Catalytic Gas Conditioning: Application to Biomass and Waste

Aug 1, 1997 - Catalytic gas conditioning is a key step in producing clean syngas via gasification of ... In gas conditioning, a major challenge is the...
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Ind. Eng. Chem. Res. 1997, 36, 4184-4192

Catalytic Gas Conditioning: Application to Biomass and Waste Gasification Denis N. Bangala,† Nicolas Abatzoglou,‡ Jean-Pierre Martin,§ and Esteban Chornet*,| Department of Chemical Engineering, Universite´ de Sherbrooke, Que´ bec, Canada J1K 2R1

Catalytic gas conditioning is a key step in producing clean syngas via gasification of heterogeneous materials. Our work has focused on the steam re-forming of naphthalene and orthodichlorobenzene as prototypes of polyaromatic hydrocarbons (PAHs) and halogenated aromatics. Subsequently, we have studied the conversion of tar present in the syngas derived from biomass and waste gasification. Steam re-forming of naphthalene was initially studied over a UCI GB-98 commercial catalyst in a fixed bed reactor operated at atmospheric pressure, in the temperature range 873-1123 K, with residence times of 0.31-0.82 s and steam to naphthalene molar ratios of 10-22. Although the catalyst is efficient, it suffers from a progressive drop in activity due to coke formation as well as weight loss (35% weight loss after 24 h on stream). To overcome deactivation and catalyst weight loss, a robust catalyst formulation has been developed. It has demonstrated excellent activity as well as reasonable time-on-stream and easy regeneration without significant loss of activity. Total conversion of naphthalene and dichlorobenzene has been observed at 1023 and 1123 K, respectively. The yields of dry gas in both cases have been higher than 90%. After 60 h on stream, the catalyst weight loss is less than 5%. This catalyst has also performed efficiently in the conversion of tar present in the producer gas from air gasification of biomass and mixed wastes. Introduction In gas conditioning, a major challenge is the conversion or destruction of molecules either that are considered serious pollutants (e.g., dioxins, furans, or PAH in thermochemical conversion processes) or that can be an obstacle for the use of the gas in downstream operations (e.g., tar derived from gasification). The deposition and the decomposition of tar in injection systems can lead to improper functioning of compressors and combustion devices. In the case of synthesis operations, the presence of tar can interfere with the catalyst performance. In all cases the presence of tar means lower gas yields. In gasification, the syngas produced contains significant amounts of water vapor depending upon the composition of the organic raw material and the oxidizing media used for the partial oxidation (e.g., gasification) step. Since the catalytic conversion of tar is highly steam dependent, additional steam may be needed to ensure the highest yields of CO and H2, the desired products. The tar conversion operations are similar in concept to the re-forming of hydrocarbons as practiced in the petroleum and natural gas sectors. The major difference is in the nature of the compounds to be reformed since tar from gasification is a mixture of organic molecules having most often a polyaromatic structure with some heteroatoms (such as O, S, or N) present as remnants of the initial molecular structure of the gasified raw material (lignocellulosics, urban and industrial wastes). * To whom correspondence should be addressed. † Telephone: (819) 821-2754. E-mail: dbangala@ coupal.gcm.usherb.ca. ‡ Kemestrie Inc., Sherbrooke, Que ´ bec, Canada J1L 2C8. Telephone: (819) 569-4888. E-mail: [email protected]. § Alcan International Ltd., Kingston, Ontario, Canada K7L 5L9. Telephone: (613) 541-2237. E-mail: Jean-Pierre_Martin@ CCKRDC.ALCAN.CA. | National Renewable Energy Laboratory, Golden, CO 804013393. E-mail: [email protected] or chornete@ tcplink.nrel.gov. S0888-5885(96)00785-3 CCC: $14.00

