Applied Catalysis B: Environmental 31 (2001) 195–207

Destruction efficiency of catalytic filters forpolychlorinated dibenzo-p-dioxin and dibenzofurans in

laboratory test and field operation — insightinto destruction and adsorption behavior of

semivolatile compounds

Roland Weber a,∗, Marc Plinke b, Zhengtian Xu b, Michael Wilken c

a Environmental Process Development Department, Ishikawajima-harima Heavy Industries Co. Ltd.,1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan

b W.L. Gore & Associates, Inc., Elkton, MD 21922, USAc Michael Wilken Umwelt Consulting, Berlin, Germany

Received 1 September 2000; received in revised form 20 November 2000; accepted 20 November 2000

Abstract

Catalytic destruction of chlorinated dibenzodioxins and dibenzofurans for environmental protection is one of the key subjectsof applied catalysis in combustion facilities. For catalytic filters, the removal and destruction efficiencies (RE and DE) forpolychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) were tested in the laboratory and compared with datafrom field operation. The comparison shows very similar values of laboratory measurements and actual field measurementsfor fresh samples, used samples without catalyst deactivation, and used samples with varying degrees of deactivation.

The non-poisoned catalytic filter showed destruction and removal efficiencies for PCDD/PCDF and the “toxic equivalents”(TEQ) of more than 99%. The laboratory comparison confirmed this activity did not decrease after 2 years of operation in amunicipal waste incinerator.

The laboratory test provides a deep insight into the dependence of both the removal and destruction of semivolatilecompounds in relation to their physical and chemical properties because it excludes any interfering adsorption effect of dustparticles from the flue gas stream. The catalytic decomposition of PCDD/PCDF strongly depends on their volatility andoxidative behavior, both related to the degree of chlorination.

In addition, four different non-chlorine containing polyaromatic hydrocarbons (PAHs) varying in their number of rings andboiling points were tested. For these, the destruction was dependent only on the volatility of the respective molecule and wasin the similar order of magnitude as the PCDD/F. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Catalytic filter; V2O5/WO3-TiO2; PCDD/PCDF; PAH; POP; Semivolatile compound; Destruction; Volatility; Adsorption

∗ Corresponding author. Tel.: +81-45-7592164;fax: +81-45-7592149.E-mail address: roland weber@ihi.co.jp (R. Weber).

1. Introduction

Due to their toxicity, polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzo-p-furans(PCDF) have caused much environmental concern

0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0 9 2 6 -3 3 73 (00 )00278 -2

196 R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207

during the last two decades (for a review, see Van denBerg et al. [1]).

PCDD/PCDFs are emitted in trace amounts alongwith other chlorinated aromatics and polyaromaticcompounds (e.g. from municipal [2], hazardous andmedical [3] waste incinerators, crematories [4], sinter-ing plants [5,6] or from the metalurgic industry [7]).Concentrations without control technologies rangefrom 0.1 ng “toxic equivalents” 1 (TEQ)/Nm3 up toseveral 100 ng TEQ/Nm3. To minimize the emissionsinto the air, stringent limiting values for PCDD/PCDFof 0.1 ng (TEQ)/Nm3 for some emission sources (e.g.municipal and hazardous waste incinerators) havebeen in effect in several European countries, andalso for new municipal waste incinerators (MWIs) inJapan [8–10].

Catalytic oxidation with direct destruction ofPCDD/PCDFs in the flue gas is a proven technologyto reduce PCDD/PCDF stack gas concentrations tobelow 0.1 ng TEQ/Nm3 [11,12]. It has been shownby Hagenmaier and co-workers that the TiO2-basedV2O5/WO3 catalysts originally designed for theremoval of nitrogen oxides (NOx) by selective cat-alytic reduction (SCR) [13–17] are very effective inthe decomposition of PCDD/PCDF at the same tem-peratures used for the DeNOx reaction [11,12,18].The TiO2-based V2O5/WO3 catalyst is a true sinkfor the PCDD/PCDF due to their oxidation which re-sults in the production of carbon dioxide, water, andHCl [18]. The complete destruction of the moleculebackbone was also reported for the oxidation of1,2-dichlorobenzene by Krishnamoorthy and Amiridisover V2O5/TiO2 or V2O5/Al2O3 catalyst where car-bon dioxide and carbon monoxide were the onlycarbon-containing products detected after the catalystin the exhaust [19].

