Deliverable Report
INGAS
Grant agreement N°
218447
Project acronym
Project title
INGAS
Integrated GAS powertrain – Low
emissions, CO2 optimised and efficient
CNG engines for passengers cars (PC) and
light duty vehicles (LDV)
Instrument
Theme
Integrated Project
SST – 2007 – RTD 1
“Sustainable Surface Transport”
Start date of project
Duration
01.10.2008
36 Months
IP Co-ordinator
IP Project manager
Massimo Ferrera, CRF
Stefania Zandiri, CRF
Subproject
Sub-project Co-ordinator
SPB2
Michel Weibel
Deliverable
CH4/NOx operation strategy
D.B2.8
Due date of deliverable
Actual submission date
31/03/2010
02/12/2010
Organisation name of lead contractor
for this deliverable
Michel Weibel
Daimler AG
Report status
Consortium confidential
Revision version
1.0
Deliverable Report
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Contract N.:218447
Revisions table
Version
Date
1.0
02.12.2010 First version
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Reason
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Table of contents
Executive summary .................................................................................................... 4
1 Research activities .............................................................................................. 6
1.1
Introduction NOx/CH4 strategies ........................................................................... 6
1.1.1
NOx abatement technologies ........................................................................ 6
1.1.2
Methane catalyst reactivity ............................................................................. 7
1.2
Experimental set-up ................................................................................................ 9
1.3
Strategies for NOx abatement under lean conditions ...................................... 13
1.3.1
Laboratory ....................................................................................................... 13
1.3.2
Engine test bench .......................................................................................... 14
1.4
Strategies for CH4 abatement under stoichiometric conditions ...................... 17
1.4.1
Steady state conditions................................................................................. 17
1.4.2
Transient conditions – Lambda-sweep ...................................................... 18
2 Conclusions ....................................................................................................... 23
3 References ........................................................................................................ 24
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Executive summary
Sub-Project B2 deals with the development of an aftertreatment technology for
natural gas vehicles, with special focus on the reduction of methane emissions under
stoichiometric and lean conditions. An assessment of the potential of a lean-NOx trap
to remove NOx under lean conditions is also considered. In WPB2.2 one objective
consists in the development of control strategies for improving the methane
conversion under stoichiometric conditions and for demonstrating the suitability of a
NOx storage catalyst for lean application.
For improving the light-off temperature and the activity of methane catalysts, not only
the catalyst formulation is of relevance but also the surface state of the catalyst after
synthesis, preconditioning and under operating conditions. Therefore the operation
mode of the engine plays an important role in the stabilisation and the activation of
the catalytic surface.
Indeed, the nature of active sites in Pd-based catalysts for methane oxidation has
been a matter of debate for a long time, the key question being: what state of the
catalyst is more active: metallic or oxidized? A short literature survey came to the
conclusion that the best catalytic surface is represented by a cohabitation of oxidized
and reduced Pd-particles.
Under steady state conditions, it appears that for each operating temperature, a
slightly rich average lambda leads to the best CH4 conversion. But a continuous
operation under slightly rich conditions would certainly also lead to a slow
deactivation of the catalyst and is in term of fuel consumption not the best strategy.
The best conversion performances were obtained by operating the catalyst under a
-sweep regime, obtained by periodically switching the feed from slightly rich
(=0.98) to slightly lean (=1.02) conditions. Applying this control strategy, a
regeneration of the catalyst previously deactivated under both stoichiometric and
slightly lean conditions was observed. Widely, but regularly oscillating CH4
conversions occurred with conversion minima and maxima appearing in the lean
period and in the rich period respectively, the latter ones being associated with the
presence of significant amounts of H2 and CO. Such a behaviour is likely associated
with reversible transformation of palladium from the oxide to the metallic state which
may result in an higher activity associated with an average optimal oxidation state of
palladium. Such a control strategy can easily be implemented on a lambda=1 engine
operating naturally around the stoichiometric point. This strategy will be implemented
on the engine in WPB2.4 and further optimized with regard to amplitude and
frequency in the lambda oscillation.
Concerning the removal of NOx under lean operation conditions it has been
demonstrated that a standard NOx storage catalyst is suitable for CNG engine
application. It has been shown that an efficient regeneration of the NOx storage
catalyst can be achieved only if H2 is present in the gas phase during a rich spike.
Indeed, methane is not able to regenerate the catalyst at temperatures below 400°C
due to its low reactivity. In the presence of H2, the catalyst can be easily and fully
regenerated even at temperatures as low as 200°C.
Tests performed on the CNG engine test bench showed that as soon as the engine is
operated under rich conditions, high amounts of CO and H2 are generated without
simultaneous increase of the CH4 concentration. The concentrations of CO and H2
are mainly dependent on the lambda value and increase strongly with lowering
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lambda towards the rich side. Therefore, an implementation of a NOx storage
catalyst on a lean-burn CNG engine is conceivable and would allow the removal of
NOx in a broad temperature window.
