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An experiment to purify diesel exhaust gas using an electric trap and three types of catalysers
Journal of
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 723-728
An Experiment to Purify Diesel Exhaust Gas Using an Electric Trap and Three Types of Catalysers S. Watanabe*, K. Kinoshita**, N. Hayashi*, A. Ohashi*, Y. Uchida*, D. Dykes***, G. Touchard****, * Aichi Institute of Technology, Yakusa-cho, Toyota, 470-03 Japan. ** Nakanihon Automotive College, Sakahogi-cho, Gifu, 505 Japan. *** Yokkaichi University, Kayo-cho, Yokkaichi, 512 Japan. ****Universit~ de Poitiers, 40 av. du Recteur Pineau, Poitiers, 86022 France.
ABSTRACT Diesel engine exhaust gas is known to be one cause of photochemical smog, which is so d a m a g i n g to city environments. However, because of its high t h e r m a l efficiency and economic advantages, the diesel engine is not easily dispensable. The authors have previously conducted a series of experiments to assess the purifying effects of a diesel gas purification device employing an electric trap. It has been confirmed that an extraction rate of 6 0 ~ 7 5 % for black smoke can be achieved using this apparatus, but the extraction effect for nitrogen oxides (NOx) is insufficient if the electro-trap device is used on its own. To improve its performance, experiments have now been u n d e r t a k e n using the same device in combination with various types of attached catalysers to oxidise and remove the nitrogen in the exhaust gas. Utilising a commercial 3 - w a y c a t a l y s e r , it p r o v e d d i f f i c u l t to m a i n t a i n t h e gas at a s u i t a b l e temperature, so that no outstanding result could be obtained. Nor was any significant result observed from the use of an ozoniser. However, with a zeolyte catalyser, it was possible to achieve a NOx reduction of 20~30 %. This latter experiment is reported in the present paper.
1. INTRODUCTION Currently, atmospheric pollution through diesel exhaust is becoming ever worse, particularly in large city areas. Diesel engines have a high thermal efficiency, and from an economic point of view also offer the a d v a n t a g e of 0304-3886/97/$17.00 © Elsevier ScienceB.V. All rights reserved. S0304-3886(97)00122-8
PII
724
,7. Watanabe et aL /Journal o f Electrostatics 40&41 (1997) 723-728
efficient fuel combustion. However, compared with a gasoline engine, a diesel engine emits 2 to 20 times more nitrogen oxides (NOx) and 30 to 100 times more black smoke into the atmosphere. The principal means used to combat this problem hitherto was design improvement in the engine itself. But the prospects of satisfying new and more stringent exhaust regulations by this means alone seem poor. I n t h e case of b l a c k smoke, a r e p o r t e d e x p e r i m e n t c o n d u c t e d at the National Environmental Research Institute of the Environment Agency, in which black smoke from diesel engine exhaust was injected into rats, showed that half of the animals died when 0.6 mg was administered, while at doses of 0.9 mg and above none survived [1]. A different report concerning nitrogen oxides concludes that levels of 150 ppm and above can have a fatal influence, and that concentrations of 300 ppm or more will lead to chest and throat pains followed by death from pulmonary edema [2]. Bearing these facts in mind, the need to fred a fast solution to the diesel gas pollution problem is very urgent. Judging from the previous example of gasoline vehicles, one effective answer might be the use of catalysers. The authors of this paper have succeeded in removing black smoke and NOx from diesel engine exhaust gas by the use of a silencer incorporating an electric trap [3]. Experiments have confirmed that the use of this device allows over 60 % of the emitted black smoke to be removed. This means that it ought to be possible to employ an ordinary catalyser of the sort used with gasoline engines to deal with the NOx emissions from a diesel engine, too. Using their electric trap and several other types of catalyser, the authors have carried out further experiments to remove NOx from diesel exhaust gas. The two possible methods available for this are reduction and oxidation. For the reduction method, the a u t h o r s used two kinds of catalyser : a 3-way and a zeolyte type. For the oxidation method, they used an ozoniser. The present paper reports the results of these NOx removal experiments.