Several studies (Longwel et al., 1985; Alden et al., 1988; Corella and Monzon, 1988) have shown that basic matrices, such as CaO and MgO and their mixtures as derived from dolomite, can indeed re-form tarry compounds at relatively high temperatures (>1173 K) and within quite a large range of steam to carbon and oxide to tar ratios. An alternate catalytic process is based on the approach used industrially for hydrocarbon re-forming. Nibased catalysts on ceramic-like supports such as alumina are the universal catalysts for hydrocarbon reforming at temperatures < 1173 K (Baker and Mudge, 1985; Ekstro¨m et al., 1985). The control of the acidity of the support by addition of basic oxides, such as K2O, CaO, or La2O3 in the catalyst structure, limits the extent of coke formation (Chen and Fan, 1990). In this paper we discuss our work on the re-forming of naphthalene (surrogate molecule for PAHs) and o-dichlorobenzene (surrogate molecule for halogenated tar that could be produced when gasifying mixed wastes such as those containing PVC) using a commercial catalyst (UCI GB-98) and a novel Ni-based robust formulation catalyst [UdeS] that incorporates a rare earth oxide in the alumina matrix and a metal promoter to facilitate the “work” of the nickel in the difficult “tar environment”. Application of the novel catalyst to real syngas derived from the gasification of biomass and RDF has proven the possibility of re-forming the tar to additional H2 and CO. Formation and Nature of Tar In the context of this paper, we will use the generic word tar to define a heterogeneous group of organic compounds including phenol, naphthalene, polyaromatic hydrocarbons, chlorinated aromatic compounds, and their substituted derivatives from thermochemical conversion of waste and biomass. The mechanisms of formation of these compounds are not yet well understood. If we consider halogenated aromatics, Lusten© 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4185

Figure 1. Schematic of the re-forming unit. Table 1. Commercial Steam Re-forming Catalysts (Mainly Developed for CH4) composition, wt % catalyst

source

active metals

support

NCM G90C G98B C20-07-02 ICI-46-1

W. R. Grace United Catalysts United Catalysts United Catalysts Imperial Chemical Industries

NiO, 12%; CuO, 4.25%; MoO3, 9.25% NiO, 19% NiO, 54.7%; CuO, 5%; MoO3, 6% NiO, 3-4%; MoO3, 15-18%; P, 1.5% 16.5% Ni (21% NiO)

NT 506 NT 550

Katalco Katalco

5-8% NiO; 25-30% MoO3 5-7% NiO; 18-22% WO3

SiO2-Al2O3 60-70% Al2O3; 5-8% CaO SiO2 31%/alumina alumina 14% SiO2; 29% Al2O3; 13% MgO; 13% CaO; 7% K2O; 3% Fe2O3 alumina alumina

houver and Hutzinger (1980) first postulated three possible mechanisms of PCDD and PCDF formation during combustion processes: (i) survival of trace levels of PCDD in the fuel, (ii) generation of PCDD from precursors such as polychlorinated benzene, polychlorinated phenols, and poly(vinyl chloride) (PVC) present in the fuel, and (iii) the “de novo” synthesis of PCDD as a consequence of a complex array of thermal reactions of chemically unrelated nonchlorinated organic compounds and inorganic forms of chlorine. Recent discussions of the possible mechanisms of PCDD/F formation have centered on two hypotheses: (i) PCDD/F are formed from chloroaromatic precursors such as polychlorophenols and polychlorobenzenes by reactions that have been shown to occur via heterogeneous catalysis on the surface of fly ash particulates at 523-673 K (Karasek and Dixon, 1987); (ii) Stieglitz and Klaus (1991) have shown the synthesis of PCDD/F from char-containing particulates by reactions involving oxygen from air, moisture, and inorganic chlorides, catalyzed by Cu(II). This does not require chloroaromatic precursors to be present in the fly ash or in the gas stream. Altwicker and Behrooz (1995) have reported the low-temperature (523-623 K)/long time scale (reaction times of many minutes to hours) formation of PCDD/F from a fixed bed of fly ash. The rates of formation are typically less than 0.1 mg of PCDD/F per g of fly ash per minute, while the reactions from precursors can occur at rates higher than 10 mg of PCDD/F per g of fly ash per min in the same temperature range.

