Testing Total Oxidation Catalysts for Gas Cleanup ... - ACS Publications

Ind. Eng. Chem. Res. 2002, 41, 1171-1181

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Testing Total Oxidation Catalysts for Gas Cleanup in Waste Incineration at Pilot Scale Jose´ Corella* and Jose´ M. Toledo Department of Chemical Engineering, University Complutense of Madrid, 28040 Madrid, Spain

Seven different total oxidation catalysts (PRO*CLEAN*500; EF 258 H/D; Siemens A; ZERONOX; and Ru, Pd, and V2O5 on a TiO2 support) have been tested for abatement of principal organic hazardous compounds (POHCs) in the flue gas from a waste incinerator of the fluidized-bed type. These catalysts were placed in a slip flow downstream from the waste incinerator. The catalytic reactors used were both metallic and glass, made for monoliths and for particulates. The temperatures used in the catalytic reactors ranged from 240 to 510 °C, and the volumetric gas hourly space velocities (GHSVs) ranged from 1200 to 5700 [(mgas,normal conditions3/h)/mcatalyst3]. The catalysts operated under a realistic flue gas, which was sampled before and after the catalytic reactor at different times on stream. Condensates after the catalytic reactor were also analyzed for organics and for compounds sometimes lost from the catalyst. Gas sampling and analysis were carried out using standarized methods. Conversions (destructions) of most of the POHCs were 99.99%, but small GHSV values were required. Experimental conditions for an even more refined catalyst comparison are also provided. Introduction Waste-to-energy processes (such as incineration and combustion) are well-known methods for waste disposal but are hindered by the formation and emission of products of incomplete combustion (PICs) or principal organic hazardous compounds (POHCs), which include the famous polychlorinated dibenzo dioxins and furans (PCDD/Fs) and polycyclic aromatic hydrocarbons (PAHs). Many good studies have been performed on the types and amounts of these PICs in coal combustion1-5 and in waste incineration plants.6-20 Much is currently known about the influence of operating variables on emissions from combustors and/or incinerators. This field is so broad that this paper is devoted only to the incineration of residues and wastes containing chlorine, such as municipal solid waste (MSW) or refuse-derived fuel (RDF) and some sludges. When chlorine is present in the feedstock to be incinerated, PCDD/Fs can be present in the exit or stack gas. Several methods for their elimination have been developed of which the most widely used are absorption with some liquids or slurries and adsorption by activated cokes. Nevertheless, these methods do not destroy the PCDD/Fs but simply transfer them from the flue gas to another phase or flow, not solving at all the problem of the disposal or destruction of the formed PCDD/Fs.21,22 For this reason, some people, including these authors, believe that the best method for PCDD/F elimination, in an incineration flue or stack gas, is their catalytic total oxidation or destruction. This idea is again well-known, and it is applied in some countries in which MSW incineration plants have DeNOx units with one layer, or even another unit, of catalyst for the so-called DeDioxins. These DeNOx and DeDioxins units normally operate in MSW combustion plants between 300 and 330 °C (when the flue gas is reheated after the filter or the absorption unit) and in coal power plants * Author to whom correspondence should be addressed. Fax: +34-91-394 41 64. E-mail: [email protected].

between 320 and 400 °C (normally 340 °C). Because the flue gas has particulates in suspension, these catalysts are usually monoliths with typical values of 64 cells/ in.2 Catalysts for DeDioxins are similar to, if not the same as, those used for DeNOx units, whose typical formula is V2O5-WO3/ TiO2 (often called V2O5-based or simply V-W-Ti catalysts). Nevertheless, many significant doubts about using these V-W-Ti catalysts for dioxin abatement exist. For instance, first, if V2O5-based catalysts are active for the reaction NOx + NH3 (+ O2) f ..., why are they also active for the simultaneous reaction PCDD/Fs + O2 f ... if it is intrinsically and chemically quite different from the first one? Second, V2O5-based monoliths are not very much used (for dioxins abatement) in MSW incineration plants because they require large units (which imply high costs and large surfaces in the whole plant) as a result of the low volumetric gas hourly space velocities (of about 2000 h-1) or high residence times required for the gas. This would mean that the existing catalysts would not be very active for the DeDioxins application. Third, in the market, many other types of catalysts are also available, such as those based on noble metals (Pt, Pd, etc.) or on chromia, which can also be manufacturated as monoliths. Why are such catalysts not being used (excepting in Korea23) for the DeDioxins application? These important and even basic questions led these authors 10 years ago to start an exhaustive research program aimed at obtaining scientifically and technically based answers and finding a good catalyst, if possible, for the mentioned application. If a good (including inexpensive) catalyst could be identified for DeDioxins, one of the main problems or constraints in the use and installation of new waste incineration plants would be eliminated. Despite the high social and economic impact of the research on the catalytic abatement of dioxins, only a few institutions worldwide are carrying out some research (or, at least, publishing it) in this field. Apart from the pioneering and well-known work of Hagen-

