Ind. Eng. Chem. Res. 1992,31, 1963-1970
mation of alumina and copper sulfate species. After extended testa of reaction-regeneration more complex phenomena occur, which are under investigation.
Acknowledgment This work was sponsored by the European Community BRITE program (Contract RI 18B-0078-1). €&&try NO.SO,, 12624-32-7; NO,, 11104-93-1;SO,, 744609-5; CU, 7440-50-8; SO3, 7446-11-9.
Literature Cited Buzzi-Ferraris, G. An Optimization Method for Multivariable Functions. Presented at the Working Party on Routine Computer Programs and the Use of Computers in Chemical Engineering, Florence, Italy, 1970. Centi, G.; Riva, A.; Passarini, N.; Brambilla, G.; Hodnett, B. K.; Delmon. B.: Ruwet. M. Simultaneous Removal of SOJNO. from Flue GAS.. Sorbent/Catalyst Design and Performances. ?!hem. Eng. Sci. 1990,45,2679. Centi, G.; Passarini, N.; Perathoner, S.; Riva, A. Combined DeSOJDeNO, Reactions on a Copper on Alumina Sorbent-Cat-
1963
alyst. 1. Mechanism of SO2 Oxidation-Adsorption. Znd. Eng. Chem. Res. 1992,preceding paper in this issue. Himmelblau, D. M. Process Analysis and Statistical Methods; Wiley: New York, 1968. Kartheuser, B.; Hodnett, B. K.; Riva, A.; Centi, G.; Matralii, H.; Ruwet, M.; Grange, P.; Passarini, N. Temperature Programmed Reduction and X-ray Photoelectron Spectroscopy of Copper Oxide on Alumina Sorbent/Catalyst following Exposure to SO2and 0%Znd. Eng. Chem. Res. 1991,30,2105. Pollack, S. S.;Chisholm, W. P.; Obermyer, R. T.; Hedges, S. W.; Romanathan, M.; Montano, P. A. Properties of Copper/Alumina Sorbents Used for the Removal of Sulfur Dioxide. Znd. Eng. Chem. Res. 1988,27,2276. Waqif, M.; Saur, 0.;Lavalley, J. C.; Perathoner, S.; Centi, G. Nature and Mechanism of Formation of Sulfate Species on Copper/Alumina SorbentrCatalyst for SO2Removal. J. Phys. Chem. 1991,95, 4051. Yeh, J. T.; Dnunmond, C. J.; Joubert, J. I. Process Simulation of the Fluidized-Bed Copper-Oxide Process Sulfation Reaction. Environ. Prog. 1987,6,44. Received for review December 30, 1991 Revised manllscript received April 21, 1992 Accepted May 12, 1992
Combined DeSO,/DeNO, Reactions on a Copper on Alumina Sorbent-Catalyst. 3. DeNO, Behavior as a Function of the Surface Coverage with Sulfate Species Gabriele Centi,* Nello Passarini,+Siglinda Perathoner, Alfred0 Riva, and Giuseppina Stella Department of Industrial Chemistry and Materials, V.le Risorgimento 4, 40136 Bologna, Italy
The catalytic behavior of the copper on alumina sorbent-catalyst in NO reduction with NH3/ O2 during the simultaneous oxidation-adsorption of SO2 was studied in a series of consecutive cycles of reaction-regeneration and as a function of the surface coverage with sulfate species. The behavior of a CuO/Si02 system with simultaneous DeSOJDeNO, reaction is also reported. Tests of thermal desorption for the analysis of residual adsorbed species after reaction and thermogravimetric tests of ammonia adsorption also were carried out. Results indicate that (i) there is an initial competition between the adsorption of ammonia to form an ammonium bisulfate species and the reaction of NH3 with NO, (ii) ammonia adsorption continues up to a complete 1:l reaction with the surface sulfate species, (iii) these competition and adsorption phenomena give rise to the presence of an induction time in reaching the steady-state catalytic behavior, with a negative influence on the efficiency in NO removal, and (iv) the sulfate species induces a decrease in the activity of the copper-based active phase, with a reduction in NO conversion, but especially an increase in the formation of unwanted N20. Furthermore, desorption testa showed that (i) the presence of adsorbed ammonia induces a desorption of SO2 when the catalyst temperature is increased from about 570 to 720K and (ii) adsorbed ammonia thermally desorbs at higher reaction temperatures as such or as products of further transformation, specifically N2.
