- Email: [email protected]
CaO catalyst
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Hydrogen production by biomass gasification in a fluidizedbed reactor promoted by an Fe/CaO catalyst Bing-Shun Huang, Hsin-Yi Chen, Kui-Hao Chuang, Ren-Xuan Yang, Ming-Yen Wey* Department of Environmental Engineering, National Chung Hsing University, 250, Kuo Kuang Rd, Taichung 402, Taiwan, ROC
article info
abstract
Article history:
Biomass gasification for hydrogen production was performed in a continuous-feeding
Received 20 September 2011
fluidized-bed with the use of Fe/CaO catalysts. The relationship between catalyst proper-
Received in revised form
ties and biomass gasification efficiencies was studied. The findings indicated that only CaO
11 January 2012
was involved in the enhancement of char gasification, resulting in an increased hydrogen
Accepted 17 January 2012
production. However, CaO was also easily deactivated by biomass tar. The characterization
Available online 11 February 2012
results indicated that when CaO was impregnated with Fe, Ca2Fe2O5 formed on the surface of the support. Ca2Fe2O5 decomposed polyaromatic tar but was not effective in char gasi-
Keywords:
fication. The synergistic effects between Fe and CaO that effectively enhanced biomass
Biomass
gasification mainly involved combustion and pyrolysis, and the biomass gasification
Catalyst
products, i.e., char and tar, were further gasified, indicating that tailor-made Fe/CaO
Char
catalysts prevented CaO deactivation by tar, thus promoting biomass gasification and
Tar
hydrogen production.
Gasification
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen
1.
Introduction
Natural resources are being rapidly exhausted, resulting in the release of large amounts of pollutants. Hydrogen has been proposed as a potential energy vector owing to its cleanliness and high-energy yield. However, the predominant method of producing hydrogen from natural gas or other fossil fuel sources requires abundant energy and results in the emission of a significant amount of CO2 into the atmosphere [1]. Biomass energy, a type of renewable energy, is an abundant and environmentally friendly resource. Thus, the use of biomass as an energy source through thermochemical conversion technology is expected to play an important role in hydrogen production [2,3]. Increasing hydrogen production rates in biomass pyrolysis/gasification processes mainly involves (1) steam gasification, (2) reaction temperature, and (3) catalyst assistance.
reserved.
First, the introduction of steam effectively enhances the rate of hydrogen production but requires temperatures above 1073 K [4,5]. However, previous studies have reported that steam gasification of biomass occurs at mild temperatures (<1073 K) with the assistance of catalysts [6e10]. Second, increasing the reaction temperature promotes hydrogen production. In case the direct biomass gasification process cannot attain the desired temperature, more energy from an indirect heating source such as an electrical or fuel supply is necessary. Some studies have recently reported that the addition of a catalyst enhances reaction rates and increases hydrogen production rates under mild conditions [11,12]. For this reason, biomass pyrolysis/gasification with the assistance of a catalyst is a promising alternative for hydrogen-rich gas production. Asadullah et al. [13] used Rh-based catalysts at very mild temperatures (723e823 K) in biomass catalytic gasification.
* Corresponding author. Tel.: þ886 4 22852455; fax: þ886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.071
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The Rh/CeO2 catalyst showed excellent catalytic performance by increasing carbon conversion and H2 production. Chen et al. [14] reported the effect catalysts (e.g., FeO, Al2O3, MnO, Cr2O3, and CuO) have on catalytic biomass pyrolysis. Their results showed that the catalysts positively influenced hydrogen production and pyrolytic gas yields. For sawdust catalytic biomass pyrolysis, Cr2O3 and CaO showed higher gas yields than MnO, FeO, or Al2O3. However, the use of Cr2O3 in the gasification reaction is detrimental to the process because of the risk that Cr3þ from Cr2O3 may change to toxic Cr6þ, resulting in environmental pollution. Ohtsuka and Tomita [15] reported that gasification rates increased with increasing calcium loading. Steam gasification of coal was completed within 25 min at 973 K, which was 150 K lower than gasification with no catalysts. In addition, CaO was identified as an effective sorbent for CO2 capture [16,17]. Previous studies [18,19] reported on a thermodynamic equilibrium analysis of H2 production from biomass gasification by in situ CO2 capture. The results indicated that the H2 concentration was enhanced by the presence of CaO. In addition, CaO was an effective catalyst for the decomposition of tar species during biomass gasification [16]. However, CaO easily suffers deactivation under high concentrations of tar (>2 g/N m3) [20]. One of the main problems in biomass gasification is the formation of tar, which results in catalyst deactivation, operation interruption, and the production of carcinogenic elements [21,22]. Thus, alkali metal-based catalysts have been extensively used for hot-gas tar removal and enhanced hydrogen production during gasification reactions [23]. However, previous studies [24,25] indicated that alkali metalbased catalysts have lower activity for carbon conversion, regeneration difficulties, and easy deactivation because of particle agglomeration. Transition metal-based catalysts have recently shown potential in tar conversion, and they are cheaper than metals such as Pt, Ru, and Rh. Many research studies [26e29] have reported on the use of such catalysts for tar conversion, demonstrating their decomposition mechanism, catalytic effect, and reaction parameters. CaO is easily deactivated by biomass tar, resulting in the suppression of the CaO functionality during biomass gasification; therefore, the present study aimed to design a novel catalyst for preventing CaO deactivation by loading Fe on the CaO as a protection. The effect of the catalysts on biomass gasification and H2 production yield were studied using a continuous-feed fluidized-bed. By combining product analyses with the characterization results from X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray energy dispersive spectrometry (EDS), and the BrunauereEmmetteTeller (BET) method, the roles of Fe and CaO in the biomass gasification system were determined.