10.1021/ie010297p CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002

maier in Germany,21,28 these authors know only of the work done in Japan by Kobelco24-26 and IshikawajimaHarima Heavy Industries Co.,27,28 in Sweden by Umeå University,29 and in Germany by the FZK research center30,31 and BASF AG32. These five institutions have recently provided some information on the use of V2O5based catalysts for the catalytic abatement of dioxins. The incinerators used by Kobelco and Umeå University are fluidized beds, whereas a grate-based incinerator is used by FZK. As the feedstock, Kobelco and FZK use MSW, and Umeå University uses an artificial or simulated MSW. At the University Complutense of Madrid (UCM), research on the catalytic abatement of dioxins started 10 years ago.22,33,34 A small bubbling fluidized-bed incinerator was used in the first years of the research. Several commercial catalysts for the total oxidation of volatile organic compounds (VOCs) were tested then in a full-flow catalytic reactor downstream from the incinerator.34 Because of the difficulties associated with testing catalysts in a waste incineration plant and with the large number of catalysts to be tested, during 19941999, tests were carried out at UCM only with some targeted chlorinated VOCs (Cl-VOCs), using a synthetic flue gas. Such catalyst screening tests were performed with chromia-based,35 noble-metal- (Pt-, Pd-, Ru-) based,36,37 and V2O5-based38 catalysts. Much useful information concerning the best operating conditions and catalyst lifetime, for instance, was then obtained. The best catalysts (in terms of activity, selectivity, and lifetime for Cl-VOC abatement) were then selected from those laboratory tests (using synthetic gas mixtures). The next and last step would be their testing at the pilot scale with a realistic flue gas composition. This is precisely the objective of this paper: A few selected catalysts are tested at a small pilot plant downstream from a waste incinerator of the fluidized-bed type. The intention is to obtain data under realistic conditions on the temperatures and GHSV values that have to be used with these catalysts and their respective usefulness. Catalyst manufacturers claim that these catalysts are very useful for dioxin abatement, but very little concrete data have been published in the open literature. The catalysts were tested and compared at full size, not ground. This is not a comparison of the intrinsic chemical activities of the catalysts, because some internal diffusion control might exist. Catalysts are compared on the basis of the temperature and GHSV [(mgas,normal conditions3/h)/mbed of catalyst3] values, independently of whether the catalysts are spheres or a monolith, at which more than 99% conversion (destruction) of POHCs is obtained. From both an economical and scientific or reaction engineering point of view, a “good” catalyst should work at high GHSV values (and thus low reactor volumes) and low temperatures to avoid flue gas reheating. Previously, several tests of this type had been already performed,34,38 and from such previous experience, the key factor for accuracy in this research was known to be the sampling and analysis of the POHCs from the flue gas. If the upstream incinerator has a good design and is well operated, as was intended, the POHC concentrations in the flue gas should be very, very low so that their sampling and characterization is not easy. Thus, much care concerning this point has been taken in this work.

Experimental Section Incineration Plant. The small pilot plant used was based on a fast bubbling fluidized bed of 15-cm i.d. in the feeding zone and 5.2-m total height. It has been described previously.35,39 The feedstock used contained 1.0 wt % Cl. This feedstock was a mixture of 98 wt % small pinewood chips (1-4 mm in diameter) and 2.0 wt % PVC (50 wt % Cl) chips. A full characterization of the wood chips used can be found in ref 40. These two solids were mixed by hand before being placed in the feeding hopper. To avoid large plumes in the chimney, located on the campus of the university, the incinerator operated at a capacity of only 1.0-2.0 kg of waste/h, well below its maximum capacity. Feeding was carried out continuously by two in-series screw feeders.35,39 The feeding point was located just at the entrance of the incinerator bed. To maintain a stationary bed to sustain the fluidization, about 4.0 kg of silica sand (new in each test) of 1.0-1.6 mm diameter was added to the bed. The temperature in the bed at the bottom of the incinerator varied between 840 and 940 °C. The superficial gas velocity at the bed inlet was approximately 1.5 m/s. The O2 content in the flue gas, after the catalytic reactor, was continuously monitored and was between 9 and 11 vol % on a dry basis. The other main operating parameters are indicated for some tests in Table 1. Because of the feedstock preparation, exhaustive cleaning of the pilot plant after each test, connection of all vessels in the plant, preparation of the gas sampling devices, etc., one complete incineration test run (coded FBI-xx) in UCM’s pilot plant required an average of 1.8 man-months (excluding chemical analysis performed outside UCM). Thus, these tests required significant effort and expense. For this reason, only nine tests were carried out in this pilot plant. The incineration test itself (with catalyst testing) lasted 10-20 h. Stationary state in the incinerator was achieved in 2-4 h, and under such stationary state, the catalysts were tested Catalytic Reactor(s). Downstream from the incinerator was a hot ceramic filter operating at 500-600 °C and then, in a bypass or slip flow, was the catalytic reactor. Initially, this reactor was made of stainless steel and had a 40-mm inner diameter, the flow was upward, and the gas distributor was a bubble cap, as shown in refs 35 and 38. This metallic reactor had a relatively large wall-to-volume ratio, and after some tests, it was observed (as described below in the text) that the hot wall, together with the gas distributor, had significant catalytic effects, thus masking the intrinsic activity of the catalyst located in the reactor. For this reason, this catalytic reactor was removed, and another new and glass reactors were used in the slip flow. Greater confidence can be placed in the results obtained with the glass reactors than in those obtained with the former metallic reactor because the glass had no catalytic activity at the temperatures used. The (catalytic) glass reactors had downward flow and inner diameters of 30-60 mm (the monoliths used had several sizes and shapes). The total length of these catalytic reactors was 70 cm. A scheme of the location of these glass reactors is shown in Figure 1. Because of its size, the pilot plant encountered severe thermal and mechanical stresses. The connection of the glass reactors to the pilot plant and their operation were not easy, and some problems had to be solved.

