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Research on exhaust emission control characteristics of a natural gas vehicle — Characteristics of methane oxidation reaction of three-way catalysts
ELSEVIER
JSAE Review 17 (1996) 259-265
Research on exhaust emission control characteristics of a natural gas vehicle - Characteristics of methane oxidation reaction of three-way catalysts Yasunari Hanaki, Toru Sekiba, Mitsunori Ishii, Akihide Okada, Shizuo Ishizawa Central Engineering Laboratories. Nissan Motor Co., Ltd., Natsusima-cho 1, Yokosuka-shi, Kanagawa, 237 Japan Received 4 October 1995
Abstract
Model gas reaction experiments were conducted to analyze the factors causing the conversion rate of hydrocarbons (consisting mainly of CH 4) to decline in the lean-mixture region, using a natural gas engine fitted with a three-way catalyst. The results showed that there was no decline in the conversion rate of methane in CH4-O 2 reactions in the lean-mixture region. However, it was observed that oxidation of CH 4 was suppressed when either H20 or NO was also present. It is thought that prior adsorption of H20 and NO inhibits adsorption of CH 4 at active sites, resulting in a lower conversion rate.
1. Introduction Natural gas is considered to be one of the most promising candidates for a clean substitute fuel and a great amount of research on the compressed natural gas (CNG) fueled vehicle has been performed. Some of these researchers reported emission characteristics of the CNG engine [1-3] and efficiency of catalyst [4-6], and it is known that the HC conversion efficiency decreases rapidly in the lean-mixture region of the CNG engine system equipped with a three-way catalyst. The authors have also been advancing studies and development of the CNG vehicle from the standpoint of reducing the environmental and energy issues, and have been investigating the output and emission characteristics of the CNG engine, the causes of reduction of the HC conversion efficiency in the lean-mixture region, etc. In this paper, the influence of emission of the CNG engine on the conversion characteristics of the three-way catalyst was investigated and the causes of reduction in HC conversion efficiency in the lean-mixture region was analyzed through the model gas experiment in order to understand more clearly the exhaust emission control characteristics of the natural gas vehicle.
2. Overview of experimental equipment
2.1. Fueling / exhaust system of the tested CNG vehicle The general specifications of the CNG engine used in bench tests are given in Table 1, and an overview of the system is shown in Fig. 1. The engine was modified from a gasoline engine with fuel system to a gas injector and the shape of the piston crown was modified to obtain a high compression ratio of 12.0: 1.
2.2. Catalyst performance evaluation device using model gas The apparatus used for reaction analysis was a fixed-bed reactor which consists of a gas supply unit, a preheater, a catalytic reactor and a gas analyzer as shown in Fig. 2. Various reaction gases of variable concentration were supplied from high-pressure gas cylinders. The flow of each gas was controlled by a mass flow controller to produce the desired model gas mixture, which was heated by the preheater, introduced into the catalytic reactor and then exhausted after reaction. Water was vaporized in the vaporizer and supplied into the model gas. The air/fuel ratio of each model gas was controlled by
0389-4304/96/$15.00 © 1996 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved PII S 0 3 8 9 - 4 3 0 4 ( 9 6 ) 0 0 0 1 6- 1
JSAE9631588
260
Y. Hanaki et al./JSAE Reuiew 17 (1996) 259-265
Table l General specification of the test CNG engine Gasoline engine Engine model Number of cylinders Displacement vol. Bore × stroke Compression ratio Fuel supply system Pressure regulator Fuel supply pressure Fuel control
VG30E 6 cyl. 2960 cc 87 × 83 mm 9.0:1 MPI
PressureRegulator t~, • Valve l~'~'2/~rage' 20MPalMax) ~ ~ (0.TMPa) Shot-off
Natural Gas
Natural-gas engine
Fuel
12.0:1 (gas injector) 2 stage 0.7 MPa
closed loop
ExhaustGas Table 2 Evaluation conditions using model gas Model gas specifications Species
Vol. (%)
CH 4 02 CO H2 NO H20 N2
0.16 1.05-1.38 1.20- 1.92 0.40-0.64 0.15 15 balance
Fig. 1. Schematic diagram of fueling/exhaust system of the test CNG vehicle.
passing through the catalyst was controlled to keep SV at 60,000 h - E. The composition of gas mixture which simulates the exhaust emission of the CNG engine was determined by the values obtained by the engine bench test (Table 2).
SV = 60,000 h ' Catalyst inlet temperature = 400°C
3. Conversion characteristics of CNG engine emission by three-way catalysts
adjusting the oxygen concentration in the gas. The conversion efficiency was calculated from the gas concentration values measured both upstream and downstream of the catalyst reactor. Evaluation was done at 400°C (temperature of the model gas at catalyst inlet), and the flow rate of the model gas
MassFlo' Con rolle,
3.1. Engine-out emission characteristics HC, NO x and CO concentration values in the CNG and gasoline engine emission under varied excess air ratio in the engine bench tests are shown (Fig. 3). The exhaust
II II
~
-,,~- Preheater
~ (
~umace
) ~ Catalyst
Gas C linder (CH4, NO, CO, H2, 02, CO, N2)
.............
Gas Supply Unit . . . . . . . . . . . .
Exhaust Gas Fig. 2. Schematic diagram of evaluation system of catalyst efficiency using model gas.
261
Y. Hanaki et a / . / J S A E Review 17 (1996) 259-265
39.2Nm/ 2000=
6000
MBT VG30E
Catalyst: FreshPt/Rh Inlet temperature 400"(2 100,
Gaoli(MPI)/
E
o. 4000
¢3.
80'[ E
O
.o 60
-r 20O0 ~--__~ ~ i . ~
.'~~N~altu ra.