2.
Materials and methods
2.1.
Materials and catalyst preparation
Sawdust was used as a biomass feedstock in the biomass gasification experiments. The basic chemical compositions of the feedstock were analyzed using elemental analysis, as shown in Table 1. The sample was milled, sieved to a size
fraction of 500e600 mm (30e35 mesh), and packed into a cellulose capsule (size number ¼ 1, weight ¼ 0.71 g, length ¼ 19.2 mm, thickness ¼ 0.10 mm, Dah Feng Capsule Corporation). The Fe/CaO catalysts were prepared by impregnating CaO (99%, Hayashi Pure Chemical Industries) with an iron precursor. First, CaO was ground into powder and sieved to a size fraction of 500e600 mm. The powder was used as a support and mixed with an iron nitrate precursor (Fe(NO3)3H2O, purity 99%, Fluka RDH, Germany) in ethanol solution. The mixture was continuously stirred at 343 K until all the liquid had evaporated, after which the samples were calcined in air for 3 h at 773 K. The weight ratio of Fe/CaO was controlled to 10%, 15%, and 20% by the concentration of the iron nitrate solution used during impregnation. Because the distribution of the catalyst particle size was easily affected by the impregnationecalcination steps, the processing of these samples was controlled under the same conditions. The samples were denoted as X Fe/CaO catalysts, where X denotes the weight ratio of Fe/CaO.
2.2.
Characterization of Fe/CaO catalyst
The crystal structure of the Fe/CaO catalyst was determined by XRD (M18XHF, Mac Science Company). The morphology and elemental mapping of the catalysts were observed by an FE-SEM system (JEOL JSM-6700F scanning electron microscope) equipped with energy dispersive spectrometer (EDS) capability. The specific surface area and total pore volume of the catalysts were measured at 77 K by a nitrogen absorption apparatus (BET-201-AEL).
2.3.
Experimental apparatus and procedure
Biomass gasification was performed in a bubbling fluidizedbed reaction system; the setup is shown in Fig. 1. The reaction system was composed of a preheated chamber (30 cm long) and a main chamber (80 cm high) with an inner diameter of 2.5 cm and equipped with a stainless steel porous plate (15% open area) for distributing gas. The reaction system was electrically heated. The temperature of the reaction system was monitored using a K-type thermocouple and was adjusted using a proportionaleintegralederivative (PID) controller. Silica sand was employed as the bed material and sieved to a size fraction of 500e600 m by a 30e35 mesh (rp ¼ 2600 kg/m3; Si, 94.88%; Al, 1.62%; K, 1.62%; Ca, 1.66%; and Fe, 0.22%). Variable parameters included blank experiments, and the application of the catalyst partially mixed with the bed material (catalyst: silica sand ¼ 1:16). The reaction temperature was adjusted to 933 K, and the freeboard chamber was measured at around 1003 K. The gasifying agent, air, was supplied into the reactor. The gasification system was in an insufficient air atmosphere. The equivalence ratio (actual air flow/the stoichiometric air flow) was controlled to 0.62 by air flow supply. Biomass feeding rate was 3.9 g/min (0.161g/ pill 24 pill/min) and the stoichiometric air flow was calculated as 12.1 L/min, so the air flow supply was controlled at 7.5 L/min by a variable area rotameter (2-20 NL/min, FONG JEI COMPANY., LTD). When the system was heated to the desired
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Table 1 e Analysis data of biomass feedstock. Materials
Sawdust capsule
Elemental analysis (wt.%) C
H
O
N
Moisture
combustibles
Ash
46.72 47.52
5.99 7.28
46.92 44.56
0.36 e
6.28 4.50
93.16 95.50
0.57 e
gasification temperature, the capsules containing the biomass were dropped into the gasifier one at a time through a lock hopper for 50 min. After 8 min, the system stabilized and the gas products were initially sampled.
2.4.
Proximate analysis (wt.%)
Product analyses
The majority of the char particles from the combustion chamber were collected using a cyclone connected to bag filters. The bag filters were also used to collect elutriated particles from the incinerator. Small portions of char in the reactor were collected, and the total amount of char particles was calculated by weighing these two parts. Condensable products were collected in four trapping tubes and two bottles connected to the backup adsorber, and the temperature of the cooling system was controlled to 274 K. Non-condensable gases, which were mainly composed of H2, CO, CO2, CH4, and a small amount of polyaromatic tar, were conducted through a rotameter for calculating the flow rate; they were collected in Teflon sample bags for gas composition analysis. The main gas sample concentrations of H2, CO, CO2, and CH4 were analyzed by gas chromatography (GC, Clarus 500 with
a Carboxen 1000 column, Perkin Elmer) equipped with a thermal conductivity detector (TCD), and polyaromatic tar was determined by GC/mass spectroscopy (TRACE GC ultra with an Equity-1 column, Thermo Finnigan). Accurate quantitative analyses of polyaromatic tar were difficult to conduct using the existing gas chromatography equipment. The peak area of the GCeMS chromatogram was considered a good approximation of the various chemical compounds, thus enabling the calculation of the conversion rate.
3.
Results and discussion
Fig. 2 shows the XRD patterns of the catalysts with Fe/CaO ratios varying from 0 to 20 wt%. When Fe was loaded onto the CaO support, the phases of the resulting catalyst were mainly CaO and Ca2Fe2O5. The results indicate that Fe reacted with CaO to form a Ca2Fe2O5 phase (JCPD Card No. 02-0937). With increasing Fe content, the intensity of the CaO peak decreased, indicating that Ca2Fe2O5 had spread throughout the surface of the CaO support and gradually became the main phase of the catalyst.
