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Hydrogen production from catalytic steam reforming of benzene as tar model compound of biomass gasification
Fuel Processing Technology 148 (2016) 380–387
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Research article
Hydrogen production from catalytic steam reforming of benzene as tar model compound of biomass gasification Ningbo Gao a,b,⁎,1, Xiao Wang b,1, Aimin Li b, Chunfei Wu c, Zhifan Yin b a b c
School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China School of Engineering, University of Hull, Hull HU6 7RX, UK
a r t i c l e
i n f o
Article history: Received 25 January 2016 Received in revised form 15 March 2016 Accepted 18 March 2016 Available online 2 April 2016 Keywords: Hydrogen production Catalyst Steam reforming Benzene Tar
a b s t r a c t Tar reduction is an important issue for the development of biomass gasification process. In this work, a NiO/ceramic foam catalyst was developed and studied for catalytic steam reforming of tar model compound (benzene) using a fixed-bed reactor. Different reaction temperatures, equivalent ratios (ER), and steam/carbon (S/C) molar ratios were investigated with a space velocity of 5.6 h−1. The introduction of the NiO/ceramic foam catalyst showed excellent production of hydrogen and carbon conversion. With the increase of reaction temperature from 700 to 900 °C, the yield of hydrogen increased from 140.67 to 182.06 (g H2 kg−1 benzene). The increase of ER resulted in the decrease of the H2 yield. A stability test (including regeneration of reacted catalyst) showed that the catalyst was deactivated by the deposition of carbons (confirmed from scanning electron microscopy), which could be removed using air oxidation at 750 °C. The catalytic activity of the catalyst in relation to the hydrogen production could be regained after the regeneration process. A kinetic model study of the process showed that the apparent activation energy and the pre-exponential factor were 73.38 kJ/mol and 1.18 × 105 (m3 kg−1catalyst h−1), respectively. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tar, as a mixture of condensable organic components in gasification process, consists of aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene and phenol [1,2]. The condensation of tar at low temperature causes several technical problems such as coking catalysts, fouling, and choking downstream pipelines and equipment. These problems result in a reduction of total process efficiency and an increase of the costs of equipment management and maintenance [3–6]. Therefore, it is essential to find an efficient method to converting these tarry materials into valuable products. Catalytic steam reforming is an attractive technique for tar destruction since it produces hydrogen and carbon monoxide, which are high-value gas products [2,6,7]. Until now, several metal catalysts, including Fe [8,9], Ni [10–13], Co [11,14,15], Pt [5], Pd [16] or combinations of metals [7,17] have been studied by a large amount of publications, dedicating to develop efficient catalysts for tar steam reforming. In particular, Ni-based catalyst has been extensively investigated for tar conversion due to its wide availability, low cost and effective catalytic reactivity in cracking aromatic hydrocarbons [6,18–20]. ⁎ Corresponding author at: School of Energy and Power Engineering, Xi’'an Jiaotong University, Xi’'an 710049, China. E-mail address: [email protected] (N. Gao). 1 The first two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.fuproc.2016.03.019 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Tomishige and Li developed several transition metal catalysts such as Rh, Ni, and Co and compound metal catalyst for systemic biomass tar and model compounds reforming. It is found that the Ni/Mg/Al, Ni-Cu/ Mg/Al and Co/Mg/Al catalysts exhibited much higher activity and resistance to coke deposition [21,22]. Alumina (Al2O3) has been used as a support of Ni-based catalyst by many studies [11,23]. However, Ni/ Al2O3 catalyst is known to be rapidly deactivated by carbon deposition on the surface of the catalyst, which can block the active sites, and result in the deactivation of catalyst [2,24,25]. Natural minerals have been used as supports such as dolomites and olivine for Ni-based catalysts [26]. However, due to their low specific surface area and easy abrasion at high temperature, tar reduction using catalysts with dolomite or olivine support is not efficient enough for the downstream applications of the produced gas. Ceramic foam has spherical-like cells connected to each other through openings or windows, and high porosities (85–90%), making it an attractive catalyst support [27]. It is noted that there are few reports of catalytic reforming of biomass gasification tar using Ni-based catalyst with ceramic foam as catalyst support. Since the ceramic foam has large porosity, high temperature stability, very low operation pressure drop and large surface area, it is worthwhile to investigate the catalytic performance of NiO/ceramic foam catalyst for steam reforming of biomass gasification tar compounds. The catalytic steam reforming of real biomass tar over NiO/ceramic foam catalyst was investigated in our previous studies [28]. Due to the complicated components of real biomass tar, it is difficult to perform
N. Gao et al. / Fuel Processing Technology 148 (2016) 380–387
mechanism studies. To better understand the reaction behaviors, model compound, such as phenol [29,30], benzene [31,32], toluene [6,25] or naphthalene [24,33] has been used for fundamental studies of catalytic tar steam reforming. Benzene was chosen as a tar model compound because it is a main constituent of high-temperature gasification tar, as well as it represents a stable aromatic structure in polyaromatics [28]. In addition, Josuinkas et al. [34]investigated the steam reforming of three tar model compounds, including benzene, toluene and naphthalene with nickel catalysts and it was reported that benzene is more reactive over a wide temperature range compared to toluene or naphthalene. In this study, the steam reforming of tar using benzene as a model compound was carried out over a NiO/ceramic foam catalyst in a fixed bed reaction system. Various process conditions including reaction temperature, equivalent ratio (ER) and steam/carbon (S/C) molar ratio were studied. Furthermore, particular attention was paid to the catalyst stability and reusability by carrying out a stability test (4 h) including a catalyst regeneration step, in relation to hydrogen production and carbon formation on the surface of the reacted catalysts. 2. Materials and methods 2.1. Catalyst preparation and characterization Nickel nitrate hexahydrate (Ni (NO3)2·6H2O) were purchased from Tianjin Kermel Company, China. Ceramic foam (Beisi New Materials Company, China) is a cylinder with diameters of 38 mm and lengths of 50 mm. The chemical compositions of ceramic foam are 79.24% Al2O3, 19.29% P2O5, 0.77% SiO2, which have been reported in our previous study [28]. Prior to the catalyst development, the ceramic foam was washed using hydrochloric acid solution (0.1 mol L−1) in order to eliminate the effect of the impurities on the experiments, and then dried at 110 °C for 3 h. An impregnation method was used to prepare the NiO/ceramic foam catalyst. Initially, certain amount of Ni (NO3)2·6H2O was dissolved completely into the deionized water to form a 1 mol L−1 of solution. The pre-treated ceramic foam was soaked into the aqueous solution for 1 h. Then, the Ni2 +/ceramic foam was dried in an oven at 105 °C for 12 h and calcined subsequently in a muffle furnace at 900 °C for 3 h at a heating rate of 7.5 °C min−1. The average amount of NiO loaded on the ceramic foam after calcination was around 3.50% ± 0.30% by weight. The morphology of the blank ceramic foam, the NiO/ceramic foam catalyst and the reacted catalyst were characterized using a Hitachi S-4800 scanning electron micrographs (SEM). A FEI Tecnai
381
G220 S-Twin transmission electron microscope (TEM) was used to observe the size and morphology of the reacted catalysts. 2.2. Reaction system and procedures The reaction system consists of a feeding system, a reactor, a product collector and a gas analysis system as shown in Fig. 1. Two double channel micro-infusion pumps (Smith Medical, Model WZS-50F6) were used to feed the pure benzene and water into the reactor, respectively. Mass flow meters (Sevenstar, Model D07-7B) were employed to provide accurate flows of nitrogen and air to the reaction system (air was used for providing oxygen to adjust the ER value and the regeneration of the reacted catalyst). The fixed-bed reactor is made of stainless steel (40 mm i.d. × 500 mm length). The reactor was heated by a tubular furnace (1.5 kW, 220 V), which was connected with a temperature controller (Yudian, Model AI-518P). The NiO/ceramic foam catalyst was placed in the middle of the reactor. The temperature of the reactor was measured by a K-type thermocouple with a precision of 1 °C. The products at the outlet of the reactor were recovered by a glass condenser and subsequently collected in a Tedlar gas bag for analyzing off-line by a gas chromatograph (Techcomp, Model 7890II), which is coupled with a thermal conductivity detector (TCD) to obtain the concentrations of H2, O2, N2, CO, CH4 and CO2. During the experiment, about 27 g ± 2 g catalyst was used. The whole system was guaranteed as inert using N2 as carrier gas with a flow rate of 100 mL min−1. Benzene was injected into the reactor at a feed flow rate of 5.3 g h−1, which is equivalent to a weight hourly space velocity (WHSV) of 5.6 h− 1. In this work, oxygen equivalent ratio (0.0–0.4), steam/carbon molar ratio (0.0–3.0), reaction temperature (700–900 °C) were investigated. Each test lasted 30 min. Experiments were repeated to ensure the reliability of the results. 2.3. Data analysis method H2 (CO, CO2 or CH4) concentration, H2 yield, Carbon conversion, H2/ CO and CO/CO2 molar ratio were calculated using the following Eqs. (1)–(5): H2 ðCO; CO2 or CH4 Þ concentration ð%Þ ¼
H2 ðCO; CO2 ; or CH4 Þ concentration in the product by GC The sum of the concentrations of H2 ; CO; CO2 and CH4 in the product by GC
ð1Þ
Fig. 1. The schematic diagram of a lab-scale catalytic steam reforming apparatus: (1) Air compressor; (2) Micro-infusion pump for benzene; (3) Micro-infusion pump for water; (4) Mass flow meter; (5) Nitrogen cylinder; (6) Tubular furnace; (7) Fixed-bed reactor; (8) NiO/ceramic foam catalyst; (9) Temperature controller; (10) K-type thermocouple; (11) Condenser. (12) Wet type gas meter; (13) Gas bag.