CO
/
THC
t-
Gas (MPI)_ _
~ 4o 0
0
0.5
1.0 1.5 Excess Air Ratio
2.0
0
201i"
3000
0
0.950 0.975 1.00 1.025 (NF=I 5.93)(A/F=t 6.35)(A/F=1 6.77)(A/F=t 7.1 9)
EO_2000 O.
Excess
r~
Fig. 5. Conversion efficiency of Pt/Rh-based three-way catalyst in the CNG engine emission.
~C /Natur?l Gas(MPI)
z 1000
0
0.5
6
1.0
1.5 Excess Air Ratio
I
Air Ratio
2.0
I
fuel. The water content of the emission was 15 to 17% for the CNG and 10 to 13% for the gasoline engine (both values were obtained under the condition close to the theoretical air/fuel ratio).
~)Gasoline(MPI)
3.2. Conversion characteristics of three-way catalysts O O
tural Gas (MPI) 2
0
0.5
1.0
1.5 Excess Air Ratio
2.0
Fig. 3. Emission concentration of the CNG and gasoline engines.
emission level of the CNG engine was considerably lower than that of the gasoline engine, and therefore it was found that the CNG engine has high potential as a low-pollution
Catalyst: FreshPt/Rh ~rature: 400"(2
100 80
The conversion characteristics of three-way catalysts were studied. The conversion efficiency of three-way catalysts with the gasoline engine emission and that of CNG engine emission are shown in Figs. 4 and 5, respectively. A Pt/Rh-based three-way catalyst currently used for the gasoline vehicles was employed as the test catalyst. Comparing the conversion efficiency of the gasoline engine emission with that of the CNG engine emission, it is found that a significant difference exists. In the lean-mixture region, whereas the HC conversion efficiency of the gasoline engine emission increases, that of the CNG engine emission decreases considerably. Furthermore, the HC conversion efficiency of the CNG emission is also low,
co
Catalyst : Fresh Pd Inlet temperature 400"£:
1 0 q0 , . ~ - ~ A 80
Ox
60
6o ,I
> 40 tO 0
.g
>= 4o
o~
t-
..E .£
O 20
20
0 0.950 0.975 1.00 1.025 (NF=13.97)(NF=14.33) (NF=14.7)(A/F=15.07) Excess
Air Ratio
Fig. 4. Conversion efficiency of Pt/Rh-based three-way catalyst in the gasoline engine emission.
THC
o
O
03
0.950 0.975 1.00 1.025 (PJF=I 5.93)(A/F=I 6.35) (MF=I 6.77) (A/F=17.1 9) Excess
Air
Ratio
Fig. 6. Conversion efficiency of Pd-based three-way catalyst in the CNG engine emission.
262
Y. Hanaki et a l . / J S A E Review 17 (1996) 259-265
Catalyst: Fresh Pd
Road Load (50krn/h) 100
A
80
',i!i',i i!!i
60
ii,iii!i,iii!!iiiiiii 6o
O3 t.-
•~ iii~iiiii 4o
>c " 40 0
L) 2O
i!/
80
iiiiii!iliiiiiiiiii
o--t v
"i :5: . m .
I
•
.........
)
. - - , " i::~(Mean)
i
• ' :
~" m " ~
:e-
~0
Q-
,
~
!!:!::i
=---
"~
"
:i!!!!!i:
20
I
2 I=
0_ I~1..%! o A =0.984 (A/F=16.5)
), =1.020 (A/F=17.1)
=1.038 (A/F= 17.4)
Excess Air Ratio Fig. 7. Conversion efficiency of the Pd-based catalyst for each HC species in the CNG emission.
even in the excessively rich mixture region, so that the width of the window (the width of the air/fuel ratio band for obtaining high conversion efficiency) becomes small. Figure 6 shows the conversion efficiency for the CNG engine emission by the Pd-based three-way catalyst which is increasingly used for gasoline engines. Compared with the Pt/Rh-based catalyst, this new catalyst exhibits improved HC conversion efficiency in the excessively rich mixture region, making the window larger. Therefore, the Pd-based catalyst is considered to be promising as a catalyst for CNG vehicles. However, in the same manner as the Pt/Rh-based catalyst, the Pd-based catalyst also exhibits poor HC conversion efficiency in the lean-mixture region. Consequently, it is necessary to improve the HC emission characteristics in the lean-mixture region by improving the engine system or catalyst for the practical use of a CNG vehicle with much lower emission.
3.3. Conversion characteristics of each HC species in the lean-mixture region
Figure 7 shows the conversion efficiency of catalysts for each HC species in the case of a vehicle equipped with the Pd-based three-way catalyst, running at a steady speed of 50 k m / h with varied excess air ratio. Though the conversion efficiency is almost equal for all HCs in the excessive rich mixture region, it is found that in the lean-mixture region, the lower the carbon number of HCs, for example, methane and ethane, the larger the reduction in the HC conversion efficiency becomes. Consequently, it is considered that the reduction of the HC conversion efficiency in the lean-mixture region is caused by the low HC conversion efficiency for HCs of lower carbon numbers, mainly methane.
4. Analysis of oxidation reaction of methane by using model gas
The cause of reduced conversion efficiency of methane, which is the main component of the CNG emission, in the lean-mixture region was analyzed by using model gas. 4.1. Comparison between CNG bench test evaluation and evaluation using model gas
The result of the reaction experiment performed with the gas which simulates the CNG engine emission (hereafter called mixture model gas) is shown in Fig. 8. A Pd-based three-way catalyst of the same specification as that employed in the CNG engine bench test was used as the test catalyst. In the evaluation using the mixture model gas, the phenomenon of reduced conversion efficiency for methane in the lean-mixture region similar to that in the CNG bench test was reproduced. Consequently, it was confirmed that the conditions of experiment are appropriate.