Polycyclic aromatic hydrocarbons (PAH) are generally formed by the incomplete oxidation of organic material (Menzie and Santodonato, 1992). Attempts have been made to derive mechanisms for PAH formation (Bittner and Howard, 1981; Cole et al., 1984) but have suffered from the lack of kinetic and thermodynamic data. Diels-Alder addition of butadiene to itself, to acetylenes, and to other olefins has been proposed as the primary route for the six-carbon ring closure and aromatic formation (Smith, 1981). Kinney and Crowly (1954) also found that the yields and formation rates of aromatics in the pyrolysis of C2 and C4 paraffins and olefins could be explained by a free radical mechanism. Literature Review on Tar Conversion The producer gas formed upon gasification of organic matter when air is used as the gasifying medium contains (a) the main gaseous components (CO, H2, CO2, CH4, N2), (b) H2O, (c) tar, (d) acidic and basic compounds (NH3, HCN, H2S) considered as impurities for the subsequent catalytic re-forming, and (e) particulate solid matter (fly ash). Many producer gas applications require complete tar elimination. Such is the case when producer gas needs cooling; tar condenses in conduits and valves, thereby causing blockages (Alden et al., 1992). Thermal cracking of tar is one possible method of reducing its concentration, but it requires high temperature (>1373 K) and it also produces soot (Jo¨nsson, 1985). Pekka and Bredenberg (1990) have reported that

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Figure 2. Composition of tar from gasification of biomass (mixed sawdust) via the Biosyn air-blown atmospheric gasifier. Temperature of gasification: 1053 K.

after thermal cracking at high temperatures (11731273 K), the producer gas still contains condensable polyaromatic hydrocarbons (PAH) to such an extent that it would not be suited for applications requiring clean gas. Catalytic oxidation for the destruction of PCDD/F has been reported by Freidel (1992). The low temperatures required for this process (573-673 K) will force cooling of the gas. In addition, the efficiency of the gasification process (e.g., the heating value of gas) is reduced, since excess oxygen or air is required to carry out the oxidation reactions.

Several studies (Wen, 1983; Baker and Mudge, 1985) have shown that aluminum silicate materials, such as zeolites having large pores and large specific surface areas crack tar efficiently. Cracking is enhanced by increasing the temperature. At temperatures above 1123 K, however, most of the aluminum silicates tested seem to lose their catalytic activity. Baker and Mudge (1985) have indicated that aluminum silicate catalysts have also little effect on other gas-phase reactions, such as water-gas-shift, re-forming, and methanation. Catalytic steam re-forming offers a number of advantages for the destruction and conversion of tar: (i) the treatment temperature is essentially the same as in the gasifier; (ii) the clean syngas can be enriched in H2 if so desired; (iii) additional steam may be added to ensure complete re-forming of the tar. Alden et al. (1992) have reported that basic matrices, such as CaO and MgO and their mixtures as derived from dolomite, can indeed re-form tarry compounds at relatively high temperatures (>1173 K) and at residence times between 0.1 and 0.8 s. This observation has also been reported by other investigators (Longwell et al., 1985; Alden et al., 1988; Sjo¨stro¨m et al., 1988). Although dolomite catalysts are efficient, they also have several drawbacks, such as thermal instability and the loss of surface area by sintering. At 1053 K, dolomite undergoes phase changes and eventually “melts”. The melting is likely to destroy the pore structure and thereby makes the available surface area smaller. HCl, if present in the producer gas, may react with CaO present in dolomite producing CaCl2 and thereby deactivating the surface of the dolomite (Alden et al., 1988). Commercial steam re-forming supported nickel catalysts (Table 1), have been found to decompose tar very

Figure 3. Conversion and gas yield (a), syngas composition (b), catalyst weight loss (c), and coke yield (d) as a function of steam/ naphthalene molar ratio. T: 1023 K. Residence time: 0.35 s. GHSV: 10080 h-1. Initial catalyst weight: 45 g.

