I n d . Eng. Chern. R e s . 1993,32, 826-834
Oxidation of SO2 to SO3 over Honeycomb DeNoxing Catalysts Jiri Svachula; Louis J. Alemany,* Natale Ferlazzo, Pi0 Forzatti,' and Enrico Tronconi Dipartimento di Chimica Zndustriale e Ingegneria Chimica "G. Natta" del Politecnico, Piazza Leonard0 da Vinci 32, 20133 Milano, Italy
Fiorenzo Bregani Centro di Ricerca Termica e Nucleare, ENEL-DSR, Via Monfalcone 18, 20132 Milano, Italy
A systematic study addressing the effects of the operating conditions (contact time, temperature), of feed composition ( 0 2 , SOZ, H20, "3, NO,, NH3 NO, concentrations), and of catalyst design parameters (wall thickness, V content) in the oxidation of SO2 to SO3 over honeycomb commercialtype DeNoxing catalysts is described. Data are presented which refer to transient behavior of the catalysts, indicating that long conditioning times are required associated with the buildup of surface sulfate species. The steady-state reaction rate is of variable order in SOz, the order increasing with SOz concentration as long as this is below 200 ppm and then decreasing; it is asymptotically independent of oxygen, depressed by water, strongly inhibited by ammonia, and slightly enhanced by NO,. The apparent activation energy changes from -50 t o -20 kcal/mol on increasing the reaction temperature. A redox steady-state kinetic model is presented which accommodates qualitatively all of the observed effects. Once properly modified the same model has the potential to explain also transient effects during conditioning of the catalyst.
Introduction The selective catalytic reduction (SCR) of nitrogen oxides with ammonia is widely used for the control of NO, emission in flue gases from thermal power plants (Bosch and Janssen, 1988; Nakatsuji and Miyamoto, 1991). Commercial SCR catalysts consist of homogeneous mixtures of anatase TiO2, tungsta, and vanadia, along with minor amounts of silicoaluminates as mechanical promoters, and are employed in the form of monoliths or plates. Commercial catalysts are required to have several characteristics, including high DeNO, activity in a wide temperature window, high stability, and low SO2oxidation activity. In particular sulfur trioxide produced through the oxidation of SO2 along with SO3 already present in flue gases is known to react with ammonia and water to form ammonium bisulfate (NH4HSO4) or ammonium sulfate ((NH4)2S04) which may deposit in the cold equipment downstream of the SCR reactor causing corrosion problems and reduced performances, the deposition being controlled by the equilibrium between ammonium sulfates and NH3 + SO3 + H2O. This problem is so important that industrial specifications for SCR processestypically include upper bounds both on the NH3 slip and on the concentration of SO3 exiting the SCR reactor. The admissible limits of outlet SO3concentration (of the order of 10 ppm) correspond to practical SO2 conversions as low as 1-2 % . While several scientific and technical publications deal with DeNO, activity and stability of tungsta-vanadiatitania catalysts (EPAJEPRI, 1991; Bosch et al., 1986; Tuenter et al., 1986; Binder-Begsteiger et al., 1990; Chen and Yang, 1992; Tronconi et al., 1992; Svachula et al., 19931,papers addressing the oxidation of SO2 to SO3 over the same catalytic systems are scarce in the scientific literature.
* To whom correspondence should be addressed.
On leave from Department of Physical Chemistry, University of Chemical Technology, 53210 Pardubice, Czechoslovakia. f On leave from Department of Chemical Engineering, Campus Teatinos, 29071 University of Malaga, Spain.
In this paper we present a systematic study of the effects of the operating conditions (contact time, temperature), of feed composition (02, S02, H20, "3, NO,, NH3 + NO, concentrations), and of catalyst design parameters (wall thickness, V content) on the oxidation of SO2 to SO3 over honeycomb commercial-type SCR catalysts. Data are presented which refer to the conditioning of the catalysts and to the steady-state behavior of the catalysts. A kinetic model is derived to describe in a comprehensive way the results obtained by changing the operating conditions, the feed composition, and the catalyst characteristics. The capability of the model to explain transient effects during the conditioning of the catalyst is also addressed.