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Mass of hydrogen product −1 H2 yield g H2 kg benzene ¼ Mass of benzene fed Carbon conversion ð%Þ ¼
Moles of carbon in the gas product Moles of carbon fed 100
Moles of hydrogen product H2 =CO molar ratio ¼ Moles of carbon monoxide product CO=CO2 molar ratio ¼
Moles of carbon monoxide product : Moles of carbon dioxide product
ð2Þ
ð3Þ ð4Þ
Table 1 Experimental results during the catalytic reforming reaction of benzene with the ceramic foam (control experiment) and the NiO/ceramic foam catalyst; Catalytic reforming conditions: temperature was 750 °C; S/C molar ratio was 1.0; ER molar ratio was 0.1; and WHSV was 5.6 h−1. Catalyst −1
H2 yield (g H2 kg benzene) Carbon conversion (%) H2/CO molar ratio
Ceramic foam
NiO/ceramic foam
22.38 44.15 0.67
177.62 77.03 2.21
ð5Þ
3. Results and discussion 3.1. Comparison to control experiment without NiO loading The ceramic foam has metal impurities such as Fe2O3 and Na2O. In order to eliminate the effect of these impurities, control experiment using only ceramic foam support without NiO loading was carried out for benzene steam reforming at 750 °C, S/C molar ratio of 1.0, ER of 0.1 and WHSV of 5.6 h− 1. As shown in Fig. 2, the major components of the produced gases were H2, CO, CO2 and CH4. When the NiO/ceramic foam catalyst was used, H2 formation remarkably increased from 24.78 to 59.98% and CH4 concentration slightly increased from 0.72 to 0.96%, and CO and CO2 decreased from 36.93 to 27.15% and 37.57 to 11.91%, respectively, comparing to the control experiment. H2 yield, carbon conversion and H2/CO molar ratio from the experiments with and without NiO loading are shown in Table 1. H2 yield and carbon conversion were much higher in the presence of the NiO/ceramic foam compared to the control experiment. It can be explained that benzene and steam were adsorbed dissociatively on the surface of the NiO/ceramic foam resulting in the promoting of the catalytic reforming reactions. Therefore the high activity of the NiO/ceramic foam during benzene reforming process could be attributed to the excellent ability of oxygen transfer of the catalyst [32]. In addition, the ceramic foam support plays a significant role in increasing steam adsorption and surface carbon gasification rates during steam reforming reaction [12]. 3.2. Influences of process parameters Fig. 3 illustrated the distribution of main product gases (H2, CO, CO2 and CH4) and H2 yield with the increase of the reaction temperature
Fig. 2. Gas concentrations of the catalytic reforming reaction of benzene with the ceramic foam support and the NiO/ceramic foam catalyst; Catalytic reforming conditions: temperature was 750 °C; S/C molar ratio was 1.0; ER molar ratio was 0.1; and WHSV was 5.6 h−1.
from 700 to 900 °C, when the S/C molar ratio was 1.0 and the oxygen equivalent ratio (ER) was 0.1 during the catalytic reforming of benzene. The catalyst exhibited an excellent catalytic activity showing that H2 and CO was the dominant produced gases. The concentrations of H2 and CO2 decreased slightly from 62.69 to 55.75% and 15.15 to 4.76%, respectively, when the reaction temperature was increased from 700 to 900 °C. However, carbon monoxide showed an opposite trend that its concentration increased from 20.81 to 39.28%. It is indicated that benzene was favor to be decomposed at high temperature. It is reported that water-gas shift reaction (WGSR) (Reaction (7)) and steam reforming reactions were promoted when the reaction temperature was higher than 700 °C [35]. Park et al. [32] found that an appropriate benzene conversion temperature was above 700 °C, which was efficient for tar reduction using a Ni-based catalyst. High temperature facilitates endothermic reactions; however, at too high reaction temperature, it is easy to form carbidic carbon, which is a precursor of graphitic carbon resulting in the deactivation of catalyst. Furthermore, partial oxidation of benzene (Reaction (6)) and WGSR (Reaction (7)) are inhibited with the increase of reaction temperature due to both reactions are exothermic. Partialoxidization : 2C6 H6 þ 9O2 ¼ 6H2 O þ 12CO; ΔH ¼ −3267:5kJ=mol
ð6Þ
Water gasshift : CO þ H2 O ¼ CO2 þ H2 ; ΔH ¼ −41kJ=mol:
ð7Þ
Thus, hydrogen concentration was slightly reduced with the increase of temperature from 700 to 900 °C, as shown in Fig. 3. In addition, it was noted that because of high heat capacity and large surface area of the ceramic foam, the temperature of the reaction field could be
Fig. 3. Effect of reaction temperature on the distribution of main product gases and H2 yields; Catalytic reforming conditions: temperature increased from 700 to 900 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
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maintained in a stable condition, which could avoid the fluctuation of the reaction temperature during catalytic reactions [28]. Carbon conversion, H2/CO and CO/CO2 molar ratios from the experiments with different reaction temperatures were shown in Table 2. It is indicated that the carbon conversion of benzene was significantly increased from 54.43 to 93.92% when the temperature was increased from 700 to 900 °C. The hydrogen yield and carbon conversion reached maximum values of 182.06 (g H2 kg−1 benzene) and 93.92% at 900 °C, respectively. However, the H2/CO molar ratio was decreased with the increase of reaction temperature, due to the increase of the concentration of CO when the reaction temperature was high than 750 °C. These results are consistent with the data reported by Josuinkas et al. [34], who investigated steam reforming of benzene and toluene with a Ni catalyst. They reported that the concentrations of small molar gases were significantly affected by reaction temperature. The equivalent ratio (ER) is one of the key factors in the process of tar reforming. In this work, the effect of ER on product distribution and H2 yield was investigated by varying ER ratios from 0.0 to 0.4 by adjusting the flow rate of air. As shown in Fig. 4, the concentration of H2 declined dramatically from 71.11 to 42.81% when the ER was increased from 0.0 to 0.4, and a decrease of the concentration of CH4 was also observed with the increase of the ER. However, CO2 concentration was increased from 8.89 to 28.65% when the ER was increased from 0.0 to 0.4. The content of CO showed a maximum value of 32.17% at ER of 0.3. Due to the promoted combustion reactions with the increase of the ER, H2 content declined significantly. For example, the H2 yield decreased dramatically from 179.70 to 77.39 (g H2 kg−1 benzene) when the ER was increased from 0.0 to 0.4. As shown by Reactions (8) and (9), the formations of CO, H2O and CO2 were favored at high ER ratio due to the promoting of the oxidization of benzene, CO and CH4. In addition, H2 can be easily consumed with the increase of oxygen content in the process. It is noted that the reactor temperature can be increased with the increase of ER due to the exothermic reactions. However, higher ER leads to the promotion of CO oxidation, resulting in the decrease of CO concentration when the ER was increased to 0.4 (Fig. 4). Dryreforming : C6 H6 þ 6CO2 ¼ 12CO þ 3H2 ; ΔH ¼ þ953:6kJ=mol
ð8Þ
Boudouard : CO2 þ C ¼ 2CO; ΔH ¼ þ172kJ=mol:
ð9Þ
When the ER was changed from 0.0 to 0.4, the carbon conversion increased from 47.44 to 67.21%. Carbon conversion reached a maximum of 79.08% when the ER was 0.2. The variation in product with varying ER ratios at constant temperature is shown in Table 3. This is because that the increased ER caused more H2O content in the product stream. The catalytic effect of S/C molar ratio on product gas composition was investigated for the reforming of tar model compound, when the steam reforming temperature was 750 °C and the ER was 0.1. As shown in Fig. 5, the S/C molar ratio has significant influence on the catalytic activity. For example, the content of CO decreased from 34.03 to 19.83%, while CO2 increased from 3.27 to 20.09%, when the S/C molar ratio was increased from 0.0 to 3.0, however, the concentration of hydrogen was stabilized around 59%. The yield of H2 increased from
Table 2 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios at different reaction temperatures; Catalytic reforming conditions: temperature increased from 700 to 900 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
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Fig. 4. Effect of oxygen equivalent ratio on the distribution of main product gases and H2 yields; Catalytic reforming conditions: ER increased from 0.0 to 0.4; temperature was 750 °C; S/C molar ratio was 1.0; and WHSV was 5.6 h−1.
102.38 to 192.36 (g H2 per kg benzene) when the S/C molar ratio was increased from 0.0 to 1.5. The hydrogen yield was kept stable with the further increase of S/C molar ratio to 3.0. It is suggested that the increasing S/C molar ratio leads to the promotions of Reactions (10) and (7) favoring the production of H2. Although excess water content promotes the production of hydrogen, too high S/C molar ratio is not encouraged in industrialization process, due to the sintering possibility of the active metal sites on the catalyst, the high energy inputs for steam generation and the associated cost of liquid separation. Benzenecompletereforming : 1=6C6 H6 þ 2H2 O ¼ CO2 þ 2:5H2 ; ΔH ¼ þ544:5kJ=mol:
ð10Þ
Carbon conversion and molar ratios of H2/CO and CO/CO2 of the experiments using different S/C molar ratios were shown in Table 4. The carbon conversion was increased from 46.30 to around 83.00% when the S/C molar ratio was increased to 1.5 and stabilized with the further increase of the S/C molar ratio to 3.0. The result was consistent to the hydrogen yield as shown in Fig. 5. The carbon conversion was increased from 46.30 to 83.84% when the S/C molar ratio was increased from 0.0 to 3.0. The result is in agreement with the results obtained by Li et al. [36] who has studied steam reforming of toluene over Ni/mayenite and found that a higher S/C molar ratio favored carbon conversion. When the S/C molar ratio was increased from 0.0 to 3.0, the H2/CO molar ratio was increased from 1.73 to 3.01 (shown in Table 4), and the CO/CO2 molar ratio was decreased significantly from 10.39 to 0.99. The large decrease of the molar ratio of CO/CO2 might be ascribed to the significant promotion of WGSR with the increase of S/C molar ratio.
3.3. Investigation of catalyst stability and characterizations of the fresh and reacted catalyst 3.3.1. Catalytic stability and reusability The above discussion highlighted the excellent catalytic activities of reforming of model tar compound (benzene) using a NiO/ceramic foam in terms of hydrogen production. It is reported that the stability and reusability of catalyst are important for catalytic steam reforming of tar
Table 3 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios at different oxygen equivalent ratios; Catalytic reforming conditions: ER increased from 0.0 to 0.4; temperature was 750 °C; S/C molar ratio was 1.0; and WHSV was 5.6 h−1.
Reaction temperature (°C)
700
750
800
850
900
ER
0.0
0.1
0.2
0.3
0.4
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
54.43 3.01 1.37
77.03 2.21 2.28
82.83 1.57 5.95
90.01 1.50 6.88
93.92 1.42 8.25
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
47.44 3.85 2.08
77.03 2.21 2.28
79.08 1.79 1.85
70.04 1.47 1.58
67.21 1.50 1.00
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Fig. 5. Effect of S/C molar ratio on the distribution of main product gases and H2 yields; Catalytic reforming conditions: S/C molar ratio increased from 0.0 to 3.0; temperature was 750 °C; ER was 0.1; and WHSV as 5.6 h−1.