It has been a common practice to use honey-comb catalysts placed in multi-layers [12,20] for thesimultaneous destruction of NOx and PCDD/F. Dueto their additional space requirements, these systemsare normally not used solely for dioxin destruction.During the last 5 years, it has been demonstrated thata catalytic filter system can also destroy PCDD/PCDF

1 The TEQs are calculated using the international toxic equivalentfactors according to NATO/CCMS.

below the regulatory limit of 0.1 ng/Nm3 TEQ [21].During 3 years of performance in MWI plants, thiscatalytic filter’s long-term activity did not decrease.

The primary goal of this study was to establishand validate a laboratory testing method with freshand used, non-poisoned and poisoned catalytic filterssupplied by W.L. Gore & Associates, Inc. These testswere conducted using PCDD/PCDF in a flow stream.Results from the laboratory were then compared tothe removal and destruction efficiencies from fieldoperation. This work is important because a reliablelaboratory test will alleviate part of the expense ofPCDD/PCDF measurements in flue gases of combus-tion plants.

A secondary goal was to further the understand-ing of how the chemical and physical properties ofthe semivolatile compounds affected their adsorp-tion and destruction. Only a few catalytic laboratorystudies using semivolatile compounds (includingPCDD/PCDFs) at temperatures below 250◦C (thetemperature region commonly used for removal ofPCDD/PCDFs in plant operation) have been reported[22–24]. All published studies have some experimen-tal shortcomings. In our own previous study [22] andin the study of Kajikawa et al. [23], the importantevaluation of adsorption and desorption effects is im-possible because the PCDD/PCDF were adsorbed onthe catalyst prior to the experiment. Anderson et al.[24] used a flow system and applied a real flue gasfrom a pilot incinerator, containing 540 mg dust/Nm3.While Anderson’s test method yields useful infor-mation with respect to plant operation, it does notallow an unbiased catalyst evaluation. It is knownthat incinerator dust adsorbs dioxins to a varying de-gree depending on the operation condition and thetemperature of dust collection. Using Anderson’smethod, dust particle adsorption and catalysis cannotbe examined independently. Testing of catalytic fil-ters in a flow-through situation can overcome theseshortcomings. Such a test offers the possibility toinvestigate the effect of the catalyst destruction andadsorption behavior during the removal of PCDD/PCDF.

This paper also introduces a first description of anew approach to expose catalysts on fibers to feedstreams. This system has the advantage of high masstransfer efficiency between the gas stream and thecatalyst with low pressure drop.

R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207 197

Fig. 1. SEM picture of the expanded PTFE filter including the catalyst particles.

2. Materials and methods

2.1. Catalytic filter

The catalytic filter system is manufactured by W.L.Gore & Associates, Inc. (Gore) and sold under thetrade name REMEDIA D/F Catalytic Filter System[25]. During the manufacturing process, the catalyst isincorporated into a dispersion of PTFE. After drying,the dispersion is extruded into a thin tape. The tapeis stretched and chopped into short staple fibers. Thestaple fibers are needle-punched into a RASTEX® 2

PTFE scrim to form a coherent felt. In the last step ofthis process, a micro-porous membrane is laminatedto the felt forming the final product. This new systemconsists, therefore, of a GORE-TEX®2 membranelaminated to a catalytically-active filter.