Introduction The combined dry removal of SO2and NO from flue gas is an economically interesting technological possibility in comparison to not-integrated approaches for separate removal of these two pollutants (Kohl and Riesenfeld, 1979; Moser, 1981; Rosenberg et al., 1980; Siddiqui and Tenini, 1981). Interest has been focused on the copper on alumina system as a sorbent/catalyst for the removal of SO2 (DeSO,), acting as an oxidant for SO2and as a sorbent to form surface sulfates, and at the same time as a catalyst for the selective reduction of NO with ammonia in the presence of gaseous oxygen (DeNO,) (Centi et al., 1990; Lowell et aL,1971; Mc Crea et al., 1970; Pollack et al., 1988; Yeh et al., 1985,1987). There are three main aspects which 'Div. Servizi Awiliari, E N C H E M ANIC, S.D o m b Milanew, Italy.
characterize the interest for this catalytic material: 1. The first is the possibility of formation of sulfate species by reaction with SOz and O2 at temperatures compatible with those of flue gas (around 570 K) (Centi et al., 1991a,b; Mc Crea et al., 1970; Pollack et al., 1988; Yeh et al., 1985, and 1987). 2. The second is the regeneration by reduction that is less demanding than that required for many other metal sulfates and which can be realized in reasonable reaction conditions, especially in terms of influence on the resistance to deactivation of the systems during several cycles of reaction-regeneration (Lowell et al., 1971; Kartheuser et al., 1991; Waqif et al., 1991; Uysal et al., 1988). 3. The third is the catalytic activity in the reduction of ammonia in the presence of O2that is not strongly influenced by the parallel process i f formation of sulf& species on the surface (Centi et d.,1990; Yeh et d.,1985, 1987).
0888-5885/92/2631-1963$03.00/00 1992 American Chemical Society
1964 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
In fact, several very active oxide systems are known to catalyze the selective reduction of NO with NH3/O2 (Bosh and Jamsen, 1988), and in particular optimal performances were obtained using vanadium oxide supported on Ti02, which also constitutes the active component for commercial applications (Beeckman and Hegedus, 1991; Siddiqui and Tenini, 1981). However, this catalyst strongly deactivates in the presence of sulfur oxide, due to fouling by the deposition of ammonium bisulfate and the formation of an inactive vanadium sulfate species (Bosh and Janssen, 1988 and references cited therein; Centi et al., 1991; Matauda et al., 1982; Nam and Eldridge, 1986). The activity of copper on oxide supports such as alumina, Ti02, and Si02 or of Cu(I1)zeolites has been reported (Blanco et al., 1986; Iizuka et al., 1986; Mizumoto et al., 1978; Williarpon and Lunsford, 19761, but few data are available on the behavior of these catalytic systems in the DeNO, reaction in the presence of SO2,an aspect of critical importance for the development of a combined process of removal of SO2and NO pollutants from flue gas. In fact, Stelman (1987) found that copper sulfate catalyzes the reaction of NO reduction with NH3, whereas CuO is inactive. The aim of the work reported in this paper was therefore to analyze the behavior of copper on alumina sorbent-catalyst in the removal of NO with ammonia in the presence of S02/02. In the previous papers in which the analysis of the process of combined S02/N0removal from flue gas on copper on alumina is reported (Centi et al., 1990, Mc Crea et al., 1970; Yeh et al., 1985,1987),the possibility of obtaining a removal of NO of over 90% during the simultaneous oxidation-adsorption of SO2 was shown, but without a detailed analysis of the nature of the active phase and the dependence of the catalytic performance on the surface coverage with sulfate species. Experimental Section . Sample Preparation. The copper on alumina sample was prepared by incipient wet impregnation of pure yN2O3(117 m2/g) using a solution containing copper acetate in such an amount as to have 4.8 w t % copper oxide in the f d catalpt (3.9% as Cu). After impregnationand drying, the particles were calcined at 773 K to decompose the acetate anions. Further details on the preparation of the samples have been reported previously (Centi et al., 1990, 1992a,b). Hereinafter, this sample will be referred to as F-Cu/A1203 (fresh). Samples representative of the conditions after some cycles of reaction-regeneration were obtained from this starting catalyst by treatment in a quartz microreactor using conditions simulating those of flue gas (reaction stage; temperature 573 K) up to complete exhaustion of the oxidation-sorption capacity for SO2 and by a consecutive reduction treatment (regenerationstage; temperature 723 K using a flow containing 2% H2 in helium). Three complete cycles of reaction-regeneration were carried out. The sample after these three complete cycles will be referred to hereinafter as R-Cu/A1203 (regenerated). This sample was then further treated in the same conditions as those for the reaction step up to complete exhaustion of the oxidation-sorption capacity for SO2. This sample will be referred to hereinafter as S-Cu/A1203(sulfated). Other samples were prepared in a similar way, but with a higher number of reaction-regeneration cycles. The results obtained with these samples were found to be very comparable to those obtained with the R-and S-Cu/A1203 samples. The CuO/Si02 sample was prepared in the same manner as the fresh copper on alumina and with the same amount of copper (4.8 wt % CuO) and using a commercial SiOz
