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Influence of geometrical parameters of honeycomb commercial SCR-DeNOx-catalysts on DeNOx-activity, mercury oxidation and SO 2 /SO 3 -conversion T. Schwämmle a,, F. Bertsche a , A. Hartung b , J. Brandenstein c , B. Heidel a , G. Scheffknecht a a University of Stuttgart, Institute of Combustion and Power Plant Technology – IFK, Department Fuels and Flue Gas Cleaning, Pfaffenwaldring 23, 70569 Stuttgart, Germany b IBIDEN Porzellanfabrik Frauenthal GmbH, Gamserstr. 38, 8523 Frauental, Austria c E.ON New Build & Technology GmbH, Alexander-von-Humboldt-Straße 1, 45896 Gelsenkirchen, Germany highlights " Catalysts with different geometrical parameters have been tested. " Tests have been performed and compared in a micro reactor and bench reactor. " We investigated DeNOx-activity, SO 2 /SO 3 -conversion and mercury oxidation. " Small pitch favours DeNOx-activity due to enhanced external mass transfer. " Tests showed higher mercury oxidation at catalysts with increased wall thickness. article info Article history: Received 27 November 2012 Received in revised form 11 February 2013 Accepted 15 February 2013 Available online 26 February 2013 Keywords: SCR-DeNOx-catalysts DeNOx-activity Mercury oxidation SO 2 /SO 3 -conversion abstract A systematic study on the influence of geometrical parameters (pitch and wall thickness) of commercial high-dust honeycomb SCR-DeNOx catalysts on DeNOx-activity, mercury oxidation and SO 2 /SO 3 -conver- sion is described. All catalysts had an identical chemical composition of 0.6 wt.% V 2 O 5 . The study was con- ducted in a laboratory micro reactor and a technical scale bench reactor and focuses on the effect of different honeycomb geometrical parameters on the reactions at the catalysts. The combined variation of pitch and wall thickness showed lower DeNOx-activity at catalysts with larger channel openings. This indicates mass transfer limitations when the flow regimes are developed in the catalyst’s channels due to the relatively fast reaction kinetics of the DeNOx-reaction. Results showed that the SO 2 /SO 3 -conversion is linearly dependent on catalysts wall thickness. Mercury oxidation increased slightly linear with increas- ing wall thickness of the catalyst, indicating that the reaction takes also place in the catalysts bulk because of its slow chemical reaction kinetics in contrast to DeNOx-reaction being controlled by diffu- sion. The importance of flue gas HCl-concentration on mercury oxidation was pointed out. Additionally, research on the co-influence of DeNOx-reaction and SO 2 /SO 3 -conversion was performed. A strong inhibi- tion of SO 2 /SO 3 -conversion by flue gas ammonia was shown. However, DeNOx-activity is enhanced by conditioning with SO 2 due to superior acidity of active sites. Summarising the results, due to the draw- back of elevated SO 2 /SO 3 -conversion, higher risk of channel blocking and material costs, increasing wall thickness cannot be considered a reasonable strategy to enhance mercury oxidation by SCR catalysts. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Over 40% of the global electricity is produced in coal fired power stations, either by firing hard coal or lignite [1]. The total amount of coal consumed globally will increase over the next years. Coal derived flue gas consists of several pollutants, such as nitrogen oxides (NO x ), sulphur oxides (SO 2 , SO 3 ), particulate matter (PM) and trace elements e.g. mercury (Hg). Particulate matter is controlled by electrostatic precipitators or baghouse filters. SO 2 is removed by dry or wet flue gas desulphurisation (FGD). During technical combustion, nitrogen oxides are mainly formed by two different mechanisms, namely by oxidation of the nitrogen content of the fuel and depending on combustion temper- ature via oxidation of air-nitrogen [2]. For the reduction of NO x (DeNOx), the selective catalytic reduction process (SCR) is the most common applied technology since it was commercially vended in the 1980s [3,4]. The overall reaction can be expressed as: 4NO þ 4NH 3 þ O 2 ! 4N 2 þ 6H 2 O ð1Þ 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.02.057 Corresponding author. Tel.: +49 711 685 67760; fax: +49 711 685 63491. E-mail address: [email protected] (T. Schwämmle). Chemical Engineering Journal 222 (2013) 274–281 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

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  • Inuence of geometrical parameters of honeycomb commercial SCR-DeNOx-catalysts on Dand SO2/SO3-conversion

    T. Schwmmle a,, F. Bertsche a, Aa University of Stuttgart, Institute of Combustion and Po

    serstr. 3er-von-H

    arametared inSO3-cone to en n at ca

    condit ioning with SO2 due to superior acidity of active sites. Summarising the results, due to the draw- sts, increasing wall by SCR catalysts.All rights reserved.

    Over 40% of the global electricity is produced in coal red power stations, either by ring hard coal or lignite [1]. The total amount ofcoal consumed globally will increase over the next years. Coal derived ue gas consists of several pollutants, such as nitrogen oxides (NOx), sulphur oxides (SO2, SO3), particulate matter (PM)and trace elements e.g. mercury (Hg). Particulate matter is

    use ltersremoved by dry or wet ue gas desulphurisation (FGD).

