Ind. Eng. Chem. Res. 1993,32, 1805-1810
1805
Removal of Nitric Oxide by Its Reduction with Hydrocarbons over Alumina under Lean Conditions Sam Subramanian,’ Robert J. Kudla, Woosang Chun, and Mohinder S. Chattha MD 3179 SRL,Chemical Engineering Department, Ford Motor Company, P.O.Box 2053, Dearborn, Michigan 48121 -2053
The objective of this study is to investigate the removal of nitric oxide (NO) from a net oxidizing gas mixture using hydrocarbons (HC) as reductants over alumina. Studies were conducted to investigate the effect of operating variables such as catalyst temperature, space velocity, and HC concentration on NO conversion. The durability of the catalyst was also investigated. Propane (C3H8) and propylene (C3H6) were used as reductants during the flow reactor experiments. The study shows that the removal of NO over A1203occurs in the 500-700 O C temperature range when either C3H6 or C3H8 is used as reductant. Maximum NO conversion is observed around 600 “C. The NO conversions observed using C3H8 and C3H6 are comparable when NO is being removed from a NO-HC-O~-SO~-CO-HZ-H~O stream at space velocities lower than 38 570 h-l. At space velocities higher than 38 570 h-l, higher NO conversion is observed when C3H6 is used as reductant. Durability investigations conducted on the pulse flame combustor show that the durability of A1203is better than that of the zeolite-based catalysts for removing NO under oxidizing conditions.
Introduction One of the promising technologies being investigated to improve the fuel economy of engines is the “lean-burnn technology (Gomez and Reinke, 1988). While conventional gasoline-fueled engines operate around the stoichiometric point [air-fuel (A/F) ratio of 14.61, lean-burn engines operate with an A/F ratio of 19-27. The exhaust from lean-burn engines contains excess oxygen and is net oxidizing. Conventional automotive three-way catalysts (TWCs) are not effective in reducing nitric oxide (NO) when the composition of the gas is net oxidizing (Kummer, 1980). Therefore, TWCs cannot be used to reduce the NO, present in the exhaust of lean-burn engines. The development of catalysts for the selective reduction of NO with hydrocarbons (HC) in oxidizing atmospheres has recently attracted considerable attention (Held and Koenig, 1987; Fujitani et al., 1988). Investigators have reported that the selective reduction of NO occurs very efficiently over copper (e.g., Cu-ZSM-5) and other metalexchanged zeolites (Held et al., 1990; Iwamoto et al., 1990, 1991; Sato et al., 1991; Hamada et al., 1990). It has been shown that hydrocarbons such as propane (C3H8), propylene (C3H6), and ethylene (CzH4) are effective in reducing NO over zeolite-based catalysts (Held et al.,1990). Iron silicates, which are structurally similar to zeolites, have also been found to be effective (Kikuchi et al., 1991). Recently, it has been reported that the reduction of NO with C3H8 occurs over oxides such as alumina (A1203), silica-alumina (SiOz-A1203), titania (TiOZ), and zirconia (ZrO2) (Kintaichi et al., 1990). The catalytic activity of these oxides has been attributed to their acidity. Among oxide catalysts, A1203has been found to be the most effective. Alumina is inexpensive and thermally stable. Therefore, A1203 is an attractive catalyst for removing NO from lean-burn engine exhaust. A survey of the literature suggests that only a limited number of studies have been conducted to investigate the NO-HC reaction over Al2O3. The mechanism of this reaction (C3H8 as reductant) has been studied (Hamada et al., 1991a). It has been proposed that NO2 (formed by the oxidation of NO) reacts with C& to form Nz. To whom correspondence should be addressed. Telephone: (313)323-1486. FAX: (313)248-5627.