280 1.43 6.5 40 856 12.2 10.8 1.19

340 1.44

6.8 40 889 12.2

11.2

1.27

metallic

PRO*CLEAN*500 spheres 2-4

5170 468 477 494 -

throughput [kg/(h m2)] superficial gas velocity at inlet (m/s) total air flow rate (Nm3/h) excess air ratio (%) incineration temp (°C) moisture in the flue exit gas (vol %) O2 in flue exit gas, dry basis (vol %) pressure (primary air) (atm)

wall

name or code shape size (mm)

GHSV (h-1, normal cond.) temperatures used (°C) 5670 328 352 435 477

EF 258 H/D spheres 2.4-4

metallic

14.3 1.0

14.3 1.1

moisture (wt %) flow rate (kg/h)

FBI-35 4/6/00

FBI-34 3/27/00

test date

5010 302 403 508 -

silica sand particulates 2-3

metallic

1.25

8.5

7.8 80 879 12.4

240 1.61

17.0 0.9

FBI-36 4/28/00

280 1.47

incinerator 260 1.40

5050 281 316 370 418

catalyst Ru3/SM1 extrudates 1×5

catalytic reactor metallic

1.23

9.8

4970 310 337 372 418

silica sand particulates 2-3

metallic

1.35

9.8

7.3 55 943 11.9

9.3 1.0

waste feed 18.0 0.9

7.4 70 835 12.5

FBI-38 6/12/00

FBI-37 5/23/00

Table 1. Main Experimental Conditions in the Incinerator and in the Catalytic Reactor

3900 300 328 369 -

Siemens A monolith 60 × 60 × 230

glass

1.36

8.4

7.9 65 940 12.1

320 1.60

12.0 1.0

FBI-39 7/5/00

V2O5/SM1 monolith 44 (φ), 110 (length) 1230 262 285 321 -

glass

1.33

9.4

7.2 50 830 11.8

325 1.40

9.0 1.0

FBI-40 7/20/00

Pd/SM1 monolith 44 (φ), 110 (length) 5030 256 295 339 389

glass

1.30

10.6

6.9 50 870 11.7

320 1.46

9.0 1.0

FBI-41 7/26/00

5000 240 270 300 330

ZERONOX monolith 35 × 35 × 300

glass

1.36

8.4

7.9 75 940 12.1

320 1.60

12.0 1.0

FBI-42 9/6/00

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Figure 1. Small pilot plant at UCM for waste incineration with catalytic hot gas cleaning.

During the testing of monoliths, the space between the monolith and the wall of the reactor was filled with carborundum (CSi) to avoid gas bypassing through this zone. During operation with monoliths, the temperature was measured with two thermocouples located at the inlet and exit of the monolith. During operation with spheres or particulates, the temperature was measured in the center of the bed and at the wall (inner side). In all cases, temperature differences were below 10 °C, so the catalyst (bed or monolith) can be considered isothermal. Each catalytic reactor was externally heated, and its temperature could be modified during an incineration test run. Thus, during a single incineration experiment, the catalyst was tested at different temperatures. These temperatures are shown at the bottom of Table 1. Temperatures and GHSV values were chosen from the previous tests performed with the same catalysts but with pure Cl-VOCs.36-38 For noble-metal-based catalysts, relatively high temperatures (up to 494 °C) and space velocities (of approximately 5000 h-1) were used. For vanadia-based catalysts (tests FBI-39, -40, and -42), relatively low temperatures (up to 370 °C) and space velocities (between 1200 and 3900 h-1) were used. All catalysts were tested under stationary state in the catalytic reactor(s) and in a stable way (which, in this process, is not a major problem). Because of the relatively small size of the catalytic reactor, stationary state was achieved within approximately 30 min after the temperature had been changed. (Once sampling had

been carried out before and after the catalytic bed, its temperature was increased to the new desired level, at which it was held for about 1 h prior to gas sampling at the new temperature.) Because the incineration pilot plant was not allowed to run during the night, no “longterm” tests were performed. Thus, the catalyst lifetime or deactivation was not studied in this facility. Nevertheless, the authors simultaneously ran another laboratory-scale facility using the same catalysts but with synthetic mixtures.36,38 In this facility, tests of several hundred hours in length were performed, and deactivation was studied at several temperatures and levels of Cl-VOCs. Such important deactivation results, to be published elsewhere, have to be considered together with the catalysts activities shown in this paper. Catalysts Tested. Seven different catalysts were tested in the pilot plant. The catalysts used in the form of particulates (spheres and extrudates) were PRO*CLEAN*500 (0.15% Pt, 0.15% Pd/Al2O3) from Su¨dChemie AG, EF 258 H/D (0.15% Pt, 0.15% Pd/Al2O3) from Degussa Hu¨ls AG, and Ru3/SM1 (0.1% Ru/SM1) developed by the Universities of Budapest (HU) and Wroclaw (PL). Four other catalysts were used as monoliths: “A” catalyst (4%V2O5, 7% WO3/TiO2) from Siemens AG, V2O5/SM1 (8% V2O5) from the University of Wroclaw (PL), Pd/SM1 (0.3% Pd/SM1) from the University of Wroclaw (PL), and ZERONOX (6% V2O5, 4% WO3/ TiO2) from KWH GmbH (now Allied Resource Corp.). The detailed chemical and physical characterizations of these catalysts (as far as known for commercial

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1175 Table 2. Effectiveness of the PRO*CLEAN*500 Catalyst for POHC Abatement at a GHSV of 5170 h-1 (Test FBI-34) sample (quality) after catalyst bed pollutant VOCs

before catalyst bed Cl-VOCs

benzene styrene benzene, ethynyl benzaldehyde benzonitrile phenol benzofuran benzene, 1-propynyl naphthalene 1,1′-biphenyl acenaphthylene dibenzofuran phenanthrene pyrene methane, dichloro ethene, tetrachloro benzene, chloro phenol, 2-chloro phenol, 2,4-dichloro benzene, 1,2-dichloro benzene, 1,3-dichloro benzaldehyde, 3-chloro benzene, 1,2,4-trichloro benzene, 1,2,3-trichloro benzene, 1,2,3,4-tetrachloro phenol, 2,4,6-trichloro

first

second

third

first 468 °C

X (91) X (95) X (90) X (91) X (91) X (90) X (94) X (93) X (94) X (70) X (94) X (94) X (95)

