Soot trapping and continuously oxidizing behavior by flow-through
Metallic PM filter *
Seiji Okawara1)
Shinji Tsuji2)
Mikio Inoue3)
Toshiro Itatsu4)
Tetsuo Nohara5)
Kazunari Komatsu6)
We studied flow-through type metallic Particulate Matter(PM) filter for diesel after treatment system. The
structure could be verified advantage of soot oxidation due to tridimensional gas diffusion structure compared
with wall-flow type PM filter.
As a result, this metallic PM filter can be the simple vehicle application for soot reduction technology,
and we got a improvement way of continuous re-generation system.
Key Words: Metallic PM Filter, Flow through, NO2 Consumption, Soot Oxidation
1. INTRODUCTION
A diesel vehicle has advantages of fuel economy and low CO2
emission. On the other hand, Particulate Matter (PM) and
nitrogen oxide (NOx) in the exhaust gas has been required to
reduce around the world. Particularly being popular in Europe,
EU has enacted the PM regulation in the air from concern for the
environment. Hereby, PM reduction is one of the critical issues
including existence vehicles. One of PM reduction technology
has come into practical use with aftertreatment system1), which
are wall-flow DPF with precise regeneration control.
In these years, purification of emission gas is progressed by
improvement of engine combustion. If the simple PM reduction
technology would exist such as Flow-through oxidation catalyst
and diesel particulate filter (DPF) without forced regeneration
control, it would have been possible to spread the clean diesel
Fig.1 Scheme of flow-through metallic PM filter
vehicle including existence vehicle. In this time, it’s focused
attention on tridimensional gas flow structure of metallic PM filter
2. CONCEPT
that has flow-through channel and wall-flow unit.
2.1. Structure and soot trapping mechanism
In this paper, the flow-through metallic PM filter (M-PMF)2)
Fig.1 shows M-PMF system, appearance and inside enlarged picture.
was verified by PM trapping efficiency, pressure loss and soot
M-PMF is structured as layers stack, which are corrugation foil with
oxidation behaviors. And this paper also reports that the
shovels and porous fleece. Exhaust gas will be introduced to up and
uniqueness of gas flow structure can be an improvement of
down side of fleeces by shovels, and then soot can be trapped when
continuous soot oxidation
the gas goes through into the fleece. In addition, another air spaces
are available before introducing to shovels, by which left and right
cells can be secured as a flow through passage.
Fig.2 shows soot trapping mechanism compared with conventional
wall-flow DPF(WF-DPF) and M-PMF. In the great difference
*Accepted at Jul.29 , 2005Presented at 2005 JSAE Annual Congress
between WF-DPF and M-PMF, gas flow of WF-DPF has only one
(Sep.28th, 2005).
chance to go through wall of filter. However, gas flow of M-MPF
th
1)-4 Toyota Motor Corporation (1 Toyota-cho, Toyota- shi, Aichi
can go through the fleece layer repeatedly. As described later, in
471-8572, Japan)
this point, it is expected that the soot oxidation is accelerated by
5) 6
EMITEC Japan
(SIGMA Bldg. 6F, 3-7-12 Shibaura,
Minato-ku, Tokyo 108-0023,Japan)
effectiveness of an oxidizer of soot.
Sintered Metal Fleece
compared with trapping efficiency and pressure loss by time. The
Outlet
Outlet
Inlet
M-PMF showed 20~30% of trapping efficiency in Fig.3. Unless
DOC was set before the M-PMF, the trapping efficiency gradually
decreased, but the system with DOC could keep the trapping
Inlet
efficiency. In Fig.4, the pressure loss was a saturation tendency. It
Porous Wall
Corrugated Mixing Foil
was considered that the trapped soot was oxidized in M-PMF by
Fig.2 Flow direction of M-PMF and Wall flow DPF
NO2. The NO2/Soot ratio of the test was 14.4 at the gas flow.
2.2. M-PMF system
As shown Fig.1, M-PMF is placed after an oxidation catalyst, and
trapped soot is mainly oxidized with NO2, which is oxidized NOx in
an exhaust gas. Also, the soot oxidation with O2 can be occurred by
over 600C of the exhaust gas.