Automotive exhaust typically contains slow-reacting as well as fast-reacting hydrocarbons, typical representatives of which are C3H8 and C3H6, respectively. Carbon monoxide (CO), hydrogen (Hz), sulfur dioxide (SOz),and water vapor (HzO) are usually present as well. The objective of this study is to investigate the removal of NO from a net oxidizing gas mixture over A1203.The effects of operating variables such as space velocity, catalyst temperature, and HC (C3H8and C3Hs) concentration and the effect of the presence of SO2 and CO and H2 on the NO reduction activity of A1203 have been studied. The removal of NO from NO-HC-O~-SOZ-CO-HZ-HZO reaction mixtures and the durability of A1203 for lean-burn engine exhaust treatment have also been investigated. The performance of A1203 has been compared with that of CuZSM-6.
Experimental Section A. Flow Reactor Studies. These investigations were conducted on powder samples of -pA1203 (Alumina-C, Degussa Corp.) with a nominal surface area of 100 m2/g. The activity measurements were conducted using an integral reactor (Subramanian et al., 1992). The reactor consisted of a horizontal Pyrex glass tube. The temperature was measured just downstream of the catalyst bed. The CO, HC, and NO concentrations were determined using infrared (Beckman Model 868), flame ionization (Beckman Model 400),and chemiluminescence (Beckman Model 951A) detectors, respectively. (The chemiluminescence detector was used in the NO mode to determine the NO concentration.) The 0 2 concentration was determined using a membrane cell detector (Sensormedics OM-11EA). All studies were conducted using a total gas flow rate of 3000 standard (STP)cm3/min. Nitrogen was used as the carrier gas. 1. Effect of Temperature. A feed gas containing 1000 ppm NO, 40 000 ppm 0 2 , and 2025 ppm C3H8 or 2250 ppm C3Hs was used in this study. The feed gas mixture had a redox ratio ( R ) of 0.25. The redox ratio is the ratio of the reducing components to the oxidizing components in the feed gas (Subramanian et al., 1992). The catalyst temperature was increased in steps of 50 OC between 400 and 700 OC. Sufficient time was allowed for steady state
0888-588519312632-1805$04.00/0 0 1993 American Chemical Society
1806 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
Table I. Space Velocities Corresponding to the Amounts of Alumina Used space vel based on space vel for amt of packed density (0.6 g/cm3) A1203 washcoated A 1 2 0 3 (g) of r-A1203 powder (h-9 monolith" (h-l) 45 725 0.6 180 OOO 16 140 1.7 63 530 9 800 2.8 38 570 7 035 3.9 27 692 5 485 5.0 21 600
I""
, Feed Gas Composltlon: C3H6 = 2250 ppm NO = 1000 ppm
80
Gas Flow Rate
Space Veloclty
a Based on a monolith having a bulk density of 0.585 g/cm3 and an AZO3loading of 35% by weight of the cordierite substrate.
to be attained at each temperature. The NO conversions observed at steady state are reported here. 2. Effect of HC Concentration. The NO and 0 2 concentrations were maintained constant at 1000 and 40 000 ppm, respectively. The catalyst temperature was held constant at 600 "C, and the HC concentration was varied to change the redox ratio between 0.05 and 0.55. These experimental conditions (i.e., 1000ppm NO, 40 000 ppm 0 2 , and variable amounts of HC) are designated condition A. 3. Effect of SO2 and CO and H2. These studies were conducted using 1000 ppm NO and 40000 ppm 0 2 . Variable amounts of HCs (C3Ha or C3Hd were used. The catalyst temperature was adjusted to 600 "C in a HC, NO, 0 2 stream. The feed gas was then modified by adding gases (SO2 and CO and H2) to obtain the following compositions: (i) Condition B: 20 ppm of SO2 was added to the NOHC-02 mixture. (ii) Condition C: 15 000 ppm CO and 5 000 ppm H2 were added to the NO-HC-02 mixture. CO and H2 normally coexist in the automotive exhaust in the ratio 3:l (Heywood, 1988). The effect of CO and H2 was not assessed separately. It may be noted that the addition of CO and H2 led to an increase in the catalyst temperature due to the exothermic nature of CO and Ha oxidation reactions. The increase in temperature was between 3 and 8 "C. 4. Removal of NO from N O - H C - O ~ - S O Z ~ O - H ~ H2O Reaction Mixtures. These studies were conducted using 1000ppm NO and 40 000 ppm 0 2 . Variable amounts of HCs (C3He or C3H6) were used. The catalyst temperature was adjusted to 600 "C