with a surface area of 416 m2/g. Catalytic Tests. Catalytic testa were carried out in a quartz microreactor with a computer-controlled massquadrupole detector (VG Micromass SX200) for the continuous analysis of reagent and product composition. The reactor was placed in a copper bar externally heated by an electrical resistance in order to ensure an isothermal axial profile. A thermocouple sliding inside the reactor allowed control of the isothermicity of the axial profile. Samples with particle dimensions in the 0.25-0.42-mm range and a high linear gas velocity were used in order to avoid inter- and intraparticle heat- and mass-transfer limitations. Suitable experimental testa and calculations also were made to verify that these effects can be considered negligible in our experimental conditions. The following components were simultaneously analyzed using the mass-quadrupole detector (the relative m / e values utilized for the quantitative analysis are reported in parentheses): reaction step, He (41, NO (30), NH3 (17, 161, H20 (181, N2 (28L 0 2 (321, N20 (44)s NO2 (46), SO2 (48,64),and SO3 (80); regeneration step, He (4), SOz (48, 64), SO3 (801,H2S (341, and H2 (2). The intensity of the helium peak was used as an internal standard for correction of the vacuum chamber pressure. Furthermore, a suitable computer program was used which takes into account the possible interference in the peak intensities due to fragmentation of other compounds, in particular for the contribution to the intensity of the peak at m / e 17 (NH,) of the water (23% on the intensity of the peak at m / e = 18). The composition in the reagent and product stream was then calculated using sensitivityfactors derived from a calibration with standard mixtures of known composition. N, 0, and H balances also were made to ensure the correctness of the determinations. The following composition was usually employed for the reaction step: 0.123% SO2, 0.072% NO, 0.070% NH3, 3.0% 02,and 96.735% helium. For the regeneration step a flow of 2.0% H2 in helium was used. Desorption tats were carried out on the same apparatus used for the catalytic testa with a continuous analysis of product stream by the mass-quadrupole detector. The testa were made as follows: After the sorbent-catalyst capacity toward SO2 removal was exhausted (reactant composition as in the reaction step, temperature 573 K), the sample was flushed with He in order to remove gasphase compounds. The temperature was then increased up to 723 K maintaining the sorbent-catalyst under the helium flow and analyzing the products of desorption as a function of the reactor temperature. Ammonia adsorption testa were carried out on a Perkin-Elmer TGS-2 thermobalance. Before the ammonia was introduced into the system, the copper on alumina sample was treated at 573 K with a S 0 2 / 0 2flow (0.8% and 3% v/v, respectively, in helium) until a S02/Cu molar ratio of 1.0 was reached. The temperature was then raised to 723 K in a helium flow to remove adsorbed sulfur oxide species and then cooled in the 323-423 K temperature range. A helium flow containing dried ammonia in the 0.1-0.8% v/v range was successively sent to the sorbentcatalyst and the weight changes as a function of time on stream were followed.
Results DeNO, Behavior during the Simultaneous Oxidation-Adsorption of SO2. Reported in Figure 1 is the behavior of F-Cu/A1203at 573 K during the simultaneous removal of SO2 (DeSO,) and NO (DeNO,). The SO2 removal is complete during the fmt 20 min of time on stream (this time is a function of the amount of catalyst, the flow
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1965 Removal,% 7
Products, PPm
ppm N20 (after 100 min)
100
lW.-
40
rem SO2
++
remNOx
A
ppmN20
*
r
# ou:-*+ 20
0
\ +-
4
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SO
75 1c
-
ppmNH3 - .
_____
1 /--__
CUO on 50 t
25
i~
+
i
r---/ Alumina
I
0. 0
0
100
Time, min
Figure 1. Behavior of F-Cu/A1209in the simultaneousremoval at 573 K of SO2and NO. Experimental conditions: helium flow containing 724 ppm NO, 703 ppm NH9,3% 02,and 1230 ppm SO,; total flow rate (STP) 6 L/h; 0.35 g of sorbent-catalyst.
2
-_L1
4
6
8
12
10
nr. cycles reaction-regeneration
Figure 3. Dependence on the formation of N20 at 573 K in the reaction step during consecutive cycles of reaction-regeneration for copper on alumina or silica samples. Experimental conditione as in Figure 1. Removal, % _ ___ _ - ~ ~ - _Products, _ _ _ ppm 1600
Conv. NOX,%
-
80
1
60
i 4o -
0
~
L
-
20
40
-
L A -
~
60
SO
100
120
Time, min
Figure 2. DeNO, behavior at 573 K of copper on alumina sorbent-catalyst in the reaction step during consecutive cycles of reaction-regeneration. Experimental conditions as in Figure 1.