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1
Research activities
1.1
Introduction NOx/CH4 strategies
Contract N.:218447
1.1.1 NOx abatement technologies
Burning lean fuel mixture with excess of air brings an important benefit in the form
of lower fuel consumption. However, abatement of the NOx emissions from Diesel
and gasoline lean-burn engines is a complicated task because direct NOx reduction
is hindered under the oxidizing conditions in the exhaust gas (in contrast to classical
gasoline engines with balanced air/fuel and red/ox ratios). Nowadays there exist two
commercially available technologies for the elimination of the NOx emissions from
lean-burn engines: NOx storage and reduction catalyst (NSRC), and selective
catalytic reduction of NOx by NH3 generated from urea solution stored in a special
tank (NH3-SCR, urea-SCR).
The selective catalytic reduction of NOx by NH3 (SCR) was originally developed for
stationary emission sources, mainly power plants (Ref [1] Forzatti et al., 2002),
however it soon turned out to be a promising technology for the deNOx in automotive
industry as well (Ref [2] Heck et al., 2002). It was introduced in Europe for
commercial heavy-duty vehicles in 2005, and more recently also for passenger cars
(Ref [3] Enderle et al., 2008). Different types of catalysts, mainly V2O5/WO3/TiO2 or
zeolite based formulations (Fe-ZSM5 and Cu-ZSM5) are used in the automotive
industry. The NH3-SCR converter needs an external source of the selective reducing
agent (ammonia). In standard configuration, ammonia is generated from urea
solution injected in a controlled way into the exhaust line, where it is thermally
decomposed into NH3 and CO2. Ammonia is able to react selectively with NOx under
lean (oxidizing) conditions, giving N2 as the final product. The vehicle needs to be
equipped with a special tank containing urea solution (marketed under different
names, e.g., AdBlue), which needs to be re-filled periodically. Therefore for small
vehicles this technology seems not to be the most adequate one since the second
well known technology (NOx storage catalyst) described below enables an operation
without a secondary tank.
The NOx storage and reduction catalyst has been derived from classical three-way
catalyst by adding the NOx adsorbing components into the active washcoat layer.
Typical formulation of the NSRC catalyst is NM/EA/OS/γ-Al2O3; here NM denotes
noble metals (Pt,Rh,Pd), EA denotes earth alkaline or alkali metals serving as active
components for the NOx chemisorption (Ref [4] Kobayashi et al., 1997), and OS
denotes oxygen storage compounds (typically Ce oxides). The NSRC needs to be
operated with periodic regenerations in the form of the increased concentration of
reducing components (CO, H2, HC, normally present in the exhaust gas). In the
course of a longer lean phase (economical engine operation with lean fuel mixture,
oxidizing conditions, lasting several minutes) the NOx are adsorbed on the catalyst
surface. The accumulated NOx then need to be reduced within a controlled short rich
phase (enrichment of fuel mixture, reducing conditions, lasting several seconds).
While the NSRC is primarily being used in passenger cars, the SCR is well
established for heavy-duty vehicles. For application on a CNG engine it has been
claimed that the regeneration of a NOx storage catalyst is not feasible due to the fact
that methane is not able to react efficiently with stored nitrates in the considered
temperature range. Objective of the laboratory investigations is to analyse the
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reactivity of different reductants during a rich spike for regenerating the NOx storage
catalyst. The results will be used as input in WPB2.4 in order to find the right engine
calibration on the test bench.
The main reactions involved in the NOx storage catalyst are summarized below:
Adsorption phase under lean conditions:
BaCO3 + 2NO2 + ½ O2  Ba(NO3)2 + CO2
Regeneration phase (with CO, H2 or HC):
Ba (NO3)2 + 3CO  BaCO3 + 2NO + 2CO2
2NO + 2CO  N2 + 2CO2
1.1.2 Methane catalyst reactivity
For improving the light-off temperature of the methane conversion not only the
catalyst formulation is of relevance but also the surface state of the catalyst after
synthesis, preconditioning and under operating conditions. Therefore the operation
mode of the engine plays an important role in the stabilisation and the activation of
the catalytic surface. To better understand the complexity of the reactions and the
role of the oxidative state of the catalyst surface, some aspects from literature studies
are reported in the following paragraph.
The nature of active sites in Pd-based catalysts for methane oxidation has been a
matter of debate for a long time, the key question being: what state of the catalyst is
more active: metallic or oxidized? The views on this issue evolved over the years
[Ref [5] Cullis and Willat, 1983; Ref [6] [7] Hicks et al. 1990a, 1990b; Ref [8] Farrauto
et al., 1992, Ref [9] 1995; Ref [10] Groppi et al., 2000; Ref [11] Persson K. et al.,
2007; Ref [12] Gabasch et al., 2007] and some aspects are summarized below.