in the HC-NO-02 stream. SOz, CO, Ha, and H2O were added to obtain a feed gas containing 20 ppm SOP, 15 000 ppm CO, 5000 ppm H2, and 20 OOO ppm H2O. 5. Effect of Space Velocity. The space velocity was varied by changing the amount of catalyst used. The amounts of yA1203 used and the corresponding space velocities are presented in Table I. Two space velocities are reported. The space velocity based on the packed density of the yA1203 powder (0.6 g/cm3) is reported in column 2. Washcoated cordierite monoliths having a honeycomb structure are typically used as substrates for automotive exhaust treatment catalysts (Kummer, 1980). Space velocities were also calculated assuming a 400 cells per square inch (cpsi) monolith to be coated with 35% by weight yA1203. Such a monolith typically has a bulk density of 0.585 g/cm3. The total volume of the monolith was used to calculate the space velocity, and the values are reported in column 3 (Table I). B. Pulse Flame Combustor Evaluation. The durability of A1203was evaluated on a pulse flame combustor (PFC). A detailed description of this apparatus has been previously reported (Siegl et al., 1992). Briefly, the PFC consists of two major sections: a combustor,?kwhich the exhaust gases are generated and a catalytic reactor, in
3000 cm3/mln
= 27,690 hr.'
0 Space Velocity 38,570 hr.' 0 Space Velocity = 63,530 hr.' 0 Space Velocity = 180,000 hr.'
1
0
200
400
600
800
Temperature, "C Figure 1. Effect of temperature on NO conversion as a function of space velocity when C3He is used as reductant. (The solid lines represent a smooth fit through the experimentally observed data points.)
which catalyst aging is carried out at selected temperatures and exhaust stoichiometries. The PFC is also equipped with a set of analyzers similar to those used in the flow reactor apparatus. The HC, CO, NO, and 0 2 concentrations of the gases entering and leaving the catalyst are monitored. A 400 cpsi cordierite monolith washcoated with 7-A1203 was used during this investigation. The studies were conducted at space velocities of 4000 and 6667 h-1. Pure isooctane and contaminant-doped isooctane (isooctane with 0.03 % S, 3 mg/gal Pb, and 0.8 mg/gal P) were used in the combustor. The redox ratio of the gas stream at the catalyst inlet was 0.44. Studies were first conducted to investigate NO and HC conversions as a function of temperature. The catalyst temperature was later held constant at 600 "C at 6667 h-' space velocity, and its performance was monitored as a function of time.
Results and Discussion A. Flow Reactor Evaluation. Nitric oxide conversions as high as 25 ?6 were observed when the NO-HC-02 feed gas mixture was allowed to flow through the empty reactor at 600 "C. Prolonged use of the quartz reactor tube did not result in a change in the NO conversion. The NO conversions observed from the empty reactor were similar to those observed when 5.0 g of Si02 (500 m2/g surface area) was placed inside the reactor. This suggests that the NO conversion results from homogeneous gas phase reaction. No nitric oxide conversion was observed, however, when CO and Hz were added to the NO-HC-02 gas mixture. The homogeneous gas phase reaction may be suppressed by the presence of CO and H2. The NO conversions were not "correctedn for the conversions observed from the empty reactor; observed NO conversions are reported here. On the basis of repetition of experimental runs, the error in the conversions reported was assessed to be f3.5 percentage points. The effects of temperature, feed gas HC concentration, space velocity, and presence of CO and H2, SO2, and H2O in the feed gas on NO conversions are discussed in the following sections. 1. Effect of Temperature and Space Velocity on Removal of NO from NO-HC-02 Mixture. In Figure 1, the effect of temperature on NO conversion is shown as a function of space velocity when C3H6 is used as reductant. The NO-C3H6-02 reaction mixtures were used
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1807 loo
-r
100 NO = 1000 ppm O2 = 40,000 ppm
-c
Feed 0 1 8 Composltlon:
80
0
Gas Flow Rate = 3000 cm3/mIn
I
0
f
r
s Flow Rate = 3000 cm3/mln
I
60
0 Spaca Velocity = 38,570 hr" 0 Space Veloclty
= 63,530 hr"
0 Space Veloclly = 180,000 hr"
0 0
0.1 810
0.2 1620
0.3 2430
0.4 3240
0.5 4050
0.6 4860
0.1 810
0 0
0.2 1620
R Value C3Hs (PPm)
loo
T
'.