    During technical combustion, nitrogen oxides are formed by two different mechanisms, namely by oxidation of the nitrogen content of the fuel and depending on combustion temper- ature via oxidation of air-nitrog en [2]. For the reduction of NOx(DeNOx), the selective catalytic reduction process (SCR) is the most common applied technology since it was commerciall y vended inthe 1980s [3,4]. The overall reaction can be expresse d as:

    4NO 4NH3 O2 ! 4N2 6H2O 1 Corresponding author. Tel.: +49 711 685 67760; fax: +49 711 685 63491.

    Chemical Engineering Journal 222 (2013) 274281

    Contents lists available at

    n

    w.E-mail address: [email protected] (T. Schwmmle).back of elevated SO2/SO3-conversion, higher risk of channel blocking and material cothickn ess cannot be considered a reasonable strategy to enhance mercury oxidation

    2013 Elsevier B.V.

    1. Introduction controlle d by electrostatic precipitators or bagho1385-8947/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.02.057. SO2 is

    mainly Article history:Received 27 November 2012 Received in revised form 11 February 2013 Accepted 15 February 2013 Available online 26 February 2013

    Keywords:SCR-DeNOx-catalystsDeNOx-activityMercury oxidation SO2/SO3-conversion

    A systematic study on the inuence of geometrical parameters (pitch and wall thickness) of commercial high-dust honeycomb SCR-DeNOx catalysts on DeNOx-act ivity, mercury oxidati on and SO2/SO3-conver-sion is described. All catalysts had an identical chemical composition of 0.6 wt.% V2O5. The study was con- ducted in a laboratory micro reactor and a technical scale bench reactor and focuses on the effect ofdifferent honeycomb geometrical parameters on the reactions at the catalysts. The combined variation of pitch and wall thickness showed lower DeNOx-activity at catalysts with larger channel openings. This indicates mass transfer limitations when the ow regimes are developed in the catalysts channels due tothe relatively fast reaction kinetics of the DeNOx-reaction. Results showed that the SO2/SO3-conversion islinearly depende nt on catalysts wall thickness. Mercury oxidation increased slightly linear with increas- ing wall thickness of the catalyst, indicating that the reaction takes also place in the catalysts bulk because of its slow chemical reaction kinetics in contrast to DeNOx-r eaction being controlled by diffu- sion. The importance of ue gas HCl-concentr ation on mercury oxidation was pointed out. Additionally,research on the co-inuence of DeNOx-reaction and SO2/SO3-conversion was performe d. A strong inhibi- tion of SO2/SO3-conversion by ue gas ammonia was shown. However, DeNOx-activity is enhanced bya r t i c l e i n f o a b s t r a c tb IBIDEN Porzellanfabrik Frauenthal GmbH, Gamc E.ON New Build & Technology GmbH, Alexand

    h i g h l i g h t s

    " Catalysts with different geometrical p" Tes ts have been performed and comp" We investigated DeNOx-activity, SO2/" Small pitch favou rs DeNOx-activity du" Tests showed higher mercury oxidatioeNOx-activity, mercury oxidation

    . Hartung b, J. Brandenstein c, B. Heidel a, G. Scheffknecht a

    wer Plant Technology IFK, Department Fuels and Flue Gas Cleaning, Pfaffenwaldring 23, 70569 Stuttgart, Germany 8, 8523 Frauental, Austria umboldt-Strae 1, 45896 Gelsenkirchen, Germany

    ers have been tested.a micro reactor and bench reactor.version and mercury oxidation.hanced external mass transfer.talysts with increased wall thickness.Chemi cal Engi

    journal homepage: wwSciVerse ScienceDi rect

    eering Journal

    elsevier .com/locate /ce j

  • Since combustion of coal accounts for almost 50% of the global mercury emission s [18] and due to its high toxicity, mercury was identied as a chemical of global concern. There are global efforts to reduce mercury emissions from coal red power plants [18].Driven by the reduction of the overall CO2-emissions in coal redpower plants, co-ring of biomass and refuse derived fuels is analternativ e way to CCS-tech nology to reduce CO2-emissions [19].

    However , standard commercial high-dust SCR-DeNOx -catalysts have not initially been designed for increased mercury oxidation over a long time at various fuels. Addition ally, high-dust SCR-

    by kneading and extrusion followed by calcination in full scale

    ineering Journal 222 (2013) 274281 275Ammonia (NH3) is therefore injected upstream of the catalyst unit and reacts with NOx at the catalyst to nitrogen (N2) and water (H2O). SCR-catalyst s are design ed either as homogeneous extruded ceramic honeycom bs, plate catalysts or corrugated catalysts. Vari- ous catalyst materials have been investi gated in the last decades [5]. Today, the most common carrier mater ial for power plant appli- cation is TiO 2 in its anatase modication and vanadia (V2O5) as ac- tive phase [6]. Certai n promoters, such as WO3 or MoO 3, are added in order to increase the therma l stability of the TiO 2 phase [7].

    As an undesired side reaction, sulphur dioxide is heteroge- neously oxidized to sulphur trioxide at the SCR catalysts, which is called SO2/SO3-conversion:

    SO2 12 O2 ! SO3 2

    The rate of this reaction is linked to the vanadia content of the cat- alyst [5]. There are various reactions of SO3 with other ue gas com- ponents, causing corrosion or deposit s downstre am of the SCR-unit.In SCR-DeN Ox-cataly sts, WO3 is introduce d to suppress the SO2/SO3conversio n [4].