2. PURIFICATION OF DIESEL EXHAUST GAS USING A 3-WAY CATALYSER The 3 way catalyser used in the experiment was of a normal commercial type. A 3-way c a t a l y s e r is a device to remove CO, HC and NOx c o n s t i t u e n t s by s i m u l t a n e o u s oxidation and reduction. For these processes to work, the reducible gases CO, H 2 and HC and the oxidising agent O 2 must be present in the exhaust gas in appropriate quantities. To meet this requirement, the a i r - f u e l r a t i o m u s t fall w i t h i n a v e r y n a r r o w r a n g e . The e x p e r i m e n t a l apparatus is shown in Fig. 1. The engine used was a Nissan LD20, with a turbo charger. The engine specifications are shown in Table 1. The experimental procedure was as follows. The engine was first started and allowed to warm up. It was then kept running at a fixed rate of revolutions and u n d e r a c o n s t a n t load, a n d a h e a t e r was s w i t c h e d on to r e g u l a t e the
S. Watanabe et al. /Journal o f Electrostatics 40&41 (1997) 723-728
temperature of the emitted exhaust gas.
~
ENG I NE
~
/"
NOx METE~
ELECTRIC TRAP
I='-÷: ..............H ..... : : ; E .
S,L~CER
~ / . . ~ ~ J HEATER
/ THREE--WAY CATALYSER
Fig. 1 Experimental apparatus for testing of 3-way catalyser Table 1 Dimensions of engine Engine models Total piston displacement (cc) Combustion equipment Piston Bore × Stroke (mm) Compression ratio Mamraum output (PS/rpm) Idle runing (rpm)
20 lO
LD20T. II 1952 Swirl chanber type 85 × 86 21.3 79/4400(net) 650 Engine load' • : 5kgf • : 10kgf
..
• : 15kgf • : 20kgf
o (3
~0
o -----~----~-
g~ Z
lo'oo ,s'0o 20'00 2s'0o Revolution speed[rpm]
Fig. 2 Removal of NOx using a 3-way catalyser
725
726
S. Watanabe et al./Journal of Electrostatics 40&41 (1997) 723-728
Finally, amounts of NOx contained in the exhaust gas were measured. The NOx measurements were taken by means of a portable-type NOx analyser produscd by the firm Shimazu Seisakusho. M e a s u r e m e n t s of NOx concentration were made at the inlet of the electric trap and at the outlet of the catalyser. The extraction rate was found by subtracting and comparing. An example of the measurement results is given in Fig. 2. Looking at these results, it can be confirmed, once again, that the NOx reduction rate shows a negative value. Where the air-fuel ratio exceeds 14.7, there is a decrease in the extraction rate. In the case of a gasoline engine, an air-fuel ratio of 14. 7 is maintained, so that this problem does not arise. However, in a diesel engine the amount of air is kept constant, while the a m o u n t of fuel is varied. As a result, the air-fuel ratio is impossible to control, and varies greatly between about 20 and 80. Diesel e x h a u s t gas c o n t a i n s large a m o u n t s of residual oxygen, and therefore a great deal of oxidation occurs in the catalyser. The NOx increase recorded in the experiment is probably due to the oxidation of N 2 atoms in the exhaust gas. Market 3-way catalysers are designed so as to produce an appreciable effect only at exhaust t e m p e r a t u r e s above 300 °C. The exhaust t e m p e r a t u r e in the present case, without the use of the heater, was approximately 120 °C. No effect of NOx reduction was apparent at this temperature. With the use of the heater, the e x h a u s t t e m p e r a t u r e rose and changes in the NOx c o n c e n t r a t i o n could be discerned. Even so, no very considerable NOx reduction could be found at temperatures of around 240~C. The reason, no doubt, is that the heater used in the e x p e r i m e n t does not raise the e x h a u s t t e m p e r a t u r e into the r a n g e within which the 3-way catalyser works effectively. Another important consideration is engine load. The greater the load, the higher the rate of NOx extraction. The probable reason for this is that as the engine load increases, a decrease occurs in the air-fuel ratio, so t h a t it approaches closer to the optimal air-fuel ratio at which the 3-way catalyser can work effectively. An increase in the number of engine revolutions caused another drop in the NOx extraction rate. The increased number of revolutions no doubt leads to an increased amount of exhaust gas, with which the catalyser is unable to cope.
