- Email: [email protected]
Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas
Studies in Surface Science and Catalysis 143 E. Gaigneaux et al. (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
159
Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao, G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium To deposit the nickel catalyst in the filter, two preparation methods, depositionprecipitation of nickel nitrate with urea and conventional impregnation with nickel nitrate, were used and compared. In the deposition-precipitation with urea, experimental parameters such as reaction time and urea/nickel molar ratio were investigated to obtain a high controlled fixation of precursor during the slow decomposition of urea. SEM-EDX characterization showed that the urea method gave a fairly uniform spatial distribution of nickel throughout the filter substrate. On the contrary, with the conventional impregnation method, most of the precursor was deposited on the outer surface of the substrate. Tar cracking over nickel-modified catalytic filter prepared by the urea method was studied with a synthetic biomass gasification gas (free of H2S and dust). Benzene and Naphthalene were used as the model compounds, respectively. It was found that both benzene and naphthalene could be completely converted at 800-900~ over the catalytic filter with 1 wt% nickel. 1. INTRODUCTION Catalytic filters and membranes as multifunctional reactors, coupling a catalytically promoted reaction and a separation allowed by the filter or membrane itself, have gained an increasing attention in recent years due to demand for process efficiency improvement as well as environmental concerns [1-6]. The applications of catalytic filters in hot gas cleaning by the simultaneous removal of solid particles and some pollutants such as tar and ammonia from biomass gasification gas are also attracting public interest [1]. Apparently, the novel idea to combine reaction and separation in a single process unit may lead to a higher energy efficiency but also simplify the entire gas cleaning process with a potential reduction in investment costs and space saving, which is an important factor to make biomass gasification a feasible alternative energy source. Tars are always present in gasification gas as a side product and can easily plug downstream process equipment. Catalytic high-temperature gas cleaning is one of the potential solutions to the operational problems caused by tars. It has been demonstrated that nickel-based catalysts are very efficient in decomposing tars in biomass gasification To whom correspondenceshould be addressed. Email: [email protected]; Tel: +32-2-6293250; Fax: +322-6293248.
160 gas at 900 ~ [7]. Regarding the particulate removal, the ceramic candle filters can be applied to remove particles down to micrometer size at high temperature. So, a catalytic filter for simultaneous removal of solid particles and tar from biomass gasification can be achieved if a nickel catalyst for tar decomposition is placed on the pore walls of the inorganic filters through optimised deposition techniques. The most straightforward method is conventional impregnation with a nickel salt solution into the support body, followed by drying to bring about the deposition of salt inside the pores of the support. However, the drying is critical since the solution migrates and the precursor is deposited mainly where the solvent evaporates. Even when performed carefully, some nonuniformity must always be expected [8]. An improvement can be achieved by depositionprecipitation to immobilize the precursor in the pores of the substrate with a precipitant before the drying step [9]. On the other hand, it is required that the precursor and the precipitant are distributed uniformly throughout the pores before the onset of precipitation. Therefore, urea can serve well as a precipitant because it decomposes slowly at 90~ in an aqueous solution with generation of hydroxyl ions to make the precursor precipitate homogeneously throughout the pores [8, 10]. In this work, catalytic ceramic filters with uniform nickel-distribution were attained by the urea precipitation method, which was compared with the conventional impregnation method. To gain more insight on the urea method, the influence of the reaction time of urea decomposition and urea/nickel molar ratio on the fixation of the precursor on the support prior to drying was investigated. Finally, the catalytic performance of the nickel-modified filter in hot gas cleaning was tested on lab-scale using a simulated biomass gasification gas with benzene and naphthalene as mr model compounds. 