    Depending on their origin, coals contain 0.02 up to 10 mg/kg mercury [8]. At high temperatures in the combusti on chamber,mercury is released in its elemental form Hg0 [9]. When ue gas cools down, gas-phase transformat ions occur and elemental mer- cury is homogeneousl y or heteroge neously oxidized to Hg2+. Due to adsorption processes , mercury is partially bound on y ash par- ticles (HgP). Elemental mercury is considered as inert and is almost insoluble in water. In ue gases of combustion processes, the dom- inant oxidized mercury compound is HgCl 2, as it was shown bytheoretical calculations performed by Martel [10]. The overall oxi- dation reaction of elemental mercury in ue gas can be described as:

    2Hg0 4HCl O2 $ 2HgCl2 2H2O 3The impact of SCR-DeN Ox-cataly sts on the oxidation of mercury was investigate d by several authors in lab- and full scale measure- ments [9,11,12 ]. They demonstrat e consistently that the oxidatio nof mercury is enhanced in the presence of SCR-cata lysts. The results were highly dependent on coal compositio n, especially on haloge ncontent (Cl, Br), but also on sulphur content. Catalyst deactivatio nstate plays a key role in mercury oxidation. There are various reac- tion-pat hways discussed for mercury oxidatio n at SCR-cata lysts.Niksa and Fujiwa ra [13] and Hong et al. [14] proposed EleyRideal reaction mechani sms, in which HCl adsorbs on the active sites ofthe cataly st surface, competi ng with ammonia. Either in the gas phase or as weekly adsorbe d species , mercury reacts with the adsorbed HCl. Another possible mecha nism is the Deacon process,proposed by [11] and described as:

    4HCl O2 $ 2Cl2 2H2O 4

    Hg0 Cl2 $ HgCl2 5In this possible reaction pathway for mercury oxidation atSCR-cata lysts, Cl2 is produce d by the reaction of HCl with cataly stactive sites (Eq. (4)). Highly reactive Cl2 subsequent ly oxidises Hg0

    in the gas phase (Eq. (5)). It was shown by [15] that the direct oxidation of elemental mercury with Cl2 and Br2 in the gaseous phase is faster than the indirect reaction of Eq. (3).

    Due to its physical and chemical properties [16], HgCl 2 adsorbson y ash and can be absorbed in wet ue gas desulphurisati onunits [9,17], leading to lower overall mercury concentrations inthe gas phase. Therefore, the combinati on of SCR-DeNO x-catalyst,

    T. Schwmmle et al. / Chemical EngESP and wet-FGD, which is a common set-up for installed air pollution control devices in Western Europe, forms an ideal arrangement with the co-benet of mercury removal.monolith size (150 mm 150 mm) by IBIDEN Porzellan fabrik Frauenthal GmbH, Austria. All catalysts had the same chemical compositi on, which was veried by surface and bulk XRF-analysis.Geometri cal properties of honeycomb catalysts can be character- ized by pitch, wall thickness and length. Fig. 1 shows a honeycomb catalyst sample, with the homogeneous catalyst wall (w) and the clear width (d) of the ow channel. The catalyst pitch is equal toone clear width and one wall thickness. A combined variation ofpitch and wall thickness has been performed, in order to get anoverview on both: channel opening and wall thickness of the cat- alyst. In Table 1, the dimensions of the tested catalysts are listed:

    The catalysts are named according to their pitch. Reference is a7 mm pitch catalyst with 21 21 channels of the full size mono- lith, which is the typical high-dust catalyst pitch. Variation of pitch was performed in the range of 6 mm up to 9.7 mm and accordingly catalysts are facing further challenges, such as erosion and blocking by y ash [20], leading to lower efciency and lifetime of the catalyst. Standard commerc ial honeycomb catalysts are produced with a 7 mm pitch. Increasing the catalyst pitch athoneycomb catalysts would reduce the risk of blocking of the channels. Thicker walls of the catalysts honeycomb structure would lead to an increased lifetime of the catalyst monolith inthe ue gas. It is assumed, that the modication of the catalysts geometri cal parameters will also have an effect on the reactions catalysed. In order to quantify the extent of this effect, a systematic study at different test conditions is performed, giving an overall insight in the most important reactions for emission reduction oncommerc ial SCR-DeNO x-catalysts. Finally, the economic aspects of different catalyst geometries of honeycomb catalysts are addresse d.

    2. Experimen tal method

    2.1. Catalysts

    In this study, the behaviou r of standard commercial honeycomb catalysts is addressed, which are one of the major catalyst types for power plant application. The catalysts were of V2O5WO3/TiO2-type with a V2O5-content of 0.6 wt.%. This is a typical value for high-dust catalysts in coal-red power plants located downstream of the boiler [5,20,21]. Honeycomb catalysts have been produced Fig. 1. Geometrical properties of honeycomb catalysts.

  • wall thickness was varied between 0.8 mm and 1.4 mm, covering the whole reasonable range for high-dust application.

