Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
123
Parametric and Durability Studies of NO, Reduction with NH3 on Fe-Cr Oxide Catalysts George L. Bauerle, S. C. Wu, and Ken Nobe' School of Engineering and Applied Science, University of California, Los Angeles, California 90024
The reduction of NO, with NH3 on a series of alumina-supported iron oxide-chromium oxide catalysts has been studied in flow reactors using simulated flue gas temperatures from 200 to 500 OC. The optimum catalyst contained 10 wt % of the active metal oxides at a weight ratio of 911 Fe:Cr. Conversion of NO,, which maximized at 400 OC, was not affected by the presence of C02 or H20 but was sharply increased with increase in oxygen between 0 and 0.5%; between 0.5 and 5.0% 02, however, NO, conversion did not undergo as substantial a relative increase with increase in oxygen concentration. Continuous operation of the flow reactor for over 1000 h showed that the 10% Fe-Cr (9/1) catalyst was effective for the selective removal of NO, (1000 ppm nominal concentration) with NH3 in simulated flue gas containing up to 1500 ppm of SO,. An intrinsic rate expression was developed for design use between 250 and 450 'C, up to 1500 ppm of NO, NH3/N0 ratios of 0.5 to 3.0, and 0.5 to 5 % 02.
Introduction Previously, preliminary studies of iron-chromium oxide catalysts for the selective reduction of NO with NH3 have been reported (Bauerle et al., 1975). I t was shown that 0 2 accelerated the reduction of NO on an alumina-supported catalyst containing a n 11%mixture of FepOa and Crz03 (5/1 Fe:Cr) between 220 and 450 "C. At 400 "C the conversion of NO was essentially constant during a short-term durability test (71 h) in the presence of SO*. Conversion of NO, (1000 ppm) in simulated flue gas was typically about 70% a t a space velocity of 20 000 h-1 (STP)with about 50% excess NH3 for the complete reduction of NO. Subsequently, a more comprehensive study of the Fe-Cr oxide-alumina catalysts has been conducted, and t h e results are reported in this paper. T h e optimum catalyst composition and the Fe-Cr oxide loading on the A1203support have been determined. In addition, an intrinsic rate expression has been developed from parametric investigations of the optimum catalyst. Finally, the effect of SO, on the catalytic activity was examined for a period of operation of over 1000 h. T h e results of this work should be directly applicable to the selective removal of NO, from power plant exhaust streams. Experimental Section Details of the bench-scale reactor system, gas analyses, and experimental procedure have been presented in part 3 of this series (Bauerle e t al., preceding article in this issue). Catalysts. T h e catalysts were prepared by impregnating carrier pellets with aqueous solutions of Fe(N0&9H20 and Cr03. The moist pellets were dried a t 160 "C and then calcined a t 500 "C. Filtrol Grade 86 alumina pellets (l/s in. diameter X 3/16 in. length) were used as support material in most of the formulations. A catalyst was prepared with the American Cyanamid Aeroban ZW-1470 A1203-Si02 spheres (3/16 in. diameter) a t the optimum FepOz and Crz03 concentrations, and its activity was compared with the Filtrol supported catalyst. T h e composition and weight percent of active metal oxides of the laboratory-prepared catalysts and two commercial catalysts examined in this work are listed in Table I. R e s u l t s and Discussion Optimization S t u d i e s of C a t a l y s t Composition. Preliminary studies involved those catalysts listed in the test matrix depicted in Table 11. T h e standard flue gas mixture 0019-7890/78/1217-0123$0.100/0
Table I. List of Catalysts Tested Catalyst no.