The essential factor in discussion on the nature of active phase in palladium catalysts
is the relative instability of palladium metal in the presence of oxygen. Palladium
oxidizes in air to PdO between ca. 300-400oC, being stable in air at atmospheric
pressure up to about 800oC. Above this temperature the stable species is metallic
palladium. A number of research groups observed that the combustion rates are
different when the catalyst is either cooled or heated in the reaction mixture. This
unusual kinetic behaviour, referred to as an activity hysteresis, was assigned to the
decomposition of PdO to Pd and its re-formation, which also shows a hysteresis [Ref
[8] Farrauto et al., 1992; Ref [13] Groppi et al., 2001]. It was suggested that strongly
bound chemisorbed oxygen is formed on the palladium surface during cooling and
that this oxygen species passivate the surface and inhibit further oxidation. Hicks et
al., Ref [6], Ref [7] and Oh et al. Ref [14], while relating the catalytic activity to
metallic Pd, considered also the role of oxidized palladium. Hicks et al. Ref [7]
conducted a series of methane oxidation tests on Pd/alumina catalysts at 573 K.
Based on the experimental results, the authors claimed that PdO dispersed on Pd
crystallites was more active for methane oxidation than PdO dispersed over alumina.
According to Oh et al. Ref [14], a thin layer of PdO on metallic Pd was the active form
of the catalyst, while bulk PdO was inactive in the methane combustion.
A thorough study aiming at the identification of the palladium active phase was
carried out by Lyubovsky and Pfefferle Ref [15]. Bearing in mind that not only catalyst
preparation, pretreatment, type of support and precursor, but also the reaction
conditions affect the catalyst activity, the authors employed three different
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experimental procedures to address the problem. Catalyst activity was studied as a
response to the a) change in the process temperature, b) variation in oxygen
concentration in the gas mixture, c) in situ hydrogen reduction of the catalyst. To
minimize the influence of metal-support interaction on the experimental results, the
low surface area -Al2O3 was used as a support. The activation energy for complete
methane oxidation over the PdO phase of the catalyst was estimated at 73 kJ/mol
and over the Pd phase at 147-167 kJ/mol. The authors found that the pre-exponential
factor for the Pd state is 5-6 orders of magnitude higher than that for the PdO state of
the catalyst, depends strongly on the process conditions and the sample history, and
can change during the reaction. If the methane oxidation over the metallic Pd has
higher activation energy and higher pre-exponential factor than the reaction over the
oxidized form of the catalyst then at some temperature the graphs of the temperature
dependence of activity in the Arrhenius plot would intersect as illustrated in Fig.1.
This means that while in the low temperature regime the PdO state of the catalyst is
more active than the reduced form, the opposite is true in the high temperature
range.
Fig. 1 Comparative activation energies and pre-exponential factors for reaction
over the Pd and the PdO oxidation states.
In a more recent paper, Ferrer et al. Ref [16] studied a supported Pd catalyst having
similar characteristics to those currently used in purifying the pollutant emissions
generated by automobile engines, and presented yet an other evidence that methane
oxidation activity is strongly influenced by reduction treatments and the nature of the
PdOx phase is modified by the oxygen partial pressure.
In view of the difficulties in relating the methane oxidation activity to specific catalyst
properties by conventional methods (continuous flow reactor studies), Choudhary et
al. Ref [17] employed methodical pulse reactor studies to obtain correlations between
the initial methane combustion activity and the catalyst properties (Pd 0/PdO content
and path of PdO formation). The authors observed that the initial methane
combustion activity (at 160–280 °C) continuously increased with increasing PdO
concentration (0–100%) in the catalyst, and continuously decreased with increasing
Pd0 content (0–100%), which confirmed the importance of PdO as active phase
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component. Additionally, the authors demonstrated that along with the relative
concentration of PdO, the PdO formation pathway is also critical in determining the
methane combustion activity of the catalyst. In a carefully designed experiment the
catalysts with identical PdO content were obtained by two pathways: (i) by controlled
partial oxidization of Pd0/Al2O3 and (ii) by controlled partial reduction of PdO/Al2O3.
Catalytic tests demonstrated that for a given PdO content, the catalysts obtained by
partial oxidation of Pd0/Al2O3 showed a significantly superior performance to the
catalyst obtained by partial reduction of PdO/Al2O3 for all the temperatures
investigated.
Therefore the oxidation state of the catalyst surface and especially the oxidation state
of Pd plays an important role in the activity of the catalyst. Additionally the catalytic
combustion of hydrocarbons over Pd-catalysts is a complex process which proceeds
via a number of surface reactions. The exact reaction mechanism is difficult to
determine and depends on parameters such as catalyst composition, fuel/air ratio
and temperature.
Therefore different strategies based on lambda variations have been investigated at
Daimler and Polimi in order to determine what could be the best lambda strategy for
getting the best activated catalyst surface and therefore the best methane
conversion.
1.2
Experimental set-up
Laboratory set-up for CH4 strategy (Polimi):
In Fig.2 is reported the scheme of the rig which includes three different sections: the
feed section (red line), the reaction section (green line), and the analysis section
(blue line).
Fig. 2 Experimental rig: red line = feed section, green line = reaction section, blue line
=analysis section.