    2.2. Test setup

    Investigations on the reactions at the catalyst have been per- formed at two different test rigs: The micro reactor and the bench reactor, both constructed and operated accordin g to the guideline for testing of DeNOx catalysts [22]. Fig. 2 describes the general set- up of the reactors.

    In both test rigs, ue gas is doped with the relevant compo- nents, which are added from gas bottles or by evaporati on of liquid solutions of these compounds . Table 2 summari zes the test condi- tions. Water content is adjusted either by evaporation or by con- densation. The gas mixture is heated up in the reactor.Thermocoupl es up- and downstream of the catalyst ensure a con- stant temperature of 380 C. Samples are taken at ports up- and downstream of the catalyst and analysed as described below. For

    Table 1Dimensions of the tested catalysts.

    Pitch (mm) Wall (mm)

    P6 6.0 0.8 P6.7 6.7 0.9 RC 7.0 0.9 P8.2 8.2 1.0 P9.2 9.2 1.2 P10 9.7 1.4

    276 T. Schwmmle et al. / Chemical EngineeTable 2Test conditions.the operation of the test reactors and testing of the catalysts, the linear velocity (LV), describin g the ue gas ow rate divided bythe catalyst surface area exposed to ow, and area velocity (AV),which is the quotient of ue gas ow rate and geometric surface

    Fig. 2. Test reactor design.Micro reactor Bench reactor

    LV (m/s @ STP,wet) 1 2.5 AV (m/h @ STP,wet) 27.5 1018Channels Up to 9 Up to 625 Flue gas ow rate (m3/h @ STP) 1.2 150 Temperature (C) 380 380 O2 (vol.%) 4 5CO2 (vol.%) 15 14N2 Balance Balance H2O (vol.%) 7 6NO (ppmv; STP dry) 300 300 NH3 (ppmv; STP dry) 360 300 SO2 (ppmv; STP dry) 500 500 Hg (lg/m3; STP dry) 50 HCl (mg/m3; STP dry) 10100 area of the catalyst, are of great importance . These values are cal- culated at standard temperat ure and pressure (STP).

    2.2.1. Micro reactor In micro reactors, samples of 3 3 channels of the full scale

    monolith s are tested in laboratory in synthetic ue gas. The reactor is constructed of glass. Flue gas ow rate is adjusted to a LV of 1 m/s and an AV of 27.5 m/h. Due to different geometries of the cata- lysts, the length of the catalysts has been adjusted to meet the test condition s, resulting in catalyst samples with differing length.

    2.2.2. Bench reactor This technical scale reactor is constructed of stainless steel and

    ue gas is generated by a propane burner. In a bench reactor, full scale catalyst monoliths of 150 mm 150 mm are tested in their original manufactur ed dimensions . Flue gas ow rate is set to150 m3/h. Results of the bench reactor test can directly be trans- ferred to full scale catalyst reactors and bench reactor measure -ments are a reference for guarantee values of manufactur ers [20].Due to safety reasons, no mercury oxidation measureme nts were performed at this reactor.

    3. Determined parameters

    In order to fully cover all relevant reactions at the SCR-DeNOx -catalysts, DeNOx-a ctivity, SO2/SO3-convers ion and mercury oxida- tion have been researched in this study.

    3.1. DeNOx-ac tivity

    DeNOx activity describes the performance of the catalyst re- lated to NOx-reduction with ammonia, considering the AV. It isdetermined at dened and xed NH3/NO-ratios of a = 1.0 in bench reactor and a = 1.2 in micro reactor. There is a higher NH3/NO ratio applied at the micro reactor in order to minimise the effect of pos- sibly unequal distribution of ammonia in the small sample size. Atthis test condition, there is always excess ammonia in the ue gas,showing the maximum value of NOx-reduction. DeNOx-a ctivity iscalculated according to the rst order reaction equation in the fol- lowing formula [22] with the AV and NOx-reduction g(a):

    KNOx amh

    h i AV ln1 ga 6

    Nitrogen oxide concentrat ion up- and downstre am of the catalyst ismeasured by continuou s monitor s, working on the principle ofchemilu minescen ce [22].

    3.2. SO2/SO3-conversion

    SO2/SO3-convers ion is an indicator for the amount of SO3 pro-duced over the catalyst. SO3-concentration is measured up- and downstre am of the catalysts in the ue gas according to the VDI method 2462 [23] by controlled condensation of sulphuric acid aerosols in a glass condenser at a temperat ure of 85 C. The tem- perature is high enough to avoid condensation of water in uegas, but low enough to condense SO3 as sulphuric acid aerosols.The conversion is calculated considering the ue gas SO2-concen-tration, which was measured by continuous monitors working onthe principle of UV absorption, and the SO3-inlet- and outlet con- centration accordin g to [22]:

    SO2=SO3 conversionj23% cSO3out cSO3incSO2in 80 7

    ring Journal 222 (2013) 274281The differe nt molar ratios of SO2 and SO3 accoun t for the factor of 80[%]. SO2/SO3-conversion coefcient is calculate d similar to the DeNOx -activity.