Prepared catalyst
Carrier"
5% Fe-Cr (1/1) 5% Fe-Cr (3/1) 5% Fe-Cr ( 9 h ) 10%Fe-Cr (1/1) 10% Fe-Cr (3/1) 10%Fe-Cr (9/1) 10% Fe-Cr ( 9 h ) 10% Fe-Cr (20/1) 10%Fe-Cr (50/1) 10% Fez03 11%Fe-Cr (5/1) 20% Fe-Cr (111') 20% Fe-Cr (3/1) 20% Fe-Cr (9/1) 20% Fez03
Filtrol 86 Filtrol86 Filtrol86 Filtrol86 Filtrol 86 Filtrol86 Aeroban Filtrol 86 Filtrol 86 Filtrol 86 Filtrol 86 Filtrol 86 Filtrol86 Filtrol 86 Filtrol86
1 2 3 4 5
6 6a 7 8 9 10 11 12
13 14
Catalyst A
Commercial catalysts 87% Fe-Cr (12/1) Girdler G-3A (MgO, Si02 and graphite carrier). 10%Fe203 Harshaw Fe-0301-T1/8 (A1203 carrier)
B Alumina.
Table 11. Iron-Chromium Test Matrix % active material, wt % oxides
5 10 20
Fe203/Cr203, wt ratio 1/1
3/1
1/1
311 3/1
1/1
9i1 9/1 9/1
(12% CO2,5% H20, and 3% 0 2 in Nz) with nominally 700,800, or 1100 ppm of NH3 was employed in these experiments. Table I11 summarizes typical results. All catalysts showed maximum activity a t 400 "C, except for the three catalysts with 20% active material. Large amounts of NzO were produced on these 20% catalysts, maximizing between 385 and 450 "C (Le., a t temperatures greater than that of maximum conversion of NO, which was about 300 "C). A t the temperature of maximum conversion, the activity of the 20% catalysts increased with increasing iron content. Since the data indicate that conversion is too low for operation 0 1978 American Chemical Society
124
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17,No. 2, 1978
Table 111. Summary of Activity Tests with Fe-Cr Catalysts a Catalyst no.
Catalyst active material
Fe-Cr ratio
Temp, "C
Inlet gas, ppm NO "3
111
Outlet gas, ppm NO NHa NzO
300 1036 610 940 400 940 1036 315 465 1036 450 940 300 980 775 683 400 980 510 730 478 984 775 515 4 10% 295 960 1100 430 111 400 1140 1040 325 530 450 1100 930 1040 465 758 300 400 550 1090 850 11 20% 305 940 1060 560 111 645 390 940 1060 460 940 1060 880 300 780 580 1020 780 690 385 1025 460 1025 780 960 730 2 5% 990 1095 300 311 990 1095 365 395 1095 305 455 990 760 780 1050 300 7 80 465 400 1050 7 80 1050 425 455 1120 1106 465 5 10% 300 311 1120 1106 395 390 1120 1106 560 450 520 1100 735 300 530 400 1100 735 640 735 445 1100 1140 335 12 20% 910 300 311 1140 690 390 910 1140 900 465 1110 480 1000 7 80 300 665 780 400 1000 975 1000 780 465 1120 805 5% 1110 3 300 911 1120 335 1110 395 1120 425 1110 475 763 690 1130 310 430 763 1130 400 510 1130 763 470 1106 410 10% 300 980 6 911 1152 235 970 400 1152 450 970 460 480 1000 750 300 515 750 1000 400 605 750 1000 460 225 1010 1118 20% 911 295 13 1118 475 1010 390 765 1118 455 1010 395 753 940 300 570 753 400 940 745 753 940 455 S.V.= 20 000 h-1. Simulated flue gas contains also 12% C o n , 3% 02,5% H20 in Nr. 1
5%
with near-stoichiometric N H 3 levels, catalyst selection should be based on operation with excess "3. Figure 1 summarizes activity at 300 "C (open points) and 400 "C (closed points) as a function of catalyst composition A t 400 "C and excess "3, a n Fe/Cr for 1100 ppm of "3. ratio of 1/1appears slightly better than an Fe/Cr ratio of 3/1. T h e 9/1 catalyst is superior to both and maximum activity in excess NH3 occurs at 10% loading. At 300 "C, however, a 20% catalyst with high iron content is favored. Activities of the 9/1 catalysts with 10% active material a t 400 "C and the 9/1 cat-
600 94 0 345
NO conversion, %
35 66 52
412
0 50 50 0 0 0 0
0 0 0 0
0 0 0 0
0
190 420 330 180 220 165 0
68 44 55 49 46 38 15 43 33 6 26 63 69 28 56 57 58 65 48
0 0
0 0 0
0 0
848 340 0
118 0 0