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Feed section
The feed section consists of 6 lines, each equipped with a mass flow controller. It is
designed for simultaneous feeding of a multi-component stream (CH4, CO, H2),
together with oxygen, NO and large amounts of H2O (5-15vol.%) and CO2 (5-15%).
In order to investigate sulfur ageing effect, a line is also dedicated to the feeding of
SO2. Intervals of flow rate and gas composition fed by each line are reported in
Table1.
Gas composition
Flow rate [Ncc/min]
N2
120-1200
5000ppm NO in N2
20-200
Air
5-50
510 ppm SO2 in N2
5-50
12%CO+3%CH4+2%H2 in N2 10-100
CO2
15/150
Air or H2 ( -sweep)
5-50
Table 1. Gas composition and intervals of flow rate in the different feed lines.
A HPLC pump/evaporator system is adopted to regulate the H 2O flow. H2O
concentration is continuously monitored by means of a humidity sensor (Vaisala
HUMICAPP –HMT334) and periodically checked by gas chromatography.
Lambda-sweep tests can be performed by step variations of both oxygen and H 2
concentrations through a dedicated additional line equipped with a programmable
mass flowmeter, as shown in Fig. 3. The lambda value is monitored by an ETAS
LA4-4.9 Lambda Meter, placed just upstream the reaction section.
1,05
Lambda Sweep
Stoichiometric conditions
1,04
1,03
Lambda
1,02
1,01
1,00
0,99
0,98
0,97
0,96
0,95
0
1
2
3
4
5
6
Time [min]
Fig. 3 Schematic of λ-sweep experiments.
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Reaction section
The reaction section is designed to test monolithic honeycomb samples (6x6
channels, Fig. 4) charged into a stainless steel reactor externally heated by a tubular
oven.
Fig. 4 Sliding termocouple into the monolith channels.
The configuration of the reactor is schematically illustrated in Fig. 5. The upstream
section of the reactor is filled by quartz spheres (2.6mm of diameter) to allow
complete mixing and preheating of gas feed. A square 6x6 channels sample,
obtained from the ceramic honeycomb washcoated with the Pd-based reference
catalyst provided by Ecocat, is wrapped by a quartz wool tape and located in a
properly designed holder to avoid by-pass phenomena.
A sliding thermocouple (TC2) is inserted into one of the central channels of the
monolith in order to measure the axial temperature profile of the catalyst during the
experiments. This allows to take into account T-profiles associated with strongly
exothermic combustion reactions in the kinetic analysis of the experimental data. A
second fixed thermocouple (TC1) is located just before the entrance of the monolith
sample.
Flow
direction

36 cm
TC2
1.63 cm
15.2 cm
15.2 cm
1.3-4.0 cm
Fig. 5 Stainless steel reactor set up.
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Analysis section
The analysis section consists of a Micro GC (3000 A, Agilent Technologies) equipped
by TCD detectors and a Molecular Sieve 5 Å column for separation of N2, H2, O2,
CH4 and CO (Ar carrier) and a Plot Q column for separation of CO 2 and H2O. Gas
analysers (ABB A02020) were also installed for continuous monitoring of H2, O2,
CH4, CO and CO2 outlet concentrations during testing under dynamic conditions (e.g
-sweep). The rig was not equipped for analysis of NOx.
Laboratory set-up for NOx and CH4 strategies (Daimler):
The investigations have been performed on a laboratory apparatus able to simulate
gas compositions close to real exhaust compositions. As represented on Fig. 6 three
main sections can be identified:
- A dosing section enabling the mix of all the gas species present in the exhaust of a
combustion engine including the dosing of different amounts of H2O. The injection of
all individual gases is done by mass flow controllers enabling a precise mixture in
front of the reactor.
- A reactor section equipped with a bypass line in order to control the gas
composition in front of the catalyst. The test rig is equipped with three reactors giving
also the possibility to test several catalysts connected in a serial way. Before entering
the reactor section the gas flow is preheated on 180°C in order to avoid deposits or
condensation.
- An analytic section enabling an accurate detection of the standard components of
an exhaust gas.
Exhaust air
Gas dosing section
Reactors
Analytical
Analytics
train
H2OEvaporator
Detector
detector
Synth. gases
Exhaust air
Fig. 6 Laboratory experimental set-up.
The whole test rig is controlled by computer and allows the succession of different
characterisation steps corresponding to different temperature levels and/or gas
concentrations. Since the test rig is equipped only with one dosing section, transient
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tests consisting of variation of the catalyst inlet concentration can be done only with
step by step changes. Indeed, a real transient mode would require a second dosing
section with a fast three way valve enabling a rapid switch between the two lines.
1.3
Strategies for NOx abatement under lean conditions
1.3.1 Laboratory
The NOx regeneration ability in presence of CH4 and other reducing agents is
investigated on a laboratory scale under simplified conditions representative of a
CNG engine. A commercial NOx storage catalyst with a loading of 142 g/ft3 was
used.