Fig. 1 e Bubbling fluidized-bed reactor: (1) pump, (2) flow meter, (3) thermocouple, (4) PID controller, (5) feeder, (6) sand bed, (7) cyclone, (8) bag filter, (9) cooler, (10) trapping tube, (11) gas washing bottles, (12) backup adsorber, (13) GC/TCD, and (14) GC/MS.
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Fig. 2 e XRD patterns of CaO supports with various Fe loads.
Fig. 3 shows the FE-SEM images of the CaO catalysts with varying Fe loads, and the EDS mapping images of the Fe/CaO catalysts confirmed elemental Fe distribution. The inlay in Fig. 3(a) shows the pure CaO-supported material appearing as spherical particles when analyzed under high-resolution conditions. When Fe was loaded onto the CaO surface, the particle shapes transformed into squares, results that are ascribed to the formation of Ca2Fe2O5, as indicated by the XRD results. When the Fe loads increased from 0 to 20 wt%, the number of square structures that appeared increased, as shown in Fig. 3(b)e(d). When the Fe content was increased to 20 wt%, Ca2Fe2O5 spread over almost the entire CaO support. The elemental mapping images indicate that Fe elements
were well distributed in all the samples. The Fe element intensities increased with increasing Fe loading, spreading over almost the entire CaO support when the Fe load was 20 wt%; these findings are consistent with the FE-SEM images. Table 2 lists the BET specific surface areas and pore volumes of the catalysts with different Fe/CaO weight ratios. The specific surface areas and pore volumes of the catalysts showed no significant changes with respect to the Fe loads. The scheme of biomass gasification is as follows [13,30]. The biomass is first pyrolyzed to obtain gas products, tar, H2O, and char. The products are subsequently converted to gases and condensable hydrocarbons. Fig. 4 shows the weight ratios of char, condensable products, and non-condensable gases obtained from biomass catalytic gasification. The results indicate that biomass gasification in the presence of CaO causes the char fraction to decrease, which is in contrast to the reaction without catalyst, indicating that CaO enhances biomass gasification and produces more condensable products and less residual char. However, when Fe was loaded onto CaO, condensable products consisting mainly of tar and water decreased, especially at increasing Fe/CaO ratios from 0% to 15%. The decrease in the amount of condensable products can be ascribed to the fact that iron oxide is beneficial for tar conversion and wateregas shifting reactions [31]. The percentage of char produced also decreased with increasing Fe/CaO ratio in the range of 0%e15%. To determine whether the decrease in the amount of char was caused by Ca2Fe2O5, the Fe/CaO ratio was increased to 20%. The characterization results show that Ca2Fe2O5 spread over the entire support, rendering fewer CaO active sites available for biomass gasification. However, char gasification over Ca2Fe2O5 did not decrease, and the three-phase ratio
Fig. 3 e FE-SEM images showing the Fe elemental mapping of Fe/CaO catalysts with varying catalyst compositions: (a) CaO, (b) 10 FeCaO, (c) 15 FeCaO, and (d) 20 FeCaO.
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Table 2 e Characterizations of various catalyst samples. sample
CaO
10Fe/CaO
15Fe/CaO
20Fe/CaO
2
5 0.014
5 0.010
5 0.009
7 0.012
SBET (m /g) pore volume (cm3 g1)
remained the same as in non-catalytic gasification. Thus, the use of only Ca2Fe2O5 for biomass gasification is ineffective, and a synergistic effect between Fe and CaO in the catalyst must exist in order to enhance gasification. The results thus indicate that CaO plays an important role in biomass gasification. Fig. 5 shows the peak area of non-condensable hydrocarbons from the biomass gasification process; the analysis was performed by GC/MS. The results indicate that the products were mainly composed of 1,3-cyclopentadiene, benzene, toluene, ethyl benzene, p-xylene, and styrene. Biomass catalytic gasification using only CaO showed no significant polyaromatic tar decomposition compared with the noncatalytic process. Since CaO is a well-known catalyst for the decomposition of tar [16], the results indicate that CaO was possibly deactivated by biomass tar. When Fe was loaded onto the CaO surface, the peak area of polyaromatic tar gradually decreased, indicating that the iron oxide catalyst reacted with the biomass tar and that the tar was possibly converted to H2, CO, CO2, CH4, and other lighter hydrocarbons [31]. At a 20% Fe/CaO ratio, the main reactive sites shifted from CaO to Ca2Fe2O5, which spread over the entire support surface, but the amount of polyaromatic tar only partially decreased. At a 15% Fe/CaO ratio, a significant decomposition of tar was observed, and the conversions of 1,3-cyclopentadiene, benzene, toluene, ethyl benzene, and pxylene were found to be 11.3%, 41.1%, 22.3%, 35.6%, and 41.3%, respectively. These synergistic effects suggest that a suitable Fe loading on the CaO surface can prevent the suppression of the capabilities of CaO during biomass gasification and tar removal. An Fe/CaO ratio of 15% appears to be adequate for polyaromatic tar decomposition. The results thus indicate
Fig. 4 e Three-phase ratios of gasification products against different types of catalysts at 933 K.
