(5)Catalytic Properties in DeNOx and SO2-SO3 Reactions

Catalysis Today 56 (2000) 431–441

Catalytic properties in deNOx and SO2–SO3 reactions

Pio Forzatti (I-3)∗, Isabella Nova (I-3), Alessandra Beretta (I-3)Dipartimento di Chimica Industriale, Ingegneria Chimica “G. Natta” Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy

Contributors: Isabella Nova, Alessandra Beretta, Pio Forzatti (I-3); Vincenzo Palma, Paolo Ciambelli (I-4); Elena M. Slavinskaya, Bair S.Bal’zhinimaev (RU-1); Raffaele Colafato, Fiorenzo Bregani (I-6)

Abstract

SCR-deNOx reaction and SO2–SO3 oxidation tests were carried out by different research groups over fresh and usedEUROCAT oxide samples in order to characterize the reactivity of the catalysts and to compare data obtained in severallaboratories (Politecnico of Milan, Università of Salerno, ENEL of Milan, Boreskov Insitute of Catalysis).

Data are presented which indicate that the used EUROCAT catalyst is slightly more active both in the deNOx reaction andSO2–SO3 oxidation than the fresh sample.

An analyses of data collected over honeycomb catalysts by means of a 2D, single-channel model of the SCR monolithreactor has been performed to evaluate the intrinsic kinetic constant of the deNOx reaction; a satisfactory comparison hasbeen obtained between estimation of the intrinsic kinetic constant and estimation of the intrinsic catalyst activity from datacollected over powdered catalysts. A good agreement has been found in the experimental results collected in the differentlabs, both for the deNOx reaction and SO2–SO3 oxidation. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:DeNOx ; SO2–SO3 reactions; Selective catalytic reduction

1. Introduction

The selective catalytic reduction (SCR) process isthe most widely spread technology for the controlof NOx emissions from stationary sources (includingpower-plants and incinerators), due to its efficiencyand selectivity [1,2].

The deNOx process is based on the reaction be-tween NO and ammonia to produce nitrogen and wa-ter according to the reaction

4NO+ 4NH3 + O2 → 4N2 + 6H2O

∗ Corresponding author. Tel.:+39-2-23993238;fax: +39-2-70638173.E-mail address:[email protected] (P. Forzatti (I-3)).

In the treatment of sulfur-containing flue gases,SO2 is also fed to the SCR reactor; here it is partlyoxidized to SO3 which can react with water andunconverted ammonia to form sulfuric acid and am-monium sulfates. Accordingly, SO2 oxidation repre-sents a highly undesirable side reaction of the SCRprocess; catalyst formulation and operating variableshave to be designed in order to minimize its extentand, as a consequence, to avoid the risk of depositionof ammonium-sulfates or corrosion in the cold sec-tions of equipment of the plant downstream from thereactor.

Commercial SCR catalysts are a homogeneousmixture of vanadium pentoxide, tungsten (or molyb-denum) trioxide, supported on a high-surface areaanatase TiO2 carrier. It is well accepted that tung-sten (molybdenum) trioxide acts as a promoter of the

0920-5861/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.PII: S0920-5861(99)00302-8

432 P. Forzatti (I-3) et al. / Catalysis Today 56 (2000) 431–441

deNOx reaction: it lowers the temperature thresholdat which the reaction proceeds, and confers superiorthermal stability and better mechanical properties tothe catalyst [3–8]. V2O5 is the active component. Inthe case of the more traditional power-plant applica-tions, the vanadium oxide content is generally keptlow, in order to minimize SO3 formation; when usedin waste incineration plants, however, the catalystV2O5 content may be higher, in order to accomplishthe effective simultaneous abatement of dioxins andNOx .

For this study of a commercial deNOx catalyst forthe treatment of incinerator flue gases, the EUROCAToxide supplied by Austrian Energy, has been the objectof an international research project involving a largenumber of research and R & D labs from EuropeanUniversities and Industries. In this working party reac-tivity tests in the deNOx and SO2–SO3 reactions havebeen undertaken simultaneously by different partnersof the project.

Each laboratory received samples of the fresh andused EUROCAT catalyst; the used samples had beensubject to 9000 h of operation in a full-scale SCRreactor.