rate, and SO2 concentration), and then the amount removed decreases progressively up to complete exhaustion of the oxidation-adsorption capacity of the sorbent-catalyst in about 90-100 min. At the same time, it is possible to observe a reproducible evolution of the conversion of NO which passes through a minimum after about 10 min and then progressively increases up to a conversion of about 9697%. The formation of N20 also evolves with time on stream. During the first 10 min N20 does not form, but later N20 starts to be formed and continually increases up to a value of about 20 ppm. In all ranges of the time on stream, no ammonia slip is detected. After exhaustion of the SO2oxidation-sorption capacity this sorbent-catalyst was regenerated at 723 K with a stream of helium containing 2% Hz, and then, after cooling at 573 K, the behavior in the combined DeSOJDeNO, reactions was tested again. No significant changes in the trend with time on stream of the removal of SO2 were noted, but a significant modification in the NO conversion w time was observed. Reported in Figure 2 is the dependence on time on stream at 573 K of the conversion of NO during the reaction step for consecutive cycles of reaction-regeneration. The change in the dependence of the NO conversions on time is clearly evident in going from the first to the second cycle, whereas less significant modifications are observed for further cycles. In the second or third cycle the conversion of NO is initially about 50% but continually increases up to nearly total conversion. In the first cycle, the conversion is 100% initially, then decreases down to a minimum conversion of about 78%, and finally increases to nearly the same value as that found in the consecutive cycles. The dependence of the formation of NzO on time also
0
10
XI Time, rnin
30
40
Figure 4. Behavior of F-CuO/Si02 in the simultaueous removal at 573 K of SO2 and NO. Experimental conditions as in Figure 1.
changes in going from the first to consecutive cycles of reaction-regeneration. As seen before, the N20formation in the first cycle increasesfrom nearly 0 to around 20 ppm, whereas the N20 formation in the consecutive cycles is nearly constant with respect to time on stream, but higher than the final value found in the first cycle (Figure 3). The formation of N20,however, is usually less than 50 ppm, whereas in testa carried out using a CuO/Si02 sample the formation of N20 is about 2 times higher (Figure 3). Not only the formation of N20 is higher using silica instead of alumina as the support for copper, but ala0 the efficiency in SO2 removal is lower as well as the conversion of NO which is about half of that found using copper on alumina (compare Figure 4 with Figure 1). These resulta provide further evidence for the role of the support oxide in stabilizing a special active surface configuration of copper (Centi et al., l990,1992a,b). It should be noted, furthermore, that unreacted ammonia starta to be detected only after about 15 min of time on stream, indicating that during this time the unreactd ammonia remains adsorbed on the surface, probably in the form of ammoniumsulfate. In agreement, in the infrared spectra of the copper on alumina sorbent-catalyst after reaction, besides the bands due to sulfate species in the 1050-1200-~m-~ region, a clear band is present at about 1400 cm-l. This band is not present in the copper on alumina sample treated with S02/N0/02in absence of NH3and can be thus attributed to u4 (6,) of ammonium ion in ammonium sulfates. No evidence of the presence of ammonium sulfate species, on the contrary, can be derived from X-ray diffraction analysis of the sample, due probably both to the low amount and absence of crystallinity. Surface Residual Species after the Combined DeS0,-DeNO, Reaction. The adsorption of ammonia
1966 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 Desprption Products, ppm
n
2500,
2000
7 lo
i\
+
XI000
500
0 570
630
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660
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720
Temperature, K Desorptm products, ppm
XI000
2500
4
I\
2000 k
I
B
]lo
a
/A\
-175
-
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so2 N2
X NU3 ~4N X ,
- 5
'
-8 H X , 8 N c
5
570
600
630
660
690
720
Temperature, K
Figure 5. Producta of thermal desorption in a helium flow (see text) for copper on alumina samples after the reaction step during the f i t (A) or third (B)cycle of reaction-regeneration. Experimental conditions for the reaction step IU in Figure 1.
evidenced in Figure 4 and confirmed from infrared analysis indicates the importance of the analysis of the adsorbed species formed on the sorbent-catalyst during the DeNO,/DeSO, tests. In order to obtain information on these species, the products of thermal desorption after reaction were analyzed. The desorption testa were carried out as follows: After the reaction step at 573 K the sorbent-catalyst was flushed with a helium flow at the same temperature in order to remove the physisorbed and gas-phase species, and then the temperature of the reactor was linearly increased (10 K/min) up to 723 K while following the evolution of desorption products with the mass-quadrupoledetector. The results are shown in Figure 5 for the copper on alumina sample after the reaction step during the first (A) or during the third (B)reaction-regeneration cycle. In both cases relatively similar results were found, apart from an increase in the concentration of NO and NzOin desorption products after the third cycle (Figure 5B). The main products that desorb from the sorbent-catalyst after reaction are as follows, listed in order of decreasing quantity: water, nitrogen, ammonia, and sulfur dioxide. The amount of water, in particular, is about 4-5 times higher than that of the other components. The inmated area of the water desorption peak indicates that around 2-3 mol of water desorb per mole of copper present in the sorbent catalyst. A large fraction of the surface, therefore, operates in a hydrated state during the catalytic reaction. The N2which desorbs from the sample reasonably may be attributed to a consecutivetransformation of adsorbed ammonia, which is selectively oxidized to Nz during desorption. The copper on alumina sample, in fact, catalyzes the selective oxidation of ammonia to Nz in the presence or absence of gaseous oxygen (Stella, 1991). In the latter case, lattice oxygen may act as the source of