259 0 0
118 0 0
153 0
0 0 0 0
753 0 0
553 0 0
0
0 0 0 0 0 150 0 0 0 0
0 400 140 140 200 190 0 100 100 0 0 0
448
0
0
150 0
0 165 0 0 330 118 0
0 0
0
0 0
0 0 200 200 0 90 130
35
48 48 55
52
52 42 63 24 19 52 34 3 27 70 62 39 62 55 58 76 54 52 49 40 78 53 24 62 45 28
alyst with 20% active material at 300 "C were essentially equivalent. Operation at 400 "C appears to be somewhat more amenable to existing power plant operation since economizer outlet temperatures are typically on the order of 400 "C. Therefore, operation of the catalytic reactor a t 300 "C may require cooling of the flue gas before introduction into the catalytic reactor. Since Figure 1 suggests that additional improvement in activity a t Fe/Cr ratios greater than 911 may be possible, a
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
125
CONVERSION OF NO (9J
'"1
{
0 8 3/1 A A 911 OPEN POINTS-300.C CLOSED P O I N T S - 4 W T :
30
4 '
i " 'IO
1 ; '
PERCENT ACTIVE MATERIAL
20 '
Figure 1. Effect of Fe-Cr ratio and percent active material on NO conversion at 300 and 400 "C; 1000 ppm of NO, 1100 ppm of NH3 (s.v. = 20 000 h-l).
Figure 3. Conversion of NO as a function of Fe203 and Cr203 consee Table centrations at 400 "C: 1000 ppm of NO, 1100 ppm of "3; I for catalysts denoted by numbered points (s.v. = 20 000 h-l).
0 9/1, 109. Fe-Cr 0
m/1. IO9.Fe-Cr
A GIRDLER G 3A
0 0 AEROBAN SUPPORT 0 8 FILTROL SUPPORT OPEN SYMBOLS-NO
I O % F e p 0 3 MARSHAW FE-0301-
TEMPERATURE ('CI
Figure 2. Conversion of NO on Fe and Fe-Cr catalysts in simulated flue gas; 1000 ppm of NO, 1100 ppm of NH3 (s.v. = 20 000 h-l). I
M
300
400
TEMERATTLRE ('C
second series of tests with catalysts containing FeiCr ratios of 20/1 and 50/1, both a t a 10%loading, were conducted. In addition, catalysts containing Cr-free iron oxide were studied, 10% F e 2 0 3on A1203,20% Fen03 on A1203, a commercial 10% F e 2 0 3 on A1203 catalyst (Harshaw Fe-0301-T1/8), and the Fe-Cr composition examined earlier (Bauerle et al., 1975) which contained about 11%active material with an Fe/Cr ratio of 5/1 on A1203. Figure 2, which summarizes the results, shows t h a t t h e commericial FesOS catalyst has significantly lower activity t h a n t h e laboratory-prepared catalysts, with t h e 200/0 Fe203 catalyst distinctly superior to t h e 10% Fe20:i catalyst. I t is apparent that optimum operation of the pure FepO:, catalysts will require temperatures of 450 "C or higher. With 10%catalysts there is a definite increase in activity when a small amount of chromium is added to Fez03 (the 20/1 Fe/Cr and 50/1 Fe/Cr catalysts are of equivalent activity). It is considered that 400 "C is t h e maximum desirable temperature for operation of the NO, catalysts. Figure 3 summarizes results of conversion of NO as a function of absolute F e 2 0 3 and C r 2 0 3concentrations a t t h a t temperature to facilitate the selection of the optimum composition. T h e marked improvement in activity near 1OOh loading by addition of small amounts of C r 2 0 3 is clearly evident in the plot. Incorporation of additional chromium to lower t h e Fe/Cr ratio to less than 5/1 results in reduced activity. An initial cursory examination of the results suggest t h a t t h e single-component, 20% Fe203 formulation is the most economical catalyst. However, a more careful examination indicates that a catalyst of equivalent activity can be prepared
c
J
5w
I
Figure 4. Conversion of NO and NHB on Fe-Cr (9/1) in simulated flue gas; 1000 pprn of NO, 1100 ppm of NH3 (s.v. = 20 000 h-l).
