter
flow direction would result in an inherently save operation. This
conclusion will be taken up in Section 4.2.
If the necessity to add a total oxidation catalyst and a NOXconversion device to the exhaust train is taken into account, the
diesel exhaust purification is even more complicated and
challenging than the examples discussed before. Summarizing,
forthcoming engine exhaust purification regulations make it
more and more difficult to fulfill the legal requirements with
add-on solutions and elaborate engine control measures without
sacrificing engines performance and fuel economy.A new conceptual approach seems to be necessary where
exhaust purification should be decoupled from engine control.
Integrated reactor concepts, developed in chemical engineering
during the last decades, in combination with a targeted
development and improvement of appropriate catalyst systems,
could provide appropriate solutions. Such an approach will be
outlined in the next section.
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163026
Fig. 13. Simulation results for the regeneration of a uniformly loaded soot filter at two different feed temperatures, using soot combustion kinetics [21] of old
(graphitized) soot (left) and photo of a diesel oxidation catalyst, positioned in front of the soot filter and molten during the initiation of the filter regeneration.
Fig. 14. Combustion front temperature increase over reaction front velocity for
decoking of fixed beds or for soot filter regeneration.
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4. A new, integrated engine emission control concept
The requirements for an optimized engine emission control
concept which overcomes the present add-on solutions can be
specified as follows:
lower total energy consumption as presently, equal or reduced pressure loss of the exhaust train as
presently,
no additional fuels or additives for De-NOX or soot filterregeneration,
thermally safe soot filter regeneration concept, lower cost and space requirements than present add-on
solutions.
Following, it will be demonstrated that the above goals are
indeed attainable. The basic idea is to combine all emission
relevant components into one unit which may even contain the
muffler and devices for heating the passenger compartment.
The key element of the proposed solution is again an optimalenergy integration concept, comprising of an efficient counter-
current heat exchanger reactor. It will minimize the additional
heat consumption, if the engine exhaust is too cold and may
also be used to protect the exhaust catalysts from excessive
temperatures if the exhaust is too hot. The purification stages
requiring strongly differing temperature levels can be
incorporated at appropriate positions in the reactor.
4.1. An example for diesel exhaust purification
Fig. 15 shows the proposed concept for the case of diesel
exhaust purification. The engine exhaust gas enters the counter-current heat exchanger section axially, is led through the up-
flow channels to the top, turned around into the down-flow
channels and leaves the device laterally close to the inlet. Heat
can be added into the top chamber via an electrically ignited
catalytic burner, which is fed with the engine fuel and air (or, in
case of lean burn engines, with oxygen containing exhaust). It is
used for starting-up and maintaining the required temperature
level. The catalytic burner can be operated under fuel lean
conditions as a conventional catalytic burner, or under fuel-rich
conditions, if generation of reducing agents (CO, H2) is
required. The purification stages are integrated in the heat
exchanger channels at positions which correspond with their
optimal operation temperature window. A suitable set-up for
diesel exhaust purification consists of a NO oxidation catalyst
in the upstream channels with a soot filter at the top in order to
utilize the CRT effect. In the down-flow channels a NOXstorage
catalyst can follow, which may be regenerated either through
engine control or, preferably, through the CO/H2-mixture
created in the catalytic burner under partial oxidation
conditions. An oxidation catalyst with oxygen storage capacity
may follow to prevent CO-leakage.
A modification for lean-burn Otto-engines is quite straight-forward. Here the oxi-cat in the entrance channels may be
replaced by a three-way cat and the soot filter can be omitted. In
a further step, it would also be possible to integrate muffler
components into the unit and to use (part of) the hot exhaust at
the units top for heating of the passenger compartment, even
without engine running.
For the heat exchanger section the parallel metal plate or
folded sheet reactor concept discussed in Sections 1 and 2 can
be used. A recent modification allows for an axial inflow with
low pressure drop and prevents blocking through particulates
deposition which could be a problem in alternative designs
with flow deflection at the entrance. In addition, appropriatecatalysts can be placed at different positions in the inflow
and outflow channels if they are deposited on the spacers.
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 27
Fig. 15. Sketch of the proposed integrated emission control concept. Left: detail of the counter-current parallel plate design; middle: cross-sectional cut through the
up- and down-flow channels; right: catalyst placement in the down-flow channels.
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The design is comparatively light weight because of the thin
metal walls and provides an excellent heat recovery. This
allows maintaining the optimal catalyst temperatures with a
minimum of additional fuel consumption in case the exhaust
is too cold. In case the exhaust is too hot, the catalysts are
prevented from overheating through cold air injection at the
top. Due to the optimal temperature control a considerably
reduced catalyst volume and an extended catalyst lifetime can
be expected (present exhaust catalysts are largely oversized
for normal operation conditions to provide sufficient capacity
for low temperature operation and catalyst deactivation).
Concerning space requirements, the exhaust catalyst needs
no longer be squeezed into the engine compartment, but can be
placed under the car floor since its temperature is controlled
independently from the engine exhaust.
4.2. Soot filtering options
The new design concept also opens new alternatives for soot
filtering and its regeneration. As explained in Fig. 15, the sootfilter will be positioned at the top, either before the exit of the
entrance channels or at the beginning of the exit channels. The
latter option has the advantage to also prevent possible soot
emissions from the catalytic burner. The filter temperature can
either be maintained continuously at a temperature sufficient
for a continuous combustion of the soot deposited (above
350 8C if the CRT-effect of NO2 oxidation is exploited). This
option has been examined in detail in [21]. The necessary fuel
input through the catalytic burner is small, if the heat exchanger
is as effective as the designs discussed in Sections 1 and 2.