[26]. In this work, a stability test was conducted at temperature of 750 °C, S/C molar ratio of 1.0, ER of 0.1, and WHSV of 5.6 h−1. During the stability test, the catalytic steam reforming of benzene was carried out for 150 min initially in the presence of the NiO/ceramic foam catalyst (Step I). Then the reacted catalyst was regenerated through the oxidation under air (100 mL min−1) for 5 min (Step II). The regenerated catalyst was tested for the reforming of benzene for another 85 min (Step III). A total of 240 min was used for the stability test, as shown in Fig. 6. It is demonstrated that the NiO/ceramic foam catalyst exhibited decreasing tendency of catalyst activity, as the concentration of H2 decreased from 59.48 to 56.25% after 150 min of reaction (Step I). It is suggested that the catalyst was deactivated slightly after 150 min reaction. The decreasing trend of the NiO/ceramic foam catalytic activity can be attributed to the deposition of carbon that covering the surface of the catalyst. The deactivation of a C12A7-O catalyst has been reported during catalytic steam reforming of bio-oil [37]. Carbon deposition on the surface of reacted catalyst is known as a key factor for the deactivation of catalyst during catalytic steam reforming process. Therefore, catalyst regeneration through the oxidation of the reacted catalyst using air was carried out in this work to remove the deposited carbons and regenerate the catalyst. From Fig. 6, for the Step III process using the regenerated catalyst, the concentration of H2 was returned back to around 60%, which is similar to the initial hydrogen production in the Step I. It is noted that the concentrations of CO2 and CH4 were stabilized at about 11.82% and 0.95%, respectively, during the whole 240 min test. It is therefore suggested that the regeneration of the reacted catalyst carried in this work is efficient to maintain the catalytic performance of the developed NiO/ceramic foam catalyst for the catalytic steam reforming of tar model compound (benzene). 3.3.2. SEM and TEM analysis The results of scanning electron microscope (SEM) of the fresh, reacted and regenerated catalysts are shown in Fig. 7. The structure of the ceramic foam support is shown in Fig. 7(a). It is found that the
Table 4 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios with different S/C molar ratios; Catalytic reforming conditions: S/C molar ratio increased from 0.0 to 3.0; temperature was 750 °C; ER was 0.1; and WHSV was 5.6 h−1. S/C
0.0
0.5
1.0
1.5
2.0
3.0
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
46.30 1.73 10.39
57.24 2.11 3.06
77.03 2.21 2.28
83.37 2.51 1.55
85.35 2.64 1.22
83.84 3.01 0.99
Fig. 6. The distribution of main product gases during the stability test ( : H2, : CO, : CO2, : CH4); Catalytic reforming conditions: reforming temperature was 750 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
surface of the ceramic foam is scraggy and multilayer, which is suitable for loading metals. Fig. 7(c) shows the presence of filamentous carbons deposited on the surface of the reacted catalyst after 150 min testing of benzene steam reforming (Step I, stability test). The amount of carbon formed on the surface of the reacted catalyst was about 1.02 g (after 150 min of reaction) by measure the weight of the difference of the fresh and reacted catalyst. The deposition of carbons on the surface of the catalyst is suggested to deactivate the catalyst, and this is confirmed from the reduction of hydrogen production, as shown in the Step I of the stability test (Fig. 6). From Fig. 7(d), the regenerated catalyst presents similar morphology compared to the fresh catalyst as shown in Fig. 7(b). Furthermore, it is suggested that the deposited carbons including filamentous carbons shown in Fig. 7(c) were oxidized during the regeneration step of the stability test (Step II), where air was introduced at temperature of 750 °C for 5 min. TEM characterization of the reacted catalyst after stability testing was carried out and the results are shown in Fig. 8. Tubular carbon nanotubes (CNTs) are clearly observed on the surface of the reacted NiO/ceramic foam catalyst (Fig. 8(a)). It is indicated that the deactivation of catalyst might be mainly due to carbon deposition on the surface of reacted catalyst reducing the availability of active sites (Ni). CNTs with narrow diameter distribution could be observed from TEM image (Fig. 8(b)), where CNTs are found with outer diameters of around 23 nm and inner diameter of around 9 nm. In addition, it can be found that the average particle diameter of catalysts is about 100 nm. Similar production of tubular CNTs has been obtained at temperature of 800 °C in the presence of a Ni-Mg-Al catalyst, when toluene were used as feedstock [38]. 3.4. Kinetic modeling In this study, a kinetic model is developed in the conditions where heat and mass transfer resistances are absent. The reactor with a limited gas expansion has been considered as plug flow (constant flow rate it is assumed) [12]. In addition, the fluid channeling and back mixing were not considered during the model development. The model of the process is described with the following equation: −r benzene ¼ kapp C nbenzene C m H2 O
ð11Þ
where rbenzene is the disappearance rate of benzene, kapp is the apparent
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Fig. 7. SEM images of the catalysts: (a) ceramic foam support, (b) fresh NiO/ceramic foam catalyst, (c) reacted catalyst after catalytic reforming of benzene after 150 min stability test (Step I, Fig. 6), (d) regenerated catalyst by air oxidation with a flow rate of 100 mL min−1 at 750 °C for 5 min (Step II, Fig. 6).
rate constant, Cbenzene is the concentration of benzene and CH2O is the concentration of H2O. According to previous studies [6,35,36], water concentration showed little influence on the conversion of benzene when the S/C value was higher than stoichiometric ratio. Therefore, we have assumed that the power-law type general equation (Eq. (11)) is a pseudo-order reaction with respect to only benzene concentration, which can be expressed as follow, assuming a first-order reaction [39,40]: −r benzene ¼ kapp C benzene :
ð12Þ
The temperature dependency on the rate constant of the kapp is determined by the Arrhenius equation. The apparent activation energy (Eapp) and apparent pre-exponential factor (A(k0 , app)) are calculated
from Eq. (13), derived from an integral plug flow reactor model [6]:
kapp ¼
−Eapp ½− ln ð1−X c Þ Wc ¼ k0;app e RT r ; τ ¼ ; τ v0
ð13Þ
where Xc is the carbon conversion equaled to benzene conversion in this study, τ is the space time, k0,app is the apparent pre-exponential factor, Eapp is the apparent activation energy, R is the universal gas constant, Tr is the reaction temperature, Wc is the catalyst weight and v0 is the gas flow rate. Based on the benzene conversion shown in Table 2, the calculated apparent activation energies using an Arrhenius plot (Fig. 9) is 73.38 kJ/mol and the pre-exponential factor is 1.18 × 105 (m3 kg−1catalyst h−1), which are in agreement with the values reported in the literature [41], where a study of catalytic upgrading of fuel gas was studied.