During operation, the upstream filter membranecollects flue gas particles, whereas the catalytic fil-

2 RASTEX and GORE-TEX are registered trademarks of W.L.Gore & Associates, Inc.

ter reacts with gaseous flue gas components. Thecatalytic filter is composed of fibers containingfine catalyst particles. The catalyst particles areattached within the polymeric node and fibril struc-ture in the expanded polytetrafluorethylene (PTFE)(Fig. 1), resulting in a three-dimensional highlyporous structure. Unlike other filter materials consist-ing of solid fibers, the gas flows through the porousPTFE fibers, resulting in highly efficient gas–catalystcontact.

Another advantage of this system is that the up-stream filter membrane collects particles with a veryhigh efficiency (>99.95%), therefore, protecting thecatalyst from potential flue gas poisons. This is espe-cially important during the critical startup and shut-down procedures in which plants pass through dewpoints. During these procedures, water/acid dropletsare formed which contain heavy metals and othercontaminants. These droplets cannot penetrate thehydrophobic PTFE structure and, therefore, cannotpoison the catalyst.

The catalyst used is a V2O5/WO3-TiO2 catalyst,specially developed for low temperature destruction

198 R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207

of PCDD/PCDF. According to the manufacturers,the BET surface of the catalyst was between 70 and100 m2/g. The vanadia content was less than 8% andtungsten below 8%. The filter velocity in all fieldapplications and in the laboratory test was approxi-mately 1 m/min, which is typical for industrial clothfilters. When recalculating the flow to the volume ofthe catalytic active fiber, this corresponds to a spacevelocity (SV) of 25,000/h.

2.2. Chemicals

2.2.1. PCDD/PCDFA complete mixture of PCDD and PCDF was used

in the experiments (Fig. 2): monochlorodibenzodi-oxin/furan (MCDD/MCDF), dichlorodibenzodioxin/furan (DCDD/DCDF), trichlorodibenzodioxin/furan(T3CDD/T3CDF), tetrachlorodibenzodioxin/furan(T4CDD/T4CDF), pentachlorodibenzodioxin/furan(P5CDD/P5CDF), hexachlorodibenzodioxin/furan(H6CDD/H6CDF), heptachlorodibenzodioxin/furan(H7CDD/H7CDF), and octachlorodibenzodioxin/furan(OCDD/OCDF).

The mixture contained all 17 toxic 2,3,7,8-substitutedcongeners, allowing a detailed analysis of destructionefficiency for the TEQ values.

2.2.2. Polyaromatic hydrocarbons (PAHs)Naphthalene, pyrene, phenanthrene, benzo[a]pyrene

were chosen as representative PAHs (all purchasedfrom GL Science Inc., Japan, or Aldrich, Germany)(Fig. 2).

Fig. 2. Molecular structure of PCDD, PCDF, and of the tested PAHs.

2.3. Laboratory experiments

A temperature of 200◦C was chosen for the com-parison of laboratory results with field performance.At this temperature, most plant operation data havebeen collected.

In the laboratory, the catalytic filters were testedin a stainless steel reactor (Fig. 3). First, a filter wasloaded into the stainless steel reactor. Then the reactorwas surrounded by a heating mantel which maintainedthe reactor temperature within ±2◦C. Heating tapewas used to maintain all ancillary tubing at the sametemperature as the reactor.

Before closing the reactor, the PCDD/PCDF mix-ture (or the PAHs) were placed in the evaporating zone.After the reactor reached temperature, the gas flowwas started. The volumetric flow through the catalyticfilter was 300 cm3/min (20% O2, 80% N2) and corre-sponded to a face velocity of about 1 m/min. The flowrate was regulated using mass flow controllers (Shi-nagawa Seiki, Japan). When equilibrium was reached,the experiment was started by heating the evaporationzone for 10 min and continuing the flow through thefilter for 30 min.