2. EXPERIMENTAL 2.1. Preparation and characterisation of catalytic filter Some a-A1203-based filter discs (Schumacher, Germany) were vacuum impregnated with a solution containing appropriate amounts of nickel nitrate and urea. After the excess solution was drained off, the discs were placed in a closed vessel and kept at 90 ~ for a certain period, resulting in precipitation of nickel precursor by the slow hydrolysis of urea in the pores of the discs. After reaction, the filter discs were dried at 110 ~ for a few hours and calcined at 450 ~ for 4 h. Then the nickel-modified ceramic filter discs were obtained. For the conventional impregnation method, other steps and experimental conditions were identical to the preparation procedure with the urea method except for the absence of urea in the impregnation solution. The two-dimensional distribution of nickel throughout the modified filter disc was examined with SEM/DEX (energy dispersive X-ray) on a polished radial cross section of the disc. The analysis was performed on a JSM 6400 (JEOL) equipped with a NORAN Xray analysis system. The K line of nickel was used as the X-ray analysis line. The electron probe worked at an acceleration voltage of 15-25 kV and a current intensity of 3 x 109 A. 2.2. Test of reaction performance For reaction tests, the catalytic filter disc was fixed in the middle of a reactor tube (internal diameter 3 cm and length 50 cm), which was made of dense c~-A1203. The
161 catalytic performance was tested in a laboratory reaction setup, which has been described in detail in a previous publication [ 11 ]. In studies of tar cracking using a separate catalyst bed, two types of tar sources are applied, one directly drawn from a biomass gasifier and the other from model compounds. According to VTTs work [ 12], the tar consists mainly of highly stable compounds such as benzene (60-70 wt %), naphthalene (10-20 wt %), and other polyaromatic hydrocarbons (10-20 wt %), which can amount to 15-20 g of tar/Nm 3 in biomass gasification. So, benzene and naphthalene were used in this work as tar model compounds with a fixed concentration of 15 g/Nm 3 (4300 ppm) for benzene and 5 g/Nm 3 (875 ppm) for naphthalene, respectively. The gas composition used was 50 vol % N2, 12 vol % CO, 10 vol % HE, 11 vol % CO2, 12 vol % H20, 5 vol % CH4, 4300 ppm benzene (or 875 ppm naphthalene), which is a typical composition of the product gas from a biomass fluidised bed gasifier operated with air. The reaction tests were performed under three filtration gas velocities 2.5, 4 and 6 cm/s. All experimental points were monitored for at least 60 min after the reaction reached an apparent steady state at the selected operation condition. 3. RESULTS AND DISCUSSION 3.I. Influence of reaction time and urea/nickel molar ratios Urea decomposes slowly at 90~ in aqueous solution [ 10] according to reaction (1). Consequently, hydroxyl groups are slowly generated, uniformly throughout the pores and the precipitation takes place homogeneously. CO(NH2)2 + H2O ~ 2OH- + 2NH4 + + CO2
(1)
In the deposition-precipitation method with urea, experimental parameters such as the reaction temperature, the reaction time and the concentrations of urea and Ni 2+ in the impregnation solution determine the fixation degree of nickel, which is the amount of nickel precursor fixed by precipitation on the support before drying relative to the maximum amount that can be deposited. At present, there is no general agreement in the literature [9, 13-14] about the choice of these experimental parameters. So, it is necessary to investigate the urea method first, to determine adequate values of reaction time and urea/nickel molar ratio for the impregnation of the ceramic substrates. The study of the influence of the reaction time for urea decomposition and the urea concentration in the urea method was performed on ct-A12O 3 powder rather than a ceramic filter disc since the supply of the ceramic discs was limited. Experimental details have been reported elsewhere [ 11]. To investigate the role of the reaction time, the urea/nickel molar ratio was fixed at a value of 1.3. The results are shown in Fig. 1. The precipitation amount of nickel precursor and the suspension pH both increased with reaction time. The amount of nickel precipitate sharply increased during the first 6 h reaction time and increased slowly after 6 h. This suggested that at least 6 h reaction time was needed to fix a reasonable amount of precursor on the support during the wet stage of the urea method. The nickel precipitate was identified By XRD analysis as Ni3(NOa)2(OH)4 in all cases. The effect of the urea/nickel molar ratio on the precipitation was investigated in a range of ratios from 1 up to 3 for a reaction time of 24 h (see Fig. 2). It was found that the
162 amount of the nickel precipitate hardly increased when the molar urea/nickel ration was varied between 1 and 1.7, while for higher urea/nickel molar ratios, the amount of nickel precipitate surprisingly decreased although the pH continued to increase. This decrease of nickel precipitate is due to the increase of complex-formation of the Ni 2+ ion with NH3. The detail explanation for the behaviour in Fig. 2 can be found in our previous publication [15]. In addition, a maximum fixation degree of nickel precursor precipitated on the support (about 75 %) was obtained when the urea/nickel molar ratio is 1.7 after 24 h reaction. Therefore, such optimum experimental parameters as reaction time of 24 h and urea/nickel moral ratio of 1.7 were used to prepare the nickel-activated catalytic filter. 8