In addition, this paper had studied on no coated M-PMF due to
verification of characteristic of its structure. And basically, the no
coated M-PMF could trap soot in PM, at the same time SOF in PM
went through. (Skipped a figure)
Fig.3 Soot trapping efficiency of M-PMF w/wo DOC
3 Test results of fundamental performance
3.1. Test procedure
Trapping efficiency, pressure loss and oxidation performance of soot
were tested at engine bench. Table.1 shows specification of M-PMF,
diesel oxidation catalyst (DOC) and the engine condition for this
study. The engine was run by steady state or EC mode driving cycle
at each test, and the fuel with less than 50ppm of sulfur was used.
The trapping efficiency of soot was estimated by the rate of the
3
Fig.4 Pressure drop of M-PMF w/wo DOC
3
difference of inlet and outlet smoke(mg/m ) to inlet smoke(mg/m ),
those were measured by smoke meter. And the oxidation
Measured Soot
3g/unit
performance of soot was estimated by consumption rate of NO2 and
soot oxidation rate. The former was estimated by the inlet and outlet
NO2 concentration, the latter was estimated by the total trapping
soot mass and actual accumulated soot mass in M-PMF.
Here, the total trapping soot mass is the calculated soot amount in
which inlet soot amount of M-PMF was multiplied by the soot
trapping rate, that is, total mass of trapped soot including oxidized
soot mass in M-PMF at each condition.
Fig.5 Trap rate and pressure drop for M-PMF volume
Table.1 Test condition at Engine bench
Measured Soot
3g/unit
3.2. Soot trapping efficiency and pressure loss
(1) Trapping efficiency, pressure loss behavior
Fig.3 and Fig.4 show M-PMF (Dia.127mm, Length: 75mm) test
results at 300C of inlet gas with/without DOC, which were
Fig.6
Trap rate and pressure drop for M-PMF L/D
(2) Effect of volume and figures
2 Correlation of NO2 consumption and soot oxidation
Four kinds of M-PMF (Dia105mm and 127mm, length: 75mm and
According to the results of 3.3 (1), the correlation of NO2
150mm) were tested for trapping efficiency and pressure loss, which
consumption and soot oxidation were investigated.
were related to volume and length/diameter. Fig.5 and 6 show the
Fig.8 shows the correlation between NO2 consumption rate and soot
data in the same soot accumulation per M-PMF(3g/unit).
oxidation velocity at predetermined time in the steady condition at
By the same volume, smaller diameter were obtained better
300C of inlet temperature. The soot oxidation velocity increased as
trapping efficiency and higher pressure loss.
NO2 consumption was increased with increasing of soot trapping
By the same diameter, longer length were obtained better trapping
mass. Besides, NO2 and soot reaction is 2NO2+C 2NO+CO2;
efficiency without rising pressure loss.
while the theoretical reaction rate is 2:1 by the mol ratio (the weight
By same L/D ratio, larger volume were obtained better trapping
ratio, 7.7:1), it is considered that NO2 is needed in the practical
efficiency and low pressure loss.
reaction more than the theoretical ratio. Then, the calculated reaction
Based on the above results, it has been considered as the
ratio of NO2 and soot at each point in this test is added in Fig.8. It
improvement factor of trapping efficiency that the gas volume into
was suggested that NO2 / soot reaction ratio was also improved with
filter layers by shovels was increased by the higher velocity of gas
progress of test time, specifically increasing of soot trapping mass.
flow, and the filter surface area was increased.
In this way, M-PMF can be controlled soot trapping efficiency by
volume and design. Hereafter, increasing trapping efficiency and
reducing pressure loss at large L/D ratios remain to be solved.
3.3. Oxidation performance of soot
(1) Behavior of NO2 consumption on M-PMF
Trapped soot on M-PMF is oxidized mainly by NO2 in inlet gas.