Fig. 8. TEM images of the reacted catalyst after reacting 150 min for catalytic steam reforming of benzene (Step I, Fig. 6): (a) image of the tubular carbon nanotubes; (b) Enlarged image of the tubular carbon nanotube in the circle of Fig. 8(a).
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Fig. 9. Arrhenius plot for the calculation of apparent activation energy. Catalytic reforming conditions: temperature increased from 700 to 900 °C, S/C molar ratio of 1.0, ER of 0.1, and WHSV of 5.6 h−1.
4. Conclusion A NiO/ceramic foam catalyst was evaluated for catalytic steam reforming of benzene (a model tar compound) using a fixed-bed reactor. The results showed that H2 yield and carbon conversion in catalytic reaction was much higher than the experiment using a ceramic foam without catalyst loading. The investigation of operating parameters (reaction temperature, ER and S/C molar ratio) showed that the yield of hydrogen increased from 140.67 to 182.06 (g H2 kg−1 benzene) when the temperature increased from 700 to 900 °C, and the yield of hydrogen increased from 102.38 to 191.65 (g H2 kg−1 benzene) when the S/C molar ratio increased from 0.0 to 3.0. With the increase of ER from 0.0 to 0.4, the H2 yield decreased from 179.70 to 77.39 (g H2 kg−1 benzene). The stability and reusability of the NiO/ceramic foam catalyst were studied by conducting a stability test. It was found that the NiO/ceramic foam catalyst was deactivated slightly in terms of the production of hydrogen after 150 min reaction, due to the deposition of carbons (as confirmed from the SEM and TEM analysis). The catalytic activity in terms of hydrogen production was return back to the initial level, when the reacted catalyst was regenerated using air at 750 °C for 5 min. A kinetic model with zero order reaction for water and one order reaction for benzene was proposed. The apparent activation energy and the preexponential factor are 73.38 kJ/mol and 1.18 × 105 (m3 kg−1catalyst h−1), respectively. Acknowledgement The authors would like to appreciate the support of the National Natural Science Foundation of China (NSFC) (51476023, 51306029) and Open Project of State Key Laboratory of Urban Water Resource and Environment (No. QA201532). References [1] T. Phuphuakrat, T. Namioka, K. Yoshikawa, Tar removal from biomass pyrolysis gas in two-step function of decomposition and adsorption, Appl. Energy 87 (2010) 2203–2211. [2] D. Mukai, S. Tochiya, Y. Murai, M. Imori, Y. Sugiura, Y. Sekine, Structure and activity of Ni/La0·7Sr0.3AlO3−δ catalyst for hydrogen production by steam reforming of toluene, Appl. Catal. A Gen. 464-465 (2013) 78–86. [3] C. Xu, J. Donald, E. Byambajav, Y. Ohtsuka, Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification, Fuel 89 (2010) 1784–1795. [4] C. Zuber, C. Hochenauer, T. Kienberger, Test of a hydrodesulfurization catalyst in a biomass tar removal process with catalytic steam reforming, Appl. Catal. B Environ. 156-157 (2014) 62–71. [5] M.M. Yung, W.S. Jablonski, K.A. Magrini-Bair, Review of catalytic conditioning of biomass-derived syngas, Energy Fuel 23 (2009) 1874–1884.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Research article
Hydrogen production from catalytic steam reforming of benzene as tar model compound of biomass gasification Ningbo Gao a,b,⁎,1, Xiao Wang b,1, Aimin Li b, Chunfei Wu c, Zhifan Yin b a b c
School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China School of Engineering, University of Hull, Hull HU6 7RX, UK
a r t i c l e
i n f o
Article history: Received 25 January 2016 Received in revised form 15 March 2016 Accepted 18 March 2016 Available online 2 April 2016 Keywords: Hydrogen production Catalyst Steam reforming Benzene Tar
a b s t r a c t Tar reduction is an important issue for the development of biomass gasification process. In this work, a NiO/ceramic foam catalyst was developed and studied for catalytic steam reforming of tar model compound (benzene) using a fixed-bed reactor. Different reaction temperatures, equivalent ratios (ER), and steam/carbon (S/C) molar ratios were investigated with a space velocity of 5.6 h−1. The introduction of the NiO/ceramic foam catalyst showed excellent production of hydrogen and carbon conversion. With the increase of reaction temperature from 700 to 900 °C, the yield of hydrogen increased from 140.67 to 182.06 (g H2 kg−1 benzene). The increase of ER resulted in the decrease of the H2 yield. A stability test (including regeneration of reacted catalyst) showed that the catalyst was deactivated by the deposition of carbons (confirmed from scanning electron microscopy), which could be removed using air oxidation at 750 °C. The catalytic activity of the catalyst in relation to the hydrogen production could be regained after the regeneration process. A kinetic model study of the process showed that the apparent activation energy and the pre-exponential factor were 73.38 kJ/mol and 1.18 × 105 (m3 kg−1catalyst h−1), respectively. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Tar, as a mixture of condensable organic components in gasification process, consists of aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene and phenol [1,2]. The condensation of tar at low temperature causes several technical problems such as coking catalysts, fouling, and choking downstream pipelines and equipment. These problems result in a reduction of total process efficiency and an increase of the costs of equipment management and maintenance [3–6]. Therefore, it is essential to find an efficient method to converting these tarry materials into valuable products. Catalytic steam reforming is an attractive technique for tar destruction since it produces hydrogen and carbon monoxide, which are high-value gas products [2,6,7]. Until now, several metal catalysts, including Fe [8,9], Ni [10–13], Co [11,14,15], Pt [5], Pd [16] or combinations of metals [7,17] have been studied by a large amount of publications, dedicating to develop efficient catalysts for tar steam reforming. In particular, Ni-based catalyst has been extensively investigated for tar conversion due to its wide availability, low cost and effective catalytic reactivity in cracking aromatic hydrocarbons [6,18–20]. ⁎ Corresponding author at: School of Energy and Power Engineering, Xi’'an Jiaotong University, Xi’'an 710049, China. E-mail address: [email protected] (N. Gao). 1 The first two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.fuproc.2016.03.019 0378-3820/© 2016 Elsevier B.V. All rights reserved.