3. PURIFICATION USING A ZEOLYTE CATALYSER For this experiment, the 3-way catalyser was removed and replaced by a zeolyte one. The chemical composition of zeolyte is SiO ~(91.7%), A1 2 O 3 (8.23%) and Na 2 O(0.03%). The alkaline cations in this compound have the property of exchanging ions relatively easily. The catalyser was manufactured to take advantage of this property to effect an exchange between copper and sodium ions. The experimental procedure was the same as with the 3-way catalyser. The apparatus is shown in Fig. 3.
S. Watanabe et al./Journal of Electrostatics 40&41 (1997) 723-728
727
The NOx concentration was measured at the inlet of the electric trap and at the outlet of the catalyser, and, as before, the extraction rate was found from the difference. Fig. 4 gives an example of the results obtained. A NOx r e d u c t i o n r a t e of b e t w e e n 24 % and 33 % was recorded. No c h a n g e was produced in this rate by variations in either the revolution rate or the engine load. Fig. 5 shows the reduction m e c h a n i s m for the NOx substances. In general, the temperature of the exhaust gas appears to be an important factor in the performance of any kind of catalyser. In the case of the zeolyte catalyser, a t e m p e r a t u r e of 250 ~ 500 °C is found to be necessary. However, in the p r e s e n t e x p e r i m e n t , the e x h a u s t gas t e m p e r a t u r e in the v i c i n i t y of the catalyser was approximately 90 ~ 160 °C. At such low temperatures, the performance of the catalyser will not be satisfactory. At a higher temperature, a better extraction rate would certain|y be obtained. NOx
METER
................
HEATER
i
J ZEOLYTE CATALYSER
Fig. 3 Experimental apparatus for testing of zeolyte catalyser 100 • • • • •
o o
: Skgf : lOkgf 15kgf : 20kgf. : 25kgf
50
Z
°Iooo
Isbo 2o'oo 2s'oo Revolution speed [rpm
Fig. 4
]
Removal of NOx using a zeolyte catalyser
728
X Watanabe et aL/Journal of Electrostatics 40&41 (1997) 723-728
Fig. 5 Model of purification mechanism using a zeolyte catalyser
4. SUMMARY
The use of catalysers with diesel engines has encountered difficulties because of the l a r g e a m o u n t s of black smoke contained in the e x h a u s t gas. I t is to remedy this problem that the authors have designed an electric trap. Among existing devices on the market, the 3-way catalyser appears unable to purify diesel exhaust gas adequately, partly because the air-fuel ratio displays such a wide range of variation. The ozoniser gives poor results[4], too, as is further verified by the chemical reaction equation. In general, such devices require high operating t e m p e r a t u r e s if they are to work effectively. For this reason, it is common practice to attach catalysers immediately to the rear of the engine. However, in the authors' experiments, the electric trap was fitted to the exhaust pipe, as a result of which there was a lowering in the exhaust gas temperature, m3king it i m p o s s i b l e to m a i n t a i n the gas within the o p t i m a l t e m p e r a t u r e range. This probably explains why the NOx removal rate did not show an improvement. In future experiments it will be necessary to use a heating device in order to raise the temperature. ACKNOWLEDGEMENTS The authors express their w a r m e s t t h a n k s to Tosoh Co. Ltd, Sales Co. Ltd., for their kind provision of test materials.
and Sumial
REFERENCES 1. Sagai, F u r u y a m a , Ichinose. J a p a n Society Air of Pollution Vol. 28, No. 4, (1993) pp. 220-230. 2. Osa, Sato, Koda, Yoshida, Takahashi, Tominaga. Chemistry ofNOx, p. 15, Kyoritsushuppan, 1978. 3. Watanabe, Suzuki, Kinoshita, Hayashi. 1lth International Conference on Gas Discharges and their Applications, Vol. 2, (1995) pp. 434-436. 4. W a t a n a b e , Kinoshita, H a y a s h i , Uchida, Touchard. The 3rd I n t e r n a t i o n a l C o n f e r e n c e on A d v a n c e d O x i d a t i o n T e c h n o l o g i e s for W a t e r a n d Air Remediation, (1996)
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 723-728
An Experiment to Purify Diesel Exhaust Gas Using an Electric Trap and Three Types of Catalysers S. Watanabe*, K. Kinoshita**, N. Hayashi*, A. Ohashi*, Y. Uchida*, D. Dykes***, G. Touchard****, * Aichi Institute of Technology, Yakusa-cho, Toyota, 470-03 Japan. ** Nakanihon Automotive College, Sakahogi-cho, Gifu, 505 Japan. *** Yokkaichi University, Kayo-cho, Yokkaichi, 512 Japan. ****Universit~ de Poitiers, 40 av. du Recteur Pineau, Poitiers, 86022 France.