Fig. 5 e Non-condensable tars produced from biomass catalytic gasification analyzed by GC/MS.
that iron oxide plays an important role in preventing CaO deactivation. Table 3 lists the main product yields and efficiencies of biomass gasification over the various catalysts used. The H2 production yield from biomass gasification increased in the presence of the CaO additive. According to the mass balance results (Fig. 4), a higher H2 production can be ascribed to the enhancement of biomass gasification by CaO owing to less char residual. However, the CO2 production yield should increase instead of decrease, as indicated by CaO capture according to the following equation [14,16,19]: CaO þ CO2 /CaCO3
(1)
The H2 production yield further increased when Fe was loaded onto the CaO surface in the Fe/CaO ratio range of 0%e 15%. According to the mass balance (Fig. 4) and the polyaromatic tar analysis (Fig. 5) results, the increase in H2 production was attributed to the enhancement in biomass gasification and biomass tar conversion brought about by the Fe/CaO catalyst. In particular, an Fe/CaO ratio of 15% effectively gasified the biomass and further converted tar and char, thereby forming more gas products. It appears that a suitable Fe load on the CaO surface can form Ca2Fe2O5, thus preventing the suppression of the capabilities of CaO during biomass gasification and tar conversion. Better biomass gasification generally results in more gas production, higher cold gas efficiency, and higher carbon conversion values. However, the carbon conversion value of biomass gasification does not increase because of the amount of CO2 captured by CaO when the Fe/CaO ratio was varied from 0% to 10%. This is in contrast with the Fe/CaO ratio of 15%, in which the carbon conversion of biomass gasification significantly increased, which can be ascribed to excellent biomass gasification. An Fe/CaO ratio of up to 20% resulted in the overspreading of Ca2Fe2O3 on the CaO surface, reducing the number of CaO active sites involved in the reaction and thus lowering biomass gasification and H2 production yields. As the main reactive site, Ca2Fe2O3 was not as effective in CO2 capture as CaO was. A previous study [26] indicated that Ca2Fe2O5 has less CO2-capturing ability, as confirmed by a CO2 sorption test. Their result indicated that
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Table 3 e Performance of various catalysts in the gasification of sawdust in fluidized-bed reactor. Catalyst
None CaO 10FeCaO 15FeCaO 20FeCaO
Yield of gases (mol/kg) H2
CO
CO2
CH4
8.1 10.4 12.4 13.8 10.1
17.5 19.5 24.5 23.0 20.1
26.5 24.4 19.1 34.2 25.8
5.6 5.3 5.7 7.0 5.7
C conversion (%)
Cold gas efficiency (%)
H2 promotion ratio (%)
75.0 74.2 74.3 97.0 78.0
42.4 45.5 53.6 57.8 47.1
e 28.4 53.1 70.4 24.7
Fig. 6 e Schematic diagram of the mechanism of biomass catalytic gasification over various catalysts: (a) CaO, (b) 10 Fe/CaO, (c) 15 Fe/CaO, and (d) 20 Fe/CaO.
the release of free Fe2O3, as shown in the following equation, rarely occurs: Ca2 Fe2 O5 þ 2CO2 /2CaCO3 þ Fe2 O3
(2)
Therefore, biomass gasification with the assistance of the 20% Fe/CaO catalyst shows higher carbon conversion values than biomass gasification with only the CaO additive. The H2 production yields were enhanced by biomass catalytic gasification. The main reactive sites, CaO and Ca2Fe2O5, increased the H2 promotion ratio to approximately 28.4% and 24.7%, respectively. However, at a ratio of 15%, the Fe/CaO exhibited synergistic effects, increasing the H2 promotion ratio by up to 70.4%. Further study needs to be conducted to clarify the mechanism of this synergistic effect. A schematic representation of catalytic gasification based on the experimental results is shown in Fig. 6. CaO effectively enhanced the gasification of biomass to gas products. However, the functions of CaO were subsequently suppressed by biomass tar. When Fe was loaded onto CaO, Ca2Fe2O5 formed; this material protected the support and prevented CaO deactivation by biomass tar. When the Fe/CaO ratio was increased up to 20%, overspreading of Ca2Fe2O3 occurred on CaO and a few CaO active sites were involved in the reaction. Although Ca2Fe2O3 can decompose tar, it has no significant
effect on biomass gasification; this catalytic property results in higher residual char contents and lower hydrogen production. However, at an Fe/CaO ratio of 15%, Ca2Fe2O5 exhibited optimal performance in preventing CaO deactivation by biomass tar. Therefore, the Fe/CaO catalyst can enhance both char gasification and tar conversion.
4.
Conclusions
Biomass catalytic gasification was performed in a continuousfeed fluidized-bed reactor. The Fe/CaO catalyst was fabricated via impregnation of iron oxide on CaO. The relationship between catalyst properties and biomass gasification efficiencies was investigated. Based on the results obtained, the following conclusions were drawn: (1) CaO was impregnated with Fe and a new compound, Ca2Fe2O5, was formed on the CaO surface. When the Fe/ CaO ratio was increased to 20%, Ca2Fe2O5 spread over the entire surface of CaO and replaced it as the main reactive site. (2) Only CaO was involved in biomass gasification, and a smaller amount of char was formed during gasification
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because of catalytic assistance. The H2 promotion ratio during CaO biomass catalytic gasification was enhanced by 28.4%, as compared to biomass catalytic gasification without catalysts. However, the functions of CaO for biomass gasification and tar conversion were easily suppressed by biomass tar. (3) As the main reactive sites, Ca2Fe2O5 particles decomposed polyaromatic tar; however, biomass gasification was not enhanced. The H2 promotion ratio of Ca2Fe2O5 was enhanced by 24.7%. (4) The synergistic effect between Fe and CaO effectively increased the H2 promotion ratio by approximately 70.4%, indicating that the Fe/CaO catalyst prevented the CaO deactivation by tar and enhanced biomass gasification. The current study investigated the performance in biomass gasification of a low-cost catalyst fabricated from Fe and CaO via simple impregnation. An effective approach to hydrogen-rich gas production was demonstrated, and the catalyst exhibited high potential in biomass-to-energy industrial applications.
