Catalytic Hot Gas Cleaning of Fuel Gas from an Air-Blown Pressurized

Department of Chemical Engineering II, Lund University, P.O. Box 124, 221 00 Lund, Sweden. Catalytic ammonia decomposition and tar reduction by a Ni ...
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Ind. Eng. Chem. Res. 2000, 39, 4075-4081

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Catalytic Hot Gas Cleaning of Fuel Gas from an Air-Blown Pressurized Fluidized-Bed Gasifier Wuyin Wang, Nader Padban, Zhicheng Ye, Go1 ran Olofsson, Arne Andersson, and Ingemar Bjerle* Department of Chemical Engineering II, Lund University, P.O. Box 124, 221 00 Lund, Sweden

Catalytic ammonia decomposition and tar reduction by a Ni catalyst were studied using a feed gas from a pilot-scale pressurized fluidized-bed gasifier. Tests were conducted in a tubular fixedbed reactor with a space time of about 3 s at 800-900 °C and 12 atm. Ammonia removals of 35-95% and light tar conversions of 90-95% were observed. The amount of the light hydrocarbons was found to have a negative effect on the ammonia decomposition. An ammonia concentration in the fuel gas, gas residence time, and catalytic bed temperature also had a significant influence on the ammonia removal efficiency. After the catalyst, CO2 and CO approached equilibrium values, but the content of H2 and H2O was lower because of reactions with tar. The heating value of the fuel gas remained the same. The gasification efficiency increased by about 10%, mainly because of catalytic tar cracking. Deactivation of the catalyst was not observed in the fuel gas containing 50-150 ppm H2S and about 10 g/Nm3 tar. Introduction Increasing interest in the use of biomass gasification has arisen in the power generation industry recently because of awareness of the possibility of utilizing renewable biomass energy as a means of abating greenhouse emissions.1 Use of pressurized biomass gasification and a gas turbine represents an advance in power plant design and provides an efficiency of over 40%.1 This approach has been considered to be promising for the small-scale production of electricity.2-4 The promotion of plant investment and the subsidization of power appear essential as the technology becomes mature and widely used for improving the competitiveness of the cycle.3,4 In pressurized gasification, hot gas cleaning is of vital importance for maintaining a high level of efficiency of the process. The main contaminants produced in gasification, namely, ammonia and tar, need to be held under control during gas turbine combustion. Ammonia converts to NOx by combustion. Tar can block the particulate filters or deposit downstream and be converted to soot that clogs pipes, engine valves, and turbochargers, leading to a decrease in performance and an increase in the need for maintenance.1,2,5,6 The acceptable levels for a gas turbine are 50 ppm for NH3 and 50-100 mg/m3 for tar.7,8 In fluidized-bed biomass gasification, about 10 g/m3 of tar tends to be produced,8 with the amount of NH3, which depends on the fuel-N and the operating conditions, lying in the range of 3001000 ppm for the biomass alone and of up to 6000 ppm when it is mixed with wastes.9 High temperatures are required for the thermal conversions of ammonia and of tar. For ammonia, less than 10% conversion can be achieved at 900 °C and 3 s residence time.9 For tar more than 1200 °C is needed at 1 s residence time, with soot precursors and soot being formed as undesirable products.6,8,10 Catalytic reductions of NH3 and tar in a secondary bed after the * Corresponding author. Tel.: 00 46 46 2228268. Fax: 00 46 46 2228268. E-mail: [email protected].