ABSTRACT Diesel engine exhaust gas is known to be one cause of photochemical smog, which is so d a m a g i n g to city environments. However, because of its high t h e r m a l efficiency and economic advantages, the diesel engine is not easily dispensable. The authors have previously conducted a series of experiments to assess the purifying effects of a diesel gas purification device employing an electric trap. It has been confirmed that an extraction rate of 6 0 ~ 7 5 % for black smoke can be achieved using this apparatus, but the extraction effect for nitrogen oxides (NOx) is insufficient if the electro-trap device is used on its own. To improve its performance, experiments have now been u n d e r t a k e n using the same device in combination with various types of attached catalysers to oxidise and remove the nitrogen in the exhaust gas. Utilising a commercial 3 - w a y c a t a l y s e r , it p r o v e d d i f f i c u l t to m a i n t a i n t h e gas at a s u i t a b l e temperature, so that no outstanding result could be obtained. Nor was any significant result observed from the use of an ozoniser. However, with a zeolyte catalyser, it was possible to achieve a NOx reduction of 20~30 %. This latter experiment is reported in the present paper.
1. INTRODUCTION Currently, atmospheric pollution through diesel exhaust is becoming ever worse, particularly in large city areas. Diesel engines have a high thermal efficiency, and from an economic point of view also offer the a d v a n t a g e of 0304-3886/97/$17.00 © Elsevier ScienceB.V. All rights reserved. S0304-3886(97)00122-8
PII
724
,7. Watanabe et aL /Journal o f Electrostatics 40&41 (1997) 723-728
efficient fuel combustion. However, compared with a gasoline engine, a diesel engine emits 2 to 20 times more nitrogen oxides (NOx) and 30 to 100 times more black smoke into the atmosphere. The principal means used to combat this problem hitherto was design improvement in the engine itself. But the prospects of satisfying new and more stringent exhaust regulations by this means alone seem poor. I n t h e case of b l a c k smoke, a r e p o r t e d e x p e r i m e n t c o n d u c t e d at the National Environmental Research Institute of the Environment Agency, in which black smoke from diesel engine exhaust was injected into rats, showed that half of the animals died when 0.6 mg was administered, while at doses of 0.9 mg and above none survived [1]. A different report concerning nitrogen oxides concludes that levels of 150 ppm and above can have a fatal influence, and that concentrations of 300 ppm or more will lead to chest and throat pains followed by death from pulmonary edema [2]. Bearing these facts in mind, the need to fred a fast solution to the diesel gas pollution problem is very urgent. Judging from the previous example of gasoline vehicles, one effective answer might be the use of catalysers. The authors of this paper have succeeded in removing black smoke and NOx from diesel engine exhaust gas by the use of a silencer incorporating an electric trap [3]. Experiments have confirmed that the use of this device allows over 60 % of the emitted black smoke to be removed. This means that it ought to be possible to employ an ordinary catalyser of the sort used with gasoline engines to deal with the NOx emissions from a diesel engine, too. Using their electric trap and several other types of catalyser, the authors have carried out further experiments to remove NOx from diesel exhaust gas. The two possible methods available for this are reduction and oxidation. For the reduction method, the a u t h o r s used two kinds of catalyser : a 3-way and a zeolyte type. For the oxidation method, they used an ozoniser. The present paper reports the results of these NOx removal experiments.