  • 3.3. Mercury oxidation

    The concentration of elemental mercury is measured by acontinuous mercury analyser (Hg2010, Semtech Metallurgy AB)working with cold vapour atomic adsorption and a Zeeman background correction. Oxidised mercury is determined as the difference between total mercury, which is measured by wet chemical reduction with tin(II)chloride solution upstream of the analyser, and the concentration of elemental mercury. Mercury oxidation is therefore calculated according to:

    jHg0=Hg2 % cHg2out cHg2in

    cHg0in 100 8

    tioning is essential because there is a differenc e in activity compar-

    3000 mg/m 3 SO2 (not shown here) showed no further increase inDeNOx-a ctivity.

    The comparison of catalysts with different pitch at constant area velocity leads to the conclusio n, that there is no inuence ofthe catalysts geometry on DeNOx-acti vity evident, regardles sof the sulphating of the active sites. An inuence of the thickness of the wall cannot be seen from these measurements , which can be referred to the fast reaction kinetics of the NOx-reduction.According to [27], the reaction takes place only in the rst 50 lmof the catalyst walls.

    Fig. 4 shows the DeNOx-a ctivity measureme nt of all tested catalyst determined at the bench reactor.

    When comparing the results at the micro reactor and bench reactor of the measure ments of the reference catalyst RC, it isobvious that the DeNOx-a ctivities are approximately the same.Deviation s can be explained by the higher NH3/NO-ratio in micro

    T. Schwmmle et al. / Chemical Engineering Journal 222 (2013) 274281 277ing fresh and preconditioned results. For commercial application,sulphating is performed during the rst start-up of the plant with installed new catalysts, so the test with ue gas SO2 shows the more realistic test conditions. Nevertheless, measure ments atIt has to be stressed , that the in- and outlet total mercury concen- tration has to be identica l, in order to ensure steady state operation of the catalyst. These conditio ns are not reached before sufcientprecond itioning time.

    4. Results

    4.1. DeNOx-activ ity

    The most important reaction, which SCR-DeNOx -catalysts are originally designed for, is the NOx-reduction. First of all, the effect of the different pitch size on NOx-reduction was studied in micro reactor at selected fresh catalyst samples. Measurem ents were per- formed with the fresh catalyst sample in absence of sulphur diox- ide in the ue gas. Following, the catalyst was exposed to SO2 forsome days and the measure ments were repeated. Fig. 3 showsthe results of these measurements .

    The results of the experime nts in the presence and in the ab- sence of sulphur dioxide differ signicantly. Fresh catalyst samples show a limited sulphating of active sites. When exposed to ue gas containing SO2, the Lewis and Brnsted acidity of the active sites isincreased due to the adsorption of SO2 [24]. Being a Brnsted base,NH3 is adsorbed at the acidic active sites, leading to an increased DeNOx-acti vity. At TiO 2-based industrial SCR-catal ysts, sulphating is only partially and reversibly and does not cause any deactiva- tion, rather leading to enhanced DeNOx-a ctivity [25]. In contrast to Svachula et al. [26], it can be stated here, that for measuring the DeNOx-activity of commercial SCR-DeNOx-ca talysts, precondi- Fig. 3. DeNOx-activity of catalysts with different pitch at NH3/NO = 1.2 in micro reactor.reactor measurement, leading to an excess of ammonia and slightly higher DeNOx-acti vity. Catalyst reactors of full scale power plants are designed with DeNOx-activity values of fresh honey- comb catalysts of around 40 m/h [20], thus the determined values are quite representat ive for standard commercial honeycomb catalysts.

    These measureme nts also conrm the fast reaction kinetics ofNOx-reduction, since there is no dependence on wall-thickness .In contrast to the results of the micro reactor, a decrease of De- NOx-acti vity was measure d with increasing pitch. This can be ex- plained by differing ow conditions. The reaction process in the catalyst is the result of external mass transfer (from bulk gas phase to the surface of the catalyst), internal mass transfer (diffusion ofreactants through internal pores to and from active sites) and chemical reaction kinetics. Fig. 5 gives an insight in ue gas owregimes through catalyst channels.

    In the inlet-zon e, ue gas enters the channel, while the owprole is not yet fully developed. There is mass exchange from the bulk phase to the surface due to convection and diffusion.Downstr eam the inlet zone, the laminar ow prole has been fully develope d. In this prole, mass exchange is only due to diffusion,being limited when compared to convection. In literature [20],the inlet-zone of a catalyst channel is often called turbulent inlet zone. Therefore, the rate of NOx-reduction is much greater in the inlet zone of the catalyst. The dimensio ns of the test samples for micro reactor and bench reactor result in different ratios of free width (d) to length (l). For micro reactor catalysts, the d/l ratio equals to 0.03. In contrast to that, the d/l ratio of full scale mono- liths, investigated with the bench reactor, varied between 0.005 and 0.008. Due to the short length of the micro reactor samples and the relatively high area velocity, the fraction of the turbulentFig. 4. DeNOx-activity of all 6 tested catalysts measured in bench reactor at NH3/NO = 1.

  • It can be seen, that there is an increase in the SO2/SO3-conver-sion for higher pitch in micro reactor measureme nts. Unfortu- nately, the results gained in bench reactor measureme nts, aspresente d in Fig. 6 show a less distinctive interrelation. Further correlations have been performed and are shown in Figs. 7 and 8.