gasifier primarily involve two types of catalysts, namely, nonmetallic ones, such as dolomite, and metallic oxides, such as nickel-based catalysts.8 The latter catalysts, Ni catalysts in particular, are usually more effective than the nonmetallic ones, especially for NH3 removal. Of the metallic catalysts, the Ni catalysts used in steam reforming have received the greatest attention. Various Ni catalysts for ammonia decomposition and for tar conversion have been investigated. Studies comparing the ammonia decomposition occurring in the product gas from gasifiers and in simulated gasification gases have shown a much lower degree of decomposition of ammonia to be obtained in the product gas from gasifiers,2,11 where an ammonia reduction of only 4080% has been reported, in contrast to the nearly complete ammonia removal obtained in laboratory-scale experiments using simple synthetic gases. Possible reasons for this can be partial deactivation of the catalyst by such product gas contaminants as H2S, HCl, alkali metals, trace elements, and tar compounds and/ or poor heat and mass transfer to the catalyst surface. Thus, longer gas residence times may be necessary to achieve higher ammonia decomposition efficiencies. Decomposition of tar by Ni catalysts has been studied both for model compounds from methane to naphthalene or even pyrene and for product gas from gasifiers. In the absence of H2S, naphthalene is always converted completely.6,10 Benzene and methane can be removed by steam reforming but can also be formed as intermediate products from the steam reforming of higher hydrocarbons. In testing of catalysts on the product gas from gasifiers, it was found, in contrast to the case of ammonia, that the efficiency of the catalysts remained high. Catalysts for steam reforming of naphthas provided very high activity, with 97-99.5% removal efficiencies usually being achieved.12 In some tests, the catalytic reactor was found to reduce the tar content from 1-5 to below 50 mg/Nm3.13 Bangala et al. 6 concluded that at 800-850 °C a steam/tar weight ratio > 4 could guarantee total conversion of the tar from biomass and refuse-derived fuel (RDF) gasification.

10.1021/ie000275n CCC: $19.00 © 2000 American Chemical Society Published on Web 10/07/2000

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With 5-15 g/Nm3 tar in the product gas, a catalyst generally decomposed less than 30% of the benzene and 50% of the naphthalene but more than 60% of the other light tar compounds from benzene to pyrene.11 Deactivation due to fouling of the catalyst and to H2S has been examined in most of the studies. Corella et al.,12,13 Abatzoglou et al.,14 Bangala et al.,6 and Mojtahedi et al.11 observed no catalyst deactivation without regeneration at times on stream of up to about 60 h. Simell et al.2 also reported no decline of Ni catalyst activity during 100 h long test runs at 900 °C and 5 bar. Abatzoglou et al.14 found as well that the global tar conversion efficiency began to decrease slowly after 60 h of operation. After regeneration, however, the catalyst returned to its initial performance level in terms of catalytic activity. At a higher H2S content, such as at 3000 ppm, on the other hand, and/or with the presence of tar, the activity of Ni catalysts was often substantially reduced.10,15 Sulfur affected less the decomposing activity of toluene and methane than that of ammonia.16 When the temperature was increased or the pressure decreased, the effect of sulfur poisoning diminished. It was proposed that the effect of H2S could be totally reversed through catalyst regeneration by oxygen.7 Mojtahedi and Abbasian7 also observed a gradual decrease in the activity of the catalyst over an 8 h period in the absence of H2S, apparently because of the formation and deposition of a very small quantity of soot on the surfaces of the catalysts. In a previous study, the screening of catalysts and the modeling of ammonia decomposition were carried out in laboratory-scale studies.9 A kinetic model for ammonia decomposition was developed based on Temkin’s equation.17 Ammonia removals of 50-90% in the fuel gas were estimated by the model at 800-900 °C, 3 s space time, and 16 atm. Temperature and space time were the dominant factors determining the decomposition efficiency. In the estimates, the effect of light hydrocarbons (LHCs) was not accounted for. The present study concerns the testing of a Ni-based catalyst in a high-pressure fixed-bed reactor (HPFB), using a slipstream from a pressurized fluidized-bed gasifier (PFBG). Results for ammonia decomposition, reduction of LHCs, and changes in the major gaseous components will be presented. The effects of the bed temperature, residence time, amount of ammonia and LHCs in the fuel gas, deactivation of the catalyst, and heating value of the gas will be discussed. Experimental Section The HPFB employed was a 25 cm long tubular reactor with a 1.8 cm diameter quartz liner (Figure 1), described in detail elsewhere.9 A commercial methanation Nibased catalyst found to be suitable for the efficient decomposition of ammonia was used.9 The catalyst was in pellet form and was 4 mm in size. In the tests, a 15 cm catalyst bed was employed. According to a previous kinetic study of the ammonia decomposition needed to achieve suitable removal efficiency, the experimental temperatures were set at 800-900 °C. The space time for the gas in the empty reactor was about 3 s at 12 atm, 800-900 °C, or a space velocity of 3500-4200 h-1 through the reactor. A slipstream of fuel gas was introduced in the HPFB from a 0.1 MW pilot-scale PFBG (Figure 2).18 The gasifier operated under bubbling fluidized conditions

Figure 1. Schematic diagram of HPFB.