Experimental Methods Apparatus. Figure 1shows the schematic diagram of the apparatus used for the measurements of the oxidation of SO2 to S03. The reactant gas typically consisted of 1000 ppm SOz, 2% Oz,lO% H20, and balance Nz. The SO2 concentration is typical for flue gasesfrom combustion of fuels with 1.5-2 % w/w S content. In some experiments the reactant gas also contained NO, and/or NH3 as specified in the text; when employed, NH3 was directly injected at the top of the reactor and mixed to prevent side reactions such as the direct oxidation of ammonia. The flow rates of the individual gaseous streams were controlled by Brooks mass flowmeters; water was supplied by a metering micropump (GILSON Model 302). The reaction mixture was preheated and premixed. The reactor was an electrically heated stainless steel tube provided with an internal glass tube (i.d. 3 cm) to prevent the reactor wall from catalyzing the oxidation of SO2 to S03. The reactor was loaded with catalyst samples with square cross section consisting of 9 or 16 channels and with 15-cm length and was operated isothermally, as confirmed by a thermocouple sliding inside a capillary tube. The catalyst sample was wrapped with quartz wool and a bandage of ceramic material that prevented the outer surface of the catalyst from catalyzing the reaction and then was forced into the reactor to secure that no gas would
0888-5885/93/2632-0826$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 827
Table I. Catalyst Characteristics Vcontent
wall thickness (mm)
no. of channels
A B C D E
medium medium medium low high
=7 =7 -7 -7 =6
=1.4 =1.4 -1.1 -1.4 -1.1
9 9 9 9 16
Table 11. Results of SCR Activity catalyst
A B C D
Figure 1. Schematic diagram of the apparatus used for the measurements of the oxidation of SO2 to SOs: F, mass flowmetere; M, mixer; P, pump; R, reactor; C, catalyst bed; S, ammonia scrubber.
bypass the catalyst. The area velocity was 10 m/h(NTP) in most experiments. The area velocity AV is defined as AV = Q/(V&),where Q is the reactant gas flow rate in m3/h (NTP), V , is the total catalyst volume in m3, and a is the catalyst geometric surface area per unit volume in m2/m3. SO3 concentration in the product gas was determined through condensation of sulfuric acid at 90 "C in a glass spiral, followed by analysis with a Ionic Chromatograph Dionex Model Quic. It was found impossible to directly measure the very low conversions of Son, since they were typically limited to a very few percent. Blank experiments confirmed that the contribution of the apparatus to the formation of SO3could be safelyneglected. Special care was taken to ensure that the measurements refer to true equilibrium conditions rather than to transient conditions: transient effects were generally observed after changes in the experimental conditions and particularly in the reaction temperature and in the SO2 concentration, and typically lasted for several hours. The same apparatus was used to study the activity of the catalysts in the selective catalytic reduction of NO, with NH3 (Svachula et al., 1992). In this case the reaction mixture consisted of 500 ppm N,, 550 ppm "3,500 ppm S02,2% 02,10% HzO, and Nz balance, and the system was operated with AV = 33 m/h(NTP). In this case the gas exiting the reactor was scrubbed with a 6% aqueous solution of phosphoric acid to trap unconverted ammonia, cooled to 4 - 1 0 "C to condense water vapor, split into two streams, and eventually analyzed for NO/NO,, SOZ,and 02 contents using a chemiluminescence NO/NO, analyzer (Beckman, Model 955), a ND IR analyzer (Beckman, Model 865), and a paramagnetic oxygen analyzer (Beckman, Model 755). Catalysts. Different honeycomb commercial-type SCR catalysts were used in this study. The V loading of commercial SCR catalysts, expressed as Vz05 content, ranges between 0.3 and 2% w/w, and is homogeneously distributed across the thickness of the monolith walls by virtue of the preparation procedure. Such a uniform distribution of V is generally required in SCR applications in order to secure a constant level of activity in spite of possible abrasion by the particulate in the gas stream. The relevant characteristics of the catalysts, including V content, pitch, and wall thickness, are listed in Table I.
