- Email: [email protected]
Catalytic conversion of tars in biomass gasification fuel gases with nickel-activated ceramic filters
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1595
Catalytic conversion of tars in biomass gasification fuel gases with nickelactivated ceramic filters D.J. Draelants, H.-B. Zhao and G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussel, Belgium, Phone: +32-2-6293262, Fax: +32-2-6293248 E-mail: [email protected], [email protected] The problem of efficient, reliable and environmentally sound tar and particle removal is common to most applications of biomass gasification technology. In this work, a novel catalytic reactor for simultaneous tar and particle removal is introduced. It consists of a ceramic candle filter, which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process with a potential reduction in investment costs. The catalytic feasibility of the concept was screened on a laboratory-scale with a sulphur- and dust-free synthetic biomass gas and with benzene and naphthalene as tar model compounds. The catalytic filter was represented by a disc-shaped ix-alumina cartridge, activated with nickel through a deposition-precipitation technique with urea. This paper gives an overview of the screening experiments that included variation of reaction temperature, flow rate and nickel loading. Above 850~ a high performance for converting benzene and naphthalene was found using gas velocities typically encountered in candle filtration. 1. INTRODUCTION Tars (benzene, naphthalene and heavy aromatics) are one of the undesirable co-products in production of fuel gas for power production by biomass gasification, because they can cause fouling of equipment and are an environmental hazard, if released [1]. Hence, in many applications the tars must be removed before the fuel gas can be utilised. Preferably, this gas cleaning is performed at temperatures close to the ones at the gasifier exit (700-900~ since this may lead to a higher energy efficiency but more importantly to simplified processes and lower cost, avoiding several high temperature heat exchangers. In addition, this cleaning needs to be performed with the smallest possible extra investment so as to make its use economically viable, especially for small-scale units. The previously proposed or existing solutions for gas cleaning, like wet scrubbing methods, do not fulfil these conditions. The use of catalysts to eliminate tars in biomass fuel gas is a good alternative to wet scrubbing because the tars can be directly converted to useful components of the fuel gas (H2 and CO), avoiding a loss of the thermal value of biomass fuel gas. Two types of catalysts (naturally available dolomites and steam reforming nickel-based catalysts) have been used, usually in a packed bed reactor at 800-900~ with the commercial nickel-based catalysts more active than calcined dolomites [2]. Nickel-based catalysts can however be deactivated by coking when the amount of the tars is high and by sulphur compounds in the fuel gas like H2S [3]. It is recognised that the catalytic bed can work under severe internal diffusion
1596 limitations, which prevents the efficient use of the catalyst [4]. In general, the reaction schemes of catalytic tar conversion are based on gasification of adsorbed hydrocarbon species on the catalyst surface by H20 and/or CO2 [2]. Another important fuel gas cleaning step involves the removal of the particles, causing plugging and abrasion of downstream equipment. At present, this is performed with commercially available ceramic candle filters for hot gas cleaning, which are usually made of silicon carbide or aluminium oxide. Mostly, those filters are formed of two layers, i.e., a thin filtration membrane supported on a mechanically stable large-pore support body [ 1]. In this work, a novel catalytic reactor for tar removal is studied. It consists of a classic ceramic candle filter, but which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process by integration of the removal of particles and tars in one unit, with a potential reduction in investment costs [5]. In addition, due to the intrinsic pore structure of the filter, the gas flows now through the catalytic active pores and internal diffusion is no longer a limiting factor for the tar conversion as in the conventional packed bed reactors. Figure 1 gives a schematic representation of such a catalytic candle filter. Tar components in gaseous form flow through the catalytic filter (1 m long, 1 cm wall thickness), and are converted in the interior of the filter support body. The particles are trapped on the outside thin filtering membrane where a dust cake is formed, which should not affect the catalytic activity, because the catalytic material is situated only in the interior of the porous support structure [6]. This project focuses on the catalytic performance of the catalytic filter, since this is the intrinsic new addition to the candle filter. It involves preparation chemistry for incorporation of nickel into lab-scale filter cartridges similar to the porous support body of a candle filter, determination of their tar cracking ability and modelling of the mass transfer. At present, a preparation route to incorporate pure nickel into the preformed filter substrates has been developed and some activated filter substrates were screened with major tar model components like benzene and naphthalene in a sulphur- and dust-free biomass fuel gas. This paper gives an overview of the results of this first batch of catalyst screening experiments. Dust and tar removed fuel gas