Acknowledgments The authors would like to thank the National Science Council (NSC), Taiwan, R.O.C. for the financial support under Grant No. NSC 98-2221-E-005-014-MY3.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen production by biomass gasification in a fluidizedbed reactor promoted by an Fe/CaO catalyst Bing-Shun Huang, Hsin-Yi Chen, Kui-Hao Chuang, Ren-Xuan Yang, Ming-Yen Wey* Department of Environmental Engineering, National Chung Hsing University, 250, Kuo Kuang Rd, Taichung 402, Taiwan, ROC
article info
abstract
Article history:
Biomass gasification for hydrogen production was performed in a continuous-feeding
Received 20 September 2011
fluidized-bed with the use of Fe/CaO catalysts. The relationship between catalyst proper-
Received in revised form
ties and biomass gasification efficiencies was studied. The findings indicated that only CaO
11 January 2012
was involved in the enhancement of char gasification, resulting in an increased hydrogen
Accepted 17 January 2012
production. However, CaO was also easily deactivated by biomass tar. The characterization
Available online 11 February 2012
results indicated that when CaO was impregnated with Fe, Ca2Fe2O5 formed on the surface of the support. Ca2Fe2O5 decomposed polyaromatic tar but was not effective in char gasi-
Keywords:
fication. The synergistic effects between Fe and CaO that effectively enhanced biomass
Biomass
gasification mainly involved combustion and pyrolysis, and the biomass gasification
Catalyst
products, i.e., char and tar, were further gasified, indicating that tailor-made Fe/CaO
Char
catalysts prevented CaO deactivation by tar, thus promoting biomass gasification and
Tar
hydrogen production.
Gasification
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Hydrogen
1.
Introduction
Natural resources are being rapidly exhausted, resulting in the release of large amounts of pollutants. Hydrogen has been proposed as a potential energy vector owing to its cleanliness and high-energy yield. However, the predominant method of producing hydrogen from natural gas or other fossil fuel sources requires abundant energy and results in the emission of a significant amount of CO2 into the atmosphere [1]. Biomass energy, a type of renewable energy, is an abundant and environmentally friendly resource. Thus, the use of biomass as an energy source through thermochemical conversion technology is expected to play an important role in hydrogen production [2,3]. Increasing hydrogen production rates in biomass pyrolysis/gasification processes mainly involves (1) steam gasification, (2) reaction temperature, and (3) catalyst assistance.
reserved.
First, the introduction of steam effectively enhances the rate of hydrogen production but requires temperatures above 1073 K [4,5]. However, previous studies have reported that steam gasification of biomass occurs at mild temperatures (<1073 K) with the assistance of catalysts [6e10]. Second, increasing the reaction temperature promotes hydrogen production. In case the direct biomass gasification process cannot attain the desired temperature, more energy from an indirect heating source such as an electrical or fuel supply is necessary. Some studies have recently reported that the addition of a catalyst enhances reaction rates and increases hydrogen production rates under mild conditions [11,12]. For this reason, biomass pyrolysis/gasification with the assistance of a catalyst is a promising alternative for hydrogen-rich gas production. Asadullah et al. [13] used Rh-based catalysts at very mild temperatures (723e823 K) in biomass catalytic gasification.
* Corresponding author. Tel.: þ886 4 22852455; fax: þ886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.01.071
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The Rh/CeO2 catalyst showed excellent catalytic performance by increasing carbon conversion and H2 production. Chen et al. [14] reported the effect catalysts (e.g., FeO, Al2O3, MnO, Cr2O3, and CuO) have on catalytic biomass pyrolysis. Their results showed that the catalysts positively influenced hydrogen production and pyrolytic gas yields. For sawdust catalytic biomass pyrolysis, Cr2O3 and CaO showed higher gas yields than MnO, FeO, or Al2O3. However, the use of Cr2O3 in the gasification reaction is detrimental to the process because of the risk that Cr3þ from Cr2O3 may change to toxic Cr6þ, resulting in environmental pollution. Ohtsuka and Tomita [15] reported that gasification rates increased with increasing calcium loading. Steam gasification of coal was completed within 25 min at 973 K, which was 150 K lower than gasification with no catalysts. In addition, CaO was identified as an effective sorbent for CO2 capture [16,17]. Previous studies [18,19] reported on a thermodynamic equilibrium analysis of H2 production from biomass gasification by in situ CO2 capture. The results indicated that the H2 concentration was enhanced by the presence of CaO. In addition, CaO was an effective catalyst for the decomposition of tar species during biomass gasification [16]. However, CaO easily suffers deactivation under high concentrations of tar (>2 g/N m3) [20]. One of the main problems in biomass gasification is the formation of tar, which results in catalyst deactivation, operation interruption, and the production of carcinogenic elements [21,22]. Thus, alkali metal-based catalysts have been extensively used for hot-gas tar removal and enhanced hydrogen production during gasification reactions [23]. However, previous studies [24,25] indicated that alkali metalbased catalysts have lower activity for carbon conversion, regeneration difficulties, and easy deactivation because of particle agglomeration. Transition metal-based catalysts have recently shown potential in tar conversion, and they are cheaper than metals such as Pt, Ru, and Rh. Many research studies [26e29] have reported on the use of such catalysts for tar conversion, demonstrating their decomposition mechanism, catalytic effect, and reaction parameters. CaO is easily deactivated by biomass tar, resulting in the suppression of the CaO functionality during biomass gasification; therefore, the present study aimed to design a novel catalyst for preventing CaO deactivation by loading Fe on the CaO as a protection. The effect of the catalysts on biomass gasification and H2 production yield were studied using a continuous-feed fluidized-bed. By combining product analyses with the characterization results from X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray energy dispersive spectrometry (EDS), and the BrunauereEmmetteTeller (BET) method, the roles of Fe and CaO in the biomass gasification system were determined.