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efficiently (Mudge and Gerber, 1988). However these catalysts are limited by deactivation via (i) poisoning in the presence of sulfur, chlorine, metals, and basic nitrogen compounds; (ii) phase transformations; (iii) sintering; (iv) loss of active components by volatilization, or attrition; (v) fouling by deposition of coke or carbonaceous materials (Corella and Monzon, 1988). Moreover, under high-temperature reaction conditions, conventional nickel catalysts lose their metal surface area quite rapidly (Bartholomew and Sorenson, 1983). The major challenge in many processes involving nickel catalysts is the loss of activity (Mirodatos and Praliaud, 1987). Conventional nickel catalysts, which have been essentially designed for paraffinic hydrocarbon re-forming, are not necessarily well suited for the molecular structures present in tar compounds, often containing heteroatoms.We thus felt that improved formulations and methods of preparation of catalysts had to be developed. Our conceptual strategy is summarized as follows: Ni is necessary since it provides dehydrogenation/hydrogenation ability; γ-Al2O3 is also necessary for supporting the Ni crystallites and providing OH species from H2O splitting; MgO cements the alumina support by forming the MgAl2O4 spinel; Cr enhances the resistance to S and Cl; La lowers coke formation. Controlled thermal treatment of the mixed oxides is key to structure stabilization. In what follows we discuss the results obtained with a typical commercial re-forming catalyst and with the improved catalyst formulation developed in the course of this work. Experimental Section Catalysts Regeneration. The catalysts examined were the UCI GB 98 commercial obtained from United Catalysts Inc. and the [UdeS] catalyst, which incorporates a rare earth oxide in the alumina matrix and a metal promoter. Two methods were used for catalyst regeneration: “in situ” and “out situ” regeneration. In situ regeneration is carried out in a fixed bed containing coked catalyst. The procedure is to regenerate with an inert recirculating gas containing 0.5-1 mol % of oxygen, and the ignition temperature is kept fixed at 773 K. The coke burn-off is controlled by the air injection rate in order to avoid localized burning and to keep the temperature rise through the bed to less than 73 K. The gas products after regeneration were analyzed in order to determine the remaining carbon dioxide and carbon monoxide. The gas sampling was done every 30 min until the complete disparution of carbon oxides peaks, indicating the completion of coke burn-off. It has been observed that the burning of the coke during the regeneration in a fixed bed is limited to a long time of reaction, 3 to 4 h to remove coke on catalyst surface. Out situ regeneration is based on the approach used for catalyst calcination. The coked catalyst pellets were placed in a furnace and were calcined at 773 K for 2 h and then cooled to room temperature. After regeneration, the catalyst pellets recovered their initial color, indicating the completion of coke burn-off. Apparatus and Procedure. Steam re-forming runs were conducted in a bench scale tubular reactor. A schematic diagram of the experimental setup is shown in Figure 1. Naphthalene or dicholorobenzene and water, at the required flow rates, were mixed and preheated to approximately 773 K. The preheated mixed stream entered a tubular reactor made of SS 316

and having the following dimensions: 55 cm length and 2.5 cm i.d. The reactor is heated by a three zone electric furnace. The catalyst bed, placed at the center of the tubular reactor had a typical height of 12.0 cm. Temperatures at the bottom and top sections of the catalyst bed were measured by thermocouples. The gas leaving the reactor was flown through two condensers, connected in series. Most of the unconverted feed, liquid products, and steam were recovered in the first condenser. The noncondensible gases, which included C1C4 hydrocarbons, carbon monoxide, carbon dioxide, and hydrogen were analyzed by gas chromatography using molecular sieve (5Å) and Porapak Q columns. Determination of C1-C4, CO, and CO2 was done via the Porapak Q column. Hydrogen in the product gases was determined via the molecular sieve column. Conversion and yields (illustrated for naphthalene) are defined as follows:

conversion )

gas yield )

o f - wnaph wnaph o wnaph

o f - wnaph - wcoke wnaph o wnaph

or gas yield )

coke yield )

o f - wnaph - wgas wnaph o wnaph

wgas o wnaph

or coke yield )

wcoke o wnaph

o f where wnaph , wnaph , wgas, and wcoke are respectively the weights of naphthalene fed into the reactor, weight of naphthalene at the reactor outlet, weight of total gas produced, and weight of coke produced.

Results and Discussion Steam Re-forming of Naphthalene over Commercial GB-98 Catalyst. We initiated our study using naphthalene as the prototype model compound for PAH. Naphthalene is predominant in the composition of tar present in the flue gas from gasification (Figure 2). The catalyst initially used, provided by United Catalysts, had as composition NiO, 54.7%; CuO, 5%; MoO3, 6%; supported on SiO2 31%/γ-alumina). In our study we used a water/naphthalene molar ratio between 10 and 22 (steam to carbon mole ratio of 1.6-3.6) in the temperature range 873-1123 K, and at residence times in the catalyst bed ranging from 0.3 to 0.8 s (WHSV: 1.2-0.2 (gnaph/gcat)‚h-1). All runs were conducted at atmospheric pressure. These conditions were chosen according to reported work on steam re-forming of paraffinic hydrocarbons (Steel and Ross, 1973; Rostru¨pNielsen, 1984). Figure 3 shows the effects of the steam/naphthalene molar ratio on the GB-98 catalyst activity: naphthalene conversion and yield of dry gas (Figure 3a); product gas distribution (Figure 3b); catalyst weight loss (Figure 3c) and coke yield (Figure 3d). The tests were run at a space velocity of GHSV ) 10080 h-1 and at a fixed temperature of 1023 K. The following points can be