2.4. Sampling, clean-up and analysis

2.4.1. Work-up of laboratory samplesAfter each experiment, the glass tubes downstream

of the catalyst were rinsed with toluene. These rinseswere combined with the toluene in the washing bottle(impinger). Separately, the catalysts were extracted by

R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207 199

Fig. 3. Schematic of the reactor. (1) O2/N2 source, (2) flow controller, (3) preheating system, (4) evaporation port for compounds, (5)furnace, (6) stainless steel reactor, (7) catalytic filter, (8) washing bottles for sampling (empty/toluene), (9) active carbon trap, (10) gas meter.

12 h Soxhlet extraction with toluene. The toluene inthe washing bottle and the extracts of the catalystswere analyzed separately. The clean-up procedures aredescribed elsewhere [26].

2.4.2. Measurements of plantsThe field samples that were used for compari-

son with the laboratory studies were taken fromfull-scale incineration plants and also pilot testsin non-incineration plants (Table 1). A, B, and Cwere full-scale incinerators completely equippedwith the catalytic filters. The removal efficiency ofPCDD/PCDF at these incinerators was high enoughto ensure PCDD/PCDF outlet emissions of less than0.1 ng TEQ/Nm3. In the non-incineration plants Dand E (Table 1), only pilot units were equipped withcatalytic filters because of uncertainties in extendedcatalyst performance (see Section 3.3).

For all plants, the PCDD/PCDF content of theparticle filters and the condensate/XAD fractionwere analyzed separately to best evaluate the actualgas-phase concentration before and after the filtermaterial. All raw gas and stack measurements wereconducted by MPU (Mess- und Prüfstelle TechnischerUmweltschutz GmbH) according to the Euro NormEN 1948, with sampling times of at least 3 h.

(PCDD, PCDF)inlet − [(PCDD, PCDF)outlet + (PCDD, PCDF)on catalyst]

(PCDD, PCDF)inlet.

2.4.3. GC/MS analysis and quantificationAnalysis of the laboratory experiments was carried

out on a HP 6890 gas chromatograph coupled to aHP 5973 mass selective detector. Field measurementswere analyzed on an Autospec Ultima. PCDD/PCDFquantification was carried out by isotope dilutionmass spectrometry with 13C-labeled standards. TheGC columns used were a CP-SIL 88 column (50 m,0.25 mm i.d., 0.2 �m film thickness, CHROMPACK,Frankfurt/FRG) and a DB-5 fused-silica column(30 m, 0.32 mm i.d., 0.25 �m film thickness, J&WScientific, Folsom/USA).

2.4.4. Calculation of removal efficiency anddestruction efficiency

Removal efficiency (RE) describes the ability of thefilter to remove dioxins from the gas stream. It is cal-culated as

(PCDD, PCDF)inlet − (PCDD, PCDF)outlet

(PCDD, PCDF)inlet.

Destruction efficiency (DE) describes the ability of thefilter to destroy dioxins during the experimental time.It is calculated as

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The PCDD/PCDF recovery in the impinger inpercent is calculated as

Recoveryimpinger = 100 − RE.

The recovery on the catalyst in percent is calculated as

Recoverycatalyst = (PCDD, PCDF)on catalyst

(PCDD, PCDF)inlet.

3. Results and discussion

3.1. Blank test

One difficulty with experiments using semivolatilecompounds are adsorption effects. These phenomenacannot be estimated without blank tests. Therefore,blank tests were performed with the filter-less stain-less steel reactor. At 200◦C, the stainless steel reactoritself did not show any destruction effect. A secondblank test was conducted using a PTFE filter withoutcatalyst. These filters contained the same PTFE andwere manufactured in the same fashion as the catalystfilters, with the exception that they did not contain anycatalyst. At 200◦C, the PTFE filter itself showed noadsorption of any PCDD/PCDF, and all of the spikedcompounds were recovered in the impinger. There-fore, all measured destruction and adsorption effects

Fig. 4. Reproducibility of removal efficiency of T4CDD/F–OCDD/F on fresh catalyst filters in laboratory tests (runs 1–4), and comparisonto field operation in two municipal waste incinerators (plants A, B).

in the laboratory experiments were caused by the ef-fect of the TiO2-based V2O5/WO3 catalyst alone, andnot influenced by the PTFE or the reactor itself.