This paper reports the results obtained by theresearch groups in charge with the characteriza-tion of EUROCAT oxide activity in the deNOx andSO2–SO3 reactions. Bal’zhinimaev and coworkers atthe Boreskov Institute of Catalysis, Ciambelli andcoworkers at Univertità di Salerno, Professor Forzattiand coworkers at Politecnico di Milano and Breganiand coworkers at the research laboratory of ENELhave independently performed deNOx and SO2–SO3tests over both fresh and used EUROCAT samples.Most of the authors have tested the catalysts in theform of monoliths, as received by the supplier. Ex-periments from the Boreskov Institute of Catalysiswere carried out in a continuously stirred tank reactorby using the catalysts in the form of powders. Theresults are collected herein and discussed with thetwofold aim of fully characterizing the performanceof the EUROCAT oxide and of verifying the match inthe results from remote and independent laboratorieswhere state-of-the-art procedures are in use.

In the following, a brief description of the testingunits is given first. The results from activity dataover fresh and used monoliths are then presented andcompared. These have been analyzed by means of a

2D, single-channel model of the SCR monolith reac-tor developed previously [9,10]; the model analysisprovided a better insight about the relative importanceof intrinsic reactivity of the catalyst and mass transferin EUROCAT oxide.

Finally, data from the Boreskov Institute of Cataly-sis are presented. As they were obtained under kineticregime, and influences from inter-phase and intra-porous resistances were absent, these results weresuitable to the direct estimation of the deNOx intrinsicactivity of the catalyst.

A comparison between kinetic results obtained overpowders and estimation of the catalyst activity con-stant obtained from the model fit to the monolith-datawas then performed.

2. Experimental

2.1. Apparatus for activity test

2.1.1. Monolith catalystFig. 1 shows a schematic of the apparatus used

for the measurements of NOx reduction with ammo-nia (configuration A) over honeycomb catalysts at Po-litecnico of Milano; the same apparatus was modified(configuration B) to perform activity tests of SO2 ox-idation. A detailed description of both configurationshas been reported elsewhere [11,12].

The gas feed consisted of NOx /N2, O2/N2, H2O andN2. Mass flow-meters (F) were used to measure andcontrol the single gaseous streams, while a meteringmicro-pump (P) was used to inject water.

The feed stream was preheated and premixed inthe mixer (M); to avoid side reactions, ammonia wasadded to the gas mixture immediately before enteringthe reactor.

The reactor (R) consisted of a stainless steel tubeheated with four electric resistors; the inner wall of thereactor was lined with a glass tube in order to preventundesired oxidation reactions over the metallic partsupstream from the reaction zone. The temperature wasmeasured and controlled by a thermocouple slidinginside a capillary tube inserted in the catalyst (C).

Before loading, the monolith was wrapped withquartz wool and a bandage of ceramic material thatprevented the outer surface of the catalyst fromcatalyzing the reaction; it was then forced into the

P. Forzatti (I-3) et al. / Catalysis Today 56 (2000) 431–441 433

Fig. 1. Politecnico of Milan experimental apparatus for deNOx activity tests.

reactor taking care that no gas could bypass thecatalyst.

The gas exiting the reactor was scrubbed (S) withan aqueous solution of phosphoric acid to trap uncon-verted ammonia, then cooled in aniso-propylic alco-hol cooler to condense water vapor.

In the configuration A the gases were analyzed by achemiluminescence NO/NOx analyzer and by a para-magnetic oxygen analyzer.

In the configuration B, used to study the oxidationof SO2 to SO3, the SO3 concentration in the productgas was determined through condensation of sulfuricacid at 363 K in a glass spiral, followed by analysiswith an Ionic Chromatograph.

Blank experiments had been performed to confirmthat contributions of the apparatus to the deNOx andSO2–SO3 reactions were absent.

The apparatuses developed at the University ofSalerno and ENEL for the deNOx experiments andfor the SO2–SO3 oxidation tests had similar configu-rations; their description is thus omitted.

2.1.2. Powdered catalystThe apparatus used at the Boreskov Institute of

Catalysis consisted of a continuously stirred tank reac-

tor (operated within temperature region 523–623 K),wherein gases were fed by stainless steel lines, and aspecially designed membrane circulating pump usedto recycle the gas stream. The pump had a capacity of1000 l/h at 408 K and provided a recycle ratio equalto 200. All parts were heated at 408 K to avoid saltsformation.