Figure 2. Schematic diagram of the PFB gasifier. Table 1. Gasification Conditions for the PFB Gasifier fuel bed material freeboard temp, °C bed temp, °C pressure, atm equivalent ratio (ER) mixing ratio, % air input, NL/min

sawdust

plastics/sawdust

MgO 800-950 800-900 12 0.30-0.40

MgO 850-900 800-850 12 0.25-0.40 20 350

350

using air as the gasification agent. The test rig consisted of three sections: a fluidized-bed gasifier, a filtration unit, and a catalytic reactor. The fluidized-bed reactor had an inner diameter of 80-102 mm and could perform at a fuel-feeding rate of 10-20 kg/h. In the filtration unit, a SiC candle filter was installed. Since a satisfactory separation result was obtained using the filter, the particulates ceased to be a problem for the catalytic unit. The gasification conditions for the tests using the catalyst are listed in Table 1. In the experiments, two kinds of feedstock were gasified: sawdust and sawdust mixed with 20 wt % plastic waste. The plastic waste was generally of polyethylene (PE) and polypropylene (PP) origin. The results of analysis for the fuels are given in Table 2. The compositions of the fuel gas, depending on the equivalent ratio (ER), the feedstocks, and the gasification temperatures, differed for the various experiments. The range of gas compositions is shown in Table 3.

Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 4077 Table 2. Elementary Analysis of the Feedstock HHV, MJ/kg LHV, MJ/kg volatile, % fixed carbon, % water, % bulk density, ×1000 kg/m3 dry wt % ash C H N O S Cl Na K a

sawdust

plastic waste

19.412 18.001 78.25 15.65 5.42 0.25

naa naa 89.00 7.50 0.60 0.4

0.72 51.33 6.21 0.33 41.41 0 naa naa naa

3.40 81.10 13.40 0.10 1.70 0.10 0.31 0.19 0.05

na ) not analyzed.

Table 3. Fuel-Gas Composition ER CH4, % CO, % CO2, % H2, % a

0.29-0.37 4.9-8.4 7.5-10.1 14.9-16.3 3.0-7.8

NH3, ppm LHCs,a % tar, g/m3 H2O, % N2, %

340-1140 0.35-1.71 8.5-18.2 8.7-12.8 47-54

LHCs: C2H6, C2H4, C6H6, C7H8, and C9H8.

The fuel gas left the gasifier at about 200 °C, passing through a heat-traced tube at 150 °C, before entering the HPFB (Figure 1). Measurements on the gases were carried out on-line by a mass spectrometer (MS; Balzers) and a Fourier transform infrared (FTIR) gas analyzer (GASMET) either before or after the HPFB. The levels of CH4, CO, CO2, H2O, NH3, HCN, and LHCs were determined by FTIR, whereas the CH4, CO, CO2, H2, N2, O2, and H2S levels were determined by MS. The gasphase LHCs included ethane, ethylene, benzene, toluene, and indene. C3 and C4 hydrocarbons were neglected in the analysis because they were present in lesser amounts and their determination could have interfered with the determination of the other hydrocarbons. Xylene was not measured. Tar in the fuel gas was sampled through condensing tars by use of a tar cooler in an ice bath. The total tar amount was determined by a gravimetric method.18 After the experiments, the condensed tar was analyzed by gas chromatography (GC)/MS (Varian). Results and Discussion Catalytic Ammonia Decomposition. The Ni-based catalyst was tested by attaching the HPFB to a slipstream of fuel gas from the gasifier. The aim here was to observe the effect of the catalyst on ammonia in the presence of other gas components in “real” fuel gas. Because Ni-based catalysts tend to lose their catalytic effect because of carbon deposition, the effect of the fuel gas tar was also considered. The Ni-based catalyst provided 35-95% ammonia removals and 90-95% removal of LHCs at 790-880 °C, 2.9-3.4 s space time, and 12 atm (Table 4). The ammonia removal efficiency did not simply increase with the fixed-bed temperature. The levels of LHCs and ammonia in the fuel gas also showed notable effects (Table 4). Ammonia concentrations at the outlet of the HPFB were mostly around 200 ppm, which was higher than the equilibrium value. Similar results have been reported by Simell et al.,2 who suggested this to be based on partial deactivation of the catalyst by tar