NO = 1000 ppm 0. = 40,000 BDm
Feed Gas Composltlon: NO = 1000 ppm O2 = 40,000 ppm C3Hl = Varlable
2ol--ml-J 0
900
0.6 4860
Feed Gas Composltlon:
0
0.1
0.5 4050
R Value C3Hg (PPm)
, crn3/rnln
0 0
0.4 3240
0.3 2430
0.2
1800
0.3
2700
0.4
3600
0.5
4500
0.6
5400
R Value C3H6 (PPm)
~~s ,-iow ~~t~
0
Space Veloclly Space Veloclly 0 Spaca Veloclly 0 Space Voloclty 0 space velocity
= 3000 cm~/mln
I
= 21,600
hrl
= 27,690 hr-' = 38,570 hr-'
= 83,530 hr.) = 180,000 hr.'
204 0
1
0 0
,
I , 0.1 0.2 900 1800
I
I
0.3 2700
I I I I 0.4 0.5 0.6 3600 4500 5400
R Value C3Hg (PPm)
Figure 2. (a, top) Effect of C3He concentration on NO conversion with space velocity as parameter. (b, bottom) Effect of C3Hs concentration on NO conversion with space velocity as parameter. (The solid lines represent a smooth fit through the experimentally observed data points.)
Figure 3. (a, top) Effect of C3Hs concentration on C3Hs conversion with space velocity as parameter. (b, bottom) Effect of CS& concentration on C3& conversion with space velocity as parameter. (The solid lines represent a smooth fit through the experimentally observed data points.)
here. It is observed that the reduction of NO occurs in the 500-700 "C temperature range. The temperature range over which the reduction of NO is observed is similar (Le., 500-700 "C) when C3H8 is used as reductant. Maximum NO conversion is observed around 600 OC when either C3H6 or C3H8 is used as reductant. These results are in general agreement with those reported in the literature (Kintaichi et al., 1990). These investigators have observed maximum NO conversion around 500 "C when C3H8 is used as reductant. It was also observed during the present investigation that the fraction of NO converted to NH3 is negligible. 2. Effect of Feed Gas HC Concentration and Space Velocity on Removal of NO from NO-HC-02 Mixture. The effect of HC concentration on NO conversion was investigated as a function of space velocity. NO-HC-02 reaction mixtures were used. The NO conversion,the C3H8 conversion, and the percentage of C3H8 converted to CO (calculated by dividing the concentration of CO at the exit of the reactor by 3 times the inlet concentration of C3H8 and multiplying by 100) are shown as a function of R as well as C3H8 concentration in Figures 2a, 3a, and 4a, respectively. Similar results observed with C3H6 as reductant are shown in Figures 2b, 3b, and 4b, respectively. It is observed that the NO conversion increases with an increase in the HC concentration and a decrease in the space velocity (Figure 2). The.NO conversions observed using a feed gas HC concentration of 1625ppm are shown as a function of space velocity in Figure 5. I t is observed that C3Hs and C3H6 provide generally similar levels of NO conversion over the range of space velocities investigated.