The glass reactor was placed inside an electricalfurnace. To avoid internal diffusion, the catalyst wascrushed and sieved to 0.009–0.016 cm particles. Priorto the reaction, the catalyst was calcined at 623 K for2 h in a flow of 20 vol.% of O2 in He. Then NO, NH3and O2 in He were fed into the reactor. The inlet reac-tants concentrations were varied in each experimentsin order to keep constant the composition of the outletstream, which is detailed in the following.

The concentration of all the species exiting thereactor were determined using a three-column gaschromatograph with an automatic analysis system(Tsvet 500) combined with a combustion gas analysiscomputer “Ecom-omega”. A Cromosorb 104 columnwas used for NH3 and H2O separation. A Poropak QScolumn was used for the analysis of N2O and NO2.The sensitivity limit for N2O was 3.0× 10−3 vol.%NO2 and NO concentrations were determined byEcom-omega.


Table 1Geometry of the EUROCAT monolith catalyst

Channel number 9Pitch 3.6 mmWall thickness 0.6 mmLength 15 cmGeometrical surface area 194.400 cm2

The gas mixture was diluted 17 times by nitrogenand after complete ammonia adsorption with concen-trated H3PO4 it was directed to NO and NO2 sensors,working within 0–2000 ppm.

2.2. Operating conditions

Table 1 lists the geometrical characteristics of thefresh and the used EUROCAT catalyst samples pro-vided to the different project partners.

The standardized operating conditions adopted bythe research groups for the deNOx activity tests overmonoliths were suggested by Austrian Energy andare representative of industrial application. They arelisted in Table 2. In comparison with the operatingconditions generally used to study SCR catalysts forpower-plant applications [11,12], the present condi-tions are characterized by high oxygen concentration.Forzatti and coworkers performed additional experi-ments under experimental conditions (also listed inTable 2) traditionally used at Politecnico of Milanoand common in the literature for the screening ofcommercial SCR catalysts.

Table 2DeNOx activity tests over fresh and used EUROCAT monolithsa

Austrian Energy Politecnico di(conditions A) Milano (conditions B)

Area velocity (N m/h) 12.8 33α (=NH3/NOx ) 1 1.1NO (ppm) 300 500NH3 (ppm) 300 550O2 (vol.%) 11 2H2O (vol.%) 20 10N2 Balance BalanceSO2 Absent Absent

a Operating conditions: Universita di Salerno, Politecnico diMilano and ENEL have carried out experiments under conditionsA. Politecnico di Milano and ENEL performed additional experi-ments under conditions B.

Table 3SO2 oxidation tests over fresh and used EUROCAT monoliths:operating conditions of the experiments performed by Universitadi Salerno and Politecnico di Milano

Area velocity (N m/h) 10O2 (vol.%) 2H2O (vol.%) 10SO2 (ppm) 300N2 (ppm) Balance

In the case of the deNOx activity tests over pow-ders in CSTR reactor, a total flow rate of 75 cm3/min(STP) was used. The outlet concentrations of NO, NH3and O2 were kept constant at 0.17, 0.2 and 2 vol.%,respectively, in all the experiments.

SO2–SO3 oxidation tests over monoliths were per-formed by Università di Salerno and Politecnico diMilano under the experimental conditions listed inTable 3.

3. DeNOx activity data

In this section a full report is given of the deNOx

activity tests performed by the four research groups,using both the fresh and the used EUROCAT samples.

3.1. Tests over monoliths

3.1.1. Comparison of data from different laboratoriesTable 4 shows the results obtained over the fresh

EUROCAT monolith by different laboratories underoperating conditions A; these are compared with ref-erence data provided by the same supplier. The re-sults are presented in terms of NOx conversion as afunction of temperature within the range 420–560 K.The activity of the catalyst was remarkable, as about30–40% NO conversion was accomplished already at420–430 K; NO conversions higher than 90% werethen observed at 500 K.

The results obtained at Politecnico of Milano andthose obtained at the University of Salerno were ingood agreement; both the sets of data also comparewell with the result provided by Austrian Energy at553 K.