Raw fuel gas
m
m
Fig. 1. Schematic representation of a catalytic candle filter
Pore-wall catalytically modified pore
1597 2. EXPERIMENTAL
2.1. Catalyst preparation The porous alumina filter substrates used (Schumacher, Germany) were disc-shaped (1 cm thickness, 3 cm diameter) and consisted of non-porous ~-A1203 grains (100-350 ~tm). They had a mean pore radius of 26 ~tm, a pore volume of 0.1 ml/g and a BET specific surface area of 0.33 m2/g and were similar to the support structure of alumina candle filters. These substrates were catalytically activated with nickel using the precipitation-deposition method with urea. The substrates are vacuum impregnated with a solution containing appropriate amounts of urea and nickel nitrate. After the excess solution is drained off, the substrates are put in a closed glass vessel and placed in an oven at 90~ during a certain period for reaction, resulting in precipitation of the nickel precursor by the slow decomposition of urea in the pores of the disc [7]. After reaction, the disc was dried at 110~ and subsequently calcined at 450~ for 4 h in an air atmosphere to decompose the precipitated nickel precursor to nickel oxide. This technique allows us to deposit up to 2 wt% of nickel by one impregnation cycle. We have demonstrated that a fairly uniform distribution of the nickel precursor throughout the substrates can be obtained and that one impregnation cycle hardly changes the porosity of the filter cartridges. More details about the catalyst preparation can be found in another publication [8]. Before being used for reaction tests, the activated substrates were reduced in 10 to 50 v% H2 in N2 overnight at 900~
2.2. Reaction set-up A laboratory-scale reaction set-up has been constructed to perform catalyst screening tests, long-term tests, deactivation studies and reaction kinetic studies. The gas mixing zone allows to feed a representative dust-free synthetic biomass fuel gas (N2, HE, CO, CO2, H20 and CH4) to the reactor, with or without addition of impurities like NH3, HES and tar (benzene and naphthalene). The inlet gas is preheated till 150~ before and after introduction of water, benzene and naphthalene to prevent condensation. The total gas flow rate can be set as such that the superficial gas velocity in the reactor is comparable to the face velocity used in candle filtration (1 - 4 cm/s). The reactor consists of a horizontal, dense alumina tube (i.d. 30 mm, length 500 mm), to minimise catalytic wall effects. The catalytic filter substrate is fixed in the middle of the tube by means of an alumina powder cement. The reactor is heated in a tube oven. A differential pressure sensor measures the pressure drop across the reactor because an increase in pressure drop can be an indication for e.g. carbon deposition. The gas leaving the reactor remains heated till 150~ to prevent condensation and finally led either into a tar sampling device or into a by-pass line. In the latter, tar and water are removed from the gas by a cold trap (0~ and sulphuric acid before the gas is led to an online gas chromatograph (Varian 3400 GC with TCD) to determine the content of the main gas components (N2, HE, CO, CO2 and CH4). The tar sampling train is composed of two washing bottles in a bath of cooling liquid at -20~ The first bottle is empty to trap most of the water and naphthalene, while the second bottle contains dichloromethane as a solvent to absorb the benzene and other tars. After sampling, the bottles are rinsed with dichloromethane and the content of the tar compounds in the solvent is off-line analysed by gas chromatography (HP 6890 GC with FID). To monitor an immediate change in behaviour of the catalytic filter substrates during the tests, the outlet gas composition was qualitatively followed by an on-line mass spectrometer (Balzers QMG 420) for N2, HE, CO, CO2, H20, CH4 and benzene.
1598 2.3. Reaction experiments In this study, a typical sulphur- and dust-flee biomass feed gas contained 51 v% N2, 12 v% CO, 10 v% HE, 5 v% CH4, 11 v% CO2 and 11 v% H20, 10-30 g/Nm 3 benzene and 4.5 g ~ m 3 naphthalene. The screening tests involved variation of temperature, inlet flow rate and nickel loading. The temperature was varied from 900~ till 750~ in decreasing steps of 50~ Table 1 gives an overview of the inlet flow rates used, together with their respective space velocity (at normal conditions and based on the reaction volume) and superficial gas velocity (in the reactor tube at 900~ for the two nickel loadings studied. The gas velocity is similar to the face velocity typically used in candle filtration (1-4 cm/s). A gas velocity of 6 cm/s is maybe not realistic for filtration, but was selected to test if it was still possible to get high conversions with such a low contact time. The reaction data were obtained at steady state of the reactor, based on the MS analysis.