X (91) X (95) X (91) X (91) X (83) X (91) X (94) X (94) X (95) X (64) X (93)

X (91)

X (91)

X (95) X (98) X (93) X (95) X (98) X (97) X (95)

X (95) X (97) X (93) X (94) X (97) X (97) X (95) X (98) X (97)

X (94) X (97) X (96)

X (99) X (98)

X (98) X (98)

X (98) X (98) X (98) X (98)

catalysts) have been published previously.36-38 The support called SM1, manufactured by the University of Wroclaw (PL), in the form of both extrudates and monoliths, is a mixture of 84 wt % TiO2 and 11 wt % SiO2.41 The sizes and shapes of all of these catalysts are indicated at the bottom of Table 1, together with the volumetric gas hourly space velocities and temperatures used with each catalyst. Two more tests (FBI-36 and -38) were performed by filling the catalytic reactor with only 2-4 mm silica sand to determine the catalytic activity of the metallic reactor and the importance of thermal reactions on an “inert” solid in the abatement of POHCs. Chemical Analysis of the POHCs in the Flue Gas. Gas sampling at the UCM facility was performed periodically before and after the catalytic reactor following U.S. EPA Method 0030, using tubes with an adsorbent bed (0.1 g of 60/80-mesh Tenax TA). Both the gas sampling and the gas analysis were performed, under contract, by an external institution (CIEMAT, Madrid, Spain) that is an expert in this matter. The gas analysis (TD/GC/MS) done at CIEMAT was carried out following the US-TO-14 reference method in an analytical system formed of three parts. First was a thermal desorption unit (Perkin-Elmer, ATD-400) coupled with a gas chromatograph and a mass-selective detector. The desorption conditions were as follows: T (desorption 1), 250 °C, 10 min; T (desorption 2, trap), 250 °C, 10 min; desorption flow, 72 mL/min; outlet flow, 40 mL/min. Second was a gas chromatograph (Hewlett-Packard, 5890) equipped with an HP-5 column (30-m length, 0.25mm i.d.) operated with 2 psi of He gas with the following temperature program: hold 50 °C for 2 min, heat at 10 °C/min, hold at 250 °C for 3 min. Third was a MSD (Hewlett-Packard, 5971-A) that scanned the range 34300 amu. Analysis of Condensates. The incineration pilot plant has two exit flows: main and slip. In both streams,

X (91) X (90) X (91) X (91) X (93) X (95) X (91) X (94) X (96) X (93) X (95)

X (96)

second 477 °C

third 494 °C

X (95)

X (90)

X (91)

X (95)

X (93) X (95) X (98) X (93)

X (95) X (91)

X (96)

the flue gas was cooled before being sent to the stack. During this cooling, some condensates were generated. They were collected in two different vessels (see Figure 1), and samples were taken at different times on stream and temperatures in the catalytic reactor. Condensates are thus obtained from the flue gas without the catalytic reactor (main flow) and after the catalytic reactor (slip flow). Because of previous experience in this process and the existence of excellent centralized services for some chemical analysis at UCM, where these incineration tests were carried out, samples of the condensates were sent to UCM’s laboratories for analysis. Analysis for POHCs in Condensates. This analysis was carried out at UCM’s Mass Spectrometry Service and involved standarized gas chromatography (GC) and mass spectrometry (MS). The equipment used was an HP 5890 series II gas chromatograph with a 30-m carbowax M-20 HP column and an HP 5989 A mass spectrometer operating at 250 °C at the ionization source and 100 °C at the analyzer. Analysis for Metals in Condensates. In previous tests at the laboratory scale with some targeted Cl-VOCs, chromium and vanadium had been detected in the condensates35,38 when chromia- or vanadia-based catalysts were used with flue gas containing significant amounts of Cl-VOCs. Nascent Cl2 then forms by the reaction of HCl with O2, and it attacks the vanadium (and/or chromium) at the catalyst surface, thus forming low-boiling-temperature compounds that are lost from the catalyst and can be detected in the condensates downstream from the catalytic reactor. This type of catalyst deactivation can thus be detected by chemical analysis for the targeted metals in the condensates. Samples of condensates obtained in the incineration tests at different temperatures in the catalytic reactor were sent to UCM’s centralized Atomic Spectrometry Center, where they were analyzed by standarized ICP-

1176


Table 3. Effectiveness of the EF 258 Degussa Catalyst for POHC Abatement at GHSV ) 5670 h-1 (Test FBI-35) sample (quality) after catalyst bed pollutant VOCs


benzene benzene, 1-ethenyl-3-ethyl benzene, 1-ethenyl-4-ethyl benzene, 1,3-diethenyl styrene benzaldehyde benzonitrile benzofuran naphthalene methane, dichloro ethyne, dichloro acetonitrile, chloro ethene, tetrachloro benzene, chloro phenol, 2-chloro phenol, 2,4-dichloro benzene, 1,2-dichloro benzene, 1,3-dichloro benzene, 1,2,4-trichloro benzene, 1,3,5-trichloro benzene, 1,2,3-trichloro benzene, 1,2,3,4-tetrachloro phenol, 2,4,6-trichloro

first

second

third

fourth

X (91)

X (91)

X (91)

X (90) X (94) X (93) X (90)

X (91) X (91) X (94) X (95)