8
6
6
A
4== -o
"r:
2
~
5 r
2
suspension pH
nickel
precipitate
--a--- suspension pH tr ,
l
I
,
I
,
I
,
I
l
I
,
0.8 4
8
12
16
2O
|
,
|
1.2
1.6
|
2
,
i
2.4
,
|
2.8
,
3.2
24 Urea/nickel
Reaction t i m e
,
I
molar
ratio
(hr}
Fig. 1. Precipitation amount of the nickel precursor and the suspension pH as a function of reaction time (urea/nickel molar ratio: 1.3)
Fig. 2. Precipitation amount of the nickel precursor and the suspension pH as a function of urea/nickel molar ratio (reaction time/24 h)
3.2. Nickel distribution throughout the catalytic filter disc To compare the nickel distribution obtained with the urea method, some catalytic filter discs were prepared by conventional impregnation with a solution containing only nickel nitrate. The distribution of NiO throughout the filter disc was investigated by SEM/DEX. Their Ni element mappings are shown in Fig. 3 (urea method) and Fig. 4 (conventional impregnation), respectively. The X-ray area scanning was made of part of a polished radial cross section of the discs till a depth of about 3.5 mm (vertical direction) in the disc. The white dots represent the EDX-mapping for the element Ni and the black background is from both the pores and the ~-A1203 particles of the filter disc. It appears that nickel displays a fairly uniform spatial distribution throughout the filter disc prepared by the urea method. However, throughout the disc prepared with the conventional impregnation method, most of the NiO is situated in the pores close to the outer surface (top area of Fig. 4), while less NiO is detected near the middle of disc (bottom area of Fig. 4). To increase the NiO loading further, the deposition cycle was performed several successive times on the filter disc under the same conditions previously used. SEM/EDX analyses were performed on a cross-section of the twofold impregnated filter discs. The Ni
163 element mapping showed that the urea method still gave a fairly homogeneous distribution of nickel throughout the disc after two cycles. However, the nickel distribution throughout the disc seemed to be less uniform with the conventional impregnation, since more precipitation occurred in the outer surface of the substrate. Therefore, the urea method is reliable and reproducible to deposit nickel catalyst on the filter with a uniform distribution of nickel.
Fig. 3. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the urea method.
Fig. 4. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the conventional impregnation method.
3.3. Catalytic performance of the nickel-activated filter discs Fig. 5 shows the benzene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. As a reference, the benzene conversions using a blank disc (no catalyst inside) in the same conditions are displayed in the figure. It was found that full conversion of benzene was obtained at typical filtration gas velocities (2.5 and 4 crn/s) and in a temperature range from 750~ to 900~ Even at a higher filtration velocity such as 6 cm/s, a complete conversion is still reached above 800 ~ Fig. 6 shows the naphthalene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. The blank disc was also used as a reference. As shown in Fig. 6, almost complete naphthalene cracking was achieved above 800 ~ with any gas velocity lower than 4 crn/s. Even with a gas velocity of 6 cm/s, the conversion still remains about 97 %. However, below 800 ~ the conversion of naphthalene significantly decreased as the reaction temperature decreased and the gas velocity increased. In addition, benzene was identified as one reaction product of naphthalene cracking at 750 and 800 ~ Table 1 lists the amount of benzene in the outlet gas after reaction over the nickel-modified filter disc. It is evident that for all gas velocities at 750 ~ benzene is present in the outlet gas with a significant amount. Above 800 ~
164 nearly no benzene was found in the outlet gas. Since no benzene formation was detected during the tests with the blank disc, it can be concluded that benzene was derived from the catalytic naphthalene cracking. Apparently, the nickel-activated filter disc displayed an excellent catalytic performance for the decomposition of either benzene or naphthalene as tar model compound in the simulated biomass gasification gas at a typical filtration gas velocity when the reaction temperature was in the range of 800-900 ~ Table 1 Yield of benzene as a reaction product of catalytic naphthalene cracking over a 1 wt % nickel-modified filter disc at different temperatures and gas velocities. Gas velocity Temperature (~ 2.5 cm/s 4 cm/s 6 cm/s 750 800 850 900 a Not detectable.