Thus, the oxidation of trapped soot has been investigated from
behavior of NO2 consumption. The tests were carried out with inlet
gas of temperature from 200C to 350C, that was considered around
Fig.8 NO2 consumption and Soot oxidation with NO2/Soot
oxidation start temperature with NO23) and general range of exhaust
temperature by vehicles. Then, the behavior of NO2 consumption on
Fig.9 shows the behaviors of exhaust temperature and NO2
M-PMF were measured by steady state condition of the inlet gas
consumption rate in EC mode on engine bench. This result indicated
temperature and smoke for 6 hours that were adjusted by engine
that the soot was oxidized at EUDC (High speed driving cycle) due
load for each test condition.
to rapid increasing of NO2 consumption at more than 250C. In the
The result was shown in Fig.7. NO2 consumption was hardly
meantime, although the exhaust gas was less than 200C in UDC
observed on 200C while 6 hours of the test. NO2 consumption was
(Low speed driving cycle), some behavior of NO2 consumption
observed over 250C and that increased with progress of test time.
could be observed. It was observed at the points where HC and CO
Although the amount of soot and NO2 are different by each
emission was more after oxidation catalyst at less than 200C. This
temperature condition, it is considered that these phenomena
result indicates that NO2 in M-PMF oxidized the residual HC and
occurred because the contact probability of NO2 and soot increase
CO through an oxidation catalyst. Such a phenomenon has been
with increasing of soot accumulation.
reported by recent paper4).
Fig.7 Transition of NO2 consumption with inlet temp.
Fig.9 NO2 consumption and temp. on EC mode
(3) Compared with wall-flow DPF on soot oxidation efficiency
Therefore, in Fig.11 the soot oxidation rate(%) except time factor is
Commonly, ceramics WF-DPF is used as high efficient structure of
shown for total soot trapping mass (outlined in 3.1) to each point.
soot trapped. By the difference of structures as shown in Fig.2, soot
In case of the same soot trapping an amount, the soot oxidation in
oxidation characteristic was compared with NO2 as oxidizer. The
M-PMF was approx. two times of that in WF-DPF. NO2/soot
behavior of soot oxidation velocity or rate was investigated on
reaction ratio of this test condition was around 2/1 at mol ratio
M-PMF and cordierite DPF (2Liter, porosity=65%, uncoated) at the
(weight ratio:7.7:1), that was calculated from oxidized soot and
same steady condition. Fig.10 shows the average of soot oxidation
consumed NO2. This means the reaction might be done around
velocity for measured soot amount of each test point 6hr at 350C of
theoretical ratio.
inlet gas temperature in each filter. According to Fig.10, oxidation
Secondly, the similar comparison has been investigated with lower
velocity of WF-DPF reached higher than M-PMF. However in the
temperature and more little soot accumulation amount more than the
oxidation velocity to same soot accumulation, that of M-PMF was
case of Fig.11. According to Fig.12, in case of 280C and 2g/L of
higher than WF-DPF. As a result, the advantage of soot oxidation
soot accumulation, in M-PMF approx. 25% of oxidation rate by
performance the both filters could not be decided by the oxidation
NO2 to total trapping soot amount, while no soot oxidation was
velocity because the filters have different soot trapping rate, namely
confirmed in cordierite WF-DPF. This would be caused by that the
velocity of soot accumulation.
contact frequency of NO2 and each trapped soot on the fleece was
higher in M-PMF because of its tri-dimensional gas diffusion
structure. As a result, even though low soot trapping efficiency in
comparison with WF-DPF, it was suggested M-PMF has an
advantage as the item for continuous regeneration of soot due to
high performance of soot oxidation by NO2.
4. Discussion with soot oxidation
4.1. Calculation model of soot oxidation velocity
With regard to the previous test results, the behavior of soot
Fig.10 Soot oxidation g/h
compared with uncoated DPF
based on remained Soot 350
oxidation was demonstrated about increase of oxidation by NO2 due
to soot accumulation. It was tried to make the way to estimate actual
soot oxidation rate based on theoretical equation.
Actual NO2 and soot reaction on M-PMF in actual system would
occur because the surface of soot (Carbon) is contacted with NO2 in
exhaust gas via several elementary reactions. Therefore, the
calculation equation of soot oxidation velocity has been developed
from reaction formula in consideration of surface reaction.