oxygen to form water during the oxidation of ammonia. Considering the sum of the integrated areas of N2and NH3 as the moles of adsorbed ammonia (taking into account that 2 mol of ammonia are required per mole of N2), a value of about 1-1.5 mol of ammonia per mole of Cu in the sorbent-catalyst is obtained. This indicates that about all the sulfate species present on the surface of the sorbent-catalyst have reacted with ammonia to form an ammonium sulfate species. Indeed, as shown previously (Centi et al., 1992a,b) for a reaction temperature of 573 K, a limiting value for the moles of sulfate species formed per mole of Cu present of about 1-1.2 is found. It should also be noted that SO2 is found in the products of desorption, but not SOB. This may suggest that part of the SOz also remains chemically adsorbed on the surface during reaction, in the form of ammonium bisulfate or an ammonium sulfite/sulfate complex; however, these compounds are thermally not stable at temperatures of around 573 K. It should be noted that when SOz oxidation-adsorption is carried out on Cu/Al2O3 in the absence of ammonia in the gas stream, sulfate species form, but no SO2 or SO3 desorption is observed with increasing reaction temperature (Centi et al., 1992a,b). A more reasonable interpretation of the observation of SO2 in the products of desorption is to consider that adsorbed ammonia is partially oxidized during desorption forming N2 Ammonia may thus reduce sulfate to sulfite species that desorb as SO2 and H20. However, the amount of SOzthat desorbs is about half of that of N2 It should mentioned that infrared characterization of the nature of the adsorbed species on copper on alumina (Stella, 1991) indicates the presence of nitrate species due to oxidation of NO. Probably, nitrate and adsorbed ammonia (in the form of ammonium ions) react during thermal desorption to form N2 and H20,which may explain the difference observed between the moles desorbed of N2 and of SO2. It should be noted that probably the amount of adsorbed nitrate species increases on the sample after three cycles of reaction-regeneration (Figure 5B)as compared to that after one cycle (Figure 5A), in agreement with the increase in the difference between the integrated areas of the N2and SO2 desorption curves (from about 1:2 to about 1:3). This observation is also in agreement with the increase in the desorbed amount of NO and NzO. Effect of the Number of Cycles of Reaction-Regeneration on the Catalytic Behavior in the Absence of SO2. Desorption experiments (Figure 5 ) show that a series of chemisorbed species remain on the sorbent-catalyst surface after the combined DeSOJDeNO, tests, in addition to the different sulfate species, discussed previously (Centi et al., 1992a,b), which are stable against thermal desorption. In order to distinguish better the role of the surface reactivity toward the DeNO, reaction induced by the presence of these sulfate species, catalytic tests were carried out in the absence of SO2 in the reagent stream, but using copper on alumina samples pretreated in a series of consecutive cycles of reaction-regeneration. Results of the catalytic behavior in NO conversion with NH3/02 are reported in Figure 6 as a function of the number of cycles of reaction-regeneration for the two situations referring to the sulfated (S) [after the reaction step] or regenerated (R) [after the regeneration step] sorbent-catalyst. In order to avoid interference due to other adsorbed species, pretreatment before the DeNO, reaction was carried out using a stream containing only 0.15% SOz and 3% O2 in helium for the reaction step. It should also be noted that in order to have a reasonable time scale for the exhaustion of the oxidation-absorption
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1967 PPm
*
N20(R)
1
soot
Conversion
1
120 250
0
2
4
6
-
8
nr cycles reaction-regeneration
OC
'0 0
Figure 6. Conversion of NO and formation of NzO in DeNO, testa at 566 K on fresh Cu/A1203(zero cycles) and on sulfated (S)or regenerated (R) samples after consecutive cycles of reaction/regeneration. Experimental conditions: helium flow containing 800 ppm NO, 807 ppm NH3, 2.8% 0,;total flow rate (STP) 6 L/h; 0.11g of sorbent-catalyst.
25
75
50
100
Time, min Ppm
Conversion ,% 100
750
capacity toward SO2of the copper on alumina sample (see, for example, Figure l),contact times higher than those necessary to have a nearly complete conversion of NO during the simultaneous DeSO,/DeNO, reactions are usually required. Longer contact times, however, may alter the analyses of the changes in the activity of copper on alumina samples as a function of the coverage with sulfate species, and therefore the catalytic testa of Figure 6 were carried out using a shorter contact time, corresponding to that necessary to obtain around 95% of conversion at 573 K. Reference testa using sulfated alumina without copper indicate the absence of reactivity toward NO reduction with NH3 of the sulfated species on the alumina and only some activity toward NH3 adsorption. The resulta of Figure 6 indicate some important aspects: 1. The fresh copper on alumina (zero cycles) shows the higher activity in NO reduction, but especially N20 does not form. 2. In the samples after regeneration (R)a limited decrease in the conversion of NO is noted (about l-2%)) but an increase in the formation of N20 in the order of 10 ppm is found. 3. In the sulfated samples after the reaction step (S) a significant decrease in the conversion of NO is observed (about 10-12%) and the formation of N20 increases considerably, reaching values of around 40 ppm. It should be mentioned that, as shown previously (Centi et al., 1992a,b;Kartheuser et al., 1991;Waqif et al., 1991) during the first cycles of reaction-regeneration, sulfate species linked to alumina form that may be only partidy removed during the regeneration step. Thus after regeneration, some residual sulfate species are also present, even though in much lower amounts as compared to sulfated samples. However, the presence of sulfated species also induces a third effect which is illustrated in Figure 7 where the time-on-stream changes in the catalytic DeNO, behavior of regenerated R-Cu/A1203(A) and sulfated SCu/A1203(B)are reported. It is evident that, besides a decrease in the limiting value of NO conversion, there also is a considerable increase in the time required to reach to this limiting value. This induction time increases from about 15-20 min (A) to around 50-70 min (B). Parallel to this evolution, the ammonia conversion decreases. Also reported in Figure 7 are the ppm of NH3 and H20 adsorbed from the catalyst, as calculated from maas balances. Their trend suggests that the time-on-stream evolution of the catalytic behavior may be related to the parallel effect of adsorption of these reagenta/producta on the catalyst
75
500
250
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I!