    Unlike the characterist ics of the DeNOx-reac tion, a depende ncy of the SO2/SO3-conversion on pitch cannot be traced back to the clear width of the catalysts channels. Taking into account that NOx-reduction is limited by external mass transfer and diffusion,mass transfer controlled SO2-conversion would lead to lower SO2/SO3-convers ion values. As shown in Fig. 7, there is a linear depende ncy of the SO2/SO3-conversion on the catalysts wall thick- ness. This effect can be explained by the relatively low kinetics ofthis chemical reaction [32]. In contrast to NOx-reduction, which proceeds rapidly when the reactants occupy the active sites, the reaction of SO2 is compara tively slow. Thus, SO2 diffuses in the catalysts bulk and penetrates into the macroporou s system ofthe catalyst wall. Thus, the reaction takes place within the whole catalyst material. According to Dunn et al. [34], SO2 oxidation has

    278 T. Schwmmle et al. / Chemical Engineering Journal 222 (2013) 274281zone is dominating, thus no dependence on pitch of DeNOx- activity can be observed. In bench reactor (long samples), where mass exchange is mainly controlled by diffusion, the impact of owregime on DeNOx-activity is obvious, resulting in reduced activity at larger channel openings. This effect of external mass-transfer onNOx-reduction at honeycomb catalysts was also observed and described by [26,28,2931]. They considered not only the inter- phase mass transfer, but also the intra-phase mass transfer in the catalyst porous system. Inter-phase mass transfer can be described by correlations of the Sherwood number and is described in detail in the given references.

    4.2. SO2/SO3-conversion

    SO2/SO3-conversion was determined by micro reactor and bench reactor measureme nts in the absence of ammonia in uegas after at least 48 h of preconditioning. Hence, the values presented in Fig. 6 show the maximum values at the measured SO2-concentrat ion.

    For decreasing ue gas SO2-concentrat ions, the relative SO2/SO3-conversion will increase, however the absolute SO3-concentratio n downstream of the SCR is decreasing. This is in good agreement with previous ndings in [32,33]. Differences between the micro reactor and bench reactor values can be referred totemperature deviations. The effect of temperat ure on SO2/SO3-conversion was discussed in a previous work [33], pointing out the exponential dependence. After nishing the test series in the

    Fig. 5. Flow conditions in a catalyst channel.micro reactor, a deviation in temperature of 14 C above 380 Cduring the SO3-measurement es was determined. This explains the deviations between the measureme nts in micro- and bench reactor.

    Fig. 6. SO2/SO3-conversion coefcient determined in micro- and bench reactor without NH3, NO.Fig. 7. Correlations of SO2/SO3-conversion measured in micro reactor.Fig. 8. Correlation of SO2/SO3-conversion measured in bench reactor.

  • a very low turnover frequency, caused by the inefcient adsorptio nof SO2 on the acidic surface vanadia species under reaction condi- tions. Correlations with limited amount of variations have also been performed by Svachula et al. [32], who concluded, that there is a linear dependency of SO2 oxidation on catalyst wall thickness.Since the catalysts wall and therefore the whole volume of the cat- alyst is responsible for the increase in SO2/SO3-convers ion, a corre- lation of the SO2/SO3-conversion over the weight per channel has been performed in Fig. 7. The weight per channel ratio was calcu- lated by dividing the total mass of the sample by its amount ofchannels and provides plausible results, tracing back SO2/SO3-con-version directly to the catalysts mass. For bench-reactor SO2/SO3-conversion measureme nts, a similar correlation with catalysts

    lyst, due to fewer active sites occupied by the DeNOx-reac tion because of superior mass exchange . Different values of mercury oxidation in Seniors study compared to the values presented here can be referred to lower residence time in the catalyst at the exper- iments, leading to lower mercury oxidation at the samples.

    4.4. Combined reactions

    In addition to the research on single reactions at the catalyst,which were performed in order to study the reactions in detail,experime nts observing the interaction of the reactants involved were conducted. Fig. 11 shows the SO2/SO3-convers ion during par- allel NOx-reduction measured at catalyst P8.2 in the micro reactor.

    There is a linear increase of NOx-reduction for increasing ammonia /nitrogen oxide ratio, proving that the availabili ty ofammonia is the limiting factor of the reaction up to the stoichiom- etric ratio of 1. At this point, reaction order with regard to NH3 be-comes zero. NOx-reduction is not exactly proportional to NH3/NO-

    T. Schwmmle et al. / Chemical Engineemass was performed (Fig. 8).For a comparative presentation , the SO2/SO3-convers ion was di-

    vided by the catalysts sample mass. These mass-specic values show very similar results for all catalysts investiga ted. However,the value of catalyst RC is quite high, which can possibly be traced back to a measure ment error. All these ndings indicate that SO2/SO3-conversion is directly dependent on catalysts mass. However ,referring SO2/SO3-conversion to catalysts surface, as performed atthe conversion coefcient in Fig. 6, is expected to lead to an in- crease with increasing wall thickness. As a further consequence,comparison of non-mass-s pecic values of SO2/SO3-conversionfor catalyst materials with different chemical compositi on can only be performed for identical catalyst wall thickness.

    4.3. Mercury oxidation

    As the third reaction, mercury oxidation at the catalysts with different pitch was studied. In Fig. 9, mercury oxidation at the com- mercial reference catalyst at micro reactor test conditions (withoutNH3, NO and SO2) is related to ue gas HCl-concen tration.