Table 1 Flows used during the screening experiments Nickel loading of substrate
inlet flow (Nml/min)
space velocity (h -1)
Superficial velocity (cm/s)
0.5 wt% Ni
195
1655
2
585
4965
6
245
2080
2.5
395
3350
4
590
5008
6
1 wt%Ni
3. RESULTS AND DISCUSSION Figure 2 gives an overview of the conversion of the tar model compounds benzene and naphthalene for the different flow rates, temperatures and nickel loadings studied. 3.1. Effect of gas flow rate To limit the pressure drop across a candle filter, face velocities between 1 and 4 cm/s are normally used. Consequently, the contact time with the catalyst in a catalytic candle filter is imposed by the filtration step. It was not known if this limitation was compatible with the catalytic reactions that have to take place on the low surface catalyst. Our experiments show that, on the condition that the temperature is high enough (850~ the range of tested filtration gas velocities (2-4 cm/s) is adequately to obtain very high conversions of benzene and naphthalene. This indicates a high intrinsic catalytic activity for tar elimination. Mass balance calculations with the inlet and outlet gas compositions, confirmed that benzene and naphthalene were converted to gaseous components like H2 and CO. Below a certain temperature, the conversions decrease as the flow increases, which means that the reaction is then limited by contact time. Nevertheless, this decrease in conversion remains limited in comparison with the variation of the flow rate itself. As already mentioned, a velocity of 6 cm/s (590 Nml/min) is not so realistic for candle filters, but the naphthalene conversion remains very high above 850~ with this high flow rate. However, this velocity is too high to obtain full conversion of benzene (more stable molecule than naphthalene), even at 900 ~
1599 1 wt% nickel
0.5 wt% nickel
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800
850
900
950
.-.o-- 590 Nml/min'
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.
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2o
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20-
700
850
I
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Reaction temperature (~
Reaction temperature (~
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----o-- 245 Nml/min - ~ - - 395 Nml/min
~" 2 0 rn I~
0 700
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60
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950
I
0
700
750
800
I
850
I 900
I 950
Reaction temperature (~
Fig. 2. Conversions of the tar model compounds benzene and naphthalene 3.2. Effect of temperature The benzene conversion is strongly affected by the temperature, and this for all tested flow rates. The experiments show that at least a temperature around 850~ is necessary to obtain a high activity for benzene conversion with typical filtration velocities. Further improving the catalyst can of course decrease this temperature. The working temperature of the catalytic filter is a very important parameter since a lower working temperature certainly improves the life time of the candle filter itself and is beneficial to oppose sintering of the nickel catalyst. It also allows operation of the gasifier at lower temperature such as with lower grade biomass. The naphthalene conversion is less affected by temperature and it seems that 800~ is sufficient for the typical filtration velocities used. 3.3. Effect of nickel loading There seems to be no important difference in activity between the two tested substrates. The conversions with the 1 wt% Ni substrate are only slightly higher compared with the 0.5 wt% Ni substrate, despite of the fact that the nickel loading was doubled. Naturally, the total nickel loading doesn't give any information about the nickel dispersion, which indicates how much of the deposited nickel atoms are really available at the surface for the catalytic reactions. In our case, increasing the loading did not seem to really improve the dispersion and hence the activity. This indicates that the original available surface for nickel deposition
1600 (i.e. the outer surface of the non-porous a-alumina grains) is nearly completely covered with nickel for a loading of only 0.5 wt%. The fact that a low loading of nickel is sufficient simplifies the catalytic filter preparation since multiple impregnation steps with the urea method to increase the loading do not need to be considered. The commercial nickel on (xalumina catalysts, tested for tar conversion in packed beds, had a nickel loading of about 10 20 wt% [2, 4]. In practice, they consist of porous mm-sized particles. Consequently, almost all the nickel is situated inside these particles but can only be reached by the gas by diffusion into the small pores of the particles. In practice, this leads to a low efficient use of the catalyst due to internal diffusion limitations. In a catalytic filter, all the nickel is situated on the outer surface of a-alumina grains of a few 100 ~tm diameter and the gas can instantly react with the nickel when it flows through the filter. 