3.2. Removal efficiency of fresh catalytic filters

In the first measurement series in the laboratory,fresh catalytic filter samples were tested for theirPCDD/PCDF removal efficiency at 200◦C. Four mea-surements were made and in each one the catalystshowed high removal of PCDD and PCDF (Fig. 4,runs 1–4). The total removal efficiency for tetra- tooctachlorinated dibenzo-p-dioxins and dibenzofuranswas above 99%. The average RE for the total TEQwas more than 99.6%. The reproducibility in thelaboratory experiments was very good, proving thereliability of the experimental procedure (Fig. 4, runs1–4).

The PCDDs were removed with a slightly higherefficiency than the PCDF. The same trends can be ob-served in actual municipal waste incinerators (Fig. 4,plants A and B). The REs in the laboratory and thefield are in good agreement (Fig. 4). The slightlyincreased variability of the field measurements ver-sus the laboratory measurements may be caused byinterfering adsorption effects of flue gas dust on thefilter bag surface. The influence of this dust dependson its carbon content (and other factors), even though

202 R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207

Fig. 5. Recovery of PCDD, PCDF, and TEQ adsorbed on the catalytic filter in the laboratory study at 200◦C.

only the gas-phase dioxins were analyzed. The car-bon content of the dust changes, because the fieldmeasurements were done in different facilities wherecombustion conditions varied significantly. These ad-sorption effects are excluded in the laboratory study.The good agreement of laboratory and field results,however, proves that the measurement method appliedin the field is highly reliable when used to measurethe actual performance of the catalytic filter.

The good agreement of laboratory and fieldmeasurements may indicate that the catalyst filter isnot at its kinetic limit. While raw gas values in MWIof PCDD/PCDF are around 3–30 ng TEQ/Nm3 (cor-responding to about 200–4000 ng/Nm3 total PCDD/PCDF), the raw gas values in the laboratory were ashigh as 200,000 ng/Nm3. Therefore, a 10–100-foldincrease in inlet concentration still resulted in a simi-lar destruction efficiency. Therefore, the catalytic filtermaterial should reduce excessively high PCDD/PCDFinlet concentrations with a high destruction efficiency.

3.2.1. Adsorption of PCDD/PCDF on the freshcatalytic filter

To prove catalytic PCDD/F destruction, thePCDD/F adsorbed on the catalyst was measured foreach experiment. For 40 min experiments at 200◦C,only 0.1% of the total TEQ was detected on the cat-alytic filter (Fig. 5). When closing the mass balancefor these 40 min experiments while including the 0.4%

TEQ (average of laboratory test runs 1–4, Fig. 4)found in the impinger, it was concluded that 99.5% ofthe TEQ were destroyed by the catalyst (Figs. 4 and5). For 2 h experiments, no PCDD/Fs were detectedon the catalyst, while the outlet emissions remainedthe same.

This is in agreement with the detection of onlynegligible amounts of PCDD/PCDF on the catalyst fil-ter after 2 years of operation in the field (0.1 ng TEQ/gfilter material). 3

When comparing the homolog-specific catalyticdestruction, it was found that no lower chlorinatedhomologs (T4CDD/F and P5CDD) were detected onthe catalyst (Fig. 5). Hexachlorinated dioxins weredestroyed more than 99.99%, while hexachlorinatedfurans only by 99.65%. For the OCDD, 99.6% wasoxidized, compared to 98% of the OCDF (Fig. 5).This data shows that the destruction kinetics decreasewith increasing chlorination degree. The highly-chlorinated congeners are not completely oxidizedand stay adsorbed on the catalyst for 30 min. Thisshows that destruction rates and desorption rates ofPCDD/PCDF at 200◦C on TiO2-based V2O5/WO3catalysts are in the minute range.