100
80
Catalyst : Fresh Pd Inlet temperature : 400"(2
~H
'~
CH4=1600ppm H2=0.4~0,64% CO=1.2~1.92%
>¢ 40 ~-
NO=1500ppm H20=15%
60
4
o 0 2o
0 0.6
NOx
0.8
1.0
1.2
N2 balance 1.4
Excess Air Ratio Fig. 8. Conversion efficiency of Pd-based three-way catalyst for the mixture model gas.
263
Y. Hanaki et a l . / J S A E Review 17 (1996) 259-265 Catalyst : FreshPd IO0
O( .~I
!'°
40
O tO
20
c
0 0.6
Gas A
CO=1.2~1.92'% NO,,1500ppm H20-15% N2 balance
F',o
CH4 - 02" Model Gas
v
r-
> e- 6O
8
N2 balance
1.0
1.2
"+H2(1.0!o) i ~ ~.... . .' . ~:"................ i ~ .....
80
O ¢/)
CH4,3200ppm O2,,0.45~0.83%--
0.8
Catalyst : Fresh Pd Temperature : 400"C
100
CH4=I600ppm H2=0.4~0.64%
O0
•
I n f l u e n c e of H2 c o n c e n t r a t i o n
Temperature : 400"C
CH,I- 0 2
1.4 4O 0.6
Excess Air Ratio
0.8
1.0
1.2
Catalyst : Fresh Pd Temperature : 400"C
i
loo
4.3. Influence of each component on the methane oxidizing reaction 4.3.1. Influence of H 2 and CO Figure 10 shows the influence of the concentration of H 2 and CO in the model gas on the methane oxidizing reaction. When H 2 and CO are added into the simple model gas, it is noted that the conversion efficiency for methane increases with the increase in the concentration of added H 2 or CO. This may be caused by the accelerated methane oxidizing reaction through the activation of the redox reaction by reaction of H 2 and CO with the oxygen-adsorbing species on the surface of the catalyst.
._o > tO O
!
~ 60
.
.
.
i
05N
.
.
.
.
.
.
i
.
.
.
.
40 0.6
0.8
1.0
1.2
1.4
1.6
Excess Air Ratio
Fig. 10. Influenceof H2 and CO concentrationon CH~-02 reaction. 3000ppm. The reduction of conversion efficiency for methane is observed when NO coexists in the methaneoxygen system. This may be caused by the inhibition of methane adsorption by the NO adsorbed species. In the NO concentration range from 100 to 3000ppm, the reduction of conversion efficiency for methane was constant. It is considered that in the NO concentration of 100ppm or above, the inhibitory effect of NO on the methane conversion was saturated.
4.3.3. Influence of H e 0 concentration Figure 12 shows the influence of H 2° concentration on the conversion efficiency for methane. When H 20 coexists Catalyst : Fresh Pd Temperature : 400"(
1 O0
I
sc
CH4-
i
+ [NO]"~
02~
• O
I00 pprn I 500ppm I
[]
~ooop ~ I
4c 0
4.3.2. Influence of NO concentration The influence of NO concentration on the methaneoxygen reaction is shown in Fig. 11. Here, the concentration of added NO was varied in the range from 100 to
1.6
I n f l u e n c e of C O c o n c e n t r a t i o n
4.2. Conuersion efficiency of methane in the simple model gas and mixture model gas Figure 9 shows the conversion efficiency of the Pd-based catalyst for methane in the reaction gas composed of methane and oxygen (hereafter called simple model gas) and in the mixture model gas. In the simple model gas, contrary to the mixture model gas, the conversion efficiency for methane increases in the lean-mixture region. Thus it is considered that the reduction of conversion efficiency for methane observed in the mixture model gas is due to the influence of one of the other components in the mixture model gas, namely H2, CO, NO or H20, and not to the characteristics of the methane-oxygen reaction. In the region ranging from the theoretical air/fuel ratio to the lean-mixture, the conversion efficiency for methane in the mixture model gas shows high values. This may also be caused by the effect of other component gases. Therefore, the individual influence of each component (H 2, CO, NO and H20) of the mixture model gas on the methaneoxygen reaction was studied.
1.4
Excess Air Ratio
Fig. 9. Conversion efficiency for methane in simple model gas and mixture model gas.
2c
00.6
~+NO
0.8
(100 ~ 3 0 0 0 p p m ) - -
I
1.0
I
1.2
I
1.4
.6
Excess Air Ratio
Fig. 11. Influenceof NO concentrationon CHa-O 2 reaction.
264
Y. Hanaki et al./JSAE Review 17 (1996) 259-265
100
I
....
8o
.9
Catalyst : Fresh Pd Ten" ~era~re: 400"C
CH4 - O2~.
60
Q)
1 L
,
=0 ¢.-)
20 o
~ ~:~
0.6
5% !