4. CONCLUSIONS AND FUTURE WORK In view of the application background of this project, the first catalyst screening tests have positively demonstrated the principle of activating candle filters with nickel to eliminate tars from a sulphur-free biomass fuel gas. Above 850~ a high performance for converting benzene and naphthalene was found with gas velocities typically encountered in candle filtration. These results are encouraging to further develop this system and continue more screening. However, biomass gasification gas may contain 50-200 ppmv HaS, which is a known poison for nickel catalysts. Additives like Ca and Mg may increase the sulphur resistance of the catalyst and its long-term stability. This implies an adjustment of the catalyst preparation procedure and this will be implemented in the near future in collaboration with a catalyst manufacturer. The operation time was too short to extrapolate to long term activity, but some experiments with a larger time-on-stream will be performed. In addition, future experiments may include the study of the conversion of the NOx-precursor ammonia in the biomass fuel gas, which is present in concentrations of a few thousand ppmv and can also be decomposed with nickel. REFERENCES 1. E. Kurkela, "Formation and removal of biomass-derived contaminams in fluidized-bed gasification processes", VTT Publications, Espoo, 1996. 2. P. Simell, "Catalytic hot gas cleaning of gasification gas", VTT Publications, Espoo, 1997. 3. J. Hepola and P. Simell, Applied Catalysis B: Environmental, 14 (1997) 287. 4. I. Narv~ez, J. Corella and A. Orio, Ind. Eng. Chem. Res., 36 (1997) 317. 5. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 6. K. Hiibner, A. Pape and E.A. Weber, "High temperature gas cleaning", E. Schmidt et al. (Eds.), Institut Ftir Mechanische Verfahrenstechnik und Mechanik, Karlsruhe, (1996) 267. 7. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, Stud. Sur. Sci. Catal., 63 (1991) 165. 8. H. Zhao, D.J. Draelants, and G.V. Baron, Catalysis Today, in press.
1595
Catalytic conversion of tars in biomass gasification fuel gases with nickelactivated ceramic filters D.J. Draelants, H.-B. Zhao and G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussel, Belgium, Phone: +32-2-6293262, Fax: +32-2-6293248 E-mail: [email protected], [email protected] The problem of efficient, reliable and environmentally sound tar and particle removal is common to most applications of biomass gasification technology. In this work, a novel catalytic reactor for simultaneous tar and particle removal is introduced. It consists of a ceramic candle filter, which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process with a potential reduction in investment costs. The catalytic feasibility of the concept was screened on a laboratory-scale with a sulphur- and dust-free synthetic biomass gas and with benzene and naphthalene as tar model compounds. The catalytic filter was represented by a disc-shaped ix-alumina cartridge, activated with nickel through a deposition-precipitation technique with urea. This paper gives an overview of the screening experiments that included variation of reaction temperature, flow rate and nickel loading. Above 850~ a high performance for converting benzene and naphthalene was found using gas velocities typically encountered in candle filtration. 1. INTRODUCTION Tars (benzene, naphthalene and heavy aromatics) are one of the undesirable co-products in production of fuel gas for power production by biomass gasification, because they can cause fouling of equipment and are an environmental hazard, if released [1]. Hence, in many applications the tars must be removed before the fuel gas can be utilised. Preferably, this gas cleaning is performed at temperatures close to the ones at the gasifier exit (700-900~ since this may lead to a higher energy efficiency but more importantly to simplified processes and lower cost, avoiding several high temperature heat exchangers. In addition, this cleaning needs to be performed with the smallest possible extra investment so as to make its use economically viable, especially for small-scale units. The previously proposed or existing solutions for gas cleaning, like wet scrubbing methods, do not fulfil these conditions. The use of catalysts to eliminate tars in biomass fuel gas is a good alternative to wet scrubbing because the tars can be directly converted to useful components of the fuel gas (H2 and CO), avoiding a loss of the thermal value of biomass fuel gas. Two types of catalysts (naturally available dolomites and steam reforming nickel-based catalysts) have been used, usually in a packed bed reactor at 800-900~ with the commercial nickel-based catalysts more active than calcined dolomites [2]. Nickel-based catalysts can however be deactivated by coking when the amount of the tars is high and by sulphur compounds in the fuel gas like H2S [3]. It is recognised that the catalytic bed can work under severe internal diffusion
1596 limitations, which prevents the efficient use of the catalyst [4]. In general, the reaction schemes of catalytic tar conversion are based on gasification of adsorbed hydrocarbon species on the catalyst surface by H20 and/or CO2 [2]. Another important fuel gas cleaning step involves the removal of the particles, causing plugging and abrasion of downstream equipment. At present, this is performed with commercially available ceramic candle filters for hot gas cleaning, which are usually made of silicon carbide or aluminium oxide. Mostly, those filters are formed of two layers, i.e., a thin filtration membrane supported on a mechanically stable large-pore support body [ 1]. In this work, a novel catalytic reactor for tar removal is studied. It consists of a classic ceramic candle filter, but which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process by integration