Lean/rich cycles were performed at constant temperatures ranging from 150°C to
550°C. For all tests the NOx concentration in the lean phase was kept constant at
2000 ppm and the space velocity was equal to 32.000 h-1. The succession of the
lean/rich cycles is represented in Fig. 8 (a). The test procedure consisted in 12
periodic lean rich cycles at constant temperatures where the NOx concentration
downstream of the NOx storage catalyst is continuously monitored. The adsorbed
NOX mass was balanced for the last 4 cycles of the procedure and corresponds to
the converted NOx mass.
The lambda value in the rich phase as well as the methane concentration were kept
constant at 0.94 and 2800 ppm resp., while the H2 and CO concentrations were
varied in order to analyse the reducing potential of each component . The different
concentrations of reducing agent used during a rich spike are summarized in Fig. 7.
CO and H2 concentrations are progressively added from mixture 2 to 4. Therefore
the role and efficiency of each reducing agent (CH4, CO, H2) for regenerating a NOx
storage catalyst can be investigated.
Rich mixture 1 at Lambda=0.94
CH4H2OCO2N2
2800ppm 12Vol.% 10.7Vol% balance
Rich mixture 2 at Lambda=0.94
CH4COH2O2800ppm 0.35Vol% 0Vol%
CO2N2
10.7Vol% balance
Rich mixture 3 at Lambda=0.94
CH4COH2H2OCO2N2
2800ppm 0.35Vol% 0.1Vol% 12Vol.% 10.7Vol% balance
Rich mixture 4 at Lambda=0.94
CH4COH2H2OCO2N2
2800ppm 0.35Vol% 0.5Vol% 12Vol.% 10.7Vol% balance
Fig. 7 Gas compositions during rich spike at constant lambda value of 0,94.
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Fig. 8 (b) summarizes the NOx conversion (expressed in g NOx converted) obtained
at different temperatures with different gas concentrations in the rich phase. It
appears that regenerations performed w/o or with low amount of H2 are not effective
below 400°C and exhibit a lower NOx conversion than in the presence of high
amount of H2 at temperatures above 400°C. Below 400°C the NOx storage catalyst
can be regenerated only in the presence of high amount of H2 in the rich phase. As a
consequence, a detailed characterisation of the exhaust gas composition in the rich
phase will be necessary on the test bench in order to evaluate the H2 composition of
the exhaust gas. Depending on the measured H2 concentration, a shaping of the rich
spike could be necessary for supplying the catalyst with enough H2 during the rich
spike, especially at low temperatures where CO and CH4 are not active.
4000
a)
3500
QI11_NOx
QI13_NO
CH4
concentration [ppm]
3000
2500
2000
1500
1000
500
0
500
700
900
1100
1300
1500
time [s]
0.16
b)
0.14
NOx conversion [g]
0.12
Reg. with CH4 with H2O
Reg. with CH4, CO w/o H2O
Reg. with CH4, CO with H2O
Reg. with CH4, CO, H2 with H2O
0.1
0.08
0.06
0.04
0.02
0
150°C
200°C
250°C
300°C
350°C
400°C
450°C
500°C
550°C
Fig. 8 a) Lean/rich cycles (lean-time t=3min, rich-time t=20s) with gas
concentrations of NOx, NO, CH4, b) NOx conversion for different gas mixtures in the
rich spike. SV = 32.000h-1.
1.3.2 Engine test bench
In order to characterize the exhaust gas composition of the CNG engine developed in
SPA2, different tests have been performed under rich conditions at different lambda
values.
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The engine was operated at steady state load points from n=750min-1 up to
n=1800min-1.
Fig. 9 represents the different points from the engine map, which have been
investigated. Since for all the points a similar behaviour of the exhaust composition
has been observed, only the point n=1600min-1 / 2bar will be described in detail.
LFNR
1
2
3
4
5
6
7
N
1/min
750
1200
1600
1600
1600
1800
1800
PMEFF
bar
0.72
0.72
2.01
3.41
4.71
3.4
6.02
MD
Nm
10.3
10.3
28.66
48.76
67.37
48.55
86.08
PEFF
kW
0.809
0.809
4.801
8.169
11.287
9.153
16.229
Fig. 9 Stationary operation points for engine raw emissions characterisation.
Fig. 10 a) represents the raw H2 and CO concentrations in the exhaust as a function
of the lambda and of the injection timing (220° and 70°CA). It appears that as soon
as the engine is operated under rich conditions, CO and H2 are produced in the
combustion process. A nearly linear increase of CO (factor 4,25) and H2 (factor 4,35)
can be observed as function of the lambda value on the rich side. In contrast to CO
an H2, the CH4 concentration gradient is with factor 1,31 significantly lower and the
NOX concentration decreases by factor -2 (Fig. 10 b)).
This result shows that it is possible to increase the CO and H2 concentrations in the
exhaust by adjusting the engine calibration, without increasing significantly the CH4
concentration.
Furthermore the injection timing can be used for adjusting the CO and H2
concentrations to the desired value as represented on Fig. 10.