As shown in Figure 3, the HC conversion at a given space velocity is generally lower when C3H8 (slowburning) is used as reductant. When C3H6 (fast burning) is used as reductant, the HC conversion is close to 100% over the range of HC concentrations and space velocities investigated. As shown in Figure 4a, a t a given redox ratio, the fraction of C3H8 converted to CO is highest in the 38 570-63 530-h-' space velocity range. In contrast, when C3H6 is used as reductant, the fraction of HC converted to CO increases or remains constant with an increase in the space velocity (Figure 4b). Thus NO conversion and the fraction of C3H8 converted to CO increase when the space velocity decreases from 180 OOO to 63 530 h-l, and they remain relatively constant in the 38 570-63 530-h-l space velocity range. The NO conversion increases and the fraction of C3Ha converted to CO decreases when the space velocity decreases below 38 570 h-l. When C3H6 is used as reductant, the NO conversion increases and the fraction of C3H6 converted to CO decreases when the space velocity decreases below 180 000 h-l. 3. Effect of SO2 and CO and H2 on NO Removal. No nitric oxide conversion was observed when a NO-02CO-Hz feed gas mixture was flowed over Al2O3. (It may be noted that a part of the CO introduced was converted to C02.) These experiments showed that CO and H2 are not selective in reducing NO under net oxidizing conditions. The effect of the presence of SO2 and CO and HZ on the NO conversion was studied as a function of space velocity. The NO conversion observed a t different space velocities a t constant C3H8 and C3H6 feed gas concentrations (1620 ppm) under conditions A, B, and C is shown
1808 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Feed Gar CompOSlllOn: C3Hl = Variable NO = 1000 ppm o2= 40,000 ppm T = 600 'C
0 0 0
Gas FIOW Rate = 3000 cm3/mln
60-
0 NO-C3H8-OZ(ConditionA)
A 0
0 0
0.1
810
0.3
0.2 1620
0.4 3240
2430
0.5 4050
R Value C3Hs ( P P ~ )
I
I
I
I
I
I
I
10
20
30
40
50
60
70
60
70
'--I I
Feed Gao Composlllon: NO = 1000 ppm O2 = 40,000 ppm C3Hs = Varleble T = 600
0 Space Veloclly = 21,600 hr"
+ Space Veloclty = 27,890 hr-' 0 Space Veloclty 0 Spice Vdoclty
oc
P
0
0.6 4860
NO-C3H8-O2-SO2(Condltlon B) NO-C3Hs-02-CO-H2(Condition C)
= 38,570 hr.'
63,530 hr.'
I
0 NO-C3H6-02(CondltlonA)
A NO-C3H6-02-S02(Condltlon8) NO-C3H6-02-CO-HZ (Condltlon C)
0
0.1 900
0 0
0.2 1800
0.3 2700
0.4 3600
0.5 4500
0.6 5400
R Value C3Hs (ppm)
Figure 4. (a, top) Effect of CsH8 concentration on the fraction of CsHa converted to CO with space velocity as parameter. (b, bottom) Effect of (23% concentration on the fraction of C3H6 converted to CO with space velocity as parameter. (The solid lines represent a smooth fit through the experimentally observed data points.) 100
n
21,600
=
38,570 27,690 Space Veloclty (hr-l)
NO-C,Hs-02
63,530
0NO-CsHg-02
Figure 5. Comparison of NO conversions observed using CsHe and CS& as reductants. (lo00ppm NO, 1625ppm C3H8, and 40 OOO ppm 02; 1OOO ppm NO,1625 ppm C3&, and 40 OOO ppm 0 2 . )
in Figure 6 a and 6b, respectively. It should be noted that the catalyst temperature was 600 OC under condition A. The catalyst temperature under condition C was slightly higher (up to 8 "C) than 600 OC. It will be shown later that the effect of these relatively small changes in catalyst temperature on NO conversion is negligible. (a) Effect of SOz. The addition of SO2 to the feed gas leads to a decrease in NO conversion over the entire space velocity range investigated when C3Hs is used as reductant
10
20
30
40
50
Space Veloclty (hr-1) (Thousands)
Figure 6. Effect of SO2 and CO and Hz on NO removal. (a, top) Condition A 1OOO ppm NO, 1625 ppm CsHa, and 40 OOO ppm 0 2 . Condition B: 1OOO ppm NO,1625 ppm C&, 40 OOO ppm 02,and 20 ppm SO2. Condition C: 1OOO ppm NO, 1625 ppm CsH8,40 OOO ppm 02,15OOO ppm CO, and 5000ppm Hz. (b, bottom) Condition A 1OOO ppm NO,1625 ppm C s h , and 40 OOO ppm 02. Condition B: 1OOO ppm NO,1625ppm c a s , 40 OOO ppm 02,and 20 ppm so^. Condition C: 1OOO ppm NO, 1625 ppm Cs&, 40 OOO ppm 02,and 15 OOO ppm CO, and 5000 ppm Hz.