of the removal of particles and tars in one unit, with a potential reduction in investment costs [5]. In addition, due to the intrinsic pore structure of the filter, the gas flows now through the catalytic active pores and internal diffusion is no longer a limiting factor for the tar conversion as in the conventional packed bed reactors. Figure 1 gives a schematic representation of such a catalytic candle filter. Tar components in gaseous form flow through the catalytic filter (1 m long, 1 cm wall thickness), and are converted in the interior of the filter support body. The particles are trapped on the outside thin filtering membrane where a dust cake is formed, which should not affect the catalytic activity, because the catalytic material is situated only in the interior of the porous support structure [6]. This project focuses on the catalytic performance of the catalytic filter, since this is the intrinsic new addition to the candle filter. It involves preparation chemistry for incorporation of nickel into lab-scale filter cartridges similar to the porous support body of a candle filter, determination of their tar cracking ability and modelling of the mass transfer. At present, a preparation route to incorporate pure nickel into the preformed filter substrates has been developed and some activated filter substrates were screened with major tar model components like benzene and naphthalene in a sulphur- and dust-free biomass fuel gas. This paper gives an overview of the results of this first batch of catalyst screening experiments. Dust and tar removed fuel gas
Raw fuel gas
m
m
Fig. 1. Schematic representation of a catalytic candle filter
Pore-wall catalytically modified pore
1597 2. EXPERIMENTAL
2.1. Catalyst preparation The porous alumina filter substrates used (Schumacher, Germany) were disc-shaped (1 cm thickness, 3 cm diameter) and consisted of non-porous ~-A1203 grains (100-350 ~tm). They had a mean pore radius of 26 ~tm, a pore volume of 0.1 ml/g and a BET specific surface area of 0.33 m2/g and were similar to the support structure of alumina candle filters. These substrates were catalytically activated with nickel using the precipitation-deposition method with urea. The substrates are vacuum impregnated with a solution containing appropriate amounts of urea and nickel nitrate. After the excess solution is drained off, the substrates are put in a closed glass vessel and placed in an oven at 90~ during a certain period for reaction, resulting in precipitation of the nickel precursor by the slow decomposition of urea in the pores of the disc [7]. After reaction, the disc was dried at 110~ and subsequently calcined at 450~ for 4 h in an air atmosphere to decompose the precipitated nickel precursor to nickel oxide. This technique allows us to deposit up to 2 wt% of nickel by one impregnation cycle. We have demonstrated that a fairly uniform distribution of the nickel precursor throughout the substrates can be obtained and that one impregnation cycle hardly changes the porosity of the filter cartridges. More details about the catalyst preparation can be found in another publication [8]. Before being used for reaction tests, the activated substrates were reduced in 10 to 50 v% H2 in N2 overnight at 900~
2.2. Reaction set-up A laboratory-scale reaction set-up has been constructed to perform catalyst screening tests, long-term tests, deactivation studies and reaction kinetic studies. The gas mixing zone allows to feed a representative dust-free synthetic biomass fuel gas (N2, HE, CO, CO2, H20 and CH4) to the reactor, with or without addition of impurities like NH3, HES and tar (benzene and naphthalene). The inlet gas is preheated till 150~ before and after introduction of water, benzene and naphthalene to prevent condensation. The total gas flow rate can be set as such that the superficial gas velocity in the reactor is comparable to the face velocity used in candle filtration (1 - 4 cm/s). The reactor consists of a horizontal, dense alumina tube (i.d. 30 mm, length 500 mm), to minimise catalytic wall effects. The catalytic filter substrate is fixed in the middle of the tube by means of an alumina powder cement. The reactor is heated in a tube oven. A differential pressure sensor measures the pressure drop across the reactor because an increase in pressure drop can be an indication for e.g. carbon deposition. The gas leaving the reactor remains heated till 150~ to prevent condensation and finally led either into a tar sampling device or into a by-pass line. In the latter, tar and water are removed from the gas by a cold trap (0~ and sulphuric acid before the gas is led to an online gas chromatograph (Varian 3400 GC with TCD) to determine the content of the main gas components (N2, HE, CO, CO2 and CH4). The tar sampling train is composed of two washing bottles in a bath of cooling liquid at -20~ The first bottle is empty to trap most of the water and naphthalene, while the second bottle contains dichloromethane as a solvent to absorb the benzene and other tars. After sampling, the bottles are rinsed with dichloromethane and the content of the tar compounds in the solvent is off-line analysed by gas chromatography (HP 6890 GC with FID). To monitor an immediate change in behaviour of the catalytic filter substrates during the tests, the outlet gas composition was qualitatively followed by an on-line mass spectrometer (Balzers QMG 420) for N2, HE, CO, CO2, H20, CH4 and benzene.