15.4 % ND a ND ND
15.7 % 2.1 % ND ND
100
i
100 /
i
,,
1 wt%Ni
r
A
12.3 % 1.6 % ND ND
I wt% Ni A
8o r
.o 0 >
8
~ 4
4 cm/s
I1 era
--D-- 2.5 cm/s
- o - - 2.5 cm/s
cm/s
--o-- 6 crn/s
--o-- 6 cm/s
40
blank dis4z
m
blank dis c
Z
20
20
0
0
'
70O
75O
80O
850
9O0
95
Fig. 5. Benzene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.
|
700
'
750
'
800
'
850
i
900
950
Fig. 6. Naphthalene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.
4. CONCLUSIONS The urea method can be applied to obtain nickel-modified catalytic filter with a uniform spatial distribution of nickel. The study of urea method showed that a reaction time of at least 6 h for urea decomposition is necessary and that a higher urea/nickel molar ration than 1.7 that led to less fixation of nickel precipitation. A maximum fixation of 75 % of the
165 precursor was found for a reaction time of 24 h and an urea/nickel molar ratio of 1.7. Moreover, the complete conversion of tar model compounds, such as benzene or naphthalene in a simulated sulfur-free and dust-free biomass gasification gas over a nickelmodified filter disc was obtained at 800-900 ~ using typical filtration gas velocities. This demonstrated that it is potential to develop a nickel-activated catalytic filter for simultaneous tar and particle removal from biomass gasification gas. This is encouraging to further develop more improved catalysts by the urea method for the removal of more complex tar mixtures and for the tar removal from real biomass gasification gas. ACKNOWLEDGEMENT
This work was financed by Research in Brussels (grant No. RIB-96/32) and the EU 5th Framework Programme (Contract No. ENK5-2000-00305). REFERENCES
1. A. Cybulski and J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc.: New York, 1998. 2. S.R. Ness, G.E. Dunham, G.F. Weber and D.K. Ludlow, Environ. Prog., 14 (1995) 69. 3. G. Saracco and V. Specchia, Catal. Rev. Sci. Eng., 36 (1994) 305. 4. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 5. G. Saracco and V. Specchia, Chem. Eng. Sci., 55 (2000) 897. 6. J.N. Armor, Appl. Catal. A: General, 49 (1989) 1. 7. A.V. Bridgwater, Appl. Catal. A, 116 (1994) 5. 8. J.T. Richardson, Principles of Catalyst Development, Plenum Press, New York, 1989. 9. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, in B. Delmon, P. Grange, P. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 166. 10. W.H.R. Shaw and J.J. Bordeaux, J. Am. Chem. Soc., 77 (1955) 4729. 11. H. Zhao, D.J. Draelants and G.V. Baron, Ind. Eng. Chem. Res., 39 (2000) 3195. 12. P. Simell, Catalytic hot gas cleaning of gasification gas. Espoo 1997, VTT Publications 330; Technical Research Center of Finland: Finland, 1997. 13. K.B. Mok, J.R.H. Ross and R.M. Sambrook, in: G. Poncelet, P. Grange, P. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdmn, 1983, p. 291. 14. X. Xu, H. Vonk, A. Cybulski and J.A. Moulijn, in G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds.), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 1069. 15. H. Zhao, D.J. Draelants and G.V. Baron, Catal. Today, 56 (2000) 229.