or by H2S. Because the other reactions, such as the steam reforming of LHCs, were also catalyzed by the catalyst, a competition for active sites may have decreased the NH3 conversion in our case. Evidence for this can be seen in studying the relationship between the ammonia conversion and the LHC levels in the fuel gas (Table 4). It can be noted that at similar bed temperatures an increase in the LHCs in the fuel gas leads to a drop in the ammonia conversion. In addition, at similarly low LHC levels, the ammonia removal efficiency increases as the temperature rises. A trend of increasing ammonia conversion with increasing temperature (at higher NH3 levels) and increasing NH3 concentrations in the fuel gas was observed (Table 4). One can note, nevertheless, that there are figures in Table 4 that do not conform to this tendency. Obviously, the degree of ammonia conversion that occurred is determined by the combined effects of several factors. To illustrate how the ammonia conversion was affected, a set of variables comprised of the catalytic bed temperature, LHC level in the fuel gas, ammonia concentration in the fuel gas, and space time were chosen. A simple first-order equation is formulated to show the dependence of ammonia conversions on the set of variables

y ) a0 + a1*x1 + a2*x2 + a3*x3 + a4*x4

(I)

where y ) ammonia conversion (%), x1 ) catalyst bed temperature (°C), x2 ) LHCs in the fuel gas (%), x3 ) NH3 in the fuel gas (ppm), x4 ) space time (s), and a0, a1, a2, a3, a4 ) coefficients. The data in Table 4 are fitted to the eq I by a leastsquares method. The resulting coefficients are shown in Table 5, with the fitted values being compared with the experimental ammonia conversion that is shown in Figure 3. The lowest two rows in Table 5 indicate how an increase in the variables affects the conversion of ammonia in the range studied in the experiments. Evidently, all of the variables chosen are of importance. The significance of the catalyst bed temperature here was less than that in laboratory-scale experiments using synthetic gases.9 The amount of LHCs seems to play an important role. Mojtahedi et al.11 and Simell et al.2 observed an invariable decline in the ammonia decomposition efficiency when Ni catalysts were applied to “real” fuel gas. Another possible reason for this in addition to the reasons they suggested, such as deactivation of the catalysts, could be the competition for active sites when LHCs and tar are present in the fuel gas. LHCs and Tar Reduction. The fuel gases were produced by gasifying two types of feedstock: sawdust and sawdust containing 20 wt % plastic waste. The most distinctive differences in the composition of the fuel gases were that less H2 and H2O but more CH4, C2H6, and C2H4 were produced in gasification of the plastic waste (Figures 4a-c and 5c,d). Because the plastic waste was of PE and PP origin, it is reasonable to assume that H2 and H2O were consumed by addition and reforming reactions and that CH4, C2H6, and C2H4 were formed by decomposition of the plastic waste. By gravimetric methods, the amounts of tar in the fuel gas were determined to be around 10 g/m3 (Table 4). The condensed tar samples were analyzed by GC/ MS. It was found that about 20% was naphthalene and 20% polyaromatic hydrocarbons (PAH) between naphthalene and benzo[g,h,i]perylene.18 The remaining 60%

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Table 4. Ammonia and LHC Conversions before and after the Catalyst Bed temp, °C space time, s ammonia conv, % NH3 in fuel gas, ppm NH3 after HPFB, ppm NH3 equilibrium, ppm LHC conv, % LHCs in fuel gas, % tar in fuel gas, g/m3

1

2

3

4

5

6

7

8

9

870 3.2 34 365 241 1.4 95 1.7

880 2.9 46 339 182 1.0 93 0.8 9.7

820 3.4 59 526 215 1.8 94 1.3

789 3.1 66 675 231 2.2 95 0.8 16.7

830 3.0 66 647 217 1.7 91 1.1

809 2.9 73 1136 303 1.5 91 0.5 7.5

877 3.1 73 660 176 1.7 95 0.9

819

874 3.0 95 1136 60 2.1 89 0.5 7.5

79 974 209 1.9 89 0.3 11.0

Table 5. Fitted Coefficients a0 ) -153

x1, °C a1 ) 0.102

x2, % a2 ) -26.3

x3, ppm a3 ) 0.0344

x4, s a4 ) 42.7

change in xi change in y

10 °C 1.0%

0.1% -2.6%

100 ppm 3.4%

0.1 s 4.3%

Figure 3. Comparison of experimental and fitted ammonia conversions.