3 In our opinion, most of this small amount of PCDD/PCDFis adsorbed on the fly ash particles attached to the filter (notcompletely removed even after severe brushing).

R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207 203

Fig. 6. Removal efficiency of the catalytic filter at 200◦C in dependence of chlorination degree in laboratory tests (MCDF–OCDF) andfield measurements (T4CDF–OCDF).

This data further demonstrates that destruction ratesfor PCDD on this catalyst are higher compared toPCDF. The same destruction behavior was observed inthe catalytic study with a honeycomb-type TiO2-basedV2O5/WO3 catalyst [22]. This finding explains thehigher RE (and DE) for PCDD in the laboratory tests(and plant operation) compared to PCDF (Fig. 4).

3.2.2. Dependence of removal efficiency onchlorination degree — competition of volatility andoxidation rate

The dependence of the removal efficiency on chlori-nation degree of PCDD/PCDF is similar between fieldand laboratory tests. Starting with OCDF, the RE is de-creasing with decreasing chlorination degree (Fig. 6).However, it reaches a minimum for P5CDF/T4CDF,and then the RE increases with decreasing chlori-nation degree. Therefore, two effects are competingwith each other depending on the chlorination degree.

As seen in Section 3.2.1 and [22], the oxidationrate decreases with increasing chlorination degree.

Table 2Boiling points of some polychlorinated dibenzo-p-dioxin, polychlorinated dibenzofurans [27,28]

PCDD DD 23-D2CDD 124-T3CDD 2378-T4CDD 123678-H6CDD H7CDD O8CDDBoiling point (◦C) 279 358 375 447 487 507 510

PCDF DF 23-D2CDF 238-T3CDF 2378-T4CDF 123678-H6CDF H7CDF O8CDFBoiling point (◦C) 287 375 408 438 488 507 537

This can be derived, in this study, from the adsorbedcompounds on the catalyst filter (Fig. 5). In the for-mer study [22], the isolated effect of chlorinationdegree on destruction rates was investigated by apply-ing stationary conditions. Under these circumstances,interfering adsorption and desorption effects wereeliminated. According to this effect, lower chlori-nated PCDD/PCDF are better destroyed than higherchlorinated PCDD/PCDF during the same residencetime on the catalyst surface.

The opposing effect is due to the changing volatil-ity of the PCDD/PCDF with chlorination degree. Thevolatility of PCDD/PCDF decreases with increasingchlorination degree as can be derived from the boilingpoints of the PCDD/PCDF (Table 2) [27,28]. Higherchlorinated PCDD/PCDFs are retained on the catalystsurface longer, therefore, increasing the chance of oxi-dation. Overlaying both effects leads to a maximum re-moval efficiency for octa/hepta- and monochlorinatedcongeners, and a minimum for the tetra- to hexachlo-rinated furans under the applied conditions (Fig. 6).

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Table 3Removal efficiency and boiling points of the selected PAHs (200◦C, SV 25,000/h)

Naphthalene Phenanthrene Pyrene Benzo[a]pyrene

Boiling point (◦C) 217.9 340 404 495Removal efficiency (%) 75 98 99.82 99.94

In former studies using honeycomb catalysts, thesecompeting effects were discussed for the chloro-benzenes [22,29]. However, with chlorobenzenes theeffect of lowering the volatility with increasing chlo-rine substitution overcompensated the effect of anincreasing “redox potential”. This resulted in a dec-reasing removal efficiency with decreasing chlorinedegree only [22,29].