X (94) X (91) X (94) X (91) X (96)

X (94) X (91) X (95)

X (93) X (97)

X (94)

X (91) X (91) X (95) X (94) X (94)

X (95) X (91)

first 328 °C

second 352 °C

third 435 °C

X (91) X (94)

X (87)

X (91)

X (95)

X (95)

fourth 477 °C

X (91) X (91) X (95) X (95)

X (95)

X (90) X (97) X (97)

X (95) X (97) X (98)

X (97) X (98)

X (98) X (99)

X (98) X (98) X (99) X (98)

Table 4. Effectiveness of the Ru3/SM1 Catalyst for POHC Abatement at GHSV ) 5050 h-1 (Test FBI-37) sample (quality) after catalyst bed pollutant VOCs


acetic acid benzoic acid benzene, methyl naphthalene naphthalene, 2-phenyl dibenzofuran acenaphthylene pyrene anthracene fluoranthene

first

second

third

X (91) X (90) X (86)

fourth X (72)

first 281 °C

second 316 °C

third 370 °C

fourth 418 °C

X (90) X (64)

X (95)

X (91)

X (86) X (72) X (90)

X (90) X (72) X (80) X (72) benzene, 1,3-dichloro benzene, 1,2,4-trichloro benzene, 1,2,3-trichloro

AES (inductively coupled plasma atomic emission spectrometry) technology. Catalyst Testing in the Pilot Plant. Results from the Flue Gas Analysis Noble Metals and Particulate-Shaped Catalysts. Results from the analyses of trace organics or POHCs before and after the catalytic reactor for the PRO*CLEAN*500, EF 258 H/D, and Ru/SM1 catalysts are shown in Tables 2-4, respectively. The numbers in parentheses in these and the following tables indicate the probability that the indicated compound was the one detected, as determined by standard mass spectroscopy. Downstream from the incinerator (and before the catalytic reactor), some compounds/pollutants appeared in only some gas samples because of fluctuations in the upstream incinerator. This is a well-known, reported, and accepted fact. The POHCs in the flue gas were divided into two broad groups: VOCs and Cl-VOCs. At the incinerator exit, most (if not all) of the VOCs in the flue gas are benzene, naphthalene, polyaromatic hydrocarbons

X (91)

X (86) X (86) X (94)

(PAHs), and their derivatives. Most of the Cl-VOCs in the flue gas (at the incinerator exit) are chlorinated benzenes and phenols, in addition to CH2Cl2 (which also appears very often in the flue gas after the catalytic bed). The results in Tables 2-4 indicate that all of the noble-metal catalysts studied are very active, the most active being the Ru3/SM1 catalyst developed by the Universities of Wroclaw and Budapest. The next most active catalyst is EF 258 H/D from Degussa AG. PRO*CLEAN*500 from Su¨d-Chemie AG is similar but not quite as active. These two commercial catalysts destroy most of the VOCs and Cl-VOCs in the flue gas, except for benzene and CH2Cl2 (see Tables 2 and 3), which still appear in the flue gas after the catalytic reactor. The average conversions (destructions) at 470-490 °C of all VOCs and Cl-VOCs by the PRO*CLEAN*500 catalyst, just as an example, are shown in Table 5. At these temperatures and GHSV values, most of the pollutants are completely destroyed (conversions above 99.99%). The only pollutants not completely destroyed were naphthalene (90% conversion), chlorobenzene (70%),

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1177

oxidation catalyst35). Thus, two incineration tests (FBI36 and -38) were performed at a similar space-velocity using silica sand (2-4-mm diameter) in the catalytic reactor instead of the catalyst. The temperature in this inert bed of silica sand was varied from 300 to 508 °C, and the results are shown in Tables 6 and 7. One can see that several VOCs and Cl-VOCs are eliminated (destroyed) from the flue gas. This means that the bed of silica sand and the metallic walls exhibited significant catalytic activity, mainly at relatively high temperatures (above 370 °C). For this reason, for a better comparison of the catalysts, further experiments were performed using glass reactors, which were difficult to connect to the pilot plant but which had no catalytic activity. Tests Performed with Monoliths Using Glass Reactors. The last tests performed using inert (glass) reactors are considered to be the most reliable. The catalytic reactors used were of different diameters because of the different sizes of the four monoliths tested: “A” monolith from Siemens AG-KWU, V2O5/SM1 monolith from the University of Wroclaw, Pd/SM1 monolith from the University of Wroclaw, ZERONOX monolith from KWH GmbH (now Allied Resource Corp.). The results are shown in Tables 8-11, respectively. Noticeable differences among these four catalysts can be appreciated. Siemens’ A Monolith. The Siemens’ A monolith (Table 8), operated at GHSV ) 3900 h-1, gives conversions of nearly all of the VOCs and Cl-VOCs of 100%. At the exit of the catalytic reactor, traces of only three pollutants can be detected but only at low temperature (300 °C). Above 300 °C, even these three pollutants disappear. One-hundred-percent conversion is good for technical purposes, of course, but for a comparison of catalysts, it is not necessarily as good. A higher space velocity or shorter residence time, generating conversions of less than 100%, is recommended for further research. V2O5/SM1 Monolith. Using the V2O5/SM1 monolith at a low space velocity (1230 h-1), all pollutants except benzoic acid were converted/destroyed (Table 9). For technical and environmental objectives, this result is very good, but for a detailed comparison of catalysts, higher space velocities are recommended for future research. The space velocity (SV) was quite low in this test run, of course.