of the tar was thus composed of lighter hydrocarbons smaller than naphthalene, such as indene, toluene, and benzene, and heavier hydrocarbons larger than benzo[g,h,i]perylene. The concentrations of methane, ethane, ethylene, and benzene are plotted in Figure 4. The values before and after the catalytic bed were compared with equilibrium values, calculated for those compounds that could be analyzed by FTIR and MS, i.e., without consideration of polycyclic aromatic hydrocarbons (PAHs) that were larger than naphthalene. The equilibrium values for ethane, ethylene, and benzene were far below 0.1 ppm. As can be seen in Figure 4, significant reductions in the LHCs were achieved, although the results are still quite high compared with the equilibrium values, particularly for the sawdust containing 20% plastics. This can be due to the concentrations of hydrocarbons being higher at the inlet of the HPFB and/or to the LHCs being the intermediate cracking products of higher hydrocarbons. The finding of Mojtahedi et al.11 that the heavier the tar compound, the greater the percent that was decomposed is probably due to the steam reforming of high hydrocarbons producing lighter ones as intermediate products. The greater methane concentrations for the sawdust/plastics mixture than for the sawdust after application of the catalyst indicated methane to be an intermediate product (Figure 4a). In addition, the levels of methane after the catalyst showed the methane level to decrease with increasing bed temperature. This was a sign that the methane reforming was controlled by the rate of the reaction. For ethane, ethylene, and benzene, the results here can be seen as representing the combined effects of both mechanisms. Main Gas Composition. Utilization of the catalyst also altered the main gas composition by converting

y, % norm(y) ) 17

hydrocarbons and water vapor to CO and H2. The levels of H2 and CO increased whereas the amounts of H2O and CO2 decreased as a result of steam- and CO2reforming reactions (Figure 5and Table 6). When the values are compared to the equilibrium values shown in Figure 5, only CO and CO2 can be seen to have approached equilibrium. H2 and H2O are still much lower than the equilibrium values. Because the catalytic cracking of tars larger than naphthalene was not included in the equilibrium calculations, the results indicate the occurrence of tar-cracking reactions. Tar reforming and the addition of intermediate products such as olefins consumed considerable amounts of H2 and H2O. Heating Value and Gasification Efficiency. Low heating values (LHV) for the fuel gas were computed, with the contributions of hydrocarbons higher than naphthalene being excluded. There was no clear variation in LHV after the catalyst bed (Figure 6). In contrast, Corella et al.5 found the LHV of the flue gas to increase in the catalytic reactor (on average 0.3 MJ/ Nm3). The difference here could be due to LHCs being taken into account in our calculations of LHV. The gasification efficiency (GE) was improved by about 13%, where by definition GE ) chemical heat in the product gas/chemical heat of the fuel. The improvement was caused by an increase in the gas yield after passing the catalyst. The equilibrium calculation suggested there to be a GE increase of about 5%. The greater increase than this that was obtained can be attributed mainly to tar-cracking reactions and to some extent to external heating. Corella et al.5 also observed a gas yield increase of 10% at ER ) 0.25, attributing this to most of the tars that were not included in the calculation being converted into H2 and CO. Deactivation of the Catalyst. The content of H2S in the fuel gas lay between 50 and 150 ppm. At this level of H2S, deactivation of the catalyst by sulfur was found negligible under the experimental conditions. The tar content in the fuel gas was around 10 g/m3 (Table 4), which was higher than the tar limit of 0.5-1.0 g/m3 that Corella et al.12 proposed. However, no carbon deposit on the catalyst was found. According to Bangala et al.,6 a steam/tar weight ratio > 4 can guarantee total conversion of the tar from biomass and RDF gasification at 800-850 °C. In our tests, the weight ratio of steam/ tar was between 4 to 11. This was likely to ensure that there was no carbon deposition on the catalyst. No regeneration of the catalyst between experiments occurred. The catalyst was taken out and examined.

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Figure 4. Content of hydrocarbons before and after the catalytic bed: (a) CH4; (b) C2H6; (c) C2H4; (d) C6H6.

Figure 5. Concentrations of the main gas components before and after the catalytic bed: (a) CO; (b) CO2; (c) H2; (d) H2O.

Neither any visible change in the catalyst nor any carbon deposit was found. The ammonia concentration after the catalyst seemed to stabilize at around 200 ppm,

and the conversion of LHCs, which remained at 9095%, provided no clear signs of deactivation of the catalyst.