V content medium medium medium low high
T ("C) 320 320 320 320 320
NO, (m/h(NTP)) 33.4 35.5 35.2 23.5 49.1
Results DeNO, Activity. The DeNO, activity of the investigated catalysts is quoted in Table I1in terms of first-order overall catalyst activity constant NO, according to the expression k,ol = -AV ln(1- xNO,) (1) where
XNO, represents the fractional conversion of NO, = [([NOxlin - [NOxlout)/[NOxlin)l. With reference to the experimental conditions of this study the reactor operates under combined intraparticle and external diffusion control so that (Svachula et al., 1993; Beeckman and Hegedus, 1991; Lefers et al., 1991) (XNO,
l/kNOz = l/kc + l/kg
where k, is the pseudo-first-order rate constant of the surface chemical reaction and k, represents the interphase gas-solid mass-transfer coefficient. Notice that k, is an effective rate constant incorporating the influence of mass transfer in the catalyst pores, the DeNO, reaction being strongly limited by intraparticle diffusion (Tronconi et al., 1992;Beeckman and Hegedus, 1991;Lefers et al., 19911, and that it is referred to the geometric catalyst surface. The results indicate that the SCR activity depends primarily on the vanadium content and that it increases on increasing the V loading. A more detailed discussion of the effects of the operating parameters and of the catalyst characteristics on NO, removal over honeycomb SCR catalysts is presented by Svachula et al. (1993). Oxidation of SO2 to SO3 1. Conditioning of the Catalysts. To obtain significant and reproducible results in the oxidation of SO2 to SO3, the catalyst must be conditioned. The conditioning procedure implies operation at the reaction temperature with gaseous mixtures containing sulfur dioxide, oxygen, water, and nitrogen. Figure 2 shows the results of a typical conditioning experiment: the concentration of SO3 at the reactor exit increaseswith time until a steady-state level is approached. When the flow of sulfur dioxide is stopped, the concentration of SO3 decreases slowly, indicating that SO3 is released from the catalyst (Figure 3). If the SO2 supply is resumed, the outlet concentration of SO3 gradually recovers its original value. These data prove that the conditioningof the catalyst is associatedwith a slow process of buildup of metal oxide sulfates onto the catalyst surface (it may take several hours up to -70 h depending on the experimental conditions and the catalyst) and proceeds until a steady-state concentration of sulfates is reached. The steady-state concentration of sulfates appears to be limited by the formation-decomposition equilibrium of metal oxide sulfates via so3 adsorption-desorption processes.
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Figure 3. Concentration of SO3 at the reactor exit after stopping the flow of SO2 (standard conditions): catalyst E, T = 330 "C. 30 -I4
O2 CONCBNTRATION (w1.S)
Figure 2. Results of a typical conditioning experiment during the oxidation of SO2 (standard conditions): catalyst E, T = 330 O C .
Figure 4. Effect of SO2 concentration on the oxidation of SO2 (standard conditions): catalyst B, T = 380 "C.
2. Effect of SO2. Figure 4 shows the results of experiments performed with different SO2 concentrations
Figure 5. Effect of 0 2 concentration on the oxidation of SO2 (standard conditions): catalyst B, T = 380 "C.
in a wide range over catalyst B. Diagnostic calculations performed by invoking the analogy with the GraetzNusselt heat-transfer problem to compute local masstransfer coefficients for SO2 (Hawthorn, 1974; Tronconi and Forzatti, 1992) showed that negligible SO2 concentration gradients are present at the gas-olid interface. Likewise, also intraporous limitations can be neglected on the basis of the Weisz-Prater criterion (Froment and Bishoff, 19791, since the SO2 oxidation reaction is considerably slower than SO2 diffusion within the catalyst pores. Accordingly the process is controlled by the chemical reaction, which is in line with very low measured SO2 conversions. The plot of SO2 conversion in Figure 4 exhibits a maximum, suggesting an apparent kinetic order in SO2 higher than 1for low SO2 concentrations (0-200 ppm) and a decreasing fractional order at higher inlet SO2 concentrations. Such a peculiar behavior is typical of SCR catalysts, since it has been observed in our laboratory in the case of several honeycomb catalysts when the effect of SO2 concentration was carefully investigated. It is worth noticing that, for practical purposes, a firstorder dependence on SO2 may provide a reasonable approximation in the range 0-1O00 ppm SOZ,in line with several technical reports (EPA/EPRI, 1991; Bosch et al., 1986). 3. Effect of 0 2 . The effect of oxygen concentration on the oxidation of SO2 is plotted in Figure 5. It appears that the rate of reaction is almost independent of oxygen concentration above 0.5-1 % v/v oxygen levels, due to the excess of oxygen. A dependence becomes manifest only for 02 concentrations comparable to those of SO2 (=lo00 ppm = 0.1% v/v), as expected. However, it is worth stressing that the specific oxygen concentrations where a dependence of the SO2 conversion becomes apparent depend both on the experimental conditions and on the catalyst redox properties. In any case the conversion of SO2 is almost independent of oxygen partial pressure over SCR commercial catalysts for representative operating conditions (02> 2% v/v). 4. Effect of Water Vapor. The addition of water to the reaction mixture results in a significant decrease of , 9 0 2 conversion (Figure 6). The inhibiting effect of water is apparent at low water concentrations and diminishes above 5% vlv water content: accordingly the rate of reaction is practically independent of the concentration of water vapor in the range of practical interest ( 6 1 5 % v/v). The original activity measured in the absence of water is restored within -1 h and then keeps constant if
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H20 CONCENTRATION (%r)
Figure 6. Effect of H20 concentration on the oxidation of SO2 (standard conditions): catalyst A, T = 380 O C . the flow of water is stopped: this time is much shorter than the characteristic time of catalyst conditioning as deduced from Figure 1 (1h