(1)
C(Soot) + 2NO2 → CO2 + 2NO
r '=
p
1 . 1 ξm
mol
ξ=
= k ′ c NO 2 = k ′ x NO 2 ges [ 2 ] (2)
A
A Mc
RT
m s
k '=
1 8 RT
49800
m
1,1*10 −4 exp(−
) [ ] (3)
4 π M NO 2
RT
s
.
Fig.11 Soot oxidation %
compared with uncoated DPF
based on total trapping Soot
350
A = A'soot _ specific msoot
.
ξ m = k ′ c NO 2 A Mc
C: mol concentration [mol/m³]
X: NO2 concentration[ppm]
[m2 ]
(4)
g
[ ]
s
5
T: Temperature [K]
M: weight per mol [g/mol]
P: Pressure [N/m²]
Fig.12 Soot oxidation %
compared with uncoated DPF
based on total trapping Soot
280
A’ soot _specific constant of soot surface area [m²/g]
R: Universal gas constant[J/mol K]
m: soot mass[g]
As discussed above, equation (1) indicates stoichiometric formula
regard to calculation model, it was used the exhaust gas in engine
with soot and NO2, equation (2) indicates r’ which is mol
bench tests that was based on as described above gas flow condition
consumption rate (=surface reaction velocity) from carbon
with in fundamental experiment. And soot accumulated mass of this
consumption rate per an unit area5). Equation (3) indicates k’ in
M-PMF was used the range that identified correlation (Up to
equation (2); it is constant of surface reaction velocity which consists
5.41g/m2). Above this soot mass, it is possible that the surface area
of mean molecular speed and Arrenius equation including NO2
coefficient per unit soot mass in equation (5) changes.
6)
activation energy from Kamm . Equation (4) indicates A which is
soot surface area per soot mass weight. Equation (5) indicates
Fig. 14 shows the comparison of the accumulated soot mass
as
calculated with the mathematical model and the measured
soot oxidation velocity that consists of discussed above formulas.
accumulative soot mass in M-PMF along with measured total
Here the k'as constant of surface reaction velocity is constant of
trapped soot mass at both 350C and 280C. The results of 350C and
oxidation velocity by NO2, but a factor caused by structure peculiar
280C were appeared a certain level of correlation with actual
m
of M-PMF which was described contact frequency with soot is not
measured values of M-PMF and the calculation model. However, in
yet included.
both temperature condition, remaining the soot accumulation mass;
To begin with, reaction of NO2 in synthetic gas and soot on the tiny
measured values of M-PMF was smaller than the calculation results.
filters were tested in order to confirm the effectiveness of as
From soot oxidation velocity, average of oxidation velocity on
discussed above equations. The conditions for the tests were as
M-PMF was 1.3~1.4 times faster than the calculation results with
follows; the piece of fleece (=3.14cm2) from filter of M-PMF, small
both temperature conditions. The one of the main factor is suggested
2
soot mass: 1.70mg(=5.41g/m ) which was deposited on the fleece,
a lot of contact with NO2 and soot by the filter of gas flow structure
gas flow in micro reactor: 140ln/min. It was demonstrated various
as described before. The gas goes through only once in the tiny
NO2 concentration and temperature under each steady state;
fleece which has a good correlation with the calculation model. But
measured variety of soot weight on the fleece with an electronic
exhaust gas including NO2 on M-PMF could be introduced several
measure because of calculation of oxidation velocity. Fig.13 is
times and several positions by shovels on the internal metallic foils.
shown comparison with the test results and calculated results from
Therefore, the chance of soot oxidation is considered increasing
equation (5), which is based on NO2 concentration to soot oxidation
more than in one piece of tiny fleece. In real engine, the actual
velocity. According to Fig.13, the calculated results are indicated
oxidization speed of soot is affected by conditions such as soot
that temperature range of over 200C grows the reaction velocity
trapping speed, soot accumulation condition, and change in soot
exponentially, and NO2 concentration grows linearly. Also, the
diameter due to different engine load. For the future, the frequency
actual measurement of oxidation velocity by test piece tests and
factor including soot and NO2 contact frequency and other main
calculated results are generally correlated.
factors will be added to the calculation model of k’ (constant of
surface reaction velocity). As a results, it is considered that further
precisely calculation model can be made.