(Figure 6a). Although lower NO conversions are observed in the presence of SO2 (condition B), the activity quickly returns to that observed under condition A when the SO2 flow is switched off. Similar observations have been reported during the reduction of NO using C3H6 over a Cu-ZSM-5 catalyst (Iwamoto et al., 1991). These investigators speculate that SO2 competes with NO for the adsorption sites, and this lowers NO conversion (Teraoka et al., 1987). When CsH6 is used as reductant (Figure 6b), the addition of SO2 to the feed gas does not appear to lead to a significant decrease in NO conversion. ( b ) Effect of CO and H2. When C3H6 is used as reductant, the addition of CO and Hz lowers the NO conversion over the entire range of space velocities investigated (Figure 6b). When C3Hs is used as reductant, the effect of addition of CO and H2 on NO conversion depends on the catalyst space velocity (Figure 6a). It is known that the oxidation of CO (to COZ)and Hz (to HzO) are exothermic reactions (Reid et al., 1977). During the present study, it was observed that the catalyst temperature increased by 3-8 "C when CO and Hz were added to the feed gas. In other words, the catalyst temperature was 600 OC under condition A and in the 603-608 OC range under condition C. Recall that the NO conversion drops when the temperature increases above 600 "C (Figure 1). The increase in the catalyst temperature could in principle result in lower NO conversion. This effect was investigated in greater detail as outlined in the following paragraphs. Experiments were conducted at temperatures above and below 600 "C, i.e., 520 and 620 OC (designated TOC), to determine the effect of addition of CO and Hz on NO
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1809 Table 11. Effect of CO and HZon NO Removal reactanta 2250 ppm C&, 1000 ppm NO, 40 OOO ppm 02 2250 ppm CS&, 1000 ppm NO, 40 000 ppm 0~,15 000 ppm CO, 5000 ppm Hz 2250 ppm Cab, 1000 ppm NO, 40 OOO ppm 02 2250 ppm C&, 1OOO ppm NO, 40 OOO ppm 02 2250 ppm C&, 1000 ppm NO, 40 OOO ppm 0 ~ ~ 000 1 5ppm CO,5000 ppm Hz 2250 ppm C&, 1OOO ppm NO, 40 OOO ppm 02 a Denotes TOC as described in test. Denotes TlOC as described in test.