159
Preparation of nickel-modified ceramic filters by the urea precipitation method for tar removal from biomass gasification gas D.J. Draelants, Y. Zhang, H. Zhao, G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium To deposit the nickel catalyst in the filter, two preparation methods, depositionprecipitation of nickel nitrate with urea and conventional impregnation with nickel nitrate, were used and compared. In the deposition-precipitation with urea, experimental parameters such as reaction time and urea/nickel molar ratio were investigated to obtain a high controlled fixation of precursor during the slow decomposition of urea. SEM-EDX characterization showed that the urea method gave a fairly uniform spatial distribution of nickel throughout the filter substrate. On the contrary, with the conventional impregnation method, most of the precursor was deposited on the outer surface of the substrate. Tar cracking over nickel-modified catalytic filter prepared by the urea method was studied with a synthetic biomass gasification gas (free of H2S and dust). Benzene and Naphthalene were used as the model compounds, respectively. It was found that both benzene and naphthalene could be completely converted at 800-900~ over the catalytic filter with 1 wt% nickel. 1. INTRODUCTION Catalytic filters and membranes as multifunctional reactors, coupling a catalytically promoted reaction and a separation allowed by the filter or membrane itself, have gained an increasing attention in recent years due to demand for process efficiency improvement as well as environmental concerns [1-6]. The applications of catalytic filters in hot gas cleaning by the simultaneous removal of solid particles and some pollutants such as tar and ammonia from biomass gasification gas are also attracting public interest [1]. Apparently, the novel idea to combine reaction and separation in a single process unit may lead to a higher energy efficiency but also simplify the entire gas cleaning process with a potential reduction in investment costs and space saving, which is an important factor to make biomass gasification a feasible alternative energy source. Tars are always present in gasification gas as a side product and can easily plug downstream process equipment. Catalytic high-temperature gas cleaning is one of the potential solutions to the operational problems caused by tars. It has been demonstrated that nickel-based catalysts are very efficient in decomposing tars in biomass gasification To whom correspondenceshould be addressed. Email: [email protected]; Tel: +32-2-6293250; Fax: +322-6293248.
160 gas at 900 ~ [7]. Regarding the particulate removal, the ceramic candle filters can be applied to remove particles down to micrometer size at high temperature. So, a catalytic filter for simultaneous removal of solid particles and tar from biomass gasification can be achieved if a nickel catalyst for tar decomposition is placed on the pore walls of the inorganic filters through optimised deposition techniques. The most straightforward method is conventional impregnation with a nickel salt solution into the support body, followed by drying to bring about the deposition of salt inside the pores of the support. However, the drying is critical since the solution migrates and the precursor is deposited mainly where the solvent evaporates. Even when performed carefully, some nonuniformity must always be expected [8]. An improvement can be achieved by depositionprecipitation to immobilize the precursor in the pores of the substrate with a precipitant before the drying step [9]. On the other hand, it is required that the precursor and the precipitant are distributed uniformly throughout the pores before the onset of precipitation. Therefore, urea can serve well as a precipitant because it decomposes slowly at 90~ in an aqueous solution with generation of hydroxyl ions to make the precursor precipitate homogeneously throughout the pores [8, 10]. In this work, catalytic ceramic filters with uniform nickel-distribution were attained by the urea precipitation method, which was compared with the conventional impregnation method. To gain more insight on the urea method, the influence of the reaction time of urea decomposition and urea/nickel molar ratio on the fixation of the precursor on the support prior to drying was investigated. Finally, the catalytic performance of the nickel-modified filter in hot gas cleaning was tested on lab-scale using a simulated biomass gasification gas with benzene and naphthalene as mr model compounds. 