2.
Materials and methods
2.1.
Materials and catalyst preparation
Sawdust was used as a biomass feedstock in the biomass gasification experiments. The basic chemical compositions of the feedstock were analyzed using elemental analysis, as shown in Table 1. The sample was milled, sieved to a size
fraction of 500e600 mm (30e35 mesh), and packed into a cellulose capsule (size number ¼ 1, weight ¼ 0.71 g, length ¼ 19.2 mm, thickness ¼ 0.10 mm, Dah Feng Capsule Corporation). The Fe/CaO catalysts were prepared by impregnating CaO (99%, Hayashi Pure Chemical Industries) with an iron precursor. First, CaO was ground into powder and sieved to a size fraction of 500e600 mm. The powder was used as a support and mixed with an iron nitrate precursor (Fe(NO3)3H2O, purity 99%, Fluka RDH, Germany) in ethanol solution. The mixture was continuously stirred at 343 K until all the liquid had evaporated, after which the samples were calcined in air for 3 h at 773 K. The weight ratio of Fe/CaO was controlled to 10%, 15%, and 20% by the concentration of the iron nitrate solution used during impregnation. Because the distribution of the catalyst particle size was easily affected by the impregnationecalcination steps, the processing of these samples was controlled under the same conditions. The samples were denoted as X Fe/CaO catalysts, where X denotes the weight ratio of Fe/CaO.
2.2.
Characterization of Fe/CaO catalyst
The crystal structure of the Fe/CaO catalyst was determined by XRD (M18XHF, Mac Science Company). The morphology and elemental mapping of the catalysts were observed by an FE-SEM system (JEOL JSM-6700F scanning electron microscope) equipped with energy dispersive spectrometer (EDS) capability. The specific surface area and total pore volume of the catalysts were measured at 77 K by a nitrogen absorption apparatus (BET-201-AEL).
2.3.
Experimental apparatus and procedure
Biomass gasification was performed in a bubbling fluidizedbed reaction system; the setup is shown in Fig. 1. The reaction system was composed of a preheated chamber (30 cm long) and a main chamber (80 cm high) with an inner diameter of 2.5 cm and equipped with a stainless steel porous plate (15% open area) for distributing gas. The reaction system was electrically heated. The temperature of the reaction system was monitored using a K-type thermocouple and was adjusted using a proportionaleintegralederivative (PID) controller. Silica sand was employed as the bed material and sieved to a size fraction of 500e600 m by a 30e35 mesh (rp ¼ 2600 kg/m3; Si, 94.88%; Al, 1.62%; K, 1.62%; Ca, 1.66%; and Fe, 0.22%). Variable parameters included blank experiments, and the application of the catalyst partially mixed with the bed material (catalyst: silica sand ¼ 1:16). The reaction temperature was adjusted to 933 K, and the freeboard chamber was measured at around 1003 K. The gasifying agent, air, was supplied into the reactor. The gasification system was in an insufficient air atmosphere. The equivalence ratio (actual air flow/the stoichiometric air flow) was controlled to 0.62 by air flow supply. Biomass feeding rate was 3.9 g/min (0.161g/ pill 24 pill/min) and the stoichiometric air flow was calculated as 12.1 L/min, so the air flow supply was controlled at 7.5 L/min by a variable area rotameter (2-20 NL/min, FONG JEI COMPANY., LTD). When the system was heated to the desired
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Table 1 e Analysis data of biomass feedstock. Materials
Sawdust capsule
Elemental analysis (wt.%) C
H
O
N
Moisture
combustibles
Ash
46.72 47.52
5.99 7.28
46.92 44.56
0.36 e
6.28 4.50
93.16 95.50
0.57 e
gasification temperature, the capsules containing the biomass were dropped into the gasifier one at a time through a lock hopper for 50 min. After 8 min, the system stabilized and the gas products were initially sampled.
2.4.
Proximate analysis (wt.%)
Product analyses
The majority of the char particles from the combustion chamber were collected using a cyclone connected to bag filters. The bag filters were also used to collect elutriated particles from the incinerator. Small portions of char in the reactor were collected, and the total amount of char particles was calculated by weighing these two parts. Condensable products were collected in four trapping tubes and two bottles connected to the backup adsorber, and the temperature of the cooling system was controlled to 274 K. Non-condensable gases, which were mainly composed of H2, CO, CO2, CH4, and a small amount of polyaromatic tar, were conducted through a rotameter for calculating the flow rate; they were collected in Teflon sample bags for gas composition analysis. The main gas sample concentrations of H2, CO, CO2, and CH4 were analyzed by gas chromatography (GC, Clarus 500 with
a Carboxen 1000 column, Perkin Elmer) equipped with a thermal conductivity detector (TCD), and polyaromatic tar was determined by GC/mass spectroscopy (TRACE GC ultra with an Equity-1 column, Thermo Finnigan). Accurate quantitative analyses of polyaromatic tar were difficult to conduct using the existing gas chromatography equipment. The peak area of the GCeMS chromatogram was considered a good approximation of the various chemical compounds, thus enabling the calculation of the conversion rate.
3.
Results and discussion
Fig. 2 shows the XRD patterns of the catalysts with Fe/CaO ratios varying from 0 to 20 wt%. When Fe was loaded onto the CaO support, the phases of the resulting catalyst were mainly CaO and Ca2Fe2O5. The results indicate that Fe reacted with CaO to form a Ca2Fe2O5 phase (JCPD Card No. 02-0937). With increasing Fe content, the intensity of the CaO peak decreased, indicating that Ca2Fe2O5 had spread throughout the surface of the CaO support and gradually became the main phase of the catalyst.