Table 5. Pollutant Conversions by the PRO*CLEAN*500 Catalyst at 470-490 °C and GHSV ) 5170 h-1 (Test FBI-34) VOCs

avg conversion (%)

Cl-VOCs methane, dichloro

benzene styrene ethene, tetrachloro benzene, chloro benzene, ethynyl benzaldehyde benzonitrile phenol phenol,2-chloro phenol,2,4-dichloro benzofuran benzene, 1,2-dichloro benzene, 1,3-dichloro benzene, 1-propynyl benzaldehyde, 3-chloro benzene, 1,2,4-trichloro naphthalene benzene, 1,2,3-trichloro benzene, 1,2,3,4-tetrachloro phenol, 2,4,6-trichloro 1,1′-biphenyl acenaphthylene dibenzofuran phenanthrene pyrene a

-13a 20 99.99 -9.5a 40 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99 90 99.99 99.99 99.99 99.99 99.99 99.99 99.99 99.99

Negative conversion indicates formation.

benzene (20%), and CH2Cl2 (-13%). The negative (overall) conversion (yield in this case) of CH2Cl2 and the low (overall) conversion of benzene are clearly due to two facts: First, they are quite refractory, and second, these two POHCs are simultaneously formed by several inseries or consecutive reactions from most of the VOCs and Cl-VOCs present in the flue gas. (Interesting and detailed reaction schemes or networks that produce these POHCs in the catalytic oxidation of several PAHs are given in the recent work by Bru¨ckner and Baerns42.) Metallic Reactor Filled with Silica Sand. Tests without Catalyst. Because of the surprisingly very high conversions of nearly all of the POHCs present in the flue gas (Table 5), the effects of both thermal reactions and the catalytic activity of the (inner side) wall of the catalytic reactor were considered (stainless steel contains chromium, which is known to be a good

Table 6. Effectiveness of the Catalytic Reactor Filled with Silica Sand as Inert Material (Test FBI-36) sample (quality) after catalyst bed pollutant VOCs


hydrochloric acid benzene benzaldehyde benzene, methyl benzoic, acid Naphthalene styrene benzene, 1-ethenyl-3-ethyl benzene, 1,3-diethenyl

first

second

third

X (78) X (72) X (83)

X (83)

X (78)

first 302 °C

second 403 °C

X (91)

X (90)

third 508 °C X (78)

X (91) X (90) X (90)

X (95)

X (95)

X (95) X (97) X (94) X (96)

ethene, trichloro ethene, tetrachloro benzene, chloro benzene, 1,2-dichloro benzene, 1,3-dichloro benzene, 1,3,5-trichloro benzene, 1,2,4-trichloro

X (94) X (97) X (80)

X (97)

X (91) X (90) X (97) X (91)

X (88) X (97) X (95)

X (90) X (96)

X (93)

X (94) X (94) X (86) X (95)

1178


Table 7. Effectiveness of the Catalytic Reactor Filled with Silica Sand as Inert Material (Test FBI-38) sample (quality) after catalyst bed pollutant VOCs


hydrochloric acid acetic acid 2-butanone naphthalene benzoic acid hexadecanoic acid ethyne, dichloro ethene, trichloro ethene, tetrachloro benzene, 3,4-dichloro benzene, 1,2,4-trichloro benzene, 1,3,5-trichloro benzene, 1,2,3,4-tetrachloro benzene, 1,2,3,5-tetrachloro benzene, pentachloro

first

second

third

fourth

first 310 °C

X (78) X (90) X (72) X (95) X (91) X (99)

X (78)

X (72) X (72) X (93) X (91)

X (72) X (91)

X (92)

X (80) X (90) X (96)

X (99)

X (97)

X (97) X (98) X (99)

X (97) X (99) X (98)

third 372 °C

fourth 418 °C

X (99)

X (64) X (97) X (96)

second 337 °C

X (91) X (98) X (98)

X (98) X (98)

X (96) X (98)

X (98) X (98)

X (98)

X (99)

X (98)

Table 8. Effectiveness of the Siemens A Monolith at GHSV ) 3900 h-1 (Test FBI-39) sample (quality) after catalyst bed pollutant VOCs


first

acetic acid 2-butanone benzaldehyde phenol naphthalene benzoic acid dibenzofuran benz[a]anthracene acenaphthylene

second

X (53) X (90) X (72) X (91) X (90)

X (47) X (64) X (93) X (91)

third

first 300 °C

second 328 °C

X (80)

X (86)

third 369 °C

X (43) X (86) X (72) X (86) X (91) X (60)

benzofuran, 5,7-dichloro-2-methyl ethyne, dichloro ethene, trichloro benzene, chloro benzene, 1,3-dichloro benzene, 1,4-dichloro benzene, 1,2,3-trichloro benzene, 1,2,4-trichloro benzene, 1,3,5-trichloro benzene, 1,2,34-tetrachloro benzene, pentachloro

X (80) X (36) X (38)

X (96) X (97) X (99)

X (96) X (83) X (91) X (91) X (97) X (91)

X (95) X (96)

X (99) X (96)

X (98) X (90)

X (58)

Table 9. Effectiveness of the V2O5/SM1 Monolith at GHSV ) 1230 h-1 (Test FBI-40) sample (quality) after catalyst bed pollutant VOCs


benzene 2-butanone naphthalene benzoic acid benzonitrile benzaldehyde benzofuran biphenylene

first

second

third

first 262 °C

second 285 °C

third 321 °C

X (83)

X (91)

X (83) X (53) X (91)

X (91) X (83) X (64)

X (91)

X (78) ethyne, dichloro ethene, trichloro benzene, chloro benzene, 1,2-dichloro benzene, 1,2,4-trichloro benzene, 1,2,3,4-tetrachloro benzene, pentachloro

X (47)

Pd/SM1 Monolith. The Pd/SM1 monolith did not provide good results (Table 10). This might be a result of the relatively high space velocity used (5000 h-1) or a low catalytic activity for this application.