2. EXPERIMENTAL 2.1. Preparation and characterisation of catalytic filter Some a-A1203-based filter discs (Schumacher, Germany) were vacuum impregnated with a solution containing appropriate amounts of nickel nitrate and urea. After the excess solution was drained off, the discs were placed in a closed vessel and kept at 90 ~ for a certain period, resulting in precipitation of nickel precursor by the slow hydrolysis of urea in the pores of the discs. After reaction, the filter discs were dried at 110 ~ for a few hours and calcined at 450 ~ for 4 h. Then the nickel-modified ceramic filter discs were obtained. For the conventional impregnation method, other steps and experimental conditions were identical to the preparation procedure with the urea method except for the absence of urea in the impregnation solution. The two-dimensional distribution of nickel throughout the modified filter disc was examined with SEM/DEX (energy dispersive X-ray) on a polished radial cross section of the disc. The analysis was performed on a JSM 6400 (JEOL) equipped with a NORAN Xray analysis system. The K line of nickel was used as the X-ray analysis line. The electron probe worked at an acceleration voltage of 15-25 kV and a current intensity of 3 x 109 A. 2.2. Test of reaction performance For reaction tests, the catalytic filter disc was fixed in the middle of a reactor tube (internal diameter 3 cm and length 50 cm), which was made of dense c~-A1203. The
161 catalytic performance was tested in a laboratory reaction setup, which has been described in detail in a previous publication [ 11 ]. In studies of tar cracking using a separate catalyst bed, two types of tar sources are applied, one directly drawn from a biomass gasifier and the other from model compounds. According to VTTs work [ 12], the tar consists mainly of highly stable compounds such as benzene (60-70 wt %), naphthalene (10-20 wt %), and other polyaromatic hydrocarbons (10-20 wt %), which can amount to 15-20 g of tar/Nm 3 in biomass gasification. So, benzene and naphthalene were used in this work as tar model compounds with a fixed concentration of 15 g/Nm 3 (4300 ppm) for benzene and 5 g/Nm 3 (875 ppm) for naphthalene, respectively. The gas composition used was 50 vol % N2, 12 vol % CO, 10 vol % HE, 11 vol % CO2, 12 vol % H20, 5 vol % CH4, 4300 ppm benzene (or 875 ppm naphthalene), which is a typical composition of the product gas from a biomass fluidised bed gasifier operated with air. The reaction tests were performed under three filtration gas velocities 2.5, 4 and 6 cm/s. All experimental points were monitored for at least 60 min after the reaction reached an apparent steady state at the selected operation condition. 3. RESULTS AND DISCUSSION 3.I. Influence of reaction time and urea/nickel molar ratios Urea decomposes slowly at 90~ in aqueous solution [ 10] according to reaction (1). Consequently, hydroxyl groups are slowly generated, uniformly throughout the pores and the precipitation takes place homogeneously. CO(NH2)2 + H2O ~ 2OH- + 2NH4 + + CO2
(1)
In the deposition-precipitation method with urea, experimental parameters such as the reaction temperature, the reaction time and the concentrations of urea and Ni 2+ in the impregnation solution determine the fixation degree of nickel, which is the amount of nickel precursor fixed by precipitation on the support before drying relative to the maximum amount that can be deposited. At present, there is no general agreement in the literature [9, 13-14] about the choice of these experimental parameters. So, it is necessary to investigate the urea method first, to determine adequate values of reaction time and urea/nickel molar ratio for the impregnation of the ceramic substrates. The study of the influence of the reaction time for urea decomposition and the urea concentration in the urea method was performed on ct-A12O 3 powder rather than a ceramic filter disc since the supply of the ceramic discs was limited. Experimental details have been reported elsewhere [ 11]. To investigate the role of the reaction time, the urea/nickel molar ratio was fixed at a value of 1.3. The results are shown in Fig. 1. The precipitation amount of nickel precursor and the suspension pH both increased with reaction time. The amount of nickel precipitate sharply increased during the first 6 h reaction time and increased slowly after 6 h. This suggested that at least 6 h reaction time was needed to fix a reasonable amount of precursor on the support during the wet stage of the urea method. The nickel precipitate was identified By XRD analysis as Ni3(NOa)2(OH)4 in all cases. The effect of the urea/nickel molar ratio on the precipitation was investigated in a range of ratios from 1 up to 3 for a reaction time of 24 h (see Fig. 2). It was found that the
162 amount of the nickel precipitate hardly increased when the molar urea/nickel ration was varied between 1 and 1.7, while for higher urea/nickel molar ratios, the amount of nickel precipitate surprisingly decreased although the pH continued to increase. This decrease of nickel precipitate is due to the increase of complex-formation of the Ni 2+ ion with NH3. The detail explanation for the behaviour in Fig. 2 can be found in our previous publication [15]. In addition, a maximum fixation degree of nickel precursor precipitated on the support (about 75 %) was obtained when the urea/nickel molar ratio is 1.7 after 24 h reaction. Therefore, such optimum experimental parameters as reaction time of 24 h and urea/nickel moral ratio of 1.7 were used to prepare the nickel-activated catalytic filter. 8