Fig. 1 e Bubbling fluidized-bed reactor: (1) pump, (2) flow meter, (3) thermocouple, (4) PID controller, (5) feeder, (6) sand bed, (7) cyclone, (8) bag filter, (9) cooler, (10) trapping tube, (11) gas washing bottles, (12) backup adsorber, (13) GC/TCD, and (14) GC/MS.
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Fig. 2 e XRD patterns of CaO supports with various Fe loads.
Fig. 3 shows the FE-SEM images of the CaO catalysts with varying Fe loads, and the EDS mapping images of the Fe/CaO catalysts confirmed elemental Fe distribution. The inlay in Fig. 3(a) shows the pure CaO-supported material appearing as spherical particles when analyzed under high-resolution conditions. When Fe was loaded onto the CaO surface, the particle shapes transformed into squares, results that are ascribed to the formation of Ca2Fe2O5, as indicated by the XRD results. When the Fe loads increased from 0 to 20 wt%, the number of square structures that appeared increased, as shown in Fig. 3(b)e(d). When the Fe content was increased to 20 wt%, Ca2Fe2O5 spread over almost the entire CaO support. The elemental mapping images indicate that Fe elements
were well distributed in all the samples. The Fe element intensities increased with increasing Fe loading, spreading over almost the entire CaO support when the Fe load was 20 wt%; these findings are consistent with the FE-SEM images. Table 2 lists the BET specific surface areas and pore volumes of the catalysts with different Fe/CaO weight ratios. The specific surface areas and pore volumes of the catalysts showed no significant changes with respect to the Fe loads. The scheme of biomass gasification is as follows [13,30]. The biomass is first pyrolyzed to obtain gas products, tar, H2O, and char. The products are subsequently converted to gases and condensable hydrocarbons. Fig. 4 shows the weight ratios of char, condensable products, and non-condensable gases obtained from biomass catalytic gasification. The results indicate that biomass gasification in the presence of CaO causes the char fraction to decrease, which is in contrast to the reaction without catalyst, indicating that CaO enhances biomass gasification and produces more condensable products and less residual char. However, when Fe was loaded onto CaO, condensable products consisting mainly of tar and water decreased, especially at increasing Fe/CaO ratios from 0% to 15%. The decrease in the amount of condensable products can be ascribed to the fact that iron oxide is beneficial for tar conversion and wateregas shifting reactions [31]. The percentage of char produced also decreased with increasing Fe/CaO ratio in the range of 0%e15%. To determine whether the decrease in the amount of char was caused by Ca2Fe2O5, the Fe/CaO ratio was increased to 20%. The characterization results show that Ca2Fe2O5 spread over the entire support, rendering fewer CaO active sites available for biomass gasification. However, char gasification over Ca2Fe2O5 did not decrease, and the three-phase ratio
Fig. 3 e FE-SEM images showing the Fe elemental mapping of Fe/CaO catalysts with varying catalyst compositions: (a) CaO, (b) 10 FeCaO, (c) 15 FeCaO, and (d) 20 FeCaO.
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Table 2 e Characterizations of various catalyst samples. sample
CaO
10Fe/CaO
15Fe/CaO
20Fe/CaO
2
5 0.014
5 0.010
5 0.009
7 0.012
SBET (m /g) pore volume (cm3 g1)
remained the same as in non-catalytic gasification. Thus, the use of only Ca2Fe2O5 for biomass gasification is ineffective, and a synergistic effect between Fe and CaO in the catalyst must exist in order to enhance gasification. The results thus indicate that CaO plays an important role in biomass gasification. Fig. 5 shows the peak area of non-condensable hydrocarbons from the biomass gasification process; the analysis was performed by GC/MS. The results indicate that the products were mainly composed of 1,3-cyclopentadiene, benzene, toluene, ethyl benzene, p-xylene, and styrene. Biomass catalytic gasification using only CaO showed no significant polyaromatic tar decomposition compared with the noncatalytic process. Since CaO is a well-known catalyst for the decomposition of tar [16], the results indicate that CaO was possibly deactivated by biomass tar. When Fe was loaded onto the CaO surface, the peak area of polyaromatic tar gradually decreased, indicating that the iron oxide catalyst reacted with the biomass tar and that the tar was possibly converted to H2, CO, CO2, CH4, and other lighter hydrocarbons [31]. At a 20% Fe/CaO ratio, the main reactive sites shifted from CaO to Ca2Fe2O5, which spread over the entire support surface, but the amount of polyaromatic tar only partially decreased. At a 15% Fe/CaO ratio, a significant decomposition of tar was observed, and the conversions of 1,3-cyclopentadiene, benzene, toluene, ethyl benzene, and pxylene were found to be 11.3%, 41.1%, 22.3%, 35.6%, and 41.3%, respectively. These synergistic effects suggest that a suitable Fe loading on the CaO surface can prevent the suppression of the capabilities of CaO during biomass gasification and tar removal. An Fe/CaO ratio of 15% appears to be adequate for polyaromatic tar decomposition. The results thus indicate
Fig. 4 e Three-phase ratios of gasification products against different types of catalysts at 933 K.