by substituting over half of the Fe203content of a 20% catalyst with a n amount of C r 2 0 3equal to only 9% of t h e Fe20:) replaced (Le., the catalyst weight of active material is decreased from 20% Fe20a to a 10% catalyst with Fe/Cr ratio of 9/1). By assuming that commercial quantities of catalyst are prepared following t h e procedure currently employed in the laboratory and assigning an arbitrary cost of 100 to the 20% Fe10:3catalyst, the calculated relative cost of 9/1 Fe/Cr catalyst with 10% loading would be about 45. T h e relative cost is based on current, moderate-volume prices for Fe(NO:j),).SH20 and CrOy. T h e only other support material for the Fe-Cr catalysts examined in this study was Aeroban A1203-Si02. Tests were performed to determine the efficacy of the Aeroban substrate for t h e 9/1 Fe-Cr catalyst which has been selected for parametric tests. Figure 4 shows that in simulated flue gas, conversion of 1000 ppm of NO with 1100 ppm of NH3 is significantly higher on t h e catalyst with the Filtrol support. Conversion of NO maximized a t 300 "C with Aeroban compared to about 400 "C for Filtrol support. T h e low net conversion of NO on Aeroban appears to he related to enhanced preferential removal of N H 3 by oxidation with 0 2 or by decomposition; the NH3 conversion rate was substantially higher on the Aeroban catalyst as shown in the figure. T h e Aerobansupported catalyst apparently promotes the formation of NO
126
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 2, 1978
120-
00
320-500
4
'
8o -
400-560 A A 745-800 0 a 0 0 1000
-
0, H
1350, 1300
395pc
600
b
I I$ -5 4 0 -
20
20
0
n
200'C
0
0 '
0.5
1.0
1.5
'
2.0
'
(
'
2.5
CONCof NO
(ppml
390-400
TEMPI'CI 440-445
460-SI5 770-785
A
A
A
0 0
0
0
890-1040 1500-1435
0
0
480-490
0
Q
I
3.0
INLET REACTANT RATIO Ippm NH,/ppmNO)
Figure 5. Conversion of NO on Fe-Cr (9/1) in simulated flue gas (s.v. = 20 000 h-l).
by oxidation of NH3 with 02 a t substantially lower temperatures than the Filtrol catalyst. Tests with the commercial Girdler G3A Fe-Cr catalyst were conducted a t temperatures varying from 200 t o 400 "C using stoichiometric "3. At 400 "C, total conversion of NH3 was observed, but conversion of N O was zero as shown in Figure 4. At 300 "C, NO conversion was a maximum and only 26%. Considerable amounts of N 2 0 , which appeared to be formed reaction, were produced on this primarily by the "9-02 commercial catalyst, in contrast t o the laboratory-prepared Fe-Cr-Al203 catalysts. Parametric Studies of Fe-Cr (9/1) Catalyst. T h e experimental approach in t h e parametric tests was t o first use the standard simulated flue gas mixture (12% COz, 5% H20, 3% 0 2 )varying concentrations of N O from 250 t o 1500 ppm, N H 3 concentrations between about 0.5 to 1.5 times the stoichiometric requirement for complete reduction of NO a t each concentration and temperatures from 200 to 500 "C. Then, at the temperature of maximum conversion with a selected NO/NH3 ratio, the CO2 and H20 concentrations were perturbed from the standard flue gas concentrations (e.g., from 0 to 20% C 0 2 and from about 0 to 18% H20). T h e effect of oxygen on reduction of NO was investigated by varying 0 2 concentrations from 0 to 5% a t