3.2.3. Destruction of PAH — the effect of volatilityTo demonstrate the removal efficiency of semivola-

tile compounds without the effect of chlorine substitu-tions, a destruction study with non-chlorinated PAHswas performed. These semivolatile compounds arealso of ecotoxicological relevance. They are generallyamong the most abundant organic micro-pollutantsin incineration processes, and present in considerablyhigher concentrations as compared to PCDD/PCDF[30–32]. Due to their high concentrations, the PAHscause an even higher carcinogenic risk in flue gasesfrom thermal processes compared to PCDD/PCDF[33,34]. For this study, four representative PAHs com-monly found in incineration processes were chosen,naphthalene, phenanthrene and pyrene (some of themost abundant PAHs [30–32]) and benzo[a]pyrene,because of its elevated carcinogenic potential andhigh toxic equivalence factor [35]. A second criteriafor choosing these four compounds was to comparePAHs that have differences in volatility (Table 3) andcontain two-, three-, four-, and five-rings (Fig. 2). ThePAHs are removed effectively by the catalytic filter.The highest removal was detected for the low volatilebenzo[a]pyrene (boiling point 495◦C) with a RE above99.9%. The RE decreased with increasing volatility.For both pyrene (99.8%) and phenanthrene (98%),the RE was very high. Only for the highly-volatilenaphthalene (boiling point 217◦C) was the removalefficiency as low as 75%. Therefore, for PAHs, thevolatility is the main determining factor for RE, sincethe “redox” potential of the non-chlorinated PAHs

are about the same. Similar to the low chlorinatedPCDD/PCDF, no PAHs were detected on the catalyst.

3.2.4. Removal efficiency of used catalytic filtersamples from the field

In a second experimental series with field samples,the level of agreement of the laboratory tests withfield results was investigated for filters with differ-ent catalytic activities. Three catalytic filter samples(CC, DD, EE) were chosen after exposure to differentflue gases in a variety of plants (C, D, E). SampleCC was from a municipal waste incinerator after2 years of operation. The two other samples (DDand EE) were deactivated during field operation innon-incineration plants during 1 week tests using pi-lot filters. The flue gas temperatures of plants D andE varied widely (between 120 and 250◦C (Table 1))within 1 day of operation. Nevertheless, these plantsoperated for an extended period of time below 150◦C.In addition, the total organic carbon (TOC) in the gasphase was extremely high with peak values of severalhundred ppm.

The CC sample from the MWI shows a removalefficiency of more than 99% in the field and the lab-oratory measurements (Fig. 7). The activity remainedunchanged compared to the fresh catalytic filter(Fig. 4). The 2-year-exposure to waste incinerationflue gas did not decrease the activity of the catalyst.

The partially deactivated sample DD showed lowRE during field operation using a pilot plant (Fig. 7).The REs in the laboratory test and the field mea-surement showed very good agreement (Fig. 7). ThePCDFs were removed in the field to 69.5% and in thelaboratory to 74.6%. The RE measurements for PCDDwere 95.2% in the field and 96.6% in the laboratory.

Catalyst sample EE showed no gas-phase removalof PCDD/PCDF in the field after poisoning (Fig. 7).In the laboratory, a removal efficiency of 51% forPCDD and 26.7% for PCDF was observed (Fig. 7).However, the analysis of the PCDD/PCDF on the filter

R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207 205

Fig. 7. Comparison of removal efficiency in the laboratory and the field for used catalytic filters from plants C, D, and E. The filters inthe non-incinerator plants D and E were tested using pilot filters.

proved that most of the removed PCDD/PCDFs in thelaboratory were adsorbed on the catalyst and not de-stroyed (Figs. 7 and 8a and b). The catalytic activityof the filter was completely reduced. Only the easiestcompounds to oxidize — the low-chlorinated PCDDs— were removed up to 25%. Within the measurementerror, no PCDFs were destroyed. Therefore, catalystfilter EE showed only a negligible destruction effi-ciency in the laboratory as also seen for field operation.

This finding demonstrates once more the impor-tance of analyzing adsorbed compounds on a cata-lyst at operation temperatures below 250◦C [22,29].Before a final conclusion regarding destructionefficiency of persistent semivolatile compounds withboiling points above 300◦C can be made, the catalystrequires to be analyzed.