Fig. 5 e Non-condensable tars produced from biomass catalytic gasification analyzed by GC/MS.
that iron oxide plays an important role in preventing CaO deactivation. Table 3 lists the main product yields and efficiencies of biomass gasification over the various catalysts used. The H2 production yield from biomass gasification increased in the presence of the CaO additive. According to the mass balance results (Fig. 4), a higher H2 production can be ascribed to the enhancement of biomass gasification by CaO owing to less char residual. However, the CO2 production yield should increase instead of decrease, as indicated by CaO capture according to the following equation [14,16,19]: CaO þ CO2 /CaCO3
(1)
The H2 production yield further increased when Fe was loaded onto the CaO surface in the Fe/CaO ratio range of 0%e 15%. According to the mass balance (Fig. 4) and the polyaromatic tar analysis (Fig. 5) results, the increase in H2 production was attributed to the enhancement in biomass gasification and biomass tar conversion brought about by the Fe/CaO catalyst. In particular, an Fe/CaO ratio of 15% effectively gasified the biomass and further converted tar and char, thereby forming more gas products. It appears that a suitable Fe load on the CaO surface can form Ca2Fe2O5, thus preventing the suppression of the capabilities of CaO during biomass gasification and tar conversion. Better biomass gasification generally results in more gas production, higher cold gas efficiency, and higher carbon conversion values. However, the carbon conversion value of biomass gasification does not increase because of the amount of CO2 captured by CaO when the Fe/CaO ratio was varied from 0% to 10%. This is in contrast with the Fe/CaO ratio of 15%, in which the carbon conversion of biomass gasification significantly increased, which can be ascribed to excellent biomass gasification. An Fe/CaO ratio of up to 20% resulted in the overspreading of Ca2Fe2O3 on the CaO surface, reducing the number of CaO active sites involved in the reaction and thus lowering biomass gasification and H2 production yields. As the main reactive site, Ca2Fe2O3 was not as effective in CO2 capture as CaO was. A previous study [26] indicated that Ca2Fe2O5 has less CO2-capturing ability, as confirmed by a CO2 sorption test. Their result indicated that
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Table 3 e Performance of various catalysts in the gasification of sawdust in fluidized-bed reactor. Catalyst
None CaO 10FeCaO 15FeCaO 20FeCaO
Yield of gases (mol/kg) H2
CO
CO2
CH4
8.1 10.4 12.4 13.8 10.1
17.5 19.5 24.5 23.0 20.1
26.5 24.4 19.1 34.2 25.8
5.6 5.3 5.7 7.0 5.7
C conversion (%)
Cold gas efficiency (%)
H2 promotion ratio (%)
75.0 74.2 74.3 97.0 78.0
42.4 45.5 53.6 57.8 47.1
e 28.4 53.1 70.4 24.7
Fig. 6 e Schematic diagram of the mechanism of biomass catalytic gasification over various catalysts: (a) CaO, (b) 10 Fe/CaO, (c) 15 Fe/CaO, and (d) 20 Fe/CaO.
the release of free Fe2O3, as shown in the following equation, rarely occurs: Ca2 Fe2 O5 þ 2CO2 /2CaCO3 þ Fe2 O3
(2)
Therefore, biomass gasification with the assistance of the 20% Fe/CaO catalyst shows higher carbon conversion values than biomass gasification with only the CaO additive. The H2 production yields were enhanced by biomass catalytic gasification. The main reactive sites, CaO and Ca2Fe2O5, increased the H2 promotion ratio to approximately 28.4% and 24.7%, respectively. However, at a ratio of 15%, the Fe/CaO exhibited synergistic effects, increasing the H2 promotion ratio by up to 70.4%. Further study needs to be conducted to clarify the mechanism of this synergistic effect. A schematic representation of catalytic gasification based on the experimental results is shown in Fig. 6. CaO effectively enhanced the gasification of biomass to gas products. However, the functions of CaO were subsequently suppressed by biomass tar. When Fe was loaded onto CaO, Ca2Fe2O5 formed; this material protected the support and prevented CaO deactivation by biomass tar. When the Fe/CaO ratio was increased up to 20%, overspreading of Ca2Fe2O3 occurred on CaO and a few CaO active sites were involved in the reaction. Although Ca2Fe2O3 can decompose tar, it has no significant
effect on biomass gasification; this catalytic property results in higher residual char contents and lower hydrogen production. However, at an Fe/CaO ratio of 15%, Ca2Fe2O5 exhibited optimal performance in preventing CaO deactivation by biomass tar. Therefore, the Fe/CaO catalyst can enhance both char gasification and tar conversion.
4.
Conclusions
Biomass catalytic gasification was performed in a continuousfeed fluidized-bed reactor. The Fe/CaO catalyst was fabricated via impregnation of iron oxide on CaO. The relationship between catalyst properties and biomass gasification efficiencies was investigated. Based on the results obtained, the following conclusions were drawn: (1) CaO was impregnated with Fe and a new compound, Ca2Fe2O5, was formed on the CaO surface. When the Fe/ CaO ratio was increased to 20%, Ca2Fe2O5 spread over the entire surface of CaO and replaced it as the main reactive site. (2) Only CaO was involved in biomass gasification, and a smaller amount of char was formed during gasification
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because of catalytic assistance. The H2 promotion ratio during CaO biomass catalytic gasification was enhanced by 28.4%, as compared to biomass catalytic gasification without catalysts. However, the functions of CaO for biomass gasification and tar conversion were easily suppressed by biomass tar. (3) As the main reactive sites, Ca2Fe2O5 particles decomposed polyaromatic tar; however, biomass gasification was not enhanced. The H2 promotion ratio of Ca2Fe2O5 was enhanced by 24.7%. (4) The synergistic effect between Fe and CaO effectively increased the H2 promotion ratio by approximately 70.4%, indicating that the Fe/CaO catalyst prevented the CaO deactivation by tar and enhanced biomass gasification. The current study investigated the performance in biomass gasification of a low-cost catalyst fabricated from Fe and CaO via simple impregnation. An effective approach to hydrogen-rich gas production was demonstrated, and the catalyst exhibited high potential in biomass-to-energy industrial applications.
Acknowledgments The authors would like to thank the National Science Council (NSC), Taiwan, R.O.C. for the financial support under Grant No. NSC 98-2221-E-005-014-MY3.
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