The ENEL experimental results show a certain de-gree of deviation from the other data: ENEL observedhigher NO conversions in the whole temperature rangeinvestigated.


Table 4Results of deNOx tests over the fresh EUROCAT monolith with operating conditions A

Temperature (K) NOx conversion (%)

Austrian Energy Politecnico di Milano ENEL University of Salerno

429 25 45 32443 64449 45 50453 71463 82473 85479 73 75483 89493 90503 96512 88 89533 93 95553 97 96 97

The same results are represented in Fig. 2 in termsof overall kinetic constantk (N m/h), estimated by theexpression

k = AV ln

(1 − χ

100

)(1)

wherein AV is the area velocity (N m/h) andχ thepercentage NO conversion. This equation is derivedon the assumption of plug-flow reactor, isothermal

Fig. 2. Results obtained by the different partners under experimental conditions suggested by the supplier over the fresh EUROCAT sample.

conditions, first order kinetics in NO concentrationand zero order kinetics in ammonia concentration. Anapparent activation energy as low as 20 kJ/mol can beestimated from the temperature dependence ofk.

Again, it appears that the data from Politecnicoof Milan and University of Salerno were in goodagreement.

Table 5 reports the results of the deNOx activitytests performed over the used EUROCAT catalyst


Table 5Results of the activity tests performed over the used EUROCAT catalyst under experimental conditions suggested by the supplier (conditionsA)


Politecnico di Milano ENEL University of Salerno

424 45429 43 43443 67451 66 61463 80473 88480 83 90 79514 91 91531 96 95555 97 98

using the experimental conditions A. The same rangeof temperature was investigated as for the freshsample (420–560 K), in order to facilitate a directcomparison of the performances of fresh and usedcatalysts. As previously, a satisfying agreement wasfound among the three sets of data of different centers;the match was especially good between the resultsfrom Università di Salerno and those from Politecnicodi Milano, while NO measurements at ENEL wereslightly higher.

As mentioned above, additional deNOx activitytests were performed by some of the authors underoperating conditions B. The results obtained over thefresh EUROCAT catalyst and used EUROCAT cata-lysts are reported in Tables 6 and 7, respectively. Awider temperature range was investigated under theseconditions, from 420 to 650 K, due to the higher AVvalue (33 vs. 12.8 N m/h) and the consequent low-ering of NO conversion at equal temperature withrespect to conditions A. The NOx conversions mea-sured by ENEL were still higher than those observedat Politecnico di Milano.

3.1.2. Fresh EUROCAT catalyst vs. used EUROCATcatalyst

Under both operating conditions A and operatingconditions B, the fresh EUROCAT sample showed alower activity in the deNOx reaction than the usedEUROCAT catalyst. No deactivation seems to accom-pany the 9000 h operation under industrial conditionof the present catalyst: this results in line with the factthat commercial catalysts are generally guaranteed for16 000–24 000 h operation.

On the contrary, there is the possibility that mod-ifications of the catalyst surface enhance the catalystactivity. It is known, for instance, that the formationof vanadium agglomerates leading to a loss of disper-sion (e.g. sintering of the catalyst [13]) increases sig-nificantly both the deNOx and the SO2–SO3 activityin SCR catalysts. This seems a possible explanationfor the behavior observed over the EUROCAT cata-lyst, given its rather high vanadium content. However,only a focused surface characterization of the freshand used samples (which is beyond the scope of thispaper) could provide the relevant pieces of evidence.

Table 6Results obtained under Politecnico standard experimental condi-tions (conditions B) over the fresh EUROCAT sample


Politecnico of Milan ENEL

424 15431 4453 29461 16483 48490 30513 63519 45543 72548 55572 79578 67604 85609 76629 78633 88


Table 7Results obtained under Politecnico standard experimental condi-tions (conditions B) over the used EUROCAT catalyst


Politecnico of Milan ENEL

424 20443 27453 32464 36473 42493 54513 56 61533 63543 71550 67565 71573 79584 75602 78 84627 81644 82

3.1.3. Effect of operating conditionsFig. 3 compares the overall intrinsic activityk of

the fresh EUROCAT catalyst measured under operat-ing conditions A (representative of the industrial ap-plication) and operating conditions B, at Politecnicodi Milano. It was found that at equal reaction temper-ature conditions A were associated with a much bet-

Fig. 3. Results obtained at Politecnico of Milan over fresh catalysts using different sets of operating conditions.

ter deNOx performance of the catalyst with respect toconditions B. The same effect was observed over theused catalyst and was confirmed by the experimentsperformed by ENEL. The increase of catalyst activ-ity can be most likely associated to the higher oxygencontent which characterizes the operating conditionssuggested by Austrian Energy with respect to condi-tions B. Such an increase has been quantified withthe model analysis presented in the following sectionwherein this effect is further discussed.