Fig. 8. Recovery of PCDD (a) and PCDF (b) on catalyst and in the impinger of poisoned sample EE.

3.3. Discussion of poisoning of catalytic filters DDand EE

A detailed analysis of the poisoned catalyst filtersDD and EE was carried out. Problems arise due tothe difficulty of separating fly ash particles from theused filter. Even after brushing, these fly ash parti-cles remain partly on the membrane, and influencesurface and chemical composition measurementsof the catalyst, which is incorporated in the filtermaterial.

A first indication that incomplete combustion prod-ucts reduced the activity of the catalytic filters wasobserved in testing of filters DD and EE using temper-ature program desorption (TPD). These tests showedthat both catalyst filters adsorbed TOC. This indicates

206 R. Weber et al. / Applied Catalysis B: Environmental 31 (2001) 195–207

a deactivation of the catalyst through products ofincomplete combustion.

It has been proposed that exposure of an oxidationcatalyst over a long period of time to both partially-oxidized and incomplete-combustion products maydeactivate the catalyst [36,37]. In another laboratorystudy, it was shown that coking could be a reason forthe deactivation of transition metal oxide catalystsduring destruction of chlorinated VOC [38].

This deactivating effect does not seem to be presentin municipal waste incinerators or medical wasteincinerators. Both of these have smaller variations intemperature. In these plants, no decrease in activityof the catalytic filter was observed.

4. Conclusion

A method was developed to predict field perfor-mance of catalytic filters in laboratory studies usingsemivolatile compounds. For the destruction rate ofPCDD/PCDF for catalytic filters, there is close agree-ment in the established laboratory test and actual fieldperformance for both poisoned and non-poisonedsamples. The reproducibility of the laboratory testswas good. Having such a laboratory test allows futuretesting of different catalysts and more complete stud-ies of destruction mechanisms. In addition, filter per-formance monitoring is possible, reducing the needfor costly field dioxin measurements. Furthermore,this testing method should allow simulation and pre-diction of the performance of the catalytic filter inthe field (e.g. testing of the effects of temperature orspace velocity on the catalyst performance).

The laboratory test showed that the catalyst inthe filters destroys PCDD/PCDF by more than99%. This could be shown without the interferingadsorption effect of dust. The analysis of the cat-alyst itself proved that at 200◦C, the dioxins aredestroyed.

Laboratory tests with PAHs showed that the filteralso effectively removes other semivolatile com-pounds. For example, dibenzo[a]pyrene was destroyedby more than 99.9%. Therefore, the catalytic filtershould be useful not only for removal of PCDD/PCDFin municipal waste incinerators and other applications,but also in processes where semivolatile compoundssuch as PAHs must be removed.

The investigation of semivolatile compounds in flowopens a new option for insight into catalytic destruc-tion processes. The key is that both the retention timeof these compounds on the catalyst and the destructionrates are in the minute range. This allows enough timeto investigate adsorption/desorption and destructioneffects that are dependent on the physical and chemicalproperties of semivolatile molecules. Similar investi-gations are not possible for more volatile compounds.For these compounds, a destruction temperature ofmore than 300◦C is required where retention timeson the catalyst surface are in the “non-measurable”sub-second range.

This study has shown that the catalytic decompo-sition and the removal of semivolatile compounds,strongly depends on volatility and oxidative behav-ior of the compounds. Both factors are related to thedegree of chlorination.

Combining laboratory studies and field operationcreates the potential of fundamentally understand-ing the important details of catalytic destruction ofsemivolatile compounds. This is important for theadvancement of applied catalysis in this field.

Acknowledgements

The authors want to express their thanks to MPU(Mess- und Prüfstelle Technischer UmweltschutzGmbH) for sampling and analysis of the plants, andTakaaki Ohno for his great care in the analysis of thelaboratory experiments. Additional thanks to DianeOrndorff, Dinah Jones, John DiMeo, and Keith Frit-sky for critical reading of the manuscript.

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