• -----"~ ~ ~
0.8
10%
1.0
1.2
1.4
1.6
Excess Air Ratio Fig. 12. Influenceof H20 concentration on CH~t-O 2 reaction.
in the simple model gas, the conversion efficiency for methane decreases in the whole region ranging from the excessive rich to lean mixture. In addition, it is seen that the higher the H 2 0 concentration, the more the methane oxidizing reaction is inhibited. This is considered to be caused by the inhibition of methane adsorption to the active point by the H20-adsorbed species. As described above, the individual influence of H2, CO, NO and H 2 0 on the methane oxidation reaction in the simple model gas was investigated. As a result, it is estimated that H 2 0 and NO, especially H20, greatly contribute to the reduction of HC conversion efficiency in the lean-mixture region with regard to the CNG engine emission control by the three-way catalyst. 4.4. Influence o f H 2 0 concentration in the mixture model gas
In order to confirm the inhibitory effect of H 2 0 , the influence of H 2 0 concentration was investigated using the mixture model gas which has a composition similar to the actual CNG engine emission. The result is shown in Fig. 13. When H 2 0 is not present in the mixture model gas, the conversion efficiency for methane is high in the entire
H2OiJ~
v
,
~
~
i
r
'
~-
0% 5% lO% 12% 15%
o
........~ = =
-760
iN --o-_
tO
40
(l) > E 0
20
It is considered that the significant reduction of conversion efficiency for methane in the lean-mixture region under the presence of H 2 0 is caused by the phenomenon that H 2 0 is adsorbed instead of methane due to poor adsorbability of methane, and this adsorbed H 2 0 inhibits the adsorption of methane. Therefore, the influence of the hydrocarbon species on the HC conversion efficiency in the presence of H 2 0 was studied with the same experimental procedure, employing propylene and propane as HC species which have higher adsorbability on Pd than methane. The result is shown in Fig. 14. Comparing the conversion efficiency of propylene, propane and methane,
~
•1r"
o
r.
E
0
o !
0.6
i
0.8
Excess
!
1 .0
Air
,
1 .2
~"'~,
80
.4
Ratio
Fig. 13. Influenceof H20 on the reaction of mixture model gas.
CO=1.2~1.92%
C3H8
NO=1500ppm N2 balance
b.
i
40
CH4=1600ppm C3HS=650pprn
¢
20
C3H6=722ppm
f.....
0
i 0.6
i
H20=15% H2=0.4~0.64%
N
f
60
z.~ .,.~H 6
CH4
I
o
0
Catalyst : Fresh Pd Temperature : 400"C
lOO
v
o
4.5. Influence o f hydrocarbon species on the H C conversion efficiency
Catalyst : Fresh Pd Temperature : 400"(2
100
80
range of the air/fuel ratio. Under the presence of H20, however, the reduction of conversion efficiency for methane is observed only in the lean-mixture region. The extent of this reduction increases with the increase of H 2 0 concentration in the model gas, and a very low conversion efficiency is observed when the H 2 0 concentration in the actual CNG emission is about 15% (the value at the region close to the theoretical air/fuel ratio). Whereas the simple model gas is affected by H 2 0 in the entire range of the air/fuel ratio, the influence of H 2 0 on the mixture model gas is smaller in the region ranging from rich to theoretical air/fuel ratio. This is presumably caused by the phenomenon that H z or CO contained in the mixture model gas eliminates the H~O-adsorbed species so that the adsorption of methane is not inhibited. Even if the H 2 0 concentration in the mixture model gas is varied, the conversion efficiency for CO and NO does not change significantly under the same condition. Consequently, it is considered that CO and NO are easily adsorbed and have high reactivity so that it is difficult to inhibit the reaction of these species even under the presence of H20.
i
l
0.8
Excess
i 1.0
Air
I 1.2
1.4
Ratio
Fig. 14. Comparisonof the oxidizing reaction behaviorof individual HCs in the presence of H20.
Y. Hanaki et al./JSAE Review 17 (1996) 259-265
all HCs show high conversion efficiency at the theoretical air/fuel ratio. In the lean-mixture region, on the other hand, the efficiency varied in the order shown below: C 3 H 6 > C 3 H 8 > C H 4.
This order is the same as that of the adsorbability of these HCs on Pd. This is thought to be so because, unlike methane, propylene and propane are adsorbed on the catalyst surface without being inhibited by H20, and hence, can react on the catalyst easily. This reduction of HC conversion efficiency is not observed in the gasoline emission. This may be due to the fact that the major HC components of the gasoline emission are olefins and aromatic compounds which have higher adsorbability than methane.
5. Conclusion (1) Comparing the Pt/Rh-based three-way catalyst with the Pd-based three-way catalyst for the exhaust emission control of CNG vehicles, the Pd-based three-way catalyst is considered to be promising for the CNG vehicle because it can provide a higher HC conversion efficiency in the over-rich mixture region and produces a wider window in comparison with the Pt/Rh-based catalyst. However, the
265
conversion efficiency of both Pt/Rh and Pd-based catalysts decreases similarly in the lean-mixture region. (2) The reduction of THC conversion efficiency in the lean-mixture region is mainly caused by the decrease in conversion efficiency for methane. (3) The reduction of conversion efficiency for methane in the lean-mixture region is assumed to be caused by the inhibition of methane adsorption by the adsorption of H 2 0 or NO.
References [1] Park, P.W. et al., Evaluation of 3-Way Catalytic Converters for the Reduction of CH 4 in the Exhaust Stream of a CNG Fueled Vehicle, SAE paper 931990 (1993). [2] Creamer, K.S. and Saunder, J.S., Evaluation of a Catalytic Converter for a 3.75 kW Natural Gas Engine, SAE paper 930221 (1993). [3] Ishii, M. et al., Experimental Studies on a Natural Gas Vehicle, SAE paper 942005 (1994). [4] Siewert, R.M., Methane Oxidation Over Alumina - Supported Noble Metal Catalysts with and without Cerium Additives, J. Catal., Vol. 132, pp. 287-301 (1990). [5] Subramanian, S. et al., Treatment of Natural Gas Vehicle Exhaust, SAE paper 930223 (1993). [6l Ribeiro, F.H. et al., Kinetics of the Complete Oxidation of Methane over Supported Palladium Catalysts, J. Catal., Vol. 146, pp. 537-544 (1993).