    Despite mercury ue gas concentratio n is tremend ously lower compared to the ue gas HCl-concen tration (cHCl/cHg = 1000), there is a dependency on the HCl-content. At HCl concentratio ns higher than 50 mg/m 3, the increase in mercury oxidation reaches alimiting value. Therefore, the concentratio n of 100 mg/m 3 was cho- sen as the upper limit of the experiments. Thus, the studied range ofHCl-concen tration covers almost the whole range of full scale power plant HCl-concen trations, which is strongly depended oncoal origin. Kolker et al. [35] presented a range of 105000 mg Cl/ kg coal, resulting in ue gas concentratio ns of about 1500mg HCl/m 3. However, most of the coals red in boilers globally have low Cl-content, resulting in ue gas HCl-concen trations lower than 100 mg/m 3. It is commonly accepted, that HCl plays an important role in mercury oxidation [13,14,36], but no evidence was given,why there is such an abundan ce of HCl needed, even if there are no other possibly disturbing ue gas components involved.Fig. 9. Effect of ue gas HCl-concentration on mercury oxidation.Fig. 10 shows the correlation of the mercury oxidation with cat- alysts wall thickness at different HCl-concentra tions.

    For all investigated HCl-concen trations, there is a slight increase of mercury oxidation with catalysts wall thickness and pitch. Asshown at the DeNOx-reactio n, a fully diffusion controlle d reaction results in decreasing or at least constant values for increasing pitch/wal l thickness at the micro reactor. The results reveal that mercury oxidation does not only take place at the surface of the catalyst, indicating slow chemical reaction kinetics. Thus, the reac- tants (mercury and/or HCl) are able to penetrate into the catalysts macropo rous system. These results are in contrast to the theoreti- cal calculations performed by Niksa and Fujiwara [13], showing in- creased mercury oxidation at lower pitch sizes respectivel y low channel openings . This was explained by mass transport phenom- ena. Senior [37] developed a model for mercury oxidation at SCR- DeNOx-c atalysts and concluded, based on data of [9], that there islower mercury oxidation at high dust catalysts with greater pitch compare d to low dust catalysts with lower pitch. However, it has to be kept in mind that in [37], only two catalysts with a difference in wall thickness of 0.4 mm were considered. In this study, more catalysts with a range of over 0.6 mm were measured. Addition ally,Seniors calculations have been performed at a NH3/NO-ratio of 1.Taking into account the results gained at the DeNOx-activity mea- suremen ts above and the assumption, that DeNOx-activity and mercury oxidation takes place at the same active sites, it is obvi- ous, that there is higher mercury oxidation at the low pitch cata-

    Fig. 10. Effect of catalysts wall thickness on mercury oxidation of SCR-DeNOx catalyst.

    ring Journal 222 (2013) 274281 279ratio, which can also be referred to the low residence time of the gas inside the catalyst. DeNOx-acti vity was also measured with ue gas mercury and HCl (100 mg/m 3). The results showed no

  • adsorbed ammonia constrain s the diffusion of SO2 through the cat-

    the main relevant reactions at the SCR-DeNOx-ca talyst, but for

    280 T. Schwmmle et al. / Chemical Engineealysts walls. Measurements and calculatio ns by Orsenigo et al. [24]showed the same behaviour, but the inhibition occurs rstlyaround NH3/NO = 1.0 at an area velocity of 10 m/h with a slight tendency to lower ratios at increased area velocities. Due to the high area velocities (AV = 27.5 m/h) in this study, the inhibition occurs at relatively low NH3/NO ratios because of the high NH3-coverage of the active sites. The interaction of DeNOx- reaction and SO2/SO3-conversion will also take place in full scale SCR-reactor s of coal red power plants. In the rst catalysts layer,correspondi ng to high NH3/NO ratios, the SO2/SO3-conversion islow and increases signicantly at the last catalyst layer where noammonia is present. Thus, the values of SO2/SO3-conversion inFig. 6 represent conditions in the last catalyst layer of the full scale high-dust SCR-DeNOx reactor.

    4.5. Pressure drop

    In Fig. 12, pressure drop over the tested catalysts measured inbench reactor and normalised to 1000 mm length is shown.

    Pressure drop increases with decreasing clear width respec- tively cell opening. This will directly inuence the operating costs of the power plant by increased power consumptio n of the induced inuence of the reactants of the mercury oxidation reaction onDeNOx-acti vity, which can be explained by the low mercury con- centration and the acidic nature of the active sites. However,SO2/SO3-conversion is inhibited by parallel DeNOx-reactio n. For the NH3/NO-ratio of 0.8, the rate of SO3 formation is almost zero.Active sites are preferent ially occupied by ammonia, hence SO2cannot be converte d to SO3. Moreover, it can be assumed, that

    Fig. 11. SO2/SO3-conversion at parallel NOx-reduction.draught. Due to combined variation of pitch and wall thickness, the void fraction, which is also relevant for pressure drop, stays almost

    paramete rs of honeycomb catalysts has been performed . Although

    Fig. 12. Pressure drop over tested catalysts measured at bench-reactor and normalised to 1000 mm length.there are commonl y accepted correlations for the comparison ofdifferent test conditions based on reaction kinetics (e.g. DeNOx- the operation of SCR units of full scale power plants, there are sev- eral other factors to be considered. Due to the fact, that in this study a combined variation of pitch and wall thickness has been performed , the effect of the two factors has to be discussed separately .