8
6
6
A
4== -o
"r:
2
~
5 r
2
suspension pH
nickel
precipitate
--a--- suspension pH tr ,
l
I
,
I
,
I
,
I
l
I
,
0.8 4
8
12
16
2O
|
,
|
1.2
1.6
|
2
,
i
2.4
,
|
2.8
,
3.2
24 Urea/nickel
Reaction t i m e
,
I
molar
ratio
(hr}
Fig. 1. Precipitation amount of the nickel precursor and the suspension pH as a function of reaction time (urea/nickel molar ratio: 1.3)
Fig. 2. Precipitation amount of the nickel precursor and the suspension pH as a function of urea/nickel molar ratio (reaction time/24 h)
3.2. Nickel distribution throughout the catalytic filter disc To compare the nickel distribution obtained with the urea method, some catalytic filter discs were prepared by conventional impregnation with a solution containing only nickel nitrate. The distribution of NiO throughout the filter disc was investigated by SEM/DEX. Their Ni element mappings are shown in Fig. 3 (urea method) and Fig. 4 (conventional impregnation), respectively. The X-ray area scanning was made of part of a polished radial cross section of the discs till a depth of about 3.5 mm (vertical direction) in the disc. The white dots represent the EDX-mapping for the element Ni and the black background is from both the pores and the ~-A1203 particles of the filter disc. It appears that nickel displays a fairly uniform spatial distribution throughout the filter disc prepared by the urea method. However, throughout the disc prepared with the conventional impregnation method, most of the NiO is situated in the pores close to the outer surface (top area of Fig. 4), while less NiO is detected near the middle of disc (bottom area of Fig. 4). To increase the NiO loading further, the deposition cycle was performed several successive times on the filter disc under the same conditions previously used. SEM/EDX analyses were performed on a cross-section of the twofold impregnated filter discs. The Ni
163 element mapping showed that the urea method still gave a fairly homogeneous distribution of nickel throughout the disc after two cycles. However, the nickel distribution throughout the disc seemed to be less uniform with the conventional impregnation, since more precipitation occurred in the outer surface of the substrate. Therefore, the urea method is reliable and reproducible to deposit nickel catalyst on the filter with a uniform distribution of nickel.
Fig. 3. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the urea method.
Fig. 4. SEM/DEX mapping for Ni of a cross section of a nickel-modified filter disc after a single deposition cycle with the conventional impregnation method.
3.3. Catalytic performance of the nickel-activated filter discs Fig. 5 shows the benzene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. As a reference, the benzene conversions using a blank disc (no catalyst inside) in the same conditions are displayed in the figure. It was found that full conversion of benzene was obtained at typical filtration gas velocities (2.5 and 4 crn/s) and in a temperature range from 750~ to 900~ Even at a higher filtration velocity such as 6 cm/s, a complete conversion is still reached above 800 ~ Fig. 6 shows the naphthalene conversion as a function of the reaction temperature and gas velocity with a 1 wt % nickel-modified filter disc. The blank disc was also used as a reference. As shown in Fig. 6, almost complete naphthalene cracking was achieved above 800 ~ with any gas velocity lower than 4 crn/s. Even with a gas velocity of 6 cm/s, the conversion still remains about 97 %. However, below 800 ~ the conversion of naphthalene significantly decreased as the reaction temperature decreased and the gas velocity increased. In addition, benzene was identified as one reaction product of naphthalene cracking at 750 and 800 ~ Table 1 lists the amount of benzene in the outlet gas after reaction over the nickel-modified filter disc. It is evident that for all gas velocities at 750 ~ benzene is present in the outlet gas with a significant amount. Above 800 ~
164 nearly no benzene was found in the outlet gas. Since no benzene formation was detected during the tests with the blank disc, it can be concluded that benzene was derived from the catalytic naphthalene cracking. Apparently, the nickel-activated filter disc displayed an excellent catalytic performance for the decomposition of either benzene or naphthalene as tar model compound in the simulated biomass gasification gas at a typical filtration gas velocity when the reaction temperature was in the range of 800-900 ~ Table 1 Yield of benzene as a reaction product of catalytic naphthalene cracking over a 1 wt % nickel-modified filter disc at different temperatures and gas velocities. Gas velocity Temperature (~ 2.5 cm/s 4 cm/s 6 cm/s 750 800 850 900 a Not detectable.