JSAE Review 17 (1996) 259-265
Research on exhaust emission control characteristics of a natural gas vehicle - Characteristics of methane oxidation reaction of three-way catalysts Yasunari Hanaki, Toru Sekiba, Mitsunori Ishii, Akihide Okada, Shizuo Ishizawa Central Engineering Laboratories. Nissan Motor Co., Ltd., Natsusima-cho 1, Yokosuka-shi, Kanagawa, 237 Japan Received 4 October 1995
Abstract
Model gas reaction experiments were conducted to analyze the factors causing the conversion rate of hydrocarbons (consisting mainly of CH 4) to decline in the lean-mixture region, using a natural gas engine fitted with a three-way catalyst. The results showed that there was no decline in the conversion rate of methane in CH4-O 2 reactions in the lean-mixture region. However, it was observed that oxidation of CH 4 was suppressed when either H20 or NO was also present. It is thought that prior adsorption of H20 and NO inhibits adsorption of CH 4 at active sites, resulting in a lower conversion rate.
1. Introduction Natural gas is considered to be one of the most promising candidates for a clean substitute fuel and a great amount of research on the compressed natural gas (CNG) fueled vehicle has been performed. Some of these researchers reported emission characteristics of the CNG engine [1-3] and efficiency of catalyst [4-6], and it is known that the HC conversion efficiency decreases rapidly in the lean-mixture region of the CNG engine system equipped with a three-way catalyst. The authors have also been advancing studies and development of the CNG vehicle from the standpoint of reducing the environmental and energy issues, and have been investigating the output and emission characteristics of the CNG engine, the causes of reduction of the HC conversion efficiency in the lean-mixture region, etc. In this paper, the influence of emission of the CNG engine on the conversion characteristics of the three-way catalyst was investigated and the causes of reduction in HC conversion efficiency in the lean-mixture region was analyzed through the model gas experiment in order to understand more clearly the exhaust emission control characteristics of the natural gas vehicle.
2. Overview of experimental equipment
2.1. Fueling / exhaust system of the tested CNG vehicle The general specifications of the CNG engine used in bench tests are given in Table 1, and an overview of the system is shown in Fig. 1. The engine was modified from a gasoline engine with fuel system to a gas injector and the shape of the piston crown was modified to obtain a high compression ratio of 12.0: 1.
2.2. Catalyst performance evaluation device using model gas The apparatus used for reaction analysis was a fixed-bed reactor which consists of a gas supply unit, a preheater, a catalytic reactor and a gas analyzer as shown in Fig. 2. Various reaction gases of variable concentration were supplied from high-pressure gas cylinders. The flow of each gas was controlled by a mass flow controller to produce the desired model gas mixture, which was heated by the preheater, introduced into the catalytic reactor and then exhausted after reaction. Water was vaporized in the vaporizer and supplied into the model gas. The air/fuel ratio of each model gas was controlled by
0389-4304/96/$15.00 © 1996 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved PII S 0 3 8 9 - 4 3 0 4 ( 9 6 ) 0 0 0 1 6- 1
JSAE9631588
260
Y. Hanaki et al./JSAE Reuiew 17 (1996) 259-265
Table l General specification of the test CNG engine Gasoline engine Engine model Number of cylinders Displacement vol. Bore × stroke Compression ratio Fuel supply system Pressure regulator Fuel supply pressure Fuel control
VG30E 6 cyl. 2960 cc 87 × 83 mm 9.0:1 MPI
PressureRegulator t~, • Valve l~'~'2/~rage' 20MPalMax) ~ ~ (0.TMPa) Shot-off
Natural Gas
Natural-gas engine
Fuel
12.0:1 (gas injector) 2 stage 0.7 MPa
closed loop
ExhaustGas Table 2 Evaluation conditions using model gas Model gas specifications Species
Vol. (%)
CH 4 02 CO H2 NO H20 N2
0.16 1.05-1.38 1.20- 1.92 0.40-0.64 0.15 15 balance
Fig. 1. Schematic diagram of fueling/exhaust system of the test CNG vehicle.
passing through the catalyst was controlled to keep SV at 60,000 h - E. The composition of gas mixture which simulates the exhaust emission of the CNG engine was determined by the values obtained by the engine bench test (Table 2).
SV = 60,000 h ' Catalyst inlet temperature = 400°C
3. Conversion characteristics of CNG engine emission by three-way catalysts
adjusting the oxygen concentration in the gas. The conversion efficiency was calculated from the gas concentration values measured both upstream and downstream of the catalyst reactor. Evaluation was done at 400°C (temperature of the model gas at catalyst inlet), and the flow rate of the model gas
MassFlo' Con rolle,
3.1. Engine-out emission characteristics HC, NO x and CO concentration values in the CNG and gasoline engine emission under varied excess air ratio in the engine bench tests are shown (Fig. 3). The exhaust
II II
~
-,,~- Preheater
~ (
~umace
) ~ Catalyst
Gas C linder (CH4, NO, CO, H2, 02, CO, N2)
.............
Gas Supply Unit . . . . . . . . . . . .
Exhaust Gas Fig. 2. Schematic diagram of evaluation system of catalyst efficiency using model gas.
261
Y. Hanaki et a / . / J S A E Review 17 (1996) 259-265
39.2Nm/ 2000=
6000
MBT VG30E
Catalyst: FreshPt/Rh Inlet temperature 400"(2 100,
Gaoli(MPI)/
E
o. 4000
¢3.
80'[ E
O
.o 60
-r 20O0 ~--__~ ~ i . ~
.'~~N~altu ra.