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Table 6. Reactions Involved in the Catalyst Beda,2,16 steam reforming CO2 reforming dealkylation hydrocracking water-gas shift methanation Boudouard ammonia

CnHm + nH2O f nCO + (n + m/2)H2 CnHm + nCO2 f 2nCO + m/2H2 C6H6CH3 + H2O f C6H6 + CH4 + CO C6H6CH3 + H2O f C6H6 + CH4 C6H6CH3 + 10H2 f 7CH4 CO + H2O T CO2 + H2 CH4 T C + 2H2 2CO T C + CO2 2NH3 T N2 + 3H2

(1) (2) (3) (4) (5) (6) (7) (8) (9)

a Under the experimental conditions, thermal cracking and thermal carbon formation reactions are ignored.

by about 10% because of the increase in the gas yield. Catalytic tar cracking appears to be the major reason for this. No deactivation of the catalyst and no decline in the catalytic effectiveness for either ammonia decomposition or tar reduction were observed. In the temperature range studied, a H2S content of 50-150 ppm in the fuel gas shall not lead to many problems of sulfur poisoning of the catalyst. Although about 10 g/Nm3 tar was present in the fuel gas, with the steam/tar weight ratios being larger than 4, no clear carbon deposition occurred. Acknowledgment The research was funded by the “Swedish National Board for Industrial and Technical Development (NUTEK)” and by the following Swedish companies: Helsingborg Energi, Plastkresten AB, Svenska Kartonga˚tervinning AB, Sydkraft AB, SYSAV AB, Tetra Pak Fibre Systems AB, and Vattenfall Utveckling. Nomenclature

Figure 6. LHV of the fuel gas before and after the catalytic bed.

Because the test rig is inside the university, it was difficult to run the gasifier overnight. The catalyst was only tested for up to 6 h. Simell et al.19 proposed that catalyst lifetimes of 2-5 years were necessary to ensure an economical and feasible cleaning of the gas. Abatzoglou et al.14 estimated that the useful on-stream life of the Ni catalyst could be some 4000-5000 h, depending on the gasification feedstock. Milne et al.8 recommended that emphasis be placed on long-term tests as well as on catalyst poisoning and regeneration. In the future it will be of interest to test the catalyst for longer periods of time and with considerations to the possible poisoning effects of contaminants such as sulfur, chlorine, alkali, and tars. Conclusions Ammonia removals of 35-95% were observed, results that were rather low compared with those obtained for synthetic gas. For light tars that are smaller than naphthalene, 90-95% conversions can always be achieved. The amount of LHCs was found to have a negative effect on the ammonia decomposition. At higher levels of LHCs, when plastic waste was added to the gasification, the effect became considerable. The main reason for this, apart from the possible fouling of the catalyst by tars, was assumed to be a competition for active sites between ammonia decomposition and steam reforming of the hydrocarbons. The concentration of ammonia in the fuel gas, the gas residence time, and the catalyst bed temperature were also found to have a significant influence on the ammonia removal efficiency. The effect of bed temperature was of lesser importance than in the case of synthetic gases. After the catalyst bed, CO2 and CO approached equilibrium values, but the concentrations of H2 and H2O were lowered because of the tar-cracking reactions. CH4, C2 hydrocarbons, and C6H6 were the intermediate products. The heating value of the fuel gas remained the same after the catalyst. The gasification efficiency increased

y ) ammonia conversion, % x1 ) catalytic bed temperature, °C x2 ) LHCs in fuel gas, % x3 ) NH3 in fuel gas, ppm x4 ) space time, s a0, a1, a2, a3, a4 ) coefficients