Additionally the presence of a TWC in front of the NOx storage catalyst can improve
the generation of H2 through the water gas shift reaction over the TWC. Indeed
under rich conditions CO is able to react with water to form H2 and CO2. Fig. 11
represents the H2 and CO2 formation as a function of temperature when CO (6000
ppm) and H2O (10%) are present in the gas mixture. The lambda value is kept
constant at 0,997. It appears that CO starts to be converted into CO2 at around
300°C. For temperatures above 350°C all the CO is converted into CO2 and the
equivalent amount of H2 is produced through the water gas shift reaction.
These results demonstrate that operating a CNG engine under rich conditions leads
to generation of high amounts of H2 and CO, depending on the lambda value.
Therefore the use of a NOx storage catalyst for removing NOx from the exhaust of a
lean running CNG engine is fully appropriate.
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3.5
H2_Engine out_220°
a)
3
H2_Engine out_70°
concentration [Vol.%]
dX_CO = +4.25
CO_Engine out_220°
2.5
CO_Engine out_70°
2
1.5
dX_H2 = +4.35
1
0.5
0
0.85
0.9
0.95
1
Lambda [-]
1.05
1.1
1.15
6000
CH4_Engine out_220°
dX_CH4 = +4.3
b)
CH4_Engine out_70°
5000
NOx_Engine out_220°
concentration [ppm]
NOx_Engine out_70°
4000
3000
2000
1000
dX_NOx = -2
0
0.85
0.9
0.95
1
Lambda [-]
1.05
1.1
1.15
Fig. 10 Raw exhaust gas concentrations for n=1600min-1 and 2bar, injection timing
70° and 220°CA - a) CO, H2 concentrations as a function of lambda b) CH4, NOx
concentrations as a function of lambda.
7000
CO2 downstream catalyst
concenration [ppm]
6000
H2_downstream catalyst
5000
4000
3000
2000
1000
0
200
250
300
350
400
temperature [°C]
450
500
550
Fig. 11 H2 and CO2 concentrations downstream a TWC catalyst. Constant lambda
= 0,997.
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1.4
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Strategies for CH4 abatement under stoichiometric conditions
For improving the light-off temperature of the methane conversion not only the
catalyst formulation is of relevance but also the operation strategy of the engine as
reported in the introduction. The oxidation state of the catalyst surface and especially
the oxidation state of Pd plays an important role in the activity of the catalyst.
Therefore different strategies based on lambda variations have been investigated at
Daimler and Polimi in order to determine the role of the reducing agents present in
the exhaust.
1.4.1 Steady state conditions
In WPB2.2 the development of control strategies for enhancing the reactivity of
methane oxidation has been considered at Daimler. For the removal of methane,
stoichiometric and lean gas mixtures have to be considered as well as lambda
variations based on step-changes between different A/F ratios.
All tests have been performed with the reference catalyst from Ecocat. Prior to all
tests the catalyst has been pre-treated at 600°C for 5h according to the standard
procedure.
Fig. 12 represents the inlet lambda value as well as the CO concentration during a
lambda step-change test. Lambda ranged from 1.05 to 0.97 while the CO
concentration was varied from 6300 ppm (lambda = 1,05) to 2,4Vol% (lambda =
0,97). The concentration of CO has been varied in constant steps of 0,6Vol% while
the concentration of the other gas components has been kept constant (Table 2).
T(°C)
520
SV(h-1)
100000
CH4(ppm)
1000
CO(ppm)
6300
NOx(ppm)
2600
O2(%)
1,1
CO2(%)
10,7
H2O(%)
10
Table 2: Gas concentrations for a step-change test.
Lambda upstream
CO upstream
Fig. 12. Lambda and CO-profile upstream of the catalyst during a lambda-sweep
experiment.
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It can be seen on Fig. 13 a) that under stoichiometric conditions, the lightoff
temperature of CH4 is around 320°C with a maximum of 80% conversion at higher
temperatures. On Fig. 13 b) the highest CH4 conversion is represented as a function
of temperature and for an optimized lambda value. It appears that the lightoff
temperature for CH4 can be improved in optimizing the lambda value at each
temperature. In the whole temperature range the best CH4 conversion has been
obtained with slightly rich lambda values. On the other hand NOx can be easily
converted to 100% under stoichiometric and rich conditions.
NOX lambda=1
CH4 conversion [%]
CH4 lambda=1
a)
NOX ideal Operation Point
CH4 conversion [%]
CH4 ideal Operation Point
b)
Fig. 13. a) CH4 and NOx conversion under stoichiometric conditions, b) max. CH4
and NOx conversion as function of lambda and temperature.
1.4.2 Transient conditions – Lambda-sweep
At Polimi the influence of lambda oscillations on methane conversion has been
investigated.
Procedures
According to indications in Deliverable DB2.3 Boundary/testing conditions, at the
beginning of the experimental campaign, each monolithic sample undergoes a
conditioning treatment (“degreening”) in a standard mixture flow for 5h @ 600°C.