CO
/
THC
t-
Gas (MPI)_ _
~ 4o 0
0
0.5
1.0 1.5 Excess Air Ratio
2.0
0
201i"
3000
0
0.950 0.975 1.00 1.025 (NF=I 5.93)(A/F=t 6.35)(A/F=1 6.77)(A/F=t 7.1 9)
EO_2000 O.
Excess
r~
Fig. 5. Conversion efficiency of Pt/Rh-based three-way catalyst in the CNG engine emission.
~C /Natur?l Gas(MPI)
z 1000
0
0.5
6
1.0
1.5 Excess Air Ratio
I
Air Ratio
2.0
I
fuel. The water content of the emission was 15 to 17% for the CNG and 10 to 13% for the gasoline engine (both values were obtained under the condition close to the theoretical air/fuel ratio).
~)Gasoline(MPI)
3.2. Conversion characteristics of three-way catalysts O O
tural Gas (MPI) 2
0
0.5
1.0
1.5 Excess Air Ratio
2.0
Fig. 3. Emission concentration of the CNG and gasoline engines.
emission level of the CNG engine was considerably lower than that of the gasoline engine, and therefore it was found that the CNG engine has high potential as a low-pollution
Catalyst: FreshPt/Rh ~rature: 400"(2
100 80
The conversion characteristics of three-way catalysts were studied. The conversion efficiency of three-way catalysts with the gasoline engine emission and that of CNG engine emission are shown in Figs. 4 and 5, respectively. A Pt/Rh-based three-way catalyst currently used for the gasoline vehicles was employed as the test catalyst. Comparing the conversion efficiency of the gasoline engine emission with that of the CNG engine emission, it is found that a significant difference exists. In the lean-mixture region, whereas the HC conversion efficiency of the gasoline engine emission increases, that of the CNG engine emission decreases considerably. Furthermore, the HC conversion efficiency of the CNG emission is also low,
co
Catalyst : Fresh Pd Inlet temperature 400"£:
1 0 q0 , . ~ - ~ A 80
Ox
60
6o ,I
> 40 tO 0
.g
>= 4o
o~
t-
..E .£
O 20
20
0 0.950 0.975 1.00 1.025 (NF=13.97)(NF=14.33) (NF=14.7)(A/F=15.07) Excess
Air Ratio
Fig. 4. Conversion efficiency of Pt/Rh-based three-way catalyst in the gasoline engine emission.
THC
o
O
03
0.950 0.975 1.00 1.025 (PJF=I 5.93)(A/F=I 6.35) (MF=I 6.77) (A/F=17.1 9) Excess
Air
Ratio
Fig. 6. Conversion efficiency of Pd-based three-way catalyst in the CNG engine emission.
262
Y. Hanaki et a l . / J S A E Review 17 (1996) 259-265
Catalyst: Fresh Pd
Road Load (50krn/h) 100
A
80
',i!i',i i!!i
60
ii,iii!i,iii!!iiiiiii 6o
O3 t.-
•~ iii~iiiii 4o
>c " 40 0
L) 2O
i!/
80
iiiiii!iliiiiiiiiii
o--t v
"i :5: . m .
I
•
.........
)
. - - , " i::~(Mean)
i
• ' :
~" m " ~
:e-
~0
Q-
,
~
!!:!::i
=---
"~
"
:i!!!!!i:
20
I
2 I=
0_ I~1..%! o A =0.984 (A/F=16.5)
), =1.020 (A/F=17.1)
=1.038 (A/F= 17.4)
Excess Air Ratio Fig. 7. Conversion efficiency of the Pd-based catalyst for each HC species in the CNG emission.
even in the excessively rich mixture region, so that the width of the window (the width of the air/fuel ratio band for obtaining high conversion efficiency) becomes small. Figure 6 shows the conversion efficiency for the CNG engine emission by the Pd-based three-way catalyst which is increasingly used for gasoline engines. Compared with the Pt/Rh-based catalyst, this new catalyst exhibits improved HC conversion efficiency in the excessively rich mixture region, making the window larger. Therefore, the Pd-based catalyst is considered to be promising as a catalyst for CNG vehicles. However, in the same manner as the Pt/Rh-based catalyst, the Pd-based catalyst also exhibits poor HC conversion efficiency in the lean-mixture region. Consequently, it is necessary to improve the HC emission characteristics in the lean-mixture region by improving the engine system or catalyst for the practical use of a CNG vehicle with much lower emission.
3.3. Conversion characteristics of each HC species in the lean-mixture region
Figure 7 shows the conversion efficiency of catalysts for each HC species in the case of a vehicle equipped with the Pd-based three-way catalyst, running at a steady speed of 50 k m / h with varied excess air ratio. Though the conversion efficiency is almost equal for all HCs in the excessive rich mixture region, it is found that in the lean-mixture region, the lower the carbon number of HCs, for example, methane and ethane, the larger the reduction in the HC conversion efficiency becomes. Consequently, it is considered that the reduction of the HC conversion efficiency in the lean-mixture region is caused by the low HC conversion efficiency for HCs of lower carbon numbers, mainly methane.
4. Analysis of oxidation reaction of methane by using model gas
The cause of reduced conversion efficiency of methane, which is the main component of the CNG emission, in the lean-mixture region was analyzed by using model gas. 4.1. Comparison between CNG bench test evaluation and evaluation using model gas
The result of the reaction experiment performed with the gas which simulates the CNG engine emission (hereafter called mixture model gas) is shown in Fig. 8. A Pd-based three-way catalyst of the same specification as that employed in the CNG engine bench test was used as the test catalyst. In the evaluation using the mixture model gas, the phenomenon of reduced conversion efficiency for methane in the lean-mixture region similar to that in the CNG bench test was reproduced. Consequently, it was confirmed that the conditions of experiment are appropriate.