3.2. Data over powders

Table 8 reports the experimental results obtainedat the Boreskov Institute of Catalysis over the pow-dered fresh and the used EUROCAT samples. NOx

conversion measurements were performed in the range430–560 K.

In the following section these data are presented andcompared to those obtained over monoliths.

3.3. Monolith reactor modeling and data analysis

Some of the authors have previously developed a2D, single-channel model of the SCR monolith reac-tor [9,10,14]. The model relies upon some simplifyingassumptions: same conditions inside the monolith


Table 8Results obtained over the fresh and the used EUROCAT catalystat the Boreskov Institute of Catalysis


Fresh EUROCAT Used EUROCAT

427 47 48448 48 47476 68 69557 60 59

channels, isothermal reactor, negligible axial diffu-sion. It accounts for the single steps of inter-phasediffusion, intraporous diffusion and reaction on thecatalyst surface which the deNOx process kineticsinvolves. Gas–solid mass transfer is described on thebasis of the analogy with the Graetz–Nusselt problemof heat transfer in square ducts with laminar flow andconstant wall temperature [9]. An Eley–Ridel mech-anism is assumed to govern the reaction between NO(gas phase) and ammonia (strongly adsorbed) withkinetic expression

rNOx = kNOx CNOx

KNH3CNH3

1 + KNH3CNH3

(2)

Eq. (2) reduces to first order kinetics with re-spect to NO in the case of excess ammonia, i.e.KNH3CNH3 � 1. Effective diffusivity coefficients forNO and NH3 inside the monolith wall are evaluatedwith the “random-pore model” proposed by Wakaoand Smith [15]. Also, the model includes an analyti-cal approximate expression of the catalyst effective-ness factor; this is valid both for first order and forEley–Rideal kinetics, under the assumption of largeThiele moduli (i.e. the concentration of the limitingreactant is zero at the centerline of the catalytic wall).

Model equations include the following dimension-less material balances for NO and NH3 in the gas phase

dC∗NO

dz∗ = −4ShNO(C∗NO − C∗

NO, wall),

z∗ = 0, C∗NO = 1 (3)

α − C∗NH3

= 1 − C∗NO, z∗ = 0, C∗

NH3= α (4)

ShNO = 2.977+ 8.827(1000z∗)−0.545exp(−48.2z∗)(5)

and in the solid-phase

Table 9Geometrical data and morphological characteristics of the freshEUROCAT catalyst as assumed for model analysis

Wall half-thickness= 0.03 cmChannel opening= 0.3 cmPore volume, range: rp ≤ 100 Å= 0.14 cm3/g from N2

adsorption–desorptionPore volume, range 100≤ rp ≤ 500 Å= 0.24 cm3/g from Hg intru-sionMean pore radius= 150 ÅCatalyst bulk density= 1.75 g/cm3

ShNO(C∗NO − C∗

NO, wall)

= ShNH3(C∗NH3

− C∗NH3, wall)

De, NH3

De, NO(6)

ShNO(C∗NO − C∗

NO, wall) = Da[C∗2

NO − Y 20

+2(S1 − S2)

(C∗

NO − Y0 − S2 lnC∗

NO + S2

Y0 + S2

)]1/2

(7)

S1 = De, NH3

De, NOC∗

NH3− C∗

NO,

S2 = S1 + De, NH3

De, NO

1

K∗NH3

(8)

if S1 ≥ 0, Y0 = 0, elseY0 = −S1 (9)

In Eqs. (3)–(9), concentrations of NO and NH3are normalized with respect to the inlet NO concen-tration C0

NO, the dimensionless axial coordinate isz∗ = (z/dh)/Re Sc (dh being the hydraulic channeldiameter),Da = (kNODe, NO)1/2dh/DNO is a mod-ified Damköhler number,K∗

NH3= KNH3C

0NOis the

dimensionless NH3 adsorption constant,Di (cm2/s)the molecular diffusivity of speciesi, and De,i theeffective intraporous diffusivity of speciesi.

