Ind. Eng. Chem. Res. 1993,32, 1053-1060
1063
Selective Reduction of NOx by NH3 over Honeycomb DeNOxing Catalysts Jiri Svachula,+Natale Ferlazzo, Pi0 Forzatti,' and Enrico Tronconi Dipartimento di Chimica Industriale e Zngegneria 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 Data are presented for the effects of feed composition ( 0 2 , NO,, "3, HzO, SO2 concentrations) and of the operating conditions (area velocity, reaction temperature, and Re number) on the NO, removal efficiency over commercial-type honeycomb DeNOxing catalysts with different compositions and geometric characteristics. All of these data are explained by a Rideal type kinetic expression, derived assuming reaction between strongly adsorbed ammonia and gaseous or weakly bonded NO. The significanceof interphase and intraparticle diffusion limitations is discussed, and a comprehensive model of the SCR monolith reactor is illustrated which fully accounts for both external and internal transport of the reactants aswell as for the effects of the operating variables and of the feed composition under representative SCR conditions.
Introduction Among flue gas treatments for the control of NO, emissions from thermal power stations selective catalytic reduction (SCR) is best developed and most widely used all over the world due to its efficiency, selectivity, and economics (Bosch and Janssen, 1988; Nakatsuji et al., 1991). SCR technology is based on the reduction of NO, (mixture of 95% NO and 5% NO21 with NH3 into innocuous water and nitrogen according to the following reactions:
-
4N0 + 4NH3 + 0, 4N2 + 6H20
(1)
6N02+ 8NH3 -* 7N2 + 12H2
(2)
I
F----ml
~~
The reactions occur in the presence of parallel flow catalysts (honeycomb monoliths or plates) which operate with very small pressure drops. Commercial catalysts consist of homogeneousmixtures of anatase TiOz, tungsta, and vanadia, along with minor amounts of silicoaluminates as mechanical promoters. Although publications dealing with the catalytic behavior in the removal of NO, of vanadia, titania-vanadia, titania-tungsta, and titania-tungsta-vanadia catalysts are available in the literature (Bosch and Janssen, 1988; Nakatsuji et al., 1991; and references cited therein), systematic investigations over commercial SCR catalysts in the form of monoliths or plates addressing the effects of the operating variables and of the catalyst design parameters are scarce in the scientific literature. In this paper we present a systematicstudy of the effects of the operating conditions (contact time, Re number, temperature), of the feed composition ( 0 2 , NO,, SO2, HzO, NH3 concentrations),and of the catalyst design parameters (Vcontent and pore size distribution) on the reduction of NO, by NH3 over honeycomb commercial-type SCR catalysts. The significance of interphase and intraparticle diffusion limitations is discussed and a comprehensive
* To whom correspondenceshould be addressed. FAX: +392-70638173. + On leave from Department of Physical Chemistry, University of Chemical Technology, 53210 Pardubice, Czecoslovakia.
"3
Na
x%
'N
N8
Figure 1. Schematic diagram of the experimental apparatus for NO, removal catalytic teste: F, m u flowmeters;M, mixer; P, pump; R, reactor; C, catalyst bed; S, ammonia scrubber; A, analyzers.
model is presented, which fully accounts for both external and intraporous transport phenomena and for the effects of the operating variables and of the feed composition under representative SCR conditions. A similar investigation devoted to the undesired oxidation of SO2 to SO3 over commercial honeycomb SCR catalysts is reported in a previous paper (Svachula et al., 1993).
Experimental Methods Apparatus. Figure 1shows the schematic diagram of the apparatus used for the measurements of the reduction of NO, by "3. The reactant gas typically consisted of 500 ppm NO, (5 % v/v NO2 + 95 9% v/v NO), 500 ppm SOZ, 2 5% v/v 0 2 , l O 5% v/v HzO, and balance N2. The flow rates of the individualgaseous streams were controlledby Brooks mass flowmeters; water was fed through a metering micropump (GILSON, Model 302). The reactant mixture was preheated and premixed before entering the reactor; ammonia was injected directly at the top of the reactor to prevent side reactions. The reactor was a stainless steel tube electrically heated with four thermoresistances to provide isothermal conditions and was equipped with a thermocouple sliding inside a capillary tube. The reactor
QSSS-5885/93/2632-1053$04.00/0 0 1993 American Chemical Society
1054 Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 Table I. Characteristics of the Honeycomb Catalysts Tested in This Study catalyst V content pitch (mm) no. of channels A medium =7 1/9 B low =I 9 C medium =4 1/16 D medium =6 9 E high =6 16
65
W
w
was typically loaded with catalyst samples with square cross section consisting of 9 or 16channels 15cm in length. When the effect of Re number was investigated, the catalyst sample consisted of a single channel in order to achieve Re numbers as high as possible in the limit of the available total gas flow rates. The catalyst sample was wrapped with quartz wool and a bandage of ceramic material to prevent the outer surfaces of the catalyst sample from catalyzing the reaction, and then forced into the reactor to guarantee that no gas would bypass the catalyst. The area velocity was 33 m/h (NTP) in most cases. 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 catalyst volume in m3,and a is the geometric surface area per unit volume of the catalyst in m2/m3. 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,, S02, and 02 contents using a chemiluminescence NO/NO, analyzer (Beckman, Model 955), a ND IR analyzer for SO2 (Beckman, Model 865), and a paramagnetic oxygen analyzer (Beckman, Model 755). Analysis of the inlet reactant mixture indicated that NO was always more than 95 % of total NO,, the balance being NO2. Catalysts. Honeycombcommercial-type SCR catalysts were employed in this study. The V 2 0 ~loading of commercial catalysts is typically low (