Tomishige and Li developed several transition metal catalysts such as Rh, Ni, and Co and compound metal catalyst for systemic biomass tar and model compounds reforming. It is found that the Ni/Mg/Al, Ni-Cu/ Mg/Al and Co/Mg/Al catalysts exhibited much higher activity and resistance to coke deposition [21,22]. Alumina (Al2O3) has been used as a support of Ni-based catalyst by many studies [11,23]. However, Ni/ Al2O3 catalyst is known to be rapidly deactivated by carbon deposition on the surface of the catalyst, which can block the active sites, and result in the deactivation of catalyst [2,24,25]. Natural minerals have been used as supports such as dolomites and olivine for Ni-based catalysts [26]. However, due to their low specific surface area and easy abrasion at high temperature, tar reduction using catalysts with dolomite or olivine support is not efficient enough for the downstream applications of the produced gas. Ceramic foam has spherical-like cells connected to each other through openings or windows, and high porosities (85–90%), making it an attractive catalyst support [27]. It is noted that there are few reports of catalytic reforming of biomass gasification tar using Ni-based catalyst with ceramic foam as catalyst support. Since the ceramic foam has large porosity, high temperature stability, very low operation pressure drop and large surface area, it is worthwhile to investigate the catalytic performance of NiO/ceramic foam catalyst for steam reforming of biomass gasification tar compounds. The catalytic steam reforming of real biomass tar over NiO/ceramic foam catalyst was investigated in our previous studies [28]. Due to the complicated components of real biomass tar, it is difficult to perform
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mechanism studies. To better understand the reaction behaviors, model compound, such as phenol [29,30], benzene [31,32], toluene [6,25] or naphthalene [24,33] has been used for fundamental studies of catalytic tar steam reforming. Benzene was chosen as a tar model compound because it is a main constituent of high-temperature gasification tar, as well as it represents a stable aromatic structure in polyaromatics [28]. In addition, Josuinkas et al. [34]investigated the steam reforming of three tar model compounds, including benzene, toluene and naphthalene with nickel catalysts and it was reported that benzene is more reactive over a wide temperature range compared to toluene or naphthalene. In this study, the steam reforming of tar using benzene as a model compound was carried out over a NiO/ceramic foam catalyst in a fixed bed reaction system. Various process conditions including reaction temperature, equivalent ratio (ER) and steam/carbon (S/C) molar ratio were studied. Furthermore, particular attention was paid to the catalyst stability and reusability by carrying out a stability test (4 h) including a catalyst regeneration step, in relation to hydrogen production and carbon formation on the surface of the reacted catalysts. 2. Materials and methods 2.1. Catalyst preparation and characterization Nickel nitrate hexahydrate (Ni (NO3)2·6H2O) were purchased from Tianjin Kermel Company, China. Ceramic foam (Beisi New Materials Company, China) is a cylinder with diameters of 38 mm and lengths of 50 mm. The chemical compositions of ceramic foam are 79.24% Al2O3, 19.29% P2O5, 0.77% SiO2, which have been reported in our previous study [28]. Prior to the catalyst development, the ceramic foam was washed using hydrochloric acid solution (0.1 mol L−1) in order to eliminate the effect of the impurities on the experiments, and then dried at 110 °C for 3 h. An impregnation method was used to prepare the NiO/ceramic foam catalyst. Initially, certain amount of Ni (NO3)2·6H2O was dissolved completely into the deionized water to form a 1 mol L−1 of solution. The pre-treated ceramic foam was soaked into the aqueous solution for 1 h. Then, the Ni2 +/ceramic foam was dried in an oven at 105 °C for 12 h and calcined subsequently in a muffle furnace at 900 °C for 3 h at a heating rate of 7.5 °C min−1. The average amount of NiO loaded on the ceramic foam after calcination was around 3.50% ± 0.30% by weight. The morphology of the blank ceramic foam, the NiO/ceramic foam catalyst and the reacted catalyst were characterized using a Hitachi S-4800 scanning electron micrographs (SEM). A FEI Tecnai
381
G220 S-Twin transmission electron microscope (TEM) was used to observe the size and morphology of the reacted catalysts. 2.2. Reaction system and procedures The reaction system consists of a feeding system, a reactor, a product collector and a gas analysis system as shown in Fig. 1. Two double channel micro-infusion pumps (Smith Medical, Model WZS-50F6) were used to feed the pure benzene and water into the reactor, respectively. Mass flow meters (Sevenstar, Model D07-7B) were employed to provide accurate flows of nitrogen and air to the reaction system (air was used for providing oxygen to adjust the ER value and the regeneration of the reacted catalyst). The fixed-bed reactor is made of stainless steel (40 mm i.d. × 500 mm length). The reactor was heated by a tubular furnace (1.5 kW, 220 V), which was connected with a temperature controller (Yudian, Model AI-518P). The NiO/ceramic foam catalyst was placed in the middle of the reactor. The temperature of the reactor was measured by a K-type thermocouple with a precision of 1 °C. The products at the outlet of the reactor were recovered by a glass condenser and subsequently collected in a Tedlar gas bag for analyzing off-line by a gas chromatograph (Techcomp, Model 7890II), which is coupled with a thermal conductivity detector (TCD) to obtain the concentrations of H2, O2, N2, CO, CH4 and CO2. During the experiment, about 27 g ± 2 g catalyst was used. The whole system was guaranteed as inert using N2 as carrier gas with a flow rate of 100 mL min−1. Benzene was injected into the reactor at a feed flow rate of 5.3 g h−1, which is equivalent to a weight hourly space velocity (WHSV) of 5.6 h− 1. In this work, oxygen equivalent ratio (0.0–0.4), steam/carbon molar ratio (0.0–3.0), reaction temperature (700–900 °C) were investigated. Each test lasted 30 min. Experiments were repeated to ensure the reliability of the results. 2.3. Data analysis method H2 (CO, CO2 or CH4) concentration, H2 yield, Carbon conversion, H2/ CO and CO/CO2 molar ratio were calculated using the following Eqs. (1)–(5): H2 ðCO; CO2 or CH4 Þ concentration ð%Þ ¼
H2 ðCO; CO2 ; or CH4 Þ concentration in the product by GC The sum of the concentrations of H2 ; CO; CO2 and CH4 in the product by GC
ð1Þ
Fig. 1. The schematic diagram of a lab-scale catalytic steam reforming apparatus: (1) Air compressor; (2) Micro-infusion pump for benzene; (3) Micro-infusion pump for water; (4) Mass flow meter; (5) Nitrogen cylinder; (6) Tubular furnace; (7) Fixed-bed reactor; (8) NiO/ceramic foam catalyst; (9) Temperature controller; (10) K-type thermocouple; (11) Condenser. (12) Wet type gas meter; (13) Gas bag.