This model was successfully compared with labo-ratory data of NO conversion over commercial honey-comb SCR catalysts. It was also applied to the anal-ysis of deNOx activity data obtained over plate-typemonoliths.

The same model has been used to analyze the datacollected over the EUROCAT catalyst in the shape ofmonoliths. The analysis has been circumscribed to theperformance of the fresh catalyst and, for simplicity,the deNOx activity data obtained by Politecnico of


Milano were used for model fit, only. Such data weretaken as fully representative of the experiments per-formed by the other partners of the project.

The geometrical and morphological characteris-tics assumed as input parameters are summarized inTable 9.

It has been observed above that the tests over mono-lith showed quite different activity depending on theoperating conditions adopted. Namely, the referenceAustrian Energy operating conditions (characterizedby high oxygen concentration, 11 vol.%) resulted inhigher deNOx activity than the conditions B (with lowoxygen concentration, 2 vol.%). Notably, low O2 feedcontent (3.5 vol.%) was also adopted by the researchgroup at the Boreskov Institute of Catalysis for the ac-tivity tests over powders. Accordingly both situationswere analyzed by the model.

The catalyst intrinsic activity and effectiveness fac-tor were estimated by model fit to the data listed inTable 4 (conditions A), in order to characterize theEUROCAT oxide performance under industrial con-ditions. Model fit to the data listed in Table 6 (condi-tions B) provided elements for a more homogeneouscomparison between estimated activity of the mono-lith and measured activity of powders.

Since experiments were performed at NH3/NO feedratios≥1, first order kinetics were assumed. The re-sults of model fit are shown in Fig. 4. In both the casesof high and low O2 concentration, a good match be-tween experimental and calculatedT-dependence ofNO conversion was obtained based on the estimated

Fig. 4. Model fit (solid line) to activity data obtained under AustrianEnergy conditions (solid symbols) and model fit (dashed line) toactivity data obtained under conditions B.

intrinsic activation energy of 66.99 kJ/mol. This valueis more than three times higher than the apparent ac-tivation energy estimated in a previous section.

In the case of O2-rich feed stream (conditions A),the following optimal estimation of EUROCAT intrin-sic activity was obtained

kdeNOx= 199.817 exp

(−8052.34

(1

T− 1

523

))

(s−1) (10)

As part of the model analysis results, it was foundthat the fresh EUROCAT guarantees good intraporousdiffusion of reactants; the catalyst is in fact charac-terized by a high pore volume fraction in the range100–500 Å, and, from application of Wakao–Smithformula, it has been estimated that NO effective dif-fusivity at the reaction temperature of 533 K amountsto 2.6 cm2/s. This value is comparable with the high-est values of effective diffusion coefficients reportedin the literature for optimized SCR catalyst morpholo-gies [16,17]. As a consequence, it was estimated bythe model that the effectiveness factor of the EURO-CAT catalyst was as high as 30% at 533 K. The cata-lyst effectiveness increased significantly with decreas-ing reaction temperature, so that the activity data col-lected below 473 K were associated to monolith walleffectiveness higher than 50%.

Concerning the catalyst behavior in the presenceof 2% oxygen (conditions B), the following intrin-sic activity constant was estimated within theT-range431–629 K:

kdeNOx= 65.565 exp

(−8052.34

(1

T− 1

523

))

(s−1) (11)

At the reference temperature of 533 K, it was thusfound that by changing the operating conditions fromA to B, the catalyst activity was reduced to almostone third. As already mentioned, this effect was likelyassociated to the decrease of oxygen concentration. Itis usually reported in the literature that SCR reactionrate is affected by oxygen only at low O2 concentra-tions, while it reaches an asymptotic trend for O2 con-centrations greater than 2–5 vol.% [2,12]. Apparently,this does not apply to the present EUROCAT catalystat least in the range of relatively low temperatures atwhich it has been tested. Indeed, low temperatures and


Fig. 5. Comparison between the estimation of the intrinsic activityconstantkdeNOx (s−1) obtained from experiments over powdersby Boreskov Institute of Catalysis (symbols) and that obtainedthrough model fit to data obtained over monolith under conditionsB by Politecnico of Milan.

high vanadium concentrations are expected to empha-size the kinetic effect of oxygen, which is intrinsicallyrelated to the redox reaction mechanism governing thedeNOx process [18].