100
80
Catalyst : Fresh Pd Inlet temperature : 400"(2
~H
'~
CH4=1600ppm H2=0.4~0,64% CO=1.2~1.92%
>¢ 40 ~-
NO=1500ppm H20=15%
60
4
o 0 2o
0 0.6
NOx
0.8
1.0
1.2
N2 balance 1.4
Excess Air Ratio Fig. 8. Conversion efficiency of Pd-based three-way catalyst for the mixture model gas.
263
Y. Hanaki et a l . / J S A E Review 17 (1996) 259-265 Catalyst : FreshPd IO0
O( .~I
!'°
40
O tO
20
c
0 0.6
Gas A
CO=1.2~1.92'% NO,,1500ppm H20-15% N2 balance
F',o
CH4 - 02" Model Gas
v
r-
> e- 6O
8
N2 balance
1.0
1.2
"+H2(1.0!o) i ~ ~.... . .' . ~:"................ i ~ .....
80
O ¢/)
CH4,3200ppm O2,,0.45~0.83%--
0.8
Catalyst : Fresh Pd Temperature : 400"C
100
CH4=I600ppm H2=0.4~0.64%
O0
•
I n f l u e n c e of H2 c o n c e n t r a t i o n
Temperature : 400"C
CH,I- 0 2
1.4 4O 0.6
Excess Air Ratio
0.8
1.0
1.2
Catalyst : Fresh Pd Temperature : 400"C
i
loo
4.3. Influence of each component on the methane oxidizing reaction 4.3.1. Influence of H 2 and CO Figure 10 shows the influence of the concentration of H 2 and CO in the model gas on the methane oxidizing reaction. When H 2 and CO are added into the simple model gas, it is noted that the conversion efficiency for methane increases with the increase in the concentration of added H 2 or CO. This may be caused by the accelerated methane oxidizing reaction through the activation of the redox reaction by reaction of H 2 and CO with the oxygen-adsorbing species on the surface of the catalyst.
._o > tO O
!
~ 60
.
.
.
i
05N
.
.
.
.
.
.
i
.
.
.
.
40 0.6
0.8
1.0
1.2
1.4
1.6
Excess Air Ratio
Fig. 10. Influenceof H2 and CO concentrationon CH~-02 reaction. 3000ppm. The reduction of conversion efficiency for methane is observed when NO coexists in the methaneoxygen system. This may be caused by the inhibition of methane adsorption by the NO adsorbed species. In the NO concentration range from 100 to 3000ppm, the reduction of conversion efficiency for methane was constant. It is considered that in the NO concentration of 100ppm or above, the inhibitory effect of NO on the methane conversion was saturated.
4.3.3. Influence of H e 0 concentration Figure 12 shows the influence of H 2° concentration on the conversion efficiency for methane. When H 20 coexists Catalyst : Fresh Pd Temperature : 400"(
1 O0
I
sc
CH4-
i
+ [NO]"~
02~
• O
I00 pprn I 500ppm I
[]
~ooop ~ I
4c 0
4.3.2. Influence of NO concentration The influence of NO concentration on the methaneoxygen reaction is shown in Fig. 11. Here, the concentration of added NO was varied in the range from 100 to
1.6
I n f l u e n c e of C O c o n c e n t r a t i o n
4.2. Conuersion efficiency of methane in the simple model gas and mixture model gas Figure 9 shows the conversion efficiency of the Pd-based catalyst for methane in the reaction gas composed of methane and oxygen (hereafter called simple model gas) and in the mixture model gas. In the simple model gas, contrary to the mixture model gas, the conversion efficiency for methane increases in the lean-mixture region. Thus it is considered that the reduction of conversion efficiency for methane observed in the mixture model gas is due to the influence of one of the other components in the mixture model gas, namely H2, CO, NO or H20, and not to the characteristics of the methane-oxygen reaction. In the region ranging from the theoretical air/fuel ratio to the lean-mixture, the conversion efficiency for methane in the mixture model gas shows high values. This may also be caused by the effect of other component gases. Therefore, the individual influence of each component (H 2, CO, NO and H20) of the mixture model gas on the methaneoxygen reaction was studied.
1.4
Excess Air Ratio
Fig. 9. Conversion efficiency for methane in simple model gas and mixture model gas.
2c
00.6
~+NO
0.8
(100 ~ 3 0 0 0 p p m ) - -
I
1.0
I
1.2
I
1.4
.6
Excess Air Ratio
Fig. 11. Influenceof NO concentrationon CHa-O 2 reaction.
264
Y. Hanaki et al./JSAE Review 17 (1996) 259-265
100
I
....
8o
.9
Catalyst : Fresh Pd Ten" ~era~re: 400"C
CH4 - O2~.
60
Q)
1 L
,
=0 ¢.-)
20 o
~ ~:~
0.6
5% !