Literature Cited (1) Reed, T. B.; Gaur, S. A Survey of Biomass Gasification 2000sGasifier Projects and Manufacturers around the World. Prime Contract No. DE-AC36-83CH10093, Subcontract No. ECG6-16604-01 (BEF) of DOE, 1999. (2) Simell, P.; Kurkela, E.; Stahlberg, P.; Hepola, J. Catalytic Hot Gas Cleaning of Gasification Gas. Catal. Today 1996, 27, 5562. (3) Solantausta, Y.; Beckman, D.; Podesser, E.; Overend, R. P.; Ostman, A. IEA Bioenergy Feasibility Studies. In Biomass, Proceedings of the 4th American Biomass Conference; Overend, R. P., Chornet, E., Eds.; Elsevier Science: Oxford, U.K., 1999; CODEN 68IQAG, Vol. 1, pp 463-469. (4) Thielen, W. Concepts for the Decentralised Energetic Utilisation of Biomass and Residues. Chem. Eng. Technol. 1999, 21 (8), 671-674. (5) Corella, J.; Orio, A.; Aznar, P. Biomass Gasification with Air in Fluidised Bed: Reforming of the Gas Composition with Commercial Steam Reforming Catalysts. Ind. Eng. Chem. Res. 1998, 37, 4617-4624. (6) Bangala, D. N.; Abatzoglou, N.; Martin, J. P.; Chornet, E. Catalytic Gas Conditioning: Application to Biomass and Waste Gasification. Ind. Eng. Chem. Res. 1997, 36, 4184-4192. (7) Mojtahedi, W.; Abbasian, J. Catalytic Decomposition of Ammonia in a Fuel Gas at High Temperature and Pressure. Fuel 1995, 74 (11), 1698-1703. (8) Milne, T. A.; Abatzoglou, N.; Evans, R. J. Biomass Gasifier “Tar”: Their Nature, Formation, and Conversion. NREL/TP-57025357, 1998. (9) Wang, W. Y.; Padban, N.; Ye, Z. C.; Andersson, A.; Bjerle, I. Kinetics of Ammonia Decomposition in Hot Gas Cleaning. Ind. Eng. Chem. Res. 1999, 38, 4175-4182. (10) Depner, H.; Jess, A. Kinetics of Nickel-Catalyzed Purification of Tarry Fuel Gases from Gasification and Pyrolysis of Solid Fuels. Fuel 1999, 78, 1369-1377. (11) Mojtahedi, W.; Ylitalo, M.; Maunula, T.; Abbasian, J. Catalytic Decomposition of Ammonia in Fuel Gas Produced in Pilot-Scale Pressurized Fluidised-Bed Gasifier. Fuel Process. Technol. 1995, 45, 221-236. (12) Corella, J.; Orio, A.; Toledo, J. M. Biomass Gasification with Air in A Fluidised Bed: Exhaustive Tar Elimination with Commercial Steam Reforming Catalysts. Energy Fuels 1999, 13, 702-709.

Ind. Eng. Chem. Res., Vol. 39, No. 11, 2000 4081 (13) Corella, J.; Caballero, M. A.; Aznar, M. P.; Gil, J. Biomass Gasification with Air in Fluidized Bed: Hot Gas Cleanup and Upgrading with Steam-Reforming Catalysts of Big Size. In Biomass, Proceedings of the 4th American Biomass Conference; Overend, R. P., Chornet, E., Eds.; Elsevier Science: Oxford, U.K., 1999; CODEN 68IQAG, Vol. 2, pp 933-938. (14) Abatzoglou, N.; Bangala, D.; Chornet, E. An Integrated Modular Hot Gas Conditioning Technology. In Biomass, Proceedings of the 4th American Biomass Conference; Overend, R. P.; Chornet, E., Eds.; Elsevier Science: Oxford, U.K., 1999; CODEN 68IQAG, Vol. 2, pp 953-959. (15) Krishnan, G. N.; Wood, W. J.; Tong, G. T.; McCarty, J. G. Study of Ammonia Removal in Coal Gasification Processes; Report DOE/MC/23087-2667; SRI International: Menlo Park, CA, 1988. (16) Hepola, J.; Simell, P. Sulphur Poisoning of Nickel-Based Hot Gas Cleaning Catalysts in Synthetic Gasification Gas. I. Effect of Different Process Parameters. Appl. Catal. B 1997, 14, 287303.

(17) Temkin, M. I.; Pyzhev, V. Acta Physicochim. (U.R.S.S.) 1940, 12, 327. (18) Padban, N.; Wang, W. Y.; Ye, Z. C.; Bjerle, I.; Odenbrand, I. Tar Formation in Pressurized Fluidized Bed Air Gasification of Woody Biomass. Energy Fuels 2000, 14 (3), 603-611. (19) Simell, P.; Stahlberg, P.; Solantausta, Y.; Hepola, J.; Kurkela, E. Gasification Gas Cleaning with Nickel Monolith Catalyst. In Development Thermochemical Biomass Conversions; Bridgwater, A. V., Boockock, D. G. B., Eds.; Blackie Academic & Professional: London, U.K., 1997; Vol. 2, pp 1103-1116.

Received for review February 29, 2000 Revised manuscript received July 11, 2000 Accepted August 10, 2000

IE000275N