15.4 % ND a ND ND
15.7 % 2.1 % ND ND
100
i
100 /
i
,,
1 wt%Ni
r
A
12.3 % 1.6 % ND ND
I wt% Ni A
8o r
.o 0 >
8
~ 4
4 cm/s
I1 era
--D-- 2.5 cm/s
- o - - 2.5 cm/s
cm/s
--o-- 6 crn/s
--o-- 6 cm/s
40
blank dis4z
m
blank dis c
Z
20
20
0
0
'
70O
75O
80O
850
9O0
95
Fig. 5. Benzene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.
|
700
'
750
'
800
'
850
i
900
950
Fig. 6. Naphthalene conversion as a function of reaction temperature and gas velocity over a 1 wt% nickel-modified and blank filter disc.
4. CONCLUSIONS The urea method can be applied to obtain nickel-modified catalytic filter with a uniform spatial distribution of nickel. The study of urea method showed that a reaction time of at least 6 h for urea decomposition is necessary and that a higher urea/nickel molar ration than 1.7 that led to less fixation of nickel precipitation. A maximum fixation of 75 % of the
165 precursor was found for a reaction time of 24 h and an urea/nickel molar ratio of 1.7. Moreover, the complete conversion of tar model compounds, such as benzene or naphthalene in a simulated sulfur-free and dust-free biomass gasification gas over a nickelmodified filter disc was obtained at 800-900 ~ using typical filtration gas velocities. This demonstrated that it is potential to develop a nickel-activated catalytic filter for simultaneous tar and particle removal from biomass gasification gas. This is encouraging to further develop more improved catalysts by the urea method for the removal of more complex tar mixtures and for the tar removal from real biomass gasification gas. ACKNOWLEDGEMENT
This work was financed by Research in Brussels (grant No. RIB-96/32) and the EU 5th Framework Programme (Contract No. ENK5-2000-00305). REFERENCES
1. A. Cybulski and J.A. Moulijn, Structured Catalysts and Reactors, Marcel Dekker Inc.: New York, 1998. 2. S.R. Ness, G.E. Dunham, G.F. Weber and D.K. Ludlow, Environ. Prog., 14 (1995) 69. 3. G. Saracco and V. Specchia, Catal. Rev. Sci. Eng., 36 (1994) 305. 4. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 5. G. Saracco and V. Specchia, Chem. Eng. Sci., 55 (2000) 897. 6. J.N. Armor, Appl. Catal. A: General, 49 (1989) 1. 7. A.V. Bridgwater, Appl. Catal. A, 116 (1994) 5. 8. J.T. Richardson, Principles of Catalyst Development, Plenum Press, New York, 1989. 9. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, in B. Delmon, P. Grange, P. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, p. 166. 10. W.H.R. Shaw and J.J. Bordeaux, J. Am. Chem. Soc., 77 (1955) 4729. 11. H. Zhao, D.J. Draelants and G.V. Baron, Ind. Eng. Chem. Res., 39 (2000) 3195. 12. P. Simell, Catalytic hot gas cleaning of gasification gas. Espoo 1997, VTT Publications 330; Technical Research Center of Finland: Finland, 1997. 13. K.B. Mok, J.R.H. Ross and R.M. Sambrook, in: G. Poncelet, P. Grange, P. Jacobs (Eds.), Preparation of Catalysts III, Elsevier, Amsterdmn, 1983, p. 291. 14. X. Xu, H. Vonk, A. Cybulski and J.A. Moulijn, in G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs, P. Grange (Eds.), Preparation of Catalysts VI, Elsevier, Amsterdam, 1995, p. 1069. 15. H. Zhao, D.J. Draelants and G.V. Baron, Catal. Today, 56 (2000) 229.