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Figure 4. Conversion and gas yield (a), syngas composition (b), coke yield (c), and catalyst weight loss (d) as a function of temperature. GHSV: 10080 h-1. Steam/naphthalene molar ratio: 16. Residence time: 0.35 s. Initial catalyst weight: 45 g.

made: (i) At high steam to naphthalene molar ratios (18-22), the conversion changes only slightly. For example, at 1023 K, WHSV of 1.2 (gnaph/gcat)‚h-1, and steam/naphthalene molar ratios of 18, 20, and 22, the conversions are 64.8, 65.4, and 64.5, respectively. (ii) The steam/naphthalene molar ratio has a slight effect on the product gas distribution. (iii) The catalyst weight loss increases with the molar ratio. (iv) Coke formation decreases with increasing steam/naphthalene ratios (Figure 3d). The suppression of coke-forming reactions (such as condensation and polymerization) is probably due to the reduction of the partial pressure of coke precursors. This reduction decreases their adsorption rate on the active sites (Bakhshi and Eager, 1984). It is also possible that some of the coke formed on the catalyst might be removed via the coke + steam reactions (Corella et al., 1992). Figure 4a-d shows the effects of reaction temperature on the GB-98 catalyst activity. The tests were run at a fixed steam/naphthalene molar ratio of 16. As expected, the naphthalene conversion increased with reaction temperature; total conversion of naphthalene was observed at temperatures higher than 1123 K. Another observation is the increase of catalyst weight loss with increasing temperature, probably due to the softening of the SiO2 support under the high-temperature and high steam concentration environment. As can be seen in Figure 5, the conversion increases with residence time in the range 0.3-0.6 s. High residence times (>0.6 s) have a slight effect on conversion of naphthalene and catalyst weight loss. The coke yield increases, however, considerably. A “long-run” was carried out at 1023 K, a residence time of 0.55 s,

and a steam/naphthalene molar ratio of 16. It was found that the gas yield decreased rapidly while the yield of coke increased with time-on-stream (Figure 6). After 22 h, the yield of gas decreased from 88% to 38%. The drop in activity was, most probably, caused by fouling of the catalyst. The GB-98 catalyst showed relatively low time-onstream activity (24 h) and its rapid disintegration into powder-like material and 35% of weight loss led us to develop an improved catalyst, denoted hence-to-forth as [UdeS] catalyst. Steam Re-forming of Naphthalene over the [UdeS] Catalyst. The [UdeS] catalyst has been prepared by impregnation and tested in pelleted form. The method of preparation has been described elsewhere (Bangala and Chornet, 1994). The formulation used in this work comprises a Ni-based catalyst that incorporates a rare earth oxide in the alumina matrix and a metal promoter. The catalyst was prepared as pellets and pretreated with H2O just below the reaction temperature. Figure 7a,b shows long term activity before and after regeneration. After a 60 h run it was found that the new catalytic formulation resulted in both excellent activity and a reasonable time-on-stream before regeneration was needed. The latter can be conveniently done without any loss in activity. Steam Re-forming of Dichlorobenzene over [UdeS] Catalyst. The goal for PCDD and PCDF emission levels of 0.1 ng of TEQ/Nm3 will necessitate the development of new and costly technology (Jones et al., 1994). It has been suggested that emissions at such low levels may be attained by use of appropriate

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Figure 5. Conversion and gas yield (a), syngas composition (b), coke yield (c), and catalyst weight loss (d) as a function of residence time. T: 1023 K. Steam/naphthalene molar ratio: 16. Initial catalyst weight: 45 g.