    SCR-DeNO x-catalysts are -regarding the material usage- very ineffective related to DeNOx-perfor mance. Only a very thin surface layer of around 50 lm is needed for the reaction to take place. Nev- ertheless, walls of commerc ial SCR-DeNOx -catalysts have a thick- ness of around 1 mm. As a result, the monolith wall thickness could be reduced signicantly without affecting the DeNOx-perfor -mance. Furthermor e, it would be benecial to reduce the wall thickness in order to reduce the undesired SO2/SO3 conversionand catalyst cost. The pressure drop will also be lower due to high- er void fraction. Unfortunate ly, mercury oxidation at the catalysts with lower wall thicknesses would be reduced simultaneou sly. Onthe other hand, regarding the pitch of the catalyst, it would bedesirable for increased NOx-reduction to apply small pitch cata- lysts due to shorter distances of diffusion and gassolid mass transfer limitations of this fast reaction.

    However , there are several constrain ts from practical point ofview, mainly related to applicati on limitations and the catalyst production process. The applicable channel clear width (or pitch)is mainly determined by the particulate content of the ue gas,the characterist ics of the dust, and the allowable pressure drop across the SCR reactor. The more dust, the larger the pitch size inorder to avoid or minimize dust deposition and catalyst plugging .Catalyst channels in high-dust applicati ons (where the catalyst islocated immediatel y after the boiler, processing the full dust load- ing leaving the boiler) will therefore be signicantly larger than ina tail-end installati on (the catalyst is located downstre am of a par- ticulate collection device and a ue gas desulphuriz ation FGD sys- tem). The honeycomb catalyst wall thickness for high-dust applicati ons at coal-red boilers shall be within a certain range (typically between 0.8 and 1.1 mm) to maintain mechanical resis- tance in operation and during a possible catalyst regeneration pro- cess. For tail-end or gas red applicati ons small pitch catalysts (24 mm) with a reduced wall thickness (0.250.5 mm) can be used.

    Honeyco mb catalysts are obtained by extruding a ceramic paste made by catalytic material followed by a delicate drying step.Therefore a certain ratio between inner wall thickness and channel clear width or respectively a certain range of the monolith void fraction (typically between 70% and 80%) shall be maintain ed for successfu l production. Also the (transversal) compression strength of the nal honeycomb catalyst is strongly affected by the number of cells and the inner wall thickness. Sufcient compress ion strength is required for packing of the catalyst monoliths into the steel frames.

    6. Conclusion

    A systematical study on the effect of the variation of geometri cal constant . Larger channels at P10 lead to lower pressure drop com- pared to P6.

    5. Discussion

    There is a great inuence of honeycomb catalysts geometry on

    ring Journal 222 (2013) 274281activity) available, the specic test condition s have to be consid- ered. Micro reactor studies in laboratory with small samples are asuitable tool for systemati c studies, especially regarding still not

  • fully claried reaction mechanism s and interactions of mercury

    reactants use the whole catalysts bulk and not only the surface

    reduced and the wall thickness increased to a certain extent. How-

    [7] J.P. Chen, R.T. Yang, Role of WO3 in mixed V2O5WO3/TiO2 catalysts for

    T. Schwmmle et al. / Chemical Engineering Journal 222 (2013) 274281 281ever, this would increase the material costs due to higher amount of catalyst material needed and would also lead to higher pressure drop and a higher risk of catalyst blocking by y ash. Unfortu- nately, this would also boost SO2/SO3-conversion at the catalysts,which is highly undesired by power plant operators. Therefore,increasing of mercury oxidation by variation of catalysts pitch orwall thickness is not possible without drawbacks. However, further investigatio ns on the interaction of the reactants with the catalysts material will be performed in order to gain a greater insight into the reaction mechanis m and its dependencies. Still, there are methods available for selectively increasing mercury oxidation byvariation of (chemical) composition of the catalyst, which are cur- rently under research in the ongoing project.

    Acknowled gements

    The authors greatly thank IBIDEN Porzellanfabrik Frauenthal GmbH for providing the catalyst samples and E.ON New Build and Technolo gy GmbH for performing the bench-react or tests.The work was funded by the European Commission within the DEVCAT project under the Research Fund for Coal and Steel ofthe European Commission (RFCR-CT-2010-00012).

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    Influence of geometrical parameters of honeycomb commercial SCR-DeNOx-catalysts on DeNOx-activity, mercury oxidation and SO2/SO3-conversion1 Introduction2 Experimental method2.1 Catalysts2.2 Test setup2.2.1 Micro reactor2.2.2 Bench reactor

    3 Determined parameters3.1 DeNOx-activity3.2 SO2/SO3-conversion3.3 Mercury oxidation

    4 Results4.1 DeNOx-activity4.2 SO2/SO3-conversion4.3 Mercury oxidation4.4 Combined reactions4.5 Pressure drop

    5 Discussion6 ConclusionAcknowledgementsReferences