conversion. The catalyst temperature was held constant at TOC in a NO-C3H& mixture, and the C3H6 and NO conversions were monitored. CO (15 OOO ppm) and Hz (5000 ppm) were added to the feed gas. The catalyst temperature increased to TlOC, and NO and C3H6 conversions were measured. The CO and Hz flow was then cut off. The catalyst temperature was increased to TIOC, and the NO and C3H6 conversions were determined. The results of these studies are shown in Table 11. It is observed that the effect of addition of CO and Hz on NO conversion is dependent on the initial catalyst temperature, i.e., TOC. While NO conversion increases when CO and Hz are introduced at 520 "C, NO conversion decreases when CO and Hz are introduced at 620 "C. However the NO conversion does not approach the values observed when CO and Hz are cut off and the temperature is increased to T1"C. This indicates that the increase in temperature resulting from the CO and Hz oxidation reactions does not solely account for the observed decrease in NO conversion when CO and Hz are added to the NOC3H6-02 mixture at 600 "c. It is interesting to note that the C3Hsconversionsremain unaffected either when CO and Hz are added or when the catalyst temperature is increased from TOC to T1"C. 4. Removal of NO from NO-HC-02-SOz-CO-HzHzO Mixtures. The removal of NO from NO-HC-02SOZ-CO-HZ-HZO mixtures was studied. The effect of changes in space velocity and feed gas C3H8 and C3H6 concentration are shown in Figure 7a and 7b, respectively. The NO conversion increases with an increase in feed gas HC concentration. It is observed in Figure 7a that at a constant C3H8 concentration the NO conversion at a space velocity of 27 690 h-l is higher than that observed at 21 600 and 63 530 h-l. When C3H6 is used as reductant, the NO conversion uniformly decreases with an increase in the space velocity. At constant HC concentration, the NO conversion observed using C3H8 and C3H6 as reductants are comparable at space velocities lower than 38 570 h-l. At space velocities higher than 38570 h-l, higher NO conversion is observed when C3H6 is used as reductant. B. Pulse Flame Combustor Evaluation. The HC and NO conversions at two space velocities, 4000 and 6667 h-1, are shown as a function of temperature in Figure 8. The HC conversion increases with an increase in temperature and does not vary significantly with a change in the space velocity. NO conversion is observed in the 450750 OC range, and it peaks a t 600 "C. The temperature range over which NO conversion is observed is wider than that observed with a flow reactor where C3H6 or C3H8 is used as reductant. The peak NO conversion decreases from 78 to 57% when the space velocity increases from 4000 to 6667 h-l. The composition of the HC emissions under fuel-lean conditions has been previously reported (Siegl et al., 1992). The exhaust resulting from the fuellean combustion of isooctane contains several hydrocarbons including isooctane (50.2% of total HC emissions), 2-methylpropene (22.2%), propylene (3.9%), ethane
catalyst temp ("C) 5200 5286 52ab 6200 62Tb 62Tb
conversion ( % ) CaHa NO 47.44 11.89 46.89 25.87 49.11 12.19 99.78 48.90 100.00 37.00 100.00 48.70
I00
2
80
c
.$
eo
Y
6
40
V
p
20 0
-;p
100
1
:
60
E
60
8
40
0
20 0
'pace VOlOClly (hr.7)
Figure 7. (a, top) Effect of space velocity and C& concentration on NO removal from NO-C&I-OZSOZCO-HZH~O mixtures. (b, bottom) Effect of space velocity and CS& concentration on NO removal from N O - C S H ~ O ~ S O & O - H ~ H mixtures. ~O
(3.8%1, and ethyne (2.3%1. Different hydrocarbons have different activity for reducing NO, Therefore NO conversion is observed over a wider temperature range compared to that observed during the studies conducted on the flow reactor. The durability of the catalyst was investigated at 600 "C and 6667 h-l space velocity, and the data are presented in Figure 9. The data observed with isooctane fuel are shown by hollow symbols; the solid symbols denote data observed using the contaminant-dopedisooctane (contains S, P, and Pb). The NO conversion is close to 60% when isooctane fuel is used. The NO and HC conversions remain constant as the catalyst is aged to 22 OOO mi. The NO conversion drops to about 45% when the contaminantdoped isooctane is used. The NO conversion increases back to 60% when the fuel is switched back to isooctane. C. Comparison of A1203 and Zeolite-Based Catalysts for Lean-Burn Engines. It is known that zeolites are unstable under severe hydrothermal conditions (Scherzer, 1984). Zeolite-based catalysts deactivate rapidly when exposed to automotive exhaust. Dealumination of the zeolite framework has been proposed to be the cause of this deactivation (Sano et al., 1987). Studies conducted
1810 Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 100
-
90
0 NO Converi~on(4000 hrl) 0 NO Converrlon (6667 h r ' ) A HC Converrlon (4000 h r ' )
iA
4.