(Buser, 1979):

Figure 6. Conversion, gas yield, and coke yield as a function of time-on-stream. Catalyst: GB-98. T: 1023 K. GHSV: 6588 h-1. Steam/naphthalene molar ratio: 16. Residence time: 0.55 s.

flue gas cleaning devices, for example the catalytic destruction of these substances (Kilgroe et al., 1990). Dichlorobenzene was chosen as a model compound for dioxins and furans on the following basis: (1) The o-dichlorobenzene is structurally similar to the portion of 2,3,7,8-TCDD that is the most difficult to destroy (Freidel, 1992):

(2) It has been shown that the dichlorobenzene is a precursor of PCDD/F formation, which occurs as follows

(3) According to the relative thermal stability of a larger number of organic compounds (Philip et al., 1990), the dichlorobenzene is more thermally stable than dioxin and it can thus be considered as a convenient surrogate molecule (Hanson, 1993). The activity of the [UdeS] catalyst during steam reforming of dichlorobenzene was studied in the temperature range 773-1123 K. For these runs, the weight ratio of steam to dichlorobenzene was kept fixed at 6.5. The residence time was also kept fixed at 0.5 s. All the experiments were performed at atmospheric pressure. The conversion of dichlorobenzene, the yields of gas, and the coke yield are shown in Figure 8. The production of CO and HCl decreased significantly with an increase in temperature. The decrease of CO is due to the watergas-shift reaction (CO + H2O T CO2 + H2). This

4190 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 2. Role of H2O/Tar Weight Ratio (Biomass as Feedstock) regime

H2O/tar (weight)

H2

CO

G G+R G+R G+R G+R

2.00 2.00 4.00 6.00 8.00

2.70 18.00 29.30 35.80 44.90

26.00 17.00 10.20 10.40 13.10

gas composition, vol % CO2 CH4 C2 + C3 17.50 16.50 18.50 19.40 15.60

5.80 2.70 1.80 0.00 0.00

3.40 0.00 0.00 0.00 0.00

N2 + O2

coke yield, %

gas yield, %

44.60 45.80 41.20 34.40 26.40

8.00 1.60 0.80 0.60

92.00 98.40 99.20 99.40

a Notes: gasification (G); 780 °C; re-forming (R), 850 °C, 0.55 s as residence time; WHSV, 0.67 (g /g )‚h-1; GHSV, 6588 h-1; negligible tar cat coke formation has been observed at H2O/tar >4; O2 in the gas entering the re-former was always below 1 vol %.

Figure 9. Gas composition as a function of re-forming temperature for RDF derived syngas. Gasification at 1053 K. Residence time: 0.55 s. GHSV ) 6588 h-1. Steam/tar weight ratio in reformer: 4.

Figure 7. (a) Conversion, gas yield, and coke yield as a function of time-on-stream. Catalyst: [UdeS]. T: 1023 K. GHSV: 6588 h-1. Steam/naphthalene molar ratio: 16. Residence time: 0.55 s. (b) Conversion, gas yield, and coke yield as a function of time-onstream. Catalyst: regenerated [UdeS]. Temperature: 1023 K. GHSV: 6588 h-1. Steam/naphthalene molar ratio: 16. Residence time: 0.55 s.

Figure 8. Conversion, gas yield, and coke yield as a function of reaction temperature for steam re-forming of dichlorobenzene. Catalyst: [UdeS]. GHSV ) 13088 h-1. Steam/DCB weight ratio: 6.5. Residence time: 0.55 s.

observation is in agreement with the trends reported for steam gasification of lignocellulosic residues with commercial steam re-forming catalysts (Corella and Aznar, 1993). Total conversion of dichlorobenzene occurs at 1123 K. At temperatures higher than 1123 K, the yield of gas decreases due to increased coking.

Applicability of [UdeS] Catalyst to Conditioning Gas from Biomass and Waste Gasification. The Biosyn gasifier used to generate the producer gas for this work has been described by Abatzoglou and coworkers (1997). Four types of feedstock were used: (i) wood shavings of mixed softwoods and hardwoods; (ii) a mixture of 90% of the previous shavings and 10% waste polyethylene; (iii) a refuse-derived fuel (RDF) mixture composition of 57% commercial paper waste, 21% wood residues, 9.7% inorganics, 9% polyethylene waste, 2.7% compostable material; (iv) wood residues purposedly contaminated with Pb and Hg. The gasification was conducted at 1053 K. The activity of the improved catalyst was studied in the temperature range 873-1123 K. At each temperature, runs were conducted at identical conditions. The residence time of the producer gas in the re-former was maintained at 0.55 s. The steam/tar weight ratio varied from 2 to 8. In all runs, the steam/tar ratio comprises the water present in the crude gas from the gasifier (0.2 cm3 water/LSTP of gas) and additional water was introduced to adjust the H2O/tar ratio; typical results are shown in Table 2. The hydrogen content increases while CO decreases with the H2O/tar ratio. For steam/tar weight ratios >6, methane and olefinic hydrocarbons were not detected among the gaseous products. It is clear that the high steam/tar ratio has an appreciable effect on methane, ethylene, and propylene re-forming. These trends are in good agreement with the data presented by Biswadip and Kunzru (1992). Figure 9 shows the significance of re-forming temperature on gas composition. H2 and CO2 are predominant at the highest re-forming temperatures. Conclusions From this study the following conclusions can be drawn: (1) We have developed a robust catalyst formulation that has shown excellent activity as well as a reasonable time-on-stream (>60 h) and easy regeneration without significant loss of activity.