2. PURIFICATION OF DIESEL EXHAUST GAS USING A 3-WAY CATALYSER The 3 way catalyser used in the experiment was of a normal commercial type. A 3-way c a t a l y s e r is a device to remove CO, HC and NOx c o n s t i t u e n t s by s i m u l t a n e o u s oxidation and reduction. For these processes to work, the reducible gases CO, H 2 and HC and the oxidising agent O 2 must be present in the exhaust gas in appropriate quantities. To meet this requirement, the a i r - f u e l r a t i o m u s t fall w i t h i n a v e r y n a r r o w r a n g e . The e x p e r i m e n t a l apparatus is shown in Fig. 1. The engine used was a Nissan LD20, with a turbo charger. The engine specifications are shown in Table 1. The experimental procedure was as follows. The engine was first started and allowed to warm up. It was then kept running at a fixed rate of revolutions and u n d e r a c o n s t a n t load, a n d a h e a t e r was s w i t c h e d on to r e g u l a t e the
S. Watanabe et al. /Journal o f Electrostatics 40&41 (1997) 723-728
temperature of the emitted exhaust gas.
~
ENG I NE
~
/"
NOx METE~
ELECTRIC TRAP
I='-÷: ..............H ..... : : ; E .
S,L~CER
~ / . . ~ ~ J HEATER
/ THREE--WAY CATALYSER
Fig. 1 Experimental apparatus for testing of 3-way catalyser Table 1 Dimensions of engine Engine models Total piston displacement (cc) Combustion equipment Piston Bore × Stroke (mm) Compression ratio Mamraum output (PS/rpm) Idle runing (rpm)
20 lO
LD20T. II 1952 Swirl chanber type 85 × 86 21.3 79/4400(net) 650 Engine load' • : 5kgf • : 10kgf
..
• : 15kgf • : 20kgf
o (3
~0
o -----~----~-
g~ Z
lo'oo ,s'0o 20'00 2s'0o Revolution speed[rpm]
Fig. 2 Removal of NOx using a 3-way catalyser
725
726
S. Watanabe et al./Journal of Electrostatics 40&41 (1997) 723-728
Finally, amounts of NOx contained in the exhaust gas were measured. The NOx measurements were taken by means of a portable-type NOx analyser produscd by the firm Shimazu Seisakusho. M e a s u r e m e n t s of NOx concentration were made at the inlet of the electric trap and at the outlet of the catalyser. The extraction rate was found by subtracting and comparing. An example of the measurement results is given in Fig. 2. Looking at these results, it can be confirmed, once again, that the NOx reduction rate shows a negative value. Where the air-fuel ratio exceeds 14.7, there is a decrease in the extraction rate. In the case of a gasoline engine, an air-fuel ratio of 14. 7 is maintained, so that this problem does not arise. However, in a diesel engine the amount of air is kept constant, while the a m o u n t of fuel is varied. As a result, the air-fuel ratio is impossible to control, and varies greatly between about 20 and 80. Diesel e x h a u s t gas c o n t a i n s large a m o u n t s of residual oxygen, and therefore a great deal of oxidation occurs in the catalyser. The NOx increase recorded in the experiment is probably due to the oxidation of N 2 atoms in the exhaust gas. Market 3-way catalysers are designed so as to produce an appreciable effect only at exhaust t e m p e r a t u r e s above 300 °C. The exhaust t e m p e r a t u r e in the present case, without the use of the heater, was approximately 120 °C. No effect of NOx reduction was apparent at this temperature. With the use of the heater, the e x h a u s t t e m p e r a t u r e rose and changes in the NOx c o n c e n t r a t i o n could be discerned. Even so, no very considerable NOx reduction could be found at temperatures of around 240~C. The reason, no doubt, is that the heater used in the e x p e r i m e n t does not raise the e x h a u s t t e m p e r a t u r e into the r a n g e within which the 3-way catalyser works effectively. Another important consideration is engine load. The greater the load, the higher the rate of NOx extraction. The probable reason for this is that as the engine load increases, a decrease occurs in the air-fuel ratio, so t h a t it approaches closer to the optimal air-fuel ratio at which the 3-way catalyser can work effectively. An increase in the number of engine revolutions caused another drop in the NOx extraction rate. The increased number of revolutions no doubt leads to an increased amount of exhaust gas, with which the catalyser is unable to cope.