Fig.13 Comparison of basic experimental data
and calculation model
Fig.14 Comparison of M-PMF results and calculation
4.2. Correlation with calculation model and measured value
In the next place, M-PMF was investigated comparison with
5. Verification of vehicle tests
measured value and as discussed above calculation model.
5.1. Test procedures
The measured value was used with the results of 3.3 (3), which were
Finally, the test results of initial performance in vehicle were shown
steady state tests by 350C and 280C of inlet gas temperatures. In
on soot continuous oxidation with M-PMF. The test was used
M-PMF that was half volume of engine displacement in series with
DOC. The soot reduction behavior was investigated with M-PMF by
driving cycle of continuous EC mode.
**% : the oxidation rate
at each period
5.2. Soot reduction rate by EC mode
In a temperature condition as shown in Fig.15, the test vehicle was
driven by approx. 900km with EC mode continuously. No
68%
deterioration of PM emission after 900km driving, and the soot
85%
0%
reduction rate was 33%. Also, Fig.15 shows a behavior of Smoke
emission by Opacimeter. No deterioration of opacity after 900km
driving, too. From this test, it can be stated stability of the soot reduction.
Fig.16 Transition of Soot mass in M-PMF by vehicle test
6. Conclusion
For spreading clean diesel vehicles, not only wall-flow type of DPF
system but also mainly flow-through type of metallic DPF(M-PMF)
was discussed with various characteristics because of necessity of
simplified PM reduction technology. The main results obtained are
shown below:
1. Flow-through structure of M-PMF was identified the soot
trapping efficiency more than 30% due to modified shovels and
blades on metallic foil, which was introduced a gas flow into a
Fig.15 Temp. and smoke condition on EC mode by vehicle
filter layers.
2. The trapping efficiency of M-PMF can be varied along with
5.3.Behavior of the soot oxidation by EC mode
volume and figure; a smaller diameter and longer length are more
A behavior of soot oxidation on the vehicle was investigated
increasing the trapping efficiency.
about actual oxidation with trapped soot in M-PMF. The oxidized
soot amount was determined in certain interval after start driving
3. NO2/soot reaction ratio and soot oxidation velocity were
confirmed to increase by progress of soot accumulation.
with the temperature condition shown in Fig. 15. Transition of total
4. The oxidation efficiency by NO2 to the trapping soot mass on
soot trapping mass (see 3.1) and actual measured soot mass on
M-PMF was higher than that of wall-flow DPF due to
M-PMF are shown in Fig.16. In addition, the soot oxidation rate of
tridimensional gas diffusion structure.
each measured point was described in the figure. No oxidation
5. From surface reaction with soot and NO2 of viewpoint, making of
behavior was observed until 250km by 3g of that accumulated
the calculation model was attempted for prediction of oxidation
trapping soot. After that, the oxidation rate was increased with
velocity; the actual measurement value and calculation model
increasing of driving distance; it rose to 85% of interval oxidation
ware generally correlated. To improve the calculation precision, it
rate at 900km and the total soot accumulated mass showed a
is necessary to reflect the constant of surface reaction velocity
saturation tendency. This result has been considered that the
(=k’) including the frequency contact of soot and NO2 because of
accumulated soot mainly started to be oxidized over 3g of soot due
to the contact with required amount of NO2.
In the vehicle tests, the facilitatory effect of the oxidation of trapped
soot was identified same as described before engine bench test and
unique structure of M-PMF.
6. Possibility of the continuous soot oxidation on the EC mode was
identified by vehicle tests.
As a future challenge with flow-through PM filter, improvement
calculation model. The acceleration effect of soot oxidation in the
of further trapping efficiency, reduction of a pressure loss and
vehicle test was also confirmed as well as it did in the bench test
further contact frequency by soot and oxidizer are proposed.
result and the simulation result which were described above, and it
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