1598 2.3. Reaction experiments In this study, a typical sulphur- and dust-flee biomass feed gas contained 51 v% N2, 12 v% CO, 10 v% HE, 5 v% CH4, 11 v% CO2 and 11 v% H20, 10-30 g/Nm 3 benzene and 4.5 g ~ m 3 naphthalene. The screening tests involved variation of temperature, inlet flow rate and nickel loading. The temperature was varied from 900~ till 750~ in decreasing steps of 50~ Table 1 gives an overview of the inlet flow rates used, together with their respective space velocity (at normal conditions and based on the reaction volume) and superficial gas velocity (in the reactor tube at 900~ for the two nickel loadings studied. The gas velocity is similar to the face velocity typically used in candle filtration (1-4 cm/s). A gas velocity of 6 cm/s is maybe not realistic for filtration, but was selected to test if it was still possible to get high conversions with such a low contact time. The reaction data were obtained at steady state of the reactor, based on the MS analysis.
Table 1 Flows used during the screening experiments Nickel loading of substrate
inlet flow (Nml/min)
space velocity (h -1)
Superficial velocity (cm/s)
0.5 wt% Ni
195
1655
2
585
4965
6
245
2080
2.5
395
3350
4
590
5008
6
1 wt%Ni
3. RESULTS AND DISCUSSION Figure 2 gives an overview of the conversion of the tar model compounds benzene and naphthalene for the different flow rates, temperatures and nickel loadings studied. 3.1. Effect of gas flow rate To limit the pressure drop across a candle filter, face velocities between 1 and 4 cm/s are normally used. Consequently, the contact time with the catalyst in a catalytic candle filter is imposed by the filtration step. It was not known if this limitation was compatible with the catalytic reactions that have to take place on the low surface catalyst. Our experiments show that, on the condition that the temperature is high enough (850~ the range of tested filtration gas velocities (2-4 cm/s) is adequately to obtain very high conversions of benzene and naphthalene. This indicates a high intrinsic catalytic activity for tar elimination. Mass balance calculations with the inlet and outlet gas compositions, confirmed that benzene and naphthalene were converted to gaseous components like H2 and CO. Below a certain temperature, the conversions decrease as the flow increases, which means that the reaction is then limited by contact time. Nevertheless, this decrease in conversion remains limited in comparison with the variation of the flow rate itself. As already mentioned, a velocity of 6 cm/s (590 Nml/min) is not so realistic for candle filters, but the naphthalene conversion remains very high above 850~ with this high flow rate. However, this velocity is too high to obtain full conversion of benzene (more stable molecule than naphthalene), even at 900 ~
1599 1 wt% nickel
0.5 wt% nickel
//~__r / j,L
o~" 100 E
o
80
> E O O
60
|E
40
. m
/ /
tD N
'tD m
~V 100 /
/-
80
//2
/
O O
I --o-- 195 Nml/min
,.
40
~"
9
(
20
.
'
,
750
800
850
900
950
.-.o-- 590 Nml/min'
I 700
o
100
60-
9
40-
'-
z
950
900
.