X (90) X (64) X (72) X (91) X (90) X (91) X (95) X (98) X (78)

X (95) X (96)

ZERONOX Monolith. The ZERONOX monolith at 5000 h-1 and temperatures of 240-330 °C provided very good results (Table 11). The temperatures used with this monolith were lower than those used in previous tests

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1179 Table 10. Effectiveness of the Pd/SM1 Monolith at GHSV ) 5030 h-1 (Test FBI-41) sample (quality) after catalyst bed pollutant VOCs

before catalyst bed

Cl-VOCs

first

acetic acid 2-butanone benzene naphthalene benzoic acid benzaldehyde benzofuran phenol

second

third

fourth

first 256 °C

second 295 °C

X (64) X (42)

third 339 °C

fourth 389 °C

X (42) X (56)

X (83) X (91)

X (91) X (90)

X (91) X (91)

X (83)

X (78)

X (83)

X (83)

X (90)

X (91)

X (90)

X (90)

X (94) X (90)

X (91) X (91)

X (91) X (47) ethyne, dichloro ethene, trichloro benzene, chloro benzene, 1,2-dichloro benzene, 1,3-dichloro benzene, 1,4-dichloro benzene, 1,2,4-trichloro benzene, 1,3,5-trichloro benzene, 1,2,3,4-tetrachloro benzene, 1,2,3,5-tetrachloro

X (90) X (83)

X (91)

X (90)

X (72) X (64)

X (86) X (91) X (91) X (91) X (97)

X (98) X (60)

X (90) X (97) X (94) X (50)

X (94) X (98)

Table 11. Effectiveness of the ZERONOX monolith from KWH at GHSV ) 5,000 h-1 (Test FBI-42) sample (quality) after catalyst bed pollutant VOCs


benzene benzene, 1-ethenyl-3-ethyl benzene, 1,3-diethenyl benzoic acid Styrene benzaldehyde benzonitrile benzofuran naphthalene phenol

first

second

third

X (87) X (77) X (79) X (90) X (93)

ethyne, dichloro ethene, trichloro benzene, chloro phenol, 2,4-dichloro benzene, 1,2-dichloro benzene, 1,3-dichloro benzene, 1,2,4-trichloro benzene, 1,3,5-trichloro benzene, 1,2,3-trichloro benzene, 1,2,3,4-tetrachloro benzene, pentachloro

X (99) X (98) X (98)

to study it in the kinetics-controlled regime and not at equilibrium (99.99% conversion) as in some previous tests. Even at these relatively low temperatures, this monolith abated most of the POHCs present in the flue gas. On the other side, it was also concluded that the experimental conditions used in testing this catalyst were the best, and they were recommended for a possible further and more detailed comparison of the catalysts. The results obtained with V2O5-based catalysts (Siemens A, V2O5/SM1 and ZERONOX) are absolutely promising thus. Further tests should be performed with these monoliths at higher GHSV values (smaller gas residence times) if discrimination between these competitive catalysts be required. (Complementary) Results from an Analysis of the Condensates Analysis of Condensates for POHCs. The analysis of the condensates for POHCs by GC-MS with and

second 270 °C

third 300 °C

X (90)

X (91) X (90)

X (90)

X (89)

X (96) X (89) X (91)

X (92)

fourth 330 °C

X (81) X (82) X (90) X (91) X (81)

X (85) X (91) X (77) X (90) X (85)

fourth

first 240 °C

X (91)

X (91) X (82) X (89) X (91) X (89)

X (98) X (91)

X (98)

X (98)

X (96) X (94)

X (91)

X (98) X (81) X (91)

X (92) X (98)

X (93)

X (95) X (96) X (98)

X (97)

without the catalytic bed did not provide any important information. The most abundant compounds clearly detected in the condensates after the incinerator (main flue gas, without the catalytic reactor) were benzoic acid, methyl and ethenylbenzene, and some others benzyl derivates, together with a substantial amount of HCl. After the catalytic reactor (condensates obtained in the slip stream), HCl was always detected, but all of the other indicated compounds had disappeared or significatively decreased. Analyses of the total organic content (TOC) in the condensateswere also carried out (with a Shimazu model 5050 TOC instrument) to quantify this reduction, but these results were considered not very significant and are omitted here. Analysis of Condensates for Metals. The analysis of the condensates for metals, carried out by ICP-AES, did not detect Pt, Pd, or Ti. Nevertheless, this analysis always detected 1-3 ppm of V and 0.1-1 ppm of W in the condensates when the catalyst being used was V2O5based and the temperature (in the catalytic reactor) was