concentrations of NO of 750 ppm and 1000 ppm with a t least two concentration levels of NH3 (stoichiometric and 1.5 times stoichiometric). Initial experiments established the effects of NO and NH3 concentrations in the standard flue gas mixture on conversion of NO which is a function of the NH3/NO mole ratio, as shown in Figure 5 , for temperatures u p t o 395 "C. There is little variation in NO conversion above NH3/NO ratios of 1.1(0.67 is the stoichiometric value for conversion of NO to N2). For temperatures above 400 "C similar dependency on t h e reactant ratio is observed, as seen in Figure 6. T h e data indicate t h a t a broad maximum in conversion on t h e Fe-Cr catalyst occurred between 400 and 490 "C. Figure 6 shows t h a t conversion at 490 "C (half-filled points) approximate that at 400 "C for low N H 3 / N 0 ratios and that at 440 "C for higher NH3/NO ratios. T h e effects of H 2 0 and C 0 2 on N O conversion were determined a t 400 and 450 "C with both stoichiometric and excess NH3 for the CO2 tests and with excess NH3 for the H20 tests. Figures 7a and 7b show that neither CO2 nor Ha0 had any appreciable effect on the conversion of NO,. Figure 8 shows that stoichiometric or excess NH3 and a t all temperatures studied, the presence of a small amount of 0 2 produces substantial enhancement of the conversion of NO. Above 0.5% 02,further increases in the 0 2 concentration affect
Y
60 5 IO INCET WATER CCI(C€NTRATW
I5
20
(%I
5 % H20, 3 % O z , 1000 wm NO IN N2 I hl
INLET C 0 2 CCWCENTRATON (%I
Figure 7. Effect of water vapor (a) and CO:! (b) on conversion of NO on Fe-Cr (9/1) in simulated flue gas (s.v. = 20 000 h-l). TEMPI'C)
0 0
0 0 A
300 305 400 450 305
NO Ippml 750 740 750 750 950
NH31ppml 525 875 900 9 20 780
v i (
OXYGEN COHCEhTRATlW (Val
%I
Figure 8. Effect of 0 2 on conversion of NO on Fe-Cr (9/1) in simulated flue gas (s.v. = 20 000 h-l).
conversion only slightly. By assuming t h a t typical 0 2 levels for actual power plant use (low excess air firing is not required with catalytic NO, control) will be above 0.5%,t h e data indicate a positive, near-zero reaction order for 0 2 . T h e nearly parallel NO conversion-02 concentration curves above 0.5% 0 2 indicate t h a t the reaction order for 0 2 is essentially independent of temperature and NO/NH3 ratio. The kinetic data were assembled in computer input format for determination of the kinetic parameters in the global rate
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17,No. 2, 1978
127
Table IV. Typical Experimental a n d Calculated Conversion of NO on Fe-Cr (9/1) Based on Intrinsic Rate Expression a Inlet gas concentrations
a
Temp, "C
porzo, (atm x 106)
(atm x lo6)
200 200 300 300 445 300 300 300 300 305 305 305 305 400 400 400 400 450 450 495 495
955 740 505 1375 1020 735 745 760 750 1011 1015 1030 1060 720 760 760 800 990 1050 1020 1040
1305 1340 536 897 1198 545 506 506 506 1259 1283 1283 1283 908 905 905 905 1331 1331 1331 1331
P0r\,H39
poop (atm x 10') 20 000 h-' 3 3
Exptl,
Conversion Calcd,
YO
043
1
12
16 18 43 35 72 35 37 49 41 35 37 40 40 61 62 65 66
0.4,5 0.43 0.13 0.17 0.04 0.17 0.16 0.15 0.14
20 45 33
3
3 3 0.5
72
32 36 39 42 29 33 37 40 58 61 65 67 66 74 68 74
1 3
5 03 1
3 5 0.