i
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0 0
25
50
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Time, rnin Figure 7. Dependence of DeNO, behavior at 566 K on time on stream for regeneratedR-Cu/A1203(A) and for sulfated S-Cu/Alz08 (B). Also reported on the graph are the ppm of NH3 and H20 adsorbed on the catalyst as estimated from the lack of N, 0, and H mass balances in the outlet streams. Experimental conditions as in Figure 6. Samples were pretreated before the catalytic tests in a helium flow at the same reaction temperature, in order to remove adsorbed species.
surface. Reasonably, more critical is the adsorption of the reagent ammonia. In fact, the decrease in conversion of NH3 follows a trend opposite that of the NO conversion. The difference between the NH3 and NO conversion may be attributed mainly to NH3 species which remain adsorbed on the catalyst (Figure 7). In order to verify better this indication, the dependence of the NO and NH3 conversions, the induction time, and the N20 formation on sulfated S-Cu/A1203samples were studied as a function of the NH3/N0 molar ratio in the reacting mixture (Figure 8). Results indicate that the time required to reach steady-state behavior (induction time) is related to the rate of adsorption of NH3 on the surface which, in turn, depends on the concentration of NH3 and on the NH3/NO molar ratio. It is worthwhile to note that, with increasing NH3/N0 ratio, two other effects are observed: (a) the NO conversion paeses through a maximum centered at a value of around NH3/N0 = 1 and (b) N20formation increases with increasing NH3/N0 ratio. Both effects may be attributed to a very strong adsorption of NH3on the catalyst related to the increase in ammonia concentration in the feed, which causes on one hand a self-induction on activily in NO conversion and on the other hand the formation of unwanted N20. Effect of the Reaction Temperature. The effect of the reaction temperature on the steady-state (after 1 h of time on stream) conversion of NO with NH3/02is reported in Figure 9 for the fresh (F),sulfated (S)[after the reaction
1968 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
75
-
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I
i o 2 weight change, _____ %
I-d Time fmin) or NX) (own)
conv.. Y.
>:
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* *
NH3Conv
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IrdTime
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N20ppm
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*
623
7-
25
%
-
J
r
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~j
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3
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96
NH3/NO Molar Ratio
0
Figure 8. Dependence of DeNO, behavior and induction time to reach nearly constant NO conversion on NH3/N0 molar ratio in the reagent mixture for sulfated S-Cu/A1203.Experimental conditions as in Figure 6. NO Conv.. %
F CuiA1203 + S CuiA1203 + R CuiA1203 +
2 540 560 580 600 620 640
820
Temperature, K
Figure 9. Effect of reaction temperature on conversion of NO using F-Cu/A1203,S-Cu/A1203and R-Cu/A1203samples. Experimental conditions as in Figure 6.
step] or regenerated (R) [after the regeneration step] sorbent-catalyst. In all range of temperatures the fresh and regenerated sorbent--catalyst are more active than the sulfated sample, in agreement with the previous data (see Figure 7). For reaction temperatures higher than about 580 K the conversion of NO begins to decrease for all samples, due to an increase in the parallel rate of NH3 oxidation to N2which causes a lowering of the surface ammonia available for the reaction with NO. An optimal temperature of around 570-580 K thus exists for all samples as regards NO conversion. Thermogravimetric Tests of Adsorption of Ammonia on Sulfated Copper on Alumina. In order to investigate further the adsorption of ammonia on sulfated copper on alumina, the interaction was studied in a thermogravimetric apparatus as a function of time on stream and of reaction temperature using a constant NH3 concentration (Figure 10) in a helium flow. For temperatures in the 473-523 K range, NH3 adsorption gives rise to a continuous weight increase up to a limiting value of about 101.5%. This limiting weight increase corresponds to an unitary ratio between moles of ammonia adsorbed and moles of sulfate species, suggesting that all surface sulfate group react with ammonia to form an ammonium bisulfate species. For temperatures higher than about 600 K, on the contrary, a continuous weight loss is observed in all ranges of time on stream. As shown by Matsuda et al. (19821,ammonium bisulfate is not stable at temperatures higher than about 600 K. This explains why a continuous weight loss is observed at 623 K upon NH3 adsorption. Indeed, ammonia reacting with surface sulfate species forms an ammonium bisulfate like species which
25
50
75
io0
Time mm
Figure 10. Thermogravimetric testa of adsorption of NH3 on sulfated S-Cu/A1203as a function of reaction temperature. Experimental conditions: helium flow containing 0.91% NH3, with constant ammonia concentration as a function of time on stream; sample pretreated in a helium flow at 653 K up to constant weight.