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Mass of hydrogen product −1 H2 yield g H2 kg benzene ¼ Mass of benzene fed Carbon conversion ð%Þ ¼
Moles of carbon in the gas product Moles of carbon fed 100
Moles of hydrogen product H2 =CO molar ratio ¼ Moles of carbon monoxide product CO=CO2 molar ratio ¼
Moles of carbon monoxide product : Moles of carbon dioxide product
ð2Þ
ð3Þ ð4Þ
Table 1 Experimental results during the catalytic reforming reaction of benzene with the ceramic foam (control experiment) and the NiO/ceramic foam catalyst; Catalytic reforming conditions: temperature was 750 °C; S/C molar ratio was 1.0; ER molar ratio was 0.1; and WHSV was 5.6 h−1. Catalyst −1
H2 yield (g H2 kg benzene) Carbon conversion (%) H2/CO molar ratio
Ceramic foam
NiO/ceramic foam
22.38 44.15 0.67
177.62 77.03 2.21
ð5Þ
3. Results and discussion 3.1. Comparison to control experiment without NiO loading The ceramic foam has metal impurities such as Fe2O3 and Na2O. In order to eliminate the effect of these impurities, control experiment using only ceramic foam support without NiO loading was carried out for benzene steam reforming at 750 °C, S/C molar ratio of 1.0, ER of 0.1 and WHSV of 5.6 h− 1. As shown in Fig. 2, the major components of the produced gases were H2, CO, CO2 and CH4. When the NiO/ceramic foam catalyst was used, H2 formation remarkably increased from 24.78 to 59.98% and CH4 concentration slightly increased from 0.72 to 0.96%, and CO and CO2 decreased from 36.93 to 27.15% and 37.57 to 11.91%, respectively, comparing to the control experiment. H2 yield, carbon conversion and H2/CO molar ratio from the experiments with and without NiO loading are shown in Table 1. H2 yield and carbon conversion were much higher in the presence of the NiO/ceramic foam compared to the control experiment. It can be explained that benzene and steam were adsorbed dissociatively on the surface of the NiO/ceramic foam resulting in the promoting of the catalytic reforming reactions. Therefore the high activity of the NiO/ceramic foam during benzene reforming process could be attributed to the excellent ability of oxygen transfer of the catalyst [32]. In addition, the ceramic foam support plays a significant role in increasing steam adsorption and surface carbon gasification rates during steam reforming reaction [12]. 3.2. Influences of process parameters Fig. 3 illustrated the distribution of main product gases (H2, CO, CO2 and CH4) and H2 yield with the increase of the reaction temperature
Fig. 2. Gas concentrations of the catalytic reforming reaction of benzene with the ceramic foam support and the NiO/ceramic foam catalyst; Catalytic reforming conditions: temperature was 750 °C; S/C molar ratio was 1.0; ER molar ratio was 0.1; and WHSV was 5.6 h−1.
from 700 to 900 °C, when the S/C molar ratio was 1.0 and the oxygen equivalent ratio (ER) was 0.1 during the catalytic reforming of benzene. The catalyst exhibited an excellent catalytic activity showing that H2 and CO was the dominant produced gases. The concentrations of H2 and CO2 decreased slightly from 62.69 to 55.75% and 15.15 to 4.76%, respectively, when the reaction temperature was increased from 700 to 900 °C. However, carbon monoxide showed an opposite trend that its concentration increased from 20.81 to 39.28%. It is indicated that benzene was favor to be decomposed at high temperature. It is reported that water-gas shift reaction (WGSR) (Reaction (7)) and steam reforming reactions were promoted when the reaction temperature was higher than 700 °C [35]. Park et al. [32] found that an appropriate benzene conversion temperature was above 700 °C, which was efficient for tar reduction using a Ni-based catalyst. High temperature facilitates endothermic reactions; however, at too high reaction temperature, it is easy to form carbidic carbon, which is a precursor of graphitic carbon resulting in the deactivation of catalyst. Furthermore, partial oxidation of benzene (Reaction (6)) and WGSR (Reaction (7)) are inhibited with the increase of reaction temperature due to both reactions are exothermic. Partialoxidization : 2C6 H6 þ 9O2 ¼ 6H2 O þ 12CO; ΔH ¼ −3267:5kJ=mol
ð6Þ
Water gasshift : CO þ H2 O ¼ CO2 þ H2 ; ΔH ¼ −41kJ=mol:
ð7Þ
Thus, hydrogen concentration was slightly reduced with the increase of temperature from 700 to 900 °C, as shown in Fig. 3. In addition, it was noted that because of high heat capacity and large surface area of the ceramic foam, the temperature of the reaction field could be
Fig. 3. Effect of reaction temperature on the distribution of main product gases and H2 yields; Catalytic reforming conditions: temperature increased from 700 to 900 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
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maintained in a stable condition, which could avoid the fluctuation of the reaction temperature during catalytic reactions [28]. Carbon conversion, H2/CO and CO/CO2 molar ratios from the experiments with different reaction temperatures were shown in Table 2. It is indicated that the carbon conversion of benzene was significantly increased from 54.43 to 93.92% when the temperature was increased from 700 to 900 °C. The hydrogen yield and carbon conversion reached maximum values of 182.06 (g H2 kg−1 benzene) and 93.92% at 900 °C, respectively. However, the H2/CO molar ratio was decreased with the increase of reaction temperature, due to the increase of the concentration of CO when the reaction temperature was high than 750 °C. These results are consistent with the data reported by Josuinkas et al. [34], who investigated steam reforming of benzene and toluene with a Ni catalyst. They reported that the concentrations of small molar gases were significantly affected by reaction temperature. The equivalent ratio (ER) is one of the key factors in the process of tar reforming. In this work, the effect of ER on product distribution and H2 yield was investigated by varying ER ratios from 0.0 to 0.4 by adjusting the flow rate of air. As shown in Fig. 4, the concentration of H2 declined dramatically from 71.11 to 42.81% when the ER was increased from 0.0 to 0.4, and a decrease of the concentration of CH4 was also observed with the increase of the ER. However, CO2 concentration was increased from 8.89 to 28.65% when the ER was increased from 0.0 to 0.4. The content of CO showed a maximum value of 32.17% at ER of 0.3. Due to the promoted combustion reactions with the increase of the ER, H2 content declined significantly. For example, the H2 yield decreased dramatically from 179.70 to 77.39 (g H2 kg−1 benzene) when the ER was increased from 0.0 to 0.4. As shown by Reactions (8) and (9), the formations of CO, H2O and CO2 were favored at high ER ratio due to the promoting of the oxidization of benzene, CO and CH4. In addition, H2 can be easily consumed with the increase of oxygen content in the process. It is noted that the reactor temperature can be increased with the increase of ER due to the exothermic reactions. However, higher ER leads to the promotion of CO oxidation, resulting in the decrease of CO concentration when the ER was increased to 0.4 (Fig. 4). Dryreforming : C6 H6 þ 6CO2 ¼ 12CO þ 3H2 ; ΔH ¼ þ953:6kJ=mol
ð8Þ
Boudouard : CO2 þ C ¼ 2CO; ΔH ¼ þ172kJ=mol:
ð9Þ