1180


above 330 °C. (Details about the deactivation of the V2O5-based catalysts under synthetic flue gas compositions will be published elsewhere.) For these authors, it was conclusive that vanadium (and also tungsten) was being lost from the catalysts. This means that these catalysts would become significantly deactivated (above 330 °C) in long-term tests. It should also be stated that, in these incineration tests, the chlorine content in the flue gas might be higher than that encountered in some MSW incineration plants, so that, in such commercial plants, deactivation might not occur or might occur at a lower rate. What can be concluded in this respect is that, above 330-350 °C, V2O5-based catalysts do not support flue gases with relatively high contents of chlorine. Conclusions The amounts and types of pollutants in the flue gas downstream from an incinerator depend on its design and operation. A well-designed fluidized-bed incinerator, operated under optimized conditions, can generate a very clean exit flue gas, with only small traces of pollutants (a few parts per million of VOCs and ClVOCs). Catalysts have to operate (and so were tested) under these optimized conditions. Accurate (standarized) sampling and analysis of the pollutants in flue gas under these circumstances is not an easy task. Thus, this comparison of catalysts has been difficult and expensive. Most of the catalysts tested proved to be useful (active) for POHC abatement by total oxidation, and ranking the catalysts according to their activities at the pilot-plant scale was not easy. Under the experimental conditions used here (relatively low values for the space velocity), most of the catalysts tested completely destroyed (99.99%) nearly all of the POHCs present in the flue gas. Further tests, using higher values for the space velocity in the catalytic reactor, are needed for a detailed ranking of the catalysts. Conclusions that can be drawn from these tests at the pilot scale include the following: 1. Most of the pollutants present in the flue gas at the incinerator exit are dangerous mono-, di-, tri-, tetra-, ..., polychlorinated benzenes and phenols. These compounds are present in amounts on the order of very few parts per million, making their analysis difficult. 2. Most of the VOCs and Cl-VOCs are destroyed (99.99% conversion) with nearly all of the catalysts tested. Only CH2Cl2, benzene, and chlorobenzene still appear after the catalytic reactor when operated at relatively low temperatures. At relatively high temperatures, these refractory compounds are completely destroyed as well. 3. When noticeable amounts of chlorine or Cl-VOCs are present in the flue gas, noble-metal- (Pt-, Pd-) based catalysts can work at temperatures as high as 450 °C without deactivation, but V2O5-WO3-TiO2 and V2O5/ SM1 catalysts become deactivated above 330-350 °C. This temperature limit mainly depends on the chlorine content in the flue gas (although a mathematical correlation among these two variables has not yet been found because of the large number of variables involved). 4. For the above reason, the V2O5-based catalysts have to be used (if significant amounts of Cl-VOCs are present) at relatively low temperatures (220-300 °C) to avoid deactivation. This implies that they must be

used at relatively high gas residence times or, equivalently, low values of the space velocity (GHSV). Recommended values of the GHSV for the vanadia-based catalysts, at the industrial or commercial scale, are 1500-4000 h-1 (normal conditions) or less. For catalyst testing at the pilot scale, the recommended GHSV value is 5000 h-1 with temperatures always below 300 °C if some chlorine is present in the flue gas. 5. Comparing the GHSV values used in this and similar works24-32 with the usually much higher GHSV values in another related catalytic processes, these authors now understand why this catalytic gas cleaning is hardly economically feasible and not widely used in commercial municipal and industrial waste incineration plants. Because of the quite low temperatures to be used (below 300 °C) and the extremely low concentrations of Cl-VOCs (in good waste incineration plants), the overall rate of disappearance of the Cl-VOCs (or POHCs or PCDD/Fs, etc.) is very, very low. The volume (and cost) required for the catalytic reactor, even using good catalysts, is thus very high. Until much more active catalysts (with higher GHSV values) or concepts are developed, this catalytic abatement approach will not have much future (although it is still much better than the typical and not at all ecological wet absorption, with its transfer of PCDD/Fs to aqueous exit streams). This might be a somewhat negative conclusion, but it is the main conclusion that can be drawn from this work. Acknowledgment This work was carried out under the EC, DGXII, Contract INCO-DEMO-2094-1996. The authors thank the European Commission, INCO Programme, for its financial support. We also acknowledge Dr. Janusz Trawczynski from the Technical University of Wroclaw (PL), Prof. Szećhy Ga´bor from the Technical University of Budapest, Dr. Reisinger from Degussa AG, Dr. Ohlrogge from Su¨d-Chemie AG, Dr. S. Fisher from Siemens-KWU in Redwitz (GE), Dr. Ollenik from KWH GmbH in Marl (GE) and BASF AG (for the supply of samples of catalysts for their testing at UCM), Mr. Fernando Gutie´rrez Tamayo (for carrying out the gas sampling and the gas analysis at CIEMAT), Dra. Begonã Fabrellas for leading the gas analysis at CIEMAT (Madrid, Spain), Dr. Nour Kayali for the GCMS analysis at UCM’s Mass Spectrometry Service, and Dra. Maite Larrea et al. for the ICP-AES analysis at UCM’s Atomic Spectrometry Center, all of whom made important contributions to this work. Literature Cited (1) Chagger, H. K.; Jones, J. M.; Pourkashanian, M. Emission of volatile organic compounds from coal combustion. Fuel 1999, 78, 1527-1538. (2) Mastral, A. M.; Calleń, M. S. A review on polycyclic aromatic hydrocarbon (PAH) emissions from energy generation. Environ. Sci. Technol. 2000, 34 (15), 3051-3057. (3) Mastral, A. M.; Calleń, M. S.; Mayoral, C. Polycyclic aromatic hydrocarbon emissions from fluidized bed combustion of coal. Fuel 1995, 74 (12), 1762-1766. (4) Smith, I. M. PAHs from Coal UtilisationsEmissions and Effects; Report ICTIS/TR29; IEA Coal Research: London, U.K., Dec1984. (5) Liu, K.; Xie, W.; Zhao, Z.-B.; Pan, W.-P. Investigation of polycyclic aromatic hydrocarbons in fly ash from fluidized bed combustion systems. Environ. Sci. Technol. 2000, 34 (11), 22732279. (6) Lin, C.; Wu, T. C.; Liu, S. F. Correlation between the combustion performance indices and the products of incomplete

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Received for review April 2, 2001 Revised manuscript received November 30, 2001 Accepted November 30, 2001 IE010297P

Testing Total Oxidation Catalysts for Gas Cleanup ... - ACS Publications

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