decomposes, leading to a net loss of sulfate species for the sorbent-catalyst and thus a decrease in weight. On the basis of desorption experiments (Figure 5 ) this effect can be interpreted as a reduction in sulfate species induced by the oxidation of ammonia to form NP At the intermediate temperature of 573 K, which is the same as that of the catalytic tests, a nearly constant weight change is noted, which derives, however, from the combination of the two above effects. In fact, an initial slight increase in weight which passes through a maximum can be observed. The maximum derives from the initial adsorption of ammonia by sulfate species (weight increase) with progressive exhaustion of the adsorption capacity. This leads to increasing amounts of ammonium bisulfate whose rate of loss/decomposition (a weight loss) is a function of its concentration and of the ammonia concentration. As a function of time on stream, therefore, a maximum in the weight change is noted at this intermediate temperature, whereas due to the opposite dependence on the temperature of the rate of ammonia adsorption and of ammonium bisulfate volatilization, different effects prevail at lower or higher temperatures. The net prevailing effect, therefore, depends considerably on the reaction temperature, but also on the ammonia concentration which, in turn,also influences the relative concentration of the ammonium bisulfate species formed as a function of time on stream. This is illustrated in Figure 10, which reports the influence of the ammonia concentration on the weight changes at 573 K after sending the flow with ammonia to sulfated copper on alumina. These results may be interpreted as a higher reaction order in the rate of ammonium bisulfate loss/decomposition as compared to the rate of ammonia adsorption, which linearly depends on the ammonia concentration. However, it is more interesting to observe that, for concentrations such as those usually employed in the DeNO, tests (around 0.074.08% NHJ at 573 K, the effect of ammonia adsorption prevails over that of ammonium bisulfate loss/decomposition, which certainly may reduce the efficiency in SO2 removal during the combined DeSO,/DeNO, tests.
Discussion The results reported in Figure 2 evidence that the activity in NO removal of a copper on alumina sorbentcatalyst shows an evolution with time on stream during exhaustion of its oxidation-sorption capacity toward SOz. T w o different trends can be observed depending on the starting situation, a fresh Cu/A1203or a regenerated sam-
Ind. Eng. Chem. Res., Vol. 31, No. 8,1992 1969 weight change, %
101 r ~
I
I
I
Time, min
Figure 11. Thermogravimetric testa of adsorption of ammonia at 573 K on sulfated S-Cu/A1203am a function of concentration of NH3 in a helium flow. Other experimental conditions am in Figure 10.
pie. The activity in both cases tends toward a similar NO conversion of about 96-97%, but the mean conversion (about 8590%) is lower due to the transient evolution. This indicates that the efficiency in NO removal of copper on alumina sorbent-catalyst in an industrial reactor application with solid transport [such as a mobile bed reactor] will be negatively affected by the presence of this transient evolution. This evolution in the reaction step of the first cycle of reaction-regeneration can be associated with two parallel effects: (a) the slight inhibition of copper oxide activity in NO reduction with NH3/02due to the formation of sulfate species on the sorbent-catalyst (Figure 6) and (b) the adsorption of ammonia on the sulfate groups to form an ammonium bisulfate species. In fact, catalytic tests (Figure 7), in agreement with the desorption (Figure 5 ) and the thermogravimetric teats (Figures 10 and 11) and infrared data, clearly show the adsorption of ammonia during the DeNO, reaction up to a nearly 1:l NH3 to sulfate ratio. This effect leads to competition between the reaction of ammonia with NO to form N2and the reaction of ammonia with surface sulfates which in turn leads to a net reduction in the amount of ammonia available for the first reaction. The specific rate of NO conversion is thus lower on a copper on alumina surface fully covered with sulfate species and increases progressively to the formation of ammonium bisulfate. This explains the continuous evolution of NO conversion with time on stream (Figures 2 and 7). In the first cycle, however, the specific activity of fresh copper on alumina is higher than that of sulfated samples and, thus the NO conversion is 100% at the beginning of reaction, then decreases down to a minimum value, and then increases later. On the contrary, not all sulfate groups can be removed from the regenerated sample, due to the formation of some less reducible sulfate species connected to Al sitea (Centi et al., 1990;Kartheuser et al., 1991;Waqif et aL, 1991). For these samples only the net subtraction of ammonia available for reaction with NO due to interaction with residual not-regenerated sulfate species prevails. In these samples, therefore, the NO conversion continues to increase. A second negative effect on the DeNO, behavior of copper on alumina can be associated with the presence of sulfate species and is related to the enhanced formation of unwanted N20 (Figures 3 and 6). The results reported in Figure 8 may suggest that the formation of N20 can be related to a very strong adsorption of ammonia that also strongly inhibits the activity in NO conversion. This aspect requires more investigation to be confiied. It should be noted, however, that a study of the mechanism of the DeNO, reaction on copper-baaed oxides (Andersson et al.,