When the ER was changed from 0.0 to 0.4, the carbon conversion increased from 47.44 to 67.21%. Carbon conversion reached a maximum of 79.08% when the ER was 0.2. The variation in product with varying ER ratios at constant temperature is shown in Table 3. This is because that the increased ER caused more H2O content in the product stream. The catalytic effect of S/C molar ratio on product gas composition was investigated for the reforming of tar model compound, when the steam reforming temperature was 750 °C and the ER was 0.1. As shown in Fig. 5, the S/C molar ratio has significant influence on the catalytic activity. For example, the content of CO decreased from 34.03 to 19.83%, while CO2 increased from 3.27 to 20.09%, when the S/C molar ratio was increased from 0.0 to 3.0, however, the concentration of hydrogen was stabilized around 59%. The yield of H2 increased from
Table 2 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios at different reaction temperatures; Catalytic reforming conditions: temperature increased from 700 to 900 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
383
Fig. 4. Effect of oxygen equivalent ratio on the distribution of main product gases and H2 yields; Catalytic reforming conditions: ER increased from 0.0 to 0.4; temperature was 750 °C; S/C molar ratio was 1.0; and WHSV was 5.6 h−1.
102.38 to 192.36 (g H2 per kg benzene) when the S/C molar ratio was increased from 0.0 to 1.5. The hydrogen yield was kept stable with the further increase of S/C molar ratio to 3.0. It is suggested that the increasing S/C molar ratio leads to the promotions of Reactions (10) and (7) favoring the production of H2. Although excess water content promotes the production of hydrogen, too high S/C molar ratio is not encouraged in industrialization process, due to the sintering possibility of the active metal sites on the catalyst, the high energy inputs for steam generation and the associated cost of liquid separation. Benzenecompletereforming : 1=6C6 H6 þ 2H2 O ¼ CO2 þ 2:5H2 ; ΔH ¼ þ544:5kJ=mol:
ð10Þ
Carbon conversion and molar ratios of H2/CO and CO/CO2 of the experiments using different S/C molar ratios were shown in Table 4. The carbon conversion was increased from 46.30 to around 83.00% when the S/C molar ratio was increased to 1.5 and stabilized with the further increase of the S/C molar ratio to 3.0. The result was consistent to the hydrogen yield as shown in Fig. 5. The carbon conversion was increased from 46.30 to 83.84% when the S/C molar ratio was increased from 0.0 to 3.0. The result is in agreement with the results obtained by Li et al. [36] who has studied steam reforming of toluene over Ni/mayenite and found that a higher S/C molar ratio favored carbon conversion. When the S/C molar ratio was increased from 0.0 to 3.0, the H2/CO molar ratio was increased from 1.73 to 3.01 (shown in Table 4), and the CO/CO2 molar ratio was decreased significantly from 10.39 to 0.99. The large decrease of the molar ratio of CO/CO2 might be ascribed to the significant promotion of WGSR with the increase of S/C molar ratio.
3.3. Investigation of catalyst stability and characterizations of the fresh and reacted catalyst 3.3.1. Catalytic stability and reusability The above discussion highlighted the excellent catalytic activities of reforming of model tar compound (benzene) using a NiO/ceramic foam in terms of hydrogen production. It is reported that the stability and reusability of catalyst are important for catalytic steam reforming of tar
Table 3 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios at different oxygen equivalent ratios; Catalytic reforming conditions: ER increased from 0.0 to 0.4; temperature was 750 °C; S/C molar ratio was 1.0; and WHSV was 5.6 h−1.
Reaction temperature (°C)
700
750
800
850
900
ER
0.0
0.1
0.2
0.3
0.4
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
54.43 3.01 1.37
77.03 2.21 2.28
82.83 1.57 5.95
90.01 1.50 6.88
93.92 1.42 8.25
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
47.44 3.85 2.08
77.03 2.21 2.28
79.08 1.79 1.85
70.04 1.47 1.58
67.21 1.50 1.00
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Fig. 5. Effect of S/C molar ratio on the distribution of main product gases and H2 yields; Catalytic reforming conditions: S/C molar ratio increased from 0.0 to 3.0; temperature was 750 °C; ER was 0.1; and WHSV as 5.6 h−1.
[26]. In this work, a stability test was conducted at temperature of 750 °C, S/C molar ratio of 1.0, ER of 0.1, and WHSV of 5.6 h−1. During the stability test, the catalytic steam reforming of benzene was carried out for 150 min initially in the presence of the NiO/ceramic foam catalyst (Step I). Then the reacted catalyst was regenerated through the oxidation under air (100 mL min−1) for 5 min (Step II). The regenerated catalyst was tested for the reforming of benzene for another 85 min (Step III). A total of 240 min was used for the stability test, as shown in Fig. 6. It is demonstrated that the NiO/ceramic foam catalyst exhibited decreasing tendency of catalyst activity, as the concentration of H2 decreased from 59.48 to 56.25% after 150 min of reaction (Step I). It is suggested that the catalyst was deactivated slightly after 150 min reaction. The decreasing trend of the NiO/ceramic foam catalytic activity can be attributed to the deposition of carbon that covering the surface of the catalyst. The deactivation of a C12A7-O catalyst has been reported during catalytic steam reforming of bio-oil [37]. Carbon deposition on the surface of reacted catalyst is known as a key factor for the deactivation of catalyst during catalytic steam reforming process. Therefore, catalyst regeneration through the oxidation of the reacted catalyst using air was carried out in this work to remove the deposited carbons and regenerate the catalyst. From Fig. 6, for the Step III process using the regenerated catalyst, the concentration of H2 was returned back to around 60%, which is similar to the initial hydrogen production in the Step I. It is noted that the concentrations of CO2 and CH4 were stabilized at about 11.82% and 0.95%, respectively, during the whole 240 min test. It is therefore suggested that the regeneration of the reacted catalyst carried in this work is efficient to maintain the catalytic performance of the developed NiO/ceramic foam catalyst for the catalytic steam reforming of tar model compound (benzene). 3.3.2. SEM and TEM analysis The results of scanning electron microscope (SEM) of the fresh, reacted and regenerated catalysts are shown in Fig. 7. The structure of the ceramic foam support is shown in Fig. 7(a). It is found that the
Table 4 Experimental results of carbon conversion, H2/CO and CO/CO2 molar ratios with different S/C molar ratios; Catalytic reforming conditions: S/C molar ratio increased from 0.0 to 3.0; temperature was 750 °C; ER was 0.1; and WHSV was 5.6 h−1. S/C
0.0
0.5
1.0
1.5
2.0
3.0
Carbon conversion (%) H2/CO molar ratio CO/CO2 molar ratio
46.30 1.73 10.39
57.24 2.11 3.06
77.03 2.21 2.28
83.37 2.51 1.55
85.35 2.64 1.22
83.84 3.01 0.99
Fig. 6. The distribution of main product gases during the stability test ( : H2, : CO, : CO2, : CH4); Catalytic reforming conditions: reforming temperature was 750 °C; S/C molar ratio was 1.0; ER was 0.1; and WHSV was 5.6 h−1.