As already noted, Bal’zhinimaev and coworkersalso operated at lower oxygen concentration; as theirdata were obtained over powders (absence of intra-porous diffusion resistances) and in a perfectly mixedreactor (concentration of reacting species inside thereactor equal to outlet concentrations), an estima-tion of the intrinsic activity can be derived directlyfrom the conversion measurements, according to theequation:

F inletNO conversion= kdeNOx

CoutletNO Vcat (12)

wherein first order kinetics are assumed.Fig. 5 shows the comparison between (i) estimation

of EUROCAT intrinsic activity obtained from model

Table 10SO3 conversion values obtained over the fresh and the used EUROCAT sample

Temperature (K) Fresh EUROCAT Used EUROCAT

Politecnico of Milano University of Salerno Politecnico of Milano University of Salerno

653 >3.7 3.5643 – 3.8623 2.5 2.0 2.7 2.8610 1.8 1.7 2.3 1.9593 1.3 0.9 1.6 1.5578 0.9 0.7 1.0 1.1

fit to activity data over monolith under conditions B(Eq. (11)) and (ii) estimation of the catalyst intrinsicactivity from application of Eq. (12) to the activity dataover powders. The match is really satisfactory, given:(1) the completely independent nature of the sets ofdata, (2) the different reactor configurations used bythe two research groups, (3) the possible inaccuraciesof estimation ofkdeNOx

related to the description ofthe catalyst morphology (in the case of data obtainedover monolith) and to the simplifying character of Eq.(12) (in the case of data obtained over powders), (4)differences in operating conditions (e.g. water con-tent). Such agreement further supports the evidence ofa pronounced O2-effect over the EUROCAT catalystwith a significant decrease of activity in the presenceof lower oxygen concentrations.

4. SO2–SO3 activity data

Table 10 shows the values of SO2 conversion to SO3measured by Politecnico of Milano and University ofSalerno over the fresh and used EUROCAT samples.

SO3 conversion values obtained over both the cata-lysts are quite higher than those measured over otherSCR catalysts: this fact can be attributed to the highervanadia loading present in the EUROCAT catalyst.

It is also clear that the SO2–SO3 activity tests resultsfor the Politecnico of Milan and University of Salernodata are in good agreement.

The table also shows that the SO2–SO3 oxidationreaction over the used EUROCAT sample gives aslightly higher conversion than the fresh EUROCATcatalyst. As mentioned above, this effect may beattributed to modifications of the catalyst surface oc-curred under industrial operations (e.g. formation ofvanadium agglomerates).


5. Conclusions

The following conclusions can be drawn from theresults obtained over the EUROCAT oxide catalystby different research groups and presented in thisstudy:• deNOx data collected at Politecnico of Milan and

University of Salerno over both the fresh and usedmonolith samples are in good agreement, and com-pare well with the results provided by the catalystsupplier; the ENEL experimental results are slightlyhigher.

• Under both sets of experimental conditions underwhich the deNOx tests have been carried out, theused EUROCAT catalyst showed a higher reactivitythan the fresh sample. This confirms that 9000 h ofoperation did not deactivate the EUROCAT catalyst.

• A 2D, single-channel model of the SCR monolithreactor was used to analyze data collected at Po-litecnico of Milano over monolith samples and anestimation of the intrinsic kinetic constant and ofthe effectiveness factor was made; these intrinsicactivity data well compare with those obtained overpowdered catalyst by the Boreskov Institute.

• SO2–SO3 activity tests were also performed: activ-ity tests carried out by Politecnico of Milan andUniversity of Salerno are in good agreement. Theused EUROCAT sample presented a slightly higherconversion than the fresh EUROCAT catalyst.

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(5)Catalytic Properties in DeNOx and SO2-SO3 Reactions

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