After the degreening, tests are carried out performing two kind of experimental runs:
programmed temperature ramps (heating and cooling the oven @ 10°C/min) in
dynamic conditions (referred as “T-ramp”) and temperature steps under dynamic
conditions in lambda-sweep mode (referred as “step-up”) (Fig. 14).
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a)
b)
T-ramp
600
550
Temperature [°C]
550
Temperature [°C]
Step-up
600
500
450
400
350
500
450
400
350
300
300
250
250
200
200
0
30
60
90
120
time [min]
150
180
0
50
100
150
200
250
300
350
time [min]
Fig. 14. Different typologies of experimental runs: a) T-ramp; b) step-up.
λ-sweep tests
As reported previously, operating under dynamic λ-sweep conditions can strongly
increase the activity of the catalyst. Fig. 15 shows the results of a T-steps run
performed by changing the feed mode from λ=1 to λ-sweep around stoichiometric
conditions. λ was swept between 0.98 and 1.02 by 30s cycling of the O2 feed
concentration between 0.2% and 0.95% v/v according to the dynamics sketched in
Fig. 3, while keeping all the other species constant at the concentration level reported
in Table 1.
An activity increase is evident when switching from λ=1 to λ-sweep regime, which is
particularly marked at 400°C, but still present at all the investigated temperatures.
Operations under λ-sweep conditions result in wide conversion oscillations.
Inspection of Fig. 16 reveals that conversion minima correspond to the leanest part of
the cycle, whereas a more complex trend is observed in the rich part where
conversion maxima (CH4 concentration minima) are anticipated with respect to the
achievement of the richest point which corresponds to complete O2 consumption
paralleled by peaks of H2 and CO concentration possibly due to the onset of steam
reforming and water gas shift reactions (Net H2 production).
On the other hand only limited temperature oscillations (±2°C) were observed in
correspondence with the conversion ones (Fig. 17). This indicates that oscillations
are not thermally driven but depend on chemical modifications of the catalyst. It is
likely that palladium reversibly undergoes to PdO ↔ Pd° transformation when
switching forth and back from rich to lean conditions. Apparently the reduction
process is fast as suggested by the immediate appearance of CO and H 2. On the
other hand according to literature indications, the oxidation process is slower being
controlled by oxygen diffusion in the Pd° (small particles) and PdO (large particles)
lattice Ref [18] and by reconstruction of PdO lattice Ref [19]. Accordingly an
incomplete re-oxidation may occur during the lean part of the cycle resulting in a
mixed PdO/Pd° or partially oxidised state. Such a state has been indicated by many
authors Ref [20] as the most active one in CH4 combustion and might be responsible
for the high conversion values observed in the λ-sweep regime.
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100
600°C
600
550°C
550
80
500°C
70
500
60
450°C
50
450
40
400°C
400
30
20
 = 1.00
 sweep
10
Temperature [°C]
CH4 conversion [%]
90
350
0
300
0
50 100 150 200 250 300 350 400 450
Time [min]
Fig. 15 Step-up from 400 to 600°C (Toven) with T-steps of 50°C @ λ=1.00 and λ –
sweep.
0,7
CH4
O2
Molar fraction [%]
0,6
H2
CO
0,5
0,4
0,3
0,2
0,1
0,0
16
18
20
22
24
Time [min]
Fig. 16 Concentrations of CH4, O2, H2 and CO during the sweep @ 400°C
(Toven).
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1.04
416
1.02
Lambda
1.00
412
0.98
0.96
10
11
12
13
14
Temperature [°C]
414
410
15
Time [min]
Fig. 17 and temperature trends during l-sweep @ 400°C (Toven).
Note that during oscillations an average CH4 conversion of about 50% is obtained at
a catalyst temperature of 410-420°C, corresponding to a gas inlet temperature of
390°C.
In order to check if such activated state could be maintained under stable feed
conditions the λ-sweep experiments were repeated performing a temperature hold at
constant λ before and after the λ-sweep cycles. Results are reported in Fig. 18 and
19.
100
90
600
550
80
500°C
70
500
60
50
450
40
400
400°C
30
20
Temperature [°C]
CH4 conversion [%]
600°C
 = 1.00
 sweep
350
10
0
300
0
50 100 150 200 250 300 350 400 450 500
Time [min]
Fig. 18 Step-up from 400 to 600°C with T-steps of 100°C and λ=1.00 / λ –sweep/
λ=1.00 holds.
At λ=1.00 (Fig. 18) a significant activity enhancement is retained at 400°C, although
conversion at constant lambda was always lower than during λ-sweep. On the other
hand no significant activity variation is observed at 500°C and a dramatic activity loss
occurs at 600°C. This trend suggests that at λ=1.00 over-reduction of palladium may
occur at high temperature being the endothermic PdO Pd° process
thermodynamically favoured on increasing the temperature.