surface of the ceramic foam is scraggy and multilayer, which is suitable for loading metals. Fig. 7(c) shows the presence of filamentous carbons deposited on the surface of the reacted catalyst after 150 min testing of benzene steam reforming (Step I, stability test). The amount of carbon formed on the surface of the reacted catalyst was about 1.02 g (after 150 min of reaction) by measure the weight of the difference of the fresh and reacted catalyst. The deposition of carbons on the surface of the catalyst is suggested to deactivate the catalyst, and this is confirmed from the reduction of hydrogen production, as shown in the Step I of the stability test (Fig. 6). From Fig. 7(d), the regenerated catalyst presents similar morphology compared to the fresh catalyst as shown in Fig. 7(b). Furthermore, it is suggested that the deposited carbons including filamentous carbons shown in Fig. 7(c) were oxidized during the regeneration step of the stability test (Step II), where air was introduced at temperature of 750 °C for 5 min. TEM characterization of the reacted catalyst after stability testing was carried out and the results are shown in Fig. 8. Tubular carbon nanotubes (CNTs) are clearly observed on the surface of the reacted NiO/ceramic foam catalyst (Fig. 8(a)). It is indicated that the deactivation of catalyst might be mainly due to carbon deposition on the surface of reacted catalyst reducing the availability of active sites (Ni). CNTs with narrow diameter distribution could be observed from TEM image (Fig. 8(b)), where CNTs are found with outer diameters of around 23 nm and inner diameter of around 9 nm. In addition, it can be found that the average particle diameter of catalysts is about 100 nm. Similar production of tubular CNTs has been obtained at temperature of 800 °C in the presence of a Ni-Mg-Al catalyst, when toluene were used as feedstock [38]. 3.4. Kinetic modeling In this study, a kinetic model is developed in the conditions where heat and mass transfer resistances are absent. The reactor with a limited gas expansion has been considered as plug flow (constant flow rate it is assumed) [12]. In addition, the fluid channeling and back mixing were not considered during the model development. The model of the process is described with the following equation: −r benzene ¼ kapp C nbenzene C m H2 O
ð11Þ
where rbenzene is the disappearance rate of benzene, kapp is the apparent
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Fig. 7. SEM images of the catalysts: (a) ceramic foam support, (b) fresh NiO/ceramic foam catalyst, (c) reacted catalyst after catalytic reforming of benzene after 150 min stability test (Step I, Fig. 6), (d) regenerated catalyst by air oxidation with a flow rate of 100 mL min−1 at 750 °C for 5 min (Step II, Fig. 6).
rate constant, Cbenzene is the concentration of benzene and CH2O is the concentration of H2O. According to previous studies [6,35,36], water concentration showed little influence on the conversion of benzene when the S/C value was higher than stoichiometric ratio. Therefore, we have assumed that the power-law type general equation (Eq. (11)) is a pseudo-order reaction with respect to only benzene concentration, which can be expressed as follow, assuming a first-order reaction [39,40]: −r benzene ¼ kapp C benzene :
ð12Þ
The temperature dependency on the rate constant of the kapp is determined by the Arrhenius equation. The apparent activation energy (Eapp) and apparent pre-exponential factor (A(k0 , app)) are calculated
from Eq. (13), derived from an integral plug flow reactor model [6]:
kapp ¼
−Eapp ½− ln ð1−X c Þ Wc ¼ k0;app e RT r ; τ ¼ ; τ v0
ð13Þ
where Xc is the carbon conversion equaled to benzene conversion in this study, τ is the space time, k0,app is the apparent pre-exponential factor, Eapp is the apparent activation energy, R is the universal gas constant, Tr is the reaction temperature, Wc is the catalyst weight and v0 is the gas flow rate. Based on the benzene conversion shown in Table 2, the calculated apparent activation energies using an Arrhenius plot (Fig. 9) is 73.38 kJ/mol and the pre-exponential factor is 1.18 × 105 (m3 kg−1catalyst h−1), which are in agreement with the values reported in the literature [41], where a study of catalytic upgrading of fuel gas was studied.
Fig. 8. TEM images of the reacted catalyst after reacting 150 min for catalytic steam reforming of benzene (Step I, Fig. 6): (a) image of the tubular carbon nanotubes; (b) Enlarged image of the tubular carbon nanotube in the circle of Fig. 8(a).
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Fig. 9. Arrhenius plot for the calculation of apparent activation energy. Catalytic reforming conditions: temperature increased from 700 to 900 °C, S/C molar ratio of 1.0, ER of 0.1, and WHSV of 5.6 h−1.
4. Conclusion A NiO/ceramic foam catalyst was evaluated for catalytic steam reforming of benzene (a model tar compound) using a fixed-bed reactor. The results showed that H2 yield and carbon conversion in catalytic reaction was much higher than the experiment using a ceramic foam without catalyst loading. The investigation of operating parameters (reaction temperature, ER and S/C molar ratio) showed that the yield of hydrogen increased from 140.67 to 182.06 (g H2 kg−1 benzene) when the temperature increased from 700 to 900 °C, and the yield of hydrogen increased from 102.38 to 191.65 (g H2 kg−1 benzene) when the S/C molar ratio increased from 0.0 to 3.0. With the increase of ER from 0.0 to 0.4, the H2 yield decreased from 179.70 to 77.39 (g H2 kg−1 benzene). The stability and reusability of the NiO/ceramic foam catalyst were studied by conducting a stability test. It was found that the NiO/ceramic foam catalyst was deactivated slightly in terms of the production of hydrogen after 150 min reaction, due to the deposition of carbons (as confirmed from the SEM and TEM analysis). The catalytic activity in terms of hydrogen production was return back to the initial level, when the reacted catalyst was regenerated using air at 750 °C for 5 min. A kinetic model with zero order reaction for water and one order reaction for benzene was proposed. The apparent activation energy and the preexponential factor are 73.38 kJ/mol and 1.18 × 105 (m3 kg−1catalyst h−1), respectively. Acknowledgement The authors would like to appreciate the support of the National Natural Science Foundation of China (NSFC) (51476023, 51306029) and Open Project of State Key Laboratory of Urban Water Resource and Environment (No. QA201532). References [1] T. Phuphuakrat, T. Namioka, K. Yoshikawa, Tar removal from biomass pyrolysis gas in two-step function of decomposition and adsorption, Appl. Energy 87 (2010) 2203–2211. [2] D. Mukai, S. Tochiya, Y. Murai, M. Imori, Y. Sugiura, Y. Sekine, Structure and activity of Ni/La0·7Sr0.3AlO3−δ catalyst for hydrogen production by steam reforming of toluene, Appl. Catal. A Gen. 464-465 (2013) 78–86. [3] C. Xu, J. Donald, E. Byambajav, Y. Ohtsuka, Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification, Fuel 89 (2010) 1784–1795. [4] C. Zuber, C. Hochenauer, T. Kienberger, Test of a hydrodesulfurization catalyst in a biomass tar removal process with catalytic steam reforming, Appl. Catal. B Environ. 156-157 (2014) 62–71. [5] M.M. Yung, W.S. Jablonski, K.A. Magrini-Bair, Review of catalytic conditioning of biomass-derived syngas, Energy Fuel 23 (2009) 1874–1884.
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