100 ID >
g 60
O I11
--o-- 195 Nml/min
I
0-
750
800
850
ReacUon temperature (~
I 900
4O
-<>--245 N m l / m i n -
2o
- 0 - - 5 9 0 Nml/min
395 Nml/min - -
c--
---o-- 585 Nml/min
20-
700
850
I
.9 o~ 80
"3 80tD o
800
I
E
c O
O (D E
750
I
Reaction temperature (~
Reaction temperature (~
> E
----o-- 245 Nml/min - ~ - - 395 Nml/min
~" 2 0 rn I~
0 700
~'/
60
Z
950
I
0
700
750
800
I
850
I 900
I 950
Reaction temperature (~
Fig. 2. Conversions of the tar model compounds benzene and naphthalene 3.2. Effect of temperature The benzene conversion is strongly affected by the temperature, and this for all tested flow rates. The experiments show that at least a temperature around 850~ is necessary to obtain a high activity for benzene conversion with typical filtration velocities. Further improving the catalyst can of course decrease this temperature. The working temperature of the catalytic filter is a very important parameter since a lower working temperature certainly improves the life time of the candle filter itself and is beneficial to oppose sintering of the nickel catalyst. It also allows operation of the gasifier at lower temperature such as with lower grade biomass. The naphthalene conversion is less affected by temperature and it seems that 800~ is sufficient for the typical filtration velocities used. 3.3. Effect of nickel loading There seems to be no important difference in activity between the two tested substrates. The conversions with the 1 wt% Ni substrate are only slightly higher compared with the 0.5 wt% Ni substrate, despite of the fact that the nickel loading was doubled. Naturally, the total nickel loading doesn't give any information about the nickel dispersion, which indicates how much of the deposited nickel atoms are really available at the surface for the catalytic reactions. In our case, increasing the loading did not seem to really improve the dispersion and hence the activity. This indicates that the original available surface for nickel deposition
1600 (i.e. the outer surface of the non-porous a-alumina grains) is nearly completely covered with nickel for a loading of only 0.5 wt%. The fact that a low loading of nickel is sufficient simplifies the catalytic filter preparation since multiple impregnation steps with the urea method to increase the loading do not need to be considered. The commercial nickel on (xalumina catalysts, tested for tar conversion in packed beds, had a nickel loading of about 10 20 wt% [2, 4]. In practice, they consist of porous mm-sized particles. Consequently, almost all the nickel is situated inside these particles but can only be reached by the gas by diffusion into the small pores of the particles. In practice, this leads to a low efficient use of the catalyst due to internal diffusion limitations. In a catalytic filter, all the nickel is situated on the outer surface of a-alumina grains of a few 100 ~tm diameter and the gas can instantly react with the nickel when it flows through the filter. 4. CONCLUSIONS AND FUTURE WORK In view of the application background of this project, the first catalyst screening tests have positively demonstrated the principle of activating candle filters with nickel to eliminate tars from a sulphur-free biomass fuel gas. Above 850~ a high performance for converting benzene and naphthalene was found with gas velocities typically encountered in candle filtration. These results are encouraging to further develop this system and continue more screening. However, biomass gasification gas may contain 50-200 ppmv HaS, which is a known poison for nickel catalysts. Additives like Ca and Mg may increase the sulphur resistance of the catalyst and its long-term stability. This implies an adjustment of the catalyst preparation procedure and this will be implemented in the near future in collaboration with a catalyst manufacturer. The operation time was too short to extrapolate to long term activity, but some experiments with a larger time-on-stream will be performed. In addition, future experiments may include the study of the conversion of the NOx-precursor ammonia in the biomass fuel gas, which is present in concentrations of a few thousand ppmv and can also be decomposed with nickel. REFERENCES 1. E. Kurkela, "Formation and removal of biomass-derived contaminams in fluidized-bed gasification processes", VTT Publications, Espoo, 1996. 2. P. Simell, "Catalytic hot gas cleaning of gasification gas", VTT Publications, Espoo, 1997. 3. J. Hepola and P. Simell, Applied Catalysis B: Environmental, 14 (1997) 287. 4. I. Narv~ez, J. Corella and A. Orio, Ind. Eng. Chem. Res., 36 (1997) 317. 5. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 6. K. Hiibner, A. Pape and E.A. Weber, "High temperature gas cleaning", E. Schmidt et al. (Eds.), Institut Ftir Mechanische Verfahrenstechnik und Mechanik, Karlsruhe, (1996) 267. 7. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, Stud. Sur. Sci. Catal., 63 (1991) 165. 8. H. Zhao, D.J. Draelants, and G.V. Baron, Catalysis Today, in press.