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 = 1.02
 sweep
600°C
600
80
550
500°C
70
500
60
50
450
400°C
40
400
30
20
Temperature [°C]
CH4 conversion [%]
90
350
10
0
300
0
100
200
300
400
Time [min]
Fig. 19 Step-up from 400 to 600°C with T-steps of 100°C and λ=1.02 / λ –sweep/
λ=1.02 holds
Under slightly oxidising conditions at λ=1.02 (Fig. 19) a lower activity enhancement is
obtained at 400°C, but no dramatic activity losses occur at 600 °C. This again
suggests that palladium over-reduction can occur at high temperature under
stoichiometric conditions and can be avoided by operation under slightly oxidising
atmosphere. On the other hand operations at λ=1.02 may result in an over-oxidation
of the catalyst with respect to the optimal state and, consequently, in a lower activity
level.
Further evidences are obtained by comparing the cooling ramps at λ=1 and λ=1.02
performed after the step-up runs in Fig. 18 and 19 respectively. The presence of a
reverse conversion trend (negative apparent activation energy) typically due to the
Pd → PdO transition can be noticed between 575°C and 500°C only in the λ=1 case
(see Fig. 20), suggesting that palladium undergoes to reduction at 600°C and it is reoxidised on decreasing temperature. Noteworthy a highly active state is obtained
upon re-oxidation.
100
CH4 conversion [%]
90
80
70
=1
60
50
40
30
20
10
0
300
=1.02
350
400
450
500
550
600
Temperature [°C]
Fig. 20 Cooling ramps after step-up runs @ λ=1 and @ λ=1.02.
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Conclusions
The work performed in WPB2.2 within Deliverable DB2.8 was focussed on the
development of control strategies for:
- ensuring the regeneration of a NOx storage catalyst (lean burn application)
- improving the methane light-off temperature and catalytic activity of the Pdbased reference catalyst (stoichiometric application).
Concerning the removal of NOx under lean operation conditions it has been
demonstrated that a standard NOx storage catalyst is suitable for CNG engine
application. Adsorption of NOx in the lean phase can occur without restrictions like on
diesel or lean-burn gasoline engines since CO and HC don’t play any important role.
On the contrary, it has been shown that an efficient regeneration of the NOx storage
catalyst can be achieved only if H2 is present in the gas phase during a rich spike.
Indeed, methane is not able to regenerate the catalyst at temperatures below 400°C
due to its low reactivity. In the presence of H2, the catalyst can be easily and fully
regenerated even at temperatures as low as 200°C. Therefore a systematic
characterisation of the exhaust gas composition under rich conditions has been
carried out with the CNG engine M271 developed in SPA2. The results showed that
as soon as the engine is operated under rich conditions, high amounts of CO and H2
are generated without simultaneous increase of the CH4 concentration. The
concentrations of CO and H2 are mainly dependent on the lambda value and
increase strongly with lowering lambda towards the rich side. Therefore, an
implementation of a NOx storage catalyst on a lean-burn CNG engine is conceivable
and would allow the removal of NOx in a broad temperature window.
For improving the performance of the methane reference catalyst different strategies
based on lambda variations have been investigated. Indeed, the nature of active
sites in Pd-based catalysts for methane oxidation has been a matter of debate for a
long time, the key question being: what state of the catalyst is more active: metallic or
oxidized? A short literature survey came to the conclusion that the best catalytic
surface is represented by a cohabitation of oxidized and reduced Pd-particles.
Therefore it was of great importance to characterize the activity of the reference
catalyst under different gaseous environments.
Under steady state conditions, it appears that for each operating temperature, a
slightly rich average lambda leads to the best CH4 conversion. But a continuous
operation under slightly rich conditions would certainly also lead to a slow
deactivation of the catalyst and is in term of fuel consumption not the best strategy.
Therefore investigations under lambda-sweep conditions have been undertaken.
The best conversion performances were obtained by operating the catalyst under a
-sweep regime, obtained by periodically (30 s) switching the feed from slightly rich
(=0.98) to slightly lean (=1.02) conditions. Using this test mode, a regeneration of
the catalyst previously deactivated under both stoichiometric and slightly lean
conditions was observed. Widely, but regularly oscillating CH4 conversions occurred
with conversion minima and maxima appearing in the lean period and in the rich
period respectively, the latter ones being associated with the presence of significant
amounts of H2 and CO. Such a behaviour is likely associated with reversible
transformation of palladium from the oxide to the metallic state which may result in an
higher activity associated with an average optimal oxidation state of palladium. It is
worth noting that the -sweep mode roughly corresponds to actual engine operation,
although a much longer cycling time was adopted in the experiments (30 s vs <1s).
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Nevertheless the results demonstrate that an oscillation of the lambda value around
lambda=1 leads to an improvement of the methane conversion. Such a control
strategy can be easily implemented on a lambda=1 engine. This strategy will be
implemented on the engine in WPB2.4 and further optimized with regard to lambda
amplitude and frequency.
3
References
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Ref [16] V. Ferrer, A. Moronta, J. Sánchez, R. Solano, S. Bernal, D. Finol, 2005:
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