H C Converilon (6667 hr-'1
E
401
Feed Ge8 Composltlon: H C = 450 ppm NO = 450 ppm CO = 20,000 ppm 0, = 40,000 ppm
30
i;
I
I
I
I
I
I
I
400
450
500
550
600
650
700
750
2o 10
800
Temperature ("C)
Figure 8. Effect of temperature on NO and HC conversion at 4000 and 6667 h-1 over a y-AlzO8 monolith. The study was conducted using isooctane fuel. The feed gas contained 450 ppm HC, 450 ppm NO, 20 OOO ppm CO, and 40 OOO 02.
,
Feed Q a l Compollllon: HC = 450 ppm NO :450 ppm o1 ==4o.ooo CO 20,000ppm ppm
io '" 3
uQ&=w-?p%l9-$
60 40
.(reoe3
O0
NO
is used as reductant. The NO conversion is maximum around 600 "C. The removal of NO from a NO-HC-02 stream increases with an increase in the feed gas HC concentration. When C3H6 is used as reductant, the NO conversion is adversely affected by the addition of CO and Hz to the feed gas mixture. When C3H8 is used as reductant, the NO conversion is suppressed by the presence of SO2 over the entire range of space velocitiesinvestigated. The NO conversion observed using C3H8 and C3H6 as reductants are comparable at space velocities lower than 38 570 h-l when NO is being removed from a NO-HCO~SOZ-CO-HZ-H~O stream. At space velocities higher than 38 570 h-l, higher NO Conversion is observed when C3H6 is used as reductant. Zeolite-based catalysts reduce NO a t temperatures lower than those required over Al2O3. Pulse flame combustor studies show that A1203 has better durability than zeolite-based catalysts for removing NO under oxidizing conditions.
Acknowledgment The authors thank Drs. H. S. Gandhi, R. W. McCabe, and K. Otto for their helpful suggestions and for critically reading the manuscript.
Literature Cited
0
~ , ( , , , , , , , 1 1 , , ~ 1 , , , , ( , , , , 1 , , , , 1 , , , , ( , , , ~
0
5
10
15
20
25
30
35
40
Miles (X 1000)
Figure 9. Effect of catalyst aging and fuel additives (S, P, and Pb) on NO and HC conversion. The studies were conducted using isooctane and contaminant-doped isooctane (0.03% S, 3 mg/gal Pb, and 0.8 mg/gal P in isooctane) as fuels. The feed gas contained 450 ppm HC, 450 ppm NO, 20 OOO ppm CO, and 40 OOO 02.
during the present investigation show that A1203 is robust and maintains its activity. Studies conducted during the present investigation show that NO removal occurs in the 450-700 OC temperature range over Al2O3. Maximum NO conversion is achieved a t 600 OC. In contrast, Cu-ZSM-5 catalysts show NO removal activity a t lower temperatures, Le., in the 300500 OC range; maximum NO conversion is observed around 400 "C (Truex et al., 1992). The combustion temperature of a lean-burn engine is lower than that of an engine operating around the stoichiometric point (Heywood, 1988). The Cu-ZSM-5 catalyst offers more flexibility in packaging the catalyst in the exhaust system since it requires a lower temperature for removing NO. In summary, zeolite-based catalysts require lower temperature than A1203 for providing NO removal. A1203 possesses better durability than zeolite-based catalysts. This suggests that efforts should be directed at modifying A1203 to increase its activity so that the temperature required for NO removal can be lowered.
Conclusions The study shows that the removal of NO over A1203 occurs in the 500-700 "C range when either C3H6 or C3H8
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Received for review January 11, 1993 Revised manuscript received April 26, 1993 Accepted May 4, 1993