3. PURIFICATION USING A ZEOLYTE CATALYSER For this experiment, the 3-way catalyser was removed and replaced by a zeolyte one. The chemical composition of zeolyte is SiO ~(91.7%), A1 2 O 3 (8.23%) and Na 2 O(0.03%). The alkaline cations in this compound have the property of exchanging ions relatively easily. The catalyser was manufactured to take advantage of this property to effect an exchange between copper and sodium ions. The experimental procedure was the same as with the 3-way catalyser. The apparatus is shown in Fig. 3.
S. Watanabe et al./Journal of Electrostatics 40&41 (1997) 723-728
727
The NOx concentration was measured at the inlet of the electric trap and at the outlet of the catalyser, and, as before, the extraction rate was found from the difference. Fig. 4 gives an example of the results obtained. A NOx r e d u c t i o n r a t e of b e t w e e n 24 % and 33 % was recorded. No c h a n g e was produced in this rate by variations in either the revolution rate or the engine load. Fig. 5 shows the reduction m e c h a n i s m for the NOx substances. In general, the temperature of the exhaust gas appears to be an important factor in the performance of any kind of catalyser. In the case of the zeolyte catalyser, a t e m p e r a t u r e of 250 ~ 500 °C is found to be necessary. However, in the p r e s e n t e x p e r i m e n t , the e x h a u s t gas t e m p e r a t u r e in the v i c i n i t y of the catalyser was approximately 90 ~ 160 °C. At such low temperatures, the performance of the catalyser will not be satisfactory. At a higher temperature, a better extraction rate would certain|y be obtained. NOx
METER
................
HEATER
i
J ZEOLYTE CATALYSER
Fig. 3 Experimental apparatus for testing of zeolyte catalyser 100 • • • • •
o o
: Skgf : lOkgf 15kgf : 20kgf. : 25kgf
50
Z
°Iooo
Isbo 2o'oo 2s'oo Revolution speed [rpm
Fig. 4
]
Removal of NOx using a zeolyte catalyser
728
X Watanabe et aL/Journal of Electrostatics 40&41 (1997) 723-728
Fig. 5 Model of purification mechanism using a zeolyte catalyser
4. SUMMARY
The use of catalysers with diesel engines has encountered difficulties because of the l a r g e a m o u n t s of black smoke contained in the e x h a u s t gas. I t is to remedy this problem that the authors have designed an electric trap. Among existing devices on the market, the 3-way catalyser appears unable to purify diesel exhaust gas adequately, partly because the air-fuel ratio displays such a wide range of variation. The ozoniser gives poor results[4], too, as is further verified by the chemical reaction equation. In general, such devices require high operating t e m p e r a t u r e s if they are to work effectively. For this reason, it is common practice to attach catalysers immediately to the rear of the engine. However, in the authors' experiments, the electric trap was fitted to the exhaust pipe, as a result of which there was a lowering in the exhaust gas temperature, m3king it i m p o s s i b l e to m a i n t a i n the gas within the o p t i m a l t e m p e r a t u r e range. This probably explains why the NOx removal rate did not show an improvement. In future experiments it will be necessary to use a heating device in order to raise the temperature. ACKNOWLEDGEMENTS The authors express their w a r m e s t t h a n k s to Tosoh Co. Ltd, Sales Co. Ltd., for their kind provision of test materials.
and Sumial
REFERENCES 1. Sagai, F u r u y a m a , Ichinose. J a p a n Society Air of Pollution Vol. 28, No. 4, (1993) pp. 220-230. 2. Osa, Sato, Koda, Yoshida, Takahashi, Tominaga. Chemistry ofNOx, p. 15, Kyoritsushuppan, 1978. 3. Watanabe, Suzuki, Kinoshita, Hayashi. 1lth International Conference on Gas Discharges and their Applications, Vol. 2, (1995) pp. 434-436. 4. W a t a n a b e , Kinoshita, H a y a s h i , Uchida, Touchard. The 3rd I n t e r n a t i o n a l C o n f e r e n c e on A d v a n c e d O x i d a t i o n T e c h n o l o g i e s for W a t e r a n d Air Remediation, (1996)