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(2) Total conversion of naphthalene is observed at 1023 K. (3) Dichlorobenzene was completely destroyed at 1123 K. (4) Water (as steam)/tar weight ratios higher than 4 can guarantee total conversion of the tar present from biomass and RDF gasification when re-forming is conducted at 1073-1123 K with the [UdeS] catalyst formulation. Nomenclature PAH: polyaromatic hydrocarbons RDF: refuse derived fuel PVC: poly(vinyl chloride) PCDD: polychlorodibenzodioxin PCDF: polychlorodibenzofuran WHSV: weight hourly space velocity [(gnaph/gcat)‚h-1] GHSV: gas hourly space velocity (h-1) TCDD: tetrachlorodibenzodioxin TEQ: toxic equivalence i.d.: interior diameter DCB: dichlorobenzene

Acknowledgment Technical assistance provided by Jacques Bureau, Gilles Phaneuf and Jacques Gagne´ and the support of other laboratory technicians at the G.R.T.P.C (Sherbrooke University) throughout the research program are highly appreciated. Financial assistance of the Bioenergy Development Program (Canmet), Centre Que´becois de Valorisation de la Biomasse (CQVB), and Kemestrie Inc. is gratefully acknowledged. Literature Cited Abatzoglou, N.; Fernandez, J. C.; Larame´e, L.; Jollez, P.; Chornet, E. Application of Gasification to the Conversion of Wood, Urban and Industrial Wastes. In Developments in Thermochemical Biomass Conversion; Bridgewater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional, An Imprint of Chapman & Hall: London, 1997; Vol. 1, pp 960-972. Alden, H.; Bjo¨rkman, E. Laboratory and Pilot Scale Gasification: Hot Gas Clean Up Process for RDF Disposal and Energy recovery. In Energy From Biomass and Wastes XV; U.S. GPO: Washington, DC, March 1991. Alden, H.; Espena¨s, B. G.; Rensfelt, E. Conversion of Tar in Pyrolysis Gas from Wood Using a Fixed Dolomite Bed. In Research in Thermochemical Biomass Conversion; Bridgewater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science Publishers: New York, 1988; pp 987-1001. Alden, H.; Bjo¨rkman, E.; Magnus, C. Catalytic Cracking of Naphthalene on Dolomite. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie & Professional, An Imprint of Chapman & Hall: London, 1992; Vol. 2, pp 216-230. Altwicker, E. R.; Behrooz, G. Formation of polychlorinated Dioxins, Furans, Benzenes and Phenols in the Post-combustion Region of a Heterogeneous Combustor. Effect of Bed material and PostCombustion Temperature. Environ. Sci. Technol. 1995, 29, 2156-1162. Baker, E. G.; Mudge, L. K. Catalysis of Gas-Phase Reactions in Steam Gasification of Biomass In Fundamentals of Thermochemical Biomass Conversion; Elsevier Applied Science Publishers: New York, 1985; pp 863-874. Bakhshi, N. N.; Eager, R. L. Effect of Steam Addition on Catalytic Upgrading of Canola Oil. In Catalysis on the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier Science Publishers: New York, 1984; pp 85-92. Bangala, N. D.; Chornet, E. High Temperature Stable Catalyst for Steam Reforming of Polycyclic Hydrocarbons. Catalyst and Method of their Preparation. Canadian Patent Application, 2,114,965, 1994.

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Received for review February 10, 1996 Revised manuscript received May 19, 1997 Accepted May 27, 1997X IE960785A X Abstract published in Advance ACS Abstracts, August 1, 1997.