• -----"~ ~ ~
0.8
10%
1.0
1.2
1.4
1.6
Excess Air Ratio Fig. 12. Influenceof H20 concentration on CH~t-O 2 reaction.
in the simple model gas, the conversion efficiency for methane decreases in the whole region ranging from the excessive rich to lean mixture. In addition, it is seen that the higher the H 2 0 concentration, the more the methane oxidizing reaction is inhibited. This is considered to be caused by the inhibition of methane adsorption to the active point by the H20-adsorbed species. As described above, the individual influence of H2, CO, NO and H 2 0 on the methane oxidation reaction in the simple model gas was investigated. As a result, it is estimated that H 2 0 and NO, especially H20, greatly contribute to the reduction of HC conversion efficiency in the lean-mixture region with regard to the CNG engine emission control by the three-way catalyst. 4.4. Influence o f H 2 0 concentration in the mixture model gas
In order to confirm the inhibitory effect of H 2 0 , the influence of H 2 0 concentration was investigated using the mixture model gas which has a composition similar to the actual CNG engine emission. The result is shown in Fig. 13. When H 2 0 is not present in the mixture model gas, the conversion efficiency for methane is high in the entire
H2OiJ~
v
,
~
~
i
r
'
~-
0% 5% lO% 12% 15%
o
........~ = =
-760
iN --o-_
tO
40
(l) > E 0
20
It is considered that the significant reduction of conversion efficiency for methane in the lean-mixture region under the presence of H 2 0 is caused by the phenomenon that H 2 0 is adsorbed instead of methane due to poor adsorbability of methane, and this adsorbed H 2 0 inhibits the adsorption of methane. Therefore, the influence of the hydrocarbon species on the HC conversion efficiency in the presence of H 2 0 was studied with the same experimental procedure, employing propylene and propane as HC species which have higher adsorbability on Pd than methane. The result is shown in Fig. 14. Comparing the conversion efficiency of propylene, propane and methane,
~
•1r"
o
r.
E
0
o !
0.6
i
0.8
Excess
!
1 .0
Air
,
1 .2
~"'~,
80
.4
Ratio
Fig. 13. Influenceof H20 on the reaction of mixture model gas.
CO=1.2~1.92%
C3H8
NO=1500ppm N2 balance
b.
i
40
CH4=1600ppm C3HS=650pprn
¢
20
C3H6=722ppm
f.....
0
i 0.6
i
H20=15% H2=0.4~0.64%
N
f
60
z.~ .,.~H 6
CH4
I
o
0
Catalyst : Fresh Pd Temperature : 400"C
lOO
v
o
4.5. Influence o f hydrocarbon species on the H C conversion efficiency
Catalyst : Fresh Pd Temperature : 400"(2
100
80
range of the air/fuel ratio. Under the presence of H20, however, the reduction of conversion efficiency for methane is observed only in the lean-mixture region. The extent of this reduction increases with the increase of H 2 0 concentration in the model gas, and a very low conversion efficiency is observed when the H 2 0 concentration in the actual CNG emission is about 15% (the value at the region close to the theoretical air/fuel ratio). Whereas the simple model gas is affected by H 2 0 in the entire range of the air/fuel ratio, the influence of H 2 0 on the mixture model gas is smaller in the region ranging from rich to theoretical air/fuel ratio. This is presumably caused by the phenomenon that H z or CO contained in the mixture model gas eliminates the H~O-adsorbed species so that the adsorption of methane is not inhibited. Even if the H 2 0 concentration in the mixture model gas is varied, the conversion efficiency for CO and NO does not change significantly under the same condition. Consequently, it is considered that CO and NO are easily adsorbed and have high reactivity so that it is difficult to inhibit the reaction of these species even under the presence of H20.
i
l
0.8
Excess
i 1.0
Air
I 1.2
1.4
Ratio
Fig. 14. Comparisonof the oxidizing reaction behaviorof individual HCs in the presence of H20.
Y. Hanaki et al./JSAE Review 17 (1996) 259-265
all HCs show high conversion efficiency at the theoretical air/fuel ratio. In the lean-mixture region, on the other hand, the efficiency varied in the order shown below: C 3 H 6 > C 3 H 8 > C H 4.
This order is the same as that of the adsorbability of these HCs on Pd. This is thought to be so because, unlike methane, propylene and propane are adsorbed on the catalyst surface without being inhibited by H20, and hence, can react on the catalyst easily. This reduction of HC conversion efficiency is not observed in the gasoline emission. This may be due to the fact that the major HC components of the gasoline emission are olefins and aromatic compounds which have higher adsorbability than methane.
5. Conclusion (1) Comparing the Pt/Rh-based three-way catalyst with the Pd-based three-way catalyst for the exhaust emission control of CNG vehicles, the Pd-based three-way catalyst is considered to be promising for the CNG vehicle because it can provide a higher HC conversion efficiency in the over-rich mixture region and produces a wider window in comparison with the Pt/Rh-based catalyst. However, the
265
conversion efficiency of both Pt/Rh and Pd-based catalysts decreases similarly in the lean-mixture region. (2) The reduction of THC conversion efficiency in the lean-mixture region is mainly caused by the decrease in conversion efficiency for methane. (3) The reduction of conversion efficiency for methane in the lean-mixture region is assumed to be caused by the inhibition of methane adsorption by the adsorption of H 2 0 or NO.
References [1] Park, P.W. et al., Evaluation of 3-Way Catalytic Converters for the Reduction of CH 4 in the Exhaust Stream of a CNG Fueled Vehicle, SAE paper 931990 (1993). [2] Creamer, K.S. and Saunder, J.S., Evaluation of a Catalytic Converter for a 3.75 kW Natural Gas Engine, SAE paper 930221 (1993). [3] Ishii, M. et al., Experimental Studies on a Natural Gas Vehicle, SAE paper 942005 (1994). [4] Siewert, R.M., Methane Oxidation Over Alumina - Supported Noble Metal Catalysts with and without Cerium Additives, J. Catal., Vol. 132, pp. 287-301 (1990). [5] Subramanian, S. et al., Treatment of Natural Gas Vehicle Exhaust, SAE paper 930223 (1993). [6l Ribeiro, F.H. et al., Kinetics of the Complete Oxidation of Methane over Supported Palladium Catalysts, J. Catal., Vol. 146, pp. 537-544 (1993).