1989) haa led to the following suggested mechanism: oxidation of NO to form an adsorbed nitrate-like species that reads with ammonium ions to form N20 which is further decomposed to N2. This general reaction pattern agrees with our indications on the reaction mechanism on a copper on alumina sorbent-catalyst (Stella, 1991) and suggests that the enhanced formation of N20 can be associated with the reduced activity of sulfated copper on alumina as compared to the behavior before sulfation, in agreement with the reduction in NO conversion. The formation of N20 is therefore a further index of the negative influence of sulfate species on the activity of the copper active phase in NO conversion. In agreement, a CuO/SiOzsample shows a lower rate of NO conversion and a higher formation of N20 (Figures 3 and 4). The comparison of the results of CuO/Si02 (Figure 4) with those of copper on alumina (Figure 1) evidence the role of the formation of a specific compound of copper on the surface. According to literature indications (Friedman and Freeman, 1978; Millar et d., 1991;Strohmeier et d., 1985; Wolberg and Roth, 1965) copper is present mainly as microcrystalline copper oxide particles on silica and as a surface defect spinel-like CuA1204phase on alumina, at least for copper surface coverage such as those in the present case. This suggests that a suitable modification of the catalyst in order to enhance the specific rate of copper activity toward NO conversion may enhance the rate of reaction of ammonia with NO as compared to the reaction with sulfate groups, with reduction of N20 formation and of induction time and an increase in the efficiency of NO removal. On the contrary, the increase in the reaction temperature in order to enhance the rate of NO conversion has a negative effect (Figure 9) due to an increase in the parallel rate of NH3conversion to N2that limits the rate of reaction of NO with NH3. This fact also indicates that the optimal temperatures for the combined removal of SOz and NO is in the 570-620 K range. This range of temperatures is also suitable for limiting the deep sulfation of the alumina support that reduces the resistance of the support itself to extensive reaction-regeneration cycling. The reduction in the adsorption of ammonia may reduce a further negative aspect related to the presence of sulfate species on the sorbent-catalyst during the DeNO, reaction. In fact, as shown by desorption tests (Figure 5), during the heating of the sorbent-catalyst from 573 K (temperature of reaction step) to 723 K (temperature of regeneration step) significant amounts of several species desorb. The presence of ammonia in the desorption products, in particular, creates problems in the operations of the regeneration limit in the industrial process; desorption of ammonia therefore must be avoided or minimized as much as possible. It should be noted, however, that adsorbed ammonia may itself act as a reducing agent for sulfate species (see discussion of the product evolution during desorption; Figure 5) forming Nz and SO2 as reaction products. This effect may be suitably utilized to minimize the desorption of ammonia as such when the sorbentcatalyst is transported from the reaction unit to the regeneration unit. An increase in the specific DeNO, activity of the catalyst would certainly improve the rate of NH3 oxidation to N2 with formation of SOz. Finally, it should be mentioned that the comparison of the activity of fresh, sulfated, and regenerated Cu/Al2O3 samples (Figures 7 and 8) clearly indicates that copper oxide is more active than copper sulfate, even though the deactivation effect is very limited. These data are in contrast with the findings of Stelman (1987)reporting that
1970 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
sulfate is needed for NO reaction to occur. It may be observed that these data refer to a higher reaction temperature (670K)that may affect the comparison; for example, the reported low activity of CuO may be due to a too high rate of the parallel reaction of NH3 conversion to Nz. It should be also observed that our data on the activity of supported CuO are in agreement with those reported by other authors about the behavior of supported CuO in NO conversion with NH3 (Blanco et al., 1986; Iizuka et al., 1986;Mizumoto et al., 1978; Williamson and Lunsford, 1976). In conclusion,present data indicate that even though copper on alumina may effectively operate in the simultaneous DeSOJDeNO, reactions and more efficiently than copper on silica, the influence of sulfate species on the activity of the copper-active phase has a negative influence on the behavior in terms of induction time, NO conversion, NzO formation, and adsorption of ammonia. An improvement in the sorbent-catalyst DeNO, performance in order to reduce these effects and an optimization of reaction conditions to limit the desorption of ammonia during the regeneration step are thus required to develop further this syatem and the technology of simultaneous SOz and NO removal from flue gases.
Acknowledgment Financial support for this work from ENICHEM ANIC is gratefully acknowledged. Registry No. SO,, 12624-32-7; NO,, 11104-93-1;NH3, 766441-7;Cu, 7440-50-8;AZOa,1344-28-1;SiOz, 7631-86-9;SO2, 7446-09-5;NO, 10102-43-9;NzO,10024-97-2.
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