Then, under steady state conditions, an adiabatic temperature
rise of 50 8C at the top of the unit increases the filter
temperature by about 300 8C.
The second option is the conventional periodic regeneration
of the filter once the soot loading increased the filter pressure
drop beyond an acceptable threshold. The filter is attached to
the exit channels at the top as shown in Fig. 16 (left). This
enforces a counter-current flow at the feed and the permeate
side of the filter. The internal feedback of counter-current heat
exchange leads to a substantially different regeneration
characteristic as compared with the behavior described in
Figs. 12 and 13. Fig. 16 (right) shows the simulation of the
regeneration process. If initiating regeneration through
increasing filter feed temperature to 900 8C a combustion
front is rapidly carried into the filter, but the maximum
temperature stays at a moderate level except for the final stage(t> 120 s), when it rises to about 1500 8C (Fig. 16, top right).This is obviously a much more favorable behavior than
observed in Fig. 13.
In addition, the new design provides the option for initiating
a safer regeneration procedure by raising the rear end
temperature with the catalytic burner. Fig. 16(c) shows the
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 163028
Fig. 16. Integratedsoot filter: (a) flowstructure and (b) regeneration profiles for ignitionvia exhaust gas temperature increase and (c) regeneration profiles for ignition
via burner gas temperature increase. In case (c) the shaded soot loading is not burnt off.
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simulated temperature and soot loading profiles for rear end
ignition. The temperature overshoot completely disappears.
However, soot combustion dies out, being quenched by the cold
feed. A steady state is reached where approximately 60% of the
filter area is free of soot. However, this mode is susceptible to
blocking the entrance cross section. A reasonable alternative
would therefore consist in combining the advantages of both
regeneration strategies shown in Fig. 16. This can be achieved, if
the catalytic burner during soot filter regeneration is operated in
the partial oxidation mode, producing a CO- and H2-containing
gas, which is oxidized with the lean engine exhaust at a catalyst
positioned somewhere in the middle of the downstream filter
channels. The heat of combustion will then be released at an
optimal location in the filter itself, causing a milder soot
combustion without substantial temperature overshoots.
These examples indicate, that the integrated exhaust
purification concept offers a multitude of options for optimizing
automotive exhaust purification. Besides improved controll-
ability of the process it provides favorable, well-defined
conditions to different catalytic stages. This should enablecatalyst developers to optimize catalysts with respect to their
specific purpose, free of secondary constraints imposed by
inappropriate operating conditions.
Presently, a prototype of the heat-integrated after-treatment
unit is under test (Fig. 17). In this prototype, the NOX storage
catalyst and the catalytic burner are not yet included. The unit is
suited for the exhaust treatment of a 2.2 l diesel engine.
5. Summary and conclusions
In the first part of this contribution several new concepts for
the efficient coupling of endothermic and exothermic reactions
have been discussed both through simulations and experimental
results. The examples included methanol steam reforming in a
counter-current flow reactor with distributed side feed for the
combustion reaction and methane steam reforming in a co-
current reaction section with counter-current heat exchangers
attached. Both concepts require an excellent heat transfer
between the reforming and combustion channels, leading to a
small channel width but sufficient channel length for a good
heat recovery. The so-called folded sheet reactor concept
proved to be both sufficiently simple and efficient for this
purpose. It is well suited for compact, small-scale reformers for
decentralized hydrogen production for fuel cells, since it shows
a rapid start-up and load change behavior.
The second part has been devoted to automotive exhaust
purification. In automotive exhaust purification three-way
catalysts for l-controlled engines are a well-established
standard. Lean-burn Otto as well as diesel engines, however,
require new concepts, in which different purification steps have
to be combined. They include total oxidation of combustibles,
NOX reduction and soot filtering with subsequent sootcombustion. For NOX reduction the presently favored option
for passenger cars is the NOX-storage catalyst which requires a
frequent regeneration under fuel rich (reducing) conditions and
an occasional high temperature desulphurization. Since the
above steps have their favorable operation windows at strongly
different temperatures, it becomes more and more difficult to
operate the purification system through engine control.
Therefore, a new approach to automotive exhaust purifica-
tion has been proposed, where the exhaust purification unit can
be operated independently from engine control. Its main
ingredient is an efficient heat exchange section for which the
design concepts from the first part of the paper can be used.
G. Kolios et al. / Applied Catalysis B: Environmental 70 (2007) 1630 29
Fig. 17. Designsketch andphoto of a soot filter/heat exchanger. The sketchis a cutthroughthe up-goingchannels (left side) andthe down-goingchannels (right side).
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The unit contains a counter-current heat exchanger, where the
different components are situated at positions at which the
necessary operation temperatures can be provided. A small
catalytic fuel burner at the hot end of the heat exchanger can be
controlled independently to provide the required temperature
level and (reducing or oxidizing) gas composition. An
integrated new soot filter design allows for a safe regeneration
without excessive temperatures. This design will enable to
operate the different exhaust purification catalysts under well
defined, optimal temperature conditions and will, therefore,
facilitate exhaust catalyst development.
Acknowledgments
Support of the first parts of this work through the Deutsche
Forschungsgemeinschaft and through Adam Opel AG is
gratefully acknowledged.
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